Plant life, considered with special reference to form and function

Preface

IN recognition of the fact that the study of botany in the past has been too much a study of books about plants, numerous laboratory manuals have been published which make possible the study of plants themselves. Laboratory work has now become well-nigh universal. With the strenuous insistence that this method should be used in the secondary schools, there has been a growing danger that such study would degenerate into mere memory training, unless the relation of the facts, often entirely isolated in the pupil's mind, were clearly brought out. Since laboratory study soon came to include the examination of the lower plants as well as seed plants, and has now begun to include some experiments in their physiology, the absence of an elementary account of the form and functions of plants of all groups has made itself felt. I am not aware that any book at present attempts to meet this need.

To the proper teaching of botany in secondary schools such a book is indispensable. However capable the teacher may be to gather up the facts observed in the laboratory and to relate them with others so as to produce a clear conception of plant life, he cannot wisely rely upon the lecture for pupils of 13 to 1 8 years. They need the printed page, which appeals to eye as well as ear, if the principles and facts are to be firmly grasped.

• • •

in

IV PREFACE.

PLANT LIFE is an attempt to exhibit the variety and progressive complexity of the vegetative body; to discuss the more important functions ; to explain the unity of plan in both the structure and action of the reproductive organs ; and finally to give an outline of the more striking ways in which plants adapt themselves to the world about them. I have made an effort to treat these subjects so that, however much the student may have still to learn, he will have little to unlearn ; for eradication of false notions is the despair of the college teacher of science.

This is not a book to be memorized and recited. If so used it is abused. It aims to be intelligible to pupils 13 to 18 years of age who are engaged in genuine laboratory study — not at irregular hours, without supervision, in a school desk, or at home, but in a suitable laboratory, with regular time (an hour and a half daily if possible), under the direction of a live teacher who has studied far more botany than he is trying to teach. I am aware that such conditions are yet unrealized in many schools ; but they may be gradually reached. That PLANT LIFE may prove useful in botanical instruction even under the most unfavorable conditions, I permit myself to hope rather than to expect.

This book may be used to supplement any laboratory guide or the directions prepared by the teacher. For the sake of teachers who may not wish to use two books, or who lack time and facilities for preparing laboratory directions, I have outlined a course of study in Appendix I which can be carried out with the equipment listed in Appendix III. A description of the material needed and of suitable methods of preserving it forms Appendix II. Each teacher will, of course, need to modify the directions to suit the material available. I have always found it easier to prepare directions to fit the material than to create the material to fit the directions. The "demonstrations " of Appendix I are intended as suggestions to the

PREFA CE. V

teacher of things which it is advisable to show to pupils under the compound microscope, in case inadequate equipment, lack of time, or difficulty of preparation forbid class study.

I have made the directions fullest in relation to cryptogams and physiology because these fields are most unfamiliar to teachers at present. Further directions for the study of seed plants can readily be provided from the books listed in Appendix IV.

For laboratory study it is necessary to select certain illustrative types and observe their structure. In the text I have not specifically discussed these plants, but have treated general topics so as to correlate the facts gained by a study of types with others which can be readily interpreted by means of the experience in the laboratory. References from the bare directions of Appendix I to the paragraphs and figures of the text are abundant and are intended to aid the student in the comprehension of the type under observation. It may be objected that he is thus aided too much. But I believe that in his first steps in laboratory training the student requires a large amount of help, and that its results are more often nullified by too little assistance than by too much.

In the text I have neither sought nor avoided the use of technical terms. I have refrained from making the book a mere illustrated glossary. Yet I see no advantage, even for young students, in repeated circumlocution, for which a single word might stand. Definitions of most of the technical terms used may be found by means of the index ; others are defined in the standard dictionaries. The careful teacher will insist upon a clear understanding of the meaning of terms and the accurate use of language.

I have refrained from frequent citation of plants by name as examples of facts stated, chiefly because beginners are rarely familiar with any plants except the commonest domesticated ones and a few forest or shade trees.

VI PREFA CE.

No apology is necessary for the exclusive use of the metric system. If pupils lack familiarity with it, the actual handling of metric measures and weights will soon remedy this. A useful chart showing the units may be obtained of the American Metrological Society, 41 East Forty-ninth Street, New York, for ten cents.

Very few of the illustrations are original. In the main they have been selected from a wide range of standard works with especial care to secure accuracy and clearness. Whenever possible the source of the figure and its magnification have been given. The attention of the teacher is invited to the very full description which accompanies each figure. In these explanations will be found much matter which is often put into subordinate paragraphs in other books. I have observed that students are prone merely to " look at ' ' figures, and rarely study them. I therefore suggest that real study of the illustrations as supplementing the text be insisted upon. Sections are apt to be puzzling to beginners unless they are taught how to interpret them. This can be done by requiring them to sketch on paper or blackboard imaginary sections of common objects in different planes. Articles of regular form, such as a pencil, book, slate, ink bottle, desk, etc., may be " sectioned," until from sketches of sections in three planes at right angles the student can construct a mental image of the object.

Although divided into four parts, it has not been possible to keep the subject matter of each wholly distinct, since morphology, physiology and ecology are so interrelated. Indeed it has been thought best to combine the morphology and physiology of the reproductive organs to form Part III, rather than to divide it between two. The teacher will do well to see that the pupil does not neglect the abundant cross references.

While the whole book is simply a restatement of widely

PREFACE. VI 1

known facts, for which I am mainly indebted to general treatises, monographs and shorter papers, 1 am constrained to acknowledge for Part IV my special indebtedness to NVarming' s Lehrbuch dcr okologischen Pflanzengeographie and Ludwig's Lehrbuch der Pflanzenbiologie.

C. R. B.

UNIVERSITY 01- CHICAGO,

Contents

PREFACE

PAGE

iii

Part I: The Vegetative Body

INTRODUCTION : THE UNIT OF STRUCTURE CHAPTER I. SINGLE-CELLED PLANTS AND COLONIES II. LINEAR AND SUPERFICIAL AGGREGATES

III. THE THALLUS OF THE HIGHER ALG/E .

IV. THE FUNGUS BODY OF HYPHAL ELEMENTS .

V. LIVERWORTS AND MOSSES .

VI. FERNWORTS AND SEED PLANTS

VII. THE ROOT

VIII. THE SHOOT

IX. THE STEM ....;..

X. THE LEAVES ,

17 26

39 49 60

"7

Part II : Physiology. Chapter XL Introduction: The Unit Of Function. . 143

XII. THE MAINTENANCE OF BODILY FORM . . 147

XIII. NUTRITION 151

XIV. GROWTH 178

XV. THE MOVEMENTS OF PLANTS l88

Part Iii: Reproduction

CHAPTER XVI. INTRODUCTION : REPRODUCTIVE STRUCTURES . 209 XVII. VEGETATIVE REPRODUCTION .... 211

XVIII. SEXUAL REPRODUCTION 268

ix

CONTENTS.

Part IV : Ecology. Page

CHAPTER XIX. FORMS OF VEGETATION 308

XX. MESOPHYTES 312

XXI. XEROPHYTES AND HALOPHYTES . . . 318

XXII. HYDROPHYTES 327

XXIII. ADAPTATIONS TO OTHER PLANTS AS SUPPORTS 330

XXIV. SYMBIOSIS. . 333

XXV. ANIMALS AS FOOD, FOES, OR FRIENDS . . 342

XXVI. PROTECTION AND DISTRIBUTION OF SPORES

APPENDIX I. DIRECTIONS FOR LABORATORY STUDY . . . 369 II. DIRECTIONS FOR COLLECTING AND PRESERVING

III. APPARATUS AND REAGENTS 409

IV. REFERENCE BOOKS ...... 413

V. OUTLINE OF CLASSIFICATION .... 415

INDEX 419

PLANT LIFE.

Introduction

1. Units of structure. — An examination of any plant by proper methods reveals the fact that it is made up of one or more units of structure. The unit of structure of a brick wall is the individual brick. Each has a definite shape and relation to others, upon which the form of the wall depends. The unit of structure of a plant is called a cell. The cells have each a definite form and relation to others, and upon these two factors the form of the entire plant depends.

But between the plant and the brick wall there is this important difference. The bricks, after being perfectly formed, were put together. The cells of the plant are produced where they lie and gradually grow to a mature form and size. The bricks are originally disconnected ; the plant-cells are connected by origin, and only as they become mature do they separate, if at all.

2. The cell. — A plant-cell is a minute portion of living matter, called protoplasm or plasma, generally surrounded by a membrane, called the cell-wall (fig. i). If the brick in the previous illustration be taken to represent the protoplasm, the mortar may be considered as the cell-Wall.*

* This illustration must be carried no further than to show the relation of position of these two parts of a cell.

PLANT LIFE.

3. Protoplasm.- -The protoplasm is the essential part of the cell. It constructs the cell-wall. Rarely, if ever, is it uniform throughout, but is differentiated into distinct members, each having special work to do. In the most com-

pletely differentiated active cells the greater part of the protoplasm consists of a finely granular or nearly transparent, colorless portion, called cytoplasm. Embedded in the cytoplasm are the nucleus, centrospheres (figs, i and 2), and plastids (figs. 3 to 8).

4. Cytoplasm. - • This is not a single substance, but a mixture of several different substances, so in-

±"'roasnpdhearCeCs°Tm The* line timatcly mixed and so unstable that

it is not possible to analyze it.

FIG. I.-A cel (the megaspore)

"io dianu- Moreover, the nature and amount

After Guignand. Qf ^ components are probably

variable. Most of the substances belong to the class of compounds called proteids, so that cytoplasm responds to proteid tests and is often spoken of as a mixture of proteids. In addition there are frequently present other organic substances (such as amides, carbohydrates, fats, and enzymes), and always small quantities of mineral matters which appear as ash when cytoplasm is completely burned. The minute granules embedded in the cytoplasm are of various nature. Most of them are solid substances.

5. Vacuoles. — Scarcely distinguishable from these at first are the minute cavities, called vacuoles, filled with dilute watery solutions of many different substances, the cell- sap. In all but the youngest cells more or fewer of these bubbles of water may unite to form larger ones (fig. 7). These often increase so as to occupy the greater part of the space within

IXTRODUCTIOX. 3

the cell-wall, being separated only by plates of protoplasm. When all vacuoles fuse into one the cytoplasm is crowded as a thin layer against the wall, with sometimes strands of it crossing the vacuole as the remnants of the plates at an earlier stage (fig. T88).

6. Nucleus.- -The nucleus varies much in shape. In cells whose diameters are nearly equal, it is generally spherical or ovoid, but in elongated cells it may become spindleshaped or cylindric. It is surrounded by a very delicate membrane, and is composed of two sorts of substances, one of which can be readily stained by certain liquid dyes, while the other usually remains uncolored (fig. 2). The nucleus

A B

FIG. 2. — A part of the same cell as in fig. i. but older, with the nucleus beginning to divide. The dark thread in A, separated into pieces in £, represents the chromatin of the nucleus deeply stained, the rest of the nuclear material being unstained, a, centrospheres. Magnified 600 diam. — After Guignard.

may divide into two, a regular succession of changes in the arrangement of the materials composing it characterizing this process, which is commonly followed by the formation of a partition-wall separating the cell into two parts, each containing one of the daughter-nuclei.

PLANT LIFE.

7. Centrospheres. — The centrospberes are intimately related to the nucleus. They are two very minute spherical bodies lying in contact with it (figs, i, 2). When the nucleus is about to divide one centrosphere goes to each pole (fig. 2, B), and the separation of the nuclear material occurs near the nuclear equator. Just as this occurs the centrospheres divide, forming a pair at each pole. Two accompany each daughter-nucleus. Their purpose is not yet fully understood.

FIG.

FIG. 5.

FIG. 3. — A cell from the interior of the leaf of the oat, showing its wall, and some inclusions of the cytoplasm, z, the nucleus; c, chloroplasts; 0, an oil-drop. Magnified about 100 diam. — After Zimmermann.

FIG. 4. — A, chloroplasts from the skin of the petiole of ivy; B, from the inner leafcells of morning-glory; C, from the same cells of Achyranthes. The shaded parts are protoplasm in which are embedded starch-granules, j, and proteid crystalloids, k. Magnified about 1000 diam. — After Zimmermann.

FIG. 5. — Leucoplasts from a young shoot of Canna. The shaded part is protoplasm, in which are embedded starch-grains, s, and proteid crystalloids, k. Magnified about 1000 diam. — After Schimper.

8. Plastids.--In most cells there are also other protoplasmic structures, the plastids. In young cells these are small, rounded, colorless bodies. As the cell grows older they increase in size and number. At maturity, in cells which lie near the surface of green plants, they are commonly roundish or biscuit-shaped, of spongy texture, and colored yellowish-

INTRODUCTION.

green by a substance known as chlorophyll. These are consequently known as chloroplasts or chlorophyll-bodies (figs. 3, 4). In other cells, particularly those for the storage of food, they may develop into smaller, denser, flattened or roundish, uncolored bodies, called leucoplasts (figs. 5, 6, 7). These may act either as starch-accumulators, or in case

FIG. 6.

FIG. 7.

FIG g — Part of the cell-contents of an inner cell of white potato, z, nucleus; j, starch-grains, each having been formed by a leucoplast, /, which is still attached to one side of the grain; £, crystalloid. Magnified abont 1000 diam. — After Zimmermann.

FIG. 7. — Leucoplasts in place in a young cell of a leaf of vanilla. /, leucoplasts; z, nucleus; e. an oil-former or elaioplast. The unshaded spaces surrounded by protoplasm are vacuoles. Magnified about 1000 diam. — After Wakker.

of need, in young cells, may even be converted into chloroplasts. In other cells, particularly in highly colored parts, the plastids may become of most diverse form and size, and colored red or yellow, whence they are called chromoplasts or color-bodies (figs. 8, 9).

9. Wall.- -The cell-wall is formed by the protoplasm. In green plants when first formed it consists chiefly of cellulose, with which, as it grows older, various other substances may be mixed.' Some of these, such as pectin, are present even in the young wall, and may increase with age ; others

PLANT LIFE.

are characteristic of special changes which the wall may undergo. The most noticeable changes are four: (i) Some cell-walls contain suberin or cu/m, fat -like substances by the presence of which water and gases are hindered from passing

FIG. 8. — I, chromoplasts from flower-leaves of an orchid; II, from the root of carrot; III, from the fruit of mountain-ash. Embedded in the protoplasmic body of the chromoplast are sometimes proteid crystalloids, /, pigment-crystals, _/", or starch-grains, s. Magnified about 1000 diam. — After Schimper.

FIG. 9. — Chromoplasts from the flesh-colored shoots of the horsetail, containing the coloring matter in the form of granules embedded in colorless protoplasm. Magnified 1400 diam. — After Zimmermann.

through. The cell-walls of 'bottle-cork are suberized, and those in the skin of the apple are cutinized. (2) Some cell-walls are lignified, as, for example, those of wood by reason of

FIG. io.— A part of a thin slice lengthwise through the centre of the stem of gardenbalsam. The cells and vessels are elongated and are here seen from the side, showing the thickened lines on the side walls of z>, z/, z/", v"1 ', z/"", and v'"" . Magnified about 400 diam. — After Duchartre.

the presence of certain substances (vanillin, coniferin, etc.). They allow the ready passage of water and gases. (3) Others are so transformed that, in contact with water, they swell enormously, forming a mucilage or gum. These swelling

IN TROD UCTION.

substances are produced by the alteration of the cellulose or other constituents of the original wall. (4) An excessive deposit of mineral matters in the wall is known as mineralization.

Such walls may even retain their form after all organic matter is burned out, as in the skin of the scouring rush or horsetail.

10. Growth of the cell-wall.— As the cells become older the wall may increase in thickness. It must also increase in area as the cells grow in size. The growth in area is usually accomplished by putting new particles between the older ones. Growth in thickness is rarely Fu? "..-Cells from a liverwort

J showing thickened walls A^

uniform. When the wall grows haifaaeiat ^' art re

highly magnified; B, a cell from

thicker except at certain spots, these th.e, ^wer part of the thaiius,

with reticulate thickenings

remain as pits or pores in the (shaded); c, A rhizoids with

isolated branched thickenings.

thickening layers. When only cer- Highly magnified. - After

Sachs.

tain spots or lines grow thicker,

the wall shows projecting spikes, bands, or threads, which

give it the appearance in figs. 10, n.

Chapter I. Single-celled Plants And Colonies

IN the lakes and pools, in ditches and slow streams, on the surface of damp rocks and wood, may be found many sorts of microscopic plants, whose entire body is merely a single cell.

Blue-green algae.

11. Fission-algae. — The simplest forms of these, the fission-algae, have the protoplasm only slightly differentiated. The central part becomes the nucleus, while the whole of the remaining protoplasm is colored by the chlorophyll and a blue coloring matter called phycocyanin, so that in mass these algae look bluish-green or even blackish. For this reason they are called blue-green algae to distinguish them from those in which only the yellow-green of chlorophyll is present.

12. Gelatinous colonies.- -The cell-wall may be a thin

sheet of cellulose, but commonly it is composed of several layers, of which the outer are changed into mucilage.

This swells into a transparent jelly when wet, either becoming homogeneous or showing distinct stratification. When a number of such forms grow in company (fig. 12), this jelly-like material blends into a single

FIG. 12.— A blue-green alga J

Single indi- mass jn which the protoplasm of the

and colonies

to be emviduals, A,

^~E^-f. 5f vaj,!ous afes associated plants seems

Magnified 300 diam. — After

Sachs- bedded.

13. Gelatinous filament-colonies. — In other cases, instead of being associated only by the adhesion of the mucilaginous portion of the cell-wall, the cells, still practically independent the one of the other, remain connected by the

SINGLE-CELLED PLANTS AND COLONIES.

firmer portions of the wall into rows, forming irregularly coiled or serpentine filaments, which are embedded in a profuse gelatinous material (fig. 13). The essential independence of the individual cells, even though they remain connected, is shown by the fact that such a chain may be il^lfe^^lf A

FIG. 13. — Nostoc. A, a gelatinous colony, irregularly lobed. Natural size. /?, a portion of a serpentine filament with five heterocysts (one at each end by which it \vasseparatedfromtherestof the cells composing the filament, and three intermediate ones") and the jelly belonging to it. Magnified about 400 diam. — After Thuret and Janczevvski.

broken up into any number of pieces and each piece will retain all its powers. Here and there in the chain there occur cells unlike the rest (/*, fig. 14), called heterocysts, whose function seems to be to break the chain into pieces, from the growth of which independent colonies may arise. The association of considerable numbers of these plants in colonies gives rise to masses of jelly which vary from the size of a pin-head to 2—5 centimeters in diameter. They may be found adhering to water-weeds as clear- or dirty-green masses, or sometimes floating free (A, fig. 13).

14. Filaments of loose organization. — Of very near kin to these plants are the oscillarias, which have received this name from the pendulum-like swinging of their tips (fig. 15). In them the cells remain connected more extensively and more firmly, so that each is disk-shaped, and the filament is much less easily separated into its component cells. More-

PLANT LIFE.

over the gelatinous part of the wall is much less prominent, so that often it is only seen with difficulty. Even though invisible, it may be detected by the slippery feel of the plants when rubbed gently between the fingers.

FIG. 14. — Part of a filament of Anabcena. /i, heterocyst; a-d, successive stages in the division of a cell of the filament. Magnified 540 diam. — After Strasburger.

FIG. 15. — Oscillaria. a, the tip; b, a portion of the middle of a filament. Magnified 540 diam. — After Strasburger.

15. Feeding habits. --The feeding habits of the oscillarias are worth notice. They are found in permanent puddles and ditches where organic matter is decaying. The significance of this is that some of the ancestors of the green oscillarias probably had offspring which, instead of living upon food prepared by means of the green coloring matter (see \ 230), learned to utilize the organic matter in the water, at first perhaps no more than the present oscillarias do ; but gradually they came to live exclusively upon it. As a consequence, they lost their color and became incapable of existing where organic food cannot be had.

Bacteria.

16. Fission-fungi. — Along with the loss of color and change of habit went a diminution in size. They have thus

SINGLE-CELLED PLANTS AND COLONIES.

I I become so different that they are now known as fission -fungi, and popularly as bacteria, bacilli, microbes, germs, etc. These plants, probably the descendants of common ancestors with the fission-algre, are the smallest known organisms (figs. 1 6, 17). The diameter of many sorts does not o o a

OS ooooooj e

FIG. 16. — Various bacteria, «, Micrococcus, the " blood-portent " ; <5, zoogloea form of the same ; c, Bacterium acetz, the ferment of vinegar ; d, Sarcina, a harmless parasite of the human intestine, a, b, magnified 300 diam.; c, 2000 diam.; d, 800 diam. — After Kerner.

exceed .0005 of a millimeter. That would allow 175 to lie side by side upon the edge of the paper on which this book is printed. In many the successive divisions are parallel, in others they divide the cells in two planes, and in others again in three. The cells, when they divide, separate readily, in most sorts never cohering at all, but living as independent cells as soon as produced. Other sorts remain connected into two- to several-celled chains, sheet, or packets (a, d, fig. 1 6). A few have their cells firmly coherent into a filament. As the cells are either spherical or rod-like, the shape of the colony depends upon the shape of the component cells and the way in which they divide (see 1~ 24).

17. Gelatin. — In the fission-fungi, as in the fission-algae, considerable masses of gelatinous material are produced, in which the cells may lie embedded. The films, sometimes smooth, sometimes wrinkled, which appear on an infusion of organic matter, are formed by the masses of bacteria which become embedded in the gelatinous material produced by the alteration of their cell-walls (b, fig. 16).

18. Cilia. — Most species are furnished with locomotor organs consisting of fine threads of cytoplasm protruded through the wall, which, by their sudden contraction on one side, lash about like whips, and propel the cell by jerky, darting motions through the fluid in which it swims. These lashes, called cilia, may be single at the ends of the cell (C, fig. 17), or many at ends or sides (A, fig. 17), or the

FIG. 17. — Bacteria stained to show cilia. A, cilia tufted at one end; /?, cilia irregularly distributed over body; C, cilium single at one or both ends. B. the bacillus of typhoid fever; C, the bacillus of Asiatic cholera. Magnified 775 diam. — After Migula.

whole cell may be covered with them like hairs (B, fig. 17). They may be withdrawn or drop off when the plant comes to rest, as when they form the scums previously mentioned.

These plants are most interesting on account of their economic relation to health and disease, decay, fermentation, etc., which cannot be discussed here.*

'- For further information on these plants, see Frankland ' : Our Secret Friends and Foes ; Pritdden : Story of the Bacteria, Dust and

SINGLE-CELLED PLANTS AND COLONIES.

Yellow-green algae.

19. Single-celled plants with chloroplasts. — Among the single-celled green plants, one of the most common groups

FIG. 18. — Pleurococcus viridis. A, a single individual; B, a colony shortly after division; C, the same after separation. Magnified 540 diam. — After Strasburger.

is that represented by fig. 18, which shows a representative of an extensive series in which the vegetative body consists of a single cell with its wall, cytoplasm, nucleus, and a few relatively large chloroplasts. In this greater specialization of the protoplasm, these plants show the only advance upon the blue-green algoe. The wall in such as this Pleurococcus is almost uniform and quite thin.

20. Colonies. — The cells are fre-

,1 j i FIG. 10. — Volvo.r. a colony.

quently associated in colonies, em- The individuals are repre-

, ill- -11 T<T_ sented by the minute circles,

bedded in jelly or not. Ine most between which the protopias-

., . 111 r ^t i mic strands form a network.

striking and elaborate of these colo- The large bails in the interior

,. i i TT- 7 /• r \ are daughter-colonies to be

nies is formed by Volvox (fig. 19).

In this plant the colony is a hollow sphere, often large enough to be seen by the naked eye as a minute green ball, composed of thousands of individuals, embedded in a common jelly, arranged in a single layer at the surface. Each is connected with its immediate neighbors by strands of protoplasm, and two

set free upon the rupture and death of the mother-colony.

Magnified about 45 diam. — From Bessey.

PLANT LIFE.

cilia are protruded into the water outside. The lashing of these rolls the whole colony about. Each vegetative individual is entirely like the others, but those connected with reproduction become specialized.

Diatoms and desmids,

21. Shelled plants. — Other one-celled plants constitute a group known as diatoms, found in both fresh and salt waters,

a b c d

k

FIG. 20. — Various diatoms, a, Synedra ; b, Pleurpsigma ; c, d, Grammatofihora, side and top views ; e, colony of Gomphonema, with branched stalks attached to an alga ; _/", g; single cells of same, more magnified, top and side views; A, colony of Diatorna, the cells connected into a zigzag band ; z, /£, colony and individuals (top and side views) of Frngiilaria : /, w, «, Cocconema. In m the pair is surrounded by jelly preliminary to the escape of the protoplasm and the formation of two new cells (auxospores) which has been completed in n. — After Kerner.

either attached or free-swimming (figs. 20, 21). The diatoms are very various in form, and present two different aspects. When seen from the side they are generally elongated-rectangular. When looked at from above they are short-cylindric, disk-shaped, boat -shaped, or variously curved

SINGLE-CELLED PLANTS AND COLONIES. I 5

or angular. They are peculiar in having the cell-wall so impregnated with silica that scarcely any organic matter is left. Indeed the plants may be heated to a red heat and boiled in acid without destroying the form and markings of the cell-wall, so completely has it become silicified. To permit growth this rigid cell-wall is constructed in two pieces which fit together like the two parts of a pill-box (fig. 21). Each of these pieces, or valves, is sculptured into regular patterns in lines and dots, which are often so excessively minute or close together as to be barely visible with the highest powers of the microscope (b, fig. 20). Seen in mass, as they may often be on the sides of a glass aquarium, living diatoms appear yellowish-brown. The chloroplasts, which are sometimes single and always few, contain a brownish pigment (diatomin] in addition to the green chlorophyll.