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TWENTIETH CENTURY TEXT-BOOKS
EDITED BY A. F. NIGHTINGALE, Pu. D. SUPERINTENDENT OF HIGH SCHOOLS, CHICAGO AND CHARLES H. THURBER, A. M.
ASSOCIATE PROFESSOR OF PEDAGOGY IN THE UNIVERSITY OF CHICAGO
TWENTIETH CENTURY TEXT-BOOKS
gd Oe ts Me
A TEXT-BOOK OF BOTANY
BY JOHN M. COULTER, A.M., Pu. D.
HEAD OF DEPARTMENT OF BOTANY UNIVERSITY OF CHICAGO
NEW YORK D. APPLETON AND COMPANY 1900
(@ GRY] CO
|4 00
CopyRiGHT, 1899 By D. APPLETON AND COMPANY
Citi)
Pipe NES A TEXT-BOOK OF BOTANY
PREFATORY NOTE
AutHoven Plant Relations and Plant Structures have been prepared as independent volumes, chiefly to meet the needs of those schools which can give but one half year to Botany, they form together a natural introduction to the science. With this in view, the simple title Plants seems suitable, with the understanding that this volume is an introduction to the study of plants.
. Either part of this combined volume may be used first, according to the views or needs of the teacher. In many cases it may be wise not to observe the order of the book, but to organize laboratory work as seems best, and to assign the appropriate readings wherever they may occur in the volume. The author isa stickler for independent teaching, and would not presume to prescribe an order or a method for teachers. His purpose is simply to offer those facts and suggestions which may be helpful to them in organizing and presenting their work. He would urge that intelligent contact with plants is the essential thing; that a clear understanding of a few large facts is better than the collec- tion of numerous small ones; and that “ getting through” should never sacrifice the leisure needed for digestion.
The two parts of this work are indexed separately, and references to indexes are to be made at the end of each part.
JouHn M. Counter.
Tue University or Cutcaco, November, 1899.
TWENTIETH CENTURY TEXT-BOOKS
PLANT RELATIONS
A FIRST BOOK OF BOTANY
BY JOHN M. COULTER, A.M., Px.D.
HEAD PROFESSOR OF BOTANY UNIVERSITY OF CHICAGO
NEW YORK D. APPLETON AND COMPANY
1900
COPYRIGHT, 1899, By D. APPLETON AND COMPANY.
PREFACE,
THE methods of teaching botany in secondary schools are very diverse, and in so far as they express the experience of successful teachers, they are worthy of careful considera- tion. As the overwhelming factor in successful teaching is the teacher, methods are of secondary importance, and may well vary. It is the purpose of the present work to contribute another suggestion as to the method of teach- ing botany in secondary schools. The author does not intend to criticise other methods of teaching, for each teacher has his own best method, but it may be well to state the principles which underlhe the preparation of this work,
The botany is divided into two parts, each representing work for half a year. The two books are independent, and opinions may differ as to which should precede. The first book, herewith presented, is dominated by Ecology, and also contains certain fundamentals of Physiology that are naturally suggested. The second book will be domi- nated by Morphology, but plant structure, function, and classification will be developed together in an attempt to trace the evolution of the plant kingdom. In the judg- ment of the author Ecology should precede Morphology, but this order brings to Ecology no knowledge of plant structures and plant groups, which is of course unfortu- nate. The advantages which seem to overbalance this dis- advantage are as follows:
1. The study of the most evident life-relations of
plants gives a proper conception of the place of plants in 1*
vi PREFACE.
nature, a fitting background for subsequent more detailed studies.
2. Such a view of the plant kingdom is certainly of the most permanent value to those who can give but a half year to botany, for the large problems of Ecology are con- stantly presented in subsequent experience, when details of structure would be forgotten.
3. The work in Ecology herein suggested demands Lht- tle or no use of the compound microscope, an instrument il adapted to first contacts with nature.
The second book will demand the use of the compound microscope, and those schools which possess such an equip- ment may prefer to use that part first or exclusively.
In reference to the use of this part something should be said, although such cautions are reiterated in almost every recent publication. .A separate pamphlet containing “Suggestions to Teachers” who use this book has been prepared, but a few general statements may be made here. This book is intended to present a connected, readable account of some of the fundamental facts of botany, and may serve to give a certain amount of information. If it performs no other service in the schools, however, its pur- pose will be defeated. It is entirely too compact for any such use, for great subjects, which should involve a large amount of observation, are often merely suggested. It is intended to serve as a supplemeut to three far more im- portant factors: (1) the teacher, who must amplify and suggest at every point; (2) the laboratory, which must bring the pupil face to face with plants and their strue- tures; (3) field-work, which must relate the facts observed in the laboratory to their actual place in nature, and must bring new facts to notice which can be observed nowhere else. Taking the results obtained from these three fac- tors, the book seeks to organize them, and to suggest explanations. It seeks to do this in two ways (1) by means of the tect, which is intended to be clear and un-
PREFACE. vii
technical, but compact ; (2) dy means of the illustrations, which must be studied as carefully as the text, as they are only second in importance to the actual material. Espe- cially is this true in reference to the landscapes, many of which cannot be made a part of experience.
Thanks are due to various members of the botanical staff of the University, who have been of great service in offering suggestions and in preparing illustrations. In this first book I would especially acknowledge the aid of Professor Charles R. Barnes and Dr. Henry C. Cowles.
The professional botanist who may critically examine this first book knows that Ecology is still a mass of incho- ate facts, concerning which we may be said to be making preliminary guesses. It seems to be true, nevertheless, that these facts represent the things best adapted for pres- entation in elementary work. The author has been com- pelled to depend upon the writings of Warming and of Kerner for this fundamental material. From the work of the latter, and from the recent splendid volume of Schim- per, most useful illustrations have been obtained. The number of original illustrations is large, but those obtained elsewhere are properly credited.
Joun M. COULTER. Tae University oF Curcaco, /ay, 1899.
CONTENTS.
CHAPTER, PAGE. I.—INTRODUCTION . é : : ‘ , 1 TI.—FouiaGr LEAVES: THE LIGHT-RELATION : 6
If{]..—FoulaGr LEAVES: FuNcTION, STRUCTURE, AND PROTECTION 23
IV.—Snoots 53 V.—Roors 80 VI.—REPRODUCTIVE ORGANS 109 VIT.—FLoweERS AND INSECTS 123 VIIL.—AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS 188 IX.—THE STRUGGLE FOR EXISTENCE 142 X.—THE NUTRITION OF PLANTS 149 XI.—PLANt’ SOCIETIES: ECOLOGICAL FACTORS ‘ 162 ATI.—HypRopPHYTE SOCIETIES : 170 XIII.—XeEROPHYTE SOCIETIES g 193 NIV.—MesopoHyre sSocreTIES . 230 XV.—HALOPHYTE SOCIETIES : 3 249
InDEX : ) ‘
BOTANY PART I.—PLANT RELATIONS
CHAPTER I. INTRODUCTION.
1. General relations.—Plants form the natural covering of the earth’s surface. ‘So generally is this true that a land surface without plants seems remarkable. Not only do plants cover the land, but they abound in waters as well, both fresh and salt waters. They are wonderfully varied in size, ranging from huge trees to forms so minute that the microscope must be used to discover them. They are also exceedingly variable in form, as may be seen by comparing trees, lilies, ferns, mosses, mushrooms. lichens, and the green thready growths (a/g@) found in water.
2. Plant societies—One of the most noticeable facts in reference to plants is that they do not form a monotonous covering for the earth’s surface, but that there are forests in one place, thickets in another, meadows in another, swamp growths in another, ete. In this way the general appear- ance of vegetation is exceedingly varied, and each appear- ance tells of certain conditions of hving. These groups of plants living together in similar conditions, as trees and other plants ina forest, or grasses and other plants in a meadow, are known as plunt societies, These societies are as
2 PLANT RELATIONS.
numerous as are the conditions of living, and it may be said that each society has its own special regulations, which ad- mit certain plants and exclude others. The study of plant societies, to determine their conditions of living, is one of the chief purposes of botanical field work.
3. Plants as living things—Before engaging in a study of societies, however, one must discover in a general way how the individual plant lives, for the plant covering of the earth’s surface is a living one, and plants must always be thought of as living and at work. They are as much alive as are animals, and so far as mere living is concerned they live in much the same way. Nor must it be supposed that animals move and plants do not, for while more animals than plants have the power of moving from place to place, some plants have this power, and those that do not can move cer- tain parts. The more we know of living things the more is it evident that life processes are alike in them all, whether plants or animals. In fact, there are some living things about which we are uncertain whether to regard them as plants or animals.
4, The plant body.—Every plant has a body, which may be alike throughout or may be made up of a number of different parts. When the green thready plants (a/g@), so common in fresh water, are examined, the body looks like a simple thread, without any special parts ; but the body of a lily is made up of such dissimilar parts us root, stem, leaf, and flower (see Figs. 75, 144, 155, 168). The plant without these special parts is said to be stmple, the plant with them is called complex. The siniple plant lives in the same way and does the same kind of work, so far as living is concerned, as docs the complex plant. The differ- ence is that in the case of the simple plant its whole body does every kind of work; while in the complex plant different kinds of work are done by different regions of the body, and these regions come to look unlike when differ- ent shapes are better suited to different work, as in the
INTRODUCTION. 3
case of a leaf and a root, two regions of the body doing different kinds of work.
5. Plant organs—These regions of the plant body thus set apart for special purposes are called organs. The sim- plest of plants, therefore, do not have distinct organs, while the complex plants may have several kinds of organs. All plants are not either very simple or very complex, but beginning with the simplest plants one may pass to others not quite so simple, then to others more complex, and so on gradually until the most complex forms are reached. This process of becoming more and more complex is known as differentiation, which simply means the setting apart of different regions of the body to do different kinds of work. The advantage of this to the plant becomes plain by using the common illustration of the difference between a tribe of savages and a civilized community. The savages all do the same things, and each savage does everything. In the civilized community some of the members are farmers, others bakers, others tailors, others butchers, etc. This is what is known as “ division of labor,” and one great advan- tage it has is that every kind of work is better done. Ditf- ferentiation of organs in a plant means to the plant just what division of labor means to the community ; it results in more work, and better work, and new kinds of work. The very simple plant resembles the savage tribe, the com- plex plant resembles the civilized community. It must be understood, however, that in the case of plants the differ- entiation referred to is one of organs and not of individuals.
6. Plant functions—Whether plants have many organs, or few organs, or no organs, it should be remembered that they are all at work, and are all doing the same essential things. Although many different kinds of work are being carried on by plants, they may all be put under two heads, nutrition and reproduction. Every plant, whether simple or complex, must care for two things: (1) its own support (nutrition), and (2) the production of other plants like
4 PLANT RELATIONS.
itself (reproduction). To the great work of nutrition many kinds of work contribute, and the same is true of repro- duction. Nutrition and reproduction, however, are the two primary kinds of work, and it is interesting to note that the first advance in the differentiation of a simple plant body is to separate the nutritive and reproductive regions. In the complex plants there are nutritive organs and reproductive organs ; by which is meant that there are distinct organs which specially contribute to the work of nutrition, and others which are specially concerned with the work of reproduction. The different kinds of work are conveniently spoken of as functions, each organ having one or more functions.
7. Life-relations—In its nutritive and reproductive work the plant is very dependent upon its surroundings. It must receive material from the outside and get rid of waste material ; and it must leave its offspring in as favorable conditions for living as possible. As a consequence, every organ holds a definite relation to something outside of it- self, known as its life-relation. For example, green leaves are definitely related to light, many roots are related to soil, certain plants are related to abundant water. some plants are related to other plants or animals (living as parasites), etc. A plant with several organs, therefore, may hold a great variety of life-relations, and it is quite a complex problem for such a plant to adjust all of its parts properly to their necessary relations. The study of the life-relations of plants is a division of Botany known as Ecology, and presents to us many of the most important problems of plant life.
It must not be supposed that any plant or organ holds a perfectly simple life-relation, for it is affected hy a great variety of things. A root, for instance, is affected by light, gravity, moisture, soil material, contact, etc. Every or- gan, therefore, must adjust itself to a very complex set of life-relations, and a plant with several organs has so many
INTRODUCTION. 5
delicate adjustments to care for that it is really impossi- ble, as yet, for us to explain why ull of its parts are placed just as they are. In the beginning of the study of plants, only some of the most prominent functions and life-rela- tions can be considered. In order to do this, it seems bet- ter to begin with single organs, and afterwards these can be put together in the construction of the whole plant.
CHAPTER II. FOLIAGE LEAVES: THE LIGHT-RELATION.
8. Definition.—A foliage leaf is the ordinary green leaf, and is avery important organ in connection with the work of nutrition. It must not be thought that the work done by such a leaf cannot be done by green plants which have no leaves, as the algw, for example. A leaf is simply an or- gan set apart to do such work better. In studying the work of a leaf, therefore, we have certain kinds of work set apart more distinctly than if they were confused with other kinds. For this reason the leaf is selected as an in- troduction to some of the important work carried on by plants, but it must not be forgotten that a plant does not need leaves to do this work ; they simply enable it to work more effectively.
9. Position It is easily observed that foliage leaves grow only upon stems, and that the stems which bear them always expose them to light; that is, such leaves are aerial rather than subterranean (see Figs. 1, 75,169). Many stems grow underground, and such stems either bear no foliage leaves, or are so placed that the foliage leaves are sent above the surface, as in most ferns and many plants of the early spring (see Figs. £5, 46, 144).
10. Color.—Another fact to be observed is that foliage leaves have a characteristic green color, a color so universal that it has come to be associated with plants, and espe- cially with leaves. It is also evident that this green color holds some necessary relation to light, for the leaves of plants grown in the dark, as potatoes sprouting in a cellar,
FOLIAGK LEAVES: THE LIGHT-RELA'TION. 7
do not develop this color. Even when leaves have devel- oped the green color they lose it if deprived of light, as is shown by the process of blanching cclery, and by the effect on the color of grass if a board has lain upon it for some time. It seems plain, therefore, that the green color found in working foliage leaves depends upon light for its existence.
We conclude that at least one of the exseutial life-rela- tions of a foliage leaf is what may be called the light-rela- fion. his seems to explain satisfactorily why such leaves are not developed in a subterranean position, as are many stems and most roots, aud why plants which produce them do not grow in the dark, asin caverns. The sume green, and hence the sume light-relation, is observed in other parts of the plant as well, and in plants without leaves, the only difference being that leaves display it most conspicu- ously. Another indication that the green color is con- nected with light may be obtained from the fact that it is found only in the surface region of plants. If one cuts across a living twig or into a cactus body, the green color will be seen only in the outer part of the section. The con- clusion is that the leaf is a special organ for the lght-re- lation. Plants sometimes grow in such situations that it would be unsafe for them to display leaves, or at least large leaves. Insuch a case the work of the leaves can be thrown upon the stem. .\ notable illustration of this is the cactus plant, which produces no foliage leaves, hut whose stem dis- plays the leaf color.
11. An expanded organ.—Another general fact in refer- ence to the foliage leaf is that in most cases it is an expanded organ. This means that it has a great amount of surface exposed in comparison with its mass. As this form is of such common occurrence it is safe to conclude that it is in some way related to the work of the leaf, and that whatever work the leaf doves demands an exposure of surface rather
than thickness of body. It is but another step to say that 2
8 PLANT RELATIONS.
the amount of work un active leaf can do will depend in part upon the amount of surface it exposes.
THE LIGHT-RELATION,
12. The general relation —'The ordinary position of the foliage leaf is more or less horizontal. This enables it to receive the direct rays of light upon its wpper surface. In
Fig. 1. The leaves of this plant (Wiews) are in general horizontal, but it will be seen that the lower ones are directed down- ward, and that the leaves hecome more horizontal as the stem is ascended. It will also be seen that the leaves are so broad that there are few vertical rows.
this way more rays of light strike the leaf sur- face than if it stood ob- liquely or on edge. It is often said that leaf blades are so directed that the flat surface is at right angles to the ieident rays of light. While this may be true of horizon- tal leaves in a_ general way, the observation of almost any plant will show that it is a very general statement, to which there are numerous exceptions (see Fig. 1). Leaves must be arranged to receive as much light as possible to help in their work, but too much light will destroy the green substance (chloro- phyll), which is essential to the work. The adjust- ment to light, therefore, is a delicate one, for there must be just enough
FOLIAGE LEAVES: THE LIGHT-RELATION. 9
and not too much. The danger from too much light is not the same in the case of all leaves, even on the same plant, for some are more shaded than others. Leaves also have « way of protecting themselves from too intense light by their structure, rather than by a change in their posi- tion. It is evident, therefore, that the exact position which any particular leaf holds in relation to ight depends upon many circumstances, and cannot be covered by a general rule, except that it seeks to get all the light it can without danger.
13. Fixed position.— Leaves differ very much in the power of adjusting their position to the direction of the light.
Fie, 2. The day and night positions of the leaves of a member (Amicia) of the pea family.—After STRASBURGER.
Most leaves when fully grown are in a fixed position and cannot change it, however unfavorable 1t may preye to he, except as they are blown about. Such leaves are said to have fized light positions. This position is determined by the light conditions that prevailed while the leaf was grow- ing and able to adjust itself. If these conditions continue, the resulting fixed position represents the best one that can be secured under the circumstances. The leaf may not receive the rays of light directly throughout the whole period of daylight, but its fixed position is such that it probably receives more light than it would in any other position that it could secure.
10 PLANT RELATIONS.
14. Motile leaves—There are leaves, however, which have no fixed light position, but are so constructed that they can shift their position as the direction of the light changes. Such leaves are not in the same position in the afternoon as in the forenoon, and their night position may be very different from either (see Figs. 2, 3, 3b, £). Some of the common house plants show this power. In the cause of the com- mon Qralis the night
Fie. 3a. The day position of the leaves of redbud position of the leaves (Cervix). —After ARTHUR. is remarkably different
from the position in light. If such a plant is exposed to the light ina window and the positions of the leaves noted, and then turned half way around, so as to bring the other side to the light, the leaves may be observed to adjust them- selves gradually to the changed light-relations. Fic. 3b, The night position of the leaves 15. Compass plants.—.\ of redbud (Cervés),—After ARTHUR.
striking illustration of a
special light position is found in the so-called * compass plants.” The best known of these plants is the rosin-weed of the prairie region. Growing in situations exposed te intense light, the leaves are turned edeewise, the flat faces being turned away from the intense rays of midday, and directed towards the rays of less intensity ; that is, those of
FOLIAGE LEAVES: THE LIGHT-RELATION. 11
. bY N N \ N \¥
we ST” y
Fig. 4. Two sensitive plants, showing the motile Jeaves. The plant to the left has its leaves and numerous leaflets expanded ; the one to the right shows the leaflets folded together and the leaves drooping.— After KERNER,
the morning and evening (see Fig. 165). As a result, the apex of the leaf points in a general north or south direction. It is a significant fact that when the plant grows in shaded places the leaves do not assume any such position. It seems evident, therefore. that the position has something to do with avoiding the danger of too intense light. It
12 PLANT RELATIONS.
must not be supposed that there is any ac- curacy in the north or south direction, as the edgewise position seems to be the signifi- cant one. In the ros- in-weed probably the north and south direc- tion is the prevailing one; but in the prickly lettuce, a very common weed of waste grounds, and one of the most striking of the compass plants, the edgewise position is frequently assumed without any special reference to the north or south direc- tion of the apex (see Fig. 5).
16. Heliotropism.— The influence of light upon the positions of leaves and other or- gans is known as /eli- otropism, and it is one of the most important of those external influ- ences to which plant organs respond (see Figs. 6, 43).
Fie, 5. The common prickly lettuce (Lactuca It should be under-
Scariola), showing the leaves standing edge- : ‘ : wise, and in a general north and south plane, stood cl early that. this
—After AnrHUR and MacDouea. is but a slight glimpse
FOLIAGE LEAVES: THE LIGHT-RELATION, 13
Tia. 6. These plants are growing near a window. It will be noticed that the stems bend strongly towards the light, and that the leaves face the light.
of the most obvious relations of foliage leaves to light, and that the important part which heliotropism plays, not only in connection with foliage leaves, but also in connection with other plant organs, is one of the most important and extensive subjects of plant physiology.
RELATION OF LEAVES TO ONE ANOTHER.
A. On erect stems.
In view of what has been said, it would seem that the position of foliage leaves on the stem, and their relation to one another, must be determined to some extent by the necessity of a favorable light-relation. It is apparent that the conditions of the problem are not the same for an erect as for a horizontal stem.
17. Relation of breadth to number of vertical rows.— Upon an erect stem it is observed that the leaves are usu-
14 PLANT RELATIONS.
ally arranged in a definite number of vertical rows. It is to the advantage of the plant for these leaves to shade one another as little as possible. Therefore, the narrower the leaves, the more numerous may be the vertical rows (see Figs. 7, 8); and the broader the leaves the fewer the vertical rows (xee Fig. 1). A relation exists, therefore, be- tween the breadth of leaves and the number of verti- cal rows, and the meaning of this becomes plain when the light-re- lation ix consid- ered.
18. Relation of length to the dis- tance between Fic. 7. An Easter lily, showing narrow leaves and leaves of the same
numerous vertical rows. row.—The leaves
in a vertical row
may he close together or far apart. If they should be close together and at the same time long, it is evident that they will shade each other considerably, as the light cannot well strike in between them and reach the surface of the lower leaf. Therefore, the closer together the leaves of a verti- cal row, the shorter are the leaves; and the farther apart the leaves of a row, the longer may they be. Short leaves permit the light to strike hetween them cyen if they are close together on the stem; and long leaves permit the same thing only when they are far apart on the stem, A
FOLIAGE LEAVES: THE LIGHT-RELATION. 15
relation is to be observed, therefore, between the length of leaves and their distance apart in the same vertical row.
The same kind of relation cun be observed in reference to the breadth of leaves, for if leaves are not only short but narrow they can stand very close together. It is thus seen that the length and breadth of leaves, the number of ver- tical rows on the stem, and the distance between the leaves
Fig. 8. A dragon-tree, showing narrow leaves extending in all directivus, and numer- ous vertical rows.
of any row, all have to do with the light-relation and are answers to the problem of shading.
19. Elongation of the lower petioles—There is still another common arrangement by which an effective light- relation is secured by leaves which are broad and placed close together on the stem. In such a case the stalks (petioles) of the lower leaves become longer than those above and thus thrust their blades beyond the shadow (sce Fig. 9). It may be noticed that it is very common to
16 PLANT RELATIONS.
find the lowest leaves of a plant the largest and with the longest petioles, even when the leaves are not very close together on the stem.
It must not be supposed that by any of these devices shading is absolutely avoided. This is often impossible and sometimes undesirable. It simply means that by these
Fie. 9. A plant (Sein(paulic) with the lower petioles elongated, thrusting the blades beyond the shadow of the upper leaves. A loose rosette.
arrangements the most favorable light-relation is sought by avoiding too great shading.
20. Direction of leaves—Not only is the position on the stem to be observed, but the direction of leaves often shows a definite relation to light. It is a very common thing to find a plant with a cluster of comparatively large leaves at or near the lase, where they are in no danger of shading other leaves, and with the stem leaves gradually becoming
FOLIAGE LEAVES: THE LIGHT-RELATION. 17
smaller and less horizontal toward the apex of the stem (see Figs. 10, 13). The common shepherd's purse and the mullein may be taken as illustrations. By this arrange- ment all the leaves are very completely exposed to the light.
21. The rosette habit.— The habit of producing a cluster or rosette of leaves at the base of the stem is called the rosette habit. Often this rosette of leaves at the hase, frequently lying flat on the ground or on the rocks, includes the only fo- liage leaves the plant pro- duces. It is evident that a rosette, in which the leaves must overlap one another more or less, is not a very favorable light arrange- ment, and therefore it must be that something is being
Fie. 10. A plant (Echereria) with fleshy
provided for besides the leaves, showing large horizontal ones light-relation (see Figs. 1a; at base, endo thsits becomine, smaller
‘ . feo ‘ and more directed upward as the 12, 13). What this is will ators is-necerdad.
appear later. but even in
this comparatively unfavorable light arrangement, there is evident adjustment to secure the most ight possible under the circumstances. The lowest leaves of the rosette are the longest, and the upper (or inner) ones become gradu- ally shorter, so that all the leaves have at least a part of the surface exposed to light. The overlapped base of such leaves is not expanded as much as the exposed apex, and hence they are mostly narrowed at the base and broad at the apex. This narrowing at the base is sometimes
18 PLANT RELATIONS.
carried so far that most of the part which is covered is but a stem (petiole) for the upper part (blade) which is exposed.
In many plants which do not form close rosettes a gen-
Hie, 11. A eroup of live-for-evers, illustrating the rosette habit and the light-relation. In the rosettes it will be observed how the leaves are fitted together and diminish in size inwards, so that excessive shading is avoided. The individual leaves also become narrower where they overlap, antl are broadest where they are exposed to light. In the background is a plant showing leaves in very definite vertical rows.
eral rosette arrangement of the leaves may be observed by looking down upon them from aboye (sce Fig. 9), ax in some of the early buttercups which are so low that the large leaves would seriously shade one another, except that the lower leaves have longer petioles than the upper, and so reach beyond the shadow.
FOLIAGE LEAVES: THE LIGHT-RELATION. 19
Fic. 12. Two clumps of roscttes of the house leek (Semperrirum), the one to the right showing the compact winter condition, the one to the left with rosettes more open after being kept indoors for several days.
22. Branched leaves.—Another notuble feature of foliage leaves, which has something to do with the light-relation, is that on some plants the blade does not consist of one piece, but is lobed or even broken up into separate pieces. When the divisions are distinct they are called leaflets, and every gradation in leaves can be found, from distinct leaf- lets to lobed leaves, toothed leaves, and finally those whose margins are not indented at all (ev/irc). This difference in leaves probably has more important rea- sons than the light- relation, but its sig- nificance may be ob- served in this connec- tion. In those plants whose leaves are un- divided, the leaves generally either di- minish in size toward the top of the stem, or the lower ones de- Fie. 18. The leaves of a bellflower (Campanula),
velop longer petioles. showing the rosette arrangement. The lower
Bs tg Peri petioles are successively longer, carrying their In this Case the Celt blades beyoud the shadow of the blades above. eral outline of the —After Kernen,
dangerous shading, It will be seen that the larger blades or less-branched leaves are towards the bottom of the group,
FOLIAGE LEAVES: THE LIGHT-RELATION. 21
plant is conical, a form very common in herbs with entire or nearly entire leaves. In plants whose leaf blades are broken up into leaflets (compound or branched leaves), however, no such diminution in size toward the top of the stem is necessary (see Fig. 17), though it may frequently
Be x % 2 y at
Fig. 15. A plant showing much-branched leaves, which occur in great profusion with- out cutting off the light from one another.
occur. When a broad blade is broken up into leaflets the danger of shading is very much less, as the light can strike through between the upper leaflets and reach the leaflets below. On the lower leaves there will be splotches of light and shadow, but they will shift throughout the day, so that probably a large part of the leaf will receive light at some time during the day (see Fig. 14). The
22 PLANT RELATIONS.
general outline of such a plant, therefore, is usually not conical, as in-the other case, but cylindrical (sce Figs. 4, 15, 16, 22, 45, 83, 96, 155, 162, 169 for branched leaves). Many other factors enter into the light-relation of foli- age leaves upon erect stems, but those given may suggest
Fie. 16. A cycad, showing much-branched leaves and palm-like habit.
observation in this direction, and serve to show that the arrangement of leaves in reference to light depends upon many things, and is by no means a fixed and indifferent thing. The study of any growing plant in reference to this one relation presents a multitude of problems to those who know how to observe.
B. On horizontal stems.
23. Examples of horizontal stems, that is. stems exposed on one side to the direct light, will be found in the ease of many branches of trees, stems prostrate on the ground, and
FOLIAGE LEAVES: THE LIGHT-RELATION. 23
stems against a support, as the ivies.
It is only necessary
to notice how the leaves are adjusted to Lght on an erect
stem, und then to bend the stem into a horizontal posi- tion or against a support, to realize how unfavorable the sume arrangement would be. and how many new ad- justments must be made. The leaf blades must. all be brought to the light side of the stem, so far as possible, and those that belong to the lower side of the stem must be fitted imto the spaces left by the leaves which belong to the upper This may be brought about by the twisting of the stem, the twisting of the petioles, the bending of the blade on the petiole, the lengthening of petioles, or in some other way. Every horizontal stem has its own special problems of leaf adjustment which may be observed (see Figs. 18, 50).
Sometimes there is not space enough for the full development of every blade, and smaller ones are fitted
side.
into the spaces left by the larger ones (see Fig. 21).
Fig. 17. lobed leaves, the rising of the petioles to adjust the blades to light, and the general cylindrical habit.
A chrysanthemum, showing
This
sometimes resultsin what are called unequally paired leaves, where opposite leayes develop one large blade and one small
24 PLANT RELATIONS.
one. Perhaps the most complete fitting together of leaves is found in certain ivies, where a regular layer of angular interlocking leaves is formed, the leaves fitting together like
as SB TRL REL EE — — ss eee
Fie. 18. A plant (Pellionia) with drooping stems, showing how the leaves are all brought to the lighted side and fitted together.
the pieces of a mosaic. In fact such an arrangement is known as the mosate arrangement, and involves such an amount of twisting, displacement, elongation of petioles,
“SUIPLYS ploav 07 JYIA50) payy are Lay] MOY SUTMOYS ‘saaval BIUOseg Jo o1vsoUl Y “EL “OMT
26 PLANT RELATIONS.
Fre. 20. A spray of maple, showing the adjustment of the leaves in size and position of blades and length of petioles to secure exposure to light on « horizontal stem.— After KEnNER.
etc., as to give ample evidence of the effort put forth by plants to secure a favorable light-relation for their foliage
= SASS Ss ~ = = Fig. 21. Two plants showing adjustment of Jeaves on a horizontal stem. The plant to the left is nightshade, in which small blades are fitted into spaces left by the large ones. The plant to the right ix Sclaginella, in which small leaves are dis- tributed along the sides of the stem, and oticrs are displayed along the upper sur- face.—After Kerner.
FOLIAGE LEAVES: THE LIGHT-RELATION. OW.
leaves (see Figs. 19, 22). In the case of ordinary shade trees every direction of branch may be found. and the resulting adjustment of leaves noted (see Fig. 20).
Looking up into a tree in full foliage, it will be noticed that the horizontal branches are comparatively bare be-
Fic. 22. A mosaic of fern (Adiantum) leaflets.
neath, while the leaf blades have been carried to the upper side and have assumed a mosaic arrangement.
Sprays of maidenhair fern (see Fig. 22) show a remark- able amount of adjustment of the leaflets to the light side. Another group of fern-plants, known as club-mosses, has horizontal stems clothed with numerous very small leaves. These leaves may he seen taking advantage of all the space on the lighted side (see Fig. 21).
CHAPTER III.
FOLIAGE LEAVES: FUNCTION, STRUCTURE, AND PROTEO- TION.
A. Functions of foliage leaves.
24, Functions in general We have observed that foliage leaves are light-related organs, and that this relation is an important one is evident from the various kinds of adjust- ment used to secure it. We infer, therefore, that for some important function of these leaves light is necessary. It would be hasty to suppose that light is necessary for every kind of work done by a foliage leaf, for some forms of work might be carried on by the leaf that light neither helps nor hinders. Foliage leaves are not confined to one function, but are concerned in « variety of processes, all of which have to do with the great work of nutrition. Among the variety of functions which belong to foliage leaves some of the most important may he selected for mention. It will be possible to do little more than indicate these functions until the plant with all its organs is considered, but some evidence can be obtained that various processes are taking place in the foliage leaf.
25. Photosynthesis—The most important function of the foliage leaf may be detected by a simple experiment. If an active leaf or a water plant be submerged in water ina glass vessel, and exposed to the hght, bubbles may be seen coming from the leaf surface and rising through the water (see Fig. 23). The water is merely a device by which the bubbles of gas may be seen. If the leaf is very active the
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETc. 29
bubbles are numerous. That this activity holds a definite relation to light may be proved by gradually removing the vessel containing the leaf from the light. As the light diminishes the bubbles diminish in number, and when a
Fig. 23. An experiment to illustrate the giving off of oxygen in the process of photo. synthesis.
certain amount of darkness has been reached the bubbles will cease entirely. If now the vessel be brought back gradually into the light. the bubbles will reappear, more and more numerous as the light increases. That this gas being given off is oxygen may be proved by collecting the
30 PLANT RELATIONS.
bubbles in a test tube, as in an ordinary chemical experi- ment for collecting gas over water, and testing it in the usual way.
Some very important things are learned by this experi- ment. It is evident that some process is going on within the leaf which needs light and which results in giving off oxygen. It is further evident that as oxygen is eliminated, the process indicated is dealing with substances which contain more oxygen than is needed. The amount of oxygen given off may be taken as the measure of the work. The more oxygen, the more work; and, as we have observed, the more light, the more oxygen; and no light, no oxygen. Therefore, light must be essential to the work of which the elimination of oxygen is an external indication. That this process, whatever it may be, is so essentially related to light, suggests the idea that it is the special process which demands that the leaf shall be a light-related organ. If so, it is a dominating kind of work, as it chiefly determines the life-relations of foliage leaves.
The process thus indicated is known as photosynthesis, and the name suggests that it has to do with the arrange- ment of material with the help of ight. It is really a pro- cess of food manufacture, by which raw materials are made into plant food. This process is an exceedingly important one, for upon it depend the lives of all plants and animals. The foliage leaves may be considered, therefore, as spectu/ organs of photosynthesis. They are special organs, not ex- clusive organs, forany green tissue, whether on stem or fruit or any part of the plant body, may do the same work. It is at once apparent, also, that during the night the process of photosynthesis is not going on, wnd therefore during the night oxygen is not being given off.
Another part of this process is not so easily observed, but is so closely related to the elimination of oxygen that it must be mentioned. Carbon dioxide occurs in the air to which the foliage leaves are exposed. It is given off from
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 81
our lungs in breathing, and also comes off from burning wood or coal. It is a common waste product, being a com- bination of carbon and oxygen so intimate that the two elements are separated from one another with great dif- ficulty. During the process of photosynthesis it has been discovered that carbon dioxide is being absorbed from the air by the leaves. As this gas is absorbed chiefly by green parts and in the light, in just the conditions in which oxy- gen is being given off, it is natural to connect the two, and to infer that the process of photosynthesis involves not only the green color and the light, but also the absorption of carbon dioxide and the elimination of oxygen.
When we observe that carbon dioxide is a combination of carbon and oxygen, it seems reasonable to suppose that the carbon and oxygen are separated from one another in the plant, and that the carbon is retained and the oxygen given back to the air. The process of photosynthesis may be partially defined, therefore, as the breaking up of carbon dioxide by the green parts of the plants in the presence of light, the retention of the carbon, and the elimination of the oxygen. The carbon retained is combined into real plant food, in a way to be described later. We may con- sider photosynthesis as the most important function of the foliage leaf, of which the absorption of carbon dioxide and the evolution of oxygen are external indications ; and that light and chlorophyll are in some way essentially connected with it.
26. Transpiration.—One of the easiest things to observe in connection with a working leaf is the fact that it gives off moisture. .A simple experiment may demonstrate this. If a glass vessel (bell jar) be inverted over a small active plant the moisture is seen to condense on the glass, and even to trickle down the sides. A still more convenient way to demonstrate this is to select a single vigorous leaf with a good petiole ; pass the petiole through a perforated card- board resting upon a tumbler containing water, and invert
32 PLANT RELATIONS.
a second tumbler over the blade of the leaf, which projects above the cardboard (see Fig. 24). It will be observed that moisture given off from the surface of the working leaf is condensed on the inner surface of the inverted tumbler. The cardboard is to shut off evaporation from the water in the lower tumbler.
When the amount of water given off by a single leaf is noted, some vague idea may be formed as to the amount ot moisture given off by a great mass of vegetation, such as a meadow or a forest. It is evident that green plants at work are contributing a very large amount of moisture to the air in the form of water vapor, moisture which has been absorbed by some region of the plant. The foliage leaf, therefore, may be regarded as an organ of transpiration, not that the leaves alone are engaged in transpiration, for many parts of the plant do the same thing, but because the foliage leaves are the chief seat of transpiration.
The important fact in connection with transpiration is not that moisture is given off by active foliage leaves, but that this escaping moisture is the external indication of some work going on within the leaf. Transpiration, therefore, may not be regarded so much as work, as the result, and hence the indication of work. In case the leaves are submerged, as is true of many plants, it is evi- dent that transpiration is practically checked, for the leaves are already bathed with water, and under such cir- cumstances water vapor is not given off. The same is true of green water plants without leaves (such as alge). It is evident that under such circumstances leaf work must be carried on without transpiration.
2%. Respiration ——.\nother kind of work also may be de- tected in the foliage leaf, but not so easily described. In fact it escaped the attention of hotanists long after they had discovered photosynthesis and transpiration. Itis work that goes on so long as the leaf is alive, never ceasing day or night. The external indication of it is the absorption
Fra. 24. Experiment illustrating transpiration.
34 PLANT RELATIONS.
of oxygen and the giving out of carbon dioxide. It will be noted at once that this is exactly the reverse of what takes place in photosynthesis. During the day, therefore, carbon dioxide and oxygen are both being absorbed and evolved. It will also be noted that the taking in of oxygen and the giving out of carbon dioxide is just the sort of exchange which takes place in our own respiration. In fact this pro- cess is also called respiration in plants. It does not depend upon light, for it goes on in the dark. It does not depend apon chlorophyll, for it goes on in plants and parts of plants which are not green. It is not peculiar to leaves, but goes on in every living part of the plant. A process which goes on without interruption in all living plants and animals must be very closely related to their living. We conclude, therefore, that while photosynthesis is peculiar to green plants, and only takes place in them when light is present, respiration is necessary to all plants in all conditions, and that when it ceases life must soon cease. The fact is, respiration supplies the energy which enables the living substance to work.
Once it was thought that plants differ from animals in the fact that plants absorb carbon dioxide and give off oxygen, while animals absorb oxygen and give off carbon dioxide. Tt is seen now that there is no such difference, but that respiration (absorption of oxygen and evolution of carbon dioxide) is common to both plants and animals. The difference is that green plants have the added work of photosynthesis.
We may also call the foliage leaf, therefore, an organ of respiration, because so much of such work is done hy it, but it must be remembered that respiration is going on in every living part of the plant.
This by no means completes the list of functions that might he made out for foliage leaves, but it serves to indicate both their peculiar work (photosynthesis) and the fact that they are doing other kinds of work as well.
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 85
B. Structure of foliage leaves.
28. Gross structure.—It is evident that the essential part of a foliage leaf is its expanded portion or blade. Often the leaf is all blade (sce Figs. 7, 8, 18) ; frequently there is a longer or shorter leaf-stalk (petiolv) which helps to put
ia CUE iegeaaey
sh sy :
QS
A
Fie. 25. Two types of leaf venation. The figure to the left is a leaf of Solomon’s seal (Polygonatum), and shows the principal veins parallel, the very minute cross veinlets being invisible to the naked cye, being a monocotyl type. The figure to the right is a leaf of a willow, and shows netted veins, the main central vein (mid- rib) sending out a series of parallel branches, which are connected with one another by a network of veinlets, being a dicotyl type.—After ErrINGsHAUSEN.
the blade into better light-relation (see Figs. 1, 9,17, 20, 26); and sometimes there are little leaf-like appendayes (stip- ules) on the petiole where it joins the stem, whose func- tion is not always clear. Upon examining the blade it is seen to consist of a green substance through which a
36 PLANT RELATIONS,
framework of veins is variously arranged. The large veins which enter the blade send off smaller branches, and these send off still smaller ones, until the smallest veinlets are
Fic. 26. A leaf of hawthorn, showing a short petiole, and a broad toothed blade with a conspicuous network of veins. Note the relation between the veins and the tecth.— After SrRASBURGER.
invisible, and the framework is a close network of branching veins. This is plainly shown bya ‘‘skel- eton” leaf, one which has been so treated that all the green sub- stance has disap- peared, and only the network of veinsremains. It will be noticed that in some leaves the veins and yeinlets are very prominent, in others only the main veins are prominent, while in some it is hard to detect any veins (see Figs. 25, 26).
20. Significance of leaf veins,—It is clear that the
framework of veins is doing at least two things for the blade: (1) it mechanically supports the spread out green sub- stance ; and (2) if conducts material to and from the green substance. No complete is the network of veins that this
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 37
support and conduction are very perfect (see Fig. 27). It is also clear that the green substance thus supported and supplicd with material is the important part of the leaf, the part that demands the light-relation, Study the various plans of the vein systems in Figs. 3, 4, 13, 18, 19, 20, 21,
2d, 20, d1, 70, 76, 52, 83, 92, 161.
Hs ; ees guia
CONS
Fie. 27. A plant (Fittonia) whose leaves show a network of veins, and also an adjust- ment to one another to form a mosaic.
30, Epidermis.—If a thick leaf be taken, such as that of a hyacinth, it will be found possible to peel off from its surface a delicate transparent skin (epidermis). This epidermis completely covers the leaf, and generally shows no green color. It is a protective covering, but at the same time it must not completely shut off the green substance beneath from the outside. It is found, therefore, that three important parts of an ordinary foliage leaf are: (1)
38 PLANT RELATIONS.
a network of veins; (2) a green substance (mesophyll) in the meshes of the network ; and (3) over all an epidermis.
31. Stomata.—If a compound microscope is used, some very important additional facts may be discovered. The thin, transparent epidermis is found to be made up of a layer of cells which fit closely together, sometimes dovetailing with each other. Curious openings in the epidermis will also be discovered, sometimes in very great numbers. Guarding each opening are two crescent-shaped cells, known as Hi Ba alk oP eae guard-cells, and between them a
of Maranta, showing the Slit-like opening leads through the interlocking walls, and a enidermis. The whole apparatus stoma (s) with its two guard-, cells. is known as a stoma (plural stomata), Which really means “mouth,” of which the guard-cells might he called the lips (see Figs. 28, 29). Sometimes stomata are found only on the under side of the leaf, sometimes only on the upper side, and sometimes on both sides.
The important fact about stomata is that the guard-cclls can change their shape, and so regulate the size of the opening. Itis not certain just how the guard-cells change their shape and just what stomata do for leaves. They are often called ‘breathing pores,” but the name is very inappropriate. Stomata Fi. 29. A single
: . ‘ stoma from the are not peculiar to the epidermis of foliage epidermis of a leaves, for they are found in the cpidermis lily leat, show- of any green part, as stems, young fruit, ae on ete. It is evident, therefore, that they hold of chlorophyl, an important relation to green tissue which Tee is covered by epidermis. Also, if we examine between,
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 39
foliage leaves and other green parts of plants which live submerged in water, we find that the epidermis contains no stomata. Therefore, stomata hold a definite relation to green parts covered by epidermis only when this epider- mis is exposed to the air.
It would seem that the stomata supply open passage- ways for material from the green tissue through the epider- mis to the air, vr from the air to the green tissue, or both. It will be remembered. however, that quite a number of substances are taken into the leaf and given out from it, so that it is hard to determine whether the stomata are specially for any one of these movements. For instance, the leaf gives out moisture in transpiration, oxygen in photosynthesis, and carbon dioxide in respiration ; while it takes in carbon dioxide in photosynthesis, and oxygen in respiration. It is thought stomata specially favor transpira- tion, and, if so, “‘ breathing pores” is not a happy phrase, for they certainly assist in the other exchanges.
32. Mesophyll.—If a cross-section be made of an ordi- nary foliage leaf, such as that of a lily, the three leaf regions can be secn in their proper relation to cach other. Bounding the section above and below is the layer of trans- parent epidermal cells. pierced here and there by stomata, marked by their peculiar guard-cells. Between the epi- dermal layers is the green tissue, known as the mesophyll. made up of cells which contain numerous small green bodies which give color to the whole leaf, and are known as chlorophyll bodies oy chloroplasts.
The mesophyll cells are usually arranged differently in the upper and lower regions of the leaf. In the upper region the cells are elongated and stand upright, present- ing their narrow ends to the upper leaf surface. forming the palisade tissue. In the lower region the cells are irreg- ular, and so loosely arranged as to leave passageways for air between, forming the spongy tissue. The air spaces among the cells communicate with one another, so that a system of
4
40 PLANT RELATIONS.
air chambers extends throughout the spongy mesophyll. It is into this system of air chambers that the stomata open, and so they are put into direct communication with the mesophyll or working cells. The peculiar arrangement of the upper mesophyll, to form the palisade tissue, has to do with the fact that that surface of the leaf is exposed to the direct rays of light. This light, so necessary to the mesophyll, is also dangerous for at least two reasons. If
Oy st
Fig. 30. A section through the leaf of lily, showing upper epidermis (we), lower cpi- dermis (Ze) with its stomata (sé), mesophyll (dotted cells) composed of the palisade region (p) and the spongy region (sp) with air spaces among the cells, and two veins (v) cut across.
the light is too intense it may destroy the chlorophyll, and the heat may dry out the cells. By presenting only nar- row ends to this direct light the cells are less exposed to intense light and heat. Study Fig. 30.
33. Veins,—In the cross-section of the leaf there will also be seen here and there, embedded in the mesophyll, the cut ends of the veinlets, made up partly of thick- walled cells, which hold the leaf in shape and conduet material to and from the mesophyll (see lig. 30).
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 41
C. Leuf protection.
34. Need of protection—Such an important organ as the leaf, with its delicate active cells well displayed, is ex- posed to numerous dangers. Chief among these dangers are intense light, drought, and cold. ATI leaves are not exposed to these dangers. For example, plants which grow in the shade are not in danger from intense light ; many
rater plants are not in danger from drought ; and plants of the tropical lowlands are in no
“Zs.
Fig. 31. Sections through leaves of the same plant, showing the effect of exposure to light upon the structure of the mesophyll. In both cases os indicates upper surface, and ws under surface. In the section at the left the growing leaf was exposed to direct and intense sunlight, and, as a consequence, all of the mesophyll cells have assumed the protected or palisade position. In the section at the right the leaf was grown in the shade, and none of the mesophyll cells have organized in palisade fashion.—After STAHL.
danger from cold. The danger from all these sources is be- cause of the large surface with no great thickness of body, and the protection against all of them is practically the same. Most of the forms of protection can be reduced to two general plans: (1) the development of protective structures between the endangered mesophyll and the air ; (2) the diminution of the exposed surface.
35. Protective structures—The palisade arrangement of mesophyll may be regarded as an adaptation for protection,
42 PLANT RELATIONS.
but it usually occurs, and does not necessarily imply ex- treme conditions of any kind. However, if the cells of the palisade tissue are unusually narrow and elongated, or
C. LY yyy : Sas Oh ae ti Ge
Fie. 82. Section through a portion of the leaf of the yew (7axus), showing cuticle (c), epidermis (e), and the upper portion of the palisade cells (p).
form two or three layers, we might infer the probability of exposure to intense light or drought. The accompanying illustration (Fig. 31) shows in a striking way the effect of light intensity upon the structure of the mesophyll, by contrasting leaves of the same plant exposed to the extreme conditions of ight and shade.
The most usual structural adaptations, however, are connected with the epidermis. The outer walls of the epi- dermal cells may become thickened, sometimes excessively so; the other epidermal walls may wlso become more or less thickened; or even what seems to be more than one epi- dermal Javer is found protecting the meso- phyll. If the outer
Fig. 33. Section through a portion of the leaf of walls of the epidermal carnation, showing the heavy cuticle (ew) HELLS ti formed by the outer walls of the epidermal eclls continue to
eclls (ep). Through the cuticle a pussageway thicken, the outer re-
leads to the stoma, whose two guard-cells are ane
secu lying between the two epidermal cells Son of the thick wall shown in the figure. Below the epidermal loses its structure cells some of the palisade cells (pad) are shown e ie containing chloroplasts, and below the stoma and forms the cuticle, is seen the air chamber into which it opens. which is one of the
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 48
Fie. 34. A hair from the leaf of Potentilla. It is seen to grow out from the epi- dermis,
best protective substances (see Fig. 32). Sometimes this cuticle be- comes so thick that the passage- ways through it leading down to the stomata become regular canals (see Fig. 33).
Another very common protective structure upon leaves is to be found in the great variety of hairs de- veloped by the epidermis. These may form but «a slightly downy covering, or the leaf may be cov- ered by a woolly or felt-like mass so that the epidermis is entirely concealed. The common mullein is a good illustration of a felt- covered leaf (see Fig. 36), In cold or dry regions the hairy covering of leaves is very noticeable, often
giving them a brilliant silky white or bronze look (see Figs. 34, 35). Sometimes, instead of a hair-like cover- ing, the epidermis develops scales of various patterns, often overlapping, and forming an excellent protection (see Fig. 37). In all these cases it should be remembered that these hairs and scales may serve other purposes also, as well as that of protection.
36, Diminution of exposed surface.— It will be impossible to give more than a few illustrations of this large subject. In very dry regions it has always been noticed that the leaves are small and
Fie, 35. A section through the leaf of bush clover (Lespedeza), showing upper and lower epidermis, palisade cells, and cells of the spongy region. The lower epidermis produces numerous hairs which bend sharply and lie along the leaf surface (appressed), forming a close covering.
44 PLANT RELATIONS.
Fig. 36. A branching hair from the leaf of common mullein. The whole plant has a felt-like covering composed of such hairs.
comparatively thick, although they may be very numerous (see Figs. 4, 167). In this way each leaf exposes a small
Fie. 37. A scale from the leaf of Shepherdia. These scales overlap and form a complete covering.
surface to the dry- ing air and intense sunlight. In our southwestern dry regions the cactus abounds, plants which have reduced their leaves so much that they are no longer used for chlorophyll work, and are not usually recognized as leaves. In their stead the globular or eylin- drical or flattened stems are green and
do leaf work (Figs,
‘stoond aes aq 0} adv 4] JO apIS Jay}LO UO pus ‘qanid jrasop PEAo[-[[CUIS B ST SOSNJOVO IBMUIN[OS OA} YI WAOMJO|T “sNyORO vad ATYord B SL PuNoASyorg atwonsa ay} ul puv { saioy snjovo
[volayds [[etus ov PUNOIS ayy UO Yor puL qs oy YB ! sulsoy snyovo TVUWULOD OIG Taj}UId OY NT ‘saavat YoryI Asda UTA ‘SoABSU OIU JJo[ PUB AYSII owlejxe 94} YW ‘“oovjans F89[ popes SurMoysS ‘s}1asap suyovo oyy WOIT sjuuid yo dnois y ‘ge ‘ong
Fic. 89, A group of cactus forms (slender cylindrical, columnar, and globular), all of them spiny and without leaves ; an agave in front ; clusters of yucca flowers in the background.
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 47
38, 89, 40, 185, 186, 187, 188). In the same regions the agaves and yuccas retain their leaves, but they become so thick that they serve as water reservoirs (see Figs. 38, 39,
A Fig. 40. A globular cactus, showing the ribbed stem, the strong spines, and the entire absence of leaves.
189). In all these cases this reduced surface is supple- mented by palisade tissue, very thick epidermal walls, and an abundant cuticle.
37. Rosette arrangement.—The rosette arrangement of leaves is a very common method of protection used by
48 PLANT RELATIONS.
small plants growing in exposed situations, as bare rocks and sandy ground. ‘he cluster of leaves, flat upon the ground, or nearly so, and more or less overlapping, is very effectively arranged for resisting intense light or drought or cold (see Figs. 11, 12, 48).
3x. Protective positions—In other cases, a position is assumed by the leaves which directs their flat surfaces so that they are not exposed to the most intense rays of light. The so-called “‘ com-
Fie. 41. A leaf of a sensitive plant in two conditions. In the figure to the left the leaf is fully expanded, with its four main divisions and numerous leaflets well spread. In the figure to the right is shown the same leaf aftcr it has been “‘shocked”? by » sndden touch, or by sudden heat, or in some other way. The leaflets have been thrown together forward aud upward ; the four main divisions have been moved together; and the main leaf-stalk has been directed sharply downward. The whole change has very much reduced the surface of exposure.— After DUCHARTRE.
pass plants,” already mentioned, are illustrations of this, the leaves standing elgewise and receiving on their surface the less intense rays of light (see Figs. 5, 165). In the dry regions of Australia the leaves on many of the forest, trees and shrubs have this characteristic edgewise position, known as the profile posi/ion, giving to the foliage a very curious appearance.
Some leaves have the power of shifting their position according to their necds, directing their flat surfaces to- ward the light, or more or less inchning them, according
| Q
Fie. 42. The telegraph plant (Desmodium gyrans). Each leaf is made up of three leaflets, a large terminal one, and a pair of small lateral ones. In the lowest figure the large leaflets are spread out in their day position ; in the central figure they are turned sharply downward in their night position. The name of the plant refers to the peculiar and constant motion of the pair of lateral leaflets, each one of which describes a curve with u jerking motion, like the second-hand of a watch, as indicated in the uppermost figure.
50 PLANT RELATIONS.
to the danger. Perhaps the most completely adapted leaves of this kind are those of the ‘‘sensitive plants,” whose leaves respond to various external influences by changing their positions. The common sensitive plant abounds in dry regions, and may be taken as a type of such plants (see Figs. 4, £1, 166). The leaves are divided into very numerous small leaflets, sometimes very small, which stretch in pairs along the leaf branches. When drought approaches, some of the pairs of leaflets fold to- gether, slightly reduc- ing the surface expo- sure. As the drought continues, more leaflets fold together, then still others, until finally all the leaflets may be folded together, and the leaves themsclyes may Fig. 43. Colyledons of squash seedling, show- bend. aginst the stem. darkness Gight Ngure)—Atter Arkanon. It i8 like a sailing vessel gradually tuking in sail as a storm approaches, until finally nothing is exposed, and the vessel weathers the storm by presenting only bare poles. Sensitive plants can thus regulate the exposed sur- face very exactly to the need.
Such motile lewves not only behave in this manner at the coming of drought, but the positions of the leaflets are shifted throughout the day in reference to light, and at night a very characteristic position is axsumed (sce Pigs. 2, 3, 42), once called a “sleeping position.” The danger from night exposure comes from the radiation of heat which oceurs, which may chill the leaves to the danger point. The night position of the leaflets of Ovel/x hag heen re- ferred to already (see $14). Similar changes in the direc- tion of the leaf planes at the coming of night may be observed in most of the Leyuminose, even the common
FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 51
white clover displaying it. It can be observed that the expanded seed leaves (codyledons) of many young germinat- ing plants shift their positions at night (see Fig. 43), often assuming a vertical position which brings them in contact with one another, and also covers the stem bud (pluie).
Certain leaves with well-developed protective structures are able to en- dure the winter, as in the case of the so-called evergreens. In the case of juniper, however, the winter and summer positions of the leaves are quite different (see Fig. 44). In the winter the leaves le close against the stem and overlap one another; while with the coming of warmer conditions they become widely spreading.
39. Protection against rain.—It is
also necessary for leaves to avoid Fis: 44 Two twiss of junt- ‘ per, showing the effect of
becoming wet by rain. If the water heat and cold upon the is allowed to soak in there is danger positions of the leaves.
Sree 7 = The ordinary protected of filling the stomata and interfering qintee osition “of the with the air exchanges. Hence it leaves is shown by 4; i ] Newel that ay aed ane while in B, in response to will be noticed that most leaves are varmer sconditious) the able to shed water, partly by their leaves have spread apart
and have become freely ex-
positions, partly by their structure. Sa Se
In many plants the leaves are so ar-
ranged that the water runs off towards the stem and so reaches the main root system: in other plants the rain is shed outwards, as from the eaves of a house.
Some of the structures which prevent the rain from soaking in are a smooth epidermis, a cuticle layer, waxy secretions. felt-like coverings, ete. Interesting experi- ments may be performed with different leaves to test their power of shedding water. If a gentle spray of water is allowed to play upon different plants, it will be observed
52 PLANT RELATIONS.
that the water glances off at once from the surfaces of some leaves, runs off more slowly from others, and may be more or less retained by others.
In this same connection it should be noticed that in most horizontal leaves the two surfaces differ more or less in appearance, the upper usually being smoother than the lower, and the stomata occurring in larger numbers, some- times exclusively, upon the under surface. While these differences doubtless have a more important meaning than protection against wetting, they are also suggestive in this connection.
CHAPTER IV. SHOOTS.
40. General characters.—The term shoot is used to include both stem and leaves. Among the lower plants, such as the alge and toadstools, there is no distinct stem and leaf. In such plants the working body is spoken of as the thallus, which does the work done by both stem and leaf in the higher plants. These two kinds of work are separated in the higher plants, and the shoot is differentiated into stem and leaves.
41. Life-relation—In seeking to discover the essential life-relation of the stem, it is evident that it is not neces- sarily a light-relation, as in the case of the foliage leaf, for many stems are subterranean. Also, in general, the stem is not an expanded organ, as is the ordinary foli- age leaf. his indicates that whatever may be its essential life-relaution it has little to do with exposure of surface. It becomes plain that the stem is the great leaf-hbearing organ, and that its life-relation is a leaf-relation. Often stems branch, and this increases their power of producing leaves.
In classifying stems, therefore, it seems natural to use the kind of leaves they bear. From this standpoint there are three prominent kinds of stems: (1) those bearing foli- age leaves ; (2) those bearing scaly leaves ; and (3) those bearing floral leaves. There are some peculiar forms of stems which do not bear leaves of any kind, but they need not be included in this general view.
b4 PLANT RELATIONS.
A. Stems bearing foliage leaves.
42. General character.—.\s the purpose of this stem is to display foliage leaves, and as it has been discovered that the essential life-relation of foliage leaves is the light-relation, it follows that a stem of this type must be able to relate its leaves to light. Itis, therefore, commonly aerial, and that it may properly display the leaves it is generally elongated, with its joints (xodes) bearing the leaves well separated (see Figs. 1, 4, 18, 20).
The foliage-bearing stem is generally the most conspicu- ous part of the plant and gives style to the whole body. Qne’s impression of the forms of most plants is obtained from the foliage-bearing stems. Such stems have great range in size and length of life, from minute size and very short life to’ huge trees which may endure for centuries. Branching is also quite a feature of foliage-bearing stems ; and when it occurs it is evident that the power of displiuy- ing foliave is correspondingly increased. Certain promi- nent types of foliage-bearing stems muy be considered.
+5. The subterranean type.—It may seem strange to in- clude any subterranean stem with those that bear foliage, as such a stem seems to be away from any light-relation. Ordinarily subterranean stems send foliage-bearing branches above the surface, and such stems are not to be classed as foliage-bearing stems. But often the only stem possessed by the plant is subterranean, and no branches are sent to the surface. In such cases only foliage leaves appear above ground, and they come directly from the subterranean stem. The ordinary ferns furnish a conspicuous illustration of this habit, all that is seen of them above ground being the characteristic leaves, the commonly called ++ stem” being only the petiole of the leaf (see Figs. 45, £6, 144). Many seed plants can also be found which show the same habit, especially those which flower early in the spring. This cannot be regarded as avery favorable type of stem for
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Fig. 45. A fern (Aspidium), showing three large branching leaves coming from a hori- zontal subterranean stem (rootstock) ; growing leaves are also shown, which are gradually unrolling. The stem, young leaves, and petioles of the large leaves are thickly covered with protecting hairs. The stem gives rise to numerous small roots from its lower surface. The figure marked 3 represents the under surface of a portion of the leaf, showing seven groups of spore cases; at 5 is represented a section through one of these groups, showing how the spore cases are attached and protected by a flap ; while at 6 is represented a single spore case opening and dis- charging its spores, the heavy spring-like ring extending along the back and over the top.—After WossIDLo,
5
56 PLANT RELATIONS.
leaf display, and as a rule such stems do not produce many foliage leaves, but the leaves are apt to be large.
Fie. 46. A common fern, showing the underground stem (rootstock), which sends the few large foliage leaves above the surface.—After ATKINSON.
The subterranean position is a good one, however, for purposes of protection against cold or drought, and when the foliage leaves are killed new ones can be put out by
SHOOTS. 57
the protected stem. This position is also taken advantage of for comparatively safe food storage, and such stems are apt to become more or less thickened and distorted by this food deposit.
44. The procumbent type.—In this case the main body of the stem lies more or less prostrate, although the advanc- ing tip is usually erect. Such stems may spread in all directions, and become interwoven into a mat or carpet. They are found especially on sterile and exposed soil,
G Fie. 47. A strawberry plant, showing a runner which has devel- oped a new plant, which in turn has sent out another run- ner.—After SEUBERT.
and there may be an important relation between this fact and their habit, as there may not be sufficient building material for erect stems, and the erect position might result in too much exposure to light, or heat, or wind, etc. Whatever may be the cause of the procumbent habit, it has its advan- tages. As compared with the erect stem, there is economy of building material, for the rigid structures to enable it to stand upright are not necessary. On the other hand, such a stem loses in its power to display leaves. Instead of being free to put out its leaves in every direction, one side is against the ground, and the space for leaves is diminished at least one-half. All the leaves it bears are necessarily directed towards the free side (see Fig. 18).
We may be sure, however, that any disadvantage com- ing from this unfavorable position for leaf display is over- balanced by advantages in other respects. The position is
58 PLANT RELATIONS.
certainly one of protection, and it has a further advantage in the way of migration and vegetative propagation. As the stem advances over the ground, roots strike out of the nodes into the soil. In this way fresh anchorage and new soil supplies are secured ; the old parts of the stem may
Fie. 48. Two plants of a saxifrage, showing rosette habit, and also the numerous runners sent out from the base, which strike root at tip and produce new plants. —After Kerner.
die, but the newer portions have their soil connection and continue to live. So effective is this habit for this kind of propagation that plants with erect stems often make use of it, sending out from near the base special prostrate branches, which advance over the ground and form new plants. A very familiar illustration is furnished by the straw- berry plant, which sends out peculiar naked ‘ runners” to strike root and form new plants, which then become
SHOOTS. 59
independent plants by the dying of the runners (see Figs. ae,
45. The floating type.—In this case the stems are sus- tained by water. Numerous illustrations can be found in small inland lakes and slow-moving streams (see Fig. 41). Beneath the water these stems often seem quite erect, but
Fie, 49. A submerged plant (Ceratophyllum) with floating stems, showing the stem joints bearing finely divided leaves.
when taken out they collapse, lacking the buoyant power of the water. (irowing free and more or less upright in the water, they seem to have all the freedom of erect stems in displaying foliage leaves, and at the same time they are not called upon to build rigid structures. Economy of building material and entire freedom to display foliage would seem to be a happy combination for plants. It must be noticed, however, that another very important condition is introduced. To reach the leaf surfaces the light must pass through the water, and this diminishes its intensity so
60 PLANT RELATIONS.
greatly that the working power of the leaves is reduced. At no very great depth of water a limit is reached, beyond which the light is no longer able to be of service to the leaves in their work. Hence it is that water plants are
Fig. 50. A vine or liana climbing the trunk of a tree. The leaves are all adjusted to face the light and to ayoid shading one an- other as far as possible,
restricted to the surface of the water, or to shoal places ; and in such places vegetation is very abundant. Water is so serious an impediment to light that very many plants bring their working leaves to the surface and float them, as seen in water lilies, thus obtaining light of undiminished intensity.
46. The climbing type.—Climb- ing stems are developed especially in the tropics, where the vegeta- tion is so dense and overshadow- ing that many stems have learned to climb upon the bodies of other plants, and so spread their leaves in better light (see Figs. 50, 55, 98, 201). Great woody vines fairly interlace the vegetation of tropical forests, and are known as ‘‘lianas,” or ‘‘lianes.*” The same habit is noticeable, also, in our temperate vegetation, but it is by no means so extensively dis- played as in the tropics. There are a good many forms of climb- ing stems. Remembering that the habit refers to one stem de- pending upon another for mechanical support, we may in- clude many hedge plants in the
SHOOTS. 61
list of climbers. In this case the stems are too weak to stand alone, but by interlacing with one another they may keep an upright position. There are stems, also, which climb by twining about their support, as the hop vine and
Fie, 51. A cluster of smilax, showing the tendrils which enable it to climb, and also the prickles.—After KERNER.
morning glory : others which put out tendrils to grasp the support (see Figs. 51, 52), as the grapevine and. star cucumber ; and still others which climb by sending out suckers to act as holdfasts, as the woodbine (see Figs. 53, 54). In all these cases there is an attempt to reach towards
62 PLANT RELATIONS.
the light without developing such structures in the stem as would enable it to stand upright.
47. The erect type—This type seems altogether the best adapted for the proper display of foliage leaves. Leaves
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Fig. 52. Passion-flower vines climbing supports by means of tendrils, which may be seen more or less extended or coiled, The two types of leaves upon a single stem may also be noted.
can be sent out in all directions and carried upward to- wards the light ; but it is at the expense of developing an elaborate mechanical system to enable the stem to retain this position. There is an interesting relation between these erect bodies and zones of temperature. At high alti-
SHOOTS.
st
Fie. 538. Woodbine (.4
mpelopsis) in a deciduous forest. The tree trunks are almost
covered by the dense masses of woodbine, whose leaves are adjusted so as to form compact mosaics. A lower stratum of vegetation is visible, composed of shrubs and tall herbs, showing that the forest is somewhat open.—After SCHIMPER.
tudes or latitudes the subter- ranean and prostrate types of foliage-bearing stems are most common ; and as one passes to lower altitudes or latitudes the erect stems become more nu- merous and more lofty. Among stems of the erect type the tree is the most impressive, and it has developed into a great vari- ety of forms or ‘‘ habits.” Any one recognizes the great differ- ence in the habits of the pine and the elm (see Figs. 56, 57, 48, 59), and many of our
Fig. 54. A portion of a woodbine (Ampelopsis). The stem tendrils have attached themselves to a smooth wall by means of disk-like suckers.—After STRASBURGER.
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Fie. 56.
A tree of the pine type (larch), showing the continuous central shaft and the horizontal branches, which tend to become more upright towards the top of the tree. The general outline is distinctly conical. such trees in periodically shedding its leaves,
The larch is peculiar among
Fie. 57. A pine tree, showing the central shaft and also the bunching of the needle leaves toward the tips of the branches where there is the best exposure to light.
SHOOTS. 67
common trees may be known, even at a distance, by their characteristic habits (see Figs. 60, 61, 62). The difficulty of the mechanical problems solved by these huge bodies is very great. They maintain form and position and en- dure tremendous pressure and strain.
Fie. 58. An elm in its winter condition, showing the absence of a continuous central shaft, the main stem soon breaking up into branches, and giving a spreading top. On each side in the background are trees of the pine type, showing the central shaft and conical outline,
68 PLANT RELATIONS.
48, Relation to light.—As stems bearing foliage leaves hold a special relation to light, it is necessary to speak of the influence of light upon the direction of growth, an
the spreading top.
influence known as heliotropism, already referred to under foliage leaves. In the case of an erect stem the tendency is to grow towards the source of light (see Figs. 1, 64).
SHOOTS. 69
This has the general result of placing the leaf blades at right angles to the rays of light, and in this respect the heliotropism of the stem aids in securing a favorable leaf position (see Figs. 63, 63a). Prostrate stems are differently affected by the light, however, being directed transversely to the rays of light. The same is true of many foliage
se
Fic. 60. An oak in its winter condition, showing the wide branching. The various directions of the branches have been determined by the light-relations.
branches, as may be seen by observing almost any tree in which the lower branches are in the general transverse posi- tion. These branches generally tend to turn upwards when they are beyond the region of shading. Subterranean stems are also mostly horizontal, but they are out of the influence of light, and under the influence of gravity, known as geotropism, which guides them into the trans- verse position, The climbing stem, like the erect one,
70 PLANT RELATIONS.
et
Fie. 61.
habit, and the tendency to grow in groups.
grows towards the light, while floating stems may be either erect or transverse.
B. Stems bearing scale leuves.
49, General character.—.A scale leaf is one which does not serve as foliage, as it does not develop the necessary chlorophyll. This means that it does not need such an exposure of surface, and hence scale leaves are usually much smaller, and certainly are more inconspicuous than foliage leaves. A good illustration of scale leaves is furnished hy the ordinary scaly buds of trees, in which the covering of overlapping scaly leaves is very conspicuous (see Fig. 65). As there is no development of chlorophyll in such leayes,
SHOOTS. 71
they do not need to be exposed to the light. Stems bearing only scale leaves, therefore, hold no necessary light-relation, and may be subterranean as well as aerial. For the same
Fie. 62. A group of weeping birches, showing the branching habit and the peculiar hanging branchlets. The trunks also show the habit of birch bark in peeling off in bands around the stem.
reason scale leaves do not need to be separated from one another, but may overlap, as in the buds referred to.
Sometimes scale leaves occur so intermixed with foliage 6
a <a
Fia. 63. Sunflowers with the upper part of the stem sharply bent towards the light, giving the leaves better exposure.—After SCHAFFNER.
SHOOTS. 73
leaves that no peculiar stem type is developed. In the pines scale leaves are found abundantly on the stems which are developed for foliage purposes. In fact, the main stem axes of pines bear only scale leaves, while short spur-like branches bear the characteristic needles, or foliage leaves, but the form of the stem is controlled by the needs of the foliage. Some very distinct types of scale-bearing stems may be noted.
50. The bud type. —TIn this case the nodes bearing the leaves remain close together, not sepa- rating, as is neces- sary in ordinary foliage-bearing stems, and the leaves overlap. In J a stem of this char- Fie. 63a. Cotyledons of castor-oil bean ; the seedling acter the later joints to the left showing the ordinary position of the may become s epa- cotyledons, the one to the right showing the curva-
j . ture of the stem in response to light from one rated and bear foli- side.—After ATKINSON.
age leaves, so that one finds scale leaves below and foliage leaves above on the same stem axis. This is always true in the case of branch buds, in which the scale leaves serve the purpose of protection, and are aerial, not because they need a light-relation, but because they are protecting young foli- age leaves which do.
Sometimes the scale leaves of this bud type of stem do not serve so much for protection as for food storage, and become fleshy. Ordinary bulbs, such as those of lilies, etc.,
Fie. 64. An araucarian pine, showing the central shaft, and the regular clusters of branches spreading in every direction and bearing numerous small leaves. The low- ermost branches extend downwards and are the largest, while those above become more horizontal and smaller. These dif- ferences in the size and direction of the branches secure the largest light expo- sure,
74 PLANT RELATIONS.
are of this character ; and as the main pur- pose is food storage the most favorable position is a subter- ranean one (see Fig. 66). Sometimes such scale leaves become very broad and not merely overlap but en- wrap one another, as in the case of the onion.
51. The tuber type. —The ordinary potato may be taken as an il- lustration (see Fig. 67). The minuie scale leaves. to be found at the ‘‘eyes” of the potato, do not overlap, which means that the stem joints are farther apart than in the bud type. The whole form of the stem results from its use as a place of food storage. and hence such stems are generally subterra- nean. Food storage, subterranean position, and reduced scale leaves are facts which seem to follow each other naturally.
SHOOTS.
52. The rootstock type.—This is prob- ably the most common form of subter- ranean stem. It is elongated, as are foli- age stems, and hence the scale leaves are well separated. It is prominently used for food storage, and is also admirably adapted for subterranean migration (see Fig. 68). It can do for the plant, in the way of migration, what prostrate foliage- bearing stems do, and isinamore protected position. Advancing beneath the ground, it sends up a succession of branches to the surface. It is a very efficient method for the “‘spreading” of plants, and is extensively used by grasses in coy- ering areas and forming turf. The persist- ent continuance of the worst weeds is often due to this habit (see Figs. 69, 70). It is impossible to remove all of the indefinitely branching rootstocks from the soil,
75
Fie. 65. Branch buds
of elm. Three buds (k) with their over- lapping scales are shown, each just above the scar (d) of an old leaf.— After BEHRENS.
Fie. 66. A bulb, made up of overlap- ping scales, which are fleshy on account of food storage. — After Gray.
and any fragments that remain are able to send up fresh crops of aerial branches.
53. Alternation of rest and activity.—In all of the three stem types just mentioned, it is important to note that they are associated with a remark- able alternation between rest and vigorous activity. From the branch buds the new leaves
76
Fie. 67. A potato plant, showing thesubterranean tubers.—
After STRASBURGER.
be covered suddenly with young vegetation.
PLANT RELATIONS.
emerge with great rapidity, and trees be- come covered with new foliage in a few days. From the sub- terranean stems the aerial parts come up so speedily that the surface of the ground seems to This sudden
change from comparative rest to great activity has been well spoken of as the ‘“‘awakening ” of vegetation.
C. Stems bearing floral leaves.
54. The flower.—The so-called “flowers” which certain plants produce represent another type of shoot, being stems with peculiar leaves. So attractive are flowers that they have been very much studied; and this fact has led many people to believe that flowers are the only parts of plants worth studying. Aside from the fact that a great many plants do not produce flowers, even in those that do the flowers are connected with only one of the plant pro- cesses, that of reproduction. Every one knows that flowers are exceedingly variable, and names
Fig. 68. The rootstock of Solo- mon’s seal; from the under side roots arc developed ; and on the upper side are seen the scars which mark the positions of the successive aerial branches which bear the leaves. The advanc- ing tip is protected by scales (forming a bud), and the posi- tions of previous buds are in- dicated by groups of ring-like scars which mark the attach- ment of former scales. Advanc- ing in front and dying behind such a rootstock may give rise to an indefinite succession of acrial plants.—After Gray.
SHOOTS. vue
have been given to every kind of variation, so that their study is often not much more than learning the definitions of names. However, if we seek to discover the life-rela- tions of flowers we find that they may be stated very simply.
55. Life-relations—The flower is to produce seed. It must not only put itself into proper relation to do this, but
Fic. 69. The rootstock of a rush (Juncus), showing how it advances beneath the ground and sends above the surface a succession of branches. The breaking up of such a rootstock only results in so many separate individuals.—After CowLes.
there must also be some arrangement for putting the seeds into proper conditions for developing new plants. In the production of seed it is necessary for the flower to secure a transfer of certain yellowish, powdery bodies which it pro- duces, known as pollen or pollen-grains, to the organ in which the seeds are produced, known as the pisti/. This transfer is called pollination. One of the important things, therefore, in connection with the flower, is for it to put
78 PLANT RELATIONS.
Fie. 70. An alpine willow, showing a strong rootstock developing aerial branches and roots, and capable of long life and extensive migration.—After SCHIMPER.
itself into such relations that it may secure pollination.
Fig. 71. A flower of peony, showing the four sects of floral organs: 4, the sepals, together called the calyx ; c, the petals, together called the corolla ; u, the numerous stamens; g, the two carpels, which contain the ovules.—After STRASBURGER.
Besides pollination, which is necessary to the production of seeds, there must be an arrangement for seed distribution. It is always well for seeds to he scattered, so as to be separated from one another and from the parent plant. The two great external prob- lems in connection with the flower, therefore, are polli-
SHOOTS.
nation and seed-distribution. It is necessary to call attention to certain peculiar features of this type of stem.
56. Structures.—The joints of the stem do not spread apart, so that the peculiar leaves are kept close together, usually forming a rosette-like cluster (see Fig. 71). These leaves are of four kinds: the lowest (outermost) ones (indi- vidually sepals, collectively calyx) mostly resemble small foliage leaves ; the next higher (inner) set (individually petals, collectively corolla) are usually the most conspicuous, delicate in texture and brightly col- ored; the third set (stamens) produces the pollen; the highest (innermost) set (car- pels) form the pistil and pro- duce the ovules, which are to become seeds. These four sets may not all be present in the same flower; the members of the same set may be more or less blended with one another, forming tubes, urns, etc. (see Figs. 72, 78, 74); or the dif- ferent members may be modi- fied in the greatest variety of ways.
Another peculiarity of this
type of stem is that when the ©
79
Fie.72. A group of flowers of the rose
family. The one at the top (Poten- tilla) shows three broad sepals, much smaller petals alternating with them, a group of stamens, and a large receptacle bearing numer- oussmall carpels. The central one (Alchemilla) shows the tips of two small sepals, three larger petals united below, stamens arising from the rim of the urn, and.a single pe- culiar pistil. The lowest flower (the common apple) shows the sepals, petals, stamens, and three styles, all arising from the ovary part of the pistil.—After Focke.
80 PLANT RELATIONS.
Fie. 73. <A flower of the tobacco plant: u, a complete flower, showing the calyx with its sepals blended below, the funnelform corolla made up of united petals, and the stamens just showing at the mouth of the corolla tube ; 6, acorolla tube split open and showing the five stamens attached to it near the base ; ¢, a pistil made up of two blended carpels, the bulbons base (containing the ovules) being the ovary, the long stalk-like portion the style, and the knob at the top the stigma.—After STRASBURGER.
last set of floral leaves (carpels) appear, the growth of the stem in length is checked and the cluster of floral leaves
a €
Fie. 74, A group of flower forms: a, a flower of harebell, showing a bell-shaped corolla composed of five petals ; }, a flower of phlox, showing a tubular corolla awith its five petals distinct above and sharply spreading ; c, a flower of dead-nettle, showing an irregular corolla with its five petals forming two lips above the funnel- form base ; d, a flower of toad-flax, showing a two-lipped corolla, and also a spur formed by the base of the corolla; e, a flower of the snapdragon, showing the two lips of the corolla closed.—After Gray,
SHOOTS. 81
appears to be upon the end of the stem axis. It is usual, also, for the short stem bearing the floral leaves to broaden
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Fie. 75. The Star-of-Bethlehem ( Ornithogalum), showing the loose cluster of flowers at the end of the stem. The leaves and stem arise from a bulb, which produces a cluster of roots below.—After STRASBURGER.
at the apex and form what is called a receptacle, upon which the close set floral leaves stand. __ Although many floral stems are produced singly, it is
82 PLANT RELATIONS. very common for them to branch, so that the flowers appear
in clusters, sometimes loose and spray-like, sometimes com- pact (see Figs. 75, 76, 77). For example, the common
AS TRY
Fie. 76. A flower cluster from a walnut tree.—After STRASBURGER.
dandelion ‘“ flower” is really a compact head of flowers. All of this branching has in view better arrangements for pollination or for seed-distribution, or for both.
The subject of pollination and seed-distribution will be considered under the head of reproduction.
SHOOTS. 88
STRUCTURE AND FUNCTION OF THE STEM.
57. Stem structure—The aerial foliage stem is the most favorable for studying stem structure, as it is not distorted by its position or by being a depository for food. If an active twig of an ordinary woody plant be cut across, it will
Fie. 77. Flower clusters of an umbellifer (Sitz2).—After STRASBURGER.
be seen that it is made up of four general regions (see Fig. 78): (1) an outer protecting layer, which may be stripped off as a thin skin, the epidermis ; (2) within the epider- mis a zone, generally green, the cortez ; (3) an inner zone of wood or vessels, known as the vascular region; (+4) a central pith.
58. Dicotyledons and Conifers—Sometimes the vessels
84 PLANT RELATIONS.
Fie. 78. Section across a young twig of box elder, showing the four stem regions: e, epidermis, represented by the heavy bound- ing line ; ¢c, cortex ; w, vascular cylinder ; D, pith.
are arranged in a hollow cylinder, just inside of the cortex, leaving what is called pith in the center (see Fig. 78). Sometimes the pith dis- appears in older stems or parts of stems and leaves the stem hollow. When the vessels are arranged in this way and the stem lives more than a year, it can increase in diameter by adding new vessels outside of the old. In
the case of trees these additions appear in cross-section like a series of concentric rings, and as there is usually but one growth period during the year, they are often called annual rings (see Fig. 79), and the age of a tree is often estimated
by counting them. This method of ascer- taining the age of a tree is not absolutely certain, as there may be more than one growth period in some years. In the case of trees and shrubs the epidermis is replaced on the older parts by layers of cork, which sometimes hecomes
very thick and makes Fie. 79, Section across a twig of box elder three years old, showing three annual rings, or growth
up the outer part of rings, in the vascular cylinder. The radiating what is commonl y lines (7m) which cross the vascular region (2¢) rep-
resent the pith rays, the principal ones extending called bark. from the pith to the cortex (c).
SHOOTS. 85
Stems which increase in diameter mostly belong to the great groups called Dicotyledons and Conifers. To the former belong most of our common trees, such as maple, oak, beech, hickory, etc. (see Figs. 58, 59, 60, 61), as well as the great majority of common herbs; to the latter belong the pines, hemlocks, etc. (see Figs. 46, 57, 192, 193, 194). This annual increase in diameter enables the tree tc put out an increased number of branches and hence foliage leaves each year, so that its capacity for leaf work be- comes greater vear after year. A reason for this is that the stem is conducting important food sup- plies to the leaves, and if it in- creases in diameter it can conduct more supplhes each year and give work to more leaves.
59. Monocotyledons—In other stems, however, the vessels are arranged differently in the central Fis. 80. A corn-stalk, showing
Gj : cross-section and longitudinal
region. Instead of forming a hol- esilon, Wheat mpneeat low cylinder enclosing a pith, they the scattered bundles of ves- are scattered through the central Bi paced ey region, as may be seen in the cross- fiber-like strands. section of a corn-stalk (see Fig. 80). Such stems belong mostly to a great group of plants known as Monocotyledons, to which belong palms, grasses. lilies, etc. For the most part such stems do not increase in diameter. hence there is no branching and no increased foliage from year to year. A palm well illustrates this habit, with its columnar, unbranching trunk, and its crown of foliage leaves, which are about the same in number from year to vear (see Figs. 81, 82).
60. Ferns,—The same is true of the stems of most fern- plants, as the vessels of the central region are so arranged that there can be no diameter increase, though the ar-
Fig. 81. A date palm, showing the unbranched columnar trunk covered with old leaf bases, and with a cluster of huge active leaves at the top, only the lowest portions of which are shown, Two of the very heavy fruit clusters are also shown,
SHOOTS. 87
rangement is very different from that found in Monocotyle- dons. It will be noticed how similar in general appearance is the habit of the tree fern and that of the palm (see Fig. 83).
61. Lower plants.—In the case of moss-plants, and such alge and fungi as develop stems, the stems are very much
ONO Was |
Pa a festa, PIAS Bey all as ns mn mi Pt Fie, 82. A palm of the palmetto type (fan palm), with low stem and acrown of large leaves.
simpler in construction, but they serve the same general purpose.
62. Conduction by the stem.—Aside from the work of producing leaves and furnishing mechanical support, the stem is a great conducting region of the plant. This sub- ject will be considered in Chapter X., under the general
head of ‘‘ The Nutrition of Plants,” 7
Fig. 83. <A group of tropical plants. To the left of the center is a tree fern, with its slender columnar stem and crown of large leaves. The large-leaved plants to the right are bananas (monocotyledons).
CHAPTER V.
ROOTS.
63. General character—The root is a third prominent plant organ, and it presents even a greater variety of rela- tions than leaf or stem. In whatever relation it is found it is either an absorbent organ or a holdfast, and very often both. For such work no light-relation is necessary, as in the case of foliage leaves ; and there is no leaf-relation, as in the case of stems. Roots related to the soil may be taken as an illustration.
It is evident that a soil root anchors the plant in the soil, and also absorbs water from the soil. If absorption is considered, it is further evident that the amount of it will depend in some measure upon the amount of surface which the roots expose to the soil. We have already noticed that the foliage leaf has the same problem of exposure, and it solves it by becoming an expanded organ. The question may be fairly asked, therefore, why are not roots expanded organs? The receiving of rays of light, and the absorbing of water are very different in their demands. In the former case a flat surface is demanded, in the latter tubular pro- cesses. The increase of surface in the root, therefore, is obtained not by expanding the organ, but by multiplying it. Besides, to obtain the soil water the roots must burrow in every direction, and must send out their delicate thread- like branches to come in contact with as much soil as pos- sible. Furthermore, in soil roots absorption is not the only thing to consider, for the roots act as holdfasts and must grapple the soil, This is certainly done far more effectively
90 PLANT RELATIONS.
by numerous thread-like processes spreading in every direc- tion than by flat, expanded processes.
It should also be noted that as soil roots are subterra- nean they are used often for the storage of food, as in the case of many subterranean stems. Certain prominent root types may be noted as follows :
64, Soil roots—These roots push into the ground with
great energy,
and their ab- sorbing sur- eat {iy ——~y_ faces are en-
ol eee tirely covered. Only the voung- est parts of a root system absorb actively, the older parts transporting the absorbed material to the stem, and help- ing to grip the
5 mm _ Fie. 84. Root tips of corm, showing root hairs, their position soil. The soil in reference to the growing tip, and the effect of the root is the most
surrounding medium upon their development : 1, in soil ; 2, in air; 3, in water. common root
type. being used by the great majority of seed plants and fern plants, and among the moss plants the very simple root-like pro- cesses are mostly soil-related. T'o such roots the water of the soil presents itself either as free wafer—that is, water that can be drained away—or as films of water adhering to each soil particle, often called water of adhesion. To come in contact with this water, not only does the root system usually branch profusely in every direction, but the youngest branches develop abundant absorbing hairs, or root hairs (see Fig. 84), which crowd in among the soil particles and
ROOTS.
91
absorb moisture from them. By these root hairs the absorb- ing surface, and hence the amount of absorption, is greatly
Fie. 85. Apparatus to show the influence of water (hydrotropism) upon the direction of roots. The ends (a) of the box have hooks for hanging, while the box proper is a cylinder or trough of wire netting and is filled with damp sawdust. In the sawdust are planted peas (g), whose roots (h, i, 4, m) first descend until they emerge from the damp sawdust, but soon turn back toward it— After Sacus.
increased. Individual root hairs do not last very long, but new ones are constantly ap- pearing just behind the advancing root tips, and the old ones are as constantly disap- pearing.
(1) Geotropism and hydrotropism. — Many outside influences affect roots in the direction of their growth, and as soil roots are espe- cially favorable for ob- serving these influ- ences, two prominent
ones may be mentioned. The influence of gravity, or the earth influence, is very strong in directing the soil root.
Fie. 86. Araspberry plant, whose stem has been bent down to the soil and has ‘‘ struck root.”—After BEAL.
92 PLANT RELATIONS.
As is well known, when a seed germinates the tip that is to develop the root turns towards the earth, even if it has come from the seed in some other direction. This earth influence is known as geotropism. Another directing in. fluence is moisture, or the water influence, known as hydrot-
Fie. 87. A section through the leaf-stalk of a yellow pond-lily (Vuphar), showing the numerous conspicuous air passages (s) by means of which the parts under water are aerated ; h, internal hairs projecting into the air passages; v, the much reduced and comparatively few vascular bundles.
ropism. By means of this the root is directed towards the most favorable water supply in the soil.
Ordinarily, geotropism and hydrotropism direct the root in the same general way, and so reinforce each other ; but the following experiment may be arranged, which will separate these two influences. Bore several small holes in the bottom of a box (such as a cigar box), suspended as in- dicated in Figure 85, and cover the bottom with blotting paper. Pass the root tips of several germinated seeds
ROOTS. 93
through the holes, so that the seeds rest on the paper, and the root tips hang through the holes. If the paper is kept moist germination will continue, but geotropism will pull the root tips downwards, and hydrotropism (the moist paper) will pull them upwards. In this way they will pursue a devious course, now directed by one influence and now by the other.
If a root system be examined it will be found that when there is a main axis (tap root) it is directed steadily downwards, while the branches are directed differently. This indicates that all parts of a root system are not alike in their response to these influ- ences. Several other influences are also con- cerned in directing soil
S Soe leae nee roots, and the path of conn. eatsses 0s: any root branch is a Fie. 88. Asection through the stem of a water- result of all of them. wort (Elatine), showing the remarkably large ; and regularly arranged air passages for root How variable they are aeration. The single reduced vascular bundle may be seen by the is central and connected with the small cor- : ; f tex by thin plates of cells which radiate like numerous directions the spokes of a wheel.—After ScHENCK.
which the branches travel, and the whole root system preserves the record of these numerous paths.
(2) The pull on the stem.—Another root property may be noted in connection with the soil root, namely the pull on the stem. When a strawberry runner strikes root at tip (see Fig. 47), the roots, after they obtain anchorage in the soil, pull the tip a little beneath the surface, as if they had gripped the soil and then slightly contracted. The same thing may be observed in the process known as
94 PLANT RELATIONS,
“layering,” by which a stem, as a bramble, is bent down and covered with soil. The covered joints strike root, and the pulling follows (see Fig. 86). A very plain illustration of the same fact can be obtained from many crevice plants. These plants send their root systems into the crevices of rocks, and spread a rosette of leaves against the rock face. In the next year a new rosette of leaves, developed further
Fie. 89. Section through the leaf of a quillwort (Zsoetes), showing the four large air chambers («), the central vascular region (b), and the very poorly developed cortex.
up the stem, is also found against the rock face. It is evident that the stem has been pulled back into the crevice enough to bring the new leaves against the rock, and this pulling has been effected by the new roots, which have laid hold of the crevice soil, or walls.
(3) Soil dangers.—In this connection certain soil dan- gers und the response of the roots should be noted. The soil may become poor in water or poor in certain essential materials, and this results in an extension of the root sys-
ROOTS. 95
tem, as if seeking for water and the essential materials. Sometimes the root system becomes remarkably extensive, visiting a large amount of soil in order to procure the necessary supplies. Sometimes the soil is poor in heat, and root activity is interfered with. In such cases it is very common to find the leaves massed against the soil, thus slightly checking the loss of heat.
Most soil roots also need free air, and when water covers the soil the supply is cut off. In many cases there is some way by which a supply of free air may be brought down into the roots from the parts above Fre. 90. Longitudinal section
? ti a fle through a young quillwort leaf, water ; sometimes y farge am showing that the four air cham-
passages in leaves and stems bers shown in Fig. 89 are not con- ‘ tinuous passages, but that there
y) P _ ges, (see iE igs. 87, 88, 89, 90) » some are four vertical rows of promi- times by developing special root nent chambers. The plates of structures which rise above the Cells separa tines the! chambers . a vertical row very soon become water level, as prominentl dead and full of air. In addition
? y
shown b the cypress in the to the work of aeration these air y y ; chambers are very serviceable in development of knees. These enabling the leaves to float when knees are outgrowths from roots _ they break off and carry the com-
paratively heavy spore cases. beneath the water of the cypress
swamp, and rise above the water level, thus reaching the air and aerating the root system (see Fig. 91). It has been shown that if the water rises so high as to flood the knees for any length of time the trees will die, but it does not follow that this is the chief reason for their development. 65. Water roots.—A very different type of root is devel- oped if it is exposed to free water, without any soil relation. If a stem is floating, clusters of whitish thread-like roots usually put out from it and dangle in the water. Ifthe water level sinks so as to bring the tips of these roots to the mucky
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ROOTS. 97
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Fie. 92. A tropical aroid (Anthurium), showing its large leaves, and bunches of aerial roots.
soil they usually do not penetrate or enter into any soil re- lation. Such pure water roots may be found dangling from the under surface of the common duck weeds, which often cover the surface of stagnant water with their minute, green, disk-like bodies.
98 PLANT RELATIONS. F
Plants which ordinarily develop soil roots, if brought into proper water relations, may develop water roots. For instance, willows or other stream bank plants may be so close to the water that some of the root system enters it. In such cases the numerous clustered roots show their water
van
Fic. 93. An orchid, showing aerial roots,
character. Sometimes root systems developing in the soil may cuter tile drains, when water roots will devclop in such clusters as {o choke the drain, The same bunching of water roots may he noticed when a hyacinth bulb is grown in a vessel of water.
60, Air roots—In certain parts of the tropics the air is so moist that it is possible for some plants to obtain sufti-
ROOTS. 99
cient moisture from this source, without any soil-relation or water-relation. Among these plants the orchids are most notable, and they may be observed in almost any green- house. Clinging to the trunks of trees, usually imitated in the greenhouse by nests of sticks, they send out long roots which dangle in the moist air (see Digs. 93, 94). It is necessary to have some special absorbing and condens- ing arrangement, and in the orchids this is usually pro- vided by the development of a sponge-like tissue about the root known as the velamen, which greedily absorbs the moisture of the air. Examine also Figs. 92, 95, 96, 7.
67. Clinging roots.—These roots are developed to fasten the plant body to some sup- port, and do no work of ab- sorption (see Fig. 98). Very common illustrations may be obtained from the ivies, the trumpet creeper, etc. These Hoots Gag to-vanous puppets, Beat danerciad, sowie world stone walls, tree trunks, etc., rooisand thicie. leaves. by sending minute tendril- like branches into the crevices. The sea-weeds (alga) develop grasping structures extensively, a large majority of them being anchored to rocks or to some rigid support beneath the water, and their bodies floating free. The root-like processes by which this anchorage is secured are very prominent in many of the common marine sea-weeds (see Fig. 157).
68. Prop roots—Some roots are developed to prop stems or wide-spreading branches. In swampy ground, or in tropical forests, it is very common to find the base of
or
#1e. 95. Astughorn fern (Platycerium), an aerial plant of the tropics. About it is a
Fig. 96. Selaginella, showing dangling aerial roots und nuely divided leaves.
Fig. 97. Live oaks, in the Gulf States, upon which are growing masses of long moss or black moss (Ti//andsia), a common aerial plant.
like holdfasts developed by certain
showing the cord
which pass around the tree trunks like ti
KERNER.
a)
Fie. 98. A tropical forest.
ghtly bound ropes.—After
lianas,
ROOTS. 103
tree trunks buttressed by such roots which extend out over and beneath the surface, and divide the area about the tree into a series of irregular chambers (see Fig. 100). Some-
x oe ee. ——s <= h LEW a i e-. 4 Fie. 99. A screw-pine (Pandanus), from the Indian Ocean region, showing the prominent prop roots put out near the base.
times a stem, either inclined or with a poorly developed
primary root system, puts out prop roots which support
it, as in the screw-pine (see Fig. 99). A notable case is 8
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106 PLANT RELATIONS.
that of the banyan tree, whose wide-spreading branches are supported by prop roots, which are sometimes very numerous (see Fig. 101). The immense banyans usually
Fie. 102. A dodder plant parasitic on a willow twig. The leafless dodder twines about the willow, and sends out sucking processes which penetrate and absorb,—After STRASBURGER.
illustrated are especially culti- vated as sacred trees, the prop roots being as- sisted in pene- trating the soil. There is record of such a tree in Ceylon with 350 large and 3,000 small prop roots, able to cover a village of 100 huts.
69. Parasites, —Besides roots related to soil, water, air, and various mechani- cal supports, there are others related to hosts. A host is a liv- ing plant or animal upon which some other plant or animal is living as a parasite,
The parasite gets its supplies from the host, and must be related to it properly. If the parasite grows upon the surface of its host, it must penetrate the body to obtain
ROOTS.
food supplies. Therefore, pro- cesses are devel- oped which pene- trate and absorb. The mistletoe and dodder are seed- plants which have this habit, and both have such processes (see Figs. 102, 103). This habit is much more extensively devel- oped, however, in a low group of plants known as the fungi. Many
107
Fic. 103. A section showing the living connection between dodder and a golden rod upon which it is
of these parasitic growing. The penetrating and absorbing organ (/)
fungi live upon plants and animals,
has passed through the cortex (c), the vascular zone (6), and is disorganizing the pith (yp).
common illustrations being the mildews of lilac leaves and many other plants, the rust of wheat, the smut of corn, ete.
Fic. 104. Section through the thallus of a liver- wort (Marchantia), showing the hair-like pro- cesses (rhizoids) which come from the under surface and act as roots in gripping and ab- sorbing. In the epidermis of the upper surface a chimney-like opening is seen, leading into a chamber containing cells with chloroplasts.
70. Root structure. —In the lowest groups of plants (alge, fungi, and moss-plants) true roots are not formed, but very simple struc- tures, generally hair- like (see Fig. 104). In fern-plants and seed- plants, however, the root is a complex structure, so different from the root-like pro-
108 PLANT RELATIONS.
cesses of the lower groups that it is regarded as the only true root. It is quite uniform in structure, consisting of a tough and fibrous central axis surrounded by a region of more spongy structure. The tough axis is mostly made up of vessels, so called because they conduct material, and is called the vascular axis, The outer more spongy region is the cortez, which covers the vascular axis like a thick skin (see Fig. 105).
One of the peculiarities of the root, in which it differs from the stem, is that the branches come from the vascular axis and burrow through the cortex, so that when the latter is peeled off the ranches are left attached to the axis, und the cortex ees ase - ‘anew aaaal shows the holes through which they
section through the root passed. It is evident that when such tip of shepherd’s purse, 4 root is absorbing, the absorbed ma- showing the central vas- e ‘ é cularaxis(p ),surroundea terial (water with various materials by the cortex (p), outside in solution) is received into the of the cortex the epi- 2 .
dermis (e) which disap. Cortex, through which it must pass pears in the older partsof {to the vascular axis to be conducted the root, and the promi- Pete shar
nent root-cap (¢).
Another pecuharity of the root is that it elongates only by growth at the tip, while the stem usually continues to elongate some distance behind its growing tip. In the soil this delicate growing tip is protected by a little cap of cells, known as the root-cap (see Fig. 105).
ep pt
fe Leas
a
i
CHAPTER VI.
REPRODUCTIVE ORGANS.
Ir will be remembered that nutrition and reproduction are the two great functions of plants. In discussing foliage leaves, stems, and roots, they were used as illustra- tions of nutritive organs, so far as their external relations are concerned. We shall now briefly study the reproductive organs from the same point of view, not describing the processes of reproduction, but some of the external relations. f Fa
71. Vegetative multiplica- f) 3 tion Among the very lowest plants no special organs of
reproduction are developed, B
Fic. 106. A group of spores: 4. but most plants have them. spores: fem. Common old a There is a kind of reproduc- fungus), which are so minute and : : : light that they are carried about by tion by which a portion of the air ; B, two spores from a com-
the parent body is set apart to mon alga (Ulothrix), which can . swim by means of the hair-like produce anew plant, as when processes; ’, the conspicuous dotted a strawberry runner produces cell is a spore developed by a com- a mon mildew (a fungus), which is n r i) a i HEY strawber ty pla t, e carried about by currents of air, when a willow twig or a grape cutting is planted and produces new plants, or when a potato tuber (a subterranean stem) produces new potato plants, or when pieces of Begonia leaves are used to start new Begonias. This is known as vegetative multiplication, a kind of repro-
duction which does not use special reproductive organs.
110 PLANT RELATIONS.
72. Spore reproduction.—Besides vegetative multiplica- tion most plants develop special reproductive bodies, known as spores, and this kind of reproduction is known as spore reproduction. These spores are very simple bodies, but have the power of producing new individuals. There are two great groups of spores, differing from each other not at all in their powers, but in the method of their production by the parent plant. One kind of spore is produced by dividing certain organs of the parent; in the other case two special bodies of the parent blend together to form the spore. Although they are both spores, for convenience Wwe may call the first kind spores (see Figs. 106,
Fic. 107. Fragments of « common alga (Spi- rogyra). Portions of two threads are shown, which have been joined together by the grow- 109), and the second
ing of connecting tubes. In the upper thread la - = four cells are shown, three of which contain kind eggs (see ue 1g.
eggs (z), while the cell marked g, and its mate 107). * The two special of the other thread each contain a gamete, a . the lower one of which will pass through the bodies which blend to- tube, blend with the upper one, and form gether to form an egg another!eee. are called gametes (see Figs. 107, 108, 109). These terms are necessary to any discussion of the external relations. Most plants develop both spores and eggs, but they are not always equally con- spicuous. Among the alge, both spores and eggs are prom- inent; among certain fungi the same is true, but many fungi are not known to produce eggs ; among moss-plants the spores are prominent and abundant, but the eve is concealed and not generally noticed. What has been said * Tt is recognized that this spore is really a fertilized egg, but in the absence of any accurate simple word, the term egg is used for con- venience,
REPRODUCTIVE ORGANS.
of the moss-plants is still more true of the fern-plants; while among the seed-plants certain spores (pol- len grains) wre conspicuous (see Fig. 110), but the eggs can be ob- served only by special manipulation in the luboratory. Seeds are neither spores nor eggs, but peculiar repro- ductive bodies which the hidden egg has helped to produce.
73. Germination,— Spores and eggs are expected to germinate ; that is, to begin the development of a new plant. This germination needs certain external conditions, prominent among which are defi- nite amounts of heat, moisture, and oxygen, and sometimes light. Conditions of germination may be observed most easily in connection with seeds. It must be understood, however, that what is called the germination of seeds is something
111
Fie. 108.
A portion of the body of a common alga
(Gdogonium), showing gametes of very unequal size and activity; a very large one (0) is lying in a globular cell, and a very small one is entering the cell, another similar one (s) being just outside. The two small gametes have hair-like pro- cesses and can swim freely. The small and large gam- etes unite and form an egg.
Fie. 109. A group of swim- ming cells: A, a spore of Edogonium (an alga); B, spores of Ulothrix (an alga); C, a gamete of Equisetum (horse-tail or scouring rush).
very different from the germination of spores and eggs. In the latter cases, germination includes the very beginnings of the young plant. In the case of a seed, germination begun by an egg has been checked, and seed germination is its renewal. In other words, an egg has germinated and produced a young plant called the ‘‘embryo,” and the germination of the seed simply consists in the continued growth and the escape of this embryo.
112 PLANT RELATIONS.
It is evident that for the germination of seeds light is not an essential condition, for they may germinate in the ght or in the dark ; but the need of heat, moisture, and oxygen is very apparent. The amount of heat re- quired for germination
varies widely with different Fo 210A pollen gain pore\from te seeds, some germinating in its transportation by currents of air. at much lower tempera- tures than others. Every kind of seed, or spore, or egg has a special temperature range, below which and above which it cannot germinate. The two limits of the range may be called the lowest and highest points, but be- tween the two there is a best point of temperature for germination. The same general fact is true in reference to the moisture supply.
V4. Dispersal of reproductive bodies, —Among the most striking external relations, however, are those con- nected with the dispersal of spores, gametes, and seeds. Spores and seeds must be carried away from the parent plant, and separated from each other, out of the reach of rivalry for nutritive material ; and gametes must come together and ee ne ee eee blend to form the eggs, Conspicuous (Epilobiun) opening and among the means of transfer are the —°XPosing its plumed seeds
7 which are transported by following. the wind.—After Brat.
REPRODUCTIVE ORGANS. 118
75. Dispersal by locomotion.—The common method of locomotion is by means of movable hairs (cilia) developed upon the reproductive body, which propel it through the
water (see Fig. 109). Swimming spores are very common among the alga, and at least one of the gametes in alge, moss-plants, and fern-plants has the power of swim- ming by means of cilia.
76. Dispersal by water,—It is very common for repro- ductive bodies to be transported by cur- rents of water. The spores of many water plants of all groups, not constructed for locomotion, are thus floated about. This method of transfer is also very common among seeds. Many seeds are buoyant, or become so after soak- ing in water, and may be carried to great distances by
Fie. 112. The upper figure to the left isan opening pod of fireweed discharging its plumed seeds. The lower figure represents the seed-like fruits of Clematis with their long tail-like plumes.— After KERNER.
currents. For this reason the plants growing upon the banks or flood-plains of streams may have come from a wide area. Many seeds can even endure prolonged soak- ing in sea-water, and then germinate. Darwin estimated
114 PLANT RELATIONS.
that at least fourteen per cent. of the seeds of any country can re- tain their vital- ity in sea-water for twenty- eight days. At the ordinary rate of move- ment of ocean currents, this length of time would permit such seeds to be transported
Fic, 113. A ripe dandelion head, showing the mass of plumes, a few seed-like fruits with their plumes still over a thou- attached to the receptacle, and two fallen off.—After gand miles, KERNER.
thus making possible a very great range in distribution.
77. Dispersal of spores by air.—This is one of the most common methods of transport- ies ing spores and seeds. In most 4a 4 cases spores are sufficiently ee ST small and light to be trans- ported by the gentlest move- ments of air. Among the fungi this is a very common method of spore dispersal (see Fig. 106), and it is extensively used in scattering the spores of moss-plants, fern-plants (see Fig. 45), and seed-plants. Among seed-plants this is one Fis 14. Seed-like fruits of Senecio
. 2 with plumes for dispersal by air.— method of pollination, the After KERNER.
REPRODUCTIVE ORGANS. 115
ae SN
Fie. 115. A winged seed of Bignonia.—After STRASBURGER.
spores called pollen grains being scattered by the wind, and occasionally falling upon the right spot for germination. With such an agent of transfer the pollen must be very light and powdery, and also very abun- dant, for it must come down al- most like rain to be certain of reaching the right places. Among the gymno- sperms (pines, hem- locks, ete.) this is the exclusive method of pollination, and when a pine forest is shedding pollen the air is full of the spores, which may be carried to a great
Nee distance before being Fig. 117. Winged fruit of Ptelea.—After a 7 KERNER. deposited. Occasional
Fic. 116. Winged fruit of maple.—After KERNER.
116
Fie. 118. Winged fruit of Ailanthus.—After Krer- NER.
PLANT RELATIONS.
reports of ‘‘ showers of sulphur” have arisen from an especially heavy fall of pollen that has been carried far from some gymnosperm forest. In the case of pines and their near relatives, the pollen spores are assisted in their dis- persal through the air by developing a pair of broad wings from the outer coat of the spore (see Fig. 110). This same method of pollination—that is, carrying the pollen spores by currents of air—is also used by many mono- cotyledons, such as grasses; and by many dicotyledons, such as our most
common forest trees (oak, hickory, chest- nut, ete.).
78. Dispersal of seeds by air,—Seeds are very rarely light enough to be carried by currents of air without some special adaptation. Wings and plumes of very many and often very beautiful patterns are exceedingly com- mon in. connection with seeds or seed- like fruits (see Figs. 15, Wb. Ta as: 119). Wings are de- veloped by the fruit of maples and of ash, and by the seeds
Fie. 119. Fruit of basswood (7%lia), showing the peculiar wing formed by a leaf.—After Kerner.
REPRODUCTIVE ORGANS. 117
Fie. 120. A common tumbleweed (Cycloloma).
of pine and catalpa. Plumes and tufts of hairs are devel- oped by the seed-like fruits of dandelion, thistle, and very many of their relatives, and by the seeds of the milkweed (see Figs. 111, 112,113, 114). On plains, or level stretches, where winds are strong, a curious habit of seed dis- persal has been de- veloped by certain plants known as ““tumbleweeds ” or ‘‘field rollers.” These plants are profusely branching
ith a small annuals Ww % Fie. 121. The 3-valved fruit of violet discharging root system in a ita seeds.—After Brat.
Fig. 122. A fruit of witch hazel discharging its seeds.— After BEAL.
PLANT RELATIONS.
light or sandy soil (see Fig. 120). When the work of the season is over, and the absorbing rootlets have shriveled, the plant is easily blown from its anchorage by a gust of wind, and is trundled along the surface like a light wicker ball, the ripe seed ves- sels dropping their seeds by the way. In case of an obstruction, such as a fence, great masses of these tumble- weeds may often be seen lodged against the windward side.
79. Discharge of spores.—In many plants the distribution of spores and seeds is not provided for hy any of
the methods just mentioned, but the vessels containing them are so constructed that they are discharged with
more or less violence and are some- what scattered.
Many spore cases, especially those,
of the lower plants, burst irregularly, and with sufficient violence to throw out spores. In the liverworts pecu- har cells, called eaters or ** jumpers,” are formed among the spores, and when the wall of the spore case is ruptured the elaters are liberated, and by their active motion assist in discharging the spores.
In most of the true mosses the spore case opens by pushing off a lid at the apex, which exposes a delicate fringe of teeth covering the mouth of the urn-like case. These teeth bend in and out of the open spore case as they become moist or
Fie. 123. A pod of wild bean bursting, the two valves violently twisting and dis- charging the seeds.—After Brat.
REPRODUCTIVE ORGANS. 119
dry, and are of considerable service in the discharge of spores.
In the common ferns a heavy spring-like ring of cells encircles the delicate-walled spore case. When the wall becomes dry and comparatively brittle the spring straightens with considerable force, the delicate wall is suddenly torn and the spores are discharged (see Fig. 45).
Even in the case of the pollen- spores of seed-plants, a special layer of the wall of the pollen-sac usually develops as a spring-like layer, which assists in opening widely the sac
{ i Fie. 125. A fruit of beggar ticks, showing the two barbed append- ages which lay hold of animals. —After BEAL.
9
5 é a when the wall be- Fig. 124. Fruits of Spanish needle, showing barbed ap-
gins to yield along pendages for grappling.
the line of break- The figure to the left is one i of the fruits enlarged.— ing. After KERNER.
80. Discharge of seeds,— While seeds are generally carried away from the parent plant by the agency of water currents or air currents, as al- ready noted, or by animals, in some in- stances there ix a mechanical discharge provided for in the structure of the seed- case. In such plants as the witch hazel and violet, the walls of the seed-vessel press upon the contained seeds, so that when rupture occurs the seeds are pinched out, as a moist apple-seed is discharged by being pressed between the thumb and finger (see Figs. 121, 122). In the touch- me-not a strain is developed in the wall of the seed-vessel, so that at rupture it
120 PLANT RELATIONS.
suddenly curls up and throws the seeds (see Fig. 123). The squirting cucumber is so named because it becomes very much distended with water, which is finally forcibly ejected along with the mass of seed. An *‘ artillery plant” common in cultivation discharges its seeds with considerable vio- lence ; while the detonations resulting from the explosions of the seed-vessels of Hura crepitans, the ‘‘monkey’s din- Fig. 126. The fruit of carrot, showing Neer bell,” are often remarked
the grappling appendages.—After by travelers in tropical
BEAL.
forests.
81. Dispersal of seeds by animals—Only a few illustra- tions can be given of this very large subject. Water birds are great carriers of seeds which are contained in the mud clinging to their feet and legs. This mud from the borders of ponds is usually completely filled with seeds and spores of various plants. One has no conception of the number until they are actually com- puted. The following ex- tract from Darwin's Origin of Species illustrates this point :
“T took, in February, three 2 a . tabl fuls of d:from thr Fig. 127. The fruit of cocklebur, showing ablespoontuls OF Mud trom three the grappling appendages.—After BEAL.
different points beneath water,
on the edge of a little pond. This mud when dried weighed only 64 ounces ; I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; and yet the viscid mud was all contained in a breakfast cup!”
Water birds are generally high and strong fliers, and the seeds and spores may thus be transported to the margins of distant ponds or lakes, and so very widely dispersed.
In many cases seeds or fruits develop grappling append-
REPRODUCTIVE ORGANS. 121
ages of various kinds, which lay hold of animals brushing past, and so the seeds are dispersed. Common illustrations are Spanish needles, beggar ticks, stick seeds. burdock, etc. Study Figs. 124, 125, 126, 127, 128, 129, 130.
Fie. 128. Fruits with grappling appendages. That to the left is agrimony ; that to the right is Galiwm.—After KERNER.
In still other cases the fruit becomes pulpy, and attrac- tive as food to certain birds or mammals. Many of the seeds (such as those of grapes) may be able to resist the attacks of the digestive fluids and escape from the alimen- tary tract in a condition to germinate. As if to attract the attention of fruit-eating animals, fleshy fruits usually become brightly col- ored when ripe, so that they are plainly seen in contrast with the foliage.
82. Dispersal of pol- len spores by insects.—
The transfer of pollen, the name applied to Fic. 129. Fruits with grappling appendages.
; The figure to the left is cocklebur ; that to the certain spores of seed- right is burdock.—After KERNER.
122 PLANT RELATIONS,
plants, is known as pollination, and the two chief agents of this transfer are currents of air and insects. In $77 the transfer by currents of air was noted, such plants being known as anemophilous plants. Such plants seldom produce what are generally recognized as true flowers. All those seed-plants which produce more or less showy flowers, however, are in some way related to the visits of oa cee ae insects to bring about pollination,
grappling appendages— and are known as entomophilous
eee plants. This relation between in- sects and flowers is so important and so extensive that it will be treated in a separate chapter.
CHAPTER VII.
FLOWERS AND INSECTS.
83. Insects as agents of pollination The use of insects as agents of pollen transfer is very extensive, and is the pre- vailing method of pollination among monocotyledons and dicotyledons. All ordinary flowers, as usually recognized, are related in some way to pollination by insects, but it must not be supposed that they are always successful in securing it. This mutually helpful relation between flow- ers and insects is a very wonderful one, and in some cases it has become so intimate that they cannot exist without each other. Flowers have been modified in every way to be adapted to insect visits, and insects have been variously adapted to flowers.
84. Self-pollination and cross-pollination.—The advantage of this relation to the flower is to secure pollination. The pollen may be transferred to the carpel of its own flower, or to the carpel of some other flower. The former is known as self-pollination, the latter as cross-pollination. In the case of cross-pollination the two flowers concerned may be upon the same plant, or upon different plants, which may be quite distant from one another. It would seem that cross-pollination is the preferred method, as flowers are so commouly arranged to secure it.
85. Advantage to insects.—The advantage of this relation to the insect is to secure food. This the flower provides either in the form of nectar or pollen ; and insects visiting flowers may be divided roughly into the two groups of nectar-feeding insects, represented by butterflies and moths,
124 PLANT RELATIONS.
and pollen-feeding insects, represented by the numerous bees and wasps. When pollen is provided as food, the amount of it is far in excess of the needs of pollination. The presence of these supplies of food is made known to the insect by the display of color in connection with the flowers, by odor, or by form. It should be said that the attraction of insects by color has been doubted recently, as certain experiments have suggested that some of the com- mon flower-visiting insects are color-blind, but remarkably keen-scented. However this may be for some insects, it seems to be sufficiently established that many insects rec- ognize their feeding ground by the display of color.
86. Suitable and unsuitable insects.—It is evident that all insects desiring nectar or pollen for food are not suit- able for the work of pollination. For instance, the ordi- nary ants are fond of such food, but as they walk from plant to plant the pollen dusted upon them is in great danger of being brushed off and lost. The most favorable insect is the flying one, that can pass from flower to flower through the air. It will be seen, therefore, that the flower must not only secure the visits of suitable insects, but must guard against the depredations of unsuitable ones.
87. Danger of self-pollination.—There is still another problem which insect-pollinating flowers must solve. If cross-pollination is more advantageous to the plant than self-pollination, the latter should be prevented so far as possible. As the stamens and carpels are usually close to- gether in the sume flower, the danger of self-pollination is constantly present in many flowers. In those plants which have stamen-producing flowers upon one plant and carpel- producing flowers upon another, there is no such danger.
88. Problems of pollination.—In most insect-pollinating flowers, therefore, there are three problems: (1) to prevent self-pollination, (2) to secure the visits of suitable insects, and (3) to ward off the visits of unsuitable insects. It must not be supposed that flowers are uniformly successful
FLOWERS AND INSECTS.
in solving these problems.
125
They often fail, but succeed
often enough to make the effort worth while. 80. Preventing self-pollination—It is evident that this danger arises only in those flowers in which the stamens
and carpels are associ- ated, but their separa- tion in different flowers may be considered as one method of prevent- ing self-pollination. In order to understand the various arrangements to be considered, it is nec- essary to explain that the carpel does not re- ceive the pollen indif- ferently over its whole surface. There is one definite region organ- ized, known as the stigma, upon which the pollen must be deposited if it is to do its work. Usually this is at the projecting point of the carpel, very often at the end of a stalk- like prolongation from the ovary (the bulbous part of the carpel), known as the style;
most
Fig. 131. Parts of the flower of rose acacia (Robinia hispida). In 1 the keel is show n pro- jecting from the hairy calyx, the other more showy parts of the corolla haying been re- moved. Within the keel are the stamens and the carpel, asseen in3. The keel forms the natural Janding place of a visiting bee, whose weight depresses the kee] and causes the tip of the style to protrude, as shown in 2. This style tip bears pollen upon it, caught among the hairs, seen in 3, and as it strikes the body of the bee some pollen is brushed off. If the bee has previously visited another flower and received some pollen, it will be seen that the stigma, at the very tip of the style, striking the body first, will very probably receive some of it. The nectar pit is shown in 3, at the base of the uppermost stamen.—After Gray.
When
sometimes it may run down one side of the style. the stigma is ready to receive pollen it has upon it a sweetish, sticky fluid, which holds and feeds the pollen. In this condition the stigma is said to be mature: and the pollen is mature when it is shedding, that is, ready to fall
126
PLANT RELATIONS.
out of the pollen-sacs or to be removed from them. The devices used by flowers containing both stamens and carpels to prevent self-pollination are very numerous, but most of them may be included under the three following heads :
Fie. 132. A portion of the flower of an iris, or flag. The single stamen shown is standing between the petal to the right and the petal-like style to the left. Near the top of this style the stigmatic shelf is seen extending to the tight, which must receive the pollen upon its upper sur- face. The nectar pit is at the junc- tion of the petal and stamen. While ob- taining the nectar the insect brushes the pollen-bearing part of the stamen. and pollen is lodged upon its body. In visiting the next flower and entering the stamen chamber the _ stig- matic shelf is apt to be brushed.—After GRAY,
(1) Pos/tion.—In these cases the pollen and stigma are ready at the same time, but their position in reference to each other, or in reference to some con- formation of the flower. makes it un- likely that the pollen will fall upon the stigma. The stigma may be placed above or beyond the pollen sacs, or the two may be separated by some mechan- ical obstruction, resulting in much of the irregularity of flowers.
In the flowers of the rose acacia and its relatives, the several stamens and the single carpel are in a cluster, en- closed in the keel of the flower. The stigma is at the summit of the style, and projects somewhat beyond the pollen-sacs shedding pollen. Also there is often a rosette of hairs, or bristles, just beneath the stigma, which acts as a barrier to the pollen (see Fig. 131).
In the iris, or common flag, each stamen is in a sort of pocket between the petal and the petal-like style, while the stigmatic surface is on the top of a flap, or shelf, which the stvle sends out asa roof to the pocket. With such an arrangement, it would seem impossible for the pollen to reach the stigma un- aided (see Fig. 132).
In the orchids, remarkable for their strange and beautiful flowers, there are
FLOWERS AND INSECTS. 127
usually two pollen-sacs, and stretched between them is the stigmatic surface. In this case, however, the pollen grains are not dry and powdery, but cling together in a mass, and cannot escape from the sac without being pulled out (see Fig. 133). The same sort of pollen is developed by the milkweeds.
(2) Consecutive maturity.—In these cases the pollen and
Fie. 183. A flower of an orchid (Habenaria). At 1 the complete flower is shown, with three sepals behind, and three petals in front, the lowest one of which has developed a long strap-shaped portion, and a still longer spur portion, the opening to which is seen at the base of the strap. At the bottom of this long spur is the nectar, which is reached by the long proboscis of a moth. The two pollen sacs of the single stamen are seen in the centre of the flower, diverging downwards, and between them stretches the stigma surface. The relation between pollen sacs and stigma surface is more clearly shown in 2. Within each pollen sac is a mass of sticky pollen, ending below in a sticky disk, which may be seenin 1 and 2. When the moth thrusts his proboscis into the nectar tube, his head is against the stig- matic surface and also against the disks. When he removes his head the disks stick fast and the pollen masses are dragged out. In 3 a pollen mass (q@) is shown sticking to each eye of a moth. Upon visiting another flower these pollen masses are thrust against the stigmatic surface and pollination is effected.—After GRAY.
128 PLANT RELATIONS.
stigma of the same flower are not mature at the same time. It is evident that this is a very effective method of prevent- ing self-pollination. When the pollen is being shed the stigma is not ready to receive, or when the stigma is ready to receive the pollen is not ready to be shed. In some cases the pollen is ready first, in other cases the stigma, the former condition being called protandry, the latter protogyny. This is a very common method of preventing self-pollination, and is com- monly not associated with irregularity.
The ordinary figwort may be taken as an example of protogyny. When the flowers first open, the style, bearing the stigma at its tip, is found Fie, 134. Flowers of fireweed (Zpi- protruding from the urn-like
lobium), showing protandry. Inithe flower, while the four stamens are thrust forward, and the style is sharply turned downward and stamens are curved down backward. In 2 the style is thrusts into the tube, and not ready forward, with its sigmiatic branches to shed their pollen. At spread, An insect in passing from 1 to 2 will almost certainly transfer po- some later time the style nea ai ha ltothestig bearing the stigma wilts, and the stamens straighten up and protrude from the tube. In this way, first the receptive stigma, and afterwards the shedding pollen-sacs. occupy the same position.
Protandry is even more common, and many illustrations can be obtained. For example, the showy flowers of the common fireweed, or great willow herb, when first opened display their eight shedding stamens prominently, the style being sharply curved downward and backward, carrying the four stigma lobes well out of the way. Later, the stamens bend away, and the style straightens up and ex- poses its stigma lobes, now receptive (see Fig. 154).
(3) Difference in pollen.—In these cases there are at
FLOWERS AND INSECTS. 129
least two forms of flowers, which differ from one another in the relative lengths of their stamens and styles. In the accompanying illustrations of Housfonta (see Fig. 135) it is to be noticed that in one flower the stamens are short and included in the tube, and the style is long and pro- jecting, with the four stigmas exposed well above the tube. In the other flower the relative lengths are exactly re- versed, the style being short and in- cluded in the tube, and the stamens long and projecting. It appears that the pollen from the short sta- mens is most effective upon
the stigmas of Fic. 135, Flowers of Houstonia, showing two forms of the sl tatvles flowers. In 1 there are short stamens and a long style ; CS ORES TTS: in 2 long stamens and short style. An insect visiting 1 and that the will receive a band of pollen about the front part of its "5 body ; upon visiting 2 this band will rub against the
pollen from the stigmas, and a fresh pollen band will be received upon
long stamens 1s the hinder part of the body, which, upon visiting another
g P
most effective flower like No. 1, will brush against the stigmas.— After GRayY.
upon the stig- mas of the long styles; and as short stamens and long stvles, or long stamens and short styles, are associated in the same flower, the pollen must be transferred to some other flower to find its appropriate stigma. This means that there is a difference between the pollen of the short stamens and that of the long ones.
In some cases there are three forms of flowers, as in one
130 PLANT RELATIONS.
of the common loosestrifes. Each flower has stamens of two lengths, which, with the style, makes possible three combinations. One flower has short stamens, middle-length stamens, and long style ; another has short stamens, middle- length style, and long stamens; the third has short style, middle-length stamens, and long stamens. In these cases also the stigmas are intended to receive pollen from stamens
Fic. 136. Yueca and Pronuba. In the lower figure to the right an opened flower shows the pendent ovary with the stigma region at its apex. The upper figure to the right shows the position of Pronuba when collecting pollen, The figure to the left represents a cluster of capsules of Yucca, which shows the perforations made by the larvee of Pronuba in escaping.—After RibEyY and TRELEASE.
of their own length, and a transfer of pollen from flower to flower is necessary.
90, Self-pollination.—In considering these three general methods of preventing self-pollination, it must not be sup- posed that self-pollination is never provided for. It is pro- vided for more extensively than was once supposed. — It is found that many plants, such as violets, in addition to the usual showy, insect-pollinated flowers, produce flowers that are not at all showy, in fact do not open, and are often not prominently placed. The fact that these flowers are often closed has suggested for them the name cletstogamous
FLOWERS AND INSECTS. 131
flowers. In these flowers self-pollination is a necessity, and is found to be very effective in producing seed.
91. Yueca and Pronuba.—There can be no doubt, also, that there is a great deal of self-pollination effected in flowers adapted for pollination by insects, and that the in- sects themselves are often responsible for it. But in the remarkable case of Yucca and Pronuba there is a definite arrangement for self-pollination by means of an insect (see Fig. 136). Yuccais a plant of the southwestern arid regions of North America, and Pronuba is a moth. The plant and the moth are very dependent upon each other. The bell- shaped flowers of Yucca hang in great terminal clusters, with six hanging stamens, and a central ovary ribbed lengthwise, and with a funnel-shaped opening at its apex, which is the stigma. The numerous ovules occur in lines beneath the furrows. During the day the small female Pronuba rests quietly within the flower, but at dusk becomes very active. She travels down the stamens, and resting on the open pollen-sac scoops out the somewhat sticky pollen with her front legs. Holding the little mass of pollen she runs to the ovary, stands astride one of the furrows, and _pierc- ing through the wall with her ovipositor, deposits an egg in an ovule. After depositing several eggs she runs to the apex of the ovary and begins to crowd the mass of pollen she has collected into the funnel-like stigma. These actions are repeated several times, until many eggs are deposited and repeated pollination has been effected. As a result of all this the flower is pollinated, and seeds are formed which develop abundant nourishment for the moth larve, which become mature and bore their way out through the wall of the capsule (Fig. 136).
92. Securing cross-pollination—In very many ways flow- ers are adapted to the visits of suitable insects. In ob- taining nectar or pollen as food, the visiting insect receives pollen on some part of its body which will be likely to come in contact with the stigma of the next flower visited.
Fie. 137. A clump of lady-slippers (Cypripedium), showing the habit of the plant and the general structure of the flower.—After Gipson.
FLOWERS AND INSECTS. 1838
Illustrations of this process may be tuken from the flowers already described in connection with the prevention of self-pollination.
In the flowers of the pea family, such as the rose acacia (see Fig. 151), it will be noticed that the stamens and pistil are concealed within the keel, which forms the natural land- ing place for the bees which are used in pol- lination. This keel is so inserted that the weight of the insect de- presses it, and the tip of the style comes in contact with its body. Not only does the stigma strike the body, but by the glancing blow the surface of the style is rubbed against the insect, and on this
style, below the stigma, : 7
x Fig. 188. Flower of Cypripedium, showing the the pollen has been de- flap overhanging the opening of the pouch, posited and is rubbed into which a bee is crowding its way. The off against the insect small figure to the right shows a side view of
the flap; that to the left a view beneath the At th e next fl ower flap, showing the two dark anthers, and be- visited the stigma is oS ae ann ikely to strike the pol-
len cptained from the previous flower, and the style will deposit a new supply of pollen.
In the flower of the common flag (see Fig. 132) the nectar is deposited in a pit at the bottom of the chamber formed by each style and petal. In this chamber the stamen is found, and more or less roofing it over is the flap, or shelf,
134 PLANT RELATIONS.
upon the upper surface of which the stigma is developed. As the insect crowds its way into this narrowing chamber, its body is dusted by the pollen, and us it visits the next flower and thrusts aside the stigmatic shelf, it is apt to deposit upon it some of the pollen previously received.
The story of pollination in connection with the orchids is still more complicated (see Fig. 133). Taking an ordi- nary orchid for illustration, the detailsare as follows. Each of the two pollen masses terminates in a sticky disk or button ; between them extends the concave stigma sur- face, at the bottom of which is the opening into the long tube-like spur in which the nectar is found. Such a flower is adapted to the large moths, with long probosces which can reach the bottom of the tube. As the moth thrusts its pro- boscis into the tube, its head touches the sticky button on each side, so that when it flies away these buttons stick
to its head, sometimes directly to its
arcs rn a eyes, and the pollen masses are torn
away) of Cypripedium. out. These masses are then carried
a to the next flower and are thrust against the stigma in the attempt to get the nectar.
In the lady-shpper (Cypripedium), another orchid, the flowers have « conspicuous pouch (see Fig. 137), in which the nectar is secreted. A peculiar structure, like a flap, overhangs the opening of the pouch, beneath which are the two anthers, und between them the stigmatic surface (see Fig. 138). Into the pouch a bee crowds its way and be- comes imprisoned (see Fig. 139). The nectar which the bee obtains is in the bottom of the pouch (see Fig. 140). When escaping, the bee moves towards the opening oyer- hung by the flap and rubs first against the stigmatic sur- face (see Fig. 141), and then against the anthers, receiving pollen on its back (see Fig. 142). A visit to another flower
FLOWERS AND INSECTS, 135
will result in rubbing some of the pollen upon the stigma, and in receiving more pollen for another flower.
In cases of protandry, as the common figwort, flowers in the two condi- tions will be visited by the pollinating insect, and as the shedding stamens and receptive stig- mas occupy the
same relative posi- SSE tion, the pollen Fie. 140. A bee obtaining nectar in the pouch of 5 Cypripedium.—After Gipson.
from one flower will be carried to the stigma of another. It is evident that exactly the same methods prevail in the case of protogyny, as the fireweed (see Fig. 134).
The Houstonia (see Fig. 135), in which there are sta- mens and styles of different lengths, is visited by insects whose bodies fill the tube and pro- trude above it. In visiting flowers of both kinds, one re- gion of the body receives pollen from the short sta-
Fic. 141. A bee escaping from the pouch of Cypri- mens, and another pedium, and coming in contact pale the stigma. region from the Advancing a little further the bee will come in con- tact with the anthers and receive pollen.—After long stamens, In
Cieeon this way the insect will carry about two bands of pollen, which come in con- tact with the corresponding stigmas. When there are three forms of flowers, as mentioned in the case of one of the loosestrifes, the insect receives three pollen bands, one for each of the three sets of stigmas.
93. Warding off unsuitable insects——Prominent among 10
136 PLANT RELATIONS,
the unsuitable insects, which Kerner calls ‘‘ unbidden guests,” are ants, and adaptations for reducing their visits to a minimum may be taken as illustrations.
(1) Hatrs.—A common device for turning back ants, and other creeping insects, is a barricr of hair on the stem, or in the flower cluster, or in the flower.
(2) Glandular secretions.—In some cases a sticky secretion is exuded from the surface of plants, which effectively stops the smaller creep- ing insects. In certain species of catch-fly a sticky ring girdles each joint of the stem.
(3) Lsolation.— The leaves of cer- tain plants form water reservoirs about the stem. To ascend such a stem, therefore, a _ creeping insect Fia. 142. A bee escaping from the pouch of Cypri- must cross a series
pedium, and rubbing against an anther.—After of such reservoirs. GIpson.
Teasel furnishes a common illustration, the opposite leaves being united at the base and forming a series of cups. More extensive water reservoirs are found in Bilbergia, sometimes called ‘“‘traveler’s tree,” whose great flower clusters are pro- tected by large reservoirs formed by the rosettes of leaves, which creeping insects cannot cross.
(4) Later.—This is a milky secretion found in some plants, as in milkweeds. Caoutchoue is a latex secretion of certain tropical trees. When latex is exposed to the air it stiffens immediately, becoming sticky and _ finally
FLOWERS AND INSECTS. 137
hard. In the flower clusters of many latex-secreting plants the epidermis of the stem is very smooth and deli- cate, and easily pierced by the claws of ants and other creeping insects who seek to maintain footing on the smooth surface. Wherever the epidermis is pierced the latex gushes out, and by its stiffening and hardening glues the insect fast.
(5) Protective forms.—In some cases the structure of the flower prevents the access of small creeping insects to the pollen or to the nectar. In the common snapdragon the two lips are firmly closed (see Fig. 74), and they can be forced apart only by some heavy insect, as the bumble-bee, alighting upon the projecting lower lip, all lighter insects being excluded. In many species of Pentstemon, one of the stamens does not develop pollen sacs, but lies like a bar across the mouth of the pit in which the nectar is secreted. Through the crevices left by this bar the thin proboscis of a moth or butterfly can pass, but not the whole body of a creeping insect. Very numerous adaptations of this kind may be observed in different flowers.
(6) Protective closure.—Certain flowers are closed at certain hours of the day, when there is the chief danger from creeping insects. For instance, the evening prim- roses open at dusk, after the deposit of dew, when ants are not abroad ; and at the same time they secure the visits of moths, which are night-fliers.
Numerous other adaptations to hinder the visits of unsuitable insects may be observed, but those given will serve as illustrations.
CHAPTER VIII. AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS.
For the purpose of summarizing the general life-rela- tions detailed in the preceding chapters, it will be useful to apply them in the case of a single plant. Taking a com- mon seed-plant as an illustration, and following its history from the germination of the seed, certain general facts become evident in its relations to the external world.
‘4, Germination of the seed.—'The most obvious needs of the seed for germination are certain amounts of moisture and heat. In order to secure these to the best advantage, the seed is usually very definitely related to the soil, either upon it and covered by moisture and heat-retaining debris, or embedded in it. Along with the demand for heat and moisture is one for air (supplying oxygen), which is essen- tial to life. The relation which germinating seeds need, therefore, is one which not only secures moisture and heat advantageously, but permits a free circulation of air.
45. Direction of the root.—The first part of the young plantlet to emerge from the seed is the tip of the axis which is to develop the root system. It at once appears to be very sensitive to the earth influence (geotropisim) and to moisture influence (hydrotropism), for whatever the direction of emergence from the seed, a curvature is devel- oped which directs the tip towards and finally into the soil (sce Fig. 143). When the soil is penetrated the primary root may continue to grow vigorously downward, showing a strong geotropic tendency, and forming what is known as the tap-root, from which lateral roots arise, which are
AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS. 189
much more influenced in direction by other external causes, especially the presence of moisture. As a rule, the soil is not perfectly uniform, und contact with different substances induces curvatures, and as a result of these and other causes, the root system may become very intricate, which is extremely favor- able for absorbing and gripping.
9. Direction of the stem. —as soon as the stem tip is extricated from the seed, h it exhibits sensitiveness to the light influence (hel¢ot- ropism), being guided in a general way towards the light (see Fig. 145¢). Direction towards the light, the source of the in- fluence, is spoken of as positive heliotropism, us distinguished from direc- tion away from the light, called negative heliotro-
Fie. 143. Germination of the seed of arbor-vite (Y7huja). B shows the emergence of the axis (7) which is to develop the root, and its turning to- wards the soil. (‘shows a later stage, in which the root (7) has been some- what developed, and the stem of the
pism. If the main axis continues to develop, it continues to show this posi- tive heliotropism strongly, but the branches may show
embryo (2) is developing a curve pre- paratory to pulling out the seed leaves (cotyledons). 2 shows the young plant- let entirely free from the seed, with its root (7) extending into the soil, its stem (hk) erect, and its first leaves (¢) hori- zontally spread.—After STRASBURGER.
every variation from positive to /ransverse heliotropism ; that is, a direction transverse to the direction of the rays of light. In some plants certain stems, as stolons, run- ners, etc., show strong transverse heliotropism, while other stems, as rootstocks, etc., show a strong transverse geot- ropism.
97. Direction of foliage leaves—The general direction of foliage leaves on an erect stem is transversely heliotropic ;
140 PLANT RELATIONS.
if necessary, the parts of the leaf or the stem itself twisting to allow the blade to assume this position. The danger of the leaves shading one another is reduced to a minimum by the elongation of internodes, the spiral arrangement, short- ening and changing direction upwards, or lobing.
This outhnes the general nutritive relations, the roots
Fie. 143a. Germination of the garden bean, showing the arch of the seedling stem above ground, its pull on the seed to extricate the cotyledons and plumule, and the final straightening of the stem and expansion of the young leaves.—After ATKINSON.
and leaves being favorably placed for absorption, and the latter also favorably placed for photosynthesis.
WS. Placing of flowers——The purposes of the flower seem to be served best by exposed positions, and consequently flowers mostly appear at the extremities of stems and branches, a position evidently favorable to pollination and seed dispersal. The flowers thus exposed are very com- monly massed, or, if not, the single flower is apt to be large and conspicuous. The various devices for protecting nec- tar and pollen against too great moisture, and the more
AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS. 141
delicate structures against chill ; for securing the visits of suitable insects, and warding off unsuitable insects; and for dispersing the seeds, need not be repeated.
99. Branch buds—If the plant under examination be a tree or shrub, branch buds will be observed to develop at the end of the growing season (see Fig. 65). This device for protecting growing tips through a season of dangerous cold is very familiar to those living in the temperate regions. The internodes do not elongate, hence the leaves overlap ; they develop little or no chlorophyll, and become scales. The protection afforded by these overlapping scales is often increased by the development of hairs. or by the secretion of mucilage or gum.
CHAPTER IX. THE STRUGGLE FOR EXISTENCE.
100. Definition. The phrase “struggle for existence” has come to mean, so far as plants are concerned, that it is usually impossible for them to secure ideal relations, and that they must encounter unfavorable conditions. The proper light and heat relations may be difficult to obtain, and also the proper relations to food material. It often happens, also, that conditions once fairly favorable may be- come unfavorable. Also, multitudes of plants are trying to take possession of the same conditions. <All this leads to the so-called “struggle,” and vastly more plants fail than succeed. Before considering the organization of plant societies, it will be helpful to consider some of the possible changes in conditions, and the effect on plants.
101. Decrease of water.—This is probably the most com- mon factor to fluctuate in the environment of a plant. Along the borders of streams and ponds, and in swampy places, the variation in the water is very noticeable, but the same thing is true of soilsin general. However, the change chiefly referred to is that which is permanent, and which compels plants not merely to tide over a drought, but to face a permanent decrease in the water supply.
Around the margins of ponds are very commonly seen fringes of such plants as bulrushes, cat-tail flags, reed- grasses, etc., standing in shoal water. As these plants grow close together, silt from the land is entangled by them, and presently it accumulates to such an extent that there is no more standing water, and the water supply for the
THE STRUGGLE FOR EXISTENCE. 148
bulrushes and their associates has permanently decreased below the favorable amount. In this way certain lake margins gradually encroach upon the water, and in so doing the water supply is permanently diminished for many plants. By the same process, smaller lakelets are gradually being converted into bogs, and the bogs in turn into drier ground, and these unfavorable changes in water supply are a menace to many plants.
The operations of man, also, have been very effective in diminishing the water supply for plants. Drainage, which is so extensively practiced, while it may make the water- supply more favorable for the plants which man desires, cer- tainly makes it very unfavorable for many other plants. The clearing of forests has a similar result. The forest soil is receptive and retentive in reference to water, and is somewhat like a great sponge, steadily supplying the streams which drain it. The removal of the forest destroys much of this power. The water is not held and gradually doled out, but rushes off in a flood; hence, the streams which drain the cleared area are alternately flooded and dried up. This results in a much less total supply of water available for the use of plants.
102. Decrease of light.—It is very common to observe tall, rank vegetation shading lower forms, and seriously interfering with the light supply. If the rank vegetation is rather temporary, the low plants may learn to precede or follow it, and so avoid the shading ; but if the over-shading vegetation is a forest growth, shading becomes permanent. In the case of deciduous trees, which drop their leaves at the close of the growing season and put out a fresh crop in the spring, there is an interval in the early spring, before the leaves are fully developed, during which low plants may secure a good exposure to light (see Fig. 144). In such places one finds an abundance of ‘‘spring flowers,” but later in the season the low plants become very scarce. This effective over-shading is not common to all forests, for
Fie. 144. A common spring plant (dog-tooth violet) which grows in deciduous forests. The large mottled leaves and the conspicuous flowers are sent rapidly above the surface from the subterrancan bulb (see cut in the left lower corner), where are also seen dissected out some petals and stamens and the pistil.
THE STRUGGLE FOR EXISTENCE. 145
there are ‘light forests,” such as the oak forest, which permit much low vegetation, as well as the shade forests, such as beech forests, which permit very little.
In the forest regions of the tropics, however, the shad- ing is permanent, since there is no annual fall of leaves. In such conditions the climbing habit has been extensively cultivated.
103. Change in temperature.—In regions outside of the tropics the annual change of temperature is a very im- portant factor in the life of plants, and they have previded for it in one way or another. In tracing the history of plants, however, back into what are culled ** geological times.” we discover that there have een relatively per- manent changes in temperature. Now and then glacial conditions prevailed, during which regions before temperate or eyen tropical were subjected to arctic conditions. It is very evident that such permanent changes of temperature must have had an immense influence upon plant life.
104. Change in soil composition.—One of the most ex- tensive agencies in changing the compositious of soils in certain regions has been the movement of glaciers of vonti- nental extent, which have deposited soil material over very extensive areas. Areas within reach of occasional floods, also, may have the soil much changed in character by the new deposits. Shifting dunes are billow-like masses of sand, developed and kept in motion hy strong prevailing winds, and often encroach upon other areas. Besides these changes in the character of soil by natural agencies. the yarious operations of man have been influential. Clearing, draining, fertilizing, all change the character of the soil, both in its chemical composition and its physical properties.
105. Devastating animals.—The ravages of animals form an important factor in the life of many plants. For example, grazing animals are wholesale destroyers of vegetation. and may seriously affect the plant life of an area. The various leaf feeders among insects have frequently done a vast
146 PLANT RELATIONS.
amount of damage to plants. Many burrowing animals attack subterranean parts of plants, and interfere seriously with their occupation of an area.
Various protective adaptations against such attacks have been pointed out, but this subject probably has been much exaggerated. The occurrence of hairs, prickles, thorns, and spiny growths upon many plants may discourage the attacks of animals, but it would be rash to assume that these protections have been developed because of the danger of such attacks. One of the families of plants most com- pletely protected in this way is the great cactus family, chiefly inhabiting the arid regions of southwestern United States and Mexico. In such a region succulent vegetation is at a premium, and it is doubtless true that the armor of thorns and bristles reduces the amount of destruction.
In addition to armor, the acrid or bitter secretions of certain plants or certain parts of plants would have a tendency to ward off the attacks of animals.
106. Plant rivalry.—It is evident that there must be rivalry among plants in occupying an area, and that those plants which can most nearly utilize identical conditions will be the most intense rivals. For example, a great many young oaks may start up over an area, and it is evident that the individuals must come into sharp competition with one another, and that but few of them succeed in establish- ing themselves permanently. This is rivalry between in- dividuals of the same kind ; but some other kind of trees, as the beech, may come into competition with the oak, and another form of rivalry will appear.
As w consequence of plant rivalry, the different plants which finally succeed in taking possession of an area are apt to be dissimilar, and a plant society is usually made up of plants which represent widely different regions of the plant kingdom. It is sometimes said that any well de- veloped plant society Is an epitome of the plant kingdom.
A familiar illustration of plant rivalry may be observed
THE STRUGGLE FOR EXISTENCE. 147
in the case of what are called ‘‘ weeds.” Every one is fa- miliar with the fact that if cultivated ground is neglected these undesirable plants will invade it vigorously and seri- ously affect the development of plants under cultivation.
107. Adaptation—When the changes mentioned above occur in the environment of plants to such an extent as to make the conditions for living very unfavorable, one of three things is likely to occur, adaptation, migration, or destruction.
The change in conditions may come slowly enough, and certain plants may be able to endure it long enough to adjust themselves to it. Such an adjustment may involve changes in structure, and probably no plants are plastic enough to adjust themselves to extreme and sudden changes which are to be comparatively permanent. There are plants, such as the common cress, which may be called amphibious, which can live in the water or out of it without change of structure, but this is endurance rather than adaptation. Many plants, however. can pass slowly into different conditions, such as drier soil, denser shade, etc., and corresponding changes in their structure may be noted. Very often, however, such plants are given no opportunity to adjust themselves to the new conditions, as the area is apt to be invaded by plants already better adapted. While adaptation may be regarded as a real result of changed con- ditions, it would seem to be by no means the common one.
108. Migration —This is a very common result of changed conditions. Plants migrate as truly as animals, though, of course, their migration is from generation to generation. It is evident, however, that migration cannot be universal, for barriers of various kinds may forbid it. In general, these barriers represent unfavorable conditions for living. Ifa plant area with good soil is surrounded by a sterile area, the latter would form an efficient barrier to migration from the former. Plants of the lowlands could not cross mountains to escape from unfavorable conditions.
148 PLANT RELATIONS.
To make migration possible, therefore, it is necessary for the conditions to be favorable for the migrating plants in some direction. In the case of bulrushes, cat-tail flags, etc., growing in the shoal water of a lake margin, the building up of soil about them results in unfavorable con- ditions. As a consequence, they migrate further into the lake. If the lake happens to be a small one, the filling up process may finally obliterate it, and a time will come when such forms as bulrushes and flags will find it impossible to migrate.
In glacial times very many arctic plants migrated south- ward, especially along the mountain systems, and many alpine plants moved to lower ground. When warmer con- ditions returned, many plants that had been driven south returned towards the north, and the arctic and alpine plants retreated to the north and up the mountains. The history of plants is full of migrations, compelled by changed con- ditions and permitted in various directions. It must be remembered, also, that migrations often result in changes of structure.
109. Destruction.—Probably this is by far the most com- mon result of greatly changed conditions. Even if plants adapt themselves to changed conditions, or migrate, their structure may be so changed that they will seem like quite different plants. In this way old forms gradually disappear and new ones take their places.
CHAPTER X. THE NUTRITION OF PLANTS.
110. Physiology.—In the previous chapters plants have been considered in reference to their surroundings. It was observed that various organs of nutrition hold certain life-relations, but it is essential to discover what these rela- tions mean to the life of the plant. The study of plants from the standpoint of their life-relations has been called Ecology ; the study of the life-processes of plants is called Physiology. These two points of view may be illustrated by comparing them to two points of view for the study of man. Man may be studied in reference to his relation to his fellow-men and to the character of the country in which he lives ; or his bodily processes may be studied, such as digestion, circulation, respiration, etc. The former cor- responds to Ecology, the latter is Physiology.
All of the ecological relations that have been mentioned find their meaning in the physiology of the plant, for life- relations have in view life-processes. The subject of plant physiology is a very complex one, and it would be impossi- ble in an elementary work to present more than a few very general facts. Certain facts in reference to plant move- ments. an important physiological subject, have been men- tioned in connection with life-relations, but it seems neces- sary to make some special mention of nutrition.
111. Significance of chlorophyll—Probably the most im- portant fact to observe in reference to the nutrition of plants is that some plants are green or have green parts, while others, such as toadstools, do not show this green
150 PLANT RELATIONS.
color. It has been stated that this green color is due to the presence of a coloring matter known as chlorophyll (see §12). The two groups may be spoken of, therefore, as (1) green plants and (2) plants without chlorophyll. The presence of chlorophyll makes it possible for the plants containing it to manufacture their own food out of such materials as water, soil material, and gases. For this reason, green plants may be entirely independent of all other living things, so far as their food supply is concerned.
Plants without chlorophyll, however, are unable to manufacture food out of such materials, and must obtain it already manufactured in the bodies of other plants or animals. or this reason, they are dependent upon other living things for their food supply, just as are animals. It is evident that plants without chlorophyll may obtain this food supply either from the living bodies of plants and ani- mals, in which case they are called parasites, or they may obtain it from the substances derived from the bodies of plants and animals, in which case they are called sapro- phytes. For example, the rust which attacks the wheat, and is found upon the leaves and stems of the living plant ; is a parasite, while the mould which often develops on stale bread is a saprophyte. Some plants without chlorophyll can live either as parasites or saprophytes, while others are always one or the other. By far the largest number of parasites and saprophytes belong to the group of low plants called fungi, and when fungi are referred to, it must be understood that it means the greatest group of plants with- out chlorophyll.
112. Photosynthesis.—The nutritive processes in green plants are the same as in other plants, and in addition there is in green plants the peculiar process known as photoxyn- thesis (see §25). In plants with foliage leaves, these are the chief organs for this work. It must be remembered, however, that leaves are not necessary for photosynthesis, for plants without leaves, such as alge, perform it. The
THE NUTRITION OF PLANTS. 1651
essential thing is green tissue exposed to light, but in this brief account an ordinary leafy plant growing in the soil will be considered.
As the leaves are the active structures in the work of photosynthesis, the raw materials necessary must be brought to them. Ina general way, these materials are carbon di- oxide and water. The gas exists diffused through the atmosphere, and so is in contact with the leaves. It also occurs dissolved in the water of the soil, but the gas used is absorbed from the air by the leaves. The supply of water, ou the other hand, in soil-related plants, is obtained from the soil. The root system absorbs this water, which then ascends the stem and is distributed to the leaves.
(1) Ascent of water.—The water does not move up- wards through all parts of the stem, but is restricted to a certain definite region. This region is easily recognized as the woody part of stems. Sometimes separate strands of wood, looking like fibers, may be seen running lengthwise through the stem ; sometimes the fibrous strands are packed so close together that they form a compact woody mass, as in shrubs and trees. In the case of most trees new wood is made each year, through which the water moves. Hence the very common distinction is made between sap-wood, through which the water is moving, and heart-wood, which the water current has abandoned. Just how the water ascends through these woody fibers, especially in tall trees, is a matter of much discussion, and cannot be regarded as definitely known. In any event, it should be remembered that these woody fibers are not like the open veins and arteries of animal bodies, and no ‘‘ circulation ” is possible. These same woody strands are seen branching throughout the leaves, forming the so-called vein system, and it is evi- dent, therefore, that they form a continuous route from roots to leaves.
It is easy to demonstrate the ascent of water in the
stem, and the path it takes, by a simple experiment. If 11
152 PLANT RELATIONS.
an active stem be cut and plunged into water stained with an aniline color called eosin,* the ascending water will stain its pathway. After some time sections through the stem will show that the water has traveled upwards through it, and the stain will point out the region of the stem used in the movement.
In general, therefore, the carbon dioxide is absorbed directly from the air by the leaves, and the water is ab- sorbed by the root from the soil, and moves upwards through the stem into the leaves. An interesting fact about these raw materials is that they are very common waste products. They are waste products because in most life-processes they cannot be taken to pieces and used. The fact that they can be used in photosynthesis shows that it is a very re- markable life process.
(2) Chloroplasts.—Maving obtained some knowledge of the raw materials used in Fig. 145. Some mesophyll cells from photosynthesis, and their
the leaf of Fitlonia, showing chloro- SOUTCCS, it 1s necessary to
piaste: consider the plant machinery arranged for the work. In the working leaf cells it is discovered that the color is due to the presence of very smiull green bodies, known as chlorophyll bodies or chloro- plasts (see Fig. 145). These consist of the living substance, known as protoplasm, and the green stain called chloro- phyll; therefore, each chloroplast is a living body (plastid) stained green. It is in these chloroplasts that the work of photosynthesis is done. In order that they may work it is necessary for them to obtain a supply of energy from some outside source, and the source used in nature is sun- light. The green stain (chlorophyll) seems to be used in absorbing the necessary energy from sunlight, and the
* The commoner grades of red ink are usually solutions of eosin.
THE NUTRITION OF PLANTS. 153
plastid uses this energy in the work of photosynthesis. It is evident, therefore, that photosynthesis goes on only in the sunlight, and is suspended entirely at night. It is found that any intense light can be used as a substitute for sunlight, and plants have been observed to carry on the work of photosynthesis in the presence of electric light.
(3) Result of photosynthesis.—The result of this work can be stated only in a very general way. Carbon dioxide is composed of two elements, carbon and oxygen, in the proportion one part of carbon to two parts of oxygen. Water is also composed of two elements, hydrogen and oxy- gen. In photosynthesis the elements composing these sub- stances are separated from one another, and recombined in anew way. In the process a certain amount of oxygen is liberated, just as much as was in the carbon dioxide, and a new substance is formed, known as a carbohydrate. The oxygen set free escapes from the plant, and may be re- garded as waste product in the process of photosynthesis. It will be remembered that the external changes in this process are the absorption of carbon dioxide and the giving off of oxygen (see §25).
(4) Carbohydrates und proteids.—The carbohydrate formed is an organic substance; that is, a substance made in nature only by life processes. It is the same kind of substance as sugar or starch, and all are known as carbohy- drates ; that is, substances composed of carbon, and of hy- drogen and oxygen in the same proportion as in water. The work of photosynthesis, therefore, is to form carbohy- drates. The carbohydrates, such as sugar and starch, rep- resent but one type of food material. Proteids represent another prominent type, substances which contain carbon, hydrogen, and oxygen, as do carbohydrates, but which also contain other elements, notably nitrogen, sulphur, and phosphorus. The white of an egg may be taken as an ex- ample of proteids. They seem to be made from the carho-
154 PLANT RELATIONS.
hydrates, the nitrogen, sulphur, and other necessary additional elements being obtained from soil substances dissolved in the water which is absorbed and conveyed to the leaves.
113. Transpiration.—The water which is absorbed by the roots and passes to the leaves is much more abundant than is needed in the process of photosynthesis. It should he re- membered that the water is not only used as a raw material for food manufacture, but also acts as a solvent of the soil materials and carries them into the plant. The water in excess of the small amount used in food manufacture is given off from the plant in the form of water vapor, the process being already referred to as (ranspiration (sev $26).
114. Digestion. —Carbohydrates and proteids may be re- garded as prominent types of plant food which green plants are able to manufacture. These foods are trans- ported through the plant to regions where work is going on, and if there is a greater supply of food than is needed for the working regions, the excess is stored up in some part of the plant. Asa rule, green plants are able to manufac- ture much more food than they use, and it is upon this ex- cess that other plants and animals live. In the transfer of foods through the plant certain changes are often neces- sary. For example, starch is msoluble, and hence cannot be carried about in solution. It is necessary to transform it into sugar, which is soluble. These changes, made to facilitate the transfer of foods, represent digestion.
115. Assimilation—When food in some form has reached a working region, it is organized into the living substance of the plant, known us protoplasm, and the protoplasin builds the plant structure. This process of organizing the food into the living substance is known is axsimilation.
116. Respiration —The formation of foods, their diges- tion and assimilation are all preparatory to the process of respiration, which may be called the use of assimilated food. The whole working power of the plant depends
THE NUTRITION OF PLANTS. 155
upon respiration, which means the absorption of oxygen by the protoplasm, the breaking down of protoplasm, and the giving off of carbon dioxide and water as wastes. The im-
Fic. 146. The common Northern pitcher plant. The hollow leaves, each with a hood and a wing, form a rosette, from the center of which arise the flower stalks,— After KERNER,
portance of this process may be realized when it is remem- bered that there is the same need in our own living, as it is essential for us also to ‘‘ breathe in” oxygen, and asa result we ‘** breathe out” carbon dioxide and water. This breaking down or ‘‘ oxidizing * of protoplasm releases the
156 PLANT RELATIONS.
power by which the work of the plant is carried on (see
$27).
117. Summary of life-processes—T'o summarize the nu- tritive life-processes in green plants, therefore, photosyn-
Fie. 147, The Southern pitcher plant, showing the funnelform and winged pitcher, and the overarching hood with translu- cent spots.—After KERNER.
thesis manufactures carbohydrates, the materials used being carbon dioxide and water, the work being done by the chloroplast with the aid of light ; the manufacture of proteids uses these carbohydrates, and also substances containing nitrogen, sulphur, etc.; digestion puts the insoluble carbohydrates and the proteids into a soluble form for transfer through the plant; assimilation converts this food material into the living sub- stance of the plant, protoplasm ; respiration is the oxidizing of the protoplasm which enables the plant to work, oxygen being ab- sorbed, and carbon dioxide and water vapor being given off in the process.
118. Plants without chlorophyll. —Remembering the life-processes described under green plants, it is evident that plants without chlo- rophyll cannot do the work of photosynthesis. This means that they cannot manufacture carbo- hydrates, and that they must de-
pend upon other plants or animals for this important food.
Mushrooms, puff-balls,
molds, mildews, rusts, dodder,
corpse plants, beech drops, etc., may be taken as illustra-
tions of such plants.
THE NUTRITION OF PLANTS, 157
Although plants without chlorophyll cannot manufac- ture carbohydrates, the other processes, proteid manufac- ture, digestion, assimilation, and respiration, are carried on. It is true, however, that in obtaining carbohydrates from
other plants and ani- mals, proteids are ob- tained also, so that proteid manufacture is not go prominent as in green plants.
119. “ Carnivorous” plants.—This name has been given to plants which have developed the curious habit of capturing insects and using them for food. They are green plants and, therefore, can man- ufacture carbohydrates. But they live in soil poor in nitrogen com- pounds, and hence pro- teid formation is inter- fered with. The bodies of captured insects sup- plement the proteid supply, and the plants have come to depend upon them. Many, if not all of these carniy- orous plants, secrete a digestive substance which acts upon the
Fie. 148. The Californian pitcher plant (Darling- tonia), showing twisted and winged pitcher, the overarching hood with translucent spots, and the fish-tail appendage to the hood which is attractive to flying insects.—After KERNER.
bodies of the captured insects very much as the diges- tive substances of the alimentary canal act upon proteids
158 PLANT RELATIONS.
swallowed by animals. Some common illustrations are as follows :
(1) Pitcher plants.—In these plants the leaves form tubes, or urns, of various forms, which contain water, and to which insects are attracted and drowned (see Fig. 146). A pitcher plant common throughout the Southern States may be taken as a type (see Fig. 147). The leaves are shaped like slender, hollow cones, and rise in a tuft from the swampy ground. The mouth of this conical urn is over- arched and shaded by a hood, in which are translucent spots, like small windows. Around the mouth of the urn are glands, which se- crete a sweet hquid (nectar), and nectar drops form a_ trail down the outside of Ky theurn. Inside, just below the rim of the at 3 : urn, is a glazed zone,
‘ TAS so smooth that insects
LE NRE cannot walk wpon it.
Fie. 149. A sun-dew, showing rosette habit of Below the glazed zone
the insect-catching leayes. is unother zone Pi
thickly set with stiff,
downward-pointing hairs, and below this is the liquid in the bottom of the urn.
If a fly is attracted by the nectar drops upon this curious leaf, it naturally follows the trail wp to the rim of the urn, where the nectar is abundant. If it attempts to descend within the urn, it slips on the glazed zone, and falls into
THE NUTRITION OF PLANTS. 159"
the water, and if it attempts to escape by crawling up the sides of the urn, the thicket of downward-pointing hairs prevents. If it seeks to fly away from the rim, it flies towards the translucent spots in the hood, which look like the way of escape, as the direction of entrance is in the shadow of the hood. Pounding against the hood, the fly falls into the tube. This Southern pitcher plant is known
Bile Ce rs a gd
aoe
Silke a
Fie. 150. Two leaves of a sun-dew. The one to the right has its glandular hairs fully expanded ; the one to the left shows half of the hairs bending inward, in the position assumed when an insect has becn captured.—After KERNER.
as a great fly-catcher, and the urns are often well supplied with the decaying bodies of these insects.
A much larger Californian pitcher plant has still more elaborate contrivances for attracting insects (see Vig. 148).
(2) Drosera.—The droseras are commonly known as “‘sun-dews,” and grow in swampy regions, the leaves form- ing small rosettes on the ground (see Fig. 149). In one form the leaf blade is round, and the margin is beset by prominent bristle-like hairs, each with a globular gland at its tip (see Fig. 150). Shorter gland-bearing hairs are
160 PLANT RELATIONS.
scattered also over the inner surface of the blade. These glands excrete a clear, sticky fluid, which hangs to them in denies like dew-drops. If a small insect becomes entangled
Fic. 151. Plants of Dionwa, showing the rosette habit of the leaves with terminal traps, and tbe erect flowering stem,—After KERNER.
in the sticky drop, the hair begins to curve inward, and presently presses its vietim down upon the surface of the blade. In the case of larger insects, several of the marginal hairs may join together in holding it, or the whole blade may become more or less rolled inward.
THE NUTRITION OF PLANTS. 161
(3) Dionea.—This is one of the most famous and re- markable of fly-catching plants (see Fig. 151). It is found only in swamps near Wilmington, North Carolina. The leaf blade is constructed like a steel trap, the two halves snapping together, and the marginal bristles interlocking like the teeth of a trap (see Fig. 152). A few sensitive hairs, like feelers, are developed on the leaf surface, and when one of these is touched by a small flying or hover- ing insect, the trap snaps shut and the in- sect is caught. Only after digestion does the trap open again.
There are certain green plants, not called carnivorous plants, which show the same general habit of sup- plementing their food supply, and so reduc- ing the necessity of food manufacture. Fie. 152. Three leaves of Dionwa, showing
Site ig * Bo vai
i hee RK
Taal
‘a
The istletoe is the details of the trap in the leaves to right ners k : and left, and the central trap in the act of green plant, growing capturing an insect.
upon certain trees, from which it obtains some food, supplementing that which it is able to manufacture.
In rich soil, the organized products of the decaying bodies of plants and animals are often absorbed by ordinary green plants, and so a certain amount of ready-made food is obtained.
CHAPTER NI. PLANT SOCIETIES: ECOLOGICAL FACTORS.
120. Definition of plant society.—From the previous chapters it has been learned that every complex plant is a combination of organs, and that each organ is related in some special way to its environment. It follows, therefore, that the whole plant, made up of organs, holds a very com- plex relation with its environment. The stem demands certain things, the root other things, and the leaves still others. To satisfy all of these demands, so far us possible, the whole plant is delicately adjusted.
The earth’s surface presents very diverse conditions in ref- erence to plant life, and as plants are grouped according to these conditions, this leads to definite associations of plants, those adapted to the same general conditions being apt to live together. Such an association of plants living together in similar conditions is a plant society, the conditions for- bidding other plants. It must not be understood that all plants affecting the same conditions will be found living together. For example, a meadow of a certain type will not contain all the kinds of grasses associated with that type. Certain grasses will be found in one meadow, and other grasses will be found in other meadows of the same type.
Very closely related plants generally do not live in the same society, as their rivalry is apt to be intense. Closely related plants are likely to occur, however, in different socicties of the same type. A plant society, therefore, may contain a wide representation of the plant kingdom, from plants of low rank to those of high rank.
PLANT SOCIETIES: ECOLOGICAL FACTORS. 163
Before considering some of the common societies, it is necessary to note some of the conditions which determine plant societies. ‘Those things in the environment of the plant which influence the organization of a society are known as ecological factors.
121. Water.—Water is certainly one of the most im- portant conditions in the environment of a plant, and has great influence in determining the organization of societies. If all plants are considered, it will be noted that the amount of water to which they ure exposed is exceedingly variable. At one extreme are those plants which are completely submerged; at the other extreme are those plants of arid regions which can obtain very little water ; and between these extremes there is every gradation in the amount of available water. Among the most striking adaptations of plants are those for living in the presence of a great amount of water, and those for guarding against its lack.
One of the first things to consider in connection with any plant society is the amount of water supply. It is not merely a question of its total annual amount, but of its distribution through the year. Is it supplied somewhat uniformly, or is there alternating flood and drought? The nature of the water supply is also important. Are there surface channels or subterranean channels, or does the whole supply come in the form of rain and snow which fall upon the area ?
Another important fact to consider in connection with the water supply has to do with the structure of the soil. There is what may be called a water level in soils, and it is important to note the depth of this level beneath the sur- face. In some soils it is very near the surface ; in others, such as sandy soils, it may be some distance beneath the surface.
Not only do the amount of water and the depth of the water level help to determine plant societies, but also the substances which the water contains. Two areas may have
164 PLANT RELATIONS.
the same amount of water and the same water level, but if the substances dissolved in the water differ in certain particulars, two entirely distinct societies may result.
122. Heat.—The general temperature of an area is im- portant to consider, but it is evident that differences of temperature are not so local as differences in the water supply, and therefore this factor is not so important in the organization of the local associations of plants, called socie- ties, as is the water factor. In the distribution of plants over the surface of the earth, however, the heat factor is probably more important than the water factor. The range of temperature which the plant kingdom, as a whole, can endure during active work may be stated in a general way as from 0° to 50° C.; that is, from the freezing point of water to 122° Fahr. There are certain plants which can work at higher temperatures, notably certain alge growing in hot springs, but they may be regarded as exceptions. It must be remembered that the range of temperature given is for plants actively at work, and does not include the tem- perature which many plants are able to endure in a specially protected but very inactive condition. For example, many plants of the temperate regions endure a winter tempera- ture which is frequently lower than the freezing point of water, but it is a question of endurance and not of work.
It must not be supposed that all plants can work equally well throughout the whole range of temperature given, for they differ widely in this regard. Tropical plants, for in- stance, accustomed to a certain limited range of high tem- perature, cannot work continuously at the lower tempera- tures. Tor each kind of plant there is what may be called a zero point, below which it is not in the habit of working.
While it is important to note the general temperature of an arca throughout the year, it is also necessary to note its distribution. Two regions may have presumably the same amount of heat through the year, but if in the one case it is uniformly distributed, and in the other great extremes
PLANT SOCIETIES: ECOLOGICAL FACTORS. 165
of temperature occur, the same plants will not be found in both. It is, perhaps, most important to note the tempera- ture during certain critical periods in the life of plants, such as the flowering period of seed-plants.
Although the temperature problem may be compara- tively uniform over any given area, the effect of it may be noted in the succession of plants through the growing sea- son. In our temperate regions the spring plants and summer plants and autumn plants differ decidedly from one another. It is evident that the spring plants can endure greater cold than the summer plants, and the succession of flowers will indicate somewhat these relations of temperature.
It should be remarked, also, that not only is the tem- perature of the air to be noted, but also that of the soil. These two temperatures may differ by several degrees, and the soil temperature especially affects root activity, and hence is a very important factor to discover.
At this point it is possible to call attention to the effect of the combination of ecological factors. For instance, in reference to the occurrence of plants in any society, the water factor and the heat factor cannot be considered each by itself, but must be taken in combination. For example, if in a given area there is a combination of maximum heat and minimum water, the result will be a desert, and only certain specially adapted plants can exist. It is evident that the great heat increases the transpiration, and tran- spiration when the supply of water is very meager is pe- culiarly dangerous. Plants which exist in such conditions, therefore, must be specially adapted for controlling tran- spiration. On the other hand, if in any area the combina- tion is maximum heat and maximum water, the result will be the most luxuriant vegetation on the earth, such as grows in the rainy tropics. It is evident that the possible com- binations of the water and heat factors may be very numer- ous, and that it is the combination which chiefly affects plant societies.
166 PLANT RELATIONS.
123. Soil—The soil factor is not merely important to consider in connection with those plants directly related to the soil, but is a factor for all plants, as it determines the substances which the water contains. There are two things to be considered in connection with the soil, namely, its chemical composition and its physical properties. Per- haps the physical properties are more important from the standpoint of soil-related plants than the chemical com- position, although both the chemical and physical nature of the soil are so bound up together that they need not be considered separately here. The physical properties of the soil, which are important to plants, are chiefly those which relate to the water supply. It is always important to de- termine how receptive a soil is. Does it take in water easily or not ? It is also necessary to determine how re- tentive it is; it may receive water readily, but it may not retain it.
For convenience in ordinary field work with plants, soils may be divided roughly into six classes: (1) rock, which means solid uncrumbled rock, upon which certain plants are able to grow ; (2) sand, which has small water capacity, that is, it may receive water readily enough, but does not retain it ; (3) l’me soil ; (4) clay, which has great water capacity ; (5) humus, which is rich in the products of plantand animal decay ; (6) salé sotl, in which the water contains various salts, and is generally spoken of as alka- line. These divisions in a rough way indicate both the structure of the soil and its chemical composition. Not only should the kinds of soil on an area be determined, but their depth is an important consideration. It is very common to find one of these soils overlying another one, and this relation between the two will have a very important effect. For instance, if a sand soil is found lying over a clay soil, the result will be that the sand soil will retain far more water than it would alone. If «humus soil in one area overlies a sand soil, and in another area
PLANT SOCIETIES: ECOLOGICAL ‘ACTORS. 167
overlies a clay soil, the humus will differ very much in the two cases in reference to water.
The soil cover should also be considered. The common soil covers are snow, fallen leaves, and living plants. It will be noticed that all these covers tend to diminish the loss of heat from the soil, as well as the access of heat to the soil. In other words, a good soil cover will very much diminish the extremes of temperature. All this tends to increase the retention of water.
124. Light—It is known that light is essential for the peculiar work of green plants. However, all green plants cannot have an equal amount of light, and some have learned to live with a less amount than others. While no sharp line can be drawn between green plants which use intense light, and those which use less intense light, we still recognize in a general way what are called light plants and shade plants. We know that certain plants are chiefly found in situations where they can be exposed freely to light, and that other plants, as a rule, are found in shady situations.
Starting with this idea, we find that plants grow in strata. In a forest society, for example, the tall trees rep- resent the highest stratum ; below this there may be a stratum of shrubs, then tall herbs, then low herbs, then forms like mosses and lichens growing close to the ground. In any plant society it is important to note the number of these strata. It may be that the highest stratum shades so densely that many of the other strata are not represented at all. An illustration of this can be obtained from a dense beech forest.
125. Wind.—It is generally known that wind has a dry- ing effect, and, therefore, it increases the transpiration of plants and tends to impoverish them in water. This factor is especially conspicuous in regions where there are pre- vailing winds, such as near the sea-coast, around the great
lakes, and on the prairies and plains. In all such regions 12
168 PLANT RELATIONS.
the plants have been compelled to adapt themselves to this loss of water ; and in some regions the prevailing winds are so constant and violent that the force of the wind itself has influenced the appearance of the vegetation, giving what is called a characteristic physiognomy to the area.
These five factors have been selected from a much larger number that might be enumerated, but they may be re- garded as among the most important ones. It will be noticed that these factors may be combined in all sorts of ways, so that an almost endless series of combinations seems to be possible. This will give some idea as to the possible number of plant societies, for they may be as numerous as are the combinations of these factors.
126. The great groups of societies.—It is possible to re- duce the very numerous societies to three or four great groups. For convenience, the water factor is chiefly used tor this classification. It results in a convenient. classitica- tion, but one that is probably more or less artificial. The selection of any one factor from among the many for the purpose of classification never results in a very natural classification when the combination of factors determines the group. Ilowever, for general purposes, the usual classification on the basis of water supply will be used. On this basis there are three great groups of societies, as follows :
(1) Hydrophytes.—The name means ** water plants,” and suggests that such societies are at that extreme of the water supply where it is very abundant. Such plants may grow in the water, or in very wet soil, but in any event they are exposed to a large amount of water.
(2) Yerophytes.—The name means ** drought plants,” and suggests the other extreme of the water supply. True xerophytes are exposed to dry soil and dry atmosphere.
(3) Mesophytes.—Between the two extremes of the water supply there is a great middle region of medium water supply, and plants which occupy it are known as meso-
PLANT SOCIETIES: ECOLOGICAL FACTORS. 169
phytes, the plants of the middle region. It is evident that mesophytes gradually pass into hydrophytes on the one side, and into xerophytes on the other; but it is also evi- dent that mesophyte societies have the greatest range of water supply, extending from a large amount of water to a very small amount.
It should be understood that these three groups of socie- ties, which are distinguished from one another by the amount of the water supply, are artificial groups rather than natural ones, for they bring together unrelated societies, and often separate those that are closely related. For example, a swampy meadow is put among hydrophyte societies by this classification ; and it may shade into an ordinary meadow, which belongs among the mesophytes. Probably the largest fact which may be used in grouping plant societies is that certain societies are so situated that they seek for the most part to reduce transpiration, and that others are so situated that they seek for the most part to increase transpiration.
However, the factors which determine societies are so numerous that they cannot be presented in an elementary book, and the simpler artificial grouping given above will serve to introduce the societies to observation.
Upon a different basis another great group of socicties has been suggested as follows :
(4) Halophytes.—The word means “salt plants.” The basis of classification here depends not so much upon the water supply as upon the fact that the water contains certain salts which make it impossible for most plants to live. Such societies may be found near the sea-coast, around salt springs, on alkaline flats, or wherever the soil contains these characteristic salts.
CHAPTER NII. HYDROPHYTE SOCIETIES.
127. General character.— Hydrophytes are related to abundant water, either throughout their whole structure or in part of their structure. It is a well-known fact that hydrophytes are among the most cosmopolitan of plants, and hydrophyte societies in one part of the world look very much like hydrophyte societies in any other region. It is probable that the abundant water makes the condi- tions more uniform.
It is evident that for those plants, or plant parts, which are submerged, the water affects the heat factor by dimin- ishing the extremes. It also affects the light factor, in so far as the light must pass through the water to reach the chlorophyll-containing parts, as light is diminished in intensity by passing through the water. Before consider- ing a few hydrophyte sucieties, it is necessary to note the prominent hydrophyte adaptations.
128. Adaptations——In order that the illustration may be as simple as possible, a complex plant completely exposed to water is selected, for it is evident that the relations of a swamp plant, with its roots in water and its stem and leaves exposed to air, are complicated. A number of adaptations may be noted in connection with the submerged or floating plant.
(1) Thin-walled epidermis.—In the case of the soil-re- lated plants, the water supply comes mainly from the soil, and the root system is constructed to absorb it. In the case of the water plant under consideration, however, the
HYDROPHYTE SOCIETIES. 171
whole plant body is exposed to the water supply, and there- fore absorption may take place through the whole surface rather than at any particular region such as the root. In order that this may be done, however, it is necessary for the epidermis to have thin walls, which is usually not the case in epidermis exposed to the air, where a certain amount of protection is needed in the way of thickening.
(2) Roots much reduced or wanting.—-It must be evident that if water is being absorbed by the whole free surface of the plant, there is not so much need for a special root region for absorp- tion. Therefore, in such water plants the root sys- tem may be much re- duced, or may even disap- pear entirely. It is often retained, however, to act as a holdfast, rather than as an absorbent organ, for
Fie. 153. Fragment of a common seaweed
most water plants anchor (Fucus), showing the body with forking
themselves to some sup- branching and bladder-like air cavities.— After LUERSSEN.
port.
(3) Reduction of water-conducting tissues.—In the ordi- nary soil-related plants, not only is an absorbing root sys- tem necessary, but also a conducting system, to carry the water absorbed from the roots to the leaves and elsewhere. It has already been noted that this conducting system takes the form of woody strands. It is evident that if water is being absorbed by the whole surface of the plant, the
172 PLANT RELATIONS.
work of conduction is not so extensive or definite, and therefore in such water plants the woody bundles are not so prominently developed as in land plants.
(4) Reduction of mechanical tissues.—In the case of ordinary land plants, certain firm tissues are developed so
Fig. 154. Gulfweed (Sargassum), showing the thallus differentiated into stem-like and leaf-like portions, and also the bladder-like floats.—After BENNETT and MURRAY.
that the plant may maintain its form. These supporting tissues reach their culmination in such forms as trees, where massive bodies are able to stand upright. It is evi- dent that in the water there is no such need for rigid sup- porting tissues, as the buoyant power of water helps to support the plant. This fact may be illustrated by taking
HYDROPHYTE SOCIETIES. 173
out of water submerged plants which seem to be upright, with all their parts properly spread out. When removed they collapse, not being able to support themselves in any way. (5) Development of air cavities.—The presence of air in the bodies of water plants is necessary for two reasons: (1),
Fig. 155. Bladderwort, showing the numerous bladders which float the plant, the finely divided water leaves, and the erect flowering stems. The bladders are also effective ‘‘insect traps,’ Utricularia being one of the ‘carnivorous plants.” —After KERNER.
to aerate the plant ; (2), to increase its buoyancy. In most complex water plants there must be some arrangement for the distribution of air containing oxygen. This usually takes the form of air chambers and passageways in the body of the plant (see Figs. 87, 88, 89, 156). Of course such air chambers increase the buoyancy of the body. Sometimes, however, a special buoyancy is provided for by the development of regular floats, which are bladder-
174 PLANT RELATIONS.
like bodies (see Figs. 153, 154). These floats are very common among certain of the seaweeds, and are found among. higher plants, as the utricularias or bladderworts, which have received their name from the numerous blad- ders developed in connection with their bodies (see Fig. 155).
Such adaptations as the above may be regarded as the most prominent general adaptations. There are many other special ones in connection with certain groups of water plants which will be mentioned in considering the societies.
A. Free-swimming societies.
120. Definition.—In these societies there is the largest exposure to water, and no relation at all to the nutrient or mechanical support of the soil, the plants being completely supported by the water. In such plants all of those mate- rials which lund plants obtain from the water of the soil are obtained from the water in which they are submerged ; and in the case of completely submerged plants the materials usually obtained from the air are also supplied by the water. Free swimming plants may be either submerged or floating, and they are free to move either by locomo- tion or by water currents. Two prominent societies are selected as types.
130. The plankton.—This term is used to designate the minute organisms, both plants and animals, which are found in the water. The plankton is composed of indi- viduals invisible to the naked eye, but taken together they represent an enormous organic mass. The plankton socie- ties are especially well represented in the colder oceanic waters, but they are not absent from any waters. Among the most prominent plants in these societies are the dia- toms. Diatoms are minute plants of various forms, and all have a wall very full of silica. This makes their bodies
HYDROPHYTE SOCIETIES. 175
extremely enduring, and therefore diatoms are often found in great deposits in the rocks, in some cases forming the whole mass of rock. Associated with the diatoms are numerous other plant and animal forms.
131. Pond societies—The word pond is used to indicate stagnant or slow-moving waters. In such waters free- swimming plants of all groups are associated. Of course the alge are well represented, but even the highest plants are repre- sented by the duckweeds, which are very commonly seen in the form of small green disks floating on the surface of the Fig. 156. A section through the body of a duckweed (Lemna), water, which showing the air spaces (@) which make it buoyant, the they frequent- origin (7) of the simple dangling root, and the pockets
(s and Z) from which new plants bud out. and in which ly cover with flowers are developed.
great masses
(see Fig. 156). It should be observed that the floating and submerged positions result in a difference in light-relations. The floating forms may be regarded as light forms, being exposed to the greatest amount of light. The submerged forms are shade plants, and the shading becomes greater as the depth of the water is greater. It must not be sup- posed that submerged plants can live at any depth, for soon a limit is reached, beyond which the light is not intense enough to enable plants to work.
It has been noticed that this complete water habit has affected plants in many ways. For instance, the duck- weeds are related to land plants with root, stem, and leaves, but they have lost the distinction between stem and leaf, and the body is merely a flat leaf-like disk floating upon
176 PLANT RELATIONS.
the water, with a few roots dangling from the under side, or with no roots at all (see Fig. 156). This same duck- weed also shows some interesting modifications in its hab- its of reproduction. Althongh related to plants which pro- duce flowers and make seed, the duckweeds have almost lost the power of producing flowers, and when they do produce them, seeds are very seldom formed. In other words, the ordinary method of reproduction employed by flowering plants has been more or less abandoned. Replacing this method of reproduction is a great power of vegetative propagation. From the disk-like body of the plant other disk-like bodies bud out, and this bud- ding continues until a large group of disks, more or less connected with each other, may be formed. These plants also form what are known as winter buds—well protected bud-lke bodies which sink to the bottom of the pond when the floating plants are destroyed, and remain protected by the mucky bottom until the waters become warm again in the next growing season.
In examining the pond societies, therefore, attention should be paid to the floating forms and the submerged forms, and also to the varying depths of the latter. It will also be noted that the leaves of floating forms are com- paratively broad, while those of submerged forms are narrow.
B. Pondweed societies.
132. Definition.—These are societies fixed to the soil but with submerged or floating leaves. In this case there is still great exposure to water, but there is also a definite soil relation. 'T'wo prominent societies are selected from this group for illustration.
133. Rock societies—The term rock is used in this con- nection in a very general way, meaning simply some firm support beneath the water ; it is just as likely to be a stick
HYDROPHYTE SOCIETIES, 177
as a stone. Probably the most prominent group of plants affecting these conditions are alge, both fresh water and marine. In the fresh waters very many of the alge will be
Fie. 157. A group of marine seaweeds (Laminarias). Note the various habits of the plant body and the root-like holdfasts.—After KERNER.
found anchored to some support. The largest display of such forms, however, is found among the marine alge, which abound along all seacoasts (see Fig. 157). It will
Fic. 158. A natural, but nearly overgrown lily pond, The lily pads may be seen rising more or less above the water where they are thickest. The forest growth in the background is probably a tamarack (larch) swamp. It is to be noticed that as the lily pond loses its water it is being invaded by the coarse sedge and grass growth of a swamp-moor, Between the lily pond and the forest is a swamp- thicket. At least four distinct societies are represented in this view, A fifth is probably represented in the form of plants of the reed-swamp type, which form a transition between the lily pond and the swamp-thicket.
HYDROPHYTE SOCIETIES. 179
be noticed that the habit of anchorage demands the development of special organs of attachment, which usu- ally take the form of root-like structures, often associated with sucker-like disks. Associated with the anchoring structures is often a development of floats, which is es- pecially characteristic of seaweeds, enabling the working body to float freely in the water (see Figs. 153, 154). It is evident that while free-swimming forms may be suitable for stagnant waters, anchored forms are better adapted for moving waters. Therefore, where there are currents of water, or wave action, the anchored forms predominate. The ability to live in moving waters, and often in those that become violently agitated, has its advantage to the plant in the more rapidly renewed food material. In such a situation free-swimming forms would soon be stranded or disposed of in quieter waters.
In the case of the marine seaweeds there is an interest- ing relation between the depth of the water and the color of the plants. While the fresh water algee are prevailingly green, it will be remembered that the prevailing colors of the alge of the seashore are brown and red. The brown often passes into some shade of yellow, and the red may merge into purple or violet, but in general the two types of color may be called brown and red. It has been noticed that the brown forms are found at less depth than the red forms, so that ina general way there are two zones of dis- tribution in relation to depth, the red zone being the lower one and the yellow zone the upper. Just what this means in the economy of the plants is not clear, but it has been suggested that the yellow and the red colors assist the chlorophyll in its work, which is more or less interfered with by the diminished intensity of the light passing through sea water.
134. Loose soil societies —This phrase is used merely to contrast with rock societies, referring to the fact that the anchorage is not merely for mechanical support, but that
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HYDROPHYTE SOCIETIES. 181
there is a definite relation to soil in which roots or root-like structures are embedded. Societies of this type contain the greatest variety of plants of all ranks. In these soci- eties are found alge, mosses, fern plants, pondweeds, water lilies, etc. (see Figs. 158, 159, 160, 161). Pondweeds and water lilies may be taken as convenient types of high grade plants which grow in such conditions.
In the first place, it will be noticed that they are in- clined to social growths, great numbers of individuals growing together and forming what are known as lily ponds or pondweed beds, although in the small lakes of the interior where pondweeds abound in masses, they are more commonly known as “ pickerel beds.” If the petiole of a lily pad be traced down under the water, it will be found to arise from an intricate mass of thick, knotted stems. So extensively do these stems (rootstocks) in the mucky bottom branch that they are able to give rise to close set masses of leaves.
Water lilies and pondweeds may ulso be compared, to show the effect of the floating habit in vontrast with the submerged habit. The leaves of water lilies float on the surface, and therefore are broad ; and being exposed to light are a vivid green, indicating the abundant development of chlorophyll. Many of the pondweeds, however, are com- pletely submerged. As one floats over one of these “ pick- erel beds,’ the leafy plants may be seen, at considerable depths, and have a pallid, translucent look. It will be seen that in these causes the leaf forms are narrow rather than broad, often being ribbon-like, or in some submerged plants even cut up into thread-like forms. It is evident that such narrow leaf forms can respond more easily to water movements than broad forms. The pallid look of these submerged leaves indicates that there has not been an abundant development of chlorophyll. Some pondweeds, however, have both types of leaves, some being submerged and others floating. In these cases it is interesting to notice
Fie. 160.—A group of pondweeds, The stems are sustained in an erect position by the water, and the narrow leaves are exposed to a light whose intensity is dimin- ished by passing through the water.—After KERNER,
HYDROPHYTE SOCIETIES. 183
the corresponding change of form; on the same individual the submerged leaves are very narrow, or divided into very narrow lobes, while the floating ones are broad (see Fig. 162). The relation of the plant to the water, therefore, has determined the leaf form. The advantage of the floating habit of leaves is not merely a better rela- tion to light, but the carbon dioxide used in photosynthesis and the oxygen used in respiration may be obtained freely from the air, rather than from the water. It will also be noticed that these water plants usually send their flowers to the surface, indicating that such a position is more fav- orable for the work of the flower than a submerged position. Any society of this type will furnish abundant material for observation, and it is, perhaps, the most valuable type of society for study that has been mentioned so far.
C. Swamp societies.
135. Definition.—In swamp societies the plants are rooted in water, or in soils rich in water, but the stems bearing the leaves rise above the surface. Among the hydrophytes, swamp plants are least exposed to water, and as the stem and its leaves are exposed to the air, there is no such reduc- tion of the root system and of conducting and mechanical tissues as in the other hydrophytes. Also the epidermis is not thin, and there is no development of floats to increase the buoyancy. However, the root must be aerated, and hence air chambers and passageways are abundant. This aeration of the root system reaches a very high develop- ment in such swamp trees as the cypress. In cypress swamps the so-called ‘‘ knees” are abundant, and they are found to be special growths from the root system, which rise above the surface of the water, both for bracing and to admit air to the roots (see Fig. 91). It has been shown that if such swamps are flooded above the level of the knees, many of the trees are killed. In ordinary cases the air is admitted
13
Fie. 161. Eel grass (Vallisneria), a common pondweed plant. The plants are anchored and the foliage is submerged. The carpel-bearing flowers are carried to the surface on long stalks which allow a variable depth of water. The stamen- bearing flowers remain submerged, as indicated near the lower left corner, the flowers breaking away and rising to the surface, where they float and effect pollina- tion,—After KERNER.
HYDROPHYTE SOCIETIES. 185
through openings in the epidermis of the stem and leaves, and so enters the air passageways and reaches the roots. Another habit of swamp plants is called turf-building, which means that new individuals arise from older ones, and so a dense mat of roots and rootstocks is formed. Very prominent among these turf-building swamp plants are
Fic. 162, Two leaves of a water buttercup, showing the difference in the forms of submerged and aerial leaves on the same plant, the former being much more finely divided.—After STRASBURGER.
the sedges. Some of the prominent swamp societies may be enumerated as follows :
136, Reed swamps.—The reed-swamp plants are tall wand- like forms, which grow in rather deep, still water (see Fig. 163). Prominent as types are the cat-tail flag, bulrushes, and reed grasses. Such an assemblage of forms usually characterizes the shallow margins of small lakes and ponds. In such places the different plants are apt to be arranged according to depth, the bulrushes standing in the deepest water, and behind them the reed grasses, and then the
186 PLANT RELATIONS.
cat-tails. This regular arrangement in zones is so often
interfered with, however, that it is not always evident. The reed-swamp societies have been called ‘‘ the pioneers
of land vegetation,” for the detritus collects about them,
% if: BY 2
Tic. 168. A reed swamp, fringing the low shore of a lake or a sluggish stream. The plants are tall and wand-like, and all are monocotyls. Three types are prominent, the reed grasses (the tallest), the cat-tails (at the right), and the bulrushes(a group standing out in deeper water near the middle of the fringing growth). The plant in the foreground at the extreme right is the arrow-leaf (Sagitlaria), recognized by its characteristic leaves.—After Kerner.
the water becomes more and more shallow, until finally the reed plants are compelled to migrate into deeper water (see §108). In this way small lakes and ponds may be completely reclaimed, and become converted first into ordinary swamps, und finally into wet meadows. Instances
HYDROPHYTE SOCIETIES. 187
of nearly reclaimed ponds may be noticed, where bulrushes, cat-tail flags, and reed grasses still occupy certain wet spots, but are shut off from further migration. The social growth of these plants, brought about by extensive root- stock development, is especially favorable for detaining detritus and building a land surface.
Reed-swamp plants also have in general a tall and un- branched habit of body. They may be bare and leafless, with a terminal cluster of flowers, as in the bulrushes; or the wand-like stems may bear long, linear leaves, us in the cat-tails; or the stem may be a tall stalk with two rows of narrow leaves, as in the reed grasses. No more charac- teristic group of forms is found in any socicty. Of course, associated with these forms are also free and fixed hydro- phytes, which characterize the other societies.
137. Swamp-moors.—The word moor is used to designate the meadow-like expatses of swampy ground. Here belong the ordinary swamps, marshes, bogs, etc. There is less water than in the case of the reed swamps, and often very little standing water. One of the peculiarities of the swamp-moor is that the water is rich in the soil materials used in food manufacture, notably the nitrates from which nitrogen is obtained for proteid manufacture. In such conditions, therefore, the vegetation is dense, and the soil is black with the humus derived from the decaying plant bodies.
Typical swamp-moors border the reed swamps on the land side, and slowly encroach upon them as the reed plants build up land. Probably the most characteristic plant forms of the swamp-moor are the sedges. and asso- ciated with them are certain coarse grasses. These give the meadow-like aspect to the swamp, although these grass- like forms are very coarse. Along with the dominant sedges and grasses are numerous other plants adapted to such conditions, such as some of the buttercups. It would be impracticable to give a list of swamp-moor plants, as the
188 PLANT RELATIONS.
forms associated with sedges and grasses may vary widely in different societies.
In almost all swamp-moors there is a lower stratum of vegetation than that formed by the sedges. This lower stratum is made of certain swamp mosses, which grow in very dense masses. Towards the north, where the temper- ature conditions are not so favorable for the sedge stratum, it may be lacking almost entirely, and only the lower moss stratum left. In these cases the swamp-moor becomes little more than a great bed of moss. It is in such con- ditions that peat may be formed, a coal-like substance formed by the dead plant bodies which have been kept from complete decay by submergence in water.
138. Swamp-thickets—Swamp-thickets are very closely associated with swamp-moors, and are doubtless derived from them. If aswamp-moor, with its sedge stratum and moss stratum, be invaded by shrubs or low trees, it be- comes a swamp-thicket. This means that the general con- ditions of food supply which belong to the swamp-moor also favor certain shrubs and trees. It will be noticed that these shrubs and trees are of very uniform type, being mainly willows, alders, birches, etc. Such willow and alder thickets are very common in high latitudes.
139. Sphagnum-moors.—The sphagnum-moor is a very peculiar type of swamp society. It is sonamed because the common bog or peat moss, known as sphagnum, gives a peculiar stamp to the whole area. Sphagnums are large, pale mosses, whose lower parts may die, and whose upper parts continue to live and put out new branches, so that a dense turf is formed. In walking over such a bog the moss turf seems springy, and sometimes trembles so as to sug- gest the name “‘ quaking bog.” These are the great peat- forming bogs. It is interesting to know what conditions keep the swamp-moor plants out of the sphagnum-moor. The plants of the sphagnum-moor seem to be entirely dif- ferent from those of the swamp-moor, although the amount
UYDROPHYTE SOCIETIES. 189
of water is approximately the same. Not only are the plants different in the sphagnum-moor, but they are not so numerous, and, with the exception of the moss, do not grow so densely. It is to be noticed that creeping plants are abundant, and also many forms which are known to obtain their food material already manufactured, and there- fore are saprophytes. Certain kinds of sedges and grasses are found, but generally not those of the swamp-moor, while heaths and orchids are especially abundant. It is in these sphagnum-moors, also, that the curious forms of car- niyorous plants are developed, among which the pitcher plants, droseras, and dionwas have been described. In considering this strange collection of forms, it is evident that there must be some peculiarity in the food supply, for the heaths and orchids are notorious for their partial sap- rophytic habits, and the carnivorous plants are so named because they capture insects to supplement their food sup- ply. The fact, also, that the peculiar sphagnum mosses, rather than the mosses of the swamp-moor, are the prevalent ones, indicates the same thing.
It has been discovered that the water of the sphagnum- moor is very poor in the food materials which are abundant in the water of the swamp-moor. There is a special lack of the materials which are used in the manufacture of pro- teids. and hence this process is seriously interfered with. It is necessary. therefore, to obtain proteids already formed in animals. or in other plants. This will account for the necessity of the saprophytic habit, and of the carnivorous habit, and for the sphagnum mosses which can endure such conditions. Of course, it also accounts for the exclusion of the characteristic plants of the swamp-moor.
Another peculiarity in connection with the sphagnum- moor, aside from its poverty in food material, is the lack of those low plant forms (dacteria) which induce decay. Bacteria are very minute plants, some of which are active agents in processes of decay, and when these are absent
190 PLANT RELATIONS.
decay is checked. As a consequence, the sphagnum-moor waters are strongly antiseptic; that is, they prevent decay by excluding certain bacteria, It is a well-known fact that bodies of men and animals which have become submerged in sphagnum-hogs may not decay, but have been found preserved after a very long period. This will also indicate why such bogs are especially favorable for peat formation.
These two types of moors, therefore, may be contrasted as follows: The swamp-moor is rich in plant food, and is characterized chiefly by grassy plants ; the sphagnum-moor is poor in food material, and is characterized chiefly by sphagnum moss. It will be noted that peat may be formed in connection with both of these moors, but in the swamp- moor the plant forms cannot be distinguished in the peat, as they have been more or less disorganized through decay, while in the peat of the sphagnum-moor the plant forms are well preserved. The peat of the swamp-moor, also, yields a great amount of ash, for the swamp-moor is rich in soil materials, while the peat of the sphagnum-moor yields very little ash.
140. Swamp forests.—It was noted that the special types of shrub or tree growth associated with the swamp-moor conditions are willows, alders, birches, etc. In the same way there is a peculiar tree type associated with the sphagnum-moor. It is very common to have a sphagnum area occupied by trees, and the area becomes a swamp forest, rather than a sphagnum-moor. The chief tree type which occupies such conditions is the conifer type, popularly known as the evergreens. The swamp forests, therefore, with a sphagnum-moor foundation, are made up of larches, certain hemlocks and pines, junipers, ete., and towards the south the cypress comes in (sce Fig. 164). The larch is » very common swamp tree of the northern regions, where such an area ig commonly called a “ tama- rack swamp’ (see Fig. 158). The larch forests are apt to be in the form of small patches, while the larger swamp
‘B9SSOUI JO WINILI}S U SSA[}QNOp SI a0} T}.Mows.o9pun at) MOfa_ “SUJOs PUT SQNAYS Jo YJMOISIOpuN osuap at[} pUL ‘sidJTWOO Jo SyuNIP aBIe] Moy B Uoos aq ABUT YI ul ysadoj dures Y “POT “SLA
192 PLANT RELATIONS.
forests are made of dense growths of hemlocks, pines, etc. In the densest of these forests the shade is so complete that there may be very few associated plants occurring in strata between the sphagnum moss and the trees. In the larch forests, however, the undergrowth may be very dense.
CHAPTER NII. XEROPHYTE SOCIETIES.
141. General character.—Strongly contrasted with the hydrophytes are the xerophytes, which are adapted to dry air and soil. The xerophytic conditions may be regarded in general as drouth conditions. It is not necessary for the air and soil to be dry throughout the year to develop xerophytic conditions. These conditions may be put under three heads: (1) possible drouth, in which a season of drouth may occur at irregular intervals, or in some seasons may not occur at all; (2) pertodic drouth, in which there is a drouth period as definite as the winter period in cer- tain regions ; (3) perennial drouth, in which the dry con- ditions are constant, and the region is distinctly an arid or desert region.
However xerophytic conditions may occur, the problem of the plant is always one of water supply, and many strik- ing structures have been developed to answer it. Plants in such conditions must provide, therefore, for two things: (1) collection and retention of water, and (2) prevention of its loss. It is evident that in these drouth conditions the loss of water through transpiration (see $26) tends to be much increased. This tendency in the presence of a very meager water supply is a menace to the life of the plant. It is impracticable to stop transpiration entirely, for it must take place in connection with a necessary life-process. The adaptations on the part of the plant, therefore, are directed towards the regulation of transpiration, that it
194 PLANT RELATIONS,
may occur sufficiently for the life-processes, but that it may not be wasteful.
The regulation of transpiration may be accomplished in two general ways. It will be remembered that the amount of transpiration holds some relation to the amount of leaf exposure or exposure of green tissue. Therefore, if the amount of leaf exposure be diminished, the total amount of transpiration will be reduced. Another general way for regulating transpiration is to protect the exposed surface in some way so that the water does not escape so easily. In a word, therefore, the general method is to reduce the extent of exposed surface or to protect it. It must be understood that plants do not differ from each other in adopting one or the other of these methods, for both are very commonly used by the same plant.
aldaptations.
142. Complete desiccation Some plants have a very re- markable power of completely drying up during the drouth period, and then reviving upon the return of moisture. This power is strikingly illustrated among the lichens and mosses, some of which can become so dry that they may be crumbled into powder, but revive when moisture reaches them. <A group of club mosses, popularly known as ‘ res- urrection plants,” illustrates this same power. The dried up nest-like bodies of these plants are common in the markets, and when they are placed in a bowl of water they expand and may renew their activity. In such cases it can hardly be said that there is any special effort on the part of the plant to resist drouth, for it seems to yield completely to the dry conditions and loses its moisture. The power of reviving, after being completely dried out, is an offset, however, for protective structures.
143. Periodic reduction of surface—In regions of periodic
XEROPHYTE SOCIETIES.
drouth it is very com- mon for plants to diminish the exposed surface in a very de- cided way. In such cases there is what may be called a peri- odic surface decrease. For example, annual plants remarkably diminish their ex- posed surface at the period of drouth by being represented only by well-pro- tected seeds. The whole exposed sur- face of the plant, root,stem,and leaves, has disappeared, and the seed preserves the plant through the drouth.
Little less remark- able is the so-called geophilous habit. In this case the whole of the plant surface ex- posed to the air dis- appears, and only underground parts, such as bulbs, tubers, etc., persist (see Figs. 45, 46, 66, 67, U8, 69, 70, 75, 144, 164a, 1642). At the re-
195
: one ) SS | i aah (hi (eas | ON | NAAN An | \ Hey of Ne } | Cesta WI } TiS ay \P, Mm | | \ { |) HH \ \ | \Y Nl . \\ \ " SSS KS ee \ Sa aA Sa TSS) “ re - es I, eA ve i a Ie ee (CA SAG / Fie. 164a. The bloodroot (Sanguinaria), showing
the subterranean rootstock sending leaves and flower above the surface.—After ATKINSON.
196
Fic, 164, The spring beauty ( Claytonia), showing subterranean tuber-like stem sending leaf and flower-bearing stem above the surface.—After ATKINSON.
PLANT RELATIONS.
turn of the moist season these underground parts develop new exposed surfaces. In such cases it may be said that at thecoming of the drouth the plant seeks a sub- terranean retreat.
A little less decrease of exposed surface is shown by the deciduous habit. It is known that certain trees and shrubs, whose bodies remain exposed to the drouth, shed their leaves and thus very greatly reduce the amount of exposure ; with the return of mois- ture, new leaves are put forth. It will be re- marked, in this connec- tion, that the same habits serve just as well to bridge over a period of cold as a period of drouth, and perhaps they are more familiar in connection with the cold period than in con- nection with the drouth period.
144. Temporary reduc- tion of surface.—While the habits above have to do with regular drouth
XEROPHYTE SOCIETIES. 197
periods, there are other habits by which a temporary re- duction of surface may be secured. For instance, at the approach of a period of drouth, it is very easy to observe certain leaves rolling up in various ways. As a leaf be- comes rolled up, it is evident that its exposed surface is reduced. ‘The behavior of grass leaves, under such cir- cumstances, is very easily noted. .\ comparison of the grass blades upon a well-watered lawn with those upon a dried-up lawn will show that in the former case the leaves are flat, and in the latter more or less rolled up. The same habit is also very easily observed in connection with the larger- leaved mosses, which are very apt to encounter drouth periods.
145. Fixed light position.—In general, when leaves have reached maturity, they are unable to change their position in reference to light, having obtained what is known as a fixed light position. During the growth of the leaf, how- ever, there may be changes in direction so that the fixed light position will depend upon the light direction during growth. The position finally attained is an expression of the attempt to secure sufficient, but not too much light (see §13). ‘The most noteworthy fixed positions of leaves are those which have been developed in intense light. A very common position in such cases is the profile posi- tion, in which the leaf apex or margin is directed upwards, and the two surfaces are more freely exposed to the morn- ing and evening rays—that is, the rays of low intensity— than to those of midday.
Illustrations of leaves with one edge directed upwards can be obtained from the so-called compass plants. Prob- ably most common among these are the rosin-weed of the prairie region, and the prickly lettuce, which is an intro- duced plant very common in waste ground (see Fig. 165). Such plants received their popular name from the fact that many of the leaves, when edgewise, point approximately north and south, but this direction is very indefinite. It is
198 PLANT RELATIONS.
evident that such a position avoids exposure of the leaf surface to the noon rays, but obtains for these same sur- faces the morning and evening rays. If these plants are developed in the shade, the ‘‘ compass” habit does not
Fie. 165. Two compass plants. The two figures tothe left represent the same plant (Silphium) viewed from the east and from the south. The two figures to the right represent the same relative positions of the leaves of Lactuca.—After KERNER.
appear (see $15). The profile position is a very common one for the leaves of Australian plants, a fact which gives much of the vegetation a peculiar appearance. All these positions are serviceable in diminishing the loss of water, which would occur with exposure to more intense light. 146. Motile leaves—Although in most plants the mature
XEROPHYTE SOCIETIES. 199
leaves are in a fixed position, there are certain ones whose leaves are able to perform movements according to the need. Mention has been made already of such forms as Oralis (see §14), whose leaves change their position readily in reference to light. Motile leaves have been developed most extensively among the Leguminose, the family to which
Fic. 166. Two twigs of asensitive plant. The one to the left shows the numerous small leaflets in their expanded position ; the one to the right shows the greatly reduced surface, the leaflets folded together, the main leaf branches having approached one another, and the main leaf-stalk having bent sharply downwards. —After STRASBURGER.
belong peas, etc. In this family are the so-called ‘‘sen- sitive plants,” which haye received their popular name from their sensitive response to light as well as to other influences (see Fig. 166). The acacia and mimosa forms are the most notable sensitive plants. and are especially developed in arid regions. The leaves are usually very large, but are so much branched that each leaf is com- posed of very numerous small leaflets. Each leaflet has 14
200 PLANT RELATIONS.
the power of independent motion, or the whole leaf may move. If there is danger from exposure to drouth, some of the leaflets will be observed to fold together ; in case
the danger is prolonged, more leaflets will fold together ; and if the danger persists, the surface of exposure will be still further reduced, until the whole plant may have its leaves completely folded up. In this way the amount of
XEROPHYTE SOCIETIES. 201
reduction of the exposed surface may be accurately regu- lated to suit the neud (see $3s).
147. Reduced leaves—In regions that are rather per- manently dry, itis observed that the plants in general pro- duce smaller leaves than in other regions (see Fig. 1s), That this holds a direct relation to the dry conditions is
Fic. 168. Leaves from the common basswood (Zilia), showing the effect of environ- ment; those at the right being from a tree growing in ariver bottom (mesophyte conditions) ; those at the left being from a tree growing upon a dune, where it is exposed to intense light, heat, cold, and wind. Not only are the former larger, but they are much thinner. The lcaves from the dune tree are strikingly smaller, much thicker, and more compact.—After CowLEs.
evident from the fact that the same plant often produces smaller leaves in xerophytic conditions than in moist con- ditions. One of the most striking features of an arid region is the absence of large, showy leaves (see Fig. 167). These reduced leaves are of various forms, such as the needle leaves of pines, or the thread-like leaves of certain sedges and grasses, or the narrow leaves with inrolled margins such as is common in many heath plants. The
202 PLANT RELATIONS.
at. es
St wt
Fic. 169. Two species of Achillea on different soils. The one to the left was grown in drier conditions and shows an abundant development of hairs.— After SCHIMPER,
extreme of leaf reduction has been reached hy the cactus
plants, whose leaves, so far as follage is concerned, have disappeared entirely, and the leaf work is done by the
XEROPHYTE SOCIETIES. 2038
surface of the globular, cylindrical, or flattened stems (see §36).
14s. Hairy coverings.—A covering of hairs is an effective sun screen, and it is very common to find plants of xerophyte regions character- istically hairy (see §35). The hairs are dead struc- tures, and within them there is air. This causes them to reflect the light, and hence to ap- pear white or nearly so. This reflection of light by the hairs dimin- ishes the amount which reaches the working region of the plant (see Fig. 169).
149. Body habit. —Besides the ya- rious devices for diminishing ex- posure or leaf sur- tes, aril ence ee, ee loss of water, unbranched one having grown in normal mesophyte enumerated abore, conditions ; the short, bushy branching, more slender
k form having grown on the dunes (xerophyte condi- the whole habit of tions).—After CowLEs.
the plant may em-
phasize the same purpose. In dry regions it is to be observed that dwarf growths prevail. so that the plant as a whole does not present such an exposure to the dry air as in regions of greater moisture (see Fig. 170). Also the pros-
2.04 PLANT RELATIONS.
trate or creeping habit is a much less exposed one in such regions than the erect habit. In the same manner, the very characteristic rosette habit, with its cluster of overlapping leaves close against the ground, tends to diminish loss of water through transpiration.
One of the most common results of xerophytic conditions upon body habit is the development of thorns and spiny
Fie, 171. Young plants of Huphorbia splendens, showing a development of thorns characteristic of the plants of dry regions,
processes. Ax 2 consequence, the vegetation of dry regions is characteristically spiny. In many cases these spiny pro- cesses Cun be made to develop into ordinary stems or leaves in the presence of more favorable water conditions. It is probable, therefore, that such structures represent reduc- tions in the growth of certain regions, caused by the unfavor- able conditions. Incidentally these thorns and spiny pro- cesses ure probably of great service as a protection to plants in regions where vegetation is peculiarly exposed to the
XEROPHYTE SOCIETIES. 205
ravages of animals (see §105). Examine Figs. 171, 172, 173, 174, 175, 176.
150. Anatomical adaptations—It is in connection with the xerophytes that some of the most striking anatomical adaptations have been developed. In such conditions the epider- mis is apt to be coy- ered by layers of cuticle, which are de- veloped by the walls of the epidermal cells, and being constantly formed beneath the cuticle, may become yery thick. This forms a very efficient protective covering, and has a tendency to diminish the loss of water (see §35). It is also to be observed that among xerophytes there is a strong de- velopment of palisade
tissue. The working Fie. 172. Two plants of common gorse or furze
Ils of the 1 <t (Ulex), showing the effect of environment : } Cells 0 ne leaves Nex is a plant grown in moist conditions; a is a
to the exposed surface plant grown in ary conditions, the leaves and are elongated, snd are anehehving em amos etry develope directed endwise to
the surface. In this way only the ends of the elongated cells are exposed, and as such cells stand very closely to- gether, there is no drying air between them. In some cases there may be more than one of these palisade rows (see §32). It has been observed that the chloroplasts in these palisade cells are able to assume various positions in
206 PLANT RELATIONS.
Fig. 173. A branch of Cytisus, showing the reduced leaves and thorny branches.—After KERNER.
regulation of transpiration, but also to the storage of water, as it is received at rare inter- vals. It is very common to find a certain re- gion of the plant body given over to this work, forming what is known as water tissue. In
the cell, so that when the light is very intense they move to the more shaded depths of the cell, and when it be- comes less intense they moye to the more exter- nal regions of the cell (see Fig. 177). The stomata, or breathing pores, which are devel- oped in the epidermis, are also great regulators of transpiration, as has been mentioned already (see §31).
151. Water reservoirs. —In xero- phytes at- tention must be given not only to the
many leaves this water tissue may be distin- guished from the ordinary working cells by being a group of colorless cells (see Figs. 178, 179, 180). In plants of the drier regions leaves may become thick and fleshy through acting as water reservoirs, as in the case of the agave, sedums, etc. Fleshy or ‘succulent ” leaves are regarded as adaptations of prime impor-
Fie. 174. A leaf of traga- canth, show- ing the re- duced leaf- lets and the thorn-like tip.—After KKERNER.
XEROPHYTE SOCIETIES. 207
tance in xerophytic conditions. In the cactus plants the peculiar stems have become great reservoirs of moisture. The globular body may be taken to represent the most com- plete answer to this general problem, as it is the form of body by which the least amount of surface may be exposed and the greatest amount of water storage secured. In the case of fleshy leaves and fleshy bodies it has long been noticed that they not only contain water, but also have a
Fie. 176. Twig of com- mon locust, showing the thorns.—After KERNER.
berry, showing the thorns. —After KERNER.
great power of re- taining it. Plant collectors have found great difficulty in drying these fleshy forms, some of which seem to be able to retain their moisture in- definitely, even in the driest conditions.
152. Xerophytic structure.—The adap- tations given above are generally found in plants growing in drouth conditions, and they all imply an effort to diminish transpiration. It must not be supposed, however, that only plants living in drouth conditions show these adapta- tions. Such adaptations result in what is known as the xerophytic structure, and such a structure may appear even in plants growing in hydrophyte condi- tions. For example, the bulrush grows in shallow water, and is a prominent member of one of the hydrophyte socie- ties (see §136); and yet it has a remark- ably xerophytic structure. This is prob- ably due to the fact that although it
208 PLANT RELATIONS,
stands in the water its stem is exposed to a heat which is often intense.
The ordinary prairie (see $163) is included among mesophyte societies on account of the rich, well-watered soil; and yet many of the plants are very xerophytic in structure, probably on account of the prevailing dry winds.
The ordinary sphagnum-bog (see $139), or ‘‘peat-bog,” is included among hydrophyte societies. It has an abundance of water, and is not ex- posed to blazing heat, as in the case of the bulrushes, or to drying wind, as in the case of prairie plants; and yet its plants show uw xerophytic struc-
Fig. 177. Cells from the leaf of a quillwort (Jsoetes). The light is striking the cells from the direction of one looking at the illus-
tration. If it be some- 7 ture.
what diffuse the chloro- plasts distribute them- selves through the shal- low cell, as in the cell to the left. If the light be intense, the chloroplasts move to the wall and as- sume positions less ex-
This is found to be due, proba- bly, to a lack of certain important soil materials.
It is evident, therefore, that xero- phytic structures are not necessarily confined to xerophytic situations. It
posed, as in the cell to
: is probably true that all societies which the right. =
show xerophytic structures belong to- gether more naturally than do the societies which are grouped ac- cording to the water
supply.
Societies.
No attempt will be made to classify these very numerous socie- ties, but a few prom-
Fie. 178, Asection through a Begonin leaf, show- ing the epidermis (ep) above and below, the water-storage tissue (ws) above and below, and the central chlorophyll region (as).
XEROPHYTE SOCIETIES. 209
inent illustra- tions will be given.
153. Rock societies. — Vari- ous plants are able to live up- on exposed rock surfaces, and \ therefore form
Fie. 179. A section through a fleshy leaf (Clinia), show-
distinct associa- ing the chlorophyll region on the outside (shaded and tions of xero- marked as), and the large interior water-storage region (ws).
phytes. In gen- eral they are lichens, mosses, and crevice plants (see Fig. 181). The crevice plants are those which send their roots into the rock crevices and so gain a foothold. The crevice plants also commonly show a rosette habit, the rosette of overlapping leaves being against the rock face, and therefore in the most favorable position for checking loss of water.
154. Sand societies—In general sand societies may be roughly grouped as beach societies, dune societies, and sandy field societies. These three hold a certain definite relation to one another. This natural relation- Fig. 180. Asectionthrough = ship appears on the borders of the
a leaf of an epiphyte,
showing avery large de. large lakes, and on seacoasts. The pee oe tissue heach is nearest the water, the dunes dermis and the chloro, are next, and behind them stretch phyll region, which is the sandy fields. When the three restricted nies types are thus associated. the plants
under surface of the leaf. —After ScHmmrEn. of the different areas pass gradually
210 PLANT RELATIONS.
into one another. It is very common to find the dunes omitted in the series, and to have the beaches pass gradu- ally into the sandy fields.
The beach society is usually quite characteristic, and in general it is a poor flora, the beach being characteristically bare. ‘The plants which grow in such conditions are apt to occur in tufts, or are creeping plants. It is evident that
Fie. 181. A rock covered with lichens.
while the water may seem to be abundant, it disappears quickly, so that plants must adapt themselves to a dry condition of the soil, which is poor and with little or no accumulation of humus. At the same time, the exposure to intense light is extreme. This combination results in a poor display of individuals and of species. Here and there along beaches, where special conditions have favored the accumulation of humus, dense vegetation may spring up, but it should not be confused with the ordinary beach type.
“SHTMOD JO) PY—'STOIT JSIOF OY] SPARMOZ OALVLOD PUR svadT ALUTAMS OY] SPANAOY XOAUOS BL JUOIF DUNP Ol} JVYY PooTpOU oY [PTA YL osvo anpHoyaLd SIG] UP ‘Wert [BOJF POXTUL JOYJOUR ST pPUUOADYOVG VY UL ANUQATZ [|S puv ‘popwaul Sung st sjood dwuas Jo verve uv qysatoy oy} pully = "ySatoy yRo‘puv oud poxtw v uodn sf pUSUMTPIVOIIUD OY} YOG AYR = “SOYSUI -[Ing Suzurmjuos divas vw uodn Suryovossue st Jt punoiswos0y oy} uy “satqotoos yur snomea uodn Stiqovodguo ouNp Y ‘“B8T “OMT
212 PLANT RELATIONS.
The dune societies are subjected to very peculiar con- ditions. Dunes are billows of sand that have been devel- oped by prevailing winds, and in many cases they are con- tinually changing their form and are frequently moving
ae Pay ae : ey ty
ae a ae kes DOW ec ae ma eer a MSc
Fie. 183. A sandy field type, showing the development of vegetation upon an old beach. The vegetation is low, often tufted and heath-like, being composed chiefly of grasses, bearberry (Arctostaphylos) and Hudsonia. In the background to the right is a conifer forest, and between it and the old beach is seen a dense mass of bearberry, a very characteristic heath plant, and forming here what is called a transition zone between the beach and the forest.—After CowLEs.
landward (see Fig. 182). The moving dunes should be distinguished from the fixed ones, where the billow form is retained, but the dunes have ceased their motion. In the case of the active dunes a peculiar type of vegetation is de- manded, As is to be expected, the flora is very scanty, and
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214 PLANT RELATIONS.
has two remarkably developed characters. The plants are what are known as ‘‘ sand-binders,” that is, the underground structures become extremely developed, reaching to great distances horizontally and vertically, so that one is always surprised at the extent of the underground system. This wide searching for water results in giving the plants a deep anchorage in the shifting soil, and at the same time helps to prevent the shifting. As soon as enough of the sand- binders have established themselves, a shifting dune becomes a fixed one. Another characteristic that must be strongly developed by these plants is the ability to grow up through the sand after they have been engulfed. The plants of the shifting dunes are often buried as the dune shifts, and unless the burial has been too deep, they are able to continue their development until leaves may be exposed to the air. In this way plants have often developed a length of stem which is far beyond anything they attain when growing in ordinary conditions.
The sandy field societies are represented by a much more abundant flora than the beach or the dune societies, the general character being tufted grasses and low shrubby growths (see Fig. 183).
155. Shrubby heaths—The shrubby heaths are very characteristic of the more northern regions, and are closely related to the sandy field societies. The heath soil is apt to be a mixture of coarse sand, or gravel and rock, with an occasional deposit of humus, and would be regarded in general as a sterile soil. The flora of the shrubby heaths shows well-marked strata, the upper one being the low shrubby plants of the heath family, most prominent among which are huckleberries and bearberries (see Fig. 167). The lower stratum is made up of mosses and l- chens. A branching lichen, usually spoken of as the “reindeer moss,” often occurs in immense patches on such heaths. While these shrubby heaths occur most extensively towards the north, small areas showing the
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216 PLANT RELATIONS.
same general character are common in almost all temper- ate regions.
156. Plains.—Under this head are included great areas in the interior of continents, where dry air and wind prevail. The plains of the United States extend from about the one hundredth meridian westward to the foot- hills of the Rocky Mountains. Similar great areas are represented by the steppes of Siberia, and in the interior of all continents. These regions have been regarded as semi- desert areas, but they are found for the most part to be far from the real desert conditions. They are certainly areas of comparative dryness, on account of the dry winds which prevail.
Taking the plains of the United States as a type, a very characteristic plant physiognomy is presented (see Fig. 184). In general, there is a meadow-like expanse, but the vegetation is much more sparse than in meadows, and is much more dense than in deserts. The two characteristic plant forms are the bunch grasses, that is, grasses which grow in great tufts; and low grayish shrubs, predomi- nantly ‘‘sage brush.” Under the shelter of the sage brush or other bush forms, many low herbs succeed in growing. In such areas the growing scasou is very short, during which time the vegetation looks vigorous and fresh; but during the rest of the year it is very dull. In some parts the plain is dry enough to permit the growth of the prickly- pear cactus (Opuntia), which may take possession of ex- tensive areas (see Fig. 185).
Usually there are two rest periods during the year, developed by the summer drouth and the winter cold. As a consequence, the plants of the area are partly spring plants, which are apt to he very brilliant in flower; and partly the later, deep-rooted forms. Over such areas the transportation of seeds by the wind is very prominent, as the force of the wind and the freedom of its sweep make possible very wide distribution. It is in such areas that
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220 PLANT RELATIONS.
the tumbleweed habit is prominently developed. Certain low and densely branching plants are lightly rooted in the soil, so that at the close of their growing period they are easily uprooted by the wind, and are rolled to great
Fira. 189. Tree-like yuccas from the arid regions of Africa, showing the very numer- ous thick and pointed, sword-like leaves.
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D929: PLANT RELATIONS.
distances. Where some barrier, such as a fence, lies across the track of the wind, these tumbleweeds may accumulate in great masses. This tumbling over the surface results in an extensive scattering of seeds (see Fig. 120).
The prairies, so characteristic of the United States, are regarded by some as belonging to the plains. They cer- tainly are closely related to them in origin, but can hardly be regarded as being included in xerophyte conditions, as the conditions of water supply and soil are characteristically mesophyte, under which head they will be considered.
157. Cactus deserts—In passing southward on the plains of the United States, it is to be noted that the con- ditions become more and more xerophytic, and that the bunch grasses and sage brush, peculiar to the true plains, gradually merge into the cactus desert, which represents a region whose conditions are intermediate between true plains und true deserts (see Fig. 186). In the United States this characteristic desert region begins to appear in West- ern Texas, New Mexico, Arizona, and Southern California, and stretches far down into the Mexican possessions. This yast arid region has developed a pecuhar flora, which con- tains most highly specialized xerophytic forms. The va- rious cactus forms may be taken as most characteristic, and associated with them are the agaves and the yuccas. Not only are the adaptations for checking transpiration and for retaining water of the most extreme kind, but there is also developed a remarkable armature. It is eyi- dent that such succulent bodies as these plants present might speedily disappear through the attacks of animals, were it not for the armor of spines and bristles and rigid walls. Study Pigs. 38, 39, 40, 187, 18s. 189.
15s. Tropical deserts —In such areas xerophyte con- ditions reach the greatest extreme in the combination of maximum heat and minimum water supply. It is evident that such a combination is almost too difficult for plants to endure. That the very scanty vegetation is due to lack
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224 PLANT RELATIONS.
of water, and not to lack of proper materials in the soil, is shown by the fact that where water does occur oases are developed, in which luxuriant vegetation is found.
The desert which extends from Egypt across Arabia may be regarded as a typical one. It is to be noted that the vegetation is so scanty that the soil is the conspicuous feature, and really gives the characteristic physiognomy (see Fig. 191). Accordingly the appearance of the deserts will depend upon whether the desert soil is rocky, or of small stones, or gravel (as in the Desert of Sahara), or of red clay, or of the dune type. As is to be expected, such vegetation as does occur is of the tuft and bunch type, as developed by certain grasses, or of the low irregular bush type (see Fig. 190).
In the South African deserts certain remarkable plants have been noted which have attained a certain amount of protection through mimicry, rather than by means of armor, as in the case of the cactus forms. Some of these plants resemble the ordinary stones lying about upon the desert. With the tropical deserts should not be confused such areas as those about the Dead Sea, or in the Death’s Valley in Southern California, as the barrenness of these areas is due to the strongly alkaline soils, and therefore they belong to the halophyte areas.
159. Thickets.—The xerophyte thicket is the most strongly developed of all thicket growths. Mention has been made of willow and alder thickets in hydrophyte con- ditions, but these are not to be compared in real thicket characters with the xerophyte thickets. These thickets are especially developed in the tropics and subtropies, and may be described as growths which are scraggy. thorny, and impenetrable. Warming speaks of these thickets as ‘the unsuccessful attempt of Nature to form a forest.” Hvidently the conditions are not quite favorable for for- est development, and un extensive thicket is the result. Such thickets are well developed in Texas, where they are
Fig. 192., A xerophyte conifer forest in the mountains. The peculiar conifer habit of body is recognized, the trees finding foothold in the crevices of rocks or in areas of rock débris,
22.6 PLANT RELATIONS.
spoken of as ‘‘ chaparral.” These chaparrals are nota- bly composed of mesquit bushes, acacias and mimosas of various sorts, and other plants. Similar thickets in Af- rica and Australia are frequently spoken of as ‘* bush ™ or ‘scrub.’ In all of these cases the thicket has the same general type, and probably represents one of the most for- bidding areas for travel.
160. Forests—The xerophyte forest societies may be roughly characterized under three general heads :
(1) Contferous forests.—These forests are very common in xerophyte conditions to the north, and also in the more sterile regions towards the south (see Figs. 192, 193, 194). They are generally spoken of as evergreen forests, although the name is not distinctive. These forests are of several types, such as true pine forests, in which pines are the prevailing trees and the shade is not dense; the fir and hemlock forests, which are relutively dark ; and the mixed forests, in which there is a mingling of various conifers. In such forests the soil is often very bare, and such under- growth as docs occur is largely composed of perennial plants. Many characteristic shrubs with fleshy fruits oc- cur, such as huckleberries, bearberries, junipers, ete. It will be noted that in these forests a characteristic adapta- tion to xerophyte conditions is the development of needle leaves, which are not only narrow, thus presenting a small exposure of surface, but also have heavy walls, which further prevents excessive transpiration.
(2) Foliage foresis.—These are more characteristic of tropical and subtropical xerophyte regions. Illustrations may be obtained from the eucalyptus, a characteristic Australian forest tree, the live oaks, oleanders, ete. It will be noticed that in these cases the leaves are not so narrow as the needles of conifers, but are generally lance- shaped, and stiff and leathery, indicating heavy walls to reduce transpiration. :
(3) Leafless forests.—In Java and other oriental regions
Fie. 193. A pine forest, showing the slender, tall, continuous trunks and compara- tively little undergrowth.—After ScHIMPER,
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KEROPHYTE SOCIETIES, 229
areas of dry naked soil are sometimes occupied by forest growths which show no development of leaves, the tree-like forms appearing continually bare. ‘The oriental leafless tree form is mostly a Casuarina. Bordering the Gulf of California, both in Mexico proper and in Lower California, there are leafless forests composed of various kinds of giant cactus (see Fig. 187), known as the ** cardon forests.” These leafless forests represent the most extreme xerophyte conditions occupied by plant forms which may be regarded as trees.
CHAPTER XIV. MESOPHYTE SOCIETIES.
161. General characters.— Mesophytes make up the com- mon vegetation of temperate regions, the vegetation most commonly met and studied. The conditions of moisture are medium, precipitation is in general evenly distributed, and the soil is rich in hunus. The conditions are not ex- treme, and therefore special adaptations, such as are neces- sary for xerophyte or hydrophyte conditions, do not appear. This may be regarded as the normal plant condition. It is certainly the arable condition, and most adapted to the plants which men seek to cultivate. When for purposes of cultivation xerophyte areas are irrigated, or hydrophyte areas are drained, it is simply to bring them into mesophyte conditions.
In looking over a mesophyte area and contrasting it with a xerophyte area, one of the first things evident is that the former is far richer in leaf forms. It is in the meso- phyte conditions that foliage leaves show their remarkable diversity. In hydrophyte and xerophyte arcas they are apt to be more or less monotonous in form. Another contrast is found in the dense growth over mesophyte areas, much more so than in xerophyte regions, and even more dense than in hydrophyte areas.
Among the mesophyte societies must be included not merely the natural ones, but those new societies which have been formed under the influence of man, and which do not appear among xcrophyte and hydrophyte societies.
Fic. 195. Alpine vegetation, showing the low stature, dense growth, and conepicu- ous flowers.-—After KERNER. 16
232 PLANT RELATIONS.
These new societies have been formed by the introduction of weeds and culture plants.
162. The two groups of societies—Two very prominent types of societies are included here under the mesophytes, although they are probably as distinct from one another as are the mesophyte and xerophyte societies. One group is composed of low vegetation, notably the common grasses and herbs ; the other is a higher woody vegetation, com- posed of shrubs and trees. The most characteristic types under each one of these divisions are noted as follows.
A. Grass and herb societies.
It should not be inferred from this title that most grasses are not herbs, but it is convenient to consider grasses and ordinary herb forms separately.
163. Arctic and alpine carpets.—These are dense mats of low vegetation occurring beyond forest growth in arctic regions, and above the tree limit in high mountains. These carpet-like growths are a notable feature of such regions. In such positions the growing season is very short, and the temperature is quite low at times, especially at night. It is evident. therefore. that there must be provision for rapid growth, and also for preventing dangerous radiation of heat, which might chill the active plant below the point of safety. It is further evident that the short season and the low temperature form a combination which prevents the growth of trees or shrubs, or even tall herbs, because the season is too short for them to reach a protected condition, and their more exposed young structures are not in a posi- tion to withstand the daily fall of temperature.
These carpets of vegetation are notably fresh-looking, indicating rapid growth ; green, indicating an abundance of chlorophyll and great activity; thick, as they are mostly perennials, developed from abundant underground structures ; low, on account of the short season and low
Fic. 196. Two plants of a rock-rose (J/elianthemum), showing the effect of low ground and alpine conditions. The low-ground plant (@) shows an open habit, and elongated stems with comparatively large and well-separated leaves. The same plant in alpine conditions is drawn to the same scale in 6, and magnified in c, the very short and compact habit being in striking contrast with that of the low- ground form.—After BoNNIER.
234 PLANT RELATIONS.
temperature ; and soft, the low stature and short life not involving the development of specially rigid structures for support or resistance. In such conditions, as would be expected, annuals are in the minority, the plants being mostly perennial and geophilous. Geophilous plants are those which have the habit of disappearing underground when protection is needed. This is probably the best adap- tation for total disappearance from the surface and for rapid reappearance (see $143). In such conditions, also, rosette forms are very common, the overlapping leaves of the rosette closely pressed to the ground diminishing the loss of heat by radiation. It has also been noticed that these arctic and alpine carpets show intense color in their flowers, and often a remarkable size of flower in proportion to the rest of the plant. Wherever the area is relatively moist, the carpet is prevailingly a grass mat; in the drier and sandier spots the herbs predominate (see Fig. 1115).
In the case of plants which can grow both in the low ground and in the alpine region, a remarkable adaptation of the plant body to the different conditions may be noted. The difference in appearance is sometimes so great that it is hard to realize that the two plants belong to the same species (see Fig. 196).
164. Meadows.—This term must be restricted to natural meadow areas, and should not be confused with those arti- ficial areas under the control of man, which are commonly called meadows. The appearance of such an area hardly needs definition, as it is a well-known mixture of grasses and flowering herbs, the former usually being the pre- dominant type. Such meadow-like expanses are common in connection with forest areas, and it is an interesting question to consider what. conditions permit forest growth and meadow growth side by side (see Fig. 197).
The greatest meadows of the United States are the well- known prairies, which extend from the Missouri castward to the forest regions of Illinois and Indiana (see Fig. 198).
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236 PLANT RELATIONS.
The prairie is regarded by some as a xerophyte area, and this is a natural conclusion when one examines only the struc- tures of the plants which occupy it. It is certainly a tran- sition area between the plains of the West and the true mesophyte areas of the Hast. However, an examination of the soil reveals a deep, rich humus, with the water sup- ply decidedly greater than that which characterizes a true xerophyte area. On the other hand, the prevailing winds are dry. There is a mixture, therefore, of mesophyte con- ditions in the soil and xerophyte conditions in the air, which leads to a peculiar association of structures.
The vegetation of the prairies in general is composed of tufted grasses and perennial flowering herbs. Unfortu- nately, most of the natural prairie has disappeared, to be replaced by farms, and the characteristic prairie forms are not easily seen, The flowering herbs are often very tall and coarse, but with brilliant flowers, such as species of aster, goldenrod, rosin-weed, indigo plant, Inpine, bush clover, etc. The most characteristic of these forms show their xerophyte adaptations by their rigidity and roughness.
It has long been a vexed question as to the absence of trees in a soil which seems to be most suitable for their development. Probably the most ancient explanation was the occurrence of prairie fires, but it seems evident that some general natural condition rather than an artificial one is responsible for such an extensive area. <A possible explanation is as follows: The extensive plains of the West develop the strong and dry winds which prevail over the prairie region, and this brings about extremes of heat and drouth, in spite of the character of the soil. In such conditions a tree in a germinating condition could not establish itself. The prairies, therefore, represent a sort of broad beach between the Western plains and the East- ern forests. The eastward limit of the prairie has proba- bly depended upon the limit of the dry winds, which are gradually modified as they move eastward, until they
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238 PLANT RELATIONS.
cease to be unfavorable to forest growth. The forest does not begin abruptly upon the eastern limit of the prairie, but appears first as clumps of trees, with interspersed meadows, and finally as a dense forest mass. Of course, the forest display of the eastern border of the prairie has been immensely interfered with by man.
165. Pastures,—This term is applied to areas drier than
very dense, excluding all other vegetation, and the grass or pasture areas are too dry to form real meadows.—After CowLEs.
natural meadows, and includes the meadows formed or con- trolled by man (see Fig. 199). They may be natural, or derived from natural meadow areas, or from forest clear- ings; therefore they are often maintained in conditions which, if not interfered with, would not produce a meadow. In general, the pasture differs from the natural meadow in being drier, a fact often due to drainage, and in develop- ing lower and more open vegetation. Naturally the plant
MESOPHYTE SOCIETIES. 239
forms are prevailingly grasses, and their cultivation is the purpose of the artificial pasture, but the meadow tendency is shown by the coming in of perennial weeds. The inva- sion of pastures by weeds suggests many interesting ques- tions. Are the weeds natives or foreigners? Are they annuals or perennials ? What is the relative success of the different invaders, and why are some more successful than others? <A study of pastures will also reveal the fact that there is great difference in the vegetation of mowed and grazed pastures. The same effects are noted when natural meadows are used for grazing.
B. Woody societies.
These societies include the various shrub and tree as- sociations of mesophyte areas, associations entirely distinct from the grass and herb societies.
166. Thickets—The mesophyte thickets are not so abun- dant or impenetrable as the xerophyte thickets. They seem to be developed where the conditions are not quite favor- able for forests. An illustration of this fact may be ob- tained by noting the succession of plants which appear on a cleared area. After such an area has been cleared of its trees, by cutting or by fire, it is overrun by herbs which develop rapidly from the seed. Sometimes these herbs are tall and with showy flowers, as the so-called fire-weed or great willow herb. Following the herb societies there is a gradual invasion of coarser herbs and shrubby plants, forming thickets, and finally a forest growth may appear again.
In arctic and alpine mesophyte regions the willow is the great thicket plant, often covering large areas. In tem- perate regions willow thickets are confined to stream banks and boggy places, the plants evidently needing moist and cool soil. Although the willow may be regarded as the characteristic mesophyte thicket plant, there are other
240 PLANT RELATIONS.
well-known thicket plants, such as hazel, birch, alder, etc. Although pure thickets frequently occur, that is, thickets in which willow, or hazel, or alder, is the prevailing type, mixed thickets are probably more common. One who is familiar with mesophyte thickets will recognize that the ordinary mixed thickets are composed of various kinds of shrubs, brambles, and tall herbs.
167. Deciduous forests—Deciduous forests are especially characteristic of temperate regions. The deciduous habit, that is, the habit of shedding leaves at a certain period, is an adaptation to climate. In the temperate regions the adaptation is in response to the winter cold, when a vast reduction of delicate exposed surface is necessary. Instead of protecting delicate leaf structures from the severe cold of winter, these plants have formed the habit of dropping them and putting out new leaves when the favorable season returns.
It is instructive to notice how differently the conifers (pines, etc.) and the deciduous trees (oaks, maples, etc.) have answered the problem of adaptation to the cold of winter. The conifers have protected their leaves, giving them a small surface and heavy walls. In this way pro- tection has been secured at the expense of working power during the season of work. Reduced surface and thick walls are both obstacles to leaf work. On the other hand, the deciduous trees have developed the working power of their leaves to the greatest extent, giving them large sur- face exposure and comparatively delicate walls. It is out of the question to protect such an amount of surface dur- ing the winter, and hence the deciduous habit. The coni- fers are saved the annual renewal of leaves, but lose in working power; the deciduous trees must renew their leaves annually, but gain greatly in working power.
It should be remarked that leaves do not fall because they are broken off, but that in a certain sense it is a process of growing off. Often at the base of the leaves,
MESOPHYTE SOCIETIES. 241
where the separation is to occur, a cleavage region is gradu- ally developed until the leaf is entirely separated from the stem except by a woody strand or two, which is easily broken (see Fig. 200). In this way the scar which remains has really been formed before the leaf falls.
In this process of sloughing off leaves, the plant cannot afford to lose the living substance present in the working leaves. This substance, during the prep- aration for the fall, has been graid- ually withdrawn into the perma- nent parts of the plant.
It will be noticed that in general deciduous leaves are thin, exceedingly variable in form, and in a general horizontal position, nor do they have the firm, leathery texture of the xerophyte leaves. All this indicates great leaf ac- tivity, for, the necessity of pro- tection being removed, the leaf is not impeded in its work by the development of protective struc-
Fie, 200. A section through the base of a leaf of horse-chest- nut preparing to fall off at the end of the growing sea- son. A cleavage plate (s) has
tures.
One of the most prominent features associated with the de- ciduous habit is the autumnal col-
developed between the woody bundle (0) and the surface. Presently this reaches the surface, and only the woody strand fastens the leaf to the
* aoe % stem. oration. The vivid colors which
appear in the leaves of many trees, just before the time of falling, is a phenomenon which has attracted a great deal of attention, but although it is so prominent, the causes for it are very obscure. It will be noticed that this autumnal coloration consists in the development of various shades of two typical colors, yellow and red. These colors are often associated together in the same leaf, and sometimes a leaf may show a pure color.
242 PLANT RELATIONS,
The two colors hold a very different relation in the leaf cell. It is known that the yellow is due to the break- ing down of chlorophyll, so that the chloroplasts, which are green when active, become yellow when disorganizing, and finally bleach out entirely. That yellow may indicate a post mortem change of chlorophyll may be noticed in con- nection with the blanching of celery, in which the leaves and wpper part of the stem may be green, the green may shade gradually into yellow, and finally into the pure white of complete blanching.
The red shades, however, do not seem to hold any such relation to the disorganization of chlorophyll. The red coloring matter appears as a stain in the cell sap, so that what might be called the atmosphere of the active cell is suffused with red. Certain experiments upon plant colors have indicated that the presence of the red color slightly increases the temperature by absorbing more heat. This has suggested that the red color may be a slight protec- tion to the living substance, which has ceased working and which is in danger of exposure to cold. If this be true, it may he that the same explanation will cover the case of the red flush so conspicuous in buds and young leaves in the early spring. It must not be supposed that the need of protection has developed the color, but that since it is developed it may be of some such service to the plant. The whole subject, however, is too indefinite and obscure to be presented in any other form than as a bare suggestion.
Even the conditions which determine autumnal colora- tion have not been made out certainly. To many the an- tumnal coloration is associated with the coming of frost, which simply means a reduction of temperature ; others associate it with diminishing water supply: still others associate if with the change in the direction of the rays of light, which are more oblique in autumn than during the active growing season. It is certainly true that the colors
MESOPHYTE SOCIETIES. 243
are far more brilliant in certain years than in others, and that the coloration must be connected in some way with the food relations of the plants. Recent experiments have shown that the red coloration is largely dependent upon low temperature, which affects certain of the food-stuffs, and the red stain is one of the products.
The autumnal colors are notably striking in American forests on account of the fact that in these forests there is the greatest display of species, and hence not only are more colors produced, but they are usually strikingly associated.
Not only is protection during the cold period secured by deciduous forests through the falling of leaves, but the development of scaly buds is an adaptation to the same end. By means of these overlapping. often hairy, and even varnished structures, delicate growing tips are pro- tected during the cold season. The development of cork, also, on the older parts, is a measure of protection.
As in the case of thickets, deciduous forests may be pure or mixed. A very common type of pure forest is the beech forest, which is a characteristic dark forest. The wide-spreading branches of neighboring beeches overlap each other, so as to form dense shade. As a consequence, ina pure beech forest there is little or no undergrowth ; in fact, no lower strata of vegetation until the lowest ones are reached, made up of grasses and mosses. An- other type of pure forest, which belongs to the drier re- gions, is the oak forest, which forms a sharp contrast to the beech, in that it isa Lght forest, permitting access of light for lower strata of plants. lence in such a forest there is usually more or less undergrowth, consisting of shrubs, etc., which may develop regular thickets. The typical American deciduous forest, however, is the great mixed forest, made up of many varieties of trees, such as beech, oak, elm, walnut, hickory, gum, maple, etc. These great mixed forests, with their remarkable autumnal
244 PLANT RELATIONS.
coloration, reach their culmination in the Central West, in Southern Illinois, Central and Southern Indiana, Ohio, and Kentucky.
168. Evergreen foliage forests—The word foliage is in- troduced to distinguish these forests from the ordinary evergreen forests which are coniferous, and which do not display broad leaves. The evergreen foliage forests are chiefly characteristic of tropical regions, but occasionally they are represented in temperate regions, notably of South America. The conditions which especially favor them are abundant precipitation and great heat. These rainy forests of the tropics may be regarded, as Warming says, ‘‘as the climax of the world’s vegetation,” for the conditions in which they are developed favor constant plant activity at the highest possible pressure. Such great forest growths are found within the region of the trade winds, where there is heavy rainfall, great heat, and rich black soil. So abundant is the precipitation that the air is often saturated and the plants drip with moisture. In such conditions pure forests may occur, characterized by such tree forms as the tree ferns, palms, or bamboos. Only the great mixed tropical forest will be considered. The main characteris- tics are as follows :
(1) Absence of simultaneous pertodicity.—Perhaps the most. striking feature, in contrast with the deciduous forests, is that there is no regular period for the develop- ment or fall of leaves. Leaf activity is possible through- out the year, and there is no time of bare forest, or of forests just putting out leaves. This does not mean that the leaves persist indefinitely, but that there is no regular time for their fall and formation. Leaves are continually being shed and formed, but the trees always appear in full foliage.
(2) Density of growth.—Such an arca is remarkably filled with vegetation, stratum after stratum occurring, resulting in gigantic jungles. The higher strata may be
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246 PLANT RELATIONS.
made up of trees of different heights, below them are shrubs of varying heights, then tall and low herbs, and finally mosses and liverworts. Among these close-set standing forms, great vines or lianas climb and bind the
Ll aah.
Fie. 202. A group of aerial plants (epiphytes) from a tropical forest. Note the vari- ous habits of the epiphytes attached to the tree-trunks, and the dangling roots.— After ScHimpER.
standing vegetation into an inextricable tangle (see Figs. 55, 201). In addition to these, hosts of aerial plants find lodging places upon the tree-trunks and vines (sce Fig. 202). These rainy forests of the tropics furnish the very best conditions for the development of the numerous epi- phytic orchids, bromelias, etc. In such conditions also
MESOPHYTE SOCIETIES. 247
numerous saprophytes occur. Such an assemblage of vege- tation is to be found nowhere else.
(3) Mumber of species.—Not only is there an immense number of individuals, but an extraordinary number of species occur. <A list of plants growing in these forests would show a re- markable representation of the plant kingdom.
(+) Forms of trees.— The dense vegetation re- sults in straight leafless tree-trunks, so that the leaves of trees are mainly clustered at the tops of high branches. The shade is so dense and the inter- ference is so great that the development of low branches is impossible. It is common, also, for the larger trees to develop a system of buttresses near the base, and also fre- quently to send out prop roots (see Figs. 100, 101).
(5) Absenceofbud scales. —In the deciduous forest bud scales are necessary to Fie. 203. A gutter-pointed leaf from a
; tropical plant.—After ScHimpER. protect the tender growing tips during the period of ccld. The same device would be sufficient to protect against a period of drouth. In the tropical forest there is danger neither from cold nor drouth, and in such conditions bud scales are not developed, and the buds remain naked and unprotected.
(6) Devices against too abundant rain.—The abundance
VG
248 PLANT RELATIONS.
of rain is in danger of checking transpiration, and as this process is essential to plant activity, there are often found devices to prevent the leaves from becoming saturated. Many leaves have cuticles so smooth and glazed that the water glances off without soaking in; in other cases a velvety covering of hairs answers the same purpose; in still other cases leaves are gutter-pointed, that is, the tip is prolonged as asort of gutter, and the veins are depressed, the whole surface of the leaf resembling a drainage system, so that the rain is conducted rapidly from the surface (see Fig. 203). These are only a few illustrations of many devices against dangerous wetting.
CHAPTER XV. HALOPHYTE SOCIETIES.
169, General characters——The hydrophytes, xerophytes, and mesophytes are distinguished from one another by the amount of water accessible. This classification must be regarded as largely artificial, often resulting in the natural separation of closely related societies. For example, the sphagnum-moor is a well-marked hydrophyte society, but it holds a very close relation to the shrubby heath, which is a xerophyte society. These two societies, however, are kept separate on the basis of the water supply, but they are brought together by similarity in the food material sup- plied by the water. It becomes evident, therefore, that a natural classification properly depends not so much upon the amount of water, as upon what the water contains. However, the three groups of societies already considered have been used for the sake of simplicity.
The halophytes, however, are characterized in a very different way, for the condition which determines them is not the amount of water supply, but the fact that the water contains certain salts, notably common salt, gypsum, and magnesia. The water may be abundant enough to rep- resent hydrophyte conditions, or it may be scanty enough to represent xerophyte conditions, but if.these salts are present in the soil in sufficient abundance to strongly affect the water, the plants are halophytes. Such soils are recognized in popular language as salt soils or alkaline soils.
Such areas occur in various positions: (1) in the
250 PLANT RELATIONS.
vicinity of the seashore, where there are salty beaches, and swamps and meadows ; (2) the margins of salt lakes, such as the Great Salt Lake, the Dead Sea, or Caspian Sea, and a host of smaller lakes: (3) about saline springs, which are common among the numerous medicinal springs of water- ing places ; (4) certain interior arid wastes, which probably mark the position of old sea basins. An extensive area of this last kind is known as the Bad Lands, which stretch over certain portions of Nebraska and Dakota. In these Bad Lands the waters are strongly alkaline.
Comparatively few plants wre able to endure such con- ditions. The family which has been able to develop most halophyte forms is the family of chenopods, which contains such prominent halophyte forms as the sam- phire, seablight, saltwort, greasewood, etc. Associated with these chenopods are certain portulacas, spurges, sedges, grasses, etc. Such plants do not seem to be very sensitive to climate, for the same hulophyte species are found everywhere, in all latitudes and at all altitudes. Probably the so-called Russian thistle, which is not a thistle at all, may be cited as a notable illustration of a chenopod which ranges through all climates.
Halophyte vegetation is very open, and the ground rarely seems to be covered. If the soil is always moist, some plants which are not true halophytes may grow in connection with the halophyte plants. If the soil dries up easily, even a small percentage of salt presently be- comes very conspicuous, and from such places every other plant is driven out but the pure halophytes.
There are many great families of plants which are never known to grow in halophyte conditions, as for example the great groups represented by oaks, hickories, walnuts, etc., the nettle family, the rose family. the heath family, and the whole display of mosses and lichens. On the other hand, halophytes often grow outside of halophyte condi- tions. To be wv halophyte does not mean that other condi-
— = St Se LE
|
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HALOPHYTE SOCIETIES. 261
tions are not possible, but that halophytes are plants which have succeeded in living in halophyte conditions. If they find other conditions, they may grow with even greater vigor.
Halophytes are mostly succulent plants, with the leaves thick and often translucent. This indicates the presence of water reservoirs, and also the fact that the plants are poor in chlorophyll. The succulent habit is common also
Fic. 204. A mangrove forest advancing into the water.—After SCHIMPER.
among xerophytes, a group which halophytes further re- semble in the small leaves and often prostrate habit. If halophytes with such adaptations are transplanted into more favorable conditions, as into a mesophyte area, the plants become taller and thin-leaved.
The evidence seems to show that the presence of the salts in the soil, at least in the amount in which they occur, interferes with the nutritive work of the plant. Certainly the plant seems to make food with difficulty, a
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HALOPHYTE SOCIETIES. 253
difficulty that means impossibility to all but the compara- tively few forms that have succeeded in living in halophyte conditions.
Numerous halophyte societies have been recognized, but attention is called only to a few.
170. Mangrove swamps.—This is certainly the most vigor- ous of the halophyte societies. Mangrove swamps occur along flat tropical sea coasts, where the waters are quiet. The mangrove is a tree of curious habit, which advances slowly out into the water and extends back landwards as low woods or thickets (see Figs. 204. 205). The whole surroundings appear forbidding, for the water is sluggish and mucky, covered with scum, rich in bacteria, and with bubbles constantly breaking upon the surface from decay- ing matter beneath the water. The mangrove has the pe- culiarity of germinating its seeds while still upon the tree, so that embryos hang from the trees. and then drop like plumb-bobs into the muck beneath, where they stick fast and are immediately in a condition to establish them- selves. In these mangrove swamps the species are few, and the adaptations chiefly in the way -of developing various kinds of holdfasts for anchoring in the uncertain soil, and also various devices for carrying air to the submerged parts.
171. Beach marshes and meadows.—The salt marshes and meadows near the seacoast are very well known. They lie beyond the reach of ordinary flood tide, but the waters are brackish. In these marshes and meadows occur certain characteristic halophyte grasses and sedges. Such forms being the dominant type give the general appearance of a coarse meadow. The difference between a marsh and meadow is simply a question of the amount of water.
172. Salt steppes—These areas are often large in extent, and belong to the interior of continents (see Fig. 206). So far as water supply is concerned, they hold the same relation to other halophyte societies as do the plains to mesophyte societies. In the United States one of the most
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HALOPHYTE SOCIETIES. 255
extensive of the salt steppes is the Great Salt Lake basin. It is here that the halophyte chenopod forms are especially developed, and there is a rich display of greasewoods, seablights, samphires, etc. The Bad Lands, already re- ferred to, represent another such area.
173. Salt and alkaline deserts.—In these areas the water supply reaches its minimum, and therefore the water be- comes saturated with the characteristic salts of the soil. No worse combination for plant activity can be imagined than the combination of minimum water and maximum salts. In consequence, such areas are almost, if not abso- lutely, devoid of vegetation. As illustrations, the exten- sive desert of the Dead Sew region and the Death’s Valley may be cited. Death's Valley is an area in Southern Cali- fornia upon the borders of Arizona, in the general region of the cactus desert. The soil is so strongly impregnated with the so-called alkaline salts, and the water supply is so scanty, that the surface of the soil is covered by a thick crust of salt. In such conditions vegetation becomes impossible.
INDEX TO PLANT RELATIONS.
(The italicized numbers indicate that the subject is illustrated on the page cited. In such case the subject may be referred to only in the illustration, or it may be
referred to also in the text.]
A
Acacia, 199.
Achillea, 202.
Adaptation, 147.
Adiantum, 27,
Aeration, 92, 93, 95, 188.
Agave, 45, 47. :
Agrimony, 121.
Ailanthus, 116.
Air, 95, 98, 114, 122, 138.
Air cavities, 171, 172, 178, 176.
Air passages, 92, 93, 94, 95.
Air plants, 97, 98, 99, 100, 101, 246.
Air roots, 97, 98, 99, 100.
Alchemilla, 79.
Alge, 1, 2, 87, 99, 107, 109, 110, 111, 118, 150, 171, 172, 177.
Alkaline deserts, 255.
Alpine plants, 148, 291, 282, 233.
Amicia, 9.
Ampelopsis, 63,
Anemophilous, 122.
Animals, 119, 121, 122, 128, 145, 205.
Annual habit, 195.
Annual rings, 84.
Anthurium, 97.
Apple, 79.
Araucaria, 74.
Arbor vite, 139.
Arctic plants, 148, 232. Arrow-leaf, 186. Artillery plant, 120. Ash, 116.
Aspidium, 55. Assimilation, 154, 156, Autumn coloration, 241.
B
Bacteria, 189. Banana, 88.
Banyan, 105, 106. Barberry, 207.
Bark, 84.
Basswood, 116, 201. Beach, 209, 210. Beach marshes, 253. Beach meadows, 253 Beach pea, 118.
Bean, 140.
Bearberry, 212, 214. Beech drops, 157. Beech forest, 145, 243. Beggar ticks, 119, 121. Begonia, 25, 208. Bellflower, 19, 80. Bidens, 719. Bignonia, 715.
258
Bilbergia, 136.
Birches, 71.
Black moss, 96, 101. Bladderwort, 173.
Blade, 35
Bloodroot, 195.
Bogs, 148.
Box elder, 84.
Bramble, 94.
Branched leaves, 19, 20, 21, 23. Buds, 70, 73, 75, 141, 248, 247. Bulbs, 73, 75, 81.
Bulrush, 142, 148, 185, 186, 207. Burdoek, 121, 122.
Bush, 226.
Bush clover, 43.
Buttercup, 185.
Buttresses, 108, 104, 247.
C
Cactus deserts, 277, 222.
Cactus forms, 45, 46, 47, 146, 202, 218, 219,
207,
222, Calyx, 78, 79, 80, 128. Campanula, 19, 80. Caoutchoue, 186. Carbohydrates, 153, 156. Carbon, 153. Carbon dioxide, 30, 151, 153. Cardon forests, 229. Carnation, 42.
215, 216, 217,
Carnivorous plants, 155, 156, 157,
1738, 189, Carpel, 78, 79, 80, £25. Carrot, 120. Castor-oil bean, 73. Casuarina, 229. Catalpa, 117. Catchfly, 186. Cat-tail flag, 142, 148, 185, 286.
INDEX.
Cercis, 10.
Chaparral, 226.
Chenopods, 250.
Chlorophyll, 6, 8, 149, 162.
Chloroplasts, 39, 107, 152, 208, 209, 242.
Chrysanthemum, .23.
Cilia, 109, 111.
Claytonia, 190.
Cleistogamons, 130.
Clematis, 272.
Climbing stems, 60, 61, 62, 63, 64, 102, 248.
Clinging roots, 99, 10.2.
Clinia, 209.
Coeklebur, 720, 171.
Compass plants, 10, 12, 197, 198.
Compound leaves, 19, 2, 21, 23.
Conducting tissue, 171.
Conifer forests, 225, 226, 227, 228.
Conifers, 83, 190, 191, 223, 226,
Cork, 243.
Corn, 85, 90.
Corolla, 78, 79, 80.
Cortex, 88, 84, 93, 94, 107, 108.
Cottonwood, 70.
Cotyledons, 50, 51, 73, 139, 140.
Crevice plants, 94, 209.
Cuticle, 42, 205.
Cyead, 22.
Cycloloma, 177.
Cypress knees, 95, 96, 183.
Cypripedium, 132, 133, 134, 135, 156,
Cytisus, 20.
205,
D
Dandelion, 82, 774, 117. Darlingtonia, 157. Date palm, 87. Dead-nettle, 80.
INDEX.
Deciduous forests, 240.
Deciduous habit, 148, 196, 240, lel,
Deserts, 771,
Desiccation, 194.
Desmodium gyrans, 49.
Destruction of plants, 148.
Diatoms, 174.
Dicotyledons, 35, 83, 116.
Differentiation, 3.
Digestion, 154, 156.
Dionea, 160, 161.
Dodder, 106, 107, 157.
Dog-tooth violet, 244.
Dragon tree, 15.
Drainage, 143, 145.
Drosera, 158, 150.
Drouth, 193.
Duckweed, 97, 175.
Dunes, 145, 201, 209, 212, 212.
Dwarf growths, 203.
999
Nay
223, 255.
E
Easter lily, 14.
Echeveria, 17.
Ecological factors, 163.
Ecology, 4, 149.
Eel grass, 184.
Ege, 110, 111.
Elaters, 118.
Elatine, 92.
Elm, 63, 67, 68, 75.
Embryo, 111, 139.
Entomophilous, 122, 123.
Epidermis, 87. 40, 41, 42, 88, 84, 107, 170, 205, 208, 209.
Epilobium, 712, 113, 128, 185.
Epiphyte, 209.
Equisetum, 111, 203.
Erect stems, 62, 65, 66, 67, 68, 69, 70, 71.
258
Erica, 200. Erythronium, 244. Euphorbia, 04.
PF
Ferns, 55, 56, 85, 88, 100, 111, 118, 119.
Fertilizing, 145.
Ficus, 8.
Figwort, 128, 135.
Fireweed, 11?, 113, 128, 135, 239.
Fittonia, 37, 152.
Fixed light position, 197.
Flag, 124, 138.
Floating stems, 59.
Floats, 171. 172, 173.
Flowers, 76, 78, 140.
Foliage forests, 226, 244.
Foliage leaves, 6, 28, 139.
Forest clearing, 143. 145.
Forests, 190, 226, 240, 245.
Fruit. 173, 114, 115, 116, 117, 118, A190, 120, 12d, D2,
Fucus, 171.
Functions, 3.
Fungi, 87, 107, 709, 110.
Furze, 25.
G
Galium, 17.
Gamete, 710, 117, 112, 113.
Geophilous habit, 55, 56, 73, 74, Toy, PB, TT, FS OL, 108, L9G, 234:
Geotropism, 69, 91, 188.
Germination, 111, 188, 739, 140.
Gorse, 205.
Grape vine, 61.
Grass, 187, 197, 216, 236.
Gravity, 91.
Guard cells, 38.
Gymnosperms, 115.
260
H
Habenaria, 727.
Hairs, 43, 92, 136, 146, 202, 208.
Halophytes, 169, 249.
Harebell, 19, 80.
Hawthorn, 36.
Heart-wood, 151.
Heat, 112, 1388, 145, 164.
Heath plants, 189, 200, 214.
Helianthemum, 232.
Helotropism, 12, 13, 68, 72, 73, 139.
Hemlock, 190.
Horse-chestnut, 242.
Hosts, 106.
House leek, 79.
Houstonia, 729, 185.
Huckleberry, 214.
Hudsonia, 212.
Hura crepitans, 120.
Hydrogen, 153.
Hydrophytes, 168, 170, 174.
Hydrotropism, 91, 188.
ui
Insects and flowers, 123. Tris, 126, 188.
Isoetes, 94, 95, 208.
Ivy, 99.
J
Juncus, 77. Juniper, 51, 238,
L
Lactuca, 12, 197, 198.
Lady-slipper, 132, 183, 134, 135, 186.
Lakes, 143, 148.
INDEX.
Laminaria, 277. Larch, 178, 190. Latex, 136. Leafless forests, 226. Leaflet, 19. Leaf-relation, 538. Lemna, 97. Lespedeza, 43. Lianas, 60, 61, 62, 63, 64. 102, 245, 246, Lichens, 194, 209, 214. Life-relations, 4, 7, 8, 53, 77. Light, 148, 167, 197. Light-relation, 7, 8. Lily, 38, 40. Live-for-ever, 18. Live oak, 101. Liverworts, 118. Locomotion, 118. Locust, 207. Long moss, 96, 101. Loosestrife, 1380, 135. Lotus, 180.
M
Mangroves, 251, 252, 253.
Maple, 26, 115, 116.
Maranta, 38.
Marchantia, 107.
Meadows, 234, 235.
Mechanical tissue, 172.
Mesophyll, 38, 39, 40, 41, 42, 152.
Mesophytes, 168, 280.
Migration, 58, 75, 147.
Mildew, 109, 157.
Milkweed, 117.
Mimosa, 199.
Mistletoe, 107.
Mold, 109.
Monocotyledons, 35, 85, 88, 116, ISG,
Moors, 187, 188,
INDEX.
Mosaic arrangement, 25, 27, 37.
Mosses, 87, 107, 110, 118, 118, 188, 194, 209, 214.
Motile leaves, 9, 10, 11, 49, 198, 199.
Mould, 109.
Mullein, 48, 44.
Mushrooms, 157.
N
Nectar, 123, 158. Nelumbium, 780. Nicotiana, 80. Nightshade, 26. Nitrogen, 153. Nodes, 54.
Nuphar, 92. Nutrition, 3, 149. Nymphexa, 178, 180.
ce)
Oak, 69, 101. Oak forest, 145, 248. (Edogonium, 111. Orchids, 98, 99, 126, 127, 132, 133, 134, 185, 136, 189. Organs, 8. Ornithogalum, 81. Ovary, 79, 80, 125. Ovules, 78, 79, 80. Oxalis, 10, 50, 199. Oxygen, 29, 138, 158.
P
Palisade tissue, 39, 40, 42, 205. Palms, 86, 87, 228.
Pandanus, 103.
Parasites, 106, 150.
Passion vine, 62.
Pastures, 238.
261
Pellionia, 24.
Pentstemon, 137.
Peony, 78.
Petals, 78, 79, 80.
Petioles, 15, 26, 35, 55.
Phlox, 80.
Photosynthesis, 28, 29, 150, 153, 156.
Physiology, 149.
Pickerel weed, 181, 182.
Pines, 63, 65, 66, 112, 115, 190, 227, 228.
Pirus, 79.
Pistil, 77, 79, 80.
Pitcher plant, 155, 156, 157,
Pith, 88, 84, 107.
Plains, 213, 215, 216.
Plankton, 174.
Plant body, 2.
Plant societies, 1, 146, 162, 174.
Plastid, 152.
Platycerium, 100.
Plumes, 112, 113, 114, 116, 117.
Plumule, 51, 140.
Pollen, 111,
123.
Pollination, 77, 115, 122, 123.
Polygonatum, 35.
Ponds, 142, 175, 178, 180, 184.
Pondweed, 176, 181, 25.7.
Potato, 74, 76.
Potentilla, 43, 79.
Prairies, 208, 222, 284, 237.
Prickles, 146.
Prickly lettuce, 12, 197, 198.
Primrose, 187.
Procumbent stem, 57.
Profile position, 197, 198.
Pronuba, 130, 131.
Prop roots, 99, 108, 104, 105, 106, 247.
152,
117,
158.
168,
pad vt,
112, 115, 121,
262
Protandry, 128, 185.
Protection of leaves, 9, 10, 11, 12, 41, 42, 43, 48, 49.
Proteids, 158, 156, 189.
Protogyny, 128, 135.
Protoplasm, 154, 156.
Ptelea, 115.
Puff-balls, 157.
Q Quillwort, 94, 95, 208.
R
Rain, 51, 247.
Ranunculus, 18.7.
Raspberry, 92.
Receptacle, 77, 81, 114.
Redbud, 0.
Reed grass, 142, 185, 186.
Reed swamps, 185.
Reproduction, 3, 109.
Respiration, 82, 154, 156.
Rhizoids, 107.
Rivalry, 146.
Robinia, 125, 126, 1238, 207.
Rock-rose, 233.
Rock societies, 209, 210.
Roots, 89, 90, 95, 98, 99, 188, 139, AA.
Root-cap, 108.
Root-hairs, 90.
Rootstalk, 55, 56, 75, 76, 77, 78, D6,
Rose acacia, 175, 126, 133.
Rosette habit, 76, 17, 78, 19, 47, 158, 160, 209, 234.
Rosinweed, 10, 197, 198.
Rubber tree, 74.
Runners, 57, 3.
Rusts, 157.
OA,
INDEX.
8
Sage brush, 216.
Sagittaria, 186,
Saintpaulia, 16.
Sult deserts, 255.
Salt steppes, /54.
Sand societies, 209.
Sandy fields, 209, 272.
Sanguinaria, 195.
Saprophytes, 130, 189.
Sap-wood, 151.
Sargassum, 17.
Sarracenia, 165, 156, 158.
Saxifrage, 58.
Seale leaves, 70, 78.
Scales, 141.
Scouring rush, 203.
Serew pine, 70:3.
Scrub, 226.
Seaweeds, 1, 2, 87, 99.
Sedges, 187. ;
Seed-dispersal, 22”, 118, 774, 116, 117, 118, 119, 120.
Seed-plants, 111, 119, 121.
Seeds, 111, 122, 123, 115, 138, 139, 140.
Selaginella, 20, 100, 194.
Semperviyum, 19.
Senecio, 174.
Sensitive plants, 71, 48, 50, 299.
Sepals, 78, 79, 80.
Shepherdia, 44.
Shoots, 53.
Silphium, 10, 197, 798.
Sinilax, 67.
Snapdragon, 80, 137.
soil, 90, 94, 145, 151, 166, 214 ed,
Solomon’s seal, 35, 70.
Spanish needle, 779, 121.
Sphagnum, 18s.
INDEX. 26
Sphagnum-bogs, 208.
Sphagnum-moors, 188.
Spines. 146, 204.
Spiregyra, 110.
Spongy tissue. 89, 40.
Spore cause, 55, 118, 119.
Spore-dispersal, 10, 711,112, 118, 114, 11s
Spores, 209, 110, 112. 112.
Spring beauty. 296.
Spring plants, 148, 144.
Squash seedlings, 39.
Squirting cucumber, 120.
staghorn fern, 100.
Brame, 78, 79, 80, 125,
Starch, 153.
star cucumber, 61.
Star-of-Bethlehem, 82.
Stem, 54, 85, 139.
Steppes, 216, 203.
Stigma. 80, 123.
Supules. 35.
Stomata, 8. 40, 206.
Strawberry plant, 57, 58, 98.
Struggle for existence, 142.
Style. 80, 128.
Subterranean stems, 54, 55, 56. 76, Pip 8s
Succulent plants, 251.
Sugar, 158.
Sundew, 738, 159.
Sunflower, 72.
Swamp-forest, 190, 252.
Swamp-imoers, 187,
Swamp-thickets, 18s.
Swamps, 183.
£
Tamarack, 178, 190. Tap root. 93. AR 42,
axus, 4 6
qo
Teasel, 136.
Telegraph plant, 49.
Temperature, 145.
Tendrils, 62. 62, 63.
Thallus, 107.
Thickets, 188, 224, 239.
Thistle, 117.
Thorns, 146, 204. 205, 206, 207, 224.
Thuja, 209.
Tilia, 126, 201.
Tillandsia, 96, 101.
Toad-flax, 80.
Toadstools, 149.
Tobacco, 80.
Touch-me-not, 119.
Tragacanth, 206.
Transpiration, 31, 33, 154. 193, 2A,
Tropical forest. 245.
Trumpet creeper. 99.
Tubers, 74. 76. 196.
Tumbleweeds. 277, 220.
Turf-building, 185.
U
Ulex, 208. Ulothrix, 109, 111. Utricularia, 173, 174.
Vv
Vallisneria, 154. Vascular bundles, 88, 84, 92, 938, 94, 107, 108, 151, 171. Vegetative multiplication. 109. Veins, 335, 36. 387, 40, 151. Velamen, 99. Venation, 33, Victoria, 180. Violet, 117, 119.
56, 37.
264 INDEX.
WwW Witch hazel, 118, 119. Woodbine, 61, 62. Walnut, 82.
Water, 90, 92, 94, 95, 118, 188, x
142, 151, 163, 193, 206, 244. 7 Water lily, 178, 180, 181, Xerophytes, 168, seed Water reservoirs, 206, 208, 209. ERODING SRRELELIE), Weeds, 147. Willow, 3. 239. “ Wind, 95, 98, 114, 122, 167. Yew, 42. Wings, 712, 115, 116. Yucea, 45, 47, 180, 131, 220.
TWENTIETH CENTURY TEXT-BOOKS
PLANT SIRUGIURES
A SECOND BOOK OF BOTANY
BY JOHN M. COULTER, A.M., Pu. D.
HEAD OF DEPARTMENT OF BOTANY UNIVERSITY OF CHICAGO
NEW YORK D. APPLETON AND COMPANY 1900
Copyricut, 1899, By D. APPLETON AND COMPANY.
PREFACE
Iy the preface to Plant Relations the author gave his reasons for suggesting that the ecological standpoint is best adapted for the first contact with plants. It may be, how- ever, that many teachers will prefer to begin with the mor- phological standpoint, as given in the present book. Rec- ognizing this fact, Plant Structures has been made an independent volume that may precede or follow the other, or may provide a brief course of botanical study in itself.
Although in the present volume Morphology is the domi- nant subject, it seems wise to give a somewhat general view of plants, and therefore Physiology, Ecology, and Taxonomy are included in a general way. For fear that Physiology and Ecology may be lost sight of as distinct subjects, and to introduce important topics not included in the body of the work, short chapters are devoted to them, which seek to bring together the main facts, and to call attention to the larger fields.
This book is not a laboratory guide, but is for reading and study in connection with laboratory work. An accom- panying pamphlet for teachers gives helpful suggestions to those who are not already familiar with its scope and purpose. It is not expected that all the forms and sub- jects presented in the text can be included in the labora- tory exercises, but it is believed that the book will prove a useful companion in connection with such exercises. It is very necessary to co-ordinate the results of /aboratory work, to refer to a larger range of material than can be handled, and to develop some philosophical conception of
Vv
vi PREFACE
the plant kingdom. The learning of methods and the collection of facts are fundamental processes, but they must be supplemented by information and ideas to be of most service.
The author does not believe in the use of technical terms unless absolutely necessary, for they lead frequently to mistaking definitions of words for knowledge of things. But it is necessary to introduce the student not merely to the main facts but also to the literature of botany. Ac- cordingly, the most commonly used technical terms are introduced, often two or three for the same thing, but it is hoped that familiarity with them will enable the student to read any ordinary botanical text. Care has been taken to give definitions and derivations, and to call repeated attention to synonymous terms, so that there may be no confusion. The chaotic state of morphological terminology tempted the author to formulate or accept some consistent scheme of terms; but it was felt that this would impose upon the student too great difficulty in reading far more important current texts.
Chapters I-XII form a connected whole, presenting the general story of the evolution of plants from the lowest to the highest. The remaining chapters are supplementary, and can be used as time or inclination permits, but it is the judgment of the author that they should be included if possible. The flower is so conspicuous and important a feature in connection with the highest plants, that Chapter ATII seems to be a fitting sequel to the preceding chapters. It also seems desirable to develop some knowledge of the great Angiosperm families, as presented in Chapter XIV, since they are the most conspicuous members of every flora. In this connection, the author has been in the habit of directing the examination of characteristic flowers, and of teaching the use of ordinary taxonomic manuals. Chap- ter XV deals with anatomical matters, but the structures included are so bound up with the form and work of plants
PREFACE vil
that it seems important to find a place for them even in an elementary work. The reason for Chapters XVI and AX VII has been stated already, and even if Plant Relations is stud- ied, Chapter A VII will be useful either as a review or as an introduction. In the chapter on Plant Physiology the author has been guided by Noll’s excellent résumé in the ““Strasburger ” Botany.
The illustrations have been entirely in the charge of Dr. Otis W. Caldwell, who for several years has conducted in the University, and in a most efficient way, such labo- ratory work as this volume implies. Many original illus- trations have been prepared by Dr. Caldwell and his assist- ants, and some are credited to Dr. Chamberlain and Dr. Cowles, of the University, but it is a matter of regret that pressure of work and time limitation have forbidden a still greater number. The authors of the original illustrations are cited, and where illustrations have been obtained else- where the sources are indicated. The descriptions given in connection with each illustration are unusually full, and should be studied carefully, as frequently they contain important material not included in the text.
The author would again call attention to the fact that this book is merely intended to serve as a compact supple- ment to three far more important factors: the teacher, the laboratory, and field work. Without these it can not serve its purpose.
Joun M. CouLter.
Tue University oF Cuica@o, Vovember, 1899.
CONTENTS
CHAPTER I.—Iytropuction
Il.—THaLuopuytes: ALG& III.—TueE EVOLUTION OF SEX IV.—THE GREAT GROUPS OF ALG.E V.—THALLOPHYTES: FUNGI VI—THE FOOD OF PLANTS VIL—BryYoruyteEs VIII—THE GREAT GROUPS oF BRYOPHYTES . IX.—PTeERIDOPHYTES X.—THE GREAT GROUPS OF PTERIDOPHYTES XI.—SPERMATOPHYTES : GYMNOSPERMS XTI.—SpPERMATOPHYTES : ANGIOSPERMS XIUL—THE FLOWER XIV.—MonocoryLEDONS AND DICOTYLEDONS . XV.—DIFFERENTIATION OF TISSUES XVI.—P.LaNT PHYSIOLOGY XVII—PuLaxt ECOLOGY GLOSSARY
INDEX
ix
PAGE
TO ae NEY. PART IL.—PLANT STRUCTURES
CHAPTER I
INTRODUCTION
1. Differences in structure—It is evident, even to the casual observer, that plants differ very much in structure. They differ not merely in form and size, but also in com- plexity. Some plants ure simple, others are complex, and the former are regarded as of lower rank.
Beginning with the simplest plants—that is, those of lowest rank—one can pass by almost insensible grada- tions to those of highest rank. At certain points in this advance notable interruptions of the continuity are dis covered, structures, and hence certain habits of work, chang- ing decidedly, and these breaks enable one to organize the vast array of plants into groups. Some of the breaks ap- pear to be more important than others, and opinions may differ as to those of chief importance, but it is customary to select three of them as indicating the division of the plant kingdom into four great groups.
2. The great groups—The four great groups may be indicated here, but it must be remembered that their names mean nothing until plants representing them have been studied. It will be noticed that all the names have the
1
2 PLANT STRUCTURES
constant termination phytes, which is a Greek word mean- ing “plants.” The prefix in each case is also a Greek word intended to indicate the kind of plants.
(1) Thallophytes—The name means “thallus plants,” but just what a “thallus” is can not well be explained until some of the plants have been examined. In this great group are included some of the simplest forms, known as Alge and Fungi, the former represented by green thready growths in fresh water and the great host of sea- weeds, the latter by moulds, mushrooms, etc.
(2) Bryophytes—The name means “moss plants,” and suggests very definitely the forms which are included. Every one knows mosses in a general way, but associated with them in this great group are the allied liverworts, which are very common but not so generally known.
(3) Pteridophytes—The name means * fern plants,” and ferns are well known. Not all Pteridophytes, however, are ferns, for associated with them are the horsetails (scouring rushes) and the club mosses.
(4) Spermatophytes.—The name means “ seed plants ”— that is, those plants which produce seeds. In a general way these are the most familiar plants, and are commonly spoken of as “flowering plants.” They are the highest in rank and the most conspicuous, and hence have received much attention. In former times the study of botany in the schools was restricted to the examination of this one group, to the entire neglect of the other three great groups.
3. Increasing complexity.—.\t the very outset it is well to remember that the Thallophytes contain the simplest plants—those whose bodies have developed no organs for special work, and that as one advances through higher Thallophytes, Bryophytes, and Pteridophytes, there is a con- stant increase in the complexity of the plant body, until in the Spermatophytes it becomes most highly organized, with numerous structures set apart for special work, just as in the highest animals limbs, eyes, cars, bones, muscles, nerves, etc.,
INTRODUCTION 3
are set apart for special work. The increasing complexity is usually spoken of as /ifterentiation—that is, the setting apart of structures for different kinds of work. Hence the Bryophytes are said to be more highly differentiated than the Thallophytes, and the Spermatophytes are regarded as the most highly differentiated group of plants.
4. Nutrition and reproduction.—However variable plants may be in complexity, they all do the sume general kind of work. Increasing complexity simply means an attempt to do this work more effectively. It is plant work that makes plant structures significant, and hence in this book no at- tempt will be made to separate them. All the work of plants may be put under two heads, nutrition and repro- duction, the former including all those processes by which a plant maintains itself, the latter those processes by which it produces new plants. In the lowest plants, these two great kinds of work, or functions, as they are called, are not set apart in different regions of the body, but usually the first step toward differentiation is to set apart the re- productive function from the nutritive, and to develop special reproductive organs which are entirely distinct from the general nutritive body.
5. The evolution of plants.—It is generally supposed that the more complex plants have descended from the simpler ones; that the Bryophytes have been derived from the Thallo- phytes, and so on. All the groups, therefore, are supposed to be related among themselves in some way, and it is one of the great problems of botany to discover these relation- ships. This theory of the relationship of plant groups is known as the theory of descent, or more generally as evo- lution. To understand any higher group one must study the lower ones related to it, and therefore the attempt of this book will be to trace the evolution of the plant king- dom, by beginning with the simplest forms and noting the gradual increase in complexity until the highest forms are reached.
CHAPTER II
THALLOPHYTES: ALG
6. General characters—Thallophytes are the simplest of plants, often so small as to escape general observation, but sometimes with large bodies. They occur everywhere in large numbers, and are of special interest as representing the beginnings of the plant kingdom. In this group also there are organized all of the principal activities of plants, so that a study of Thallophytes furnishes a clew to the structures and functions of the higher, more complex groups.
The word “thallus” refers to the nutritive body, or vegetative body, as it is often called. This body does not differentiate special nutritive organs, such as the leaves and roots of higher plants, but all of its regions are alike. Its natural position also is not erect, but prone. While most Thallophytes have thallus bodies, in some of them, as in certain marine forms, the nutritive body differentiates into regions which resemble leaves, stems, and roots ; also cer- tain Bryophytes have thallus bodies. The thallus body, therefore, is not always a distinctive mark of Thallophytes, but must be supplemented by other characters to determine the group.
%. Alge and Fungi—It is convenient to separate Thallo- phytes into two great divisions, known as Alge and Fungi. It should be known that this is a very general division and not a technical one, for there are groups of Thallophytes which can not be regarded as strictly either Algw or Fungi, but for the present these groups may be included.
4
THALLOPHYTES: ALGH 5
The great distinction between these two divisions of Thallophytes is that the Alge contain chlorophyll and the Fungi do not. Chlorophyll is the characteristic green color- ing matter found in plants, the word meaning “leaf green.” It may be thought that to use this coloring material as the basis of such an important division is somewhat superficial, but it should be known that the presence of chlorophyll gives a peculiar power—one which affects the whole structure of the nutritive body and the habit of life. The presence of chlorophyll means that the plant can make its own food, can live independent of other plants and animals. Algae, therefore, are the independent Thallophytes, so far as their food is concerned, for they can manufacture it out of the inorganic materials about them.
The Fungi, on the other hand, contain no chlorophyll, can not manufacture food from inorganic material, and hence must obtain it already manufactured by plants or animals. In this sense they are dependent upon other or- ganisms, and this dependence has led to great changes in structure and habit of life.
It is supposed that Fungi have descended from Alge— that is, that they were once Algae, which gradually acquired the habit of obtaining food already manufactured, lost their chlorophyll, and became absolutely dependent and more or less modified in structure. Fungi may be regarded, there- fore, as reduced relatives of the Algez, of equal rank so far as birth and structure go, but of very different habits.
ALG
8. General characters—As already defined, Alge are Thallophytes which contain chlorophyll, and are therefore able to manufacture food from inorganic material. They are known in general as “ seaweeds,” although there are fresh-water forms as well as marine. They are exceedingly variable in size, ranging from forms visible only by means
19
6 PLANT STRUCTURES
of the compound microscope to marine forms with enor- mously bulky bodies. In general they are hydrophytes—that is, plants adapted to life in water or in very moist places. The special interest connected with the group is that it is supposed to be the ancestral group of the plant kingdom— the one from which the higher groups have been more or less directly derived. In this regard they differ from the Fungi, which are not supposed to be responsible for any higher groups.
9. The subdivisions—Although all the Alge contain chlorophyll, some of them do not appear green. In some of them another coloring matter is associated with the chlo- rophyll and may mask it entirely. Advantage is taken of these color associations to separate Algw into subdivisions. As these colors are accompanied by constant differences in structure and work, the distinction on the basis of colors is more real than it might appear. Upon this basis four sub- divisions may be made. The constant termination phycec, which appears in the names, is a Greek word meaning “ sea- weed,” which is the common name for Alge; while the pre- fix in each case is the Greek name for the color which char- acterizes the group.
The four subdivisions are as follows: (1) Cyanophycea, or “ Blue Algve,” but usually called ‘ Blue-green Alew,” as the characteristic blue does not entirely mask the green, and the general tint is bluish-green ; (2) Chlorophycecw, or “ Green Algee,” in which there is no special coloring matter associ- ated with the chlorophyll; (3) Phaaphyceew, or * Brown Alge”; and (4) Rhodophycee, or “ Red Alge.”
It should be remarked that probably the Cyanophycee do not belong with the other groups, but it is convenient to present them in this connection.
10. The plant body.—By this phrase is meant the nutri- tive or vegetative body. There is in plants a unit of struc- ture known as the cell. The bodies of the simplest. plants consist of but one cell, while the bodies of the most com-
THALLOPHYTES: ALG q
plex plants consist of very many cells. It is necessary to know something of the ordinary living plant cell before the bodies of Alge or any other plant bodies can be under- stood.
Such a cell if free is approximately spherical in outline, (Fig. 6), but if pressed upon by contiguous cells may become yariously modified in form (Fig. 1). Bounding it there is a thin, elastic wall, com- posed of a substance called cellulose. The cell wall, therefore, forms a delicate sac, which contains the liv- ing substance known as pro- toplasm. This is the sub- stance which manifests life, and is the only substance in the plant which is alive. It is the protoplasm which j has organized the cellulose Fig. 1. Cells from a moss leaf, showing wall about itself, and which nucleus (B) in which there is a nucle- does all the plant work. It eens a ci is a fluid substance which varies much in its consistence, sometimes being a thin vis- cous fluid, like the white of an egg, sometimes much more dense and compactly organized.
The protoplasm of the cell is organized into various structures which are called organs of the cell, each organ having one or more special functions. One of the most conspicuous organs of the living cell is the single nucleus, a comparatively compact and usually spherical protoplasmic body, and generally centrally placed within the cell (Fig. 1). All about the nucleus, and filling up the general cavity within the cell wall, is an organized mass of much thinner protoplasm, known as cytoplasm. The cytoplasm seems to form the general background or matrix of the cell, and the
8 PLANT STRUCTURES
nucleus lies imbedded within it (Fig. 1). Every working cell consists of at least cytoplasm and nucleus. Sometimes the cellulose wall is absent, and the cell then consists sim- ply of a nucleus with more or less cytoplasm organized about it, and is said to be naked.
Another protoplasmic organ of the cell, very conspicuous among the Alge and other groups, is the plastid. Plastids are relatively compact bodies, commonly spherical, variable in number, and lie imbedded in the cytoplasm. There are various kinds of plastids, the most common being the one which contains the chlorophyll and hence is stained green. The chlorophyll-containing plastid is known as the chloro- plastid, or chloroplast (Fig. 1). An ordinary alga-cell, there- fore, consists of a cell wall, within which the protoplasm is organized into cytoplasm, nucleus, and chloroplasts.
The bodies of the simplest Alge consist of one such cell, and it may be regarded as the simplest form of plant body. Starting with such forms, one direction of advance in complexity is to organize several such cells into a loose row, which resembles a chain (Fig. 4); in other forms the cells in a row become more compacted and flattened, form- ing a simple filament (Figs. 2, 5); in still other forms the original filament puts out branches like itself, producing a branching filament (Fig. 8). These filamentous bodies are very characteristic of the Alge.
Starting again with the one-celled body, another line of advance is for several cells to organize in two directions, forming a plate of cells. Still another line of advance is for the cells to organize in three directions, forming a mass of cells.
The bodies of Algz, therefore, may be said to be one- celled in the simplest forms, and in the most complex forms they become filaments, plates, or masses of cells.
11. Reproduction.—In addition to the work of nutrition, the plant body must organize for reproduction. Just as the nutritive body begins in the lowest forms with a single cell
THALLOPHYTES: ALGA 9
and becomes more complex in the higher forms, so repro- duction begins in very simple fashion and gradually be- comes more complex. Two general types of reproduction are employed by the Algx, and all other plants. They are as follows:
(1) Vegetative multiplication.—This is the only type of reproduction employed by the lowest Alga, but it persists in all higher groups even when the other method has been introduced. In this type no special reproductive bodies are formed, but the ordinary vegetative body is used for the purpose. For example, if the body consists of one cell, that cell cuts itself into two, each half grows and rounds off as a distinct cell, and two new bodies appear where there was one before (Figs. 3,6). This process of cell division is very complicated and important, involving a division of nucleus and cytoplasm so that the new cells may be organized just as was the old one. Wherever ordinary nutritive cells are used directly to produce new plant bodies the process is vegetative multiplication. This method of reproduction may be indicated by a formula as follows: P—P—P—P-—P, in which P stands for the plant, the formula indicating that a succession of plants may arise directly from one another without the interposition of any special structure.
(2) Spores.—Spores are cells which are specially organ- ized to reproduce, and are not at all concerned in the nutri- tive work of the plant. Spores are all alike in their power of reproduction, but they are formed in two very distinct ways. It must be remembered that these two types of spores are alike in power but different in origin.
-lserual spores.—These cells are formed by cell divi- sion. A cell of the plant body is selected for the purpose, and usually its contents divide and form a variable number of new cells within the old one (Fig. 2,8). These new cells are asexual spores, and the cell which has formed them within itself is known as the mother cell. This peculiar kind of cell division, which does not involve the wall of the
10 PLANT STRUCTURES
old cell, is often called internal division, to distinguish it from fission, which involves the wall of the old cell, and is the ordinary method of cell division in nutritive cells.
If the mother cell which produces the spores is different from the other cells of the plant body it is called the sporan- gium, which means “spore vessel.” Often a cell is nutri- tive for a time and afterward becomes a mother cell, in which case it is said to function as a sporangium. The wall of a sporangium usually opens, and the spores are dis- charged, thus being free to produce new plants. Various names have been given to asexual spores to indicate certain peculiarities. As Alge are mostly surrounded by water, the characteristic asexual spore in the group is one that can swim by means of minute hair-like processes or cilia, which have the power of lashing the water (Fig. 7, (). These ciliated spores are known as zoospores, or “animal- like spores,” referring to their power of locomotion ; some- times they are called swimming spores, or swarm spores. It must be remembered that all of these terms refer to the same thing, a swimming asexual spore.
This method of reproduction may be indicated by a for- mula as follows: P—o—P—o—P—o-—P, which indi- cates that new plants are not produced directly from the old ones, as in vegetative multiplication, but that between the successive generations there is the asexual spore.
Serual spores.—These cells are formed by cell union, two cells fusing together to form the spore. This process of forming a spore by the fusion of two cells is called the sexual process, and the two special cells (sexual cells) thus used are known as gametes (Fig. 2, (, d, e). It must be noticed that gametes are not spores, for they are not able alone to produce a new plant; it is only after two of them have fused and formed a new cell, the spore, that a plant can be produced. The spore thus formed does not differ in its power from the asexual spore, but it differs very much in its method of origin.
THALLOPHYTES: ALG 11
The gametes are organized within a mother cell, and if this cell is distinct from the other cells of the plant it is
called a gametangium, which means “ gamete vessel.” This method of reproduction may be indicated by a for- mula as follows: P=%>0—P=%>0—P=3>07-—P,
which indicates that two special cells (gametes) are pro- duced by the plant, that these two fuse to form one (sexual spore), which then produces a new plant.
It must not be supposed that if a plant uses one of these three methods of reproduction (vegetative multiplication, asexual spores, sexual spores) it does not employ the other two. All three methods may be employed by the same plant, so that new plants may arise from it in three differ- ent ways.
CHAPTER III THE EVOLUTION OF SEX
12. The general problem.—In the last chapter it was re- marked that the simplest Algee reproduce only by vegetative multiplication, the ordinary cell division (fission) of nutri- tive cells multiplying cells and hence individuals. Among other low Alge asexual spores are added to fission as a method of reproduction, the spores being also formed by cell division, generally internal division. In higher forms gametes appear, and a new method of reproduction, the sexual, is added to the other two.
Sexual reproduction is so important a process in all plants except the lowest, that it is of interest to discover how it may have originated, and how it developed into its highest form. Among the Alge the origin and develop- ment of the sexual process seems to be plainly suggested ; and as all other plant groups have probably been derived more or less directly from Algewe, what has been accom- plished for the sexual process in this lowest group was probably done for the whole plant kingdom.
13. The origin of gametes—One of the best Alge to illustrate the possible origin of gametes is a common fresh- water form known as Ulothrir (Fig. 2). The body consists of a simple filament composed of a single row of short cells (Fig. 2, 4). Each cell contains a nucleus, and a single large chloroplast which has the form of a thick cyl- inder investing the rest of the cell contents. Through the microscope, if the focus is upon the center of the cell, an optical section of the cylinder is obtained, the chloro-
12
THE EVOLUTION OF SEX 13
plast appearing as a thick green mass on each side of the central nucleus. As no other color appears, it is evident that Ulothriz is one of the Chlorophycee.
Fie. 2. Ulothrix, a Conferva form. A, base of filament, showing lowest holdfast cell and five vegetative cells, each with its single conspicuous cylindrical chloro- plast (seen in section) inclosing a nucleus; B, four cells containing numerous small zoospores, the others emptied; C, fragment of a filament showing one cell (a) containing four zoospores, another zoospore (%) displaying four cilia at its pointed end and just having escaped from its cell, another cell (¢) from which most of the small biciliate gametes have escaped, gametes pairing (d@), and the resulting zygotes (e); D, beginning of new filament from zoospore; #, feeble filaments formed by the small zoospores; F, zygote growing after rest; G, zoospores produced by zygote.—CALDWELL, except F and G, which are after DopEL-Port.
The cells are all alike, excepting that the lowest one of the filament is mostly colorless, and is elongated and more or less modified to act as a holdfast, anchoring the filament to some firm support. With this exception the cells are all nutritive ; but any one of them has the power of organizing for reproduction. This indicates that at first nutritive and
14 PLANT STRUCTURES
reproductive cells are not distinctly differentiated, but that the same cell may be nutritive at one time and reproductive at another.
In suitable conditions certain cells of the filament will be observed organizing within themselves new calls by internal division (Fig. 2, C, a, 0). The method of forma- tion at once suggests that the new cells are asexual spores, and the mother cell which produces them is acting as a sporangium. The spores escape into the water through an opening formed in the wall of the mother cell, and each is observed to have four cilia at the pointed end, by means of which it swims, and hence it is a zoospore or swarm spore. After swimming about for a time, the zoospores “settle down,” lose their cilia, and begin to develop a new filament like that from which they came (Fig. 2, D).
Other cells of the same filament also act as mother cells, but the cells which they produce are more numerous, hence smaller in size than the zoospores, and they have but two cilia (Fig. 2, C, ¢). They also escape into the water and swim about, except in size and in number of cilia resem- bling the zoospores. In general they seem to be unable to act as the zoospores in the formation of new filaments, but occasionally one of them forms a filament much smaller than the ordinary one (Fig. 2, #’'). This indicates that they may be zoospores reduced in size, and unable to act as the larger ones. The important fact, however, is that these smaller swimming cells come together in pairs, each pair fusing into one cell (Fig. 2, (, d,e). The cells thus formed have the power of producing new filaments more or less directly.
It is evident that this is a sexual act, that the cell pro- duced by fusion is a sexual spore, that the two cells which fuse are gametes, and that the mother cell which produces them acts as a gametangium. Cases of this kind suggest that the gametes or sex cells have been derived from zoo- spores, and that asexual spores have given rise to sex cells.
THE EVOLUTION OF SEX 15
The appearance of sex cells (gametes) is but one step in the evolution of sex. It represents the attainment of sexuality, but the process becomes much more highly developed.
1t. Isogamy.— When gametes first appear, in some such way as has been described, the two which fuse seem to be exactly alike. They resemble each other in size and activ- ity, and in every structure which can be distinguished. This fact is indicated by the word isogamy, which means “similar gametes,” and those plants whose pairing gametes are similar, as Ulothriv, are said to be isogamous.
The act of fusing of similar gametes is usually called conjugation, which means a “yoking together” of similar bodies. Of course it is a sexual process, but the name is convenient as indicating not merely the process, but also an important character of the gametes. The sexual spore which results from this act of conjugation is called the zygote or zygospore, meaning “ yoked spore.”
In isogamy it is evident that while sexuality has been attained there is no distinction between sexes, as obtains in the higher plants. It may be called a wufserual condition, as opposed to a dixerwal one. The next problem in the evolution of sex, therefore, is to discover how a bisexual condition has been derived from a unisexual or isogamous one.
15. Heterogamy.—Beginning with isogamous forms, a series of plants can be selected illustrating how the pairing gametes gradually became unlike. One of them becomes less active and larger, until finally it is entirely passive and very many times larger than its mate (Fig. 7). The other retains its small size and increases in activity. The pairing gametes thus become very much differentiated, the larger passive one being the femule gamete, the smaller active one the male gamete. This condition is indicated by the word heterogamy, which means “ dissimilar gametes,” and those plants whose pairing gametes are dissimilar arc said to be heterogamous.
16 PLANT STRUCTURES
In order to distinguish them the large and passive female gamete is called the oosphere, which means “egg sphere,” or it is called the egg ; the small but active male gamete is variously called the spermatozoid, the antherozoid, or simply the sperm. In this book egg and sperm will be used, the names of similar structures in animals.
In isogamous plants the mother cells (gametangia) which produce the gametes are alike; but in heterogamous plants the gametes are so unlike that the gametangia which produce them become unlike. Accordingly they have re- ceived distinctive names, the gametangium which produces the sperms being called the entheridium, that producing the egg being called the oogoniwm (Fig. 10).
The act of fusing of sperm and egg is called fertiliza- tion, which is the common form of the sexual process. The sexual spore which results from fertilization is known as the oospore or “ egg-spore,” sometimes called the fertilized egg.
It is evident that heterogamous plants are bisexual, and bisexuality is not only attained among Alge, but it prevails among all higher plants. Among the lowest forms there is only vegetative multiplication ; higher forms added sexu- ality ; then still higher forms became bisexual.
16. Summary.—Isogamous forms produce gametangia, which produce similar gametes, which by conjugation form zygotes. Heterogamous forms produce antheridia and oogonia, which produce sperms and eggs, which by fertiliza- tion form oospores.
CHAPTER IV THE GREAT GROUPS OF ALG
17. General characters—The Alge are distinguished among Thallophytes by the presence of chlorophyll. It was stated in a previous chapter that in three of the four great groups another coloring matter is associated with the chlorophyll, and that this fact is made the basis of a division into Blue-green Algew (Cyanophycee), Green Alga (Chloro- phycez), Brown Alge (Pheophycez), and Red Alge (Rhodo- phycee). In our limited space it will be impossible to do more than mention a few representatives of each group, but they will serve to illustrate the prominent facts.
1. CyaNnoPHYcE®£ (Blue-green Alg@)
18. Gleocapsa.—These forms may be found forming blue-green or olive-green patches on damp tree-trunks, rock, walls, etc. By means of the microscope these patches are seen to be composed of multitudes of spherical cells, each representing a complete Gleocapsa body. One of the pecul- larities of the body is that the cell wall becomes mucilagi- nous, swells, and forms a jelly-like matrix about the work- ing cell. Each cell divides in the ordinary way, two new Gleocapsa individuals being formed, this method of vegeta- tive multiplication being the only form of reproduction (Fig. 3).
When new cells are formed in this way the swollen mucilaginous walls are apt to hold them together, so that presently a number of cells or individuals are found lying
17
18
PLANT STRUCTURES
together imbedded in the jelly-like matrix formed by the
wall material (Fig. 3).
Fi. 8. Gleocapsa, a blue- green alga, showing single cells, and small groups which have been formed by division and are held together by the enveloping mucilage.— CALDWELL.
These imbedded groups of individ- uals are spoken of as colonies, and as colonies become large they break up into new colonies, the individual cells composing them continuing to divide and form new individuals. This rep- resents a very simple life history, in fact a simpler one could hardly be imagined.
19. Nostoc.—These forms occur in jelly-like masses in damp places. If the jelly be examined it will be found to contain imbedded in it numerous cells like those of Glevcapsa, but they are strung together to form chains of varying lengths (Fig. 4). The jelly in which these chains are imbedded is the same as that found in Glwocapsa, being
formed by the cell walls becoming mucilaginous and swollen. One notable fact is that all the cells in the chain are not
alike, for at irregu- lur intervals there oc- cur larger colorless cells, an illustration of the differentiation of cells. These larger cells are known as hef- crocysts (Fig. 4, 4), which simply means “other cells.” It is observed that when the chain breaks up into fragments each fragment iscomposed of the cells between
Fi. chain-like filameuts, and the heterocysts (.1) which determine the breaking up of the chain, - CALDWELL,
4. Nosfoc, a blue-green alga, showing the
THE GREAT GROUPS OF ALG.E 19
two heterocysts. The fragments wriggle out of the jelly matrix and start new colonies of chains, each cell dividing to increase the length of the chain. This cell division, to form new cells, is the characteristic method of repro- duction.
At the approach of unfavorable conditions certain cells of the chain become thick-walled and well-protected. These cells which endure the cold or other hardships, and upon the return of favorable conditions produce new chains of cells, are often called spores, but they are better called “resting cells.”
20. Oscillaria—These forms are found as bluish-green slippery masses on wet rocks, or on damp soil, or freely floating. Theyare simple filaments, composed of very short flattened cells (Fig. 5), and the name Oscillaria refers to the fact that they exhibit a peculiar oscillating move- ment. These motile filaments are is- olated, not being held together in a jelly-like matrix as are the chains of Vostoc, but the wall develops a cer- tain amount of mucilage, which gives the slippery feeling and sometimes forms a thin mucilaginous sheath about the row of cells.
The cells of a filament are all alike, = except that the terminal cell has its pene uivianle ible. free surface rounded. If a filament ae rete ey, breaks, and a new cell surface ex- and a single filament
a more enlarged (B).— posed, it at once becomes rounded. Oupawein If a single cell of the filament is freed from all the rest, both flattened ends become rounded, and the cell becomes spherical or nearly so. These facts indicate at least two important things: (1) that the cell wall is elastic, so that it can be made to change its form, and (2) that it is pressed upon from within, so that if free
90 PLANT STRUCTURES
it will bulge outward. In all active living cells there is this pressure upon the wall from within.
Each cell of the Oscillaria filament has the power of dividing, thus forming new cells and elongating the fila- ment. A filament may break up into fragments of varying lengths, and each fragment by cell division organizes a new filament. Here again reproduction is by means of vegeta- tive multiplication.
21. Conclusions—Taking Gleocapsa, Nostoc, and Oscil- laria as representatives of the group Cyanophycee, or “ oreen slimes,” we may come to some conclusions concern- ing the group in general. The plant body is very simple, consisting of single cells, or chains and filaments of cells. Although in Nostoc and Osctllaria the cells are organized into chains and filaments, each cell seems to be able to live and act independently, and the chain and filament seem to be little more than colonies of individual cells. In this sense, all of these plants may be regarded as one-celled.
Differentiation is exhibited in the appearance of hetero- cysts in .Vostoc, peculiar cells which seem to be connected in some way with the breaking up of filamentous colonies, although the Oscillvria filament breaks up without them.
The power of motion is also well exhibited by the group, the free filaments of Osr//lvr‘a moving almost continually, and the imbedded chains of \Vostoc at times moving to es- cape from the restraining mucilage.
The whole group also shows a strong tendency in the cell-wall material to become converted into mucilage and much swollen, a tendency which reaches an extreme expres- sion in such forms as Nos/oc and Gleocapsa.
Another distinguishing mark is that reproduction is exclusively by means of vegetative multiplication, through ordinary cell division or fission, which takes place very freely. Individual cells are organized with heavy resistant walls to enable them to endure the winter or other unfavor- able conditions, and to start a new series of individuals
THE GREAT GROUPS OF ALG a1
upon the return of favorable conditions. These may be regarded as resting cells. So notable is the fact of repro- duction by fission that Cyanophycee are often separated from the other groups of Alge and spoken of as “ Fission Alge,” which put in technical form becomes Schizophycee. In this particular, and in several others mentioned above, they resemble the “ Fission Fungi” (Schizomycetes), com- monly called “bacteria,” so closely that they are often associated with them in a common group called “ Fis- sion plants” (Schizophytes), distinct from the ordinary Alge and Fungi.
2. CHLOROPHYCE® ((reen .tlg@).
22. Pleurococcus.—This may be taken as a type of one- celled Green Alge. It is most commonly found in masses covering damp tree-trunks, etc., and looking like a green stain. These fine- ly granular green masses are found to be made up of multitudes of spherical cells re- sembling those of Gleocapsa, except that there is no blue with the chlo- rophyll, and the cells are not im-
bedded in such Fie. 6. Pleurococcus, a one-celled green alga : A, show-
jelly-like masses. ing the adult form with its nucleus; B, (, D, E,
various stages of division (fission) in producing new The cells may be cells; #, colonies of cells which have remained in solitary, or may contact.—CALDWELL.
cling together in
colonies of various sizes (Fig. 6). Like Gleorapsi, a cell
divides and forms two new cells, the only reproduction 20
99 PLANT STRUCTURES
being of this simple kind. It is evident, therefore, that the group Chlorophycee begins with forms just as simple as are to be found among the Cyanophycee.
Pleurococcus is used to represent the group of Protococ- cus forms, one-celled forms which constitute one of the subdivisions of the Green Alge. It should be said that Pleurococeus is possibly not a Protococcus form, but may be a reduced member of some higher group; but it is so common, and represents so well a typical one-celled green alga, that it is used in this connection. It should be known, also, that while the simplest Protococcus forms re- produce only by fission, others add to this the other meth- ods of reproduction.
23. Ulothrix—This form was described in § 13. It is very common in fresh waters, being recognized easily by its simple filaments composed of short squarish cells, each cell containing a single conspicuous cylindrical chloroplast (Fig. 2). This plant uses cell division to multiply the cells of a filament, and to develop new filaments from fragments of old ones; but it also produces asexual spores in the form of zoospores, and gametes which conjugate and form zygotes. Both zoospores and zygotes have the power of germination— that is, the power to begin the development of a new plant. In the germination of the zygote a new filament is not pro- duced directly, but there are formed within it zoospores, each of which produces a new filament (Fig. 2, 7, @). All three kinds of reproduction are represented, therefore, but the sexual method is the low type called isogamy, the pair- ing gametes being alike.
Ulothriv is taken as a representative of the Conferva forms, the most characteristic group of Chlorophycee. All the Conferva forms, however, are not isogamous, as will be illustrated by the next example.
24. Edogonium.—This is a very common green alga, found in fresh waters (Fig. 7). The filaments are long and simple, the lowest cell acting as a holdfast, as in Ulolhrix
Fie. 7. Edogonium nodosum, a Conferva form: A, portion of a filament showing a vegetative cell with its nuclens (@), an oogonium (@) filled by an egg packed with food material, a second oogonium (c) containing a fertilized egg or oospore as shown by the heavy wall, and two antheridia (5), each containing two sperms; B, another filament showing antheridia (a) from which two sperms (0) have escaped, a vegetative ceil with its nucleus, and an oogonium which a sperm (c) has entered and is coming in contact with the egg whose nucleus (@) may be seen; C, a zoo- spore which has been formed in a vegetative cell, showing the crown of cilia and the clear apex, as in the sperms; J. a zoospore producing a new filament, putting out a holdfast at base and elongating; #, a further stage of development; F, the four zoospores formed by the oospore when it germinates.—CALDWELL, except Cand F, which are after PRINGSHEIM.
4 PLANT STRUCTURES
(§ 13). The other cells are longer than in Ulothriz, each cell containing a single nucleus and apparently several chloroplasts, but really there is but one large complex chloroplast.
The cells of the filament have the power of division, thus increasing the length of the filament. Any cell also may act as a sporangium, the contents of a mother cell organizing a single large asexual spore, which is a zoospore. The zoospore escapes from the mother cell into the water, and at its more pointed clear end there is a little crown of cilia, by means of which it swims about rapidly (Fig. 7, C). After moving about for a time the zoospore comes to rest, attaches itself by its clear end to some support, elongates, begins to divide, and develops a new filament (Fig. 7, D, /).
Other cells of the filament become very different from the ordinary cells, swelling out into globular form (Fig. 7, A, B), and each such cell organizes within itself a single large egg (oosphere). As the egg is a female gamete, the large globular cell which produces it, and which is differen- tiated from the other cells of the body, is the oogonium. A perforation in the oogonium wall is formed for the entrance of sperms.
Other cells in the same filament, or in some other fila- ment, are observed to differ from the ordinary cells in being much shorter, as though an ordinary cell had been divided several times without subsequent elongation (Fig. 7, .1, f, B, a). In each of these short cells one or two sperms are organized, and therefore each short cell is an antheridium. When the sperms are set free they are seen to resemble very small zoospores, having the same little crown of cilia at one end.
The sperms swim actively about in the vicinity of the oogonia, and sooner or later one enters the oogonium through the perforation provided in the wall, and fuses with the egg (Fig. 7, Bc). As a result of this act of fer- tilization an oospore is formed, which organizes a firm wall
THE GREAT GROUPS OF ALGA 95
about itself. This firm wall indicates that the oospore is not to germinate immediately, but is to pass into a resting condition. Spores which form heavy walls and pass into the resting con- dition are often spoken of as “ rest- ing spores,” and it is very common for the zygotes and oospores to be resting spores. These resting spores enable the plant to endure through unfavor- able conditions, such as failure of food supply, cold, drought, etc. When favorable conditions return, the protected rest- ing spore is ready
for germination. Fie. &. Cladophora, a branching green alga, a very When the small part of the plant being shown. The branches arise at the upper ends of cells, and the cells are
oospore of Edogo- coenocytic.—CALDWELL.
nium germinates
it does not develop directly into a new filament, but the contents become organized into four zoospores (Fig. 7, /), which escape, and each zoospore develops a filament. In this way each oospore may give rise to four filaments.
It is evident that Hdogonium is a heterogamous plant, and is another one of the Conferva forms. Conferva bodies are not always simple filaments, as are those of UVlothrix and Edogonium, but they are sometimes extensively branch- ing filaments, as in Cladophora, a green alga very common
26 PLANT STRUCTURES
in rivers and lakes (Fig. 8). The cells are long and densely crowded with chloroplasts ; and in certain cells at the tips of branches large numbers of zoospores are formed, which have two cilia at the pointed end, and hence ure said to be biciliute.
25. Vaucheria—This is one of the most common of the Green Alge, found in felt-like masses of coarse filaments in shallow water and on muddy banks, and often called “ green
Fie. 9. Vaueheria geminata, a Siphon form, showing a portion of the ccenocytic body (4) which has sent out a branch at the tip of which a sporangium (B) formed, within which a large zoospore was organized. and from which (D) it is discharged later as a large multiciliate body ((), which then beyins the develop- ment of a new ccenocytic body (# ).—CALDWELL.
felt.” The filament is very long, and usually branches ex- tensively, but its great peculiarity is that there is no parti- tion wall in the whole body, which forms one long continuous cavity (Figs. 9,11). This is sometimes spoken of as a one- celled body, but it is a mistake. Imbedded in the exten- sive cytoplasm mass, which fills the whole cavity, there are not only very numerous chloroplasts, but also numerous nuclei. As has been said, a single nucleus with some cyto-
THE GREAT GROUPS OF ALG oF
plasm organized about it is a cell, whether it has a wall or not. Therefore the body of Vuucheria is made up of as many cells as there are nuclei, cells whose protoplasmic structures have not been kept separate by cell walls. Such a body, made up of numerous cells, but with no partitions, is called a cenocyte, or it is said to be cenarylic. Vancheria represents a great group of Chlorophycew whose members have coenocytic bodies, and on this account they are called the Siphon forms.
Vaucherta produces very large zoospores. The tip of a branch becomes separated from the rest of the body by a partition and thus acts as a sporangium (Fig. 9, 2). In this improvised sporangium the whole of the contents or- ganize a single large zoospore, which is ciliated all over, escapes by squeezing through a perforation in the wall
(Fig. 9, C), swims about for a time, and finally develops another Vawrheria body (Figs. 9, #, 10). It should be said that this large body, called a zoospore and acting like one, is really a mass of small biciliate zoospores, just as the
Fie. 10. A young Vareheria germinating from a spore (sp), and showing the holdfast (2#).— After Sacus.
apparently one-celled vegetative body is really composed of many cells. In this large compound zoospore there are many nuclei, and in connection with each nucleus two cilia are developed. Hach nucleus with its cytoplasm and two cilia represents a small biciliate zoospore, such as those of Cladophora, § 24.
Antheridia and oogonia are also developed. In a com- mon form these two sex organs appear as short special branches developed on the side of the large ccenocytic body,
28 PLANT STRUCTURES
and cut off from the general cavity by partition walls (Fig. 11). The oogonium becomes a globular cell, which usually
Fig. 11. Vaucheria sessilis, a Siphon form, show- ing a portion of the ceenocytic body, an an- theridial branch (4) with an empty anthe- ridium (a) at its tip, and an oogoninm (B) containing an oospore (¢) and showing the opening (/) through which the sperms passed to reach the egg.—CaLDWELL.
develops a perforated break for the entrance of the sperms, and organizes within itself a single large egg (Fig. 11, B). The an- theridium is a much smaller cell, within which numerous very small sperms are formed (Fig. 11, .1, a). Fie. 12. Botrydium, one of the The sperms are discharged, swarm penne aes
y containing
about the oogonium, and finally one continuous cavity, with a bulbous, chlorophyll-con-
one passes through the break and uisiiniee ‘peeitOn, “amet Soak fuses with the egg, the result be- like branches which pene- 7 trate the mud in which ing an oospore. The oospore or- noire Sirantnernad en ganizes a thick wall and becomes WELL.
a resting spore.
It is evident that Vaucheria is heterogamous, but all the other Siphon forms are isogamous, of which Botrydiwm may be taken as an illustration (Fig. 12).
26. Spirogyra.—This is one of the commonest of the “pond scums,” occurring in slippery and often frothy masses of delicate filaments floating in still water or about
THE GREAT GROUPS OF ALGE& 99
springs. The filaments are simple, and are not anchored by a special basal cell, as in Ulothrix and Edogonium. The
Fic. 18. Spirogyra, a Conjugate form, showing one complete cell and portions of two others. The band-like chloroplasts extend in a spiral from one end of the cell to the other, in them are imbedded nodule-like bodies ( pyrenoids), and near the center of the cell the nucleus is swung by radiating strands of cytoplasm.— CALDWELL.
cells contain remarkable chloroplasts, which are bands pass- ing spirally about within the cell wall. These bands may
Fie. 14. Spirogyra, showing conjugation: 4, conjugating tubes approaching each other; B, tubes in contact but end walls not absorbed: C, tube complete and con- tents of one cell passing through; D, a completed zygospore.—CaLDWELL.
30 PLANT STRUCTURES
be solitary or several in a cell, and form very striking and conspicuous objects (Figs. 13, 14).
Spirogyra and its associates are further peculiar in pro- ducing no asexual spores, and also in the method of sexual reproduction. Two adjacent filaments put out tubular processes toward one another. A cell of one filament sends out a process which seeks to meet a corresponding process from a cell of the other filament. When the tips of two such processes come together, the end walls disappear,
Fig. 15. Spirogyra, showing some common exceptions. At .1 two cells have been connected by a tube, but without fusion a zygote has been organized in each cell; also, the upper cell to the left has attempted to conjugate with the cell to the right. At B there are cells from three filaments, the cells of the central one hay- ing conjugated with both of the others.—CaLDWELL,
and a continuous tube extending between the two cells is organized (Figs. 14,15). When many of the cells of two parallel filaments become thus united, the appearance is that of a ladder, with the filaments as the side pieces, and the connecting tubes as the rounds.
While the connecting tube is being developed the con- tents of the two cells are organizing, and after the comple- tion of the tube the contents of one cell pass through and enter the other cell, fuse with its contents, and a sexual
THE GREAT GROUPS OF ALGE 31
spore is organized. As the gametes look alike, the process is conjuga- tion, and the sex spore is a zygote, which, with its heavy wall, is rec- ognized to be a resting spore. At the beginning of each growing season, the well-protected zygotes which have endured the winter germinate directly into new Spi- rogyra filaments.
On account of this peculiar style of sexual reproduction, in which gametes are not discharged, but reach each other through spe- cial tubes, Spiroyyra and its allies are called Conjugate forms—that is, forms whose bodies are ‘ yoked together” during the fusion of the gametes.
In some of the Conjugate forms the zygote is formed in the connect- ing tube (Fig. 16, .1), and some- times zygotes are formed without conjugation (Fig. 16, B). Among the Conjugate forms the Desmids are of great interest and beauty, being one-celled, the cells being organized into two distinct halves (Fig. 17).
27. Conclusions. — The Green Alge, as indicated by the illustra- tions given above, include simple one-celled forms which reproduce by fission, but they are chiefly fila-
Fic.16 Two Conjugate forms : al (Vongeotia), showing for- mation of zygote in conjuga- ting tnbe; B, ( (Gonatone- ma), showing formation of zygote without conjugation. —After WITTROCK.
mentous forms, simple or branching. These filamentous bodies either have the cclls separated from one another
39 PLANT STRUCTURES
by walls, or they are ccenocytic, as in the Siphon forms. The characteristic asexual spores are zoospores, but these may be wanting, as in the Conjugate forms. In addition to asexual reproduction, both isogamy and heterogamy are developed, and both zygotes and oospores are resting spores.
Fia. 17. A group of Desmids, one-celled Conjugate forms, showing various pat- terns, and the cells organized into distinct halves.—After KERNER.
The Green Algex are of special interest in connection with the evolution of higher plants, which are supposed to have been derived from them.
3. PHmopnycn.s (Brown -{lge)
28. General characters—The Blue-green Algw and the Green Alge are characteristic of fresh water, but the Brown Alge, or “kelps,” are almost all marine, being very charac-
THE GREAT GROUPS OF ALG 383
teristic coast forms. All of them are anchored by holdfasts, which are sometimes highly developed root-like structures; and the yellow, brown, or olive-green floating
bodies are buoyed in the water usually by the aid of floats or air-bladders, which are often very conspicuous. The kelps are most highly developed in the colder waters, and form much of the “wrack,” “tangle,” etc., of the coasts.
The group is well adapted to live exposed to waves and cur- rents with its strong holdfasts, air-bladders, and tough leathery bodies. It is what is known as a specialized group—that is, one which has become highly organ- ized for certain special condi-
tions. It is not our purpose to consider such a specialized group in any detail, as it does not usual- ly help to explain the structures of higher groups.
29. The plant body.—There is very great diversity in the structure of the plant body. Some of them, as Fctocar- pus (Fig. 18), are fil- amentous forms, like the Confervas among the Green Alge, but
Fig. 18. A brown alga (£etocarpus), showing a
body consisting of a simple filament which puts out branches (A), some sporangia (B) contain- ing zoospores, and gametangia ((') containing gametes.—CALDWELL.
others are very much more complex. The thallus of Lam- inarta is like a huge floating leaf, frequently nine to ten
Fic. 18a. A group of brown seaweeds (Laminarias). Note the various habits of the plant body with its leaf-like thallus and root-like holdfasts. —Aftcr KERNER,
THE GREAT GROUPS OF ALGE 35
feet long, whose stalk develops root-like holdfasts (Fig. 18a). The largest body is developed by an Antarctic Laminaria form, which rises to the surface from a sloping bottom with a floating thallus six hundred to nine hundred feet long. Other forms rise from the sea bottom like trees, with thick trunks, numerous branches, and leaf-like appendages.
The common Fucus, or “rock weed,” is rib- bon-form and constantly branches by forking at the tip (Fig. 19). This method of branching is called dichotomous, as dis- tinct from that in which branches are put out from the sides of the axis (monopodial). The swol- len air-bladders distrib- uted throughout the body are very conspicuous.
The most differenti- ated thallus is that of Sargassum (Fig. 20), or “oulf weed,” in which there are slender branch- ing stem-like axes bearing lateral members of various kinds, some of them like re. 19. Fragment of a common brown
rdinar foliage leaves: alga (Fucus), showing the body with . J 8 Ate dichotomous branching and bladder-like others are floats or air- air-bladders.—After LUERSSEN.
bladders, which sometimes
resemble clusters of berries; and other branches bear the sex organs. All of these structures are but different regions of a branching thallus. Sargassum forms are often torn from their anchorage by the waves and carried away from the coast by currents, collecting in the great sea eddies
36 PLANT STRUCTURES
produced by oceanic currents and forming the so-called “Sargasso seas,” as that of the North Atlantic.
Fie. 20. A portion of a brown alga (Sargassum), showing the thallus differentiated into stem-like and leaf-like portions, and also the bladder-like floats.—After BEN- NETT and Murray. ,
30. Reproduction The two main groups of Brown Alge differ from each other in their reproduction. One, repre- sented by the Laminarias and a majority of the forms, pro- duces zoospores and is isogamous (Fig. 18). The zoospores and gametes are peculiar in having the two cilia attached at one side rather than at an end; and they resemble each other very closely, except that the gametes fuse in pairs and form zygotes.
Fie. 21. Sexual reproduction of Fwevs, showing the eight eggs (six in sight) dis- charged from the oogonium and surrounded by a membrane (A), eggs liberated from the membrane (£), antheridium containing sperms ((’), the discharged lat- erally biciliate sperms (@), and eggs surrounded by swarming sperms (7, H).— After SINGER.
21
88 PLANT STRUCTURES
The other group, represented by Fucus (Fig. 21), pro- duces no asexual spores, but is heterogamous. A single cogonium usually forms eight eggs (Fig. 21, 1), which are discharged and float freely in the water (Fig. 21, #). The antheridia (Fig. 21, C’) produce numerous minute laterally biciliate sperms, which are discharged (Fig. 21, @), swim in great numbers about the large eggs (Fig. 21, &, H), and finally one fuses with an egg, and an oospore is formed. As the sperms swarm very actively about the egg and impinge against it they often set it rotating. Both an- theridia and oogonia are formed in cavities of the thallus.
4. Ruopopnyce.e (Red .1lg@)
31. General characters—On account of their red colora- tion these forms are often called Mloridew. They are mostly marine forms, and are anchored by holdfasts of various kinds. They belong to the deepest waters in which Alge grow, and it is probable that the red coloring matter which character- izes them is associated with the depth at which they live. The Red Alge are also a high- ly specialized line, and will be mentioned very briefly.
32. The plant body.
Bie, 22. A ted alga (Gigartina), showing —The Red Alge, in branching habit, and ‘fruit bodies.”— ; After SCHENCK. general, are more deli-
cate than the Brown Alge, or kelps, their graceful forms, delicate texture, and brightly tinted bodies (shades of red, violet, dark purple,
A-red alga (Cadlophyllis), with a greatly branched body composed of thin plates of cells.
a3
Fra,
Iie, 24. A red alga (Dasyu), showing a finely divided thallus body. CALDWELL.
Fie. 25. A red alga (Raddonia), showing holdfasts and branching thallus body. CALDWELL.
Vie. 26. A red alga (Ptilota). whose branching body resembles moss.— CALDWELL.
THE GREAT GROUPS OF ALGH 43
and reddish-brown) making them very attractive. They show the greatest variety of forms, branching filaments, ribbons, and filmy plates prevailing, sometimes branching very profusely and delicately, and resembling mosses of fine texture (Figs. 22, 23, 24, 25, 26). The differentiation of the thallus into root and stem and leaf-like structures is also common, as in the Brown Alge.
33. Reproduction.— Red Alge are very peculiar in both their asexual and sexual reproduction. A sporangium pro- duces just four asexual spores, but they have no cilia and no power of motion. They can not be called zoospores, therefore, and as each spo-
pia Fie. 27. A red alga(Cullithamnion), show- Fic. 28. A red alga (Nemalion); A, ing sporangium (4), and the tetraspores sexual branches, showing antheri- discharged (B),.—After THURET. dia (a), oogonium (0) with its trich- ogyne (ft), to which are attached two spermatia (s): B, beginning of a
rangium always produces just cystocarp (0), the trichogyne (¢) still four, they have been called showing; C, an almost mature cys- RNG. 1 tocarp (0), with the disorganizing tetraspores (Fig. 27). frichiaganie tne sid arene: Red Alge are also heterog- amous, but the sexual process has been so much and so variously modified that it is very poorly understood. The antheridia (Fig. 28, 4,@) develop sperms which, like the tetraspores, have no cilia and no power of motion. To dis-
44 PLANT STRUCTURES
tinguish them from the ciliated sperms, or spermatozoids, which have the power of locomotion, these motionless male gametes of the Red Alge are usually called spermatia (singular, spermatium) (Fig. 28, A, s).
Fie. 29. A branch of Polysiphonia, one of the red alge, showing the rows of cells composing the body (A), small branches or hairs (B), and a cystocarp (C’) with escaping spores (2) which have no cilia (car- pospores).—CALDWELL.
The oogonium is very pe- culiar, being differentiated into two regions, a bulbous base and a hair-like process (trichogyne), the whole struc- ture resembling a flask with a long, narrow neck, excepting that it is closed (Fig. 28, 4, o, t). Within the bulbous part the egg, or its equivalent, is organized; a spermatium at- taches itself to the trichogyne (Fig. 28, .1, s); at the point of contact the two walls become perforated, and the contents of the spermatium thus enter the trichogyne, and so reach the bulbous base of the oogo- nium. The above account rep- resents the very simplest con- ditions of the process of fer- tilization in this group, and gives no idea of the great and puzzling complexity exhibited by the majority of forms.
After fertilization the trich- ogyne wilts, and the bulbous base in one way or another develops a conspicuous struc-
ture called the rystocarp (Figs. 28, 29), which is a case con- taining asexual spores; in other words, a spore case, or kind of sporangium. In the life history of a red alga, there-
THE GREAT GROUPS OF ALG.E 45
fore, two sorts of asexual spores are produced: (1) the tetraspores, developed in ordinary sporangia; and (2) the caurpospores, developed in the cystocarp, which has been produced as the result of fertilization.
OTHER CHLOROPHYLL-CONTAINING THALLOPHYTES
34. Diatoms—These are peculiar one-celled forms, which occur in very great abundance in fresh and salt waters.
Fie. 30. A group of Diatoms : cand d, top and side views of the same form; e, colony of stalked forms attached to an alga; f and g, top and side views of the form shown ate; h, acolony; i, a colony, the top and side view shown at x.—After KERNER.
They are either free-swimming or attached by gelatinous stalks; solitary, or connected in bands or chains, or im- bedded in gelatinous tubes or masses. In form they are rod-shaped, boat-shaped, elliptical, wedge-shaped, straight or curved (Fig. 30).
46 PLANT STRUCTURES
The chief peculiarity is that the wall is composed of two valves, one of which fits into the other like the two parts of a pill box. This wall is so impregnated with silica that it is practically indestructible, and siliceous skeletons of dia- toms are preserved abundantly in certain rock deposits. They multiply by cell division in a peculiar way, and some
of them have been observed to con-
ts jugate.
(Ys They occur in such numbers in the hth Uy} | ocean that they form a large part of a Wh the free-swimming forms on the sur-
a face of the sea, and doubtless showers
of the siliceous skeletons are constant- ly falling on the sea bottom. There are certain deposits known as “si- liceous earths,” which are simply masses of fossil diatoms.
Diatoms have been variously placed in schemes of classification. Some have put them among the Brown Algew because they contain a brown coloring matter; others have placed them in the Conjugate forms among the Green Alge on account of the occasional conjugation that has been observed. They are so different from other forms, however, that’ it seems best to keep them separate from all other Algz.
35. Characeee.—These are common- ly called “stoneworts,” and are often Fig. 31. A common Chara, included as a group of Green Alga,
showing tip of main axis. ‘ —After Srraspurarr, a8 they seem to be Thallophytes, and have no other coloring matter than chlorophyll. However, they are so peculiar that they are better kept by themselves among the Alge. They are such
THE GREAT GROUPS OF ALG 44
specialized forms, and are so much more highly organized than all other Alga, that they will be passed over here with a bare mention. They grow in fresh or brackish waters, fixed to the bottom, and forming great masses. The cylin- drical stems are jointed, the joints sending out circles of branches, which repeat the jointed and branching habit (Fig. 31).
The walls become incrusted with a deposit of lime, which makes the plants harsh and brittle, and has sug- gested the name “stoneworts.” In addition to the highly organized nutritive body, the antheridia and oogonia are peculiarly complex, being entirely unlike the simple sex organs of the other Alge.
CHAPTER V
THALLOPHYTES: FUNGI
36. General characters.—In general, Fungi include Thal- lophytes which do not contain chlorophyll. From this fact it follows that they can not manufacture food entirely out of inorganic material, but are dependent for it upon other plants or animals. This food is obtained in two general ways, either (1) directly from the living bodies of plants or animals, or (2) from dead bodies or the products of living bodies. In the first case, in which living bodies are at- tacked, the attacking fungus is called a parasite, and the plant or animal attacked is called the host. In the second case, in which living bodies are not attacked, the fungus is called a saprophyte. Some Fungi can live only as parasites, or as saprophytes, but some can live in either way.
Fungi form a very large assemblage of plants, much more numerous than the Alge. As many of the parasites attack and injure useful plants and animals, producing many of the so-called ‘“ diseases,” they are forms of great interest. (Governments and Experiment Stations have ex- pended a great deal of money in studying the injurious parasitic Fungi, and in trying to discover some method of destroying them or of preventing their attacks. Many of the parasitic forms, however, are harmless; while many of the saprophytic forms are decidedly beneficial.
It is generally supposed that the Fungi are derived from the Alge, having lost their chlorophyll and power of inde- pendent living. Some of them resemble certain Alge so closely that the connection seems very plain; but others
48
THALLOPHYTES: FUNGI 49
have been so modified by their parasitic and saprophytic habits that they have lost all likeness to the Alge, and their connection with them is very obscure.
3%. The plant body.—Discarding certain problematical forms, to be mentioned later, the bodies of all true Fungi are organized upon a uniform general plan, to which they can. all be referred (Fig. 32). A set of colorless branching
Fig. 32 A diagrammatic representation of cor. showing the profusely branching mycelium. and three vertical hyphe (sporophores), sporangia forming on } and c. —After Zopr. :
filaments, either isolated or interwoven, forms the main working body, and is called the mycelium. The interweay- ing may be very loose, the mycelium looking like a delicate cobweb; or it may be close and compact, forming a felt-like mass, as may often be seen in connection with preserved fruits. The individual threads are called hyphe (singular, hypha) or hyphal threads. The mycelium is in contact with its source of food supply, which is called the sudstratum.
50) PLANT STRUCTURES
From the hyphal threads composing the mycelium verti- cal ascending branches arise, which are set apart to produce the asexual spores, which are scattered and produce new mycelia. These branches are called ascending hyphe or sporophores, meaning “ spore bearers.”
Sometimes, especially in the case of parasites, special descending branches are formed, which penetrate the sub- stratum or host and absorb the food material. These spe- cial absorbing branches are called haustoria, meaning “ ab- sorbers.”
Such a mycelial body, with its sporophores, and perhaps haustoria, lies either upon or within a dead substratum in the case of saprophytes, or upon or within a living plant or animal in the case of parasites.
38. The subdivisions—The classification of Fungi is in confusion on account of lack of knowledge. They are so much modified by their peculiar life habits that they have lost or disguised the structures which prove most helpful in classification among the Alge. Four groups will be pre- sented, often made to include all the Fungi, but doubtless they are insufficient and more or less unnatural.
The constant termination of the group names is mycetes, a Greek word meaning “fungi.” The prefix in each case is intended to indicate some important character of the group. The names of the four groups to be presented are as follows: (1) Phycomycetes (“ Alga-Fungi’”), referring to the fact that the forms plainly resemble the Alge ; (2) Ascomycetes (“ Ascus-Fungi”); (3) @#eidiomycetes (“Aicidium-Fungi ”) ; (4) Basidiomycetes (“ Basidium-Fungi”). Just what the prefixes ascus, ecidium, and basidiwm mean will be ex- plained in connection with the groups. The last three groups are often associated together under the name My- comycetes, meaning “ Fungus-Fungi,” to distinguish them from the Phycomycetes, or “ Alga-Fungi,” referring to the fact that they do not resemble the Algw, and are only like themselves.
THALTOPHY TES? FUNGL 1
ar
One of the ordinary life processes which seems to be seriously interfered with by the saprophytic and parasitic habit is the sexual process. At least, while sex organs and sexual spures are about as cyident in Phycomycetes as in Alew, they are either obscure or wanting in the Mycomycete groups.
1. Puycomycetes (Alga-Fung!)
39. Saprolegnia.—This is a group of “ water-moulds,” with aquatic habit like the Alge. They live upon the dead bodies of water plants and animals (Fig. 33), and some- times attack living fish, one kind being very destructive to young fish in hatcheries. The hyphe composing the mycelium are c:enocytes, as in the Siphon forms.
Sporangia are organized at the ends of branches by forming a partition wall separating the cavity of the tip from the general cavity (Fig. 33, B). The tip becomes more or less swollen, and within it are formed numerous biciliate zoospores, which are discharged into the water (Fig. 33, ('), swim ubout for a short time, and rapidly form new mycelia. The provess is very suggestive of Claduphora and Taucheria. Oogonia and antheridia are also formed at the ends of the branches (Fig. 33, 4"), much as in Vaw- cheria. The oogonia are spherical, and form one and some- times many eggs (Fig. 33, D, £). The antheridia are formed on branches near the oogonia. An antheridium comes in contact with an oogonium, and sends out a deli- cate tube which pierces the oogonium wall (Fig. 33, F’). Through this tube the contents of the antheridium pass, fuse with the ecg, and a heavy-walled oospore or resting spore is the result.
It is an interesting fact that sometimes the contents of an antheridium do not enter an oogonium, or antheridia may not even be formed, and still the egg, without fertiliza- tion, forms an oospore which can germinate. This peculiar
59 PLANT STRUCTURES
habit is called parthenogenesis, which means reproduction by an egg without fertilization.
Fig. 33. A common water mould (Saprolegnia): A, a fly from which mycelial fila- ments of the parasite are growing; B, tip of a branch organized as a sporangium, C, sporangium discharging biciliate zoospores; F, oogonium with antheridium in contact, the tube having penetrated to the egg; D and #, oogonia with several eggs.— 4-Cafter Tourgt, D-F after DeBary.
40. Mucor.—One of the most common of the Mucors, or “black moulds,” forms white furry growths on damp bread, preserved fruits, manure heaps, ete. It is therefore a saprophyte, the ccenocytic mycelium branching extensively through the substratum (Fig. 34).
THALLOPHYTES: FUNGI 58
Erect sporophores arise from it in abundance, and at the top of each sporophore a globular sporangium is formed, within which are numerous small asexual spores (Figs. 35,
Fig. 34. Diagram showing mycelium and sporophores of a common Mucor.—
CALDWELL.
36). The sporangium wall bursts (Fig. 37), the light spores are scattered by the wind, and, falling upon a suitable sub-
stratum, germinate and form new mycelia. It is evident that these asex- ual spores are not z00- spores, for there is no water medium and swim- ming is impossible. This method of transfer being impossible, the spores are scattered by currents of air, and must be corre- spondingly light and pow- dery. They are usually spoken of simply as “spores,” without any prefix. 22
Fie. 35. Forming sporangia of ucor, show- ing the swollen tip of the sporophore (A), and a later stage (B), in which a wall is formed separating the sporangium from the rest of the body.—CaLDWELL.
54 PLANT STRUCTURES
While the ordinary method of reproduction through the growing season is by means of these rapidly germinating spores, in certain conditions a sexual process is observed, by which a heavy-walled sexual spore is formed as a resting spore, able to outlive unfavorable conditions. Branches arise from the hyphe of the mycelium just as in the forma-
Fig. 36. Mature sporangium of Mucor, showing Fic. 37. Bursted sporangium of
the wall (A), the numerous spores ((), and Mucor, the ruptured wall not the columella (8)—that is, the partition wall being shown, and the loose pushed up into the cavity of the sporangium. spores adhering to the colu- —CALDWELL. mella.—CALDWELL,
tion of sporophores (Fig. 38). Two contiguous branches come in contact by their tips (Fig. 38, .1), the tips are cut off from the main ccenocytic body by partition walls (Fig. 38, 2), the walls in contact disorganize, the contents of the two tip cells fuse, and a heavy-walled sexual spore is the result (Fig. 38, ('). It is evident that the process is conjugation, suggesting the Conjugate forms among the
THALLOPHYTES: FUNGI 55
Alge ; that the sexual spore is a zygote; and that the two pairing tip cells cut off from the main body by partition walls are gametangia. J/ueor, therefore, is isogamous.
Fic. 38. Sexual reproduction of J/vcor, showing tips of sex branches meeting (A), the two gametangia cut off by partition walls (2), and the heavy-walled zygote (C).—CALDWELL.
41. Peronospora.—These are the “downy mildews,” very common parasites on seed plants as hosts, one of the most common kind attacking grape leaves. The mycelium is cceeno- eytic and entirely internal, ramifying among the tissues within the leaf, and piercing the living cells with haustoria which rapidly absorb their contents (Fig. 39). The pres- ence of the parasite is made known by discolored and
56 PLANT STRUCTURES
finally deadened spots on the leaves, where the tissues have been killed.
From this internal mycelium numerous sporophores arise, coming to the surface of the host and securing the scattering of their spores, which fall upon other leaves and germinate, the new mycelia pene- trating among the tissues and begin-
ning their ravages. The sporophores af- Fie. 39. A branch of Peronospora in contact with J
oo two cells of a host plant, and sending into them ter rising above the its large haustoria.—After DEBary.
surface of the leaf,
branch freely; and many of them rising near together, they form little velvety patches on the surface, suggesting the name “ downy mildew.”
Fie. 40. Peronospora, one of the Phycomycetes, showing at @ an oogonium (0) con- taining an egg, and an antheridium (7) in contact; at b the antheridial tube pene- trating the oogonium and discharging the contents of the antheridium into the egg; at ¢ the oogonium containing the oospore or resting spore.—After DEBary.
In certain conditions special branches arise from the mycelium, which organize antheridia and oogonia, and remain within the host (Fig. 40). The oogonium is of the usual spherical form, organizing a single egg. The an-
THALLOPHYTES: FUNGI 57
theridium comes in contact with the oogonium, puts out a tube which pierces the oogonium wall and enters the egg, into which the contents of the antheridium are discharged, and fertilization is effected. The result is a heavy-walled oospore. .\s the oospores are not for immediate germina- tion, they are not brought to the surface of the host and scattered, as are the asexual spores. When they are ready to germinate, the leaves bearing them have perished and the oospores are liberated.
42. Conclusions—The ccenocytic bodies of the whole group are very suggestive of the Siphon forms among Green Alge, as is also the method of forming oogonia and antheridia.
The water-moulds, Suprolegnia and its allies, have re- tained the aquatic habit of the Alge, and their asexual spores are zoospores. Such forms as Jdwcor and Perono- spora, however, have adapted themselves to terrestrial con- ditions, zoospores are abandoned, and light spores are de- veloped which can be carried about by currents of air.
In most of them motile gametes are abandoned. Even in the heterogamons forms sperms are not organized within the antheridium, but the contents of the antheridium are discharged through a tube developed by the wall and pene- trating the oogonium. It should be said, however, that a few forms in this group develop sperms, which make them all the more alga-like.
They are both isogamous and heterogamous, both zygotes and oospores being resting spores. Taking the characters all together, it seems reasonably clear that the Phycomycetes are an assemblage of forms derived from Green Algz (Chlo- rophycez) of various kinds.
2. ASCOMYCETES (Lscus- or Suc-Fungi)
43, Mildews.—These are very common parasites, growing especially upon leaves of seed plants, the mycelium spread- ing over the surface like a cobweb. A very common mil-
58 PLANT STRUCTURES
dew, Microsphera, grows on lilac leaves, which nearly always show the whitish covering after maturity (Fig. 41). The branching hyphe show numerous partition walls, and are not ceenocytic as in the Phycomycetes. Small disk-like haustoria penetrate into the superficial cells of the host, anchoring the mycelium and absorbing the cell contents.
Sporophores arise, which form asexual spores in a pe culiar way. The end of the sporophore rounds off, almost separating itself from the part below, and becomes a spore or spore-like body. Below this another organizes in the same way, then another, until a chain of spores is developed, easily broken apart and scat- tered by the wind. Falling upon other suitable leaves, they germinate and form new mycelia, enabling the fungus to spread rapidly. This meth- od of cutting a branch into sections to form spores is called adstriction, and the spores formed in this way are called conidia, or conidi- ospores (Fig. 43, B).
At certain times the myce- lium develops special branches which develop sex organs, but they are seldom seen and may Fra. 41. Lilac leaf covered with mi Ot always occur. An 00go0-
dew (Microsphwra), the shaded ree pium and an antheridium, of
and the back dots theaceoearpe the usual forms, but probably
CALDWELL. without organizing gametes,
come into contact, and as a result an elaborate structure is developed—the ascocarp, sometimes called the “spore fruit.” These ascocarps ap- pear on the lilac leaves as minute dark dots, each one being
THALLOPHYTES: FUNGI 59
a little sphere, which suggested the name Microsphera (Fig. 41). The heavy wall of the ascocarp bears beauti- ful branching hair-like appendages (Fig. 42).
Bursting the wall of this spore fruit several very delicate, bladder-like sacs are extruded, and through the transparent wall of each sac there may be seen several spores (Fig. 42). The ascocarp, therefore, is a spore case, just as is the cystocarp of the Red Alge (§$ 33). The delicate sacs within are the asci, a word meaning “sacs,” and each ascus is evidently a mother cell within which asexual spores are formed. These spores are distinguished
from other asexual spores by the name ascospore.
It is these peculiar moth- er cells, or asci, which give
Fic. 42. Ascocarp of the lilac mildew, showing branching appeudages and two asci protruding from the rup- tured wall and containing ascospores. —CALDWELL.
name to the group, and an
Ascomycete, Ascus-fungus, or Sac-fungus, is one which pro- duces spores in asci; and an ascocarp is a spore case which contains asci.
In the mildews, therefore, there are two kinds of asexual spores: (1) conidia, formed from a hyphal branch by abstric- tion, by which the mycelium may spread rapidly; and (2) ascospores, formed in a mother cell and protected by a heavy case, so that they may bridge over unfavorable conditions, and may germinate when liberated and form new mycelia. The resting stage is not a zygote or an oospore, as in the Alge and Phycomycetes, no sexual spore probably being formed, but a heavy-walled ascocarp.
44. Other forms.—The mildews have been selected as a simple illustration of Ascomycetes, but the group is a very
60 PLANT STRUCTURES
large one, and contains a great variety of forms. All of them, however, produce spores in asci, but the asci are not always inclosed by an ascocarp. Here belong the common blue mould (Penicillium), found on bread, fruit, etc., in which stage the branching chains of conidia are very con- spicuous (Fig. 43); the truffle-fungi, upon whose subter-
Fie. 43. Penicillium, a common mould: 4, mycelium with numerous branching sporophores bearing conidia; B, apex of a sporophore enlarged, showing branch- ing and chains of conidia.—After BREFELD.
ranean mycelia ascocarps develop which are known as “truffles”; the black fungi, which form the diseases known as “ black knot ” of the plum and cherry, the “ergot ” of rye (Fig. 44), and many black wart-like growths upon the bark of trees; other forms causing * witches’-brooms ” (ab- ‘normal growths on various trees), “ peach curl,” etc., the cup-fungi (Figs. 45, 46), and the edible morels (Fig. £7).
THALLOPHYTES: FUNGI 61
Fie. 45. Two species of cup-fungus (Peziza).—After LinDav.
Fie. 44. Head of rye attacked by “‘er- Fic. 46. A cup-fungus (Pitya) grow- got” (a), peculiar grain-like masses ing on a spruce (Picea).— After replacing the grains of rye; also a REM. mass of ‘‘ergot’’ germinating to form spores (%).—After TULASNE.
In some of these forms the ascocarp is completely closed, as in the lilac mildew; in others it is flask-shaped; in others, as in the cup-fungi, it is like a cup or disk; but in all the spores are inclosed by a delicate sac, the ascus.
62 PLANT STRUCTURES
Here must probably be included the yeast-fungi (Fig. 48), so commonly used to excite alcoholic fermentation.
Fie. 47. The common edible morel (Morchella Fic. 48. Yeast cells, reprodu- esculenta). The structure shown and used cing by budding, and form- represents the ascocarp, the depressions of ing chains.—CALDWELL.
whose surface are lined with asci contain- ing ascospores.—After Gipson.
The “ yeast cells ” seem to be conidia having a peculiar bud- ding method of multiplication, and the remarkable power of exciting alcoholic fermentation in sugary solutions.
3. AZECIDIOMYCETES (.2eidium-Fung?)
45. General characters.—This is a large group of very destructive parasites known as “rusts” and “ smuts.” The rusts attack particularly the leaves of higher plants, pro- ducing rusty spots, the wheat rust probably being the best known. The smuts especially attack the grasses, and are very injurious to cereals, producing in the heads of oats, barley, wheat, corn, etc., the disease called smut.
THALLOPHYTES: FUNGI 63
No indication of a sexual process has been obtained, and the life histories are so complicated and obscure that the position of the group is very uncertain. The forms should probably be included with the Basidiomycetes, but they are so unlike the ordinary forms of that group that they are here kept distinct.
Most of the forms are very polymorphic—that is, a plant assumes several dissimilar appearances in the course of its life history. These phases are often so dissimilar that they have been described as different plants. This polymorphism is often further complicated by the appearance of different phases upon entirely different hosts. For example, the wheat-rust fungus in one stage lives on wheat, and in an- other on barberry.
46. Wheat rust.—This is one of the few rusts whose life histories have been traced, and it may be taken as an illus- tration of the group.
The mycelium of the fungus is found ramifying among the leaf and stem tissues of the wheat. While the wheat is growing this mycelium sends to the surface numerous spo-
Fie. 49. Wheat rust, showing sporophores breaking through the tissues of the host and bearing summer spores (uredospores).—After H. MansHALL WaRD.
rophores, each bearing at its apex a reddish spore (Fig. 49). As the spores occur in great numbers they form the rusty- looking lines and spots which give name to the disease. The spores are scattered by currents of air, and falling upon other plants, germinate very promptly, thus spreading the
64 PLANT STRUCTURES
disease with great rapidity (Fig. 50). Once it was thought that this completed the life cycle, and the fungus received the name (redo. When it was known that this is but one
Fic. 50 —Wheat rust, showing a young hypha forcing its way from the surface of a leaf down among the nutritive cells.—After H. MarsuaLL Warp.
stage in a polymorphic life history it was called the Uredo- stage, and the spores wredospores, sometimes “summer
spores.”
Fie. 51. Wheat rust, showing the winter spores (telentospores).—After H. MarsHaui WARD.
Toward the end of the summer the same mycelium develops sporophores which bear an entirely different kind of spore (Fig. 51). It is two-celled, with a very heavy black
THALLOPHYTES: FUNGI 65
wall, and forms what is called the “black rust,” which ap- pears late in the summer on wheat stubble. These spores are the resting spores, which last through the winter and germinate in the following spring. They are called felewto- spores, meaning the “last spores” of the growing season. They are also called “ winter spores,” to distinguish them from the uredospores or ‘summer spores.” At first this teleutospore-bearing mycelium was not recognized to be identical with the uredospore-bearing mycelium, and it was called Puccinia. This name is now retained for the whole polymorphous plant, and wheat rust is Puccinia graminis. This mycelium on the wheat, with its summer spores and winter spores, is but one stage in the life history of wheat rust.
In the spring the teleutospore germinates, each cell developing a small few-celled filament (Fig. 52). From each cell of the filament a little branch arises which develops at its tip a small spore, called a spo- ridium, which means “ spore-like.” This little filament, which is not a parasite, and which bears sporidia, ee en Bes is a second phase of the wheat rust, ino'a teleatosporewermins:
really the first phase of the growing _tingand forming a short fil- ament, from four of whose
season. cells a spore branch arises, The sporidia are scattered, fall the lowest one bearing at 5 its tip a sporidium.—After upon barberry leaves, germinate,and yy yfansnarz Wanp. develop a mycelium which spreads through the leaf. This mycelium produces sporophores which emerge on the under surface of the leaf in the form of chains of reddish-yellow conidia (Fig. 53). These chains of conidia are closely packed in cup-like receptacles,
and these reddish-yellow cup-like masses are often called
66 PLANT
STRUCTURES
“cluster-cups.” This mycelium on the barberry, bearing cluster-cups, was thought
to be a distinct plant, and was
Above is a flask-shaped mass discharging
af, with two ecidia below, one of them having broken through
nidia (zecidiospores)
very minute bodies, which are probably still other spores of the parasite.—After H. MarsnaLi Warp.
Fie. 53. Wheat rust, showing section of barberry the epidermis and exposed it» chains of
called &cidium. The name now is applied to the cluster-cups, which are called eridia, and the conidia-like spores which they produce are known as cecidiospores.
It is the wcidia which give name to the group, and Aicidiomycetes are those Fungi in whose life history ecidia or cluster-cups appear.
The wxcidiospores are scattered by the wind, fall upon the spring wheat, germinate, and develop again the myce- lium which produces the rust on the wheat, and so the life cycle ix com- pleted. There are thus at least three distinct stages in the life history of wheat rust. Begin- ning with the vrowing season they are as fol- lows: (1) The phase bear- ing the sporidia, which is not parasitic; (2) the ecidium phase, pirasitic
on the barberry; (3) the uredo-teleutospore phase, para-
sitic on the wheat.
In this life cycle at least four kinds of asexual spores
THALLOPHYTES: FUNGI 67
appear: (1) sporidia, which develop the stage on the barber- ry; (2) ecidiospores, which develop the stage on the wheat ; (3) wredospores, which repeat the mycelium on the wheat ; (4) teleutospores, which last through the winter, and in the spring produce the stage bearing sporidia. It should be said that there are other spores of this plant produced on the barberry (Fig. 53), but they are too uncertain to be included here.
The barberry is not absolutely necessary to this life cycle. In many cases there is no available barberry to act as host, and the sporidia germinate directly upon the young wheat, forming the rust-producing mycelium, and the cluster-cup stage is omitted.
Fic. 54. Two species of ‘cedar apple” (Gymnosporangium), both on the common juniper (Juniperus Virginiana).—A after Fartow, B after ENGLER and PRANTL.
47. Other rusts—Many rusts have life histories similar to that of the wheat rust,in others one or more of the stages are omitted. In very few have the stages been con-
68 PLANT STRUCTURES
nected together, so that a mycelium bearing uredospores is called a Uredo, one bearing teleutospores a Puccinia, and one bearing ecidia an .Leidiwm ; but what forms of Uredo, Puccinia, and .£cidiwn belong together in the same life cycle is very difficult to discover.
Another life cycle which has been discovered is in con- nection with the “cedar apples” which appear on red cedar (Fig. 54). In the spring these diseased growths be- come conspicuous, especially after a rain, when the jelly- like masses containing the orange-colored spores swell. This corresponds to the phase which produces rust in wheat. On the leaves of apple trees, wild crab, hawthorn, ete., the ecidium stage of the same parasite develops.
4. BasIDIOMYCETES (Basidiwmn-Fungt).
48. General characters——This group includes the mush- rooms, toadstools, and puffballs. They are not destructive parasites, as are many forms in the preceding groups, but mostly harm- less and often useful sap- rophytes. They must also be regarded as the most highly organized of the Fungi. The popular distinction between toad- stools and mushrooms is not borne out by botan- ical characters, toadstool and mushroom being the same thing botanically, and forming one group, puffballs forming an- other.
Ss] wastrel Fr. 55. The common edible mushroom, Asin Ec idiomycetes, Agaricus canupestris.—Afler GrBsoNn. no sexual process has
THALLOPHYTES: FUNGI 69
been discovered. The life history seems simple, but this apparent simplicity may represent a very complicated his- tory. The structure of the common mushroom (.lyari- cus) will serve as an illustration of the group (Fig. 55). 49. A common mushroom, — The mycelium, of white branching threads, spreads extensively through the decay- ing substratum, and in cultivated forms is spoken of as the “spawn.” Upon this myce- lium little knob- like protuberances begin to arise, grow- ing larger and larger, until they are organized into the so-called ‘‘mushrooms.”’ The real body of the plant is the white thread -lke mycelium, while the “mushroom ” part seems to rep-
resent a great num- f sporophores Fis. 56. A common Agaricus : A, section through one hen . : P side of pileus, showing sections of the pendent gills; organized together B, section of a gill more enlarged, showing the cen- i i he ba- m ing] tral tissue, and the broad border formed by th a aa is sidia: (, still more enlarged section of one side of com pl ex spore- a gill, showing the club-shaped basidia standing at ir 5 re. tight angles to the surface, and sending out a pair Dearing SEeHavIn of small branches, each of which bears a single ba- The mushroom sidiospore.—After Sacus.
23
“NOSAIY IdJJY—'alqipa | (s2ppwoo snur -dog) snsuny ,, ouvur ASSvys,, aL 69 “81
“ITAMATYO— STIS pus ‘snoid ‘ads Surmoys “(pseipuepy [ooys -pvo} snouosiod wouttl0d WY "8g "PL
"NOSED Id1py—‘olqrpa $ (sapnaso snwuspapyy) susung , Sula Aiey,, Yo "LG ‘DIT
THALLOPHYTES: FUNGI 71
has a stalk-like portion, the stipe, at the base of which the slendér mycelial threads look like white rootlets; and an expanded, umbrella-like top called the pilews. From the under surface of the pileus there hang thin radiating plates, or gills (Fig. 55). Each gill is a mass of interwoven fila- ments (hyphe), whose tips turn toward the surface and form a compact layer of end cells (Fig. 56). These end
Fic. 60. A bracket fungus (Polyporus) growing on the trunk of a red oak.— CALDWELL.
cells, forming the surface of the gill, are club-shaped, and are called dasidia. From the broad end of each basidium two or four delicate branches arise, each bearing a minute spore, very much as the sporidia appear in the wheat rust.
"9 PLANT STRUCTURES
These spores, called basidiospores, shower down from the gills when ripe, germinate, and produce new mycelia. The peculiar cell called the basidium gives name to the group Basidiomycetes.
50. Other forms—Mushrooms display a great variety of form and coloration, many of them being very attractive
Fra. 61. A toadstool of the bracket form which has grown about blades of grass without interfering with their activity CALDWELL,
(Figs. 57, 58, 59). The “ pore-fungi” have pore-like depres- sions for their spores, instead of gills, as in the very com- mon “bracket-fungus” (Polyporus), which forms hard shell-like outgrowths on tree-trunks and stumps (Figs. 60,
Fic. 62. The common edible Boletus (B. edu- Fig. 68. Another edible Boletus (B. stro- dis), in which the gills are replaced by bilaceus).— After Gipson. pores,—After GIBSON.
Fie. 64. The common edible ‘coral fun- Fie. 65. Hydrum repandum, in which gills gus’’ (Clavaria).—After Gipson. are replaced by spinous processes; edi- ble.—After Gipson.
q4 PLANT STRUCTURES
61), and the mushroom-like Bolett (Figs. 62, 63). The “ear-fungi” form gelatinous, dark-brown, shell-shaped masses, and the “coral fungi” resemble branching corals (Fig. 64). The Hydnum forms have spinous processes instead of gills (Fig. fo 65). The puffballs or- : ganize globular bodies (Fig. 66), within which the spores develop, and are not liberated until ripe; and with them belong also the “ bird’s nest fungus,” the “earth star,” the ill-smelling “stink-horn,” etc.
OTHER THALLOPHYTES WITHOUT CHLOROPHYLL
51. Slime - moulds. — These perplexing forms, named Aywrumiyretes, do Fie. 66, Puffballs, in which the basidia ana Ot seem to be related
spores are inclosed ; edible.—After Gmson. to any group of plants,
and it is a question whether they are to be regarded as plants or animals. The working body is a mass of naked protoplasm called a plius- modium, suggesting the term “slime,” and slips along like a gigantic ameeba. They are common in forests, upon black soil, fallen leaves, and decaying logs, the slimy yel- low or orange masses ranging from the size of a pmhead to as large as a man’s hand. They are saprophytic, and are said to engulf food as do the amuwhbas. So suggestive of certain low animals is this hody and food habit that slime-moulds have also been called A/ycedozod or fungus- animals.”
THALLOPHYTES: FUNGI 45
In certain conditions, however, these slimy bodies come to rest and organize most elaborate and often very beau- tiful sporangia, full of spores (Fig. 67). These varied and easily preserved sporangia are used to classify the
Fie. 67. Three common slime moulds (Myxomycetes) on decaying wood: to the left above, groups of the sessile sporangia of Trichia ; to the right above, a group of the stalked sporangia of Stemonitis, with remnant of old plasmodium at base; below, groups of sporangia of Hemiarcyria, with a plasmodium mass at upper left hand.—CALDWELL,
forms. Slime-moulds, or “ slime-fungi,” therefore, seem to have animal-like bodies which produce plant-like spo- rangia.
52. Bacteria.—These are the “ Fission-Fungi,” or Schizo- mycetes, and are popularly known as “ bacteria,” “ baci.li,” “ microbes,” “germs,” etc. They are so important and pe- culiar in their life habits that their study has developed a special branch of botany, known as “ Bacteriology.” In many ways they resemble the Cyanophycewx, or “ Fission- Algez,” so closely that they are often associated with them in classification (see § 21).
Fie 68. A group of Bacteria, the bodies being black, and bearing motile cilia in
various ways. .L, the two to the left the common hay Pacil/us (B. subtilis), the one to the right a Spiriflam ; By a Cocens form (Planocaccus); C.D. E. species of Pseudomonas ; 7, @, species of Bacillus, # being that of typhoid fever; ZZ, Jfiero- spira; J, WK, L, M, species of Spirtdium.—After ENGLER and PRANTL.
THALLOPHYTES: FUNGI 44
They are the smallest known living organisms, the one- celled form which develops on cooked potatoes, bread, milk, meat, etc., forming a blood-red stain, having a diameter of but 0.0005 mm. (g5)57 in.). They are of various forms (Fig. 68), as Coccus forms, single spherical cells; Bacterium forms, short rod-shaped cells; Bacillus forms, longer rod- shaped cells; Leptothrix forms, simple filaments; Spirillum forms, spiral filaments, etc.
They multiply by cell division with wonderful rapidity, and also form resting spores for preservation and distri- bution. They occur everywhere—in the air, in the water, in the soil, in the bodies of plants and animals; many of them harmless, many of them useful, many of them dan- gerous.
They are intimately concerned with fermentation and decay, inducing such changes as the souring of fruit juices, milk, etc., and the development of pus in wounds. What is called antiseptic surgery is the use of various means to exclude bacteria and so prevent inflammation and decay.
The pathogenic forms—that is, those which induce dis- eases of plants and animals—are of great importance, and means of making them harmless or destroying them are being searched for constantly. They are the causes of such diseases as pear-blight and peach-yellows among plants, and such human diseases as tuberculosis, cholera, diphtheria, typhoid fever, etc.
LICHENS
53. General character.— Lichens are abundant every- where, forming various colored splotches on tree-trunks, rocks, old boards, etc., and growing also upon the ground (Figs. 69, 70, 71). They have a general greenish-gray color, but brighter colors may also be observed.
The great interest connected with Lichens is that they are not single plants, but each Lichen is formed of a fungus and an alga, living together so intimately as to appear like a single
“WIEMGTIVO— [IL ‘YlVd Joe IwoN “(osafiq¢ng sqiaydopshg) susay Jo WMOoIs do} Uo puv ‘suayaI[ Jo [Mord asuep v Aq padaacd JYSIA oy} 07 oJ oY] Furmoys ‘poor Jo espa, WV 69 “OL
THALLOPHYTES: FUNGI 79
plant. In other words, a Lichen is not an individual, but a firm of two individuals very unlike each other. This habit
Fie. 70. A common lichen (Physcia) growing on bark, showing the spreading thallus and the numerons dark disks (apothecia) bearing the asci— CALDWELL.
of living together has been called symbiosis, and the indi- viduals entering into this relation are called symbiouts.
Fig. 71. A common foliose lichen (Parmelia) growing upon a board, and showing apothecia.—CaLDWELL.
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82 PLANT STRUCTURES
bles an incrustation upon its substratum of rock, soil, ete. ; (2) Foliose Lichens, with flattened, leaf-like, lobed bodies, at-
Fie. 74. Much enlarged section of a portion of the apothecium of Anaptychia, show- ing the fungus mycelium (7m), which is massed above (y), just beneath the layer of asci (1, 2,3, 4), im which spores in various stages of development are shown.— After Sacus.
tached only at the middle or irregularly to the substratum ; (8) Fruticose Lichens, with filamentous bodies branching like shrubs, either erect, pendulous, or prostrate.
CHAPTER VI
THE FOOD OF PLANTS
54. Introductory.— All plants use the same kind of food, but the Alge and Fungi suggest that they may have very different wavs of obtaining it. The Algez can manufacture food from raw material, while the Fungi must obtain it already manufactured. Between these two extreme condi- tions there are plants which can manufacture food, and at the same time have formed the habit of supplementing this by obtaining elsewhere more or less manufactured food. Besides this, there are plants which have learned to work together in the matter of food supply, entering into a con- dition of symbiosis, as described under the Lichens. These various habits will be presented here briefly.
55. Green plants—The presence of chlorophyll enables plants to utilize carbon dioxide (CO,), a gas present in the atmosphere and dissolved in waters, and one of the waste products given off in the respiration of all living organisms. This gas is absorbed by green plants, its constituent ele- ments, carbon and oxygen, are dissociated, and with the ele- ments obtained from absorbed water (H,O) are recombined to form a carbohydrate (sugar, starch, etc.), which is an organ- ized food. With this as a basis other foods are formed, and so the plant can live without help from any other organism.
This process of utilizing carbon dioxide in the formation of food is not only a wonderful one, but also very important. It is wonderful, because carbon dioxide and water, both of
them very refractory substances, are broken up at ordinary 83
84 PLANT STRUCTURES
temperatures and without any special display of energy. It is important, because the food of all plants and animals de- pends upon it, as it is the only known process by which inor- ganic material can be organized.
The process is called photosynthesis, or photosyntir, words indicating that the presence of light is necessary. The mechanism on the part of the plant is the chloroplast, which when exposed to light is able to do this work. The process is often called * carbon assimilation,” ‘ chlorophyll assimilation,” “ fixation of carbon,” etc. It should be noted that it is not the chlorophyll which does the work, but the protoplasmic plastid stained green by the chlorophyll. The chlorophyll manipulates the light in some way so that the plastid may obtain from it the energy needed for the work. Further details concerning it may be obtained by reading §$ 112 of Plant Relations.
It is evident that green plants must expose their chloro- phyll to the light. For this reason the Alge can not live in deep waters or in dark places. In the case of the large marine kelps, although they may be anchored in considera- ble depth of water, their working bodies are floated up toward the light by air-bladders. In the case of higher plants, specially organized chlorophyll-bearing organs, the foliage leaves, are developed.
56. Saprophytes—Only cells containing chloroplasts can live independently. In the higher plants, where bodies be- come large, many living cells are shut away from the hght, and must depend upon the more superficial green cells for their food supply. The habit of cells depending upon one another for food, therefore, is a very common one.
When none of the cells of the plant body contain chloro- phyll, the whole plant becomes dependent, and must live as a saprophyte or a parasite. In the case of saprophytes dead bodics or body products are attacked, and sooner or later all organic matter is attacked and decomposed by them. The decomposition is a result of the nutritive processes of plants
THE FOOD OF PLANTS 85
without chlorophyll, and were it not for them “the whole surface of the earth would be covered with a thick deposit of the animal and plant remains of the past thousands of years.”
The green plants, therefore, are the manufacturers of organic material, producing far more than they can use, while the plants without chlorophyll are the destroyers of organic material. The chief destroyers are the Bacteria and ordinary Fungi, but some of the higher plants have also adopted this method of obtaining food. Many ordinary green plants have the saprophytic habit of absorbing organic material from rich humus soil; and many orchids and heaths are parasitic, attaching their subterranean parts to those of other plants, becoming what are called “root parasites.” The cultivated plants, also, may be regarded as partially saprophytic, in so far as they use the organic material sup- plied to them in fertilizers.
57. Parasites—Certain plants Hthonk chlorophyll are not content to obtain organic material from dead bodies, but attack living ones. As in the case of saprophytes, the vast majority of plants which have formed this habit are Bacteria and ordinary Fungi. Parasites are not only modi- fied in structure in consequence of the absence of chloro- phyll, but they have developed means of penetrating their hosts. Many of them have also cultivated a very selective habit, restricting themselves to certain plants or animals, or even to certain organs.
The parasitic habit has also been developed by some of the higher plants, sometimes completely, sometimes par- tially. Dodder, for example, is completely parasitic at maturity (Fig. 75), while mistletoe is only partially so, doing chlorophyll work and also absorbing from the tree into which it has sent its haustoria.
That saprophytism and parasitism are both habits grad- ually acquired is inferred from the number of green plants
which have developed them more or less, as a supplement to pea
86 PLANT STRUCTURES
the food which they manufacture. The less chlorophyll is used the less is it developed, and a green plant which is obtaining the larger amount of its food in a saprophytic or parasitic way is on the way to losing all of its chlorophyll and becoming a com- plete saprophyte or parasite.
Certain of the low- er Alge are in the habit of living in the body cavities of high- er plants, finding in such situations the moisture and protec- tion which they need. They may thus have brought within their reach some of the organic products of the higher plant. If they can use some of these, as is very like- ly, a partially para- sitic habit is begun, which may lead to loss of chlorophyll and complete para-
Fie. 75. A dodder plant purasitic on a willow twig. sitism.
The leafless dodder twines a bout the willow, and 58. Symbionts. ae sends out sucking processes which penctrate and absorb.—After STRASBURGER. The phenomenon of
symbiosis has already been referred to in connection with Lichens ($ 53). In its broadest sense the word includes any sort of depend- ence between living organisms, from the vine and the tree
THE FOOD OF PLANTS 87
upon which it climbs, to the alga and fungus so intimately associated in a Lichen as to seem a single plant. In a nar- rower sense it includes only cases in which there is an inti- mate organic relation between the symbionts. This would include parasitism, the parasite and host being the sym- bionts, and the organic relation certainly being intimate. In a still narrower sense symbiosis includes only those cases in which the symbionts are mutually helpful. This fact, however, is very difficult to determine, and opinions vary widely as to the mutual advantage of the relation. How- ever large a set of phenomena may be included under the term symbiosis, we use it here in this narrowest sense, which is often distinguished as mutualism.
(1) Lichens.—The main facts of symbiosis in connection with Lichens were presented in § 53. That the fungus- symbiont can not live without the alga has been demon- strated, but whether the alga-symbiont derives any benefit from this association is a question in dispute. The latter can live independently of the former, but enmeshed by the fungus the alga seems to thrive and to live in situations which would be impossible to it without the protection and moisture supplied by the fungus-thallus. Those who lay stress on the first fact regard the Lichen merely as a pecul- iar case of parasitism, which has been called helotism, or a condition of slavery, indicating that the alga is enslaved and even cared for by the fungus for its own use. Those who see an advantage to the alga in this association regard a Lichen as an example of mutualism.
It may be of interest to know that artificial Lichens have been formed, not only by cultivating together spores of a Lichen-fungus and some Lichen-alga, but also by using “wild ” Alga—that is, Alge which are in the habit of living independently.
(2) Mycorrhiza.—The name means “root-fungus,” and refers to an association which exists between certain Fungi of the soil and roots of higher plants, such as orchids, heaths,
Fic. 76. Mycorrhiza: to the left is the tip of a rootlet of beech enmeshed by the fungus; A, diagram of longitudinal section of an orchid root, showing the cells of the cortex (7) filled with hyphe; ZB, part of longitudinal section of orchid root much enlarged, showing epidermis (e), outermost cells of the cortex (p) filled with hyphal threads, which are sending branches into the adjacent cortical cells (@, i). —After Frank.
Fig. 77. Mycorrhiza: A, rootlets of white poplar forming mycorrhiza; B, enlarged section of single rootlets, showing the hyphe penetrating the cells.—After KERNER.
THE FOOD OF PLANTS 89
oaks and their allies, etc. (Figs. 76, 77). The delicate branching filaments (hyphz) of the fungus spread through the soil, wrap the rootlets with a mesh of hyphe, and pene- trate into the cells. It seems clear that the fungus obtains food from the rootlet as a parasite; but it is also thought that the hyphal threads, spreading widely through the soil, are of great service to the host plant in aiding the rootlets in absorbing. If this be true, there is mutual ad- vantage in the association, for the small amount of nourishment taken by the fungus is more than compen- sated by its assistance in absorption.
(3) Root-tubercles.—On the roots of many legume plants, as clovers, peas, beans, etc., little wart-like outgrowths are frequently found, known as “root-tubercles” (Fig. 78). It is found that these tuber- cles are caused by certain Bacteria, which penetrate the roots and in- duce these excrescent growths. The tubercles are found to swarm with Bacteria, which are doubtless ob- taining food from the roots of the host. At the same time, these Bac- teria have the peculiar power of laying hold of the free nitrogen of the air circulating in the soil, and of supplying it to the host plant
7 ' : Fie. 78. Root-tubercles on in some usable form. Ordinarily Vicia Faba.—after Nout.
plants can not use free nitrogen, although it occurs in the air in such abundance, and this power of these soil Bacteria is peculiarly interesting.
This habit of clover and its allies explains why they are useful in what is called “restoring the soil.” After ordi-
90 PLANT STRUCTURES
nary crops have exhausted the soil of its nitrogen-contain- ing salts, and it has become comparatively sterile, clover is able to grow by obtaining nitrogen from the air through the root-tubercles. If the crop of clover be “plowed under,” nitrogen-containing materials which the clover has organ- ized will be contributed to the soil, which is thus restored to a condition which will support the ordinary crops again. This indicates the significance of a very ordinary “rotation of crops.”
(4) Ant-plants, etc—In symbiosis one of the symbionts may he an animal. Certain fresh-water polyps and sponges become green on account of Alge which they harbor with- in their bodies (Fig. 79). Like the Lichen-fungus, these ani- mals use the food manufactured by the Alge, which in turn find a congenial situation for living. By some this would also be re- garded as a case of helotism, the animal enslaving the alga.
Very definite arrangements are made by certain plants for harboring ants, which in turn Fic. 79. A fresh-water polyp (y- guard them against the attack
dra) attached toa twig and feed- Of leaf-cutting insects and oth- ing upon alge (C), which may — er foes. These plants are called be seen through the transparent body wall (#).—-CALDWELL. Myrmecophytes, which means “ant-plants,” or myrmecophtlous plants, which means “ plants loving ants.” These plants are mainly in the tropics, and in stem cavities, in hollow thorns, or elsewhere, they provide dwelling places for tribes of warlike ants (Fig. 80). In addition to these dwelling places they provide special kinds of food for the ants.
(5) Flowers and insects.—A very interesting and impor- tant case of symbiosis is that existing between flowers and insects. The flowers furnish food to the insects, and the
THE FOOD OF PLANTS 91
latter are used by the flowers as agents of pollination. An account of this relationship is deferred until seed-plants are
Fig. 80. An ant plant (Hydnophytvm) from South Java, in which an excrescent growth provides a habitation for ants.—After SCHIMPER.
considered, or it may be found, with illustrations, in Plant Relations, Chapter VII.
99 PLANT STRUCTURES
59. Carnivorous plants—Certain green plants, growing in situations poor in nitrogen-containing salts, have learned to supplement the proteids which they manufacture by cap- turing and digesting insects. The various devices employed for securing insects have excited great interest, since they do not seem to be associated with the ordinary idea of plant activities. Prominent among these forms are the bladder- worts, pitcher-plants, sundews, Venus’s fly-trap, etc. For further account and illustrations of these plants see Plant Relations, § 119..
CHAPTER VII
BRYOPHYTES (MOSS PLANTS)
60. Summary from Thallophytes.—Before considering the second great division of plants it is well to recall the most important facts connected with the Thallophytes, those things which may be regarded as the contribution of the Thallophytes to the evolution of the plant kingdom, and which are in the background when one enters the region of the Bryophytes.
(1) Inereasing complexity of the body.—Beginning with single isolated cells, the plant body attains considerable complexity, in the form of simple or branching filaments, cell-plates, and cell-masses.
(2) Appearance of spores.—The setting apart of repro- ductive cells, known as spores, as distinct from nutritive cells, and of reproductive organs to organize these spores, represents the first important differentiation of the plant body into nutritive and reproductive regions.
(3) Differentiation of spores.—After the introduction of spores they become different in their mode of origin, but not in their power. The asexual spore, ordinarily formed by cell division, is followed by the appearance of the sexual spore, formed by cell union, the act of cell union being known as the sexual process.
(4) Differentiation of gametes.—At the first appearance of sex the sexual cells or gametes are alike, but after- ward they become different in size and activity, the large
passive one being called the egg, the small active one the 93
94 PLANT STRUCTURES
sperm, the organs producing the two being known as oogo- nium and antheridium respectively.
(5) Alge the main line-—The Alge, aquatic in habit, appear to be the Thallophytes which lead to the Bryophytes and higher groups, the Fungi being regarded as their de- generate descendants; and among the Alge the Chloro- phycex seem to be most probable ancestors of higher forms. It should be remembered that among these Green Alge the ciliated swimming spore (zoospore) is the characteristic asexual spore, and the sexual spore (zygote or oospore) is the resting stage of the plant, to carry it over from one growing season to the next.
61. General characters of Bryophytes——The name given to the group means “‘ moss plants,” and the Mosses may be regarded as the most representative forms. Associated with them in the group, however, are the Liverworts, and these two groups are plainly distinguished from the Thallo- phytes below, and from the Pteridophytes above. Starting with the structures that the Alge have worked out. the Bryophytes modify them still further, and make their own contributions to the evolution of the plant kingdom, so that Bryophytes become much more complex than Thallo- phytes.
62, Alternation of generations— Probably the most im- portant fact connected with the Bryophytes is the distinct alternation of generations which they exhibit. So impor- tant is this fact in connection with the development of the plant kingdom that its general nature must be clearly under- stood. Probably the clearest definition may be obtained by tracing in bare outline the life history of an ordinary moss.
Beginning with the asexual spore, which is not ciliated, as there is no water in which it can swim, we may imagine that it has been carried by the wind to some spot suitable for its germination. It develops a branching filamentous growth which resembles some of the Conferva forms among the (rreen Alge (Fig. 81). It is prostrate, and is a regu-
BRYOPHYTES 95
lar thallus body, not at all resembling the ‘‘moss plant” of ordinary observation, and is not noticed by those una- ware of its existence.
Presently one or more buds appear on the sides of this alga-like body (Fig. 81, 4). A bud develops into an erect
Fic, 81. Protonema of moss: A, very young protonema, showing spore (8) which has germinated it; B, older protonema, showing branching habit, remains of spore (s), rhizoids (7), and buds (2) of leafy branches (gametophores).—After MULLER and THURGAU.
stalk upon which are numerous small leaves (Figs. 82, 102). This leafy stalk is the ‘“‘moss plant” of ordinary observa- tion, and it will be noticed that it is simply an erect leafy branch from the prostrate alga-like body.
At the top of this leafy branch sex-organs appear, cor- responding to the antheridia and oogonia of the Alge, and within them there are sperms and eggs. A sperm and egg fuse and an oospore is formed at the summit of the leafy branch.
The oospore is not a resting spore, but germinates im- mediately, forming a structure entirely unlike the moss
96 PLANT STRUCTURES
plant from which it came. This new leafy body consists of a slender stalk bearing at its summit an urn- like case in which are developed nu- merous asexual spores (Figs. 82, 107). This whole structure is often called the ‘‘spore fruit,” and its stalk is imbedded at base in the summit of the leafy branch, thus obtaining firm anchorage and absorbing what nour- ishment it needs, but no more a part of the leafy branch than is a para- site a part of the host.
When the asexual spores, pro- duced by the ‘‘ spore fruit,” germi- nate, they reproduce the alga-like body with which we began, and the life cycle is completed.
In examining this life history, it is apparent that each spore produces a different structure. The asexual spore produces the alga-like body with its erect leafy branch, while the oospore produces the ‘‘ spore fruit” with its leafless stalk and spore case. These two structures, one produced by the asexual spore, the other by the oospore, appear in alternating succession, and this is ee ae what is meant by alfernalion of yen-
(Polytrichum commune), erattons.
showing the leafy gameto- These two “ generations » differ phore with rhizoids (rh), 2 :
and twosporophytes (sporo. strikingly from one another in the gonia), with seta (s), calyp- spores which they produce. The tra (c), and operculum (@), m1 i the calyptra having been re. generation composed of alga-like moved.—After Scuenck. hody and erect leafy branch pro-
BRYOPHYTES 97
duces only sexual spores (oospores), and therefore pro- duces sex organs and gametes. It is known, therefore, as the gametophyte—that is, ‘‘the gamete plant.”
The generation which consists of the ‘‘spore fruit ”— that is, leafless stalk and spore case—produces only asexual spores, and is called the sporophyte—that is, ‘‘the spore plant.”
Alternation of generations, therefore, means the alter- nation of a gametophyte and a sporophyte in completing a life history. Instead of having the same body produce both asexual and sexual spores, as in most of the Algz, the two kinds of spores are separated upon different structures, known as “ generations.” It is evident that the gameto- phyte is the sexual generation, and the sporophyte the asexual one; and it should be kept clearly in mind that the asexual spore always produces the gametophyte, and the sexual spore the sporophyte. In other words, each spore produces not its own generation, but the other one.
The relation between the two alternating generations may be indicated clearly by the following formula, in which G and § are used for gametophyte and sporophyte respectively :
G—§>0—S—o—G= > 0—S—o—G, ete.
The formula indicates that the gametophyte produces two gametes (sperm and egg), which fuse to form an oospore, which produces the sporophyte, which produces an asexual spore, which produces a gametophyte, ete.
That alternation of generations is of great advantage is evidenced by the fact that it appears in all higher plants. It must not be supposed that it appears first in the Bryo- phytes, for its beginnings may be seen among the Thallo- phytes. The Bryophytes, however, first display it fully organized and without exception. Just what this alterna- tion does for plants may not be fully known, but one advantage seems prominent. By means of it many gameto- phytes may result from a single oospore ; in other words,
98 PLANT STRUCTURES
it multiplies the product of the sexual spore. A glance at the formula given above shows that if there were no sporo- phyte (S) the oospore would produce but one gametophyte (G). By introducing the sporophyte, however, as many gametophytes may result from a single oospore as there are asexual spores produced by the sporophyte, which usually produces a very great number.
In reference to the sporophytes and gametophytes of Bryophytes two peculiarities may be mentioned at this point: (1) the sporophyte is dependent upon the gameto- phyte for its nourishment, and remains attached to it; (2) the gametophyte is the special chlorophyll-generation, and hence is the more conspicuous. It follows that, in a general way, the sporophyte of the Bryophytes only pro- duces spores, while the gametophyte both produces gametes and does chlorophyll work.
It is important also to note that the protected resting stage in the life history is not the sexual spore, as in the Alge, but is the asexual spore in connection with the sporophyte. These spores have a protecting wall, are scattered, and may remain for some time without germi- nation.
If the ordinary terms in reference to Mosses be fitted to the facts given above, it is evident that the ‘moss plant” is the leafy branch of the gametophyte; that the “‘moss fruit” is the sporophyte ; and that the alga- like part of the gametophyte has escaped attention and a name.
The names now given to the different structures which appear in this life history are as follows: The alga-like part of the gametophyte is the protonema, the leafy branch is the gametophore (‘‘ gamete-bearer ”) ; the whole sporophyte is the sporogonium (a name given to this peculiar leafless sporophyte of Bryophytes), the stalk-like portion is the seta, the part of it imbedded in the gametophore is the foot, and the urn-like spore-case is the capsule,
BRYOPHYTES 99
63. The antheridium.—The male organ of the Bryophytes is called an antheridium, just as among Thallophytes, but it has a very different structure. In general among the
Fig. 83. Sex organs of a common moss (Funaria): the group to the right represents an antheridium (1) discharging from its apex a mass of sperm mother cells (a), a single mother cell with its sperm (J), and a single sperm (¢). showing body and two cilia; the group to the left represents an archcgonial cluster at summit of stem (4), showing archezonia (@), and paraphyses and leaf sections (0), and also a single archegonium (ZB), with venter (0) containing egg and ventral canal cell, and neck (f) containing the disorganizing axial row (neck canal cells).—After Sacus.
Thallophytes it is a single cell (mother cell), and may be called a simple antheridium, but in the Bryophytes it is a many-celled organ, and may be regarded as a compound antheridium. Itis usually a stalked, club-shaped, or oval to
100 PLANT STRUCTURES
globular body (Figs. 83, 84, 103). A section through this body shows it to consist of a single layer of cells, which forms the wall of the antheridium, and within this a com- pact mass of small cubical (square in section) cells, within each one of which there is formed a single sperm (Fig. 84). These cubical cells are evidently moth- er cells, and to distinguish them from others they are called sperm mother cells. An antheridium, therefore, aside from its stalk, is a mass of sperm mother cells surrounded by a wall consisting of one layer of cells.
The sperm is a very small cell with two long cilia (Fig. 83). The two parts are spoken of as ‘‘ body” and cilia, and the body may be straight or somewhat curved. These small bicili- Fie. 84, Antheridium of @¢€ sperms are one of the distinguish-
aliverwort in section, ing marks of the Bryophytes. The showy single layer existence of male gametes in the form ing the mass of moth- _ of ciliated sperms indicates that fertil- er cells.—After STRAS- : : : Bee ization can take place only in the pres- ence of water, so that while the plant has become terrestrial, and its asexual spores have respond- ed to the new conditions and are no longer ciliated, its sexual process is conducted as among the Green Alge. It must not be supposed, however, that any great amount of water is necessary to enable sperms to swim, even a film of dew often answering the purpose.
When the mature antheridia are wet they are ruptured at the apex and discharge the mother cells in a mass (Figs. 83, 105, #), the walls of the mother cells become mucilagi- nous, and the sperms escaping swim actively about and are attracted to the organ containing the egg.
64. The archegonium.—This name is given to the female sex organ, and it is very different from the oogonium of
BRYOPHYTES 101
Thallophytes. Instead of being a single mother cell, it is a many-cellcd structure, shaped like a flask (Figs. 83, 98). The neck of the flask is more or less elongated, and within the bulbous base (venter) the single egg is organized. The archegonium, made up of neck and venter, consists mostly of a single layer of cells. This hollow flask is solid at first, there being a central vertical row of cells surrounded by the single layer just referred to. All of the cells of this axial row, except the lowest one, disorganize and leave a passageway down through the neck. The lowest one of the row, which lies in the venter of the archegonium, or- ganizes the egg. In this way there is formed in the arche- gonium an open passageway through the neck to the egg lying in the venter.
To this neck the swimming sperms are attracted, enter and pass down it, one of them fuses with the egg, and this act of fertilization results in an oospore.
It is supposed that archegonia have been derived in some way from oogonia, but no intermediate stages suggest the steps. In any event, the presence of the archegonia is one strong and unvarying distinction between Thallophytes and Bryophytes. Pteridophytes also have archegonia, and so characteristic an organ is it that Bryophytes and Pteri- dophytes are spoken of together as Archegoniates.
65. Germination of the oospore.—The oospore in Bryo- phytes is not a resting spore, but germinates immediately by cell division, forming the sporophyte embryo, which presently develops into the mature sporophyte (Fig. 85, 4). The lower part of the embryo develops downward into the gametophore, forming the foot, which penetrates and ob- tains a firm anchorage in the gametophore (Fig. 85, B, C). The upper part of the embryo develops upward, organizing the seta and capsule. In true Mosses, when the embryo becomes too large for the venter of the archegonium in which it is developing, the archegonium is broken near the
base of the venter and is carried upward perched on the top 25
102 PLANT STRUCTURES
of the capsule like a loose cap or hood, known as the calyp- tra (Figs. 82, c, 107), which sooner or later falls off. As
Fie. 85. Sporogonium of Fuvaria: A, an em- bryo sporogonium (7, 7”), developing within the venter (0, b) of an archegonium ; B, C, tips of leufy shoots bearing young sporo- gonia, pushing up calyptra (¢) and archego- nium neck (7), and sending the foot down into the apex of the gametophore.—After GOEBEL.
stated before, the ma- ture structure devel- oped from the oospore is called a sporogoni- um, a form of sporo- phyte peculiar to the Bryophytes.
66. The sporogonium. —In its fullest devel- opment the sporogoni- um is differentiated into the three regions, foot, seta, and capsule (Figs. 82, 107); but in some forms the seta may be lacking, and in others the foot also, the sporogonium in this last case being only the capsule or spore case, which, after all, is the essential part of any sporogonium.
At first the capsule is solid, and its cells are all alike. Later a group of cells within begins to differ in ap- pearance from those about them, being set apart for the produc- tion of spores. This
initial group of spore-producing cells is called the arche- sportwm, « word meaning ‘‘the beginning of spores.” It
BRYOPUYTES 103
does not follow that the archesporial cells themselves pro- duce spores, but that the spores are to appear sooner or later in their progeny. Usually the archesporial cells divide and form a larger mass of spore-producing cells. Such cells are known as sporogenous (‘ spore-producing ””) cells, or the group is spoken of as sporogenous tissue. Spo- rogenous cells may divide more or less, and the cells of the last division are mother cells, those which directly produce the spores. The usual sequence, therefore, is archesporial cells (archesporium), sporogenous cells, and mother cells ; but it must be remembered that they all may be referred to as sporogenous cells.
Each mother cell organizes within itself four spores, the group being known as a tetrad. In Bryophytes and the higher groups asexual spores are always produced in tetrads. After the spores are formed the walls of the mother cells disorganize, and the spores are left lying loose in a cavity which was formerly occupied by the sporoge- nous tissue. All mother cells do not always organize spores. In some cases some of them are used up in supplying nour- ishment to those which form spores. Such mother cells are said to function as nutritive cells. In other cases, certain mother cells become much modified in form, being organ- ized into elongated, spirally-banded cells called elaters (Figs. 97, 101), meaning “‘ drivers” or “‘hurlers.” These elaters lie among the loose ripe spores, are discharged with them, and by their jerking movements assist in scattering them.
The cells of the sporogonium which do not enter into the formation of the archesporium, and are not sporoge- nous, are said to be sterzle, and are often spoken of as sterile tissue. Hvery sporogonium, therefore, is made up of sporogenous tissue and sterile tissue, and the differences found among the sporogonia of Bryophytes depend upon the relative display of these two tissues.
The sporogonium is a very important structure from the standpoint of evolution, for it represents the conspicu-
Lot PLANT STRUCTURES
ous part of the higher plants. The “fern plant,” and the herbs, shrubs, and trees among ‘flowering plants” correspond to the sporogonium of Bryophytes, and not to the leafy branch (gametophore) or ‘‘moss plant.” Conse- quently the evolution of the sporogonium through the Bryophytes is traced with a great deal of interest. It may be outlined as follows:
In a liverwort called Riccia the simplest sporogonium is found. It is a globular capsule, without seta or foot
Fie. 86. Diagrammatic sections of sporogonia of liverworts: A, Riccia, the whole capsule being archesporium except the sterile wall layer; B, Marchantia, one half the capsule being sterile, the archesporium restricted to the other half; D, ulnthoceros, archesporium still more restricted, being dome-shaped and capping a central sterile tissue, the columella (col).—After GoEBEL.
(Fig. 86, 1). The only sterile tissue is the single layer of cells forming the wall, all the cells within the wall be- longing to the archesporium. The ripe sporogonium, therefore, is nothing but a thin-walled spore case. It is well to note that the sporophyte thus begins as a spore case, and that any additional structures that it may de- velop later are secondary.
In another liverwort (Marchantia) the entire lower half of the sporogoniun is sterile, while in the upper half there
BRYOPHYTES 105
is a single layer of sterile cells as a wall about the arche- sporium, which is composed of all the remaining cells of the upper half (Fig. 86, 8). It will be noted that the sterile tissue in this sporogonium has encroached upon the arche- sporium, which is restricted to one half of the body. In this case the archesporium has the form of a hemisphere. In another liverwort (Jungermannia) the archesporium is still more restricted (Fig. 87). The sterile tissue is organ-
Fie. 87. Diagrammatic section of spo- Fie. 88. Section through sporogonium of
rogonium of a Jungermannia form, Sphagnum, showing capsule (k) with showing differentiation into foot, old archegoninm neck (ah), calyptra (ca), seta, and capsule, the archesporium dome-shaped mass of sporogenons tissue restricted to upper part of sporogo- (spo), and columella (co), also the bulb- nium.—After GoEBEL. ous foot (spf) imbedded in the pseudo-
podium (ps) —After SCHIMPER.
ized into a foot and a seta, and the archesporium is a com- paratively small mass of cells in the upper part of the sporogonium.
In another liverwort (Anthoceros) the sterile tissue or- ganizes foot and seta, and the archesporium is still more restricted (Fig. 86, D). Instead of a solid hemispherical
106
PLANT STRUCTURES
mass, it is a dome-shaped mass, the inner cells of the hemi- sphere having become sterile. This central group of sterile
Fre. 89. Young sporogoni- um of a true moss, show- ing foot, scta, and young capsule, in which the ar- chesporium (darker por- tion) is harrel-shaped, and through it the columella is continuous with the lid.— After CAMPBELL.
cells which is surrounded by the ar- chesporium is called the columella, which means ‘‘a small column.”
In a moss called Sphagnum there is the same dome-shaped archespori- wn with the columella, as in -Ln- thoceros, but it is relatively smaller on account of the more abundant sterile tissue (Fig. 88).
In the highest Mosses the arche- sporium becomes very small as com- pared with the sterile tissue (Fig. 89). A foot, a long seta, and an elaborate capsule are organized from the sterile tissue, while the arche- sporium is shaped like the walls of a barrel, as though the dome-shaped archesporium of Sphagnum or An- thoceros had become sterile at the apex. In this way the columella is continued through the capsule, and is not capped by the archesporium.
This series indicates that after the sporogonium begins as a simple spore case (irei/a), its tendency is to increase sterile tissue and to re- strict sporogenous tissue, using the sterile tissue in the formation of the organs of the sporogonium body, as foot, seta, capsule walls, ete.
Among the Green Algw there is a form known as Coleorhete, whose body resembles those of the sim- plest Liverworts (Fig. 90). When
BRYOPHYTES 107
its oospores germinate there is formed a globular mass of cells, every one of which is a spore mother cell (Fig. 90, C). If an outer layer of mother cells should become sterile and form a wall about the others, such a spore case as that of
Fie. 90..—Coleochete, one of the green alge: A, a portion of the thallus, showing oogonia with trichogynes (og), antheridia (a1), and two enlarged biciliate sperms (z); B, a fertilized oogonium containing oospore and invested by a tissue (7) which has developed after fertilization; C, an oospore which has germinated and formed a mass of cells (probably a sporophyte), each one of which organizes a biciliate zoospore (D).—After PrinasHEIm.
Riecia would be the result (Fig. 86, 4). For such reasons many believe that the Liverworts have been derived from such forms as Coleochete.
67. The gametophyte—Having considered the sporo- phyte body as represented by the sporogonium, we must consider the gametophyte body as represented by proto- nema and leafy branch (gametophore). The gametophyte results from the germination of an asexual spore, and in the Mosses it is differentiated into protonema and leafy gametophore (Figs. 81, 82, 102). Like the sporophyte,
108 PLANT STRUCTURES
however, it shows an interesting evolution from its sim- plest condition in the Liverworts to its most complex con- dition in the true Mosses.
In the Liverworts the spore develops a flat thallus body, one plate of cells or more in thickness, which generally branches dichotomously (see § 29) and forms a more or less extensive body (Fig. 92). This thallus is the gametophyte, there being no differentiation into protonema and. leafy branch.
In the simpler Liverworts the sex organs (antheridia and archegonia) are scattered over the back of this thallus (Fig. 92). In other forms they become collected in certain definite regions of the thallus. In other forms these defi- nite sexual regions become differentiated from the rest of the thallus as disks. In other forms these disks, bearing the sex organs, become short-stalked, and in others long- stalked, until a regular branch arises from the thallus body (Figs. 96, 97). This erect branch, bearing the sex or- gans, is, of course, a gametophore, but it is leafless, the thallus body doing the chlorophyll work.
In the Sphagnum Mosses the spore develops the same kind of flat thallus (Fig. 104), but the gametophore be- comes leafy, sharing the chlorophyll work with the thallus. In the true Mosses most of the chlorophyll work is done by the leafy, gametophore, and the flat thallus is reduced to branching filaments (the protonema) (Fig. 102).
The protonema of the true Mosses, therefore, corre- sponds to the flat thallus of the Liverworts and Sphagnum, while the leafy branch corresponds to the leafless gameto- phore found in some Liverworts. It also seems evident that the gametophore was originally set apart to bear sex organs, and that the leaves which appear upon it in the Mosses are subsequent structures.
CIIAPTER VII
THE GREAT GROUPS OF BRYOPHYTES Heratice (Liverworts)
68. General character—Liverworts live in a variety of conditions, some floating on the water, many in damp places, and many on the bark of trees. In general they are moisture-loving plants (hydrophytes), though some can en- dure great dryness. The gametophyte body is prostrate, though there may be erect and leafless gametophores.
This prostrate habit develops a dorsiventral body—that is, one whose two surfaces (dorsal and ventral) are expused to different conditions and become unlike in structure. In Liverworts the ventral surface is against the substratum, and puts out numerous hair-like processes (rhizoids) for ab- sorption and anchorage. The dorsal region is exposed to the light and its cells develop chlorophyll. If the thallus is thin, chlorophyll is developed in all the cells; if it be so thick that the light is cut off from the ventral cells, the thallus is differentiated into a green dorsal region doing the chlorophyll work, and a colorless ventral region producing absorbing rhizoids. This latter represents a simple differ- entiation of the nutritive body into working regions, the ventral region absorbing material and conducting it to the green dorsal cells which use it in making food.
There seems to have been at least three main lines of development among Liverworts, each beginning in forms with a very simple thallus, and developing in different di-
rections. They are briefly indicated as follows: 109
110
PLANT STRUCTURES
69. Marchantia forms.—In this line the simple thallus
gradually becomes changed into a very complex one.
Fie. 91. A very small species of Riccia, one of the Marchantia forms: A, a group of thallus bodies slightly en- larged ; B, section of a thallus, show- ing rhizoids and two sporogonia im- bedded and communicating with the outside by tubular passages in the thallus.—After STRASBURGER.
ventral regions (Fig. 94).
The thallus retains its simple outlines, but becomes thick and differentiated in tissues (groups of similar cells). The line may be distin- guished, therefore, as one in which the differentia- tion of the tissues of the gametophyte is emphasized (Figs. 91-93). In Mar- chantia proper the thallus becomes very complex, and it may be taken as an illus- tration.
The thallus is so thick that there are very distinct green dorsal and colorless
The latter puts out numerous
rhizoids and scales from the single layer of epidermal cells. Above the ventral epidermis are several layers of colorless
Fia. 92.
Rieciocarpus, 2 Marchantia form, showing numerous rhizoids from ventral
surface, the dichotomous branching, und the position of the sporogonia on the dorsal surface along the ‘‘ midribs.’— CALDWELL.
Fie. 98. Two common liverworts: to the left is Conocephalus, a Marchantia form, showing rhizoids, dichotomous branching, and the conspicuous rhombic areas (areola) on the dorsal surface; to the right is Anthoceros, with its simple thallus and pod-like sporogonia.—C'ALDWELL,
Fra. 94. Cross-sections of thallus of J/archantia: A, section from thicker part of thallus, where supporting tissue (y) is abundant, and showing lower epidermis giving rise to rhizoids (%) and plates (0), also chlorophyll tissue (¢//) organized into chambers by partitions (0); B, section near margin of thallus more magnified, showing lower epidermis. two layers of supporting tissue (gy) with reticulate walls, a single chlorophyll chamber with its bounding walls (s) and containing short, often branching filaments whose cells contain chloroplasts (¢//), overarching upper epidermis (0) pierced by a large chimney-like air-pore (sp).—After GoEBEL.
Fig. 95. Section through eupule of Warchantia, showing wall in which are chloro- phyll-bearing air-chambers with air-pores, and gemmu (a) in various stages of development,—After Knr.
Fra. 96. Murehantia polymorpha: the lower figure represents a gametophyte bear- ing a mature antheridial branch (@), some young antheridial branches, and also some cupules with toothed margins, in which the gemma may be seen; the upper figure represents a partial section through the antheridial disk, and shows antheridia within the antheridial cavities (a, 0, ¢, a, e, f).—After Kny,
THE GREAT GROUPS OF BRYOPHYTES 113
cells more or less modified for conduction. Above these the dorsal region is organized into a series of large air cham- bers, into which project chlorophyll-containing cells in the
Fie. 97. Marchantia polymorpha, a common liverwort: 1, thallus, with rhizoids, bearing a mature archegonial branch (f) and several younger ones (a, 0, ¢, @, €); 2 and 3, dorsal and ventral views of archegonial disk; 4 and 5, young sporophyte (sporogonium) embryos; 6, more mature sporogonium still within enlarged venter of archegonium; 7, mature sporogonium discharging spores; 5, three spores and an elater —After Kwy.
form of short branching filaments. Overarching the air chambers is the dorsal epidermis, and piercing through it into each air chamber is a conspicuous air pore (Fig. 94, B).
114 PLANT STRUCTURES
The air chambers are outlined on the surface as small rhombic areas (aveole), each containing a single air pore.
Peculiar reproductive bodies are also developed upon the dorsal surface of Murchantia for vegetative multiplica-
Fig. 98. Marchantia potymorpha: 1, partial section through archegonial disk, show- ing archegonia with long necks, and venters containing eges; 9, young archego- nium showing axial row; 10, superficial view at luter stage; 12, mature archego- nium, with axial row disorganized and leaving an open passage to the large egg; 12, cross-section of venter; 13, cross-section of neck.—After Kyy.
tion. Little cups (cvpiles) appear, and in them are numer- ous short-stalked bodies (gemm), which are round and flat (biscuit-shaped) and many-celled (Figs. 95, 96). The
THE GREAT GROUPS OF BRYOPHYTES 115
gemme fall off and develop new thallus bodies, making rapid multiplication possible.
Marchantia also possess remarkably prominent gameto- phores, or ‘*sexual branches” as they are often called. In this case the gametophores are differentiated, one bear- ing only antheridia (Fig. 96), and known as the ‘anthe- ridial branch,” the other bearing only archegonia (Figs. 97, 98), and known as the ‘‘archegonial branch.” The scal- loped antheridial disk and the star-shaped archegonial disk, each borne up by the stalk-like gametophore, are seen in the illustrations. Not only are the gametophores sexually dif- ferentiated, but as only one appears on each thallus, the thal- lus bodies are sexually differentiated. When the two sex organs appear upon different individuals, the plant is said to be diecious, meaning ‘‘two households”; when they both appear upon the same individual, the plant is wonwcious, meaning “one household.” Some of the Bryophytes are mo- neecious, and some of them are dicecious (as Aurchantia).
Another distinguishing mark of the line of Marchantia forms is that the capsule-like sporogonium opens irregu- larly to discharge its spores (Fig. 97, 7).
70. Jungermannia forms.—This is the greatest line of the Liverworts, the forms being much more numerous than in the other lines. They grow in damp places; or in drier situations on rocks, ground, or tree-trunks ; or in the tropics also on the leaves of forest plants. They are gen- erally delicate plants, and resemble small Mosses, many of them doubtless being commonly mistaken for Mosses.
This resemblance to Mosses suggests one of the chief features of the line. Beginning with a simple thallus, as in the Marchantia line, the structure of the thallus re- mains simple, there being no such differentiation of tissues as in the Marchantia line; but the form of the thallus becomes much modified (Figs. 99,100). Instead of a flat thallus with even outline, the body is organized into a cen- tral stem-like axis bearing two rows of small, often crowded
116 PLANT STRUCTURES
leaves. There are really three rows of leaves, but the third is on the ventral side against the substratum, and is often so much modified as not to look like the other leaves. In consequence of this the Jungermannia forms are usually called ‘leafy liverworts,” to distinguish them from the
af
Fic. 99. Two liverworts, both Jungermannia forms: to the left is Blasia, which re- tains the thallus form but has lobed margins; to the right is Scupunia, with dis- tinct leaves and sporogonia (A).—CALDWELL.
other Liverworts, which are ‘‘thallose.” They are also often called ‘‘ scale mosses,” on account of their moss-like appearance and their small scale-like leaves.
The line may be distinguished, therefore, as one in which the differentiation of the form of the gametophyte is emphasized. Another distinguishing mark is that the sporogonium has a prominent seta, and the capsule splits down into four pieces (elves) when opening to discharge the spores (Fig. 100, ().
71. Anthoceros forms.—This line contains comparatively few forms, but they are of great interest, as they are sup- posed to represent forms which have given rise to the
THE GREAT GROUPS OF BRYOPHYTES a ta bv
Fie. 100. Species of Lepidozia, a genus of leafy liverworts, showing different leaf forms, and in .4 and (‘the dehiscence of the sporogonium by four valves. In @ rhizoids are evident; and in B, D, and # the three rows of leaves are seen, the leaves of the ventral row being comparatively small.—After ENGLER and PRANTL.
Mosses, and possibly to the Pteridophytes also. The
thallus is very simple, being differentiated neither in
structure nor form, as in the two other lines; but the 26
118 PLANT STRUCTURES
special development has been in connection with the
sporogonium (Figs. 93, 101).
This complex sporogonium (sporophyte) has a large bulbous foot imbedded in the simple thallus, while above there arises a long pod-like capsule. The com-
Fie. 101. Anthoceros gracilis: A, several gametophytes, on which sporogonia have developed ; B, an enlarged sporogonium, showing its elongated character and de- hiscence by two valves leaving exposed the slender columella on the surface of which are the spores; (, D, H, F, cla- ters of various forms; G, spores.—After SCHIFFNER.
plex walls of this cap- sule contain chlorophyll and air pores, so that the sporogonium is or- ganized for chlorophyll work. If the foot could send absorbing processes into the soil, this sporo- phyte could live inde- pendent of the gameto- phyte. In opening to discharge spores the pod- like capsule splits down into two valves.
Another peculiarity of the .Lnthocervs forms is in connection with the antheridia and arch- egonia. These organs, instead of growing out free from the body of the thallus, as in other Liv- erworts, are imbedded in it. The significance of this peculiarity lies in the fact that it is a char- acter which belongs to the Pteridophytes.
The chief direction of development of the three liv- erwort lines may be summed up briefly as follows: The Murchantia line has differentiated the structure of the
THE GREAT GROUPS OF BRYOPIYTES 119
gametophyte; the Junxgermannia line has differentiated the form of the gametophyte; the -lnthoceros line has differentiated the structure of the sporophyte. It should be remembered that other characters also serve to distin- guish the lines from one another.
Muscr (Josses)
72. General character.—Mosses are highly specialized plants, probably derived from Liverworts, the numerous forms being adapted to all conditions, from submerged to very dry, being most abundantly displayed in temperate and arctic regions. Many of them may be dried out com- pletely and then revived in the presence of moisture, as is true of many Lichens and Liverworts, with which forms Mosses are very commonly associated.
They also have great power of vegetative multiplica- tion, new leafy shoots putting out from old ones and from the protonema indefinitely, thus forming thick carpets and masses. Bog mosses often completely fill up bogs or small ponds and lakes with a dense growth, which dies below and continues to grow above as long as the conditions are favorable. These quaking bogs or “‘ mosses,” as they are sometimes called, furnish very treacherous footing unless rendered firmer by other plants. In these moss-filled bogs the water and silt shut off the lower strata of moss from complete disorganization, and they become modified into a coaly substance called peat. which may accumulate to con- siderable thicknéss by the continued upward growth of the mass of moss.
The gametophyte body is differentiated into two very distinct regions: (1) the prostrate dorsiventral thallus, which is called protonema in this group, and which may be either a broad flat thallus (Fig. 104) or a set of branching filaments (Figs. 81, 102); (2) the erect leafy branch or gametophore (Fig. 82). This erect branch is said to be
120 PLANT STRUCTURES
radial, in contrast with the dorsiventral thallus, referring to the fact that it is exposed to similar conditions all around, and its organs are arranged about a central axis
like the parts of a radiate animal.
Fig. 102. A moss (Bryum), showing base of a leafy branch (gametophore) attached to the protonema, and having sent out rhizoids. On the protoncmal filament to the right and be- low is the young bud of another leafy branch. —MULLER.
This position is much more favorable for the chlorophyll work than the dorsiventral posi- tion, as the special chlorophyll organs (leaves) can be spread out to the light freely in all directions.
It should be re- marked that the gam- etophyte im all groups of plants is a thallus, doing its chlorophyll work, when it does any, in a dorsiventral position ; the only ex- ception being the ra- dial leafy branch that arises from the thal- lus of Mosses. From Mosses onward the gametophyte becomes less conspicuous, so that the prominent leafy plants of the higher groups hold no
relation to the little erect leafy branch of the Mosses, which is put out by the gamctophyte, and which is the best the gametophyte ever does toward getting into a bet-
ter position for chlorophyll work.
The leafy branch of the Mosses usually becomes inde- pendent of the thallus by putting out rhizoids at its base
THE GREAT GROUPS OF BRYOPIYTES 191
(Fig. 102), the thallus part dying. Sometimes, however, the filamentous protonema is very persistent, and gives rise to a perennial succession of leafy branches.
ie) ie) ie) °
og [oxe)
Fie. 103. Tip of leafy branch of a moss (Fwvaria), bearing a cluster of sex organs, showing an old antheridium (A), a younger one (B), some of the curious associated hairs (p), and leaf sections (2).—After CAMPBELL.
At the summit of the leafy gametophore, either upon the main axis or upon a lateral branch, the antheridia and archegonia are borne (Figs. 83, 103). Often the leaves at the summit become modified in form and arranged to form
129 PLANT STRUCTURES
a rosette, in the center of which are the sex organs. This rosette is often called the ‘* moss flower,” but it holds no relation to the flower of Seed-plants, and the phrase should not be used. <A rosette may contain but one kind of sex organ (Figs. 83, 103), or it may contain both kinds, for Mosses are both dicecious and monecious. The two prin- cipal groups are as follows:
73. Sphagnum forms.—These are large and pallid bog mosses, found abundantly in marshy ground, especially of temperate and arctic regions, and are conspicuous peat- formers (Fig. 105, 1). The leaves and gametophore axis are of peculiar struc- ture to enable them to suck up and hold a large amount of wa- ter. This abundant water -storage tissue and the comparative- ly poor display of chlorophyll - contain- ing cells gives the peculiar pallid ap- pearance.
They resemble the Liverworts in the broad thallus body of the gametophyte, Fie. 104, Thallus body of gametophyte of Sphag- from which the large
num, giving rise to rhizoids (7) and bnds (h)
which develop into the large leafy branches leafy gametophore
(gametophores).—After CAMPBELL. arises (Fig. 104).
They also resemble «lnthoceros forms in the sporogonium, the archesporium being a dome-shaped mass (Fig. 105, (’). On the other hand, they resemble the true Mosses, not only in the leafy gametophore, but also in the fact that the capsule opens at the apex by a circular lid, called the operculum (Fig.
THE GREAT GROUPS OF BRYOPHYTES 123
105, D), which means a “cover” or “lid.” This may serve to illustrate what is called an “intermediate” or “transition” type, Sphagnum showing characters which ally it to Anthoceros forms on the one side, and to true Mosses on the other.
A peculiar feature of the sporogonium is that it has no long stalk-like seta, as have the true Mosses, although it appears to have one. This false appearance arises from the
alin aa B C
Fie. 105. Sphagnum; A, a leafy branch (gametophore) bearing four mature sporo- gonia; B, archegonium in whose venter a young embryo sporophyte (em) is de- veloping; (, section of a young sporogonium (sporophyte), showing the bulbous foot (spf) imbedded in the apex of the pseudopodium (ps), the capsule (x), the columella (co) capped by the dome-shaped archesporium (so). a portion of the calyptra (ca), and the old archegonium neck (ah); D, branch bearing mature sporogonium and showing pseudopodium (ps), capsule (4), and operculum (@); 2, antberidium discharging sperms; F, a single sperm, showing coiled body and two cilia.—After SCHIMPER.
fact that the axis of the gametophore is prolonged above its leafy portion, the prolongation resembling the seta of an ordinary moss (Fig. 105, D). This prolongation is
124 PLANT STRUCTURES
called a pseudopodium, or ‘false stalk,” and in the top of it is imbedded the foot of the sporogonium carrying the globular capsule (Fig. 105, C’).
74, True Mosses—This immense and most highly organ- ized Bryophyte group contains the great majority of the Mosses, which are sometimes called the Brywm forms, to distinguish them from the Sphagnum forms. They are
Fic. 106. Different stages in the development of the leafy gametophore from the pro- tonema of acommon moss (Fvnaria): A, the first few cells and a rhizoid (7); B, C, later stages, showing apical cell (7) and young leaves (2); D, later stage much less magnified, showing protonemal filaments and the young gametophore (gam) —After CAMPBELL.
the representative Bryophytes, the only group vying with them being the leafy Liverworts, or Jiiagermaniic forms. They grow in all conditions of moisture, from actual sub- mergence in water to dry rocks, and they also form exten- sive peat deposits in bogs.
The thallus body of the gametophyte is made up of branching filaments (Figs. 81, 102), those exposed to the
THE GREAT GROUPS OF BRYOPHYTES 195
light containing chlorophyll, and those in the substratum being colorless and acting as rhizoids. The leafy gameto- phores are often highly organized (Figs. 102, 106), the leaves and stems showing a certain amount of differentia- tion of tissues.
It is the sporophyte, however, which shows the great- est amount of specialization (Fig. 107). The sporogonium
Fie. 107. A common moss (Faria): in the center is the leafy shoot (gametophore), with rhizoids, several leaves, and a sporogonium (sporophyte), with a long seta, capsule, and at its tip the calyptra (cal); to the right a capsule with calyptra re- moved, showing the operculum (0); to the left a young sporogonium pushing up the calyptra from the leafy shoot.—After CAMPBELL.
has a foot and a long slender seta, but the capsule is espe- cially complex. The archesporium is reduced to a small hollow cylinder (Fig. 88), the capsule wall is most elabo- rately constructed, and the columella runs through the
Fie. 108, Longitudinal section of moss capsule (Funaria), showing its complex character: d, operculum; p, peristome: c, c’, columel- la; s, sporogenous tissne; outside of s the complex wall consisting of layers of cells and large open spaces (/) traversed by strands of tissue.—After GOEBEL.
Fig. 110. Sporogonia of Grimmia, from all of which the operculum has fallen, displaying the peristome teeth : 4, position of the teeth when dry; /, position when moist.—After KERNER.
eoofac
ae
2
Fie. 109. Partial longitudinal section through a moss cap- sule: A, younger capsule, showing wall cells (@), cells of columella (é), and sporag- enous cells (sw); B, some- what older capsule, @ and ¢ same as before, and sm the spore mother cells, — After GOEBEL.
THE GREAT GROUPS OF BRYOPHYTES 127
center of the capsule to the lid-like operculum (Figs. 108, 109). When the operculum falls off the capsule is left like an urn full of spores, and at the mouth of the urn there is usually displayed a set of slender, often very beau- tiful teeth (Fig. 110), radiating from the circumference to the center, and called the per/stome, meaning ‘* about the mouth.” These teeth are hygroscopic, and by bending inward and outward help to discharge the spores.
CHAPTER IX PTERIDOPHYTES (FERN PLANTS)
75. Summary from Bryophytes—In introducing the Bryo- phytes a summary from the Thallophytes was given (see § 60), indicating certain important things which that group has contributed to the evolution of the plant kingdom. In introducing the Pteridophytes it is well to notice certain important additions made by the Bryophytes.
(1) -llternation of gencrations—The great fact of alter- nating sexual (gametophyte) and sexless (sporophyte) gen- erations 1s first clearly expressed by the Bryophytes, although its beginnings are to be found among the Thallophytes. Each generation produces one kind of spore, from which is developed the other generation.
(2) Gametophyte the chlorophyll generation.—On account of this fact the food is chiefly manufactured by the gameto- phyte, which is therefore the more conspicuous generation. When a moss or a liverwort is spoken of, therefore, the gametophyte is usually referred to.
(3) Gametophyte and sporophyte not independent.—The sporophyte is mainly dependent upon the gametophyte for its nutrition, and remains attached to it, being commonly called the sporogonium, and its only function is to produce spores.
(4) Differentiation af thallus into stem and leaves,— This appears incompletely in the leafy Liverworts (./iuger- maunia forms) and much more clearly in the erect and radial leafy branch (gametophore) of the Mosses.
128
PTERIDOPHY TES 129
(5) Many-celled sex organs —The antheridia and the flask-shaped archegonia are very characteristic of Bryo- phytes as contrasted with Thallophytes.
76. General characters of Pteridophytes.— The name means ‘fern plants,” and the Ferns are the most numerous and the most representative forms of the group. Associated with them, however, are the Horsetails (Scouring rushes) and the Club-mosses. By many the Pteridophytes are thought to have been derived from such Liverworts as the .lntho- ceros forms, while some think that they may possibly have been derived directly from the Green Algw. Whatever their origin, they are very distinct from Bryophytes.
One of the very important facts is the appearance of the vascular system, which means a ‘‘system of vessels,” organized for conducting material through the plant body. The appearance of this system marks some such epoch in the evolution of plants as is marked in animals by the appearance of the ‘* backbone.” As animals are often grouped as “‘ vertebrates ” and “invertebrates,” plants are often grouped as ‘‘ vascular plants” and ‘‘non-vascular plants,” the former being the Pteridophytes and Spermato- phytes, the latter being the Thallophytes and Bryophytes. Pteridophytes are of great interest, therefore, as being the first vascular plants.
“7. Alternation of generations—This alternation con- tinues in the Pteridophytes, but is even more distinct than in the Bryophytes, the gametophyte and sporophyte be- coming independent of one another. An outline of the life history of an ordinary fern will illustrate this fact, and will serve also to point out the prominent structures. Upon the lower surface of the leaves of an ordinary fern dark spots or lines are often seen. These are found to yield spores, with which the life history may be begun.
When such a spore germinates it gives rise to a small, green, heart-shaped thallus, resembling a delicate and sim- ple liverwort (Fig. 111, -1). Upon this thallus antheridia
130 PLANT STRUCTURES
and archegonia appear, so that it is evidently a gameto- phyte. This gametophyte escapes ordinary attention, as it is usually very small, and lies prostrate upon the substra- tum. It has received the name prothalliwm or prothallus, so that when the term prothallium is used the gametophyte of Pteridophytes is generally referred to ; just as when the term sporogonium is used the sporophyte of the Bryophytes isreferred to. Within an archegonium borne upon this little prothallium an oospore is formed. When the oospore ger-
Fig. 111. Prothallium of a common fern (Aspidivm): A, ventral surface, showing rhizoids (72), antheridia (@7), and archegonia (ar); B, ventral surface of an older gametophyte, showing rhizoids (7) and young sporophyte with root (zw) and leaf (b).—After SCHENCK.
minates it develops the large leafy plant ordinarily spoken of as ‘“‘the fern,” with its subterranean stem, from which roots descend, and from which large branching leaves rise above the surface of the ground (Fig. 111, 2). It is in this complex body that the vascular system appears. No sex organs are developed upon it, but the leaves bear numer- ous sporangia full of asexual spores. This complex vascular plant, therefore, is a sporophyte, and corresponds in this life history to the sporogonium of the Bryophytes. This
PTERIDOPHY TES 131
completes the life cycle, as the asexual spores develop the prothallium again.
In contrasting this life history with that of Bryophytes several important differences are discovered. The most striking one is that the sporophyte has become a large, leafy, vuscular, and independent structure, not at all re- sembling its representative (the sporogonium) among the Bryophytes.
Also the gametophyte has become much reduced, as compared with the gametophytes of the larger Liverworts and Mosses. It seems to have resumed the simplest liver- wort form, even the gametophore being suppressed, and represented, if at all, by a rudiment. The conspicuous leafy branch of the Mosses, commonly called ‘* the moss plant,” corresponds to nothing in the Pteridophytes, there- fore, except possibly the rudiment referred to, the prothal- lium representing only the protonema part of the gameto- phyte of the true Mosses.
This reduction of the gametophyte seems to be associ- ated with the fact that the chlorophyll work has been trans- ferred to the sporophyte, which hereafter remains the conspicuous generation. The ‘‘fern plant” of ordinary observation, therefore, is the sporophyte ; while the ‘* moss plant” is a leafy branch of the gametophyte.
Another important contrast indicated is that in Bryo- phytes the sporophyte is dependent upon the gametophyte for its nutrition, remaining attached to it; while in the Pteridophytes both generations are independent green plants, the leafy sporophyte remaining attached to the small gametophyte only while beginning its growth (Fig. iy By,
Among the Ferns some interesting exceptions to this method of alternation have been observed. Under certain conditions a leafy sporophyte may sprout directly from the prothallium (gametophyte) instead of from an oospore. This is called apogamy, meaning ‘‘ without the sexual act.”
132 PLANT STRUCTURES
Under certain other conditions prothallia are observed to sprout directly from the leafy sporophyte instead of from a spore. This is called apospory, meaning “without a spore.”
78. The gametophyte—The prothallium, like a simple liverwort, is a dorsiventral body, and puts out numerous
Fig. 112. Stag-horn fern (Platycertum grande), an epiphytic tropical form, showing the two forms of leaves: a and 2, young sterile leaves ; c, leaves bearing spo- rangia ; d, an old sterile leaf.—CaLDWELL.
rhizoids from its ventral surface (Fig. 111). It is so thin that all the cells contain chlorophyll, and it is usually short- lived. In rare cases it becomes quite large and permanent,
Fig, 113. Archegonium of Pteris at the time of fertilization, showing tissue of gam- etophyte (1), the cells forming the neck (2B), the passaceway formed by the dis- organization of the canal cells (C'), and the egg (D) lying exposed in the venter. —CALDWELL.
Fie. 114. Antheridium of Peris (B), showing wall cells (a), opening for escape of sperm mother cells (e), escaped mother cells (¢), sperms free from mother cells (0), showing spiral and multiciliate character.—CALDWELL.
27
134 PLANT
STRUCTURES
being a conspicuous object in connection with the sporo-
phyte.
At the bottom of the conspicuous notch in the prothal-
lium is the growing point, representing the apex of the plant. This notch is always a conspicuous feature.
The antheridia and arch- egonia are usually developed on the under surface of the prothallium (Fig. 111, .1), and differ from those of all Bryophytes, except the .fv- thocerox forms, in being sunk in the tissue of the prothal- lium and opening on the sur-
ae
Fie. 115. Development of gamctophyte of Plerix; the figure to the left shows the old spore (B), the rhizoid (4, and the thallus (4); that to the right is older, showing the same parts, and also the apical cell (D).—CaLDWELL,
Fig, 116.
Young gametophyte of P/r7is, showing old spore wall (2B), rhizoids ((), apical cell (7), a young anther. idium (/’), and an older one in which sperms have organized (#').—Ca.p- WELL.
PTERIDOPHYTES 135
face, more or less of the neck of the archegonium projecting (Fig. 113). The eggs are not different from those formed within the archegonia of Bryophytes, but the sperms are very different. The Bryophyte sperm has a small body and two long cilia, while the Pteridophyte sperm has a long spirally coiled body, blunt behind and tapering to a point in front, where numerous cilia are developed (Fig. 114). It is, therefore, a large, spirally-coiled, multiciliate sperm, and is quite characteristic of all Pteridophytes excepting the Club-mosses. It is evident that a certain amount of water is necessary for fertilization—in fact, it is needed not only
Fig. 117. Sections of portions of the gametophyte of Pteris, showing development of archegonium: A, young stage, showing cells which develop the neck (a), and the cell from which the egg cell and canal cells develop (4); B, an older stage, showing neck cells (a), neck canal cell (2). and cell from which is derived the egg cell, and the ventral canal cell (c); Ca still older stage, showing increased num- ber of neck cells (a), two neck canal cells (4), the ventral canal cell (c), and the cell in which the egg is organized (¢@).—CALDWELL.
by the swimming sperm, but also to cause the opening of the antheridium and of the archegonium neck. There seems to be a relation between the necessity of water for fertilization and a prostrate, easily moistened gametophyte.
Prothallia are either moncecious or dicecious (see § 69). When the prothallia are developing (Fig. 115) the anther-
SEAMHOHR
Fic. 118. A fern (lspidium), showing three large branching leaves coming from a horizontal subterranean stem (rvotstock); young leaves are also shown, which show circinate vernation. The stem, young leaves, and petioles of the large Icaves are thickly covered with protecting hairs, The stem gives rise to numerous small roots from its lower surface. The figure marked 3 represents the under sur- face of a portion of the leaf, showing seven sori with shield-like indusia; at 4 is represented a section through a sorus. showing the sporangia attached and pro- tected by the indusium; while at 6 is represented a single sporangium opening and discharging its spores, the heavy annulus extending along the back and over the top.—After Woss1DLo.
PTERIDOPHYTES 137
idia begin to appear very early (Fig. 116), and later the archegonia (Fig. 117). If the prothallium is poorly nour- ished, only antheridia appear; it needs to be well developed and nourished to develop archegonia. There seems to be a very definite relation, therefore, between nutrition and the development of the two sex organs, a fact which must be remembered in connection with certain later develop- ments.
79. The sporophyte.—This complex body is differentiated into root, stem, and leaf, and is more highly organized than any plant body heretofore mentioned (Fig. 118). The development of this body and its three great working regions must be considered separately.
(1) Development of embryo.—The oospore, from which the sporophyte develops, rests in the venter of the arche- gonium, which at this stage resembles a depression in the
Fig. 119. Embryos of a common fern (Pferis): A, young embryo, showing direction of basal wall (7), and of second walls (77), which organize quadrants, each of which subsequently develops into foot (f), root (w), leaf (6), and stem (s); B, an older embryo, in which the four regions (lettered as in A) have developed further, also showing venter of archegonium (a7), and some tissue of the prothallium (pr). —A after Krenirz-Geriorr; B after HoFMEISTER.
lower surface of the prothallium (Fig. 119, B). It germi- nates at once, as in Bryophytes, not being a resting spore asin Green Alge. The resting stage, as in the Bryophytes,
138 PLANT STRUCTURES
is in connection with the asexual spores, which may be kept for a long time and then germinated.
The first step in germination is for the oospore to di- vide into two cells, forming a two-celled embryo. In the ordinary Ferns this first dividing wall is at right angles to the surface of the prothallium, and is called the dasal wall (Fig. 119, A). One of the two cells, therefore, is anterior (toward the notch of the prothallium), and the other is posterior.
The two cells next divide by forming walls at right angles to the basal wall, and a four-celled embryo is the result. This is called the ‘‘ quadrant stage” of the em- bryo, as each one of the four cells is like the quadrant of a sphere.
With the appearance of the quadrant, four body regions are organized, each cell by its subsequent divisions giving rise to a distinct working region (Fig. 119, 4). Two of the cells are inner (away from the substratum) ; also one of the inner and one of the outer (toward the substratum) cells are anterior ; while the two other inner and outer cells are posterior. The anterior outer cell develops the first leaf of the embryo, generally called the cotyledon (Fig. 119, &) ; the anterior inner cell develops the stem (Fig. 119, s) ; the pos- terior outer cell develops the first (primary) root (Fig. 119, w); the posterior inner cell develops a special organ for the use of the embryo, called the foot (Fig. 119, f). The foot remains in close contact with the prothallinm and absorbs nourishment from it for the young embryo. When the young sporophyte has developed enough to become in- dependent the foot disappears. It is therefore spoken of as a temporary organ of the embryo. It is necessary for the leaf to emerge from beneath the prothallium, and it may be seen usually curving upward through the notch. The other parts remain subterranean.
(2) The root.—The primary root organized by one of the quadrants of the embryo is a temporary affair (Figs.
PTERIDOPHY TES 139
111, 119), as it is in an unfavorable position in reference to the dorsiventral stem, which puts out a series of more favor- ably placed secondary roots into the soil (Fig. 118). The mature leafy sporophyte, therefore, has neither foot nor primary root, the product of two of the quadrants of the embryo having disappeared.
The secondary roots put out by the stem are small, and do not organize an extensive system, but they are interest- ing as representing the first appearance of true roots, which therefore come in with the vascular system. In the lower groups the root function of absorption is conducted by sim- ple hair-like processes called rhizoids; but true roots are complex in structure and contain vessels.
(3) Zhe stem.—In most of the Ferns the stem is sub- terranean and dorsiventral (Fig. 118), but in the “tree ferns” of the tropics it forms an erect, aérial shaft bearing a crown of leaves (Fig. 120). In the other groups of Pteri- dophytes there are also aérial stems. both erect and pros- trate. The stem is complex in structure, the cells being organized into different “‘ tissue systems,” prominent among which is the vascular system. These tissue systems of vas- cular plants are described in Chapter AV.
The appearance of the vascular system in connection with the leafy sporophyte is worthy of note. The leaves are special organs for chlorophyll work, and must receive the raw material from air and soil or water. The leaves of the moss gametophyte are very small and simple affairs, and can be supplied with material by using very little ap- paratus. In the leafy sporophyte, however, the leaves are very prominent structures. capable of doing a great deal of work. To such working structures material must be brought rapidly in quantity, and manufactured food ma- terial must be carried away, and therefore a special con- ducting apparatus is needed. This is supplied by the vas- cular system. These vessels extend continuously from root- tips, through the stem, and out into the leaves, where they
Fig. 120. A group of tropical plants. To the left of the center is a tree fern, with its slender columnar stem and crown of large leaves. The laree-leayed plants to the right are bananas (Monocotyledons).—From “ Plant Relations,”
PTERIDOPHYTES 141
are spoken of as ‘leaf veins.” Large working leaves and a vascular system, therefore, belong together and appear together; and the vascular plants are also the plants with leafy sporophytes.
(4) The leaf.—lLeaves are devices for spreading out green tissue to the light, and in the Ferns they are usually large. There is a stalk-like portion (petiole) which rises from the subterranean stem, and a broad expanded portion (Slade) exposed to the light and air (Fig. 118). In Ferns the blade is usually much branched, being cut up into segments of various sizes and forms.
The essential structure consists of an expansion of green tissue (mesophyll), through which strands of the vascular system (veins) branch, forming a supporting framework, and over all a compact layer of protecting cells (epidermis). A surface view of the epidermis shows ca ) that it is pierced by numer- +5 gs i ous peculiar pores, called : ae, * Y 1s stomata, meaning “mouths.” § ‘ ls i aly U The surface view of a stoma + a y_ ¢ f shows two crescentic cells * (guard cells) in contact at the ends and leaving be- tween them a lens-shaped opening (Fig. 121).
A cross-section through a leaf gives a good view of the three regions (Fig. 122). Fic. 121. Some epidermal cells from leaf
Above and below is the col- of Pleris, showing the interlocking
s : . walls and three stomata, the guard orless epidermis, pierced cells containing chloroplasts.—CaLp- here and there by stomata ; WELL,
between the epidermal lay-
ers the cells of the mesophyll are packed; and among the mesophyll cells there may be seen here and there the cut ends of the veins. The leaf is usually a dorsiventral
149 PLANT STRUCTURES
organ, its two surfaces being differently related to light. To this different relation the mesophyll cells respond in their arrangement. Those in contact with the upper epi- dermis become elongated and set endwise close together, forming the palisade tissue ; those below are loosely ar-
Soot)
BE Go Sose2.80
@\OS, ag
5 SEX
Fig. 122. Cross-section through a portion of the leaf of Preris, showing the heavy- walled epidermis above and below, two stomata in the lower epidermis (one on each side of the centcr) opening into intercellular passages, the mesophyll cells containing chloroplasts, the upper row arranged in palisade fushion, the other cells loosely arranged (spongy mesophyll) and leaving large intercellular passages, and in the center a scetion of a veinlet (vascular bundle), the xylem being repre- sented by the central group of heavy-walled cells. —CALDWELL.
ranged, leaving numerous intercellular spaces, forming the spongy tissue, These spaces form a system of inter- cellular passageways among the working mesophyll cells, putting them into communication with the outer air through the stomata. The freedom of this communication
P'TERIDUPHYTES 143
is regulated by the guard cells of the stomata, which come together or shrink apart as occasion requires, thus dimin- ishing or enlarging the opening between them. ‘The sto- mata have well been called ‘‘ automatic gateways” to the system of intercellular passageways.
One of the peculiarities of ordinary fern leaves is that the vein system branches dichotomously, the forking veins being very conspicuous (Figs. 123-126). Another fern habit is that the leaves in expanding seem to unroll from the base, as though they had been rolled from the apex downward, the apex being in the center of the roll (Fig. 118). This habit is spoken of as circinate, from a word meaning ‘‘circle” or ‘‘ coil,” and circinate leaves when unrolling have a crozier-like tip. The arrangement of leaves in bud is called vernation (‘‘ spring condition”), and therefore the Ferns are said to have circinate verna- tion. The combination of dichotomous venation and cir- cinate vernation is very characteristic of Ferns.
80. Sporangia— Among Thallophytes sporangia are usu- ally simple, mostly consisting of a single mother cell ; among Bryophytes simple sporangia do not exist, and in connec- tion with the usually complex capsule of the sporogonium the name is dropped; but among Pteridophytes distinct sporangia again appear. They are not simple mother cells, but many-celled bodies. Their structure varies in different groups of Pteridophytes, but those of ordinary Ferns may be taken as an illustration.
The sporangia are borne by the leaves, generally upon the under surface, and are usually closely associated with the veins and organized into groups of definite form, known as sort. A sorus may be round or elongated, and is usually covered by a delicate flap (¢ndusiwm) which arises from the epidermis (Figs. 118, 123, 124). Occasionally the sori are extended along the under surface of the margin of the leaf, as in maidenhair fern (4d/antum), and the common brake (Pteris), in which case they are protected by the inrolled
Fig. 123. Fragrant shield fern (Aspid- ium fragrans), showing gencral habit, and to the left (a) the under surface of a leaflet bearing sori covered by shield-like indusia.— After MARION SATTERLEE.
Fig, 124. The bladder fern ( Cystopteris bulh- ifera), showing general habit, and to the right (a) the under surface of a leaflet, showing the dichotomons venation, and five sori protected by pouch-like indusia, —After MARION SATTERLEE.
P'TERIDOPILYTES 145
margin (Figs. 125, 126), which may be called a ‘false in- dusium.”
It is evident that such leaves are doing two distinct kinds of work—chlorophyll work and spore formation. This is true of most of the ordinary Ferns, but some of them show a tendency to di- vide the work. Certain leaves, or certain leaf-branches, pro- duce spores and do no chloro- phyll work, while others do chlorophyll work and produce no spores. This differentia- tion in the leaves or leaf-re- gions is indicated by appro- priate names. Those leaves which produce only spores are called sporophylls, meaning “spore leaves,” while the leaf branches thus set apart are called sporophyll branches.
Fic. 125. Leaflets of two common
Those leaves which only do ferns: 1, the common brake : (Pteris); B, maidenhair (Adian- chlorophyll work are called So- tm), both showing sori borne liage leaves ; and such branch- at the margin and protected by es are foliage branches. As the infolded margin, which thus 2 forms a false indusium.—CaLp-
sporophylls are not called upon weit
for chlorophyll work they often
become much modified, being much more compact, and not at all resembling the foliage leaves. Such a differentiation may be seen in the ostrich fern and sensitive fern ( Onoclea) (Figs. 127, 128), the climbing fern (Zygodium), the royal fern (Osmunda), the moonwort (Botrychium) (Fig. 129), and the adder’s tongue (Ophioglossum) (Fig. 130).
An ordinary fern sporangium consists of a slender stalk and a bulbous top which is the spore case (Fig. 118, 6). This case has a delicate wall formed of a single layer of cells, and extending around it from the stalk and nearly to
146 PLANT STRUCTURES
the stalk again, like a meridian line about a globe, is a row of peculiar cells with thick walls, forming a heavy ring, called the annulus. The annulus is hke a bent spring, and when the delicate wall becomes yielding the spring straightens violently, the wall is torn, and the spores are discharged with considerable force (Fig. 131). This dis-
Fig. 126.—The purple cliff brake (Pred atropurpurea), showing general habit, and at @a single leaflet showing the dichotomous venation and the infolded margin covering the sori.— After MARION SATTERLEE.
charge of fern spores may be seen by placing some sporangia upon a moist slide, and under a low power watching them as they dry and burst.
Within this sporangium the archesporium (sce § 66) consists of a single cell, which by division finally produces
PTERIDOPHYTES 147
numerous mother cells, in each of which a tetrad of spores is formed. The disorganization of the walls of the mother
POPES OI
DWMUHELLLLL Ey LLM AAI VA SH _esseerg OTN ae Lede WAS
pre SEPERENO NTS TPS EA 4 y
Metin
DOP DEON
C { =
| c
Fig. 127. The ostrich fern (Onoclea struthiopteris), showing differentiation of foliage leaf (a) and sporophyll (4).—After Marion SATTERLEE.
cells sets the spores free in the cavity of the sporangium, and ready for discharge.
Fie. 128. The sensitive fern (Onoclea sensibilis), showing differentiation of foliage leayes and sporophylls.—From ‘“ Field, Forest, and Wayside Flowers.”
PTERIDOPHYTES 149
Among the Bryophytes the sporogenous tissue appears very early in the development of the sporogonium, the pro- duction of spores being its only function ; also there is a
Fie. 129. A moonwort (Botrychi- um), showing the leaf differen- tiated into foliage and sporophyll branches.—After STRASBURGER.
28
Fie. 130, The adder’s tongue (Ophioglossum vulgatum), showing two leaves, each with a foliage branch anda much longer sporophyll branch.—After Marion Sat- TERLEE,
150 PLANT STRUCTURES
tendency to restrict the sporogenous tissue and increase the sterile tissue. It will be observed that with the introduc- tion of the leafy sporophyte among the Pteridophytes the sporangia appear much later in its development, sometimes not appearing for several years, as though they are of
Fic. 131. A series showing the dchiscence of a fern sporangium, the rupture of the wall, the straightening and bending back of the annulus, and the recoil.—After ATKINSON,
secondary importance as compared with chlorophyll work ; and that the sporogenous tissue is far more restricted, the sporangia forming a very small part of the bulk of the sporophyte body.
PTERIDOPHYTES 151
81. Heterospory.—This phenomenon appears first among Pteridophytes, but it is not characteristic of them, being en- tirely absent from the true Ferns, which far outnumber all other Pteridophytes. Its chief interest lies in the fact that it is universal among the Spermatophytes, and that it rep- resents the change which leads to the appearance of that high group. It is impossible to understand the greatest group of plants, therefore, without knowing something about heterospory. As it begins in simple fashion among Pteridophytes, and is probably the greatest contribution they have made to the evolution of the plant kingdom, unless it be the leafy sporophyte, it is best explained here.
In the ordinary Ferns all the spores in the sporangia are alike, and when they germinate each spore produces a prothallium upon which both antheridia and archegonia appear. It has been remarked, however, that some pro- thallia are dicecious—that is, some bear only antheridia and others bear only archegonia. In this case it is evident that the spores in the sporangium, although they may ap- pear alike, produce different kinds of prothallia, which may be called male and female, as each is distinguished by the sex organ which it produces. As archegonia are only produced by well-nourished prothallia, it seems fair to sup- pose that the larger spores will produce female prothallia, and the smaller ones male prothallia.
This condition of things seems to have developed finally into a permanent and decided difference in the size of the spores, some being quite small and others relatively large, the small ones producing male gametophytes (prothallia, with antheridia), and the large ones female gametophytes (prothallia with archegonia). When asexual spores differ thus permanently in size, and give rise to gametophytes of different sexes, we have the condition called heterospory (‘‘spores different’), and such plants are called heterospo- rous (Fig.139). In contrast with heterosporous plants, those in which the asexual spores appear alike are called homos-
152 PLANT STRUCTURES
porous, or sometimes tsosporous, both terms meaning ‘spores similar.” The corresponding noun form is homos- pory or isospory. Bryophytes and most Pteridophytes are homosporous, while some Pteridophytes and all Spermato- phytes are heterosporous.
It is convenient to distinguish by suitable names the two kinds of asexual spores produced by the sporangia of heterosporous plants (Fig. 139). The large ones are called megaspores, or by some writers macrospores, both terms meaning “large spores”; the small ones are called micro- spores, or **small spores.” It should be remembered that megaspores always produce female gametophytes, and mi- crospores male gametophytes.
This differentiation does not end with the spores, but soon involves the sporangia (Fig. 139). Some sporangia produce only megaspores, and are called megasporangiu ; others produce only microspores, and are called microspo- rangia. It is important to note that while microsporangia usually produce numerous microspores, the megasporangia produce much fewer megaspores, the tendency being to diminish the number and increase the size, until finally there are megasporangia which produce but a single large megaspore.
The differentiation goes still further. If the sporangia are born upon sporophylls, the sporophylls themselves may differentiate, some bearing only megasporangia, and others only microsporangia, the former being called megasporo- phylls, the latter microsporophylls. In such a case the sequence is as follows: megasporophylls produce megaspo- rangia, which produce megaspores, which in germination produce the female gametophytes (prothallia with archego- nia); while the microsporophylls produce microsporangia, which produce microspores, which in germination produce male gametophytes (prothallia with antheridia).
A formula may indicate the life history of a heteros- porous plant. The formula of homosporous plants with
PTERIDOPHYTES 153
alternation of generations (Bryophytes and most Pterido- phytes) was given as follows (§ 62) : Gx > 0—S—o—G==8 > o—S—o— G23 > 0—n, ete.
In the case of heterosporous plants (some Pteridophytes and all Spermatophytes) it would be modified as follows :
G_-0 > 0S §—8 > 0- AGT i=3 > 04, ete.
In this case two gametophytes are involved, one pro- ducing a sperm, the other an egg, which fuse and form the oospore, which in germination produces the sporophyte, which produces two kinds of asexual spores (megaspores and microspores), which in germination produce the two gametophytes again.
One additional fact connected with heterospory should be mentioned, and that is the great reduction of the gam- etophyte. In the homosporous ferns the spore develops a small but free and independent prothallium which pro- duces both sex organs. When in heterosporous plants this work of producing sex organs is divided between two gam- etophytes they become very much reduced in size and lose their freedom and independence. They are so small that they do not escape entirely, if at all, from the embrace of the spores which produce them, and are mainly dependent for their nourishment upon the food stored up in the spores (Figs. 140, 141). As the spore is produced by the sporo- phyte, heterospory brings about a condition in which the gametophyte is dependent upon the sporophyte, an exact reversal of the condition in Bryophytes.
The relative importance of the gametophyte and the sporophyte throughout the plant kingdom may be roughly indicated by the accompanying diagram, in which the
c Ss
shaded part of the parallelogram represents the gameto- phyte and the unshaded part the sporophyte. Among the
154 PLANT STRUCTURES
lowest plants the gametophyte is represented by the whole plant structure. When the sporophyte first appears it is dependent upon the gametophyte (some Thallophytes and the Bryophytes), and is relatively inconspicuous. Later the sporophyte becomes independent (most Pteridophytes), the gametophyte being relatively inconspicuous. Finally (heterosporous Pteridophytes) the gametophyte becomes dependent upon the sporophyte, and in Spermatophytes is so inconspicuous and concealed that it is only observed by means of laboratory appliances, while the sporophyte is the whole plant of ordinary observation.
CHAPTER X
THE GREAT GROUPS OF PTERIDOPHYTES
82. The great groups—At least three independent lines of Pteridophytes are recognized: (1) filicales (Ferns), (2) Equisetales (Scouring rushes, Horsetails), and (3) Ly- copodiales (Club-mosses). The Ferns are much the most abundant, the Club-mosses are represented by a few hun- dred forms, while the Horsetails include only about twenty- five species. These three great groups are so unlike that they hardly seem to belong together in the same division of the plant kingdom.
FrnicaLes (/erns)
83. General characters—The Ferns were used in the preceding chapter as types of Pteridophytes, so that little need be added. They well deserve to stand as types, as they contain about four thousand of the four thousand five hundred species belonging to Pteridophytes. Although found in considerable numbers in temperate regions, their chief display is in the tropics, where they form a striking and characteristic feature of the vegetation. In the trop- ics not only are great masses of the low forms to be seen, from those with delicate and filmy moss like leaves to those with huge leaves, but also tree forms with cylindrical trunks encased by the rough remnants of fallen leaves and sometimes rising to a height of thirty-five to forty-five feet, with a great crown of leaves fifteen to twenty feet long (Fig. 120).
155
‘TITMAIVO —"(punru 2Uojhv)/) DpunuseE) surdj Jo yueq vy" EE OL
THE GREAT GROUPS OF PTERIDOPHYTES 157
There are also epiphytic forms (air plants)—that is, those which perch “upon other plants” but derive no nourishment from them (Fig. 112). This habit belongs chiefly to the warm and moist tropics, where the plants can absorb sufficient moisture from the air without send- ing roots into the soil. In this way many of the tropical ferns are found growing upon living and dead trees and other plants. In the temperate regions the chief epi- phytes are Lichens, Liverworts, and Mosses, the Ferns be- ing chiefly found in moist woods and ravines (Fig. 132), although a number grow in comparatively dry and exposed situations, sometimes covering extensive areas, as the com- mon brake (Péeris) (Fig. 125).
The Filicales differ from the other groups of Pterido- phytes chiefly in having few large leaves, which do chloro- phyll work and bear sporangia. In a few of them there isa differentiation of functions in foliage branches and sporo- phyll branches (Figs. 127-180), but even this is excep- tional. Another distinction is that the stems are un- branched.
84. Origin of sporangia—An important feature in the Ferns is the origin of the sporangia. In some of them a sporangium is developed from a single epidermal cell of the leaf, and is an entirely superficial and generally stalked affair (Fig. 118, 5) ; in others the sporangium in its devel- opment involves several epidermal and deeper cells of the leaf, and is more or less of an imbedded affair. In the first case the ferns are said to be leptosporangiate ; in the sec- ond case they are eusporangiate.
The leptosporangiate Ferns are overwhelmingly abun- dant as compared with the Eusporangiates. Back in the Coal-measures, however, there was an abundant fern vege- tation which was probably all eusporangiate. The Lep- tosporangiates seem to be the modern Ferns, the once abundant Eusporangiates being represented now in the temperate regions only by such forms as moonwort (Bo-
158 PLANT STRUCTURES
trychium) (Fig. 129) and adder’s tongue (Ophioglossum) (Fig. 130). It is important to note, however, that the Horsetails and Club-mosses are Eusporangiates, as well as all the Seed-plants.
Another small but interesting group of Ferns includes the ‘‘ Water-ferns,” floating forms or sometimes on muddy flats. The common Jursilia may be taken as a type (Fig. 133). The slender creeping stem sends down numerous roots into the mucky soil, and at intervals gives rise to a comparatively large leaf. This leaf has a long erect petiole and a blade of four spread-
Fig. 133.—A water-fern (Marsilia), Fie. 184, One of the floating water-ferns (Sa?-
showing horizontal stem, with vinia), showing side view (4) and view from descending roots, and ascend- above (B). The dangling root-like processes ing leaves; a, a young leaf are the modified submerged leaves. In A, showing circinate vernation ; near the top of the cluster of submerged 8,8, 8porophyl]l branches (‘‘spo- leaves, some sporophyll branches (‘ sporo- rocarps”"’).—After BIscHorr. carps’’) may be seen.—After BiscHorF.
ing wedge-shaped leaflets like a “ four-leaved clover."’ The dichotomous venation and circinate vernation at once sug- gest the fern alliance. From near the base of the petiole
THE GREAT GROUPS OF PTERIDOPHYTES 159
another leaf branch arises, in which the blade is modified as a sporophyll. In this case the sporophyll incloses the sporangia and becomes hard and nut-like. Another com- mon form is the floating Salvinia (Fig. 134). The chief interest lies in the fact that the water-ferns are heteros- porous. As they are leptosporangiate they are thought to have been derived from the ordinary leptosporangiate Ferns, which are homosporous.
Three fern groups are thus outlined: (1) homosporous- eusporangiate forms, now almost extinct ; (2) homosporous- leptosporangiate forms, the great overwhelming modern group, not only of Filicales but also of Pteridophytes, well called true Ferns, and thought to be derived from the pre- ceding group; and (3) heterosporous-leptosporangiate forms, the water-ferns, thought to be derived from the pre- ceding group.
EQuiseraues (Horsetatls or Scouring rushes)
85. General characters—The twenty-five forms now rep- resenting this great group belong to a single genus (Aquwise- tum, meaning ‘‘horsetail”), but they are but the linger- ing remnants of an abundant flora which lived in the time of the Coal-measures, and helped to form the forest vegeta- tion. The living forms are small and inconspicuous, but very characteristic in appearance. They grow in moist or dry places, sometimes in great abundance (Fig. 135).
The stem is slender and conspicuously jointed, the joints separating easily; it is also green and fluted with small longitudinal ridges ; and there is such an abundant deposit of silica in the epidermis that the plants feel rough. This last property suggested its former use in scouring, and its name ‘‘scouring rush.” At each joint is a sheath of minute leaves, more or less coalesced, the individual leaves some- times being indicated only by minute teeth. This arrange- ment of leaves in a circle about the joint is called the cyclic
Fie. 135. Equisetum arvense, a common horsetail: 1, three fertile shoots rising from the dorsiventral stem, showing the cycles of coalesced scale-leaves at the joints and the terminal strobili with numerous sporophylls, that at a@ being mature; 2, a sterile shoot from the same stem, showing branching; 3, a single peltate sporo- phyll bearing sporangia; 4, view of sporophyll from beneath, show ing dehiscence of sporangia; 5, 6, 7, spores, showing the unwinding of the outer coat, which aids in dispersal.—After WossIDLo,
THE GREAT GROUPS OF PTERIDOPIHYTES 161
arrangement, or sometimes the whorled arrangement, each such set of leaves being called a cycle or a whorl. These leaves contain no chlorophyll and have evidently abandoned chlorophyll work, which is carried on by the green stem. Such leaves are known as scales, to distinguish them from foliage leaves. The stem is either simple or profusely branched (Fig. 135).
st. The strobilus—One of the distinguishing characters of the group is that chlorophyll-work and spore-formation are completely differentiated. Although the foliage leaves
Fie. 136. Dicecious gametophytes of guisetum: A. the female gametophyte, show- ing branching, rhizoids. and an archegonium (ar); B, the male gametophyte, showing several antheridia ( 2 ).—After CAMPBELL.
are reduced to scales, and the chlorophyll-work is done by the stem, there are well-organized sporophylls. The sporo- phylls are grouped close together at the end of the stem in a compact conical cluster which is called a strodilus, the Latin name for “pine cone,” which this cluster of sporo- phylls resembles (Fig. 135).
Each sporophyll consists of a stalk-like portion and a shield-like (peltate) top. Beneath the shield hang the
162 PLANT STRUCTURES
sporangia, which produce spores of but one kind, hence these plants are homosporous ; and as the sporangia origi- nate in eusporangiate fashion, Lyuisetum has the homospo- rous-eusporangiate combination shown by one of the Fern groups. It is interesting to know, however, that some of the ancient, more highly organized members of this group were heterosporous, and that the present forms have dicecious gametophytes (Fig. 136).
LycopopiaLeEs (('Jub-mosses)
87. General characters—This group is now represented by about five hundred species, most of which belong to the two genera Lycopodium and Selaginella, the latter being much the larger genus. The plants have slender, branching, prostrate, or erect stems completely clothed with small foliage leaves, having a general moss-like appearance (Fig. 137). Often the erect branches are terminated by conspicuous conical or cylindrical strobili, which are the ‘‘ clubs” that enter into the name ‘ Club- mosses.” There is also a certain kind of resemblance to miniature pines, so that the name ‘‘ Ground-pines” is sometimes used.
Lycopodiales were once much more abundant than now, and more highly organized, forming a conspicuous part of the forest vegetation of the Coal-measures.
One of the distinguishing marks of the group is that the sperm does not resemble that of the other Pteridophytes, but is of the Bryophyte type (Fig. 140, 7’). That is, it consists of a small body with two cilia, instead of a large spirally coiled body with many cilia. Another distinguish- ing character is that there is but a single sporangium pro- duced by each sporophyll (Fig. 137). This is in marked contrast with the Filicales, whose leaves bear very numer- ous sporangia, and with the Equisetales, whose sporophylls bear several sporangia.
THE GREAT GROUPS OF PTERIDOPHYTES 163
Fic. 187. A common club-moss (Lycopodium clavatum): 1, the whole plant, showing horizontal stem giving rise to roots and to erect branches bearing strobili; 2, a single sporophyll with its sporanginm; 3, spores, much magnified.—After Wos- SIDLO.
88. Lycopodium.—This genus contains fewer forms than the other, but they are larger and coarser and more charac- teristic of the temperate regions, being the ordinary Club- mosses (Fig. 137). They also more commonly display conspicuous and distinct strobili, although there is every
164 PLANT STRUCTURES
gradation between ordinary foliage leaves and distinct sporophylls.
The sporangia are borne either by distinct sporophyils or by the ordinary foliage leaves near the summit of the stem. At the base of each of these leaves, or sporophylls, on the upper side, is a single sporangium (Fig. 137). The sporangia are eusporangiate in origin, and as the spores are all alike, Lycopodium has the same homosporous-eusporan- giate combination noted in Equisetales and in one of the groups of Filicales.
89. Selaginella—This large genus contains the smaller, more delicate Club-mosses, often being called the “ little Club-mosses.” They are especially displayed in the trop-
Fie. 188, Selaginelia, showing general spray-like habit, and dangling leafless stems which strike root (rhizophores).—From “ Plant Relations.”
ics, and are common in greenhouses as delicate, mossy, decorative plants (Fig. 138). In general the sporophylls are not different from the ordinary leaves (Fig. 139), but sometimes they are modified, though not so distinct as in certain species of Lycopodium.
THE GREAT GROUPS OF PTERIDOPHYTES 165
The solitary sporangium appears in the ails (upper angles formed by the leaves with the stem) of the leaves and sporophylls, but arise from the stem instead of the
Fic. 189. Selaginella Martensii; A, branch bearing strobili; B, a microsporophy]] with a microsporangium, showing microspores through a rupture in the wall; C, a megasporophyll with a megasporangium; J, megaspores; Z, microspores.— CALDWELL.
29
166 PLANT STRUCTURES
leaf (Fig. 139). This is important as showing that sporan- gia may be produced by stems as well as by leaves, those being produced by leaves being called foliar, and those by stem cauline.
The most important fact in connection with Selaginella, however, is that it is heterosporous. Megasporangia, each usually containing but four megaspores, are found in the axils of a few of the lower leaves of the strobilus, and more numerous microsporangia occur in the upper axils, con- taining very many microspores (Fig. 139). The character of the gametophytes of heterosporous Pteridophytes may be well illustrated by those of Selaginella.
The microspore germinates and forms a male gameto- phyte so small that it is entirely included within the spore
Fig. 140 Male gametophyte of Selaginella; in each case p is the prothallial cell, w the wall cells of the antheridium, s the sperm tissue: /, the bicilfate sperms.— After BELAJEFF.
wall (Fig. 140). A single small cell is all that represents the ordinary cells of the prothallium, while all the rest is an antheridium, consisting of a wall of a few cells sur- rounding numerous sperm mother cells. In the presence
THE GREAT GROUPS OF PTERIDOPHYTES 167
of water the antheridium wall breaks down, as also do the walls of the mother cells, and the small biciliate sperms are set free.
The much larger megaspores germinate and become filled with a mass of numerous nutritive cells, representing the ordinary cells of a prothallium (Fig. 141). The spore wall is broken by this growing prothallium, a part of which thus protrudes and becomes exposed, although the main part of it is still invested by the old megaspore wall. In this exposed portion of the female gameto- phyte the archegonia appear, and thus be- come accessible to the sperms. In the case of Isoetes (see § 90) the reduction of the female gametophyte is even greater, as it does not project from the megaspore wall at all, and the archegonia are made accessible through cracks in the wall immediately over
Fie. 141. Female gametophyte of a Selaginella: spm, wall of megaspore ; pr, gametophyte;
them. ar, an archegonium; emd, and eh, em- The embryo of Se- bryo sporophytes ; «f, suspensors ; the gam- 5 a a etophyte bas developed a few rhizoids,— laginella is also impor- ‘After PrnkeR:
tant to consider. Be-
ginning its development in the venter of the archegonium, it first hes upon the exposed margin of the prothallium, while the mass of nutritive cells lie deep within the mega- spore (Fig. 141, emd,, emb,). It first develops an elongated cell, or row of cells, which thrusts the embryo cell deeper among the nutritive cells. This cell or row of cells, formed by the embryo to place the real embryo cell in better rela-
168 PLANT STRUCTURES
tion to its food supply, is called the suspensor, and is a temporary organ of the embryo (Figs. 141, 142, et). At the end of the suspensor the real embryo develops, and when its regions become organized it shows the following parts: (1) a large foot buried among the nutritive cells of the prothallium and absorbing nourishment; (2) a root stretching out toward the substratum ; (3) a stem extend-
C3 eae, Siisttingr nme eee OT LL
oe, a ee r
Fig. 142. Embryo of Selaginella removed from the gametophyte, showing suspensor (et), root-tip (w), foot (f), cotyledons (0/), stem-tip (s¢), and ligules (/ég).—After PFEFFER.
ing in the other direction, and bearing just behind its tip (4) a pair of opposite leaves (cotyledons) (Fig. 142).
As the sporangia of Selayinella are eusporangiate, this genus has the heterosporous-eusporangiate combination—a combination not mentioned heretofore, and being of special interest as it is the combination which belongs to all the Spermatophytes. For this and other reasons, Selaginella is one of the Pteridophyte forms which has attracted special attention, as possibly representing one of the an- cestral forms of the Seed-plants.
THE GREAT GRUUPS OF PTERIDOPHYTES 169
90. Isoetes—This little group of aquatic plants, known as “‘quillworts,” is very puzzling as to its relationships among Pteridophytes. By some it is put with the Ferns, forming a distinct division of Filicales ; by others it is put
\
!
TA cr i i e '
AS
I fe RAYA Fig. 143. A common quillwort (Isoetes lacus- Fie. 144. Sperm of Isoetes, show- tris), showing cluster of roots dichoto- ing spiral body and seven long mously branching, and cluster of leaves cilia arising from the beak.— each enlarged at base and inclosing a sin- After BELAJEFF.
gle sporangium.—After ScHENCK.
with the Club-mosses, and is associated with Selayinella. It resembles a bunch of fine grass growing in shoal water or in mud, but the leaves enlarge at the base and overlap one another and the very short tuberous stem (Fig. 143). Within each enlarged leaf base a single sporangium is formed, and the cluster contains both megasporangia and microsporangia. The sporangia are eusporangiate, and therefore Jscetes shares with Selaginella the distinction of
170 PLANT STRUCTURES
having the heterosporous-eusporangiate combination, which is a feature of the Seed-plants.
The embryo is also peculiar, and is so suggestive of the embryo of the Monocotyledons (see § 114) among Seed- plants that some regard it as possibly representing the ancestral forms of that group of Spermatophytes. The peculiarity lies in the fact that at one end of the axis of the embryo is a root, and at the other the first leaf (cotyledon), while the stem tip rises as a lateral outgrowth. This is exactly the distinctive feature of the embryo of Monocoty- ledons.
The greatest obstacle in the way of associating these quillworts with the Club-mosses is the fact that their sperms are of the large and spirally coiled multiciliate type which belongs to Filicales and Equisetales (Fig. 144), and not at all the small bicihate type which characterizes the Club- mosses (Fig. 140). To sum up, the short unbranched stem with comparatively few large leaves, and the coiled multi- ciliate sperm, suggest Filicales; while the solitary spo- rangia and the heterosporous-eusporangiate character sug- gest Selaginella.
CHAPTER XI SPERMATOPHYTES: GYMNOSPERMS
91. Summary from Pteridophytes.—In considering the important contributions of Pteridophytes to the evolution of the plant kingdom the following seem worthy of note :
(1) Prominence of sporophyte and development of vuscu- lar system.—This prominence is associated with the display of leaves for chlorophyll work, and the leaves necessitate the work of conduction, which is arranged for by the vas- cular system. This fact is true of the whole group.
(2) Differentiation of sporuphylls.—The appearance of sporophylls as distinct from foliage leaves, and their or- ganization into the cluster known as the strobilus, are facts of prime importance. This differentiation appears more or less in all the great groups, but the strobilus is distinct only in Horsetails and Club-mosses.
(3) Introduction of heterospory and reduction of gameto- phytes.—Heterospory appears independently in all of the three great groups—in the water-ferns among the Fili- cales, in the ancient horsetails among the Equisetales, and in Seluginella and Jsoetes among Lycopodiales. All the other Pteridophytes, and therefore the great majority of them, are homosporous. The importance of the appear- ance of heterospory lies in the fact that it leads to the development of Spermatophytes, and associated with it is a great reduction of the gametophytes, which project little, if at all, from the spores which produce them.
92. Summary of the four groups—It may be well in this
connection to give certain prominent characters which will 171
172 PLANT STRUCTURES
serve to distinguish the four great groups of plants. It must not be supposed that these are the only characters, or even the most important ones in every case, but they are convenient for our purpose. Two characters are given for each of the first three groups—one a positive character which belongs to it, the other a negative character which distinguishes it from the group above, and becomes the positive character of that group.
(1) Thallophytes.—Thallus body, but no archegonia.
(2) Bryophytes.—Archegonia, but no vascular system.
(3) Pteridephytes.—Vascular system, but no seeds.
(+) Spermatophytes.—Seeds.
93. General characters of Spermatophytes.—This is the greatest group of plants in rank and in display. So con- spicuous are they, and so much do they enter into our experience. that they have often been studied as ‘‘ botany,” to the exclusion of the other groups. The lower groups are not meiely necessary to fill out any general view of the plant kingdom, but they are absolutely essential to an understanding of the structures of the highest group.
This great dominant group has received a variety of names. Sometimes they are called .{xthophytes, meaning “Flowering plants,” with the idea that they are distin- guished by the production of “flowers.” A flower is diffi- cult to define, but in the popular sense all Spermatophytes do not produce flowers, while in another sense the strobilus of Pteridophytes is a flower. Hence the flower does not accurately limit the group, and the name Anthophytes is not in general use. Much more commonly the group is called Phanerogams (sometimes corrupted into Phenogams or even Phenogams), meaning “evident sexual reproduc- tion.” At the time this name was proposed all the other groups were called Crypfogams, meaning “hidden sexual reproduction.” It is a curious fact that the names ought to have been reversed, for sexual reproduction is much more evident in Cryptogams than in Phanerogams, the mistake
SPERMATOPHYTES: GYMNOSPERMS 1%3
arising from the fact that what were supposed to be sexual organs in Phanerogams have proved not to be such. The name Phanerogam, therefore, is being generally abandoned ; but the name Cryptogam is a useful one when the lower groups are to be referred to; and the Pteridophytes are still very frequently called the Vascular Cryptogams. The most distinguishing mark of the group seems to be the production of seeds, and hence the name Spermatophytes, or ** Seed-plants,” is coming into general use.
The seed can be better defined after its development has been described, but it results from the fact that in this group the single megaspore is never discharged from its megasporangium, but germinates just where it is devel- oped. The great fact connected with the group, therefore, is the retention of the megaspore, which results in a seed. The full meaning of this will appear later.
There are two very independent lines of Seed-plants, the Gymnosperms and the Angiosperms. The first name means ‘‘ naked seeds,” referring to the fact that the seeds are always exposed; the second means ‘‘inclosed seeds,” as the seeds are inclosed in a seed vessel.
GYMNOSPERMS
94. General characters——The most familiar Gymnosperms in temperate regions are the pines, spruces, hemlocks, cedars, etc., the group so commonly called ‘‘ evergreens.” It is an ancient tree group, for its representatives were associated with the giant club-mosses and horsetails in the forest vegetation of the Coal-measures. Only about four hundred species exist to-day as a remnant of its for- mer display, although the pines still form extensive forests. The group is so diversified in its structure that all forms can not be included in a single description. The common pine (Pinus), therefore, will be taken as a type, to show the general Gymnosperm character.
174 PLANT STRUCTURES
95, The plant body.—The great body of the plant, often forming a large tree, is the sporophyte; in fact, the gametophytes are not visible to ordinary observation. It should be remembered that the sporophyte is distinctly a sexless generation, and that it develops no sex organs. This great sporophyte body is elaborately organized for nutritive work, with its roots, stems, and leaves. These organs are very complex in structure, being made up of various tissue systems that are organized for special kinds of work. The leaves are the most variable organs, being differentiated into three distinct kinds—(1) foliage leaves, (2) scales, and (3) sporophylls.
96. Sporophylls—The sporophylls are leaves set apart to produce sporangia, and in the pine they are arranged in a strobilus, as in the Horsetails and Club-mosses. As the group is heterosporous, however, there are two kinds of sporophylls and two kinds of strobili. One kind of strobilus is made up of megasporophylls bearing mega- sporangia ; the other is made up of microsporophylls bear- ing microsporangia. These strobili are often spoken of as the “‘ flowers” of the pine, but if these are flowers, so are the strobili of Horsetails and Club-mosses.
97. Microsporophylls—In the pines the strobilus com- posed of microsporophylls is comparatively small (Figs. 145, /, 164). Each sporophyll is like a scale leaf, is nar- rowed at the base, and upon the lower surface are borne two prominent sporangia, which of course are microspo- rangia, and contain microspores (Fig. 146).
These structures of Need-plants all received names before they were identified with the corresponding struc- tures of the lower groups. The microsporophyll was called a stumen, the microsporangia pollen-saes, and the microspores pollen grains, or simply pollen, These names are still very convenient to use in connection with the Spermatophytes, but it should be remembered that they are simply other names for structures found in the lower groups.
Fie. 145.
Pinus Laricio, showing tip of branch bearing needle-leaves, scale-leaves, and cones (strobili): @, very young carpellate cones. at time of pollination, borne at tip of the young shoot upon which new leaves are appearing; 5, carpellate cones one year old; ¢, carpellate cones two years old, the scales spreading and shedding the seeds; d, young shoot bearing a cluster of staminate cones.—CaLDWELL.
176 PLANT STRUCTURES
The strobilus composed of microsporophylls may be called the staminate strobilus—that is, one composed of stamens; it is often called the staminate cone, “cone” being the English translation of the word “strobilus.” Frequently the staminate cone is spoken of as the ‘‘ male cone,” as it was once supposed that the stamen is the
Fic. 146. Staminate cone (strobilus) of pine (Pinus): A, section of cone, showing microsporophylls (stamens) bearing microsporangia; B, longitudinal section of a single stamen, showing the large sporangium beneath ; (C, cross-section of a sta- men, showing the two sporangia; D, a single microspore (pollen grain) much en- larged, showing the two wings, and a male gametophyte of two cells, the lower and larger (wall cell) developing the pollen tube, the upper and smaller (genera- tive cell) giving rise to the sperms.—After SciimMPER.
male organ. This name should, of course, be abandoned, as the stamen is now known to be a microsporophyll, which is an organ produced by the sporophyte, which never pro- duces sex organs. It should be borne distinctly in mind that the stamen is not a sex organ, for the literature of botany is full of this old assumption, and the beginner is in
SPERMATOPHYTES: GYMNOSPERMS 7%
danger of becoming confused and of forgetting that pollen grains are asexual spores.
98. Megasporophylls—The strobili composed of mega- sporophylls become much larger than the others, forming
Fig. 147. Pinus sylvestris, showing mature cone partly sectioned, and showing car- pels (sg, sg}, sg?) with seeds in their axils (g), in which the embryos (e771) may be distinguished; 1, a young carpel with two megaspcrangia; B, an old carpel with mature seeds (ch), the micropyle being below (J/).—After BEssEY.
the well-known cones so characteristic of pines and their allies (Figs. 145, a, 6, c, 163). Each sporophyll is some- what leaf-like, and at its base upon the upper side are two megasporangia (Fig. 147). It is these sporangia which are peculiar in each producing and retaining a solitary large megaspore. This megaspore resembles a sac-like cavity in
178 PLANT STRUCTURES
the body of the sporangium (Fig. 148, 7), and was at first not recognized as being a spore.
These structures had also received names before they were identified with the corresponding structures of the lower groups. The megasporophyll was called a carpel, the megasporangia ovules, and the megaspore an embryo- sac, because the young embryo was observed to develop within it (Fig. 147, em).
The strobilus of megasporophylls, therefore, may be ealled the carpellate strobilus or curpellate cone. As the carpel enters into the organization of a structure known as the pistil, to be described later, the cone is often called the pistillate cone, As the staminate cone is sometimes wrongly called a ‘*male cone,” so the carpellate cone is wrongly called a ‘‘female cone.” the old idea being that the carpel with its ovules represented the female scx organ.
The structure of the megaspo- rangium, or ovule, must be known. The main body is the nucellus (Figs. 148, ¢, 149, nc); this sends out from near its base an outer membrane (ufegument) which is distinct above (Figs. 148 6, 149 7), covering the main part of the nucellus and projecting seetie: Beae arate beyond its apex as a prominent neck,
carpel structures of pine, the passage through which to the apex showin the heavy seale of the nucellus is called the micropyle (A) which Dears the a = A ovule CB). inwhich ame (Little. gate) (Mie. 44s; @). Oeus seen the micropyle «@, {rally placed within the body of the integument (/), nncellus Fi : (©), embryo sue or megn. TUCellus is the conspicuous cavity spore ().—CarpwetL. called the embryo-sac (Fig. 148, «/), in reality the retained megaspore. The relations between integument, micropyle, nucellus, and embryo-sac should be kept clearly in mind. In the
SPERMATOPHYTES: GYMNOSPERMS 179
pine the micropyle is directed downward, toward the base of the sporophyll (Figs. 147, 148).
99. Female gametophyte—The female gametophyte is always produced by the germination of a megaspore, and
therefore it should be produced by the sc- called embryo-sae with- in the ovule. This im- bedded megaspore ger- minates, just as does the megaspore of v- laginella or Isoetes, by cell division becoming filled with a compact mass of nutritive tissue representing the ordi- nary cells of the female prothallium (Fig. 149, e). This prothallium naturally does not protrude beyond the boundary of the mega- spore wall, beg com- pletely surrounded by the tissues of the sporangium. It must be evident that this gametophyte is abso- lutely dependent upon the sporophyte for its nutrition, and remains not merely attached to it, but is actually im- bedded within its tis-
Fie. 149. Diagrammatic section through ovule (megasporangium) of spruce (Picea), showing integument (i), nucellus (7c), endosperm or female gametophyte (e) which fills the large megaspore imbedded in the nucellus, two archegonia (@) with short neck (¢) and. venter containing the egg (0), and position of ger- minating pollen grains or microspores (p) whose tubes (f) penetrate the nucellus tissue and reach the archegonia,—Aftet “i HIIPER.
sues like an internal parasite. So conspicuous a tissue within the ovule, as well as in the seed into which the
180 PLANT STRUCTURES
ovule develops, did not escape early attention, and it was called endosperm, meaning ‘‘ within the seed.” The endo- sperm of Gymnosperms, therefore, is the female gameto- phyte.
At the margin of the endosperm nearest the micropyle regular flask-shaped archegonia are developed (Fig. 149, a), making it sure that the endosperm is a female gameto- phyte. It is evident that the necks of these archegonia (Fig. 149, c) are shut away from the approach of sperms by swimming, and that some new method of approach must be developed.
100. Male gametophyte.—The microspores are developed in the sporangium in the usual tetrad fashion, and are pro- duced and scattered in very great abundance. It will be remembered that the male gametophyte developed by the microspore of Selaginella is contained entirely within the spore, and consists of a single ordinary prothallial cell and one antheridium (see § 89). In the pine it is no bet- ter developed. One or two small cells appear, which may be regarded as representing prothallial cells, while the rest of the gametophyte seems to be a single antheridium (Fig. 146, D). At first this antheridium seems to consist of a large cell called the wal? cell, and a small one called the generative cell. Sooner or later the generative cell divides and forms two small cells, one of which divides again and forms two cells called mule cells. which seem to represent the sperm mother cells of lower plants. The three active cells of the completed antheridium, therefore, are the wall cell, with a prominent nucleus, and two small male cells which are free in the large wall cell.
These sperm mother cells (male cells) do not form sperms within them, as there is no water connection be- tween them and the archegonia, and a new method of transfer is provided. This is done by the wall cell, which develops a tube, known as the podlen-fube. Into this tube the male cells enter, and as it penetrates among the cells
SPERMATOPHYTES: GYMNOSPERMS 181
which shut off the archegonia it carries the male cells along, and so they are brought to the archegonia (Fig. 150).
Fic. 150. Tip of pollen tube of pine, Fic. 151. Pollen tube passing through the
showing the two male cells (4, B), neck of an archegonium of spruce (Picea), two nuclei ((’) which accompany and containing near its tip the two male them, and the numerous food nuclei, which are to be discharged into the granules (D): the tip of the tube egg whose cytoplasm the tube is just en- is just about to enter the neck of tering.—After STRASBURGER.
the archegonium,—CaLDWELL.
101. Fertilization.—Before fertilization can take place the pollen-grains (microspores) must be brought as near as possible to the female gametophyte with its archegonia. The spores are formed in very great abundance, are dry and powdery, and are scattered far and wide by the wind. In the pines and their allies the pollen-grains are winged (Fig. 146, D), so that they are well organized for wind dis- tribution. This transfer of pollen is called pollination, and those plants that use the wind as an agent of transfer are said to be anemophilous, or ‘* wind-loving.”
The pollen must reach the ovule, and to insure this it must fall like rain. To aid in catching the falling pollen the scale-like carpels of the cone spread apart, the pollen
grains slide down their sloping surfaces and collect in a 30
182 PLANT STRUCTURES
little drift at the bottom of each carpel, where the ovules are found (Fig. 147, A, B). The flaring lips of the micro- pyle roll inward and outward as they are dry or moist, and by this motion some of the pollen-grains are caught and pressed down upon the apex of the nucellus.
In this position the pollen-tube develops, crowds its way among the cells of the nucellus, reaches the wall of the embryo-sac, and penetrating that, reaches the necks of the archegonia (Fig. 149, p, ¢); crowding into them (Fig. 151), the tip of the tube opens, the male cells are
Fie. 152. Fertilization in spruce (Picea): B is an egg, in the tip of which a pollen tube (p) has entered and has discharged into the cytoplasm a male nucleus (sz), which is to unite with the egg (female) nucleus (on); C, a later stage in which the two nuclei are uniting.—After ScHIMPER.
discharged, one male cell fuses with the egg (Fig. 152), and fertilization is accomplished, an oospore being formed in the venter of the archegonium.
It will be noticed that the cell which acts as a male gamete is really the sperm mother cell, which does not organize a sperm in the absence of a water connection. This peculiar method of transferring the male cells by means of a special tube developed by the antheridium is
SPERMATOPHYTES: GYMNOSPERMS 183
called siphonogamy, which means “sexual reproduction by means of a tube.” So important is this character among Spermatophytes that some have proposed to call the group Siphonogams.
102. Development of the embryo—The oospore when formed lies at the surface of the endosperm (female gameto- phyte) nearest to the micropyle. As the endosperm is to supply nourishment to the em- bryo, this position is not the most favorable. Therefore, as in Selaginella, the oospore first develops a suspensor, which in pine and its allies becomes very long and often tortuous (Fig. 153, d,s), At the tip of the suspensor the cell or cells (em- bryo cells) which are to develop the embryo are carried (Fig. 153, dA, ka), and thus become deeply buried, about centrally placed, in the endosperm.
Fie. 153. Embryos of pine: A,
Several suspensors may start very young embryos (ka) at the
from as many archegonia in the tips of long and contorted sus-
pensors (s); B, older embryo,
same ovule, and several embryos showing attachment to snspen-
may begin to develop, but as a sor (x), the extensive root sheath
I A d th (wh), root tip (ws), stem tip
rule only one survives, an e Go) anil eoigledons qi Aten solitary completed embryo (Fig. STRASBURGER.
153, B) les centrally imbedded
in the endosperm (Fig. 153). The development of more than one embryo in a megasporangium (ovule)is called polyembryony, a phenomenon natural to Gymnosperms with their several archegonia upon a single gametophyte.
103. The seed.—While the embryo is developing some important changes are taking place in the ovule outside of the endosperm. The most noteworthy is the change which transforms the integument into a hard bony covering,
184 PLANT STRUCTURES
known as the seed coat, or testa (Fig. 153a). The devel- opment of this testa hermetically seals the structures with- in, further development and activity are checked, and the living cells pass into the resting condition. This pro- Fie. 158. Pine seca, tected structure with its dormant cells is the seed.
In a certain sense the seed is a transformed ovule (mega-
sporangium), but this is true only as to its outer configura-
Fra. 154. Pine seedlings, showing the long hypocotyl and the numerous cotyledons, with the old seed case still attached.—After ATKINSON,
SPERMATOPHYTES: GYMNOSPERMS 185
tion. If the internal structures be considered it is much more. It is made up of structures belonging to three gen- erations, as follows: (1) The old sporophyte is represented by seed coat and nucellus, (2) the endosperm is a gameto- phyte, while (3) the embryo is a young sporophyte. It can hardly be said that the seed is simple in structure, or that any real conception of it can be obtained without approach- ing it by way of the lower groups.
The organization of the seed checks the growth of the embryo, and this development within the seed is known as
Fie. 155. A cycad, showing the palm-like habit. with much branched leaves and scaly stem.—From “ Plant Relations.”
the intra-seminal development. In this condition the em- bryo may continue for a very long time, and it is.a ques- tion whether it is death or suspended animation. Is a seed alive ? is not an easy question to answer, for it may be kept in a dried-out condition for years, and then when placed in suitable conditions awaken and put forth a living plant.
“TIEAMMTVY Q— SOA RO] asBI[OJ plo Surpwaids Lpopra oy} ase asoyy MOTaq pur ‘png Ul Wey paiaaod YOT][ i S9avol o[VOS OY} 2B MOL ‘soALT BBBI[OS Sunod JU Lo}shfd Joo APAWoU otf} SE solos oY] UT “Ulays oy] JO JLUITUNS oY] 7B oStIOF OY BuULMOYS “Ysjouas svg “9ST “OTA
SPERMATOPHYTES: GYMNOSPERMS 187
This ‘‘ awakening ” of the seed is spoken of as its ‘‘ ger- mination,” but this must not be confused with the germi- nation of a spore, which is real germination. In the case of the seed an oospore has germinated and formed an embryo, which stops growing for a time, and then resumes it. This resumption of growth is not germination, but is what
Fie. 157. Tip of pollen tube of Cycas revoluta, containing the two spiral, multiciliate sperms.—After IKENO.
happens when a seed is said to ‘‘ germinate.” This second period of development is known as the exfra-seminal, for it is inaugurated by the escape of the sporophyte from the seed (Fig. 154).
104. The great groups of Gymnosperms.—There are at least four living groups of Gymnosperms, and two or three
Fig. 158. A pine (Pinus) showing the central shaft and also the bunching of the needle leaves toward the tips of the branches.—From “ Plant Relations.”
SPERMATOPHYTES: GYMNOSPERMS 189
extinct ones. The groups differ so widely from one an- other in habit as to show that Gymnosperms can be very much diversified. They are all woody forms, but they may be trailing or straggling shrubs, gigantic trees, or high-climbing vines ; and their leaves may be nee- dle-like, broad, or ‘‘ fern- like.” For our purpose it will be only necessary to define the two most prom- inent groups.
105. Cycads, — Cycads are tropical, fern - like forms, with large branched (compound) leaves. The stem is either a columnar shaft crowned with a ro- sette of great branching leaves, with the general habit of tree-ferns and palms (Figs. 155, 156); or they are like great tu- bers, crowned in the same way. In ancient times (the Mesozoic) they were very abundant, forming a conspicuous feature of the vegetation, but now they are represented only by about eighty forms scattered through both the oriental and occiden- tal tropics. Fie. 159. The giant redwood (Seguoia gi-
They are very fern- gantea) of California: the relative size
: J is indicated by the figure of a man stand- like in structure as well ing at the right.—After WILL1amson.
190 PLANT STRUCTURES
as in appearance, but they produce seeds and must be associated with Spermatophytes, and as the seed is ex- posed they are Gymnosperms. A discovery has been made
Fie. 160. An araucarian pine (Araucaria), showing the central shaft, and the regular cycles of branches spreading in every direc- tion and bearing numerous small leaves.— From “ Plant Relations.”
recently that strikingly emphasizes their fern- like structure. In fer- tilization a pollen-tube develops, as described for pine and its allies, but the male cells (sperm mother - cells) which it contains or- ganize sperms, and these sperms are of the coiled multiciliate type (Fig. 157) charac- teristic of all the Pter- idophytes except Club- mosses. This associa- tion of the old ciliated sperm habit with the new pollen-tube habit is a very interesting in- termediate or transition condition. It should be said that these sperms havebeen actually found in but two species of the Cycads, but there are reasons for suppos- ing that they may be found in all. Another one of the Gymnosperm groups, represented to- day only by the com- monly cultivated maid-
SPERMATOPHYTES: GYMNOSPERMS 191
enhair tree (Gingko), with broad dichotomously veined leaves, also develops multiciliate sperms.
The testa of the seed, instead of being entirely hard as described for pine and its allies, develops in two layers, the inner hard and bony, and the outer pulpy, making the ripe fruit resemble a plum.
106. Conifers—This is the great modern Gymnosperm group, and is characteristic of the temperate regions, where it forms great forests. Some of the forms are widely dis- tributed, as the great genus of pines (Pinus) (Fig. 158), while some are now very much restricted, although for- merly very widely distributed, as the gigantic redwoods (Sequoia) of the Pacific slope (Fig. 159). The habit of the body is quite charac- teristic, a central shaft extending continuously to the very top, while the lateral branches spread horizontally, with dimin- ishing length to the top, forming a conical outline (Figs. 160, 162). This habit of firs, pines, etc.,
Fie. 161.—Cross-section of a needle-leaf of
gives them an appearance very distinct from that of other trees.
Another peculiar fea- ture is furnished by the characteristic ‘“‘needle-
pine, showing epidermis (¢) in which there are sunken stomata (sp), heavy- walled hypodermal tissue (es) which gives rigidity, the mesophyll region (7) in which a few resin-ducts (2) are seen, and the central region (s¢ede) in which two vascular bundles.are developed.— After Sacus.
leaves,” which seem to be
poorly adapted for foliage. These leaves have small spread of surface and very heavy protecting walls, and show adaptation for enduring hard conditions (Fig. 161). As they have no regular period of falling, the trees are always clothed with them, and have been called ‘‘ evergreens.” There are some notable exceptions to this, however, as in
tate . ieee PEN Wola a
Hie. 162. A larch (Zaria), showing the continuous central shatt and horizontal branches, the general outline being distinctly conical. The larch is peculiar among Conifers in periodically shedding its leaves.—From *t Plant Relations,”
, Ce Tal hax
SPERMATOPHYTES: GYMNOSPERMS 193
the case of the common larch or tamarack, which sheds its leaves every season (Fig. 162). There are Conifers, also, which do not produce needle-leaves, as in the com- mon arbor-vite#, whose leaves consist of small closely-over- lapping scale-like bodies (Fig. 163).
The two types of leaf arrangement may also be noted. In most Conifers the leaves are arranged along the stem in spiral fashion, no two leaves being at the same level. This is known as the spi- ral or alternate arrange- ment. In other forms, as the cypresses, the leaves are in cycles, as was men- tioned in connection with the Horsetails, the ar- rangement being known as the cyclic or whorled.
The character which gives name to the group is the ‘‘cone”—that is, the prominent carpellate cone which becomes so conspicuous in connec- Fie. 163. Arbor-vite (Thuja), showing a
tion with the ripening of branch with scaly overlapping leaves, and some carpellate cones (strobili).— the seeds. These cones After EICHLER.
generally ripen dry and hard (Figs. 145, 147, 163), but sometimes, as in junipers, they become pulpy (Fig. 164), the whole cone forming the so-called ‘ berry.”
There are two great groups of Conifers. One, repre- sented by the pines, has true cones which conceal the
194 PLANT STRUCTURES
ovules, and the seeds ripen dry. The other, represented by the yews, has exposed ovules, and the seed either ripens fleshy or has a fleshy investment.
Fia. 164. The common juniper (.Jivniperus communis), the branch to the left bearing staminate strobili; that to the right bearing staminate strobili above and earpel- late strobili below, which latter have matured into the fleshy, berry-like fruit. —After Bere and Scumiprt.
CHAPTER XII SPERMATOPHYTES: ANGIOSPERMS
107. Summary of Gymnosperms.—Before beginning An- giosperms it is well to state clearly the characters of Gym- nosperms which have set them apart as a distinct group of Spermatophytes, and which serve to contrast them with Angiosperms.
(1) The microspore (pollen-grain) by wind-pollination is brought into contact with the megasporangium (oyule), and there develops the pollen-tube, which penetrates the nucellus. This contact between pollen and ovule implies an exposed or naked ovule and hence seed, and therefore the name “‘ Gymnosperm.”
(2) The female gametophyte (endosperm) is well organ- ized before fertilization.
(3) The female gametophyte produces archegonia.
108. General characters of Angiosperms,— This is the great- est group of plants, both in numbers and importance, being estimated to contain about 100,000 species, and forming the most conspicuous part of the vegetation of the earth. It is essentially a modern group, replacing the Gymnosperms which were formerly the dominant Seed-plants, and in the variety of their display exceeding all other groups. The name of the group is suggested by the fact that the seeds are inclosed in a seed case, in contrast with the exposed seeds of the Gymnosperms.
These are also the true flowering plants, and the ap-
pearance of true flowers means the development of an 195
196 PLANT STRUCTURES
elaborate symbiotic relation between flowers and insects, through which pollination is secured. In Angiosperms, therefore, the wind is abandoned as an agent of pollen transfer and insects are used; and in passing from Gym- nosperms to Angiosperms one passes from anemophilous to entomophilous (‘‘insect-loving”) plants. This does not mean that all Angiosperms are entomophilous, for some are still wind-pollinated, but that the group is prevailingly ento- mophilous. This fact, more than anything else, has re- sulted in a vast variety in the structure of flowers, so char- acteristic of the group.
109. The plant body.—This of course is a sporophyte, the gametophytes being minute and concealed, as in Gym- nosperms. Thesporophyte represents the greatest possible variety in habit, size, and duration, from minute floating forms to gigantic trees; herbs, shrubs, trees; erect, pros- trate, climbing ; aquatic, terrestrial, epiphytic ; from a few days to centuries in duration.
Roots, stems, and leaves are more elaborate and vari- ously organized for work than in other groups, and the whole structure represents the high- est organization the plant body has attained. As in the Gymnosperms, the leat is the most variously used organ, showing at least four distinct modifications: (1) foliage leaves, (2) scales, (3) sporophylls, and (4) floral leaves. The first three are present in Gymnosperms, and even in Pteri- dophytes, but floral leaves are pecul- Fie. 105. Stamens of hen. iar to Angiosperms, making the true
bane (Zyoseyamus): A, flower, and being associated with en- ce view, pune. Hla: tomophily.
ment (7) and anther (p);
B, Wack view, showing 110. Microsporophylls—The micro- the connective () be snorophyll of Angiosperms is more
tween the pollen-sacs. ~ —After ScumreEn. definitely known asa ‘‘ stamen ” than
SPERMATOPHYTES: ANGIOSPERMS 197
that of Gymnosperms, and has lost any semblance to a leaf. It consists of a stalk-like portion, the filament; and a sporangia -bearing portion, the anther (Figs. 165, 167a).
Fig. 166. Cross-section of anther of thorn apple (Datura), showing the four imbedded sporangia (@, p) containing microspores; the pair on each side will merge and dehisce along the depression between them for the discharge of pollen.—After FRANK.
The filament may be long or short, slender or broad, or variously modified, or even wanting. The anther is simply the region of the sporophyll which bears sporangia, and is
Fie. 167. Diagrammatic cross-sections of anthers: A, younger stage, showing the four imbedded sporangia, the contents of two removed, but the other two con- taining pollen mother cells (pm) surrounded by the tapetum (¢); B, an older stage, in which the microspores (pollen grains) are mature, and the pair of sporangia on each side are merging together to form a single pollen-sac with longitudinal dehiscence.—After BaiLLon and LUERSSEN.
therefore a composite of sporophyll and sporangia and is often of uncertain limitation. Such a term is convenient,
but is not exact or scientific. al
198 PLANT STRUCTURES
.If a young anther be sectioned transversely four sporan- gia will be found imbedded beneath the epidermis, a pair on each side of the axis (Figs. 166, 167). When they reach maturity, the paired sporangia on each side usually merge to- gether, forming two spore-containing cavities (Fig. 167, B). These are generally called ‘‘ pollen-sacs,” and each anther is said to consist of two pollen-sacs, although each sac is made up of two merged sporangia, and is not the equivalent of the pollen-sac in Gymnosperms, which is a single sporangium.
Fig. 1674. Various forms of stamens: A, from Solanum, showing dehiscence by terminal pores; B, from Ardufus, showing anthers with terminal pores and “horns”; C, from Berberas; D, from Atherosperma, showing dehiscence by uplifted valves; £, from Aqvilegia, showing longitudinal dehiscence ; #, from Popowia, showing pollen-sacs near the middle of the stamen,—After ENGLER and PRANTL.
SPERMATOPHYTES: ANGIOSPERMS 199
The opening of the pollen-sac to discharge its pollen- grains (microspores) is called dehiscence, which means “‘a
splitting open,” and the methods of dehiscence are various (Fig. 167a). By far the most common method is for the wall of each sac to split lengthwise (Fig. 168), which is called longitudinal dehiscence ; an- other is for each sac to open by a terminal pore (Fig. 167a), in which case it may be prolonged above into a tube.
111. Megasporophylls, — These are the so-called *‘ carpels” of Seed- plants, and in Angiosperms they are organized in various ways, but always so as to inclose the mega- sporangia (ovules). In the simplest cases each carpel is independent (Fig.
Fig. 168. Cross-section of
anther of a lily (Butomus), showing the separating walls between the members of each pair of sporangia broken down at z, forming a con- tinuous cavity (pollen sac) which opens by a longitudi- nal slit.—After Sacus.
169, 4), and is dif-
ferentiated into three regions: (1) a hollow bulbous base,
Fie. 169. Types of pistils: .4, three simple pistils (apocarpous), each showing ovary and style tipped with stigma; 2B, a compound pistil (syncarpous), showing ovary (f), separate styles (g), and stigmas (n); OC, a compound pistil (syncarpous), showing ovary (f), single style (g), and stigma ().—After Bere and ScHmiptT.
which contains the ovules and is the real seed case, known as the ovary; (2) sur- mounting this is a slender more or less elongated process, the style; and (3) usually at or near the apex of thestyle a special receptive surface for the pol- len, the stigma.
In other cases several carpels to-
2000 PLANT STRUCTURES
gether form a common ovary, while the styles may also combine to form one style (Fig. 169, (’), or they may remain more or less distinct (Fig. 169, 2). Such an ovary may contain a single chamber, as if the carpels had united edge to edge (Fig. 170, 4); or it may contain as many chambers as there are constituent carpels (Fig. 170, 4), as though each carpel had formed its own ovary before coalescence. In ordinary phrase an ovary is either ‘‘one-celled” or ** several-celled,” but as the word “cell” has a very differ- ent application, the ovary chamber had better be called a loculus, meaning “‘a compartment.” Ovaries,
A B C
Fic. 170. Diagrammatic sections of ovaries: 1, cross-section of an ovary with one loculus and three carpels, the three scts of ovules said to be attached to the wall (parietal); B, cross-section of an ovary with three loculi and three carpels, the ovules being in the center (central); C, longitudinal section of B.—After Scuim- PER.
therefore, may have one loculus or several loculi. Where there are several loculi each one usually represents a con- stituent carpel (Fig. 170, B); where there is one loculus the ovary may comprise one carpel (Fig. 169, 1), or several (Fig. 170, 4).
There is a very convenient but not a scientific word, which stands for any organization of the ovary and the accompanying parts, and that is p/sti7. A pistil may be one carpel (Fig. 169, .!), or it may be several carpels or- ganized together (Fig. 169, B, ('), the former case being a stmple pistil, the latter a compound pistil. Tn other words,
SPERMATOPHY TES:
any organization of carpels which ap- pears as a single organ with one ovary is a pistil.
The ovules (megasporangia) are developed within the ovary (Fig. 170) either from the carpel wall, when they are foliar, or from the stem axis which ends within the ovary, when they are cauline (see § 89). They are similar in structure to those of Gymnosperms, with integument and micropyle, nu- cellus, and embryo-sac (megaspore), except that there are often two integu- ments, an outer and an inner (Fig. 171).
ANGIOSVERMS
Fic. 171. A diagrammatic section of an ovule of Angiosperms, showing outer integument (ai), inner integument (ii), micropyle (m), nucellus (k), and embryo sac or megaspore (em).—After Sacus.
112, The male gametophyte.— When the pollen-grain (microspore) germinates there is formed within it the sim-
plest known gametophyte (Fig. 172).
No trace of the
Fie. 172. Germination of microspore (pollen grain) in duckweed (Zemna): A, mature
spore with its nucleus; B, nucleus of spore dividing;
(. twe nuclei resulting from
the division; D, a large and small cell following the nuclear division, forming the two-celled male gametophyte; #, division of smaller cell (generative) to form the two male cells; F, the two male cells completed and lying near the large tube
nucleus. —CALDWELL.
902 PLANT STRUCTURES
ordinary nutritive cells of the gametophyte remains, and the whole structure seems to represent a single antherid- ium. At first it consists of two cells, the large wall cell and the small free generative cell (Fig. 172, 2). Later
g 9. i Te ti ‘ Ey i b ai ae Ba ) \ 5 2 Mi 2 ‘> : se i k
Fig. 173. Diagram of a longitudinal section through a carpel, to illustrate fertilization with all parts in place: s, stigma; g, style; 0, ovary ; ai, ii, outer and inner integuments; 7, base of nucel- lus ; f, funiculus ; 4, antipodal cells; ¢, endo- sperm nucleus; 4, egg and one synergid; p, pol- len-tube, having grown from stigma and passed through the micropyle (m) to the egy.—After LUERSSEN.
the generative cell di- vides (Fig. 172, £), either while in the pollen-grain or after entrance into the pol- len-tube, and two male cells (sperm mother- cells) are formed (Fig. 172, #), which do not organize sperms, but which function direct- ly as gametes.
When _ pollination occurs, and the pollen has been transferred from the pollen-sacs to the stigma, it is de- tained by the minute papille of the stig- matic surface, which also excretes a sweet- ish sticky fluid. This fluid is a nutrient so- lution for the micro- spores, which begin to put out their tubes. A pollen-tube pene- trates through the stigmatic surface, en- ters among the tissues
of the style, which is sometimes very long, slowly or rap- idly traverses the length of the style supplied with food by
SPERMATOPHYTES: ANGIOSPERMS 903
its cells but not penetrating them, enters the cavity of the ovary, passes through the micropyle of an ovule, penetrates the tissues of the nucellus (if any), and finally reaches and pierces the wall of the embryo-sac, within which is the egg awaiting fertilization (Fig. 173).
This remarkable ability of the pollen-tube to make its way through so much tissue, directly to the micropyle of an inclosed ovule, can only be explained by supposing that it is under the guidance of some strong attraction.
113. The female gametophyte—The megaspore (embryo- sac) occupies the same position in the ovule as in Gymno- sperms, but its germination is remarkably modified. The development of the female gametophyte shows two distinct
Fic. 174. Lilium Philadelphicum; to the left a young megasporangium (ovule), showing integuments (C’), nucellus (A), and megaspore (B) containing a large nu- cleus. To the right a megaspore whose nucleus is undergoing the first division in the formation of the gametophyte.—CALDWELL.
periods, separated from one another by the act of fertiliza- tion. If fertilization is not accomplished the second stage of the gametophyte is usually not developed.
First period.—The megaspore nucleus divides (Fig. 174), and one nucleus passes to each end of the embryo-
204 PLANT STRUCTURES
sac (Fig. 175, at left). Each of these nuclei divide (Fig. 175, at right), and two nuclei appear at each end of the sac (Fig. 175, at middle). Each of the four nuclei divide
Fie. 175. Liliwimn Philadelphicum; to the left is an embryo-sac with a gametophyte nucleus in each end; to the right these two nuclei are dividing to form the two nuclei shown in each end of the sac in the middle figure. —CaLDWELL.
(Fig. 176, at left), and four nuclei appear at each end (Fig. 176, at middle). When eight nuclei have appeared, nuclear division stops. Then a remarkable phenomenon occurs. One nucleus from each end, the two being called ‘> polar nuclei,” moves toward the center of the sac, the two meet and fuse (Fig. 176, at right, C’), and a single large nucleus is the result.
The three nuclei at the end of the sac nearest the micro- pyle are organized into cells, each being definitely sur- rounded by cytoplasm, but there is no wall and the cells remain naked but distinct. These three cells constitute the egg-apparatus (Fig. 176, at right, .!), the central one, which usually hangs lower in the sac than the others, being the egg, the two others being the sywergids, or ** helpers.” Here, therefore, is an egg without an archegonium, a dis- tinguishing feature of Angiosperms.
SPERMATOPILYTES: ANGIOSPERMS 905
The three nuclei at the other end of the sac are also or- ganized into cells, and usually have walls. These cells are known as antipodal cells (Fig. 176, at right, B). The large nucleus near the center of the sac, formed by the fusion of the two
Fie. 176. Lilium Philadelphicum, showing last stages of germination of megaspore before fertilization: the embryo sac to the left contains the pair of nuclei in each end in a state of division preparatory to the stage represented by the middle figure, in which there are four nuclei at each end; the figure to the right shows an embryo- sac containing a gametophyte about ready for fertilization, with the egg apparatus (A) composed of the two synergids and egg (central and lower), the three antipo- dal cells (B), and the two polar nuclei fusing ((’) to form the primary endosperm nucleus.—CALDWELL.
polar nuclei, is known as the primary endosperm nucleus or the definitive nucleus.
206 PLANT STRUCTURES
Fic. 177. Fertilization in the cotton plant, a Dicotyledon, showing the pollen tube (P) passing throngh the micropyle and con- taining a single sperm (male cell), and hay- ing entered the embryo-sac is in contact with one of the synergids (.S) on its way to the egg (#).—After Duaaar.
This completes the first period of gametophyte de- velopment, and it is ready for fertilization.
Fertilization. — The pollen-tube, carrying the two male cells, has passed down the style and en- tered the micropyle (Fig. 173). It then reaches the wall of the embryo-sac, pierces it, and is in con- tact with the egg-appa- ratus. Usually it passes along the side of one of the synergids (Fig. 177), feeding upon and disor- ganizing it. When it comes near the conspicu- ous nucleus of the egg, the tip of the tube breaks and one male cell is dis- charged into the cyto- plasm of the egg (Fig. 178). The egg and the male cell now fuse, and an oospore, which invests itself with a wall, is the result.
Second period.—After fertilization the gameto- phyte begins its second period of development. The primary endosperm nucleus begins a series of divisions, and as a result
SPERMATOPHYTES: ANGIOSPERMS DT
the sac becomes more or less filled with nutritive cells, which are often organized into a compact tissue (Fig. 179). These nutri- tive cells correspond to the endo- sperm of Gymnosperms, and re- ceive the same name. In Gymno- sperms, therefore, the endosperm (the nutritive body of the female gametophyte) is mainly formed before fertilization, while in An- glosperms it is mainly formed after fertilization. This means that in Angiosperms eggs are formed and fertilization takes
lace in a very youn ameto- Fie. 178. End of embryo-sac of P ey 8 8 lily (Lilium Philadelphicum):
phyte, while in Gymnosperms and a pollen tube has entered the
heterosporous Pteridophytes the sac and has discharged a male
cell, whose nucleus is seen
egss appear much later. uniting with the nucleus of
The antipodal cells also proba- the egg; near the tip of the
. wat lls of th tube is the disorganizing nu-
bly represent nutritive cells o e cleus of one of the synergids. gametophyte. Sometimes they dis- —CALDWELL.
Fig. 179. One end of the embryo-sac in wake-robin (77rillium). showing endosperm (shaded cells) in which a young embryo is imbedded.—After ATKINSON.
208 PLANT STRUCTURES
appear very soon after they are formed; but sometimes they become very active and even divide and form a con- siderable amount of tissue, aiding the endosperm in nour- ishing the young embryo.
114. Development of embryo.—While the endosperm is forming, the oospore has germinated and the sporophyte embryo is developing (Fig. 180). Usually a suspensor, more or less distinct, but never so prominent as in Gymnosperms, is formed; at the end of it the embryo is developed (Fig. 181), which, when completed, is more or less surrounded by nourish- ing endosperm (Fig. 183).
The two groups of Angio- sperms differ widely in the struc- ture of the embryo. In Mono- cotyledons the axis of the em- bryo develops the root-tip at one end and the ‘seed-leaf ” (coty- ledon) at the other, the stem-tip arising from the side of the axis as a lateral member (Fig. 182). rem coed ames, ot This relation of orgs Teva
ing in the upper right enda the embryo of Jsoetes (see § 90). young embryo, in the other Naturally there can be but one end the antipodal cells cut off s
by a partition, and seatterea COtyledon under such cireum- through the sac a few free en- stances, and the group has been Se eeene nn aaptiatee Monocotyledons.
In Dicotyledons the axis of the embryo develops the root-tip at one end and the stem- tip at the other, the cotyledons (usually two) appearing as a pair of opposite lateral members on either side of the stem-tip (Fig. 181). This recalls the relation of parts in the embryo of Se/uyinella (see § 89). As the cotyledons are lateral members their number may vary. In Gymno- sperms, whose embryos are of this type, there are often
SPERMATOPHYTES: ANGIOSPERMS 209
several cotyledons in a cycle (Fig. 154); and in Dicotyle- dons there may be one or three cotyledons; but as a pair of opposite cotyledons is almost without exception in the group, it is named Dicotyledons.
The axis of the embryo between the root-tip and the cotyledons is called the hypocotyl (Figs. 154, 193,194), which
Fie. 181. Development of embryo of shepherd's purse (Capsella), a Dicotyledon: beginning with J, the youngest stage, and following the sequence to VJ, the old- est stage, v represents the suspensor, ¢ the cotyledons, s the stem-tip, w the root, h the root-cap. Note the root-tip at one end of the axis and the stem-tip at the other between the cotyledons.—After HANSTEIN.
means ‘‘ under the cotyledon,” a region which shows pecul- iar activity in connection with the escape of the embryo from the seed. Formerly it was called either cawlicle or radicle. In Dicotyledons the stem-tip between the coty-
910 PLANT STRUCTURES
ledons often organizes the rudiments of subsequent leaves, forming a little bud which is called the plumule.
Embryos differ much as to com- pleteness of their development within the seed. In some plants, especially those which are parasitic or sapro- phytic, the embryo is merely a small mass of cells, without any organiza- tion of root, stem, or leaf. In many cases the embryo becomes highly de- veloped, the endosperm being used up and the cotyledons stuffed with food material, the plumule contain- ing several well-organized young leaves, and the embryo completely filling the seed cavity. The com- mon bean is a good illustration of this last case, the whole seed within Fie. 188. Young embryo of | the integument consisting of the two
Water plantain (Alisma@),€ Jaroe, fleshy cotyledons, between
Monocotyledon, the root a c
being organized at one Which le the hypocotyl and a plu-
end (next the suspensor), mule of several leaves.
atthe other and thestem, 115. The seed,— As in Gymno-
tip arising from a lateral sperms, while the processes above
notch (#).— After HAN” deseribed are taking place within
the ovule, the integument or integu-
ments are becoming transformed into the testa (Fig. 183). When this hard coat is fully developed, the activities within cease, and the whole structure passes into that con- dition of suspended animation which is so little under- stood, and which may continue for a long time.
The testa is variously developed in seeds, sometimes being smooth and glistening, sometimes pitted, sometimes rough with warts or ridges. Sometimes prominent append- ages are produced which assist in seed-dispersal, as the wings in Catulpa or Bignonia (Fig. 184), or the tufts of
SPERMATOPHYTES; ANGIOSPERMS O11
Fig. 183. The two figures to the left are seeds of violet, one showing the black, hard testa, the other being sectioned and showing testa, endosperm, and imbedded embryo; the figure to the right is a section of a pepper fruit (Piper), showing modified ovary wall (pe), seed testa (sc), nucellus tissue (py), endosperm (en), and embryo (em).—After ATKINSON.
hair on the seeds of milkweed, cotton, or fireweed (Fig. 185). For a fuller account of the methods of seed-dispersal see Plant Relations, Chapter VI.
Fie. 184. A winged seed of Bignonia.—After STRASBURGER.
116. The fruit—The effect of fertilization is felt beyond the boundaries of the ovule, which forms the seed. The ovary is also involved, and becomes more or less modified. It enlarges more or less, sometimes becoming remarkably enlarged. It also changes in structure, often becoming hard or parchment-like. In case it contains several or numerous seeds, it is organized to open in some way and discharge them, as in the ordinary pods and capsules (Fig. 185). In case there is but one seed, the modified ovary
212
¥ Ze "Yj =
Fig. 185. A pod of fireweed (Epilobium) opening and exposing its plumed seeds which are transported by the wind.-—After Brat.
PLANT STRUCTURES
wall may invest it as closely as another integument, and a seed-like fruit is the result—a fruit which never opens and is practically a seed. Such a fruit is known as an akene, and is very characteristic of the greatest Angiosperm family, the Composite, to which sunflowers, asters, golden- rods, daisies, thistles, dandelions, ete., belong. Dry fruits which do not open to discharge the seed often bear appendages to aid in dispersal by wind (Figs. 186, 187), or by animals (Fig. 188).
Capsules, pods, and akenes are said to be dry fruits, but in many cases fruits ripen fleshy. In the peach, plum, cherry, and all ordinary ‘‘ stone fruits,” the modified ovary wall or- ganizes two layers, the inner being very hard, forming the ‘‘stone,” the outer being pulpy (Fig. 189), or vari-
ously modified (Fig. 190). In the true berries, as the grape, currant, tomato, etc., the whole ovary becomes a thin-skinned pulpy mass in which the seeds are imbedded.
In some cases the effect of ferti- lization in chang- ing structure is felt beyond the
ovary. Intheap- /
ple, pear, quince, and such fruits, the pulpy part is the modified calyx (one of the
Fie. 186. Winged fruit of maple.—After KERNER,
SPERM
ATOPHYTES: ANGIOSPERMS 913
floral leaves), the ovary and its contained seeds being repre- sented by the “core.” In other cases, the end of the stem bearing the ovaries (receptacle) becomes enlarged and pulpy, as in the strawberry (Fig. 191). This effect some-
times involves even more than the parts of a single flower, a whole flower-cluster, with its axis and bracts, becoming an enlarged pulpy mass, as in the pineapple (Fig. 192).
The term “fruit,” therefore,
Fig. 188. An akene of beg- gar ticks, showing the two barbed appendages which lay hold of animals.—Af- ter BEAL.
32
Fig. 187. A ripe dandelion head, showing the mass of plumes, a few seed-like fruits (akenes) with their plumes still attached to the receptacle, and two fallen off.—After KERNER.
Fie. 189. To the left a section of a peach (fruit), showing pulp and stone formed from ovary wall, and the contained seed (kernel); to the right the fruit of almond, which ripens dry.—After Gray.
214
PLANT STRUCTURES
is a very indefinite one, so far as the structures it includes are concerned. It is simply an effect which follows fer- tilization, and involves more or less of the structures adja-
Fie. 190. Fruit of nutmeg (I/yristica): A, section of fruit, showing seed within the heavy wall; B, section of seed, showing peculiar convoluted and hard endosperm (m) in which an embryo (7) is imbedded.—After BERG and ScumiptT.
cent to the seeds.
As has been seen, this effect may extend
only to the ovary wall, or it may include the calyx, or it
Fie, 191. Fruit of straw- berry, showing the per- sistent calyx, and the en- larged pulpy receptacle in which numerous sim- ple and dry fruits (a- kenes) are imbedded.— After BaILEy.
lodgment. If the
may be specially directed toward the receptacle, or it may embrace a whole flower-cluster. It is what is called a physiological effect rather than a defi- nite morphological structure.
117. Germination of the seed.—lIt has been pointed out (§ 103) that the so-called “ germination of the seed” is not true germination like that of spores. It is the awakening and es- cape of the young sporophyte, which has long before passed through its germination stage.
By various devices seeds are sepa- rated from the parent plant, are dis- persed more or less widely, and find
lodgment is suitable, there are many
devices for burial, such as twisting stalks and awns, bur-
SPERMATOPHYTES: ANGIOSPERMS 215
rowing animals, etc. The period of rest may be long or short, but sooner or later, under the influence of moisture, suitable temperature, and oxygen the quiescent seed begins to show signs of life.
The sporophyte within begins to grow, and the seed coat is broken or penetrated through some thin spot or
Fig. 192, Pineapple: A, the cluster of fruits forming the so-called ‘fruit’; B, single flower, showing small calyx and more prominent corolla; (, section of flower, showing the floral organs arising above the ovary (epigynous).—4, B after Koc; C after LE Maovt and DECAISNE.
opening. The root-tip emerges first, is protruded still farther by the rapid elongation of the hypocotyl, soon curves toward the earth, penetrates the soil, and sending out rootlets, becomes anchored. After anchorage in the
216 PLANT STRUCTURES
soil, the hypocotyl again rapidly elongates and develops a strong arch, one of whose limbs is anchored, and the other is pulling upon the cotyledons (Fig. 193). This pull finally frees the cotyledons, the hypocotyl straightens, the cotyle-
Fie. 193. Germination of the garden bean, showing the arch of the hypocotyl] above ground, its pull on the seed to extricate the cotyledons and plumule, and the final straightening of the stem and expansion of the young leaves.—After ATKINSON.
dons are spread out to the air and light, and the young sporophyte has become independent (Fig. 194).
In the grain of corn and other cereals, so often used in the laboratory as typical Monocotyledons, but really excep- tional ones, the embryo escapes easily, as it is placed on one side of the seed near the surface. The hypocotyl and stem split the thin covering, and the much-modified cotyle- don is left within the grain to absorb nourishment.
In some cases the cotyledons do not escape from the seed, either being distorted with food storage (oak, buck- eye, etc.), or being retained to absorb nourishment from the endosperm (palms, grasses, etc.). In such cases the stem-tip is liberated by the elongation of the petioles of the
SPERMATOPHYTES: ANGIOSPERMS O17
cotyledons, and the seed coat containing the cotyledons remains like a lateral appendage upon the straightened axis.
It is also to be observed in many cases that the young root system, after gripping the soil, contracts, drawing the young plant deeper into the ground.
118. Summary from Angio- sperms.—At the beginning of this chapter (§ 107) the characters of the Gymnosperms were summar- ized which distinguished them from Angiosperms, whose con- trasting characters may be stated as follows:
(1) The microspore (pollen- grain), chiefly by insect pollina- tion, is brought into contact with the stigma, which is a receptive region on the surface of the car- pel, and there develops the pollen- tube, which penetrates the style to reach the ovary cavity which contains the ovules (megasporan- gia). The impossibility of con- tact between pollen and ovule im- plies inclosed ovules and hence seeds, and therefore the name “« Angiosperm.”
(2) The female gametophyte is but slightly developed before fertilization, the egg appearing very early.
Fic. 194. Seedling of hazel ( Car- pinus), showing primary root (iw) bearing rootlets (sw) upon which are numerous root hairs (r), hypocotyl (%), cotyledons (¢), young stem (e), and first (/) and second (/’) true leaves.—After Scumm- PER.
(3) The female gametophyte produces no archegonia,
but a single naked egg.
CHAPTER AIII THE FLOWER
119. General characters——In general the flower may be regarded as a modified branch of the sporophyte stem bear- ing sporophylls and usually floral leaves. Its representa- tive among the Pteridophytes and Gymnosperms is the stro- bilus, which has sporophylls but not floral leaves. Among Angiosperms it begins in a simple and somewhat indefinite way, gradually becomes more complex and modified, until it appears as.an elaborate structure very efficient for its purpose.
This evolution of the flower has proceeded along many lines, and has resulted in endless diversity of structure. These diversities are largely used in the classification of Angiosperms, as it is supposed that near relatives are indi- cated by similar floral structures, as well as by other fea- tures. The significance of these diversities is supposed to be connected with securing proper pollination, chiefly by insects, and favorable seed distribution.
Although the evolution of flowers has proceeded along several lines simultaneously, now one feature and now another being emphasized, it will be clearer to trace some of the important lines separately.
120. Floral leaves.—In the simplest flowers floral leaves do not appear, and the flower is represented only by the sporophylls. Both kinds of sporophylls may be associated, in which case the flower is said to be perfect (Fig. 195); or they may not both occur in the same flower, in which case
one flower is staminate and the other pistillute (Fig. 196). 218
THE FLOWER 919
When the floral leaves first appear in connection with the sporophylls they are inconspicuous, scale-like bodies. In higher forms they become more prominent and inclose
Fie. 196. Naked flowers of dif- ferent willows (Salix), each from the axil of a bract: a, 6, ¢c, staminate flowers ; d, e, f, pistillate flowers, the pistil composed of two car- pels (syncarpous). — After WARMING.
Fic. 195. Lizard’s tail(Saururus): A, tipofbranch Pye, 197, Flower of calamus
bearing leaves and elongated cluster of flowers; (Acorus), showing simple B, a single naked flower from A, showing sta- perianth, stamens, and syn- mens and four spreading and stigmatic styles; carpous pistil: a hypogynous C, flower from another species, showing sub- flower without differentiation tending bract, absence of floral leaves, seven of calyx and corolla.—After stamens, and a syncarpous pistil; the flowers . Byer.
naked and perfect.—After ENGLER.
Fie. 199. Common flax (Linum): 1. entire flower, showing calyx and corolla; B, floral leaves re- moved, showing stamens and syncarpous p‘stil; @, a mature
Fie. 198. Flowers of elm (Ulmus): A, branch capsule splitting open. —After hearing clusters of flowers and scaly buds ; ScHIMPER.
B, single flower, showing simple perianth and stamens, being a stamirate flower; (', flower showing perianth, stamens. and the two divergent styles stigmatic on inner surface, being a perfect flower; DJ, section through perfect flower, showing peri- anth, stamens, and pistil with two loculi each with a single ovule —After ENGLER.
Fie. 200. A flower of peony, showing the four sects of floral organs: k, the sepals, to- gether called the calyx; vc, the petals, together called the corolla; a, the numerous stamens; g, the two carpels, which contain the ovules.—After STRASBURGER.
THE FLOWER 991
the young sporophylls, but they are all alike, forming what is called the pertanth (Figs. 197, 198).
In still higher forms the perianth differentiates, the inner floral leaves become more delicate in texture, larger and generally brightly colored (Fig. 199, 1). The outer set may remain scale-like, or become like small foliage leaves. When the dif- ferentiation of the peri- anth is distinct, the outer set of floral leaves is called the calyz, each leaf being a seval; the inner set is the corolla, each leaf being a petal (Fig. 200). Sometimes, as in the lily, all the floral leaves become uniformly large and brightly colored, in which case the term perianth is retained (Fig. 201). In other cases, the calyx may be the large and colored set, but whenever there is a clear distinction between sets, the outer is the calyx, the inner the corolla.
Both floral sets may not appear, and it has become the custom to regard the missing set Fie. 201.—An easter-lily. a Monocotyledon, as the corolla, such showing perianth (a), stamens (4), stigma (c),
e c flower bud (d), and a carpel after the peri- o flowers being called anth has fallen (7), with its knob-like stigma,
ape talou 8, meaning long style, and slender ovary.—CALDWELL.
229, PLANT STRUCTURES
“without petals.” It is not always possible to tell whether a flower is apetalous—that is, whether it has lost a floral set which it once had—or is simply one whose perianth has not yet differentiated, in which case it would be a ‘ primi- tive type.”
The line of evolution, therefore, extends from flowers without floral leaves, or naked flowers, to those with a dis- tinctly differentiated calyx and corolla.
121. Spiral to cyclic flowers——In the simplest flowers the sporophylls and floral leaves (if any) are distributed about an elongated axis in a spiral, like a succession of leaves. That part of the axis which bears the floral organs is for convenience called the receptacle (Fig. 202). As the recep-
Fie. 202. A buttercup (Ranwneulus): a, complete flower, showing sepals, petals, sta- mens, and head of numerous carpels on a large receptacle; 6, section showing relation of parts; a hypogynous, polypetalous, apocarpous, actinomorphic flower. —After Barton.
tacle is elongated and capable of continued growth, an in- definite number of each floral organ may appear, especially of the sporophylls. With the spiral arrangement, there- fore, there is no definiteness in the number of floral organs ; there may be one or very many floral leaves, or stamens, or carpels. The spiral arrangement and indefinite numbers are features of the ordinary strobilus, and therefore such flowers are regarded as more primitive than the others.
In higher forms the receptacle becomes shorter, the spiral more closely coiled, until finally the sets of organs
THE FLOWER 993
appear to be thrown into rosettes or cycles. This change does not necessarily affect all the parts simultaneously. For example, in the common buttercup the sepals and petals are nearly in cycles, while the carpels are spirally arranged and indefinitely numerous on the head-lke recep- tacle (Fig. 202). On the other hand, in the common water-
I
Fie. 203. Flower of water-lily (Vymphea), showing numerous petals and stamens.— After Caspary.
lily the petals and stamens are spiral, and indefinitely re- peated, while the sepals and carpels are approximately cyclic (Fig. 203).
Finally, in the highest forms, all the floral organs are in definite cycles, and there is no indefinite repetition of any part. All through this evolution from the spiral to the cyclic arrangement there is constantly appearing a tend- ency to ‘‘settle down” to certain definite numbers. When the complete cyclic arrangement is finally established these numbers are established, and they are characteristic of great groups. In cyclic Monocotyledons there are nearly always just three organs in each cycle, forming what is called a ¢rimerous flower (Fig. 204) ; while in cyclic Dicot-
994 PLANT STRUCTURES
4
yledons the number five prevails, but often four appears, forming pentumerous or tetramerous flowers (Fig. 199). This does not mean that there are necessarily just three, four, or five of each organ in the flower, for there may be two or more cycles of some one organ. For example, in the common lily there are six floral leaves in two sets, six sta-
mens in two sets, and three carpels (Fig. 204). In the cyclic flowers it is also to be noted that each set alternates with the next set outside (Fig. 204). The petals are not directly opposite the se-
fillies pals, but are opposite the spaces
Tams between sepals; the stamens in
OS turn alternate with the petals; if
(o =) there is a second set of stamens,
iy 2 it alternates with the outer set,
% and so on. If two adjacent sets
are found opposing one another,
it is usually due to the fact that
Fic. 204. Diagram of such a @ Set between has disappeared.
flower as the lily, showing re- Foy example, if a set of stamens lation of parts: uppermost . : .
oraan is the bract intheaxil 18 opposite the set of petals, either
of which the flower occurs; an outer stamen set or an inner
black dot below indicates po- . sition of stem ; floral parts in petal set has disappeared.
threes and in five alternating This line of evolution, there- cyeles (two stamen sets), being — fore, extends from flowers whose a trimerous, pentacyclic flow- 2 .
cr.—After SCHIMPER. parts are spirally arranged upon
an elongated receptacle and in- definite in number, to those whose parts are in cycles and definite in number.
122. Hypogynous to epigynous flowers—In the simpler flowers the sepals, petals, and stamens arise from beneath the ovary (Figs. 197, 202, 205, 7). As in such cases the ovary or ovaries may be seen distinctly above the origin (insertion) of the other parts, such a flower is often said to have a ‘‘ superior ovary.” The more usual term, however, is hypogynous, meaning in effect “‘ under the ovary,” refer-
THE FLOWER
lo
25
ring to the fact that the insertion of the other parts is under the ovary.
Hypogyny is very largely displaved among flowers, but there is to be observed a tendency in some to carry the insertion of the outer parts higher up. When the outer parts arise from the rim of an urn-like outgrowth from the
Fiz, 205. Flowers of Rose family: 1, a hypogynous flower of Pofentilia, sepals. petals, and stamens arising from beneath the head of carpels; 2, a perigynous flower of Alchemilia. sepals. petals, and stamens arising from rim of urn-like pro- longation of the receptacle. which surrounds the carpel; 3. an epizynons flower of the common apple, in which all the parts seem to arise from the top of the ovary, two of whose loculi are seen.—After FocKE.
receptacle, which surrounds the pistil or pistils, the flower is said to be perigynous (Figs. 205, 2, 206), meaning * around the pistil.” Finally, the insertion is carried above the ovary. and sepals. petals. and stamens seem to urise from the top of the ovary (Fig. 205, 2), such a flower being epigynozs. the outer parts appearing “‘upon the ovary.” In such a case the ovary does not appear within the flower. but below it (Figs. 205, 252, 261), and the flower is often said to have an ‘* inferior ovary.”
123. Apocarpous to syncarpous flowers——In the simpler flowers the carpels are entirely distinct, each carpel organ-
926 PLANT STRUCTURES
izing a simple pistil, a single flower containing as many pistils as there are carpels, as in the buttercups (Figs. 200, 202). Such a flower is said to be apocarpous, meaning “carpels separate.” There is a very strong tendency,
Fie. 206. Sweet-scented shrub (Calycanthus): A, tip of branch bearing flowers; B, section through flower, showing numerous floral leaves, stamens, and carpels, and also the development of the receptacle about the carpels, making a perigynous flower.—After THIEBAULT.
however, for the carpels of a flower to organize together and form a single compound pistil. In such a flower there may be several carpels, but they all appear as one organ (Figs. 195, C, 197, 198, D, 199, B), and the flower is said to be syncarpous, meaning ‘‘ carpels together.”
124. Polypetalous to sympetalous flowers—The tendency for parts of the same set to coalesce is not confined to the carpels. Sepals often coalesce (Fig. 208), and sometimes stamens, but the coalescence of petals seems to be more important. Among the lower forms the petals are entirely separated (Figs. 109, .f, 202, 203, 207), a condition which
THE FLOWER
has received a variety of names, but probably the most common is poly- petalous, meaning “petals many,” although elewtheropetalous, meaning ‘petals free,” is much more to the point.
In the highest Angiosperms, how- ever, the petals are coalesced, form- ing a more or less tubular organ (Figs. 208-210). Such flowers are said to be sympetalous, meaning “petals united.” The words gamo- petalous and monopetalous are also
Fic, 207. Flower of straw- berry, showing sepals, pet- als, numerous stamens, and head of carpels; the flower is actinomorphic, hypogynous, and with no coalescence of parts.—Af- ter BaILEY.
much used, but all three words refer to the same condition of the flower. Often the sympetalous corolla is differenti-
Fie. 208. A flower of the tobacco plant: a, a complete flower, showing the calyx with its sepals blended below, the funnelform corolla made up of united petals, and the stamens just showing at the month of the corolla tube; 5. a corolla tube split open and showing the five stamens attached to it near the base; c, a syncarpous pistil made up of two carpels, showing ovary, style, and stigma.—After STRASBURGER.
298 PLANT STRUCTURES
ated into two regions (Fig. 210, 6), a more or less tubular portion, the ¢wbe, anda more or less flaring portion, the limb.
125. Actinomorphic to zygomorphic flow- ers.—In the simpler flowers all the mem- bers of any one cycle are alike; the petals are all alike, the stamens are all alike, etc. Looking at the center of the flower, all the parts are re- peated about it like the parts of a radi- ate animal. Such a
flower is actinomor- Fie 209. Flower of morning-glory (Ipomea), with phic, meaning ra. sympetalous corolla split open, showing the five 3 ” ie attached stamens, and the superior ovary with diate, and 1S often prominent style and stigma; the flower is hy- called ai ‘“* regular
pogynons, sympetalous, and actinomorphic.— » After MuIssNER. flower. Although
the term actinomor- phic strictly applies to all the floral organs, it is especially noteworthy in connection with the corolla, whose changes will be noted.
Fie. 210. A group of sympetalous flower forms: @, a flower of harebell, showing a bell-shaped corolla; }, a lower of phlox, showing a tube and spreading limb; ¢, a flower of dead-nettle, showing a zygomorphic two-lipped corolla; @, a flower of toad-flax, showing a two-lipped corolla, and also a spur formed by the base of the corolla; é, a flower of the snapdragon, showiug the two lips of the corolla closed. —After Gray.
THE FLOWER 929
In many cases the petals are not all alike, and the radi- ate character, with its similar parts repeated about a cen-
ter, is lost. In the common violet, for example, one of the petals develops a spur (Fig. 211); in the sweet pea the petals are remarkably un- like, one being broad and erect, two small- er and drooping downward, and the other two much modi- fied to form together a boat-like structure which incloses the
Fie. 211. The pansy (Violu tricolor): A, section showing sepals (/,/’), petals (¢) one of which produces a spur (cs). the flower being zygomor- phic; B, mature fruit (a cap-ule) and persistent calyx (k%); C, the three boat-shaped valves of the fruit open, most of the seeds (s) having been discharged.— After Sacus.
sporophylls. Such flowers are called zygomorphir, meaning ‘“ yoke-form,” and they are often called ‘irregular flowers.”
When zygomorphic flowers are also sympetalous the corolla is often curiously shaped. A very common form
Fig. 212. Flower of a mint (Jfwtha aquaticai: A, the entire flower. showing calyx of united sepals, unequal petals. stamens, and style with two stigma lobes; B, a corolla split open, showing petals united and the four stamens attached to the tube; the flower is sympetalous and zygomorphic.—After WERMING.
33
230
PLANT STRUCTURES
is the dilabiate, or ** two-lipped,” in which two of the petals usually organize to form one lip, and the other three form
Fic. 213. Flower of a Labiate (Teucrium),
showing the calyx of coalesced sepals, the sympetalous and two-lipped (bilabi- ate) corolla with three petals (middle one largest) in the lower lip and two small ones in the upper, and the stamens and style emerging through a slit on the up- per side of the tube; a sympctalous and zygomorphic flower.—After Brique.
the other lip (Figs. 210, ¢, d, é, 212, 213). The two lips may be nearly equal, the upper may stand high or overarch the lower, the lower may project more or less conspicuously, etc. 126. Inflorescence— Very often flowers are soli- tary, either on the end of a stem or branch (Figs. 231, 236), or in the axil of a leaf (Fig. 258). But
such cases grade insensibly into others where a definite region of the plant is set aside to produce flowers (Figs. 253, 260). Such a region forms what is called the znflo- rescence. The various ways in which flowers are arranged in an inflorescence have received technical names, but they do not enter into our purpose here. They are simply dif- ferent ways in which plants seek to display their flowers so as to favor pollination and seed distribution.
There are several tendencies, however, which may be noted. Some groups incline to loose clusters, either elon- gated (Fig. 260) or flat-topped (Fig. 253); others prefer large and often solitary flowers (Fig. 258) to a cluster of smaller ones; but in the highest groups there is a distinct tendency to reduce the size of the flowers, increase their number, and mass them into a compact cluster. This ten- dency reaches its highest expression in the greatest family of the Angiosperms, the Composite, of which the sunflower or dandelion can be taken as an illustration (Figs. 261, 262), in which numerous small flowers are closely packed together in a compact cluster which resembles a single large flower. It does not follow that all very compact inflorescences in-
THE FLOWER 231
dicate plants of high rank, for the cat-tail flag (Fig. 221) and many grasses have very compact inflorescences, and they are supposed to be plants of low rank. It is to be noted, however, that the very highest groups have settled upon this as the best type of inflorescence.
127, Summary.—In tracing the evolution of flowers, therefore, the following tendencies become evident: (1) from naked flowers to those with distinct calyx and corolla ; (2) from spiral arrangement and indefinite numbers to cyclic arrangement and definite numbers; (3) from hypogynous to epigynous flowers; (+) from apocarpous to syncarpous pistils ; (5) from polypetalous to sympetalous corollas ; (6) from actinomorphic or regular to zygomorphic or irregular flowers ; (7) from loose to compact inflorescences.
These various lines appear in all stages of advancement in different flowers, so that it would be impossible to deter- mine the relative rank in all cases. However, if a flower is naked, spiral, with indefinite numbers, hypogynous, and apocarpous, it would certainly rank very low. On the con- trary, the flowers of the Composite have calyx and corolla, are cyclic, epigynous, syncarpous, sympetalous, often zygo- morphic, and are in a remarkably compact inflorescence, indicating the highest possible combination of characters.
128. Flowers and insects.— The adaptations between flowers and insects, by which the former secure pollination and the latter food, are endless. Many Angiosperm flowers, especially those of the lower groups, are still anemophilous, as are the Gymnosperms, but most of them, by the presence of color, odor, and nectar, indicate an adaptation to the visits of insects. This wonderful chapter in the history of plants will be found discussed, with illustrations, in Plant Relations, Chapter VII.
CHAPTER XIV MONOCOTYLEDONS AND DICOTYLEDONS
129. Contrasting characters—The two great groups of Angiosperms are quite distinct, and there is usually no dif- ficulty in recognizing them. The monocotyledons are usually regarded as the older and the simpler forms, and are represented by about twenty thousand species. The Dicotyledons are much more abundant and diversified, con- taining about eighty thousand species, and form the domi- nant vegetation almost everywhere. The chief contrasting characters may be stated as follows:
Monocotyledons. — (1) Embryo with terminal cotyledon and lat- eral stem-tip. This character is practically without exception.
(2) Vascular bundles of stem scattered (Hig. 214). This means that there is no annual increase in
re the diameter of the woody stems,
Fig. 214. Section of stem of : : corn, showing the scatterea @Nd no extensive branching, but bundles, indicated by black to this there are some exceptions.
dots in cross-section, and by : :
lines in Jongitudinal section, (3) Leat veins forming a closed —From ‘Plant Relations.” system (Fig. 215, figure to left). As a rule there is an evident set of veins which run approximately parallel, and intricately branching between them is a system of minute veinlets not
readily seen. ‘The vein system does not end freely in the 282
MONOCOTYLEDONS AND DICOTYLEDONS 933
margin of the leaf, but forms a ‘‘ closed venation,” so that the leaves usually have an even (ev/ire) margin. There are some notable exceptions to this character.
(4) Cyclic flowers trim- erous. The “ three-parted ”
i s
CN
A
Fig. 215. Two types of leaf venation: the figure to the left is from Solomon’s seal, a Monocotyledon, and shows the principal veins parallel, the very minute cross veinlets being invisible to the naked eye; that to the right is from a willow, a Dicotyledon, and shows netted veins, the main central vein (midrib) sending out a series of parallel branches, which are connected with one another by a network of veinlets.—After ETTINGSHAUSEN.
flowers of cyclic Monocotyledons are quite characteristic, but there are some trimerous Dicotyledons. Dicatyleduns.—(1) Embryo with lateral cotyledons and terminal stem-tip. (2) Vascular bundles of stem forming a hollow cylinder (Fig. 216, wv). This means an annual increase in the diam-
234 PLANT STRUCTURES
Fig. 216. Section across a young twig of box elder, showing the four stem regions: é, epidermis, represented by the heavy bounding line; ¢, cortex; w, vascular cyl- inder; p, pith. From ‘‘ Plant Relations.”
eter of woody stems (Fig. 217, w), and a possible increase of the branch system and foliage dis- play each year.
(3) Leaf veins form- ing an open system (Fig. 215, figure to right). The network of smaller veinlets between the larger veins is usually very evident, especially on the under surface of the leaf, suggesting the name ‘‘net-veined”
leaves, in contrast to the “‘ parallel-veined ” leaves of Mono-
cotyledons.
this, although the leaf may remain entire, it very commonly be- comes toothed, lobed, and divided in various ways. Two main types of venation may be noted, which influence the form of leaves. In one case a single very prominent vein (77d) runs through the mid- dle of the blade. and is called the midrib. From this all the mi- nor veins arise as branches (Figs. 218, 219), and such a leaf
The vein system ends freely in the margin of the leaf, forming an ‘* open venation.”
In consequence of
Fra. 217. Section across 1 twig of box elder
three years old, showing three annual rings, or growth rings, in the vascular cylinder, the radiating lines (m) which cross the vascular region (w) represent the pith rays, the princi- pal ones extending from the pith to the cor- tex (c).—From ‘ Plant Relations.”
MONOCOTYLEDONS AND DICOTYLEDONS 93°
Or
is said to be pinnate or pinnately veined, and inclines to elongated forms. In the other case several ribs of equal prominence enter the blade and diverge through it (Fig. 218). Such a leaf is palmate or palinately veined, and in- clines to broad forms.
(4) Cyclic flowers pentamerous or tetramerous. The flowers ‘‘in fives” are greatly in the majority, but some
Fic. 218. Leaves showing pinnate and palmate branching; the one to the left is from sumach, that to the right from buckeye.—CALDWELL.
very prominent families have flowers “‘in fours.” There are also dicotyledonous families with flowers ‘‘in threes,” and some with flowers ‘in twos.”
It should be remembered that no one of the above char- acters, unless it be the character of the embryo, should be depended upon absolutely to distinguish these two groups.
a) PLANT STRUCTURES
It is the combination of characters which determines a group. MonocoTyLEDONS
130. Introductory.—This great group gives evidence of several distinct lines of development, distinguished by what may be called the working out of different ideas. In this way numerous families have resulted—that is, groups of
Fie. 219, A leaf of honey locust, to show twice pinnate branching (bipinnate leaf).— CALDWELL.
forms which seem to belong together on account of similar structures. This similarity of structure is taken to mean relationship. A family, therefore, is made up of a group of nearly related forms. Opinions may differ as to what forms are so nearly related that they deserve to consti- tute a distinct family. <A single family of some botanists may be ‘‘split up” into two or more families by others. Despite this diversity of opinion, most of the families are fairly well recognized.
MONOCOTYLEDONS AND DICOTYLEDONS 237
Within a family there are smaller groups, indicating closer relationships, known as genera (singular, genus). For example, in the great family to which the asters belong, the different asters resemble one another more than they do any other members of the family, and hence are grouped together in a genus .{stev. In the same family the golden- rods are grouped together in the genus Solidago. The different kinds of Aster or of Solidago are called species (singular also species). A group of related species, there- fore, forms a genus ; and a group of related genera forms a family.
The technical name of a plant is the combination of its generic and specific names, the former always being written first. For example, Quercus alba is the name of the com- mon white oak, Quercus being the name of the genus to which all oaks belong, and a/ba the specific name which distinguishes this oak from other oaks. No other names are necessary, as no two genera of plants can bear the same name.
In the Monocotyledons about forty families are recog- nized, containing numerous genera, and among these genera the twenty thousand species are distributed. It is evident that it will be impossible to consider such a vast array of forms, even the families being too numerous to mention. A few important families will be mentioned, which will serve to illustrate the group.
131. Pondweeds,—These are submerged aquatics, found in most fresh waters (some are marine), and are regarded as among the simplest Monocotyledons. They are slender, branching herbs, growing under water, but often having floating leaves, and sending the simple flowers or flower clusters above the surface for pollination and seed-distri- bution. The common pondweed (Potamogeton) contains numerous species (Fig. 220), while .Va‘as (naiads) and Zannichellia (horned pondweed) are common genera in. ponds and slow waters.
938 PLANT STRUCTURES
The simple character of these forms is indicated by their aquatic habit and also by their flowers, which are mostly naked and with few sporophylls. A flower may consist of a single stamen, or a single carpel; or there may be several stamens and carpels associated, but without any coalescence (Fig. 220, B).
In the same general line with the pondweeds, but with more complex flowers, are the genera Sagittaria (arrow-
Fia. 220. Pondweed (Potamogeton): A, branch with cluster (spike) of simple flowers, showing also the broad floating leaves and the narrow submerged ones; B, a sin- gle flower, showing the inconspicuous perianth lobes (¢), the short stamens (a), and the two short styles with conspicuous stigmatic surfaces.—A after REIOHEN- BacH; B after Le Maour and Drcatsne.
Fic. 221. Cat-tails (Typha), showing the dense spikes of very simple flowers, each showing two regions, the lower the pistillate flowers, the upper the staminate.— From “ Field, Forest, and Wayside Flowers.”
240 PLANT STRUCTURES
leaf) and disma (water-plantain), in which there is a dis- tinct calyx and corolla. The genus Typha (cat-tail) is also an aquatic or marsh form of very simple type, the flow-
Fie, 222. A common meadow grass (Fi s/vea): A, portion of flower cluster (spikelet), showing the bracts, in the axils of two of which flowers are exposed ; B, a single flower with its envelop- ing bract, showing three stamens, and a_pistil whose ovary bears two style branches with much branched stigmas.—After STRASBURGFR.
ers being in dense cylindrical clusters (spikes), the upper flowers consisting of stamens, the lower of carpels, thus forming two very distinct re- gions of the spike (Fig. 221).
132. Grasses,— This is one of the largest and probably one of the most use- ful groups of plants, as well as one of the most peculiar. It is world-wide in its dis- tribution, and is re- markable in its dis- play of individuals, often growing so densely over large areas as to form a close turf. If the grass-like sedges be associated with them there are about six thousand species, representing nearly one third of the Mon- ocotyledons. Here belong the various cereals, sugar canes,
MONOCOTYLEDONS AND DICOTYLEDONS O41
4
bamboos, and pasture grasses, all of them immensely use- ful plants.
The flowers are very simple, having no evident perianth (Fig. 222). Most commonly a flower consists of three sta- mens, surrounding a single carpel, whose ovary ripens into the grain, the characteristic seed-like fruit of the group. The stamens, however, may be of any number from one to six. The flowers, therefore, are naked, with indefinite num- bers, and hypogynous, indicating a comparatively simple type. It is also noteworthy that the group is anemophilous.
One of the noteworthy features of the group is the prominent development of peculiar leaves (4racts) in con- nection with the flowers. Each flower is completely pro- tected or even inclosed by one of these bracts, and as the bracts usually overlap one another the flowers are invisible until the bracts spread apart and permit the long dangling stamens to show themselves. These bracts form the so- called *‘ chaff” of wheat and other cereals, where they persist and more or less envelop the grain (ripened ovary). As they are usually called glumes, the grasses and sedges are said to be glumaceous plants.
Grasses are not always lowly plants, for in the tropics the bamboos and canes form growths that may well be called forests. The grasses constitute the family Gramineae, and the sedges the family Cyperacee.
133. Palms.—More than one thousand species of palms are grouped in the family Palmucew. These are the tree Monocotyledons, and are very characteristic of the tropics, only the palmetto getting as far north as our Gulf States. The habit of body is like that of tree-ferns and C'yeads, a tall unbranched columnar trunk bearing at its summit a crown of huge leaves which are pinnate or palmate in char- acter, and often splitting so as to appear lobed or compound (Figs. 223, 224).
The flower clusters are usually very large (Fig. 223), and each cluster at first is inclosed in a huge bract, which
Fig, 223. A date palm, showing the unbranched columnar trunk covered with old leaf bases, and with a cluster of huge pinnate leaves at the top, only the lowest por- tions of which are shown; two of the very heavy fruit clusters are also shown.— From ‘“ Plant Relations.”
MONOCOTYLEDONS AND DICOTYLEDONS IAB
is often hard. Usually a perianth is present, but with no differentiation of calyx and corolla, and the flower parts are quite definitely in “‘ threes,” so that the cyclic arrangement with the characteristic Monocotyledon number appears.
split so as to appear palmately branched.—From * Plant Relations.”
134. Aroids—This is a group of nearly one thousand species, most of them belonging to the family 4racee. In our flora the Indian turnip or Jack-in-the-pulpit (4risema) (Fig. 225), sweetflag (Acorws), and skunk-cabbage (Symplo- carpus), may be taken as representatives ; while the culti- vated Calla-lily is perhaps even better known. The great display of aroids, however, is in the tropics, where they are endlessly modified in form and structure, and are erect, or climbing, or epiphytic.
944 PLANT STRUCTURES
The flowers are usually very simple, often being naked, with two to nine stamens, and one to four carpels (Fig.
i b) f 2 '
case a side view shows the naked tip of the projecting spadix.—After ATKINSON. ~
197). They are inconspicuous and closely set upon the lower part of a fleshy axis, which is naked above and often
MONOCOTYLEDONS AND DICOTYLEDONS O45
modified in a remarkable way into a club-shaped or tail-like often brightly colored affair. This singular flower-cluster with its fleshy axis is called a spudir. The flowers often include but one sort of sporophyll, and staminate and pistillate flowers hold different positions upon the spadix (Fig. 226).
The spadix is enveloped by a great bract, which sur- rounds and overarches like a large loose hood, and is called the spathe. The spathe is exceedingly variable in form, and is often conspic- uously colored, forming in the Calla- lily the conspicuous white part, within which the spadix may be seen, near the base of which the flowers are found. In Jack-in-the-pulpit (Fig. 225) it is the overarching spathe which suggests the *‘ pulpit.” The spadix and spathe are the characteristic features of the group, and the spathe is variously modified in form, structure, and color
for insect pollination, as is the peri- “yim with spathe ro- anth of other entomophilous groups. moved, showing cluster « cee of naked pistillate flow- Aroids are further peculiar in hav- Gard tatey Wek dot ing broad net-veined leaves of the Di- a cluster of staminate flowers, and the club- cotyledon type. Altogether they form shaned tip’ 08 the ph: a remarkably distinct group of Mon- — aix.—atter Wosstpio. ocotyledons.
135. Lilies—The lily and its allies are usually regarded as the typical Monocotyledon forms. The perianth is fully developed, and is very conspicuous, either undifferen- tiated or with distinct calyx and corolla, and the flower is well organized for insect pollination. The flowers are either solitary or few in a cluster and correspondingly large, or in more compact clusters and smaller. In any event, the perianth is the conspicuous thing, rather than spathes or glumes.
246 PLANT STRUCTURES
In the general lily alliance, composed of eight or nine families, there are more than four thousand species, repre- senting about one fifth of all the Monocotyledons, and they are distributed everywhere. They are almost all terrestrial herbs, and are prominently geophilous (** earth-lovers “)— that is, they develop bulbs, rootstocks, etc., which enable them to disappear from above the surface during un- favorable conditions (cold or drought), and then to reappear rap- idly upon the return of favorable conditions (Figs. 227, 225, 231, 233).
In the regular lly family (Lilivcew) the flowers are hypogy- nous and actinomor- phic (Fig. 231), the six perianth parts are mostly alike and some- times sympetalous (as in the lily-of-the-val-
Fie. 227. Wake-robin (Zrillimn), showing root- ley, hyacinth, easter stock, from which two branches arise, each bear- 7 ‘3 Ia: 290 ing a cycle (whorl) of three leayes and a single lily) (Figs. 201, x 20), trimerons flower.—After ATKINSON. the stamens are usu-
ally six (two sets), and the three carpels are syncarpous (Figs. 204, 230). This is a higher combination of floral characters than any of the preceding groups presents. Hypogyny and actinomorphy are low, but a conspicuous perianth, syn- carpy, and occasional sympetaly indicate considerable ad- vancement,
MONOCOTYLEDONS AND DICOTYLEDONS 944
In the amaryllis family (4maryllidacee), a higher fam- ily of the same general line, represented by species of .Var- rissus (jonquils, daffodils, etc.), gave, etc., the flowers are distinctly epigynous.
Fig. 228. Star-of-Bethlehem (Ornithogalum): a, entire plant with tuberous base and trimerous flowers; 0, a single flower; c, portion of flower showing relation of parts, perianth lobes and stamens arising from beneath the prominent ovary (hy- pogynous); d, mature fruit; ¢, section of the syncarpous ovary, showing the three carpels and loculi.—After SCHIMPER.
In the iris family (Jridacee), the most highly specialized family of the lily line, and represented by the various spe-
Fie. 229. The Japan lily, showing a tubular perianth, the parts of the perianth distinct above —From “ Field, Forest, and Wayside Flowers.”’
MONOCOTYLEDONS AND DICOTYLEDONS 9AQ
cies of Iris (flags) (Fig. 232), Crocus, Gladiolus (Figs. 233, 234), etc., the flowers are not only epigynous, but some of them are zygomorphic. When a plant has reached both epigyny and zygomorphy in its flowers, it may be re- garded as of high rank.
136. Orehids—In number of species this (Orehidacee) is the greatest family among the Monocotyledons, the species being yvari- ously estimated from Fic. 230. Diagrammatic cross-section of ovary
s tl sand. 46:4 of Lilium Philadedphicum, showing the three S1x Lousan 0 en loculi, in each of which are two ovules (mega-
thousand, representing sporangia); -, ovule; B, integuments; (, nu- : cellus; D, embryo-sac (megaspore),—C'aLp- between one third and die
one half of all known
Monocotyledons. In display of individuals, however, the orchids are not to be compared with the grasses, or even with lilies, for the various species are what are called ‘rare plants ’—that is, not extensively distributed, and often very much restricted. Although there are some beautiful orchids in temperate regions, as species of Hubenariu (rein- orchis) (Fig. 255). Poyonta, Calapagon, Calypso, Cypripe- dium (lady-slipper, or moccasin flower) (Fig. 236), ete., by far the greatest display and diversity zre in the tropics, where many of them are brilliantly flowered epiphytes (Fig. 237).
Orchids are the most highly specialized of Monocoty- ledons, and their brilliant coloration and bizarre forms are associated with marvelous adaptation for insect visitation (see Plant Relations, pp. 134, 135). The flowers are epigy- nous and strongly zygomorphic. One of the petals is re- markably modified, forming a conspicuous ly which is
\—
Fie. 231. The common dog-tooth violet, showing the large mottled leaves and con- spicuous flowers which are sent rapidly above the surface from the subterranean bulb (see cut in the left lower corner), also some petals and stamens and the pistil dissected out.—From “‘ Plant Relatious,”
MONOCOTYLEDONS AND DICUTYLEDONS 951
modified in a great variety of ways, and a prominent, often very long, sywr, in the bottom of which nectar is secreted, which must be reached by the proboscis of an insect (Fig. 235). The stamens are reduced to one or two, and welded with the style
Fic. 232. Flower of flag (Z7is), showing some of the sepals and petals, one of the three stamens, and the distinctly in- ferior ovary, being an epigy- nons flower.—After Gray.
Fic. 233 Gladiolus, showing tuberons subter-
Fie. 234. Flower cluster of Gla- ranean stem from which roots descend, grass- diolus, showing somewhat zrgo- like leaves, and somewhat zygomorphic flow- morphic flowers.—CaLDWELL, ers.—After REICHENBACH,
252 PLANT STRUCTURES
and stigmatic surface into an indistinguishable mass in the center of the flowers. The pollen-grains in each sac are sticky and cohere in a club-shaped mass (pollinium), which is pulled out and carried to another flower by the
Fic. 235. A flower of an orchid (Habena- via): at 1 the complete flower is shown, with three sepals behind and three pet- als in front, the lowest one of which has developed a long strap-shaped portion (ip) and a still longer spur portion, the opening to which is seen at. the base of the strap, and behind the spur the long inferior ovary (epigynous character) ; the two pollen sacs of the single stamen are sccn in the center of the flower, di- verging downward, and between them stretches the stigma surface; the rela- tion between pollen sacs and stigma sur- face is shown in 2; within each pollen sac is a mass of sticky pollen (pollini- um), ending lhclow in a sticky disk,
which may be seen in 7 and 2; in 3 a pollen mass (a) is shown sticking to each
cye of a moth.—After Gray.
visiting insect. The whole structure indicates a very highly specialized type, elaborately organized for insect pollination.
Another interesting epigynous and zygomorphic trop- ical group, but not so elaborate as the orchids, is repre- sented by the cannas and bananas (Fig. 120). common in cultivation as foliage plants, and the aromatic gingers.
From the simple pondweeds to the complex orchids the evolution of the Monocotyledons has proceeded, and be- tween them many prominent and successful families have been worked out.
LS ee rs
Fig. 236. A clump of lady-slippers (Cypripedium), showing the habit of the plant and the general] strncture of the zygomorphic flower.—After GiBson,
254 PLANT STRUCTURES
Fie. 237. A group of orchids (Cattleya), showing the very zygomorphic flowers, the lip being well shown in the flower to the left (lowest petal).—-CALDWELL.
DiIcoTYLEDONS
137. Introductory.— Dicotyledons form the greatest group of plants in rank and in numbers, being the most highly organized, and containing about eighty thousand species. They represent the dominant and successful vegetation in all regions, and are especially in the preponderance in tem- perate regions. They are herbs, shrubs, and trees, of every variety of size and habit, and the rich display of leaf forms is notably conspicuous.
Two great groups of Dicotyledons are recognized, the Archichlamydew and the Sympetale. In the former there is either no perianth or its parts are separate (polypeta- lous) ; in the latter the corolla is sympetalous. The Archi- chlamydez are the simpler forms, beginning in as simple a fashion as do the Monocotyledons ; while the Sympetale
MONOCOTYLEDONS AND DICOTYLEDONS 955
are evidently derived from them and become the most highly organized of all plants. The two groups each con- tain about forty thousand species, but the Archichlamydex contain about one hundred and sixty families, and the Sympetale about fifty.
To present over two hundred families, containing about eighty thousand species. is clearly impossible, and a very few of the prominent ones will be selected for illustrations.
Archichlamydee
138. Poplars and their allies—This great alliance repre- sents nearly five thousand species, and seems to form an isolated group. It is a notable tree assemblage, and appar- ently the most primitive and ancient group of Dicotyledons, containing the most important deciduous forest forms of
fe
Fie. 238. An oak in winter condition.—From ‘Plant Relations,”
256 PLANT STRUCTURES
temperate regions, for here belong the oak (Fig. 238), hick- ory, walnut, chestnut, beech, poplar, birch, elm (Figs. 198, 239), willow (Fig. 240), etc. The primitive character is in- dicated not merely by the floral structures, but also by the general anemophilous habit.
In the poplar (Populus) and its allied form, the willow (Salix), the flowers are naked and hypogynous (Fig. 16),
Fie. 239. An elm in foliage.—From ‘‘ Plant Relations,”
MONOCOTYLEDONS AND DICOTYLEDONS 257
the stamens are indefinite in number (two to thirty), and the pistil is syncarpous (two carpels). The stamens and
Fiu. 240. Flower clusters of willow (aments); that to the left is pistillate, the other staminate.—After WaRMING.
pistils are not only separated in different flowers, but upon different plants, some plants being staminate and others pistillate (Fig. 240). The flowers are clustered upon a long axis, and each one is protected by a promi- nent bract. It is these scaly bracts which give character to the cluster, which is called an ament or cathin, and the plants which produce such clusters are said to be wmenta- ceous. These aments of poplars, ‘“‘pussy willows,” and the Fie. 241. Aments of alder (Alnvs): a, branch
alders and birches are with staminate aments (n), pistillate aments
very familiar obj ects (m), and a young bud (x); 0, pistillate ament. at ‘ time of discharging seeds, showing the promi- (Figs. wt), 241). nent bracts.—After WaRMINa.
958 PLANT STRUCTURES
The only advanced character in the flowers as described above is the syncarpous pistil, but in the great allied pepper family (Piperacee) of the tropics, with its one thousand species, and most nearly represented in our flora by the
Fie. 242. Ovule of hornbeam (Carpinvs), showing chalazogamy: m, the micropyle; pt, the pollen tube, which may be traced to its entrance into the embryo-sac at its antipodal end, and thence upward through the sac toward the egg.—Aftcr Mary Ewart.
lizard-tail (Saururvs) of the swamps (Fig. 195), the flowers are not merely naked, but also apocarpous, and the whole structure is much like that of the simplest Monocotyle-
MONOCOTYLEDONS AND DICOTYLEDONS 959
dons. The peppers seem to represent the simplest of the Dicotyledons, and this great line may have begun with some such forms.
A very interesting fact in connection with the fertiliza- tion of certain amentaceous plants has been discovered. In birch, alder, walnut, hornbeam, and some others, the pollen-tube does not enter the ovule by way of the micro- pyle, but pierces through in the region of the base of the ovule and so penetrates to the embryo-sac (Fig. 242). As the region of the ovule where integument and nucellus are not distinguishable is called the chalaza, this phenomenon is known as chalazogamy, meaning “fertilization through the chalaza.”
139. Buttercups and their allies—This is a great assem- blage of terrestrial herbs, including nearly five thousand species, and is thought by many to be the great stock from which most of the higher Dicotyledons have been derived. The alliance includes the water-lilies, buttercups, and pop- pies, the specialized mustards, and certain notable tree forms, as magnolias, custard-apples. and the tropical laurels with one thousand species represented in our flora only by the sassafras. Here also is the strange group of *: car- nivorous” plants (Sarracenia, Drosera, Dionea, ete.). The group is distinctly entomophilous, in striking contrast with the preceding one.
Taking the buttercup (Ranunculus) as a type (Fig. 202), the flower is hypogynous, the calyx and the corolla are dis- tinctly differentiated and actinomorphic, and adapted for insect-pollination, but the spiral arrangement and indefinite numbers are very apparent, notably in connection with the apocarpous pistils, which are very numerous upon a promi- nent receptacle, but involving more or less all the parts. The stamens are also very numerous (Figs. 200, 243, 24+). In the water-lilies the petals and stamens are indefinitely numerous (Fig. 203), and in the poppies there is no definite number. In many of the forms. however, in connection
Fre. 243. Marsh marigold ((a/thi), a member of the Buttercup family, also showing floral diagram, in which the floral leaves are five, but the stamens and apocarpous pistils are indefinitely numerous —After ATKINSON.
Fig 244. Zygomorphie flower of larkspur Fie, 245. Diagram of the zygomorphic
(Delphinium), with sepals removed, show- flower of larkspur (De/phinium), show- ing two petals with prominent spurs, and ing the spur developed by a sepal and numerous stamens.—After BATLLON. inclosing the two petal spurs.—After
BAtrLion.
MONOCOTYLEDONS AND DICOTYLEDONS 961
with one or more of the parts, the Dicotyl number (five) appears (Figs. 243, 245), but with no special constancy.
In certain genera of the buttercup family (Renuncula- cew) zygomorphy appears, as in the larkspur (Delphiniun) with its spurred petals and sepals (Figs. 244, 245), and the monkshood (Aconitum) with its hooded sepal; and in the
Fic. 246. The common cabbage (Brassica), a member of the mustard family: 1, flower cluster, showing buds at tip, open flowers below with four spreading petals, and forming pods below; B, mature pod, with the persistent style; (. pod opening by two valves, and showing seeds attached to the false partition.—After WaRMING.
water-lily family (Vympheacee) and poppy family (Papa- veracee) syncarpy appears. In this alliance, also, belong the sweet-scented shrubs (Calycanthus), with their perigy- nous flowers containing numerous parts (Fig. 206).
35
252 PLANT STRUCTURES
The most specialized large group in this alliance is the mustard family (Crucifere), with twelve hundred species, to which belong the mustards, cresses, shep- herd’s purse, peppergrass, radish, cabbage (Fig. 246), etc.
The sepals are four in two sets, the
petals four in one set, the stamens
ao six with two short ones in an outer
fe, = \ set and four long ones in an inner
¢ @) 9 set, and one carpel whose ovary be-
Ks 8 comes divided into two loculi by
pS what is called a ‘false partition ”
(Figs. 246, C, 247), and usually be-
oe gpm: temas comes an elongated pod (Fig. 246,
of parts; four sepals, four A, B). This specialized structure
— abs stamens, and one of the flower distinctly marks the
pel with a false partition.
--After Waraine. family, whose name is suggested
by the fact that the four spreading
petals often form a Maltese cross (Fig. 246, 4). The pecul-
iar stamen character, four long and two short stamens, is called fefradynamous (‘four strong”).
140. Roses—This family (Rosacer) of one thousand species is one of the best known and most useful groups of the temperate regions. In it are such forms as NSpired, five-finger (Poten- tilla), strawberry (Fragaria) (Figs. 191, 207), raspberry (Fig. 248), and blackberry (Ru- bus), rose (usa), hawthorn § ('rute-
gus), apple, and = .
Ria < 1a, 248. The common raspberry: the figure to the pear (Pir us) (Fig. left showing flower-stalk, calyx, old stamens 249), plum, cherry, ({s), and prominent receptacle, from which the
“fruit” (a cluster of small stone fruits, each almond, and peach representing a carpel) has been removed.— After
(Pranus). Baier,
MONOCOTYLEDONS AND DICOTYLEDONS 963
Many of the true roses have a strong resemblance (Fig. 207) to the buttercups (Ranunenlvs), with their hypogy- nous regular flowers, and indefinite number of stamens and carpels, but the sepals and petals are much more frequently five, the Dicotyl number being better established. The
Fie. 249, The common.pear (Pirus communis), showing branch with flowers (1). sec- tion of a flower (?) showing its epigynous character, section of fruit (3) showing the thickened calyx outside of the ovary or ‘‘core”’ (indicated by dotted outline), and flower diagram (4) showing all the organs in fives except the stamens. —After WossIDLo.
whole family remains actinomorphic, but perigyny and epigyny appear in certain forms (Fig. 205), giving rise to the peculiar fruit (pome) of apples and pears (Fig. 249), in which the calyx and ovary ripen together. Another spe- cialized group of roses is that which develops the stone-
264 PLANT STRUCTURES
fruits (drupes), as apricots, peaches (Fig. 189), plums, cherries.
141. Legumes,—This is far the greatest family (Legumi- nose) of the Archichlamydew, containing about seven thou- sand species, distributed everywhere and of every habit. It is the great zygomorphic group of the Archichlamydee, being elaborately adapted to insect pollination. The more
Fig. 250. A legume plant (Lofus), showing flowering branch (1), a single flower (2) showing zygomorphic corolla, the cluster of ten stamens (3) which with the carpel is included in the keel, the solitary carpel (4) which develops into the pod or le- gume (5), the petals (6) dissected apart and showing standard (qa), wings (0), and the two lower petals (¢) which fold together to form the keel, and the floral dia- gram (7).—After WossIDLo.
primitive forms of the Leguminose, the mimosas, acacias (Fig. 251), etc., very much resemble true roses and the but- tercups, with their hypogynous regular flowers and nu- merous stamens, but the vast majority are Payilio forms with very irregular (zygomorphic) flowers and few stamens
MONOCOTYLEDONS AND DICOTYLEDONS 265
(Fig. 250). The petals are very dissimilar, the upper one (standard) being the largest, and erect or spreading, the two lateral ones (7ings) oblique and descending, the two lower ones coherent by their edges to form a projecting boat-shaped body (Keel), which incloses the sta- mens and pistil. From a fancied re- semblance to a but- terfly such flowers are said to be papil- tonaceous.
The whole fam- ily is further char- acterized by the sin- gle carpel, which after fertilization develops a pod (Fig. 250, 5), which
compared with the carpel. It is this peculiar pod (/e- gume) which has given to the family its technical name Fie. 251. A sensitive-plant (Acacia), showing the Leguminose and flowers with inconspicuous petals and very nu-
merous stamens, and the pinnately branched sen- the common name sitive leaves.—After Meyer and ScHuMANN. “Legumes.”
Well-known members of the family are lupine (Lupi- nus), Clover (Trifolium), locust (Robinia), Wistaria, pea (Pisum), bean (Phaseolus), tragacanth (1s¢ragalus), vetch (Vicia), redbud ((ere/s), senna (Cassia), honey-locust (Gleditschia), indigo (Indigofera), sensitive-plants (Acacia, Mimosa, etc.) (Fig. 251), ete.
266 PLANT STRUCTURES
142. Umbellifers—This is the most highly organized family (Umbellifere) of the Archichlamydex, which may be said to extend from Peppers to Umbellifers. The Le- gumes adopt zygomorphy, but remain hypogynous ; and in some of the Roses epigyny appears; but the Umbellifers with their fifteen hundred species are all distinctly epigy-
Fig. 252. The common carrot (Daucus Carota): 4. branch bearing the compound umbels; B, u single epigynous flower, showing inferior ovary, five spreading petals, five stamens alternating with the petals, and the two styles of the bicarpel- lary pistil; ( section of flower, showing relation of parts, and also the minute sepals near the top of the ovary and just beneath the other parts, —After WARMING.
nous (Fig. 252, B, (), being one of the very few epigy- nous families among the Archichlamydee. In addition to epigyny, the cyclic arrangement and definite Dicotyl number is established, there being five sepals, five petals. five stamens, and two carpels, the highest known floral
MONOCOTYLEDONS AND DICOTYLEDONS 267
formula, and one that appears among the highest Sym-
petale.
The name of the family is suggested by the character- istic inflorescence, which is also of advanced type. The
flowers are reduced in size and massed in flat-
topped clusters called He
umbels (Figs. 252, 4, 253). The branches of the clus- ter arise in cycles from the axis like the braces of an umbrella. As a re- sult of the close approxi- mation of the flowers the sepals are much reduced in size and often obsolete (Fig. 252, C).
The Umbellifers are mainly perennial herbs of the north temperate re- gions, forming a very dis- tinct family, and contain- ing the following familiar forms: carrot (Daucus) (Fig. 252), parsnip (Pasti- naca), hemlock (Conium) (Fig. 253), pepper-and- salt (Erigenia), caraway (Carum), fennel (Fente- ulum), coriander (Cori- andrum), celery (Apt- um), parsley (Petroseli- num), etc. Allied to the Umbellifers are the Ara- lias (Araliacee), and the Dogwoods (Cornacee).
Fie. 253. Hemlock (Conium), an Umbellifer, showing the umbels, with the principal rays rising from a cycle of bracts (invo- lucre), and each bearing at its summit a secondary umbel with its cycle of second- ary bracts (énvolucel).—After SCHIMPER.
268 PLANT STRUCTURES
Sympetale
143. Introductory.—These are the highest and the most recent Dicotyledons. While they contain numerous shrubs and trees in the tropics, they are by no means such a shrub and tree group in the temperate regions as are the Archi- chlamydee. The flowers are constantly cyclic, the num- ber five or four is established, and the corolla is sympeta- lous, the stamens usually being borne upon its tube (Figs. 208, 209, 212).
There are two well-defined groups of Sympetale, distin- guished from one another by the number of cycles and the number of carpels in the flower. The group containing the lower forms is pexfacyclic, meaning ‘ cycles five,” there being two sets of stamens. In it also there are five carpels, the floral formula being, Sepals 5, Petals 5, Stamens 5 + 5, Carpels 5. As the carpels are the same in number as the other parts, the flowers are called dsocarpic, meaning * car- pels same.” The group is named either Pentacyrl or [sv- carpe, and contains about ten families and 4,000 species.
The higher groups, containing about forty families and 36,000 species, is fetracyclic, meaning ** cycles four,” and anisocarpic, meaning *‘carpels not the same,” the floral formula being, Sepals 5, Petals 5, Stamens 5, Carpels 2. The group name, therefore, is Tefrarycle or Anisocurpe.
144. Heaths.—The Heath family (ricacer) and its allies represent about two thousand species. They are mostly shrubs, sometimes trailing, and are displayed chiefly in temperate and arctic or alpine regions, in cold and damp or dry places, often being prominent vegetation in bogs and heaths, to which latter they give name (Fig. 254). The flowers are pentacyclic and isocarpic, as well as mostly hyp- ogynous and actinomorphic. It is interesting to note that some forms are not sympetalous, the petals being distinct, showing a close relationship to the Archichlamydex. One of the marked characteristics of the group is the dehiscence
MONOCOTYLEDONS AND DICOTYLEDONS 969
of the pollen-sacs by terminal pores, which are often pro- longed into tubes (Fig. 255).
Fie. 254. Characteristic heath plants: 4, B. (. Lyonia, showing sympetalous flowers and single style from the lobed syncarpous ovary; D, two forms of Cassiope, showing trailing habit, small overlapping leaves, and sympetalous flowers, but in the smaller form the petals are almost distinct.—After DRUDE.
Common representatives of the family are as follows: huckleberry ((@aylussacia), cranberry and blueberry ( Vuc- cinium), bearberry (i retostaphylos), trailing arbutus (£p/-
270 PLANT STRUCTURES
gea), wintergreen (Gaultheria), heather (Calluna), moun- tain laurel (Aalmia), Azalea, Rhododendron (Fig. 256), Indian pipe (Monotropa), etc.
Fig, 255. Flowers of heath plants (#rica), showing complete flowers (4), the sta- mens with ‘‘ two-horned”’ anthers which discharge pollen throngh terminal pores, and the lobed syncarpous ovary with single style and prominent terminal stigma (B, C, D).—After DruveE.
145. Convolvulus forms.—The well-known morning-glory (Jpomea) (Fig. 209) may be taken as a type of the Convol-
MOSOCOTYLEDONS AND DICOTYLEDORKS 971
vulus family (Convolvulacew). Allied with it are Polemo- nium and Phlow (Fig. 210, b) (Polemoniacee), the gentians (Gentianacew), and the dog-banes (lpocynacee) (Fig. 257). It is here that the regular sympetalous flower reaches its highest expression in the form of conspicuous tubes, fun-
Fie. 256. A cluster of Rhododendron flowers.—After HooKER.
nels (Fig. 258), trumpets, etc. The flowers are tetracyclic and anisocarpic, besides being hypogynous and actinomor- phic. These regular tubular forms represent about five thousand species, and contain many of the best-known flowers.
979 PLANT STRUCTURES
146. Labiates—This great family (Zadiate) and its alli- ances represent more than ten thousand species. The con- spicuous feature is the zygomorphic flower, dif- fering in this regard from
the Convolvulus forms, which they resemble in being tetracyclic and ani- socarpic, as well as hypogy- nous. The irregularity consists in organizing the mouth of the sympetalous corolla into two ‘‘ lips,” resulting in the /adiate or
Fie. 257. A common dogbane (.fpocynum).—From “ Field, Forest, and Wayside Flowers.”
Fig. 258. The hedge bindweed ( Conro/eudus), showing the twining habit and the con- spicuous funnelform corollas.—From “ Field. Forest, and Wayside Flowers.”
O74 PLANT STRUCTURES
bilabiate structure (Fig. 210, ¢, d, ¢), and suggesting the name of the dominant family. The upper lip usually con- tains two petals, and the lower three ; the two lips are some- times widely separated, and sometimes in close contact, and differ widely in relative prominence.
Associated with zygomorphy in this group is a frequent reduction in the number of stamens, which are often four (Fig. 212) or two. The whole structure is highly special- ized for the visits of insects, and this great zygomorphic alliance holds the same relative position among Sympetale as is held by the zygomorphic Le- gumes among Archi- chlamydee.
In the mint family, as the Labiates are often called, there are about two thousand seven hun- dred species, including mint (Mentha) (Fig. 212), dittany (Cunila), hyssop (Hyssopus), mayr- joram (Origanum),
Fie. 259. Flowers of dead nettle (Za- Fre. 260. A labiate plant (Zeucrium), show- mium): A, entire bilabiate flower ; ing branch with flower clusters (4), and B, section of flower, showing rela- side view of a few flowers (B), showing tion of parts.—After Warmina, their bilabiate character.—After Briquet,
MONOCOTYLEDONS AND DICOTYLEDONS 975
thyme (Thymus), balm (Jelissa), sage (Salvia), catnip (-Vepeta), skullcap (Scutelluria), horehound (Murrubium), lavender (Lavandula), rosemary (Rosmarinus), dead nettle (Lamium) (Fig. 259), Teuerium (Figs. 2138, 260), ete., a remarkable series of aromatic forms.
Allied is the Nightshade family (Solanacew), with fif- teen hundred species, containing such common forms as the nightshades and potato (So/anwm), tomato (Lycoper- sicum), tobacco (.Vicotiana) (Fig. 208), etc., in which the corolla is actinomorphic or nearly so; also the great Fig- wort family (Scrophulariacee), with two thousand species, represented by mullein ( Verbascum), snapdragon (.intir- rhinum) (Fig. 210, ¢), toad-flax (Linarta) (Fig. 210, d), Pentstemon, speedwell (Veronica), Gerardia, painted cup (Castilleta), etc.; also the Verbena family (Verbenacee), with over seven hundred species; and the two hundred plantains (Pluntaginucee), etc.
147. Composites—This greatest and ranking family (Composite) of Angiosperms is estimated to contain at least twelve thousand species, containing more than one seventh of all known Dicotyledons and more than one tenth of all Seed-plants. Not only is it the greatest family, but it is the youngest. Composites are distributed everywhere, but are most numerous in temperate regions, and are mostly herbs.
The name of the family suggests the most conspicuous feature—namely, the remarkably complete organization of the numerous small flowers into a compact head which resembles a single flower, formerly called a ‘‘ compound flower.” Taking the head of an Arnica as a type (Fig. 261), the outermost set of organs consists of more or less leaf-like bracts or scales (‘nvolucre), which resemble sepals ; within these is a circle of flowers with conspicuous yellow corollas (rays), which are zygomorphic, being split above the tubular base and flattened into a strap-shaped body, and much resembling petals (Fig. 261, 1, 2); within the
Fie. 261. Flowers of Arnica? .1, lower part of stem, and upper part bearing a head, in which are seen the conspicuous rays and the disk; D, single ray flower, showing the corolla, tubular at base and strap-shaped above, the two-parted style, the tuft of pappus hairs, and the inferior ovary which develops into a seed-like fruit (akene); Z#. single disk flower, showing tubular corolla with spreading limb, the two-parted style emerging from the top of the stamen tube, the prominent pappus, and the inferior ovary or akene; (', a single stamen,—Aftcr IlorFMAN.
276
MONOCOTYLEDONS AND DICOTYLEDONS OWT
ray-flowers is the broad expanse supplied by a very much broadened axis, and known as the dish (Fig. 261, 4), which is closely packed with very numerous small and regular tubular flowers, known as disk-flowers (Fig. 261, e).
Fic. 262. The common dandelion ( Tararacuvm): 1, two flower stalks; in one the head is closed, showing the double involucre, the inner erect, the outer reflexed, in the other the head open, showing that all the flowers are strap-shaped; 2, a single flower showing inferior ovary, pappus, corolla, stamen tube, and two-parted style; 3, a mature akene; 4, a head from which all but one of the akenes have been re- moved, showing the pitted receptacle and the prominent pappus beak.—After STRASBURUER.
The division of !abor among the flowers of a single head is plainly marked, and sometimes it becomes quite com- plex. The closely packed flowers have resulted in modity- ing the sepals extremely. Sometimes they disappear en-
36
278 PLANT STRUCTURES
tirely ; sometimes they become a tuft of delicate hairs, as in Arnica (Fig. 261, D, #), thistle (Caicus), and dandelion (Taravacum) (Fig. 263), surmounting the seed-like akene and aiding in its transportation through the air ; sometimes they are converted into two or more tooth-like and often
Fie. 263. Flowers of dandelion, showing action of style in removing pollen from the stamen tube: 7, style having elongated through the tube and carrying pollen; 2, style branches beginning to recurve; 3, style branches completely recurved.— From ‘ Field, Forest, and Wayside Flowers.”
barbed processes arising from the akene, as in tickseed (Coreopsis) and beggar-ticks (Fig. 188) or Spanish needles (Bidens), to lay hold of passing animals ; sometimes they become beautifully plumose bristles, as in the blazing star (Liatris) ; sometimes they simply form a more or less con- spicuous cup or set of scales crowning the akene. In all of these modifications the calyx is called pappus.
The stamens within the corolla are organized into a tube by their coalescent anthers (Fig. 263), and discharge their pollen within, which is carried to the surface of the
MONOCOTYLEDONS AND DICOTYLEDONS 979
head and exposed by the swab-like rising of the style (Fig. 263). The head is thus smeared with pollen, and visiting insects can not fail to distribute it over the head or carry it to some other head.
In the dandelion and its allies the flowers of the disk are like the ray-flowers, the corolla being zygomorphic and strap-shaped (Figs. 262, 263).
The combination of characters is sympetalous, tetracyc- lic, and anisocarpic flowers, which are epigynous and often zygomorphic, with stamens organized into a tube and calyx modified into a pappus, and numerous flowers organized into a compact involucrate head in which there is more or less division of labor. There is no group of plants that shows such high organization, and the Composite seem to deserve the distinction of the highest family of the plant kingdom.
The well-known forms are too numerous to mention, but among them, in addition to those already mentioned, there are iron-weed (Vernonia), Aster, daisy (ellis), goldenrod (Solidago), rosin-weed and compass-plant (Si/ph- tum), sunflower (Helianthus), Chrysanthemum, ragweed (Ambrosia), cocklebur (.Yanthiwm), ox-eye daisy (Leucan- themum), tansy (ZLanacetum), wormwood and sage-brush (Artemisia), lettuce (Lactuca), etc.
CHAPTER XV DIFFERENTIATION OF TISSUES
148. Introductory— Among the simplest Thallophytes the cells forming the body are practically all alike, both as to form and work. What one cell does all do, and there is very little dependence of cells upon one another. As plant bodies become larger this condition of things can not continue, as all of the cells can not be put into the same relations. In such a body certain cells can be related to the external food supply only through other cells, and the body becomes differentiated. In fact, the relating of cells to one another and to the external food-supply makes large bodies possible.
The first differentiation of the plant body is that which separates nutritive cells from reproductive cells, and this is accomplished quite completely among the Thallophytes. The differentiation of the tissues of the nutritive body, however, is that which specially concerns us in this chapter.
A tissue is an aggregation of similar cells doing similar work. Among the Thallophytes the nutritive body is prac- tically one tissue, although in some of the larger Thallo- phytes the outer and the inner cells differ somewhat. This primitive tissue is composed of cells with thin walls and active protoplasm, and is called parenchyma, meaning “parent tissue.”
Among the Bryophytes, in the leafy gametophore and in the sporogonium, there is often developed considerable dissimilarity among the cells forming the nutritive body, but the cells may all still be regarded as parenchyma. It
280
DIFFERENTIATION OF TISSUES O81
isin the sporophyte of the Pteridophytes and Spermato- phytes that this differentiation of tissues becomes extreme, and tissues are organized which differ decidedly from parenchyma. This differentiation means division of labor, and the more highly organized the body the more tissues there are.
All the other tissues are derived from parenchyma, and as the work of nutrition and of reproduction is always retained by the parenchyma cells, the derived tissues are for mechanical rather — than for vital purposes. )
There is a long list of <
these derived and me- chanical tissues, some of them being of general’ occurrence, and others more restricted, and
there is every gradation NA A between them and the gre. 264. Parenchyma and sclerenchyma from parenchyma from which the pa of Pteris, in cross-section.—CHAmM- they have come. We ~
shall note only a few which are distinctly differentiated and which are common to all vascular plants.
149. Parenchyma,—The parenchyma of the vascular plants is typically made up of cells which have thin walls and whose three dimensions are approximately equal (Figs. 264, 265). though sometimes they are elongated. Until abandoned, such cells contain very active protoplasm, and it is in them that nutritive work and cell division are carried on. So long as these cells retain the power of cell division the tissue is called meristem, or it is said to be meristematic, from a Greek word meaning ‘to divide.” When the cells stop dividing, the tissue is said to be permanent. The growing points of organs, as stems, roots, and leaves, are composed of parenchyma which is meristematic (Figs. 266, 274), and meristem occurs wherever growth is going on.
982 PLANT STRUCTURES
150. Mestome and stereome—When the plant body be- comes complex a conductive system is necessary, so that the different regions of the body may be put into communi- cation. The material absorbed by the roots must be carried to the leaves, and the food manu- factured in the leaves must be carried to regions of growth and storage. This business of transportation is provided for by the specially organized ves- sels referred to in preceding chapters, and all conducting tis- sue, of whatever kind. is spoken of collectively as mestome.
If a complex body is to main- tain its form, and especially if it is to stand upright and be- come large, it must develop structures rigid enough to fur- nish mechanical support. All the tissues which serve this pur- pose are collectively known as
Fie. 265. Same tissues as in pre- stereome. ceding figure, in longitudinal sec- The sporophyte body of
mele —CHametius. Pteridophytes and Spermato-
phytes, therefore, is mostly
made up of living and working parenchyma, which is traversed by mechanical mestome and stereome.
151. Dicotyl and Conifer stems.—The stems of these two groups are so nearly alike in general plan that they may be considered together. In fact, the resemblances were once thought to be so important that these two groups were put together and kept distinct from Monocotyledons ; but this was before the gametophyte structures were known to bear very different testimony.
DIFFERENTIATION OF TISSUES 283
At the apex of the growing stem there is a group of active meristem cells, from which all the tissues are de- rived (Fig. 266). This group is known as the apical group. Below the apical group the tissues and regions of the stem begin to appear, and still farther down they become dis- tinctly differentiated, passing into permanent tissue, the apical group by its divisions continually adding to them and increasing the stem in length.
Just behind the apical group, the cells begin to give the appearance of being organized into three great embryonic re-
gions, the cells still
remaining meristem- Fic. 266. Section through growing point of stem of 3 : Hippuris : below the growing point, composed
atic (Fig. 266). At of a uniform meristem tissue, the three embry-
the surface there is a onic regions are outlined, showing the dermato- “ gen (ad, d), the central plerome (,/, p), and be- single layer of cells tween them the periblem.—After DE Bary.
distinct from those within, known as the dermatagen, or “ skin-producer,” as farther down, where it becomes permanent tissue, it is the epidermis. In the center of the embryonic region there is organized a solid cylinder of cells, distinct from those around it, and called the plerome, meaning “that which fills up.” Farther down, where the plerome passes into permanent tissue, it is called the central cylinder or stele (‘‘column”). Between the plerome and dermatogen is a tissue region called the periblem, meaning “that which is put around,” and when it becomes permanent tissue it is called the cor/ex, meaning “ bark” or “rind.”
Putting these facts together, the general statement is that at the apex there is the apical group of meristem cells ;
”
284 PLANT STRUCTURES
below them are the three embryonic regions, dermatogen, periblem, and plerome; and farther below these three regions pass into permanent tissue, organizing the epider- mis, cortex, and stele. The three embryonic regions are usually not so distinct in the Conifer stem as in the Dico- tyl stem, but both stems have epidermis, cortex, and stele. Epidermis.—The epidermis is a protective layer, whose cells do not become so much modified but that they may be regarded as parenchyma. It gives rise also to super- ficial parts, as hairs, etc. In the case of trees, the epidermis does not usually keep up with the increasing diameter, and disappears. This puts the work of protection upon the cortex, which organizes a superficial tissue called cork, a prominent part of the structure known as bark. Cortee.—The cortex is characterized by containing much active parenchyma, or primitive tissue, being the chief seat of the life activities of the stem. Its superficial cells, at least, contain chlorophyll and do chlorophyll work, while its deeper cells are usually temporary storage places for food. The cortex is also char- acterized by the development of stereome, or rigid tissues for me- chanical support. The stereome may brace the epidermis, forming the hypodermis ; or it may form bands and strands within the cor- tex; in fact, its amount and ar- rangement differ widely in differ- ent plants. The two principal stereome tis- Fr ee mn em sues are collenchyma and seleren- mon dock (umes), showing ehyma, meaning *‘ sheath-tissue ” aie ee me and “hard-tissue” respectively. In collenchyma the cells are thick- ened at the angles and have very elastic walls (Fig. 26%), making the tissue well adapted for parts which are growing
DIFFERENTIATION OF TISSUES 985
in length. The chief mechanical tissue for parts which have stopped growing in length is sclerenchyma (Figs. 264, 265). The cells are thick-walled, and usually elongated and with tapering ends, including the so-called “ fibers.”
Via ZN Fs SE san SZ) )
A, cross-section; B, longitudinal section; the letters in both referring to the same structures; J/, pith; XY, xylem, containing spiral (s, s’) and pitted (¢, ¢’) vessels; ¢, cambium; P, phloem, containing sieve vessels (sb); 6, a mass of bast fibers or sclerenchyma; ic, pith rays between the bundles; e, the bundle sheath; 7, cor- tex.—After VINES.
Stele.—The characteristic feature of the stele or central cylinder is the development of the mestome or vascular
286 PLANT STRUCTURES
tissues, of which there are two prominent kinds. The tracheary vessels are for water conduction, and are cells with heavy walls and usually large diameter (Fig. 268). The thickening of the walls is not uniform, giving them a very characteristic appearance, the thickening taking the form of spiral bands, rings, or reticulations (Fig. 268, B). Often the reticulation has such close meshes that the cell wall has the appearance of being covered with thin spots, and such cells are called ‘ pitted vessels.” The vessels with spirals and rings are usually much smaller in diameter than the pitted ones. The true tracheary cells are more or less elongated and without tapering ends, fitting end to end and forming a continuous longitudinal series, suggesting a trachea, and hence the name. In the Conifers there are no true tracheary ceils, as in the Dicotyledons, except a few small spiral vessels which are formed at first in the young stele, but the tracheary tissue is made up of ¢racherds, mean- ing “trachea-like,” differing from truchee or true tracheary vessels in having tapering ends and in not forming a continu- ous series (Fig. 269). The walls of these tracheids are ** pitted” in a way which is characteristic of Gymnosperms, the ‘pits ” appearing as two concentric rings, called “ bordered pits.” Fra. 269. ‘Tracheids from wood of The other prominent mes- pine, sng teri ovis tome tissue developed inthe stele is the steve vessels, for the conduction of organized food, chiefly proteids (Fig. 208). Sieve cells are so named because in their walls special areas are organized which are perforated like the lid of a pepper-
DIFFERENTIATION OF TISSUES IST
box or a “sieve.” These perforated areas are the szeve- plates, and through them the vessels communicate with one another and with the adjacent tissue.
The tracheary and sieve vessels occur in separate strands, the tracheary strand being called .rylem (“ wood”), the sieve strand phloem (“bark”). A xylem and a phloem strand are usually organized together to form a vascular bundle, and it is these fiber-like bundles which are found traversing the stems of all vascular plants and appearing conspicuously as the veins of leaves. Among the Dicotyls and Conifers the vascular bundles appear in the stele in such a way as to outline a hollow cylinder (Fig. 216), the xylem of each bundle being toward the center, the phloem toward the circumference of the stem. The undifferenti- ated parenchyma of the stele which the vascular cylinder incloses is called the pith. In older parts of the stem the pith is often abandoned by the activities of the plant, and either remains as a dead spongy tissue, or disappears en- tirely, leaving a hollow stem. Between the bundles form- ing the evsrulur cylinder there is also undifferentiated parenchyma, and as it seems to extend from the pith out between the bundles like ‘‘rays from the sun,” the rays are called pith rays.
Such vascular bundles as described above, in which the xylem and phloem strands are ‘‘ side-by-side ” upon the same radius, are called collatvral (Fig. 270). One of the pecul- iarities of the collateral bundles of Dicotyls and Conifers, however. is that when the two strands of each bundle are organized some meristem is left between them. This means that between the strands the work of forming new cells can goon. Such bundles are said to be open; and the apen collateral bundle is characteristic of the stems of the Dico- tyls and Conifers.
The meristem between the xylem and phloem of the open bundle is called cambium (Figs. 268, 270). The cam- bium also extends across the pith rays between the bundles,
288 PLANT STRUCTURES connecting the cambium in the bundles, and thus forming
acambium cylinder, which separates the xylem and phloem of the vascular cylinder. This cambium continues the for-
PE
Fig. 270. Cross-section of open collateral vascular bundle from stem of castor-oil plant (Ricinus), showing pith cells (a), xylem containing spiral (2) and pitted (q) vessels, cambium of bundle (c) and of pith rays (cb), phloem containing sieve ves- sels (y), three bundles of bast fibers or sclerenchyma (), the bundle sheath con- taining starch grains, und outside of it parenchyma of the cortex (r),—After Sacits.
mation of xylem tissue on the one side and phloem tissue on the other in the bundles, und new parenchyma between the bundles, and so the stem increases in diameter. If the stem lives from year to year the addition made by the cam- bium each season is marked off from that of the previous season, giving rise to the so-called growth rings or annual rings, 80 conspicuous a feature of the cross-section of tree
DIFFERENTIATION OF TISSUES 989
trunks (Fig. 217). This continuous addition to the vessels increases the capacity of the stem for conduction, and per- mits the further extension of branches and a larger display of leaves.
The annual additions to the xylem are added to the in- creasing mass of wood. The older portions of the xylem mass are gradually abandoned by the ascending water (<‘sap”), often change in color, and form the heart-wood. The younger portion, through which the sap is moving, is the sup-wood. It is evident, however, that the annual ad- ditions to the phloem are not in a position for permanency. The new phloem is deposited inside of the old, and this, to- gether with the new xylem, presses upon the old phloem, which becomes ruptured in various ways, and rapidly or very gradually peels off, being constantly renewed from within. It is the protecting layers of cork (see this section under Cortez), the old phloem, and the new phloem down to the cambium, which constitute the so-called bark of trees, a structure exceedingly complex and extremely vari- able in different trees.
The stele also frequently develops stereome tissue in the form of sclerenchyma. These thick-walled fibers are often closely associated with one or both of the vascular strands of the bundles (Fig. 270), and lead to the old name jidro- vascular bundles.
To sum up, the stems of Dicotyledons and Conifers are characterized by the development of a vascular cylinder, in which the bundles are collateral and open, permitting increase in diameter, extension of the branch system, and a continuous increase in leaf display.
152. Monocotyl stems—In the stems of Monocotyledons there is the same apical development and differentiation (Fig. 266). The characteristic difference from the Dicotyl and Conifer type, just described, is in connection with the development of the vascular bundles in the stele. Instead of outlining a hollow cylinder, the bundles are scattered
290 PLANT STRUCTURES
through the stele (Fig. 214). This lack of regularity would interfere with the organization of a cambium cylinder, and we find the bundles collateral but closed—that is, with no meristem left between the xylem and phloem (Fig. 271).
Rs
evar: }
Fig. 271. Cross-section of a closed collateral bundle from the stem of corn, showing the xylem with annular (7), spiral (s), and pitted (g) vessels: the phloem contain- ing sieve vessels (v), and separated from the xylem by no intervening cambium; both xylem and phloem surrounded by 2 mass of sclerenchyma (fibers); and in- vesting vessels and fibers the parenchyma (p) of the pith-like tissue through which the bundles are distributed.—After Sacus.
This lack of cambium means that stems living for sey- eral years do not increase in diameter, but become columnar
DIFFERENTIATION OF TISSUES 291
shafts, as in the palm, rather than much elongated cones. It also means lack of ability to develop an extending branch system or to display more numerous leaves each year. The palm may be taken as a typical result of such a structure, with its columnar and unbranched trunk, and its foliage crown containing about the same number of leaves each year.
The lack of regular arrangement of the bundles also prevents the outlining of a pith region or the organization of definite pith rays. The failure to increase in diameter also precludes the necessity of bark, with its protective cork constantly renewed, and its sloughing-off phloem.
To sum up, the stems of the Monocotyledons are characterized by the vascular bundles not developing a cylinder or any regular arrangement, and by collateral and closed bundles, which do not permit increase in diameter, or a branch system, or increase in leaf display.
153. Pteridophyte stems.—The stems of Pteridophytes are quite different from those of Spermatophytes. While the large Club-mosses (Lyco- podium) and Jscetes usually have an apical group of meris- tem cells, as among the Seed- plants, the smaller Club-mosses (Selaginella), Ferns, and Horse- tails usually have a single api- cal cell, whose divisions give rise to all the cells of the stem.
Fig. 272. Diagram of tissnes in cross- section of stem of a fern (Pferis),
Generally also a dermatogen is showing two masses of scleren- Site * . chyma (sf), between and about not organized, and in such which are vascular bundles. —
cases there isno true epidermis, — Cuamperrary.
the cortex developing the ex-
ternal protective tissue. In the cortex there is usually an extensive development of stereome. in the form of scleren- chyma (Fig. 272), the stele furnishing little or none, and the vascular bundles not adding much to the rigidity, as they do in the Seed-plants.
299 PLANT STRUCTURES
In ELquasetum and Isocetes the vascular bundles may be said to be collateral, as in the Seed-plants, but the charac- teristic Pteridophyte type is very different. In fact, the vascular masses can hardly be compared with the bundles of the Seed-plants, although they are called bundles for convenience. In the stele one or more of these bundles are organized (Fig. 272), the tracheary vessels (xylem) being in the center and completely invested by the sieve vessels
Fic. 278. Cross-section of concentric vascular bundle of a fern (Pteris): the single row of shaded cells investing the others is the bundle sheath; the large and heavy- walled cells within constitute the xylem; and between the xylem and the bundle sheath is the phloem.—CHAMBERLAIN.
(phloem). This is called the concentric bundle (Fig. 273), as distinguished from the collateral bundles of Seed-plants, and is characteristic of Pteridophyte stems.
DIFFERENTIATION OF TISSUES 993
154. Roots—True roots appear only in connection with the vascular plants (Pteridophytes and Spermatophytes) ; and in all of them the structure is essentially the same, and quite different from stem structure. A single ap- ical cell (in most Pteridophytes) (Fig. 274) or an apical group (in Spermatophytes) usually gives rise to the three embryonic regions—dermatogen, periblem, and plerome (Fig. 275).
A fourth region, how- ever, peculiar to root, is usually added. The apical
Fie. 274. Section through root-tip of Fig. 275. A longitudinal section through Pteris; the cell with a nucleus is the the root-tip of shepherd’s purse,
single apical cell, which in front has showing the plerome (p/), surround- cut off cells which organize the root- ed by the periblem (p), outside of cap.—CHAMBERLAIN. periblem the epidermis (e) which disappears in the older parts of the
cell or group cuts off a tis- root, and the prominent root-cap (¢).
—From ‘Plant Relations.”
sue in front of itself (Fig. 274), known as the calyptrogen, or “cap producer,” for it organizes the root-cap, which protects the delicate meri- stem of the growing point.
Another striking feature is that in the stele there is organized a single solid vascular cylinder, forming a tough central axis (Fig. 277), from which the usually well-devel- oped cortex can be peeled off as a thick rind. In this vas- cular axis, which is called ‘‘a bundle ” for convenience but does not represent the bundle of Seed-plant stems, the ar- rangement of the xylem and phloem is entirely unlike that
37
294 PLANT STRUCTURES
Fie. 276. Cross-section of the vascular axis of a root, showing radiate type of bundle the xylem (j) and phloem (pf) alternating. —After Sacus.
found in stems. The xylem is in the center and sends out a few radiating arms, between which are strands of phloem,
Fie, 277. Endogenous origin of root branch- es, showing them (7) arising from the cen- tral axis (7) and breaking through the cortex (7).—After VINEs.
forming the so-called radiate bundle (Fig. 276). This arrangement brings the tracheary vessels (xylem) to the surface of the bundle region, which is not true of either the concentric or collateral bundle. This seems to be associated with the fact that the xylem is to receive and conduct the water absorbed from the soil. It should be said that this characteristic
DIFFERENTIATION OF TISSUES 295
bundle structure of the root appears only in young and active roots. In older ones certain secondary changes take place which obscure the structure and result in a resem- blance to the stem.
The origin of branches in roots is also peculiar. In stems branches originate at the surface, involving epi- dermis, cortex, and vascular bundles, such an origin being called exogenous (** produced outside“); but in roots branches originate on the vascular cylinder, burrow through the cortex, and emerge at the surface (Fig. 277). If the cortex be stripped off from a root with branches, the branches are left attached to the woody axis, and the cor- tex is found pierced with holes made by the burrowing branches. Such an origin is called endogenous, meaning + produced within.”
To sum up the peculiarities of the root, it may be said to develop a root-eap. to have a solid vascular cylinder in which the xylem and phloem are arranged to form a bundle of the radiate type, and to branch endogenously.
Z ra sh St Fig. 278. A section through the leaf of lily, showing upper epidermis (we), lower epi- dermis (/e) with its stomata (sf), mesophyll (dotted cells) composed of the palisade region (p) and the spongy region (sp) with air spaces among the cells. and two veins (v) cut across.—From * Plant Relations.”
296 PLANT STRUCTURES
155. Leaves—Leaves usually develop from an apical region in the same general way as do stems and roots, modified by their common dorsiventral character. Com- paring the leaf of an ordinary seed-plant with its stem, it will. be noted that the three regions are represented (Fig. 278): (1) the epidermis ; (2) the cortex, represented by the mesophyll ; (3) the stele, represented by the veins.
In the case of collateral bundles, where in the stem the xylem is always toward the center and the phloem is toward the circumference, in the leaves the xylem is toward the upper and the phloem toward the lower surface.
CHAPTER XVI PLANT PHYSIOLOGY
156. Introductory.—Plants may be studied from several points of view, each of which has resulted in a distinct division of Botany. The study of the forms of plants and their structure is MoRPHOLOGY, and it is this phase of Bot- any which has been chiefly considered in the previous chap- ters. The study of plants at work is PaysioLoGy, and as structure is simply preparation for work, the preceding chapters have contained some Physiology, chiefly in refer- ence to nutrition and reproduction. The study of the clas- sification of plants is Taxonomy, and in the preceding pages the larger groups have been outlined. The study of plants as to their external relations is EcoLoey, a subject which will be presented in the following chapter, and which is the chief subject of Plant Relations. The study of the diseases of plants and their remedies is ParnoLoey ; their study in relation to the interests of man is Economic Borany.
Besides these general subjects, which apply to all plants, the different groups form the subjects of special study. The study of the Morphology, Physiology, or Taxonomy of the Bacteria is Bacteriology; of the Alge, Algology; of the Fungi, Mycology; of the Bryophytes, Bryology ; of the fossil plants, Paleobotany or Paleophytology, etc.
In the present chapter it is the purpose to give a very brief outline of the great subject of Plant Physiology, not with the expectation of presenting its facts adequately, but with the hope that the important field thus presented may
297
298 PLANT STRUCTURES
attract to further study. It is merely the opening of a door to catch a fleeting glimpse.
A common division of the subject presents it under five heads: (1) Stability of form, (2) Nutrition, (3) Respira- tion, (4) Movement, (5) Reproduction.
STABILITY OF FORM
157. Turgidity.—It is a remarkable fact that plants and parts of plants composed entirely of cells with very thin and delicate walls are rigid enough to maintain their form. It has already been noted (see § 20) that such active cells exert an internal pressure upon their walls. This seems to be due to the active absorption of liquid, which causes the very elastic walls to stretch, as in the ‘‘ blowing up ” of a bladder. In this way each gorged and distended cell be- comes comparatively rigid, and the mass of cells retains its form. It seems evident that the active protoplasm greedily pulls liquid through the wall and does not let it escape so easily. If for any reason the protoplasm of a gorged cell loses its hold upon the contained liquid the cell collapses.
158. Tension of tissues—The rigidity which comes to active parenchyma cells through their turgidity is increased by the tensions developed by adjacent tissues. For exam- ple, the internal and external tissues of a stem are apt to increase in volume at different rates; the faster will pull upon the slower, and the slower will resist, and thus be- tween the two a tension is developed which helps to keep them rigid. This is strikingly shown by splitting a dande- lion stem, when the inner tissue, relieved somewhat from the resistance of the outer, elongates and causes the strip to become strongly curved outward or cven coiled. Experi- ments with strips from active twigs, including the pith, will usually demonstrate the same curve outward. Tension of tissues is chiefly developed, of course, where elongation is taking place.
PLANT PHYSIOLOGY 299
159. Stereome—When growth is completed, cell walls lose their elasticity. turgidity becomes less, and therefore tensions diminish, and rigidity is supplied by special ster- eome tissues, chief among which is sclerenchyma. An- other stereome tissue is collenchyma, which on account of its elastic walls can be used to supplement turgidity and tension where elongation is still going on. For a fuller account of stereome tissues see § 150.
NUTRITION
160. Food—Plant food must contain carbon (C), hydro- gen (H), oxygen (Q), and nitrogen (\). and also more or less of other elements, notably sulphur, phosphorus, potassium, calcium, magnesium, and iron. In the case of green plants these elements are obtained from inor- ganic compounds and food is manufactured ; while plants without chlorophyll obtain their food already organized. The sources of these elements for green plants are as follows: Carbon from carbon dioxide (CQ,) of the air; hydrogen and oxygen from water (H,0):; and nitrogen and the other elements from their various salts which occur in the soil and are dissolved in the water which enters the plant.
All of these substances must present themselves to plants in the form of a gas or a liquid, as they must pass through cell walls: and the processes of absorption have to do with the taking in of the gas carbon dioxide and of water in which the necessary salts are dissolved.
161. Absorption—Green plants alone will be considered, as the unusual methods of securing food have been men- tioned in Chapter VII. For convenience also, only terres- trial green plants will be referred to, as it is simple to modify the processes to the aquatic habit, where the sur- rounding water supplies what is obtained by land plants from both air and soil.
300 PLANT STRUCTURES
In such plants the carbon dioxide is absorbed directly from the air by the foliage leaves, whose expanse of surface is as important for this purpose as for exposing chlorophyll to light. When the work of foliage leaves is mentioned it must always be understood that it applies as well to any green tissue displayed by the plant.
The water, with its dissolved salts, is absorbed from the soil by the roots. Only the youngest parts of the root- system can absorb, and the absorbing capacity of these parts is usually vastly increased by the development of numerous root hairs just behind the growing tip (Fig. 194). These root hairs are ephemeral, new ones being continu- ally put out as the tip advances, and the older ones disap- pearing. They come in very close contact with the soil particles, and ‘suck in” the water which invests each particle as a film.
162. Transfer of water.—The water and its dissolved salts absorbed by the root-system must be transferred to the foli- age leaves, where they are to be used, along with the carbon dioxide, in the manufacture of food.
Having entered the epidermis of the absorbing rootlets the water passes on to the cortex, and traversing it enters the xylem system of the central axis. In some way this transfer is accompanied by pressure, known as root pres- sure, which becomes very evident when an active stem is cut off near the ground. The stump is said to “bleed,” and sends out water (‘‘sap”) as if there were a force pump in the root-system. This root pressure doubtless helps to lift the water through the xylem of the root into the stem, and in low plants may possibly be able to send it to the leaves, but for most plants this is not possible.
When the water enters the xylem of the root it is ina continuous system of vessels which extends through the stem and out into the leaves. The movement of the ab- sorbed water through the xylem is called the transpiration current, or very commonly the ‘‘ascent of sap.” An ex-
PLANT PHYSIOLOGY 301
periment demonstrating this ascent of sap and its route through the xylem will be found described in Plant [ela- tions, p. 151. How it is that the transpiration current moves through the xylem is not certainly known.
163. Transpiration— When the water carrying dissolved salts reaches the mesophyll cells, some of the water and all of the salts are retained for food manufacture. However, much more water enters the leaves than is needed for food, this excess having been used for carrying soil salts. When the soil salts have reached their destination the excess of water is evaporated from the leaf surface, the process being called transpiration. For an experiment demonstrating transpiration see Plant Relations, § 26.
This transpiration is regulated according to the needs of the plant. If the water is abundant, transpiration is encouraged ; if the water supply is low, transpiration is checked. One of the chief ways of regulating is by means of the very small but exceedingly numerous stomata (see § 79 [4]), whose guard cells become turgid or collapse and so determine the size of the opening between them. It has been estimated that a leaf of an ordinary sunflower contains about thirteen million stomata, but the number varies widely in different plants. In ordinary dorsiventral leaves the sto- mata are much more abundant upon the lower surface than upon the upper, from which they may be lacking entirely. In erect leaves they are distributed equally upon both sur- faces ; in floating leaves they occur only upon the upper surface ; in submerged leaves they are lacking entirely.
The amount of water thus evaporated from active leaves is very great. It is estimated that the leaves of a sunflower as high as a man evaporate about one quart of water in a warm day; and that an average oak tree in its five active months evaporates about twenty-eight thousand gallons. If these figures be applied to a meadow or a forest the result may indicate the large importance of this process.
802 PLANT STRUCTURES
164. Photosynthesis—This is the process by which car- bon dioxide and water are “broken up,” their elements recombined to form a carbohydrate, and some oxygen given off as a waste product, the mechanism being the chloroplasts and light. It has been sufficiently described in § 55, and also in Plant Relations, pp. 28 and 150.
165. Formation of proteids—The carbohydrates formed by photosynthesis, such as starch, sugar, etc., contain car- bon, hydrogen, and oxygen. Ont of them the living cells must organize proteids, and in the reconstruction nitrogen and sulphur, and sometimes phosphorus, are added. This work goes on both in green cells and other living cells, as it does not seem to be entirely dependent upon chloroplasts and light.
166. Transfer of carbohydrates and proteids——These two forms of food having been manufactured, they must be carried to the regions of growth or storage. In order to be transported they must be in soluble form, and if not already soluble they must be digested, insoluble starch being con- verted into soluble sugar, etc. In these digested forms they are transported to regions where work is going on, and there they are assimilated—that ‘is, transformed into the enormously complex working substance protoplasm ; or they are transported to regions of storage and there they are reconverted into insoluble storage forms, as starch, etc.
These foods pass through both the cortex and phloem in every direction, but the long-distance transfer of pro- teids, as from leaves to roots, seems to be mainly through the sieve vessels.
RESPIRATION
167. Respiration.—This is an essential process in plants as well as in animals, and is really the phenomenon of “breathing.” The external indication of the process is the absorption of oxygen and the giving out of carbon di- oxide ; and it goes on in all organs, day and night. When
PLANT PHYSIOLOGY 303
it ceases death ensues sooner or later. By this process energy, stored up by the processes of nutrition, is liberated, and with this liberated energy the plant works. It may be said that oxygen seems to have the power of arousing pro- toplasm to activity.
It is not sufficient for the air containing oxygen to come in contact merely with the outer surface of a complex plant, as its absorption and transfer would be too slow. There must be an ‘internal atmosphere” in contact with the living cells. This is provided for by the intercellular spaces, which form a labyrinthine system of passageways, opening at the surface through stomata and lenticels (pores through bark). In this internal atmosphere the exchange of oxygen and carbon dioxide is effected, the oxygen being renewed by diffusion from the outside, and the carbon dioxide finally escaping by diffusion to the outside.
MOVEMENT
168. Introductory.—In addition to movements of mate- rial, as described above, plants execute movements depend- ent upon the activity of protoplasm, which result in change of position. Naked masses of protoplasm, as the plas- modium of slime-moulds (see § 51), advance with a sliding, snail-like movement upon surfaces ; zoospores and ciliated sperms swim freely about by means of motile cilia; while many low plants, as Bacteria (§ 52), Diatoms (§ 34), Oseil- laria (§ 20), ete., have the power of locomotion.
When the protoplasm is confined within rigid walls and tissues, as in most plants, the power of locomotion usually disappears, and the plants are fixed ; but within active cells the protoplasm continues to move, streaming back and forth and about within the confines of the cell.
In the case of complex plants, however, another kind of movement is apparent, by which parts are moved and variously directed, sometimes slowly, sometimes with great
304 PLANT STRUCTURES
rapidity. In these cases the part concerned develops a curvature, and by various curvatures it attains its ultimate position. These curvatures are not necessarily permanent, for a perfectly straight stem results from a series of cur- vatures near its apex. Curvatures may be developed by unequal growth on the two sides of an organ, or by unequal turgidity of the cells of the two sides, or by the unequal power of the cell walls to absorb water.
169. Hygroscopic movements.—These movements are only exhibited by dry tissues, and hence are not the direct result of the activity of protoplasm. The dry walls absorb mois- ture and swell up, and if this absorption of moisture and its evaporation is unequal on two sides of an organ a curva- ture will result. In this way many seed vessels are rup- tured, the sporangia of ferns are opened, the operculum of mosses is lifted off by the peristome, the hair-like pappus of certain Composites is spread or collapsed, certain seeds are dispersed and buried, etc. One of the peculiarities of this hygroscopic power of certain cells is that the result may be obtained through the absorption of the moisture of the air, and the hygroscopic awns of certain fruits have been used in the manufacture of rough hygrometers (‘‘ measures of moisture ”).
170. Growth movements.—Growth itself is a great physi- ological subject, but certain movements which accompany it are referred to here. Two kinds of growth movements are apparent.
One may be called nutation, by which is meant that the growing tip of an organ does not advance in a straight line, but bends now toward one side, now toward the other. In this way the tip describes a curve, which may be a circle, or an ellipse of varying breadth; but as the tip is advancing all the time, the real curve described is a spiral with circular or elliptical cross-section. The sweep of a young hop-vine in search of support, or of various tendrils, may be taken as extreme illustrations, but in most cases
PLANT PHYSIOLOGY 305
the nutation of growing tips only becomes apparent through prolonged experiment.
The other prominent growth movement is that which places organs in proper relations for their work, sending roots into the soil and stems into the air, and directing leaf planes in various ways. For example, in the germina- tion of an ordinary seed, in whatever direction the parts emerge the root curves toward the soil, the stem turns upward, and the cotyledons spread out horizontally.
The movement of nutation seems to be due largely to internal causes, while the movements which direct organs are due largely to external causes known as stimuli, Some of the prominent stimuli concerned in directing organs are as follows:
Heliotropism.—tIn this case the stimulus is light, and under its influence acrial parts are largely directed. Plants growing in a window furnish plain illustration of helio- tropism. In general the stems and petioles curve toward the light, showing positive heliutropism (Fig. 279); the leaf blades are directed at right angles to the rays of light, showing transverse heliotropism ; while if there are hold- fasts or aérial roots they are directed away from the light, showing negative heliotropism. The thallus bodies of ferns, liverworts, etc., are transversely heliotropic, as ordinary leaves. a position best related to chlorophyll work. If the light is too intense, leaves may assume an edgewise or pro- file position, a condition well illustrated by the so-called “compass plants.” (See Plant Relations, p. 10.)
Geotropism.—In this case the stimulus is gravity, and its influence in directing the parts of plants is very great. All upward growing plants, as ordinary stems. some leaves, etc.. are negatively geotropic, growing away from the center of gravity. Tap-roots are notable illustrations of positive geotropism, growing toward the source of gravity with con- siderable force. Lateral branches from a main or tap-root, however. are usually transversely geotropic.
Fig, 279. Sunflower stems with the upper part of the stem sharply bent toward the light, giving the leaves better exposure, the stem showing positive heliotropism,— After SCHAFPNER.
PLANT PITYSIOLOGY 307
That these influences in directing are very real is testi- fied to by the fact that when the organs are turned aside from their proper direction they will curve toward it and overcome a good deal of resistance to regain it. Although these curvatures are mainly developed in growing parts, even mature parts which have been displaced may be brought back into position. For example, when the stems of certain plants, notably the grasses, have been prostrated by wind, etc., they often can resume the erect position under the influence of negative geotropism, a very strong and even angular curvature being developed at certain joints.
Hydrotropism.—The influence of moisture is very strong in directing certain organs, notably absorbing systems. Roots often wander widely and in every direction under the guidance of hydrotropism, even against the geotropic influence. Ordinarily geotropism and hydrotropism act in the same direction, but it is interesting to dissociate them so that they may “‘pull” against one another. For such an experiment see Plant Relutionsx, p. 91.
Other stimuli.—Other outside stimuli which have a directive influence upon organs are chemical substances (chemotropism), such as direct sperms to the proper female organ ; heat (¢hermofrapism) ; water currents (rheotrapism) 5 mechanical contact, etc. The most noteworthy illus- trations of the effect of contact are furnished by tendril- climbers. When a nutating tendril comes in contact with a support a sharp curvature is developed which grasps it. In many cases the irritable response goes further. the ten- dril between the plant axis and the support developing a spiral coil.
171. Irritable movements.—The great majority of plants can execute movements only in connection with growth, as described in the preceding section, and when mature their parts are fixed and incapable of further adjustment. Cer- tain plants, however, have developed the power of moving mature parts, the motile part always being a leaf, such as
308 PLANT STRUCTURES
foliage leaf, stamen, etc. It is interesting to note that these movements have been cultivated by but few families, nota- ble among them being the Legumes (§ 141).
These movements of mature organs, some of which are very rapid, are due to changes in the turgidity of cells. As already mentioned (§ 157), turgid cells are inflated and rigid, and when turgidity ceases the cells collapse and the tissue becomes flaccid. A special organ for varying tur- gidity, known as the pulvinus, is usually associated with the motile leaves and leaflets. The pulvinus is practically a mass of parenchyma cells, whose turgidity is made to vary by various causes, and leaf-movement is the result.
The causes which induce some movements are unknown, as in the case of Desmodium gyrans (see Plant Relations, p. 49), whose small lateral leaflets uninterruptedly de- scribe circles, completing a cycle in one to three minutes.
In other cases the inciting cause is the change from light to dark, the leaves assuming at night a very dif- ferent position from that during the day. Dur- ing the day the leaflets are spread out freely,
Fig. 280. A leaf of a sensitive plant in two conditions: in the figure to the left the leaf is fully expanded, with its four main divisions and numcrous leaflets well spread; in the figure to the right is shown the same leaf after it has been ‘‘ shocked” by a sudden touch, or by sudden heat, or in some other way; the leaflets have been thrown together forward and upward, the four main divisions have been moved together, and the main leaf-stalk has been directed sharply downward,—After DucuaRtTRE.
PLANT PHYSIOLOGY 309
while at night they droop and usually fold together (see Plant Relutions, pp. 9, 10). These are the so-called nycti- tropic movements or *‘ night movements,” which may be ob- served in many of the Legumes, as clover, locust, bean, etc.
In still other cases, mechanical irritation induces move- ment, as sudden contact, heat, injury, etc. Some of the “carnivorous plants” are notable illustrations of this, es- pecially Dionea, which snaps its leaves shut like a steel trap when touched (see Plunt Relutivns, p. 161). Among the most irritable of plants are the so-called *‘ sensitive plants,” species of Mimosa, Acacia, etc., all of them Le- gumes. The most commonly cultivated sensitive plant is Uinosa pudica (Fig. 280), whose sensitiveness to contact and rapidity of response are remarkable (see Plant Rela- tions, p. 48).
REPRODUCTION
172. Reproduction.—The important function of repro- duction has been considered in connection with the various plant groups. Among the lowest plants the only method of reproduction is cell division, which in the complex forms results in growth. In the more complex plants va- rious outgrowths or portions of the body, as gemme, buds, bulbs, tubers, various branch modifications, etc., furnish means of propagation. All of these methods are included under the head of vegetative multiplication. as the plants are propagated by ordinary vegetative tissues.
When a special cell is organized for reproduction, dis- tinct from the vegetative cells. it is called a spore, and re- production by spores is introduced. The first spores devel- oped seem to have been those produced by the division of the contents of a mother cell. and are called axerval spores. These spores are scattered in various ways—by swimming (zoospores), by floating, by the wind, by insects.
Another type of spore is the serval spore. formed by the union of two sexual cells called gametes. The gametes
38
310 PLANT STRUCTURES
seem to have been derived from asexual spores. At first the pairing gametes are alike, but later they become differ- entiated into syerms or male cells, and eggs or female cells.
With the establishment of alternation of generations, the asexual spores are restricted to the sporophyte, and the gametes to the gumetophyte. With the further introduction of heterospory, the male and the female gametes are sepa- rated upon different gametophytes, which become much reduced.
With the reduction of the megaspores to one in a spo- rangium (ovule), and its retention, the seed is organized, and the elaborate scheme of insect-pollination is developed.
CHAPTER XVII PLANT ECOLOGY
173. Introductory.— Ecology has to do with the external relations of plants, and forms the principal subject of the volume entitled Plant Relations, which should be consulted for fuller descriptions and illustrations. It treats of the adjustment of plants and their organs to their physical surroundings, and also their relations with one another and with animals, and has sometimes been called ‘plant sociology.”
LIFE RELATIONS
174. Foliage leaves.—The life relation essential to foliage leaves is the relation to light. This is shown by their positions and forms, as well as by their behavior when deprived of light. This light relation suggests the answer to very many questions concerning leaves. It is not very important to know the names of different forms and differ- ent arrangements of leaves, but it is important to observe that these forms and arrangements are in response to the light relation.
In general a leaf adjusts its own position and its relation to its fellows so as to receive the greatest amount of light. Upon erect stems the leaves occur in vertical rows which are uniformly spaced about the circumference. If these rows are numerous the leaves are narrow; if they are few the leaves are usually broad. If broad leaves were associ-
ated with numerous rows there would be excessive shading ; 311
312 PLANT STRUCTURES
if narrow leaves were associated with few rows there would be waste of space.
It is very common to observe the lower leaves of a stem long-petioled, those above short-petioled, and so on until the uppermost have sessile blades, thus thrusting the blades of lower leaves beyond the shadow of the upper leaves. There may also be a gradual change in the size and direc- tion of the leaves, the lower ones being relatively large and horizontal, and the upper ones gradually smaller and more directed upward. In the case of branched (compound) leaves the reduction in the size of the upper leaves is not so necessary, as the light strikes between the upper leaflets and reaches those below.
On stems exposed to light only or chiefly on one side, the leaf blades are thrown to the lighted side in a variety of ways. Inivies, many prostrate stems, horizontal branches of trees, etc., the leaves brought to the lighted side are observed to form regular mosaics, each leaf interfering with its neighbor as little as possible.
There is often need of protection against too intense light, against chill, against rain, etc., which is provided for in a great variety of ways. Coverings of hairs or scales, the profile position, the temporary shifting of position, rolling up or folding, reduction in size, etc., are some of the common methods of protection.
175. Shoots—The stem is an organ which is mostly related to the leaves it bears, the stem with its leaves being the shoot. In the foliage-bearing stems the leaves must be displayed to the light and air. Such stems may be sub- terranean, prostrate, floating, climbing, or erect, and all of these positions have their advantages and disadvantages, the erect type being the most favorable for foliage display.
In stems which bear scale leaves no light relation is necessary, so that such shoots may be and often are sub- terranean, and the leaves may overlap, as in scaly buds and bulbs. The subterranean position is very favorable
PLANT ECOLOGY 313
for food storage, and such shoots often become modified as food depositories, as in bulbs, tubers, rootstocks, ete. In the scaly buds the structure is used for protection rather than storage.
The stem bearing floral leaves is the shoot ordinarily called ‘the flower,” whose structure and work have been sufficiently described. Its adjustments have in view polli- nation and seed dispersal, two very great ecological sub- jects full of interesting details.
176. Roots.—Roots are absorbent organs or holdfasts or both, and ‘they enter into a variety of relations. Most common is the soil relation, and the energetic way in which such roots penetrate the soil, and search in every direction for water and absorb it, proves them to be highly organized members. Then there are roots related to free water, and others to air, each with its appropriate struc- ture. More mechanical are the clinging roots (ivies, etc.), and prop roots (screw pines, banyans, etc.), but their adap- tation to the peculiar service they render is none the less interesting.
The above statements concerning leaves, shoots, and roots should be applied with necessary modifications to the lower plants which do not produce such organs. The light relation and its demands are no less real among the Alge than among Spermatophytes, as well as relations to air, soil, water, mechanical support, etc.
PLANT SOCIETIES
177. Introductory—Plants are not scattered at hap- hazard over the surface of the earth, but are organized into definite communities. These communities are deter- mined by the conditions of living—conditions which admit some plants and forbid others. Such an association of plants living together in similar conditions is a plant so- ciety. Closely related plants do not usually live together
314 PLANT STRUCTURES
in the same society, as their rivalry is too intense; but each society is usually made up of unrelated plants which can make use of the same conditions.
There are numerous factors which combine to deter- mine societies, and it is known as yet only in a vague way how they operate.
178. Ecological factors— Water.—This is a very impor- tant factor in the organization of secieties, which are usu- ally local associations. Taking plants altogether, the amount of water to which they are exposed varies from complete submergence to perpetual drought, but within this range plants vary widely as to the amount of water necessary for living.
Heat.—In considering the general distribution of plants over the surface of the earth, great zones of plants are out- lined by zones of temperature ; but in the organization of local societies in any given area the temperature condi- tions are nearly uniform. Usually plants work only at temperatures between 32° and 122° Fahr., but for each plant there is its own range of temperature, sometimes extensive, sometimes restricted. Even in plant societies, however, the effect of the heat factor may be noted in the succession of plants through the working season, spring plants being very different from summer and autumn plants.
Soil.—The great importance of this factor is evident, even in water plants, for the soil of the drainage area deter- mines the materials carried by the water. Soil is to be considered both as to its chemical composition and its physical properties, the latter chiefly in reference to its disposition toward water. Soils vary greatly in the power of receiving and retaining water, sand having a high recep- tive and low retentive power, and clay just the reverse, and these factors have large effect upon vegetation.
Light.—All green plants can not receive the same amount of light. Hence some of them have learned to live with a
PLANT ECOLOGY 315
less amount than others, and are ‘‘shade plants” as dis- tinct from “light plants.” In forests and thickets many of these shade plants are to be seen, which would find an exposed situation hard to endure. In almost every society, therefore, plants are arranged in strata, dependent upon the amount of light they receive, and the number of these strata and the plants characterizing each stratum are im- portant factors to note.
Wind.—This is an important factor in regions where there are strong prevailing winds. Wind has a drying effect and increases the transpiration of plants, tending to impoverish them in water. In such conditions only those plants can live which are well adapted to regulate tran- spiration.
The above five factors are among the most important, but no single factor determines a society. As each factor has a large possible range, the combinations of factors may be very numerous, and it is these combinations which de- termine societies. For convenience, however, societies are usually grouped on the basis of the water factor, at least three great groups being recognized.
179. Hydrophyte societies—These are societies of water plants, the water factor being so conspicuous that the plants are either submerged or standing in water. A plant completely exposed to water, submerged, or floating, may be taken to illustrate the usual adaptations. The epi- dermal walls are thin, so that water may be absorbed through the whole surface; hence the root system is very commonly reduced or even wanting ; and hence the water- conducting tissues (xylem) are feebly developed. The tis- sues for mechanical support (stereome) are feebly devel- oped, the plant being sustained by the buoyant power of water. Such a plant, although maintaining its form in water, collapses upon removal. Very common also is the development of conspicuous air passages for internal aéra- tion and for increasing buoyancy ; and sometimes a special
316 PLANT STRUCTURES
buoyancy is provided for by the development of bladder- like floats.
Conspicuous among hydrophyte societies may be men- tioned the following: (1) Free-swimming societies, in which the plants are entirely sustained by water, and are free to move either by locomotion or by water currents. Here belong the “plankton societies,” consisting of minute plants and animals invisible to the naked eye, conspicuous among the plants being the diatoms ; also the ‘‘ pond so- cieties,” composed of alge, duckweeds, etc., which float in stagnant or slow-moving waters.
(2) Pondweed societies, in which the plants are an- chored, but their bodies are submerged or floating. Here belong the ‘‘rock societies,” consisting of plants anchored to some firm support under water, the most conspicuous forms being the numerous fresh-water and marine alge, among which there are often elaborate systems of holdfasts and floats. The ‘loose-soil societies” are distinguished by imbedding their roots or root-like processes in the mucky soil of the bottom (Figs. 281, 282). The water lilies with their broad floating leaves, the pondweeds or pickerel weeds with their narrow submerged leaves, are conspicuous illus- trations, associated with which are alge, mosses, water ferns, etc.
(3) Swamp societies, in which the plants are rooted in water, or in soil rich in water, but the leaf-bearing stems rise above the surface. The conspicuous swamp societies are ‘reed swamps,” characterized by bulrushes, cat-tails and reed-grasses (Figs. 283, 28+), tall wand-like Monocoty- ledons, usually forming a fringe about the shallow margins of small lakes and ponds; ‘‘swamp-moors,” the ordinary swamps, marshes, bogs, etc., and dominated by coarse sedges and grasses (Fig. 282); ‘‘swamp-thickets,” consist- ing of willows, alders, birches, etc. ; ‘* sphagnum-moors,” in which sphagnummoss predominates, and is accompanied by numerous peculiar orchids, heaths, carnivorous plants, etc. ;
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PLANT ECOLOGY 319
““swamp-forests,” which are largely coniferous, tamarack (larch), pine, hemlock, etc., prevailing.
showing the reed swamp growth of rushes and sedyes.—Cow Les.
Shore of Calionet Lake, OL,
283.
Kia.
180. Kerophyte societies—These societies are exposed to the other extreme of the water factor, and are composed of plants adapted to dry air and soil. To meet these
320 PLANT STRUCTURES
drought conditions numerous adaptations have been de- veloped and are very characteristic of xerophytic plants. Some of the conspicuous adaptations are as follows: peri-
water, eventually leading to filimg up.—Cow es.
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odic reduction of surface, annuals bridging over a period of drought in the form of seeds, geophilous plants also dis- appearing from the surface and persisting in subterranean
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3824 PLANT STRUCTURES
parts, deciduous trees and shrubs dropping their leaves, etc. ; temporary reduction of surface, the leaves rolling up or folding together in various ways; profile position, the leaves standing edgewise and not exposing their flat sur- faces to the most intense light; motile leaves which can shift their position to suit their needs ; small leaves, a very characteristic feature of xerophytic plants; coverings of hair; dwarf growth; anatomical adaptations, such as cuticle, palisade tissue, etc. Probably the most conspicu- ous adaptation, however, is the organization of ‘‘ water- reservoirs,” which collect and retain the scanty water sup- ply, doling it out as the plant needs it.
Some of the prominent societies are as follows: ‘‘rock- societies ” composed of plants living upon exposed rock sur- faces, walls, fences, etc., notably lichens and mosses ; “sand societies,” including beaches, dunes, and sandy fields ; ‘‘shrubby heaths,” characterized by heath plants ; “plains,” the great areas of dry air and wind developed in the interiors of continents; “‘ cactus deserts,” still more arid areas of the Mexican region, where the cactus, agave, yucca, etc., have learned to live by means of the most ex- treme xerophytic modifications ; ‘‘ tropical deserts,” where xerophytic conditions reach their extreme in the combina- tion of maximum heat and minimum water ; ‘‘ xerophyte thickets,” the most impenetrable of all thicket-growths, represented by the “chaparral” of the Southwest, and the “bush” and “scrub” of Africa and Australia; ‘‘ xero- phyte forests,” also notably coniferous. (See Figs. 285, 286, 287.)
181. Mesophyte societies—Mesophytes make up the com- mon vegetation, the conditions of moisture being medium, and the soil fertile. This is the normal plant condition, and is the arable condition—that is, best adapted for the plants which man seeks to cultivate. If a hydrophytic area is to be cultivated, it is drained and made mesophytic ; if a xerophytic area is to be cultivated, it is irrigated and
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PLANT ECOLOGY 327
made mesophytic. As contrasted with hydrophyte and xero- phyte societies. the mesophyte societies are far richer in leaf forms and in general luxuriance. The artificial soci- eties which have been formed under the influence of man, through the introduction of weeds and culture plants. are all mesophytic.
Among the mesophyte grass and herb societies are the ‘arctic and alpine carpets.” so characteristic of high lati- tudes and altitudes where the conditions forbid trees. shrubs, or even tall herbs : ‘‘ meadows,” areas dominated by grasses, the prairies being the greatest meadows, where crasses and flowering herbs are richly displaved ; ‘* pastures.” drier and more open than meadows.
Among the woody mesophyte societies are the ‘ thick- ets.” composed of willow. alder. birch, hazel, etc., either pure or forming a jungle of mixed shrubs. brambles, and tall herbs: ‘ deciduous forests.” the glory of the temperate regions, rich in forms and foliage display. with annual fall of leaves. and exhibiting the remarkable and conspicuous phenomenon of autumnal coloration : ‘rainy tropical for- ests.” in the region of trade winds, heavy rainfalls, and great heat, where the world’s vegetation reaches its climax, and where in a saturated atmosphere gigantic jungles are developed, composed of trees of various heights, shrubs of all sizes. tall and low herbs, all bound together in an inex- tricable tangle by great vines or lianas, and covered by a luxuriant growth of numerous epiphytes. (See Figs. 233, 289.)
GLOS 5A RY
(The definitions of a glossary are often unsatisfactory. It is much better to con- sult the fuller explanations of the text by means of the index. The following glos- sary includes only frequently recurring technical terms. Those which are found only in reasonably close association with their explanation are omitted. The number fol- lowing each definition refers to the page where the term will be found most fully defined. }
ACTINOMORPHIC: applied to a flower in which the parts in each set are similar; regular. 228.
AKENE: a one-seeded fruit which ripens dry and seed-like. 212.
ALTERNATION OF GENERATIONS: the alternation of gametophyte and sporophyte in a life history. 94,
ANEMOPHILOUS: applied to flowers or plants which use the wind as agent of pollination, 181.
ANISOCARPIC: applied to a flower whose carpels are fewer than the other floral organs. 268.
ANTHER: the sporangium-bearing part of astamen. 197.
ANTHERIDIUM: the male organ, producing sperms. 16.
ANTIPODAL CELLS: in Angiosperms the cells of the female gametophyte at the opposite end of the embryo-sac from the egg-apparatus. 205.
APETALOUS : applied to a flower with no petals. 221.
APocARPOUS: applied to a flower whose carpels are free from one an- other. 226.
ARCHEGONIUM: the female, egg-producing organ of Brrophytes, Pteri- dophrtes, and Gymnosperms. 100.
ARCHESPORIUM : the first cell or group of cells in the spore-producing series. 102.
AscocaRP: a special case containing asci. 58.
AASCOSPORE : a spore formed within an ascus. 59.
Ascus: a delicate sac (mother-cell) within which ascospores develop. 59.
ASEXUAL SPORE: one produced usually by cell-division, at least not by cell-union. 9.
829
330 GLOSSARY
CaLyx : the outer set of floral leaves. 221.
CapsuLE: in Bryophytes the spore-vessel ; in Angiosperms a dry fruit which opens to discharge its seeds. 98, 211.
Carpe: the megasporophyll of Spermatophytes. 178.
CHLOROPHYLL: the green coloring matter of plants. 5.
CuLoropiast: the protoplasmic body within the cell which is stained green by chlorophyll. 7.
CoLUMELLA: in Bryophytes the sterile tissue of the sporogonium which is surrounded by the sporogenous tissue. 106.
Conrpium : an asexual spore formed by cutting off the tip of the sporo- phore, or by the division of hyphe. 58.
ConsuGation : the union of similar gametes. 15.
Corot: the inner set of floral leaves. 221.
CoryLEpon : the first leaf developed by an embryo sporophyte. 188.
Cyciic: applied to an arrangement of leaves or floral parts in which two or more appear upon the axis at the same level, forming a cycle, or whorl, or verticil. 159.
DeuiscENcE: the opening of an organ to discharge its contents, as in sporangia, pollen-sacs, capsules, ete. 199.
Dicuotomous: applied to a style of branching in which the tip of the axis forks. 35.
Diaecious : applied to plants in which the two sex-organs are upon dif- ferent individuals. 115.
DorsiventRaL: applied to a body whose two surfaces are differently exposed, as an ordinary thallus or leaf. 109.
Eee: the female gamete. 16.
Eac-apparatus : in Angiosperms the group of three cells in the embryo- sac composed of the egg and the two synergids. 204.
Exater: in Liverworts a spore-mother-cell peculiarly modified to aid in scattering the spores. 103.
Enpryo: a plant in the earliest stages of its development from the spore. 187.
Empryo-sac: the megaspore of Spermatophytes, which later contains the embryo. 178.
EyposprrM : the nourishing tissue developed within the embryo-sac, and thought to represent the female gametophyte. 180.
ENposPerM NUCLEUS: the nucleus of the embryo-sac which gives rise to the endosperm. 205.
EntomopiiLous : applied to flowers or plants which use insects as agents of pollination. 196.
GLOSSARY 331
Epieynovs: applied to a flower whose outer parts appear to arise from the top of the ovary, 225.
Evsporanerate: applied to those Pteridophytes and Spermatophytes whose sporangia develop from a group of epidermal and deeper cells, 157.
Famity: a group of related plants, usually comprising several genera. 236.
Fertinizatioy : the union of sperm and egg. 16.
FILAMENT: the stalk-like part of a stamen. 197.
Fission: cell- division which includes the wall of the old cell. 10.
Foot: in Bryophytes the part of the sporogonium imbedded in the gametophore; in Pteridophytes an organ of the sporophyte embryo to absorb from the gametophyte. 98, 188,
GAMETANGIUM: the organ within which gametes are produced. 11.
GaAMETE: a sexual cell, which by union with another produces a sexual spore. 10.
GAMETOPHORE: a special branch which bears sex organs. 98,
GAMETOPHYTE: in alternation of generations, the generation which bears the sex organs. 97.
GENERATIVE CELL: in Spermatophytes the cell of the male gameto- phyte (within the pollen grain) which gives rise to the male cells. 180.
GeEnvs: a group of very closely related plants, usually comprising sev- eral species. 237.
Havstoricm: a special organ of # parasite (usually a fungus) for ab- sorption. 90.
Herrerogamovs: applied to plants whose pairing gametes are un- like. 15.
Hererosporots : applied to those higher plants whose sporophyte pro- duces two forms of asexual spores. 151.
Ilomosporous: applied to those plants whose sporophyte produces simi- lar asexual spores. 151.
Host: a plant or animal attacked by a parasite. 48.
Hypua: an individual filament of a mycelium. 49.
Hypocoryn: the axis of the embryo sporophyte between the root-tip and the cotyledons. 209.
Hyrpoernovs: applied to a flower whose outer parts arise from beneath the ovary. 224.
832 GLOSSARY
Inpusium : in Ferns a flap-like membrane protecting a sorus. 148.
INFLORESCENCE: a flowet-cluster. 280.
Insertion: the point of origin of an organ, 224,
InteGuMENT: in Spermatophytes a membrane investing the nucellus. 178.
InvoLucre: a cycle or rosette of bracts beneath a flower-cluster, as in Umbellifers and Composites. 275.
Isocarpic: applied to a flower whose carpels equal in number the other floral organs. 268.
Isocamous: applied to plants whose pairing gametes are similar. 15.
LEPTOSPORANGIATE: applied to those Ferns whose sporangia develop from a single epidermal cell. 157.
MALE cELL.: in Spermatophytes the fertilizing cell conducted by the pollen-tube to the egg. 180.
MrGasporaNGium: asporangium which produces only megaspores. 152.
Mecaspore: in heterosporous plants the large spore which produces a female gametophyte. 152.
MeGasporopHyLL: a sporophyll which produces only megasporangia. 152.
MesopnyLi: the tissue of a leaf between the two epidermal layers which usually contains chloroplasts, 141.
MicRosPORANGIUM: a sporangium which produces only microspores. 152.
Microspore: in heterosporous plants the small spore which produces a male gametophyte. 152.
MicRosPoROPHYLL : a sporophyll which produces only microsporangia. 152.
MicRopyLe: the passageway to the nucellus left by the integument. 178.
Monacrous: applied to plants in which the two sex organs are upon the same individual. 115.
MoyopopiaL: applied to a style of branching in which the branches arise from the side of the axis. 35.
Moruer ceLn: usually a cell which produces new cells by internal divi- sion. 9.
Mycetium: the mat of filaments which composes the working body of a fungus. 49.
NAKED FLOWER: one with no floral leaves. 222. Nucenuus: the main body of the ovule. 178.
GLOSSARY 333
Ooco1um : the female, egg-producing organ of Thallophytes. 16.
OospHERE: the female gamete, or egg. 16.
OosporeE: the sexual spore resulting from fertilization. 16.
Ovary: in Angiosperms the bulbous part of the pistil, which contains the ovules, 199.
OvuLE: the megasporangium of Spermatophytes. 178.
Pappus : the modified calyx of the Composites. 278.
PaRasiTE: a plant which obtains food by attacking living plants or ani- mals. 48.
PENTACYCLIC : applied to a flower whose four floral organs are in five cycles, the stamens being in two cycles. 268.
Prrianru: the set of floral leaves when not differentiated into calyx and corolla. 221.
Prricynous: applied to a flower whose outer parts arise from a cup surrounding the ovary. 225.
Peta: one of the floral leaves which make up the corolla. 221.
PnorosynrueEsis: the process by which chloroplasts, aided by light, manufacture carbohydrates from carbon dioxide and water. 84.
Pisin: the central organ of the flower, composed of one or more car- pels. 200.
PIsTILLATE : applied to flowers with carpels but no stamens. 218.
PoLLen : the microspores of Spermatophytes. 174. PoLLEN-TUBE: the tube developed from the wall of the pollen grain which penetrates to the egg and conducts the male cells. 180. PoLLination: the transfer of pollen from anther to ovule (in Gymno- sperms) or stigma (in Angiosperms). 181.
PoLyPETaLous: applied to flowers whose petals are free from one an- other. 227.
ProtHaLLium : the gametophyte of Ferns. 130.
Protoxema: the thallus portion of the gametophyte of Mosses. 98.
Raprau; applied to a body with uniform exposure of surface, and pro- ducing similar organs about a common center. 120.
RecepracLe: in Angiosperms that part of the stem which is more or less modified to support the parts of the flower. 222.
Rauizorw : a hair-like process developed by the lower plants and by inde- pendent gametophytes to act as a holdfast or absorbing organ, or both. 109.
SaPROPHYTE: a plant which obtains food from the dead bodies or body products of plants or animals. 48.
334 GLOSSARY
ScaLeE: a leaf without chlorophyll, and usually reduced in size, 161,
SEpaL: one of the floral leaves which make up the calyx. 221.
Sera: in Bryophytes the stalk-like portion of the sporogonium. 98.
SEXUAL SPORE: one produced by the union of gametes. 10.
Species : plants so nearly alike that they all might have come from a single parent. 287.
Sperm: the male gamete. 16.
SPIRAL: applied to an arrangement of leaves or floral parts in which no two appear upon the axis at the same level; often called alter- nate. 193.
SporanGium: the organ within which asexual spores are produced (ex- cept in Bryophytes). 10.
Spore: a cell set apart for reproduction. 9.
Sporocontum : the leafless sporophyte of Bryophytes. 98.
SPoROPHORE : a special branch bearing asexual spores. 49.
SpoROPHYLL: a leaf set apart to produce sporangia. 145.
Sporopuyte: in alternation of generations, the generation which pro- duces the asexual spores. 97.
Sramen: the microsporophyll of Spermatophytes. 174.
STAMINATE: applied to a flower with stamens but no carpels. 218.
Sriema: in Angiosperms that portion of the carpel (usually of the style) prepared to receive pollen. 199.
Stroma (pl. Sromava): an epidermal organ for regulating the communi- cation between green tissue and the air. 141.
Srropiius: a cone-like cluster of sporophylls. 161.
SryLe: the stalk-like prolongation from the ovary which bears the stigma, 199.
Suspensor : in heterosporous plants an organ of the sporophyte embryo which places it in a more favorable position in reference to food
supply. 168. SymBionT: an organism which enters into the condition of symbio- sis. 79.
Symprosts: usually applied to the condition in which two different organisms live together in intimate and mutually helpful rela- tions, 7.
Symprratous: applied to a flower whose petals have coalesced. 227,
Syycarpous: applied to a flower whose carpels have coalesced. 226.
Synerer: in Angiosperms one of the pair of cells associated with the egg to form the egg-apparatus. 204.
GLOSSARY 335
Testa: the hard coat of the seed. 184.
TETRACYCLIc: applied to a flower whose four floral organs are in four cycles. 268.
TETRAD: a group of four spores produced by a mother-cell. 103.
ZoosPoRE : a motile asexual spore. 10.
Zycomorpuic: applied to a fluwer in which the parts in one or more sets are not similar; irregular, 229.
ZyGore: the sexual spore resulting from conjugation. 15.
INDEX TO PLANT STRUCTURES
[The italicized numbers indicate that the subject is illustrated upon the page cited. In such case the subject may be referred to only in the illustration, or it may be
referred to also in the text.]
A
Absorption, 299.
Acacia, 265.
Aconitum, 261.
Acorus, 219, 248.
Actinomorphy, 228.
Adder's tongue : see Ophioglossuin.
Adiantum, 148, 145.
ZEcidiomycetes, 50, 62.
AKicidiospore, 66.
ZEcidium, 66.
Agaricus, 68, 69.
Agave, 247.
Air pore: see Stoma.
Akene, 212, 213, 214, 276, 277.
Alchemilla, 225.
Alder: see Alnus,
Algw, 4, 5, 17.
Alisma, 210, 240.
Almond: see Prunus.
Alnus, 257.
Alternation of generations, 94, 129.
Amanita, 70.
Amaryllidacex, 247,
Amaryllis family: see Amarylli- dace.
Ambrosia, 279.
Ament, 257.
Anaptychia, 87, 82,
Anemophilous, 181. Angiosperms, 178, 195, 217. Anisocarpx, 268. Annulus, 136, 146, 150. Anther, 196, 197, 199. Antheridium, 16, 99, 100, 112, 121,
133, 134, 161, 166. Antherozoid, 16. Anthoceros, 104, 105, 111, 116, 118. Anthophytes, 172. Antipodal cells, 202, 205, 208. Antirrhinum, 228, 275, Ant-plants, 90, 92. Apical cell, 234. Apical group, 283. Apium, 267. Apocarpy, 199, 222, 225. Apocynacee, 271, Apocynum, 272, Apogamy, 181, Apospory, 182. Apothecium, 79, 81, 82. Apple: see Pirus. Aquilegia, 198. Aracer, 243, Araliacez, 267. Araucaria, 190. Arbor vite: see Thuja. Arbutus, 198: see Epigea, Archegoniates, 101.
337
338
Archegonium, 99, 100, 713, 114, 133, 185, 161, 167, 179.
Archesporium, 102, 104, 105, 146.
Archichlamydew, 255.
Arctostaphylos, 269.
Areolie, 111, 114.
Ariswma, 248, 244,
Arnica, 275, 276, 278.
Aroids, 248.
Artemisia, 279.
Arum, 245.
Ascocarp, 58, 59.
Ascomycetes, 50, 57.
Ascospore, 59.
Ascus, 59.
Asexual spore, 9.
Aspidium, 180, 1/6, 144.
Assimilation, 802.
Aster, 279.
Astragalus, 265.
Atherosperma, 198.
Azalea, 270.
B
Bacillus, 76.
Bacteria, 21, 75. 76. Balm: see Melissa. Banana, 140.
Bark, 284, 289. Basidiomycetes, 50, 68 Basidiospore, 6.9, 72. Basidium, 69, 71.
Bean: see Phaseolus. Bearberry : see Arctostaphylos. Beech, 256.
Bellis, 279.
Berberis, 198.
Bidens, 278. Beggar-ticks, 213. Bignonia, 211.
Birch, 256.
Blackberry: see Rubus.
INDEX
Black knot, 60.
Black mould, 52.
Blasia, 116.
Blueberry: see Vaccinium.
Blue-green alge, 6, 17.
Blue mould, 60.
Boletus, 73, 74.
Botrychium, 145, 749.
Botrydium, 28.
Box elder, 234,
Bracket fungus, 72.
Brake: see Pteris.
Brassica, 261.
Bryophytes, 2, 98, 172.
Brown alge, 6, 32.
Bryum, 720, 124.
Buckeye, 235.
Butomus, 199.
Buttercup: see Ranunculus.
Buttercup family: see Ranuncu- lacea.
C
Cabbage: see Brassica. Calamus: see Acorus. Calla-lily, 248. Callithamnion, 43. Callophyllis, 39. Calluna, 270. Calopogon, 249.
Caltha, 260. Calycanthus, 226, 261, Calypso, 24.
Calyptra, 102, 125. Calyptrogen, 293. Calis, 22 oT. Cambium, 285, 287, 288. Capsella, 209, 29:3. Capsule, 98, 123, 125, 126, 211, 212. Caraway: sce Carum, Carbohydrate, 3802. Carbon dioxide, 83.
Carnivorous plants, 92.
Carpel, 177, 178, 199, 219, 220.
Carpinus, 217, 258. Carpospore, 44, 45. Carrot: see Daucus, Carum, 267.
Cassia, 265, Cassiope, U9. Castilleia, 275, Catkin, 257. Catnip: see Nepeta. Cat-tail: see Typha. Cattleya, 254. Caulicle, 209, Cauline, 166.
Cedar apple, 67, 68. Celery: see Apium. Cell, 6, 7.
Cellulose, 7.
Cercis, 265. Chalazogamy, 258, 259, Characew, 46. Chemotropism, 307. Cherry: see Prunus, Chestnut, 256, Chlorophycee, 6, 21. Chlorophyll, 5, 88. Chloroplast, 7, 8. Chrysanthemum, 279. Cilia, 10.
Circinate, 736, 143. Cladophora, 25. Clavaria, 72.
Climbing fern: see Lygodium.
Closed bundle, 290. Clover: see Trifolium. Club mosses, 162.
Cnicus, 278.
Cocklebur: see Xanthium. Cenocyte, 27. Coleochete, 106, 707. Collateral bundle, 287,
INDEX 339
Collenchyma, 284.
Columella, 104, 105, 106, 126
Compass plant: see Silphium,
Composite, 275.
Composites, 275, 276, 277, 278.
Concentric bundle, 292.
Conferva forms, 22.
Conidia, 58, 60.
Conifers, 191, 282.
Conium, 267,
Conjugate forms, 31.
Conjugation, 15.
Connective, 196.
Conocephalus, 122.
Convolvulaces, 271.
Conyolvulus forms, 270.
Convolvulus, 272,
Coprinus, 7”.
Coral fungus, 73, 74.
Coreopsis, 278.
Coriandrum, 267.
Cork, 284.
Corn, 216, 28.2, 290.
Cornacee, 267.
Corolla, 22u, 221.
Cortex, 285, 284, 288.
Cotton, 206.
Cotyledon, 737, 188, 168, 184, 209, BLO dls OL
Cranberry: see Vaccinium.
Crategus, 262.
Crocus, 249,
Crucifer, 262.
Crucifere, 262.
Cryptogams, 172.
Cunila, 274.
Cup fungus, 60, 62.
Cupule, 722, 114.
Cyanophycee, 6, 17.
Cycads, 185, 186, 187, 189.
Cyclic, 159, 193.
Cyperacee, 241.
840
Cypripedium, 249, 253. Cystocarp, 43, 44.0 Cystopteris, 78, 144. Cytoplasm, 7.
D
Daisy: see Bellis.
Dandelion: see Taraxacum.
Dasya, 40.
Datura, 197.
Daucus, 266, 267.
Dead-nettle, 228.
Definitive nucleus: see Endosperm nucleus.
Dehiscence, 198, 199.
Delphinium, 260, 261.
Dermatogen, 283.
Desmids, 81, 32.
Desmodium, 308.
Diatoms, 45.
Dichotomous, 35.
Dicotyledons, 208, 288, 254, 282.
Differentiation, 3, 280.
Dogbane: see Apocynum.
Dog-tooth violet: see Erythronium.
Dogwood family: see Cornace.
Dorsiventral, 109.
Downy mildew, 55.
Drupe, 264.
Digestion, 302.
Dicecious, 115,
Disk, 276, 277.
Dodder, 86.
E
Ear-fungus, 74.
Easter lily, 2:27.
Ecology, 297, 311. Economic botany, 297. Ectocarpus, 33. Edogonium, 22, 23.
Egg, 16, 202, 204, 205, 206.
INDEX
Egg-apparatus, 204, 205, 206.
Elater, 103, 113, 118.
Elm: see Ulmus.
Embryo, 137, 167, 168, 170, 183, 207, 208, 209, 210, 211.
Embryo-sac, 178, 179, 201, 205, 208.
Endosperm, 179, 180, 207, 208, 211.
Endosperm nucleus, 202, 205.
Entomophilous, 196.
Epidermis, 141, 142, 191, 288, 284, 295.
Epigea, 269.
Epigyny, 224, 228.
Kpilobium, 212.
Epiphyte, 157.
Equisetales, 159.
Equisetum, 159, 260, 261.
Ergot, 60, 62.
Erica, 270.
Ericacer, 268.
Erigenia, 267.
Krythronium, 250.
Eusporangiate, 157.
Evolution, 3.
F
Fennel: see Foeniculum.
Ferns, 155, 156.
Fertilization, 16, 181, 206, 207.
Festuca, 240.
Figwort family: see Scrophula- riacer.
Filament, 8, 796, 197.
Filicales, 155.
Fireweed: see Epilobium.
Fission, 10.
Flax: see Linum.
Floral leaves, 218,
Floridex, 38.
Flower, 218.
Flowering plants, 172.
Feeniculum, 267.
INDEX
Foliar, 166.
Food, 83, 299.
Foot, 98, 102, 137, 138, 168. Fragaria, 214, 2.77, 262.
Fruit, 211, 222, 213, 214, 215 Fucus, 03, 37.
Funaria, 99, 102, 121, 124, 125, 126. Fungi, 4, 48.
G
Gametangium, 11. Gamete, 10, 12. Gametophore, 9%, 1172, 120, 124.
Gametophyte, 97, 107, 182, 134, 162. 166, 167, 176, 179, 180, 201, v8,
204, 205. Gaultheria, 270. Gaylussacia, 260. Gemma, 712, 114. Generative cell, 180, 201. Gentianacex, 271. Geophilous, 246. Geotropism, 305. Gerardia, 275. Germination, 187, 214. Gigartina, 38. Gills, 71. Ginkgo, 191. Gladiolus, 249, 257. Gleditschia, 236, 265. Gleocapsa, 17, 18. Glume, 241. Goldenrod: see Solidago. Gonatonema, 31. Graminee, 241. Green alge, 6. 21. Green plants, 88. Green slimes, 20. Grimmia, 176. Growth movement, 304, Growth ring, 234, Grain, 241.
40
Grasses, 240.
Grass family: see Graminee. Gymnosperms, 171, 178, 195. Gymnosporangium, 7.
H Habenaria, 249, 252. Harebell, 7/8. Haustoria, 50. Hazel: see oo Heart-wood, 28 Heat, 314.
: Heath family: see Ericacez.
Heaths, 268, 269, 27 Helianthus, 279, 283, 306. Heliotropism, 305. Hemiarcyria, 75.
Hemlock: see Conium. Henbane: see Hyoscyamus. Hepatic, 109.
Heterocyst, 18.
Heterogamy, 15. Heterospory, 151,
Hickory, 256,
Hippuris, 283,
Homospory, 151.
Honey locust: see Gleditschia. Horehound: see Marrubium. Hornbeam : see Carpinus. Horsetail, 159.
Host, 48.
Huckleberry: see Gaylussacia. Hydnum, 73, 74
Hydra, 90.
Hydrophytes. 6, 315. Hydrophytum, 91. Hydrotropism, 307. Hygroscopie movement, 304. Hyosceyamus, 156.
Hypha, 49.
Hypocotyl, 184, 209, 216, 217.
342
Hypodermis, 284. Hypogyny, 224, 225. Hyssopus, 274.
ii
Indigo: see Indigofera. Indigofera, 265. Indusium, 136, 148, 144. Inflorescence, 230. Insects and flowers, 90. Integument, 178, 179, 201, 202, 208. Involucre, 267, 275, 277. Ipomoea, 228, 270, Tridacev, 247.
Tris, 248, 251.
Iris family: see Iridacee. Irritable mavement, 307. Isocarpa, 268.
Isoetes, 169.
Isogamy, 15.
Japan lily, 248. Jungermannia, 105, 115, 226, 117. Juniper, 194.
k Kalmia, 270.
Labiate, 272.
Lahiates, 272.
Lactuca, 279.
Laminaria, 838, 34. Lamium, 274, 275.
Larch: see Larix.
Larix, 192.
Larkspur: see Delphinium. Laurel: see Kalmia. Lavandula, 275.
Leaf, 141, 142, 295, 296, 811. Legumes, 250, 251, 264.
INDEX
Leguminose, 264.
Lemna, U1.
Lepidozia, 127.
Leptosporangiate, 157,
Lettuce: see Lactuca.
Leucanthemum, 279.
Liatris, 278.
Lichens, 77, 78, 79, 87.
Life relations, 311.
Light, 314.
Ligule, 168,
Liliacew, 246.
Lilies, 245.
Lilium, 208, 204, 205, 207, 224, 249, 2a5.
Lily: see Lilium.
Lily family: see Liliacez.
Linaria, 228, 275.
Linum, 220.
Liverworts, 109.
Loculus, 200.
Locust: see Robinia.
Lotus, 204.
Lupinus, 265,
Lycopersicum, 275.
Lycopodiales, 162.
Lycopodium, 162, 763.
Lygodium, 145,
Lyonia, 269.
M
Macrospore, 152.
Maidenhair fern: see Adiantum.
Male cell, 180, 187, 201, 206, 207.
Maple, 272.
Marasmius, 7.
Marchantia, 704, 110, 271, 112, 113, 11h.
Marguerite: see Leucanthemum.
Marjoram : see Origanum.
Marrubium, 275.
Marsh marigold: see Caltha,
INDEX.
Marsilia, 158. Megasporangium, 152, 177, 170.
Megaspore, 152, 165, 167, 179, 201.
BUS
Megasporophyll, 152, 165, 177, 109.
Melissa, 275.
Mentha, 229, 274.
Meristem, 281.
Mesophyll, 141, 142, 191, 295. Mesophytes, 324.
Mestome, 282.
Micropyle, 178, 201, 20.2, 200. Microspira, 76.
Microsphera, 58. Microsporangium, 152, 176, 197.
Microspore, 152. 105, 166, 779, 197.
201.
Microsporophyll, 152, 163, 174, 196,
198. Midrib, 234. Mildews, 57. Mimosa, 265, 308, 309. Mint: see Mentha. Mint family: see Labiate.
Monocotyledons, 208, 252, 286, 280.
Moneecious, 115. Monopodial, 35.
Monotropa, 270.
Moonwort: see Botrychium. Morels, 60, 6? Morning-glory: see Ipomeea. Morphology, 297.
Mosses, 98, 119, 124.
Mother cell, 9.
Mougeotia, 31.
Movement, 303.
Mucor, 49, 52, 53, 54, 58. Mullein: see Verbascum. Musci, 119. Mushrooms, 68. Mustard family: Mycelium, 49.
see Crucifere.
343
Mycomycetes, 50. Mycorrhiza, 87, 83. Muyristica, 214. Myrmecophytes. 90, 91. Myxomycetes, 74, 75.
N Naias, 237. Narcissus, 247, Nemalion, 42. Nepeta, 275. Nicotiana, 227, 275. Nightshade family : see Solanacee, Nostoe, 18. Nueellus, 178, 179, 2v1, Nucleus, 7
202, 203.
' Nutation, 304.
Nutmeg, 214.
Nutrition, 3, 299. Nyctitropic movement, 309. Nympheacee, 261.
O Oak, 255, 256. (Edogonium : see Edogonium, Onoclea, 145, 147, 148. Oogonium, 16. Oosphere, 16. Oospore, 16, 101. Open bundle, 287. Operculum, 122, 125. Ophioglossum, 145, 249. Orchidaceew, 249. Orchids; 249; 252. 25.2. 254. Orchid family: see Orchidacez. Origanum, 274. Ornithogalum, 247. Oscillaria, 29. Osmunda, 145. 156. Ostrich fern: see Onoclea, Ona 299, 200: 202: Ovule, 178, 179, 201, 203.
344
P
Palisade tissue, 742, 295. Palmacer, 241.
Palm family: see Palmacew. Palms, 241, 242, 243. Papaveraceex, 261.
Pappus, 270, 277, 278. Parasites, 48, 85.
Parenchyma, 280, 281, 282, 288. Parmelia, 70.
Parsley: see Petroselinum. Parsley family: see Umbelliferw. Parsnip: see Pastinaca. Parthenogenesis, 52. Pastinaca, 267.
Pathology, 297.
Pea: see Pisum.
Peach: see Prunus.
Peach curl, 60.
Pea family: see Leguminose, Pear: see Pirus.
Peat, 119.
Pellewa, 146.
Penicillium, 60.
Pentacycle, 268.
Pentstemon, 275.
Peony, 20.
Pepper, /17, 258.
Pepper family: see Piperacex. Perianth, 219, 22, 221. Periblem, 283.
Perigyny, 225, 220.
Peristome, 16, 127. Peronospora, 55, 56.
Petal, 220, 221.
Petiole. 141.
Petroselinum, 267. Phophycew, 6, 32. Phanerogams, 172.
Phaseolus, 216, 265.
Phloem, 286, 287, 288, 290, 20.2, 204
INDEX
Phlox, 228, 271. Photosyntax, 84. Photosynthesis, 84, 302. Phycomycetes, 50, 51. Physcia, 79. Physiology, 297. Picea, 179, 181, 18. Pileus, 71. Pine: see Pinus. Pineapple, 215. Pinus, 173, 175, 176, 177, 178, 181, 183, IN4, ISS, 101, 280. Piperace, 258. Pirus, 225, 262, 263. Pistil, 299, 200, 219, 220. Pisum, 265. Pith, 285, 287, 288. Planococeus, 74. Plantaginacee, 275. Plant body, 6. Plant societies, 313. Plasmodium, 74, 7. Plastid, 7, 8. Platycerium, 132. Plerome, 283. Pleurococeus, 21. Plum : see Prunus. Plumule, 210. Pod, 211, 212. Pogonia, 249. Polemoniacee, 271. Polemonium, 271. Pollen, 174, 176, 197, 201. Pollen-tube, 179, 180, 181, 187, 202, 206, 207. Pollination, 181. Polyembryony, 183. Polymorphism, 63. Polypetaly, 226. Polyporus, 71, 7.2. Polysiphonia, 44. Polytrichum, 96,
INDEX
Pome, 268.
Pondweeds, 237.
Poplars, 255.
Popowia, 108.
Poppy, 261.
Poppy family: see Papaveracee.
Populus, 256.
Pore-fungus, 72.
Potamogeton, 237, 238.
Potato: see Solanum.
Potentilla, 2/5, 262.
Proteid, 302.
Prothallium, 130, 132, 1/4.
Protococcus forms, 22.
Protonema, 95, 98.
Protoplasm, 7.
Prunus, 71, 262.
Pseudomonas, 76.
Pseudopodium, 105, 123, 124.
Pteridophytes, 2, 128, 172, 201.
Pteris, 133, 134, 135, 137, 141, 132, 148, 145, 281, 291, 292, 298.
Ptilota, 42.
Puccinia, 63, 64, 65, 66.
Puff-balls, 68, 74.
Pulvinus, 308.
Q
Quillwort: see Isoetes.
R Rabdonia, 41. Radiate bundle, 294. Radicle, 209. Radish, 120. Ragweed: see Ambrosia. Ranunculaceex, 261. Ranunculus, 222, 259. Raspberry: see Rubus. Rays, 275, 276. Receptacle, 222. Red alge, 6, 38.
Redbud : see Cercis. Redwood: see Sequoia. Reproduction, 3. 8. 309, Respiration, 302. Rheotropism, 307, Rhizoid, 109, 170, 134. Rhizophores, 164. Rhododendron, 270. 271. Rhodophycee, 6, 3%. Riccia, 104, 110. Ricciocarpus, 110. Ricinus. 288,
Robinia, 265.
Root, 138. 217, 293, 294, 818. Root-cap, 29.2. Root-fungus, $7. 88. Root-hairs, 217, 300. Root-pressure, 300. Root-tubereles, $9. Rosacea, 262.
Rose family: see Rosacew. Rosin-weed : see Silphium. Rosmarinus, 275.
Royal fern: see Osmunda. Rubus, 202.
Rumex, 284.
Rust, 62, 63, 64. 65, 66.
8 Sac-fungi, 57. Sage: see Salvia. Sage-brush: see Artemisia. Sagittaria, 208, 388. Salix, 219, 233, 256, 257. Salvia, 275. Salvinia, 158. Saprolegnia, 51. 52. Saprophyte, 48, 84. Sap-wood, 289. Sargassum, 35, 36. Saururus, 219, 258. Seales, 161.
346
Seapania, 116.
Schizomycetes, 21.
Schizophytes, 21.
Sclerenchyma, 281, 282, 284, 255, 288, 290, 201.
Scouring rush, 159.
one
Scrophulariacee, 279.
Scutellaria, 275,
Sedge family: see Cyperacee.
Seed, 183, 284, 210, 211, 212, 214.
Selaginella, 162, 264, 105, 100, 168.
Sensitive fern: see Onoclea.
Sensitive-plant: see Acacia.
sepal, 220, 221.
Sequoia, 789.
Seta, 98, 125.
Sex, 12.
Sexual spore, 10.
Shepherd’s purse: see Capsella.
Shield fern: see Aspidium.
Shoot, 812.
Sieve vessels, 285, 286.
Silphium, 279.
Siphon forms, 27.
Siphonogams, 183.
Siphonogamy, 183.
Slime moulds, 74, 75.
mt, 62:
Snapdragon: see Antirrhinum.
Soil, 314.
Solanacew, 275.
Solanum, 798, 275.
Solidago, 279.
Solomon’s seal, 2.3.3.
Sorus, 196, 143, 144.
Spadix, 244, 245.
Spathe, 244, 245.
Sperm, 16, 100, 7.33, 169, 187, 190.
Spermatia, 43, 44.
Spermatophytes, 2, 171, 172.
Spermatozoid, 16.
35, 162, 166,
INDEX
Sperm mother cell, 100.
Sphagnum, 105, 106, 122, 123.
Spike, 240.
Spirea, 262.
Spiral, 193.
Spirillum, 76.
Spirogyra, 28, 29, 30.
Spongy tissue, 142.
Sporangium, 10, 236, 148, 145, 150, 157, 163, 179.
Spore, 9.
Sporidium, 65.
Sporogenous tissue, 103.
Sporogonium, 98, 102, 104, 105, 106, £5, dou.
Sporophore, 49, 50.
Sporophyll, 145, 247, 148, 149, 174, 176.
Sporophyte, 97, 102, 137.
Spruce: see Picea.
Stability of form, 298.
Stamen, 174, 176, 196, 198, 219, 220.
Stele, 191, 288, 285.
Stem, 189, 282, 289, 291, 312.
Stemonitis, 75.
Stereome, 282, 299.
Sterile tissue, 103.
Sticta, 80.
Stigma, 199, 202.
Stomata, 141, 142, 191, 295, 301.
Strawberry: see Fragaria.
Strobilus, 160, 161, 163, 165, 174, 175, 106, 198, 14,
Style, 199, 202.
Substratum, 49.
Sumach, 225,
Sunflower: see Helianthus.
Suspensor, 107, 168, 183, 210.
Symbiont, 79, 86.
Symbiosis, 79, 86.
209,
Sympetale, 268. Sympetaly, 226, 227. Symplocarpus, 243. Synearpy, 199, 219, 225,
Synergid, 2u2, 204, 205, 2uc.
E,
Tanacetum, 279.
Tansy: see Tanacetum. Taraxacum, 210, 277, 278. Taxonomy, 207. Teleutospore, 64, 65. Tension of tissues, 298. Testa, 184, 211. Tetracyclie, 268,
Tetrad, 103.
Tetraspore, 43. Teucrium, 230, 274, 275. Thallophytes, 2, 4, 172. Thermotropism, 307. Thistle: see Cnicus. Thorn apple: see Datura. Thuja, 192.
Thymus, 274.
Tickseed: see Coreopsis. Tissues, 280.
Toad-flax: see Linaria. Toadstools, 68. Tobacco: see Nicotiana,
Tomato: see Lycopersicum.
Trachew, 285, 286. Tracheids, 286. Transfer of water, 300. Transpiration, 301. Tree fern, 140. Trichia, 74. Trichogyne, 43, 44. Trillium, 207, 246, 265. Truffles, 60.
Turgidity, 298.
Typha, 239, 240.
INDEX
347
U
Umbel, 266, 267. Umbelliferee, 266, Umbellifers, 266. Ulmus, 710, 256. Ulothrix, 12, 13, 22. Uredo, 64. Uredospore, 63, 64,
Vv
Vaccinium, 269,
Vascular bundle, 22.2, 23.4, 287, 291. Vascuiar cylinder, 234, 287. Vascular system, 129, 139. Vaucheria, 26, 27, 28. Vegetative multiplication, 9. Veins, 141, 142.
Venation, 233.
Verbascum, 275. Verbenacer, 275. Vernation, 148.
Vernonia, 279.
Veronica, 275,
Vicia, 265.
Violet, 211, 229.
WwW
Wall cell, 180.
Walnut, 256.
Water, 83, 314.
Water ferns, 158.
Water-lily, 223, 261.
Water-lily family: see Nymphea- cer.
Water moulds, 51.
Wheat rust, 63, 64, 65, 66.
Willow: see Salix.
Wind, 315.
Wintergreen: see Gaultheria.
348 INDEX
Wistaria, 265. Y Witches’-broom, 60. Yeast, 62. Wormwood: see Artemisia. y
Zannichellia, 237.
Xx Zoospore, 10. Xanthium, 279. Zygomorphy, 228, 229. Xerophytes, 319. Zygospore, 15,
Xylem, 285, 287, 288, 290, 292, 294. | Zygote, 15.
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