%> •& p. ^Ctll pkarg ^orilj Carolina ^iaie College QK4-7 C63 v.l N?5.™ f *R.?"NA STATE UNIVERSITY LIBRARIES SO 1949072 W Date Due Abi./a>| DEC - £ 1987 - - -•.• ' 8 ;,iar34' 13H0V34 - i9^ajlt 13 5'50X QK47 C89 Coulter ^H ■ ^^^^^ v.l 12 730 £l_-2 A^ textbook of ootan # a TEXTBOOK OF BOTANY FOR COLLEGES AND UNIVERSITIES BY MEMBERS OF THE BOTANICAL STAFF OF THE UNIVERSITY OF CHICAGO JOHN MERLE COULTER, Ph.D. PROFESSOR OF PLANT MORPHOLOGY CHARLES REID BARNES, Ph.D. LATE PROFESSOR OF PLANT PHYSIOLOGY HENRY CHANDLER COWLES, Ph.D. ASSOCIATE PROFESSOR OF PLANT ECOLOGY VOL. I.. PART II. PHYSIOLOGY NEW YORK •:• CINCINNATI •:• CHICAGO AMERICAN BOOK COMPANY Copyright, 1910, by AMERICAN BOOK COMPANY. Entered at Stationers' Hall, London. a textbook of botany, vol. i., pt. n. W. p. 6 Li**ARY Na c- s*<*te College PREFACE The study of plants has assumed so many points of view that every laboratory has developed its own method of undergraduate instruc- tion. No laboratory attempts to include all the phases of work that may be regarded as belonging to botany ; and therefore each one selects the material and the point of view that seem to it to be the most appropriate for its own purpose. During the last ten years the Hull Botanical Laboratory at the University of Chicago has been developing its undergraduate instruction in botany to meet its own needs. Freed from the necessity of laying special stress upon the economic aspects of the subject, and compelled to prepare students for investigation, it seemed clear that its selection must be the funda- mental facts and principles of the science. Its endeavor has been to help the student to build up a coherent and substantial body of knowledge, and to develop an attitude of mind that will enable him to grapple with any botanical situation, whether it be teaching or investigation. It has been thought useful to present this point of view in the present volume. The material of course is common to all laboratories, but its selection, its organization, and its presenta- tion bear the marks of individual judgment. The three parts of the book represent the three general divisions of the subject as organized at the Hull Botanical Laboratory. They are felt to be the fundamental divisions which should underlie the work of most subdivisions of botanical investigation. For example, a study of the very important subject of plant pathology must pre- suppose the fundamentals of morphology and physiology ; paleobotany is, in part, the application of morphology and ecology to fossil plants ; and scientific plant breeding rests upon the foundations laid by morphology, physiology, and ecology. In our selection for under- graduate instruction, therefore, we believe that there has been in- 12720 iv PREFACE eluded the essential foundation for most of the varied work that is included to-day under botany. We recognize that the presentation of the three great subjects here included is very compact, but the book is not intended for reading and recitation. The teacher is expected to use it for suggestive material and for its organization ; the student is expected to use it in relating his observations to one another and to the general points of view that the book seeks to develop. There is a continuity of presentation in each part, so that random selection may miss the largest meaning. For example, in the part on morphology, the thread upon which the facts are strung is the evolution of the plant kingdom, and each plant introduced has its peculiar application in illustrating some phase of this evolution. When certain groups are selected for laboratory study, therefore, the intervening text should be read. It is important to call attention to the fact that the book has been prepared for the use of undergraduate students. It does not repre- sent our conception of graduate work, which should include much that is omitted here. For example, the graduate student should be introduced to the original sources of information, which would involve an extensive citation of literature far beyond the needs of the undergraduate. Still less has this book been written for our profes- sional colleagues, who will notice what they may regard as glaring omissions. Such omissions must be taken to express a deliberate judgment as to what may be omitted with the least damage to the undergraduate student. The motive is to develop certain general conceptions that are felt to be fundamental, rather than to present an encyclopedic collection of facts. This purpose has demanded occasionally also a greater apparent rigidity of form in general state- ments than is absolutely consistent with all the facts ; but it was a choice between a clear and important conception for one with no perspective and a contradiction of large truths by isolated facts, result- ing in confusion. For the same reasons, the extensive terminology of the subject has been kept in the background as much as possible. Definitions usually are made an incident to the necessary introduc- tion of terms. It is assumed that in so far as the definite application of a term may not seem clear, the student will find a compact defini- tion in the current dictionaries. PREFACE V For the benefit of the teacher and of our professional colleagues, it should be stated that much attention has been given to the avoid- ance of any phraseology that might involve a teleological implication. It has not been possible to avoid such phrases in all cases without introducing clumsiness of expression or breaking the continuity of some important series of structures or events. It should be kept in mind, therefore, that all teleological implications of language that remain are disavowed. It seems hardly necessary to say that most of the material presented in the book has been worked over by classes repeatedly. Some new matter has been developed incidentally in all the parts in connection with ordinary laboratory and field work; and especially in Part III have many scattered observations and some new points of view been included. There has been no intention to include any formal con- tribution, but merely to present in general outline some of the material worked over by undergraduates, some of the results of investigation already published in contributions from the laboratory, and some ob- servations and conclusions that hardly seemed to justify separate pub- lication. Provision has been made for students with more interest or more time than usual to get a somewhat larger view, by including in smaller type further details of structure, additional illustrative material, and suggestive theories. Most of the illustrations are origi- nal, in the sense that they have been prepared especially for this book or have appeared in our own contributions. Those that have been copied or adapted are credited ; the former usually being indi- cated by " from," the latter by " after." The three authors are individually responsible only for their own parts, and, while they had the advantage of mutual criticism, it could not be expected that they would agree absolutely at every point. This will explain any lack of harmony that may be discovered in the three parts. A morphologist, a physiologist, and an ecologist look at the same material from different angles, and lay emphasis upon different features ; but all their points of view should be included in any general consideration of plants. It is for this reason, also, that the parts contain a certain amount of repetition, which is abso- lutely necessary when the same structures or functions are being considered from different points of view. vi PREFACE The selection and preparation of the illustrations for Part I were under the efficient direction of Dr. W. J. G. Land, and most of the original drawings of the book were made by Miss Anna Hamilton, an artist to whom great credit is due. We owe certain original illus- trations to the cooperation of our colleagues, who are named in con- nection with the figures; and also some of the drawings in Part III to Miss Anna M. Starr. In addition to the mutual criticism of the authors, Dr. C. J. Chamberlain, Dr. William Crocker, and Mr. George D. Fuller made helpful suggestions in reading the proof. For such errors as remain, after all our efforts to eliminate them, the authors themselves assume full responsibility. In correcting them, we shall welcome the help of the wider circle of users to whom the book now goes. JOHN M. COULTER. CHARLES R. BARNES. HENRY C. COWLES. The University of Chicago. CONTENTS Vol. I., Part II. Physiology CHAPTER Introduction I. The material income of plants 1. The plant cell . . . . 2. Diffusion and osmosis 3. Turgor and its conse- quences 4. The permeable regions of root and shoot . . . II. The material outgo of plants . 1. Transpiration . . . . 2. Exudation of water . . 3. The movement of water 4. Other losses III. Nutrition 1. The nature of plant food 2. Photosynthesis .... (1) The raw materials . (2) The laboratories . (3) The energy . . . (4) The products and the process . . 3. The synthesis of proteins 4. Other ways of getting food ( II AT I KK (III) IV. V. 5. The storage and translo cation of food 6. Digestion . . Destructive metabolism 1. Respiration . . 2. Fermentations . 3. Waste products and ash Growth and movement . 1. Growth .... 2. Irritability . . . 3. Morphogenic stimuli Nastic curvatures Locomotion and stream mg Turgor movements Tropisms .... (1) Geotropism . (2) Thigmotropism (3) Traumatropism (4) Rheotropism (5) Chemotropism (6) Phototropism (7) Other tropisms w radiant energy The death of plants 3S8 397 403 403 409 412 417 417 426 435 442 444 4s '" 458 459 469 472 473 4 73 475 479 4S0 PART II — PHYSIOLOGY INTRODUCTION The relation between the form and structure of a plant and its behavior is very intimate and to a large extent reciprocal. Form and structure in general determine behavior, and behavior, especially as it is itself controlled by external agents, to a great degree determines form and structure. It is not possible at present to discover all these reciprocal relations, much less to describe them in terms of physics and chemistry. Nor is the behavior of plants sufficiently known to be explained in these terms. Morphology, concerned with form and structure, is particularly in- terested in how each plant comes to be what it is in the short history of its own life (ontogeny), and also seeks to form a conception of how plants have come to be what they are in the long course of their history since they began to develop on the earth (phylogeny). The former topic is clearly open to experimental study and constitutes the field of experimental morphology. But the latter is much less open to experi- ment ; scarcely at all, indeed, except for the determination of the laws of heredity, a field which has been called " experimental evolution." Obviously such experiments, whether in the field or laboratory, cannot be wisely planned or executed without a thorough knowledge of plant physiology. A wide range of facts is open also to mere observation, because the ordinary changes in climate and soil, some of which are produced by other plants and animals, affect the form and structure of plants. This field is part of that distinguished from physiology proper as Ecology (Part III). Naturally even the most careful observations need to be confirmed or corrected by experiments. Thus this portion of ecology and experimental morphology are mutually related, and both really form a part of physiology in the broadest sense, and depend upon it. Physiology, in its turn, seeking to expound the phenomena of plant life in terms of matter and force, depends upon the data of chemistry and 295 296 PHYSIOLOGY physics. In certain directions present knowledge is almost Or quite sufficient to permit the framing of physical and chemical explanations. In others the data of chemistry and physics are not yet adequate for this; and in still others it seems now quite improbable that the phenomena can ever be analyzed in terms of matter and force. It must not be forgotten, however, that this is the direction of all recent advances, and that what is hopelessly obscure often becomes beautifully clear as some new van- tage point widens the view. In its broadest sense, then, plant physiology includes the study of the behavior of plants of all sorts, and of all the ways in which this is affected by external agents of every sort. On the one hand it overlaps morphology, and on the other it includes a large part of ecology. In this book, however, it is restricted in the main to a consideration of the behavior of the larger plants, especially seed plants, though in certain cases reference is made to others. In this part no section on reproduc- tion will be found. That topic is relegated to Morphology (Part I), since the purely physiological processes are relatively simple, so far as known, and very much alike, whereas the reproductive organs are very different in different groups of plants and are most significant for their morphology. For convenience, also, the effect of external agents on plants is treated so as to develop and illustrate general principles, whereas the more extended account of specific cases will be found in Part III, on Ecology. CHAPTER I— THE MATERIAL INCOME OF PLANTS [. THE PLANT CELL An organ. — At a glance one sees that the body of an ordinary green plant, such as a bean, is segmented, certain parts being clearly marked off by form from others. The colorless root grows in the soil; the green shoot grows in the air and consists of a distinct stem with lateral out- growths, the leaves. Anatomically, these parts are members; but as the work of the plant is distributed among them, each has its functions, and physiologically each is an organ. A cell. — When one of the organs of the bean, such as a leaf, is inspected, one sees that it, too, is made up of parts, the petiole and the leaflets. The latter are composed of ribs and veins, with green tissue, or mesophyll, between. These parts also have certain functions and hence may be called organs. A microscopic examination of the mesophyll reveals that it is composed of minute bits of material which has come to be known as living, and is called pro- toplasm. Each, individualized, is a protoplast, separated more or less completely from its neigh- bors by membranes which it and they have a mesophyll cell of a leaf; formed. The membrane and protoplast con- c> cbloroplast; n, nucleus; 11 v, vacuole ; w, cell wall. stitute a cell (fig. 619). Organs of a cell. — When the protoplast is examined more closely, a general translucent material, the cytoplasm, may be distinguished from various inclusions. There are (a) many very minute particles, whose nature is obscure, which tend to make the cytoplasm opaque; (b) minute clear spaces, more fluid and sometimes watery, the vacuoles, many of which coalesce as they enlarge with age, and form a few relatively very large water spaces or only one; (c) a roundish nucleus; (d) numer- 297 298 PHYSIOLOGY ous oval green bodies, the chloroplasts. Of these, the nucleus and cbloroplasts, having definite though only partly known functions, are often called organs of the cell. The unit of function. — The word "organ," then, is applied to parts most diverse as to size and complexity; it designates merely a part when its work is thought of rather than its structure. Since the various parts of a cell do not work properly when separated, the cell may be con- sidered as the unit of function, as it is, for convenience, known as the unit of structure. Naturally cells accustomed to association with others do not work properly when separated; but there are plants whose whole body is a single cell. This fact has influenced the conception of the cell as a unit. Work of the protoplast. — What a plant or any part of a plant can do depends primarily upon the protoplasts, since they alone are com- posed of living substance; but not all protoplasts have the same organs. For example, the protoplasts of the leaf mesophyll, furnished with chloro- plasts, can make certain food when properly lighted and supplied with carbon dioxid. But in the higher plants protoplasts which lack these organs cannot form food of this kind under any conditions. The pro- I toplasts of a tuber, having organs known as amyloplasts (starch-formers), are able from suitable material to organize the large starch grains that constitute a form of reserve food of much importance. These grains are not produced except by such special organs. The cell wall. — Each protoplast jackets itself with a membrane, which usually shuts it off completely from the outer world and from its neighbors, except for some exceedingly minute threads of cytoplasm by which it remains connected with them. These threads, traversing the cell wall, persist from the time of its formation. The protoplasts are much hampered by these walls in certain ways, though compensating advantages doubtless accrue. For instance, the movement of the pro- toplast is restricted, and it cannot engulf food particles, but is limited to the substances which can dissolve in water and so migrate through the wall. Thus the cell wall becomes a factor of prime importance to the plant. The cell wall is the most easily observed and striking part of the cell ; in fact the word itself commemorates the discovery of the empty chambers of cork and charred wood which Hooke and Malpighi and Grew saw (1667-1671) with their primitive microscopes, and thought the fundamental feature of plant structure. THE MATERIAL INCOME OF PLANTS 299 Removal and alteration of the wall. — The cell wall, formed by the protoplast, is subject to partial or complete removal by it. In green plants it is usually composed at first of cellulose; but pectic substances early appear in it, and with increasing age it is subject to various modi- fications, which alter its relation to water and thus profoundly affect the conditions of life of the protoplast within. One alteration to which the wall is subject is known as cutinization. because cutin is deposited or formed within it. Sometimes, as on the outer face of superficial cells, this takes place to such an extent as to form the cuticle, a layer which may be loosened and removed entire from the rest of the wall. Parts of the outer wall adjacent to the cuticle may also become impregnated with cutin to varying degrees. The cuticle and these cutinized layers repel water, so that a minimum only is found in the wall and little can pass through. By another modification portions of the wall may become gelatinous. When wetted, they take up great quantities of water (sometimes as much as 98 per cent of their wet weight) and swell so enormously as to lose altogether their usual firmness. Again, the wall may become lignified, a condition characteristic of the walls of woody tissues, whence the name. Lignified walls do not swell so remarkably as gelatinized ones, but they allow water to pass through them with comparatively little resistance. Water of the plant. — From what has been said it is evident that water forms an important part of the cell ; but it is necessary to comprehend its intimate relations to every part in order to understand its full significance. In ordinary land plants water constitutes always over one half and usu- ally about three fourths of their weight. Of the least watery parts, such as wood, it forms one half, and of the most watery parts, such as the pulp of juicy fruits, as much as 95 per cent. In ordinary speech it is common to indicate the general character of an object by naming its most abun- dant component; as, a wooden table, a brick wall, wood and brick being respectively the dominant but not the only material in the struc- ture. If the water of the plant were visible to the eye, distinct from the other constituent materials, on the same principle a plant might be spoken of justly as water, held in form by other substances mingled with it. This is quite the reverse of the ordinary conception, but its essential truth becomes evident when we consider not merely the quantity of water relative to other constituents, but attempt to picture the relations of water to the various parts of the cell. 3°° PHYSIOLOGY Imbibition. — When a plant is placed in dry air, water evaporates from it and its various parts shrink and shrivel. A little shrinkage occurs when plants wilt on a hot, dry day. When water again enters in suffi- cient quantity, they swell and regain their fresh look. The water may even be driven out entirely from some plants,as certain mosses, and when again wetted, the parts swell and regain partly or wholly their original dimensions. The most obvious of these changes are due to the collapse or expansion of the cells; but that they are not limited to alterations in the dimensions of the cells may be shown by measuring a dry bit of cell wall or a dry starch grain under the microscope, and after wetting, remeasuring it. On examination it appears that almost every substance in the plant body is capable of imbibing water, and of swelling or shrinking as the proportion of imbibed water increases or diminishes. The smaller the quantity of water the more difficult, and the larger the amount the more easy it is to remove it. From the fully swollen gelati- nous body of a sea weed, Laminaria, some water maybe extracted by the pressure of the ringers, while the greatest pressure does not suffice to squeeze it all out, and even by heating it is most difficult to remove the last traces of water. Theoretical structures of organized bodies. — A study of the phe- nomena of swelling by imbibition, and of the way in which cell walls and starch grains affect polarized light, permits some inferences either as to the form and position of the particles, or as to the existence of strain or tension between them, by which they are slightly deformed or displaced. These inferences lead to theories of the invisible structure of the cell parts. The particles of which wall and protoplast are composed, it seems probable, are surrounded by water. Whether these particles are the chemist's molecules, linked together in a tense network, or aggre- gates of molecules (micellae) having a crystalline form, which are features of the two prominent theories, is of only remote significance. In either case the water between them may increase or diminish in amount; correspondingly, the particles approach or recede from one another. When any water is present, it forms a connected whole, how- ever irregular its distribution may be. The particles of the swollen stuff also cohere, and remain so related to one another that when the water is all removed, they regain the form they had before it entered. Swelling and solution. — In the recovery of the original form is a practical but only a partial difference between the behavior of merely swollen and of dissolved substances. In both cases water wanders in THE MATERIAL INCOME OF PLANTS 301 among the particles and separates them more or less widely. But there comes a limit to the swelling, and no more water enters. If it is removed, the body regains its form and the particles, presumably, their identical position. In solution there is no limit to separation, except by the amount of water present; and when it is removed, the particles rearrange themselves in forms which may be similar to those of the original body, but are obviously not identical with them. Yet swelling may become excessive, as when starch grains are put into hot water or alkalies, and after certain limits are passed the swollen grain will not regain its normal form. By such transitions imbibition merges almost insensibly into solution. Relations of inner and outer water. — For further understanding it is useful to attempt to picture the relations of the water to the other com- ponents of a young cell immersed in natural water. The outside water has particles of many sorts scattered through it ; for no matter how pure, in nature all water is really a dilute solution of various substances. The water of the cell wall has so many par- ticles of cell-wall stuff scattered through it that nearly half the volume is cellulose; but it is con- tinuous with the water outside. The water of the cytoplasm and of its inclusions is freer of these substances, i.e. it is more nearly pure, because the cytoplasmic particles form only about one fifth of the whole mass. This water, too, is con- tinuous with the water of the cell wall, and with that of the solution outside. The water of the vacuole is still less encumbered with other particles, only one or two per cent, perhaps, but these are of diverse kinds, for the cell sap is a solution of many things. The water here is likewise continuous with that outside through the cytoplasm and wall (fig. 620). Continuity of water. — The picture sketched above may be applied to any plant cell by modifying it to fit special features, and may furnish a working hypothesis, crude though it be, of the invisible structure of organic bodies in general. This hypothesis is conceived to coordinate awe p t v Fig. 620. — Diagram of an imaginary sec- tion through the cell wall and protoplast to show the possible relations of water to the cell ; a, outer water ; w, cell wall ; e, ectoplast ; p, general cytoplasm ; t, tonoplast ; v, vacuole (inner water); e, p, t, belong to the protoplast. 302 PHYSIOLOGY the observed facts of structure and of the migration of substances into the plant. The continuous cell wall determines that only substances soluble in water can enter the body. But according to this picture a continuous waterway is provided along which water-soluble substances may travel. Now in order to conceive how this migration occurs, one must have a mental picture of the behavior of watery solutions. To get such a picture it is necessary to bring to mind certain ideas of physi- cists regarding matter in its various states. 2. DIFFUSION AND OSMOSIS For convenience, matter is said to exist in three states: gaseous, liquid, and solid. Gases. — One characteristic of gases is that their particles tend to separate and to occupy to its utmost limits any receptacle in which the gas is placed. If unconfined by impermeable walls on one side, they form no free surface, but show unlimited capacity for diffusion, and their particles may become so dispersed among the other gases constituting our atmosphere as to be unrecognizable by any means at our disposal. This distribution of the particles is independent of any mixing by mass movements, such as those which show as currents or arise by jarring or stirring. On the contrary, it is assumed to be due to the energy of the gas molecules themselves, being hastened by any means which imparts energy, as by the application of heat. Liquids. — The molecules of liquids are much less mobile than those of gases. When placed in a container, they shape themselves to it and form a free surface that is horizontal under the action of gravity, from which particles may fly off as vapor into the air. In volatile liquids this takes place at ordinary temperatures to such an extent that the process is easily measurable; in others, called non- volatile, the move- ment is too slight to be observed, or is masked by other changes. In- creasing the molecular energy of the liquid, as by heating it (unless it dissociates too rapidly), hastens its conversion into vapor, which behaves nearly or quite as a gas. Solids. — The particles of solids are still less mobile than those of liquids, so that solids retain more or less perfectly their own shape, except under stress. Some solids, like ice and iron, can be liquefied and then vaporized ; others, like camphor, may vaporize without passing through the liquid state. THE MATERIAL INCOME OF PLANTS 3°3 Solution. — In every state of matter there exists a tendency of the particles to separate, hampered more or less by their cohesion or mutual attraction. Even very dense solids, such as lead and gold, when placed in contact, show intermingling along the line of contact, though this is so slow as to be actually measurable only after a long time.1 But when certain solids and liquids are brought together, the intermingling occurs so speedily as to attract attention, and the solid is said to dissolve in the liquid. The liquid then is known as the solvent, and the former solid as the solute. Gases also dissolve in liquids. In like manner when two liquids can be mixed (i.e. are miscible), their particles become intermingled; then one may be considered as the solvent and the other as the solute; e.g. glycerin and water. All gases are miscible and in all proportions; but not all liquids (e.g. oil and water), nor all solids and liquids. Otherwise stated, when one substance dissolves another, the two do not always mix in all proportions; usually there is a limit to the ratio of solvent to solute, and when the limit of intermingling is reached (a condition called saturation), any excess of the solute remains undissolved. Nature of solution. — It is not necessary to the idea of a solution that the mixture should be liquid, though this is the popular usage. A solid, a liquid, or a gas may " dissolve " in a solid and the solution be a solid. So a gas may " dissolve " in a gas and the solution be gaseous. For our purposes, then, a solution is a mixture of substances so intimate that they cannot be mechanically separated; as, for example, by filtration. The actual chemical state of the substances is not certainly known. Moreover, by mingling finely divided but insoluble substances, such as lamp black, with a solution, many particles of the solute may be taken out, probably by adhesion, so that this sort of partial mechanical separation is possible. Water as a solvent. — Almost the only liquid which is of much sig- nificance in plant life as a solvent is water, and this is capable of dis- solving more different substances than any other known; whence it is said to be the most general solvent in nature. In water solutions the par- ticles of the solute behave as those of a gas ; they may diffuse to the limits of the solvent, for its boundary forms the only limit to their movements. Natural solutes. — Water is widely distributed in nature, and comes in contact with many things; first, as it falls in a spray through the 'In an experiment in which a rod of lead and a disk of gold were kept in contact for four years, the gold had diffused over 7 millimeters from the contact surface, in amounts appreciable by assaying. 3°4 PHYSIOLOGY atmosphere, and then as it percolates through the soil and rocks or flows over their surface. Hence, natural waters hold many solutes, and are almost always in position to acquire more if any are removed by chemi- cal action. Thus, the water in arable soils contains everywhere much the same amounts and kinds of mineral salts; for, though soils differ greatly in the proportion of their constituents, the quantities are kept nearly constant by the steady dissociation of the dissolved minerals, by the further solution of any substance which has disappeared from the water for any reason, and by the movement of solutes from one point to another. Diffusion. — If solutes are free to diffuse through the water to its utmost limits, what determines the direction and rate of this movement? Im- agine a crystal of a soluble salt placed in a tumbler of water (fig. 621). The particles fly off from the surface and become numerous in the water immediately adjacent. Here, freed partly from the mutual constraint of the crystalline condition, they may be conceived to be in rapid movement to and fro, colliding often with their fellows where these are most numerous and less often where they are fewer. Hence, in regions towards the crystal, rebuffs are most frequent; con- sequently the particles are continually work- ing out into parts of the solvent more and more remote from the crystal and the crowd Fig. 621. — Imaginary sec- of salt particles, the final result being an equal tion of a tumbler of water with d;stribution throughout the solvent. The a soluble crystal, showing by ° arrows the direction of diffusion, movement is from the region where the par- and by dotted circles the lines of tjcles are most numerous to that where they equal concentration. . . are less numerous, i.e. from the regions of higher concentration of the solute to regions of lower. Or, since gas pres- sure is conceived to be due to the impact of the molecules on the sides of the container, and since the solute behaves as a gas, it is from regions of higher to regions of lower pressure. For convenience, the ten- dency of solutes to diffuse may be called diffusion pressure or diffusion tension. Rate of diffusion. — The rate of movement of diffusing particles of any solute depends on the difference in concentration, or the gradient of the pressure. Thus, when a very soluble crystal is put into a solvent, the rate of diffusion is at first rapid, because an infinitely high concentration THE MATERIAL INCOME OF PLANTS 305 of solution is adjoined by a zero concentration; the gradient is " steep " because the solute at infinite pressure adjoins the pure solvent of zero pressure. But the rate constantly falls as diffusion progresses, since the difference at any two points is becoming less and less. The rate is also greatly influenced by temperature, an increase accelerating and a de- crease retarding the rate, exactly as in gases.1 Osmosis. — Returning now to the conception of the relation of water to the plant cell: it might seem that, given waterways in cell wall and protoplast, any solute, inside the plant or out, might diffuse in any direc- tion in which its concentration is lower. And this would be the case were there no relation existing between the solutes and the material of the separating membranes, the cell wall and protoplasm. These modify the free diffusion; diffusion through membranes or partitions is distinguished as osmosis, and the pressure which solutes may exert on the container is known as osmotic pressure. Unlike gas pressure, to which it is comparable, osmotic pressure cannot be mea- sured directly except with great difficulty. It is calculable from the amount by which a solute lowers the freezing point and raises the boiling point of the solvent. Permeable and impermeable membranes. — Suppose in a closed glass vessel (fig. 622) a glass partition divide A, pure water, from B, a watery solution of salt. No interchange of water or salt between A and B is possible through such a parti- tion, whence it is said to be im- permeable. But if the partition be made of some substance with whose particles salt particles can mingle — a substance, that is, with which salt forms a solid or semi-solid solution — then the salt particles which by diffusion reach the A side of the partition may fly off thence into the water, a; and they will do so, provided the attraction of the water for the salt is greater than that of the partition stuff for the salt. The nature of the partition, then, determines whether any substance may pass through it, and of course modifies the rate of its diffusion. Fig. 622. — Diagram: A, pure water; B, watery solution of salt, or sulfuric acid; p, portion of the partition supposed to be removable ; a, b, air. 1 To avoid misunderstanding it maybe necessary to add that under like conditions each solute diffuses at a rate- peculiar to itself. C. B. & C. BOTANY — 20 306 PHYSIOLOGY This is well illustrated by using air as the partition. In fig. 622, suppose A to be jmre water and B sulfuric acid, with the impermeable glass partition reaching only a little bevond the top of the two liquids, the space above them being filled with air. Water (as vapor) can mingle with air, ss quantities of other substances diffuse from the root into the soil-water films. Solution of carbonates is increased by the pres encc of C02 in water, as is shown by the readiness with which a polished marble plate may be etched by roots traversing its surface and giving off C02. Reactions due to other solutes which diffuse from the root, or to excretions from it, may determine the solution of other sorts of soil, particles, and the substances so dissolved may then enter the root. It is not known that these changes so produced in the soil are of any con- siderable importance in plant life. Whether by diffusion from the roots of live plants or by the decomposition of dead roots, or by both, it is certain that various complex organic compounds, not yet fully known, exist in soils, which may interfere seriously with the growing of plants thereon. In certain soils the character and quantity of these little known substances are so injurious that the soils are almost sterile Even a watery extract from them proves harmful. In such cases the 3' PHYSIOLOGY soil can be improved by mechanical and chemical treatment designed to remove or destroy the harmful compounds. The rotation of crops may find partial explanation herein; the excretions and decomposition products of a given crop may be injurious to the same plants, but less so or not at all to others. Even manuring may prove to have its value less in the compounds put into the soil than in the improvement of soil texture and the destruction of the deleterious compounds in it. Entry of water. — The cells bearing root hairs and the adjacent ones are so constructed as to facilitate the immigration of water and various solutes. The cell walls are thin and the protoplast apparently forms only a thin sheet over the inner surface, the greater part of the cell being occupied by a huge sap cavity. The cell sap is usually a more concentrated solution than the water outside; the internal pressure of the water is consequently less (p. 308), and water enters, distending the cell until the elastic recoil of the stretched wall is sufficient to balance the osmotic pressure of the solutes, or to exude as much water as enters. Entry of solutes. — At the same time, if any solutes to which the pro- toplast is permeable exist in the soil water, but either not at all or in less amount in the cell sap, they will diffuse into the cell. But their move- ment is as independent of the movement of the water as are the condi- tions of such movement ; water and solutes move independently. If any solute which enters thus is not changed or stored in the plant, i.e. if it is not removed as such from solution, it may attain equilibrium inside and outside the plant, so that no more enters ; but if it is removed by being chemically changed or by being stored, more constantly enters. Entry and exit via roots. — The root therefore possesses permeable surface cells always in contact with soil water, through which water and a variety of solutes, chiefly oxygen and mineral salts, make their way, under the conditions already set forth regarding osmosis. At the same time, the root permits through these same surfaces the outgo of any solute formed in the cells, to which the cytoplasm is permeable, that does not exist at equal or greater pressure in the soil water. It is even conceivable that water would pass out thus, were it possible for the soil to become sufficiently dry. Artificially this can be demonstrated; it has not been shown that it occurs in nature. When the roots are exposed to air, as in transplanting, especially if the plants are to be transported far, it is necessary to guard against excessive loss of water by evaporation from the roots; and the quick drying of exposed roots is a most obvious danger in transplanting. THE MATERIAL ENCOME OF PLANT* 317 Aerial permeable regions. — Land plants possess also certain per- meable regions on the aerial parts of- the shoot. Small plants that grow in wet places, where the air is very moist or nearly saturated, might safely have all aerijl parts permeable, because evaporation is slow and the distance from root to aerial surface short. Moreover, spray or rain falling on such parts may enter there, as well as soil water by the roots. But larger plants could not exist in ordinary dry air were their permeable aerial surfaces freely exposed; for if accessible to rain, the evaporation would be dangerously great. So far as protection is concerned, large plants with aerial shoots might thrive (1) if they were completely water- proofed, thus checking all evaporation, or (2) if their damp surfaces were shielded by drier partial coverings, thus reducing evaporation and necessarily excluding water. Waterproofing vs. salts. — There seems to be no a priori reason re- lated to the necessary supply of water and salts why the first of these alternatives should not have appeared in land plants. Structurally, it would be quite possible to waterproof the aerial parts completely, since plants do check water loss by such means in certain places. In such a case, enough water for other purposes might undoubtedly enter, since enough to supply the great evaporation now enters by the roots alone. But, it is objected, this would haVe prevented the intake of sufficient salts. As to that, it is not probable that stopping evaporation, and therefore the large inflow of water at the roots, would interfere with the supply of salts. This is rendered probable, because diffusion of solutes is independent of the movement of water; and to assume, as this objec- tion does, that the solutes are carried along by the entering water which replaces that evaporated, contravenes all that is known about osmotic movement. Further, it is supported by the observation that in the rain forests of Ceylon (and doubtless elsewhere) there are regions of luxuri- ant vegetation where for months at a time the rain ceases only to be replaced by a mist. In such conditions evaporation is almost impos- sible. It cannot, therefore, be necessary to the adequate supply of solutes from the soil. It is difficult or impossible to create such con- ditions experimentally; and ordinary plants, accustomed to evapo- ration, are so upset by being grown in a saturated atmosphere that most culture experiments to ascertain the role of evaporation have failed. The few that have resulted in healthy development indicate also that evaporation is not necessary, so far as a supply of salts is concerned. 318 PHYSIOLOGY Waterproofing vs. gases. — Though water and salts might still be admitted, a complete waterproofing of aerial surfaces would exclude the gases of the air, because all substances must enter in solution. So, as a matter of fact, plants possess aerial surfaces of large extent, freely per- meable, but shielded by covers which, while more or less waterproof, are perforate, so that gases have access to the moist cells underneath. There is one gas, oxygen, needed by almost every plant for respiration, which the terrestrial plants can get satisfactorily only from the atmos- phere. There is another gas, carbon dioxid, which is absolutely essen- tial for the food making of green plants, and this likewise can enter land plants only from the air. As the food made by green plants is the sole supply for them and for most other living things, even for man, and further is the chief source of energy for doing the world's work, it is evidently of some importance that the aerial parts of green plants should expose wet surfaces to the air and so make possible the solution and admission of oxygen and carbon dioxid. Protective tissues. — The admission of oxygen and carbon dioxid by the smaller plants, mosses, liverworts, and the like, is made possible by the fact that the whole surface of the body is moist and therefore permeable. But the larger plants expose wet cell walls only as the bound- ing surfaces of internal chambers that constitute an aerating system, shielded by a nearly waterproof epidermis or a layer of cork tissue. ^ The outer wall of the epidermis has its outermost layer so completely cutinized as to constitute a continuous sheet, the cuticle; and the sub- jacent layers are often infiltrated with cutin to a greater or less extent. Besides this, the epidermal cells not infrequently form wax, resin, and similar substances which are secreted in granules or continuous sheets on the outer wall. These substances all repel water, so that only minute amounts occupy these parts of the wall; consequently very little can escape into the air as vapor. On the older parts of the stem, the epidermis is at first underlaid, and later, sloughing off, is replaced by layers of cells, which, before losing their living contents, impregnate the walls with suberin, so that they become nearly impermeable to water (cork). Both these superficial waterproof tissues, epidermis and cork, are perforate at numerous points (stomata and lenticels), which com- municate with and indeed form a part of the aerating system. (See Part III on cutin and cork.) Aerating system. — This is a network of canals and spaces, of the utmost irregularity in land plants, and connected throughout. The * THE MATERIAL [NCOME OF PLANTS 319 passages are formed gradually among the parenchyma cells by partial separation as they enlarge. At first all cells are coherent with their neighbors, a necessity of the mode of division; hut unequal growth and turgor produce strains which split the common wall at the corners and sometimes along whole faces (fig. 627). In submersed water plants the aerating system attains its most marked development; huge canals arise in the softer tissues of the stems and leaf-stalks (tig. 628), and in Fig. 627. — Cross section of leaf of lily, somewhat diagrammafiMp^^fftr epidermis ; <•', lower epidermis, with stomata, s , in cross section; />, palisad •; l^veowi^iml e', spongy tissue, with large intercellular spaces (i) below .stoma (s) anil vcflSpa^. — From PARI 1. other parts branched cells, the branches in contact only by their tips, leaving large space for gases. These inner chambers in submersed aquatics do not communicate with the atmosphere directly; they con- tain gases which have come out of solution in the adjacent cells and constitute an internal atmosphere into which gases may diffuse or from which gases may migrate into the living cells (of course in solution) (See further, Part III, p. 551.) 3 20 PHYSIOLOGY Fig. 628. — Cross section of stem of Myri- ophyllum, with air canals.— From Part III. Stomata. — The aerating system of the terrestrial plants, and of water plants not normally completely submersed, communicates with the at- mosphere freely, because certain cells of the epidermis, predeter- mined by the mode of their de- velopment, break apart through the central portion of their last- formed division wall. As imme- diately beneath them an air space of some size develops, this estab- lishes a passage to the outer air. These two crescentic cells of the epidermis are the lips of a mouth- like slit called a stoma; the two lips are called guard cells (fig. 629). The guard cells differ from other epidermal cells in their crescentic form and smaller size, and in having chloroplasts which are usually absent from other epidermal cells. Their walls are also peculiarly and unequally thickened (see also Part III, figs. 794-806). Their turgor variations, the unequally thick walls, and their position with respect to the adjacent cells make them change shape, with increasing turgor becom- ing more, arcuate and with lessening turgor straighter. The effect of these changes is to widen or narrow the slit between them, so making more free or restricted the passage of gases either by flow or diffusion. Size and number of stomata. — A stoma is very minute; the area of the pore when open, in thirty-seven sorts of cultivated plants, averages 0.000092 sq. mm. But their great number on those organs (such as leaves) in which the admission and exit of gases is most free, makes up for their small size. Both features will be grasped better by this statement : in an area equal to that of the dot here printed (•), there are on the under side of the apple leaf over 1400 Fig. 629. — ■ Stoma of Scdum ; a, a, a, first wall, cutting off mother cell of stoma ; b, b, b, second; c, c, c, third; d, d, fourth; e , e , final wall ; the latter, forming the two guard cells, g, g, partially splits to form the slit (s); 1, 2, 3, subsidiary cells. THE MATERIAL INCOME OF PLANTS 321 stomata, and on the under side of the olive leaf about 3700. The following table (after Weiss) shows the numbers per square millimeter in various common plants. Name of plant Number of stomata Name of plant Number of STOMATA Upper side Under side Upper side Under side Olca curopaea (olive) . Castalia odorata (white water lily) Helianthus animus (sun- flower) Syringa vulgaris (lilac) Solarium Dulcamara (bit- tersweet) Pisum sativum (pea) . . Ficus elastica (rubber plant) O 460 175 0 60 1 or 0 625 O 32 5 330 26s 216 145 Zca Mays (Indian corn) . Bctula alba (white birch) Berberis vulgaris (bar- berry) Populus deltoides (Cot- tonwood) Pinus Strobus (white pine) A vena saliva (oats) . . Lilium bulbiferum (tiger Hiy) 94 0 O 89 142 48 O 158 237 229 I31 0 27 62 So far as plants have been examined, it appears that a large majority of mesophytes have less than 200 stomata to the square millimeter, and a fair average is perhaps 150. (See Part III, p. 556, on variations in the structure and distribution of stomata, and the causes thereof.) Transpiration. — Since the intercellular spaces arc bounded by moist cell walls, freely permeable to water, they are always filled with air which contains more or less water vapor. This vapor diffuses through the stomata into the drier outer air, and being lost from the plant will be replaced in whole or in part by water entering the root. At the same time, since the walls of the epidermal cells contain a little water, some evaporation takes place directly from them. The total evapora- tion of water under these conditions is designated as transpiration (see p. 323). Exit but no entry for water. — The aerial parts are constantly losing water because they are permeable ; at the same time, there is practically no opportunity for the admission of water, even when such parts are deluged by it. Ordinarily rain comes into contact only with a nearly waterproof surface, the cuticle. It cannot easily penetrate the minule stomata, even when they occur on the upper surface of leaves, for there 322 PHYSIOLOGY are usually some special substances or structures that repel water ; and so it does not come into contact with the wet and permeable walls of the internal cells. Here then is an arrangement, not found elsewhere in the plant, by which water may leave the body rather freely, yet practically cannot enter it when conditions are reversed. It may be assumed that there may enter the cuticle, when wet, amounts of water corresponding to those that evaporate from it when dry. The re- vival of wilted plants after the foliage is sprinkled, however, is due chiefly to checking the evaporation ; yet the trifling amount of water entering tends to the same result. Entry and exit of gases. — The aerial parts facilitate the entry and exit of gases. The external atmosphere communicates freely with the internal atmosphere of the intercellular spaces by way of the stomata. Any oxygen or carbon dioxid in the air of the intercellular spaces may dissolve in the water of the cell walls and then migrate into the adjacent cells, if the pressure of these solutes is less in the cells than in the internal atmosphere. In like manner either may diffuse into the internal at- mosphere when the reverse conditions exist. The solubility of C02 and 02 in water under like conditions is very unequal, the former being about 30 times as soluble at ordinary temperatures as the latter. The rate of diffusion is also unequal. The quantity of each used or produced by the plant likewise differs. These factors all play a part in determining the amount of gas which enters or leaves. As the composition of the internal air fluctuates on account of subtraction or addition of COa or 02, a dif- ference is created between the internal and external atmosphere, which leads at once to diffusion through the stomata in a direction determined by the existing inequality in pressure of either gas.1 Nitrogen, the only other considerable constituent of air, is neither used nor produced; hence practical equilibrium between the N2 of the air and the N2 in solution in the plant is early attained, and this equilibrium is scarcely disturbed thereafter. In submersed plants the oxygen and carbon dioxid are dissolved in the water and find admission at any permeable surface, like other solutes. 1 Further discussion of the r61e of these gases will be found in the sections on Photo- synthesis (p. 363) and Respiration (p. 403). CHAPTER II — THE MATERIAL OUTGO OF PLANTS i. TRANSPIRATION The term transpiration. — Frequent reference has already been made to the most important outgo of material from the plant body — the water evaporated from the aerial parts. This was long ago called transpira- tion, after the analogy of the exhalation of water vapor from the lungs, with whose movements, however, it has nothing in common. It is considered by many to be a function of the aerial parts, something which they actively do, in which case a special name would be quite appropriate. It is better, however, to look upon it as a process in which they are passive. In this case evaporation is no more a " function" of a wet leaf than it is of a wet towel, and the need of a special term is less evident. Yet the word is convenient as a short form for the expression, the evaporation of water from live plants. Evaporation. — When a dish of water is exposed to air which contains less water vapor than it can hold, more water particles will fly off into the air in a given time than will fall into the water from the air; hence the volume of liquid will be diminished; the water evaporates. The rate of evaporation is determined by the temperature of the water, the temperature and pressure of the air, and the relative amount of water vapor in the air (humidity). Decreased humidity, higher temperature, or lower pressure increases the rate of evaporation, and vice versa. The presence of any solutes in the water retards evaporation. Likewise water adherent to any substance, or imbibed by it, is held there and evaporates less readily than if in contact with water particles only. Thus the water evaporates from a dish of wet sand or from a wet towel or sponge more slowly than from an equal surface of free water. since the actual exposed surface may lie greatly increased by spreading out the water over sand grains or linen fibers, the evaporation from a given area of the material is not comparable with that from an equal area of water. Because the evaporation from a green leaf and that from a like area of water are not equal is no reason for giving a special name in the evaporation from leaves, as has been urged. If it were, we should need one term for evaporation from a .123 324 PHYSIOLOGY towel, another for evaporation from a sponge, etc., for the rate varies always accord ing to the material with which the water is in contact. Adhesion. — The water which is part of a plant body adheres to the particles of cell wall, cytoplasm, and its inclusions, and is held with un- equal tenacity according to the amount of each substance and its rela- tion to water. As a rule, the greater the proportion of water in any sub- stance, the less firmly it is held. The attractions between the water particles and plant substance are altered when the plant is " killed." Thus, if a living and a dead leaf be placed side by side in dry air, the dead leaf loses its water much more rapidly than the living one, and shrivels in a few hours. Probably this is in large part due to changes that the cytoplasm undergoes, which we call death; but these cannot be accurately described, beyond certain gross visible changes that do not help us to understand the matter. Cytoplasmic changes. — There are many changes that the cytoplasm may undergo, which, though not visible, occur in the course of daily living. The nature of these changes is not known, and the precise way in which they affect water loss is not known. Some of them may be produced by the very diminution of the water content itself and thus at any moment may operate to alter suddenly the rate of evaporation. A somewhat analogous action is known in the case of a number of salts which form hydrates with variable quantities of water. Thus, copper sulfate forms a pentahydrate, a trihydrate, and a monohydrate. In drying at 500 the pentahydrate (CUSO4, 5H2O) maintains a vapor pressure of 47 mm. (mercury) as long as any pentahydrate remains; then the vapor pressure suddenly drops to 30mm., that of the trihydrate (CUSO4, 3H2O). With further desiccation it again suddenly falls, as soon as the trihydrate is all decomposed, to 4.5 mm., the vapor pressure of the monohydrate (CuS04, H20), and there it remains until all the water is driven off. In this case there would be at each point a sudden fall in the rate of evaporation. Just such sudden alterations have been observed in transpiration. Regulation. — To say that the living protoplast " regulates " the loss of water from a plant is only to say that as the nature of the living material may change, its water relations change, and the rate of evapo- ration changes in consonance. But this is not " regulation " in the sense of adjusting the loss to the income, so that no harm may come to the plant. It is regulation only in the sense that the crystal, when heated, " regulates " the loss of its component water. In both cases evaporation becomes increasingly difficult, and for the plant this may avert death from excessive water loss. THE MATERIAL OUTGO OF PLANTS 325 Influx of water. — Transpiration has been called a function because it creates a current of water through the plant, whi< h was falsely sup- posed to sweep in with it the needful mineral salts. But it is impossible to reconcile this conception with present ideas of osmotic movement. The only condition under which more water can enter is when, by the concentration of solutes in the plant, the internal pressure of the water of these solutions has been reduced; and this is precisely the tendem yof evaporation. If the water and plant substance were in equilibrium, evaporation from aerial parts would upset this equilibrium by reducing the amount of water, which would be replaced by the entrance of water at any permeable region in contact with it. Hut this would by no means furnish an adequate reason for the entrance of any solute which was in equilibrium before evaporation took place. On the contrary, by con- centration of the solution, the tendency would be in the opposite direc- tion; the solutes to which the protoplasts were permeable would emigrate. And the mineral salts in question, being admissible by hypothesis, would do this. Transpiration, therefore, may occasion an influx of water, but not of salt ; indeed it might easily cause an outgo of salts. Transpiration and salts. — Transpiration has been called a function, also, because it was supposed to be useful in concentrating the dilute solu- tions of salts brought up to the leaves.1 That evaporation of water from the leaves would tend to do this is true, of course. But the loss of water is at once compensated, under favorable conditions, by the entry of more water, and the solutions are again diluted. If equilibrium were assumed for the moment, then the disturbance of equilibrium by evaporation would determine a movement of water to readjust it, and the solution would again be brought to the same concentration. Were a liter of water containing a gram of cooking salt set on the fire to boil, and were pure water added as fast as it boiled away, no concentration of the salt solu- tion could occur. But if salt solution were added as water evaporated, the concentration of the salt would be constantly increasing. This idea of the concentration of dilute solutions in the leaves by evaporation in- volves, therefore, the same assumption as the other " function " assigned to transpiration; namely, that water carries along with it the dissolved salts, as a river current sweeps along suspended mud. But this is a mere assumption, and contradicts both theory and observation of osmotic movement. 1 One popular book for children even speaks of leaves as the plant's "kitchens," where the thin "soups" arc boiled down. 326 PHYSIOLOGY A possible advantage. — There is only one region in the plant where solutes may move with the water; that is, where solutions move as a whole, namely, in the conducting tissue, which extends from root cortex to leaf cortex. But solutions cannot enter this tissue in the live plant without first passing through several live cells of the cortex, where os- motic movement only is possible; nor can they usually reach the evap- orating surface of a leaf (the wet walls of the aerating passages) without passing several live cells, where again the solutes and water must move independently. (See movement of water, p. 341.) It is conceivable that the relatively rapid movement of solutions along this portion of the path from root to leaf may be advantageous to the plant by placing a greater supply of salts within reach of the leaves ; but there is no proof that plants depend on this arrangement for an adequate amount of salts. Moreover, this is rendered improbable by the fact that many plants grow most luxuriantly with practically no transpiration for months at a time to set up such a stream of solutions along the conducting tissue. A menace to life. — Transpiration, far from being a function of plants, is an unavoidable danger. That it is a danger, a real menace to life, is almost a matter of common observation. Millions of plants perish annually because the outgo of water is greater than the income. A loose soil and an exposed situation, sudden extreme evaporation due to a hot dry wind and a blazing sun, or prolonged drought, are causes of death only too well known to farmers in some regions. Scarcely a plant escapes the loss of some parts by reason of shortage in the water supply; and in temperate regions, with the average rainfall (say 100 cm. annually), few plants attain the development of which they are capable with a larger water supply. The luxuriant weed of well-watered ground compared with the same weed, meager and dwarfed on the dry wayside, illustrates what a menace to life and vigor is the evaporation from plants. Transpiration and growth. — There are, of course, other causes of stunting and meager development than transpiration. If some of these operate to reduce vigor and growth, transpiration is affected thereby. In fact, growth and transpiration, in seedlings at least, seem to be recip- rocally related, and the one varies directly as the other, when an ample supply of water is available, as in a water culture. It is not improbable that a like relation exists under these conditions in mature plants. Transpiration unavoidable. — Dangerous as transpiration is, it is unavoidable, because moist cell walls must be exposed to permit solu- THE MATERIAL OUTGO OF PLANTS 327 tion and entrance of the gases absolutely indispensable to life. To be sure, the outer walls of the surface cells arc relatively dry, esp© ially in plants of dry regions, where water loss is to be reduced to a minimum. Of the total water lost scarcely more than 20 per cent, and as little as 3 per cent, escapes through the epidermis. This evaporation is sometimes distinguished as cutii ular transpiration. The remaining 80-97 Per cent diffuses through the stomata and constitutes stomatal transpiration. The efficiency of this arrangement in reducing transpiration and yet admit- ting gases freely is more obvious when one observes that the actual evaporation surface — i.e. of the cells bounding the intercellular spaces — is several times that of the leaf itself. The place of maximum rutirular evaporation has hern shown In some leaves to be that part of the outer wall of the epidermis where the side walls abut. In these cases water of imbibition is more abundant there than elsewhere. It is impossible to determine the actual surface exposed in the very irregular air passages. If a leaf 1 mm. thick had an epidermis 0.1 mm. thick of coherent cubical cells on each face, and if the remaining cells were spheres each 0.1 mm. in diameter, tangent to each other, the internal surface would be about fifteen times th< the corresponding outer faces of the leaf. This, of course, does not pretend to pii - ture the actual state of affairs; but it will give an idea of the relative magnitudes involved. Stomata. — The guard cells of the stomata are different from the rest of the epidermal cells in form, in the peculiar unequal thickening of their walls, and generally in the possession of chloroplasts. These characters and the position of the guard cells with reference to the ad- jacent subsidiary cells determine simultaneous differences in turgor and make them behave differently from the others. In general when turgid, they become arcuate, and when flaccid, they straighten. The mechanics of these movements differs considerably with differences of form, structure, and position, and in none of the several types that have been distinguished is it fully understood. The chloroplasts are supposed, but on no very good grounds, to impart power to make osmotically active substances that do not exist in adjacent cells (or are presenl in smaller amount), so that these cells may be more turgid than the others with the same water supply. The longitudinal thickenings are elasti and are supposed to straighten the cells when they become flaccid. The auxiliary cells are supposed to offer proper bracing for the guard cells so that turgor will arch them. Regulation by stomata. — Naturally the guard cells are most likely to be turgid when the water supply is good; then the opening of the slit 328 PHYSIOLOGY between them permits free diffusion of the water vapor into the outer air. Conversely, the guard cells become flaccid with scant water, straighten elasticaily, and practically close the slit. This sort of adjust- ment is held to " regulate " the transpiration, permitting it when water is abundant, reducing it when the supply is inadequate. Yet if the assumption of the existence of special osmotically active substances in the guard cells were correct, they should be the last to feel the slackening of the water supply; and so one must assume further that they are ad- ' justed to water much as are the other green cells of the leaf — an assump- tion which is hardly justifiable in view of their position and connections. Of course the immediate effect of the reduction in area of the stomatal slits is to reduce the amount of vapor diffusing through them. But this in turn would increase the relative humidity of the internal atmos- phere (i.e. that of the intercellular spaces), would cause the accumula- tion of a " head " of pressure, so to speak, that would accelerate the dif- fusion through the narrower slit, and the system would tend to reach an equilibrium again. Thus the closure of the stomata is rendered par- tially or wholly ineffective. Were the internal atmosphere saturated with moisture when stomata are open (as has been assumed) , the closure could not have this effect. But this assumption has not proved correct. Other changes in the external world (i.e. stimuli) affect the guard cells. Of these light is the most notable. In general the guard cells curve in light and straighten in darkness; the tendency, then, is for the stomata to open at a time when the evaporation is greatest and to close when it is least. It is difficult to reconcile the facts with the commonly accepted view that the stomata are " delicately balanced valves " which adjust trans- piration to the " needs " of the plant. If the logic on which that idea rests were valid, it would prove rather that the stomata regulate the admission of gases, since any diminution of the size of the slit must diminish the amount of C02 admitted to the air passages, no " backing up " and accumulation of a " head " of pres- sure being possible in this case, whereas it does occur with water vapor diffusing from the plant. In the absence of any apparent advantage in regulating the movement of gases, and the " need " of some control over evaporation, it has been assumed that the stomata are able to adjust matters so that enough water will flow through the plant, carry- ing with it needed salts, while at any time these governors can check loss when danger threatens. Many cases, however, have been reported THE M \ I I : l< I \l. i »l K',( » < »i PLANTS 329 in which the guard cells are immobile, or respond very sluggishly to external stimuli. Further, the more exael become the studies on plants of desert regions, where the need of an effective regulating mechan- ism seems most obvious, the less efficient do the stomata appear. Rather it appears that they are scarcely more than a tardily acting mech- anism which may save the plant in extremity, hut does not produce any exacl adjustment. Factors in transpiration. — The amount of water losl from a given surface of plant tissue is extremely variable. The humidity of the air, its temperature and pressure, which also affect humidity, and the tem- perature of the plant are the chief factors which cause the rate of evapora- tion to vary. The simplest mode of determining evaporation quanti- tatively is by weighing potted plants at intervals, having prevented evaporation from the surface of pot and soil by some impervious cover- ing of rubber, metal, or wax. It is not justifiable, however, to apply these data to plants in nature. Humidity. — In a saturated atmosphere there can be no water loss. Yet experimentally this is very difficult to establish. The reason is to be sought in two directions. First, it has been found practically im- possible to maintain an atmosphere absolutely saturated at all times, for that means an invariable temperature, which, under other conditions necessary to the experiment, is unattainable. Second, even were the proper external conditions attained, the plant by respiration would be a little warmer than the air, and the air next the plant, therefore, would not be quite saturated; so some small amount of evaporation might take place. Yet during rain, mist, or fog, practically no evaporation occurs; and as the humidity decreases from ioo per cent to the 70 per cent of a moderate day or to the 50 per cent of a dry day, evaporation increases. As the humidity fluctuates from day to day or even from hour to hour, the evaporation varies likewise. The most marked changes in relative humidity are due to the rising or falling temperature of the air. As temperature rise-, relative humidity becomes less, the heat energy im- parted to the plant is greater, arid evaporation is increased by both causes. Barometric pressure. — As the air pressure is reduced the boiling point of water falls; so fluctuations in the barometer indi. ate inverse changes in the rate of transpiration. Yet these variation- at any locality are insignificant; the reduction in air pressure becomes important only in comparing plants at high and low altitudes. In alpme regions, when 330 PHYSIOLOGY low barometer may coincide with low humidity and therefore intense light, the excessive evaporation often becomes a powerful factor in dwarfing plants and in controlling their distribution. Temperature. — The temperature of the plant itself tends normally to equal that of the air, since its extended surface permits quick gain or loss of heat toward equilibrium. A rise of temperature in the air, therefore, is quickly followed by a rise of temperature in the plant, and (even with no change in the relative humidity of the air) by increased evaporation. But the temperature of the plant depends also upon the energy absorbed by the green pigment in diffuse light or direct sunlight. In diffuse light the greater part of this energy is used in food making, and only a small portion exerts a heating effect. But in sunlight two thirds to three fourths of that absorbed is free to heat the tissues, and as soon as that begins, evaporation is thereby much accelerated. This tends to dissipate the heat. It has been proposed to call the evaporation due to the excess of energy absorbed by the chlorophyll, chlorovaporization. The term has its only value in promoting recognition of the fact; but chlorovaporization cannot be distinguished practically from the rest. Were it not for this transfer of energy to the water vapor, the tempera- ture of the tissues would rise to the danger point, or at least to a degree which retards food making. When transpiration is greatly reduced by enclosing a shoot in a glass chamber whose air quickly becomes nearly saturated while the light is absorbed, death quickly ensues. The " scalding " of leaves by sunshine after a summer shower is an example of the same effect. If a plant derives no other advantage from tran- spiration, this prevention of injury by overheating in direct sunlight is certainly one. For even temporary interference with food making might be serious, and permanent stoppage of it by the killing of any considerable area of leaves might be fatal to the whole plant. How- ever possible it might be for plants to meet this difficulty by other methods, if transpiration could be eliminated for other reasons, under the present organization transpiration is of real advantage in this particular. Amount transpired. — Because of the extreme variation, from zero to the maximum, a quantitative statement of the amount of evaporation is of little value, though a voluminous literature records an enormous number of observations and calculations. The following will serve as illustrative examples. THE MATERIAL OUTGO OF PLANTS 331 Measured evaporation from 100 sq. cm. of leaves (200 sq. cm. of surface) in bright hii fuse light, at about 20° c, with humidity about 50 PER CENT I hr. 24 hr. Phaseolus vulgaris 0.117 gm. 2.81 gm. Hedera Helix 0.17 4.09 Begonia argentea 0.19 4.57 Colcus Blumei 0.21 1 5.06 Cucurbita Pcpo 0.224 5-39 Ficus elastira 0.262 6.3 Helianthus annuus 0.5 12.0 Lupinus albus 0.594 14.27 Chrysanthemum frutescens 0.681 16.35 Vicia Faba 0.683 16.4 Hemp plants in a season of 140 days were estimated to evaporate (each) 27 kg. and sunflowers 66 kg. of water. It is estimated that if the water evaporated by the following cereals were again condensed on the area occupied by each sort, say 1 sq. m., it would cover the ground in the case of rye to a depth of 83 mm., wheat, 11S mm., and oats, 127 mm. The average annual rainfall in the north < en- tral states is in the neighborhood of 1000 mm., so that one twelfth to one eighth of the total passes through such cereals. A birch tree with 200,000 leaves is estimated to evaporate on a hot day 300 to 400 kg. A beech, 15 years old, is said to average about 75 kg. per day in the months from June to September, inclusive. At thai rale a he< tare of bee< h forest contain- ing 400-600 trees would evaporate some 20,000 barrels. In all these calculations and estimates a liberal allowance must be made for errors. Reduction of water loss. — Among all the agencies that affect the form and mode of development of plants none has more influence than water, and the relation between the available supply and the loss by evapora- tion. In the peculiarities of form and structure which seem related particularly to water, many see " adaptations " to a habitat with much water, a moderate amount, or a scanty supply. Thus the cutinization of the epidermis, the formation of a waxy or resinous coating, and the development of cork are structures which reduce the loss of water. In other plants the scanty or fleshy foliage, the complete absence of leaves, the development of water-holding tissues, the short cylindric or globular fleshy body, the deep-running roots, and many other peculi- arities (treated more fully in Part III, Ecology) are considered as " adaptations " to a dry climate. It would be better to look upon them as effects of climate and similar factors, since experiments indicate that such " adaptations" can be produced at will, even in one generation, by cultivation under appropriate conditions. 332 PHYSIOLOGY 2. EXUDATION OF WATER Forms of exudation. — Besides the vapor which constantly exhales from plants, liquid water exudes from certain regions intermittently. The places whence it issues are, first, certain specially permeable areas of the permeable regions in an uninjured plant; second, the conducting tissue when opened by some wound. Guttation is the escape of water in drops from uninjured plants. It occurs especially in leaves in the vicinity of the tips of main veins, where there are stomata, often enlarged, called water pores, through which water exudes. Bleeding is the oozing of water from the water-conducting tissues when ruptured. It is espe- cially notable in the spring, before the foliage is fully developed. Secre- tion consists in the exudation of water and solutes from certain special- ized cells, constituting a gland, and found on various parts of plants, but especially on foliage and flower leaves. All these processes are essen- tially similar, with minor differences. Guttation. — Guttation may be readily observed by inverting a glass jar over grass seedlings growing in well-watered soil and thus checking the evaporation. In a short time a water drop accumulates at the tip of the blade and enlarges until it runs down or falls off. Leaves of vigor- ous plants of many species {e.g. aroids, fuchsia, cabbage, nasturtium) under like conditions show droplets of water at the tips, or at marginal teeth, or near the end of main ribs. Accessory structures. — In all these cases an examination shows es- sentially the same features: (a) a rift in the epidermis, or one or more water pores, over (b) a rather large chamber, which is bounded by (r) more or less specialized colorless parenchyma cells (epithem), and near by (d) the tracheids at the end of a vein. The rift in the epidermis may be due (as in grasses) to growth and consequent stretching and rupture. The water pore is simply a deformed stomatal apparatus whose dilated slit is always wide open because the distorted guard cells are no longer motile. When the water pore is single, it is usually greatly enlarged and deformed ; when there are a number together, each is more nearly like an ordinary wide-open stoma. The cells lining the substomatal chamber differ from the mesophyll cells chiefly in lacking chloroplasts. They resemble the sheath of colorless cells, the so-called transfusion tissue, that adjoins the tracheids, which form the endings of the water-conducting bundles of the leaves. In some cases this epithem seems to be a water-secreting THE MATERIAL OUTGO OF PLANTS 333 tissue and to deserve the name water gland; in others it seems to be passive. Guttation in fungi. — Guttation is ool confined to the higher plant-, nor are there always sueh elaborate accessory structures. It occurs in its simplest form in many fungi. Thus Pilobolus crystallinus owes its specific name to the droplets of water which appear on its sporangio- phores (fig. 630), and Mendius lacrymans, the dry rut fungus, likewise, " weeps " so much water that it accum- ulates in big drops on the surface of its sheetlike mycelium. Nightly guttation. — In nature the checking of evapo- ration, which results in guttation, occurs chiefly at night, when many young plants exude water. What remains adherent at the water pore may be partly resorbed when transpiration begins. This seems to be the way in which a destructive bacterial dis- ease of cabbage infects the plants. By contamination of the hanging drop the bacteria find their way into the chamber as the drop evaporates or is resorbed, there develop and so kill the adjacent cells, whence they enter the xylcm bundles and work backward, killing and rotting the bundles. When the crop is gathered and stored, they develop further, until the head is spoiled by the extension of the blackened and rotted tracts in the blanched leaves. Fig. 630. — Sporangiophore of Pilobolus, showing ex- uded water. — Adapted from ZOPF. One may easily observe the exudation of water from the leaves of lawn grasses early in the evening, when the " dew " is said to be " fall- ing." The warm soil conduces to the entry of water; the cooler air checks evaporation; these conditions permit maximum turgor; gutta- tion at the tips of uninjured Leaves, or, more often and more promptly, bleeding from the cut ends of the leaves is the result. Dew, of course, may form under proper conditions; but exuded water forms a great part of what passes as dew. Artificial guttation. — Guttation may be produced artificially by injecting water under pressure into the stem of a plant known to have water pores, as by attaching the end of a cut shoot to a water tap. Presently droplets exude at the usual places. It is usually assumed that the water is thereby forced through the plant tissues, but as city water pressure varies from 2—3 atmospheres (seldom more, and less will often answer), it is doubtful if so low a pressure ias compared with the 3—10 atmospheres of common turgor pressure) would be adequate to do this (see further, P- 3&)- 334 PHYSIOLOGY Quantity exuded. — In a few plants, especially in aroids, guttation under favorable conditions is so rapid that water drips from leaf tips or is even ejected. Thus a vigorous leaf of Colocasia has yielded 1008 cc. of water in 9 days, the water drop- ping at the rate of 85-100 drops per minute at times. C. nymphaeoides has been observed to eject a stream of minute droplets (at a rate of 195 per minute, so that it seemed almost a continuous jet of water) to a height of about 1 cm. Advantage? — Seeing the structural features which permit guttation, one naturally asks, Is it advantageous? To that question no certain answer can be given. It is assumed that the free escape of water at these points prevents its escape elsewhere, and therefore prevents the infiltration of the aerating system with water, which would greatly retard the entry of gases and so the manufacture of food. But there are so many plants which lack the arrangements for guttation that one must doubt if this answer be adequate. Bleeding. — Bleeding may be observed when vines are pruned rather late, or in many cases when a potted plant is decapitated. It must be distinguished from exudation due to heating the water and especially the gases contained in the woody parts of a plant, which has the same general effect. Thus, when a green stick is put on the fire, the scanty sap presently boils out of the ends; for the expansion of the gases and of the water, and later the steam generated by the fire, drive it out forcibly. Or if on a cold day in winter, one bring into a warm room a branch of a shrub or tree, water will soon ooze out at the cut surface. Here the gases in the wood are warmed (for though fuller of water in winter than at other times, the wood is never free from gases, else no green wood would float); they expand, and press upon the free water, forcing it out at the nearest opening. True bleeding, however, is restricted to live plants and is quite independent of any gas pressure due to heat. Industrial applications. — Collecting maple sap for sugar or sirup making is partly an industrial application of bleeding. The work is often begun when only the heating of the twigs on a warm sunny day is active in forcing out the water through the wound made in the trunk; but a great part of the later exudation is dependent on other causes and must be accounted to this extent as true bleeding. Another commercial application of bleeding is found in the collection of the sap of various species of Agave in Mexico and Central America for the manufacture of fermented and distilled liquors. The process begins with cutting out the huge bud at the time when the plant, at the end of 5-15 years' growth, is about to send up the great flower stalk, 12-20 cm. in diameter and 6-10 m. high. Into the basin formed by removing the bud, the plant exudes several liters of water a day, for two months or more; this is collected daily, and after the addition of milk and fermentation is esteemed as a beverage, called pulque. Extensive plantations are devrted to rais- THE MATERIAL OUTGO OF PLANTS 335 ing the agave or maguey, and pulque trains run into lli«' large I ilics, as milk trains do in tin's country. The fermented sap is also distilled to make various fiery alco- holic drinks. Conditions. — The conditions under which bleeding occurs are like those for guttation, a liberal water supply and limited transpiration; that is, the conditions which permit maximum turgor. Even so, not all plants bleed; hence it cannot be at all necessary, nor can the causes be universally active. Cause of exudation. — The cause of bleeding and guttation is to be sought in the development of high turgor in certain cells (on account of the osmotic pressure of the solutes in them to which the protoplast is impermeable), which is made possible by adequate water supply. To stop evaporation by making the air about the aerial parts very moist, or by cutting away the aerial parts, or to have limited evaporation because the foliage is not yet fully developed, are merely ways by which a water supply, that might otherwise be barely enough to cover the evaporation, is made ample; and this permits high turgor when other conditions are- met. When the turgor rises to a certain point in the active cells, it seems that water is exuded. This may be mere filtration under pressure. But we may also conceive it to be due to a sudden alteration of the permeability of the cytoplasm, wrought by the very pressure itself. In that event, upon the relief of pressure when the outgo oc- curs, there would be a gradual recovery of impermeability and consequently of turgor to the maximum; then another automatic change of permeability, a conse- quent outrush of water, and so on. This outflow naturally cannot be pure water : but on the theory of filtration the water will contain at least the substances to which the pro- toplast is permeable; and on the second hypothesis, any or all solutes might be released, the sap as a whole escaping. In the water there are often substances in small amount, regarding whose osmetic relations we are ignorant, though the general assumption is that they could not pan- tile cytoplasm without some special modification of its permeability. When that is demonstrated, it will be necessary to adopt the second hypothesis, which is also used to account for the presence of such ^lb- stances in secretions (see p. 340). Until then it will suffice to assume that they issue with the water because they are free to do so. Tissues concerned. — In the case of Pilobolus and like plans, the tur- gor which causes the escape of water evidently arises in the verv cell or 336 PHYSIOLOGY coenocyte from which it escapes. This may also be the case in guttation among seed plants. The epithem of the water chamber, receiving an adequate supply of water from the adjacent vein, may develop turgor sufficient to cause water to pass the cytoplasm and the wall. It is ob- vious that to issue from the free surface it will encounter less resistance than elsewhere; consequently it takes this direction. The chamber fills and water soon oozes from the water pore. But the epithem can- not develop an adequate turgor unless the water supply is sufficient. That may be made sufficient either by checking the transpiration, or by forcing water up to these cells so that they may get enough, even though transpiration is unchanged. Water may be supplied thus artificially by cutting the stem and attaching it to a water tap; or the same end would be accomplished in nature if the root cortex had a supply adequate to enable it to become fully turgid and exude water under pressure into the conducting system. " Root pressure." — The condition just mentioned often exists in the root cortex, and perhaps always when plants are not flaccid. The loca- tion of this turgor has suggested for it the name " root pressure." This is unfortunate, because it tends to obscure the fact that any live thin- walled cells with like conditions may develop a turgor which will cause water to exude. Thus, bleeding was found to occur in the inflorescence of some tall palms, but the root cortex had no part in so distant an exuda- tion; the pressure originated near the base of the flower stalk. The " root pressure " being a frequent cause of bleeding, the phrase " bleed- ing pressure " has been suggested as a substitute; but this is little better, since whether or not bleeding results is purely incidental. No special term is needed other than turgor pressure; that is general and is specific enough. (See also p. 349.) Amount and pressure. — Experiments on bleeding are often con- ducted with potted plants, which are decapitated, and to the stump is affixed apparatus for measuring the amount of water exuded, or the pressure with which it is forced out. With trees, the trunk is bored and the receptacles or gages attached. A few examples will give an idea of the maximum quantity of the sap and the pressures involved. A calla lily bled 39 cc. in 24 hours. A vigorous European grape some- times exudes nearly a liter per day. The Mexican agaves, cultivated for this purpose, are said to give out 5-6 liters daily for several months. Under favorable conditions, the sugar maple yields 5-8 liters in the course of a day, and the birches give out about as much. THE MATERIAL OUTGO OF PLANTS 637 The pressures, recorded in millimeters of mercury (700 = 1 atmosphen from o to Ribcs rubrum (red currant) 358 Acer platanoides (sycamore maple) ,347 Acer saccharum (sugar maple) 1033 Psedera quinquefolia (Virginia creeper) 615 Betula alba (white birch) 1390 Betula lutea (yellow birch) 1.S15 Betula lenta (black birch) 2040 Vitis vinifera (European grape) 860 Much study has been given to variations in the amount and pressure of bleeding; seasonal and possibly diurnal fluctuations have been dis- covered; but inasmuch as turgor pressure must be influenced by tran- spiration, itself of infinite variability, the precise results of these studies are not important. The limited movement of water through submersed aquatics which has been described cannot be due to transpiration, and is probably not a case of guttation. The experimental evidence is scanty and the movement may be referable to the larger heating effects on the Leaves as compared with the stems. This should create a slow movement of water out of the leaf, to be supplied from below. Secretion. — Secretion is a much more general and varied phenomem >n than guttation or bleeding. It is performed by more limited and specialized tissues, called glands, and the variety of substances which escape is much greater, though the amounts lost are much smaller. Many of the secretions are of such a nature that they play an important part in the life of the plant; others are of no use so far as we know and are therefore called waste products. No dis- (l£^y tinction can be made in plants between useful secretions |g| and waste excretions. £**• Glands. — There are some glands which secrete water, „ F10-6^- — ,. . , ,., , . , . ^oung gland- with no distinctive solutes, like that which escapes in uiar hair oi guttation and bleeding ; and because there are no dis- Pdarpmium : tinctive solutes these are called water glands. Glands volatile oil.— are named usually according to the most abundant or lrom 1>;Ur characteristic material they -circle. Thus those in whose secretion calcium salts become conspicuous by concentration are called lime glands; digestive glands secrete water containing enzymes. Most common of all are the nectar glands or nectaries, abundant in flowers, but found also on oilur parts as extra-floral nectaries (fig. 1183), whose 338 PHYSIOLOGY water is sweet with sugar and often fragrant. Not all glands, however, secrete water and its solutes. There are glands whose secretion is an essential oil,1 of which a great variety are formed. Still others secrete resin, which may be formed from an essential oil. Form of glands. — The form of glands is various. A single epidermal cell may differ from its neighbors; it may be level with them, or sunk, or raised upon a shorter or longer stalk, like the glandular hairs (fig. 631). A filament or a cluster of such cells may form a stalked gland (fig. 632); the gland cells may form a rather indefinite mass, or they may line a shal- low cavity (fig. 633), or a deep pouch, as in the nectary of the nasturtium (fig. 634); or they may be the epi- thelium of a sim- ple or branched duct, as in the lilies (fig. 635). Nor do all glands pour out their secretion on the surface. The gland cells may part when young, forming intercel- lular spaces into which the secretion exudes to escape through water pores. Or a single intercellular space may develop in the center of the group (fig. 636) which receives the secretion; then as the gland and space grow, the secreting cells form an epithelium for a closed reservoir, larger or smaller, containing the secretion. Or, later, by the destruction of the gland cells loaded with the secretion, it finally occupies their place as well as the intercellular space, and reaches the surface only by mechanical rupture of enveloping tissues (fig. 637). Emission of secretions. — Very little is known of the chemical pro- cesses by which the peculiar materials of the secretion are formed. Each sort of gland doubtless pursues a different course. Nor is it pos- sible to account for the emission of the various substances. Some, like 1 Not true oils, from which they may be distinguished by making only a transient grease spot on paper. Fig. 632. — Gland (g) from the upper surface of the leaf of lilac (Syringa vulgaris): e, epidermis; c, cuticle; p, p, palisade cells ; i, intercellular space. THE MATERIAL OUTGO OF PLANTS 339 cane sugar, are known to be re- tained ordinarily by the cyto- plasm ; yet nectar glands se< rete sugar one or more times. Others, for example enzymes, have a com- position which, though imper- fectly known, is such as to suggest that the cytoplasm would usually be impermeable to them; yet di- gestion occurs in such places as to make it certain that enzymes are able to pass out of the cells in which they arise. Fig. 633. — Section through a petal of buttercup (Ranunculus), showing nectar gland (n) and shallow receptacle formed by the "nectary" (a). Note bundle of conducting tissues (x). — After Bonnier. Fig. 634. — Flower of nastur- tium (Tropin rolum mains) cut through the middle to show the spur (5) and the nectar («). Fig. 635. — Net tar gland in the ovary of day lily {HemerocaUis Jiava). — After ScHNIl WIND- Tim S. 34o PHYSIOLOGY The problem, therefore, is: How can solutes pass the ectoplast usually impermeable to them ? The answer is merely in the form of a hypothe- sis, like the one already proposed to account for guttation and bleeding. If the accumulation of the solute causes a rise of turgor, it is conceivable that the very pressure itself might work such a change in the cytoplasmic membranes that they alter their permeability and permit the outrush of water and its solutes in the direction of least resistance, which will be toward the free surface. Whether a renewed secretion will take place de- pends on the further activity of the cell. Given a repeated formation of the secretion, it might escape again. The hypothesis then suggests a rhythmic variation in the permeability of the cell membranes, the secretion being formed inside the cell. Fig. 636. — Young resin gland of fir (Abies): a, duct, an intercellular space formed by the sepa- ration of the four nucleate cells. — After Tschirch. This hypothesis is clearly inapplicable to secretions which are not miscible with water, like essential oils and resins. They are probably formed, however, in the very wall itself, and thus the material may not have to traverse the ecto- plast as resin or oil. Unfortunately, even the place of their origin is still obscure. Role of certain secretions. — Nec- tar is gathered by many insects, some of which store it, after partial digestion, as honey. While the floral glands are being explored for nectar, the visitors become dusted with pollen and transfer this to ripe stigmas of the same or other flowers, thus insuring pollination in many cases where otherwise it might not occur (see Part III on pollination). The role of extrafloral nectar is not clear. Digestive glands, most defi- nite in insectivorous plants (p. 386), secrete enzymes (p. 399) by which the soft parts of captured insects are dissolved. Essential oils (p. 413) sometimes prevent plants from being eaten by animals. Fig. 637. — Oil receptacle (a) in orange (Cit- rus Aurantium), formed partly by splitting, but chiefly by destruction of secreting cells and their neighbors (t) ; 0,0, drops of essential oil. — After Tschirch. THE MATERIAL OUTGO OF PLANTS 341 3. THE MOVEMENT OF WATER Transpiration stream. — In the two foregoing sections it has appeared clearly that the region where water enters a plant and the region whence it leaves are rarely identical, but that these parts arc more or less widely separated. There must be, therefore, movement of wakr through the body. Small quantities of water are used in the body for saturating new-made materials and parts, and for food making by green plants. Somewhat larger quantities arc exuded by guttation, bleeding, or secre- tion. But the dominant cause of movement is to be found in evaporation, for the amount thus leaving the body is often many times greater than all other quantities combined. So considerable is it that the How through the body is figuratively known as the transpiration stream. Transfer in small plants. — In the smaller land plants, whose bodies are composed of living cells throughout, as in many liverworts and mosses, the water has to travel but a short distance, and the movement can be osmotic only. Evaporation at an exposed surface concentrates the solutions in those cells, thereby reducing the internal pressure of the water, which moves from an adjacent cell to reestablish equilibrium, and so the disturbance soon reaches the surface cells in contact with free water, which enters the plant. Origin of a conducting system. — We might infer that these osmotic movements are too slow to afford a proper supply to larger plants, be- cause, as an actual fact, they are in operation for only relatively short distances; the larger the plant and the more necessary a large supply of water, the more perfect and extensive becomes the special system of tissues for conducting water by avoiding osmotic transfer. This is especially striking when one follows the development of such a plant as a sunflower from the embryo, a stage when there is no water-conducting tissue, to maturity, observing how the extent and amount of this tissue increases as the foliage develops and so increases the evaporating sur- face. It may be assumed that somewhat similar has been the history of the evolution of land plants. As the early aquatics became more and more exposed to evaporation, there probably came about the develop- ment of structures which limit the water loss, and simultaneously the development of the water-conducting strands, which greatly facilitate water movement. Elongation of cells. — Presumably one of the simplest expedients to accelerate movement is to reduce the number of membranes which the 342 PHYSIOLOGY water must pass osmotically. This could be accomplished to a certain extent by elongating the cells in the main direction of travel; and it may be that elongation of the cells was one of the early steps in the evolution of a conducting system. To-day there are plants in which such strands exist, as in the stalk of the sporophyte of liverworts and mosses, and these are often accounted rudimentary conducting tissues.1 Lignified traeheids. — The complete elimination of the cytoplasmic membranes may well have been a second step in evolution. This would make movement more easy by removing just so much resistance from the path. If in addition the walls were altered so as to be more freely permeable to water, movement would thus be further facilitated. That change, known as lignification, is indeed common. Then by thickening the wall only in parts, leaving the rest thin, passage of water through it by way of the thinner areas would be still easier. Strands of elongated cells of this sort constitute the endings of the conducting system in the leaves of almost all plants, and they form almost the whole of the characteristic wood in gymnosperms and the conducting strands in pteridophytes. Tracheae. —One further step attains the condition in the most per- fectly developed conducting tissues, namely, the resorption of the greater number of the transverse partition walls between the elements, forming cell fusions of great length, known as ducts, or vessels, or tracheae, the latter from their occasional resemblance to the human trachea and the air tubes of insects. Resorption does not usually occur near the endings of the strands in the leaves; and in gymnosperms it fails except in the primary strands. But the other changes do occur, and the elements being cells and not cell fusions are distinguished as traeheids. Following the history of any row of cells which is to become a duct, there is first the elongation of the cells; then the unequal thickening of the wall and its lignification, together with the resorption of most of the end walls; and finally the disappearance of the protoplasm. Some such steps as these may also have marked the evolution of the conducting system through earlier ages. Xylem. — The conducting system in the larger plants now consists of a series of strands known as xylem strands or as the xylem regions of the vascular bundles (p. 242). Physiologically it is more satisfactory to treat the xylem as independent of the phloem (p. 242), for although they are usually closely associated in their course, they may be independent, and the functions of the two are quite unlike. The xylem strands form a 1 This is based too much on analogy and inference; the experimental evidence is weak. TIIK M VI i:KI AL OUT(H) OF PLANTS 343 connected >eries, extending from the tool hair region t<> the mesophyU of the leaves, among which they branch so extensively thai there is s< an ely a cell which is separated from a strand by more than a half dozen of its neighbors. Here the first branches end blindly (fig. 638) or join their fellows. A section of the root in the root- hair region shows likewise that only a few cells intervene between the free surface and the young xylem strands, which, nearer the root tip, are being differentiated from the plerome (p. 239). Like- Fig. 638. —Ending of a xylem strand among the cells of the mesophyU in a leaf of lilac (Syringa vulgaris) : I, trachcid ; i, in- tercellular space. Fig. 630. — Skeletonized cdpe of a leaf of a Finis, shoving the mode of branching of the smaller ribs ; the smallest are completely gone. — From a photograph by Land. wise, a section of the leaf (fig. 627, p. 319) shows the relations of this water-conducting tissue to the surface, and an examination of the vena- tion of various leaves (of which only the larger veins are visible to the 344 PHYSIOLOGY unaided eye) shows how extensive is the branching (fig. 639). Be- tween these extremes the bundles run, with lateral connections here and there, especially at the nodes, and more or less variation in size and branching. Tracheal markings. — The walls of the tracheae are always peculiarly thickened, the thick regions being in the form of rings, or spirals, or a network (figs. 640, 641). The thin parts may be more extensive than the thick, as in annular and spi- ral tracheae (figs. 640,0,5; 641, s); or they may be mere spots in the midst of the thick wall, as in pitted tracheae (figs. 640, p; 641, p, ;-). The thick and thin parts in adjacent tracheae or tracheids correspond; and thus the movement of water laterally, when conditions require it, is facilitated. In scalariform tracheids the parts of the wall not thickened are resorbed, and the neighbor- ing cavities communicate freely. W2 u 3 s 3 p y C Fig. 640. — Longitudinal section (diagrammatic) of a young xylem strand : c, cambium ; y, young trachea, undifferentiated except as to size ; p, pitted trachea; 5, s, s, spiral tracheae; a, annular trachea; m, pith. — After Haberlandt. If water in which some cinnabar has been rubbed up be passed through filter paper, to remove all but the very finest particles, and then the fil- trate is driven under pressure through a piece of fresh pine wood, the pits become choked with cinnabar, showing that water filters through them more easily and so in greater quantity than elsewhere. Secondary thickening. — The primary xylem, i.e. that differentiated from the young tissue near the growing points (fig. 642), is adequate to supply only the first leaves. As with age the foliage increases, each primary xylem strand may undergo secondary thickening; i.e. it has added to it similar tissues, originating from a layer of dividing cells which adjoins its outer face (fig. 643). In addition, this meristem (cam- bium), arising between the primary strands, may originate new strands of xylem tissue between the primary ones. These secondary strands may then increase in thickness in the same manner as the primary THE MATERIAL ()UT(!() OF PLANTS 345 f — s •" - ■ r Fig. 641. — Enlarged details of spiral (s), pitted (/>), and reticulate (r) tracheae; at d, traces of original partition walls. — Adapted from IIabeklanut and TSCHERCH. ones. When numerous primary and secondary strands are produced, they may form a column of xylem, with pith in the center, interrupted by thin radiating plates of parenchyma, the pith rays. Such is the condition in the sun- flower, castor bean (fig. 644), and many other di( otyledons. In case the xylem strands do not undergo individual secondary thickening (as is the case in most monocoty- ledons), there may be a cylinder of meristem which repeatedly produces new bundles, as in asparagus. But in all plants which produce numerous leaves the increasing evaporation is Fig. 642. — Young vas- cular bundle: />, primary phloem ; .v, primary xylem ; r, first divisions of cambium cells. — After Bonnikk. Diagrammatic. Fig. 643.— Older vas- cular bundle, with second- ary thickening in , p, phloem ; e, cambium, forming by division both secondary phloem and xylem; .v, xylem, com- post '1 of .Vj and .v., the primary and secondary xybm. — Ai't.-r Bonnier. 346 PHYSIOLOGY accompanied by an increase of the conducting tissues (see Part I, p. 243). Annual thickening. — In trees and shrubs the xylem undergoes sec- ondary thickening in the first season of growth, and this is resumed in the second season, and so on, from the persistent cambium. Thus arises a great cylinder of xylem, which constitutes the wood of the trunk and Fig. 644. — Cross section of stem of Ricinus communis, showing ring of secondary xylem; for description, see fig. 541. — From Part I. branches. In many trees the xylem formed in the course of the growing season gradually changes its character. The first formed tissues con- tain many large ducts and less mechanical tissue, while the later formed xylem has small ducts and much mechanical tissue. In these cases the open tissues produced in the spring abut on the denser ones last pro- duced in the summer or autumn, and the sharp contrast marks visibly the periodicity in growth. As these differences in the tissue depend upon growth, and as this is most affected by the annual seasonal changes, the growth rings are usually annual rings, and make possible an estimate of THE MATERIAL OUTGO I >l PLANTS 347 the age of the tree. But annual rings may show subordinate rings, due to some pronounced climatic change whi< h affe< ted the rale of growth more than once in the year. These rings may be so pronounced as to make the age estimate uncertain, but in temperate regions the annual rings are usually well defined. In some trees the differences between spring and autumn wood are slight, and the annual rings are discerned with more difficulty. The definite annual rings are responsible in large part for the " grain " of wood. (See also Pari III on annual rings.) Heart wood and sap wood. — With age the xylem loses its capacity to conduct water, and sooner or later may so change in color and com- position as to distinguish the older heart wood from the newer sap wood. These changes, however, do not coincide with the annual rings, nor do they exactly correspond with the differences in conductivity, since in some plants the whole of the sap wood, hut in others only the youngest portion of it, is traversed by the transpiration stream. Xylem is water path. — The evidence that the xylem is the path of the transpiration stream rests in part upon direct observation, hut mainly upon inference from the effects of cutting the xylem strands or hloi king the tracheae. Relative development. — In the first place, one finds a general relation between the amount of transpiration and the development of the xylem. In most submersed water plants the xylem is very poorly differentiated, its place being occupied by some elongated cells, slightly different from their neighbors, which are morphologically equivalent to xylem, but physiologically they are negligible. On the other hand, in climbing plants, whose spread of foliage is large and their stems slender, the xylem reaches its best development, occupying a large proportion of the cross section of the stem, and having ducts of relatively large diameter. Not much reliance could he placed upon such a loose and general relation, were it not for more direct evidence. Girdling. — Girdling experiments show more clearly the path of the water. It is a matter of common knowledge that by tutting through the sap wood of a tree the foliage promptly wilts and dies; and in earlier days it was commoner than now to see the trees in some piece of woodland "girdled," preparatory to clearing the ground for cultivation. But removing only the hark does not produ< e wilting, except after weeks or months, for thus only the phloem strands an- interrupted. More exact experiments may he performed. By selecting a herbaceous plant whose vascular bundles are distinct, one may cut the pith, the vascular 348 PHYSIOLOGY bundles, and the cortex in different specimens and compare the effect. It will be found that only in the specimens whose bundles have been cut do the leaves wilt, and the fact that in woody plants the bark may be removed without causing wilting eliminates the phloem strands. Such experiments permit the inference only that the xylem strands are the chief paths of the transpiration stream, not that they are the sole path ; for wilting implies merely an inadequate water supply. Water moves in the lumina. — But the path can be localized more exactly. A shoot of a climber, such as Clematis, may be cut off under water, and the end sliced very obliquely, so as to open wide the ends of the ducts. If this shoot be fastened to a microscope slide, and the end covered with water, into which has been introduced some finely divided carbon, as from Chinese ink, one may watch the water swirling into the open ends of the ducts, its course being made evident by the opaque particles it carries. Under such circumstances it is evident that the water enters and probably traverses the lumen of the trachea. But this was for a long time a disputed point. When the extraordinary freedom of movement of water in lignified tissues was discovered, it was held that the water traveled in the substance of the walls and not in the lumina (the chambers they enclose). This opinion, however, rested upon inac- curate experimentation. Closing the lumina. — Attempts were made by compressing the stem in a vise to collapse the tracheae, and so to close their lumina. In the earlier experiments of this sort, wilting did not occur, and the inference was plain, therefore, that the water traveled in the wall itself. Repeated studies showed that the difficulty of compress- ing the tracheae had been underestimated, and that when they were actually closed mechanically, the leaves did wilt. A better method of closing them is by plugging them with paraffin or gelatin which melts at a low temperature. By cutting a shoot under the melted material, it is carried up instantly to some distance in the tracheae. When cooled, it solidifies and a fresh surface of wall can be exposed by removing a thin slice, while the lumina remain plugged. The leaves of such a shoot promptly wilt when exposed to dry air. Path of least resistance. — On the whole, therefore, it is fairly certain that the transpiration stream traverses the xylem strands, and that it is the lumina of the tracheae that form the chief conduits for the water. That some travels in the walls is quite probable, especially when the tracheae are partly blocked, as they often are, by gas, the path of least resistance being followed here as always. Nor is it impossible that some water moves in the cortex; but this is never enough to cover any considerable loss by evaporation. THE MATERIAL OUTGO OF PLANTS 349 Ascent of water. — As to the forces concerned in the ascent of water, little that is definite can be said, for the problem is one of extraordinary complexity, and knowledge of the exact physical conditions is very difficult to attain. Nor is it likely that the problem i ould be solved wen- all the factors in the plant body known, simply for lack of knowledge of the physical principles involved. Capillarity. — Some " causes " frequently assigned and popularly current may be definitely discarded. The first of these is capillarity, as commonly understood. The xylem ducts are narrow tubes. Water rises in capillary glass tubes above the level outside, and the smaller the bore the higher it rises. Oil rises in a twisted lamp wick by capillarity. What more simple than to " explain " the rise of water in the ducts of the xylem strands by ascribing it to capillarity, since here are " strands " and " tubes"? But surface tension (which is a better name for capil- larity) implies a free surface, and within the duct there can be no free surface which is lifting, as in an open glass tube. If one appeals to the surfaces bounding the bubbles of gas so common in tracheae (see p. 350), it must be remembered that for every meniscus concave upwards there is one concave downwards to balance it. Nor can one neglect the numer- ous transverse walls in the xylem of angiosperms,1 and the fact that all the effective xylem of gymnosperms is composed of tracheids. How- surface tension forces may operate at the evaporating surfaces in the leaves is not known; but these are not the ones referred to when capil- larity is invoked as the cause of the ascent of water, or at least an aid to it. Root pressure. — Root pressure (see p. 336) is frequently alleged to be active in forcing water up; and it is even held to be adequate in the case of the herbaceous plants and low shrubs, though confessed to be insuf- ficient in the taller trees. The radical difficulty with turgor in the root cortex as a cause of the ascent of water, or at least an aid to it, is that it does not exist when it is most needed. In the very nature of the case the root cortex can be fully turgid only when it has an abundance of water; and it is not likely to have that when evaporation is active. To develop root pressure it is necessary to check evaporation, as by decapitation, and only after a time does water begin to ooze from the xylem in conse- quence of turgor. ( >ften water at first enter- the stump of a da apitated plant, showing clearly that there was no surplus of water under previous 1 The longest continuous ducts found exceed 5 m., but those 1 m. long are ran-, and the average is probably less than 10 cm. 3 so PHYSIOLOGY conditions. Nor can root pressure be invoked even as an aid. For unless maximum turgor can be attained no extrusion of water from cortical cells is possible. If a boy could push a wagon while the horse walked, he would be unable to push as soon as the horse's speed exceeded his own. If he clung to the wagon, he would be merely a drag, though if he ran he would be less of a drag than if he made no exertion. The transpiration horse often goes too fast with the water wagon for the root pressure boy to push. Then his grip is broken at once and he is no drag on the load, for root pressure cannot even hold on like the boy and " help " by not being wholly a drag. Atmospheric pressure. — Atmospheric pressure has been invoked as an explanation. It is found that the gases which develop in the tracheae are often under a pressure less than one atmosphere. Indeed they develop there readily because this is the case. The tracheae, it must be remembered, are dead cells; their lumina therefore are as free to be occupied by gases as are intercellular spaces. Whenever the concen- tration of gases dissolved in a free liquid exceeds the amount normal at one atmosphere pressure, the gas particles escape from solution and form bubbles. This happens when any bottle of liquid " charged " with CO2 is opened. The gas is dissolved in the liquid under a pressure greater than one atmosphere; on un- corking it the pressure is reduced immediately to that of the outer air, the gas flashes at once into bubbles, and portions of the liquid are often forced out of the bottle by the violence of effervescence. Bubbles would inevitably form in the water of the tracheae, whenever that water has the pressure on it reduced below one atmosphere. If this pressure were equal to half an atmosphere, it is argued that such tension could " lift " water about 5 m. So it could, if the lower end of the water columns were open to the pressure of the atmosphere and there were no resistance. If one took away half an atmosphere of pressure from the upper end of a water column and left a whole atmosphere of pressure to act on the lower end, of course the water would rise to the point of equi- librium. But these conditions do not exist in the plant. Evaporation may reduce the pressure on the water in the tracheae, but the lower end of the water column is not open. The living cells of the root cortex are interposed, and water cannot be driven through them by a difference of half an atmosphere or even a whole atmosphere of pressure; nor has the pressure in the tracheae ever been found to fall to zero. If it were zero, and there were no resistance to the movement, water could be pressed up to a height of only 10 m., a small fraction of the 100 m. I ill. MATERIAL * HJTGO OF PLANTS 351 which the tallest trees attain. Atmospheric pressure therefore is utterly inadequate at best. The most that can be allowed is> this: by how much the difference in atmospheri* pressure in the tracheae and in the air tends to make it easier for water to pas> through the root hair and the root cortex, by so much atmospheri< pressure may be said to help in the entry of water. But the very fact thai these differences exist shows that they are not compensated by the movement of the water. In fad the difference between inner and outer pressure seems to be rather a result than a cause of water movement. Role of living cells. — The ultimate cause of the ascent of sap is tran- spiration; but how it acts is entirely unknown. The energy employed in vaporizing the water is adequate to lift it miles high; but how i> it ap- plied so as to keep a continuous stream rising? One link in the chain is the osmotic relations of the living cells of the leaf; for if the leaves be killed, evaporation continues from their cells, but the supply from the xylem strands is interrupted and the leaf dries up promptly. It was also proposed, first many years ago, to ascribe the ascent of water to the action of living cells along the course of the xylem strands, and this theory is being advocated again to day. One notion of their action was that it is like that of relay pumps, which take water in at one level and force it up to a higher level. It is difficult to conceive the physics of such an operation, and there is no anatomical evidence of such a mechanism, unless the cells of the pith rays are the active cells. The experimental evidence as to the cooperation of live cells in the process is contradictory, to say the least, and by its very nature the theory must be rather vague. That the living cortex and wood parenchyma are neces- saryto keep the xylem in proper condition for conduction is assumed. Cohesion theory. — A current theory, which also is confronted by many difficulties and leaves much to be explained, is based upon the fact of the cohesion of water. That serins, at first blush, like talking of the strength of a rope of sand; but it is actually very difficult to break a small column of water, if sidewise or shearing strains are eliminated. I'hc cohesive strength of water is variously estimated by physicists at 10-150 atmospheres. The rupture of sporangia <>f ferns and the anthers of flowering plants, and the collapse of cells on drying, have now been shown to depend upon the cohesion of water. The mechanism for spore scattering in the sporangium >>f a fern, for ex- ample, is illuminating. It consists of thi< k walled 1 ells around the edge, the annul us 352 PHYSIOLOGY (a, figs. 645-647), which contain water. As the water evaporates it pulls the cell walls together, and in doing so straightens the ring and tears open the weak side. The thick elastic C~shaped walls of the cells resist this compression, until finally the cohesion of water in the wall with the free water in the lumen is overcome, and the sudden elastic recoil of the annulus hurls the spores as from a sling. Figs. 645-647. — Rupture of sporangium of a fern (Polystichum acrostichoides): 645, the sporangium cracked; a, the annulus; 646, position of complete reversion, many of the spores adherent to the upper part of the sporangium; 647, position after recoil, the sporangium emptied; dotted lines in this figure show the position as in 646. — After Atkinson. This cohesion is predicated of the columns of water which occupy the tracheids and tracheae of the xylem, and it is coherent even through the end and side partitions (see theory of relation of water and cell wall, p. 301). If now any adequate lifting force could be applied at the upper end, the cohesion of the water is sufficient to enable it to hold together even to the roots of the tallest trees. That lifting force is evaporation, and the osmotic relations of water in the live cells of the leaf furnish the connection. Why the water columns do not break wherever bubbles of gas appear (and they must appear whenever the column is under any considerable strain), is not satisfactorily explained; and other like difficulties appear. Yet this theory at least faces in the right direction, seeking to give an account of the rise of water in purely physical terms. However, as this phenomenon has baffled investigators for more than a century, it may be a long while before it can be satisfactorily described. 4. OTHER LOSSES Gases from the shoot. — Quite apart from the liquids and water vapor which escape from the aerial parts, there are gases which are constantly set free and leave the plant as such. These are carbon dioxid and oxygen; the former is one of the usual end products of respiration, and THE MATERIAL OUTGO OF PLANTS 353 the latter is a by-product of food making, bul is used by all live parts in respiration. Carbon dioxid i> continually produced in all live part-; but in green parts, when adequately lighted, it < an be used for making food, and therefore in these parts under such conditions it never accu- mulates to an amount which permits it to diffuse out. Oxygen is only intermittently produced. When the green parts are making certain foods, its production is a measure of their activity; l>ut that takes plat e only in the light. Since, therefore, the leaves are the green parts par excellence, oxygen escapes chiefly from them, because the amount pro- duced is in excess of that used in their respiration. When it has accu- mulated in the cell sap to a concentration whose osmotic pressure is greater than its pressure in the air (i.e. about 0.2 of an atmosphere, or 152 mm. of mercury), it will fly off as a gas from the surface of the cell into the internal atmosphere of the aerating system. Likewise when carbon dioxid has accumulated to a suitable pressure (less than 0.0003 A., or about 0.22 mm. Hg.), it begins to diffuse into the air. Diffusion from the root. — Oxygen can be formed only in green parts and hence escapes only from aerial parts; carbon dioxid, being formed in all live cells, can also escape through the other permeable region, the root. Its escape there may be directly into the soil water, whenever it has accumulated to a greater pressure in the cell sap. To demonstrate dif- fusion it is only necessary to grow the roots in contact with a polished mar- ble plate (calcium carbonate), whose surface will be etched along the lines of contact because water, " carbonated " by the C02 escaping from the roots, converts the calcium carbonate (CaC03) into calcium bicarbonate [Ca(HC03)2], which is readily soluble. Or by growing seedling in water with phenolphthalein (an indicator which is rose red in weak alkaline and colorless in acid solution), the water will be decolorized by the roots; but the color will return upon boiling, thus driving off the COj which had united with the indicator. Were any mineral or organic- acids the cause of the decoloration, the color would not return. I'.ui l>esidesC02 other substances may l.avc the planl by wayof the roots. At present these are not accurately known. Water i ultures made with soil extra* ts indicate that organic compounds, often very deleterious to the culture plants, are frequently present. These may have come into the soil by diffusion from roots (see p. 315). Acid salts, such as hydrogen potassium phosphate, are probably not among the exudates, as once they were believed to be. Yel any substani e in the root cortex, to which the cells are permeable, may ea ape ; and when the matter is studied further, many compounds, now unsuspected, may be found diffusing into the soil water. C. B. & C. BOTANY — 23 354 PHYSIOLOGY Mechanical losses. — Mechanical losses must also be taken into ac- count. In all plants the drying of leaves, flower parts, rootlets, and even larger parts of the body, is followed sooner or later by their falling off. In annuals, the whole body perishes at the end of the growing season; hence the perennials offer the best examples. In woody peren- nials, particularly, the partial fall of the leaves in summer, due to heat, drought, or other causes, and the complete autumnal fall, are striking losses of material. Yet this is not so expensive to the plant as it might seem at first sight, for a large part of the available foods have been trans- ferred from the leaves before their fall, and what is left is chiefly cell-wall stuff, unavailable organic matter, and ash. Nevertheless, much of that represents past expenditure of energy and is a dead loss; though by decay some of the materials again become available for rebuilding. Fall of leaves. — The once active food-making machines go to the scrap heap in autumn and have no value except as junk. Their deterio- ration is progressive. In the leaves of woody plants as compared with other parts, there is with age, as a rule, a steady accumulation of dry matter and a rising proportion of ash. Thus in the leaves of the European beech (Fagus sylvatica) : May June July Per cent dry matter . . 23.35 40.21 43-04 Per cent of ash . . . 4.67 5.20 7.45 In black locust (Robinia Pseudacacia): May Per cent dry matter 26.50 Per cent of ash 6.25 In 500 leaves of the plane tree (Platanus orirntalis): June July Grams dry matter . . . 142.53 184.70 Grams of ash .... 8.70 14.62 Contrast with these figures the average ash content of the wood of such trees, which is about 0.7 per cent, with a minimum of 0.2 per cent and an occasional maximum of about 3 per cent. This high ash content of leaves is not due merely to the retention of mineral matters when the water evaporated, as lime scale accumulates in a tea kettle.1 Rather the using of certain constituents of the salts, particularly the nitrogen, sulfur, and phosphorus, left behind the bases, calcium, magnesium, etc., ready to enter into new combinations and to 1 This is further evidenced by the fact that the ribs of leaves are usually richer in ash than the mesophyll. Aug. Sept. Oct. Nov. 5°-74 47.42 40.37 45-55 9-03 8.90 10.80 11.42 July Sept. Oct. 35-90 44-30 44.60 7-75 8.22 11.74 is): Aug. Sept. Oct. 182.80 I93-85 196.24 17.81 20.12 2i-33 THE MATERIAL OUTGO OF PLANTS 355 reappear in the ash, when the organic matter is burned away, as Ca< », MgO, etc. Moreover, certain mineral salts maybe stored in the walls, as silica often is; and these reappear as oxids in the ash. Fall of branches. — In woody perennials the competition between brain hes is so severe that many more die than survive. Thousands of rudimentary branches (as buds) never develop at all, and other thousands, after growing for a year or two, are outstripped by their more fortunately situated fellows, die, and drop off. The mortality is vastly greater than is realized without close observation, such as was made on a volunteer black cherry, and described in figurative language thus: Tin' first year it made a straight shoot nineteen inches high, which producec twenty-seven buds. It also sent out a branch eight in< lies long w hi< li Wore twelve buds. The little tree had, therefore, enlisted thirty-nine soldiers for the coming conflict. The second year twenty of these buds did not grow. Nineteen of them made an effort, and these produced three hundred and seventy buds. In two years it made an effort, therefore, at four hundred and nine branches, bul at the close of the sec ond year there were only twenty-seven branches upon the tree. At the close of the third year the little tree should have produced about thirty-live hundred buds or branch germs. It was next observed in July of its fourth year, when it stood just eight feet high ; instead of having between three and four thousand branches, it bore a total of two hundred and ninety-seven, and most of them were only weak spurs from one to three inches long. It was plain that not more than twenty, at the outside, of even this small number could long persist. The main stem or trunk bore forty-three branches, of which only eleven had much life in them, and even some of this number showed signs of weakness. In other words, in my little cherry tree, standing alone and having things all its own way, only one bud out of every hundred and seventy-live succeeded in making even a fair start towards a perma- nent branch. And this struggle must have proceeded with greater severity as the top became more complex, had I not put an end to its travail with the axe ! — Bailey: Survival of the unlike, p. 88. Loss of bark. — The constant flaking-off of bark, when the warping due to wetting and drying loosens the outer portions, or the steady weathering of the solid bark, occasions further losses of a relatively inexpensive kind. As in some cases waste products accumulate in the bark, this may be accounted <>ne way by which the plant gets rid of wastes. Hark also contains a very large percentage of ash. Fruits and seeds. — Fruits and seeds are separated annually from the body. These are loaded with surplus food for the embryo, and 50 consti tute a most expensive loss - one that not infrequently distinctly impairs the vitality of the plant. The intermittent bearing of orchard tree-. vines, etc., may herein find a partial explanation. CHAPTER III — NUTRITION I. THE NATURE OF PLANT FOOD Fool in general is organic. — The question, what is food for plants, elicits very different answers according to the point of view. The term food is not one which admits of accurate definition, and the difficulty increases the wider the range of organisms to which it refers. A lion obviously lives upon flesh, and the general constituents of his food can be determined. A sheep feeds on herbage, and that can be analyzed. A man consumes meat and vegetables of the most varied sorts. A fungus like PenicUlium, which will grow on a glass of jelly or an orange or a piece of cheese or a plate of gelatin, obviously feeds upon vegetable or animal substances indifferently. The nutritive constituents of flesh and vege- tables are many and diverse; plainly the term which is to include them must be most general. That term by common consent is food. It represents the totality of substances which nourish an organism and enable it to pass successively through the phases of its normal develop- ment. Now all the substances referred to belong to a category known as organic, because they are all produced by the chemical processes in a living organism. Food, therefore, for the lion, the sheep, the man, the mold, is composed of organic substances. It is true that there are also, in the very organic substances themselves and dissolved in the juices which make part of them, mineral salts of various kinds, and that these are indispensable to living beings; but their amount is very small in- deed, and alone they are quite incapable of sustaining life. For the present, therefore, they may be left out of account. Is the food of green plants inorganic ? — The beings enumerated represent all sorts of organisms except the green plant. When we ask, "On what does the green plant feed?" the answer, based on analogy, has been, "On the substances that enter it — water, mineral salts, and carbon dioxid; for with these alone it can develop from embryo to maturity." These are inorganic substances; and if the answer be true, the food of green plants is inorganic and that of all other beings organic. 356 NUTRITION 357 Is " food " food only for certain cells? — The first thing that awakens suspi< ion as ta the wisdom of this answer is that the living matter of green plains is like that of all other living things, and it would be very strange if in them protoplasm could be nourished with inorganic substances, when in all others it requires organic material. Yet the green plant might be differently constituted; and it is said by way of explanation thai this peculiarity is due to the presence of the green pigment, < hlorophyll. On examining this point, it is found that only a part of the plant has chlorophyll. .Most roots entirely lack it; only the outer cells of the stem ever contain it; and there are many cells, even in a thin leaf, and a great mass of them in a fleshy leaf, which are not green. Then we are forced to state the matter thus: the green parts of green plants use inorganic " food "; the colorless parts require organic food, for it is conceded on all hands that the colorless cells are unable to utilize any carbon dioxid and water. Whence it would seem that one cell might nourish itself with inorganic " food " and its next neighbor he unable to do so. That would certainly be a confusing situation if it could not be better described. Is "food" food only at certain times? — It appears, further, that carbon dioxid and water can be " foods " only part of the time; namely, when the green cells are adequately lighted; So except in the day, even the green cell would require organic food! The situation would have to be stated thus: The " food " of the green cells only, and only by day, consists of carbon dioxid and water; the rest of the plant all the time and the whole of the plant at night must have organic food like all other living things. Antithesis avoidable. — A little consideration shows that the apparent antithesis between green plants and other creatures is of our own making; it is produced solely by the application of the term food to the sub- stance- which enter the body, irrespective of their role. This antithesis can be avoided, and the confusion and contradiction eliminated, merely by avoiding this inept use of the term food and by applying it to organic substances only. By this expedient we es< ape a different use of the same terms in plant and animal physiology, with its resultant confusion of idea-, and we bring the green plant- into line with all other beings, SO far as nutrition is concerned. Excluding the inorganic substances from the category of foods, we need to recognize thai one power possessed by green plant- is unique: they alone make their own food, and not their own only, but food for the whole world. What they use for this i><>"\ making — carbon dioxid and water— may be distinguished as food ma- 358 PHYSIOLOGY terials. What they make is universally known as food for their colorless cells, for non-green plants, and for animals. Why should it not also be recognized as their own food? Food for plants is organic. — Food for plants, then, is like food for animals, always organic, the product of living beings; and in the last analysis, all food is made by green plants, for they alone among living beings have the power of making it out of the simple compounds C02 and FLO.1 They make it only in the green cells by the aid of light; and they make so much that they feed not only themselves, but all other creatures. The lion may live exclusively on flesh, but the flesh was built up by the herbivorous animal from the herbage it grazed, and the herbage was nourished by the foods it could itself make in sunlight. Man grows plants and appropriates the leaves, the roots, the stems, the fruits, or the seeds, improved by his selection and loaded with surplus food, for his own nourishment; or he feeds the steer, the sheep, and the hog with grass or grain that he may later use their flesh for his food. What are the plant foods ? — Having established a general meaning for the word food, the next question is: To what specific substances is it to be applied? Foods come from many sources and are of many kinds; and because they are so various, only the principal classes can be named, and a few examples briefly described. The four most important sorts are carbohydrates, fats, amides, and proteins. Carbohydrates. — Some carbohydrates are directly made by green plants; but there are also many that are secondary products. The name is no longer used in chemical classification; it is rather convenient than exact, just as " cryptogam " among plants or " invertebrate " among animals. Here belong the sugars, the starches, and the celluloses, each probably comprehending an indefinite number of different in- dividuals. This is certain among the simpler sugars, whose composi- tion is known; but only hypothetical ft starch and cellulose, whose complexity has hitherto baffled all analysis. All these substances have a composition like this : C„H2»On, or CnH2(n-i)0(„_i), or C„H2(n_2) 0(n-2), in which the value of n is 6 or a multiple of 6 where known, but may run up to several hundred. Thus grape sugar and its allied hexoses all contain C6H12O6; while cane sugar and its allies consist of C12H22O11. Starch and cellulose can be represented only as n (CeHioOs), with the value of n quite uncertain, but large. These empirical formulas, however, cannot convey any idea of the 1 Certain bacteria also seem to be able to utilize these substances to form foods; but so far as is known the product is utterly trivial in amount, and the fact is entirely without significance, were it not for its exceptional character. NUTRITION 359 complexity of even the amplest i arbohydrates, aor of the fact that a mere difference in the position of certain atoms or groups of atoms, whi< li does qoI affet I the per- centage composition at all, gives « holly different < hemi< al and physic al i haracters to the substance Thus, grape sugar (glucose) exists in two forms, one of which rotates a beam of polarized light to the right and the other to the left; the one, i-glu< ose, is abundant in plants; the other, /-glucose, does not o< i ur in nature but has been made art i- ficially. The difference is shown partly in the three following structural formulas, which all sum up (V,l I pj< »« : OH H OH OH ! I I I COH — C C — C — C CH2OH =<*-glucose I I I I H OH H H H OH H H I ! I I COH — C — C — C C CH2OH =/-glucose I I I I OH H OH OH Further, fruit sugar ((/-fructose) is abundant in plants, and its structure is quite different from glucose: H OH OH I I I CHoOH — CO — C — C — C — CHoOH =H and resolves itself tntoa molei ule of glui ose and a mole* ule of fru< lose. These two hexose sugars, glucose and fructose, and the disaccharide, cane sugar, are the only sugars which occur in abundance in plants; though mannose, galactose, and maltose are formed in the ( ..u isc> of digestion. 360 PHYSIOLOGY The simplest carbohydrate which has been detected in plants is formaldehyde, HCOH. This group will be recognized in the makeup of all the more complex ones above (but see p. 375). While it has only a transient existence and does not occur free, except in minute amounts, it has its special significance in that it is probably the first substance formed by the green cells from water and carbon dioxid. Fats. — Fats are apparently always secondary products, and consti- tute a common form of surplus food. These storage products furnish various commercial oils; e.g. olive oil, cotton oil, linseed oil, castor oil, corn oil, etc. They occur usually in fluid form as minute droplets in the protoplast, only occasionally being solid at ordinary temperatures, as in the seed of cacao. They are of very complex structure, being com- pounds of glycerin and three molecules of fatty acid. Their structure may be understood from these formulas: CH2OH CH2 • R I I Glycerin is: CHOI I A fat is: CH • R I I CH2OH CH2 • R in which R may represent oleic acid (CisH3402), linoleic acid (Ci8H3202), hypogaeic acid (Ci6H3o02), or any other member of a considerable series of fatty acids, minus the acid ion H. The R radicals may be all alike or different. When digested, fats break up into glycerin and the fatty acid or acids. The fats contain a notably small proportion of oxygen. The lecithins are substances allied to the fats in their constitution, containing phosphoric acid and cholin in place of one of the fatty acid radicals, R. They are very widely distributed in plants, and probably play an important role in the protoplasm, but just what is not known at present. It may be that they determine what substances may pass through the membranes; and it may be also that they are connected with the formation of chlorophyll. Amides. — The name is here used loosely and not in its strict chemical sense, for a group of substances of which none are popularly known. For convenience, they may be distinguished as nitrogenous compounds intermediate between carbohydrates and proteins. On the one hand, they are derivatives of proteins, among whose decomposition products various amino-acids always figure. On the other hand, they are deriv- atives of the carbohydrates and their allies, from which, with proper additions, they are readily formed. In addition to the carbon, hydrogen , and oxygen of carbohydrates, they contain nitrogen, always combined NUTRITION 36] with hydrogen as a definite radical, XII,, known as the amide radical. It may replace an II « >r < ) 1 1 group in the various carbohydrates and their allied acids, converting them by this slight change into quite different substances. Thus, either aceti< acid, CHa COOH, or glycolic acid, CH9OH COOH, be- comes amido-acetic acid (glycin), CHa(NH2) COOH, by the substitution of the amide radical Nil- for hydrogen (II) or hydroxy! (OH), respectively. Glucose, (Mil ciloll CHOH CHOH CHOH— CH2OH, becomes glucosamin COH CIIiNHj)— CIIOII— CIIOII— CHOH— CII.jOH, by a like substitution. On the other hand, some of the constant decomposition products of the more complex CH3X proteins are glycin (ante); leucin, yCH.—CHz—CH.(Nlh)—<:OOU; tyrosin, CH3/ HC— CH OH— c/ V;— CH2— CH(Nh2)— COOH; asparagin, CO(NH2)— CH(NHS) HC=CH — CHj — COOH; in all of whi< h the amide radical has replaced II orOII of an allied substance. Proteins. — Proteins are the substances which compose the larger part of the cytoplasm; protein foods, therefore, are those which can be most directly used for nourishment, and so represent the end of food making. To define proteins is quite impossible; they are so numerous and so varied that scarcely any characteristic is universal. Within this huge group are included some substances which are relatively simple, and others whose complexity defies all analysis. Even the simplest are scarcely known chemically, the actual knowledge permitting only theo- ries of their construction. It has been possible in most cases to deter- mine only the percentage composition, which with a study of the decom- position products sometimes permits the establishment of an empirical formula. The more complex proteins contain sulfur, and some have also phosphorus in addition to the carbon, hydrogen, oxygen, and nitrogen of amides, with traces of ash, which may or may not be struc- turally a part of the protein. One nearly pure protein is familiarly known, the albumin or " white " of eggs; perhaps the best known plant protein is the one longest known, the gluten of wheat grains. To illustrate the complexity of these substam es and, as well, the uncertainty re- garding their composition, the following formulas, though hardly more than guesses, are quoted. A crystalline vitelfin from squash: C jgd 1 ( _,\ ■.,.,< >s.s.. ^" albumin: CnoHiiMNgMOMsSs. Hemoglobin (of the blood): CnaHiwoNsiiOMjFeSa; the same, another guess, C6,„,l [geoN ui1 'l7»FeS|. 362 PHYSIOLOGY The most familiar physical characteristic of many proteins is that they coagulate; heat, prolonged shaking, the action of acids, alcohol, salts, etc., cause the protein to change from a liquid or semi-liquid form to a firmer " clot," which by pressure can be separated into a fluid and a more solid portion. The coagulation of white of egg by heat, of milk on souring, and of the fibrin of blood on contact with a vessel are familiar examples. Ordinarily the coagulum is insoluble in water. But the neu- tral salts act differently, producing a soluble clot. Advantage is taken of this fact to separate various mixed proteins and purify them partially for analysis by " salting out." Other physical peculiarities are their high resistance to the electric current, their large molecular weight (prob- ably 15,000 and more in many cases) and hence slow diffusibility, so slow usually as to be negligible. Some proteins crystallize, but most do not. When first discovered such crystals were called " crystalloids," because it was not believed that true crystals could be formed by organic matter. They are regularly present in the protein grains of the Brazil nut, castor bean, etc. (fig. 664). Plant foods again. — Plant foods, then, are specifically these complex organic compounds — not the simple inorganic substances out of which green plants alone can make food. This is practically implied in the terms proposed by authors who reject this use of the term food, and used frequently to distinguish plants as to their mode of nutrition, viz. autotrophic, or self-nourishing, plants, and heterotrophic plants. The ob- vious objection to these two terms, if they are anything more than con- venient and figurative ones, is that only some parts of most so-called autotrophic plants are strictly self-nourishing. Only the plants whose every cell contains chlorophyll are actually autotrophic. If the term be used in the wide sense, green plants are not merely self-nourishing — they nourish all living things. - Kinds of food needed. — However, there is a wide difference among plants as to the kind of food that they require. The known variety is so great that it is impracticable to state it in detail here, and only a small number of plants, chiefly fungi, have been carefully studied in this respect. Some thrive best on comparatively simple compounds; others require the most complex proteins. Some flourish on material which is useless or even highly injurious to others. The proverb " what is one man's meat is another man's poison," is quite applicable to plants. Among the lowest and simplest plants, the bacteria, there are some which live upon substances almost as simple as the food materials of higher NUTRITION 363 plants; bul they manage to secure energy in ways unknown to us, and build these substances into their bodies. Food a source of energy. —After all, foods arc of value to plants, as we conceive things, because they supply them with energy as well as with material. The energy income in this way is indeed the important feature. The green plant locks up in the food it constructs a fraction of the solar energy which reached it as light; and thus this energy becomes available to other organisms, since after further transforma- tions of the foods they can release it by decomposition and apply it to other reactions. Food and growth. — Because with our best appliances we are unable to know yet the real nature of nutrition, the use which a plant makes of food can be determined only by the extent to which it promotes growth and development of the body. The term economic coefficient has been used to express the ratio which the increase in the weight of a crop (say of a fungus) bears to a given quantity of a particular food. Manifestly there are other ways in which the plant uses a food besides incorporating it into the permanent structure of the body, and many complicated rela- tions may be disturbed by too limited nutrition. Yet this economic co- cfli. ient expresses, in a crude way, die differences in the availability of foods for body building, and so impresses the fact that the processes of nutrition differ widely in different plants. 2. PHOTOSYNTHESIS The fundamental fact in the nutrition of all living things is the capacity of green plants to mnke certain complex organic compounds, carbohy- drates namely, out of carbon dioxid and water, by the aid of light. This unique process is known as photosynthesis. The term used. — When the food of green plants was described as inorganic, this transformation of inorganic- materials into carbohydrates, which was taken to be their incorporation into the body, was called assimi- lation, after the analogy of the transformations undergone by the food of animals. As the radical differences between the food making of a green plant and true assimilation in both plants and animals began to appear, an attempt was made to obviate the confusion by using the term carbon assimilation. These terms are still in common use in other countries, but will gradually disappear.1 Clearness demands the use of the (lis- 1 For sximple, a recent hybrid is "photosynthi 1 i. carbon 1 imitation"' 364 PHYSIOLOGY tinctive term photosynthesis for the process that is peculiar to green plants, leaving the term assimilation to be applied to the same process in both plants and animals; namely, to the transformation of foods of all kinds into the actual living stuff. As photosynthesis requires a supply of certain substances, which re- appear in more elaborate form, and acts through certain structures, which require a supply of energy for doing the work, the making of carbohy- drates may be described appropriately in terms of a manufacturing pro- cess. There are (1) the raw materials, (2) the laboratories, (3) the en- ergy, (4) the products and the process. (1) The Raw Materials Carbon dioxid. — The raw materials needed have already been named, carbon dioxid and water. Carbon dioxid exists everywhere in the air, in the ratio of about 3 parts in 10,000, and its nearly uniform distribution is assured by the convection currents (winds) that stir the atmosphere. Only in the neighborhood of cities or other places where C02 is being produced in quantity is there temporarily an excess. By decomposition of rocks, burning of fuel, decay of organic matter, and respiration of plants and animals, the supply of C02 is maintained, though great quantities are removed from the air by green plants. The amount is constant, so far as can be known historically, though there is geological evidence that in earlier periods of the earth's development COs existed in much larger and also in smaller quantities than now, since enormous amounts have been fixed in beds of limestone, and later released by weathering. C02 near the ground. — On quiet days there is a layer of air near the ground where the proportion may rise much higher (10 to 12 times as much), owing to the diffusion of C02 from the soil, where it is being evolved by the decomposition of organic matter through the agency of bacteria, etc. Perhaps turf- forming and rosette plants profit from the lowly position of their leaves, since the more C02 in the air, within limits, the more can enter them and be used for food making. CO, in water. — In the water of quiet pools and lakes, as well as in slow streams, the amount of C02 dissolved is much greater than in the air. It is produced by the host of organisms living in the waters and by decay, and is also dissolved from the air. As C02 is very soluble in water (up to volume for volume at ordinary temperatures), it may NUTRITION 365 thus accumulate to 25 or even 100 times as much as in the air. This puts water plains in a very advantageous position so far as a supply of ( -( ). is 1 "iii erned. Admission of COo. — Of course in all plants that present an uncutin- ized (ami consequently a wet) surfa< e t<> the air, the ('< >_. enter- direi th- at the surface; in fact it can enter, in proportion, wherever water can evaporate. As the cuticular evaporation in most of the higher plants is small, the quantity of C02 entering through the epidermis is trifling. Into some epiphytic seed plants which have no stomata (e.g. Tillamhia), the leaves of mosses, the thallus of liverworts, etc., C02 enters directly. The supply for the great majority of the larger land plants, however, passes through the stomata. These openings are ample to admit not only what is net essary, but five or six times more than actually passes through them in nature. It has been shown that C02 will diffuse through a multiperforate partition, placed over some ready solvent like sodium hydroxid, as freely as it would enter the solvent were the partition absent, provided the perforations are not farther apart than ten times their diameter. The epidermis is like such a multiperforate parti- tion in which the area of the openings is scarcely more than I per cent of the total surface. Hut the CO2 dissolves so readily in the wet cell walls bounding the inter- cellular spaces that its pressure in the internal passages is usually o; so it may traverse the stomata as rapidly as is permitted by the gradient of pressure, 0.228 mm. outside to o inside. The speed of the molecules is found to he greatly accelerated as they swirl through the narrow passage of a stoma; in fact, they traverse it at a speed about 50 times as great as when diffusing freely into sodium hydroxid. Even when the orifice of the stoma is partly closed, though this reduces proportionally the amount of gas passing, the supply of C02 is not likely to fall below the maximum that can be used. As in good light the sto- mata are usually more than half opened, even though the evaporation i- excessive, an adequate supply of C02 is thus assured, so far as admis- sion to the aerating system is concerned. Deficiency in C0L.. A- a matter of fact, however, the supply of O », is often less than muld be utilized by the chloroplasts. This is shown by the fact that photosynthesis is increased when, in good light, the amount of COs in the air around tin- plant is artificially increased. The increase may go to a hundredfold or more with positive benefit, at least so far as brief experiments -how. Any increase in the air mean- in- creased pressure of COs in the aerating passages; and this mean- the solution of more C( >.. in the wet walls, and consequently faster diffusion 366 PHYSIOLOGY toward the chloroplasts, where the COa is actually utilized. Here, indeed, is the point at which the normal pressure of C02 usually limits the process of photosynthesis. The main-line transportation through stomata and intercellular spaces is adequate, but the switching facilities in the terminal yards (from cell wall to chloroplast) are not; hence when otherwise capable of operating to full capacity, the laboratories are hindered by the impossibility of securing enough of this raw material. There are other factors which may limit the output, to be discussed later; but the shortage of C02 due to low diffusion pressure is the com- monest. Water. — Water, the other of the raw materials, is never lacking when plants are active. Its source for most land plants is the soil water that enters through the roots. The little that may enter via the leaves (com- parable with the amount leaving in the same time by cuticular evapora- tion, p. 327) is practically negligible. Only in mosses, liverworts, and a few epiphytes, i.e. plants with practically uncutinized surfaces, may it freely enter aerial parts. In many such cases there are special struc- tures that hold water until it can enter. Relation of C02 and H20. — The carbon dioxid and water enter into a double relation. In part, the C02 is merely dissolved in the water; in part the two form a loose chemical combination, carbonic acid, H2C03. This three-phase system, solute, solvent, compound, is in equilibrium, and if the amount of any member is altered, corresponding changes take place in others and equilibrium is again reached. (2) The Laboratories Chloroplasts. — The laboratories in which photosynthesis proceeds are the chloroplasts. These are organs of various form and size, found only in superficial parenchyma cells, chlorenchyma, of stems and foliage. (For a discussion of this tissue and its relations to external agents, see Part III, p. 530.) The chloroplasts are embedded in the cytoplasm just within the ectoplast and marked by their green color. In a few algae (especially the Conjugates, p. 37) they have various and sometimes fantastic forms, but in almost all the higher plants they are shaped like a bun or a thick round cake; that is, two diameters are nearly equal, and the other is shorter, with the convexity greater on one face than the other (see fig. 619, p. 297). Their form is subject to change from internal causes, and in moving about with the cytoplasm they are easily distorted NUTRITION 367 by pressure, showing thai they arc of a soft, elastic, and semi-fluid con- sistent v. Pigment and stroma. — In fact, the body or stroma of the chloroplasts seems to be like the cytoplasm, but dyed by the green pigment. The precise relation between the pigment and the stroma lias not been satis- factorily made out, even in the killed chloroplast, and in the live un- altered chloroplasts it can only be conjectured. In some cases, when the pigment has been dissolved out by alcohol, the stroma (of course coagulated by the alcohol) presents a spongy appearance, and it has been inferred that the meshes of the sponge throughout were occupied by pigment. In others, especially in the larger chloroplasts which can be sectioned, the pigment seems to be restricted to a spongy shell of measur- able thickness at the surface, while the interior is colorless. Pigments. — The yellow-green pigment is tailed chlorophyll; but it is not a single substance. Several pigments can be separated more or less completely, of which only two are abundant and constant in all higher plants, the one bluish green and the other pale yellow. The names applied to these are confusing. To distinguish them we shall employ the terms chlorophyllin and carotin. To the bluish green one no dis- tinctive term has been generally applied, but it has been usually called chlorophyll (not distinguishing it from the combination), or chlorophyll proper. For the yellowish one, xanthophyll, etiolin, and carotin have been used. The last is preferable. The term xanthophyll is descriptive, but it has also been used for other minor yellow pigments. Etiolin was applied to tin- pale yellow pigment which appears when plants have been "etiolated" by being grown or kept for a time in darkness. It seems to be identical with the yellow pigment named from the carrot, carotin, which proves to be very widely distributed in plants. Chlorophyllin and carotin may be partially separated by their unequal solubilities. If to a fresh solution of chlorophyll in 80 per cent alcohol, benzene be added, the mixture shaken, and then allowed to stand, the ben- zene ri>es, carrying the greater part of the chlorophyllin, while the alco- hol retains the greater part of the carotin. Chlorophyllin. The chemical composition of chlorophyllin is not known. It is very easily altered ami i- certainly very complex, contain- ing N as well a- C, H, and ( ). Whether phosphorus or magnesium is an essential constituent is in contention. Iron does not seem to be an integral part of it, though considered essential to it- formation. The red coloring matter of the blood, hemoglobin, yield- do omposition prod- 368 PHYSIOLOGY ucts very like those of chlorophyllin, suggesting that the two pigments have structural similarities. That both have peculiar relations with carbon dioxid is interesting, but cannot yet be explained. When chlorophyllin disappears in the autumn, the yellow pigments become prominent, and some of its decomposition products have a share in reddening the tissues. The red pigments are then dissolved in the cell sap ; the yellows are still in the chloroplasts. The autumnal coloring, however, is not yet fully understood. Carotin. — The chemical composition of carotin is certainly very different from that of chlorophyllin. Its formula, probably C^H^ or C4oH56, shows that it lacks both O and N. It is widely distributed in plants, and to it chiefly the orange and yellow tints of flowers, fruits, seeds, roots, etc., are due. (3) The Energy Light. — While the intricate chemical relations of chlorophyll are yet unknown, one of its physical features is known to be of the greatest importance. That is its capacity to absorb radiant energy. When the radiant energy coming from the sun is passed through prisms of rock salt, glass, or other appropriate media, or is reflected from a minutely striate surface, the various wave lengths are unequally refracted or reflected, so that the physiological and other effects of energy of dif- ferent wave lengths can be studied. Certain of these wave lengths (if they were sound waves one might say about 1 octave out of 11) affect our eyes, and this physiological effect is what we know as light. By a figure of speech the cause is likewise so named, and the waves them- selves are called " light." But they differ only in length and frequency from the much greater number, both longer and shorter, slower and faster, which we cannot perceive with our eyes. Other physiological effects, such as inflammation of the skin and the development of pig- ment ("sunburn "), are produced by light waves. On the plant, like- wise, waves of different lengths produce different effects according as certain parts are attuned to them (see p. 449). Absorption spectrum. — The chlorophyll is so constituted that it can absorb waves of certain lengths, all falling within the range of our vi- sion. On the plant this energy cannot produce the effect that it does on our eyes, and hence for the plant it is " light " only by a convenient figure of speech. There are seven separated groups of waves whose absorption is more or less complete. When we look at a spectrum of NUTRITION 369 sunlight, i.e. a narrow liar of light dispersed into a band of different wave lengths, each group of waves produces its appropriate effeel and we see a band of blending colors, 'lark red at one end, running through ted, orange, yellow, green, blue, indigo, violet, and ending in the dark- est violet. On interposing a leaf in the path of the light, there appear across the spectrum dark strips due to the partial or complete stoppage of the energy. Similar absorption hands, slightly displaced, are seen by using in the same way an alcoholic solution of chlorophyll (tig. 648). a B C D E b F G h Fig. 648. — Absorption spectra: A, chlorophyll of Allium ursittum in alcohol; B, chlorophyll of English ivy (II edera Helix) in alcohol; C, chlorophyll of Oscili alcohol; D, carotin, i, 2, 3, 4, absorption bands of chlorophyllin; /, //, ///, absorp- tion bands of carotin; EA, end absorption. The Intend broken lines mark tin- position of the ['rincipal absorption lines of the solar spectrum (Fraunhofer lines); the numbered solid lines form a si ale from which wave lengths (\) in millionths of a millimeter may be found by adding a cipher; note the increasing dispersion from left (red) to right (violet). — After Koiil. These absorption bands arc as follows: 1, in the red a wide black "tie. its wave lengths i\i being 67c— 635 nn' ; 2, a narrower and less intense one in the orange, X=622-5o; mm; 3, in the yellow, a band much lighter than a, and shading out on the sides, \ ^S;-^,^ uix; (, a faint band in the green, not always to be seen, and probably due to decomposition products, X = 544-5,^0 mm- ( >r- dinarily the other three blend into one, and there are no visible waves left beyond the blue (X= 495-420). By very careful manipulation, using dilute solutions in- stead of a leaf, they .an be distinguished, their limits not being sharply marked. 1 The exact location of the bands varies. 1 /xfi — 0.000001 nun. C. B. .v C. no 1 \\y j 1 370 PHYSIOLOGY The bands 1-3, and possibly 4, belong to chloropbyllin, while the indefinite three, I-III, belong to carotin. These three are much better seen in the absorption spectrum of carotin alone (fig. 648, D). Fluorescence. — Chlorophyll has another physical character, which it shares with some other dyes ; its solution is fluorescent. When a strong solution in alcohol is held between the eye and the light, the color is a vivid green; but if examined by bright reflected light, it appears deep blood-red. While this is a useful recognition mark, the physiological significance of fluorescence, if any, cannot be explained. The absorbed energy. — The energy that drives the machinery is de- rived from light, for if a green plant be kept in darkness, it is entirely unable to make any carbohydrates. Furthermore, it is only the chloro- plast directly illuminated that receives this energy. A lighted portion of a leaf cannot communicate the energy to a darkened area. If a por- tion of a leaf be covered with an opaque plate, while C02 is allowed free access, the rest of the leaf may show evidence of active photosynthesis, but the darkened area shows none. Moreover, it is the energy absorbed by the chlorophyll that does the work. The following experiment shows this: A plant was kept in the dark until its leaves showed no trace of starch. Then on a sunny day a spectrum of sunlight, as bright as possible, was cast on a leaf and kept steadily in the same place for some hours. Thus the chlorophyll could absorb energy only in those regions along the band of light where fell the waves of lengths that it can stop; on the leaf these re- gions of course corresponded in position to the absorption bands before described. If, therefore, the leaf works with the absorbed energy, photosynthesis can occur only in these strips and not elsewhere. After the exposure, on testing the leaf for starch (the accumulation of which is a mark of active photosynthesis), it was found in abundance where lay absorption band 1 (fig. 648), and scantily in others; but it was wholly lacking in other parts of the spectrum. This is what would be expected; but there was once an idea that chl< rophyll acted merely as a screen, shading the protoplasm from harm- ful rays of light; and that the protoplasm could work properly only behind such a screen. There is now evidence that the protoplasm is unnecessary in the first stages of carbohydrate synthesis, those strictly called photosynthesis. It is probably light transformed to electricity that reduces the H2COs to formaldehyde (see p. 375), which then con- denses into more complex carbohydrates. Exposure to light. — Plainly the light which has passed through a chlomplast is unlike that which has not; and the more chloroplasts it passes through, the more complete is the absorption of effective waves. The upper cells of a leaf, therefore, are in a more favorable position with respect to light than the lower, especially in weak or diffuse light ; but NUTRITION 371 if the stomata are only on the under surface, as they often arc, the lower cells arc more favorably placed with respect to ('()„; and the more soas the looser arrangement of these cells permits freer diffusion. The very structure of the leaf is in large measure a response to these different factors, and so perhaps the advantages and disadvantages balance one another. A, leaf which is directly shaded by another is obviously in a decidedly disadvantageous situation; and we observe various arrange- ments and positions that reduce shading. These result in leaf mosaics of various kinds (sec Part III, p. 543). A plant that grows in shade is different from the same species grown in the sun; indeed shade plants have peculiarities which depend in large part on the difference in the illumination (see Part III, p. 531). Energy obtained. — An ordinary thin leaf reflects and absorbs 40-70 per cent of the sunlight which falls upon it ; but of diffuse light it absorbs about 95 percent. The chlorophyll itself seems to absorb something like 20-30 per cent, but of this only a small part can be used for photo- synthesis and so stored as potential energy in the carbohydrate made. That amount is variously estimated from 0.5 to 3 per cent. The balance is free to heat the leaf, whose internal temperature in the sun sometimes rises 10-150 above that of the air. This surplus heat, of course, is partly transferred to the air adjacent, but the greater part becomes latent in the water, whose vaporization is accelerated thereby. This is the so-called " chlorovaporization " (see p. 330). Deficiency in light. — It will be evident from the foregoing that in nature light is seldom lacking to drive the machinery rapidly enough to dispose of all available C02. Yet it may be reduced to an intensity at which light, instead of the small supply of C02, limits the output. For example, some plants are so situated that they get only 2 per cent of the total sunlight in the vicinity. From the point at which the effective energy of the light absorbed is just equal to disposing of the available CO..., whether this is greater than natural or not, lessening the intensity of the light results in a proportional diminution of the amount of the product. Efficiency. — It will be further evident that the plant is a very in- efficient machine-, considering the relation of energy received to the energy stored in the produ( t. A -team engine which delivers as mechan- ical power less than 10 per c cut of the energy of the fuel consumed under tin- boilers is lit for the s( rap heap, and the best types are delivering above 1 5 per tent. ( lontrast this with the O.5-3 l"-'r l rnl "' ''u' |''anl ''' oiiomy. 372 PHYSIOLOGY Yet in spite of this relative inefficiency, the total product is enormous and invaluable, because of the limitless store of energy pouring upon the earth constantly from the sun, beside which the artificially released energy of fuel is absolutely a negligible quantity. The solar energy received by the earth in a second is represented by 250 X io15 calories. The coal consumed in the whole world in a year, reported in 1906 as about 1000 million metric tons,1 represents 8 X io15 calories. The plant can afford, so to speak, to be inefficient. Source of light. — The source of light is quite a matter of indifference. In nature, of course, the primary source, the sun, is alone to be con- sidered, since the light of even the full moon (only -g^oVo^ ^at °f tne sun) is too weak to effect photosynthesis to a measurable extent. Va- rious secondary sources may be used in experiments, some electric lamps and the incandescent mantles (with gas) giving light of sufficient inten- sity when near the plants. Attempts to " force " plants, by enabling them to make food by night with electric arc illumination, have been successful with certain sorts, showing that there is no need for rest at night, and that a greater supply of food permits more rapid develop- ment; but there will be no incentive for commercial application of this result until the cost of electric energy is vastly less than now. Temperature. — A suitable temperature has usually been considered merely a condition of photosynthesis, and not a source of energy for the process. This is evidence that our knowledge of the energy rela- tions of this process is vague, and that the matter needs investigation. At present, however, it is not possible to describe in terms of energy the effect of heat upon photosynthesis, so we must be content with a brief statement on temperature as a condition. Experiments show that even at temperatures approaching o° C. some plants can make carbohydrates; the algae of arctic waters are conspicuous examples. Yet for most plants such a low temperature practically stops photosynthesis; while even at several degrees higher it may be the limiting factor, less food being made than the COL, and light would permit. Likewise in direct sunlight the temperature may rise so high in the interior of a leaf as to retard photosynthesis2; and in tropical deserts, where the heat of the air itself may run to 450 C, it is probable that photosynthesis is reduced thereby. 1 The metric ton about equals the English "long " ton, 2200 lbs. 2 But these heating effects of direct sun are compensated in a measure by evaporation. NUTRITION 373 (4) The Products and the Process The products. — The first product of photosynthesis is not known with entire certainty, and the process, therefore, cannot be described ac- curately. The product of later synthesis whi< h is most general and has been longest known is starch. The fart that it is so generally present and that it is so universally used as evident e of photosynthesis be< ause it can be so easily detected, tend to confirm the common impression that starch is the producl of photosynthesis. But there are many plants in which starch is either not formed at all, or appears only under excep- tional conditions, and in no plants is it the exclusive product. Thus, in most fungi no starch is formed when they are fed on carbohydrates; in the kelps fucosarj takes its place, and in many monocotyledons, oil; while even in the plants which produce starch abundantly, much of the earlier product is diverted into amides and possibly other nitrogenous compounds. In any event starch is a secondary product, and represents the surplus in the manufacture of primary carbohydrates over immediate use, re- moval, transformation into amides, etc. That starch does not appear under certain conditions, in a leaf in which it is usually formed, is no evidence, therefore, that no photosynthesis has occurred, but only that it has not gone on at a rate rapid enough to yield enough excess to appear as starch. Amount of product. ■ — A method of estimating the amount of photo- synthesis under various conditions is based upon the relative weight of equal, hut necessarily small, areas of leaves, taken at the be- ginning and end of the experimental time, allowances being made for migration1 and use by data from other experiments. The results at best cannot be exact, and the introduction and multiplication of small initial errors make the calculations based on these data quite unreliable. - When accurate data for photosynthesis are needed, the only reliable method is to determine the amount of ('< )., used. This requires rather complicated apparatus, skillful manipulation, and accurate gas analysis. This method is obviously independent of the products and their use or migration. 1 <>r this may 1m- rendered impossible by severing the leaf from the plant 2 Tlic results obtained by this method an twotothrei times .1- large as those in the table on the following p.iyc. 374 PHYSIOLOGY The best estimates as to the amount of photosynthesis carried on by thin-leaved plants are given in the following table: Carbohydrate made in i hr. by i sq.m. of leaf surface Name of plant Condition OF LEAF Light Temp. °C. co2 USED, CC. co2 USED, MG. Carb. MADE, i. Helianthus animus . . attached diffuse 21. 1 312.6 6l2 392 2. Helianthus annuus . . detached diffuse 19.0 439-9 S62 551 3. Helianthus annuus . . detached J strong to [ diffuse 26.S 385-3 755 483 4. Helianthus annuus . . attached bright sun 47-1 21.9 43 27 5. Tropaeolum ma jus . . detached diffuse 21.7 158.3 3io I98 6. Tropaeolum majus . . detached diffuse 25-9 243-7 487 305 7. Catalpa bignonioides . detached interm. sun 20.0 373-2 737 468 8. Petasites albus . . . detached in term . sun 17.0 208.4 408 26l 9. Polygonum Weyrichii . detached 21.0 473-2 927 593 10. Prunus Laurocerasus . detached 10.0 2S1 11. Prunus Laurocerasus . detached 37-5 810 12. Helianthus annuus . . detached 19.0 569 13. Helianthus annuus . . detached 29.0 650 14. Helianthus annuus . . detached 3S-o 73° Nos. 1-9, after Brown and Escombe, in part recalculated ; nos. 10-14, after Blackman and Matthaei, especially intended to show the effects of temperature on photosynthesis. An effect of excessive temperature is to be seen also in no. 4. Using such results as the basis of calculation, it would be easy to show how enormous a weight of food is made in a growing season by the foli- age of meadows and forests. But unknown allowances must be made for leaves unfavorably situated or lacking in vigor, and such estimates are of little value except for their impressiveness. The value and volume of the annual crops of cultivated plants is even more impressive; and to this must be added in imagination the unknown but huge volume of wild vegetation, all dependent upon photosynthesis for at least 85 per cent of its dry substance. The following are the approximate values of some of the more important crops of 1909 in the United States: corn, $1,720,000,000; wheat, oats, rye, and barley, $1,280,000,000; cotton, $850,000,000; hay, $665,000,000; potatoes, $212,000,000. Together the weight of these marketable products is something like 175,000,000 metric tons ; and of course this is but a small fraction of the vegetation that pro- NUTRITION 375 duced them. In addition to the staple crops just named, whose aggregate value in 1909 was about $^,000,000,000, other farm crops add nearly as much more, being estimated at Sj, 700,000,000. Su< li are the values that plants annually pro- duce in this country, chiefly from the air and water, by photosynthesis. Process. — The process of photosynthesis is not certainly known; but all the evidence points Strongly in one direction; so that the hypoth- esis of von Baeyer may be considered as highly probable. It appears that the carbonic acid (C02 + H^O ^OHCOOH) is by some means reduced, perhaps first to formic acid (HCOOH), and later to the sim- plest carbohydrate, formaldehyde (H-COH). In the course of this reduction a molecule of oxygen, O,, is set free and appears as a by- product. The reduction of KfoCOs to formaldehyde lias lately been accomplished artifi- cially, though much less efficiently than in plants. A thin layer of chlorophyll on gelatin or floating on the surface of water (to which has been added an enzyme that will break up hydrogen peroxid, EI3O2, into water and oxygen), when supplied with CO2 in light permits the accumulation of formaldehyde and oxygen to a measurable extent in the apparatus, The formaldehyde molecule so quickly combines with others of its kind thai it has been difficult to prove its formation in leaves. Free, it is a powerful poison, even in dilute solution (1 : 20,000); but its prompt conden- sation into some hexose sugar prevents accumulation to a harmful extent. The /H details are probably as follows: six molecules of formaldehyde, II— C^. , unite into a chain. This union engages two of the four bonds of each C atom, except at the ends, where only one is concerned. This consequently either releases one of the twoO bonds or leaves one H atom free, or does both. The free II immediately joins its neighboring half-free O, and together they form < >II, bound t<> C by only one bond. At one end no II is freed ; but the half-freed ( ) takes up H and the group becomes CH2OH, characteristic of an alcohol. At the other end, the loss of one II leaves the aldehyde group C^ as in formaldehyde. In glucose a further % O transposition occurs in group 4, II and OH exchanging places. H H H OH H I I I I I /H H— C— C— C-C- C C< = ./-glucose (p. 159), OHOHOH II OH Glucose and starch. — Glucose probably represents the first stable carbohydrate formed in most plants; yet there i^ some variation in this respect in different plants, and there is evidence thai in some cases cane sugar, saccharose, is the thief product. It is quite possible, moreover, 376 PHYSIOLOGY to divert some of the product into amides by a simple substitution of the amide radical, NH2, for some H or OH radical. Thus, if the fifth group in the glucose chain became HC(NH2), the product would be glucosamin, a substance of quite different properties (see p. 360). Like diversion by substitution might readily occur if only two or three formaldehyde mole- cules had come together. Such processes seem to be the initial steps in protein synthesis (p. 380). The common main product, glucose, usually accumulates in the cells because it is formed faster than it can move away. Finally starch or some other stable product appears. The intervening steps are hypo- thetical. It seems that at a certain concentration glucose molecules show a tendency to combine with each other to form a compound sugar, maltose (CioHwOn), which promptly compounds itself in like manner into a dextrin and finally into starch. The combinations occur rapidly, and the intermediate products are hence obscure. Perhaps the process takes place under the influence of third bodies, called enzymes; maltase and diastase in the cases here cited being the possible agents (but see enzymes, p. 399). The Fig. 649.-TW0 chioToplasts starch accumulates in minute granules of Rhipsalis, with grains of starch within the chloroplasts (fig. 649), so their (5) and minute oil droplets. — stroma may be the direct agent in organ- izing the starch, or at least may be the seat for the formation of the enzymes which bring this about. These grains have a definite structure and a rather uncertain composition (see starch, p. 358), for both of which the chloroplast itself may be responsible (see leucoplasts, p. 389). Removal of products. — If a leaf is isolated, the accumulation of the synthetic products may reach a point where it interferes with further photosynthesis; but in nature this does not occur. Use on the spot, or diffusion of such products as remain simple and soluble, or the digestion of the more complex and the insoluble ones by enzymes (p. 399) and subsequent diffusion, is constantly removing the new materials from the leaves and stems to other places where they may accumulate or be used (see translocation, p. 393). In darkness or weak light, the transporta- tion facilities, temporarily overtaxed in full light, overtake the manu- facturing; the laboratories are cleared, consumers are supplied, and the warehouses and distributing centers are filled with the surplus awaiting future use. NUTRITION 377 The by-product. —The by-] >rod- uct, oxygen, is used to some extent in respiration (p. 406); the excess diffuses to the surface, whence it escapes into the aerating system and theme into the air. The final step in its exit i an be observed in water plants readily, because the constant accumulation in the air chambers leads to its escape as bubbles when the passages are opened by a cut or break (fig. 650). If the canals are intact, 02 may become abundant enough in bright light to form bubbles on the sur- face, which rise as they become larger. The rising gases can be conducted by an inverted funnel into a test tube and analyzed; they are about 85 per cent oxygen, the remainder being other gases produced in other processes. So uniform is the evolution of 02 by water plants that with precautions the number of bubbles given off in unit time can be used to exhibit the general effect of the three ex- ternal fat tors, intensity of light, temperature, and supply of C02, on photosynthesis. It is not sat- isfactory for quantitative deter- minations. Fir.. 650. — Upper part of a plant of ton attached to a glass rod and submersed, showing escape of gas bubbles (mostly oxygen) from cut end of stun in sunlight. 3. THE SYNTHESIS OF PROTEINS Proteins the end-product. —The formation of carbohydrates is by no mean- the only process of food making. Indeed it may be looked upor a> merely the firsl stage in the construction of proteins, of which carbo- hydrates are important components. As the living protoplasm appears 378 PHYSIOLOGY to be composed chiefly of proteins (probably more complex and labile than in the non-living state), it is evident that protein foods are of the highest importance — indeed indispensable — for nutrition, since it is the protoplasm which grows, wastes, and needs repair. Proteins are, as it were, the highest type of foods; they represent the final stage of food making. Inasmuch as the carbohydrates contain only carbon, hydrogen, and oxygen, while proteins contain in addition nitrogen and sulfur and in many cases phosphorus also, it is plain that they cannot be formed from carbohydrates alone. A strict carbohydrate diet is as unsuitable for plants as it is for animals. Some materials must be supplied from which nitrcgen, sulfur, and phosphorus can be obtained. Source of nitrogen. — As the air contains 78 per cent of nitrogen, the atmosphere would appear to be a natural source of this element. But though the nitrogen is everywhere dissolved in the water of the plant, and can enter and leave it freely, no plants are known to be able to use it in this uncombined form, except certain bacteria, some of which live in the soil and in some waters. Certain soil species enter the roots of various plants, especially the Leguminosae, causing them to form tuber- cles. (See below, p. 379.) Almost all plants, therefore, must get com- bined nitrogen. This is found in soils as nitrates of various bases, e.g. calcium, magnesium, potassium, and sodium; and when a soil is deficient in nitrogen, such compounds are important constituents of the fertilizers, natural and artificial, which are added to it. The nitrates in the soil result mainly from the decay of organic matter in it. The later steps in the process are controlled by certain bacteria in the soil which bring about the oxidation of ammonia to nitrites, whereupon others oxidize the nitrites to nitrates. The very fertility of arable soils, therefore, depends on the microscopic organisms living in them, which prepare the way for the larger plants. Loss of N. — The soil of cultivated areas is constantly losing its com- bined nitrogen by solution and drainage, and this loss is only partially made good by the ammonia and nitrous and nitric acids washed into it from the air by rains. Under natural conditions the dying vegetation ultimately returns its constituents to soil and air; but crops are carried off, their nitrogen with them. Gardens and fields, therefore, require replacement of this nitrogen sooner or later. When they lie fallow, certain bacteria of the soil, associated with algae and perhaps with other plants, slowly increase the nitrogen content of the soil by fixing NUTRITION 379 the free N2 from the air in their bodies, which, dying, restore it to the soil. Leguminosae. — The case of the Leguminosae and a few other plants i- peculiar. Certain soil bacteria cuter the young root hairs, grow and multiply, and work gradually into the cortex, where, as they in< rease, they stimulate the rootlet to multiply and enlarge the cortical cells, so that a local swelling or tubercle is formed. The largest of these a an civ exceed- the size of a hazelnut, and most are smaller than a pea or even a grain of wheat. The relations are probably as follows: The bat teria depend on their host for carbohydrate food, but can use the free nitrogen (presumably that nearest them in solution, which is replaced from the air) in their protein making. Being favorably situated, many of the bac- teria become excessively enlarged, and often branch into X and Y forms. The host sooner or later gets the better of the parasite and consumes these fat bacteria (" bacteroids "), their proteins proving valuable foods. By reason of this peculiar relation, leguminous crops can be grown in soils which contain no combined nitrogen whatever, provided the proper bacteria be present.1 If the crop be then plowed under (a process called green manuring), the soil is enriched in nitrogen at the expense of the air.2 (See further Part III on root tubercles.) Source of S and P. — The sulfur and phosphorus needed are obtained by the green plants from sulfates and phosphates which dissolve in the soil water. Few soils lack these, though for cropping the phosphates may be insufficient or may be so reduced as to interfere with full devel opment. " Land plaster " (gypsum, or calcium sulfate) is sometimes applied to fields; but it probably has more beneficial effects on other qualities than on the composition of the soil. Phosphates are an impor- tant part of artificial manures.2 In the case of both nitrogen and phos- phorus it is highly important, if immediate effects are desired, that the compounds be such as are " available," and compounds can be available only when they are soluble or readily become so. Raw materials. — The nitrates, sulfates, and phosphates enter the larger plants through the roots. These are the mineral salts which are most necessary for the wclbbcing of the plant, because they are needed for 1 If qoI the soil may be info ted by scattering on it soil in which such a crop has been previously thrown. Commercial attempts t<> supply pure cultures of appropriate bacteria for infecting the -oil through the seed sown have not been very successful. ■The whole Subject of the relation of manures and fertilizers to the soil and crop is m a very unsatUfat tory state and needs further investigation before the practice and results ran he explained. 380 PHYSIOLOGY protein synthesis. LikeCOo and H20, they have been called " foods "; but it is far better to look upon them as raw materials out of which, with others, food can be made. Given carbohydrates (finished and partly torn up again, or " in the making ") plus nitrates, sulfates, and phosphates, most plants can make proteins. There is no set of plants to whiclv protein synthesis is re- stricted, as is photosynthesis to the green plants. Yet there are plants (certain bacteria for example) which require their nitrogen supplied in other forms than nitrate, and some even which can use nothing less complex than proteins. Here we may properly speak of assimilation rather than of synthesis. No special organs. — In the larger plants protein synthesis is not re- stricted to a particular organ. Neither chlorophyll nor light is essential to it, for it is carried on freely by fungi which have no chlorophyll, and it is doubtful, in spite of much experimenting, whether light has any in- fluence upon its rate. Since carbohydrates are usually the basis of pro- tein synthesis, the leaves, in green plants, are the chief seat of this pro- cess; for in the leaves carbohydrates are being made, and to them stream the dilute watery solutions of salts, brought via the xylem bundles by evaporation. Process. — So long as the constitution of proteins remains unknown it will be impossible to describe the process by which they are made. Inasmuch as all proteins on decomposition yield amides (amino-acids), and the simpler ones are certainly formed from them by condensation, it is supposed that carbohydrates are converted into amides first, by the introduction of NH2-groups here and there, and that these amides link themselves together, some becoming modified by the incorporation of sulfur and phosphorus molecules, and so form proteins of various kinds. But the details are all uncertain and only the vaguest statements can be made. 4. OTHER WAYS OF GETTING FOOD Dependent plants. — The green plants are sometimes distinguished from others by the term autotrophic, meaning that they nourish them- selves by their ability to make in their own bodies the most important foods, the carbohydrates. All others are heterotrophic plants, signifying that they secure food in a different way. (But see p. 362.) The more important ways are now to be described. NUTRITION 3«i Among the many thousand species of heterotrophic plants, the bac- teria and fungi hold the dominant place. A few seed plants lack chlorophyll entirely, such as the Indian pipe (Motwtropa), beech drops (Epifagusvirginiana), dodder (Cuscuta), etc.; and some have only par- tially lost it, or with a good supply nevertheless have the nutritive habits of the non-green plants. The families in which such dependent species are prominent are theLoranthaceae, Rafflesiaceae, Scrophulariaceae, < >rol>am hat ear, and Halanophora eae. If a plant cannot make carbohydrates, it must of necessity get food directly or indirectly from some plant that can. The direct way of doing this is to live on or in a live green plant. The indirect way differs only in that the food secured is more remote from the original food maker. Thus, a plant may live upon or in some animal or some non- green plant, or upon the dead bodies of these, more or less decayed and disintegrated. Indeed, decay and disintegration are only the obvious evidence that plants (chiefly the minute bacteria and fungi) are living upon such a dead body. And not infrequently death itself is simply the result of the vigorous development of such creatures on or in the body of a once healthy organism. Parasitism. — An association between two live organisms is known as symbiosis. When one obtains its food from the other, the rela- tion is called parasitism, and the two are known respectively as parasite and host. As a rule the food maker is called the host, and the other the parasite; if neither or both be food makers, the larger is distinguished as the host. Thus, fungi are parasitic on leaves or twigs or in the wood of trees, or on animals; " beech- drops " (Epifagus virginiana, a small flower- ing plant) is parasitic on the roots of the beech tree; mistletoe is parasitic on elms, etc. This relation requires the closest contact between the cells of parasite and host, and the parasite even penetrates the 1 ells of the host in many cases. The smaller parasites, such as fungi, may grow bodily through cells, doubtless dis- FlG. 651. — An epidermal cell of a grass | Poo) penetrated l>y a branched naustorium ^1) of a fundus (Erysiphe grami- nis); the mycelial hypha to which the slender penetrating tube (a) is attached is not shown. — After Smith. 382 PHYSIOLOGY Fig. 652. — Section of stem penetrated by haustorium (h) of dodder (Cuscuta). — From Part III. (For ex- planation of letters, see fig. 1082.) solving the wall by some enzyme (see digestion, p. 399), or it may send into them short branches, called haustoria (tig. 651; see also figs. 1079, 1080, Part III), through which the food enters the parasite. A vascu- lar parasite, the dod- der, which twines exten- sively over coarse herbs, sends into its host short branches, likewise called haustoria (fig. 652), whose vascular strands come into the most inti- mate contact with those of the host. (See Part III on parasitism.) Partial parasites. — When such complete contact has been estab- lished, it is difficult to determine what or how much material migrates from host to parasite. Colorless parasites, of course, must get all their food from the host. Certain green parasites undoubtedly could live by getting merely water and its dissolved salts, for they can make food for them- selves. Hence they are known as partial parasites. But that they completely restrict themselves to such food materials and do not admit any real food is quite improbable, in view of the intimate union between the two. Mutualism. — The support of the parasite by the host may result in no considerable injury or even weakening. Indeed, many cases have been described in which the association suggested a partnership, whence the term mutualism. From another point of view the relation resembles that of master and European beech (Fa- slave, whence the term helotism (see Part III). £"s ^^a);h, hy- ' v ' phae. — After Frank. The lichens (p. 78) furnish the classical example. Yet even here the algae are somewhat restricted in development by the constant drain upon them, though perhaps they can work at food Fig. 653. — Ecto- trophic mycorhiza of NUTRITIi >N 3*3 making longer because the encompassing fungus by its spongy tex- ture retains rainwater longer than would the algae alone. Mycorhiza i> another instance of so-called mutualism, in which fungi associate themselves with the roots of certain plants, especially the oaks (Cupuliferae), the heaths (Kricaceae), and the orchids (Orchidaceae). Sometimes they jacket the rootlets with a weft of filaments (cctotrophic mycorhiza, fig. 653), and sometimes they penetrate the corti- cal cells, forming a tangle about the nucleus (endotrophic mycorhiza, fig. 654). The fungi are supposed to aid the root in acquiring water and food materials (especially nitrogen compounds, which they themselves may form from the free nitrogen of the air) from the soil. Certainly they derive some food from F.G. 6j4 _ Endotrophic the root, and injury to the root is suggested mycorhiza of Neotiia: a, host , . 11 r 1 1 e .1 cell with active fungus hyphae; by its stubby form and the frequent absence b> ccll with dcgcnerating hy_ of root hairs. In fact, the more the cases of phae.— After Magnus. (See SO >alled mutualism are studied, the more it a ° gs" becomes evident that they are only cases of modified parasitism, with minor injury to the host. (See Part III on reciprocal parasitism.) Injury by parasites. — On the other hand, the drain on the food re- sources of the hist may be severe, so weakening it that it succumbs to adverse conditions which otherwise could be overcome. Quite apart from this weakening for lack of food, the parasite may act as a stimulus to local growth, or it may produce injurious substances which cause local or even general death. The location of a parasite is often marked by deformities; leaves are crinkled or thickened, as in peach curl; circum- si ril>e<| swellings of peculiar and fantastic or beautiful forms (galls) grow on leaves or stems (tig. 655); even large tumors are formed, as in the Maik knot of ( herry and plum trees. Local death is another common mark of the presence of a parasite. The fire blight of apple and pear tree-, .hie to parasitic bacteria, gets its name because young shoots are killed for a distance of 20 to 50 cm., and the withered brown leaves make the tree look as though it had been scorched by a tire. General death in large plants is seldom produced by a parasite unless it inter- feres with the water supply or invades the entire organism In wilt disease the parasite blocks the tracheae, interfering with the supply of 384 PHYSIOLOGY water to the leaves, and death follows with surprising suddenness. In other cases, since in plants there are no means for quick distribution of poisons locally produced, nor any regulatory centers whose injury up- sets the whole system, death is likely to be merely local. In animals, on the contrary, a parasitic plant, restricted to a limited region, may produce poisons which are quickly spread through the body by the blood, attack the central nervous system or important viscera, and soon cause death. Thus, in diphtheria, the bacteria flourish chiefly Fig. 655. — Galls: a, on leaf of rose; b, on stem of grape. — From Part III. in the throat, where they may produce no serious lesion, but the toxins produced reach the heart and kidneys and sometimes fatally injure them. Saprophytes. — The association of a plant with a dead organism or organic debris is called saprophytism, and the live member is a sapro- phyte. Since a parasite may kill its host and then continue to live upon the body, the distinction between parasites and saprophytes is not always clear. Thus there are obligate parasites and obligate saprophytes; plants, namely, that are obliged to live in one relation or the other. Cor- respondingly there are facultative parasites and facultative saprophytes, which may pass part of their lives in one way and part in the other or wholly in either. Often the full cycle can be completed only if the given plant can establish the preferred relation. NUTRITION 385 Saprophytes are very numerous and varied. They may be superfii ial, or may penetrate the substratum thoroughly, showing finally at the sur- face only the reproductive bodies. The very fact that they are getting food from the dead organism indicates that they are con- suming it. In- asmuch as they often must digest the food before it can enter their bodies, they disintegrate the body on which they feed. In the course of this digestion and disin- tegration, many and varied chemical reactions occur, some incited by the saprophyte, some in- cidental to the changes it produces. These are summed up for fluid media under the term fer- mentation, and for solids under the terms decay or putrefaction. Certainly in fermentation (p. 409), and probably also in putrefaction and decay, some of the most striking reactions are not connected \]k • with food getting, though apparently they are en- tire! v similar thereto. Organic debris. — It is not necessary that the If ' ','U'u1 dead body retain any semblance of its original iljj rMjil form. It may even be so far destroyed as to be m^ if Vh'j^| mere particles of a soil; yet the saprophyte relies wL* *" U on these for its food. Thus, the common mush- W \ $ room of comment' (Agaricus campestris) is grown \\| v upon a compost of soil and horse dung, the par- \l \ v_Lea| of jy* minute flora, whose operations are often indispen- penthes Mastersiatia.— Froma sable to the welfare of larger plants. photograph by G.W.Ouver. Succession. — Nothing is more striking than the succession of sap- rophytes that live upon a dead organism and finally dispose of all its organic matter, each appropriating a suitable part and reducing that to the most simple and stable compounds, until finally it " returns to the dust whence it came." This emphasizes, too, the striking differences C. B. & C. HOT ANY — 25 386 PHYSIOLOGY between saprophytes in their use of offered foods — differences which at present are quite inexplicable. A classification of saprophytes accord- ing to the sort of food on which they thrive best has been made; but this expresses only in a summary way our very imperfect knowledge of their nutrition. Insectivorous plants. — Besides the ordinary parasites and sapro- phytes, there are a few rather isolated cases of green seed plants which Fig. 657. — A rosette of leaves of Venus's flytrap (Dionaea muscipula) seen from above. — From a photograph by G. W. Oliver. have special apparatus for capturing small animals and digesting them. Some are submersed water plants, some grow on land. They are col- lectively known as insectivorous or carnivorous plants, but the methods of capture are quite diverse. Pitcher plants. — The pitcher plants, Sarracenia, Darlingtonia, Nepen- thes (fig. 656), and Cephalotus, have part or all of the leaf trumpet-like, pitcher-like, or cuplike, holding more or less water. The sides have stiff downward-pointing hairs, slippery areas of treacherous footing, decep- NUTRITION 387 tive translucent spots away from the concealed opening, one or all, whi< h prevent the escape of insects that wander in and sooner or later drown in the fluid; whence nitrogenous compounds derived from their bodies by decay or digestion enter the tissues of the pitcher. Flytrap. — Venus's flytrap (Dionaea, fig. 657) has leaves with two terminal lobes about 1 cm. long, hinged about the midrib, and fringed with long slender teeth, which interlock when the lobes shut together (figs. 658, 659) . On the surface of each lobe are three large sensitive bris- tles, and if one of these be bent so as to compress the basal cell, the lobes shut like the two jaws of a trap. Insects, flying or crawling, which come into contact with the bristles are often caught. Then the glands upon the upper (in- ner) surface pour out a digestive fluid, the proteins are reduced to such simplicity that they can enter the tissues, and after a few days the leaf opens again. Its water mate, Aldrovanda, has a similar but smaller trap, by which minute swimming crustaceans, Dapknia, Cyclops, etc., arc often caught. Sundew. — Drosera, the sun- dew, has its leaves (fig. 685) fringed and stalked glands that secrete viscid transparent fluid, in which small insects alighting may become enveloped by their own struggles, and further (in our species) on account of the inflection of the >talks of the glands. When an insect i- caught, the character of the secretion changes; it becomes more watery and Figs. 658, 659. — ( tass set tions of the termi- nal Id1.cs forming the "trap" of Dionaea: 658, enlarged view, dosed position, diagram- matic; g, digestive glands; />, ^parenchymatous tissues whose varying turgor opens and doses tin- "trap"; s, sensitive bristles; 659, outline, »vered above with on a smaller scale, of same in an open position. \fter Kn'V. 388 PHYSIOLOGY contains an enzyme which digests proteins. That the products enter the plant and are advantageous has been shown by comparing fed and unfed plants in the same pot. Those on whose leaves tiny bits of meat and egg were placed were larger and thriftier, and had more flowers, as well as more and larger seed, than the ones which grew under identi- cal conditions without feeding. The capture of insects probably supplements a scanty supply of nitro- gen obtained from the soil nitrates; but too little is known of the ecology of such plants to establish this explanation as at all conclusive. A fuller discussion of most of the topics of this chapter will be found in Part III. 5. THE STORAGE AND TRANSLOCATION OF FOOD Surplus food. — A part of the food made by a plant is promptly util- ized in the making of new tissues (growth) and in the repair of the pro- toplasm which has undergone changes in the course of its activity. It is often said, also, that a part of it is oxidized directly to furnish energy for growth and other work; but it is at least doubtful whether this is true.1 However that may be, most plants, at least at some period of their existence, make more food than they actually use at the time. The surplus is then stored for a longer or shorter time, until it is required. But it may never be used. Storage places. — Accumulation may take place in the very part where the food is made; but usually, if there is any room there, it is insufficient; and to judge from the infrequent storage in food-forming organs these two functions are not fully compatible. So when there is any considerable surplus of food, it migrates to some more or less spe- cialized storage organ. In the lower plants these are relatively simple, for ordinarily such plants make little excess food. In Marchantia, for instance, the colorless parenchyma of the lower part of the thallus is accounted the storage region. In the pteridophytes and sperma- tophytes, any one of the larger organs, root, stem, or leaf, may become the seat of food accumulation. In many cases there is marked change in structure and form. Parenchyma increased. — The characteristic change in structure con- sists of an exaggerated development of parenchyma, in which chiefly the 1 The matter wjll be discussed further in the section on Respiration, p. 403. NUTRITION 3»9 food accumulates. This may l>c the parenchyma of the cortex, or of the vascular bundles, or of the pith; <>r all may be involved. One note- worthy point is that the storage tissues arc composed of live cells, even though, as in -nine ferns, they arc very thick walled. It is to be observed also that the reservoirs of food are usually Located in part- thai persist through a dry or cold season unfavorable to growth, and that have rudi- mentary growing points capable of quick and vigorous development by using the adjacent suqilus. So the seeds, bulbs, tubers, rhizomes, etc., are organs of propagation, and by way of attaining that end become also organs of storage. (See Part III on seeds, bulbs, and tubers.) Storage cells active. — The storage of food is not merely a stuffing of passive cells with surplus food; it involves the activity of the storage cells themselves, at least for the ac- cumulation of the food, and usually also for the mobilization when this food is about to travel to growing regions where it is subse- quently used. The process of mobilization is commonly called digestion (see p. 397), and J^ JSSirfKS seems to be the reverse of the process by onia: 660, simple starch grain which the storage forms of food are pro- £* '^^'f in ***».! & » 661, leucoplast alone of asimi- duced. lar grain; 662, leucoplast of ■After The storage forms of food a Uv,n sram- x 9°°-- Meykr. Storage forms. are chiefly starches, sugars, hemi-celluloses, inulin, fats, and proteins. From this list it will be apparent that carbo- hydrates predominate, and quantitatively they form much the greater part of stored food. Starches. — Starches are stored in the form of grains, many having a form characteristic of the plant in which they are found. The grains are organized by the activity of cell organs called leucoplasts or amyloplasts (figs. 600-062), which seem to take the material as it comes to the cells, perhaps as glucose, and combine it into larger and more complex mole- cules, that finally become Stan h. This is disposed in the interior of the leucoplast as one or more grains, which at length stretch it enormously, or even rupture it. The actual structure of the grain is believed to be that of a spherite; thai is, il is composed of a multitude of mil roscopi- cally minute, threadlike crystals, radiating from its organic center. If 390 PHYSIOLOGY more than one such crystal starts in the leucoplast, a compound or aggre- gate grain may result (fig. 662). The grains may show irregular layers (fig. 660), this appearance signifying differences in the proportion of water, composition of material, etc., doubtless determined by variations in the available sugars and other conditions during the growth of the grain. The starchy reservoirs are sources of important foods for men and animals, as well as plants. Many of our farm and garden crops are such storage organs, greatly improved and enlarged by breeding. Potatoes, sweet potatoes, yams, all the cereals, peas and beans, arrowroot, sago, and tapioca are widely used plant products, whose most abundant constituent is starch. The extraction of starch for commercial purposes, especially from potatoes and corn, is an industry of consider- able magnitude, as is also the production of alcohol by the fermentation of glucose derived from the starch of these plants. The following table shows the approxi- mate starch content of some common food reservoirs, in percentages of their dry weight. In seeds of rice .... 68 In seeds of navy beans . 45 In seeds of wheat .... 68 In seeds of flax .... 23 In seeds of corn .... 60 In seeds of almond ... 8 In seeds of pea .... 52 In tuber of potato ... 80 Sugars. — The chief storage form of the sugars is saccharose, or cane sugar. While glucose and fructose may be counted as constituents of almost every active cell, they do not accumulate in nature to any great extent, whereas saccharose in some plants, such as sugar cane and beet, is almost the only form of surplus food, and in many it accompanies the reserves of starch. The commercial supply of sugar is obtained chiefly from cane and beet, while sorghum, maple, and certain palms furnish a relatively small or local supply. Sugar is extracted from cane by crushing and washing, clarifying the liquor and concentrating it. Beets are finely sliced and the sugar is extracted by diffusion, then recovered by clarification and concentration of the solution. The cultivated races of beet now average nearly 15 per cent of sugar, with some samples going over 20 per cent, as against less than 7 per cent when breeding began. Cane juice yields 10-18 per cent, and maple sap 2-5 per cent of saccharose. The refining of sugar by redissolving and purifying removes the coloring and flavoring matters which give to crude sugars from different plants their distinctive taste. " Reserve cellulose." — This name has been applied to food accumu- lated upon the walls of cells; yet the substances are quite different from the cellulose which forms the permanent part of the wall, and should rather be called hemi-celluloses. They consist often of mannans and galactans, which on digestion yield mannose and galactose, sugars that are quickly M TKITION 391 transformed into other compounds. The hemi-celluloses art- especially common in the endosperm of seeds, and are used as food by the embryo in germination. They are deposited in layers on the interior of the cell wall-, sometimes to the greal reduction of the lumen; yet through the pits in the thickened walls the protoplast in each chamber maintains communication by slender threads with its neighbor. This excessive thickening imparts to such seeds a hornlike toughness, as in the coffee "bean," or even a bony hardness, as in the date "stones." Sometimes cotyledons and even bud scales have like deposits on their cell walls. Inulin. — Inulin is comparatively restricted, being characteristic of a few large families (and occasional elsewhere). It occurs dissolved in the cell sip. especially of subterranean organs. It is a very complex carbohydrate, though less so than starch, having a formula w(C6H10O5), where n is probably as much as 12 or 18. Whereas starch is built from glucose units, inulin is formed by the condensation of fructose units, and is comparable in complexity with some of the dextrins, which starch yields by digestion. When inulin-containing tissues are put into strong alcohol, the inulin is deposited as spherites (see Part III, fig. 1209). Fats. — Fats are among the most important and valuable of surplus foods. In most plants they exist as small drops of oil in the protoplast; but in some cases, as in cacao, they are solid at ordinary temperatures. The most universal storage place for fats is the seed, where it is in some cases the dominant form of food, and in almost all it is present in greater or less quantity. It is by no means confined to seeds, but occurs in the flesh of fruits (olive), in rhizomes (potato, iris, and sedges), in bulbs (onion), and in roots (carrot). In almost every part of a plant, indeed, small quantities of oil may be found, and from many reservoirs it can be extracted in commercial quantities. True oils must be distinguished from volatile or essential oils, which are common in leaves and flower parts. The latter usually have a distinct odor and make a temporary translucent spot on writing paper, whereas that made- by true oils is lasting. Accumulated oils are obtained for commercial uses by crushing and pressure; lint as only a portion of the oil i whi< h forms 2 to 68 per c ent of the dry weight) > an l.c recovered thus, the " cake " remaining, with its residue of oil and other sub- may still be valuable food for animals, as is the case with cotton and flax seed. Proteins. - Proteins, unless they take on a specific solid form, cannot readily be distinguished from resting protoplasm. Thus, the " gluten " 392 PHYSIOLOGY of wheat is apparently a part of the network of protoplasm in which the starch grains are imbedded. The best known storage forms appear in vacu- oles of the endosperm in seeds. The proteins accumulate in the small vacuoles, and upon the loss of water, characteristic of maturation for a resting period, become more and more con- centrated, until finally they solidify, forming the " aleu- rone " or protein grains. These Fig. 663. — Outer portion of a cross section are very commonly associated ' m QO't of a wheat grain: h, various integuments of the ovary and seed, forming the husk; a, cells of "aleuronc layer" of endosperm, loaded with protein grains; b, starch-bearing cells. — After Cobb. with reserve starch, either in the same cells, as in the pea and bean, or the protein grains are characteristic of certain cells, as in wheat and other cereals, where they abound in the outer layer of the endosperm (fig. 663). In large grains some proteins may crystallize out, as in the castor bean (fig. 664) and the Brazil nut, but oftener they remain apparently homogeneous. Amides. — Amides occur in such quantities, especially in some sappy reservoirs, that they may be considered as stored food. There they may form 40-70 per cent of the nitroge- nous materials. Alkaloids. — Some recent studies of cacao ("cocoa " ) and coffee make it probable that their alkaloids (see p. 415), which are of a different type from most, may be a form of surplus nitrogenous food, since they come again into use. They constitute a very compact source of available nitrogen. Combination of food. — It must not be sup- posed that the foods above named accumulate independently. On the contrary, they always occur associated, though one form is likely to be dominant. Rarely, if ever, are they so re- FlG. 664. — Cell from en- dosperm of castor bean {Ricinus communis'): p, />, protein grains, made up of amorphous proteins, crys- talline proteins (c) (" erys- talloids")i and globular compounds of -proteins with calcium and magnesium, the globoids (g). — Adapted. NUTRITION 393 lated to one another in amount as to form what animal feeders call a balanced ration. Tin's is shown by the fact that, when growth is resumed, food of one sort is not used in the ratio which it hears to others stored with it. often indeed the reserves are not exhausted until the plant or shoot, having begun independent manufai ture, is able to supplement the deficiencies in the stored ration. Tims, finally, it may utilize all the accumulated reserve, hut often this is not done, and the excess is again stored elsewhere Traveling forms. — Since the plates of storage are seldom the places of food making or use, translocation of food usually precedes and fol- lows storage. Unfortunately, little is known about the translocation of foods. It seems clear that the traveling forms must he relatively simpler than those in which they an' stored. Obviously, they can travel only in solution, and, as a rule, the protoplasm does not permit the pas- sage of the foods in their storage forms. Thus, cane sugar probably travels as glucose and fructose; the fats a- glycerin and fatty acids; the proteins as amides. For in all translocation of foods, whether in small plants or large, it is necessary that they he able finally to diffuse through live cells, and the more complex compounds are usually un- ahle to do this. Diffusion. — In the smaller plants osmotic differences alone must account for the transfer from cell to cell. This may he facilitated by the delicate protoplasmic connections which commonly exist and would make it unnecessary for all the food to pass through the coll wall itself. In fungi which have coenocytic hyphae, the absence of transverse parti- tions probably facilitates transfer; while the surging movements that have heen observed in the contents of certain molds (Mucorales) would certainly do so. Vet actual knowledge regarding the translocation of food iii even the simplest plant is scant}-. Food obviously get- from place to plate, and there is apparently no way for it to do so except by diffusion. Conducting system. — In the larger plants a conducting system is developed; and it is evidently advantageous that the -lower movement of diffusion he- supplemented by a more rapid one along the- chief lines of travel when the factories are separated by considerable distances fn m the places of use or storage. This conducting system in all the vascular plant- consists of the phloem strands. It may he supplemented in certain large families by the latex system, though the fun< ti<>n of the latex is somewhat uncertain. 394 PHYSIOLOGY Phloem strands. — The phloem strands are usually definitely related to the xylem strands (which carry water), though they occur also inde- pendent of them. In most seed plants there is a phloem strand lying along the outer face of a xylem strand, and except in the monocotyledons there is generally between them a meristem (cambium), which may add to the radial diameter of both xylem and phloem. It may also, if it extend from one strand to another around the axis, produce new second- ary phloem strands between the old ones. The phloem strands form a continuous system, and may be traced from the stem outward into the leaves and downward into the roots. So followed, they usually dis- appear before the xylem strands end; that is, their differentiation does not begin so early in the rootlets nor extend so far in the leaves. Elements of phloem. — The elements of the phloem strands are sieve tubes, companion cells, cambiform cells, and parenchyma, with some- times mechanical tissues, though the latter belong more commonly to the adjacent tissue systems. It is impossible to specify the precise role of each of the elements; but among them all the sieve tubes may be considered the chief lines of conduction, the others being supplemen- tary thereto.1 In a way the sieve tubes are analogous to the tracheae of the xylem; particularly in that, having their end walls partially resorbed, they constitute tubes through which the foods may move without the delay necessitated by osmotic transfer from cell to cell. Evidence of conductivity. — The reasons for assigning conductive functions to the phloem strands are chiefly these: (i) The pith is so commonly dead and its cells filled with gases that it may be excluded from consideration. (2) The cortex, too, is often dead; particularly is this almost universally true of the older parts of shrubs and trees in which it is frequently sloughed off after a few years; yet there is an active trans- fer of foods. Moreover, the movement of food through the protoplasmic membranes of live cells is apparently too slow to meet the needs of plant growth. (3) When the cortex is removed by surgical operation, the supply of food seems to be quite adequate to permit development; but if the phloem strands are interrupted, transfer of foods is almost or quite stopped. This is particularly noticeable when girdling occurs in nature, as when birds destroy a zone of bark in conifers whose wood remains able to conduct water. The tops and roots (if one or more circles of branches below the injury remain, 1 It is as though the sieve tubes were the main railway lines and the adjacent tissue sidetracks temporarily occupied. NUTRITION 395 keeping the latter supplied with food) may continue t<> live for years (fig. 665), vet the vigorous growth is above the injury. Girdling experiments with willow shoots arc often cited as adequate proofs of the conductive function of phloem. For example, by removing a ring of cortex g mm. wide, a few centimeters from the lower end in one case and several times as far in another, and plat ing both shoots in water, lateral roots and slmots develop in lx>th cases. Their vigor is somewhat proportional to the relative lengths of stem below and above the girdling, and this is taken to indicate that the new parts can draw only upon food stored in the part of the stem above and below the girdling, transfer being prevented by the interruption of the phloerru But if bridges of bark be left across the gap, the differences of development tend to disappear; and the more numerous the bridges the less the differ- ences. While such experiments agree fairly well with other observations, they are in themselves not con- elusive, since the results are complicated with obscure phenomena of regeneration, and perhaps with wound irritability. (4) The content of the sieve tubes, which is a coagulable slime, consists more largely of foods than would be at all likely unless the sieve tubes were organs of either conduction or storage, and the latter supposition is unlikely because the foods are almost entirely in solution. In atypi- cal case analysis showed that, excluding water, FlG- 665. — Portion of . . 111 the trunk of a pine, the the constituents were: carbohydrates, 30 per bark completely destroyed cent; amides, 38 per cent; proteins, 20 per cent. hv hirds :lt «• A single c- -i 1 r 1 1 1 r j ui 11 circle of branches below So rich a supply of soluble foods could hardly I!- be found anywhere else. (5) A bit of merely corroborative evidence is derived from the dis- tribution and relative development of the phloem. No plants need more facile movement of foods keeping all tin- parts lower than a scantily supplied with food, the upper part made .111 excessive growth, especially in the neighbor- hood of the wound, but food could not pass a than vines, whose stems are necessarily slender freely (perhaps not at all) and long, a.id in none is there better develop- Original in the museum of ' I'urduel niversitv. — rrom ment of the phloem. Indeed, when the anatomist photograph supplied bj wishes to study the largesl and most specialized Stanley Coulter. sieve tubes, vine- are almost invariably selected. Moreover, where the requirements for food transfer are the greatest, a- in flower clusters and in tlie brain lies of inflorescences, the phloem strands are particularly well developed, 396 PHYSIOLOGY Rhythmic translocation. — Since leaves are the principal regions of food making, which is distinctly rhythmic by reason of the alternation of light and darkness, the translocation of food shows a corresponding rhythm. The transfer of any soluble food is continuous, and the rate is determined by the usual factors ; but, as the transportation facilities are overtaxed during the day, there is on the whole an accumulation of food in the leaves then; only after the nightly slackening does emptying of the leaf become obvious. That a leaf which shows starch near the close of a day may show none in the early morning does not necessarily indicate that carbohydrates have been carried off during the night, though they doubtless are, but only that they have been re- duced in amount in some way, probably by migration and by conversion into other foods. Causes of movement. — Nothing is satisfactorily known as to the causes of movement in the phloem. In the sieve tubes the absence of protoplasmic membranes closing the ends surely permits more rapid diffusion, which may be further facilitated by mechanical mixing due to bending and other compression of parts of the system. That the con- tents are under pressure is shown by the rapid oozing of material from cut sieve tubes, an amount being reported in Cucurbita which indicates that one or even two internodes had been emptied, and so the material must have passed 75 to 100 of the sieve plates (the perforate end walls of the sieve cells). The source of this pressure and the effect of it on translo- cation is not known. Latex system. — In certain families,1 it may be that translocation of foods takes place through the latex vessels, as well as by the phloem. Latex vessels form a system of branched or anastomosing tubes run- ning through the cortex (more rarely elsewhere), and ending blindly in the leaves and roots. Histologically, they are coenocytes or cell fusions (see Part I, p. 27). They approach very near to the growing points, and in the leaves have close relations with the manufacturing cells, the very arrangement sometimes suggesting its fitness for collect- ing foods. The latex which fills these tubes is the cell sap of a huge vacuole, the protoplasmic contents being reduced to a very thin layer. Latex is in part a watery solution of many substances, such as proteins, sugars, gums, tannins, alkaloids, and salts; in part an emulsion of oils and tannins in droplets; and in part suspended granules of starch, gum, 1 Particularly the Papaveraceae, Compositae (Cichorieae), Lobeliaceae, Campanu- la! i .11 , Asclepiadaceae, Apocynaceae, Euphorbiaceae, Moraceae, Araceae, and Musaceae. NUTRITION 397 resin, and caoutchouc. Some latex is translucent, but usually it is an opaque, white, yellow, or orange liquid, Familiar to many as the milky " juice " of dandelion, poppy, milkweed, or the orange " blood " of the bloodroot. Latex is commercially important as the source of opium and its alkaloids, of India rubber, and of gutta peri ha. Function. — The principal reasons for ascribing to latex vessels the function of a conducting system are the abundance <>f foods in the latex, and the peculiar structural relations of the latex vessels t<> the nutritive cells of the leaves. The carbohydrate and nitrogenous foods of the latex run as high as 30 per cent of the dry matter therein; they are most abun- dant when active growth and development are beginning, and least so when growth is checked and a resting period i- at hand. In some leaves the latex vessels look as though they were favorably arranged to receive materials collected from the nutritive cells. Yet for the conduc- tive function the evidence is rather presumptive than convincing. It may be that the latex has to do rather with storage and protection. For further details on latex and accumulation of foods, see Part III. 6. DIGESTION Nature of digestion. — Whenever foods are insoluble in water (as are some of the mosl valuable ones), they cannot be used by plants until transformed into a soluble substance. Whenever soluble foods are un- able to diffuse readily through protoplasmic membranes, they (an scarcely mow from one point to another, and are available, if at all, chiefly in the cell where they happen to be. Every transformation of food by the agency of a third body from an insoluble to a soluble and from an indiffusible to a diffusible condition, whatever the precise chemical nature of the change, is summed up in the term digestion. This use of the term is in exact accord with its long use in animal physiology. The pn>< e— es in plant and animal, indeed, are essentially the- same; they are wroughl by the same sorts of agents, affect the same orts of substances, and result in the same sorts of products. No special digestive organs. — Plants differ from the' larger animals in having 110 pou< hed tube wherein food is lodged, and in which some of the more striking digestive processes take place, before the food truly enters the- body. This digestive trait, its parts and accompanying gland , constitute the special digestive organs of the animal, though mm h important digestion takes phu e elsewhere. Plants have no spe< ial 398 PHYSIOLOGY digestive organs comparable to these; but places of food making and food storage must be places where digestion is also particularly active. Misleading comparisons of the leaves to the stomach not rarely occur in primary books, which thus seek to " explain " the work of a leaf. When, as in one notable instance, a leaf is compared to a kitchen, where the dilute " soups," coming up from the roots, are " boiled down"; later to a stomach, where the food is made ready; and finally to the lungs, by which the dear little plant breathes, the child would have a truly appalling notion of a leaf were he not usually immune to such bad pedagogy, by reason of his ignorance of at least the stomach and lungs. Extra-cellular digestion. — In plant as in animal, many foods must be digested before they can enter the cells at all, while others are digested as they lie in the cells. So one may distinguish, as to location, extra- cellular and intra-cellular digestion; but agents, processes, and results are essentially alike in both. In a fungus which merely pushes its way among the intercellular spaces of another plant, it is impossible to say whether any food is being digested or whether only what is already soluble and diffusible is being used. But when a fungus sends a branch, as a haustorium, through the cell wall (fig. 651), or when, as in certain wood-destroying fungi, the mycelium penetrates the walls freely in all directions, it is obvious that by some means the wall is actively dissolved at the point of contact. Chemical changes. — The changes characteristic of digestion result in the cleaving of compounds into two or more simpler substances, with or without the taking up of water. In case water is incorporated the cleavage is called hydrolysis. Thus when cane sugar is digested: Ci2H22Oii + H20 ^> C6H12Oe + C6H1206 saccharose water glucose fructose Starch when digested takes up water, and four fifths of it breaks up into maltose units (Ci2H22On), the other fifth resisting full digestion for a longtime. The mal- tose is further digested into two units of glucose, with assumption of another mole- cule of water. Other foods split up into simpler compounds without adding anything to their members. Thus sinigrin, a glucoside characteristic of the plants in the mustard family, cleaves thus: Ci0H18NKS2Oio ^t C3H6CNS + C6Hi2Oc + KHSO4 sinigrin allyl thiocyanate glucose potassium-hydrogen (mustard oil) sulfate The chemical changes of digestion represent only a few of the mul- titudinous reactions going on in the plant. The rate of these reactions, like all others, depends on temperature, concentration, etc., and espe- NUTRITION 399 i iallyon the effect of other substances whi« li arc present. It is not always evident just how a third body affects the rate at which one substance is converted into another in a chemical reaction, and so doubtless many effects of this sort pass unnoticed. Hut when the effect i> pronounced, the third body is spoken of as a catalyst, and the effect of the catalyst on the reaction is known as catalysis. By such agents reaction-, so slow as to be unnoticed, may be greatly accelerated and become evident; and others, which might be very rapid, arc retarded, even until they are negligible. Enzymes. — Among the catalytic agents (which arc varied and not at all confined to living beings) are certain substances produced by organ- isms and called enzymes. These are widely different in their action, though they all seem to be of protein nature, so far as their chemical < har- acter is made out. The great difficulty in doing this lies in the impossi- bility, up to date, of separating them from the other protein- of the cell and obtaining them in any certain state of purity. In general they act best within certain narrow- limits of temperature, such as 30-450 C, and most are totally destroyed at such temperatures as 60-750 C. Small quantities of free acid or alkali may facilitate their action; while certain metallic ions, e.g. Hg, Cu, Ag, may retard or inhibit their ordinary effect, just as they " poison " a live cell. There seems to be a great variety of enzymes, each producing an ap- propriate effect upon certain foods; but others are known which have to do with reactions quite apart from the digestive changes. The di- gestive enzymes, then, are only part of a larger class of bodies, whose number and variety are only imperfectly known. Reversible action. — The action of a number of enzymes is known to be reversible; i.e. they not only, under certain conditions, hasten the otherwise imperceptible decomposition of a particular substance into two or more simpler compounds, but also, under other condition-, ac- celerate the combination of the simpler substances into the more com- plex one. Indeed, it seems likely that the constructive action of enzymes may soon be shown to be as important as the destructive. This action would be of the greatest importance in the making of complex food- from simpler ones, such as the formation of Starch from glucose, of 1 ane sugar from glucose and fructose, of proteins from amido-compounds, etc. But the knowledge of this constructive action is yet very -canty. Carbohydrate enzymes. — Diastase is one of the most important and widespread enzymes. It is found in practically all part- of plants, but 400 PHYSIOLOGY especially in leaves and storage organs. It partly digests starch into maltose, a residue, representing about 20 per cent of the grain, resisting its action for a long time. In the course of decomposition, various dextrins are produced by successive cleavage, presently becoming simple enough to be analyzed. The last member of the series breaks into mal- tose and isomaltose, C1L.H220n. There are at least two forms (possibly more), secretion diastase and translocation diastase, differing in the mode of dissolution of the starch grain. The former erodes the surface irregularly, whence narrow canals penetrate the interior, and the grain often falls into fragments; the latter corrodes the grain almost evenly, reducing it gradually in size until it disappears. It is probable th.it what is here called diastase consists of at least two enzymes; amylase, which digests starch to a dextrin, and dextrinase, which breaks the dextrin into maltose; this, maltase (see below) cleaves into glucose. Inveriase, in like manner, can hasten the hydrolysis of cane sugar into two hexose sugars, glucose and fructose. Trehalose and several other enzymes in fungi attack trehalose and other sugars peculiar to them, and digest them into the hexoses of which they were originally built. Maltase, an enzyme which is often associated with diastase, carries the process of starch digestion further, cleaving each maltose molecule into two molecules of glucose. Inulase likewise attacks inulin, breaking it up into levulins and finally into fructose. Perhaps there is here also more than one enzyme at work. Cytase is responsible for digesting hemi-celluloses (chiefly mannans and galactans) of seeds, while enzymes under the same name, but prob- ably different, have been found in wood-destroying fungi, and have been assumed present whenever a tissue is penetrated by a hypha, or by a more massive member, as in the sinking of the foot of bryophytes into the gametophyte (see Part I, p. 108) and in the emergence of the branches of roots through the cortex (fig. 667; see also Part I, p. 250, and fig. 558). Fat enzymes. — Lipase, perhaps of several different forms and so deserving distinctive names, has been found in organs where fats are present, especially in seeds and many fungi. Lipase breaks up fats into their components, fatty acids and glycerin, which are then readily dif- fusible. Glucoside enzymes. — These are common, setting free glucose from many dif- ferent compounds. Emulsin, for example, breaks amygdalin, a glucoside common NUTRITION 401 in peach, almond, and apple Beeds, into hydrocyanic acid, glucose, and benzoic aldehyde, thus; Ca>Hs7NOii - 1 HaO -> C7H4O I- IICN + 2(C«HuC amygdalin water benzoic aldehyde hydro i>. mi. .1. id glucose The so-called "mustard oil " is produced, along with glucose and two other 1 (im- pounds (see p. 398) from sinigrin, a glucoside < hara< teristi of the mustard family. These actions are very rapid, as shown by the formation of the peculiar flavor or pungency almost as soon as the parts are crushed by the teeth and the enzyme thus brought into contact with the glucoside. Protein enzymes. — Several enzymes are known which digest proteins. In animals their digestion proceeds by two prominent stages: first, the peptic enzymes (i.e. those like pepsin of the stomach) convert proteins into peptones, which arc soluble and diffusible; second, the trypsin of the intestine converts proteins and peptones alike into amino-acids and other compounds, still more freely soluble and diffusible. At first protein digestion in plants was ascribed to peptic enzymes; later, be- cause of its completeness, it was referred to tryptic enzymes and the presence of peptic enzymes was denied. Now, however, it is possible to distinguish the two classes of enzymes, though they act together and carry forward the processes to completion without a pause at any par- ticular stage of simplification. Inasmuch as the proteins are not prominent among surplus foods, it might seem at first sight that protein digestion was unimportant in plants. But aside from the stored food, many instances where such digestion must occur may be cited. Thus, the exhaustion of proteins to a large extent from the foliage of annuals as the seeds ripen (e.g. as shown in cereals), and the partial recovery of proteins from leaves of trees before their fall, presuppose protein digestion. So, also, the action of a plant parasite or saprophyte on animal bodies, and of the curious pitchers and traps of carnivorous or insectivorous plants involve protein digestion. Assimilation. — All the digestive changes are preliminary to the trans- location of foods from places of manufacture to places of storage or use, or from places of storage to places of use. And before foods arc of real use they must be incorporated into the living substances of the body,1 which grows thereby. This final step in the chemical progress of foods, by which they become a pari of the living protoplasm, is known 1 This view is only partly shared by those physiologists who believe that f<«»l can be "oxidized" use the term energesis for the < hemi< al . hanges iii the tissues, whose end seems t<> be the setting free of energy. It remains to lie seen whether or not this distinction is .n < eptable or important. It may prove, indeed, that the release of energy is quite incidental to other more essential processes. 403 404 PHYSIOLOGY tion of plants, or of green plants at least, is exactly the reverse of that of animals. This misconception is clue to confusing the effect produced upon a limited volume of air by the respiration of animals and by the photosynthesis of plants, two processes which are as little comparable in their results as are walking and eating. Neither gaseous exchange nor combustion. — The striking change that most organisms produce in the air of a limited space is the reduction in the amount of oxygen and the increase in the amount of carbon dioxid. This can readily be demonstrated by putting a considerable quantity of germinating seeds or opening flowers into a fruit jar and sealing it for a few hours. On then lowering a lighted taper into the jar, the flame will be extinguished; and a cup of baryta water will be covered quickly with a film of barium carbonate. This has led to a superficial concep- tion of respiration, current in text-books and encyclopedias, as an ex- change of the gases, oxygen and carbon dioxid, between the air and the organism. Because in the burning of wood and other carbon com- pounds oxygen is consumed and carbon dioxid is produced, respiration has been assumed to be a process of oxidation, in which foods undergo " combustion " in the same sense as the fuel in a furnace, the energy being liberated as heat and in other forms, when the carbon of the com- pounds is combined with the oxygen of the air. One striking difference between " combustion " inside an organism and outside is that the former occurs at low temperatures, while the latter takes place commonly at high temperatures. To escape this difficulty the term " physiological combustion " was invented. But the conception of respiration as an exchange of gases accompanying oxidation of carbonaceous foods is inadequate, and comparing it to any sort of combustion is more mislead- ing than helpful. Aerobic and anaerobic respiration. — In the first place, though or- dinarily oxygen is fixed, oxygen is not indispensable to respiration; and in the second place, though ordinarily C02 is evolved, carbon dioxid is not a necessary product and probably in no case does the 02 combined with the C come directly from the air. That being so, it is obvious that the above-mentioned conceptions as to respiration cannot be valid. That respiration sometimes goes on in the absence of free oxygen, makes it necessary to distinguish normal or aerobic respiration and intramo- lecular or anaerobic respiration.1 Aerobic respiration proceeds only 1 Inasmuch as under the conditions one is as really normal as the other, and as the term intramolecular expresses an interpretation of anaerobic respiration which is no DESTRUCTIVE METABOLISM 405 when Ojis presenl insufficient quantities, and among the end products two, COs and H^O, arc characteristic, though formed in very variable quantities in proportion to the Oa taken up. Anaerobic may replace aerobic respiration in any organism when I )s is cut off, and may proceed for a long time; bul the end products are various and quite different from those of aerobic respiration. Among them are <<>mmonly ethyl alcohol and hydrogen, and less C02. Certain minute organisms may pass their whole existence without oxygen, which indeed hinders or alto- gether stops their development, and they are thus restricted to anaerobii respiration. In most organisms, however, anaerobic respiration can be considered only as a makeshift. Nature. — What then is the fundamental feature of a process that goes on under such different conditions and results in such diverse prod- ucts? So far as now appears, respiration consists in the decomposition of the protoplasm or some of its constituent proteins, either directly, or as a result of the action of an enzyme or of some internal force (stim- ulus) upon it. Inasmuch as the inciting cause is rarely apparent, spon- taneous or self-decomposition is often spoken of, but this merely means that the reason is unknown. The view here presentee! is not the one most generally held at present, hut appeals to the author as most consistent with the known fa< ts. Many physiologists consider respiration to consist primarily in the decomposition of foods by the protoplasm or l>v enzymes, without their assimilation into the living substance. In this case f Is arc a kind of fuel for the body (see p. 406). It is not denied that some de- composition of protoplasm <" the plant. Hut the mosl definite reason for con- necting the release of energy with respiration Is that those tissues in whit b growth or other work is proi eeding rapidly arc- also i harai terized longer tenable, the words aerobic and anaerobic (aer, air; bios, life; ■'", not), applied first to organisms that live in air or nourish only when it is excluded, are preferable. 4o6 PHYSIOLOGY by rapid respiration. Thi.^ is in harmony with numberless observa- tions in animals, in which the work can be increased at will, when a corresponding increase in the products of respiration, the consumption of nutritive materials, and the evolution of heat is readily shown. It is perhaps better to consider all those phenomena of respiration as its re- sults, the decomposition of the protoplasm being the primary and essen- tial feature. Indeed the phenomena of respiration may all be directed to ridding the body of the products of an inevitable decomposition of the unstable proteins of the living protoplasm. Role of oxygen. — When energy is released from chemical compounds, the more completely they are decomposed the more energy is liberated, as a rule. In anaerobic respiration the decomposition does not go so far as in aerobic, for the resulting substances are not so simple, and probably therefore the energy released is far less. The fact that growth either does not occur at all, or is very limited, when oxygen is cut off from plants accustomed to it, also indicates this. Herein, indeed, appears the prob- able role of oxygen in respiration. It seems to be necessary not to com- bine with carbon compounds, but, by combining with and so removing substances whose presence interferes with the usual reactions, to enable the respiratory processes to go on to completion. The common idea is that oxygen combines directly with carbon and so causes " combustion." But chemical studies of the combustion of certain gases show that it does not do this, even at high temperatures. Water vapor, which yields II and OH ions by dissociation, furnishes the necessary OH ions for facilitating the decomposition of the carbon compounds, and this decomposition does not proceed at all in the absence of water, not even in pure oxygen. The oxygen does combine, however, with hydrogen to regenerate water, so that a small quantity of water serves, provided O2 is continually supplied. In this, O2 behaves somewhat as the depolar- izer does in a galvanic battery, wherein its function is that of an oxidizing agent to convert into water the hydrogen that otherwise would accumulate on the cathode and stop the chemical action. Undoubtedly other "depolarizers" than oxygen are present in the cells ; and in some organisms the long continuance of anaerobic respiration without serious harm may be thus explicable. The presence of oxidiz- ing enzymes, also, may be essential to the fixation of oxygen. End products. — When, therefore, O is supplied, the end products of decomposition are in large part the most stable ones, C02 and H20. When 02 is not available, these are less prominent, while ethyl and higher alcohols, organic acids, aromatic compounds, hydrogen, etc., are the more abundant end products. In the one case certain parts of the pro- toplasm break into simpler and simpler compounds; in the other the DESTRUCTIVE METABOLISM 407 decomposition stops while yet the materials arc complex, and hydrogen appears because do oxygen is available to combine with it. Why carbohydrates disappear.- — The end products, however, prob- ably do not represent in any case the whole of the protein molecule. Certain fragments of it, under suitable conditions, go down into COs and H20; but others are not so far split up that they cannot be rebuilt, with necessary additions, into protein again. It seems to be the com- ponents of the protein molecule derived from carbohydrates, which are particularly liable to complete decomposition. If this nucleus alone were broken up, the ratio of free Ol, fixed to C02 produced should have a value of unity. This is not by any means true: the average is below 1 and the value varies from 0.3 to 5.0; so it is probable that the process is complicated by the interaction of other substances. The repair of the proteins requires chiefly carbohydrates, because the nitrogenous losses in the plant are quite inconsiderable as compared with those of an animal. So a marked effect of respiration is a disappearance of the accumulated carbohydrates. The assumption that carbohydrates are directly decomposed in respiration rests largely on the fact that the value of the ratio ( )2:C< >a is affected by the food supplied to non-green plants. Thus, in Aspergillus it ranges from 0.4,^ with 10 per cent tannin, to 1.7S with 10 per cent glucose, indicating that not composition alone but other and unknown factors are concerned. And composition, as well as these un- known factors, may produce this result indirectly, through their influence on assimi- lation, quite as effectively as by directly modifying the " combustion " of foods. Loss of weight. — The transformation of carbohydrates in the repair of proteins can have little effect on the weight of the plant; but the escape of C02 as a gas and the evaporation of the water produced does result in a loss of weight. If the total dry weight of seeds be calculated (the percentage of water in like seeds having previously been determined), and these seeds be grown for some weeks in the dark, plants of consider- able size can be raised. But on drying them, the residue will be found to weigh less than the calculated dry weight of the original seeds. This difference corresponds to the combined COs and H20 produced and lo>t in the course of respiration. Production of heat. — The heat produced by respiration is often not observable at all, unless some means are used to prevent its radiation and its transfer to the air by the evaporating water, [f a ma— of wheat seeds be germinated, a thermometer thrust into the mass will show a temperature considerably higher than that of the air ; but this is duo 408 PHYSIOLOGY largely to microorganisms, whose active respiration, and especially the fermentation they cause, liberates much heat.1 If, however, the surface of the seeds is carefully sterilized before germinating, the difference is much less, in many cases with ordinary insulation only 1-1.50 C. By using Dewar flasks, which afford very perfect protection against loss of heat by radiation and conduction, differences of 200 C. or more hare lately been found with 80 gm. of peas (weighed dry). The opening of flowers crowded into a compact cluster within a bract, as in the calla, causes a decided rise of temperature, differences of 5-100 C. having been noted. This production of heat is continuous, though its rate varies. It is said that a kilogram of seedlings may produce heat enough per minute to warm 1 gm. of water from o to 50 or even ioo° C. Yet under ordinary circumstances this heat is steadily dissipated. Comparative activity. — It is commonly supposed that at best the aerobic respiration of plants is weak compared with that in animals. This is a mistake. The respiratory rate for active tissues of plants compares well, weight for weight, with that of even warm-blooded ani- mals, and in some cases far exceeds it, if the gaseous changes may be taken as a fair measure of the process. Thus, if a man of 75 kg. pro- duces at light work about 900 gm. C02 in 24 hours, the output of C02 equals 1.2 per cent of his weight. By the buds of lilac the output of C02 equals 1.8 per cent of their weight; by those of horse chestnut, 3 per cent; by seedlings of poppy, 2 per cent; by molds, 6 per cent. While a man may use in 24 hours 1 gm. of oxygen for each 100 gm. of his weight, young leaves of wheat use it at the same rate; opening flowers use 4 times as much, and some bacteria 200 times as much. The stage of developmtnt, the general activity, and the rate of growth influence decidedly the rate of respiration. The younger and more active the tissues or organs, the more rapid, as a rule, is the respiration. Life. — It has already been indicated that anaerobic respiration begins like aerobic, but that the decompositions cease before they attain the same extent. It may very well be, also, that they pursue a somewhat different course, on account of the lack of oxygen. Growth ordinarily ceases when growing tissues are forced to do without 02, though some- 1 When moist plants or manures are piled up, very high temperatures may he produced in the midst of the mass by the combined activities of many different fungi and bacteria. This "heating" may even suppress or kill off all species except those that flourish at 55-65* c. DESTRUCTIVE METABOLISM 409 times it continues for a time; whence it Is inferred that the energy re- leased by anaerobic respiration is usually inadequate lor growth. Life, however, persists for a variable time, sometimes for weeks or months, though in active parts the functions are much disturbed after a few hours, and death shortly ensues. 2. FERMENTATION Microorganisms. — The fact that anaerobic respiration gives rise, among other things, to alcohol and carbon dioxid, suggests at om 1 relation to a process long known to occur in sugary juices, like those of grapes and apples, when they are allowed to stand unsterilized and un- sealed. The sugar disappears, bubbles of gas (COo) rise through the liquid, and considerable alcohol is formed in it. This process is known as fermentation. It was shown long ago to be due to the presence of yeast plants, for it does not occur when they are excluded. Further study has shown that analogous changes which take place in organic substances, many of them (like the souring of milk and the spoiling of meat) being familiarly known, are due to the action of other micro- organisms. The application of the term fermentation has now been extended to cover all these changes. Names. — Fermentations are named after the most prominent or de- sirable substance produced, or sometimes after the substance destroyed. Thus, the fermentation of glucose (grape sugar) is alcoholic fermenta- tion; that of lactose (milk sugar) is lactic fermentation; that of alcohol is acetic fermentation; because alcohol, lactic acid, and acetic acid, respectively, are formed. On the contrary, the cellulose fermentation is so named because cellulose is destroyed. When proteins are attacked, evil-smelling gases are among the products, and such fermentations are frequently distinguished as putrefactions; but they are not essentially different from others. Only a few of the better known and more impor- tant fermentations can be treated here. Alcoholic fermentation. — The alcoholic fermentation is produced in different sugars by various organisms. The sugars that arc now known to be fermentable are only those the number of whose carbon atoms is 3 or a multiple of 3; thus, the trioses < 11 I ' . hexoses (C6Hi206), and nonnoses (C,H1809) are directly atta< ked; while the more complex carbohydrates (di- and polysaccharides), su< h as cane and malt sugar (CuHaOu) and starch [5n(C8HioOj)], are fermented .inly after 4io PHYSIOLOGY they have been simplified by cleavage into hexoses. Why this limitation exists, and why within this there are others even more specific, is not known. The organisms concerned are chiefly those known as yeasts (see Saccharomycetes, p. 70), but certain molds and bacteria also give rise to ethyl alcohol, though the latt r more commonly produce higher alcohols (propyl alcohol, butyl alcohol, etc.). In this connection it is to be remembered that even the higher plants produce ethyl alcohol in the course of anaerobic respiration. The sugar is split up in large measure into C02 and ethyl alcohol, but there are other products, such as glycerin, succinic acid, etc., in smaller quantity. Fermentation proceeds very slowly when the yeasts are abundantly supplied with 02; then, however, they grow and mul- tiply rapidly, and apparently use the sugar chiefly as food. But when the supply of 02 is small, so that their vegetative processes are hindered, fermentative action is increased. Though alcohol is produced at all times, its quantity is in a sort of inverse ratio to the favorablencss of the conditions for life. When 12 per cent have accumulated in the liquid, the action is retarded, and by 14 per cent it is stopped. Fermentation by yeasts was long believed to be due to the direct action of their protoplasm on the sugar; now it has been proved that an extract, made by grinding the yeast with sand and filtering the juice under high pressure through porcelain, can produce the same effect. The active substance, known as zymase, is soon destroyed, unless protected from digestion by accompanying enzymes. Similar substances have been iso- lated in higher plants, which are believed to act upon carbohydrates in anaerobic respiration,1 giving rise to alcohol and C02 in the same propor- tions as in fermentation. The economic uses of alcoholic fermentation are many. It plays a prominent rule in the lightening of bread, in which, however, other organisms share with yeast the production of the gases that raise the dough; it is the source of commercial ethyl alcohol, which is distilled from fermented liquids, in which hexose sugars are first produced from corn and potato starch; it gives rise to the alcohol in a host of fermented liquids used as beverages : wine, beer, koumiss, pulque, sake, etc. Lactic fermentation. — The lactic fermentation, giving rise to lactic acid, is best known in the souring of milk, and may be produced whenever lactose is present in a solution to which the lactic acid bacterium has 1 The source of these carbohydrates is uncertain. They may be either the unassimi- lated carbohydrates of the food; or, equally well, a carbohydrate nucleus from the decomposition of the protoplasm. DESTRUCTIVE METABOLISM (i i access. As in the alcoholic fermentation, the accumulation of tne products brings the action to a standstill. When 8 per cenl of la. i i . acid has accumulated (or less in milk), the bacterium becomes inactive. Acetic fermentation.— The aceti< fermentation is due to bacteria, which oxidize ethyl and other alcohols to acids. The commonest form converts ethyl alcohol into acetic acid, ( 1 1 . • CI I .< )I I + ( ).^± (I [8 • COOH + Il.O. In the quick process for the manufacture of vinegar, in which this fermentation is applied, dilute alcohol (6-xo per cent) is allowed to trickle over beech shavings in a deep vat, which have become covered with a slimy coating of the organisms. By the time the alcohol has reached the bottom it has been oxidized completely to at eti< acid. Butyric fermentation. — Butyric fermentation, by which butyri< acid is produced from various sugars, especially lactose, and indirectly from polysaccharides, through the agency of bacteria, underlies the production of desirable flavors in butter and cheese. Putrefactions. — The putrefaction of proteins is wrought by various bacteria, but little is known of the details. Among the numerous end products are the disagreeable gases hydrogen sulfid, mercaptans, skatol, etc. So a multitude of fermentations might be named, each concerned with a particular compound and due to a particular organism. By the single or successive action of such organisms, complex organic matter is gradually reduced to simple forms, like those from which it was con- structed, which then may enter again into the cycle and be built up, through the agency of green plants, into foods. Advantage. — The precise role of fermentations in the life history of the organism that produces them is not certainly known. It is possible that they are, as respiration is supposed to be, a source of energy. The minuteness of the organisms would make possible the appropriation of this energy, even though, in contrast to that set free by respiration, it i- released outside the body. From this point of view it would seem that fermentation might be considered as a substitute for respiration, though a rather ineffective one. and hence requiring an exaggerated decom- position of organic matter. On the other hand, it has been suggested that fermentation serves for the production of substances in which the producers can live, but by which other organisms are injured and so prevented from competing with them for food and room. This sugges- tion, however, seems forced and inadequate. Yet again, it may be 4I2 PHYSIOLOGY that all fermentations are effected by enzymes, as some are known to be, and that the formation of these enzymes is not so much a matter of advantage to the organism as an inevitable result of the conditions under which it develops. If tins be true, to seek for explanation through ad- vantage is a fruitless quest. 3. WASTE PRODUCTS AND ASH Wastes not useless. — In the course of the many and varied chemical changes which take place in plants, there arise, especially in consequence of the destructive metabolism, a great number of compounds which are not usable for the building of new parts, and are not again drawn into the metabolism. Some of these are nevertheless of considerable service to the plant, and in varied ways; as, for example, in protecting it from predatory animals by disagreeable tastes or odors, in covering wounds by gummy or resinous exudations, in attracting by color or odor insects which effect pollination, etc. In spite of the usefulness of some of them, these substances are often called waste products, and this word may well be retained instead of the more technical term, aplastic products, which has been applied to them. For in every household there are like products, properly " waste," so far as the direct economy is concerned, some of which may nevertheless be collaterally serviceable. Number. — Of the reactions by which these waste products are pro- duced, not much is known, and they need not be considered at all here. The number of the products is very great, and it is possible to name only a few of the more important groups and examples of them. An im- pression of their number may be gained from the fact that in a recent work on plant chemistry more than 4000 are mentioned, and the book does not pretend to enumerate all known substances. Thus there are over 200 known alkaloids, and a single firm lists some 200 essential oils of commercial value. Yet the knowledge of the chemistry of plants is very incomplete and lags far behind that of animals. No true excretion. — Almost all of the wastes accumulate in the tis- sues, for actual excretion by plants is very imperfect. Except for those which are got rid of in the fragments of bark, roots, twigs, and leaves that are shed, and the relatively minute quantities that are secreted by surface glands, or diffuse out into the water from roots and other im- mersed parts, there is no provision for doing more than storing these substances in some out-of-the-way place. In no case is there any ar- DESTRUCTIVE METABOLISM 413 rangement for continuous riddance, such as is found in the excretory organs of animals. It is also particularly noteworthy that among the wastes there are few or none except the alkaloids that contain nitrogen. Even these are not necessary products of metabolism, for the very plants that produce alkaloids most abundantly may be so grown, and healthily, as not to contain any. Gaseous wastes. — Among gaseous wastes, the most important, C< >_. and 02, have already been mentioned; and the water resulting from respiration, while not produced as a gas, leaves the body mostly in this form. In a few plants, notably in the stinking goosefoot and flowers of hawthorns, a very disagreeable odor makes known the escape of a gas, trimethylamin; but this is formed only in trilling amounts. Essential oils. — Most of the odors of plants, fragrant or not, are due to the essential (volatile) oils, which are distinguishable from true oils, to which they are not at all allied chemically, by leaving only a transient spot on paper. They are especially abundant in the foliage and flowers, though there is no part but may be the seat of their production or storage. They are the more volatile constituents of complex mixtures, secreted by glands of various forms (see p. 337), whose solid residues, after the " oils " have been driven off, are resins (see below). These secretions may escape at once upon the surface, or they may be stored in inter- cellular receptacles and released only by crushing. In the flower leaves they are curiously distributed, being formed in the epidermis of both petals and sepals, or only in one, or only in the cells of one face, or only in lines or patches of cells. From such parts, even when in very -mall amounts, they may be distilled, and when more abundant they may be expressed and purified. Some are medicinal, and some an' commer- cially valuable as perfumes for soaps, ointments, and other toilet arti< les. Chemically they are quite diverse; many of their constituents belong to the class of compounds known as terpenes. Gums and resins.- — Gums and resins occur in great variety, and often in mixtures called gum-resins and balsams. These term- are rather loosely used, and do not designate definite chemical groups. The true gums are in large part l arbohyt Irate-, arabinose being especially abundant ((',-,! I,,,* )-), and arise from the transformation of the cell wall and growing tissues in woody plants. They swell readily in water. Gum arabic and gum tragacanth are well known commercially, and the gum of cherry and peach trees is familiar. Resins are yellowish solids, usually derivatives of essential oils, that occur dissolved in essential oils. 414 PHYSIOLOGY Thus, turpentine consists of colophony or resin dissolved in "oil of turpen- tine," itself a mixture of several terpenes. " Canada balsam," as used for mounting sections, consists of a resin solidified by driving off the volatile oil and redissolved in a more volatile solvent. The gum-resins or bal- sams are variable mixtures of gums and resins, with many other acci- dental constituents. The best known are asafetida, as distinguished for its disagreeable odor as are galbanum, myrrh, and frankincense, the chief components of incense from time immemorial, for their fragrant smoke. They exude from wounds in various oriental shrubs and solid- ify in drops and irregular masses. Organic acids. — The organic acids are also numerous, but four pre- dominate. These four, oxalic, malic, tartaric, and citric acids, are all very widely distributed and are not infrequently associated. Oxalic acid (COOH • COOH) is not certainly known to occur in the free state, but is abundant in salts of calcium, potassium-hydrogen, and magnesium. Calcium oxalate is found in every large group of plants except bryo- phytes. It crystallizes in long slender needles (raphides) or as " crystal sand," with two molecules of water ; or it forms large single crystals or crystal aggregates, of octahedral form, when it combines with six mole- cules of water. (See Part III, fig. 919.) Magnesium oxalate forms spherites. Malic acid (COOH • CH2 • CHOH • COOH), which is almost as widely distributed as oxalic, occurs in the juice of many unripe fruits, especially the apple, pear, cherry, etc., either free or in salts of calcium and potassium. Tartaric acid (COOH • CHOH • CHOH • COOH) is closely allied to malic acid. It is found abundantly in the juice of f CH2 • COOH ] grapes as potassium-hydrogen tartrate. Citric acid OH • C • COOH 1 CH2 • COOH I occurs in the juice of many plants, being especially abundant in the fruits of the citrus family (lemon, lime, orange, etc.). Tannins. — The tannins are numerous and widely distributed, occur- ring especially in bark, wood, leaves, fruits, and galls. They are bitter and astringent substances, which form insoluble compounds with pro- teins and gelatin, and so are used for converting hides into leather. Tea leaves contain 14-16 per cent or more (dry weight), various barks up to 40 per cent, and galls up to 60 per cent. Some substances included in the loose term tannins are glucosides, and such as can be made to yield glucose by digestion may be considered as plastic substances rather than wastes. DESTRUCTIVE METABOLISM 415 Alkaloids. — The alkaloids arc numerous, and very Important medi- cinally, as they are dangerous poisons <>r useful local stimulants, ac< ording to circumstances. A few, such as caffein from tea and coBee,lheobromin from the seeds of cacao ("cocoa "),and the deadly muscarin from the poisonous mushroom (Amanita muscaria), are not related to the alka- loids proper, which are for the most part derivatives of pyridin and 1 bin- olin. The true alkaloids are found in fungi and various seed plants, but are* most common in certain families of dicotyls. For example, in the Papaveraceae, the oriental poppy alone yields more than twenty alka- loids, of which morphin, narcotin, and codein are best known ; in the Solanaceae, tobacco contains nicotin and others, and most of the other genera yield atropin and a number allied to it; a great number of the Apocynaceae have alkaloids in their latex, at least twenty different ones being known; in the Rubiaceae, the cinchonas and their allies produce more than thirty alkaloids, of which quinin and rim honin are widely known; in the Loganiaceae, seeds of Strychnos nux-vomica yield strych- nin and brurin, while another species yields several " curare" alkaloid-,; and in the Erythroxylaceae, coca yields among others rorain, at once highly useful as a local anesthetic and utterly destructive to body and mind when used habitually. Coloring matters of flowers, fruits, barks, seeds, etc., are too numerous and varied to be discussed here. Ash. — Mineral salts are present, sometimes amorphous, incrusting or incorporated in the cell walls, as is the case with silica; sometimes crystallized, as is the case with calcium oxalate. The ash of plants consists of the total mineral matter left as oxids when completely burned. Analysis shows that the amount and content of the ash varies much in the same plant in different situations, thus indicating that in part (and doubtless in large part) these materials are determined not by the " needs" of the plant but by the solutions which have opportunity to wander into it. Cultures under special conditions have shown that plants may be deprived of many of the chemical elements ordinarily found, and no evil effects follow; but the absence of others has obvious ill effects. Thus silica is an abundant material in the cell walls of the epidermis of most cereals; yet corn has been cultivated through four generations with practically no silica. Necessary elements. A list of the elements that have been found in the ash of one plant or another would be almost a li>t of the commoner 4i6 PHYSIOLOGY elements themselves, over thirty out of the present total of seventy- eight having been recorded. Yet of this large number only a few seem to be indispensable. These are usually reckoned as calcium, potassium, magnesium, and iron; while chlorin and sodium are present in all and may be necessary. Many attempts have been made to determine the precise role of each of these indispensable elements, with rather conflict- ing results. It does not seem possible by cultures which omit a par- ticular element to reach reliable conclusions; nor is it at all likely that the role of any particular element is simple, and the withdrawal of one may permit others to act in a wholly different way. Thus if plants be grown in solutions of calcium chlorid or of magnesium chlorid of a cer- tain concentration, they will die; but if the two be mixed in the same concentration, the plants will grow well. Singly both are injurious, together they are not, though no reaction occurs between them. When therefore it is said that a definite amount of each " indispen- sable " element is needed by a plant, and that the minimum determines the crop (" law of the minimum "); that on potassium depends the for- mation of new organs at the growing point; that calcium is required for the transfer of starch, and so on, all such statements must be con- sidered as extremely doubtful and liable to complete reversal when a deeper insight is gained into the processes concerned. CHAPTER V. — GROWTH AND MOVEMENT i. GROWTH Ideas involved. — Nothing about plants as a whole is more readily seen than that they grow, and in due course unfold new organs. How- ever small and simple, however large and complex, growth is almost always obvious, and sometimes it becomes striking because of its ra- pidity or its long duration. Two ideas arc involved in the term growth as ordinarily used, () the formation of new organs. The latter is sometimes distinguished under the term develop- ment, and if one speaks of growth and development, the term growth must be limited to the enlargement of already formed cells. But the terms are nearly synonymous; though growth may be restricted for a time to cells already formed, it normally leads to the formation of new organs; and though development is possible without enlargement, it is usually accompanied by an increase in size. The production of new organic material is not essential; when the corn seedling, raised in the dark, grows into a plant many times larger, the stored organic material has been merely rearranged, with the addition of water, and when the surplus food has been fully used for growth, there is actually a smaller total of dry matter than when growth began. Additional organic matter can be produced only when the conditions for photosynthesis are fulfilled. Few plants have so definite a cycle of development as most animals. In some cases leaves produced in the juvenile period differ from those i f later stages.1 Again, leaves developed at certain periods are so different in form and texture as to be really different organs, as in the case of bud si ales, floral parts, etc. But these periods of flowering or seed formation or other reproductive process are determined largely by external con- ditions, and little or not at all by the fact that the plant has reai hed a certain stage of maturity, though of course the formation of the special organs, as of all others, is conditioned by the supply of constructive 'These juvenile forms, however, may appear later under suitable conditions. Sec Part III. p. 596. 417 4i8 PHYSIOLOGY material. Plants, therefore, do not in general have a definite stage of maturity, and a corresponding form. They do have, however, periods characterized by growth, including the formation of new organs and their development. These periods occur once, being limited to a single season or less, as in the case of annuals; or twice, as in biennials; or they are repeated, season after season, as in perennials. This periodicity is less marked in equable tropical climates, but is rarely, if ever, entirely absent. Phases. — If the history of any limited portion of a plant be followed (and the more limited the better, even to a single cell), it can be observed to pass through a development in which may be recognized three phases. The first phase may be called the formative phase; the second, the phase of enlargement; and the third the phase of maturation. These phases are characterized clearly enough by certain peculiarities of structure and behavior, but they are not sharply delimited. On the contrary, the first passes by imperceptible gradations into the second, and the second into the third; then growth finally ceases, unless some unusual stimulus brings the cells again into an active state. Formative phase. — The formative phase is the earliest. Every plant begins its existence as a single cell, and even when this one has increased to many, they usually remain practically alike. The embryo in seed plants, at the time when it resumes its interrupted growth, usually consists of cells all in the formative stage. They are characterized by a relatively large nucleus, abundant cytoplasm with only minute vacuoles, and thin walls. In this phase the frequent division of the cells is a feature, and in consequence of the more rapid production of new cells by division at certain points, the primordia of new organs appear (fig. 666). Some of the simpler plants never get beyond this phase, except as to their reproductive organs. Even in the larger plants, some of the cells permanently retain these characters, and so constitute formative centers or growing points; but far the greater number pass gradually into the second phase and the third, assuming quite a different aspect and behavior. In particular, the power of division is given up. Fig. 666. — Growin •After De Bary. point of Hippuris. GROWTH AND MOVEMEN1 419 Primary meristem. — The formative regions in thallophytes an- often rather indefinite, with a tendency in the higher forms to Derestricted to the apex of the body. In the bryophytes they arc found only at the apex, while in the vascular plants they persist commonly at both apex and base, i.e. at the tip of each axis and of each root. Here the active division of the formative cells and the differentiation of their progeny adds to the length of the body at one or both ends. There may be a single cell acting as the source of all, as in ferns, or a group of initial-, as in seed plants (fig. 666). The repeated division of these initial-, and their progeny being the important feature, the formative tissue is des- ignated as meristem, and because this meristem persists from the earliest stage in the life history, it is the primary meristem. Secondary meristem. — Regularly in certain regions and accidentally in others, tissues that have passed beyond the formative phase regain the power of division and exercise it for a longer or shorter time. Thus, in all plants whose xylem and phloem bundles show secondary thickening, a layer of cells between the two becomes a secondary meristem (cambium), and these initials may produce new cells on either face or both, which are gradually transformed into elements like their neighbors, while the in- itials continue to divide through the season, or function year after year. Again, a certain zone of the cortex or even the epidermis itself may resume active division, becoming a secondary meristem called the phellogen, whose offspring, the suberized periderm, constitutes a layer of cork protecting the surface (see fig. 539). Wounds, the presence of a parasite, or other stimuli may call again into active division almost any live cells, and the resulting tissues will cover the wound with a callus, or produce the deformity characteristic of the particular injury or parasite. Origin of branches. — In the primary meristem of the stem the primor- dia. of new organs are produced at the surface, the first indication of a new lateral branch, whether a shoot or a leaf, being a slight elevation of the surface, due to more rapid growth of cells at that point. This mode of origin is known as exogenous (fig. 666) and is characteristic of branches of the shoot axis. In the root, on the contrary, the lir>t ap- pearance of a lateral branch is not at the surface, nor in the primary meristem, but at the limit of the stele or central cylinder (within the cor- tex), and among cells which have given over for a time ai live division and growth (fig. 667). The new branch must break through the cortex, since it is endogenous in origin; and this is characteristic of the root axis. Adventitious growing points, giving rise to new shoot-, may appear in 420 PHYSIOLOGY this endogenous fashion upon roots, and likewise on old shoots or leaves. They commonly owe their origin to some external stimulus (see p. 428). Many of the growing points that are formed regularly (exogenously) on the shoot do not develop, for one reason or another. They may then be overgrown completely in woody plants, and so lie dormant for years, to be called into activity when some accident has checked the growth of others, formerly more favor- ably situated. Not every shoot, then, that appears to come from the interior is really endogenous in origin. Phase of enlargement. — As cells newly formed in the meristem grow older, they enter gradually upon the second phase of development. This is characterized by enlargement, oftentimes so great and so rapid as to be very remarkable. In this period the volume of the cell not infrequently in- creases a thousandfold or more, though ordinarily much less. Of course this involves rapid growth of the cell wall in area, and if the cytoplasm were relatively as abundant as in the earliest stage, it would require the formation of a large mass of costly material. But while the cytoplasm does actually increase considerably, much the greater part of the cell is occupied by the water which en- ters it. Hence an indispensable condition for growth is an adequate supply of water; and the dwarfing which results from a deficiency of water is partly a direct consequence of the non-distension of the cells in this stage. The water enters the protoplasm, doubtless as a result of the formation of substances having a high osmotic pressure. It enlarges the minute vacu- oles everywhere through the cytoplasm, until some become so distended as to merge, forming fewer but larger ones. This process continues until in the center a few large vacuoles, or often only one, occupy the greater part of the space, while the major portion of the cytoplasm lies next the cell wall as a relatively thin layer, containing the nucleus, plastids, and other inclusions (see diagram, fig. 619). It will be apparent that since this many-fold enlargement is attained so largely at the expense of water, plant growth is relatively economical. Unequal enlargement. — The young cell has its three dimensions nearly equal. Enlargement takes place in all dimensions, but to different Fig. 667. — Endoge- nous origin of a lateral root (r) of ice plant [Me- sembryanthemun crystal- linum) : c, primary cor- tex, and e, endodermis, ruptured by young root; p, pericycle, from which it arises; X, primary xy- lem element. — After Van Tieghem and Douliot. GROWTH AND MOVEMENT 421 degrees, according to circumstances. Thus, cells which arc part of an elongated organ like a stem, arc likely to grow mm h more in the longi- tudinal diameter than the transverse. The real reason for these ine- qualities of growth is obscure. To say that they are due to " inherent causes" or are determined by "heredity" in no wise enlightens the inquirer. In a few cases they are referable to definite agencies. Thus, the < ells near the upper surface of a leaf are influent ed, mainly by light, to grow longer in the axis at right angles to the surface than in the other two.1 The sum total of growth in the individual cells determines in large measure the final form of the organ in which they lie. In most cases the causes which determine the general course of growth can be analyzed at present as little as those which determine the form of the single cell; but the effect of external agents is often detected, and in many < ases it is dominant (see section 3, p. 435). Grand period. — Enlargement proceeds at an unequal pace, even though the external conditions which affect the rate are kept uniform. In the earlier portion of the period it is slow, then it becomes more and more rapid until it attains a maximum, when it quickly falls off and gradually comes to an end. If the progress is graphically represented by plotting the increment from day to day, a curve is obtained of which fig. 668 is an example. This is the history, indeed, of the growth in length of a short portion of a stem, which is made up of a multitude of cells in the phase of enlargement. In a similar way the growth in volume of a fruit, such as an apple or a pumpkin, might be described. The total period of enlargement is named the grand period of growth, to dis- tinguish it from periodic variations in the rate within the grand period, some of which are due to periodically acting external agents, such as light and heat (daily period, see p. 436), and others to causes unknown and hence called " spontaneous " variations. The same features of the course of growth may lie seen when the in. remenl <>f successive small portions of an axis is rec orded. Thus if a root is marked into milli- meter spa is, or a stem into longer spaces ami the in< remenl of ea h is ro orded for a number <>f hours, it will appear that certain spaces are growing more rapidly than others, respe< lively more or less distant from tin- tip, ;'./•. older or younger. The increment in twenty-four hours of each of ten 1 mm. spa esof a root is here shown: I II III IV V VI VII VIII IX X ■s 5-8 82 3-5 1.6 1.3 0.5 0..? 0.2 O.I 1 Transpiration may Ik- another f.utor; the precise relation of tin- two is uncertain. See fart III, p. 536. 422 PHYSIOLOGY Similarly the increase in forty hours of twelve 3.5 mm. spaces of a stem of Phaseolus: 1 II III IV V VI VII VIII IX X XI XII 2 2.5 4.5 6.5 5.5 3.0 1.8 1.0 1.0 0.5 0.5 0.5 Inspection of these records shows that the two younger millimeters of the root and the seven older are growing less rapidly than the third ; in the stem the four- teenth to the seventeenth millimeters (space IV) are growing most rapidly, and beyond this the older a division is the more slowly it grows. Growing regions. — Comparison of the total length of root and stem still growing appreciably shows a striking difference. About 1 cm. 75° 70° , S \ y'' \ '"" \ ^ --J "-> — 70 60 1 / 50 / _J / 40 I \ / \ 30 f > \ 20 t MM DAYS-* 1 10 »2 13 14 15 Fig. 663. — Grand curve of growth (solid line): the first day of the observation was evidently after fairly rapid growth had begun; it attained a maximum on the fifth day, with an increment of 72 mm.; thence the rate falls off rapidly, and on the sixteenth day is only 18 mm.; growth rate magnified 10 times. The temperature curve (broken line) for the same days runs between 71 and 770 F. — From data by Spoehr. of the root and more than 4 cm. of the stem is shown to be growing by the record above. In general the total elongating portion of a root scarcely exceeds this; but in many stems 10-20 cm. are found elon- gating, and in twining plants 40-60 or even 80 cm. may be growing. GROWTH AND MOVEMENT 423 The growth of aerial stems is not hindered by the medium. When they grow underground, the apex is protet ted by a duster of overarching s< ale-. Growth of such stems is seldom rapid, but when it is, as in the extensive running rootstocks of couch grass, the terminal hud is sharp-pointed and smooth, so that it offers the least resistance to being driven through the soil; at the same time the firm scales protect the primary meristrm behind. In the root it is obviously advantageous to have the growth zone restricted, and to have the zone of most rapid growth as near the apex as possible; for, so much as any part behind it elongates, so far is the tip actually driven through the soil. The sloughing and slimy surface of the root cap lubricates the advancing apex, thus facilitating its pas- sage. For good growth of roots (which makes for good growth above also), it is desirable that the soil have an optimum content of water, since it has been shown that its resistance to penetration is then at a minimum. Drought, indeed, hinders root growth doubly; it not only retards enlargement directly by lack of water, but also, by compacting most soils, mechanically opposes the extension of the root system, and so intensifies the difficulty of procuring the necessary water. Nutations. — The rate of elongation is not only different in different sections along the axis; it is also unequal in different segments around the axis. This is especially marked in bilateral organs, such as leaves, and varies from one face to another at different periods of development. Thus, most leaves when young grow more rapidly on the back (later the under surface), so that they are appressed to the stem; or they arch over its apex when they outgrow it, as they commonly do, forming a " bud " there. Later, growth becomes more rapid on the inner face (at matur- ity the upper surface) and the bud opens. Local differences in rate lead to the folding and rolling so characteristic of young leaves in the bud. In radially symmetrical organs, such as stems, inequality of growth on different radii leads to bending, so that the tip is not erect but more or less declined. As the most rapid growth shifts to different segments around the axis, the tip nods successively to all points of the compass, and so describes a very irregular ellipse ,,r ( in le, or, considering also its upward growth, a very irregular ascending spiral. Plotting successive observations on a plane shows tracings like fig. 669. The nodding of leaves or stems or roots on account of unequal growth is 1 ailed nutation. The inequalities in the rate of growth may be due to unknown causes, assumed to be internal, when the corresponding nutation is (ailed spon- taneous or autonomic; or they may be due to external causes (stimuli), 424 PHYSIOLOGY when the nutations are said to be induced. The latter will be particu- larly discussed later (see section 4, p. 442, and section 7, p. 458). Rapidity. — The absolute rate of growth in the period of enlargement is, of course, extremely different in different plants and under different conditions. A few cases may give an idea of the upper limits. The filaments of wheat stamens at the time of blooming grow for a brief time at the rate of 1.8 mm. per minute, which is about the rate at which the minute hand of a man's watch travels. If such a rate continued for 24 hours, they would become 2.5 m. long. The leaf sheath of the banana grows at the rate of 1.1 mm. and that of bamboo 0.6 mm. per min- ute. When the century-plant blooms (as it does in 10-25 years), a shaft about 15 mm. in diameter rises to a height of 6-8 m. at the rate of about 15 cm. per day. Phase of maturation. — The phase of maturation is the final phase of growth. This phase is entered upon only when enlargement has prac- tically ceased; therefore its progress is not measurable, though it is quite as important as the preceding. During this phase the cells attain their mature form and character. In all cases the thickening of the cell wall is obvious, though often slight; but sometimes it proceeds to such an extreme as to be the most notable change. The thickening is never uni- form, and sometimes thin and thicker spots in patterns produce an effect of sculpturing that is characteristic, as in the tracheae and tracheids (figs. 640, 641). Conversely the resorption of certain parts of the wall may occur, as the end partitions of sieve tubes and of the components of Fig. 669. — Nutations of a young sunflower plant: 1 position at 9 A.M., 2 9:15, 3 9:30, 4 9:45. 5 10:00, 6 10:15, 7 10:30, 8 11 :oo, 9 11:30,70 12 M., 11 1:00 p.m., 12 2:00; from point 12 the plant made a deep nod to the west till 4 P.M., then again eastward till 5:00, again westward till 6:00, and finally to original meridian at 9:00 p.m. — From data by Land. GROWTH AND MOVEMENT 425 tracheae and the thin portions of the wall in the scalariform tracheids of ferns. In case great thickening occurs, the death of the protoplast is likely to follow, and tin's is regularly the case in tracheary tissue. When that occurs, further modification of the wall is possible only by the agency of adjacent live cells, by chemical reaction in the wall sub- stances, or by mere impregnation with solutes which may be precipitated or absorbed. So proceed such changes as the coloring and other altera- tions which mark the heart wood of trees. Tension of tissues. — When growth is finally at an end in any region, it is found that the various tissues have not grown equally. Hence there exist strains or tensions; one region is compressed, another is stretched. These inequalities tend to adjust themselves if tin- regions are parted anitu tally, as when the pith, the bark, and the wood are separated from one another. Similarly, tensions due to unequal turgor exist (see p. 310). All these strains acting in different directions within the structure tend to increase its rigidity, just as do like strains in a latticed girder or a bridge truss. Conditions. — The conditions for growth are first of all an adequate supply of water, for unless turgor of a meristem region is maintained, division of the cells is impossible, and unless an adequate amount of water be present, enlargement of formed cells is limited. Secondly, there must be a sufficient supply of constructive materials ; for though water plays an extraordinary part in enlargement, there is needed much food for making new cytoplasm as new cells arise by division and en- large. Nuclear material, cell-wall stuff, and much besides must be steadily constructed by the protoplasts, and the growing region is there- fore the seat of intense chemical activity. Thirdly, oxygen is necessary, probably to permit the metabolism in general, and especially the res- piratory changes, to proceed properly. For though growth has been observed in the absence of oxygen, it is quite limited, and, having been detected only by measurement, was probably due solely t>> the disten- tion by water. Cell division also is checked by lack of Oj. Lastly, growth, like all other phenomena, goes on only within certain limits of temperature, other conditions being suitable. The optimum (dif- ferent for different plants and for the same plant under different con- ditions) usually lies between 250 and 320 C, and the extremes are near o° and 420 C. Any one of the conditions named may likewise vary within rather wide limits, and any one being unfavorable may retard or stop growth. Yet when all the condition^ are favorable, periodic 426 PHYSIOLOGY variations still mark the rate of growth, indicating clearly that there are unknown factors that operate with or against the known factors to affect it. The existence of such unknown influences is further shown by the fact that growth ceases, sooner or later, in individual cells, and often in the whole plant, in spite of all efforts to supply appropriate conditions. External agents. — A study of growth shows that external agents produce obvious effects. They do, indeed, affect every function, and much investigation is still necessary before the full extent of their influ- ence is known. But growth is at once so fundamental and so easy to observe, that it affords the best means for showing how extraordinary a part external agents play in determining the form and behavior of plants. To this phase of plant life attention must now be directed. 2. IRRITABILITY External agents. — It is a matter of common observation that the size and form of plants is affected by the conditions under which the) are grown. The luxuriance of weeds in a neglected garden, in contrast with their stunted forms on a dry roadside ; the rich green corn of a high prairie, in contrast with the yellowish and starved plants on a wet clay field ; the thrifty trees of a park, in contrast with the struggling and dying ones along a paved street, can hardly fail of notice by the most unobservant. These differences show clearly that the complex of con- ditions external to the plant profoundly affects its internal processes. As all functions center in the living stuff, protoplasm, the conclusion is that protoplasm is sensitive to the various agents that act upon it (or irritable); that is, that it reacts or responds to these by altering its be- havior in some way. In that event the agent producing the reaction is a stimulus. These three topics, stimulus, response, and sensitiveness or excitability, require consideration. Variety of stimuli. — The forces that act upon any plant are many, and varied in direction and intensity; and their combinations are almost infinite. Consider a tree, growing in a Chicago park. Every day the light which falls on it varies both in direction and in intensity from hour to hour, and is almost lacking at night; furthermore it varies from day to day and season to season. The temperature is hardly the same from one hour to another, and in this climate occasionally changes io° C. within twice as many minutes, while the seasonal changes range GROWTH AND MOVEMENT 427 over some 700 C. The humidity of the air shows like hourly, daily, and seasonal fluctuations, and the tree may he thrashed by a pan hing wind or wrapped in a dripping fog. A gentle shower, torrential rain, or hail may fall upon it within the hour; and with a change of season it may be weighed down by sleet and snow. The underground parts suffer less extreme variations of temperature than the top. The water ion tent of the soil swings from the drought of summer to the saturation of late winter and spring, and the solutes vary more or less in concentration with the rains and evaporation. Combine all these in as many ways as possible, and some idea is obtained of the variations in external con- ditions which may affect the plant. Adjustment.' — To many of these a plant must be able to adjust itself on pain of death, and suitable response to others is advantageous. The plant is indeed a self-adjusting ' mechanism, whose reactions are often- times more delicate than those of our own bodies, with all their special senses and complicated sense organs. Thus, many a tendril is sensitive to a mechanical stimulus which we cannot perceive, even by the tip of the tongue, the portion of the body most sensitive to contact ; and some plants distinguish differences of illumination which are inappreciable to the eye. On the whole, it is perhaps fair to say that plants are more responsive than animals. The plant has mostly to take what comes and make the best of it; the animal often takes shelter from unfavorable conditions or migrates to a gentler climate. Intricate relations. — It is extremely difficult to disentangle the com- plex of forces acting on a plant and to assign to each its special influence. Out of them all only a few have yet been isolated. What are known as general or formative stimuli, namely, the totality of physical conditions, external and internal, which determine the general course of develop- ment and consequently the form of the plant as a whole or of any par- ticular organ, furnish especially intricate problems, because it is so dif- ficult to alter only one condition experimentally, or to evaluate the influence of those which cannot be controlled. Experience is showing, too, that so-called special stimuli, i.e. those which act locally, such as gravity, light, heat, etc., are interrelated, and their effects are unexpei 1 edly interwoven. No phase of plant life requires more cartful experi- mentation and more caution in inference than the study of stimuli and the responses to them. 1 This term must be understood as if it were applied to a steam engine or a dynamo, both of which adjust themselves automatically to their " load." 428 PHYSIOLOGY Definition. — A stimulus is any change in the intensity or direction of application of energy which produces an appreciable effect upon living protoplasts. Of course when no appreciable effect is produced, the energy may differ neither in amount nor form from that which does arouse a reaction; and effects may be produced which are not perceived because improper tests are applied. A stimulus, thus, has no absolute value; it implies not a definite amount of energy measured in physical units, but merely enough applied suddenly enough to call forth a reaction as revealed by some arbitrary test. Therefore, what is a stimulus under certain conditions, is not a stimulus under others.1 Nor need the stimu- lus arise or act outside the plant as a whole. It may originate in one part and act upon an adjacent part, even in one protoplast and act upon another. These stimuli, in one sense external and in another internal, are most difficult to study. They are in part, and perhaps wholly, the occasion for the reactions that are called autonomic, or less properly " spontaneous." Kinds. — Stimuli may be classified for convenience as mechanical, chemical, and ethereal. Under mechanical stimuli are grouped those which depend upon mass movements, resulting in contact, impact, friction, pressure, etc., upon the plant. For lack of definite knowledge of the nature of gravitation, the stimulus of gravity may be conveniently included here, since it depends upon mass attraction and induces mass movements. Under chemical stimuli are included those whose action depends on their chemical quality — their composition and molecular structure — rather than on their mass. Ethereal stimuli comprise those propagated as vibrations in the ether and distinguished according to the length of the waves as light, heat, and electricity. Modes of reaction. — The action of a stimulus results in stimulation or excitation, and this may or may not lead to an observable reaction, depending upon the state of the protoplasm and the means used to detect a change in its behavior. Thus, immediately upon excitation a change in the electrical condition of the protoplast occurs, but this does not mani- fest itself to our senses, unless the stimulated region and an unstimulated one are put into electrical connection with the poles of a sensitive gal- vanometer (fig. 670). At the same moment a contraction of the proto- 1 No sharp distinction can be drawn between the stimuli which are followed by a prompt and easily observable response and those external agents whose very gradual change has no early apparent effect, but produces ultimately some deviation from the usual course of development. In the broad sense both are stimuli, but the term is usually applied only to the former, in which sense it is here defined. GROWTH AND M< >VI MINT 429 plasts occurs, and this may or may not be apparent. It expres 1 by a change of position in the leaf of Biophytum (fig. 070), or of Mimosa because there is at the base of the leaf a cushion of cells, whose lower ones, on account of the stimulation, exude some of the water that kept them tense mi. re readily than do the upper ones. Again, upon stimu- lation there may be a < hange in the rate or amount of some function or, more rarely, a change in the character of a function. Thus, the proto- plasm of a gland may be caused to secrete more or less rapidly than before, or the protoplasm in a growing cell may have its growth accel- erated or retarded. Further, a gland may have the character of its 0 l' V Z' FlG. (>~o. — Records of simultaneous mechanical (M) and electrical (E) response in Biophytum; the figures are seconds; dotted lines show the moment of application of a stimulus, ami the solid lines the deflection of the leaflet or of the galvanometer needle. -After Bose. secretion profoundly altered by excitation, or a part not growing may have its cells set again into active division and growth. Sensitive plants. — The fact that certain plants, having a special mechanism, respond to a stimulus quickly by a mechanical movement has given them an undeserved reputation as " sensitive plants " par ex- cellence; but they are not really more sensitive than others. Whether a plant exhibits movements or not depends on whether it has an ap- propriate mechanism to permit the protoplasmic contractions to propel it through the water, or the changed turgor to displace an organ, or the changed rate of growth to cause a curvature. Movements, then, are favorable for a study of sensitiveness merely because they are obvious reactions that can often be observed without apparatus. They do not signify unusual sensitiveness, nor does immobility imply its lack. Every plant responds appropriately to a sufficient stimulus, and every plant is therefore a sensitive plant. Propagation of the excitation. — The reaction specially observed is nut usually the only one. It may be only one of a st ties, and curvature, 430 PHYSIOLOGY resulting in movement, is most likely to be merely the end reaction. Thus if a primary root of a bean be set horizontal, the first reaction occurs instantly and in the very tip of the root, but it is not visible; only after a half an hour or more, at a distance of 2-3 mm. from the tip, does a growth reaction set in that starts to turn the root tip downward. Between the first reaction and the last there must have been a series of changes, each of which was a reaction to a preceding stimulus and a stimulus to a succeeding reaction. By a rough analogy the process may be com- pared to the tumbling of a row of blocks, each falling by reason of the impulse from its predecessor and impelling its successor to fall. The push that displaced the first one is the primary stimulus, and if the last were properly connected mechanically, it might, for the end reaction, ring a bell or fire a gun. Such a series of reactions is often spoken of as the transmission of the stimulus. More properly it is the propagation of the excitation. It is equally the propagation of a reaction. None of these phrases nor the above analogy should be understood to require that the reactions in a series are necessarily alike, nor is the end reaction the only one to which the term properly belongs, though it is usually so applied unless the contrary is indicated. Perceptive region. — The region where the first reaction occurs is often called the receptive or perceptive x region, particularly if a later and ob- vious end reaction occurs at another place. Since in animals a similar localization of sensitiveness for special stimuli marks the peripheral por- tion of sense organs, these regions in plants, especially when very cir- cumscribed, may be looked upon as sensory organs of the simplest sort.2 Regions of this sort, sensitive to gravity and light as stimuli, will be described later (pp. 463, 477). In the great majority of cases, however, perception is not strictly localized, and the condition resembles rather that in the diffuse senses of animals, like those of touch and temperature. Transmission. — Special tracts, the nerves, exist in almost all animals, along which the excitation is propagated, but nothing at all comparable has been found in plants, though this claim has been made more than once. The most that can be said is that propagation is more rapid lengthwise than crosswise of the cells of a tissue and in some tissues is easier than in others. Presumably the propagation is from protoplast to protoplast by way of the slender threads that connect them, traversing 1 These words are used in a figurative sense, and the last must not be understood to have its usual psychological implication. 2 Here again it is necessary to point out that in no sense is consciousness implied. GROWTH AND MOVEMENT 43] the walls. It is do! at all certain that there are not other more me- chanical means of transmitting the disturbance thai eventuates in move- ment.1 Responsive region.- — Corresponding to the perceptive region, the place where the final reaction occurs is called the active or responsive region. Of course it is not more active or responsive than the inter- vening regions; but attention is fixed on it as the seat of the selei ted reaction. Thus, in the root above referred to, the perceptive region is in the root cap, the excitation is propagated backwards through several millimeters of meristematie cells to those in the phase of enlargement, and the region of most rapid growth is the responsive region, because there the growth rate is unequally affected on the upper and under side, and so a curvature appears in that zone, which turns the tip downward again. Mechanism of reaction. — Consideration of even one such curvature shows that the nature of the reaction is in no way determined by the nature of the stimulus, since the same stimulus produces a number of reactions differing entirely from the end reaction, curvature. When many movements are studied, this feature appears most strikingly, for it is seen that the same stimulus may produce curvatures in exactly opposite directions in different parts, such as a root and a shoot, while different stimuli may call forth identical responses. Further, stimuli of the same sort at different intensities may call forth opposite reactions. The mode of action is determined in fact by the mechanism concerned. Just as an electric current may ring a doorbell, Mart an engine, or ex- plode a mine, according to the mechanism at the end of the wire; so an electric current may shorten a stamen, drop a leaf, or curve a tendril, according to the mechanism set into operation in the plant. Vet prob- ably there is some effect, fundamentally similar in each case, which works out to a different final result, just as, in the comparison, the magnetizing of an iron bar underlies the varied results. Tropic, nastic, taxic movements. — In some cases, however, the stimu- lus in a measure controls the rea< tion. A stimulus that acts upon plants from a definite dire, tion, and consequently from one side, may deter- mine by that fact the plane of the consequent curvature, provided the organ be physiologically radial, i.e. capable of response in any plane. 1 The "nerves" of leaves are so called only b ■ relative terms, "veins" and "rilis," imlii ate. They probably have nothing to '1" with transmitting an ezi itation in ordinary . ases, though some r« ent ob • i ration alb ge the i ontrary, 432 PHYSIOLOGY Such curvatures are called in general tropic and the phenomena tropisms. To these terms is often prefixed a word indicating the stimulus which calls forth the tropism, as geotropism (ge, the earth = gravity), photo- tropism (photos, light), etc. (see p. 458). When a curvature evoked by either a uniform or a one-sided stimulus is restricted to a single plane by the bifacial structure of the organ, the curvatures are called nastic, and the phenomena nasties. This term is also applied to like curvatures due to unknown (" internal " or " inherent ") causes. Thus we have epinasty and hyponasty, photonasty, photepinasty, etc. (see further, p. 442). In the organisms capable of locomotion, a one-sided stimulus may determine the direction of creeping or swimming. These phenom- ena are taxic, collectively taxies, and individually chemotaxy, phototaxy, geotaxy, etc., according to the stimulus (see p. 446). Energy relations. — Not only is the mode of reaction independent of the kind of stimulus, but its energy is disproportionate to the amount of energy expended in excitation. The stimulus, therefore, cannot be the sole cause of the reaction, though the two stand related to each other apparently as cause and effect. On the unexpected pricking of the finger, little energy is expended; the sudden jerking away of the hand involves many times as much. Somewhere this energy must have been released and applied; and this is one reaction of the series, whose final one was movement. So in the plant, stimulation often involves a mere fraction of the energy expended in the final movement; it is released, presumably from the protoplasm or some part of it that is particularly unstable, and is applied to the work. If this be so, the .chemical changes (metabolism) ought to be different in a stimulated and unstimulated organ. This hypothesis, however, has not yet been verified experimentally. Reinvesti- gation of the one case in which such a result was reported has produced a conflict of evidence. Another hypothesis, that stimulation results in molecular strain only, from which there is gradual recovery, sufficiently accounts for fatigue (see next para- graph), but does not account for the disparity in energy between stimulus and re- action, the existence of which its advocates merely ignore or deny. Fatigue, tetanus, and summation. — After an organ is stimulated once and the response occurs, the original state is presently regained, and the organ is ready to respond again as at first (fig. 671). If several stimuli follow, each before complete recovery, the responses are of less extent than before. This effect is described by the term fatigue, and in many cases the responses gradually become smaller and smaller until they GROWTH AND MOVEMENT 433 cease entirely. When the stimuli recur very frequently, the responses become f<>r a time combined, so thai the organ assumes a fixed position unlike the unstimulated <>nc. This quite resembles the condition of a mil i le in tetanus, as can be seen by comparing the records in fig. 672. After a period of tetanus, however, the reactions (case until rest from excitation permits recovery. If stimulation, too brief to produce the Fig. 671. — Uniform electrical response in radish to repeated stimulation. — After Bose. end reaction, he repeated at proper intervals, the separate effects be- come combined and suffice presently to call forth the end reaction. This summation of stimulation seems to be a sort of tetanic piling up of the earlier excitations of the series, which finally becomes sufficient to transmit its effects to the active region. rn^rx Fig. 672. Records of tetanic contraction in muscle (<;, b) and in style of Datura (c, d): a, c, incomplete; b, d, more complete. — After Bose. Reaction time. — Some time elapses between the beginning of stimu- lation and the end reaction, and this is appropriately called reaction time. Whereas in animals this is usually measured by a fraction of a -cioiid, in plant- it is much longer, occasionally a few seconds, but often minutes or even hours. This tardiness is due not so mm li to a low degree of sensitiveness, for the first reaction (perception) take- place almost instantly, a- to -low propagation and especially to the sluggish- ness of the met hanism ol growth. By contrast, turgor mechanisms usu- ally re-pond quickly. Naturally the reaction time i- made up of the perception time (a -mall fraction of a second), the transmission time (the rate varies commonly from o to 4 cm. per second), and the growth time, which is far the greater part of the whole period. C. B. & C. BOTANY — 28 434 PHYSIOLOGY Presentation time. — In order to produce any reaction a stimulus of given intensity must act for a definite time, called the presentation time. For the primary reaction this is extremely brief — practically instan- taneous. But end reactions, especially those due to growth, require some minutes or even an hour or more. Thus, roots must be kept hori- zontal for 15-30 minutes or even longer (depending upon the plant and its condition), in order that gravity may cause a curvature. This means, apparently, that the excitation must reach a given pitch through con- tinuous or summated stimulation, before it can be propagated to the active region and affect the growth mechanism. Once that pitch is attained, the end reaction will follow; and if the initial stimulus cease to act, it will follow as an after effect. If the intensity of the stimulus be increased, presentation time is correspondingly shortened (within limits, the ratio is inverse). Excitability. — To obtain a reaction it is not enough that a stimulus act upon a plant. The protoplasm must be in a certain condition, or excitation cannot follow. This is clearly recognized when it is said that a " dead " plant no longer responds to stimulation as before. It was once said: " The dead organism is ' dead ' merely because it has lost its irritability; " but this is true only by an extension of the term irritability beyond its usual sense. Closer study reveals the fact that many agents that do not produce death temporarily abolish or reduce or even exalt excitability. When protoplasm is in a condition of excitability, it is also in a condition to carry on well its usual activities; irritability there- fore is associated with other normal physiological qualities covered by the term lone. One experiences the feeling of well-being and vigor ; it comes when all the functions of the body are proceeding properly. So under favorable conditions the plant's functions are all effective and this tonic condition may be assumed as the norm,1 the result of the com- bined responses to many simultaneous external and internal stimuli. Retardation or acceleration of particular functions may then be brought about by the intensification or weakening of particular stimuli of this complex, or by the application of unusual ones. Loss of irritability. — Excitability may be diminished or abolished temporarily by a dose of anesthetics, like chloroform and ether, certain other functions being also interfered with. The precise mode of action is not known. After a time the effect passes away and tonic irritability 1 Note that this is not a fixed or well-defined condition; it is merely the usual, the ordi- nary; and it is assumed purely for convenience. GROWTH AND MOVEMENT 435 is regained. By a larger dost' irritability may be permanently abolished (that is, it kill:-*), while by a smaller dose it may become heightened. Various narcotics act in a similar way. Substances that kill are usually called poisons; really they are poisons only in certain doses. Their modes of action are doubtless as different as the poisons themselves. In the following sections, the foregoing general principles will find specific illustrations in the movements of locomotion, in the nastic ami tropic < urvatures of various organs, in the displacement of leaves by motor organs, and in the effects of stimuli upon form. It is important that the principles just set forth be constantly referred to and kept in mind in reading these sections. 3. MORPHOGENIC STIMULI The most general fashion in which various external agents affect growth appears in the way they control the form of the body through local alterations in the development of various parts. The varied and diffuse stimuli are termed formative or morphogenic. The reactions to them are extremely difficult to study because both stimuli and reactions are so general, and particularly because experimental alteration of one factor is almost certain to alter others to an unsuspected or an uncontrollable extent; wherefore the analysis of the factors operating is rendered very uncertain. It will be possible, therefore, to mention here only the simpler and best attested examples. Light and growth. — It is well known that the rate of growth rises and falls with the temperature, and since heat and light are both forms of 6 4 1 T-,r,, 674 L t— 3 1 Figs. 673, 674. — Graphs showing growth in millimeters in alternating periods of dark- ness (shaded) and light: 673, sporangiophore <>f Mueor Mucedo, periods 15 minutes; ''74. rhi/.oiiis of Marcluinlia polymorpha, periods 20 minutes. — Based on data by Si ameroff. radiant energy, it might be expected that the shorter and faster light waves wmild also affect the rate of growth. This proves to be true. In general the effect of light is to retard growth, particularly in elongating 436 PHYSIOLOGY organs. This is very clearly seen in the sporangiophores of Mucor and the rhizoids of Marchantia, as will appear from the graphic representa- tion of the observations (figs. 673, 674). It comes out also in the auto- graphic records of the growth of elongating stems when plotted so as to show the increment during the day and during the night, the temperature and other conditions, of course, being kept as constant as practicable. Daily period. — In nature the retardation due to light is doubtless accentuated by the greater evaporation of the daytime; but it is more ' , / V \ /\ / \ \ I \ \ / \ 1 * / \ ,..-->''"■- •-.. \ ^" ■*\ V 7 6 5 4 3 2 t mmL 60 T F° 8 10 Mm. 2 4 6 8 10 12;/. 2 4 6 8 10 Fig. 675. — Curve of daily period (solid line) and of temperature (broken line) : each vertic al interval corresponds to i mm. increment for the growth curve and to 50 F. for the temperature curve; horizontal intervals are hours, the region of close-set lines showing the night. Note rapid growth during the first day, beginning to fall off before the tem- perature falls (probably a transpiration effect) and then rising in the night in spite of falling temperature (partly also a moisture effect). — From data by Spoehr. or less compensated by the acceleration due to the rising temperature. Contrariwise, the acceleration upon the coming of darkness and a moister air is partly offset by the retardation due to the lower temperature of the night. Nevertheless, a periodic variation in growth in length, corre- sponding to the day and night, and hence called the daily period, can be traced, unless the fluctuations of temperature are excessive. This means that as certain conditions act antagonistically upon the rate of growth, they may be balanced or one set may overcome the other. The difference between the darkness of night and the light of day is so much greater than the usual differences of temperature and moisture in these hours, that the light effect is likely to be dominant (fig. 675). GROWTH AND MOVEMENT 437 Light and form. — The form of the aerial part-- of most plant- is pro- foundly influenced by light, dim dyor indirectiy. This is shown by the striking changes that ensue {etiolation) when they are grown in darkness. Without starvation this is pos- sible only with plants that have already stored a sufficient amount of surplus food. One who has observed the long pallid shoots of a potato which has sprouted in the dark will have seen the general effects. The stems tend to elongate much more than usual, though they are not necessarily more slender; the branching is at a different angle; and the leaves remain small and imperfectly developed. (The pallor from lack of chlorophyll and the presence of carotin are features already mentioned.) On the whole, elongation is likely to be accentuated, breadth is likely to be repressed (fig. 676). Though these are the common results of the lack of light during develop- ment, they are by no means universal. Thus, there are plants whose stems do not elongate, and others whose leaves arc not reduced. But if not these, other characteristics may be altered; e.g. reduction of the mechanical elements of the tissues is one of the less obvious effects. Scarcely a plant escapes but those that pass all their lives in darkness, and only those parts that are buried in the soil are exempt from the formative influence of light. Dorsiventrality. — In plant organs not grown in darkness, but of which one side is better illumi- nated than the other, light effects can be observed. One effect is the development of a distinctly different structure in the better lighted surface ;is compared with the shaded one, and since these are naturally the upper and under surface-, an organ showing such difference- is termed dorsiventrol.1 Thus the pali sade portion of the mesophyll of leaves owes its exist- ence chiefly to light.2 Dorsiventrality in the liverworts is likewise due mainly to light. None shows this better than the common Man Jnwtiii t l re 676.— Plant of Phaseolus grown in darkness. - AiU r MacDoogal. 1 Dorsiventral organs may owe the difference of their fai es t<> other formative stimuli, ■ g. to gravity. Se< footnote, p. 4a 1. 438 PHYSIOLOGY If a gemma (p. 98), which when separated from the parent is just alike on the two sides, be grown in a moist chamber with the lower side illuminated and the upper dark, air chambers will be developed on the lighted side and rhizoids on the dark one, exactly the reverse of the usual relation. Gravity, if it furnish any stimulus, as is prob- able, is clearly overcome by light. In like manner light determines the formation of the sex organs upon the under side of fern prothallia. A striking example of light effects among the seed plants is to be found in the dorsiventrality of the rootstocks of the spatter dock (Nympheca advene). These great rhizomes develop at the surface of the mud at the bottom of pools, and are of the length and thickness of a man's arm. From the upper side numerous leaves arise, and from the under side roots. This distribution of organs is found to be determined by differences in lighting. Electric waves. — Of the same class as heat and light waves are the electric waves; and they too have considerable formative influence. It has been shown that the germination of many seeds is hastened by suit- able electric stimuli, and for a considerable time the growth of seedlings is also accelerated. When crops of barley, wheat, beets, and other economic plants are frequently subjected to a quiet discharge of high- tension currents from wires, with many pendent points, strung over the experimental fields, it has been found by several observers that the plants grow better, come to maturity earlier, show increased productiv- ity, and are of better quality than on control plots. Thus, an electrified wheat plot of 3 acres yielded a crop 39 per cent greater than the control plot, sold at 7.5 per cent higher prices, and the flour was of a higher grade on account of its baking quality. Beets (for the table) on an electrified plot showed ^^ per cent increase and contained an average of 8.8 per cent sugar, against 7.7 per cent on the control plot. Chemical agents. — Chemical stimuli are also extremely important in determining the form of plants. The presence or absence of particular substances in the cells, whether foods or wastes, doubtless exerts a pro- found influence. But the precise influence of the different compounds cannot be determined satisfactorily, because the chemical processes within the plant are so imperfectly known. It is in this region that the role of the so-called necessary elements of the ash, calcium, magnesium, potassium, and iron are to be sought, in all probability. How far the xerophytic structure of plants is to be ascribed to the lack of water is not certain. The deficiency of available water may be in itself a chemical GROWTH AND MOVEMENT 439 stimulus, or it may make possible the stimulating action of other sub- stances within the plant, which, but for their increased concentration, would not act so. Unquestionably other causes than lack of water around the roots of a plant may call forth such structures, as is well seen in the case of bog plants. Indeed it has become customary to speak of "physiological" drought as the cause of serophytic structure, when physical drought is obviously out of the question. This may be taken as a convenient expression for some difficulty which prevents the plant from admitting a sufficient amount of water, such as the poor develop- ment of the root system. Whatever does this will tend to dwarf or other- wise transform the aerial parts, either as the plain lack of water does, or possibly in quite different and unrecognized ways. (See further, Part III on dwarfing in bogs.) Recent investigations are bringing to light some new causes for the imperfect development of plants, which probably is due primarily to an effect on the roots. It is found that the sterility of some soils is due to the presence in these soils of organic substances, which are partly soluble, so that a watery extract of such soils, when used as a water-culture medium, acts as badly as the soil itself. Furthermore, these substances can be removed in large part by adding some finely divided ma- terial like lampblack to the liquid and then filtering it out. The filtrate may then be used without detriment to the cultures. Still further study makes it probable that these substances originate in large part from the plants which have previously grown in the soil. The necessity for the rotation of crops on any field has long been known. The reason has been assumed to lie in the exhaustion of the materials which are supposed ti> be nei essary fur the nutrition of the plants. Without denying that there may be something in this assumption (it is nothing else at present, because the ex- perimental evidence upon which it rests is faulty), it seems now much more likely that the chief cause is to be found in the excretions from the roots of the previous crops and the products of their decay in the soil. It has been shown that though the mineral salts of a culture solution be maintained unchanged, the water becomes more and more unfit for use with repeated cultures of the same species, and that this impairment may be remedied by treatment with lampblack as above desc ribed, though the content of salts be not altered. Water cultures, to which have been added various orgai ic substances that might be produced, or are known to occur in plants, have shown like injuries to the plants, and though the amount of the deleterious sulistam :es occurring in nature is too small for direct analysis, their general i harm ter may be ascertained by further experimentation in this way. Mechanical agents. — Pressure and tension have evident influence on the development of mechanical tissues. The encasing of a stem in a plaster cast, SO that as it thickens it will compress itself, leads to changes in the stru< ture within the zone of compression and especially just beyond the margin. Continuous tension seems to bring little if any 440 PHYSIOLOGY increase in mechanical tissues, but ilexure, with its alternating compres- sion and tension, such as the wind in certain regions produces, beyond doubt increases the proportion of mechanical tissues and thickens their walls. When combined with excessive evaporation and perhaps other unfavorable factors, the effect on bodily form is astonishing (see Part III on stem-dwarfing). Deformities. — Noteworthy local modifications of form are produced by the attacks of parasites, either plant or animal. When specific deformities are produced, the structures are called galls (fig. 655, p. 384). Just how far these are due to chemical substances excreted by the para- site, and how far to the mechanical pressure, to the punctures, or to the movements of the larvae of animal parasites, remains at present quite uncertain. Whether chemical or mechanical stimuli act upon the host, its response might be first an altered metabolism, which produces ap- propriate effects upon the division and course of development of the cells, resulting in the deformation of the region. Profound alterations in the relative development of the tissues and in the character of their elements accompany the deformity. Injuries. — Injuries of various sorts call forth growth in tissues which have long passed the ordinary period of cell-division. This gives rise to a callus at the edges of the wound which tends to close it, a fact that is of great practical service in the grafting and budding so indispensable in fruit growing. Desirable sorts, too tender for a given climate, may thus be united with stocks that are hardy, but have no good qualities in their fruit. In practice, smoothly cut surfaces are opposed and kept in close contact, with the exclusion of water and spores by wrappings and wax. The healing tissues blend, as they form at the junction, and an organic union is established, permitting the passage of water and foods freely. If a wound be allowed to heal, the callus may give rise to new growing points, from which the regeneration of removed organs may proceed. Thus, if a root be decapitated, a new apex may be regenerated, if the cut be near enough the tip, or new lateral roots may arise that would not otherwise have been produced, or old roots may be incited to more active growth. In either case of the formation of new organs, the reac- tion to the wound stimulus is complicated with unknown factors named polarity, and with the influence of other organs called correlations. Polarity. — Since the opposite ends of an egg cell give rise to unlike structures (for example, in seed plants, suspensor cells from one end and GROWTH AND MOVEMENT 441 the embryo initial from the other), it is assumed that the two hemi- spheres arc unlike, oven though no structural differences arc visible. This is expressed by the term polarity, after the analogy of the invisible differences in the two ends or poles of the magnet. A like- polarity must be imputed to all other cells, its progeny, so that the embryo initial, when it develops, produces at the one cud a rool and at the other a shoot. Later in life, any piece of the shoot cut away from the rest shows a ten- den< y to produce shoots at the apical end and routs at the basal end, when put under conditions to regenerate lost organs. The conception of polarity in the cells is thus extended to aggregates of cells of any size. because they show such differences at the apical and basal ends. All attempts to ascertain the nature of polarity have so far proved futile, so that there is nothing to " explain " the phenomena but the word and the assumption for which it stands. Correlations. — The term correlation designates the reciprocal influ- ence of organs. Of this little is known beyond the fact that the suppres- sion or the removal of one organ exercises a marked effect upon some or all of the remaining ones. Many examples might be cited, but no ade- quate explanation of the effects can be given. It is known, however, that at least some of them are not due merely to differences in the InA or water supply, or to like conditions. Examples will make clear what is meant by correlations. Quantitative correlations. — In the axil of each cotyledon of the bean there is present a bud, neither of which develops into a shoot unless the main axis is cut off or prevented from developing. If one desires sweet peas and such plants to continue flowering, it is necessary to cut away the older flowers or the youm,' pods, so as to prevent the formation of fruit. If this is done, the plants go on flowering till frost, whereas their season is quickly over when allowed to sel seed. The gametophyte of ferns is shortdived, as a rule; but if the fertilization of the egg be prevented, its life may he prolonged for months, and it proliferates, forming nil again and again. The possibility of shaping a tree by judicious prun- ing, and of increasing the production of fruits by ore hard trees in the same way rests upon like reactions. Qualitative correlations. — Correlations are not merely quantitative, as the above examples might seem to imply; they are also qualitative. That is, the whole be- havior and even the structure of an organ may he altered according as other organs are present or absent. Thus, the central axis of most conifers is strictly radial in structure and in branching, while the lateral branches are distinctly dorsiventral. But if the terminal shoot be cut away, one (<>r more) of the laterals may become erei t, losing entirely the dorsi ventralit y, and becoming radial like the leader. The aerial shoots of the potato, which hear foliage leaves and Bo vers, are very different from the subterranean ones, which hear the scales and tubers. But if the aerial 442 PHYSIOLOGY shoots be cut away, some of the subterranean shoots will (urn up into the air, be- come green, and develop foliage and flowers as though never inclined to be subter- ranean. The sporophylls of certain ferns, notably Onoclea, arc entirely different in aspect from the nutritive leaves, and have so many sporangia crowded on the sur- face that they seem entirely covered. If all the nutritive leaves be cut away, leaves that ordinarily would have become sporophylls will then becomo foliage leaves and bear no sporangia. In like manner the tendrils of the pea leaf may be made to develop into leaflets. In all these cases transformation is possible only before the primordia have gone too far in any determined course, though the point at which new influences may affect them is very different in the different cases. Usually the stimulus must be applied very early, while the primordia are still undifferentiated. Many of the problems of regeneration are com- plicated by these phenomena of correlation, if they are not wholly de- termined by them. 4. NASTIC CURVATURES Epinasty and hyponasty. — A somewhat less general manner in which stimuli of various sorts affect plants is to be found in their effects upon the rate of growth on the two faces of bilateral organs, such as thalli, foliage leaves, bud scales, perianth leaves, etc. It is very common to find that such organs grow at different rates on the two faces, so that they are distinctly curved thereby. Thus, in their earliest stages, the leaves grow fastest on the back or outer side, so that the inner face is pressed close to the axis, and as they usually outgrow it, they curve together over it in a protective fashion, forming a bud. The scales, especially, long maintain this form, as the longitudinal section of any bud will show. Later, the relative rate changes; the inner face grows more rapidly than the outer, and the bud opens because the curvature carries the leaf or scale away from the axis. Thalli often show the same thing; the upper surface may be so tense from greater growth that the thallus is tightly appressed to the ground. Such curvatures are described briefly by the terms epinasty or hyponasty, according as the greater growth is on the upper (inner) or lower (outer) face. The greater number of these nastic curvatures are due to unknown (internal?) causes, but some have been found to be reactions to external stimuli (paratonic). The former are not unlike those autonomic curvatures of radial organs described as nutations (p. 423), only in this case the bilateral structure of the organ determines that the nutations shall be in one plane only. The latter are also allied to tropisms, but differ from them in that net the direction GROWTH AND Ml >VEMENT 443 from whi< li (lie stimulus acts but the structure of the organ predetermines the plane of the movement. Light and temperature. — Examples of paralonic nastic curvature are seen when light and temperature act as stimuli upon foliage and flower leaves, and less plainly in tendrils. Temperature changes arc espo ially effective with the perianth leaves of tulip, crocus, snowdrop, colchi< um, and other plants whose blossoms appear very late in the autumn or very early in the spring. In the crocus a rise of half a degree suffices to bring about a curvature that opens the flower; while the tulip can be made to open and close as many as eight times in the course of an hour by raising and lowering the temperature. Tendrils respond to a tempera- ture change, whether a rise or a fall, by curving in one direction only, the upper side being stimulated to accelerated growth. In this they differ from the perianth leaves cited, for in these a rise of temperature tends to accelerate growth on the inner face and thus to open the flower, and a fall to accelerate growth of the outer face and so to close the flower. Very many, perhaps the majority of foliage leaves, show nastic curvatures in response to alterations in temperature and light as long as the petiole is still capable of growing; finally curvature ceases && tfae fixed light position of maturity is attained. Such bending movefn^n© remind one of the photeolic movements executed throughout life by le^jes that have motor organs (see p. 451). Among flowers those nest ^trikingly re- sponsive to light are the heads of some Compositae, s^h as the dande- lion. Here the flowers and the bracts about the flfT^rgJ-luster, the involucre, curve so as to close the head when the light S^dminished, as in cloudy days, and to open it in sunshine (Part III, fiq£i&, 1194). In countries where the climate is equable it is possible to select plants whose Bowers open at particular hours of the day on account of light and temperature stimuli, and by planting them in a circle to have a sort of floral clock. Naturally it is not very reliable. Gravity. — Nastic curvatures are also produced in plants in response to gravity, which, however, usually cooperates with or antagonizes the light reactions. In all cases the stimulus at work may be indicated by (he nrvlix. Thus we have photonasty, thermonasty, etc., and still more specifically photepinasty, geohyponasty, etc. Mechanism. — In all these curvatures the mechanism of response is the same. The growth of the outer or inner surface i- accelerated, as can be shown by making equidistant marks upon the two faces and mea- suring the changes. This observation shows, too, that under frequent 444 PHYSIOLOGY stimulation the total growth is much greater than it is under uniform conditions. 5. LOCOMOTION AND STREAMING Locomotion limited. — Locomotion is restricted among plants to the simplest forms (with a few exceptions to those that are unicellular), and to the gametes, especially the male gametes, of the multicellular plants. The reason for this is doubtless to be found in the restriction of freedom to move imposed by the cell wall — in effect a sort of strait- jacket — in which the protoplast incases itself. Even when the proto- plast moves, as it often does, within this case, its movements do not bear against the outer medium and therefore do not propel it about. The only exception to this restriction occurs in those plants whose wall is perforate; then the protoplasm protrudes through the opening so as to operate against the outer medium, or in a few cases it excretes mucilage forcibly against the medium or the substratum and so pushes itself slowly along. Rate. — When the protoplast changes its shape suddenly, quick swimming and darting movements result ; when slowly, the move- ment is perceptible only because magnified by the microscope. In the very swiftest move- ments the absolute translation is small, say 50 mm. per minute; and in the sperms of ferns, which under the microscope seem to be going fast, the rate is only 0.1 to 0.2 mm. per minute. Measured relatively, as in terms of size, and taking account of the resistance of the medium, the translation is seen to be very rapid. The very fast human runners cover about 50 times their own length (100 yards) in 10 seconds; the swarm spores of Viva can travel 100 times their own length in the same time; and the spiral sperms of a fern (N ephrodiiim) can do 50 to 100 times their length (as coiled) in 10 seconds (fig. 677). Amoeboid movements. — The slow movements are a kind of creeping, and are of two sorts, amoeboid and excretory. Amoeboid movements (so called because characteristic of Amoeba, a genus of infusoria) are found rarely among plants, being known only in the plasmodia of Myxo- mycetes, a group of organisms with so many animal characters that they Fig. 677. — Sperm of Ne- phrodium, with flagella — After Yamanouchi. GROWTH AND MOVEMENT 445 arc often included in the animal kingdom (see p. i). The plasmodium is a naked mass of protoplasm (sometimes like* a thin cake, often a richly anastomosed network), which during its vegetative period lives in wet places among decaying wood, leaves, etc. The creeping is a< complished by the protrusion of marginal lobes of the protoplast along one side, and toward these the rest slowly flows. In this way the whole mass advani es in a definite direction, which is frequently changed and is subject to control by external agents. Thus, by varying the temperature, the mois- ture, or the illumination, the plasmodium may be made to creep in one direction or another. Its response to these stimuli, however, differs with its own stage of development. Whereas during a considerable vegetative period it avoids light and drier places, later it creeps out from the substratum and ascends to drier and exposed situations, where it produces sporangia with a casing and framework of cellulose and a multitude of spores. Excretory movements. — Excretory movements are executed by some diatoms and desmids, and those of Oscillatoria and Spirogyra are probably of this sort. The diatoms and desmids forcibly excrete muci- lage through slits or pores in the wall against the substratum (a glass slide, the wall of an aquarium, the bottom of a pool, or the surface of a water plant) over which they creep slowly with a majestic and mysterious motion, which is not yet fully under- stood (see also p. 451). Ciliary movement. — The more rapid movements are called ciliary-, because executed by the lashing of slender threads of protoplasm through the water, in which alone such organisms can move. The motile threads are known as cilia or flagella.1 They arise from different places on the protoplast, often at the pointed apex or along a band, where the special organ which produces them, the blepharoplast, is lo< ated (fig. 678). The flagellates (unicellular organ- isms of uncertain relationship, p. 20), bacteria, the zoospores and gametes of certain algae and fungi, and the sperms of bryophytes, pteridophytes, and Fig. 678.— Swarm spore of Hydrodit tyon, with two cilia ari ing from a blepharoplast with nu< lear CODDl l - lions.— After Timbi k- LAKE. 1 No constant distinction can be made between cilia, which arc typically short, hair- like, and numerous, and Bagella, which arc long, whiplike, and few (1 4) for 1 Yet a cell sometimes has a single (.ilium, OT tWO, and llayella are numerous on the sperms of ferns. 446 PHYSIOLOGY cycads, exhibit ciliary locomotion. The cilia are so slender, and when magnified sufficiently their movements are so rapid, that the details of the strokes are difficult to follow. In the thicker cilia of infusoria the forward stroke (fig. 679) consists of a progressive bending, which begins below the free tip and advances to the base, where it is most powerful. At the moment of greatest efficiency (fig. 679, 2), the curve bears against the water like the blade of an exaggerated spoon oar (though, of course, the cilium is not flattened). The return stroke (fig. 680) is slower and consists of a reverse and some- what different curvature, advancing from base to apex. Cause. — The cause of these repeated lashings is completely hidden. They con- tinue for a time and then cease. Though they cannot be initiated, they can be stopped or modified in rate by appropriate stimuli, and their duration can be pro- Figs. 679, 680. — Diagram- longed. Thus, if zoospores of algae be matic representation of sucessive rdeased in Hght they may swim about positions (as numbered) of cilia of Urostyla grandis ; 679, in for- for a few hours, then attach themselves ward stroke; 680, in recovery, and germinate. But if they be kept in -After Verworn. darkness, the swimming may continue for two or three days, until the zoospore seems entirely exhausted and perishes without settling down. Taxies. — The direction of swimming may also be controlled by ex- ternal agents. The phenomena of directed locomotion are compre- hensively called taxies, and with a prefix, designating the directive agent, we have phototaxy, thermotaxy, chemotaxy, etc. These responses, apparently simple, are really very difficult to interpret, and experiments, seemingly quite conclusive, may lead to false inferences through the operation of some overlooked factor. Thus, if a dish containing zoo- spores of algae be placed on a window ledge so that one side is more brightly illuminated than the other, the swarm spores will be seen to accumulate on the side with brighter light, and this movement was described at first as a positive response to light. Later it was found that the droplets in an oil emulsion would behave in the same way because of the previously unnoticed differences in temperature, making convection currents in the dish. Two factors were therefore involved and more rigid tests were needed to demonstrate phototaxy. CROW III AM) Mi »VI Ml A I •I i; Chemotaxy. — Chemotaxy has been most extensively investigated, but is not yet fully elucidated. If a soluble crystal be introduced into water undisturbed by currents, the molecules gradually diffuse from its surface in a constantly enlarging sphere; or if the water be the film under a cover glass, in an increasing /.one-. By using a glass tube drawn out to a very fine capillary and closed at one end, liquids of any sort may be used. A short capillary is filled with the solution and placed on a microscope slide with its open end under the cover glass. Slow diffusion takes place from the mouth, while the behavior of the organisms is watched under the microscope. As a rule the rate of their movement is not affected, except by substances that are directly injurious. It appears that the directive effect of such stimuli is exercised in two dif- ferent way-. i. Orienting reaction. — In the first case, the direction is altered because the organism, in response to the stimulation, orients itself, so that with continued movement the body will be carried toward or away from the source of the diffusing molecules. It is assumed that this orientation is determined by the unequal or one-sided action of the molecules, the end (less probably the flank) toward the source being most powerfully affected, whereupon the creature turns, and according as it brings the anterior or the posterior end toward the source of stimulus, and swims, it will approach or recede from that source. 2. Recoil reaction. — The second case is quite different. The move- ments of sperms and zoospores are too rapid to be followed easily; but if large and slow-moving organisms are observed, they may be seen to swim about quite indifferently, passing in close proximity to the crys- tal or capillary tube from which the molecules are diffusing, without showing any tendency to swim towards it. But when they reach by chance the limits of the diffusion /one, they suddenly reverse their direc- tion and back away, as though they had encountered an obstacle and had rebounded from it. This reaction is repeated at every side, and having one e chani ed to swim into the diffusion /.one, they are imprisoned within it, because the attempt to pass out of it results always in the re^ action of ret oil. So. ;i^ mure and more an- thus caught, there is an accumulation within the diffusion zone, as though it were a trap. Not all substances, however, permit the first accidental entry, for the recoil may be produced at the attempt to cuter this zone, while any such organ- isms placed within it would be free to swim out without recoil. In such a i ase the final result is the accumulation of the organisms in the regions 448 PHYSIOLOGY outside the diffusion zone. Besides the reaction of recoil, there are accompanying minor reactions which cannot be discussed here. Attraction and repulsion. — Many different substances have been tested with respect to chemotactic control. Some prove to be attractive, some indifferent, and some repellent. That responses occur to substances that are never met in nature, as well as to those that are not foods, and further, that they do not prevent the organisms from coming to serious or even fatal injury, indicates that chemotaxy depends upon some fundamental property of the protoplasm and is not a mere adaptation to secure special ends, however well it may occasionally serve such a purpose. In many cases a substance which is attractive at a low con- centration proves to be repellent at a higher. In such a case the ques- tion arises whether the repellent action is due to the chemical constitu- tion of the stimulant or to the osmotic pressure of its solution. As the latter seems to be the reason for the action in certain cases, the phe- nomenon is named osmotaxy. It has not yet been sufficiently investi- gated, but is in many ways parallel to chemotactic irritability. Amount effective. — The amount of a substance which can act di- rectively upon motile organisms is infinitesimal. Thus it was found that a minute capillary into which the sperms of a fern crowded, contained, all told, less than three hundred-millionths of a milligram (0.000000028 mg.) of malic acid. Of this, certainly, only a very small fraction could have reached any one of the sperms. Yet relatively the amount is not at all insignificant; for the estimated weight of one of the sperms is only ten times greater than the total weight of the acid, and if only i/roo,ooo of the total acted upon a sperm, the ratio would be 1 : 1,000,000, which is still 10 times the ratio of a minimum effective dose of morphin for the human body. Weber's law. — The phenomena of chemotaxy offer an excellent illus- tration of a general law of response known as Weber's law. If a fern sperm is swimming in water, it will be diverted toward a capillary con- taining malic acid whose concentration is 1 part in 100,000 of water. But if it is brought into a solution too weak to evoke a response, say 1 : 200,000, it is so affected by the enveloping acid that it does not respond unless the solution in the capillary is 30 times as strong as that by which it is surrounded, i.e. 30 : 200,000. If again the concentration of the acid in the medium be raised, say from 1 : 200,000 to 1 : 100,000, the concentration of the stimulant in the tube must be 30 times greater, i.e., 30 : 100,000, in order to evoke response; and so on. It GROWTH AND MOVEMENT 449 appears from this that a sensitive organism becomes adjusted to a con- stant non-directive stimulus, and then is unresponsive to an intensity of one sided stimulus of the same sort, to which in the unaccustomed state it reacts. Thus accommodation is really a lowering of irritability toward a particular stimulus. The noteworthy point is that it is a proportional lowering; for, after each adjustment has occurred, it requires a definite increase in intensity (in this particular case a large one — 30 times the constant) to call forth a response. Some ratio of this kind, whether it be an increase of 3 times or 30 times the constantly acting stimulus, ha- been found to hold good for many forms of response and in many sorts of organisms. In all cases the law is valid for moderate stimuli only; an intensity is soon reached where it ceases to express the facts. The law was formulated in 1834, with reference to touch and sight. It has been stated lately thus: "The smallest change in the magnitude of a stimulus whi< h will call forth a response always bears the same proportion to the whole stimulus." Aerotaxy. — One form of chemotaxy has received a special name, aeroiaxy, which signifies that the air, or more exactly the oxygen of the air, is the excitant. Certain forms of bacteria are motile only when they are in contact with oxygen, and cease to move when they are de- prived of it. In so far, this also might be due merely to respiratory disturbance, just as many functions cease when no oxygen is supplied. But these forms also swim in the direction from which the oxygen is diffusing, and accumulate about its source. Such forms, if evenly dis- tributed under a cover-glass, soon desert the center and gradually ac( umu- late at the edge, where the 02 is diffusing into the water. These species, motile in oxygen, can be used as indicators of photosynthesis, because 02 is a by-product. Ionic stimuli. — All chemotactic reactions to substances that dissociate in water probably rest upon the specific action of the various ions and molecules present in the solution, ami attempts have l>ecn made to correlate the action of the various sails and a< ids. Hut the phenomena are too complex to permit satisfactory analy- sis yet; and since undissociable substances also art as stimuli, it is probable that the undissociated molecules, as well as the ions, have a stimulating action in many cases. Phototaxy. — Phototaxy is particularly characteristic of those organ isms that have chlorophyll, such as the zoospores of algae and the ciliated colonial algae like Volvox, Eudorina, etc1 That they swim towards light of moderate intensity is not to be doubted; but it has been very 1 Some fungus swarm spurus also are sensitive to liv;lit. C. B. & C. BOTANY 29 45° PHYSIOLOGY difficult to determine whether this response is due to the direction of the light rays, or to the fact that one region is more brightly illuminated than another. Accumulation certainly occurs in regions of moderate light, with avoidance of the more shaded or the more brightly illuminated portions. The most exact of the recent studies of Volvox shows that its orientation is controlled by the relative intensity of the illumination on different sides of the colony, and as it swims with a definite pole forward, swimming after orientation causes it to move nearly parallel with the rays, some deflections from this course being due to certain minor disturbing factors. In phototaxy, as in chemotaxy, organisms respond both by orientation and by recoil, though, so far as known, the latter is much less common. The light waves vary in action according to their length, the reds and yellows, though the brightest, being quite unstimulating, whereas the blues are most effective. Yet this gives no clew to the real nature of the excitation or of the organs by which it is perceived. Geotaxy. — Certain organisms have also been found to be geotactic. This prop- erty is quite distinct from others; for organisms that respond alike to other stimuli, such as light and oxygen, may react differently to gravity, the one being positively, the other negatively geotactic. Upon such irritability may depend the al ility of the creatures to rise or sink through the water on occasion. Motion of cell organs. — Not unrelated to the movements of free- swimming organisms that have been described are the movements of organs of the cell which take place within the limits set by the wall. Such, particularly, are the movements of the chloroplasts and the nu- cleus. The former are known to be in part responses to light stimuli. Certain algae of the genus Mougeotia (Mesocarpus) have a single platelike chloroplast, which lies in the axis of the cell, facing the incident light, when this is of appropriate intensity. But if the light becomes more intense, the plate rotates until the edge is presented to the light. The numerous rounded chloroplasts of seed plants, mosses, etc., alter their distribution and their shape according to the illumination (figs. 68 1, * 682, and in Part III, figs. 758, 759). This trophe. — After Schimper. suggests a sort of escape from too bright light, Figs. 681, 682. — Two leaf cells of a moss (Atrichum undulatum) seen from above : the chloroplasts in 68 epistrophe; in 682, in apos GROW 111 AND MOVEMENT 451 in idea thai agrees with what is known of the intensity of light required for photosynthesis (see p. 371)- Yet tne arrangement is seldom as regu- lar or complete as it is sometimes described, and effective protection from light is secured mainly in other ways. Aside from their own amoeboid movements the chloroplasts are subject to displacement by movements of the protoplast, as in streaming (below). The nucleus also changes its position in the cell " spontaneously " or in response to certain stimuli, notably to wounding. Nothing is known as to the significance or mechanism of such movements. Streaming. — In very many active cells a streaming movement of portions of the protoplasm has been observed. The layer closest to the wall does not participate in the movement, and though the chloro- plasts, when any are present, are not necessarily involved, they are often swept along when they lie deeper. The rate of the motion varies with temperature and with other conditions that affect the general activity of the protoplasm, and the movement may be entirely stopped by appro- priate stimuli. Nothing is known as to the causes or the effects of these movements, though they are extremely common and perhaps universal. The idea that they facilitate the more rapid distribution of foods and solutes in the cells and so hasten osmotic transfer of materials would be more plausible were streaming less common and vigorous in those cells, e.g. in hairs, where such a process seems of slight importance. In some diatoms the protoplasm partly protrudes through a longitudinal median slit (the raphe) in the valves, and streaming movements in this outer belt, reacting against the water or the substratum, propel the cell slowly in the direction opposite to the outer streaming. The counter-stream, of course, moves within the cell wall. Surging movements of the protoplasm in the coenocytic hyphae of Mucor and other fungi have been seen, but their causation and significance are unknown. 6. TURGOR MOVEMENTS Motor organs. — In a considerable number of plants thin-walled turgid cells are so arranged thai the position of the organ of which they form a part depends upon the relative turgor of these cells. In mosl cases the organs are leaves, either foliage or flower leaves, and the structure is such that the motor organ curves only in one plane, the distal part rising or falling with the variations of turgor. Examples of these motor organs arc afforded by the leaves and leaflets of the Leguminosae and the Oxalidaceae, by the Stamens of Bcrbcrls, and by the stigmas of Stimulus, 45: PHYSIOLOGY they are also found in a considerable number of families allied to the Berberidaceae and Scrophulariaceae. Structure. — The leaves of Leguminosae are usually much branched, and the primary motor organ, when present, is located at the base of the main petiole. In many cases there are also motor organs (secondary) at the origin of the secondary petioles, and if the leaf is ternately com- pound the petiolules or stalks of the leaflets are motor organs. Thus Mimosa has primary, second- ary, and tertiary motor organs (fig. 683) ; but the red and sweet clovers have only one set, the stalks of the leaflets. The motor organ consists of all or a portion of the petiole or peti- olule, modified by changes in the position of the vascular bundles, and by an excessive development of the paren- chyma of the cortex. Through the greater part of the leaf stalk the vascular bundles lie at some distance from the center, surrounding a distinct pith, and within a cortex of moderate thickness. In the motor organ, however, they ap- Leaf of Mimosa in open and closed proach one another so closely positions. — From Part III. ,, , ,, . , that there is scarcely any cen- tral pith, and they form a shaft, elliptical or kidney-shaped in section. Outside, the cortex is correspondingly larger, and its cells are usually somewhat different from the rest. As a whole the motor organ is some- times thicker than the other part of the petiole, but it is quite as likely to be smaller ; in all cases, however, the relative increase of the cortex in cross section gives the impression of a cushion of parenchyma.1 In this region the cells are rather regular in form, approximately cylindric, and with smaller intercellular spaces than in the nutritive regions. Intercellular spaces are present, however, at the junction of three or more cells. 1 This is the reason for a technical name applied to the motor organ, the pulvinus. Fig. 683. GROWTH WD MOVEMEN V 453 Mechanism. — It is evident that the central position of the vascular bundles permits flexure more readily than if they wire Mattered and more peripheral; while the peripheral position <>f the thin-walled cells of the cortex is such that any variation in their turgor will produce a cur- vature, the side with less turgor becoming concave, since its cell- no longer oppose fully the turgid cells of the opposite side. Correspond ingly, the parts beyond the curving motor organ will be displaced by it. These turgor variations, due to modified permeability, being usually restricted to the upper and lower sides of the motor organ, the distal parts arc moved up and down. Since the relaxed cells may recover tur gidity and the turgid cells become flaccid, the notable feature of all such movements is that the changes in the cells are reversible; whereas the cell changes involved in growth are irreversible (or soon become so). The motor organs of stigmas and stamens are essentially similar to those of foliage leaves, but simpler, since vascular tissues are slightly or not at all developed, and almost the whole tissue is parenchymatous. Autonomic movements. — The variations in turgor are sometimes autonomic, that is, determined by causes unknown and apparently in- ternal to the plant, but more commonly they are controlled by external stimuli. Autonomic movements are not at all uncommon, but they are mostly too slow to be observed easily without apparatus, and, when sought, are often masked by more obvious movements (see p. 457). The classical and almost the only striking example of easily seen move- ments is offered by Desmodium gyrans, whose lateral leaflets (fig. 684) are constantly rising and falling under favorable conditions. These movements, sometimes uniform, but usually jerky, are not very rapid, for a complete up- and-down movement requires 2-4 minutes. The fall is more rapid than the rise (for ex- ample, 45 sec. as against 70); and as the tur- gor variations tend to fluctuate regularly to right and left of the vertical plane, the tip of each leaflet describes a narrow ellipse. The reason for these move- ment- is unknown, nor are they known to be of any value to the plant. Fie. 684. — Leaf of tele- graph plant (Desmodium gyrans), natural si/.o: /, /, lateral leaflets which show autonomous movements; the terminal lea fie t in the depressed position. — After I'l 1 111 K. 454 PHYSIOLOGY Under unfavorable conditions they cease, but the plant may still be able to respond to external stimuli like others about to be described. Paratonic movements. — The terminal leaflet of Desmodium gyrans, like leaves of other members of the 'bean family, exhibits paratonic movements {i.e. those due to special stimuli, not tonic; opposed to auto- nomic). Moreover, some plants whose leaflets ordinarily exhibit only paratonic movements, may make autonomic ones under exceptionally favorable conditions. Thus it would seem that there is no fundamental difference in the two, and when the precise stimuli that initiate the move- ment are discovered, autonomic movements may all be relegated to the paratonic category. Turgor movements due to external stimuli are numerous and easily observed, but except in a few striking cases they are not rapid enough to be seen by watching for a brief time. The stimuli initiating the move- ments are of the most varied character; contact, gravity, and changes of light and temperature being the most common. Contact movements. — If the stamens of the barberry (Berberis) be touched near the base at the time when they are shedding pollen, they suddenly fly up and inward, carrying the anthers close to the stigma. After a short time they resume their former position against the petals. The filaments of the Cynareae, a tribe of Compositae, shorten instantly on being touched (the reaction time is less than i sec), dragging the coherent anthers quickly down over the style, whose hairs scrape out the pollen like a pipe cleaner. In Centaurea americana, this contraction continues for 7-13 seconds, and after a minute the rest position is again reached. Probably the best known of the rapid contact movements are those of the species of Mimosa and Biophytum, the " sensitive plants." In Mimosa the leaflets are carried by the motor organs forward and upward until the upper faces are pressed together, while the primary motor organ drops the whole leaf (fig. 683, p. 452). Another famous example is the quick closure of the " fly-trap " of Dionaea (figs. 657, 658, p. 386). Here Fig. 685. — Leaf of sundew (Drosera rotundifol ia) with half of the tentacles inflexed from stimulation. — Adapted from Keener. CROW I'll AND MOVEMENT 455 tlu- motor organ lies along the central rib, between the two lobes of the leaf, and when an insect tone lies one of the three sensitive bristles on either face, these lobes shut together quickly like the jaws of a trap, and their interlocking teeth prevent the prey from crawling out easily. After a time the superficial glands pour out a secretion containing an enzyme that digests the proteins, and these are absorbed and utilized as food. After several days the trap again opens. Somewhat slower movements are made by the " tentacles " of Drosera (fig. 685). When an insect is entangled in the viscid secretion at the tips of these leaf lobes, its struggles furnish a stimulus which results in the incurving of all, until it is completely enveloped in their secretion, which then changes char- acter, bediming digestive, and so prepares the proteins for absoq)tion (see p. 388). Gravity movements. — Gravity cannot act as a stimulus unless the plant be displaced. If a potted bean plant be turned upside down or laid on the side, in a few hours the motor organs become curved so as to bring the leaves again into the usual position, or as near to it as possible. Photeolic movements. — The most striking movements are the regular ones produced by motor organs under periodic stimulation by variations in the intensity of light (and temperature). These have been known under the misleading name of " sleep movements," because they are notable at nightfall. However, they have no similarity whatever to tin- relaxed position assumed by animals in sleep, nor do they bring any recovery from fatigue. On the contrary, the nocturnal position is one of precisely as much strain as the diurnal one, since the resistance of the motor organ to bending is measurably the same; and even the position is as likely to be erect as drooping. Technically they have been called nyctitropic movements, but as the curvature is not a tropic one this term is objectionable, and the more so as the movements are quite as much associated with day as with night. They are best called photeolic i/r. light variation) movements, because the illumination is chiefly responsible for them, though corresponding fluctuations in temperature accompany the changes in light and sometimes cooperate in selling up the movement. Photeolic movements consist of a rising or falling, a forward or back- ward movement, of the entire leaf and (if the leaf be compound) of all the leaflets as well; or the leaflets alone of a compound leaf may exhibit such movements. The change in the leaves of the common purslane (figs. 686, 687) will make clear the general < haracter of these changes 456 PHYSIOLOGY of position, which are executed by differences of turgor on opposite sides of motor organs appropriately situated. Inasmuch as the changes in illumination are not sudden (in nature), it should be expected that the movements would not be restricted to morning and nightfall. In fact it can be shown that there is really a slow variation, so that in the brightest hours of the day the blades reach their highest or lowest posi- tion, the opposite being attained in the maximum darkness. As the changes in the inten- sity of the light are most marked at dawn and at dusk, the changes of position are then most rapid and so attract atten- tion. Persistence. — To these periodic vari- ations in light the plant becomes habit- uated, and even if they are not allowed to occur, as when a plant is kept in con- tinuous darkness or Figs. 686, 687. — Shoot of the purslane {Portulaca olcra- . ,. , , cea), photographed from identical position at 2 p.m. (686) continuous llgnt, tne and at 8.30 p.m. (687); note that the older leaves show little movements continue, change of position. - From photograph by Land. ^ diminishing am_ plitude, for a considerable time (3-5 days) before they cease entirely. The normal periodic stimulation seems to have impressed upon the protoplast a rhythmic variation in turgor, so that it cannot at once cease the customary action even when no stimulus demands a reaction (fig. 688). When these movements are ceasing, there come to view similar ones which are usually masked by the photeolic reactions. These, however, are autonomous ; they are much less extensive and have a much shorter period than the others. When sought, they can be observed even in the presence of the photeolic movements. They consist of a pendulum-like swinging of the leaf or leaflets, up and down (some- what as in Desmodium, fig. 684; see also fig. 689), whose advantage and effects are alike obscure. GROWTH AND M< >VEMENT 457 Advantage. — The benefits of photeolic movements have been vari ously imagined. They have been supposed to prevent injury to the leaves by frost, since the folded position diminishes radiation; or to prevent the formation of dew, so that transpiration may begin promptly in the morning. The difficulty with the firsl of these ideas is thai frost does not occur in the regions where Leguminosae, which exhibit them more strikingly than any other family, most abound; furthermore, a temperature approaching o° C. would render response impossible. The second explanation involves the assumption that transpiration is a valu- FiGS. 688,689. — Autographic rec- ords of leaf movements: dates and hours of the day are given below; 12 noon, 24 midnight; the horizontal median line represents the line the recording point would have described had the leaf remained quiet, moving neither toward the diurnal (day) nor tin nocturnal (i.igki) positions; the bla< k strips mark the periods of dark- ening, which have no relation to the natural alternation of day ami night; 688, photeolic movements of leaf of Albizzia lophantha; after a period of constant illumination the plant was subjected to l> hi. period- of alternating darkness and light, then to continuour light, and finally to 3 hr periods of alternate darkness and light; note the persistence in light t< ). t. 25-27) of the movements, which gradually disappear; 6S0, photeolic and autonomous movements of leaf of Phaseolus vitcllinus, the latter restricted to the reversed periods (lf illumination (<> P.M. to 6 A.M.); note the l.ii' of the response in the former. — After PFEFFER. able function which the plant promote?, instead of a danger that menaces its very life. It is difficult to conceive the significance of these movements in terms of welfare, and it is quite possible that they have tlotie. Other stimuli. - Changes in temperature, which often coincide and cooperate with changes of light in producing photeolic movements, may acl alone, and, when Sufficient, call forth like responses. Severe injury, even when wrought without mechanical disturbance, as by burning with a lens, will also stimulate the motor organs to curvature. So will a variety of other stimuli. 458 PHYSIOLOGY Growth movements and turgor movements. — The intimate relations that exist between turgor and growth, as well as the suddenness of their response under favor- able conditions, make it possible that the first curvature of tendrils (see p. 471) is due to quick alterations in the turgor of the cortical parenchyma. If this be true, the turgor curvature is followed promptly by unequal growth, to which irreversible process the more permanent and more important of the tendril movements are due. Their behavior will therefore be discussed in connection with growth movements. Indeed it is not improbable that turgor changes underlie all such movements, though they are not apparent. In many plants whose leaves have no well-defined motor organs there exist slight modifications of structure looking in the same direction, with movements of less extent than those executed by well-developed motor organs. Moreover, there are to be found similar movements in young leaves that have no trace of motor organs, but these movements cease by the time the leaves have attained mature size. (Compare young and old leaves in figs. 686, 687.) Doubtless growth, that is, irre- versible changes in the size of the cells, as contrasted with the reversible changes produced by turgor, cause these movements. From the foregoing it is evident that no hard and fast line can be drawn between the displacements due to turgor and those due to growth. In fact there are all gradations between them. Therefore, the separate treatment must not be per- mitted to establish in the mind too sharp distinctions ; for distinctions are valuable chiefly as conveniences to the memory ; they have usually slight basis in nature. 7. TROPISMS Growth curvatures. — It is a matter of common observation that the various parts of a plant have definite positions. If they are mechani- cally displaced, the usual position often is resumed after a time by cur- vature. Again, if some external force acts upon them from an unac- customed direction, a curvature may result, restoring the customary relations so far as may be. Some of these curvatures have been con- sidered; namely, those that are due to changes of turgor. But a much larger number are due to growth, because few plants have such a struc- ture as to permit turgor variations to move an organ. On the contrary, every plant has some part where growth is either in progress or can be initiated, and consequently a curvature can be induced, if by appropriate mechanisms the amount or rate of growth can be modified locally. Practically all plants have such mechanisms, which are set into operation by various external stimuli. It will be most convenient to consider these tropic curvatures according to the stimulus that induces the reaction. Parallelotropic and plagiotropic organs. — Observation shows that in certain plants the main axes respond to a tropic stimulus by placing them- selves parallel to the direction from which the stimulus acts, while other GROWTH AND MOVEMENT 459 parts, such as the branches or leaves, set themselves at a definite angle to the line of the stimulus. Other plain- may place even the main axes at an angle to the stimulus. This difference of behavior i- expressed by the terms parallelotropic and plagiotropic, applied to the organ concerned. Because responses to tropic stimuli lead so often to the erect position oi ax< , such axes were first called orthotopic organs, and their correlates were called plagiotropic, with reference merely to position. No confusion can arise from the substitution of the more specific term parallelotropic, and the use of plagiotropic in a somewhat modified sense. (1) Geotropism The stimulus. — No force acts so constantly and so equally in all parts of the earth and in all situations as gravity. It might he expected, there- fore, that it would have some influence upon the position that the parts of plants assume. If then- were nothing more to be observed than that the main stems of so many plants in all countries are directed away from the center of the earth, this would suggest the agency of some general stimulus. But it is easy to observe that as soon as a plant stem which usually grows erect is overthrown, curvatures occur in the younger parts that again direct the apex upward, though the older parts are unable to erect themselves. Fallen trees, and corn or other cereals beaten down by wind and rain, offer many examples, and the simplest experiments suffice to demonstrate the main facts; namely, that gravity is the stimulus, and unequal growth the end reaction. The first demonstrative experiments were conducted at the beginning of the last century, by affixing boxes to the rim of a wheel, which could be rotated either in the vertical or the horizontal plane, and planting seeds in these boxes. When the seedlings appeared on the vertically placed wheel, they seemed to have quite lost their way, growing in any direction in which they happened to be pointed when they broke through the soil; and some did not even emerge. On the horizontal wheel, however, no difference was apparent when it was rotated slowly; but when it was turned rapidly enough to introduce a considerable centrifu- gal acceleration (" centrifugal force "), the usual position of the axes was changed, the stems which would normally grow erect tending to direct themselves toward the center of the wheel, and the primary root-. which usually grow downwards, glowing toward the periphery ; and these tendencies were the more pronounced the more rapid the rotation. This mode of experimentation is universally used when one wishes to equalize or modify the u> tion of any one-sided stimulus. In all such experiments it is essen 460 PHYSIOLOGY tial to arrange the plants so that the only factor in their environment that is altered is the direction from which the stimulus acts. The clumsy wheel has been replaced by the modern cliiwstat, a disk to which potted plants can be conveniently attached and capable of rotation in any plane, continuously or intermittently, at a very even speed • by strong clockwork or by a water or electric motor. The centrifuge is a modi- fication whose disk is driven at a high speed when centrifugal acceleration is to be compared with gravitational. Parallelotropic organs. — The behavior of parallelotropic and plagio- tropic organs differs in certain particulars. The former will first be considered. Parallelotropic stems in responding to gravity curve so as to erect their apices when displaced. Primary roots, which are usu- ally directed straight downwards, when displaced respond by turning the tip toward the earth. These responses, in quite opposite directions, arise from an identical original stimulus. By some mechanism within the plant body the end reaction is made different. It is convenient to dis- tinguish the difference by assuming some difference in the sensitiveness. So the special term positive geotropism or progeotropism is used to desig- nate the property by which the growing point is directed toward the center of the earth, and negative geotropism or a po geotropism that by which the tip is turned away from it. The curvature might be due (a) to unequal retardation of growth along both sides; or (b) to unequal acceleration of growth along both sides; or (c) to an unchanged rate of growth on one side with either acceleration or retardation on the other ; or finally (d) to simultaneous retardation on one side and acceleration on the other. It has been determined that usually, in both stems and roots, gravity accelerates growth, but the segments are unequally affected according to position (case b). In the one case (apogeotropism), the lower side is caused to grow more rapidly than the upper ; in the other (progeotropism), the upper side grows more rapidly than the lower. How this difference in action is brought about is quite unknown. Course of curvature. — The course of curvature in a parallelotropic stem continuously stimulated by being laid horizontal shows an interest- ing example of " after-effects." The reaction time is usually some hours in length. When the apex has reached the erect posture again, it might be supposed that it would go no further. On the contrary, it is carried past the vertical, responding to the excitation set up some hours before. Being thus carried beyond the position of equilibrium, it is stimulated 1 Otherwise any exact experiments may be vitiated by errors due to unequal stimula- tion, a common fault with makeshift clock clinostats, which suffice, however, for elemen tary demonstrations. GR< 'Will AND M< <\ i Mi N T 461 to a reverse curvature, ami tliis also, by rea on of continued stimulation during the long reaction time, may again carry the tip past tin- vertical; thus, only by a series of pendulum-like swings is tlu' position of equilibrium at- tained. The succes- sive positions of the stem of Impatiens shows the way in which such a stem erects it- self (fig. 690). It shows also that the curvature begins in the region of most active growth and gradually affe< ts 1 h.. 6()o. — Successive positions, from photographs, <>f Impatiens glanduligera in erecting itself from tin- horizon- tal.— After PFEFFER. less active regions, becoming permanent finally as the tissues of the growing region most remote from the apex cease to grow. That the curvature appears in the region of most active elongation is clearly shown by the behavior of certain roots. If a suitable one be marked at intervals of 1 mm. ami then fixed in a horizontal position, it will be found aftersome hours ^-*- — ",_~~' that curvature is taking place in the third and fourth of these f?C 692 divisions; after twenty-four hours it is easy to see that the i/ V. sci ond and third divisions have grown most, though the c hief urvature still persists in the fourth division that was grow- ing most rapidly (figs. 691-603). Presentation time. — It is not necessary to con- tinue stimulation until the reaction appears. In other words reaction time is longer than presenta- tion time. These periods are, of course', very sari- able. The shortest presentation time recorded for geotropic curvature is 2-3 minutes (cut shoots of Capsella, hypocotyls of Helianthus, and peduncles placed horizon- <>f PlatUago). In many plants it is 15 25 minutes; t.d; 69a, seven hours jn |t._ sensitive plants it is double or treble this, or later ; 6,,;,, twenty- ' three hours later.— even extends to several hours. Moth periods are After S.miis. greatly inlhicnced by temperature. Thus, a seedling of Vicia Faba, having at 140 C. a presentation time of 70 minute- and a reaction time of 120 minutes, hail these periods at 300 C. respectively Figs. 601-60 3. — ( reol ropii 1 urvatureof a root of Vit iii Faba 60 4(>2 PHYSIOLOGY 10 minutes and 48 minutes. The longer the stimulation, other things being equal, the more marked the curvature ; from which it is evident that there is an increase of the excitation with continued stimulation, and thereby the end reaction becomes more marked. Summation. — Contrariwise, it should be expected that stimulation too short to result in curvature would not be without effect. That it does produce excitation is shown by the fact that if a plant be placed alter- nately horizontal and erect, each period of stimulation being shorter than the presentation time for that particular plant, and the interval of rest shorter than is needed for recovery, curvature will finally occur. Evidently this is a cumulative effect; yet it is not a summation of the total successive excitations that occur during the times of horizontality, a j- but only of the re- sidual excitation. For, if a suitable plant be placed horizontal for 30 minutes continu- ously, the reaction curvature is more C pronounced than Diagram: for explanation, see text. .? .. , 1 j 5 ^ if it be so placed for ten 3-minute periods at 10-minute intervals. Clearly, while erect, the preceding excitation is slowly disappearing, and if the interval before the next stimulation is too long, recovery will be complete and no evidence of the excitation will appear in the form of curvature.1 In such experiments, therefore, it is necessary to apportion properly the intervals of rest and stimulation. Rotation. — From the above considerations it will be evident that when a plant is rotated in the horizontal plane on a clinostat, its failure to exe- cute any curvature is not at all due to a lack of excitation, for while the side a of the stem is passing through quadrant A of its rotation (fig. 694), quadrants a and c are under stimulation almost as though for a corre- sponding time the stem were at rest. But these sides remain under stimu- lation for less than the presentation time and so the excitation does not suffice for the end reaction. When side a has passed into quadrant C 1 It has been suggested that during the periods of no stimulation a counter-excitation is set up; but simple recovery from excitation seems sufficient to account for all the facts known. The process is apparently analogous to recovery from fatigue. GROWTH AND MOVEMENT of its rotation an the greater the more opportunity there is for summation; and if the responses win- not contrary the one to the other, curvature would show itself. The net result upon the rotated plant is that growth is at first accelerated as compared with a control plant rotated in the vertical plane; but long- continued rotation leads to fatigue and no response. Position of equilibrium.— In order that a parallelotropic axis be in a position of stable equilibrium with respect to gravity, it must not only be parallel to its direction, but the stem must be erect and the root pointed downward. There is a polarity which must be conserved. Though the strictly inverted position for either roots or stems is one of little stimulation or possibly of none, it is a position of such instability that the slightest deviation leads to stimulation, which results in decided < ur- vatures and recovery of the normal position. Much study has been given also to the position of maximum stimulation. The general results are most strongly in favor of a oo° deviation from the normal, as agai-Hst 1 3 5° or any intermediate angle. Variable intensity. — By comparing centrifugal acceleration with that due to gravity, it has been shown that it produces the same curva- tures. So while it is not possible to alter appreciably the intensity of gravity, it is possible to vary this corresponding stimulus. Experiments in this line show that as the centrifugal acceleration is increased or di- minished, the reaction time is shortened or lengthened, but whether pro- portionately or not is uncertain. Thus, in earlier experiments with a root of Vici gravity, 464 PHYSIOLOGY The most thorough experiments, however, have been made upon roots, and these seem to show that perception takes place mainly in the very tip, within a zone little more than a millimeter long, including the root cap. Indeed, the inner portions of the root cap itself are believed to be the cells most concerned. But the results also show that the growing region has some perceptivity. This conclusion rests upon evidence derived mainly from three modes of experi- mentation: (a) Decapitation. Cutting off the terminal millimeter or two leaves the root still capable of weak response, after recovery from the shock, (b) Me- chanical deformation. If root tips are made to grow into glass slippers (figs. 695, 696), or against a glass plate, so that the terminal millimeter is bent at right angles to Figs. 695, 696. — Roots of Vicia Faba with tips in glass slippers : 695, a, b, c, three stages in the curvature of the same root, o to 20 hours ; 696, a, b, two stages of the same root ; b, 1 8 hours after being placed in position a. — After Czapek. the body of the root and therefore can be placed in the position of stimulation while the rest is not (or vice versa), responses show the dominance of the excitation at the tip over that in the growing region ; but the conclusion that the tip alone is per- ceptive is not warranted, (c) Rotation. Experiments in which the roots are fixed on a centrifuge, deviating 1350 from their normal position, permit responses to be varied at will, according to the extent of the root tip beyond the axis of rotation. In all cases, if the stimulus to the growing region is to determine the response, it must be several times greater than is needed at the tip. Anatomical facts, in con- nection with the statolith theory of geoperception, support the physiological evidence above cited (fig. 697). Statolith theory. — In its original form this theory was purely specu- lative. It postulated in the protoplasts of perceptive cells minute vacu- oles, beyond the limits of microscopic vision, filled with a fluid in which there lay granules of slightly greater specific gravity, that would fall to the bottom of the vacuole, whatever position it occupied, and rest against the cytoplasmic membrane bounding it. In the normal position of parallelotropic organs this would lead to no excitation; but if the cells GROWTH AND MOVEMENT 465 were displaced, the granules would settle upon a new and excitable side of the vacuole wall, starting into action the mechanism of the end re- sponse. There are many objections l<> this form of the theory, which was suggested by the visible otocysts of Crustacea, and the appearance of the centrosomes, which were then supposed to be common in the cells of seed plants. In a more concrete form the theory has much to commend it, though it cannot yet be considered as firmly established. In this form no in- visible structures are predicated, but the principle is the same. Certain Figs. 697, 6q8. — Perceptive regions: 697, median longitudinal section of the rootcap of Roripa amphibia ; d, dermatogen; 698, apex of the coleoptile of the plumule of Panicum miliaceum. — After Ni'.mi.c. cells, notably those of the inner median portions of the root cap (fig. 697), the tip of the coleoptile in grasses (fig. 698), and a layer around the vas- cular cylinder in stems, contain rather large starch grains in such abun- dance as to attract attention. Moreover, these starch grains are freely movable, and in whatever position the organ rests they accumulate on the physically lower side of the tells. They seemed to answer the re- quirement for bodies Heavier than the fluid in which they lie, and there- fore capable of setting up an excitation by coining to rest upon a part of the protoplast unaccustomed to their contact. It is assumed thai cer- tain areas of the protoplast are properly sensitive; thai their excitation will start into activity the mechanism of curvature, which will eventually restore the organ to its normal position and so remove the irritating starch C. B. & C. HOT ANY — 30 466 PHYSIOLOGY grains from excitable areas, tumbling them again upon the side corre- sponding to the position of equilibrium. These mobile grains are called statoliths and the cells containing them statocysts, after the analogy of the otocysts of the Crustacea, once thought to be organs of hear- ing, but now recognized as organs of equilibrium. The semicircular canals of the ear of vertebrates, with their fluid and mineral granules, have a similar function, giving the animal a sense of position or equilibrium. Extensive anatomical studies have shown a remarkable parallelism between the presence of such grains and geotropic sensitiveness. Almost without exception, geotropic organs have mobile starch grains, while non-geotropic organs lack them. Moreover, when an organ, placed under unfavorable conditions (e.g. low tem- perature), has lost its mobile starch, it seems at the same time to have lost its geo- tropism, which is regained simultaneously with the rebuilding of the starch grains and not until then, although conditions favorable for response (had perception been possible) may have existed for much longer than the usual reaction time. This method of experimentation is, indeed, open to some objections ; but the most serious one, namely, that the unfavorable temperature which determines the removal of the starch at the same time suppresses the geotropic irritability, is largely obviated by the fact that perception and the end reaction can be separately interfered with. Thus, by a temporary reduction of temperature, perception is not interfered with, for upon again raising the temperature with no further stimulation the end reaction proceeds as usual. Further, it is executed more promptly after the restoration of favorable temperature than it is when the low temperature is first used to eliminate the starch, and then at a favorable temperature stimulation is attempted. This in- dicates that the failure to obtain the curvature when there is no mobile starch is due to an interference with the mechanism of perception rather than with the mechanism of transmission or of growth. Plagiotropic organs. — The erect position of certain organs is not necessarily determined by gravity alone, but may be due to the coopera- tive action of other stimuli. In like manner the oblique or horizontal position may be determined wholly by a response to gravity, or by some other single stimulus, or by simultaneously acting stimuli. Experiment alone can determine the agents in each case. Among plagiotropic or- gans which owe their position to gravity, some rhizomes that run hori- zontally beneath the surface of the ground are noteworthy. When such a rootstock is displaced by directing the tip obliquely upward or down- ward, curvatures ensue, precisely as in the case of parallelotropic roots, though, of course, the growth is much slower. This mode of reaction is known as transverse geotropism or diageotropism, corresponding to the positive and negative geotropism of parallelotropic organs. Quite similar behavior is to be seen in some peduncles, which are pendulous while the flower is in bud, but become in bloom horizontal, and in fruit GROWTH AND M<»\ I MEN I' 407 erect (figs. 1105-1197). When the change of position can be shown to be due wholly to gravity, this indicates that the peduncle undergoes with age a change in its mode of response. Well known examples are offered by the snowdrop and the wind flower. Less generally known are like changes in direction when certain stems, erect in the seedling stage, develop into horizontal rhizomes in an older stage. Diageotropism. — Diageotropism of a somewhat modified type is seen in the branches of the primary roots of some plants. These grow out at a definite angle, and, if displaced, they will curve until the normal angle is again attained. Similarly the oblique branches of trees some times are decidedly geotropic, and even the pendent ones may show it. Only by the most cautious and precise experimentation in each « ase can it be ascertained whether the positions assumed are due to gravity. Unwarranted generalizations in this direction are particularly seductive. In far the greater number of cases the position of organs is determined by a complex of stimuli most difficult of analysis. Twiners. — Among the most interesting of the complex phenomena are those exhibited by twining plants, in which geotropic reaction of a peculiar kind plays a most important part. Twiners have slender stems with a very long growing region, and a tardy development of the lateral organs (leaves and branches), so that the long tips often look quite naked. These ends seem to travel in a spiral fashion around some suit- able, slender support, and the mature plant is thus wound around it and clasps it tightly. At the outset the seedling, say of a morning glory, grows quite erect, and seems like a parallelotropic plant, as, indeed, a study of its reactions with a clinostat shows it to be at this period. After reaching a certain height the tip no longer grows erect, but declines to one side, and then a movement begins, quite like the irregular nutation that every erect plant makes, except that it is regular and more striking. The tip, standing in a nearly horizontal line, swings steadily around and is directed successively to every point of the compass. This may bring it into contact with a suitable support, around which it then proceeds to twine, the free tip continuing the swinging movement from the point of contact with the support. The fundamental feature of the twining, therefore, is the swinging motion. Lateral geotropism. — Since the swinging movement does not con- tinue when 'a twiner is properly rotated on a clinostat, it must be con- sidered a response to gravity. As growth thai can swing the tip sidewise can be effective only if it takes place on the Hank, the inference is made 468 PHYSIOLOGY that the stimulus, instead of finally affecting the side of the stem next the earth, as it does in the younger stages of development, now affects the flank, determining there more rapid growth. According as the right flank or the left grows faster, the tip will be swung like the hands of a clock or in the opposite direction. The twining may then be desig- nated as clockwise or counter-clockwise (see Part III, fig. 957). There is no fundamental reason, apparently, for one direction rather than the other. While usually the same species of plant twines always in the same fashion, closely allied species will differ in this; there are some species that twine indifferently in either direction; and there are a few in which the individual plant may change the direction of twining in the course of its development. Rotation and revolution. — When growth of a given flank has swung the free tip around, this very act, by twisting the stem on its own axis, brings a new segment of the stem into the flank position and so exposes it to excitation. This may be understood by representing the stem by a hexagonal pencil. If the side on which the name is stamped face the right with the pencil horizontal and the point away from the body, then this right flank may be imagined to be the one whose growth is accelerated; by that the point would be swung to the left, and by the time it has passed over oo° the pencil would be rotated on its axis through 90°, so that the stamped side would now face upward and the angle that was first at the bottom would now be the flank. This rotation may be imitated, if it cannot be seen to be a mechanical necessity when a horizontal portion of an erect stem is so rotated, by sticking the end of a pencil into a piece of rubber tubing just stiff enough to bend into a quadrant under its weight. Now upon swinging this apparatus without torsion, as can be done by holding the end of the tube and pushing the test pencil around with another, the rotation will become at once evident, being complete when one revolution is completed. The new flank thus brought under the influence of gravity has its rate of growth increased, which swings the tip further, rotates the free part of the axis, and so brings another segment into the flank position. Given the sensitiveness of the flank to gravity, the revolving movement follows as a necessity. The support. — When a stem is swinging thus, if it come into contact with some obstacle near the tip, flexure may carry it past the object ; but if it strikes the obstruction further back, the bending may not be sufficient to carry the axis past the obstacle, particularly if it be of moder- ate size. Instead, curvature will soon occur in the part projecting beyond it, and the revolving movement will be continued by the apical GROWTH AND MOVEMENT 469 portion, which steadily wraps itself around the support. In the nature of the case it is not possible for twiners to wrap about large supports, nor those that are too nearly horizontal. Plants differ much in their capacity in these two poim-, a difference which depends chiefly upon the relative length of the growing portion of their stems, and consequently upon the precise distance of the most actively growing region from the apex. Few twiners encircle supports more than 15 cm. in diameter, or those that lie nearer the level than 450. Straightening. — The coils that a twiner forms at first are loose and of low inclination. Later they become steeper and hug the support tightly. This seems to be due to a return, in the last stages of growth, to the apogeotropism that they possessed in the seedling period, so that the stem starts to erect itself, with the effect stated. Very commonly the surface of the stem is rough, being ridged or angled or furnished with stiff hairs, which prevents slipping from a support too easily or sliding along it. Inspection of the stem in the regions no longer growing shows that it is twisted, the longitudinal ridges coursing spirally around the axis in a direction the reverse of the twining. This torsion is mainly the result of the final erection of the stem, though other causes cooperate to increase or diminish it. This also is a mechanical necessity of the behavior. It can be imitated by coiling a long piece of rubber tubing on a tabic, marking a crayon line along the upper surface, and then lifting the inner end of the ( oil while the other end is held on the table, both ends being prevented from twisting in the fingers. Then it will be seen that the line apparently passes spirally around the tube, because the latter is twisted by the steepening of its coils. The tardy development of the leaves and branches is very evidently an advantage in twining, for they would greatly impede the revolving movement and the subsequent tightening of the coils. When tin- branches do develop, they show the same behavior as the main axis. This explanation of twining is not wholly satisfactory, because there are details of the process, and some features that appear only under experiment, that are no! clearly accounted for ; but it is far the best of the many theories that have been pro- posed, and in the major outlines that have been presented here it iscertainU (2) THIGMOTROPISM Tendrils.. — Many plant- are sensitive to mechanical stimuli such as contact <>r friction, as shown by the alteration- of the rate of growth thai lead to curvature. This phenomenon i- tkigmotropism. The 470 _ PHYSIOLOGY tendrils of climbing plants exhibit the most remarkable sensitiveness to mechanical stimuli, and it is by this means that their attachment to supports is secured. Tendrils are slender, even threadlike, lateral or- gans, branched or not, sometimes occupying the usual place of a branch, sometimes that of a leaf or of one or more leaflets of a compound leaf. They are therefore formed successively with the development of the main axis and its chief branches, so that the plant is constantly laying hold of a support by younger and younger tendrils. It may thus climb to great heights, while the main axes remain very slender and wholly unable to support their own weight, much less that of foliage, flowers, and fruit. The most important feature of the tendril is its irritability to contact, and the curvatures which follow as end reactions. Behavior. — When a tendril is young and only about one fourth grown, it may be either straight or curled up into a loose spiral, of which the convex surface corresponds to the under side. If coiled, it unrolls as its period of rapid growth begins, at which time also begin nutating movements that are almost as regular as the revolving movements just de- scribed in the twiners. These tendril movements, however, are not due to any known external stimulus, but must be called at present autonomic. The tip is thereby swung in all directions and is thus likely to come into contact with some suitable support. When it does so, it quickly wraps around it. After a time, through continued and unequal growth in length, spiral coils appear in the region between the axis and the attached part, increase in number and closeness, and become more and more firm, until this part has become a veritable spiral spring by which the plant is slung to its support. These results are attained in the following way: Stimulus : friction. — The tendril is sensitive to contact, usually throughout its whole length, and on all sides, but most so towards the tip. Yet it is not sensitive to contact in the narrow sense; it is because things come into contact with the tendril in more than one place when they touch, so that it is only by multiple and successive contacts, and usually by shifting contact or friction, that the tendril is excited. Liquids (even the heaviest, mercury), if entirely free from solid particles, and perfectly smooth solids, like gelatin, do not produce excitation. Rain, therefore, does not cause useless movements of tendrils. But very slight rubbing movements of excessively light objects suffice to start them. It has been found, for example, that a bit of thread, weighing by estimate only 0.00025 mg., if moved by the wind over a very sensitive tendril, will induce curvature. CROW III AND MOVEMENT 471 While the tendril may be sensitive throughout, the responses evoked by excita- tion ditTcr sometimes according to the region stimulated. Thus, a stimulus applied to the " under " side, which at the time <>f greatest sensitiveness has usually grown near the apex a little less than the other, so that at the tip it is slightly concave, results in a curvature. So also does stimulation of the Banks, and in some tendrils that of the upper side too. But there are some others whi< h give no sign if rubbed on the upper side, except that stimulation there will inhibit a simultaneous stimula- tion on the under side, which ordinarily would result in a curvature. Primary response. — The first result of slight rubbing contact with a suitable support (that is, one that is small enough fur the tendril to en- circle, no matter in what position it stands) is a prompt curvature. In sensitive tendrils under favorable conditions this follows in the course of a few seconds (5-30), but in others in a few minutes. The facts ob- served are that the cells on the convex side become suddenly consider- ably elongated, while those on the concave side become somewhat short- ened. This and the promptness of the end reaction suggest a turgor change, and many observers have concluded that such is the mechanism of the primary curvature, and that it becomes fixed later by growth. Others attribute these results to a very rapid and extraordinarily sudden growth of the cells of the convex side, and to the consequent compression of those on the concave side. It is not improbable that the truth in this, as in many similar recondite and much controverted matters, will prove to lie between the contentions. So it may very well be that a turgor varia- tion begins the movement, whereupon growth follows it up more promptly than usual, and extends and completes the encircling of the support. Secondary response. — After the tendril has become firmly attached, the excitation extends toward the base of the tendril, producing an in- equality of growth on the opposite sides (in this case the " upper " side becomes the convex one) that throws this part of the tendril into coils (see Part III, fig. 958). This coiling may be rudely but essentially imitated by placing in a pan of water a narrow strip, slit from the scape of a fruiting dandelion which has not attained it. full height, and by pinching eat h end in a short folded piece of sheet lead to prevent twisting. After a few hours the strip will be found coiled into a spiral, with one or more reversals of direction just as in the tendril, though more irregularly. Here the tissue next the pith cavity grows and becomes more turgid than the epidermal and cortical tissues. The reversal of coil is a mechanical necessity if the ends are not free to rotate. These coils are not merely the result of continued growth of the tendril; for if one not full grown becomes attached, it does nol rea< It it- possible 472 PHYSIOLOGY maximum length, but from that time grows only in such a way as to throw it into the spiral coils. One which does not become attached grows longer and longer, but finally shrivels, usually without coiling. Soonei or later, upon the cessation of this second phase of growth, the phase of maturation is marked by the development of mechanical tissues, which add strength to the elastic coils. The nature of the stimulus that brings about the final coiling is uncertain. It may be the strain from the weight of the plant after becoming fastened, or the spreading stimulation from the contact pressure (for the attachment coils compress the support), or some unsuspected stimulus may be brought into action. There are many other stimuli which will evoke reactions from the tendrils, but none which in nature has any importance. Sensitive petioles. — There are other plant organs that behave in a similar way to the tendrils, though none of them is so sensitive. The petioles of Clematis and of the climbing Tropaeolum, or "nasturtium," are familiar examples. While such petioles do not wrap themselves around the support nor form spiral coils as well as a tendril does, nevertheless they are efficient prehensile organs, enabling the plants to climb high. Dodders. — Any account of twining and climbing plants would be incomplete without mention of the dodders (Ciiscuta), leafless yellowish parasites that wind their stems around and clamber over erect herbaceous plants, sending haustoria into their stems, whence they obtain food and water. In the first stages of development, the species that have been studied germinate in the soil, and the young seedling behaves as a twiner; but shortly after it has found a suitable host and begun to twine around it, the lower part of the stem dies away, while the upper part continues its growth at the expense of the host. The further twining, however, in- stead of being dependent upon gravity, is the result of a contact stimulus like that which enables tendrils to secure a hold, so that the parasite enwraps supports in all sorts of positions. In the possession of these two modes of response at different periods of development, the dodder? are unique (see further Part III, fig. io8t). (3) Traumatropism The wounding of plants produces immediate reactions, mostly invisible, but root tips may be so wounded as to lead to curvature. If an active tip be branded on one side with a hot iron or glass rod, or if it be similarly cut or otherwise injured, the tip will turn to one side. When the injury is severe, this may so seriously impair the tissues on the injured side that their growth will cease, and the injured side will become concave near the point of injury, because there the tissues shrivel and the growth of the other side goes on. This is not a true reaction, since the result is GROWTH AND MOVEMENT 473 due merely to mechanical interference with growth. On the other hand, if the in- jurs- is one that <1y the wound lias spread thence to the region of most rapid growth, inducing a true tropic curvature. After experiments by attaching bits of cinder, paper, and the like to root tips by means of gum, ii was believed that the root tip, by its sensitive- ness to contact, was a sort of direi live organ, whii b 1 ould feel its way through the soil, and avoid injury. I'.ut in these experiments the gum injured the cells, and it, not the attached particle, was the stimulating agent, so that the response was ac- tually to injury and not to contact. It is not probable that sensitiveness to injury is of any advantage to the plant, as it undoubtedly is to a conscious organism. ( Occasionally, of course, traumatropism might he advantageous to a plant in getting a root tip once injured out of immediate danger of further injury. (4) Rheotropism Roots grown in a current of water of adequate veloi it y may respond by directing their tips against the current. In this case the stimulus might he the strains set up by the pressure of the current, or the impact and friction of the water parti* les against the surface. Its precise nature is not satisfactorily determined, hut it seems to be the pressure of the water and the resulting strains rather than mere contai t or impact. The whole of the growing region seems to he sensitise, and not the tip ali tne. It is not apparent that this reaction can have any significance for the plant in nature. (5) Chemotropism Of fungi. — Chemical compounds may not only be usable in repair and constructive work, but may so affect the living substance and its chemism as to act upon it as stimuli. Since by diffusion they may act from one side, these stimuli may be directive, causing curvatures toward or away from the source, which are manifestations of chemotropism. Very striking reactions to chemical compounds of many sorts have been ascribed to the hyphae of fungi and to pollen tubes. Chemotropism of the latter may be maintained still, as it has not been seriously im- peached; but that of fungus hyphae has been brought under suspicion by the latest researches, and may be cither established or disproved by further study. For the hyphae to be sensitive, especially to carbohy- drate and other foods, would be of much service in inducing them to grow in directions that would bring them into favorable feeding regions, and precisely this power has been as< ribed to them. For instance, when certain fungus spores are sown in a layer of gelatin containing no nutritive materials, between layers of gelatin, on the one side with nutritive ma- terial and on the other side without, it is reported that the hyphae turn toward the layer of nutritive gelatin. The same reaction was found to 474 PHYSIOLOGY occur when the central layer contained food, provided the outer layer had enough more of the same to act as a stimulus. (In this case the ratio had to be about 10 : i. See Weber's law, p. 448.) Likewise the hyphae grew through fine perforations in thin plates of mica or celluloid, when the nutritive gelatin was thus separated from the other, suggesting the way in which fungus hyphae, arising from spores on a leaf, turn into a stoma and so find their way into the interior of a leaf of their host. In fact, when leaves were injected with a solution of food, like sugar, fungus hyphae of many kinds are reported to turn into the stomata, though they do not naturally grow on the leaves used. A great variety of substances were tested in similar ways. Some proved to be attractive, some repel- lent; and the reaction varied according to the concentration of the solute, though generally the hyphae were injured before the limits of concen- tration for repelling effects had been reached. On the other hand, an apparently careful repetition of many of these experiments gave negative results, in that the numbers of hyphae reacting positively is so slightly in excess of the number indifferent or negative, that the results seem scarcely more than chance, or ascribable to other than the cause assigned heretofore. A complete restudy of the matter will be necessary. Of pollen tubes. — When pollen tubes are developed under a cover glass in company with a bit of the stigma of the same plant, they turn toward it, from whatever direction they first issue. An ovule or a bit of the wall of the ovary is likewise attractive. Investigation shows that soluble carbohydrates and proteins are here the attractive substances. It seems likely, therefore, that the growth of the pollen tube toward the ovules is directed by the diffusion of such substances, which are always found in these organs. (See the chemotaxy of sperms, p. 448.) Aerotropism. — A special form of chemotropism has been called aerotropism, and was first ascribed to roots. When certain gases, especially oxygen, diffuse against young roots from one side, it is reported that the root curves toward the Source of the gas. These results also have fallen under suspicion. Recent investi- gations are conflicting ; and one is left in some doubt whether to ascribe the curva- tures to a true reaction to gases, in accordance with the weight of evidence, or to moisture, in which case they belong to the following special category of chemotropic response. Stems also have shown sensitiveness to O2 and CO2, and it may be that aero- tropism is more general than has heretofore appeared. It is not evident that it can be of any great advantage to either roots or stems, except, perhaps, those of swamp plants. GROWTH AND MOVEMENT 475 Hydrotropism. — Another special form of chemotropism, which has been named hydrotropism, designates the sensitiveness of root-, the hyphae of some fungi, the rhizoids of liverwort-, et< ., exhibited by turn- ing toward or away from the source of diffusing water vapor, or capil- lary water in soils. When seedlings are grown in an atmosphere less than saturated with water vapor, so that the roots, as they grow, pass further and further away from a wet surface,1 it will be found that they deviate presently from the perpendicular, inclining toward the wet surface; soon again they turn downwards, but once more return to the moisture, and this may be repeated many times. Plainly the root- are subject here to two stimuli acting nearly at right angles, gravity and the diffusing vapor. First the one dominates and then the other. Were it not for the long reaction times the root might be expected to take an inter- mediate direction, the resultant of the effects of the two stimuli; but as in the case of gcotropism alone (see p. 460), the after-effects carry the root tip past the position of equilibrium, whereupon the other stimulus gives it such strong and long excitation that its after-effects carry the root again past the equilibrium point; then the gravity stimulus comes upon it again; and so it weaves back and forth. The vegetative hyphae of the mold fungi may show positive hydro- tropism and their sporangiophores negative hydrotropism. It can easily be shown that the rhizoids of Marchantia, which normally grow straight downward, will deviate toward a moist surface in the same way as roots; only the moisture stimulus is dominant over gravity. Roots in the soils also grow towards the moister regions, and especially do they tend toward tile drains, into which they may penetrate, often branching profusely enough to plug up the drain completely. Part of this direc- tive effect may be due, and probably most of the branching is due,2 to chemical stimulation by the solutes. (6) Phototropism Stimulus. — Of all the external conditions that act upon plants, light is one of the most variable, for from time to time it differs in direction, in intensity, and in quality. Quite apart from its fundamental relation to all life jn furnishing the energy for food making, arc it- effects as a stimulus. Whereas the most effective quality of light for food making 1 As by planting them in < oarse sawdust held in plat e on the under Burfai e of an ini lined board by bobbinrt. 2 In which case this is .1 morphogenic effect See p. 1.35. 476 PHYSIOLOGY is the red-yellow, the most effective light as a stimulus is that near the violet end of the spectrum. Since this is the region of least energy, the shortness and frequency of the waves are the important features of light as a stimulus. In this respect the red end of the spectrum, though its energy is far greater, behaves as darkness. Response. — In general the response of plants to light differs according to the usual attitude of the organ and its mode of growth, for which indeed light is largely determinative. Parallelotropic organs respond by directing their tips toward or away from the source of light, while plagiotropic organs place themselves more or less at right angles to the direction of the rays. Primary stems, therefore, are mostly positively phototropic, and some roots, particularly aerial roots, are negatively phototropic; while leaves are mostly transversely phototropic or diapho- to tropic. These phenomena were first known as heliotropism, etc., and are often still so called, because the sun in nature is the source of all light. It seems better, however, to use the wider term, since plants respond in the same way to artificial light, which is so largely used in experimental work. The general result of these reactions is the same as of those to gravity, so far as the same organs are sensitive to both stimuli, though the two act from opposite directions in nature. Intensity. — The intensity of the light may determine either a positive or a negative curvature, and within certain limits between these two there is a range of intensity which calls forth no visible reaction ; this is the point of phototropic indifference. It is by no means the point of no excitation. At high intensities that call forth negative curvature, injury soon appears. Near the lower limit of intensity that can produce an end reaction, plants show themselves very sensitive to light. Thus, radish seedlings respond to the light of a single candle at a distance of about 8 m., the broad bean (Vicia Faba) at 22 m., and a cress (Lepidium sativum) at about 55 m. The differences that plants can distinguish are within the limits of error for the unaided eye, and are not very easily distinguishable even with the photometer. Time relations. — The presentation time, of course, depends upon the intensity of light used, and is approximately inversely proportional to it. The greatest range of presentation time recorded is that for etiolated seedlings of oats, being 0.001 second with light intensity of 26,520 Hefner candles, and 13 hours with light intensity of 0.000439 Hefner candle. Intermediate light intensities give corresponding inverse proportional intermediate presentation times. As a rule the younger an organ is, the GROWTH AND MOVEMENT 477 more sensitive it is; but this is by no means universally true. The re- action time varies fn>m a few minutes in some hours, depending upon tlu- temperature, the intensity of the light, and the general condition of the plant. Reversal. — The reactions to light also are often reversed with age. This is especially seen in flower Stalks, which at the time of blooming are likely to be positively phototropic, but later, during the ripening of the fruit, many become negatively phototropic, carrying the fruit under the leaves or even into crevices of the soil or rocks on which the species grows. Mechanism. — The mechanism of the response is the same as in geo- tropism, and occurs in the same region; namely, that of most active growth, where one side grows more rapidly than the other, leading to a curvature whose tendency is to direct the axis into the line of the light rays. This inequality of growth is brought about by its acceleration on the convex side and by simultaneous retardation on the concave side. These changes in rate are not due to the fact that the rate of growth is retarded by light (see p. 435), for this (apparently applicable to posi- tive phototropism and once an accepted explanation) could not ac< ount for the acceleration on the convex side, nor for any of the changes in negative phototropism. The reaction is determined by the mechanism of the parts concerned and not by the direct influence of the stimulus. Perceptive region. — In many phototropic reactions there is a distinct perceptive region, a propagation of the excitation, and an end reaction in a different region. Thus when seedlings of millet raised in the dark are exposed to lateral illumination, the sharp curvature that presently appears in the axis (" hypocotyl "), which is rapidly growing, can be shown by appropriate shading to owe its origin to the stimulus perceived by the leaf at the tip ("cotyledon ") and not to excitation of the axis itself. In a similar way the seedlings of oats show that though the w hole of the subaerial part is sensitive t<> light, the tip is much the most so, and that excitation, spreading thence downward, dominates even con- trary excitation set up in the lower parts. What is perceived? — Nothing is known as to the mode of perception or the structure of the perceptive organ. Indeed, it is not entirely cer- tain what sort of stimulus the plants perceive; whether it is the direc- tion of the rays, that is, the- line of propagation of the waves, or whether it is inequality of the illumination of different -ides. It ha- even been suggested, in casting about for something tangible, that plants distin- guish between the different pressures in the lighted and shaded portions! 478 PHYSIOLOGY It has been shown that the impact of the ether waves of full sunlight produces a pressure equal to about half a milligram per square meter. In a seedling of oats at this rate the plant would have to be sensitive to a difference of five millionths of a milligram and probably to one tenth of this infinitesimal amount. This is simply inconceivable! It seems most likely that it is the difference in the lighting that is per- ceived, for the intensity of the stimulus has an important bearing on the form of the reaction, and plants are able to respond to differences of illumination coming from different sides that are too small for the eye to distinguish. Plagiotropic organs. — The behavior of plagiotropic organs toward light is especially interesting, because it seems to be usually of the very greatest importance for the welfare of the plant in food making by leaves, thalli, etc. The fact that the leaves of most common plants, set before a window, place themselves at right angles to the incident light, attracts attention at once. If the pots be turned around, the position of the leaf blades will soon be changed, and they face the window again. Thus the leaves obviously come into a position most advantageous for receiving the maximum of energy for photosynthesis. The corresponding orientation in the open shows that it is not the direct sunlight alone to which the leaves respond, but rather what may be distinguished as sky light; that is, the brightest diffused or reflected light. Indeed in some cases the direct sunlight is evidently too intense, and the plane of the blades is set at an angle to the direct light, the edge in some plants being directed upward. Compass plants. — When the position of leaves is uniform or nearly so, and cor- responds approximately with the plane of the principal meridian, the plants are known as compass plants. The wild lettuce, Lactuca Scariola, is the most widely distributed of these, and on the prairies and along railways, the compass plant, Silphium laciniatum, which illustrates the habit far better, is common. Other plants in this and other countries have the same habit. That this is a response to intense light is seen easily in the lettuce, for when this plant grows in the shade, its meridional position is not assumed. Fixed light position. — The reaction of a leaf to light can occur only while it (especially the petiole, which is the seat of most curvatures) is still growing or capable of growing. During this period the habitual responses lead finally to a position known as the fixed light position, a sort of resultant, which on the whole gives the blade the most advan- tageous illumination. One result of this is the arrangement of blades in such a way as to avoid shading one another. This produces the so-called leaf mosaics (see Part III, p. 543.) The movements of the leaf GROWTH AND Ml >VEMENT 479 Fig. <><)<)■ Ordinary epidermis and "ocella1 (c) of leaf of Dioscorca. — -After Habkri.andt. in attaining these positions may involve curvature, lengthening, and twisting of the petiole and even of the blade itself. Perceptive region.- Perception in most cases eems i cur in the blade, whence the excitation is propagated to the petiole, whose upper parts grow for the longest time, and even after elongation has ceased may be started into growth again by the light. In some cases, however, the petiole itself may be sensitive to light, and may either cooperate with the blade, or alone be responsible for both perception and curvature. The mechanism of perception has been sought in the epidermis of the blades. It lias been found in some cases that the epidermal cells are domed and that they act as lenses (fig. 699), focusing the light upon the lower side of the cell, so that a spot in the center is much more brightly illuminated when the light strikes at right angles. The position of this area is shifted when the leaf blade is oblique to the rays. Correspondingly, it is assumed that the protoplast is excited when the bright spot rests on any but the central area. There is no doubt that the structures de- scribed concentrate the light, for that ran be shown photographically ; but there are sensitive blades in which domed epidermal cells are wanting, and experiments do not yet unequivocally sustain the assumed distribution of irritability. The per- ceptive organs of leaves have not been located other than by this still doubtful hypothesis. (7) Other tropisms with radiant energy Electrotropism. — Currents of electricity passing through the medium in which plants are growing, and presumably through the organs themselves, evoke various curvatures according to the density of the < urrents used. By nature roots lend them- selves especially well to experiment. Some of these responses, and possibly all of them, are due to one sided injury of the roots. The effects appear to be due to ele< - trolysis of the solutions used ; but whether by the dire, t ai tion of the ions outside or by the withdrawal of ions from the protoplasl is not certain. Electrotropism or galvanotropism may therefore be hardly more than a spe< ial form of chemotropism. It does not seem likely that such stimuli ad to any important extent in nature. The more important effects of galvanic and static currents upon development have already been dest ribed see p. 438). Thermotropism. — Thermotropism is also of little importance. Both roots and stems of particular plants turn toward or away from a bku lined plate radiating heat, according to the temperature. In a similar way mots growing in sawdust will grow toward or away from a source of conducted heat. Wither form of n- it tion can be of nun h importance in nature. The same may be said of reactions to radium and its sails, as well as those to X-rays. The in jurious effects of these .ire more pronounced than the tropisms. 480 PHYSIOLOGY 8. THE DEATH OF PLANTS The cycle ends. — From the foregoing it has become evident that the growth and development of plants does not proceed uniformly, but that it is profoundly influenced — one may even say controlled — by external conditions; and since many of these external conditions evince a de- cided periodicity, growth and development exhibit a corresponding periodicity. But it has also become apparent that growth and develop- ment are likewise affected, and in many particulars as profoundly affected or controlled, by factors that are wholly internal, so far as is known at present. It is found, further, that these factors may give rise to periodicity in growth and development; for, however uniform the exter- nal conditions may be, neither proceeds uniformly. In nothing is this more impressively shown than in the fact that the cycle of development, in spite of all that can be done, sooner or later comes to an end, and the plant perishes, leaving behind comparatively few living cells, if indeed it leaves any, out of the unnumbered millions that may have constituted its body. No inherent reason for death. — There does not seem to be any in- herent reason why a plant should die. The material of which it is com- posed is all the while undergoing decomposition and repair. In a per- ennial plant, like a tree, the tissues in great part are renewed annually, so that though the living and the dead stand together as a sort of unity, which may have occupied the place for centuries, the oldest of the living parts is only a minute fraction of these centuries old. In such a plant, however, it becomes increasingly difficult to supply the extremities with the needful materials, because they are steadily becoming separated by greater and greater distances. The leaves are yearly further from the ports of entry for water, and the roots are yearly further from the source of food. With expanse of branching, mechanical overthrow threatens more and more. Thus the physical conditions are steadily becoming more severe, and it is easy to imagine why the plant must finally suc- cumb. Yet the long persistence, even after it has become evident that a tree has reached the practical limit of growth, shows that there is nothing in the living parts themselves which determines the end; and still more is this shown by the fact that cuttings may be taken from an old tree and successfully started upon a new cycle which may be as long as the parent's. Thus, the Washington elm at Cambridge has been struggling against adversity for more than a quarter of a century, slowly GROWTH AND MOVEMEN1 481 succumbing in a losing fight; but a cutting from it is now a thrifty, well- grown tree on the Boston Common. Reproduction. — In the smaller plants the inception of unfavorable- conditions is often a signal for the gathering together of all the living material into a form that can endure adversity, as with the encystment in lai teria, fungi, and algae. Under these circumstances also the pro- toplasm is divided into several or many parts, each appropriately pro- tected; thus multiplication becomes possible if more than one part es- capes injury and finds suitable conditions again for development (see Botrydium, p. 33, and many other illustrations in Part I). This simple situation has been worked out, in the higher plants, into elaborate mech- anisms of reproduction, which are now not always obviously related to the inception of unfavorable conditions. Yet methods of cultivation in- dicate that the formation of spores, even in the seed plants, in which naturally it often far precedes the period of flowering, may be initiated by conditions unfavorable for vegetative growth. Until these conditions can be more exactly designated and analyzed, it is unprofitable to con- sider them more in detail. At present, then, all that can be said is that unfavorable conditions bring about a redistribution of the living mate- rial, of which as much as possible resists and persists. Thus, since the beginning of things, we assume, there has been an unbroken chain of living matter, shaping itself for a time into organisms more or less com- plex, and then retiring into the simplest and least exposed forms, to begin another cycle of development when the conjunction of internal and external forces permitted. What is death? — The abandonment by the living protoplasm of a body previously constructed, or the destruction of the protoplasm wholly or in great part, is what is usually meant by the death of a plant. Since plants conspicuously lack individuality whenever they become more complex than a single cell, the severance of a plant, even the highest, into two or more parts may not bring death, as it does to so many of the higher animals, but rather renewed vigor. Correspondingly, the death « I even a large part of the body does not necessarily bring death to the whole, but often likewise renewed vigor to the parts that persist. Local and general death. — Extensive local death, as this may be called for convenience, i-- possible in plants without the serious consequences that follow in the- higher animals, fir~i because plants have so little spe- cialization <>f organs and so many of the same kind ; second, because they have no circulatory system that might rapidly distribute to other parts C. B. Sc C. BOTANY — }I 482 PHYSIOLOGY deleterious substances arising in the dead region, and so cause their injury or death; and third, because they have no nervous system, (tut- ting into quick communication sound distant organs with hurtful stimuli from the dead ones. Yet these differences, on the surface so marked, are in reality not fundamental, for what is general death in the animal is in reality only an extension of local death to the several tissues and organs more rapidly than in plants. But each part dies at its own rate and only because the interruption of the activity of one organ has created conditions unfavorable to the other. Irreversible reaction. — The phenomena of death are not easily de- scribed. Certain changes in the appearance of the cytoplasm are visible under the microscope (such as are familiar in fixed cells and are too com- monly thought of as the normal appearance of cytoplasm), chiefly ag- gregation and vacuolation; but the significance of these is not known. Alteration in the chemical processes and different behavior, especially permanent insensitiveness to external stimuli, are the most important marks of death. During life the protoplasm is constantly adjusting itself to new conditions, each response suited to the stimulus, whether in a favorable or unfavorable direction. These responses of normal life are assumed to be reversible, as are many chemical reactions. But when the responses to severe stimuli become irreversible in too great measure, the possibility of readjustment to new stimuli is past ; this con- dition is death. Diseases. — Plants are often killed by diseases which may arise from the disturbance of function wrought by external agents, such as the ele- ments of climate, the solutes of the soil, gases in the air, etc. Or disease may be due to the invasion of the body by parasites, which rob the host of food, interfere with its water supply, or upset some necessary function. A study of diseases forms a great field in itself, plant pathology, under which name therapeutics, the study and application of remedial measures, is also usually comprehended. It is one of the divisions of botany which is of great economic importance, and one whose study has reached its highest level in this country, where the remedial and preventive meas- ures devised save annually many millions of dollars. The knowledge of infectious diseases has been most extensively developed, but therein a great field for investigation still lies open, and a still greater one in the more difficult study of functional disorders. Mechanical injury. — Mechanical injuries often lead to death, es- pecially because they expose the plant to infection by bacteria and fungi. GROWTH AND MOVEMENT 483 Unwise pruning <>f trees in our cities, much more the heedless hacking at the hands of linemen stringing telegraph and telephone wires, and the gnawing by horses carelessly bitched t<> the trees, frequently open the way for infection by some deadly parasite. Ice storms, bail, wind-, and lightning all contribute to serious mechanical injuries at times, whose direct effects are less to be feared than the indiret t. Heat and cold. — High temperature is a fruitful cause of local death, for this is often associated with a deficiency in the water supply. There has been recognized a falling of the leaves, espei ially of trees, in mid- summer, which is due to the heat, and may amount to a large per cent of the total foliage. The older leaves, and those least favorably situated for receiving sufficient water (the latter are at the same time most ex- posed to the direct rays of the sun) are the ones that suffer most. Low temperatures kill tender plants by direct injury to the protoplasts, t ven before the freezing point is reached. Others are killed only by the freez- ing itself, probably because this withdraws water from the protoplast and vacuoles, thus concentrating the solutions, perhaps to a point where certain solutes may become poisonous. There are many plants, how- ever, which are able to withstand freezing, and on gradual thawing the water is taken back into the protoplast again. All the trees and shrubs and the persistent parts of herbaceous perennials are liable to be solidly frozen, often more than once, in the winters of the northern states and Canada, but they usually bear this unharmed, though the trees then have almost a maximum water content. The most serious danger in the northern winters, especially to the evergreens, is that during a warm period the evaporation will surpass the income from the shaded and frozen soil. Temperature and water. — In general the proportion of water present determines the resistance to injury by low and high temperatures, other things being equal. Thus air-dry seeds withstand the lowest tempera- ture yet tried, that of liquid hydrogen (— 25o°(\),' and germinate freely when planted; while the same seeds, if soaked in water until swollen, will be killed at a very much higher temperature. In like manner tempera- tures short of absolute charring an' borne by dry seeds, while a few min- utes' exposure at 700 C. will kill soaked ones. Similarly, plants of firm texture and little sap withstand unfavorable temperatures better than watery ones. 1 Doubtless they will endure the temperature of liquid helium (probably within five or six degrees of the absolute zero, — 2730) if enough is ever obtained for such a test. 484 PHYSIOLOGY Poisons. — Various substances, comprehensively known as poisons, kill the protoplasts, when their concentration is sufficient. At lower con- centrations many of the very same substances accelerate growth or develop- ment or special functions. The action of these substances may depend upon their dissociation in solution into ions, if they are electrolytes, or upon the molecules themselves, or both. Some act by coagulating the protoplasm and others induce changes of a different sort, not accurately known. Ionic hydrogen, silver, copper, and mercury are remarkably injurious. A solution of only one part per million of a silver salt is quickly fatal to the roots of lupines, and still less of mercury kills. Some very important economic measures depend upon the extreme sensitiveness of protoplasm to such substances. For microscopic study it makes possible the almost instant killing of the protoplasts, and by combining a fixing with the killing agent, the preserving of the protoplast in a form which approaches closely the condition in life ; so far, at least, as can be judged from what can be seen of minute structures in the living condition. Further, the poisonous nature of such substances makes it possible to employ them against the agents of infectious diseases, par- ticularly those that grow on the surface of the host. The poisons act at lower dilutions upon the parasite, because its protoplasm is more ac- cessible than that of the host, whose epidermis prevents injury in great measure. The usual form in which they are employed is in solution, which can be sprayed at appropriate times over the host. Many most destructive diseases are thus held in check. Where a disease is trans- mitted with the seed, they may be disinfected by short soaking in a suitable solution, without materially injuring their germinative power. The modern methods of antiseptic surgery, personal and municipal hygiene, and the treatment of infectious diseases rest essentially upon like principles, for in nearly all these cases the organisms to be com- batted are plants. ^* *vj The death of plants appropriately terminates#a|di9<^ssion of their behavior. ^&r ~C* INDEX [Figures in italics indicate pages upon which illustrations occur.] Abies, resin gland, 340. Abietineae, 219- Absorption bands, 369; spectrum, 368, 369. Abstraction, of spores, 65. Acetic fermentation, 411. Achene, 282. Acids, organic, 414. Acorus, vascular bundle, 246. Acrocarpae, i.m. Acrogynae, 103. Acropctal succession, in flowers, 256. Actinomorphic flowers, 256. Adder's tongue, 149. Adhesion, 324. Adiantum, stem section, 160. Aecidiospores, 83. Aecidium, 83, 84. Aerating system, 318. Aerotaxy, 449. Aerotropism, 474. Aestivation, 282. Art hi ilium, 3. Agaricaceae, 88. Agaricales, 87. Agaricus, 89. Albizzia, leaf movements, 456. Albugo, 65, 66. Alcoholic fermentation, 409. Aleurone grains, 3Q2. Algae, 14. Alkaloids, 392, 415. Allium, absorption spectra, 369. Alternation of generations, Coleochaete, 31 ; Polysiphonia, 60; rusts, 84; higher plant:, 92. Ament, 279. Amides, s»o, 392. Amphigastria, 104. Amphithecium, 95, 97, 99, 108, 113, 115, 11S, 110. Am phi vasal bundles, 245, 246. Amylase, 400. Anabaena, 9. Anabolism, 402. Vnacrogynae, 101 ; conclusions, 10?. Anaptychia, 80. Anatropous ovules, 261. Andreaeales, 114. Androspores, 30. Aneimia, sporangium, 137. Aneimites, 183. Aneura, 101. Angiosperms, 180, 238; classification, 276. Anisocarpic flowers, 281. Annual thickening, 346. Annular vessels, 2 / ,'. Annulus, mushrooms, 89; ferns, 155, 156. 157, 163, 164, 165, 352. Anther, 256. Antheridium, green algae, 27, 30, 31, 32, 36, 42, 43; Fucus, 50; Nemalion, 56; Poly- siphonia, so; fungi, 64, 66, 73, 75, 78; liverworts, 92, Q4, 93, 99, 102, 104, 103, 106, 107 ; mosses, 112, 115, 117; lycopods, 127, 120, 133, 140; equisetums, 148; ophioglossums, 154; ferns, 166, 167; water ferns, 174, 175, 178. Antherozoid, 17, 56. Anthoceros, 106, 107, 108, ioq. Anthocerotales, 100; conclusions, 109. Antipodal cells, 265. Apical cell, 42, 46, 98. Aplanospores, 34. Aplastic products, 412. Apocarpous flowers, 279. Apogamy, Urns, 169; angiosperms, 275. Apogeotropism, 460. Apophysis, 119, 120. Apospory, ferns, 169. Apostrophe, 450. Apothecium, 71, 72, 78, 79, 80. Araks, 277. Araucarineae, 220. Archegonium, 92; liverworts, 94, 06, 09, 103, 104, 107; mosses, 112, r/j, 115, • • 7 . 1X8; lycopods. 128, ;,•«', [36, / 37, 140; equisetums, [48; ophioglossums, 153, 131; ferns, 107, 168; water ferns, 175, I7q; gymnosperms, 197, ig8, IffQ, 205, 210, 211. 214, J/5, 216, 223, 233; complex, 223 ; jacket, 198. Archicarp, 79. Archichlamydeae, 239; classification, 278. Arthrospores, g. INDEX Ascobolus, 73. Ascocarp, 70, 71, 72, 73, 75, 76. Ascogenous hyphae, 72, 73. Ascogonium, 73. Ascomycetes, 70. Ascospores, 70, 72, 73, 76, 77. Ascus, 70, 72, 73, 76, 77, 78, 80. Ash, 412, 415. Aspergillus, 74. Aspidium, habit and sporangia, 163; gamc- tophyte, 166. Assimilation, 364, 401. Atrichum, leaf cells, 450. Atropin, 415. Auriculariales, 86. Autobasidiomycetes, 86. Autonomic movements, 453. Autotrophic plants, 362, 380. Auxiliary cells, Polysiphonia, sg, 60. Auxospores, 53. Azolla, 171, 172, 173, 174, 175. Bacillus, 11. Bacteria, 10, //; aerobic, 13 ; anaerobic, 13 ; iron, 14 ; nitrifying, 13 ; nitrogen, 13 ; pathogenic, 13 ; saprophytic, 13 ; sulphur, 14. Bark, loss of, 355. Basidiomycetes, 80. Basidiospore, 80, 83, 8g. Basidium, 80, 82, S3, 8g. Batrachospermum, 57. Bennett itales, 185. Bennettites, strobilus and seed, i8g. Bilabiate flowers, 282. Biophores, 291. Biophytum, records of responses, 42Q. Black fungi, 76. Black knot, 76. Black mold, 67. Bleeding, 332, 334. Blepharoplast, 200, 211. Blue-green algae, 4. Blue mold, 74. Body cell, igg, 200, 211, 215, 217, 224, 235- Bog mosses, no. Boletus, 88. Botrychium, 149 ; habit, 150; gametophyte and archegonium, 134. Botrydium, 33, 34. Bowenia, 102. Box elder, section of stem, 245. Branches, fall of, 355 ; origin of, 419. Brown algae, 44. Brucin, 415. Bryales, 115. Bryophytes, 92. Bulbochaete, 30, 31. Butyric fermentation, 411. Caffein, 415. Callus, 419. Calymmatothcca, 184. Calyptra, 95, 07, 100, 113, 118. Calyptrogen, 247. Calyx, 252. Cambium, 150, 192, 243, 24g. Campanales, 282. Campy lot ropous ovule, 261, 262. Canna, megasporangium, 263. Capillarity, and ascent of water, 349. Capillitium, puffballs, 90 ; slime molds, 3. Capsella, embryo, 272, 273. * Capsule, liverworts, 100, 103, 105, 108, iog; mosses, 113, 114, 115, 118, iiq, 120; ferns, 164. Carbohydrates, 358, 374. Carbon assimilation, 363. Carbon dioxid, admission of, 365 ; as raw material, 364. Carnivorous plants, 386. Carotin, 367, 368. Carpel, 260. Carpogonium, 56, 57, 59, 60. Carpospore, 56, 57, sg, 60. Catabolism, 402. Catalysis, 399. Catalyst, 399. Catkin, 279. Cell, organs, 297; role of living, 351 ; wall, 297, 298, 306. Cellulose, 299, 359; "reserve," 390. Central body, blue-green algae, 5. Central cylinder, 124. Centripetal succession, flowers, 256. Ceratozamia, megasporophyll, 197. Chaetophora, 26. Chalaza, 266, 269. Chalazogamy, 268. Chantransia, 58. Chara, apical cell, 42; habit, 41; sex organs, 43- Charales, 41. Chemical stimuli, 438. Chemotaxy, 447. Chemotropism, 473 ; fungi, 473 ; pollen tubes, 474. Chlamydomonas, 15. Chlamydospore, 81. Chlorenchyma, 366. Chlorophyceae, 15. Chlorophyll, 2, 367. Chlorophyllin, 367. Chloroplast, 297, 366, 376, 450. Chlorovaporization, 330. B i.\i)i;x Chromosomes, .52, 51, 60. 92. 170, 275. Chytridiales, 63. Chy Iridium, <$J. Cilia. 445 ; in action, 7V<5. Cim bonin, 415- ('initiate vernation, 159, 163, 176. Cilrii add, )i 1 Citrus, oil receptacle, 340 ■ Cladophora, 26, 27 ; walls, 308. Cladosiphonic, 159. Clavariales, 87. Cleistocarpac, 1 jo. Cleistothedum, 74, 75. ( llinostat, 462. I lo terium, 37, 38. ( lull mosses, 122. Clustercup, 83, 84. Cocain, 415. Coccus, 11. Codein, 415. Codonotheca, 184. Coenocytes, 26, 27, 33, 34, 35, 36, 62. Cohesion theory, 351. Cold, cause of death, 483. Coleochaete, 31, 32. Coleoptile, and starch grains, 465. Colony, blue-green algae, 6 ; green algae, 1 7, 21. Columella, Anthoceros, 108, 100; black mold, 67, 68; mosses, 113, 118, 119, 120. Companion cells, 24 3; role, 394. Compass plants, 478. Concentration, 304. Concentric bundles, 125, 161. Conceptade.jo. Conducting system, 393 ; origin, 341. Conducting tissue, in style, 260. Confervales, 24 ; conclusions, a- Conidia, 63, 74, 75. Coniferales, 212. Conjugates, 37 ; conclusions, 40. Conjugation, 16, 38, 39, 40. Contact movements, 454. Coprinus, 88, 89. Coral fungi, 87. Corallina, 55. Cordaitales, 203, 204, -'"5, 206. Cork, cambium, 240; cells, 240; role, 318. ( lorn, set tion of stem, 245. Cornaceae, 280. Corn smut, 81. Corolla, 252, 253. Correlations, 441. Cortex, angiosperms, 239, 240, 242; gymno- sperms. 192, 194, 2x9; kelps, 48; pteri- dophytes, 124, X2$, 145, 139, /e><>, iru, 162. Crossing of |H>llen, 268. Crossotheca, 184. Cryptogams, 180. Cup fungi, 71. Cupressineae, 220. Cupules, 98, 99. Curvatures, growth, 458 ; nastic, 442. Cuscuta, haustorium, 382. Cutin, 299. Cutleriaceae, 49. Cyanophyceae, 4. Cyatheaceae, 156. Cycadales, 190. Cycadella, 185. Cycadeoidea, habit, 183; strobilus, 186, 187, iSS; synangia, 189. Cycadofilicales, 181. Cycadofilices, 181. Cycas, habit, 190; microsporophyll, 105; megasporophyll, 197; male gametophyte, 199; embryo, 202. Cystocarp, 56, 57, 59. Cytase, 400. Cytoplasm, 2, 297. Dacromycetales, 86. Dacrydium, male gametophyte, 217. Daily period, 436. Death, 480. Decay, 385. Deformities, 440. Dendroceros, 106. Dermatogen, 239, 240, 247, 465. Desmidiaceae, 37. Desmids, 37. Desmodium, leaflets, 453. Determinants, 291. Development, 417. Deztrinase, 400. Diageotropism, 467. Diaphragm, water ferns, 175. Diastase, 399. Diatomin, 53. Diatoms, 52, 34. Dichotomous, branching, 49; venation, 159, 163, 164. Dicotyledons, 238; classification, 279; embryo, 271 ; vascular system, 243. Dictyotales, 55. I diffusion, 302, 304, 393 ; rate, 304. Digestion, 307 ; extracellular, 398. Dioedous, 30, 105. I lioedsm, 30. Dionaea, 386, 387. Dioon, embryo sac. roo; habit, 191; ovule, 198; staminate strobilus, 195. Dioscorea, ocella, 479. Discomycetes, 72. Disease, 482. Disk flowers, 282. INDEX Dodders, 472. 1 >orsiventrality, 437. I (orj cordaites, 204. Dotted ducts, 241, 243. Double fertilization, 269. Downy mildews, 65. Drosera, 387, 454. Ear fungi, 86. Earth star, 90. Ebenales, 281. Ecology, 295. Ectocarpus, 45. Ectoplast, 306. Efficiency, 371. Egg, thallophytes, 17, 18, 19, 28, 30, 32, 36, 51, 64; bryophytes, 96, 107, 118; pteri- dophytes, 130, 137, 140, 154, 168, 179; spermatophytes, 198, 216, 217, 226, 265, 266, 269. Egg apparatus, 265. Eichhomia, megaspore tetrad, 26 3. Elaterophore, 103. Elaters, 100. Electric waves, 438. Electrotropism, 479. Eligulatae, 132. Embryo, angiosperms, 271, 272, 273, 274, 275; Botrychium, 153, 154; Equisetum, 149; ferns, 168, 169; gymnosperms, 189, 200, 201, 202, 211, 212, 218, 225, 226, 227, 236, 237; Isoetes, 141; Lycopodium, 130, 131; Selaginella, 137. Embryo sac, 197, 198, 199, 234, 261, 264, 265, 266. Endarch, 157. Endodermis, 240. Endogenous, 250 ; root branches, 248. Endo^>erm, 202, 211, 223, 270. Endothecium, anthers, 257, 238, 259; bryo- phytes, 95, 97, 99, 108, 113, 115. n8, Tig. Energy, 368 ; absorbed, 370. Entomophthorales, 68. Environment, 284. Enzymes, 3g ; carbohydrate, 399 ; fat, 400 ; glucoside, 400 ; protein, 401. Ephedra, archegonia, 233; embryo, 236, 237; habit, 228; male gametophyte, 233. Epidermis, angiosperms, 239, 240, 242, 248, 250, 251, 319; bryophytes, g4, 98, 109, 118, 120; pteridophytes, 145, 160, 161. Epigyny, 255. Epinasty, 442. Epistrophe, 450. Epithem, 332. Equilibrium, position of, 463. Equisetales, 143 ; conclusions, 149. Equisetum, 143 ; antheridium, 148; embryo, 149; gametophyte, 147; habit, 144; sporangium, 146; stem section, 145. Ergot fungus, 77. Ericales, 281. Erysiphaceae, 75. Erysiphe, haustorium, 381. Etiolin, 367. Eudorina, 17, 18. Euglcna, 20. Eumycetes, 62. Eurotium, 74. Eusporangiates, 126. Evaporation, 323. Evolution, 283. Exarch, 157. Excitability, 434. Exine, 147, 258. Exoascus, 71. Exobasidiales, 86. Exogenous, origin of branches, 419. Exudation, 332, 333; cause, 335. Fagus, mycorhiza, 382. Farinales, 278. Fat enzymes, 400. Fatigue, 432. Fats, 360, 391. Fermentation, 385, 409; acetic, 411; alco- holic, 409 ; butyric, 411; lactic, 410. Ferns, 155. Fertilization, gymnosperms, 201, 211, 215, 217, 225, 226, 235, 268, 269; mosses, 116; pteridophytes, 136, 16S ; thallophytes, 18, 28, 66. Ficus, leaf skeleton, 343. Filament, of anther, 256, 257. Filicales, 155. Filicineae, 155; conclusions, 170. Filmy ferns, 155. Flagella, 445. Flagellates, 20. Florideae, 55. Flower, 180, 251. Flowering plants, 180. Fluorescence, 370. Flytrap, 386, 3$7- Food, 356 ; and growth, 363 ; source of energy, 363 ; storage, 388 ; translocation, 388. Foot, bryophytes, wo, 103, 108, 109, 113, 1 14; pteridophytes, 130, 131, 137, 141. 149, 154, 168. Formaldehyde, 360. Form and light, 437. Formative stimuli, 435. Fragmentation, blue-green algae, 7. Friction, as stimulus, 470. Fronds, 159. [NDEX Fructose, 3Sg. Fruits, loss of, 355- I males, 4Q. Fucus, fertilization and embryo, 32; habit, to; sex organs, 50, 51. Fuligo, aethalium, 3 ; Plasmodium, -'. I 1111. linn, 2D7 ; unit of, 2g8. Fungi, 01 ; chemotropism of, 473. Calls, ]Q4, .(40. Gametangium, 43, 46. Gamete, t6, 17, \6, 40- Gametophyte, 32; thallophytes, 52, 60. 85; liverworts, 92, pj, 07, 101, 103, 10O ; mosses, in, 115; pteridophytes, 127, 128, m, 147, 148, tS3, i').S, 166, 167; female, 136, 140, 174, 175, 17S, T7Q, [96, /pp, 205, 210, 211, 214, 2/(5, 223, 232, 234, 264, 26s, 266; male, 133, /^o, 174, 175, 178, iqq, 205, aotf, 210, 215, 217, 223, 224, 235, 267. Gases, diffusion, 302 ; entry and exit, 322 ; exclusion, 318 ; from shoot, 352. « iasteromycetes, 89. ( ieaster, go. Gemmae, gS, 104, 117. Generative cell, iqq, 224, 267. < ientianales, 282. Geotaxy, 450. Geotropism, 4sg ; lateral, 467. Geranium, section of cortex, 240. ( lerminal selection, 2gi. Gill, of mushrooms, 88, Sq. Gill fungi, 88. Ginkgo, leaf, 207; female gametophyte, 211; ovule, 210; procmbryo, 212; stro- bili, 208, Ginkgoales, 207. Girdles, leaf trace, 193, 10 /. Girdling, 347. ( Hands, geranium, 337; form, 338; Syringa, as,- nectar, yo; resin, ,•/<>. ( ileba, ( iasteromyi otes, 00. Gleichenia, sori, 136; stele, i$q. Gleicheniaceae, 155. Glochidia, 174, '75- < lloeoi apsa, 5. ( lloeothece, .=;. Glucose, 350, .575- Glucoside enzymes, 400. Glumales, 276. Glumes, 277. Gnetales, 228. Gnetum, embryo, 236; female gametophyte, 234; ovule, 234; strobili, 231, 232. Gonidia, ig. Gradient, 304. (".rand period, 421, 422. ( irape mildew, 00. Gravity, movements, 45s; nastic curvatures, 1 1 |. Green algae, is- Growing point, angiosperms, 239, 240, 247. ( Irowing regions, 422. Growth, 417; curvatures, 458; and food, 363; and light, 435; movements, i s 7 : rapidity, 424; and transpiration and turgor, 510. Guard (ells, 251, 320. Gulfweed, 52, 53. ( iums, 1 1 ,- Guttation, 332; artificial, 353; in fungi, 333 ; nightly, , Gymnosperms, 180, 181. llaustorium, fungi, 61, 381, 3g8 ; pollen tube, 201. Heat, cause of death, 483 ; from respiration, 407. Ilelminthostachys, i4g; habit, 151. Helobiales, 276. llelolism, 382. Helvellales, 71. Hemerocallis, nectar gland, 33Q. Hemitelia, si>orangium, 57. Hepaticae, g3. Herbarium mold, 74. Heredity, 2g3. Heterangium, 182. Heterocysts, 7, 8, q. Heterogamy, 17. Heterospory, 132, 133, 134. Heterotrophic plants, 362, 380. Hippuris, stem ti|>, 240, 41S. Homospory, 134. Hormogonia, S. Horsetails, 143. Host, "i, 381. Humidity, and transpiration, 329. Hybrids, 268, 2g3. eae, 88. Hydnum, 88. Hydrodictyon, 21, 22, 23, (45. Hydropteridineae, 170; conclusions, i7g. Hydrotropism, 475. 1 [ymenium, 71. I [ymenogastrales, w. 1 1> menomj 1 etes, 86. ll> menophj llaceae, 1 55. 1 1\ menophyllum, sorus, 136. Hyphae, 61. Hypogyny, 255. Hyponasty, 1 1.-. Hypophysis, 271, 273. Imbibition, 300. [mpatiens, geotropic curvature, 461, INDEX Income, material, 297. Indusium, 156, 157, 163, 165, 172, 175, 177- Injury, 440 ; mechanical, 482. Insectivorous plants, 386. Integument, 183, 196, 205, 209, 210, 213, 214, 230, 232, 233, 261, 280. Internodes, 41, 145. Intine, 147, 258. Inulase, 400. Inulin, 391. Invertase, 400. Involucre, 280. Irregularity, flowers, 256, 278, 280, 282. Irritability, 426 ; loss of, 434. Isocarpic flowers, 281. Isoetes, embryo, 141; gametophytes, 140; habit, 138; sporangia, 139. Isogamy, 16. Isolation, 292. Jungermanniales, 101 ; contrast with Mar- chantiales, 105. Laboulbeniales, 77. Lactic fermentation, 410. Lactuca, root hairs, 312. Lagenostoma, 182. Laminariaceae, 47. Laminaria, 46. Latex system, 396. Leaf, angiosperms, 250, 251 ; fall of, 354 ; gaps, 159; gymnosperms, 100, 101, 102, 103, 203, 204, 207, 208, 220, 229 ; liver- worts, 101, 104; mosses, m, 112, 116; pteridophytes, 122, 133, 138, 150, 151, 158, 163, 164, 171, 176; traces, 125, 192, 104. Leafy liverworts, 101. Lecithins, 360. Leguminosae, 280 ; relation to nitrogen, 379. Lenticel, 240, 241. Leptosporangiates, 162. Lessonia, 48. Leucoplast, 380. Lichens, 78, 91. Life, 408. Light, exposure to, 370; and form, 437; and growth, 435; and nastic curvatures, 443 ; photosynthesis, 368; position, 478 ; source of, 372. Ligulatae, 132. Ligule, Lycopodiales, 132, 134, 137, 130, 141. Liliales, 278. Lily, anther section, 2 so; leaf epidermis, 231 ; leaf section, 250, 31 9. Liquids, 302. Liverworts, 93. Locomotion, 444. Lycoperdales, 00. Lycoperdon, 90. Lycopodiaceae, conclusions, 132. Lycopodiales, 122. Lycopodium, 122 ; antheridium, 120; arche- gonium, 130; embryo, 1 31 ; gametophyte, 127, 128, 1 20; habit, 12 3, 134; sporan- gium, 123, 126; stele, 125. Lyginodendron, stem section? 182. Lygodium, sporangium, 157. Lyngbya, 6. Macrocystis, habit, 47. Malic acid, 414. Maltase, 400. Manubrium, 43. Maple sap, 334. Marattia, embryo, 169; habit, 158; leaflet, 164. Marattiaceae, 155 ; antheridium, 166 ; spo- rangia, 160. Marchantia, 08, 00, 100, 435. Marchantiaceae, 97. Marchantiales, 93 ; contrast with Junger- manniales, 105. Marsilea, female gametophyte, 170; habit, 176; male gametophyte, 178; sporocarp, 177. Marsileaceae, 176. Massulae, water ferns, 173, 175. Material income, 297. Material outgo, 323. Mechanical stimuli, 439. Medulla, kelps, 48. Medullosa, 182. Megaceros, 106. Megasporangium, 134, 135, 139, 172, 173, 174, 177, 261, 262, 263. Megaspore, 135, 172, 196, 262. Megasporocarp, 171, 172. Megasporophyll, 135, 107. Members, 297. Membrane, cell wall, 306 ; cytoplasmic, 306 ; impermeable, 305 ; permeable, 305. Mendel's law, 292. Merismopedia, 6. Meristem, 133 ; primary, 419 ; secondary, 419. Mesarch, 157. Mesembryanthemum, origin of lateral root, 420. Mesocarpaceae, 38. Mesophyll, 250. Metabolism, 402 ; destructive, 403. Metaxylem, 157, 241. Micellae, 300. Microcycas, 201. Microorganisms, 409. [NDEX Micropyle, 183, 261. Microsphaera, 73, 76. Microsporangium, 134, 135, 130, 172, 173, '74. '77. 184, l89, -"->. -'-". »57i -' Microspore, 133, 175. Microsporocarp, 171, 172, 173, 174. Mi. rosporophyu, 135, 795, 214, 220, 257. Mi. Ml.- layers, anthers, 257. Mildews, 75, 76. Mimosa, leaf, 432. Miscible, 303. Monadelphous stamens, 255. Monoblepharis, 64. Monocotyledons, classification, 276; em- bryo, 273, 274, 275; vascular system, 244, 245, 246. Monosiphonous, algae, 45. Moonwort, i4g, 150. Morchella, 71. Morel, 71. Morphin, 415. Morphogeny: stimuli, 435. Morphology, 1, 295. Mosses, no. Mother cell, 127. Motor organs, 451. Movement, 417 ; amoeboid, 444; autono- mic, 453; of cell organs, 430; ciliary, 445, 446; contact, 432, 434; excretory, //<,- gravity, 455 ; growth, 457 ; leaf, 455, fs6; nyctitropic, 436 ; paratonic, 454 ; photeo- lic, 455, 456; turgor, 451, 457. Mucilage, blue-green algae, 6. Mucor, 67, 68, 6g, 435. Mucorales, 67. Muscarin, 415. Must i. no. Mushrooms, 87. Mutation, 288. Mutualism, 382. Mycelium, 61. Mycetozoa, 2. Mycorhiza, 74, 382, 383. Myriophyllum, stem section, 320. Myxobacteriaceae, 14. Narcotin, 415. Nastic movements, 431. Nasties, 432. Natural selection, 285. Nectary, 339. Nemalion, 36. Neottia, mycorhiza, 383. Nepenthes, leaf, 385. Nephrodium, sperm, 444. Nereocystis, 47, 48. igi, 90. Nicotiana, flower, 253. 114, 119, 120. 49; conclusions, 154. ^9 ; habit, 150; sporangium Nicotin, 415. Nidulariales, 90. Nitella, 42. Nitrogen, source of, 378. Nodes, 41, 145. Nostoc, 7, 8. Notothylas, 106, 108. Nucellus, 183. Nucleus, 6, 15, 16, 2Q7. Nutation, 423, 424. Nutrition, 356. Nutritive mechanism, in ovule, 266. Oedogonium, 27, 29, 30. Oil receptacle, 340. Oils, essential, 413. Ontogeny, 295. Oogonium, 27, 28, 30, 31, 36. Oomycetes, 62. Ooplasm, 66. Oosphere, 17. Oospore, 18. Operculum, 113 Ophioglossales, Ophioglossum, 1 *J2, i53- Orchidales, 278. Organ, 297. Organized bodies, structures, 300. Organogeny, flowers, 256. Orthogenesis, 289. Orthotropic organs, 459. Orthotropous ovules, 261. Oscillatoria, 6, 7. Osmosis, 302, 305. Osmotic pressure, 309. Osmunda. sporangium, 136; stele, 162. Osmundaceae, 155. Outgo, material, 323. Ovary, 260. Ovule, 183; angiosperms, 260, 261; gym- oosperms, 196, 107, 198, 205, 206, 20Q, 210, 213, 214, 221, 222, 232, 233, 234. Oxalic acid, 414. Palisade, leaf, 250, 251, 31 0. Palmales, 277. Palmella, 26. Pandanates, 276. Pandorina, 17. Panicum, coleoptile, 465. Panmixia, 29). Pappus, 282. Parallelotropic organs, 458, 460. Paraphyses, 30. 31, 117. Parasite. 6l, 381 ; injury by, 383. Parasitism. ;8i. Paratonic movements, 454. INDEX Parmelia, 79. Parthenogenesis, 40, 64, 169, 275. Peach curl, 71. Pecopteris, seeds, 1S3. Pediastrum, 21, 22. Pelargonium, capitate hairs, 337. Pallia, thallus, iot. Pellionia, starch grains, 38Q. Penicillium, 74. Pentacyclic flowers, 281. Peony, flower, 252. Perceptive region, 430, 463, 465, 477, 479. Perianth, 252. Periblem, 239, 240, 247. Pericentral cell, 59, 60. Pericycle, 241. Periderm, 419. Peridineae, 54. Peridium, 90. Perigyny, 255. Perinium, 144, 147, 174, 175. Periplasm, 66. Perisperm, 270. Perithecium, 76, 77, 78. Peronospora, 67. Peronosporales, 65. Persistence, 457. Petal, 252. Petioles, sensitive, 472. Peziza, 72. Pezizales, 71. Phaeophyceae, 44. Phaeosporales, 45. Phallales, 91. Phanerogams, 180. Phaseolus, in darkness, 437; leaf movements, 456. Phellogen, 240, 419. Phloem, 1 24, 345, 394. Phosphorus, source of, 379. Photeolic movements, 455, 456. Photosynthesis, 363 ; process, 375 ; prod- ucts, 373. Phototaxy, 449. Phototropism, 475. Phycocyanin, 4. Phycoerythrin, 55. Phycomycetes, 62 ; conclusions, 69. Phycophaein, 44. Phycoxanthin, 44. Phylloglossum, 131, 132. Phyllosiphonic, 159. Phylogeny, 295. Physcia, 78. Physiology, 295. Phytophthora, 66. Pileus, 87, 88, 89. Pilobolus, 68, 333. Pilularia, 176. Pinaceae, 219. Pine, archegonium, 223; embryo, 226; male gametophyte, 224; needle, 220; pollen, 221; pollen tube, 225; stem sec- tion, 219; strobili, 220, 221; wounded, 305- Pistil, 253, 234, 260. Pitcher plants, 385, 386. Pith rays, 150. Pitted vessels, 241. Placenta, 260. Plagiotropic organs, 458, 466, 478. Plantaginales, 282. Plasmodium, 2. Plasmolysis, 309. Plasmopara, 66. Plectascales, 74. Plerome, 239, 240, 247. Pleurocarpae, 121. Pleurococcus, 20, 21. Plowrightia, 76. Plum pockets, 71. Poa, penetrated by fungus, 381. Podocarpineae, 212. Podocarpus, 212; microsporophylls, 214. Poisons, 4S4. Polarity, 440. Pollen, 199; chamber, 183, 198,210; chemo- tropism of, 474; sac, 260; tube, 201, 211, 216, 217, 225, 235, 268, 269. Pollination, 268. Pollinium, 259. Polyembryony, 275. Polyhedra, 23. Polymorphism, rusts, 83. Polypodiaceae, 156; antheridium, 167, sporangia, 162. Polyporaceae, 88. Polyporus, 88. Polysiphonia, 58, 59, 60. Polysiphonous, algae, 45. Polystele, 157. Polystichum, sporangium, 352. Pore fungi, 88. Porella, 103, 104, 105. Porogamy, 269. Portulaca, photeolic movements, 455. Postelsia, habit, 48. Potamogeton, escape of gas bubbles, 377. Potato rot, 66. Presentation time, 434, 461. Pressure, atmospheric, 350 ; barometric, 329 ; diffusion, 304 ; reot, 349. Primary tubercle, Lycopodium, 127. Primulales, 281. Procarp, 36, sg, 60. Proembryo, cycads, 200 201, 202 ; Ginkgo, IX'DKX 212; Torreya, 218; Pinus, 2251 2*6 1 Ephedra, 2,30", 237; angiosperms, --71 Prog< otropism, 400. Promycelium, £j. Prosenchyma, 1 1 2. Protective tissues, 318. Protein enzymes, 401. Proteins, 361, 39] ; synthesis of, 377. Prothallial tubes, Tumboa, 235. Prothallium, 165, 166. Protoascales, 70. Protobasidiomycetes, Sr. Protococcales, 20 ; conclusions, 24. Protodiscales, 71. Protonema, 1 15, 116. Protoplast, 7, 207 ; work of, 298. Protosiphon, 34. Protostele, 125, 157, 139. Protoxylem, 157. 241. Pseudopodium, slime molds, 2; mosses, 113, //./. "S- Psilotales, 142. Psilotum, 142, 143. Pteritlophytes, 122. Pteris, stem section, 161. Pucdnia, 82, 83, 84, 85. Puffballs, 00. Pulque, 334. Pulvinus, 452. Purslane, photeolic movements, 455. Putrefaction, 355, 411. Pycnidia, 83. I'y< nidiospores, 83. Pyrenoid, 16, 30, 40. Pyrenomycetales, 75. Pyronema, 72, 73. Quillworts, 138. Radial bundles, 248, 24Q. Ramentum, 185. Ranales, 279. Ranunculus, nectar gland, 330; root section, 2 n- Ray Bowers, 282. Reaction, mechanism, 431; modes, 428; time. 433. Receptat le, flower, 253 ; Man hantia, 99. Red algae, 54. Regular flowers, 256. Reproduction, 481. Resins, 413. Respiration, t.\; : aerobic and anaerobic, 404; products, 4011; rule of oxygen, 400. Reversible ai tion, 109, Revolution, twiners, 40S. Rheotropi n Rhipsalis, chloroplasts, 376. Rhizopus, 67. Rhodophyceae, 54. 94, 95, 96, 97- Riii iaceae, 93 ; con< lusions, 96. I\i. I k>l ,11 [HI-., i) ;. Kuiniis, endosperm 1 ell, 392; stem section, -'./-'. 346. RingleSS terns, 155. Rivularia, 8, 9. Rockweed, 10. Km., 1. angiosperm, 247, 248, 249; branches, 248; cap, 246, j 17. f6s; diffusion from, 353; effect on soil, 315; hair>, J47, 248, ji2] permeable regions, ;n ; "pressure," 336; pt endophytics, 122, 1.50, 131, 137, 1 /'■ 163, 168; system, 311 ; tip, 247. Roripa, rootcap, if>5- Rosales, j.So. Rotation, ( linostat, 462; twiner, 468. Rubiales, 282. Rusts, 82, 83, 84, 85. Saccharomycetes, 70. Saccharose, 359. Sac fungi, 70. Sagittaria, embryo, 273, 274, 275. Salix, megasporangium, 262. Salts, and transpiration, 325; and water- proofing, ,w~. Salvinia, /,-/. Salviniaceae, 171. Sa[> pressure and turgor, 311. Saprolegnia, 64. Saprolegniales, 63. Saprophytes, 2, 61, 384. Sargassum, 52, 53. Sarracenia, iS'>. Scalariform vessels, 241, 244. Scale mosses, 101. Scenedesmus, 21. Sceptridium, 149. Schizaeaceae, 155. S( hizomyi etes, 10. Schizophyceae, 4. Schizophytes, |. Si itaminales, 278. s< lerodermales, <.*>. Sclerotium, 2, 77. Scouring rushes, 143. Si 5 tonem Secondary sylem, 241, 144, J45. Se. retion, , ,.■. 1 .7 ; emission, 338. Sedum, stoma, Seeds, r8o, r*2, 183, 188, 189, 212, 23a; loss of, 355. Seed plants, 1S0. INDEX Selaginefla, 132 ; archegonium, 137; embryo, 137; female gametophyte, 136; habit, 133; sporangia, 134; spores, 135. Selection, variable, 307. Selective action, 307. Sensitive plants, 429. Sepal, 252. Seta, 100, 101, 103, 104, 105, 113, 116, 120. Shoot, permeable regions of, 311. Sieve plates, 242, 243. Sieve vessels, 242, 243; role, 394. Silphium, fertilization, 26q; male gameto- phyte, 267; microsporangium, 258. Siphonales, 33 ; conclusions, 37. Siphonostele, 133 ; amphiphloic, 157, 160; ectophloic, 157, 162. Sleep movements, 455. Soil, 312; capacity for water, 313 ; effect of roots, 315; water of, 313. Solids, 302. Solutes, 303 ; entry of, 316 ; natural, 303. Solution, 300, 303. Solvent, 303. Soredia, 79. Sorus, 156, 163, 165. Spadix, 277. Spathe, 277. Spectrum, absorption, 368, 36Q. Sperms, 1 7 ; thallophy tes, 18, 19, 28, 30, 36, 43, 50, 52, 56; bryophytes, 92, 93, 102, 112, 117; pteridophytes, 128, 129, 135, 140, 148, 167, 178, 444; gymnosperms, 199, 201, 211. Spermatium, red algae, 56 ; rusts, 83. Spermatophytes, 180. Spermatozoid, 17, 56. Spermogonium, lichens, 79 ; rusts, 83. Sphacelaria, 46. Sphaerella, 75. Sphaerocarpus, 105. Sphaeroplea, 27, 28. Sphaerotheca, 75. Sphagnales, no; conclusions, 114. Sphagnum, antheridia, 112; archegonia, 113; gametophyte, 111; habit, 112, 113; leaf, in; sporophytes, 113, 114. Sphenophy Hales, 143. Sphenophyllum, 143. Spiral vessels, 241, 243. Spirillum, 11. Spongy region, leaf, 250, 251. Sporangiophore, 143, 146, 333. Sporangium, 3 ; thallophytes, 3, 27, 45, 58, SO, 67, 68; pteridophytes, 125, 126, 133, 134, 130, 142, 143, 144, 146, 152, 153, 156, 157, 160, 163, 164, 165, 352. Spores, 3, 16, 62, 147. Sporidia, 83. Sporocarp, 171, 172, 173, 174, 176, 177. Sporogonium, 95. Sporophores, 62. Sporophylls, 122. Sporophyte, 32 ; thallophytes, 32, 60, 84 bryophytes, 92, 95, 97, 99, 100, lot, 103 104, 105, 108, 100, 113, 114, 115, II0» 118, iiq, 120; pteridophytes, 122, 132, 138. 143, 149, 156, 171, 176 ; gymnosperms, 181 185, 191, 203, 207, 213, 220, 229. Spirogyra, 30. Sprout chains, yeast, 70, 71. Squirting fungus, 68. Stalk cell, 200. Stamens, 183, 184, 186, 187, 195, 205, 208, 213, 214, 220, 22i, 230, 231, 252, 253, 256, 257- Staminodia, 281. Starch, 358, 375, 389. Starch grain, 38Q. Statolith theory, 464. Stegocarpae, 121. Stele, 124, 239, 241. Stem, angiosperms, 239. Stemonitis, 3. Sterigmata, 80, 89. Stigeoclonium, 26. Stigma, 260. Stigmatomyces, 78. Stigonema, 10. Stimulus, 426 ; chemical, 438 ; formative, 435 ; mechanical, 439 ; morphogenic, 435 ; tonic, 449. Stink horns, 91. Stipe, mushroom, 87. Stomata, 109, 146, 250, 251, 319, 320, 327; regulation of, 327. Stomium, 164. Stoneworts, 41. Storage, 388. Straightening, twiners, 469. Strains, sexual, 68. Streaming, 444, 451. Strobilus, 122 ; pteridophytes, 124, 132, 133, 144; gymnosperms, 186, 187, 188, i8q, iq2, 193, iqs, ig6, 204, 205, 206, 208, 209, 213, 214, 220, 221, 222, 228, 229, 230, 231, 232; theory of, 123. Stroma, chloroplasts, 367 ; fungi, 76, 77. Style, 260. Substratum, fungi, 61. Sugar, 390 ; cane, 359 ; fruit, 359 ; grape, 359- Sulfur, source of, 379. Summation, 432, 462. Sundew, 387, 454. Sunflower, nutation, 424. Suspensor, angiosperms, 271, 272, 273, 274; INDEX gymnosperm Mu... i. 68, 69; pteridophytes, 130, / ,'/, / .7- Swarm spores, r6, 11 >. Swelling, 300. Swimming spores, 10. Sympetalae, 239; classification, 280. Sympetalous corolla, 255. Sympetaly, 255. Symphyogyna, roi. Synangium, t6i, r<5 i- Synanthales, 277. Syncarpous pistils, 255. Syncarpy, 255. Synchj trium, 63. Synsepalous calyx, 255. Syringa, leaf gland, 338; leaf xylem, 343. Tannins, in. Tapetum, 126, 153, 164, 174, 177, 257, 258, 259. Tartaric add, 414. Taxaceae, 212. Taxic movements, 431. Taxies, 432, 446. Taxineae, 212. Taxodineac, 220. Taxus, 213; microsporophyll, 214. Telegraph plant, y f. Teleutospore, 82, 83. Temperature, and death, 4S3 ; and nastic curvatures, 44.5 ; and photosynthesis, .572 ; and transpiration, 3.->o. Tendrils. 469. I ension of tissues, t-'5. 1 erpenes, 11 ;. Testa, [83, id'), 205, 209, 213. Tetanus, 432, 433- Tetracyc lit Bowers, 281. Tetraspores, 55, 59, 60. Thallophytes, 1. Thelephorales, 87. Theobromin, 415. Thermotropism, 479. Thigmotropism, y»>. Thuja, archegonium complex, 222. ris, habit and sporangia, 142. Toadstools, 87. Tobai co, Bower, Tolypothrix, 9. Tone, 434. Tooth fungi, 88. Torreya, archegonium, 215; embryo, 218; fe- male gametophyte, 216; fertilization, 2/7; male gametophyte, 217; microsporophyll, 21 1; strobilus, si ;. 21 /. Tr.il.e. 11I u Tracheae, .|i. /;. ;i-. .;/;. Trai lieids. 1 50, 220. 241, .■//. I ; nili.i, r.M.I tips, 247. ["ran lot ation ..1 1 1, (88 ; rhyihmi. . Transmission of stimuli, 430. Transpiration, 321, 323; factors, 320; and growth, ,s -'*> ; and salts, 325. il ropism, 472. Tree ferns, [56. Trehalase, 400. Tremellales, 86. ne, 56, 57, 80. Trichomanes, sporangia, 156. Triple fusion, 270. Tropaeolum, nectary, 339. Tropic movements, 431. Tropisms, 432, 458. True mosses, 1 15. Truffles, 7 \. Twiners, 4(17. Tube cell, [99. Tuberales, 74- Tubifloralcs, 282. Tumboa, embryo, 236 ; female gametophyte, 234; "flowers," 230; habit, 229; strobi- lus, 22Q. Turgidity, 308. Turgor, 309; and growth, 310 ; movements, 451, 457 ; rigidity from, 310. Ulothrix, 24, 25. Ulva, 26. I'mbellales, 280. Umbelliferae, 280. Uncinula, 75, 76. Uredinales, 82. 1 redo, 84. Uredospore, 82. Urostyla, movement of cilia, 446. Use and disuse, 284. Ustilaginales, 81. Vacuole, 297. Yasi ular anatomy, Bennettitales, 185 ; Coniferales, 219; Cordaitales, 203 ; Cyca- dales. 102, ;,;/,• Cycadofilicales, t8i, 182; Dicotyledons, 242, 245; Equisetum, 145; Filicales, 156, 159, /<•>", 161, 162; Ginkgo, 207; Gnetales, 229; Isoetes, 138; Lyco podium, 124, 125; Monocotyledons, 244, 245; Ophioglossales, 149; root, 247,240; Ha, / y. Vaucheria, 14, Vegetative multiplication, algae, 0, 10; mOSSt ■-,. 1 in. Velum, Norte-, 1 jo; mushroon \'ieia, geotropit rool curvature, f>i. \'oU a, mushrooms, 88. INDEX Yolvocales, 15. Vol vox, 1 8, iq. Wastes, 412. Water, ascent, 349 ; capillary ascent, 314; continuity, 301 ; and death, 483 ; entry, 316; exudation, 332; immigration, 308; influx, 325 ; loss, 331 ; migration into roots, 314; movement, 341; and plants, 299, 311; raw material, 366; relations, 301; soil, 313 ; solvent, 303. Water ferns, 170. Water molds, 63. Water proofing, 317, 318. Weber's law, 448. Weismannism, 200. Welwitschia, 228. Wheat, aleurone grains, 392. Wheat rust, 82, 83, 84, 85. White rust, 65. Witch brooms, 71. Wood, heart and sap, 347. Xanthophyll, 367. Xenia, 271. Xylaria, 77. Xylem, 124, 241, 342, 343, 344, 345; water path, 347. Yeast, 70 ; fermentation, 410. Zamia, embryo, 200, 201 ; habit, 193; sta- men, 195; stem section, 104; strobilus, 196. Zonal development, 254. Zoospore, 16, 22, 23, 25, 26, 28, 2Q, 30, 32, 34, 35, 45- Zygnema, 38, 40. Zygnemaceae, 38. Zygomorphic (lowers, 256. Zygomycetes, 67. Zygospore, 16, 17, 22, 23, 25, 3S, 39, 40. Zygote, 16. Zymase, 410. ADVERTISEMENTS PLANT LIFE A N L> PLANT USE S By JOHN GAYLORD COULTER, Ph. D. 5 i. 20 AN elementary textbook providing a foundation for the study of agriculture, domestic science, or college botany. But it is more than a textbook on botanv — it is a hook about the fundamentals of plant life and about the relations between plants and man. 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In order to aid and stimulate the development of the pupil's powers of observa- tion, questions have been introduced under each experiment. The directions for making and handling the apparatus, and for performing the experiments, are simple and clear, and are illustrated by diagrams accurately drawn to scale. AMERICAN BOOK COMPANY (.'RAY'S NEW MANUAL ( ) F B O T AN Y — S E V E N T H EDITION, ILLUSTRATED Thoroughly revised and largely rewritten bv BENJAMIN LINCOLN ROBINSON, Ph.D., Asa Gray Pi of Systematic Botany, and MERRITT LYNDON FERNALD, S.B., Assistant Professor of Botany, Harvard University, assisted by specialists in certain groups. Regular edition. Cloth, 8vo, 026 pages $2.50 Tourist's edition. Flexible leather, 1 :mo, 926 pages. . . . 300 Largely rewritten and rearranged, with its scope consider- ably widened. The nomenclature follows the code of inter- national rules recently adopted in Europe. As now pub- lished, it presents in clear and well-ordered form the scattered results of diffuse publication, and treats its subject with due consideration for the results of the latest investigation. AMES'S TEXT-BOOK OF G E N E R A L PHYSICS For use in colleges. By JOSEPH S. AMES, Ph.D., Pro- fessor ol Physics and Director of the Physical Labora- tory, [ohns Hopkins University. Cloth, 8vo, -68 pages, illustrated . #3.5° A one year college course, stating the theory of the sub- ject clearly and logically, and giving a concise summary or the experimental tacts on which the science of physics is based. Every important experiment and observation is men- tioned and explained, and the few great principles of nature- are given the prominence they deserve. A M E RICAN BOOK C O M PA N V INTRODUCTION TO POLITICAL SCIENCE By JAMES WILFORD GARNER, Ph. D., Professor of Political Science, University of Illinois I2.5O THIS systematic treatise on the science of government covers a wider range of topics on the nature, origin, organization, and functions of the state than is found in any other college textbook published in the English lan- guage. The unusually comprehensive treatment of the various topics is based on a wide reading of the best literature on the subject in English, German, French, and Italian, and the student has opportunity to profit by this research work through the bibliographies placed at the head of each chapter, as well as by means of many additional references in the footnotes. ^J" An introductory chapter is followed by chapters on the nature and essential elements of the state ; on the various theories concerning the origin of the state ; on the forms of the state; on the forms of government, including a discussion of the elements of strength and weakness of each; on sov- ereignty, its nature, its essential characteristics, and its abiding place in the state; on the functions and sphere of the state, including the various theories of state activity; and on dis- organization of the state. In addition there are chapters on constitutions, their nature, forms, and development; on the distribution of the powers of government; on the electorate; and on citizenship and nationality. ^j Before stating his own conclusions the author gives an im- partial discussion of the more important theories of the origin, nature, and functions of the state, and analyzes and criticises them in the light of the best scientific thought and practice. Thus the pupil becomes familiar with the history of the science as well as with its principles as recognized to-day. AMERICAN BOOK COMPANY