SIS Bete 7s a Hl i tl [ Es cay = : ih Fir tessitih lial alate ety tiles Bibatte URE Sree ah WHE @ QK4+ 7 AA ha Vy SB CopyYRIGHT, 1917, By D. C. Heatu & Co. Ij7 PREFACE THERE are such wide differences of opinion regarding the proper content of an elementary course in botany that no teacHer would venture to put forth a particular selection of facts and principles as the only one wisely to be made from the great body of knowledge concerning plants. The present writers are quite uninclined to adopt such an attitude toward their own choice; still less would they insist upon the precise order in which the materials they have selected should be presented. We are convinced, however, that an introductory course can best be centered about a fairly intensive study of a small number of plants, and that an attempt at a more desultory treatment of the subject must prove unsatisfactory. The plants used as types should be easily available; and so far as possible they should be selected from among those that already have definite meanings to the pupil, either because of their familiarity or because of their util- ity. In addition, the predominance and the practical significance of the seed plants may be recognized by devoting some time to a study of their divers forms and uses; and in this study, too, illus- trations should be drawn chiefly from those plants of which the pupil sees or hears most in his everyday life. At whatever point in the course this latter work is done, it should have been preceded by an intensive study of the life of at least one particular seed plant. The present book offers a sufficient amount of work for a year’s course. Probably all teachers will agree that a secondary school course in botany should not occupy less time than this, although, unfortunately, the ideal is often impossible of attainment. The order of presentation here adopted is that which will probably, in general, be most satisfactory in schools that begin the study of botany in the fall. It involves, first, a short study of a familiar seed plant (Chapter I); this is followed by the examination of a 11 iv PREFACE series of plant types (Chapters II-XIII), arranged in an ascending series; then follows (Chapters XIV-XVIII) a discussion of the varied forms and uses of the conspicuous organs of seed plants and (Chapter XIX) of the classification of angiosperms; and finally (Chapters XX-XXIV) there is a brief consideration of some of the more important practical aspects and applications of botany. If the course in botany occupies less than a year, or if it begins at midyear or in the spring, it is plain that the work here outlined will require abridgment, or modification in the order of topics, or both. The teacher should, therefore, feel free to select from and to rearrange the material given in the textbook. There are other reasons why the teacher should not be too closely bound by the arrangement of matter in the text; local needs and conditions must be carefully considered if the greatest possible effectiveness is to be attained. For courses which begin at other times than in the fall, or for which less than a year is allowed, the following modifi- cations are suggested : For a year beginning in February, Chapter I may be studied first, with the substitution if necessary of the flowers of some of the plants suggested in the laboratory directions (page 391); this may be followed by Chapters XIV to XIX, then by Chapters II to XIII, and these in turn by Chapters XX to XXIV. For a course of two terms only, beginning in the spring (‘spring and fall botany’’), the outline given in the preceding paragraph may be followed, with the omission of Chapters III, VII (or VIII), XIII, and XX to XXIV. For a half-year’s course beginning in the fall, we recommend the following order: Chapters I, II, IV, V, VI, IX, X, XI (§§139- 153), XII, XIV to XVIII (abbreviating the laboratory work out- lined in connection with the latter five chapters). For a half-year’s course starting in February, we recommend beginning with Chapter I, substituting the flowers of other plants for those of the cucumber; this may be followed by Chapters XIV to XVIII, abbreviating the laboratory work suggested in connection with these chapters, then by Chapters II, IV, V, VI, IX, X, XI ($§ 139-153), and XII. Special ernphasis should be laid upon the importance of laboratory -PREFACE Vv work. Directions for laboratory study, if included at all in a textbook, are necessarily so placed as unfortunately to suggest the relative unimportance of the laboratory. Secondary school work in a natural science should invariably have laboratory study as its central feature, and the textbook should be considered an accessory which helps to elaborate and to tie together the knowledge obtained from the study of the objects themselves. Ample time should be allowed for laboratory work, and whenever possible double periods should be arranged for this purpose. In general, too, the labora- tory study of a topic should precede the textbook study and recita- tion upon that particular topic; but this rule cannot always be rigidly insisted upon, and here as elsewhere the judgment of the teacher must play an important réle. The laboratory directions given in the present book (Appendix I) are intended to be only suggestive. It may well be that in places the teacher will find it advisable to abbreviate. In many more places it will be necessary to supplement by additional directions and by the demonstration of structures which cannot well be studied, or of experiments which cannot well be carried on, by the pupils individually. Much the greater part of the work outlined can be done with very simple apparatus, including hand lenses or dissecting microscopes, of which there should be one for each student. Some of the study necessarily involves the use of a compound microscope. We do not feel, however, that the secondary school student should spend any considerable part of his time with the compound microscope, and in general we recommend that this instrument be used only for demonstrations arranged by the teacher. If the school is suf- ficiently equipped, there are certain points, for example in the study of bacteria, at which individual study with the aid of the compound microscope is helpful. Even in the study of bacteria, however, this individual work may be omitted and yet with the aid of a few demonstrations the student may obtain a very satisfactory conception of the nature and activities of the organisms in question. A word may be added regarding work out-of-doors. There is no way in which an enthusiastic teacher can more successfully impart his enthusiasm to his pupils than by means of field work. The successful teacher of botany is the one who induces his stu- dents to explore the world of plants for themselves. Field work, vi PREFACE however, meets certain practical difficulties. In the crowded condition of the average curriculum it is difficult to find time for trips long enough for real accomplishment; moreover, climatic conditions in large portions of the country make field work pos- sible only for a limited period at the beginning and the end of the year. However, within the limits set by these practical conditions, field work is to be encouraged; and, assuming that the teacher is gifted with enthusiasm of the right sort, even voluntary excursions at out-of-class periods will be productive of valuable results. Es- pecially in connection with the work at the beginning of the year there should be some trips, which may also be utilized by the teacher in collecting material for later study. But the mistake must not be made of substituting, in any large measure, field work for laboratory work. Nothing can replace the individual study of plants in the laboratory under careful direction and supervision. So far as possible, this laboratory study should be of live plants. There is perhaps no way in which a teacher can add more to the interest and consequently to the value of a course in botany than by the actual growing of plants in the schoolroom. If the course extends throughout the year, some time may well be devoted in the spring and early summer to the collection and identification of wild plants. The exact amount of work of this nature must be left to the discretion of the teacher; it will neces- sarily depend upon much the same factors that govern field work in general. If circumstances allow, the ability to identify unknown plants by means of a manual is an accomplishment to the attain- ment of which a reasonable amount of time may profitably be devoted. The interest and value of laboratory and textbook study may be greatly enhanced by carefully selected supplementary reading. Lists of references are given in Appendix II for each of the more important topics to be studied, and a fair proportion of the books cited in these lists should be in the school library. Large use may profitably be made of the bulletins and circulars issued by the United States Department of Agriculture and by the experiment stations of the various states. We have included references to many of these publications which will be helpful to the teacher of botany, but the list is by no means complete. They may be PREBACE vil obtained in most cases from the respective experiment stations or from the Superintendent of Documents at Washington, either gratis or at a nominal price. ; It is a pleasure to the authors to acknowledge their indebtedness to Professor R. A. Harper, now of Columbia University. Whatever value this book may have is due in large measure to the stimulus that came through their years of association with him and to the inspiration furnished by his discussions of the problems of botanical teaching, and by his example as a teacher. Of the many others to whom the authors are indebted for assistance, special mention should be made of Dr. C. A. Fuller, now of Providence, Rhode Is- -land, whose valuable suggestions aided in the preparation of Chapter II; Professor E. T. Harper, of Geneseo, Illinois, upon whose col- lection of photographs of the fleshy fungi we have freely drawn; Mr. F. B. Moody of the Wisconsin State Conservation Commis- sion, who furnished several of the photographs used in illustrating Chapter XXII; President V. E. McCaskill of the Superior State Normal School, who has read and criticized several of the chapters ; Professors L. R. Jones, G. W. Keitt, and A. G. Johnson, of the Department of Plant Pathology of the University of Wisconsin, for assistance especially in the preparation of Chapter XXIV; Professor Alban Stewart of the Florida College for Women; and to the various members of the Department of Botany of the Uni- versity of Wisconsin, especially Professors J. B. Overton, R. H. Denniston, E. T. Bartholomew, and G. M. Smith, Dr. W. N. Steil, Dr. G. S. Bryan, and Mrs. P. M. Smith, for assistance and sug- gestions in connection with all parts of the work. Mapison, WISCONSIN. May, 1917. Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http:/Awww.archive.org/details/cu31924001797129 CHAPTER CONTENTS A. Brier Stupy oF A FamItiar PLANT The Squash, Pumpkin, or Cucumber BACTERIA Protrococcus . YEASTS A Ponp Scum THE BREAD MoLtp THE WHEAT Rust A MusHrRoom . A Moss . THE BRACKEN FERN THE PINE THE BEAN THE INDIAN CORN Roots AND THEIR USES ‘ STEMS AND BRANCHES AND THEIR USES LEAVES AND THEIR USEs FLOWERS AND THEIR USES FRuITS AND SEEDS AND THEIR USES Some IMPORTANT FAMILIES OF ANGIOSPERMS SOME USEFUL PLANTS AND PLANT PRODUCTS WEEDS AND Poisonous PLANTS FORESTRY AND Forrest MANAGEMENT PLANT BREEDING PLANT DISEASES ix x CONTENTS AppENDIX I. — Directions for Laboratory and Field Work APPENDIX II. — Reference Books AppENDIXx III. — The State and Territorial Agricultural Ex- periment Stations AppeNDIx IV. — Botanical Supplies Glossary Index. : : es 443 451 A TEXTBOOK OF BOTANY CHAPTER I A BRIEF STUDY OF A FAMILIAR PLANT THE SquasH, PuMPKIN, OR CucUMBER! 1. The Squash Seed (Fig. 1). — This is a hard, flat, white object, rounded at one end and more or less pointed at the other. At the pointed end is a scar marking the part of the seed by which it was attached to the inside of the fruit (which we commonly call the ‘‘ squash’). In this scar and a little to one side of the point is a conspicuous depression or hole, the micropyle. If a seed has first been soaked overnight, it is easier to remove the outer hard covering, the seed coat. Just inside this is a thin, greenish, skin-like layer, the en- dosperm. Within this is the embryo, or young plant, which makes up the larger part of the seed. One end of the embryo is rather pointed and is called the radicle; this is turned toward the micropyle. The greater part of the embryo is made up of two thick seed leaves, in which food is stored that is to be used by the embryo when the seed germinates. The seed leaves are attached to the radicle, and between the seed leaves, at the point where they join the radicle, is a small swelling, the plumule. The plumule bears two very small secondary leaves, which, however, can hardly be made out by 1 Although any one of the three plants mentioned may be used for the work of the present chapter, the cucumber will usually be found most convenient because of its smaller size. But the cucumber seed is not so favorable for study as is the larger seed of the squash or pumpkin. Specific reference is made, therefore, to the seed and seedling of the squash, and to the mature plant, flowers, and fruit of the cucumber. I 2 TEXTBOOK OF BOTANY the use of an ordinary lens. The important fact for us to notice is that the seed consists of a small plant (the embryo) which is ready to continue its growth as soon as conditions Fic. 1. — A, side view of a squash seed; B, a lengthwise section through the middle of a squash seed; a, scar, showing the place at which the seed was attached; 6b, seed coat; c, endosperm; d, seed leaves; e, plumule; f, radicle. are favorable; and also of some surrounding layers which protect the embryo. 2. Germination of the Seed. — If some squash seeds are placed in moist sand or sawdust and are examined from time to time, it will be found that they gradually swell. This is because water is taken in both by the seed coat and by the embryo. After a few days the seed coat begins to crack; the crack starts at the micropyle and extends down the two edges of the coat. Then the end of the radicle begins to push out through the opening in the seed coat (Fig. 2, A). This is because all parts of the embryo, and especially the radicle, are beginning to grow, and there is no longer room for the whole embryo within the seed coat. As soon as the end of the radicle is well outside the seed, it turns downward and grows in that direction. This downward growth pushes the tip of the radicle farther into the soil. From the radicle, therefore, is developed the primary root of the young plant, STUDY OF A FAMILIAR PLANT 3 as well as a short part of the very base of the stem. Before the young root has grown very far, branch roots begin to grow from it (Fig. 2, B). At this time we can see that the seed leaves and the part of the radicle still within the seed coat are also swelling, and that just within the edge of the splitting seed coat there is a flat projection (a peg) which has grown out of one side of the radicle. Now the upper portion of the Fic. 2.— A, a germinating squash seed; B, C, and D, stages in the development of the seedling. Notice how the “peg” and the lengthening upper part of the radicle help to free the seed leaves from the seed coat. radicle grows still more and bends so as to push out of the seed. This lengthening of the radicle finally pulls the seed leaves out of the seed coat, the latter being held firmly in place by the peg (Fig. 2, C). The seed coat has now been thrown off entirely, and the radicle straightens so that the end bearing the seed leaves and the plumule is turned upward. 3. Growth of the Seedling. — The young plant just be- ginning to grow is called a seedling. When the growth of the radicle has pushed the seed leaves well above the surface of the soil, they spread apart, grow, and become green like ordinary leaves (Fig. 2, D), although they are different in 4 TEXTBOOK OF BOTANY shape and not so large as the leaves formed later. The plu- mule, which has already begun to grow, now grows more rapidly ; the two secondary leaves which it bears, and which were already formed within the seed, spread out and become large and green; they are shaped much like the later leaves of the plant. By the further growth of the plumule into a long stem and by the production of more leaves upon the stem, the above-ground portion of the seedling develops into that of the mature plant at the same time that the root and its branches are growing and pushing into the soil. 4. The Mature Cucumber Plant: Roots. — The primary root is developed from the lower end of the radicle, which continues to grow farther and farther into the soil. After a time, however, the primary root grows more slowly, and some of the branch roots grow fast enough so that they come to be fully as long as the primary root. These branch roots occasionally branch also, so that the root system of an older plant is made up of a great many slender roots. It is by means of this system of roots that the plant is firmly an- chored in the soil. The roots also take up.from the soil large amounts of water as well as some other substances that the plant needs, and these are carried through the roots to the stem and thence to all parts of the plant above ground. The water and other materials are not taken in from the soil by all parts of the roots, but only through very short, slender root hairs (see Fig. 2). These root hairs are borne in large numbers, but only on the youngest parts of the root system — that is, close to the growing ends of the primary root and of each branch root. 5. The Stem. — We have seen that the plumule grows into the stem of the mature plant. The stem branches, like the primary root, though less frequently, and each branch of the stem may branch again. The stem and its branches, like the primary root and its branches, may continue to grow in length as long as the plant lives. The stem at first grows STUDY- OP -A-PAMECIAR PLANT 5 upward. But it is not very strong, and when it is a few inches in length its weight and that of the leaves make it bend over ; from this time on, unless care is taken to train it upward on a trellis or other support, the stem sprawls on the surface of the ground. 6. Leaves. — The secondary leaves (a name given to all the leaves that are formed after the two seed leaves) are pro- duced at intervals along the stem. They are alternately arranged — that is, at one point on the stem only one leaf arises, and the leaf next above will be not on the same side but on a different side of the stem. Each leaf has two parts, the leaf-stalk and the blade (Fig. 3). The blade is the large, flat portion of the leaf. It has a lobed outline, the three or five lobes being pointed, and ; ? the edge of each lobe Fic. 3. — ee ay pat | stem bearing irregularly toothed. There are many hairs growing on both the upper and lower surfaces of the leaf, as well as on the leaf-stalk and on the stem and branches of the plant. If the leaf be held up between the eye and a window, one can see in the blade many light green lines, the veins. Five large veins spread out from the point where the blade is attached to the leaf-stalk ; as these main veins spread out in the blade they give off branches, the branches branch, and so on until the whole system of veins looks like a network. It is in the leaf that much of the food is manufactured which is to be used in the growth of the plant, and the veins are useful in two ways: 6 TEXTBOOK OF BOTANY first, they carry to all parts of the leaf the water and other substances that have been brought up from the roots; and second, they carry from’all parts of the blade to the leaf- stalk and so finally to other parts of the plant the food that has been manufactured in the leaf. 7. Tendrils. — Here and there, growing from the same level on the stem as one of the leaves, is a slender structure that looks like a branch, but is more or less curved or coiled (Fig. 3). If the end portion of a young tendril is gently stroked on one side with the finger or with a pencil, it will often begin to bend toward the object that is stroking it. It is in this way that tendrils coil about the objects that they touch. Some of the relatives of the cucumber climb by means of their tendrils. But the tendrils of the cucumber are not numerous enough or strong enough to support the weight of any large part of the plant; for this reason the plant usually sprawls on the ground, although sometimes the ends of branches clamber up a short distance on neighbor- ing objects. However, with a little assistance, a cucumber plant can be induced to climb. 8. Terminal Buds. — The tip of the stem is covered by young leaves which have not yet unfolded. The stem tip and the young leaves that cover it are together called a bud. This bud includes not only the few leaves that one sees on the outside; for if these are stripped off, several smaller leaves will be found within. As the part of the stem inclosed within . the bud grows, the distance between the older leaves of the bud increases, and these older leaves unfold, leaving the next younger leaves as the outer covering of the bud; at the same time new leaves are being formed within the bud as minute swellings upon the stem, just back of its very end. It is by means of these changes within the bud that the stem grows in length and that new leaves are formed. A part of the stem for a short distance back of the bud is also growing longer, but the older parts do not grow in length. There STUDY OF=2=FHNTLIAR PLANT 7 is a bud at the end of each branch like that at the end of the stem, and each branch grows in the way that has just been described for the stem. 9. Axillary Buds. — In the avil of each leaf — that is, in the angle where the leaf joins the stem or branch — there is also a small bud. Now and then one of these buds grows out into a branch. This explains why a branch always grows from the axil of a leaf. As a rule most of the axillary buds remain very small, and only here and there one grows farther. But if the bud at the end of the stem or of a branch is killed or removed, the axillary buds are stimulated to grow, and several or even all of them for some distance back from the end may grow into branches. This fact is often taken ad- vantage of by gardeners, because by pinching off the terminal buds one can stimulate the growth of branches and so in the’ end induce the plant to bear more fruit than it otherwise would. 10. Flower Buds and Flowers.— As the plant grows older, we find on examination that in many cases an axillary bud contains not only the growing point of a branch and sev- eral young leaves, but the beginnings of flowers as well. Such a bud is called a flower bud; one that contains only a growing point and young leaves is a leaf bud. When the flowers open (Fig. 4), we see that they are of two kinds. In some, below the yellow part of the flower there is a swollen, spiny region which seems to be a part of the flower-stalk, but which really belongs to the flower; flowers with this swollen portion are pistillate flowers. Others have no such swollen part; they are staminate flowers. Both kinds grow from axillary buds. The pistillate flowers are usually single, the staminate flowers usually in clusters. 11. A Pistillate Flower (Fig. 4, A). — The outermost part of the flower consists of five hairy, greenish-yellow, leaf-like structures called sepals, which are grown together except at their tips. ' Next within comes another series of five leaf- 8 TEXTBOOK OF BOTANY like structures, the yellow petals, which are the largest and most conspicuous parts of the flower; they are attached be- low to the sepals and are grown together with each other about half-way up, their outer ends being free and spreading. In the very center of the flower is the pistil. The upper end of the pistil is the stigma, composed of three lobes having a very rough surface; each lobe is two-parted. Below the stigma is the style, a short stalk that connects the stigma with the green, swollen, spiny lower part of the pistil, the ovary. ‘The ovary seems to be below the petals and sepals, Fic. 4.— A, a pistillate flower of the cucumber; a, sepal; 6, petal; c, stigma; d, style; ¢, ovary. B,astaminate flower; f, filament; g, anther. as though these parts of the flower were growing out of its top, but really the lower parts of the sepals and petals have grown closely together with the wall of the ovary. Below the ovary is the short flower-stalk. A section cut across the ovary shows that it is divided by cracks, usually three in number, each of which begins at the center of the ovary and, as it approaches the outer wall, divides into two branches that curve inward toward the center. In the branch cracks are found a number of small rounded swellings, the ovtles. The way in which the ovules are attached to the solid flesh of the ovary cannot well be made out except in thin, specially colored sections. STUDY -OF. 4 BaMYLIAR PLANT 9 12. A Staminate Flower (Fig. 4, B). — This looks much like the pistillate flower, and like it has sepals and petals. But within the petals, instead of a pistil are three stamens, which are attached near the base of the petals. Each stamen has a short stalk or filament, and a larger head or anther. Growing out from the upper end of each anther is a forked leaf-like projection. When the anthers are ripe, they open by long, curved slits and allow the dust-like pollen to escape. This pollen is made up of small round grains. Some of the pollen grains must fall upen the stigma of a pistillate flower if seeds are to be formed.! The history of the germination of the pollen grain 6n the stigma and of the growth of the pollen tube from the pollen grain is a difficult one to follow and will be taken up more fully in a later chapter. At the center of the staminate flower is a whitish, three-lobed swell- ing that loons like a small stigma. This really is a rudimen- tary pistil which is quite useless in the staminate flower. In the same way, the pistillate flower has a circle of rudimentary stamens, which are very small and easily overlooked. 13. The Fruit (Fig. 5).—The fruit of the cucumber is the part of the plant that is of use to us, and so it is with the fruit that we are likely to be most familiar. It pe. 5.— Portion is developed from the ovary of the pistillate of a cucumber fruit, flower. As this part of the flower grows ss peer cad rapidly, the other parts (sepals and petals, style and stigma) wither away and may be seen for some time as little dried-up structures at the outer end of the fruit. Ordinarily the fruit is picked for use while it is still green. But if it is allowed to remain on the vines it becomes yellow 1 As a rule, plants will not form seeds or fruits unless pollen grains land upon their stigmas. Cucumbers of some varieties are exceptions to this rule, for they may form fruits (but fruits without seeds) though thcir stigmas receive no pollen. Io TEXTBOOK OF BOTANY and, as we say, ripe. The spines that were noticed on the surface of the ovary remain hard and sharp while the fruit is growing, but many or all of them (in most varieties of cucum- ber) dry and fall or are rubbed off before it is ripe. A cross section of the ripe fruit shows about the same structure as one through the ovary, except that all the parts are much larger. The ovary wall has become thicker and hard; it is now the fruit coat or rind. The tissues that made up the inside of the ovary have become soft and juicy, and the ovules have grown into seeds, whose structure we have studied. The fruit of the cucumber, unlike many other fruits, has no means of opening or of scattering its seeds. Therefore, unless the fruit is broken open in some way, the seeds remain inside, protected by the fruit coat until the latter decays; then they fall upon the ground and are in a position to germinate. 14. Length of Life. — The whole history that we have followed is passed through in one season. The cucumber is an annual; that is, no part of the plant lives through the winter, but new plants must be raised from seed each year. 15. Relatives of the Cucumber. — The cucumber is a representative of the genus Cucumis. The botanical name of the species is Cucumis sativus. Another species of the same genus is Cucumis melo, the musk- melon. These plants belong to the gourd family, which includes about 700 species, the greater part of them natives of the tropics. The pump- kin and the summer squash (both forms of Cucurbita pepo) are mem- bers of the same family ; so are the Hubbard squash (Cucurbita maxima), the winter crookneck squash (Cucurbita moschata), the watermelon (Citrullus vulgaris), and gourds of various forms (Lagenaria vulgaris). 16. Historical Note.— The cucumber is thotight to have been originally a native of East India. It has been cultivated from the earliest times, and is said to have been introduced into China about 200 B.C. Many different varieties have been produced. The pumpkin is probably a native either of Central or of South America. Pumpkins were under cultivation by North American Indians when the continent was discovered by white men; but the pumpkins then grown seem to have been more like the gourd pumpkins than like the field pumpkins of the present day. The origin of the different squashes is uncertain. CHAPTER II BACTERIA 17. A Hay Infusion. — If we place some hay in water and allow the mixture to stand in a warm place, after two or three days we shall find on examination that it contains a great number of small living organisms. Some of these swim actively; others are quiet except as they are carried about by currents in the liquid. Among the most numer- ous forms are likely to be rod-shaped bodies, some of them single, others joined end to end Fic. 6.— Bacillus subtilis. A, in the in chain-like rows. Some motile condition; B, in the resting con- of the single bodies as dition, the vibrating threads having been wall as eome-oF Che tows lost; C, in the spore condition. After Brefeld. of bodies are in active motion; others are quiet. The rod-shaped bodies belong to the group of bacteria. Even among the rod-shaped bacteria in the infusion there may be two or three different kinds; but the differences between them are slight, and we may con- sider them under the name of what will probably be the most abundant species, Bacillus subtilis (Fig. 6). 18. The Structure of a Bacillus. — Each rod-shaped body isa cell; Bacillus subtilis is a plant composed of a single cell. Il 12 TEXTBOOK OF BOTANY A living bacillus seems under a microscope even of the high- est power to be merely a colorless body, two or three times as long as wide, with rounded ends. If two or more cells are attached, the ends by which they join one another are flat- tened rather than rounded. Something more of the structure of the cell may be made out if it is killed and stained by the use of suitable dyes. The process of stain- ing is a delicate one and can be carried on successfully only after con- siderable experience. A cell prepared in this way (Fig. 6, A) is seen to be surrounded by a thin wall; within the wall is the protoplasm, through which are scattered a few dark granules. Extending outward from the wall are a number of long threads. These threads were part of the living matter of the cell when it was alive, and it was by means of the rapid vibration of the threads that the cell moved about in the water. 19. Food. — This bacillus is commonly found in solutions which, like the hay infusion, contain dead and more or less decaying organic matter — that is, matter which has been a part of the bodies of plants or animals. From this fact we may infer that the bacillus needs ready-made organic food just as animals do, and this is the case. We shall see later that green plants can build up their living matter out of very simple materials; but this is not true of bacteria. Many bacteria live, like Bacillus subtilis, on the dead substances of animals and plants. Bacteria and other plants which use dead food are spoken of as saprophytes. Other bacteria, which live upon or in the living tissues of plants or animals, are parasites. 20. How the Bacillus Obtains Food. — The wall of the bacillus cell is firm and rigid, and has no opening through which food materials can be taken into the cell. Therefore it is necessary that anything which the cell is to take in as food must first be dissolved, since water and many substances dissolved in water can pass through the cell wall. Such a substance as the hay with which we started is, of course, not soluble in water; so the bacillus must change some or all of BACTERIA 13 the material of the hay into a soluble form before it can use it. This is one of the things that bacteria do on a large scale. They are able to change many organic materials with which they come in contact into a soluble form — or, as we commonly say, the materials are digested — and then the bacteria absorb such parts of the solution as they can use. If we keep the hay infusion supplied with water so that it does not dry out, the solid substance of the hay will gradu- ally disappear, and finally there will remain only a slimy solution with a disagreeable smell. The same thing would happen to bread or meat or to a potato that was left in water under similar conditions. The organisms that would be at work in the digestion of meat would not be altogether the same as those we find in the hay infusion; but the most numerous and active among them would be bacteria, although Bacillus subtilis might not be one of them. Bacteria digest solid foods like hay or meat by means of special substances which they form within their cells and pass out through the walls, just as the cells that line the human stomach form the gastric juice which digests meat and other foods within the stomach. Such substances, formed inside living cells, which can produce changes in the chemical nature of other stib- stances, are called enzyms. We shall learn that very many different enzyms are produced by plant and animal cells, and that each kind of enzym can cause one particular kind of ‘chemical change. 21. Respiration. — Like ourselves, the bacillus must respire; that is, it must have oxygen if it is to remain active, and this oxygen it takes in from the air. The oxygen is used, not in building up, but in tearing down the substances within the cell wall. This tearing down is just as necessary as the building up; it supplies the energy that the cell must have in order to dowork. One form of the energy so obtained is heat. 14 TEXTBOOK OF BOTANY 22. Growth. — In the hay infusion we find bacillus cells of different lengths. This is because a cell does not remain the same size, but if it is supplied with food, and if other conditions, such as moisture and temperature, are favorable, it grows. Growth goes on because the cell builds up into living matter some of the food that it takes in, and this be- comes a part of the cell body, just as the food that we eat is in part built up into the substance of our own bodies. It is true, as we have just seen, that the opposite process also goes on; that is, the living matter is at the same time being broken down into simpler substances by respiration, and these sim- pler substances are given off to the outside through the cell wall as waste matter. If the building up and tearing down should just balance each other, the cell might remain of the same size. But if food is abundant, as it is in a hay infusion, the building up goes on more rapidly than the tearing down, and the cell grows. 23. Reproduction. — The growth of a cell does not go on indefinitely. When it reaches a certain size (which is always about the same for any particular species so long as the condi- tions remain unchanged) it can grow no longer, but divides. The division of the cell is always crosswise, so that the two smaller cells thus formed are attached end to end. These may remain attached, and each one then grows to full size and divides, and by this alternate growth and division rows or colonies of cells are formed. The cells, however, are easily broken apart, so that in an infusion we always find single cells as well as colonies of various lengths. It is by division that the bacillus reproduces, and cell division is the only kind of reproduction that is found among bacteria. Many bac- teria are able to grow and divide very rapidly; some kinds have been observed to divide about every half-hour. In such a case a single cell might give rise in the course of ten hours to 1,048,576 cells; and in the course of twenty-four hours, if the food supply were abundant and all other con- BACTERIA Is ditions remained favorable, to an almost inconceivable number. It is not surprising, therefore, that bacteria are as numerous as they are and that they are found practically everywhere ; nor that, although each one is so extremely small, they are able to accomplish tremendous results. The “scum ” that rises to the surface of the hay infusion contains great num- bers of separate cells and rows of cells in a quiescent condi- tion held together by a sticky, slimy substance which is secreted by the bacteria. Although the cells in this scum are not moving (because they have lost their vibrating threads), they may still be actively dividing (Fig. 6, B). 24. Spore Formation. — If the liquid in which the bac- teria live is allowed to dry up slowly, or if we keep it from dry- ing up until the supply of food for the bacteria is exhausted, a change takes place in the appearance of many of the cells. The protoplasm of each cell shrinks away from the wall and rounds up into a body much smaller than the original cell (Fig. 6, C), which lies at the center or toward one side or one end of the old cell cavity and is closely surrounded by a thick new wall. This shrinkage is at least partly due to the loss of water, and the spore so formed is much less easily af- fected by unfavorable conditions, such as dryness, heat, and cold, than is the ordinary cell. Many kinds of bacteria (though by no means all) are able to pass into the form of spores, in which condition they may remain unaffected by surrounding conditions for a long time. In this form they may be blown about in the air, and when they land in a place where conditions are favorable they can return to their origi- nal form and then grow and multiply as before. This abil- ity to change from one form to another is of great advantage to bacteria, since it enables them to adapt themselves to great changes in their surroundings. 25. Distribution and Forms of Bacteria. — More than two thousand kinds or species of bacteria have been recognized, and new species are 16 TEXTBOOK OF BOTANY discovered every year. The majority of them are saprophytes, like the one we have studied. They are found in every conceivable situa- tion —in the air, in both salt and fresh water, %, _ and in the soil. They live in the food we eat, in the water we drink, and upon and within many parts of our own bodies, as well as of the bodies Yeo, of lower animals and plants. Three classes of Fic. 7.— A pus- Pacteria are distinguished which differ from one forming bacterium, another in the general shape of their cells; but Streptococcus pyog- within each class there is considerable variety. enes. Those of one class are nearly or quite globular. They form colonies of different kinds, depending upon whether the divisions take place all in the same direction or in different directions. Some of the globular species‘form chains, as for ~ eS Sal ee ‘SF . ea. 4 . ; “ io eee Votwy ad \ (re mF wy ey t % 2 ~ a wet OP _™é 2 & & a = "Fee NS oe ah Ray = é 7 at Z ¢: im Oa ‘ * om bos ad A Hi Pennoni A Fro. 8. = Disease-producing bacteria. A, the bacillus of Asiatic cholera ; B, the bacillus of chicken cholera; C, the bacillus of splenic fever; D, the spirillum of recurrent fever. After Giinther. BACTERIA 17 example Streptococcus pyogenes (Fig. 7), one of the pus-forming bac- teria; others form plates or masses of cells. Another class includes the rod-shaped bacteria, many of which form chain-like colonies; Bacillus subtilis is a type of this group. Those of the third class are spiral or corkscrew-shaped. The spiral may be very short, as in the “comma bacillus '’ that causes Asiatic cholera (Fig. 8, A); or it may be composed of several coils, as in the species of Spirillum, one of which (Fig. 8, D) is the germ of recurrent fever. Bacterial cells differ in size as well as in form, although they are all very small; in general they are the smallest known living organisms. One of the smallest is zz75g9 inch long and z33\;55 inch wide. One of the largest is z/5g to z45 inch long and 37:3 to z+;5 inch wide. It is estimated that it would require 16,800,000,000,000 bacilli of average size to weigh an ounce. 26. The Activities of Bacteria. — We have seen that in the process of obtaining food for their own use, bacteria by means of enzyms change the nature of organic substances such as meat and hay and make them soluble. One of the most striking, and from our point of view one of the most important, characteristics of bacteria is their power of thus changing the chemical nature of substances with which they come in contact. All such changes are, of course, of direct benefit to the bacteria; but they are also in many cases of great conse- quence to other plants and to animals. If, as is very com- monly the case, the change is a breaking down of organic substances — that is, of materials which have once been parts of the living bodies of plants or animals — the process is called decay. Not all forms of decay are due to bacteria ; but bacteria are more largely concerned in decay and produce more kinds than all other sorts of organisms taken together. If in the process of decay bubbles of gas are formed, we often speak of the process as fermentation; if an unpleasant odor arises, we call it putrefaction. Bacteria are often thought of as a group of useless and harmful creatures; as a matter of fact, only a small number of species are injurious to man, and a much greater propor- tion are useful in one way or another. Processes of decay, due largely to bacteria, are constantly destroying the dead 18 TEXTBOOK OF BOTANY bodies of plants and animals and returning the elements of which they are made to the soil, water, and air to be used again in building up the bodies of living animals and plants. If it were not for decay, the surface of the earth would long ago have been covered with dead organic matter, and the life of the higher plants and animals, including man, would now be impossible. Most bacteria, including many that cause decay, like Bacillus subtilis, can obtain the oxygen that they need for respiration only from the air. For this reason, the decay of a substance can often be stopped or prevented if air is entirely excluded from it. This is one reason why fruits and vege- tables are preserved in air-tight cans and jars. On the other hand, some bacteria cannot grow in the presence of air; so they thrive only in places, for example deep in the soil, to which air does not reach. Such bacteria need oxygen for respiration just as all living organisms do; but they obtain it from the organic substances that they decompose, and not directly from the air. An example of this class is Bacillus tetanus (Fig. 9, D), which causes lockjaw. Since this bacillus cannot multiply in the presence of air, it is important not to allow wounds into which the lockjaw germ may have entered to heal over too soon upon the surface, and not to keep them covered so tightly as to keep out the air. There is a third group of bacteria that can take their oxygen from the air, but which, if they are shut away from the air, can obtain it by the breaking down of organic materials. 27. Soil Bacteria. — Great numbers of bacteria are found in the soil, especially in the upper layers to a depth of six inches or a foot. Various countings of bacteria in the surface layers of different kinds of soil have shown from 15,000,000 to 300,000,000 bacteria per ounce of soil. At greater depths the numbers are smaller, and relatively few are found more than five or six feet below the surface. Among those in the deeper soil layers are many kinds that cause decay and putrefaction. BACTERIA 19 Another group of soil bacteria that are of great importance, particularly to the farmer, are the nitrifying bacteria. The decay-producing bacteria, as we have seen, decompose the remains of plants and animals into simpler substances, but not for the most part into materials which higher plants, like the squash and the cucumber, can use for food. Among the substances formed by decay are compounds containing ammonia. The nitrifying bacteria change these ammonia compounds first into a class of compounds called nitrites, and then into nitrates, and the nitrates are substances which the cucumber and other garden and farm plants can absorb through their root hairs and can use in the building up of their own bodies. It’ is not sufficient, therefore, that the soil be fertilized by the addition of organic substances, such as manures; it must contain also both the decay-producing and the nitrifying bacteria to change these substances into a form suitable for plant food. 28. Bacteria in Milk. — One of the means used to measure the cleanliness of milk is to count the bacteria contained in a certain amount. The number of bacteria present varies enormously with the care taken to secure clean milk and to prevent the entrance of bacteria while handling and trans- portingit. Ifextreme care is taken, the number may be kept down to a few hundred per cubic centimeter. In certified milk the number of bacteria is frequently limited to 10,000 per cubic centimeter, and in some cities milk cannot be sold that contains more than 250,000 bacteria per cubic centi- meter. On the other hand, the bacterial content of dirty milk offered for sale has been found to run in some cases to more than 15,000,000 per cubic centimeter. Of the many kinds of bacteria found in milk, some are harmless and pro- duce no particular effect ; some cause disease in persons who drink the milk, and these, of course, are the forms that it is especially important to keep out ; and some produce fermen- tations of various kinds, most of which make the milk unfit 20 TEXTBOOK OF BOTANY for use. Of the forms producing fermentation, the lactic acid bacteria are of most interest. These grow and multiply rapidly in milk at ordinary temperatures, so that if only a few get in they will cause the milk to sour in a short time. However, lactic acid bacteria multiply very slowly at lower temperatures (40°-s0° F.), and this is the reason why milk remains sweet longer if it is kept cool. While lactic acid fer- mentation spoils milk for most purposes, it is necessary in order to prepare the milk for butter-making. 29. Sterilization and Pasteurization. — Milk as well as other liquids may be sterilized by heating it three or four times on successive days to the boiling point (212° F.). This kills all bacteria whether in the active or the spore condition, but it also produces changes in the chemical make-up of the milk which render it less desirable for some purposes. By heating the milk to about 155° F.! for twenty minutes and then cooling it, the great majority of the bacteria in the active condition (but not those in the spore condition) are killed and little change is caused in the milk itself. This process is called pasteurization, and properly pasteurized milk, kept in a cool place, should remain sweet for several days. In time, however, some of the bacteria which were not killed will grow, divide, and become numerous, and the milk will sour or change in other ways, depending upon the kinds of bacteria that it contains. 30. Other Saprophytic Bacteria. — Among the many sap- rophytic bacteria that occur in milk are those which are con- cerned in the “ripening” of cheese. The characteristic flavors of many different kinds of cheese are due to substances produced in the cheese by the action of particular species of bacteria. To make one of these cheeses, therefore, it is necessary not only to treat the materials in the proper way, but also to make sure that the right kinds of bacteria are present. Another 1 The temperature actually used in the pasteurization of milk varies considerably, but is usually between 140° F. and 160° F, BACTERIA 27 familiar process due to bacteria is the fermentation that produces vinegar. 31. Ptomains.— We have seen that bacteria decompose organic substances into a great number of simpler compounds. Some of these simpler compounds are useful to the bacteria as food; but many of them are merely by-products. Among the latter are a class of substances called pfomains. Some of the ptomains, produced for example in decaying meat, cheese, or milk, are extremely poisonous to human beings. Many deaths are caused by eating partly spoiled foods, especially canned meats, in which ptomains have been formed by the action of bacteria. 32. Disease-producing Bacteria. — Although only a small proportion of the known species of bacteria are parasitic, the j Bs Fic. 9. — Disease-producing bacteria. 4, the tuberculosis bacterium; B, the diphtheria bacterium; C, the bacillus of typhoid fever; D, the bacillus that causes lockjaw; some of the cells are forming spores. parasitic forms include those that cause most of the con- tagious and infectious diseases of man and animals. Bac- teria are not responsible for all these diseases ; some, such as malaria and the African sleeping sickness, are produced by one-celled animals; and the organisms which cause some common diseases (for example, smallpox) are unknown. Many important plant diseases, too, are due to bacteria, though the number of such diseases thus far known is smaller than the number of bacterial diseases in animals. Some 22 TEXTBOOK OF BOTANY of the most important bacterial plant diseases will be dis- cussed in Chapter XXIV. 33. Tuberculosis. — This name is given to diseased con- ditions which are caused in various parts of the body by Bacterium tuberculosis (Fig. 9, A). The most frequent form is tuberculosis of the lungs, often called consumption. This is the most serious and widespread disease that attacks the human race. Each year about 150,000 persons are killed by tuberculosis in the United States, and about 1,500,000 in the world. Although the fight now being waged against tuberculosis is reducing the death rate from year to year, it is still true that of the persons now living in the United States more than 5,000,000 will die of tuberculosis unless more successful methods than are now known are found for treat- ing the disease. 34. How Tuberculosis is Caused. — The lung cells that are attacked by the bacteria are destroyed. Then there is a growth of new cells around the diseased region, forming a tubercle, which gave the disease its name. By the de- composition of the lung tissues, the bacteria produce, among other substances, one which passes into solution in the blood of the patient and there acts as a poison. This is one of a class of poisons, or toxins, which many parasitic bacteria produce. The toxin of the tubercle bacteria is responsible for many of the serious features of the disease. Eventually, if the disease is not in some way checked, the tissues of the lungs are largely destroyed. In many cases, however, in otherwise healthy and vigorous persons, the disease does not progress far. This is because the white corpuscles of the blood attack and kill the bacteria. The patient then recovers. The possibility of a cure of tuberculosis in its early stages shows the importance of keeping in good general health as a precaution against the disease, and also the necessity of be- ginning the treatment of tuberculosis at the earliest possible ROBERT KOCH Born at Klausthal, in Hanover, Germany, 1843; died at Baden-Baden, to10. One of the greatest investigators of the disease-producing bacteria, many of which he first discovered by means of methods that he devised. Among his discoveries are those of the organisms which cause tuberculosis and Asiatic cholera. BACTERIA 23 moment. Thus, it is said that about 90 per cent of all adult Europeans have at one time or another been afflicted with tuberculosis, although only 15 per cent die of the disease. There is no doubt that every one of us has many times taken the tubercle bacteria into his throat and lungs; but if they were not too numerous, and if we were in vigorous health, the bacteria were quickly killed and no harm resulted. Al- though tuberculosis most commonly affects the lungs, it oc- curs also in the skin, in the spinal column, the hip joint, the glands of the neck, the intestines, and the brain. 35. Tuberculosis in Cattle. — Tuberculosis is a disease of some of the lower animals, including cattle, as well as of man. The bacteria pass into the milk of diseased cows, and it has been shown that tuberculosis may be conveyed to babies by such milk. This is one reason for the adoption of laws by virious states requiring the testing of cattle for tuberculosis _d the killing of those found to be diseased. There is also a ossibility of getting the disease by eating the flesh of affucted cattle, but since most of the bacteria are killed in the cool ing of the meat, this danger is not so serious as is that from milk. 36 How Tuberculosis is Spread. — The bacteria enter the body by being breathed into the lungs, or by being taken into the stomach with food, or througha wound. The first method is proLably by far the most common. The disease germs are distributed in the sputum of diseased persons. If, therefore, all the sputum were destroyed, the spread of the disease would be ve.y largely checked, and the immediate destruction of the sputum is one of the most important means to be used in combating tuberculosis. Ifit isnot destroyed but is allowed to dry, the bacterial cells also become dry and are carried about like dust in currents of air (see Fig. 10). In this dried condition the germs can live for a long time. They are killed, however, by prolonged exposure to sunlight, and they live longer in bad air, ‘as in close, poorly ventilated rooms, 24 TEXTBOOK OF BOTANY than in fresh air. Plenty of fresh air and sunshine are there- fore very important, and especially so in the house where a tuberculous person lives or has lived. It is in crowded, unventi- lated, and poorly lighted tene- ments that a large proportion of cases of tuberculosis occur, both because conditions there are more favorable for the bacteria, and because persons living under such conditions are less healthy and so less able to throw off the Fic. 10.— Photograph show- disease. ing the growth of bacterial 37 Diphtheria. —Thisis caused colonies on an agar plate that : had been exposed for a short by a bacterium. that finds lodg- time to the air. The bacteria ment in the throat (Fig. 9, B). to Living and multiplying here, the and multiplied. bacterium gives off a toxin which is carried by the blood to other ee of the body and there produces the serious symptoms of the disease. The presence of the toxin in the blood of the sufferer causes the blood to form an antitoxin — a sub- stance which counteracts the poisonous effects of the toxin. If the antitoxin is formed in large enough quantity, the most serious effects of the disease do not appear; the patient finally gets rid of the bacteria that are producing the poison and recovers. If the antitoxin is not formed rapidly enough, the disease grows worse and the patient dies. Fortunately it has been found that a horse or mule inoculated with diphtheria toxin produces a large amount of antitoxin, and that this antitoxin, taken from the animal and injected under the skin of the human sufferer, has the same effect as the human antitoxin. To be effective in curing diphtheria, the antitoxin must be given early in the course of the disease; so it is most im- BACTERIA 25 portant that diphtheria be recognized and treated at the earliest possible moment. The use of this antitoxin has greatly reduced the proportion of deaths from diphtheria. For instance, in New York the average annual death rate from diphtheria fell from 15.19 in each 10,000 of the popu- lation before the introduction of antitoxin to 6.62 in 10,000 after its introduction ; and in Vienna it fell from 8.14 to 2.95 in 10,000. Ifa person recovers from an attack of diphtheria, he will not as a rule have the disease again, no matter how often he may be exposed; he is immune. Exceptions to this rule, however, occur. Similar immunity is known in the case of many other diseases, such as measles, smallpox, and yellow fever. 38. Typhoid Fever.— This is due to a bacillus which seems to be taken in only with food or drink (Fig. 9,C). The bacillus lives and multiplies chiefly in the small intestine, but also in other parts of the body, as the blood and the marrow of the bones. As in the case of the diphtheria bac- terium, its evil effects are due to a toxin which is absorbed by the blood. This toxin, however, is not given off by the living typhoid bacilli, but is a part of their living matter, and it passes into the blood of the human victim only after the bacilli have died. The most common source of typhoid in- fection is impure water, and the purity of the water supply of a city can be quite accurately judged by the number of cases of typhoid in that city. Flies, which breed and seek their food in filthy places, carry the typhoid germs about upon their bodies and deposit them in milk, cream, or other food that they touch. Flies carry many other germs besides that of typhoid, and they are now well recognized as dangerous distributors of disease. Persons who have been cured of typhoid fever may still ‘carry large numbers of actively multiplying typhoid bacteria in their intestines or bile ducts; this condition often lasts for several months, and in some cases for years. Such carriers, 26 TEXTBOOK OF BOTANY as they are called, as well as persons who are suffering from a light attack which is not recognized as typhoid, are a source of danger to others, especially if they are careless or un- cleanly in their habits. One case of a typhoid carrier that has been carefully studied was that of a cook; in several families in which she worked for a term of years typhoid cases occurred, and she was the cause of a number of deaths. An outbreak of typhoid fever involving forty-one cases and three deaths occurred at Madison, Wisconsin, in the fall of 1908, among those who lived or ate at a certain boarding house. The source was found to be a student who was employed in cleaning and wiping dishes and who had returned to Madison in the fall in the early stages of typhoid fever. Sufferers from typhoid are not immune to the disease after recovering from it; they may be repeatedly attacked. However, an artificial immunity can be produced by vaccination with typhoid bacilli that have been killed by heat or other means ; and the use of this method of vaccination has practically eliminated typhoid from the armies of many nations. 39. Lockjaw. — The germ of lockjaw or tetanus (Fig. 0, D), is a bacillus that lives in the soil and is found practically everywhere. The disease, like diphtheria, is caused by a toxin which is given off by the bacillus and is taken up by the blood. Infection occurs from soil that gets into cuts and wounds. It follows that a cut should always be thoroughly cleaned and then treated with an antiseptic, such as iodine, which will check the growth and reproduction of any bacteria that may be present. We have already seen that the wound should not be so tightly covered as to prevent the entrance of air, because it is in the absence of air that the tetanus bacillus thrives. 40. Anthrax. — This is a serious disease of cattle and sheep. It is highly contagious and difficult to combat, partly because the spores of the anthrax bacillus are very resistant to drying and can live in the soil for many years. BACTERIA 27 For this reason, anthrax sometimes appears suddenly and without any apparent cause when cattle are turned into a pasture where diseased animals had fed years before. The disease is largely due to the rapid multiplication of the bacteria in the blood vessels so that the circulation is stopped. A toxin is also produced. Animals are made immune to anthrax for about a year by vaccinating them with a culture of anthrax bacilli that have been weakened by exposure to air at a rather high temperature. 41. Nitrogen-fixing Bacteria (Fig. 11). — Disease-produc- ing bacteria live within the tissues of the host and are Fic. 11. — Nitrogen-fixing bacteria. A, roots of a leguminous plant (such as clover), bearing many small swellings. B, the method of entrance of the bacteria through a root-hair, and their progress through the tissues of the clover. C, a single cell of a clover root containing many bacteria, which appear as small dark dots. D, single bacteria as they appear in an artificial culture. E, as they appear in a cell of the clover. injurious to it. A quite different relation exists between clover, alfalfa, beans, peas, and other plants of the same family, and the bacteria which live within their roots. If we pull up a clover plant we find many small swellings on all parts of the roots. The swellings are outgrowths of the root 28 TEXTBOOK OF BOTANY tissues and contain immense numbers of a particular sort of bacillus. These bacilli can use nitrogen, which makes up about four-fifths of the air, in manufacturing complex sub- stances and ultimately in building up the living matter of their own cells. The power of using the nitrogen of the air in this way is possessed by some other bacteria and possibly by some fungi, but not by green plants. Now, nitrogen is necessary to the building up of all living matter, and green plants as a rule obtain it from the soil in the form of the compounds called nitrates. A soil that contains too small an amount of nitrates will raise only poor crops of grain or potatoes or of almost any useful plant. But in the case of clover and related plants, when the bac- teria in their root swellings die, as they do in large numbers, the nitrogen-containing compounds in the bodies of the bacteria are taken up by the host plant and are used by it as food. Thus clover and alfalfa are able with the help of the bacteria in their roots to use the nitrogen of the air, and this is the reason why crops of these plants can be raised in soil that is poor in nitrates. It also explains why soil is enriched by growing clover or alfalfa upon it, especially if the crop is plowed under, because then the nitrogen-containing sub- stances of the plant are broken down by the soil bacteria into nitrates which can be used by other plants. The root bacteria of clover and alfalfa are not parasites, but are in a sort of partnership with the host plant; the host supplies a moist place for development and also probably some forms of food for the bacteria, and the bacteria supply nitrogen- containing compounds for the host plant. 42. Historical Note.— Perhaps the first observation of bacteria was in 1659 by Kirchner, who saw “ minute living worms ” in putrefy- ing meat and other foods. Leeuwenhoek first described and figured bacteria in a lctter to the Royal Society of London in 1683. The modern study of bacteria began with the work of Cohn, published chiefly between the years 1853 and 1872. Pasteur in 1857 found that the souring of milk is due to bacterial action, and he showed the BACTERIA 29 practical importance of bacteria in producing fermentations of various sorts. A paper by Koch in 1876 on anthrax was the first thorough study of the production of a disease by bacteria, although the anthrax bacillus had been suspected as early as 1863 of being the cause of the disease, and Pasteur in 1870 had shown that a disease of silkworms is due to bacteria. Koch devised ways of isolating and cultivating bacteria and of staining them with anilin dyes, and by means of his staining methods he discovered the tuberculosis bacterium in 1882. CHAPTER III PROTOCOCCUS 43. General Appearance. — On the sides of fences or of old buildings, especially in spots that are shaded much of the time, we often see a layer of a dark-green, powdery sub- stance which becomes bright green when moist.