LT)
m m i-q CD o
TWENTIETH CENTURY TEXT-BOOKS
EDITED BY
A. F. NIGHTINGALE, PH. D.
SUPERINTENDENT OF HIGH SCHOOLS, CHICAGO AND
CHARLES H. THURBER, A. M.
ASSOCIATE PROFESSOR OF PEDAGOGY IN THE UNIVERSITY OF CHICAGO
' • '
TWENTIETH CENTURY TEXT-BOOKS
PLANT STRUCTURES
A SECOND BOOK OF BOTANY
BY
JOHN M. COULTER, A.M., PH.D.
HEAD OF DEPARTMENT OF BOTANY UNIVERSITY OF CHICAGO
NEW YORK D. APPLETON AND COMPANY
1900
COPYRIGHT, 1899, BY D. APPLETON AND COMPANY.
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. Rec- ognizing this fact, Plant Structures has been made an independent volume that may precede or follow the other, or may provide a brief course of botanical study in itself.
Although in the present volume Morphology is the domi- nant subject, it seems wise to give a somewhat general view of plants, and therefore Physiology, Ecology, and Taxonomy are included in a general way. For fear that Physiology and Ecology may be lost sight of as distinct subjects, and to introduce important topics not included in the body of the work, short chapters are devoted to them, which seek to bring together the main facts, and to call attention to the larger fields.
This book is not a laboratory guide, but is for reading and study in connection with laboratory work. An accom- panying pamphlet for teachers gives helpful suggestions to those who are not already familiar with its scope and purpose. It is not expected that all the forms and sub- jects presented in the text can be included in the labora- tory exercises, but it is believed that the book will prove a useful companion in connection with such exercises. It is very necessary to co-ordinate the results of laboratory work, to refer to a larger range of material than can be handled, and to develop some philosophical conception of
yi PREFACE
the plant kingdom. The learning of methods and the collection of facts are fundamental processes, but they must be supplemented by information and ideas to be of most service.
The author does not believe in the use of technical terms unless absolutely necessary, for they lead frequently to mistaking definitions of words for knowledge of things. But it is necessary to introduce the student not merely to the main facts but also to the literature of botany. Ac- cordingly, the most commonly used technical terms are introduced, often two or three for the same thing, but it is hoped that familiarity with them will enable the student to read any ordinary botanical text. Care has been taken to give definitions and derivations, and to call repeated attention to synonymous terms, so that there may be no confusion. The chaotic state of morphological terminology tempted the author to formulate or accept some consistent scheme of terms ; but it was felt that this would impose upon the student too great difficulty in reading far more important current texts.
Chapters I-XII form a connected whole, presenting the general story of the evolution of plants from the lowest to the highest. The remaining chapters are supplementary, and can be used as time or inclination permits, but it is the judgment of the author that they should be included if possible. The flower is so conspicuous and important a feature in connection with the highest plants, that Chapter 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
rKK,rA\ i'-. Yii
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 Xoll'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 Dr. Caldwell and his assist- ants, and some are credited to Dr. Chamberlain and Dr. Cowles, of the University, but it is a matter of regret that pressure of work and time limitation have forbidden a still greater number. The authors of the original illustrations are cited, and where illustrations have been obtained else- where the sources are indicated. The descriptions given in connection with each illustration are unusually full, and should be studied carefully, as frequently they contain important material not included in the text.
The author would again call attention to the fact that this book is merely intended to serve as a compact supple- ment to three far more important factors : the teacher, the laboratory, and field work. Without these it can not serve its purpose.
JOHX M. COULTER.
THE UXIVKKSITY OF CHICAGO. ^Yocember, 1899.
CONTENTS
CHAPTER PAGE
I. — INTRODUCTION . . 1
II. — THALLOPHYTES : AL.GJE 4
III. — THE EVOLUTION OF SEX 12
IV. — THE GREAT GROUPS OF ALG^E ... . . 17
V. — THALLOPHYTES: FUNGI 48
VI. — THE FOOD OF PLANTS 83
VII. — BRYOPHYTES . . .93
VIII. — THE GREAT GROUPS OF BRYOPHYTES 109
IX. — PTERIDOPHYTES 128
X. — THE GREAT GROUPS OF PTERIDOPHYTES .... 155
XI. — SPERMATOPHYTES : GYMNOSPERMS 171
XII. — SPERMATOPHYTES : ANGIOSPERMS 195
XIIL— THE FLOWER 218
XIV.— MONOCOTYLEDONS AND DICOTYLEDONS 232
XV. — DIFFERENTIATION OF TISSUES 280
XVI. — PLANT PHYSIOLOGY 297
XVII. — PLANT ECOLOGY . . . 311
GLOSSARY . 329
INDEX ' . 337
ix
BOTANY
PART II.— PLANT STRUCTURES
CHAPTER I
INTRODUCTION
1. Differences in structure,— It is evident, even to the casual observer, that plants differ very much in structure. They differ not merely in form and size, but also in com- plexity. Some plants 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 Avill be noticed that all the names have the
J
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 Algcv and Fungi, the former represented by green thready growths in fresh water and the great host of sea- weeds, the latter by moulds, mushrooms, etc.
(2) Bryophytes. — The name means " moss plants," and suggests very definitely the forms which are included. Every one knows mosses in a general way, but associated with them in this great group are the allied liverworts, which are very common but not so generally known.
(3) Pteridophytes. — The name means " fern plants," and ferns are well known. Xot 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 theory of descent, or more generally as evo- lution. To understand any higher group one must study the lower ones related to it, and therefore the attempt of this book will be to trace the evolution of the plant king- dom, by beginning with the simplest forms and noting the gradual increase in complexity until the highest forms are reached,
CHAPTER II
THALLOPHYTES :
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. AVhile 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 AlycB and Fuiiyi. 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: ALG.fi 5
The great distinction between these two divisions of Thallophytes is that the Algae 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 Algae, which gradually acquired the habit of obtaining food already manufactured, lost their chlorophyll, and became absolutely dependent and more or less modified in structure. Fungi may be regarded, there- fore, as reduced relatives of the Algae, of equal rank so far as birth and structure go, but of very different habits.
ALG^E
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
Q PLANT STRUCTURES
of the compound microscope to marine forms with enor- mously bulky bodies. In general they are hydrophytes — that is, plants adapted to life in water or in very moist places. The special interest connected with the group is that it is supposed to be the ancestral group of the plant kingdom — the one from which the higher groups have been more or less directly derived. In this regard they differ from the Fungi, which are not supposed to be responsible for any higher groups.
9. The subdivisions. — Although all the 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 phycece, 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) Cyanophycecz, 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) Chlorophycece, or " Green Algae," in which there is no special coloring matter associ- ated with the chlorophyll ; (3) Phceophycece, or " Brown Algae " ; and (4) Rhodophycece, or " Eed Algae."
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-
THALLOPHYTES : ALG^E
B- C
plex plants consist of very many cells. It is necessary to know something of the ordinary living plant cell before the bodies of Algae or any other plant bodies can .be under- stood.
Such a cell if free is approximately spherical in outline, (Fig. 6), but if pressed upon by contiguous cells may become variously modified in form (Fig. 1). Bounding it there is a thin, elastic wall, com- posed of a substance called cellulose. The cell wall, therefore, forms a delicate sac, which contains the liv- ing substance known ^pro- toplasm. This is the sub- stance which manifests life, and is the only substance in the plant which is alive. It is the protoplasm which 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 oTganized into various structures which are called organs of the cell, each organ having one or more special functions. One of the most conspicuous organs of the living cell is the single nucleus, a comparatively compact and usually spherical protoplasmic body, and generally centrally placed within the cell (Fig. 1). All about the nucleus, and filling up the general cavity within the cell wall, is an organized mass of much thinner protoplasm, known as cytoplasm. The cytoplasm seems to form the general background or matrix of the cell, and the
FIG. 1. Cells from a moss leaf, showing nucleus (B) in which there is a nucle- olus, cytoplasm (C), and chloroplasts (A). — 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 Algae and other groups, is the plastid. Plastids are relatively compact bodies, commonly spherical, variable in number, and lie imbedded in the cytoplasm. There are various kinds of plastids, the most common being the one which contains the chlorophyll and hence is stained green. The chlorophyll-containing plastid is known as the chloro- plastid, or chloroplast (Fig. 1). An ordinary alga-cell, there- fore, consists of a cell wall, within which the protoplasm is organized into cytoplasm, nucleus, and chloroplasts.
The bodies of the simplest 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, o) ; 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 Algae.
Starting again with the one-celled body, another line of advance is for several cells to organize in two directions, forming a plate of cells. Still another line of advance is for the cells to organize in three directions, forming a mass of cells.
The bodies of 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
THALLOPHYTES: ALGJ5 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 Algae, and all other plants. They are as follows :
(1) Vegetative multiplication. — 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 spores, and the cell which has formed them within itself is known as the mother cell. This peculiar kind of cell division, which does not involve the wall of the
IQ PLANT STRUCTURES
old cell, is often called internal division, to distinguish it from fission, which involves the wall of the old cell, and is the ordinary method of cell division in nutritive cells.
If the mother cell which produces the spores is different from the other cells of the plant body it is called the sporan- gium, which means " spore vessel." Often a cell is nutri- tive for a time and afterward becomes a mother cell, in which case it is said to function as a sporangium. The wall of a sporangium usually opens, and the spores are dis- charged, thus being free to produce new plants. Various names have been given to asexual spores to indicate certain peculiarities. As 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 swimming spores, or swarm spores. It must be remembered that all of these terms refer to the same thing, a swimming asexual spore.
This method of reproduction may be indicated by a for- mula as follows : P — o — P — o — P — o — P, which indi-. cates that new plants are not produced directly from the old ones, as in vegetative multiplication, but that between the successive generations there is the asexual spore.
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^E 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.
CHAPTER 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 Algae 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 Algae 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, ,4). Each cell contains a nucleus, and a single large chloroplast which has the form of a thick cyl- inder investing the rest of the cell contents. Through the microscope, if the focus is upon the center of the cell, an optical section of the cylinder is obtained, the chloro-
12
THE EVOLUTION OF SEX
13
plast appearing as a thick green mass on each side of the central nucleus. As no other color appears, it is evident that Ulothrix is one of the Chlorophycese.
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 (6) 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; 6r, 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
14- PLANT STKUCTUEES
reproductive cells are not distinctly differentiated, but that the same cell may be nutritive at one time and reproductive at another.
In suitable conditions certain cells of the filament will be observed organizing within themselves new calls by internal division (Fig. 2, (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 the/r cilia, and begin to develop a new filament like that from which they csme (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, E). 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, (7, d, e). The cells thus formed have the power of producing new filaments more or less directly.
It is evident that this is a sexual act, that the cell pro- duced by fusion is a sexual spore, that the two cells which fuse are gametes> and that the mother cell which produces them acts as a gametangium. Cases of this kind suggest that the gametes or sex cells have been derived from zoo- spores, and that asexual spores have given rise to sex cells.
THE EVOLUTION OF SEX 15
The appearance of sex cells (gametes) is but one step in the evolution of sex. It represents the attainment of sexuality, but the process becomes much more highly developed.
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 isogamous.
The act of fusing of similar gametes is usually called conjugation, which means a " yoking together " of similar bodies. Of course it is a sexual process, but the name is convenient as indicating not merely the process, but also an important character of the gametes. The sexual spore which results from this act of conjugation is called the zygote or zygospore, meaning " yoked spore."
In isogamy it is evident that while sexuality has been attained there is no distinction between sexes, as obtains in the higher plants. It may be called a unisexual 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 heterogamy, which means " dissimilar gametes," and those plants whose pairing gametes are dissimilar are said to be heterogamous.
16 PLANT STRUCTURES
In order to distinguish them the large and passive female gamete is called the oosphere, which means " egg sphere," or it is called the egg ; the small but active male gamete is variously called the sperniatozoid, 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 antheridium, that producing the egg being called the oogonium (Fig. 10).
The act of fusing of sperm and egg is called fertiliza- tion, which is the common form of the sexual process. The sexual spore which results from fertilization is known as the oospore or " egg-spore," sometimes called the fertilized egg.
It is evident that heterogamous plants are bisexual, and bisexuality is not only attained among Algae, but it prevails among all higher plants. Among the lowest forms there is only vegetative multiplication ; higher forms added sexu- ality ; then still higher forms became bisexual.
16. Summary. — Isogamous forms produce gametangia, which produce similar gametes, which by conjugation form zygotes. Heterogamous forms produce antheridia and oogonia, which produce sperms and eggs, which by fertiliza- tion form oospores.
CHAPTER IV
THE GREAT GROUPS OF
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 Algae (Chloro- phyceae), Brown Alga? (Phaeophyceae), and Red Algae (Rhodo- 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. CYANOPHYCEAE (Blue-green Algce]
18. Glceocapsa. — 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 Glceocapsa 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 Glceocapsa individuals being formed, this method of vegeta- tive multiplication being the only form of reproduction (Fig. 3).
When new cells are formed in this way the swollen mucilaginous walls are apt to hold them together, so that presently a number of cells or individuals are found lying
17
18
PLANT STRUCTURES
together imbedded in the jelly-like matrix formed by the wall material (Fig. 3). 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 Glwocapsa, 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 het- erocysts (Fig. 4, A), which simply means "other cells." It is observed that when the chain breaks up
into fragments each FIG. 4. Nostoc, & blue-green alga, showing the
frao-TnPTit i<3pnTnr>n<jprl chain-like filaments, and the heterocysts (A)
-0mP°' which 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
19
two heterocysts. The fragments wriggle out of the jelly matrix and start new colonies of chains, each cell dividing to increase the length of the chain. This cell division, to form new cells, is the characteristic method of repro- duction.
At the approach of unfavorable conditions certain cells of the chain become thick-walled and well-protected. These cells which endure the cold or other hardships, and upon the return of favorable conditions produce new chains of cells, are often called spores, but they are better called " resting cells."
20. Oscillaria. — These forms are found as bluish-green slippery masses on wet rocks, or on damp soil, or freely floating. They are simple filaments, composed of very short flattened cells (Fig. 5), and the name Oscillaria refers to the fact that they exhibit a peculiar oscillating move- ment. These motile filaments are is- olated, not being held together in a jelly-like matrix as are the chains of 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
FIG. 5. Oscillaria, a blue- green alga, showing a group of filaments (A), and a single filament more enlarged (B). — CALDWEI,L.
20 PLANT STKUCTDKES
it will bulge outward. In all active living cells there is this pressure upon the wall from within.
Each cell of the Oscillaria filament has the power of dividing, thus forming new cells and elongating the fila- ment. A filament may break up into fragments of varying lengths, and each fragment by cell division organizes a new filament. Here again reproduction is by means of vegeta- tive multiplication.
21. Conclusions. — Taking Glwocapsa, Nostoc, and Oscil- laria as representatives of the group Oyanophyceae, or " green slimes," we may come to some conclusions concern- ing the group in general. The plant body is very simple, consisting of single cells, or chains and filaments of cells. Although in Nostoc and Oscillaria 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 Oscillaria filament breaks up without them.
The power of motion is also well exhibited by the group, the free filaments of Oscillaria moving almost continually, and the imbedded chains of Nostoc at times moving to es- cape from the restraining mucilage.
The whole group also shows a strong tendency in the cell-wall material to become converted into mucilage and much swollen, a tendency which reaches an extreme expres- sion in such forms as Nostoc and Glwocapsa.
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^E
21
upon the return of favorable conditions. These may 'be regarded as resting celis. 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 Algae and Fungi.
2. CHLOROPHYCE^: (Green Alga).
22. Pleurococcus. — This may be taken as a type of one- celled Green Algae. 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 Gloeocapsa, 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. 6). Like Glceocapsa, 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 ; S, C, D, E, various stages of division (fission) in producing new cells ; F, colonies of cells which have remained in contact. — C \LDWELI,.
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 Algag. 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.
23. THothrix. — 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. Edogonimn. — 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. Edogoninm nodosiim. a Conferva form : A, portion of a filament showing a vegetative cell with its nucleus (d), an oogoninm (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 (A), each containing two sperms; /?, another filament showing antheridia (a) from which two sperms (6) have escaped, a vegetative ceil with its nucleus, and an oogonium which a sperm (c} has entered and is coming in contact with the egg whose nucleus (d) 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; D. a zoospore producing a new filament, putting out a holdfast at base and elongating: K a further stage of development; F, the four zoospores formed by the oospore when it germinates. — CALDWELL. except (7 and F, which are after PRINGSHEIM,
24 PLANT STRUCTURES
(§13). The other cells are longer than in Ulofhrix, each cell containing a single nucleus and apparently several chloroplasts, but really there is but one large complex chloroplast.
The cells of the filament have the power of division, thus increasing the length of the filament. Any cell also may act as a sporangium, the contents of a mother cell organizing a single large asexual spore, which is a zoospore. The zoospore escapes from the mother cell into the water, and at its more pointed clear end there is a little crown of cilia, by means of which it swims about rapidly (Fig. 7, C). After moving about for a time the zoospore comes to rest, attaches itself by its clear end to some support, elongates, begins to divide, and develops a new filament (Fig. 7, Z>, E).
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 GKEAT GROUPS OF ALG^E
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. W hen 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
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 c oen ocy tic . — C ALD WELL .
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 biciliate.
25. Vaucheria. — This is one of the most common of the Green Algas, found in felt-like masses of coarse filaments in shallow water and on muddy banks, and often called " green
FIG. 9. Vaucheria geminata, a Siphon form, showing a portion of the ccenocytic body (A) 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 (Z>) it is discharged later as a large multiciliate body (C), which then begins the develop- ment of a new ccenocytic body (E).— CALDWEI.L.
felt." The filament is very long, and usually branches ex- tensively, but its great peculiarity is that there is no parti- tion wall in the whole body, which forms one long continuous cavity (Figs. 9, 11). This is sometimes spoken of as a one- celled body, but it is a mistake. Imbedded in the exten- sive cytoplasm mass, which fills the whole cavity, there are not only very numerous chloroplasts, but also numerous nuclei. As has been said, a single nucleus with some cyto-
THE GREAT GKOUPS OF ALG^E 27
plasm organized about it is a cell, whether it has a wall or not. Therefore the body of Vauclieria is made up of as many cells as there are nuclei, cells whose protoplasmic structures have not been kept separate by cell walls. Such a body, made up of numerous cells, but with no partitions, is called a cc&nocyte, or it is said to be cwnocytic. Vauclieria represents a great group of Chlorophyceae whose members have ccenocytic bodies, and on this account they are called the Siphon forms.
Vauclieria 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, (7), swims about for a time, and finally develops another Vaucheriabody (Figs. 9, E, 10). It should be said that this large body, called a zoospore and acting like one, is really a mass of small biciliate zoospores, just as the
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 Cladophora, § 24.
Antheridia and oogonia are also developed. In a com- mon form these two sex organs appear as short special branches developed on the side of the large coenocytic body,
PLANT STRUCTURES
and cut off from the general cavity by partition walls (Fig. 11). The oogonium becomes a globular cell, which usually
FIG. 11. Vaucheria sessilis, a Siphon form, show- ing a portion of the coenocytic body, an an- theridial branch (A) with an empty anthe- ridinm (a) at its tip, and an oogouium (S) containing an oospore (c) and showing the opening (/) through which the sperms passed to reach the egg.— CALDWELL.
develops a perforated break for the entrance of the sperms, and organizes within itself a single large egg (Fig. 11, B}. The an- theridium is a much smaller cell, within which numerous very small sperms are formed (Fig. 11, J, a). The sperms are discharged, swarm about the oogonium, and finally one passes through the break 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 " 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 GREAT GROUPS OF ALG^E 29
springs. The filaments are simple, and are not anchored by a special basal cell, as in Ulothrix and Edogonium. The
FIG. 13. Spirogyra, a Conjugate form, showing one complete cell and portions of two others. The band-like chloroplasts extend in a spiral from one end of the cell to the other, in them are imbedded nodule-like bodies ( pyrenoids), and near the center of the cell the nucleus is swung by radiating strands of cytoplasm. — CALDWELL.
cells contain remarkable chloroplasts, which are bands pass- ing spirally about within the cell wall. These bands may
FIG. 14. Spirogyra, showing conjugation : A, conjugating tubes approaching each other; B, tubes in contact but end walls not absorbed: C, tube complete and con- tents of one cell passing through; D, a completed zygospore. — CALDWELL.
30
PLANT STRUCTURES
be solitary or several in a cell, and form very striking and conspicuous objects (Figs. 13, 14).
Spirogyra and its associates are further peculiar in pro- ducing no asexual spores, and also in the method of sexual reproduction. Two adjacent filaments put out tubular processes toward one another. A cell of one filament sends out a process which seeks to meet a corresponding process from a cell of the other filament. When the tips of two such processes come together, the end walls disappear,
FIG. 15. Spirogyra, showing some common exceptions. At A two cells have been connected by a tube, but without fusion a zygote has been organized in each cell; also, the upper cell to the left has attempted to conjugate with the cell to the right. At B there are cells from three filaments, the cells of the central one 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^E
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 Algae, 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 (Mougeotia), showing for- mation of zygote in conjuga- ting tube ; B, C ( Ganatone- ma), showing formation of zygote without conjugation. — After WITTROCK.
32 PLANT STRUCTURES
*
by walls, or they are coanocytic, 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.
FIG. 17. A group of Desmids, one-celled Conjugate forms, showing various pat- terns, and the cells organized into distinct halves. — After KEBNER.
The Green Algae are of special interest in connection with the evolution of higher plants, which are supposed to have been derived from them.
3. PH^OPHYCE^: (Brown Algce)
28. General characters. — The Blue-green Algae and the Green Algae are characteristic of fresh water, but the Brown Algae, or " kelps," are almost all marine, being very charac-
THE GREAT GROUPS OF ALGJE
33
teristic coast forms. All of them are anchored by holdfasts, which are sometimes highly developed root-like structures ; and the yellow, brown, or olive-green floating bodies are buoyed in the water usually by the aid of floats or air-bladders, which are often very conspicuous. The kelps are most highly developed in the colder waters, and form much of the " wrack," " tangle," etc., of the coasts. The group is well adapted to live exposed to waves and cur- rents with its strong holdfasts, air-bladders, and tough leathery bodies. It is what is known as a specialized group — that is, one which has become highly organ- ized for certain special condi- tions. It is not our purpose to consider such a specialized group in any detail, as it does not usual- ly help to explain the structures of higher groups.
29. The plant body. — There is very great diversity in the structure of the plant body. Some of them, as Ectocar- pus (Fig. 18), are fil- amentous forms, like the Confervas among the Green Algae, 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 (B) contain- ing zoospores, and gametangia (C) containing gametes.— CALD WELL.
FIG. 18a. A group of brown seaweeds (Laminarias). Note the various habits of the plant body with its leaf-like thallus and root-like holdfasts.— After KERNER.
THE GREAT GROUPS OF ALG.E
35
feet long, whose stalk develops root-like holdfasts (Fig. 18a). The largest body is developed by an Antarctic Laminaria form, which rises to the surface from a sloping bottom with a floating thallus six hundred to nine hundred feet long. Other forms rise from the sea bottom like trees, with thick trunks, numerous branches, and leaf-like appendages.
The common Fucus, or " rock weed," is rib- bon-form and constantly branches by forking at the tip (Fig. 19). This method of branching is called dichotomous, as dis- tinct from that in which branches are put out from the sides of the axis (monopodial). The swol- len air-bladders distrib- uted throughout the body are very conspicuous.
The most differenti- ated thallus is that of Sargassum (Fig. 20), or " 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 the 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 bladder-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 (Sargassurri), showing the thallus differentiated into stem-like and leaf -like portions, and also the bladder-like floats.— After BEN- NETT and MURRAY.
30. Reproduction. — The two main groups of Brown Algae differ from each other in their reproduction. One, repre- sented by the Laminarias and a majority of the forms, pro- duces zoospores and is isogamous (Fig. 18). The zoospores and gametes are peculiar in having the two cilia attached at one side rather than at an end ; and they resemble each other very closely, except that the gametes fuse in pairs and form zygotes.
FIG. 21. Sexual reproduction of Fucus, showing the eight eggs (six in sight) dis- charged from the oogonium and surrounded by a membrane (A), eggs liberated from the membrane (#), antheridium containing sperms (C), the discharged lat- erally biciliate sperms (G), and eggs surrounded by swarming sperms (F, H). — After SI\GRR.
21
38
PLANT 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, G), swim in great numbers about the large eggs (Fig. 21, F, H), and finally one fuses with an egg, and an oospore is formed. As the sperms swarm very actively about the egg and impinge against it they often set it rotating. Both an- theridia and oogonia are formed in cavities of the thallus.
4. RHODOPHYCE.E (Red Alga)
31. General characters. — On account of their red colora- tion these forms are often called Floridea. They are mostly
marine forms, and are anchored by holdfasts of various kinds. They belong to the deepest waters in which Algae grow, and it is probable
Ty/tf'&fi: ^^ ^6 r6^ C°l°ring
^~"^™- 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,
FIG. 22. A red alga (Gigartind), showing branching habit, and "fruit bodies."— After SCHENCK.
FIG. 24. A red alga (Dasya), showing a finely divided thallus body. — CALDWELL.
FIG. 25. A red alga (Jtabdonia), showing holdfasts and branching thallus body
CALDWELL.
FIG. 26. A red alga (Ptilota), whose branching body resembles moss. — CALDWELL.
THE GKEAT GROUPS OF ALG^E
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 Algas.
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- |] J3 j: *
FIG. 27. A red alga ( Callithamnion), show- ing sporangium (A), and the tetraspores discharged (B).— After THURET.
FIG. 28. A red alga (Nemation} ; 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 (t) still showing; (7, an almost mature cys- tocarp (o), with the disorganizing trichogyne (t).— After VINES.
rangium always produces just four, they have been called tetraspores (Fig. 27).
Red Algae 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, A, a) develop sperms which, like the tetraspores, have no cilia and no power of motion. To dis-
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 spermatia (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 (trichogyne)) the whole struc- ture resembling a flask with a long, narrow neck, excepting that it is closed (Fig. 28, A, o, t). Within the bulbous part the egg, or its equivalent, is organized ; a spermatium at- taches 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 cystocarp (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 Polytiphoiria, one of the red algae, showing the rows of cells composing the body (A), small branches or hairs (B). and a cystocarp (C) with escaping spores (D) which have no cilia (car- pospores). — CALDWEI-L.
THE GKEAT GROUPS 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 CHLOROPHYLL-CONTAINING THALLOPHYTES
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 KEENER.
They are either free-swimming or attached by gelatinous stalks; solitary, or connected in bands or chains, or im- bedded in gelatinous tubes or masses. In form they are rod- shaped, boat-shaped, elliptical, wedge-shaped, straight or curved (Fig. 30).
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 Algae on account of the occasional conjugation that has been observed. They are so diiferent 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 Algaa, 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 Chara, showing tip of main axis. —After STRASBURGER.
THE GREAT GROUPS OF ALG^E 47
specialized forms, and are so much more highly organized than all other Algae, 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 Algae.
CHAPTEE V
THALLOPHYTES : FUNGI
36. General characters,— In general, Fungi include Thal- lophytes which do not contain chlorophyll. From this fact it follows that they can not manufacture food entirely out of inorganic material, but are dependent for it upon other plants or animals. This food is obtained in two general ways, either (1) directly from the living bodies of plants or animals, or (2) from dead bodies or the products of living bodies. In the first case, in which living bodies are at- tacked, the attacking fungus is called a parasite, and the plant or animal attacked is called the host. In the second case, in which living bodies are not attacked, the fungus is called a saprophyte. Some Fungi can live only as parasites, or as saprophytes, but some can live in either way.
Fungi form a very large assemblage of plants, much more numerous than the Algae. As many of the parasites attack and injure useful plants and animals, producing many of the so-called " diseases," they are forms of great interest. Governments and Experiment Stations have ex- pended a great deal of money in studying the injurious parasitic Fungi, and in trying to discover some method of destroying them or of preventing their attacks. Many of the parasitic forms, however, are harmless ; while many of the saprophytic forms are decidedly beneficial.
It is generally supposed that the Fungi are derived from
the Algae, 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
THALLOPHYTES : 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 plan, to which they can all be referred (Fig. 32). A set of colorless branching
FIG. 32. A diagrammatic representation of Jfucor, showing the profusely branching mycelium, and three vertical hyphse (sporophores), sporangia forming on b and c. — After ZOPP.
filaments, either isolated or interwoven, forms the main working body, and is called the mycelium. 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 hypJice (singular, hypha) or liyplial threads. The mycelium is in contact with its source of food supply, which is called the substratum.
50 PLANT STRUCTURES
From the hyphal threads composing the mycelium verti- cal ascending branches arise, which are set apart to produce the asexual spores, which are scattered and produce new mycelia. These branches are called ascending hyplice or sporophores, meaning " spore bearers."
Sometimes, especially in the case of parasites, special descending branches are formed, which penetrate the sub- stratum or host and absorb the food material. These spe- cial absorbing branches are called haustoria, meaning " ab- sorbers."
Such a mycelial body, with its sporophores, and perhaps haustoria, lies either upon or within a dead substratum in the case of saprophytes, or upon or within a living plant or animal in the case of parasites.
38. The subdivisions. — The classification of Fungi is in confusion on account of lack of knowledge. They are so much modified by their peculiar life habits that they have lost or disguised the structures which prove most helpful in classification among the Algae. Four groups will be pre- sented, often made to include all the Fungi, but doubtless they are insufficient and more or less unnatural.
The constant termination of the group names is mycetes, a Greek word meaning "fungi." The prefix in each case is intended to indicate some important character of the group. The names of the four groups to be presented are as follows : (1) Phycomycetes (" Alga-Fungi "), referring to the fact that the forms plainly resemble the Algae ; (2) Ascomycetes (" Ascus-Fungi ") ; (3) JScidiomycetes ("^Ecidium-Fungi ") ; (4) Basidiomycetes (" Basidium-Fungi "). Just what the prefixes ascus, cecidiitm, and basidium 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 Algae, 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 Algae. They live upon the dead bodies of water pla'nts and animals (Fig. 33), and some- times attack living fish, one kind being very destructive to young fish in hatcheries. The hyphae composing the mycelium are coanocytes, 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 Cladophora and Vaucheria. 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 egg, and a heavy-walled oospore or resting spore is the result.
It is an interesting fact that sometimes the contents of an antheridium do not enter an oogonium, or antheridia may not even be formed, and still the egg, without fertiliza- tion, forms an oospore which can germinate. This peculiar
52
PLANT STRUCTURES
habit is called parthenogenesis, which means reproduction by an egg without fertilization.
FIG. 33. A common water mould (Saprolegnia): A, a fly from which mycelial fila- ments of the parasite are growing; B, tip of a branch organized as a sporangium: C, sporangium discharging biciliate zoospores; F, oogonium with antheridium in contact, the tube having penetrated to the egg; D and E, oogonia with several eggs.— A-C after THURET, D-F after DEBARY.
40. Mucor. — One of the most common of the Mucors, or " black moulds," forms white furry growths on damp bread, preserved fruits, manure heaps, etc. It is therefore a saprophyte, the crenocytic 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,
It is asex- are not zoo- there is no
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. evident that these ual spores spores, for
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
FIG. 35. Forming sporangia of Mucor, show- ing the swollen tip of the sporophore (A), and a later stage (J5), in which a wall is formed separating the sporangium from the rest of the body.— CALDWELL.
54 PLANT STRUCTURES
While the ordinary method of reproduction through the growing season is by means of these rapidly germinating spores, in certain conditions a sexual process is observed, by which a heavy-walled sexual spore is formed as a resting spore, able to outlive unfavorable conditions. Branches arise from the hyphae of the mycelium just as in the forma-
FIG. 36. Mature sporangium of Mucor, showing the wall (A), the numerous spores (C), and the columella (B)— that is, the partition wall pushed up into the cavity of the sporangium. — CALDWELL.
FIG. 37. Bursted 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, ^4), the tips are cut off from the main coenocytic body by partition walls (Fig. 38, #), 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
THALLOPHYTE8 : FUNGI
55
Algae ; that the sexual spore is a zygote ; and that' the two pairing tip cells cut off from the main body by partition walls are gametarigia. Mucor, therefore, is isogamous.
FIG. 38. Sexual reproduction of Mucor, showing tips of sex branches meeting (A), the two gametangia cut off by partition walls (B), and the heavy-walled zygote (f).— 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 cceno- 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 this internal mycelium numerous sporophores arise, coming to the surface of the host and securing the scattering of their spores, which fall upon other leaves and germinate, the new mycelia pene- trating among the tissues and begin- ning their ravages. The sporophores, af- 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 Peronospora in contact with two cells of a host plant, and sending into them its large haustoria.— After DEBARY.
FIG. 40. Peronosp&ra, one of the Phycomycetes, shewing at a an oogonium (o) con- taining an egg, and an antheridium (n) 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 egg. The an-
THALLOPHYTES: FUNGI 57
theridium comes in contact with the oogonium, puts out a tube which pierces the oogonium wall and enters the egg, into which the contents of the an theridium 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.
±2. Conclusions. — The ccenocytic bodies of the whole group are very suggestive of the Siphon forms among Green Algae, 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 Algae (Chlo- rophyceae) of various kinds.
2. ASCOMYCETES (Ascus- or Sac-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, Microsphcera, grows on lilac leaves, which nearly always show the whitish covering after maturity (Fig. 41). The branching hyphse show numerous partition walls, and are not ccenocytic as in the Phycomycetes. Small disk-like haustoria penetrate into the superficial cells of the host, anchoring the mycelium and absorbing the cell contents.
Sporophores arise, which form asexual spores in a pe- culiar way. The end of the sporophore rounds off, almost separating itself from the part below, and becomes a spore or spore-like* body. Below this another organizes in the
same way, then another, until a chain of spores is developed, easily broken apart and scat- tered by the wind. Falling upon other suitable leaves, they germinate and form new mycelia, enabling the fungus to spread rapidly. This meth- od of cutting a branch into sections to form spores is called abstriction, and the spores formed in this way are called conidia, or conidi- ospores (Fig. 43, B).
At certain times the myce- lium develops special branches which develop sex organs, but they are seldom seen and may 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 (MicroaphoKra), the shaded re- gions representing the mycelium, and the black dots the ascocarps. — CALDWELL.
THALLOPHYTES: FUNGI
59
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 Algae (§ 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) conidia, formed from a hyphal branch by abstric- tion, by which the mycelium may spread rapidly ; and (2) ascospores, formed in a mother cell and protected by a heavy case, so that they may bridge over unfavorable conditions, and may germinate when liberated and form new mycelia. The resting stage is not a zygote or an oospore, as in the 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. — CALDWELL.
60
PLANT STRUCTUKES
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-
Fio. 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. 45. Two species of cup-fungus (Peziza).— After LINDAU.
FIG. 44. Head of rye attacked by "er- got " («), peculiar grain-like masses replacing the grains of rye ; also a mass of "ergot" germinating to form spores (b). — After TULASNE.
FIG. 46. A cup-fungus (Pitya) grow- ing on a spruce (Picea). — After REHM.
In some of these forms the ascocarp is completely closed, as in the lilac mildew ; in others it is flask-shaped ; in others, as in the cup-fungi, it is like a cup or disk ; but in all the spores are inclosed by a delicate sac, the ascus.
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 structure shown and used
represents the ascocarp, the depressions of
whose surface are lined with asci contain-
• ing ascospores.— After GIBSON.
FIG. 48. Yeast cells, reprodu- 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. JEciDiOMYCETES (indium- Fung i)
45. General characters. — This is a large group of very destructive parasites known as " rusts " and " smuts." The rusts attack particularly the leaves of higher plants, pro- ducing rusty spots, the wheat rust probably being the best known. The smuts especially attack the grasses, and are very injurious to cereals, producing in the heads of oats, barley, wheat, corn, etc., the disease called smut.
THALLOPHYTES: FUNGI
63
No indication of a sexual process has been obtained, and the life histories are so complicated and obscure that the position of the group is very uncertain. The forms should probably be included with the Basidiomycetes, but they are so unlike the ordinary forms of that group that they are here kept distinct.
Most of the forms are wry 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-
FIG. 49. Wheat rust, showing sporophores breaking through the tissues of the host and bearing summer spores (uredospores). — 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
PLANT STRUCTURES
disease with great rapidity (Fig. 50). Once it was thought that this completed the life cycle, and the fungus received the name Uredo. 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 WAIID.
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 H. MARSHALL WARD.
66
PLANT STRUCTURES
"cluster-cups." This mycelium on the barberry, bearing cluster-cups, was thought to be a distinct plant, and was
called ^Ecidium. 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 a3cidia which give name to the group, and ^Ecidiomycetes are those Fungi in whose life history ascidia or cluster-cups appear.
The aecidiospores 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 aecidium 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
THALLOPHYTES : FUNGI
67
appear : (1) sporidia^ which develop the stage on the barber- ry ; (2) cecidiospores, which develop the stage on the wheat ; (3) ttredosporeS) which repeat the mycelium on the wheat ; (4) teleutospores,v?\iich last through the winter, and in the spring produce the stage bearing sporidia. It should be said that there are other spores of this plant produced on the barberry (Fig. 53), but they are too uncertain to be included here.
The barberry is not absolutely necessary to this life cycle. In many cases there is no available barberry to act as host, and the sporidia germinate directly upon the young wheat, forming the rust-producing mycelium, and the cluster-cup stage is omitted.
FIG. 54. Two species of "cedar apple" (Gymnosporangium), both on the common juniper (Junipents Virginiand).—A after FARLOW, B after ENGLER and PRANTL.
47. Other rusts, — Many rusts have life histories similar to that of the wheat rust, in others one or more of the stages are omitted. In very few have the stages been con-
68
PLANT STRUCTURES
nected together, so that a mycelium bearing uredospores is called a Vredo, one bearing teleutospores a Puccinia, and one bearing aecidia an ^cidium ; but what forms of Uredo, Puccinia, and jEcidium belong together in the same life cycle is very difficult to discover.
Another life cycle which has been discovered is in con- nection with the " cedar apples " which appear on red cedar (Fig. 54). In the spring these diseased growths be- come conspicuous, especially after a rain, when the jelly- like masses containing the orange-colored spores swell. This corresponds to the phase which produces rust in wheat. On the leaves of apple trees, wild crab, hawthorn, 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 campestrte.—Afier GIBSON. HO SCXUal prOCCSS has
TIIALLOPTIYTES: FUNGI
69
been discovered. The life history seems simple, but this apparent simplicity may represent a very complicated his- tory. The structure of the common mushroom (Agari- cux) will serve as an illustration of the group (Fig. 55).
49. A common mushroom. — The mycelium, of white branching threads, spreads extensively through the decay- ing substratum, and in cultivated forms is spoken of as the " spawn." Upon this myce- lium little knob- like protuberances begin to arise, grow- ing larger and larger, until they are organized into the so-called " mushrooms." The real body of the plant is the white thread - like mycelium, while the " mushroom " part seems to rep- resent a great num- ber of Sporophores FlG- 56> A common Agrarians : A, section through one side of pileus, showing sections of the pendent gills;
Organized together s, section of a gill more enlarged, showing the cen-
tral tissue, and the broad border formed by the ba- sidia : f, still more enlarged section of one side of a gill, showirg the club-shaped basidia standing at right angles to the surface, and sending out a pair
to form a single complex spore- bearing structure. The mushroom 23
of small branches, each of which bears a single ba- sidiospore.— After SACHS.
£ t
T <
THALLOPHYTES : 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 pileux. 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 (hyphae), whose tips turn toward the surface and form a compact layer of end cells (Fig. 56). These end
FIG. 60. A bracket fungus (Poly]X)ruti) growing on the trunk of a red oak. — CAT.DWKI.I..
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 the sporidia appear in the wheat rust.
Y2 PLANT STRUCTURES
These spores, called basidiospores, shower down from the gills when ripe, germinate, and produce new mycelia. The peculiar cell called the basidium gives name to the group Basidiomycetes.
50. Other forms. — Mushrooms display a great variety of form and coloration, many of them being very attractive
FIG. 61. A toadstool of the bracket form which has grown about blades of grass without interfering with their activity.— CALDWELL.
(Figs. 57, 58, 59). The " pore-fungi " have pore-like depres- sions for their spores, instead of gills, as in the very com- mon "bracket-fungus" (Polyporus), which forms hard shell-like outgrowths on tree-trunks and stumps (Figs. 60,
FIG. 62. The common edible Boletus (B. edu- FIG. 63. Another edible Boletus (B. stro- lls), in which the gills are replaced by bilaceus).— After GIBSON. pores.— After GIBSON.
FIG. 64. The common edible "coral fun- gus11 ( Clararia).— After GIBSON.
FUJ. 65. Hydrum repandum, in which gills are replaced by spinous processes ; edi- ble.—After GIBSON.
74
PLANT STRUCTURES
61), and the mushroom-like Boleti (Figs. 62, 63). The "ear-fungi" form gelatinous, dark-brown, shell-shaped masses, and the " coral fungi " resemble branching corals (Fig. 64). The Hydnum forms have spinous processes
instead of gills (Fig. 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.
FIG. 66. Puffballs. in which the basidia and spores are inclosed : edible.— After GIBSON.
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 pjfix- m odium, 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 amo3bas. So suggestive of certain low animals is this body and food habit that slime-moulds have also been called Mycetozoa or " fungus- animals."
THALLOPHYTES : 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. 67). These varied and easily preserved sporangia are used to classify the
FIG. 67. Three common slime moulds (Myxomycetes) on decaying wood: to tin- 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 plasmodiuin at base; below, groups of sporangia of Ilemiarcyria, with a plasmodium mass at upper left hand.— CALDWKI.L.
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," " bacilli," " 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 Cyanophyceae, or "Fission- Algae," so closely that they are often associated with them in classification (see § 21).
FIG. 68. A group of Bacteria, the bodies being black, and bearing motile cilia in various ways. A , the two to the left the common hay Bacillus (B. mbtilis}, the one to the right a Spirillum ; B, a Coccus form (Planococcus)\ C, D, E, species of Pseudomonas : F, G, species of Bacillus, F being that of typhoid fever: ff. Micro- *inra ; J, K, L, M, species of Spirillum.— After ENGLER and PBANTL.
THALLOPIIYTES : FUNGI 77
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. (-g^fo^ 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
TIIALLOPHYTES : FUNGI
79
plant. In other words, a Lichen is not an individual, but a tirm of two individuals very unlike each other. This habit
FIG. 70. A common lichen (Pliyscia] growing on bark, showing the spreading thallue and the numerous dark disks (apothecia) bearing the asci.— OAI.DWET.L.
of living together has been called symbiosis, and the indi- viduals entering into this relation are called symbionts.
FIG. 71. A common foliose lichen (Paiinelid) growing upon a board, and showing apothecia.— CALDWEI.I..
80
PLANT STRUCTURES
If a Lichen be sectioned, the relation 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 Algae, sometimes scattered, some-
FIG. 72. Section through thallus of a lichen (Sticta), showing holdfasts (r), lower (t/) and upper (o) surfaces, fungus hyphae (m), and enmeshed algae (<?). — After SACHS.
times massed. It is these enmeshed Algae, 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-
TIIALI.ol'HYTES: FUNGI
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.
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 Alga3. 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 shows 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.
Fu;. 73. Section through an apothccium of Anaptycfria, showing stalk of the cup (m), masses of algal cells (g), outer margin of cup (>•), overlapping edge ((. t), layer of asci (/n. and massing of hyphae beneath asci (y).— After SACHS.
Certain Ascomycetes, 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) Crustaceans 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 Anapttjchia. show- ing the fungus mycelium \.m), which is massed above (y), just beneath the layer of asci (1, 2, 3, A), in which spores in various stages of development are .-hown.— After SACHS.
tached only at the middle or irregularly to the substratum ; (3) FruticoKf. Lichens, with filamentous bodies branching like shrubs, either erect, pendulous, or prostrate.
CHAPTER VI
THE FOOD OF PLANTS
54. Introductory. — All plants use the same kind of food, but the Algae and Fungi suggest that they may have very different ways of obtaining it. The Algae 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.
oo. Green plants, — The presence of chlorophyll enables plants to utilize carbon dioxide (C02), 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 (H20) are recombined to form a carbohydrate (sugar, starch, etc.), which is an organ- ized food. With this as a basis other foods are formed, and so the plant can live without help from any other organism.
This process of utilizing carbon dioxide in the formation of food is not only a wonderful one, but also very important. It is wonderful, because carbon dioxide and water, both of them very refractory substances, are broken up at ordinary
83
84 PLANT STKUCTURES
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 photosyntax, 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 many orchids and heaths are parasitic, attaching their subterranean parts to those of other plants, becoming what are called "root parasites." The cultivated plants, also, may be regarded as partially saprophytic, in so far as they use the organic material sup- plied to them in fertilizers.
57. Parasites. — Certain plants 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
86
I'LANT STRUCTURES
m
the food which they manufacture. The less chlorophyll is used the less is it developed, and a green plant which is obtaining the larger amount of its food in a saprophytic
or parasitic way is on the way to losing all of its chlorophyll and becoming a com- plete saprophyte or parasite.
Certain of the low- er 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. — The phenomenon of symbiosis has already
been referred to in connection with Lichens (§ 53). In its broadest sense the word includes any sort of depend- ence between living organisms, from the vine and the tree
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 STRASBUBGER.
THE FOOD OF PLANTS 87
upon which it climbs, to the alga and fungus GO intimately associated in a Lichen as to seem a single plant. In a nar- rower sense it includes only cases in which there is an inti- mate organic relation between the symbionts. This would include parasitism, the parasite and host being the sym- bionts, and the organic relation certainly being intimate. In a still narrower sense symbiosis includes only those cases in which the symbionts are mutually helpful. This fact, however, is very difficult to determine, and opinions vary widely as to the mutual advantage of the relation. How- ever large a set of phenomena may be included r ider the term symbiosis, we use it here in this narrowest sense, which is often distinguished as mutualism.
(1) Lichens. — The main facts of symbiosis in connection with Lichens were presented in § 53. That the fungus- symbiont can not live without the alga has been demon- strated, but whether the alga-symbiont derives any benefit from this association is a question in dispute. The latter can live independently of the former, but enmeshed by the fungus the alga seems to thrive and to live in situations which would be impossible to it without the protection and moisture supplied by the fungus-thallus. Those who lay stress on the first fact regard the Lichen merely as a pecul- iar case of parasitism, which has been called heloti*m, or a condition of slavery, indicating that the alga is enslaved and even cared for by the fungus for its own use. Those who see an advantage to the alga in this association regard a Lichen as an example of mutualism.
It may be of interest to know that artificial Lichens have been formed, not only by cultivating together spores of a Lichen-fungus and some Lichen-alga, but also by using " wild " Algae — that is, Algae which are in the habit of living independently.
(2) Mycorrhiza. — The name means "root-fungus," and refers to an association which exists between certain Fungi of the soil and roots of higher plants, such as orchids, heaths,
FIG. 76. Mycorrhiza: to the left is the tip of a rootlet of beech enmeshed by the fungus; A, diagram of longitudinal section of an orchid root, showing the cells of the cortex (p) filled with hyphae; B, part of longitudinal section of orchid root much enlarged, showing epidermis («), outermost cells of the cortex (p) filled with hyphal threads, which are sending branches into the adjacent cortical cells (a, i). —After FRANK.
FIG. 77. Mycorrhiza: A, rootlets of white poplar forming mycorrhiza; B, enlarged section of single rootlets, showing the hyphae penetrating the cells.— After KEENER.
THE FOOD OF PLANTS
89
oaks and their allies, etc. (Figs. 76, 77). The delicate branching filaments (hyphae) of the fungus spread through the soil, wrap the rootlets with a mesh of hyphae, and pene- trate into the cells. It seems 'clear that the fungus obtains food from the rootlet as a parasite ; but it is also thought that the hyphal threads, spreading widely through the soil, are of great service to the host plant in aiding the rootlets in absorbing. If this be true, there is mutual ad- vantage in the association, for the small amount of nourishment taken by the fungus is more than compen- sated by its assistance in absorption.
(3) Root-tubercles. — On the roots of many legume plants, as clovers, peas, beans, etc., little wart -like outgrowths are frequently found, known as " root-tubercles " (Fig. 78). It is found that these tuber- cles are caused by certain Bacteria, which penetrate the roots and in- duce these excrescent growths. The tubercles are found to swarm with Bacteria, which are doubtless ob- taining food from the roots of the host. At the same time, these Bac- teria have the peculiar power of laying hold of the free nitrogen of the air circulating in the soil, and of supplying it to the host plant 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-
Fio. 78. Root-tubercles on Vicia Faba.-A.fter NOLL.
90
PLANT STRUCTURES
nary crops have exhausted the soil of its nitrogen-contain- ing salts, and it has hecome 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 Alga3 which they harbor with- in their bodies (Fig. 79). Like the Lichen -fungus, these ani- mals use the food manufactured by the Algae, 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 Mynnecophytes, which means " ant-plants," or myrmecopMlous
plants, which means "plants loving ants." These plants are mainly in the tropics, and in stem cavities, in hollow thorns, or elsewhere, they provide dwelling places for tribes of warlike ants (Fig. 80). In addition to these dwelling places they provide special kinds of food for the ants.
(5) Flowers and insects. — A very interesting and impor- tant case of symbiosis is that existing between flowers and insects. The flowers furnish food to the insects, and the
FIG. 79. A fresh-water polyp (Hy- dra) attached to a twig and feed- ing upon algae (C), which may be seen through the transparent body wall (B).— CALDWKLL.
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
r-
FIG. 80. An ant plant (Hydnophytum) from South Java, in which an excrescent growth provides a habitation for ants.— After SCHIMPER.
considered, or it may be found, with illustrations, in Plant Relations, Chapter VII.
92 PLANT STRUCTURES
59. Carnivorous plants, — Certain green plants, growing in situations poor in nitrogen-containing salts, have learned to supplement the proteids which they manufacture by cap- turing and digesting insects. The various devices employed for securing insects have excited great interest, since they do not seem to be associated with the ordinary idea of plant activities. Prominent among these forms are the bladder- worts, pitcher-plants, sundews, Venus's fly-trap, etc. For further account and illustrations of these plants see Plant Relations, § 119.
CHAPTER VII
BRYOPHYTES (MOSS PLANTS)
60. Summary from Thallophytes. — Before considering the second great division of plants it is well to recall the most important facts connected with the Thallophytes, those things which may be regarded as the contribution of the Thallophytes to the evolution of the plant kingdom, and which are in the background when one enters the region of the Bryophytes.
(1) 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 egg, the small active one the
93
94 PLANT STRUCTURES
sperm, the organs producing the two being known as oogo- nium and antheridium respectively.
(5) Algae the main line. — The Algae, aquatic in habit, appear to be the Thallophytes which lead to the Bryophytes and higher groups, the Fungi being regarded as their de- generate descendants ; and among the Algae the Chloro- 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 (Fig. 81). It is prostrate, and is a regu-
BRYOPIIYTES
95
lar thallus body, not at all resembling the "moss plant" of ordinary observation, and is not noticed by those una- ware of its existence.
Presently one or more buds appear on the sides of this alga-like body (Fig. 81, V). A bud develops into an erect
FIG. 81. Protonema of moss: A, very young protonema, showing spore (S) which has germinated it; B, older protonema, showing branching habit, remains of spore (s), rhizoids (r), and buds (b) of leafy branches (gametophores). — After MULI.ER 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 STRUCTURES
.rh
FIG. 82. A common moss (Polytrichum commune), showing the leafy gameto- phore with rhizoids (rh), and two sporophytes (sporo- gonia), with seta (a), calyp- tra (c), and operculum (d), the calyptra having been re- moved.—After SCHENCK.
plant from which it came. This new leafy body consists of a slender stalk bearing at its summit an urn- like case in which are developed nu- merous asexual spores (Figs. 82, 107). This whole structure is often called the " spore fruit/7 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 alternation 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 gametophyte — that is, "the gamete plant."
The generation which consists of the "spore fruit "- that is, leafless stalk and spore case — produces only asexual spores, and is called the sporophyte — that is, "the spore plant."
Alternation of generations, therefore, means the alter- nation of a gametophyte and a sporophyte in completing a life history. Instead of having the same body produce both asexual and sexual spores, as in most of the Algae, 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>o— S— o— G=g>o— S— o— G, etc.
The formula indicates that the gametophyte produces two gametes (sperm and egg), which fuse to form an oospore, which produces the sporophyte, which produces an asexual spore, which produces a gametophyte, 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 PLANT STRUCTURES
it multiplies the product of the sexual spore. A glance at the formula given above shows that if there were no sporo- phyte (S) the oospore would produce but one gametophyte (G). By introducing the sporophyte, however, as many gametophytes may result from a single oospore as there are asexual spores produced by the sporophyte, which usually produces a very great number.
In reference to the sporophytes and gametophytes of Bryophytes two peculiarities may be mentioned at this point : (1) the sporophyte is dependent upon the gameto- phyte for its nourishment, and remains attached to it ; (2) the gametophyte is the special chlorophyll -generation, and hence is the more conspicuous. It follows that, in a general way, the sporophyte of the Bryophytes only pro- duces spores, while the gametophyte both produces gametes and does chlorophyll work.
It is important also to note that the protected resting stage in the life history is not the sexual spore, as in the 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 name.
The names now given to the different structures which appear in this life history are as follows : The alga-like part of the gametophyte is the protonema, the leafy branch is the gametophore ("gamete-bearer") ; the whole sporophyte is the sporogonium (a name given to this peculiar leafless sporophyte of Bryophytes), the stalk-like portion is the seta, the part of it imbedded in the gametophore is the foot, and the urn-like spore-case is the capsule.
BKYOPHYTES
99
63. The antheridium. — The male organ of the Bryophytes is called an antheridium, just as among Thallophytes, but it has a very different structure. In general among the
Fi«. 83. Sex organs of a common moss (Funaria): the group 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 group to the left represents an archegonial cluster at summit of stem (A), showing arche<jonia (a), and paraphyses and leaf sections (6), and also a single archegonium (B), with venter (b) containing egg and ventral canal cell, and neck (h) containing the disorganizing axial row (neck canal cells). — Afier 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 ruptured 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 egg.
64. The archegonium, — This name is given to the female sex organ, and it is very diiferent 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 STBAS-
BURGER.
BRYOPHYTKS
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 egg is organized. The archegonium, made up of neck and venter, consists mostly of a single layer of cells. This hollow flask is solid at first, there being a central vertical row of cells surrounded by the single layer just referred to. All of the cells of this axial row, except the lowest one, disorganize and leave a passageway down through the neck. The lowest one of the row, which lies in the venter of the archegonium, or- ganizes the egg. In this way there is formed in the arche- gonium an open passageway through the neck to the egg lying in the venter.
To this neck the swimming sperms are attracted, enter and pass down it, one of them fuses with the egg, and this act of fertilization results in an oospore.
It is supposed that archegonia have been derived in some way from oogonia, but. no intermediate stages suggest the steps. In any event, the presence of the archegonia is one strong and unvarying distinction between Thallophytes and Bryophytes. Pteridophytes also have archegonia, and so characteristic an organ is it that Bryophytes and Pteri- dophytes are spoken of together as Archegoniates.
65. Germination of the oospore, — The oospore in Bryo- phytes is not a resting spore, but germinates immediately by cell division, forming the sporophyte embryo, which presently develops into the mature sporophyte (Fig. 85, A). The lower part of the embryo develops downward into the gametophore, forming the foot, which penetrates and ob- tains a firm anchorage in the gametophore (Fig. 85, B, C). The upper part of the embryo develops upward, organizing the seta and capsule. In true Mosses, when the embryo becomes too large for the venter of the archegonium in which it is developing, the archegonium is broken near the base of the venter and is carried upward perched on the top 25
102
PLANT STRUCTURES
of the capsule like a loose cap or hood, known as the calyp- tra (Figs. 82, c, 107), which sooner or later falls off. As
stated before, the ma- ture structure devel- oped from the oospore is called a sporogoni- um, a form of sporo- phyte peculiar to the Bryophytes.
66. The sporogonium. — In its fullest devel- opment the sporogoni- um is differentiated into the three regions, foot, seta, and capsule (Figs. 82, 107) ; but in some forms the seta may be lacking, and in others the foot also, the sporogonium in this last case being only the capsule or spore case, which, after all, is the essential part of any sporogonium.
At first the capsule is solid, and its cells are all alike. Later a group of cells within begins to differ in ap- pearance from those about them, being set apart for the produc- tion of spores. This
initial group of spore-producing cells is called the arche- sporium, a word meaning "the beginning of spores." It
FIG. 85. Sporogonium of Fun aria : A, an em- bryo sporogonium (/,/'), developing within the venter (6, b) of an archegonium ; B, 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.
BRYOPI1YTES 1Q3
does not follow tliat 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-
104
PLANT STRUCTURES
ous part of the higher plants. The "fern plant," and the herbs, shrubs, and trees among "flowering plants" correspond to the sporogonmm of Bryophytes, and not to the leafy branch (gametophore) or "moss plant." Conse- quently the evolution of the sporogonium through the Bryophytes is traced with a great deal of interest. It may be outlined as follows :
In a liverwort called Riccia the simplest sporogonium is found. It is a globular capsule, without seta or foot
'cot
Jf
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 colnmella (col).— After GOEBET,.
(Fig. 86, ^4). The only sterile tissue is the single layer of cells forming the wall, all the cells within the wall be- longing to the archesporium. The ripe sporogonium, therefore, is nothing but a thin-walled spore case. It is well to note that the sporophyte thus begins as a spore case, and that any additional structures that it may de- velop later are secondary.
In another liverwort (Marchantia) the entire lower half of the sporogonium is sterile, while in the upper half there
BRYOPHYTES
105
is a single layer of sterile cells as a wall about the arche- sporium, which is composed of all the remaining cells of the upper half (Fig. 86, #). It will be noted that the sterile tissue in this sporogonium has encroached upon the arche- sporium, which is restricted to one half of the body. In this case the archesporium has the form of a hemisphere.
In another liverwort (Jungermannia) the archesporium is still more restricted (Fig. 87). The sterile tissue is organ-
FIG. 87. Diagrammatic section of spo- rogonium of a Junrjermannia form, showing differentiation into foot, seta, and capsule, the archesporium restricted to upper part of sporoso- nium.— After GOKBKI,.
FIG. 88. Section through sporogonium of Sphagnum, showing capsule (k) with old archeironium neck (ah), calyptra (ca), dome-shaped mass of sporogenous tissue (spo), and colnmella («>). also the bulb- ous foot (spf) imbedded in the pseudo- podium (ps).— After SCHIMPEK.
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 (Antlwceros) the sterile tissue or- ganizes foot and seta, and the archesporium is still more restricted (Fig. 86, D). Instead of a solid hemispherical
106
PLANT STRUCTURES
mass, it is a dome-shaped mass, the inner cells of the hemi- sphere having become sterile. This central group of sterile cells which is surrounded by the ar- chesporium is called the columella, which means "a small column."
In a moss called Sphagnum there is the same dome-shaped archespori- um with the columella, as in An- thoceros, but it is relatively smaller on account of the more abundant sterile tissue (Fig. 88).
In the highest Mosses the arche- sporium becomes very small as com- pared with the sterile tissue (Fig. 89). A foot, a long seta, and an elaborate capsule are organized from' the sterile tissue, while the arche- sporium is shaped like the walls of a barrel, as though the dome-shaped archesporium of Sphagnum or An- thoceros had become sterile at the apex. In this way the columella is continued through the capsule, and is not capped by the archesporium.
This series indicates that after the sporogonium begins as a simple spore case (Riccia), 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 Coleochcete, whose bod resembles - those of the sim-
J .^^
plest Liverworts (Fig. 90). u hen
FIG. 89. Young sporogoni- um 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.
BKYOPIIYTES
107
its oosporcs 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. W.-Coleocttcete. one of the green algae: A, a portion of the thallns, showing oogonia with trichogynes (og), antheridia (an), and two enlarged biciliate sperms (2): B, a fertilized oogonium containing oospore and invested by a tissue (r) which has developed after fertilization ; C, an oospore which has germinated and formed a mass of cells (probably a sporophyte), each one of which organizes a biciliate zoospore (D),— After PRINGSHEIM.
Riccia would be the result (Fig. 86, A). For such reasons many believe that the Liverworts have been derived from such forms as Coleochmte.
67. The gametophyte. — Having considered the sporo- phyte body as represented by the sporogonium, we must consider the gametophyte body as represented by proto- nema and leafy branch (gametophore). The gametophyte results from the germination of an asexual spore, and in the Mosses it is differentiated into protonema and leafy gametophore (Figs. 81, 82, 102). Like the sporophyte,
10$ PLANT STRUCTUKES
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 garnet ophyte, there being no differentiation into protonema and leafy branch.
In the simpler Liverworts the sex organs (antheridia and archegoiiia) are scattered over the back of this thallus (Fig. 92). In other forms they become collected in certain definite regions of the thallus. In other forms these defi- nite sexual regions become differentiated from the rest of the thallus as disks. In other forms these disks, bearing the sex organs, become short-stalked, and in others long- stalked, until a regular branch arises from the thallus body (Figs. 96, 97). This erect branch, bearing the sex or- gans, is, of course, a gametophore, but it is leafless, the thallus body doing the chlorophyll work.
In the Sphagnum Mosses the spore develops the same kind of flat thallus (Fig. 104), but the gametophore be- comes leafy, sharing the chlorophyll work with the thallus. In the true Mosses most of the chlorophyll work is done by the leafy gametophore, and the flat thallus is reduced to branching filaments (the protonema) (Fig. 102).
The protonema of the true Mosses, therefore, corre- sponds to the flat thallus of the Liverworts and Sphagnum, while the leafy branch corresponds to the leafless gameto- phore found in some Liverworts. It also seems evident that the gametophore was originally set apart to bear sex organs, and that the leaves which appear upon it in the Mosses are subsequent structures.
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 numerous hair-like processes (rhizoids) for ab- sorption and anchorage. The dorsal region is exposed to the light and its cells develop chlorophyll. If the thallus is thin, chlorophyll is developed in all the cells ; if it be so thick that the light is cut off from the ventral cells, the thallus is differentiated into a green dorsal region doing the chlorophyll work, and a colorless ventral region producing absorbing rhizoids. This latter represents a simple differ- entiation of the nutritive body into working regions, the ventral region absorbing material and conducting it to the green dorsal cells which use it in making food.
There seems to have been at least three main lines of development among Liverworts, each beginning in forms with a very simple thallus, and developing in different di- rections. They are briefly indicated as follows :
109
110
PLANT STRUCTURES
69. Harchantia forms. — In this line the simple thallus gradually "becomes changed into a very complex one. The
thallus retains its simple outlines; but becomes thick and differentiated in tissues (groups of similar cells). The line may be distin- guished, therefore, as one in which the differentia- tion of the tissues of the gametophyte is emphasized (Figs. 91-93). In Mar- chantia proper the thallus becomes very complex, and it may be taken as an illus- tration.
The thallus is so thick that there are very distinct green dorsal and colorless
ventral regions (Fig. 94). The latter puts out numerous rhizoids and scales from the single layer of epidermal cells. Above the ventral epidermis are several layers of colorless
FIG. 91. A very small species of Jticcia, one of the Marchantia forms : A, a group of thallus bodies slightly en- larged ; B, section of a thallus, show- ing rhizoids and two sporogonia im- bedded and communicating with the outside by tubular passages in the thallus.— After STRASBURGER.
FIG. 92. Ricciocarpvs, a Marchantia form, showing numerous rhizoids from ventral surface, the dichotomous branching, and the position of the sporogonia on the dorsal surface along the " midribs."— CALDWELL.
FIG. 93. Two common liverworts : to the left is Conocephalus, a Marchantia form, showing rhizoids, dichotomous branching, and the conspicuous rhombic areas (areolse) on the dorsal surface; to the right is Anthoceros, with its simple thallus and pod-like sporogonia. — CALDWELL.
rid
FIG. 94. Cross-sections of thallus of Marchantia: A, section from thicker part of thallus, where supporting tissue (p) is abundant, and showing lower epidermis giving rise to rhizoids (h) and plates (J), also chlorophyll tissue (chl) organized into chambers by partitions (o)\ B, section near margin of thallus more magnified, showing lower epidermis, two layers of supporting tissue (p) with reticulate walls, a single chlorophyll chamber with its bounding walls ($) and containing short, often branching filaments whose cells contain chloroplasts (chl), overarching upper epidermis (o) pierced by a large chimney-like air-pore (sp). — After GOEBEI-.
FIG. 95. Section through cupule of Marchcmtia, showing wall in which are chloro- phyll-bearing air-chambers with air-pores, and gemmae (a) in various stages of development.— After KNY.
FIG. 96. Jfarchanfia poh/morpha : the lower figure represents a gametophyte bear- ing a mature aiitheridial branch (d), some young antheridial branches, and also eome cupules with toothed margins, in which the gemmae may be seen ; the upper figure represents a partial section through the antheridial disk, and shows antheridia %vithin the antheridial cavities (a, b, c, d, f.f). — After KNY.
THE GREAT GROUPS OF BRYOPHYTES
113
cells more or less modified for conduction. Above these the dorsal region is organized into a series of large air cham- bers, into which project chlorophyll-containing cells in the
FIG. 97. Marchantia polymorpha, a common liverwort : 1. thallns. with rhizoide, bearing a mature archegonial branch (/) and several younger ones (a, b, c, d, e)\ 2 and 3, dorsal and ventral views of arcliegonial disk; A and 5, young sporophyte (sporogonium) embryos; 6. more mature sporogonium still within enlarged venter of archegoninm; 7, mature sporogonium discharging spores; 8, 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 STKUCTDKES
The air chambers are outlined on the surface as small
rhombic areas (areolce), each containing a single air pore.
Peculiar reproductive bodies are also developed upon
the dorsal surface of Marchantia for vegetative multiplica-
FIG. 98. Marchantia polymorpha : 1, partial section through archegonial disk, show- ing archegonia with long necks, and venters containing eggs: 9, young archego- n in m 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 KNY.
tion. Little cups (citpules] appear, and in them are numer- ous short-stalked bodies (gemmce), which are round and flat (biscuit-shaped) and many-celled (Figs. 95, 96). The
THE GREAT GROUPS OF BRYOPHYTES H5
gemmae fall off and develop new thallus bodies, making rapid multiplication possible.
Marchantia also possess remarkably prominent gameto- phores, or "sexual branches" as they are often called. In this case the gametophores are differentiated, one bear- ing only antheridia (Fig. 96), and known as the "anthe- ridial branch," the other bearing only archegonia (Figs. 97, 98), and known as the " archegonial branch." The scal- loped antheridial disk and the star-shaped archegonial disk, each borne up by the stalk-like garnet oph ore, are seen in the illustrations. Xot 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"; when they both appear upon the same individual, the plant is monwcious, meaning " one household." Some of the Bryophytes are mo- noecious, and some of them are dioecious (as Marchantia).
Another distinguishing mark of the line of Marchantia forms is that the capsule-like sporogonium opens irregu- larly to discharge its spores (Fig. 97, 7).
70. Jungermaimia forms.— This is the greatest line of the Liverworts, the forms being much more numerous than in the other lines. They grow in damp places ; or in drier situations on rocks, ground, or tree-trunks ; or in the tropics also on the leaves of forest plants. They are gen- erally delicate plants, and resemble small Mosses, many of them doubtless being commonly mistaken for Mosses.
This resemblance to Mosses suggests one of the chief features of the line. Beginning with a simple thallus, as in the Marchantia line, the structure of the thallus re- mains simple, there being no such differentiation of tissues as in the Marchantia line ; but the form of the thallus becomes much modified (Figs. 99, 100). Instead of a flat thallus with even outline, the body is organized into a cen- tral stem-like axis bearing two rows of small, often crowded
116
PLANT STRUCTURES
leaves. There are really three rows of leaves, but the third is on the ventral side against the substratum, and is often so much modified as not to look like the other leaves. In consequence of this the Jungermannia forms are usually called "leafy liverworts," to distinguish them from the
FIG. 99. Two liverworts, both Jungermannia forms: to the left is Blasia, which re- tains the thallus form but has lobed margins; to the right is Scapania, with dis- tinct leaves and sporogonia (A).— CALDWELL.
other Liverworts, which are "thallose." They are also often called " scale mosses," on account of their moss-like appearance and their small scale-like leaves.
The line may be distinguished, therefore, as one in which the differentiation of the form of the gametophyte is emphasized. Another distinguishing mark is that the sporogonium has a prominent seta, and the capsule splits down into four pieces (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 GREAT GKOUPS OF BRYOPHYTES
FIG. 100. Species of Lepidnzia. a genus of leafy liverworts, showing different leaf forms, and in A and Cthe 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 PRANTL.
Mosses, and possibly to the Pteridophytes also. The thallus is very simple, being differentiated neither in structure nor form, as in the two other lines ; but the 26
118
PLANT STRUCTURES
special development has been in connection with the
sporogonium (Figs. 93, 101).
This complex sporogonium (sporophyte) has a large
bulbous foot imbedded in the simple thallus, while
above there arises a long pod-like capsule. The com- plex walls of this cap- sule contain chlorophyll and air pores, so that the sporogonium is or- ganized for chlorophyll work. If the foot could send absorbing processes into the soil, this sporo- phyte could live inde- pendent of the gameto- phyte. In opening to discharge spores the pod- like capsule splits down into two valves.
Another peculiarity of the 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 t]iat jt j^ 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
THE GEEAT GKOUPS OF BKYOPHYTES
garnet ophyte ; the Jungermannia line has differentiated the form of the garnet ophyte ; the AntJioceros 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 and silt shut off the lower strata of moss from complete disorganization, and they become modified into a coaly substance called peat, which may accumulate to con- siderable 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 STRUCTURES
radial, in contrast with the dorsiventral thallus, referring to the fact that it is exposed to similar conditions all around, and its organs are arranged about a central axis like the parts of a radiate animal. 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 (Bryum), showing base of a leafy branch (gametophore) attached to the protonenia. and having sent out rhizoids. On the protonemal filament to the right and be- low is the young bud of another leafy branch. — MULLER.
THE GREAT GROUPS OF BRYOPHYTES
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.
A
\
:P
FIG. 103. Tip of leafy branch of a moss (Funaria), bearing a cluster of sex organs, showing an old antheridium (A), a younger one (B), some of the curious associated hairs (/>), and leaf sections (I).— 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 STRUCTURES
a rosette, in the center of which are the sex organs. This rosette is often called the "moss flower," but it holds no relation to the flower of Seed-plants, and the phrase should not be used. A rosette may contain but one kind of sex organ (Figs. 83, 103), or it may contain both kinds, for Mosses are both 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 large 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 (f) and buds (k) which develop into the large leafy branches (gametophores). — After CAMPBELL.
THE GREAT GROUPS OF BRYOPHYTES
123
105, D), which means a "cover" or "lid." This may serve to illustrate what is called an "intermediate" or "transition" type, Sphagnum showing characters which ally it to Anthcceros 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
em
B
FIG. 105. Sphagnum : A, a leafy branch (gametophore) bearing four mature sporo- gonia; B, archegonium in whose venter a young embryo sporophyte (em) is de- veloping; C, section of a young sporogonium (sporophyte), showing the bulbous foot (spf) imbedded in the apex of the pseudopodium (ps), the capsule (k), the columella (co) capped by the dome-shaped archesporium (spo), a portion of the calyptra (ca), and the old archegonium neck (ah); D, branch bearing mature sporogonium and showing pseudopodium (ps), capsule (k), and operculum (rf); E, antberidium discharging sperms; F, a single sperm, showing coiled body and two cilia. — After SCHIMPER.
fact that the axis of the gametophore is prolonged above its leafy portion, the prolongation resembling the seta of an ordinary moss (Fig. 105, D). This prolongation is
124
PLANT STRUCT UKES
called a pseudopodium, or " false stalk," and in the top of it is imbedded the foot of the sporogonium carrying the globular capsule (Fig. 105, C).
74. True Mosses. — This immense and most highly organ- ized Bryophyte group contains the great majority of the Mosses, which are sometimes called the Bryum forms, to distinguish them from the Sphagnum 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); J5, C. later stages, showing apical cell (1) and young leaves (2); D, later stage much less magnified, showing protonemal filaments and the young gametophore (gam) —After CAMPBELL.
the representative Bryophytes, the only group vying with them being the leafy Liverworts, or Jungermannia forms. They grow in all conditions of moisture, from actual sub- mergence in water to dry rocks, and they also form exten- sive peat deposits in bogs.
The thallus body of the gametophyte is made up of branching filaments (Figs. 81, 102), those exposed to the
THE GREAT GROUPS OF BKYOPHYTES
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 differentia- 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 (col); to the right a capsule with calyptra re- moved, showing the operculum (o); to the left a young sporogonium pushing up the calyptra from the leafy shoot.— After CAMPBELL.
has a foot and a long slender seta, but the capsule is espe- cially complex. The archesporium is reduced to a small hollow cylinder (Fig. 88), the capsule wall is most elabo- rately constructed, and the columella runs through the
FIG. 108. Longitudinal section of moss capsule (Funaria), showing its complex character: d, operculum; p, peristome: c, c', columel- la; $, sporogenous tissue; outside of s the complex wall consisting of layers of cells and large open spaces (h) traversed by strands of tissue.— After GOEBEL.
A
B
FIG. 110. Sporogonia of Grimmia, from all of which the operculum has fallen, displaying the peristome teeth : A, position of the teeth when dry ; B, position when moist.— After KEENER.
sin
FIG. 109. Partial longitudinal section through a moss cap- sule : A, younger capsule, showing wall cells (a), cells of columella (i\ and sporog- enous cells (su) ; B, some- what older capsule, a and i same as before, and sm the spore mother cells. — After GOEBEL.
THE GREAT GROUPS OF BRYOPHYTES ^97
center of the capsule to the lid-like operculum (Figs. 108, 109). When the operculum falls off the capsule is left like an urn full of spores, and at the mouth of the urn there is usually displayed a set of slender, often very beau- tiful teeth (Fig. 110), radiating from the circumference to the center, and called the peristome, meaning "about the mouth." These teeth are hygroscopic, and by bending inward and outward help to discharge the spores.
CHAPTER IX
PTERIDOPHYTES (FERN PLANTS)
75. Summary from Bryophytes. — In introducing the Bryo- phytes a summary from the Thallophytes was given (see § 60), indicating certain important things which that group has contributed to the evolution of the plant kingdom. In introducing the Pteridophytes it is well to notice certain important additions made by the Bryophytes.
(1) 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 manufactured by the gameto- phyte, which is therefore the more conspicuous generation. When a moss or a liverwort is spoken of, therefore, the gametophyte is usually referred to.
(3) Gametophyte and sporophyte not independent. — The sporophyte is mainly dependent upon the gametophyte for its nutrition, and remains attached to it, being commonly called the sporogonium, and its only function is to produce spores.
(4) Differentiation of tliallus into stem and leaves.— This appears incompletely in the leafy Liverworts (Junger- mannia forms) and much more clearly in the erect and radial leafy branch (gametophore) of the Mosses.
128
PTERIDOPHYTES. 129
(5) Many-celled sex organs. — The antheridia and the flask-shaped archegonia are very characteristic of Bryo- phytes as contrasted with Thallophytes.
76. General characters of Pteridophytes.— The name means "fern plants/'' and the Ferns are the most numerous and the most representative forms of the group. Associated with them, however, are the Horsetails (Scouring rushes) and the Club-mosses. By many the Pteridophytes are thought to have been derived from such Liverworts as the Antho- ceros forms, while some think that they may possibly have been derived directly from the Green Algse. Whatever their origin, they are very distinct from Bryophytes.
One of the very important facts is the appearance of the vascular system, which means a "system of vessels," organized for conducting material through the plant body. The appearance of this system marks some such epoch in the evolution of plants as is marked in animals by the appearance of the "backbone." As animals are often grouped as "vertebrates" and "invertebrates," plants are often grouped as "vascular plants" and "non-vascular plants," the former being the Pteridophytes and Spermato- phytes, the latter being the Thallophytes and Bryophytes. Pteridophytes are of great interest, therefore, as being the first vascular plants.
77. Alternation of generations. — This alternation con- tinues in the Pteridophytes, but is even more distinct than in the Bryophytes, the gametophyte and sporophyte be- coming independent of one another. An outline of the life history of an ordinary fern will illustrate this fact, and will serve also to point out the prominent structures. Upon the lower surface of the leaves of an ordinary fern dark spots or lines are often seen. These are found to yield spores, with which the life history may be begun.
When such a spore germinates it gives rise to a small, green, heart-shaped thallus, resembling a delicate and sim- ple liverwort (Fig. Ill, A). Upon this thallus antheridia
130
PLANT STRUCTURES
and archegonia appear, so that it is evidently a gameto- pliyte. This gametophyte escapes ordinary attention, as it is usually very small, and lies prostrate upon the substra- tum. It has received the name prothallium or prothallus, so that when the term prothallium is used the gametophyte of Pteridophytes is generally referred to ; j ust 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. Prothallmm of a common fern (Aspidium): A, ventral surface, showing rhizoids (rh), antheridia (an), and archegonia (ar) ; B, ventral surface of an older gametophyte, showing rhizoids (rh) and young sporophyte with root (w) and leaf (b).— After SCUENCK.
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, B). 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
PTERIDOPHYTES 131
completes the life cycle, as the asexual spores develop the prothallium again.
In contrasting this life history with that of Bryophytes several important differences are discovered. The most striking one is that the sporophyte has become a large, leafy, vascular, and independent structure, not at all re- sembling its representative (the sporogonium) among the Bryophytes.
Also the gametophyte has become much reduced, as compared with the gametophytes of the larger Liverworts and Mosses. It seems to have resumed the simplest liver- wort form, even the gametophore being suppressed, and represented, if at all, by a rudiment. The conspicuous leafy branch of the Mosses, commonly called " the moss plant," corresponds to nothing in the Pteridophytes, there- fore, except possibly the rudiment referred to, the prothal- lium representing only the protonema part of the gameto- phyte of the true Mosses.
This reduction of the gametophyte seems to be associ- ated with the fact that the chlorophyll work has been trans- ferred to the sporophyte, which hereafter remains the conspicuous generation. The "fern plant" of ordinary observation, therefore, is the sporophyte ; while the "moss plant " is a leafy branch of the gametophyte.
Another important contrast indicated is that in Bryo- phytes the sporophyte is dependent upon the gametophyte for its nutrition, remaining attached to it ; while in the Pteridophytes both generations are independent green plants, the leafy sporophyte remaining attached to the small gametophyte only while beginning its growth (Fig. Ill, /y).
Among the Ferns some interesting exceptions to this method of alternation have been observed. Under certain conditions a leafy sporophyte may sprout directly from the prothallium (gametophyte) instead of from an oospore. This is called apogamy, meaning " without the sexual act."
132 PLANT STIit:CTUKES
Under certain other conditions prothallia are observed to sprout directly from the leafy sporophyte instead of from a spore. This is called apospory, meaning "without a spore."
78. The gametophyte. — The prothallium, like a simple liverwort, is a dorsiventral body, and puts out numerous
FIG. 112. Stag-horn fern (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. Archegonium of P/sri* at the time of fertilization, showing tissue of gam- etophyte (A), the cells forming the neck (#». the passageway formed by the dis- organization of the canal cells (V), and the egg (D) lying exposed in the venter. — CALUWELL.
FIG. 114. Antheridium of Pteris (£). showing wall cells («), opening for escape of sperm mother cells (e\ escaped mother cells (c), sperms free from mother cells (6), showing spiral and multiciliate character. — CALDWELL.
27
134
PLANT STRUCTURES
being a conspicuous object in connection with the sporo- phyte.
At the bottom of the conspicuous notch in the prothal- lium is the growing point, representing the apex of the plant. This notch is always a conspicuous feature.
The antheridia and arch- egonia are usually developed on the under surface of the prothallium (Fig. Ill, .1), 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 gametophytc of Pteris: the figure to the left shows the old spore (B), the rhizoid ( O, and the thallus U); that to the right is older, showing the same parts, and also the apical cell (D).— CALDWELL.
FIG. 116. Young gametophytc of Pteris, showing old spore wall (B), rhizoid* (Ct). apical cell (D). a young author idium (K), and an older one in which sperms have organized (F).— WELL.
PTEKIDOPHYTES
135
face, more or less of the neck of the archegonium projecting (Fig. 113). The eggs are not different from those formed within the archegonia of Bryophytes, but the sperms are very different. The Bryophyte sperm has a small body and two long cilia, while the Pteridophyte sperm has a long spirally coiled body, blunt behind and tapering to a point in front, where numerous cilia are developed (Fig. 114). It is, therefore, a large, spirally-coiled, multiciliate sperm, and is quite characteristic of all Pteridophytes excepting the Club-mosses. It is evident that a certain amount of water is necessary for fertilization — in fact, it is needed not only
FIG. 117. Sections of portions of the gainetopliyte ()f Pteris, showing development of archegonium: A, young stage, showing cells which develop the neck (a), and the cell from which the egg cell and canal cells develop (6); B, an older stage, showing neck cells (a), neck canal cell (£>). 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), the ventral canal cell (c), and the cell in which the egg is organized (rf). — CALDWELL.
by the swimming sperm, but also to cause the opening of the antheridiuin and of the archegonium neck. There seems to be a relation between the necessity of water for fertilization and a prostrate, easily moistened gametophyte. Prothallia are either monoecious or dioecious (see § 69). When the prothallia are developing (Fig. 115) the anther-
FIG. 118. A fern (Aspidium), 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 ." is represented a section through a sorns. showing the sporangia attached and pro- tected by the indusium; while at « is represented 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. 116), and later the archegonia (Fig. 117). If the prothallium is poorly nour- ished, only antheridia appear ; it needs to be well developed and nourished to develop archegonia. There seems to be a very definite relation, therefore, between nutrition and the development of the two sex organs, a fact which must be remembered in connection with certain later develop- ments.
79. The sporophyte. — This complex body is differentiated into root, stem, and leaf, and is more highly organized than any plant body heretofore mentioned (Fig. 118). The development of this body and its three great working regions must be considered separately.
(1) Development of embryo. — The oospore, from which the sporophyte develops, rests in the venter of the arche- goninm, which at this stage resembles a depression in the
w
FIG. 119. Embryos of a common fern (Pteris): A, young embryo, showing direction of basal wall (I), and of second walls (77), which organize quadrants, each of which subsequently develops into foot (/), root (w), leaf (b). and stem (*); B, an older embryo, in which the four regions (lettered as in ^4) have developed further, also showing venter of archegonium (aw), and some tissue of the prothallium (pr). —A after KIENITZ-GKRLOPP; B after HOPMKISTER.
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,
138 PLANT STRUCTURES
is in connection with the asexual spores, which may be kept for a long time and then germinated.
The first step in germination is for the oospore to di- vide into two cells, forming a two-celled embryo. In the ordinary Ferns this first dividing wall is at right angles to the surface of the prothallium, and is called the basal wall (Fig. 119, A). One of the two cells, therefore, is anterior (toward the notch of the prothallium), and the other is posterior.
The two cells next divide by forming walls at right angles to the basal wall, and a four-celled embryo is the result. This is called the "quadrant stage" of the em- bryo, as each one of the four cells is like the quadrant of a sphere.
With the appearance of the quadrant, four body regions, are organized, each cell by its subsequent divisions giving rise to a distinct working region (Fig. 119, A). Two of the cells are inner (away from the substratum) ; also one of the inner and one of the outer (toward the substratum) cells are anterior ; while the two other inner and outer cells are posterior. The anterior outer cell develops the first leaf of the embryo, generally called the cotyledon (Fig. 119, b) ; 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.
PTERIDOPHYTES 139
111, 119), as it is in an unfavorable position in reference to the dorsiventral stem, which puts out a series of more favor- ably placed secondary roots into the soil (Fig. 118). The mature leafy sporophyte, therefore, has neither foot nor primary root, the product of two of the quadrants of the embryo having disappeared.
The secondary roots put out by the stem are small, and do not organize an extensive system, but they are interest- ing as representing the first appearance of true roots, which therefore come in with the vascular system. In the lower groups the root function of absorption is conducted by sim- ple hair-like processes called rhizoids ; but true roots are complex in structure and contain vessels.
(3) 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 the vas- cular system. These vessels extend continuously from root- tips, through the stem, and out into the leaves, where they
FIG. 120. A group of tropical plants. To the left of the center is a tree fern, with its slender columnar stem and crown of large leaves. The large-leaved plant* to the right are bananas (Monocotyledons).-From "Plant Relations."
PTEHIDOPHYTES
141
are spoken of as "leaf veins." Large working leaves and a vascular system, therefore, belong together and appeal- together; and1 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. There is a stalk-like portion (petiole) which rises from the subterranean stem, and a broad expanded portion (blade) exposed to the light and air (Fig. 118). In Ferns the blade is usually much branched, being cut up into segments of various sizes and forms.
The essential structure consists of an expansion of green tissue (mesophyll), through which strands of the vascular system (veins) branch, forming a supporting framework, and over all a compact layer of protecting cells (epidermis). A surface view of the epidermis shows that it is pierced by numer- ous peculiar pores, called xtnmata, 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 epidermis, pierced here and there by stomata ; between the epidermal lay- ers the cells of the mesophyll are packed : and among the mesophyll cells there may be seen here and there the cut ends of the veins. The leaf is usually a dorsiventral
FIG. 121. Some epidermal cells from leaf of Pleris, showing the interlocking walls and three stomata, the guard cells containing chloroplasts.— CAI.D- WEI.I..
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-
Fio. 122. Cross-section through a portion of the leaf of Pteris, showing the heavy- walled epidermis above and below, two stomata 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. — CAI.DWKLL.
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
TTERIDOPHYTES 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 scid 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 ma,y 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 sori. A sorus may be round or elongated, and is usually covered by a delicate flap (indushnn) 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 (Adiantum), and the common brake (Pteris), in which case they are protected by the inrolled
FIG. 123. Fragrant shield fern (Aspid- i>/m fragrans), showing general habit, and to the left (a) the under surface of a leaflet bearing sori covered by shield-like indusia.— After MARION SATTERI.EE.
FIG. 134. The bladder fern ( Oygtopterit >»/!/>- ifera), showing general habit, and to the right (a) the under surface of a leaflet, showing the dichotomous venation, and five sori protected by pouch-like indusia. —After MARION SATTERLKE.
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 fo- 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 fe'Tn ( Onoclea) (Figs. 127, 128), the climbing fern (Lygodinm), the royal fern (Osmunda), the moonwort (Botryclihim) (Fig. 129), and the adder's tongue (Ophioglossum) (Fig. 130).
An ordinary fern sporangium consists of a slender stalk and a bulbous top which is the spore case (Fig. 118, 6). This case has a delicate wall formed of a single layer of cells, and extending around it from the stalk and nearly to
FIG. 125. Leaflets of two common ferns : A. the common brake (Pterw): />. maidenhair (Adian- t a in}: both showing sori borne at the margin and protected by the infolded margin, which thus forms a false indusinm.— CALD-
WEl.L.
146 PLANT STKUCTURES
the stalk again, like a meridian line about a globe, is a row of peculiar cells with thick walls, forming a heavy ring, called the annulus. The annulus is like a bent spring, and when the delicate wall becomes yielding the spring straightens violently, the wall is torn, and the spores are discharged with considerable force (Fig. 131). This dis-
FIG. 126. — The pm-ple cliff brake (Pelliea atropiirpnrea\ showing general habit, and at a a single leaflet showing the dichotomoiis 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
PTERIDOPHYTES
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 fern ( Onodea striithiopteris), showing differentiation of foliage leaf (a) and sporophyll (6).— After MARION SATTERLKE.
cells sets the spores free in the cavity of the sporangium, and ready for discharge.
FIG. 128. The sensitive f ern ( Onodea sensibilis), showing differentiation of foliage leaves and sporophylls.— From "Field, Forest, and Wayside Flowers."
PTEBIDOPHYTE8
149
Among the Bryophytes the sporogenous tissue appears very early in the development of the sporogonium, the pro- duction of spores being its only function ; also there is a
*^x^
r\
FIG. 129. A moonwort (Botrychi- tim), showing the leaf differen- tiated into foliage and sporophyll branches.— After STHASBUHGEK.
28
FIG. 130. The adder's tongue ( Ophiogtoesum vulgaturri), showing two leaves, each with a foliage branch and a much longer sporophyll branch.— After MAKIOX SAT-
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
<~cs.
—
-'.CD .^
FIG. 131. A series showing the dehiscence of a fern sporangium, the rupture of the wall, the straightening and bending back of the annulus, and the recoil.— After ATKINSON.
secondary importance as compared with chlorophyll work ; and that the sporogenous tissue is far more restricted, the sporangia forming a very small part of the bulk of the sporophyte body.
PTERIDOPIIYTES 151
81. Heterospory. — This phenomenon appears first among Pteridophytes, but it is not characteristic of them, being en- tirely absent from the true Ferns, which far outnumber all other Pteridophytes. Its chief interest lies in the fact that it is universal among the Spermatophytes, and that it rep- resents the change which leads to the appearance of that high group. It is impossible to understand the greatest group of plants, therefore, without knowing something about heterospory. As it begins in simple fashion among Pteridophytes, and is probably the greatest contribution they have made to the evolution of the plant kingdom, unless it be the leafy sporophyte, it is best explained here.
In the ordinary Ferns all the spores in the sporangia are alike, and when they germinate each spore produces a prothallium upon which both antheridia and archegonia appear. It has been remarked, however, that some pro- thallia are 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 diiferent kinds of prothallia, which may be called male and female, as each is distinguished by the sex organ which it produces. As archegonia are only produced by well-nourished prothallia, it seems fair to sup- pose that the larger spores will produce female prothallia, and the smaller ones male prothallia.
This condition of things seems to have developed finally into a permanent and decided difference in the size of the spores, some being quite small and others relatively large, the small ones producing male gametophytes (prothallia with antheridia), and the large ones female gametophytes (prothallia with archegonia). When asexual spores differ thus permanently in size, and give rise to gametophytes of different sexes, we have the condition called heterospory ("spores different"), and such plants are called heterospo- rous (Fig. 139). In contrast with heterosporous plants, those in which the asexual spores appear alike are called homos-
152 PLANT STRUCTURES
porous, or sometimes isosporous, both terms meaning "spores similar." The corresponding noun form is homos- pory or isospory. Bryophytes and most Pteridophytes are homosporous, while some Pteridophytes and all Spermato- phytes are heterosporous.
It is convenient to distinguish by suitable names the two kinds of asexual spores produced by the sporangia of heterosporous plants (Fig. 139). The large ones are called megaspores, or by some writers macrospores, both terms meaning "large spores" ; the small ones are called micro- spores, or "small spores." It should be remembered that megaspores always produce female gametophytes, and mi- crospores male gametophytes.
This differentiation does not end with the spores, but soon involves the sporangia (Fig. 139). Some sporangia produce only megaspores, and are called 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 megaspore.
The differentiation goes still further. If the sporangia are born upon sporophylls, the sporophylls themselves may differentiate, some bearing only megasporangia, and others only microsporangia, the former being called megasporo- phylls, the latter microsporophylls. In such a case the sequence is as follows : megasporophylls produce megaspo- rangia, which produce megaspores, which in germination produce the female gametophytes (prothallia with archego- nia) ; while the microsporophylls produce microsporangia, which produce microspores, which in germination produce male gametophytes (prothallia with antheridia).
A formula may indicate the life history of a heteros- porous plant. The formula of homosporous plants with
PTERIDOPHYTES
alternation of generations (Bryophytes and most Pterido- phytes) was given as follows (§ 62) :
G=8> o— S— o— Gzzg> o— S— o— G=g> o— S, etc. In the case of heterosporous plants (some Pteridophytes and all Spermatophytes) it would be modified as follows :
G O^^ O O G O\ n C— — O G O^^ Q ^.4.^
G. — o > 0 — £> — o — G — o> 0 — o — o — G — o > O — £>> 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
154 PLANT STRUCTURES
lowest plants the gametophyte is represented by the whole plant structure. When the sporophyte first appears it is dependent upon the gametophyte (some Thallophytes and the Bry ophytes), 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.
CHAPTEE X
THE GREAT GROUPS OF PTERIDOPHYTES
82. The great groups. — At least three independent lines of Pteridophytes are recognized : (1) Filicales (Ferns), (2) Equisetales (Scouring rushes, Horsetails), and (3) Ly- copodiales (Club-mosses). The Ferns are much the most abundant, the Club-mosses are represented by a few hun- dred forms, while the Horsetails include only about twenty- five species. These three great groups are so unlike that they hardly seem to belong together in the same division of the plant kingdom.
FILICALES (Fern*}
83. General characters.— The Ferns were used in the preceding chapter as types of Pteridophytes, so that little need be added. They well deserve to stand as types, as they contain about four thousand of the four thousand five hundred species belonging to Pteridophytes. Although found in considerable numbers in temperate regions, their chief display is in the tropics, where they form a striking and characteristic feature of the vegetation. In the trop- ics not only are great masses of the low forms to be seen, from those with delicate and filmy moss like leaves to those with huge leaves, but also tree forms with cylindrical trunks encased by the rough remnants of fallen leaves and sometimes rising to a height of thirty-five to forty-five feet, with a great crown of leaves fifteen to twenty feet long (Fig. 120).
155
THE GREAT GROUPS OF PTERIDOPHYTES
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 eusporangiate.
The leptosporangiate Ferns are overwhelmingly abun- dant as compared with the Eusporangiates. Back in the Coal-measures, however, there was an abundant fern vege- tation which was probably all eusporangiate. The Lep- tosporangiates seem to be the modern Ferns, the once abundant Eusporangiates being represented now in the temperate regions only by such forms as moon wort (Bo-
158
PLANT STRUCTURES
trycMum) (Fig. 129) and adder's tongue (Opldoglossum) (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 comparatively large leaf. This leaf has a long erect petiole and a blade of four spread-
FIG. 133.— A water-fern (jfarsilia). showing horizontal stem, with descending roots, and ascend- ing leaves ; a, a young leaf showing circinate vernation : «,*,sporophyll branches ("spo- rocarps "). — After BISCHOFF.
FIG. 134. One of the floating water-ferns (Sal- vinia), 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.— Aft er BISCHOFF.
ing wedge-shaped leaflets like a " four-leaved clover. " The dichotomous venation and circinate vernation at once sug- gest the fern alliance. From near the base of the petiole
THE GREAT GROUPS OF PTERIDOPHYTES ^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 ruxlte*}
85. General characters. — The twenty-five forms now rep- resenting this great group belong to a single genus (Equise- tum, meaning "horsetail"), but they are but the linger- ing remnants of an abundant flora which lived in the time of the Coal-measures, and helped to form the forest vegeta- tion. The living forms are small and inconspicuous, but very characteristic in appearance. They grow in moist or dry places, sometimes in great abundance (Fig. 135).
The stem is slender and conspicuously jointed, the joints separating easily ; it is also green and fluted with small longitudinal ridges ; and there is such an abundant deposit of silica in the epidermis that the plants feel rough. This last property suggested its former use in scouring, and its name " scouring rush." At each joint is a sheath of minute leaves, more or less coalesced, the individual leaves some- times being indicated only by minute teeth. This arrange- ment of leaves in a circle about the joint is called the cyclic
FIG. 135. Equisetum arvense, a common horsetail: 1, three fertile shoots rising from the dorsiventral stem, showing the cycles of coalesced scale-leaves at the joints and the terminal strobili with numerous sporophylls, that at a being mature; 2, a sterile shoot from the same stem, showing branching; 3, a single peltate sporo- phyll bearing sporangia; 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 PTERIDOPHYTES
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).
86. The strobilus. — One of the distinguishing characters of the group is that chlorophyll-work and spore-formation are completely differentiated. Although the foliage leaves
--ar
FIG. 136. Dioecious gametophytes of Eqmsetum : A, the female gametopnyte, show- ing branching, rhizolds. and an archegonium (ar); B, the male gametophyte, showing several antheridia ( $ ).— After CAMPBELL.
are reduced to scales, and the chlorophyll-work is done by the stem, there are well-organized sporophylls. The sporo- phylls are grouped close together at the end of the stem in a compact conical cluster which is called a strobilus, 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
1(52 PLANT STRUCTURES
sporangia, which produce spores of but one kind, hence these plants are homosporous ; and as the sporangia origi- nate in eusporangiate fashion, Equisetum 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 (Club-mosses)
87. General characters. — This group is now represented by about five hundred species, most of which belong to the two genera Lycopodium and SelagineUa, 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 PTERIDOPHYTES
163
FIG. 137. A common club-moss (Lycopodium davatutn): 1, the whole plant, showing horizontal stem giving rise to roots and to erect branches bearing strobili; 2, a single sporophyll with its sporangium; 3, spores, much magnified. — After Wos-
8IDLO.
88. Lycopodium. — This genus contains fewer forms than the other, but they are larger and coarser and more charac- teristic of the temperate regions, being the ordinary Club- mosses (Fig. 137). They also more commonly display conspicuous and distinct strobili, although there is every
164
PLANT STRUCTURES
gradation between ordinary foliage leaves and distinct sporophylls. '
The sporangia are borne either by distinct sporophyils or by the ordinary foliage leaves near the summit of the stem. At the base of each of these leaves, or sporophylls, on the upper side, is a single sporangium (Fig. 137). The sporangia are eusporangiate in origin, and as the spores are all alike, Lycopodiitm has the same homosporous-eusporaii- 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-
FIG. 138. Selaginella, showing general spray-like habit, and dangling leafless stems which strike root (rhizophores).— From "Plant Relations."
ics, and are common in greenhouses as delicate, mossy, decorative plants (Fig. 138). In general the sporophylls are not different from the ordinary leaves (Fig. 139), but sometimes they are modified, though not so distinct as in certain species of Lycopodium^
THE GREAT GROUPS OF PTERIDOPI1YTE8
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. Selac/inella Martemii : A, branch bearing strobili; B, a microsporophyll with a microsporangium, showing microspores through a rupture in the wall; (', a megasporophyll with a megapporangium ; D, megaspores ; E, microspores. — CALDWELL.
29
166
PLANT STRUCTURES
leaf (Fig. 139). This is important as showing that sporan- gia may be produced by stems as well as by leaves, those being produced by leaves being called foliar, and those by stem cauline.
The most important ffact in connection with Selaginella, however, is that it is heterosporous. Megasporangia, each usually containing but four megaspores, are found in the axils of a few of the lower leaves of the strobilus, and more numerous microsporangia occur in the upper axils, con- taining very many microspores (Fig. 139). The character of the gametophytes of heterosporous Pteridophytes may be well illustrated by those of Selaginella.
The microspore germinates and forms a male gameto- phyte so small that it is entirely included within the spore
FIG. 140. Male gametophyte of Selaginella: in each case p is the prothallial cell, 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 PTERIDOPHYTES
167
ar
of water the antheridium wall breaks down, as also do the walls of the mother cells, and the small biciliate sperms are set free.
The much larger megaspores germinate and become filled with a mass of numerous nutritive cells, representing the ordinary cells of a prothallium (Fig. 141). The spore wall is broken by this growing prothallium, a part of which thus protrudes and becomes exposed, although the main part of it is still invested by the old megaspore wall. In this exposed portion of the female gameto- phyte the archegonia appear, and thus be- come accessible to the sperms. In the case of Isoetes (see § 90) the reduction of the female gametophyte is even greater, as it does not project from the megaspore wall at all, and the archegonia are made accessible through cracks in the wall immediately over them.
The embryo of Se- laginella 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, emb^ embj. It first develops an elongated cell, or row of cells, which thrusts the embryo cell deeper among the nutritive cells. This cell or row of cells, formed by the embryo to place the real embryo cell in better rela-
FIG. 141. Female gametophyte of a Selaginella : sjmi, wall of megaspore ; pr, gametophyte ; ar, an archegonium ; embl and emb%, em- bryo sporophytes ; et, suspensors ; the gam- etophyte has developed a few rhizoids. — After PFEFFER.
168
PLANT STRUCTURES
tion to its food supply, is called the suspensor, and is a temporary organ of the embryo (Figs. 141, 142, et). At the end of the suspensor the real embryo develops, and when its regions become organized it shows the following parts : (1) a large foot buried among the nutritive cells of the prothallium and absorbing nourishment ; (2) a root stretching out toward the substratum ; (3) a stem extend-
FIG. 142. Embryo of Selaginella removed from the gametophyte, showing suspensor (et), root-tip (w), foot (/), cotyledons (bl), stem-tip (fit), and ligules (lig).— After PFEFFEB.
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 Pteridophyte forms which has attracted special attention, as possibly representing one of the an- cestral forms of the Seed-plants.
THE GREAT GROUPS OF PTERIDOPHYTES
169
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 SCHENCK.
FIG. 144. Sperm of Isoetes, show- ing spiral body and seven long cilia arising from the beak. — After BELAJEFP.
with the Club-mosses, and is associated with Selaginella. It resembles a bunch of fine . grass growing in shoal water or in mud, but the leaves enlarge at the base and overlap one another and the very short tuberous stem (Fig. 143). Within each enlarged leaf base a single sporangium is formed, and the cluster contains both megasporangia and microsporangia. The sporangia are eusporangiate, and therefore Isoetes shares with Selaginella the distinction of
170 PLANT STRUCTURES
having the heterosporous-eusporangiate combination, which is a feature of the Seed-plants.
The embryo is also peculiar, and is so suggestive of the embryo of the Monocotyledons (see § 114) among Seed- plants that some regard it as possibly representing the ancestral forms of that group, of Spermatophytes. The peculiarity lies in the fact that at one end of the axis of the embryo is a root, and at the other the first leaf (cotyledon), while the stem tip rises as a lateral outgrowth. This is exactly the distinctive feature of the embryo of Monocoty- ledons.
The greatest obstacle in the way of associating these quillworts with the Club-mosses is the fact that their sperms are of the large and spirally coiled multiciliate type which belongs to Filicales and Equisetales (Fig. 144), and not at all the small 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.
CHAPTEK XI
SPERMATOPHYTES : GYMNOSPERMS
91. Summary from Pteridophytes. — In considering the important contributions of Pteridophytes to the evolution of the plant kingdom the following seem worthy of note :
(1) Prominence of sporophyte and development of vascu- lar system. — This prominence. is associated with the display of leaves for chlorophyll work, and the leaves necessitate the work of conduction, which is arranged for by the vas- cular system. This fact is true of the whole group.
(2) Differentiation of sporophylls. — The appearance of sporophylls as distinct from foliage leaves, and their or- ganization into the cluster known as the strobilus, are facts of prime importance. This differentiation appears more or less in all the great groups, but the strobilus is distinct only in Horsetails and Club-mosses.
(3) Introduction of heterospory and reduction of gameto- phytes. — Heterospory appears independently in all of the three great groups — in the water-ferns among the Fili- cales, in the ancient horsetails among the Equisetales, and in 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 heterospory lies in the fact that it leads to the development of Spermatophytes, and associated with it is a great reduction of the gametophytes, which project little, if at all, from the spores which produce them.
92. Summary of the four groups. — It may be well in this connection to give certain prominent characters which will
171
PL AST 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) ThallopJiytes. — Thallus body, but no archegonia.
(2) Bryophytes.— 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 Antliopliytes, meaning "Flowering plants," with the idea that they are distin- guished by the production of "flowers." A flower is diffi- cult to define, but in the popular sense all Spermatophytes do not produce flowers, while in another sense the strobilus of Pteridophytes is a flower. Hence the flower does not accurately limit the group, and the name Anthophytes is not in general use. Much more commonly the group is called Phanerogams (sometimes corrupted into 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
SPERMATOPHYTES : GYMNOSPERMS
arising from the fact that what were supposed to be sexual organs in Phanerogams have proved not to be such. The name Phanerogam, therefore, is being generally abandoned ; but the name Cryptogam is a useful one when the lower groups are to be referred to ; and the Pteridophytes are still very frequently called the Vascular Cryptogams. The most distinguishing mark of the group seems to be the production of seeds, and hence the name Spermatophytes, or " Seed-plants," is corning into general use.
The seed can be better defined after its development has been described, but it results from the fact that in this group the single megaspore is never discharged from its megasporangium, but germinates just where it is devel- oped. The great fact connected with the group, therefore, is the retention of the megaspore, which results in a seed. The full meaning of this will appear later.
There are two very independent lines of Seed-plants, the Gymnosperms and the Angiosperms. The first name means "naked seeds," referring to the fact that the seeds are always exposed; the second means " inclosed seeds," as the seeds are inclosed in a seed vessel.
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, although the pines still form extensive forests. The group is so diversified in its structure that all forms can not be included in a single description. The common pine (Pinus), therefore, will be taken as a type, to show the general Gymnosperm character.
174 PLANT STRUCTURES
95. The plant body. — The great body of the plant, often forming a large tree, is the sporophyte ; in fact, the gametophytes are not visible to ordinary observation. It should be remembered that the sporophyte is distinctly a sexless generation, and that it develops no sex organs. This great sporophyte body is elaborately organized for nutritive work, with its roots, stems, and leaves. These organs are very complex in structure, being made up of various tissue systems that are organized for special kinds of work. The leaves are the most variable organs, being differentiated into three distinct kinds — (1) foliage leaves, (2) scales, and (3) sporophylls.
96. Sporophylls. — The sporophylls are leaves set apart to produce sporangia, and in the pine they are arranged in a strobilus, as in the Horsetails and Club-mosses. As the group is heterosporous, however, there are two kinds of sporophylls and two kinds of strobili. One kind of strobilus is made up of megasporophylls bearing mega- sporangia ; the other is made up of microsporophylls bear- ing microsporangia. These strobili are often spoken of as the " flowers " of the pine, but if these are flowers, so are the strobili of Horsetails and Club-mosses.
97. Microsporophylls. — In the pines the strobilus com- posed of microsporophylls is comparatively small (Figs. 145, 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 pollen-sacs, and the microspores pollen grains, or simply pollen. These names are still very convenient to use in connection with the Spermatophytes, but it should be remembered that they are simply other names for structures found in the lower groups.
FIG. 145. Pinus Laricio, showing tip of branch bearing needle-leaves, scale-leaves, and cones (strobili): a. very young carpellate cones, at time of pollination, borne at tip of the young shoot upon which 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 starainate cones.— CALDWBLL.
176
PLANT STRUCTURES
The strobilus composed of microsporophylls may be called the staminate strobilus — that is, one composed of stamens ; it is often called the staminate cone, " cone " being the English translation of the word "strobilus." Frequently the staminate cone is spoken of as the " male cone/' as it was once supposed that the stamen is the
FIG. 146. Staminate cone (strobilus) of pine (Pinus): A, section of cone, showing microsporophylls (stamens) bearing microsporangia; J5, longitudinal section of a single stamen, showing the large sporangium beneath ; C, cross-section of a sta- men, showing the two sporangia; D, a single microspore (pollen grain) much en- larged, showing the two wings, and a male gametophyte of two cells, the lower and larger (wall cell) developing the pollen tube, the upper and smaller (genera- tive cell) giving rise to the sperms. — After SCHIMPER.
male organ. This name should, of course, be abandoned, as the stamen is now known to be a microsporophyll, which is an organ produced by the sporophyte, which never pro- duces sex organs. It should be borne distinctly in mind that the stamen is not a sex organ, for the literature of botany is full of this old assumption, and the beginner is in
SPERMATOPHYTES : GYMNOSPEKMS
177
danger of becoming confused and of forgetting that pollen grains are asexual spores.
98. Megasporophylls.— The strobili composed of mega- sporophylls become much larger than the others, forming
FIG. 147. Pinus sylvestris, showing mature cone partly sectioned, and showing car- pels (sq, gq1, sq*) with seeds in their axils (g), in which the embryos (em) may be distinguished; A, a young carpel with two megaspcrangia ; B, an old carpel with mature seeds (cK), the micropyle being below (M). — After BESSEY.
the well-known cones so characteristic of pines and their allies (Figs. 145, #, b, c, 163). Each sporophyll is some- what leaf -like, and at its base upon the upper side are two megasporangia (Fig. 147). It is these sporangia which are peculiar in each producing and retaining a solitary large megaspore. This megaspore resembles a sac-like cavity in
ITS
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 embryo- sac, because the young embryo was observed to develop within it (Fig. 147, em).
The strobilus of megasporophylls, therefore, may be called the carpellate strobilus 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 (integument) which is distinct above (Figs. 148 b, 149 i), covering the main part of the nucellus and projecting beyond its apex as a prominent neck,
FIG. 148. Diagram of the J "
carpel structures of pine, the passage through which to the apex showing the heavy scale of t^e nuceHUs is called the micropiile
(A) which bears the /,,,..., ,,N /T,. x 'J
ovule CB), in which are ("little gate") (Fig. 148, a). Cen- seen the micropyie (a), trally placed within the body of the
integument (6), nucellus ,,
(c), embryo sac or mega- HUCellus IS the COnSplCUOUS Cavity
spore (c?).— CALDWELL. called the embryo-sac (Fig. 148, d), in reality the retained megaspore.
The relations between integument, micropyie, nucellus, and embryo-sac should be kept clearly in mind. In the
8PEBMATOPHYTES : GYMNO8PERMS
179
nc—
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 Se- 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, within the ovule, as well as in
FIG. 149. Diagrammatic section through ovule (megasporangium) of sprnce (Picea), showing integument (i), nucellns (nc), endosperm or female gametophyte (e) which fills the large megaspore imbedded in the nucellus, two archegonia (a) with short neck (c) and venter containing the egg (0). and position of ger- minating pollen grains or microspores (p) whose tubes (t) penetrate the nucellns tissue and reach the archegonia. — After SCHIMPER.
So conspicuous a tissue the seed into which the
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. 149, a), making it sure that the endosperm is a female gameto- phyte. It is evident that the necks of these archegonia (Fig. 149, c) are shut away from the approach of sperms by swimming, and that some new method of approach must be developed.
100. Male gametophyte. — The microspores are developed in the sporangium in the usual tetrad fashion, and are pro- duced and scattered in very great abundance. It will be remembered that the male gametophyte developed by the microspore of Selaginella is contained entirely within the spore, and consists of a single ordinary prothallial cell and one antheridium (see § 89). In the pine it is no bet- ter developed. One or two small cells appear, which may be regarded as representing prothallial cells, while the rest of the gametophyte seems to be a single antheridium (Fig. 146, D). At first this antheridium seems to consist of a large cell called the 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-tube. Into this tube the male cells enter, and as it penetrates among the cells
SPERMATOPHYTES : GYM^OSPERMS
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 (D) : the tip of the tube is just about to enter the neck of the archegonium.— CALDWELL.
V
FIG. 151. Pollen tube passing through the neck of an archegonium of spruce (Picea), and containing near its tip the two male nuclei, which are to be discharged into the egg whose cytoplasm the tube is just en- tering.—After STRASBURGEK.
101. Fertilization. — Before fertilization can take place the pollen-grains (microspores) must be brought as near as possible to the female gametophyte with its archegonia. The spores are formed in very great abundance, are dry and powdery, and are scattered far and wide by the wind. In the pines and their allies the pollen-grains are winged (Fig. 146, Z>), so that they are well organized for wind dis- tribution. This transfer of pollen is called pollination, and those plants that use the wind as an agent of transfer are said to be anemophilous, or "wind-loving."
The pollen must reach the ovule, and to insure this it
must fall like rain. To aid in catching the falling pollen
the scale-like carpels of the cone spread apart, the pollen
grains slide down their sloping surfaces and collect in a
30
182
PLANT STRUCTURES
little drift at the bottom of each carpel, where the ovules are found (Fig. 147, A, B). The flaring lips of the micro- pyle roll inward and outward as they are dry or moist, and by this motion some of the pollen-grains are caught and pressed down upon the apex of the nucellus.
In this position the pollen-tube develops, crowds its way among the cells of the nucellus, reaches the wall of the embryo-sac, and penetrating that, reaches the necks of the archegonia (Fig. 149, p, t) ; crowding into them (Fig. 151), the tip of the tube opens, the male cells are
FIG. 152. Fertilization in spruce (PiceaY B is an egg, in the tip of which a pollen tube (p) has entered and has discharged into the cytoplasm a male nucleus (m), which is to unite with the egg (female) nucleus (on); C, a later stage in which the two nuclei are uniting.— After SCHIMPER.
discharged, one male cell fuses with the egg (Fig. 152), and fertilization is accomplished, an oospore being formed in the venter of the archegonium.
It will be noticed that the cell which acts as a male gamete is really the sperm mother cell, which does not organize a sperm in the absence of a water connection. This peculiar method of transferring the male cells by means of a special tube developed by the antheridium is
SPEEMATOPHYTES : GYMNOSPERMS
183
Tfa
called siphonoga-my, which means " sexual reproduction by means of a tube." So important is this character among Spermatophytes that some have proposed to call the group Siphonogams.
102. Development of the embryo. — The oospore Avhen formed lies at the surface of the endosperm (female gameto- phyte) nearest to the micropyle. As the endosperm is to supply nourishment to the em- bryo, this position is not the
most favorable. Therefore, as in Selaginella, the oospore first develops a suspensor, which in pine and its allies becomes very long and often tortuous (Fig. 153, 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, vl, Jed), 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. 153a). The development of more than one embryo in a megasporaiigium (ovule) is called polyembryony, a phenomenon natural to Gymnosperms with their several archegonia upon a single gametophyte.
103. The seed. — While the embryo is developing some important changes are taking place in the ovule outside of the endosperm. The most noteworthy is the change which transforms the integument into a hard bony covering,
I- a
FIG. 153. Embryos of pine : .1. very young embryos (kg) at the tips of long and contorted sus- pensors (s) ; B, older embryo, showing attachment to suspen- sor (s), the extensive root sheath (wK), root tip (ws), stem tip (v), and cotyledons (c).— After STRASBURGER.
184
PLANT STRUCTURES
known as the seed coat, or testa (Fig. 153&). The devel- opment of this testa hermetically seals the structures with- in, further development and activity are checked, and the living cells pass into the resting condition. This pro- 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. 153a. Pine seed.
FIG. 154. Pine seedlings, showing the long hypocotyl and the numerous cotyledons, with the old seed case still attached.— After ATKINSON.
SPERM ATOPHYTES : 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 habit, with ranch branched leaves and scaly stem. — From " Plant Relations."
the intra-seminal development. In this condition the em- bryo may continue for a very long time, and it is a ques- tion whether it is death or suspended animation. Is a seed alive ? is not an easy question to answer, for it may be kept in a dried-out condition for years, and then when placed in suitable conditions awaken and put forth a living plant.
SPERMATOPHYTES : GYMNOSPERMS
187
This " awakening " of the seed is spoken of as its " ger- mination/' but this must not be confused with the germi- nation of a spore, which is real germination. In the case of the seed an oospore has germinated and formed an embryo, which stops growing for a time, and then resumes it. This resumption of growth is not germination, but is what
FIG. 157. Tip of pollen tube of Cycas revoluta, containing the two spiral, multiciliate sperms. — After IKENO
happens when a seed is said to "germinate." This second period of development is known as the extra-seminal, for it is inaugurated by the escape of the sporophyte from the seed (Fig. 154).
104. The great groups of Gymnosperms. — There are at least four living groups of Gymnosperms, and two or three
- ,-_-:- -
FIG. 158. A pine (Pinus) showing the central shaft and also the bunching of the needle leaves toward the tips of the branches.— From " Plant Relations."
SI'KKMATOIMIVTKS: GYMNOSPERMS
189
extinct ones. The groups differ so widely from one an- other in habit as to show that Gymnosperms can be very much diversified. They are all woody forms, but they may be trailing or straggling shrubs, gigantic trees, or high-climbing vines ; and their leaves may be nee- dle-like, broad, or "fern- like." For our purpose it will be only necessary to define the two most prom- inent groups.
105. Cycads. — Cycads are tropical, fern -like forms, with large branched (compound) leaves. The stem is either a columnar shaft crowned with a ro- sette of great branching leaves, with the general habit of tree-ferns and palms (Figs. 155, 156) ; or they are like great tu- bers, crowned in the same way. In ancient times (the Mesozoic) they were very abundant, forming a conspicuous feature of the vegetation, but now they are represented only by about eighty forms scattered through both the oriental and occiden- tal tropics. FIG. 159. The giant redwood (Sequoia gi-
TheV are VerV fern- ffantea) of California : the relative size
J * is indicated by the figure of a man stand-
like in Structure as Well ing at the right.— After WILLIAMSON.
190
PLANT STRUCTURES
as in appearance, but they prod associated with Spermatophytes, posed they are Gymnosperms. A
FIG. 160. An araucarian pine (Araucaria), showing the central shaft, and the regular cycles of branches spreading in every direc- tion and bearing numerous small leaves. — From "Plant Relations."
uce seeds arid must be and as the seed is ex- discovery has been made recently that strikingly emphasizes their fern- like structure. In fer- tilization a pollen-tube develops, as described for pine and its allies, but the male cells (sperm mother - cells) which it contains or- ganize sperms, and these sperms are of the coiled multiciliate type (Fig. 157) charac- teristic of all the Pter- idophytes except Club- mosses. This associa- tion of the old ciliated sperm habit with the new pollen-tube habit is a very interesting in- termediate or transition condition. It should be said that these sperms have been actually found in but two species of the Cycads, but there are reasons for suppos- ing that they may be found in all. Another one of the Gymnosperm groups, represented to- day only by the com- monly cultivated maid-
SPERMATOPHYTES : GYMXOSPERMS
191
enhair tree (Oingko), with broad dichotomously veined leaves, also develops multiciliate sperms.
The testa of the seed, instead of being entirely hard as described for pine and its allies, develops in two layers, the inner hard and bony, and the outer pulpy, making the ripe fruit resemble a plum.
106. Conifers. — This is the great modern Gymnosperm group, and is characteristic of the temperate regions, where it forms great forests. Some of the forms are widely dis- tributed, as the great genus of pines (Pinus) (Fig. 158), while some are now very much restricted, although for- merly very widely distributed, as the gigantic redwoods (Sequoia) of the Pacific slope (Fig. 150). 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 (.<?/>), 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 general outline being distinctly conical. The larch is peculiar among Conifers in periodically shedding its leaves.— From " Plant Relations."
SPERMATOPHYTE8 : GYMJfQSPEBMS
193
the case of the common larch or tamarack, which sheds its leaves every season (Fig. 162). There are Conifers, also, which do not produce needle-leaves, as in the com- mon arbor-vitae, whose leaves consist of small closely-over- lapping scale-like bodies (Fig. 163).
The two types of leaf arrangement may also be noted. In most Conifers the leaves are arranged along the stem in spiral fashion, no two leaves being at the same level. This is known as the spi- ral or alternate arrange- ment. In other forms, as the cypresses, the leaves are in cycles, as was men- tioned in connection with the Horsetails, the ar- rangement being known as the cyclic or whorled.
The character which gives name to the group is the "cone" — that is, the prominent carpellate cone which becomes so
FIG. 163. Arbor-vitae (Thuja}, showing a branch with scaly overlapping leaves, and some carpellate cones (strobili). — After EICHLER.
conspicuous in connec- tion with the ripening of the seeds. These cones generally ripen dry and
hard (Figs. 145, 147, 163), but sometimes, as in junipers, they become pulpy (Fig. 164), the whole cone forming the so-called "berry/'
There are two great groups of Conifers. One, repre- sented by the pines, has true cones which conceal the
194 PLANT 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.
FIG. 164. The common juniper (Juniperus communis); the branch to the left bearing staminate etrobili; that to the right bearing stamina tc strobili above and carpel- late strobili below, which latter have matured into the fleshy, berry-like fniit. — After BERG and SCHMIDT.
CHAPTER XII
SPERMATOPHYTES : ANGIOSPERMS
107. Summary of Gymnosperms, — Before beginning An- giosperms it is well to state clearly the characters of Gym- nosperms which have set them apart as a distinct group of Spermatophytes, and which serve to contrast them with Angiosperms.
(1) The microspore (pollen-grain) by wind-pollination is brought into contact with the megasporangium (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 entomophilous ("insect-loving") plants. This does not mean that all Angiosperms are entomophilous, for some are still wind-pollinated, but that the group is prevailingly ento- mophilous. This fact, more than anything else, has re- sulted in a vast variety in the structure of flowers, so char- acteristic of the group.
109. The plant body. — This of course is a sporophyte, the gametophytes being minute and concealed, as in Gym- nosperms. 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 ,c 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-
FIU. 165. stamens of hen- iar to Angiosperms, making the true bane (Hyotcyamuft) : A, flower, and being associated with en-
front view, showing fila- ^V^l •,
rnent,/) and anther („,; tomophlly.
110. Microsporophylls. — The micro-
JL back view, showing
the connective (c) be- tween the pollen-sacs.
the connective (c) be- sporopliyll Qf Angiosperms is
J
-After SCHIMPBR.
definitely known as a " stamen " than
SPERMATOPHYTES : AXGIOSPERMS
19'
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, 167«).
FIG. 166. Cross-section of anther of thorn apple (Datura), .showing the four imbedded sporangia (a. p) containing microspores?; the pair on each side will merge and dehisce along the depression between them for the discharge of pollen. — After FRANK.
The filament may be long or short, slender or broad, or variously modified, or even wanting. The anther is simply the region of the sporophyll which bears sporangia, and is
JJIH
FIG. 167. Diagrammatic cross sections of anthers: A, younger stage, showing the four imbedded sporangia, the contents of two removed, but the other two con- taining pollen mother cells (pm) surrounded by the tapetum (t); 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 LI-KKSSKX.
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. 166, 167). When they reach maturity, the paired sporangia on each side usually merge to- gether, forming two spore-containing cavities (Fig. 167, B). These are generally called " pollen-sacs," and each anther is said to consist of two pollen-sacs, although each sac is made up of two merged sporangia, and is not the equivalent of the pollen-sac in Gymnosperms, which is a single sporangium.
FIG. I67a. Various forms of stamens : A, from Solannm, showing dehiscence by terminal pores; J5, from Arbutus, showing anthers with terminal pores and "horns"; C, from Berbens; D, from Atherosperma, showing dehiscence by uplifted valves; E, from Aquilegia, showing longitudinal dehiscence; F, from Popowia, showing pollen-sacs near the middle of the stamen. — After ENGLER and FRANTIC
SPERMATOPIIYTES : ANGIOSPEKMS
199
FIG. 168. Cross - section of anther of a lily (Butomus), showing the separating walls between the members of each pair of sporangia broken down at z, forming a con- tinuous cavity (pollen sac) which opens by a longitudi- nal slit.— After SACHS.
The opening of the pollen-sac to discharge its pollen- grains (microspores) is called dehiscence, which means "a splitting open," and the methods of dehiscence are various (Fig. 167$). By far the most common method is for the wall of each sac to split lengthwise (Fig. 168), which is called longitudinal dehiscence; an- other is for each sac to open by a terminal pore (Fig. 1670), in which case it may be prolonged above into a tube.
111. Megasporophylls. — These are the so-called " carpels " of Seed- plants, and in Angiosperms they are organized in various ways, but always so as to inclose the mega- sporangia (ovules). In the simplest
cases each carpel is independent (Fig. 169, A), and is dif- ferentiated into three regions : (1) a hollow bulbous base,
which contains the ovules and is the real seed case, known as the ovary ; (2) sur- mounting this is a slender more or less elongated process, the style; and (3) usually at or near the apex of the style a special receptive surface for the pol- len, the stigma.
In other cases several carpels to-
FIG. 169. Types of pistils : A, three simple pistils (apocarpous), each showing ovary and style tipped with stigma ; B, a compound pistil (syncarpous), showing ovary (/), separate styles (g), and stigmas (n) ; C, a compound pistil (syncarpous). showing ovary (f), single style (g), and stigma (n). — After BERG and SCHMIDT.
200
PL AST STRUCTURES
gether form a common ovary, while the styles may also combine to form one style (Fig. 169, C), or they may remain more or less distinct (Fig. 169, 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.— After SCHIM-
PER.
therefore, may have one loculus or several loculi. Where there are several loculi each one usually represents a con- stituent carpel (Fig. 170, B) ; where there is one loculus the ovary may comprise one carpel (Fig. 169, ^1), or several (Fig. 170, .4).
There is a very convenient but not a scientific word, which stands for any organization of the ovary and the accompanying parts, and that is pistil. A pistil may be one carpel (Fig. 169, A), or it may be several carpels or- ganized together (Fig. 169, B, (7), the former case being a simple pistil, the latter a compound pistil. In other words,
SPEKMATOPHYTES : AKGlOSPEEMS
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 tire 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). Xo trace of the
FKJ. 171. A diagrammatic section of an ovule of Angiosperms, showing outer integument (ai), inner integument (ii), micropyle (m), nucellus (&), and embryo sac or megaspore (em). — After SACHS.
FIG. 172. Germination of microspore (pollen grain) in duckweed (Lemna): A, mature spore with its nucleus; B. nucleus of spore dividing: C, two nuclei resulting from the division; Z>, 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. — CALPWELI..
202
PLANT STRUCTURES
ordinary nutritive cells of the gametophyte remains, and the whole structure seems to represent a single antherid- ium. At first it consists of two cells, the large wall cell and the small free generative cell (Fig. 172, D). Later
the generative cell di- vides (Fig. 172, 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 through the
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, ii, 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 stigmatic surface, en-
LUERSSEN. >
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
SPERM ATOPIIYTES : 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 shows two distinct
FIG. 174. Lilium Philadelphicum : to the left a young megasporangium (ovule), showing integuments ( C). nucellus (A), and megaspore (B) containing a large nu- cleus. To the right a megaspore whose nucleus is undergoing the first division in the formation of the gametophyte.— CALDWELL.
periods, separated from one another by the act of fertiliza- tion. If fertilization is not accomplished the second stage of the gametophyte is usually not developed.
First period. — The megaspore nucleus divides (Fig. 174), and one nucleus passes to each end of the embryo-
204
PLANT STRUCTURES
sac (Fig. 175, at left). Each of these nuclei dhide (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. Lilium Phtiadelphicum : to the left is an embryo-sac with a gametophyte nucleus in each end; to the right these two nuclei are dividing to form the two nuclei shown in each end of the sac in the middle figure.— CALDWELL.
(Fig. 176, at left), and four nuclei appear at each end (Fig. 176, at middle). When eight nuclei have appeared, nuclear division stops. Then a remarkable phenomenon occurs. One nucleus from each end, the two being called "polar nuclei," moves toward the center of the sac, the two meet and fuse (Fig. 176, at right, C), and a single large nucleus is the result.
The three nuclei at the end of the sac nearest the micro- pyle are organized into cells, each being definitely sur- rounded by cytoplasm, but there is no wall and the cells remain naked but distinct. These three cells constitute the egg-apparatus (Fig. 176, at right, A), the central one, which usually hangs lower in the sac than the others, being the egg, the two others being the synergids, or "helpers." Here, therefore, is an egg without an archegonium, a dis- tinguishing feature of Angiosperms.
SPERMAT OPLIYTES : ANGIO8PERMS
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 antipodal 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 gametophyte about ready for fertilization, with the egg apparatus (A) composed of the two synergids and egg (central and lower), the three antipo- dal cells (B), and the two polar nuclei fusing (C) to form the primary endosperm nucleus. — CALDWELL.
polar nuclei, is known as the primary 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 (E).— After DUGGAR.
This completes the first period of gametophyte de- velopment, and it is ready for fertilization.
Fertilization. — The pollen-tube, carrying the two male cells, has passed down the style and en- tered the micropyle (Fig. 173). It then reaches the wall of the embryo -sac, pierces it, and is in con- tact with the egg -appa- ratus. Usually it passes along the side of one of the synergids (Fig. 177), feeding upon and disor- ganizing it. When it comes near the conspicu- ous nucleus of the egg, the tip of the tube breaks and one male cell is dis- charged into the cyto- plasm of the egg (Fig. 178). The egg and the male cell now fuse, and an oospore, which invests itself with a wall, is the result.
Second period. — After fertilization the gameto- phyte begins its second period of development. The primary endosperm nucleus begins a series of divisions, and as a result
SPEKMATOPHYTES : ANGIOSPEKMS
L><>7
the sac becomes more or less filled with nutritive cells, which are often organized into a compact tissue (Fig. 179). These nutri- tive cells correspond to the endo- sperm of Gymnosperms, and re- ceive the same name. In Gymno- sperms, therefore, the endosperm (the nutritive body of the female gametophyte) is mainly formed before fertilization, while in An- giosperms it is mainly formed after fertilization. This means that in Angiosperms eggs are formed and fertilization takes place in a very young gameto- phyte, while in Gymnosperms and heterosporous Pteridophytes the eggs appear much later.
The antipodal cells also proba- bly represent nutritive cells of the gametophyte. Sometimes they dis-
FIG. 178. End of embryo-sac of lily (Lilium Philadelphia/m): a pollen tube has entered the sac and has discharged a male cell, whose nucleus is seen uniting with the nucleus of the egg ; near the tip of the tube is the disorganizing nu- cleus of one of the synergids. — CALDWELL.
FIG. 179. One end of the embryo-sac in wake-robin (Trittium). showing endosperm (shaded cells) in which a young embryo is imbedded. — After ATKINSON.
208
PLANT STRUCTURES
appear very soon after they are formed ; but sometimes they become very active and even divide and form a con- siderable amount of tissue, aiding the endosperm in nour- ishing the young embryo.
114. Development of embryo. — While the endosperm is forming, the oospore has germinated and the sporophyte embryo is developing (Fig. 180). Usually a suspensor, more or less distinct, but never so prominent as in Gymnosperms,
is formed ; at the end of it the embryo is developed (Fig. 181), which, when completed, is more or less surrounded by nourish- ing endosperm (Fig. 183).
The two groups of Angio- sperms differ widely in the struc- ture of the embryo. In Mono- cotyledons the axis of the em- bryo develops the root-tip at one end and the " seed-leaf " (coty- ledon) at the other, the stem-tip arising from the side of the axis as a lateral member (Fig. 182). This relation of organs recalls the embryo of Isoetes (see § 90). Xaturally 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 Selaginella (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 off by a partition, and scattered through the sac a few free en- dosperm cells. — After SCHAFF-
NER.
SPERM ATOPHYTE8 : ANGIOSPEKMS
209
several cotyledons in a cycle (Fig. 154) ; and in Dicotyle- dons there may be one or three cotyledons ; but as a pair of opposite cotyledons is almost without exception in the group, it is named Dicotyledons.
The axis of the embryo between the root-tip and the cotyledons is called the hypocotyl (Figs. 154, 193, 194), which
FIG. 181. Development of embryo of shepherd's purse (Capsella), a Dicotyledon, beginning with 7, the youngest stage, and following the sequence to VI, the old- est stage, v represents the stispensor, 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 caulide or radicle, In Dicotyledons the stem-tip between the coty-
210
PLANT STKUCTURES
ledons often organizes the rudiments of subsequent leaves, forming a little bud which is called the plumule.
Embryos differ much as to com- pleteness of their development within the seed. In some plants, especially those which are parasitic or sapro- phytic, the embryo is merely a small mass of cells, without any organiza- tion of root, stem, or leaf. In many cases the embryo becomes highly de- veloped, the endosperm being used up and the cotyledons stuffed with food material, the plumule contain- ing several well - organized young leaves, and the embryo completely filling the seed cavity. The com- mon bean is a good illustration of this last case, the whole seed within 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 integument or integu- ments are becoming transformed into the testa (Fig. 183). When this hard coat is fully developed, the activities within cease, and the whole structure passes into that con- dition of suspended animation which is so little under- stood, and which may continue for a long time.
The testa is variously developed in seeds, sometimes being smooth and glistening, sometimes pitted, sometimes rough with warts or ridges. Sometimes prominent append- ages are produced which assist in seed-dispersal, as the wings in Catalpa or Bignonia (Fig. 184), or the tufts of
PIG. 182. Young 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 ATOPHYTES : 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 (po, seed testa (sc), nucellus tissue (p), endosperm (en), and embryo (em).— After ATKINSON.
hair on the seeds of milkweed, cotton, or fireweed (Fig. 185). For a fuller account of the methods of seed-dispersal see Plant Relations, Chapter VI.
FIG. 184. A winged seed of Bignonia.— After STRASBURGER.
116. The fruit. — The effect of fertilization is felt beyond the boundaries of the ovule, which forms the seed. The ovary is also involved, and becomes more or less modified. It enlarges more or less, sometimes becoming remarkably enlarged. It also changes in structure, often becoming hard or parchment-like. In case it contains several or numerous seeds, it is organized to open in some way and discharge them, as in the ordinary pods and capsules (Fig. 185). In case there is but one seed, the modified ovary
212
PLAXT STRUCTURES
wall may invest it as closely as another integument, and a seed-like fruit is the result — a fruit which never opens and is practically a seed. Such a fruit is known as an akene, and is very characteristic of the greatest Angiosperm family, the Composite, to which sunflowers, asters, golden- rods, daisies, thistles, dandelions, 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 modifie d
Calyx (one Of the FIQ. 1S6. Winged fruit of maple.— After RERNI;R,
FIG. 1&5. A pod of fireweed (Epilobium) opening and exposing its plumed seeds which are transported by the wind.— After BKAL.
SPERMATOPIIYTES : ANGIOSPEKMS
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 ^j flower, a whole flower-cluster, with its axis and bracts, becoming j j an enlarged pulpy mass, as in the pineapple (Fig. 192).
The term "fruit," therefore,
FIG. 187. A ripe dandelion head, showing the mass of plumes, a few seed-like fruits (akenes) with their plumes still attached to the receptacle, and two fallen off.— After KERNEK.
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
PLANT STRUCTURES
is a very indefinite one, so far as the structures it includes are concerned. It is simply an effect which follows fer- tilization, and involves more or less of the structures adja-
A
FIG. 190. Fruit of nutmeg (Myristica) : A, section of fruit, showing seed within the heavy wall ; B, section of seed, showing peculiar convoluted and hard endosperm (m) in which an embryo (n) is imbedded — After BERG and SCHMIDT.
cent to the seeds. As has been seen, this effect may extend only to the ovary wall, or it may include the calyx, or it 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.
SPEKMATOPHYTES : ANGIOSPEKMS
215
rowing animals, etc. The period of rest may be long or short, but sooner or later, under the influence of moisture, suitable temperature, and oxygen the quiescent seed begins to show signs of life.
The sporophyte within begins to grow, and the seed coat is broken or penetrated through some thin spot or
FIG. 192. Pineapple: A, the cluster of fruits forming the so-called "fruit"; B, single flower, showing small calyx and more prominent corolla; C, section of flower, showing the floral organs arising above the ovary (epigynous). — A, B after KOCH; C after LB MAOUT and DECAISNE.
opening. The root-tip emerges first, is protruded still farther by the rapid elongation of the hypocotyl, soon curves toward the earth, penetrates the soil, 'and sending out rootlets, becomes anchored. After anchorage in the
216
PLANT STRUCTURES
soil, the hypocotyl again rapidly elongates and develops a strong arch, one of whose limbs is anchored, and the other is pulling upon the cotyledons (Fig. 193). This pull finally frees the cotyledons, the hypocotyl straightens, the cotyle-
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
SPERMATOPHYTES : AXtHOSPERMS
cotyledons, and the seed coat containing, the cotyledons remains like a lateral appendage upon the straightened axis.
It is also to be observed in many cases that the young root system, after gripping the soil, contracts, drawing the young plant deeper into the ground.
118. Summary from Angio- sperms. — At the beginning of this chapter (§ 107) the characters of the Gymnosperms were summar- ized which distinguished them from Angiosperms, whose con- trasting characters may be stated as follows :
(1) The microspore (pollen- grain), chiefly by insect pollina- tion, is brought into contact with the stigma, which is a receptive region on the surface of the car- pel, and there develops the pollen- tube, which penetrates the style to reach the ovary cavity which contains the ovules (megasporan- gia). The impossibility of con- tact between pollen and ovule im- plies inclosed ovules and hence seeds, and therefore the name " Angiosperm."
(2) The female gametophyte is but slightly developed before fertilization, the egg appearing very early.
(3) The female gametophyte produces no archegonia, but a single naked egg.
FIG. 194. Seedling of hazel ( Car- pinus), showing primary root (hw) bearing rootlets (sw) upon which are numerous root hairs (r), hypocoiyl (h), cotyledons (c). young stem (e), and first (I) and second (I') true leaves. — After SCUIM-
* PEH.
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 efficient for its purpose.
This evolution of the flower has proceeded along many lines, and has resulted in endless diversity of structure. These diversities are largely used in the classification of Angiosperms, as it is supposed that near relatives are indi- cated by similar floral structures, as well as by other fea- tures. The significance of these diversities is supposed to be connected with securing proper pollination, chiefly by insects, and favorable seed distribution.
Although the evolution of flowers has proceeded along several lines simultaneously, now one feature and now another being emphasized, it will be clearer to trace some of the important lines separately.
120. Floral leaves. — In the simplest flowers floral leaves do not appear, and the flower is represented only by the sporophylls. Both kinds of sporophylls may be associated, in which case the flower is said to be perfect (Fig. 195) ; or they may not both occur in the same flower, in which case one flower is staminate and the other 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. 195. Lizard's tail (Saururus}: A, tip of branch bearing leaves and elongated cluster of flowers; B, a single naked flower from A, showing sta- mens and four spreading and stigmatic styles; (7, flower from another species, showing sub- tending bract, absence of floral leaves, seven stamens, and a syncarpous pistil ; the flowers naked and perfect.— After ENGLEK.
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. 197. Flower of calamus (Acorus), showing simple perianth, stamens, and syn- carpous pistil: a hypogynous flower without differentiation of calyx and corolla.— After ENGLER.
B
FIG. 199. Common flax (Linum) : A. entire flower, showing calyx and corolla ; B. floral leaves re- moved, showing stamens and syucarpous pistil ; C, a mature capsule splitting open. — After
SCHIMPER.
FIG. 198. Flowers of elm (Vlmns) : A, branch bearing clusters of flowers and scaly buds ; B, single flower, showing simple perianth and stamens, being a stamii ate flower ; C,
flower showing perianth, stamens, and the two divergent styles stigmatic on inner surface, being a perfect 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 STRASBURGEU.
THE FLOWER
221
the young sporophylls, but they are all alike, forming what is called the perianth (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 regard the missing set FIG 201 _An easter.niy. a Monocotyledon,
as the Corolla, SUCh showing perianth (a), stamens (6), stigma (c),
flowers being called flowe^ bu<Yrf)' »nd .\carPel a^rt the. peri'
anth has fallen (/), with its knob-like stigma, long style, and slender ovary. -CALDW ELL.
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-
a/ '"' *
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 FLO WEE
223
appear to be thrown into rosettes or cycles. This change does not necessarily affect all the parts simultaneously. For example, in the common buttercup the sepals and petals are nearly in cycles, while the carpels are spirally arranged and indefinitely numerous on the head-like recep- tacle (Fig. 202). On the other hand, in the common water-
FIG. 203.
Flower of water-lily (Nymphced), showing numerous petals and stamens. — After CASPARY.
lily the petals and stamens are spiral, and indefinitely re- peated, while the sepals and carpels are approximately cyclic (Fig. 203).
Finally, in the highest forms, all the floral organs are in definite cycles, and there is no indefinite repetition of any part. All through this evolution from the spiral to the cyclic arrangement there is constantly appearing a tend- ency to " settle down " to certain definite numbers. When the complete cyclic arrangement is finally established these numbers are established, and they are characteristic of great groups. In cyclic Monocotyledons there are nearly always just three organs in each cycle, forming what is called a trimerous flower (Fig. 204) ; while in cyclic Dicot-
224
PLANT STRUCTURES
yledons the number five prevails, but often four appears, forming pentamerous or tetramerous flowers (Fig. 199). This does not mean that there are necessarily just three, four, or five of each organ in the flower, for there may be two or more cycles of some one organ. For example, in the common lily there are six floral leaves in two sets, six sta- mens in two sets, and three carpels (Fig. 204).
In the cyclic flowers it is also to be noted that each set alternates with the next set outside (Fig. 204). The petals
are not directly opposite the se- pals, but are opposite the spaces between sepals ; the stamens in turn alternate with the petals ; if there is a second set of stamens, 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 between 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 epigynous flowers. — In the simpler flowers the sepals, petals, 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 (insertion) of the other parts, such a flower is often said to have a "superior ovary." The more usual term, however, is hypogynous, meaning in effect "under the ovary," refer-
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 scts\ being a trimerous, pentacyclic flow- er.— After SCHIMPER.
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
Fi(i. 205. Flowers of Rose family: /, a hypogynous flower of Potentilla, sepals, petals, and stamens arising from beneath the head of carpels; 2, a pi-rigynous flower of Alchemilla, sepals, petals, and stamens arising from rim of urn-like pro- longation of the receptacle, which surrounds the carpel ; 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 be perigynous (Figs. 205, .2, 206), meaning " around the pistil." Finally, the insertion is carried above the ovary, and sepals, petals, and stamens seem to arise from the top of the ovary (Fig. 205, 3), such a flower being epigynous, the outer parts appearing "upon the ovary." In such a case the ovary does not appear within the flower, but below it (Figs. 205, 252, 261), and the flower is often said to have an "inferior ovary."
123. Apocarpous to syncarpous flowers. — In the simpler flowers the carpels are entirely distinct, each carpel organ-
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 apocarpous, meaning "carpels separate." There is a very strong tendency,
FIG. 206. Sweet-scented shrub (Calycanthus): A, tip of branch bearing flowers; B, section through flower, showing numerous floral leaves, stamens, and carpels, and also the development of the receptacle about the carpels, making a perigynous flower. — After THIEBAULT.
however, for the carpels of a flower to organize together and form a single compound pistil. In such a flower there may be several carpels, but they all appear as one organ (Figs. 195, C, 197, 198, A 199, B), and the flower is said to be syncarpous, meaning "carpels together."
124. Polypetalous to sympetalous flowers. — The tendency for parts of the same set to coalesce is not confined to the carpels. Sepals often coalesce (Fig. 208), and sometimes stamens, but the coalescence of petals seems to be more important. Among the lower forms the petals are entirely separated (Figs. 199, A, 202, 203, 207), a condition which
THE FLOWER
227
has received a variety of names, but probably the most common is poly- 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 sympetalous, meaning "petals united." The words gamo- petalous and monopetalous 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 straw- berry, showing sepals, pet- als, numerous stamens, and head of carpels ; the flower is actinomorphic, hypogynous, and with no coalescence of parts. — Af- ter BAILEY.
FIG. 208. A flower of the tobacco plant: a, a complete flower, showing the calyx with its sepals blended below, the funnelform corolla made up of united petals, and the stamens just showing at the mouth of the corolla tube; b. 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 STRASBTJRGER.
228
PLANT STRUCTUEES
ated into two regions (Fig. 210, b), a more or less tubular portion, the tube, and a more or less flaring portion, the limb.
125. Actinomorphic to zygomorphic flow- ers. — In the simpler flowers all the mem- bers of any one cycle are alike ; the petals are all alike, the stamens are all alike, etc. Looking at the center of the flower, all the parts are re- peated about it like the parts of a radi- ate animal. Such a flower is actinomor-
phic meaning " ra- "T . „ , . „ Cliate, and IS Olten
called a ' ' regular
fl » Although
LOWer.
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. Flower of morning -glory (Ipomced), with sympetalous corolla split open, showing the five attached stamens, and the superior ovary with prominent style and stigma ; the flower is by- pogynous, sympetalous, and actinomorphic.- After MEISSNEB.
FIG. 210. A group of sympetalous flower forms: a. a flower of harebell, showing a bell-shaped corolla; 6, 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 zygomorphic., 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 tricolor): A, section showing sepals (I, /')• petals (c) one of which produces a spur (oo. the flower being zygomor- phic; B, mature fruit (a capsule) and persistent calyx (£); C, the three boat-shaped valves of the fruit open, most of the seeds (*) having been discharged. — After SACHS.
FK;. -21-2. Flower of a mint (Mtut/ta aquaticar. A, the entire flower, showing calyx of united sepals, unequal petals, stamens, and style with two stigma lobes: B. a corolla split open, showing petals united and the four stamens attached to the tube; the flower is sympetalous and zygomorphic.— After WERMING,
33
230
PLANT STRUCTURES
FIG. 213. Flower of a Labiate ( Teucrium), showing the calyx of coalesced sepals, the sympetalous and two-lipped (bilabi- ate) corolla with three petals (middle one largest) in the lower lip and two small ones in the upper, and the stamens and style emerging through a slit on the up- per side of the tube; a sympetalous and zygomorphic flower.— After BIUQUET.
is the bilabiate) 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 less conspicuously, etc.
126. Inflorescence. — Very often flowers are soli- tary, either on the end of a stem or branch (Figs. 231, 236), or in the axil of a leaf (Fig. 258). But
such cases grade insensibly into others where a definite region of the plant is set aside to produce flowers (Figs. 253, 260). Such a region forms what is called the 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 Compositse, 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 FLOWEK 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 Composite have calyx and corolla, are cyclic, epigynous, syncarpous, sympetalous, often zygo- morphic, and are in a remarkably compact inflorescence, indicating the highest possible combination of characters.
128. Flowers and insects. — The adaptations between flowers and insects, by which the former secure pollination and the latter food, are endless. Many Angiosperm flowers, especially those of the lower groups, are still anemophilous, as are the Gymnosperms, but most of them, by the presence of color, odor, and nectar, indicate an adaptation to the visits of insects. This wonderful chapter in the history of plants will be found discussed, with illustrations, in Plant Relations, Chapter VII.
CHAPTER XIV
MONOCOTYLEDONS AND DICOTYLEDONS
129. Contrasting characters. — The two great groups of Angiosperms are quite distinct, aud 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 233
margin 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- erous. The " three-parted "
FIG. 215. Two types of leaf venation: the figure to the left is from Solomon's seal, a Monocotyledon, and shows the principal veins parallel, the very minute cross veinlets being invisible to the naked eye; that to the right is from a willow, a Dicotyledon, and shows netted veins, the main central vein (midrib) sending out a series of parallel branches, which are connected with one another by a network of veinlets.— After ETTINGSHAUSEN.
flowers of cyclic Monocotyledons are quite characteristic, but there are some trimerous Dicotyledons.
Dicotyledons. — (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 in the diam-
234
PLANT STRUCTURES
eter of woody stems (Fig. 217, w), and a possible increase of the branch system and foliage dis- play each year.
(3) Leaf veins form- ing an open system (Fig. 215, figure to right). The network of smaller veinlets between the larger veins is usually very evident, especially on the under surface of the leaf, suggesting the name "net- veined" leaves, in contrast to the " parallel-veined " leaves of Mono- cotyledons. The vein system ends freely in the margin of the leaf, forming an "open venation." In consequence of this, although the leaf _____
C
rrj
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; p, pith. — From "Plant Relations/'
may remain entire, it very commonly be- comes toothed, lobed, and divided in various ways. Two main types of venation may be noted, which influence the form of leaves. In one case a single very prominent vein (rib} runs through the mid- dle of the blade, and is called the midrib. From this all the mi- nor veins arise as branches (Figs. 218, 210), and such a leaf
US
FIG. 217. Section across a twig of box elder three years old, showing three annual rings, or growth rings, in the vascular cylinder; the radiating lines (m) which cross the vascular region (w) represent the pith rays, the princi- pal ones extending from the pith to the cor- tex (c).— From " Plant Relations."
MONOCOTYLEDONS AND DICOTYLEDONS
235
is said to be pinnate or pinnately veined, and inclines to elongated forms. In the other case several ribs of equal prominence enter the blade and diverge through it (Fig. 218). Such a leaf is palmate or palmately veined, and in- clines to broad forms.
(4) Cyclic flowers pentamerous or tetramerous. The flowers " in fives " 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.
very prominent families have flowers " in fours." There are also dicotyledonous families with flowers "in threes," and some with flowers " in twos."
It should be remembered that no one of the above char- acters, unless it be the character of the embryo, should be depended upon absolutely to distinguish these two groups.
236
PLANT STRUCTURES
It is the combination of characters which determines a group.
MONOCOTYLEDONS
130. Introductory. — This great group gives evidence of several distinct lines of development, distinguished by what may be called the working out of different ideas. In this way numerous families have resulted — that is, groups of
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 forms. Opinions may differ as to what forms are so nearly related that they deserve to consti- tute a distinct family. A single family of some botanists may be " split up " into two or more families by others. Despite this diversity of opinion, most of the families are fairly well recognized.
MONOCOTYLEDONS AND DICOTYLEDONS 337
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 species (singular also species). A group of related species, there- fore, forms a genus ; and a group of related genera forms a family.
The technical name of a plant is the combination of its generic and specific names, the former always being written first. For example, Quercus alba is the name of the com- mon white oak, Quercus being the name of the genus to which all oaks belong, and alba the specific name which distinguishes this oak from other oaks. Xo 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 Naias (naiads) and Zannichellia (horned pondweed) are common genera in ponds and slow waters.
238
1'LANT STKUCTI KKs
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-
FIQ. 220. Pondweed (Potamogeton): A, branch with cluster (spike) of simple flowers, showing also the broad floating leaves and the narrow submerged ones; B, a sin- gle flower, showing" the .inconspicuous perianth lobes (c\ the short stamens (a). and the two short styles with conspicuous stigmatic surfaces.— A after REICHEN- BACH; B after LE MAOUT and DECATPNE.
FIG. 221. Cat-tails (Typhd), showing the dense spikes of very simple flowers, each showing two regions, the lower the pistillate flowers, the upper the staminate.— From " Field. Forest, and Wayside Flowers."
2-k)
PLANT STRUCTURES
leaf) and Alisma (water-plantain), in which there is a dis- tinct calyx and corolla. The genus Typlia (cat-tail) is also an aquatic or marsh form of very simple type, the flow- ers being in dense cylindrical clusters (spikes), the upper flowers consisting of stamens, the lower of carpels, thus forming two very distinct re- gions of the spike (Fig. 221).
132. Grasses.— This is one of the largest and probably one of the most use- ful groups of plants, as well as one of the most peculiar. It is world-wide in its dis- tribution, and is re- markable in its dis- play of individuals, often growing so densely over large areas as to form a close turf. If the grass -like sedges be associated with them there are about six thousand species, representing nearly one third of the Mon- ocotyledons. Here belong the various cereals, sugar canes,
FIG. 222. A common meadow grass (Festuca): A, portion of flower cluster (epikelet), 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
bamboos, and pasture grasses, all of them immensely use- ful plants.
The flowers are very simple, having no evident perianth (Fig. 222). Most commonly a flower consists of three sta- mens, surrounding a single carpel, whose ovary ripens into the grain, the characteristic seed-like fruit of the group. The stamens, however, may be of any number from one to six. The flowers, therefore, are naked, with indefinite num- bers, and hypogynous, indicating a comparatively simple type. It is also noteworthy that the group is anemophilous.
One of the noteworthy features of the group is the prominent development of peculiar leaves (bracts) in con- nection Avith the flowers. Each flower is completely pro- tected or even inclosed by one of these bracts, and as the bracts usually overlap one another the flowers are invisible until the bracts spread apart and permit the long dangling stamens to show themselves. These bracts form the so- called " chaff" of wheat and other cereals, where they persist and more or less envelop the grain (ripened ovary). As they are usually called glumes, the grasses and sedges are said to be ylumaceous 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 Graminew, and the sedges the family Cyperacem.
133. Palms. — More than one thousand species of palms are grouped in the family Palmacea>. 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 crown of huge leaves which are pinnate or palmate in char- acter, and often splitting so as to appear lobed or compound (Figs. 223, 224:).
The flower clusters are usually very large (Fig. 223), and each cluster at first is inclosed in a huge bract, which
FIG. 223. A date palm, showing the unbranched columnar trunk covered with old leaf bases, and with a cluster of huge pinnate leaves at the top, only the lowest por- tions of which are shown ; two of the very heavy fruit clusters are also shown.— From " Plant Relations,11
MONOCOTYLEDONS AND DICOTYLEDONS
24:3
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, which have split so as to appear palmately branched.— From " Plant Relations."
134. Aroids. — This is a group of nearly one thousand species, most of them belonging to the family Aracece. In our flora the Indian turnip or Jack-in-the-pulpit (Ariscemd) (Fig. 225), sweetflag (Acorus), and skunk-cabbage (Symplo- carpus), may be taken as representatives ; while the culti- vated Calla-lily is perhaps even better known. The great display of aroids, however, is in the tropics, where they are endlessly modified in form and structure, and are erect, or climbing, or epiphytic.
24:4:
PLANT STRUCTURES
The flowers are usually very simple, often being naked, with two to nine stamens, and one to four carpels (Fig.
FIG. 225. Jack-in-the-pulpit (Aristema). showing the overarching spathes; 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. 226).
The spadix is enveloped by a great bract, which sur- rounds and overarches like a large loose hood, and is called the spathe. The spathe is exceedingly variable in form., and is often conspic- uously colored, forming in the Calla- lily the conspicuous white part, within which the spadix may be seen, near the base of which the flowers are found. In Jack-in-the-pulpit (Fig. 225) it is the overarching spathe which suggests the "pulpit." The spadix and spathe are the characteristic features of the group, and the spathe is variously modified in form, structure, and color for insect pollination, as is the peri- 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 club- 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 geophilous ('( earth -lovers")—
that is, they develop bulbs, rootstocks, etc., which enable them to disappear from above the surface during un- favorable conditions (cold or drought), and then to reappear rap- idly upon the return of favorable conditions (Figs. 227, 228, 231, 233).
In the regular lily family (LiUacece) 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- ley, hyacinth, easter lily) (Figs. 201, 229), the stamens are usu- ally six (two sets),
and the three carpels are syncarpous (Figs. 204, 230). This is a higher combination of floral characters than any of the preceding groups presents. Hypogyny and actinomorphy are low, but a conspicuous perianth, syn- carpy, and occasional sympetaly indicate considerable ad- vancement.
Fi(i. 227. Wake-robin {Trillium}, showing root- stock, from which two branches arise, each bear- ing a cycle (whorl) of three leaves and a single trimerotis flower. — After ATKINSON.
.MONOCOTYLEDONS AND DICOTYLEDONS
24'
In the amaryllis family (AmaryUidacece), a higher fam- ily of the same general line, represented by species of Nar- cissus (jonquils, daffodils, etc.), Agave, etc., the flowers are distinctly epigynous.
FIG. 228. Star-of -Bethlehem (Ornithogalvm) : a, entire plant with tuberous base and trimerous flowers; b, a single flower; c, portion of flower showing relation of parts, perianth lobes and stamens arising from beneath the prominent ovary (hy- pogynons); d, mature fruit; e. section of the syncarpous ovary, showing the three carpels and loculi.— After SCHIMPER.
In the iris family (frirfacece), the most highly specialized family of the lily line, and represented by the various spe-
FIG. 25J9. The Japan lily, showing a tubular perianth, the parts of the perianth distinct above.— From " Field, Forest, and Wayside Flowers."
MONOCOTYLEDONS AND DICOTYLEDONS
249
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.
130. Orchids.— In number of species this (OrcJiidacece) 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, Cypripe- dium (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 Liliitm PhiladelpMcum, showing the three loculi, in each of which are two ovules (mega- sporangia): A, ovule; B, integuments; (', nu- cellus ; I), embryo-sac (megaspore).— CAI.D- WEI.L.
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 disst-cted out. — From " Plant Relations."
MONOCOTYLEDONS AND DICOTYLEDONS
251
modified in a great variety of ways, and a prominent, often very long, spur, 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 GRAT.
FIG. 234. Flower cluster of Gla- diolus, showing somewhat zygo- morphic flowers. — CALDWELL.
FIG. 233. Gladiolus, showing tuberous subter- ranean stem from which roots descend, grass- like leaves, and somewhat zygomorphic flow- ers.—After REICHENBACH.
252
PLANT STRUCTURES
and stiginatic 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 (polliniuni). which is pulled out and carried to another flower bv the
which may be seen in 1 and f ; in eye of a moth. — After OKAY.
FIG. 235. A flower of an orchid Ha): at 1 the complete flower is shown, with three sepals behind and three pet- als in front, the lowest one of which has developed a long strap-shaped portion (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 irt 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, 3 a pollen mass (a) is shown sticking to each
visiting insect. The whole structure indicates a very highly specialized type, elaborately organized for insect pollination.
Another interesting epigynous and zygomorphic trop- ical group, but not so elaborate as the orchids, is repre- sented by the cannas and bananas (Fig. 120), common in cultivation as foliage plants, and the aromatic gingers.
From the simple pondweeds to the complex orchids the evolution of the Monocotyledons has proceeded, and be- tween them many prominent and successful families have been worked out.
FIG. 236. A clump of lady-slippers (Cypripedium), showing the habit of the plant and the general structure of the zygoinorphic flower.— After GIBSON.
254
PLANT STRUCTURES
FIG. 287. A group of orchids (C'attleya), showing the very zygomorphic flowers, the lip being well shown in the flower to the left (lowest petal).— CALIMVELL.
DICOTYLEDONS
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 Arcliichlamydece 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 Sympetalae
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 Archichlamydese 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.
A rclt icldamydecf}
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
FIG. 238. An oak in winter condition. — From " Plant Relations.1
256
PLANT STRUCTURES
temperate regions, for here belong the oak (Fig. 238), hick- ory, walnut, chestnut, beech, poplar, birch, elm (Figs. 198, 239), willow (Fig. 240), etc. The primitive character is in- dicated not merely by the floral structures, but also by the general anemophilous habit.
In the poplar (Populus) and its allied form, the willow (Salix), the flowers are naked and hypogynous (Fig. 196),
FIG. £J(J. An elm in foliage. — From " Plant Rela
MONOCOTYLEDONS AND DICOTYLEDONS
the stamens are indefinite in number (two to thirty), and the pistil is syncarpous (two carpels). The stamens and
FIG. 240. Flower cluster 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 FIG 241 Amentg of alder ( . ^ branch
with staminate aments (ra), pistillate aments (m), and a young bud (&); b. pistillate ament at time of discharging seeds, showing the promi-
alders and birches are very familiar objects (Figs. 240, 241).
nent bracts.— After WARMING.
258 PLANT STRUCTURES
The only advanced character in the flowers as described above is the syncarpous pistil, but in the great allied pepper family (Piperacece) of the tropics, with its one thousand species, and most nearly represented in our flora by the
FIG. 242. Ovule of hornbeam (Carpinvf>\ 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 MARY 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. 24=2). As the region of the ovule where integument and nucellus are not distinguishable is called the chalaza, this phenomenon is known as chalazoyamy, meaning "fertilization through the chalaza.'1
139. Buttercups and their allies. — This is a great assem- blage of terrestrial herbs, including nearly five thousand species, and is thought by many to be the great stock from which most of the higher Dicotyledons have been derived. The alliance includes the water-lilies, buttercups, and pop- pies, the specialized mustards, and certain notable tree forms, as magnolias, custard-apples, and the tropical laurels with one thousand -species represented in our flora only by the sassafras. Here also is the strange group of " car- nivorous" plants (Sarracenia, Drosera, 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 adapted for insect-pollination, but the spiral arrangement and indefinite numbers are very apparent, notably in connection with the apocarpous pistils, which are very numerous upon a promi- nent receptacle, but involving more or less all the parts. The stamens are also very numerous (Figs. 200, 243, 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. 245. Diagram of the zygomorphic flower of larkspur (Delphinium), show- ing the spur developed by a sepal and inclosing the two petal spurs. — After BAIIJ.ON.
.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- cecc) zygomorphy appears, as in the larkspur (Delphinium) with its spurred petals and sepals (Figs. 244, 245), and the monkshood (Aconitum) with its hooded sepal ; and in the
FIG. 246. The common cabbage (Brassicd), a member of the mustard family: A, flower cluster, 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 (Nymphaaceci) 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 of 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 (Crucifera), 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 carpel 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 (Rosacem) of one thousand species is one of the best known and most useful groups of the temperate regions. In it are such forms as Spircva, five-finger (Poten- tilla), strawberry (Fragaria) (Figs. 191, 207), raspberry (Fig. 248), and blackberry (Ru- bus), rose (Rosa), hawthorn ( CratcB- 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 («). 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
263
Many of the true roses have a strong resemblance (Fig. 207) to the buttercups (Ranunculus), with their hypogy- nous regular flowers, and indefinite number of stamens and carpels, but the sepals and petals are much more frequently five, the Dicotyl number being better established. The
FIG. 249. The common pear (Pirus Comments), showing branch with flowers (1), sec- tion of a flower (2) showing its epigynons character, section of fruit (3) showing the thickened calyx outside of the ovary or "core" (indicated by dotted outline), and flower diagram (k) showing all the organs in fives except the stamens. — After
WOSSIDLO.
whole family remains actinomorphic, but perigyny and epigyny appear in certain forms (Fig. 205), giving rise to the peculiar fruit (pome) of apples and pears (Fig. 249), in which the calyx and ovary ripen together. Another spe- cialized group of roses is that which develops the stone-
264
PI. A NT STRUCTURES
fruits (drupes), as apricots, peaches (Fig. 189), plums, cherries.
141. Legumes, — This is far the greatest family (Legumi- nosm) of the Archichlamydeae, containing about seven thou- sand species, distributed everywhere and of every habit. It is the -great zygomorphic group of the Archichlamydese, being elaborately adapted to insect pollination. The more
FIG. 250. A legume plant (Lotus), showing flowering branch (J), a single flower (2) showing zygomorphic corolla, the cluster of ten stamens (3) which with the carpel is included in the keel, the solitary carpel (A) which develops into the pod or le- gume (5), the petals (6) dissected apart and showing standard (a), wings (6), 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 Leguminosae, 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 Papilio forms with very irregular (zygomorphic) flowers and few stamens
MONOCOTYLEDONS AND DICOTYLEDONS
265
(Fig. 250). The petals are very dissimilar, the upper one (standard) being the largest, and erect or spreading, the two lateral ones (wings) oblique and descending, the two lower ones coherent by their edges to form a projecting boat-shaped body (keel], which incloses the sta- mens and pistil. From a fancied re- semblance to a but- terfly such flowers are said to be papil- 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- nus)9 clover (Tri folium), locust (Robinia), Wistaria, pea (Pimm), bean (PJiaseolus), tragacanth (Astragalus), vetch (Vicia), redbud (Cere is), senna (Cassia), honey-locust (Gleditschid), indigo (Indigofera), sensitive-plants (Acacia, Mimosa, etc.) (Fig. 251), etc.
FIG. 251. A sensitive-plant (Acacia), showing the flowers with inconspicuous petals and very nu- merous stamens, and the pinnately branched sen- sitive leaves.— After MEYER and SCHUMANN.
266
PLANT STRUCTURES
142. ITmbellifers. — This is the most highly t organized family ( Umbellifera) of the Archichlamydeas, which may be said to extend from Peppers to Umbellifers. The Le- gumes adopt zygomorphy, but remain hypogynous ; and in some of the Roses epigyny appears ; but the Umbellifers with their fifteen hundred species are all distinctly epigy-
FIG. 252. The common carrot (Daucus Carota): 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, (7), 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- petalse.
The name of the family is suggested by the character- istic inflorescence, which is also of advanced type. The flowers are reduced in size and massed in flat- topped clusters called umbels (Figs. 252, A, 253). The branches of the clus- ter arise in cycles from the axis like the braces of an umbrella. As a re- sult of the close approxi- mation of the flowers the sepals are much reduced in size and often obsolete (Fig. 252, 0).
The Umbellifers are mainly perennial herbs of the north temperate re- gions, forming a very dis- tinct family, and contain- ing the following familiar forms : carrot (Daucus) (Fig. 252), parsnip (Pasti- naca), hemlock (Conium) (Fig. 253), pepper-and- salt (Erigenia), caraway (Carum), fennel (Fcenic- ulum), coriander (Cori- andrum), celery (Api- 11 m), parsley (Petroseli-
\ , AIT rl f fli FIG. 253. Hemlock (Conium), an Umbellifer,
n), etc. All] showing the umbels, with the principal
Umbellifers are the Ara- rays rising from a cycle of bracts (invo-
lucre), and each bearing at its summit a secondary umbel with its cycle of second- ary bracts (involucel).— After SCHIMPER.
lias (Araliacea), and the Dogwoods (Cornacece).
2(58 PLANT STRUCTURES
Sympetalce
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- chlamydese. 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 Sympetalae, 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 pentacyclic, meaning "cycles five," there being two sets of stamens. In it also there are five carpels, the floral formula being, Sepals 5, Petals 5, Stamens 5 + 5, Carpels 5. As the carpels are the same in number as the other parts, the flowers are called isocarpic, meaning " car- pels same." The group is named either Pentacydce or Iso- carpce, and contains about ten families and 4,000 species.
The higher groups, containing about forty families and 36,000 species, is tetr acyclic, 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 TetracyclcB or Anisocarpce.
144. Heaths. —The Heath family (Ericacew) 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 Cassiope, showing trailing habit, small overlapping leaves, and sympetalous flowers, but in the smaller form the petals are almost distinct. — After DRUDE.
Common representatives of the family are as follows : huckleberry (Gaylussacia), cranberry and blueberry ( Vac- cinium), bearberry (Arctostaphylos), trailing arbutus (Epi-
270
PLANT STRUCTURES
gcea), wintergreen (Gaultheria), heather (Calluna), moun- tain laurel (Ealmia), Azalea, Rhododendron (Fig. 256), Indian pipe (Monotropa), etc.
FIG. 255. Flowers of heath plants (Erica}, showing complete flowers (A), the sta- mens with " two-horned " anthers which discharge pollen through terminal pores, and the lobed syncarpous ovary with single style and prominent terminal stigma (B, C, D).— After DRUDE.
145. Convolvulus forms, — The well-known morning-glory (Tpomcea) (Fig. 209) may be taken as a type of the Convol-
MONOCOTYLEDONS AND DICOTYLEDONS
2T1
vulus family (Convolvulacece). Allied with it are Polemo- nium and Phlox (Fig. 210, b) (Polemoniacece}, the gentians (Gentianacece), and the dog-banes (Apocynacece) (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
PLANT STRUCTURES
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 common dogbane (Apoc-ynuin). — Froir
Flowers.'"
" P'ield, Forest. ;ind Wayside
FIG. 258. The hedge bindweed ( Convolvulus), showing the twining habit and the con- spicuous funnelform corollas.— From " Field, Forest, and Wayside Flowers."
274
PLANT STKUCTUEES
bilabiate structure (Fig. 210, c, 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 differ widely in relative prominence.
Associated with zygomorphy in this group is a frequent reduction in the number of stamens, which are often four (Fig. 212) or two. The whole structure is highly special- ized for the visits of insects, and this great zygomorphic alliance holds the same relative position among 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 (Mentlia) (Fig. 212), dittany (Cunila), hyssop (Hyssopus), mar- joram (Origanum),
FIG. 259. Flowers of dead nettle (La- mium) : A, entire bilabiate flower ; B, section of flower, showing rela- tion of parts.— After WARMING,
FIG. 260. A labiate plant ( Teucrium), show- ing branch with flower clusters (4), and side view of a few flowers (B), showing their bilabiate character. — After BRIQUET.
MONOCOTYLEDONS AND DICOTYLEDONS 275
thyme (Thymus), balm (Melissa), sage (Salvia), catnip (Nepeta), skullcap (Scutellaria), horehound (Marrubium), lavender (Lavandula), rosemary (Rosmarinus) , dead nettle (Lamium) (Fig. 269), Teucrium (Figs. 213, 260), etc., a remarkable series of aromatic forms.
Allied is the Xightshade family (Solanacece) , with fif- teen hundred species, containing such common forms as the nightshades and potato (Solatium), tomato (Ly coper - sicum), tobacco (Nicotiana) (Fig. 208), etc., in which the corolla is actinomorphic or nearly so ; also the great Fig- wort family (Scroplmlariacece), 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 ( Verbenacece) , with over seven hundred species ; and the two hundred plantains (Plant aginacetB), etc.
147. Composites. — This greatest and ranking family ( Composite) of Angiosperms is estimated to contain at least twelve thousand species, containing more than one seventh of all known Dicotyledons and more than one tenth of all Seed-plants. 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 strap-shaped 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 top 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 disk (Fig. 261, A), which is closely packed with very numerous small and regular tubular flowers, known as disk-floivers (Fig. 261, e).
FIG. 262. The common dandelion ( Taraxacum): 1, two flower stalks; in one the head is closed, showing the double involucre, the inner erect, the outer reftexed, 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; It, a head from which all but one of the akenes have been re- moved, showing the pitted receptacle and the prominent pappus beak. — After STRASBURGER.
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-
278
PLANT STRUCTURES
tirely ; sometimes they become a tuft of delicate hairs, as in Arnica (Fig. 261, />, 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 through the tube and carrying pollen; 2, style branches beginning to recurve; 3, style branches completely recurved. — From " Field, Forest, and Wayside Flowers."
barbed processes arising from the akene, as in tickseed (Coreopsis) and beggar-ticks (Fig. 188) or Spanish needles (Bidens), to lay hold of passing animals ; sometimes they become beautifully plumose bristles, as in the blazing star (Liatris) ; sometimes they simply form a more or less con- spicuous cup or set of scales crowning the akene. In all of these modifications the calyx is called pappus.
The stamens within the corolla are organized into a tube by their coalescent anthers (Fig. 263), and discharge their pollen within, which is carried to the surface of the
MONOCOTYLEDONS AND DICOTYLEDONS 279
head and exposed by the swab-like rising of the style (Fig. #63). The head is thus smeared with pollen, and visiting insects can not fail to distribute it over the head or carry it to some other head.
In the dandelion and its allies the flowers of the disk are like the ray-flowers, the corolla being zygomorphic and strap-shaped (Figs. 262, 263).
The combination of characters is sympetalous, tetracyc- lic, and anisocarpic flowers, which are epigynous and often zygomorphic, with stamens organized into a tube and calyx modified into a pappus, and numerous flowers organized into a compact involucrate head in which there is more or less division of labor. There is no group of plants that shows such high organization, and the Composite seem to deserve the distinction of the highest family of the plant kingdom.
The well-known forms are too numerous to mention, but among them, in addition to those already mentioned, there are iron-weed ( Vernonia), Aster, daisy (Bellis), goldenrod (Solidago), rosin-weed and compass-plant (Silph- ium), sunflower (Helianthus), Chrysanthemum, ragweed (Ambrosia), cocklebur (Xanthium), ox-eye daisy (Leucan- themum), tansy (Tanacetum), wormwood and sage-brush (Artemisia), lettuce (Lactuca), etc.
CHAPTEE XV
DIFFERENTIATION OF TISSUES
148. Introductory. — Among the simplest Thallophytes the cells forming the body are practically all alike, both as to form and work. What one cell does all do, and there is very little dependence of cells upon one another. As plant bodies become larger this condition of things can not continue, as all of the cells can not be put into the same relations. In such a body certain cells can be related to the external food supply only through other cells, and the body becomes differentiated. In fact, the relating of cells to one another and to the external food-supply makes large bodies possible.
The first differentiation of the plant body is that which separates nutritive cells from reproductive cells, and this is accomplished quite completely among the Thallophytes. The differentiation of the tissues of the nutritive body, however, is that which specially concerns us in this chapter.
A tissue is an aggregation of similar cells doing similar work. Among the Thallophytes the nutritive body is prac- tically one tissue, although in some of the larger Thallo- phytes the outer and the inner cells differ somewhat. This primitive tissue is composed of cells with thin walls and active protoplasm, and is called parenchyma, meaning "parent tissue."
Among the Bryophytes, in the leafy gametophore and
in the sporogonium, there is often developed considerable
dissimilarity among the cells forming the nutritive body,
but the cells may all still be regarded as parenchyma. It
380
DIFFEKENTIATION OF TISSUES 281
is in the sporophyte of the Pteridophytes and Spermato- phytes that this differentiation of tissues becomes extreme, and tissues are organized which differ decidedly from parenchyma. This differentiation means division of labor, and the more highly organized the body the more tissues there are.
All the other tissues are derived from parenchyma, and as the work of nutrition and of reproduction is always retained by the parenchyma cells, the derived tissues are for mechanical rather than for vital purposes. There is a long list of these derived and me- chanical tissues, some of them being of general occurrence, and others more restricted, and there is every gradation
between them and the FIG. 264. Parenchyma and sclerenchyma from
parenchyma from which the 8tem of pteris> in cross-section.-CHAM-
f J BEKLAIN.
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 meristein, or it is said to be meristematic, from a Greek word meaning "to divide." When the cells stop dividing, the tissue is said to be permanent. The growing points of organs, as stems, roots, and leaves, are composed of parenchyma which is meristematic (Figs. 266, 274), and meristem occurs wherever growth is going on.
282
PLANT STRUCTURES
150. Mestome and stereome. — When the plant body be- comes complex a conductive system is necessary, so that the different regions of the body may be put into communi- cation. The material absorbed by the roots must be carried to the leaves, and the food manu- factured in the leaves must be carried to regions of growth and storage. This business of transportation is provided for by the specially organized ves- sels referred to in preceding chapters, and all conducting tis- sue, of whatever kind, is spoken of collectively as mestome.
If a complex body is to main- tain its form, and especially if it is to stand upright and be- come large, it must develop structures rigid enough to fur- nish mechanical support. All the tissues which serve this pur- pose are collectively known as 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 hecome dis- tinctly differentiated, passing into permanent tissue, the apical group by its divisions continually adding to them and increasing the stem in length.
Just behind the apical group, the cells begin to give the appearance of being organized into three great embryonic re- gions, the cells still remaining meristem- 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 uthat which fills up." Farther down, where the plerome passes into permanent tissue, it is called the central cylinder or stele ("column"). Between the plerome and dermatogen is a tissue region called the periblem, meaning "that which is put around," and when it becomes permanent tissue it is called the 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 ;
FIG. 266. Section through growing point of stem of Hippuris : below the growing point, composed of a uniform meristem tissue, the three embry- onic regions are outlined, showing the dermato- gen (d, d), the central plerome (p, p), and be- tween them the periblem. — After DK BARY.
PLANT STRUCTURES
below them are the three embryonic regions, dermatogen, periblem, and plerome ; and farther below these three regions pass into permanent tissue, organizing the epider- mis, cortex, and stele. The three embryonic regions are usually not so distinct in the Conifer stem as in the Dico- tyl stem, but both stems have epidermis, cortex, and stele. Epidermis. — The epidermis is a protective layer, whose cells do not become so much modified but that they may be regarded as parenchyma. It gives rise also to super- ficial parts, as hairs, etc. In the case of trees, the epidermis does not usually keep up with the increasing diameter, and disappears. This puts the work of protection upon the cortex, which organizes a superficial tissue called cork, a prominent part of the structure known as bark.
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 Jiypodermis ; 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 scleretir chyma, 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
FIG. 267. Some collenchyma cells from the stem of a com- mon dock (Rumex), showing the cells thickened at the angles.— CHAMBERLAIN.
DIFFERENTIATION OF TISSUES
285
in length. The chief mechanical tissue for parts which have stopped growing in length is sclerenchyma (Figs. 264, 265). The cells are thick-walled, and usually elongated and with tapering ends, including the so-called "fibers."
FIG. 268. 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; JT, xylem, containing spiral (s, s') and pitted (t, t') vessels; C, cambium; P, phloem, containing sieve vessels ($b); b, a mass of bast fibers or sclerenchyma; ic, pith rays between the bundles; e, the bundle sheath: It, cor- tex.—After VINES.
Stele. — The characteristic feature of the stele or central cylinder is the development of the mestome or vascular
286
PLANT STRUCTURES
tissues, of which there are two prominent kinds. The tracheary vessels are for water conduction, and are cells with heavy walls and usually large diameter (Fig. 268). The thickening of the walls is not uniform, giving them a very characteristic appearance, the thickening taking the form of spiral bands, rings, or reticulations (Fig. 268, B). Often the reticulation has such close meshes that the cell wall has the appearance of being covered with thin spots, and such cells are called " pitted vessels." The vessels with spirals and rings are usually much smaller in diameter than the pitted ones. The true tracheary cells are more or less elongated and without tapering ends, fitting end to end and forming a continuous longitudinal series, suggesting a trachea, and hence the name. In the Conifers there are
no true tracheary cells, 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 tracheae or true tracheary vessels in having tapering ends and in not forming a continu- ous series (Fig. 269). The walls of these tracheids are " pitted" in a way which is characteristic of Gymnosperms, the "pits" appearing as two concentric rings, called "bordered pits."
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-
Fio. 269. Tracheids from wood of pine, showing tapering ends and bordered pit?.— CHAMBERLAIN.
DIFFERENTIATION OF TISSUES 287
box or a "sieve." These perforated areas are the sieve- plates, and through them the vessels communicate with one another and with the adjacent tissue.
The tracheary and sieve vessels occur in separate strands, the tracheary strand being called xylem (" 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 sfcele which the vascular cylinder incloses is called the pith. In older parts of the stem the pith is often abandoned by the activities of the plant, and either remains as a dead spongy tissue, or disappears en- tirely, leaving a hollow stem. Between the bundles form- ing the vascular cylinder there is also undifferentiated parenchyma, and as it seems to extend from the pith out between the bundles like "rays from the sun," the rays are called pith rays.
Such vascular bundles as described above, in which the xylem and phloem strands are " side-by-side " upon the same radius, are called 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 open ; 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 camHum cylinder, which separates the xylem and phloem of the vascular cylinder. This cambium continues the f or-
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 (g) vessels, cambium of bundle (c) and of pith rays (cb), phloem containing sieve ves- sels («/), three bundles of bast fibers or sclerenchyma (6). 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 growth rings or annual rings, so conspicuous a feature of the cross-section of tree
DIFFERENTIATION OF TISSUES 289
trunks (Fig. 217). This continuous addition to the vessels increases the capacity of the stem for conduction, and per- mits the further extension of branches and a larger display of leaves.
The annual additions to the xylem are added to the in- creasing mass of wood. The older portions of the xylem mass are gradually abandoned by the ascending water ("sap"), often change in color, and form the heart-wood. The younger portion, through which the sap is moving, is the sap-wood. It is evident, however, that the annual ad- ditions to the phloem are not in a position for permanency. The new phloem is deposited inside of the old, and this, to- gether with the new xylem, presses upon the old phloem, which becomes ruptured in various ways, and rapidly or very gradually peels off, being constantly renewed from within. It is the protecting layers of cork (see this section under Cortex), the old phloem, and the new phloem down to the cambium, which constitute the so-called bark of trees, a structure .exceedingly complex and extremely vari- able in different trees.
The stele also frequently develops stereome tissue in the form of sclerenchyma. These thick-walled fibers are often closely associated with one or both of the vascular strands of the bundles (Fig. 270), and lead to the old name fibro- vascular bundles.
To sum up, the stems of Dicotyledons and Conifers are characterized by the development of a vascular cylinder, in which the bundles are collateral and open, permitting increase in diameter, extension of the branch system, and a continuous increase in leaf display.
152. Monocotyl steins. — In the stems of Monocotyledons there is the same apical development and differentiation (Fig. 266). The characteristic difference from the Dicotyl and Conifer type, just described, is in connection with the development of the vascular bundles in the stele. Instead of outlining a hollow cylinder, the bundles are scattered
290
PLANT STRUCTURES
through the stele (Fig. 214). This lack of regularity would interfere with the organization of a cambium cylinder, and we find the bundles collateral but closed — that is, with no meristem left between the xylem and phloem (Fig. 271).
FIG. 271. Cross-section of a closed collateral bundle from the stem of corn, showing the xylem with annular (r), spiral (s), and pitted (g) vessels; the phloem contain- ing sieve vessels (v), and separated from the xylem hy no intervening cambium; both xylem and phloem surrounded by a mass of sclerenchyma (fibers); and in- vesting vessels and fibers the parenchyma (p) of the pith-like tissue through which the bundles are distributed. — After 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- podium] and Isoetes usually have an apical group of meris- tem cells, as among the Seed- plants, the smaller Club-mosses (Selaginella), Ferns, and Horse- tails usually have a single api- cal cell, whose divisions give rise to all the cells of the stem. 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 crops- section of stem of a fern (Pteris), showing two masses of scleren chyma (st), between and about which are vascular bundles. — CHAMBERLAIN.
292 PLANT STRUCTURES
In Equisetum 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
X
FIG. 273. Cross-section of concentric vascular bundle of a fern (Pteris): the single row of shaded cells investing the others is the bundle sheath; the large and heavy- walled cells within constitute the xylem; and between the xylem and the bundle ehealh 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 vascular plants (Pteridophytes and Spermatophytes) ; and in all of them the structure is essentially the same, and quite different from stem structure. A single ap- ical cell (in most Pteridophytes) (Fig. 274) or an apical group (in Spermatophytes) usually gives rise to the three embryonic regions — dermatogen, periblem, and plerome (Fig. 275).
A fourth region, how- ever, peculiar to root, is usually added. The apical
FIG. 274. Section through root-tip of Pteris: the cell with a nucleus is the single apical cell, which in front has cut off cells which organize the root- cap. — CHAMBERLAIN.
Fi<i. 275. A longitudinal section through the root - tip of shepherd's purse, showing the plerome (pi), surround- ed by the periblem (/», outside of periblem the epidermis (e) which disappears in the older parts of the root, and the prominent root-cap (c). —From "Plant Relations."
cell or group cuts off a tis- sue in front of itself (Fig. 274), known as the calyptrogen, or "cap producer." for it organizes the root-cap, which protects the delicate meri- stem of-the growing point.
Another striking feature is that in the stele there is organized a single solid vascular cylinder, forming a tough central axis (Fig. 277), from which the usually well-devel- oped cortex can be peeled off as a thick rind. In this vas- cular axis, which is called " a bundle " for convenience but does not represent the bundle of Seed-plant stems, the ar- rangement of the xylem and phloem is entirely unlike that 37
294
PLANT STRUCTURES
FIG. 276. Cross-section of the vascular axis of a root, showing radiate type of bundle the xylem (p) and phloem (ph) 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 associated with the fact that the xylem is to receive and conduct the
FIG. 277. Endogenous origin of root branch- water absorbed from the es, showing them (») arising from the cen- -i T- i ^ i .,
tral axis (/) and breaking through the
cortex (r).-After VINES. that this characteristic
DIFFERENTIATION OF TISSUES
295
bundle structure of the root appears only in young and active roots. In older ones certain secondary changes take place which obscure the structure and result in a resem- blance to the stem.
The origin of branches in roots is also peculiar. In stems branches originate at the surface, involving epi- dermis, cortex, and vascular bundles, such an origin being called exogenous ("produced outside7'); but in roots branches originate on the vascular cylinder, burrow through the cortex, and emerge at the surface (Fig. 277). If the cortex be stripped off from a root with branches, the branches are left attached to the woody axis, and the cor- tex is found pierced with holes made by the burrowing branches. Such an origin is called endogenous, meaning " produced within."
To sum up the peculiarities of the root, it may be said to develop a root-cap, to have a solid vascular cylinder in which the xylem and phloem are arranged to form a bundle of the radiate type, and to branch endogenously.
FIG. 2~8. A section through the leaf of lily, showing upper epidermis (ue), lower epi- dermis (le) with its stomata (st). mesophyll (dotted cells) composed of the palisade region (/>) and the spongy region <'/?/?) with air spaces among the cells, and two veins (v) cut across. — From "Plant Relations.'"
296 PLANT STRUCTURES
155. Leaves. — Leaves usually develop from an apical region in the- same general way as do stems and roots, modified by their common dorsiventral character. Com- paring the leaf of an ordinary seed-plant with its stem, it will be noted that the three regions are represented (Fig. 278) : (1) the epidermis ; (2) the cortex, represented by the mesophyll ; (3) the stele, represented by the veins.
In the case of collateral bundles, where in the stem the xylem is always toward the center and the phloem is toward the circumference, in the leaves the xylem is toward the upper and the phloem toward the lower surface.
CHAPTER XVI
PLANT PHYSIOLOGY
156. Introductory. — Plants may be studied from several points of view, each of which has resulted in a distinct division of Botany. The study of the forms of plants and their structure is MORPHOLOGY, and it is this phase of Bot- any which has been chiefly considered in the previous chap- ters. The study of plants at work is 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 BOTAXV.
Besides these general subjects, which apply to all plants, the different groups form the subjects of special study. The study of the Morphology, Physiology, or Taxonomy of the Bacteria is Bacteriology ; of the Algae, Algology ; of the Fungi, Mycology ; of the Bryophytes, Bryology ; of the fossil plants, Palceobotany or Palwophytology, etc.
In the present chapter it is the purpose to give a very brief outline of the great subject of Plant Physiology, not with the expectation of presenting its facts adequately, but with the hope that the important field thus presented may
297
298 PLA.NT STRUCT! KKS
attract to further study. It is merely the opening of a door to catch a fleeting glimpse.
A common division of the subject presents it under five heads : (1) Stability of form, (2) ^Nutrition, (3) Respira- tion, (4) Movement, (5) Reproduction.
STABILITY OF FORM
157. Turgidity. — It is a remarkable fact that plants and parts of plants composed entirely of cells with very thin and delicate walls are rigid enough to maintain their form. It has already been noted (see § 20) that such active cells exert an internal pressure upon their walls. This seems to be due to the active absorption of liquid, which causes the very elastic walls to stretch, as in the "blowing up " of a bladder. In this way each gorged and distended cell be- comes comparatively rigid, and the mass of cells retains its form. It seems evident that the active protoplasm greedily pulls liquid through the wall and does not let it escape so easily. If for any reason the protoplasm of a gorged cell loses its hold upon the contained liquid the cell collapses.
158. Tension of tissues. — The rigidity which comes to active parenchyma cells through their turgidity is increased by the tensions developed by adjacent tissues. For exam- ple, the internal and external tissues of a stem are apt to increase in volume at different rates ; the faster will pull upon the slower, and the slower will resist, and thus be- tween the two a tension is developed which helps to keep them rigid. This is strikingly shown by splitting a dande- lion stem, when the inner tissue, relieved somewhat from the resistance of the outer, elongates and causes the strip to become strongly curved outward or 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.
PLANT PHYSIOLOGY 299
159. Stereome. — When growth is completed, cell walls lose their elasticity, turgidity becomes less, and therefore tensions diminish, ard 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 8 150.
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 (C02) of the air; hydrogen and oxygen from water (H20) ; and nitrogen and the other elements from their various salts which occur in the soil and are dissolved in the water which enters the plant.
All of these substances must present themselves to plants in the form of a gas or a liquid, as they must pass through cell walls ; and the processes of absorption have to do with the taking in of the gas carbon dioxide and of water in which the necessary salts are dissolved.
161. Absorption. — Green plants alone will be considered, as the unusual methods of securing food have been men- tioned in Chapter VII. For convenience also, only terres- trial green plants will be referred to, as it is simple to modify the processes to the aquatic habit, where the sur- rounding water supplies what is obtained by land plants from both air and soil.
300 PLANT STRUCTURES
In such plants the carbon dioxide is absorbed directly from the air by the foliage leaves, whose expanse of surface is as important for this purpose as for exposing chlorophyll to light. When the work of foliage leaves is mentioned it must always be understood that it applies as well to any green tissue displayed by the plant.
The water, with its dissolved salts, is absorbed from the soil by the roots. Only the youngest parts of the root- system can absorb, and the absorbing capacity of these parts is usually vastly increased by the development of numerous root hairs, just behind the growing tip (Fig. 194). These root hairs are ephemeral, new ones being continu- ally put out as the tip advances, and the older ones disap- pearing. They come in very close contact with the soil particles, and "suck in" the water which invests each particle as a film.
162. Transfer of water. — The water and its dissolved salts absorbed by the root-system must be transferred to the foli- age leaves, where they are to be used, along with the carbon dioxide, in the manufacture of food.
Having entered the epidermis of the absorbing rootlets the water passes on to the cortex, and traversing it enters the xylem system of the central axis. In some way this transfer is accompanied by pressure, known as root pres- sure, which becomes very evident when an active stem is cut off near the ground. The stump is said to "bleed," and sends out water ("sap") as if there were a force pump in the root-system. This root pressure doubtless helps to lift the water through the xylem of the root into the stem, and in low plants may possibly be able to send it to the leaves, but for most plants this is not possible.
When the water enters the xylem of the root it is 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 transpiration current, or very commonly the "ascent of sap." An ex-
i'LANT PHYSIOLOGY
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 Plant Relations, § 26.
This transpiration is regulated according to the needs of the plant. If the water is abundant, transpiration is encouraged ; if the water supply is low, transpiration is checked. One of the chief ways of regulating is by means of the very small but exceedingly numerous stomata (see § 79 [4]), whose guard cells become turgid or collapse and so determine the size of the opening between them. It has been estimated that a leaf of an ordinary sunflower contains about thirteen million stomata, but the number varies widely in different plants. In ordinary dorsiventral leaves the sto- mata are much more abundant upon the lower surface than upon the upper, from which they may be lacking entirely. In erect leaves they are distributed equally upon both sur- faces ; in floating leaves they occur only upon the upper surface ; in submerged leaves they are lacking entirely.
The amount of water thus evaporated from active leaves is very great. It is estimated that the leaves of a sunflower as high as a man evaporate about one quart of water in a warm day ; and that an average oak tree in its five active months evaporates about twenty-eight thousand gallons. If these figures be applied to a meadow or a forest the result may indicate the large importance of this process.
302 PLANT STKU'TtkKS
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 oif 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.
BESPIRATION
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. AVhen
PLANT I'liYSlOLOdY
it ceases death ensues sooner or later. By this process energy, stored up by the processes of nutrition, is liberated, and with this liberated energy the plant works. It may be said that oxygen seems to have the power of arousing pro- toplasm to activity.
It is not sufficient for the air containing oxygen to come in contact merely with the outer surface of a complex plant, as its absorption and transfer would be too slow. There must be an "internal atmosphere" in contact with the living cells. This is provided for by the intercellular spaces, which form a labyrinthine system of passageways, opening at the surface through stomata and lenticels (pores through bark). In this internal atmosphere the exchange of oxygen and carbon dioxide is effected, the oxygen being renewed by diffusion from the outside, and the carbon dioxide finally escaping by diffusion to the outside.
MOVEMENT
168. Introductory.— In addition to movements of mate- rial, as described above, plants execute movements depend- ent upon the activity of protoplasm, which result in change of position. Naked masses of protoplasm, as the plas- modium of slime-moulds (see § 51), advance with a sliding, snail-like movement upon surfaces ; zoospores and ciliated sperms swim freely about by means of motile cilia ; while many low plants, as Bacteria (§52), Diatoms (§34), Oscil- laria (§ 20), etc., have the power of locomotion.
When the protoplasm is confined within rigid walls and tissues, as in most plants, the power of locomotion usually disappears, and the plants are fixed ; but within active cells the protoplasm continues to move, streaming back and forth and about within the confines of the cell.
In the case of complex plants, however, another kind of movement is apparent, by which parts are moved and variously directed, sometimes slowly, sometimes with great
;>Q4 PLANT STRUCTUKKS
rapidity. In these cases the part concerned develops a curvature, and by various curvatures it attains its ultimate position. These curvatures are not necessarily permanent, for a perfectly straight stem results from a series of cur- vatures near its apex. Curvatures may be developed by unequal growth on the two sides of an organ, or by unequal turgidity of the cells of the two sides, or by the unequal power of the cell walls to absorb water.
169. Hygroscopic movements. — These movements are only exhibited by dry tissues, and hence are not the direct result of the activity of protoplasm. The dry walls absorb mois- ture and swell up, and if this absorption of moisture and its evaporation is unequal on two sides of an organ a curva- ture will result. In this way many seed vessels are rup- tured, the sporangia of ferns are opened, the operculum of mosses is lifted off by the peristome, the hair-like pappus of certain Composites is spread or collapsed, certain seeds are dispersed and buried, etc. One of the peculiarities of this hygroscopic power of certain cells is that the result may be obtained through the absorption of the moisture of the air, and the hygroscopic awns of certain fruits have been used in the manufacture of rough hygrometers (" measures of moisture ").
170. Growth movements. — Growth itself is a great physi- ological subject, but certain movements which accompany it are referred to here. Two kinds of growth movements are apparent.
One may be called nutation, by which is meant that the growing tip of an organ does not advance in a straight line, but bends now toward one side, now toward the other. In this way the tip describes a curve, which may be a circle, or an ellipse of varying breadth ; but as the tip is advancing all the time, the real curve described is a spiral with circular or elliptical cross-section. The sweep of a young hop-vine in search of support, or of various tendrils, may be taken as extreme illustrations, but in most cases
PLANT PHYSIOLOGY 395
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 tire due largely to external causes known as stimuli. Some of the prominent stimuli concerned in directing 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 positive heliotropism (Fig. 279) ; the leaf blades are directed at right angles to the rays of light, showing trans verve heliotropism ; while if there are hold- fasts or aerial roots they are directed away from the light, showing negative heliotropism. The thallus bodies of ferns, liverworts, etc., are transversely heliotropic, as ordinary leaves, a position best related to chlorophyll work. If the light is too intense, leaves may assume an edgewise or pro- file position, a condition well illustrated by the so-called "compass plants." (See Plant Relations, p. 10.)
Geotropism. — In this case the stimulus is gravity, and its influence in directing the parts of plants is very great. All upward growing plants, as ordinary stems, some leaves, etc., are negatively geotropic, growing away from the center of gravity. Tap-roots are notable illustrations of positive geotropism, growing toward the source of gravity with con- siderable force. Lateral branches from a main or tap-root, however, tire usually transversely yeotrupic.
FIG. 279. Sunflower stems with the upper part of the stem sharply bent toward the light, giving the leaves better exposure, the stem showing positive heliotropism. - After SCHAFFNEK.
PLANT 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 (chemotropism), such as direct sperms to the proper female organ ; heat (thermotropism) ; water currents (rheotropism) ; mechanical contact, etc. The most noteworthy illus- trations of the effect of contact are furnished by tendril- climbers. When a nutating tendril comes in contact with a support a sharp curvature is developed which grasps it. In many cases the irritable response goes further, the ten- dril between the plant axis and the support developing a spiral coil.
171. Irritable movements.— The great majority of plants can execute movements only in connection with growth, as described in the preceding section, and when mature their parts are fixed and incapable of further adjustment. Cer- tain plants, however, have developed the power of moving mature parts, the motile part always being a leaf, such as
308
•LANT STRUCTURES
foliage leaf, stamen, etc. It is interesting to note tliat 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 arc very rapid, are due to changes in the turgidity of cells. As already mentioned (§ 157), turgid cells are inflated and rigid, and when turgidity ceases the cells collapse and the tissue becomes flaccid. A. special organ for varying tur- gidity, known as the pulvinus, is usually associated with the motile leaves and leaflets. The pulvinus is practically a mass of parenchyma cells, whose turgidity is made to vary by various causes, and leaf -movement is the result.
The causes which induce some movements are unknown, as in the case of Desmodium gyrans (see Plant Relation*, 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 movod together, and the main leaf-stalk has been directed sharply downward. — After
DUCHARTRE.
PLANT PHYSIOLOGY 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 Dioncva, which snaps its leaves shut like a steel trap when touched (see Plant Relations, 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 pudica (Fig. 280), whose sensitiveness to contact and rapidity of response are remarkable (see Plant Rela- tions, p. 48).
REPBODUCTION
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.
AVhen a special cell is organized for reproduction, dis- tinct from the vegetative cells, it is called a spore, and re- production by spores is introduced. The first spores devel- oped seem to have been those produced by the division of the contents of a mother cell, and are called 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 gametes. The gametes 38
310 PLANT STRUCT UKES
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 sporophyte, and the gametes to the gametophyte. With the further introduction of heterospory, the male and the female gametes are sepa- rated upon different gametophytes, which become much reduced.
With the reduction of the megaspores to one in a spo- rangium (ovule), and its retention, the seed is organized, and the elaborate scheme of insect-pollination is developed.
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 RELATIONS
174. Foliage leaves. — The life relation essential to foliage leaves is the relation to light. This is shown by their positions and forms, as well as by their behavior when deprived of light. This light relation suggests the answer to very many questions concerning leaves. It is not very important to know the names of different forms and differ- ent arrangements of leaves, but it is important to observe that these forms and arrangements are in response to the light relation.
In general a leaf adjusts its own position and its relation to its fellows so as to receive the greatest amount of light. Upon erect stems the leaves occur in vertical rows which are uniformly spaced about the circumference. If these rows are numerous the leaves are narrow ; if they are few the leaves are usually broad. If broad leaves were associ- ated with numerous rows there would be excessive shading ;
an
312 PLANT STKUCTURES
if narrow leaves were associated with few rows there would be waste of space.
It is very common to observe the lower leaves of a stem long-petioled, those above short-petioled, and so on until the uppermost have sessile blades, thus thrusting the blades of lower leaves beyond the shadow of the upper leaves. There may also be a gradual change in the size and direc- tion of the leaves, the lower ones being relatively large and horizontal, and the upper ones gradually smaller and more directed upward. In the case of branched (compound) leaves the reduction in the size of the upper leaves is not so necessary, as the light strikes between the upper leaflets and reaches those below.
On stems exposed to light only or chiefly on one side, the leaf blades are thrown to the lighted side in a variety of ways. 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 shoot. In the foliage-bearing stems the leaves must be displayed to the light and air. Such stems may be sub- terranean, prostrate, floating, climbing, or erect, and all of these positions have their advantages and disadvantages, the erect type being the most favorable for foliage display.
In stems which bear scale leaves no light relation is necessary, so that such shoots may be and often are sub- terranean, and the leaves may overlap, as in scaly buds and bulbs. The subterranean position is very favorable
PLANT ECOLOGY 313
for food storage, and such shoots often become modified as food depositories, as in bulbs, tubers, rootstocks, etc. In the scaly buds the structure is used for protection rather than storage.
The stem bearing floral leaves is the shoot ordinarily called "the flower," whose structure and work have been sufficiently described. Its adjustments have in view polli- nation and seed dispersal, two very great ecological sub- jects full of interesting details.
176. Roots. — Eoots are absorbent organs or holdfasts or both, and they enter into a variety of relations. Most common is the soil relation, and the energetic way in which such roots penetrate the soil, and search in every direction for water and absorb it, proves them to be highly organized members. Then there are roots related to free water, and others to air, each with its appropriate struc- ture. More mechanical are the clinging roots (ivies, etc.), and prop roots (screw pines, banyans, etc.), but their adap- tation to the peculiar service they render is none the less interesting.
The above statements concerning leaves, shoots, and roots should be applied with necessary modifications to the lower plants which do not produce such organs. The light relation and its demands are no less real among the Algae than among Spermatophytes, as well as relations to air, soil, water, mechanical support, etc.
PLANT SOCIETIES
177. Introductory. — Plants are not scattered at hap- hazard over the surface of the earth, but are organized into definite communities. These communities are deter- mined by the conditions of living — conditions which admit some plants and forbid others. Such an association of plants living together in similar conditions is a plant so- ciety. Closely related plants do not usually live together
314 PLANT STRUCTURES
iu the same society, as their rivalry is too intense ; but each society is usually made up of unrelated plants which can make use of the same conditions.
There are numerous factors which combine to deter- mine societies, and it is known as yet only in a vague way how they operate.
178. Ecological factors. — Water.— This is a very impor- tant factor in the organization of societies, which are usu- ally local associations. Taking plants altogether, the amount of water to which they are exposed varies from complete submergence to perpetual drought, but within this range plants vary widely as to the amount of water necessary for living.
Heat. — In considering the general distribution of plants over the surface of the earth, great zones of plants are out- lined by zones of temperature ; but in the organization of local societies in any given area the temperature condi- tions are nearly uniform. Usually plants work only at temperatures between 32° and 122° Fahr., but for each plant there is its own range of temperature, sometimes extensive, sometimes restricted. Even in plant societies, however, the effect of the heat factor may be noted in the succession of plants through the working season, spring plants being very different from summer and autumn plants.
Soil. — The great importance of this factor is evident, even in water plants, for the soil of the drainage area deter- mines the materials carried by the water. Soil is to be considered both as to its chemical composition and its physical properties, the latter chiefly in reference to its disposition toward water. Soils vary greatly in the power of receiving and retaining water, sand having a high recep- tive and low retentive power, and clay just the reverse, and these factors have large effect upon vegetation.
Light. — All green plants can not receive the same amount of light. Hence some of them have learned to live with a
PLANT ECOLOGY
less amount than others, and are " shade plants " as dis- tinct from " light plants." In forests and thickets many of these shade plants are to be seen, which would find an exposed situation hard to endure. In almost every society, therefore, plants are arranged in strata, dependent upon the amount of light they receive, and the number of these strata and the plants characterizing each stratum are im- portant factors to note.
Wind. — This is an important factor in regions where there are strong prevailing winds. Wind has a drying effect and increases the transpiration of plants, tending to impoverish them in water. In such conditions only those plants can live which are well adapted to regulate tran- spiration.
The above five factors are among the most important, but no single factor determines a society. As each factor has a large possible range, the combinations of factors may be very numerous, and it is these combinations which de- termine societies. For convenience, however, societies are usually grouped on the basis of the water factor, at least three great groups being recognized.
179. Hydrophyte societies.— These are societies of water plants, the water factor being so conspicuous that the plants are either submerged or standing in water. A plant completely exposed to water, submerged, or floating, may be taken to illustrate the usual adaptations. The epi- dermal walls are thin, so that water may be absorbed through the whole surface ; hence the root system is very commonly reduced or even wanting ; and hence the water- conducting tissues (xylem) are feebly developed. The tis- sues for mechanical support (stereome) are feebly devel- oped, the plant being sustained by the buoyant power of water. Such a plant, although maintaining its form in water, collapses upon removal. Very common also is the development of conspicuous air passages for internal aera- tion and for increasing buoyancy ; and sometimes a special
316 PLANT STRUCTURES
buoyancy is provided for by the development of bladder- like floats.
Conspicuous among hydrophyte societies may be men- tioned the following : (1) Free-swimming societies, in which the plants are entirely sustained by water, and are free to move either by locomotion or by water currents. Here belong the " plankton societies," consisting of minute plants and animals invisible to the naked eye, conspicuous among the plants being the diatoms ; also the " pond so- cieties," composed of algae, duckweeds, etc., which float in stagnant or slow-moving waters.
(2) Pondweed societies, in which the plants are an- chored, but their bodies are submerged or floating. Here belong the " rock societies," consisting of plants anchored to some firm support under water, the most conspicuous forms being the numerous fresh-water and marine algae, among which there are often elaborate systems of holdfasts and floats. The "loose-soil societies" are distinguished by imbedding their roots or root-like processes in the mucky soil of the bottom (Figs. 281, 2S2). The water lilies with their broad floating leaves, the pondweeds or pickerel weeds with their narrow submerged leaves, are conspicuous illus- trations, associated with which are algae, mosses, water ferns, etc.
(3) Swamp societies, in which the plants are rooted in water, or in soil rich in water, but the leaf-bearing stems rise above the surface. The conspicuous swamp societies are "reed swamps," characterized by bulrushes, cat-tails and reed-grasses (Figs. 283, 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 societies. — These societies are exposed to the other extreme of the water factor, and are composed of plants adapted to dry air and soil. To meet these
320
PLANT STRUCTURES
drought conditions numerous adaptations have been de- veloped and are very characteristic of xerophytic plants. Some of the conspicuous adaptations are as follows : peri-
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 societies are as follows : "rock- societies " composed of plants living upon exposed rock sur- faces, walls, fences, etc., notably lichens and mosses ; "sand societies," including beaches, dunes, and sandy fields ; " shrubby heaths," characterized by heath plants ; "plains," the great areas of dry air and wind developed in the interiors of continents; "cactus deserts," still more arid areas of the Mexican region, where the cactus, agave, yucca, etc., have learned to live by means of the most ex- treme xerophytic modifications ; " tropical deserts," where xerophytic conditions reach their extreme in the combina- tion of maximum heat and minimum water ; " xerophyte thickets," the most impenetrable of all thicket-growths, represented by the " chaparral " of the Southwest, and the "bush" and "scrub" of Africa and Australia; "xero- phyte forests," also notably coniferous. (See Figs. 285, 286, 287.)
181. Mesophyte societies. — Mesophytes make up the com- mon vegetation, the conditions of moisture being medium, and the soil fertile. This is the normal plant condition, and is the arable condition — that is, best adapted for the plants which man seeks to cultivate. If a hydrophytic area is to be cultivated, it is drained and made mesophytic ; if a xerophytic area is to be cultivated, it is irrigated and
39
PLANT ECOLOGY 327
made mesopliytic. As contrasted with hydrophyte and xero- phyte societies, the mesophyte societies are far richer in leaf forms and in general luxuriance. The artificial soci- eties which have been formed under the influence of man, through the introduction of weeds and culture plants, are all mesophytic.
Among the mesophyte grass and herb societies are the '•arctic and alpine carpets," so characteristic of high lati- tudes and altitudes where the conditions forbid trees, shrubs, or even tall herbs ; " meadows," areas dominated by grasses, the prairies being the greatest meadows, where grasses and flowering herbs are richly displayed ; " pastures," drier and more open than meadows.
Among the woody mesophyte societies are the " thick- ets," composed of willow, alder, birch, hazel, etc., either pure or forming a jungle of mixed shrubs, brambles, and tall herbs ; " deciduous forests," the glory of the temperate regions, rich in forms and foliage display, with annual fall of leaves, and exhibiting the remarkable and conspicuous phenomenon of autumnal coloration ; " rainy tropical for- ests," in the region of trade winds, heavy rainfalls, and great heat, where the world's vegetation reaches its climax, and where in a saturated atmosphere gigantic jungles are developed, composed of trees of various heights, shrubs of all sizes, tall and low herbs, all bound together in an inex- tricable tangle by great vines or lianas, and covered by a luxuriant growth of numerous epiphytes. (See Figs. 288, 289.)
GLOSSARY
[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.
A KENE : 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. AXEMOPHILOUS : applied to flowers or plants which use the wind as agent
of pollination. 181. AN*SOCARPIC : 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. ApocAKi'ors: applied to a flower whose carpels are free from one an- other. 226. ARCHEGONIUM : the femnle, egg-producing organ of Bryophytes. Pteri-
dophytes, and Gymnosperms. 100. ABCHESPOEIDM : the first cell or group of cells in the spore-producing
series. 102.
Asi OCARP: a special case containing asci. 58. ASCOSI'ORE : a spore formed within an ascus. 59. Asrrs: 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 Bryophytes the spore-vessel ; in Angiosperms a dry fruit
which opens to discharge its seeds. 98, 211. CARPEL : the megasporophyll 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 hyphae. 58. CONJUGATION : the union of similar gametes. 15. COROLLA : the inner set of floral leaves. 221.
COTYLEDON : the first 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.
DKECIOUS : 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. ENTOMOPHILOUS : applied to flowers or plants which use insects as agents
of pollination. 196.
GLOSSARY 331
EPIGYNOUS : applied to a flower whose 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 cells. 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.
PIETEROGAMOUS : 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 Spermatophytes a membrane investing the nucellus.
178. INVOLUCRE : a cycle or rosette of bracts beneath a flower-cluster, as iu
Umbellifers and Composites. ~7~>. 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 bodv of the ovule. 178.
GLOSSARY 333
OOGONIUM : the female, egg-producing organ of Thallophytes. 16.
OOSPHERE : 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. OvrLE : the megasporangium of Spermatophytes. 178.
PAPPUS : the modified calyx of the Composites. 278.
PARASITE : a plant which obtains food by attacking living plants or ani- mals. 48.
PENTACYCLIC : applied to a flower whose four floral organs are in five cycles, the stamens being in two cycles. 268.
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 Gymno- sperms) or stigma (in Angiosperms). 181.
POLYPETALOUS : applied to flowers whose petals are free from one an- other. 227.
PROTHALLIUM : the gametophyte of Ferns. 130.
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. 237.
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.
SPOROGONIUM : 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. 103.
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 page cited. In such case the subject may be referred to only in the illustration, or it may bn referred to also in the text.]
Absorption, 299. Acacia, 265. Aconitum, 261. Acorus, 219. 243. Actinomorphy, 228. Adder's tongue : see Ophioglossmn. Adiantum, 143, 145. ^Ecidiomycetes, 50, 62. yEcidiospore, 66. ^Ecldium, 66. Agaricus, 68, 69. Agave, 247. Air pore : see Stoma. Akene, 212, 213. 814, 276, 177. Alchemilla, 225. Alder : see Alnus. Algae, 4, 5, 17. Alisma, 210, 240. Almond : see Prunus. Alnus. 257.
Alternation of generations, 94, 129. Amanita, 70. Amaryllidaceae, 247. Amaryllis family: see Amarylli- daceae.
Ambrosia, 279. Ament, 257. Anaptychiii. Hi. ,s' .'.
Anemophilous, 181. Angiosperms, 173, 195, 217. Anisocarpae, 268. Annulus, 136, 146, 150. Anther, 106, 197, 199. Antheridium, 16, 99, 100, 11J, 121.
i,:.;. 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. -'$.:. Apium, 267. Apocarpy, 199. 222, 225. Apocynaceae. 271. Apocynum, 272. Apogamy, 131. Apospory, 132. Apothecium, 79, 81. $ .'. Apple : see Pirus. Aquilegia, 198. Araceae, 243. Araliaceae, 267. Araucaria, 190. Arbor vitae: see Thuja. Arbutus, 198 : see Epigjea. Archegoniates, 101. 337
338
INDEX
Archegonium, 99, 100, US, 114, 133,
135, 161, 167, 179. Archesporiuin, 102, 104, 105, 146. Archichlamydeae, 255. Arctostaphylos, 269. Areolae, 111, 114. Arisasma, 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, 144. Assimilation, 302. Aster, 279. Astragalus, 265. Atherosperma, 198. Azalea, 270.
B
Bacillus, 76.
Bacteria, 21, 75. 76.
Balm : see Melissa.
Banana, 140.
Bark, 284, 289.
Basidiomycetes, 50, 68.
Basidiospore, 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 Vaccinium. Blue-green algae, 6, 17. Blue mould, 60. Boletus, 73, 74. Botrychium, 145, 149. Botrydium, 28. Box elder, 234. Bracket fungus, 72. Brake : see Pteris. Brassica, 2 61. Bryophytes, 2, 93, 172. Brown algae, 6, 32. Bryum, 120, 124. Buckeye, 235. Butomus, 199.
Buttercup : see Ranunculus. Buttercup family : see Ranuncu- laceae.
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,
Caraway : see Carum.
Carbohydrate, 302.
Carbon dioxide, 83.
INDEX
339
Carnivorous plants, 92.
Carpel, 111, 178, 199, 219, 220.
Carpinus, 217, 258.
Carpospore, 44i 45.
Carrot : see Daucus.
Carum, 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 Prunus.
Chestnut, 256.
Chlorophyceae, 6, 21.
Chlorophyll, 5, 83.
Chloroplast, 7, 8.
Chrysanthemum, 279.
Cilia, 10.
Circinate, 136, 143.
Cladophora, 25.
Clavaria, 13.
Climbing fern : see Lygodium.
Closed bundle, 290.
Clover : see Trifolium.
Club mosses, 162.
Cnicus, 278.
Cocklebur : see Xanthium.
Ccenocyte, 27.
Coleochaete, 106. 101.
Collateral bundle, 287.
Collenchyma, 284.
Columella, 104, 105, 106, 126.
Compass plant : see Silphium.
Composite, 275.
Composites, 275, 216, 211, 218.
Concentric bundle, 292.
Conferva forms, 22.
Conidia, 58, 60.
Conifers, 191, 282.
Conium, 261.
Conjugate forms, 31.
Conjugation, 15.
Connective, 196.
Conocephalus, 111.
Convolvulaceae, 271.
Convolvulus forms, 270.
Convolvulus, 273.
Coprinus, 70.
Coral fungus, 13, 74.
Coreopsis, 278.
Coriandrum, 267.
Cork, 284.
Corn, 216, 282, 290.
Cornacese, 267.
Corolla, 220, 221.
Cortex, 283, 284, 288.
Cotton, 206.
Cotyledon, 137, 138, 168, 184, $09,
210, 216, 211.
Cranberry: see Vaccinium. Cratsegus, 262. Crocus, 249. Crucifer, 262. Cruciferae, 262. Cryptogams, 172. Cunila, 274. Cup fungus, 60, 61. Cupule, 112, 114. Cyanophyceae, 6, 17. Cycads, 185, 186, 181, 189. Cyclic, 159, 193. Cyperaceae, 241.
340
INDEX
Cypripedium, 249, 253. Cystocarp, 43, 44. Cystopteris, 78, 144. Cytoplasm, 7.
D
Daisy: see Bellis.
Dandelion : see Taraxacum.
Dasya, 40.
Datura, 197.
Daucus, 266, 267.
Dead-nettle, 228.
Definitive nucleus : see Endosperm
nucleus.
Dehiscence, l'J8, 1!M). Delphinium, 260, 261. Dermatogen, 283. Desmids, 31, 32. Desmodium, 308. Diatoms, 45. Dichotomous, 35. Dicotyledons, 208, 233, 254, 282. Differentiation, 3, 280. Dogbane: see Apocynum. Dog-tooth violet : see Erythronium. Dogwood family : see Cornacea\ Dorsiventral. 109. Downy mildew, 55. Drupe, 264. Digestion, 302. Dioecious, 115. Disk, 276, 277. Dodder, 86.
E
Ear-fungus, 74. Easter lily, 221. Ecology, 297, 311. Economic botany, 297. Ectocarpus, 33. Edogonium, 22, £8. Egg, 16. 202, 204, 205, 206.
Egg-apparatus, 204, 205, 206.
Elater, 103, 113, 118.
Elm : see Ulmus.
Embryo, 137, 167, 168, 170, 183, 207,
208, 209, 210, 211. Embryo-sac, 178, 179, 201, 203, 208. Endosperm, 179, 180, 207, 208, 211. Endosperm nucleus, 202, 205. Entomophilous, 196. Epidermis, 141, 142, 191, 283, 284,
295.
Epigaaa, 269. Epigyny, 224, MS. Epilobium, 212. Epiphyte, 157. Equisetales, 159. Equisetum, 159, 160, 161. Ergot, 60. 61. Erica, 270. Ericaceae. 268. Erigenia, 267. Erythronium. 250. Eusporangiate. 157. Evolution, 3.
Fennel : see Foeniculum.
Ferns, 155, 156.
Fertilization, 16, 181, 206, 207.
Festuca, 2jQ.
Figwort family: see Scrophula-
riaceae.
Filament, 8. 196, 197. Filicales, 155. Fireweed : see Epilobium. Fission. 10. Flax : see Linum. Floral leaves. 218. Florideae, 38. Flower, 218. Flowering plants, 172. Fcenk-ulum, 267.
INDEX
341
Foliar, 166.
Food, 83, 299.
Foot, 98, 102, 137, 138, 168.
Fragaria, 214, 227, 262.
Fruit, 211, 21$, 213, 214,215.
Fucus, 35, 37.
Funaria, 99, 102, 121, 124, 125, 126.
Fungi, 4, 48.
G
Gametangiura, 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. Gentianaceae, 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. Gramineae, 241. Green algne, 6. 21. Green plants, 83. Green slimes, 20. Grimmia, 126. Growth movement, 304. Growth ring, 234. Grain, 241.
40
Grasses, 240.
Grass family : see Grarnineae. Gymnosperms, 171, 173, 195. Gymnosporangium, 67.
H
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 Conium.
Henbane : see Hyoscyamus.
Hepaticae, 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
Hypodermis, 284. Hypogyny, 224, 225. Hyssopus, 274.
I
Indigo : see Indigofera.
Indigofera, 265.
Iiidusium, 136, 143, 144.
Inflorescence, 230.
Insects and flowers, 90.
Integument, 178, 179, 201, 202,
Involucre, 267, 275, 277.
Ipoinoea, 228, 270.
Iridaceae, 247.
Iris, 248, 251.
Iris family : see Iridaceae.
Irritable movement, 307. -
Isocarpa?, 268.
Isoetes, 169.
Isogamy, 15.
Japan lily, 21$.
Jungermannia, 105, 115, 116, 117.
Juniper, 194-
K
Kalmia, 270.
Labiatae, 272.
Labiates, 272.
Lactuca, 279.
Lam in aria. 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.
Leguminosa?, 264.
Lemna, 201.
Lepidozia, 117.
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, 205, 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. Lycopodiurn, 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, 104, HO, 111, 112, 113,
114.
Marguerite: see Leucanthemum. Marjoram : see Origanum. Marrubium, 275. Marsh marigold : see Caltha.
INDEX
343
Marsilia, 158.
Mega-sporangium, 152, 177, 179. Megaspore, 152, 165, 167, 179, 201,
203.
Megasporophyll, 152, 165, 177, 199. Melissa, 275. Mentha, 229, 274. Meristem, 281.
Mesophyll, 141, 142, 191, 295. Mesophytes, 324. Mestome, 282.
Micropyle, 178, 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, 808, 309. Mint : see Mentha. Mint family : see Labiatas. Monocotyledons, 208, 232, 236, 289. Monoecious, 115. Monopodial, 35. Monotropa, 270. Moon wort: see Botrychium. Morels, 60, 62.
Morning-glory : see Ipomoea. 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.
Mustard family : see Cruciferae. Mycelium, 49.
Mycoinycetes, 50. Mycorrhiza, 87, 88. Myristica, 214. Myrmecophytes, 90, 91. Myxomycetes, 74, 75.
N
Naias, 237. Narcissus, 247. Nemalion, 43. Nepeta, 275. Nicotiana, 227, 275. Nightshade family : see Solanacea?. Nostoc, 18.
Nucellus, 178, 179, 201, 202, 203. Nucleus, 7. Nutation, 304. Nutmeg, 214. Nutrition, 3, 299. Nyctitropic movement, 309. Nymphaeaceae, 261.
0
Oak, 255, 256.
CEdogonium : see Edogonium. Onoclea, 145, 147, 148. Oogonium, 16. Oosphere, 16. Oospore, 16, 101. Open bundle, 287. Operculum, 122, 125. Ophioglossum, 145, 149. Orchidacea3, 249. Orchids, 249, 252. 253, 254. Orchid family : see Orchidacea?. Origanum, 274. Ornithogalum, 247. Oscillaria, 19. Osmunda, 145. 156. Ostrich fern : see Onoclea. Ovary, 199, 200, 202. Ovule, 178, 179, 201, 203.
344
INDEX
Palisade tissue, 142, 295.
Palmaceae, 241.
Palm family : see Palmaceae.
Palms, 241, 242, 243.
Papaveraceae, 261.
Pappus, 276, 277, 278.
Parasites, 48, 85.
Parenchyma, 280, 281, 282, 288.
Parmelia, 79.
Parsley: see Petroselinum.
Parsley family : see Umbellifera3.
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.
Pellasa, 146.
Penicillium, 60.
Pentacycla?, 268.
Pentstemon, 275.
Peony, 220.
Pepper, 211, 258.
Pepper family : see PiperaceaB.
Perianth, 219, 220, 221.
Periblem, 288.
Perigyny, 225, 226.
Peristome, 126, 127.
Peronospora, 55, 56.
Petal, 220, 221.
Petiole, 141.
Petroselinura, 267.
Phaeophyceae, 6, 32.
Phanerogams, 172.
Phaseolus, 216, 265.
Phloem, 285, 287, 288, 290, 292, 29+
Phlox, 228, 271.
Photosyntax, 84.
Photosynthesis, 84, 302.
Phycomycetes, 50, 51.
Physcia, 79.
Physiology, 297.
Picea, 179, 181, 182.
Pileus, 71.
Pine : see Pin us.
Pineapple, 215.
Pinus, 173, 175, 176, 177, 178, 181,
183, 184, 188, 191, 286. PiperaceaB, 258. Pirus, 225, 262, 263. Pistil, 199, 200, 219, 220. Pisum, 265. Pith, 285, 287, 288. Planococcus, 76. Plantaginaceae, 275. Plant body, 6. Plant societies, 313. Plasmodium, 74, 75. Plastid, 7, 8. Platycerium, 182. 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. 207. Pollination, 181. Polyembryony, 183. Polymorphism, 63. Polypetaly, 226. Polyporus, 71, 72. Polysiphonia, 44- Polytrichum, 96.
INDEX
345
Pome, 263.
Pondweeds, 237.
Poplars, 255.
Popowia, 198.
Poppy, 261.
Poppy family : see Papaveraceae.
Populus, 256.
Pore-fungus, 72.
Potamogeton, 237, 238.
Potato : see Solanuin.
Potentilla, 225, 262.
Proteid, 302.
Prothallium, 130, 132, 134.
Protococcus forms, 22.
Protonema, 95, 98.
Protoplasm, 7.
Prunus, 218, 262.
Pseudomonas, 76.
Pseudopodium, 105, 123, 124.
Pteridophytes, 2, 128, 172, 291.
Pteris, 133, 134, 135, 137, 141, 142,
143, 145, 281, 291, 292, 293. Ptilota, 42.
Puccinia, 63, 64, 65, 66. Puff-balls, 68, 74. Pulvinus, 308.
Q
Quillwort : see Isoetes.
R
Rabdonia, 41. Radiate bundle, 294. Radicle, 209. Radish, 120.
Ragweed : see Ambrosia. Ranunculaceae, 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, 164.
Rhododendron, 270, 271.
Rhodophyceae, 6, 38.
Riccia, 104, 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.
Rosaceae, 262.
Rose family : see Rosaceae.
Rosin- weed : see Silphium.
Rosmarinus, 275.
Royal fern : see Osmunda.
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. Sclerenchyma, 281, 282, 284, 285,
288, 290, 291. Scouring rush, 159. Scrophulariacea?, 275. Scutellaria, 275. Sedge family : see Cyperaceas. Seed, 183, 184, 210, 811, 212, 214. 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. Solanaceae, 275. Solanum, 198, 275. Solidago, 279. Solomon's seal, 233. Sorus, 136, 143, 144. 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.
Spiraa, 262.
Spiral, 193.
Spirillum, 76.
Spirogyra, 28, 29, 30.
Spongy tissue, 142.
Sporangium, 10, 136, 143, 145, 150,
157, 163, 179. Spore, 9. Sporidium, 65. Sporogenous tissue, 103. Sporogonium, 98, 102, 104, 105, 106,
125, 126.
Sporophore, 49, 50. Sporophyll, 145, 147, 148, 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, 141, 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
Sympetalae, 268. Sym petaly, 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. Tetracyclae, 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. Tracheae, 285, 286. Tracheids, 286. Transfer of water, 300. Transpiration, 301. Tree fern, 140. Trichia, 75. Trichogyne, 43, 44. Trillium, 207, 246, 265. Truffles, 60. Turgid ity, 298. Typha, 239, 240.
U
Umbel, 266, 267. Umbelliferae, 266. Umbellifers, 266. Ulmus, 210, 256. Ulothrix, 12, 13, 22. Uredo, 64. Uredospore, 63, 64.
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, 142.
Venation, 233.
Verbascum, 275.
Verbenaceae, 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-
cese.
Water moulds, 51. Wheat rust, 63, 64, 65, 66. Willow : see Salix. Wind, 315. Wintergreen : see Gaultheria.
348
Wistaria, 265. Witches'-broora, 60. Wormwood : see Artemisia.
Xanthium, 279.
Xerophytes, 319.
Xylem, 285, 287, 288, 290,
INDEX
Yeast, 62.
Zannichellia, 237. Zoospore, 10. Zygomorphy, 228, Zygospore, 15. Zygote, 15.
THE END
TWENTIETH CENTURY TEXT BOOKS.
The closing years of the present century are witnessing the beginning of a remarkable awakening of interest in our American educational problems. There has been repeated and elaborate discussion in every part of our land on such topics as the co- ordination of studies, the balancing of the different contending elements in school programmes, the professional training of teachers, the proper age of pupils at the different stages of study, the elimina- tion of pedantic and lifeless methods of teaching, the improvement of text books, uniformity of college-entrance requirements, and other questions of like character.
I n order to meet the new demands of the country along the higher plane of educational work with a complete and correlated series of text books fully embodying the latest advances in our education, the Twentieth Century Text Books are now offered.
At every step in the planning of the series care has been taken to secure the best educational advice, in order that the books issued may really meet the increasing demand that now comes alike from academies, high schools, and colleges for text books that shall be pedagogically suitable for teach- ers and pupils in the schools, sound in modern scholarship, and adequate for college preparation.
The editors and the respective authors of the Twentieth Century Text Books have been chosen with reference to their qualifications for the special work assigned to them. These qualifications are, first, that the author should have a thorough knowl- edge of his subject in its latest developments, espe- cially in the light of recent educational discussions ; second, that he should be able to determine the relative importance of the subjects to be treated in a text book ; third, that he should know how to present properly his topics to the ordinary student.
The general editorial supervision of the series has been placed in the hands of Dr. A. F. Night- ingale, Superintendent of High Schools, Chicago, and Professor Charles H. Thurber, of the Univer- sity of Chicago, men thoroughly conversant with every phase of education.
The offer of a complete series of text books for these higher grades of schools, issued under auspices so favorable, concentrating and co-ordinating such a force of able writers all working with one end in view, is an event worthy of the twentieth century, and a good omen for the educational welfare of the future.
Nearly one hundred volumes are in prepara- tion, and several are now ready. Others will follow rapidly, the issue of which will be duly announced.
D. APPLETON AND COMPANY, NEW YORK.
TWENTIETH CENTURY TEXT BOOKS.
A History of the American Nation.
By ANDREW C. MCLAUGHLIN, Professor of American History in the University of Michi- gan. With many Maps and Illustrations. i2mo. Cloth, $1.40 net.
" One of the most attractive and complete one- volume his- tories of America that has yet appeared." — Boston Beacon.
" Complete enough to find a -place in the library as well as in the school." — Denver Republican.
"This excellent work, although intended for school use, is equally good for general use at home." — Boston Transcript.
"It should find a place in all historic libraries." — Toledo Blade.
" Clearness is not sacrificed to brevity, and an adequate knowledge of political causes and effects may be gained from this concise history." — New York Christian Advocate.
"A remarkably good beginning for the new Twentieth Cen- tury Series of text-books. . . . The illustrative feature, and especially the maps, have received the most careful attention, and a minute examination shows them to be accurate, truthful, and illustrative." — Philadelphia Press.
"The work is up to date, and in accord with the best modern methods. It lays a foundation upon which a superstructure of historical study of any extent may be safely built." — Pittsburg Times.
"A book of rare excellence and practical usefulness." — Salt Lake Tribune.
"The volume is eminently worthy of a place in a series des- tined for the readers of the coming century. It \* highly creditable to the author." — Chicago Evening Post.
D. APPLETON AND COMPANY, NEW YORK.
TWENTIETH CENTURY TEXT BOOKS.
Plant Relations.
A First Book of Botany. By JOHN M. COULTER, A. M., Ph. D., Head of Department of Botany, University of Chicago. 1 2mo. Cloth, $ i . i o net.
"A pretty full and very interesting knowledge of the general relations of plant life — the conditions under which plants grow, and the way in which their anatomical and physiological charac- teristics testify to the circumstances of their environment, and their needs as regards nutrition and reproduction. . . . The name and position of the author certify to the high quality of the book." — The Illustrated Buffalo Express.
" This beautifully printed volume is a striking example of the vast advance which has been made in recent years in the mechan- ical features of text-books. The illustrations are a special feature of the book, and a great number of them are made from photo- graphs, beautifully reproduced in half-tone. We are acquainted with no manual of botany that can more usefully serve to supple- ment work in the laboratory and the field by organizing the re- sults obtained from investigation and suggesting explanations of them." — Philadelphia Press.
ft The work is beautifully printed and illustrated, and the subjects are treated in a manner at once entertaining and instruc- tive. Professor Coulter is an enthusiast with the rare quality of imparting his enthusiasm to his students in class and out of it." — Chicago Journal.
"A class-room book of great excellence, calculated to give a fair botanical knowledge to a student in six months' study or to prepare him for the more detailed studies which Professor Coulter will embody in a second book. The illustrations are verv nu- merous and of supreme excellence." — Chicago Interior.
D . APPLETON AND COMPANY, NEW YORK.
INTERNATIONAL EDUCATION SERIES.
12mo, cloth, uniform binding.
'T'HE INTERNATIONAL EDUCATION SERIES was projected for the pur- •*• pose of bringing together in orderly arrangement the beet writings, new and old, upon educational subjects, and presenting a complete course of reading and training for teachers generally. It is edited by WILLIAM T. HARRIS, LL. D., United States Commissioner of Education, who has contributed for the different volumes in the way of introduction, analysis, and commentary. The volumes are tastefully and substantially bound in uniform style.
VOLUMES NOW READY.
1. The Philosophy of Education. By JOHANN K. F. ROSENKRANZ, Doc tor of Theology and Professor of Philosophy. University of KOnigsberg. Translated by ANNA C. BRACKETT. Second edition, revised, with Com-
mentary and complete Analysis. $1.50.
2. A History of Education. By F. V. N. PAINTER, A.M., Professor of Modern Languages and Literature, Roanoke College, Va. $1.50.
3 The Rise and Early Constitution of Universities. WITH A SUR- VEY OP MEDLEVAL EDUCATION. By S. S. LAURIE, LL. D., Professor of the Institutes and History of Education, University of Edinburgh. $1.50.
4. The Ventilation and Warming of School Buildings. By GILBERT
B. MORRISON, Teacher of Physics and Chemistry, Kansas City High School. $1.00.
5. The Education of Man. By FRIKDRICH FROEBEL. Translated and an
notated by W. N. HAILMANN, A.M., Superintendent of Public Schools, La Porte, Ind. $1.50.
6. Elementary Psychology and Education. By JOSEPH BALDWI. ,
A. M., LL. D., author of " The Art of School Management.11 $1.50.
7. The Senses and the Will. (Part I of "THE MIND OF THE CHILD.")
By W. PREYER, Professor of Physiology in Jena. Translated by H. W. BROWN, Teacher in the State Normal School at Worcester. Mass. $1.50.
8. Memory : What it is and How to Improve it. By DAVID KAY,
F. R. G. S., author of " Education and Educators," etc. $1.50.
9. The Development of the Intellect. (Part II of " THE MIND OP THE
CHILD.") By W. PREYER, Professor of Physiology in Jena. Translated by H. W. BROWN. $1.50.
10. How to Study Geography. A Practical Exposition of Methods and
Devices in Teaching Geography which apply the Principles and Plans of Ritter and Guyot. By FRANCIS W. PARKER, Principal of the Cook County (Illinois) Normal School. $1.50.
11. Education in the United States : Its History from the Earliest
Settlements. By RICHARD G. BOONE, A.M., Professor of Pedagogy, Indiana University. $1 50.
12. European Schools ; OR, WHAT I SAW IN THE SCHOOLS OP GERMANY,
FRANCE, AUSTRIA, AND SWITZERLAND. By L. R. KLEMM, Ph. D., Principal of the Cincinnati Technical School. Fully illustrated. $2.00.
13. Practical Hints for the Teachers of Public Schools. By GEORGE
HOWLAND, Superintandent of the Chicago Public Schools. $1.00.
14. Pestalozzi : His Life and Work. By ROGER DE GUIMPS. Authorized
Translation from the second French edition, by J. RUSSELL, B. A. With an Introduction by Rev. R. H. QUICK, M. A. $1.50.
15. School Supervision. By J. L. PICKARD, LL. D. $1.00.
16. Higher Education of Women in Europe. By HELENS LANGB, Berlin
Translatedand accompanied by comparative statistics by L.R. KLEMM. $1.00.
17. Essays on Educational Keformers. By ROBERT HERBERT QUICK,
M. A., Trinity College. Cambridge. Only authorized edition of the work as rewritten in 1890. $1.50.
18. A Text-Book in Psychology. By JOHANN FRIEDRICH HERBART. Trans-
lated by MARGARET K. SMITH. $1.00.
THE INTERNATIONAL EDUCATION SERIES.— (Continued.)
19. Psychology Applied to the Art of Teaching. By JOSEPH BALDWIN,
A.M.,LL.D. $1.50.
20. Rousseau's Emile ; OR, TREATISE ON EDUCATION. Translated and an-
notated by W. H PAYNE, Ph. D., LL. D. $1.50.
21. The Moral Instruction of Children. By FELIX ADLER. $1.50.
22. English Education in the Elementary and Secondary Schools^
By ISAAC SHARPLESS, LL. D., President of liaveiford College. $1.00.
23. Education from a National Standpoint. By ALFRED FOUILLEE. $1.50
24. Mental Development of the Child. By W. PREYER, Professor ol
Physiology in Jena. Translated by H. W. BROWN. $1.00
25. How to Study and Teach History. By B. A. HINSDALE, Ph. D., LL. D.,
University of Michigan. $1.50.
26. Symbolic Education. A COMMENTARY ON FROEBEL'S " MOTHER-PLAY."
By SUSAN E. BLOW. $1.50.
27. Systematic Science Teaching. By EDWARD GARDNIER HOWE. $1.50.
28. The Education of the Greek People. By THOMAS DAVIDSON. $1.50.
29. The Evolution of the Massachusetts Public-School System. By
G.H.MARTIN, A.M. $1.50.
30. Pedagogics of the Kindergarten. By FRIEDRICH FROEBEL. 12mo. $1.50.
31. The Mottoes and Commentaries of Freidrlch Froebel's Mother^
Play. By SUSAN E. BLOW and HENRIETTA R. ELIOT. $1.50.
32. The Songs and Music of Froebel's Mother-Play. By SUSAN E.
BLOW. $1.50.
33. The Psychology of Number, and its Application to Methods of
Teaching Arithmetic. By JAMES A. MCLELLAN, A.M., and JOHN DEWEY, Ph. D. $1.50.
34. Teaching the language-Arts. SPEECH, READI> G. COMPOSITION. By
B. A. HINSDALE, Ph. D., LL. D. $1.00.
35. The Intellectual and Moral Development of the Child. PART I.
Containing Chapters on PERCEPTION, EMOTION. MEMORY, IMAGINATION, and CONSCIOUSNESS. By GABRIEL COMPAYRE. Translated from the French by MARY E. WILSON. $1.50.
36. Herbart's A B C of Sense-Perception, and Introductory Works
By WILLIAM J. ECKOFF, Ph. D., Pd. D. $1.50.
37. Psychologic Foundations of Education. By WILLIAM T. HARRIS
A.M,LL. D. $1.50.
38. The School System of Ontario. By the Hon. GEORGE W. Ross, LL. D.,
Minister of Education for the Province of Ontario. $1.00.
39. Principles and Practice of Teaching. By JAMES JOHONNOT. $1.50.
40. School Management and School Methods. By JOSEPH BALDWIN.
$1.50.
41. Froebel's Educational L.aws for all Teachers. By JAMES L.
HUGHES, Inspector of Schools, Toronto. $1.50.
42. Bibliography of Education. By WILL S. MONFOE, A. B. $2.00.
43. The Study of the Child. By A. R. TAYLOR, Ph. D. $1.50.
44. Education by Development. By FRIEDRICH FROEBEL. Translated by
JOSEPHINE JARVIS. $1.50.
45. Letters to a Mother. By SUSAN E. BLOW. $1.50.
46. Montaigne's The Education of Children. Translated by L. E. REC-
TOR, Ph. D. $1.00.
47. The Secondary School System of Germany. Bv FREDERICK E.
BOLTON $1.50.
OTHER VOLUMES IN PREPARATION.
D. APPLETON AND COMPANY, NEW YORK.
The Races of Europe.
A Sociological Study. By WILLIAM Z. RIPLEY, Ph. D., Assistant Professor of Sociology, Massa- chusetts Institute of Technology ; Lecturer in An- thropology at Columbia University, in the City of
New York. Crown 8vo, cloth; 650 pages, with 85 Maps and 235 Portrait Types. With a Supplementary Bibliography of nearly 2,000 Titles, separately bound in cloth, issued by the Boston Public Library. 178 pages. - - Price, $6.00
"One of the most fascinating sociological and anthropo- logical studies that have been offered of late to the public. . . . The book is one to be studied with care, and it is a pleasure to commend it as most helpful to sociological students."
Chicago Evening Post.
"Will win the approval of all thoughtful readers; and the care, patience, skill, and knowledge with which it is planned, and the highly satisfactory manner in which the plan is car- ried out, call for the very highest praise."
Boston Saturday Evening Gazette.
"One of the most important works of the year."
New York Mail and Express.
"A valuable and interesting book. . . . Will attract the attention of all students of anthropology and all its kindred subjects. While it will most deeply interest advanced schol- arly readers, it at the same time abounds in value for those not among the learned classes. ' ' Chicago Inter- Ocean.
"An important work in the domain of anthropology and a book of supreme interest at the present moment."
Chicago Times-Herald.
"Not only a profound sociological study but a scholarly contribution to the science of anthropology and ethnology by an eminent authority." Philadelphia Press.
D. APPLETON AND COMPANY, NEW YORK.
LITERATURES OF THE WORLD.
Edited by
EDMUND GOSSE, Hon. M. A. of Trinity College, Cambridge. J2mo. Cloth, $1.50 each.
A succession of attractive volumes dealing with the history of literature in each country. Each volume will contain about three hundred and fifty I2mo pages, and will treat an entire literature, giving a uniform impression of its development, history, and character, and of its relation to previous and to con- temporary work.
NOW READY.
Bohemian Literature. By FRANCIS, Count LUT-
zow, author of "Bohemia : An Historical Sketch."
Japanese Literature. By W. G. ASTON, C. M. G.,
M. A., late Acting Sec'y at the British Legation at Tokio. Spanish Literature. By J. FITZMAURICE-KELLY,
Member of the Spanish Academy. Italian Literature. By RICHARD GARNETT, C. B.,
LL. D., formerly Keeper of Printed Books in the British
Museum.
Ancient Greek Literature. By GILBERT MURRAY,
M. A., Professor of Greek in the University of Glasgow. French Literature. By EDWARD DOWDEN, D. C. L., LL. D., Professor of English Literature at the University of Dublin.
Modern English Literature. By the EDITOR.
IN PREPARATION. American Literature. By Prof. W. P. TRENT, of the University
of the South. German Literature. Hungarian Literature. By Dr. ZOLTAN BEOTHY, Professor of
Hungarian Literature at the University of Budapest. Latin Literature. By Dr. ARTHUR WOOLGAR VERRALL, Fellow
and Senior Tutor of Trinity College, Cambridge. Modern Scandinavian Literature. By Dr. GEORG BRANDES,
of Copenhagen. Sanskrit Literature. By A. A. MACDONELL, M. A., Deputy
Boden Professor of Sanskrit at the University of Oxford.
Russian Literature. By R. WALISZEWSKI. D. APPLETON AND COMPANY, NEW YORK.