Introduction

A TEXTB@@K W <DF GENERAL BOTANY By GILBERT M. SMITH STANFORD T JAMES B. OVERTON, EDWARD M. GILBERT, ROLLIN H. DENNISTON, GEORGE S. BRYAN, AND CHARLES E. ALLEN T7NIVEBSITY OF WISCONSIN Third Edition NEW YORK THE MACMILLAlf COMPANY 1937 THIRD EDITION COPYRIGHTED, 1935 BY THE MACMILLAN COMPANY All rights reserved no part of this book may be reproduced in any form without permission in writing from the publisher, except by a reviewer who wishes to quote brief passages in connection with a review written for inclusion in magazine or newspaper. Printed in the United States of America Set up and electrotyped. Published April, 1935, Reprinted January, 1936. Copyrighted, 1923, By Gilbert M. Smith First edition copyrighted, 1924; second edition, 1928, By Tt}e Mftcmillan Company. First edition published Ju|y, 1024; second edition, September, 1928. FROM THE PREFACE TO THE FIRST EDITION This book is an outgrowth of the experience of the authors in the teaching of elementary botany at the University of Wisconsin. For the past three years the text, in successively revised form, has been used in our first-year courses. In its preparation, we have been guided by the view that the subject of botany should be presented as a unit. The beginning student is not interested in, and should not be burdened with, distinctions between the artificially abstracted phases of the subject morphology, physiology, ecology, and the like distinctions which have their place in defining and limiting the scope of more advanced and special courses. Especially should the study of structure and that of function be intimately correlated in an elementary course. It is hardly necessary, in the present state of development of the teaching of science, to point out that forms selected for study should, whenever possible, be such as are already known to the student, either because of their widespread occurrence in nature or in cultivation, or because of their economic importance; or that general conceptions should be illustrated by familiar facts. Particularly in botany should the beginning of the study be an observation of everyday plants. Considerations such as these have guided us in the choice of material to be used in an elementary course. In a subject the teaching of which involves the introduction of the student to many new concepts, the use of a new terminology is unavoidable. However, the authors realize that each new term imposes an additional burden upon the student and correspond- We ingly handicaps him in the mastery of the subject matter. have attempted, therefore, to avoid technical terms except those which were found indispensable to a clear presentation. Only such facts and conceptions have been introduced as our experience has shown can be successfully treated in the Bourse of the beginning year. Necessarily the subject matter has been arranged in what seems to the authors a logical order assumption of a continuous year's course. Howev&n vii viii FROM THE PREFACE TO THE FIRST EDITION institutions, conditions necessitate the offering of a briefer elemen- We tary course in botany. have tried to provide for the possibility of such a course by so treating various topics that, within reason- able limits, certain chapters and portions of chapters may be omitted without destroying the continuity of the course or im- pairing the utility of the later parts of the book. CONTENTS 7HAPTER I. THE MAKE-UP OF4BBB ^- PAGM .W^IWWV ! 11 * ^ . 1 III. How MATERIALS ENTER AND LEAVE ^Ca^SSt^JL5 IV. ROOTS . fflBP^ 25 V STEMS . ^rs?^ 50 Vx. .bUDS A/^u^/^p^-r r 83 VII. LEAVES 94 Vllly RELATIONS OF PLANTS TO WATER ** r r . . 114 IX.VTHE MANUFACTURE OF FOODS 136 X/THE UTILIZATION OF FOODS 152 XI.^STIMULUS AND RESPONSE 166 -XII. NUCLEAR AND CELL DIVISION .--rr r : r 183 XHI.-THE CLASSIFICATION OF PLANTS ^^--T- . . 195 2vIV.'v"CHLOROPHYCEAE (GREEN ALGAE) . ^^^^ . . 199 XV. vMYXOPHYCEAE (BLUE-GREEN ALGAE) ^ * . 225 XVI. ^HAEOPHYCEAE (BROWN ALGAE) . , . . 231 XVII. RHODOPHYCEAE (RED ALGAE) 243 SVIIL BACTERIA i x . \S , XIX. PHYCOMYCETES ^. 250 . 267 XX. ASCOMYCETES ^"'. 280 XXL BASIDIOMYCETES tx 290 XXII. LICHENS . 310 XXIII. HEPATICAE (LIVERWORTS) . 317 XXIV. Musci (MOSSES) . 340 XXV. REDUCTION OF THE NUMBER OF CHROMOSOMBS v- 356 XXVI. FILICINEAE (FERNS) . 36B XXVII. SOME OTHER PTERIDOPHYTES . . . . . . ISlI ix x CONTENTS .... CHAPTER XXVIII. GYMNOBPERMS PAGE ^-rr~~. . . 397 XXIX. ANGIOSPERMS 423 XXX. SEEDS AND FRUITS . ~ r ..,- T""' 443 XXXI. FLORAL TYPES AND THE FAMILIES OF ANGIOSPERMS - 462 XXXII. INHERITANCE AND VARIATION 486 XXXIII. EVOLUTION 510 XXXIV. THE GEOGRAPHIC DISTRIBUTION OF PLANTS IN NORTH AMERICA 522 XXXV. THE ECONOMIC SIGNIFICANCE OF PLANTS y' . . 533 INDEX 561 A TEXTBOOK OF GENERAL BOTANY A TEXTBOOK OF GENERAL BOTANY CHAPTER I THE MAKE-UP OF A PLANT A 1. Plant and Its Parts. It is helpful to begin the study of plants by a consideration of one which is familiar, and at the same time large and easily handled. Such a plant is the sunflower (Fig. 1). The sunflower is not only a member of the group of most complex plants (the seed plants) ; it is also one of the most highly developed members of that group. One noticeable characteristic of the sunflower plant is that it is made up of distinct parts. These parts of which the plant is composed are called organs. The organs that are present at almost any stage in the development of the plant are leaves, stem, and roots. Certain other organs may or may not be present; occasionally, for example (especially in certain varieties of the sunflower), branches, which, as will appear, can conveniently be classed with the stem; and flowers and fruits organs whose study may better be left until later. In order to remain alive, to grow, and to reproduce that is, to give rise to new plants the sunflower plant must do certain work. The different kinds of work that a plant carries on are spoken of as its functions. In order to understand a plant, it is necessary to learn as much as possible about its structure that is, about the parts of which it is composed, their form and arrangement; and also about its functions the work that it does. It is always true that, in a general way, a plant is so constituted as to be able to perform its functions successfully; and so the structure of a plant can not be understood without a familiarity with its functions, nor can its functions be understood without a knowledge of its structure. What has just been said of a plant as a whole applies also to its separate organs. The work of a plant is in large measure 1 GENERAL BOTANY divided between the organs of which it is composed. Each organ is so constituted that it is fitted to carry on a certain function or certain functions better than other organs of the plant can perform them. There is a consider- Flower, Cluster able degree of division of labor between the organs, each doing especially the work for which its struc- ture best fits it. It be- 'LeafAxil comes necessary, therefore, to study each separate organ likewise from the standpoints of structure and of function. 'Nod -Intemode 2. Leaf of the Sunflower. A leaf of the sunflower (Fig. 2) is made up of two parts: a slender stalk, or petiole, and a broad blade. A leaf blade held between the eye and the light is seen to be marked by many light green lines which are called veins. There is one large central vein (midrib), from both sides of which run smaller branch veins; these branch veins send off finer branches, these in turn still finer ones, and so on; the smallest branches run to- gether here and there, so jRoot A FIG. 1. sunflower plant. that the whole blacje is penetrated by a close network of veins large and small. The positions of the larger veins are marked by ridges on the under surface of the blade. The parts of the leaf petiole, blade, and veins have, to some extent, different functions; that is, just as there is a division of labor between the organs that THE MAKE-UP OF A PLANT make up the plant, so there is a division of labor between the parts of a single organ such as a leaf. Similarly, a stem or a root is made up of different parts, each doing its share of the work of the organ as a whole. 3. Other Types of Leaves. Although the more familiar and larger plants are in general composed of the same organs as is the sunflower, these organs differ greatly in form in different A plants. leaf of the Indian corn (Fig. 3) is long and slender, and is divided, not into blade and petiole as is the sunflower leaf, but into blade and sheath. The sheath, or basal part of the leaf, is a clasping structure which surrounds the stem for some distance above the level at, which the leaf is really attached. There is a difference also in the ar- rangement of the veins. Whereas FIG. 2. Leaf of a sunflower, showing the conspicuous veins of the sun- the arrangement of veins. flower leaf are much branched and form a network, those of the corn leaf run approximately parallel from the base to the apex of the blade. The appearance of the vein systems in the two cases is very different; in reality both leaves have branch veins, but the branch veins in the corn leaf are very fine and not easily seen. 4. Stems and Branches. Both sunflower and corn (Fig. 4) have upright stems each consisting of nodes, or joints, at which the leaves are borne, and internodes (the portions of each stem between successive nodes). The stem of the corn is commonly thought of as unbranched; there are, however, occasional branches (suckers) which grow from near its base. The tassels and ears, which bear flowers, are also branches, or systems of branches. An important characteristic of the sunflower plant is in the fact that older parts of the stem or branches gradually increase in thickness as long as the plant is growing. The method of growth GENERAL BOTANY by which this secondary thickening takes place will be described in Chapter V. The corn stem, on the other hand, has no such means of secondary thickening. 5. Roots. The original (primary) root of a sunflower plant is a direct continuation of the stem. Sooner or later this primary root produces lateral branches (secondary roots) which may themselves branch. Production of sec- ondary roots results in a rather widespread root system in which the branches are, for the most part, progressively smaller, the ultimate branches being very slender. The roots of the sunflower have a method of secondary thickening similar to that of stems and branches. The primary root of a corn plant, like the pri- mary root of a sunflower, is a direct continuation of the stem. It does not, at least under ordinary con- ditions, give rise to sec- FIG. 3. Leaf of the corn, showing the sheath ondary roots. Often this ^ primary root dies early. In 8Uch a Case the root tern of the corn then con- sists chiefly of roots which have grown, not from the primary root as in the sunflower, but from the lower nodes of the stem. Many of these adventitious roots ( 31) arise at the underground nodes; but others commonly grow from one or more of the aboveground nodes of the corn stem. Some of the adventitious roots that arise above ground extend downward into the soil, so serving as props to the stem; others remain too short to reach the surface of the soil. The roots of the corn, like its stem, do not undergo secondary thickening. THE MAKE-UP OF A PLANT 6. Functions of Organs. Two important functions of a root system are, in most cases, anchorage of the plant in the soil and absorption from the soil of substances that are needed by the plant. The substances so taken in must be transported to the parts of the plant above ground, so that the conduction of absorbed sub- stances is also a part of the work of roots. The storage of reserve food, too, is a function of many roots, and this function is especially im- portant in such thick roots as those of the carrot, radish, and beet. The chief functions of the stem, and of the branches (if any), are usually the conduction of materials from roots to leaves and from leaves to roots, and the support of the leaves, as well as of the flowers, in a position favorable to the performance of their work. The leaves are, in the majority of seed plants, the chief food-manufacturing organs; but some food is made also in the green parts of stem, branches, and flowers. 7. Adaptation. It was suggested in 1 that in general a plant and its organs are so constituted that they are able to perform their func- tions successfully. It may be added that these functions can be per- formed most satisfactorily under the conditions, such as those of temperature, light, and moisture, A FIG. 4. corn plant. to which the plant is ordinarily exposed. These facts are summed up by saying that the plant is adapted to its environment. Adaptation in this sense is widely characteristic of living organisms, whether plants or animalsAdaptation is never perfect; but if organisms were not fairlj 6 GENERAL BOTANY well adapted to their environment, obviously they would not long survive; nor will they usually survive if the environment How is greatly altered. the adaptation of organisms to their environment has come about is one of the major biological prob- lems. Something of what is known regarding this problem will be discussed in Chapters XXXII and XXXIII. CHAPTER II THE STRUCTURE OF A CELL 8. Units of Structure. Every plant and every organ of a plant is made up of small parts, each of which is a cell. Cells are the unite of structure of plants and animals, much as bricks or stones may be the units of structure of the wall of a house. As we must know the nature of bricks or of stones in order to understand the strength and durability of the house which is built of them, so to understand the nature of a plant or of an animal we must ft know something of the cells of which it is composed. Some very simple organisms consist each of a single cell; but in general any one of the larger plants and animals is com- posed, like the sunflower, of many cells too small to be seen with the naked eye. The word cell commonly means a cavity or chamber which may be quite empty. But in speaking of the cells that compose a living organ- ism, the word is used in a different sense. These units of structure were first called cells by Robert Hooke (1635-1703). FIG. 5. The cellular structure of cork. This, the first published illustration showing a tissue composed of ceils, Hooke was interested in ex- amining various objects with appeared in Hooke's "Micrographia" in 1665. the aid of the compound microscope, then a new toy, very crude as compared with present-day instruments, which had recently been introduced into England. Among other objects, as reported in his "Micrographia" (1665), he examined a thin slice of cork and, much to his surprise, found that it contained many little "pores 8 GENERAL BOTANY or cells" (Fig. 5). Years later it was recognized that cork, such as Hooke had studied, is made up of the walls of dead cells, and that the really living part of any cell is the liquid or jelly-like substance within its walls. Indeed, as is now known, many cells consist entirely of this liquid or semi-liquid substance and have no walls at all. However, the name cell has persisted and is ap- plied to all these units of living matter, whether or not they are surrounded by walls. Although the mature cells present in a plant are alike in fun- damental characteristics, they may differ greatly in size, form, and function. The cells of any organ (such as a root, stem, or A leaf) are organized into tissues. tissue may be composed of cells all of which are much alike. However, the term tissue is also often applied to groups of cells which differ in structure but which cooperate in the performance of a common function. Thus, a root or stem contains conduct- ing tissues each composed of cells of several different sorts as to size and form. Just as every plant, except some of the simpler ones, is composed of organs, so these organs are made up of tissues, and the tissues in turn are composed of cells. 9. Organization of a Cell. In the study of the living cells of one of the more complex plants, seri- ous difficulty results from the fact that an organ of such a plant is usually of considerable thickness and composed of numerous cells. FIG. 6. Elodea plants. iividiml cells in such an organ. The microscope gives at best only a confused idea of any of the inFor this reason the cells can often DC seth more clearly in a leaf, because of its thinness, than in a stem or root, and the thinner the leaf the more distinctly can the A structure of an individual cell be made out. favorable leaf for ;uch a study of a mature cell is that of Elodea (Fig. 6), an aquatic )lant sometimes called the "water pest/' a pative of North Amer- ca which grows in sluggish streams and in ponds throughout the THE STRUCTURE OF A CELL 9 continent, except in the extreme northern portions. The plant has a slender, branching, submerged stem varying in length, according to the depth of the water in which it grows, from a few inches to several feet. Slender roots growing from the stem anchor the plant more or less firmly in the soil at the bottom of the water. The many leaves are small, narrow, and pointed, and are usu- ally borne in circles of three, four, or more. A leaf of Elodea is only one cell in thickness at its margin, and for the greater part two cells in thickness elsewhere. Viewed from above (Fig. 7), the cells of several rows near the margin of the leaf appear narrow and rectangular. At intervals, pointed cells project from the edge. Occupying the greater part of the surface of the leaf are a larger number of rows of wider and shorter FIG. 7. Portion of an Elodea leaf. cells which, as will be seen later, are concerned largely in food- manufacture. The cells of the lower layer are about hatfas wide as those forming the upper layer. In the central are several layers of narrow, elongated cells which squarei midrib. Although a cell appears rectangular or seen under the microscope (which shows only one plane), it S|gt be remembered that the cell has thickness as well, and is therefore box-shaped. The cells of the leaf (Fig. 8) are separated from one another, as well as botinded above and below, by transparent, ceM watts. The wall between any two adjacent cells is composed of sev- eral layers, of which the middle one is the oldest; during the 10 GENERAL BOTANY development of the cells the other layers were deposited successively on either side of this original layer through the activity of the living matter adjoining the wall on either side. All the material within a cell wall is referred to as protoplasm. Protoplasm is never homogeneous; it consists of numerous substances differing in nature, which have a definite arrangement within the cell. The whole structure made up by the protoplasm Cell Wall Plasma Membrane Intercellular Space Dense Cytoplasm Nudeolus Nucleus Chloroplast A FIG. 8. living cell of a leaf of Elodea, as seen in optical section. that is, the body of the cell exclusive of the wall is sometimes called ihe protoplast. The protoplasm is divided into cytoplasm and nucleus. Each of these two main divisions is in turn composed of various different substances which are definitely arranged. The arrangement of the substances that compose cytoplasm and nucleus is the organization of the cell. Because dells, as well as plants and animals which consist of many cells, have a definite organization, a singlap&ll living alone, or a many-celled plant or animal, is an organism. 10. Structure of Cytoplasm. Just within the wall on all sides of a mature cell, including top and bottom, is a thin layer of the cytoplasm which appears relatively dense and often finely granular; this thin layer will be referred to as the dense cytoplasm. Included in it are many ovoid, or at times somewhat flattened, green bodies, the chloroplasts. These, which are also parts of the THE STRUCTURE OF A CELL 11 dense cytoplasm, are the most conspicuous structures in most of the cells of a leaf. In the central part of the cell, enclosed by the layer of dense cytoplasm, is a large, transparent central vacuole. The cell sap which fills the central vacuole is a rather dilute solution of food substances, salts, and numerous other materials. The very outermost film of the dense cytoplasm, next the cell wall, is the plasma membrane; a similar film next the central vacuole is the vacuolar membrane. Under certain conditions the layer of dense cytoplasm with the chloroplasts (but not including the plasma membrane) is in motion. The movement is mainly one of rotation (Fig. 9), usually about FIG. 9. Diagram showing the direction of rotation of the layer of dense cytoplasm in a cell of an Elodea leaf. the vertical axis of the cell. Commonly the movement is in the same direction in all the cells of a leaf; but frequent exceptions to this rule occur. Occasionally a cross strand of dense cytoplasm cuts from one side to another through the central vacuole. The dense cytoplasm is the active substance in this movement; the chloroplasts are carried along by the current, much as pieces of ice may be carried in a river. The cell wall is perforated by pores, usually too minute to be seen with the highest powers of the microscope. These pores offer means by which the protoplasts of adjacent cells are either continuous or in contact with one another. The similarity in color and in transparency between most parts of the cytoplasm, such as the dense cytoplasm and the 12 GENERAL BOTANY vacuoles, makes it impossible to distinguish accurately the bound- aries of these parts when a cell is alive. On account of these dif- ficulties it is necessary, in order to study the finer details of structure, to subject a leaf to a rather lengthy series of processes. These processes are, in brief: (a) killing and fixing the leaf in a poison or combination of poisons so selected as to kill the cells at once but to leave all parts of each cell in as nearly their original positions as possible; (6) hardening by means of alcohol; (c) cutting into thin sections; and (d) staining of the sections. The stains used in the last-named process are, with a few exceptions, aniline dyes. Advantage is taken of the fact that protoplasmic substances in general show an affinity for aniline dyes, and that different parts of the protoplast have varying affinities for different dyes. If, therefore, a section of a leaf is subjected to the successive action of two or three properly selected dyes of different colors, various parts of the cell may take on contrasting colors and thus stand out distinctly one from another. The appearance of a cell in an Elodea leaf treated as just de-