84
PLANT STRUCTUKES
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 ijliotosyntliesis^ or plioto^yntax^ 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 Algae- 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 light, 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 bodies 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 some plants (as broom rapes) are parasitic, attaching their subterranean parts to those of other plants, becoming what are called " root para- sites." In cases of mycorhiza (see p. 87), which are now thought to include great numbers of green plants, it is sup- posed that some organic material is brought in by the fungus.
57. Parasites. — Certain plants without 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 24
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S01 949071 V
134844
This book may be kept out TWO WEEKS ONLY, and is subject to a fine of SHBI CENTS a day thereafter. It is due on the day indicated below:
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TWENTIETH CENTURY TEXT-BOOKS
EDITED BY
A. F. NIGHTINGALE, Ph.D., LL.D.
SUPERINTENDENT OF SCHOOLS, COOK COUNTY, ILLINOIS
TWENTIETH CENTURY TEXT-BOOKS
PLANT STRUCTURES
A SECOND BOOK OF BOTANY
BY
JOHN M. COULTER, A.M., Ph.D. ><^
HEAD OF DEPARTMENT OF BOTANY UNIVERSITY OF CHICAGO
^
/:^-^
^ - >^'
SECOND EDITION REVISED
NEW YORK
D. APPLETON AND COMPANY
1904
Copyright, 1899, 1904,
bt d. appleton and company.
aK47
PREFACE
In" 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. Eec- 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 lahoratorij uwrlc. 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 laboratory work, to refer to a larger range of material than can be handled, and to develop some ^jhilosophical conception of
134844
^ 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 fiower is so conspicuous and important a feature in connection with the highest plants, that Chapter XIII 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
VU
that it seems important to find a place for them even in an elementary work. The reason for Chapters XVI and XVII has been stated already, and even if Plant Relations is stud- ied, Chapter XVII 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 resume 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 him, and under his direction by Messrs. S. M. Coulter, B. A. Goldberger, W. J. G. Land, and A. 0. Moore, 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 for- bidden a still greater number. The authors of the original illustrations are cited, and where illustrations have been obtained elsewhere the sources are indicated.
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, a?id field work. John M. Coulter.
The University of Chicago, November, 1899.
PREFACE TO THE REVISED EDITIOX
During the last five years the science of Botany has made rapid progress, both in the addition of new facts and in changed points of view. Some of this progress affects Plant Structures., and it is recorded in this revised edition so far as it can be without a complete rewriting of the volume. Changes will be found, therefore, in statements of fact, in points of view, in terminology, in illustrations, and also in the addition of new material.
John M. Coulter. The University of Chicago, April, 1904.
CONTENTS
CHAPTER I. — Introduction |
PAGE 1 |
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II.— Thallophytes : Alg^ |
4 |
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III.— The evolution of sex |
. 13 |
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IV.— The great groups of Alg^ . |
. 17 |
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v.— Thallophytes : Fungi |
48 |
|||
VI. — The food of plants .... |
83 |
|||
VII. — Bryophytes |
93 |
|||
VIII.— The great groups of Bryophytes . |
109 |
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IX.— Pteridophytes |
128 |
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X. — The great groups of Pteridophytes |
155 |
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XI. — Spermatophytes : Gymnosperms |
171 |
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XII. — Spermatophytes : Angiosperms |
195 |
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XIII.— The flower |
218 |
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XIV.— Monocotyledons and dicotyledons . |
232 |
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XV.— Differentiation of tissues |
280 |
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XVI. — Plant physiology .... |
297 |
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XVII. — Plant ecology |
311 |
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Glossary |
329 |
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Index |
337 |
IX
BOTANY
PART II.— PLANT STRUCTURES
CHAPTEK 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 are 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
D. H. r iiLL LIBRARY North Carolina State Colleg*
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 Algce 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) Bryopliytes. — 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. — At 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, ears, bones, muscles, nerves, etc.,
INTRODUCTION 3
are set apart for special work. The increasing complexity is usually spoken of as differentiation — 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 same 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 tlieory 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.
CHAPTEE 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.
7. Algae and Fungi. — It is convenient to separate Thallo- phytes into two great divisions, known as Algce 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 Algae or Fungi, but for the present these groups may be included.
4
THALLOPHYTES: ALGiE 5
The great distinction between these two divisions of Thallophytes is that the Algse 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 Algae — that is, that they were once Alg^e, 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 Algae, of equal rank so far as birth and structure go, but of very different habits.
ALG^
8. General characters. — As already defined. Algae 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
g PLANT STRUCT [JRES
of the compound microscope to marine forms with enor- mously bulky bodies. In general they are liydropliytes — 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 Algae 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 Algae 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 phycem^ which appears in the names, is a Greek word meaning " sea- weed," which is the common name for Algae ; 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) Cyanoiiliycece^ or " Blue Algae," but usually called " Blue-green Algae," as the characteristic blue does not entirely mask the green, and the general tint is bluish-green ; (2) CMorophycece^ or " Green Algae," in which there is no special coloring matter associ- ated with the chlorophyll ; (3) PhcBophycecB^ or " Brown Algae " ; and (4) Rliodophycece, or " Red Alga?."
It should be remarked that probably the Cyanophyceae 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-
THALLOPIIYTES: ALG^
C''
plex plants consist of very many cells. It is necessary to know something of the ordinary living plant cell before the bodies of Alga3 or any other plant bodies can be under- stood.
Such a cell if free is approximately spherical in outline, (Fig. G), but if pressed upon by contiguous cells may become variously modifiod in form (Fig. 1). Bounding it there is a thin, elastic Avail, com- posed of a substance called cellulose. The cell wall, therefore, forms a delicate sac, which contains the liv- ing substance known aspro- tojjlasm. This is the sub- stance which manifests life, and is the only substance in the plant which is alive. It is the protoplasm which has organized the cellulose wall about itself, and which does all the plant work. It 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 nucletis, 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 ciffoplasm. The cytoplasm seems to form the general backgrouiul or matrix of the cell, and the
Fig. 1. Cells from a moss loaf, showing nucleus (B) in which there is a nucle- olus, cytoplasm (C), and chloroplasts (.4).— Caldwell.
g 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 Alga3 and other groups, is the pla slid. 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- plasHd, or clilorojolast (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 Algae 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 Alg^e.
Starting again with the one-celled body, another line of advance is for several cells to organize in two directions, forming 2^, 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 Algae, 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
TIIALLOl'IIYTES: ALG^ 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 Algse, and all other plants. They are as follows :
(1) Vegotative mnlti^Mcatlon. — This is the only type of reproduction employed by the lowest Algae, 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.
Asexual 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, B). These new cells are asexual sjxjres, 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 STRUCTUKES
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- ginni, 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 j^roduce new plants. Various names have been given to asexual spores to indicate certain peculiarities. As Algae 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, C). These ciliated spores are known as zoospores, or " animal- like spores," referring to their power of locomotion ; some- times they are called swi7nming spores, or sivarm 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.
Sexual 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, C, 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^ H
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 = °>o — P = ^>o — P = :>o — 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.
CHAPTEE III
THE EVOLUTION OF SEX
12. The general problem. — In the last chapter it was re- marked that the simplest Algae reproduce only by vegetative multiplication, the ordinary cell division (fission) of nutri- tive cells multiplying cells and hence individuals. Among other low Algae 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 Algse 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 Algae, 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 Alg^ to illustrate the possible origin of gametes is a common fresh- water form known as Ulothrix (Fig. 2). The body consists of a simple filament composed of a single row of short cells (Fig. 2, A). Each cell contjiins 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-
13
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 Ulotlirix is one of the Chlorophycege.
Fig. 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 {b) displaying four cilia at its pointed end and just having escaped from its cell, another cell (c) 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; E, 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
DODEL-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
l^ PLANT STKUCTURES
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 cells by internal division (Fig. 2, (7, «, b). 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, 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, F). 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, C\ 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 ][5
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.
14. 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 Ulothrix^ are said to be isoga7nous.
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 tinisexual condition, as opposed to a bisexual 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 female gamete, the smaller active one the male gamete. This condition is indicated by the word lieterogamy^ which means " dissimilar gametes," and those plants whose pairing gametes are dissimilar are said to be lieterogamous.
IQ PLANT STRUCTUKES
In order to distinguish them the hirge and passive female gamete is called the oosjjJi ere, which means " egg sphere," or it is called the egg ; the small hut active male gamete is variously called the sjiermatozoid, 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 antlieridmm, that producing the Qgg being called the oogonium (Fig. 10).
The act of fusing of sperm and Qgg 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 Alg^e, 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.
CHAPTEE IV
THE GREAT GROUPS OF ALG^
17. General characters. — The Algae 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 Algae (Cyanophyceae), Green Alg^ (Chloro- phyceae), Brown Algae (Ph£eophyce£e), and Eed Algge (Ehodo- phyceae). 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. CYAKOPHYCE.E {Blue-gveen Algm)
18. Gloeocapsa. — 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 Glmocapsa body. One of the pecul- iarities 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 Gloeocapsa individuals being formed, this method of vegeta- tive multiplication being the only form of reproduction (Pig. 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). 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 Glmoccq^sa^ 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 Glceocapsa^ 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- lar intervals there oc- cur larger colorless cells, an illustration of the differentiation of cells. These larger cells are known as liet- erocysts (Fig. 4, A), which simply means "other cells." It is observed that when the chain breaks up
into fragments each Fig. 4. Nostoc, a blue-green alga, showing the frao-mPTif iscomnosed chain-like filaments, and the heterocysts {A)
iragment IS composed ^^,^.^j^ determine the breaking up of the chain.
of the cells between -caldwell.
Fig. 3. Glceocapsa, 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.
THE GREAT GROUPS OF ALG^
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. Oscillatoria. — These forms are found as bluish-green slippery masses on wet rocks, or on damp soil, or freely floating. They are simple filaments, composed of very short flattened cells (Fig. 5), and the name Oscillatoria refers to the fact that they exhibit a peculiar oscillating move- ment. These motile filaments are iso- lated, not being held together in a jelly-like matrix as are the chains of Nostoc^ 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 free surface rounded. If a filament breaks and a new cell surface ex- posed, it at once becomes rounded. 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
Fio. 5. Oscillatoria, a blue-green alga, show- ing a group of filaments (^•1), and a single fila- ment more enlarged {B). —Caldwell.
20 PLANT STRUCT LIKES
it will bulge outward. In all active living cells there is this pressure upon the wall from within.
Each cell of the Oscillatotna 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 Gloeocapsa, JVostoc, and Oscil- lator ia as representatives of the group Cyanophyceae, or " green slimes," we may come to some conclusions concern- ing the groap in general. The plant body is very simple, consisting of single cells, or chains and filaments of cells. Although in Nostoc and Oscillatoria 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 Nostoc, peculiar cells which seem to be connected in some way with the breaking up of filamentous colonies, although the OscUlatoria filament breaks up without them.
The power of motion is also well exhibited by the group, the free filaments of Oscillatoria moving almost continu- ally, and the imbedded chains of Kostoc at times moving to escape 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 Nostoc and Gloeocapsa.
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^
21
upon the return of favorable conditions. These may be regarded as resting cells. So notable is the fact of repro- duction by fission that Cyanophyceae are often separated from the other groups of Algae and spoken of as " Fission Algae," which put in technical form becomes Schizophyceae. 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 Alg£e and Fungi.
2. Chlorophyce^ {Green Algm),
22. Pleurococcus. — This may be taken as a type of one- celled Green Algse. 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 Glceocapsa^ except that there is no blue with the chlo- rophyll, and the cells are not im- bedded in such jelly-like masses. The cells may be solitary, or may cling together in
colonies of various sizes (Fig. G). Like Glwocapsa, a cell divides and forms two new cells, the only reproduction 20
Fig
. 6. Pleurococcus, a one-celled green alga : A, show- ing the adult form with its nucleus ; B, C, D, E, various stages of division (fission) in producing new cells ; F, colonies of cells which have remained in contact.— C aldwell.
22 PLANT STRUCTURES
being of this simple kind. It is evident, therefore, that the group Chlorophyceae begins with forms just as simple as are to be found among the Cyanophyceae.
Pleurococcus is used to represent the group of Protococ- cus forms, one-celled forms which constitute one of the subdivisions of the Green Alg^e. It should be said that Pleurococcus 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. S. 23. TJlothrix. — 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, F^ G). All three kinds of reproduction are represented, therefore, but the sexual method is the low type called isogamy, the pair- ing gametes being alike.
Ulothrix is taken as a representative of the Conferva forms, the most characteristic group of Chlorophyceae. 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 Ulothrix
Fig. 7. CEdogonlum nodosum, a Conferva form : A^ portion of a filament showing a vegetative cell with its nucleus id), an oogonium (a) 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 {b), each containing two sperms; B, another filament showing antheridia {a) from which two sperms (6) have escaped, a vegetative cell 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; 1). a zoospore producing a new filament, putting out a holdfast at base and elongating; E, a further stage of development; F, the four zoospores formed by the oospore when it germinates.— Caldwell, except C'and F, which are after Pkingsueim.
24: PLANT STRUCTURES
(§ 13). The other cells are longer than in Ulothrix, 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, G). 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, i), B).
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, A, /, 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, B, c). As a result of this act of fer- tilization an oospore is formed, which organizes a firm wall
THE GREAT GROUPS OF ALG^
25
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.
When the oospore of (Edogo- nium germinates
it does not develop directly into a new filament, but the contents become organized into four zoospores (Fig. 7, F\ which escape, and each zoospore develops a filament. In this way each oospore may give rise to four filaments.
It is evident that (Edogonium is a heterogamous plant, and is another one of the Conferva forms. Conferva bodies are not always simple filaments, as are those of Ulothrix and (Edogonium^ but they are sometimes extensively branch- ing filaments, as in Cladopliora^ a green alga very common
D. h. . LIBRARY Horth Carolina State College
Fig. 8. Cladophora, a branching green alga, a very small part of the plant being shown. The branches arise at the upper ends of cells, and the cells are coenocytic— Caldwell.
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 are said to be Mciliate. \ 25. Vaucheria. — This is one of the most common of the Green Alga?, found in felt-like masses of coarse filaments in shallow water and on muddy banks, and often called " green
Fig. 9. Vaucheria geminafa, 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 (/>) it is discharged later as a large multiciliate body (C), which then begins the develop- ment of a new ccenocytic body (E ).— Caldwell.
felt." The filament is very long, and usually branches ex- 4 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^
27
plasm organized about it is a cell, whether it has a wall or not. Therefore the body of Vaucheria is made up of as many cells as there are nuclei, cells whose protoplasmic structures have not been kept separate by cell w^Jls. Such a body, made up of numerous cells, but with no partitions, is called a coenocyte^ or it is said to.be coemocytic. Vaucheria represents a great group of Chlorophyceas whose members have coenocytic bodies, and on this account they are called the Siphon forms.
Vaucheria 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, B). 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, 6'), swims about for a time, and finally develops another Vaucheria body (Figs. 9, £J, 10). It should be said that this large body, called ^ a zoospore and acting like one, is really a ma§s of small biciliate zoospores, just as the
Fig. 10. A young Vaucheria germinating from a spore (sp), and showing the holdfast {w}.— After Sachs.
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. Each nucleus with its cytoplasm and two cilia represents a small biciliate zoospore, such as those of Cladojihora^ § 24.
Antheridia and oogonia are also developed. In a com- mon form these tAVO sex organs appear as short special branches developed on the side of the large coenocytic body,
28
PLANT STRUCTURES
and cut off from the general cavity by partition walls (Fig. 11). The oogonium becomes a globular cell, which usually
^3=SiSS3^-^
Fig. 11. Yaucheria sessilis, a Siphon form, show- ing a portion of the crenocytic body, an an- theridial branch (A) with an empty anthe- ridium (a) at its tip, and an oogonium (B) containing an oospore (c) and showing the opening (/) through which the sperms passed to reach the egg.— Caldwell.
develops a perforated beak for the entrance of the sperms, and organizes within itself a single large egg (Fig. 11, i^). The an- theridium is a much smaller cell, within which numerous very small sperms are formed (Fig. 11, A^ a). The sperms are discharged, swarm about the oogonium, and finally one passes through the beak and fuses with the egg, the result be- ing an oospore. The oospore or- ganizes a thick wall and becomes a resting spore.
It is evident that Vaucheria is heterogamous, but all the other Siphon forms are isogamous, of which Botrydium may be taken as an illustration (Fig. 12).
26. Spirogyra. — This is one of the commonest of the
i " pond scums," occurring in slippery and often frothy
masses of delicate filaments floating in still water or about
Fig. 12. Botrydium, one of the Siphon forms of green algae, the whole body containing one continuous cavity, with a bulbous, chlorophyll-con- taining portion, and root- like branches which pene- trate the mud in which the plant grows. — Cald- well.
THE GKEAT GRODPS OF ALG^
29
springs. The filaments are simple, and are not anchored by a special basal cell, as in Ulothrix and (Eclogoiiium. The
Fig. 13. Spirogyi^a, a Conjugate form, showing one complete cell and portions of iwo 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
Fia. 14. Spirogyra. showing conjugation : A, conjugating tubes api)roaching 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 A two colls 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 hav- 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 ALG^
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, Spirogyra 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, A)^ 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 Alg?e, as indicated by the illustra- tions given above, include simple one-celled forms which reproduce by fission, but they are chiefly fila- mentous forms, simple or branching. These filamentous bodies either have the cells separated from one another
Fig. 16 Two Conjugate forms : A (.}foi(geotia), showing for- mation of zygote in conjuga- ting tube ; B, C {Gonatone- ma), showing formation of zygote without conjugation. —After WiTTUocK.
32
PLANT STRUCTURES
by walls, or they are coenocytic, 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.
17. A group of Desmids, one-celled Conjugate forms, showing various pat- terns, and the cells organized into distinct halves.— After Kerneb.
The Green Algae are of special interest in connection with the evolution of higher plants, which are supposed by some to have been derived from them.
3. Pn.TiopnYCE.T^: [Broivn Algce)
28. General characters. — The Blue-green Algae and the Green kV^dd are characteristic of fresh water, but the Brown Algae, or "kelps," are almost all marine, being very charac-
THE GREAT GROUPS OF ALGiE
33
teristic coast forms. All of them are anchored by holdfasts, which are sometimes highly developed root-like structures ; and the yellow, broAvn, 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. Certain Brown Algae, as Uctocar2ms (Fig. 18), Sire of great interest on account of their pos- sible relation to the evolution of higher plants. It is in this group that we have found our only suggestions as to the origin of the complex sex-organs occurring in Bryophytes and Pteridophytes.
29. The plant body. — There is very great diversity in the structure of the plant body. Some of them, as Edocar- pus (Fig. 18), are fil- amentous forms, like the Confervas among the Green Alg^e, but
others are very much more complex. The thallus of Lam- inaria is like a huge floating leaf, frequently nine to ten
Fig. 18. A brown alga (Ectocarpus), showing a body consisting of a simple filament which puts out branches (A), some sporangia (^) contain- ing zoospores, and gametangia ((') containing gametes.— Caldwell.
Fig. 18a. A group of brown scuw L'L-d.-> {Lui/ii/ai/ius). Note the various habits of the plant body with its leaf -like thallus and root-like holdfasts.— After Keuneb.
THE GREAT GROUPS OF ALG^E
35
feet long, whose stalk develops root-like holdfasts (Fig. 18^^). 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 dicliotomous^ as dis- tinct from that in which branches are put out from the sides of the axis {monoijodial). 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 " gulf weed," in which there are slender branch- ing stem-like axes bearing lateral members of various kinds, some of them like ordinary foliage leaves ; others are floats or air- 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 tlie waves and carried away from the coast by currents, collecting in the great sea eddies
Fig. 19. Fragment of a common brown alga {Fucus), showing the body with dichotomous branching and bUidder-like air-bladders.— After Luerssen.
36
PLANT STRUCTURES
produced by oceanic currents and forming the so-called " Sargasso seas," as that of the North Atlantic.
Fig. 20. A portion of a brown alga (Sargassum), showing the thalhis 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 Algae differ from each other in iheir reproduction. One, repre- sented by the Laminarias nd 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 a. an end ; and they resemble each other very closely, except that the gametes fuse in pairs and form zygotes.
^ /
T/l/i^
Fig. 21. Sexual reproduction of Fnais, sh' wing the eight eggs (six in sight)
charged from tlie oogonium and surrounded by a membrane (A), eggs liberated from the membrane (E), antheridium containing sperms (C), the discharged lat- erally biciliate sperms (G), and eggs surrounded by swarming sperms (F, H).— After Singer.
21
38
t»LANT STRUCTURES
The other group, represented by Fucus (Fig. 21), pro- duces no asexual spores, but is heterogamous. A single oogonium usually forms eight eggs (Fig. 21, A), which are discharged and float freely in the water (Fig. 21, E). The antheridia (Fig. 21, C) produce numerous minute laterally biciliate sperms, which are discharged (Fig. 21, 6^), swim in great numbers about the large eggs (Fig. 21, F^ R), 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. 'RB.OJyoFKYCEJE (Red Algw) 31. General characters. — On account of their red colora- tion these forms are often called Floridem. They are mostly
marine forms, and are
rRS^r-
anchored by holdfasts of various kinds. They belong to the deepest waters in which Algae 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 Algae are also a high- ly specialized line, and will be mentioned very briefly.
32. The plant body. — The Red Algae, in general, are more deli- cate than the Brown Algae, or kelps, their graceful forms, delicate texture, and brightly tinted bodies (shades of red, violet, dark purple,
'iG. 22. A red alga {Gigartina), showing branching habit, and "fruit bodies."— After ScHENCK.
Fig. 24. A red alga {Dasrja), showing a finely divided thalliiB body.- Caldwell.
Fig. 25. A red alga {Habdonia), A\o\\\\\\i holdfasts and branching thalhis body.
Caldwell.
Fig. 26. A red alga {Ptilota), whose branching body resembles moss. Caldwell.
THE GREAT GROUPS OF ALG^
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 Algae.
33. Reproduction. — Eed Algae 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-
FiG. 27. A red alga {Callithamnioti), show- ing sporangium (A), and the tetraspores discharged {B). — After Thuret.
rangium always produces just four, they have been called tetraqmres (Fig. 27).
Red Alg^e 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, ^, «) develop sperms which, like the tetraspores, have no cilia and no power of motion. To dis-
FiG. 28. A red alga (Nemalion) ; A, sexual branches, showing antheri- dia (a), oogonium (o) with its trich- ogyne {t), to which are attached two spermatia (s); B, beginning of a cystocarp (o), the trichogyne (0 still showing; C, an almost mattire cj-s- tocarp (o), with the disorganizing trichogyne (^).— After Kny.
44
PLANT STRUCTURES
tinguish them from the ciliated sperms, or spermatozoids, which have the power of locomotion, these motionless male gametes of the Red Algae are usually called s;permatia (singular, spermatium) (Fig. 28, A, s).
The oogonium is very pe- culiar, being differentiated into two regions, a bulbous base and a hair-like process (trichoc/pie), the whole struc- ture resembling a flask with a long, narrow neck, excepting that it is closed (Fig. 28, A, 0, t). Within the bulbous part fertilization usually takes place ; a spermatium attaches itself to the trichogyne (Fig. 28, A, 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 mjstocarp (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-
FiG. 29. A branch of Polysiphonia, one of the red algae, showing the rows of cells composing the body (A), small branches or hairs (B), and a cystocarp iC) with escaping spores (Z>) which have no cilia (car- pospores).— Caldwell.
THE (4REAT GEOUPS OF ALG.E
45
fore, two sorts of asexual spores are produced: (1) the tetraspores^ developed in ordinary sporangia; and (2) the carpospores^ developed in the cystocarp, which has been produced as the result of fertilization.
OTHER CHLOROPHTLL-CONTAINIIs^G TIIALLOPHYTES
34. Diatoms. — These are peculiar one-celled forms, which occur in very great abundance in fresh and salt waters.
Fig. 30. A group of Diatoms : c and d, top and side views of the same form; e, colony of stalked forms attached to an alga;/ and g, top and side views of the form shown at e\ h, a, colony; i, a colony, the top and side view shown at A;.— After Kernek.
They are either free-swimming or attached by gelatinous stalks; solitary, or connected in bands or cliains, 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- jugate.
They occur in such numbers in the ocean that they form a large part of the free-swimming forms on the sur- 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 Algae because they contain a brown coloring matter; others have placed them in the Conjugate forms among the Green Algse 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 Algae.
35. Characeae. — These are common- ly called " stoneworts," and are often included as a group of Green Alg«, as 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 Algae. They are such
Fig. 31. A common CJiara, allowing tip of main axis. —After Stuasburger.
THE GREAT GROUPS OF ALG^ 47
specialized forms, and are so much more highly organized than all other Alg«, 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 Algse.
CHAPTEE V
THALLOPHYTES : FUNGI
36. General characters. — In general, Fungi include Thal- lophytes whicli do not contain chlorophyll. From this fact it follows tliat tliey 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 iiarasite^ 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 Alg^. 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 ; wliile many of the saprophytic forms are decidedly beneficial.
It is generally supposed that the Fungi are derived from the Algee, having lost their chlorophyll and power of inde- pendent living. Some of them resemble certain Algae so closely that the connection seems very plain ; but others 48
THALLOPIIYTES: FUNGI
49
have been so modified by their parasitic and saprophytic habits that they have lost all likeness to the Algae, and their connection with them is very obscure.
37. The plant body. — Discarding certain problematical forms, to be mentioned later, the bodies of all true Fungi are organized upon a uniform general jAan, to which they can all be referred (Fig. 32). A set of colorless branching
^^;
Fig. 32. A diagrammatic representation of Mvco7', showing the profusely branching mycelium, and three vertical hyphee (sporophores), sporangia forming on b and c. —After Zopp.
filaments, either isolated or interwoven, forms the main working body, and is called the inijceUum. The interweav- 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 liyplicB (singular, liyplia) or hyplial threads. The mycelium is in contact with its source of food supply, which is called the suhstratum.
50
PLANT STRUCTDRES
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 liyplm or sjuirophores, 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 siibdivisions. — 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 Algae. Four groups will be pre- sented, often made to include all the Fungi, but doubtless they are insufificient 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) Pliycomycetes ("Alga-Fungi"), referring to the fact that the forms plainly resemble the Algse ; (2) Ascomycetes ("Ascus-Fungi"); (3) ^cidiomycetes ("^cidium-Fungi ") ; (4) Basidiomycetes (" Basidium-Fungi "). Just what the prefixes ascus, cecidium, and hasidium 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 Algae, and are only like themselves.
THALLOPHYTES: FUNGI 5;[
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 spores are about as evident in Phycomycetes as in Alga3, they are either obscure or wanting in the Mycomycete groups.
1. Phycomycetes (Alga-Fungi)
39. Saprolegnia. — This is a group of "water-moulds," with aquatic habit like the Algas. They live upon the dead bodies of water plants and animals (Pig. 33), and some- times attack living fish, one kind being very destructive to young fish in hatcheries. The hyphse composing the mycelium are coenocytes, 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, (7), swim about for a short time, and rapidly form new mycelia. The process is very suggestive of Cladoplioi^a and Yauclieria. Oogonia and antheridia are also formed at the ends of the branches (Fig. 33, F)^ much as in Vau- cheria. The oogonia are spherical, and form one and some- times many eggs (Fig. 33, />, E). 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 Qgg^ 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 Qgg^ without fertiliza- tion, forms an oospore which can germinate. This peculiar
52
PLANT STRUCTURES
habit is called partlienogenesis, which means reproduction
by aw e^g without fertilization.
Fig. 33. A common water mould {Saiwolegnia): 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 E, oogonia with several eggs.— ^-C after Thuret. Z>-/^ 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, etc. It is therefore a saprophyte, the co^nocytic mycelium branching extensively through the substratum (Fig. 34).
THALLOPHYTES : FUNGI
53
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 zoo- 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
Fio. 35. Foriping sporangia of Mucor\ show- ing the swollen tip of the sporophore (.1), and a later stage (B), in which a wall is formed separating the sporangium from the rest of the body.— Caldwell.
64
PLANT STRUCTURES
While tlie ordinary inetliod of reproduction through the growing season is by means of these rapidly germinating spores, in certain conditions a sexual process is observed, by wliich a heavy-walled sexual spore is formed as a resting spore, able to outlive unfavorable conditions. Branches arise from the hyph^ of the mycelium just as in the forma-
FiG. 3G. Mature sporangium of Miicor, showing the wall {A), the numerous spores (C), and the columella (i?)— that is, the partition wall pushed up into the cavity of the sporangium. —Caldwell.
Fig. 37. Bnrsted sporangium of Mucor, the ruptured wall not being shown, and the loose spores adhering to the colu- mella.—Caldwell.
tion of sporophores (Fig. 38). Two contiguous branches come in contact by their tips (Fig. 38, J), the tips are cut off from the main coenocytic body by partition walls (Fig. 38, B)^ the walls in contact disorganize, the contents of the two tip cells fuse, and a heavy-walled sexual spore is the result (Fig. 38, C). It is evident that the process is conjugation, suggesting the Conjugate forms among the
THALLOPHYTES: FUNGI
55
Algse ; 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. Mucor^ therefore, is isogamous.
ZT^Tm^^^ /^"^??^$
Fig. 38. Sexual reproduction of Mucor, showing tips of sex branches meeting {A), the two gametangia cut oflf by partition walls {B), 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 coeno- cytic 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 tliis 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- ter rising above the 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."
Fig. 39. A branch of Peronospoi'a in contact with two cells of a host plant, and sending into them its large haustoria.— After DeBary.
Fig. 40. Peronospm^a, one of the Phycomycetes, showing at a an oogonium (o) con- taining an egg, and an antheridinm in) in contact; at b the antheridial tube pene- trating the oogonium and discharging the contents of the antheridium into the egg; at c 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 Qgg. 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. As 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 coenocytic bodies of the whole group are very suggestive of the Siphon forms among Green Alg?e, as is also the method of forming oogonia and antheridia.
The water-moulds, Saprolegnia and its allies, have re- tained the aquatic habit of the Algae, and their asexual spores are zoospores. Such forms as Mucor 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 heterogamous 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 Alg^ (Chlo- rophyceae) of various kinds.
2. AscoMYCETES {Ascus- ov Scic-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 STKUCTURES
dew, Microsphara, grows on lilac leaves, which nearly always show the whitish covering after maturity (Fig. 41). The branching hyplia? show numerous partition Avails, and are not coenocytic as in the Phycomycetes. Small disk-like haustoria penetrate into the superficial cells of the host, anclioring 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 ab strict mi, and the spores formed in this way are called co7iidia, 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 not always occur. An oogo- nium and an antheridium, of the usual forms, but probably 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
Fig. 41. Lilac leaf covered with mil dew {Microsph(era)y the shaded re gions representing the mycelium, and the black dots the ascocarps.— S. M. Coulter.
THx\LLOriIYTES: FUNGI
50
a little sphere, which suggested the name Microsphcera (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 Algge (§ 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 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) co?iidia, 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 Algae 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
Fig. 42. Ascocarp of the lilac mildew, showing branching appendages and two asci protruding from the rup- tured wall and containing ascospores. — S. M. Coulter.
60
PLANT STKDCTURES
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-
PiG. 43. Penicillium, a common mould : A, 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. 47).
THALLOPHYTES : FUNGI
61
Fig. 44. Head of rye attacked by " er- got" (a), peculiar grain-like masses replacing the grains of rye ; also a mass of "ergot" germinating to form spores (6).— After Tulasne.
Fig. 45. Two species of cup-fangu8 {Peziza). — After Lindau.
Fig. 46. A cup-fungus (Pitya) grow- ing on a spruce (Picea). — Aiter Rehm.
In some of these forms the ascocarp is completely closed, as in the lilac mildcAV ; in others it is flask-shaped ; in others, as in the cup-fungi, it is like a cup or disk ; hut 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.
Fig. 47. The common edible morel {Morchella esculenta). The strncture shown and used represents the ascocarp, the depressions of whose surface are lined with asci contain- ing ascospores. — After Gibson.
Fig. 48. Yeast cells, reprodti- cing by budding, and form- ing chains.— Caldwell.
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. ^ciDiOMYCETES (^cidium-Fungi)
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.
TIlALLOrilYTES: FUNGI
63
In some forms an obscure sexual process has been de- scribed, but it is beyond the reach of ordinary observation. The ^cidiomycetes do not form an independent and nat- ural group, but are now generally placed under the Basi- diomycetes, but they are so unlike the ordinary forms of that group that they are here kept distinct.
Most of the forms are yqvj 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-
c^VlioliW'W^a'S^ lii/L
Fig. 49. Wheat rust, showing sporophores breaking through the tissues of the host and bearing summer spores (urcdospores).— After II. Marshall 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 Credo. When it was known that this is but one
Fig. 50— Wheat rust, showing a young hypha forcing its way from the surface of a leaf down among the nutritive cells.— After H. Marshall Ward.
stage in a polymorphic life history it was called the Uredo- stage, and the spores uredospores., sometimes "summer
spores.
Fig. 51. Wheat rust, showing the winter spores (teleutospores).- After H. Marshall 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 teleuto- 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, is a second phase of the wheat rust, really the first phase of the growing season.
The sporidia are scattered, fall upon barberry leaves, germinate, and 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
Fig. 52. Wheat rust, show- ing a teleutospore germina- ting and forming a short fil- ament, from four of whose cells a spore branch arises, the lowest one bearing at its tip a sporidium. — After n. Marshall Ward.
Q6
PLANT STRUCTURES
"cluster-Clips." This mycelium on the barberry, bearing cluster-cups, was thouglit to be a distinct plant, and was
called j^cidium. The name now is applied to the cluster-cups, which are called cecidia, and the conidia-like spores which they produce are known as cecidiospores.
It is the aecidia which give name to the group, and ^cidiomycetes are those Fungi in whose life history ^ecidia or cluster-cups appear.
The secidiospores 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 is com- pleted. There are thus at least three distinct stages in the life history of wheat rust. Begin- ning with the growing season they are as fol- lows : (1) The phase bear- ing the sporidia, which is not parasitic ; (2) the gecidium phase, parasitic on the barberry; (3) the uredo-teleutospore phase, para- sitic on the wheat.
In this life cycle at least four kinds of asexual spores
TIIALLOPIIYTES : FUNGI
67
appear : (1) sporidia^ which develop the stage on the barber- ry ; (2) cecicUospores^ which develop the stage on the wheat ; (3) nredos2)ores^^Yh\G\\ repeat the mycelium on the wheat ; (4) teUutos2)ores^^\\i(^\\ last through the winter, and in the spring produce the stage bearing sporidia. It should be said that there are other structures of this plant produced on the bar- berry (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.
Fig. 54. Two species of "cedar apple" {Gymnosporangium), both on the common juniper {Juniperus Virgi)}iana).—A after Farlow, B after Engleu 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-
08
PLANT STRUCTURES
nected together, so that a mycelium bearing uredospores is called a Ureclo^ one bearing teleutospores a Puccinia^ and one bearing »cidia an j^cidium ; but what forms of Uredo^ Pucci)iia^ and ^Fcidium belong together in tlie 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, etc., the aecidium stage of the same parasite develops.
4. Basidiomycetes {Basidium- Fungi). 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.
As in ^cidiomycetes,
Fig. 55. The common edible mushroom, Agaricus campestris.— After Gibson. an obsCUrC SCXUal prOCCSS
TIIALLOPIIYTES: FUNGI
60
is reported. The life history seems simple, but this appar- ent simplicity may represent a very complicated history. The structure of the common mushroom (Agaricus) 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, gro w- ing larger and larger, until they are organized into the so-called "mushrooms." The real body of the plant is the white thread - like mycelium, while the " mushroom " part seems to rep- resent a great num- ber of sporophores organized together to form a single complex spore- bearing structure.
The mushroom 28
Fig. 56. A common Agaricus : A, section through one side of pileus, showing sections of the pendent gills; 7?, section of a gill more enlarged, showing the cen- tral tissue, and the hroad border formed by the ba- sidia: T. still more enlarged section of one side of a gill, showing the club-shaped basidia standing at right angles to the surface, and sending out a pair of small branches, each of which bears a single ba- sidiospore.— After Sacus.
w O
S Si
be 5
§ o
hn I
THALLOPIIYTES : FUNGI
71
has a stalk-like portion, the stipe^ at the base of which the slender mycelial threads look like white rootlets ; and an expanded, umbrella-like top called the pileus. 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 (liyph^), whose tips turn toward the surface and form a compact layer of end cells (Fig. 56). These end
'^^1% |
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Fig.
A bracket fungus {Polyporits} growing on the trunk of a red oak. Caldwell.
cells, forming the surface of the gill, are club-shaped, and are called basidia. From the broad end of each basidium two or four delicate branches arise, each bearing a minute spore, very much as tlie sporidia appear in the wheat rust.
72
PLANT STRUCTURES
These spores, called hasidiospores^ shower down from the gills when ripe, germinate, and produce new mycelia. The peculiar cell called the hasidium gives name to the group Basidiomycetes.
50. Other forms. — Mushrooms display a great variety of form and coloration, many of them being very attractive
Fig. G1. 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,
;'^^%^^.-«r;
Fig. 62. The common edible Boletus (B. edu- lis), in which the gills are replaced by pores.— After Gibson.
Fig. 63. Another edible Boletus (B. stro- bilaceus).— After Gibson.
Fig. 64. The common edible " coral fun- gus " {Ciavaria).— After Gibson.
Fig. 65. Hydninn trjia/idum, in which gills are replaced by .ipiuous processes ; edi- ble.—After Gibson.
74
PLANT STKUCTUKES
61), and the mushroom-like Boleti (Figs. 02, 63). The "ear-fungi" form gelatinous, dark-brown, shell-shaped masses, and the " coral fungi " resemble branching corals (Fig. 6J:). The Hydnum forms have spinous processes
instead of gills (Fig. 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
5 1 . Slime - moulds. — These perplexing forms, named Myxomycetes^ do not seem to be related 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 j-j^^s- modium, suggesting the term "slime," and slips along like a gigantic amoeba. 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 pinhead to as large as a man's hand. They are saprophytic, and are said to engulf food as do the amoebas. So suggestive of certain low animals is this body and food habit that slime-moulds have also been called Mycetozoa or " fungus- animals."
Fig. 66. Pnffballs, in which the basidia and spores are inclosed ; edible.— After Gibson.
TIIALLOPIIYTES: FUNGI
Y5
In certain conditions, however, these slimy bodies come to rest and organize most elaborate and often very beau- tiful sporangia, full of spores (Fig. G7). These varied and easily preserved sporangia are used to classify the
Fig. 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 Ilemiarcyria, with a Plasmodium mass at upper left hand.— GOLDBEKUER.
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 Cyanophycese, or " Fission- Algae," so closely that they are often associated Avith them in classification (see § 21).
\v^^y//
v^
?//v
V «
Fig. 68. A group of Bacteria, the bodies being black, and bearing motile cilia in various ways. .1, the two to the left the common hay BacUhis (B. srihtilis), the one to the right a Spirilliwi ; B, a Coccus form {Planococcus)\ C, D, E, species of Pseudomonas : F, G, species of Bacillus, i^" being that of tyjjhoid fever; II, Micro- spira ; J, A', L, M, species of Spirillum.— After Engler and Pkantl.
TIIALLOPHYTES: FUNGI Y7
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. (^oioo 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
TIIALLOPIIYTES: FUNGI
79
plant. In other words, a Lichen is not an individual, but a firm of two individuals very unlike each other. This habit
Fig. 70. A common lichen {PMjscia) growing on bark, showing the spreading thallus and the numerous dark disks (apothecia) bearing the asci.— Goi.dberger.
of living together has been called spnbiosis, and the indi- viduals entering into this relation are called symhionts.
Fig. 71. A common foliose lichen (Pannelia) growing upon a board, and showing apothecia.— GoLUBERGEu.
80
PLANT STRUCTURES
If a Lichen be sectioned, the rehition between the sym- bionts will be seen (Fig. 72). The fungus makes the bulk of the body with its interwoven mycelial threads, in the meshes of which lie the Algge, sometimes scattered, some-
FiG. 72. Section through thaHus of a lichen (Sticta), showing holdfasts (r), lower (w) and upper (o) surfaces, fungus hyphse (?«), and enmeshed algoe (g).— After Sachs.
times massed. It is these enmeshed Algas, showing through the transparent mycelium, that give the greenish tint to the Lichen.
In the case of Lichens the symbionts are thought by some to be mutually helpful, the alga manufacturing food for the fungus, and the fungus providing protection and water containing food materials for the alga. Others do not recognize any special benefit to the alga, and see in a Lichen simply a parasitic fungus living on the products of an alga. In any event the Algae are not destroyed but seem to thrive. It is discovered that the alga symbiont can live quite inde-
THALLOPIIYTES: FUNG]
81
pendently of the fungus. In fact, the enmeshed Algae are often recognized as identical with forms living independ- ently, those thus used being various Blue-green, Protococ- cus, and Conferva forms (see p. 87).
On the other hand, the fungus symbiont has become quite dependent upon the alga, and its germinating spores do not develop far unless the young mycelium can lay hold of suitable Algas. At certain times cup-like or disk-like bodies appear on the surface of the lichen thallus, with brown, or black, or more brightly-colored lining (Figs. 70, 71). These bodies are the apothecia^ and a section through them show^s that the colored lining is largely made up of delicate sacs containing spores (Figs. 73, 74). These sacs are evidently asci, the apothecia correspond to ascocarps, and the Lichen fungus proves to be an Ascomycete.
Fig. 73. Section through an apothecium of AnaptycMa, showing stalk of the cup (m), masses of algal cells {g), outer margin of cup (/■), overlapping edge {t, i), layer of asci (h), and massing of hyphre beneath asci (y).— After Sachs,
Certain x^scomycetes, therefore, have learned to use cer- tain Algae in this peculiar way, and a Lichen is the result. Some Basidiomycetes have also learned the same habit, and form Lichens.
Various forms of Lichen bodies can be distinguished as follows : (1) Crustaceous Lichens^ in which the thallus resem-
82
PLANT STRUCTUKES
bles an incrustation upon its substratum of rock, soil, etc. ; (2) Foliose Lichens^ with flattened, leaf -like, lobed bodies, at-
FiG. 74. Much enlarged section of a portion of the apothecium of Anaptychia, show- ing the fungus mycelium (r«), which is massed above (y), just beneath the laj'er of asci {1, 2, 3, //), in which spores in various stages of development are shown. — After Sachs.
tached only at the middle or irregularly to the substratum ; (3) Fruticose Licliens^ with filarnentous bodies branching like shrubs, either erect, pendulous, or prostrate.
CHAPTEK VI
THE FOOD OF PLANTS
54. Introductory. — All plants use the same kind of food, but the Alg^ and Fungi suggest that they may have very different ways of obtaining it. The Algag 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 (COg), 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 (HoO) are recombined to form a carbohydrate (sugar, starch, etc.), which is an organ- ized food. AVith 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
34 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 photos i/ntax, 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 Plcmt Ihlations.
It is evident that green plants must expose their chloro- phyll to the light. For this reason the Algae- 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 light, 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 bodies 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 35
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 some plants (as broom rapes) are parasitic, attaching their subterranean parts to those of other plants, becoming what are called " root para- sites." In cases of mycorhiza (see p. 87), which are now thought to include great numbers of green plants, it is sup- posed that some organic material is brought in by the fungus.
57. Parasites. — Certain plants without 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 24
^LA^'T STKUCTURES
the food wliicli, 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 Algae 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- sitism.
58. Symbionts. — Symbiosis means "living together," and two organisms thus related are called S7jmlio7its. In its broadest sense symbiosis includes any sort of depend- ence between living organisms, from the vine and the tree
Fig. 75. 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.
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 in certain cases. 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. — A lichen is a complex made up of a fun- gus and an alga living together. It is certain that the fun- gus cannot live without the alga, but the alga can live without the fungus. Hence it seems plain that this rela- tion is not one of mutual helpfulness, but that the fungus is living upon the alga, as any other parasite lives upon its host (see § 194).
(2) MycorMza. — The name means "root-fungus," and refers to an association which exists between certain Fungi of the soil and roots of higher plants. It was formerly thought that mycorhiza occurred only in connection with a limited number of liigher plants, such as orchids, heaths, oaks, etc., but more recent study indicates that probably the large majority of vascular plants (that is, plants with true roots) possess it, the water plants being excepted (Figs. 149, 150). It has been found that the humus soil of forests is in large part " a living mass of innumerable filamentous fungi." It is clearly of advantage to roots to relate them- selves to this great network of filaments, which are already in the best relations for absorption, and those plants which are unable to do this are at a disadvantage in the competi- tion for the nutrient materials of the forest soil. It is doubtful whether many vascular green plants can absorb
Fig. 76. Mycorhiza : 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 (p) filled with hyphte; B, 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 (a, i). —After Fkank.
Fig. 77. Mycorhiza : A, rootlets of white poplar forming mycorhiza ; B, enlarged section of single rootlets, showing the hypha3 penetrating the cells.— After
KEBNEIi.
THE FOOD OF PLANTS
enough for their needs from the soil without this assistance, and, if so, the fungus becomes of vital importance in the nutrition of such plants. In the case of some of these plants it seems that the soil fungus is not merely passing into their bodies the soil water with its dissolved salts, but is contributing to them organized food, thus diminishing the amount of necessary food manu- facture. The delicate branching fila- ments (hyphse) of the fungus wrap the rootlets with a mesh of hyphae and penetrate into the cells, and it is evident that the fungus obtains food from the rootlet as a parasite.
(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 in some usable form. Ordinarily 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-
FiG. 78. Root-tubercles on Vicia Faba.— After Noll.
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 be an animal. Certain fresh-water polyps and sponges become green on account of Algae which they harbor with- in their bodies (Eig. 79). Like the Lichen-fungus, these animals are benefited by the presence of the Algge, 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 guard them against the attack of leaf-cutting insects and oth- er foes. These plants are called Myrmecopliytes^ which means " ant-plants," or myrmecopliilous 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) Floicers and itisects. — 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
Fig. 79. A fresh-water polyp (ZTy- d?'a) attached to a twig and con- taining algfe (C), which may be seen through the transparent body wall (5).— Goldberger.
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
yr>>^T;iV/^\'
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Fig. 80. An ant plant (Hydnophytum) from South Java, in which an excrescent growth provides a habitation for ants.— After Schimpeu.
considered, or it may be found, with illustrations, in Plant Relations, Chapter VII.
92 PLANT STRUCTURES
59. Carnivorous plants. — Certain green plants, growing in situations poor in nitrogen-containing salts, have learned to supplement the proteicls 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 j^lant 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
BRYOPHYTBS (MOSS PLANTS)
60. Summary from ThaUophytes. — Before considering the second great division of plants it is well to recall the most important facts connected with the ThaUophytes, those things which may be regarded as the contribution of the ThaUophytes to the evolution of the plant kingdom, and which are in the background when one enters the region of the Bryophytes.
(1) Increasing 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 Qgg, the small active one the
93
94 PLANT STRUCTURES
sperm, the organs producing the two being known as oogo- nium and antheridium respectively.
(5) Algce the main line. — The Algag, aquatic in habit, appear to be the Thallophytes which lead to the Bryophytes and higher grouj^s, the Fungi being regarded as their de- generate descendants ; and among the Algae the Cliloro- phyceae seem to be most probable ancestors of higher forms. It should be remembered that among these Green Algae 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 Algae 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 Green Algae (Pig. 81). It is prostrate, and is a regu-
BRYOPHYTES
95
lar thalliis 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, ^). A bud develops into an erect
Fig. 81. Protonema of mops : A, very young protonema, showing spore (-S*) which has germinated it; B, older protonema, showing branching habit, remains of spore (s), rhizoids (?'), and buds {b) 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 Algae, 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 STKUCTURES
.rh
Fig. 82. A common moss (Polytrichtim catnminie), showing the leafy gameto- phore with rhizoids (rh), and two sporophytes (eporo- gonia), with seta (s), calyp- tra (c), and opercuhim (d), the calyptra having been re- moved.—After SCHENCK.
plant from wliich it came. This new leafless 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 what is meant by aUernatio7i of gen- erations.
These two ^^generations" differ strikingly from one another in the spores which they produce. The generation composed of alga -like body 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 gametopliyte — 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 spor'opliyte — 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 Algge, 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 S are used for gametophyte and sporophyte respectively :
G=g> 0— S— 0— G=8> 0— S— 0— G, etc.
The formula indicates that the gametophyte produces two gametes (sperm and Qgg), which fuse to form an oospore, which produces the sporophyte, which produces an asexual spore, which produces a gametophyte, etc.
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 TLANT 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 Algae, 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 popular 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 protonenia, the leafy branch is the gametopliora (^^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 gametopliore is the foot, and the urn-like spore-case is the capsule.
BKYOPIIYTES
99
63. The antheridium.— The male organ of the Bryophytes is called an antheridium, just as among Thallophytes, but it has a very diHerent structure. In general among the
Fig. 83. Sex organs of a common moss {Funaria): the gronp to the right represents an antheridium {A) discharging from its apex a mass of sperm mother cells (a), a single mother cell with its sperm (6), and a single sperm (c), showing body and two cilia; the gronp to the left represents an archegonial cluster at summit of stem (.1), showing archeu^onia (a), and paraphyses and leaf sections ib), and also a single archegonium \B), with venter (b) containing egg and ventral canal cell, and neck Ui) containing the disorganizing axial row (neck canal cells).— After Sachs.
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. It is 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- ate sperms are one of the distinguish- ing marks of the Bryophytes. The existence of male gametes in the form of ciliated sperms indicates that fertil- 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 Algae. 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 opened at the apex and discharge the mother cells in a mass (Figs. 83, 105, E), the walls of the mother cells become mucilagi- nous, and the sperms escaping swim actively about and are attracted to the organ containing the Qgg.
64. The archegonium. — This name is given to the female sex organ, and it is very different from the oogonium of
Fig. 84. Antheridium of a liverwort in section, showing single layer of wall cells surround- ing the mass of moth- er cells.— After Stuas-
BURGER.
BRYOPHYTES 101
Thallophytes. Instead of being a single mother cell, it is a many-celled 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 Qgg 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 Qgg. In this way there is formed in the arche- gonium an open passageway through the neck to the Qgg lying in the venter.
To this neck the swimming sperms are attracted, enter and pass down it, one of them fuses with the Qgg, and this act of fertilization results in an oospore.
Archegonia and antheridia are supposed to have been derived from a many-celled gametangium, such as occurs in certain Brown Algae (Fig. 18). The presence of the archegonia is one strong and unvarying distinction between Thalloj^hytes and Bryophytes. Pteridophytes also have archegonia, and so characteristic an organ is it that Bryo- phytes and Pteridophytes are spoken of together as Arche- goniate^.
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 the foot, which ob- tains a firm anchorage in the gametophore by the latter growing up around it (Fig. 85, B^ C). The upper part of the embryo develops the seta and capsule. As the embryo increases in size, the venter of the archegonium grows also, forming what is called the c(dy2)tra ; and in true Mosses the embryo presently breaks loose the calyptra at its base 25
102
PLANT STEUCTUEES
and carries it upward perched on the top of the capsule like a loose cap or hood (Figs. 82, c, 107), which sooner or later
falls oif. As stated be- fore, the mature struc- ture developed from the oospore is called a sporogonium, a form of sporophyte peculiar to the Bryophytes.
6G. The sporogoniuni. — 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 aj^- pearance from those about them, being set apart for the produc- tion of spores. This initial group of spore-producing cells is called the arclie- sporium, a word meaning ^'the beginning of s23ores." It
Fig. 85. Sporogonium of Funaria : ^, an em- bryo sporogonium if,/'), developing within the venter (b, b) of an archegonium ; li, C, tips of leafy shoots bearing young sporo- gonia, pushing up calyptra (c) and archego- nium neck (h), and sending the foot down into the apex of the gametophore.— After
GOEBEL.
BRYOPIIYTES 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 sterile, and are often spoken of as sterile tissue. Every 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-
lO-i
PLANT STRUCTURES
ous part of the higher phmts. The ^' fern plant," and the herbs, shrubs, and trees among *^ flowering plants" correspond to the sporogoninm of Bryophytes, and not to the leafy branch (ganietophore) 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 IHccia the simplest sporogonium is found. It is a globular capsule, without seta or foot
Fig. 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, Anthoceros, archesporium still more restricted, being dome-shaped and capping a central sterile tissue, the columella (co^).— After Goebel.
(Fig. 86, A). 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 {Marchfuifia) the entire lower lialf of the sporogonium is sterile, while in the upper half there
BRYOrilYTES
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. 8Q, B). 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 {Jungermamiia) the archesporium is still more restricted (Fig. 87). Tlie sterile tissue is organ-
FiG. 87. Diagrammatic section of spo- Fig. 88. Section through sporogonium of
rogonium of a Jun germannia form, showing differentiation into foot, seta, and capsule, the archesporium restricted to upper part of sporogo- nium.—After GOEBEL.
Sphag/ivm, showing capsule (k) with old archegoninm neck (ah), calyptra {ca), dome-shaped mass of sporogenous tissue (spo), and columella (co). also the bulb- ous foot (spf) imbedded in the pseudo- podium (;?,<).— 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 (AnfJioceros) the sterile tissue or- ganizes foot and seta, and the archesporium is still more restricted (Fig. 86, B). 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 cells which is surrounded by the ar- chesporium is called the columella, which means -^^a small column."
In a moss called Sj^liagnum there is the same dome-shaped archespori- um with the columella, as in Aii- 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 Spliagnnm or An- tlioceros had become sterile at the apex. In this way the columella is continued through the capsule, and is not capped by the archesiDorium.
This series indicates that after the sporogonium begins as a simjDle spore case {Biccia), 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, etc.
Among the Green Algae there is a form known as Coleochcnte, whose body resembles those of the sim- plest Liverworts (Fig. 90). AVhen
Fig. 89. Young sporoirnni- nm of a true moss, show- ing foot, seta, and young capsule, in which the ar- chesporium (darker por- tion) is barrel -shaped, and through it the columella is continuous with the lid.— After Campbell.
BRYOPIIYTES
lor
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
Fig. %.-Coleoch^te, one of the green algfe : A, a portion of the thallus, showing oogonia with trichogynes {og), antheridia (an), and two enlarged biciliate sperms (zY B a fertilized oogonium containing oospore and invested by a tissue (r) which' has developed after fertilization; (7, an oospore which has germinated and formed a mass of cells (probably a sporophyte), each one of which organizes a biciliate zoospore (2)).— After Pringsheim.
Riccia would be the result (Fig. 86, A). For such reasons many believe that the Liverworts have been derived from such forms as CoUoclia'fe.
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 m the Mosses it is differentiated into protonema and leafy gametophore (Figs. 81, 82, 102). Like the sporophyte,
IQg PLANT STKUCTUKKS
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 diiferentiated 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 S^jJiagnum 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 tliallus of the Liverworts and SpJiagnmn, while the leafy branch corresponds to the leafless gameto- phore found in some Liverworts. It also seems evident that the gametojohore was originally set apart to bear sex organs, and that the leaves whicli appear upon it in the Mosses are subsequent structures.
I
CHAPTEE VIII
THE GREAT GROUPS OF BRYOPHYTES
Hepatic.^ {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 exposed to different conditions and become unlike in structure. In Liverworts the ventral surface is against the substratum, and puts out hair-like processes {rhizoids) for anchorage and possibly absorption. 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 anchoring 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 seem to have been at least three main lin«s 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
TLA NT STKUCTUKES
69. Marchantia forms. — In this line tlie simple thallus gradually becomes changed into a very complex one. The
thallus retains its simple outlines, but becomes thick and differentiated in tissnes (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 3Iar- cliaiitia proper the thallus becomes very comjolex, and it may be taken as an illus- tration.
The thallus is so thick
that there are very distinct
green dorsal and colorless
ventral regions (Fig. 94). The latter puts out numerous
rhizoids and scales from the single layer of ej)idermal cells.
Above the ventral epidermis are several layers of colorless
Fig. 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 Strasbuuger.
Fig. 92. Ricciocarpvs. a Marchantia form, showing ntmierous rhizoids from ventral surface, the dichotomous branching, and the position of the sporogonia on the dorsal surface along the "midribs."— Goldberger.
Fig. 93. Two common liverworts : to the left is Conocephahis^ a Marchantia form, showing rhizoicls, dichotomous branching, and the conspicuous rhombic areas (areolae) on the dorsal surface; to the right is Anthoceros, with its simple thallus and pod-like sporogonia.— Goldberger.
Fio. 94. Cross-sections of thallus of Marchantia: A, section from thicker part of thallus, where supporting tissue (p) is a])undant. and showing lower epidermis giving rise to rhizoids {h) and plates (6), also chlorophyll tissue (r///) organized into chambers by partitions (O); B, section near margin of thallus more magnified, showing lower epidermis, two layers of supporting tissue {])) with reticulate walls, a single chlorophyll chamber with its bounding walls (s) and containing short, often branching filaments whose cells contain chloroplasts {chl), overarching upper epidermis (o) pierced by a large chimney-like air-pore (s/j).— After Goebkl.
1
Fig. 95. Section through cnpulc oi Marchantia, showing wall in which are chloro- phjil-bearing air-cliambers with air-pores, and gemmie (a,) in various stages of development.— DoDEL- Port.
Fia. 96. Marchantin polymorpha : the lower figure represents a gametophyte bear- ing a mature antheridial branch id), some j^oung antheridial branches, and alpo 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, b, c, d, e,/).— After Kny.
THE GREAT GROUPS OF BRYOPIITTES
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
^^Ji
Fig. 97. Marchantia polyinorphn. a common liverwort : U thallns. with rhizoide, bearing a mature archegonial branch if) and several younger ones (fl, b. c. d, e)\ 2 and 5, dorsal and ventral views of archegonial disk; h and ,-7. young sporophyte (sporogoninm) embryos; G. more mature sporogonium still within enlarged venter of archegonium: 7. mature sporogonium discharging spores; S, three spores and an elater. — After Kny.
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
ivtl
The air chambers are outlined on the surface as small
rhombic areas {areolcp), ea6h containing a single air pore.
Peculiar reproductive bodies are also developed u^n
the dorsal surface of MarchcDitia for vegetative multipliciifc
Fig. 98. Marchantia polymorpha : 1, partial section thronph archegonial di^k, show- ing archcgonia with long necks, and venters containing eggs; .'/. young archego- nium showing axial row; 10. superficial view at later stage; 11, 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 Knt.
tion. Little cups {cnpules) appear, and in them are numer- ous short-stalked bodies (geinmce), which are round and flat (biscuit-shaped) and many-celled (Figs. 95, 96). The
THE GREAT GROUPS OF BRYOPIIYTES II5
gemmae fall off and develop new thallus bodies, making rapid multiplication possible.
Marchantia also possess remarkably prominent gameto- pliores, 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 game toph ore, 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 dioecious, meaning ^^two households"; Avhen they both appear upon the same individual, the plant is monmcious, meaning " one household." Some of the Bryophytes are mo- noecious, and some of them are dioecious (as Marclumtici).
Another distinguishing mark of the line of Marclmntia 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 Marcliantia line, tlie structure of the thallus re- mains simple, there being no such differentiation of tissues as in the Marcliantia 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 Juugvrmannia forms are usually called ** leafy liverworts," to distinguish them from the
Fig. 99. Two liverworts, both Jungermannia forms: to the left is Blasia, which re- tains the thailus form but has lobed margins; to the right is Scapania, with dis- tinct leaves and sporogonia (.4).— Goldbergek.
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 (valves) when opening to discharge the spores (Fig. 100, C).
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 GKEAT GROUPS OF BEYOPHYTES
117
Fig. 100. Species of Lepidozia, a genus of leafy liverworts, showing different leaf forms, and in A and C the dehiscence of the sporogonium by four valves. In C rhizoids are evident; and in B, D, and E the three rows of leaves are seen, the leaves of the ventral row being comparatively small.— After Engler and Frantl.
Mosses, and possibly to the Pteridopbytes also. The thallus is very simple, being differentiated neither in structure nor form, as in the two other lines ; but the 26
118
TLANT" 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- plex walls of this cap- sule contain chlorophyll and air pores, so that the sporogonium is or- ganized for chlorophyll work. If it could send absorbing roots into the soil, this sporophyte could live independent of the gametophyte. In opening to discharge spores the pod-like cap- sule splits down into two valves.
Another peculiarity of the Anthoceros 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 Marchantia line has differentiated the structure of the
Fig. 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 o^ which are the spores; C, D, E, F, ela- ters of various forms ; G, spores.— After
SCHIFFNER.
THE GREAT GROUPS OF BRYOPIIYTES HQ
gametophyte ; the Jungermannia line has differentiated the form of the gametophyte ; the A^ithoceros 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.
Musci (Mosses)
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 shuts off the lower strata of moss from complete disorganization, and they become modified into a coaly sub- stance called peat, which may accumulate to considerable thickness 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 STKUCTURES
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. 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 in 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 gametophyte, 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
Fig. 102. A moss {Bryvm), showing base of a leafy branch (gametophore) attached to tlie protonema. and having sent out rhizoids. On the protonemal filament to the right and be- low is the young bud of another leafy branch.
— MULLEU.
THE GREAT GROUPS OF BRYOPIIYTES
121
(Fig. 102), the thallus part dying. Sometimes, however, the filamentous protonema is very persistent, and gives rise to a perennial succession of leafy branches.
Fig. 103. Tip of leafy branch of a moss (.Funana), bearing a cluster of sex organs, showing an old antheridium {A), a yonnger one (7?), some of the curious associated hairs (p), and leaf sections (/).— 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
122
PLANT STRUCTUKES
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 dioecious and monoecious. 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, A). 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, from which the larg3 leafy gametophore arises (Fig. 104). They also resemble Anthoceros forms in the sporogonium, the archesporium being a dome-shaped mass (Fig. 105, C). 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.
Fig. 104. Thallus body of gametophyte of Sphag- num, giving rise to rhizoids (r) and buds (k) which develop into the large leafy branches (gametophores).— After Campbell.
THE GREAT GROUPS OF BRYOPHYTES
123
^nid." This may "intermediate" or
105, D), which means a "cover" or
serve to illustrate what is called an
"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
B C /; a ^
Fig 105 Svhagmim : A, a leafy branch (gametophore) bearing four mature eporo- gona; f archegorlium in whose venter a young embryo sporophyte (em) ,s de- vdop in^' C, section of a young sporogonium (sporophyte), shown.g the b« bous foX/) imbedded in the apex of the pseudopodium (;;.), the caps^xle U. he cohmella (CO) capped by the dome-shaped archesporium (.;;(» a portion of the ca vptra ica) and the old archegonium neck (aA); D, branch bcanng mature po'rogonJum'and showing pseudopodium (;.), capsule (.•), and ^^^^^^^^ antheridium discharging sperms; F, a single sperm, showmg coded body and i^^o cilia.— After Schimpek.
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 STKUCTUKES
called a jjseudopodium, or ''false stalk," andjivilie top of it is imbedded the foot of the sporogormH^Tcarrying 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 Brijum forms, to distinguish them from the Si^lmgnum forms. They are
Fig. 106. Different stages in the development of the leafy gametophore from the pro- tonema of a common moss {Funaria): A, the first few cells and a rhizoid {r)\ B, C, later stages, showing apical cell (I) and yonng leaves' (i?); 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 Jungprmannia ioi'va^. 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
125
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 difEerentia- tion of tissues.
It is the sporophyte, however, which shows the great- est amount of specialization (Fig. 107). The sporogonium
Fig. 107. A common moss (Funaria): 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)x to the right a capsule with calyptra re- moved, showing the operculum (o); to the left a yonng 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 tlirougli the
Fig. 108. Lougitudiiial section of moss capsule {Funaria), showing its complex character: d, operculum; p, peristome: c, c', columel- la; s, sporogenous tissue; outside of s the complex wall consisting of layers of cells and large open spaces {h) traversed by strands of tissue,— After Goebei,.
Fig. 110. Sporogonia of Grimmia, from all of which the operculum has fallen, displaying the peristome teeth : A, position of the teeth when dry; J5, position when moist.— After Kerkeb.
Fig. lO'J. Partial longitudinal section through a moss cap- sule : .4, younger capsule, showing wall cells (a), cells of columella (i), and sporog- enous cells (sv) ; B, some- what older capsule, a and i same as before, and sm the spore mother cells. —After
GOEBEL.
THE GREAT GROUPS CF BRYOPIIYTES 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 beautiful teeth (Fig. 110), converging from the circumference toward the center, and called the peristome^ meaning " about the mouth." These teeth are hygroscopic, and by bending inward and outward help to discharge the spores.
CHAPTEE 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) Alternation of generations. — The great fact of alter- nating sexual (gametophyte) and sexless (sporophyte) gen- erations is 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 m!inufactured by the gameto- phyte, which is therefore the more conspicuous generation. When a moss or a liver Avort is spoken of, therefore, the gametophyte is usually referred to.
(3) Gametophyte and sporophyte not independent. — The sporo2:)hyte 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) Differentiatio7i of thallus into stem and leaves. — This appears incompletely in the leafy Liverworts {Junger- maniiia forms) and much more clearly in the erect and radial leafy branch (gametophore) of the Mosses.
128
I>TERII)0PI1YTES 129
(5) Many-celled sex organs—fX^^ antheridia and the flask-shaped archegonia are very characteristic of Bryo- phvtes as contrasted with Thallophytes.
76 General characters of Iteridophytes.-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 Antho- ceros forms, while some think that they may possibly have . been derived directly from the Green Alg« Whatever their origin, they are very distinct from Bryophytes.
•One of the very important facts is the appearance ot the vascular system, which means a "system of vessels, organized for conducting material through the plant body. Tlie appearance of this system marks some such epoch m 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. , . ,,
77 Alternation of generations.-This alternation con- tinue in the Pteridophytes, but is even more distinct than in the Bryophytes, the gametophyte and ^porophyte be- coming independent of one another. An outline of the life history of an ordinary fern will illustrate this f-t and wm serve also to point out the prominent structures. Upon the lower surface of the leaves of an ordinary fern dark spots clines 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, ereen heart-shaped thallus, resembling a delicate and sim- ple IWerwort (Fig. Ill, -I). I'pon 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 lyrotliallium or i^rothallusy so that when the term prothallium is used the gametophyte of Pteridoi^hytes is generally referred to ; just as when the term sporogonium is used the sporophyte of the Bryophytes is referred to. Within an archegonium borne upon this little prothallium an oospore is formed. When the oospore ger-
FiG. 111. Prothallium of a common fern (Aspidium): A, ventral surface, showing rhizoids (rh), antheridia icm), and archegonia (a?) ; B, ventral surface of an older gametophyte, showing rhizoids (r/i) and young sporophyte with root (w) and leaf (6).— After Sciienck.
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. Ill, 7i). 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
PTEKIDOPllYTES 131
completes the life cycle, as the asexual spores develop the prothallium again.
In contrasting this life history with that of Bryophytes several important dilferences are discovered. The most striking one is that the sporophyte has become a large, leafy, vascular, and independent structure, not at all re- sembling its representative (the sporogonium) among the Bryophytes.
Also the gametophyte is much less prominent than the gametophytes of the larger Liverworts and Mosses. If Ferns have been derived from the Liverworts, therefore, it is probable that they came from those with very simple bodies rather than from those in which the gametophyte had become large and complex. The conspicuous leafy branch of the Mosses, commonly called " the moss plant/^ corresponds to nothing in the Pteridophytes, the prothal- lium representing only the protonema part of the gameto- phyte of the true Mosses.
The small size of the gametophyte seems to be associ- ated with the fact that the chlorophyll work has been transferred 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 most of the Pteridophytes both generations are independent green plants, the- leafy sporophyte remaining attached to the small gametophyte only while beginning its growth (Fig. Ill, /y).
Among tlie 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 apfxjionn^ meaning " without the sexual act."
132
PLANT STKUCTUKES
Under certain other conditions prothallia are observed to sprout directly from the leafy sporophyte instead of from a spore. This is called apospoi'y, meaning " without a spore."
78. The gametophyte. — The prothallium, like a simple liverwort, is a dorsiventral body, and puts out numerous
Fig. 112. Stas-horn fern {Platycerium grande), an epiphytic tropical form, showing the two forms of leaves : a and b, 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. Arcliegonium of Pteris at the time of fertilization, showing tissue of gam- etophyte (A), the cells forming the neck {B), the passageway formed by the dis- organization of the canal cells (C), and the egg {D) lying exposed in the venter. —Caldwell.
Fig. 114. Anthcridiiim of Pteris {B), showing wall cells (a), opening for escape of sperm mother cells (e), escaped mother cells (c), sperms free from mother cells (b), showing spiral and multiciliate character. — Caldwell.
27
134
PLANT STEUCTUEES
being a conspicuous object in connection with the spore- phyte.
At the bottom of the conspicuous notch in the prothal- Hum 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. Ill, A), and differ from those of all Bryophytes, except the An- thoceros forms, in being sunk in the tissue of the prothal- _ lium and opening on the sur- ^
Fig. 115. Development of gametophyte of Pteris: the figure to the left shows the old spore {B), the rhizoid (C), and the thallns (.4); that to the right is older, showing the same parts, and also the apical cell (Z>).— Caldwell.
Fig. 116. Young gametophyte of Pteris, showing old spore wall {B), rhizoids (C), apical cell (/)), a young anther- idium {E), and an older one in which sperms have organized (i?').— Cald- well.
PTEKIDOPUYTES
135
face, more or less of the neck of the archegonium projecting (Fig. llo). 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
Pig. 117. Sections of portions of the gametophyte of PteHs, 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 (6^; 7?, an older stage, showing neck cells (a), neck canal cell ib), and cell from which is derived the egg cell, and the ventral canal cell (c); C, a still older stage, showing increased num- ber of neck cells {a), two neck canal cells {b),i\\c ventral canal cell (c), and the cell in which the egg is organized (rf).— Caldavell.
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 eitlier monoecious or dioecious (see § 69). When the prothallia are developing (Fig. 115) the anther-
Fig. 118. A fern (Aspidinm), showing three large branching leaves coming from a horizontal subterranean stem (rootstock); young leaves are also shown, which show circinate vernation. 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 .? represents the under sur- face of a portion of the leaf, showing seven sori with shield-like indusia; at 5 is represented a section through a sorus. showing the sporangia attached and pro- tected by the indusium; while at 6' is rei)resented a single sporangium opening and discharging its spores, the heavy annulus extending along the back and over the top.— After Wossidlo.
PTERIDOPHYTES
137
idia begin to appear very early (Fig. IIG), 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 witli the development of heterospory.
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) Develoimient 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 {Pltris): A, younj,' embryo, showina: direction of basal wall (7). and of second walls (7Z), which organize quadrants, each of which subsequently develops into foot (/), root (?r). 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 (a?/'), and some tissue of the prothallium (pr). —A after Kiknitz-CJekloff; B after Hof.meisteu.
lower surface of the prothallium (Fig. 119, B). It germi- nates at once, as in Bryophytes, not being a resting spore as in Green Algae. The resting stage, as in the Bryophytes,
;[38 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 hasal ivall (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, yl). 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 cotyledo7i (Fig. 119, Z*) ; 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, /). The foot remains in close contact with the prothallium 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.
PTERIDOPIIYTES I39
111, 119), as it is in an unfavorable position in reference to the dorsiventral stem, which puts out a seizes 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 not assumed by any special organ, unless rhizoids sometimes absorb ; but true roots are complex in structure and contain vessels.
(3) The 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, aerial shaft bearing a crown of leaves (Fig. 120). In the other groups of Pteri- dophytes there are also aerial 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 XV.
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 tlie 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 lett of the center i^^ a tree fern, with its slender colnmnar stem and crown of large leaves. The large-leaved plants to the right are bananas (Monocotyledons).— From "Plant Relations,"
PTEKIDOPUYTES
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. — Leaves are devices for spreading out green tissue to the light, and in the Ferns they are usually large, ^here is a stalk-like portion {petiole) which rises from the subterranean stem, and a broad expanded portion {Made) 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 {77ieso2)liyll)^ 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 that it is pierced by numer- ous peculiar pores, called stomata., meaning " mouths." The surface view of a stoma 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). Above and below is the col- orless e23idermis, pierced here and there by stomata ; between the epidermal lay- ers the cells of tlie mesopliyll are packed; and among the mesophyll cells there may be seen here and tliere tlie cut ends of the veins. The leaf is usually a dorsiventral
Fig. 121. Some epidermal cells from leaf of Pteris, sjhowiiifr the inter- locking walls and three stomata, the guard cells containing chloroplasts. —Land.
142
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-
FxG. 122. Cross-seclion through a portion of the leaf of Pteris, showing the heavy- walled epidermis above and below, two stoniata in the lower epidermis (one on each side of the center) opening into intercellular passages, the mesophyll cells containing chloroplasts, the upper row arranged in palisade fashion, the other cells loosely arranged (spongy mesophyll) and leaving large intercellular passages, and in the center a section of a veinlet (vascular bundle), the xylem being repre- sented by the central group of heavy-walled cells. — Land.
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
PTERIDOPIIYTES 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 (indnsiym) 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 {Adiantnm), and the common brake {Pteris), in which case they are protected by the inroUed
mw
Fig. 123. Fragrant i^hield ft>ru(Aspid- ium fragrans), showing general habit, and to the left ia) the under surface of a leaflet bearing sori covered by shield-like indusia.— After Makion Satterlee.
Fig. 184. The bladder fern ( Cystoptens bulb- ifera), showing general habit, and to the right (a) the under surface of a leaflet, showing the dichotomous venation, and five sori protected by j)ouch-like indusia. —After Marion Satterlee.
PTERIDOPHYTES
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. Those leaves which only do chlorophyll work are called /o- liage leaves ; and such branch- es are foliage branches. As sporophylls are not called upon 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 (Li/godinm), the royal fern {Osmimda), the moon wort {Botriicliium) (Fig. 129), and the adder's tongue {OpJiioghjssu^n) (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
Fig. 125. Leaflets of two common ferns : A, the common brake (Pteris) ; B, maidenhair (Adian- tum); both showing sori borne at the margin and protected by the infolded margin, which thus forms a false indiisium.— Cald-
AVELL.
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 anmilus. The annulus is like a bent spring, and when the delicate wall becomes yielding the spring straightens violently, the wall is torn, and in the recoil the spores are discharged with considerable force (Fig. 131). This dis-
FiG. 126.— The purple cliff brake (Pellcea atropurpurea), showing general habit, and at a a single leaflet showing the dichotomons 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 (see § 66) consists of a single cell, which by division finally produces
PTEEIDOPHYTES
147
numerous mother cells, in each of which a tetrad of spores is formed. The disorganization of the walls of the mother
Fig. 127. The ostrich iaxu^Onodea struthioptej'is), showing differentiation of foliage leaf (a) and sporophyll (6).— After Marion Satteulee.
cells sets the spores free in the cavity of the sporangium, and ready for discharge.
Fig. 128. The sensitive ievTx {Onodea smsibUis), showing dilTereTitiation of foh'age leaves and sporophylls.— From "Field, Forest, and Wayside Flowers."
PTERIDOPIIYTES
149
Among the Bryophytes the sporogenons tissue appears very early in the development of the sporogonium, the pro- duction of spores being its only function ; also there is a
Fig. 129. A niooiiwort {Botrychi- nm), showing the leaf differen- tiated into foliage and pporophyll branches.— After Stuasburgek.
28
Fig. 130. The adder's iongna {Ophioglossum tulgatum), showing two leaves, each with a foliage branch and a 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
^^-^^^i^^-^m
^ ,^^^\
■\\\^
.\
.\\0
Fig. 131. A series showing the dehiscence of a fern sporangium, the rapture 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 ^^-^
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 tliat 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 dioecious — 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 heterof<j.)ori/ (''spores different"), and such plants are called lieterospo- rous (Fig. 139). In contrast with heterosporous plants, those in which the asexual spores appear alike are called homos-
152 PLANT STRUCTURES
porouSy or sometimes isosporous, both terms meaning "spores similar." The corresponding noun form is liomos- 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 m^■cro- 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 megasporangia ; 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 functioning 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- phyllSy 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) :
Gizg> 0— S— 0— G=8> 0— S— o— Gz:8> 0— S, etc.
In the case of heterosporous plants (some Pteridophytes and all Spermatophytes) it would be modified as follows : g=8>o— Sizg=g=8>o— S=:g=g=8>o— S, etc.
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
shaded part of the parallelogram represents the gameto- phyte and the unshaded part the sporophyte. Among the
1^4: PLANT STKUCTURES
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 Pteridopliytes are recognized : (1) Filicales (Ferns), (2) Equisetales (Scouring rushes, Horsetails), and (3) Ly- copodiaUs (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.
Filicales {Ferns)
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. x\lthough 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
THE GREAT GROUPS OF PTERIDOPIIYTES 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 (Pteris) (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 is a differentiation of functions in foliage branches and sporo- phyll branches (Figs. 127-130), 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 eiisporangiate.
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 STKUCTUKES
trychium) (Fig. 129) and adder's tongue (OpJiioglosstim) (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 Marsilia 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 comj)aratively large leaf. This leaf has a long erect petiole and a blade of four spread-
■/V
\i
Fig. 133.— a water-fern (Marsilia), showing horizontal stem, with descending roots, and ascend- ing leaves ; a, a young leaf showing circinate vernation ; s, s, sporophy 11 branches ( ' ' spo- rocarps "). — After Bischofp.
Fig. 134. One of the floating water-ferns (Sal- Dinia), showing side view (A) and view from above (B). The dangling root-like processes are the modified submerged leaves. In A, near the top of the cluster of submerged leaves, some sporophyll branches ("sporo- carps ") may be seen. — After Bisciioff.
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 PTERIDOPIIYTES ^59
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.
Equisetales {Horsetails or Scouring rushes)
85. General characters. — The twenty-five forms now rep- resenting this great group belong to a single genus {Equise- tum^ meaning ^Hiorsetail "), 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
Fig. 135. Equisetum arvense, a common horsetail: ;, three fertile tfhoots rising from the dorsiventral etem, showing the cycles of coalesced scale-leaves at the joints and the terminal strobili with numerous sporoph}il8, that at a being mature; 2, a sterile shoot from the same stem, showing branching; 3, a single peltate sporo- phyll bearing sporangia; U, view of sporophyll from beneath, showing dehiscence of sporangia; 5, 6, 7, spores, showing the unwinding of the outer coat, which aids in dispersal.— After Wossidlo.
THE GREAT GROUPS OF PTERIDOPIIYTES
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).
80. The strobilus. — One of the distinguishing characters of the group is that chlorophyll-work and spore-formation are completely differentiated. Although the foliage leaves
;. 130. Dicecious gametophytes of Eqvisefum: A, the female gametophyte, show- ing branching, rhizolds, and an archegoninm {ar)\ B, the male gametophyte, showing several antheridia ( 6 ).— After Campbell.
are reduced to scales, and tlie 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 strohilus, 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 STKUCTUKES
sporangia, which produce spores of but one kind, hence these plants are homosporous ; and as the sporangia origi- nate in eusporangiate fashion, Eqvisetum 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 dioecious gametophytes (Fig. 136).
Lycopodiales {Cluh-mosses)
87. General characters. — This group is now represented by about five hundred species, most of which belong to the two genera Lycojjodmm and Selcfginella, 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, F). 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 I'TEKIDOPIIYTES 163
^
S^
Fig. 137. A common club-moss (Lycopodium davatum): 1, the whole plant, showing horizontal stem giving rise to roots and to erect branches bearing strobili; 2, a single sporopiiyll with its sporangium; 3, spores, much magnified.— After Wos-
SIDLO.
88. Lycopodium. — This genus contains fewer forms than the other, hut they are larger and coarser and more charac- teristic of the temperate regions, heing the ordinary Chib- 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 sporophylls 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, Lycopodhim 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-
mm:
FiQ. 138. Sdaginella, showing general spray-like habit, and dangling leafless stems which strike root (rA^^o/)^o^'es).— 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 GKOUPS OF PTEKIDOPIIYTES
165
The solitary sporangium appears in the axils (upper angles formed by the leaves with the stem) of the leaves and sporophylls, but arise from the stem instead of the
Fig. 139. Selaginella Martensii: A, branch bearing etrobili; B, a microsporophyll with a microsporancium, showing microspores through a rupture in the wall; C, a megasporophyll with a megasporangium ; D, megaspores : E, microspores.—
GOLUBEUGEU.
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 Selagiudl((, 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
tt'^
Fig. 140. Male gametophyte of Selaginella : in each case p is the prothallial cell, the wall cells of the antheridium, s the sperm tissue: F, the biciliate 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 PTERIDOPIIYTES
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- m n ^ ^^
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 them.
The embryo of Se- laginelJa is also impor- tant to consider. Be- ginning its development in the venter of the archegonium, it first lies upon the exposed margin of the prothallium, while the mass of nutritive cells lie deep within the mega- spore (Fig. 141, €7111)^, emh^). It first develops an elongated cell, or row of cells, which tlirusts 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-
spm
Fig. 141. YemuXe gsimeto^\iyte oi a Selaginella : spm, wall of megaspore ; pr, gametophyte ; ar, an archegonium ; e?nf)^ and e/nb^, em- bryo 8i)orophytes ; ef. suspensors : the gam- etoi)hyte has developed a few rhizoids. — After Pfepfer.
1(58 PLANT STRUCTUKES
tion to its food sujjply, is called the suspe?isor, 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 stretcliing out toward the substratum ; (3) a stem extend-
Fi6. 142. Embryo of Selaginella removed from the gametophyte, showing suspensor {et), root-tip {w), foot (/), cotyledons (jbl), stem-tip {st), and ligules (/i^).— After Pfefper.
ing in the other direction, and bearing just behind its tip (4) a pair of opposite leaves (cotyledons) (Fig. 142).
As the sporangia of Selaginella 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 Pteridoj)hyte forms which has attracted special attention, as possibly representing one of the an- cestral forms of the Seed-plants.
THE GREAT GROUPS OF PTERIDOPIIYTES
100
90. Isoetes. — This little group of aquatic plants, known as ^^ quill worts," 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
Fig. 143. A common quillwort {Isoetes lacus- tris), showing cluster of roots dichoto- mously branching, and cluster of leaves each enlarged at base and inclosing a sin- gle sporangium.— After ScnENCK.
Fig. 144. Sperm of Isoetes, show- ing spiral body and seven long cilia arising from the beak. — After Be LA JEFF.
with the Club-mosses, and is associated with Scli(fjineUa. It resembles a bunch of fine grass growing in shoal water or in mud, but the leaves enlarge at tlie 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 Isoetes shares with Selagiuella 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 biciliate 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 : GYIVENOSPERMS
91. Summary from Pteridophytes. — In considering tho important contributions of Pteridophytes to the evolution of the plant kingdom the following seem worthy of note :
(1) Prominence of sporopliyte and development of vascii- 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) Differe7itiation of sporojihylls. — 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 lieterospory and reduction of gameto- phytes. — lieterospory 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 Selaginella and Isoetes among Lycopodiales. All the other Pteridophytes, and therefore the great majority of them, are homosporous. The importance of the appear- ance of lieterospory 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
1^2 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) ThaUopJiytes. — Thallus body, but no archegonia.
(2) Bryojiliytes. — Archegonia, but no vascular system.
(3) Pteridophytes. — Vascular system, but no seeds.
(4) 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 Anthophytes, meaning "Flowering plants," with the idea that they are distin- guished by the production of "flowers." A flower is difii- 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 Phaenogams or even Phenogams), meaning "evident sexual reproduc- tion." At the time this name was proposed all the other groups were called Cryptogams, 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
SPERMATOrilYTES : GYMNOSPERMS 173
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 Spermatopliytes, 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 grouj) 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.
Gymnospekms
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, altliough the pines still form extensive forests. The group is so diversified in its structure tliat all forms can not be included in a single description. Tlie common pine {Pinus), therefore, will be taken as a type, to show the general Gymnosperm character.
]^Y4 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 jDine, 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, d, 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 Seed-plants all received names before they were identified with the corresponding struc- tures of the lower groups. The microsporophyll was called a stamen, the microsporangia j)ollen-sacs, and the microspores foUen 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.
Fig. 145. Pinv,s Laricio, showing tip of branch bearing needle-leaves, scale-leaves, and cones (strobili): rt, very yoinii,' carpellate cones, at time of pollination, borne at tip of the young shoot ui)on whicli new leaves are appearing; b, carpellate cones one year old; c, carpellate cones two years old, the scales spreading and shedding the seeds; d, young shoot bearing a cluster of staminate cones. — Caldwell.
176
PLANT STRUCTUKES
The strobilus composed of microsporophylls may be called the stamiuate strohlhis — 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
Fig. 146. Staminate cone (strobilus) of \)mG {Pimts): A, section of cone, showing microsporophylls (stamens) bearing microsporangia; ^, longitudinal section of a single stamen, showing the large sporangium beneath ; C, cross-section of a sta- men, showing the two sporangia; Z>, 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 Strasburgek.
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 sjiorophyte, which never pro- duces sex organs. It should be boriie 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
srEKMATOPIlVTE.S : (iYMNOSPElLMS
m
danger of becoming confused and of forgetting that pollen grains are asexual spores.
98. Megasporophylls.— The strobili composed of mcga- sporophylls become much larger than the others, forming
,j'
f\ J
Fig. 147. Pimis sylvestris, showing mature cone i)artly sectioned, and showing car- pels {sq, sq^, sq^) with seeds in their axils (g), in which the enibr3'os (e?n) may be distinguished; A, a young carpel with two megasjx.rangia; J], an old carpel with mature seeds (ch), the micropyle being below (3/").— After Bessey.
the well-known cones so characteristic of pines and their allies (Figs. 145, a, b, c, 1G3). 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, d), 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 emhryo- sac, because the young embryo was observed to develop within it (Fig. 147, em).
The strobilus of megasporophyll s, therefore, may be called the carpellate sfrohilus or carpellate 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 sex organ.
The structure of the megaspo- rangium, or ovule, must be known. The main body is the nucellus (Figs. 148, c, 149, nc) ; this sends out from near its base an outer membrane {ijitegument) which is distinct above (Figs. 148 J), 149 ^), covering the main part of the nucellus and projecting beyond its apex as a prominent neck, the passage through which to the apex of the nucellus is called the 7nicropyle (^^ little gate") (Fig. 148, a). Cen- trally placed within the body of the nucellus is the conspicuous cavity called the embryo-sac (Fig. 148, d), in reality the retained megaspore. The relations between integument, micropyle, nucellus, and embryo-sac should be kept clearly in mind. In the
Fig. 148. Diagram of the carpel structures of pine, showing the heavy scale {A) which bears the ovule {B), in which are seen the micropyle {a), integument (h), nucellus id), embryo sac or mega- spore (C?).— MoORE.
SPERMATOPIIYTES : GYMNOSPEKMS
179
ne L z_
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 so- called embryo-sac with- in the ovule. This im- bedded megaspore ger- minates, just as does the megaspore of 8e- 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, being 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- sues like an internal parasite. So conspicuous a tissue within the ovule, as well as in the seed into which the
Fig. 149. Diagrammatic section through ovule (megasporangiiim) of spruce {Picea), showing integument (i), nucelhis inc), endosperm or female gametophyte (e) which fills the large megaspore imbedded in the nucelhis, two archegonia (a) with short neck {c) and venter containing the egg io). and position of ger- minating pollen grains or microspores (p) whose tubes (0 penetrate the nucelhis tissue and reach the archegonia. — After Schimper.
180 PLANT STKUCTURES
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. 14:9, «), 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 hy 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 wall 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 male 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 pollen-tuhe. Into this tube the male cells enter, and as it penetrates among the cells
SPEEMATOPHYTES: GYMNOSPEEMS
181
which shut off the archegonia it carries the male cells along, and so they are brought to the archegonia (Fig. 150).
Fig. 150. Tip of pollen tube of pine, showing the two male cells {A, B), two nuclei (C) which accompany them, and the numerous food granules (Z>) : the tip of the tube is just about to enter the neck of the archegonium.— Caldavell.
Fig. 151. Pollen tube passing through the neck of an archegonium of spruce (Picea), and containing near its tip the two male nuclei, w^hich are to be discharged into the egg whose cytoplasm the tube is just en- tering.—After Strasburger.
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. 116, />>), 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
PLxVNT STKUCTURES
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, t) ; crowding into them (Fig. 151), the tip of the tube opens, the male cells are
Mi
-^^Siiif
Fig. 152. Fertilization in spruce {Piceaw B is an egg. in the tip of which a pollen tube {p) has entered and has discharged into the cytoplasm a male nucleus (sn). which is to unite with the egg (female) nucleus (o?i); C, a later stage in which the two nuclei are uniting.— After Schimper.
discharged, one male cell fuses with the ^gg (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
cdled siplionogamy, which means " sexual reproduction by means of a tube." So important is this character among Spermatophytes that some have proposed to call the group Siphonogmns.
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 8elaginella, the oospore first develops a suspensor, which in pine and its allies becomes very long and often tortuous (Fig. 153, A, s). At the tip of the suspensor the cell or cells (em- bryo cells) which are to develop the embryo are carried (Fig. 153, A^ lea), and thus become deeply buried, about centrally placed, in the endosperm.
Several suspensors may start from as many archegonia in the same ovule, and several embryos may begin to develop, but as a rule only one survives, and the solitary completed embryo (Fig. 153, B) lies centrally imbedded
in the endosperm (Fig. 153r/). The development of more than one embryo in a megasporangium (ovule) is called poliiembryony, 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 develop- ment of a special tissue that forms a hard bony coverino-,
.1.
Fig. 153. Embryos of pine very young embryos {ka) at the tips of long and contorted sus- pensors (5); B, older embryo, showing attachment to suspen- sor (s), the extensive root sheath {wh), root tip itcs), steiy tip {v\ and cotyledons (c).— After Strasburger.
184
PLANT STRUCTURES
known as the seed coat, or testa (Fig. 153a). The devel- opment of this testa hermetically seals the structures with- in, farther development and activity are checked, and the living cells pass into the resting condition. This pro- 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-
FiG. 153«. Pine seed.
Fig. 154. Pine seedlinp^s. ehowing 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
Fig. 155. A cycad, showing the palm-like h.ihu. wnn mma scaly 8tein.— From " Plant Relations."
)r:uHiu'n leaves and
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.
SPERMATOPHYTES : GYMNOSPERMS
18"
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
Fig. 157. Tip of pollen tube of Crjcas revoliita. containing the two spiral, multiciliate spt'ims. — After Ikeno.
happens when a seed is said to " germinate." This second period of development is known as the extra-seminal, for it is inaugurated by the escape of the sporophyte from the seed (Fig. 15-4).
104. The great groups of Gynmosperms. — There are at least four living groups of Gymnosperms, and two or three
Fig. 158. A pine (Pinus) ehowing 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 Avidely 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. Fig. 159. Tlic jxiant roilwood {Sv<ji/oia gi-
TheV are VerV fern- ^^''^'"^ ""^ California : the relative size
i.ll^\l tlie vei^ itJin is indicated by the figure of a man stand-
like in structure as well Ing at the right.— After Williamson.
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
recently that strikingly * ^^^^;^^^'-p^j^ 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 have been actually found in but few species of the Cycads, but there are reasons for suppos- ing that they may be found in all. Another one of the Gymnosperm
Fig. 160. An araucarian pine (Araucaria), ^ \^
showing the central shaft, and the regular groupS, represented to-
cycles of branches spreading in every direc- (Jg^y Q-^ly "^jy ^J^g com- tion and bearing numerous small leaves.— *^ i , • i -i • i
From " Plant Relations." mouly Cultivated maid-
SPERMATOPHYTES : GYMNOSPERMS
191
enhair tree {Gingho), with broad dichotomously veined leaves, also develops mnlticiliate 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 (Finus) (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., gives them an appearance very distinct from that of other trees.
Another peculiar fea- ture is furnished by the characteristic "needle- 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
Fig. 161.— Cross-section of a needle-leaf of pine, showing epidermis (e) in which there are sunken stomata (sp), heavy- walled hypodermal tissue (es) which gives rigidity, the mesophyll region (p) in which a few resin-ducts (h) are seen, and the central region {stele) in which two vascular bundles are developed.— After Sachs.
Fig. 162. A larch (Larix), showing the continuous central shaft and horizontal branches, the geneial outline being distinctly conical. The larch is peculiar among Conifers in periodically shedding its leaves.— From •' Plant Relations."
SPERMATOPHYTES: GYMNOSPEEMS I93
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-vitse, 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 ivliorled.
The character which gives name to the group is the " cone " — that is, the prominent carpellate cone which becomes so
conspicuous in COnneC- Fig. 103. Arbor-vitfe (T/wja), showing a
tion with the ripening of ^"-^V''^ ''""' scaly overlapping loaves,
^ *^ and some carpellate cones (strobili).—
the seeds. These cones After eichlbr.
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
19i
PLANT STRUCTUEES
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.
Pig. 164. The common juniper (Jvniperus commtmix); the branch to the left bearing staminate strobili; tliat to the right bearing staminate strobili above and carpel- late strobili below, which latter have matured into the fleshy, berry-like fruit. —After Berg and Schmidt.
CHAPTEE XII
SPERMATOPHYTES : ANGIOSPERMS
107. Summary of Gjrmnosperms. — 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 (ovule), 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 e?ito?nophilo2cs ('^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. The sporophyte 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.
Eoots, 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 leaf 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- Fio. 165. stamens of hen- i^r to Augiosperms, making the true bane {Hyoscyamus) : A, flowcr, and being associatcd with en- front view, showing fila- ^Q^lOnhilv ment(/) and anther (;»; LUmupiUl^/. , „ ^^
B, back view, showing 110. Microspoiophylls. — The micro-
the connective (c) be- gporophvll of Au^iospcrms is more
tween the pollen-sacs. ,^ „ .^ / , 5 i » i i
-After scHiMPBR. definitely kuowu as a " stamcu 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, 167rt).
Fig. 1G6. Cross-section of anther of thorn apple (Datura), showing the four imbedded sporangia (a, 2^ 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
Fig. 167. Diagrammatic cross-sections of anthers: .1, younger stage, showing the four imbedded sporangia, the contents of two removed, but the other two^con- taining pollen mother cells (;)m) surrounded by the tapetum (0; 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. 31
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. 1G6, 167). When they reach maturity, the paired sporangia on each side usually merge to- gether, forming two spore-containing cavities (Fig. 167, ^). 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. 167a. Various forms of stamens : A, from Solanmn, showing dehiscence by terminal pores; B, from Arbutus, showing anthers witii terminal pores and "horns"; C, from Berberis; Z>, from Atherosperjna, showing dehiscence by uplifted valves; E, from Aqinlegia, showing longitudinal dehiscence; F, from Popoivin. showing pollen-sacs near the middle of the stamen.— After Engleb and I'ltANTL.
SPKKMAT0P11YTP:S : ANGIOSPEKMS
199
The opening of the pollen-sac to grains (microspores) is called splitting open," and the methods of dehiscence are various (Fig. lG7ft). By far the most common method is for the wall of each sac to split lengthwise (Fig. 1G8), which is called longitudinal dehiscoice ; an- other is for each sac to open by a terminal pore (Fig. 167«), in which case it may be prolonged above into a tube.
111. Megasporophylls. — These are the so-called " carpels " of Seed- plr.:^.ts, 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 ferentiated into three regions : (1) a
discharge its pollen- which means ^^a
Fuj. 1G9. Types of pistils : .1, tliree simple pistils (apocarpous), each sliowing ovary and style tijjped with stigma ; B, a comi)oiiii(l pistil (syncarpoiis\ showing ovary (/), separate styles (g), and stigmas (») ; O, a compound pistil (syncarpous), showing ovary (/). single style (gr), and stigma (yt)— After Berg and Schmidt.
Fig. 168. Cross - section of anther of a lily {Butoimts), 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 Sachs.
. 169, A), and is dif- hollow bulbous base, ^ which contains the ovules and is the real seed case, known as the 0 V a r y ; (3) sur- mounting this is a slender more or less elongated process, the style; and (3) usually at or near the apex of the style a special receptive surface for the pol- len, the stigma.
In other cases several carpels to-
200
PLANT STRUCTUKES
gether form a common ovary, while the styles may also combine to form one style (Fig. 1G9, C), or they may remain more or less distinct (Fig. 1G9, B). Such an ovary may contain a single chamber, as if the carpels had united edge to edge (Fig. 170, A) ; or it may contain as many chambers as there are constituent carpels (Fig. 170, B), 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,
Fig. 170. Diagrammatic sections of ovaries: A, cross-section of an ovary with one loculus and three carpels, the three sets 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, showing ovules attached to free axis (" free central ").— After Schimper.
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, A)^ or several (Fig. 170, A).
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 pistil. A pistil may be one carpel (Fig. 169, A), or it may be several carpels or- ganized together (Fig. 169, B, O), the former case being a simple pistil, the latter a compound pistil. In other words.
SPERMATOniYTES : ANGIOSPERMS
201
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).
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
Fig. 171. A diagrammatic section of an ovule of Angiosperms, showing outer integument (ai), inner integument (ii), micropyle (m), nucelius {k), and embryo sac or megaspore (em) . —After Sacks.
Fig. 172. Germination of microspore (pollen grain) in duckweed {Leinna): A, mature spore with its nucleus; B, nucleus of spore dividing; C, two nuclei resulting from the division; D, a large and small cell following the nuclear division, forming the two-celled male gametophyte; E, 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.
202
PLANT STRUCTURES
ordinary nutritive cells of the gametopliyte remains, and the whole structure seems to represent a single antherid- At first it consists of two cells, the large wall cell
lum.
and the small free generative cell (Fig. 172, D). Later
the generative cell di- vides (Fig. m, E), 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, F), 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 papillae 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 t h r 0 u g h th e 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
Fig. 173. Diagram of a longitudinal section through a carpel, to illustrate fertilization with all parts in place : s, stigma ; g, style ; o, ovary ; ai, U, outer and inner integuments; n, base of nucel- lus ; /, funiculus ; b, antipodal cells ; c, endo- sperm nucleus; k, egg and one synergid; p, pol- len-tube, having grown from stigma and passed through the micropyle (m) to the egg.— After
LUEUSSKN.
SPERMATOPIIYTES : ANGIOSPEKMS
203
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, the act of fertil-
FiG. 174. Lilium PliiladelpMcxmi : to the left a young megasporangium (ovule), showing integuments (C), nucellus (.1), 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.
ization, and the development of endosperm are the three subjects to be considered. If fertilization is not accom- plished the endosperm is usually not developed.
Development. — The megaspore nucleus divides (Fig. 174), and one nucleus passes to each end of the embryo-
204
PLANT STKUCTURES
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
Fig. 175. Liliiim 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 8ho^vn 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 tvro 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-cfp^aratus (Fig. 176, at right. A), the central one, which usually hangs lower in the sac than the others, being the Qgg, the two others being the synergids, or "helpers." Here, therefore, is an egg without an archegonium, a dis- tinguishing feature of Angiosperms.
SPERMATOPIIYTES : ANGIOSPERMS
205
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 anti2)odal cells (Fig. 176, at right, B). The large nucleus near the center of A
the sac, formed by the fusion of the two / \
Fig. 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 gametopliyte about ready for fertilization, with the egg apparatus {A) composed of the two syncrgids and egg (central and lower), the three antipo- dal cells {B), and the two polar nuclei fusing (C) to form the primary endosperm nucleus.— Caldwkli..
polar nuclei, is known as the primari/ endosperm nucleus or the definitive nucleus.
206
PLANT STRUCTURES
Fig. 177. Fertilization in the cotton plant, a Dicotyledon, showing the pollen tube (P) passing through the micropyle and con- taining a single sperm (male cell), and hav- ing entered the embryo-sac is in contact with one of the synergids (S) on its way to the egg (£').— After Duggar.
Fertilization. — The pollen-tube, carndng 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 (Fig. 177). When it comes near the egg., the tip of the tube breaks and the two male cells are dis- charged into the embryo- sac. One male cell passes to the Qgg and the two nuclei fuse, the resulting cell being the oospore, which develops the em- bryo. The other male cell passes to the endo- sperm nucleus and fuses with it, the cell resulting from this triple fusion of a male cell and two polar nuclei developing the endosperm (Fig. 178). These two simultaneous acts of fertilization are spoken of as " double fer- tilization."
Endosperm. — After fertilization, the primary endosperm nucleus begins a series of divisions, and as a result the sac becomes
SPEEMATOPIIYTES : ANG108 PERMS
207
more or less filled with nutritive cells, which are often organized into a compact tissue (Fig. 179). These nutritive cells do not cor- respond to the endosperm of Gymnosperms, although they re- receive the same name. In Gym- nosperms the endosperm is main- ly formed before fertilization and is the nutritive body of the female gametophyte; while in Angio- sperms it is formed after fertiliza- tion and is probably not a part of the gametophyte. As the endosperm of Angiosperms is a product of what appears to be fertilization, it would seem proper to regard it as sporo-
phyte tissue, but its real character is still under discussion.
The antipodal cells probably represent nutritive cells
of the gametophyte. Sometimes they disappear very soon
after they are formed; but sometimes they become very
Fig. 178. End of embryo-sac of Silphhmi, showing double fer- tilization : sy, synergid, the other having been destroyed by the pollen-tube ; o, egg with coiled male cell (spi) lying against its nucleus ; e, endo- sperm cell, with large coiled male cell {sp^) iying against it. —After Land.
Fig. 179. One end of the embryo-sac in wake-robin {THllvim). showing endosperm (shaded cells) in which a young embryo is imbedded.-Aftcr Atkinson.
208
TLANT STRUCTURES
active and even divide and form a considerable amount of tissue, which usually nourishes the embryo until endosperm tissue is developed, and then becomes disorganized; or even invades the tissue of the nucellus.
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). This relation of organs recalls the embryo of Isoetes (see § 90). Naturally there can be but one cotyledon under such circum- stances, and the group has been named 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 Sehfginella (see § 89). As the cotyledons are lateral members their number may vary. In Gymno- sperms, whose embryos are of this type, there are often
Fig. 180. Curved embryo-sac of arrowhead (Sagittaria), show- ing in the upper right end a young embryo, in the other end the antipodal cells cut oflf by a partition, and scattered through the sac a few free en- dosperm cells.— After Schaff-
NER.
SPERMATOPIIYTES . ANGIOSPERMS
209
several cotyledons in a cycle (Fig. 154) ; and in Dicotyle- dons there may be one or several 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 liypocotyl (Figs. 154, 193, 194), which
Fig. 181. Development of embryo of shepherd's purse {Caj)sella), a Dicotyledon: beginning with /, the youngest stage, and following the sequence to VI, the old- est stage, V represents the suspensor, c 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 caulicle or radicle. In Dicotyledons the stem-tip between the coty-
210
PLANT STRUCTURES
ledons often organizes the rudiments of subsequent leaves, forming a little bud which is called ih.Q 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 the integument consisting of the two large, fleshy cotyledons, between which lie the hypocotyl and a plu- mule of several leaves.
115. The seed. — As in Gymno- sperms, while the processes above described are taking place within the ovule, the tissue is developing that forms the hard seed-coat or testa (Fig. 183). When this hard coat is fully developed, the activities within cease, and the whole structure passes into that condition of suspended animation which is so little understood, and wliich 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 tlie wings in CataJpa or Bif/nonia (Fig. 184), or the tufts of
Fig. 182. YoHng embryo of water plantain {Alisma), a Monocotyledon, the root being organized at one end (next the suspensor), the single cotyledon (C) at the other, and the stem- tip arising from a lateral notch {V). — After Han- stein.
SPERM ATOPII YTES : ANGIOSPERMS
211
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 (;;c), seed testa {sc), nucellus tissue (7;), endosperm {en), and embryo (e?«).— After Baillon.
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.
Fig. 184. A winged seed of Bignnma.—.\.^icv 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
PLANT STKUCTUKES
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 ake7ie, and is very characteristic of the greatest Angiosperm family, the Composita?, to which sunflowers, asters, golden- rods, daisies, thistles, dandelions, etc., 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. In the ap- ple, pear, quince, and such fruits, the pulpy part is the modified
calyx (one of the Fio. ise. winged fruit of maple.— After Kerner.
Fig. 183. A pod of fireweed {Epilobium) opening and exposing its plumed seeds which are transported by the wind.— After Beal.
SPERMATOPHYTES: ANGIOSPERMS
213
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 fl 0 w e r - c 1 u s t e r, : with its axis and bracts, becoming , an enlarged pulpy mass, as in the pineapple (Fig. 192).
The term "fruit," therefore.
Fig. 18r. A ripe dandelion head, showing the mass of plumes, a few seed-like fruits (akenes) witli their plumes still attached to the receptacle, and two fallen off.— After Kerner.
Fig. 188. An akene of beg- gar ticks, showing the two barbed appendages which lay hold of animals.— Af- ter Beal.
32
Fig. 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
TLANT 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-
Fio. 190. Fruit of nutmeg (Myristica) : A, section of fruit, showing sc-t-d within the heavy wall ; B, section of seed, showing peculiar convoluted and hard endosperm (m) in which an embryo (n) is imbedded.— After Berg and Schmidt.
cent to the seeds. As has heen seen, this effect may extend only to the ovary wall, or it may include the calyx, or it 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. — It 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. If the lodgment is suitable, there are many devices for burial, such as twisting stalks and awns, bur-
FiG. 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.
SPEKMATOPIIYTES : ANGIOSPEKMS
215
rowing animals, etc. The period of rest may be long or short, but sooner or later, under the influence of moisture, suitable tem2)erature, 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
\ \
:sMls
Fk;. 102. Pineapple: A. the cluster of friiits forming,' the .so-called "fruit"; 7?. 8ini,He flower, showing small calyx and more prominent corolla; (\ section of flower, showing the floral organs arising above the ovary (epigynous).— .1, B after Kocu; 6' after I.e Maout and Decaisne.
opening. The root-tip emerges first, is protruded still farther by the rapid elongation of tlie hypocotyl, soon curves toward the earth, penetrates the soil, and sending out rootlets, becomes anchored. After anchorage in the
216
PLANT STRUCTUEES
soil, the hypocotyl again rapidly elongates and develops a strong arch, one of whose limbs is anchored, and the other is pulling npon the cotyledons (Fig. 193). This pull finally frees the cotyledons, the hypocotyl straightens, the cotyle-
FiG. 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
SPERMATOPIIYTES : ANGIOSPERMS
217
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 Anglo- 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- tnbe, which penetrates the style tu reach the ovary cavity w^hich 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 at the time of fertilization con- sists of only a few free nuclei and cells, usually seven in number.
(3) The female gametophyte produces no arcliegonia, but a single naked egg.
Fig. 194. Seedling of hornbeam (C'arpinus), showing primary root {hiv} bearing rootlets {^w) upon wliich are numerous root hairs (/•), hypocotyl (A), cotyledons (c), young stem ((?), and first (l) and second (/') true leaves.— After Scuim-
PEU.
")
CHAPTEE XIII
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 efiicient 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 sta7ninate and the other pistillate (Fig. 196).
218
THE FLOWER
219
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
Fig. 196. Naked flowers of dif- ferent willows (Salix), each from the axil of a bract : a, b, c, staminate flowers ; d, e, /, pistillate flowers, the pistil composed of two car- pels (syncarpous). — After Warming,
Fig. 195. Lizard's tail (SawrMn/s): ^, tip of branch yiq. 197. Flower of calamus
bearing leaves and elongated cluster of flowers;
B, a single naked flower from A, showing sta- mens and four spreading and stigmatic styles;
C, flower from another species, showing sub- tending bract, absence of floral leaves, seven stamens, and a syncarpous pistil ; the flowers naked and perfect.— After Englbr.
(Acori/s), showing simple perianth, stamens, and syn- carpous pistil: a hypogynous flower without differentiation of calyx and corolla.— After Engler.
Fig. 199. Common flax (Limtm) : A, entire flower, showing calyx and corolla ; B, floral leaves re- moved, showing stamens and syi.carpous pistil ; C, a mature Fig. 198. Flowers of elm (77/m?/.9) : ^1, branch capsule splitting open. —After
bearing clusters of flowers and scaly buds ; Schimper.
J3, single flower, showing simple jjcrianth and stamens, being a stamii ate flower ; C,
flower showing perianth, stamens, and the two divergent styles stigmatic on inner surface, being a jjcrfcct flower; I), section through perfect flower, showing peri- anth, stamens, and pistil with two loculi each with a single ovule — After Engler.
Fig. 200. A flower of peony, showing the four sets of floral organs: k, the sepals, to- gether called the calyx; c, the petals, together called the corolla; a, the numerous stamens; g, the two carpels, which contain the ovules.— After Strasburger.
THE FLOWER
221
the young sporophylls, but they are all alike, forming what is called the j^erianth (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, A). 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 calyx^ each leaf being a sepal ; 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 resrard the missin"^ set t^, „ai * . ,-, ,r . , ^
& o Fig. 201.— An easter-hly. a Monocotyledon,
as the corolla, such showing perianth (a), stamens (6), etigma(c),
flowers being called flower bud (cZ) and a carpel after the peri-
° . anth has fallen (/), with its knob-like stigma,
apetaloUS^ meaning long style, and slender ovary.— Caldwell.
222
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-
FiG. 202. A buttercup {Ranunculus): a, complete flower, showing sepals, petals, sta- mens, and head of numerous carpels on a large receptacle; b, section showing relation of parts; a hypogynous, polypetalous, apocarpous, actinomorphic flower. —After Baillon.
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 223
appear to be throAvn 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-like recej)- tacle (Fig. 202). On the other hand, in the common water-
FiG. 203. Flower of water-lil^ {NymjJhiBa), showing numerons 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 tlie 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 establislied, 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 trimerous flower (Fig. 204) ; while in cyclic Dicot-
224
PLANT STRUCT URES
yledons the number five prevails, but often four appears, forming jjentamerous or tetramcrous 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 tw^o 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- pals, but are opposite the spaces between sepals ; the stamens in ^^sXP">^ \ turn alternate with the petals ; if
T c9/^'u) 1 V there is a second set of stamens,
\ ^ ^& ^ ' I 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 a set betAveen has disappeared. For example, if a set of stamens is opposite the set of petals, either an outer stamen set or an inner petal set has disappeared.
This line of evolution, there- fore, extends from flowers whose 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 epigjmous flowers. — In the simpler flowers the sepals, j^etals, and stamens arise from beneath the ovary (Figs. 197, 202, 205, 1). As in such cases the ovary or ovaries may be seen distinctly above the origin {insertio7i) 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-
FiG. 204. Diagram of such a flower as the lily, showing re- lation of parts : uppermost organ is the bract in the axil of which the flower occurs ; black dot below indicates po- sition of stem ; floral parts in threes and in five alternating cycles (two stamen sets), being a trimerous, pentacyclic flow- er.—After SCHIMPBR.
THE FLOWER
225
ring to the fact that the insertion of the other parts is under the ovary.
Hypogyny is very largely displayed 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
Fig. 205. Flowers of Rose family: 1, ahypogynons flower of Potentilla, sepals, petals, and stamens arisino; from beneath the head of carpels; 2, a perigynous flower of Alchemilla, sepals, petals, and stamens arising from rim of urn-like pro- longation of the receptacle, which surrounds the car.pel ; 3, an epigynous 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 he perigi/)ious (Figs. 205, ^,206), meaning "around the pistil." Finally, the insertion is carried above the ovary, and sepals, petals, and stamens seem to arise from the top of the ovary (Fig. 205, S), such a flower being epignnous, 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, 2G1), 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-
226
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 apoca?'pous,mesimng "carpels separate." There is a very strong tendency.
Fig. 206. Sweet-scented shrub {Cahjcanthus): 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 perigjmous 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, A 199, B), and the flower is said to be syncarpon^, 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. 199, A, 202, 203, 207), a condition which
THE FLOWER
227
has received a variety of names, but probably the most common is 2)oly- petalous, meaning "petals many,*' although eleutheropetalous, 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 symjjetalous, meaning "petals united." The words gamo- petalous and moiwpetalous are also
much used, but all three words refer to the same condition of the flower. Often the sympetalous corolla is differenti-
FiG. 207. Flower of Ptraw- berry, showing sepals, pet- als, numerous stamens, and head of carpels ; the flower is actinomorphic, hypogj'nous, and with no coalescence of parts. — Af- ter Bailey.
Fig. 208. A flower of the tobacco plant: «, 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, 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 Sthasbuuger.
228
PLANT STRUCTUEES
ated into two regions (Fig. 210, h), a more or less tubular portion, the tube, and a more or less flaring portion, the Jimh.
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- pMc, meaning ^^ ra- diate," and is often called a " regular 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.
Fig. 209. FJower of morning-glory {Ipomcea), with sympetalous corolla split open, showing the five attached stamens, and the superior ovary with prominent style and stigma ; the flower is hy- pogynous, sympetalous, and actinomorphic. — After Meissner.
c d e
Fig. 210. A group of sympetalous flower forms: a, a flower of harebell, showing a bell-shaped corolla; b. a flower of phlox, showing a tube and spreading limb; c. a flower of dead-nettle, showing a zygomorphic two-lipped corolla; 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.
THE FLOWER
229
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 sporophylls. Such flowers are called zygomorpMc, 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. 211. The pansy {Viola tricoloi^) : A, section showing sepals {I, I'), petals (c) one of which produces a spur (cs), the flower being zygomor- phic; B, mature fruit (a capsule) and persistent calyx (k); C, the three boat-shaped valves of the fruit open, most of the seeds (s) having been discharged.— After Sachs.
Fig. 212. Flower of a mint {Mentha aquatica): 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 Werminq, 33
230 PLANT STRDCTUKES
is the hilabiate, or " two-lipped," in which two of the petals usually organize to form one lip, and the other three form
the other lip (Figs. 210, c, d, e, 212,213). The two lips may be nearly equal, the upper may stand high or overarch the lower, the lower may project more or '-'^^ZlZ'f^^ZZ^. le«^ conspiouondy, etc.
the sympetalous and two-lipped (bilabi- 126. Infl 01 CSC eilCe. — «
ate) corolla with three petals (middle one y^ ^f ^^^^ jSoWCrS are Soli- largest) m the lower lip and two small "^
ones in the upper, and the stamens and tary, either On the end of
style emerging through a slit on the up- ^ ^^^^ ^^ branch (Fis^S. per side of the tube; a sympetalous and . i •!
zygomorphic flower.— After Bkiquet. /Col, /Cobj, Or in tlie axii
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 inflo- 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 Compositae, 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 ; (4) 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 Compositae 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.
CHx\PTER 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 (Fig. 214). This means that there is no annual increase in the diameter of the woody stems, and no extensive branching, but to this there are some exceptions.
(3) Leaf veins forming a closed 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 232
Fig. 214. Section of stem of corn, showing the scattered bundles, indicated by black dots in cross-section, and by lines in longitudinal section. —From "Plant Relations.''
MONOCOTYLEDONS AND DICOTYLEDONS
nuirgiii of the leaf, but forms a ^^ closed venation/*' so that the leaves usually have an even {entire) margin. There
are some notable exceptions to this character.
(4) Cyclic flowers trim- crous. The *^ three-parted"
Fig. 215. Two tj'pes 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, wliich are connected with one auothcr by a network of veinlets. — After Ettingshausen.
flowers of cyclic Monocotyledons are quite characteristic, but there are some trimerous Dicotyledons.
Dicotijledojis. — (1) Embryo with lateral cotyledons and terminal stem-tip.
(2) Vascular bundles of stem forming a hollow cylinder (Fig. 216, w). This means an annual increase iii tlie (liiim-
234
PLANT STRUCTUKES
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, esj)ecially on the under surface of the leaf, suggesting the name "^^ net- veined" leaves, in contrast to the " parallel-veined" leaves of Mono- cotyledons. The vein system ends freely in the margin of the leaf, forming an *^open venation." In consequence of this, although the leaf
^:^,^.^\ \ iT:i :/y:T?^:-cK ^W
Fig. 216. Section across a young twig of box elder, showing the four stem regions: e, epidermis, represented by the heavy bounding line; c, cortex; w, vascular cyl- inder; ;?, pith.— From "Plant Relations."
w
may remain entire, it very commonly be- comes toothed, lobed, and divided in various ways. Two main types of venation may be noted, Avhich influence the form of leaves. In one case a single very prominent vein {ril)) runs through the mid- dle of the blade, and
is called the midrih. Fig. 217. section across a twig of box elder
From this all the mi- nor veins arise as branches (Figs. 218, 219), and such a leaf
three years old, showing three annual rings, or growth rings, in the vascular cylinder; the radiating lines (771) 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
235
is said to be pi7i7iate 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 yalmate or palmately veined^ and in- clines to broad forms.
(4) Cyclic flowers pentamerous or tetramerous. The flowers '' in flves " are greatly in the majority, but some
Fig. 218. Leaves showing pinnate and palmate branching; the one to the left is from sumach, that to the right from buckeye.— Caldwell.
veryi prominent families have flowers ^'in fours." Tliere 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.
236
PLANT STKUCTUKES
It is the combination of characters which determines a group.
MON'OCOTYLEDONS
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
Fig. 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 form? 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 Aster. In the same family the golden- rods are grouped together in the genus Solidago. The different kinds of Aster or of Solidago are called sjyecies (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 alia is the name of the com- mon white oak, Quercus being the name of the genus to which all oaks belong, and aWa 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 Kaias (naiads) and Zannichellia (horned pondweed) are common genera in ponds and slow waters.
238
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-
FiG. 220. Pondwecd (Potamogeton): A, branch with chistor (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 (c), the short stamens (a), and the two short styles with conspicuons stigmatic surfaces.— J. after Rbiohen- bach; B after Le Maout and Decaisne.
Fig. 221. Cat-tails (Typha), showing the dense epikes of very simple flowers, each showing two regions, the lower the pistillate flowers, the upper the staminate.— From " Field, Forest, aud Wayside Flowers."
240
PLANT STKUCTUKES
leaf) and Alisma (water-plantain), in which there is a dis- tinct calyx and corolla. The genus Tiji)lia (cat-tail) is also an aquatic or marsh form of very simple type, the flow- ers being in dense cylindrical clusters {sjnhes), 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.
Fig. 222. A common meadow grass (Festuca) : 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 Strasburger.
MONOCOTYLEDONS AND DICOTYLEDONS 241
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 notcAVorthy features of the group is the prominent development of peculiar leaves {bracts) 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 (rij^ened ovary). As they are usually called glumes, the grasses and sedges are said to be ghimaceous 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 GraminecB^ and the sedges the family Cyperacem.
133. Palms. — More than one thousand species of palms are grouped in the family Palmacece. 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 Cycads, a tall unbranched columnar trunk bearing at its summit a croAvn 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 unbranchcd columnar trunk (jo\Lrijd .\.,L -lu .Lai 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
243
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.
Fig. 224. A fan palm, with low stem and crown of large palmate leaves, w hich hav( split 80 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 AracecB. In our flora the Indian turnip or Jack-in-the-pulpit {Aris(P7na) (Fig. 225), sweetflag (Acorus), and skunk-cabbage (Sijinplo- 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.
24:4:
PLANT STRUCTURES
The flowers are usually very simple, often being naked, with two to nine stamens, and one to four carpels (Fig.
Fio. 2'2b. Jack-iu-the-piilpit (Arisivma). showiiij,' tlie overarcliing spathos; in one 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
245
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 spadix. The flowers often include but one sort of sporophyll, and staminate and pistillate flowers hold different positions upon the spadix (Fig. 22G).
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- anth of other entomophilous groups.
"Aroids are further peculiar in hav- ing broad net-veined leaves of the Di- cotyledon type. Altogether they form a remarkably distinct group of Mon- 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.
34
Fig. 226. Spadix of an Arum, with spathe re- moved, showing cluster of naked pistillate flow- ers at base, just above a cluster of staminate flowers, and the chib- shaped tip of the spa- dix.—After WOSSIDLO.
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 geopMlotis (''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, 228, 231, 233).
In the regular lily family [Liliacew) 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-
FiG. 227. W^ake-robin {Trillhmi), showing root- ley, hyacinth, Castcr stock, from which two branches arise, each bear- ]n^\ /'TTio-q 201 229^ ing a cycle (whorl) of three leaves and a single J ) \ o ' ' /'
trimerous flower.— After Atkinson. the stamCUS 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
247
In the amaryllis family (AmaryUidacecB), a higher fam- ily of the same general line, represented by species of iVY/r- cissus (jonquils, daffodils, etc.), Agave, etc., the flowers are distinctly epigynous.
Fig. 228. Star-of-Bothlehcm iOrnithogalum) : a, entire plant with tuberous base and trimerons flowers; b, a single flower; c, portion of flower !?howiiig relation of parts, perianth lobes and stamens arising from beneath the i)rominent ovary (hy- (l^ mature fruit; e, section of the syncarpous ovary, showing the three
pogynous); carpels and loculi.
-After ScHiMPEH.
In the iris family (Iridacece), the most highly specialized family of the lily line, and represented by the various spe-
Fig. 25iy. The Japan lily, showing a tabular perianth, the parts of the perianth distinct above. — From "Field, Forest, and Wayside FlowerSc"
MONOCOTYLEDONS AND DICOTYLEDONS
240
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. Orchids.— In number of species this {Orcliidacece) is the greatest family among the Monocotyledons, the species being vari- ously estimated from six thousand to ten thousand, representing between one third and 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 Habenaria (rein- orchis) (Fig. 235), Pogonia^ Calopogon^ Calypso, Cijpripe- diiim (lady-slipper, or moccasin flower) (Fig. 236), etc., by far the greatest display and diversity are 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 lip> which is
Fig. 230. Diagrammatic cross-section of ovary of Lilium Philadelphician, showing the three loculi, in each of which are two ovules (mega- sporangia); A, ovule; B, integuments; C, nu- cellus ; D, embryo-sac (megaspore).— Cald- well.
Fig. 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 Relations.'"
MONOCOTYLEDONS AND DICOTYLEDONS
251
modified in a great variety of ways, and a prominent, often very long, S2mr, 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
Fig. 232. Flower of flag {Iris), 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.
Fig. 234. Flower cluster of Gla- diolvs, showing somewhat zygo- morphic flowers.— Caldwell.
Fi(i. 2SS. Gladiolus, showing tuberous subter- ranean stem from which roots descend, grass- like leaves, and somewhat zygomorphic flow- ers.—After Keicuenbacu.
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 (polUmtcm), which is pulled out and carried to another flower by the
Fig. 235. A flower of an orchid {Habena- ria): at 1 the comi)lete 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 (lip) 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 seen 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 below in a sticky disk,
which may be seen in 1 and 3 ; in 3 a pollen mass («) is shown sticking to each
eye of a moth.— After Grat.
visiting insect. The whole structure indicates a very highly specialized type, elaborately organized for insect pollination.
Another interesting epigynous and zygomorphic tro})- 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.
Fig. 236. A clump of lady-slippers {Ci/pripedium), hhowiu- the habil of the plant and the general structure of the zygomorphic flower.— After Gibson.
254
PLANT STRUCTURES
FlG. 237. A group of orchids (Cattlerja), showing the very zygomorphic flowers, the lip being well shown in the flower to the left (lowest petal).— Caldwell.
DICOTYLEDON'S
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 Arcliiclilamydem and the Sympetalce. In the former there is either no perianth or its parts are separate (polypeta- lous) ; in the latter the corolla is sympetalous. The Archi- chlamydeae are the simpler forms, beginning in as simple a fashion as do the Monocotyledons ; while the SympetalaB
MONOCOTYLEDONS AND DICOTYLEDONS
255
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 Archichlamydea3 contain about one hundred and sixty families, and the Sympetalae 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.
ArcMchlamydecB
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
Fio. 238. An oak in winter condition.— From " Plant Relations."
256
TLANT STRUCTURES
temperate regions, for here belong the oak (Fig. 238), hick- ory, wahmt, cliestnut, beech, poplar, birch, elm (Figs. 198, 239), willow (Fig. 2-1:0), etc. The primitive character is in- dicated not merely by the floral structures, but also by the general anemophilous liabit.
In the poj^lar (Populus) and its allied form, the willow (Salix), the flowers are naked and liy})ogynous (Fig. lOG),
Fig. 239. An elm in foliage. — From "Plant KeluLionB.
MONOCOTYLEDONS AND DICOTYLEDONS
257
the stamens are indefinite in number (two to thirty), and the pistil is syncarpous (two carpels). The stamens and
Ficj. 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 catkin, and the plants which produce such clusters are said to be amenta- ceous. These aments of poplars, "pussy willows," and the alders and birches are very familiar objects (Figs. 240, 241).
Fio. 241. Aments of alder (Almis) : a, branch with staminate aments (/i), pistillate aments (m), and ayoiinp; bud {k)\ b. pistillate ament at time of discharging seeds, showing the promi- nent bracts.— After Warming.
258
PLANT STRUCTURES
The only advanced character in the flowers as described above is tlie syncarpous pistil, but in the great allied pepper family {Pipcrarece) of the tropics, with its one thousand species, and most nearly represented in our flora by the
Fig. 242. Ovule of hornbeam (Carpimis), 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.— After Mart
EWART.
lizard-tail {Saururus) 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 259
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 {8arrace7iia, Drosera, Dioncea, etc.). 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 adajited for insect-pollination, but the spiral arrangement and indefinite numbers are very apparent, notably in connection with tlie 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, 244). 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
Fig. 243. Marsh marigold ( Caltha), 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. Zygomorphic flower of larkspur (Delphinium), with sepals removed, show- ing two petals with prominent spurs, and numerous stamens. — After Baillon.
Fig. 24.5. Diagram of the zygomorphic flower of larkspur {DelphiniU7n), show- ing the spur developed by a sepal and inclosing the two petal spurs.— After Baillon.
MONOCOTYLEDONS AND DICOTYLEDONS
261
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 {Ranuncula- cew) zygomorphy appears, as in the larkspur {Del^^ihininm) with its spurred petals and sepals (Figs. 244, 245), and the monkshood {Aconitum) with its hooded sepal ; and in the
Fig. 240. The common cabbage (Brassica), a member of the mustard family: A, flower chister, showing buds at tip, open flowers below with four spreading petals, and forming pods below; B, mature pod, with the persistent style; C. pod opening by two valves, and showing seeds attached to the false partition.— After Warming.
water-lily family (Nymphceacea') and poppy family {Papa- veracece) syncarpy appears. In this alliance, also, belong the sweet-scented shrubs {Calycanthus)^ with their perigy- nous flowers containing numerous parts (Fig. 206). 35
262
PLANT STRUCTURES
Fig. 247. Diagram ot crucifer flower, showing the relations of parts ; four sepals, four petals, six stamens, and one carpel with a false partition. —After Warming.
The most specialized large group in this alliance is the mustard family (CVucifercB), 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 six with two short ones in an outer set and four long ones in an inner set, and one pistil whose ovary be- comes divided into two loculi by what is called a '^ false partition" (Figs. 246, C, 247), and usually be- comes an elongated pod (Fig. 246, A, B). This specialized structure of the flower distinctly marks the family, whose name is suggested by the fact that the four spreading petals often form a Maltese cross (Fig. 246, A). The pecul- iar stamen character, four long and two short stamens, is called tetradynamous (^^four strong").
140. Roses. — This family {Bosacece) of one thousand species is one of the best known and most useful groups of the temperate regions. In it are such forms as Spircsa, five-finger {Poten- tilla), strawberry {Fragaria) (Figs. 191, 207), raspberry (Fig. 248), and blackberry (Ru- bus), rose (Rosa), hawthorn ( Cratce- gus), apple, and pear (Pirus) (Fig. 249), plum, cherry, almond, and peach {Primus).
Fig. 248. The common raspberry: the figure to the left showing flower-stalk, calyx, old stamens {s), and prominent receptacle, from which the "fruit" (a cluster of small stone fruits, each representing a carpel) has been removed.— After Bailey.
MONOCOTYLEDONS AND DICOTYLEDONS
2G3
Many of the true roses have a strong resemblance (Fig. 207) to the buttercups {Ranunculus), with their hypogy- nous reguhir 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
Fig. 249. The common pear (Pirns commvnis), ehowing branch with flowers (1), sec- tion of a flower {2) showing its epigynoiis character, section of fruit (,J) showing tlie 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 (jiomp) 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
TLANT STRUCTURES
fruits {drupes), as apricots, peaches (Fig. 189), plums, cherries.
141. Legumes. — This is far the greatest family {Legumi- noHce) of the Arcliichlamydeae, containing about seven thou- sand species, distributed everywhere and of every habit. It is the great zygomorphic group of the Archichlamydeae, being elaborately adapted to insect pollination. The more
Fig. 250. A legume plant {Lodn^), showing flowering ))ranch (1), a single flower {S) showing zygomorphic corolla, the cluster of ten stamens (S) which with the carpel is included in the keel, the solitary carpel ik) which develops into the pod or le- gume {[)), the petals {(]) dissected apart and showing standard (a), wings {b), and the two lower petals (c) which fold together to form the keel, and the floral dia- gram (7).— After Wossidlo.
primitive forms of the Leguminosse, 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 Pajnlio forms with very irregular (zygomorphic) flowers and few stamens
MONOCOTYLEDONS AND DICOTYLEDONS
265
(Fig. 250). The petals are very dissimilar, the upper one {stiindard) being the largest, and erect or spreading, the two lateral ones {icings) oblique and descending, the two lower ones coherent by their edges to form a projecting boat-shaped body {heel), which incloses the sta- mens and pistil. From a fancied re- semblance to a but- terfly such flowers are said to be papil- ionaceous.
The whole fam- ily is further char- acterized by the sin- gle carpel, which after fertilization develops a pod (Fig. 250, 5), which often becomes re- markably large as compared with the carpel. It is this peculiar pod {le- gume) which has given to the family its technical name LeguminoscB and the common name ^' Legumes."
Well-known members of the family are lupine {Lupi- mt. 9), clover {TrifoUum), locust {J?ol)inia), Wislan'((, pea {Pisum), bean {Phascolus), tragacanth {Astragalus), vetch {Vicia), redbud {Cercis), senna {Cassia), honey-locust (GMitscJiia), indigo {Indigofera), sensitive-plants {Acacia, Mimosa, etc.) (Fig. 251), etc.
Fig. 251. A sensitive-plant {Acacia), showing the flowers with inconsi)iciious petals and very nu- merous stamens, and the i)innately hranched sen- sitive leaves.— After ]Metp:u and Schumann.
266
PLANT STRUCTURES
142. TJmbellifers. — This is the most highly organized family ( Umhelliferm) of the Archichlamydeae, 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 (Daueiis Carota): A, branch bearing the compound umbels; B, a single epigynous flower, showing inferior ovary, five spreading petals, five stamens alternating with the petals, and the two styles of the bicarpel- lary pistil; C, 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, C), being one of the very few epigy- nous families among the Archichlamydeae. 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- petalae.
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- j^^%f^- topped clusters called U7nbels (Figs. 252, A, 263), 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 (Dmtcus) (Fig. 252), parsnip (Pasti- naca), hemlock {Conium) (Fig. 253), pepper-and- salt {Erigenia), caraway (Carum), fennel {Fcenic- uhim), coriander {Cori- andriim), celery {Api- lun), parsley (Petroseli- 7mm), etc. Allied to the Umbellifers are the Ara- lias {Araliacem), and the Dogwoods (ConiacecB).
Fio. 253. ITomlock {Con'umi), an Umbollifer, show-ills tlic umbels, with the principal rays rising from a cycle of bracts {invo- luc7'e), and each bearing at its summit a secondary umbel with its cycle of second- ary bracts {involucel).— After Scuimper.
268 PLANT STRUCTURES
SympetalcB
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- chlamydeae. 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 Sympetal^, 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 pe^itacycUc, meaning " cycles flve," 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 isocarpic, meaning " car- pels same." The group is named either Fentacydce or Iso- carpce, and contains about ten families and 4,000 species.
The higher groups, containing about forty families and 36,000 species, is tetracyclic, 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 Tetracyclce or Anisocarpce.
144. Heaths.— The Heath family {Ericaceoi) 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 Archichlamydeae. One of the marked characteristics of the group is the dehiscence
MONOCOTYLEDONS AND DICOTYLEDONS 269
of the pollen-sacs by terminal pores, which are often pro- longed into tubes (Fig. 255).
Fig. 254. Characteristic heath plants: A, B. C\ Lyonia, showing sympetalous flowers and single style from the lobed syncarpous ovary; Z>, two forms of Cas/^iope, showing trailing habit, small ovorlapi)ing leaves, and symi)etal()ns flowers, but in the smaller form the petals are almost distinct.— After Duude.
Common representatives of the family are as follows : huckleberry (Gaylussacia), cranberry and blueberry ( Vac- cinium), bearberry (Arctostaphylos), trailing arbutus {Epi-
270
PLANT STKUCTUKES
gcEo), wintergreen (GmiWieria), heather (Calluna), moun- tain laurel (Kahnia), Azalea, Rhododendron (Fig. 25G), Indian pipe (Monotrojya), etc.
Pig. 255. Flowers of hcatli plants {Erica), phowinj^ coinijlotc flowers (.4), the sta- mens wiih "two-horned" anthers which dischar<^e pollen through terminal pores, and the lobed syncarpous ovary with single style and prominent terminal stigma {li, C, />).— After Dkude.
145. Convolvulus forms. — The well-known morning-glory {Ipomma) (Fig. 209) may be taken as a type of the Convol-
MONOCOTYLEDONS AND DICOT i'LEDONS
271
villus family {ConvolviilacecB). Allied with it are Polemo- 7iiu7n and Phlox (Fig. 210, h) {Polemoniacem), the gentians (Gentianacece), and the dog-banes {Apocpiacew) (Fig. 257). It is here that the regular sympetalous flower reaches its highest expression in the form of conspicuous tubes, fun-
Fig. 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.
272
TLANT STKUCTUKES
146. Labiates. — This great family {LaUatce) 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 labiate or
Fig. 257. A commuu dogbane {Apoajnum).—Fx:om "Field, Forest, and Wayside
Flowers."
Fig. 258. Thr Iml-r huulw. ( d . ( n,iru/ruliiti). showing tho twiniiiL: hahit and the con- spicuous funnt'lform corollas.— From " Field, Forest, and Wayside Flowers.""
274
PLANT STKUCTURES
hilaliate structure (Fig. 210, f, d, e), 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 diifer 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 Sympetalae as is held by the zygomorphic Le- gumes among Archi- chlamydeae.
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), mar- joram {Origanum),
Fig. 259. Flowers of dead nettle {La- miutn) : A, entire bilabiate flower ; B, section of flower, showing rela- tion of parts.— After Wakming.
Fig. 260. A labiate plant ( Tencrium), show- ing branch with flower clusters {A), and side view of a few flowers (/?), showing their bilabiate character.— After Bkiquet.
MONOCOTYLEDONS AND DICOTYLEDONS 275
thyme {Thymus), balm {Melissa), sage {Salvia), catnip {Xepeta), skullcap {Scutellaria), horehound {Marruhium), lavender {Lavandula), rosemary {Rosmarinus), dead nettle {Lamium) (Fig. 259), Teucrium (Figs. 213, 260), etc., a remarkable series of aromatic forms.
Allied is the Nightshade family {Solanacece), with fif- teen hundred species, containing such common forms as the nightshades and potato {Solanum), tomato {Lycojjer- sicum), tobacco {Mcotiana) (Fig. 208), etc., in which the corolla is actinomorphic or nearly so ; also the great Fig- wort family {Scrophulariacem) , with two thousand species, represented by mullein ( Verbascum), snapdragon {Antir- rhinum) (Fig. 210, e), toad-flax {Linaria) (Fig. 210, d), Pentstemon^ speedwell ( Veronica), Gerardia, painted cup {Castilleia), etc.; also the Verbena family {Verbenacew), with over seven hundred species ; and the two hundred plantains {Plant aginacew), etc.
147. Composites. — This greatest and ranking family {Comjjositw) 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. Xot 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 {involucre), 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, A, D) ; within the
Fig. 261. Flowers of Arnica: A, 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 stra})-shapcd above, the two parted style. the tuft of pappus hairs, and the inferior ovary which develops into a seed-like fruit (akene); E. single disk flower, showing tubular corolla with spreading limb, the two-parted style emerging from the toj) of the stamen tube, the prominent pappus, and the inferior ovary or akene; C, a single stamen.— After Hoffman. 276
MONOCOTYLEDONS AND DICOTYLEDONS 277
ray-flowers is the broad expanse supplied by a very much broadened axis, and known as the dish (Fig. 261, ^), which is closely packed with very numerous small and regular tubular flowers, known as disk-flowers (Fig. 261, e).
Fig. 262. The common dandelion (yaraxac?///*): 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; /,, a head from which all but oue of the akenes have been re- moved, showing the pitted receptacle and the promiueut i)appus beak.— After Strasburgkr.
The division of labor among the flowers of a single head is plainly marked, and sometimes it becomes quite com- plex. The closely packed flowers have resulted in modify- ing the sepals extremely. Sometimes they disappear en- 3G
278
PLANT STRUCTURES
tirely ; sometimes iliej become a tuft of delicate hairs, as in Arnica (Fig. 261, D, E), thistle (Cnicus), and dandelion (Taraxacum) (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
Fig. 263. Flowers of dandelion, showing action of style in removing pollen from the stamen tube: 1, style having elongated throngh 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 279
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 Compositae 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 ( Verno7iia), Asfer, daisy (Bellis), goldenrod {SoUdago), rosin-weed and compass-plant {Silph- ium), sunflower (HeUantJms), Chrijsanthemwn, ragweed (Ambrosia), cocklebur (XcmfJmwi), ox-eye daisy (Leuccm- thenium), tansy {Tanaceliim)^ wormwood and sage-brush [Artemisia), lettuce (Lactuca), etc.
CHAPTER XV
DIFFERENTIATION OF TISSUES
148. Introductory.— Among the simplest Tliallopbytes 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 Tliallophytes 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, composed of cells with thin walls and ac- tive protoplasm, and to be regarded as the parent tissue, is called parenchyma.
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
281
is in the sporopliyte of the Pteridophytes and Spcrmato- 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 t: 3ues, some of them being of general occurrence, and others more restricted, and there is every gradation between them and the parenchyma from which 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 ineristein, or it is said to be mcristematic, 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 mcristematic (Figs. 266, 274), and meristem occurs wherever growth is going on.
Fig. 264. Parenchyma and sclerenchyma from the stem of Pteris, in cross-section.— Cham- berlain.
282
PLANT STRUCTURES
150. Mestome and stereome. — When the plant body be- comes complex a conductive system is necessary, so that tlie 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 growtli 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 stereome.
The sporophyte body of
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.
Fig. 265. Same tissues as in pre- ceding figure, in longitudinal sec- tion, the parenchyma showing nuclei.— Chamberlain.
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
Fig. 266. Section through growing point of stem of Hipjniris : below the growing point, composed of a uniform meristem tissue, the three embry- onic regions are outlined, showing the dermato- gen id, d), the central plerome ip,p), and be- tween them the periblem.— After De Bary.
atic (Fig. 266). At the surface there is a single layer of cells distinct from those
within, known as the dermatogen, 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 jo^r/Z'/e;/?, meaning "that which is put around," and when it becomes permanent tissue it is called the cortex, 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. Ejndermis. — 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 corJc, a prominent part of the structure known as harh.
Cortex. — 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 liyjjoderrnis ; 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- sues are collenchyma and scleren- cliyma, meaning '^ sheath-tissue " and " hard-tissue " respectively. In collenchyma the cells are thick- ened at the angles and have very elastic walls (Fig. 267), making the tissue well adapted for parts which are growing
i^'iG. 267. Some collenchyma cells from the stem of a com- mon dock (liumex), showing the cells thickened at the anfrles.— Chamberlain.
DIFFERENTIATION OF TISSUES
285
in length. The chief mechanical tissue for parts which have stopped growing in length is sclerenchyma (Figs. 264, 2G5). The cells are thick-walled, and usually elongated and with tapering ends, including the so-called "fibers."
Fig. 2G8. Sections through an open collateral vascular bundle from a sunflower stem; A, cross-section; B. longitudinal section; the letters in both referring to the same structures; M, pith; A', xylem, containing spiral (.«, s') aiid pitted (t, V) vessels; C, cambium; P, phloem, containing sieve vessels {sb)\ b, a mass of ])ast fibers or sclerenchyma; ic, pith rays between the bundles; e, tlie bundle sheath; ^, 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. 2G8, 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 tracheids^ mean- ing "trachea -like," differing from trachecB or true tracheary vessels in having tapering ends and in not forming a continu- ous series (Fig. 260). The walls of these tracheids are ^ Spitted" in a way which is characteristic of Gymnosperms, the '^pits" appearing as two concentric rings, called "bordered pits."
The other prominent mes- tome tissue developed in the stele is the sieve vessels, for the conduction of organized food, chiefly proteids (Fig. 268). Sieve cells are so named because in their walls special areas are organized which are perforated like the lid of a pepper-
FiG. 269. Tracheids from wood of pine, showing tapcrins; ends and bordered pits.— Chambeklain.
DIFFERENTIATION OF TISSUES , 287
sieve-
box or a "sieve." These perforated areas are the jjlates, 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 xijlem (" 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 jjitJi. 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 vascular ci/Under 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 collateral (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 go on. Such bundles are said to be opoi : and the open 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 a camhium cylinder^ which separates the xylem and phloem of the vascular cylinder. This cambium continues the for-
FiG. 270. Cross-section of open collateral vascular bundle from stem of castor-oil plant {Ricinus), showing pith cells (m), xylem containing spiral {t) and pitted ig] vessels, cambium of bundle (c) and of pith rays icb), phloem containing sieve ves- sels (y), three bundles of bast fibers or sclerenchyma (b), the bundle sheath con' taining starch grains, and outside of it parenchyma of the cortex (r).— After Sachs
mation of xylem tissue on the one side and phloem tissue on the other in the bundles, and 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 groivth rings or annual rings, so conspicuous a feature of the cross-section of tree
DIFFERENTIATION OF TISSUES
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 lieart-ivood. The younger portion, through which the sap is moving, is the sap-iuood. 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 Cortex), the old phloem, and the new phloem down to the cambium, which constitute the so-called barh 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 Jibro- vascular hundles.
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. 200). 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).
Fig. 271. Cross-section of a closed collateral biindle from the stem of corn, showinp: the xylem with annular (r), spiral (s), and pitted {(i) vessels; the jihloem contain- ing sieve vessels {v), and separated from the xylem by no interveninn cambium; both xylem and phloem surrounded by a mnss of sclerenchyma (fibers); and in- vesting vessels and fibers the parenchyma (p) of the pith-like tissue through which the bundles are distributed.— After Sachs.
This lack of cambium means that stems living for sev- 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- 2)0clium) and Isoetes usually have an apical group of meris- tem cells, as among the Seed- plants, the smaller Club-mosses (Selac/ineUa), Ferns, and Horse- tails usually have a single api- cal cell, whose divisions give rise to all the cells of the stem. Generally also a dermatogen is not organized, and in such cases there is no true epidermis, 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.
Fig. 272. Diagram of tissues in cross- section of stem of a fern ( Pfe7is), showing two masses of scleren- chyma {st), between and about which are vascular bundles. — Chamberlain.
292 PLANT STRUCTURES
In EquiseUim and Isoetes 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
Fig. 273. Cross-section of concentric vascular bundle of a fern {Pteris): the single row of shaded cells investin-,' the others is the bundle sheath; the large and heavy- walled cells within constitute the xylein; and between the xylem and the bundle shealh 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
293
154. Roots.— True roots appear only in connection with the vascLihir phmts (Pteridophytes and Spermatophytes) ;
P P^ P ?
Fig. 274. Section through root- tip oiPteris: the cell with a nucleus is the single apical cell, which in front has cut off cells which organize the root-cap. — Chamberlain.
and in all of them the structure is essential- ly the same, and quite different from stem structure. A single apical cell (in most Pteridophytes) (Fig. 274) or an apical group (in Spermato- phytes) usually gives rise to the three em- bryonic regions — der- matogen, periblem, and plerome (Fig. 275). A fourth re- gion, however, pecul- iar to root, is usually added. The apical cell or group cuts off a tissue in front of itself (Fig. 274), known as the cahjp- trogen^ or " cap producer," for it organizes the root-cap^ which protects the delicate mcristem of the growing point. 87
Fig. 275. A longitudinal section throngh the root- tip of spiderwort, showing the plerome {j)l\ surrounded by the periblem { p), outside of periblem the epidermis (f) which disappears in the older parts of the root, and the prominent root-cap (c).— Land.
294
PLANT STKUCTUKES
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
Fig. 276. Cross-section of the vascular axis of a root, showing radiate type of bundle, the xylem {2d and phloem (pA) alternating. —After Sachs.
found in stems. The xylem is in the center and sends out a few radiating arms, between which are strands of phloem, 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 asso- ciated with the fact that the xylem is to receive and conduct the water absorbed from the soil. It should be said that this characteristic bundle structure of the root appears only
DIFFEKENTIATION OF TISSUES
295
in young and active roots. In older ones certain secondary changes take place whicli obscure the structure and result in a resemblance to the stem.
The origin of branches in roots is also peculiar. In stems branches originate at the sur- face, involving epidermis, cor- tex, and vase alar 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 cor!:ex be stripped off from a root with branches, the branches are left attached to the woody axis, and the cortex is found pierced with holes made by the burrowing branches. Such an origin is called endogenous^ meaning " produced within."
Fig. 277. Endogenous origin of root branches, showing them {n) arising from tlie central axis (/) and breaking through the cortex (r).— After Vines.
Fig. 278. A section through tlie liaf of lily, showing upper epidormis (ut), lower epi- dermis (/< ) with its stomata (,»0. mesopliyll (dotted cells) composed of the palisade region (;>) and the spongy region (.s/;) with air spaces among the ceils, and two veins (,v) cut across. — From "Plant Relations."
29G PLANT STRUCTURES
To sum up the peculiarities of the root, it may be said to develop a root-cap, to have a solid vascular cylinder in wliich the xylem and pliloem are arranged to form a bundle of the radiate type, and to branch endogenously.
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 w^ork is Physiology, 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 Ecology, 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 Pathology ; their study in relation to the interests of man is Economic Botany.
Besides these general subjects, which apply to all plants, the ditt'erent groups form the subjects of special study. The study of the Morphology, Physiology, or Taxonomy of the Bacteria is Bacteriology ; of the Alg^e, Algology ; of the Fungi, Mycology ; of the Bryophytes, Bryology ; of the fossil plants, Palwohotany or Palwophytology, etc.
In the present chapter it is the purpose to give a very brief outline of tlie 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) Eespira- tion, (4) Movement, (5) Keproduction.
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 even 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.
TLANT 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.
NUTEITION-
160. Food. — Plant food must contain carbon (C), hydro- gen (H), oxygen (0), and nitrogen (N), 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 (COo) of the air ; hydrogen and oxygen from water (HgO) ; 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 STKUCTURES
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 7'oot 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 in a 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 trnuspiration 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 Rela- 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 Pla7it Belatio7is, § 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 Avarm 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.
302 PLANT STRUCTDRES
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 ojff 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. Out 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.
EESPIRATIOK
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 3Q3
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.
MOVEMEll^T
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), Oscil- laria (§ 20), etc., have the power of locomotion.
When the protoplasm is confined within rigid walls and tissues, as in most plants, tlie power of locomotion usually disappears, and the plants are fixed ; but within active cells the protoplasm continues to move, streaming back and fortli 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
30-1. 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 C 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 responses to stimuli concerned in direct- ing organs are as follows :
Heliotropism. — In this case the stimulus is light, and under its influence aerial 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 ijositive lieliotrojnsm (Fig. 279) ; the leaf blades are directed at right angles to the rays of light, showing transverse lieliotrojnsm ; while if there are hold- fasts or aerial roots they are directed away from the light, showing negative lieliotrojnsm. 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 negativeUi 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 hcliotropism.—
After SCHAFFNER.
TLANT PHYSIOLOGY 397
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 Relations, p. 91.
Other stimuli. — Other outside stimuli which have a directive influence upon organs are chemical substances {chemotro2nsm) , such as direct sperms to the proper female organ ; heat (thermotropism) ; water currents (rheotropisiu) ; mechanical contact, etc. The most noteworthy illus- trations of the effect of contact are furnished by tendril- climbers. AVhen 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 collajise 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 DesynocUum gijrans (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 numerous 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
DUCUARTRK.
PLANT rilYSlOLOGY 309
while at night they droop and usually fold together (see Plant Relations, pp. 9, 10). These are the so-called nycti- tropic movements or " night movements/' which maybe 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 Dioncea, which snaps its leaves shut like a steel trap when touched (see Phmt lielations, 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 Mimosa jnidica (Fig. 280), whose sensitiveness to contact and rapidity of resj)onse are remarkable (see Plant Rela- tions, p. 48).
KEPEODUCTIOK
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 gemmae, 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 lij spiores 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 asexual spores. These spores are scattered in various ways — by swimming (zoospores), by floating, by the wind, by insects.
Another type of spore is the sexual spore, formed by the union of two sexual cells called gaf/wtes. 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 sperms or male cells, and eggs or female cells.
With the establishment of alternation of generations, the asexual spores are restricted to the spoivjjhyte, and the gametes to the gametophyte. With the further introduction of lieterospory, the male and the female gametes are sepa- rated upon different gametophytes, which become much reduced.
With the reduction of the functioning megaspores to one in a sporangium (ovule), and its retention, the seed is organized, and the elaborate scheme of insect-pollination is developed.
CHAPTEE 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 EELATIOlSrS
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 tlie 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 woiiM be waste of space.
It is very common to observe the lower leaves of a sliin 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. In ivies, 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 slioot. 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, etc. In the scaly buds the structure is used for protection rather than storage.
The stem bearing floral leaves is the shoot ordinarily called ^Hhe 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 appro})riate 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 Algae than among Spermatophytes, as well as relations to air, soil, water, mechanical support, etc.
PLAINT ASSOC lATIOI^S
177. Introductory.— Plants are not scattered at hap- hazard over tlie surface of the earth, but are organized into definite communities. These communities are determined by the conditions of living — conditions wliieli admit some plants and forbid others. Such an assemblage of plants living together in similar conditions is a plant assoru(tfO)f. Closely related plants are the most intense rivals, as they
314 PLANT STKUCTUEES
make almost identical demands upon their surroundings. Hence it is usual for a plant association to be made up of a large number of unrelated plants.
There are numerous factors which combine to deter- mine associations, 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 associations, which are usually local assemblages. 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 temj^erature ; but in the organization of local associations 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 associations, however, the effect of the heat factor may be noted in the succession of i^lants 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 com]30sition 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 associa- tion, therefore, plants are arranged in strata, dependent upon the amount of light they receive, and the number of these strata and the phmts characterizing each stratum are important 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 trans- piration.
The above five factors are among the most important, but no single factor determines an association. As each factor has a large possible range, the combinations of fac- tors may be very numerous, and it is these combinations which determine associations. For convenience, however, associations are usually grouped on the basis of the water factor, at least three great groups being recognized.
179. Hydrophyte associations. — These are associations 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 adajDtations. 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 aera- tion and for increasing buoyancy ; and sometimes a special
316 PLANT STKUCTUKES
buoyancy is provided for by the development of bladder- like floats.
Conspicuous among hydrophyte associations may be mentioned the following : (1) Free-swimming associations, 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 associations," consisting of minute plants and animals invisible to the naked eye, conspicuous among the plants being the diatoms ; also the " pond associations," composed of algas, duckweeds, etc., which float in stagnant or slow-moving waters.
(2) Pondweed associations, in Avhich the plants are an- chored, but their bodies are submerged or floating. Here belong the "rock associations," consisting of plants an= chored to some firm support under water, the most conspic- uous forms being the numerous fresh-water and marine algae, among which there are often elaborate systems of holdfasts and floats. The "loose-soil associations" are dis- tinguished by imbedding their roots or root-like |)i'ocesses in the mucky soil of the bottom (Figs. 281, 282). The wa- ter lilies with their broad floating leaves, the pondweeds or pickerel weeds with their narrow submerged leaves, are conspicuous illustrations, associated with which are algae, mosses, water ferns, etc.
(3) Swamp associations, in which the plants are rootea in water, or in soil rich in water, but the leaf-bearing stems rise above the surface. The conspicuous swamp associations are "reed swamps," characterized by bulrushes, cat-tails and reed-grasses (Figs. 283, 284), 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. ;
PLANT ECOLOGY
319
'' swamp-forests," which are largely coniferous, tamarack (larch), pine, hemlock, etc., prevailing.
180. Xerophyte associations.— These associations are ex- posed to the other extreme of the water factor, and are com- posed 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-
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
324 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 associations are as follows : "rock-associations," composed of plants living upon exposed rock surfaces, walls, fences, etc., notably lichens and mosses ; " sand associations," 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 associations. — Mesophytes make up the common vegetation, the conditions of moisture being me- dium, and the soil fertile. This is the normal plant condi- tion, 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
PLANT ECOLOGY 39^
made mesophytic. As contrasted with hydrophyte and xerophyte associations, the mesophyte associations are far richer in leaf forms and in general luxuriance. The arti- ficial associations which have been formed under the influ- ence of man, through the introduction of weeds and culture plants, are all mesophytic.
Among the mesophyte grass and herb associations are the " arctic and alpine carpets," so characteristic of high latitudes and altitudes where the conditions forbid trees shrubs, or even tall herbs ; " meadows," areas dominated by grasses, the prairies being the greatest meadows, where grasses and flowering herbs are richly displayed; "pas- tures," drier and more open than meadows.
Among the woody mesophyte associations 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 Fiffs. 2SS 289.)
0. n. nILL LIBRARY North Carolina <;fat^ r^iU^.
GLOS S AR Y
[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 a stamen. 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 Bryophytes, Pteri-
dophytes, and Gymnosperms. 100. Archesporium : the first cell or group of cells in the spore-producing
series. 102. Ascocakp: a special case containing asci. 58. AscospoRE : 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.
329
330 GLOSSARY
Calyx : the outer set of floral leaves. 221.
Capsule : in Bry()[)hytes the spore-vessel ; in Angiosperms a dry fruit
which opens to discharge its seeds. 98, 211. Carpel : the raegasporophyll of Spermatophytes. 178. Chlorophyll : the green coloring matter of plants. 5. Chloroplast : 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. CoNiDiUM : an asexual spore formed by cutting off the tip of the sporo-
phore, or by the division of hypha3. 58. Conjugation : the union of similar gametes. 15. Corolla : the inner set of floral leaves. 221.
CoTtLEDON : the flrst leaf developed by an embryo sporophyte. 138. Cyclic : 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.
Dehiscence : the opening of an organ to discharge its contents, as in sporangia, pollen-sacs, capsules, etc. 199.
DiCHOTOMOus : applied to a style of branching in which the tip of the axis forks. 35.
DicECious : 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.
Egg : the female gamete. 16.
Egg-apparatus : in Angiosperms the group of three cells in the embryo- sac composed of the egg and the two synergids. 204. Elater : in Liverworts a spore-mother-cell peculiarly modified to aid
in scattering the spores. 103. Embryo : a plant in the earliest stages of its development from the
spore. 137. Embryo-sac : the megaspore of Spermatophytes, which later contains
the embryo. 178. Endosperm : the nourishing tissue developed within the embryo-sac, and
thought to represent the female gametophyte. 180. Endosperm nucleus : the nucleus of the embryo-sac which gives rise to
the endosperm. 205. Entomopuilous : applied to flowers or plants which use insects as agents
of pollination. 196.
GLOSSARY 331
Epigynous : applied to a flower wliose outer parts appear to arise from
the top of the ovary. 225. EuspoRANGiATE : applied to those Pteridophytes and Spermatophytes
whose sporangia develop from a group of epidermal and deeper
cells. 157.
Family : a group of related plants, usually comprising several genera.
236. Fertilization : 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, 138.
Gametangium : the organ within which gametes are produced. 11.
Gamete : 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 cdls. 180.
Genus : a group of very closely related plants, usually comprising sev- eral species. 237.
Haustorium : a special organ of a parasite (usually a fungus) for ab- sorption. 50.
IIeterogamous : applied to plants whose pairing gametes are un- like. 15.
Heterosporous : applied to those higher plants whose sporophyte pro- duces two forms of asexual spores. 151.
HoMOSPORous : applied to those plants whose sporophyte produces simi- lar asexual spores. 151.
Host : a plant or animal attacked by a parasite. 48.
Hypha : an individual filament of a mycelium. 49.
Hypocotyl : the axis of the embryo sporophyte between the root-tip and the cotyledons. 209.
Hypogynous : applied to a flower whose outer parts arise from beneath the ovary. 224.
332 GLOSSARY
Indusium : in Ferns a flap-like membrane protecting a sorus. 143.
Inflorescence : a flower-cluster. 230.
Insertion : the point of origin of an organ. 224.
Integument : in S[)ermatophytes 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. IsoGAMOUS : 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 Sperm atophytes the fertilizing cell conducted by the
pollen-tube to the egg. 180. Megasporangium : a sporangium which produces only megaspores. 152. Megaspore : in heterosporous plants the large spore which produces a
female gametophyte. 152. Megasporophyll : a sporophyll which produces only megasporangia.
152. Mesophyll : 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. Moncecious : applied to plants in which the two sex organs are upon
the same individual. 115. Monopodial : applied to a style of branching in which the branches
arise from the side of the axis. 35. Mother cell : usually a cell which produces new cells by internal divi- sion. 9. Mycelium : the mat of filaments which composes the working body of
a fungus. 49.
Naked flower : one with no floral leaves. 222. Nucellus : the main body of the ovule. 178.
GLOSSARY 333
Oogonium : the female, egg-producing organ of Thallophytes. 16.
OospiiERE : the female gamete, or egg. 16.
Oospore : 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. 378.
Parasite : a i)lant 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.
Perianth : the set of floral leaves when not differentiated into calyx and corolla. 221.
Perigynous : applied to a flower whose outer parts arise from a cup surrounding the ovary. 225.
Petal : one of the floral leaves which make up the corolla. 221.
Photosynthesis : the process by which chloroplasts, aided by light, manufacture carbohydrates from carbon dioxide and water. 84.
Pistil : 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 Gyrano- 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.
Protonema : the thallus portion of the gametophyte of Mosses. 98.
Radial : applied to a body with uniform exposure of surface, and pro- ducing similar organs about a common center. 120.
Receptacle : in Angiosperms that part of the stem which is more or less modified to support the parts of the flower. 222.
Rhizoid : 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
Scale : a leaf without chlorophyll, and usually reduced in size. 161.
Sepal : one of the floral leaves which make up the calyx. 221.
Seta : 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.
Sporogoxium : the leafless sporophyte of Bryophytes. 98.
Sporophore : a special branch bearing asexual spores. 49.
Sporophyll : a leaf set apart to produce sporangia. 145.
Sporophyte : in alternation of generations, the generation which pro- duces the asexual spores. 97.
Stamen : the microsporophyll of Spermatophytes. 174.
Staminate : applied to a flower with stamens but no carpels. 218.
Stigma : in Angiosperms that portion of the carpel (usually of the style) prepared to receive pollen. 199.
Stoma (pi. Stomata) : an epidermal organ for regulating the communi- cation between green tissue and the air. 141.
Strobilus : a cone-like cluster of sporophylls. 161.
Style : 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.
Symbiosis : usually applied to the condition in which two different organisms live together in intimate and mutually helpful rela- tions. 79.
Sympetalous : applied to a flower whose petals have coalesced. 227.
Syncarpous : applied to a flower whose carpels have coalesced. 226.
Synergid : 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. 108.
Zoospore : a motile asexual spore. 10.
Zygomorphic : applied to a flower in which the parts in one or more
sets are not similar ; irregular. 229. Zygote : the sexual spore resulting from conjugation. 15.
INDEX
[The italicized numbers indicate that the subject is illustrated upon the pasje cited. In such case the subject may be referred to only in the illustration, or it may be referred to also in the text.]
Absorption, 299.
Acacia, S65,
Aconitum, 261.
Acorus, £19, 243.
Actinomorpliy, 228.
Adder's tongue : see Ophioglossum.
Adiantura, 143, 145.
^cidiomycetes, 50, 62.
^cidiospore, 66.
jEcidium, 66.
Agaricus, 6S, 69.
Agave, 247.
Air pore : see Stoma.
Akeiie, 212, ns, 2U, 216, 217.
Alchemilla, 225.
Alder : see Alnus.
Alga3, 4, 5, 17.
Alisraa, 210, 240.
Almond : see Primus.
Alnus, 251.
Alternation of generations, 94,
129. Aniaryllidacea?, 247. Amaryllis family: see Amarylli-
dacea?. Ambrosia, 279. Ament, 257. Anaptychia, SI, S2.
Anemophilous, 181. Angiosperms, 173, 195, 217. Anisocarpje, 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, 91. Apical cell, 134. Apical group, 283. Apium, 267. Apocarpy, 199, 222, 225. Apocynacea3, 271. Apocynum, 272. Apogamy, 131. Apospory, 132. Apothecium, 79, 81, 82. Apple : see Pirus. Aquilegia, 198. Ariiceiv, 243. Araliacea>, 267. Araucaria, 190. Arbor vita^: see Thuja. Arbutus, 198 : see Epiga^a. Archegoniates, 101. 337
338
INDEX
Archegonium, 99, 100, 113, 114, 133,
135, IGl, 167, 179. Archesporium, 102, 104, 105, 146. Archichlarayde:^', 255. Arctostaphylos, 269. Areola?, Ill, 114. Arisa^ma, 243, 244. Arnica, 275, 276, 278. Aroids, 243. Artemisia, 279. Arum, 245. Ascocarp, 58, 59. Ascomycetes, 50, 57. Ascospore, 59. Ascus, 59. Asexual spore, 9. Aspidium, 130, 136, I44 Assimilation, 302. Aster, 279. Astragalus, 265. Atherosperma, 198. Azalea, 270.
B
Bacillus, 76.
Bacteria, 21, 75, 76.
Balm : see Melissa.
Banana, I40.
Bark, 284, 289.
Basidiomyeetes, 50, 68
Basidiospore, 69, 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.
Black knot, 60. Black mould, 52. Blasia, 116.
Blueberry : see Vacciniura. Blue-green alga3, 6, 17. Blue mould, 60. Boletus, 73, 74. Botrychium, 145, I49. Botrydium, 28. Box elder, 234. Bracket fungus, 72. Brake : see Pteris. Brassica, 261. Bryophytes, 2, 93, 172. Brown algae, 6, 32. Bryum, 120, 124. Buckeye, 235. Butomus, 199.
Buttercup : see Ranunculus. Buttercup family : see Ranuncu- laceae.
C
Cabbage : see Brassica.
Calamus : see Acorus.
Calla-lily, 243.
Callithamnion, 43.
Callophyllis, 39.
Calluna, 270.
Calopogon, 249.
Caltha, 260.
Calycanthus, 226, 261.
Calypso, 249.
Calyptra, 102, 125.
Calyptrogen, 293.
Calyx, 220, 221.
Cambium, 285, 287, 288.
Capsella, 209, 293.
Capsule, 98, 123, 125, 126, 211, 212.
Caraway : see Carum.
Carbohydrate, 302.
Carbon dioxide, 83.
INDEX
3^9
Carnivorous plants, 92.
Carpel, 177, 178, 109, 219, 220.
Carpi nus, 217, 258.
Carpospore, 44» 45.
Carrot : see Daucus.
Carurn, 267.
Cassia, 265.
Cassiope, 269.
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.
Characeae, 46.
Chemotropism, 307.
Cherry : see Primus.
Chestnut, 256.
Chlorophyceae, 6, 21.
Chlorophyll, 5, 83.
Chloroplast, 7, 8.
Chrysanthemum, 279.
Cilia, 10.
Circinate, 136, 143.
Cladophora, 25.
Clavaria, 73.
Climbing fern : see Lygodium.
Closed bundle, 290.
Clover : see Trifoliura.
Club mosses, 162.
Cnicus, 278.
Cocklebur: see Xanthium.
Coenocyte, 27.
ColeoehcTte, 106, 107.
Collateral bundle, 287.
Collenchyma, 284.
Columella, 104, 105, 106, 126.
Compass plant : see Silphium.
Composita?, 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, 111.
Convolvulaceap, 271.
Convolvulus forms, 270.
Convolvulus, 2^3.
Coprinus, 70.
Coral fungus, 73, 74.
Coreopsis, 278.
Coriandrum, 267.
Cork, 284.
Corn, 216, 282, 290.
Cornacea3, 267.
Corolla, 220, 221.
Cortex, 283, 284, 288.
Cotton, 206.
Cotyledon, 137, 138, 168, 184,
210, 216, 217. Cranberry: see Vaccinium. Crataegus, 262. Crocus, 249. Crucifer, 262. Crucifera% 262. Cryptogams, 172. Cunila, 274. Cup fungus, 60, 61. Cupule, 112, 114. Cyanophycea\ 6, 17. Cycads, 185, 186, 187, 189. Cyclic, 159, 193. Cyperacea3, 241.
340
INDEX
Cypripedium, 249, 253. Cystocarp, 43, 44. Cystopteris, ^.S", 144. Cytoplasm, 7.
D
Daisy: see Bellis.
Dandelion : see Taraxacum.
Dasya, 40.
Datura, 197.
Daucus, 2GG, 267.
Dead-nettle, 22S.
Definitive nucleus : see Endosperm
nucleus. Dehiscence, 198, 199. Delphinium, 260, 261. Dermatogen, 2S3. Desmids, 31, 32. Desmodium, 308. Diatoms, 45'
Dichotomous, 35. ^.^ »fT
Dicotyledons, 208^^) 254, 282. Differentiation, 3, 280. Dogbane: see Apocynura. Dog-tooth violet: see Erythroniura. Dogwood family : see CornaceiP. Dorsiventral 109. Downy mildew, 55. Drupe, 264. Digestion, 302. Dioecious, 115. Disk, 276, 277. Dodder, S6.
E
Ear-fungus, 74. Easter lily, 221. Ecology, 297, 311. Economic botany, 297. Ectocarpus, 33. Edogonium, 22, 23. Egg, 16, 202, 204, 205, 206.
Egg-apparatus, 204, 205, 206.
Elater, 103, 113, US.
Elm : see Ulmus.
Embryo, 137, 167, 168, 170, 183, 207,
208, 209, 210, 211. Embryo-sac, 178, 179, 201, 203, 208. Endosperm, 179, 180, 207, 208, 211. Endosperm nucleus, 202, 205. Entomophilous, 196. Epidermis, 14I, 142, 191, 283, 284,
295. Epiga^a, 269. Epigyny, 224, 225. Epilobium, 212. Epiphyte, 157. Equisetales, 159. Equisetum, 159, 160, 161. Ergot, 60, 61. Erica, 270. Ericaceae, 268. Erigenia, 267. Erythronium, 250. Eusporangiate, 157. Evolution, 3.
Fennel : see Foeniculum.
Ferns, 155, 156.
"Fertilization, 16, 181, 206, 207.
Festuca, 240.
Figwort family : see Scrophula-
riacea\ Filament, 8, 196, 197. Filicales, 155. Fireweed : see Epilobium. Fission, 10. Flax : see Linum. Floral leaves, 218. FloridefP, 38. Flower, 218. Flowering plants, 172. Fceniculum, 267.
INDEX
341
Foliar, 166.
Food, 83, 299.
Foot, 98, 102, 137, 138, 168.
Fragaria, 214, ^^7, 262.
Fruit, 211, 212, 213, 214, 215.
Fucus, 35, 37.
Funaria, 99, 102, 121, 124, 125, 126.
Fungi, 4, 48.
G
Gametangium, 11.
Gamete, 10, 12.
Gametophore, 98, 112, 120, 124.
Gametophyte, 97, 107, 132, 134, 161.
166, 167, 176, 179, 180, 201, 203,
204, 205. Gaultheria, 270. Gaylussacia, 269. Gemma, 112, 114. Generative cell, 180, 201. Gentianaceffi, 271. Geophilous, 246, Geotropism, 305. Gerardia, 275. Germination, 187, 214. Gigartina, 38. Gills, 71. Ginkgo, 191. Gladiolus, 249, 251. Gleditschia, 236, 265. GlcEocapsa, 17, 18. Glume, 241.
Goldenrod : see Solidago. Gonatonema, 31. Gramine.T, 241. Green alga?, 6, 21. Green plants, 83. Green slimes, 20. Grimmia, 126. Growth movement, 304. Growth ring, 234. Grain, 241.
40
Grasses, 240.
Grass family : see Gramineae. Gymnosperms, 171, 173, 195. Gymnosporangium, 67.
Habenaria, 249, 252.
Harebell, 228.
Haustoria, 50.
Hazel : see Carpinus.
Heart-wood, 289.
Heat, 314.
Heath family : see Ericaceae.
Heaths, 268, 269, 270.
Helianthus, 279, 285, 306.
Heliotropism, 305.
Hemiarcyria, 75.
Hemlock : see Coniura.
Henbane : see Hyoscyamus.
HepaticaB, 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.
Hygroscopic movement, 304.
Hyoscyamus, 196.
Hypha, 49.
Hypocotyl, 184. 209, 216, 217.
342
INDEX
Hypoderrais, 284. Hypogyny, 224, 225. llyssopus, 274.
Indigo : see Indigofera.
Indigofera, 265.
Iiidusium, 136, 143, UI^.
Inflorescence, 230.
Insects and flowers, 90.
Integument, 178, 179, 201, 202, 203.
Involucre, 267, 275, 277.
Ipomoea, 228, 270.
Iridacea?, 247.
Iris, 248, 251.
Iris family : see Iridaceae.
Irritable movement, 307.
Isocarpap, 268.
Isoetes, 169.
Isogamy, 15.
Japan lily, 21,8.
Jungermannia, 105, 115, 116, 117.
Juniper, 194.
K Kalmia, 270.
L
Labiatae, 272.
Labiates, 272.
Lactuca, 279.
Laminaria, 33, 34.
Lamium, 274, 275.
Larch : see Larix.
Larix, 192.
Larkspur : see Delphinium.
Laurel: see Kalmia.
Lavandula, 275.
Leaf, 141, 142, 295. 296, 311.
Legumes, 250, 251, 264.
Leguminosag, 264.
Lemna, 201.
Lepidozia, 117.
Le))iota, 70,
Leptosporangiate, 157.
Lettuce : see Lactuca.
Leucanthemum, 279.
Liatris, 278.
Lichens, 77, 78, 79, 87.
Life relations, 311.
Light, 314.
Ligule, 168.
Liliacea% 246.
Lilies, 245.
Lilium, 203, 204, ^05, 207, 224, 249,
295. Lily : see Lilium. Lily family : see Liliaceae. Linaria, 228, 275. Linum, 220. Liverworts, 109. Loculus, 200. Locust : see Robinia. Lotus, 264. Lupinus, 265. Lycopersicum, 275. Lycopodiales, 162. Lycopodium, 162, 163. Lygodium, 145. Lyonia, 269.
M
Macrospore, 152.
Maidenhair fern : see Adiantum.
Male cell, 180, 181, 201, 206, 207.
Maple, 212.
Marasmius, 70.
Marchantia, IO4, 110, 111, 112, 113,
114. Marguerite: see Leucanthemum. ^larjoram : see Origanum. Marrubium, 275. Marsli marigold: see Caltha.
INDEX
343
Marsilia, 158.
Megasporangium, 152, 177, 179. Megaspore, 152, 165, 167, 179, 201,
203. Megasporophyll, 152, 165, 177, 199. Melissa, 275. Mentha, 229, 274. Meristem, 281.
Mesophyll, 141, I42, 191, 295. Mesophytes, 324. Mestome, 282.
Micropyle, 17S, 201, 202, 206. Microspira, 76. Microspha^ra, 58.
Microsporangium, 152, 176, 197. ■ Microspore, 152, 165, 166, 179, 197,
201. Microsporophyll, 152, 165, 174, 196,
198. Midrib, 234. Mildews, 57. Mimosa, 265, 308, 309. Mint : see Mentha. Mint family : see Labiatfe. Monocotyledons, 208, 232, 236, 289. Monoecious, 115. Monopodial, 35. Monotropa, 270. Moonwort: see Botrychium. Morels, 60, 62.
Morning-glory : see Ipomosa. Morphology, 297. Mosses, 93, 119, 124. Mother cell, 9. Mougeotia, 31. Movement, 303. Mucor, 49, 52, 53, 54, 55. Mullein : see Verbascum. Musci, 119. Mushrooms, 68.
IVfustard family : see Crucifera^. Mycelium, 49.
Mycomycetes, 50. Mycorrhiza, 87, 88. Myristica, 214. Myrmecophytes, 90, 91. Myxomycetes, 74, 75.
N Naias, 237. Narcissus, 247. Nemalion, 4S. Xepeta, 275. Nicotiana, 227, 275. Nightshade family : see Solanaceae. Nostoc, 18.
Nucellus, 178, 179, 201, 202, 203. Nucleus, 7. Nutation, 304. Nutmeg, 214. Nutrition, 3, 299. Nyctitropic movement, 309. Nymphaeaceee, 261.
Oak, 255, 256.
(Edogonium : see Edogonium.
Onoclea, 145, 147, I48.
Oogonium, 16.
Oosphere, 16.
Oospore, 16, 101.
Open bundle, 287.
Operculum, 122, 125.
Ophioglossum, 145, 149.
Orchidacea?, 249.
Orchids, 249, 252, 253, 254.
(Orchid family: see Orchidaceap.
Origanum, 274.
Ornithogalum, 247.
Oscillatoria, 19.
Osmunda, 145, 156.
Ostrich fern : see Onoclea.
Ovary, 199, 200, 202.
Ovule, 178, 179, 201, 203.
344
INDEX
Palisade tissue, 11,2, 295.
Palinaccffi, 241.
Palm family : see Palmacejc.
Palms, 241, 2J^2, 2J,3.
Papaveracea?, 261.
Pappus, 276, 277, 278.
Parasites, 48, 85.
Parenchyma, 280, 281, 282, 288.
Parmelia, 79.
Parsley: see Petroselinum.
Parsley family : see Umbellifera?.
Parsnip : see Pastinaca.
Parthenogenesis, 52.
Pastinaca, 267.
Pathology, 297.
Pea : see Pisum.
Peach : see Prunus.
Peach curl, 60.
Pea family : see Leguminosae.
Pear : see Pirus.
Peat, 119.
PelLnea, U6.
Penicillium, 60.
Pentacyclae, 268.
Pentstemon, 275.
Peony, 220.
Pepper, 211, 258.
Pepper family : see Piperaceae.
Perianth, 219, 220, 221.
Periblem, 283.
Perigyny, 225, 226.
Peristome, 126, 127.
Peronospora, 55, 56.
Petal, 220, 221.
Petiole. 141.
Petroselinum, 267.
Phaeophyceae, 6, 32.
Phanerogams, 172.
Phaseolus, 216, 265.
Phloem, ^^5, 287, 288, 290, 292, 20Jf.
Phlox, 228, 271.
Photosyntax, 84.
Photosynthesis, 84, 302.
Phycomycctes, 50, 51.
Physcia, 79.
Physiology, 297.
Picea, 179, 181, 182.
Pileus, 71.
Pine : see Pinus.
Pineapple, 215.
Pinus, 173, 175, 176, 177, 178, 181,
183, 184, 188, 191, 286. Piperaceae, 258. Pirus, 225, 262, 263. Pistil, 199, 200, 219, 220. Pisum, 265. Pith, 285, 287, 288. Planococcus, 76. Plantaginacea?, 275. Plant body, 6. Plant associations, 813. Plasmodium, 74, 75. Plastid, 7, 8. Platycerium, 132. Plerome, 283. Pleurococcus. 21. Plum : see Prunus. Plumule, 210. Pod, 211, 212. Pogonia, 249. Polemoniaceae, 271. Polemonium, 271. Pollen, 174, 176, 197, 201. Pollen-tube, 179, 180, 181, 187, 202,
206, W7. Pollination, 181. Polyembryony, 183. Polymorphism, 63. Polypetaly, 226. Polyporus, 71, 72. Polysiphonia, 44. Polytrichura, 96.
INDEX
345
Pome, 263.
Pondweeds, 237.
Poplars, 255.
Popowia, 198.
Poppy, 261.
Poppy family: see PapaveracciT.
Populus, 256.
Pore-fungus, 72.
Potamogeton, 237, 23S.
Potato : see Solanum.
Potentilla, 225, 262.
Proteid, 302.
Prothallium, 130, 132, 134.
Protococcus forms, 22.
Protonema, 95, 98.
Protoplasm, 7.
Prunus, 213, 262.
Pseudomonas, 76.
Pseudopodium, 105, 123, 124.
Pteridophytes, 2, 128, 172, 291.
Pteris, 133, 134, 135, 137, I4I, I42,
143, 14s, 281, 291, 292, 293. Ptilota, 42.
Puccinia, 63, 64, 65, 66. Puff-balls, 68, 74. Pulvinus, 308.
Quillwort : see Isoetes.
R
Rabdonla, 4I. Radiate bundle, 294. Radicle, 209. Radish, 120.
Ragweed : see Ambrosia. RanunculaceJB, 261. Ranunculus, 222. 259. Raspberry : see Rubus. Rays, 275, 276. Receptacle, 222. Red algae, 6, 38.
Redbud : see Cercis.
Redwood : see Sequoia.
Reproduction, 3, 8, 309.
Respiration, 302.
Rheotropism, 307.
Rhizoid, 109, 110, 134.
Rhizophores, I64.
Rhododendron, 270, 271.
Rhodophycea% 6, 38.
Riccia, I04, 110.
Ricciocarpus, 110.
Ricinus, 288.
Robinia, 265.
Root, 138, 217, 293, 294, 313.
Root-cap, 293.
Root-fungus, 87, 88.
Root-hairs, 217, 300.
Root-pressure, 300.
Root-tubercles, 89.
Rosacea% 262.
Rose family : see Rosaceae.
Rosin-weed : see Silphium.
Rosmarinus, 275.
Royal fern : see Osraunda.
Rubus, 262.
Rumex, 284.
Rust, 62, 63, 64, 65, 66.
Sac-fungi, 57. Sage: see Salvia. Sage-brush : see Artemisia. Sagittaria, 208, 338. Salix, 219, 233, 256, 257. Salvia, 275. Salvinia, 158. Saprolegnia, 51, 52. Saprophyte, 48, 84. Sap-wood, 289. Sargassum, 35, 36. Saururus, 219, 258. Scales, 161.
346
INDEX
Scapania, 116. Schizomycetes, 21. Schizophytes, 21. Sclerenchynia, 281, 282, 284, 285,
288, 290, 291. Scouring rush, 159. Scrophulariacea% 275. Scutellaria, 275. Sedge family: see Cyperacea?. Seed, 183, 184, 210, 211, 212, 2U. Selaginella, 162, 164. 165, 166, 168. Sensitive fern : see Onoclea. Sensitive-plant : see Acacia. Sepal, 220, 221. Sequoia, 189. Seta, 98, 125. Sex, 12.
Sexual spore, 10. Shepherd's purse : see Capsella. Shield fern : see Aspidium. Shoot, 312.
Sieve vessels, 285, 286. Silphium, 279. Siphon forms, 27. Siphonogams, 183. Siphonogamy, 183. Slime moulds, 74, 75. Smut, 62.
Snapdragon : see Antirrhinum. Soil, 314. Solanacea^, 275. Solanum, 198, 275. Solidago, 279. Solomon's seal, 233. Sorus, 136, 143, I44. Spadix, 244, 245. Spathe, 244, 245. Sperm, 16, 100, 133, 135, 162, 166,
169, 187, 190. Spermatia, 43, 44. Spermatophytes, 2, 171, 172. Spermatozoid, 16.
Sperm mother cell, 100.
Sphagnum, 105, 106, 122, 123.
Spike, 240.
Spiraea, 262.
Spiral, 193.
Spirillum, 76.
Spirogyra, 28, 29, 30.
Spongy tissue, I42.
Sporangium, 10, 136, 143, 145, 150,
157, 163, 179. Spore, 9. Sporidium, 65. Sporogenous tissue, 103. Sporogonium, 98, 102, 10 4, 105, 106,
125, 126. Sporophore, 49, 50. Sporophyll, 145, I47, I48, 149, 174,
176. Sporophyte, 97, 102, 137. Spruce : see Picea. Stability of form, 298. Stamen, 174, 176, 196, 198, 219,
220. Stele, 191, 283, 285. Stem, 139, 282, 289, 291, 312. Stemonitis, 75. Stereome, 282, 299. Sterile tissue, 103. Sticta, 80. Stigma, 199, 202. Stomata, I4I, 142, 191, 295, 301. Strawberry : see Fragaria. Strobilus, 160, 161, 163, 165, 174,
175, 176, 193, 194. Style, 199, 202. Substratum, 49. Sumach, 235.
Sunflower : see Helianthus. Suspensor, 167, 168, 183, 209,
210. Symbiont, 79, 86. Symbiosis, 79, 86.
INDEX
347
Sympetal.T, 268. Sympetaly, 226, 227. Symplocarpus, 243. Syncarpy, 199, 219, 225. Synergid, 202, 204, 205, 206.
Tanacetum, 279. Tansy : see Tanacetum. Taraxacum, 213, 277, 278. Taxonomy, 297. Teleutospore, 64, 65. Tension of tissues, 298. Testa, 184, 211. Tetracycla?, 268. Tetrad, 103. Tetraspore, 43. Teucrium, 230, 274, 275. Thallophytes, 2, 4, 172. Thermotropism, 307. Thistle : see Cnicus. Thorn apple : see Datura. Thuja, 193. Thymus, 274. Tickseed : see Coreopsis. Tissues, 280. Toad-flax : see Linaria. Toadstools, 68. Tobacco : see Nicotiana. Tomato : see Lycopersicum. TrachejT, 285, 286. Tracheids, 286. Transfer of water, 300. Transpiration, 301. Tree fern, 140. Trichia, 75. Trichogyne, 43, 44. Trillium, 207, 246, 265. Truffles, 60. Turgidity, 298. Typha, 239, 240.
U
Umbel, 266, 267. Umbelliferffi, 206. Umbellifers, 260. Ulmus, 210, 256 Ulothrix, 12, 13, 22. Uredo, 64. Uredospore, 63, 04.
Vaccinium, 269.
Vascular bundle, 232, 234, 287, 291.
Vascular cylinder, 234, 287.
Vascular system, 129, 139.
Vaucheria, 26, 27, 28.
Vegetative multiplication, 9.
Veins, 141, I42.
Venation, 233.
Verbascum, 275.
VerbenaceaB, 275.
Vernation, 143.
Vernonia, 279.
Veronica, 275.
Vicia, 265.
Violet, 211, 229.
W
Wall cell, 180.
Walnut, 256.
Water, 83, 314.
Water ferns, 158.
Water-lily, 223, 261.
Water-lily family : see Nymphaea-
ceae. Water moulds, 51. Wiieat rust, 63, 64, 65, 66. Willow : see Salix. Wind, 315. Wintergreen : see Gaultheria.
348
INDEX
Wistaria, 265. Witches'-broora, 60. Wormwood : see Artemisia.
Xanthiura, 279.
Xeropliytes, 319.
Xylem, 2S5, 287, 288, 290, 2D2, 204.
Yeast,
Zannichellia, 237. Zoospore, 10. Zygomorphy, 228, Zygospore, 15. Zygote, 15.
C4)
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"A very attractive bo .k, which contains a mass of useful information and curious anecdote. — San /^rancisco Chronicle.
"The book is one that is apt to please the young naturalist, as it is not over- crowded with scientific words of such dimensions as are usually a bugbear to the young student. The information is given in a pleasant way that is attractive as well as instructive. — Minneapolis Tribune.
Familiar Features of the Roadside.
With 130 Illustrations by the Author. i2mo. Cloth, $1.75.
" Which one of us, whether afoot, awheel, on horseback, or in comfortable carriage, has not whiied axyay the time by glancing about ? How many of us, however, have taken in the details of what charms us? We ^ee the flowering fields and budding woods listen to the notes of birds and fro?s, the hum of some big bumblebee but how much do we know of what we sense? These questions, these doubts have occurred to a 1 of us, and it is to answer them that Mr Mathews sets foith. It is to his credit that he succeeds so well He puts before us in chronological order the flowers, birds, and beasts we meet on our highway and byway travels, tells us how to recognize them, what they are really like, and gives us at once charming drawings in words and lines, lor Mr. Mathews is his own illustrator."— i9^j/<7« Journal.
APPLETON AND COMPANY, NEW YORK.
By WILLIAM C. EDGAR.
The Story of a Grain of Wheat.
By William C. Edgar, Editor of "The North-^ western Miller." Illustrated. Cloth, $i.oo net ; post- age, lo cents additional.
The story of wheat is a marvelous one, and is here told with all the interest of a narrative. A short chapter dealing with the character of the berry itself, and its ene- mies, diseases, and pests, precedes its earlier history from its probable birthplace in the valley of the Euphrates to its cultivation in modern times. Then follows a review of Britain's supplies and requirements, with a brief review of the fields of France, Germany, and other European countries. India is considered as a wheat producer, and Russia's ability to compete in the world's markets is dis- cussed.
This book will merit the attention of the general reader who may not be practically interested in wheat and its products, because of its direct and lucid narrative, telling the story which appeals to all human kind — the story of man's long-continued struggle for plenty and his final triumph over savagery and want. Its special and exceptional value, however, beyond its intrinsic worth, will be to those who are concerned directly or remotely in the making of flour, its handling and sale, or its man- ufacture into bread. By these it will be welcomed as a book of record and reference, an exponent of the funda- mental principles of their particular industry and an im- partial history of its achievements, written by one who is in full sympathy with its broader and higher aspirations.
D. APPLETON AND COMPANY, NEW YORK.
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