A brown textured background. The D. H. Hill Library A circular emblem with a design of trees and a building. North Carolina State College QK47 S6 SE 49 S6 Date Due
4F26 NOV 10 1965
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5869 10 BOTANY PRINCIPLES AND PROBLEMS McGRAW-HILL AGRICULTURAL AND BIOLOGICAL PUBLICATIONS CHARLES V. PIPER, CONSULTING EDITOR 1. THE LAW OF CHANGE IN RELATION TO AGRICULTURE JAMES C. CARTER 2. THE LABORATORY MANUAL HENRY LE BOURG 3. PRINCIPLES OF ANIMAL BIOLOGY EDWARD W. HARRISON 4. LABORATORY DIRECTIONS IN PRINCIPLES OF ANIMAL BIOLOGY EDWARD W. HARRISON 5. CHEMISTRY OF PLANT LIFE EDWARD W. HARRISON 6. BREEDING CROP PLANTS EDWARD W. HARRISON 7. AN INTRODUCTION TO CYTOLOGY FREDERICK E. BURTON 8. APPLIED ENTOMOLOGY JAMES C. CARTER 9. FARM MANAGEMENT JAMES C. CARTER 10. THE PHYSIOLOGY OF FRUIT PRODUCTION JAMES C. CARTER 11. LABORATORY MANUAL OF FRUIT AND VEGETABLE PRODUCTS JAMES C. CARTER 12. THE SOYBEAN JAMES C. CARTER 13. THE BEGINNINGS OF AGRICULTURE IN AMERICA JAMES C. CARTER 14. TEXTBOOK OF AGRICULTURAL BACTERIOLOGY JAMES C. CARTER 15. VEGETABLE CROPS JAMES C. CARTER 3 A forest scene with various plants and trees. All four main divisions of the plant kingdom are represented in this picture. (Photograph) BOTANY PRINCIPLES AND PROBLEMS BY EDMUND W. SINNOTT Professor of Botany, Connecticut Agricultural College FIRST EDITION McGRAW-HILL BOOK COMPANY, INC. NEW YORK: 379 SEVENTH AVENUE LONDON: 6 & 8 Bouverie St., E. C. 4 1923 Handwritten note: 8/14/49 156 Copyright, 1923, by the McGraw-Hill Book Company, Inc. Printed in the United States of America THE MAPLE PRESS - YORK PA To My Mother PROPERTY LIBRARY N. C. State College 9869 A blank, light peach-colored page. PREFACE The present volume, an outgrowth of experience in presenting to college freshmen a course in elementary botany, endeavors to set forth the facts concerning the morphology, physiology and classification of plants, and to provide a body of problem material which may be of assistance in stimulating thought and in provoking class discussion. The consideration of structure and function in the earlier portions of this book is confined to the higher plants, and the distinctive characteristics of the other members of the plant kingdom are discussed in the last five chapters. Should the course be continued beyond the present volume, Chapter XIII, which deals with the main events in the history of the plant kingdom and the important features of its various divisions, may prove useful in providing a background for consideration as a whole. In view of the importance of the soil in the life of plants, and in order to emphasize the fact that living things cannot be understood without reference to their environment, an entire chapter has been devoted entirely to the soil itself. The increased interest in matters pertaining to inheritance has warranted a special chapter on heredity and variation, and a chapter is also devoted to the study of plant diseases, both general and specific aspects. The chapters have been so written as to be separately understandable and may readily be taken up in some other order than that indicated by their arrangement. This book was not primarily designed for agricultural students but many of the questions naturally involve a practical application of botanical principles, and it is hoped that those interested in botany will apply themselves to these questions with especial facility. The rather extensive list of "Questions for Thought and Discussion" at the end of each chapter is intended to result from the present volume, have resulted from an attempt to stimulate within the student an attitude of interest, of curiosity and of critical judgment. It is hoped that they will serve as a stimulus present, and thus to provide him with a clearer insight into the way in which plants are constructed and function, and a firmer ix X PREFACE command of botanical science in general, than can be given him merely through a series of lectures and recitations. Anyone who has been in the habit of teaching botany knows what he has at his command a reasonably broad body of experience, ought to be able, through the exercise of a little thought, to prepare satisfactory answers to the questions which are "Reference Problems" are by no means new in botanical pedagogy, but it is believed that no previous text has employed them to quite such an extent as does the present one. It is hoped that their inclusion here may encourage students to think more deeply and more satisfactorily than the common but somewhat outworn practice where-in a monopoly of thinking and talking is enjoyed by the instructor, while the student is left to follow along with a series of what often seem to him to be unrelated facts. The "Reference Problems" are designed to send the student occasion- ally to consult the text, and thus to make use of it. They are thus to broaden his point of view and distil from his mind the com- fortable assurance that any particular authority may be infallible. To all those who have contributed in any way to the preparation of the book the author desires to express his sincere thanks. Many of his colleagues have contributed helpful suggestions and information during the course of this work. Professor C. H. Turrey and Professor G. S. Turrey is he especially indebted for advice and assistance during the course of the work. Professor Torrey and Dr. L. C. Torrey have kindly given permission to quote certain portions of the manuscript. A. L. Weinsteins has been helpful in many ways. Professor M. L. Fernand of the Gray Herbarium kindly supplied the data used in Figs. 132 and 133. To Professor W. M. Keen, Professor J. H. Bower, and Professor Pen- jassor J. W. Harsberger of the University of Pennsylvania the author is under obligation for their courtesy in providing material and facilities for making illustrations for this work. He is also much indebted to his wife for frequent assistance during the preparation of text and illustrations. The original drawings for this work were original. They are the work of several individuals, to whom the author is grateful for hearty cooperation. Mrs. Grace Graham Hooking is responsi- ble for Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, PREFACE xi 180, 190, 191, 193, 209, 211, and 213; Mort Uyehara for Figs. 154 and 155; E. J. Shantz for Fig. 18, Mr. H. L. Weinstein for Fig. 19, and Professor W. F. Ganong for Figs. 111 and 112 from his unpublished work. The other original drawings, some ninety in all, are by the author. He is indebted to Professor W. F. Ganong and Henry Holt and Co. for the use of two figures from "The Living Plant" (Holt); to the United States Department of the Interior's Sons and Co. for the use of four figures from "The Fundamentals of Botany". To the United States Forest Service, the United States Forest Products Laboratory, the New York Botanical Garden, the editors of the Journal of Heredity and of Genetics, Dr. A. J. Grout, Professors E. B. Balcock and R. E. Clausen, Dr. M. A. Howe, Professor C. H. Blackman, Professor G. S. Torrey, Drs. M. L. Fernand, and Professor G. S. Torrey the author is indebted for original illustrations or permission to use published ones. A few familiar figures have also been taken from some of the older texts. The author will be very glad to welcome criticisms and suggestions, especially those that relate to the arrangement of the Questions and Problems in the conduct of class work. CONNECTICUT AGRICULTURAL COLLEGE July, 1923 E. W. SINNOTT A plain, light beige background. CONTENTS | Page | Text | |------|------| | 1 | Preface | | 1 | To the Student | | xxii | xxiii | CHAPTER I The Science of Botany The subdivisions of botany—The history of botany. CHAPTER II Introductory Survey The plant kingdom—The structure and functions of plants—the root and its functions—the leaf and its functions—the stem and its functions—the reproductive organs and their function—Metabolic processes. CHAPTER III The Soil and Its Influence on Plants. Basic particles—Water—air—Organic matter—Dissolved substances—Organisms. CHAPTER IV The Root and Its Functions Excretion—Respiration—The absorbing region—the plant cell—Internal structure of roots—Diffusion and osmosis—Diffusion and osmosis in the plant cell—The absorption of water and salts—Other chemical phenomena in the root—Other functions of the root. CHAPTER V The Leaf and Its Functions The structure of the leaf—Photosynthesis—Transpiration. CHAPTER VI The Stem and Its Functions The external structure of the stem—the internal structure of the stem—the growth of wood—the mesocarp in stems—Translocation of food. CHAPTER VII Metabolism Plant foods—Digestion—Anaerobic respiration. xii xiv CONTENTS CHAPTER VIII Growth—The production of new cells—Growing-points and their function—Terminal growing-point—Lateral growing-point or cambium—Differentiation. 138 CHAPTER IX The Plant and Its Environment—Sunlight and response—Temperature-Light-Gravity-Moisture—Chemical substances—Living organisms. CHAPTER X Reproduction—Sexual reproduction—Pollination—Fertilization—Seed development—The fruit—Seed dispersal—Seed germination. CHAPTER XI HEREDITY AND VARIATION—Heredity—Variation—Law of inheritance—inheritance of acquired characters—Inheritance of acquired characters by mutation—Dominance-Segregation-Independent assortment-Spontaneous mutations. CHAPTER XII EVOLUTION—Evolution for evolution-Lamarck's theory-Darwin's theory-Natural Selections-De Vries' theory-Organoism. CHAPTER XIII The Plant Kingdom—Plant life—Phylogenetic stages in plant evolution-1. The multicellular plant-2. Differentiation-3. Sexual reproduction-4. Alteration of generality-5. The invasion of the land-6. The evolution of the seed and its functions-Nonembolism. CHAPTER XIV The Thallophyta—Cyanophyceae or blue-green algae-Chlorophyceae or green algae-Fungi-Protozoa or lower animals-Bacteria or red algae-Bacteria-Phycomycetes or algae-like fungi-Amycetes or sea fungi-Bacillariophyceae or diatoms-Lichens. CHAPTER XV The Eucaryota—Alterations of generation-Multicellular sexual organs-Hepatic vascular system-Mutus or mosses CONTENTS CHAPTER XVI The Pteridophyta. Page 325 The advance from bryophytes to pteridophytes—Filicinaceae or ferns—Lycopodiaceae or club-mosses—Equisetaceae or horsetails. CHAPTER XVII The Spermatophyta. Page 514 The origin of the seed—The advance from pteridophytes to seed plants—Pinophyta—Gymnosperms or gymnospermous spermatous or angiosperms—Dicotyledonous or dicotyledonous—Monocotyledonous or monocotyledonous. Index Page 371 [API_EMPTY_RESPONSE] TO THE STUDENT Botany has sometimes been spoken of rather lightly as the "feminine" science, and indeed in the minds of many people it is chiefly associated with pleasant flowers, with the plants, and with the animals, of which we see quaint individuals who wander aimlessly in the woods, or eat quantities of food which they cannot possibly digest. The use of botany is wholly inadequate, a study of the following pages should simply prove. Plants are far more than interesting playthings. They are a source of food, clothing, fuel, shelter, and many other necessities. More important still, they are also sources of knowledge. The knowledge which has always made all living things eagerly stalked by man, not only for their own sake but for the light which they throw on many of the mysteries of life, is largely due to the understand- ing of our surroundings, surely no one should pretend to be educated well who is unfamiliar with plants and their activities. A college student who has never seen a plant is a technologist, however, ought to do something more than give familiarity with a mass of facts, what matter how important they may be. It should develop in us a scientific attitude—a both critical and inqui- sitive. It should enable us to find our way to the truth through a mass of facts. This is the attitude which distinguishes a truly educated man from one who is merely well-informed. A man who is well-inform- ed, and a lack of it is responsible for much of the loose thinking and false reasoning with regard to biological problems which is in- liable to him. Let us see what this attitude demands and how it may best be attained. We are all obliged to acquire a mass of information if we are to be able to think clearly about any problem. A mass of the various things which we eat and wear, the operation of countless devices which we use, the rules and habits of other people, the are... xviii TO THE STUDENT manifest concerns of our particular occupations, together with countless other facts are the same in the world. We are continually asking us "what?" "where?" "when?" and "how?"; and we are continually answering it as best we may. For most people the ability to reply successfully to these questions is sufficient to satisfy their curiosity. But this is not true in elementary botany, such would be content merely to learn the facts about plants so that they might pass the inevitable examina- tion. They do not ask why plants grow, nor how they are supplied thus to take everything for granted, as a lesson to be learned. They want to ask the world a question in their turn, and their questing spirit will not be satisfied until they have not only merely knowledge but understanding. They want to penetrate the array of facts to the laws upon which these facts rest. To this honesty of inquiry they owe much of their success. The adven- turers, all discoverers and explorers, all who have helped to bring mankind upward from thoughtless savagery to rational civili- zation, have been men of curiosity. Some of them have been even persecuted for their presumptuous curiosity, but they have persisted through the ages in that insatiable inquisitiveness which has led them to ask questions which others would not accept it; to penetrate into things rather than take all for granted. Their spirit animates every true scientist and every really educated man today. But the scientific attitude is not mere inquisitiveness. Brutes often have plenty of that. Many people are idle curious and will accept any explanation offered them. The scientist must go beyond this. He must ask himself why he asks what he asks him when he asks "why?" he must be able to discriminate, to separate the true from the false. He must weigh accurately and without prejudice the evidence before him, and when he has put forward to answer his questions. He must remain skeptical and unconvincned until adequate proof is at hand. He must satisfy his reason by reasoning, and not by accepting what is "good." This attitude of critical curiosity is hard to gain and still harder to maintain actively. It is a useful practice, particularly when one is young, but it is easy to become bored and feed ourselves continually with questions and problems which it brings up, not merely accepting someone else's assertion but trying to find out whether it is true or false. When we are all men, should then learn to be his own Socrates! It is for this TO THE STUDENT purpose that the list of "Questions for Thought and Discussion," which will be found at the end of each chapter of the following text, has been gathered. The student can try his hand at these, and if he has first mastered the comparatively few facts presented in the text he should have no great difficulty. They refer to some of the most important subjects. Others may be harder to find but will be worth more in the finding. By frequent practice in this sort of exercise he will not only develop and keep sharp his curiosity, but also learn how to apply his knowledge to the most important ends for him to gain in such a course as this—but he will acquire almost unconsciously a far more thorough understanding of the subject than could be gained by merely attempting to memorize a list of facts. The tonic of curiosity and the fresh air of skepticism are sovereign aids for maintaining interest and the rate of health and vigor in which they can steadily assimilate a rich diet of knowledge without becoming sluggish and over-fed. six A light beige background with a subtle gradient effect. BOTANY: PRINCIPLES AND PROBLEMS CHAPTER 1 THE SCIENCE OF BOTANY Botany may be defined as that field of precise and classified knowledge—that science, in other words—which deals with plants. Together with the members of the animal kingdom, plants are distinguished from all other objects by the fact that they are alive. Botany and Zoology, the sciences which treat of these two great groups of living things, are therefore closely related to one another, and many of their problems are common to both; and together they constitute the broader scientific field of Biology, which is concerned with the study of Life in its various manifestations. The study of plants is perhaps the least understood and the most important. Between lifeless things and living things exists a great gap which, despite the efforts of botanists, has not yet been bridged. Inanimate machines and complex chemical substances are familiar products of manufacture, but never have we succeeded in producing anything comparable to the evolving material world. The sun's rays strike us, multiply and die before our eyes, but as to many of the processes which take place behind these outward activities we are still in almost complete ignorance. We may describe the sun as a ball of fire, but what is burning? Living things are not fixed and constant in their characteristics but have undergone profound changes, from simpler beginnings in ages long past to the complex organisms which we see around us today; but what has been their origin, or how and why they have progressed to their present condition, are questions for the future to answer. What is the meaning of life? What is the purpose in life which always has keenly stimulated the curiosity of man; and 1 2 BOTANY: PRINCIPLES AND PROBLEMS since he himself is a living being, much of his speculation and philosophy has been directed to the study of that product which, in a broad sense, are biological in their nature. The task of the biologist in extending the field of our scientific knowledge of plants and animals therefore assumes added significance from its relation to some of the most important problems of human life. Man is confronted by an integral part of the science of biology, botany has already made notable contributions to our knowledge of living things. It is this fact which has given us considerable curiosity toward Nature which has distinguished him as a thinking being, he will always in some measure be a student of plants. Aside from the practical importance of plants, it is of very great practical importance to mankind because plants touch human activities so intimately and in so many ways. All food which men consume is derived from plants. The air we breathe comes originally from a union of water and a simple gas, carbon dioxide, in the leaves of green plants. Our clothing, our fuel, our drugs and countless other products are derived directly or indirectly con- tracted, directly or indirectly, by members of the plant kingdom. As a means of quickening that intelligent appreciation of his surroundings which is characteristic of man, it is clear that man, a scientific knowledge of plants is therefore of the utmost value. The Subdivisions of Botany—Owing to the great mass of diverse facts which have been collected concerning many points of view from which its problems have been attacked, it is evident that major science has necessarily become divided into sub-sciences, each of which makes its particular contribution to the whole. Thus botany may be subdivided into three main branches—the laws of these—Systematic Botany, Plant Morphology and Plant Physi- ology—are most worthy of note and are themselves subdivided and recounted below. Systematic Botany or Plant Taxonomy is concerned with the names of plants and the classification of the vegetative kingdom. By this term I mean the arrangement into groups or kinds of plants which can be distinguished, and to arrange them, according to their natural relationships, into those groups which at any particular time are considered to be valid. These relationships can be determined only after a knowledge of evolutionary history, the science of Plant Phylogeny, which endeavors to trace the general history of the plant kingdom, is an important adjunct to systematic botany. A diagram showing different types of plants arranged in a hierarchy. THE SCIENCE OF BOTANY 3 Plant Morphology is concerned with the form and structure of plants. Its object is to describe the construction and organisation of the plant, and to compare the various structural peculiarities in form between various plant groups. Under morphology are included Anatomy, dealing with internal structure in general; Histology, dealing with the structure of tissues; Embryology, with the structure of the cell; Embryology, with the development of the individual, and Experimental Morphology, with the causes which determine form and structure. Plant Physiology is concerned with the functions of plants. Its objects are to describe and explain the various activities by which the life of the plant is maintained and transmitted to its offspring. It includes also that part of the other subdivisions of botany since it touches the very process of living. A branch of physiology particularly active today is Gnotobiology, which deals with the life of micro-organisms. Aside from these three major sub-sciences there are other fields of botany which deserve mention. Plant Ecology is con- cerned with the distribution of plants in relation to various fac- tors of their environment such as soil, climatic conditions and living organisms; and, in particular, with the modifications of structure and function which plants undergo as they adapt them- selves to changes in their surroundings. Ecology necessarily involves both morphology and physiology, as well as certain of the phy- sical sciences. The study of ecology has led to a better understanding of the geographical distribution of plants, and is intimately related both to systematic botany and to ecology, as well as to geology and geography. Paleobotany is concerned with the study of fossil remains, fossil plants, and thus touches systematic botany, morphology, and geology. In addition to all these aspects of botany, most of which are theoretical rather than practical in their bearing, we should not fail to mention the great group of sciences concerned with the utilization and culture of plants. Economic Botany, in its nar- row sense, deals with those plants which are useful to man and of use to the uses which they are put. The various sciences commonly grouped under Agriculture (Soil Science, Agronomy, Horticulture) are closely connected with botany. So too is Forestry. Pharmacology and their subsidiaries, are eminently practical, and their close relationship to botany is sometimes overlooked. They are nevertheless integral parts of our activity. 4 BOTANY: PRINCIPLES AND PROBLEMS and for their successful pursuit demand a sound knowledge of botanical principles. Everyone who is concerned with plants grows a botanist, whether he pursues his purpose or profession, is rightfully to be called a botanist. The History of Botany. The Classical Period.—Botany as we know it today is the result of a long term of observation and inquiry. As do so many other sciences, it looks back to the fertile speculations of the Greeks for the first definite expres- sion of its principles. The study of the structure of plants was studied by Aristotle (384-322 B.C., Fig. 1) who saw clearly certain of the broader problems of plant and animal life, and whose work has been the basis of all subsequent worthy of our attention. It is his disciple Theophrastus of Eronea (327-257 B.C.), however, whom botanists have generally regarded as the father of their science. This keen naturalist accumulated a great wealth of information on the plants which possessed their various characteristics. Rome also had her share in the development of plant science, notably through the contributions of Pliny the Elder (23-79 A.D.), whose "Natural History," A bust of Aristotle. Fig. 1.—Aristotle, 384-322 B.C. and inquiry. THE SCIENCE OF BOTANY 5 a comprehension of facts and fancies about living things, was long a storehouse of botanical information. Discoveries, living at almost the time of their discovery, and their medicinal properties and benefits an important place historically in both botany and medicine. The Herbal Period—after this, its classical period, botany went into that profound eclipse suffered by all sciences during the Middle Ages. The teachings of the ancient masters were largely preserved, but the methods of acquiring knowledge were the thought of extending knowledge by direct observation and experiment was held to be almost unheard of. About the begin- ning of the 16th century, a number of men lived in the Rhine valley and its adjacent regions under- took to explore the plant kingdom afresh for themselves. They were not content with what had been written, nor was it found therein, and paying scant attention to the doctrines and dogmas of the ancients, they went about describing and drawing with fidelity the various plants which they found in their native countries. From these numerous herb-books or "Herball" in which the resulting discoveries were published, these pioneers have been known as the "Herballists". They endeavored to distinguish between different species of plants by means and proposed certain crude methods of classifying the plant kingdom. So unprejudiced and free from the conventional dogmatism of the age did these Herballists act that many of their Herballists are generally regarded as the fathers of modern botany. The Modern Period.—The first extensive and thorough classifi- cation of plants was made by Carl Linnaeus (1707-1778), the father of Linnaean botanist Celsusplio (1540-1603). Combining an acquaintance with the ancients and an intimate first-hand knowledge of plant life, he laid down a system which has been followed in all subsequent specific botany for many years. Modern taxonomy, however, dates from the publication of the "Species Plantarum" by the great Swedish naturalist Carl Linnaeus (1707-1778) in 1753. In this monumental book all the plant species known at that time were named, carefully described, and arranged according to a definite system. Although the early work in botany was thus concerned chiefly with taxonomy, the study of plant structures was not neglected. Grew (1628-1712) in England, and Muhlygh (1628-1694) in Italy, respectively, made valuable contributions to this field. 6 BOTANY: PRINCIPLES AND PROBLEMS plant body, and their works on "phytostomy" laid the foundation for our modern knowledge of plant anatomy. The continued improvement in the compound microscope made possible more complete and accurate knowledge of the way plants are con- structed, and led to the formulations by Schleiden, in 1838, of the Cell Theory, which states that all living things are composed of cells and that protoplasm is its essential constituent. From this beginning, modern anatomy and cytology have added a great body of facts to our knowledge of the structure, growth, and reproduction of the plant body. The nineteenth and the early modern botanists for the most part had fanciful and fanciful ideas as to the way in which the plant carried on its various functions, an ignorance largely due to the undeveloped state of the sciences of physics and chemistry at the time. It was not until the latter half of the eighteenth century that modern plant physiology became definitely estab- lished. Oxygen was discovered by Priestley in 1774, and five years later it was shown by Priestley that green plants give off oxygen, and that all plants give off a certain amount of carbon dioxide. These gas analyses were followed by those of Gay-Lussac in the early years of the nineteenth century, and some of the Carl von Linné (Carl von Linné), 1707-1778. Fig. 2. —Carl von Linné (Carl von Linné), 1707-1778. THE SCIENCE OF BOTANY 7 important facts of plant physiology thus became established. Since that time the development of modern chemistry and physics has greatly extended our knowledge of the physiological processes of living things. The publication of the "Origin of Species" by Charles Darwin (Fig. 3) in 1859 resulted in a general acceptance among biologists of the theory of evolution. A recognition of the fact that the Figure 3 - Charles Darwin, 1809-1882. plants of today have been slowly developed from simpler ancestors has had a profound effect upon botanical science and has stimu- lated a great interest in reconstructing the "family tree" of the plant kingdom. The need for a more natural system of classification, based on actual relationship, to replace the artificial systems of Linnaeus and his predecessors. It has led also to a more thorough study of the structure and function of plants and the causes and method of evolution. This has been encouraged still further by the discovery of Mendel's Law of Inheritance, proposed independently by Darwin and by Hutton, which finally brought to the attention of botanists again in 1900. The present state of the science of botany, then, is the result of a long series of advances, each advance having been gradually separated from error and our present vast store of facts amassed. With each advance new questions have arisen and new fields of investigation have opened until the science has BOTANY: PRINCIPLES AND PROBLEMS Incorporated from a mere discussion of the names and properties of medicinal herbs to an attack upon the fundamental problems of life itself. QUESTIONS FOR THOUGHT AND DISCUSSION 1. Name three important differences between a typical plant and a typical animal. 2. Which do you think is the most important of all the various configurations of plants to man's welfare? 3. What great industries are founded primarily on plants? 4. Could man get along better without his domestic animals or without his cultivated plants? Explain. 5. What is the difference between a botanist and a person who is merely interested in plants? 6. Would you consider the following to be botanists: * A cabinet maker? * A florist? * A sailor? * An automobile engineer? * A landscape architect? 7. If a person makes a careful study of all the species and varieties of wheat, classifying them and finding their correct names, in what field of botany is he at work? 8. If he studies the structure of the wheat stem, in what field of botany is he at work? 9. If he studies the manner in which food is manufactured by the wheat plant, in what field of botany is he at work? 10. If he studies the ways in which wheat plants respond to changes in their environment, in what field is botany at he at work? 11. If he studies inheritance in wheat, in what field of botany is he at work? 12. If he studies the geographical distribution of wheat, in what field of botany is he at work? 13. If he studies the various uses to which wheat may be put, in what field of botany is he at work? 14. In what ways may a knowledge of botany be of value in everyday life? 15. Name a definite and specific problem, of practical, "dull-and-ends" importance, in agriculture or any other line of industry, for THE SCIENCE OF BOTANY The solving of which a knowledge of systematic botany would be valuable. 16. Name such a problem in which a knowledge of plant morphology would be valuable. 17. Name such a problem in which a knowledge of plant physiology would be valuable. 18. Name such a problem in which a knowledge of plant genetics would be valuable. 19. Name such a problem in which a knowledge of plant ecology would be valuable. 20. Give an instance of a practical utilization in which man with a knowledge of both the scientific and the practical sides of agriculture would have an advantage over a man who knew only the practical side. Why would he have this advantage? 21. How is a knowledge of chemistry important to a botanist? 22. How is a knowledge of physics important to a botanist? 23. How is a knowledge of meteorology important to a botanist? 24. How is a knowledge of geology important to a botanist? 25. Why was systematic botany the first aspect of botany to be studied scientifically? 26. Plants have always been used far more for food than for medicine, but despite this fact, the science of botany in its early days was almost entirely concerned with the medicinal properties of plants and their use in agriculture or any other branch of practical science. How do you explain this? 27. Why was a knowledge of the minute structure of plants impossible for the ancients to acquire? PROBLEMS REFERENCES 1. What is meant by a science? State briefly the doctrine of Spontaneous Generation and explain why it no longer holds good in our time. 2. The following "equal" sciences are all closely related to botany and are founded upon it. With what does each deal? Heredity Agrology Pharmacology 10 BOTANY: PRINCIPLES AND PROBLEMS 4. Summarize the life and work of Aristotle and state his important contributions to botany. 5. Summarize the life and work of Linnaeus and state his important con- tributions to botany. 6. Summarize the life and work of Darwin and state his important con- tributions to botany. 7. What change in geological theory occurred at about the same time that the cell theory of Darwin was wide acceptance (in the latter half of the nineteenth century)? 8. Give the derivation of each of the following terms and explain in what way it is an appropriate one:
Taxonomy Physiology Ecology
Morphology Phylogeny Genetics
CHAPTER II INTRODUCTORY SURVEY Before commencing an intensive study of any aspect of botani- cal science or of any branch of biology, it is well to acquaint oneself with plants, it will be well for us to make a brief survey of the plant kingdom as a whole, and of some of the more important structures and functions of the plant body. The Plant Kingdom.—About 250,000 different kinds or species of plants have been discovered and described, and every year botanical exploration and careful study bring more of them to our knowledge. The first step in the classification of these plants is to name this host of plants and to arrange and classify its members in a logical system. Over much of the details of such a classification, however, agreement still fails to exist. There is now rather general agreement as to the main groups into which the plant kingdom should be divided. Four such divisions are commonly recognized: A. The Thallophytae.—These are lowly plants, various in their structure, activities, and methods of reproduction, but agreeing in the fact that they are all unicellular or contain cells and in multiplying by single-celled spores. The majority of Thallophytes inhabit water or moist places and are small and soft-bodied plants. There are two main series of Thallophytae: The Algae (Fig. 4), which possess the green pigment chlorophyll; and they are thereby able to manufacture food by photosynthesis; the Fungi (Fig. 5), which lack chlorophyll and consequently are obliged to obtain their food from living organisms and plants or from dead organic material. Here belongs the vast variety of bacteria, molds, lichens, roots, tomatoes, mushrooms, and similar plants, many of which live as parasites and are often the cause of serious diseases of man and the lower animals. B. The Bryophytae or Moss Plants.—These plants are dis- tinguished from the Thallophytae chiefly by their more highly A diagram showing the structure of a moss plant. BOTANY: PRINCIPLES AND PROBLEMS developed sexual structures and their more complicated methods of reproduction. The plant body of the Bryophytes has no roots, and in many cases consists of only a flat, strap-like mass of green tissue, but the higher members of the group possess very simple stems and leaves. The Bryophytes are mostly epiphytes, and generally thrive best in moist situations. Bryophytes are subdivided into the simple and lowly Liverworts (Hepaticae) and the commoner and more highly specialized Mosses (Mosses, Fig. 6). An Alga. One of the rock-woods (Ferns sporophyte) growing on a rock between tide-marks. (Photo by M. S. Reed). C. The Bryophytes or Fern Plants These possess true roots, stems, and leaves, but they are similar in structure to those of the Seed Plants, but they still reproduce by spores rather than by seeds. Compared with Bryophytes, the plant body is large and vigorous, and is called the sporophyte (Fig. 7). The three important subdivisions of the Pteridophytes are: The Ferns (Filicales, Fig. 7), possessing large and feathery leaves on the stalks of slender stems; the Clubmosses (Lycopodiales), with short-stemmed leaves; and the Horsetails (Equisetaceae), which have long-stemmed cones but with jointed, hollow stems and minute, whorled leaves. 12 INTRODUCTORY SURVEY D. The Spore-morphyde or Seed Plants--The dominant and familiar portion of the earth's vegetation today consists of these plants, which are well adapted for life on land and often attain great size. Their distinctive feature is the production of a complex, many-celled reproductive body, the seed, in which is contained an embryo plant and a supply of stored food. Seed Plants are very numerous, and vary greatly in form and structure, ranging from small and delicate herbs to huge trees over three hundred feet tall. They are the most conspicuous and best known of all the divisions of the plant kingdom, and provide the greatest variety of food for man and other vegetable products which form the basis of our civilization. A fungus, one of the gill fungi, Pleurotus, growing on a log. Fig. 5.--A Fungus. One of the gill fungi, Pleurotus, growing on a log. 14 BOTANY: PRINCIPLES AND PROBLEMS Fig. 6.—A Mace. A hard-seeded mace, Polygala polygama, with ripe capsules. (From A. J. Grout, "Maces with Lewis and Clark," Copyright by the author.) Fig. 7.—A Fern. One of the shield-ferns (Lophium spinulosum). A plant with large, fan-shaped leaves, growing on a forest floor. A close-up of a fern frond, showing the delicate structure of the leaves. INTRODUCTORY SURVEY 15 Two major subdivisions of the Seed Plants are recognized: The Gymnosperma (Fig. 8), which have primitive, often cone- like flowers and bear their seeds openly exposed on scales as A tree with cones, likely a gymnosperm. Fig. 8.—A Seed Plant (Gymnosperma). For true Loblolly pine. in our common coniferous trees; and the Angiosperma (Fig. 9), in which there is usually a typical flower with its various floral parts, including an ovary in which the seeds are enclosed during their development. There are only about 150 species of Gymno- sperms growing in this country. The Angiosperms are an exhorbi- 16 BOTANY: PRINCIPLES AND PROBLEMS group of more than 130,000 species and are our most familiar plants. They are divided again into two main groups, the A Seed Plant (Angiosperm). Hawthorn tree (Crataegus). Fig. 8.--A Seed Plant (Angiosperm). Hawthorn tree (Crataegus). Dicotyledons and the Monocotyledons, which differ from each other in the structure of the seed, leaf, stem, and flower. INTRODUCTORY SURVEY 17 Underlying the differences by which these various groups are distinguished from one another, there are many fundamental similarities in structure and function which are common to all plants; but the marked changes which appear as we pass from the lowest to the highest types make very difficult a concise description of the general nature of plant life. It will therefore, be profitable for us to confine our attention, at first, mainly to those plants which are most familiar to every one and which have been the subject of much study—namely, the Seed Plants. The sciences of morphology and physiology, as exemplified by the Seed Plants, will accordingly be the chief object of our attention. In this chapter we shall see that only chapters we should be careful to remember that only one division of the plant world—all but the most important one—is being considered, namely, that of the seed-bearing plants. This is estab- lished of course, valid for all plants. In the last chapters of the text we shall discuss in some detail the lower members of the vegetative kingdom, the green plants, in which they differ from each other and from the Seed Plants. The great variety of plant types and the diversity of condi- tions under which they live renders it difficult to make general statements about them. But it is possible to make certain assertions to any such statement may usually be found. Indeed, varia- bility is one of the most notable characteristics of all life. The students of biology have learned that many facts and principles act forth briefly and simply in an elementary text are to be taken as true for typical cases and under ordinary con- ditions. But it must be remembered that these are not necessarily and universally true for all plants and under all conditions. Living things are too complicated to be described completely in simple terms. The Structures and Functions of Plants.—A notable characteris- tic of plants, which they share with all other living things, is their organization into definite parts, each serving a specific external or accidental but is the mark of a fundamental organization or "division of labor" within the plant itself. The individual is made up of a series of distinct and visibly different parts, each of which has a definite function (Fig. 10). These parts are called organs. The root, the stem, the leaf, the flower, the fruit, and the seed, as well as 18 BOTANY: PRINCIPLES AND PROBLEMS A diagram illustrating the important structures and functions of a seed plant, illustrated diagrammatically. Flower - Reproduction Fruit - Photosynthesis Transpiration Respiration (In Part) Leaf - Support Conduction Stem - Anchorage Root - Absorption Fig. 10.—The important structures and functions of a seed plant, illustrated diagrammatically. INTRODUCTORY SURVEY the various subcellular parts of which each may be composed, such as head, pedicel, or stamen, are sense of these organs. The organ, in turn, is composed of tissues. These are diverse and construction but is made up of a group of tissues, each of which performs a particular task contributory to the general function of the entire organism. The tissues are classified into three classes: parenchymatous tissue, hard tissue, and vascular tissue. Parenchymatous tissue, such as leaf, cambial tissue, woody tissue, and pith tissue, each of them playing some role in the economy of the stem or root. Hard tissue is found in the shell of seeds and into the construction of several organs; for there is woody tissue, for example, in the root, the stem, and the leaf. A characteristic feature of all living organisms is that these ultimate units of structure and function in all organisms. Each plant a minute but distinct bit of living substance, or protoplasm, with its own life processes and its own functions apart from itself, and is usually enclosed in a cell wall. The individual plant may, indeed, be regarded as a huge colony of minor indivi- duals. Each individual plant consists of a number of cells grown in the whole and all bound together to their mutual advantage. A knowledge of the structure and activities of cells is the founda- tion upon which our understanding of plants and animals must be built. The Root and Its Functions.—The root is that organ which anchors the plant in the soil and which absorbs water from the soil and mineral substances from the soil. As a result of these primary functions, it frequently serves as a storehouse for reserve food and often assumes other secondary duties. The root system may be simple or complex. It may consist of one weak lateral branch or of a much-branched series of smaller fibrous roots. The function of absorption takes place in minute root-hairs, delicate structures on the surface of the root. These root-hairs are young and growing root-tips. Into these root-hairs, by the process of osmosis, pass water and dissolved substances (chiefly nutrient minerals) from the soil. The Leaf and Its Functions.—The typical leaf is a broad and thin structure, green in color and freely exposed to air and light. Its essential parts are the blade (or lamina), the midrib, and the petiole. From the leaf tissues, the water which has entered the root and ascended the stem is evaporated in the process of transpiration. This evaporation is aided by chlorophyll, a green pigment, chlorophyll, which can utilize the energy of light 20 BOTANY: PRINCIPLES AND PROBLEMS to combine carbon dioxide (coming from the air) and water (com- ing from the soil) into grape sugar, a simple carbohydrate food. This process of photosynthesis is the only way to grow plants. By it is nourished all the food which sustains the lives of plants and animals and of man himself, for all the complex foods with which we humans have been built up by progressive modifications of grass sugar, which is in its turn built up by The Stem and Leaf Functions. The stem normally holds aloft in the air the leaves and reproductive organs, and serves as a highway for the transport of materials. It also supports fruits from the root to the leaf, and of manufactured food from the leaf to other parts of the plant body. Stems may be comparatively small and thin, as in grasses, or they may be large and woody, as trees and shrubs. They are occasionally modified for special functions, such as food-storage or photosynthesis. The Reproductive Organs. — The sole function of these organs is the production of offspring, through which the life of the plant may be transmitted to succeeding generations. The male organ is called a pollen grain; the female, stamen, and pistil, is concerned with effecting fertilization, or the union of male and female sex cells; the fruit protects the growing seed until it is ripe; and the seeds themselves are young plants themselves in embryo, protected by a coat, provided with a supply of food, and ready to begin their independent growth and development whenever favourable conditions appear. Metabolic Functions. — The functions of this plant are not confined to any one organ but are characteristic of living substance or protoplasm wherever it may be. Notable among these functions are digestion, respiration, and photosyn- thesis, whereby such digested food is incorporated into protoplasts, and respiration, whereby the supply of energy necessary for life is obtained through the break- ing down of living tissue, with the consequent absorption of oxygen and liberation of carbon dioxide. QUESTIONS FOR THOUGHT AND DISCUSSION 23. There are almost as many species of Thallophytes as of Seed Plants, but why are there so much more familiar to most people than are the former? Why? 29. Which of the four main divisions of the plant kingdom do you believe is the oldest? Why? A diagram showing the basic structure of a plant cell. INTRODUCTORY SURVEY 21 30. Which of these four divisions contains the most plants useful for food? 31. In which of these four divisions are the largest plants found? In which the smallest? 32. To what extent do the four divisions do plants growing in the ocean chiefly belong? To what do land plants chiefly belong? 33. Name some Thallophytes which are useful to man and some which are harmful. Name some Spermatophytes which are useful and some which are harmful. 34. Which of the four divisions contains the largest number of plant species which are harmful to man and to his domestic animals and plants? Which contain the largest number of useful species? 35. Bryophytes and Pteridophytes have much fewer species than other Thallophytes or Spermatophytes. What reasons can you suggest for this fact? 36. Under what climatic conditions are Pteridophytes most conspicu- ous? What climatic conditions are Thallophytes most conspicuous? 37. What are the advantages of organization (the "division of labor") in a plant? 38. Which of the four main divisions of plants do you think shows the highest degree of organization within its plant body? 39. In general, do you think that it has been the most highly organized or the least highly organized plants which have been most successful? Explain your answer. 40. Why are living things called "organisms"? 41. Are the two important agricultural crops derived from the root, the leaf, or both? Explain your answer. 42. Roots are confined almost entirely to land plants. Explain. 43. Give an example of a root which serves as a store-house for a large amount of food. 44. What advantage has the root over the stem as a place for food storage? 45. Name ten cultivated plants in which the root is the organ useful to man. 46. Name a few plants in which the stem is very much reduced. Under what important disadvantage are such plants? 32 **BOTANY: PRINCIPLES AND PROBLEMS** **47.** Name ten cultivated plants in which the stem is the organ useful to man. **48.** The blade of an ordinary leaf is broad and thin. Explain the advantage of this to the plant. **49.** Name ten cultivated plants in which the leaf is the organ useful to man. **50.** What makes reproduction necessary among plants and animals? **51.** Name ten cultivated plants in which the fruit or seed is the organ useful to man. **52.** What important resemblances are there between the physiology of a typical plant and of a typical animal? What important physiological differences are there between these two groups of organisms? **REFERENCE PROBLEMS** **9.** Are there more species of plants or of animals on the earth today? **10.** Are there any plants which lack roots? Which lack leaves? Which lack a stem? Which lack reproductive organs? **11.** Give the derivation of each of the following terms and explain in what way it is an appropriate one: Thallophyte Bryophyte Pteridophyte Spermatophyte A diagram showing three types of plants: Thallophyte, Bryophyte, and Pteridophyte. CHAPTER III THE SOIL AND ITS IMPORTANCE TO PLANTS It is impossible to study the plant as a living organism without an understanding of the surroundings or environment in which it grows. The environment in which the plant lives and grows may be divided roughly into the soil and the atmosphere. Of these two, the soil is much the more complex—physically, chemically, and biologically—than the atmosphere, together with the profound effect produced upon the plant by changes within it. Diagram showing composition of soil. Rock particles 40%, air 15%, water 35%. Fig. 21.—Composition of soil. Graph showing the percentage composition. Warrants us in devoting to the soil a brief preliminary discussion before we consider in detail the structures and functions of the plant itself. The soil has three main uses in the plant's economy: It provides an anchorage and support whereby the plant may be held firmly in position; it furnishes the supply of water which the plant uses; and it contributes certain mineral salts essential to the plant's economy. 23 24 BOTANY: PRINCIPLES AND PROBLEMS Soils vary much in physical texture, element composition, depth, origin, richness and other respects, but all are normally made up of a mixture of distinct components, each of which has its particular influence upon the life of plants. These com- ponents are rock particles, water, air, humus, dissolved sub- stances, and living organisms (Fig. 13). **Rock Particles.** The bulk and the basic material of a soil is composed of small fragments of rock. These fragments may have been formed by disintegration of rocks or they may have been carried by water to their present location. The weight of ordinary good soil furnishes the necessary anchorage for the plant, and, through the substances dissolved from their surfaces, provides the essential nutrients for plant growth. The particles vary greatly in size, from those fine of clay to those of coarse gravel. They differ also in their chemical composition according to the type of rock from which they came. The irregularity of contour which characterizes many rock particles makes them difficult for plants to fit very closely together, and a consider- able amount of space (porosity) is thus left between them. Soils which are in good condition for the growth of ordinary plants possess particles in groups to form craters or fissures (Fig. 13). This allows water to penetrate into the soil by water-films or by such a convenient substance as clay. One important purpose of this is to impart this crumby structure or flocculation to a soil. At the soil surface, by the direct action of the raindrops, these particles are broken apart and mixed into their constituent particles, which then pack closely together and on drying harden into a firm, clay-like crust. **Water.** Water is one of the most important elements in many ways. It constitutes the great bulk of the bodily material; it enters into the manufacture of food; it assists in maintaining the plump- ness and vitality of plants; it is essential for plant respiration; it serves as the general medium in which most physiological processes are carried on. The chief source of water is water, and in most cases the only one, is the rain which falls upon the soil surface. Various rates await this water (Fig. 13). A considerable part of it may not enter the soil at all because it is too heavy; some may be heavy, but may drain away instead. This raw-off is best for plants, and PROPERTY LIBRARY N. C. State College THE SOIL AND ITS IMPORTANCE TO PLANTS 25 may even do much harm by washing away a portion of the soil itself. The water which does enter the soil may either percolate downward between the particles under the influence of gravity, or may be held on the surface by capillary attraction. Percolating or gravitational water passes downward rapidly if the soil particles are coarse, more slowly if they are finer, until if the soil particles are clayey, none may pass at all below a level where all the air spaces are filled with standing A diagram showing the different types of water movement in soil. The top left shows rain-water falling onto the surface of the soil. The top right shows water percolating through the soil. The bottom left shows water being drawn up by capillary action. The bottom right shows water being held on the surface by capillary attraction. Fig. 3.—The various forms of rain-water which falls upon the soil. It may run off without entering the soil, or it may percolate downwards, or it may enter the roots and be transpired from the leaves; it may be held in the soil by capillarity, or it may percolate downwards to the water-table. or hygroscopic water. This level is known as the water-table. It is possible to give a formula for stating the height at which water will stand in a well dug at that point, and its distance below the surface varies from place to place and is subject to much variation. The water-table is usually found to rise up to the upper soil layers after heavy rains, but persists there for only a short time. When water has percolated downwards to this level it is often beyond the reach of roots, and is thus quite unavailable to plants. Capillary water is water held in the soil by the force of capilarity. Capillary attraction is exerted between any two objects (such as one's hand) immersed in water and then lifted out again, some water still adheres to its surface in this thin film, or web, of water. This is due to the fact that there is greater attraction between molecules of water than between air and water than is excited by the force of gravity or by cohesion of the water particles themselves. Any material with a large amount of 26 BOTANY: PRINCIPLES AND PROBLEMS surface, internal or external, which may be wetted (such as a sponge, blotting paper or coarse fabric) will therefore hold within itself, and will not drain out under gravity. For exactly the same reason, much of the water entering a soil will fail to percolate through it but will remain on its surface as a film, and this is true even in total area, which are presented by the multitude of soil particles. If the amount of rainfall is small, all of it may thus be retained A diagram showing different layers of soil: Air-Space, Water-Film, Rock-Particle. Fig. 14.--Section through soil, much enlarged. A capillary water-film remains on the surface of the soil particles (see Fig. 13). The larger spaces are occupied by air. Much enlarged. and none lost through percolation. Each particle is such a model and so acts as a reservoir for water (see Fig. 14). The films about adjacent particles coalesce, filling the minute spaces and lining those that are large, and a continuous film-system is thus set up. It is this continuous capillary water which enables plants with the great bulk of their weight to grow. One of the important objects in manipulating a soil is to increase, by one means or other, this water-holding capacity, and thus prevent waste through evaporation. The principle of capillarity is of further importance in deter- mining all movements of water in the soil other than the down- ward one due to gravity. In a glass tube placed in water, the narrow glass tube is placed in water, the water will rise inside the tube to a point somewhat higher than its level outside, is due to the attraction between the surface of the glass and the water; an THE SOIL AND ITS IMPORTANCE TO PLANTS attraction which is sufficient to lift water against gravity. The lifting force will be proportional to the exposed surface of the tube, and therefore where the volume of water is small in relation to this surface (as is the case inside the tube) the water will rise only a short distance. But when the surface area counter-balances the pull exerted by the surface attraction. Obviously, the narrower the tube, the higher the column of water will rise, since the volume of liquid to be lifted will be smaller in proportion to the area of the attracting surface. Thus, in any material the A diagram showing a series of hexagonal cells with varying sizes of particles within them. The largest particle is on the left, and each subsequent cell has a smaller particle than the one before it. Fig. 35.—The relation between the size of soil particles and the amount of surface which they present. Diagrams of sections through equal volumes of sand, gravel, and clay. The total surface is evidently greater where the size of the particles is less. structure which presents a great amount of surface surrounding small but communicating tubes, pores or other narrow spaces, as in leaf-scales, or in the root-hairs of plants, can very conveniently be carried to a considerable distance in all directions by capillarity. Just such a material as this is the soil. The multitude of its minute channels and passages forms a vast and powerful capillary system which is able to carry water far. This water tends to surround each particle in a thin, capillary film, but if the soil particles are too large, or if they are too closely packed together, and under such conditions the ascent of water eventually stops. Water moves readily within the films and when those at the top of the soil particles become saturated, water passes down into cavities or through the attraction exerted by still higher and unwetted surfaces, the films below are drawn up, and water passes upward through these films until it reaches a point where it balances the weight of the water lifted balancing the surface attraction at the top of the column. The height to which water will rise by capil- 38 BOTANY: PRINCIPLES AND PROBLEMS larity is determined chiefly on the size of the soil particles; for the smaller the particles, the greater will be their number in proportion to the space between them (Fig. 15), and thus the higher will be the rise of water (Fig. 16). In ordinary soil this rise varies roughly from two to four feet. In clay, however, therefore, that water which has been mobilized far below the surface cannot be made available to plants again through capillary ascent. In most soils there is a capillary movement of water toward the soil surface, where it evaporates. If the particles at the surface are very close together, as they are where the soil has been plowed, a very efficient capillary system is produced which connects the soil surface with the deeper water-holding layers, and thus greatly hastens the evaporation of water. Where this is not so, it may be evaporated (Fig. 17). An important purpose of tillage is to prevent such waste of water by breaking up the capillary system at the surface and forming there a layer of loose, coarse material called a mellow. Capillary movement of water is by no means always vertical but may occur plane to plane, as in clay soil, or as ink spreads on all directions in a piece of blotting paper. This move- A diagram showing the structure of a soil particle and its effect on water movement. Fig. 16.—Rise of water by capillarity. In the two glass tubes at the left, the size of water drops is shown in relation to the size of the particles in which they are contained. The two chambers at the right, which are filled with cobwebs, show how much water can be held by a single cobweb. Fig. 17.—Capillary movement of water in clay soil. The upper part shows how water rises in a clay soil when it is plowed; the lower part shows how it may be prevented from rising by breaking up the capillary system at the surface. THE SOIL AND ITS IMPORTANCE TO PLANTS ment tends to continue until the water films are of equal thickness throughout the entire soil mass, causing it to be uniformly moist. When water is removed at any particular point, as by surface evaporation, the moisture content of the soil is drawn thither (whether from all other points until equilibrium is restored). Fig. 17.—A foot-plot in brown soil. A vertical slice through the soil under and near a foot-plot, showing how the particles which are under the foot have been partly dried out, while those which are over the foot remain moist. The lower soil layer is shown to have a near normal supply of water to the surface, thus indicating that the plant has obtained sufficient water. In soils which have kept all their capillary water by evaporation, there still remains around each particle an exceedingly thin film of aqueous water, which clings so tenaciously that it may be driven off only by very high pressure or by great temperature changes. When the air is very dry, this water is present in minimum amount, but when humidity rises, more water may be taken up directly from the atmosphere. This supply of water is removed with such difficulty from the soil particles, however, that the plant is able to obtain little or none of it. Air.—Since oxygen is essential for the healthy growth of ordi- nary plants, and since it is necessary for the proper supply of air is a matter of vital importance. If the spaces between the soil particles become filled with water, most of the air is necessarily driven out. In order to maintain a proper supply of air, main- tained, ordinary plants suffer. We have seen, however, that such excess of water normally passes downward by percolation, and as it does so it carries with it some air. In sandy and well-drained soils, from 20 to 35 per cent of the volume consists of air spaces. The composition of the air which fills these is often somewhat different from that of the atmosphere; the proportion of carbon 29 30 BOTANY: PRINCIPLES AND PROBLEMS dioxide being relatively high. Powing tends to increase greatly the air content of a soil, since the structure of the whole mass is loosened. In this condition, the air is more easily retained by the soil; in this condition is said to be in good tilth. When water occurs only in capillary form much air is present in the larger spaces, and under these conditions the soil is not so favorable for plant growth since then, and then only, is a plentiful supply of water combined with a plentiful supply of oxygen. Organic Matter.---The organic matter consists of the amount of material derived from the dead bodies of organisms, particu- larity of plants. Roots which die and remain in the soil, and leaves and other plant parts which fall upon the surface, are the sources from which this matter is necessarily derived by nature. In the practice of agriculture it is increased in amount by various artificial means. After entering the soil it soon begins to undergo decomposition, and its products are largely converted into simple end-products—carbon dioxide, water, and ammonia. As this organic material decays it becomes characteristically dark in color and emits a putrid odor when exposed to air. In this condition it is known by the general name of humus. Humus is of importance to plants in many ways. It improves the physical condition of the soil by increasing its porosity and fragmentary character it tends to separate the particles and thus to increase materially the air-content of the soil. Since humus absorbs water readily, it increases the capacity of the soil for water- holding capacity. The decomposition of humus liberates certain nutrient materials, notably an abundant supply of nitrogen com- pounds, which ultimately enter into plant nutrition. Humus also has the effect of supplying to the soil bacteria, minute organi- zms which are indispensable in plant nutrition. Any treatment of the soil which will increase its humus content will therefore tend to improve its fertility. On the other hand, excessive humus content will impoverish the soil. Disolved Substances.—Soil water is not merely pure water but contains dissolved within it a great variety of substances. Anything which is to be taken in by the roots of plants must be in solution, and it is consequently obvious that these dissolved substances are essential to their nutrition. These include, first, which is directly available as nutrient material for plants. Their origin and chemical composition are therefore of much importance botanically. THE SOIL AND ITS IMPORTANCE TO PLANTS 31 The solvent power of soil water is increased by the presence within it of carbon dioxide, liberated in the respiration of plant roots and of the lower organisms. Thus reinforced, water not only dissolves more readily the mineral substances which form any soluble material which may appear in the humus or as a product of bacterial activity. There is a great variety of substances present in the soil solution, and we know from chemical analyses of the ash of plants that very many of them may be taken into the plant body. Components of nitrogenous matter—ammonia, urea, uric acid, magnesium, manganese, phosphorus, phosphorus, sulphur, chlorine, and silicon are commonly absorbed by the roots, and many others may be taken up occasionally. Certain of these elements are far more important to the plant than others. The fact that it has been clearly proven by experiment that seven are essential for normal plant growth: Sulphur, phosphorus, calcium, magne- sium, potassium, iron, and boron. It is evident that all of these mineral nutrients taken up by the plant is exceedingly small in proportion to the size of the plant body, but in the activities of vegetative growth and reproduction they are used up so quickly that their supply must be returned to the soil if abundant plant growth is to be maintained permanently thence. This necessitates the common practice of manuring with fertilizers containing those materials which renew the supply of essential salts there available to plants. Organisms—Aside from its service as a medium for the root-growth of higher plants, the soil provides a dwelling-place for a great variety of lower forms of life. These exert an important effect on the composition of the soil and on the processes which go on therein. Rodents, insects, and anguillomorpha all modify the physical condition of the soil by their activities. These minute and lowly of living things, however, which we group together as micro-organisms are of far greater importance, *The task of plants is the residue left after complete combustion of the ash in the soil. The ash contains all the mineral substances present in mineral substances present in the plant.* 31 32 BOTANY: PRINCIPLES AND PROBLEMS for experiment has shown that without their presence the soil would soon become unfit to support a vegetation of higher plants. Most notable among these micro-organisms are the bacteria (Fig. 10). Many of them are capable of generating their own chlorophyll. Many of these—the bacteria of decay—decompose the complex organic substances found in humus into simple end-products, which are digested by rumen anaerobes, thus releasing great quantities of nutrient materials which would otherwise be locked up and useless in dead bodies of animals and plants. Still other bacteria in the soil cause chemical changes of various sorts there, the results of which are of great moment to the higher plants. The most important of these concerns with the transformations of nitrogen and its compounds, for through their activity alone is the available supply of this necessary element increased. This process is known as the assimilation of nitrogen through its various successive stations in organisms, air, and soil is known as the Nitrogen Cycle (Fig. 19). Complex organic compounds such as proteins and nucleic acids of dead animals and plants are broken down by the bacteria of decay into simpler compounds, which are finally reduced to ammonia, which is then converted into nitrate. This occurs in the form of nitrate soils, however, this ammonia is not directly available to them but must first be converted into nitrate soil's through the process of nitrification. This is carried out by A: A B: B C: C D: D Fig. 19. Some important soil bacteria. A, nitrite bacteria; N. virens; × 2000. B, nitrate bacteria; N. virens; × 2000. C, a common decay-bacterium; Agrobacterium; × 500. D, nitrogen-fixing bacteria; Rhizobium; × 750. from Agarostomum. THE SOIL AND ITS IMPORTANCE TO PLANTS 33 two types of nitrifying bacteria: the nitrite bacteria, which change ammonia to nitrites, and the nitrate bacteria, which in turn convert the nitrites to nitrates. These nitrates can be readily absorbed and assimilated by plants, and is ultimately returned to the soil again in the bodies of plants or animals, thus complet- ing the cycle. Through the activity of another group of micro- minute organisms, certain of the seed plants are also able to take advantage of the enormous supply of nitrogen in the atmosphere, which is ordinarily quite unavailable. These are the nitrogen- fixing bacteria, which live in symbiosis with such plants as legumes. The development of the tubercles or nodules usually found on the roots of plants belonging to the Legume family (Fig. 20), which include beans, peas, clover, alfalfa, etc., is due to these bacteria. These bacteria are able to absorb the free gaseous nitrogen of the air and to build it into nitrogenous compounds in their bodies, where they are stored until needed by the plant. The plant then utilizes these nitrogenous compounds in its growth. Without drawing at all upon the nitrogen compounds in the soil, a leguminous plant is consequently able to acquire an abundant supply of this impor- tant element. 3 Fig. 19.—The Nitrogen Cycle. 20 BOTANY: PRINCIPLES AND PROBLEMS In the case of many species of plants, particularly those which grow in forests, we find situated within their roots thread-like filaments of fungi as indicated associated with the smaller roots, entering their outer tissues and surrounding the root with a web-like jacket of fungus threads. These very largely take the place of the mycorrhizal association of the higher plants and A diagram showing the characteristic tubercles upon the roots, caused by the presence of intergrading bacteria. Fig. 51. - Root infestation. A part of the root system of a continuous plant showing the characteristic tubercles upon the roots, caused by the presence of intergrading bacteria. nutrient material from the soil, and the fungus, as well, is evidently dependent upon such relationship. This root-fungus association is known as a mycorrhiza. Certain plants have become so dependent in this way upon particular species of fungi that they cannot live without them. In conclusion, we may emphasize again the extreme complexity of the soil and the vital significance to plants of its composition and of the processes which go on therein. The study of this remarkable material has required the collaboration of almost THE SOIL AND ITS IMPORTANCE TO PLANTS 35 all of the sciences, but we still lack a precise knowledge of many of its aspects and fail to understand clearly the manner in which it affects the life of plants growing in it. QUESTIONS FOR THOUGHT AND DISCUSSION 53. Through what processes does bare rock gradually become covered with soil? 54. Soil always forms by the rock lying directly under it? What are the origins of soil in this region? 55. In what various ways is soil being impoverished or "used up"? 56. In what various ways is soil being replenished? 57. What is the surface of a bit of gravel which is in the form of a solid cube, the sides of which are 1 cm. long? What would be the surface area of such a cube? How many times greater is the surface with sides of 1 mm. each? What conclusion can you draw from this as to the importance of the size of soil-particles? 58. What disadvantages to plants are there in a soil where the particles are very small? What disadvantages when they are large and coarse? 59. Why are soils in which the particles are small usually more productive than soils in which the particles are large? 60. Is it important to have the soil very loose and "allowed" to such "root-space," as potting-soil, currant, beets, and turnips so yield heavily. Explain. 61. The surface of land after plowing is a few inches higher than it was before plowing. Why? 62. In what respect is cutting off of forests bad for the soil? 63. Is a gentle shower or a hard, beating rain better for the soil and for plants growing therein? Explain. 64. It is harmful to the soil to till (or "work") it immediately after a rain while the soil is very wet. Why? 65. Why is extensive removal of forests in a region often followed by floods thereon? 66. What various means of soil treatment can you suggest for insuring that the largest possible proportion of the rainfall is utilized by plants growing on this soil? 36 BOTANY: PRINCIPLES AND PROBLEMS 76. Which will absorb and hold a larger amount of water: Clay or sand? Why? 77. Will the rise of water by capillarity be higher in sand or in clay? Why? 78. Which is better for plant growth in a dry season, a clay soil or a sandy soil? 79. A layer of coarse gravel a few feet below the surface of the soil makes the soil above it much drier than similar soil not underlain by gravel. Why? 71. Why does the surface of soil which has been perfectly dry during the day become moist at night? 72. Why is the surface of the soil in arid regions often caked with a crust of salts? 73. Why is a well-forested region apt to suffer less from drought than one with no forests? 74. Why is it generally be colder, a soil with fine particles or one with coarse? Why? 75. A potted plant will dry out the soil in its pot very uniformly. Explain. 76. Is it better to water house plants by pouring water over the soil or by letting the pots stand in water for a little while? Why? 77. Explain exactly why a mulch on the soil surface reduces loss of water from the soil through evaporation. 78. What basic fact is there for the saying, "Water your garden with a straw."? 79. Why is it advisable to scratch over a garden with hoe or rake as soon after a rain as the soil is workable? 80. Why is it advisable to plow as early in the spring as the soil is workable? 81. Why is it important to water a plant before it is transplanted? 82. Give three reasons for preserving the soil firmly about the roots of a plant after it has been transplanted. 83. After a plant has been transplanted, it is well to scratch over the ground around it with hoe or rake. Why? 84. What advantage is gained by pressing down the soil above seeds after planting? THE SOIL AND ITS IMPORTANCE TO PLANTS 37 55. After sowing seed for certain crops, a farmer often rolls the soil with a heavy roller. Why? What is gained by this? 56. Why is irrigation necessary in some regions but not in others? 57. What is the best time of day to water a garden? Why? 58. Why is it necessary to drain wet land before ordinary crops can be grown thereon? 59. In what ways is the air in the soil constantly being changed and renewed? 60. Of what use is the air in the soil in addition to providing oxygen for plant roots? 61. Give two reasons why the formation of a "crust" on the soil surface is harmful to plants. 62. Is it necessary to remove plants frequently and lightly or infre- quently and heavily? Why? 63. How is the supply of humus maintained in soils which are not under cultivation? 64. Dark-colored soil is usually richer than light-colored soil. Explain. 65. Why is soil from low land usually darker in color than soil from a side hill? 66. The continual addition to the soil of no other fertilizer than the ordinary "commercial" fertilizers is generally found to be harm- ful. Why? 67. What conditions are there under which the addition of humus to the soil might be actually injurious to plants? 68. How do you know that a particular element is, or is not, essen- tial to plant life? 69. Most commercial fertilizers contain nitrate, phosphoric acid, and potash. Why? 70. A good crop of corn, wheat or potatoes requires about 38 pounds of nitrogen per acre from the soil. Nitrogen compounds about four-fifths of the weight of the atmosphere. At this rate of renewal by crops, how many years would it take to replenish all the nitrogen lost, provided that it could be made available to the plants growing thereon? 71. Name several ways in which chemical substances essential to plant growth are wasted in modern civilization. What methods can you suggest to prevent this waste? 38 BOTANY: PRINCIPLES AND PROBLEMS 102. Why are wood ashes so valuable as fertilizer? 103. Why do swarms often become flagged with mites? 104. What two important contributions to human welfare are made by the bacteria of decay? 105. What various organisms living in the soil may be harmful to plants? 106. What is the chief importance of azaleas in the growth of plants? 107. Plants native to the woods will often fail to thrive when transplanted into fields from which the soil is rich and the conditions of shade and temperature are similar to those in the forest. Can you suggest a reason for this difficulty? 108. At what point in the nitrogen cycle, and in what form, is loss of available nitrogen most apt to occur? 109. Manure left freely exposed to the air will lose much of its fertilizing value. Why? 110. For many plants, rather old manure is better than that which is absolutely fresh. What reason can you suggest for this? 111. Plants which have very deep roots (such as certain weeds and cover-crops) have a decided advantage to crops which are subsequently grown on the soil. 112. A "cover-crop" is a crop (such as rye) planted in late summer or early fall which grows up enough to cover the ground before winter. What are some advantages of this practice? 113. Why is flood-plain or "river-bottom" soil usually very productive? 114. In what ways may the productivity of a soil be diminished other than by the presence of crops grown upon it? 115. It has long been recognized that land which is left uncropped or "fallow" for a few years proves to be more productive after-ward. How do you explain this? 116. In what ways is the productivity of a soil maintained in nature, before it becomes exhausted of nutrients? 117. What other factors may the addition of fertilizer to the soil perform aside from that of providing nutrient materials for plants? THE SOIL AND ITS IMPORTANCE TO PLANTS 118. State at least three advantages which are gained by plowing the soil. 119. Cultivating the soil around growing crop plants (by hoeing or otherwise) is not necessary or advantageous after the crop has covered the ground fairly well. Explain. 120. How can soil be a state of nature, entirely without cultivation, support plant life? 121. Give several reasons for the desirability of keeping the soil around a young fruit tree well "mulched." 122. Vegetable growers often sterilize by steam, fertilizer or other means, the soil in which they grow their vegetables before putting their young plants for later transplanting outside. What advantages has this process? What disadvantages would it have if applied in the field? 123. Farmers sometimes build a bag bender over the spot where they are going to start their young plants of cabbage, tomato, and similar crops. What two advantages are gained by this? REFERENCE PROBLEMS 12. What is meant by tillage? 13. What is irrigation and how is it practiced? 14. Soils which are poorly aerated are apt to be "soar." Explain. 15. Distinguish between animal manure, green manure, and com- mercial fertilizers, and explain their value in the garden. 16. What is meant by the formation of aluminum nitrate? Of ammonium of potashammonium nitrate? Of ammonium complex compounds? 17. May organic substances dissolved in the soil solution enter directly into growing plant roots and used by them? Explain. 18. Give an example of a crop that will thrive on an alkaline soil; one that thrives best on acid soil. Which of these two crop types do most crop plants favor? 19. Algae and many bacteria are usually present in 1 c.c. of rich soil? 20. What is "rotation of crops" and what is its value? 21. Give the derivation of each of the following terms and explain in what way it is an appropriate one: Capillary Humus Mycorrhizae CHAPTER IV THE ROOT AND ITS FUNCTIONS The portion of the plant most intimately related to the soil is the root. This organ has two major functions—to anchor the plant firmly and to absorb water and certain important nutrient materials from the soil. Beyond this, the root often serves as a A diagram showing a plant's root system with roots spreading outwards. Fig. 21.—A fibrous root-system (Broad). The roots are all rather slender and much branched. storage reservoir for food, and may perform various other functions. External Structure.—The most common type of root is a rather slender and profusely branched structure, penetrating the soil in 40 THE ROOT AND ITS FUNCTIONS all directions and forming a fibrous root-system (Fig. 21). Its advantages for anchorage and absorption are obvious. Somewhat less common are types which possess a single main root or tap- A diagram showing a tap-root system (Dandelion). The tap-root sends out lateral roots which penetrate deeply into the soil and much deeper than the lateral roots which arise from it (Fig. 22). Tap-roots lend themselves readily to storage purposes and frequently become large and heavy. The root-systems of many plants are intermediate. Fig. 22.—A tap-root system (Dandelion). From the tap-root a large number of lateral roots penetrate deeply into the soil and much deeper than the lateral roots which arise from it (Fig. 22). Tap-roots lend themselves readily to storage purposes and frequently become large and heavy. The root-systems of many plants are intermediate. root, penetrating deeply into the soil and much stouter than the lateral roots which arise from it (Fig. 22). Tap-roots lend themselves readily to storage purposes and frequently become large and heavy. The root-systems of many plants are intermediate 42 BOTANY: PRINCIPLES AND PROBLEMS between these two main types. Others may sometimes depart radically from the normal forms in response to certain special and unusual conditions. The Absorbing Region.—Absorption of water and nutrient material is carried on only by the younger portions of the root, near its tip. The very tip itself is covered with a sheathing root- A diagram showing the structure of a root. Fig. 23.—Tip of a root, showing root-cap (A), growth zone (B), and root-hair zone (C). (From Darwin's "Fruitful of Beauty," copyrighted by the Macmillan Company.) cap of cells, which protects the delicate underlying tissues as the root pushes its way through the soil (Fig. 71). Back of this is a short region of growth, the only place where elongation of the root occurs. This is followed by a somewhat longer zone, the surface of which is covered with thousands of exceedingly delicate filaments, the root-hairs (Fig. 20). Each hair is a prolongation of one of the surface cells of the root (Fig. 21), its sap-cavity and lining of cytoplasm being continuous with those of the root-cell of which it forms a part (Fig. 26). The root-hair may reach a length 42 THE ROOT AND ITS FUNCTIONS 43 of several millimeters and forces its way into the minute crevices of the soil, thus coming into most intimate contact with the soil particles (Fig. 26). Through the enormous surface which these root-hairs expose to the soil, absorption of water and mineral A diagram showing the structure of a root hair. The diagram includes labels for "Cell Wall," "Sap Cavity," "Cytosol," and "Exocytosis." The root hair is depicted as a thin, elongated structure with a central nucleus surrounded by cytoplasm. The sap cavity is shown as a small space within the cell wall. The exocytosis is indicated as a process occurring at the tip of the root hair. Fig. 24—Root-hair and their relation to the root. A transverse section across the root shows the root-hair (right) and the cell wall (left). In the root may be distinguished the fibre-vascular cylinder (in the center), surrounded by the cortex. A diagram showing a cross-section of a typical root-chub, showing its various components. The diagram includes labels for "Cell Wall," "Sap Cavity," "Cytoplasm," and "Exocytosis." The root-chub is depicted as a long, thin structure with a central nucleus surrounded by cytoplasm. The sap cavity is shown as a small space within the cell wall. The exocytosis is indicated as a process occurring at the tip of the root-chub. Fig. 25—A root-chub, section through a typical root-chub, showing its various components. The diagram includes labels for "Cell Wall," "Sap Cavity," "Cytoplasm," and "Exocytosis." The root-chub is depicted as a long, thin structure with a central nucleus surrounded by cytoplasm. The sap cavity is shown as a small space within the cell wall. The exocytosis is indicated as a process occurring at the tip of the root-chub. suits take place. Root-hairs are generally short-lived, dying away as the corky bark begins to appear. A root-chub rose of fairly constant length thus follows behind the growing root-tip, new hairs appearing in its younger portion to replace the oldest ones, until the continuously growing twig. 14 BOTANY: PRINCIPLES AND PROBLEMS The Plant Cell. The root-hair (including its basal portion) is a plant cell. Some knowledge of the structure and functions of cells is obviously essential if we are to understand how the root-hair is constructed and does its work; or, indeed, how any other part of the plant is put together and functions. It will be necessary at this stage to describe briefly some of the more important characteristics of cells in general and of plant cells in particular. We have already spoken of the remarkable living material which is called protoplasm, the seat of all the various activities which are maintained in animals and plants, and the only portion of body which can be said to be alive. Protoplasm is a thin, jelly-like, cellular substance, but its exact structure is not clearly known. Chemically, it is a mixture of very complex proteins and is thus composed chiefly of carbon, oxygen, hydrogen, and nitrogen. It is the most extraordinary material known to man. The protoplasm of the plant body is not a continuous mass but is made up of many separate units or protoplasts, each of which is a distinct and mere or less independent unit, possessing a definite structure and carrying on within itself a variety of functions. In the root-hair cell the protoplasm secretes a dead cell-wall composed of the characteristic 41 Fig. 20.—Diagrammatic section through a portion of a young root and the different layers composing it. The outermost layer consists of rock particles, water droplets, and air spaces in the soil are shown. Much enlarged. THE ROOT AND ITS FUNCTIONS plant substance cellulose. This is firm in texture but easily penetrated by water. The protoplasm of the cell has two distinct portions—the nucleus, a dense, somewhat spherical body which appears to be the direct center for the cell's activities; and the cytoplasm, through which the watery in texture, while within the inner surface of the wall in which the cell is bounded. A diagrammatic drawing showing the con- textual arrangement of the cell parts. Embedded in the cytoplasm is the nucleus. Portions of the walls of six adjacent cells are shown. Fig. 27.—A typical plant cell. Diagrammatic drawing showing the con- textual arrangement of the cell parts. Embedded in the cytoplasm is the nucleus. Portions of the walls of six adjacent cells are shown. without and within by a very delicate membrane. Embedded in the cytoplasm frequently appear small, somewhat denser bodies, the plastids. These perform special functions, such as carrying on the manufacture of C.-v., building up starch-grains or producing chlorophyll. In some cells, however, even though the cytoplasm is not passive and immobile, but that within it a slow, streaming movement often takes place. In mature cells, the entire contents of the cell may be dissolved by strong acids and reconstituted filled with water in which various substances are dissolved, and surrounded externally by the layer of cytoplasm. A typical plant cell is about one-fifth inch in diameter. As in a well-worn football or basketball, the firm leather covering corresponding to the cell-wall; the inner slender bladder of rubber, tightly pressed against the outer bladder, corresponding to the sap-cavity; and finally, in the sap-cavity. A comparison to an automobile tire, with its stout shoe or casing, its definite inner tube, and its circular air-cavity, might be made. Cells are normally very small objects, averaging about 0.01 mm. in diameter, and varying widely in shape and character according to their function. They are so small that they can be support, absorption, conduction, storage, protection, food-maintain **BOTANY: PRINCIPLES AND PROBLEMS** structure, growth, or reproduction. The plant body is composed of a multitude of cells, bound firmly together by cementing sub- stances to form an entire, coherent organism. As we consider the various tissues and organs in detail, we shall have occasion to describe their structure and function in the following display. **Internal Structure of Roots.** Internally, roots show a very marked structural differentiation. In a young root, three main cell-systems or tissues are distinguishable—the *epidermis*, the *cortex*, and the *fibro-vascular cylinder* (Fig. 28). Fig. 28.--Transverse section of the fibro-vascular cylinder of a young root. The lamellae of the epidermis are shown on the left side of the section, and the radially arranged root-hairs on the right side. The whole is surrounded by an endodermis, outside of which is the cortex and finally the vascular cylinder. The epidermis of the root, like that of all other plant organs, is a single layer of cells in thickness. These cells are normally protective in function, but in the root-hair zone many of them produce on their outer surface a characteristic projection, the root-hair. The cortex lies just under the epidermis and is of varying thickness. In the young root, its cells serve to transmit water and dissolved food from the soil to the vascular cylinder, and, in the older roots, to store food. Most of the flabby portion of storage-roots consists of enlarged cortex. The innermost layer of cortex is often especially modified and is then known as the *endodermis*. 46 THE ROOT AND ITS FUNCTIONS The fibre-vascular cylinder occupies the core of the root, furnishing mechanical strength and serving as a highway for con- ducting water and dissolved substances. It consists of two main tissues, the wood or xylem and the bast or phloem. The wood, which forms the central axis of this cylinder, is usually represented by a single layer of cells, but in some cases a part of thick-walled and much elongated dead cells, the walls of which have become woody. It provides rigidity for the root and conducts upward the water and dissolved substances which Diffusion of a dissolved substance. Diagram representing the outward diffusion of a substance being absorbed into the surface of a edible substance immersed in water. enter from the soil. Between the points of the star are patches of bast, formed of thin-walled cells which transport manufactured food materials to other parts of the plant. In older roots the fibre-vascular cylinder, particularly as to its woody portion, increases greatly in thickness through the activity of cambium tissue (see p. 30). When the stem; and a corky bark is usually developed on the outside. **Diffusion and Osmosis.**—The most important function of the root is to absorb water and dissolved substances needed for the plant's life and growth. This involves the physical processes of diffusion and osmosis, a consideration of which we shall see where we can understand clearly this primary activity of the root. **Diffusion.**—Diffusion may be defined as the tendency of any substance, when it occurs as a gas or in solution, to become A diagram showing the outward diffusion of a substance being absorbed into the surface of a edible substance immersed in water. 48 BOTANY: PRINCIPLES AND PROBLEMS evenly distributed throughout the whole space available to it by moving from points of greater to points of lesser concentration. Its operation is illustrated in Fig. 27. For the minute particles given off by any strongly scented substance will move outward rapidly, even in perfectly still air, and will soon become equally dispersed over the whole space, so that no point of origin. Two gases liberated within a closed space soon diffuse throughout its whole extent and become thoroughly mixed. In the case of water, however, the molecules of water will in time have its molecules dispersed so uniformly, even though the liquid is free from moving currents, that samples taken from any part of the liquid will show identical concentrations (Fig. 28). This constant tendency toward diffusion is explained by the fact that in gases and liquids the molecules are constantly in motion, colliding with each other and striking against another and rebounding. There are obviously fewest collisions, in those directions where there are fewest molecules, and hence the greatest number of collisions of the molecules therefore necessarily takes place until they are present everywhere in uniform abundance. The principle of diffusion is operative in all living organisms, including plants, but in plants that it must be thoroughly grasped if these processes are to be understood. When two liquids are separated by a membrane through which they can pass, diffusion between them will still take place. Such diffusion through a membrane is called osmosis, and it tends to continue until the concentration of solute on one side of the membrane is equal to the composition of the liquids on both sides of the membrane in the same. If a solution of salt in water, for example, is present on one side of a membrane and pure water on the other, water will tend to diffuse through the membrane until its concentration is the same throughout. It is important to note that the concentration (the amount of substance per unit volume) on either side of the membrane rather than the total amount of the substance or the bulk of the solution, is the factor which determines the direction and rate of diffusion. It is by diffusion through the cytoplasmic membranes of the cells that water enters into plant cells. This inward movement of any given salt will continue so long as its concentration is greater in the soil water than in the sap of the root hair. THE ROOT AND ITS FUNCTIONS Osmosis Movement of Solutes.—This phenomenon of osmosis is complicated, however, by the fact that the dissolving liquid or solvent (water in this case) as well as the dissolved substance will pass through a membrane. In order to understand this circum- stance that where such movement occurs, it is always more rapid in one direction than in the other. Experiment has shown that if two solutions of different concentrations are separated by a membrane, a movement of under takes place from the less concentrated to the more concentrated solution, and that the rate of this movement is proportional to the difference in concentration. The more concentrated solution will therefore tend to equalize this access of water, and if it is confined within a closed space, the result will be considerable magni- tude, will develop. As to whether a movement of water occurs no complete agreement of opinion yet exists, for the process of osmotic inter- change involves some of the less clearly understood phenomena of physical chemistry. It is true that the molecules of the dissolved sub- stance are attractive or attractive power for water, that this attraction increases with the concentration of the substance; or that the molecules of the dissolved substance interfere with the free motion of water molecules. But it is also true that there is little material in solution the water molecules strike the membrane and pass through it sooner than they can where much material is present. This may be regarded as an explanation as really a manifestation of the fundamental principle of diffusion, since the tendency is for the solutions on both sides of the mem- brane to become alike. The following explanations have been brought about by a movement of the dissolving liquid as well as of the dissolved substance itself. None of these explanations is entirely satisfactory, but they do show how we can explain the process more clearly to our minds. The essential fact remains that water will pass through a membrane toward the denser solution, and that this passage depends on the power of the plant to withdraw water from the source. Permeability of the Membrane.—Thus far, we have assumed that both the dissolved substance and water may pass with per- meability through a membrane. But it must be remembered that since water is the ubiquitous solvent in plants, we shall confine our at- tention only to those osmotic processes in which it is involved. 49 50 BOTANY: PRINCIPLES AND PROBLEMS fect freedom through the membrane, or that the membrane is permeable to them. All osmotic membranes are readily perme- able to substances which are soluble in water, but they are not the case with which dissolved substances of various sorts can diffuse through them. One membrane may be perfectly permeable to a given substance, while another membrane is impermeable to it, and difficulty, and snailace may exclude it altogether. Nor does even the same membrane display an equal degree of permeability to all substances, for some may pass through it easily, while others with difficulty, and so on. The reason why certain substances are more permeable than are others we do not know, but they are presumably caused by the relations between the structure of the membrane and the size and character of the molecules of the dissolved substance. A membrane which allows only one substance through it but does not allow any other dissolved substance to do so is said to be permeable, and it is a highly important biological fact that all membranes in living cells seem to belong to this class. The membrane of the cell wall of a plant cell is permeable to water, but it is impermeable to such substances as sugar, which are easily dissolved in the sap solution. The cell is thus able not only to retain those substances which are necessary for its life processes by means of outward diffusion, but to use them as a permanent means of drawing in osmotically a supply of water from the soil, since their presence within the root-hairs minimally diminishes the size of these cells at a higher concentration than would occur otherwise. This same membrane, however, is permeable to most of the mineral salts present in the soil, which are thus able to diffuse readily into the roots. Other Principles of Osmotic Action.—Before we attempt to apply the principles of osmosis to the living plant, however, we should observe that there are two distinct kinds of osmotic phenomena in general about which confusion frequently arises. First, substances which are not soluble or for which any reason are not in solution cannot diffuse across a membrane by any effect whatever. Sugar, for instance, is soluble and is osmotically active, but the moment it is converted into starch, which is an insoluble substance, it ceases to be active. Second, the osmotic strength of a solution, and consequently its power to attract water, is determined not by the chemical nature of the dissolved substances but by the total concentration of material, A diagram showing a plant cell with a semi-permeable membrane around it. THE ROOT AND ITS FUNCTIONS 51 of whatever kind, which in its solution. A solution of sugar, one of salt, one of a mixture of the two, or one containing half a dozen substances, may be changed into another by diffusion and osmosis. Third, the diffusion of water through a membrane and the diffusion of solutes through the same membrane occur quite independently of one another. The diffusion of water from a solution of greater total concentration, but a dissolved substance, following the general law of diffusion, will pass from a point where that substance is abundant to one where it is scarce, without any change in the concentration of the substance to it. Given the proper conditions, it is quite possible for a dissolved substance to pass through a membrane osmotically with water, but not at all by diffusion. It is also possible for water to move without a movement of dissolved substances, or even for water to pass in one direction and dissolved substances in the other. In short, each process occurs independently of the other, each will tend to diffuse quite independently of all others. Differences in the concentration of each substance, considered by itself, will determine the rate and direction of diffusion of that substance. Diffusion and Osmosis in the Plant Cell.—It is upon the principles here explained that the absorption of water and mineral substances takes place in the plant cell. Not only for the absorption of water and mineral substances from the soil, but for most of the circulation of materials which goes on within the plant body. This is clearly outlined briefly by the structure of the animal plant cell and may now be seen in the osmotic interchanges which go on therein. The cell wall in plants is ordinarily composed of cellulose. Like muscle tissue, it has great capacity for absorbing water vigorously by imbibition and will therefore swell considerably if placed, when dry, in contact with water. This property is utilized by plants in many ways. For certain of the plant's activities, as in the germination of the seed, but the wall of an ordinary living cell is moist and has imbued water sufficient to permit it to expand. The cell wall can absorb all substances in solution, pass through this cellulose wall with great readiness, and since it thus offers practically no resistance to diffusion or osmosis, it becomes a very efficient medium for transporting materials throughout the plant. We have noted that, in the mature plant cell, the cytoplasm is dispersed in a thin layer closely pressed against the inner surface of the cell-wall, and that it is completely surrounded by a large central 52 BOTANY: PRINCIPLES AND PROBLEMS vacuole or sap-cavity, filled with water in which various sub- stances, sugar usually prominent among them, are dissolved. On its outer surface, the cell membrane is continuous with that next the sap-cavity, the cytoplasm is bounded by a delicate membrane, so that we find here fulfilled all conditions necessary for osmotic activity. The cell may therefore take up water from the mem- brane or membrane from another solution, which may be the soil water (in the case of a root-hair) or the sap-elimination of an adjacent cell. These cytoplasmic membranes, unlike the cell-wall, offer resistance to the diffusion of certain things and are thus highly important in cell physiology. We find that they are character- istically permeable to certain substances but not to others, such sub- stances as sugars, which are dissolved in the sap-cavity; and we have already noted that the cell is thus able to retain these valuable materials when they are present in the soil. This is due to its being inocommodiously a continuous supply of water from the soil or from adjacent cells. To the essential mineral salts and to many other dissol- vables, however, these membranes are generally permeable, though in varying degrees, and the cell is therefore readily able to absorb a supply of such substances from any adjacent solution. It is also true that we can assume that the degree of permeability of the cell membrane is not a fixed and constant one but is subject to change from moment to moment in response to changes in the environment and in the protoplasm itself. A cell which at first admits a given substance very readily may at another place it enter but slowly, or may exclude it altogether. Many of the physio- logical changes are closely associated with corresponding changes in the permeability of its membranes. The rapidity with which a substance passes through a mem- brane is directly related to its permeability in permeability but to differences in the concentration of the solutions on the two sides of the membrane. Where this difference is great (other things being equal), the rate of passage will be great; where it is slight, it will be slow. Therefore if the concentration of a dissolved substance within a cell is reduced, either by its diffusion into an adjacent cell or by its elimination from it, then the rate at which it enters into the construction of a complex organic molecule (in the protoplasm) the rate at which a new supply enters the cell from without is at once correspondingly increased; but the THE ROOT AND ITS FUNCTIONS 53 moment its concentration becomes equal, within and without the cell, the movement of this particular substance ceases, even though others are passing rapidly through the membranes. The Absorption of Water and Solutes by the Root. The root of the cell as an osmotic system evidently controls its most important functions. Let us first consider the role played by osmosis in that process which is the immediate subject of this chapter, the A diagram showing the entrance of water and dissolved solutes into the root. The diagram shows the entrance of water and dissolved salts into two root-hairs and their exit from another root-hair. The outermost membrane is readily permeable to water and salts but prevents the passage of sugar. The innermost membrane is impermeable to water and salts but allows the passage of sugar. Fig. 30. Diagram showing the absorption of water and nutrient materials from the soil (Fig. 30). Each root-hair, as we have seen, is merely a projection from one of the epidermal cells of the root. The cytoplasm and surrounding protoplasmic material form a tube-like whole of which is thus lined by a thin cytoplasmic layer, with its membranes (Fig. 25). The root-hair penetrates the soil and comes in intimate contact with the soil-particles, to the surface of each of which is attached a root-hair. The water in this water is dissolved a great variety of substances, but the total concentration of the soil-solution is normally less than that in 54 BOTANY: PRINCIPLES AND PROBLEMS sap-avity of the root-hair, where sugar and other materials are dissolved. In obedience to the law of osmosis, therefore, water will pass from the solution into the cell, and the mem- branes of the root-hair and into its sap-avity. This flow of water will continue so long as there is a difference in total density between the solution and the sap-avity. When the soil becomes dry and the film around each particle grows so thin that the surface attraction of the particles equals the atomic attrac- tion of the root-hair solution, the flow will necessarily cease; and if the contact with air throughout the length of the plant, the plant will suffer from drought. Sulfs and other substances in the solution diffuse through the root-hair cells independently of the passage of water, and the rate at which they enter depends upon the factors which we have above considered. Any substance which is in greater concentration outside than within a cell, and to which the membranes are permeable, will diffuse outwards into the soil; but except for carbon dioxide, which is given off by respiration, no other substances diffuse in from the plant in this manner. As water enters through its sap-avity of the root-hair, the solution there becomes less dense; and the first cell of the cortex is consequently able in turn to withdraw water osmotically from the root-hair cell. The second row of cortical cells may now withdraw water from their first neighbors until they reach a point where the water reaches the central cylinder. The water-ducts here, however, are nothing but dead cells, their living cytoplasm having died away when they were cut off from their supply. They are filled with water, and it is hard to understand why water should move into them from the cells of the cortex rather than in any other direction. It is evident that in these cortical cells, a considerable pressure is probably developed by osmosis, and water may simply be squeezed through the cytoplasm and into those parts of the cortex which are not connected up through the ducts under a good deal of pressure. This root- pressure may be measured by a gauge attached to an opening in the stem. In some trees, such as oaks, great heights in the trunks of trees we shall speak later; but root-pressure is apparently only one of the factors involved. THE ROOT AND ITS FUNCTIONS 55 Other Osmotic Phenomena in the Plant.--Not only the absorption of water from the soil, but the whole process of circulation within the plant, is dependent on osmotic action; and one case, for the salts taken in by the root-hairs, and any dissolved sub- stances in taken other cells throughout the plant, move from cell to cell by diffusion, is due to osmotic pressure. Still another contribution of osmosis to the plant's activi- ties in maintaining the turgidity of the tissues. It is evident that if a plant is placed in a solution of salt, or sugar, or some other substance which it can absorb, it will take up this water vigorously, it will become plump and fully expanded and will press tightly against its neighbors. If all the cells become turgid in this way, the plant will stand erect and rigid, like a well-watered tree. In fact, most of parts which possess a firm skeleton, such as the leaf blades, floral organs, or other comparatively stiff structures, this turgidity is necessary to main- tain their shape. But if a cell is exposed to a solution of greater concentration than its cell-sap, water will be withdrawn from it, it will collapse, and in consequence lose its turgor. If a plant is placed in such a condition of *plasmolysis*, if extreme or long-continued, will result in the death of the cell; and, if extensive, in the death of the plant. Osmosis is also responsible for the growth of roots. In any growing region we find at points where the cells are multiplying in number but are still small, and another point behind this where each cell has grown larger and more numerous. This is due to the consequent stretching of the cell-walls and growth of the tissues, is due to rapid absorption of water by the young and delicate cells, through osmosis. The force exerted by any growing region is thus primarily due to osmotic pressure. Other Functions of the Root.--We have briefly discussed the root as an organ of anchorage and of storage, and now more detail as an organ of absorption. It has less frequently certain other functions which should be mentioned here. Roots may arise from almost any part of the stem or from any part of the leaves. In many climbing plants they are produced abundantly on these aerial organs and serve to hold the plant firmly to its support. In some cases they may be used as prop roots, growing above the ground and pass into the soil, thus acting as props or gay- ropes for the tall plant. In epiphytes, the roots are sent out directly into the air and possess a characteristic spongy envelope. 56 BOTANY: PRINCIPLES AND PROBLEMS which absorbs and holds rain-water and dew. In parasitic plants the roots are converted into short, surging organs, penetrating the surface and withdrawing thencefrom the food upon which the parasite feeds. The root and the leaf are the two most important vegetative organs of the plant, and it is therefore the leaf which we shall next discuss. **QUESTIONS FOR THOUGHT AND DISCUSSION** 124. Which is apt to be more regular and symmetrical in shape, the root-system or the stem-system of a plant? Why? 125. In a young plant growing from the seed, the root is larger and better developed than the shoot. Explain why. 126. Taproots are usually tapering in shape, being broadest near the surface of the ground and gradually narrowing below. Explain. 127. Do all roots grow directly downward, provided that there are no obstacles in the way? Give evidence for your answer. 128. It has been found that for most species of plants there is a rather definite order in which the roots develop. This is shown by the bulk of the roots are formed. Some species are deep-rooted, some shallow, and others intermediate. What physiological differences between the plants can you suggest which might account for these structural differences? 129. Why does the planting of grass on sand-dunes stop the drifting of the sand? 130. Why does a covering of grass turf or other like plant growth prevent a layer of soil on steep banks and other exposed situations? 131. A is a "soil-binder" to prevent erosion would a crop of carrots or a cover of grass be better? Why? 132. If from around a tree which has been growing in the forest all its neighboring trees are cut down, it is then much more likely to blow over than it was before they were cut down. Why? 133. If a tree-trunk is partly buried, for instance through raising the level of the soil about it by grading or other means, why is the tree likely to die? 134. Which type of plant do you think would withstand a drought better, one with a fibrous root or one with a taproot? Why? THE ROOT AND ITS FUNCTIONS 57 135. Which do you think will tend to have the deeper root-system, a bog plant or a desert plant? Why? 136. Do you think that a root-system will tend to spread farther in the soil or at the surface of the soil? 137. Roots will usually grow toward a supply of nutrient material in the soil. Can you suggest what causes them to do this? 138. Root-hairs are entirely absent on the older portions of a root. Why? 139. Root-hairs are absent from the growing region at the tip of the root. Of what advantage is this fact to the plant? 140. Of what advantage are root-hairs to the plant aside from their function in the absorption of water and salts? 141. Root-hairs are commonly absent in water plants. Explain. 142. The cells of the root hairs are usually found in the form of a solid rod, and that of the shoot in the form of a hollow tube. Of what advantage to the plant are these arrangements of the tissues? 143. Why is the soil immediately around the base of a tree trunk somewhat moistened? What is this called? 144. Give an example (other than those cited in the text) of a plant which has a fibrous root-system: a tap-root, climbing roots; parasitic roots. 145. Why is protoplasm expanded as the only really "living" substance in the plant? 146. What are some of the advantages of cellular structure in plants and animals? 147. Give all the reasons you can think of for the fact that cells should be very small objects. 148. Why do animal cells have a decided appearance? 149. Of what advantage is the streaming of protoplasm which commonly takes place in the cell? 150. What important functions does the cell-wall perform? 151. Are there any cells of the plant which are useful to the plant after they die? 152. If a leaf made of bladders or a similar connective membrane is filled with moisture, tied up, and placed in a vessel of water, what will happen? 58 **153.** If this same ear is filled with water, tied up and placed in a vessel of sugar solution, will it take up sugar? **154.** Under what conditions will water pass through a membrane osmotically without any movement of dissolved substances taking place? When would this be likely to happen in the root-hairs of plants? Explain. **155.** Under what conditions will a dissolved substance pass through a membrane without any movement of water taking place? When would this be likely to happen in the root-hairs of plants? Explain. **156.** Under what conditions will water pass through a membrane in one direction and a dissolved substance in the other? When would this be likely to happen in the root-hairs of plants? Explain. Note: Assume in both questions 153 and 154 that the ear is made of an elastic material which is semi-permeable so that it allows the free diffusion of water through it but entirely prevents the passage of sugar. Assume in both questions 155 and 156 that the ear is made of an impermeable salt. Under the conditions set forth in each question, state clearly what will happen, carry on the process through to its conclusion and noting the differences between the first and later stages. **157.** What will happen if the ear is filled with a solution of sugar and placed in a vessel of pure water? **158.** What will happen if the ear is filled with a solution of sugar and the liquid in the vessel is a solution of salt which is of lesser concentration than the sugar solution in the ear? **159.** What will happen if the ear is filled with a solution of sugar and the liquid in the vessel is a solution of salt which is equal in concentration to the sugar solution in the ear? **160.** What will happen if the ear is filled with a solution of sugar and the liquid in the vessel is a solution of salt which is greater in concentra- tion than the sugar solution in the ear? **161.** What will happen if the ear is filled with a mixture of sugar and salt solutions of equal concentration and that the liquid in the vessel is pure water? **162.** Can a plant take out all the water from the soil around its roots? Explain. **163.** The stems of most woody plants will "bleed" if cut in the spring, but will not do so if cut in the summer. Explain. **164.** Why do tomatoes and other soft and juicy fruits tend to split open at low temperatures? THE ROOT AND ITS FUNCTIONS 59 165. How does it happen that a plant can take up such large amounts of salts when these salts occur in such very weak concentrations in the soil? 166. Many plants thrive for long periods without excelling their root-systems into fresh regions of soil. How are they able to obtain an unlimited supply of water and mineral salts? 167. A plant grown in "water-culture" (on a jar of water containing the necessary nutrient salts in solution) will almost completely remove those salts from the jar, even though its roots still only a small part of the jar. 168. Iodine is much more abundant in the tissues of certain sea- weeds than it is in the sea water. Explain how this can be. 169. How is it possible for a group of cells in the middle of a tissue, surrounding a cell, to absorb large amounts of a substance which is rare or absent in the other cells? 170. The text states that the cell-membrane of a root-hair is imperme- able to sugar, and that sugar therefore cannot get out of the root-hair into the cell. What do you think the sugar is able to get into the root-hair in the first place? 171. A crop plant which removes a large amount of nutrient material from the soil is known as a "heavy feeder" and one which removes little and no nutrient material from the soil is known as that which would cause plants to differ in this respect? 172. The salts taken from the soil by one plant are often very differ- ent in kind and amount from those taken in by another plant. What to factor do you think might account for this? 173. One crop often needs a different fertilizer from another. To what physiological differences in the two crop-plants may this be due? 174. How do submerged water-plants get their salts? 175. Some fertilizers, when applied very abundantly, will often kill plants. 176. A spray-solution which is strongly concentrated will often kill plants to which it is applied. Why? 177. A spray-solution which will kill one plant may not kill another. Explain. 178. Strong spray will often kill the young and growing parts of a plant but not its older portions. Why? 60 **BOTANY: PRINCIPLES AND PROBLEMS** **179. If a very strong spray (such as leaf-sprayer), used against cer- tain bark-insect pests, is applied to a tree, why must this be done only when the tree is leafless?** **180. In such places as gravel walks and tennis courts it is often customary to kill weeds by sprinkling salt upon them. Why is this practice effective? What other methods are available for killing weeds?** **181. How is it possible for some plants to live on salt-marshes and sea-beaches while others cannot?** **182. Desert plants and salt-march or sea-beach plants frequently show similar modifications in structure. Explain.** **183. Why do salt-marsh or sea-beach plants usually die if subjected to flood?** **184. The sap concentration in the cells of parasitic plants has been found to be higher than in the cells of the plants upon which they are parasitic. Explain.** **185. Why are dried currants, raisins, and prunes useful so much when placed in water?** **186. When berries are cooked with little sugar they are apt to burst. When cooked with much sugar they are not apt to collapse. Explain.** **187. Vegetables usually cook more quickly if they are not salted while cooking. Explain.** **188. Celery, cucumbers and similar vegetables are often placed in water for a while before they are served. What effect does this produce and why?** **189. Why are we thirsty after eating much salt or sugar?** Note.—In the five following questions, remember that decay is due to the activity of bacteria, which are nearly every small plant orilis (p. 25). **190. Why is salt such a good preservative of vegetables, meat, fish, and other foods?** **191. Which will "keep" better if exposed freely to the air, grape juice or grape jelly? Why?** **192. Old-fashioned preserving of fruit was done by the "pound for pound" method, a pound of sugar being used for every pound of fruit. Why was this method successful even in the absence of boiling or any other means of sterilization?** A diagram showing the process of making preserves. THE ROOT AND ITS FUNCTIONS 61 135. What is the fundamental difference between preserving food by salt and preserving it by heat? Explain by means of a similar substance. 134. A little salt placed in the water in which cut flowers are standing will often cause them to keep fresh longer than they otherwise would. Explain. Would a large amount of salt in the water have the same result? 133. What causes the root-hairs to force themselves among the soil particles? 132. Growing roots and stems often exert tremendous pressures, sometimes sufficient to split and lift big heavy rocks. What causes this expansive power? 137. Roots are sometimes split apart, in quarrying, by the insertion of dry wooden wedges into drills or cracks. These wedges are then pulled out with great force. Why do these wedges work so well? Is it different in its origin is this pressure from that exerted by a tree-root which splits open a rock? 138. Roots growing below, tumips, and similar fibrous plant parts sometimes crack open during growth. Why? 139. Water is sometimes forced from the leaves of a plant in the form of droplets. Why is this and under what conditions is it likely to take place? **REFERENCE PROBLEMS** 22. Are the "root-crope" (such as carrot, parsnip, turnip, and beet) usually annual, biennial, or perennial? Why? 23. Are there any deeply-rooted plants which do not have tap-roots? 24. What important crop-plants are propagated by buds formed on its roots? 25. When and by whom was the Cell Theory first formulated? 26. When and by whom was the term "protoplasm" in its present sense first used? 27. Who discovered that every cell has a nucleus? 28. What do we mean by saying that protoplasm is a colloidal substance? 29. State two fundamental differences between the typical plant cell and the typical animal cell. With what general differences between animals and plants can you compare these two cells? 30. What prevents the cells of a plant from falling apart? 62 BOTANY: PRINCIPLES AND PROBLEMS 31. Distinguish between physical drought and physiological drought. In what does this difference consist? 32. What is the chemical composition of the ash of three important crop-plants? (Take figures from any reliable determination.) How does it appear that the substances and their proportions are different in the different crops? 33. What is the difference between a saturator and a balanced solution for plant growth? 34. What relation is there between the osmotic pressure of a substance and the gas pressure exerted by this substance in its gaseous state? 35. Give the derivation of the following terms and explain in what way each is expressed: - Protoplasm - Cytoplasm - Nucleolus - Vacuole - Plastid - Osmosis - Lipid-bilayer - Enzyme - Phospholysis CHAPTER V THE LEAF AND ITS FUNCTIONS The vegetative organs of the plant naturally fall into two groups: The root-system, situated in the soil and concerned primarily with the absorption therofore of water and certain nutritive materials; and the shoot-system, which includes the stem (or shoot), which unfold in the air and are concerned primarily with the manufacture of food, the raw materials for which they derive from both air and soil. Of the two members of the shoot-system the leaf is the primary and more important one, the stem serving merely as a support to it. It is also provided with pores, or stomata, which provide a means of communication between them and the root-system. It is logical, therefore, for us to follow our study of the root with that of the leaf. The Structure of the Leaf.--Before we can understand clearly the functions which the leaf performs, we shall need to observe with some care its rather complicated structure. A diagram showing three types of leaf-formation. At left, netted-veined leaf of Linden. A, blade; B, petiole; C, stipules. At right, parallel-veined leaf of Solomon's Seal. D, blade; E, petiole. Fig. 31.--Two types of leaf-formation. At left, netted-veined leaf of Linden. A, blade; B, petiole; C, stipules. At right, parallel-veined leaf of Solomon's Seal. D, blade; E, petiole. 64 BOTANY: PRINCIPLES AND PROBLEMS External Structure (Figs. 31 and 32).—Externally, the typical leaf consists of a broad, flat, and thin portion, the blade, which is the seat of all the functions of the leaf. It is green in color and provided with a system of ribs or veins of stouter texture than the rest of the tissue. The blade may sometimes be attached directly to the stem, as in the case of the simple leaves of the petiole, which holds it out in a place favorable for the performance of its functions and serves as a highway for transportation of water and food between blade and stem. At the base of the petiole are often found two small appendages, the stipules, the function of which in many cases obscure. Leaves vary widely in size, shape, texture, margin, venation, and other characters. They may be entire or lobed or even, or it may be lobe'd or sometimes actually divided into separate portions, the leaflets, in which state the leaf is said to be compound (Fig. 33). The margin is sometimes quite smooth, but A simple leaf of Mountain Ash, with eleven leaflets. B simple leaf of Apple. Fig. 32.—Simple and compound leaves. A, compound leaf of Mountain Ash, with eleven leaflets. B, simple leaf of Apple. THE LEAF AND ITS FUNCTIONS is more commonly broken into teeth of various sizes. The vein-system (Fig. 31), is either parallel where the veins run side by side, or divergent, where they spread out at right angles to each other. The petiole and stipules vary greatly in their development. Foliar anatomy. Internally, the structure of the leaf is highly differentiated. A transverse section cut at right angles to the surface of the blade (Fig. 34) displays three important tissues: the epidermis, or protective covering; the mesophyll, A diagram showing a cross-section of a leaf, with labels indicating different layers. Fig. 35.—A small piece of a typical leaf-Made, seen in three planes and highly magnified, showing the epidermis, mesophyll, and vascular bundles. A, epidermis layer. B, spongy layer. C, lower epidermis, covered by exudate. D, stem (in one plane). E, mesophyll (in another plane). F, vascular bundles constituting the major portion of the leaf substance, and the reinae, each of which is a separate fibre-vascular bundle and represents a final branch of the vascular system which runs through root and stem. The epidermis covers the entire leaf surface and is usually but one cell-layer in thickness. Its cells are generally thin-walled and contain little or no chlorophyll. Spaces over the outside wall is a thin, waxy, water-proofing layer; the cells extend from cell to cell and form a continuous skin which covers the epidermis. It is usually much thicker on the upper than on the lower surface of the leaf. The mesophyll is not an unbroken layer but is provided with minute openings, the stomata (singular, stoma), through which an exchange of gases between the interior of the leaf and the outside air may take place. 65 66 BOTANY: PRINCIPLES AND PROBLEMS place (Fig. 35). These are much more numerous in the lower epidermis than in the upper, and, indeed, are often absent from the latter altogether. Each stoma is a slit-like pore formed by A cross-section through the blade of a typical leaf. A, upper epidermis, covered with cuticle. B, lower epidermis also covered with cuticle. C, guard-cells between two stomata. D, stoma. E, stoma. T, vein. LARS KONG From Gansing, "Textbook of Botany", copyrighted by the Macmillan Company. Reprinted by permission. Fig. 35.—A stoma. A, face view, showing the two guard-cells containing chloroplasts. B, transverse section, with the two guard-cells, several adjacent cells of the epidermis, and a vein. C, transverse section of a stoma. T, vein. From Gansing, "Textbook of Botany", copyrighted by the Macmillan Company. Reprinted by permission. the pulling apart of two modified epidermal cells, the guard-cells, which are unlike other cells of the epidermis in containing chloroplasts. These guard-cells are so constructed that when A stoma. A, face view, showing the two guard-cells containing chloroplasts. B, transverse section, with the two guard-cells, several adjacent cells of the epidermis, and a vein. LARS KONG From Gansing, "Textbook of Botany", copyrighted by the Macmillan Company. Reprinted by permission. THE LEAF AND ITS FUNCTIONS 67 plump and turgid with water they tend to pull apart, thus enlarging the opening. On becoming limp and partially collapsed, however, they spring together again and close it. The degree of stretching of the guard-cells is determined by the water supply of the guard-cells rises and falls in response to changing internal or external conditions. The mesophyll consists of tissue which is characteristically thin-walled, soft, and green. The cytoplasm within its cells contains very small, roundish bodies, denser than the rest of the living substance, and give rise to the characteristic chloroplasts which are found in them. These the green pigment chlorophyll, to which the characteristic color of foliage is due. The mesophyll is not a homogeneous tissue but in typical leaves is divided into two main regions: (a) the palisade layer, which lies on the upper side of the leaf is composed of cells which are elongated at right angles to the leaf surface, packed rather closely together, and provide a large surface area for gas-exchange (Fig. 30). This region is known as the palisade layer and here is carried on most actively the process of food-manufacture or photo-synthesis. It is made up of long, narrow cells which are so very irregular in shape that large air-spaces occur between them and a very loose, sponge-like tissue, the spongy layer, is formed. The cells of this layer communicate directly with the outside air through the stomata. Chlo-roplasts are present in the spongy layer, but not abundantly. There is no evidence that these chloroplasts have a surface, opportunity is provided in this portion of the mesophyll for those gas-exchanges which are continuously taking place between the leaf and atmosphere. In addition to this there is a continuous loss of both carbon dioxide and oxygen in the process of photosynthe-sis and respiration, and the evaporation of water in the process of transpiration. Remaining through the blade are the fibre-vascular bundles or veins, the channels which the leaf tissues are kept in communi-cation with the rest of the plant. The main veins are stout, often presenting a distinct ridge along their length. They are thick. These break up into smaller and smaller veins, and finally into minute veinslets which consist of only a few cells. Each vein is surrounded by a sheath of parenchyma tissue, a tissue in which most of its rigidity is seen. Within this are two tissues: The wood, consisting largely (as elsewhere in the plant body) of clen- 68 BOTANY: PRINCIPLES AND PROBLEMS gated, water-conducting cells, the *tubuloid* and *dacta*, which dis- tribute the water and dissolved substances brought up through the stem from the root; and the bast, consisting of especially modified cells, the *xerocella*, which collect from the mesophyll. A diagram showing the structure of a plant cell. The cell is divided into three main parts: the outermost layer is labeled "Chloroplast," the middle layer is labeled "Nucleus," and the innermost layer is not labeled but is implied to be the cell wall. The chloroplasts are shown as circular structures within the nucleus. The xerocella are shown as elongated cells surrounding the nucleus. Fig. 30.—A palisade-cell. The *chloroplast* are somewhat biconcave bodies, being roughly circular in front-view and elliptical when seen from the side. They are usually arranged in two rows parallel to each other, with their long axes forming parallel to the cell wall. The *chloroplasts* which lie nearer circular are situated between the *xerocella* and near the *nucleus*. Those which appear elliptical are lying next to the cell wall. the food manufactured there and convey it to the bast of the stem, along which it is transported to other parts of the plant. The petiole, usually circular in cross section, has within it a cylinder or half-cylinder of fibro-vascular bundles which are con- THE LEAF AND ITS FUNCTIONS 69 timous with the main veins of the blade above and which enter directly into the vascular cylinder of the stem below. **Phosphorus.**—The leaves are the **source** of food in the **manufacture** of food from certain simple inorganic materials—carbon dioxide and water—by energy derived from light. This process is called photosynthesis, and it is evident that for it is not only an essential function of green plants themselves but is of the utmost significance to animals and man, because it constitutes the sole ultimate source of food in the world. Food is primarily necessary for life, and it is also necessary for prosperity for living things. In the green parts of plants, and nowhere else among organisms, is active or kinetic energy—in this case the energy derived from light—available to living organisms for use in maintaining their vital activities; and, moreover, in green plants alone are produced those substances which are indispensable to all other living bodies are constructed. All the complex metabolic changes which later take place in the organic world are simply elaborations or simplifications of the processes which occur in green plants. A more detailed account of the various types of foods and their uses, and of the energy-relations of the plant, will be given in our chapter on nutrition. **Materials.**—The materials combined by the plant in this proc- ess are but two—water and carbon dioxide. Water is absorbed from the soil by means of the roots, through the epidermis, the petiole, and the veins of the leaf, and thence enters the mesophyll cell. None is obtained by the leaf directly from the atmosphere. It should be remembered that only a relatively small portion of the water used by a plant is transpired; most of it is used for much larger part soon leaves the plant again, passing out of the leaf into the air by transpiration. The carbon dioxide used in photosynthesis is present in the air at all times, but it is always present, but in such small quantities that it constitutes only about three parts in ten thousand of the atmosphere or three hundred millionths (300/1000) per cent. If its higher concentration would be advantageous to plant growth, since up to a certain point the rate of photosynthesis rises if the proportion of carbon dioxide increases; but even if we assume that through this comparatively rare gas alone that the plant derives its supply of carbon, that element so vitally necessary to all living organ- isms. No other carbon compound, not even the abundant 70 BOTANY: PRINCIPLES AND PROBLEMS supplies present in the complex organic materials of humus, can apparently be drawn upon by ordinary green plants. Carbon, oxygen, and hydrogen are obtained from water and carbon dioxide, elements derived from the soil, constitute the necessary chemical basis for plant life. **Mechanism.** The mechanism or saprophytes by which water and carbon dioxide are combined is the remarkable green pigment chlorophyll. This is present only in the *chloroplasts*, portions of the cytoplasm, digested by the cell wall. These chloroplasts may be few and large or certain lower plants but in the higher ones are almost always small, numerous, and more or less spherical in shape. They are located in the path of light entering the leaf of the plant. As chlorophyll itself we know comparatively little. We know that it is a complex protein and contains magnesium. Iron is essen- tial for its productions but apparently does not enter into the constructive process. It is also known that light is necessary; light is also necessary for the full development of chlorophyll, as is shown by the pale color of leaves which have grown in darkness. We are ever more convinced that chlorophyll is not merely a pigment or agent in bringing about the union of carbon dioxide and water, nor have we yet succeeded in imitating this process in the labora- tory. We know that chlorophyll is a very unstable compound and con- tribute material to the product formed and that it is not used up itself in the process, and we may therefore infer that it acts some- what like a catalyst. Associated with chlorophyll is usually another pigment or group of pigments, yellow in color instead of green, to which the general term "xanthophyll" has been applied. These are not concerned with photosynthesis and their function is poorly understood. To them are due most of the yellow colors which occur in plants. Chlorophyll is a very unstable compound and tends to break down when exposed to light from below head or when the leaf loses its vitality, but the yellow pigments are much more resistant and often survive long after chlorophyll has disintegrating. **Energy.** Energy is necessarily expended in the process of breaking up the molecules of water and carbon dioxide and recon- stituting them into carbohydrates. It has been shown that this energy is derived not from heat, as in so many cases, but entirely from light, which thus plays an essential part in the physiology of plants. According to the most widely accepted theory light THE LEAF AND ITS FUNCTIONS 71 is due to minute and enormously rapid vibrations, the length of the vibration--its one-length--determining the color of the light produced. Sunlight, therefore, is a mixture of all the visible colors, each wave-length being composed of a great variety of different wave-lengths, but when such light is passed through a prism these become sorted out into a mixture of all the colors. The red rays, which have the longest wave-length, are approximately 750 millimeters in a millimeter, to the violet ones, where it is approximately 400. These visible radiations are by no means the only ones which occur, however. Rays of longer wave-length than red-- A diagram showing light breaking up a plane beam into its constituent parts, which form a spectrum. A shows a plane beam of white light. B shows the same beam after passing through a prism. The upper part shows the dark absorption bands in the red and the blue. (From Gomori, "The Living Plant," Henry Holt and Co.) Fig. 27.--Diagram of a spectrum showing light breaking up a plane beam into its constituent parts, which form a spectrum. A shows a plane beam of white light. B shows the same beam after passing through a prism. The upper part shows the dark absorption bands in the red and the blue. (From Gomori, "The Living Plant," Henry Holt and Co.) The infra-red rays--waves gradually into heat-waves, and those shorter than them, the ultra-violet rays--are active chemically. When falling upon different objects, light behaves differently. All of it may be absorbed by the object and converted into heat or some other form of energy; or it may be reflected back; or all may be reflected, the object then appearing white; or certain wave-lengths may be absorbed and certain others reflected, the object in such a case displaying to our eyes the color of 72 BOTANY: PRINCIPLES AND PROBLEMS the light which it reflects. We know, for example, that a green substance like chlorophyll absorbs in general those wave-lengths which have a red color, and that it reflects the green or greenish-yellow. We can determine more accurately, however, the kind of light which is absorbed by a substance, if we break up into a spectrum the light which falls upon it, and observe the absorption bands, in those portions which correspond to the particular kinds of waves which are reflected. The absorption spectrum of chlorophyll (Fig. 37) shows a narrow, sharp, black band in the orange-red and a wider, less definite one in the blue-green. This indicates that chlorophyll absorbs the kind of light which chlorophyll absorbs, and suggests that the red and blue rays in sunlight, and no others, furnish the energy used in the process of photosynthesis. Chlorophyll possesses the remarkable property of utilizing energy from this source in the manufacture of fuel, an ability that is unique in the organic world. The intensity as well as the character of the light affects the rate at which photosynthesis proceeds. The process begins at illuminations of very low intensity, reaches its maximum at about this intensity, and then decreases with further gain in light which is so bright as to injure protoplasm. Photosynthesis may be readily accomplished in artificial light of the proper intensity, as shown by experiments. Given a supply of the necessary raw materials, a sufficient temperature, the presence of chlorophyll, and light of proper character and intensity, photosynthesis will take place wherever there is a plant. Although these conditions are predominantly fulfilled in the mesophyll of the leaves, they may also be present to a lesser extent in other parts, epipods, edaphic layers, and other organs, thus inducing a utilization of carbon dioxide even when a small supplementary food supply. **Products.**—Let us turn now from a consideration of the neces- ary components of photosynthesis to a study of its products. The details of the process whereby carbon dioxide unites with water are not yet known, but the formation of formicablehyde (CH$_2$O) is certain. It is probable that this is the first product which can be recognized, however, and a substance which is therefore of unique interest, is glucose or sugar, C$_6$H$_{12}$O$_6$, formed according to the following equation: $$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$ THE LEAF AND ITS FUNCTIONS 73 Glucose is present in the sap of practically all plant cells. It is the fundamental carbohydrate and the basis for all other foods, freely derived from it. Moreover, through the action of enzymes and by various additions and chemical modifications, all the organic compounds of plants and animals. The product of a large number of reactions is a chlorophyll-becoming result of the stoppage of its manufacture there, and is disadvantageous for other reasons. We find, accordingly, that before photosynthesis has long continued, the resulting sugar becomes a very important food material for the plant (C6H12O6). Starch is complex and inisible, occurring in both definite bodies or grains, the size, shape and markings of which vary with different kinds of plants and their species. The starch molecule is very large—just how large we do not know—and is derived through the combination of many glucose molecules, with the liberation of a molecule of water from each, thus: $$\text{C}_6\text{H}_{12}\text{O}_6 + \text{H}_2\text{O} \rightarrow 2\text{C}_6\text{H}_{12}\text{O}_6$$ Neither sugar nor starch are consumed in very great quantities in the leafchase, but most of the products of photosynthesis are removed shortly after their production to those regions of the plant where they are to be used or stored. Respiration is the process by which the atoms of carbon dioxide and water out of which glucose is produced there is evidently a surplus of oxygen, and we find this oxygen given off as a by-product. This oxygen is necessary for respiration, obtainable only from green plants during daylight. This is of little significance to the plant itself but is often important to other organisms. Photosynthesis is the process by which the food of the plant is manufactured from very simple inorganic materials, through the agency of a characteristic substance chlorophyll, and by energy from light. The significance of photosynthesis lies in the fact that it is the only process among living things whereby organic compounds are built up from simple inorganic substances, with the resultant storage of energy. All other processes are merely changes already encountered either with the transformation of one type of organic material *stands for the unknown number of smaller molecules which are united into one of the large and complex units of each a substance mentioned.* 74 BOTANY: PRINCIPLES AND PROBLEMS into another or with the breaking down of complex organic compounds into simpler ones. Photosynthesis alone is fundamen- tally constructive, and the activity of green plants thus underlies that of all other organisms. Transpiration is the evaporation of the mesophyll, or the epoxy layer, is not concerned primarily with photosynthesis but with the interchange of gases between plant and atmosphere. Notable among these interchanges is the evaporation of water from the tissues and its replacement by air which we know as transpiration. The Importance of Water. The water-circulation of a plant is of the utmost importance to it and profoundly influences its structure and activities. We have seen that water constitutes the major portion (75 to 90 per cent) of plant tissues in general, and a very large proportion of the total weight of the plant. The abun- dance of water keeps the cells plump, and by maintaining the rigidity of the tissues, enables the soft parts of the plant to preserve their shape. The water also serves as a medium in which one of the raw materials entering into the process of photosynthe- sis. It is the solvent of the mineral nutrients, which can enter the plant only when they are dissolved in water, and in watery solutions all the important physiological processes of the plant take place. The maintenance of an abundant supply of water in all its tissues is therefore essential for the life and growth of the plant. To this end the primary requisites are evidently the presence of sufficient amounts of mineral salts in solution and its abundant absorption throught by the roots. Of no less signifi- cance in the water-circulation of the plant is the process by which this water evaporates from the leaves and passes into the air. Absorption must equal or exceed transpiration, for a plant to thrive, for should there be a deficiency in income or an excess of outgo, a shortage of water will result in the tissues, and the plant will die. Only a very small fraction of the water which enters the root- hairs and passes upward to the leaves takes part in the manu- facture for food. Most of it is lost through evaporation through the cells of the epoxy layer and evaporates from their moistened wells, departing through the stomata as water vapor (Fig. 38). A smaller amount escapes through pores on the epidermal surface of the epidermal cells. During the growing season a constant THE LEAF AND ITS FUNCTIONS stream of water is thus passing through the plant body, entering at the stomata and passing through the stomata. The total quantity of this water entering the plant may be hundreds times as much as the final dry weight of the plant itself (Fig. 39). Fig. 39.—Transpiration from the leaf-stalk. Cross-section of a blade sectioned transversely to show the passage of water from the leaf through the mesophyll cells into the air-spaces and out through the stomata. Solid circles represent water molecules. The Rate of Transpiration.—The rate of water-loss varies greatly according to the kind of plant, the soil conditions, the season of the year, the time of day, and various environmental factors. As a general rule, we find that the rate tends to increase under conditions which favor rapid evaporation, such as high temperature, bright light, rapid air movement and low humidity; and to decrease under environments of the opposite character. Transpiration is influenced by many factors which influence evaporation alone. The rate of water-loss from a given leaf-surface and from an equal area of free water do not rise and fall together. For example, in a given leaf, living leaf sometimes being relatively higher and sometimes relatively lower. There must, therefore, be factors in the leaf itself (as opposed to those outside) which either tend to accelerate or to retard transpiration. The most important of these is doubtless the opening and closing of the stomata, which we have already discussed. The second factor is secretion of the sap in the mesophyll cells also probably determine to some extent the rate at which water evaporates from their surfaces. The amount of transpiration during any growing season may be large or small, depending on the size of the plant, 75 76 BOTANY: PRINCIPLES AND PROBLEMS Its leaf-area, its transpiration-rate, and the moisture and fertility of the soil. Of most significance to the plant, however, is not the actual bulk of transpiration, but the efficiency with which the water is used. This is determined by comparing the weight of A corn plant and a barrel. Fig. 39.—The water-representation of a corn plant. The amount of water transpired with the weight of the dry plant material ultimately produced, their quotient being known as the water-requirement of the plant. Thus when we say that the water-requirement of corn is 50 lbs., this means that every gram of dry weight of corn plant produced the total water transpired with the weight of the dry plant material ultimately produced, their quotient being known as the water-requirement of the plant. Thus when we say that the water-requirement of corn is 50 lbs., this means that every gram of dry weight of corn plant produced THE LEAF AND ITS FUNCTIONS 77 at maturity, there have been transpired through its leaves 800 grams of water. Species vary markedly in their water-require- ment, and so do plants of the same species when grown under different conditions. The Transpiration. Transpiration—Excessive water-loss is an ever-present danger to land plants, and many structural modi- fications have been developed by various species, or may appear in particular cases, to counteract this loss of water, which tend to reduce this loss. The question therefore arises as to whether transpiration is an unevolved mode necessary by the fact that the stomata are open to permit the exchange of gases with the atmosphere; or whether it really is a func- tion of the leaf and performs a useful part in the plant's economy. It was long thought that water must be taken in through the leaves, and that it was lost through the stomata, absorption of nutrient materials from the soil, but a fuller un- derstanding of the phenomena of osmosis and root absorption shows the failure of these views. It has been shown that transpiration is useful in concentrating the very dilute solutions of nutrient salts taken from the soil—boiling them down, so to speak—and thus making them more available to those which determine the entrance of nutrient materials into the plant preclude such an explanation; and, indeed, experiment shows that the amount of water transpired is not related to the amount of water absorbed. Transpiration from the leaves, however, is evidently what causes the transpiration stream, or condensation of moisture on the surface of the leaves through the lifeless ducts in the wood of the stem. We shall consider this movement more fully when discussing the functions of the stem; but it is sufficient here to state that certain dissolved in- substances are transported bodily from the central cylinder of the root upward throughout the plant as far as the ramifications of the dead conducting elements of the wood extend. This movement is due partly to diffusion from cell to cell, and in tall plants, particularly, the transpiration stream probably forms a distinct service in distributing water throughout the plant; but it does not supply the nutrient materials absorbed from the soil. Transpiration is also of distinct usefulness in regulating the temperature of the plant. The sun's rays contain much energy than it uses in photosynthesis, particularly in bright light; and 78 BOTANY: PRINCIPLES AND PROBLEMS The excrese, as heat, would sometimes raise the temperature of the tissuedangerously high. It not absorbed in evaporating water from the mesophyll cells. Transpiration is carried on primarily in the leaves, but may occur in any other part of the plant. The stomata are the pores through which transpiration takes place. The leaves lose much water is often prevented in such regions by the development of cell layers with corky walls. QUESTIONS FOR THOUGHT AND DISCUSSION 200. Are leaves generally more variable in their area or in their thickness? Explain. 201. What general difference in shape is there between net-veined and parallel-veined leaves? 202. Leaves on the same plant often differ markedly in the length of their petioles. How do you explain these differences? 203. Name three functions which the veins of a leaf perform. 204. In leaves where the veins are much stoutner than the thickness of the blade, they usually stand out on the under side of the leaf and thus leave the blade smooth. What are the advantages of this arrangement to the plant? 205. Of what advantage, and of what disadvantage, is it to plants of temperate climates to shed their leaves in the winter? 206. The leaves of "evergreen" trees do not remain permanently on the tree but fall off during winter and then drop off. Why should these leaves continue to live and function indefinitely? 207. The upper epidermis and its cuticle are almost always thicker than the lower. Explain. 208. The cells of the palisade-epidermis usually have transparent walls and a colorless protoplasm. What advantage is this to the plant? 209. Some parenchyma layer attack only the epidermis of the leaf, but they often cause the death of the leaf and even of the entire plant. Explain. 210. How do you explain the fact that the palisade layer is next the upper surface of the leaf and the spongy layer next to the lower surface? 211. What does the plant gain by having the cells of the palisade layer elongated at right angles to the leaf surface? THE LEAF AND ITS FUNCTIONS 79 **212.** The upper surface of each epidermal cell of the leaf is often slightly convex. How may this perhaps be of advantage to the plant? **213.** Do you think that photosynthesis is carried on in the 'epoxy layer' at all? Let evidence prove your for or against. **214.** Name at least two advantages which the plant derives from having its stomata more abundant on the lower surface of the leaf than on the upper. **215.** Sinusata are usually absent in the leaves of submerged water-plants. Explain. **216.** Why does washing the leaves of house plants often improve the health of the plant? **217.** In cities where soft coal is burned and there is much coal-dust in the air, all trees, and even grasses in particular find it very difficult to live. Explain these facts. **218.** In what ways is the supply of carbon dioxide in the atmosphere being replenished? **219.** Do you think that the continual removal of carbon dioxide from the atmosphere by green plants in photosynthesis will reduce the amount of this gas which is present there? Explain. **220.** The great deposits of coal formed in the Coal Period, millions of years ago, are now being used as fuel. Do you think that carbon dioxide was much more abundant in the atmosphere then than now? Is there basis for such a conclusion? **221.** As the carbon dioxide in the leaf is removed by photosynthesis, what remains in the air? Where does it come from the outside air? **222.** What factors determine the rate at which carbon dioxide enters the leaf? **223.** In some greenhouses the plants are "fertilized" by pouring carbon dioxide into a chimney into the air around the plants. Why does this increase plant growth and why would it be impracticable on a large scale? **224.** The fact that carbon dioxide is more soluble in cold water than in warm has been cited as one of the reasons why salves, warts, polar bears, and even Eskimos are able to thrive in such large numbers as far north as they do. **225.** A solution of chlorophyll outside the plant, when exposed to sunlight and in the presence of carbon dioxide, will not produce sugar. How do you explain this? 80 BOTANY: PRINCIPLES AND PROBLEMS 226. Which is darker green in color, the upper or the lower surface of the leaf? Why? 227. Why is the intensity of green color in its foliage good evidence as to the health of a plant? 228. The stems and leaves of parasitic plants, such as mistletoe and dodder, are either very pale green or are some other color than green. Explain. 229. Why do autumn leaves and dying leaves often turn yellow? 230. When new trees are turned into fresh pasture in the spring, their better leaves are usually removed by grazing animals. 231. In order to "thaw" celery plants, gardeners cover them with earth or wrap them in paper. Why is this practice effective in securing the growth of these plants? 232. What is the orientation (vertical, horizontal, oblique or other) of an ordinary leaf blade? Explain. 233. The lower leaves on a plant or branch often hang downward somewhat obliquely, in contrast to the upper leaves, which are generally horizontal. 234. In many herbaceous plants the lower leaves have long petioles but higher up on the stem the petioles gradually decrease in length and often are quite absent in the upper leaves. Explain. 235. Of what advantage and of what disadvantage is it to a plant to have its leaves spread out or folded? 236. Are opposite leaves usually broader or narrower than spirally arranged ones? Explain. 237. Why are plants with upright, grass-like leaves usually more succes- sful in open, sunny situations than are plants with broad, horizontal leaves? Explain. 238. Why is it easier to maintain a lawn under elm or apple trees than under maples? 239. In just what part of a tree are most of its leaves borne? Why? 240. Why are the lower limbs of a tree growing in the forest usually dead? 241. Many plants are able to thrive in relatively deep shade on the floor of the forest, where the lower limbs of the forest trees have long since died. How can this be? A diagram showing different types of leaf arrangement. THE LEAF AND ITS FUNCTIONS 81 242. Seedlings of most forest trees will grow and thrive on the forest floor in rather deep shade, but the lower limbs of mature trees of the same species are subjected to similar light conditions, will soon die. Of what advantage to the plant is this difference between young and old individuals? 243. Most of our woodland "wild flowers" (herbaceous plants which grow in the open) are winter-blooming and bear leaves in early spring. Why? 244. Where does the light come from which is utilized by the chlorophyll of the epophyly layer? 245. Leaves exposed to bright sunlight are thicker than those growing in the shade, even on the same plant. Explain. 246. How would you determine what wave-lengths of light are of most importance to a particular plant? 247. Would you predict that a plant would thrive better in red light or in green light? Why? 248. Plants grown in winter in greenhouses rarely grow as rapidly as plants grown in summer, even though the temperature is kept as high. Explain. 249. Other things being equal, why do most plants grow faster in June than later in the summer? 250. Crops are necessarily planted much later near the southern limit of their range than they are near the northern limit, so that they are more or less much more rapid that they reach maturity almost as soon. Explain. 251. In the North Sea fisheries, it has been found that the size of the season's catch of fish tends to be greater in seasons when there has been more sunshine than in seasons which have been relatively cloudy. Explain. 252. Some plants (such as the fang) are able to thrive in the absence of light. What other important physiological differences must these be better adapted to withstand than those of other plants? 253. Of what advantage is it to the plant to have the sugar which is produced by photosynthesis converted rapidly into starch? 254. It is sometimes said that forests tend to "purify the air." What basis is fact in this for this statement? 255. Why do animals in an aquarium thrive better if green water-plants are growing all the impatient? S2 BOTANY: PRINCIPLES AND PROBLEMS 256. Compare in detail the leaf of a green plant with a manufacturing establishment. 257. From the point of view of living things, which do you think are the three most important chemical elements? Why? 258. Which do you think would be worse for a tree, the loss of half of its branches by an ice-tem or of all of its leaves through an insect attack? **Note:** By the term dry weight is meant the weight of all the material in a given body except its water. It is usually determined in the laboratory by drying the material in an oven at about 100°C. 259. Would the dry weight of a leaf be greater in the morning or in the following afternoon? 260. Will seedlings grown in the dark increase in actual weight? Explain. 261. The leaves of parasitic plants are often very small. Explain. 262. How do submerged water plants carry on photosynthesis? 263. Give at least three reasons why trees often fail to thrive in a city. 264. From your knowledge of plants, do you think that they need a rest at night, or would they thrive continuously if the light were continuous? 265. Some crops produce a much larger amount of dry weight per acre than others. Explain how this can be true. 266. The potato beetle does not eat the potato plant, but eats only the foliage of other plants. Why does it harm the crop? 267. Why does repeatedly cutting off its tops finally kill persistent perennial weeds? 268. If a man wants to clean out snakes and how snakes from pastureland, should he leave them down in summer or in winter? Why? 269. Is there any basis in fact for the belief held by some farmers that there is one particular day in summer when suckers and brush should be cut down, if you want to kill them off? 270. Which will give better and larger flowers the next season, tulip leaves which have been cut back after flowering or those left in the ground until after the leaves have withered? Why? 271. Why should a new field of squashpots not be harvested until two or three years after the young plants have been set out? THE LEAF AND ITS FUNCTIONS 83 **272.** Which should you keep mown more closely, a newly seeded lawn or an old one? Why? **273.** Should lawn grass, when cut, be raked off the lawn or not? Explain. **274.** Other things being equal, is it better to plant the rows of a garden east-west or north-south? Explain. **275.** State two reasons why a garden planted near a shade tree is apt to be unsatisfactory. **276.** Do you think that transpiration is essential for the life of the plant? Explain. **277.** What process in animals may be said to correspond roughly to the transpiration of plants? In what respects are the two processes similar? **278.** From your knowledge of combs, explain why it is that “the amount of water absorbed is practically independent of the amount of water transpired by the plant”. **279.** Just where in the leaf does evaporation take place during the process of transpiration? **280.** What makes water leave the cytoplasm of the spongy layer cells (or cut off from the cells walls)? **281.** Why is it that the air in the air spaces of the spongy layer of the leaf does not become so saturated with moisture that expansion, and consequently transpiration, will no longer take place? **282.** In general, the faster a plant loses water by transpiration, the faster it will lose its ability to grow. Explain. **283.** Do you think that a plant would be able to survive and grow permanently in an atmosphere which is completely saturated with moisture? Explain. **284.** Will transpiration be more rapid or less rapid if the sap of the one-half increases in concentration? **285.** Why does transpiration take place so much faster in wind than in still air? **286.** Why do flowers usually wall the waters and walks of a green-house as well as the plants themselves? **287.** Why does a plant wilt if its water supply fails? S4 **283. Some plants will use more readily than others. What factors may be responsible for these differences?** **289. Why is a willed plant unable to carry on its functions as well as one that is in a normal, turgid condition?** **290. Why will a plant sometimes wilt even though the soil in which it is growing is moist enough to support its growth?** **291. Why does a plant which has wilted in the daytime usually revive at night, even though no rain falls?** **292. If a tree is subjected to a severe drought, its leaves wilt but its leaves recover when the water supply is restored. Explain why this happens.** **293. In using foliage for wreaths or other decorations, would you choose young leaves or mature leaves? Why?** **294. The stomata often tend to close in the middle of a hot day. Explain why this is so and how it is of advantage to the plant.** **295. Do you think it wise to open or close the leaf if exposed to the light? Explain.** **296. How will stomata tend to close during the course of a normal period of twenty-four hours during the summer?** **297. When is the best time to cut flowers, if it is desired to keep them fresh for a long time out of water?** **298. Why will cut flowers remain fresh longer if they have been placed in water for a few hours directly after being picked?** **299. Do you think that the water requirement of a plant will be higher if it grows on moist soil than if it grows on dry soil? Why?** **300. Do you think that the water requirement of a plant will be higher if it is grown on moist soil or if it is grown on dry soil? Why?** **301. Do you think that the amount of cultivation which a plant receives will have any effect on its water requirement? Explain.** **302. Assume that the following conditions exist: 1) A bucket gives out 800 liters per hour; 2) A bucket holds 10 liters; 3) How many buckets of water have passed by transpiration through a corn plant the dry weight of which is 100 grams?** **303. Given the same conditions as in Question 302, assume further that three such corn plants are planted in each hill and that the hills are one meter apart. If all three plants are growing under similar conditions, how deep would transpiration through the season could be collected, how deep a layer would it make over the surface of the field?** THE LEAF AND ITS FUNCTIONS 85 304. Why does a shortage of water stunt a plant? 305. Is transpiration apt to be high or low when growth is most rapid? Explain. 306. Plants generally grow faster at night than in the daytime. Why? 307. How do you reconcile the fact that plants grow faster at night than in the daytime, with the fact that light is necessary for the manufacture of food? 308. Plants generally grow faster in wet weather than in dry. Why? 309. Why do plants often suffer from "seeding" after a brief summer shower in the fall? Explain. 310. Can you think of any other reason than the presence of shade which may tend to make a forest cool? 311. The planting of rank-growing species like the sunflower has sometimes been thought to reduce the amount of manure in manured fields. Is this true? Explain. 312. Name one way in which the presence of a forest tends to increase the loss of water from the soil and one way in which it tends to decrease it. In what season does the loss of water fall when it is covered with forest or if it is not? Explain. 313. Why is the effect of a killing frost in the fall usually not evident on the following morning until the sun has been up for some time? 314. If a plant has been "touched" by its frost but not killed, why is it sometimes unable to recover by sprinkling it with water and placing it in a clothed place? 315. What structural modifications do you know of in plants which tend to result in checking transpiration? 316. What characteristics must drought-resistant plants possess? 317. The leaves of corn and other grasses often roll or curl when it is very hot or dry. Why is this so and what advantage may it be to the plant? 318. The leaves of desert plants are usually leafy, feathery or very small. Explain. 319. The leaves of figwort, a tree which flourishes in warm, dry regions, hang vertically on the branches. Explain how this is of advantage to the plant. S6 BOTANY; PRINCIPLES AND PROBLEMS 320. The leaves of "compass plants" are erect and vertical, with their edges painting approximately north and south. In what two ways does this character be of advantage to the plant? 321. Plants which grow on mountain ledges are very abundant on mountain sides and exposed alpine situations. Why are they particu- larly suited to such conditions? 322. Why do epiphytes (p. 174) usually have leafy leaves? 323. The leaves of evergreens in temperate regions are usually firm and hardy. 324. Evergreen trees often suffer from "wind burn" in late winter or early spring, a part or all of their branches dying and turning brown. They do not suffer from this cause earlier or later in the season. Explain. 325. Give two reasons for the fact that plants grown in the shade are usually more tender than plants grown in the bright sunlight? 326. Why is the best lettuce grown in early spring? 327. Spring is the best season to transplant a tree? Why? 328. Why is it better to be transplanting on a cloudy or rainy day than on a bright sunny day? 329. Why is it advantageous to cut off the outer leaves of young plants before transplanting? 330. If it is necessary to transplant young trees during the summer, the plants should first be vigorously pruned. Why? 331. Why will potatoes in a storage lose weight much faster after average temperatures drop below this temperature? 332. Why is it best to store apples in a fairly moist place but beans where it is dry? 333. Tobacco growers sometimes cover their plants with cheese-cloth tents. What effect do you think this has on the structure and functions of the tobacco leaves? REFERENCE PROBLEMS 334. Give an example of a leaf which has assumed some of the functions of a stem. 335. Where is the palisade layer in leaves which stand erect, like those of fruit? THE LEAF AND ITS FUNCTIONS 38. What various functions may stipules perform? 39. Give the number of stomata found per square centimeter of leaf surface on three species of plants. 40. What is the function of ordinary air? 41. How many cubic centimeters of air needed to provide enough carbon dioxide for the manufacture of one gram of starch by photosynthesis? 42. About what proportion of the energy reaching a leaf in sunlight is used in the process of photosynthesis? Compare this with the amount utilized by a good steam-engine from the burning of coal? 43. About how much sugar is normally manufactured by one square meter of leaf surface during a day? How much would be produced if that area were doubled? 44. Are the leaves of "fading-plants," which are highly colored (not green) able to carry on photosynthesis? Explain. 45. In the case of some one common crop-plant, give the percentage of dry matter which is made up of substances which typically compose it. From what source has each been derived? 46. When and by whom was the process of photosynthesis in plants first clearly understood? 47. What is the average annual rainfall in this region? About how much of this is returned to the air by a vigorous crop through transpiration? 48. Give the derivation of the following terms and explain in what way each applies:
Petiole Stroma Xanthophyll
Epidernoia Photosynthetic Chlorophyll
Marginal Transpiration
CHAPTER VI THE STEM AND ITS FUNCTIONS We have shown that the root, which absorbs water and mineral substances from the soil, and the leaf, which carries on the manufacture of food, are the two most important organs of the plant. A third member, the stem, connects these two. It forms a conspicuous feature of most plants and in woody species constitutes a large part of their bulk. The other functions, though secondary to the major activities which we have mentioned, are nevertheless essential ones. It serves to dispose the leaves from the ground, to support them, and to provide a highway for transportation between leaf and root. In addition, the stem frequently becomes a storage-organ and may be variously modified. The External Structure of the Stem.—The stem displays a wide range of variation in size and in external and internal struc- ture, according to the habit or growth-form which the plant assumes. In some cases, such as the grasses, it grows back to the ground during periods unfavorable to vegetative activity or at the completion of a given cycle, the stem is comparatively slender and short-lived. In other cases, such as those of shore- ground parts, however, it grows thicker from year to year and becomes hard and woody, forming the stout stems characteristic of desert plants. In still others, such as those of trees, the stem is comparatively short and slender and is usually much branched, even close to the ground. In trees, it grows taller and is devel- oped for purposes of support. In all cases, however, the stem which may become very thick. Woody-stemmed transitionals between these two types often occur. Basic characteristics of the stem in length take place only at certain definite points, where the cells are thin-walled and capable of active division. In many stems, particularly those which are perennial and woody, these growing-points are protected by bursules or scales at intervals along the stem. These growing-points may be terminal, developing at the tip of the stem, or lateral, arising from 88 THE STEM AND ITS FUNCTIONS 89 the sides. Within the bud are not only the beginnings of the young stem but of the various structures which are borne upon it, such as leaves and flowers. The bud scales, which protect these delicate parts, are usually stout and impenetrable. An herb, The horsetail (Equisetum). The terminal bud governs the elongation of the stem, and through the development of lateral buds, branches arise. The shape of the aerial portion of the plant is determined primarily by the number and arrangement of these branches and by their rate of growth relative to each other and to the main stem. 90 BOTANY: PRINCIPLES AND PROBLEMS In certain herbaceous plants the terminal bud produces a flower or flower-point, and the growth of the stem in length usually ceases at this point. Such *dehiscence* growth is not common with woody plants, however, and their stems continue to elongate indefinitely. **Leaves.**—Leaves are borne throughout the length of the stem in herbaceous plants and on the twigs of the current year's A shrub. The blue (Syringa vulgaris). Fig. 41.—A shrub. The blue (Syringa vulgaris). growth in woody forms (Fig. 41). That point on a stem at which a leaf is attached is called a node and the region between two nodes, an internode (Fig. 43). The position of the node also governs the position of the leaf, which may arise either terminally or axially, only in the leaf axil, or upper angle between leaf and stem. The arrangement of leaves on the stem, or its phyllotery, may display one of three types: (1) alternate, that is, leaves at a node; the next one above it arises from the other side of the stem, and the arrangement is thus an *alternate* one (Fig. 41). These THE STEM AND ITS FUNCTIONS 91 two leaves may be exactly half way around the stem from each other, but it is much more common for their angle of divergence to be less than 180°. In this case, the leaves are arranged in series of successive leaves thus to form a loose spiral around the stem. The closeness of this spiral and the position of the leaves thereon show great diversity, but are generally constant within any par- A tree with two large branches extending outward. Fig. 42.—A tree. The Ashburk hickory (Carya ovata). (Gardner United States Forest Service.) ticular species. If two leaves arise from the same node they are always directly across the stem from each other and are said to be opposite. If three leaves arise from the same node, and more than two leaves at a node, they are disposed about the stem in a circle or whorl. **Segregation:** The surface of a young stem is protected only by an epidermis, but later this is replaced in woody plants by a characteristic layer of corky cells, the bark. The necessary exchange of gases between the air and the living tissues of the 92 BOTANY: PRINCIPLES AND PROBLEMS stem takes place through the bud scales (Fig. 43), small spots or stripe where the bark tissue is softer and looser than elsewhere. A detailed diagram of a woody twig showing various parts such as Terminal Bud, Lateral Bud, Internode, Leaf Scar, Node, Scar of Terminal Bud, Lenticel, Bundle Scars. FIG. 43.—A woody twig in winter condition (Rheum-achatum). Other Stem Types.—The typical upright, foliage-bearing stem has sometimes become radically modified for the performance of THE STEM AND ITS FUNCTIONS 93 other functions than support and conduction. Many plants have abandoned the erect habit, and their weak, slender stems climb or scramble by various means over other objects or lie prostrate on the ground. In such cases the stem may even become subterranean, in which condition it is known as a A. Summer (R) and winter (L) conditions of the same woody twig (Cherry). The leaves are shed in winter. B. Rootstock of Rhizome (Fig. 45). Typical stems give opportunity for the storage of a certain amount of food reserves, especially in pith and cortex, but in some species this function is so greatly developed that the stem itself becomes essentially a storage organ only. This condition exists in most rootstocks, and its extreme development results in a reduction of the leafy shoot to a mere stalk or scape (Fig. 46), as in potato (Fig. 46), which is morphologically a stem but now shows little obvious resemblance to that organ. The bulb and the Corm (Fig. 47) are other examples of highly modified underground stems. 44 94 BOTANY: PRINCIPLES AND PROBLEMS The Internal Structure of the Stem.—In the cross section of a typical young stem (Figs. 48, 53 and 54) there may be distinguished the same three types of tissue which are present in the Fig. 53.—Rootstock of Iris. Fig. 54.—Tuber of the potato, showing point of attachment to the parent plant (or cormelike bulb) and numerous buds or "eyes" each in the axil of a reduced and scale-like leaf. root, but they are arranged somewhat differently. Outside the whole is the epidermis, consisting of a single cell-layer, and often replaced entirely, at an early stage, by a zone of corky bark. THE STEM AND ITS FUNCTIONS 95 Beneath this is the cortex, varying in thickness but rarely occupying as prominent a place in the stem as it does in the root. Beneath the cortex lies the *fibro-vascular cylinder* which, unlike its counterpart in the root, is not a single mass but consists of two hollow tubes. The core of this tube is occupied by the *pith*, a tissue Fig. 47.—Root and stem. (A), longitudinal section through the bulb of a buttercup, showing the pith and cortex. (B), transverse section through the same, showing the vascular cylinder with its three main groups of leaf-bases. much resembling the cortex. A more detailed account of the character of the cells composing these tissues may be appropri- ately undertaken now, for although all the tissues here men- tioned have been described in connection with their greater differentiation and complexity in the stem, and in the case of the plant they can therefore most profitably be studied. The structure of the fibro-vascular tissues of a woody dicotyledonous plant can be best understood by studying a transverse or a longitudinal section through a portion of the stem shown in Fig. 48. A diagram showing a longitudinal section through a plant stem, highlighting the pith and cortex. A diagram showing a transverse section through a plant stem, highlighting the vascular cylinder with its three main groups of leaf-bases. 96 BOTANY: PRINCIPLES AND PROBLEMS Protective Layers.—The epidermal cells resemble those of the leaf epidermis and require no special comment. In stems which are covered by a protective layer, such as the stem of a sweet clover, this layer is sloughed off and its protective function is assumed by a layer of corky cells formed directly under it and constantly renewed. In these cells the protoplasm soon disappears and the normal cellulose wall becomes corky or enkerized and is thus rendered almost impermeable to air or water. The lenticels, which we have already mentioned, are openings through which the cells are somewhat loose and spongy and thus allow the passage of gases. Fig. 45.—Transverse section of a three-year-old twig of the tulip-tree (Lirio- deodora). This is a typical example of a lenticel, similar to the opening of a solid ring of wood within and lost without, surrounding a central yoth. Cortex and Pith.—The cortex and pith are very similar in constitution. Their cells usually remain alive, are roughly spherical in shape, retain their cellulose walls and function chiefly in storage. In some plants, however, especially those in which the term parenchyma is often applied. In older woody stems the pith often dries up and collapses; and the cortex, crushed by the expansion of the wood, is finally sloughed off. A diagram showing a transverse section of a stem with a lenticel. The lenticel is shown as a circular opening surrounded by corky cells. The inner part of the stem is shown as a solid ring of wood. The outer part of the stem is shown as a layer of corky cells. THE STEM AND ITS FUNCTIONS 97 Root.—The fibre-macular cylinder is composed of two distinct tissues. On the outside is the bast or phloem, the function of which is to transport the elaborated foods—the carbohydrates. A transverse section of wood and bast of the mulberry (Lorinaeolus). A portion of Fig. 50, showing the relation of the segments between the bast cells to the first annual ring of wood, together with one of the groups of bast cells, is included. Fats, and proteins—from one part of the plant to another, especially from regions of manufacture to those of storage or consumption. The cells concerned in this process are the sieve-tubes (Fig. 51), living cells with thin cellulose walls but unique... 98 BOTANY: PRINCIPLES AND PROBLEMS in their lack of a nucleus. They are elongated parallel to the main axis of the stem and their end walls (more rarely their sides) are provided with sieve-plates or definite groups of small perforations. Through these perforations extracellular threads of Radial longitudinal section of wood and bark of the apple-tree. Lenticels are seen on top, through the bark shown above. The rays are the wood, and at the bottom are the vessels which lie between them. The intercellular spaces are filled with air. The cells of the wood contain elongated markings in pits in the side walls. The ends of the sieve-tubes are occupied by sieve-plates. Fig. 50.—Radial longitudinal section of wood and bark of the apple-tree. cytoplasm from one cell to another, so that the living substance of each sieve-tube is directly continuous with that of the adjacent one. In this way, seed plants may grow next to each other without a small companion cell, provided with an abundance of cytoplasm and a nucleus. In addition to these two types, groups of long and very thin-walled cells, the cambium, differentiate continuously THE STEM AND ITS FUNCTIONS 99 A longitudinal section of a stem of a dicotyledonous plant. B Transverse section through the stem. C Transverse section through the end of the stem. D Longitudinal section of a stem of a monocotyledonous plant. E Transverse section through the end of the stem. F Transverse section through the end of the stem. G Longitudinal section of a stem of a monocotyledonous plant. H Transverse section through the end of the stem. I Longitudinal section of a stem of a dicotyledonous plant. J Transverse section through the end of the stem. K Longitudinal section of a stem of a dicotyledonous plant. L Transverse section through the end of the stem. M Longitudinal section of a stem of a dicotyledonous plant. N Transverse section through the end of the stem. O Longitudinal section of a stem of a dicotyledonous plant. P Transverse section through the end of the stem. Q Longitudinal section of a stem of a dicotyledonous plant. R Transverse section through the end of the stem. S Longitudinal section of a stem of a dicotyledonous plant. T Transverse section through the end of the stem. U Longitudinal section of a stem of a dicotyledonous plant. V Transverse section through the end of the stem. W Longitudinal section of a stem of a dicotyledonous plant. X Transverse section through the end of the stem. Y Longitudinal section of a stem of a dicotyledonous plant. Z Transverse section through the end of the stem. AA Longitudinal section of a stem of a dicotyledonous plant. AB Transverse section through the end of the stem. AC Longitudinal section of a stem of a dicotyledonous plant. AD Transverse section through the end of the stem. AE Longitudinal section of a stem of a dicotyledonous plant. AF Transverse section through the end of the stem. AG Longitudinal section of a stem of a dicotyledonous plant. AH Transverse section through the end of the stem. AI Longitudinal section of a stem of a dicotyledonous plant. AJ Transverse section through the end of the stem. AK Longitudinal section of a stem of a dicotyledonous plant. AL Transverse section through the end of the stem. AM Longitudinal section of a stem of a dicotyledonous plant. AN Transverse section through the end of the stem. AO Longitudinal section of a stem of a dicotyledonous plant. AP Transverse section through the end of the stem. AQ Longitudinal section of a stem of a dicotyledonous plant. AR Transverse section through the end of the stem. AS Longitudinal section of a stem of a dicotyledonous plant. AT Transverse section through the end of the stem. AU Longitudinal section of a stem of a dicotyledonous plant. AV Transverse section through the end of the stem. AW Longitudinal section of a stem of a dicotyledonous plant. AX Transverse section through the end of the stem. AY Longitudinal section of a stem of a dicotyledonous plant. AZ Transverse section through the end of the stem. Fig. 31.—The structure of a sieve-tube (Stomach). A, longitudinal section of a sieve-tube showing its two parts, sieve-plate and parenchyma cells, with their contents, in front and below. Its companion-cell is at left. B, transverse sections through one part only, showing sieve-plates on both sides. C, transverse section through one part only, showing sieve-plates on one side. D, transverse section through one part only, showing sieve-plates on both sides. E, transverse section through one part only, showing sieve-plates on one side. F, transverse section through one part only, showing sieve-plates on both sides. G, transverse section through one part only, showing sieve-plates on both sides. H, transverse section through one part only, showing sieve-plates on one side. I, transverse section through one part only, showing sieve-plates on both sides. J, transverse section through one part only, showing sieve-plates on one side. K, transverse section through one part only, showing sieve-plates on both sides. L, transverse section through one part only, showing sieve-plates on both sides. M, transverse section through one part only, showing sieve-plates on both sides. N, transverse section through one part only, showing sieve-plates on one side. O, transverse section through one part only, showing sieve-plates on both sides. P, transverse section through one part only, showing sieve-plates on both sides. Q, transverse section through one part only, showing sieve-plates on both sides. R, transverse section through one part only, showing sieve-plates on both sides. S, transverse section through one part only, showing sieve-plates on both sides. T, transverse section through one part only, showing sieve-plates on both sides. U, transverse section through one part only, showing sieve-plates on both sides. V, transverse section through one part only, showing sieve-plates on both sides. W, transverse section through one part only, showing sieve-plates on both sides. X, transverse section through one part only, showing sieve-plates on both sides. Y, transverse section through one part only, showing sieve-plates on both sides. Z, transverse section through one part only, showing sieve-plates on both sides. AA, AB—Longitudinal sections from different parts. AA—Transversal sections from different parts. AB—Transversal sections from different parts. AC—Transversal sections from different parts. AD—Transversal sections from different parts. AE—Transversal sections from different parts. AF—Transversal sections from different parts. AG—Transversal sections from different parts. AH—Transversal sections from different parts. AI—Transversal sections from different parts. AJ—Transversal sections from different parts. AK—Transversal sections from different parts. AL—Transversal sections from different parts. AM—Transversal sections from different parts. AN—Transversal sections from different parts. AO—Transversal sections from different parts. AP—Transversal sections from different parts. AQ—Transversal sections from different parts. AR—Transversal sections from different parts. AS—Transversal sections from different parts. AT—Transversal sections from different parts. AU—Transversal sections from different parts. AV—Transversal sections from different parts. AW—Transversal sections from different parts. AX—Transversal sections from different parts. AY—Transversal sections from different parts. AZ—Transversal sections from different parts. AA—Longitudinal sections from different parts. AB—Longitudinal sections from different parts. AC—Longitudinal sections from different parts. AD—Longitudinal sections from different parts. AE—Longitudinal sections from different parts. AF—Longitudinal sections from different parts. AG—Longitudinal sections from different parts. AH—Longitudinal sections from different parts. AI—Longitudinal sections from different parts. AJ—Longitudinal sections from different parts. AK—Longitudinal sections from different parts. AL—Longitudinal sections from different parts. AM—Longitudinal sections from different parts. AN—Longitudinal sections from different parts. AO—Longitudinal sections from different parts. AP—Longitudinal sections from different parts. AQ—Longitudinal sections from different parts. AR—Longitudinal sections from different parts. AS—Longitudinal sections from different parts. AT—Longitudinal sections from different parts. AU—Longitudinal sections from different parts. AV—Longitudinal sections from different parts. AW—Longitudinal sections from different parts. AX—Longitudinal sections from different parts. AY—Longitudinal sections from different parts. AZ—Longitudinal sections from different parts. 100 BOTANY: PRINCIPLES AND PROBLEMS occur in the phloem, and some parenchyma is usually present there also. Wood. The inner portion of the fibrous-cylinder consists of the wood or xylem, which provides mechanical rigidity for the stem and transports the stream of water and dissolved substances from root to leaf. As essential elements in the xylem A. Type of cells found in wood. A1. Fiber. A2. Tracheid. A3. Vessel. A4. With thickened walls. A5. With thin walls. B. Very short, broad, vessel-cell. C. Two ray-cells. D. Two vertical wood-microfibrils. E. Very short, broad, vessel-cell. F. Two ray-cells. G. Two vertical wood-microfibrils. we find cells which are much elongated parallel to the main axis of the stem and in which the cell-bodies walls have become very thick and strong (Fig. 52, A, B, C, D and e). Such walls are said to be lignified. As soon as one of these cells is fully developed, it dies and its protoplasmic contents disappear, so that only THE STEM AND ITS FUNCTIONS 101 the thick, woody cell-wall is left. Definite thin areas or pits occur at frequent intervals along this wall and facilitate the rapid movement of water. In simpler types of wood, each cell is able to produce its own transpiration stream and conductive capacity and is known as a *tracheid*. In the higher types, how-ever, this simple element has become specialized in structure and has been transformed into the more complex living cells, the *xylem-fibres*, in which almost no vacuum remains and which con- tribute a high degree of mechanical strength to the wood; and the remaining part of the cell-wall is converted into conducting units, and walls which are comparatively thin and are provided with large perforations in their ends. These cells, had end to end in vertical rows, form a continuous column. The characteristic of the wood many of plants, which carry the ascending stream of water through the stems. Phragmocytes cells sometimes occur among these fibres (Fig. 52, A). They are long and elongated vertically (Fig. 52, G). Other parenchyma cells are elongated at right angles to the stem (Fig. 52, F) and dispersed among the woody cells. These cells are filled with a clear fluid which passes through the xylem along the radius of the stem. These structures are known as the *wood-caps*, and in somewhat modified form extend throughout the stem. They facilitate the horizontal transfer of materials in both directions and have great importance as centres of food-storage. *Coniferae*—A narrow row of thin-walled cells, the cambium, separates the phloem from the base. Through its activity new cells are added to the outside of the wood and inside of the base, and the thickness of the stem is thereby increased. Among woody plants, however, this process is not so constant as in *Coniferae*; each season's increment, or annual ring, is easily recognizable. At each node a small but complete segment of the fibro- vascular tissue is cut off by a groove running through the cortex into the base of the petiole, causing a break, or leaf-gap, in the leaf. Into each gap may enter one, three, five or more of these leaf-gaps, depending on whether they are continuous or whether they are separated by a leaf-stalk. These gaps allow water to pass through the petiole and to form the system of veins in the blade. *Woody and Herbaceous Stems.*—The perennial woody stem in which the leaves fall off during winter is termed a *bark*; it is a continuous and rather wide raker (except for the leaf-gaps), and which receives additions in thickness year by year through cambial activity, is probably the most ancient stem-type among 102 BOTANY: PRINCIPLES AND PROBLEMS seed plants; and the herbaceous condition, where the stems are much softer and shorter-lived, has apparently been derived from it by a process of reduction, for various reasons. In herbaceous species, the amount of fibre-vascular tissue has become proportionally very much less. This may be due simply to a decrease in the activity of the entire cambium, or to the breaking up of the cylinder into separate bundles, but in general any herbaceous stem is roughly comparable to a one-year-old twig of the particular woody stem-type from which it has been evolved. The herbaceous stem differs from with its thin but continuous vascular ring, probably arising from such woody form as is shown in Fig. 53, where the vascular ring is similarly continuous and homogeneous. The stem in Fig. 36, however, shows a distinct and complete vascular ring, a distinct and completely separate bundle, is quite different in type and has probably arisen from a woody stem somewhat resembling that in Fig. 53, but differing in having more extensive increments by the development of very wide rays. Cambial activity is usually weaker opposite these rays than opposite the woody segments of the cylinder, and in the structure of this type, Fig. 53. Fig. 54. Fig. 53.—Transverse section of a one-year-old twig of the sweet gum (Liquidambar), showing the vascular ring and the separate bundles. Fig. 54.—Transverse section of a one-year-old twig of the sycamore (Platanus), showing the vascular ring and the separate bundles by the development of wide rays. (Figs. 53 and 54 from Stewart and Bailey.) THE STEM AND ITS FUNCTIONS 103 Fig. 55. Fig. 55.—The stem of an herbaceous plant (Daphne). Transverse section. In this type of herbaceous stem, the fibro-vascular bundles are scattered throughout the cortex. Fig. 56. Fig. 56.—The stem of a woody perennial plant (Delphinium). Transverse section. In this type of woody stem, the fibro-vascular bundles are arranged in a ring, which is separated into a ring of fibro-vascular bundles, each containing a group of wood-cells and a group of bast-cells. Fig. 57. Fig. 57.—The stem of a monocotyledonous plant (Coral). Transverse section, showing the fibro-vascular bundles scattered in the pith. 104 BOTANY: PRINCIPLES AND PROBLEMS the rays therefore tend to form broad constrictions in the ring. In more delicate herbaceous stems the constrictions finally become so narrow that they are no longer visible. The cylinder is thus broken up into a ring of separate segments or *phloemocambial bundles*. Each of these consists of a group of wood cells on its inner side and of bast cells on its outer, with a vestige of cambium. A diagram showing the structure of a stem bundle in a monocotyledonous plant. Fig. 56. Stem-bundle of a monocotyledonous plant. Transverse section of a *Liliaceae* stem (Ophioglossum). The bundle is composed of two phloem tubes, with companion-cells in their corners. The bundle is surrounded by a secondary xylem, which is separated from the phloem by a cambium layer. between. Connecting the cambium layers of two adjacent bundles there may be a weak *interfascicular cambium*, producing a few layers of parenchyma cells. In many herbaceous stems, however, the cambium is absent, and the bundles consist of thick-walled cells, are quite distinct and widely separated from one another, with no remnant whatever of a cambial zone between them. In still more highly specialized stems, characteristic of monocytoledonous plants, the bundles are no longer arranged in a ring but are scattered irregularly throughout the whole area of the THE STEM AND ITS FUNCTIONS stem (Fig. 37). The individual bundles are very distinctive in appearance (Fig. 58), each possessing a large air-space or nectar, surrounded by a layer of parenchyma, and traversed by long, narrow sieve tubes and companion cells. In such a stem no distinction between pith and cortex now remains. The departure of the leaf-trace from the main axis is indicated by the bundle moving outward from the center of the stem and entering the sheathing leaf-base. The Structure of Wood. —In shrubs and trees* the great bulk of the stem, particularly in its older portions, consists of bast. A cross-section of an oak log, showing the clearly-evident heart-wood at the center of the stem, surrounded by the lighter sap-wood. One tissue, the wood. Wood is so important in the economy of the plant that it has been studied with more care than any other tissue. This is justifiable in studying it a little more closely than we have the other tissues. Though all the activity of the cambium (a fuller account of which we shall reserve for the chapter on growth) a new concentric layer of wood cells is added each year to the outside of the woody cylinder, this does not mean that all parts of the plant are equally well supplied with wood. The amount of growth in the spring is usually of large diameter and are accompanied by comparatively few fibers, and it is apparently *Conifers and dicotyledons alone are discussed here. Woody mono- cotyledons are rare and their woody masses are very complex. 105 106 BOTANY: PRINCIPLES AND PROBLEMS in this spring vessel that most of the upward conduction of water takes place. In the later-formed portion of the annual ring, the water-conducting cells are more numerous than in the bulk of the tissue is composed of fibers. This summer wood is responsible for most of the rigidity and strength of the stem. In large branches and trunks, however, the summer wood is gradually replaced by the first-formed annual rings at the center of the stem, in time becomes dead throughout and ceases to perform its functions of A diagram showing transverse and longitudinal sections of a stem. The transverse section shows a cross-section of the stem with a dark line indicating the boundary between summer wood (lighter color) and winter wood (darker color). The longitudinal section shows a long, thin cylinder with a darker line running through it. Fig. 50. Transverse and longitudinal sections of a stem. Both logs have been cut transversely. In addition, the one at the left has been cut longitudinally. The dark line in each section indicates the boundary between summer wood (lighter color) and winter wood (darker color). The innermost ring is usually converted into heart-wood, which appears somewhat different from that particular plane of section. The annual rings can be seen in both sections. water-conduction and storage. It then constitutes the heart- wood (Fig. 50) and is frequently distinguished from the outer layers by its darker color. The living and functioning part of the wood is called sapwood (Fig. 50), while the heart-wood (Fig. 50). This, of course, is on the outside of the woody cylinder, and it is usually rather constant in width in any particular species, its internal ring being converted into heart-wood each year as the cut surface is exposed to light and air. All of the non-woody cells here (the parenchyma cells and ray cells) are alive. Wood is usually cut along one of three distinct planes, and the cut surface in each case presents a very different appearance (Fig. 60). In describing a given wood it is therefore customary to describe its characteristics as they appear in these three ways: 14 THE STEM AND ITS FUNCTIONS cut or sections. An ordinary "cross cut," at right angles to the length of the log, is known as a transverse section, and shows the annual rings as vertical straight lines. The wood rays running out from the center as narrow lines along the radii. Where the cut is longitudinal and made exactly along the radius of the stem, the annual rings appear as vertical straight lines and the wood rays as horizontal. A diagram showing a cross-sectional view of a tree trunk with annual rings and wood rays. Fig. 41. A segment of an oak log. At the right, the block has been cut radially, so that the annual rings are seen in a transverse section. The portion of the first and corky bark has been removed, showing a transversal view of the wood rays. The wood rays run from the center to the periphery along the radii of the stem. On the radial face, one of them is shown right open, giving the shape of a ray. The corky outer bark, however, between the large rays are many small and narrow ones. Stripes or markings. Where the rays are fairly wide, as in the oak, they may be seen as long parallel lines, which are sometimes called "silver grain" so readily seen in quartered oak. Other longitudinal sections, which do not lie in a plane passing through the center of the stem, show these rays more irregularly. Where they are perfectly regular and the cut exactly true, the annual rings are here seen as straight lines somewhat unequally distant, running up and down along the wood. The irregularity of these wood rays occurs, however, only where there is a cut to appear near two lines which produce the common "grain" of most wood surfaces. The rays are very inconspicuous in a tangential section, for only one or two rays can be seen at any one time. In these 107 108 BOTANY: PRINCIPLES AND PROBLEMS three sections and the characteristic appearance of the various wood structures are shown in Figs. 61 to 63. In the segment of an oak log (Fig. 61) and the magnified cube of pine wood (Fig. 62). Woods of various species differ from one another markedly in such gross characters as color, weight, hardness, chemical composition, etc., but they all possess certain common characteristics, size A B C Fig. 62.—Cube of pine wood, much enlarged. A, transverse section. B, radial section. C, tangential section. The three sections show parts of two others are shown. (Courtesy United States Forest Products Laboratory.) and arrangement of vessels; and in such microscopic features as the size, shape, character, and location of the different classes of wood-structures. The following figures illustrate these various markings on the cell-walls. The structure of two distinct and important woods, those of pine and of oak, are well shown in their transverse sections in Figs. 61 and 62. The various details of wood structure remain so constant that Fig. 63.—Pine wood as seen under the microscope. A, transverse section. B, radial section. C, tangential section. Note the thickened cells in the outer wood and the thinned walls in the inner wood. The large openings are readily seen in all three sections. The large openings are readily seen in all three sections. (Courtesy United States Forest Products Laboratory.) and arrangement of vessels; and in such microscopic features as the size, shape, character, and location of the different classes of wood-structures. The following figures illustrate these various markings on the cell-walls. The structure of two distinct and important woods, those of pine and of oak, are well shown in their transverse sections in Figs. 61 and 62. The various details of wood structure remain so constant that THE STEM AND ITS FUNCTIONS 109 A detailed botanical illustration showing various cross-sectional views of plant stems. The top left section shows a longitudinal view with a central pith surrounded by vascular bundles. The top right section shows a transverse view with a central pith and surrounding vascular tissues. The bottom left section shows a transverse view with a central pith and surrounding vascular tissues, but with a different pattern of tissue arrangement. The bottom right section shows a transverse view with a central pith and surrounding vascular tissues, but with a different pattern of tissue arrangement. Page 61 110 BOTANY: PRINCIPLES AND PROBLEMS A B C D Vol. 64. THE STEM AND ITS FUNCTIONS they may often be used to identify the plant species from which a piece of wood has been derived. The great diversity which wood displays, together with its abundance and the ease with which it can be removed from the soil, make it a valuable source of information, and there is consequently no other plant tissue, aside from those used in this work, which affords a great economic importance. The Amount of Sap in the Stem. - As shown by experiment that water and dissolved substances absorbed by the roots are carried upward in the wood of the stem. As to what causes this movement, we have seen that the water must pass through within the ascent of water in low-growing herbaceous plants might be fairly simple, but the factors which bring about the lifting of water in large quantities from the roots of trees and shrubs more than three hundred feet above the ground, are very hard to determine. An upward osmotic pull is of course furnished by the increased sap concentration in the leaves, but this does not account for the close thereofon in transpiration, but even granting a strong pull at the leaf, the rise obviously cannot be due to simple " suction" or atmospheric pressure. The amount of water thus conveyed to any great extent in the process, for, although water may be lifted very high in exceedingly small expiratory tubes, its move- ment is so slow that it would take many years to convey anything but could not provide the large amounts of water which we know must ascend the trunk daily. Root-pressure, if it were strong enough, might perhaps be important, but root-pressure is mani- fest only when the plant is growing rapidly and is therefore lacking at the season when transpiration is most active. It has been suggested that the living ray and wood parenchyma cells may be connected by thin water columns, but this is not very likely, perhaps furnishing a continuous series of osmotic pumps. These cells may be of some such service, but we know that for a con- siderable time after a tree has been felled, all parts of a stem where all the living cells have been killed. The most plausible hypothesis yet put forward is based on the very high cohesive power exhibited by cellulose fibers. This cohesion forms thin water columns, such as must occur in the conducting cells of stems. Fig. 64.-One oak wood under the microscope. A transverse section showing one complete annual ring and parts of two others. Note the very wide space between the rings (the bark), and also note how much smaller they are than those in Fig. 63. The wood is very light in color and has a smooth surface. Note the many differences in structure between this wood and that of pine. (Courtesy United States Forest Products Laboratory.) 112 BOTANY: PRINCIPLES AND PROBLEMS wood, this cohesive power is perhaps so strong that a pull at the top—in this case the omotic pull at the leaf—will lift the columna teddy, but it is not so strong as to prevent the plant from objections to this explanation, too, but they are not as serious as in the other hypotheses. Possibly several of the factors mentioned may be combined in one, but it is not necessary to must admit that this problem, like so many others in biology, is as yet far from a satisfactory solution. The transpiration of water by the plant most possess means only for insuring the passage of a plentiful supply of water to the leaves through the wood of the stem, petioles and veins, but also for transmitting to them a certain amount of nutritive—the manu- factured food in the form of carbohydrates, fats and proteins—to any region of the plant where food is used or stored. This function of transpiration is performed chiefly by the sieve-tubes of the leaf stalks and veins. The rate of diffusion from cell to cell is a comparatively slow process, but is the only means available in regions remote from the vascular system. Movement of water from one part of the plant to another, e.g., to the storage regions of stem and root, seems to take place almost entirely in the bast. Here the protoplasmic connections from ductate to ductate are continuous with those between cells with the necessity for diffusion through a long series of membranes and thus facilitate the rapid transfer of substances from place to place. This mode of movement has been demonstrated by experiments involving "ringing" or "girdling," in which there is removed from around the stem a continuous encircling strip of bark (fig. 3). It is a matter of common observation that a tree in which the trunk has been girdled in this way will ultimately die. Although small in amount, therefore, and rather inconspicuous when compared with that which it must be a vitally necessary time in the economy of the plant. QUESTIONS FOR THOUGHT AND DISCUSSION 334. Most stems tend to be stout below and mere slender above. Why is this, and of what advantage is it to the plant? 335. Why are young trees often somewhat cup-shaped but old trees of the same species flat or convex at the top? THE STEM AND ITS FUNCTIONS 336. Does the trunk of a tree become relatively stouter or more slender, compared with the root of the tree, as the tree grows larger? Explain. 337. What difference in method of stem-growth is responsible for the differences in shape between a spruce tree and an elm tree? 338. A group of trees of the same species, growing very close together, will often grow approximately the same shape as that of a single, well developed tree. Explain. 339. What advantages and what disadvantages does a climbing plant have as compared with an erect one? 340. What advantages and what disadvantages does a plant with a persistent leaf (evergreen) have as compared with an erect one? 341. What advantages and what disadvantages does a herbaceous plant have as compared with a tree? 342. Trees and shrubs have hard and woody stems, but herbs very soft and flexible. Explain. 343. The stems of submerged water plants are very soft and weak. Explain. 344. Give an example of a plant which is practically stemless. 345. By looking at a leafy branch which has been freshly cut out from a tree, how can you tell whether it has been growing in a vertical, oblique or horizontal position? Explain. 346. What do you think is the most important function performed by the bud-scale? Explain. 347. Do all the buds on a tree unfold and grow every season? Explain. 348. Why is a potato tuber "morphologically" a stem? 349. Why is it that a woody twig obtains air for its internal tissues through lateral rather than through stomata, so does a leaf? 350. What is there about the structure of cork which makes it such excellent material for bottle-stoppers? 351. What do we mean in saying that the cortex and pith are "confined to the tissues"? Explain. 352. What is the advantage in having the cells of the conducting tissues much elongated? 114 BOTANY: PRINCIPLES AND PROBLEMS 353. Wood has a "gram" which in general runs parallel to the axis of the tree. To what is this grain due? 354. Why does wood split easily "with the grain" but not "against the grain"? 355. What causes knots in wood? 356. A log otherwise free from knots often shows them near its center. Why? 357. Which will have more and larger knots in its wood, a tree grown in the forest or one grown in the open? Why? 358. Why is the wood of knots apt to be harder than the wood around them? 359. Which will decay faster if exposed freely to the air, heart-wood or sap-wood? Why? 360. By looking at the cut end of a board, how can you tell the position which this board held with reference to the center of the log from which it was cut? 361. How can you tell whether a piece of furniture is made of veneered wood or not? 362. What two ways do you know for telling the age of a twig? 363. What makes the annual rings in wood clearly distinct from one another? 364. What often makes it difficult to count the annual rings of trees which have grown in warm regions? 365. As a tree grows older, which increases more rapidly in thickness, its heart-wood or its sap-wood? Explain. 366. In most woody plants it is only the last year's growth of bark, or at least that of the last few years, which functions in transmitting food. Explain. 367. Of what use are the bast-binders to the plant? 368. What suggestion can you make as to the function of the compound leaf of the bass? 369. How would you prove that the ascending stream of water travels in the wood? 370. Species of trees differ markedly in the height to which they can grow. Can you suggest a factor which may be responsible for this difference? THE STEM AND ITS FUNCTIONS 371. How is it possible for a tree which is " hollow-hearted" to thrive and grow? 372. In tapping maple trees for sap, it is necessary to run the tap into the tree for only a very short distance. Why? 373. Cut flowers will keep fresh longer if the cut ends of their stems are trimmed off daily, if, after cutting, the ends are placed in boiling water for 5 minutes. Why? Why do these flowers not frequently change, or if a little salt is added to the water. Explain how it is that these various procedures tend to effect the desired result. 374. How would you prove that manufactured food travels in the best way? 375. Why will a gilded tree ultimately die? 376. Why is the chestnut-bark disease, which attacks only the outer bark, cortex and bark, so fatal to chestnut trees? 377. Why is a wire ring or other tight metal band around a tree trunk the most effective method of killing a tree? 378. If a trunk is bound tightly with wire, a swelling of the tissues finally appears above the wire. Explain. 379. In preparing a plant by "laying," a gardeners bends a branch down to the ground and covers a portion of it with earth in order that its root may penetrate the soil. Why does this procedure work? What item is bent or twisted strongly at the point where roots are desired. Why? 380. In "Chinise" laying, a ring of bark is cut off around the stem, as far as the wood, and that part of the stem is covered with moist moss. Roots eventually appear just above the ring, but not below it. Explain. 381. In order to obtain large very fruitful for exhibition, growers sometimes "train" fruit trees on wires or poles, as shown in figure 10-1, where the fruit is growing. Explain why this has the desired result. 382. What foundation in fact is there for the old belief that by driving nails into the trunk of a pine or a peach tree, larger fruit will be obtained? 383. If a tree is "girdled" while its leaves are out, will the leaves wilt or not? **REFERENCE PROBLEMS** 49. Give an example of a stem which has assumed some of the functions of a leaf; of a root. 116 BOTANY: PRINCIPLES AND PROBLEMS 50. Why does a gardener use "brash" to support pess but poles to support trees? 51. Do all buds have scales? 52. Give an example of buds which do not arise at a node. 53. Why do apple and elm trunks make better chopping blocks than do most woods? 54. What is the essential feature in the manufacture of wood-pulp? 55. Why is paper that is made from wood-pulp so much less tough than that made from straw? 56. What is the difference between a wood which is rip-porous and one which is diffuse-porous? 57. What is the process of veneering wood, and what are its advantages and disadvantages? 58. What is "quartered" oak and why is it more expensive than ordinary oak? 59. Why can oak be quartered to advantage although most woods cannot be? 60. Why cannot the ascent of water in the stem of a plant be explained on the principle of a suction pump? 61. Give the derivation of the following terms and explain in what way each applies:
Node Cortex Lenticel
Nodal Trunk Phloem
Phloem Canals Fibrousseum
A diagram showing the structure of a plant stem, including nodes, cortex, lenticels, trunk, phloem, and canals. CHAPTER VII METABOLISM The term metabolism, whether used of animals or of plants, refers to the entire series of chemical changes and processes involved in the activity of the living organism. It may be divided roughly into *constructive* and *destructive* metabolism. The constructive process is concerned with the formation of simple carbohydrates by photosynthesis, and is concerned with the construction therofrom of the more complex plant foods and with the growth of new tissues which make possible the living protoplasm, and the growth of new tissues which they make possible. The latter process involves a breaking down of the living protoplasm, and a liberation of energy, and a liberation of waste materials and a liberation of the energy which is necessary for the activity of the organism. Plants have been known since 1830 to produce the production of glucose by photosynthesis. Glucose is the basic plant food from which are ultimately derived all others—the more complex carbohydrates, proteins, fats, and the proteins—which support the life of animals and plants. Before we inquire into the characteristics of these various food types and their role in origin and development, we must consider just what constitutes by the term "food" itself. A broad definition would make "food" include everything taken into the body of the organism which is essential to its life and continued activity. Water, for example, is necessary for life, but water alone would thus be considered as the plant food, and indeed it is the host of those which in ordinary speech are most commonly referred to as "plant foods." But water is not food at all as this, however, has arisen a fallacious distinction sometimes drawn between animals and plants, namely that the former require water as well as other substances for their life, while the more strict and perhaps from our point of view a more useful employment of the term "food" limits its application to anything which supplies either of the two fundamental needs of the organism. 147 118 BOTANY: PRINCIPLES AND PROBLEMS energy and building-materials. We may therefore define food as whatever furnishes a supply of energy to the organism or constitutes the materials for the building of its body. It is the car- bohydrates, fats, and proteins which provide the materials for growth, and which, because of their somewhat unstable chemical composition, must be constantly renewed by the organism. These are the true foods. The essential mineral salts, which constitute a very small portion indeed of the A diagram showing the food cycle, with arrows indicating the flow of nutrients through various organisms. Fig. 65.—The organic food cycle. History of the construction and disintegration of the plant body. The green plants (green algae) are autotrophic organisms, that is, they can synthesize their own food from carbon dioxide and water. They are called primary producers. The green plants are not only food for other plants but also for animals. The animals are heterotrophic organisms, that is, they cannot synthesize their own food from carbon dioxide and water. They are called secondary consumers. The food of all animals is essentially the same and the difference between the two groups, therefore, lies not in the character of the food which they use but in the fact that green plants are capable of synthesizing food from inorganic material. The food of man is animal food. Man is a carnivore, that is, he eats meat. Meat is animal flesh. Meat is composed chiefly of protein and fat. Protein is a complex substance consisting of amino acids. Fat is a complex substance consisting of fatty acids and glycerol. Both protein and fat are substances which furnish energy to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plant food consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetable food. Vegetable food is plant food. Plantfood consists chiefly of carbohydrates and mineral salts. Carbohydrates are complex substances consisting of carbon, hydrogen, and oxygen. Mineral salts are substances which furnish minerals to the organism. The food of man is vegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfoodconsistsofcarbohydratesandmineralsalts. Carbohydratesarecomplexsubstancesconsistingofcarbon, hydrogen,andoxygen. Mineralsaltsaresubstanceswhichfurnishmineralsto theorganism. Thefoodofmanisvegetablefood. Vegetablefoodisplantfood. Plantfootdconstitutecarbohydrateandsalts Carbohydratearecomplexsubstanceconssistingofcarbon, hydrogen,andoxygen Mineralsaltesubstancechichfurshinmineralssto theorganism METABOLISM 119 materials and that animals are given. Given a simple food like glucose; however, both animals and plants are able to construct therefrom an endless variety of complex foods and of other organic compounds. There is thus a constant circulation of various materials through air and soil and through the bodies of green plants, which are able to convert carbon dioxide into organic substances which organic substances are continually being built up and broken down. This Organic Cycle is graphically represented in Fig. 65. Foods such as bread, potatoes, rice, wheat, etc., which we call carbohydrates, fats, and proteins. These food types differ from each other in physical structure and chemical composition as well as in their energy content. A. Carbohydrates.—Carbohydrates are substances composed entirely of carbon, hydrogen, and oxygen, in which the hydrogen atom is always in the ratio of two to one with the oxygen atom (C:H_{2}O_{4}). The product of photosynthesis, is an example of a very simple carbohydrate. To this group of foods belong the various sugars, starches, and cellulose, which comprise the great bulk of the food consumed by plants. Starches are the chief source of energy for all organisms and provide most of the building material for the plant body. The three types of carbohydrates are: carbohydrates. There are more common in plants than others. These are glucose or grape sugar, C_{6}H_{12}O_{6}, the direct product of photosynthesis; fructose or fruit sugar, C_{6}H_{12}O_{6}, which is found in fruits differing in the arrangement of its atoms and in certain physical characteristics; and sucrose (cane sugar or beet sugar), with the formula C_{12}H_{22}O_{11}, derived from the simpler sugars by the removal of a molecule of water, thus: \text{2C}_{6}\text{H}_{12}\text{O}_{6} \rightarrow \text{H}_{2}\text{O} + \text{C}_{12}\text{H}_{22}\text{O}_{11} These three types of sugar are all common in plants, though in any particular species one is usually more abundant than the others. Glucose is found in all green plants, especially in the sugars of fruits and of the nectar of flowers, from which honey is derived, and are common elsewhere, glucose probably occurring in every living thing. Fructose is found only in fruits and honey; cane sugar and beet sugar and is therefore the type of sugar with which we are most familiar. Sugars are stored in many plants as reserve foods, but they are not used directly by animals as food; because of the fact that these carbohydrates which are made up 120 BOTANY: PRINCIPLES AND PROBLEMS must be converted into sugar before they can be transported from place to place, or before they can be assimilated into living protoplasm. The starches are insoluble carbohydrates, derived from glucose but much more complex in their chemical composition. Their general formula is C_{6}H_{10}O_{5}, and they are formed by the removal of a molecule of water, thus: \text{C}_{6}\text{H}_{10}\text{O}_{5} - \text{H}_{2}\text{O} = (\text{C}_{6}\text{H}_{10}\text{O}_{5})_{n} The formation of starch is confined to certain plastids in the cell. These are the chloroplasts, in cells where photosynthesis is going on, and the leucoplasts in storage cells. In these plastids the starch is laid down in small grains which increase in number and size until in storage tissues the entire cell-cavity may become filled with them. The grains vary greatly in shape and characteristic shapes and markings distinctive of the species by which they are produced (Fig. 60). Indeed, it is often possible by this means to identify a plant from its starch alone, provided that a particular sample of starch has been derived. A definite core or kalan, often cracked and shrunken, appears in most cases in the grain and is usually surrounded by a series of more or less concentric rings or cracks. Cellulose is a carbohydrate even more complex chemically and physically than starch but with the same basic formula, A, B, C, D, E, F, G on, and the leucoplasts in storage cells. In these plastids the starch is laid down in small grains which increase in number and size until in storage tissues the entire cell-cavity may become filled with them. The grains vary greatly in shape and characteristic shapes and markings distinctive of the species by which they are produced (Fig. 60). Indeed, it is often possible by this means to identify a plant from its starch alone, provided that a particular sample of starch has been derived. A definite core or kalan, often cracked and shrunken, appears in most cases in the grain and is usually surrounded by a series of more or less concentric rings or cracks. Cellulose is a carbohydrate even more complex chemically and physically than starch but with the same basic formula, \text{C}_{6}\text{H}_{10}\text{O}_{5} - \text{H}_{2}\text{O} = (\text{C}_{6}\text{H}_{10}\text{O}_{5})_{n} **METABOLISM** (Ch_{4}H_{8}O_{3})_{n}. Because of its comparative indigestibility cellulose is not usually available as a food, though in certain plants a layer of it (a sporopollenin) is often found on the surface of seeds of food-storages and is later absorbed and used by the plant. Such reserve cellulose is in some cases an important source of food. Of far more importance to man is the plant itself, for the part that cellulose is the material out of which the cell-wall, and therefore the entire skeleton of the plant, is constructed. B. Fats.—Fats resemble carbohydrates in being composed only of carbon, hydrogen, and oxygen atoms, but differ from them in the relative proportion of these elements. The hydrogen atoms are approximately twice as numerous as those of carbon, but in comparison with water they are very few in number and very small. Three common plant fats will illustrate the chemical composition of this type of food. These are palmitin, C_{16}H_{32}O_{5}(C_{3}H_{7}O_{2}); stearin, C_{18}H_{36}O_{6}(C_{3}H_{7}O_{2}); and olein, C_{18}H_{34}O_{4}(C_{3}H_{7}O_{2}). Some fats are liquid and others solid at ordinary temperatures. Fats are insoluble in water and when moved about from cell to cell by diffusion they are broken down into their simpler components, glycerine and fatty acids. In nature, fats are readily converted into sugars and sugars into fats. Fats are also formed by the action of bacteria among plants, but are nevertheless of frequent occurrence in certain situations, particularly in seeds and other regions where food in concentrated form is advantageous. They are apparently not produced by living cells themselves, but rather by the excretion droplets in the cytoplasm. Fats are chiefly important as sources of energy. C. Proteins.—Proteins are composed not only of the basic carbon, hydrogen, and oxygen of the carbohydrate fats, but include nitrogen also, and generally a small amount of sulphur. They vary greatly in their physical properties both as such and in physical properties, and are often very unstable. Protein molecules are large and complex, as is illustrated by the calculated molecular weight of casein, $\mathrm{C}_{16}\mathrm{H}_{30}\mathrm{N}_{2}\mathrm{O}_{5}$; albumin, $\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{N}_{4}\mathrm{O}_{5}$; and globulin of wheat, $\mathrm{C}_{18}\mathrm{H}_{36}\mathrm{N}_{4}\mathrm{O}_{9}$; $\mathrm{S}$; we know less about proteins than about any other class of organic compound known to us. This is due partly to the fact that protoplasm itself is a mixture of complex proteins. Proteins undoubtedly are formed by the union of a simple carbohydrate, such as glucose which has been produced in 122 **BOTANY: PRINCIPLES AND PROBLEMS** photosynthesis, with nitrogen and sulphur which have been taken into the plant from the soil through the root-hairs. This union probably occurs in the chloroplasts, but it is a noteworthy fact (that with a few exceptions) plants alone seem to possess the ability to bring about this synthesis of protein, an ability almost as great as that possessed by animals. The reason is that which enables them to manufacture carbohydrates from inorganic substances. Animals depend almost entirely upon plants for their supply of proteins, even given the simple protein compounds, however, animals can build up more complex characteristic protein materials of all kinds. Proteins with three or more nucleotides are not produced directly but are built up by an aggregation of simpler nitrogenous compounds, the amino-acids, which have been called their "building-stones". Over twenty of these compounds have been isolated and identified, including alanine, C₆H₁₅NO₃; glycine, C₃H₇ON₂; leucine, C₄H₁₁NO₃; glutamic acid, C₃H₇NO₄; and tyrosine, C₆H₁₁NO₃. From these amino-acids, with the addition of other substances, such as water and hydrogen peroxide, and with the aid of enzymes, the following proteins are formed: - **albumina**, globulin, glutenin, prolamina, and many others, which differ in composition, solubility, stability, and other respects. Most of the protein which is stored as a reserve food in plants occurs in definite bodies, the aleuro-precums, which are secreted by the protoderm cells. These bodies are usually spherical and often fill the ex-capacity. The storage of proteins in this form is frequently confined to particular regions of the plant. Thus the aleuro-precum of the pea is found only in the cotyledons under the pericarp, is filled with aleuron and is thus rich in protein. Proteins are much less abundant as reserve foods than are carbohydrates, and are therefore less important producers, but they compose a large part of most effective foods and other foods in the construction and renewal of living substance. As "time-builders" proteins therefore play a vital part in the nutrition of animals. **Digestion.** We have already seen that the physiological processes of a plant are concerned almost entirely with substances which are absorbed from the soil or air or pass from one cell to another within it, substance must first be dissolved. Insoluble materials are of little significance in the economy of protoplasm. Most of the METABOLISM food substances which we have discussed above exist commonly in forms which are not soluble in water. The advantages of this in the case of food are obvious, but the disadvantages are also evident, that before such foods can be moved or translocated within the plant, and before they can be assimilated into living protoplasm, they must first be converted into a form which is more easily capable of converting an insoluble food into soluble form which is known as digestion. Digestion is brought about through the activity of certain highly important but little understood substances known as enzymes or ferments. Enzymes are concerned not alone in digestion but in the production of many other chemical changes in the plant. The nature of these substances is unknown in protein character, although their composition is not definitely known. Enzymes are usually present in exceedingly small quantities but are always present in sufficient quantity to bring about decomposition to their bulk. How they do this we do not understand. The enzyme apparently does not enter into the composition of the substance which it acts upon, but merely brings about a chemical process, and it is not consumed or used up. It seems merely to hasten a chemical reaction which might still take place, although very slowly, without its presence. This is called a "catalytic" or "hulare" reaction. Temperatures largely control their rate of activity, each enzyme having an optimum temperature at which it works best. Above and below this temperature it may be destroyed by heat and even by certain poisons. Aside from effecting digestion, they are concerned with the changes which take place in the plant during growth and development, the process of oxidation in living tissues, and with the synthesis and decom- position of many organic substances. Indeed, most of the metabolic processes of plants and animals are probably depend- ent, in some way or another, on the action of enzymes. It is only the digestive enzymes and their activities with which we are here concerned. Digestion is generally accom- plished by breaking down a large molecule into smaller molecules of water to a molecule of the substance to be digested. The sugars are soluble and most of them may be assimilated directly without further change. The starches are broken down into glucose and fructose through the agency of the enzyme invertase, thus: \text{C}_{6}\text{H}_{12}\text{O}_{6} + \text{H}_2\text{O} = 2\text{C}_6\text{H}_{12}\text{O}_6 124 BOTANY: PRINCIPLES AND PROBLEMS Maltoce is also converted into glucose by malase. Far more important than these changes, however, is the digestion of starch through the action of amylase and other enzymes, with the ultimate production of glucose, thus: $$\text{C}_6\text{H}_{12}\text{O}_6 + \text{H}_2\text{O} = \text{C}_6\text{H}_{10}\text{O}_6$$ The reserve carbohydrates are broken down by hydrolysis into various sugars through the action of another enzyme, cellulase. Fats, by the agency of lipase and similar enzymes, are broken up into glycerine and fatty acids, which may be absorbed into protoplasm. Proteins are broken down into their constituent elements. Of these, of them, peptin and tryptin are especially important. The former converts proteins into water-soluble peptones and pro- teoses; the latter converts proteins into amino-acids. The complete production of amino-acids. It should be noted that all these types of digestion are carried on within the protoplasm of living cells therefore, the digestive process is not necessary, but not in the cavities of special digestive organs. Assimilation.—After a food has been digested, it must then enter the protoplasmic cavity and become an integral part of the living organism. About this process, which is known as assimila- tion and which is really the central problem of metabolism, we know very little. From the activities of protocytism it is clear that this process takes place in two stages: first, absorption both chemically and physically. It is highly unstable and is contin- ually undergoing processes of construction and destruction. Although we have no direct evidence of its occurrence into protoplasm of certain comparatively simple substances and the departure thereafter of others equally simple, we must plead almost complete ignorance concerning the processes that take place between these two events. It is here that dead matter becomes alive, that inert food substances become endowed with those attributes which make them capable of life as we call life. This change never occurs spontaneously in nature, but it always brought about through the activity of living substances already existing in the body. In short, life always comes from life and in no other way. Although this process is going on continuously in every living plant and animal, we have as yet been quite unable to master its intricacies and to initiate it artificially. A diagram showing the conversion of starch to glucose. **METABOLISM** Respiration. Hitherto we have been considering those physio- logical changes which involve the progressive building up and elaboration of organic substances by means of the process which reaches its climax in the production of protoplasm. This constant upbuilding and renewal of the living substance is suc- ceeded by a process of disintegration and destruction, which results in the liberation of energy and which is usually accom- panied by the intake of oxygen and the ejection of carbon dioxide. To this point our study has been brief, but now we must give. Before we enter upon a detailed study of this important phase of plant physiology we should discuss briefly the problem which it brings before us. The living organism is a complex body in which processes of food synthesis and nutrition form an essential part. Like every living thing, the plant is centripetally active. This activity is carried on by means of the protoplasm, the col- ler of the plant body as a whole, of the substances within it, of the atoms and molecules during those chemical changes which are always taking place in living cells, or in the preparation of growth. It is necessary that these changes take place, some of which is necessary if the plant is to remain alive, require the expenditure of energy, do any movements of matter, and one of the chief problems which confronts us in connection with the function of a machine, is to obtain an adequate supply of energy and to liberate it at the proper times and in the proper places. Kinetic energy exists in two forms: Active or kinetic and stored or potential energy. Kinetic energy performs work by setting motion in matter, sometimes by moving bodies through space, sometimes by raising their temperature, sometimes by producing chemical alterations within it, and sometimes in other ways. Potential energy is inactive energy, that is, energy which is not in motion, position, or condition of that object. Potential energy exists in a stretched spring, in a bent bow, in the water of a mountain stream, in a charged battery, in a compressed gas or liquid. It is present only when it acts only as the result of previous expenditure of kinetic energy upon that object. Care should be taken course be taken not to confuse this "state" of energy with the storage of food or any other kind of energy. The supply of kinetic energy or supply of stored energy in a given body merely affects the relations between its parts and does not alter in the least the bulk of the object or the amount of matter which it contains. We need 126 BOTANY: PRINCIPLES AND PROBLEMS only to remember that a tent spring weighs no more than an un bent one or a charged battery than an uncharged one. Energy and matter are fundamentally distinct. Release of Stored Energy.—The potential energy in an object may at first be stored up in some form, but when it is con- verted again into kinetic form and do work, as when the stretched spring moves the mechanism of a watch, the bow moves the arrow, the stretched string of a bow moves the arrow, the telegraph sounder, the burning coal converts water into steam which moves a machine, or the explosive moves a projectile. In all these cases the potential energy is finally ex- hausted and motion ceases. In this process of conversion little goes into potential energy and back again, no energy is gained or lost, the total amount remaining constant. A machine can only do work and direct the expen- diture of energy so that work of a particular kind is done at a particular place and time. One of its prime necessities is an ample supply of fuel. The most common source of fuel with which we are most familiar this is available in the form of wood, coal, oil, or stored electricity. The living organism resembles a machine in that it requires a continuous supply of energy, and it therefore needs a plentiful supply of this energy in potential form which it may liberate, in the process of respiration, at any point throughout its body to perform all its many activities. The fuel which the organic machine uses in this process we know as food, and the potential energy within this food can be converted into chemical energy which was converted into potential form by photosynthesis in the green cells of the leaf. Food resembles wood, coal, or oil in being a substance which contains chemical compounds. When ad- dition of oxygen will rapidly break down and resolve itself into simpler compounds, usually carbon dioxide and water, and thus release the potential energy it contains. This process of oxidation is common to all organisms and occurs only at high temperatures and is then known as combustion. In living organisms it can go on at much lower temperatures and is here known as respiration. Both processes involve their essential features—the liberation of energy in kinetic form by the breaking down of complex and unstable chemical compounds into simple ones—and both involve addition of oxygen—respiration—combustion are precisely similar. A diagram showing the conversion of potential energy (stored in wood, coal, oil) into kinetic energy (work done by machines). The diagram shows how potential energy can be converted into kinetic energy through various means such as burning coal to produce steam power, or using a stretched spring to move a watch mechanism. METABOLISM 137
Carbon Dioxide Process of Photosynthesis
A C B D
1C 2H₂O 3CO₂ 4O₂
2C 4H₂O 6CO₂ 8O₂
3C 6H₂O 9CO₂ 12O₂
4C 8H₂O 12CO₂ 16O₂
5C 10H₂O 15CO₂ 20O₂
6C 12H₂O 18CO₂ 24O₂
Carbon Dioxide (CO₂) Hydrogen (H₂O) Oxygen (O₂)
A.
D.
Water (H₂O)
B.
Carbon Dioxide (CO₂)
C.
Oxygen (O₂)
Process of Respiration
A.B.C.D.
Degmon (C)Glucone (G)H₂O (H)O₂ (O)
A.C.D.E.
B.D.F.G.
C.E.F.G.
D.F.G.H.
Degmon (C)Glucone (G)H₂O (H)O₂ (O)
128 BOTANY; PRINCIPLES AND PROBLEMS A machine is supplied with fuel (its "food") by an operating mechanism analogous to man's by the sun, but the food which a green plant consumes must be synthesized by its own activi- ties. The significance of the food-making process we call photo- synthesis is now more evident than before. The kinetic energy which plants derive from the rays of the sun is here used to do the work of pulling together carbon dioxide and water and uniting them into the simple food glucose. This union is obviously stored up in potential form in the molecules of the glucose and of the other foods or plant materials which may be derived from it. The energy which is thus stored up is something like a clock is stored up in the compressed mainspring. Under appro- priate conditions this potential energy may be liberated anywhere and at any time. The storage of this energy in the form of food is one of the most important principles of physiology is thus brought out—that food is merely the medium by which energy received from the sun intermittently becomes available for all sorts of activities, being used up, carried to all parts of the plant, and made available for work at all times and in all places. This conception has been concisely formulated in the metaphor that "food is a storage-battery," charged when green leaves receive their energy from the sun by respiration. The Impact of Photosynthesis—The importance of photo- synthesis in the organic world lies in the fact that this process is practically the only means whereby living things can store up energy. Green plants, the ultimate providers of all foods, are the sole agents capable of converting solar energy into forms that tap the abundant supplies of energy in the universe and obtain therefore a sufficient quantity to maintain their varied activities. This energy is all stored up in the form of chemical compounds. When a flower is converted again into kinetic form may pass through scores of modifications and enter into the composition of the bodies of a does some years later. In this way, through millions of years, stram- ping upon the earth daily in untold quantities from the sun, only the chlorophyll-containing plants are able to use it directly. Even in industry, this still depends very largely upon photo- synthesis, for example, on which he rears his crops. Coal and oil was originally locked up in these substances, in some cases millions of years ago, by the photosynthetic activity of green A diagram showing a flowchart or process flow diagram. **METABOLISM** Respiration and Life.—With an understanding of the significance of the plant's energy relationships and the part which respiration plays in them, we may now turn to a study of respiration itself. This process, unlike photosynthesis, is not carried on in particular organs and under particular conditions, but is distributed throughout the whole body of the living cell. Respiration, indeed, is believed to be a necessary accompaniment of life itself, as may be inferred from the fact that living protoplasm is continuously active and is thus continually expending its energy. The presence of oxygen in the air shows no external signs of life; respiration, though very feeble, may still be detected. The amount of respiration which takes place is a rough index of the vitality of the organism being studied. The liberation of energy is the essential feature of respiration and the addition of oxygen is its usual accompaniment, particu- larly in the case of animals. In plants, however, this is not always true, notably among some of the lowest members of the plant kingdom, respiration may be carried on in the absence of oxygen. These organisms are termed anaerobic or non-fermentable respiration. They are so different as to require separate consideration. Aerobic respiration—Aerobic respiration is essentially an oxidation process. Free oxygen is added to organic substances (chiefly carbohydrates) with the consequent breaking down of the latter into their original inorganic components, carbon dioxide and water, thus: $$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 = 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O}$$ The oxygen is usually taken directly from the atmosphere through stomata, leucites, or other openings. This process is made possible by those chemical changes which are assisted by enzymes and the oxidizing enzymes or *oxidase* occur in every living cell. As to whether it is the proplasm itself or the contained substance within the protoplasm which is oxidized, this is one subject for speculation. It is certain, however, that most of the carbohydrates and fats which are taken into the protoplasm soon furnish, either directly or indirectly, all the oxygen required for their oxidation. Proteins, although their chief function is constructive, undoubt- edly also contribute to the supply of oxidizable substances. Whatever alternative source material is given off in their de- 120 integration, however, must immediately be assimilated again, for (at least among the higher plants) nitrogenous compounds do not appear as such in the tissues of the plant. Carbon dioxide is almost invariably a product both of aerobic and of anaerobic respiration. Indeed, the evolution of this gas, even in the absence of oxygen, is a necessary condition of respiration and therefore alive. The amount of oxygen taken in and the amount of carbon dioxide given off are in the long run equal, but under certain conditions one may temporarily exceed the other. Much of the energy liberated in respiration ultimately appears as heat, and the temperature will therefore tend to rise with the temperature of its surroundings. By comparing the chemical equations for photosynthesis and anaerobic respiration, it will be seen that they are essentially reverse or reciprocal of the other. Photosynthesis adds carbon dioxide to water and produces sugar and oxygen. Respiration adds oxygen to sugar and produces carbon dioxide and water. In these two processes, obviously going on in the same tissue at the same time, a circumstance which has made the study of plant metabolism peculiarly difficult, since one activity may mask the other. This is particularly true in those plants which photosynth-bearing cells and occurs in them only in the presence of light. In such cells there is a preponderance of photosynthesis in the daytime and a preponderance of respiration at night. This is so in the morning and again at night, and for longer times when illumination is low or other conditions unfavorable for photosynthesis, the two processes being carried on simultaneously in the same tissues giving off in photosynthesis just the amount of oxygen which is necessary to carry on their respiratory activity. Respiration, unlike photosynthesis, does not require light. **Comparison between Photosynthesis and Aerobic Respiration.**—A brief comparison between photosynthesis and respiration is presented in tabular form below:
A.Degmon (C)
Photosynthesis Respiration
Stores energy Consumes energy
Absorbs carbon dioxide Liberates carbon dioxide
Liberates oxygen Absorbs oxygen
Takes place only on green plants Takes place in all plants and animals
Plants also use alchylglycerol-enzymes Plants also use alchylglycerol-enzymes
Enzyme cell Enzyme cell
Consumed food Decreases weight.
Increases weight
Consumed food Decreases weight. **METABOLISM** In its essential characteristics—intake of oxygen, liberation of energy and raising of temperature—the aerobic respiration of plants is exactly comparable to the respiration of animals, a fact which has led to the erroneous belief that they are of the same structure. **Anaerobic Respiration (Fermentation).—** Anaerobic respiration, in which the breaking down of chemical compounds and the consequent liberation of energy take place without the intake of free oxygen, is characteristic of certain lowly plants and some times of higher ones when temporarily deprived of oxygen. The best known examples are the mushrooms, the yeast organisms and allied phenomena. These were named "fermentation" from the fact that the activity of fermentors (now more commonly called alcohol producers) was at first thought to be conseqently a confusion in the application of the term fermentation, the commonest usage regarding it is practically synonymous with anaerobic respiration. It is necessary to admit it to cover all activity brought about by the agency of enzymes. The most important characteristic of anaerobic respiration is in the fact that it does not require the presence of oxygen in the organic substance or food but only to its partial decomposition, with the result that a quantity of more or less complex by-products is formed which still contain a considerable amount of potential energy. This is why anaerobic respiration often stops the process itself. The fermentation of sugar by yeast is the classic example of anaerobic respiration. The yeast cells—minute, single-celled organisms—thrive in rather weak solutions of sugar. Those which have easy access to the air usually respire aerobically, but if they happen to be in a solution where oxygen is scarce (where below the surface of the liquid) or if it becomes exhausted, the yeast respires anaerobically. The cells then obtain their energy through a process known as glycolysis, in which the formation of carbon dioxide and a complex by-product, ethyl alcohol, takes place: $$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{CO}_2 + 2\text{CH}_3\text{OH} (\text{alcohol})$$ This change is effected by the activity of an enzyme, *zymase*, which is secreted by the yeast cells. It is evident that only a portion of the glucose molecule can be used up before it is oxidized, for the resulting alcohol may be absorbed by another organism, or may be burned, and will then yield a considerable amount of additional energy. When the concentration of alcohol increases... BOTANY: PRINCIPLES AND PROBLEMS a certain point, it poisons the yeast plant and fermentation ceases. The respiration of other minute plants may also bring about alcoholic fermentation, and still others produce fermentations. A diagram showing the structure and functions of a plant. It includes representations of roots, stems, leaves, and buds with their functions. The root is shown with water entering through the root hairs, along the stem, and to and from the leaves. The leaf represents photosynthesis, transpiration, and absorption of water. The bud shows growth and budding. Fig. 58.—Important structures and functions of the plant. Diagrammatic representation of roots, stems, leaves, and buds and their functions. Wood solid black, bark light brown; water enters through the root hairs, along the stem, and to and from the leaves. One leaf represents photosynthesis, transpiration, and absorption of water. Bud shows growth and budding. Roots, stems, leaves, and buds are graphically represented. METABOLISM of different types, such as those having for their by-products butyric acid (in the spoiling of butter), lactic acid (in the souring of milk), and various others, many of which are of economic importance. In all these cases energy is liberated and may be detected by the same methods as in temperature, which is often more marked than in anaerobic respiration. Certain micro-organisms are exclusively anaerobic and are actually killed by the presence of free oxygen. Others, like yeast, may respire aerobically or anaerobically, depending upon the external conditions. The decay of dead organic matter is due almost entirely to the respiratory processes of organisms. If the material is exposed freely to the air, bacteria will break it down rapidly and completely by their aerobic respiration. If free oxygen is unavailable, as is the case in a sealed bottle, anaerobic respiration sets in, the process is carried on anaerobically and is slower and more complicated. Here a whole series of micro-organisms, each specific for certain substances, work together. The first step will break down the organic substance partially, extracting a certain amount of the potential energy which it contains. In its altered chemical form it passes into solution. Then, in succession, the remaining material is now acted upon by another type of micro-organism and, entering into the anaerobic respiration of this form, is still further broken down and loses still more of its potential energy. This process continues until finally the whole of the material (except its mineral constituents) passes into the atmosphere as carbon dioxide, water and nitrogen, the original materials being thus restored to their original state. We have now completed a study of the root, the stem, and the leaf, and of the functions of absorption, conduction, photosynthesis, respiration and conservation. These are the main vegetative structures and functions of the plant body, and are graphically set forth in Fig. 68. QUESTIONS FOR THOUGHT AND DISCUSSION 384. Which of the three main types of food makes up the largest part of the diet of man? 385. What advantage is it to the plant to have its carbohydrate food stored chiefly in the form of starch rather than chiefly in the form of sugar? 134 BOTANY: PRINCIPLES AND PROBLEMS 386. What is it that makes fats the most concentrated of foods? 387. Why are proteins "far more effective than any other foods in the reconstruction and renewal of living substance?" 388. Crops belonging to the Legume family usually contain more protein than most cereals. Explain. 389. Where and when in a plant does digestion take place most vigorously? 390. What important difference is there between the process of digestion in plants and that in animals? 391. Just how does food which is stored in the endosperm of a seed, and which is therefore not in the young embryo plant itself, become available to this young plant? 392. In what way do the insects captured by an insectivorous plant become available to it as food? 393. Why do parasitica taste so much sweeter in the early spring than in the previous fall? 394. Why are vegetables like peas and sweet corn much sweeter in their yellow or green mature state than when they grow older? 395. Maple sap is very sweet in the spring but contains almost no sugar in the summer. Explain. 396. Hay harvested before its seed is ripe has much more feed value than it has after it is ripe. Why? 397. Certain fungi attack wood, their very delicate, thread-like branches penetrating readily into the hard, woody tissues. How is it possible for them to do this? 398. Certain energy examples from everyday life (aside from those mentioned in the text) illustrate how we change kinetic into potential energy and its subsequent release in kinetic form. 399. What is the ultimate source of all energy liberated in the bodies of plants and animals? 400. Just where in the plant body is kinetic energy changed into potential energy and just where is potential energy changed into kinetic? 401. What was the original source of the energy which we derive from the burning of wood? Explain. 402. What main sources of energy used by man in his industries owe their origin to photosynthesis? What do not? **METABOLISM** 135 **403.** Is photosynthesis or respiration the more active process in a normal green plant? How do you know this? **404.** Why is an excretory system, no necessary in animals, not needed in the case of plants? **405.** State all the resemblances you can think of between an organism and the flame of a candle. **406.** How is it possible for oxygen to get into a living cell? **407.** How do you think oxygen penetrates to cells deeply seated in the plant body? **408.** Through what structures does oxygen enter (1) a young, growing root, (2) a leaf, (3) a woody twig, and (4) an old trunk? **409.** How do submerged water plants get their supply of oxygen? **410.** Plants which live in bogs or very wet places usually have large air chambers in their tissues, particularly in roots or other subterranean parts. Explain. **411.** Do you think that growing plants are good things to have in a sickroom or a hospital? **412.** If a plant were to be grown in air which had been freed of oxygen, would it live longer in darkness or in light? Why? **413.** A seedling plant which has sprouted and grown in a dark place will have a dry weight which is less than that of the seed from which it grew, though the bulk of the seedling is far greater than that of the seed. Explain. **414.** Does the increase in the dry weight of a plant measure the amount of food it has assimilated? Explain. **415.** Which of three identical wooden posts will remain sound longest: One left freely exposed to the air, one driven into the soil (as a fence post), or one driven under water (as a pile)? Explain. **416.** Most of the fossils of animals and plants which have come down to us are preserved in swamps rather than on high ground. Explain. **417.** Why is it necessary to change the water in an aquarium frequently if animals are kept in it? Some, but not all, when green plants are living in it alone? **418.** Where in a plant will you try to find the highest tempera- ture? 136 **BOTANY: PRINCIPLES AND PROBLEMS** **419.** The internal temperature of plants is not far from that of their surroundings. For the lower animals it is usually much above that of their surroundings. Explain. **420.** Why do land animals need to have lungs for inhaling and exhaling air, when such structures are unnecessary in plants? **421.** Which will weigh more, a piece of wood equal in weight to a piece of aluminum? What is your name value? What will yield more heat when burned? Explain. **422.** Why do plants in glazed pots grow poorly? **423.** Why should pebbles or bits of broken pottery be placed in the bottom of a flower pot in which a plant is to be grown? **424.** In propagating a plant by "cuttings," a small shoot is cut off and the cut end is inserted into moist sand or soil. Sand is much better than a heavy clay soil for this purpose. Why? **425.** Cut flowers will keep longer in a refrigerator than at ordinary room temperatures. Why? **426.** In cranberry bog, where there have been flooded in the fall to protect them from frost, the leaves suffer much more from "shivering," due to lack of oxygen caused by the flooding, than do the mature, fully ripe ones. Why? **427.** Fermentation often generates a considerable gas pressure, but sometimes it does not. Explain this difference. **428.** What causes yeast bread to "rise"? **429.** Why is it important to boil down maple sap so soon as it is drawn from the tree rather than let it stand? **430.** When jars of preserved fruit "spoil," why do the covers sometimes blow off? **431.** A foundation of stable manure under a hot-bed will "heat," and thus keep the soil warm. Explain. **432.** Why must the manure under a hot-bed be moist before it will "heat"? **433.** Give at least two reasons for lifting the glass from a hot-bed in the middle of a sunny day? **434.** What danger is there in putting insufficiently dried hay into a hot-bed? **METABOLISM** 157 **435.** The danger mentioned in the previous question is much less if the hay is sprinkled liberally with salt and if the barn is very tightly built. Note.—A silo is a large, tank-like structure, open at the top. Green and living corn plants, chopped up into small pieces, are packed tightly into the silo. The air is excluded by airtight doors. **436.** What prevents the contents of a silo from drying? **437.** During the first few days after a silo is filled, its contents become distinctly warm and then gradually cool off. Explain. **438.** Why is it necessary to have the walls of a silo built very tightly? **439.** If the contents of a silo is not packed down tightly, it is apt to spoil. Why? **440.** Why does the upper layer in a silo usually decay? ## REFERENCE QUESTIONS **62.** Give an example of a plant rich in starch; in fat; in protein; or in fat. **63.** Which will produce more energy per unit of weight, a carbohydrate or a fat? **64.** Does fat play a more important part in animal or in plant nutrition? Explain. **65.** Why does a starchy food keep better than a fatty one? **66.** In general, how are "organic" substances to be distinguished from "inorganic" ones? Give examples. **67.** By what means does sugar become converted into starch? **68.** Explain just what has been the history of a piece of coal and why it produces so much energy when burned. **69.** What non-gaseous waste products sometimes result from plant digestion and what does the plant do with them? **70.** Do plants ever derive energy from the oxidation of other compounds than those of carbon? **71.** Give the utilization of each of the following terms and explain in what way it is appropriate:
Metabolism Enzyme Expiration
Digestion Amination Fermentation
CHAPTER VIII GROWTH We have learned that food provides the plant with the energy needed to carry on its various functions. A large part of the food which enters into the nutrition of the plant is broken down directly by respiration, to liberate energy for immediate use, or is stored up to meet requirements of this sort which may later arise. The remainder of the food is assimilated and then is necessary to maintain the activities of its living substance, and the surplus may be built into the tissues and used to pro- duce new cells. This process of growth represents an excess of constructive over destructive metabolism. A knowledge of just what it involves, however, has not yet taken place, is evidently necessary if we are to arrive at a clear understanding of the processes and development of the plant body. The term "growth" in simplest usage, refers to any increase in size either of the whole organism or of its parts. This expansion may be a mere swelling brought about by a vigor- ous absorption of water, or it may be due to an increase in bulk of actual physiological material, or it may be due to an increase in structure secreted by it. All early stages in growth are of the former type, and the swollen and succulent tissues thus produced gradually give way to more compact structures within them of large amounts of new material. Indeed, early growth may be accompanied by an actual, though temporary, decrease in dry weight. In studying this process of growth in the plant as a whole we must remember that the plant body is made up of a mass of minute cells, each cell being composed of protoplasm and within the same species is rather constant, and is believed to approximate the size which is most efficient for that particular tissue. Very large cells would be inefficient since they would evidently possess many disadvantages. It is therefore clear that growth must consist in the production of more cells rather than in the 138 GROWTH 130 enlargement of those already present; and the method by which new cells are formed and added to the plant body deserves careful study, as considerable interest attaches to this process. The Production of New Cells.—In the previous discussion of the plant cell (Chapter IV) we noted that it consists of a small mass of protoplasm surrounded by a cell wall. This may be distinguished, the undifferentiated cytoplasm and the denser A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Fig. 60.—Cell division by mitosis. A, mature cell with chromosomes of the nucleus in a fine network. B, the chromatin is gathered into a long thread. C, this thread is divided into two equal parts. D, the two threads are now straightened. (R, C and D are called prophase.) E, synapse. The split chromosome is now seen to consist of two parts, one of which is called the spindle, with its two poles, is formed. F, metaphase. The chromosome halves separate and move towards opposite poles. G, anaphase. The chromosomes have moved apart and are now at opposite poles. H, telophase. Each new group of chromosomes arranges itself into a complete new cell, each with a nuclear content equal and similar to that of and more or less spherical nucleus. About the whole is a cellulose wall, deposited by the living substance within much as a clamshell is deposited by the living shell on growing mussel where 140 BOTANY: PRINCIPLES AND PROBLEMS cell multiplication is rapidly going on, the wall is very thin, and the sap cavity, so characteristic of the plant, is absent. In the formation of cells in such growing regions three main stages may be distinguished: Cell division, in which the number of cells is increased by the division of each parent cell into two; cell elongation, in which the cells become longer; and growth, to which A B C D E Fig. 70.—Diagram of mitosis in an ordinary body-cell. d., resting nucleus; a. and c., daughter nuclei after division; b., daughter cell; d., new cell. The separate chromosomes are shown in a. and c. The arrows indicate the direction of movement of the chromosomes during the process of division. It is evident that the chromatin material is divided exactly evenly between the two daughter cells. (Modified from Sharp.) final size, and cell nutrition, in which they assume their mature structure and characteristics (Fig. 71). Cell division, commonly known as mitosis, is not in most cases a simple splitting into two of the mother cells but is accomplished in a rather complex manner (Figs. 60 and 70). The mechanism of this process has been studied by many authors. Within this body is a characteristic granular material which stains very deeply with certain dyes used in microscopical work and has hence received the name of chromatin. In the young vegetative cell the chromatin is arranged in a fine network, but when the cell is preparing to divide, the elements of the chromatin net-work come together to form distinct chromosomes. The number of chromosomes is the same in every vegetative cell of the plant and is constant for any particular species. Thus in wheat it is 28, in oats 14, and in tobacco 48. Soon after the GROWTH 141 chromosomes become evident, two poles or apparent centers of attraction arise in the middle between opposite sides of the nucleus, and from each of these poles a spindle of delicate fibers radiates inward toward the nucleus. The nuclear membrane now disappears and the chromosomes become grouped in a plane or plate-like mass at the center of the cell. From this plate extend to the two series of spindle fibers, which soon meet and form one continuous spindle reaching from pole to pole. At this point each chromosome is divided into two daughter chromosomes, one of which now moves toward one pole and the other toward the other pole. The whole chromatin mass is thus divided into two parts, each part moving independently. Each chromosome group now becomes broken up again into a network around which a new membrane forms, and two complete nuclei are formed. At this stage, however, at the point of each spindle fiber appears a thickening, and these thickenings soon enlarge and unite to form a disc or plate across the cell. About this time the nuclear membrane reappears and completes the division of the mother cell into two similar daughter cells. Why such a complicated process as mitosis should be necessary in well-nourished animals is difficult to understand. It is perhaps concerned with the need for making an exactly equal division of the chromatin material, since this part of the nucleus is known to be of great importance in directing the growth and differen- tiation of cells. Cell Enlargement.—Although two new cells have now been formed, they still occupy together a space no larger than that of one of them before division. This is because water has not yet really taken place. The abundance of sugar and other dissolved foods, however, with which a growing region is usually supplied, causes a large amount of water to be absorbed by the cells, and water is therefore vigorously drawn into them by osmosis. Since the newly-formed walls are very thin and elastic they stretch readily, but when they reach their maximum size they do not expand until their permanent bulk is attained. During this rapid expansion the amount of protoplasm does not increase; although it fills the space between the walls, it does not increase in volume. These larger ones to a thin slice which lines the wall. The bulk of the cell is now occupied by the vacuole or sap-organ; so characteristic of mature plant cells. The vacuole increases in size continuously by absorption of water, following the production of new cells by mitotic division, causes most of the obvious increase which we see in the size of plants. Cell Metabolism. The new tissue thus formed is very soft and weak, owing to the thin walls of its component cells, and the third stage in growth, elongation, is brought about by the transfer of an abundant supply of food from the older parts of the plant, and the consequent construction therefore of new living substance and of heavier walls until the cells have reached the normal, mature condition characteristic of all parts of the plant, when they form a part. Growing-points and Their Function. Such, in brief, is the history of the development of new cells by which the growth of the plant body takes place. It is evident, however, that in mature plant tissues, where the cells are surrounded by thick and firm walls, cell division does not take place. These cells are thus really looked within their own walls and can grow no further. Organs like the leaf and flower, which rapidly attain a rather definite shape and size, do not divide at all. But this does not present this problem, for here the organ develops from a small mass of growing tissue, enlarges rapidly throughout its whole extent and reaches a definite size before it begins to undergo any growth stages. In organs as the root and stem, however, where growth continues more or less indefinitely and where the great bulk of the tissues are necessarily mature and functioning, there must occur some means whereby new cells are produced from old cells. This is accomplished through the activity of growing-points or meridians, which are merely groups of cells remaining undifferentiated and capable of undergoing mitosis, thin-walled and packed with protoplasm. These groups of permanently "young" cells occupy regions where growth is to take place and are called growing-points or meridians. Such a growing-point may long remain dormant, but when it becomes active, cell division begins again within it. The newly formed cells then grow outwards and become larger; these now undergo enlargement and become themselves mature. This process does not affect all the cells of the growing-point, however; only those which are destined to become cells still remains undifferentiated and continues to serve as a manufacturing factory of young cells which are to be added to the tissue. The growing-point is thus a factory for producing new groups of cells, which grow on in bulk itself, but carried progressively out- GROWTH 143 ward on the crest of the tissue which it creates. The growing-point may perhaps be compared with the central axis, which forms only a very thin layer of the cortex of the stem, but by their activity build the root farther and farther outward and are carried out upon it; or it may be compared to a brick-layer constantly laying bricks upon a wall, so that the building being carried himself high in air by the wall which he has made. This method of growth at a definite point or layer, through the action of a central axis, is found in all higher plant tissues and so different from that employed in the growth of animals, has certain consequences worthy of mention. It writes in the leaf, for example, a line of cells which will grow and develop, for many of the first-formed tissues are still present (unless lost through decay), buried in the successively later accretions which have been added to them. A careful internal external examination of a tree trunk, for example, enables us to tell almost exactly how tall and how thick the tree was at any given time during its life. An understanding of the position and activity of growing-points is essential in the practice of the various methods of grafting and budding, for these are necessarily concerned with a manipulation of the meristematic regions. There are two general types of growing points in most plants—terminal and lateral. The former, which develop at the tips of shoots and roots, are usually associated with leaves or buds, and through their activity the stem grows tall and as roots spread further into the soil. The latter, of which the condition is characterized by bulbils, are usually associated with leaves or buds, encircling the root and stem throughout their entire extent and causing these organs to increase in thickness. Terminal growing-points at the tip of a root furnishes a good example of a terminal growing-point. In a brief study of this region we probably enable us to understand more clearly just how such points function. At first sight they appear to be like the tip of the root is the root cap, a body of dead cells containing mucilage from the growing-point within as they are detached by friction, and which protects the delicate root tip until it is forced through the soil. Just beneath this is a layer of living cells, each with small mass of tissue usually not nearer than two or three millimeters in length, and composed of small, thin-walled and richly protoplasmic cells. These cells are placed at right angles to one another along 144 BOTANY: PRINCIPLES AND PROBLEMS A diagram showing the stages of cell division in a growing root. The topmost part shows the zone of cell maturation, with cells elongating. Below that, the zone of cell enlargement shows cells becoming larger. At the bottom, the zone of cell division shows cells dividing into two smaller cells. The root cap is at the very bottom. Fig. 71. Longitudinal section of a growing root. In the zone of cell division the cells are small, rich in cytoplasm and rapidly dividing by mitosis. In the Zone of Cell Maturation Zone of Cell Enlargement Zone of Cell Division Root Cap GROWTH 145 Behind this is the zone of growth or cell enlargement, where the cells which have been formed at the growing-point stretch and elongate. This is the most active part of the root, and here only, that growth of the root in length takes place (Fig. 72); and it is the force exerted here by cell elongation which drives the root tip through the soil. Just back of this region, in turn, is the zone of maturation, where the cells, now having attained their full size, begin to differentiate into wood and bast. In this zone of maturation the cells have attained their final size, and the center of each grows out into a central canal, while the outermost cells become thick-walled and lignified. In this zone only (or very rarely) does lateral differentiation of the tissues take place. A, B, and C are cut from these three zones, showing enlargements. A diagram showing the different stages of root development. Fig. 72.—Growth of the root in length. Two squash seedlings, one at the right a day or two older than the other at the left. The change in length of the root proceeds from below upwards. In this diagram, shown that growth in length takes place only very near the tip of the root. 10 146 BOTANY: PRINCIPLES AND PROBLEMS in thickness through the subsequent activity of a cambium farther back along the root. The terminal growing-point at the apex of a stem resembles in its essential features that described for the root, but the zones in the growing region are not usually so clearly distinguishable and their activities are more closely interrelated. The cambium is at the very tip but a certain amount of cell division is going on throughout the zone of growth, which hence may extend over a distance of several inches. Lateral Growing-point or Cambium—The lateral growing-point is somewhat more complicated than the terminal one and its activities are often a little hard to follow. As the best example of this type we refer to the structure of the fibre-vascular cylinder of root and stem, highly developed in all typical woody plants. We have seen in our study of the stem (and except for the above mentioned exceptions) that the cells of the cortex are mostly parenchymatous, while those of the fibre-vascular-tissues are arranged in a cylinder, with a ring of wood made and a ring of bast outside. Between these two rings is a layer of cells which are called procambium. These only one cell in width, which is formed of the same small, thin-walled and richly protoplasmic cells which are characteristic of terminal growing-points. The procambium is surrounded by parenchyma, the fibre-vascular cylinder, and thus the whole stem, grows progressively stouter. Unlike the one-sided terminal meristems, how- ever, this cambium is not restricted to any particular side but both sides (Figs. 49 and 50). When the cambium is active and cell division is taking place, the new cells which lie on the inner edge of the outer ring of wood become elongated and differentiate into parenchyma and maturation and become the outermost wood cells; and the new cells which lie on the outer edge of the cambium, next to the bast, similarly develop into the innermost bast cells; but a zone of thin-walled cells remains between them remaining as a second cambium (Fig. 73). Thus the cambium, never growing itself, continually adds to the thickness of both its adjacent tissues. Just as the terminal meristem produces new cells on its outer surface, so the cambium ring is carried farther and farther away from the center of the stem by the growing wood; and the bast, lying out- side this ring, is also carried farther away from it by the growth of the wood but by the increase in its own thickness which has taken place at its inner edge (Fig. 74). Cambial activity may perhaps be crudely pictured by comparing it to the growth of a will GROWTH 117 of brick (the wood) surmounted by a coping of tile (the bast); and by assuming that just at the junction between brick and tile a bricklayer (the enlaimis) is able repeatedly to insert a brick next to the last one, so that each new brick is placed upward and carrying an ever-thickening coping of tile on its top. A diagram showing the growth of a tree. The trunk is shown at the bottom, with a series of horizontal lines representing the annual rings. The first ring is labeled "Old Wood," the second "New Bast," the third "Corkum," the fourth "New Bast," and the fifth "Old Wood." The diagram illustrates the process of growth, with each new ring representing a year's growth. Fig. Fig. 73.—Cymbium, a pine stem actively producing new wood and bast. The young wood is light brown, and the bast is dark brown. The old bast has expanded to its mature size. The new wood is still very thin-walled and retains a thing of exquisitely beautiful texture. As a consequence of this method of growth, the youngest layers of the wood are the innermost and the youngest layers of the bast are the innermost; and the past history of these two tissues, as it is preserved in their structure, should thus be read in opposite directions. In the wood, where the annual rings are thick and thickness is considerable and continuous, the bast, because of its rather delicate texture and the strain which is put upon it, becomes much stretched and crinkled in its outer layers. These are ultimately A diagram showing the growth of a tree. The trunk is shown at the bottom, with a series of horizontal lines representing the annual rings. The first ring is labeled "Old Wood," the second "New Bast," the third "Corkum," the fourth "New Bast," and the fifth "Old Wood." The diagram illustrates the process of growth, with each new ring representing a year's growth. 148 BOTANY: PRINCIPLES AND PROBLEMS soughed off with the bark, whereas the wood, with its much firmer structure, remains unchanged. Well-marked growing rings are developed in the wood so that an inspection of this tissue as seen A diagram showing the growth of a stem in width (diagrammatic). Three-year section of three progressively older stems. Through the activity of the cambium a new layer of cells is added to each stem each year. Dull and outer circular, wood plain, but lined. One Year Old Two Years Old Three Years Old Fig. 74.—Growth of a stem in width (diagrammatic). Three-year section of three progressively older stems. Through the activity of the cambium a new layer of cells is added to each stem each year. Dull and outer circular, wood plain, but lined. In cross section makes it possible to read with much accuracy the age and past history of the plant. Cork Condom.—Another later growing-point of importance is the cork conodum, which develops the layers of corky bark. P.O.R.T.E.R.Y. U.C.S.D. N. C. State College GROWTH 149 This may arise almost anywhere in the cortex or in the old bast, and develop on its outer face a row of corky cells which soon die and connect with each other by means of a thin layer of living tissue, **Primary and Secondary Tissues**.—The tissues laid down by a cambium are apt to be regular in the arrangement of their cells, particularly when they are formed by a single layer of cells of the same size. This is due to the fact that each cambium cell has produced a whole row of wood cells within and of bast cells without, and that these have been naturally arranged in a straight line along the radius of the stem (fig. 73). The cambium itself is the common ancestor of both these tissues, which are their common ancestor (fig. 73). Tissues produced by a cambium are known collectively as secondary tissues, and they all display in cross section this rather regular arrangement of their cells. Primary tissues are those which arise from a terminal growing-point, and in cross section their cells are irregularly arranged. The epidermis, cortex, and pith, and the first formed wood and bast, are all primary in their origin. The great bulk of the wood and bast in woody plants is formed by secondary growth. **Differentiation.**—Growth is not mere increase in size but gives rise to definite organs and organ systems, which are markedly different from one another. The differentiation of the plant body as growth takes place are not understood, but we know that the process may be somewhat similar to that which occurs during the formation of one part of the plant, such as is brought about by pruning, will stimulate the growth of the rest, and the removal of one organ or organ-system will cause the growth of some other organs of this particular type. The development of certain organs is also dependent on the fullness of certain rather definite exter- nal conditions. For example, in many plants, such as orchids, for example, the reproductive structures (flowers and fruit) develop only after the plant has succeeded in accumulating an ample supply of reserve food from which they may be built. Whatever stimulates very rapid growth is likely to be associated with abundance of water and nitrate salts, will lead to retardation of development of floral organs, and conditions of the opposite sort (providing a sufficiency of light) will lead to retardation of vegetative develop- ment. The amount of photosynthetic activity, governed chiefly by the number of hours of sunlight available per day, also has an important influence on the appearance or suppression of sec- 150 BOTANY: PRINCIPLES AND PROBLEMS reproductive structures. By skillful manipulation, the growth and differentiation of the entire plant may thus to a certain degree be brought under control. QUESTIONS FOR THOUGHT AND DISCUSSION 441. What is the chief difference in method of growth between animals and plants? 442. With what other important difference between animals and plants is this difference in method of growth associated? 443. What disadvantage would there tend to be in very large cells? Is very large cell size good or bad? 444. What factors are there which tend to limit the size to which a tree can grow? 445. In what direction, or place, with reference to the rest of the stem are the most suitable sites on which trees take place at the terminal growing-point of the stem? at the cambium? 446. What difference in shape would you expect to exist between the cells at a terminal growing-point and those at a cambium? 447. The zone in which elongation occurs in the root tip is much shorter than is that in the stem tip. Of what advantage is this fact to the plant? Growing-points of plants are usually good to eat and in a few cases are important human foods. Why is this so? 448. The wood of a tree is formed by the wood of a twig in the spring but usually at no other season. Explain. 449. If a nail is driven into a tree trunk at a point 3 feet from the ground, what position will this nail occupy in the tree 30 years later? What evidence do you have for your answer? 450. In just what part of the stem does growth of the plant take place? of the wood? of the cortex? of the bark? of the epidermis? 451. Is the pith in a one-year-old twig wider or narrower than it is in a 20-year-old branch grown from that twig? 452. Where would you find the cortex in a tree trunk? 453. What important changes in the size and character of its tissues take place during the development of a young twig into a woody branch? 454. Why is the bark of a tree almost always rough and cracked? **GROWTH** 151 **456. Why does the bark of a tree never become as thick as the wood?** **457. If you were to determine the age of a tree by counting the annual rings, where in the tree would you make the count? Why?** **458. In what way is the bark usually wider next the pith than they are far out in the trunk. Why?** **459. How can we use the annual rings of old tree trunks to study past climatic conditions? What caution must we observe in doing so?** **Note.—In grafting, a small twig (the scion) which has been cut from one plant is inserted into another plant (the stock). This may be done in several ways, but in all cases the leaves of the scion are removed before inserting it into the stock. The operation is successful only when the cambium of one touches the cambium of the other. If the operation is successfully performed, the stock and scion will unite and bear the latter's flowers and fruit.** **460. In the process of grafting, why is it necessary for the cambial layers of stock and scion to be in close contact?** **461. After a graft has been successfully made, how does water get from the cambium of the stock into those of the scion?** **462. Why is it important to use a very sharp knife in grafting operations?** **463. In grafting, why is it necessary to cover the cut surfaces with wax or a similar substance?** **464. Plants which are rather closely related to one another cannot be grafted together. What explanation can you suggest for this fact?** **465. Many woody-plant stems can almost never be grafted. Why?** **466. Numerous plants almost slit the bark longitudinally on strong and rapidly growing stems to hasten the production of new wood. Why does this practice aid in producing the desired result?** **Note.—Pruning is the process by which certain twigs or branches are removed or shortened in trees in order to attain some desired result.** **467. Why does careful pruning make a tree more vigorous and healthy?** **468. Why is pruning generally done in spring, fall or winter rather than in summer or autumn?** **469. How differently would you prune a tree if you desired fruit production from the way you would prune if you desired the produc- tion of timber?** 152 **BOTANY: PRINCIPLES AND PROBLEMS** 470. When a branch is cut out from a tree, the wound thus caused will usually heal over. How does this healing take place? 471. If a branch is cut off very close to the trunk it will heal over much more readily than if a stump is left projecting at some distance beyond the trunk. Why? 472. Why are rapidly growing plants tender than slowly growing ones? 473. Why do sapogenous stalks become tougher and less desirable to cats as they grow older? 474. What is the best season to train woody vines upon trellises and arbors or to fix the permanent shape of woody plants in other ways? Why? 475. Trees in exposed places are permanently bent in the direction of the prevailing wind. Why? 476. To develop large blossoms on a chrysanthemum plant, growers cut off all lower buds but the terminal one. Why has this effect desired? 477. If tomato plants are "topped" (the upper part of the stem, including the small leaves and the flower cluster being cut off when it has begun to bloom), why do they produce fewer flowers than would be valu- able once commercially, will grow larger than those otherwise would. Explain. 478. Which do you think will bear fruit first, a youngselling apple tree or a section of the same which has been grafted into a large tree? Why? 479. Why does pruning of some of its roots often cause a tree to bear more fruit than it would without such pruning? 480. Why do many plants flower earlier if grown in pots than if grown in the open soil? 481. Why is it that apple trees, and many other northern fruit trees, sometimes grow well in warm climates but never bear much fruit there? 482. Why is a moist season good for forage crops but poor for seed crops? 483. In most plants which produce bulbs, it generally takes several years before a plant raised from seed will begin to flower. Explain. 484. Why does an apple tree usually bear a large crop only on alter- nate years? GROWTH 153 **465.** Does it pay to take good care of an apple tree on the years in which it does not bear heavily? Explain. **466.** By what methods would you encourage a plant to flower? **REFERENCE PROBLEMS** **72.** Are all the chromosomes in a plant cell exactly alike? **73.** Give an example of a food product which is derived from a plant growing-point. **74.** What are the differences between propagating plants by cuttings, budding, and grafting? **75.** In grafted trees, does the stock have any effect on the growth and character of the branch that develops from the scion? **76.** How are "dead" fruit trees produced? **77.** What is meant by the "polarity" of a branch? **78.** At what time of the year is it determined whether a bud which is forming on a twig will become a leaf or a flower-bud? **79.** Give the derivation of the following terms and explain in what way each is appropriate:
Miosis Meristem Chromosome
CHAPTER IX THE PLANT AND ITS ENVIRONMENT The form which a plant assumes and the activities which it carries on are due to the combined effect of two major causes. These are (1) the constitution of the plant itself, determined by the specific constitution of its protoplasm and transmitted from one generation to another by heredity; and, second, the environment in which the plant lives. Plants are so diverse and environments so varied that the relations which exist between the one and the other are many and complex. In order to understand these relations, we must study both the subject-matter of the science of Plant Ecology, some of the problems of which we shall discuss briefly in this chapter. It is evident that, in any given plant, growing in the same spot, the amount of light, temperature, moisture and various soil factors may change radically. Between two plants in different places, environmental differences may be even more marked. We have seen how difficult it is to measure accurately what these various external factors may vitally affect the way in which the plant functions, and it is therefore evident that if a plant is to survive and reproduce successfully it must modify its form and activities to meet this ever-changing environment successfully. One of the most remarkable facts of biology is that organisms are able to adapt themselves to their characteristic of advantageous regulation of structure and function in conformity to the changing external world. As to what are the causes of such adaptation, we know nothing about them at all and no certain knowledge. In describing plant activities, most of which contribute so obviously to the welfare of the individual, we continually assume that they are carried out simply for our own good. It is indeed very difficult to describe the facts of form and function in simple language without tacitly assuming that there is within the plant something which directs and regulates its life so that it will do what is best for us. This assumption is fortunate. Such an assumption is, of course, quite contrary to the modern 134 THE PLANT AND ITS ENVIRONMENT 155 scientific attitude as to plant physiology, which demands for every observed fact a definite physiological cause and not a psycho- logical one; and it introduces the deeper problem that this is clearly the province of the philosopher rather than of the botan- ist. The latter should content himself with carefully recording the changes produced by the environment on the plant, and with analyzing as carefully as possible the factors which seem to be responsible for these changes. Stimulus and response. In studying such a study it is important to remember that the environmental forces do not act on the plant as they would on a lifeless body--as a stone or a drop of water, for instance--but on living matter, and that by stimulating its surface, pulling it down by gravity or affecting it in other direct ways; but, instead, that each of these forces acts as a stimulus which leads to a change in the organism's structure or function. This response may be either a change of function or a change of structure. To the same stimulus the response of one plant may be very different from that of another. For example, the effect of different parts of the same plant may also differ greatly. The stimulus is merely a trigger which releases a response. Just what it gives rise to depends upon the nature of the tissue and upon the constitution of the living substance of the plant itself. This characteristic trait of protoplasm whereby it is continually reacting or respond- ing to stimuli is known as irritability and is a distinctive quality of all living things. The irritability of plants is so great that irritability is extraordinarily developed in nervous tissue, which receives the stimuli and controls the responses of the organism. In plants, however, irritability is not confined to nerve cells alone, though some regions are much more sensitive than others and may evidently transmit the effects of a stimulus for a considerable distance. It is this property of irritability which makes it chiefly concerned in the many responses made by the plant. It should be noted that although mature parts of the plant, particularly those which have been exposed to light, will not change their form and position to some extent through regulating the turgidity of their cells, it is the young and growing regions which are most responsive to stimuli and are most able to bring about regulatory changes of structure and position. In any discussion of the effect of the environment we should consider both its physical and its chemical aspects. We should also look at the problem from the historical viewpoint and study 156 BOTANY: PRINCIPLES AND PROBLEMS these inherent characteristics which enable to thrive in a particular environment and which have been implanted in the species during the course of its evolution. Adjustments of this kind, either of structure or function, we usually speak of as adaptations. The adaptation of the plant to desert life and of orchids for insect fertilization may be cited as examples. We have called attention to the complexity of the environment in which the plant grows. The first step in an analysis of the A graph showing vertical growth (in centimeters) over time (in days). The x-axis represents time from 0 to 30 days, with intervals of 5 days. The y-axis shows height in centimeters, ranging from 0 to 10 cm. The line starts at approximately 0 cm on day 0 and increases gradually, reaching around 5 cm by day 10, then levels off until day 20, after which it continues to increase slightly until day 30. Fig. 76. Growth in length of a plant stem in a heat, when exposed to variable hours to a constant temperature. The minimum temperature for growth was in this case found to be 12° the optimum 28° and the maximum 32° (after Lehmann). relations between this environment and the plant is to isolate the separate environmental factors and to study the specific effect of each. A number of these factors are important, but particularly important and deserve consideration here, notably temperature, light, gravity, moisture, and various chemical substances, which constitute the soil and water in which the surrounding plants and animals, or the organic environment. Temperature. It is characteristic of all vital processes that their individuality depends upon certain conditions within narrow limits of temperature, and temperature changes therefore chief THE PLANT AND ITS ENVIRONMENT 157 marked responses in the activity of protoplasm. In general, active life is possible for the higher plants between 0 and 50° Centigrade. The temperature at which a plant can change its activities to another. The hottest temperature at which a given plant can continue to live is known as its minimum temperature and the highest temperature at which it can continue to live is known as its maximum temperature. The plant displays its greatest activity and this point is known as its optimum temperature (Fig. 70).* The various physiological processes which go on within the plant, such as photosynthesis, respiration, transpiration, etc., have their own minimum and maximum temperatures and these are not necessarily the same for the different processes. Members of the vegetable kingdom keep the high and deli- cately maintained body temperature which is characteristic of the higher animals, and the temperature of most plants follows either closely that of the air or water in which they are living heat as the environment becomes warmer or colder. About 25 per cent of the radiant energy from direct sunlight, and a much larger proportion of that reflected by the surface of the plant. Most of this is converted into heat, only a small fraction of the energy being used in photosynthesis. This heat would often raise the temperature of the plant to a level where it were not largely expended in evaporating water from the plant tissues, and the importance of transpiration as a cooling process is thus again to be seen. The rate of evaporation of water from a plant may sometimes fall well below that of its immediate environ- ment, owing to excessive radiation or evaporation; or it may sometimes rise above that of its immediate environment because of heat during respiration, especially in regions where growth is active. The latter phenomenon is particularly conspicuous in the case of intense bacterial action, for the amount of heat liberated by the vigorous metabolism of bacteria is sufficient to raise the surrounding temperature markedly. The problem of the temperature relations of plants is further complicated by the fact that many plants are able to accommodate or "acclimatize" to temperatures higher or lower than the usual limits for the species, if the plant is brought into the new en- vironment long enough. For example, many plants normally suffer from cold at a given temperature may often be made to be... *These terms optimum, minimum and optimum are also used for other environmental factors, notably light and moisture. 158 BOTANY: PRINCIPLES AND PROBLEMS thrive under it by lowering the temperature gradually. This resistance of plant tissues to heat and cold is also dependent to some degree on their water content. The young tissues are much more susceptible to injury therfrom than older ones. In general, within any particular species, the ability to withstand high and low temperatures is correlated with the amount of water in the cells and particularly in the protoplasm itself. Where water is abundant, resistance is low, where it is scarce, resistance is high. There are well-marked inherited differences in the temperature relations of plants. The optimum for a Alpine plant must obviously be far lower than that of a tropical species. Certain species have a normal habitat in the water of the Arctic regions and others in the frigid arctic seas. Melons have a much higher optimum than peas, and some varieties of apple, peach, and plum are distinguished by their tolerance of extreme cold. Light.--We have already noted the essential part which light plays in the life of green plants through its influence upon the process of photosynthesis. As we have seen, when heat, electricity or other sources, the plant derives the kinetic energy which it stores up in potential form in its food; and in this important respect, light is identical with heat and even essential to all plants. This influence is evidently an indirect one, however, particularly in the case of non-green plants, for most of these thrive in the absence of light as long as their food supply holds out. Quite apart from its indirect role in nutrition, light exerts certain direct influences upon the plant. For example, growth movements made by plant parts in response to the stimulus of illumination. Not all plants and not all parts of plants respond in the same way to light. Some parts of a leaf will turn towards sources of light, the leaf away from it, and the leaf, by the twisting of its petiole, will assume a position in which the broad surface of the blade is at right angles to the light rays (Fig. 27). This movement is known as phototropism; to the stimulus of light is known as a phototropism. A normal stem is therefore said to be positively phototropic, a root negatively phototropic. The leaves are negatively phototropic. Although the results of phototropism are usually much more noticeable where a plant receives its illumination from one side only, it plays an important part in the orientation of plant THE PLANT AND ITS ENVIRONMENT structures generally. The advantageous character of the ordinary phototropic response is obvious. Not only does light affect the growth of plant organs but it has a profound influence upon their structure. /The stems of green plants grown in the dark are usually slender, much elongated and provided with but little woody mass; and their leaves are A diagram showing the phototropic response of a plant stem growing towards the light. Fig. 77.—Phototropism. A mustard seedling growing with its root in water. This plant is shown in two positions, one with the light on the left (shown by direction of arrows). Note that the stem has bent toward the light and the top grew away from it. In both cases have taken up a position at right angles to the light. (After Strecker.) greatly reduced in size, long petioled and undifferentiated internally. Chlorophyll fails to develop and the plant assumes a pale yellow coloration. This condition of the plant is known as chlorosis (fig. 88) and begins to show itself whenever the supply of light falls below the optimum either in duration or intensity. If sufficiently pronounced, chlorosis ultimately results in death. The stimulus of light which causes this condition represents the abnormal environment which we seek in etiolation, but how this effect is brought about, we do not understand. Too much light may also be harmful to the plant, but too much may be equally so through its toxic effect upon protoplasm. To the blue, violet, and ultra-violet rays living substance is particularly sensitive, and in many plants the presence of Toxicity 160 BOTANY: PRINCIPLES AND PROBLEMS structure of parts may be so modified that as small a surface as possible is exposed to light when it becomes very bright. In some A diagram showing two bean seedlings. The one on the left is under normal illumination, while the one on the right is in darkness. Fig. 78.—Photocinetic. The two bean seedlings are of the same age but the one at right was grown under normal illumination, the one at left, in darkness. Note the broad surface of the blade is exposed to light of moderate intensity but only the edge to very bright illumination. THE PLANT AND ITS ENVIRONMENT 161 Toward light, as toward temperature, plants display certain inherited adaptations. Some green plants are able to thrive in light of comparatively low intensity and are said to be *fertalant* (Fig. 70). Others will grow normally only where the illumination is good, and are said to be *oblatant* (of shade). Some are sensitive to intense light and thus cannot live in the open, whereas others, either through the greater resisting power of their protoplasm or through other structural modifications, are able to withstand the most brilliant sunlight. **Gravity**—Gravity in its effect upon plants is unlike temperature and light in that it is both continuous in action and constant in amount. It affects all parts of the plant, influencing the direction of growth in plant parts and, like light, it affects different organs in different ways. Any reaction to the stimulus A small illustration showing a plant with broad leaves, possibly a fern or a similar species, growing in a shaded area. The leaves are large and broad, suggesting they may be adapted to low light conditions. is good, and are said to be *oblatant* (of shade). Some are sensitive to intense light and thus cannot live in the open, whereas others, either through the greater resisting power of their protoplasm or through other structural modifications, are able to withstand the most brilliant sunlight. Gravitropism 162 BOTANY: PRINCIPLES AND PROBLEMS of gravity is known as a geotropism (Fig. 80). Stems ordinarily tend to grow in a direction opposite to the force of gravity and are thus somewhat erect. The roots, on the other hand, tend to grow directly toward the center of the earth and are thus positively geotropic. A diagram showing the growth of corn kernels after germination. Fig. 80.—Geotropism. Four kernels of corn which have germinated in different directions. The young root in every case has grown downward and the young shoot upward. Most leaves tend to take up their position at right angles to the force of gravity and many lateral roots also grow in an approximately horizontal direction. Such organs are said to be diapne- tropic. The stem is said to be positive geotropic and the plant is obivous. Plants differ considerably in their inherited adaptations to the influence of gravity. Some of prostrate plants have lost their negative geotropism and may even develop into rootstocks or tubers which react toward gravity like roots. The responses of flowers and fruits to gravity are variable and diverse and for the most part seem to be advantageous to the plant. The mechanism of stimulus and response has been more carefully studied than that of any other plant reaction, but environ- mental factors. If very young seedlings in which the root and stem are just appearing are fixed in any position whatever, the young root will grow downward and the young shoot will grow upward (Fig. 80). By attaching such seedlings to a disc and revolving it rapidly, centrifugal force may be developed which is greater than gravity, and in such a case the young plants orient THE PLANT AND ITS ENVIRONMENT 163 themselves to this new stimulus, the roots growing outward in the direction of the force and the stems inward, in the opposite direction. The force of gravity may also be practically elimi- nated, so that the plant can grow in any direction, by a powerful force, if the dice to which the seedlings are attached is slowly revolved in a vertical plane by clockwork, thus exposing all sides of the seedling to the same stimulus. Under these artificial con- ditions, the root and stem continue to grow in the direction in which they happened to start with no reference at all to gravita- tion. It is evident that the gravitational stimulus cannot be used as a substitute for it, must therefore be able in some way to stimulate the growing regions of root and stem so that growth occurs in certain directions only. This is true of the shoot, too. The very tip of the root, for a distance of approximately 1 or 2 millimeters in length, is the only portion which is sensitive to the stimulus. If this region is removed from the plant, then bending never takes place, regardless of the position of the rest of the organ. If this region is horizontal, however, the root will bend downward until the entire plant is horizontal. If this region is vertical, bending never takes place in the tip, however, but always in the growing zone some distance behind. As to how the stimulus is perceived by the tip, and how it is transmitted to other parts of the bud or bearing, we know nothing about such facts little definitely known. **Moisture.** - Of vital importance to every plant is the main- tenance within it of a sufficient supply of water, and we have already seen that plants require moisture for their existence; this substance plays in plant functions. It comprises over ninety per cent of most living plant tissues. Practically all of the physiological processes take place within a solution of water within it. The transportation of substances from place to place through the plant body is accomplished by their diffusion (in addition, osmosis), and water itself is one of these two essential raw materials in photosynthesis. The normal form and proper functioning of the sector plant tissues is maintained by keeping them moist at all times during their life. It is therefore to be expected that the characteristic structures and activities of plants should be concerned with obtaining and con- serving adequate amounts of water. Definite movements and changes of position with reference to moisture are shown chiefly by roots. The root tip is sensitive to variations in the water-content of the soil, and will turn from a 164 BOTANY: PRINCIPLES AND PROBLEMS region of low content to one of higher. Such a response is known as an hydrotropism and results in the pursuit of moisture by roots for example, when dry conditions prevail, the surface layers of the soil are drying out and the water table is descending. It is in responses of structural change rather than those of movement, however, that the effect of moisture is most clearly manifest. Plants which have access to abundant water supply grow luxuriantly, for the most part, with large leaves and rather soft tissues. Cuticle and epidermis are thin, woody tissues somewhat weak, and parenchyma abundant. A similar plant grown where water is scarce will show a marked reduction in size with much smaller leaves. Its tissues, particularly the epidermis and cuticle, are much tougher, and the woody elements stronger and more abundant. The water balance has been upset and structural changes are produced. The water buttress, to cite a notable example, when growing on the shore produces normal buttress leaves, but when situated on a rock, its leaves are reduced in size and are dissected into long capillary segments and are thus particularly well fitted for aquatic life. Other "amphibious" plants exhibit similar structural changes (Fig. 81). A plant with long, narrow leaves. Fig. 81.--"Amphibious" plant. A shoot of mermaid wood ("Piperminax palustris"). Submerged leaves are finely divided, aerial leaves undivided, and several leaves may be seen on each stem. THE PLANT AND ITS ENVIRONMENT 165 **Xerophytes.—Inherited adaptations to abundance or dearth of water show the pronounced effects of moisture as an environ- mental factor. Many plants have developed special structures during the course of evolution that they are able to thrive under con- ditions where the available soil water is comparatively small in** A diagram showing the structure of a xerophytic plant (Xerophytes). The stem is protected by being covered with a thick cuticle and surrounded by a layer of epidermal cells. Fig. 83.—The stems of a xerophytic plant (Xerophytes). The stems are protected by being covered with a thick cuticle and surrounded by a layer of epidermal cells. amount, and where plants without special adaptive modifications would specifically perish. Such drought-loving plants are known as xerophytes, and are characterized by several types of structural and functional modifications which result in a notable ability A diagram showing the structure of a xerophytic leaf (Xerophytes). The blade is rolled back so as to reduce its surface area, and the stomata are located on the lower (darker) surface. A felt-like layer of hairs is also developed at the leaf margins, which further reduces the loss of water. Fig. 83.—Section across the blade of a xerophytic leaf (Xerophytes). The blade is rolled back so as to reduce its surface area, and the stomata are located on the lower (darker) surface. A felt-like layer of hairs is also developed at the leaf margins, which further reduces the loss of water. both to draw water from the soil and to retain it in the plant tissues. The root-system is very well developed in proportion to the size of the plant, and is usually more extensive than those usually higher than among plants growing under less arid environments. The leaf surface is reduced, sometimes very radically; and leaves may even disappear entirely. The number of stems and leaf 166 BOTANY: PRINCIPLES AND PROBLEMS Fig. 84.--One type of xerophyte. The stonecrop (Sedum), showing its very thick and flaky leaves. Fig. 85.--Hydrophyte. The type with broad, floating leaves (water lily) and that with small, delicate leaves, mostly submerged (water milfoil), are both depicted. THE PLANT AND ITS ENVIRONMENT 167 becomes extraordinarily thick. Stems are relatively few and are usually either sunk in pits (Fig. 82), covered with a layer of hairs, or are completely hidden by the leaves. They occur mostly among arctic and alpine xerophytes, the leaves and stems are covered with a felting of hairs. In others, particularly the xerophytes, the leaves and reproductive organs become leathery and succulent (Fig. 84). Internally, most xerophytes have an abundant development of woody tissue. A cross-sectional view of a plant stem showing its internal structure. Fig. 86.—Transverse section of the stem of a typical water-plant (Myriocarpa). The large air chambers in the cortex provide buoyancy to the stem. **Hydrophytes.—At quite the opposite extreme from xerophytes are those plants which are adapted to live nearly or even submerged in water. These hydrophytes (Fig. 85) have root-systems which are much reduced or may even be entirely absent. The leaves, if submerged, are usually finely cut and dissected and are very thin, soft, and delicate. The stems are also very soft and weak and possess an exceedingly small amount of woody tissue. In certain types, notably those in which the leaves or some part of them are floating on the surface of the plant are well provided with air passages (Fig. 80). **Mesophytes.—These plants with which we are most familiar belong to the great majority of all plants. They are known as mesophytes. Living under conditions especially favorable for plant growth, they possess well developed root and leaf systems and are generally large, leafy, and fast growing as compared 168 BOTANY: FRINGIPILES AND PROBLEMS with xerophytes and hydrophytes. Their leaf area is extensive, the cuticle and epidermis rather thin, and the stomata not especi- ally prominent. The roots are highly differentiated, particu- larly as to the fibro-vascular system. Through bag and swamp plants, with their typically spongy internal structure, the water is retained by capillary attraction, and at the other environmental extreme it has to draw a sharp line between mesophytes and xerophytes. Furthermore, there are many plants which are mesophytic during the summer and xerophytic during the winter. Such plants are known as ecotophytes. The selection of a plant species for reference to their water supply are legion and provide a fascinating field of investigation, for nowhere else is the regulatory character of plant structures more clearly evident. Chemical Substances.—The effects of chemical substances upon plants are many and far-reaching. In certain cases, particularly in the lower groups, the direction of growth or movement may be determined by them. In higher plants, however, new, the structural and functional changes induced by chemical substances entering the plant from the soil. We have already seen that the chemical elements which are essential to the life of plants and has called attention to certain of their specific effects. Iron, for example, has been found to be necessary for the development of chlorophyll. Iron deficiency is abundant in seeds and is believed to stimulate the development of reproductive structures. Potassium seems to have an intimate relation with the development of leaves. A low supply of this element is necessary if the leaves are to become well developed and to function vigorously. Nitrogen, especially when associated with phosphorus, is necessary for active photosynthesis, markedly stimulates the development of vegeta- tive organs and tends to delay the production of flowers and fruit of the plant but does not affect its general growth in any part. Chemical substances injected into the plant body are also able to stimulate growth in various abnormal ways. Notable instances occur where insects or fungi attack plants through insect stings or fungus attacks (Fig. 57). Plants display characteristic inherited reactions to the presence of chemical substances in the soil—Curran speculates, for example, A diagram showing a plant with different parts labeled. THE PLANT AND ITS ENVIRONMENT 169 thrive on saline soil where the majority of plants are quite unable to exist. Others grow only where the soil is markedly acid, and still others only where the soil is distinctly alkaline. Such species types are able to exist in soils of almost any chemical composition. The distribution of plant species over the earth's surface is influ- enced by the nature of their reactions to particular chemical substances in their environment. A number of species may be all adapted in a similar way to these various physical conditions of the environment which we Fig. 87.—Galls produced by the attack of a fungus (Pseudopaspargus) on red cedar. have just been discussing and may thus grow side by side together, forming an easily recognizable habitat. Such a group is known as a Plant Association. We may distinguish, for example, a Swamp Association (Fig. 88), a Desert Association (Fig. 89), a Meso- phytic Plant Association (Fig. 90), and so on, each with its characteris- tic species and its characteristic structural and functional modifications. As the environ- ment changes, so does the vegetation, and the history of the vegetation of a region may thus be gradually altered. A study of plant associations and their history is an important part of the science of ecology. 170 BOTANY: PRINCIPLES AND PROBLEMS Fig. 88.--A Swamp Association. Note the contrast between the plants which belong to this Plant Association and those in Fig 86. Fig. 89.--A Desert Association, composed of earth and other extreme xero- phytes. (Courtesy of Dr. T. MacDougall and Forrest Shores, Desert Laboratory, University of Arizona.) THE PLANT AND ITS ENVIRONMENT 171 Living Organisms.—The environmental factors which we have been discussing thus far are all lifeless ones. Of the utmost significance to the plant are also the living organisms with which it is surrounded. The number and variety of these organisms are many and varied. First in importance is the struggle for survival which is taking place continually between living things. No plant exists by itself, resorting only to the inorganic factors which surround it. It is competing with its neighbors for water, for nutrient materials, for sunlight, for insect visitors, and for other necessary elements of its environment. It is devoured or destroyed by animals of all kinds. Only the vigorous and the fortunate succeed, and they are few compared with the hosts which fall and die. A parasite. The dodder (Cuscuta) parasitic on an herbaceous plant. Note that Cuscuta is a hemiparasite, i.e., it has both photosynthetic and non-photosynthetic parts. **Fig. 30.—A parasite.** 172 BOTANY: PRINCIPLES AND PROBLEMS Parasites.--The great majority of the seed plants are indepen- dent, derive their food directly from the inorganic environment, and the struggle between them is therefore fair combat, with victory to the most efficient. Some species, however, have abandoned this passive form of competition for settling at the expense of their neighbors and have developed a ability A parasite, The American mile-a-day (Phalaris arundinacea), parasitic on a plant. Fig. 91.--A parasite. The American mile-a-day (Phalaris arundinacea), parasitic on a plant. to obtain part or all of their food directly from the tissues of other plants. Such ni egestion is known as a parasite (Figs. 90 and 91) and its victim as its host. The tissues of the host plant are pierced by small, modified roots, the sucking organs or haustoria, which penetrate into the stem, leaf sheath, or the stem of the parasite. These penetrate to the vascular bundles or storage regions of the host and draw threfrom a supply of translocated food materials. The parasite shows characteristic structural modifications, notably an absence or poor development THE PLANT AND ITS ENVIRONMENT 173 of normal roots, a reduction in size of the leaves, and a partial or complete loss of chlorophyll. Some parasitic plants, particularly those whose roots attack the roots of other plants, may be only partially parasitic, whereas others derive their entire food supply from the tissues of living hosts. The small but interesting group of insectivorous plants have gone a step further and reversed the ordinary relation between animals and plants by becoming parasites upon insects. These plants capture their prey either by a closing trap, as in the Venus's Fly Trap; a pouch of liquid, as in the Pitcher Plants; or a mass of sticky tentacles, as in the Sundews (Fig. 92). Once caught, the bodies of the insects are apparently digested by enzymes secreted by glands on the plant surface. The small single of nitrogenous food. Parasitism is comparatively rare among seed plants but is very common in the fungi, many of which attack the tissues of animals and other plant hosts, producing serious bacterial and fungal diseases. **Saprophytes** - Similar in certain respects to parasites are a group of plants which also depend upon other organisms for food and other materials. These plants do not attack living materials. This group, however, do not attack living animals and plants but live instead upon their dead bodies, drawing therefrom the necessary materials for growth and reproduction. They feed directly as food. Such plants are known as **saprophytes**. Here belong the bacteria of decay and all bacteria and other fungi which are saprophytes. In addition to these there are some flowering seed plants, of which the Indian Pipe (Fig. 93) is perhaps the Fig. 92.—An insectivorous plant, the sundew (Drosera). In the sticky tentacles. Venus's Fly Trap; a pouch of liquid, as in the Pitcher Plants; or a mass of sticky tentacles, as in the Sundews (Fig. 92). Once caught, the bodies of the insects are apparently digested by enzymes secreted by glands on the plant surface. The small single of nitrogenous food. Parasitism is comparatively rare among seed plants but is very common in the fungi, many of which attack the tissues of animals and other plant hosts, producing serious bacterial and fungal diseases. **Saprophytes** - Similar in certain respects to parasites are a group of plants which also depend upon other organisms for food and other materials. These plants do not attack living materials. This group, however, do not attack living animals and plants but live instead upon their dead bodies, drawing therefrom the necessary materials for growth and reproduction. They feed directly as food. Such plants are known as **saprophytes**. Here belong the bacteria of decay and all bacteria and other fungi which are saprophytes. In addition to these there are some flowering seed plants, of which the Indian Pipe (Fig. 93) is perhaps the 174 BOTANY: PRINCIPLES AND PROBLEMS best-known example. The structural modifications of such forms are in general similar to those which distinguish parasites. The chief difference between the two classes of cryptogams to use complex organic substances directly is nearly or quite lacking among ordinary green plants, which are able to take through their roots only simple inorganic salts. A photograph of a tree fern, Indian Pipe (Drepanocladus). Epiphytes. A third type of relationship between one plant and another, and one which is free from destructive consequences, is presented by those species which grow upon the bodies of other plants but are not parasitic theron. Such plants are known as epiphytes (Fig. 94) and some species commonly in dense tropical regions are so large that they cover the entire surface of the growth. The roots of epiphytes have no connection with the ground and do not enter the living tissues of other plants, and in consequence they are often immersed in water for long periods. THE PLANT AND ITS ENVIRONMENT 175 from the rain and dew. Structural features characteristic of these plants are a much thickened cuticle, protected stomata, and flaky stems and leaves. Speaking of the relationships between the plant and other organisms in its environment which we have mentioned, the advantage has been one-sided. There is instance, however, of true symbiosis, an intimate relation between two plants, or Fig. 94.—Equisetum growing on a tree-trunk. between a plant and an animal, where the advantage, to some extent at least, is mutual. A notable example of this is provided by the whole group of Lichens, each member of which is a com- posite organism consisting of a fungus and a green alga. The species of alga and a species of fungus (Fig. 182) and in which both seem to derive a certain amount of benefit from the union. The mycorrhizal association between the roots of a higher plant, which we have mentioned in a previous chapter, is evidently another example of the same state, as is probably the connexion between the roots of leguminous plants and the leguminous plants in which they live. Passing higher in the vegetable kingdom, we find many instances of relationship between organisms which are also apparently 176 BOTANY: PRINCIPLES AND PROBLEMS A graph with multiple lines and annotations. Fig. 52.—Graph showing the intensity curve of growth of leaves on a tomato plant. The abscissa gives the time in days after planting. The ordinate gives the relative intensity of growth. The dotted line represents the average intensity of growth for the whole period. The solid line shows the actual intensity of growth at each stage. 14 THE PLANT AND ITS ENVIRONMENT 177 of advantage to both though they are not intimate enough to class as true synoecisms. Most notable are those between flower- ing plants and insects, which are the most important source of cross-pollination and the insect a supply of nectar; and the numerous cases where fruits are attractive to animals by furnish- ing them with food. The plant has thus two relations with these animals, with consequent benefit to the plant species. These two types of relationship are very numerous and fascinating and have long been the subject of study by botanists and zoologists. We shall describe them more fully in a later chapter. In this brief discussion of the influence upon plants of the various factors affecting their growth we have not yet outlined some of the important aspects of the problem which underlie the whole science of ecology. No one of these factors is par- ticular, and all may be present at any time in varying degrees (Fig. 50). The direction of growth, for example, may be influ- enced both by gravity and by light, and the result is determined by the relative strength of these two stimuli under the particular conditions prevailing at any time. In the same way, the distribution of a given plant species over the earth's surface is determined not alone by soil conditions or moisture or tem- peratures but also by climatic conditions and other environmental factors taken together. The essential point to remember is that the structure and activities of any plant are not entirely controlled by any one factor but are determined partly by the specific and inherent characteristics of its protoplasm, but are results of an interaction between these two forces. Plants belonging to different species may show great differences very differently even under environments which are identical; and, plants entirely similar will become very different if grown under different environments. We are here facing the ancient problem of Herodotus' "How to measure," which confronts man in so many fields of inquiry. QUESTIONS FOR THOUGHT AND DISCUSSION 487. Give an example of the response of a plant to its environment which is regulatory in character. 488. What do you mean by saying the "no nervous system has been discovered" in plants? A diagram showing various factors affecting plant growth. 178 **BOTANY: PRINCIPLES AND PROBLEMS** **489.** From its structure, which of the tissues of the plant do you think might serve most readily as a means of transmitting stimuli quickly from one part of the plant to another, in somewhat the same way as do the nerves among animals? **490.** Is there anything in the plant corresponding all to the sense organs of animals? **491.** Give an example of an adaptation of a plant species to the environment in which it lives. **492.** What danger may there be for a plant species in becoming very closely adapted to a particular environment? **493.** Why are plants more susceptible to frost when water is abundant in their tissues than when it is not? **494.** Rank rapidly growing parts of plants are apt to be injured first by frost. Why? **495.** Why is a slight frost in fall or spring often more disastrous to plants than very much lower temperature in the middle of the winter? **496.** Why will a warm period in midwinter often prevent a fruit tree from bearing fruit? Why is this unusual? **497.** It is better not to fertilize trees or shrubbery heavily in the late summer and fall. Why? **498.** A frost while fruit trees are just in bloom is more harmful than it would be a little earlier or a little later. Why? **499.** Why do the first frosts of autumn usually come in low land? **500.** Why will an orchard of such fruit trees as the peach, which are rather sensitive to cold, survive better if it grows on a hilltop or on the north slope of a hill? **501.** Which do you think will withstand frosts better, plants of salt marshes or of ordinary soil? Why? **502.** Of what advantage and of what disadvantage is snow to vegetation? **503.** Many of the higher animals can thrive and function normally in the winter whereas the higher plants cannot. To what physiological difference between the two groups is this due? **504.** Of what advantage is it to the plant to have its stems positively phototropic? Explain briefly. THE PLANT AND ITS ENVIRONMENT 179 505. Since most stems are positively phototropic, why is it that all the trees in the northern hemisphere are not bent somewhat toward the south? 506. Flowers are generally positively phototropic and fruits negatively so. Of what advantage are these reactions to the plant? 507. When nurserymen grow seedlings of forest trees, they cover the seed with soil, which they later remove after a few years. Why? 508. Most plants in darkness grow abnormally long. Of what advantage may this reaction be to the plant? 509. Of what advantage is it to the plant for its roots to be positively geotropic and its stems negatively so? 510. Give an example of a stem which is not negatively geotropic. 511. Give an example of a root which is not positively geotropic. 512. Give different in the direction in which the trunk of a tree will grow if it is planted on a hillside facing north or south. What point if it is growing on level ground? Explain. 513. Just how differently does the force of gravity act when it causes a seed to fall from a tree than when it has accidentally placed piece of wood, or other dead object, to bend downwards? 514. Of what advantage to the plant is it to have its roots positively hydrotropic? 515. Why do desert plants usually have a rather high osmotic con- centration than their leaves? 516. Crocus, narcissus, and tulip plants flower and flourish in early spring and by late spring have withdrawn down, not to appear above ground but to remain underground. For what sort of climate do you think such plants are well adapted? 517. Why are arctic and alpine plants serotinous? 518. The leaves of many evergreens, such as juniper, are pressed against the stem in winter and loosely spread in summer. Explain. 519. The timberline (or line above which trees do not grow on a mountain) is higher in the tropics than in temperate zones than on an elevated peak, and in ravines than on ridges. Why? 520. The "rosette" habit of growth, where all the leaves are in a circle next to the ground and a stem is absent, is common in cold, dry regions of the world. 180 BOTANY: PRINCIPLES AND PROBLEMS 521. Why are mesophytes "generally large, thirsty and fast-growing as compared with xerophytes and hygrophytes"? 522. Which will produce a better crop of fruit, a warm, dry summer or a cool, moist one? Why? 523. Why should nitratelets and potash be applied in large amounts only early in the growth of such a crop as beans or corn, and withheld during later growth? 524. What different treatment, as to soil richness and moisture, should you give such crops as celery and lettuce from which that you give beans and corn? 525. What is the most efficient onion among a very heterogeneous group, consisting of trees, herbs, saprophytes, climbers and many other types? 526. What are the advantages and the disadvantages of a parasitic habit of life to a plant? 527. In the case of a parasitic plant, how are the haustorium able to penetrate the host's body? 528. What do you think are the means by which the parasite with- draws food and water from its host plant? 529. Which type of plant do you think appeared first in evolution, the saprophyte or the parasite? Why? 530. What important roles do saprophytes play in the economy of nature? 531. Why are epiphytes more common in dense tropical forests than anywhere else? 532. Of what value is a pine tree in its pitch? 533. Of what advantage to a plant may be poisonous substances occurring in its leaves or roots? 534. Give at least three reasons why plants which are crowded together do not grow as well as those which have plenty of room. 535. What barriers hinder the dispersal of marine plants? 536. What various reasons can you think of to explain the fact that some plant species are much more widely dispersed than others? 537. There are many more species of plants in the tropics than there are in temperate regions. Explain. THE PLANT AND ITS ENVIRONMENT 181 **REFERENCE PROBLEMS** **80.** The mean annual temperatures ("average" temperatures) of England and of New England are about the same. Why do some plants will grow in the former region but not in the latter. How do you account for this? **81.** Compare the United States and the British Isles as to rainfall and temperature. What does this comparison tell us about the climate? Explain the fact that grass grows so much better there than here, and corn so much better here than there? **82.** Why is there more vegetation, by a succession of plant associations and what causes such a succession? **83.** Although a much wider range of plants can be grown in northern Europe than in southern Europe, in the U.S., the latter region has far more native species than the former. Explain. **84.** As a general rule, which can be used in agriculture over a wider area a breed of livestock or a variety of cultivated plants? Why? **85.** Give a brief explanation of the following terms and explain in what way each is appropriate:
Phototropism Eutrophism Zoophyte
Geotropism Xerophytism Saprophytism
Hydrotropism Bryophytes Kryptophyte
Chemotropism Mesophyte Symbiosis
CHAPTER X REPRODUCTION Thus far we have studied the plant body as an individual, in which roots, stems, and leaves develop characteristically and function in accordance with the needs of the organism. The plant is assured. The individual ultimately disappears, however, and it is obviously necessary that if the species to which it belongs is to survive, some means must be found by which reproduction is secured for insuring a constant succession of new individuals by transmitting the one generation to another. This is accomplished by the process which we call reproduction. In all plants, by one means or another, produces a group of offspring. The structures and functions of all plants may thus be divided into two rather sharply defined groups: those centering around growth (the stem, leaf, and root), which are concerned primarily with the life of the individual plant; and the reproductive, centering around the flower and fruit, which are concerned primarily with the transmission of the species. It is to the latter processes which we shall now discuss. There are two main types of reproduction, fundamentally different from one another. These are sexual or reproductive reproduction, in which portions of the body of the parent become detached from it and are set apart as new individuals; and asexual reproduction, where new individuals arise directly from parts of the parent plant, from which union with a new individual arises. **Asexual Reproduction.**—The simplest type of asexual reproduc- tion consists in the production of a new individual from two or more parts, each of which becomes independent. This is a charac- teristic property of most plants that a small portion of the body, (particularly roots) when separated from the parent plant under favorable conditions will replace the missing parts and develop into a new individual. This readiness for multiplication is made use of extensively in the various arts of plant propagation, by which new plants are produced from cuttings and like processes. In a somewhat similar manner, a bud or twig from 182 REPRODUCTION 183 one plant may be united so intimately to another by budding or grafting that it thrives and grows as an integral part of the plant to which it is grafted. In this way, by means of artificial reproduction the horticulturist succeeds in producing enormous numbers of individuals from a single plant. One particular advantage of this method of propagation is to secure a high degree of uniformity among the offspring, since each one A diagram showing the process of asexual reproduction in the strawberry. The central stem (center of diagram) is shown with two lateral branches growing from it. Each branch has leaves and flowers at its tips. Fig. 96.—Asexual reproduction in the strawberry. Central stem (center of diagram), a) from the end of which two lateral branches are growing. Several reproductions, by flowers and fruit, is also shown on the same plant. them is in fact a portion of the original individual. All true Concord grape vines or Baldwin apple trees, for example, are simply detached parts of the original vine or tree in which the variety originated. In many plants, reproduction of this sort is a constant accompaniment of the slow spread and dispersal of the vegetative parts which are produced by the parent plant. A single plant may become established in one spot and gradually extend its area until it forms a considerable patch of turf. Portions of this, through萌芽 or sprout, become separated from one 184 BOTANY: PRINCIPLES AND PROBLEMS another, and a colony of plants takes the place of a single individual. In a somewhat similar fashion the beech tree multiplies itself, buds arising on the roots of the parent tree until it is surrounded by a grove of beeches. This method of reproduction, however, is not common among higher vascular plants. In many species structures have arisen which are particularly adapted for aiding vegetative dispersal of the plant body and which thus participate in the propagation of the reproductive structure. The group among the runner or creeping plants like the strawberry, the rapidly spreading rootstocks of the quack grass, and the long arching stems of the duck-berry, in which the tips continue to grow and produce new leaves, are vegetative parts sometimes modified still further as reproductive organs. Perhaps the best known example of such is the *lode* (Fig. 46), a tuberous rootstock produced by many thickened underground stem, from the buds of which new potato plants arise next the season. The bulb (Fig. 47), as in the onion and hyacinth, is another example. In these cases there are short, stout stems with their lower leaves modified as scales. They carry the plant over from one season to the next and their bulb allows them to spread widely. Sexual Reproduction.--For commoner and more important than the anecious or vegetative method of reproduction, however, is the sexual process. In this process two cells, each consisting of two specialized sexual cells, or gametes, form a single cell, called a zygote or zygospore, from which a new individual develops. To insure the successful consummation of this process is the function of a great variety of plant organs, including flowers, fruits, and seed. These in the higher plants we call the flower, fruit, and seed. We are still uncertain as to what notable advantage is gained through this process compared with vegetative reproduction as common among plants. There is evidence in some cases that an increase in vigor characterizes the offspring of a sexual plant, but just how valuable this may be to the species is not clear. It is true that many very successful species which now depend wholly or in great part upon anecious reproduction. In the present chapter we shall consider only the higher seed plants and shall reserve a study of reproduction in the lower members of the plant kingdom for later chapters. The Flower.--The flower is perhaps the most conspicuous feature of a rather complex reproductive apparatus known so far. The flower REFSODUCTION 185 This consists typically of those structures intimately concerned with the development of the sexual cells, together with others which contribute indirectly to the success of the process of reproduction (Figs. 97 and 98). *Stamens and Pistils.*—The essential organs of the flower are the stamens and pistils. Each common feature of either or pollen me. A diagram showing the structure of a flower, showing its entire of five sepals, its corolla of five petals, its two stamens, and its pistil. The sepals are green, the petals white, the stamens yellow, 1. sepals; 2. corolla; 3. anthers; 4. pistil. Fig. 97.—The structure of a dicotyledonous seed-plant illustrating stamens, pistils, and ovary. In the flower, showing its entire of five sepals, its corolla of five petals, its two stamens, and its pistil. The sepals are green, the petals white, the stamens yellow, 1. sepals; 2. corolla; 3. anthers; 4. pistil. and within this are produced a great number of minute, single-celled pollen grains (Fig. 90), from the contents of each of which two male gametes ultimately develop. The anther is commonly protected by a stalk or viscidium, and the pollen grain consists of a closed chamber containing the two cells which form the *antheroecium*, a structure adapted to catch and hold the pollen grains. The stigma is often supported by a stalk or *stigme*. In the ovary are nurse. The female gamete is produced in the ovule, which is a female gamete or egg. The fertilization of an egg by a male gamete starts one of processes which result in the development of the plant and its offspring. 97 98 186 BOTANY: PRINCIPLES AND PROBLEMS Chromosome Reduction.—Certain noteworthy differences occur between the cell division which precedes the formation of the gametes and those which we have studied in the ordinary A flower of a monocotyledonous seed-plant (Tulip). Fig. 98.—The structure of a flower of a monocotyledonous seed-plant (Tulip). A, lateral view of the flower; B, side view of the flower with one petal and stamen removed; C, longitudinal section of the ovary showing the two layers of nutritive tissue, the outer layer being very thin, the petals, stamens and ovules outlined. D, a longitudinal diagram of the flower. 1, sepals; 2, petals; 3, stamens; 4, ovules. Pollen-granule cytotypes. A, Crocus. B, Oenothera. C, Maranta. D, Cucurbita. E, Poinsettia. F, Brassica. G, Gossypium. H, Carrot. I, Vicia. Fig. 99.—Pollen-granule cytotypes. A, Crocus. B, Oenothera. C, Maranta. D, Cucurbita. E, Poinsettia. F, Brassica. G, Gossypium. H, Carrot. I, Vicia. vegetative tissues of the plant; and a knowledge of these differences is essential to a thorough understanding of the laws of 14 KEPRODI CTIOX 187 inheritance, which we shall later discuss. In this division the chromosomes, as they make their appearance out of the nuclear network, are first seen to be paired. The members of each pair separate, one going to one of the newly formed nuclei and the other to the other. The splitting A diagram showing the process of meiosis in an ordinary body-cell. A. resting nucleus. B. and C. two cells after division. D. and E. four cells after division. The separate chromosomes, each of which has an individuality of its own, are differentially distributed between the two daughter cells. (Modified from Sharp.) of chromosomes which occurs in ordinary mitosis (Fig. 100), and by which the chromosome number is maintained, does not occur here, but the daughter cells receive only half the number de- rived from them) therefore contain only half the chromosome number found in the ordinary body cells of the plant. Such a division is called meiosis (Gr., "to separate"). (Fig. 101). When the gametes later unite in fertilization, each contributes its quota of chromosomes, and in the fertilized egg the original chro- mosome number is restored. This is true for all the cells which develop therefrom. The essential differences between these two types of division are shown in the appended table. **Periodica.**—The pistil occupies the center of the flower, with the stamens in a circle around it; and outside of these, in turn, is the calyx and corolla, which enclose the pistil completely. The inner one of these is the ovule or circle of pollen. The petals are flat, somewhat leaf-like structures, usually conspicuous in color and rather delicate in texture, whose chief function is to attract A diagram showing the process of meiosis in an ordinary body-cell. A. resting nucleus. B. and C. two cells after division. D. and E. four cells after division. The separate chromosomes, each of which has an individuality of its own, are differentially distributed between the two daughter cells. (Modified from Sharp.) 188 BOTANY: PRINCIPLES AND PROBLEMS to the flower those insects which are important in effecting pollination. Finally, outside the corolla is the calyx or circle of sepals. These are usually green or greenish structures which protect the delicate inner organs of the flower while in the bud. All floral A: A circular structure with a central hole. B: A circular structure with two lobes on either side. C: A circular structure with three lobes on either side. D: A circular structure with four lobes on either side. E: A linear structure with a central hole. F: A linear structure with two lobes on either side. G: A linear structure with three lobes on either side. H: A linear structure with four lobes on either side. I: A linear structure with five lobes on either side. J: A linear structure with six lobes on either side. K: A linear structure with seven lobes on either side. L: A linear structure with eight lobes on either side. M: A linear structure with nine lobes on either side. N: A linear structure with ten lobes on either side. O: A linear structure with eleven lobes on either side. P: A linear structure with twelve lobes on either side. Q: A linear structure with thirteen lobes on either side. R: A linear structure with fourteen lobes on either side. S: A linear structure with fifteen lobes on either side. T: A linear structure with sixteen lobes on either side. U: A linear structure with seventeen lobes on either side. V: A linear structure with eighteen lobes on either side. W: A linear structure with nineteen lobes on either side. X: A linear structure with twenty lobes on either side. Y: A linear structure with twenty-one lobes on either side. Z: A linear structure with twenty-two lobes on either side. AA: A linear structure with twenty-three lobes on either side. AB: A linear structure with twenty-four lobes on either side. AC: A linear structure with twenty-five lobes on either side. AD: A linear structure with twenty-six lobes on either side. AE: A linear structure with twenty-seven lobes on either side. AF: A linear structure with twenty-eight lobes on either side. AG: A linear structure with twenty-nine lobes on either side. AH: A linear structure with thirty lobes on either side. AI: A linear structure with thirty-one lobes on either side. AJ: A linear structure with thirty-two lobes on either side. AK: A linear structure with thirty-three lobes on either side. AL: A linear structure with thirty-four lobes on either side. AM: A linear structure with thirty-five lobes on either side. AN: A linear structure with thirty-six lobes on either side. AO: A linear structure with thirty-seven lobes on either side. AP: A linear structure with thirty-eight lobes on either side. AQ: A linear structure with thirty-nine lobes on either side. AR: A linear structure with forty lobes on either side. AS: A linear structure with forty-one lobes on either side. AT: A linear structure with forty-two lobes on either side. AU: A linear structure with forty-three lobes on either side. AV: A linear structure with forty-four lobes on either side. AW: A linear structure with forty-five lobes on either side. AX: A linear structure with forty-six lobes on either side. AY: A linear structure with forty-seven lobes on either side. AZ: A linear structure with forty-eight lobes on either side. BA: A linear structure with forty-nine lobes on either side. BB: A linear structure with fifty lobes on either side. BC: A linear structure with fifty-one lobes on either side. BD: A linear structure with fifty-two lobes on either side. BE: A linear structure with fifty-three lobes on either side. BF: A linear structure with fifty-four lobes on either side. BG: A linear structure with fifty-five lobes on either side. BH: A linear structure with fifty-six lobes on either side. BI: A linear structure with fifty-seven lobes on either side. BJ: A linear structure with fifty-eight lobes on either side. BK: A linear structure with fifty-nine lobes on either side. BL: A linear structure with sixty lobes on either side. REPRODUCTION 189 For greater range of structural diversity than the other organs of the plant, and we therefore depend upon the flower very largely for those characters which distinguish genera and families of plants from one another. A diagram of various types of flowers. The transverse diagrams show the number of parts and the relations between the members of the same flower. Of these, A is a simple flower, B a double flower, C a compound flower, D a perigynous flower. In A, B, and C, the various circles, throughout dotted, petals and filaments outlined, and other structures omitted. In D, the stamens are shown in their relation to the ovary of the flower of the Rosehip (Rosa). 5-lobed petals, stamens and pistils are all free from each other. The stamens are united with the pistil by means of an epipetalous column. Each ovary is simple. C and D, Cherry (Prunus). The sepals are united into a cup-like structure, the calyx. The petals are free. The stamens and the stamens are united directly to the ovary, or are epipetalous. The pistil is compound. Fig. 102.—Diagram of various types of flowers. The transverse diagrams show the number of parts and the relations between the members of the same flower. Of these, A is a simple flower, B a double flower, C a compound flower, D a perigynous flower. In A, B, and C, the various circles, throughout dotted, petals and filaments outlined, and other structures omitted. In D, the stamens are shown in their relation to the ovary of the flower of the Rosehip (Rosa). 5-lobed petals, stamens and pistils are all free from each other. The stamens are united with the pistil by means of an epipetalous column. Each ovary is simple. C and D, Cherry (Prunus). The sepals are united into a cup-like structure, the calyx. The petals are free. The stamens and the stamens are united directly to the ovary, or are epipetalous. The pistil is compound. In number, the circles may differ considerably. Among some of the more primitive orders there are several separate pistils (Fig. 102, A), but this part of the flower is more often single, at least in our higher plants. This difference arises from the number of stigmas or chambers in the ovary we have seen to 189 190 BOTANY: PRINCIPLES AND PROBLEMS believe that in many cases there has been a fusion of several pistils, and that the many-chambered ovary is thus a compound structure. In some cases the stamens are numerous and the floral parts are generally free from one another, although in some families they may be partially fused together. The petals are usually fewer than the stamens and rarely exceed ten in number. In certain orders they are united to form a continuous or pseudopetalous (as opposed to a polypetalous) corolla (Fig. 101), but in others they are separate, like the sepals, and like them, may sometimes be fused together into a gamosepalous (as opposed to a polypetalous) calyx (Fig. 102). Not only are the members of one corolla-continuum joined together, A diagram showing the structure of a flower with five petals and five sepals. B A diagram showing the structure of a flower with two petals and two sepals. 101 Diagrams of various types of flowers. A and B, Blueberry (Vaccinium); C, Blackberry (Rubus); D, Honeysuckle (Lonicera); E, Grape (Vitis); F, Ginkgo (Ginkgo); G, Pomegranate (Punica); H, Grapefruit (Citrus); I, Tobacco (Nicotiana). Calyx is gamosepalous, corolla gamopetalous and stamens tetradynamous. Petals and sepals are hypogynous but the structures are attached to the corolla or sepalicordia. 102 Diagrams of various types of flowers. A and B, Blackberry (Rubus); C, Grapefruit (Citrus). Calyx is gamosepalous, corolla gamopetalous and stamens tetradynamous. Petals and sepals are hypogynous but the structures are attached to the corolla or sepalicordia. REPRODUCTION but two curved circles may even be united. The corolla is some- times attached to the base of the calyx, as in the poppy (Fig. 102, D). Similarly, the stamens may be papillose (attached to the corolla, Fig. 103, D) and the calyx epigynous (attached to the ovary, Fig. 103, B) and so on. In shape, floral parts vary enormously. In A black and white illustration of a flower with a long stem and multiple petals. Fig. 104.—An irregular flower, the Anemone (Anemone pulsatilla). The corolla has two lips, which are spread apart by the base but re-enter into each other, which is secured by a girdle at the end of the long spur. the higher plant groups too, some of the sepals, petals or stamens are different from the rest of the floral parts. A symmetrical or irregular flower (Figs. 104 and 240) is produced, as opposed to the more primitive regular type (Figs. 97, 98, and 100). In color, flowers are either all green or all red or all yellow except that green is comparatively rare in the corolla. In size there is also great diversity, although flowers more than a decimeter in diameter are rare. The sepals are usually soft except for the calyx, the firmness of their parts being increased by 191 192 DEUTAY: PRINCIPLES AND PROBLEMS turgidity of the calyx rather than by secreted tissue. In certain cases, however, notably in the genus *Corylus* and allied families, the perianth segments have become hard, dry, and chaffy. Any one, or more than one, of the floral circles may sometimes be absent. If both calyx and corolla are missing the flower is A diagram showing the structure of a male flower of willow (Salix) with the stamens removed. Fig. 105.—Carpels from a male plant of willow (Salix) showing the masses of stamens. Fig. 106.—Carpels from a female plant of willow. The prominent pistil can readily be seen. said to be anem. If it is either the stamens or the pistil which is absent, the flower is anisocerad and is called either "male" or "female" according to the structures which it possesses. If both male and female flowers are distinct from one another but are on the same plant, they are said to be diandrous; if they are on separate plants (as in the willow, Figs. 105 and 106) the species is termed dioecious. In studying the structure of flowers it has been found that the sepals, petals, and probably the stamens and REPRODUCTION 193 units of the pistil are morphologically leaves; and that the earliest floral type was perfectly regular, with its various parts rather successively arranged in a manner whatever between circles or between members of the same circle. Inflorescence.—The arrangement of flowers on the plant is known as the inflorescence. The flowers may be solitary, arising from the ground, or singly in the axils of the leaves (Figs. 235 A diagram showing a plant with a single flower at the top of a stem, surrounded by several smaller flowers. Fig. 102.—Wind-pollinated flowers of the alder (Alnus). The long catkins are groups of flowers just ready to open and bear their seeds. In each catkin are composed of female flowers, their stigmas ready to receive the pollen thence liberated by the wind. The male flowers are represented by short terminal spikes and have shed their seed; are also shown. and 240); or the leaves may be reduced to small leaves, the inter- nodes shorter than the leaves, so that they grow into definite clusters. Such clusters are of various types as to shape and arrangement, the commonest among them being the raceme (Fig. 237), the panicle (Fig. 286), the corymb (Fig. 235), corolla, panicule, and cyme. Pollination.—The first step in the accomplishment of reproduction is pollination, which consists in bringing together to the stigma, a process known as pollination. At about the time when the flower unfolds, the anthers open and liberate the pollen grains. In rare cases the stigmal hair so close to the anthers that the pollen is 13 194 BOTANY: PRINCIPLES AND PROBLEMS transferred thereto directly, and this may sometimes happen even before the flower opens. In the great majority of cases, however, the pollen is carried by insects, which, by sexual agency, and pollen from the flowers of one plant is thus frequently carried to the flowers of another. The insects are the chief agencies in effecting pollination are the wind and insects. Wind-pollinated or anemophyous flowers (Figs. 104 and 257) are prominently exposed on the plant but are generally small and inconspicuous. They are usually poorly developed perianths, abundant dry and light pollen, and feathery stigmas. Insect-pollinated or cataphyllous flowers (Figs. 104, 108, 246, 253), on the other hand, are usually conspicuous, possess marked odor, and are characterized by a well-developed corolla, pollen grains which tend to adhere in masses, stigma which are sticky, and in many cases by a nectar-secreting structure producing a sugary fluid. The insect is guided to the flower by its color or odor and visits it either to secure nectar, the source of honey, or pollen. Some insects have long hairs on their bodies which reach to the hairy bases of these insects, and as it is thus carried about from flower to flower it often comes in contact with a stigma, to the sticky surface of which it is transferred. Insects belonging to the same family may visit different plants more frequently than any other in effecting pollination. In many cases we have evidence that offspring which arise from a cross between two individuals who are not two different parents, are superior in vigor to those in which both gametes came from the same plant. Perhaps in response to this fact, there are many plants which produce seeds only on their own plants, which tend to insure cross-pollination, or the transfer of pollen from one flower or plant to another, and to prevent self-pollination, or the transfer of pollen from one flower or plant to itself. This is called autogamy. Anthers and stigmas, for example, may ripen at different times, with the result that the anthers liberate their pollen either before the stigma of the same flower has ripened or after it has become receptive. In many cases also, pollen from another plant is better able to effect fertilization than the plant's own pollen, and in extreme instances the plant may be actually sterile unless it receives pollen from some other plant upon its stigma. More striking than these methods, however, are the multitude of structural devices in cataphyllous flowers whereby self-pollination through sexual agency is prevented (Fig. 104). **REPRODUCTION** 195 cult or impossible and cross-pollination made easy. This is sometimes accomplished by *floral dimorphism* (Fig. 108), in which there are two types of flowers, as constructed that the points Flower with long style and short stamens in the throat of the corolla. A flower with long style and short stamens in the throat of the corolla. Flower with short style and with stamens in the throat of the corolla. B flower with short style and with stamens in the throat of the corolla. Flowers of the mountain board (Anisotis laetifolia). The anther on hold, to be used for pollination, is shown at left. When the anther is lifted by the lifting of an insect upon it, will release the stamens and they will snap upward through the tube of the corolla, where the anther and stigma touch the insect's body are exactly reversed, with the result that the pollen of one is apt to reach the stigma of the other. More common are the various and often intricate devices in which hairs, springs (Fig. 109), traps, and other agencies are employed to secure a successful developmen where the anther and stigma touch the insect's body are exactly reversed, with the result that the pollen of one is apt to reach the stigma of the other. More common are the various and often intricate devices in which hairs, springs (Fig. 109), traps, and other agencies are employed to secure a successful developmen where the anther and stigma touch the insect's body are exactly reversed, with the result that the pollen of one is apt to reach the stigma of the other. More common are the various and often intricate devices in which hairs, springs (Fig. 109), traps, and other agencies are employed to secure a successful developmen 196 BOTANY: PRINCIPLES AND PROBLEMS ment in such families of irregular-flowered plants as the legumes and orchids, and have long excited the curiosity and admiration of naturalists. Fertilization. Pollination, however, is only a step toward the union of male and female gametes which we know as fertilization. A diagram showing the process of seed-production in a flowering plant. Longitudinal diagram of flower and its parts. The stamens and ovules are shown in A. The stamens are yellow, bud, the stamens and the single ovule beginning to develop. B. bud ready to shed pollen. C. pollen grains being transferred to stigma. D. pollen grains being transferred to the stigma. Two grains have germinated, and the pollen tube has penetrated through the micropyle of the ovule and discharged its contents—the two male gametes unite with the egg and the other with the endosperm nucleus. D. Germination of pollen grain. The pollen tube has penetrated into the primary mesophyll cells, where the nuclei have become free in the cytoplasm. The other nucleus surrounds itself with a layer of protoplasm surrounding it (shown in white for the endosperm nucleus). (Fig. 110). Although the pollen grain is a single cell, it is not the male gamete. At the time of pollination, the nucleus of the pollen divides into two equal parts, each of which remains free in the cytoplasm. The other nucleus surrounds itself with a **SEEDPRODUCTION** 197 mass of cytoplasm of its own, sometimes with a separate wall, and it knowns as the generative cell. Shortly after the pollen has reached the ovule, the pollen-grain bursts at one point and out of the grain proceeds a thin-walled pollen-tube. Near the tip of this moves the tube-nucleus, followed by the generative cell (Fig. 111). This tube-nucleus through his immense ![image](image) Fig. 111—Commencing pollen of sepaloid. The pollen grain has burst and a pollen tube is starting down through the ovule. The first act of the tube is the entry into the egg-cavity which occurs behind the generative cell, from which half developed the two male gametes which enter into the egg-cavity. of the style and carries the contents of the pollen grain down into the ovary and to the mouth of an ovule (Fig. 110, C). Meanwhile the generative cell divides into two male cells, which are the true male gametes. By this time the ovule has become prepared for fertilization. It possesses one or more large vacuoles, which occupy space between the seed-coat, which cover it except at one point, the mouth or micropyle. Here the pollen-tube usually enters. Inside the integuments is a thin nutritive layer, the nucellus. Within this, in turn, is a nutritive layer, the endosperm. Outside all of the ovule, is the chorion—case. So a small, empty cavity with three cells at each end and a naked nucleus, the *endosperm*, contains food for the embryo-sac. The embryo-sac is farthest from the micropyle; play no part in fertilization or seed development. Of the three at the micropylar end, however, one is usually developing made by the greater cell and is known as A diagram showing a flower with a pistil and stamen. 188 BOTANY: PRINCIPLES AND PROBLEMS gamete or egg. With the aid of this egg cell fuses one of the male gametes which has come down the pollen tube (Fig. 110, C). This union produces the fertilized egg, and from this single cell develops the entire embryo of the seed and thus the young plant, which grows into a mature plant. The second gamete, when combined with the egg, is the sole direct link between parents and offspring, and only across this exceedingly narrow line can characteristics transmitted by inheritance from one generation to the next. The fertilization of the egg by a male cell is not the only cell union which occurs in the seed. The pollen tube also cells fuse with the endosperm nucleus (Fig. 110, C), and from the cell thus formed arises the endosperm or food-storage tissue of the seed. Fertilization effected by gametes from the same plant is known as self-fertilization; that by gametes from different plants as cross-fertilization. **Seed Development.** After fertilization has been effected, the petals and stamens drop off and the ovule gradually develops into the seed (Fig. 110, D). Various changes accompany this process. The integuments increase in thickness, become hard and woody, and close over the micropyle. In many seeds a considerable mass of endosperm is developed around the embryo but in others this is much less abundant. Within the endosperm is the embryo or young plant, which has developed from the fertilized egg. In double-seeded plants such as beans, it differen- tiates into three main portions: the hypocotyl or primitive stem and root, its tip directed toward the micropyle; the two sec- dorbs or cotyledons, which are leaf-like structures; and the hyp- cotyl, or the plumule or bud, inserted between the cotyledons (Fig. 112). The cotyledons may be very thick and serve entirely for food storage in some plants like peas and bean, and leaf-like, serving as foliage leaves from the beginning, as in the squash growing glory, or they may combine both functions, as in the squash. In monocotyledonous plants (p. 364) endosperm is always well developed; in these plants there is no hypocotyl or bud at all. At first, the scutellum (which probably represents a single cotyledon) to the face of which are attached an upward-pointing, sheathed plumule or bud, and a downward-pointing plumule or radicle (Radix) (Fig. 113). The scutellum serves to absorb food from **REPRODUCTION** 199 the endosperm and to transmit it to the growing portions of the embryo. The **fruit** seed is a structure in which the partially developed young plant, well protected and provided with an abundant supply of food for future growth, is able to pass through a more or less extended period of dormancy. A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z. Fig. 112.—Th e structure of a seed. A and B, side and face views of a bean seed. C and D, side and face views of a pea seed. E and F, two cotyledons spread apart, revealing the plasmoid within. G and H, two cotyledons spread apart. I and M are two cotyledons spread apart. N and O are two cotyledons spread apart. P and Q are two cotyledons spread apart. R and S are two cotyledons spread apart. T and U are two cotyledons spread apart. V and W are two cotyledons spread apart. X and Y are two cotyledons spread apart. Z is a bean seed. The **fruit**—The ripened ovary, together with its contents the seeds, and with any other structures intimately associated with these, is known as the fruit. The ripened wall of the ovary is A', B', C', D', E', F', G', H', I', J', K', L', M', N', O', P', Q', R', S', T', U', V', W', X', Y', Z'. Fig. 113.—A kernel (grain or fruit) of corn. A, face view, showing outline of caryopsis in the middle. B, longitudinal section. 1. Pericarpial and seed coat; fundus. 2. Endosperm; 3. Embryo; 4. Cotyledon; 5. Radicle; 6. Hypocotyl; 7. Epicotyl; 8. Seedling; 9. Root. called the pericarpium. Fruits are various and many different types are recognized by botanists; but all shall mention here only the most common and important of them. Some are dry at maturit y and split open. Such are the exocarp (as in the lily), which arises from a compound ovary, and the pod (as in the 200 BOTANY; PRINCIPLES AND PROBLEMS bean), which arises from a simple or single-chambered seed. Others are dry but do not split open. Such are the acorn (as in the hickory), the nut (as in the walnut), the pomegranate fruit, the sat (as in the hickory), in which the pericarp becomes hard and woody, and the grain (as in the corn), the characteristic fruit of the grasses, which is usually soft and pulpy, but may be hard and fused. These single-seeded fruits are often mistaken for seeds. Many fruits become fleshy, at least in part. In the berry (as A photo of a berry with a cluster of long, cottony hairs. Fig. 114.--Berries dispersed by the wind. A, ripe fruit of the milkweed, Ascle- pias. Each berry contains a single seed, which is surrounded by a cluster of the seeds by the wind. B, fruits of the cotton-grass, Eriophorum. Each fruit in this plant contains several seeds, each surrounded by a cluster of seeds of which is a cluster of long, cottony hairs. in the blueberry), the entire fruit is so except the seeds, which have thick coats. In the stone fruit or drupe (as in the cherry), the outer coat is thin and easily separated from the inner coat, enclosing the seed, is a hard and woody "stone." In the pome, represented by such fruits as the apple and pear, it is the reccip- tacle, greenish or yellowish in color, enclosing the seeds. When fleshy, the pericarp being represented here only by the tough membranes of the core. **REPRODUCTION** 201 **Seed Dispersal.** It is obvious that to have successful offspring a plant must not only develop but must provide for their dispersal; and in bringing this about, almost as great a variety of adaptive devices are employed as there are in insure cross- pollination. Dependence is placed upon various agencies, but chiefly the wind, which carries the seeds even when the fruits are light and provided with wings or tufts of long hairs, so that they present a large surface for the wind to catch, and are often Flax. 115.—Flower clusters of the barberry (Arctostaphylos). The berries shed successively, and are carried by the wind. hooked, and thus aid in the dispersal of the fruits. Compressed berries of the barberry (Arctostaphylos). waffled many males (Fig. 114). In the case of the various "tumble weeds" the entire plant breaks off at the base of the stem and is rolled along over the ground by the wind. Other fruits are provided with hooks or barbs, or with other peculiarities which enable them to adhere to the fur of animals or the feet of birds and thus to be carried for long distances. In floddy fruits, such as those of the willow (Salix), the seeds are rendered attractive to animals by its taste. Birds are particularly important in the dissemination of the seeds of such plants, and in some cases they are able to open with such force that the seeds are projected through the air for a considerable distance. The seeds and fruits of water and shore plants are frequently dispersed by means of the water and have been known to travel thus for hundreds of miles. Willow. 116.—Flowers of Salix. 202 BOTANY: PRINCIPLES AND PROBLEMS **Seed Germination** The seed remains dormant until a favorable environment appears, when the embryo begins to grow and the seed is said to germinate (Fig. 117). The conditions necessary for germination are a plentiful supply of water and oxygen, and warmth. When these are fulfilled, metabolism begins vigorously in the embryo and in the cells of the endosperm. Water is absorbed in large quantities A diagram showing the germination of a seed. A, the bean, 1, embryo of seed, 2, root,萌芽ing, 3, shoot. B, the corn seedling, 1, root,萌芽ing, 2, shoot. Fig. 117.—Germination of the seed. A, the bean, 1, embryo of seed, 2, root萌芽ing, 3, shoot萌芽ing. B, the corn seedling, 1, root萌芽ing, 2, shoot萌芽ing. and the embryo swells, bursts the seed coat, sends its root into the ground and its stem into the air, and becomes a seedling. The food stored in the endosperm is partly used up by the developing plant and used for the development of new organs. It is generally sufficient in amount to provide for the growth of the seedling to a point where the latter can begin to manufacture its own food. Indeed, as soon as the young plant begins to grow upward it may 1 REPRODUCTION 263 turn green and commence photosynthetic activity, soon supplying an abundance of food which insures the rapid development of the plant from the seedling stage to maturity, at which point the cycle of reproduction is complete. QUESTIONS FOR THOUGHT AND DISCUSSION 538. Is sexual reproduction common in animals as in plants? Explain. 539. Give an example of a plant which commonly reproduces itself annually. 540. What advantage is it to the potato to reproduce by tubers rather than by seeds? What disadvantage is there in this process? 541. What advantages, aside from the one mentioned in your text, may sexual reproduction have over asexual multiplication? 542. What connection do you think there has been between the stationary state and characteristics of plants and their reproduction by means of flowers? 543. The petals of a flower usually drop off after a seed is set but the ovule usually remain. Of what advantage are these two facts to the plant? 544. Pollen grains are often roughened. Explain. 545. Can you suggest what makes the pollen grain germinate and why it grows down the style directly to the ovule? 546. Why is pollen generally spread if it is wet by the rain? 547. How long does it take for a fruit to develop after time lapse upon the size of the subsequent apple crop? 548. Do you think that the earliest seed plants were pollinated by wind or by insects? Why? 549. What advantages and what disadvantages are there in wind-pollination? 550. Trees which are wind-pollinated usually flower early in the spring. Explain. 551. The flowers of most coniferous trees are borne near the ends of the twigs, while those of grasses are usually raised up on a tall spike. Explain these facts. A diagram showing a plant with flowers and leaves. 204 **BOTANY: PRINCIPLES AND PROBLEMS** 553. What advantages and what disadvantages are there in insect- pollination? Explain. 553. The corolla of most flowers are some other color than green. Explain. 554. Why are low-growing plants almost always pollinated by insects? 555. Alpine flowers are usually brilliant in color or otherwise conspicu- ous. Explain. 556. Night-blooming flowers are usually white. Explain. 557. Low-growing and inconspicuous flowers are often very fragrant. Explain. 558. In many plants, the flowers are arranged in clusters. Of what advantage is this to the plant? Explain. 559. In most flower clusters, the flowers open a few at a time rather than all at once. Explain. 560. Are solitary flowers usually larger or smaller than those which occur in clusters? Explain. 561. Do you think that bees are attracted to flowers by the same oils which are attractive to human beings? Do you think that the same holds true for flies? Explain. 562. Most flowers do not exceed a diameter in diameter and the great majority are less than 1 inch across. Can you explain why flowers are commonly not larger than this? 563. Many flowers are so constructed as to admit bees readily but to exclude ants. What does the plant gain by this? 564. It is a general rule that plants rich in nectar tend to have hairy stems and leaves. Explain. 565. In many plants, the removal of the stamens as soon as the bud opens often causes the flower to remain in bloom longer than it would if the stamens remained on the flower. Explain. 566. What means of dispersal have plants aside from the dispersal of their seeds? Give examples. 567. Other things being equal, which type of plant will become dis- persed more rapidly, a tree or an herb? Explain. What advantage is it to a berry-bearing plant to have its fruits bright red? Explain. REFORMATION 205 **569.** What color prevails in some fruit? Explain. **570.** Why is green such an uncommon color among ripe berries? **571.** Fleshy fruits usually have story seeds or a story layer around the seed. Explain. **572.** At what time is the flower in the fleshy fruit stored in such a fruit as that of the apple? **573.** The flower stalks of the dandelion elongate after the seeds have been formed. Of what advantage is this to the plant? **574.** As a general rule, how tall are plants in which the fruit bears seeds or fruits? Explain. **575.** Just what part of the seed develops into the new plant? **576.** Why does cracking or shapping the shell of a hard-shelled fruit or seed often hasten its germination when planted? REFERENCE PROBLEMS **586.** What relation has there been between the evolutionary history of insects and their food supply? **587.** What flowers are there which depend on flies rather than on bees for pollination? How do they differ from bee flowers? **588.** Does any one kind of flower, one after the other, on the same day, or does one confine itself to one species? Explain the importance to plants of this behavior. **589.** Some varieties of strawberries will set seeds when planted by themselves. Others will not. Explain. **901.** One corn stalk alone in a field is put to produce few or no seeds. Why? **902.** Carrots and melons will not set fruit naturally in greenhouses, at least in winter. Why not? **921.** What is Xenia? Of what economic importance is it? **931.** Give examples (other than those in the text) of a capsule, a pod, an achene, a nut, and a berry. **941.** Name three edited fruits which are essentially seedless. Give the derivation of the following terms and explain in what way each is appropriate:
Calyx Petal Hypocotyl
Corolla Sutum Micropyle
Fertum Flacca Corolla
Sepal Ovule Phyllome
CHAPTER XI As a result of the process of reproduction which we have described in the preceding chapter, a continuous succession of new individuals arises. One of the most remarkable features of this reproductive activity is that the offspring of wheat plants bears a very close resemblance to its parents. The offspring of wheat plants are always wheat plants and nothing else; and the offspring of oak trees are always oak trees and nothing else. Any particular kind or variety of wheat or of oak will produce (under proper conditions) plants of that kind or variety. This tendency for offspring to resemble their parents, and not to differ from them, which distinguishes their parents is called heredity. **Heredity.** We have already noted the exceedingly narrow physical basis upon which the offspring resembles its parents—which con- nects one generation with the next. To its offspring, one parent contributes a single male cell and the other parent a single egg cell, and these two cells combine to form a zygote. From these two gametes, the new plant develops (Fig. 18). It is evident, therefore, that the parental characteristics must in some way be transmitted in the gametes from one generation to another. Any actual physical character (such as redness of flower petals, tallness of stem) obviously cannot be found in these cells, but something representing it, and capable of producing it in the offspring, must be present in the parent. This something is called the factor or gene for the character in question. In a wheat plant, let us say, the height and strength of the stem, the color and shape of the leaves, the number or spikes in the head, the shape, color and surface of the glumes, the weight of the kernel, the character of the grain, the yield of seed, etc., all represent characters which may be transmitted together with a host of other characteristics, have all been shown to be inheritable. It is evident that in every male gamete and in every female gamete there exists a factor or gene which repre- sents each of these characteristics and which thus determines the 206 HEREDITY AND VARIATION 207 Fig. 118.—The narrow hereditary linkage. The plant at the right receives from each of its parents two male gametes, one male and one female gamete from the other. The parents, in turn, receive from each of the plants which they produce two male gametes, one male and one female gamete with the next, and over which the entire inheritance must pass, is an exceedingly small proportion to the size of the plant, than the dots by which they are here represented. A diagram showing the process of heredity linkage. It depicts a plant receiving two male gametes from each parent, and then passing on these to its offspring. 208 BOTANY: PRINCIPLES AND PROBLEMS particular kind of wheat plant which is to be produced. These minute particles of protoplasm, into which so much is packed and out of which so much emerges, are certainly among the most remarkable bits of matter in existence. A series of five leaves, each showing different degrees of variation in size, shape, and number. Fig. 119.—Variation in number, form, and size of leaflets in the blue elderberry, Sambucus caerulea. Variation.--Close as the resemblance is between parent and offspring, however, it is almost never an exact resemblance. Any individual plant or animal, if studied closely enough, will be HEREDITY AND VARIATION 209 found to differ somewhat, even though very slightly, both from its parents and from its fellow offspring. These differences are known as variations, and they are the basis of all heredity. It is fitting about that variability which is so characteristic of all living things. **Laws of Inheritance.** The close attention given to the problems of breeding by those who have been responsible for the steady improvement of our domesticated animals and plants through the centuries has been rich in practical gains, but it has contributed little to an understanding of inheritance beyond a recognition of those two laws which have been discovered by Mendel. We know that "like begets like" and that offspring differ among themselves, and he has used this knowledge in choosing the best individuals for breeding purposes. This method of selection has made steady improvement, but this improvement has been largely due to sharp-sightedness in seeing upon favorable varia- tions certain characteristics which would enable the breeder to control the process and predict its results. Within the last century, however, and particularly within the last twenty years, notable advances have been made A series of images showing different varieties of seeds or fruits. Fig. 136.—Variation in color, shape, size, and surface in the fruit of the common melon. All of these types may appear among the descendants of a single individual. 4 210 BOTANY: PRINCIPLES AND PROBLEMS in our knowledge, and we are now beginning to see that there are indeed laws of inheritance, an understanding of which will enable us to raise plants and animals with the desired qualities. This work leads to such a firm scientific basis as that upon which chemistry and physics now rest. An investigation of these laws is the purpose of this chapter. Inheritance of Acquired Characters.—We have learned, for example, that all variations do not behave alike in inheritance. Some are due to factors embodied in the constitution of the A plant with leaves and stems, labeled A. B Plant with leaves and stems, labeled B. Fig. 121.—Variation produced by the environment. A, Plant of the dandelion (Taraxacum) grown in a lawned garden. B, Portion of the same plant grown on a lawn where the grass has been cut short. The leaves of the latter plant are smaller than those of the former, but they are not smaller because they are more exposed to sunlight. They are larger because they have received more food from the soil through their roots. The roots of the latter plant are also larger than those of the former, but they are not larger because they have received more food from the soil through their roots. They are larger because they have received more food from the soil through their roots. gametes and may thus be transmitted from one generation to the next. These are clearly inheritable and are the "raw material" with which the breeder may work. Other variations, and among them many important ones, result from the direct action of the environment upon living organisms. Such variations are apparently never transmitted to offspring. Such "acquired" characters are the increased size and vigor which result from growth in favorable conditions, such as warmth, moisture supplied by many plants when exposed to bright sunlight, the galla and other malformations resulting from insect or fungus attack, the stunting effect of overcrowding, and many others. These varia- HEREDITY AND VARIATION 211 tions merely affect the individual and do not reappear in the offspring unless the particular environment which has caused them persists. This fact is of particular interest to the farmer or to anyone who is concerned with plant culture, since they can readily be controlled by a proper management of the soil and climate. The farmer must learn to recognize them and to realize that they are quite useless for his purposes. Good care and cultivation will bring out only those varieties of plants which are well adapted to the conditions under which they grow, and change a poor race into a good one. **Mendel's Law of Inheritance.** It is therefore only with those characters which are clearly inherited that we here concern ourselves. As we see in Mendel's experiments, when two parents are crossed generation after generation we notice many apparent irregularities. The same parents will transmit different characters to their different offspring, while other characters which are equally common fail to appear in the offspring at all. In other cases the offspring will develop characters possessed by neither of its parents but sometimes by both. These facts have been studied by many investigators, and facts which geneticists are endeavoring to explain and to reduce to definite laws. The most notable of these laws, and the one which is the basis of Mendel's theory, was formulated in 1865 in a paper that was published in that year in the "Archiv für die gesamte Medizin," a journal edited by Gregor Mendel (Fig. 122), an Austrian monk, in 1865. The great importance of his work was arrived at the time and even now it is still regarded as one of the most important number of investigators have diligently studied the applications of "Mendelism" in the inheritance of a wide variety of animals and plants, although much remains to be done. Much has resulted in modifications and interpretations of this law, but it still remains fundamentally intact as the cornerstone of genetics. In his cloister garden Mendel observed inheritance in peas, making frequent crosses between different varieties and noting results from generation to generation. His method of attack on the problem differed in several important ways from that of previous workers on this subject. Instead of trying to find out what Mendel would single out a particular character of the parent and follow out the behaviour by itself, rather than trying to study the whole complex, he selected a single character such as seed shape or color, seed color, or seed surface, or plant height, and of several other characteristics of the garden peas were investigated. Second, he kept accurate pedigree records, making sure that he 212 BOTANY: PRINCIPLES AND PROBLEMS knew the exact ancestry of every individual plant and the characters of its parents and grandparents and their descendants. This method involves much care and pains, both in making artificially the particular pollinations desired and in preventing all pollinations by such uncontrolled agencies as insects and the wind, so that the results may be accurately observed and recorded. Fig. 122 — Gregor Johann Mendel, 1822-1884. (From Genetics, by permission). Mendel's method is now almost universally adopted, however, by students of inheritance. In this investigation where contrasting characters appeared (both red flowers and white ones in the offspring from a single cross, let us say) he carefully counted the number of each kind of offspring produced. He then drew a mathematical statement of the facts. In short, Mendel applied the true experimental method to the problem of heredity. The results obtained by this method and the method of investigation were first reported by Mendel and his interpretations thereof have come to be known as Mendel's Law. This law, however, is not a simple proposition but really a series of different principles. Its important properties will be briefly discussed. **Unit Characters** As a general result of his hybridization experiments, Mendel discovered that the plant seems to behave in HEREDITY AND VARIATION 213 inheritance as though it were an aggregation of independent and separable characteristics which may be exactly distinct and may exist with any combination of other characters in a given individual. These traits he called "unit characters". We now know that the expression or appearance of these characters may vary considerably, but they are all dependent upon the same real unity lies under the underlying factor than in the visible (and perhaps variable) character which it produces. The essential point, however, is that when a given character is involved in inheritance is concerned, seems to be made up of distinct and independent units. Purple flower color in peas, for example, is such a unit. It may occur alone or in combination with green seed color, wrinkled or smooth seed surface, tallness or dwarfness of vine, and so on. A skilful breeder may thus combine and rearrange the various units to produce a wide variety of plants. Dominance.—Mendel's studies also brought out the fact that when plants which are dissimilar in a given feature (such as flowers or seeds) are crossed together, the offspring produced show a tendency to resemble either parent equally well in respect to this character. Such a pair of contrasting characters are known as allelohomologous. All the characters studied by Mendel happened to show complete dominance over one another in their expressions; but many instances have since been found where a hybrid plant resembles neither parent equally well with respect to a given character—e.g., the hybrid between a short-stemmed and a long-stemmed pea plant. Such cases of the incomplete or imperfect dominance of one character over another in the hybrid state are much more common than those in which complete dominance occurs. This fact must be emphasized, however, and one that of great practical import, is that the appearance of a plant (or animal) does not necessarily indicate its true constitution. In some cases, even when dominant, partial or complete, may reveal a hybrid or merge to masquerade as a pure or superior individual. Segregation.—The most important feature than this fact of dominance in the hybrid was Mendel's discovery of the manner in 214 BOTANY: PRINCIPLES AND PROBLEMS which characters are transmitted to the second and later genera- tions following a cross. The hybrid offspring resulting from a cross between one of a pure white-flowered race and one of a white- flowered race are, as we have said, all colored. In appearance they resemble rather closely the purple-flowered parents, but in most cases they are distinctly different from them in color, being pure colored types. When two of these hybrid colored plants are crossed, or when one of them is self-fertilized (which amounts to the same thing), the offspring are all white-flowered. When white- flowered plants appear in their offspring, the former consisting about three-fourths and the latter about one-fourth of the total number, we may conclude that the white-flower is a recessive charac- ter bred perfectly true when self fertilized, and purple flower color never appears in subsequent generations of their descendants when interbred with each other. This is shown by the fact that one (only) of the four offspring (one-third of them) bred perfectly true to the purple color, none of their offspring, when inbred, possessing white flowers. The rest of the offspring (two-thirds of them) bred imperfectly true to either of them and thus show about half of the total number of the offspring resemble the hybrids in color and behave when self fertilized exactly as the hybrids did, producing offspring of which three- fourths are white-flowered and one-fourth purple-flowered. These are set forth diagrammatically in Fig. 123.* This separation and sorting out of characters which occur in offspring of hybrid plants is known as segregation. It is this process of segregation which was one of the first instances in which Mendel made use of his knowledge of inheritance. The occurrence of segregation can be seen also in the behav- iour of contrasting factors when they exist together in a hybrid individual. A factor transmitted through the gametes of one parent will be found in the cells of its offspring only if it is carried by the gamete of the other parent, come together and coexist in the cells of the hybrid offspring plant without blending or losing their identity. In order to see how this happens let us consider what occurs in cells, in turn, the two factors become completely separated or segregated from one another, each of the new gametes containing either the one or the other factor. This separation is accomplished by the mechanism which we have been using. The factors for purple and for white flower color must both be present in the * The first generation following a cross is technically known as the F1 generation; succeeding generations are designated by Roman numerals. HEREDITY AND VARIATION 215 Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple (PW) White (WW) Purple (PP) Light Purple PW White WW Fig. 172. Mendel's Law of Inheritance. Longitudinal sections of flowers, showing rule of sex ratio and genetic constitution of the male and female gametes, here represented by pollen grains and egg cells respectively. The male gamete is the pollen grain, which may be either purple or white. The parents PP, GG, GG are in one case purple-flowered PP with the corolla completely purple, and in another case white-flowered GG with the corolla entirely white. The gamete in the former case carries the factor for purple and in the latter case for white. The female gamete is the egg cell, which may be either purple or white. The egg cells in this case are all white. In the first case, when both parents are purple, the purple factor is dominant over the white factor; in the second case, when both parents are white, the white factor is dominant over the purple factor. Note that in the gametes, both male and female, about half cross-hatching. Note that in the gametes, both male and female, about half 216 BOTANY: PRINCIPLES AND PROBLEMS F₁ hybrid, though only the purple expresses itself visibly in the plant. Out of this purple hybrid, when self-fertilized, some come perfectly pure white plants which exhibit no trace of purple in their descendants and some purple which exhibit no trace of white.
PARENTS (Purple) (White)
GAMETES All P All W
F₁ PW (Purple) ½ W
F₂ GAMETES ½ P ½ PP (Purple) ½ PW (Purple)
F₂ (Pure) ½ WW (White)
Fig. 131.—Diagram showing genotypes (in letters), and appearance, of parents of the F₁ and of the F₂ in a cross between a purple-flowered and a white-flowered pea, thus proving that the factor for these characters, which for a generation have been existing together in every cell of the hybrid plant, have now (in one-half of the individuals) become completely separated and have not produced the slightest effect on one another. carry the factor for purple and half for white, and thus show as "light purple." In the second hybrid generation or F₂, derived by self-fertilization of the F₁, those plants which carry both factors for purple and half for white will produce offspring carrying the factor for purple and half light purple, with still half their gametes carrying the factor for white and half light purple. Those plants which carry both factors for white and half light purple will produce offspring carrying the factor for white and half white, with still half their gametes carrying the factor for purple and half light purple. Thus all the offspring are either light purple or white, and the light purple cases besides being the light purple F₁, producing about one fourth purple, one half light purple, and one fourth white. **HEREDITY AND VARIATION** The **Genotype**. The relation between this fact of segregation and the final results which are obtained in crosses is perhaps best explained if we consider the constitution of the cells of the plant and of their gametes by simple letters or formulas (Fig. 12). I am somewhat the same way that Mendel did in his original work. Every cell of the plant contains two sets of chromosomes, one from the union of two gametes and draws half of its inheritance from one and half from the other. If we let P represent the factor for purple flower and p the factor for white flower, then for our purple-flowered parent, which received the factor P from both of its parents, we might therefore represent by the formula PP and for our white-flowered parent, which received only WW. This formula applies to every body cell of the plant. Of course P should be borne in mind that we are here representing only one of the great number of factors which determine the con- stitution of the plant. In the cell divisions just preceding the formation of the gametes, there is a reduction by half in the amount of each factor. Thus in the pollen grain, each P will be reduced to P and each p to p, and every gamete produced now carries just half of each of the factors pairs which composed the parent plant. The gametes of the purple-flowered parent in our illustration would therefore all be represented by Pp, while those of the white-flowered parent by W. When these two plants are crossed and an egg, p, fertilized by a cell, W, (for rice seed), the genetic formula of the resulting hybrid is PpW. Now since W is a purple flower, it is almost completely dominant here, this plant appears purple-flowered, but in its factorial makeup-technically known as its genotype-there are two factors present, one purple and one white. Since both factors are present in equal amounts, they are said to be heterozygous—pink. Perhaps we should come still represent it by exactly the same genotype as before, but with one factor being a different allele, i.e., in each of the parent plants between which this cross was made, the individual is said to be homozygous for the factor in question (in this case purple). When no difference in fat is in this hybrid) it is said to be heterozygous. Now the cause of the phenomenon of segregation lies in the fact that when this hetero- zygous individual produces gametes, these are not hybrid or The chromosomes of the nucleus are in all probability the actual bodies on which the genes are located. It has been shown that in "the reduction division" just preceding the production of gametes, the number of chromosomes in the cell is reduced by half.
The Genotype. The relation between this fact of segregation and the final results which are obtained in crosses is perhaps best explained if we consider the constitution of the cells of the plant and of their gametes by simple letters or formulas (Fig. 12).
Every cell of the plant contains two sets of chromosomes, one from the union of two gametes and draws half of its inheritance from one and half from the other. If we let P represent the factor for purple flower and p the factor for white flower, then for our purple-flowered parent, which received the factor P from both of its parents, we might therefore represent by the formula PP and for our white-flowered parent, which received only WW.
This formula applies to every body cell of the plant. Of course P should be borne in mind that we are here representing only one of the great number of factors which determine the constitution of the plant.
In the cell divisions just preceding the formation of the gametes, there is a reduction by half in the amount of each factor. Thus in the pollen grain, each P will be reduced to P and each p to p,
and every gamete produced now carries just half of each of the factors pairs which composed the parent plant. and every gamete produced now carries just half of each of the factors pairs which composed the parent plant.
The gametes of the purple-flowered parent in our illustration would therefore all be represented by Pp, The gametes of the purple-flowered parent in our illustration would therefore all be represented by Pp,
while those of the white-flowered parent by W. while those of the white-flowered parent by W.
When these two plants are crossed and an egg, p, When these two plants are crossed and an egg, p,
fertilized by a cell, W, fertilized by a cell, W,
(for rice seed), (for rice seed),
the genetic formula of the genetic formula of
the resulting hybrid is PpW. the resulting hybrid is PpW.
Now since W is a purple flower, Now since W is a purple flower,
it is almost completely dominant here, it is almost completely dominant here,
this plant appears purple-flowered, this plant appears purple-flowered,
but in its factorial makeup-technically known as its genotype-there are two factors present, but in its factorial makeup-technically known as its genotype-there are two factors present,
i.e., one purple and one white. i.e., one purple and one white.
Since both factors are present in equal amounts, Since both factors are present in equal amounts,
they are said to be heterozygous—pink. they are said to be heterozygous—pink.
Pink means that there is a mixture or blend between two colors.Pink means that there is a mixture or blend between two colors.
The Genotype.
The Genotype.The relation between this fact of segregation and the final results which are obtained in crosses is perhaps best explained if we consider the constitution of the cells of the plant and of their gametes by simple letters or formulas (Fig. 12).The Genotype. The relation between this fact of segregation and the final results which are obtained in crosses is perhaps best explained if we consider the constitution of the cells of the plant and of their gametes by simple letters or formulas (Fig. 12). In somewhat the same way that Mendel did in his original work. Every cell of the plant contains two sets of chromosomes, one from the union of two gametes and draws half of its inheritance from one and half from the other. If we let P represent the factor for purple flower and p the factor for white flower, then for our purple-flowered parent, which received the factor P from both of its parents, we might therefore represent by the formula PP and for our white-flowered parent, which received only WW. This formula applies to every body cell of the plant. Of course P should be borne in mind that we are here representing only one of the great number of factors which determine the constitution of the plant. In th... e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e0. e0e0e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... e... <... 218 BOTANY: PRINCIPLES AND PROBLEMS beteorous at all, but half of them are P and half W. Thus the hybrid character of a plant cannot be carried by its gametes, which must therefore be of two kinds, one carrying P and the other W. The factors P and W, brought in from the original purple and white parents, have coexisted in the hybrid without influencing each other in any way, and have been able to recombine freely, or become segregated. **Medeian Ratio.** — In a cross between two of these hybrid F1 plants (which is a case of self-fertilization of one of them), the occurrence of the three-to-one ratio in the F2 generation is thus easy to explain. Of the gametes each parent, approxi- mately half will carry P and half W. These gametes are in the perfectly free and random union which takes place between these gametes there are four possible combinations which may occur in the offspring. These are: (1) P × P, producing P eggs, pro- ducing PW plants; (2) P × W, producing P eggs, producing PW plants; (3) W × P, producing W eggs, producing PW plants; (4) W × W, producing W eggs, producing WW plants. Each of these combinations, on the basis of pure chance, is apt to occur just as often as other ways. Approximately one-quarter of the new generation, the F2 plants, will not only look purple but also produce purple seeds. Half of their purple-flowered grandparent; approximately one-half, the PW plants, will also look purple (perhaps somewhat pale) but are of some color other than white. The remaining half among themselves will behave just as did their parent, the F1 hybrid, and yield three colored-flowered plants to every white; and the two equal proportions of these two types will be found in the F2 generation. This ratio is not true to this color as did their white grandparent (Fig. 12). The characteristic Medeian ratio is therefore not three-to-one but four-to-three. This is because it should be remembered that the results of actual breeding do not always display these ratios exactly, nor more than in the teasing or testing of a few individuals they always exact and predictable results. The ratios noted in experiments may be expected on the basis of probability. Obviously when dominance is absent the F1 generation will not merely produce two kinds of plants one three times as numerous as the other but a third as well. A crimson snapdragon, for example, when crossed with a white one gives a pink F1 hybrid. When such a pink parent is crossed with another pink parent HEREDITY AND VARIATION 219 the plants are cryptic (hemizygous), one-half pink (heterozygous), and one-fourth white (hemizygous), the one-fourth heterozygous ratio which we have been considering is evident that pink is here not a true Mendelian character at all, in the sense that it is inherited and will segregate, but that it is merely the expression of two different genotypes. **Independent Assortment.**—When Mendel studied the inheri- tance of two or more factors simultaneously he discovered the nature of the law of independent assortment. He found that, in place between the members of any one factor-pair is quite independent of that which takes place in any other, so that in the second generation there was no correlation between the two factors, many of them quite unlike those found in the original parents, may occur. Let us consider a plant which is homozygous for purple flowers and also for smooth seed coat. The number of such plants represented by the formula PP SS to be crossed with a plant homozygous for white flowers and also for rough seeds, WW RR. The for- mula of this cross would be PP SS x WW RR. In this case a smooth seed coat is dominant over rough, thus this plant would look like the purple-flowered, smooth-seeded parent. When gametes are formed by this plant, half of them carry the factor P and half carry the factor S. Half of these gametes will have a cell must carry within itself not only the factors for flower color but also those for seed surface and for all other plant characters, as well. Thus half of these gametes will carry both the factor P and half the factor R. Now we find that in any given sexual cell, it is purely a matter of chance as to whether the factor for purple flowers or for smooth seed coat is carried by this cell or with that for rough seeds. The particular combination of factors which enter into the F₁ plant from each parent (purple with smooth seed coat) is entirely accidental. This is true whatever upon the way in which they are associated in the gametes produced by this plant. Their assortment is independent. Such a plant when crossed with another plant having the same genotype produce four kinds of gametes in equal numbers: P S; P R; W S; W R. If two such plants are crossed, there will be sixteen possible combinations of gametes, four of each kind. There will be four kinds of pollen grains and four kinds of egg cells and unions is quite at random. Any one of these combinations is as likely to occur as any other, and the sixteen types will thus tend to be equally frequent in the offspring. A diagram showing Mendel's Law of Independent Assortment. 220 BOTANY: PRINCIPLES AND PROBLEMS dominant, the sixteen types will not all be visibly distinguishable, for the hybrids or heterozygous plants will resemble the pure dominant type. This is shown diagrammatically in Fig. 125. Our expectation in such a population is evidently that nine- 125
Parental Pp SS ww RR ww RR
Genetics All PS (Purple, Smooth) PR (Purple, Rough) All WR (White, Smooth)
F1 PS (Purple, Smooth) SR (Purple, Rough) All WR (White, Smooth)
F2 Characters PR (Purple, Rough) WS (White, Smooth) All WR (White, Smooth)
F3 PR (Purple, Rough) PSS (Purple, Smooth) PWR (Purple, Rough)
F4 PR (Purple, Rough) PSS (Purple, Smooth) PWR (Purple, Rough)
F5 PR (Purple, Rough) PSS (Purple, Smooth) PWR (Purple, Rough)
F6 PR (Purple, Rough) PSS (Purple, Smooth) PWR (Purple, Rough)
Total F6 Purple Smooth

(Purple, Rough)                                                                 White, Smooth) Description: Diagram showing segregation of the letters and appearance of parents of Fn and of Fm, a cross between a purple-flowered pea, smooth-seeded pea and a white-flowered pea with rough seeds. Fig. 125.--Diagram showing segregation of the letters and appearance of parents of Fn and of Fm, a cross between a purple-flowered pea, smooth-seeded pea and a white-flowered pea with rough seeds. The method by which new combinations of characters are secured through hybridization is thus shown by this diagrammatic illustration of the Fn+1 generation. HEREDITY AND VARIATION 221
P1 White Disc
wx, 10		 Yellow Sphere
YC, 55
P2 White Disc
wx, 05		 White Disc
wx, 05
P3 White Disc
wy, 05		 White Sphere
wy, 55
P4 White Disc
wy, 05		 White Disc
wy, 05
P5 White Disc
wy, 05		 White Sphere
wy, 55
P6 Yellow Disc
YC, 05		 Yellow Disc
YC, 05
P7 White Disc
wy, 05		 White Sphere
wy, 55
P1, P2, P3, P4, P5, P6, and P7 denote the four pairs of heterozygous parents involved. In each case one pair of characters is white and the other yellow. The white disc and yellow sphere given as P1 generation all of which appear white discs. When instead, P1, P2, P3, P4, P5, P6, and P7 are crossed with each other, the white disc and yellow sphere give rise to white discs and yellow spheres. Many F1 plants which both alike have been observed in this experiment. The data and the ancestry of the plants from which these cases are given in each case are shown in the table above. The Mendel's law of inheritance, where two pairs of characters are involved. In each case one pair of characters is white and the other yellow. The white disc and yellow sphere given as P1 generation all of which appear white discs. When instead, P1, P2, P3, P4, P5, P6, and P7 are crossed with each other, the white disc and yellow sphere give rise to white discs and yellow spheres. Many F1 plants which both alike have been observed in this experiment. The data and the ancestry of the plants from which these cases are given in each case are shown in the table above. 222 BOTANY: PRINCIPLES AND PROBLEMS heterozygous in one or both factor pairs and so will not breed true to their present appearance. The only $F_1$ individuals which will persist unchanged when inbred are those which are completely homozygous. A photograph of two tobacco plants growing side by side. Fig. 127.—A mutation in tobacco. The Stewart Cuban variety, which produces an unusually large number of leaves per plant. (From the Journal of Heredity.) Such, in brief, are the essential features of "Mendelism". The intensive research of the past twenty years in the fields of both botany and zoology has shown that conditions in many HEREDITY AND VARIATION 223 cases are not as simple as Mendel found them in garden peas. Some factors are "linked" together and do not display the inde- pendence of their action which is characteristic of Mendelian laws; and others are influenced by conditions which are supposed to permit their expression not upon one but upon a whole series of independent factors. Others are influenced in appearance and inheritance by the presence of other factors, and consequently as eased to quality (or score) rarely show simple Mendelian segregation at all but blend more or less completely and require for their investigation the use of measurements and statistical methods. A plant with a bent stem, showing the effect of a dominant gene on the position of the leaves. Fig. 135.—"Bent" sort or mutation, arising in a portion of the plant and not from seed. At bottom left a leaf of the original Boston form, and, at right, leaves of these mutants which have arisen from it. (Courtesy Brooklyn Botanic Garden.) All of these cases, however, can be understood or explained by simple Mendelian laws, and they illustrate the importance at all destroying its fundamental principles, and it remains today as one of the most profound generalizations of biological science. Mutations may arise either directly from changes produced directly by differences in the environment to which the plant is subjected, variations which apparently are never inherited. It is now known that many mutations arise from injury to the cell, and doubtless a very common cause of variation is the recom- bination of characters which follows the crossing of two different types or races; and it is clear that such changes in genes as the inheritance of factors themselves, are clearly inheritable; A diagram showing the process of genetic recombination. 224 BOTANY: PRINCIPLES AND PROBLEMS Fig. 130. Hormogonia from the hyphae of the genus *Hypnea*. The upper one is of *Hypnea* *margarita*, the lower one of *Hypnea* *vulgaris*. (From *The Botany of the British Isles*, by J. H. Bowerman, F.R.S., published by Longmans, Green & Co., London, 1875.) HEREDITY AND VARIATION 225 according to definite laws. There is still a third type of variation, often of importance in nature and in practical breeding, which we know as "dominance." A plant or animal which is different from the rest will appear in a pure race, where there is apparently nothing in the ancestry which can explain its origin, and will spread through the population until it becomes common (see p. 127). The production of such a new and distinct type we call a mutation. Many "double-flowered" mes of plants have arisen in this way, as has been shown by Darwin in his work on the variety of other characters. When the history of such a plant type can be traced, it is often found to begin with a single individual which arose by mutation from a parent plant of normal character. The character of this mutant is transmitted to its descendants. In some cases a mutating individual is strikingly different from the normal form and is then often called a "sports." In others, the difference is so small that it may be regarded as resulting from a slight change. Such mutations have also been found where the mutation appears in a single branch or portion of the plant rather than in a whole individual grow- ing freely. These mutations are usually very rare, occurring only in coming without warning or evident cause and in being trans- mitted to offspring. By mutation have arisen some of our surprising varieties of plants, such as the hybrid tea rose, the koh-i-nor, the navel orange, the thornless cactus, the moss rose, the Shirley poppy, and others. We can understand and man- ipulate these mutations by studying their mode of occurrence and to hybridization, but mutations are as yet beyond our con- trol. The best that the plant breeder can do is to watch for them closely and to select those which he likes. The science of genetics is today one of the most intensively studied branches of biology and has not only yielded valuable information regarding the laws by which various characteristics are transmitted from parents to offspring, but has also given us an expla- nation of the chromosomes of the nucleus as the probable seat of genetic factors, it has even thrown light on the structure and behaviour of these chromosomes. **QUESTIONS FOR THOUGHT AND DISCUSSION** **57.** What is the chief practical importance of discovering laws of inheritance? **58.** In studies of inheritance in the summer season it has been found that white fruit tobacco is dominant over yellow, and that the differ- 33 226 BOTANY: PRIKULYES AND PROBLEMS ence between these two colors is due to a single factor. If a plant homozygous for yellow fruit color with plant homozygous for yellow fruit color, what will be the appearance of the $F_1$ generation? of the $F_1$ generated derived by self fertilizing one of these $F_1$ plants? What proportion of the $F_2$ generation will be yellow-fruited and what proportion white-fruited? 873. What will be the fruit color of the $F_1$ generation produced by crossing one of the $F_1$ individuals, mentioned in Question 573, with its yellow-fruited parent? with its white-fruited parent? 890. Let the factor for white fruit color in the squash be represented by W and that for yellow fruit color by Y. The number of gametes, as far as their factors for fruit color are concerned, will be produced by plants having the following genotypes: WW, WWY, WWYY, WWYY. 891. What genotypes will produce the plants involved in the fourfold cross? What proportion will be the fruit color of offspring from each cross: WW × YY; WW × YY; WW × YY; WW × YY 892. A white-fruited squash plant when crossed with a yellow-fruited one produces offspring about half of which are white and half yellow in fruit color. What are the genotypes of the parent plants? 893. If the white-fruited parent plant in the preceding question is self-fertilized, what will be the appearance of the $F_2$ generation? 894. If the white-fruited parent plant is crossed with one of its white-fruited offspring mentioned in Question 892, what chance is there of obtaining from this cross a yellow-fruited plant? 895. Two white-fruited squash plants when crossed produce about three-fourths white-fruited offspring and one-fourth yellow-fruited. What are the genotypes of these two parents, as far as fruit color? What will such a cross be like? 896. A cross between a white-fruited and a yellow-fruited squash plant produces only yellow-fruited and no white fruited. If two of these $F_1$ plants are crossed together, what will be the fruit color of their offspring? Note.—In Foose's clock flower, red flower color is incompletely domi- nant over white, the hybrids being pink-dowered. 897. If a red-flowered Four-clock plant is crossed with a white-flowered one, what will be the flower color of the $F_1$ of the $F_2$ of the $F_1$ crossed with the red-flowered parent? Of the $F_2$ crossed back on the white-flowered parent? HEREDITY AND VARIATION 227 586. In Four-o'clock flowers, let R represent the factor for red flower color and W the factor for white. What will be the flower colors of the offspring from the following four crosses, in which the parents' genotypes are given? RW × RR; WW × RW; RR × WW; RW × RW? 587. If you wanted to produce four-week seed off of which would yield pink-dowered plants when grown, how would you do it? 588. In what respect is a character which behaves like flower color in Four-o'clock plants different from one which behaves like fruit color in squash plants? Note.—In the inheritance of fruit shape in squash squash it has been found that "the type" is dominant over the "sphere" type (see Fig. 302). 589. In a cross between a squash plant homozygous for yellow fruit color and due fruit shape and a plant homozygous for white fruit color and globular fruit shape, what will be the fruit color and shape of fruit in the $F_1$? What will these be in the $F_2$ produced by crossing two of these $F_1$ plants together? 590. If one of the $F_1$ plants in the preceding question is crossed back onto its own parent, what will be the color and shape of fruit in these offspring? What will these be if the $F_1$ plant is crossed back onto its white sphere parent? 591. Let D represent the factor for disc fruit shape and S the factor for sphere shape. What will be the shape and size of fruit in offspring of the following crosses? WW SS × YY DD YY DD × WW SS YY DS × WW SS WW DS × YY LS Note.—In the following six questions, all of which deal with fruit color and shape in the summer squash, the appearance of parents and offspring is stated. Determine in each case the genotype of the parents. 592. White disc crossed with white sphere gives one-fourth white white disc, one-fourth white sphere, one-fourth yellow disc, and one-fourth yellow sphere. 593. White sphere crossed with white sphere gives three-fourths white sphere and one-fourth yellow sphere. 594. White disc crossed with yellow sphere gives one-fourth white disc, one-fourth white sphere, one-fourth yellow disc, and one-fourth yellow sphere. 228 **BOTANY: PRINCIPLES AND PROBLEMS** 597. White disc crossed with white sphere gives three-eighths white disc, three-eighths white sphere; one-eighth yellow disc and one-eighth yellow sphere. 598. Yellow disc crossed with white sphere gives all white discs. 599. White disc crossed with white disc gives 25 white disc plants, 9 white sphere plants, 10 yellow disc plants, and 3 yellow sphere plants. 600. A cross between a plant with white disc fruits and one with yellow disc fruits gives 25 white disc plants, 9 white sphere plants, 24 with yellow disc, and 5 with yellow sphere. If the white disc parent is male, what proportion of its offspring will have yellow sphere fruits? 601. Explain how can be that plants which look exactly similar may breed very differently. 602. Hybrid animals and plants notoriously fail to breed true. Why? 603. If a potato breeder desires to obtain a new variety of potatoes by hybridization, he must first find out whether his potato has any seeds. If you advise him to plant potato "seed" (pieces of the tuber) or real seed from the seed capsule, to provide plants from which he may select? Why? 604. Do you think that the characteristics of the fruits of an apple tree will be affected by the kind of pollen which fertilized the flowers? Explain. **REFERENCE PROBLEMS** 605. Summarize briefly the life and work of Mendel and tell how his discoveries were finally recognized and accepted by the world. 606. Give examples (aside from those mentioned in the text) of new plant varieties which have arisen as a result of hybridization; as a result of mutation. 607. State briefly why it is that the chromosomes, rather than any other part of the gametes, are believed to carry the hereditary factors. 608. What is a "Pure Line" of plants? Of what importance are Pure Lines in breeding? How do they arise? 609. What is meant by "bad selection" in horticultural practice? 610. Give the derivation of the following terms and explain in what way each is appropriate: - Heterozygous - Segregation - Mutantism - Homozygous - Genotype Allohomoploid CHAPTER XII EVOLUTION Among all organisms which we can carefully watch and study new variations are continually appearing and being inherited. This fact at once suggests that living things are not constant and changeless in their characteristics but that they may undergo a certain amount of variation and that this variation sometimes succeed each other. A glimpse at the remarkable development of our domestic plants and animals, since man first began to utilize them for his benefit, shows us how great the possibilities of change which exist among organisms. The difference between the small, sour prototype of the apple, for instance, and our modern varieties is so great that to the uninitiated eye we can hardly recognize the relationship between the two. In fact, many of our cultivated forms have progressed so far under human guidance that we do not know what their wild ancestors were. Since we cannot see into the past, we must rely on evidence asculated as to the possibility that the whole organic world—plants, animals and man—is reached its present state through a gradual evolution from lower to higher forms of life. This is organic matter. It is only within the last century, however, that the subject of evolution has descended from these clouds of speculation and become a matter of scientific investigation. For a little while, evidence has been accumulating that progressive change has actually taken place, and that the plants and animals which we now know as members of our species are descended from others. As to just how this has been accomplished biologists are still far from agreed, but as to the fact of evolution there is now practically no doubt whatever. The evidence is overwhelming. We find that every species was specially and suddenly created in exactly the form which it now displays has given way to the more profound conception that it has gradually changed over a long slow but steady upward progress from lower to higher types of life. **Evidences for Evolution.** The lines of evidence on which the belief in evolution by variation are based, and we shall briefly discuss them in order. 229 230 BOTANY: PRINCIPLES AND PROBLEMS Geological Evidence.--Probably of most importance is the existence of fossils, the actual remains or impressions left in the rocks by ancient plants and animals, caught and embedded in the sand or mud millions of years ago. As our knowledge of geology increases, we find that these fossils are not only indeterminate but that similar types appear in rock layers which we know, from their position, to be of about the same age. We A diagram showing a cross-section of a fossilized plant stem with leaves. Fig. 138.--A, leaf of a fossil species of the Judas Tree (Cercis), from the Excave of Tennessee. B, leaf of living species, Circis amandina, now growing throughout the United States. The two leaves are very similar in form, but are clearly distinct. Our living species has probably been evolved from an ancestor like this, and the fossil shows us how it was. We are able to assign each rock layer, or stratum, to its particular level in the great series which records geological history from a very remote past to the present, and we find as we pass upward through the series that the fossils become more numerous and more recent ones, that the fossil remains change progressively as we proceed; so as we approach modern times, the prototypes of our familiar plants and animals begin to come into view (Fig. 139). This is a very different thing from the fossils which still exist. There are enormous gaps in this record but the advance of geological science is slowly filling them in, and even now we can see that there is a definite and progressive amount of evolutionary progress. Among the members of the plant kingdom, we can witness the rise, luxuriance, and extinction of several great groups--the ferns, gymnosperms, angiosperms--from lowly, fern-like forms, and we can recognize approximately the A diagram showing a cross-section of a fossilized plant stem with leaves. **EVOLUTION** 231 point at which our modern flowering plants first appeared upon the earth. No other evidence for evolution is quite so convincing as new tangible remains of living organisms. **Taxonomic Evidence.--The general character and classification of the plant and animal kingdoms also bears testimony that their present forms have been modified by the process of modification, from earlier types. A study of the external and internal structure of living things makes it clear that they are not haphazardly scattered, but are arranged in groups, which fall into well-marked groups of similar forms, the members of which show definite resemblances to each other. All similar indivi- duals we observe, however, do not exactly resemble one another so much, and are so different from anything else, that we place them together as a genus. A number of genera, in the same way, are placed together as families. Families are united into orders, orders into classes, and so on. We can understand this grouping of similar species and their unions into progressively larger aggregations, if we regard the organic world as a large "genus." In this "genus," its members resemble one another nearly, some remotely--by ties of descent, the "types" repre- senting species, which unite into larger and larger branches as we trace them back to their common origin. But to make the science of taxonomy possible, are inexplicable otherwise. **Morphological Evidence.--Equally significant are certain facts which emerge from the comparative anatomy of plants in a state evidently useless to the plant or animal possessing them and for which it is hard to account unless we look upon them as vestiges or degenerates of organs which once existed but have lost it during the course of evolution. Vestigial stamens, petals, sepals, stipules, and leaf blades, as well as various functionsless structures in the body of plants and animals, and there are many similar instances in the animal kingdom. Their presence can be explained only if we assume that they once were developed for some functional use that evolutionary progress, which makes them useless, has rendered them useless in their gradual degeneration. **Evidence from Geographical Distribution.--Impressive evidence in favor of evolution is presented by the facts of geographical distribution. Most plant species are not widely dispersed over the earth's surface, or even over that part of it in which condi- tions are most suitable for their growth. This fact holds true for example, are practically confined to North America, the eucalyptus to Australia, the tobacco to the western hemisphere, and so on. We can explain such localized distribution only by assuming that these plants were evolved in the regions which now inhabit, and have been distributed ever since by means of various sorts (Figs. 131, 132, and 133). The same phenomena occur A map showing the distribution of four closely related species belonging to the same genus. The species are: 1. Eucalyptus globulus; 2. Eucalyptus camaldulensis; 3. Nicotiana tabacum; 4. Nicotiana rustica. The circles indicate the areas where each species is found. Fig. 131.—The distribution of four closely related species belonging to the same genus. The species are: 1. Eucalyptus globulus; 2. Eucalyptus camaldulensis; 3. Nicotiana tabacum; 4. Nicotiana rustica. The circles indicate the areas where each species is found. repeatedly throughout the animal kingdom, and it is certain that the great mass of facts which we now possess on the geographical distribution of organisms would be largely unexplainable if we did not assume that each species has its own place of origin and its own individual evolutionary history. The facts of distribution are meaningless on any hypothesis other than that of evolution. M. L. Fernald EVOUETRUS Because of such facts as these, the scientific world has become convinced that evolutionism is not only actually occurred and that the plants and animals with which we are now familiar are the most recent members of innumerable lines of descent, reaching A diagram showing four different species of Subotin, each with a different number of petals. Fig. 138.--Showing the four species of Subotin the distribution of which is mapped in Fig. 133. They are similar but quite distinct from one another. 1, S. subalpina; 2 and 5, S. alpina; 6, S. densiflora. (From M. L. Perrot.) backward for millions of years and embracing a multitude of ancestral forms entirely different from anything now alive. An admission of the fact of evolution is, however, a grave question; however, whereas at the first living thing, the 234 BOTANY: PRINCIPLES AND PROBLEMS original ancestor of all which have since evolved; and what has been the cause of the great changes in their evolutionary progress? The first question involves the origin of life, about which we must frankly admit that our ignorance is still complete. Fig. 135.—The distributions of Gaspahusia damson (line) and its variety Regisnoua damson (dots). The species is characteristically well-marked by variety regression. In New Jersey the two varieties are found together, but their ranges over- lap only slightly. Here we evidently have two groups of plants, within a single species. While this may be true of other species, it is not so with this one, which is almost far enough to be regarded as two distinct species. [Data from M. L. Fernald.] The second, which relates to the cause and method of evolution, is more nearly soluble and has been the subject of much discussion, and does not seem for any reason to be involved, it was our inability to explain why and how evolution might have taken **EVOLUTION** 235 place that so long prevented a general acceptance of the belief that it had really occurred at all. Although much progress has been made since the birth of inquiry, we are still far from a complete and convincing solution of the many problems which seriously of evolution has raised. Lamarck's theory was one of those which attempted to explain evolution was put forward by the great French biologist Lamarck about 1800. He was much impressed by the profound effect which the environment produces and noted many instances of its operation on the structure of animals. The growth of plants in rich soil is continued with their stunted growth where the soil is poor (Fig. 121). In the cases of several "amphibious" species he observed that the limbs become more powerful when grow- ing under water are very different from those formed in the air (Fig. 81). He also observed the great structural changes which are brought about by the use and disuse of certain parts of an organism. Lamarck believed that organisms possess the ability to react to their environment advantageously and to modify their structures accordingly. This view, however, does not consider a changing environment will be attained. He never questioned that all the variations which he noted were directly transmitted to offspring, and he did not believe that new species of plants and animals as being pushed along the evolutionary road by environmental forces. Lamarck's explanation, although very attractive and plausible in certain respects, has never won wide acceptance. Biologists have as rule not been willing to admit that an organism has any innate ability to guide its reactions into a favorable course. The theory of natural selection, first proposed by Charles Darwin, searches for any conclusive evidence that "acquired" character, such as those produced by the environment, are ever inherited. It is true that Lamarck's theory is not entirely wrong, but that in certain of its features Lamarck's theory comes nearer to explaining the true method of evolution than any other yet suggested, but is what Lamarck called "a mere hypothesis." Darwin's Theory. "Natural Selection."—The most notable attempt to solve the riddle of evolution is the theory of Natural Selection as developed by Charles Darwin (1809-1882) in "The Origin of Species," a book which has had a very great influ- ence on all human thinking. The effect of this theory in reade- ing the whole process of evolution phanomeric and indeterminable 256 BOTANY: PRINCIPLES AND PROBLEMS was the chief factor in convincing scientific men of the truth of the evolutionary theory in general; and whatever we may think today of the correctness of his conclusions, we cannot but recognise that the thoroughness of his scientific work and its revolutionary effect on all lines of biological thought entitle him to rank as the first great exponent of the theory. Darwin based the theory of Natural Selection upon three main facts: Variations and their inheritance; over-production of off- Picture of Charles Darwin, 1809-1882. spring with the consequent "struggle for existence"; and the survival of the fittest. He was vividly impressed by the occurrence of variations in all animals and plants and studied them exhaustively, endeavouring to discover their causes. Like other scientific men of his day, Darwin was influenced by the views of Lamarck on the laws of inheritance. He believed, at least in his earlier work, that "acquired" characters may be transmitted to offspring, but this belief did not appeal to him as a satisfactory explanation for that of Lamarck. The main fact which he emphasized was that variations in all directions are exceedingly abundant and that in many cases they are certainly transmitted by inheritance to the offspring. The overproduction of progeny in plants and animals forms the next step in the theory. If all seeds which are produced were to **EVOLUTION** 237 grow and if all animal young were to mature, there would soon be no food or room for them on the earth's surface. Only a small fraction can possibly survive. In this struggle for existence among animals, there must necessarily be a terrific life-and-death competition, a "struggle for existence", in which the few survive and the many perish. Finally, those individuals in this struggle which possess any advantage in structure or in function over their fellows, even if this advantage be only slight, will have a better chance to succeed and survive. Of the manifold variations which plants and animals display, some will naturally be helpful and some injurious to their condition. Those individuals vary in the right direction will survive and transmit their advantageous characters to their offspring. The others will perish and leave no descendants. Through this survival of the fittest" the race tends to preserve itself by producing offspring which are better and better adapted to the conditions under which it is living, and also to develop new types which can successfully invade new regions where they may find new conditions. Natural Selection for analogy to the artificial selection long practised by man with his domestic animals and plants, by which he has caused certain characters to become more common. Breeding stock or for seed production, those individuals which varied in such a way as best to meet his requirements. Objectors have often attacked Darwin on his theory Darwin brought forth a wealth of evidence so convincing that it won very wide acceptance. As knowledge has advanced, however, various objections have been raised against it. It is said that trans- taneous character appearing in only one individual not lost, by "swamping", in crosses between this individual and the rest of the population, that these characters do not appear when they are not evidently helpful in survival? Why are many species selected by differences so small that it is hard to believe that they are life- and-death differences? What cannot be understood? And survival of each individual is the development of a structure, which has become perfect and useful to the organism? If variations occur at random, as Darwin supposed, how does it happen that a complex and highly developed organ like the eye appears? This conclusion would require innumerable variations of just the right degree, in just the right place, and at just the right time? Why have we 238 BOTANY, PRINCIPLES AND PROBLEMS never been able to produce by artificial selection a group of individuals which could be considered as constituting a new species? These and other objections have been answered in whole or in part by Darwinians, but they are still of sufficient weight to convince many biologists that the theory just as Darwin left it cannot well be used without modification. The theory as it was originally eliminated vast numbers of obviously unfit individuals, but that it has been the most important factor in producing new forms, and thus in creating evolutionary progress, is denied, either generally doubtfully or positively. De Vries's Theory—Another attempt to retain the essence and method of Darwin's theory was made over twenty years by the Dutch botanist de Vries, who believes that the small and almost imperceptible variations, regarded as most important by Darwin, are merely "fluctuations" due to chance. These fluctuations have been produced by the environment and are therefore not inheritable. The real variations which lead to evolutionary change, according to de Vries, are large and conspicuous, and are hereditary and inheritable, and are often large and conspicuous. The founder of the Mutation Theory thus looks upon organic nature as advance- ing by discontinuous steps rather than by an almost infinite number of small ones. Although de Vries recognizes the importance of natural selection in evolution, he denies that certain advantages over that put forward by Darwin. If complete new varieties arise and even new varieties and species can arise by one or even a few steps, the problem of the preservation of the early stages in the development of these new forms is not solved. The relation of exis- tence side by side of distinct but very similar species is explained. The length of time necessary for evolution is also reduced. Many of the objections raised against the theory of natural selection, however, apply with equal force to that of de Vries, and although the latter has taught us the necessity of distinguishing sharply between individual variation and evolution, this distinction it has not been accepted as a complete solution of the problem. Orthogonies—These various theories lack a convincing explanation of how new forms arise from old ones, and their harmonious incorporation into the organism. The environ- ment evidently cannot produce them, and it seems unlikely that nature will ever provide such conditions as would make it any more successful. In view of all this, some biologists have **EVOLUTION** turned to the organism itself to discover the directive factor in the production of new forms. They believe that variation is not a random process but that in any given species, or succession of individuals, the variations tend always to be of a certain particular sort, characteristic of that species. The variations consequently tend to be directed in a definite direction. The advocates of such a theory of *orthogenesis*, or internally directed evolution, believe that evolutionary change tends to be undirected, and that the variations of the plant or animal, or is forced upon the organism from without. They recognize the importance of natural selection in eliminating the rare and undesirable variations, but they do not believe that create anything new or to produce the organic world as we know it today. It must be admitted that as yet we do not fully understand the manner in which evolution has taken place and the factors which have been responsible for it. In the past there has been perhaps too much reliance on the authority of the great men of little pursuit of facts. The present intensive experimental study of heredity, physiology, cytology and morphogenesis will, it is to be hoped, provide us with a fund of information wherein we may attack this central problem of biology. **QUESTIONS FOR THOUGHT AND DISCUSSION** 606. Why is it that we do not regard new strains of corn, apples, and similar plants as new species? 606. Why does the record of plant and animal evolution given us by fossils have such large gaps in it? 607. What types of plants and animals would be most likely to be preserved in fossil beds? 608. Characters which are apparently of the least functional impor- tance to the plant are often most constant throughout large plant groups and therefore very valuable in plant classification. Explain. 609. Most of the members of the Figeret family have four stamens in their flowers. How many stamens are found in other members of this family most nearly related to the Figworte have five stamens. What can you infer from these facts as to the evolution of this family? Explain. 610. The Figs (Ficus) are a selected island or island race in the ocean (such as the Hawaiian Islands) is composed very largely of species 240 **BETAYA: PRINCIPLES AND PROBLEMS** which are found nowhere in the world except on that particular island or island group. Explain. 611. The potato, tomatoes, tobacco and various other agricultural plants which were first grown in Europe were introduced there from America after the discovery of the New World. What can you conclude as to the origin of these plants? 612. Torreya, a genus of coniferous trees, is represented by two species in China, one in California, and another in the southeastern United States, but it has not been found in any other part of the world. How can you draw this conclusion? Is this fact as to the past history of this genus? 613. In the Galapagos islands very many of the plant species are confined to this group of islands, but most of the genera are the same as those found in South America. Why do you think this is so? In your opinion, the species are not only distinctive of the islands but many of the genera also are found nowhere else. Which of these two ideas groups on the basis of your knowledge would have been isolated longer and the more effectively? Why? 614. Plants living in arid and desert regions usually have small and bushy leaves. This is a real system, in some cases plants living in regions of more abundant water supply would not have evolved this way. How would Darwin? Explain. 615. Most plant species which are very common belong to genera which are very large and average in number of species. Explain. 616. Darwin noted that species belonging to large genera were usually more variable than species belonging to small genera. Explain. 617. Which species is apt to be more successful, do you think, a relatively variable one or a relatively constant one? Why? 618. What is meant by "self-fertilization"? Can more variation in evolution, a species which is always cross-fertilized or one which is always self-fertilized? Why? 619. State at least five advantages which one plant-species might have over another which would make it more widespread and successful. 620. Give an example of a physical barrier to plant distribution: of a "biological" barrier. 621. In competition generally sooner between two individuals of the same species or between two individuals belonging to different species? Explain. 622. In competition apt to be between closely related or between distantly related species? Why? **EVOLUTION** 211 633. In most cases, individual plants may be assigned to very definite species, and between these, transitional individuals are rarely or never found. If species have been developed through a gradual evolu- tion, why are such transitional forms absent? 634. What characteristics must a successful wood possess? 635. A weed introduced into a new region often becomes more wide- spread and successful there than in its home land. Explain. 636. The American chestnut introduced about 100 years ago into the United States has exterminated all the native American chestnut trees over wide areas. In China, its native home, the species of chestnut are almost immune to disease and insects. Why do you explain this difference between American and Chinese chestnut trees? 637. Some species of plants produce comparatively few seeds but are just as successful as others which produce a great many seeds. Explain. 638. During the glacial invasion, the vegetation of the northern United States was changed by the introduction of species of its original range, and as the ice retreated it migrated northward again. Doesn't many plant species were exterminated during these changes. What checks on evolution should a plant species possess to survive such a migration successfully? 639. Name at least five different causes which might lead to the extinction of a species. 630. Why is it that all ancient and primitive types of plants have not been exterminated by the competition of those which have been more recently evolved? 631. A highly specialized and complex plant species is sometimes far more abundant than one which is much simpler and more ancient in type. Compare, for example, our common bracken fern, which thrives over almost all the world, with many of our orchids, which are often rare and have a limited distribution. 632. In the evolutionary history of many groups of animals and plants, as shown by their fossils, there is a gradual change from the simple and primitive members to the more complex ones. This change does not complete itself suddenly, but when a very high degree of specialization has arrived, the group suddenly becomes extinct. How do you explain this? 633. Primitive and ancient types of animals and plants are most com- mon in comparatively isolated regions. Why? 242 **BOTANY: PRINCIPLES AND PROBLEMS** 364. Do you think that evolutionary change would take place more rapidly in a region freely exposed to immigration from without, or in a comparatively isolated one? 365. The great land mass of Europe and Asia is believed to have been the center of evolution for many types of animals and plants now found in other parts of the world. Why? 366. Are the most widely spread plant species the oldest, do you think? Explain. 367. In consequence of the "struggle for existence" and the "survival of the fittest," why is it that in a given locality one species does not exterminate all others, but rather maintains its vegetation?" 368. Name a few of the changes in the natural vegetation of the world which have been brought about by civilization. **REFERENCE PROBLEMS** 103. Give an example of a plant or animal of cultivated plant which has recently been developed by plant breeders. 103. About how long do geologists estimate that life has existed on the earth? 104. What are the great geological periods into which the ancient history of the earth has been divided by geologists? 105. Summarize the life and work of Lamarck and state his important contributions to botany. 106. Summarize the life and work of de Vries and state his important contributions to botany. 107. Give the derivation of the following terms and explain in what way each is appropriate:
Evolution Fossil Orthogenesis
CHAPTER XIII THE PLANT KINGDOM Through a period reaching back into the past for millions of years, such a long time that the entire span covered by human history seems almost negligible beside it, the evolutionary advance of plant life has been proceeding steadily onward. We may still be uncertain as to the causes which lie behind this tremendous progressive movement, but its results are unmistakable. The world is now filled with plants, with which the earth is covered today. Those of us who are familiar with the vegetation of the temperate zone, thriving and vigorous though they may be, can only marvel at the number and variety of plant life exhibited in tropical and subtropical regions. In New England there are about 4,000 species of seed plants, but probably 56,000 species of mosses and liverworts, of which three have already been described a vast array of almost 250,000 species. Nor is our knowledge by any means complete. Although for the past three hundred years botanical exploration has been active in all parts of the world, new species are still constantly being reported. It is the seed plants which constitute the dominant and conspicuous part of this multitudinous vegetation, and in these we find most of the plants which are almost exclusively to them, but we should remember that the plant king- dom includes a host of lower and simpler members. Of the ferns and their allies there are more than 13,000 species; in many parts of the earth they are an important element in the plant population. Of the liverworts and mosses there are some 16,000 species, and among the fungi there are more than 18,000 species; while with 60,000 species and the algae with 20,000. In all these groups, exploration and critical study are yearly adding many new forms and it is probable that before long we shall have over 150,000 species; we should be able to recognise not less than 300,000 species of plants. The day has passed when any one botanist can hope to become familiar with more than a small portion of the flora of the globe. 242 244 BOTANY: PRINCIPLES AND PROBLEMS That record of remote events which has survived to our day in the form of fossil plants shows us that the organization of the earth's vegetation has repeatedly changed, that group after group has arisen, flourished and disappeared, and that thousands of species have evolved only to become extinct. Plants of today are the product of a long series of changes in environment, and to understand the vegetable kingdom and its relationships we must therefore know something of the main events in its history. Plants and Animals. Plants and animals constitute the two great branches of the organic world. In their lowest representa- tives they are alike in having a definite body structure, and certain simple forms exist which clearly combine the characters of both kingdoms. The earliest living things would perhaps have been similar to the simplest plants, but they increased, animals and plants because clearly distinguishable through the development among their members of certain characteristic traits. The plant may be distinguished from the animal by means of a mouth, and to depend on other organisms as sources of food supply; the plant, to be stationary, to absorb its nutrient mate- rials in solution, and to reproduce itself sexually, by spores (except in the fungi and a few others) to manufacture its food from simple inorganic substances. Many other differences in struc- ture and function are apparent between these two groups of life. Forward Steps in Plant Evolution. During the divergent history of the two great groups, certain notable events took place in each with which the student of biology should become familiar. Before discussing these we shall consider briefly a few of the important steps which follow in the succeeding chapters, so that we shall therefore consider briefly a few of the important steps which have marked its development. The causes which led to the appearance on our earth of the first living things, and the characteristics which these primitive organisms possessed, are matters of speculation. We know that we shall probably never discover them. There is good reason to believe, however, that among the earliest of all plants, thriving in the warm waters of a shallow sea, there existed a form which multiplied by simple division or division, possessed chlorophyll or a similar substance, and in their general characters were not greatly unlike some modern seaweeds or algae. For ages they may have been the only vegetable life on the globe. THE PLANT KINGDOM I. The Multicellular Plant.--The first great forward step which, like all first steps, was probably a long time in being accomplished, ceased to be a mere simple growth into colonies (Fig. 135). The two daughter-cells formed at a division remained attached to one another instead of separating, and thus arose small cell-groups or aggrega- tions such as we still may see among the mosses. The individual cells forming these groups might become variously--in physically different shapes, etc.--sheets. Through a still more in- timiate union between their members these cells might finally de- veloped into definite multicellular plants, various in size and shape and probably much more complex than the simplest mode of today. The Fig. 135.--The beginnings of a multicellular plant. A single cell, Flora- cea, in which the cell is somewhat elongated, is seen at the bottom in which the daughter-cells are separated by a thin membrane. At the top the two daughter- cells are united by a thickened membrane. Fig. 136.--The beginnings of differentiation. A thread-like filamentous cell, Condylostylus, is seen at the bottom. It has already begun to be di- differentiated. One is modified as a budule (C), others made several organs or cells. way was thus opened for the production of those very large and complex plant bodies with which we are most familiar. 2. Differentiation.--The evolution of the many-celled plant was soon completed by the appearance of a second important feature, the beginning of differentiation (Fig. 136). The primitive single cell performed all the functions which we now associate with the entire plant, such as absorption, photosynthesis and reproduc- tion. Some of these functions were doubtless lost, how- 246 BOTANY: PRINCIPLES AND PROBLEMS ever, three began to appear within it the same tendency which manifests itself in the animal kingdom by a kind of "division of labor". Instead of the primitive condition in which all the individual cells carry off all the functions, certain cells become specialists, some of them gradually assuming the performance of one function while others assume another. This specialization is more or less conspicuous modification in struc- ture. The first activity of plants to be thus localized was prob- ably represented by the differentiation of the gametophyte very well divided and gave rise to new individuals, certain ones were set apart to produce specialized reproductive cells or spores, provided with means of reproduction, and others were set apart to be particularly well adapted to establish a multitude of new and widely scattered plants. This process of differentiation has stand- ed by progressive specialization until the present time and has resulted in the marvelously complex individuals which we have studied among the seed plants. Here the various functions have originated in different parts of the plant body. The cells far from being uniform, are grouped into definite and highly specialized tissues, each of which plays its particular part in the life of the whole organism. Differentiation has made possible the existence of the higher plants, and it is that specialization of organization or regulation to which we have often called attention. 3 Sexual Reproduction.—Another important step in the history of the plant kingdom involved the method by which reproduction took place. In the earliest plants, this process was accomplishing by means of asexual reproduction only. It forms a little more advanced, special cells became differentiated, each of which was able to produce a new plant. Following this stage, the sexual reproduction came into being and gradually made its appearance. The essential feature of this method is the fusion of two cells into one and the subsequent development therefore of a new individual. The cells which fuse, such as unise are called sexual cells or pseu\textit{do}, and the product of their union, the zygote. In early plants, the gametes were probably nothing other than ordinary cells, but later they became cells which had assumed this additional function; and in some of the algae today we find cells of this sort, which may reproduce the plant either sexually or asexually. Asexual reproduction was often THE PLANT KINGDOM given up or reverted to only under special conditions. The sexual cells themselves become further differentiated into two sets—namely, active motile male gametes or spermatozoa and larger, non- motile female gametes or eggs, a condition which now accompanies sexuality so commonly that instances of equal gametes are com- ![image](https://i.imgur.com/3Q5y8.png) Fig. 127.—The beginning of sexuality. A very simple plant, Chlorophytum. (A) in which two cells may be differentiated as gametes and unite with each other. From this stage onward the two cells are no longer identical. In (B), as illustrated here illustrated (B) the gametes are slightly different in size, foreboding the development of sexuality. paratively rare. The causes which led to the development of sexual reproduction are unknown, but the process is so nearly universal, not only among plants but throughout the animal kingdom, that we are forced to believe it must have some special significance. It is probable that the result is increased vigor, particularly when the two gametes come from different individuals, but we also know many plants which may reproduce by means of asexual reproduction without any evi- dent loss in vitality. However that may be, a considerable 247 248 BOTANY: PRINCIPLES AND PROBLEMS A diagram showing the alternation of generations in a fern. The upper part shows the gametophyte generation, with the egg cell (ovule) and the spermatozoid. The lower part shows the sporophyte generation, with the spore mother cell and the spores. Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte Sporophyte Sexual Plant Gametophyte SporophYTE THE PLANT KINGDOM 249 proportion of the activities of plants seems to be devoted to the successful accomplishment of reproduction. 4. Alternation of Generations.--The three steps in plant evolution which we have mentioned--the appearance of the multicellular individual, of functional and structural differentia- tion, and finally, the alternation of generations--belong exclusively to the plant kingdom. It is the evolution of that remarkable double life-cycle which we know as the "alternation of generations." This is a process so simple enough in its underlying principle and in its expression among the lower plants, but which in the higher groups leads to such complex results, that it is difficult to see how any other type of repro- duction is more difficult in the vegetable kingdom than among A. In Thallophytes.--In most members of that great and primi- tive group the Thallophytes, which include the algae and the fungi and which is the lowest of the four main divisions of the plant kingdom, reproduction is effected by means of a single new individual, just as is the case among animals. Beginning rather obscenely among some of the higher Thallophytes and reaching a high degree of perfection in certain green algae, we find a modification of this simple and direct method. The fertilized egg, instead of producing a new plant like the parent, divides into a group of cells which separate and are liberated as soon as they are formed, each one giving rise to a new plant. In this way a single sexual union produces a whole group of new plants identical in one, and it is this sal- vataginous method which has been developed to a high degree in the plant kingdom, which probably led to the characteristic "alter- nating" systems of reproduction in all plants above the Thallo- phytes. The fertilized egg divides immediately after its formation, each of which grows into a sexual plant, in which fertilization again takes place. B. In Bryophytes.--Such a simple condition as this, however, was evidently soon outgrown and now occurs among only a few lovely forms. The rapidly enlarging group of cells which had their origin from the fertilized egg begins to produce some- thing more than a mass of cells. The first cell becomes differenti- ated into a protecting wall, and the sporae-case or 250 BOTANY: PRINCIPLES AND PROBLEMS The sporangium was thus formed. The cells at the base of the structure had developed into a support or stalk which lengthened and carried the sporangium upinto the air, whence the spores might readily be dispersed. This general situation, where the sexual plant produced its spores, was one of the most developed stage, in evolutionary of the second main division of the plant kingdom, the Bryophyta, which include the liverworts and the mosses. C. In Pteridophytes.—The next stage in the development of the "alternating" life cycle involved a radical change, and since the plants in which this change took place have long since perished, we can only speculate on what actually happened. The sporangium and its related structures underwent a remarkable transformation, developing into a leaf-like structure that produced spores by means of food supply, and finally sending forth roots into the soil and thus establishing a new and entirely independent individual. Instead of producing two individuals, each with a different sex, many in short, a sex-bearing, non-sexual plant, the gametophyte, had been evolved, entirely distinct from and independent of, the older generation of the plant, which was called the sporophyte. This is the situation as we meet it in the third great division of the plant kingdom, the Pteridophyta, which include the ferns, club mosses, horsetails and seed ferns. Here the dominant organism is the sporophyte, with which we are most familiar is the sporophyte. It produces thousands of sporangia and perhaps millions of spores, but actual offspring from these are entirely absent. The gametophyte arises from the primitive male sporangium and its associated organs. The gametophyte, however—the structure which corresponds to the female gamete—is minute, inconspicuous and entirely dependent upon it. It possesses no true roots or leaves but bears the all-important sexual cells. Here the fusion between gametes takes place, and from a fertilized egg in time develops a young sporophyte. This is then enveloped by a young sex-bearing plant. This soon develops leaves and roots become independent of the parent gametophyte, which then withdraws from it. Thus a new generation is produced. *The terrestrial gametophyte and sporophyte are also used in Pteridophyta; the former being applied to the spore-case and its related structures and the latter to the plant itself.* A diagram showing the alternation of generations in Pteridophyta. THE PLANT KINGDOM 251 this sporophyll will germinate and grow (if conditions are favorable) and will develop in turn into a new gametophyte-plant, which will produce its own gametes by fertilization as before. In such a life-cycle as this, there are two distinct, independent and alternating plant types or "generations"—each always producing its own gametes—namely, the gametophyte and the sporophyte. The sporophytes which led to the term "alternation of generations", and which caused us to realize the significance of the periclinal method of reproduction found in both the mosses and the seed plants. Another notable distinction between gametophytes and sporophytes lies in the fact that they are found only in cells. In the former this number is only half as great as in the latter. The gametophyte really begins at the "reduction division" (p. 38), i.e., at the first meiotic division, leading to the formation of the spores, and ends with the union of the gametes in fertilization, which restores the double-chromosome numberand begins the development of the sporophyte. D. In Spermatophytes.—Finally, in the fourth and highest division of the plant kingdom, the Spermatophytes or seed plants, the alternation of generations has reached a still further stage of specialization. The gametophyte, which was once an independent structure, now remains attached to and dependent upon the sporophyte. Furthermore there are now two kinds of spores—the male and female—and these are not pollen grains which develop into much reduced male gametocytes, producing only male gametes; and the zygotee, borne in the ovule, develops into a large embryo which produces many only egg cells.* At maturity, the male gamete comes down the pollen tube and fertilizes the egg in an ovule. From this union the young plant grows up to become a mature seed plant, and will in turn grow into a plant producing thousands of spores. In the seed plants, both gametocytes are much reduced in size and function as soon as they have been used; it was long before they were recognized as gametocytes at all. In the history of the plant kingdom we thus pass from plants which are, like animals, entirely gamete-bearing (as in the Thallophyta)—meaning that they can differentiate any one of five of the higher Spermatophytes, which we shall later describe. Nowhere did we find living these plants, however, *The word "egg" is here used in its botanical sense. 252 BOTANY; PRINCIPLES AND PROBLEMS phytes) to those in which the gametangid alternates with a small, dependent, spore-bearing structure, the primitive spor- phyte (fig. 3). In the first case, the sporophyte is a sporophyte, and gametophyte is both independent plants but where the former is now the large and conspicuous member (in the Pterido- phyta), the latter is a small, dependent plant. This is the dependent and subordinate generation, and where the only plant which we know as such is the sporophyte (on the Spermatophyta), a knowledge of this fact would have been sufficient to show us that in understanding the process of reproduction in plants but is perhaps the best approach by which we can gain a clear concep- tion of one of the most important distinctions between the four great divisions of the plant kingdom. 5. The Invasion of the Land.--The fifth great forward step in plant history was that by which plants began to live on the air rather than in the water, and which thus made possible an invasion of the dry land and the establishment there of a real continuous vegetation. This invasion of the land probably began in the Devonian. Here also doubtless took place the first great steps in the evolution of the vegetable kingdom; and although the seas teemed with life, the land masses of our earth were for a very long time barren. The land plants did not begin to appear in their damper spots only with a seam of algae. This great area was freely open to whatever plant pioneer should be able to master the difficulties presented by its environment. Difficulties of Terrestrial Life.--These difficulties were many and formidable. First and most serious among them was the problem of obtaining sufficient food for nutrition and an efficient supply of water for protoplasmic activity. We have discussed in an earlier chapter the supreme importance of water in the life of plants, and we may here refer to some of its physiological pro- cesses. When the whole plant body is immersed in water, as is the case in primitive and lowly forms, an ample supply of the substance is at once available. But when it is exposed into the air, however it is at once exposed to the danger of water- loss through evaporation, which will soon result in death. This danger of drought has always faced plants which grow upon the land. It has been overcome by means of two adaptations. It must be able both to absorb water in larger amounts and to hinder the loss of water from its tissues by evaporation. Since the soil THE PLANT KINGDOM 253 provides the only source of water available to a land plant, it is evident that roots or other organs must be developed to penetrate the soil and absorb water therfrom abundantly. A successful accomplishment of photosynthesis requires a large area of chlorophyll exposed to sunlight, and hence broad sheets of chlorophyll are necessary. The loss of water by transpiration evaporation, must also be evolved. These sheets we call leaves. The leaves cannot be too close together without depriving one another of light, and they must spread out so that they may spread out and separated in some way on an axis or stem. The region where water is constantly needed to replace water loss may thus be facilitated by the presence of a root system, but this has no well developed conducting system to carry water from root to leaf must therefore be differentiated in the tissues of the stem. Aside from these difficulties, there is the problem of supply of water, the land plant also faces problems of a mechanical nature. Owing to the buoyancy of water, a plant growing submerged therefore sinks down into the water. This is not true of plants in the air, however, there is much weight to be carried and a heavy strain to be borne by the stem, especially in its lower portions. An extreme development of thick-walled cuticles and supporting tissue is thus necessary in order to prevent the plant from being kept firm and erect. In order to be able to thrive on a land plant must therefore possess organs for functioning leaves and stems, and a stem able to serve as an efficient means of conduction and support. Such structures are unknown in the Thallophytes, and these plants are unable to grow above the surface of the water and to produce a true terrestrial vegetation, although they often thrive in moist situations on land and survive long periods of dryness. It seems probable that the first land plants which used the water and develop land-inhabiting forms were probably the Bryophytes or plants like them, which may perhaps be called the "green mosses". These plants have been found growing in moist places, though a few are aquatic and many grow in situations which are dry much of the time. Even in the most highly developed mosses, however, there is little differentiation weak and consists only of delicate thread-like rhizoids; the leaves are small and very thin, and the stems weak and with little or no development of supporting and conducting tissues. The mosses, 254 therefore, do not grow more than a few centimeters high and have never succeeded in producing a strong and vigorous land vegetation. *Success of the Pteridophytes.*—It is a different matter with the Pteridophytes, however. The dominant generation here, as we have seen, is the sporophyte; and this new plant type, at least in all the forms hitherto known, has been found to be particularly well adapted to terrestrial life. Here for the first time we meet with true roots—large, vigorous, much-branching structures, which are very well supplied with vessels and are well suited for rapid absorption and strong anchorage. The leaf, instead of being a small and thin plate of tissue, is large and relatively thick. It is composed of many layers of thin-walled cells and is provided with abundant air spaces, the whole structure being covered by a strong epidermis to cut down evaporation. The stomata are placed on the upper side of the leaf and the internal tissues of the leaf takes place through characteristic pores or stomata. The stem resolates a structural complexity nowhere else met with among plants. The vascular con- struction being particularly well developed. The evolution of the true root, the true stem, and the stroma is highly differentiated stem mind it possible for Pteridophytes to produce the vigorous and abundant land-vegetation which we know so well at present times; and from this group have come the seed plants, which form the bulk of the terrestrial vegetation today. There is another important difference between those on the one side and those on the other. Transitional forms which must have existed between these two have entirely disappeared, and we can only find them in the fossil record in the form of the Bryophytes and the Pteridophytes, and what were the first steps in the evolution of the well-developed land-habiting sporophytes which we now possess. This is a most interesting fact. The union of the dry land stands out as one of the most important and dramatic events in the history of the plant kingdom. *The origin of seeds.*—Of all the progressive movements which we shall consider is the comparatively recent one which carried the process of reproduction to a still higher degree of efficiency and resulted in the development of that most perfect of reproducing organisms—the seed. The production of seeds is the distinctive feature of the THE PLANT KINGDOM Spermatophytes or seed plants, which are now the most successful of all the higher plant types. The spore has several obvious dis- advantages over the pollen grain. It is much larger than the pollen, due to its minute size. Among the lower plants these difficulties are parti- cularly overcome by the production of spores in huge quantities, but the sporophyte itself is very small. In the higher plants developed from single-celled spores, is subject to many difficulties at best. In the seed plants, as in a few of the most advanced Pteridophytes, there are (as we have seen) two distinct organs, the macro- sporangium and microsporangium—whose produce male and female gametophytes, respectively. The happy innovation introduced by these highest plants, however, is to retain the single mega- spore within a large structure, ultimately a leaf-like structure on another plant, where it germinates into a much reduced female gameto- phyte. This whole structure, with the addition of a coat or "integument," is called a megasporangium. The number of seeds with the great number of spores formerly produced, are borne by the plant; the microspores (pollen-grains) are still liberated into the air in order that they may be carried away by the wind. Instead of falling on the ground and germinating there, they are carried to the ovule or near it, where each produces two male gametes, one of which fertilizes the egg. Not only have the seed plants abdicated the delicate, free- living gametophytes, with all the consequent dangers and diffi- culties of their existence; but they have also established a much more successful method for insuring the growth of the young plant. The fertilized egg grows at once into the complete embryo, which is then enclosed in a protective directly and abundantly from the mother plant and is thus relied of the necessity of producing them by its own activity. About the endosperm deposited this supply of stored food in the forming embryo. The young plant is supplied with food after a young root and one or two primitive leaves have been formed; and embryo and endosperm tightly enclosed in the integument of the created seed. This is known as a "seed," and is so named because it will germinate and the embryo within it will begin to grow, bursting its shell, absorbing the stored food, sending forth roots 256 BOTANY: PRINCIPLES AND PROBLEMS and leaves, and rapidly developing into a new plant. The many advantages of reproduction by seeds over the old method of wind-blown spores, and the great superiority of seed-plants are obvious, and it is easy to see why the seed-plants have become so dominant and successful. Plant Classification.--As the result of these slow, progressive changes, which have been working themselves out gradually through millions of years, we see around us the plant kingdom of today; consisting of a vast number of different kinds of plants and each consisting of an enormous variety of species. The task of the science of taxonomy is not merely to list and describe these species, but also to arrange them in a logical manner, bringing together groups of species which resemble each other, thus reducing our knowledge of the plant kingdom to that orderly arrangement which is possible only when the work has been done by botanists skilled men. But even this task is not yet finished. For though the work of the early botanists was valuable as a basis of classification and divided plants into three groups--trees, shrubs, and herbs. As botanists learned more about the vegetable kingdom, such crude systems were seen to be wholly inadequate. It became clear that no single group or suggested kind began to be noted, based on a larger number of characteristics. Thus the conception of plant "families" began to take form, and many families were distinguished and described. Some were large, some small, and many others were distinguished and described. There were still wide differences of opinion as to what the groups should be called. In fact, it was found that there were almost as many "systems" as botanists. Indeed, on the theory which assumed that all plants had been created at the same time, it was difficult to find any two plants which were similar species should exist at all, and there was really no rational foundation for any system of classification. The establishment of the modern evolution in the latter part of the nineteenth century; however, threw a flood light on the whole problem, for it showed that resemblance among members of a plant group was not an arbitrary or chance thing but was due to the fact that all the members had descended from one common ancestor. Classification became therefore a definite effort to work out a genealogy or "family tree" for the plant kingdom; or for a given group of plants (e.g., apples) to apply this genealogy to its constituent THE PLANT KINGDOM 257 for any family (Fig. 138). The problem confronting the taxono- mist today, therefore, is not so much that of certain rather vague "affinities" between plants, but simply the determination of what might be called their "blood relationships"; and during the past fifty years particular emphasis has been placed on the A diagram showing the relationships between various plant families and genera. Aracaceae-Usnea Gen/Pers. Accout Conioides Dink Fig. 138.—A suggested "family tree" for the Conioides. According to this hypothesis the species conioides belongs to one group, each of which is related to the genus Usnea. The genus Usnea itself is included in the Taxusaceae and Podocarpaceae, which are closely related to each other than they are to the genus Conioides. The genus Usnea is included in two distinct groups of genera. The branch stamps represent extinct groups. The twigs are present-day groups. By means of this diagram it is possible to show graphically the inter-relationships between the various members of the plant kingdom. In order to obtain a clear picture of the evolutionary history of the plant kingdom, it is necessary to have at hand a large number of facts, obtained by a study of fossil remains, comparative anatomy, and other sources, to trace out the complicated problem of ancestry and descent throughout the whole plant kingdom. The findings of this science are of great importance for botany and zoology alike. While there are still differences of opinion as to facts, everyone is agreed that the ideal to be attained is a system of classification which is truly a system of phylogeny. As more and more knowledge of the evolutionary history of the plant kingdom becomes more 17 258 BOTANY: PRINCIPLES AND PROBLEMS completely, our systems of classification will grow more accurate and useful. **Groups within Groups.**—Plant classification is far more com- plex, however, than a mere segregation of individuals into a series of classes, each of which is further subdivided into smaller ones and each of these, in turn, into others still smaller. This system of "groups within groups" is familiar in methods of classifying all sorts of objects. An army, for example, is made up of divisions; each division is separated into a series of large groups, or divisions. Each division, in turn, is made up of brigades; each brigade, of regiments; each regiment of battalions; each battalion, of com- panies; each company of officers, and so on to the squad. In this way every soldier occupies a definite and particular place. A leaf with serrated edges. B A leaf with smooth edges. C A leaf with lobed edges. D A leaf with toothed edges. E A leaf with entire edges. F A leaf with serrated edges. G A leaf with lobed edges. H A leaf with toothed edges. I A leaf with entire edges. J A leaf with serrated edges. K A leaf with lobed edges. L A leaf with toothed edges. M A leaf with entire edges. N A leaf with serrated edges. O A leaf with lobed edges. P A leaf with toothed edges. Q A leaf with entire edges. R A leaf with serrated edges. S A leaf with lobed edges. T A leaf with toothed edges. U A leaf with entire edges. V A leaf with serrated edges. W A leaf with lobed edges. X A leaf with toothed edges. Y A leaf with entire edges. Z A leaf with serrated edges. AA A leaf with lobed edges. AB A leaf with toothed edges. AC A leaf with entire edges. AD A leaf with serrated edges. AE A leaf with lobed edges. AF A leaf with toothed edges. AG A leaf with entire edges. AH A leaf with serrated edges. AI A leaf with lobed edges. AJ A leaf with toothed edges. AK A leaf with entire edges. AL A leaf with serrated edges. AM A leaf with lobed edges. AN A leaf with toothed edges. AO A leaf with entire edges. AP A leaf with serrated edges. AQ A leaf with lobed edges. AR A leaf with toothed edges. AS A leaf with entire edges. AT A leaf with serrated edges. AU A leaf with lobed edges. AV A leaf with toothed edges. AW A leaf with entire edges. Fig. 10b.—Species belonging to the same genus. Six species of maple (Acer). The leaves are shown in different positions from the top to the bottom. In each case the differences from the normal form are indicated by the arrows. In the fundamental characteristics of the simple groups are evident. At, Acer Picea; B, Acer rubrum; C, Acer saccharum; D, Acer ginnala; E, Acer negundo; F, Acer saccharum var. americana. THE PLANT KINGDOM 259 in the organization. In the taxonomy of the plant kingdom a somewhat similar series of groups within groups has been recognized and its parts named, precisely as in a military organi- zation. The most important of these groups are the species, genus, family, order, class, and kingdom. The first two of these principal ones are often employed. Thus all the individuals which are like one another are grouped together and constitute a species, of which there may be many thousands in any one genus. This species is clearly very similar to a large number of other species, the Prairie Rose, Swamp Rose, the Sweetsering and many others. All of these are members of the same genus, as indicated from the flowers; genera originally, are grouped together as the Rose genus (plural, genera, Fig 140), the scientific name for which is *Rosa*; and each of the species also has its scientific name of flower, which for the Prairie Rose is *Rosa palustris*. From the nature of their flowers, fruit and other organs, we believe that this Rose genus is closely related to other somewhat similar genera, such as the *Cistus* (Cistaceae), the *Rubus* (Rosaceae), the *Cranberry* (*Cranberry*), the Blackberries (*Rubus*), the Strawberries (*Fragaria*), and others; and we therefore group all of these genera together into a single family Rosaceae. This is a large family, containing about 40 genera and 3,000 species. It is evidently similar in many respects to certain other families such as the *Liliaceae* (Liliaceae), the *Lupinaceae* (Lupinaceae) and the Legume family (*Leguminosae*). This group of plants, which stand somewhat by themselves and are probably all related to each other but not closely enough to be in this case the *Rosales.* The Rosales are one of a large number of orders which constitute the great class of Dicotyledoneous or monocotyledonous plants. These are divided into two classes from another great class of the Monocotyledoneous or monocotyledonous plants; and these two classes comprise the subdivision which we call the Angiosperms or angiospermae. The angiosperms, together with the gymnosperms, form what is called the Phanerogamia. Gymnospermia, or gymnosperms, make up the division known as the Spermatophyta or seed plants, with which we have already become familiar. The classification of the four main groups into which the plant kingdom is divided: *As general, the scientific name of a family has the ending -ose*, that of an order, -ace. A diagram showing a hierarchical classification system for plants. 260 BOTANY: PRINCIPLES AND PROBLEMS These units of classification which we have illustrated include the most important ones; but large groups are often subdivided still further for purposes of convenience, so that we meet with the terms variety, tribe, section, sub-family and others, each of which has its own special system of nomenclature. Every one of the thousands of groups in the plant kingdom has its individual productive characteristics in which it differs from every other group of similar grade, so that a botanist is able to place a newly discovered species in any one of these groups without hesitation, according to reference to the plant kingdom as a whole. **Nomenclature.** The technical names for these various groups are derived from Latin words and phrases, and although many plants have "common" names in the language of the country where they grow, the advantages of technical and "scientific" names are great that they are almost exclusively used by botanists. In earlier days, before our present system of naming plants had been established, it was customary to use a Latin word referred to a given species was to use a cumbersome descriptive phrase, usually consisting of several Latin nouns and adjectives. As different people did not always understand what they became a matter of doubt as to just what plant they were talking about, and much confusion resulted. It remained for the genius of the great Linnaeus to devise a method of classifying which should be simple and uniform. He invented the *binomial* system of nomenclature, so called because each species is given two names; first, the name of genus of which it is a member, or its generic name; secondly, the name of species, or its specific name. This system is now universally adopted, except in respect to the particular species in question, or its specific name. The scientific name of the Dog Rose would thus be *Rosae caninae*. This system is also applied to animals and to other living individuals, where the "name" is that of a person's family and the "given name" is distinctively his own. In plants, this order is simply reversed, so that the "name" is that of a genus and the "given name" is distinctively that of a species. The binomial system was first used extensively for plants in the *Species Plantarum*, a great work published by Linnæus in 1753, in which he described all plant species then known. This book is the foundation upon which all later work on plant nomenclature is based. In order to avoid confusion and to make perfectly clear what plant is meant, there is placed after the plant name the name THE PLANT KINGDOM261 (or its abbreviation) of the botanist who first used this name for the species in question. Thus the full name of the Dog Rose is Rosa canina L., which means that this particular name was conferred on the plant by Linnaeus. Disputes still arise as to what justifies the use of a single name for a group of plants which sometimes have given different names to the same plant. Such questions must be settled by the adoption of universal rules and precedents. The reason why we do not use them, the nomenclature of plants may in time become uniformly uniform throughout the world. With regard to the history, classification, and nomenclature of plants, we shall proceed in the next four chapters to describe briefly the main features of the four great divisions of the vegetable kingdom. **QUESTIONS FOR THOUGHT AND DISCUSSION** 538. Do you think that the first organisms to appear on the earth were animals or plants? Why? 540. It is generally agreed that the earliest plants lived in the water. What evidence can you think of for this? 541. Do you think that the earliest plants were multicell or not? Why? 542. What are the advantages and disadvantages of a many-celled as compared with a single-celled plant? 543. What basis would you use to determine whether a group of cells is a colony or a plant? 544. In what way is an animal individual more distinct and definite than a plant individual? 545. Why is the increased size of the plant body, beyond a certain point, necessarily followed by the beginning of differentiation? 546. Why does it happen that new cells can be differentiated from the ordinary body cells of the plant? 547. In general, is there a higher degree of differentiation in the body of a water plant or of a land plant? Explain. 548. State all the resemblances you can find between a plant and a coral reef. 549. What other advantages can you think of, aside from increased vigor, which sexual reproduction might possess over sexual? 262 BOTANY: PRINCIPLES AND PROBLEMS **650.** In sexual reproduction, what is the advantage in having the two types of gametes (male and female) so radically different from one another? **651.** Gametes, particularly male gametes, are often motile when all the other cells of the plant are not. How do you think that this has come about? **652.** What various advantages and disadvantages can you think of in a life-cycle which shows an alternation of generations? **653.** Why do you think it is that the alternation of generations, so well marked in lower plants, has practically disappeared in the higher ones? **654.** Do you think that plants or animals were the first organisms to survive from one generation to another? Why? **655.** When the first plants invaded the dry land, with what kind of soil did they probably find it covered? What important changes did the presence of soil make life on the soil? **656.** Have a description of the probable appearance of the land surface during the time of the Pterophytes? What region on earth today do you think it most closely resembled? **657.** What effect did the evolution of the Pterophytes probably have on the abundance of land animals? Why? **658.** Many plants now live entirely on land. Do you think that they were the first land plants? Why? **659.** What are the advantages of the seed over the spore as an agency for reproduction? **660.** Why have seed plants largely superseded pteridophytes? **661.** In what way has the evolution of seed plants probably changed the characteristics of their descendants? **662.** What is the principal use of having a definite system of classifying plants into species, genera, families, and other groups? **663.** What organs are chiefly used as a basis for the classification of plants? Why? **664.** What is an “artificial” as opposed to a “natural” system of classification. **665.** Classify the following objects into a system of “groups within groups,” stating briefly the characteristics by which each group may THE PLANT KINGDOM be distinguished from coenocytic groups, and making in this way what is commonly known as the "plant kingdom." Apple, oak, log, pomegranate, maple leaf, cotton fiber, apple blossom, potato, tulip bulb, peanut, turnip, pine cone, peach, spruce shingle, strawberry, automobile tire, squash seed, and so on. 666. In describing the plant kingdom in the later chapters of this book, much more space, relative to their number of species, has been given than many other lower groups than has been given the angiosperms. Why is this justifiable? 667. State what advantages and disadvantages the scientific name of a plant has as compared with its "common" name. 668. In popular literature we often find that when the scientific name of an animal or plant is used by an article in front of it, as, for example, "the American toadfish." Why is this incorrect? REFERENCE PROBLEMS 105. What is meant by the "difficulty" of a plant species? 106. Why do we need a new definition of the establishment of our modern conception of the Alternation of Generations? 115. In general, how do characters which distinguish the lower groups of plants, such as mosses and ferns, differ from those which distinguish higher orders? Give a genus and a species. 111. Why is it that Latin and Greek are the languages from which the scientific names of plants and animals have been chiefly derived? 112. Write out the derivation of the scientific name "Ficus" or "of the Paper Birch" of the "Apple" of the "Cowpea," of the Blue Flag. 113. Find the species, genus, family, order, class, subdivision and division to which each of the following plants belongs, and give the correct scientific name of each. The White Oak (Quercus alba), the Common Field Daisy (Tiger Lily). 114. If two botanists each give a different name to the same plant species, what is it called? Is this situation always bad or sometimes good? 115. What is meant by the "flora" of a region? by the "noose flora" of a region? by the "forest flora" of a region? 116. Give the derivation of the following terms and explain in what way each is appropriate. 263 Genus Zygote Sporophyte Gametophyte Species CHAPTER XIV THE THALLOPHYTA The most simple and primitive of the four divisions of the plant kingdom is the Thallophyta, or Algae. This is a large and heterogeneous group of species, about 80,000 in all, which display a wide variety in their structure and life histories. The name "thallus-plants" refers to the character of these vegetative bodies, which are usually leaf-like or filamentous with little differentiation into such diverse organs as we find among the higher plants. It is usually rather small, and is often minute. This simplicity of structure, together with their generally simple and primitive methods of reproduction, are the chief features which distinguish the thallophytes as a whole. To construct a truly natural classification for such a heterogeneous group is a very difficult task indeed. The division as a whole is characterized by the possession of chlorophyll, but this pigment may be absent in some species, which possess chlorenchyma or a similar substance and may thus live independently, and which include all the seaweeds, together with the pond scum and other water plants. There are also the Fungi, which lack chlorophyll and can therefore exist only as saprophytes or parasites, and to which belong the multitude of bacteria, molds, mildews, blights, rusts, toadstools, and mushrooms. The fungi have evolved from the algae, and it has been shown that no two series parallel one another somewhat in their various characteristics. It is more convenient to treat each separately however, incidentally showing how close such relationships as seem dear between various groups in the two series. THE ALGAE The algae are commonly divided into four classes, the Blue-green Algae, the Green Algae, the Brown Algae, and the Red Algae. The differences in color which have given rise to these names are incidental and are accompanied by more deeply seated distinctions. THE THALLOPHYTA. 265 Cyanophyceae or Blue-green Algae—These are the simplest and lowest of all green plants. The body consists of a simple cell, but in most species the cells tend to hold together in colonies. The cell itself is very simple, lacking the nucleus, sap-ecvndity, and chloroplasts. The cytoplasm is homogeneous, and the protoplasm may be perfectly homogeneous, with the pigment evenly dispersed, or a colored outer zone and a colorless inner one may be roughly distinguished. The cell wall is usually thin and transparent. The pigment, which seems to be dissolved directly in the cyto-plasm and never defined as definite plastids, is usually (though not always) blue-green. In some cases it is yellow, or even brown, or chlorophyll with a blue pigment, phycoerythrin. Both of these may be encountered with photosynthetic but this is a certainty, except for one or two species of the genus Chlorella. The thic-cleum algae is far less complete than it is for higher plants. The cell-wall is typically thick and mesophyllous, and in many species a group of cells become coalesced into a leaf-like mass derived from the base of a large pile-like colony resembling in the occurrence of unfavorable conditions for growth, heavy-walled "rooting cells" may be produced. Cell division is very simple and does not differ greatly from that of other algae. The cell merely becoming constructed by the growth of a new wall until complete separation into two cells takes place. Little differentia-tion in structure occurs within the cell, though variations generally appear in certain species. Reproduction consists merely in cell division or "fission," and no instances of sexuality have ever been recorded. The two groups of blue-green algae differ from other algae but resemble bacteria, and these two groups have therefore sometimes been placed together as a separate division called Cyanophyta. These algae are divided into the classes Chlorophyceae (Cyanophyceae) and Schizomycetes (Bacteria). The Thallophtya live in both salt and fresh water and are able to grow at higher temperatures than any other plants, often thriving in the water of hot springs at temperatures up to 90°C. Most species prefer water which is dirty and full of organic matter, but some are found in clear water near rocks, and other places which are exposed to the air. One of the simplest examples is Glaucocystis (Fig. 141), a minute blue-green alga which lives in fresh water. As an individual divides, the resulting cells of the first generation, and 266 BOTANY: PRINCIPLES AND PROBLEMS sometimes those of the third and fourth, are held together, embedded in their swollen cell-walls. In Nostoc (Figs. 141, C and D) the individuals are joined loosely into filaments which somewhat resemble strings of beads, and these may also be embedded in the cell-walls. In Chlorella (Fig. 141, A), each reaching a character of one centimeter and containing hundreds of filaments. Here and there along the filament are frequently found large, empty cells called *heterocytes*. Their function is not definitely known but they may be concerned in breaking up the filament into short segments. The heterocytes are very numerous (Fig. 141, A). At the cells are pressed flatly against one another and their protoplasmic wall is so poorly developed that the filaments are free in the water, where they sway or revolve slowly. It is hard to know whether to regard them as a single organism or as a colony, or as a colony of distinct individuals or as a single, many-celled plant. **Chlorophyceae or Green Algae.** This class is by far the largest of the four classes of green plants. They contain chlorophyll but no other pigment, and the bright A diagram showing various stages of filament formation in Nostoc. Fig. 141.—Cyanophyceae of various sorts. A, Chlorella; × 240. B, Gloeocapsa; × 550. C, Nostoc: colony, natural size. D, Filament of Nostoc, with a heterocyte between two filaments; × 300. E, F, Filament of *Klebsia*, with heterocyte at base; × 225. (A and F after Engler and Pringsheim.) THE THALLOPHYTA 267 green color which the plant body thus displays has given the class its name. It is well represented in both fresh and salt water and a few species thrive in damp situations on land. The cell is much more highly differentiated here than among the blue-green algae, and the chlorophyll is found in a special place, the so-called chromatophore and usually is a sap-cavity, thus resembling in its essential details the cells of the higher plants. *Pyraceae*, or even more highly developed, are found in fresh water only. The plant body may consist of a single cell, a filament, or a plate of cells. Most species (though not all) produce zoospores, motile representatives of the plant body, which are often very small, mere lashes or cilia which grow directly into new plants. These are developed in modified cells or sporangia. Various types of sexual reproduction are also found in these algae, ranging from simple gametangia to complicatedly similar ones, those where they have become markedly distinguishable as sperms and eggs. Because of all this structural diversity and of the fact that they are not closely related to any other group, apart from lower algae to bryophytes, the Chlorophyceae have received intensive study, particularly with regard to the development of the plant body and its structure. To classify this great class thoroughly it is necessary to distinguish within it a large number of orders, but for the purposes of our discussion we can conveniently group these into five: The *Prodeconocales*, the *Conjugales*, the *Siphonales*, and the *Chlorales*. **1. Prodeconocales or One-celled Green Algae.--These are chiefly microscopic plants, some of them being unicellular, others composed of two cells and they may be completely separate or loosely joined into colonies, and are either mobile or non-motile. In *Pleurococcus* (Fig. 14), which forms the green stain found on damp bark, rocks, and similar places, is perhaps the most common type. It consists of a single cell containing a large chloroplastoid and reproduces only by cell division. The daughter cells may some- times cohere for a time in small groups. This is one of the algae which is often associated with various fungi to form the peculiar group of *lichenes*. A diagram showing a single-celled green alga. Fig. 14.--One-celled green alga. Single cell and two small daughter cells. 308 **2. Conjugales or Two-celled Green Algae.--These are mostly motile and are found in fresh water. They are distinguished from the preceding order by their possession of two cells which are connected by a narrow neck-like projection. The two cells may be equal or unequal in size, but never separated from each other. The larger cell contains a large chloroplastoid and reproduces by cell division. The daughter cells may sometimes cohere for a time in small groups. This is one of the algae which is often associated with various fungi to form the peculiar group of *lichenes*. A diagram showing two-celled green algae. Fig. 15.--Two-celled green alga. Two cells connected by a narrow neck-like projection. 309 **3. Siphonales or Siphonous Green Algae.--These are mostly motile and are found in fresh water. They are distinguished from the preceding order by their possession of two cells which are connected by a long thread-like projection. The two cells may be equal or unequal in size, but never separated from each other. The larger cell contains a large chloroplastoid and reproduces by cell division. The daughter cells may sometimes cohere for a time in small groups. This is one of the algae which is often associated with various fungi to form the peculiar group of *lichenes*. A diagram showing siphonous green algae. Fig. 16.--Siphonous green alga. Two cells connected by a long thread-like projection. 310 **4. Chlorales or Chlorophycean Green Algae.--These are mostly motile and are found in fresh water. They are distinguished from the preceding order by their possession of two cells which are connected by a short thread-like projection. The two cells may be equal or unequal in size, but never separated from each other. The larger cell contains a large chloroplastoid and reproduces by cell division. The daughter cells may sometimes cohere for a time in small groups. This is one of the algae which is often associated with various fungi to form the peculiar group of *lichenes*. A diagram showing chlorophycean green algae. Fig. 17.--Chlorophycean green alga. Two cells connected by a short thread-like projection. 311 **5. Ulvales or Ulvaceous Green Algae.--These are mostly motile and are found in fresh water. They are distinguished from the preceding order by their possession of two cells which are connected by a long thread-like projection. The two cells may be equal or unequal in size, but never separated from each other. The larger cell contains a large chloroplastoid and reproduces by cell division. The daughter cells may sometimes cohere for a time in small groups. This is one of the algae which is often associated with various fungi to form the peculiar group of *lichenes*. A diagram showing ulvaceous green algae. Fig. 18.--Ulvaceous green alga. Two cells connected by a long thread-like projection. 312 268 BOTANY: PRINCIPLES AND PROBLEMS Chloroplastomane (Fig. 143) is a matule type, its cells containing a single chloroplastid, a red pyramidal spot, some contractile vacuoles, and two cilia. The cell may lose its cilia and divide into several zoospores, each of which is capable of producing a new plant. Toward the end of the growing season, however, smaller matule cells are formed which swim about and unite in pairs. These are the precursors of the zygote, their evolution from ordinary zoospores seems to be very clear. The cell formed by their union is known as a zoospore, a term applied to all cells produced by the fusion of two gametes. Such a form of sexual reproduction is known as "zygotic." * This plant is very similar to members of the interesting group of Flagel- lata, sometimes shared with mimiola and sometimes with plants, and which contain contractile vacuoles. A B Fig. 143—Chloroplastomane. A, individual plant, showing the large chloro- plastid, the pyramidal spot, and the two cilia. B, zoospore. In this case the two zoospores are equal in size, but in the case shown they are differently different. * (after Gomphocarpus.) THE THALLOPHYTA 269 There are a number of other algae similar to Cladophoraceae except that they are united into loose colonies of ellipsoid individu-als, and the group culminates in the large, hollow, spherical colonies of Volvox (Fig. 144) which often consist of thousands of cells. In these higher forms the spermatozoa are markedly different from the eggs, and the cell formed by their union is termed (as in all such cases) a fertilized egg or oospore (Fig. 145). Sexual reproduction of this type is known as heterogamy. It should be noted that both zygospores and oospores are typically thick-walled cells which are capable of resisting such unfavorable conditions as drought and low temperature, and of germinating to Fig. 144.—Volvox. Spherical colony of bi-ellipsoid individuals, with three daughter-colonies developing in its interior. Fig. 145.—Fertilized egg or oospore. Fig. 146.—Phycodendron. A group of oospore cells united into a plate-like structure. Fig. 147.—Hydrodictyon. Portion of a leaf-like colony, individual cells forming a hexagonal pattern. 270 BOTANY: PRINCIPLES AND PROBLEMS produce new plants whenever a suitable environment again appears. In another group of Protococcals the body cells are non- mobile, independent movement being limited here to the nodes and zoospores. The zoospores are motile by means of cilia, which are simple, few-celled plates in Pediocarum (Fig. 146) but form a complex network in the water-net, Typhodium (Fig. 147). The zoospores are usually free-swimming and very small, and seem about, as is usually the case, but the group of zoospores formed within a single mother-cell displays the remarkable habit of miting, which is characteristic of all the members of this family of non-motile cells which is finally liberated and grows into a mature colony. The Protococcals are of special interest because of the light they throw upon the development of the multicellular individual and the differentiation of the sexes. 2. Coelomycetous Protococcals. These include most of the common thread-like and membrane-grown algae of salt and fresh water. They all reproduce sexually by zoospores and also exhibit asexual reproduction by means of spores, simple in the lower forms but relatively complex in the higher ones. The spore is large and varied and contains many species which are but distantly related to one another. Three typical genera will give us an idea of their structure: *Ulothrix* (Fig. 148) is a common thread-like or filamentous alga, its short cells each containing a single nucleus and one large chloroplast. The chloroplasts are surrounded by a wall which divides into a group of zoospores which escape and may form such a new plant; and smaller binucleate middle cells, produced in the same manner in which the zoospores are formed. These cells divide them- self as gametes and conjugate in pairs to produce zoospores. *Ulothrix* is often cited as another good example of the origin of sexual reproduction. *Oedogonium* (Fig. 149) is also a common genus and its filament is anchored by a modified basal cell, the *holdfast*. In certain cells the constrictions between successive cells become so narrow that zoospores with a circle of cilia near one end, and this soon settles down, develops a holdfast, and grows into a new filament. Other cells also become modified, each with a circle of single rounded, nucleated cell, well supplied with chlorophyll and other food material. This female gamete or egg, and the cell which THE THALLIOPHYTTA 271 produces it (like all egg-producing structures in these lower phyla) is termed an oospore. In other cells the contents divide into two small, motile male gametes or sperms, and each of these mother-cells (like all structures which produce male gametes) is known as an antheridium. One of the sperms carries an egg. A diagram showing the structure of a thallus with a rhizoid cell (1) by which it is attached, B, portion of filament with oospore and antheridium, C, zoospores. Fig. 148 — Chetone, A, young thallus with rhizoid cell (1) by which it is attached. B, portion of filament with oospore and antheridium. C, zoospores. The zoospores (C) swim about in water until they find a suitable place to attach themselves. The antheridium (B) secretes a gelatinous substance which serves as a medium for the germination of zoospores produced by its antheridia. From Strecker, A diagram showing the structure of a thallus with a rhizoid cell (1), oospore (2), and antheridium (3). The antheridium produces zoospores (4). Fig. 149 — Oedogonium nudum. A, thallus with antheridium (a), each producing zoospores (b). B, thallus with a rhizoid cell (c) and antheridium (d) which has developed from a fertilized egg. C, thallus with a thick-walled oospore (e) which has developed from a fertilized egg. C. Chetone. and fertilizes it, and the oospore thus formed germinates into a group of zoospores. Coleochaete (Fig. 150) is a free-water alga the vegetative body of which consists of a flat plate or cushion of arising filaments. Its cells may produce single, large, biciliate zoospores. Antheri- A diagram showing the structure of a thallus with a rhizoid cell (1), oospore (2), and antheridium (3). The antheridium produces zoospores (4). Fig. 149 — Oedogonium nudum. A, thallus with antheridium (a), each producing zoospores (b). B, thallus with a rhizoid cell (c) and antheridium (d) which has developed from a fertilized egg. C, thallus with a thick-walled oospore (e) which has developed from a fertilized egg. C. Chetone. and fertilizes it, and the oospore thus formed germinates into a group of zoospores. Coleochaete (Fig. 150) is a free-water alga the vegetative body of which consists of a flat plate or cushion of arising filaments. Its cells may produce single, large, biciliate zoospores. Antheri- 272 BOTANY: PRINCIPLES AND PROBLEMS dia and oogonia are formed much as in Oedogonium, and a thick-walled zoospore is produced. Following this, however, the adjacent cells give rise to branches which grow up and surround the oospore, forming a distinct case or fruiting body. The zoospores are liberated by rupture of the wall, and subsequently form a zoospore. This reproductive cycle foretells the "alternation of generations" of the higher plants. In its struc- ture and life history, *Calochoicne* is the most specialized of the green algae and is believed by many botanists to approach the lowest tryptophytes. 3. Synechococcus Tubular Algae.--These are distinguished from all other algae by the fact that the whole plant body, whether it be a simple filament or a well-differentiated thallus, is essentially a single cell. The cell walls are absent, and either algae and all ordinary plants into small cells are absent, and the mass of cytoplasm with its thousands of nuclei is therefore able to function as a single living unit. Such a multinucleate cell, of which these plants are extreme examples, is known as a coccoid. The Synechococcus are chiefly marine forms, especially abundant in the warmer sea. They usually produce zoospores and in A: A portion of fertile thallus. B: An egg cell. C: A portion of sterile thallus. D: A zoospore. E: A zoospore. F: The contents of the frustule dividing up into zoospores. G: Zoospores. H: Zoospores. I: Zoospores. J: Zoospores. K: Zoospores. L: Zoospores. M: Zoospores. N: Zoospores. O: Zoospores. P: Zoospores. Q: Zoospores. R: Zoospores. S: Zoospores. T: Zoospores. U: Zoospores. V: Zoospores. W: Zoospores. X: Zoospores. Y: Zoospores. Z: Zoospores. AA: Zoospores. BB: Zoospores. CC: Zoospores. DD: Zoospores. EE: Zoospores. FF: Zoospores. GG: Zoospores. HH: Zoospores. II: Zoospores. JJ: Zoospores. KK: Zoospores. LL: Zoospores. MM: Zoospores. NN: Zoospores. OO: Zoospores. PP: Zoospores. QQ: Zoospores. RR: Zoospores. SS: Zoospores. TT: Zoospores. UU: Zoospores. VV: Zoospores. WW: Zoospores. XX: Zoospores. YY: Zoospores. ZZ: Zoospores. AAAAA: Zoospores. BBBBB: Zoospores. CCCCC: Zoospores. DDDDD: Zoospores. EEEEEE: Zoospores. FFFFF: Zoospores. GGGGG: Zoospores. HHHHH: Zoospores. IIIII: Zoospores. JJJJJ: Zoospores. KKKKK: Zoospores. LLLLL: Zoospores. MMMMM: Zoospores. NNNNN: Zoospores. OOOOO: Zoospores. PPPPP: Zoospores. QQQQQ: Zoospores. RRRRR: Zoospores. SSSSS: Zoospores. TTTTT: Zoospores. UUUUU: Zoospores. VVVVV: Zoospores. WWWWW: Zoospores. XXXXX: Zoospores. THE THALLOPHYTA 273 cases where sexuality has been proven are always isogamous except in the genus *Yaschera*, which is such a familiar and distinctive type that we shall describe it more fully. This alga forms the common "green felt" so often found on damp soil or in muddy pools, and consists of a tangled mass of coarse, branch- A diagram showing the structure of a filament of *Yaschera*. The tip of the filament is cut off by a wall and its contents become a large zoospore, with many nuclei and several flagella. The remainder of the filament becomes a long, thread-like organiza- ing, tubular filaments. Large zoospores are produced, each of which is merely the contents of the tip of a filament which has been cut off by a wall and has escaped (Fig. 151). The sexual organs are not simply modified vegetative cells, as in the plants. Fig. 151.—*Yaschera*. Anemal reproduction. The tip of a filament is cut off by a wall and its contents become a large zoospore, with many nuclei and several flagella. The remainder of the filament becomes a long, thread-like organiza- ing, tubular filaments. Large zoospores are produced, each of which is merely the contents of the tip of a filament which has been cut off by a wall and has escaped (Fig. 151). The sexual organs are not simply modified vegetative cells, as in the plants. Fig. 152.—*Yaschera*. Sexual reproduction. Oogonia, o., each contains a single egg, e., which is surrounded by all three sperma and no cytoplasm. A, *I. irrorata*. B, *F. sordida*. previously studied, but specialized for gametic production (Fig. 152). A cell partitioned off by a wall from the main fila- ment or from another cell becomes an oogonium containing within which a single, large egg is formed. From the tip of another A B 274 BOTANY: PRINCIPLES AND PROBLEMS small leathn sac by is cut off a cell which develops into an anthocerium. One of the spores here developed enters the odumagum and fertilizes the egg, producing a heavy-walled oospore. 4. *Conjugales, the Pond Scums, Decidus, and Douton.*—The plant body is a thin film of water on the surface of fresh water. The sporangium is a single cell. The species are all confined to fresh water and are dis- A diagram showing stages in sexual reproduction in *Chlorophyta*. (a) Two adjacent filaments showing stages in sexual reproduction. (b) A zoospore with two flagella. (c) A zoospore with one flagellum. (d) A zoospore with no flagella. (e) A zoospore with two flagella. (f) A zoospore with one flagellum. Fig. 155.—*Spirogyra.* Two adjacent filaments, showing stages in sexual reproduction in *Chlorophyta*. (a) Two adjacent filaments showing stages in sexual reproduction. (b) A zoospore with two flagella. (c) A zoospore with one flagellum. (d) A zoospore with no flagella. (e) A zoospore with two flagella. (f) A zoospore with one flagellum. tinguished from other Chlorophytes by the absence of zoospores or other means of asexual reproduction, the absence of motile cells of any sort, the occurrence of large and conspicuous chloro- plasts, and the characteristic manner in which sexual reproduc- tion is brought about. 136 THE THALLOPHYTA. 275 The Tyndal Sponna, of which *Spirogyra* (Fig. 153) is the common example, are all filamentous algae. In this genus the chloroplast is a broad, strap-shaped structure running entirely around the cell and on it appear a series of small, rounded areas, the pyrenoids. The nucleus is suspended in the middle of the cap cavity by threads of protoplasm. The two cells of a pair, during conjugation, adjacent cells of two filaments which are lying side by side send out projections or "conjugating tubes" toward one another. The tips of these touch, the wall between them breaks down, and through the channel thus formed the whole protoplasmic con- tent of each cell passes into that of its partner. The walls of the two cells fuse into a thick-walled zygospore. Occasionally the two cells conjugate in the tube itself, and sometimes two adjacent cells of two filaments fuse together. The *Desmidia* (Fig. 154) are unicellular plants of the utmost variety and beauty of form. The cell is composed of two per- fectly symmetrical halves, each containing a single nucleus which is often constricted and under which lies the nucleus. Aside A small illustration showing various types of desmidia. Fig. 154.—Desmidia of various types. 276 BOTANY: PRINCIPLES AND PROBLEMS from ordinary cell division, reproduction is effected by conjugation between two protoplasts, each of which has escaped from its wall. The Diatoms (Fig. 153), a large group of unicellular plants of somewhat unusual appearance, are among the most interesting to be considered here. They are represented by thousands of species of salt and fresh water and are the most abundant of the minute plants which make up that multitude of freely swimming or float- A diagram showing the structure of a diatom. ing algae in the open sea and in fresh water which we know as the plankton. Many species are also scale-like and may be united into filaments or other colonies. The wall is hard and flinty, being heavily filled with silica, and consists of two halves or calices one of which fits closely over the other. The calice is highly diversified in shape and are frequently ornamented with minute and intricate markings which have long been the delight of microscopists. These markings are often so complex owing to the presence of a pigment, *diatomein*, in addition to chlorophyll. 153 THE THALLOPHYTA. 277 5. Characeae or Stoonwets (Fig. 136).—This remarkable group of plantæ stands apart from all others and we have no certain knowledge as to its relationships. The vegetative body consists of long, jointed stems, and at the joints, or nodes, arise wheels of A main branch showing the circles of small, internal branches. B part of a lateral branch showing the sexual organs; the oogonium above and the antheridium below. C short branches with antheridia. Fig. 136.—Chara. A, a main branch, showing the circles of small, internal branches. B, part of a lateral branch showing the sexual organs; the oogonium above and the antheridium below. C, short branches with antheridia. short branches. No ascusp spores are produced, but along the branches are borne antheridium and oogonium, far more complicated than among any other thallophytes. The antheridium is spherical and contains numerous antherozoids; the oogonia are egg-shaped cells from the inner surface of which arise a large number of many- The Thallophyta 275 BOTANY: PRINCIPLES AND PROBLEMS celled, open-producing filaments. The zoogonium is covered with a wall or envelope of quadrate cell-walls growing up from the tissue below it, and produces a single large egg. After fer- tilization the envelope hardens, forming a net-like spore case around the egg. The egg is surrounded by a membrane, and an alternation of generations indicates that these plants should be placed among the thallophytes, but they are clearly distinct from any other group. **Phaeophyceae** or **Brown Algae**—These plants may be dis- tinguished from other algae by their characteristic brown color, the one to many cells which are often arranged in a spiral or helix, and by certain structural characters. The Phaeophyceae are the largest and rankest of all algae and display the highest degree of bodily differentiation. They are found exclusively in salt water and are best developed in the cooler seas. Those having most commonly in shallow water and the zone between tide marks, they are subjected to the buffeting of the waves and may be exposed to drying out at low tides. These plants probably represent an entirely independent line of evolution from the green algae and seem to have led no higher types. Two orders are recognized among them, the Phaeophorales and the *Fucales*. 1. **Phaeophorales** or **Kelp** and Their Allies.—In this order occur the kelps (Laminaria), which are very large and flexible, elsewhere, together with many other large algae such as the giant kelp (Macrocystis), the sea otter's cabbage (Auroraeia), the sea palmetto (Cystoseira), and the sea grape (Gracilaria). They are all isogamous and in most cases produce zoospores. **Ectocarpus** (Fig. 157), one of the best known genera, is a rather unusual member of this order. As has already been studied, the zoospores are here produced in sporangia which are modified single cells. The gametes, however, are developed in large multinucleated structures (phloeoconsporangia) which begin to develop before the zoospores appear. These sexual organs characteristic of the bryophytes. Each of the many small cells into which the contents of this structure is divided forms one gamete. The gametes unite to form zygotes which pro- duce new sporangia. Instances have been observed in which these gametes germinate directly into a new plant, and thus function essentially like motile spermatozoa. In some cases two or more gametes of different size sometimes unite, thus indicating the beginning of THE THALLOPHYTA 279 a heterogeneous condition. In genera like *Eodeocarpus* we are evidently near the beginning of sexuality in the brown algae. The larger forms or kelps may become huge plants, the giant kelp sometimes attaining a length of from two hundred to three hundred feet. The stipes or stems are often very massive holdfasts. A short stem with multicellular or pluricellular sporangia, in which gametes are produced. A young growing sporangium is shown at left. At right, a mature sporangium with four large megaspores are produced. C, mature stage in union of two gametes. Above, a single gamete (left) and a conjugating cell (right), the resulting zygote. Fig. 127. --*Eodeocarpus*. A. Short stem with multicellular or pluricellular sporangia, in which gametes are produced. A young growing sporangium is shown at left. At right, a mature sporangium with four large megaspores are produced. C, mature stage in union of two gametes. Above, a single gamete (left) and a conjugating cell (right), the resulting zygote. (♂ after Oken.) 273 frequently much-divided blades and often display a certain degree of structural differentiation. The abundant gametes were known to be malecarse, but their true nature as sexual cells is now known to be such that eozymes are rather rare in this group. 2. *Fucinae or Eodeocarpeae.*--These plants differ from the kelps in producing only sporangia and in displaying a heterogeneous type of sexual reproduction. 280 BOTANY: PRINCIPLES AND PROBLEMS Phaeo, the best known genus, is exceedingly abundant on rocks between tide marks in the temperate regions. The vegetative body of this slug is a flat, repeatedly forking thallus well provided with air bladders (Fig. 158). The swollen tips of certain A diagram showing the structure of Phaeo, a marine slug. It shows a flat, repeatedly forked thallus with numerous air bladders. The diagram also includes a section of the thallus showing the reproductive organs. Fig. 158.--Phaeo reticulatus. Portion of thallus showing air-bladders. (a) and receptacles (b). On the surface of the latter are the many small openings of the receptacles. branches are known as *receptacles* and bear the sexual organs. Scattered over these tips and just below the surface are many small chambers, each of which is connected to the outside by a pore. Lining the wall of these chambers are masses of branch- THE THALLOPHYTA 281 ing filaments among which are placed the antheridia and oogonia (Fig. 150). The antheridia are small cells arising on branches of the filaments and producing swarms of bireticule sperms. The oogonia are larger cells, the contents of each dividing into eight A: Female conceptacle, with oogonia and sterile filaments. B: Single antheridium. C: Oogonium. D: Ovum. E: Egg after discharge into the water. F: Young plant arising from an oogonium. (M, C, D, E and F after Twardi). eggs. In some species both sex organs are produced in the same conceptacle, in others they occur in different conceptacles on the same plant, and in still others the whole plant is entirely male or entirely female. Both kinds of plants may arise from the mouth of the conceptacle and fertilization takes place in the open 282 BOTANY: PRINCIPLES AND PROBLEMS water outside. The thallus settles to the bottom, attaches itself, and develops directly into a new plant. Rhodophyceae or Red Algae.--These are a very varied group, rich in species, almost exclusively marine, and reaching their best development in the warm waters of the tropics. They grow curiously submerged, below tide marks, and are therefore not particularly conspicuous or familiar. The vegetative body tends to be alderly branched and much branched, in contrast to the bulky thallus of the brown algae. The Rhodophyceae are distinguished from all other algae by their characteristic redish color (due to the presence of chlorophyll), and by the complete absence of middle cells of any kind, and by a highly specialized type of sexual reproduction. They do not include primitive types but seem to have arisen from a A: A young thallus with several branches attached at one end. B: A mature thallus with numerous branches. C: A mature thallus with a few branches. Fig. 106.--Summary. A, young at end of a filament. B, trichogyne and 4, trichogynes, to which several sperms are attached. A male gametangium has entered that contains four sperms. C, mature thallus with many branches. D, corpi- curns. Corpi-cums are developing at the ends of short filaments. C, nucharia, groups of small rods each of which contains a spermatozoon. 106 THE THALLOPHYTUM 3 point already well up in the scale of algal evolution. The higher members of the class display an unquestionable alternation of generations. Two typical genera will illustrate the complexities of reproduction in this group. Acrocladium (Fig. 160) is a common form with a branching, filamentous habit and represents the simplest of the red algae. The bimericous small anthocerium produce non-miscible, A B C Fig. 160.--Polypodium phleomum. A, Portion of a tetraploid plant showing three branches, each bearing two sporangia, in a single cell, × 96. B, portion of a single plant. Above, on immature anthocerium below, a mature sporangium containing four spores, × 180. Below, a mature sporangium containing eight spores, × 180. C, mature sporangium showing a ripe cystocarp. Its wall enclosing a group of carpogonemes. × 45. bimaculate sperms. The female sexual organ, which is here called the procaryon, consists of two cells--a basal zygospore, containing the egg nucleus, and a terminal cell, the trichogyne, which is drawn out into a hairlike tip. The sperm is carried by water currents to the trichogyne and fertilizes the egg nucleus within the carpogonium. The fertilized carpogonium does not develop a new plant directly, but produces a group of short filaments on the end of each of which is borne a monosporic carpogonium. This whole structure is known as a cystocarp. It is also known as the carpospore. A carpospore germinates directly into a new plant. Polypodium (Fig. 161), another common member of the class, displays a much more complex sexual history and is typical of 284 BOTANY: PRINCIPLES AND PROBLEMS the majority of the red algae. Three types of plants, similar in vegetative structure but differing in their reproductive organs, may be distinguished: Male plants, producing antheridia only; female plants, producing oogonia only; and hermaphroditic plants, producing sporangia in each of which are four axonal *trisporangia*. The procarp is somewhat more complex than in *Nemalia* and *Rhodophyllum*, but the process of cell-division, however, occurs in much the same way, a sperm coming in contact with the trichogyne, entering the carpogonium, and fusing with the nucleus. In *Rhodophyllum* conjugation and fusion now take place between the fertilized carpogonium and the auxiliary cells, as a result of which sixty or more carpogonia are produced. An envelope of sterile cells grows up from the base and envelops the carpogonia. Experiment has established the fact that these carpogonia produce only tetrasporangia, and that the tetrasporangia in turn produce only male plants. Thus a complete cycle of alternation of sexual and non-sexual individuals is set up. Cytological examination has further proven that the tetrasporangia have two sets of chromosomes, one set being derived from the male plant, and there seems no reason to doubt that we are dealing here with a true alternation of generations, the tetrasporant plant (plus the cytoplasm) being a gametophyte and the male plant a gameto- phyte. This is the more remarkable since all the plants are perfectly similar in their vegetative structures. The red algae form a very distinct specialized class and although they have reached a marked degree of complexity, they apparently have not been the ancestors of anything higher up in the evolutionary series. THE FUNGI The other great group of multicellular organisms are the fungi, which are distinguished from algae by the absence of chlorophyll. Their consequent inability to manufacture food therefore compels fungi to live either as saprophytes or as parasites. Like the algae, this is a heterogeneous group, but it is possible to distinguish more closely related to certain groups of algae than to other fungi, but as a matter of convenience and custom, and in the absence of any widely accepted classification, I shall consider them together. They will all be considered together. This immense army of lowly plants is much more numerous in species than the algae and THE THALLOPHYTA. 285 contains hundreds of types which are of the utmost importance to man and which form the subject matter of various special sciences, notably bacteriology and plant pathology. In the general morphology of both their vegetative and their reproductive forms, the Thallophyta show a parallelism with the algae, ranging from strictly unicellular types in the bacteria, through filamentous forms, to those which have a large and rather complex plant body; and displaying various types of A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Fig. 162.—Bacteria of various types. A, motile individual of Bacillus subtilis. B, motile individual of Bacillus megaterium. C, motile individual of P. aeruginosa. D, non-motile cell of P. aeruginosa subsp. grahamii or subsp. coelatum of a mass of bacteria. E, chloro chlamydia. sexual reproduction, both isogamous and heterogamous. In all fungi (except the Ascomycetes), a plant body is composed of one or more filaments, each of which is called a hypha (plural hyphae). The whole mass of hyphae, which are often tangled or matted together, is called the mycelium. Special absorbing organs are usually present in the mycelium, such as rhizoids or rhizopodia, through which the plant draws its food from the material or soredia on which it grows. The mycelium may be divided into four classes: the Bacteria, the Alga-like Fungi, the Sae Fungi and the Basidiomycetes. **Bacteria.**—These plants differ from other fungi in being strictly unicellular and in having no true plant body at all. The cells are usually very small, ranging from about 0.025 mm. to as low as 0.0005 mm. in length, so that bacteria are the smallest of living organisms known to us. They are also extremely hardy and survive less. There is some evidence, however, that nuclear material and perhaps a vague nuclear body may be present, but the very minute 286 BOTANY: PRINCIPLES AND PROBLEMS size of bacteria makes a cytological study of them exceedingly difficult. The cell wall rarely contains cellulose. One or more cilia are found in many species, which thus have the power of active motility. Various structural types are recognized by bacteriologists, but the most important are the rod-shaped form, which is spherical; the bacterium or bacillus, which is rod-shaped, and the spirillum, which is curved (Fig. 162). The only type of reproduction known to occur in bacteria is binary fission, which, unaccompanied by any of the complex phenomena found in the higher plants, but which in the presence of an abundant food supply may take place at a rate of one every 20 minutes and give rise to millions of individuals in a day's time. At the onset of unfavorable conditions the protoplasm of the cell draws itself together and produces a division wall between the two daughter cells, whereby a favorable environment again ensues. Actively growing bacteria will ordinarily withstand relatively high tempera- tures but their resting spores are still highly resistant and will often be found alive after being exposed to temperatures as high as boiling water. They can also survive extreme cold and dryness. In their various characteristics, therefore, the bacteria (as we have here before) are very much like those of the green algae. Although bacteria occur in countless millions of individuals and although their diverse activities make them almost omnipresent in air, water, and soil, they are so small as to be quite invisible and from a practical point of view would be entirely negligible were it not for their importance in nature and in agriculture. Their importance in agriculture, particularly through their activity in the soil and in dairy products; their role as the chief agents of fermentation; their use as food for domestic animals; their direct interest to man as the cause of some of the worst and most preva- lent of those diseases which affect him and his domestic animals and plants; have caused them to be studied with especial thorough- ness and to occupy a prominent place in modern botany. Our knowledge of bacteria dates only from the latter half of the nineteenth century. Their existence was proven by that great Frenchman, Louis Pasteur (Fig. 163), who labored for years, meeting with the ridicule and antagonism which often greet a new discovery. It was Pasteur who first showed that certain scientists that such minute objects were actually alive and were THE THALLOPHYTA. 287 responsible for so many of the happenings of daily life. Since Pasteur's day, as the result of our knowledge of bacteria, the practice of medicine (and to a considerable extent that of certain branches of agriculture and industry) has been radically changed; and the great discoveries of Pasteur, which have been possible by Pasteur's discoveries and inaugurated by the great surgeon Lister, have converted this branch of medicine from a dreaded last resort to a common and safe means of relief. The bacteriologist of today has developed a complicated and elaborate technique for the study of these organisms, for cultivating them artificially on especially prepared and sterilized foods or media of various kinds, and study the characteristic appearance of individual species, their mode of reproduction, their behaviour peculiar to each. Both as the enemies and as the allies of the human race, these lowly plants with which the bacteriologist works are among the most important members of the vegetable kingdom. Saprophytic Types.—The saprophytic bacteria live on dead plant or animal matter, and are responsible for much of the fermentation which carbohydrate substances often undergo when exposed to the air and which, as a form of incomplete respiration, Louis Pasteur, 1852-1853. Fig. 163.—Louis Pasteur, 1852-1853. 288 BOTANY: PRINCIPLES AND PROBLEMS we have discussed in our study of metabolism. Decay, essen- tially the same type of process as fermentation except that it takes place in all sorts of organic substances and is carried through to a complete break-down of these substances into carbon dioxide, water, etc., by the action of bacteria. This process occurs rapidly through the activity of the many types of putrefactive and decay-producing bacteria. All successful methods of preserving food depend on eliminating these bacteria or preventing their activity. **Pathogenic Types.—** These members of the class which are parasitic on animals are responsible for many diseases of man and other animals. A particular species is the cause of each of the various bacterial diseases, such as diphtheria, tuberculosis, typhoid fever, pseu- doma, cholera, anthrax, and others. The disease-causing bacteria, as well as pear blight, cucumber wilt, black root of cabbage, and others among plants. These diseases are often communicated from one individual to another by direct contact with the bacteria responsible for them may be easily transmitted through air, water, food, or contact. Bacteria which are very minute or otherwise difficult to recognize are also probably responsible for many diseases the cause of which is unknown. The harmful effect of pathogenic bacteria on animals is often not due to the direct attack of the parasite but to the poisonous lysozymes produced by the bacteria which destroy the blood. The affected individual will often produce antilizoxine capable of counteracting the poisonous effect of the toxins and of neutralizing them. This antitoxin is produced in large amounts by the particular parasite. The practice of vaccination consists in inoculating an individual with the parasitic bacteria and thus subjecting him to a mild attack so that he will be able to stimulate the production of sufficient antitoxin in his system to confer immunity upon him for a long time. Vaccination is particularly effective in preventing attacks of diphtheria. In some attacks of certain other disease-producing organisms may also be pre- vented or rendered less virulent by injecting into the circulation a little blood serum obtained from a dog (or cow or horse) which has had the disease and whose blood is rich in antitoxin. This serum or antilizoxin treatment has been especially successful with diphtheria, tetanus, and hog cholera. In our consideration we have dealt only with a few other groups of bacteria which are of special importance to the higher plants THE THALLOPHYTA. 289 because of their relation to nitrogen (Fig. 18). These are the nitrogen-fixing bacteria, which are found in tubercles on the roots of plants belonging to the Legume family and which are able to take nitrogen directly from the air and to incorporate it into their tissues. The most important of these are the Rhizobiums, the end product of protein decay, into nitrite salts and these, in turn, into nitrate salts, the only form in which nitrogen can generally be used by higher plants. **Phycomycetes or Alga-like Fungi.**—These fungi, as their name implies, resemble rather closely certain of the algae, particularly A diagram showing the general habit of a plant. The hyphae (a) grow out from the base of the plant (b), and penetrate into the air stolier hyphae, the sporangiohyphae (c), on which are borne the spores (d). The sporangiohyphae are connected with the main axis of the plant by means of a root-like structure (e), which still adheres to the central axis or rhizoid of the sporangium. The two arrows indicate the direction of growth. in their methods of reproduction. They seem especially near the Siphonales because of the fact that their filaments (hyphae) are coenocytic, and it is probably from this general region of the algae that they have arisen. The Phycomycetes include a great many of the forms which we know as moulds and blights. Three orders are particularly notable, not least among them being the following:—these are the *Mucoraceae*, the *Seqordomycetaceae*, and the *Pernosporaceae*. 19 290 BOTANY: PRINCIPLES AND PROBLEMS L. Macrocystis, the True Molds or Black Mold (Fig. 164).— These fungi are very common on moist, decaying organic material such as manure, decaying fruit, and the like. Their white, cob- webby mycelium, composed of much-branched hyphae, freely penetrate the surface of the substrate. The hyphae grow out on the surface, stout hyphae arise into the air and bear at their tips globular sporangia which burst and liberate masses of dark-colored spores, each of which may germinate directly into a new plant. Sexual reproduction is inconspicuous (Fig. 165). Two short branches of opposite sides arising from adjacent hyphae approach one another and come into contact end-to-end. From the tip of each is cut off a single multinucleate cell which is the gametangium, and this cell divides to form two thick-walled zoospores. It has been found that two distinct sexual strains often exist, entirely similar outwardly but functioning quite differently. One strain produces zoospores which fertilize the pier and the stigma strain and correspond to the two sexes, for A small illustration showing a magnified view of a fungal structure. Fig. 165.—Macrocytes. Formation and germination of the zoospore. A, two overlapping hyphae; B, a zoospore; C, an enlarged zoospore; D, a zoospore with a thick-walled sporangium; E, a sporangium bursting and liberating spores. Two other branches of opposite sides arise, arising from adjacent hyphae approach one another and come into contact end-to-end. From the tip of each is cut off a single multinucleate cell which is the gametangium, and this cell divides to form two thick-walled zoospores. It has been found that two distinct sexual strains often exist, entirely similar outwardly but functioning quite differently. One strain produces zoospores which fertilize the pier and the stigma strain and correspond to the two sexes, for THE THALLOPHYTA 201 Sporopores are usually produced only when a plus plant and a minus plant come into contact. These fungi furnish an interesting example of a physiological differentiation of sex which is not accompanied by morphological differences. 2. Saprolegnia (Fig. 165).—A genus of aquatic plants. In contrast to the Mycorrea, this group is entirely aquatic. Its members live on the bodies of dead insects and other animals, and a few are parasitic, attacking fish and amphibians. A cell cut off from A: A sporangium on an insert in which the two zoospores are shown. B: A zoospore. C: A zoospore with a tail. D: A zoospore with a tail. the tip of a hypha develops into a sporangium and liberates a large number of motile zoospores. Sexual reproduction is heterogamous. A single-celled spherical oögonium is developed and produces several eggs which are fertilized by the contents of an antheridium. The zygote becomes buried within spores from the main hypha near the oögonium and lives with it. In many cases the egg develops into mature spores without having been fertilized at all. *Peronosporales or Blights and Downy Mildews* (Fig. 167).— The species composing this group are all parasites on the higher A: A sporangium on an insert in which the two zoospores are shown. B: A zoospore. C: A zoospore with a tail. D: A zoospore with a tail. 292 BOTANY: PRINCIPLES AND PROBLEMS plants. Their spores germinate on the surface of the leaf or stem and, entering the tissues, branch through all in direc- tions, absorbing food and often causing the death of the host plant. Definite fruiting bodies are produced on the surface of various parts of the host plant, which are localized but become constricted to form spores, which separate and are carried away by the wind. Non-sexual, aerial spores produced in this manner are called conidia, and are of frequent occurrence among fungi. Sexual organs appear in the deeper tissues. The egg nucleus in the ascogonium is fertilized by a male nucleus from an adjacent antheridium, filaments of which are made up of hyphae formed. Such destructive plant parasites as the potato blight and the grapevine mildew belong to this order. Ascomycetes (conidial fungi) is an enormous group of plants includes over 20,000 species. They show but little resemblance to the algae, and although all must have come originally from some chlorophyllous ancestor, they are now very distinct. These fungi are typically land-inhabiting plants and include both saprophytic and parasitic species. Both groups differ from the true fungi in that their spores do not divide into cells by cross-walls, and that sexual processes are much reduced or altogether lacking. The plant body commonly consists of A section through a leaf-Mold showing portion of a "thallus" produced by the fungus. The hyphae penetrate between the cells of the leaf tissue and absorb food from it. B Under the epidermis are produced rows of conidia. R., sexual organs, the antheridia. Fig. 157.—Albinae of the Peronosporaceae. A section through a leaf-Mold showing portion of a "thallus" produced by the fungus. The hyphae penetrate between the cells of the leaf tissue and absorb food from it. Under the epidermis are produced rows of conidia. R., sexual organs, the antheridia. by wind. Sexual organs appear in the deeper tissues. The egg nucleus in the ascogonium is fertilized by a male nucleus from an adjacent antheridium, filaments of which are made up of hyphae formed. Such destructive plant parasites as the potato blight and the grapevine mildew belong to this order. Ascomycetes (conidial fungi) is an enormous group of plants includes over 20,000 species. They show but little resemblance to the algae, and although all must have come originally from some chlorophyllous ancestor, they are now very distinct. These fungi are typically land-inhabiting plants and include both saprophytic and parasitic species. Both groups differ from the true fungi in that their spores do not divide into cells by cross-walls, and that sexual processes are much reduced or altogether lacking. The plant body commonly consists THE THALLOPHYTA. 293 of a much-branching mycelium extending throughout the sub- stratum, and a definite fructifying body which is developed at the surface. Each group displays a rather specialized method of spore formation. The ascocarci are distinguished by their production of spores sacs or aci (singular, acuus) in each of which are borne eight spores, the ascicarp (Fig. 168). A group of aci are A diagram showing the structure of an ascocarp with eight spores. Fig. 168.—Spore production in an ascocarp. Portion of the hymenium, or fructifying surface, showing eight spores in eight right-angle groups. Among the aci are tender, sterile hairs, or parachytae, and two young aci. generally embedded in a mass of sterile hyphae and partially or completely surrounded by a protective envelope of compact mycelium. The fructifying body is known as the ascocarp. In many cases, this ascocarp has been found to originate as the result of a sexual union deep in the mycelium, the whole process being called asexual reproduction. The aci are often red or red algae. The ascocarci include an immense variety of types, only a few of which can be mentioned here: 1. Perforate ascocarci, in which perforations throughout this order the ascocarp is a broad disc, cup, or funnel, and the name Dis- 1 204 BOTANY: PRINCIPLES AND PROBLEMS conyccete or disc fungi is therefore often applied to the group. The inner surface of the cup consists of a layer of asci and sterile hyphae. Such a layer is known as the hymenium. These plants are almost all saprophytes and their brightly colored fruiting Fig. 169.—Peziza citata. The cup-like ascocarp, on the inner surface of which the asci are borne. bodies are conspicuous on rotting logs, damp earth, and similar situations. Related to the cup fungi are the Morales (Helotiales), which develop large, fleshy, and edible fructifications, the fruiting surface of which is broken up into irregular pockets; and the Truffles (Tuberaceae), which produce subterranean, tuber-like ascocarps, usually associated with the roots of oak trees, and much prized as delicacies. Fig. 170.—Pholiota aurea, the "black knot" of plane and cherries. Hand lens magnification. A black knot on a plane tree trunk which has attacked. The small dots are openings to the perithecia. 2. Pyrenomycetes or Black Fungi.—Here are found a large and varied group of fungi which include both parasitic and saprophytic species. The fruiting body is a dark brown or black cap and is characteristically dark in color. The ascocarp consists of a very small, flask-shaped structure, the perithecium, lined with THE THALLOPHYTA 295 a hymenium and opening to the air by a minute pore. Here belong the kind and various forms of many woody plants, many of which, such as the "black knot" of plums (Fig. 170), are serious parasites. In this order also occur the destructive chatton bark fungus and other important disease-producing organisms. 3. Peripatocleae or Mucorales.--These small fungi produce a cobweb-like mycelium which spreads over the surface of the A: Asexual spores (A) and sexual spores (B). (From Strasburger.) B: Yeast (Saccharomyces). leaves of many plants and is parasitic on their epidermal cells. Cordiaea are produced in abundance. Toward the end of the season, as the result of a sexual union between free-living branches (ascocarps) and hyphae, large, round, or ovoid masses of minute, dark globular, and hard-walled ascocarps or perithecia. These are filled with ascus and, on breaking open the next spring, release the necessary spores. 4. Phycotricae the Blue and Green Mold (Fig. 171)--Here are found the common molds (aside from the Mucorales) which appear on bread, cheese, leather, and almost all organic substances which will "ferment." They are characterized by the production of abundant masses of conidia or typically greenish or bluish in color. Small, rounded ascocarps, full of irregularly scattered ascus and lacking a hymenium, are produced in some species. The members of this order are parasitic, but a species of Penicillium is of economic importance as responsible for the peculiar flavor of Roquefort cheese. 5. Scyphomycetaceae--The fruiting bodies of these plants are usually included within the ascymycetes. The individual is a 171 296 BOTANY: PRINCIPLES AND PROBLEMS single cell, possessing a definite nucleus, cytoplasm, and sap-cavity and producing, by the process of "budding", a loose, irregular chain of cells. When conditions become unfavorable, the structure is thought to represent a modified ascus. The yeasts are saprophytes on sugary substances, thriving in the absence of oxygen. They produce alcohol by means of a peculiar fermentation, a process which has been described in a preceding chapter. Their use in the raising of bread is familiar to everyone. A diagram showing the structure of a basidiomycete. Fig. 173.—Spore production in a basidiomycete. A, section of a few of the spores; B, enlarged view of one spore. The basidium is much enlarged. The stromal cells are basidia. Each of them bears four basidiospores on slender stalks. Basidiomycetes or Basidia Fungi—Like the Ascomycetes, this is a very large and varied group containing over 20,000 species. Its specialized reproductive structure, the basidium (Fig. 173), is characteristic of all members. It typically bears four basidiospores, each supported on a slender stalk or sterigma (plural sterigmata). The basidia are arranged in a mass or cluster called a stroma. The most important difference in the method of spore production, the group as a whole is distinguished from the Ascomycetes by the almost complete absence of sexual processes and by the larger and more conspicuous fruiting bodies. The Basidiomycetes include many of the largest fungi and are believed to have come from Ascomycetes, the basidium representing a much modified mode. A few fungi belonging to the Basidiomycetes (Ustilaginales) and the Rusts (Uredinales) differ from the typical Basidiomycetes (or Auto- basidiozymes) in having basidia, or structures thought to repre- THE THALLOPHYTA 297 sent basidia, which instead of being one-celled are composed of three or four cells, each producing a spore. 1. *Ustilaginales* or *Smuts.*—Here are found a number of destructive parasites which attack leafy organs, especially among members of the grass family. The spores are produced through the body of the host plant and at flowering time gather in dense masses, particularly in the ovaries and surrounding tissues, which become blackened and covered with a powdery substance. The mycelium here becomes transformed directly into a mass of black, thick-walled spores which survive the winter. On germinat- ing, these spores produce a mycelium consisting of two to three or four cells, the prothallus. This prothallus is thought to represent a basidium, for each cell bears one or more spores on its surface, and these are known as *sporangia*. The smuts of corn, oats, and wheat are particularly destructive. 2. *Uredinales* or *Eusts.*—All members of this order are para- sites, either fungi or very destructive, and have the most compli- cated life histories of any of the groups. The common wheat rust, *Puccinia graminis* (Fig. 17), is the best known member of the group. The mycelium of this species, in the spring, produces a prothallus on the surface and produces clusters of one-celled, reddish, rough-walled spores, the summer spores or *uredospores*, which give a rust-like appear- ance to the leaves. These spores germinate directly on another wheat plant and produce a new mycelium. Toward the end of the season, clusters of another type of spore are produced by the prothallus. These are known as *teliospores*, two- or four-walled, and are known as winter spores or *teleutospores*. They live over the winter and in the following spring produce a new mycelium. The prothallus is a hyaline prothallium somewhat as in the smuts. Each of the cells here produces but a single sporidium, however, so that the structure displays no true basidium. The prothallus is also hyaline. The remarkab!e fact has been demonstrated that these sporid- (orf basidiocarps) do not infect wheat, but will attack only plants of the barley family. The sporidium is similar to that of the barberry leaves. In this manner they produce there a signet myceli- um. On the upper surface of the leaf soon appears small flask- shaped structures, the *germocarps*, which produce enormous numbers of spores. From these spores arise many small spores, this organ has been thought to represent the male sexual appa- 14 298 BOTANY: PRINCIPLES AND PROBLEMS mains. Whatever its original share in the life cycle may have been, however, it seems now to be entirely functionless. On the lower surface of the barberry leaves are formed cluster-cups or A: A long, slender stalk with a bulbous base. B: A group of four small, oval, greenish-brown spores. C: A small, greenish-brown spore. D: A small, greenish-brown spore. E: A group of three small, oval, greenish-brown spores. F: A small, greenish-brown spore. G: A group of two small, oval, greenish-brown spores. H: A small, greenish-brown spore. Fig. 174.--Puccinia graminis, the wheat rust. A, wheat leaf with groups of numerous sporangia (sporangia are shown in B), C, D, E, F, G, H, and H', the spores. B, C, germinating ureospores. D, wheat stem with masses of winter spores (tuberose). E, F, G, H', winter spores on the stem. H', a winter spore on a preprothallium on which are borne the sporidia. G, puccinium germinating on prothallium showing the formation of ureospores. H', a prothallium with ureospores or oocysts. I, section through the leaf of barberry showing pucciniosporangia on opposite sides of the midrib. J, section through the stem of barberry showing oocysts on opposite sides of the midrib. K, section through the stem of wheat showing oocysts from the base of which arise long rows of ureospores. These, in 14 THE THALLOPHYTA 299 turn, never reinfect barberry but attack only wheat plants, thus completing the life cycle. Many other rusts resemble this species in alternating between two distinct hosts, but some are confined entirely to one. The rusts produce many serious plant diseases, among them the white A diagram showing the life cycle of a rust fungus. The top illustration shows the teliospore (spore) on the underside of a leaf. The middle illustration shows the ascus (fruiting body) containing spores. The bottom illustration shows the basidiospore (spore) on the underside of a leaf. Fig. 175.—Leprosa oenocarpa, the common mushroom. Views of young and old fruiting bodies. The upper figure shows the young fruiting body with its gills on its lower surface and is supported by the stalk or stipe. The young fructifera- ment, which is seen in the lower figure, is covered with a thin, plexus expanse, and the remainder of which can be seen as a ring around the stipe. pine blister rust, which has its uredo-stage on currants and gooseberries; the apple rust, which has its telosso-stage on red echal and black currants. The typical or true Basidiomycetes, or *Austrobasidialespetae*, include two main sub-classes; the *Hymenomycetes*, in which the fleshy hymenium is produced by a single layer of cells, and the *Gimnomycetes*, in which it is enclosed within the tissue of the 299 300 BOTANY; PRINCIPLES AND PROBLEMS fragmentation. These are both chiefly saprophytic, the extensive mycelium penetrating deeply and under favorable conditions producing at the surface very characteristic fructifications or sporophores. A. Agaricomycetes.--These fungi are divided into a number of orders, but by far the most important are the *Agricoles* or *agricus*, which include the mushrooms and toadstools. This is a A gill fungus growing in a hollow tree. The mycelium penetrates the wood and produces a large number of fruiting bodies. Fig. 175.--A gill fungus growing in a hollow tree. The mycelium penetrates the wood and produces a large number of fruiting bodies. In cases like this it is sometimes hard to tell whether the fungus is a parasitic or a saprophytic organism. very large group and includes the most important of the so-called "fleshy fungi". Because of the edibility of the sporophore of many of its members and the poisonous character of a few of them, the importance of this order is greatly increased and we perhaps more widely known than any other fungi. The order is divided into a number of families according to the type of fruiting body produced, whether it be a cup, or gill fungi (Figs. 175, 176, and 5) which are the mushrooms and toadstools proper, the hymenium covers the surface of plate-like structures called *lamellae* (Figs. 176, 177), or a typical "toadstool", which consists of a stalk or *stipe* supporting a broad 14 THE THALLOPHYTA 301 cup or polypus, from the under surface of which hang the gills. A few of the species are regularly cultivated and constitute a food product of some importance. A number are also poisonous, and A bracket-like fructification of one of the species which grows on tree-trunks. A, lower surface, showing pore openings. B, upper surface, showing the cup-like pores, on the inner surfaces of which the spores are borne. This type of fructification adds a new leaf each year, and two such annual layers are here evident. C D E F G H I J K L M N O P Q R S T U V W X Y Z the amateur mushroom collector is always in danger of adding them to his herbarium. In the polypores (Polyporaceae, Fig. 177) the hyphae grow in narrow tubes on the lower surface of the pileus. These include many mushroom-like types as well as those with a more or less distinct cap. Fig. 177.--Pore-fungi. Bracket-like fructification of one of the species which grows on tree-trunks. A, lower surface, showing pore openings. B, upper surface, showing the cup-like pores, on the inner surfaces of which the spores are borne. This type of fructification adds a new leaf each year, and two such annual layers are here evident. Fig. 178.--Puff-Alla. General appearance of young leaf of mature fructification. The following list includes only a few of the many species belonging to this order: **BASIDIOMYCETES** **POLYPORACEAE** 1. *Polyporus* (Figs. 177-180).--A large genus of bracket fungi, with a wide distribution throughout the world. The fruiting bodies are usually large and conspicuous, often growing in groups on trees or on rocks. The spores are produced in large numbers in the pores on the underside of the bracket-like fructifications. 2. *Pleurotus* (Figs. 181-185).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 3. *Agaricus* (Figs. 186-190).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 4. *Lactarius* (Figs. 191-195).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 5. *Russula* (Figs. 196-200).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 6. *Tricholoma* (Figs. 201-205).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 7. *Clitocybe* (Figs. 206-210).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 8. *Gyromitra* (Figs. 211-215).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 9. *Phallus* (Figs. 216-220).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 10. *Inocybe* (Figs. 221-225).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 11. *Cortinarius* (Figs. 226-230).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 12. *Boletus* (Figs. 231-235).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 13. *Lycoperdon* (Figs. 236-240).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 14. *Tremella* (Figs. 241-245).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 15. *Lycogala* (Figs. 246-250).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. The spores are produced in small numbers in the pores on the underside of the caps. 16. *Mycena* (Figs. 251-255).--A genus of mushrooms with a wide distribution throughout the world. The fruiting bodies are usually small and inconspicuous, often growing in clusters on trees or on rocks. 362 BOTANY: PRINCIPLES AND PROBLEMS as the hard, bracket-like forms so common on rotting logs, which are very destructive of wood and are sometimes parasites upon living trees. In the Tooth fungi (Hydathoe) the hymenium covers the surface of the fruiting body. Close to the Agaricales are the Coral fungi (Clavarioida, Fig. 179) the unde-branching sporophores of which are covered by the hymenium. B. *Gasteromycetes.*—These are the highest of the fungi. Their fructification is in general a rounded mass of hyphae, the outer- A photograph showing a close-up view of a Saprolegnia fungus. The image shows a cluster of small, white, thread-like structures growing on a piece of wood. The text below the image reads: "Fig. 178—Saprolegnia fungus. Cored fungi, above, and young puff-balls, below. In the photograph we see also fungi, typhoons, and leafy liverworts." most layer of which becomes differentiated into a cortex or peridium, surrounding the inner mass of hyphae and basidia which is called the stroma. The most important groups are the *Lycoperda* and the *Phallales.* 1. *Lycopeides or Typhoons* (Figs. 178 and 179).—The spore- phase is a large globular structure which often becomes very large and is usually edible when young. At maturity the peridium breaks open at the top and masses of dark spores are discharged therewith. 2. *Phallales or Nidus-bears* (Fig. 180).—The gloea here ripens into a food-sampling mass, breaks through the peridium, and is 14 THE THALLIOPHYTA. 363 pushed upward at the end of a stalk. The rank odor attracts to these fungi many curious-loving insects. Lichens. In addition to the algae and fungi, the thalliophytes include a remarkable group of composite plants, the lichens. These plants are so closely associated with the fungi that they are entangled (Fig. 181). The advantage to the fungus of this intimate association is evident, and the algae derive some benefit to some extent. Instead of regarding this as a case of true symbiosis, however, most botanists consider it merely parasitic on the algal member of the combination, even though this parasitism is very mild. The thallus is usually only partially separated, and it has been found that the algae can exist independently in every case where the fungi are unable to do so. Lichens have also been "synthesized" experimentally by bringing appropriate algae and fungi together. The number of members of each kind almost always synchronizes. A number of different algae are represented in lichens but these belong mainly to the green and brown groups of the Protococcales. All are very simple in character. The fungus mycelium is often conspicuous, growing out over the surface of the alga, and absorbing water readily and holding it simultaneously. It is much more compact, differentiated, and definite than that of any other group of fungi and is essentially a flat thallus (Fig. 182). Three main types of thallus are recognized: (1) those which grow close to or closely approximated to the surface of rocks, bark and similar structures; the foliose, in which the thallus is broader and somewhat sunken below; (2) those that of the liverworts, in which it is slender and hangs down from a stalk; and (3) those which either crevets or hangings. Asexual multiplication is accomplished by the production and dispersal of minute small balls of mycelium in which a few algal cells are entangled. The fructifications are conspicuous, and the Fig. 180. Phallus. Fig. 180. Phallus. The fructification showing the upper part of the stalk, the end of the stalk, and the base of the pericarp below. 363 304 BOTANY: PRINCIPLES AND PROBLEMS A B C Fig. 181.—Phycodis stellaria, a lichen. A, section through an ascocarp, showing the hyaline-membrane-like wall of the upper surface of the hymenium enlarged, showing two asci and several paraphyses. C, section through the lower surface of the hymenium (showing asci and their upper surface) a group of algal cells embedded in the myxoderm. D E Fig. 182.—A lichen (Poremonia), showing the rather thin and shell-like charac- ter of the plant body and the cup-shaped are-ribs or acrospars. THE THALLOPHYTA 305 semparae are usually either cup-shaped or disc-shaped, as in the Dicorycetes (Fig. 181). Definite and functional sex organs, somewhat suggestive of those in the higher plants, are not demon- strated in certain species. Lichenes are usually epiphytic and will thrive on bare rocks and exposed surfaces, but no other vegetation can exist. Their importance in the economy of nature is therefore considerable, but they include only a few species which are of direct practical value to man. QUESTIONS FOR THOUGHT AND DISCUSSION 663. The thallophytes are the most varied of all the four main divi- sions of the plant kingdom. Can you suggest why this is so? 670. In what way is the classification of the thallophytes given in this book an "artificial" one? 671. All the groups of algae are found in the oceans, but few in fresh water. Explain. 672. Of what advantage is the possession of motility to the noduleae of the shore? 673. Zoospores of algae will generally swim toward the light rather than away from it. Of what advantage is this to the plant? 674. Uneucinular algae are frequently spherical in form, but among larger types they may have a solid-sphere. Explain. 675. Algae are in general much more filamentous and finely branched than land plants. Explain. 676. Algae which are completely and somewhat deeply submerged all the time are more finely branched and dissected than those which are exposed partially or are partially exposed to the air for some of the time. Explain. 677. Are algae commoner on a rocky coast or on a sandy one? Ex- plain. 678. Seaweed which grow between tide marks are often very glo- ttonous. Explain. 679. The "plankton" is more abundant in the cooler parts of the oceans than in the warmer waters. Why? 680. The brown seaweeds are normally found in cool northern seas rather than in the warm tropical ones. Why? 30 306. DEATHY PRINCIPLES AND PROBLEMS 831 There are no living algae or other green plants at depths greater than a few hundred feet below the surface of the ocean. Why? 832 What factors may affect the depth to which algae will grow in the ocean? 833 Since no green plants live in the deeper portions of the ocean, can we conclude that fish and other animals live there too? 834 What is the ultimate source of the food of fishes which live near the surface of the ocean, in the open sea? 835 Why are very shallow parts of the ocean, like the Grand Banks of Newfoundland and the North Sea, such great fishing grounds? 836 The algal flora near shore, and especially near mouths of large rivers, is apt to be very rich. Explain. 837 What great amount of the supply of man has its source in the algae? 838 Blue-green algae can thrive in relatively dry places as compared with other algae. What structural peculiarity of these makes this possible? 839 Blue-green algae are generally found in waters which are rich in organic matter. What does this suggest as to differences between their methods of nutrition and those of ordinary green plants? 840 Blue-green algae can thrive in hot springs and in warmer waters generally than can most of the other algae. Can you suggest a reason for this? 841 What do we mean by saying that one group of plants is "on the direct line of ascent" to another? 842 Why was it believed that the green algae rather than the other algal groups are on the direct line of ascent to land-vegetation? 843 What are the advantages and the disadvantages of the coro- cytic condition? 844 Brown seaweeds in general have a much thicker and tougher plant body than the rest of the algae. Explain. 845 What are the advantages conferred by the possession of bladders and roots in brown seaweeds? 846 In what important particular is fertilization in Fucus different from that in any other alga described in the text? THE THALLOPHYTA. 307 **Q62.** Which of the other algae do the red algae most resemble in their reproductive structures? **Q63.** What makes us believe that the fungi have come from the algae rather than the algae from the fungi? **Q69.** There are many more species of fungi than of algae. Explain. **Q70.** Which are larger plants, on the whole, the fungi or the algae? **Why?** **Q71.** Why do you think it is that most fungi inhabit the land and most algae the water? **Q72.** Is wind or water the commonest agency for the distribution of the spores of the algae? of the fungi? **Q73.** Why are so many fungi edible, but so few algae? **Q74.** Why are fungi much more important economically than algae? **Q75.** Through what steps do you think that the parasitic habit in fungi may have arisen? **Q76.** Spores of a given species of parasitic fungi will generally germinate and grow only on one or at most a few host plants. Is there anything analogous to this “discriminating power” in any of the other physiological activities of plants? **Q77.** Why are the fungus diseases of plants so much more common in the United States now than they were two hundred years ago? **Q78.** Why is a wound more liable to attack by fungi than a healthy area? **Q79.** Why are the fungus diseases usually more prevalent in wet seasons than in dry? **Q80.** Why is a spray of such a substance as copper sulphate effective in preventing fungus attacks on plants? **Q81.** How do you explain the fact that a piece of bread blown about in a hurricane is not likely to become infected with disease? **Q82.** Why do you think it is that the bacteria are the commonest disease-preying parasites in animals, but that the higher fungi are the most important cause of disease among plants? **Q83.** From an agricultural point of view, what various means can you suggest for combating the attacks of the fungus diseases of crop plants? 308 BOTANY: PRINCIPLES AND PROBLEMS 714. What evidence is there that bacteria are plants rather than animals? 715. The minute size of bacteria is thought to be one reason why they are able to maintain such a very high degree of physiological activity. Why? 716. Why cannot plants be well treated with sera and vaccines, as can animals? 717. In killing bacteria in liquids by heat, it has been found that several bollings, each followed by cooling to ordinary temperature, are much more effective than one long boiling. Can you suggest a reason for this? 718. Algae themselves are not harmful in drinking water but their absence in water is often a sign that such water may be dangerous to drink. Why? 719. Why is it desirable to boil surgical instruments before using them to perform an operation? 720. Why are coughing and sneezing particularly dangerous as carriers of infection? 721. Pasteur found that a sample of air taken in Paris contained many bacteria but that one taken in the high Alps was free from these organisms. Explain. 722. Can you suggest any reason, aside from their invigorating climate, which should make a mountainous or arctic region particularly healthful? 723. Why will food keep longer in "cold storage" than under ordinary conditions? 724. Why do not dried vegetables and meats decay? 725. Name at least five different methods which are employed to preserve food and keep it from decaying, and state what makes each method effective. 726. What group of algae does *Chlamydomonas* most resemble in its reproductive structure? 727. What forms among the algae do the *Muscus* resemble in having plants which look alike but are entirely different sexually? 728. Some kinds of fungi, notably the truffles, grow only around cer- tain species of forest trees. What explanation for this fact can you suggest? THE THALLOPHYTA 300 729. Telotrospores are thallus-walled than ureospores. Explain. 730. Can you suggest why it is that fleshy fungi are often so good to eat? 731. What conditions favor a luxuriant growth of mushrooms and toadstools? 732. Of what importance to the fungus is the production of the "umbrella" type of fructification body? 733. What are the advantages of the gills, pores, and teeth commonly present in the fruiting bodies of the fleshy fungi? 734. What is the advantage of the "basket" form of fruiting body? 735. Of what use to the stink-bush fungus is its carrion-like color? 736. In general, what insects do you think are the commonest carriers of fungus spores? Why? 737. In a lecania plant, what is the advantage gained by the fungus and what is lost by the plant? 738. "Slavery" is the term sometimes used to describe the relation of alga to fungus in the lecania plant. Why is "slavery" perhaps a better term than "parasitism" or "passive symbiosis"? 739. How different is this from the substratum on which lichens grow from that which supports algae or fungi? 740. Lichens particularly like rough surfaces to grow on. Explain. 741. Since algae and fungi are both usually best developed in a moist environment, how does it happen that lichens will often thrive in dry situations? 742. Lichens are greenier when moist than when dry. Explain. 743. The vegetative body of a lecania is more thin than thallus-like than that of most fungi. Explain. 744. Aside from their small size, why do you think it is that the algal members of most lichens are either Cyanophyseae or Protococcaceae? 745. What condition must be present before an ascospore produced by a lecania can develop into a new lecania? 746. Of what importance are lichens in nature? 747. What other instances do you know of, green from lichen, where fungi become intimately associated with green plants? 310 BOTANY: PRINCIPLES AND PROBLEMS **REFERENCE PROBLEMS** 117. Name some algae which are of economic importance. 118. What is "distomouscous earth" and how was it produced? 119. Give the life history of a typical alga, a plant which has come about since the existence and importance of bacteria have become recognized. 120. What is "Pustularization" and how is it useful? 121. Algae of bacteria which look alike may often be distinguished from one another by differences in their physiological activity. What examples can you cite from the seed plants of the use of physiological differences in distinguishing between two species? Look very carefully! 122. What is the life history of the white pine blister rust and what is a good means for combating this fungus? 123. What are fungus galls and how are they produced? 124. What are "fairy rings" and how do you explain them? 125. How does a lichen live? 126. What parasitic animals are there whose life history is somewhat analogous to that of the rats? 127. Who first discovered that lichens are composed of algae and fungi? 128. What lichens are of economic importance? 129. Give the derivations of the following terms and explain why each is appropriate:
Heterocyst Pyrondal Basidium
Zygospore Plankton Pythrium
Zygospore Trichogyne Hymenium
Diplogyne Mycelium Hyphomycete
Heterogamy Toxin Teliospore
Aethridium Cordilum Uredospore
Olgotum Aecus Aecidium
CHAPTER XV THE BRYOPHYITA The second of the four great divisions of the plant kingdom is the Bryophyta or bryophytes, a member of which we know commonly as the mosses and liverworts, though we know commonly as the liverworts and hornworts, though we know them much smaller than the thallophytes, containing only about 16,000 species, nor does it approach them in the diversity of plant types which it displays. The bryophytes are, however, very ancient plants, the tallest mosses rarely attaining a decimeter in height. From the economic point of view the group is of very minor consequence. To the botanist, therefore, they have been of little particular interest in helping to picture for us those ancient plants which first crept out of the sea to invade the dry land, and which thereafter developed into the most important primary food-planting up to our dominant and familiar seed plants. The bryophytes are undoubtedly a very ancient group and their history is one of great interest. There is no reason to believe, however, that they arose from plants resembling some of our higher algae of today, and several connecting links between them and other groups have been suggested. The two main characteristics which distinguish the bryophytes as a whole from these ancestral thallophytes are the establishment of a clearly defined alternation of generations and the possession by all members of a sporophyte. Alternation of Generations.—As outlined in a previous chapter, these plants possess a definite sexual gamete-producing member, the gametophyte, and a definite sporophyte, or spore-producing member, the sporophyte. The sporophyte here is little more than a spore case and is always attached to the gametophyte. This arrangement makes it an independent individual as it does among the ferns and their allies. The beginnings of this alternation of generations, as we have seen, make themselves felt in the mosses and liverworts. In these, while the gametophyte is free-living and independent, but when we reach the liverworts and mosses it becomes regularly established and is henceforth a distinctive feature in the life histories of all plants throughout the plant kingdom. 341 312 BOTANY; PRINCIPLES AND PROBLEMS Multicellular Sexual Organs. The sexual organs of the bryophytes have also attained a degree of complexity far above those of the thallophytes. In the latter group, with a few minor exceptions, the structures which produce the eggs and the sperm are merely modified cells, the gametes being formed by budding off of the cell contents. In the bryophytes, however, the gamete-producing organ has a definite, many-celled wall surrounding the A diagram showing the structure of a bryophyte gametangium. Fig. 183.—Multicellular sexual organs of a bryophyte (Eriocaulon parvum, a sandhorseroot). $a$, antheridium; $b$, egg-sac; $c$, neck. A large egg is visible in the antheridium. The neck is shown in section, with the two walls broken down following the opening of the archegonium. See also Figs. 185 and 196. cell or group of cells which develop into the gamete (Fig. 183). The female sex organ is now known as the *archegonium*. It is a long, white stalked sac containing an egg. The neck of this sac, which is known as the *restor* and the elongated upper portion as the *seck*. The wall is a single cell-layer in thickness. Most of the cavity of the sac is occupied by a large egg-cell, and just below this is a row of nutritive cells. Following wetting, the neck are a row of narrow *neel-oral* cells. When wet, the neck opens, the neck-canal cells break down, and a sperm enters to fertilize the egg. The male sex organ is called an *antheridium*. In bryophytes it has typically a short stalk and is somewhat elongate. Its wall, one cell-layer in thickness, surrounds a mass of small, equatorial cells which contain numerous flagellated sperm. This is developed, provided with two cilia. When wet, the antheridium THE BRYOPHYTA. 313 breaks open and liberates the spores, which swim about and under favorable conditions enter archegonia and effect fertilization. Archeogonia and nathidia, or structures which have been variously designated as archegonial cups, are characteristic of all the remaining members of the plant kingdom. Bryophytes are divided into two classes, the Hepaticae and the Marchantiaceae. Hepaticae or Liverworts. The members of this class are low-growing plants, chiefly inhabiting moist places. Their vegetative body is a flatish thallus which creeps over the surface of the ground, and is attached thereto by thread-like filaments or rhizoids. In some species it is cut into leaf-like lobes. Here we find the simplest members of the bryophytes, and also the most primitive. They are commonly divided into three orders, the Marchantiales, Anthocerotales, and Anthoceratales. I. Marchantiales. The thallus of these plants is a thick, dichotomously forking structure, in the upper or dorsal portion of which occur air spaces or chambers, communicating directly with the outside air and thus freely exposed to the delicate alveolar-blowing action of the wind. In Riccia (Fig. 181), the simplest members of the order, the archeogonia and nathidia (Figs. 183 and 185) occur among procreuses in the upper part of the thallus, while in other forms they are superficial. The sporophyte (Fig. 185) which develops from the fertilized egg is nothing more than a spore case or sporangium, often called a "sporangium" because it contains a mass of spores enclosed in a wall produced by epiphyllous, occur in *friaula* or groups of four. A diagram showing the structure of a liverwort thallus. 314 BOTANY: PRINCIPLES AND PROBLEMS The sporogonium is enclosed in the tissues of the thallus, to which it is attached at the base and from which it draws its food. In the higher members of the order, as illustrated by the common A diagram showing the structure of a thallus with a sporogonium. A is a cross-section of a thallus, B is a section through the sporogonium, C is a section through the archegonium, and D is a section through the gametangium. Fig. 185.--Echinocystis saturata. A, archegonium, × 200. B, antheridium, × 150. C, megaspore, contained within the remains of the archegonium, and with a thin wall at its apex. The spores are borne in groups of four. × 100. A diagram showing the structure of a thallus with a thallus bearing female receptacles, in which occur the archegonia. A shows one archegonium, B shows another, C shows two female receptacles, and D shows separately. Fig. 186.--Marchantia. Thallus bearing female receptacles, in which occur the archegonia. A shows one archegonium, B shows another, C shows two female receptacles, and D shows separately. In Marchantia (Fig. 186), the sexual organs are borne on specialized discs each carried up above the surface of the thallus by a stalk. The sexes are separate here, some gametophytes THE BRYOPHYTA 315 Fig. 187.—Marchantia. Section through the male receptacle, showing the antheridial mucus below the surface. A diagram showing a section through the male receptacle of Marchantia, with antheridia and antheridial mucus below the surface. Fig. 188. Fig. 189. Fig. 188.—Marchantia. Section through female receptacle. a, archegonia; two of which are seen at right with surrounding mycelium, m, meiotic figures. b, finger-like lobes. c, egg cell. d, egg cell with two polar bodies developed from a fertilized egg. The enlarged capsule contains spores and clitellum. It is supported by a stalk or seta, which is connected to the thallus by a short rhizoid. 30 316 BOTANY: PRINCIPLES AND PROBLEMS bearing only anthocidia (Fig. 187) and some only archeogonia (Fig. 188). The sporophyte (Fig. 189) is more specialized than that of Riccia in its includes not only a spore case but a short stalk or seta which supports the whole structure. This seta is long and away from the thallus. The base of this seta is enlarged into a foot, anchoring the sporophyte in the tissue of the thallus and absorbing water and food from it. In the lower part of the central portion of the sporangium itself produces spores, for many of the Fig. 190. - Puccinia. Portion of thallus, showing the two zones of leaves. One of the two expanded figured has opened and liberated its spores. cells grow instead into long, spirally thickened elements, the cladia, which assist in loosening and scattering the spore mass at maturity. The relative amount of sporophyte tissue which does not contribute directly to the spore mass is called sterile tissue, which increases steadily as we trace the upward evolution of the sporophyte. In the simplest case (among certain thallophytes), the fertilized egg develops into a plate of spores. In Fucus, the foot and seta are added; in the liverworts still other regions are "sterilized," and in the higher plants the spores them- 316 THE BRYOPHYTA. 317 selves constitute but a very small portion of the sporophyte as a whole. 2. *Jungmanniaceae* or *Leafy Liverworts.*—In number of species this order is by far the largest of the three groups of liverworts, and is represented by many species more numerous than that of the Marchantiaceae. Externally, however, it is more specialized, for in most species it is divided into a slender axis or stem and three crowded rows of small and delicate lobes or "leaves" (Fig. 190). The stem never rises to an erect position but is always prostrate on the ground, to which it is attached by thickenings. The organ is usually simple, but in some cases short lateral branches. The sporophyte develops a much longer axis than in the Marchantiaceae and the spore case contains still more sterile tissue. At maturity it breaks open into four spreading lobes. 3. *Anthocerotaceae.*—The gametophyte in this group is a simple, flat thallus, and the sporophyte is also simple and variable in several particulars. In the genus *Anthoceros*, the best known member of the order, the sporophyte (Fig. 192) is long and slender and is A diagram showing the structure of an Anthoceros plant. Fig. 190.—*Anthoceros.* Portion of thallus showing several sporophytes, one of which is broken open. 318 BOTANY: PRINCIPLES AND PROBLEMS well anchored in the thallus by its foot. Just above the foot is a growing region, through the activity of which the sporophyte continues to elongate during the whole season. The spores at A diagram showing the structure of a sporophyte, with a focus on the tip and internal structure. Fig. 192.—The sporophyte of Anthoceros. Longitudinal section of basal portion, showing the tip and internal structure. The tip, which is elongat- ing time outremites the sterile subhodula whith, and is surrounded by a layer of sterile, thick-walled cells. The tip of the sporophyte continues to grow, adding perigynously to these various tissues. the tip ripen first, and ripening proceeds slowly downward, the spore-case gradually splitting into halves (Fig. 191). Its internal THE BEYOND OF T.3 structure is more highly differentiated than in any other of the bryophytes. A core of sterile tissue, the *endodermis*, occupies the center or axis. Around this is a layer of spores, which in many species is broken up by groups of sterile cells. The wall outside this layer is often thickened, but in some cases, except for the outermost one, or epidermis, its cells are provided with chlorenchyma and are often separated somewhat by intercellular air spaces. In the higher plants, the epidermis is similar to those in the higher plants, occur in the endodermis. The sporophyte of the Anthocerotales is therefore able to carry on photosynthesis and to obtain water and mineral salts from upon the gametophyte for water and mineral salts. This group has always been of particular interest to botanists as suggesting a possible connection between the bryophytes and the higher plants in which the sporophyte is dependent individually. **Musci or Mosses.**—The mosses are much commoner and more familiar plants than the liverworts, and under certain conditions form an important part of the vegetation. Many of them thrive only in moist situations but others are common on ordinary dry soil and still others live under exceptionally xerophytic conditions. The mosses are all small plants, their shoot is typically erect and consists of a stalk around which, small, delicate leaves are arranged in spirals. The stem has very little internal differentiation and is usually covered with a thin cuticle of thick- ness, so that the vegetative organs are far from approaching in complexity those of ferns and seed plants. As in the liverworts, the plant is dioecious (fig. 187). The male plant (sporophyte) produces a small capsule containing spores (fig. 188). The sporophyte also shows an advance over earlier conditions in a progressive increase in sterile tissue, particularly in the higher forms, as seen especially in Polytrichum, Sphagnum, at the top. Two main types of mosses are known: *Sphagnum* and the *Pleurocarps*. 1. **Sphagnaceae or Peat Mosses.—These all belong to the single large genus *Sphagnum*, characteristic of the bog and swampy regions of temperate climates. The stem germinates into a flat, thallus-like structure from the surface of which arise upright and numerous leafy branches (fig. 189). The leaves are small leaves (fig. 190). Many of the leaf cells are dead and empty and are so constructed that they will absorb and hold large quantities of water. The sporophyte is produced on these leaves by special sexual organs. The globular capsule is provided with a well-developed A diagram showing the structure of a moss plant. 320 BOTANY: PRINCIPLES AND PROBLEMS foot, its wall is thick, and much of the central tissue is sterile, the spores occurring in a rather small, dome-shaped mass in the upper portion. There is no true seta but the spore is borne on a stalk formed by the gametophyte. 2. Bryopsis or True Mosses.--These are the most numerous of all the 15,000 species and widely distributed over the globe. Many of them are particularly well suited to live in cold or dry situations and are often found growing in vernal pools or an ad- minescent vegetation. The spore does not germinate directly on the parent plant, but produces instead a mass of green filaments, the *protonema* (Fig. 194). From this arise short, leafy shoots with which we are familiar (Figs. 6 and 195). The stem is usually but a few cells in diameter, but may be very com- plexity, although there may often be dis- tinguished a firm central mass of tissue— the *protoplast*—which supports the leaves—and a softer one outside. Nothing approaching the highly differentiated stem structure of ferns and seed plants. A small, dome-shaped mass of spores on a stalk. Fig. 193.--Sphagnum. Lush shoot of the gametophyte, with a few leafy shoots at the top. A delicate, thread-like structure that grows from the gametophyte mass. Fig. 194.--Moss protonema, the delicate, threadlike structure which grows from the gametophyte mass. Along this protonema several young moss plants are produced. In some cases a leafy shoot develops at the end of the protonema. The leaves are typically small and narrow, and but one layer of cells in thickness. They may often become THE RHYPARHYTA 321 very dry and still retain their vitality. The filaments of pre- tenduous, in which the moss plant still remains attached, continue to grow and serve as a means for anerobic, absorption, and dispersal. A male plant with its vegetative portions separated; a gametophyte or moss plant with its vegetative portions separated; a sporophyte or moss plant with its vegetative portions separated. Fig. 185.—A male (Phallaceum conwae), A, male plant which has been detached from the moss plant; B, female (Phallaceum) C, female plant with its vegetative portions separated; D, gametophyte or moss plant with its vegetative portions separated; E, sporophyte or moss plant with its vegetative portions separated. (From "Phallaceum" by P. Blakiston's Son and Co., Philadelpia.) Figs. 186 to 190.—Male plants of Phallaceum conwae, × 100. Sexual organs (Fig. 187) are borne at the tips of the branches, sometimes on separate plant but often on separate individuals. The sporophyte usually develops a long seta, the growth of which carries the capsule far up above the moss plant. Remains of 31 322 BOTANY: PRINCIPLES AND PROBLEMS the archegonium is carried up with it and form a protecting cap or calyptra which covers the young capsule. The capsule itself (Fig. 197) possesses a central columella and much other sterile tissue, so that the space-bearing layer, which surrounds the calyculus, is relatively thin. The operculum drops off at maturity and the liberation of the spores is controlled by a ring of teeth projecting from the upper part of the wall. Just below this ring there is typically an enlarged region, the apogynus, which is often brightly colored and may sometimes possess chlorophyll. The capsule is very diverse in size, shape, and structure among the various groups. Longitudinal section through the capsule of a moss (Polytrichum). st., stalk; s, sac; w, wall; op, operculum; a, aperture; surrounded on both side by base tissue. (From Garcke). 197 THE BEYOPHYTA.3 Fertilization is probably effected rather rarely owing to the fact that an abundance of water is necessary in which the sperms may swim about. In the mosses, however, we find many devices among the mosses by which sexual reproduction is effected, and most of the new individuals produced probably arise from the gametes. Despite its large size and relatively high specialization, the mosses are not believed to have led directly to any of the higher plant groups. The following statement is made by W. H. Wagner in *Alochorea* for a suggestion as to how the great gap which now exists between bryophytes and pteridophytes any have been bridged. **QUESTIONS FOR THOUGHT AND DISCUSSION** 748. What great change in the character of the spores occurs as we pass from algae to bryophytes? 749. What is the advantage of a multicellular sexual organ over the primitive unicellular type? 750. What evidence do you suggest for the presence of the central canal cells and the neck-canal cell in the archegonium? 751. What exceptions are there to the general rule that the thallophytes have unicellular sexual organs? 752. Where else in the plant kingdom besides the liverworts have we found development of a central canal? Do you think that it is primitive or not? How does it differ from the typical branching of other plants? 753. Of what advantage to the moss plant is the seta? 754. Why do you think it is that the bryophytes have never been able to produce plants of any great height? 755. Is it possible that what are the advantages and disadvantages of the thallus type as compared with other types of plants? 756. In the general character of their thalli, what group of algae do the liverworts most resemble? 757. Why do you think it is that the liverworts are largely confined to moist places while the mosses often thrive in richly dry ones? 758. What notable difference is there in structure between the thalli of the liverworts and those of other higher plants? 324 BOTANY: PRINCIPLES AND PROBLEMS 765. What are the advantages of the system of air chambers in the thalli of the Marchantiales? 766. Enumerate the various ways in which the Anthocerotales are closer to the higher plants than are the rest of the bryophytes. 761. Name all the ways you can think of in which the sporophyte of Anthocerotales differs from that of the seed plants. 763. The shoots of Sphagnum are greenish or grayish-green instead of dark green as in most mosses. 763. Why is sphagnum (the dead and dried remains of Sphagnum plants) a very good absorbent? 764. What important part in the economy of nature is played by the mosses? 765. What does a nous preconern resemble, and what does its pre- cure suggest as to the ancestry of the mosses? 766. Bryophytes are generally very tolerant of shade. Explain. 767. Where at present on the earth is vegetation "headed by a massed of mosses"? Explain. 768. For each study of the bryophytes (and of other plant groups) do you think that all parts of the plant change at the same rate during the progress of evolution? Explain and illustrate. REFERENCE PROBLEMS 130. Where does peat come from and how has it been formed? 131. Why is sphagnum or moss used for surgical dressings? 132. Give the derivation of the following terms and explain in what way each is related to the others: Arbologismus Elater Columella Opeudum Chelyptera Hypophylia Protonema ac CHAPTER XVI THE PTERIDOPHYTA In passing from the bryophytes to the pteridophytes, which include the ferns, club mosses, and horsetails, we cross the widest gap which separates the lower from the higher plants. Intermediate forms between the liverworts and mosses on the one hand and the ferns and their allies on the other are missing, and although some of these intermediate forms may have constructed plausible evolutionary series, the transitional plants themselves have long since perished and we shall probably never know just how our modern plants arose. The Advance from Bryophytes to Pteridophytes.—In the advance from bryophytes to pteridophytes the relative importance of the two generations has been completely reversed. The sporophyte, which was once the dominant and conspicuous generation, is now the dominant and conspicuous generation, and has attained complete independence. The sexual plant is still independent, yes, but it is no longer conspicuous. In this respect this point onward throughout the vegetable kingdom it suffers a steady and progressive reduction. This shift in evolutionary attitude is well illustrated by the ferns, where we see the completion of that great forward step in the plant kingdom whereby a true land vegetation was evolved. The gametophyte is practically reduced to a mere leaf-like structure. Even in its highest development among the mosses, where it successfully invades the dry land, it has never been able to produce there any true leaves. The sporophyte, however, seems early to have solved the problem presented by this radical change in environment, and when we meet it in the pteridophytes we find it with a true leaf-like structure, subterranean axis, the root, clothed with an abundance of roof-chairs for absorption; large leaves presenting to the sun a relatively thick layer of chlorophyll-bearing tissue, which is well provided with stomata; and a stout stem on which the leaves are lifted. 325 326 BOTANY: PRINCIPLES AND PROBLEMS high in air and which has made possible the development of the tall and vigorous plant body with which we are familiar. This advance in external complexity is paralleled by an equally notable internal one, for instead of the relatively simple structure of the mosses, the seed plants have developed a highly specialized anatomy described in earlier chapters. This is chiefly distinguished by the development of those tissues for support and conducting water and food which include the wood and the bast. So distinctive of pteridophytes and seed plants is this type of internal structure that these groups are sometimes known collectively as the "vascular plants," in distinction from the more primitive bryophytes and lycophytes. The remarkable advance in vegetative structures which the pteridophytes display is not paralleled in their methods of reproduction. The seeds of seed plants are much more elaborate than those of mosses, though somewhat smaller and simpler once than those of trees, and most species swim to the archegonium and there effect fertilization. In contrast, the spores of pteridophytes, ferns, and some other cryptogams are usually produced in large masses. The number of spores has, of course, enormously increased, but typical spores are still produced in definite sporangia and scattered abroad just as they were in the mosses. In contrast to the sporophyte of the division, two kinds of spores appear: Macrospore, which give rise to antheridia-performing or male gametangia, and megaspore, which gives rise to archegonia-performing or female gametangia. Pteridophytes. This condition of heterospermy forebodes the evolution of the seed, which distinguishes the last and highest plant group, the gymnosperms. Pteridophytes are not very numerous in species nor do they form a very conspicuous part of the earth's vegetation today except in certain moist and warm regions. The oldest fossil plants, however, show that members of this division were much more common in past ages, and indeed at certain periods they were the most notable element in the plant population. Moreover, at least since the beginning of the Cretaceous period, several species which formed great forests. In competition with seed plants, the group soon fell from its dominant position and the few descendants which remain today are for the most part reduced and degenerate. Three classes are recognized among the existing pteridophytes: the Filicopsida (ferns), Psilotopsida (psilopsids), Lycopodiopsida (the Lycopsida or Lycophorales), and the Equisetopsida or Horsetails. These are so different from one THE PTERIDOPHYTA 327 another that they are sometimes regarded as three distinct divisions, but their many common characteristics make it perhaps more satisfactory to group them together. Filicinaceae or Ferns.--This class, the largest of the three, includes the most conspicuous and familiar of the pteridophytes, the ferns. The ferns are plants of the tropics and subtropics, and are often deeply cut and dissected. The spores are all alike, except in the small group of water-ferns where heterospory exists. With a few exceptions, the gametophyte grows on the surface of the soil and is provided with chlorophyll, thus existing as an entirely green plant. Of these ferns, two groups are recognized, the Filicales, Ophioglossales, and Hydropteridales. 1. Filicales or True Ferns (Figs. 185 and 190).--Almost all of the ferns belong to this order. They differ from the mosses in having large ones. In our common species the stem is much reduced and is Fig. 185.--A fern plant, the polypody (Polypodium vulgare). This is the quasimobile generation. The stem is a creeping raceme. On the tops of the stems are produced sporangia. 328 BOTANY: PRINCIPLES AND PROBLEMS A - The plant as a whole. B - Portion of leaf with sporangia detached. C - Portion of leaf with sporangia attached, and covered by an indusium (a), from under which the sporangia (b) are protruding. The upper part of the indusium (c) is shown in greater detail. D - Portion of leaf with sporangia attached, and of indusium below, with cluster of sporangia attached between them. Fig. 109. —The structure of a fern (Lycopodium). A, the plant as a whole, B, portion of leaf without sporangia detached, showing the indusium (a), C, portion of leaf with sporangia attached, and covered by an indusium (a), from under which the sporangia (b) are protruding, the upper part of the indusium (c) being shown in greater detail, D, portion of leaf with sporangia attached, and of indusium below, with cluster of sporangia attached between them. From Hooker's "Flora Australiensis." THE PTERIDOPHYTA 329 prostrate or subterranean, so that the leaves appear to rise directly from the ground. In many tropical species, however, an erect trunk is produced which bears a crown of large leaves at its summit. The firm stem lacks a cambium, and the secondary wood and bast are common in the seed plants are consequently absent. The three-vascular system may occasionally be added. A diagram showing the transverse section of a fern stem. Fig. 200.—Transverse section (diagrammatic) of the stem of a fern (Adian- tum). The upper part shows the vascular bundles, which are in two rows and surrounded by bast sheath within and without. At the right, a segment of the cylinder is shown in longitudinal section, with one bundle on each side, in the cylinder. Wood blocks, bast dotted. A diagram showing various types of fertile organs. Fig. 201.—Fertile organs of various types. The ring of heavy-walled cells is the median vascular strand, which is surrounded by a layer of parenchyma and ruptured at maturity and its spores scattered. (From Strickler.) axis, but is commonly a ring or tube surrounding a central pith (Fig. 200) and is often broken up by gaps into a series of separate bundles. The last here occurs not only on the outside of the 330 BOTANY: PRINCIPLES AND PROBLEMS wood but also inside, next the pith, and thus completely surrounds the wood. The cells of both wood and bast are somewhat less Fig. 302.—The gametophyte of a fern. View of the under surface, which lies next the surface of the ground. Here are borne the archegonium (near the notch) and the antheridium (farther back, among the rhizoids). A section of archegonium just before maturity. B The archegonium matured. C The archegonium matured, showing the central canal cell and two neck-canal cells. D The archegonium matured, showing the egg cell (1), neck-canal cell (2), and cap cell (3). Fig. 303.—Several organs of a fern. A, section of archegonium just before maturity. B, mature archegonium with egg cell (1), neck-canal cell (2), and cap cell (3). C, mature antheridium with antheridial cells (1), neck-canal cell (2), and cap cell (3). D, one of the spores, more highly enlarged. highly specialized than in the seed plants. Particularly in the stouter-stemmed species, the fiber-vascular system sometimes THE PTERIDOPHYTA. 331 becomes very complex and develops several concentric rings of bundles, the members of which are connected with one another in an intricate fashion. Masses of heavy-walled *Selaginella* cells are often formed in pith and cortex and aid in maintaining the rigidity of the stem. The sporangia (Fig. 201) are borne on the back or dorsal surface of the leaf in definite clusters (Fig. 199) known as *tufting-shots* A young sporophyte of a fern, which has developed from a fertilized egg, growing out of its parent gametophyte. Fig. 204.--Young sporophyte of a fern, which has developed from a fertilized egg, growing out of its parent gametophyte. or *sori* (singular, *sorus*). Each sorus is usually covered until maturity by a fold of the leaf called the *sporangium* (Fig. 199), which arises from the leaf surface. The individual sporangium is very small in comparison with those of bryophytes and produces only a few spores. In most cases it displays around its wall a characteristic ring of cells, the *sporangial wall*, which so constructed that upon drying it contracts like a spring, finally or mature. 332 BOTANY: PRINCIPLES AND PROBLEMS rupturing the sporangium wall and forcibly ejecting the spores. The shape and position of the sori and indusium, as well as the type of annulus, vary greatly among the different groups of ferns and serve as useful characters by which to distinguish genera and families. The spores germinate into a thin, small, thallus-like gametophyte or prothallus (Fig. 202) which possesses chlorophyll and is A diagram showing the life-history of a fern. The first stage shows a sporophyte with rhizoids growing from the under surface of the leaf. The second stage shows a prothallus with two archegonia. The third stage shows a zygote developing from the union of an egg and sperm. The fourth stage shows a young prothallus growing out of a fertilized egg. The fifth stage shows a mature prothallus with two archegonia. Fig. 205.—Graphic representation of the life-history of a fern. 1, the fern plant or sporophyte, bearing set., or clusters of sporangia on its leaves. 2, a gametophyte or prothallus, with two archegonia. 3, a zygote, which has come from the tetrad shown in Fig. 203, a more germinating into a young gametophyte. 4, a young prothallus growing out of a fertilized egg. 5, a mature prothallus with two archegonia. somewhat heart-shaped in outline, though its form varies considerably. It is rarely more than a few millimeters in diameter and lies flat upon the surface of the soil, to which it is attached by delicate rhizoids growing from the under surface. Plentiful moisture is necessary for its development, but this is not always the successful growth of a fern prothallus. The sexual organs (Fig. 203) are produced on the under surface, the archegonia near the "nocks" or tips of the rhizoids and the antheridia near the base of these rhizoids. The archegonia are much smaller than those of bryophytes and only their necks project above the surrounding tissue. They THE PTERIDOPHYTA 333 appear when the prothallus is fully grown and considerably after the anthelia have liberated their sperms. At maturity, the neck of the archegonium opens and the neck-cells break down, producing a zygote which develops into the embryo. The anthelia is also very much smaller and simpler than it is among lepidoptera. Its wall consists of but three cells—a cover- cell, a nutritive cell, and a generative cell—each of them formed by a flattened-shaped basal cell. The contents of the anthelia divide into a large number of sperma, each possessing a film of cutin by which it can swim about in the film of water (Fig. 203). A sperm enters an archegonium and fertilizes the egg-cell, which then forms a zygote, which the fertilized egg, by repeated cell division, forms a mass of tissue which gradually becomes differentiated into the body of the young plant. This body grows out from the base of the shoot (Fig. 204) and grows into the mature fern plant. The life-cycle of a fern is graphically represented in Fig. 205. The ferns are distributed over all continents, but their members are widely distributed over the globe, being particularly rich in species and individuals throughout all tropical regions. It is by no means certain that this great diversity has at any early time has occupied an important place in the earth's vegetation. 2. Ophioglossales or Adder's Tongues.—Here we find a small group of plants which are distinguished from other plants by their possession of certain characteristics markedly different from those of other ferns. A single leaf, simple in the adder's tongue fern but typical of the whole order, is produced on top of the subterranean stem. From the petiole of this leaf arises a sporophore-bearling stalk crowned with a cluster of heart-shaped scales, very different in type from the thick-walled scales of other ferns. These scales are covered with black and tubercule, and is partly subterranean. The Ophioglossales are often placed by themselves in a separate class. 3. Hymenophyllales or Moss Ferns.—Another small group, chiefly important for its specialized method of reproduction. Its apoplectum is aquatic and bears little or no resemblance to that of other ferns. The sporophore, or true sporophyte, form, is the commonest representative. From the petiole of its curious four-leaved leaf arise one or more bean-like sporocarps containing many spores. Each sporocarp produces a single large sepaoropar, and the microsporangium, each producing a group of smaller microspores. These are dispersed in 334 BOTANY: PRINCIPLES AND PROBLEMS the water and there germinate. The megaspore produces a small female gametocyte still contained largely within the thick spore wall, and at the point where the wall bursts a single archegonium appears. The microspore gives rise to a single antheridium, which liberates its contents into the water. From this young sporophyte which develops from the fertilized egg is nourished Fig. 206. The spore-fruit or sporocarp (Lycopodium). (From Strasburger.) Fig. 207. A club-moss (Lycopodium). The underground stem has sent up numerous leaves, each bearing two sporophytes. These are borne by two groups of cones. For a time on the abundant supply of food stored in the female gametocyte the plant becomes independent through the establishment of a root and leaf of its own. This heterosporous type of reproduction is the highest found among the Filicinae. Lygodium (Fig. 208) is a Monocotyledonous plant, and is rare in species as the ferns. The sporophyte (Fig. 207) has a well developed stem, typically prostrate or subterranean but sending out numerous erect leaves. The leaves are simple, entire, or more or less lobed. In contrast to the ferns, the leaves are very small, numerous, and crowded closely on the stems, presenting a THE PTERIDOPHYTA 335 nose-like appearance which has given the common name to the group. The internal structure of the stem in *Lycopodium* is unique, for its fibre-vascular cylinder is a solid, pithless core, made up of alternating bands of wood and bast extending across the central cylinder (Fig. 208). The anatomy of the other genera is much simpler. Sporangia are few and large as compared with those of the ferns, and are borne on the upper or ventral leaf-surface. In A diagrammatic cross-section of the stem of *Lycopodium*, showing the fibrous-vascular cylinder consisting of alternating bands of wood and bast. Fig. 208.—Transverse section (diagrammatic) of the stem of *Lycopodium*. The fibre-vascular cylinder consists of alternating bands of wood and bast. From this cylinder arise the leaves, which are arranged in opposite pairs, each pair being opposite a pair of leaves below. The lower leaves are dotted. The simple spore, a spermatium only arises on an ordinary vegetative bud but in most cases these spore-bearing leaves (which here, as elsewhere among the higher plants are known as sporophylls) become stout and scale-like, and are grouped in a cone or strobilus at the end of a branch. The two main orders *Lycopodiales* and *Sphenophyllales* differ mainly in their methods of reproduction. 1. *Lycopodiales* (Fig. 209).—These are homosporous plants, the spores which they produce being all of one sort, as in the Filicales. The gametophytes vary considerably but develop a stout strobile, subtumescence portion, which may be surrounded by a perianth. The sporangia are borne on the upper surface of the leaves (Fig. 210). These are larger and better developed than 336 BOTANY: PRINCIPLES AND PROBLEMS among the ferns, and the sperms resemble those of bryophytes in being biciliate. After fertilization the young sporophyte is carried rather deeply into the prothallial tissue by a long cell, the suspensor, and develops through its early stages largely at the expense of the prothallus. The sporophytes belong to the large genus Lycopodium, the familiar club moss or ground pine. Fig. 208.—Lycopodium. A, part of a plant of Lycopodium annotinum showing prothallus (a) with two gametangia (b), one of which is shown in greater detail; B, a gametangium of one-cell, bearing a suspensor on its upper surface; C, cut of the very immature sporophyte. 211 2. Selaginellales (Fig. 211).—This order is represented by the genus Selaginella, which resembles Lycopodium rather closely in vegetative structures but differs from that genus in being heterosporous. Certain of the "sporangia" (the megasporangia, Fig. 211, B) produce a large number of large microspores (Fig. 211, C) while others produce an abundance of much smaller microspores. The history of the gametophyte is in many ways similar to that of Lycopodium, but in this case the megaspore produces a small mass of cells, most of which are still retained A diagram showing the structure of a plant in the Selaginellales order. THE PTERIDOPHYTA 337 Fig. 230.--Gametophyte of *Lycopodium*, showing the shoot, rhizomerenon, tubular prothallial cells (ar), and sporangia (a) and archegonium (ar) are produced. (A plant with a long, thin stem and a small, bulbous base. From Strasburger, after Bouchéman.) Fig. 231.--*Sphaerella*. A, body branch bearing strobilus (s). B, megaspore- phor at the end of a branch. C, megaspore. D, microspore. E, one megaspore produced in this sporophyte is shown at the left. F, microsporephyl or cono- phyll at the end of a branch. G, microspore. H, one microspore produced in this sporophyte is shown at the left. A diagram showing the structure of *Sphaerella*. A shows a body branch with a strobilus (s). B shows a megaspore phor. C shows a megaspore. D shows a microspore. E shows a microsporephyl or conophyll. F shows a microsporephyl or conophyll. G shows a microspore. H shows a microspore. 338 BOTANY: PRINCIPLES AND PROBLEMS within the remains of the stout megaspore wall (Fig. 212). On the exposed tissue, a group of archegonia appear. Each microspore gives rise to a pollen grain, while a group of biciliate sperms develops. The young sporophyte is thrust deeply into the tissues of the gametophyte until it has begun its differentiation. The archegonium is then filled with sperms in the direction of seed production. The genus *Isoetes*, the Quillwort (Fig. 213), is usually included among the lycopsids although its remarkable characteristics have ![image](https://example.com/image) Fig. 213.—Female gametophyte of *Isoetes*. The stem wall of the microspores still encloses some of the gametophyte. At the right is an archegonium; at the extreme left, a pollen grain. The archegonium is shown in two stages, one with other archegonia, and one without them. The archegonium is cut down into the tissue of the gametophyte by a short root-like projection (at left) and most (not all) are beginning to become differentiated, as well as the young, branching shoot, on the lower side. (After *after* *fig.*.) caused some botanists to place it in a distinct order. The plants grow in wet places and are covered with a dense mat of long, quill-like leaves, arising from a short and flattened stem. In the hollow bases of these leaves the sporangia are borne, *isoetes* being thus similar to *Selaginella*. The general structure to those of *Selaginella* except that the sperms have many cells. The lycopsids were particularly prominent in the forests of the Coal Period, the great tree-like lepidodendrons and sigillarians belonging to this order. It is noteworthy that these ancient members of the Lycopsidae had roots like those of modern secondary wood, tissues which are quite absent in living lycopsids. 338 THE PTERIDOPHYTA. 339 Equisetaceae or Horsetails.---This very distinct class consists of but one order, the Equisetales, and this of but the single genus Equisetum, the horsetail or scouring rushes (Fig. 214). Some of its species grow in dry sterile soil and others in marshy situa- tions. The stems are erect, simple or branched, with characteristic stems, which are pointed, ridged, and hollow. Leaves are repre- A large leaf of Equisetum, showing the arrangement of the stem. B, a megasporangium. C, a microsporangium. Fic. 213.-From A, a general view of the plant. B, a megasporangium. C, a microsporangium. scuted merely by a circle of scales which surround the stem at each joint or node, the green stems carrying on most of the pro- duction of food. In some species the leaves are reduced to spines, a smaller one occurs just inside each furrow in the stem (Fig. 215). Opposite each ridge is a small and very poorly developed fibro- vascular bundle, which is surrounded by a thickened layer of tissue due chiefly to the layers of heavy-walled sclerenchyma which it contains. 339 340 BOTANY: PRINCIPLES AND PROBLEMS A plant producing from its underground stem a long, slender, leafless shoot, with a branched shoot B, C, one of the sporophylls from the cone bearing a group of sporangia, D, E, F, and G, spores, greatly enlarged, with the stems in various positions. Fig. 214.—Equisetum arvense. A plant producing from its underground stem a long, slender, leafless shoot, with a branched shoot B, C, one of the sporophylls from the cone bearing a group of sporangia, D, E, F, and G, spores, greatly enlarged, with the stems in various positions. From Strickler's "Botany." THE PTERIDOPHYTA 341 contains. Across the stem at each node extends a solid portion or diaphragm. The stem may be branched or unbranched. As in the lyropods, the sporangia are borne in terminal cones (Fig. 214). The *sporophylls*, however, are not at all leaf-like but A diagrammatic transverse section of the stem of *Equisetum*. Note the large central vascular bundle surrounded by smaller bundles, and the smaller ones in the bundles. The fibre-tendril system is much reduced, consisting only of a few scattered fibres. A minute area of wood and a patch of bast between. Wood black, bast dotted. Fig. 213.—Transverse section (diagrammatic) of the stem of *Equisetum*. A diagrammatic transverse section of the stem of *Equisetum*. Note the large central vascular bundle surrounded by smaller bundles, and the smaller ones in the bundles. The fibre-tendril system is much reduced, consisting only of a few scattered fibres. A minute area of wood and a patch of bast between. Wood black, bast dotted. Fig. 214.—Gymnosporus of *Equisetum*. A, male gametophyte, showing extended rhizoids; B, female gametophyte, showing long, branching lobes with archegonia on their bases. (A and C after *Hedwigiana*, B after *Schultze*.) are somewhat shield-shaped and project outward at right angles to the cone axis. On their under or inner surfaces are borne a pair of sporangia. The spores are all alike externally, but a 342 BOTANY: PRINCIPLES AND PROBLEMS given spore will generally produce either a strictly male or a strictly female gametophyte. Attached to each spore are four thread-like structures, the claters (Fig. 214), which coil tightly around its mount but expand when dry, and thus aid in spore dispersion. The gametophyte (Fig. 216) is an irregular thallus which develops close to the base of the sporophyte. It has a horizontal habit of the hypocotyl prothallus, and is provided with long, branching lobes. The two sexes are usually on separate plants and the gametophytes are therefore said to be dioecious. The sexual organs are found at the tips of the lobes and the archegonia at their bases. Each spore has a tuft of cilia. In embryonic development, the archegonium is often reduced to a whole process considerably resembles that found among the ferns. The Equisetaceae, like the Lycopsidaceae, were represented in earlier times by many species, some of which were very large, and some with well developed cambium and secondary wood. Many of these were also heterosporous. **QUESTIONS FOR THOUGHT AND DISCUSSION** 765. Which of the three groups of pteridophytes is the most ancient, in your opinion? Why? 766. What other groups of plants already described (aside from the mosses) have been found probably very abundant but are now represented by only a comparatively small number of species? 767. Why is sexual reproduction so common among the pterido- phytes? 768. Do you think that heterotrophy has arisen more than once in the evolution of the plant kingdom? Explain. 770. Which do you think is more primitive among pteridophytes, thin-walled sporangia or thick-walled ones? Why? 771. Can you suggest a reason for the fact that ferns are now so much more numerous than mosses? 772. Why are there no tree-forms in temperate climates? 773. How does the trunk of a tree-fern differ from that of an ordinary tree? 774. Why does not the trunk of a tree-fern make good lumber? THE FERIDIOPHYTA 343 778. Are common ferns annuals or perennial? Why? 779. Of what advantage to the fern plant is the indusium? 780. Of what advantage is it to the fern plant to have the antheridia and the antheridium on the same sporophyte at different times? 781. In what way do the Oplopanaxes resemble the ferns? In what do they resemble the club mosses? 782. Do you think that a cone is more primitive than a group of leaf-like sporophytes or not? Why? 783. Of what advantage to the plant is it to have its sporophytes crowded together and concentrated along the stem? 784. How do a typical fern and a typical bryophyte differ in the environment to which they are best adapted? 785. What are the advantages of a subterranean, saprophytic gametophyte plant over a surface-attached, photosynthetic gametophyte form? Which form do you think is more primitive? Why? 786. In what particulars does Equisetum resemble the ferns? In what does it resemble the club mosses? 787. In what particulars do the Equisetaceae resemble the Filariae? Is it worth noting that they are non-photosynthetic? 788. Of what significance is the fact that the gametophytes of Equisetum are usually dioecious? 789. How do the elaters in the sporangium of Equisetum aid in spore dispersal? REFERENCE PROBLEMS 132. Of what common structure are the peristhylae? 134. Construct for Equisetum a graphic life-cycle similar to that given in the text for a fern (Fig. 205). 135. Construct a similar graphic life-cycle for Scapania. 136. Construct a similar graphic life-cycle for Equisetum. 137. Give the derivations of the following terms and explain in what way each is appropriate. Heteromorphic Sexual Indusium Antheridium Strobilus Prothallus Sporophyll CHAPTER XVII THE SPERMATOPHYTA This enormous group, to which the early portions of our text were largely devoted, includes the most familiar and abundant part of the plant kingdom. In the vast majority of cases directly, the seed plants furnish almost all the food supply of the human race, all of its timber and fiber plants, and a great majority of the animals that inhabit the earth. They are the basis of our civilization. It is this division of the plant kingdom which is present most intimately in the thoughts and lives of men, and which for many years has practically been the centre subject-matter for the science of botany. In vegetative characters the spermatophytes are not remarkably different from other plants. The root, stem, and leaf are "same vigorous development of root, stem, and leaf, and although they commonly show a greater specialisation and differentiation of their tissues, particularly in the higher groups, the fundamental plan established by the lower forms remains. A well developed fibro-vascular system, is retained and further developed. Growth of the stem in diameter by means of active cambium seems to be a characteristic feature, much reduced or absent in the more delicate herbaceous species. The Origin of the Seed.—The distinguishing feature of the spermatophyta is their possession of a seed. This constitutes them of a new reproductive structure, the seed. In an earlier chapter we have outlined briefly the evolution of the seed-habit and its importance to life on earth. We shall deal with the lower plants which we now possess, it will perhaps be worth while to describe the seed and its origin somewhat more fully before we proceed to detail the various plant groups in which this structure occurs. Relation to Structures in the Lower Groups.—The seed-producing habit is one which has been evolved independently in several homologous as has been attained by the higher pteridophytes. It will be remembered that in the water ferns, in Solanaceae, and in Lecodea, 344 THE SPERMATOPHYTA 345 A B C D E F G Fig. 217.--Development of the female gametophyte and seed in a gymnosperm (Thuja). A, The egg-cell with the two polar bodies. B, The first division of the ovule, consisting of an intangium (1) and a megaspore-organ or nucellus (2), within which the megaspores have been formed. C, The second division of the nucellus. D, The megaspore-organ with the four megaspores. E, The megaspore-organ after the megaspores have aborted, the fourth is enlarging. F, The megaspore-organ with the four megaspores. G, The mature seed, which now consists of a layer of free nuclei surrounding a large vacuole. D, the central part of the megaspore-organ. H, The embryo-sac, which consists of endosperm. At one end are two archegonia (4) within each of which is a fertilized egg-cell (5). At the other end is a large vacuole (6), which contains the tip of the neck of the megaspore-organ (7). Two pollen-grains have germinated, and their tubes have passed through the nucellus and into the megaspore-organ (8). From the fertilized egg have grown two young embryos (9); one larger than the other. I, The mature seed. H, Mature seed. The intangium of the ovule has developed into the outer scale (10), which is now called the integument. The inner integument has almost disappeared and the embryo has grown to its full size. The embryo-sac is surrounded by a layer of free nuclei, which will develop from the rose-scale and under favorable conditions will produce a new plant. 346 346 BOTANY: PRINCIPLES AND PROBLEMS the spores are not uniform but that microspores and megaspores, borne in separate sporangia and on separate sporophytes, germini- ate into male and female gametophytes, respectively. The sex plants are thus produced by the same plant. In some groups, they develop microspores (now called *pollen grains*). These are borne in a microsporangium (now called an *anther*, arising from a modified leaf), while the megaspore is borne in a megasporangium (from a modified leaf). Very radical change is evident here, but in the case of the ferns, we find some marked innovations. The *Orale and Its Conjugates* (Fig. 217 and 229)—The mega- sporangium of the fern *Pteris vittata* produces only one functioning megaspore, for among the other three members of the tetrad which begin to develop soon disappear. Furthermore, the sporophyte does not produce any pollen grains. Instead, it instead, and allows it to germinate and produce the female gametophyte within the tissues of the sporophyte (nucellus), combined with the megaspore. The *anther* (now called the *cystocarp*) is a small, roundish group of cells filled with food and bearing at one end one or more archegonia or structures similar to them. The archegonium is embedded in the tissue of the nucellus and is never freely exposed. Among the highest forms it suffers such reduction that resemblance to a gamete is lost. The *anther* is usually a single cell enclosed by two or three coats or *tecta* except for a small opening, the *micropyle*, which opens into the cavity. This point of entry is not neces- sary nor does it give the anther any special benefit. The whole struc- ture—integument, nucellus, and antheroecium—is known as the *oral*, and after fertilization, for which it is now prepared, the scale which covers it falls off. The pollen grain is attached to the megasporepolypodium, which is now called a *cysto*. *Polyp* (Fig. 218)—The microspore or pollen grain, produced in the anther, germinates and produces two spermatozoa and produces within itself the male gametophyte. This is greatly reduced and consists at most of very few cells, in the higher forms of plants being reduced to a single cell or even a nucleus. These are all that remains of the male sexual generation. Germi- nation of the microspore usually takes place at least in part, either in water or on land, depending upon the species involved. After this event, the pollen grain is transferred, by wind, insects or other means, either directly to the ovule or to a receptive THE SPERMATOPHYTA 347 structure (the stigma) which is nearby. Since the egg-cells are buried in the mesocarp tissue, it is evident that the male gametes cannot approach them directly, as in the lower plants, and a new structure has accordingly been developed which conveys the pollen-grain to the egg-cell. The tube through which the pollen-grains pass from the pollen-grain to a slender, thin-walled projection and into the contents of the grain passes, led by the tube-nucleus. A diagram showing the development of the scale gametophyte in a gymnosperm (pine). A longitudinal section of a scale shows its structure. B, a scale with two scales removed. C, one of the sporophytes from A, much enlarged. The sporophyte is filled with microsporangia and megasporangia. D, a megasporangium containing two megaspores which are borne on each scale. E, a mature pollen grain. F, a mature megaspore. G, a mature female gametophyte. H, a mature seed. Fecundation and Seed-production (Figs. 217 and 220.) - The pollen tubes grow rapidly and penetrate the tissues, such as a fungus filaments penetrate the tissues of its host, until it reaches the egg-cell. The pollen tube grows through a hole in the tube, often the only one at this point except the tube-nucleus, now divides into two male gametes, and as the end of the tube bursts 218 348 BOTANY: PRINCIPLES AND PROBLEMS into the sac one of these units with an egg. The fertilized egg now divides and grows into a young sporophyte, the embryo, possessing a primitive root, stem, and leaves. The embryo soon attains its growth and becomes dormant, embedded in the tissues of the seed plant. This is the first stage of the life cycle of the sporophyte. The integument in the mean time has developed into the tough seed coat and the whole structure soon separates from the mother plant to become a seed. The seed is then ready for temperature and moisture this seed will germinate, the embryo breaking out through the seed coats and establishing itself in the soil as a new individual. In due time this individual at the expense of the stored food but soon becomes independent. The Advances from Pteridophytes to Seed Plants. The essen- tial differences between seed plants over the higher pteridophytes are therefore: (1) The gametophyte is much reduced within the megasporeanum and its germination into there female gametophyte; (2) the enclosure of the sporangium and sac by a new structure, the sporophyte; (3) the development of the reduced male gametophyte directly to the vicinity of the female gametophyte, to which the male gametes obtain access by another new structure, the antheridium; (4) the union of two young sporophytes in contact with each, and at the expense of, the parent sporophyte, and (5) its final release, dormant, well supplied with food, and independent. It is evident that this reversal of the reproductive situation as it was in it the bryo- phytes is now complete, for instead of the sporophyte being attached to a gametophyte, as in all other plants except those of the succeeding sporophytes) is here attached to the parent sporo- phyte. Indeed, in the most advanced types both gametophytes are no longer necessary for reproduction. The sexual generation and the alternation of generations has practically disappeared. The Flower. The sporophytes of seed plants tend to be arranged in terminal clusters on short branches. In the lower members of the plant kingdom these sporophytes give rise to cones of some of the pteridophytes, but higher up a very specialized shoot, commonly called the flower, has been evolved. In its fully developed form it consists of a central axis bearing leaves and carpels, but modified leaf-like structures for protection of the sexual organs and for attraction of insects. From the possession of this structure the sporophytes are sometimes called the "flowering plants." The Spermatophyta 349 To describe adequately the various orders of which this huge division is composed is quite impossible within the limits of our text. Aside from its great size, it will be remembered it also includes a great number of fossil types, many of which are imper- tant in reconstructing us for the steps in the evolutionary history of the land plants. The following pages will deal with the salient characters of the more important groups only, and to indicate their probable relationship to each other and their place in the phylogenetic tree. **Gymnosperms or Gymnosperms.**—Two classes of seed plants are commonly recognized, the gymnosperms and the angiosperms, differing from each other in that the former bear their seeds on bare spores. The gymnosperms are the most ancient of seed plants, and a varied and heterogeneous group. All agree in possessing ovules and seeds, but differ in the manner in which these are enclosed by a shell or carpel, freely exposed to the air and to the direct contact of pollen grains, and not enclosed in an ovary as they are among the angiosperms. There are two main lines of development in the earliest gymnosperms, one arising from fern-like meconites, for several remark- able fossil plants from the Coal Period have been discovered which possessed typically fern-like foliage, and were long thought to be true ferns. These plants are known as cycadeoids or cycads, undoubted, though primitive, seeds. This group, sometimes called the Cycadeoidales or cycad-ferns, has long been extinct, nor have any living representatives survived. The other primitive living gymnosperms are the Cycadales. 1. **Cycadeoids or Cycads.**—Cycad stems are typically stout and unbranched, with large leaves of broad or narrow outline (fig. 210). (FIG. 210.) A few species have tall, columnar trunks and thus superficially resemble trees or palms. Internally, the stem possesses a vascular system consisting of a single cylinder of cells rather weakly developed, although the fibre-vascular system is often complicated by the occurrence of several concentric rings of bundles around the pith (fig. 211). The male and female sex structures occur on separate plants, and the sporophylls are borne in terminal cymes (fig. 220). In the genus Zamia, the male sporophylls ("female" sporophylls) are large and lobed, densely covered with a fine down similar to foliage leaves, and bear scales along their edges (fig. 221). In the other members of the order the cones are more compact and the spor- ophylls are smaller than those of Zamia (fig. 222). Each 350 BOTANY: PRINCIPLES AND PROBLEMS microsporophyll (Fig. 222) bears many sporanges of anthocer and produces a large number of pollen. The spores are generally large and thick-walled, and the megaspore, arising as one of four potentially spore-producing cells in the middle of the microsporophyll, develops into a large, liquid-filled megasporangium. In this chamber just under the micropyle, a large, liquid-filled pollen chamber arises. The pollen grain enters this chamber through the micro- Cyanea revoluta, one of the Cyandaceae. Male plant with cone.

Fig. 218.—Cyanea revoluta, one of the Cyandaceae. Male plant with cone. (Photo by G. S. Faraggi.)

pyle and there germinates. Its two male cells are each provided with a spiral band of cilia and swim about in the liquid, a remarkable preparation for their long journey to the female plants back through pteridophytes and bryophytes to their remote algal ancestry. A pollen-tube is formed and penetrates the adjacent megasporangium, where it finds abundant food than to assist in the transference of the male cell. The pollen-chamber gradually enlarges itself until it reaches the embryo-sac, where one of the sperms enters an archegonium and effects fertilization. Cyandae are confined to the warmer regions of the globe. They are an inconspicuous group today but in earlier ages, notably in the Mesozoic era, they were very abundant and widespread. Their close relatives, the Bamboidales (now extinct), were for a time. 350 THE SPERMATOPHYTTA 351 among the most conspicuous of plants and produced some complex flower-like bisexual reproductive organs. Chiefly related to the cycads are the pteridophytes, these maidenhair tree, Ginkgo, usually placed in a separate order by itself. A page from a book showing a detailed illustration of a cycad cone. Fig. 226.—Staminate cone of Cycas revoluta. (Photo by G. S. Tower.) resembles the cycads in the possession of a pollen-chamber and of mobile sperms, but differs in its tree-like habit of growth and in the absence of seeds. 2. Coniferous or Conifera.—These are familiar to us from their wide distribution in temperate zones and from the fact that they include many trees which are of great value to man. The vegetative body differs radically from that of the cycade for it is essentially a tree, with a straight trunk and spreading lateral branches which give a spikelike appearance (Fig. 227). The leaves are typically needle-shaped, evergreen scales or needles. The cones of the 352 BOTANY: PRINCIPLES AND PROBLEMS stunt woody stem is secondary wood, laid down by an active cambium. Its water-conducting cells are all tracheids, those produced in the spring being comparatively wide and thin-walled. Fig. 221. Fig. 221. Spore-shoot from ovulate cone, showing several ovules attached to its side. One of these has developed into a seed. (From Strasburger, after Schimper.) Fig. 222. Fig. 222. Stem of a pine tree, showing numerous pollen-mace or microsporangia. (From Strasburger, after Richard.) Fig. 223. Fig. 223. Staminate or "male" cones of the tree. and those in the summer much narrower and thicker-walled (Fig. 63). The structure of the vascular tissues approaches rather closely to that of the angiosperms. THE SPERMATOPHYTA. 353 As its name implies, the reproductive structures in this order are typically produced in cones. The microsporangial (staminate or "male") cones (Fig. 223) are short-lived and somewhat delicate structures, and each cone-scale (stamen or microsporo- phyll) bears two (rarely three) microspores on its lower or dorsal surface, on which the microspores or pollen grains may developed (Fig. 218). The pollen is in all cases transferred to the ovules by wind. Except for the rather small group Taxaceae, in which the pollen is carried by insects, and the megasporangial (ovulate or "female") cones (Figs. 224 and 225) usually become hard and woody. Each cone-scale bears one or two ovules, but these are much smaller than those of the cycads and contain fewer cells (Fig. 217). The pollen alights on the nucellus and there germinates (Fig. 218). The generative cell at this time divides into two, a male cell and a body cell, which together form the inner wall of an antheridium. The body-cell in time follows the tube-nucleus down the pollen tube and divides into two male cells, one of which effects fertilization of the egg-cell, while the other degenerates, but the pollen-tube conveys the male cells, which are non-motile, Fig. 224.—Ovulate or "female" cones of the pine. 218 254 **BCTALY:** PRINCIPLES AND PROBLEMS directly to the archegonium. After fertilization there are few divisions of the egg within the archegonium itself, and the young proembryo thus formed is then carried deeply into the tissues of the embryo-sac by certain of its upper cells, which rapidly elongate. In this position it develops into the mature embryo of the seed. Fig. 223. Fig. 224. Fig. 223.—Longitudinal section (diagrammatic) of a pollen-cube or "female" cone of pine. Attached to the base of each scale is seen an ovule, its microsporangium inverted. Fig. 224.—Fertilization in a conifer. Archegonium into which a pollen-cube has entered is shown at left. The male gamete is seen in the neck of the nucleus. The other, left behind in the cytoplasm, will die. (From Scudder.) Like cycads, the conifers are an ancient group and are prominent in fossil records since Mesozoic times. Although they include only about 550 species today, they are still widely distributed for in many parts of the forested regions of both the north and the south temperate zones they contribute to the vegetation such notable trees as pines, firs, larches, hemlocks, spruces, cypresses, and many others. 5. Gnetales.—Brief mention should be made of the most highly specialized plants of this order, namely those which consist of three genera only: a tropical climber, a desert shrub, and an THE SPERMATOPHYTA 553 anomalous desert plant. These are distinguished from other members of the class chiefly by the possession of vessels or ducts in the wood and by a marked reduction in the female gametophyte somewhat similar to that which occurs among angiosperms. It has been suggested that these plants may be related to the angiosperms may perhaps have arisen from the gymnosperms. Angiosperm or Angiospermae. - Angiosperms differ from gymnosperms in having their seeds enclosed in a fruit, specifically exposed to the air on the open surface of a scale but are enclosed in a definite case or vessel, the sepal. With this definition we include all the perfected and typically insect-pollinated flowers, their enormously diversified plant bodies, their successful invasion of all habitats, and their assumption of practically every habitat on earth. The flowers of angiosperms stand at the apex of the vegetable kingdom. They are a modern group and have arisen in comparatively recent geological time. Before the advent of the angiosperms, the world was a flowerless place; the older vascular plants have been swept aside; most of them to complete extinction, and the rest, with few exceptions, to comparative insignificance. It is only the thallophytes with their simple and unimpressive forms that still maintain their primitive habits of life, that can compare with angiosperms in number of species and individuals, and we must remember that were it not for these higher plants our world would be a barren waste and many would perish. The angiosperms are of primary importance as food producers for animals and man. Since we have treated each of these groups in this chapter which we have studied almost exclusively in the earlier chapters of the text, it will not be necessary to treat them here with great detail as we have the other groups already described in considerable detail. Only, however, to bring together the essential features of the class as a whole, that we may readily compare it with the other seed plants and see how they differ from one another and from the ferns. **Vegetable Structure.** - The vegetative body is much diversified. Seed plants not only include trees and smaller woody plants, (the only large group) but also include grasses, rushes, etc., which have developed a new type, the kerat, which is particularly well adapted to temperate or semi-arid regions, since it is small and soft-stemmed, produces flowers and fruit after a very short growing period. In some cases it is even possible for it to survive long periods either underground or in the form of resistant scales. 356 BOTANY: PRINCIPLES AND PROBLEMS In perennial herbs the underground parts survive and it is only the upper portions which die back; in biennial herbs, the plant lives through two seasons, storing up food the first and flowering the second, and in annual herbs the plant body lives through only one season. The herbaceous plants are very rich in species and include the majority of our food plants and many others of economic importance. A flower diagram showing the structure of a dicotyledonous angiosperm (dicot). A. Face view of the flower, showing its calyx of five sepals, its corolla of five petals, its stamens, and its pistil. B. Section through the flower, showing the relations between the parts. 1. Sepal. 2. Petal. 3. Stamens. 4. Pistil. Fig. 227.—The structure of the flower of a dicotyledonous angiosperm (dicotomous). Internally, the vascular system reaches in angiosperms its highest degree of specialization. A cambium is well-developed in the woody members but is much reduced among herbs. The wood consists not only of those general-utility elements, the tracheids, vessels, and fibers, but also of special elements to furnish rigidity to the stem; or wide, thin-walled ducts or resins, by which large quantities of water can be conveyed rapidly through the wood. In the leaf, too, a new element, the con-

vascular cambium, intimately related to the sieve-cells, makes its appearance.

14 THE SPERMATOZOHYTA. 357 Reproduction.---The gametospores are all wind-pollinated, and many of the lower angiosperms resemble them in this respect and have inconspicuous, one-like reproductive organs (fig. 18). Often differing remarkably in general appearance and function from the gymnosperms type, except in the possession of ovules. Fig. 22.---The process of seed-production in a flowering plant. Longitudinal diagrams of flower and fruit, the calyx and corolla solid black; the ovule, seed-grain, and embryo solid white. The stamens are shown in two stages: at left, the anther, the anther cap, and the single scale beginning to develop; at right, the anther, the anther cap, and the double endosperm nucleus (in center) are ready for fertilization. $C$, fully developed ovule with embryo; $D$, seed-grain being transferred to the stigma. Two grains have germinated, and the embryo has developed into the seed-grain. The pollen-grains pass through the margin of the scale and discharged in contact with two mitotic divisions of the endosperm-nucleus. The ovary is shown in three stages: $A$, ovary with two scales; $B$, ovary with three scales; $C$, ripe fruit. Sepals, petals and stamens have dropped off, the ovary wall has become seed coat and the ovule has developed into the seed. The embryo, in the same way as in fig. 21, is surrounded by a membrane which is the endosperm surrounding it (shown in white) by the endosperm-nucleus. The higher members, however, have come to depend upon insects to transport their pollen, and have evolved more elaborate structures (figs. 22-27). This is described in a previous chapter, with its protective enly, composed of scaly; its intrac- A diagram showing the reproductive process in a flowering plant. 10/22 358 BOTANY: PRINCIPLES AND PROBLEMS Fig. 229.—Development of the female gametophyte and seed in a dicotyledonous seed magnolia (Magnolia grandiflora). A, young ovule, the two integuments (1 and 2) beginning to appear and the mega- merous megaspore (3) just beginning to develop. B, C, D, four megaspores have produced a row of four megagametes. C, three megagametes have aborted, the one on the left having been fertilized by the pollen grain which now is at opposite ends of the young embryo (4). E, each zygote has again divided into two cells, one of which has become the embryo sac, the other becoming a nucellus. F, G, H, I, J, the embryo sacs. In F, one nucleus from each group has migrated to the center, and the two sec- ondary nuclei have formed the egg cell (5) and the two polar nuclei. The time of fertilization is indicated by the arrow. The embryo sac is now fully developed and the egg is ready for fertilization. H, a perfect tube has been formed between the egg and the polar nuclei. I, the two polar nuclei are now united with the egg and the other with the endosperm. One of these (N) is uniting with the egg and the other (S) with the endosperm. J, the mature seed. The integuments are shown in black. The embryo is shown in white. The seeds are of solid endo- spores, packed with food, and the endosperm has reached its full maturity. 14 THE SPERMATOZOE 359 five ovules, composed of petiole, its pollen-producing stamens, and its ovule-bearing pedicel. The pedicel may be a single carpel (which has grown about the ovules and enclosed them); a number of A: A single carpel with two ovules. B: Two carpels, each with one ovule. C: A single carpel with three ovules. D: A single carpel with four ovules. E: A single carpel with five ovules. F: A single carpel with six ovules. G: A single carpel with seven ovules. H: A single carpel with eight ovules. I: A single carpel with nine ovules. J: A single carpel with ten ovules. K: A single carpel with eleven ovules. L: A single carpel with twelve ovules. M: A single carpel with thirteen ovules. N: A single carpel with fourteen ovules. O: A single carpel with fifteen ovules. P: A single carpel with sixteen ovules. Q: A single carpel with seventeen ovules. R: A single carpel with eighteen ovules. S: A single carpel with nineteen ovules. T: A single carpel with twenty ovules. U: A single carpel with twenty-one ovules. V: A single carpel with twenty-two ovules. W: A single carpel with twenty-three ovules. X: A single carpel with twenty-four ovules. Y: A single carpel with twenty-five ovules. Z: A single carpel with twenty-six ovules. AA: A single carpel with twenty-seven ovules. AB: A single carpel with twenty-eight ovules. AC: A single carpel with twenty-nine ovules. AD: A single carpel with thirty ovules. AE: A single carpel with thirty-one ovules. AF: A single carpel with thirty-two ovules. AG: A single carpel with thirty-three ovules. AH: A single carpel with thirty-four ovules. AI: A single carpel with thirty-five ovules. AJ: A single carpel with thirty-six ovules. AK: A single carpel with thirty-seven ovules. AL: A single carpel with thirty-eight ovules. AM: A single carpel with thirty-nine ovules. AN: A single carpel with forty ovules. AO: A single carpel with forty-one ovules. AP: A single carpel with forty-two ovules. AQ: A single carpel with forty-three ovules. AR: A single carpel with forty-four ovules. AS: A single carpel with forty-five ovules. AT: A single carpel with forty-six ovules. AU: A single carpel with forty-seven ovules. AV: A single carpel with forty-eight ovules. AW: A single carpel with forty-nine ovules. AX: A single carpel with fifty ovules. AY: A single carpel with fifty-one ovules. AZ: A single carpel with fifty-two ovules. BA: A group of ten carpellary ovaries, each containing one to ten ova lles. The pedicels are united together at the base, forming a common carpel. In some cases the pedicels are separate, but in others they are fused together. In any case, the pollen is necessarily prevented from separate carpellary ovaries, which are united into a common carpel. 360 BOTANY: PRINCIPLES AND PROBLEMS alighting directly upon the microspore as it does in the gymnosperms, but instead is received upon a special projection of the pistil, the stigma. Here it germinates and sends down a pollen tube which ultimately reaches an ovule (Fig. 228). The male gamete, or spermatozoon, is prepared, for the nucleus of the microspore now divides only into the tubule nucleus and the generative cell, and the latter produces two motile male gametes, or spermatozoa, which are greatly reduced (Figs. 229 and 230). The megaspore (the successful one of four originally produced in the meiobium) becomes much enlarged. Its nucleus divides by mitosis, so that each half goes to the end of the young embryo-sac next the micropyle of the ovule and the other to the basal or antipodal end. Here each nucleus undergoes meiosis, and each of these nuclei divides, so that in the sac there are now four nuclei. One from each set then moves toward the middle and these two there form to become endosperm nuclei. The third nucleus from each set then moves upward to become definite cells, one of them the egg cell and the other two the synergids, probably the remains of an archegonium. The three antipodal cells remain at the base of the sac as a vegetative tissue of the sac as it is found in the gymnosperms. The gametophyte now consists of seven cells (or six cells and a naked nucleus) which have been formed by division of those cells pass down the pollen tube and enter the ovule. One fertilizes the egg nucleus, as usual, and from this union the embryo results. The other, however, instead of being eliminated, unites with the endosperm nuclei and develops into a large storage body (one of which, it will be remembered, is already a product of fusion) develops the endosperm of the seed. The endosperm thus differs markedly from that in angiosperms. This phenomenon is called pheno- menon of double fertilization is a distinctive feature of reproduction in all angiosperms. The development of the embryo and the ripening of the seed go on much as among the lower seed plants. The angiosperms are divided into two clearly marked subdivisions, the dicotyledons and monocotyledons. These two groups differ in many respects. These plants are characterized by the presence of two cotyledons in the embryo (Fig. 231), whereas in monocots only one cotyledon is present (Fig. 232). In some cases of monocots, and they also display several other distinctive features (Figs. 231 and 232). These are: (1) The presence of THE SKEEMATOPHYTTA 361 noted variation in the leaf as opposed to the typically parallel-veined system of the monocotyledonous leaf; (2) the distribution of the vascular system in a ring or tube separating an internal pith from an external cortex, as opposed to the irregularly arranged vascular bundles in the monocotyledonous stem; and (3) the development of the floral parts in multiples of four or five as opposed to the construction of the monocotyledonous flower, which is typically on the plan of three. The dicotyledons, which include over 100,000 species, are generally divided into two main groups; the more primitive *Archichlamydeae*, in which the calyx and corolla are either very Fig. 231.-Characteristics of a typical dicotyledonous plant. A, leaf, illustrating parallel-veined structure. B, section of leaf showing vascular bundles. C, the flower, showing the floral parts in five. D, section of seed, showing endosperm and embryo. E, section of fruit. From G. H. Bower's "Fundamentals of Botany." P. Illustration by W. H. Hurd. Fig. 232.-Characteristics of a typical monocotyledonous plant. A, leaf, parallel-veined. B, stem, showing internodal bundles irregularly scattered. C, section of seed, showing endosperm only. D, section of fruit. From G. H. Bower's "Fundamentals of Botany." 362 BOTANY: PRINCIPLES AND PROBLEMS poorly developed, or have their various members entirely separate from one another, and the Sporephora, in which the petals are typically united into one, shall be considered as a distinct order. The Arachnomy- deae include, among others, the following orders: **Aesculiferae.—This group is now commonly divided into a number of subordinate orders, but in its larger sense it includes the oaks, beeches, chestnuts, hickories, walnuts, birches, elms, One of the Betulae. Betula (Betula pubescens), belonging to the family Betulaceae. The leaves of this tree are deciduous, like those of willows, poplars, and many others, all of them trees or shrubs. The perianth is scale-like or absent, the flowers dry and chafty and arranged in a long, sometimes cone-like inflorescence, the erect or spreading branches being densely covered with the flowers. These plants were long regarded as the most ancient of the dicotyledons, but are now looked upon by many botanists as simple modifications of the true dicotyledons. **Ranales (Fig. 253).—This great and varied group contains the buttercup, magnolia, linden, and water-lily families and their allies, including also the orchids. The flowers are usually polli- nated by insects and are primitive in the sense that their parts, One of the Rosales. Choke cherry (Prunus virginiana). The flowers are usually borne in clusters on short stalks. The perianth is usually cup-shaped and consists of five free segments. The stamens are numerous and usually exceed the petals in number. The ovary is superior and consists of two carpels. The fruit is a drupe. THE SPERMATOPHYTA 363 particularly the stamens and corolla, are typically numerous, and have not become stereotyped in number and arrangement as is often the case among higher orders. The Ranales are regarded by many as the most ancient of the dicotyledons and as the center of origin of the angiosperms (see p. 508). Ranales (Fig. 231).—This large order includes the saxifrage, rose, and legume families and their allies, comprising trees, shrubs, and herbs. The flowers are for the most part conspicuous and attractively colored, but they are usually small or minute plants. Many important fruits and vegetables also fit their place here. One of the Umbelliferae. Fig. 235.—One of the Umbelliferae. Watterson (Daucus annuus, belonging to the Umbelliferae). One of the Rubiaceae. Fig. 236.—One of the Rubiaceae. Battonia-bush (Cephalanthus occidentalis, belonging to the Rubiaceae). Regularity and symmetry characterize the flowers of the lower members but in the legume family (Leguminosae) the corolla becomes markedly irregular, producing the butterfly-like or papilionaceous type. Umbelliferae (Fig. 235).—This includes the dogwood family (mostly shrubs) and the carrot family (mostly herbs). The ovary in this group is inferior, the other floral parts being fused with it into a cup-shaped structure called a calyx, and arranged in compact, flat-topped clusters called umbels. This 364 BOTANY: PRINCIPLES AND PROBLEMS order marks the highest point of development among the Arachnothylaceae. The following orders are most important among the Symptulaceae: Eriocoles (Fig. 101).—These are the heaths, mountain laurels, blackberries, and huckleberries, with many species in North America, including many evergreens. The order is intermediate, in certain of its characters, between Arachnothylaceae and Symptulaceae. Tulipiferae (Fig. 104).—Here are placed the morning-glory, phlox, borage, verbena, nightshade, mint, and fennel flowers and a number of other plants which have been used as medicaments. The corollas are conspicuous and prominently tubular, and are regular in the lower families but extremely irregular in the higher ones. The order comprises not only many of our garden flowers but a number of plants which are of great importance to man as tobacco. Rubiaceae (Fig. 298).—Here are included the analus and honeysuckle families, which are prevailing trees and shrubs, although herbaceous species also occur among them. The flowers are most commonly on the plan of four and tend to be grouped in compact clusters. Capsoverae (Fig. 229).—This order includes the gourd, bellwort, and composite families, most of the members of which are herbaceous. The order is represented by a large and important family among angiosperms. Its flowers are arranged in complex heads, each of which somewhat resembles a single flower, the flowers being either regular or irregular. The perianth is free from those in the center. The calyx is greatly reduced and becomes a scalelike pappus surrounding a single-seeded ovary. This family is one of the most important groups in the highest point in evolutionary advance among the dicotyledons. Monocotyledoneae or Monocotyledonae.—These plants, of which there are some 30,000 species, are characteristic by their possession of a single seed-leaf or cotyledon. The leaves are close or parallel venation in the leaf, scattered vascular bundles in the stem, and a floral plan based on multiples of three. The group probably arose from a common ancestor with the monocotyledons and its evolutionary advance has been more or less parallel with that of the latter group. With the exception of one order, monocots are all herbs. Among the important orders are the following:
THE SPERMATOPHYTA. 365 Glamodes. (Fig. 257.)—These are the grasses and the sedges. The small flowers have a typical calyx and corolla, are protected by chaffy bracts, and are arranged in clusters. They are wind- pollinated, except for those cases where pollination is affected directly by insects. The flowers of the grasses, like their simple condition may have come about through reduction. The grass family includes the most important of our crop plants. Fig. 227.—One of the Glamodes. One of the Glamodes. The small flowers, belonging to the family Cyperaceae. Fig. 288.—One of the Araceæ. The The Araceæ. The small flowers, belonging to the family Araceæ. Palms. —The palms are a tropical tree-like family in which the leaves are terminal and surmounted by a cluster of large leaves. The small flowers are usually hidden in a sheath, and are pollinated by wind and in others by insects. The perianth is very simple. Fig. 289.—One of the Palms. One of the Palms. Eudryas. (Fig. 288.)—These are the acacias, a family of large herbs particularly abundant in the tropics and represented with us only by the Jack-in-the-pulpit, skunk's cumber, and a few other species. The flowers are usually solitary, and are either sessile or nonsetose-lobed, are netted-veined. The very simple flowers, almost devoid of a perianth, are clustered on a flaky spathe which is enveloped by a large and often brilliantly colored bract, the spathe. A small illustration showing one of the Araceae. A small illustration showing one of the Araceæ. A small illustration showing one of the Palms. A small illustration showing one of the Palms. A small illustration showing one of the Eudryas. A small illustration showing one of the Eudryas. 366 BOTANY: PRINCIPLES AND PROBLEMS Lilioides (Fig. 98.)—These include the lily, anemone, and iris families, perennial herbs with well-developed bulbs or rootstocks. The species usually have a conspicuous perianth and provide some of our most beautiful garden flowers. Orchidaceae (Fig. 99.)—These are remarkable for the extreme irregularity and great beauty of their flowers, and their remark- able specializations for insect pollination. They are particularly abundant and brilliant in the tropics, whence they have been introduced into the greenhouses of cooler climates as the most highly prized ornamental plants. With the development of the composite and orchid families, the plant kingdom has reached the climax of its evolution. QUESTIONS FOR THOUGHT AND DISCUSSION T90. Why have plants which reproduce by seeds rather than those which reproduce by spores become the most successful part of the world's vegetation? Left: Fig. 98.—One of the Compositae, Yellow chrysanthemum (Artemisia stricta), belonging to the family Compositae. Right: Fig. 99.—One of the Orchidaceae, Showy lady's slipper (Cypripedium reginae), belonging to the family Orchidaceae. 1 THE SPERMATOPHYTA 793. Can you suggest why seed plants have so very many more species than other bryophytes or pteridophytes? 792. From a consideration of the present distribution over the earth of ferns and of primitive seed plants, what suggestion can you make as to the climatic conditions under which the earliest seed plants were evolved? 793. Why do you think it is that seed plants are not very common in the ocean? 794. The name "Flowering Plant" is sometimes applied to the seed plants. Why is it not a very satisfactory name? 795. The arthropodism is much reduced in the gametophyte of the seed plants. Explain. 796. What is meant by the ferns or the embryo-ear of seed plants compared to the pollen grain? the ovule? the meiosis? 797. To what in seed plants do the following structures in liverworts correspond: the sporcle; the spermatangia wall and seta; the thallus; the pericarp? 798. Different terms are used for the various structures in the plant's life history among seed plants than are used among bryophytes and pteridophytes. What are these terms? (Explain for spermo- zoa, and so on.) What reasons can you suggest for this? 799. What great groups of animals are there which obtain the bulk of their food directly or indirectly from other plant groups than the seed plants? 800. What similarities can you suggest in the evolutionary history of bryophytes, pteridophytes, and spermatophytes, in the plant kingdom, that of animals, reptiles, and mammals, in the animal kingdom? 802. What various abnormalities do the cycads show in the ferns? 802. In what way is the relation of the conifers to the rest of the gymnosperms similar to that of the ferns to the rest of the pteridophytes? 803. What relation can you suggest between the characteristic wood structure of the conifer and the tough and scale-like character of their leaves? 804. In such a tree as the pine, are the staminate or the ovulate cones more numerous? Explain. 805. In regions where there are large coniferous forests, "showers of sunlight" often occur in the spring. What explanations can you suggest for this? 368 BOTANY: PRINCIPLES AND PROBLEMS **805.** What advantages have the angiosperms over the gymnosperms which should have made them so much more successful? **807.** Give an example of a crop plant which is an annual herb; one which is a bimodal herb; one which is perennial herb; one which is a shrub; one which is a tree. **808.** Which of these five plant types is of most importance in agriculture? Why? **809.** There is a larger proportion of heteromorphic species in the flora of temperate regions than in the flora of a tropical one. Explain. **810.** Angiosperms, and especially herbaceous angiosperms, have apparently evolved under faster and given rise to many more species than either pteridophytes or gymnosperms in a similar length of time. Why? **811.** What evidence can you suggest for the conclusion that the dicotyledons are more ancient than the monocotyledons? **812.** We believe that families in which the flowers are irregular are usually older than evolutionary units in which these flowers are regular. Why? **813.** We believe that a flower with a large and indefinite number of floral parts, all free from one another, is more ancient in type than one in which the floral parts are united into definite constellations and where they are more or less united with one another. Why? **814.** In what respect may stamens and carpels be called "sexual organs" and in what respect is it incorrect to call them such? Why? **815.** What conclusions can we draw from the anatomical concentrication of cells in the pollen grains and in the ends of the stigma? **816.** Why do we regard the composite and the ericoid families as marking the highest points in the evolution of the plant kingdom? ## REFERENCE PROBLEMS **138.** What family is most important as a producer of food plants and what are some of the notable crop plants included in it? Name a few other families which have many important economic plants. **139.** Seed plants no sooner knowers come to the Nephrolepisgeny. Why is this main aspect appreciated by botanists? **140.** The terms **Cyperaceae** and **Phanerogamae** are sometimes used to designate two groups into which the plant kingdom may be divided, the THE SPERMATOPHYTA 369 former including thallophytes, bryophytes, and pteridophytes, and the latter including seed plants. Why is such a classification not a very good one? 141. Name five cultivated plants which belong to the rose family; five which belong to the nightshade family; five which belong to the composite family. 142. What family of plants is most important as far as the production of cultivated fruits is concerned? 143. What family of plants is most important as far as the production of timber is concerned? 144. Give the derivation of the following terms and explain in what way each is appropriate:
Antipodal Cells Syringid Pupillaeous
24 A stylized circular design with a central dot and two curved lines on either side. INDEX Figures in boldface type indicate pages on which illustrations occur. Alternation of generations, in red algae, 284 Absorption, of water and salts, 45, 1 bands, in spectrum, 72 Aconitum, genus of, 306 Acridine, 200 Argyranthene, inheritance of, 231 Artemia, 233 Artemia, fruit of, 201 Alkaloids, 295 Adder's tongue, 353 Adenine, 200 Aesculus, 298 Aesculus, fruit of, 298 Aegilops, 299 Aegilops, fruit of, 299 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, pollen of, 301 Aegilops, 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 574 57431 Bacillus, 205, 296 Barbarea, 257-260 Barberia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 257-260 Bastardia, 318 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 Botany; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Beeswax; see Beeswax. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berries; see Berries. Berriers; see Beeswax. Cactus; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance of., B. subdivisions of., C.-D. Branch., A9 Cauliflower; defined., I. history of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined., I. history, of., A. importance, of., B. subdivisions, of., C.-D. Branch., A9 Cauliflower; defined.; I. history; of.; A. importance; of.; B. subdivisions; of.; C.-D.; Branch.; A9 INDEX 373 Cell, enlargement of, 141, 144 maturation of, 142, 144 plants, 141 size, comparison of, 140 theory, c. vul., 44, 129 Cells, of wood, 105, 161 Cereals, 289, 290 Celtis, 139, 142 Celery, 139, 124 Centrifugal force, as substitute for gravity, 270 Chamaeleon, flower of, 343 Concentrate and liquid food sources of, 230 Cyanide, 274 Chamomile, flower of, 305 Cham, 277 Cherry, 277 Chemical substances as environ- mental pollutants, 289 Cherry, flower of, 293 Chlorophyll pigment, flower of, 196 Chloroplasts, plant cell organelle of, 241, 268 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Chlorophyll pigments, leaf of, 306 Circus plant (Circaea), 258 Classification based on relationship, 258 naturalist's classification based on plants' size and number per species. Club mosses (Lycopodium), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 258 Cocoon (spinnable), 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 cottonwood tree leaves. Cottonwood leaves. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutleria plant. Cutleria. Cutlevera leaves.192 variation in, 193 Food: 69, 81 food as source of potential energy, 126 compared to fuel, 126 discharge of food particles by plant roots and leaves into water by means of root hairs and trichomes on leaves and stems and by the action of wind and water currents on the surface of the water. The food particles are then ingested by the fish and other aquatic animals which feed upon them. Food: 213 Food: 305 Food: 306 Food: 307 Food: 308 Food: 309 Food: 310 Food: 311 Food: 312 Food: 313 Food: 314 Food: 315 Food: 316 Food: 317 Food: 318 Food: 319 Food: 320 Food: 321 Food: 322 Food: 323 Food: 324 Food: 325 Food: 326 Food: 327 Food: 328 Fungus: inheritance in, 220 Frost: 406 Frosty: 200-




































































370 INDEX Termination of seed, 292 Gill fungus, 293, 300 Gills, of fungus, 296, 300 Gyposis, 285 Gyrodendron, effect of, 112 Gibbaeum, 121 Gladiolus, 121 Glabrescent, 205, 206 Gleichenia, 205, 206 Gleichenia, species of, 120 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraria, 205 Glomeraeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeaeae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ae ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee eeee e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaa aaaaaa AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAA AAAAAA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA INDEX 377 I Induction, 31 Immunity, 288 Independent assortment, in inheri- tance, 173, 174 Indian Pipe, 173 Inferiority, 101 Ingression, 6 Inhibition, 225–225 Leaf, of 209 Meander, of 211–221 of archived quantities, 210 Locus, of 209 Insects, relation of to flowers, 194 Integumentary, 107, 345, 350, 358 Interfacicular cambium, 104 Intercalary meristem, 104 Intolence, of shade, 161 Intolerance of light, 161 Invertase, 123 Ion, function of chlorophyll, 70 importance of, 36, 194 Iris, rootstock of, 47 Irrigation system, 385 Irribility, of phototropism, 155 Isomerism, 288 Jaxmaney, 268 Indusium, of dendrites, 275 Judae tree and flower species of C. thunbergii Jungmarginalsis, 317 K Kadusa, flower of, 198 Kohne, 275 Kernel of corn, 199 Kinetic energy, of 63 Lady's Slipper, flower of C. thunbergii Lamarkian theory of evolution Land differential of life on earth Leaves Leaf blade, of 93–95, 63 external structure of, 63–64 internal structure of, 64–65 margin of, 64 number of leaves per stem, simple and compound, 64 leaf arrangement on stem, venation of, 63–64 Leaflet, leaflets (plural), 64 Leaf-stalks (plural), leaf stalks, leaf stems (plural), leaf stems, leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Leaves arrangement on stem, Lemnaceae (plural), water lilies, Lemna (plural), water lilies, Lemna gibba (plural), water lilies, Lemna minor (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), water lilies, Lemna gibba (plural), 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38 378 INDEX Liverworts, 313 Living organisms, as environmental factor, 105 Lysosperma, 362 Lysospermid, 335 Lysospermidin, 335 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 336 Lysospermidin, 341-500-501-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502-502- M Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, 478 Marcescence, Naked flower, Names of plants, Narcissus, Objection to theory of, No. of cells per column, Non-canal cells, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal, Nocturnal. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. Narcissus. INDEX 379 Netted cyanotea, 63, 50 Xenoria, flower of, 190 Nitrate bacteria, 32, 33 Streptococcus, 32, 33 Nitrate bacteria, 32, 33 Xanthoria, 32, 33 Xanthoria cycle, 32, 53 Xanthoria, importance of, 31, 168 Xanthoria bacteria, 32, 33, 175 Xanthoria, chlorophyll content of, 185, 254, 346 Xanthoria, evolution of, 255 Xanthoria, growth rate of, 185 Xanthoria, inactivation of, 185 Xanthoria, inactivation in respiration, 126 Organic matter in respiration, 126 Xanthoria, photosynthesis by, 126 Xanthoria, photosynthetic products of, 126 Nutrient materials, 115 Xanthoria pollen of, 196 P Phosphatase-, 27 Phosphorus in leaf, 67, 58 Price's index of leaf area ratio of leaves to stem diameter of stems of plants with leaves and stems of different sizes (see also leaf area ratio), 105-107 O Oak stem of trees of various species (see also tree), 105-107 wood of oak trees (see also tree), 105-107 Ochreous algae (see also algae), 207-209 Ochreous algae (see also algae), oxygenation of water by (see also oxygenation), 207-209 Ochreous algae (see also algae), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxygenation), oxygenation of water by (see also oxigenaion)... Ochreous algae (see also algae), photosynthesis and respiration in aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae and aquatic plants and aquatic algae... 380 INDEX Pentaclethra, 295 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Phanococcus, 278 Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubra (L.) Kuntze. Platythrix rubra var. rubра́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́́íííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììììiïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïïäääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääääänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänäänaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaanaan 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sososososososososososososososososososososososososososososososososososososososososososososososososososso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso osso o so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo soo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii ii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iii iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv iv vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi vi v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v v vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vv vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w ww www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www www wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww INDEX 5S1 Truncia, 31 Prunus, flower of, 189, 302 Pteridophyta, 12, 245-247 Rumex, flower of, 164 Puccinia, 297, 298 Furcula, 298 Purpose, in plant activity, 154 Pyrethrum, 267, 274 Pyrenocystis, 294 Rhizobium, 32 Rhizobium, 313 Rhizobium, 93, 94 Rhizobium, 299 Rhodopispora, 262 Rhodospirillum, 263 Rhodotorula, 312, 313, 314 Rice, starch grain of, 120 Rice plant, 120 Lock particles, in soil, 24 Leaf stalks, 105 Lectus, 40-49, 323 Lecithin, abscission zone of, 42 external structure of, 40 internal structure of, 40-47 Lemnaceae, 41 Leptospermum, 185 Leptospermum, leaf of, 185 Leptospermum, leaf structure of, 185 Leptospermum, leaf surface texture of, 185 Leptospermum, leaf stomata of, 185 Leptospermum, leaf trichomes of, 185 Leptospermum, leaf vein pattern of, 185 Leptospermum, leaf water loss by transpiration of, 185 Leptospermum, leaf water loss by transpiration rate of, 185 Leptospermum, leaf water loss rate by transpiration of, 185 Leptospermum, leaf water loss rate by transpiration rate of, 185 Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, leaf water loss rate by transpiration rate of stomata of, Leptospermum, Q Quillwort, R Racone, Radial section Radial section Radix Ramosus, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays, Rays， S Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae Salviae 382 INDEX de Saxeum, 6 Schimizymyra, 205 Schizomyces, 205 Schizomyces, 205 Scheidek, 6 Scheidek, 6 Scieromyza, 311 Scieromyza, 311 Secondary tissue, 199 Scidus, 240 Scidus, 240 Seed, 139, 199, 314 Seed, 139, 199, 314 development of, 201 dispersed seed, 201 evolution of, 255 origin of, 347 plants, 347–347–356 Seeding, 260 Seed-production process of, 196 Seedling, 260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 260–260 Sedum, 315 INDEX 383 Squash, variation in fruits of, 205 Stalk-cult., of pollen, 547, 552 Nineteen, 269 Stamine corn, 352 Starch, 336 formation of, from glume, 73 grain, 120 Stem, 179 Stem, 179, 88-89, 112, 323 Stems, external parts of, 89-93 herbaceous, 104, 103 internal structure of, 94-105 surface of, 11 woody, 100, 102 Stem-gland, 267 Stem-liners, of sporophytic tissue, 156 Stem-lineage, 157 Stigmata, 279 Stigmae, 279 Stipe, 183 Stipule, 183 Stolon, 183, 184 Stobhynia, 289 Stomach, flower of, 266; function of reproduction of, 183 Stomach of animals, 120 Strabismus, 255, 256 Strawgrass for existence, 250, 257 Soil-bug, 264 Subcutaneous, 265 Summer Squash Squash-fruit of, 120 fermentation of, 131 Subglumis importance of, 149 Submergence survival of the lotus, 250, 257 Submerged aquatic plant, 270 Swamp association, 170 Sweet potato root tuber, 149-150 Synanymy twigs of, 284 Synanymy, 177; 303 Synanymy twigs of, 149-150; see also Synanymy Synaplastoides, 265 Synangiums; see also Synangiums. Synanthus; see also Synanthus. Synanthus of carburettes; frutescence of, 72 T Tangential section of wood, 106, 107 Tap-root, p. 41 Taxon-plant group of plants with leaves in rows, variation in position of leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows, Taxon-plant group of plants with leaves in rows， Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetraploid plant (see also Tetraploid) Tetrahydric acid; see also Acidic substances. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Tracheid; see also Tracheid. Trachyphyllous; see also Phyllous. 384 INDEX True form, 327 nucleus, 200 nucleus, 230 Tropaeolum, 159 Trunk, 85 Trypanosoma, 114 Tuberculosis, 122 Tuberculosis, 247 Tuber, 50, 94, 194 Tuberous, 294 Tubifex, 169 Tubular algae, 272 Tulipaceae, 167, 97, 98 "Tomato weeds", 201 Turgidity, of plant cells, 55 U Umbrella, 270, 271 Unisexual flower, 192 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 212 Unisexual plant, 213 INDEX 385 X Z Namboparkia, 70 Nemophyta, Ind., 166, 170 leaf blade of, 165 stem of, 164 Xylem, 47, 100 Zein, 121 Roggen, 207 Zygospore, 208 formation of, in *Mucor*, 290 Zygote, 144, 246 Zymosin, 154 P.O.B. No. 100 M. C. State College Yeast, fermentation by, 131 Yeast, 295 [API_EMPTY_RESPONSE] . [API_EMPTY_RESPONSE] A blank, light brown page. A blank, light brown page with a few small black spots scattered across its surface. 31 North Carolina State University Libraries CH 107 BOTANY PRINCIPLES AND PROBLEMS S02776907 N