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diff --git a/78430-0.txt b/78430-0.txt new file mode 100644 index 0000000..0a01457 --- /dev/null +++ b/78430-0.txt @@ -0,0 +1,14306 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 78430 *** + + + + + TRANSCRIBER’S NOTE + + - Some minor changes to the text are noted at the end of the book. + - Italic text is denoted by _underscores_. + - Bold text is denoted by =equal signs=. + - Small-cap text has been changed to ALL-CAP text. + - Footnote anchors are denoted by [number], and the footnotes have + been placed at the end of the book. + + + + + A PRACTICAL COURSE IN + BOTANY + + WITH ESPECIAL REFERENCE TO ITS BEARINGS ON + + AGRICULTURE, ECONOMICS, AND SANITATION + + + BY + + E. F. ANDREWS + AUTHOR OF “BOTANY ALL THE YEAR ROUND” + + + WITH EDITORIAL REVISION BY + + FRANCIS E. LLOYD + + MACDONALD PROFESSOR OF BOTANY, McGILL UNIVERSITY, + FORMERLY OF ALABAMA POLYTECHNIC INSTITUTE + + [Illustration] + + + NEW YORK ·:· CINCINNATI ·:· CHICAGO + AMERICAN BOOK COMPANY + + + + + COPYRIGHT, 1911, BY + E. F. ANDREWS. + + ENTERED AT STATIONERS’ HALL, LONDON. + + + ANDREW’S PR. BOTANY. + + W. P. 7 + + + + +PREFACE + + +In preparing the present volume, the aim of the writer has been to +meet all the college entrance requirements and at the same time +to bring the study of botany into closer touch with the practical +business of life by stressing its relations with agriculture, +economics, and, in certain of its aspects, with sanitation. While +technical language has been avoided so far as the requirements of +scientific accuracy will permit, the student is not encouraged +to shirk the use of necessary botanical terms, out of a mere +superstitious fear of words because they happen to be a little new or +unfamiliar. Such a practice not only leads to careless and inaccurate +modes of expression, but tends to foster a slovenly habit of mind, +and in the long run causes the waste of more time and labor in the +search after roundabout, and often misleading, substitutes, than it +would require to master the proper use of a few new words and phrases. + +In the choice of materials for experiment and illustration, the +endeavor has been to call for such only as are familiar and easily +obtained. The specimens for flower dissection have been selected +mainly from common cultivated kinds, because their wide distribution +makes them easy to obtain everywhere, while in cities and large +towns they are practically the only specimens available. Another +important consideration has been the desire to spare our native +wild flowers, or at least not to hasten the extinction with which +they are threatened by the ravages of Sunday excursionists and +summer tourists, to whose unthinking, but none the less destructive, +incursions, the automobile has laid open the most secret haunts +of nature. The influence of the public school teacher, and more +especially the teacher of botany, is the most potent factor from +which we can hope for aid in putting a stop to the relentless +persecution that has practically exterminated many of our choicest +wild plants and is fast reducing the civilized world to a depressing +monotony of weediness and artificiality. Except for purely systematic +and anatomical work, flowers can be studied to better purpose in +their living, active state than as dead subjects for dissection; +and the best way to show our interest in them, or to get the most +rational enjoyment out of them, is not, as a general thing, to +cut their heads off and throw them away to wither and die by the +roadside. The teacher, by instilling into the minds of the rising +generation a reverence for plant life, may do a great deal to aid +in the conservation of one of our chief national assets for the +gratification of the higher esthetic instincts. The fruits and +flowers of cultivation do not stand in the same need of protection, +since they are produced solely with a view to the use and pleasure of +man, and their propagation is provided for to meet all his demands. + +To avoid too frequent interruptions of the subject matter, the +experiments are grouped together at the beginning or end of the +sections to which they belong, according as they are intended to +explain what is coming, or to illustrate what has gone before. A few +exceptions are made in cases where the experiment is such an integral +part of the subject that it would be meaningless if separated from +the context. Under no circumstances should those capable of being +performed in the schoolroom be omitted, as much of the information +which the book is intended to give is conveyed by their means. For +this reason, and also because the aim of the book is to present the +science from a practical rather than from an academic point of view, +the experiments outlined are for the most part of a simple, practical +nature, such as can be performed by the pupils themselves with a +moderate expenditure of ingenuity and money. The experience of the +writer has been that for the average boy or girl who wishes to get +a good general knowledge of the subject, but does not propose to +become a specialist in botany, the best results are often obtained +by the use of the simplest and most familiar appliances, as in +this way attention is not distracted from the experiment itself to +the unfamiliar apparatus for making it. In saying this, it is not +meant to underrate the value of a complete laboratory equipment, +but merely to emphasize the fact that the lack of it, while a +disadvantage, need not be an insuperable bar to the successful +teaching of botany. It is, of course, taken for granted that in +schools provided with a suitable laboratory outfit, teachers will be +prepared to supplement or to replace the exercises here outlined with +such others as in their judgment the subject may demand. There are as +many ideals in teaching as there are teachers, and the most that a +textbook can do is to present a working model which every teacher is +free to modify in accordance with his or her own method. + +The writer takes pleasure in acknowledging here the many obligations +due to Professor Francis E. Lloyd, of the Botanical Department of the +Alabama Polytechnic Institute, at Auburn, Ala., for his valuable aid +in the revision of the manuscript, for the highly interesting series +of illustrations relating to phototropic movements, and for advice +and information on points demanding expert knowledge which have +contributed very materially to whatever merit this volume may possess. + +Other members of the Auburn faculty to whom the author feels +especially indebted are Mr. C. S. Ridgeway, assistant in the +Botanical Department, Professor J. E. Duggar, of the Agricultural +Department, and Dr. B. B. Ross and Professor C. W. Williamson of the +Department of Chemistry. Acknowledgments are due also to Professor +George Wood of the Boys’ High School, Brooklyn, for suggestions +which have been of great assistance in the preparation of this work; +to Professor W. R. Dodson, of the University of Louisiana, for +illustrative material furnished, and to Professor William Trelease +for the loan of original material used in reproducing the beautiful +cuts from the Reports of the Missouri Botanical Garden, credit for +which is given in the proper place. + +For original photographs and drawings by the author, and familiar +selections from well-known works, which can be generally recognized, +it has not been thought necessary to give special credit. + + E. F. ANDREWS. + + AUBURN, ALABAMA. + + + + + FULL-PAGE ILLUSTRATIONS + + + PLATE PAGE + + 1. A GROVE OF LIVE OAKS NEAR SAVANNAH, GEORGIA _Frontispiece_ + + 2. CARRYING WATER OVER THE MISSISSIPPI LEVEE BY SIPHON TO + IRRIGATE RICE FIELDS 8 + + 3. AËRIAL ROOTS OF A MEXICAN STRANGLING FIG 73 + + 4. A FOREST OF BAMBOO 99 + + 5. A GROUP OF CONIFERS 108 + + 6. A WHITE OAK, SHOWING THE GREAT SPREAD OF BRANCHES 117 + + 7. A TIMBER TREE SPOILED BY STANDING TOO MUCH ALONE 125 + + 8. AN AMERICAN ELM, ILLUSTRATING DELIQUESCENT GROWTH 130 + + 9. VEGETATION OF A MOIST, SHADY RAVINE 151 + + 10. A MOSAIC OF MOONSEED LEAVES 179 + + 11. HYBRID BETWEEN A RED AND A WHITE CARNATION 227 + + 12. GOOSEBERRIES, SHOWING IMPROVEMENT BY SELECTION 251 + + 13. THE EFFECTS OF IRRIGATION 272 + + 14. A XEROPHYTE FORMATION OF YUCCAS AND SWITCH PLANTS 282 + + 15. A GIANT TULIP TREE OF THE SOUTH ATLANTIC FOREST REGION 293 + + + + + CONTENTS + + + CHAPTER I. THE SEED PAGE + + I. THE STORAGE OF FOOD IN SEEDS 1 + + II. SOME PHYSIOLOGICAL PROPERTIES OF SEEDS 10 + + III. TYPES OF SEEDS 12 + + IV. SEED DISPERSAL 21 + + FIELD WORK 28 + + + CHAPTER II. GERMINATION AND GROWTH + + I. PROCESSES ACCOMPANYING GERMINATION 29 + + II. CONDITIONS OF GERMINATION 33 + + III. DEVELOPMENT OF THE SEEDLING 40 + + IV. GROWTH 47 + + FIELD WORK 52 + + + CHAPTER III. THE ROOT + + I. OSMOSIS AND THE ACTION OF THE CELL 53 + + II. MINERAL NUTRIMENTS ABSORBED BY PLANTS 58 + + III. STRUCTURE OF THE ROOT 61 + + IV. THE WORK OF ROOTS 65 + + V. DIFFERENT FORMS OF ROOTS 72 + + FIELD WORK 80 + + + CHAPTER IV. THE STEM + + I. FORMS AND GROWTH OF STEMS 81 + + II. MODIFICATIONS OF THE STEM 88 + + III. STEM STRUCTURE + + A. MONOCOTYLS 96 + + B. HERBACEOUS DICOTYLS 102 + + C. WOODY STEMMED DICOTYLS 107 + + IV. THE WORK OF STEMS 112 + + V. WOOD STRUCTURE IN ITS RELATION TO INDUSTRIAL USES 118 + + VI. FORESTRY 124 + + FIELD WORK 128 + + + CHAPTER V. BUDS AND BRANCHES + + I. MODES OF BRANCHING 131 + + II. BUDS 138 + + III. THE BRANCHING OF FLOWER STEMS 141 + + FIELD WORK 145 + + + CHAPTER VI. THE LEAF + + I. THE TYPICAL LEAF AND ITS PARTS 147 + + II. THE VEINING AND LOBING OF LEAVES 154 + + III. TRANSPIRATION 160 + + IV. ANATOMY OF THE LEAF 164 + + V. FOOD MAKING 168 + + VI. THE LEAF AN ORGAN OF RESPIRATION 174 + + VII. THE ADJUSTMENT OF LEAVES TO EXTERNAL RELATIONS 177 + + VIII. MODIFIED LEAVES 189 + + FIELD WORK 194 + + + CHAPTER VII. THE FLOWER + + I. DISSECTION OF TYPES WITH SUPERIOR OVARY 196 + + II. DISSECTION OF TYPES WITH INFERIOR OVARY 204 + + III. STUDY OF A COMPOSITE FLOWER 210 + + IV. SPECIALIZED FLOWERS 214 + + V. FUNCTION AND WORK OF THE FLOWER 219 + + VI. HYBRIDIZATION 223 + + VII. PLANT BREEDING 230 + + VIII. ECOLOGY OF THE FLOWER + + A. THE PREVENTION OF SELF-POLLINATION 235 + + B. WIND POLLINATION 239 + + C. INSECT POLLINATION 241 + + D. PROTECTIVE ADAPTATION 245 + + FIELD WORK 249 + + + CHAPTER VIII. FRUITS + + I. HORTICULTURAL AND BOTANICAL FRUITS 250 + + II. FLESHY FRUITS 255 + + III. DRY FRUITS 260 + + IV. ACCESSORY, AGGREGATE, AND MULTIPLE FRUITS 265 + + FIELD WORK 269 + + + CHAPTER IX. THE RESPONSE OF THE PLANT TO + ITS SURROUNDINGS + + I. ECOLOGICAL FACTORS 271 + + II. PLANT ASSOCIATIONS 277 + + III. ZONES OF VEGETATION 288 + + FIELD WORK 294 + + + CHAPTER X. CRYPTOGAMS + + I. THEIR PLACE IN NATURE 296 + + II. ALGÆ 299 + + III. FUNGI 303 + + A. BACTERIA 306 + + B. YEASTS 314 + + C. RUSTS 317 + + D. MUSHROOMS 323 + + IV. LICHENS 329 + + V. LIVERWORTS 334 + + VI. MOSSES 341 + + VII. FERN PLANTS 344 + + VIII. THE RELATION BETWEEN CRYPTOGAMS AND SEED PLANTS 354 + + IX. THE COURSE OF PLANT EVOLUTION 359 + + FIELD WORK 362 + + + APPENDIX + + 1. SYSTEMATIC BOTANY 364 + + 2. WEIGHTS, MEASURES, AND TEMPERATURES 367 + + +[Illustration: PLATE 1.—Live oaks covered with Spanish moss +(_Tillandsia_).] + + + + +CHAPTER I. THE SEED + + + I. THE STORAGE OF FOOD IN SEEDS + + MATERIAL.—In addition to the four food tests described in Exps. + 1-6, there should be provided some raw starch, a solution of grape + sugar, the white of a hard-boiled egg, and any fatty substance, + such as lard or oil. For Exps. 8 and 9, a little diastase solution + will be necessary. “Taka” diastase, made from rice acted upon by a + fungus, can be obtained for a trifle at almost any drug store. + + LIVING MATERIAL.—Grains of corn and wheat, and seeds of some kind + of bean, the larger the better. The “horse bean” (_Vicia faba_), + if it can be obtained, makes an excellent object for study, as the + cells are so large that they can be seen with the naked eye. For + showing the presence of proteins (aleurone grains) and oily matter, + use thin cross sections through the kernel of a castor bean or a + Brazil nut. Specimens for the study of the individual cell will be + found in the hairs growing on squash seedlings, in the epidermis of + one of the inner coats of an onion, in the roots of oat or radish + seedlings, or in the section of a young corn root. + + A compound microscope will be required for this study. + +=1. The economic importance of seeds.=—As a source of food to both +man and the lower animals, the importance of seeds can hardly be +overrated. All the flour, meal, rice, hominy, and other breadstuffs +sold in the market come from them, to say nothing of the fleece from +the cotton seed that clothes the greater part of the world, besides +furnishing a substitute for lard and an important food for cattle. +The oils and fats stored in nuts are also to be taken into account, +the peanut alone yielding the greater part of the so-called olive +oil of commerce. Since the value of our farm crops depends largely +upon the kind and quantity of these substances furnished by them, it +is worth our while, as a matter of economic as well as scientific +interest, to learn something about the nature of the different foods +contained in plants. + +[Illustration: FIGS. 1-3.—The world’s three most important food +grains (magnified): 1, section of a rice grain; _a_, cuticle; _b_, +aleurone, or protein layer; _c_, starch cells; _d_, germ; 2, section +of a wheat grain; _k_, germ; _s_, starch; _a_, gluten; _t_, _t_, +_t_, layers of the seed coat; 3, section of a grain of corn; _c_, +husk; _e_, aleurone layer containing proteins; _eg_, yellowish, +horny endosperm, containing proteins and starch; _ew_, lighter +starchy endosperm: the darker part below is rich in oil and proteins, +and contains the _embryo_, consisting of the absorbing organ, or +_cotyledon_, _sc_; the rudimentary bud, _s_; and the root, _w_. (1, +from Circular 77, La. Exp. Station; 2, from Francé; 3, from Sachs.)] + +=2. Why food is stored in seeds.=—The one purpose for which plants +produce their seed is to give rise to a new generation and so carry +on the life of the species. The seed is the nursery, so to speak, in +which the germ destined to produce a new plant is sheltered until +it is ready to begin an independent existence. But the young plant, +like the young animal, is incapable of providing for itself at first, +and would die unless it received nourishment from the mother plant +until it has formed roots and leaves so that it can manufacture food +for itself. Plants in general require very much the same food that +animals do, and they have the power, which animals have not, of +manufacturing it out of the crude materials contained in the soil +water and in the air. Such of these foods as are not needed for +immediate consumption, they store up to serve as a provision for the +young shoot when the seed begins to germinate. + +[Illustration: FIGS. 4-7.—Sections of corn grains showing different +qualities of food contents: 4, 5, small germ and large proportion +of horny part, showing high protein; 6, 7, large germ and smaller +proportion of horny part, showing high oil content.] + +=3. Food substances contained in seeds.=—There are four principal +classes of food stored in seeds: _sugars_, _starches_, _oils_, and +_proteins_. The first are held in solution and can be detected, if +in sufficient quantity, by the taste. The most important varieties +of this group are cane and grape sugar, the latter occurring most +abundantly in fruits, the former in roots and stems. Oil usually +occurs in the form of globules. It is very abundant in some seeds, +_e.g._ flax, castor bean, and Brazil nut. In the corn grain it is +found in the part constituting the germ, or embryo (Figs. 6, 7). +Starches and proteins occur in the form of small granules, which have +specific shapes in different plants (Figs. 8, 9). Those containing +proteins are called _aleurone_ grains, and are, as a rule, smaller +than the starch grains with which they are intermixed in the bean +and some other seeds. In wheat, corn, rice, and most grains they +form a layer just inside the husk, as shown in Fig. 10. This is the +reason why polished rice and finely bolted flour are less nutritious +than the darker kinds, from which this valuable food substance has +not been removed. The two most familiar kinds of proteins are the +_albumins_, of which the white of an egg is a well-known example, and +the _glutins_, which give to the dough of wheat flour and oatmeal +their peculiar gummy or “glutinous” structure. + +[Illustration: FIGS. 8-9.—Different forms of starch grains: 8, rice; +9, wheat.] + +=4. Organic foods.=—These four substances, starch, sugar, fats, and +proteins, with some others of less frequent occurrence, are called +_organic foods_, because they are produced, in a state of nature, +only through the action of organized living bodies, or, more strictly +speaking, of living vegetable bodies. + +[Illustration: FIG. 10.—Transverse section near the outside of a +wheat grain: _e_, the husk; _a_, cells containing protein granules; +_s_, starch cells (_after_ Tschirch).] + +=5. Our dependence upon plants.=—While the animal organism can digest +and assimilate these substances after they have been formed by +plants, it has no power to manufacture them for itself, and, so far +as we know at present, is wholly dependent upon the vegetable world +for these necessaries of life. In one sense the whole animal kingdom +may be said to be parasitic on plants. The wolf that eats a lamb is +getting his food indirectly from the grains and grasses consumed by +its victim, and the lion that devours the wolf that ate the lamb is +only one step further removed from a vegetable diet. + +=6. The vegetable cell.=—If you will break open a well-soaked horse +bean and examine the contents with a lens, you will see that they +are composed of small oval or roundish granules packed together +like stones in a piece of masonry. These little bodies, called +_cells_, are the ultimate units out of which all animal and vegetable +structures are built up, as a wall is built of bricks and stones. +They differ very much from bricks and stones, however, in that they +are, or have been, living structures with their periods of growth, +activity, decline, and death, just like other living matter, as +will be seen by and by, when we come to look more particularly into +their life history. They consist usually of an inclosing membrane +which contains a living substance called _protoplasm_. This is the +essential part of the cell, and, so far as we know at present, the +physical basis of all life. Cells are commonly more or less rounded +in shape, though they take different forms according to the purpose +they serve. Sometimes, as in the fibers of cotton and the down of +young leaves, they are long and hairlike; when closely packed, they +often become angular by pressure, like those shown in Figs. 10, 11. +The cells composing the thick body of the bean are for the most +part starch and other substances stored up for food, which render +observation difficult. It will, therefore, be better to choose for a +study of the individual cell some kind that will show the essential +parts more distinctly. + +[Illustration: FIG. 11—Typical cells: _n_, nucleus; _p_, protoplasm; +_w_, cell wall; _s_, sap.] + +=7. Microscopic examination of a cell.=—Place under a high power +of the microscope a portion of fresh skin from one of the inside +scales of an onion, or a piece of the root tip of a very young corn +or oat seedling, and fix your attention on one of the individual +cells. Notice (1) the cell wall or inclosing membrane, _w_ (Fig. 11); +(2) the protoplasm, _p_, which may be recognized by its granular +appearance; (3) the _nucleus_, _n_; and (4) the cell sap, _s_. In +very young cells the protoplasm will be seen to fill most of the +interior; but in mature ones, like the large one on the right of the +figure, it forms a thin lining around the wall, with the nucleus +on one side, while the cell sap, composed of various substances in +solution, occupies the central portion. Though there is generally +an inclosing wall, this is not essential, its office being to give +strength and mechanical support by holding the contents together, as +an India-rubber bag holds water. It is the turgidity of the cell, +when distended with liquid, that gives firmness to herbaceous plants +and the tender parts of woody ones. This may be illustrated by +observing the difference between a rubber bag when quite full and +when only half full of water, or a football when partially and when +fully inflated. In its simplest form, however, the cell is a mere +particle of protoplasm, which has one part, constituting the nucleus, +a little more dense in appearance than the rest, but this kind is not +common in vegetable structures. + +=8. How food substances get into the cells.=—As there are no openings +in the cell walls, the only way substances can get into a cell or +out of it is by soaking through the inclosing membrane, as will be +explained in a later chapter. Since starch, oil, and proteins, the +most important foods stored in seeds, are none of them soluble in the +cell sap, it is clear that they could not have got into the cells in +their present state, but must have undergone some change by which +they were rendered capable of passing through the cell wall. + +=9. Digestion.=—The process by which this change is brought about +is known as _digestion_, from its similarity to the same function +in animals. Not only are foods, in the state in which we find them +stored in the seed, incapable of passing through the cell wall, +but the protoplasm, the living part of the cell, has no power to +assimilate and to utilize these substances as food until they have +been reduced to a soluble form in which they can be diffused freely +from cell to cell through any part of the plant. By _diffusion_ is +meant the gradual spread of soluble substances through the containing +medium, as when a lump of sugar or salt, dropped into a glass of +water, dissolves and slowly diffuses through the contents, imparting +a sweet or salty taste to the whole. + +[Illustration: FIG. 12.—Starch grains of wheat in different stages of +disintegration under the action of a ferment (diastase), accompanying +germination: _a_, slightly corroded; _b_, _c_, and _d_, more advanced +stages of decomposition.] + +During the process of digestion the different kinds of food are +acted upon and made soluble by certain chemical ferments, which are +secreted in plants for the purpose. The digestion of starch, the most +abundant of plant foods, is effected by diastase, a common ferment +obtained from germinating grains of barley, wheat, corn, rice, etc. +By the presence of diastase starch is converted into grape sugar, +a substance which is readily soluble in water, and which can be +diffused easily through the tissues of the plant to any part where it +is needed. In this way food travels from the leaf, where it is made, +to the seed, where the sugar is generally reconverted into starch and +stored up for future use, though sometimes, as in the sugar corn and +sugar pea, it remains in part unchanged. The kernels of this kind of +corn can be distinguished readily from those of the ordinary starch +corn, after maturity, by their wrinkled appearance, owing to their +greater loss of water in drying. + +=10. Food tests.=—In order to tell whether any of the food substances +named occur in the seeds that we are going to examine, it will be +necessary to understand a few simple tests by which their presence +may be recognized. The chemicals required can be ordered ready for +use from a druggist or may be prepared in the laboratory as needed, +according to the directions given. Write in your notebook a brief +account of each experiment made, with the conclusions drawn from it. + + EXPERIMENT 1. TO DETECT THE PRESENCE OF FATS.—Rub a small lump of + butter or a drop of oil on a piece of thin white paper. What is the + effect? + + EXPERIMENT 2. ANOTHER TEST FOR FATS.—Place some macerated alcanna + root in a vessel with alcohol enough to cover it, and leave for + an hour. Add an equal bulk of water and filter. The solution will + stain fats, oils, and resins deep red. + +[Illustration: PLATE 2.—Carrying water over the Mississippi levee +by siphon to irrigate rice fields. (_From_ Circular of La. Exp. +Station.)] + + EXPERIMENT 3. TO SHOW THE PRESENCE OF STARCH.—Put a drop of iodine + solution on some starch. What change of color takes place? To + make iodine solution, add to one part of iodine crystals 4 parts + potassium iodide and 95 parts water. It should be kept in the + dark, as light decomposes it. Iodine colors starch blue, protein + substances light brown. In testing for starch, the solution should + be diluted till it is of a pale color, otherwise the stain will be + so deep as to appear black. + + EXPERIMENT 4. A TEST FOR PROTEINS.—Place a small quantity of the + white of an egg, diluted with water, in a clean glass and add a few + drops of nitric acid; or drop some of the acid on the white of a + hard-boiled egg. What is the effect? + + Nitric acid turns proteins yellow; if the color is indistinct, add + a drop of ammonia, when an orange color will ensue. + + EXPERIMENT 5. ANOTHER TEST FOR PROTEINS.—Place on the substance to + be examined a drop of a saturated solution of cane sugar and water; + add a drop of pure sulphuric acid; if proteins are present, they + will be colored red. See also Exp. 3. + + EXPERIMENT 6. A TEST FOR GRAPE SUGAR.—Heat a teaspoonful of + Fehling’s Solution to the boiling point in a test tube (a common + glass vial can be used by heating gradually in water) and pour in a + few drops of grape sugar solution. Heat again and observe the color + of the precipitate that forms. + + Fehling’s Solution may be obtained of the druggist, or, if + preferred, it may be prepared in the laboratory as follows: (_a_) + Dissolve 173 grams of crystallized Rochelle salts and 125 grams + of caustic potash in 500 cc. of water; (_b_) dissolve 34.64 grams + crystallized copper sulphate in 500 cc. of water, and mix equal + parts as needed. (For English equivalents, see Appendix, Weights + and Measures.) The two mixtures must be kept separate till wanted + for use, or prepared fresh as needed. + + Grape Sugar causes Fehling’s Solution to form a red precipitate. + + EXPERIMENT 7. TO SHOW THE DIFFERENCE BETWEEN SUGAR AND STARCH IN + REGARD TO SOLUBILITY.—Mix some sugar with water and notice how + readily it dissolves. Try the same experiment with starch and + observe its different behavior. + + EXPERIMENT 8. TO SHOW HOW STARCH IS DISINTEGRATED IN THE ACT OF + DIGESTION.—Place a few grains of starch on a slide, add a drop or + two of diastase solution, and observe under the microscope; the + starch granules will be seen to disintegrate and melt away. Even + with a hand lens it can be seen, from the greater clearness of the + liquid in comparison with a mixture of untreated starch and water, + that the grains have been dissolved. + + EXPERIMENT 9. TO SHOW THAT DIASTASE CONVERTS STARCH INTO + SUGAR.—Make a paste of boiled starch so thin that it looks like + water. Pour a small quantity of it into each of two tubes, adding a + little diastase to one and leaving the other untreated. Keep in a + warm place for twenty-four hours, then test both tubes for starch, + as directed in Exp. 3, and note the result. If the diastase has not + acted, add a little more and watch. + + + Practical Questions + + 1. Name all the food and other economic products you can think of + that are derived from the seed of maize; from wheat; from flaxseed; + from cotton. + + 2. Mention some seeds from which medicines are procured. + + 3. Name all the seeds you can think of from which oil is obtained; + starch; some that are rich in proteins. (Exps. 1-5.) + + 4. Describe some of the ways in which these products are frequently + adulterated. + + 5. If you were raising corn to sell to a starch factory, what part + of the seed would you seek to develop? If to feed stock, what part? + Why, in each case? (3; Figs. 4-7.) + + 6. What grain feeds more human beings than does any other? + + 7. Name all the seeds you can think of that contain sugar in + sufficient quantity to be detected without chemical tests; that is, + by tasting alone. + + 8. Is “coal oil” a mineral or an organic substance? Explain, by + giving an account of its origin. + + 9. What is gluten? (3.) Name some grains that are especially rich + in it. + + 10. Which of our three chief food grains is a water plant? (See + Plate 2.) Which grows farthest south? Which farthest north? Which + one is of American origin? + + + II. SOME PHYSIOLOGICAL PROPERTIES OF SEEDS + + MATERIAL.—Seeds of squash, pumpkin, or other melon; castor bean; + any kind of common kidney bean; grains of Indian corn. + + APPLIANCES.—In the absence of gas, an alcohol or kerosene lamp may + be used for heating. A double boiler can easily be made by using + two tin vessels of different sizes. Partly fill the larger one with + water, set in it the smaller one with the substance to be heated, + and place over a burner. A pair of scales, a strong six-ounce + bottle, wire-netting, cord, and wax or paraffin should be provided. + + EXPERIMENT 10. DO SEEDS IN THEIR ORDINARY QUIESCENT STATE CONTAIN + ANY WATER?—Place a number of beans, or grains of corn or wheat in + a glass bottle, making a small perforation in the cork to allow + the air to escape, and heat gently. Does any moisture form on the + glass? + + A better test is to weigh two or three ounces of seeds, and heat + them in a double boiler or in oil to prevent scorching. Weigh at + intervals. If there is any loss of weight, to what is it due? + + EXPERIMENT 11. DO SEEDS ABSORB WATER?—Soak a number of beans or + grains of corn in water for 12 to 24 hours and compare with dry + ones. What difference do you notice? To what cause is it due? + + EXPERIMENT 12. HOW DID WATER GET INTO THE SOAKED SEEDS?—Dry gently + with a soft cloth some of the seeds used in the last experiment and + press them lightly to see if water comes out, and where. Place a + number of dry seeds of different kinds—squash, bean, castor bean, + quince, etc.—in warm water and notice whether any bubbles of air + form on them and at what point. Examine with a lens and see if this + point differs in any way from the rest of the seed cover. Does it + correspond with the point from which water exuded in the soaked + seeds? Could hard seeds like the squash, castor bean, buckeye, and + Brazil nut get water readily without an opening somewhere in the + coat? + + EXPERIMENT 13. TO FIND OUT WHETHER WATER IS ABSORBED THROUGH THE + SEED COATS.—Place in moist sand or sawdust two rows of beans as + nearly as possible of the same size and weight, with the eye + pressed down to the substratum in one row and turned up in the + other, so that no moisture can enter through it. In the same way + arrange two rows of castor beans with the little end down in one + row and uppermost in the other. In the last set carefully break + away the spongy mass near the tip, without injuring the parts about + it. Watch and see in which rows water is absorbed most readily. + What change takes place in the spongy masses at the tips of those + castor beans on which they were left? + +[Illustration: FIG. 13.—Effect of the expansion of seeds due to +absorption of water.] + + EXPERIMENT 14. IS THE RATE OF GERMINATION AFFECTED BY THE PRESENCE + OR ABSENCE OF OPENINGS?—Seal up with wax or paraffin all the + openings of a number of air-dry peas or beans, and leave an equal + number of the same size and weight untreated. Be careful that the + sealing is absolutely water-tight, since otherwise the experiment + will be worthless. Plant both sets and keep under like conditions + of soil, temperature, and moisture. Do you see any difference in + the rate of germination of the two sets? + + EXPERIMENT 15. DO SEEDS EXERT FORCE IN ABSORBING WATER?—Fill a + common six-ounce bottle as full as it will hold with dry peas, + beans, or grains of corn; then pour in water till the bottle is + full. Tie a piece of wire-netting or stout sackcloth over the top + to keep the seeds from being forced out. Bind both the neck and the + body of the bottle tightly with strong cords encircling it in both + a horizontal and vertical direction, and place under water in a + moderately warm temperature. Watch for results. + + EXPERIMENT 16. IS THE FORCE EXERTED IN THE LAST EXPERIMENT A + MERELY MECHANICAL ONE, LIKE THE BURSTING OF A WATER PIPE, OR IS IT + PHYSIOLOGICAL AND THUS DEPENDENT ON THE FACT THAT THE SEEDS ARE + ALIVE?—To answer this question try Exp. 15 with seeds that have + been killed by heat or by soaking in formalin. + + + Practical Questions + + 1. Will a pound of pop corn weigh as much after being popped as + before? (Exp. 10.) + + 2. What causes the difference, if there is any? (Exp. 10.) + + 3. Does the tuft of downy hairs at the tip of wheat and oat grains + influence their water supply? The spongy covering of black walnuts + and almonds? The pithy inside layers of pecans and English walnuts? + (Exps. 12, 13.) + + 4. Why will seeds, as a general thing, germinate more readily after + being soaked? (Exps. 11, 14, 16.) + + + III. TYPES OF SEEDS + + MATERIAL.—Dry and soaked grains of corn, wheat, or oats; bean, + squash, castor bean, and pine seed, or any equivalent specimens + showing the differences as to number of cotyledons and the + presence or absence of endosperm. Each student should be provided + with several specimens, both soaked and dry, of the kind under + consideration. Corn, beans, and wheat need to be soaked from 12 + to 24 hours; squash and pumpkin from 2 to 5 days, and very hard + seeds, like the castor bean and morning-glory, from 5 to 10. If + such seeds are _clipped_, before soaking, that is, if a small piece + of the coat is chipped away from the end opposite the scar, or eye, + they will soften more quickly. Keep them in a warm place with an + even temperature till just before they begin to sprout, when the + contents become softened. Very brittle cotyledons may be softened + quickly by boiling for a few minutes. + + No appliances are needed beyond the pupil’s individual outfit and + some of the food tests given in Section I of this chapter. + +=11. Dissection of a grain of corn.=—Examine a dry grain of corn +on both faces. What differences do you notice? Sketch the grooved +side, labeling the hard, yellowish outer portion, _endosperm_, the +depression near the center, _embryo_, or _germ_. + +Next take a grain that has been soaked for twenty-four hours. What +changes do you see? How do you account for the swelling of the +embryo? Remove the skin and observe its texture. Make an enlarged +sketch of a grain on the grooved side with the coat removed, labeling +the flat oval body embedded in the endosperm, _cotyledon_; the upper +end of the little budlike body embedded in the cotyledon, _plumule_, +the lower part, _hypocotyl_—words meaning, respectively, “seed leaf,” +“little bud,” and “the part under the cotyledon.” As this part has +not yet differentiated into root and stem, we cannot call it by +either of these names. The cotyledon, hypocotyl, and plumule together +compose the embryo. Pick out the embryo and sketch as it appears +under the lens. Crush it on a piece of white paper; what does it +contain? + +[Illustration: FIGS. 14-16.—Dissection of a grain of corn: 14, soaked +grain, seen flatwise, cut away a little and slightly enlarged, so as +to show the embryo lying in the endosperm; 15, in profile section, +dividing the grain through the embryo and cotyledon; 16, the embryo +taken out whole. The thick mass is the cotyledon; the narrow body +projecting upwards, the plumule; the short projection at the base, +the hypocotyl (_after_ GRAY).] + +Make a vertical section of another soaked grain at right angles to +its broader face, and sketch, labeling the parts as they appear in +profile. Make a cross section through the middle of another grain and +sketch, labeling the parts as before. What proportion of the grain is +endosperm and what embryo? Put a drop of iodine and of nitric acid +separately on pieces of the endosperm, and note the effects. Test the +seed coats and the cotyledon to see if they contain any starch. + +Notice that the corn grain has but one cotyledon, hence such seeds +are said to be _monocotyledonous_, or one-cotyledoned. The grains are +not typical seeds, but are selected for examination because they are +large and easy to handle, can be obtained everywhere, and germinate +readily. + +=12. Dissection of a bean.=—Sketch a dry bean as it lies in the pod, +showing its point of attachment and any markings that may appear on +its surface. Then take it from the pod and examine the narrow edge +by which it was attached. Notice the rather large scar (commonly +called the eye of the bean) where it broke away from the point of +attachment. This is the _hilum_. Near the hilum, look for a minute +round pore like a pinhole. This is called the _micropyle_, from a +Greek word meaning “a little gate,” because it is the entrance to +the interior of the seed coat. There was no micropyle observed in +the corn grain, because it is not a true seed but a fruit inclosing +a single seed. The inclosing membrane is the fruit skin, which has +become incorporated with the seed coat and taken its place as a +protective covering. Compare a soaked bean with a dry one; what +difference do you perceive? How do you account for the change in size +and hardness? Find the hilum and the micropyle in the soaked bean. +Lay it on one side and sketch, with the micropyle on top; then turn +toward you the narrow edge that was attached to the pod and sketch, +labeling all the parts. Make a section through the long diameter at +right angles to the flat sides, press it slightly open, and sketch +it. Notice the line or slit that seems to cut the section in half +longitudinally, and the small round object between the halves at one +end; can you tell what it is? + +[Illustration: FIGS. 17, 18.—A kidney bean: 17, side view; 18, front +view, showing _h_, hilum, _m_, micropyle.] + +[Illustration: FIG. 19.—Cotyledon of a bean, showing plumule.] + +Slip off the coat from a whole bean and notice its texture. Hold it +up to the light and see if it shows any signs of veining. See whether +the scar at the hilum extends through the kernel, or marks only the +seed coat. Lay open the two flat bodies into which the kernel divides +when stripped of its coats, keeping them side by side, with the part +above the micropyle toward the top. Sketch their inner face and +label them _cotyledons_. Be careful not to break or displace the +tiny bud packed away between the cotyledons, just above the hilum. +Label the round portion of this bud, _hypocotyl_, and the upper, more +expanded part, _plumule_. Which way does the base of the hypocotyl +point; toward the micropyle, or away from it? Pick out this budlike +body entire and sketch as it appears under the lens. Open the plumule +with a pin and examine it with a lens; of what does it appear to +consist? Do you find any endosperm around the cotyledons, as in the +corn and oats? Break one of the soaked cotyledons, apply the proper +tests (Exps. 2, 3, 5), and report what substances it contains. Where +is the nourishment for the young plant stored? What part of the bean +gives it its value as food? + +Notice that in the bean the embryo consists of three parts, the +hypocotyl, plumule, and the two cotyledons, which completely fill +the seed coats, leaving no place for endosperm. Seeds like the bean, +squash, and castor bean, which have two cotyledons, are said to be +_dicotyledonous_. + +=13. The castor bean.=—Lay a castor bean on a sheet of paper before +you with its flat side down; what does it look like? The resemblance +may be increased by soaking the seed a few minutes, in order to swell +the two little protuberances at the small end. Can you think of any +benefit a plant might derive from this curious resemblance of its +seed to an insect? + +Sketch the seed as it lies before you, labeling the protuberance +at the apex, _caruncle_. The caruncle is an appendage of the +seed-covering developed by various plants; its use is not always +clear. What appears to be its object in the castor bean? Refer to +Exp. 13 and see if there is any other purpose it might serve. + +Turn the seed over and sketch the other side. Notice the colored +line or stripe that runs from the large end to the caruncle. This +is the _rhaphe_, and shows the position that would be occupied by +the seed stalk if it were present. Its starting point near the large +end, which is marked in fresh seeds by a slight roughness, is the +_chalaza_, or organic base of the seed, where the parts all come +together like the parts of a flower at their insertion on the stem. +Where was it situated in the common bean? How does this differ from +its position in the castor bean? Where the rhaphe ends, just at the +beak of the caruncle, you will find the hilum. The micropyle is +covered by the caruncle, which is an outgrowth around it. + +[Illustration: FIGS. 20-22.—Castor bean (slightly magnified); +20, back view; 21, front view; _ch_, chalaza; _r_, rhaphe; _ca_, +caruncle; 22, vertical section; _en_, endosperm; _cc_, cotyledons; +_hy_, hypocotyl; _hi_, hilum; _m_, micropyle.] + +Now cut a vertical section through a seed that has been soaked +for several days, at right angles to the broad sides, and sketch +it. Label the white, pasty mass within the seed coats, endosperm. +Can you make out what the narrow white line running through the +center of the endosperm, dividing it into two halves, represents? +Make a similar sketch of a cross section. Notice the same white +line running horizontally across the endosperm, dividing it into +two equal parts. To find out what these lines are, take another +seed (always use soaked seeds for dissection) and remove the coats +without injuring the kernel. Split the kernel carefully round the +edges, remove half the endosperm, and sketch the other half with the +delicate embryo lying on its inner face. You will have no difficulty +now in recognizing the lines in your drawings as sections of the +thin cotyledons. Where is the hypocotyl, and which way does its base +point? Remove the embryo from the endosperm, separate the cotyledons +with a pin, hold them up to the light, and observe their beautiful +texture. Sketch them under the lens, showing the delicate venation. +Is there any plumule? + +Test the endosperm with a little iodine. Does it give a blue or a +brown reaction? Crush another bit of it on a piece of white paper and +see if it leaves a grease spot. What does this show that it contains? +Test the embryo in the same way, and see whether it contains any oil. + + NOTE.—It should be borne in mind that the castor bean bears no + relation whatever to the true beans. It belongs to the spurge + family, which is botanically very remote from that of the peas and + beans. + +[Illustration: FIGS. 23-25.—Seed of a squash; 23, seed from the +outside; 24, vertical section perpendicular to the broad side; +25, section parallel to the broad side, showing inner side of a +cotyledon; _a_, seed coat; _c_, cotyledons; _h_, hypocotyl; _p_, +plumule.] + +=14. Study of a squash or gourd seed.=—How does the coat of a squash +seed differ from that of the bean? At the small end, look for two +dots, or pinholes, close together. Refer to your drawing of the bean +and see if you can make out, with the help of a lens, what they are. +The bean is a curved seed, which is bent so as to bring the hilum +close to the micropyle on one side. But by far the greater number +of seeds are _inverted_, or turned over on their stalks, as you +sometimes see huckleberry blossoms and bell flowers on their stems, +so that when the stalk breaks away from its attachment, the scar and +the micropyle come close together at one end, as in the squash seed. + +[Illustration: FIG. 26.—Diagram of an inverted or anatropous seed, +showing the parts in section: _a_, outer coat; _b_, inner coat; +_c_, kernel; _d_, rhaphe; _ch_, chalaza; _h_, hilum; _m_, micropyle +(_After_ GRAY).] + +Make a drawing of the outside of a seed, labeling all the parts you +have observed; then gently remove the hard coat, or _testa_, as it +is called. The thin, greenish covering that lines it on the inside +is the endosperm. How does it compare in quantity with that in the +corn and castor bean? How do the cotyledons compare in thickness +with those of the bean? Carefully separate them and draw, labeling +the parts as you make them out. The tiny pointed object between the +cotyledons at their point of union is the plumule; is it as well +developed as in the bean? Can you see any reason why seeds like the +pea and bean, which have cotyledons too thick and clumsy to do well +the work of true leaves, should have a well-developed plumule, while +those with thin cotyledons, like the squash and pumpkin, do not, +as a general thing, form a large plumule in the embryo? The little +projection in which the cotyledons end is the hypocotyl; which way +does it point? Where did you find the micropyle to be? Test the +cotyledons and some of the endosperm for food substances; what do you +find in them? + +=15. Study of a pine seed.=—Remove one of the scales from a pine cone +and sketch the seed as it lies in place on the cone scale. Notice +its point of attachment to the scale, and look near this point for a +small opening, which you can easily recognize as the micropyle. The +seed with its wing looks very much like a fruit of the maple, but +differs from it in being a naked seed borne on the inner side of a +cone scale, without a pod or husk or outer covering of any kind, such +as beans and nuts and grains are provided with. Plants like the pine, +which bear their seed in this way, are called _Gymnosperms_, a word +that means “naked seeds,” in contradistinction to the _Angiosperms_, +which bear their seeds in pods or other closed envelopes. + +[Illustration: FIGS. 27, 28.—Pitch pine seeds: 27, scale, or open +carpel, with one seed in place; 28, winged seed, removed. (_After_ +GRAY.)] + +[Illustration: FIG. 29.—Section of pine seed, showing the +polycotyledonous embryo (GRAY).] + +Remove the coat from a seed that has been soaked for twenty-four +hours, and examine it with a lens. Does it consist of one or more +layers? Is there any difference in color between the inner and outer +layers? Look at the base of the hypocotyl for some loose, cobwebby +appendages. These are the remains of other embryos with certain +appendages belonging to them that were formed in the endosperm, but +failed to develop. Did you find remains of this kind in any of the +other seeds examined? Pick out the embryo from the endosperm and +test both for food substances. Which of these do you find? Which +are absent? How does the embryo differ from those already examined? +How many cotyledons are there? Make an enlarged sketch of a seed in +longitudinal section, labeling correctly all the parts observed. + +=16. Comparison as to food value of seeds.=—Make in your notebook a +tabular statement after the model here given, of the food contents +found in the different seeds you have examined. Indicate the relative +quantity of each by writing under it, in the appropriate column, the +words, “much,” “little,” or “none,” as the case may be. + + + MODEL FOR RECORD OF SEEDS EXAMINED + + +================+===========================================+ + | | FOODS TESTED | + | SEEDS EXAMINED +----------+----------+----------+----------+ + | | Starch | Sugar | Oil | Proteins | + +----------------+----------+----------+----------+----------+ + | Corn | | | | | + +----------------+----------+----------+----------+----------+ + | Wheat | | | | | + +----------------+----------+----------+----------+----------+ + | Bean | | | | | + +----------------+----------+----------+----------+----------+ + | Squash | | | | | + +----------------+----------+----------+----------+----------+ + | Castor bean | | | | | + +----------------+----------+----------+----------+----------+ + | Pine | | | | | + +----------------+----------+----------+----------+----------+ + +By far the greater number of seeds contain endosperm; that is, they +consist of an embryo with more or less nourishing matter stored +about it. Even in seeds which appear to have none, the endosperm is +present at some period during development, but is absorbed by the +cotyledons before germination. + +=17. Manner of storing nourishment.=—In the various seeds examined, +we have seen that the nourishment for the young plant is either +stored in the embryo itself, as in the cotyledons of the bean, acorn, +squash, etc., or packed about them in the form of endosperm, as in +the corn, wheat, and castor bean. + +=18. The number of cotyledons.=—Seeds are also classed according +to the number of their cotyledons, as having one, two, or many +cotyledons. The first two kinds make up the great class of +Angiosperms, which includes all the true flowering plants and forms +the most important part of the vegetation of the globe. The last +is characteristic of the great natural division of Gymnosperms, or +naked-seeded plants, of which we have had an example in the pine. +They are the most primitive type of living seed-bearing plants. +Though they are not so abundant now as in past ages, numbering only +about four hundred known species, they present many diversities of +form, which seem to ally them on the one hand with the lower, or +spore-bearing plants (ferns, mosses, etc.), and on the other hand +with the Angiosperms. + + + Practical Questions + + 1. Make a list of all the seeds you can find that have very thick + cotyledons, and underline those that are used as food by man or + beast. + + 2. Make a similar list of all the kinds with thin cotyledons and + more or less endosperm, that are used for food or other purposes. + + 3. Do you find a greater number of foodstuffs among the one kind + than the other? + + 4. How do the two kinds compare, as a general thing, in size and + weight? + + 5. From what part of the castor bean do we get oil? of the peanut? + of cotton seed? (Exps. 1-6.) + + 6. Is there any valid objection to the wholesomeness of peanut oil, + and of cottonseed lard as compared with hog’s lard? (1, 3.) + + 7. What is bran? Does it contain any nourishment? (11, 12; Exps. + 1-6.) + + 8. What gives to Indian corn its value as food? to oats? wheat? + rice? (3; Exps. 1-6.) + + 9. Which of these grains has the larger proportion of endosperm to + embryo? (Figs. 1-3.) + + 10. Which contains the larger amount of starch in proportion to its + bulk, rice or Indian corn? + + 11. If you wished to produce a variety of corn rich in oil, you + would select seed for planting with what part well developed? (3; + Figs. 4-7.) + + + IV. SEED DISPERSAL + + MATERIAL.—Fruits and seeds of any kind that show adaptations for + dispersal. Some common examples are: (1) Wind: ash, elm, maple, + ailanthus, milkweed, clematis, sycamore, linden, dandelion, + thistle, hawkweed. (2) Water: pecan, filbert, cranberry, lotus, + hickory nut, coconut—obtain one with the husk on, if possible. (3) + Animal agency (involuntary): cocklebur, tickseed, beggar-ticks, + burdock; (voluntary) almost all kinds of edible fruits, especially + the bright-colored ones—wild plums, cherries, haws, dogwood, + persimmons, etc. (4) Explosive and self-planting: witch-hazel, + wood sorrel, violet, crane’s-bill, wild vetch, peanut, medick, + stork’s-bill (Erodium). + + EXPERIMENT 17. TO SHOW HOW SEEDS ARE DISPERSED BY WIND.—Take a + number of winged and plumed fruits and seeds, such as those of the + maple, ash, ailanthus, dandelion, clematis, milkweed, and trumpet + creeper; stand on a chair or table in a place where there is a + draft of air and let them all go. Which travel the farther, the + winged or the plumed kinds? Which sort is better fitted to aërial + transportation? + + EXPERIMENT 18. DISPERSAL BY WATER.—Place in a bucket of water a + hazelnut, an acorn, an orange, a cranberry, a pecan, a hickory nut, + a fresh apple, and a coconut with the husk on. Which are the best + floaters? Cut open or break open the good swimmers, compare with + the non-floaters, and see to what peculiarity of structure their + floating qualities are due. In what situations do the cranberry and + the coconut grow? Can you see any advantage to a plant so situated + in producing fruits that float easily? + + EXPERIMENT 19. DISPERSAL BY EXPLOSIVE CAPSULES.—Moisten slightly + some mature but unopened capsules of witch hazel, wood sorrel, + rabbit pea, or violet, and leave in a warm, dry place for fifteen + to forty-five minutes. What happens when the pods begin to dry? + Measure the distance to which the different kinds of seeds have + been ejected. Which were thrown farthest? What was the object of + the movement? What caused the explosion? + + EXPERIMENT 20. THE USE OF ADHESIVE FRUITS.—Scatter broadcast + a handful of hooked or prickly seeds or fruits—cocklebur, + tickseed, beggar-ticks, bur grass, etc. Are they suited for wind + transportation? Drop one of them on your sleeve, or on the coat + of a fellow student; will it stay there? What would be the effect + if it became attached to the fur of a roaming animal? Is this a + successful mode of dissemination? + +[Illustration: FIGS. 30-32.—30, A pod of wild vetch, with mature +valves twisting spirally to discharge the seed; 31, pod of +crane’s-bill discharging its seed; 32, capsules of witch-hazel +exploding.] + +[Illustration: FIGS. 33-36.—Fruits adapted to wind dispersal: 33, +winged pod of pennycress; 34, spikelet of broom sedge; 35, akene of +Canada thistle; 36, head of rolling spinifex grass.] + +=19. Agencies of dispersal.=—The means at nature’s disposal for this +purpose, as shown by the experiments just made, are four; namely, +wind, water, the explosion of capsules due to the withdrawal of +water, and the agency of animals, including man. The first three +are purely mechanical. The last, animal agency, is either voluntary +or involuntary, according as it is conscious and intentional, or +accidental merely. Man, of course, is the only consciously voluntary +agent. Of the four agencies named, animals and wind are the most +effective, and the greater number of adaptations observed will be +found to have reference to these. + +[Illustration: FIG. 37.—Good quality of clover seed.] + +[Illustration: FIG. 38.—Inferior quality of clover seed mixed with +“screenings.”] + +[Illustration: FIG. 39.—Dodder on red clover, showing how the seeds +get mixed.] + +=20. Involuntary dispersal.=—The lower animals may be voluntary +agents in a way, though not designedly so, as when a squirrel buries +nuts for his own use and then forgets the location of his hoard and +leaves them to germinate; or when a jaybird flies off with a pecan +in his bill, intending to crack and eat it, but accidentally lets +it fall where it will sprout and take root. Both man and the lower +animals are not only involuntary, but often unwilling agents of +dispersal. Some of the most troublesome weeds of civilization have +been unwittingly distributed by man as he journeyed from place to +place, carrying, along with the seed for planting his crops, the +various weed seeds, or “screenings,” as these mixtures are called +by dealers, with which they have been adulterated either through +carelessness and ignorance, or from unavoidable causes. The neglected +animals, also, that are allowed by short-sighted farmers to wander +about with their hair full of cockleburs and other adhesive weed +pests, are no doubt very unwilling carriers of those disagreeable +burdens. + +=21. Tempting the appetite.=—This is the most important adaptation +to dispersal by animals. Have you ever asked yourself how it could +profit a plant to tempt birds and beasts to devour its fruit, as so +many of the bright berries we find in the autumn woods seem to do? To +answer this question, examine the edible fruits of your neighborhood +and you will find that almost without exception the seeds are hard +and bony, and either too small to be destroyed by chewing, and +thus capable of passing uninjured through the digestive system of +an animal; or, if too large to be swallowed whole, compelling the +animal, by their hardness or disagreeable flavor, to reject them. +In cases where the seeds themselves are edible and attractive, the +fruits are usually armed during the growing season with protective +coverings, like the bur of the chestnut and the astringent hulls +of the hickory nut and walnut. The acidity or other disagreeable +qualities of most unripe fruits serves a similar purpose, while their +green color, by making them inconspicuous among the foliage leaves, +tends still further to insure them against molestation. + +[Illustration: FIGS. 40-42.—Adhesive fruits: 40, fruit of +hound’s-tongue; 41, akene of bur marigold; 42, fruit of bur grass +(cenchrus).] + +=22. Voluntary agency.=—The cultivated fruits and grains owe their +distribution and survival almost entirely to the voluntary agency +of man. Dispersal by this means, whether intentional or accidental, +is purely artificial, and except in the case of a few annuals like +horseweed, bitterweed, ragweed, goosefoot, and other field pests that +have adjusted their season of growth and flowering to the conditions +of cultivation, is not correlated with any special modification of +the plants for self-propagation. On the contrary, many of the most +widely distributed weeds of cultivation, such as the oxeye daisy, the +rib grass, mayweed and bitterweed, possess very imperfect natural +means of dispersal, and are largely dependent for their propagation +on the involuntary agency of man. + +=23. Use of the fruit in dispersal.=—It will be seen from the +foregoing observations that the fruit plays a very important part in +the work of dispersal, most of the adaptations for this purpose being +connected with it. In cases where a number of seeds are contained in +a large pod that could not conveniently be blown about by the breeze, +adaptations for wind dispersal are attached to the individual seeds, +as in the willow, milkweed, trumpet creeper, and paulonia; but as a +general thing, adaptations of the seed are for protection, the work +of dispersal being provided for by the fruit. In the case of the +large class of plants known as “tumbleweeds,” the whole plant body is +fitted to assist in the work of transportation. Such plants generally +grow in light soils and either have very light root systems, or are +easily broken from their anchorage and left to drift about on the +ground. The spreading, bushy tops become very light after fruiting, +so as to be easily blown about by the wind, dropping their seeds +as they go, until they finally get stranded in ditches and fence +corners, where they often accumulate in great numbers during the +autumn and winter. + +[Illustration: FIG. 43.—A fruiting plant of winged pigweed +(_Cycloloma_), showing the bunchy top and weak anchorage of a typical +tumbleweed.] + +[Illustration: FIG. 44.—Panicle of “old witch grass,” a common +tumbleweed.] + +=24. The advantages of dispersal.=—Seed cannot germinate unless +they are placed in a suitable location as to soil, moisture, and +temperature. In order to increase the chances of securing these +conditions, it is clearly to the advantage of a species that its +seeds should be dispersed as widely as possible, both that the +seedlings may have plenty of room, and that they may not have to draw +their nourishment from soil already exhausted by their parents. The +farmer recognizes this principle in the rotation of crops, because +he knows that successive growths of the same plant will soon exhaust +the soil of the substances required for its nutrition, while they may +leave it richer in nourishment for a different crop. + +[Illustration: FIG. 45.—Self-planting pod of peanut.] + +=25. Self-planting seeds.=—Dispersal is not the only problem the +seed has to meet. The majority of seeds cannot germinate well on +top of the ground, and must depend on various agencies for getting +under the soil. Some of them do this for themselves. The seeds of the +stork’s-bill, popularly known as “filarees,” have a sharp-pointed +base and an auger-shaped appendage at the apex, ending in a +projecting arm (the “clock” of the filaree) by which it is blown +about by the wind with a whirling motion till it strikes a soft +spot, when it begins at once to bore its way into the ground. The +common peanut is another example. The blossoms are borne under the +leaves, near the base of the stem, and as soon as the seeds begin to +form, the flower stalks lengthen several inches, carrying the young +pods down to the ground, where they bore into the soil and ripen +their seeds. + + + Practical Questions + + 1. Name the ten most troublesome weeds of your neighborhood. + + 2. What natural means of dispersal have they? + + 3. Which of them owe their propagation to man? + + 4. Are there any tumbleweeds in your neighborhood? + + 5. Would you expect to find such weeds in a hilly or a well-wooded + region? (19, 23; Exp. 17.) + + 6. What situations are best fitted for their propagation? (19, 23; + Exp. 17.) + + 7. Make a list of all the fruits and seeds you can think of that + are adapted to dispersal by wind; by water; by animals. + + 8. By what means of dissemination, or protection, or both, is each + of the following distinguished: the squash; apple; fig; pecan; + poppy; bean; beggar-tick; linden; grape; rice; pepper; olive; + cranberry; jimson weed; thistle; corn; wheat; oats? + + 9. What is the agent of dispersion, or what the danger to be + provided against, in each case? + + 10. Could our cultivated fruits and grains survive in their present + state without the agency of man? (22.) + + 11. Name all the plants you can think of that bear winged seeds and + fruits; are they, as a general thing, tall trees and shrubs, or low + herbs? + + 12. Name all you can think of that bear adhesive seeds and fruits; + are they tall trees or low herbs? + + 13. Give a reason for the difference. (Exps. 17, 20.) + + 14. Why is the dandelion one of the most widely distributed weeds + in the world? (19; Exp. 17.) + + 15. Is the wool that covers cotton seed for dispersal or protection? + + 16. What advantage to the Indian shot (canna) is the excessive + hardness of its seeds? (21.) + + 17. What is the use to the species, of the bitter taste of lemon + and orange seed? (21.) + + 18. Why are the seeds of dates and persimmons and haws so hard? + (21.) + + 19. Do you find any edible seeds without protection? If so, account + for the want of it. (21, 22.) + + 20. Name some of the agencies that may assist in covering seeds + with earth. + + 21. Do you know of any seeds that bury themselves? + + 22. The seeds of weeds and other refuse found mixed with grain + sold on the market are known, commercially, as “screenings.” Wheat + brought to mills in Detroit showed screenings that contained, among + other things, seeds of black bindweed, green foxtail grass, yellow + foxtail, chess, oats, ragweed, wild mustard, corn cockle, and + pigweed. Can you mention some of the ways in which these foreign + substances may have gotten into the crop and suggest means for + keeping them out? + + + Field Work + + The subjects treated in the foregoing chapter are, in general, + better suited to laboratory than to field work. There are some + details, however, which can be observed to advantage out of doors. + Many of the seeds found in your walks will show peculiarities of + shape and external markings and color that will invite observation. + Examine also the contents of different kinds you may meet with, as + to the presence or absence of endosperm and the arrangement and + development of the embryo. Note: (1) whether, as a general thing, + there is any difference in size and weight and amount of nourishing + matter in the two kinds; (2) the greater variety in the shape and + arrangement of the cotyledons in the albuminous kind, and in the + arrangement of the embryo; (3) the differences in the development + of the plumule in the two kinds,—and give a reason for the facts + observed. + + Among the different seeds you may find, look for adaptations for + dispersal, and decide to what particular method each is suited. + Study the agencies by which various kinds may get covered with + soil. If the common stork’s-bill (_Erodium cicutarium_) grows in + your neighborhood, its seeds will well repay a little study, and if + there is a field of peanuts within reach, do not fail to pay it a + visit. + + + + +CHAPTER II. GERMINATION AND GROWTH + + + I. PROCESSES ACCOMPANYING GERMINATION + + MATERIAL.—A pint or two of corn, peas, beans, or any quickly + germinating seed. + + APPLIANCES.—Matches; wood splinters; gas jet or alcohol lamp; test + tubes; a small quantity of mercuric oxide; a thermometer; a couple + of two-quart preserve jars, and a smaller wide-mouthed bottle that + can be put into one of them; some limewater; a glass tube (the + straws used by druggists for soft drinks will answer). + +=26. Preliminary exercises.=—Before taking up the study of +germinating seeds, it is important to learn from what sources the +organic substances used by the growing plant are derived, and some of +the processes that accompany growth and development. + + EXPERIMENT 21. TO SHOW THE CHANGES THAT ACCOMPANY OXIDATION.—Strike + a match and let it burn out. Examine the burnt portion remaining + in your hand; what changes do you notice? These changes have been + caused by the union of some substance in the match with something + outside of it, in the act of burning; let us see if we can find out + what this outside substance is. + + EXPERIMENT 22. TO SHOW THE ACTIVE AGENT IN OXIDATION.—Heat some + mercuric oxide in a test tube over the flame of a burner. The + heat will cause the oxygen to separate from the mercury, and in a + short time the tube will be filled with the gas. Extinguish the + flame from a lighted splinter and thrust the glowing end into the + tube; what happens? The oxygen unites with something in the wood + and causes it to burn just as the match did. Compare your burnt + splinter with the burnt end of the match; what resemblance do you + notice between them? + + EXPERIMENT 23. TO SHOW THAT CARBON DIOXIDE IS A PRODUCT OF + OXIDATION.—Your experiment with the match showed that ignition + is accompanied by heat, and if active enough, by light, and also + that it left behind a solid substance in the form of charcoal. But + how about the part that united with the oxygen to produce these + results? Let us see what became of it. Hold a lighted candle + under the open end of a test tube, or under the mouth of a small + glass jar. Does any vapor collect on the inside? After two or + three minutes quickly invert the jar or the tube, and thrust in a + lighted match: what happens? Can the substance now in the jar be + ordinary air? Why not? (Exps. 21, 22.) Pour in a small quantity of + limewater, holding your hand over the mouth of the tube to prevent + the air from getting in; the gas inside, being heavier than air, + will not escape immediately unless agitated. What change do you + notice in the limewater? + + It has been proved by experiment that the kind of gas formed by + the burning candle has the property of turning limewater milky; + hence, whenever you see this effect produced in limewater, you may + conclude that this gas, known as _carbon dioxide_, is present; + and conversely, the presence of carbon dioxide, especially if + accompanied by some of the other effects observed, as the giving + out of heat and moisture, may be taken as evidence that some + process similar to that going on in the burning candle is, or has + been, at work. + + EXPERIMENT 24. DO THESE EFFECTS ACCOMPANY ANY OF THE LIFE PROCESSES + OF ANIMALS?—Blow your breath against the palm of your hand; what + sensation do you feel? Blow it against a mirror, or a piece of + common glass; what do you see? Blow through a tube into the bottom + of a glass containing limewater; how is the water affected? How do + these facts correspond with the results of Exp. 23? + + EXPERIMENT 25. IS THERE ANY EVIDENCE THAT A SIMILAR PROCESS GOES ON + IN PLANTS?—(1) Half fill a small, wide-mouthed jar with limewater, + place it inside a larger one (Fig. 46), and fill the space between + them, up to the neck of the smaller vessel, with well-soaked + peas, beans, or barleycorns, on a bed of moist cotton or blotting + paper. Cover with a piece of glass and keep at a moderately warm + temperature. (2) As a control experiment, place beside this another + jar arranged in precisely the same way, except that seeds must + be used whose vitality has been destroyed by heat. To prevent + the entrance of germs among the dead seeds, which might cause + fermentation and thus interfere with the experiment, set the jar + containing them in a vessel of water and boil an hour or two before + the experiment begins. Otherwise, treat precisely as in (1). + +[Illustration: FIG. 46.—Diagrammatic section, showing arrangement +of jars for Exp. 25.] + + After germination has taken place in (1), what change do you notice + in the limewater? If the effect is not apparent, gently stir with a + straw or a glass rod to mix it with the gas in the larger jar. Has + the limewater in the control experiment undergone the same change? + (It may show a slight milkiness due to the carbon dioxide in the + air.) Insert a thermometer among the seeds in both of the larger + jars, and compare their temperature with that of the outside air; + which shows the greater rise? From this experiment and the last + one, what process, common to animals, would you conclude has been + going on in the germinating seeds? + + NOTE.—Heat in germinating seeds is not always due to this cause + alone, but is sometimes increased by the presence of minute + organisms called bacteria. Germinating barley and rye in breweries + sometimes show an increase in temperature of 40 to 70 degrees, due + to these organisms, and spontaneous combustion in seed cotton has + been reported from the same cause. + +=27. Oxidation.=—The process that brought about the results observed +in the foregoing experiments, and popularly known as _combustion_, is +more accurately defined by chemists as _oxidation._ It takes place +whenever substances enter into new combinations with oxygen. The most +familiar examples of it are when oxygen enters into combination with +substances containing carbon. It was the union of a portion of the +oxygen of the air in Exp. 21, and of that in the tube in Exp. 22, +with some of the carbon in the wood, that caused the burning. The +effect was more marked in the second case because the oxygen in the +tube was pure, while in the air it is mixed with other substances. + +=28. Carbon.=—The black substance left in your hand after oxidation +of the wood in Exps. 21 and 22 is _carbon_. It composes the greater +part of most plant bodies, and, in fact, is the most important +element in the realm of organic nature. There is not a living thing +known, from the smallest microscopic germ to the most gigantic tree +in existence, that does not contain carbon as one of its essential +constituents. + +=29. Carbon dioxide.=—The gas produced by the burning candle in Exp. +23, by the germinating seeds in Exp. 25, and expelled from your own +lungs in Exp. 24, is carbon dioxide. Chemists designate it by the +symbol CO₂, which means that it consists of one part carbon to two +parts oxygen. It is an invariable product wherever the oxidation of +substances containing carbon goes on. Heat and moisture are evolved +at the same time, and if oxidation is very active, as in Exps. 21 and +22, light also. When the process takes place very slowly, no light is +evolved, and so little heat as to be imperceptible without special +observation. Hence, oxidation may go on around us and even in our own +bodies without our being conscious of the fact. + +Carbon dioxide is of prime importance to the well-being of plants. It +furnishes the material from which the greater part of their organic +food is derived, as will be seen when we take up the study of the +leaf and its work. To animals, on the contrary, its presence is so +injurious that if the proportion of it in the air we breathe ever +rises much above 1 part to 1000, the ill effects become painfully +sensible. It is not, however, as was formerly supposed, a poison, +the harm it does being to decrease the proportion of oxygen in the +atmosphere so that animals cannot get enough of it to breathe, and +die of suffocation. + +=30. Respiration in plants and in animals.=—It was shown in Exp. +24 that respiration in animals is accompanied by the products of +oxidation; hence we conclude that respiration is a form of oxidation. +And since these same products are given off by plants (Exp. 25), the +inference is clear that the same process goes on in them. But in +plants the life functions are so much more sluggish than in animals +that it is only in their most active state, during germination and +flowering, that evidence of it is to be looked for. + +=31. Respiration and energy.=—In plants, as in animals, respiration +is the expression or measure of energy. Sleeping animals breathe +more slowly than waking ones, snakes and tortoises more slowly than +hares and hawks. The more we exert ourselves and the more vital +force we expend, the harder we breathe; hence, respiration is more +active in children than in older persons and in working people than +in those at rest. It is the same with plants; respiration is most +perceptible in germinating seeds and young leaves, in buds and +flowers, where active work is going on. Hence, in this condition +they consume proportionately larger quantities of oxygen and +liberate correspondingly larger quantities of carbon dioxide, with +a proportionate increase of heat. In some of the arums,—calla lily, +Jack-in-the-pulpit, colocasia, etc.,—and in large heads of compositæ, +like the sunflower, where a great number of small flowers are brought +together within the same protecting envelope, the rise of temperature +is sometimes so marked that it may be perceived by placing a flower +cluster against the cheek. + + + Practical Questions + + 1. What is charcoal? (28.) + + 2. Is any of this substance contained in the seed? in the flour and + meal made from seed? (28; Exp. 25.) + + 3. What combination takes place when the cook lets the stove get + too hot and burns the biscuits? (27, 28.) + + 4. Of what does the burned part consist? (28.) What was it before + it was burned? (27, 28). + + 5. Which burns the more readily, an oily seed or a starchy one? + Which leaves the more solid matter behind? (Suggestion: test by + putting a bean, or a large grain of corn, and an equal quantity + of the kernel of a Brazil nut on the end of a piece of wire and + thrusting into a flame.) + + 6. Is there any rational ground for the statement that the wooden + buildings formerly used on Southern plantations as cotton ginneries + were sometimes destroyed through spontaneous combustion due to the + heat generated by piles of decaying cotton seed? (Exp. 25, Note.) + + + II. CONDITIONS OF GERMINATION + + MATERIAL.—Several ounces each of various kinds of seed. For the + softer kinds, pea, bean, corn, oats, wheat are recommended; for + those with harder coverings, squash, castor bean, apple, pear, or, + where obtainable, cotton; for still harder kinds, persimmon and + date seeds, or the stones of plum and cherry. + + APPLIANCES.—1 dozen common earthenware plates for germinators; 1 + dozen two-ounce wide-mouthed bottles; 2 common glass tumblers; + clean sand, sawdust, or cotton batting, for bedding; a double + boiler; a gas burner, or a lamp stove. + +=32. Recording observations.=—For this purpose a page should be ruled +off in the notebook of each student, after the model here given, +and the facts brought out by the different experiments set down as +observed. + + + NUMBER OF SEEDS GERMINATED + + ==============+=+==+==+==+====+====+====+====+====+=====+==== + No. of hours | |24|48|72|4 d.|5 d.|6 d.|7 d.|8 d.|10 d.|2 w. + +-+--+--+--+----+----+----+----+----+-----+---- + No. of vessel|1| | | | | | | | | | + +-+--+--+--+----+----+----+----+----+-----+---- + No. of vessel|2| | | | | | | | | | + +-+--+--+--+----+----+----+----+----+-----+---- + No. of vessel|3| | | | | | | | | | + +-+--+--+--+----+----+----+----+----+-----+---- + No. of vessel|4| | | | | | | | | | + +-+--+--+--+----+----+----+----+----+-----+---- + No. of vessel|5| | | | | | | | | | + +-+--+--+--+----+----+----+----+----+-----+---- + No. of vessel|6| | | | | | | | | | + ==============+=+==+==+==+====+====+====+====+====+=====+==== + + EXPERIMENT 26. CAN SEEDS HAVE TOO MUCH MOISTURE?—Drop a number + of dry beans or grains of corn, oats, or other convenient seed, + into a vessel with a bedding of cotton or paper that is barely + moistened, and an equal number of soaked seeds of the same kind + into another vessel with a saturated bedding of the same material. + In a third vessel place the same number of soaked seed, covering + them partially with water, and in a fourth cover the same number + entirely. Label them 1, 2, 3, and 4; keep all together in a warm, + even temperature, and observe at intervals of twenty-four hours for + a week. What condition as to moisture do you find most favorable + to germination? Would seeds germinate in the entire absence of + moisture? How do you know? + + EXPERIMENT 27. WAS IT THE PRESENCE OF TOO MUCH WATER, OR THE LACK + OF AIR CAUSED BY IT, THAT INTERFERED WITH GERMINATION IN THE LAST + EXPERIMENT?—To answer this question experimentally is not easy, + since it is difficult to obtain a complete vacuum without special + appliances. The simplest way is to fill with mercury a glass tube + 30 inches long, closed at one end, and invert it over a small + vessel—a teacup, or an egg cup will answer—containing mercury + enough to cover the bottom to a depth of two or three centimeters + (see Appendix, Weights and Measures, for English equivalents.) + The tube must be supported in such a way that its lower end will + dip into the mercury without touching the bottom of the vessel. + With a pair of forceps insert under the mouth of the tube two or + three seeds that have been well soaked in water deprived of air + by previous boiling. Being lighter than mercury, they will float + to the top, where there is a complete absence of air while other + conditions favorable to germination are present. Before releasing, + they should be well shaken under the mercury to free them from air + bubbles, and if the coats are loose fitting so that they can be + removed without injury to the parts inclosed in them, they should + be slipped off in order to get rid of any imprisoned air they + may contain. Additional moisture may be supplied, if necessary, + by injecting, by means of a medicine dropper inserted under the + mouth of the tube, a drop or two of water that has been previously + boiled. Keep in a warm, even temperature, under conditions + favorable to germination, and compare the behavior of the seeds + with those placed in the different vessels in Exp. 26. + + If appliances for this experiment are lacking, a rough + approximation can be made by using the seeds of aquatic plants, + such as the lotus, water lily, and the so-called Chinese sacred + bean, sold in the variety stores, which we know are capable of + germinating in the limited amount of air contained in ordinary + soil water. Place an equal number of such seeds, of about the same + size and weight, on a bedding of common garden soil in two glass + tumblers. Fill one vessel a little over half full of ordinary soil + water and the other to the same height with water from which the + air has been expelled by boiling. Pour over the liquid a film of + sweet oil or castor oil, to prevent the access of air, leaving the + surface of the water in the other vessel exposed. In which do the + seeds come up most freely? + +[Illustration: FIG. 47.—To find out the proper depth at which to +plant seeds.] + + Some seeds, especially those rich in proteins, as peas and beans, + will germinate in a vacuum, because oxygen is supplied for a time + by the chemical decomposition of substances in their tissues which + contain it, but when these are exhausted, respiration ceases and + death ensues. + + EXPERIMENT 28. DOES THE DEPTH AT WHICH SEEDS ARE PLANTED AFFECT + THEIR GERMINATION?—Plant a number of peas or grains of corn at + different depths in a wide-mouthed glass jar filled with moist + sand, as shown in Fig. 47, the lowest ones at the bottom, the top + ones barely covered. Try different kinds of seed and grain,—radish, + squash, cotton, or wheat,—and watch them make their way to the + surface. Do you notice any difference in this respect between large + seed and small ones? Between those with thick cotyledons and thin + ones? At what depth do you find, from your recorded observations, + that seed germinate best? + + EXPERIMENT 29. WHAT TEMPERATURE IS MOST FAVORABLE TO + GERMINATION?—Put half a dozen soaked beans on moist cotton or + sawdust in three wide-mouthed bottles of the same size or in + germinators arranged as in Figs. 48, 49, the seed also being + selected with a view to similarity of size and weight. Keep one at + a freezing temperature; the second in a temperature of 15° to 20° + C. (see Appendix for Fahrenheit equivalents); and the third, at + 30° C. If a place can be found near a stove or a register, where + an even temperature of about 125° F. is maintained, place a fourth + receptacle there. Observe at intervals of twenty-four hours for a + week or ten days, keeping the temperature as even as possible, and + maintaining an equal quantity of moisture in each vessel. Make a + daily record of your observations. What temperature do you find + most favorable to germination? + +[Illustration: FIGS. 48, 49.—Home-made germinators: 48, closed; 49, +showing interior arrangement.] + + EXPERIMENT 30. AT WHAT TEMPERATURE DO SEEDS LOSE THEIR + VITALITY?—Place about two dozen each of grains of corn, beans, + squash seed, and castor beans, with an equal number of plum or + cherry stones, in water, and heat to a temperature of 150° F. After + an exposure of ten minutes, take out six of each kind and place + in germinators made of two plates with moist sand or damp cloth + between them, as shown in Figs. 48, 49. Raise the temperature to + 175° F., and after ten minutes take out six more of each kind of + seed and place in another germinator. Raise the water in the vessel + to 200°, take out another batch of seeds; raise to the boiling + point for ten minutes more, and plant the remaining six of each + lot. Number the four germinators, and observe at intervals of + twenty-four hours for two weeks. The harder kinds should be kept + under observation for three or four weeks, as they germinate slowly. + + Try the same experiments with the same kinds of seeds at a dry + heat, using a double boiler to prevent scorching, and record + observations as before. + + EXPERIMENT 31. TIME REQUIRED FOR GERMINATION.—Arrange in + germinators seeds of various kinds, such as corn, wheat, peas, + turnip, apple, orange, grape, castor bean, etc. “Clip” some of the + harder ones and keep all the kinds experimented with under similar + conditions as to moisture, temperature, etc., and record the time + required for each to sprout. What is the effect of clipping, and + why? + + EXPERIMENT 32. ARE VERY YOUNG OR IMMATURE SEEDS CAPABLE OF + GERMINATING?—Plant some seeds from half-grown tomatoes, and grains + of wheat, oats, or barley before they are ready for harvesting. Try + as many kinds as you like, and see how many will come up. Notice + whether there is any difference in the health and vigor of plants + raised from seeds in different stages of maturity. + + EXPERIMENT 33. THE RELATIVE VALUE OF PERFECT AND INFERIOR + SEED.—From a number of seeds of the same species select half a + dozen of the largest, heaviest, and most perfect, and an equal + number of small, inferior ones. If a pair of scales is at hand, the + different sets should be weighed and a record kept for comparison + with the seedlings at the end of the experiment. Plant the two sets + in pots containing exactly the same kind of soil, and keep under + identical conditions as to light, temperature, and moisture. Keep + the seedlings under observation for two or three weeks, making + daily notes and occasional drawings of the height and size of the + stems, and the number of leaves produced by each. + +[Illustration: FIGS. 50, 51.—Stem development of seedlings: 50, +raised from healthy grains of barley; weight, 39.5 grams (about 500 +grs.); 51, raised under exactly similar conditions from the same +number of inferior grains; weight, 23 grams (about 350 grs.).] + +[Illustration: FIGS. 52, 53.—Improvement of corn by selection: 52, +original type; 53, improved type developed from it.] + +=33. Resistance to heat and cold.=—In making experiments with regard +to temperature, notice how the extremes tolerated are influenced, +first, by the length of time the seeds are exposed; second, by the +amount of water contained in them; and third, by the nature of the +seed coats. Every farmer knows that the effect of freezing is much +more injurious to plants or parts of plants when full of sap (water) +than when dry. This, in the opinion of the most recent investigators, +is because the water in the spaces outside the cells freezes first +and as moisture is gradually withdrawn from the inside to take its +place, the soluble salts which may be present in the cell sap become +more concentrated, and by their chemical action on the contained +proteins cause them to be precipitated, or “salted out,” as we see +sugar or salt precipitated from solutions of those substances when +water is withdrawn by evaporation. In this way, it is believed, the +fundamental protoplasm of the cell may be so disorganized that death +ensues if the freezing is continued long enough, since the protein +precipitates become “denatured” and cannot be reabsorbed if kept +in a solid state too long. The length of time necessary to produce +death from this cause is, of course, different in different plants, +according to the kind of salts dissolved in the sap and the nature of +the proteins acted on by them. The proteins in the sap of Begonia, +or Pelargonium, plants which are very sensitive to cold, yield a +denatured precipitate at, or a little below the freezing point of +water, while those of winter rye withstand a temperature of -15° C., +and of pine needles, -40° C. + +Mechanical injury through rupture of parts by freezing is not apt +to cause serious damage except in cases of sudden and violent cold +at a time when the tissues are gorged with sap, as not infrequently +happens during the abrupt changes of temperature which sometimes +occur in spring after the trees have put forth their leaves. In an +extreme case of this kind, the writer has seen the trunk of an oak +a foot or more in diameter split in deep seams from the effects of +freezing. + +=34. The length of time during which seeds may retain their +vitality.=—No direct experiment can be made to test this point, +since it would require months, or even years, covering in some +instances more than the lifetime of a generation. It has been stated +on good authority that seeds of the water chinquapin (Nelumbo) have +germinated after more than a hundred years, and moss spores preserved +in herbariums, after fifty. But the records in such cases are not +always trustworthy, and there is absolutely no foundation for the +statements sometimes made about the germination of wheat grains found +preserved with mummies over two thousand years old. If kept perfectly +dry, however, seed may sometimes be preserved for months, or even +years. Peas have been known to sprout after ten years, red clover +after twelve, and tobacco after twenty. Ordinarily, however, the +vitality of seeds diminishes with age, and in making experiments it +is best to select fresh ones. Those used for comparison should also, +as far as possible, be of the same size and weight. + +=35. Effect of precocious germination.=—It has been found by +experiment that plants raised from immature seed, when they will +germinate at all (Exp. 32), yield earlier and larger crops than the +same kinds from mature seed. Early tomatoes and some other vegetables +are produced in this way. The majority of seeds, however, require +a period of rest before beginning their life work. Those that are +forced to take up the burden of “child labor” show the effect of such +abnormal condition by yielding fruits that are smaller and less firm +than those raised from mature seed, so that they do not keep well and +have to be marketed quickly. Under what circumstances does it pay to +cultivate such fruits? + + + Practical Questions + + 1. What are the principal external conditions that affect + germination? (Exps. 26-29.) + + 2. What effect has cold? want of air? too much water? + + 3. Is light necessary to germination? + + 4. What is the use of clipping seeds? (Exps. 12, 13, 14, and + Material, p. 12.) + + 5. In what cases should it be resorted to? (Exp. 31.) + + 6. Why will seed not germinate in hard, sunbaked land without + abundant tillage? Why not on undrained or badly drained land? + (Exps. 26, 27.) + + 7. Will seeds that have lost their vitality swell when soaked? + (Exp. 16.) + + 8. Are there any grounds for the statement that the seeds of plums + boiled into jam have sometimes been known to germinate?[1] (33; + Exp. 30.) + + 9. Could such a thing happen in the case of apple or sunflower + seed, and why or why not? (33.) + + 10. Does it make any difference in the health and vigor of a plant + whether it is grown from a large and well-developed seed or from a + weak and puny one? (Exp. 33.) + + 11. Would a farmer be wise who should market all his best grain and + keep only the inferior for seed? + + 12. What would be the result of repeated plantings from the worst + seed? + + 13. Of constantly replanting the best and most vigorous? + + 14. Suppose seed would germinate without moisture; would this be an + advantage, or a disadvantage to agriculturists? + + 15. Why is a cool, dry place best for keeping seeds? (Exps. 26, 29.) + + 16. Why are the earliest tomatoes found in the market usually + smaller than those offered later? (35.) + + 17. Why is continued rain so injurious to wheat, oats, and other + grains before they are mature enough to be harvested? (35; Exp. 32.) + + 18. Would the same effect be likely to occur in the case of very + oily seeds, such as flax and castor beans? Why? (Suggestion: try + the effect of putting water on a piece of oiled paper.) + + 19. Explain why many seeds cannot germinate successfully without + air. (30, 31; Exp. 25.) + + 20. Mention some of the practical advantages that a farmer, a + gardener, or a careful housewife might gain from experiments like + those made in this section. + + 21. Explain why seeds can endure so much greater extremes of + temperature than growing plants. (23, 33.) + + + III. DEVELOPMENT OF THE SEEDLING + + MATERIAL.—Seedlings of various kinds in different stages of growth. + It is recommended that the same species be used that were studied + in Section III, Chapter I, or such equivalents as may have been + substituted for them. Enough should be provided to give each pupil + three or four specimens in different stages of development. Seeds, + even of the same kind, develop at such different rates that it + will probably not be necessary to make more than two plantings of + each sort, from 2 to 5 days apart. Soaked seeds of corn and wheat + will germinate in from 3 to 7 days, according to the temperature; + oats in 1 to 4; beans in 4 to 6; squash and castor beans in from + 8 to 10. Very obdurate ones may be hastened by clipping. Keep the + germinators in an even temperature, at about 70° to 80° F. + + Pine is a very difficult seed to germinate, requiring usually + from 18 to 21 days. By soaking the mast for twenty-four hours and + planting in damp sand or sawdust kept at an even temperature of 23° + C. or about 75° F., specimens may be obtained. + +[Illustration: FIGS. 54, 55.—Seedling of corn (_after_ GRAY): 54, +early stage of germination; 55, later stage.] + +=36. Seedlings of monocotyls.=—Examine a seedling of corn that has +just begun to sprout; from which side does the seedling spring, the +plain or the grooved one? Refer to your sketch of the dry grain and +see if this agrees with the position of the embryo as observed in the +seed. Make sketches of four or five seedlings in different stages of +advancement, until you reach one with a well-developed blade. From +what part of the embryo has each part of the seedling developed? +Which part first appeared above ground? Is it straight, or bent in +any way? In what direction does the plumule grow? The hypocotyl? Does +the cotyledon appear above ground at all? Slip off the husk and see +if there is any difference in the size and appearance of the contents +as you proceed from the younger to the older plants. How would you +account for the difference? + +=37. The root.=—Examine the lower end of the hypocotyl and find where +the roots originate; would you say that they are an outgrowth from +the stem, or the stem from the root? Observe that the root of the +corn does not continue to grow in a single main axis like that of +the castor bean, but that numerous adventitious and secondary roots +spring from various points near the base of the hypocotyl and spread +out in every direction, thus giving rise to the fibrous roots of +grains and grasses. + +=38. Root hairs.=—Notice the grains of sand or sawdust that cling to +the rootlets of plants grown in a bedding of that kind. Examine with +a lens and see if you can account for their presence. Lay the root +in water on a bit of glass, hold up to the light and look for root +hairs; on what part are they most abundant? + +[Illustration: FIG. 56.—Seedling of wheat, with root hairs.] + +The hairs are the chief agents in absorbing moisture from the +soil. They do not last very long, but are constantly dying and +being renewed in the younger and tenderer parts of the root. These +are usually broken away in tearing the roots from the soil, so +that it is not easy to detect the hairs except in seedlings, even +with a microscope. In oat, maple, and radish seedlings they are +very abundant and clearly visible to the naked eye. The amount of +absorbing surface on a root is greatly increased by their presence. + +[Illustration: FIG. 57.—Diagrammatic section of a root tip: _a_, +cortex; _b_, central cylinder in which the conducting vessels are +situated; _c_, root cap; _g_, growing point.] + +=39. The root cap.=—Look at the tip of the root through your lens +and notice the soft, transparent crescent or horseshoe-shaped mass +in which it terminates. This is the root cap and serves to protect +the tender parts behind it as the roots burrow their way through the +soil. Being soft and yielding, it is not so likely to be injured by +the hard substances with which it comes in contact as would be the +more compact tissue of the roots. It is composed of loose cells out +of which the solid root substance is being formed; the growing point +of the root, _g_, is at the extremity of the tip just behind the +cap, _c_ (Fig. 57). The cap is very apparent in a seedling of corn, +and can easily be seen with the naked eye, especially if a thin +longitudinal section is made. It is also well seen in the water roots +of the common duckweed (_Lemna_), and on those developed by a cutting +of the wandering Jew, when placed in water. Are there any hairs on +the root cap? Can you account for their absence? + + NOTE.—For a minute study of the structure of roots, see =67=. + +=40. Organs of vegetation.=—The three parts, root, stem, and leaf, +are called organs of vegetation in contradistinction to the flower +and fruit, which constitute the organs of reproduction. The former +serve to maintain the plant’s individual existence, the latter to +produce seed for the propagation of the species, so we find that the +seed is both the beginning and the end of vegetable life. + +[Illustration: FIG. 58.—Seedlings of bean in different stages +of growth: _cc_, cotyledons, showing the plumule and hypocotyl +before germination; _a_, _b_, _d_, and _e_, successive stages of +advancement. At _d_ the arch of the hypocotyl is beginning to +straighten; at _e_ it has entirely erected itself.] + +=41. Definitions.=—Organ is a general name for any part of a living +thing, whether animal or vegetable, set apart to do a certain work, +as the heart for pumping blood, or the stem and leaves of a plant for +conveying and digesting sap. By “function” is meant the particular +work or office that an organ has to perform. + +=42. Seedlings of dicotyls. The bean.=—Sketch, without removing it, a +bean seedling that has just begun to show itself above ground; what +part is it that protrudes first? Sketch in succession four or five +others in different stages of advancement. Notice how the hypocotyl +is arched where it breaks through the soil. Does this occur in the +monocotyls examined? Do the cotyledons of the bean appear above +ground? How do they get out? Can you perceive any advantage in their +being dragged out of the ground backwards in this way rather than +pushed up tip foremost? What changes have the cotyledons undergone +in the successive seedlings? Remove from the earth a seedling just +beginning to sprout and sketch it. From what point does the hypocotyl +protrude through the coats? Does this agree with its position as +sketched in your study of the seed? In which part of the embryo does +the first growth take place? + +Remove in succession the several seedlings you have sketched and +note their changes. How does the root differ from that of the corn +and oats? The first root formed by the extension of the hypocotyl is +the _primary_ root and should be so labeled in your drawings; the +branches that spring from it are _secondary_ roots. Look for root +hairs; if there are any, where do they occur? + +[Illustration: FIG. 59.—Stages in the germination of a typical +seedling of the squash family: _a_, a seed before germination; _b_, +_c_, _e_, the same in different stages of growth; _d_, the empty +testa, with kernel removed; _hi_, hilum; _m_, micropyle; _p_, _p_, +the peg in the heel; _h_, _h_, _h_, the hypocotyl; _ar_, arch of the +hypocotyl; _co_, cotyledons; _pl_, plumule; _pr_, primary root; _sc_, +secondary roots.] + +=43. Germination of the squash.=—How does the manner of breaking +through the soil compare with that of the bean? With the corn? +From which end of the seed, the large or the small one, does the +hypocotyl spring? Do the cotyledons come above ground? How do they +get out of the seed coat? Notice the thick protuberance developed +by the hypocotyl and pressing against the lower half of the coat at +the point where the hypocotyl breaks through. This is called the +“peg”; can you tell its use? Could the cotyledons get out of their +hard covering without it? Slip the peg below the coat in one of your +growing specimens, leave it in the soil, and see what will happen. +How do the cotyledons of the squash differ from those of the bean +as they come out of the seed cover? Do they act as foliage leaves? +Do you see any difference in the development of the plumule in the +two seeds (Figs. 19, 25) to account for the different behavior of +the cotyledons? Sketch three seedlings in different stages, labeling +correctly the parts observed. Make a similar study of the castor +bean, or other seedling selected by your teacher, and illustrate by +drawings. + +=44. Arched and straight hypocotyls.=—This difference in the manner +of getting above ground is an important one. That by means of the +arched hypocotyl is, in general, characteristic of the process of +germination in which the cotyledons come above ground, while the +straight kind, which was illustrated in the corn and wheat, is the +prevailing method when the cotyledons remain below ground. Can you +give a reason for the difference? + +[Illustration: FIG. 60.—Pine seedling (_After_ GRAY).] + +=45. Polycotyledons; germination of the pine.=—Examine a pine +seedling just beginning to sprout. What part emerges first from +the seed coat? Where does it break through? Where did you find the +micropyle in the pine seed? (15.) Can you give a reason why the +hypocotyl in seeds should break through the coats at this point? How +do the cotyledons get out of the testa? Is the hypocotyl arched or +straight in germination? How does it compare with the bean and squash +in this respect? With the corn? Is any endosperm left in the testa +after the cotyledons have come out? What has become of it? Do the +cotyledons function as leaves? How many of them have the specimen +you are studying? Notice the little knob or button at the upper +end of the hypocotyl, just above the point where the cotyledons are +attached; this is the _epicotyl_, or part above the cotyledons, here +identical with the plumule; does it develop as rapidly as in the +other seedlings you have examined? + +=46. Relation of parts in the seedling.=—Before leaving this subject, +it is important to fix clearly in mind the different parts of the +germinating seedling and their relation to both the embryo from +which they originated and the plant into which they are to develop. +The part labeled “hypocotyl” in your sketches is all that portion +of the embryo below the point of attachment of the cotyledons. In +germination its upper part will become the stem, and in the embryo +constitutes the _caulicle_, or stemlet, while its lower part, from +which the root will develop, is the _radicle_, or rootlet; hence +the term “hypocotyl” includes both the future root and stem. The +plumule is that part of the embryo between the cotyledons and _above_ +their point of attachment to the caulicle. It is the upward growing +point of the young plant, and hence the place of attachment of the +cotyledon is the first _node_, or point of leaf origin, on the stem. + +The epicotyl, in contradistinction to the hypocotyl, is all that +part of the plant _above_ the insertion of the cotyledons. Before +germination it is identical with the plumule. As the seedling grows, +the epicotyl advances its growing point by adding new nodes and +_internodes_, as the spaces between the successive points of leaf +insertion are called. + +=47. Botanical terms.=—As the prefixes _hypo_ and _epi_ are of +frequent occurrence in botanical works, it will aid in understanding +their various compounds if you will remember that _hypo_ always +refers to something below or beneath, and _epi_, to something over +or above. With this idea in mind you will see that botanical terms +are a labor-saving device, since it is much easier, in making notes, +to use a single descriptive word than to write out the long English +equivalent, such as “the part under (or over) the cotyledons.” + + + Practical Questions + + 1. Do the cotyledons, as a general thing, resemble the mature + leaves of the same plants? + + 2. Name some plants in which you have observed differences, and + account for them; could convenience of packing in the seed coats, + for instance, or of getting out of them, have any bearing on the + matter? + + 3. Does the position in which seeds are planted in the ground have + anything to do with the position of the seedlings as they appear + above the surface? + + 4. Is this fact of any importance to the farmer? + + 5. Will grain that has begun to germinate make good meal or flour? + Why? (27, 36; Exp. 25.) + + + IV. GROWTH + + MATERIAL.—Two young potted plants; some lily or hyacinth + bulbs; seedlings of different kinds,—some with well-developed + taproots,—apple, cotton, and maple are good examples. + + APPLIANCES.—A small flat dish, some mercury, and a piece of cork. + +[Illustration: FIGS. 61, 62.—Seedling of corn, marked to show +region of growth: 61, early stage of germination; 62, later stage.] + + EXPERIMENT 34. HOW DOES THE ROOT INCREASE IN LENGTH?—Mark off the + root of a very young corn seedling into sections by moistening + a piece of sewing thread with indelible ink and applying it to + the surface of the root at intervals of about two millimeters (⅒ + of an inch), or by tying a thread lightly around it at the same + intervals. Lay the seedling on a moist bedding between two panes + of glass kept apart by a sliver of wood to prevent their injuring + the root by pressure. Watch for a day or two, and you will see that + growth takes place from a point just back of the tip (Figs. 61, 62). + + Mark off a seedling of the bean in the same way and watch to see + whether it increases in the same manner as the corn. + + EXPERIMENT 35. HOW DOES THE STEM INCREASE IN LENGTH?—Mark off a + portion of the stem of a bean seedling as explained in the last + experiment, and find out how it grows. Allow a seedling to develop + until it has put forth several leaves and measure daily the spaces + between them. Label these spaces in your drawings, “internodes,” + and the points where the leaves are attached, “nodes.” Does an + internode stop growing when the one next above it has formed? When + is growth most rapid? Reverse the position of a number of seedlings + that have just begun to sprout and watch what will happen. After a + few days reverse again and note the effect. + +[Illustration: FIGS. 63, 64.—Root of bean seedling, measured to show +region of growth: 63, early stage of germination; 64, later stage. +FIGS. 65, 66.—Stem of bean seedling, measured to show region of +growth: 65, early stage of growth; 66, later stage.] + + EXPERIMENT 36. CAN PLANTS GROW AND LOSE WEIGHT AT THE SAME + TIME?—Remove the scales from a white lily bulb, weigh them, and + lay in a warm, but not too damp place, away from the light. After + a time bulblets will form at the bases of the scales. Weigh + them again, and if there has been any loss, account for it. The + experiment may be tried by allowing a potato tuber or a hyacinth + bulb to germinate without absorbing moisture enough to affect its + weight. + +[Illustration: FIGS. 67, 68.—Experiment showing the direction of +growth in stems: 67, young potato planted in an inverted position; +68, the same after an interval of eight days.] + + EXPERIMENT 37. IS THE DIRECTION OF GROWTH A MATTER OF ANY + IMPORTANCE?—Plant in a pot suspended as shown in Fig. 67, a healthy + seedling of some kind, two or three inches high, so that the + plumule shall point downward through the drain hole and the root + upward into the soil. Watch the action of the stem for six or + eight days, and sketch it at successive intervals. After the stem + has directed itself well upward, invert the pot again, and watch + the growth. After a week remove the plant and notice the direction + of the root. Sketch it entire, showing the changes in direction of + growth. + + At the same time that this experiment is arranged, lay another pot + with a rapidly growing plant on one side, and every forty-eight + hours reverse the position of the pot, laying it on the opposite + side. At the end of ten or twelve days remove the plant and + examine. How has the growth of root and stem been affected? + + What do we learn from these experiments and from Exp. 35 as to the + normal direction of growth in these two organs respectively? Can + you think of any natural force that might influence this direction? + +[Illustration: FIG. 69.—Experiment showing the root of a seedling +forcing its way downward through mercury.] + + EXPERIMENT 38. TO SHOW THAT PLANTS WILL EXERT FORCE RATHER THAN + CHANGE THEIR DIRECTION OF GROWTH.—Pin a sprouted bean to a cork and + fasten the cork to the side of a flat dish, as shown in Fig. 69. + Cover the bottom of the dish with mercury at least half an inch + deep, and over the mercury pour a layer of water. Cover the whole + with a pane of glass to keep the moisture in, and leave for several + days. The root will force its way downward into the mercury, + although the latter is fourteen times heavier than an equal bulk + of the bean root substance, and the root must thus overcome a + resistance equal to at least fourteen times its own weight. + +=48. What growth is.=—With the seedling begins the growth of the +plant. Most people understand by this word mere increase in size; but +growth is something more than this. It involves a change of form, +usually, but not necessarily, accompanied by increase in bulk. Mere +mechanical change is not growth, as when we bend or stretch an organ +by force, though if it can be kept in the altered position till such +position becomes permanent, or as we say in common speech, “till it +grows that way,” the change may become growth. To constitute true +growth, the change of form must be permanent, and brought about, or +maintained, by forces within the plant itself. + +=49. Conditions of growth.=—The internal conditions depend upon +the organization of the plant. The essential external conditions +are the same as those required for germination: food material, +water, oxygen, and a sufficient degree of warmth. It may be greatly +influenced by other circumstances, such as light, gravitation, +pressure, and (probably) electricity; but the four first named are +the essential conditions without which no growth is possible. + +=50. Cycle of growth.=—When an organ becomes rigid and its +form fixed, there is no further growth, but only nutrition and +repair,—processes which must not be confounded with it. Every plant +and part of a plant has its period of beginning, maximum, decline, +and cessation of growth. The cycle may extend over a few hours, as in +some of the fungi, or, in the case of large trees, over thousands of +years. + +=51. Geotropism.=—The general tendency of the growing axes of +plants to take an upward and downward course as shown in Exp. +37—in other words, to point to and from the center of the earth—is +called _geotropism_. It is _positive_ when the growing organs point +downward, as most primary roots do; _negative_ when they point +upward, as in most primary stems; and _transverse_, or _lateral_, +when they extend horizontally, as is the case with most secondary +roots and branches. + +=52. Gravity and growth.=—It cannot be proved directly that +geotropism is due to gravity, because it is not possible to remove +plants from its influence so as to see how they would behave in +its absence. The effect of gravity may be neutralized, however, by +arranging a number of sprouting seeds on the vertical disk of a +clinostat, an instrument fitted with a clockwork movement by means of +which they may be kept revolving steadily for several days. By this +constant change of position gravity is made to act on them in all +directions alike, which is the same in some respects as if it did not +act at all. If the disk is made to revolve rapidly, the growing root +tips turn toward the axis of motion, without showing a tendency to +grow downward. We may then conclude that geotropism is a reaction to +gravity. + +=53. Geotropism an active force.=—It must be noted, however, that the +force here alluded to is not the mere mechanical effect of gravity, +due to weight of parts, as when the bough of a fruit tree is bent +under the load of its crop, but a certain stimulus to which the plant +reacts by a spontaneous adjustment of its growing parts. In other +words, geotropism is an active, not a passive function, and the plant +will overcome considerable resistance in response to it. (Exp. 38). + +=54. Other factors.=—The direction of growth is influenced by many +other factors, such as light, heat, moisture, contact with other +bodies, and perhaps by electricity. The result of all these forces is +an endless variety in the forms and growth of organs that seems to +defy all law. + +[Illustration: FIG. 70.—A piece of a haulm of millet that has been +laid horizontally, righting itself through the influence of negative +geotropism.] + +Heat, unless excessive, generally stimulates growth; contact +sometimes stimulates it, causing the stem to curve away from the +disturbing object, and sometimes retards it, causing the stem to +curve toward the object of contact by growing more rapidly on the +opposite side, as in the stems of twining vines. Light stimulates +nutrition, but generally retards growth. The movements of plants +toward the light are effected in this way; growth being checked on +that side, the plant bends toward the light. + + + Practical Questions + + 1. Why do stems of corn, wheat, rye, etc., straighten themselves + after being prostrated by the wind? (51, 54.) + + 2. Do plants grow more rapidly in the daytime, or at night? (54.) + + 3. Reconcile this with the fact that green plants will die if + deprived of light. + + 4. Which grows more rapidly, a young shoot or an old one? (31, 50.) + + 5. Which, as a general thing, are the more rapid growers, annuals + or perennials? Herbaceous or woody-stemmed plants? + + 6. Name some of the most rapid growers you know. + + 7. Of what advantage is this habit to them? + + 8. Why do roots form only on the under side of subterraneous stems? + (51.) + + 9. Why do new twigs develop most freely on the upper side of + horizontal branches? (51.) + + + Field Work + + (1) Notice the various seedlings met with in your walks and see how + many you can recognize by their resemblance to the mature plants. + Account for any differences you may observe between seedlings + and older plants of the same species. Observe the cotyledons as + they come up and their manner of getting out of the ground, and + notice the ways in which this is influenced by moisture, light, + and the nature of the soil. Where the cotyledons do not appear, + dig into the ground and find out the reason. Notice which method + of emergence occurs in each case, the arched, or straight, and + account for it. Observe particularly the behavior of seedlings in + hard, sunbaked soil. If you see any of them lifting cakes of earth, + compare the size and weight of the cake with that of the seed; if + there is any disparity, what does this imply? What is the force + called which the plant exercises in lifting the weight? (51.) + + (2) Notice if there are any seeds germinating successfully on top + of the ground, and find out by what means their roots get into the + soil. Observe what effect sun and shade, moisture and drought, and + the nature of the soil have on the process. Find out whether roots + exercise force in penetrating the soil; what kinds they penetrate + most readily, and what kinds, if any, they fail to penetrate at + all. Notice whether seedlings with taproots, like the turnip and + castor bean, or those with fibrous roots, like corn and wheat, are + more successful in working their way downward. + + (3) Look for tree seedlings. Explain why seedlings of fruit trees + are so much more widely distributed in cultivated districts, and so + much easier to find than those of forest trees. Where do the latter + occur, as a general thing? Account for the fact that seedling + trees are so much more rare than germinating herbs, and why trees + like the oak and chestnut and black walnut propagate so much more + slowly, in a state of nature, than the pine, cedar, ash, and maple. + + (4) Observe the direction of growth in plants on the sides of + gullies and ravines, and tell how it is influenced by geotropism. + Notice whether there are other influences at work; for instance, + light, or in the case of roots, the attraction of moisture. + + + + +CHAPTER III. THE ROOT + + + I. OSMOSIS AND THE ACTION OF THE CELL + + MATERIAL.—For experiments in osmosis provide fresh and boiled + slices of red beet, a fresh egg, a piece of ox bladder or some + parchment paper; glass tubing, thread, twine, elastic bands, salt + and sugar solutions. A common medicine dropper with the small end + cut off will answer instead of tubing for making an artificial + cell; or an eggshell may be used, by blowing out the contents + through a puncture in the small end, and carefully chipping away a + portion of the shell at the big end, leaving the lining membrane + intact. The different liquids can be put into the shell and the + exposed membrane placed in contact with the liquid in the glass, + by fitting over the latter a piece of cardboard with a hole in + the center large enough for the exposed surface to protrude + sufficiently to touch the water. + +[Illustration: FIG. 71.—Artificial cell.] + +=55. Object of the experiments.=—In order to understand clearly the +action of roots in absorbing nutrients from the soil, it will be +necessary to learn something about the movement of liquids through +the cells, upon which the physiological processes of the plant +depend. For this purpose make an artificial cell by tying a piece of +ox bladder or parchment paper tightly over one end of a small glass +tube, as shown in Fig. 71. + + EXPERIMENT 39. HOW DOES ABSORPTION TAKE PLACE IN THE CELL?—(_a_) + Put some salt water in a wineglass, partly fill the tube of the + artificial cell with fresh water, and mark on the outside of both + vessels the height at which the contained liquid stands. Set the + tube in the glass of salt water and wait for results, having first + tested carefully to make sure that there are no leaks in the + membrane. After half an hour, notice whether there is any increase + of water in the glass, as indicated by the mark. If so, where did + it come from? Is there any loss of water in the tube? What has + become of it? How did it get out? Taste it to see if any of the + salt water has got in. Which is the heavier, salt water, or fresh? + (If you do not know, weigh an equal quantity of each.) In which + direction did the principal flow take place; from the heavier to + the lighter, or from the lighter to the heavier liquid? + + (_b_) Put a sugar or salt solution in the tube, and clear, fresh + water in the glass, marking the height in each as before. Does the + liquid rise or fall in the tube? Does any of it escape into the + water of the glass, and if so, is it more or less than before? + Which now contains the denser fluid, the tube or the glass? What + principle governs the course of the liquid? Try the same experiment + with (_c_), the same liquid in both vessels, and notice whether + there is a greater flow in one direction than the other, as + indicated by a comparison with the marks on the outside. (_d_) Put + in the tube some of the white of a raw egg, insert in a glass of + pure water, and note the effect. (_e_) Reverse, with water in the + tube and white of egg in the glass. Does the water rise in the tube + as before? Test the contents for proteins; has any of the albumin + passed through the membrane into the tube? + + EXPERIMENT 40. TO TEST THE BEHAVIOR OF LIVING AND DEAD CELLS.—Slice + a fresh piece of red beet into a vessel of water and of a boiled + one into another vessel of the same liquid at the same temperature. + What difference do you notice? Can you think of any reason why the + boiled one gives up its juices and the other one does not? + +=56. Osmosis.=—The passage of liquids or of solids in solution +through membranes is known as _osmosis_. Our experiments have +shown that the principles governing the osmotic movement are: (1) +the passage of water from the thinner liquid toward the denser +takes place more rapidly than in the opposite direction; (2) the +rapidity of the transfer depends on the difference in density; (3) +crystallizable substances in solution, like sugar and salt, osmose +readily; (4) albuminous or gelatinous substances, such as the white +of an egg, osmose so slowly that the cell wall may be regarded as +practically impermeable to them. + +=57. Osmosis a form of diffusion.=—Osmosis is related to diffusion +as a part to the whole. In other words, it is a name given to the +process when it takes place through a membrane, whether solid, as the +outer wall of the cell, or semi-fluid, as the inner wall of living +protoplasm. Diffusion may therefore take place without osmosis, +that is, in the absence of a membrane, as, for example, when we +sweeten our tea or coffee by allowing sugar to diffuse through it. +Many membranes offer little resistance to the osmotic movement of +crystallizable substances. Such membranes are said to be _permeable_. +Membranes which are not permeable to the dissolved solids, are called +_semi-permeable_, since they allow the diffusion of water but not of +the substances in solution. Living protoplasm is of this class. It is +only very slightly permeable to many substances toward which, when +dead, it acts as a permeable membrane. + +=58. Absorption in living and dead cells.=—There is one great +difference between the action of the artificial cell used in the +foregoing experiments and that of the cells of which a living body +is built up. The living cell always has at least two membranes. One +of these, the cell wall, is readily permeable, while the other, the +protoplasm, is semi-permeable—that is, substances in solution usually +diffuse more or less slowly, while water diffuses rapidly. Hence +in the living cell the protoplasm exercises a power of absorption +independent of the cell wall, sometimes rejecting substances admitted +by the latter, sometimes retaining others to which it is permeable, +as shown in Exp. 40. In the boiled beet the protoplasm had been +killed and the red coloring matter passed through it unhindered, +while in the living one it was held back by the protoplasmic lining, +which is thus seen to control the absorptive properties of the cell. + +=59. Plasmolysis.=—Cells can be killed or injured in other ways +than by heat; for example, by cold, by poisons, by starvation, and +by overfeeding through the use of too much fertilizer or too rich +a one. In this last case, the soil water becomes impregnated with +soluble matter from the manure, which may render it denser than the +sap in the roots. When this happens, it will cause the osmotic flow +to set outward and thus deplete the cell of its water; whence we have +the paradox that a cell, or even a whole plant, may be starved by +overfeeding. This action of osmosis in withdrawing the contents from +a cell is termed _plasmolysis_, and you can easily understand how +very important a knowledge of the principles governing it is to the +farmer in determining the application of fertilizers to his crops. + +Dead cells, although powerless to carry on the life processes of a +plant, have nevertheless important uses in serving the purposes of +mechanical support and also to some extent in assisting in the work +of absorption, though their function here is a purely mechanical one. + +[Illustration: FIG. 72.—Root of a tree enveloping a rock. The large +sycamore, whose base is partly concealed by the trumpet creeper on +the left of the picture, is growing in very hard, stony soil, and +one of its main roots has molded itself so completely to the ledge +of rock protruding on the right, that when a portion of it was torn +away, as shown where the light streak ends at _a_, the impress of its +fibers was so strongly marked on the rock as to give the latter the +appearance of a petrified root.] + +=60. Selective absorption.=—Different plants through their roots +absorb different substances from the soil water, or the same +substance in varying degrees. Hence, one kind of crop will exhaust +the soil of certain minerals while leaving other kinds intact, or +very little diminished; and _vice versa_, another kind will take +up abundantly what its predecessor has rejected. In this sense, +plants are said to exercise a selective power in the absorption +of nutrients. The expression must not be understood, however, as +implying any kind of volitional discrimination. It is merely a short +and convenient way of saying that the cells of different plants +possess different degrees of permeability to certain substances, some +being more permeable to one thing, some to another. But beyond this +rejection of untransmissible substances there is no active power of +discrimination, any substance that can pass through the cell wall and +its protoplasmic lining being taken in, whether useful, unnecessary, +or even harmful. These may, however, be got rid of by excretion, as +the superfluous water taken in with dissolved minerals is exhaled +from the leaves; or if incapable of passing out by osmosis, rendered +harmless and retained in the form of the curious “crystalloids” +found in various parts of plants. But while the kind of selection +exercised by vegetable cells implies no power of choice, as a matter +of fact those substances most used by the plant in carrying on its +life processes are absorbed in much greater quantities than others, +being transferred to parts where growth or other changes in the plant +tissues are going on, and there used up in the work of nutrition, or +excreted in part as waste products. In either case their passage from +cell to cell will give rise to a continuous osmotic current in that +direction, and the absorption of new matter will go on in proportion +to the amounts used up. + +[Illustration: FIG. 73.—Roots of elm and sycamore contending for +possession of the soil on a rocky bluff on the Potomac.] + +=61. Definition.=—Tissue is a word used to denote any animal or +vegetable substance having a uniform structure organized to perform +a particular office or function. Thus, for instance, we have bony +tissue and muscular tissue in animals; that is, tissue made of bone +substance and muscle substance and doing the work of bone and muscle +respectively. Likewise in plants, we have strengthening tissue +made up of hard, thick-walled cells, serving mainly for purposes +of mechanical support, and vascular tissue, made up of conducting +vessels for conveying sap—and so on, for every separate function. + + + Practical Questions + + 1. Why do raspberries and strawberries have a flabby, wilted look + if sugar has been put on them too long before they are served? (7, + 56.) + + 2. Where has the juice gone? What caused it to go out of the + berries? (56, 59.) + + 3. Is a knowledge of the principles governing osmosis of any + practical use to the housekeeper? + + 4. Why cannot roots absorb water as freely in winter as in summer? + (Suggestion: which is the heavier, cold or warm water?) + + 5. Why does fertilizing too heavily sometimes injure a crop? (59.) + + 6. Do you see any apparent contradiction between the action + of plasmolysis and the selective power of protoplasm? Can you + reconcile it? + + 7. If a piece of beet that has been frozen is placed in water + it will behave just as the slice of boiled beet did in Exp. 40; + explain. (58, 59.) + + + II. MINERAL NUTRIMENTS ABSORBED BY PLANTS + + MATERIAL.—A dozen or two each of different kinds of seeds and + grains. A small portion from a growing shoot of a woody and a + herbaceous land plant, and of some kind of succulent water or marsh + plant, such as arrow grass (_Sagittaria_), water plantain, etc. + + APPLIANCES.—A pair of scales; a lamp, stove, or other means of + burning away the perishable parts of the specimens to be studied. + + EXPERIMENT 41.—DO THE TISSUES OF PLANTS CONTAIN MINERAL + MATTER?—Take about a dozen each of grains and seeds of different + kinds, weigh each kind separately, and then dry them at a high + temperature, but not high enough to scorch or burn them. After they + have become perfectly dry, weigh them again. What proportion of the + different seeds was water, as indicated by their loss of weight in + drying? + + Burn all the solid part that remains, and then weigh the ash. What + proportion of each kind of seed was of incombustible material? What + proportion of the solid material was destroyed by combustion? + + EXPERIMENT 42.—DO THEY CONTAIN DIFFERENT KINDS AND QUANTITIES OF + MINERALS?—Test in the same way the fresh, active parts of any kind + of ordinary land plant (sunflower, hollyhock, pea vines, etc.), and + of some kind of succulent water or marsh plant (Sagittaria, water + lily, fern). Do you notice any difference in the amount of water + given off and of solid matter left behind? In the character of the + ashes left? Have you observed in general any difference between the + ashes of different woods; as, for instance, hickory, pine, oak? + Compare with the residue left in Exp. 21; would you judge that the + residual substances are of the same composition? + +[Illustration: FIG. 74.—Water cultures of buckwheat, showing effect +of the lack of the different food elements: 1, with all the elements; +2, without potassium; 3, with soda instead of potash; 4, without +calcium; 5, without nitrates or ammonia salts.] + +=62. Essential constituents.=—The composition of the ash of any +particular plant will depend upon two things: the absorbent capacity +of the plant itself and the nature of the substances contained in the +soil in which it grows. But chemical analysis has shown that however +the ashes may vary, they always contain some proportion of the +following substances: potassium (potash), calcium (lime), magnesium, +phosphorus, and (in green plants) iron. These elements occur in all +plants, and if any one of them is absent, growth becomes abnormal if +not impossible. + +The part of the dried substances that was burned away after expelling +the water consists, in all plants, mainly of carbon, hydrogen, +oxygen, nitrogen, and sulphur, in varying proportions. These five +rank first in importance among the essential elements of vegetable +life, and without them the plant cell itself, the physiological unit +of vegetable structure, could not exist. They compose the greater +part of the substance of every plant, carbon alone usually forming +about one half the dry weight. Other substances may be present in +varying proportions, but the two groups named above are found in all +plants without exception, and so we may conclude that (with the +possible addition of chlorine) they form the indispensable elements +of plant food. Carbon, hydrogen, oxygen, nitrogen, sulphur, and +phosphorus compose the structure of which the plant is built. The +other four ingredients do not enter into the substance as component +parts, but aid in the chemical processes by which the life functions +of the plant are carried on, and are none the less essential elements +of its food. Figure 74 shows the difference between a plant grown in +a solution where all the food elements are present, and others in +which some of them are lacking. + +[Illustration: FIG. 75.—Roots of soy bean bearing tubercle-forming +bacteria.] + +=63. How plants obtain their food material.=—Plants obtain their +supply of the various mineral salts from solutions in the soil +water which they absorb through their roots. With a few doubtful +exceptions, they cannot assimilate their food unless it is in a +liquid or gaseous form. Of the gases, carbon dioxide, oxygen, and +hydrogen can be freely absorbed from the air, or from water with +various substances in solution, but most plants are so constituted +that they cannot absorb free nitrogen from the air; they can take it +only in the form of compounds from nitrates dissolved in the soil, +and hence the importance of ammonia and other nitrogenous compounds +in artificial fertilizers. Some of the pea family, however, bear +on their roots little tubers formed by minute organisms called +bacteria, which have the power of extracting nitrogen directly from +the free air mingled with the soil; and hence the soil in which these +tuber-bearing legumes decay is enriched with nitrogen in a form ready +for use. + + + Practical Questions + + 1. Could any normal plant grow in a soil from which nitrogen was + lacking? Potash? Lime? Phosphorus? (62.) + + 2. Could it live in an atmosphere devoid of oxygen? Nitrogen? + Carbon dioxide? (62.) + + 3. Why are cow peas or other legumes planted on worn-out soil to + renew it? (63.) + + 4. Is the same kind of fertilizer equally good for all kinds of + soil? For all kinds of plants? (60, 62.) + + 5. Why does too much watering interfere with the nourishment of + plants? (Exps. 26, 27.) + + 6. Are ashes fit for fertilizers after being leached for lye? (62.) + + 7. Why will plants die, or make very slow growth, in pots, unless + the soil is renewed occasionally? (60, 62.) + + + III. STRUCTURE OF THE ROOT + + MATERIAL.—Taproot of a young woody plant not over one or two years + old; apple and cherry shoots make good specimens. For showing root + hairs, seedlings of radish, turnip, or oat are good, also roots of + wandering Jew grown in water; for the rootcap, corn, sunflower, + squash. + +[Illustration: FIG. 76.—Cross section of a young taproot; _a_, _a_, +root hairs; _b_, epidermis; _c_, cortical layer; _d_, fibrovascular +cylinder. Note the absence of medullary rays during the first year of +growth.] + +=64. Gross anatomy of the root.=—Cut a cross section of any woody +taproot, about halfway between the tip and the ground level, +examine it with a lens, and sketch. Label the dark outer covering, +_epidermis_, the soft layer just within that, _cortex_, the hard, +woody axis that you find in the center, _vascular cylinder_, and +the fine silvery lines that radiate from the center to the cortex, +_medullary rays_ (in a very young root these will not appear). Cut +a section through a root that has stood in coloring fluid for about +three hours and note the parts colored by the fluid. What portion of +the root, would you judge from this, acts as a conductor of the water +absorbed from the ground? + +Make a longitudinal section passing through the central portion of +the root and extending an inch or two into the lower part of the +stem. Do you find any sharp line of division between them? Notice the +hard, woody axis that runs through the center. This is the vascular +cylinder and contains the conducting vessels, the cut ends of which +were shown in cross section in Fig. 76. + +[Illustration: FIG. 77.—Verti-section of branching root, showing the +branches, _n_, _n_, originating in the central axis, _f_, and passing +through the cortex, _r_, _r_.] + +=65. Distinctions between root and stem.=—Pull off a branch from +the stem and one from the root; which comes off the more easily? +Examine the points of attachment of the two and see why this is so. +This mode of branching from the central axis instead of from the +external layers, as in the stem, is one marked distinction between +the structure of the two organs. In stems, moreover, branches occur +normally above the points of leaf insertion at the nodes (46), while +in the root they tend to arrange themselves in straight vertical +rows. The shoots and cions that often originate from them are not +normal root branches, but outgrowths from irregular or _adventitious_ +buds, that may occur on any part of a plant. The root is not divided +into nodes like the stem, and never bears leaves. + +[Illustration: FIG. 78.—Root of a tree on the side of a gulley, +acting as stem.] + +=66. The active part of the root.=—It is only the newest and most +delicate parts of the root that produce hairs and are engaged in the +active work of absorption, the older parts acting mainly as carriers. +Hence, old roots lose much of their characteristic structure and +take on more and more of the office of the stem, until there is +practically no difference between them. On the sides of gullies, +where the earth has been washed from around the trees, we often +see the upper portion of the root covered with a thick bark and +fulfilling every office of a true stem. + +=67. Minute structure of the root.=—(_a_) Mount in water and place +under the microscope a portion of the root of an oat or radish +seedling containing a number of hairs. In studying the thin, +transparent roots of very young seedlings a section will not be +necessary. Observe whether the hairs originate from the epidermis or +from the interior. Are they true roots, or mere outgrowths from the +cells of the epidermis? Do they consist of a single cell or a number +of cells each? Notice what very thin cell walls the hairs have; is +there any advantage in this? The interior, transparent portion of +the hair contains the sap, and the protoplasm forms a thin lining on +the inner surface of the wall; why not the sap next the wall and the +protoplasm in the interior? (58, 60.) + +[Illustration: FIG. 79.—Longitudinal section through the tip of +a young root, somewhat diagrammatic: _h_, _h_, root hairs; _ep_, +epidermis; _a_, cortex; _b_, central cylinder; _e_, sheath of the +cylinder (endodermis); _g_, growing point; _c_, root cap; _d_, dead +and dying cells loosened from the extremity of the cap.] + +(_b_) Next examine a portion of the body of the root and try to +make out the parts as shown in Fig. 79, and compare them with your +observations in 64. The light line running through the middle is +the _central cylinder_, up which the water passes, as was shown by +the colored liquid in 64. Outside this is a darker portion (_a_, +Fig. 79), corresponding to the cortex (_rr_, Fig. 77). Besides other +uses, the cortex serves to prevent the loss of water as it passes up +to the stem, and also, in fleshy roots like the carrot and turnip, +for the storage of nourishment. Its innermost row of cells is +thickened into the sheath, or _endodermis_ (_e_), which serves as an +additional protection to the conducting tissues. The extreme outer +layer, from the cells of which the root hairs are developed, is, as +already stated, the epidermis, and in the older and more exposed +parts of perennial roots is displaced by the bark, which becomes +indistinguishable from that of the stem. (66.) + +(_c_) Look at the tip of the root for a loose structure (_c_) fitting +over it like a thimble. This is the rootcap. Do you see any loose +cells that seem to have broken away from it? These are old cells that +have been pushed to the front by the formation of new growth back of +them, and, being of no further use, are rubbed off by friction as the +root bores its way through the soil. Draw a longitudinal section of +the root as it appears under the microscope, labeling all the parts. +If they cannot be made out distinctly in the specimen examined, use +sections of young corn or bean roots, which are larger and show the +parts more distinctly. + +[Illustration: FIG. 80.—Cross section of a young root, magnified: +_h_, hairs; _a_, cortex; _b_, central cylinder; _e_, sheath or +endodermis; _ep_, epidermis; _sp_, cut ends of the ducts.] + +(_d_) Place under the microscope a thin cross section through the +hairy portion of a primary root of a bean or pea seedling, and try +to make out the parts noted above and shown in cross section in +Fig. 80. Make a sketch of what you see, labeling all the parts you +can recognize. Show in your drawing the differences in the size +and shape of the cells composing the different tissues. Notice in +the central cylinder (Fig. 80) several groups of what look in the +section like little round pits, or holes, _sp_. These are the cut +ends of large-sized tubes or _ducts_ that convey the water absorbed +by the roots to the stem. Each set of these tubes, together with a +number of smaller ones belonging to the same group, constitutes a +_fibrovascular bundle_—a very important element in the structure of +all roots and stems, as these bundles make up the conducting system +of the plant body. + + + IV. THE WORK OF ROOTS + + MATERIAL.—Germinating seedlings of radish, bean, corn, etc.; + a potted plant of calla, fuchsia, tropæolum, touch-me-not + (_Impatiens_), or corn; a plant that has been growing for some time + in a porous earthen jar. + + APPLIANCES.—Glass tumblers; coloring fluid; wax; some coarse + netting; dark wrapping paper, or a long cardboard box; a sheet of + oiled paper; some half-inch glass tubing; a few inches of rubber + tubing; an ounce of mercury; some blue litmus paper; a flower pot + full of earth; a few handfuls of sand, clay, and vegetable mold; a + pair of scales; a half dozen straight lamp chimneys, or long-necked + bottles from which the bottoms have been removed as directed in + Exp. 53. + + EXPERIMENT 43. USE OF THE EPIDERMIS.—Cut away the lower end + of a taproot; seal the cut surface with wax so as to make it + perfectly water-tight, and insert it in red ink for at least half + the remaining length, taking care that there is no break in the + epidermis. Cut an inch or two from the tip of the lower piece, or + if material is abundant, from another root of the same kind, and + without sealing the cut surface, insert it in red ink, beside the + other. At the end of three or four hours, examine longitudinal + sections of both pieces. Has the liquid been absorbed equally by + both? If not, in which has it been absorbed the more freely? What + conclusion would you draw from this, as to the passage of liquids + through the epidermis? + + From this experiment we see that the epidermis, besides protecting + the more delicate parts within from mechanical injury by hard + substances contained in the soil, serves by its comparative + imperviousness to prevent evaporation, or the escape of the sap by + osmosis as it flows from the root hairs up to the stem and leaves. + + EXPERIMENT 44. TO SHOW THAT ROOTS ABSORB MOISTURE.—Fill two pots + with damp earth, put a healthy plant in one, and set them side by + side in the shade. After a few days examine by digging into the + soil with a fork and see in which pot it is drier. Where has the + moisture gone? How did it get out? + + EXPERIMENT 45. TO SHOW THAT ROOTS SHUN THE LIGHT.—Cover the top + of a glass of water with thin netting, and lay on it sprouting + mustard or other convenient seed. Allow the roots to pass through + the netting into the water, noting the position of root and stem. + Envelop the sides of the glass in heavy wrapping paper, admitting + a little ray of light through a slit in one side, and after a few + days again observe the relative position of the two organs. How is + each affected by the light? + + EXPERIMENT 46. TO FIND OUT WHETHER ROOTS NEED AIR.—Remove a plant + from a porous earthenware pot in which it has been growing for some + time; the roots will be found spread out in contact with the walls + of the pot instead of embedded in the soil at the center. Why is + this? + + EXPERIMENT 47. TO SHOW THAT ROOTS SEEK WATER.—Stretch some coarse + netting covered with moist batting over the top of an empty + tumbler. Lay on it some seedlings, as in Exp. 45, allowing the + roots to pass through the meshes of the netting. Keep the batting + moist, but take care not to let any of the water run into the + vessel. Observe the position of the roots at intervals, for twelve + to twenty-four hours, then fill the glass with water to within + 10 millimeters (a half inch, nearly) or less of the netting, let + the batting dry, and after eight or ten hours again observe the + position of the roots. What would you infer from this experiment as + to the affinity of roots for water? + + EXPERIMENT 48. WHAT BECOMES OF THE WATER ABSORBED BY ROOTS.—Cover + a calla lily, young cornstalk, sunflower, tropæolum, or other + succulent herb with a cap of oiled paper to prevent evaporation + from the leaves, set the pot containing it in a pan of tepid water, + and keep the temperature unchanged. After a few hours look for + water drops on the leaves. Where did this water come from? How did + it get up into the leaves? + + EXPERIMENT 49. TO SHOW THE FORCE OF ROOT PRESSURE.—Cut off the stem + of the plant 6 or 8 centimeters (3 or 4 inches) from the base. Slip + over the part remaining in the soil a bit of rubber tubing of about + the same diameter as the stem, and tie tightly just below the cut. + Pour in a little water to keep the stem moist, and slip in above, + a short piece of tightly fitting glass tubing. Watch the tube for + several days and note the rise of water in it. The same phenomenon + may be observed in the “bleeding” of rapidly growing, absorbent + young shoots, such as grape, sunflower, gourd, tobacco, etc., if + cut off near the ground in spring when the earth is warm and moist. + By means of an arrangement like that shown in Fig. 81, the force + of the pressure exerted can be measured by the displacement of the + mercury. This flow cannot be due to the giving off of moisture + by the leaves, since they have been removed. Their action, when + present, by causing a deficiency of moisture in certain places may + influence the direction and rapidity of the current, but does not + furnish the motive power, which evidently comes, in part at least, + from the roots, and is the expression of their absorbent activity. + +[Illustration: FIG. 81.—Arrangement for estimating the force of root +pressure: _s_, stub of the cut stem; _g_, glass tubing joined by +means of the rubber tubing, _t_, to the stem; _m_, mercury forced up +the glass tube by water, _w_, pumped from the soil by the roots.] + + EXPERIMENT 50. TO SHOW THAT ROOTS CAUSE THE OCCURRENCE OF + ACIDS.—Lay a piece of blue litmus paper on a board or on a piece + of glass slightly tilted at one end to secure drainage. Cover the + surface with an inch of moist sand and plant in it a number of + healthy seedlings. Acids have the property of changing blue litmus + to red; hence, if you find any red stains on the paper where the + roots have penetrated, what are you to conclude? + + Carbon dioxide has a slight acid reaction and is caused to form in + varying quantities by all roots. Probably other substances, and + these not a few, are actually excreted. + + EXPERIMENT 51. CAN THE ABSORBENT POWER OF ROOTS BE INTERFERED + WITH?—Place the roots of a number of seedlings with well-developed + hairs in a weak solution of saltpeter—10 grams (about ⅓ of an + ounce) to a pint of water, and others in a stronger solution—say 30 + grams, or 1 ounce, to a pint. Try the same experiment with weak and + strong solutions of any conveniently obtainable liquid fertilizer. + After 45 minutes or an hour examine the roots under a lens and note + the change that has taken place. What has gone out of them? What + caused the loss of the contained sap? + + EXPERIMENT 52. TO TEST THE WEIGHT OF SOILS.—Thoroughly dry and + powder a pint each of sand and clay, measure accurately, and + balance against each other in a pair of scales. Which weighs more, + bulk for bulk, a “light” soil, or a “heavy” one? (77.) + + EXPERIMENT 53. TO TEST THE CAPACITY OF SOILS FOR ABSORBING AND + RETAINING MOISTURE.—Arrange, as shown in Fig. 82, a number of + long-necked bottles from which the bottom has been removed. This + can be done by making a small indentation with a file at the point + desired and leading the break round the circumference with the end + of a glowing wire or a red-hot poker. The crack will follow the + heated object with sufficient regularity to answer the purpose. + Tie a piece of thin cloth over the mouth of each bottle and invert + with the necks extending an inch or two into empty tumblers placed + beneath. Fill all to the same height with soils of different + kinds—sand, clay, gravel, loam, vegetable mold, etc.—and pour over + each the same quantity of water from above. Watch the rate at which + the liquid filters through into the tumblers. Which loses its + moisture soonest? Which retains it longest? + +[Illustration: FIG. 82.—Apparatus for testing the capacity of soils +to take in and retain moisture.] + + Next leave the soils in the bottles dry, fill the tumblers up to + the necks of the bottles, and watch the rate at which the water + rises in the different ones. The power of soils to absorb moisture + is called _capillarity_. Which of your samples shows the highest + capillarity? Which the lowest? Do you observe any relation between + the capillarity of a soil and its power of retention? + +=68. Roots as holdfasts.=—One use of ordinary roots is to serve as +props and stays for anchoring plants to the soil. Tall herbs and +shrubs, and vegetation generally that is exposed to much stress of +weather, are apt to have large, strong roots. Even plants of the same +species will develop systems of very different strength according as +they grow in sheltered or exposed places. + +[Illustration: FIG. 83.—Dandelion: _a_, common form, grown in plains +region at low altitude; _b_, alpine form.] + +=69. Root pull.=—Roots are not mere passive holdfasts, but exert +an active downward pull upon the stem. Notice the rooting end of a +strawberry or raspberry shoot and observe how the stem appears to +be drawn into the ground at the rooting point. In the leaf rosettes +of herbs growing flat on the ground or in the crevices of walls and +pavements, the strong depression observable at the center is due to +root pull. (Fig. 84.) + +[Illustration: FIG. 84.—Raspberry stolon showing root pull.] + +=70. Storage of food.=—Another office of roots is to store up food +for the use of the plant. This is done chiefly in the tissues of +fleshy roots and tubers, and gives to them their great economic +value. Next to grains and cereals, roots probably furnish a larger +portion of food to the human race than any other crop. In addition to +this they are also the source of valuable drugs, condiments, and dyes. + +=71. Absorption and conveyance of sap.=—But the most important +function of roots is that of absorption. By their action the soil +water and the minerals contained in it are drawn up into the plant +body and made available for conversion by the leaves into organic +foods, as will be explained in another chapter. From the nature +of their function, most roots have naturally a strong affinity +for water, and its presence or absence has a marked influence on +their direction of growth, being often sufficient to overcome that +of geotropism (Exp. 47). There are many trees and shrubs, notably +willow, sweet bay, red birch, and the like, that grow best on the +banks of streams and ponds, where their roots can have direct access +to water. Excess of moisture, however, is injurious to most land +plants by preventing the roots from getting sufficient air for +respiration. + +=72. The conditions of absorption.=—The sap in the root cells is +normally denser than the water in the soil, so there is a continuous +flow from the latter to the former. But if, for any reason, the +density of the liquids should be reversed, the flow would set in the +opposite direction, and if continued long enough, the strength of the +plant would be literally “sapped” by the exhaustion of its tissues, +so that it would die. What is this process of cell exhaustion called? + +=73. The use of acid secretions to the root.=—It was shown in Exp. +50 that carbon dioxide and probably other substances occur in the +immediate vicinity of roots. Carbon dioxide is an active agent in +dissolving the various mineral matters contained in the soil, and as +these last can be absorbed only in a liquid or a gaseous state (63), +the advantage to the root as an absorbent organ, of being able to +secrete such active solvents, is obvious. + +[Illustration: FIG. 85.—A natural root etching, found on a piece of +slate.] + +=74. Relation of roots to the soil.=—In order to perform their work +of absorption, roots must have access to a suitable soil. To produce +the best results a soil must contain (1) all the essential mineral +constituents (62); (2) moisture for dissolving these materials; and +(3) air enough to supply the oxygen which is necessary to the life +processes of all green plants. + +=75. Composition of soils.=—Sand, clay, and humus, or vegetable +mold, with the various substances dissolved in them, constitute the +basis of cultivated soils. A mixture of sand, clay, and humus is +called loam. When the proportion of humus is very large and well +decomposed, the mixture is called _muck_. Pure sand contains but +little nourishing matter and is too porous to retain water well. Pure +clay is too compact to be easily permeable to either air or water. +Most soils are composed of a mixture of the two with vegetable mold +in varying proportions, giving a sandy loam, or a clay loam, as the +case may be. + +=76. Tillage.=—The advantages of tillage are: (_a_) that by breaking +up the hard lumps it renders the soil more permeable to air and water +and more easily penetrable by the roots in their search for food; +(_b_) the covering of loose, friable earth left by the plow and the +harrow acts as a mulch, and by shading the soil below, prevents +too rapid a loss of water by evaporation. Where the essential food +ingredients are present, good tillage counts for more in making a +crop than the original quality of the soil. + +=77. Light and heavy soils.=—These terms are used by farmers not +in relation to the weight of soils, but in reference to the ease +or difficulty with which they are worked. Light soils contain a +preponderance of sand; heavy ones, of clay. + + + Practical Questions + + 1. Will plants grow better in an earthen pot or a wooden box than + in a vessel of glass or metal? Why? (Exp. 46.) + + 2. Which absorb more from the soil, plants with light roots and + abundant foliage, or those with heavy roots and scant foliage? + (Suggestion: roots absorb from the soil; leaves, mainly from the + air.) + + 3. Why are willows so generally selected for planting along the + borders of streams in order to protect the banks from washing? (71.) + + 4. Why are the conducting tissues of roots at the center instead of + near the surface as in stems? (67, _b_.) + + 5. Why does corn never grow well in swampy ground? (74; Exp. 46.) + + 6. Why are fleshy roots so much larger in cultivated plants than in + wild ones of the same species? (74, 76.) + + 7. When the use of a particular kind of fertilizer causes the + leaves of the plants to which it has been applied to turn brown, so + that the farmer says they have been “burned” by it, to what cause + is the trouble due? (59, 72.) + + 8. Why do farmers speak of turnips and other root crops as “heavy + feeders”? (70, 71.) + + 9. Which is more exhausting to the soil, a crop of beets, or one of + oats? Onions, or green peas? (See 2, suggestion.) + + 10. Why will inserting the end of a wilted twig in warm water + sometimes cause it to revive? (Exps. 48, 49.) + + + V. DIFFERENT FORMS OF ROOTS + + MATERIAL.—Examples of taproots: bean, pea, cotton, maple seedlings, + or any kind of very young woody root. Fibrous: any kind of grass + or grain. Fleshy: parsnip, turnip, carrot, dahlia, sweet potato. + Water: duckweed, pondweed, or a cutting of wandering Jew grown + in water. Parasitic: mistletoe, dodder, beech drops. Aërial and + adventitious: the aërial roots of old scuppernong vines, climbing + roots of ivy and trumpet vine, prop roots from the lower nodes of + cornstalks and sugar cane. + +=78. Basis of distinction.=—Roots vary in form and external structure +according to their origin, function, and surroundings. In reference +to the first, they are classed as primary or secondary; in regard to +the second, as dry or fleshy; while as to surroundings, they may be +adapted to either the soil, water, air, or the parasitic habit. Soil +roots are the normal form. According to their mode of growth they are +either fibrous or axial. + +[Illustration: PLATE 3.—Aërial roots of a Mexican “strangling” fig, +enveloping the trunk of a palm (_From_ “Rep’t. Mo. Bot. Garden”).] + +=79. Taproots.=—These are the common form of the axial type. Compare +the root of any young hardwood cion a year or two old with one +of a mature stalk of corn or other grain, and with the roots of +seedlings of the same species. Notice the difference in their mode +of growth. In the first kind a single stout prolongation called a +taproot proceeds from the lower end of the hypocotyl and continues +the axis of growth straight downward, unless turned aside by some +external influence. A taproot may be either simple, as in the turnip, +radish, and dandelion, or branched, as in most shrubs and trees. In +the latter case the main axis is called the primary root, and the +branches are secondary ones. + +[Illustration: FIG. 86.—Branched taproot of maple.] + +=80. Fibrous and fascicled roots.=—Where the main axis fails +to develop, as in the corn and grasses generally, a number of +independent branches take its place, forming what are known as +fibrous roots. Both fibrous and taproots may be either hard or +fleshy. The turnip and carrot are examples of fleshy taproots, the +dahlia and rhubarb of fascicled roots. The function of both is the +storage of nourishment. The sweet potato is an example of a tuberous +root. + +=81. Practical importance of this distinction.=—The difference +between axial and fibrous roots has important bearings in +agriculture. The first kind, which are characteristic of most +dicotyls, strike deep and draw their nourishment from the lower +strata of the soil, while the fibrous and fascicled, or radial kinds, +as we may call them for want of a better name, spread out near the +surface and are more dependent on external conditions. + +[Illustration: FIG. 87.—Fibrous root.] + +=82. Roots that grow above ground.=—The kinds of roots that have +just been considered are all subterranean, and bring the plant +into relation with the earth, whether for the purpose of absorbing +nourishment, or of mechanical support, or, as in the majority of +cases, for both. Many plants, however, do not get their mineral +nutrients directly from the soil, and these give rise to various +forms suited to other conditions of alimentation. + +=83. Adventitious roots.=—This name applies to any kinds of roots +that occur on stems, or in other unusual positions. They may be +considered as intermediate between the two classes named in 81; for +while their starting point is above ground, they generally end by +fixing themselves in the soil, where they often function as normal +roots. Familiar examples are the roots that put out from the lower +nodes of corn and sugar cane stalks, and serve both to supply +additional moisture and to anchor the plant more firmly to the soil. +Most plants will develop adventitious roots if covered with earth, or +even if merely kept in contact with the ground. The gardener takes +advantage of this capacity when he propagates by cuttings and layers. + +=84. Water roots.=—These are generally white and threadlike and more +tender and succulent than ordinary soil roots, because they have less +work to do. Floating and immersed plants, such as bladderwort and +hornwort (_Ceratophyllum_) have no need of absorbent roots, since the +greater part of their surface is in contact with water and can absorb +directly what is needed. + +Land plants will often develop water roots and thrive for a time if +the liquid holds in solution a sufficient quantity of air and mineral +nutrients. Place a cutting of wandering Jew in a glass of clear +water, and in from four to six days it will develop beautiful water +roots in which both hairs and cap are clearly visible to the naked +eye. + +=85. Haustoria=, from a Latin word meaning to drain, or exhaust, +is a name given to the roots of parasitic plants, or such as live +by attaching themselves to some other living organism, from which +they draw their nourishment ready made. Their roots are adapted +to penetrating the substance of the _host_, as their victim is +called, and absorbing the sap from it. Dodder and mistletoe are the +best-known examples of plant parasites, though the latter is only +partially parasitic, as it merely takes up the sap from the host and +manufactures its own food by means of its green leaves. + +[Illustration: FIG. 88.—Beech root: _A_, grown in unsterilized wood +humus: _p_, strands of fungal hyphæ, associated at _a_, with humus; +_B_, grown in wood humus freed from fungus by sterilization—it is not +provided with fungal hyphæ, and has root hairs, _h_. (_A_ and _B_ +both several times magnified.)] + +=86. Saprophytes.=—Akin to parasites are saprophytes, which live +on dead and decaying vegetable matter. They are only partially +parasitic and do not bear the haustoria of true parasites. Many of +them, of which the Indian pipe (_Monotropa_) and coral root are +familiar examples, obtain their nourishment in part, at least, by +association with certain saprophytic fungi, which enmesh their roots +in a growth of threadlike fibers that take the place of root hairs +and absorb organic food from the rich humus in which these plants +grow. Such growths are called _mycorrhiza_, meaning “fungal roots.” +Similar associations are formed by some of the higher plants also. +The rootlets of the common beech and of certain of the pine family, +for instance, are often enveloped in a network of fungus fibers, +and in this case root hairs are developed very poorly, or not at +all. Besides greatly increasing the absorbent surface by their +ramification through the soil, the mycorrhizal threads may possibly +benefit the plant in other ways also, as, for instance, by bringing +about chemical changes that might aid in the work of nutrition. + +[Illustration: FIG. 89.—An air plant (_Tillandsia_), growing on the +underside of a bough.] + +[Illustration: FIG. 90.—A single strand of _Tillandsia usneoides_, a +rootless epiphyte belonging to the pineapple family; better known as +the “Spanish moss” that drapes the boughs of trees so conspicuously +in the warm parts of America. Two-thirds natural size. (Photographed +by C. F. O’Keefe.)] + +=87. Epiphytes, or air plants.=—In the proper meaning of the word +these are not parasitic, but use their host merely as a mechanical +support to bring them into better light relations. The name, however, +is loosely applied to all plants that find a lodgment on the trunks +and branches of trees, whether parasites or true epiphytes that draw +no nourishment from the host. Not infrequently the latter is killed +by them through suffocation, overweighting, or the constriction of +the stems by close clinging twiners. + +=88. Aërial roots= are such as have no connection at all with the +soil or with any host plant, except as they may lodge upon the trunks +and branches of trees for a support. In other than purely epiphytic +plants, which get all their nourishment from the air, they are +generally subsidiary to soil roots, like the long dangling cords that +hang from some species of old grapevines; or they subserve other +purposes altogether than absorbing nourishment, as the climbing roots +of the trumpet vine and poison ivy. A very remarkable development +of aërial roots takes place in the “strangling fig” of Mexico and +Florida, which begins life as a small epiphyte, from seeds dropped by +birds on the boughs or trunks of trees. When it gets well started, +the young plant sends down enormous aërial roots, which find their +way to the ground, and in time so completely envelop the host that it +is literally strangled to death (Plate 3, p. 73). When this support +is removed, the sheathing roots take its place and become to all +intents and purposes the stem of the fig tree, which now leads an +independent life. + +[Illustration: FIG. 91.—Root system of a tobacco plant.] + +=89. The root system.=—The entire mass of roots belonging to a +plant, with all its ramifications and subdivisions, composes a root +system. The extent of root expansion is in general about equal to +that of the crown, thus bringing the new and active parts under the +drip of the boughs where the moisture is most abundant. Some plants +have root systems out of all seeming proportion to their size. A +catalpa seedling six months old showed, by actual measurement, +250 feet of root growth, and it is estimated that the roots of a +thrifty cornstalk, if laid end to end, would extend a mile. In the +development of the root system, a great deal depends upon external +conditions. In a poor, dry soil, the roots have to travel farther in +search of a livelihood, and so a larger system has to be developed +than in a more favorable location. + + + Practical Questions + + 1. Which is better to succeed a crop of turnips on the same land, + hay or carrots? (81.) + + 2. Write out what you think would be a good rotation for four or + five successive crops based on the forms of the roots. + + 3. Study the following rotations and give your opinion about them, + on the same principle. Suggest any improvements that may occur + to you, and give a reason for the change. Beets, barley, clover, + wheat; cotton, oats, peas, corn; oats, melons, turnips; cotton, + oats, corn and peas mixed, melons; cotton, hay, corn, peas. + + 4. Give three good reasons in favor of a rotation over a + single-crop system. (24, 60, 62, 81.) + + 5. Which will require deeper tillage, a bed of carrots or one of + strawberries? (81.) + + 6. Explain why some plants keep green and fresh when the surface of + the soil is dry, while others wilt or die. (81, 89.) + + 7. Which will better withstand drought, a crop of alfalfa or one of + Indian corn? Why? (81.) + + 8. Which will interfere less with the trees if planted in an + orchard, beets or onions? (81.) + + 9. Ought a crop of hemp and tobacco to succeed each other on the + same land? (81, 89.) + + 10. Why does a gardener manure a grass plot by scattering the + fertilizer on the surface, while he digs around the roses and + lilacs and deposits it under ground? (81.) + + 11. Do the adventitious roots of such climbers as ivy and trumpet + vine draw any nourishment from the objects to which they cling? + (83-88.) + + 12. How can you tell? + + 13. Do partial dependents of this kind injure trees by climbing + upon them; and if so, how? (87, 88.) + + 14. What is the use of the aërial roots of the scuppernong grape? + (88.) + + 15. Is the resurrection fern (_Polypodium incanum_), that grows on + tree trunks in our Southern States, a parasite or an air plant? + (87.) + + 16. On what plants in your neighborhood does mistletoe grow most + abundantly? Dodder? + + 17. Is mistletoe injurious to the host? (85.) + + 18. Name some plants that are propagated mainly, or solely, by + roots and cuttings. + + 19. Where do aërial roots get their nourishment? (88.) + + 20. Would they be of any use to a plant in a very cold or dry + climate? + + 21. Where should manure be placed to benefit a tree or shrub with + wide-spreading roots? (66, 89.) + + 22. Is it a wise practice to mulch a tree by raking up dead leaves + and piling them around the base of the trunk, as is often done? + Why, or why not? (66, 89.) + + + Field Work + + (1) Examine the underground parts of hardy winter herbs in your + neighborhood, also of any weeds or grasses that are particularly + troublesome, and see if there is anything about the structure of + these parts to account for their persistence. Note the difference + between roots of the same species in low, moist places and in + dry ones; between those of the same kind of plants in different + soils; in sheltered and in exposed situations. Study the direction + and position of the roots of trees and shrubs with reference to + any stream or body of water in the neighborhood. (The elm, fig, + mulberry, and willow are good subjects for such observations.) + Notice also whether there is any relation between the underground + parts and the leaf systems of plants in reference to drainage and + transpiration. + + (2) Observe the effect of root pull upon low herbs. Look along + washes and gullies for roots doing the office of stems, and note + any changes of structure consequent thereon. Study the relative + length and strength of the root systems of different plants, with + reference to their value as soil binders, or their hurtfulness in + damaging the walls of cellars, wells, sewers, etc. Dig your trowel + a few inches into the soil of any grove or copse you happen to + visit, note the inextricable tangle of roots, and consider the + fierce competition for living room in the vegetable world that it + implies. + + (3) Tests might be made of the different soils in the neighborhood + of the schoolhouse by planting seeds of various kinds and noting + the rate of germination; first, without fertilizers, then by adding + the different elements in succession to see what is lacking. The + field for study suggested by this subject is almost inexhaustible. + + + + +CHAPTER IV. THE STEM + + + I. FORMS AND GROWTH OF STEMS + + MATERIAL.—Vigorous young hop or beau seedlings grown in pots; a + fresh dandelion stalk; a stem of pea, squash, cucumber, grape, or + passion flower vine, with tendrils. + + APPLIANCES.—A bowl of fresh water; rods of different sizes and + smoothness for testing the hold of climbers. + + EXPERIMENT 54. TO SHOW THE MOVEMENTS OF TWINING STEMS.—Raise a + young hop or bean seedling in the schoolroom and allow it to grow + about two decimeters—8 to 10 inches—in length before providing + it with a support. Does the stem form any coils? Bring it in + contact with a suitable upright support and watch for a day or + two. What happens? Notice whether it starts to coil from right to + left or from left to right and see if you can coax it to turn in + the opposite direction. When it has reached the end of its stake, + allow it to grow about five centimeters (two inches, approximately) + beyond, and watch the revolution of the tip. Cut a hole through the + center of a piece of cardboard about 14 centimeters (five to six + inches) in diameter, slip it over the loose end of the stem, and + fasten it to the stake in a horizontal position, with a pin. Note + the position of the stem tip at regular intervals and mark on the + cardboard; how long does it take to complete a revolution? Does it + continue to coil, or to coil as readily, after leaving its stake as + before? What would you infer from this as to the effect of contact + in stimulating it to coil? + + Find out by experiment if it can climb well by means of a glass or + other smooth rod; by a fine wire; a broomstick; a large, smooth + post. See whether it does better on a horizontal or an upright + support. + + EXPERIMENT 55. TO ILLUSTRATE THE COILING OF STEMS.—Run a gathering + thread in one side of a narrow strip of muslin and notice how the + ruffle thus drawn will curl into a spiral when allowed to dangle + from the needle. Can you think of any cause that might act on a + stem in the same way? Suppose, for instance, that one side should + grow faster than the other; what would be the effect? (54.) + + Split the stem of a fresh dandelion, or other herbaceous scape, + longitudinally, and immerse it in a pan of fresh water for a few + minutes. Notice how the two halves curve outward, or even coil up + like the strip of muslin. This is due to the tension caused by the + more rapid absorption of the thinner walled cells of the internal + tissues. These, when relieved of the resistance of the thicker + walled outer tissues, swell on their free side, but are held back + on the other by the non-absorbent outer parts, as one side of the + muslin ruffle was held by the gathering thread. + + EXPERIMENT 56. TO FIND OUT WHETHER THE DIRECTION OF STEM GROWTH IS + INFLUENCED BY LIGHT.—Place two rapidly growing young pea, bean, + sunflower, or squash plants, each with several well-developed + leaves, in a room or box with a light exposure on one side only. + After two or three days, notice the position of the stems in regard + to the light. Does either one show a more decided inclination + toward it than the other? + + EXPERIMENT 57. IS THE LIGHT RELATION OF THE STEM INFLUENCED BY THE + LEAVES?—Cut the leaves from one of the plants used in Exp. 56, + covering the cut surfaces with vaseline to prevent “bleeding”; + reverse the positions of both with regard to the light, and watch + for two or three days. In which is the response to light the more + rapid? What does this indicate as one object of the stem in seeking + light? What is the best position of a stem, ordinarily, for getting + its leaves into the light? + +[Illustration: FIG. 92.—Stems of red oak and hickory that have +grafted themselves.] + +=90. Classification.=—Stems are classed according to (1) duration, as +annuals, biennials, and perennials; (2) with reference to hardness +or softness of structure, as herbaceous and woody; (3) in regard to +position and direction of growth, as erect, prostrate, climbing, +inclined, declined, underground, etc. + +=91. Annuals= complete their life cycle in a single season and then +die down as soon as they have perfected their seed. Many of our most +troublesome weeds belong to this class and might be exterminated +by the simple expedient of mowing them down before their time of +flowering. + +=92. Biennials=, as the name implies, live for two years. Their +energy during the first season is spent chiefly in laying by a store +of nourishment, usually in the tissues of fleshy roots (70). By this +means they get a good start in the second season and mature their +seeds early. Many of our common garden vegetables, such as turnips, +carrots, parsnips, and cabbage, belong to this class. Where is the +nourishment stored in the cabbage? + +[Illustration: FIG. 93.—A biennial plant, mullein, in winter +condition with stem reduced to little more than a disk supporting +a rosette of leaves. Notice how close they cling to the earth, and +compare them with their fruiting condition a few months later as +shown in Fig. 237.] + +=93. Perennials= are plants that live on indefinitely, like most of +our forest trees and woody-stemmed shrubs. Woody stems are usually +perennial and may live for hundreds and even thousands of years, as +those of the giant sequoias of California, and the famous chestnut of +Mt. Etna. + +=94. Herbaceous stems= are more or less succulent and die down after +fruiting. They are usually annuals, though some kinds, like the +garden geraniums and the common St.-John’s-wort, show a tendency +to become woody, especially at the base, and live on from year to +year. Others, such as the hawkweed and dahlia, die down above ground +in winter, but are enabled to keep their underground parts alive +indefinitely, through the nourishment stored in them, and are thus +perennial below ground and annual above. Woody-stemmed annuals, such +as the cotton and castor oil plant, are not, properly speaking, +herbs. In the tropical countries to which they belong they are +perennial shrubs, or even small trees, but on being transplanted to +colder regions have been compelled to take on the annual habit as an +adaptation to climate. + +[Illustration: FIG. 94.—Orange hawkweed with runners.] + +[Illustration: FIG. 95.—Prostrate stem of Lycopodium with assurgent +branches.] + +[Illustration: FIG. 96.—Diagram of stem growth: _ps_, surface of the +ground; _e_, erect position; _d_, declined; _a_, assurgent; _p_, +prostrate; _u_, vertical direction underground.] + +=95. Direction and habit of growth.=—As to manner of growth, there +are many forms, from the upright boles of the beech and pine to the +trailing, prostrate, and creeping stems of which we have examples +in the running periwinkle, the prostrate spurge and the creeping +partridge berry (_Mitchella repens_), respectively. Trailing and +prostrate stems are very apt to become creepers by the development of +adventitious roots at their nodes wherever they come in contact with +the soil. The rooting stems of dewberries, the runners and stolons of +strawberries and currants, are familiar examples. + +Between the extremes of prostrate and upright, stems may be inclined +or bent in various degrees. As shown in Fig. 96, there are two modes +of inclination: _assurgent_, _a_, from the prostrate, _p_, toward +the upright, _e_; and _declined_, _d_, from the upright toward the +prostrate. Below the surface, _ps_, occur only underground stems. Is +the prostrate habit an advantageous one for light exposure? Can you +think of any compensating advantages a plant might derive from it; +for example, in regard to warmth and moisture? + +[Illustration: FIG. 97.—Twining stems: _A_, hop twining with the sun; +_B_, convolvulus twining against the sun.] + +=96. Climbing stems.=—These are such as lift themselves from the +ground and attain the advantages of the upright position by clinging +to supports of various kinds—usually, in a state of nature, the +stems and boughs of other plants. The means of climbing may be: +(1) by merely leaning upon or propping themselves up by the aid of +the supporting object—examples, the rose, wistaria, star jessamine +(_Jasminum officinalis_); (2) by coiling their main axes spirally +around the support—hop, bean, morning-glory; (3) by means of +adventitious roots—poison ivy, common English ivy, trumpet vine +(_Tecoma radicans_); (4) by organs specially developed for the +purpose, called tendrils—gourd, cucumber, grape, passion flower. + +[Illustration: FIG. 98.—Leaf of common pea, showing upper leaflets +reduced to tendrils.] + +=97. Tendrils.=—The part assigned to do the work of climbing may be +a secondary branch, a flower stem, a leafstalk, a leaf, a leaflet, +or a group of leaflets (Fig. 98). Tendrils behave in general very +much like twining stems, except that they are more sensitive and +respond more quickly to any cause that may influence their movement. +While young, their tips revolve just as do the tips of twining stems, +until they meet with an object round which they can coil. When this +happens, not only the part in contact with the object coils, but +the free part between it and the main axis will usually respond by +twisting itself into a helix (Fig. 99). As the distance between the +base and tip of the tendril is shortened by coiling, the body of +the plant is drawn upward proportionally. It will be observed that +the helix is interrupted at one or more points, above and below +which the coils turn in opposite directions. This is because the +tendril is attached at both ends and cannot adjust itself to the +opposite strains of torsion. Twist with your fingers a piece of tape +so attached, and you will see that on one side of your hand it turns +from right to left and on the other from left to right. + +=98. The cause of twining.=—Botanists are not fully agreed on this +point. The explanation most generally accepted at present is that +the twining of stems is due to the combined action of lateral and +negative geotropism (51). The first causes one side to grow more +rapidly than the other, thus forming a succession of coils, while +the second, by stimulating the upward growth of the axis, stretches +it into a spiral, and in this way draws it more tightly round the +support. For this reason twining stems do best on an upright support. + +[Illustration: FIG. 99.—Stems of a passion flower transformed into +tendrils. (_After_ GRAY.)] + +In tendrils, the twining is thought to be due not to gravity, but to +contact with a solid body, which, by inducing unequal development +on opposite sides of the tendril, causes it to turn about an +available object. The coiling of the free part of the twining +organ is in response to the stimulus transmitted from the part in +contact—_stimulus_, in this sense, denoting the influence of any +external agent that calls forth a responsive adjustment on the part +of the plant. + +[Illustration: FIG. 100.—Showing the economy of labor and building +material effected by the climbing habit. Notice how the grapevine +coils like an anaconda around the tree boles, and overtops their +tallest branches. Compare the diameter of the vine with that of the +trees.] + +=99. The object of the various habits of stem growth.=—To bring +the growing parts of the plant into the best possible relations +with light and air is one of the special functions of the stem, and +the various habits of growth described in this section have been +developed with reference to this function. In the case of prostrate +and underground stems other factors may intervene; can you name some +of the causes that might influence the position of the stem in such +cases? + + + Practical Questions + + 1. Why is the normal direction of most stems upright? (Exp. 56.) + + 2. Name a dozen woody-stemmed plants; a dozen with herbaceous stems. + + 3. Name all the plants you can think of that have prostrate stems, + or leaf rosettes that hug the earth, like mullein and dandelion. + Which of these are wintergreen plants? Which are hot-weather + growers? + + 4. Can you explain in what ways both hot-weather and cold-weather + plants may be advantaged by the habit of clinging close to the + earth? (94, 95.) + + 5. Is there any difference in the height of the stem of a dandelion + flower and a dandelion ball? + + 6. Of what advantage is this to the plant? (Exp. 17.) + + 7. Name all the means you can think of by which a stem may climb, + and give an example of each. + + 8. Why do we support peas with brush, and hops or beans with poles? + (98; Exp. 54.) + + 9. Are the vines of gourds, watermelons, squashes, and pumpkins + normally climbing or prostrate? How can you tell? (96, 97.) + + 10. Why does not the gardener provide them with poles or trellises + to climb on? + + 11. Do twining plants grow equally well on horizontal and upright + supports? (98; Exp. 54.) + + 12. If there is any difference, which do they seem to prefer? + + 13. Can you give any reasons for thinking that the climbing habit + might lead to parasitism? (83, 85, 87.) + + 14. What method of climbing would be most favorable to the + development of such a habit? (Suggestion: What mode of climbing + brings the stem into closest contact with its support?) + + 15. Name some plants the stems of which are used as food. + + 16. Name some from which gums and medicines are obtained. + + 17. Explain how it can benefit a plant to have its leaves, or some + of them, modified into tendrils. (99.) + + 18. In what way is the loss of the normal function of the leaves so + modified, compensated for? (Exp. 57.) + + 19. Suppose the vine shown in Fig. 100 had to lift itself without + the aid of a support; could it reach the same height and carry the + same weight of foliage and flowers with the same expenditure of + labor and building material? + + + II. MODIFICATIONS OF THE STEM + + MATERIAL.—A shoot of asparagus; thorny branches of locust, plum, or + haw; a cactus plant; bulbs of lily and hyacinth or onion; tubers of + potato; rootstocks of iris, fern, or violet. If fresh specimens are + not accessible, dried rootstocks of the sweet flag and Florentine + iris may be obtained at the drug stores under the names of calamus + and “orris” root. + +=100. How to recognize modified parts.=—Stems, like roots, are often +modified to serve other than their normal purpose, and in adapting +themselves to these new functions they sometimes undergo such changes +of form and structure that it would be impossible to recognize their +true nature from appearances alone. The safest tests in such cases +are: (1) by a comparison of the parts of the modified structure with +those of known organs of the same kind; and (2) by observing its +position in reference to other parts. For instance, we know that +the stem is the part of the plant which normally bears leaves and +flowers, and if either of these, or if the small scales which often +take the place of leaves, are found growing on any plant structure, +we may usually take for granted that it is a stem. Then, again, as +will be shown in the next chapter, buds and branches naturally appear +only at the nodes, in or near the _axil_, or inner angle made by a +leaf with the stem. Hence, if you see any growth springing from such +a position, you may generally conclude it to be a stem. + +[Illustration: FIG. 101.—Stem-leaves (cladophylls) of a ruscus, +bearing flowers.] + +=101. Stems as foliage.=—The connection between stem and leaf is so +intimate that we need not be surprised to find a frequent interchange +of function between them, the leaf, or some part of it, doing the +work of the stem (Fig. 98), the stem more often taking upon itself +the office of the leaf. A common example is the garden asparagus. +Examine one of the young shoots sold in the market, and notice that +it bears a number of small scales in place of leaves. On an older +shoot that has gone to seed, the green, threadlike appendages, which +are usually taken for foliage, will be found to spring each from the +axil of one of these scales. What, therefore, are we to conclude that +it is? + +In the butcher’s-broom of Europe, the transformation has gone so far +that the branches of the stem have assumed the flattened appearance +of leaves (Fig. 101), but their real nature is evident both from +their position in the axils of leaf scales, and from the fact that +they bear flower clusters in the axil of a scale on their upper face. +Another example of this sort of modification is seen in the pretty +little _myrsiphyllum_ of the greenhouses (wrongly called smilax), +which is so much used for decoration. The delicate green blades are +merely altered stems, shortened and flattened to simulate leaves. + +[Illustration: FIG. 102.—Thorn branches of _Holocantha Emoryi_, a +plant growing in arid regions.] + +=102. Weapons of defense.=—Conspicuous examples of these are the +bristling thorns of the honey locust. Is their frequent branching +any indication of their real nature? Does it _prove_ anything, or +must you look for other evidence? What further indications might +you expect to find, if they are true branching stems? (100.) On old +haw, plum, crab, and pear trees, stems can be found in all stages +of transition, from stubby, ill-developed branches, to well-defined +thorns. + +[Illustration: FIG. 103.—Melon cactus, showing greatly condensed stem +for the storage and preservation of moisture.] + +=103. Storage of nourishment.=—This is one of the most frequent +causes of modification in both roots and stems. Of stems that grow +above ground, the sugar cane probably comes first in economic +importance on this account. In hot, arid regions, where the moisture +drawn from the earth would, during prolonged drought, be too rapidly +dissipated by an expanded surface of leaves, the whole plant, as +in the case of the cactus, is sometimes compacted into a greatly +thickened stem, which fills the triple office of leaf, stalk, and +water reservoir. + +[Illustration: FIG. 104.—Rootstock of creeping panic grass.] + +=104. The uses of underground stems.=—It is in these that the storage +of nourishment most frequently takes place, and the modifications +that stems undergo for this purpose are in some cases so great that +their real nature becomes apparent only after a careful examination. +But while the chief function of underground stems is the storage of +nourishment, they serve other purposes also. In plants requiring a +great deal of moisture, like the ferns, and in others growing in dry +places and needing to husband moisture carefully, like the blackberry +lily, underground stems may be useful in preventing the too rapid +evaporation that would take place through aërial stems. Defense +against frost, cold, heat, and other dangers, as well as quickness of +propagation, are also attained or assisted by this means. + +=105. Rootstocks and rhizomes.=—From a prostrate stem like that +shown in Fig. 95 to a creeping rootstock like the one in Fig. 104, +the transition is so easy that we find no difficulty in accounting +for it. From the prostrate rootstock to the thickened storage +rhizome (Fig. 105) of such plants as the iris, puccoon, bulrush, and +Solomon’s-seal, is a longer step, but the bud with its leaf scales +at the growing tip, _a_, the remains of the flower stem at the node, +_b_, and the roots from the under surface sufficiently indicate its +nature. The peculiar scars from which the Solomon’s-seal takes its +name are caused by the falling away each year of the flowering stem +of the season after its work is done, leaving behind the node of the +underground stem from which it originated. In this way the rhizome +lives on indefinitely, growing and increasing at one end as fast as +it dies at the other. Test a little of the substance of the rhizome +with iodine. Of what does it consist? Of what use is it to the plant? + +[Illustration: FIG. 105.—Rhizome of Solomon’s-seal: _a_, growing bud +at the tip; _b_, remains of the past season’s flower stem; _c_, _c_, +_c_, scars of old stems. (_After_ GRAY.)] + +[Illustration: FIG. 106.—Potato tuber showing lenticels, _A_, _A_, or +pores for air on the surface; _S_, leaf scale, or scar.] + +=106. The tuber.=—A still further thickening and shortening of +the rhizome gives rise to the tuber, of which the potato and the +Jerusalem artichoke are familiar examples. Can you give any evidence +to show that the potato is a modified stem? Find the point of +attachment of the tuber to its stem and stand it on this end, which +is its natural base. Notice that the eye sits in the axil of the +little scale that forms the eyelid. What does the scale represent? +What is the eye? (100.) Do the scales occur in any regular order—that +is, opposite, or alternating with, each other, like the leaves on +a stem? Look on the surface for a number of small, lens-shaped +dots (_A_, _A_, Fig. 106) scattered irregularly over it. These are +aërating pores called _lenticels_, and are found in most dicotyl +stems. Does their presence help to throw light on the real nature +of the tuber? If any sprouts occur on your specimen, where do they +originate? Where do buds and sprouts originate on plants above +ground? Make a sketch of the outside of a potato, showing the +lenticels, eyes, and scales, or the scars left by the scales in case +they have fallen away, as has probably happened, if your specimen is +an old one. + +[Illustration: FIGS. 107, 108.—Transverse and longitudinal sections +of the potato: _A_, skin; _B_, cortical layer; _C_, outer pith layer; +_D_, inner pith layer.] + +Cut a small slice from the stem end of two potatoes, stand them in +coloring fluid for four or five hours, then divide into cross and +vertical sections, as shown in Figs. 107, 108, and draw, labeling the +parts that you can make out. Through which has the liquid ascended +most rapidly? Test with iodine and find out in which part nourishment +is most abundant. It is this abundant store of food that makes +the potato such a valuable crop in cold countries like Norway and +Iceland, where the seasons are too short to admit of the slow process +of developing the plant from the seed. + +[Illustration: FIG. 109.—Scaly bud of oak, enlarged.] + +[Illustration: FIG. 110.—Scaly bulb of lily (GRAY).] + +Compare a common potato with a sweet potato. Are there any eyes or +buds on the latter? Is there a scale below them? Do they occur in any +regular order? Do you see any lenticels? The common potato and the +sweet potato are both tubers; can you give some of the reasons why +the one is regarded as a modified branch, and the other as a root? +(100.) Compare their food contents; which contains most starch? Which +most sugar? How can you judge about the sugar without a chemical test? + +=107. The bulb= is a form of underground stem reduced to a single +bud. Get the scaly bulb of a lily, and sketch it from the outside +and in cross and vertical section. Compare it with the scaly winter +buds of the oak and hickory, or other common deciduous tree. Make an +enlarged sketch of the latter on the same scale as the lily bulb, and +the resemblance will at once become apparent. The scales of the bulb +are, in fact, only thick, fleshy leaves closely packed around a short +axis that has become dilated into a flat disk. From the center of +the disk, which is the terminal node of this transformed stem, rises +the flower stalk, or _scape_, as it is called, of the season. After +blossoming, the scape perishes with its bulb, and their place is +taken by new ones which are developed from the axils of the scales, +thus revealing their leaflike nature. + +[Illustration: FIG. 111.—Leaf of an onion divided lengthwise showing +the base enlarged into the coat of a bulb.] + +That bulbs are only modified buds is further shown by the bulblets +that sometimes appear among the flowers of the onion, and in the leaf +axils of certain lilies. They never develop into branches, but drop +off and grow into new plants just as the subterranean bulbs do. + +The bulbs of the onion and hyacinth are still further modifications, +in which the scales consist of the thickened bases of leafstalks that +are dilated until each one completely envelops the growing parts +within. + +=108. Morphology= is the part of botany that treats of the origin, +form, and uses of the different organs of plants, and of the +modifications they undergo in adapting themselves to changes of +condition or function. Organs or parts that have the same origin +but have become adapted to different functions, like the flattened +stems of the butcher’s-broom or the bulb scales of the lily, are +said to be _homologous_; those that are different in origin but +adapted to the same function, as the sweet and common potatoes, are +_analogous_. In other words, homologous organs are morphologically +alike, but may be physiologically different; analogous organs are +alike physiologically, but differ morphologically. + +=109. Economic value of stems.=—We probably get a greater variety +of economic products from the stem than from any other part of the +plant. Consider the vast amount of food stored in underground stems +like the potato; the resins, gums, and sugar found in the sap of +plants like the sugar cane, the pine, and India-rubber trees; the +medicines, dyes, and extracts obtained from the tissues; the valuable +fibers, such as flax, jute, and hemp, furnished by the bast; the wood +pulp for making paper; and the timber for building and furnishing +our houses that we get from the woody trunks of trees. When we think +of all these things, it seems hardly possible to overestimate the +importance of this part of the vegetable kingdom to man, or to exert +ourselves too strenuously to regulate and prevent the destruction of +these invaluable natural resources. + + + Practical Questions + + 1. Would you judge from the observations made in the foregoing + section, that the work of an organ determines its form, or that the + form determines its work? (99, 100, 108.) + + 2. Which is the more important, form or function? + + 3. Name some plants that are propagated by rootstocks; by runners + or stolons; by rhizomes; by tubers; by bulbs. + + 4. What is the advantage of propagating in this way over planting + the seed? (104, 106.) + + 5. Mention any other advantages that the various plants named may + gain from the development of their underground parts. (104.) + + 6. What makes the nut grass so troublesome to farmers in some parts + of the country? + + 7. Is its “nut” a root or a tuber? How can you tell? (106.) + + 8. Suggest some ways for destroying weeds that are propagated in + this way. + + 9. Could you get rid of wild onions in a pasture by mowing them + down? By digging them up? (107.) + + 10. Is it wise for farmers to neglect the appearance of such a weed + in their neighborhood, even though it does not infest their own + land? + + 11. Name any plants of your neighborhood, either wild or + cultivated, that are valued for their rhizomes; for their tubers. + + 12. What part of the plants named below do we use for food or other + purposes? Ginger, angelica, ginseng, cassava, arrowroot, garlic, + onion, sweet flag, iris, sweet potato, Cuba yam, artichoke. + + 13. Why are the true roots of bulbous and rhizome-bearing plants + generally so much smaller in proportion to the other parts than + those of ordinary plants? (89, 104.) + + 14. If the Canada thistle grows in your vicinity, examine the roots + and see if there is anything about them that will help to account + for its hardihood and persistency. + + 15. If you live in the region of the horse nettle (_Solanum + Carolinense_), explain how it is helped by its root system. (89.) + + + III. STEM STRUCTURE + + + A. MONOCOTYLS + + MATERIAL.—Fresh cornstalks with several well-developed nodes, some + of which should have stood in coloring fluid from 1 to 3 hours. If + fresh specimens cannot be obtained from the fields, a number of + seedlings may be grown in boxes of rich earth and cared for by the + pupils either at home or in the schoolroom; they should be planted + 4 or 5 weeks before needed. Asparagus and smilax sprouts may be + used, or the stem of any large grass, or of wheat and other grains, + but stalks of corn or sugar cane make the best subjects for study + where they can be obtained. + + APPLIANCES.—A compound microscope will be needed for detailed + study. Prepared slides can be used, but it is better for students + to make their own sections where practicable. + +[Illustration: FIG. 112.—Cross section of a cornstalk (reduced): _v_, +fibrovascular bundles; _c_, cortex; _p_, pith.] + +=110. Gross anatomy of a monocotyl stem.=—Obtain a fresh +cornstalk,—preferably one that has begun to tassel,—and observe its +external characters. How are the internodes divided from one another? +What is the use of the very firm, smooth epidermis? Notice a hollow, +grooved channel running down one side between the _joints_, or nodes; +does it occur in all of them? Is it on the same side or on the +opposite sides of alternate internodes? Follow one of these grooves +to the node from which it originates; what do you find there? After +studying the internal structure of the stalk, you will understand why +this groove should occur on the side of an internode bearing a bud or +fruit. + +Cut a cross section midway between two nodes, and observe the +composition of the interior; of what does the bulk of it appear to +consist? Notice the arrangement of the little dots, like the ends of +cut-off threads, that are scattered through the pith; where are they +most abundant, toward the center or the circumference? + +[Illustration: FIG. 113.—Vertical section of cornstalk (reduced): +_g_, groove; _c_, cortex; _v_, fibrovascular bundles mingled with +parenchyma; _b_, bud; _n_, node.] + +Make a vertical section through one of the nodes. Cut a thin slice +of the pith, hold it up to the light, and examine with a hand lens. +Observe that it is composed of a number of oblong cells packed +together like bricks in a wall. These are filled with protoplasm +and cell sap, and constitute what is known to botanists as the +_parenchyma_ or fundamental tissue from which all the other tissues +are derived. Apply the iodine test; in what parts does starch occur +most abundantly? + +Draw out one of the woody threads running through the pith. Break +away a bit of the epidermis, and see how very closely they are packed +on its inner surface. Trace the course of the veins in the bases +of the leaves; find their point of union with the stem; with what +part of it do they appear to be continuous? Has this anything to do +with the greater abundance of fibers near the epidermis? Can you +follow the fibers through the nodes, or do they become confused and +intermixed with other threads there? (If a stalk of sugar cane can be +obtained, the ring of scars left by the vascular bundles as they pass +from the leaves into the stem will be seen beautifully marked just +above the nodes.) + +If there is an eye or bud at the node, see if any of the threads go +into it. Can you account now for the depression that occurs in the +internode above the eye? + +Make drawings of both cross and vertical sections, showing the points +brought out in your examination of the cornstalk. + +=111. The vascular system.=—To find out the use of the threads that +you have been tracing, examine a piece of a living stem that has +stood in red ink for three to twenty-four hours. Notice the course +the coloring fluid has taken; what would you infer from this as to +the use of the woody fibers? + +These threads constitute what is called the _vascular system_ of the +stem, because they are made up of _vessels_ or _ducts_, along which +the sap is conveyed from the roots to the leaves and back from the +leaves to the parts where it is needed after it has contributed to +the elaboration of food. + +On account of this double line of communication which they have to +maintain, the vascular threads, or _bundles_, as they are technically +called, are double; one part composed of larger vessels, carrying +water up, the other consisting of smaller ones, bringing back the +food. Can you give a reason for their difference in size? + +[Illustration: FIG. 114.—Longitudinal section through the stem of a +palm, showing the curved course of the fibrovascular bundles (GRAY, +_after_ FALKENBERG).] + +=112. Woody monocotyls.=—Examine sections of yucca, smilax, or +of palmetto from the handle of a fan, and compare them with your +sketches of the cornstalk. In which are the vascular fibers most +abundant? Which is the toughest and strongest? Why? Trace the course +of the leaf fibers from the point of insertion to the interior. How +does it differ from that of the fibers in a cornstalk? + +=113. Growth of monocotyl stems.=—After tracing the course of the +leaf veins at the nodes of the cornstalk, you will have no difficulty +in identifying these veins as part of the vascular system. In jointed +stems like those of the corn and sugar cane and other grasses, their +intercalation between the vascular bundles of the stem takes place, +as we have seen, at the nodes, forming the hard rings known as +joints; but in other monocotyls the fibers entering the stem from the +leaves usually tend first downward, toward the interior (Fig. 114), +then bend outward, toward the surface, where they become entwined +with others and form the tough, inseparable cortex that gives to +palmetto and bamboo stems their great strength. Generally, monocotyl +stems do not increase in diameter after a certain point, and as +they can contain only a limited number of vascular fibers, they are +incapable of supporting an extended system of leaves and branches. +Hence plants of this class, with a few exceptions, like smilax and +asparagus, are characterized by simple, columnar stems and a limited +spread of leaves. Such plant forms are admirably adapted by their +structure to the purposes of mechanical support. It is a well-known +law of mechanics that a hollow cylinder is a great deal stronger than +the same mass would be in solid form, as may easily be tested by the +simple experiment of breaking in your fingers a cedar pencil and a +joint of cane or a stem of smilax of the same weight. In stems that +may be technically classed as solid in structure, like the corn and +palmetto, the interior is so light compared with the hard epidermis +that the result is practically a hollow cylinder. + +[Illustration: PLATE 4.—Forest of bamboo, showing the tall, straight, +branchless habit of monocotyl stems.] + +=114. Minute study of a monocotyl stem.=—Place under the microscope +a very thin transverse section of a cornstalk. The little dots that +looked like the cut ends of threads to the naked eye will now appear +as the complex group of cells shown in Fig. 115. The same parts are +shown longitudinally in Fig. 116. As seen in cross section, their +arrangement suggests a grotesque resemblance to the face of an old +woman wearing a pair of enormous spectacles and surrounded by a cap +frill of netting with very wide meshes. These are parenchyma cells, +_f_, _f_, Fig. 115, and constitute the greater portion of the living +tissues. + +[Illustration: FIG. 115.—Transverse section through the fibrovascular +bundle of a cornstalk: _a_, annular tracheid; _sp_, spiral tracheid; +_m_ and _m′_, ducts; _l_, air space; _v_, sieve tubes; _s_, companion +cells; _vg_, strengthening fibers; _cp_, bast; _f_, _f_, parenchyma.] + +[Illustration: FIG. 116.—Vertical section of the same; _a_ and _a′_, +rings of a decomposed annular tracheid; _v_, sieve tubes; _s_, +companion cells; _cp_, bast; _l_, air space; _vg_, strengthening +tissue; _sp_, spiral duct.] + +The two large openings, _m_, _m′_, that represent the spectacles, are +ducts for carrying water _up_ the stem. They are called pitted ducts +on account of the bordered pits which cover their outer surface. The +two smaller openings between and slightly below the pitted ducts are +also vessels for carrying liquids up the stem. The lower one, _a_, +is called the annular _tracheid_ because its tube is strengthened +by rings on the inside. The upper, smaller one, _sp_, is known as +the spiral tracheid, because its walls are reinforced by spiral +thickenings. Can you think what is the use of these strengthening +contrivances in the walls of conducting cells? (Suggestion: What is +the use of the spiral wire on a garden hose?) The large, irregular +opening below the ducts is an air space. What is its object? Why has +it no surrounding wall? + +Next look above the ducts for a group of rhomboidal or hexagonal +cells, _v_, _v_, with smaller ones, _s_, between them. The larger of +these are _sieve tubes_, the smaller ones, _companion cells_. The +sieve tubes carry sap _down_ the stem after it has been made into +food by the leaves. They get their name from the sievelike openings +between the connecting walls of the cells which form them—as if a row +of pepper boxes with perforations at both top and bottom were placed +end to end, so as to form a long tube divided into compartments +by perforated walls. Can you give a reason why the cells of ducts +that carry elaborated nutriment should have a more open line of +communication than those carrying crude sap? [56 (2).] Which one of +the organic food substances was shown by Exp. 39 to be unable, or +nearly so, to pass through the cell wall by osmosis? [56 (4).] The +conducting cells are surrounded by a mass of strengthening fibers +separating them from the parenchyma, _f_, and constituting with +them a _fibrovascular bundle_. The larger vessels, _m_, _m′_, _a_, +and _sp_, compose the _xylem_, the harder, more woody part of the +bundle, and the smaller ones, _v_, _s_, the _phloëm_, or softer part. +Notice also that there is no parenchyma in contact with the xylem +and phloëm in the fibrovascular bundles of a monocotyl, to supply +material for new growth, but they are entirely surrounded by a sheath +of strengthening tissue, whence such bundles are said to be _closed_, +and are incapable of further growth by the addition of new cells. + +[Illustration: FIG. 117.—Horizontal view of the sieve tube of a gourd +stem, showing perforations.] + +[Illustration: FIG. 118.—Side view of the sieve tube of a gourd +stem: _pr_, protoplasm layer; _u_, albuminous contents, forming +mucilaginous strand.] + + + B. HERBACEOUS DICOTYLS + + MATERIAL.—Young stems of sunflower, hollyhock, burdock, ragweed, + cocklebur, castor bean, or any large herbaceous plant. In schools + unprovided with compound microscopes, the minute anatomy can be + studied with some degree of profit by the aid of pictures. + +=115. Gross anatomy.=—Examine the outside of a young stem of +sunflower, burdock, or other herbaceous dicotyl. Notice whether it +is smooth, or roughened with hairs, scales, ridges, or grooves. If +hairy, observe the nature of the hairs, whether bristly, downy, +sticky, etc. Notice the color of the epidermis, whether uniform, or +splotched or striped with other colors, as, for example, jimson weed, +and pigweed (amarantus). If there are any buds, branches, or flower +stems, notice where they originate; what is the angle between the +leaf and stem called? (100.) + +Make a transverse cut through a portion of the stem that has stood +for a time in coloring fluid and examine with a lens. Four regions +can easily be distinguished: (1) the epidermis, _e_, Fig. 119; (2) +the primary cortex, _c_; (3) a ring of fibrovascular bundles, _f_; +and (4) a central cylinder of parenchyma, _p_. In some specimens +there will be a fifth region, the pith, which will appear in the +section as a white circular spot in the center of the parenchyma. + +[Illustration: FIG. 119.—Transverse section of a very young stem +of burdock, showing fibrovascular bundles not completely united +into a ring: _e_, epidermis; _c_, primary cortex; _f_, a ring of +fibrovascular bundles; _p_, central cylinder of parenchyma.] + +In specimens a little older than the one shown in Fig. 119, a narrow +circular line will be seen running through the ring of bundles nearly +midway between their inner and outer extremities, connecting them +into an unbroken circle around the central cylinder. This is the +_cambium_ layer, which supplies the vascular region with materials +for new growth, and thus enables dicotyl stems to increase in +diameter by the successive addition of fresh vascular rings from year +to year. + +Examine in the same way a vertical section, and find the parts +corresponding to those shown in Fig. 119. Make enlarged sketches of +both sections, labeling the various parts observed. + +=116. Minute structure of a dicotyl stem.=—Place successively under +a high power of the microscope thin transverse and longitudinal +sections of the stem just examined, or such other specimen as the +teacher may provide. Bring one of the fibrovascular bundles into the +field, and try to make out the parts shown in Figs. 120 and 121. +The corresponding parts in the two sections are indicated by the +same letters. Notice the cortex, _R_, on the outside and the pith, +_M_, on the inside; between these, the cambium, _C_, the _xylem_, +or woody tissue, included between the radiating lines _X_, and the +newer tissues composing the _phloëm_ between the lines _P_. The +cambium and pith, which includes the medullary rays so conspicuous +in perennial stems, are composed of live parenchyma cells, from +which alone growth can take place; they are the active part of the +stem. The xylem contains the large vessels, _t_ and _s_, that convey +water _up_ the stem, together with the wood fibers, _h_. These are +the permanent tissues. After completing their growth the cells of +the xylem gradually lose their protoplasm, and all vitality ceases. +Even the cell sap disappears, and sometimes the walls of the ducts +are disintegrated, leaving a mere air space like that shown at _l_ +in Figs. 115 and 116. The dead cells and tissues, however, are by +no means useless. They constitute the heartwood that is so valuable +for timber, and serve an important purpose as a mechanical support +for the stem. The phloëm contains on its outer face a mass of hard +fibers, _b_, called bast, and toward the interior, the sieve tubes, +_sb_, with a number of smaller vessels that convey _down_ the stem +the sap containing the food made in the leaves. It is separated from +the cortex by the bundle sheath, _e_, and on its other side, from +the exterior face of the xylem by the cambium, _C_. In this position +the growing cambium adds new cells to the inner side of the phloëm, +and to the outer side of the xylem, so that the former grows on its +inner face and the latter on its outer. In perennial plants, as new +rings are added to the xylem from season to season, the older ones +die and are changed into heartwood, which thus gradually increases in +thickness till in some of the giant redwoods and eucalypti, it may +attain a diameter of thirty-five or forty feet. In the phloëm, on +the other hand, as new cells are added from within, the older ones +are gradually changed into hard bast, _b_, then into bark, and are +finally sloughed off and fall to the ground. It is this free line of +communication with the active cambium that enables dicotyl stems to +grow on indefinitely, the sheath, _e_, being formed on the exterior +face of the bundles only, leaving the other free, whence they are +said to be _open_. + +[Illustration: FIGS. 120-121.—Transverse and longitudinal sections of +a fibrovascular bundle in the stem of a sunflower. The two sections +are lettered to correspond: _M_, pith (parenchyma); _X_, xylem +region; _P_, phloëm; _R_, cortex; _s_, spiral ducts; _s′_, annular +ducts; _t_, _t_, pitted ducts; _C_, cambium between the phloëm and +xylem regions; _sb_, sieve tubes; _b_, bast; _e_, bundle sheath; +_ic_, cambium (parenchyma) cells; _h_, wood fibers.] + +Make drawings of cross and vertical sections of a dicotyl stem as +it appears under the microscope, labeling correctly all the parts +observed. Show the shape and relative size of the different cells. +Compare your drawings with those made in your study of monocotyl +stems, and write in your notebook the essential points of difference +between the two. + +[Illustration: FIG. 122.—Internal structure of a pine stem, showing +longitudinal section of a fibrovascular bundle through a medullary +ray, _sm_, _sm′_; _s_, tracheids; _t_, bordered pits, surface view; +_c_, cambium; _v_, sieve tubes; _vt_, sieve pits, analogous to the +sieve plates in dicotyl stems.] + +[Illustration: FIG. 123.—Internal structure of a pine stem, showing +transverse section of a tracheid: _i_, cell walls; _m_, intermediate +layer between walls of adjoining cells; _m′_, intercellular space +here occupied by substance of intermediate layer; _b_, bordered pit +in section at right angles to the surface; _t_, membrane for closing +the pit canal.] + +=117. The stems of conifers=, the group of Gymnosperms to which the +pine belongs, do not differ greatly from those of dicotyls, the +chief difference being that the vascular bundles contain tracheids +only, corresponding to the smaller vessels of the phloëm, _s_ and +_s′_, shown in Fig. 121. These tracheids have large sunken places in +their walls, called bordered pits (Fig. 123), closed by a very thin +membrane through which water and dissolved food materials can more +readily percolate. In all other essentials, the internal structure of +pine stems is like that of dicotyls. (See Plate 5.) + + + C. WOODY STEMMED DICOTYL + + MATERIAL.—Elm, basswood, mulberry, leatherwood, and pawpaw show the + bast well; sassafras, slippery elm, and (in spring) hickory and + willow show the cambium; grape and trumpet vine, the ducts. Some + of the specimens used should be placed in coloring fluid from 3 to + 8 hours before the lesson begins. The rate at which the liquid is + absorbed varies with the kind of stem and the season. It is more + rapid in spring and slower in winter. If a cutting stands too long + in the fluid, the dye will gradually percolate through all parts of + it; care should be taken to guard against this. + +[Illustration: FIG. 124.—Part of a young China tree shoot, showing, +_A_, lenticels; _B_, leaf scar; _C_, _C_, traces left by the broken +ends of fibrovascular bundles that passed from the stem into the +leaf. Natural size.] + +=118. The external layer.=—While the primary structures, as shown +in the last section, are essentially the same in all dicotyl stems, +the continued yearly growth of perennials causes them to develop +a number of secondary structures and variations of detail that +differentiate them in a marked degree from soft-stemmed annuals. Take +a piece of a three-year-old shoot of cherry, horse chestnut, or any +convenient hardwood tree, and notice that the soft, green epidermis +has given place to a thicker, harder, and usually darker colored +bark. Notice the presence of lenticels (106) and their porous, corky +texture for the admission of air to the interior. They are slightly +raised above the surface of the bark, and are usually round, or more +or less elongated in different directions, according as they are +stretched vertically or horizontally by the growth of the axis. The +characteristic markings of birch bark, which make it so ornamental, +are due to the lenticels. In most trees they disappear on the older +parts, where the bark is constantly breaking away and sloughing off. + +[Illustration: PLATE 5.—Stem of a conifer, _Sequoia gigantea_, +Mariposa Grove, California. The first branch, 6 feet in diameter, +leaves the parent trunk 125 feet above the ground. The photographer +sitting on one of the exposed roots affords a good standard for +comparison. The tree is noted for its massive limbs. The smaller +trees in the background show the characteristic mode of branching in +trees of this class.] + +=119. Internal structures.=—Cut a transverse section through your +specimen, and notice under the epidermis a greenish layer of young +bark; beneath this a layer of rather tough, stringy bast fibers, and +beyond these a harder woody substance that constitutes the bulk of +the interior; within this, at the very center of the axis, we find +a cylinder of lighter texture, the pith, or medulla, occupying the +place of the soft parenchyma which fills this space in very young +stems. + +Between the woody axis and the bark notice a more or less soft and +juicy ring. + +=120. The cambium layer.=—This is not always easily distinguishable +with a hand lens, but is conspicuous in the stems of sassafras, +slippery elm, and aristolochia. If some of these cannot be obtained, +the presence of the cambium can be recognized by observing the +tendency of most stems to “bleed,” when cut, between the wood and +bark. The reason for this is because the cambium is the active part +of the stem, in which growth is taking place, and consequently it is +most abundantly supplied with sap. In spring, especially, it becomes +so full of sap that if a rod of hickory or elder is pounded, the +pulpy cambium is broken up and the bark may be slipped off whole from +the wood. + +=121. Medullary rays.=—Observe the whitish, silvery lines that +radiate in every direction from the center, like the spokes of a +wheel from the hub. These are the medullary rays, and consist of +threads of pith that serve as lines of communication between the +“central cylinder” and the growing cambium layer. In old stems the +central pith frequently disappears and its office is filled by the +medullary rays, which become quite conspicuous. + +[Illustration: FIGS. 125, 126.—Cross sections of twigs: 125, section +across a young twig of box elder, showing the four stem regions: _e_, +epidermis, represented by the heavy bounding line; _c_, cortex; _w_, +vascular cylinder; _p_, pith; 126, section across a twig of box elder +three years old, showing three annual growth rings, in the vascular +cylinder. The radiating lines (_m_), which cross the vascular region +(_w_), represent the pith rays, the principal ones extending from the +pith to the cortex (_c_). (_From_ COULTER’S “Plant Relations.”)] + +=122. Structural regions of a woody stem.=—Sketch cross and vertical +sections of your specimen, as seen under the lens, labeling the +different parts. Refer to Figs. 125, 126, if you have any difficulty +in distinguishing the parts. In a year-old shoot (Fig. 125), the +structural regions correspond closely to those shown in Fig. 119, +except that the ring of fibrovascular bundles is here compact and +woody, and crossed by the radiating lines of the medullary rays. In +a three-year-old shoot (Fig. 126), the main divisions are the same, +but the soft parenchyma of the central cylinder is replaced by the +pith, and the vascular ring is composed of three layers corresponding +to the three years of growth. In general, mature dicotyl stems may be +said to include four well-defined regions: (1) the epidermis, or the +bark; (2) the cortex, made up of bast and certain other tissues; (3) +the cambium; (4) the woody vascular cylinder, made up of concentric +rings, each representing a year’s growth. The pith, or medulla, +constitutes a fifth region, but is obvious only in young stems. +Notice the little pores or cavities that dot the woody part in the +cross section; where are they largest and most abundant? How are the +rings marked off from one another? These pores are the sections of +ducts. They are very large in the grapevine, and a cutting two or +three years old will show them distinctly. Examine sections of a twig +that has stood in red ink from three to twelve hours, and observe +the course the fluid has taken. How does this accord with the facts +observed in your study of the conducting tissues in monocotyl and +herbaceous stems? (111, 115, 116.) + +[Illustration: FIG. 127.—Diagram illustrating the annual growth of +dicotyledons.] + +=123. The rings= into which the woody cylinder is divided mark the +yearly additions to the growth of the stem, which increases by the +constant accession of new material to the outside of the permanent +tissues (116). The cambium constantly advances outward, beginning +every spring a new season’s growth, and leaving behind the ring of +ducts and woody fibers made the year before. As the work of the plant +is most active and its growth most vigorous in spring, the largest +ducts are formed then, the tissue becoming closer and finer as the +season advances, thus causing the division into annual rings that is +so characteristic of woody dicotyl stems. Each new stratum of growth +is made up of the fibrovascular bundles that supply the leaves and +buds and branches of the season. In this way we see that the increase +of dicotyl trunks and branches is approximately in an elongated cone +(Fig. 127), the number of rings gradually diminishing toward the top +till at the terminal bud of each bough it is reduced to a single one, +as in the stems of annuals. + +Sometimes a late autumn, succeeding a very dry summer, will cause +trees to take on a second growth, and thus form two layers of wood +in a single season. On this account we cannot always rely absolutely +upon the number of rings in estimating the age of a tree, though the +method is sufficiently exact for all practical purposes. + + + Practical Questions + + 1. Old Fort Moultrie near Charleston was built originally of + palmetto logs; was this good engineering or not? Why? (113.) + + 2. Explain the advantages of structure in a culm of wheat; a stalk + of corn; a reed. (113.) + + 3. Would the same quality be of advantage to an oak? Why, or why + not? + + 4. Is it of any advantage to the farmer that grain straw is so + light? + + 5. Explain why boys can slip the bark from certain kinds of wood in + spring to make whistles. (120.) + + 6. Why cannot they do this in autumn or winter? (123.) + + 7. Name some of the plants commonly used for this purpose. + + 8. Is the spring, after the buds begin to swell, a good time to + prune fruit trees and hedges? (120.) + + 9. What is the best time, and why? + + 10. Why are grapevines liable to bleed to death if pruned too late + in spring? (120, 123.) + + 11. Why are nurserymen, in grafting, so careful to make the cambium + layer of the graft hit that of the stock? (120.) + + 12. In calculating the age of a tree or bough from the rings of + annual growth, should we take a section from near the tip, or from + the base? Why? (123.) + + + IV. THE WORK OF STEMS + + MATERIAL.—Leafy shoots of grape, balsam, peach, or other active + young stems; a cutting of willow, currant, or any kind of easily + rooting stem. Two bottles of water and some linseed or cottonseed + oil. + +[Illustration: FIG. 128.—Experiment showing that moisture is thrown +off by the leaves of plants.] + + EXPERIMENT 58. DO THE LEAVES HAVE ANY ACTIVE PART IN EFFECTING THE + MOVEMENT OF SAP IN THE STEM?—Take two healthy young shoots of the + same kind—grape, peach, corn, tropæolum, calla lily absorb rapidly. + Trim the leaves from one shoot and close the cut surfaces with + a little vaseline or gardener’s wax to prevent loss of water by + evaporation. Place the lower end of each in a glass jar or tumbler + filled to the same height with water. Cut off _under water_ a + half inch from the bottom of each shoot, to get a fresh absorbing + surface. This is necessary because exposure to air for even a + second greatly hinders absorption by permitting the entrance of air + into the severed ends of the ducts. Pour a little oil on the water + in both jars to prevent evaporation. (Do not use kerosene; it is + injurious to plants.) At the end of twenty-four hours, which vessel + has lost the more water? How do you account for the difference? + + EXPERIMENT 59. WHAT BECOMES OF THE WATER THAT GOES INTO THE + LEAVES?—Cover the top of the vessel containing the leafy twig used + in the last experiment with a piece of cardboard, having first + cut a slit in one side, as shown in Fig. 128, so that it can be + slid into place without injuring the stem. Invert over the twig a + tumbler that has first been thoroughly dried, and leave in a warm, + dry place. After an hour or two, what do you see on the _inside_ of + the tumbler? Where did the moisture come from? + +[Illustration: FIG. 129.—A twig which had been kept standing in water +after the removal of a ring of cortical tissue: _a_, level of the +water; _b_, swelling formed at the upper denudation; _c_, roots.] + + EXPERIMENT 60. THROUGH WHAT PART OF THE STEM DOES THE SAP FLOW + UPWARD?—Remove a ring of the cortical layer from a twig of any + readily rooting dicotyl, such as willow, being careful to leave the + woody part, with the cambium, intact. Place the end _below_ the cut + ring in water, as shown in Fig. 129. The leaves above the girdle + will remain fresh. How is the water carried to them? How does this + agree with the movement of red ink observed in 115 and 122? + + EXPERIMENT 61. THROUGH WHAT PART DOES THE SAP COME DOWN?—Next prune + away the leaves and protect the girdled surface with tin foil, or + insert it below the neck of a deep bottle to prevent evaporation, + and wait until roots develop. Do they come more abundantly from + above or below the decorticated ring? + +=124. The three principal functions of the stem= are:—(1) to serve +as a mechanical support and framework for binding the other organs +together and bringing them into the best attainable relations with +light and air; (2) as a water carrier, or pipe line, for conveying +the sap from the roots to the parts where it is needed; and (3) as a +receptacle for the storage of foods. + +=125. Movement of water.=—It has already been shown (71, 111) that +a constant interchange of liquid is taking place through the stem, +between the roots, where it is absorbed from the ground, and the +leaves, where it is used partly in the manufacture of food. Just +what causes the rise of sap in the stem is one of the problems of +vegetable physiology that botanists have not yet been able to solve. +There are, however, certain forces at work in the plant, which, +though they may not account for all the phenomena of the movement, +undoubtedly influence them to a great extent. From experiments 58-61, +we can obtain an idea of what some of these forces may be. + +[Illustration: FIG. 130.—The stump of a large oak that was injured by +lightning many years ago. The interior is completely decayed, leaving +only a hollow shell of living tissue, from which branches continue to +put forth leaves year after year.] + +=126. Direction of the current.=—These experiments show that the +upward movement of crude sap toward the leaves is mainly through the +ducts in the woody portion of the stem, while the downward flow of +elaborated sap from the leaves takes place chiefly through the soft +bast and certain other vessels of the cortical layer. The action of +the leaves in giving off part of the water absorbed, as shown in Exp. +59, probably has also an important influence on the course of sap +movement. If loss of water takes place in any organ through growth or +other cause, the osmotic flow of the thinner sap from the roots will +set in that direction. + +=127. Ringing fruit trees.=—The course of the sap explains why +farmers sometimes hasten the ripening of fruit by the practice of +_ringing_. As the food material cannot pass below the denuded ring, +the parts above become gorged, and a process of forcing takes place. +The practice, however, is not to be commended, except in rare cases, +as it generally leads to the death of the ringed stem. The portion +below the ring can receive no nourishment from above, and will +gradually be so starved that it cannot even act as a carrier of crude +sap to the leaves, and so the whole bough will perish. + +[Illustration: FIG. 131.—Diagram showing general movement of sap.] + +=128. Sap movement not circulation.=—It must not be supposed that +this flow of sap in plants is analogous to the circulation of the +blood in animals, though frequently spoken of in popular language +as the “circulation of the sap.” There is no central organ like the +heart to regulate its flow, and the water taken up by the roots does +not make a continual circuit of the plant body as the blood does of +ours, but is dispersed by a process of general diffusion, partly into +the air through the leaves and partly through the plant body as food, +wherever it is needed. Figure 131 gives a good general idea of the +movement of sap in trees, the arrows indicating the direction of the +movement of the different substances. + +=129. Unexplained phenomena.=—Though the forces named above +undoubtedly exert a powerful influence over sap movement, their +combined action has not been proved capable of lifting the current +to a height of more than 200 feet, while in the giant redwoods of +California and the towering blue gums of Australia, it is known to +reach a height of more than 400 feet. The active force exerted by the +cell protoplasm has been suggested as an efficient cause, but as the +upward flow takes place through the cells of the xylem, which contain +no protoplasm (116), this explanation is inadequate, and we must be +content, in the present state of our knowledge, to accept the fact as +one which science has yet to account for. + + + Practical Questions + + 1. Why will a leafy shoot heal more quickly than a bare one? (125, + 126; Exp. 58.) + + 2. Why does a transverse cut heal more slowly than a vertical one? + (126, 127.) + + 3. Why does a ragged cut heal less rapidly than a smooth one? + + 4. Why does the formation of wood proceed more rapidly as the + amount of water given off by the leaves is increased? (126; Exp. + 59.) + + 5. Why do nurserymen sometimes split the cortex of young trees in + summer to promote the formation of wood? (116, 118.) + + 6. What is the advantage of scraping the stems of trees? + + 7. Explain the frothy exudation that often appears at the cut ends + of firewood, and the singing noise that accompanies it. [120, 124 + (2).] + + 8. Of what advantage is it to high climbing plants, like grape and + trumpet vine (_Tecoma_), to have such large ducts? (111, 116, 122.) + + 9. Why is the process of layering more apt to be successful if the + shoot is bent or twisted at the point where it is desired to make + it root? (127; Exps. 60, 61.) + + 10. Why do oranges become dry and spongy if allowed to hang on the + tree too long? (72, 126; Exps. 60, 61.) + + 11. Why will corn and fodder be richer in nourishment if, at + harvest, the whole stalk is cut down and both fodder and grain are + allowed to mature upon it? (126, 127; Exps. 60, 61.) + + 12. Is the injury done to plants by freezing due, as a general + thing, to mechanical, or to chemical action? (33.) + + 13. Why in pruning a branch is it best to make the cut just above a + bud? (Exps. 60, 61.) + + 14. Why is the rim of new bark, or callus, that forms on the upper + side of a horizontal wound, thicker than that on the lower side? + (126, 127; Exps. 60, 61.) + + 15. Why is it that the medicinal or other special properties of + plants are found mostly in the leaves and bark, or in the parts + immediately under the bark? (120, 126.) + + 16. Why does twisting the footstalk of a bunch of grapes, just + before ripening, make them sweeter? (127.) + +[Illustration: PLATE 6.—A white oak, one of the monarchs of the +dicotyl type. The owner of the ground on which this noble tree stands +left a clause in his will bequeathing it in perpetuity a territory +of 8 feet in every direction from its base. Refer to 89 and decide +whether such an amount of standing room is sufficient to secure the +preservation of this beautiful object.] + + 17. Is it a mere superstition to drive nails into the stems of plum + and peach trees to make them bear larger or more abundant fruit? + (126, 127.) + + 18. Why is a living corn stalk heavier than a dry one? (124.) + + 19. Why is a stalk of sugar cane heavier than one of corn? + Suggestion: Which is the heavier, pure water, or water holding + solids in solution? + + + V. WOOD STRUCTURE IN ITS RELATION TO INDUSTRIAL USES + + MATERIAL.—Select from the billets of wood cut for the fire, sticks + of various kinds; hickory, ash, oak, chestnut, maple, walnut, + cherry, pine, cedar, tulip tree, all make good specimens. Red + oak shows the medullary rays well. Get sticks of green wood, if + possible, and have them planed smooth at the ends. Collect also, + where they can be obtained, waste bits of dressed lumber from a + carpenter or joiner. If nothing better is available, any pieces of + unpainted woodwork about the schoolroom will furnish subjects for + study. + +=130. Detailed structure of a woody stem.=—Select a good-sized billet +of hard wood, and count the rings of annual growth. How old was the +tree or the bough from which it was taken? Was its growth uniform +from year to year? How do you know? Are the rings broader, as a +general thing, toward the center or the circumference? How do you +account for this? Is each separate ring of uniform thickness all the +way round? Mention some of the circumstances that might cause a tree +to grow less on one side than on the other. Are the rings of the same +thickness in all kinds of wood? Which are the more rapid growers, +those with broad or with narrow rings? Do you notice any difference +in the texture of the wood in rapid and in slow growing trees? Which +makes the better timber as a general thing, and why? + +=131. Heartwood and sapwood.=—Notice that in some of your older +specimens (cedar, black walnut, barberry, black locust, chestnut, +oak, Osage orange, show the difference distinctly) the central part +is different in color and texture from the rest. This is because +the sap gradually abandons the center (116, 123) to feed the outer +layers, where growth in dicotyls takes place; hence, the outer +part of the stem usually consists of sapwood, which is soft and +worthless as timber, while the dead interior forms the durable +heartwood so prized by lumbermen. The heartwood is useful to the +plant principally in giving strength and firmness to the axis. It +will now be seen why girdling a stem,—that is, chipping off a ring of +the softer parts all round, will kill it, while vigorous and healthy +trees are often seen with the center of the trunk entirely hollow. + +[Illustration: FIG. 132.—Cross section through a black oak, showing +heartwood and sapwood. (_From_ PINCHOT, U. S. Dept. of Agr.)] + +[Illustration: FIG. 133.—Vertical section through a black oak. +(_From_ PINCHOT, U. S. Dept. of Agr.)] + +[Illustration: FIGS. 134-136.—Diagrams of sections of timber: 134, +cross section; 135, radial; 136, tangential. (_From_ PINCHOT, U. S. +Dept. of Agr.)] + +=132. Different ways of cutting.=—In studying the vertical +arrangement of stems, two sections are necessary, a radial and a +tangential one. The former passes along the axis, splitting the stem +into halves (Fig. 135); the latter cuts between the axis and the +perimeter, splitting off a segment from one side (Fig. 136). The +appearance of the wood used in carpentry and joiner’s work is due +largely to the manner in which the planks are cut. + +=133. The cross cut.=—The section seen at the end of a log (Figs. +132, 134) is called by carpenters a cross cut. It passes at +right angles to the grain of the wood, and severs what important +structures? (116, 119, 122.) Examine a cross cut at the end of a +rough plank, or the top of a stump or an old fence post, and tell why +this kind of cut is seldom used in carpentry. + +[Illustration: FIG. 137.—Tangential section of mountain ash, showing +ends of the medullary rays.] + +=134. The tangent cut= is so called because it is made at right +angles to the radius of a log. Repeat the geometrical principle upon +which such a cut is described as “tangential.” It passes through +the medullary rays and the annual rings diagonally (Fig. 136), and +is the cheapest way of cutting timber, since the entire log is made +into planks and there is no waste except the “slabs” and “edgings,” +as shown in Fig. 138. The cut ends of the medullary rays appear on +the surface as small lines or slits (Fig. 137), and give to this kind +of plank its peculiar graining. The wavy or “watered” appearance of +the annual rings (Figs. 133, 136, 140, 141), so often seen in cheap +furniture and in the woodwork of cheaply constructed houses, is +caused by the tangential cut, which strikes them at various angles. + +[Illustration: FIG. 138.—Diagram to show the common method of sawing +a log. The circles represent rings of annual growth: _R_, _R_, +diameter of the log; _r_, _r_, _r_ and _t_, _t_, _t_, boards cut +perpendicular to it, giving for the two or three central ones radial, +for the others, tangential, cuts. The waste portions are the “slabs” +and “edgings,” shown in the dark segments at _R_, _R_, and the small +triangular blocks, _e_, _e_, _e_.] + +[Illustration: FIG. 139.—Diagram illustrating the “quartered” cut: +_d_, _d_ and _d′_, _d′_, radial cuts (diameters) by which the log is +“quartered”; _c_, center of the log; _r_, _r_, radii passing through +the middle of each quarter, parallel to which the planks _t_, _t_, +_t_ are cut. The circles represent rings of annual growth.] + +=135. The radial, or quartered cut=, familiar to most of us in the +“quartered oak” of commerce, passes through the center of the log +and cuts the rings of annual growth perpendicularly, giving it the +“striped” appearance (Fig. 135) seen in the best woodwork. It gets +its name from the practice of dealers in first sawing a log into +quarters and then cutting parallel to the radius passing through the +middle of each quarter, as shown in Fig. 139. In this way each cut +strikes the rings perpendicularly, but except in the case of very +large logs, only narrow planks can be obtained in this manner. A +better way of treating small logs is shown in Fig. 138, where the +three central planks, _r_, _r_, _r_, on and near the diameter, will +give the “quartered” effect, while the rest can be used for the +cheaper tangential cuttings. Examine a piece of quartered board, or a +log of wood that has been split down the center, and notice that the +medullary rays appear as silvery bands or plates (Figs. 140, 141). +This is because the cut runs parallel to them. It is the medullary +rays chiefly that give to commercial woods their characteristic +graining. Knots, buds, and other adventitious causes also influence +it in various degrees. + +[Illustration: FIG. 140.—Sections of sycamore wood: _a_, tangential; +_b_, radial; _c_, cross. (_From_ PINCHOT, U. S. Dept. of Agr.)] + +[Illustration: FIG. 141.—Section of white pine wood. (_From_ PINCHOT, +U. S. Dept. of Agr.)] + +[Illustration: FIG. 142.—Section of tree trunk showing knot.] + +=136. The swelling and shrinking of timber.=—The capacity possessed +by certain substances of bringing about an increase of volume by +the absorption of liquids is termed _imbibition_. Care must be +taken not to confound imbibition with capillarity. (Exp. 53.) When +liquids are carried into a body by capillary attraction, they merely +fill up vacant spaces already existing between small particles of +the substance, and therefore do not cause any swelling or increase +in size. When imbibition takes place, the _molecules_, or chemical +units of the liquid, force their way between those of the imbibing +substance, and thus, in making room for themselves, bring about an +increase in volume of the imbibing body. To this cause is due the +alternate swelling and shrinking of timber in wet and dry weather. + +[Illustration: FIGS. 143-144.—Diagrams of tree trunks, showing knots +of different ages: 143, from tree grown in the open; 144, from tree +grown in a dense forest.] + +=137. Knots.=—Look for a billet with a knot in it. Notice how the +rings of growth are disturbed and displaced in its neighborhood. +If the knot is a large one, it will itself have rings of growth. +Count them, and tell what its age was when it ceased to grow. Notice +where it originates. Count the rings from its point of origin to the +center of the stem. How old was the tree when the knot began to form? +Count the rings from the origin of the knot to the circumference +of the stem; how many years has the tree lived since the knot was +formed? Does this agree with the age of the knot as deduced from its +own rings? As the tree may continue to live and grow indefinitely +after the bough which formed the knot died or was cut away, there +will probably be no correspondence between the two sets of rings, +especially in the case of old knots that have been covered up and +embedded in the wood. The longer a dead branch remains on a tree the +more rings of growth will form around it before covering it up, and +the greater will be the disturbance caused by it. Hence, timber trees +should be pruned while very young, and the parts removed should be +cut as close as possible to the main branch or trunk. Sometimes knots +injure lumber very much by falling out and leaving the holes that are +often seen in pine boards. In other cases, however, when the knots +are very small, the irregular markings caused by them add greatly to +the beauty of the wood. The peculiar marking of bird’s-eye maple is +caused by abortive buds buried in the wood. + + + Practical Questions + + 1. Is the swelling of wood a physical or a physiological process? + + 2. Does wood swell equally with the grain and across it? + (Suggestion: test by keeping a block under water for 10 to 20 days, + measuring its dimensions before and after immersion.) + + 3. In building a fence, what is the use of “capping” the posts? + (133.) + + 4. In laying shingles, why are they made to touch, if the work is + done in wet weather, and placed somewhat apart, if in dry weather? + (136.) + + 5. What is the difference between timber and lumber? Between a + plank and a board? Between a log, stick, block, and billet? + + 6. Why does sapwood decay more quickly than heartwood? (131.) + + 7. Explain the difference between osmosis, diffusion, capillarity, + and imbibition. (9, 56, 57, 136; Exp. 53.) + + + VI. FORESTRY + +=138. Practical bearings.=—This part of our subject is closely +related to lumbering and forestry. The business of the lumberman is +to manufacture growing trees into merchantable timber, and to do this +successfully he must understand enough about the structure of wood +to cut his boards to the best advantage, both for economy and for +bringing out the grain so as to produce the most desirable effects +for ornamental purposes. + +[Illustration: PLATE 7.—Timber tree spoiled by standing too much +alone in early youth. Notice how the crowded young timber in the +background is righting itself, the lower branches dying off early +from overshading, leaving tall, straight, clean boles. (_From_ +PINCHOT, U. S. Dept. of Agr.)] + +[Illustration: FIG. 145.—After the forest fire.] + +=139. Forestry has for its object=: (1) the preservation and +cultivation of existing forests; (2) the planting of new ones, or +the reforestation of tracts from which the timber has been destroyed. +Forests may be either _pure_, that is, composed mainly of one kind of +tree, as a pine or a fir wood; or _mixed_, being made up of a variety +of different growths, as are most of our common hardwood forests. + +[Illustration: FIG. 146.—Oyster fungus on linden.] + +=140. Enemies of the forest.=-The first step in the preservation of +our forests is to know the dangers to be guarded against. The chief +of these are: (1) fires; (2) the ignorance or recklessness of man in +cutting for commercial purposes; (3) fungi; (4) injurious insects; +(5) sheep, hogs, and other animals that eat the seeds and the young, +tender growth. + +=141. How to protect the forests.=—The annual destruction of forests +by fires probably exceeds that from all other causes combined. The +only effectual safeguard against this danger is watchfulness on the +part of _everybody_. We can each one of us help in this work by at +least being careful ourselves never to kindle a fire in the woods +without taking every precaution against its spreading. A single +match, or the glowing stump of a cigar, carelessly thrown among dry +leaves or grass, may start a conflagration that will destroy millions +of dollars’ worth of standing timber. + +To prevent the spread of fungi, dead trees should be removed, and +broken or decayed branches trimmed off and the cut surfaces painted. +Birds which destroy insects should be protected; sheep and hogs +should be kept out, and dead leaves left on the ground to cover the +roots and fertilize the soil with the humus created by their decay. +Finally, none but mature trees should be cut for industrial purposes, +and the cutting ought to be done in such a way that the young +surrounding growth will not be injured by the falling trunks. + +=142. The usefulness of forests.=—Aside from the value of their +products, forests are useful in many other ways. They influence +climate beneficially by acting as windbreaks, by giving off moisture +(Exp. 58), by shading the soil, and thus preventing too rapid +evaporation. Their roots also help to retain the water in the soil, +and by this means tend to prevent the washing of the land by heavy +rains and to restrain the violence of freshets. + +=143. Forests and water supply.=—It is especially important that +the watershed of any region should be well protected by forests, +to prevent contamination of the streams and to insure an unfailing +supply of water by checking the escape of the rainfall from the soil. + + + Practical Questions + + 1. Explain the difference between a forest, grove, copse, wood, + woodland. + + 2. In pruning a tree why ought the branch to be cut as close to the + stock as possible? (137.) + + 3. Name the principal timber trees of your neighborhood. What gives + to each its special value? + + 4. Name six trees that produce timber valuable for ornament; for + toughness and strength. + + 5. Which is the better for timber, a tree grown in the open, or one + grown in a forest, and why? (Plate 7.) + + 6. What are the objects to be attained in pruning timber trees? + Orchard and ornamental trees? + + 7. Is the outer bark of any use to a tree, and if so, what? + + 8. Why should pruning not be done in wet weather? [140 (3), 141.] + + 9. Why should vertical shoots be cut off obliquely? [133, 140 (3), + 141.] + + + Field Work + + (1) Make a study of the various climbing plants of your + neighborhood with reference to their modes of ascent, and the + effect, injurious, or other, upon the plants to which they attach + themselves. Note the origin and position of tendrils, and try to + make out what modification has taken place in each case. Consider + the twining habit in reference to parasitism, especially in the + case of soft-stemmed twiners when brought into contact with + soft-stemmed annuals. Observe the various habits of stem growth: + prostrate, declined, ascending, etc., and decide what adaptation to + circumstances may have influenced each case. + + (2) Notice the shape of the different stems met with, and learn + to recognize the forms peculiar to certain of the great families. + Observe the various appliances for defense and protection with + which they are provided, and try to find out the meaning of the + numerous grooves, ridges, hairs, prickles, and secretions that are + found on stems. Always be on the alert for modifications, and learn + to recognize a stem under any disguise, whether thorn, tendril, + foliage, water holder, rootstock, or tuber. + + (3) Note the color and texture of the bark of the different trees + you see and learn to distinguish the most important kinds: + + (_a_) scaly—peeling off annually in large plates, as sycamore, + shagbark-hickory; + + (_b_) fibrous—detached in stiff threads and fibers, as grape; + + (_c_) fissured—split into large, irregular cracks by the growth + of the stem in thickness, as oak, chestnut, and most of our large + forest trees; + + (_d_) membranous—separating in dry films and ribbons, as common + birch (_Betula alba_). + + Observe the difference in texture and appearance of the bark on + old and young boughs of the same species. Try to account for the + varying thickness of the bark on different trees and on different + parts of the same tree. Notice the difference in the timber of + the same species when grown in different soils, at different ages + of the tree, and in healthy and weakly specimens. Find examples + of self-pruning trees (Plate 7), and explain how the pruning was + brought about. + + (4) Select a small plot, about a fourth of an acre, of any wooded + tract in your neighborhood, and make a study of all the trees and + shrubs it contains. Make a list of the different kinds, with the + number of each. Take note of those that show themselves, by vigor + and abundance of growth, best adapted to the situation. These are + the “climax” or dominant vegetation of the plot. Find out, if you + can, to what cause their superiority is due. + +[Illustration: PLATE 8.—The American elm—a perfect type of +deliquescent branching.] + + + + +CHAPTER V. BUDS AND BRANCHES + + + I. MODES OF BRANCHING + + MATERIAL.—For determinate growth, have twigs of an alternate and + an opposite-leaved plant showing well-developed terminal buds: + hickory, sweet gum, cottonwood, poplar, chestnut, are good examples + of the first; maple, ash, horse-chestnut, viburnum, of the second; + for the two-forked kind, mistletoe, buckeye, horse-chestnut, jimson + weed, lilac. For showing indefinite growth: rose, willow, sumach, + and ailanthus are good examples. Gummy buds, like horse-chestnut + and poplar, should be soaked in warm water before dissecting, to + soften the gum; the same treatment may be applied when the scales + are too brittle to be handled without breaking. Buds with heavy fur + on the scales cannot very well be studied in section; the parts + must be taken out and examined separately. + +[Illustration: FIG. 147.—Diagram of excurrent growth.] + +[Illustration: FIG. 148.—Diagram of deliquescent growth.] + +=144. Modes of branching.=—Compare the arrangement of the boughs +on a pine, cedar, magnolia, etc., with those of the elm, maple, +apple, or any of our common deciduous trees. Draw a diagram of each, +showing the two modes of growth. The first represents the _excurrent_ +kind, from the Latin _excurrere_, to run out; the second, in which +the trunk seems to divide at a certain point and flow away, losing +itself in the branches, is called _deliquescent_, from the Latin +_deliquescere_, to melt or flow away. The great majority of stems, +as a little observation will show, present a combination of the two +modes. + +[Illustration: FIG. 149.—Winter twig of sugar maple: _t_, terminal +bud; _ax_, axillary buds; _ls_, leaf scars; _tr_, leaf traces; _l_, +lenticels; _rs_, ring of scars left by bud scales of preceding +season.] + +=145. Terminal and axillary buds.=—Notice the large bud at the end of +a twig of hickory, sweet gum, beech, cottonwood, etc. This is called +the _terminal_ bud because it terminates its branch. Notice the +scars left by the leaves of the season as they fell away, and look +for small buds just above them. These are _lateral_, or _axillary_, +buds, so called because they spring from the axils of the leaves. How +many leaves did your twig bear? What difference in size do you notice +between the terminal and lateral buds? + +=146. The leaf scars.=—Examine the leaf scars with a hand lens, +and observe the number and position of the little dots in them. +Ailanthus, varnish tree, sumach, and China tree show these very +distinctly. They are called _leaf traces_, and mark the points where +the fibrovascular bundles from the leaf veins passed into the stem. +Look on the bark, or epidermis, for lenticels. + +=147. Bud scales and scars.=—Notice the stout, hard scales by which +the winter buds are covered in most of our hardy trees and shrubs. +Remove these from the terminal one of your specimen, and notice the +ring of scars left around the base. Look lower down on your twig +for a ring of similar scars left from last year’s bud. Is there any +difference in the appearance of the bark above and below this ring? +If so, what is it, and how do you account for it? Is there more than +one of these rings of scars on your twig, and if so, how many? How +old is the twig and how much did it grow each year? Has its growth +been uniform, or did it grow more in some years than in others? + +[Illustration: FIG. 150.—Diagram of opposite bud scales.] + +=148. Arrangement and use of the scales.=—Notice the manner in +which the scales overlap so as to “break joints,” like shingles +on the roof of a house. Where the leaves are opposite, the manner +of superposition is very simple. Remove the scales one by one, +representing the number and position of the pairs by a diagram +after the model given in Fig. 150. In the bud of an alternately +branched twig the order will be different, and the diagram must be +varied accordingly. Do you observe any difference as to size and +texture between the outer and inner scales? Notice how the former +inclose the tenderer parts within like a protecting wall. In cold +climates the outer scales are frequently coated with gum, as in the +horse-chestnut, for greater security against the weather. The hickory +and various other trees have the inner scales covered with fur or +down that envelops the tender bud like a warm blanket. + +=149. Nature of the scales.=—The position of the scales shows +that they occupy the place of leaves or of some part of a leaf. +In expanding buds of the lilac and many other plants, they can be +found in all stages of transition, from scales to true leaves. In +the buckeye and horse-chestnut, they will easily be recognized as +modified leaf stalks (Fig. 151). In the tulip tree, magnolia, India +rubber tree, fig, elm, and many others, they represent appendages +called _stipules_, often found at the bases of leaves. (See 165, +166.) In this case a pair of scales is attached with each separate +leaflet, and as the growing axis lengthens in spring, they are +carried apart by the elongation of the internodes so that the scars +are separated, a pair at each node, making rings all along the stem, +as shown in Fig. 152, instead of having them compacted into bands at +the base of the bud. These scars are sometimes very persistent, and +in the common fig and magnolia may often be traced on stems six to +eight years old. Do they furnish any indication as to the relative +age of the different parts of the stem, like the bands of scars on +twigs of horse-chestnut and hickory? Give a reason for your answer. +(Fig. 152.) + +[Illustration: FIG. 151.—Development of the parts of the bud in the +buckeye. (_After_ GRAY.)] + +[Illustration: FIG. 152.—Stem of tulip tree: _s_, _s_, scars left by +stipular scales; _l_, _l_, leaf scars.] + +=150. Different rates of growth.=—Notice the very great difference +between branches in this respect. Sometimes the main stem will have +lengthened from twenty to fifty centimeters or more in a single +season, while some of the lateral ones will have grown but an inch +or two in four or five seasons. One reason for this is because the +terminal bud, being on the great trunk line of sap movement, gets +a larger share of nourishment than the others, and being stronger +and better developed to begin with, starts out in life with better +chances of success. + +Make a drawing of your specimen, showing all the points brought out +in the examination just made. Cut sections above and below a set of +bud scars and count the rings of annual growth in each section. What +is the age of each? How does this agree with your calculation from +the number of scar clusters left by the bud scales? + +=151. Irregularities.=—Take a larger bough of the same kind that you +have been studying, and observe whether the arrangement of branches +on it corresponds with the arrangement of buds on the twig. Did +all the buds develop into branches? Do those that did develop all +correspond in size and vigor? If all the buds developed, how many +branches would a tree produce every year? + +In the elm, linden, beech, hornbeam, hazelnut, willow, and various +other plants, the terminal bud always dies and the one next in order +takes its place, giving rise to the more or less zigzag axis that +generally characterizes trees of these species. (Fig. 153.) + +[Illustration: FIG. 153.—Bud development of beech: _a_, as it is, +many buds failing to develop; _b_, as it would be if all the buds +were to live.] + +=152. Forked stems.=—Take a twig of buckeye, horse-chestnut, or +lilac, and make a careful sketch of it, showing all the points that +were brought out in the examination of your previous specimen. Which +is the larger, the lateral or the terminal bud? Is their arrangement +alternate or opposite? What was the leaf arrangement? Count the leaf +traces in the scars; are they the same in all? If all the buds had +developed into branches, how many would spring from a node? Look for +the rings of scars left by the last season’s bud scales. Do you find +any twig of more than one year’s growth, as measured by the scar +rings? + +[Illustration: FIG. 154.—Two-forked twig of horse-chestnut.] + +Look down between the forks of a branched stem for a round scar. +This is not a leaf scar, as we can see by its shape, but one left by +the last season’s flower cluster. The flower, as we know, dies after +perfecting its fruit, and so a flower bud cannot continue the growth +of its axis as other buds do, but has just the opposite effect and +stops all further growth in that direction. Hence, stems and branches +that end in a flower bud cannot continue to develop their main axis, +but their growth is usually carried on, in alternate-leaved stems, by +the nearest lateral bud, or in opposite-leaved ones, by the nearest +pair of buds. In the first case there results the zigzag spray +characteristic of such trees as the beech and elm (Fig. 155, _B_); in +the second, the two-forked, or _dichotomous_ branching, exemplified +by the buckeye, horse-chestnut, jimson weed, mistletoe, and dogwood +(Fig. 155, _A_). + +Draw a diagram of the buckeye, or other dichotomous stem, as it would +be if all the buds developed into branches, and compare it with +your diagrams of excurrent and deliquescent growth. Draw diagrams +to illustrate the branching of the elm, beech, lilac, linden, rose, +maple, or their equivalents. + +[Illustration: FIG. 155.—Diagrams of two-forked branching. The +pointed bodies in the forks shows where terminal flower buds or +flower clusters have changed the direction of growth.] + +=153. Definite and indefinite annual growth.=—The presence or absence +of terminal buds gives rise to another important distinction in +plant development—that of _definite_ and _indefinite_ annual growth. +Compare with any of the twigs just examined, a branch of rose, honey +locust, sumac, mulberry, etc., and note the difference in their +modes of termination. The first kind, where the bough completes its +season’s increase in a definite time and then devotes its energies to +developing a strong terminal bud to begin the next year’s work with, +are said to make a _definite or determinate annual growth_. Those +plants, on the other hand, which make no provision for the future, +but continue to grow till the cold comes and literally nips them in +the bud, are _indefinite_, or _indeterminate_ annual growers. Notice +the effect of this habit upon their mode of branching. The buds +toward the end of each shoot, being the youngest and tenderest, are +most readily killed off by frost or other accident, and hence new +branches spring mostly from the older and stronger buds near the base +of the stem. It is their mode of branching that gives to plants of +this class their peculiar bushy aspect. Such shrubs generally make +good hedges on account of their thick undergrowth. The same effect +can be produced artificially by pruning. + +[Illustration: FIG. 156.—A mixed wood in winter, showing the trend of +the branches.] + +=154. Differences in the branching of trees.=—We are now prepared +to understand something about the causes of that endless variety in +the spread of bough and sweep of woody spray that makes the winter +woods so beautiful. Where the terminal bud is undisputed monarch +of the bough, as in the pine and fir, or where it is so strong and +vigorous as to overpower its weaker brethren and keep the lead, as +in the magnolia, tulip tree, and holly, we have excurrent growth. In +plants like the oak and apple, where all the buds have a more nearly +equal chance, the lateral branches show more vigor, and the result is +either deliquescent growth, or a mixture of the two kinds. In the elm +and beech, where the usurping pseudo-terminal bud keeps the mastery, +but does not completely overpower its fellows, we find the long, +sweeping, delicate spray characteristic of those species. Examine a +sprig of elm, and notice further that the flower buds are all down +near the base of the stem, while the leaf buds are near the tip. The +chief development of the season’s growth is thus thrown toward the +end of the branch, giving rise to that fine, feathery spray which +makes the elm an even more beautiful object in winter than in summer +(Fig. 158). + +An examination of the twigs of other trees will bring out the various +peculiarities that affect their mode of branching. The angle, for +instance, which a twig makes with its bough has a great effect in +shaping the contour of the tree. Compare in this respect the elm and +hackberry; the tulip tree and willow; ash and hickory. As a general +thing, acute angles produce slender, flowing effects; right or obtuse +angles, more bold and rugged outlines. + +[Illustration: FIG. 157.—Winter spray of ash, an opposite-leaved +tree.] + +[Illustration: FIG. 158.—Winter spray of elm.] + + + Practical Questions + + 1. Has the arrangement of leaves on a twig anything to do with the + way a tree is branched? (145, 151, 152.) + + 2. Why do most large trees tend to assume the excurrent, or axial, + mode of growth if let alone? (150, 154.) + + 3. If you wished to alter the mode of growth, or to produce what + nurserymen call a low-headed tree, how would you prune it? (152, + 153.) + + 4. Would you top a timber tree? (152, 153.) + + 5. Are low-headed or tall trees best for an orchard? Why? + + 6. Why is the growth of annuals generally indefinite? + + 7. Name some trees of your neighborhood that are conspicuous for + their graceful winter spray. + + 8. Name some that are characterized by sharpness and boldness of + outline. + + 9. Account for the peculiarities in each case. + + + II. BUDS + + MATERIAL.—Expanding leaf and flower buds in different stages of + development; large ones show the parts best and should be used + where attainable. Some good examples for the opposite arrangement + are horse-chestnut, maple, lilac, ash; for the alternate: hickory, + sweet gum, balsam poplar, beech, elm. Where material is scarce, the + twigs used in the last section may be placed in water and kept till + the buds begin to expand. + +=155. Folding of the leaves.=—Remove the scales from a bud of +horse-chestnut nearly ready to open, and notice the manner in +which the young leaves are folded. This is called _vernation_, or +_prefoliation_, words meaning respectively “spring condition” and +“condition preceding the leaf.” Leaves are packed in the bud so as to +occupy the least space possible, and in different plants they will be +found folded in a great many different ways, according to the shape +and texture of the leaf and the space available for it in the bud. +When doubled back and forth like a fan, or crumpled and folded as in +the buckeye, horse-chestnut, and maple, the vernation is _plicate_ +(Figs. 160, 162). + +[Illustration: FIG. 159.—Expanding bud of English walnut, showing +twice conduplicate vernation.] + +[Illustration: FIG. 160.—A partly expanded leaf of beech, showing +plicate-conduplicate vernation.] + +=156. Position of the flower cluster.=—What do you find within the +circle of leaves? Examine one of the smaller axillary buds, and +see if you find the same object within it. If you are in any doubt +as to what this object is, examine a bud that is more expanded, +and you will have no difficulty in recognizing it as a rudimentary +flower cluster. Notice its position with reference to the scales and +leaves. If at the center of the bud, it will, of course, terminate +its axis when the bud expands, and the growth of the branch will +culminate in the flower. The branching of any kind of stem that +bears a central flower cluster must, then, be of what order? Compare +your drawings with the section of a hyacinth bulb or jonquil, and +note the similarity in position of the flower clusters. In a bud +of the hickory, walnut, oak, etc., the position of the flower +clusters is different from that of flowers in the buds of lilac +and horse-chestnut. Look for a bud containing them, and find out +where they occur. Can the axis continue to grow after flowering, +in this kind of stem? Give a reason for your answer. Make sketches +in transverse and longitudinal section (see Figs. 162, 163) of two +different kinds of buds, illustrating the terminal and axillary +position of the flower cluster. + +[Illustration: FIGS. 161, 162.—Buds of maple: 161, vertical section +of a twig; 162, cross section through bud, showing folded leaves in +center and scales surrounding them.] + +[Illustration: FIG. 163.—Vertical section of hickory bud: _a_, furry +inner scales; _b_, outer scales; _l_, folded leaf; _r_, receptacle.] + +=157. Dormant buds.=—A bud may often lie dormant for months or even +years, and then, through the injury or destruction of its stronger +rivals, or some other favoring cause, develop into a branch. Such +buds are said to be _latent_ or _dormant_. The sprouts that often put +up from the stumps of felled trees originate from this source. + +[Illustration: FIG. 164.—Twig of red maple, showing supernumerary +bud, _b_; _rs_, ring of scars left by last year’s bud scales. +(_After_ GRAY.)] + +=158. Supernumerary buds.=—Where more than one bud develops at a +node, as is so often the case in the oak, maple, honey locust, +etc., all except the normal one in the axil are _supernumerary_ +or _accessory_. These must not be confounded with _adventitious_ +buds—those that occur elsewhere than at a node. + + + Practical Questions + + 1. Would protected buds be of any use to annuals? Why, or why not? + + 2. Of what use is the gummy coating found on the buds of the + horse-chestnut and balm of Gilead? (148.) + + 3. Can you name any plants the buds of which serve as food for man? + + 4. How do flower buds differ in shape from leaf buds? + + 5. At what season can the leaf bud and the flower bud first be + distinguished? Is it the same for all flowering plants? + + 6. Watch the different trees about your home, and see when the buds + that are to develop into leaves and flowers the next season are + formed in each species. + + + III. THE BRANCHING OF FLOWER STEMS + + MATERIAL.—Typical flower clusters illustrating the definite and + indefinite modes of inflorescence. Some of those mentioned in the + text are:— + + Indefinite: hyacinth, shepherd’s purse, wallflower, carrot, lilac, + blue grass, smartweed (_Polygonum_), wheat, oak, willow, clover. + + Definite: chickweed, spurge (_Euphorbia_), comfrey, dead nettle, + etc. Any examples illustrating the principal kinds of cluster will + answer. + +=159. Inflorescence= is a term used to denote the position and +arrangement of flowers on the stem. It is merely a mode of branching, +and follows the same laws that govern the branching of ordinary stems. + +The stalk that bears a flower is called the _peduncle_. In a cluster +the main axis is the common peduncle, and the separate flower stalks +are _pedicels_. A simple leafless flower stalk that rises directly +from the ground, like those of the dandelion and daffodil, is called +a _scape_ (Fig. 165). + +[Illustration: FIG. 165.—Solitary terminal flower of a lily.] + +[Illustration: FIG. 166.—Indeterminate inflorescence of moneywort. +(_After_ GRAY.)] + +=160. Two kinds of inflorescence.=—The growth of flower stems, +like that of leaf stems, is of two principal kinds, definite and +indefinite, or, as it is frequently expressed, determinate and +indeterminate. The simplest kind of each is the solitary, a single +flower either terminating the main axis, as the tulip, daffodil, +trillium, magnolia, etc., or springing singly from the axils, as the +running periwinkle, moneywort, and cotton. + +[Illustration: FIG. 167.—Raceme of milk vetch (_Astragalus_).] + +[Illustration: FIG, 168.—Catkins of aspen.] + +=161. Indeterminate inflorescence= is always axillary, since the +production of a terminal flower would stop further growth in that +direction and thus terminate the development of the axis. The +_raceme_ is the typical flower cluster of the indefinite sort. In +such an arrangement the oldest flowers are at the lower nodes, +new ones appearing only as the axis lengthens and produces new +internodes. The little scale or _bract_ usually found at the base of +the pedicel in flower clusters of this sort is a reduced leaf, and +the fact that the flower stalk springs from the axil shows it to be +of the essential nature of a branch. When the flowers are sessile +and crowded on the axis in various degrees, the cluster produced may +be a _spike_, as seen in the plantain, knotweed, etc., or a _head_, +like that of the clover, buttonwood, and sycamore. The _catkins_ that +form the characteristic inflorescence of most of our forest trees +are merely pendant spikes. The _corymb_ is a modification of the +raceme in which the lower pedicels are elongated so as to place their +flowers on a level with those of the upper nodes, making a convex, or +more or less flat-topped cluster, as in the wallflower and hawthorn. +The _umbel_ differs from the corymb in having the pedicels with their +bracts all gathered at the top of the peduncle, from which they +spread in every direction like the rays of an umbrella, as the name +implies. This is the prevalent type of flower cluster in the parsley +family, which takes its botanical name, _Umbelliferæ_, from its +characteristic form of inflorescence. The pedicels of an umbel are +called _rays_, and the circle of bracts at the base of the cluster is +an _involucre_. + +[Illustration: FIG. 169.—Corymb of plum blossoms.] + +[Illustration: FIG. 170.—Umbel of milkweed.] + +[Illustration: FIG. 171.—Panicle of grass, a compound cluster of the +racemose type.] + +[Illustration: FIG. 172.—Flat-topped cyme of sneezeweed.] + +=162. Determinate, or cymose, inflorescence.=—In the _cyme_, the +typical cluster of the determinate kind, the older blossoms in the +center, being terminal, stop the axis of growth in that direction and +force the stem, in continuing its growth, to send out side branches +from the axils of the topmost leaves, in a manner precisely similar +to the two-forked branching of stems like the horse-chestnut and +jimson weed. When the older peduncles are lengthened as described +in 161, a flat-topped cyme is produced, which is distinguished +from the corymb by its order of flowering, the oldest blossoms +being at the center, while in the corymb they appear in the reverse +order. A peculiar form of cyme is found in the scorpioid or coiled +inflorescence of the pink-root (_Spigelia_), heliotrope, comfrey, +etc. Its structure will be made clear by an inspection of Figs. +174-176. + +[Illustration: FIG. 173.—Scorpioid cyme.] + +=163. The nature of flower stems.=—A comparison of the types of +inflorescence with the modes of branching in ordinary stems (144, +152, 153) will show a strict correspondence between them. Both bear +leaves and buds, and the individual flowers of a cluster usually +spring from the axils of leaves or from bracts, which are merely +reduced leaves. What, then, is the essential nature of flower stems? + +[Illustration: FIGS. 174-176.—Diagrams of cymose inflorescence, with +flowers numbered in the order of their development: 174, cyme half +developed (scorpioid); 175, a flat-topped or corymbose cyme; 176, +development of a typical cyme.] + +=164. Significance of the clustered arrangement.=—As a general thing +the clustered arrangement marks a higher stage of development than +the solitary, just as in human life the rudest social state is a +distinct advance upon the isolated condition of the savage. In plant +life it is the beginning of a system of coöperation and division of +labor among the associated members of the flower cluster, as will be +seen later when we take up the study of the flower. + + + Practical Questions + + 1. Name as many solitary flowers as you can think of. + + 2. Do you, as a rule, find very small flowers solitary, or in + clusters? + + 3. Would the separate flowers of the clover, parsley, or grape be + readily distinguished by the eye among a mass of foliage? + + 4. Should you judge from these facts that it is, in general, + advantageous to plants for their flowers to be conspicuous? + + + Field Work + + (1) In connection with 144-154, the characteristic modes of + branching among the common trees and shrubs of each neighborhood + should be observed and accounted for. The naked branches of the + winter woods afford exceptional opportunities for studies of this + kind, which cannot well be carried on except out of doors. Note + the effect of the mode of branching upon the general outline of + the tree; compare the direction and mode of growth of the larger + boughs with that of small twigs in the same species, and see if + there is any general correspondence between them; note the absence + of fine spray on the boughs of large-leaved trees, and account + for it. Account for the flat sprays of trees like the elm, beech, + hackberry, etc.; the irregular stumpy branches of the oak and + walnut; the stiff straight twigs of the ash; the zigzag switches of + the black locust, Osage orange, elm, and linden. Measure the twigs + on various species, and see if there is any relation between the + length and thickness of branches. Notice the different trend of + the upper, middle, and lower boughs in most trees, and account for + it. Observe the mode of branching of as many different species as + possible of some of the great botanical groups of trees; the oaks, + hickories, hawthorns, and pines, for instance, and notice whether + it is, as a general thing, uniform among the species of the same + group, and how it differs from that of other groups. + + (2) In connection with 155-158, buds of as many different kinds + as possible should be examined with reference to their means of + protection, their vernation and leaf arrangement, and the resulting + modes of growth. Compare the folding of the cotyledons in the seed + with the vernation of the same plants, and observe whether the + folding is the same throughout a whole group of related plants, + or only for the same species. Notice which modes seem to be most + prevalent. Select a twig on some tree near your home or your + schoolhouse, and keep a record of its daily growth from the first + sign of the unfolding of its principal bud to the full development + of its leaves. Any study of buds should include an observation of + them in all stages of development. + + (3) With 160-165, study the inflorescence of the common plants and + weeds that happen to be in season, until you have no difficulty + in distinguishing between the definite and indefinite sorts, and + can refer any ordinary cluster to its proper form. Notice whether + there is any tendency to uniformity in the mode of inflorescence + among flowers of the same family. Consider how each kind is adapted + to the shape and habit of the flowers composing it, and what + particular advantage each of the specimens examined derives from + the way its flowers are clustered. In cases of mixed inflorescence, + see if you can discover any reason for the change from one form to + the other. + + + + +CHAPTER VI. THE LEAF + + + I. THE TYPICAL LEAF AND ITS PARTS + + MATERIAL.—Leaves of different kinds showing the various modes of + attachment, shapes, texture, etc. For stipules, leaves on very + young twigs should be selected, as these bodies often fall away + soon after the leaves expand. The rose, Japan quince, willow, + strawberry, pea, pansy, and young leaves of beech, apple, elm, + tulip tree, India rubber tree, magnolia, knotweed, furnish good + examples of stipules. For the different orders of leaf arrangement, + lilac, maple, spurge, trillium, cleavers (Galium) show the opposite + and whorled kinds. Elm, basswood, grasses; alder, birch, sedges; + peach, apple, cherry, show respectively for each group the three + principal orders of alternate arrangement. + +=165. Parts of the leaf.=—Examine a young, healthy leaf of apple, +quince, or elm, as it stands upon the stem, and notice that it +consists of three parts: a broad expansion called the _blade_; a +leaf stalk or _petiole_ that attaches it to the stem; and two little +leaflike or bristle-like bodies at the base, known as _stipules_. +Make a sketch of any leaf provided with all these parts, and label +them, respectively, blade, petiole, and stipules. These three parts +make up a perfect or typical leaf, but as a matter of fact, one or +more of them is usually wanting. + +[Illustration: FIG. 177.—A typical leaf and its parts: _b_, blade; +_p_, petiole; _s_, _s_, stipules.] + +[Illustration: FIG. 178.—Spiny stipules of clotbur.] + +=166. Stipules.=—The office of stipules, when present, is generally +to subserve in some way the purposes of protection. In many cases, +as in the fig, elm, beech, oak, magnolia, etc., they appear only as +protective scales that cover the bud during winter, and fall away as +soon as the leaf expands. When _persistent_, that is, enduring, they +take various forms according to the purposes they serve. But under +whatever guise they occur, their true nature may be recognized by +their position on each side of the base of the petiole, and not in +the _axil_, or angle formed by the leaf with the stem. (149.) + +[Illustration: FIG. 179.—Adnate stipules of clover. FIG. 180.—Leaves +of smilax, showing stipular tendrils. FIG. 181.—Leafy stipules of +Japan quince.] + +=167. Leaf attachment.=—The normal use of the petiole is to secure a +better light exposure for the leaves, but, like other parts, it is +subject to modifications, and is often wanting altogether. In this +case the leaf is said to be _sessile_, that is, _seated_, on the +stem, and the leaf bases are designated by various terms descriptive +of their mode of attachment. The meaning of these terms, when not +self-explanatory, can best be learned by a comparison of living +specimens with Figs. 184-187. + +=168. Arrangement of leaves on the stem.=—The mode of attachment is +something quite distinct from the mode of leaf arrangement on the +stem, or _phyllotaxy_, as it is termed by botanists. It was seen in +148 that this takes place in two different ways, the alternate and +opposite. These two kinds of arrangement represent the principal +forms of leaf disposition on the stem, the different varieties of +each depending on the manner in which the leaves are distributed. + +[Illustration: FIGS. 182-187.—Petioles, and leaf attachment: 182, +petioles of jasmine nightshade (_Solanum jasminoides_) acting as +tendrils; 183, acacia, showing petiole transformed to leaf blade; +184, sessile leaves of epilobium; 185, clasping leaf of lactuca; 186, +perfoliate leaves of uvularia; 187, peltate leaf of tropæolum. (182 +and 186 _after_ GRAY.)] + +Where three or more occur at a node, as in the trillium and cleavers +(_Galium_), they constitute a whorl, which is only a variant of the +opposite arrangement. There is no limit to the number of leaves that +may be in a whorl except the space around the stem to accommodate +them. + +The phyllotaxy of alternate leaves is more complicated. The +different forms are characterized by the angular distance between the +points of leaf insertion around the stem. In the elm, basswood, and +most grasses, they are distributed in two rows or ranks on opposite +sides of the stem, each just half way round the circumference from +the one next in succession (Fig. 189), the third in vertical order +standing directly over the first. In most of our common trees and +shrubs five leaves are passed in making two turns round the stem, the +sixth leaf in vertical order standing over the first. This is called +the five-ranked arrangement, and is the most common order among +dicotyls. + +[Illustration: FIG. 188.—Whorled leaves of Indian cucumber.] + +[Illustration: FIG. 189.—Twig of a hackberry (_Celtis cinerea_), +showing the two-ranked arrangement. Notice how the position of the +stems and branches of the main axis corresponds to that of the +leaves.] + +=169. Relation between the shape and arrangement of +leaves.=—Phyllotaxy is of importance chiefly on account of its +influence on the light relation of leaves. A compact, close-ranked +arrangement tends to shut off the light from the lower nodes, and +hence, in plants where it prevails, the leaves are apt to be long +and narrow in proportion to the frequency of the vertical rows. +The yucca, oleander, Canada fleabane and bitterweed (_Helenium +tenuifolium_), illustrate this relation. + +[Illustration: PLATE 9.—Vegetation of a moist, shady ravine. Notice +the expanded surface of the leaf blades and the long internodes that +separate the individual leaves. (From Rep’t. Mo. Botanical Garden.)] + +[Illustration: FIG. 190.—Narrow leaves in crowded vertical rows.] + +On the other hand, when the leaves are large and rounded in outline, +as those of the sunflower, hollyhock, and catalpa, they are usually +separated by longer internodes, or their blades are cut and incised +so that the sunlight easily strikes through to the lower ones. + +=170. Other external characteristics= to be observed in leaves are:— + +(1) General Outline: whether round, oval, heart-shaped, etc. (Figs. +191-197). + +(2) Margins: whether unbroken (_entire_), or variously toothed and +indented. (Figs. 198-202.) + +(3) Texture: whether thick, thin, soft, hard, fleshy, leathery, +brittle. + +(4) Surface: smooth, shining, dull, wrinkled, hairy, or otherwise +roughened. + +[Illustration: FIGS. 191-197.—Shapes of leaves: 191, lanceolate; 192, +spatulate; 193, oval; 194, obovate; 195, kidney-shaped; 196, deltoid; +197, lyrate. (191-195 _after_ GRAY.)] + +Not only do leaves of different kinds exhibit these characteristics +in varying degrees, but young and old leaves, or those on young and +old plants of the same kind, often differ from each other in color, +size, shape, texture, mode of attachment, and the like, to such a +degree (Figs. 203, 204) that one not familiar with them in both +stages would hardly recognize them as belonging to the same species. +The young leaves of eucalyptus, mulberry, and some oaks afford +conspicuous examples of such differences, and they exist between the +cotyledons and mature leaves of most plants. + +Can you see any benefit, in the case of the plant whose leaves you +are studying, that could be derived from such of the characteristics +named above as they may exhibit? + +[Illustration: FIGS. 198-202.—Margins of leaves: 198, serrate; 199, +dentate; 200, crenate; 201, undulate; 202, sinuate. (_After_ GRAY.)] + +[Illustration: FIGS. 203, 204.—Leaves of paper mulberry tree: 203, +leaf from an old tree; 204, leaf from a two-year-old sprout.] + + + Practical Questions + + 1. Tell the nature and use of the stipules in such of the following + plants as you can find: tulip tree; fig; beech; apple; willow; + pansy; garden pea; Japan quince (_Pyrus Japonica_); sycamore; rose; + paper mulberry (_Broussonetia_). + + 2. How would you distinguish between a chinquapin, a chestnut, a + chestnut oak, and a horse-chestnut tree by their leaves alone? By + their bark and branches? Between a hickory, ash, common elder, box + elder, ailanthus, sumach? Between beech, birch, elm, hackberry, + alder? + + (Any other sets of leaves may be substituted for those named, the + object being merely to form the habit of distinguishing readily the + differences and resemblances among those that bear some general + likeness to one another.) + + 3. From the study of these or similar specimens, would you conclude + that resemblances in leaves are confined to those of closely + related kinds? + + 4. Name some causes independent of botanical relationship that + might influence them. (169, 170; Exps. 48, 57.) + + 5. Do you find, as a general thing, more leaves with stipules or + without? + + 6. Is their absence from a mature leaf always a sign that it is + really exstipulate? (166.) + + 7. Can you trace any line of development through intervening + forms from a merely sessile leaf, like that of the pimpernel or + specularia, to a peltate one? (Figs. 184-187, and observation of + living specimens.) + + 8. Does the leaf determine the position of the node, or the node + the position of the leaf? + + 9. Strip the leaves from a twig of one order of arrangement and + replace them with foliage from a twig of a different order; for + instance, place basswood upon white oak, birch upon lilac, elm upon + pear, honeysuckle upon barberry, etc. Is the same amount of surface + exposed as in the natural order? + + 10. What disadvantage would it be to a plant if the leaves were + arranged so that they stood directly over one another? (169.) + + 11. Why are the internodes of vigorous young shoots, or scions, + generally so long? (150.) + + 12. If the upward growth of a stem or branch is stopped by pruning, + what effect is produced upon the parts below, and why? (152, 153.) + + 13. Give some of the reasons why corn grows so small and stunted + when sown broadcast for forage? (60, 63, 169.) + + 14. What is the use of “chopping” (_i.e._ thinning out) cotton? + + + II. THE VEINING AND LOBING OF LEAVES + + MATERIAL.—Leaves of any monocotyl and dicotyl will show the + difference between parallel and net-veining. To illustrate the + palmate and pinnate kinds, the leaves of grasses and arums may be + used for monocotyls, and for dicotyls, those of ivy, maple, grape, + elm, peach, cherry, etc.; for division, examine lobed and compound + leaves of as many kinds as are attainable. A specimen showing each + kind of veining should be placed in coloring fluid a short time + before the lesson begins. The leafstalks of celery and plantain + are excellent for showing the relation between the leaf veins and + vascular system of the plant. + +=171. Parallel and net veining.=—Compare a leaf of the wandering +Jew, lily, or any kind of grass, with one of grape, ivy, or willow. +Hold each up to the light, and note the veins or little threads of +woody substance that run through it. Make a drawing of each so as to +show plainly the direction and manner of veining. Write under the +first, _parallel-veined_, and under the second, _net-veined_. This +distinction of leaves into parallel and net-veined corresponds with +the two great classes into which seed-bearing plants are divided, +monocotyls, as a general thing, being characterized by the first +kind, and dicotyls by the second. + +[Illustration: FIG. 205.—Parallel-veined leaf of lily of the valley +(_After_ GRAY).] + +[Illustration: FIG. 206.—Net-veined leaf of a willow.] + +[Illustration: FIG. 207.—Pinnately parallel-veined leaf of calla lily +(_After_ GRAY).] + +[Illustration: FIG. 208.—Palmately net-veined leaf of wild ginger.] + +=172. Pinnate and palmate veining.=—Next, compare a leaf of the +canna, calla lily, or any kind of arum, with one of the elm, peach, +cherry, etc. What resemblances do you notice between the two? What +differences? Which is parallel-veined and which is net-veined? Make +a drawing of each, and compare with the first two. Notice that in +leaves of this kind, the petiole is continued in a large central +vein, called the _midrib_, from which the secondary veins branch off +on either side like the pinnæ of a feather; whence such leaves are +said to be _pinnately_, or _feather_ veined, as in Figs. 206, 207. In +the cotton, maple, ivy, etc., on the other hand, the petiole breaks +up at the base of the leaf (Fig. 208) into a number of primary +veins or ribs, which radiate in all directions like the fingers from +the palm of the hand; hence, such a leaf is said to be _palmately_ +veined. Net-veined leaves—the plantain (Fig. 209), wild smilax, +beech, dogwood—are sometimes ribbed in a way that might lead an +inexperienced observer to confound them with parallel-veined ones, +but the reticulations between the ribs show that they belong to the +net-veined class. + +[Illustration: FIG. 209.—Ribbed leaf of plantain.] + +=173. Veins as a mechanical support.=—Hold up a stiff, firm leaf +of any kind, like the magnolia, holly, or India rubber, to the +light, having first scraped away a little of the under surface, and +examine it with a lens. Compare it with one of softer texture, like +the peach, maple, or clover. In which are the veins the closer and +stronger? Which is the more easily torn and wilted? Tear a blade of +grass longitudinally and then cross-wise; in which direction does it +give way the more readily? Tear apart gently a leaf of maple, or ivy, +and one of elm or other pinnately veined plant; in which direction +does each give way with least resistance? What would you judge from +these facts as to the mechanical use of the veins? + +=174. Effect upon shape.=—By comparing a number of leaves of each +kind it will be seen that the feather-veined ones tend to assume +elongated outlines (Figs. 197, 207); the palmate-veined ones, broad +and rounded forms (Figs. 195, 208). Notice also that the straight, +unbroken venation of parallel-veined leaves is generally accompanied +by smooth, unbroken margins, while the irregular, open meshes of +net-veined leaves are favorable to breaks and indentations. + +=175. Veins as water carriers.=—Examine a leaf from a stem that has +stood in red ink for an hour or two. Do you see evidence that it has +absorbed any of the liquid? Cut across the blade and examine with a +lens. What course has the absorbed liquid followed? What use does +this indicate for the veins, besides the one already noted? Observe +the point of insertion on the stem, and examine the scar with a lens: +do you see any evidence of a connection between the leaf veins and +the fibrovascular bundles of the stem? (111, 125, 126.) Notice where +and how the veins end. Are they of the same size all the way, or do +they grow smaller toward the tip? Are they separate and distinct, or +are they connected throughout their ramifications, like the veins and +arteries of the human body? How do you know? Do you see any of the +coloring fluid in the small reticulations between the veins? How did +it get there? + +=176. The nature and office of veins.=—We learn from 173 and 175 that +the veining serves two important purposes in the economy of the leaf: +first, as a skeleton or framework, to support the expanded blade; +and second, as a system of water pipes, for conveying the sap out +of which its food is manufactured. In other words the veins are a +continuation of the fibrovascular bundles into the leaves, by means +of which the latter are put in communication with the body of the +plant. + +=177. The relation between veining and lobing.=—Compare the outline +of a leaf of maple or ivy with one of oak or chrysanthemum. Do +you perceive any correspondence between the manner of lobing or +indentation of their margins, and the direction of the veins? (Figs. +210, 211.) To what class would you refer each one? + +The lobes themselves may be variously cut, as in the fennel and rose +geranium, thus giving rise to twice-cleft, thrice-cleft (Fig. 212), +four-cleft, or even still more intricately divided blades. + +=178. Compound leaves.=—Compare with the specimens just examined +a leaf of horse-chestnut, clover, or Virginia creeper, and one +of rose, black locust, or vetch. Notice that each of these last is +made up of entirely separate divisions or leaflets, thus forming a +_compound leaf_. Notice also that the two kinds of compound leaves +correspond to the two kinds of veining and lobing, so that we +have palmately and pinnately compound ones. In pinnate leaves the +continuation of the common petiole along which the leaflets are +ranged is called the _rhachis_. + +[Illustration: FIG. 210.—Pinnately lobed leaf of horse nettle. FIG. +211.—Palmately lobed leaf of grape.] + +[Illustration: FIG. 212.—Palmately parted leaf of a buttercup. FIG. +213.—Pinnately compound leaf of black locust.] + +[Illustration: FIG. 214.—Palmately compound leaf of horse-chestnut. +FIG. 215.—Pinnately trifoliolate leaf of a desmodium. FIG. +216.—Palmately trifoliolate leaf of wood sorrel.] + + + Practical Questions + + 1. In selecting leaves for decorations that are to remain several + hours without water, which of the following would you prefer, and + why: smilax or Madeira vine (_Boussingaultia_); ivy or Virginia + creeper; magnolia or maple; maidenhair or shield fern (_Aspidium_)? + (173.) + + 2. Would you select very young leaves, or more mature ones, and why? + + 3. Can you name any parallel-veined leaves that have their margins + lobed, or indented in any way? + + 4. Which are the more common, parallel-veined or net-veined leaves? + + 5. Why do the leaves of corn and other grains not shrivel + lengthwise in withering, but roll inward from side to side? (173.) + + 6. Can you name any palmately veined leaves in which the secondary + veins are pinnate? Any pinnately veined ones in which the secondary + veins are palmate? + + 7. Lay one of each kind before you; try to draw a pinnate leaf with + palmate divisions. Do you see any reason now why these so seldom + occur in nature? + + 8. Name some advantages to a plant in having its leaves cut-lobed + or compound. (169.) + + 9. Mention some circumstances under which it might be advantageous + for a plant to have large, entire leaves. (169; Plate 9.) + + 10. How would the floating qualities of the leaves of the pond lily + be affected if their blades were cut-lobed or compound? + + 11. Do the leaves of the red cedar and arbor vitæ contribute to + their value as shade trees? + + 12. Name some of the favorite shade trees of your neighborhood; do + they, as a general thing, have their leaves entire, or lobed and + compound? + + 13. Which of the following are the best shade trees, and why: pine, + white oak, mimosa (_Albizzia_), sycamore, locust, horse-chestnut, + fir, maple, linden, China tree, cedar, ash? + + 14. Which would shade your porch best, and why: cypress vine, + grape, gourd, morning-glory, wistaria, clematis, smilax, kidney + bean, Madeira vine, rose, yellow jasmine, passion flower? + + + III. TRANSPIRATION + + MATERIAL.—Leafy twigs of actively growing young plants. Sunflower, + corn, peach, grape, calla, and arums in general transpire rapidly; + thick-leaved evergreens and hairy or rough species, like mullein + and horehound more slowly. For Exp. 63, small-leaved, large-leaved, + and thick-leaved kinds will be needed. + + APPLIANCES.—Glass jars and bottles with air-tight stoppers; a + little vaseline, oil, gardener’s wax, thread, cardboard, and a pair + of scales. + + EXPERIMENT 62. TO SHOW WHY LEAVES WITHER.—Dry two self-sealing jars + thoroughly, by holding them over a stove or a lighted lamp for a + short time to prevent “sweating.” Place in one a freshly cut leafy + sprig of any kind, leaving the other empty. Seal both jars and set + them in the shade. Place beside them, but without covering of any + kind, a twig similar to the one in the jar. Both twigs should have + been cut at the same time, and their cut ends covered with wax or + vaseline, to prevent access of air. Look at intervals to see if + there is any moisture deposited on the inside of either jar. If + there is none, set them both in a refrigerator or cover with a wet + cloth and allow to cool for half an hour, and then examine again. + In which jar is there a greater deposit of dew? How do you account + for it? Take the twig out of the jar and compare its leaves with + those of the one left outside; which have withered the more, and + why? + + EXPERIMENT 63. TO MEASURE THE RATE AT WHICH WATER IS GIVEN OFF BY + LEAVES OF DIFFERENT KINDS.—Fill three glass vessels of the same + size with water and cover with oil to prevent evaporation. Insert + into one the end of a healthy twig of peach or cherry; into the + second a twig of catalpa, grape, or any large-leaved plant, and + into the third, one of magnolia, holly, or other thick-leaved + evergreen, letting the stems of all reach well down into the water. + Care must be taken to select twigs of approximately the same size + and age, since the absorbent properties of very young stems are + more injured by cutting and exposure than those of older ones. All + specimens should be cut under water as directed in Exp. 58. Weigh + all three vessels, and at the end of twenty-four hours, weigh + again, taking note of the quantity of liquid that has disappeared + from each glass. This will represent approximately the amount + absorbed by the leaves from the twigs to replace that given off. + Which twig has lost most? Which least? Note the condition of the + leaves on the different twigs; have they all absorbed water about + as rapidly as they have lost it? How do you know this? Pluck the + leaves from each twig, one by one, lay them on a flat surface + that has been previously measured off, into square inches or + centimeters, and thus form a rough estimate of the area covered by + each specimen. Make the best estimate you can of the number of + leaves on each tree, and calculate the number of kilograms of water + it would give off at that rate in a day. + + EXPERIMENT 64. THROUGH WHAT PART OF THE LEAF DOES THE WATER GET + OUT?—Take some healthy leaves of tulip tree, grape, tropæolum, + or any large, soft kind attainable. Cover with vaseline the + _leafstalk_ and _upper_ surface of one; the stalk and _under_ + surface of a second; the stalk and _both_ surfaces of a third, and + leave a fourth one untreated. Suspend all four in a dry place by + means of a thread attached to the petioles so that both surfaces + may be equally exposed. The leaves must be all of the same species, + and as nearly as possible of the same age, size, and vigor, and + care must be taken that none of the vaseline is rubbed off in + handling. Examine at intervals of a few hours. Which of the leaves + withers soonest? Which keeps fresh longest? From what part would + you conclude, judging by this experiment, that the water escapes + most rapidly? + +=179. Transpiration, nutrition, and growth.=—We learn from the +foregoing, and from Exps. 58 and 59, that plants give off moisture +very much as animals do by perspiration. The two processes must not +be classed together, however, for they are physiologically different. +The action, in plants, is called _transpiration_. It is usually +assumed that a large amount of water must pass through the plant +in order to bring to it the necessary supply of food material; but +since the entrance of mineral salts is brought about by osmosis, +conditioned by the living cells of the root; and since osmosis +of salts may take place in a direction opposite to that of the +greater movement of water, it follows that the entrance of salts is +independent of transpiration. + +Inasmuch, however, as a certain amount of water is necessary to bring +the living cells into a condition of turgor (7) so that they may +grow, it follows that there is a relation between transpiration and +growth. If transpiration exceeds absorption for any length of time, +the tissues will be depleted of their moisture, as is shown by the +wilting of crops in dry, hot weather; and if the unequal movement +continues long enough, the plant will die. Hence, a knowledge of the +laws governing this important function is necessary to all who are +interested in cultivating agricultural products. + +[Illustration: FIG. 217.—A “weeping tree,” showing the effect where +absorption exceeds transpiration. Notice the position of the tree +near the water where the roots have unlimited moisture. (_After_ +FRANCÉ.)] + +=180. Magnitude of the work of transpiration.=—Few people have any +idea of the enormous quantities of water given off by leaves. It +has been calculated that a healthy oak may have as many as 700,000 +leaves, and that 111,225 kilograms of water—equal to about 244,700 +pounds—may pass from its surface in the five active months from June +to October. At this rate 226 times its own weight may pass through it +in a year, and it would transpire water enough during that time to +cover the ground shaded by it to a depth of 20 feet![2] Lawn grass +gives off water at such a rate that a vacant lot of 150 × 50 feet, +if well turfed, would be capable of transpiring over a ton of water +a day. Compare these figures with the average yearly rainfall in our +Gulf States—53 inches, approximately—and you can form some estimate +of the injury done to a growing crop from this cause alone. The +moisture is drawn from the surface by shallow rooted weeds (81) and +dissipated through the leaves. In the case of forest trees the effect +is different. Their roots, striking deep into the soil, draw up water +from the lower strata and distribute it to the thirsty air in summer. + +As the water given off by transpiration is in the form of vapor, +it must draw from the plant the amount of heat necessary for its +vaporization, and thus has the effect of making the leaves and the +air in contact with them cooler than the surrounding medium. At the +same time the coolness and moisture of the air tend to check the +loss by evaporation from the surface soil. It is partly to this +cause, and not alone to their shade, that the coolness of forests is +due. Measurements at various weather bureau stations in the United +States show that in summer the temperature of oak woods is 4° C. +lower during the day than in the open, and as much higher at night. +In a beech wood in Germany the difference between the forest and the +general temperature amounted to as much as 7° C. + + + Practical Questions + + 1. Is there any foundation in fact for the accounts of “weeping + trees” and “rain trees” that we sometimes read about in the papers? + (180; Exp. 48.) + + 2. Can you explain the fact, sometimes noticed by farmers, that in + wooded districts, springs which have failed or run low during a dry + spell sometimes begin to flow again in autumn when the trees drop + their leaves, even though there has been no rain? (180; Exp. 63.) + + 3. Other things being equal, which would have the cooler, + pleasanter atmosphere in summer, a well-wooded region or a treeless + one? (180.) + + 4. Could you keep a bouquet fresh by giving it plenty of fresh air? + (Exp. 62.) + + 5. Why does a withered leaf become soft and flabby, and a dried one + hard and brittle? (7; Exp. 62.) + + 6. Why do large-leaved plants, as a general thing, wither more + quickly than those with small leaves? (Exp. 63.) + + 7. Is the amount of water absorbed always a correct indication of + the amount transpired? Explain. (179.) + + 8. Explain the difference between the withering caused by excessive + transpiration and the shrinkage of cells due to plasmolysis. Are + both of these physiological processes? + + 9. Why is it best to trim a tree close when it is transplanted? + (179, 180.) + + 10. Why should transplanting be done in winter or very early + spring, before the leaves appear? (180.) + + + IV. ANATOMY OF THE LEAF + + MATERIAL.—For study of the epidermis, leaves of the white garden + lily (_Lilium album_) are best, as the stomata can be seen on + their lower surface with the naked eye. Wandering Jew, Spanish + bayonet (_Yucca aloifolia_), anemone, narcissus, iris, canna, show + them under a hand lens, but less distinctly. For sections, beet, + mustard, and beech leaves may be used, or ready-mounted specimens + obtained of a dealer. + + A compound microscope is needed for a minute study of the leaf + structure. + +[Illustration: FIGS. 218, 219.—Stomata of white lily leaf: 218, +closed; 219, open. (_After_ GRAY.)] + +=181. Stomata.=—It was shown in Exp. 64 that the water of +transpiration escapes most rapidly, as a general thing, from the +under surface of leaves. To find out why this is so, a careful study +of the epidermis will be necessary. For this purpose procure, if +possible, the leaf of a white garden lily (_Lilium album_), wandering +Jew, Spanish bayonet (_Yucca aloifolia_), anemone, narcissus, iris, +or canna. The first-named is preferable, as the transpiration pores +can be seen on it with the naked eye. Examine the under surface +with a hand lens, and you will see that it is covered with small +eye-shaped dots like those shown in Figs. 218 and 219. Strip off +a portion of the epidermis, hold it up to the light on a piece of +moistened glass, and they can be seen quite clearly with a lens. +These dots are the pores through which the water vapor escapes in +transpiration, and through which air finds its way into the tissues +of the leaf. They are called _stomata_ (sing., _stoma_), from a Greek +word meaning “a mouth.” Look for stomata on the upper epidermis; do +you find any, and if so, are there as many as on the under surface? +Do you see any relation between this fact and the results obtained +from Exp. 64? Can you see any good reasons why the stomata should be +placed on the under side in preference to the upper? Are they as much +exposed to excessive light and heat, or as liable to be choked by +dust, rain, and dew here as on the upper side? + +=182. Distribution of stomata.=—While stomata are generally more +abundant on the under side of leaves, this is not always the case. +In vertical leaves, like those of the iris, which have both sides +equally exposed to the sun, they are distributed equally on both +sides. In plants like the water lily, where the under surface lies +upon the water, they occur only on the upper side. Succulent leaves, +as a general thing, have very few, because they need to conserve all +their moisture. Submerged leaves have none at all; why? + +[Illustration: FIG. 220.—A small piece of the under epidermis of +an oak leaf, highly magnified to show the stomata, _g_, and minute +hairs, _h_.] + +[Illustration: FIG. 221.—Under epidermis of an oat leaf, showing +stomata.] + +=183. Minute study of a leaf epidermis.=—Place a bit of the lower +epidermis of a leaf under the microscope, and examine with a high +power. It will appear, if a monocotyl, to be composed of long, flat, +rectangular spaces (Fig. 221); if the leaf of a dicotyl is used, +they will be more or less irregular (Fig. 220), with the outlines +fitting into each other like the tiling of a floor or the blocks of a +Chinese puzzle. These spaces are the cells of the epidermis, and the +lines are the cell walls. Can you find any of the cell contents? The +cell sap is not often visible; do you see the nuclei? Can you give +a reason why the epidermal cells are so thin and flat? Between some +of the cells you will see two kidney-shaped bodies placed with their +concave sides together so as to leave a lenticular opening between +them. This is a _stoma_, and the kidney-shaped bodies (Figs. 218, +219) are _guard cells_. They are given this name because they open +or close the mouth of the stoma. If you will imagine a toy balloon +made in the form of a hollow ring, like the tire of a bicycle, you +can easily see, from Figs. 218, 219, that when the ring is strongly +inflated, it will expand, and in enlarging its own circumference, +will at the same time increase the diameter of the opening in the +center. When the expansive force is removed, it collapses, thus +closing, or greatly reducing, the aperture. + +[Illustration: FIG. 222.—Outline of a stoma of hellebore in vertical +section. The darker lines show the shape assumed by the guard cells +when the stoma is open; the lighter lines, when the stoma is closed. +The cavities of the guard cells with the stoma closed are shaded, and +are distinctly smaller than when the stoma is open.] + +In the same way the guard cells, when there is abundance of water in +them, expand, thus opening the stoma so that the water vapor passes +out more readily. But when there is a dearth of moisture, or when, +by reason of chemical action in the soil, the roots fail to supply +it, the leaves wilt, the guard cells, losing their water, collapse, +closing the pore, and transpiration is thus prevented or greatly +retarded. (Fig. 222.) + +Sketch a portion of the epidermis as it appears under the microscope, +labeling the parts. If stomata can be found in both conditions, make +sketches showing them both open and closed. + +=184. Internal structure of a leaf.=—Roll a leaf blade, or fold it +tightly to facilitate cutting, and with a scalpel, or a very sharp +razor, cut the thinnest possible slice through the roll. This will +give a section at right angles to the epidermis. It should be so +thin as to appear almost transparent. Put a small bit of a section +in a drop of water on a slide, place under the microscope, using a +high power, and look for the parts shown in Fig. 223. Notice the +horizontally flattened cells of the upper epidermis, _e_, and of the +lower epidermis, _e′_; also the vertically elongated palisade cells, +_p_, filled with particles of green coloring matter. These particles +are the chlorophyll bodies, to which the green color of the leaf is +due. They are the active agents in the manufacture of plant food, +and in a leaf removed from the plant during the day time and viewed +under a high power, the chlorophyll bodies, on treatment with iodine, +will be seen to contain granules of starch which they are in the act +of elaborating. The collecting cells, _t_, receive the assimilated +product from the palisade cells and pass it on through the spongy +parenchyma, _sch_, to the fibrovascular bundles. Notice how much +more abundant the green matter is in the upper part of the leaf than +in the lower; has this anything to do with the deeper color of the +upper surfaces of leaves? Notice the opening, _st_, lower epidermis; +do you recognize it? (See Fig. 222.) It is a stoma, seen in vertical +section. Notice the intercellular air spaces, _i_, _i_, in the spongy +parenchyma, and the much larger one, _a_, just behind the stoma. Why +is this last so much larger? + +[Illustration: FIG. 223.—Transverse section through a leaf of beet: +_e_, upper epidermis; _e′_, lower epidermis; _st_, stoma; _a_, air +space; _p_, palisade cells; _t_, collecting cells; _sch_, spongy +parenchyma; _i_, _i_, intercellular air spaces; _Fbv_, section of a +vein (fibrovascular bundle).] + +[Illustration: FIG. 224.—Chlorophyll bodies containing starch grains +in the course of formation. Magnified 250 times.] + +Sketch the section of your specimen as it appears under the +microscope. It will perhaps differ in some details from the one shown +in the figure, but you can recognize and label the corresponding +parts. Be sure that your drawing represents accurately the relative +size and shapes of the different kinds of cells. + +It is in the upper surface, where the chlorophyll particles abound, +that the manufacture of food goes on most actively, and from the +under surface, where the stomata are situated, that transpiration +takes place and air and other gases pass to and from the interior. +These facts have important bearings on the growth and external +characters of leaves. + + + Practical Questions + + 1. Explain why a plant cannot thrive if its stomata are clogged + with foreign matter. (179; Exp. 64; 184.) + + 2. Mention some of the ways in which this might happen. (181.) + + 3. Why must the leaves of house plants be washed occasionally to + keep them healthy? (179, 181.) + + 4. Why is it so hard for trees and hedges to remain healthy in a + large manufacturing town? + + + V. FOOD MAKING + + MATERIAL.—A sprig of pondweed, mare’s-tail (_Hippuris_), hornwort + (_Ceratophyllum_), marsh St.-John’s-wort (_Elodea_), or other green + aquatic plant; bean or tropæolum, or other green leaves gathered + from plants growing in the sunshine; a healthy potted plant; a + small, fresh cutting. + + APPLIANCES.—A shallow dish of water and two glass tumblers or + wide-mouthed jars; a bent glass or rubber tube; a piece of black + cloth or paper; a half pint of alcohol; iodine solution; a glass + funnel or a long-necked bottle from which the bottom has been + removed. + + EXPERIMENT 65. IS THERE ANY RELATION BETWEEN SUNLIGHT AND THE GREEN + COLOR OF LEAVES?—Place a seedling of oats, or other rapidly growing + shoot, in the dark for a few days, and note its loss of color. + Leave it in the dark indefinitely, and it will lose all color and + die. Hence we may conclude that there is some intimate connection + between the action of light and the green coloring matter of leaves. + + EXPERIMENT 66. DO LEAVES GIVE OFF ANYTHING ELSE BESIDES + WATER?—Submerge a green water plant, with the cut end uppermost, + in a glass vessel full of water, and invert over it a glass + funnel, or a long-necked bottle from which the bottom has been + removed as directed in Exp. 53. Expel the air from the neck of the + funnel—or bottle—by submerging and corking under water so as to + make it air-tight. Place in the sunlight and notice the bubbles + that begin to rise from the cut end of the plant. When they have + partly filled the neck of the funnel, remove the stopper and thrust + in a glowing splinter. If it bursts into flame, or glows more + brightly, what is the gas that was given off? (Exp. 22.) + + As oxygen is not a product of respiration, some other process must + be at work here, during which oxygen is set free, and some other + substance used up. (Exps. 24 and 25.) + +[Illustration: FIG. 225.—Experiment showing that green plants give +off oxygen in sunlight.] + +[Illustration: FIG. 226.—Experiment for showing that leaves absorb +carbon dioxide from the atmosphere.] + + EXPERIMENT 67. WHAT IS THE SUBSTANCE TAKEN IN WHEN OXYGEN IS GIVEN + OFF?—Fill two glass jars, or two tumblers, with water, to expel the + air, and invert in a shallow dish of water, having first introduced + a freshly cut sprig of some healthy green plant into one of them. + Then, by means of a bent tube, blow into the mouth of each tumbler + till all the water is expelled by the impure air from the lungs. + Set the dish in the sunshine and leave it, taking care that the + end of the cutting is in the water of the dish. After forty-eight + hours remove the tumblers by running under the mouth of each, + before lifting from the dish, a piece of glass well coated with + vaseline (lard will answer), and pressing it down tight so that no + air can enter. Place the tumblers in an upright position, keeping + them securely covered. Fasten a lighted taper or match to the end + of a wire, plunge it quickly first into one tumbler, then into the + other, and note the result. What was the gas blown from your lungs + into the jars? (Exps. 23, 24.) Why did the taper not go out in the + second jar? What had become of the carbon dioxide? + + EXPERIMENT 68. TO SHOW THAT LIGHT IS NECESSARY FOR A PLANT TO + ABSORB CARBON DIOXIDE AND GIVE OFF OXYGEN.—Repeat Exp. 66, keeping + the plant in a dark or shady place; do you see any bubbles? Test + with a glowing match; is any oxygen formed in the tube of the + funnel? Move back into the sunlight and leave for a few hours; what + happens when you thrust a glowing splinter into the tube? + + EXPERIMENT 69. IS ANY FOOD PRODUCT FOUND IN LEAVES?—Crush a few + leaves of bean, sunflower, or tropæolum, and soak in alcohol until + all the chlorophyll is dissolved out. Rinse them in water, and soak + the leaves thus treated in a weak solution of iodine for a few + minutes, then wash them and hold them up to the light. If there + are any blue spots on the leaves, what are you to conclude? If a + test for sugar is to be made, use sap pressed from fresh leaves; + for oils and fats, leaves should be dried without being placed in + alcohol. + +[Illustration: FIG. 227.—Leaf arranged with a piece of tin foil to +exclude light from a portion of the surface.] + + EXPERIMENT 70. HAS THE PRESENCE OR ABSENCE OF LIGHT ANYTHING TO DO + WITH THE OCCURRENCE OF STARCH IN LEAVES?—Exclude the light from + parts of healthy leaves on a growing plant of tropæolum, bean, + etc., by placing patches of black cloth or paper over them. Leave + in a bright window, or preferably out of doors, for several hours, + and then test for starch as in the last experiment; do you find any + in the shaded spots? + + EXPERIMENT 71. IS THE PRESENCE OF AIR NECESSARY FOR THE PRODUCTION + OF STARCH?—Cover the blades and the petioles of several leaves with + vaseline or other oily substance so as to exclude the air, and + after a day or two test as before. + +=185. Influence of plants on the atmosphere.=—These experiments +show that leaves cannot do their work without light and air. The +particular element of the atmosphere used by them in the process of +food making is carbon dioxide. Their action in absorbing this gas +and giving off oxygen tends to counterbalance the opposite action of +respiration, decomposition, and combustion of all kinds, by which the +proportion of it in the atmosphere tends to be constantly increased. +In this way they help to regulate the quantity of it present and have +a beneficial effect in ridding the air of one source of impurity. + +=186. Photosynthesis.=—In our examination of the internal structure +of the leaf, the chlorophyll bodies (184) were found to contain small +granules of starch which the chlorophyll, under the stimulus of +light, had elaborated as a nutriment for the plant tissues. Hence, +the leaf may be regarded as a factory in which vegetable food, +mainly starch, is manufactured out of the water brought up from the +soil, and the carbon dioxide derived through the stomata from the +atmosphere. In this process carbon dioxide (CO_{2}) is combined +with water (H_{2}O) in such proportions that part of the oxygen is +returned to the surrounding air. This is a fundamental food-forming +process characteristic of green plants, and can take place only in +the light. For this reason it has been named _Photosynthesis_, a word +which means “building up by means of light,” just as _photography_ +means “drawing or engraving by means of light.” + +In carrying on the operation of photosynthesis, sunshine is the +power, the chlorophyll bodies the working machinery, carbon dioxide +and water the raw materials, and starch or oil the finished product, +while oxygen and the water of transpiration represent the waste or +by-products. + +=187. How the new combination is effected.=—It may seem strange that +a gas and a liquid should combine to make something so different +from either as starch, but their chemical constituents are the same +in different proportions. Water is made up of 2 parts hydrogen and +1 part oxygen; carbon dioxide, of 1 part carbon and 2 parts oxygen, +while starch contains carbon, hydrogen, and oxygen, in the ratios of +6, 10, and 5, respectively. Hence, by taking sufficient quantities +of water and carbon dioxide and combining them in the proper +proportions, the leaf factory can turn them into starch. If we use +the letters C, H, and O, to represent Carbon, Hydrogen, and Oxygen, +respectively, the new combination of materials can be expressed by an +equation; thus:— + + _water_ _carbon dioxide_ _starch_ _by-products_ + 5(H_{2}O) + 6(CO_{2}) = (C_{6}H_{10}O_{5}) + 6(O_{2}) = 12(O). + +The water not used up in the process is given off as a waste product +in transpiration, while the oxygen is returned to the air, as shown +by Exp. 66. This equation is not to be understood as representing the +chemical changes that actually take place in the leaf. These are too +complicated, and at present too imperfectly known, to be considered +here. It will serve, however, to give a fair idea of the final result +from the process of photosynthesis, however brought about. + +Simple as the operation appears, the chemist has not, as yet, +been able to imitate it. He can analyze starch into its original +constituents, but while he has the ingredients at hand in abundance, +and knows the exact proportions of their combination, it is beyond +his power, in the present state of our knowledge, to put them +together. Hence, both man and the lower animals are dependent on +plants for this most important food element. The so-called factories +that supply the starch of commerce do not _make_ starch any more than +the miller makes wheat, but merely separate and render available for +use that already elaborated by plants. + +=188. Proteins.=—Foods of this class are mainly instrumental in +furnishing material for the growth and repair of the tissues out +of which the bodies of both plants and animals are built up. They +embrace a great variety of substances, but their chemical nature +is very complex and very imperfectly understood. Nitrogen is an +important element in their composition, whence they are commonly +distinguished as “nitrogenous foods.” Besides nitrogen, there are +present carbon, hydrogen, oxygen, and sulphur, and traces of the +mineral salts absorbed from the soil are found in varying quantities +in the ash of different proteins. The percentages in which these +ingredients are combined and the processes concerned in their +formation are at present a matter of pure hypothesis. Botanists +are not agreed even as to whether they are made in the leaf or in +some other part or parts of the plant, though the weight of opinion +inclines to the view that their construction takes place in the leaf. + +=189. The activities of leaves.=—As there are only 4 parts of CO_{2} +to every 10,000 parts of ordinary free air, it has been estimated +that in order to supply the leaf factory with the raw material it +needs, an active leaf surface of one square meter—a little over +one square yard—uses up, during every hour of sunshine, the CO_{2} +contained in 1000 liters (1000 quarts, approximately) of air. Suppose +an oak tree to bear 500,000 leaves, each having a surface of 16 sq. +cm., or 4 sq. in., and working 12 hours a day for 6 months in the +year; you will then have some idea of the enormous quantity of air +that passes each season through its leaf system. Add to this the +almost incredible volume of water transpired in the same time (180), +and we may well stand amazed at the tremendous activities of these +silent workers that we are in the habit of regarding as mere passive +elements in the general landscape. + +=190. The economic value of leaves.=—Besides their importance as +sanitary and food-making agencies, leaves have a direct commercial +value as food products in the hay and fodder they supply for our +domestic animals, the tea and salads with which they provide our +tables, the aromatic flavors and seasonings contained in them, and +the drugs, medicines, and dyes of various kinds for which they +furnish the ingredients. + + + Practical Questions + + 1. Why do gardeners “bank” celery? (Exp. 65.) + + 2. Why are the buds that sprout on potatoes in the cellar, white? + (Exp. 65.) + + 3. Why does young cotton look pale and sickly in long-continued wet + or cloudy weather? (Exp. 65.) + + 4. Why do parasitic plants generally have either no leaves or very + small, scalelike ones? (85, 186, 187.) + + 5. The mistletoe is an exception to this; explain why, in the light + of your answer to question 4. + + 6. Could an ordinary nonparasitic plant live without green leaves? + (186, 187.) + + 7. Are abundance and color of foliage any indication of the health + of a plant? (186, 187; Exp. 65.) + + 8. Is the practice of lopping and pruning very closely, as in the + process called “pollarding,” beneficial to a tree under ordinary + conditions? (186, 189; Exp. 63.) + + 9. Name some plants of your neighborhood that grow well in the + shade. + + 10. Compare in this respect Bermuda grass and Kentucky blue + grass; cotton and maize; horse nettle (_Solanun Carolinense_) and + dandelion; beech, oak, red maple, dogwood, pine, cedar, holly, + magnolia, etc. + + 11. Name all the aromatic leaves you can think of; all that are + used as food, beverages, drugs, and dyes. + + 12. What is the use of aromatic and medicinal leaves to the plant + itself? (Suggestion: Why does the housewife put lavender or tobacco + leaves in her woolen chest?) + + 13. Which would be richer in nourishment, hay cut in the evening or + in the morning, and why? (54, 186; Exp. 70.) + + 14. Mention three important sanitary services that are rendered by + a tree like that shown in plate 6 or 8. (180, 185, 189.) + + 15. Name some of the plants employed in the manufacture of starch. + + + VI. THE LEAF AN ORGAN OF RESPIRATION + + MATERIAL.—A number of vigorous, freshly cut green leaves; a liter + or two (one or two quarts) of expanding flower or leaf buds. + + APPLIANCES.—Some wide-mouthed jars of one or two liters’ capacity; + two small open vials of limewater. + + EXPERIMENT 72. DO LEAVES GIVE OFF CARBON DIOXIDE?—Cover the + bottoms of two wide-mouthed jars with water about two centimeters + (1 inch) deep. Place in one a number of healthy green leaves with + their stalks in the water, and insert among them a small open + vial containing limewater. In the other jar place only a vial of + limewater in the clear water at the bottom, this last being merely + to make the conditions in both vessels the same. Seal both tight + and keep together in the dark for about 48 hours, and then examine. + In which jar does the limewater indicate the greater accumulation + of CO_{2}? (It may show a slight milkiness in the other vessel + due to gas derived from the inclosed air and water.) From this + experiment, what process would you conclude has been going on among + the leaves in jar No. 1? (Exp. 25.) + +[Illustration: FIG. 228.—Arrangement of apparatus to show that heat +and carbon dioxide are given off by leaf buds.] + + EXPERIMENT 73. IS THE EXHALATION OF CARBON DIOXIDE ACCOMPANIED + BY ANY OTHER CONCOMITANT OF RESPIRATION?—In Exps. 24, 25, it was + shown that respiration is accompanied by heat; hence, if the + production of carbon dioxide by the leaf is due to this cause, it + should be attended by the evolution of heat. To find out whether + this is the case, partly fill a glass jar of two liters’ capacity + with unfolding leaf buds arranged in layers alternating with damp + cotton batting or blotting paper (Fig. 228); close the jar tightly + and leave from 12 to 24 hours in the dark to prevent the action + of photosynthesis. Then insert a thermometer and note the rise in + temperature. If a lighted taper is plunged in, it will quickly be + extinguished, showing that respiration has been going on. + +=191. Respiration in leaves.=—We see from experiments like the +foregoing that the leaf, besides carrying on the functions of +digestion, photosynthesis, and transpiration, is also an active agent +in the work of respiration. In this function oxygen is used up and +carbon dioxide given off, just as in the respiration of animals; but +the process is so slow in plants that it is much more difficult to +detect than the contrary action in photosynthesis, and is, in fact, +not perceptible at all while the latter is going on, though it does +not cease even then. + +But while the leaf is the principal organ of respiration, the process +is carried on in other parts of the plant as well, else it could not +survive during the leafless months of winter. It _appears_ to be most +active at night, but this is only because it is not obscured then, +as during the day, by the more active function of photosynthesis. +Indeed, it was for a long time supposed that plants “breathed” only +at night, and it was thought to be unwholesome to keep them in a +bedroom. It is now known, however, that respiration goes on at all +times and in all living parts of the plant, but the quantity of +oxygen taken in is so small from a hygienic point of view that it may +be disregarded. + +=192. Distinctions between respiration and photosynthesis.=—While +these two functions are contrasting and antipodal, so to speak, in +their action, they are mutually complementary and interdependent, the +one manufacturing food and the other using it up, or rather marking +the activity of those life processes by which it is used up. The +difference between them will be made clear by a comparison of the two +processes as summarized in the following statement: + + PHOTOSYNTHESIS RESPIRATION + + Goes on only in sunlight and in Goes on at all times and in all + the green parts of plants. parts of the plant. + + Produces starch and sugar. Releases energy (heat and working + power). + + Gives off, as by-product, oxygen. Gives off, as by-products, CO₂ + and water. + + A constructive process, in which A destructive, or consumptive + energy is used up to make food. process, in which food is used up + in expending energy. + +=193. Metabolism.=—The total of all the life processes of plants, +including growth, waste, repair, etc., is summed up under the general +term _metabolism_. It is a _constructive_ or building-up process when +it results in the making of new tissues out of food material absorbed +from the earth and air, and the consequent increase of the plant in +size or numbers. But, as in the case of animals, so with plants, not +all the food provided is converted into new tissue, part being used +as a source of energy, and part decomposed and excreted as waste. +In this sense, metabolism is said to be _destructive_. The waste in +healthy growing plants is always, of course, less than the gain, +and a portion of the food material is laid by as a reserve store. +For this reason, photosynthesis, being a constructive process, is +usually more energetic than respiration, which is the measure of the +destructive change of materials that attends all life processes. + +It is evident also, from what has been said, that growth and repair +of tissues can take place only so long as the plant has sufficient +oxygen for respiration, since the energy liberated by it is necessary +for the assimilation of nourishment by the tissues. + +Thus we see that plants are dependent on air not only for +respiration, but for nutrition, and none of their life processes can +be carried on without it. + + + Practical Questions + + 1. Can a plant be suffocated, and if so, in what ways? (87, 193; + Exps. 26, 27.) + + 2. The roots on the palm shown in plate 3 are not drawing any sap + from it as parasites; why does their continued growth bring about + the death of the tree? (87, 193.) + + 3. Is it unwholesome to keep flowering plants in a bedroom? Leafy + ones? Why, in each case? (191.) + + 4. Would there be any more reason for objecting to the presence of + flowers by night than by day? Explain. (191.) + + 5. Why is respiration much less marked in plants than in animals? + (30, 31.) + + + VII. THE ADJUSTMENT OF LEAVES TO EXTERNAL RELATIONS + + MATERIAL.—A potted plant of oxalis, spotted medick, white clover, + or other sensitive species. The subject is better suited for + outdoor observation than for laboratory work. + + EXPERIMENT 74. TO SHOW THAT LEAVES ADJUST THEMSELVES TO CHANGES IN + INTENSITY OF LIGHT.—Keep a healthy potted plant of oxalis, white + clover, or spotted medick in your room for observation. Note the + daily changes of position the leaves undergo. Sketch one as it + appears at night and in the morning. + +[Illustration: FIGS. 229, 230.—Leaves of a peanut plant: 229, in day +position; 230, in night position.] + + In order to determine whether these changes are due to want of + light or of warmth, put your plant in a dark closet in the middle + of the day, without change of temperature. After several hours note + results. Transfer to a refrigerator, or in winter place outside a + window where it will be exposed to a temperature of about 5° C. + (40° F.) for several hours, and see if any change takes place. Next + put it at night in a well-lighted room and note the effect. If + practicable, keep a specimen for several weeks in some place where + electric lights are burning continuously all night, and watch the + results. + + EXPERIMENT 75. TO SHOW THAT THE FALL OF THE LEAF MAY RESULT FROM + OTHER CAUSES THAN COLD OR FROST.—Wrap some leaves of ailanthus, + Kentucky coffee tree, ash, walnut, or hickory in a damp towel and + keep them in the dark for several days; the leaflets will fall + away, leaving a clear scar like those on winter twigs. + + EXPERIMENT 76. TO SHOW THAT ADJUSTMENTS TO TEMPERATURE MAY BE MADE + BY CHEMICAL MEANS.—Place a small twig of oleander, laurestinus, + or other broad-leaved evergreen in a 5 to 10 per cent solution of + sugar, and transfer it at the end of a few days to a temperature of + 6° to 8° below freezing. On comparison with a similar twig that has + stood for the same length of time in pure water, it will be found + to possess a greater power of resistance to cold. + +[Illustration: FIG. 231.—A plant that has been growing near an open +window, showing the leaves all turned toward the light.] + +=194. The light relation.=—The principal external conditions to which +leaves have to adjust themselves are light, air, moisture, gravity, +temperature, and the attacks of animals. From the knowledge of their +work and function gained in the preceding sections, it will be clear +that the primary relation of the leaf is a light relation, and to +this, first of all, it must adjust itself. + +It was shown in Exps. 56 and 57 how promptly leaves respond to +changes in the direction of light, and a little observation (Exp. +74) will convince us that they are equally sensitive to changes in +intensity and periodicity of illumination. + +=195. Phototropism.=—The movement of plants in response to light is +called _phototropism_—a word that means “turning toward or away from +light.” It includes all kinds of light adjustments, and examples of +it are to be met with everywhere in the disposition of leaves with +reference to their light exposure. + +=196. Horizontal and vertical adjustment.=—Take two sprigs, one +upright, the other horizontal, from any convenient shrub or tree—and +notice the difference in the position of the leaves. Examine their +points of attachment and see how this is brought about, whether by a +twist of the petiole or of the base of the leaf blades, or by a half +twist of the stem between two consecutive leaves, or by some other +means. + +[Illustration: PLATE 10.—A mosaic of moonseed leaves, showing +adjustment for light exposure. + +(_From_ Mo. Botanical Garden Rep’t.)] + +Observe both branches in their natural position; what part of the +leaf is turned upward, the edge or the surface of the blade? Change +the position of the two sprigs, placing the vertically growing one +horizontal, and the horizontal one vertical. What part of the leaves +is turned upward in each? + +[Illustration: FIGS. 232, 233.—Adjustment of leaves to different +positions: 232, upright; 233, procumbent.] + +[Illustration: FIG. 234.—Leaf mosaic of elm.] + +=197. Leaf mosaics.=—Trees with horizontal or drooping branches, +like the elm and beech, and vines growing along walls or trailing +on the ground, generally display their foliage in flat, spreading +layers, each leaf fitting in between the interstices of the others +like the stones in a mosaic, whence this has been called the _mosaic_ +arrangement. (Plate 10.) In plants of more upright or bunchy habit, +the leaves are placed at all angles, giving the appearance of a +rosette when viewed from above, whence this is called the _rosette_ +arrangement. + +A variety of the same disposition is seen in the pyramidal shape +assumed by plants with large, undivided leaves like the mullein and +burdock (Fig. 237), in which access of light is secured by a mutual +adjustment between the size and position of leaves, the upper ones +becoming successively smaller. + +=198. Heliotropism=—“turning with the sun”—is the name given to the +daily movement of plants like the cotton and sunflower in turning +their leaves or their blossoms to face the sun. If you live where +cotton is grown, notice the leaves in a field about ten o’clock on +a bright sunny morning, and again from the same point of view at +about four or five in the afternoon. Do you perceive any difference +in their general disposition? Watch on a cloudy day and see if any +change takes place. Find out by observation whether the “heliotrope” +of the hothouses is really heliotropic. + +[Illustration: FIGS. 235, 236.—Horse-chestnut leaves: 235, leaf +rosette seen from above; 236, the same seen sidewise, showing the +formation of rosettes by the lengthening of the lower petioles.] + +[Illustration: FIG. 237.—Leaf pyramid of mullein.] + +[Illustration: FIGS. 238, 239.—A compass plant, rosinweed (_Silphium +laciniatum_): 238, seen from the east; 239, seen from the south.] + +=199. Adjustment against too great intensity of light.=—Plants +frequently have to protect themselves against excess of light and +heat. An interesting example of this kind of adjustment is furnished +by the rosinweed, or compass plant (_Silphium laciniatum_, Figs. 238, +239), which grows in the prairies of Alabama and westward, where it +is exposed to intense sunlight. The leaves not only stand vertical, +but have a tendency to turn their edges north and south so that the +blades are exposed only to the gentler morning and evening rays. The +prickly lettuce manifests the same habit in a less marked degree. + +[Illustration: FIGS. 240, 241.—A plant of the guayule (_Parthenium +argentatum_), to the leaves of which indexes have been affixed to +show their day and night position: 240, day position; 241, night +position. (_From_ photographs by Prof. F. E. Lloyd.)] + +=200. Night and day adjustments.=—These are movements in response to +changes in the degree of illumination and temperature, as evidenced +by the fact that they become feeble and soon cease altogether if +the plant is kept a sufficient time under uniform conditions as +to these two factors. (Exp. 74.) They are called “nyctitropic” or +sleep movements, because they are most obvious in certain plants +that undergo periodic adjustments to the alternations of day and +night suggestive of an imaginary likeness to the sleep of animals. +Examples are most frequently met with among members of the pea +family (_Leguminosæ_), the spurges (_Euphorbiaceæ_), and the sorrel +(_Oxalis_) family. They are found among other species also, and +indeed are much more general than is usually supposed, most plants +showing signs of them if carefully tested. A simple way of doing +this is by attaching bristles about two inches long to the tips of +two leaves on opposite sides of the stem, as in Figs. 240, 241, +and comparing the divergence of the bristles during the day and at +nightfall. In this way a change of position in the leaves, too +slight to attract attention otherwise, will be made apparent. The +positions assumed vary in different plants, and even in the parts of +the same compound leaf; in the kidney bean, for instance, the common +petiole turns up at night, while the individual leaflets turn down. +One of the common pigweeds (_Amaranthus Palmeri_, Figs. 242-244) is +heliotropic in the day time and nyctitropic at night. + +[Illustration: FIGS. 242-244.—Showing the movements of _Amaranthus +Palmeri_: 242, 243, position at sunrise and sunset (heliotropic); +244, night position (nyctitropic) half an hour after sunset. (_From_ +photographs by Prof. F. E. Lloyd.)] + +The very striking nyctitropic adjustments of the wild senna (_Cassia +tora_) photographed by Professor Francis E. Lloyd of the Alabama +Polytechnic Institute (Figs. 245-250), though obviously influenced by +the sun, are not directed toward it as in those of truly heliotropic +plants. + +[Illustration: FIGS. 245-250.—Wild senna (_Cassia tora_), showing the +nyctitropic adjustments of its leaves. The upper figures show their +horizontal arrangement; those below, the vertical: 245, 248, position +of the leaves at 9 A.M.; 246, 249, at 3 P.M.; 247, 250, at 6.30 P.M. +(_From_ photographs by Prof. F. E. Lloyd.)] + +These movements are common also among flowers, many of them having +regular hours for opening and closing, as indicated by such names as +“morning-glory” and “four-o’clock.” In these cases, however, other +causes (277, 280) than the light relation must be taken into account. + +=201. Irritability= is a general term applied to the power in plants +of receiving and responding by spontaneous movements to impressions +from without. In its widest acceptation, irritability includes, +besides the various forms of adjustment described in this section and +the next, all movements due to geotropism, those of roots seeking +air and moisture, the revolution of twining stems and tendrils, +the circulation of protoplasm in the cell—any movement, in short, +that is made in response to an impression from the environment is a +manifestation of irritability. It may be of various degrees, but is +possessed to some extent by every living vegetable organism. + +The term is usually applied, however, more especially to those +obvious and pronounced responses made by plants to their +surroundings, as exemplified in the cases just given. Still more +marked instances are to be found in the movements of the tentacles +of insectivorous plants, and the sensitive leaflets of the mimosa +that close at the slightest touch. The tendrils of the passion flower +are said to appreciate and respond to a pressure that cannot be +distinguished even by the human tongue, and many plants will detect +and respond to the ultra-violet rays of light, which are entirely +invisible to man. + +This faculty of irritability among plants corresponds, in an +imperfect, rudimentary way, to what we recognize in animals as +nervous excitability. By this it is not meant to imply that the two +things are identical in their ultimate manifestations, though we +may regard them as fundamentally the same in that they are both to +be referred to the property inherent in protoplasm of responding to +stimuli. There is no indication, however, that irritability in the +vegetable kingdom is accompanied by anything like consciousness or +volition, or that plants possess any power of initiative. While the +movements in response to stimuli are in many cases eminently adapted +to a purpose, we have no evidence of a controlling power behind them. +The movement comes automatically in response to the stimulus, whether +the effect at the moment be advantageous or the reverse. + +=202. Adjustments in relation to moisture.=—These adjustments may +be—(1) To guard against excess of moisture; _e.g._ glands for +excreting water and salts; scales, wax, down, etc., on the surface of +leaves. These may serve also for protection against cold, insects, +excess of light and heat. (2) For the conservation of moisture; +_e.g._ the revolute leaf margins of grasses and sand plants growing +along the seashore; the fleshy leaves of stonecrops and purselanes; +the hard epidermis of yuccas and aloes; the scales, scurf, and down, +by which the moisture absorbed from the soil by plants growing in +dry and barren places is prevented from escaping too rapidly through +the stomata; the leaf cups and holders sometimes formed by winged +petioles and clasping leaf bases for retaining dew or rain water. +(3) For leaf drainage, or the conduction of moisture, by means of +grooves, channels, and taper-pointed leaves, which act as natural +gutters and drain pipes. + +[Illustration: FIG. 251.—Cross sections of the leaf of sand grass: +_a_, unrolled in its ordinary position; _b_ and _c_, rolled up to +prevent too rapid transpiration.] + +[Illustration: FIG. 252.—Winged petiole of _Polymnia_. FIG. +253.—Water cups of _Silphium perfoliatum_.] + +[Illustration: FIG. 254.—Fallen leaves. Notice how they cover the +ground with a warm mulch, protecting the soil from denudation, and +the roots and seeds from frost.] + +=203. The fall of the leaf.=—This is, in effect, an adjustment to +change of temperature, but that it is not directly due to cold is +shown by Exp. 75, and also by the fact that leaves in the tropics +and those of evergreens, while they do not fall at stated periods +like the bulk of the foliage in the temperate zones, are cut off just +the same and replaced by new ones, whenever, for any reason, they +are unable to perform their function. In cold climates they fall +at the approach of winter, not because the frost loosens them, but +because the roots are not able to absorb enough moisture to supply +them with material for making food. The needles and the scale-leaves +characteristic of evergreens in cold regions are enabled to persist +indefinitely by reason of their contracted surface. This prevents the +dissipation of moisture and affords no lodging for the accumulations +of sleet and snow that would otherwise cumber and perhaps break the +boughs with their weight. Trees and shrubs that shed their leaves +in winter are said to be _deciduous_, from a Latin word meaning “to +fall.” Can you mention some advantages of the deciduous habit to a +plant with broad, expanded leaves, growing in a cold climate? + +The mechanical means by which the leaf fall is accomplished is +through the growth of a corky layer of loose cells that forms at +the base of the petiole and cuts it away from the stem, leaving a +smooth, clean scar. Tear some fresh young leaves from a growing +twig and compare the scars with those on a winter bough. Do you see +any difference? This corky layer can be made to form in some plants +artificially, by depriving them of working material. (Exp. 75.) + +=204. The protection of wintergreen leaves.=—A great many, perhaps +the majority of broad-leaved evergreens, bear no obvious protection +against cold, while a large proportion, such as chickweed, violet, +fumitory, groundsel (_Senecio_), and dead nettle (_Lamium_), would +seem peculiarly unfitted, by their delicate structure, to withstand +it. But recent investigations by the Swedish botanist, Lidforss, +have shown that all wintergreen leaves, with the exception of those +on submerged water plants, which are sufficiently protected by the +medium in which they live, lose their starch in winter and contain +instead an increased percentage of sugar. The same is true of other +vegetable structures also, where starch is present, such as roots, +stems, tubers, and winter fruits—nuts, haws, persimmons, and the +like, which, as every schoolboy knows, become perceptibly sweeter +after frost. + +The presence of certain substances, of which sugar is the most +frequent, enables plants to withstand a greater degree of cold +than they could otherwise endure (Exp. 76). This effect, as shown +by Lidforss’s experiments, is due to the action of sugar in +counteracting, or retarding, the “salting out” of proteins by cold, +as explained in 33. + +As sugar is readily reconverted into starch by exposure to a +moderately high temperature for even a few days, we may find here +an explanation of the fact that plants which have survived the +prolonged cold of winter are often killed by a single sharp night +frost following a few warm days in early spring, before the tender +new growth has appeared. The plant suffers, not from the direct +effects of cold, but from the warmth preceding it, which stimulated +the transformation into starch of the sugar that would have prevented +the loss of proteins. On the same principle we may account for the +puzzling fact that the sunny southern side of trees and shrubs +usually suffers more from the effects of sudden frost than the shaded +and colder northern face. + +In apparent conflict with this reasoning is the fact that sugar +cane and the sugar beet are peculiarly susceptible to cold. This, +however, does not invalidate the premises established by Lidforss’s +researches, but merely emphasizes the need of further investigation, +which may either reconcile all the facts, or modify their +interpretation. + +=205. The colors of autumn leaves.=—These are due to the breaking +up and disappearance of the chlorophyll when the leaf factory has +to “shut down” for want of raw material to work with (203). It is +closely connected with the appearance of frost, since the same +changes of temperature which produce frost cause the cessation of sap +flow that brings about the disorganization of the chlorophyll and the +formation of various pigments derived from it. Besides these, leaves +may contain other coloring matters that are perceptible only when the +chlorophyll disappears; and in the sap there is a reddish pigment +which becomes either a very bright red, or a dark purplish maroon, +from the effect of chemicals that combine with it in the leaves. With +these coloring materials at command it is easy to see how the autumn +woods can assume such splendid hues. + + + Practical Questions + + 1. How would you explain the fact that the outer twigs of trees + generally are the most leafy? (99, 194; Exps. 57, 74.) + + 2. Is the common sunflower a compass plant? Is cotton? + + 3. Are there any such plants in your neighborhood? + + 4. Compare the leaves of half a dozen shade-loving plants of your + neighborhood with those of as many sun-loving ones; which, as a + general thing, are the larger and less incised? + + 5. Give a reason for the difference. (169.) + + 6. Why do most leaves—notably grasses—curl their edges backward in + withering? (182.) + + 7. What advantage is gained by doing this? (202.) + + 8. Observe such of the following plants as are found in your + neighborhood, and report any changes of position that may take + place in their leaves and the causes to which such changes should + be ascribed: wood sorrel, mimosa, honey locust, wild senna, + partridge pea, wild sensitive plant, redbud, bush clover, Japan + clover, Kentucky coffee tree, sensitive brier (_Schrankia_), + peanut, kidney bean. + + 9. Which of the trees named below shed their leaves from base to + tip of the bough (centripetally), and which in the reverse order: + ash, beech, hazel, hornbeam, lime, willow, poplar, pear, peach, + sweet gum, elm, sycamore, mulberry, China tree, sumac, chinquapin? + + 10. Account for the fact that evergreen trees and shrubs have + generally thick, hard, and shiny leaves, like those of the holly + and magnolia, or scales and needles, as the cedar and pine. (203.) + + 11. Why do many plants which are deciduous at the North tend to + become evergreen at the South? (203.) + + 12. Why are evergreens more abundant in cold than in warm climates? + (203.) + + 13. There is an apparent inconsistency between questions 11 and 12; + can you reconcile it? (203.) + + 14. Why is it more important to protect the south side of trees + against exposure to frost than the northern side? (33, 204.) + + 15. Explain why peach orchards on the tops and northern slopes of + elevated areas are less liable to have their fruit destroyed by + late frost than those in the valleys and on the southern slopes. + (33, 204.) + + + VIII. MODIFIED LEAVES + + MATERIAL.—Get from a florist a potted plant of sundew, + Venus’s-flytrap, sarracenia, or, if possible, one of all three, and + keep in the schoolroom for observation. The subject can be studied + best in a well-stocked greenhouse, if one is accessible. + +[Illustration: FIG. 255.—Spearlike leaves of Spanish bayonet.] + +=206. Modification and adaptation.=—Modification is structural +adjustment, or adaptation, carried so far as to obscure the original +form of an organ. Its true nature, however, can generally be +determined by some of the tests mentioned in 100. + +Examples of the modification of leaves to do the work of other +organs have already been noticed, as also their entire disappearance +in certain cases (97, 101, 149) and replacement by other parts; it is +unnecessary, therefore, to revert to this branch of the subject here. + +=207. Protective modifications.=—The most general protective +modifications that leaves undergo are (1) for the conservation +of moisture, as explained in 202, and (2) for protection against +animals. Many of the adaptations for the former purpose serve +incidentally for defense against animals also. Spines, hairs, scales, +sticky exudations, water holders, clasping and perfoliate leaves bar +the way to crawling insects; horny cuticles, as well as offensive +odors, bitter secretions, and poisonous juices warn leaf-eating +cattle and bugs away. These devices are merely protective, however, +and adapted to a passive attitude of self-defense. + +[Illustration: FIGS. 256-258.—Protective hairs magnified: 256, +mullein; 257, cinque-foil 258, Shepherdia.] + +=208. Insectivorous leaves.=—But sometimes a plant becomes the +aggressor, and instead of standing on the defensive or suffering +itself to be quietly devoured, proceeds to capture and devour small +game on its own account, and in this case, the leaf sometimes becomes +a deadly weapon of destruction. + +=209. Pitcher plants.=—The sarracenia, or trumpet leaf, is a +familiar example of this class. The lower part of the leaf blade is +transformed into a hollow vessel for holding water, and the top is +rounded into a broad flap called the _lamina_. Sometimes the lamina +stands erect, as in the common yellow trumpets of our coast regions, +and when this is the case, it is brilliantly colored and attracts +insects (Fig. 259). Sometimes, as in the parrot-beaked and the +spotted trumpet leaf, it is bent over the top of the water vessel +like a lid, and the back of the leaf, near the foot of the lamina, is +dotted with transparent specks that serve to decoy foolish flies away +from the true opening and tempt them to wear themselves out in futile +efforts to escape, as we often see them do against a window pane. + +[Illustration: FIG. 259.—Yellow trumpets (_Sarracenia flava_). +(_From_ the Mo. Botanical Garden Rep’t.)] + +[Illustration: FIG. 260.—Plant of sundew.] + +If the contents of one of these leaves are examined with a lens, +there will generally be found mixed with the water at the bottom the +remains of the bodies of a large number of insects. The hairs on the +outside all point up, toward the rim of the pitcher, while those on +the inside turn down, thus smoothing the way to destruction, but +making return impossible to a small insect when once it is ensnared. +When we remember that these plants are generally found in poor, +barren soil, we can appreciate the value to them of the animal diet +thus obtained. + +[Illustration: FIGS. 261-263.—Leaves of sundew magnified: 261, leaf +expanded; 262, leaf closing over captured insect; 263, leaf digesting +a meal.] + +=210. Flytraps.=—The most remarkable examples of insect-catching +leaves are the Venus’s-flytrap, found in the seacoast region of +North Carolina, and the sundew (_Drosera rotundifolia_), common +on the margins of sandy bogs and ponds. The latter is a delicate, +innocent-looking little plant, and owes its poetic name to the +dewlike appearance of a shining, sticky fluid exuded from glands on +its leaves, which glitter in the sun like dewdrops. It is, however, +a most voracious carnivorous plant, the sticky leaves acting as so +many bits of fly paper by means of which it catches its prey. When a +fly has been trapped, the tentacles close upon it, the edges of the +leaf curve inward, making a sort of stomach, from the glands of which +an acid juice exudes and digests the meal. After a number of days, +varying according to the digestibility of the diet, the blades slowly +unfold again and are ready for another capture. + +[Illustration: FIG. 264.—Bladderwort, showing finely dissected +submerged leaves bearing bladders for capturing animalculæ.] + +The bladderwort, common in pools and still waters nearly everywhere, +has its petioles transformed into floats, while the finely dissected, +rootlike blades bear little bladders which, when examined under the +microscope, are found to contain the decomposed remains of captured +animalculæ. + + + Practical Questions + + 1. Can you find any kind of leaf that is not preyed upon by + something? If so, how do you account for its immunity? + + 2. Make a list of some of the most striking of the protected leaves + of your neighborhood. + + 3. What is the nature of the protective organ in each case? + + 4. For protection against what does it seem to be specially adapted? + + 5. Are the plants in your list for the most part useful ones, or + troublesome weeds? + + 6. Examine the leaves of the worst weeds that you know of and see + if these will help in any way to account for their persistency. + + + Field Work + + (1) In connection with Sections I and II, observe the effect of the + lobing and branching of leaves in letting the sunlight through. + Notice any general differences that may appear as to shape, margin, + and texture in the leaves of sun plants, shade plants, and water + plants, and account for them. Study the arrangement of leaves on + stems of various kinds, with reference to the size and shapes of + leaves and their light relations. Consider the value of the various + kinds of foliage for shade; for ornament; as producers of moisture; + as food; as insect destroyers, etc. + + Make a special study of the twelve principal deciduous trees of + your neighborhood. Compare the leaves, bark, and branches of the + same trees so that you will be able to recognize them by any one of + these means alone. + + (2) In connection with Sections III and V, consider the effects + upon soil moisture of transpiration from the leaves of forest + trees and from those of shallow-rooted herbs and weeds that draw + their water supply from the surface. Consider the value of forests + in protecting crops from excessive evaporation by acting as wind + breaks. Study the effect of the fall of leaves upon the formation + of soil. In any undisturbed forest tract turn up a few inches of + soil with a garden trowel and see what it is composed of. Notice + what kind of plants grow in it. Note the absence of weeds and + account for it. Compare the appearance of trees scattered along + windy hillsides, where the fallen leaves are constantly blown away, + or in any position where the soil is unrenewed, with those in an + undisturbed forest, and then give an opinion as to the wisdom of + hauling away the leaves every year from a timber lot. + + (3) In Section VII, observe, in different kinds of leaf mosaics, + the means by which the adjustment has been brought about and the + purpose it subserves. Make a list of plants illustrating the two + habits. Notice the form and position of petioles of different + leaves, and their effect upon light exposure, drainage, etc., and + the behavior of the different kinds in the wind. Look for compass + plants in your neighborhood, and for other examples of adjustment + to heat and light. Study the position of leaves at different times + of day and in different kinds of weather and note what changes + occur and to what they are due. + + Make a list of ten plants that seem to you to have best worked out + the problem of leaf adjustment, giving the reasons for your opinion. + + Study the drainage system of different plants and observe whether + there is any general correspondence between the leaf drainage and + the root systems. This will lead to interesting questions in + regard to irrigation and manuring. Where plants are crowded, the + growth of both roots and leaves is complicated with so many other + factors that it is best to select for observations of this sort + specimens growing in more or less isolated situations. + + Notice the time of the expansion and shedding of the leaves of + different plants, and whether the early leafers, as a general + thing, shed early or late; in other words, whether there seems to + be any general time relation between the two acts of leaf expansion + and leaf fall. + + (4) Under Section VIII, look for instances of modified leaves; + study the nature of the different modifications you find, and try + to understand their meaning and object. Make a collection (_a_) + of all the leaves you can find modified to serve other than their + normal purposes; (_b_) of all the organs of other kinds that + have been modified to serve as leaves; (_c_) of all the modified + parts of leaves—stipules and petioles—that you can find. Keep the + collections separate, labeling each specimen with the name of the + plant it belongs to, what part it is, what use it serves, when and + where found. These collections need not be made individually, but + by the class as a whole and kept for the use of the school. + + Observe also (_d_) the differences between young and old leaves of + the same kind, and the leaves of young and old plants or parts of + plants of the same kind; (_e_) resemblances between young leaves + belonging to plants of different species; (_f_) between young + leaves of one species and mature ones of one or more different + species. Make a collection of all the specimens you can find + illustrating the three points mentioned, referring each to its + proper head, and giving the name and relative age—old or young—of + all specimens collected. + + + + +CHAPTER VII. THE FLOWER + + + I. DISSECTION OF TYPES WITH SUPERIOR OVARY + + MATERIAL.—For monocotyls, any flower of the lily family, + such as tulip, dogtooth violet (_Erythronium_), trillium, + star-of-Bethlehem, yucca, bear’s grass, and the like. The large + garden lilies make particularly good examples, but they are for the + most part spring bloomers. For autumn, spiderwort (_Tradescantia_), + arrow grass (_Sagittaria_), or late specimens of colchicum and + tiger lily may be used. Any of these will meet the essential + conditions of the analysis given in the text, but care should be + taken not to select for this exercise lily-like flowers of the iris + and amaryllis families, which have the _ovary inferior_. + + For examples of hypogynous dicotyls, flax, linden, pinks, + corn cockle, wood sorrel, poppies, tomato blossoms, and other + common flowers can usually be obtained without difficulty. In + autumn, the geraniums so largely cultivated for ornament will + meet all the conditions of the analysis. Specimens of the cress + family—wallflower, cabbage, mustard, turnip—can generally be found + everywhere and at all seasons, and they possess the advantage of + having their flowers throughout the order put up on so nearly the + same pattern that a description of one species will answer, even in + details, for the rest. + + For sympetalous specimens of the hypogynous type, hyacinth, lily of + the valley, bearberry, huckleberry, or other equivalent forms may + be used. + + APPLIANCES.—A compound microscope may be needed for examining + minute objects, such as pollen grains and ovules; but for all other + purposes, a good hand lens, with the pupil’s ordinary laboratory + equipment of drawing-materials, notebook, and dissecting needles, + will be sufficient for the studies outlined in this and the four + succeeding sections. + +[Illustration: FIGS. 265-267.—Flower of a monocotyl +(star-of-Bethlehem), with superior ovary dissected: 265, entire +flower, showing the different sets of organs: _pet_, petals; _sep_, +sepals; _sta_, stamens; _pist_, pistil; _ped_, peduncle; 266, side +view with all the petals and sepals but two removed to show order +of the parts: _r_, receptacle; _o_, ovary; _sty_, style; _stig_, +stigma—parts composing the pistil; _f_, filament; _a_, anther—parts +composing the stamen; 267, cross section of the ovary: _c_, _c_, +carpels; _ov_, ovules; _pl_, placenta.] + +=211. The floral envelopes.=—Make a sketch of your specimen flower +from the outside. Is it solitary, or one of a cluster? If the latter, +refer to 160-162 and tell the nature of the cluster. Notice the +color; is it conspicuous enough to attract attention or not? Can this +have anything to do with its clustered or solitary position? Label +the head of the peduncle that supports the flower, _receptacle_; +the outer greenish leaves, _sepals_; the inner, lighter-colored +ones, _petals_. The sepals taken together form the _calyx_, and the +petals, the _corolla_. Where the petals and sepals are all separate +and distinct, as in the tulip and the star-of-Bethlehem, the corolla +is said to be _polypetalous_ and the calyx _polysepalous_, words +meaning, respectively, many-petaled and many-sepaled. _Monopetalous_ +and _monosepalous_, or _sympetalous_ and _synsepalous_, are terms +used to describe a condition in which the petals or sepals are all +united into one, as in the morning-glory and lily of the valley. In +many flowers, there is little or no difference between the two sets +of organs. In such cases the calyx and corolla together are called +the _perianth_, but the distinction of parts is always observed, the +outer divisions being regarded as sepals, the inner ones as petals. +These two sets of organs constitute the _floral envelopes_, and are +not essential parts of the flower, as it can fulfill its office +of producing fruit and seed without them. Note their number, mode +of attachment to the receptacle, and how they alternate with each +other. Remove one of the sepals and one of the petals, and notice +any differences between them as to size, shape, or color. Which is +most like a foliage leaf? Hold each up to the light and try to make +out the veining. Is it the same as that of the foliage leaves? If a +light-colored flower is used, examine a specimen that has stood in +coloring fluid. How many of each set are there? + +[Illustration: FIGS. 268-269.—Yucca blossom: 268, external view: +_br_, bract; _pd_, peduncle; _r_, receptacle; _s_, sepal; _pet_, +petal; 269, vertical section: _ped_, peduncle; _br_, bract; _r_, +receptacle; _per_, perianth; _sta_, stamen; _o_, ovary; _sty_, style; +_stg_, stigma. The last three parts named compose the pistil.] + +[Illustration: FIGS. 270-274.—Stamens: 270, a typical stamen with the +terminal anther, _b_, surmounting the filament, _a_, and opening in +the normal manner down the outer side of each cell; 271, stamen of +tulip tree, with adnate extrorse anther; 272, stamen of an evening +primrose (_Œnothera_) with versatile anther; 273, stamen of pyrola, +the anther cells opening by chinks or pores at the top; 274, stamen +of a cranberry, with the anther cells prolonged into a tube and +opening by a pore at the apex. (_After_ GRAY.)] + +=212. The essential organs.=—Next sketch the flower on its inner +face, labeling the appendages just within the petals, _stamens_, +and the central organ within the ring of stamens, _pistil_. These +are called essential _organs_ because they are necessary to the +production of fruit and seed. Note their mode of insertion, three of +the stamens in a flower like the star-of-Bethlehem alternating with +the petals, and the other three with these and with the lobes of the +base of the pistil. + +=213. The stamens.=—Notice whether the stamens are all alike, or +whether there are differences as to size, height, shape, color, etc. +Do these differences, if there are any, occur indiscriminately and +without order, or in regular succession between the alternating +stamens? Examine one of the little powdery yellow bodies at the tip +of the stamens, and see whether they face toward the pistil or away +from it. + +Remove one of the stamens and sketch as it appears under the lens, +labeling the powdery yellow body at the top, _anther_, and the +stalklike body supporting it, _filament_. Usually the filaments +are threadlike, whence their name, but sometimes, as in the +star-of-Bethlehem, they are flattened and look like altered petals. +See if you can find such a one. What would you infer from this fact +as to the possible origin of the stamens? (100.) + +[Illustration: FIGS. 275-278.—Forms of pollen: 275, from _mimulus_; +276, star cucumber; 277 wild balsam apple; 278, _hibiscus_. (_After_ +GRAY.)] + +Notice the two little sacs or pouches that compose the anther, as to +their shape and manner of opening, or dehiscing, to discharge the +powder contained in them. This powder is called _pollen_, and will +be seen under the lens to consist of little yellow grains. These are +of different shapes, colors, and sizes, in different plants, and +their surface often appears beautifully grooved and striate when +sufficiently magnified. Place some of the pollen under the microscope +and draw two of the grains, with their markings. In the hibiscus and +others of the mallow family, they are large enough to be seen with a +hand lens. + +=214. The pistil.=—Remove the stamens and sketch the pistil as it +stands on the receptacle. Label the round or oval enlargement at +the base, _ovary_, the threadlike appendage rising from its center, +_style_, and the tip end of the style, _stigma_. In some specimens +the style may be very short, or wanting. In this case the stigma is +_sessile_, and the pistil consists of stigma and ovary alone. If the +stigma is lobed or parted, count the divisions and see if there is +any correspondence between them and the number of petals and sepals, +or of the lobes of the ovary. Examine the tip with a lens and notice +the sticky, mucilaginous exudation that moistens it. Can you think of +any use for this? If not, touch one of the powdery anthers to it, and +examine it again with a lens. What do you see? Can you blow or dust +the pollen from the stigma? + +=215. Pollination=, or the transfer of pollen from the anther to +the stigma, is a matter of great importance, as the pistil cannot +develop seed without it, except in the case of a few plants like the +Alpine everlasting, some species of meadow rue (_Thalictrum_), and +_Alchemilla_, which have the unusual faculty of perfecting seeds in +the absence of pollen. Note the relative position of pistils and +stamens and see if it is such that the pollen can reach the stigma +without external agency. + +=216. The ovary.=—Observe the shape of the ovary, and the number of +ridges, or grooves, that divide the surface. Select a flower which +has begun to wither, so that the ovary is well developed, cut a cross +section near the middle, and try to make out the number of _locules_, +or internal divisions. Do you perceive any correspondence in number +between these and the ridges or lobes outside (Fig. 280)? Between +them and the lobes of the stigma? The walls that inclose the cavities +of the ovary are called _carpels_, and the ridges or depressions that +mark their point of union on the outside are the _sutures_, or seams. +The little round bodies in the locules, as the compartments of the +ovary are called, are the _ovules_, which will later be developed +into seeds. Their place of attachment is the _placenta_. If they are +attached to the walls of the carpels (Fig. 281), the placenta is +_parietal_; if to a central axis formed by the edges of the carpels +projecting inwards (Fig. 282), it is central and axial; if instead of +being attached to the carpels, the ovules are borne on a projection +from the receptacle, the placenta is a _free central_ one (Fig. 283). +If your cross section shows a central placenta, make a vertical cut +down to the receptacle and find out whether it is free, or axial. +What appears to be the primary office of the ovary? Make an enlarged +sketch of your specimen in both vertical and horizontal section, +labeling correctly all the parts observed. + +[Illustration: FIGS. 279, 280.—Ovary of yucca, a hypogynous +monocotyl, dissected: 279, vertical section; _ov_, ovules; 280, +diagram of a horizontal section of the same, enlarged, showing the +three carpels and six locules; _ds_, dorsal sutures; _vs_, ventral +sutures; _ov_, ovules; _pl_, placenta.] + +[Illustration: FIGS. 281-283.—Different kinds of placenta: 281, +parietal; 282, central and axial; 283, free central. 281 and 282 are +horizontal sections; 283, vertical.] + +[Illustration: FIG. 284.—Horizontal diagram of a flower of the lily +kind. The dot represents the growing axis of the plant.] + +=217. Numerical plan.=—Make a horizontal diagram of the plan of the +whole flower, after the model given in Fig. 284, showing the order +of attachment of the different cycles,—sepals, petals, stamens, +and pistils,—the number of organs in each set, and their mode of +alternation with the organs of the other cycles. Notice that the +parts of each set are in threes, or multiples of three. This is +called the numerical plan of the flower, and is the prevailing number +among monocotyls. It is expressed in botanical language by saying +that the flower is _trimerous_, a word meaning measured, or divided +off, into parts for three. + +=218. Vertical order.=—Next make a vertical diagram of your specimen +after the manner shown in Fig. 269, and note carefully that the +ovary stands _above_ the other organs (this is true of all the lily +family), and is entirely separate and distinct from them. In such +cases the ovary is said to be _free_, or _superior_, and the other +organs _inferior_, or _hypogynous_, a word meaning “inserted under +the pistil.” These terms should be remembered, as the distinction is +an important one in plant evolution. + +=219. Summary of observations.=—In the flower just examined, we +found that there were four sets of floral organs present—sepals, +petals, stamens, and pistil; that the individual organs in each set +were alike in size and shape; that there were the same number, or +multiples of the same number of parts in each set, and that all the +parts of each set were entirely separate and disconnected, the one +from the other, and from those of the other cycles. Such a flower is +said to be:— + +_Perfect_, that is, provided with both kinds of organs essential to +the production of seed—stamens and pistil. + +_Complete_, having all the kinds of organs that a flower can have: +viz. two sets of essential organs, and two sets of floral envelopes. + +_Symmetrical_, having the same number of organs, or multiples of the +same number, in each set. + +_Regular_, having all the parts of each set of the same size and +shape, as in the wild rose and bellflower, or if different, arranged +in regular order or pairs, so that there will be a correspondence +between the two sides of the flower, as in the violet, sweet +pea, sage, and larkspur. For convenience, the two kinds may be +distinguished as _complete_ and _bilateral regularity_, respectively. + +The opposites of these terms are: _imperfect_, _incomplete_, +_asymmetrical_ or _unsymmetrical_, and _irregular_. + +Note that regularity refers to form, symmetry to number of parts, and +that a flower may be perfect without being complete. + +[Illustration: FIGS. 285-288.—A flower of the cress family: 285, +side view; 286, view from above; 287, diagram of parts: _p_, petals; +_s_, sepals; _st_, stamens; _pi_, pistil; _cl_, claw of petal; +, +, +position of the missing stamens; 288, pistil and stamens, enlarged. +(_After_ GRAY.)] + +[Illustration: FIG. 289.—Section of a tomato flower, showing the +hypogynous arrangement: _cx_, calyx; _c_, corolla; _s_, stamens; _p_, +pistil; _o_, ovary; _st_, stigma. (Twice natural size.)] + +=220. Dissection of a typical dicotyl flower.=—(Poppy, flax, pink, +tomato, linden, etc., can be substituted for the specimen used in +the text.) Gently remove the sepals and petals from a wallflower, +stock, mustard, or other cress flower, lay them on the table before +you in exactly the order in which they grew on the stem, and sketch +them. How many of each are there, and how do they alternate with +one another? Sketch the pistil and stamens as they stand on the +receptacle; how many of the latter are there? Notice that two of the +six are outside and a little below the others, alternate with the +petals, while the other four stand opposite them, as is natural, +if they were alternating with another ring of stamens between +themselves and the corolla. Put a dot before two of the sepals in +your first drawing to indicate the position of the two outer stamens, +and a cross before the other two to show where stamens are wanting +to complete the symmetry of this set, as in Fig. 287. When parts +necessary to complete the plan of a flower are wanting, as in this +case, they are said to be _obsolete_, _suppressed_, or _aborted_. +Place dots before the petals to represent the other four stamens. +Sketch one of the anthers as it appears under a lens, showing the +arrow-shaped base, and the mode of attachment to the filament. Is it +such that the pollen can reach the stigma without external agency? In +what manner do the anthers open to discharge their pollen? Are the +anthers and stigma mature at the same time? Remove all the stamens +from a flower and sketch the pistil, showing the long, slender +ovary, the very short style, and the _capitate_ (that is, round +and knoblike) stigma. Make cross and vertical sections of one of +the older pistils lower down on the stem. How many ovules does it +contain? How are they attached? Represent the position of the pistil +by a small circle in the center of your sketch of the separate parts. +You have now a complete ground plan of the flower. Diagram a vertical +section, as in Fig. 289, showing the position of the ovary with +reference to the other parts, and report in your notebook as to the +following points:— + + Numerical plan Presence or absence of parts + Symmetry Union of parts + Regularity (complete or bilateral) Position of ovary + + + + + II. DISSECTION OF TYPES WITH INFERIOR OVARY + + MATERIAL.—For monocotyls: in spring and early summer, iris, + snowflake, freesia, crocus, narcissus, daffodil, can be used; + in autumn, gladiolus, blackberry lily, fall crocus, star grass + (_Hypoxys_). For dicotyls: in spring, flowers of apple, pear, + quince, gooseberry, squash, gourd, melon (with both male and female + flowers); in late summer and autumn, fuchsia, evening primrose + (_Œnothera_), willow-herb (_Epilobium_). + +=221. Study of a monocotyl flower.=—Compare with the specimens +examined in the last section, a narcissus, snowflake, or iris flower. +What difference do you notice in the position of the ovary? Would +you call it _inferior_ (below the other parts) or _superior_ (above +them)? How was it in the lily and the hyacinth? If your specimen +is an iris, notice that it is sessile in the axil of a large bract +called a _spathe_, which conceals the lower part of the flower. +Remove the spathe and observe that the lower part of the perianth is +united into a long, narrow tube, from the top of which the sepals and +petals extend as long, curving lobes. + +[Illustration: FIG. 290.—Iris flower: _sp_, spathes; _s_, sepals + +_p_, petals = perianth.] + +[Illustration: FIG. 291.—Vertical section of iris flower: _ov_, +ovules; _pl_, placenta; _tu_, tube of the perianth inclosing the +style; _sta_, stamen; _sti_, stigma: _o_, ovary. (_After_ GRAY.)] + +[Illustration: FIG. 292.—Vertical section of iris flower, with +perianth removed, showing a stamen and three stigmas: _su_, stigmatic +surface.] + +[Illustration: FIG. 293.—Cross section of ovary of iris flower: _c_, +_c_, carpels; _l_, _l_, locules; _ov_, ovules; _pl_, placenta.] + +=222. Arrangement of parts.=—Sketch the outside of the flower, +labeling the oblong, three-lobed enlargement at the base, _ovary_; +the prolongation above it, _tube of the perianth_; the three outer +lobes with the broad sessile bases, _sepals_; the others, with +their bases narrowed and bent inward, _petals_. Now turn the flower +over and sketch the inside, labeling the three large, petal-like +expansions in the center, _stigmas_. Do you see any stamens? Remove +one of the sepals and look under the stigma; what do you find there? +Notice the little honey pockets at the foot of the stamen. Run the +head of your pencil into them and see what would happen to the head +of an insect probing for honey. + +Remove the perianth and sketch the remaining organs in profile, +showing the position of the stamens. Do you see any advantage in +their position? Can you determine the use of the crest of hairlike +filaments on the upper side of the sepals? Remove a stamen and sketch +it. + +=223. The pistil.=—Remove as much of the upper part of the perianth +tube as you can without injuring the pistil, and with a sharp knife +cut a vertical section down through the ovary so as to show the long +style and its connection with the placenta. Make a sketch of this +longitudinal section (see Fig. 291), labeling the parts observed. +Notice whether the placenta is central or parietal. Draw a cross +section of the ovary; how many locules has it? How many ovules in +each? Where are they attached? Is the placenta free central or axial +(Fig. 293)? Examine with a lens the little flap at the base of the +two-cleft apex of one of the stigmas, and look for a moist spot to +which the pollen will adhere. Label this in your sketch, _stigmatic +surface_. No seeds can be matured unless some of the pollen reaches +this surface; can you think by what agency it is carried there? What +insects have you seen hovering about the iris? Notice that in drawing +his head _out_ of the flower, an insect would not touch the stigmatic +surface, since it is on the _upper_ side of the flap and he would be +probing _under_ it. But in entering the next flower that he visits, +he is likely to strike his head against the flap and turn it under, +thus dusting it with pollen brought from another flower. + +[Illustration: FIG. 294.—Horizontal diagram of iris flower.] + +=224. Diagrams.=—Draw diagrams showing the horizontal and vertical +arrangement of parts in the iris or other specimen examined, and +compare with those made of the monocotyl studied in the preceding +section. In what respect does it differ from them? How do you account +for the difference in the number of stamens, if there is any? (220.) + +=225. The vertical order.=—The difference in vertical arrangement is +an important one. Bear in mind that flowers of this type have the +ovary _inferior_, that is, inserted _under_ the other organs (Figs. +296, 304), which are then said to be superior, or _epigynous_, a word +which, as you know from the prefix _epi_ (47), means over or above +the pistil. To make the matter clear, the two sets of terms employed +for describing the position of the ovary are given below in parallel +columns: + + Hypogynous Epigynous + Ovary superior Ovary inferior + Calyx or perianth inferior Calyx or perianth superior + +The epigynous arrangement is considered as marking a higher stage of +floral development than the hypogynous, which is characteristic of a +more simple and primitive structure. + +[Illustration: FIGS. 295-296.—Evening primrose, dicotyl flower with +inferior ovary: 295, exterior view; 296, longitudinal section, +showing vertical arrangement of parts.] + +=226. Dissection of a dicotyl flower.=—Sketch a blossom of quince +or apple, fuchsia, evening primrose, etc., first from the outside, +then from the inside, and then in vertical section, labeling the +parts as in your other sketches. Notice in the pear or apple how the +ovary is sunk in the hollowed-out receptacle. Where are the other +parts attached? Are they inferior or superior? Hold up a petal to the +light and examine its venation through a lens. (Use for this purpose +a petal from a flower that has stood in red ink for two or three +hours.) Is it parallel-veined or net-veined? If the flowers are +clustered, what is the order of inflorescence? Does the position of +the flowers on their branch correspond to that of the leaf axils on +the same kind of plant? + +[Illustration: FIGS. 297-300.—Flower and sections of pear: 297, +cluster of blossoms, showing inflorescence; 298, vertical section +of a flower; 299, ground plan of a flower; 300, vertical section of +fruit.] + +[Illustration: FIG. 301.—Vertical section of an almond blossom with +petals removed, showing the perigynous arrangement.] + +=227. The stamens.=—Remove the petals from a flower and examine +the stamens with a lens. Notice the attachment and shape of the +anthers. Are they all of the same color? How do you account for +the difference, if there is any? Is the position of the pistil and +stamens such that the pollen from the anthers can readily reach +the stigmas without external aid? Examine the pistil in flowers of +different ages, and see if the stigma is mature (that is, moist and +sticky) at the same time that the anthers are discharging their +pollen. Make an enlarged sketch of a stamen showing the shape of the +anther and the method of opening to discharge pollen. + +=228. The pistils.=—How many pistils do you find in the apple blossom +(or other flower under examination)? Are they distinct, or united? +Find where the styles originate; what do you see there? Make a cross +section of the ovary and count the locules; how does their number +compare with that of the styles? Can you make out the number of +ovules in each? If not, use a young fruit; as it is only an enlarged +ovary, it will show the parts correctly. Compare it with a ripe fruit +and see if all the ovules matured. Can you think of any reasons why +some of them might fail? Do you see any signs of nourishment stored +in the ovary? Name all the ways you can think of in which the ovary +can benefit the ovules and seeds. Draw the ovary in cross and +vertical sections, labeling correctly all the parts. + +[Illustration: FIGS. 302-304.—Diagrams showing arrangement of parts +with reference to the ovary: _bd_, receptacle; _k_, calyx; _kr_, +corolla; _st_, stamens; _fr_, ovary; _g_, style; _n_, stigma; 302, +perigynous; 303, hypogynous; 304, epigynous.] + +=229. The numerical plan of dicotyls.=—Diagram the plan of the flower +in cross and vertical section. How many parts are there in each set? +Can you tell readily the number of stamens? When the individuals of +any set or cycle of organs are too numerous to be easily counted, +like the stamens of the apple, pear, and peach, or the petals of the +water lily, they are said to be _indefinite_. It is very seldom that +perfect symmetry is found in all parts of the flower. The stamens and +pistil, in particular, show a great tendency to variation, so that +the numerical plan is generally determined by the calyx and corolla. +Where the parts are in fives, as in the pear, quince, and wild rose, +the flower is said to be _pentamerous_, or in sets of five. This is +the prevailing number among dicotyls, though other orders are not +uncommon. In the mustard family (220) and other well-known species, +the fourfold order prevails, while some of the saxifrages have their +parts in twos, and the magnolia and the pawpaw have a threefold +arrangement. + +=230. Intermediate types.=—Flowers like the peach and rose represent +an intermediate type in which the calyx, petals, and stamens are +attached to a prolongation of the receptacle that extends above the +ovary, but is not united with it (Fig. 301). In general, a flower is +not considered as belonging to the epigynous kind unless the ovary is +more or less consolidated with the parts around it (Fig. 304). + + + III. STUDY OF A COMPOSITE FLOWER + + MATERIAL.—The largest heads attainable should be selected, as the + florets are small at best, and difficult to handle. The large + cultivated sunflower (_Helianthus annuus_) makes an ideal specimen, + if accessible. Oxeye daisy and dandelion can be obtained throughout + the season almost everywhere, but the former has no pappus, and + the latter does not show the tubular disk flowers. Other common + specimens are: for spring, mayweed, Jerusalem artichoke, coreopsis, + arnica; for late summer and autumn, China aster, golden aster + (_Chrysopsis_), sneezeweed, elecampane—and, in fact, the great + majority of flowers to be found at this season are of the composite + family. Oxeye daisy is used as a model in the text on account of + its general accessibility, but almost any specimen of the radiate + kind will meet all essential conditions of the analysis. + +[Illustration: FIGS. 305-308.—An oxeye daisy: 305, a flower head; +306, vertical section of a head; 307, disk flower; 308, ray flower, +enlarged.] + +=231. The ray flowers.=—Examine the upper side of an oxeye daisy +through a lens. Of what is the yellow button in the center composed? +Count the narrow, petal-like rays disposed around the center. To +decide what they are, look for a small two-cleft body at the base of +the ray; this is the pistil. Do you see any stamens in the ray? An +examination will show that all rays contain pistils, but no stamens; +they are, therefore, not petals, but the corollas of imperfect +flowers. Look at the upper edge of a ray of sneezeweed, coreopsis, +arnica, chicory, etc., for small teeth or notches; these represent +the lobes of a sympetalous corolla. Split one of the tubular corollas +of the disk down one side and open it out flat; does it throw any +light on the morphology of the ray? In many composite plants, as the +sunflower, coneflower, coreopsis, the rays are all _neutral_; that +is, they have neither pistil nor stamens. Are they of any use in such +cases? If you are in doubt, remove all the rays from a head; would +the disk be noticeable enough to attract attention without them? What +is the principal office of the rays? + +=232. The involucre.=—Look at the cluster of green, leafy scales on +the under side of the head. It is not a calyx, but a collection of +bracts, called an _involucre_. Have you ever noticed the bracts under +the separate flowers on a raceme? (161.) What would be the position +of the bracts if all the flowers of the raceme were compacted into a +head like the daisy or sunflower? Is the involucre of any use? Cut +it away gently so as not to disturb the other organs and see what +happens to the rays. + +=233. The disk flowers.=—Cut a vertical section through the head of a +flower and notice the broad, flat receptacle (in some cases round or +columnar) on which the tiny florets are seated. Observe whether it is +naked, or whether it bears chaffy scales inclosing the florets. Make +an enlarged drawing of this section, showing the insertion of the +different parts and labeling them all correctly. What differences do +you observe between the disk and the ray flowers? + +=234. The pappus.=—Open one of the disk flowers with a dissecting +needle and observe the small striate (in some specimens, hairy) +body to which the base of the style is attached. This is the +ovary, inclosed in the lower part of the calyx, which has become +incorporated with it. When mature, it will form a small, one-seeded +fruit called an _akene_. Can you see the ovule? Where is it +attached? (Use a mature akene for this purpose.) In most plants of +this family, the akene is surmounted by delicate hairy bristles, +as in the dandelion, wild lettuce, and groundsel; or by small +chaffy scales, as in the sneezeweed and sunflower, and sometimes by +hooks and barbed hairs, like those of the tickseed, bur marigold, +and cocklebur. These appendages constitute the _pappus_. They are +modifications of the sepals, and serve an important purpose in aiding +the distribution of the seed. Can you suggest some of the ways in +which they may aid in accomplishing this object? + +[Illustration: FIGS. 309-314.—Akenes of the composite family: +309, mayweed (no pappus); 310, chicory (pappus a shallow cup); +311, sunflower (pappus of two deciduous scales); 312, sneezeweed +(_Helenium_, pappus of five scales); 313, sow thistle (pappus of +delicate downy hairs); 314, dandelion, tapering below the pappus into +a long beak. (_After_ GRAY.)] + +[Illustration: FIGS. 315-317.—Flowers of _Arnica montana_, showing +successive stages in pollination: 315, pistil just extruding from +anther tube, covered with pollen, but with stigmatic surfaces closed; +316, stigma opened and mature; 317, stigma recurved to receive pollen +from its own or neighboring anthers if foreign pollen has not reached +it.] + +=235. The stamens and pistil.=—Remove the corolla of a disk flower +carefully so as not to disturb the inclosed organs, and notice how +the stamens are united into a tube by their anthers. Flatten out the +tube and make an enlarged sketch of it, showing the long, narrow +shape of the anthers and their mode of attachment. Can you make out +how they open to discharge their pollen? Examine one of the younger +florets near the center of the disk, and observe that the tip of the +style is inclosed in the anther tube with the lobes of the stigma +pressed tightly together by their inner faces (Fig. 315), so that it +is impossible for any of the pollen to reach the stigmatic surface. +It remains in this position till the anthers have shed their pollen, +then, as may be seen by examining an older flower, the style begins +to elongate, pushing up the pollen that has fallen on the hairy +outside of the closed stigma, and forcing it out of the corolla tube, +where it can be scattered by insects among the other flowers of the +cluster. When the pollen of its own floret has been thus disposed of, +the stigma lobes open and curl outward, ready to receive the pollen +from other flowers. This arrangement is practically universal among +plants of the composite family; can you divine its object? It will be +shown later, that much larger and stronger seeds are produced when +the pistil is pollinated from a different flower, or, better still, +from a different plant of the same species; hence, you see what a +useful adaptation this is. + +=236. Nature of a composite flower.=—It will be evident, from the +examination just made, that the daisy, dandelion, sunflower, etc., +are not single flowers, but compact heads of small blossoms so +closely united as to appear like a single individual; hence they +are said to be _composite_, or compound. They are the most numerous +and widely disseminated of all plants, comprising one seventh of +the entire flowering vegetation of the globe, and are regarded +by botanists as representing the most advanced stage of floral +evolution. Can you point out some of the adaptations to which their +success in solving the problems of plant life is due? (164.) + + + IV. SPECIALIZED FLOWERS + + MATERIAL.—For spring and early summer: sweet pea, black locust, + wistaria, lupine, or any of the characteristic butterfly-shaped + flowers of the pea family. For autumn or late summer: tropæolum, + monkshood, or a bilabiate flower—snapdragon, digitalis, dead + nettle, salvia, catalpa, etc.—of the mint or figwort family. + +=237. Irregularity and specialization.=—Irregularity and bilateral +regularity are, as a rule, indicative of specialization, or +adaptation to a particular purpose, such as the ready distribution +of pollen, or its protection against injury. These adaptations +are more noticeable in the corolla than in other parts, and hence +flowers of this kind are usually classed according to the shape of +their corollas. The most highly specialized flowers in this respect +are the orchids, but they are too rare and difficult of access +to be available objects for study. The most familiar and widely +distributed kinds of specialized corollas are the _bilabiate_, or +two-lipped, and the _papilionaceous_, or butterfly, forms. The +first is characteristic of the mint and figwort families, of which +the toadflax, sage, and catalpa are familiar examples. The second +comprises the well-known papilionaceous flowers of the pea family, +named from the Latin word _papilio_, a butterfly, on account of their +general resemblance to that insect. + +=238. Dissection of a papilionaceous flower.=—Sketch a blossom of any +kind of pea or vetch as it appears on the outside. Are the sepals +all of the same length and shape? If not, which are the shorter, the +upper or the lower ones? + +Turn the flower over and examine its inner face. Notice the large, +round, and usually upright petal at the back, the two smaller ones +on each side, and the boat-shaped body between them, formed of +two small petals more or less united at the apex. Press the side +petals gently down with the thumb and forefinger and notice how +the essential organs are forced out from the little boat in which +they are concealed. Observe how the end of the style is bent over +so as to bring the stigma uppermost when the petals are depressed. +Imagine the legs of a bee or a butterfly resting there as he probed +for honey; with what organ would his body first come in contact when +he alighted? If his thorax and abdomen had previously become dusted +with pollen when visiting another flower, where would the pollen be +deposited? Do you notice anything in the color, shape, or odor of +this flower that would be likely to attract insects? Have you ever +observed insects hovering around flowers of this kind; for example, +in clover and pea fields, and about locust trees and wistaria vines? +What kind of insects, chiefly, have you seen about them? + +[Illustration: FIGS. 318-322.—Dissection of a papilionaceous flower: +318, front view of a corolla; 319, the petals displayed: _v_, +vexillum, or standard; _w_, wings; _k_, keel; 320, side view with +all except one of the lower petals removed, showing the essential +organs protected in the keel: _l_, loose stamen; _st_, stamen tube; +321, side view, showing how the anthers protrude when the keel is +depressed; 322, ground plan. (_After_ GRAY.)] + +Remove the sepals and petals from one side, and sketch the flower in +longitudinal section, showing the position of the pistil and stamens. +Then remove all the petals, and spread in their natural order on the +table before you, and sketch as they lie (Fig. 319). Label the large, +round upper one, _standard_ or _vexillum_; the smaller pair on each +side, _wings_, and the two more or less coherent ones in which the +pistil and stamens are contained, _keel_. + +=239. The stamens.=—Count the stamens, and notice how they are united +into two sets of nine and one. Stamens united in this way, no matter +what the number in each set, are said to be _diadelphous_, that is, +in two brotherhoods. Notice the position of the lone brother, whether +below the pistil—next to the keel—or above, facing the _vexillum_. +Would the projection of the pistil, when the wings are depressed, be +facilitated to the same extent if the opening in the stamen tube were +on the other side, or if the filaments were _monadelphous_—all united +into one set? Flatten out the stamen tube, or sheath, formed by the +united filaments, and sketch it. + +=240. The pistil.=—Remove all the parts from around the pistil, and +sketch it as it stands upon the receptacle. Look through your lens +for the stigmatic surface (223). See if there are any hairs on the +style, and if so, whether they are on the front, the back, or all +around. Can you think of a use for these hairs? Notice how the long, +narrow ovary is attached to the receptacle; is it sessile, or raised +on a short footstalk? If the latter, label the footstalk, _stipe_. +Select a well-developed pistil from one of the lower flowers, open +the ovary parallel with its flattened sides, and sketch the two +halves as they appear under the lens. Notice to which side the ovules +are attached, the upper (toward the vexillum) or the lower, and label +it, placenta. How many locules has the ovary? How many carpels? How +can you tell (216)? + +=241. Plan of the flower.=—Diagram the flower in horizontal and +vertical section, and decide upon the following points:— + + Numerical plan + Symmetry + Regularity + Union of parts + Position of the ovary + +[Illustration: FIGS. 323, 324.—Salvia: 323, a newly opened flower, +showing the pollen-covered anther striking the back of a visiting +bee; 324, an older flower, with the protruding pistil rubbing against +the back of a bee covered with pollen from a younger flower.] + +=242. Significance of these distinctions.=—These distinctions are +important to remember, not only because they are very useful in +grouping and classifying plants, but because they mark successive +stages in the evolution of the flower. In general, flowers of a +primitive type and less advanced organization are characterized +by having their organs free and hypogynous, while the more highly +developed forms show a tendency to consolidation and union of parts, +and the epigynous mode of insertion. Irregularity also, since it +indicates specialization and adaptation to a particular purpose, may +be regarded as a mark of advanced evolution. + +[Illustration: FIGS. 325, 326.—Salvia: 325, longitudinal section +through a flower, showing the rocking connective which is struck at +_a_ by a visiting insect; 326, section of the same flower after being +visited, showing the lower arm of the connective pushed back and +lowering the anther.] + +=243. Dissection of a bilabiate flower.=—Make a similar study of the +flower of a salvia, dead nettle, catalpa, or other specimen of the +bilabiate kind. Make diagrams and report as to (1) numerical plan; +(2) presence or absence of parts; (3) regularity; (4) union of parts; +(5) position of ovary. Observe especially the relative position of +stigma and anthers; is it such that the pollen can reach the stigma +without external aid? Does the peculiar shape of the corolla serve +any other purpose than to attract the attention of insect visitors +by its conspicuous appearance? What is the use of the projecting +underlip? Is it any convenience to a bee, for instance, to have a +platform to rest on while gathering pollen or honey? What is the use +of the arched upper lip? Cut it away and notice the exposed condition +of the stamens and pistil. Turn a flower upside down; what would be +the effect on a visiting bee or butterfly? (Exps. 83, 84.) + +[Illustration: FIG. 327.—Staminodia, transformed stamens of canna +simulating petals: _pet_, petals; _st_, staminodia.] + +[Illustration: FIG. 328.—Flower of a cactus (_cereus greggii_), +showing transition from scales to petals.] + +=244. Morphology of the flower.=—We have seen that the venation of +petals and sepals corresponds in a general way with that of foliage +leaves of the class to which they belong, and that their arrangement +around their axis is analogous to the arrangement of foliage leaves +on the branch. In our study of inflorescence, it was observed that +flowers and flower buds occur in the same positions where leaf buds +occur, and that they are subject to the same laws of arrangement +and growth. We learned, also, in our study of leaves, something +about the wonderful modifications that these organs are capable of +undergoing; and finally, an examination of a number of different +flowers has shown them capable of undergoing modifications to an +equal or even greater extent, and examples of the transition of +almost any floral organ into another may be observed by one who will +take the trouble to look for it. Stamens and petals are found in all +stages of transformation, from the slightly flattened filament of +the star-of-Bethlehem, or the yellow pollen speck on the petal of +a rose, to the brilliant staminodia, or transformed stamens of the +canna (Fig. 327), which simulate petals so perfectly that their real +nature is never suspected by the ordinary observer. The transition +from spines and bracts to the brilliant corolla of the cactus (Fig. +328) is so gradual that we are hardly aware of it till we examine a +specimen and see it actually going on before our eyes. + +It must not be supposed, however, that an organ is ever developed as +one thing and then deliberately changed into something else. When we +speak loosely of one organ being modified into another, the meaning +is merely that it has developed into one thing instead of into +something else that it was equally capable of developing into. + +=245. The course of floral evolution.=—For the reasons mentioned, +the flower is regarded as merely a branch with modified leaves and +the internodes indefinitely shortened so as to bring the successive +cycles into close contact, the whole being greatly altered and +specialized to serve a particular purpose. With this conception +of the nature of the flower, we can readily see that the less +specialized its organs are and the more nearly they approach in +structure and arrangement to the condition of an undifferentiated +branch, the more primitive and undeveloped is the type to which it +belongs. On the other hand, if the parts are highly specialized and +widely differentiated from the crude branch, a proportionately high +stage of floral evolution is indicated. + + + V. FUNCTION AND WORK OF THE FLOWER + + MATERIAL.—For this exercise, flowers of the mallow + family—hollyhock, abutilon, mallow, hibiscus, cotton, okra, + etc.—are particularly recommended because they have pollen grains + so large that they can be studied fairly well with a hand lens. + Lily, tulip, iris, etc., will also meet all essential conditions of + the study outlined in the text. A strand of silk from a pollinated + ear of corn is an excellent example for showing the growth of the + pollen tube, under the microscope. + + APPLIANCES.—A compound microscope; a watch crystal; sugar solution + of 5 to 15 per cent. + + EXPERIMENT 77. TO SHOW THE GERMINATION OF POLLEN GRAINS.—Put a drop + of 5 per cent sugar solution into a watch crystal or a concave + slide, seal by smearing the edges with vaseline, and cover with a + glass to keep out the dust. Examine at intervals of five minutes + under the microscope (a hand lens will show the result with the + specimens recommended, though not so well), and the pollen grains + will be observed to send out long filaments or tubes into the + sirup, as a germinating seedling sends its radicle into the soil. + +=246. Office of the flower.=—The one object of the flower is the +production of fruit and seed, and all its wonderful specializations +and variations of form and color tend either directly or indirectly +to this end. + +=247. Pollination and fertilization.=—It was stated in 215 that only +in very exceptional cases can seed be developed unless some of the +pollen reaches the stigma. This act, called _pollination_, is an +essential step in seed production, but is not sufficient to secure +that end unless it leads to the process known as _fertilization_. +Successful pollination is a necessary preliminary to fertilization, +and the one begins where the other ends. + +=248. The next step toward fertilization.=—Examine with a lens the +pollinated pistil of a mallow, lily, or other large flower, and +notice the flabby, withered appearance of grains that have stood for +some time on the stigma, as compared with those of a newly opened +anther. Can you account for the difference? Touch the tip of your +tongue to the stigma, or apply the proper chemical test, and it will +be seen that the sticky fluid which it exudes, contains sugar. Refer +to Exp. 77 and say what effect this substance has on the pollen. + +[Illustration: FIG. 329.—A pollen grain emitting a tube (magnified).] + +=249. The pollen tube.=—The same thing happens when a pollen grain +falls on the moist surface of the stigma. It begins to germinate by +sending a little tube down into the substance of the pistil, and the +withered appearance of the grains on the stigma results from the +nourishment in them having been exhausted, just as the endosperm of +the seed is exhausted when the embryo begins to germinate. Here, +however, the analogy ends, for the pollen tube is not adapted, like +the radicle of the seedling, to absorb and convey nourishment up to +the other parts, but to feed and carry down to the ovary two small +bodies called _generative cells_, which it discharges there, and +then its work is done and it disappears. So it must be borne in mind +that when we speak of the germination of the pollen grains, we mean +something really very different from the germination of a seed. + +=250. The course of the pollen tube.=—Cut the thinnest possible +section through a freshly pollinated pistil and place under the +microscope. Watch the pollen tubes from the grains on the stigma as +they descend through the style toward the ovary. A pollinated strand +of corn silk—which is only a very much elongated style—is excellent +for this purpose. It is so thin and transparent that no section need +be made, and the tube can be traced as it works its way down through +the entire length of the threadlike style to the young grain, or +ovary, on the cob. The time required for the tube to penetrate to the +ovary varies in different flowers according to the distance traversed +and the rate of growth. In the crocus it takes from one to three +days; in the spotted calla, about five days; and in orchids, from ten +to thirty days. As a rule, it occupies only a few hours. Sometimes +the pistil is hollow, affording a free passage to the pollen tube; +in other cases, it is solid, and the growing tube eats its way down, +as it were, feeding on the substance of the pistil as it grows. How +is it in the flower you are examining? It takes a grain of pollen +to fertilize each ovule, and where more than one seed is produced +to a carpel, as is commonly the case, at least as many pollen +tubes must find their way to each locule of the ovary as there are +ovules—provided all are fertilized. + +[Illustration: FIG. 330.—Diagram of a simple flower, showing course +of the pollen tube: _a_, transverse section of an anther before its +dehiscence; _b_, an anther dehiscing longitudinally, with pollen; +_c_, filament; _d_, base of floral leaves; _e_, nectaries; _f_, wall +of carpels; _g_, style; _h_, stigma; _i_, germinating pollen grains; +_m_, a pollen tube which has reached and entered the micropyle of the +ovule; _n_, stalk of ovule; _o_, base of the inverted ovule; _p_, +outer integument or testa; _q_, inner integument; _t_, cavity of the +embryo sac; _u_, its basal portion; _z_, oösphere.] + +=251. Fertilization.=—When a pollen tube has penetrated to the ovary, +it next enters one of the ovules, usually through the micropyle +(Fig. 330, _m_). There it penetrates the wall of a baglike inclosure +called the _embryo sac_ (Fig. 330, _u_, _t_, _z_), where one of the +generative cells emitted by the pollen tube fuses with a large cell +contained in the embryo sac, known as the _germ cell_, or _egg cell_ +(Fig. 330, _z_). The fusion of these two bodies is what constitutes +fertilization. The cell formed by their union finally develops into +the embryo, and the other contents of the sac into the endosperm, and +the ripened ovules become seeds. + +=252. Stability of the process of fertilization.=—The phenomena +that characterize the functions of fertilization and reproduction +are the most uniform and stable of all the life processes, varying +little not only in different species and orders, but throughout the +whole vegetable kingdom. And since these functions furnish a more +reliable standard for judging of the real affinities of the different +groups than do mere external resemblances, which are more liable +to variation and may often be accidental, they have been chosen by +botanists as the ultimate basis for the classification of plants. + +=253. Embryology.=—The study of the developing plantlet, known as +_embryology_, is a comparatively recent branch of science, and has +greatly enlarged our knowledge of the life history of both plants and +animals, by bringing to light resemblances that exist between the +most widely divergent species in their earlier stages of development +and thus showing traces of a common origin. It has shown further, +that every individual plant or animal, in its development from the +embryo to the mature state, passes briefly through stages apparently +similar to those which the species has traversed in the course of +its evolution. This summary repetition, by the individual, of the +evolutionary progress of its kind is known as the _biogenetic law_, +and through its intelligent application some of the most intricate +problems in both physiology and psychology have been solved. + + + Practical Questions + + 1. Does the biogenetic law throw any light on the resemblances + sometimes observed between leaves of different ages in unlike + species; for example, the fig and the mulberry? (170; Field Work, + p. 195.) + + 2. Can you name any other examples of plants or parts of plants + which show mutual resemblances in their early stages that do not + exist at maturity? + + 3. Are there other causes than those acting under the biogenetic + law to which some of these resemblances may be referred; for + instance, the down and waxy coating on young leaves and bud scales? + (148, 207.) + + + VI. HYBRIDIZATION + + MATERIAL.—Several potted plants of tulip, lily, or any attainable + large flowered kind; or preferably a small plot in a garden or + nursery. + + APPLIANCES.—A pair of dissecting scissors, a camel’s-hair brush, + and some paper bags. + + EXPERIMENT 78. DOES IT MAKE ANY DIFFERENCE WHETHER A FLOWER HAS + ITS OVULES FERTILIZED WITH ITS OWN POLLEN OR WITH THAT OF ANOTHER + FLOWER OF THE SAME KIND?—Carefully remove the _unopened_ anthers + from a bud of a tulip, or other large flower just ready to unfold + (Fig. 331), inclose the mutilated bud in a small paper bag until + the stigma is mature, as shown by stickiness, then transfer to it + with a camel’s-hair brush some pollen from another flower. On the + stigma of a second flower of the same kind place some of its own + pollen, and cover with a paper bag until the stigma withers, to + keep foreign pollen from reaching it by means of wind or insects. + Watch until seeds are matured. Which flower produces the more + seeds or the better ones? Plant the seeds; which produce the more + vigorous progeny? + +[Illustration: FIGS. 331-333.—Flower of Lorillard tomato: 331, newly +opened bud, showing stage in which the stamens should be removed; +332, mature flower: _cx_, calyx; _c_, corolla; _s_, stamens; _st_, +stigma; 333, flower with stamens removed for pollination. (Natural +size.)] + + EXPERIMENT 79. CAN A FLOWER BE FERTILIZED WITH POLLEN OF A + DIFFERENT KIND?—Dust the stigma of a tulip or a lily, from which + the stamens have been removed, with pollen from a narcissus, iris, + or amaryllis. Cover to protect from wind and insects. Are any seeds + produced? + + Experiments of this kind, to be conclusive, ought to be performed + on a sufficient number of plants and through at least three + generations. This is hardly practicable for class work, but + students who are specially interested in the subject may carry + on experiments at home, or supply their place, to some extent, + by observations out of doors, if there are any farms or gardens + accessible. + +[Illustration: FIGS. 334-335.—Seeds of Bartlett pear, showing the +advantage of cross-fertilization: 334, cross-fertilized; 335, +self-fertilized.] + +=254. Self-fertilization= takes place when a stigma is pollinated +from the same flower. Horticulturists have long known that continued +self-fertilization, or “in-breeding” as it is called by nurserymen, +tends to deteriorate a stock; but Charles Darwin was the first to +explain, by a series of pains-taking experiments, the meaning of +those careful adjustments which the more highly organized plants, as +a rule, have developed to guard against it. + +[Illustration: FIG. 336.—Showing the effect of in-breeding on corn in +one generation. The two left-hand rows are from self-fertilized seed.] + +=255. Cross-fertilization= is effected by the pollination of a +stigma from another flower of the same variety or species. As +used by practical horticulturists, the expression means that the +two factors, pollen and ovule, belong to different plants. Since +pollination is the necessary antecedent to fertilization, and the +only means by which we can control it, the breeder’s part in crossing +is concerned with this act only and nature does the rest. Darwin’s +experiments—and they are confirmed by the experience of plant growers +everywhere—prove that the offspring from crossing different plants +of the same kind is usually stronger and more productive than that +from self-fertilized ones; and if the parent stocks are grown in +different places and under different conditions, the offspring +is more vigorous than that from the same kind of plants grown +under like conditions. For instance, plants from crossed seeds of +morning-glory vines growing near each other exceeded in height those +from self-fertilized seeds as 100:76; while the offspring of plants +growing under different conditions exceeded those of the other cross, +in height, as 100:78; in number of pods, as 100:57, and in weight +of pods, as 100:51. Knowledge of this kind, when applied to the +raising of fruits and grains for market, is of incalculable value to +gardeners and farmers, and also to the amateur who raises fruits or +flowers for pleasure. + +=256. Hybridization= is the crossing of two plants of different +species or of widely separated varieties of the same species. The +resulting offspring is a _hybrid_. Hybridization can take place only +within certain limits. If the species are too unlike, the pollen will +either not take effect at all, or the resulting offspring will be too +weak and spindling to live; or if they survive, will not be able to +set seed (Exp. 79). + +[Illustration: PLATE 11.—Hybrid between a red and a white carnation, +showing characters intermediate between the two parents.] + +=257. Effects of hybridization.=—The most important practical uses of +hybridizing are: (1) it “breaks the type” by causing plants to vary, +and thus gives the breeder a fresh starting point for a new strain; +and (2) when the parent species are not too unlike, it accentuates +the good effects of crossing, and sometimes gives rise to offspring +greatly surpassing either parent in size and vigor. In regard to +variability it may act in three ways: (1) the hybrid may wholly +resemble one parent or the other, in which case there is, of course, +no variation; (2) it may resemble one parent more than the other; or +(3) it may show a blending of the characters of the two, as when a +cross between a red poppy and a white gives rise to a light pink, or +a mixed red and white variety. In the first two cases, the characters +of the parent that manifest themselves are said to be _dominant_; +those which do not, _recessive_. + +[Illustration: FIG. 337.—Effect of hybridization between related +species in imparting superior vigor to offspring: _M_, Californian +black walnut (_Juglans californica_), male parent; _F_, Eastern black +walnut (_J. nigra_), female parent; _H_, hybrid.] + +=258. Mendel’s Law.=—So long ago as the middle of the last century +it was discovered by Gregor Mendel, an Austrian investigator, that +hybrids vary in certain cases according to a fixed law, by means +of which the proportionate share of the characteristics of the two +parent forms inherited by the offspring can be foretold with almost +mathematical precision. The controversy over Darwin’s “Origin of +Species,” which was raging at the time, caused Mendel’s discoveries +to be overlooked for a generation, and it is only within the last +few years that their importance has been realized. The principle +of variation demonstrated by him in a series of experiments, +and confirmed by later investigators is, briefly, this: If two +parents differing in some fixed characteristic be crossed, the +entire offspring, in the first generation, will be like the parent +possessing the dominant quality. If all the seed of this generation +is planted and carefully protected from foreign pollen, its offspring +composing the second generation from the parents will vary in the +proportion of ¾ dominants (_D_, _D′_, line 2 of the diagram) to ¼ +recessives (_R_). Planting _all_ the seeds of the second generation +and carefully shielding their progeny from foreign pollen, we +get from _D_, line 2, all pure dominants (_D_, line 3)—that is, +plants producing only their own type, and from _R_, line 2, all +pure recessives (_R_, line 3). But from each of the two sets of +dominants, _D′D′_, line 2, marked “impure” in the diagram, and so +called because their seeds may produce both dominants and recessives, +we get the same result as in the second generation, namely: pure +dominants (_D′D′_, line 3), pure recessives (_R′R′_, line 3), and +impure dominants (_D″D″_, _D″D″_, line 3). If it were possible to +distinguish the seeds of these impure dominants before germination +and plant them only, for no matter how many generations, the result +would always be approximately the same,—¼ pure dominants, ¼ pure +recessives, and ²⁄₄ impure dominants capable of producing both +dominants and recessives in the proportion of 3:1. + +[Illustration: Diagram illustrating Mendel’s Law.] + +=259. Practical applications.=—Four principles of great importance +to plant breeders follow from this law in cases to which it applies: +(1) the absence of variation in the first generation of hybrids is +no sign that it may not occur later; (2) pure recessives always +breed true; hence, if they show the desired character, no further +selection is necessary for that character; (3) pure dominants always +breed true, but the distinction between pure and impure is usually +not apparent in one generation; (4) the descendants of “impure” +parents cannot be depended upon to come true to either type, but +impure dominants may breed recessives, and _vice versa_, with the +presumption, however, of 3:1 in favor of dominants. + + + Practical Questions + + 1. Would hybridization account for some of the diversities + mentioned in 170? (See 257.) + + 2. To what cases would it not apply? (256; Exp. 79.) + + 3. Would it be worth while to try to hybridize the potato and + squash? The squash and pumpkin? The lily and rose? Sweetbrier and + wild rose? Apple and peach? Wild crab and sweet apple? Blackberry + and strawberry? Blackberry and raspberry? Lemon and watermelon? + Lemon and orange? Why, or why not, in each case? (256; Exps. 78, + 79.) + + + VII. PLANT BREEDING + + MATERIAL.—If practicable, visit a market garden, a florist’s + establishment, or, lacking these, the fruit and vegetable stalls of + a city market. + +=260. Fixing the type.=—It is the tendency of plants to vary under +the influence of climate, soil, food supply, crossing, and other +causes perhaps unknown to us, that makes the plant breeder’s art +possible. When a horticulturist sets out to produce a new fruit +or vegetable, he first forms in his mind a clear idea of what he +wants—whether increase of yield or size, resistance to cold, drought, +or disease, improvement in flavor, color, shape, etc., or change +in the time of maturing or flowering (early and late varieties). +Suppose, for instance, he wishes to produce an oxeye daisy with +all the disk florets changed to white ones like the rays. He will +begin by selecting plants with the greatest number of rays and the +most conspicuous ones that he can find, and sowing the seeds of +the flowers which show the greatest tendency to the development of +these qualities. He will continue this process from generation to +generation, rigorously destroying all specimens that do not approach +nearer the ideal sought, until all disposition to “rogue,” as the +tendency to revert is called, has been eliminated. When variations +cease to occur and the seed of the new variety always “come true,” +the type is said to be _fixed_; though some care will always be +necessary to keep it so, as the influence of changed surroundings and +the danger of mixture with foreign pollen must always be provided +against. + +[Illustration: FIG. 338.—A field of pumpkins grown from selected +seed.] + +=261. Survival of the fittest.=—In the fierce struggle continually +going on among both plants and animals for food, shelter, and elbow +room in the world, any individual that happens to vary in a way which +adapts it to its surroundings a little better than its rivals, has an +advantage that will enable it to survive when less favored members of +the species will perish. Its offspring, or some of them, may inherit +this quality and transmit it, with the attendant advantage, to their +posterity, and so on, till that particular breed outstrips all +competitors, and in time, as the less favored intervening forms die +out, becomes differentiated as a new species. This is, in brief, the +doctrine of natural selection and the survival of the fittest. + +=262. Artificial selection.=—Artificial selection enables the breeder +to accomplish more quickly what nature appears to do by the slow +process of natural selection. It is by this means that our choicest +fruits and vegetables have been developed from greatly inferior, and +sometimes inedible, wild forms. Plants respond so readily to the +influence of selection, and the changes brought about by it are so +rapid, that new styles of fruits and flowers succeed each other in +the market with almost as great frequency and in as ready response +to demand as the new styles of women’s bonnets and gowns in the shop +windows. + +[Illustration: FIG. 339.—Variation in blackberry leaves due to +hybridization.] + +=263. Causes of variation.=—While man cannot directly force plants +to vary in any given direction, he can hasten the process of +variation by crossing, or by changing the conditions under which +they are growing. This is called “breaking the type.” Hybridization +furnishes the readiest means to this end. Change of food supply, +especially if accompanied by excess of nourishment, is probably the +expedient that ranks next in effectiveness. Light, temperature, +moisture, character of the soil, exposure to wind, and the like, +also have their influence; and in adapting themselves to changes +in these various conditions, plants are apt to exhibit an unusual +number of variations, when removed from one locality to another, +especially if the difference in soil and climate is very marked. +Now comes the breeder’s opportunity. By taking advantage of such +variations as may occur either spontaneously, or as the result of +his efforts to break the type, he will generally find some that will +meet his requirements; and knowing the effect produced by different +conditions, he can, to a certain extent, influence the course of +variation in the direction desired, by subjecting his specimens to +the conditions that tend to produce it. If he wishes to develop a +dwarf variety, for instance, he will take notice that overcrowding, +lack of nourishment, and cold tend to produce that result in +nature, and by acting on this hint he can direct his efforts more +intelligently. He will learn, too, not to waste time in trying to +breed a plant contrary to its nature. He must not expect to gather +figs from thistles by any art of selection or skill in culture. By +attention to Mendel’s law, a still further saving of time and labor +may be effected. + +It is obvious, from what has been said, that a breeder’s chance of +finding what he wants will be greater in proportion to the number +of individual plants he has to choose from. For this reason, a +horticulturist sometimes uses thousands and hundreds of thousands of +specimens of a single kind in conducting his experiments. In this way +he compresses into a short space of time the advantage that nature +can gain only by spreading her random experiments over a long series +of years, or even centuries. + +[Illustration: FIG. 340.—Mutation in twin ears of corn, showing the +sudden variations that sometimes occur, by which a new type may be +provided without the labor of selection.] + +=264. Mutation and variation.=—There are at least two ways in which +changes in vegetable and animal forms are thought to occur: (1) +by the preservation and fixation through selection and heredity, +of slight differences that may appear from time to time, such +divergences being called “fluctuating variations”; (2) by the +appearance now and then, due to causes as yet unknown, of definite +and sudden changes creating a new form at a single, though perhaps +small, leap. When such a change is temporary and passes away with +the individual in which it first appeared, it is called a “sport,” +and leads to no important results; but when it is inherited by the +offspring, so that it is capable of giving rise to a new species, +it constitutes a “mutation.” The value of a mutation to breeders +in saving time and trouble is obvious. Professor Hugo de Vries, a +Dutch botanist, was the first to call attention to the importance of +mutation and its bearing upon the production of new species. + +=265. Factors in the evolution of species.=—Variation, heredity, and +selection are the three principal agents underlying all changes, +whether for the improvement or deterioration of living organisms. +The influence of external surroundings in keeping up a variation +once begun, or in starting new ones, is also a factor that cannot +be disregarded. It is for this reason that natural species are so +much more stable than those brought about by man. The former, being +evolved in response to natural conditions, are liable to change +only as alterations in their surroundings are brought about by the +slow operation of natural causes. But the types resulting from the +breeder’s art, produced as they often are in response to human +demands and in direct opposition to the requirements of natural +conditions, are in a sense purely artificial, and can be preserved +only by keeping up the artificial surroundings by which they were +developed. Hence, the importance of diligent cultivation and constant +care and tillage, without which the most carefully selected stocks +may quickly “run out” and degenerate into worthless forms. + + + Practical Questions + + 1. Which are the more pliable to the breeder’s art, annuals or + perennials? Why? (91, 93, 262, 263.) + + 2. What advantage is gained by using buds and grafts instead of + seedlings in making new varieties of fruit trees? (257, 259, 260.) + + 3. Would it be practicable to breed new varieties of slow-growing + forest trees, like oak, cypress, redwood, from seeds? Why or why + not? (93, 262, 263.) + + 4. Can you account for the existence of the numerous intermediate + forms between the different species of oaks found in nature? (255, + 257.) + + 5. If a breeder wished to produce a sweet-scented daisy or pansy, + how would he make his selections? (260.) + + 6. Which would be the more useful for his purpose, a plant that + showed a general tendency to variability, or one that remained + steadily fixed to its type? (260.) + + 7. What could he do to break the type? (263.) + + 8. Would an intelligent breeder set out to produce edible roots and + tubers from wheat or barley? (263.) + + 9. Would he think it worth while to try to develop a fleshy fruit + from a filbert or a walnut tree? From a haw? From sheepberry and + black haw? From tupelo (ogeechee lime)? (263.) + + 10. Suppose a florist should wish to change the color of a rose + from pink to deep red; how could he hasten the process? (257, 263.) + + 11. Explain why it is so much easier to produce new varieties of + plants when there are already many kinds in existence, as, for + example, the rose, peach, and chrysanthemum. (255, 256; Exps. 78, + 79.) + + + VIII. ECOLOGY OF THE FLOWER + + + A. THE PREVENTION OF SELF-POLLINATION + + MATERIAL.—Any kind of unisexual flowers obtainable. Some good + examples for illustrating points mentioned in the text are: for + spring and early summer, catkins of almost any of our common forest + trees,—oak, hickory, willow, poplar, etc.; tassels and young ears + of early corn; for summer and early fall, flowers of late corn, and + of melon, squash, pumpkin, or others of the gourd family. Examples + of _dichogamy_ are: evening primrose, showy primrose (_Œnothera + speciosa_), willow herb (_Epilobium_), dandelion, artichoke, + sunflower, or any of the composite family; of _dimorphism_: + English primrose (_Primula_), loosestrife (_Pulmonaria_), bluets + (_Houstonia_), partridge berry; _cleistogamic_: fringed polygala, + violets. Peanuts, while not technically classed as cleistogamic, + are strictly close-fertilized, and approach the type so nearly that + they may be used as an illustration. + +=266. Ecology= is the study of plants and animals in relation to +their surroundings. The principal modifications that flowers undergo +in this respect are in adapting themselves for (1) pollination, and +(2) protection. + +[Illustration: FIGS. 341, 342.—Unisexual flowers of willow: 341, +staminate; 342, pistillate.] + +[Illustration: FIG. 343.—Twig of oak with both kinds of flowers: +_f_, fertile flowers; _s_, _s_, staminate; _a_, pistillate flower, +enlarged; _b_, vertical section of pistillate flower, enlarged; _c_, +portion of one of the sterile aments, enlarged, showing the clusters +of stamens.] + +=267. Unisexual flowers.=—The advantages of cross fertilization were +shown in the last two sections. It was also shown that the first +step taken by the breeder to secure this result is to render the +flower incapable of self-fertilization, by removing the stamens. +Nature accomplishes the same purpose by the more effectual expedient +of providing imperfect, or _unisexual_ flowers, in which stamens +only, or pistils only, occur in the same flower. When the stamens +alone are present, the flower is said to be staminate, or _sterile_, +because it is incapable of producing seeds of its own, though its +pollen is a necessary factor in seed production. If, on the other +hand, the ovary is present and the stamens absent, the flower is +pistillate and _fertile_; that is, capable of producing fruit when +impregnated with pollen. Sometimes both stamens and pistils are +wanting, as in the showy corollas of the garden “snowball,” the +hydrangea, and the rays of the sunflower. Such blossoms are said to +be _neutral_, from the Latin word _neuter_, meaning neither, because +they have neither pistils nor stamens. They can, of course, have no +direct part in the production of fruit, but are for show merely. +(231.) + +=268. Monœcious and diœcious plants.=—When both kinds of flowers, +staminate and pistillate, are borne on the same plant, as in the oak, +pine, hickory, and most of our common forest trees, they are said +to be _monœcious_, a word which means “belonging to one household”; +when borne on separate plants, as in the willow, sassafras, and black +gum, they are _diœcious_, or “of two households.” Draw a flowering +twig of oak, pine, or willow. Where are the fertile flowers situated? +Notice how very much more numerous the staminate flowers are than the +fertile ones. Why is this necessary? (275.) + +[Illustration: FIGS. 344, 345.—Flower of fireweed (_Epilobium +angustifolium_): 344, with mature stamens and immature pistil; 345, +the same a few days older, with expanded pistil after the anthers +have shed their pollen. (_After_ GRAY.)] + +=269. Dichogamy= is the name applied to a condition where the stamens +and pistils mature at different times, as in the evening primrose, +oxeye daisy, and most of the composite family. It is a very common +method in nature for preventing self-pollination, and quite as +effective as the monœcious arrangement, since it renders the flowers +practically unisexual. + +[Illustration: FIGS. 346-347.—Flower of pulmonaria: 346, long styled; +347, short styled.] + +=270. Dimorphism= denotes a condition in which the stamens and +pistils are of different relative lengths in different flowers of +the same species, the stamens being long and the pistils short in +some, the pistils long and the stamens short in others. Flowers +of this sort are said to be _dimorphous_, or _dimorphic_, that +is, of two forms; and some species are even _trimorphic_, having +the two sets of organs long, short, and medium, respectively, in +different individuals. Examples of dimorphic flowers are the pretty +little bluets (_Houstonia cœrulea_), the partridge berry, the swamp +loosestrife, and the English cowslip. Of trimorphic flowers we have +examples in the wood sorrel and the spiked loosestrife (_Lythrum +salicaria_) of the gardens. These flowers were a great puzzle to +botanists until the celebrated naturalist, Charles Darwin, proved +by experiment that the seeds produced by pollinating a dimorphous +flower with its own pollen, or with pollen from a flower of similar +form, are of very inferior quality to those produced by impregnating +a long-styled flower with pollen from a short-styled one, and _vice +versa_. + +[Illustration: FIGS. 348-350.—Three forms of loosestrife (_Lythrum +salicaria_).] + +=271. “Nature abhors self-fertilization.”=—These are the +three principal methods by which nature provides against +self-fertilization. Other cases occur in which the relative position +of the two organs is such that self-pollination is difficult, or +impossible, as in the iris and bear’s grass; or the pollen may be +incapable of acting on the stigma of the flower that produced it. +This aversion to self-fertilization is so great that many flowers, +even when capable of it, will give preference to the pollen of +another plant of the same kind, if dusted with both. From his +observations on the behavior of plants in reference to this function, +Charles Darwin drew the conclusion that “Nature abhors perpetual +self-fertilization.” + +=272. Cleistogamic flowers.=—Apparent exceptions to this rule are +the hidden flowers found on certain plants which seem to have been +constructed with a special view to self-fertilization. They are +called _cleistogamic_, or closed, because they never open, but are +fertilized in the bud; and those of the fringed polygala do not +even rise above ground at all. Flowers of this kind can be found on +several species of violet, concealed under the leaves, close to the +ground; and the flowers of the peanut, found in the same situation, +while they open slightly, are close-fertilized and practically +cleistogamic. They are much more prolific than ordinary flowers, +but are not common, and seem to be a provision against accident, for +the plants producing them are generally provided with other flowers +of the usual kind,—some, as the violet, having elaborate special +adaptations for cross fertilization. + + + Practical Questions + + 1. Why does a strawberry bed sometimes fail to fruit well, although + it may flower abundantly? (267, 268.) + + 2. Are berries found on all sassafras trees? On all buckthorns? + Hollies? + + 3. Would a solitary hop-vine produce fruit? A solitary ash tree? + (267.) + + 4. Why is a mistletoe bough with berries on it so much harder to + find than one with foliage merely? (267, 268.) + + + B. WIND POLLINATION + + MATERIAL.—In spring, catkins of forest trees, staminate and + pistillate flowers of pine. At nearly all seasons, heads of grain + and panicles of various kinds of grass can be obtained. For + experiment, a potted plant of any kind, just about to bloom, may be + used. + + EXPERIMENT 80. TO TEST THE EFFECT OF SHUTTING OUT EXTERNAL + AGENCIES.—Tie paper bags over flower buds of different kinds when + nearly ready to open and leave until the flowers have withered. + On removing the bags, mark with colored threads the flowers that + had been covered, and watch until seed time. Do you notice any + difference in the number, size, or weight of the seed produced by + them and by those of the same kind left exposed? How do you account + for the difference, if there is any? By what agencies could foreign + pollen have been carried to the stigmas of the exposed flowers? If + any of the covered specimens wither and drop their seed vessels + without any attempt to fruit, examine a fresh flower, and see if it + is capable of self-pollination. + + As already explained, experiments of this kind, to be conclusive, + should be tried on as many specimens as possible. The greater the + number of species and individuals included, the better. Where it is + not practicable to carry on experiments by the class, pupils who + are interested can make them at home. + +=273. The problem of pollination.=—When a plant has provided against +self-pollination, its problem is only half solved, as it must now +depend upon the conveyance of pollen to the stigma by extraneous +means. + +[Illustration: FIG. 351.—Feathery stigmas of a grass adapted to wind +pollination.] + +=274. Adaptations to wind pollination.=—A very large number of +plants, among which are included nearly all our principal forest +trees, grains, and grasses of every kind, depend exclusively upon +the wind for the distribution of their pollen. This being the case, +it is, of course, an advantage to them to get rid of all unnecessary +appendages that might hinder a free play of the wind among their +flowers, and so they consist, as a rule, of essential organs only +(Figs. 341, 342). Such flowers are often distinguished, however, +especially among grasses and low herbs, by large, feathery stigmas +that are well adapted to catch and hold any stray pollen grains which +may be floating in the air. Place a stigma of oat or other grass +under the microscope and you will probably see a number of pollen +grains clinging to its branches. + +=275. The disadvantages of wind pollination.=—This is a very clumsy +and wasteful method, however, for so much pollen is lost by the +haphazard mode of distribution that the plant is forced to spend its +energies in producing a vast amount more than is actually needed, +and great masses of it are frequently seen in spring floating like +patches of sulphur on ponds and streams in the neighborhood of pine +thickets. Like those that are self-pollinated, wind-pollinated +flowers are generally very inconspicuous, devoid of odor, and of all +attractions of form or color, because they have no need of these +allurements to attract the visits of insects. Besides being wasteful, +wind pollination is very uncertain. The pollen cannot be blown about +very well unless it is dry, and in rainy weather it may all be rotted +or washed away before it can reach the stigmas that are ready to +receive it. + + + Practical Questions + + 1. Why do the flowers of oak, willow, and other wind-fertilized + plants generally appear before the leaves? (274.) + + 2. Can you account for the showers of “sulphur” sometimes reported + in the newspapers? (275.) + + 3. Do you see any connection between the feathery stigmas of most + grasses and their mode of pollination? (274.) + + 4. Why are house plants not apt to seed so well as those left in + the open? (Exp. 80.) + + 5. Why are the tassels of corn placed at the tip of the stalk? + (274.) + + 6. Can you trace any connection between the winds and the corn + crop? (274.) + + 7. If March winds should cease to blow, would vegetation be + affected in any way? (274.) + + 8. Why are wind-fertilized plants generally trees or tall herbs? + (274.) + + 9. Is it good husbandry to plant different varieties of corn or + other grain in the same field, if it is desired to keep the strain + pure? (255, 274.) + + 10. Is water a good pollen carrier? (275.) + + 11. What is the only class of plants it is likely to reach? + + 12. What is the only other agency, besides wind and water, by which + this office can be performed? + + + C. INSECT POLLINATION + + MATERIAL.—Half a dozen panes of glass, about 6 × 9; squares of + bright-colored cloth or paper; a few spoonfuls of honey or sirup; + perfumes of various kinds, preferably flower extracts; fetid and + disagreeable smelling substances, such as a bit of decaying animal + or vegetable matter. Observations on living plants can best be made + out of doors or in a greenhouse, as opportunity offers. + + EXPERIMENT 81. HAS THE COLOR OF FLOWERS ANY ATTRACTION FOR + INSECTS?—Place half a dozen panes of ordinary window glass out of + doors or in an open window to which insects can have free access. + Lay under the first pane a piece of black paper or cloth, and under + the others bright-colored pieces of red, blue, white, yellow, and + purple. Drop on the center of each pane a little honey or sirup, + and watch. Do insects show any color preferences? Which color + attracts fewest visitors? Which most? + + EXPERIMENT 82. DOES ODOR INFLUENCE INSECTS?—Try the same experiment + with different odors, removing the bright colors and sprinkling + some kind of perfume on each pane. Try also the effect of decaying + meat and other malodorous substances. Are any insects attracted + by these? What kinds? Does this account for the noisome smells + of the “carrion-flower” and skunk cabbage? What kinds of insects + are attracted by sweet-smelling substances? Do the greater number + appear to be attracted by these, or by foul odors? Are flowers of + the sweet-smelling or the foul-smelling kind more common in nature? + Do insects seem to be more strongly influenced by colors or by + odors? + +=276. The color of flowers=, being an adaptation to changing external +conditions, is a very unstable quality, and varies greatly within +the limits of the same species. Even on the same stem, flowers of +different colors are often found, due, probably, to hybridization. +Yet, notwithstanding all this apparently random intermingling of +hues, the range of color for each species is confined, approximately, +within certain limits. Nobody has ever seen a blue rose or a yellow +aster; and though the florist’s art is constantly narrowing the +application of this law, it still remains true that in a state of +nature, certain colors seem to be associated together in the floral +art gamut. Yellow is considered the simplest and most primitive color +in flowers, and blue the latest and most highly evolved. Yellow, +white, and purple, in the order named, are the commonest flower +colors in nature; blue, the rarest. Do you see any relation between +these facts and the color preferences of insects? + +=277. Advantages of insect pollination.=—It is evident that this is +a much more certain as well as a more economical method of securing +pollination than through the haphazard agency of wind or water. +In probing around for the nectar or the pollen upon which they +feed, these busy little creatures get themselves dusted with the +fertilizing powder, which they unconsciously convey from the stamen +of one flower to the pistil of another. Insects usually confine +themselves, as far as possible, to the same species during their +day’s work, and since less pollen is wasted in this way than would +be done by the wind, it is clearly to the advantage of a plant to +attract such visitors, even at the expense of a little honey, or of a +liberal toll out of the pollen they distribute. + +=278. Special partnerships.=—Some plants have adapted themselves +to the visits of one particular kind of insect so completely that +they would die out if that species were to become extinct. The +well-known alliance between red clover and the bumblebee was brought +to light when the plant was first introduced into Australia. It grew +luxuriantly and blossomed profusely, but would never set seed till +the bumblebee was introduced to keep it company. + +[Illustration: FIG. 352.—Pod of _yucca_ pierced by the _Pronuba +yuccasella_.] + +[Illustration: FIG. 353.—Pronuba pollinating pistil of yucca.] + +[Illustration: FIG. 354.—Moth resting on yucca blossom.] + +A remarkable partnership of this kind exists between the _pronuba_, +or yucca moth, and the flowering yuccas, of which the bear’s grass +and Spanish bayonet are familiar examples. The pods of these plants +are never perfect, but all show a constriction at or near the middle, +such as is sometimes seen in the sides of wormy plums and pears. This +is caused by the larvæ of the moth, which feed upon the unripe seeds. +A glance under the nodding perianth of a yucca blossom (Fig. 354) +will show that the short stamens are curved back from the pistil in +such a manner that, under ordinary circumstances, the pollen cannot +reach the stigma except by the rarest accident. But the yucca moth, +as soon as she has deposited her eggs in the seed vessel, takes care +to provide a crop of food for her offspring by gathering a ball of +pollen in her antennæ and deliberately plastering it over the stigma +(Fig. 353). In this way fertilization of the ovules and maturing of +the fruit is secured. The larvæ feed on the unripe seeds for a time, +but so few are destroyed in proportion to the number matured that the +plant can well afford to pay the small toll charged in return for the +service rendered. + +[Illustration: FIG. 355.—Upper boughs of a caprifig tree, showing an +abundant crop of spring fruit.] + +[Illustration: FIG. 356.—Female wasps issuing from the galls of +caprifigs, in which the eggs are laid.] + +=279. Caprification of the fig.=—A more complicated case of +specialization is that of the Smyrna fig of commerce—the only one +of the species that is capable of perfecting seeds. The staminate +flowers are borne on a separate tree, the caprifig, which grows wild +in the countries bordering on the Mediterranean. The caprifigs, +as the fruit of this tree is called, are worthless except as the +breeding and nesting places of a small insect, the fig wasp. This +insect is the necessary agent in conveying pollen from the stamens of +the caprifig to the pistils of the Smyrna fig, which it penetrates at +certain seasons of the year in the effort to lay its eggs. In order +to insure _caprification_, as this process is called, the caprifigs +are strung by hand on fillets of cord or raffia and hung about on the +trees which are to be fertilized. In this case we have an example +of a threefold partnership between man, the fig tree, and the wasp, +which is necessary to the existence of two of the parties. + + + D. PROTECTIVE ADAPTATION + + EXPERIMENT 83. ARE THE FLORAL ENVELOPES OF ANY USE?—Carefully + remove the calyx and corolla from a young flower bud on a growing + plant and see what will happen. Remove them from a flower just + unfolding. Mark each by tying a colored thread lightly around the + petiole and see if it sets as many seeds, or as good ones, as the + unmutilated flowers on the same plant. + + EXPERIMENT 84. IS THE POSITION OF A FLOWER ON THE STEM OF ANY + IMPORTANCE?—Invert a blossom of pea or sage, and see what parts + would come in contact with the body of a visiting insect. How would + its chances for pollination be affected? Try to make a flower grow + in an inverted position by tying or weighting it down, and watch + the effect on seed production. + + EXPERIMENT 85. IS THE POSITION OF FLOWERS ON THE STEM INFLUENCED + BY LIGHT?—Place a potted plant with expanding flower buds near a + window so that the light will reach it from one side only, and + notice the position of the buds. After a day or two reverse the + position with regard to light, and watch whether any change of + position takes place. + +[Illustration: FIGS. 357-359.—Flower of monkshood, showing the +changes by which it returns to its original position under the +influence of geotropism after the axis of inflorescence, s, has +been inverted: 357, inverted position; 358, change due to negative +geotropism; 359, change due to lateral geotropism.] + + EXPERIMENT 86. IS THE POSITION OF FLOWERS ON THE STEM INFLUENCED + BY GEOTROPISM?—Lay a potted plant of lily of the valley, larkspur, + gladiolus, or digitalis in a horizontal position, tie the main stem + to keep it from changing its direction of growth, and leave for two + or three days in a place where it is lighted equally on all sides. + How do the individual flowers behave? What part bends to turn them + up? Vary the experiment by turning the pot bottom upwards so + that the flowering axis will point downwards. This can be done by + inclosing the pot in a bag of strong cheesecloth, with the string + tied loosely but firmly around the foot of the stem to prevent the + contents from falling out, and suspending the whole bottom upwards. + In making these experiments, use flowers that grow in a long + cluster, or raceme, and hold the main axis in a vertical position + by tying or weighting it down. Watch the behavior of the individual + flowers. Arrange another pot containing the same kind of plant, in + the same way, and suspend one in a dark place, keeping the other + in the light. Does the same movement take place in both? Is it in + response to light, or to gravity? + +[Illustration: FIGS. 360, 361.—Protection of pollen in the thistle: +360, position at night, or during wet weather; 361, position in +sunshine.] + +[Illustration: FIGS. 362, 363.—A bell flower: 362, position in +daylight; 363, position at night, or during wet weather.] + +=280. Means of protection.=—Where plants have adapted themselves to +insect pollination, it is, of course, important to shut out intruders +that would not make good carriers. In general, small, creeping +things, like ants and plant lice, are not such efficient pollen +bearers as winged insects, and hence the various devices, such as +hairs, scales, and constrictions, at the throat of the corolla, by +means of which their access to the pollen is prohibited. To this +class of adaptations belong the hairy filaments of the spiderwort, +the sticky ring about the peduncles of the catchfly, the swollen +lips of the snapdragon, the scales or hairs in the throat of the +hound’s-tongue, the velvet petals of the partridge berry, and the +recurved edges of corollas like those of the morning-glory and +tobacco, over which small crawling insects cannot easily climb. + +Of flowers that are pollinated by night moths, some close during +the day, as the four-o’clock and the evening primrose; and _vice +versa_, the morning-glory, dandelion, and dayflower (_Commelyna_) +unfold their beauties only in the sunlight. For similar reasons, +night-blooming flowers are generally white or very light-colored, and +shed their fragrance only after sunset. A nodding position is assumed +by many flowers at night, or during a shower, to keep the pollen from +being injured by dew or rain. + +[Illustration: FIG. 364.—A flower of the trumpet vine (_Tecoma +radicans_) adapted to pollination by humming birds and humming bird +moths, which has been pierced by a bee or bird for honey.] + +[Illustration: FIG. 365.—Head of the swordbill, a bird adapted to +feeding on nectar from long, tubular corollas.] + +=281. Insect depredators.=—The secretion of honey is a common +means of attracting insects, and various adaptations, such as +spurs, sacs, and pockets, are provided for protecting it against +unwelcome intruders. In general, plants that have long, tubular +flowers, like the trumpet honeysuckle (_Lonicera sempervirens_) and +the trumpet vine, are reserving their sweets for humming birds, or +long-tongued moths and butterflies. This protective device is not +always successful, however, against insect depredators, for it is not +uncommon to find such corollas with a puncture near the base, made by +wasps or bees, and sometimes by humming birds themselves, in their +impatience to get at the feast before the flower is open. Through the +breach thus made, a rabble of petty thieves can then find entrance. + + + Practical Questions + + 1. Of what use is the brilliant coloring of the camellia? The large + flowers of the magnolia? The perfume of the rose and the violet? + The fetid odor of the ailanthus? (277; Exps. 81, 82.) + + 2. Are the tastes of insects in regard to odors always the same as + ours? (Exp. 82.) + + 3. Have flowers any economic value except for decorative purposes? + + 4. Can you name any that are used as food or beverages? Any that + furnish spices and flavorings? Drugs, medicines, or dyes? + + 5. What commercial food product is obtained almost entirely from + flowers? + + 6. Name some of the flowers that are most valued by the beekeeper. + + 7. Mention another important industry that is entirely dependent on + flowers. + + 8. Name some of the flowers that are most important to the perfumer. + + 9. Why do the seeds of fruit trees so seldom produce offspring true + to the stock? (256, 257, 271, 277.) + + 10. Would you place a beehive near a field of buckwheat? Of clover? + Near a strawberry bed? In a peach orchard? Near a fig tree? Under a + grape arbor? + + 11. Why are very conspicuous flowers, like the camellia, hollyhock, + and pelargoniums, so frequently without odor? + + 12. Why is the wallflower “sweetest by night”? (280.) + + 13. What advantage can flowers like the morning-glory gain by their + early closing? (280.) + + 14. Of what use to the cotton plant, Japan honeysuckle, and + hibiscus is the change of color their blossoms undergo a few hours + after opening? (277, 278, 280.) + + 15. Why does the Japan honeysuckle, which has run wild so + abundantly in many parts of our country, produce so few berries? + (278, 280.) + + 16. If the trumpet vine grows in your neighborhood, examine a + number of corollas and account for the dead ants found in them. + Account also for the large hole (sometimes three quarters of an + inch in diameter) often found near the base of the tube. (281.) + + 17. Do you see any connection between the greater freshness and + beauty of flowers early in the morning, and the activity of bees, + birds, and butterflies at that time? + + 18. The flowers most frequented by humming birds are the trumpet + honeysuckle, cardinal flower, trumpet vine, horsemint (_Monarda_), + wild columbine, canna, fuchsia, etc.; what inference would you draw + from this as to their color preferences? + + + Field Work + + 1. The ecology of the flower is so suggestive a subject and so + peculiarly appropriate to outdoor work that it seems hardly + necessary to point out the many attractive fields of inquiry it + opens to the student of nature. In this way alone can experiments + in insect pollination be carried on to the best advantage. Try + the effect of enveloping buds of various kinds in gauze so as + to exclude the visits of insects, and note the result as to the + production of fruit and seed. Envelop a cluster of milkweed + blossoms in this way and notice how much longer the flowers so + protected continue in bloom than do the others; why is this? Try + the same experiment upon the blooms of cotton and hibiscus, if you + live where they grow, and see whether the characteristic change in + color occurs in flowers from which insects have been excluded, and + whether good seed pods are produced by them. Try the effect upon + fruit production of excluding insects from clusters of apple, pear, + and peach blossoms. + + 2. Make a list of all the outdoor plants, both wild and cultivated, + that are found blooming in your neighborhood, keeping a record of + the earliest specimens of each as you find them. The best way is to + keep a sort of daily calendar, and at the end of each month give a + summary of the species found in bloom during that period. In this + way a fairly complete annual record of the flowering time of the + different plants for that vicinity will be obtained. The record + should be kept up the whole year round. Don’t stop in winter, + but go straight on through the coldest as well as the hottest + season, and you will make some surprising discoveries, especially + if the record is continued year after year. Give the common name + of each plant, adding the botanical one if you know it. Any facts + that you may know or may discover in regard to particular plants, + such as their medicinal or other uses, their poisonous or edible + properties, the insects that visit them, and in the case of weeds, + their origin and introduction, will greatly enhance the interest + and value of the record. + + + + +CHAPTER VIII. FRUITS + + + I. HORTICULTURAL AND BOTANICAL FRUITS + + MATERIAL.—Green ears of corn or wheat, fresh pods of beans, young + fruits of apple, grape, tomato, melon, buckeye, chestnut, or pecan. + A young fruiting stem of squash, gourd, or tomato. + + APPLIANCES.—Coloring fluid, glasses of water, a piece of cardboard, + tin-foil, vaseline. + + EXPERIMENT 87. WHERE DO THE FOOD SUBSTANCES CONTAINED IN FRUITS + COME FROM?—Apply your food tests to the pulp of a young apple, + squash, bean pod, chestnut, buckeye, or a “green” ear of corn or + wheat, and see what it contains. Test the stem and roots of a plant + of the same kind in the same way. Do you find the same foods in + them? Where is the food stored? + + EXPERIMENT 88. THROUGH WHAT PARTS OF THE STEM AND FRUIT DO WATER + AND NOURISHMENT TRAVEL TO THE SEED?—Cut a young squash or cucumber + from the vine, leaving stem enough to insert by its cut end in a + glass of eosin solution. Leave for two or three days, then make a + vertical section through the stem and fruit. What course has the + liquid followed? Can you trace some of it into each seed? Do you + see now a use for the seed stalk and the rhaphe? + + EXPERIMENT 89. DOES THE SURFACE OF FRUITS GIVE OFF WATER BY + TRANSPIRATION?—Try Exp. 59, using in place of leaves a young + squash, eggplant, or a bunch of grapes, and after a day or two + notice whether any moisture has been given off. If the fruit skin + gives off moisture, it is natural to expect that it would be + provided with stomata, like other transpiring organs. To find out + whether this is so, place a thin piece of the outer epidermis of + a grape, tomato, plum, or apple under the microscope. Do you find + stomata on any of them? Do you see anything else? Try the skin of + an apple, and compare the corky dots you find there with those on + the bark of a young dicotyl stem (118) and decide what they are. + + EXPERIMENT 90. WILL FRUITS RIPEN WELL IN THE ABSENCE OF LIGHT AND + AIR?—Envelop a number of immature fruits in bags of dark cloth or + paper so that no light can reach them. Keep a number of others well + coated with oil or vaseline, and watch. Do the fruits so treated + mature as quickly or develop as fully as those of the same kind + left untreated? + +[Illustration: PLATE 12.—The improvement of fruits by cultivation and +selection: 1, the common wild gooseberry; 2, Houghton gooseberry, +seedling of the wild form; 3, Downing gooseberry, seedling of the +Houghton. (All natural size, adapted from Bailey.)] + + EXPERIMENT 91. WHAT IS THE USE OF THE RIND TO THE FRUIT?—Select + two apples of equal size, peel one, and weigh both. After 12 to 24 + hours, weigh them again. Which shows the greater loss in weight? + Leave peeled and unpeeled fruits in an exposed place and see which + is the more readily attacked by insects. Which decays the sooner? + What are some of the uses of the rind? + +=282. What is a fruit?=—Horticulturally and commercially the +distinction between a fruit and a vegetable depends very much upon +the use we make of it—whether as food, or as a mere gratification of +the palate. Broadly speaking, those fruits that are lacking in sugar, +as the tomato and cucumber, are classed as vegetables. Botanically, a +fruit is any ripened seed vessel, or ovary, with such connected parts +as may have become incorporated with it; and hence, to the botanist, +a boll of cotton, a tickseed, or a cocklebur is just as much a fruit +as a peach or a watermelon. + +=283. Classification of fruits.=—For convenience of description, +fruits are classed as: + +(_a_) Dry or fleshy, according as they have a more or less hard and +bony, or soft and fleshy, texture. + +(_b_) Dehiscent, or indehiscent, according as they open at maturity +in a regular way to discharge their seed, or remain closed until the +covering wears away or is burst by the germinating embryo. + +Fleshy fruits are very seldom dehiscent, though some few, as the +balsam apple and the chayote, or one-seeded squash, discharge their +seed when mature. The banana and some other fleshy fruits, when +peeled, separate along regular lines, and in this respect behave very +much as if they were fleshy pods. + +=284. When is a fruit ripe?=—A fruit is ripe horticulturally, when +it is good to eat; it is ripe botanically, when it has set its seed. +Many of our choicest table fruits, such as the pineapple, banana, and +most varieties of fig, seldom are botanically ripe, since they rarely +produce perfect seeds. + +It is the constant effort of the horticulturist to develop those +parts of a plant that are useful to man, while in a state of nature +the plant seeks to develop such parts as best serve its own purpose +in the struggle for existence. The plants most useful to man have, +as a general thing, been subjected to a long course of artificial +breeding and selection. They are forced developments, often +monstrosities, from the plant’s point of view, if we could conceive +of it as capable of having an opinion. Nature is continually striving +to reclaim them; and if left to themselves, they must either obey +“the call of the wild,” or die out. + +[Illustration: FIG. 366.—A seedless citrange, hybrid between the +orange and the lemon.] + +=285. Seedless fruits and vegetables.=—As the seed is the most +important thing to the plant, the edible parts in wild fruits are, as +a rule, subsidiary to its development. In a state of nature, fruits +will generally wither and drop from the stem, if for any reason they +have become incapable of perfecting their seed. It is only in a few +kinds, limited to those which can successfully propagate themselves +by other means, that the production of seed does not take place. +Among cultivated species, however, where propagation is carefully +provided for by man, the seed is of less importance, and sterile +varieties that might soon die out under natural conditions, continue +their existence indefinitely under his fostering hand. The seeds +of edible fruits are, as a general thing, both indigestible and +unpalatable (21), and hence the efforts of the horticulturist are +directed largely to getting rid of them, or to very greatly reducing +their size and number in proportion to the edible parts. + +=286. How seedless fruits arise.=—The perfecting of seed requires a +great consumption of food and energy on the part of the plant, and +when it is led, for any reason, to expend an unusual amount of force +in some other function,—as for instance, in producing tubers or in +growing bulbs,—it is apt to bear few seeds and to depend more or less +completely upon other methods of reproduction. + +Among cultivated plants, selection on the part of man, whether +conscious or unconscious, has perhaps contributed more than any +other cause to bring about the same result. To this agency is +probably due the development of our common domestic fig, of which +over four hundred varieties that mature fruits without fertilization +are cultivated in the United States alone. The fig was one of the +earliest fruits known to cultivation; and the early navigators, +ignorant of the processes of fertilization, would naturally, in +choosing specimens to carry home with them, select only fruit-bearing +trees. Such of these as matured fruits would be preserved and +propagated, until, by repeated selection, hundreds of edible +varieties have been developed that ripen fruits without caprification +(279). + +=287. The use of the fruit to the plant.=—The object of the fruit is +to furnish protection to the seeds during their period of development +and inactivity, and to aid in various ways the work of dispersal. +It probably takes part also in digesting and diffusing nourishment +for the use of the developing seeds. It has been shown in previous +chapters that plants, almost without exception, are in the habit of +storing up food in various ways as a provision for fruiting. That a +large portion of the stored nourishment is used up in the performance +of this function is proved by its disappearance from those parts—for +example, from fleshy roots, such as beets and turnips, after they +have “gone to seed.” + + + Practical Questions + + 1. What is the use of the down on the peach? The bloom of the plum + and grape? [202, (1); Exp. 91.] + + 2. Why are apples, pears, plums, and other fleshy fruits nearly + always rosier on one side than on the other? (Exp. 90.) + + 3. Can annuals be improved in any other way than by seed selection? + + 4. Would a seedless annual be perpetuated under natural conditions? + + 5. Why is decrease of moisture and increase of light desirable as + the fruiting season approaches? (126, 127; Exp. 90.) + + 6. Why are turnips, carrots, and other fleshy roots unfit to eat if + left over till the plants have seeded? (92, 287.) + + + II. FLESHY FRUITS + + MATERIAL.—A specimen of each of the four principal kinds of fleshy + fruits. Examples of the pome are: apple, pear, quince, rose hip, + haw; of the berry: grape, tomato, cranberry, currant, gooseberry, + lemon; of the pepo: melon, squash, pumpkin; of the drupe: peach, + plum, cherry, dogwood. Specimens of the commoner kinds can nearly + always be found in the market; if nothing better is available, + pickled and dried ones may be used—figs, prunes, dates, raisins, + etc. + +[Illustration: FIG. 367.—Outside of an apple, showing lenticels on +the skin.] + +=288. Dissection of a pome fruit.=—Examine with a lens the outside of +an apple or a pear. Can you make out the lenticels? What difference +in color do you notice between the ripe and unripe fruit? What +difference in taste? What substance would you judge from this, do +ripe fruits contain which green ones do not? Test both kinds for +sugar and starch; which contains the more of each? Strictly speaking, +sugar and starch are merely different forms of the same chemical +compound. In ripe fruits the starch has been cooked by the sun and +converted into sugar. + +With the point of a pencil separate the little dry scales that cover +the depression in the center of the fruit at the end opposite the +stem. How many of them are there? How does this accord with the +plan of the flower as outlined in 229? They are the remains of the +sepals, as will be more apparent on comparing them with the larger +and more leaflike ones on a hip, which is clearly only the end of the +footstalk enlarged and hollowed out with the calyx sepals at the +top. Cut a cross section midway between the stem and the blossom end, +and make an enlarged sketch of it. Label the thin, papery walls that +inclose the seed, _carpels_. How many of them are there, and how many +seeds does each contain? The carpels, together with all that portion +of the fruit which surrounds and adheres to the ovary, constitute +the _pericarp_, or wall of the seed vessel. The fleshy part of the +apple is no part of the ovary proper, but consists merely of the +receptacle, or end of the footstalk, which becomes greatly enlarged +and thickened in fruit. Look for a ring of dots outside the carpels, +connected (usually) by a faint scalloped line. How many of these dots +are there? How do they compare in number with the carpels? With the +remnants of the sepals adhering to the blossom end of the fruit? + +[Illustration: FIG. 368.—Cross section of a pome: _pl_, placenta; +_c_, carpels; _f_, fibrovascular bundles.] + +[Illustration: FIG. 369.—Vertical section of a pome: _p_, peduncle; +_f_, fibrovascular bundles; _s_, seeds; _pl_, placenta; _c_, carpel.] + +Next make a vertical section through a fruit, and sketch, enlarging +it sufficiently to show all the parts distinctly. Observe the line of +woody fibers outside the carpels, inclosing the core of the apple. +Compare this with your cross section; to what does it correspond? +Where do these threads originate? Where do they end? Can you make +out what they are? (176.) Notice how and where the stem is attached +to the fruit. Label the external portion of the stem, _peduncle_; +the upper part, from which the fibrovascular bundles branch, the +_receptacle_. It is the enlargement of this which forms the fleshy +part of the fruit. Try to find out, with the aid of your lens and +dissecting pins, the exact spot at which the seeds are attached to +the carpels, and label this point, _placenta_. Notice whether it is +in the axis where the carpels all meet at their inner edges, or on +the outer side. Observe, also, whether the seed is attached to the +placenta by its big or its little end. If you can find a tiny thread +that attaches the seed to the carpel; label it, seed stalk. Fruits +of this kind are classed, botanically, as _pomes_. Write, from your +analysis, a definition of the pome. + +[Illustration: FIGS. 370, 371.—Enlarged receptacle of Carolina +allspice (_Calycanthus_), containing fruits attached to its inner +surface: 370, exterior; 371, vertical section.] + +=289. Modifications of the receptacle.=—Compare with the drawings +you have made, a haw and a hip. What points of agreement do you see? +What differences? Which of the two more closely resembles the typical +pome? The receptacle is subject to a variety of modifications, +and forms a part of many fruits, for example, the fig, lotus, and +calycanthus (Figs. 370, 371); but a fruit is not a pome unless the +containing receptacle becomes more or less soft and edible. + +=290. The pepo, or melon.=—Next examine a gourd, cucumber, squash, or +any kind of melon, and compare its blossom end with that of the apple +or pear. Do you find any remains of a calyx, or other part of the +flower? Examine the peduncle and observe how the fruit is attached +to it. Can you tell what made the outer epidermis of the rind? Put +a small piece under the microscope; do you see any stomata, or +lenticels? Cut cross and vertical sections, and sketch them, labeling +each part. There may be some difficulty in making out the carpels, +for they are not separate and distinct as in the pome, but confluent +with the enlarged receptacle, which in these fruits forms the outer +portion of the rind, and also with each other at their edges, so +as to form one unbroken circle, as if they had all grown together. +And this is precisely what has happened. The placentas are greatly +enlarged and modified, and it may be necessary to refer to the +diagram, Fig. 372, _c_, in order to make them out. How many locules, +or chambers, are there in your specimen? How many placentas? Notice +that these are central and double, but extend to the pericarp before +dividing so that they appear to be parietal, and twice their real +number, which is only three. Are the seeds vertical, as in the apple, +or horizontal? Look for the little stalk, or thread, that attaches +them to the placenta. + +_Pepo_ is the name given by botanists to this kind of fruit. Write in +your notebook a proper definition of it, from the specimens examined. + +[Illustration: FIG. 372.—Cross section of gourd: _c_, one of the +carpels in diagram. (_After_ GRAY.)] + +[Illustration: FIGS. 373, 374.—A potato berry: 373, exterior; 374, +cross section.] + +=291. The berry.=—Examine a tomato, an eggplant, a grape, cranberry, +lemon, or orange, in both cross and vertical section, and compare +it with the melon and the apple. What differences and resemblances +do you find? Cut a cross section, and draw, showing the attachment +of the seeds. How many locules are there? Normally the tomato is a +two-celled fruit, like the potato berry (Fig. 374), but it has been +so modified by cultivation that the original plan is not always easy +to distinguish. See if you can make it out. Do the seeds in your +specimen appear to be healthy and well developed, or are some of them +small and aborted? How do you account for this? (285, 286.) What +difference do you notice in color between the ripe and unripe fruit? +Write a definition of the berry from the study you have made of it. + +Berries are the commonest of all fleshy fruits, and the most variable +and difficult to define. In general, any soft, pulpy, or juicy mass, +like the grape and tomato, whether one or many seeded, inclosed in +a containing envelope, whether skin or rind, is a berry. Its typical +forms are such fruits as the grape, mistletoe, pokeberry, etc., +though such diverse forms as the eggplant, persimmon, red pepper, +orange, banana, and pomegranate have been classed as berries; and, in +fact, the melon and the pumpkin are only greatly modified kinds of +the same fruit. In popular language, any small, round, edible fruit +is called a berry. This is a good commercial classification, though +not botanically correct. + +[Illustration: FIG. 375.—Vertical section of a drupe. (_After_ GRAY.)] + +=292. The drupe, or stone fruit.=—Examine a section of a green plum, +peach, or cherry, before the stone has hardened, and tell from what +part it is formed. This stony covering, composed of the inner layer +of the pericarp, and enveloping the seed like an outer coat, is the +main distinction between the drupe and the berry, but it is not +always possible to make out its real nature except by an examination +of the young ovary. In a green drupe, before the stone has hardened, +its connection with the fleshy part is very evident, and the ripe +fruit will answer inquiries if we know how to put them. Open the +stone, and the seed will be exposed with its own coverings inside. +When a stone has more than one kernel,—for instance, an almond or +peach stone, —the stone is not a seed coat, but the hardened inner +wall of a seed vessel or ovary; for a seed coat can never contain +more than one seed, any more than the same skin can contain more than +one animal. + +All the fruits considered in this section belong to the fleshy class. +These form the bulk of the fruits sold in the market, and are of +special importance to the horticulturist. + + + Practical Questions + + 1. Is the tomato horticulturally a fruit or a vegetable? the + squash? eggplant? cranberry? olive? elderberry? pepper? date? + maypop? crab apple? black haw? To what class does each belong? + (283, 288-292.) + + 2. Of what use to the plant is the hard stone of the drupe? (21.) + + 3. Is the pulp of fleshy fruits agreeable to the taste before they + are ripe? After? What advantage is this to the plant? (21.) + + 4. Are the seeds of edible fruits, as a general thing, digestible + or agreeable to the palate? + + 5. Is this an advantage to man? To the plant? (21, 284, 285.) + + 6. What are the most common fleshy fruits in autumn? + + 7. With what vegetative parts of the plant does the skin of many + fruits present correspondences? Are these such as to indicate + homology, or analogy only, between them? (100, 118, 288, 289; Exp. + 89.) + + 8. Name six of the most watery fruits that grow in your + neighborhood. + + 9. Under what conditions as to soil, heat, moisture, etc., does + each thrive best? + + 10. Would a gardener act wisely to infer that because a fruit + contains a great deal of water it should be planted in a very wet + place? + + 11. Which contains more water, the fruit or the leaves of the apple? + + 12. Why does not the fruit, when separated from the tree, wither as + quickly as do the leaves? (Exp. 91.) + + + III. DRY FRUITS + + MATERIAL.—Some easily attainable specimens of dry fruits are (1) + nuts: acorn, hickory nut, walnut, chestnut, pecan, filbert; (2) + pods: pea and bean pods, capsules of larkspur, milkweed, jimson + weed, cotton; (3) grains: corn, wheat, oats, rice; (4) akene: + sunflower, thistle, dandelion, buckwheat, clematis. + +=293. Importance of dry fruits.=—Dry fruits are not in general so +conspicuous or so attractive as fleshy ones, but on account of their +great number and variety they offer a wide field for study. They are +also of great interest from an economic point of view: (1) because +they include the cereal grains that furnish so large a portion of our +food, and (2) because the greater part of the troublesome weeds that +infest our crops are scattered by fruits of this class. + +=294. Indehiscent fruits.=—These kinds are so simple that it will not +be necessary to give much time to them. Compare an acorn, a chestnut, +or a filbert with a ripe bean pod or with a capsule of morning-glory. +Try to open each with your fingers; which _dehisces_, or opens, the +more readily? Which is indehiscent, having no regular way of opening? +How many seeds or kernels do you find in the dehiscent pod? How many +in the indehiscent one? Would it be of any advantage for a one-seeded +pod to open? Remove the kernel from the indehiscent fruit; has it any +covering besides the shell? Which is the pericarp, and which the seed +coat? + +[Illustration: FIGS. 376, 377.—Nut of the pecan tree: 376, exterior; +377, cross section.] + +[Illustration: FIGS. 378, 379.—Nutlike seeds: 378, horse-chestnut; +379, seed of the fetid sterculia.] + +=295. The nut= is easily recognized by its hard, bony covering, +containing usually, when mature, a single large seed that fills +the interior. Care should be taken not to confound with true nuts, +large bony seeds, like those of the buckeye, horse-chestnut, date, +and the Brazil nut sold in the markets. In the true nut, the hard +covering is the seed vessel, or pericarp, and not a part of the seed +itself, though it often adheres to it so closely as to seem so. In +bony seeds, like those of the horse-chestnut and persimmon, the hard +covering is the outer seed coat. The distinction is not always easy +to make out unless the seed can be examined while still attached to +the placenta of the fruit. + +[Illustration: FIGS. 380, 381.—Akenes (magnified): 380, of buckwheat; +381, of cinque-foil.] + +[Illustration: FIGS. 382-384.—Cremocarps, fruits of the parsley +family.] + +=296. The akene=, of which we have examples in the tailed fruit of +the clematis, the tiny pits on the strawberry, and the so-called +seeds of the thistle, dandelion, and sunflower, is a small, dry, +one-seeded, indehiscent fruit, so like a naked seed that it is +generally taken for one by persons not acquainted with botany. It +is the commonest of all fruits, and there are so many kinds that +special names have been applied to some of the most marked varieties. +The akene of the composite family may generally be known by the +various appendages in the form of scales, hooks, hairs, or chaff, +that crown it (Figs. 309-314). The fruits of the parsley family are +merely a sort of double akene attached by the inner face to a slender +stalk from which it separates at maturity. + +The _samara_, or key fruit, is an akene provided with a wing to +aid in its dispersion by the wind. The maple, ash, and elm furnish +familiar examples. + +[Illustration: FIGS. 385, 386.—Samaras: 385, ailanthus; 386, maple.] + +[Illustration: FIGS. 387, 388.—Grain of wheat with husks on: 387, +front view; 388, back view.] + +=297. The grain=, so familiar to us in all kinds of grasses, is +economically the most important of all fruits. It is popularly +classed as a seed, and for practical purposes may be treated as such, +but it is really a modification of the akene in which the seed coats +have so completely fused with the pericarp that they can no longer be +distinguished as separate organs. Peel the husk from a grain of corn +that has been soaked for twenty-four hours, and you will find the +contents exposed without any covering; remove the shell of an acorn +or a hickory nut, and the seed will still be enveloped by its own +coats. Would it be of any advantage for the seed of an indehiscent +fruit, like a grain of corn or oats, to have a separate special +covering of its own? + +[Illustration: FIG. 389.—Follicle of milkweed.] + +[Illustration: FIG. 390.—Leaflike follicle of Japan varnish tree: +_S_, outer (dorsal) suture; _S′_, inner (ventral) suture.] + +=298. Dehiscent fruits.=—_Pod_, or _capsule_, is the general name +applied to all dehiscent fruits. The simplest possible kind of pod +is the _follicle_, composed of a single carpel, like those of the +larkspur, milkweed, and marsh marigold, and may be regarded as a +modified leaf. Examine one of these pods and you will find that +it splits down one side, which corresponds to the edges of the +leaf brought together and turned inward to form a placenta for the +attachment of the seed. This line of union is called a “suture,” from +a Latin word meaning a “seam.” + +=299. The legume.=—Get a pod of any kind of bean or pea, and observe +that it differs from the follicle in having two sutures or lines +of dehiscence. One of these runs along the back of the carpel and +corresponds to the midrib of the leaf; the other, corresponding to +the united edges of the carpellary leaf, always turns inward, toward +the axis of the flower, and forms the placenta. + +[Illustration: FIGS. 391-393.—Legumes: 391, legume of bean: _v_, +ventral suture; _d_, dorsal suture; 392, constricted legume of senna +(_Cassia Nelsonia_); 393, legume of a pea, with partially constricted +pod.] + +[Illustration: FIG. 394.—Loment of beggar-ticks.] + +The beggar-ticks, so unpleasantly familiar to most of us, are merely +a kind of legume constricted between the seeds and breaking up into +separate joints at maturity. What kind of indehiscent fruits do the +joints become when separated? (296.) + +[Illustration: FIG. 395.—Cross section of one-celled syncarpous +capsule of frostweed, with parietal placentæ. (_After_ GRAY.) FIG. +396.—Follicles of larkspur borne on the same torus, but distinct.] + +=300. Compound or syncarpous pods.=—The carpellary leaves may unite +either by their open edges, as if a whorl like that represented in +Fig. 188 were to grow together by the margins (Fig. 395); or each +may first roll itself into a simple follicle like the larkspur +and columbine (Fig. 396), and then a number of these may unite by +their ventral sutures into a single syncarpous capsule, with as +many locules as there are carpels (Fig. 398). The seed-bearing +sutures being all brought together in the center, the placenta +becomes _central_ and _axial_. In the first case (Fig. 395) the open +carpels form a one-chambered capsule, though the placentas sometimes +project, as in the cotton, so far as to produce the effect of true +partitions with a central axial placenta. In capsules with only one +compartment, the number of carpels can generally be determined by the +number of sutures or of placentas. + +[Illustration: FIG. 397.—Pods of Echeveria, contiguous, but distinct.] + +[Illustration: FIG. 398.—Capsule of Colchicum, with carpels united into +a syncarpous pod.] + +[Illustration: FIG. 399.—Capsule of corn cockle, with free central +placenta.] + + + Practical Questions + + 1. To what class of fruits does each of the following belong—rice; + beggar-ticks; poppy; peanut; jimson weed; chinquapin; caraway? + + 2. Is the coconut, as usually sold in the market, a fruit or a seed? + + Suggestion: carefully examine the “eyes,” from without and from + within; if you can get a specimen with the husk on, it will help to + a decision. + + 3. Can you name any syncarpous, or compound capsule, that is + single-seeded? + + 4. Can you name any indehiscent fruit that has normally more than + one seed? + + 5. Give a reason for the difference. (23.) + + 6. Name the weeds of your neighborhood that are most troublesome on + account of their adhesive fruits. + + 7. Do these fruits belong, as a rule, to the dehiscent or to the + indehiscent class? + + 8. Give a reason for the difference, if any is noted. (23.) + + + IV. ACCESSORY, AGGREGATE, AND MULTIPLE FRUITS + + MATERIAL.—For autumn and winter, examples of accessory fruits are: + pineapple, common apple, pear, rose hip; aggregate: magnolia, + tulip tree, wild cucumber, sweet flag (_Calamus_); multiple: osage + orange, sweet gum balls, pine cones, figs, fresh or dried. + + For spring and summer, examples of accessory fruits are: raspberry, + strawberry, squash, cucumber; aggregate: strawberry, blackberry, + Jack-in-the-pulpit; multiple: fig, mulberry. Most of those named + will be found to belong to more than one class; the strawberry, for + instance, is both accessory and aggregate; the fig and pineapple, + accessory and multiple. + +=301.= Besides the varieties already named, all fruits, whether +fleshy or dry, may be simple, accessory, aggregate, or collective. +Fruits of the first kind need no explanation; they consist merely +of a single ripened ovary, whether of one or more carpels, as the +peach, cherry, bean, and lemon. + +[Illustration: FIGS. 400, 401.—Vertical sections showing the relation +between a strawberry flower and fruit: 400, the flower; 401, the +fruit developed from it. The corresponding parts are indicated by +connecting lines; _r_, receptacle; _a_, sepal; _b_, petal; _s_, +stamens; _c_, carpel (akene in fruit); _p_, style of the pistil; +_pl_, pulp of the fruit.] + +=302. Accessory fruits= are so called because some other part than +the seed vessel, or ovary proper, is coherent with, or accessory to +it, in forming the fruit, as in the apple and the hip. The accessory +part may consist of any organ, but is more frequently the calyx or +the receptacle. In the strawberry, the little hard bodies, usually +called seeds, that dot the surface are the true fruits (akenes). A +vertical section through the center will show the edible part to +consist wholly of the enlarged receptacle. In the pineapple, the +edible stalk may be traced through a mass of flowers whose seed +vessels have become enlarged and ripened into fruits. + +=303. Aggregate fruits.=—Some accessory fruits, the strawberry and +blackberry for example, are, at the same time, aggregate; that is, +they are composed of a number of separate individual fruits produced +from a single flower. The cone of the magnolia and of the tulip tree +are aggregate fruits; can you name any others? + +[Illustration: FIGS. 402-404.—Multiple fruit of the pineapple: 402, +external view of a ripe fruit, showing the prolonged receptacle +growing into a new plant above, and the scaly bracted covering below; +403, vertical section through the axis of a fruit, showing _a_, the +receptacle, with _b_, _b_, the fleshy ovaries cohering around it +and forming the edible part of fruit; 404, a single “eye” or scale, +somewhat reduced, showing the scaly bract from the axil of which the +(generally) abortive flower originates.] + +=304. Collective, or multiple, fruits.=—The pineapple is an example +of both an accessory and a multiple fruit, being composed of the +ripened ovaries of a number of separate flowers that have become +more or less coherent. The osage orange, sweet gum balls, fig, and +mulberry are other examples of this class. + +=305. Dissection of a multiple fruit.=—Get one of the dried figs sold +by the grocers. Look at the small end where the skin originates; of +what part is it a modification? (289.) Can think of a reason for this +curious, urnlike enlargement of the receptacle? Is there anything +about the fig, for instance, that renders it peculiarly liable to be +preyed upon by birds and insects? Could any but a very small insect +get through the eye without injuring the fruit? Could it free itself +from the sticky mass inside and get out again without difficulty? +Would you judge from this that the caprification of the fig is easily +effected (279), even when the fig wasp is present? Can you now +account for the fact that over four hundred varieties of cultivated +figs ripen their fruit without fertilization? + +[Illustration: FIG. 405.—Vertical section of a fig, showing the +minute flowers inside the closed receptacle.] + +Open your specimen and examine the contents; what do you find? From a +dried specimen it will hardly be practicable to make out clearly that +the pulp of the fig consists of hundreds of tiny pistillate blossoms +that line the inner face of the receptacle. The little grains +usually taken for seeds are really small akenes—the ripened ovaries +of flowers that have been pollinated from the caprifig (279, 286). +Crush one gently and examine with a lens, or under a low power of the +microscope. It is these “botanically” ripe fruits (284) that give to +the dried figs of commerce their plumpness and their pleasant, nutty +flavor. Why are our native American figs lacking in these qualities +(279)? Could this defect be remedied? Do you know of any efforts +being made in that direction by American cultivators? + +[Illustration: FIGS. 406-409.—Non caprificated and caprificated +figs: 406, outside appearance of non caprificated fig; 407, outside +appearance of caprificated fig; 408, interior of caprificated fig; +409, interior of non caprificated fig.] + +=306. Fruit clusters.=—Be careful not to confound aggregate and +collective fruits with mere clusters, like a bunch of grapes or of +sumac berries. The distinction is not always easy to make out. The +clump of akenes that make up a dandelion ball, for instance, though +held on a common receptacle, like the mulberry and other collective +fruits, have so little connection with each other, and separate +so completely at maturity, as to partake more of the nature of a +cluster than of a collective fruit. The same is true of the clump +of tailed akenes that make up the fruit of the clematis. Though the +product of a single flower and thus technically an aggregate fruit, +they are really only a compact head or cluster. Some degree of +cohesion is necessary to constitute a cluster of matured ovaries into +an aggregate or a multiple fruit. + +=307. The individual fruits= that make up the various kinds just +described may belong to any of the classes mentioned in the two +preceding sections: those of the blackberry, for instance, are +drupes; of the strawberry, akenes; of the sweet gum, capsules. + + + Practical Questions + + 1. To what class of fruits would you refer the following: a banana; + a tickseed; a dewberry; a cocklebur; a string bean; a watermelon; + a cantaloupe; a pomegranate; a black haw; a dogwood berry; a red + pepper? + + 2. Tell which of the following are aggregate or multiple fruits, + and which are fruit clusters: an ear of corn; of wheat; a + buttonwood or a sycamore ball; a hop; a dewberry; a pine cone; a + prickly pear. (303, 304, 306.) + + 3. Tell the nature of the individual fruits composing the different + combinations mentioned in the last question. + + 4. Can you suggest any advantage that might accrue to a species + from having its fruits clustered or compound? (21, 23, 24, 287.) + + + Field Work + + 1. Study the various edible fruits of your neighborhood with regard + to their means of dissemination and protection. Consider the object + of the protective adaptations in each case, whether against heat, + cold, moisture, animals, etc. Notice the color of the different + kinds, and trace its significance; for example, the bright red of + the holly, the dull color of muscadine, black haw, and wild smilax. + Account for the prevalence of red among autumn fruits. Notice the + position of the fruit on the bough and explain its object; as, for + instance, the clustering of dogwood at the end of the twig, the + pendent position of grapes and honey locusts. Observe the relation + between the color and size of fruits and their grouping. What + advantage is it for sumac and bird haws to be gathered in large + clusters? + + 2. Compare wild with cultivated fruits and notice in what respects + man has altered the latter for his own benefit. Note, for instance, + the difference between cultivated apples and the wild crab, between + the cultivated grains and wild grasses. Observe the great number of + varieties of each kind in cultivation and try to account for it. + + 3. Notice the situations in which different kinds of fruits + grow, whether hot, dry, moist, windy, or sheltered, and how they + are affected by their surroundings. For example, account for + the difference between blackberries growing on a dry hillside, + and those in moist land along the borders of a stream. Give the + conclusions drawn from your observations in each case. + + 4. Notice what animals feed upon the different kinds, and whether + their visits are harmful or beneficial. Consider in what respects + the interests of the plant itself, the interests of man, and the + interests of other animals may clash or coincide. Examine the + vegetation along the hedgerows and borders of fields and old + fences. Notice the kind of plants that compose it—sumac, sassafras, + cedars, cat brier, etc. The list will be slightly different + for different localities, but this will not alter the general + conclusion. What kinds of fruits and seeds do these shrubs produce? + What kinds of living creatures frequent hedgerows and feed upon the + seeds of such plants? Do you see any relation between these facts + and one of the modes of seed dispersal? + + 5. Classify all the fruits you have collected during your + walk, under their proper heads, as fleshy or dry, dehiscent or + indehiscent, simple, accessory, aggregate, collective. Be careful + to distinguish between compact clusters, like the heads of clematis + or buttonwood, and truly compound fruits. + + + + +CHAPTER IX. THE RESPONSE OF THE PLANT TO ITS SURROUNDINGS + + + I. ECOLOGICAL FACTORS + + MATERIAL.—A number of small flowerpots filled with soils of as many + different kinds as can be found in the neighborhood. + +=308. Definition.=—By _ecology_ is meant the relation of plants to +their surroundings, which may be considered under three general +heads: their relations to inanimate nature, to other plants, and +to animals. The subject has been touched upon repeatedly in the +foregoing pages, and, in fact, it is impossible to treat of any +branch of botany without some reference to it. All that was said +about the adjustment of leaves for light and moisture, and their +adaptations for protection and food storage, about the devices for +pollination, and for fruit and seed dispersal, really belong to +ecology. + +=309. Symbiosis.=—The relations of plants to animate nature are +_biological factors_, and may act in two ways: (1) through the +destruction of vegetation by hungry animals and by parasitic and +disease-producing organisms; and (2) by associations for mutual +benefit, such as are described in section viii of chapter VII. +Associations of this kind are included under the general term +_symbiosis_, a word which means “living together.” In its broadest +sense symbiosis refers to any sort of dependence or intimate organic +relation between different kinds of individuals, and so may include +the climbing and parasitic habits; but it is usually restricted to +cases where the relation is one of mutual benefit. It may exist +either between plants of one kind with those of another, between +animals with animals, or between plants and animals, as in the case +of the clover and bumblebee, and the yucca and pronuba. + +[Illustration: PLATE 13.—Showing the quick response of vegetation to +surroundings. The upper cut shows the appearance of an irrigation +canal in the arid plains region, when first completed; the lower cut, +ten years after completion.] + +The occurrence of root tubercles on certain of the leguminosæ (63) is +a clear case of symbiosis, the microscopic organisms in the tubercles +getting their food from the plant and at the same time enabling it to +get food for itself from the air in a way that it could not otherwise +do. + +=310. Relations with inanimate nature.=—But it is to the relations of +plants with inanimate nature, and their grouping into societies under +the influence of such conditions, that the term “ecology” is more +strictly applied. The external conditions that lead to the grouping +are called _ecological factors_. The most important of these are +temperature, moisture, soil, light, and air, including the direction +and character of the prevailing winds. Each of these factors is +complicated with the others and with conditions of its own in a way +that often makes it difficult to determine just what effect any one +of them may have in the formation of a given plant society. + +=311. Temperature= may be even and steady, like that of most oceanic +regions, or it may be subject to sudden caprices and variations, +like the “heated terms” and “cold snaps” that afflict our Eastern +coast region every few years. It is not the average temperature of +a climate, but its extremes, especially of cold, that limit the +character of vegetation. + +Temperature probably has more influence than any other factor upon +the distribution of plants over the globe; but it can have little +or no effect in evolving local differences in vegetation, because +the temperature of any given locality, except on the sides of high +mountains, will ordinarily be the same within a circuit of many miles. + +=312. Moisture=, again, may be of all degrees, from the +superabundance of lakes and rivers and standing swamps, to the arid +dryness of the desert, and the water may be still and sluggish, +or in rapid motion. It may exist more or less permanently in the +atmosphere, as in moist climates like those of England and Ireland, +where vegetation is characterized by great verdure; or it may come +irregularly in the form of sudden floods, or at fixed intervals, +causing an alternation of wet and dry seasons. Moreover, the moisture +of the soil or the atmosphere may be impregnated with minerals or +gases, which may affect the vegetation independently of the actual +amount of water absorbed. + +[Illustration: FIG. 410.—The effect of cold—a Mt. Katahdin bog. +(_From_ Mo. Botanical Garden Rep’t.)] + +Snow is a form of water which may act in two entirely opposite ways: +(1) by keeping the atmospheric precipitation locked up in a solid +state and thus bringing about a condition analogous to drought—for +example, in arctic deserts and Alpine snow fields; (2) by causing +annual floods and overflows when it melts in the spring, as in the +Nile and Mississippi valleys. + +In cold temperate regions it also influences vegetation to a +considerable extent by covering the warm earth like a blanket during +winter, and thus protecting tender seeds and shoots that otherwise +would not be able to survive. + +[Illustration: FIG. 411.—Dogwood, a tree tolerant of shade, growing +and blooming in a deeply wooded glen.] + +=313. Light= may be of all degrees of intensity, from the blazing +sun of the treeless plain to the darkness of caves and cellars where +no green thing can exist. Between these extremes are numberless +intermediate stages: the dark ravines on the northern side of +mountains, the dense shade of beech and hemlock forests, and the +light, lacy shadows of the pines,—each characterized by its peculiar +form of vegetation. Absence of light, too, is usually accompanied by +a lowering of temperature and a reduction of transpiration, factors +which tend to accentuate the difference between sun plants and shade +plants, giving to the latter some of the characteristics of aquatic +vegetation. Generally, the tissues of these are thin and delicate, +and having no need to guard against excessive transpiration, they +wither rapidly when cut or exposed to too great intensity of heat and +light. + +[Illustration: FIG. 412.—A red cedar grown in a barren, wind-beaten +situation.] + +[Illustration: FIG. 413.—A red cedar grown under normal conditions.] + +=314. Winds= affect vegetation, not only as to the manner of +seed distribution and the conveyance of pollen, but directly by +increasing transpiration, and necessitating the development of strong +holdfasts in plants growing upon mountain sides and in other exposed +situations. The nature of the region from which they blow—whether +moist, dry, hot, cold, etc.—is also an important factor. In a +district open to sea breezes, live oaks, which require a salt +atmosphere, may sometimes be found as far as a hundred miles from the +coast. + +=315. Soil.=—While water is the most important, soil is perhaps the +most interesting of these factors to the farmer, because it is the +one that he has it most largely in his power to modify. It is to be +viewed under two aspects: first, as to its mechanical properties, +whether soft, hard, compact, porous, light, heavy, etc.; secondly, as +to its chemical composition and the amount of plant food-materials +contained in it. The first can be regulated by tillage and drainage, +the second by a proper use of fertilizers. + + EXPERIMENT 92. TO SHOW THE INFLUENCE OF SOIL AS AN ECOLOGICAL + FACTOR.—Fill a number of small earthen pots with all the different + kinds of soil that are to be found in your neighborhood. Keep well + moistened and make a list of the plants that appear spontaneously + in each. Is there any difference in the kinds produced by different + soils? In vigor or abundance of the same or different kinds? Do + more seedlings appear in any of the pots than could live if left + alone? What becomes of a majority of the seedlings that come up in + a state of nature? + + After a time, stop watering until all the plants are dead and new + ones cease to appear. Notice the rate at which vegetation dies out + in each and the kind of plants that can live longest without water. + Which of the different soils is capable of sustaining vegetation + longest without a fresh supply of moisture? To what quality of the + soil is this due? (Exp. 53.) + + + Practical Questions + + 1. Is the relation between man and the plants cultivated by him a + symbiosis? (309.) + + 2. Why is it that plants of the same, or closely related species + are found in such different localities as the shores of Lake + Superior, the top of Mt. Washington, and the Black Mountains in + North Carolina? (311, 330.) + + 3. Which of the five ecological factors mentioned in paragraphs + 311-315 has probably most largely influenced their distribution? + + 4. What is the prevailing character of the soil in your + neighborhood? + + 5. Is your climate moist or dry? Warm or cold? + + 6. Can you trace any connection between these factors and the + prevailing types of vegetation? + + + II. PLANT ASSOCIATIONS + + MATERIAL.—The subject is not well suited to laboratory work, + though, if time permits, it is recommended that a detailed study + be made of at least one typical hydrophyte, halophyte, and + xerophyte plant. Some good examples are: (1) Hydrophyte: pond + weed, waterlily, pipewort (_Eriocaulon_), bladderwort, arrowhead + (_Sagittaria_); (2) Halophyte: sea lavender, sea rocket, sea + lettuce, water hyacinth; (3) Xerophyte: cactus, century plant, + pineapple, stonecrop, purslane, lichen. + +=316. Modes of grouping.=—Plants group themselves in their favorite +habitats, not according to their botanical relationships, but with +regard to the predominance of one or more of the ecological factors +that influence their growth. Sometimes one or two species will +take practical possession of large areas, like the coarse grasses +that spread over certain salt marshes, or the pines that formerly +constituted the sole forest growth over extensive regions in North +Carolina and Maine. Exclusive growths of this kind over limited +areas are sometimes called plant _colonies_, and the individuals +composing them belong, as a general thing, to the hardy, pushing sort +known as “pioneers,” which are among the first to take possession +of new soil and force their way into unoccupied territory. But more +usually we find a great diversity of forms brought together by their +common requirements as to shade, soil, moisture, and other external +conditions. + +Any well-defined assemblage of plants, whether of one kind or many, +originating in such a common response to the same influences, is +called a _formation_. These associations are variously classed, +according to the nature of their habitat, as salt water, fresh water, +sand hill, swamp, bog, river bottom, or such other kinds as their +ecological character may indicate. Local conditions in limited +areas may lead to the segregation of smaller and more compact groups +called _societies_. This term, however, is used rather loosely, being +treated in some works as synonymous with formations, in others as +analogous with what have here been defined as colonies. + +[Illustration: FIG. 414.—A colony of Alabama primroses (_Œnothera +speciosa_).] + +=317. Principles of subdivision.=—The mixed associations described in +the last paragraph are quite independent of botanical relationships, +and any of the factors named in 310, or others of a different kind, +could be made the basis of their classification. They might be +grouped, for instance, according to their economic uses, or according +to origin, whether native or introduced, as best suited the purpose +of the classification in each case. The moisture factor, however, has +been generally agreed upon as the one most convenient for ordinary +purposes. Upon this principle plants are divided into three great +groups:— + +=Hydrophytes=, or water plants, those that require abundant moisture. + +=Xerophytes=, or drought plants, those that have adapted themselves +to desert or arid conditions. + +[Illustration: FIG. 415.—A water plant (_Limnophila_), with water +leaves and air leaves and transitional forms.] + +=Mesophytes=, plants that live in conditions intermediate between +excessive drought and excessive moisture. To this class belong +most of our ordinary cultivated plants and the greater part of the +vegetation of the globe. + +=Halophytes=, “salt plants,” is a term used to designate a fourth +class, based not directly upon the water factor, but upon the +presence of a particular mineral in the water or the soil which +they can tolerate. They seem to bear a sort of double relation to +hydrophytes on the one hand and to zerophytes on the other. + +=318. Hydrophyte societies.=—These embrace a number of forms, from +those inhabiting swamps and wet moors, to the submerged vegetation of +lakes and rivers. An examination of almost any kind of water plant +will show some of the physiological effects of unlimited moisture. +Take a piece of pondweed, or other immersed plant, out of the water +and notice how completely it collapses. This is because, being buoyed +up by the water, it has no need to spend its energies in developing +woody tissue. Floating and swimming plants will generally be found +to have no root system or very small ones, because they absorb their +nourishment through all parts of the epidermis directly from the +medium in which they live. That they may absorb readily, the tissues +are apt to be soft and succulent and the walls of the cells composing +them very thin. In some of the pipeworts (_Eriocaulon_), the ells +are so large as to be easily seen with the unaided eye. If you can +obtain one of these, examine it with a lens and notice how very thin +the walls are. Water plants also contain numerous air cavities, and +often develop bladders and floats, as in the common bladderwort and +many seaweeds. The leaves of submerged plants are usually either +greatly reduced in size or very much cut and divided, while the ones +that rise above water, like those of the water lily, are apt to be +large and entire, to facilitate floating, and have stomata on their +upper surface. Floating plants sometimes form such large colonies as +to be a serious menace to navigation. Well-known instances of this +are the water hyacinths in the St. John’s River, Florida, and the +vast formations of swimming gulfweed from which the Sargasso Sea +takes its name. + +[Illustration: FIG. 416.—Seaweed (_sargassum_) with bladderlike +floats.] + +[Illustration: FIG. 417.—A pioneer swamp colony of cattails. (_From_ +a photograph by Harry B. Shaw, U.S. Dept. Agr.)] + +=319. Swamp societies.=—These include what may be regarded as the +amphibious portion of the hydrophyte group. They compose the sedge +and cattail bogs, reed jungles, moss and fern thickets, forests of +cypress, magnolia, black gum, pine, tamarack, balsam, and the like. +The sedges and cattails are the pioneers of these societies, which +tend constantly to encroach upon the water and so prepare the way +for the advance of other colonists. Drawing their nourishment from +the loose soil in which they are anchored, and lacking the support +of a liquid medium, they develop roots and vascular stems. The roots +of plants growing in swamps have difficulty in obtaining proper +aëration on account of the water, which shuts off the air from them; +hence they are furnished with large air cavities, and the bases +of the stems are often greatly enlarged, as in the Ogeechee lime +(_Nyssa capitata_) and cypress, to give room for the formation of air +passages. The peculiar hollow projections known as “cypress knees” +are arrangements for aërating the roots of these trees. + +[Illustration: FIG. 418.—A Southern cypress swamp, showing on the +left the peculiar enlargements for aëration, known as “cypress +knees.” (_From_ Mo. Botanical Garden Rep’t.)] + +=320. Xerophyte societies= are adapted to conditions the reverse of +those affected by hydrophytes. The extreme of these conditions is +presented by regions of perennial drought, like our Western arid +plains and the great deserts of the interior of Asia and Africa. +Under these conditions plants have two problems to solve,—to collect +all the moisture they can and to keep it as long as they can. Hence, +plants of such regions have a diminished evaporating surface, owing +to the absence of foliage and the compacting of their tissues into +the stem, after the manner of the cactus and prickly euphorbia; or +their leaves may become thick and fleshy so as to resist evaporation +and retain large amounts of moisture, as in the case of the yucca and +century plant. They also frequently develop a thick, hard epidermis, +or cover themselves with protective hairs and scales. + +The principal types of xerophyte plants are: (1) the lichens, mosses, +and saxifrages found on bald rocks and mountain cliffs; (2) sand +plants, such as cockspur grass, sand spurry, wiregrass, and the +like, inhabiting sea beaches and pine barrens; (3) the sage brush, +greasewood, and switch plants of our Western alkali plains; (4) the +cactus and yuccas of southern California, Arizona, and Mexico; (5) +the acacias, agaves, and hardy “chapparal” thickets of southern +Texas and Mexico. The first class are of importance as the pioneers +and pathfinders of the xerophyte community. In tropical and polar +deserts alike they are the first settlers, and by aiding in the +disintegration of rocks and their gradual conversion into soil, they +pave the way for the coming of the higher plants, and it may be of +man himself. + +[Illustration: PLATE 14.—A xerophyte formation of yuccas, cacti, +and switch plants, near Zacatecas, Mexico. (_From_ a photograph by +Professor F. E. Lloyd.)] + +=321. Partial xerophytes.=—Plants exposed to periodic and occasional +droughts frequently provide against hard times by laying up stores +of nourishment in bulbs and rootstocks and retiring underground +until the stress is over. This is known as the _geophilous_, or +earth-loving, habit. Others, as some of the lichens, and the little +resurrection fern (_Polypodium incanum_, Figs. 419, 420), so common +on the trunks of oaks and elms in the Southern States, make no +resistance, but wither away completely during dry weather, only to +waken again to vigorous life with the first shower. + +[Illustration: FIGS. 419, 420.—A resurrection fern: 419, in dry +weather; 420, after a shower.] + +=322. Physiological xerophytes.=—Plants growing in thin or poor +soil, such as that on denuded hillsides, fresh railroad cuts, and +newly graded streets, are apt to take on a more or less xerophytic +character, even though there may be no lack of moisture. Such soils +are called “new” because the mineral elements in them have not +been exposed long enough to have become decomposed and mixed with +humus, and the vegetation that first populates them has to do the +pioneer work of disintegrating and impregnating the substratum with +humus. For similar reasons the vegetation of sandy bogs and sea +beaches, owing to the poverty of the soil in nitrogenous matter, +usually develops xerophyte adaptations, even though there may be a +superabundance of moisture. Plants growing on high mountain tops and +in cold arctic bogs take on the same characteristics (Fig. 410). +Such situations are said to be “physiologically dry,” because the +moisture they have is not in a condition to be readily absorbed. The +vegetation of arctic regions suffers more from physiological drought +than from cold. + +[Illustration: FIG. 421.—A halophyte swamp of mangroves. Notice the +tangle of adventitious prop roots assisting in the work of absorption +from the brackish marsh soil. (_From_ Mo. Botanical Garden Rep’t.)] + +=323. Halophytes= include plants growing by the seashore and +the vegetation around salt springs and lakes and that of alkali +deserts. Seaweeds are in a sense halophytes, since they live in salt +water, but as they are true aquatic plants and exhibit many of the +peculiarities of hydrophytes in their mechanical structure, they are +classed with them. The name _halophyte_ applies more particularly to +land plants that have adapted themselves to the presence in the soil +or in the atmospheric vapor, of certain minerals, popularly known as +salts, which cause them to take on many xerophyte characteristics. +The reason for this, as was shown in Exp. 39, is because the mixture +of salt in the water of the soil increases its density so that it +is difficult for the plant to absorb as much as it needs, and thus +halophytes are living under “physiologically” xerophyte conditions. +If you have ever spent any time at the seashore, you cannot fail +to have observed the thick and fleshy habit exhibited by many +of the plants growing there, such as the samphire, sea purslane +(_Sesuvium_), and sea rocket (_Cakile_). A form of goldenrod found by +the seashore has thick, fleshy leaves, and is as hard to dry as some +of the fleshy xerophytes. + +Another characteristic of desert plants that is common also to +seaside vegetation is the frequent occurrence of a thick, hard +epidermis, as in the sea lavender and saw grass. The live oaks, trees +that love the salt air and never flourish well beyond reach of the +sea breezes, have small, thick, hard leaves, very like those of the +stunted oaks that grow on the dry hills of California. The presence +of spines and hairs, it will be observed, is also very common; +_e.g._ the salsola, the sea oxeye, and the low primrose (_Œnothera +humifusa_). In other cases the leaf blades are so strongly involute +or revolute (202) as to make them appear cylindrical. All these, it +will be observed, are xerophyte adaptations, and the object in both +cases is the same—the conservation of moisture. + +=324. Mesophytes.=—These embrace the great body of plants growing +under the ordinary conditions of temperate regions, which may vary +from the liberal water supply of low meadows and shady forests to +the almost desert barrenness of dusty lanes and gullied, treeless +hillsides. The forms and conditions they present are so varied that +it would be impracticable to consider them all in a work like this, +but they may be summed up under the two general heads of (1) _open +ground_ and (2) _woodland_. Under the first are included: (_a_) all +cultivated grounds—fields, meadows, lawns, pastures, and roadsides, +with their characteristic shrubs, flowers, and grasses; (_b_) heaths +and plains of northern or alpine regions, with their low, stunted +perennials and bright, but fugacious, flowers. Under the second are +classed all woods, thickets, and copses, with the shrubs and herbs +that form their undergrowth. These may be grouped in three main +divisions: (_c_) mixed forests of maple, ash, oak, hickory, birch, +sweet gum, etc.; (_d_) pure forests of pine, balsam, fir, cypress, +and the like; and finally (_e_), the perennial splendors of the +tropical forest, where the vegetation of the globe reaches its climax +in luxuriance and variety of growth. + + + Practical Questions + + 1. Why do florists cultivate cactus plants in poor soil? (320.) + + 2. What would be the effect on such a plant of copious watering and + fertilizing? + + 3. Why must an asparagus bed be sprinkled occasionally with salt? + (323.) + + 4. If a gardener wished to develop or increase a fleshy habit in a + plant, to what conditions of soil and moisture would he subject it? + (320, 323.) + + 5. What difference do you notice between blackberries and + dewberries grown by the water and on a dry hillside? + + 6. Are there corresponding differences in the root, stem, and + leaves of plants growing in the two situations, and if so account + for them? + + 7. When a tract of dry land is permanently overflowed by the + building of a dam or levee, why does all the original vegetation + die, or take on a sickly appearance? (319.) + + 8. Should plants with densely hairy leaves be given much water, as + a general thing? (202, 320.) + + 9. A farmer planted a grove of pecan trees on a high, dry hilltop; + had he paid much attention to ecology? Give a reason for your + answer. + + 10. Why do the branches of trees often die, or fail to develop, on + the windward side? (314.) + + 11. Why do trees grown in dry soil have harder wood than the same + kind grown in wet soil? (123, 318.) + + + III. ZONES OF VEGETATION + +[Illustration: FIG. 422.—A pioneer colony of sumac growing on a +railroad cutting. (_From_ a photograph by J. M. Coulter.)] + +=325. The origin of vegetable zones.=—The terms “zone” and “zonation” +are used to express a general tendency of plant societies and +formations to distribute themselves in more or less regular belts +or strata, relatively to the varying intensity of the prevalent +ecological factor of their habitat. In almost every locality there +exists some special feature—a pond, a brook, a small ravine, an +isolated hilltop, a deserted quarry, a gravel pit, or a railroad +cut,—sufficiently distinct from the general surroundings to exercise +a perceptible control over the vegetation in its immediate vicinity, +and thus to become the starting point of a series of plant zones +that mark the decreasing influence of the factor concerned, by +their change of character as they recede from its point of greatest +intensity. Starting from a barren, exposed hilltop, for example, +with a covering of dry broom sedge (_Andropogon_) and fleabane, we +encounter next an almost desert zone of washed and gullied slopes +in whose hard, sunbaked soil nothing but a few scrub pines and +brambles can gain a foothold. This will, perhaps, be succeeded, by a +straggling belt of sassafras, sumac, and buckthorn, mixed with cat +brier and blackberry canes, beyond which, at the foot of the hill, +begins a stretch of meadow, or a bit of woodland crossed by a brook, +or hollowed into a boggy depression. From this new factor originates +a second series of zonations, passing through all the stages of bog, +swamp, shade, and sun plants, back to the prevailing type of the +region. Moisture is really the controlling factor in both cases, its +influence in the first being negative,—that is, inversely,—and in the +other, positive, or directly proportioned to the quantity present. + +=326. Direction of zonation.=—When the direction in which the +controlling factor changes is horizontal, as with soil and water, +the zonation will be _horizontal_; when, as in the case of light, +it is vertical, the zonation or stratification will be _vertical_. +Examples of this can be observed in the growth of almost any forest +area, the natural order of succession being: (1) a ground layer of +mosses and fungi; (2) low, creeping vines,—partridge berry, trailing +arbutus, twinflower (_Linnæa_); (3) small ferns and low flowering +herbs—pyrola, clintonia, trillium; (4) a zone of tall herbs and low +bushes—royal fern, cohosh (_Actæa_), blueberries; (5) tall herbs and +shrubs, small trees, and climbing vines—kalmia, dogwood, farkleberry, +smilax, Virginia creeper; (6) tall treetops towering up into full +sunlight. + +When the physical cause of intensity is a central area, such as a +pond or a hilltop, the zonation will be _concentric_; that is, the +different belts will succeed each other in widening circles more +or less complete. Where the controlling cause extends in a line, +as a river, or a chain of mountains, the zones run in parallel +belts on each side of it, and the zonation is _bilateral_. In any +case, however, it is seldom regular, being frequently broken and +interrupted through the intervention of other factors. Nor must +precisely the same kind of plants be always looked for in similar +situations, though their place is usually occupied by kindred species +and genera. The common pitch pine, for instance, of the Northern sand +barrens is represented in sandy districts farther south by the tall, +long-leaved pine, a kindred species. + +=327. Succession.=—Zonation is a regular succession of different +kinds of plants in space; there is also an analogous succession in +time, as, when the vegetation of a locality is killed off by fire or +other cause, plants of an entirely different character will nearly +always spring up to occupy its place. A forest of pine, for instance, +is rarely followed by conifers, but by a growth of hardwood trees, +and _vice versa_—nature thus giving an impressive example as to the +effectiveness of a rotation of crops. + +[Illustration: FIG. 423.—A thicket of pines that has succeeded a +mixed growth of hard wood trees.] + +Succession may be influenced by a variety of causes. Two of the most +efficient are: (1) the exhaustion of the soil by the long-continued +growth of one formation (60), thus causing a deficiency of mineral +material suited for the support of plants of that kind; (2) the +migration of new species into the denuded territory where those which +have different requirements as to mineral nutrients from the former +inhabitants will, other things being equal, have the best chance to +succeed. + +[Illustration: FIG. 424.—A successful invasion—Japanese honeysuckle +covering the banks of a ravine and climbing over shrubs and tree +tops.] + +=328. Invasion.=—A rapid and widespread occupation of any territory +by a new species is called an _invasion_. Notable examples of +invaders are those of the Russian thistle in the northwestern states +of the Union, and the “bitterweed” (_Helenium tenuifolium_) that +has almost driven out the hardy dog fennel (_Anthemis cotula_) +which formerly held undisputed possession of arid places throughout +the South Atlantic states. A still more remarkable instance is +the invasion of the Japanese honeysuckle (_Lonicera Japonica_), +originally introduced for ornament, but which has naturalized itself +within the last thirty years and overrun waste places everywhere, +from the Gulf to the Potomac, with a vigor and luxuriance equaled by +few of our native species. As its beauty and fragrance are even more +conspicuous in a state of nature than under cultivation, and as it +can, moreover, be made very useful in stopping gullies and washes, +its phenomenally rapid occupation of so large a territory has caused +no alarm and consequently attracted little attention. + +=329. Climatic zones.=—These are more general groupings than those +we have been considering. They follow in a rough way the parallels +of latitude, and are classed accordingly as: (1) tropical; (2) +subtropical; (3) temperate; (4) boreal or (on mountains) subalpine; +(5) arctic or (on high mountains) alpine. Taking the cultivated +plants of our own country by way of illustration, we have the +subtropical zone, embracing Florida and the southern portion of the +Gulf states, where sugar cane, rice, and tropical fruits are the +staple crops. Then comes the temperate zone, with three agricultural +subdivisions: (_a_) the great cotton belt, with Indian corn, sweet +potatoes, and the peach, melon, and fig as secondary products. +Farther north, in the Central and Middle Atlantic states, we find +(_b_) the region of maize, hemp, and tobacco, with grapes, apples, +pears, cherries, and a great variety of garden vegetables as side +crops. Finally comes (_c_) the great wheat-growing region of the +North, with buckwheat, hay, and Irish potatoes as subsidiary crops. + +Technically, the distribution of the natural zones of vegetation from +south to north is classed under the three general heads of Forest, +Grass Land, and Arctic Desert, with numerous subdivisions in each. + +=330. Boundaries of the zones.=—While the broad continental zones +of vegetation follow, in a general way, the climatic zones outlined +above, they are not sharply defined, but run into each other and +overlap in various degrees, so that a map depicting the range of +vegetation in any wide area would show a marked deviation from those +of latitude. Various other geographical factors, such as mountain +ranges and bodies of water, influence the direction and character of +the prevailing winds and rains, and through them the moisture and +temperature, to so great an extent that they become the controlling +factors over wide areas. In countries bordering on the sea, the coast +line always marks a belt of its own, and on the sides of a mountain +range, all the climatic zones from the equator to the pole may be +repeated during an ascent of a few miles. + +In our own country, where the mountain chains and coast lines run +approximately north and south, the great continental zones have been +superseded, for all practical purposes, by four regional divisions +running almost at right angles to them. These are, disregarding minor +subdivisions:— + +(1) The Forest region, occupying the eastern and south central +portion of the Union. In classifying this territory as forest, it is +not meant to imply that it is now, or ever was, one unbroken jungle, +like parts of central Africa, but that it combines the conditions +most favorable to a vigorous and varied forest growth. + +(2) The Plains region, extending from the very irregular western +boundary of the forest region to the Rocky Mountains. + +(3) The Rocky Mountain region, including the Rockies and the Sierra +Nevadas with the desert area between them. + +(4) The Pacific Slope, a narrow strip between the Sierras and the +Pacific Ocean. + +[Illustration: PLATE 15.—This giant tulip tree is a relic of the +primitive forest. It is twenty-seven feet in circumference, at a +distance of four feet from the ground. Notice the sharp elbows of +the large boughs, a mode of branching characteristic of this kind of +tree.] + +The boundaries of these regions, like those of the great continental +zones, overlap in various ways, the plants of one region often +appearing in another, like an arm of the sea projecting into the +land. But the district where any class of plants reaches its highest +development is its proper habitat, and as a general thing the one +where its cultivation pays best. It would be a waste of time and +money to try to raise cotton in Maine, or cranberries in Georgia. + + + Practical Questions + + 1. Does the native wild growth of a region furnish any indication + of the kind of crops which could be successfully grown there? (325, + 326.) + + 2. Can you give a reason why the zones of cultivation may, in some + cases, be more extensive than the natural range of wild plants in + the same region? (262, 265.) + + 3. Can you give reasons why the reverse may sometimes be true? + (261, 284.) + + 4. What crops are raised in different parts of your own state? + + 5. Name some of the native plants characteristic of different parts + of your state. What are its principal plant formations? + + + Field Work + + 1. Ecology offers the most attractive subject for field work of all + the departments of botany. It can be studied anywhere that a blade + of vegetation is to be found. In riding along the railroad, there + is an endless fascination in watching the different plant societies + succeed one another and noting the variations they undergo with + every change of soil or climate. + + 2. Students in cities can find interesting subjects for study in + the vegetation that springs up on vacant lots, around doorsteps + and area railings, and even between the paving stones of the more + retired streets. On a vacant lot near the public library in Boston, + over thirty different kinds of weeds and herbs were found, and in + the heart of Washington, D.C., on a vacant space of about twelve by + twenty feet, nineteen different species were counted. Just where + such things come from, how they get into such positions, and why + they stay there, will be interesting questions for city students to + solve. + + 3. But the country always has been and always will be the happy + hunting ground of the botanist. All the factors considered in the + two preceding sections can hardly be found in any one locality, but + by selecting areas traversed by brooks, or by gullies and ravines, + very marked changes in the character of vegetation may often be + observed. Barren, sandy, or rocky soils, the sunbaked clay of naked + hillsides, and the borders of treeless, dusty roads will offer + close approximations to xerophyte conditions. + + 4. If there are any bodies of water in your neighborhood, examine + their vegetation and see of what it consists. Notice the difference + in the shape and size of floating and immersed leaves and account + for it. Note the general absence of free-swimming plants in + running water, and account for it. Note the difference between + the swamp and border plants and those growing in the water, and + what trees or shrubs grow in or near it. Compare the vegetation of + different bogs and pools in your neighborhood, and account for any + differences you may observe. Compare the water plants with those + growing in the dryest and barrenest places in your vicinity, note + their differences of structure, and try to find out what special + adaptations have taken place in each case. Make a list of those in + each location examined that you would class as pioneers. + + 5. Draw a map of the vegetation of some locality in your + neighborhood that presents a variety of conditions, such as a + steep hillside, a field or meadow traversed by a brook, the slopes + and borders of a ravine, or the change from cultivated ground to + uncultivated moor or woodland. Represent the different zones and + formations by different colored inks or crayons, or by different + degrees of shading with the pencil. + + 6. Draw a map of your state showing the different agricultural + regions, as indicated by the character of the cultivated plants in + each; use different colors, or light and dark shading, to define + the boundaries. Notice any irregularities of outline and account + for them—whether due to soil, moisture, geological formation, + winds, or temperature. What is the controlling factor of each + region? + + + + +CHAPTER X. CRYPTOGAMS + + + I. THEIR PLACE IN NATURE + +=331. Order of development.=—All the forms that have hitherto claimed +our attention belong to the great division of Spermatophytes, or +seed-bearing plants, designated also as _Phanerogams_, or flowering +plants. They comprise the higher forms of vegetable life, and because +they are more conspicuous and better known than the other groups, +they have been taken up first, since it is more convenient, for +ordinary purposes, to work our way backward from the familiar to the +less known, rather than in the reverse order. + +But it must be understood that this is not the order of nature. The +geological record shows that the simplest forms of life were the +first to appear, and from these all the higher forms were gradually +evolved. There is no sharp line of division between any of the orders +and groups of plants, but the line of development can be traced +through a succession of almost imperceptible changes from the lowest +forms to the highest, and it is only by a study of the former that +botanists have come to understand the true nature and structure of +the latter. + +=332. Basis of distinction.=—_Cryptogams_, or seedless plants as +a whole, are distinguished from the phanerogams by their simpler +structure and by their mode of propagation, which in the former +is by means of spores, while in the phanerogams it is by seeds. A +spore is a simple organic body, consisting usually of a single cell +which separates from the parent plant at maturity and gives rise to +a new individual. A seed is a complicated, many-celled structure, +containing within itself the rudimentary structure of a new plant +already organized. + +Beginning with the simplest forms, cryptogams are grouped in three +great orders:— + +=333. I. Thallophytes=, or thallus plants.—This group takes its name +from the _thallus_ structure that characterizes its vegetation. In +its typical form, a thallus is a more or less flat, expanded body, of +which the lichens and liverworts offer familiar examples among land +plants, and the kelps and laminarias among seaweeds. It may be of any +size and shape, however, and sometimes consists of a mere filament, +as in the common brook silk, or even of a single cell (Fig. 429). +The term is applied in general to the simplest kinds of vegetable +structure, in which there is no differentiation of tissues, and no +true distinction of root, stem, and leaves. While it is not peculiar +to the thallophytes, it has attained its most typical development +among them, and the name is therefore retained as distinctive of +that group. It embraces two great divisions, the Algæ and Fungi. The +first includes seaweeds and the common freshwater brook silks and +pond scums, besides numerous microscopic forms whose presence escapes +the eye altogether, or is made known only by the discolorations and +other changes caused by them in the water. To the fungi belong the +mushrooms and puffballs, the molds, rusts, mildews, and the vast +tribe of microscopic organisms called _bacteria_, which are so active +in the production of fermentation, putrefaction, and disease. + +[Illustration: FIG. 425.—A seaweed with broad, expanded thallus.] + +[Illustration: FIG. 426.—Anthoceros, a liverwort with flat, spreading +thallus.] + +=334. II. Bryophytes=, or moss plants.—This group likewise contains +two main divisions, Mosses and Liverworts. Familiar examples of the +latter are the flat, spreading green plants, bearing somewhat the +aspect of lichens, met with everywhere on wet rocks and banks around +shady watercourses. The name is a reminiscence of their former use in +medicine as a specific for diseases of the liver, and not, as in the +case of the liver leaf, of a fancied resemblance to that organ. + +Mosses are one of the best defined of botanical orders, and are +easily recognized by their slender, leafy fruiting stalks, growing +usually in dense, spreading mats, and presenting every appearance of +a highly organized structure, well differentiated into root, stem, +and leaves. + +The liverworts represent the more primitive division of the group, +and in some of their forms approach so near the thallophytes that it +is not difficult to recognize them as connecting links in the same +chain of life. Their relationship to the next higher group is not +clear, but while they represent a more primitive stage of evolution +than the mosses, the development of the latter has followed a course +divergent from the main line of evolutionary progress. + +[Illustration: FIG. 427.—A shoot of peat moss with ripe spore fruits, +_f_, _f_.] + +[Illustration: FIG. 428.—A common fern (_Polypodium vulgare_).] + +=335. III. Pteridophytes=, or fern plants, are classed roughly in +the three divisions of ferns, horsetails, and club mosses. They +differ greatly in structure, but all possess a vascular system, and +a well-organized structure of root, stem, and leaves. They rank next +to the spermatophytes in the order of development, and the group is +of especial interest on account of its relationship to the higher +plants. One of its divisions, the club mosses, has probably given +rise to at least one section of the gymnosperms, while the ferns are +regarded as the ancestors of the true flowering plants, which make +up the great class of angiosperms, and represent the highest type of +evolution yet attained in the vegetable kingdom. + + + II. THE ALGÆ + + MATERIAL.—Simple forms of green algæ can be found on the shady side + of tree trunks, damp walls, old fence palings, and the outside of + flowerpots. _Pleurococcus_, one of the commonest kinds, occurs as a + green, powdery mat or felt in damp places, and is often accompanied + by _protococcus_, another good specimen for study. _Spirogyra_ and + other filamentous algæ can be found in stagnant pools and ditches + and in old rain barrels. + + APPLIANCES.—Eosin solution, nitric acid, alcohol, iodine solution; + a white china plate; a hand lens; a compound microscope, and slides. + +=336. Variety of forms.=—This group embraces plants of the greatest +diversity of form and structure, from the minute volvox and desmids +that hover near the uncertain boundaries dividing the vegetable from +the animal world, to the giant kelps of the ocean, which sometimes +attain a length of from six hundred to one thousand feet. They are +usually classed according to their color, as green, brown, and red +algæ, including various subdivisions of each group. They all contain +chlorophyll, by means of which they manufacture their own food, +though in the red and brown divisions it is masked by the presence of +other pigments—an adaptation to the modified light that reaches them +at various depths under water. With few exceptions they can live only +in the water, and unlike any other form of plant life, attain their +highest development in the salty depths of the ocean. The freshwater +forms are small and inconspicuous, and generally of a more simple +type than the seaweeds. The great majority of them belong to the two +classes of green and blue-green algæ. The former is believed to have +furnished the type from which the higher plants have been evolved. + +[Illustration: FIG. 429.—Three stages in the division of a one-celled +alga (_Glœocaspa polydermatica_): _A_, division of a cell just +beginning; _B_, division further advanced; _C_, four cells after +division, remaining in contact.] + +=337. Study of a one-celled alga.=—Put a little of the green algæ +in water on a glass slide. Hold up to the light, or over a sheet +of white paper, and examine with a hand lens; then place under +the microscope. It will probably be found to contain a number of +minute organisms, but the pleurococci can be recognized as small +round bodies of a bright green color, some of them separate, others +adhering together in groups of two, four, or more, with the sides +that are in contact slightly flattened. Each of these bodies is an +individual plant consisting of a single cell, whence they are said +to be _unicellular_. Draw one of the single cells and one of the +groups, or colonies, as they appear under the microscope. Try to +make out the cell wall and the nucleus, and label all the parts (see +7). If you have any difficulty in distinguishing the cell wall, drop +a little glycerine or salt water on the slide. This will cause the +cell contents to shrink by osmosis (56, 59). Can you make out the +structure of the cell colonies? They have resulted from the peculiar +mode of multiplication that prevails among this class of plants. A +cell elongates, contracts in the middle, and divides into two parts, +each of which becomes an independent plant like the mother cell. See +if you can find one in the process of division. The daughter cells +repeat the process, each one giving rise to two new individuals, and +so on indefinitely. The new cells do not always separate immediately +on their formation, but frequently adhere together for a time, in +colonies, before falling away and beginning an independent existence. + +=338. Reproduction by fission.=—This kind of reproduction is called +_fission_, or cell division, and marks a very primitive stage of +development. Under stress of adverse conditions the cells formed +by division may remain inactive for a time. They are then called +_resting spores_, and when more favorable circumstances arise, they +begin again their work of reproduction and growth as actively as ever. + +=339. Meaning of the name.=—The suffix _coccus_ is a Latin noun +(plural _cocci_) meaning a grain or berry, and is a general term +applied to any small, round organism consisting of a single cell; +hence, _micrococcus_, a minute round body; _protococcus_, a primitive +form, or prototype of one-celled bodies; and _pleurococcus_, which +may be freely translated “a one-sided little round body,” from the +flattening of the adjacent sides during fission—_pleuro_ meaning +lateral, or pertaining to the side. + +It is important to remember this definition, as the term _coccus_ +is of very frequent occurrence in works of biology, as a suffix for +designating small round bodies of various kinds. + +=340. Examination of a filamentous alga.=—Place on a white dish a +few drops of water containing some of the green pond scum common in +stagnant pools and ditches. Examine with a hand lens; of what does +it appear to consist? Are the filaments all alike, or are they of +different lengths and thickness? Soak a number of them in alcohol for +half an hour and examine again; where has the green matter gone? Do +these algæ contain chlorophyll? (336; Exp. 65.) This class are called +filamentous algæ on account of their slender, threadlike thalli, +which look like bits of fine floss floating about in the water. The +bubbles of oxygen which they sometimes give off in great abundance +cause the frothy appearance that has given rise to their popular +name, “frog spit.” + +[Illustration: FIGS. 430, 431.—_Spirogyra_ (magnified): 430, two +filaments beginning to conjugate; 431, formation of spores.] + +=341. Spirogyra.=—The filamentous algæ are very numerous, and a +drop of pond scum will probably contain several kinds. At least +one of these, it is likely, will be a _Spirogyra_, as this is one +of the commonest and most widely distributed of them all. Place a +filament under the microscope and notice the spiral bands in which +the chlorophyll is disposed within the cells. It is from this spiral +arrangement that the species takes its name. Do you notice any +roundish particles inclosed in the chlorophyll bands? Test with a +little iodine solution and see what they contain. Each filament will +be seen, when sufficiently magnified, to consist of a number of more +or less cylindrical cells joined together in a vertical row, and thus +forming the simple threadlike thallus which characterizes this class +of algæ. Physiologically, each cell is an independent individual, and +often exists as such. Can you see the cell nucleus? If not, place a +few filaments in a solution of eosin and add a drop of acetic acid to +give the solution a pale rose color. After twenty to thirty minutes, +examine again; the nucleus will be stained a deep red. If you can +find an unbroken filament, examine both ends to see whether there is +any differentiation of base and apex. + +=342. Conjugation.=—See if you can find two filaments sending out +lateral protuberances toward each other. Watch and notice that after +a time these projections come together and unite by breaking down +the cell walls dividing them, the protoplasm in each contracts, the +contents of one pass over into the other, and the two coalesce, +forming a new cell but little, if any, larger than the original +conjugating bodies. This cell germinates under favorable conditions +and produces a new individual. This method of reproduction is known +as _conjugation_. The cells thus produced by the union of the +contents of two separate cells may either germinate at once, and give +rise to new individuals, or remain quiescent for a time, as resting +spores. + + + Practical Questions + + 1. Are any of the green algæ parasitic? How do you know? (186, 336.) + + 2. Why is their presence in water regarded as denoting unhygienic + conditions? + + 3. Mention some of the ways in which their presence may contribute + to the contamination of drinking water. + + 4. Refer to Exp. 66, and account for the bubbles and froth that + usually accompany these plants in the water. + + 5. Can you suggest any other causes than the evolution of oxygen + that might produce the same effect? + + 6. Is the presence of these gas bubbles of any use to floating + plants? + + + III. FUNGI + +[Illustration: FIG. 432.—A common form of mold, magnified, showing +thallus modified into a fibrous mycelium: _a_, _a_, spore cases; _b_, +mycelium. (_After_ KOPF, in part.)] + +=343. Classification.=—In the fungi the thallus structure is greatly +modified, appearing usually as a network of fine threads called the +_mycelium_ (pl., _mycelia_), from a Greek word meaning “fungus” +(369). These plants are all, with a few doubtful exceptions, +parasites or saprophytes which contain no chlorophyll and are +incapable of supporting an independent existence. Biologists are +divided as to their position in the genealogical tree of life. +The weight of authority at present inclines to the view that they +are degenerate forms derived from the algæ, but they have been so +modified by their parasitic habits as to render their position in the +general scheme of life a doubtful one. They represent an offshoot, +or side branch, as it were, of the great evolutionary line, and so +may be considered for the present as standing apart in a class by +themselves. + +=344. Numbers and variety.=—Fungi exceed every other class of +living organisms both in the number of species and of individuals +composing them. They include such diverse forms as bacteria, molds, +rusts, mildews, mushrooms, and the like, ranging in size all the way +from the giant puffball, a foot or more in diameter, to the almost +inconceivably minute influenza bacillus, of which nearly two thousand +million can inhabit a single drop of water without inconvenient +crowding! + +[Illustration: FIG. 433.—Cephalothecium, a fungus parasitic on +rosehips—greatly magnified. (_From_ Mo. Botanical Garden Rep’t. +Photographed by Hedgcock.)] + +=345. The parasitic habit.=—But while their life history is obscure +and hard to trace, the fungi are, as a class, well differentiated by +their parasitic habit. They contain no chlorophyll, can manufacture +no food, and consequently have to obtain it ready-made from the +tissues of living or dead animals and plants. On this account they +are active agents in the production of disease and decay, especially +certain of those manifold forms that have been grouped together +under the general head of bacteria. While not responsible for all +the disease known to be caused by living organisms,—some very +serious ones, such as malaria and cattle fever, being due to animal +parasites,—the majority of those that have been most carefully +investigated are traced to the bacteria, or other fungi. After any +of these parasites have found a lodgment in the body of an organism +whose tissues furnish them a congenial habitat, they multiply with +enormous rapidity, and through the action of certain poisons called +_toxins_, which they excrete, give rise to the most destructive +diseases in both animals and plants; and no rational sanitary science +is possible without a knowledge of their habits and life history. Add +to the vast amount of human suffering that is to be laid at their +door the economic damage done by rust and smut fungi, by molds and +blights and mildews, and we shall be tempted to conclude that the +“battle of life” is largely a struggle against these invisible foes. + +[Illustration: FIGS. 434-437.—Disease-producing bacteria: 434, +bacteria of consumption (_Bacillus tuberculosis_); 435, cholera +bacillus; 436, bacilli of anthrax, showing spores; 437, typhoid +bacillus.] + +=346. Useful fungi.=—Not all fungi, however, are injurious. On the +contrary, the great majority of them are harmless, and very many +kinds are positively beneficial to man. Without the yeasts and +bacteria of fermentation we could not have our bread and cheese. +Other forms are active agents in the fertilization of soils, it +having been estimated that there are 100,000 or more of these +infinitesimal laborers at work in every cubic centimeter (about ¹⁄₁₆ +of a cubic inch) of virgin soil! Even the bacteria of putrefaction, +which we are accustomed to regard as the embodiment of all that is +foul and loathesome, are engaged in an unceasing work as scavengers, +without which life would no longer be possible on our globe, as will +be shown in the following section. + + + A. BACTERIA + + MATERIAL.—A vessel of water in which hay has been left to soak for + several hours; a freshly boiled potato. + + APPLIANCES.—A double boiler for sterilizing; a number of clean + glass jars and bottles; cotton wool for stoppers; a compound + microscope. + + CULTURE MEDIUMS.—A freshly boiled potato answers very well for + ordinary purposes. “Bread mash” can be made by drying some bread + crumbs in an oven, then mashing and mixing them to a paste with + boiling water; sterilize by three successive heatings in a double + boiler. A sterilized preparation of gelatine solution is the medium + most commonly used. + +=347. How to obtain specimens for observation.=—While bacteria are +plentiful almost everywhere, it is not always easy to capture them +just when and where you want them. For this purpose, put some hay +in water and leave in a warm place away from the light until the +liquid becomes cloudy or a film forms on the surface. This will show +that bacteria are present. If it is desired to study any particular +kind of bacterium, inoculate one of the culture mediums described +under “material,” or a few drops of sterilized extract of beef, with +a small quantity of the substance to be examined, or with dust or +scrapings from the locality under consideration. + + EXPERIMENT 93. BY WHAT MEANS ARE BACTERIA COMMONLY DISTRIBUTED?—Put + a slice of freshly boiled potato into each of three glass tumblers + and cover with a filter of cotton wool held in place by tying + tightly with a cord, or by an elastic band. Set them all in a + vessel of water, bring it to a boil, and keep at that temperature + for half an hour, to sterilize the air in the tumblers. When they + have cooled, lift the cotton from (1) for a minute or two and then + replace. Carefully pass the tip of a medicine dropper through the + filter of (2) so as to prevent the entrance of unsterilized air, + and put on the slice of potato a small quantity of the bacterial + liquid prepared as directed in the last paragraph. Leave (3) + unopened. Keep all together in a warm, dark place and observe at + intervals of from 12 to 24 hours. Do any bacteria appear in (3)? + Do any appear on the potato in (2), where the liquid was dropped? + Are they more, or less abundant than in (1)? Since cotton wool is + entirely impervious to the smallest microörganisms known, would you + judge from this experiment that bacteria can get into any place + unless carried there by the air, or by some other means? + + EXPERIMENT 94. CAN BACTERIA BE CARRIED BY PURE AIR?—On a warm (and + preferably cloudy) day, put a slice of potato on a plate, and leave + uncovered in an unused room or closet, free from dust, and kept + carefully closed. Put another slice arranged in exactly the same + way in an open window on a dusty street, or in a room that is used + and daily swept and dusted. Do bacteria appear in the first plate? + In the second? Is air free from dust a good conveyor of bacteria? + + EXPERIMENT 95. WHAT CONDITIONS ARE FAVORABLE TO BACTERIAL + GROWTH?—Strain some of your culture liquid into half a dozen small + bottles of the same size, filling each about half full. Put (1) in + a dark, cool place—on ice, if the weather is warm; (2) in a dark, + warm place; (3) in a warm, well-lighted place; into (4) put a drop + of carbolic acid, formalin, corrosive sublimate, or boracic acid, + and keep in a dark, warm place. Keep (5) in boiling water for half + an hour or more, and then place beside (2). Keep (6) in a freezing + mixture of salt and ice for several hours, then place with (2) and + (5). Examine all at intervals of from 12 to 24 hours. In which + bottles is the presence of bacteria indicated by cloudiness of the + contained liquid, or the formation of a surface film? In which do + they appear first? In which most abundantly? In which last, or not + at all? What is the effect of light and darkness on their growth? + Of heat and cold? Of disinfectants? Name the circumstances that + tend to hinder their growth, in the order of their efficacy. + +=348. Microscopic study of bacteria.=—Put a drop of hay infusion on a +slide and examine with the highest power of the microscope. You will +see a multitude of very small glistening bodies including different +kinds of bacteria, a majority of which are probably the hay bacillus, +_B. subtilis_, shown in Figs. 443, 444. Notice that some forms move +about freely, while others are non-motile. Which kind are the more +numerous? The motion may be either mechanical, resembling that of +the small dust particles we see dancing about in the sunshine, or +apparently voluntary, and caused by the vibration of little whiplike +cilia. Can you distinguish the two kinds? Try to make out clearly +the different shapes you see. Some appear as slender chains or +filaments, but this is due to the individual cells’ adhering together +for a time before breaking up and beginning an independent existence. +The small, rounded bodies, like a period (Fig. 438), are _cocci_; the +slender, rod-shaped ones—sometimes slightly curved (Fig. 440)—are +_bacilli_ (sing., _bacillus_); the comma-shaped ones, and those +generally showing a slight spiral curvature, are _vibrios_ (Fig. +441); the spirally twisted ones, like a corkscrew (Fig. 442), are +_spirilli_ (sing., _spirillum_). These are the principal forms which +it is important to distinguish and remember. The names are applied +very loosely, however, in practice, bacillus being often used as a +general term applicable to almost any kind,—the spirillum of cholera, +for instance, being commonly known as the cholera bacillus, while by +some authors vibrios are ranked as a variety of spirillum. + +[Illustration: FIGS. 438-442.—Typical forms of bacteria: 438, coccus +type; 439, the same, hanging together in chains; 440, rod-shaped +bacteria (bacillus type), the clear areas in some of these are +spores; 441, forms of vibrio; 442, forms of spirillum.] + +=349. Life history of a typical bacterium.=—A pure culture of the +_Bacillus subtilis_ can easily be obtained by boiling some of the +hay infusion for half an hour and then leaving in a warm place till +the usual indications of the presence of bacteria appear (347). +The spores of this micro-organism are so resistant that they can +withstand the temperature of boiling water for several hours, while +those of most other forms of bacteria are killed by a few minutes’ +exposure to it; hence, the crop that develops after boiling will +consist of a pure culture of the hay bacillus. + +[Illustration: FIGS. 443, 444.—Hay bacillus (_B. subtilis_): 443, a +portion of the film from the culture liquid, the black lines, _e_, +being bacteria in the vegetative state; 444, spore formation; _a_, +_d_, motile cells and chain of cells: _b_, non-motile cells; _c_, +spores and chain of spores from the film _e_.] + +In their active state these organisms will be seen to consist of +single-celled, rod-shaped bodies, about three or four times as long +as broad, and generally cohering in bands or filaments, as shown in +Fig. 444, _c_. The black dots within the cells are the spores. Each +individual bacterium produces but a single spore, or rather becomes +a spore itself, by the contraction of its contents and the formation +around them of a strong inclosing membrane. On germinating, the +spores give rise to little ciliated, one-celled organisms called +“swarm spores,” that swim about freely in the containing medium and +multiply rapidly for a time by cell division. After this they pass +again into the quiescent state, ready, whenever favorable conditions +arise, to begin anew the repetition of their life cycle, which is an +irregular alternation of cell division and spore formation. + +=350. Resistance of spores.=—Bacteriologists are not fully agreed as +to the cause of spore formation, some holding that it takes place +only when conditions are most favorable for bacterial growth, others +claiming the reverse. The consensus of opinion at present is toward +the view that the spores are a provision for tiding over periods of +stress and difficulty. They are capable of retaining their vitality +for a long time, and are much harder to kill than the bacterial +cells in their ordinary vegetative state, as was seen in the case of +the hay bacillus. The spores of one species of potato bacillus have +retained their vitality after four hours of boiling, and those of the +typhoid bacillus after continuous exposure to a freezing temperature +for more than three months. The majority of bacteria, in their +vegetative state, are, however, either killed or rendered inert by +temperatures ranging below 10° or above 50° centigrade—equivalent to +about 50° and 122° Fahrenheit, respectively. It is easy to see what +important bearing these facts have on the process of disinfection. + +=351. Reproduction and multiplication.=—The ordinary mode of +reproduction in bacteria, as in other unicellular organisms, is by +fission (337, 338). As each individual forms but a single spore, no +increase in numbers could take place by this means alone. Hence, +while the spores are an important factor in the preservation of +the species by continuing its existence under conditions which the +active organisms could not survive, their successful propagation +depends on their power of rapid multiplication by division. If +this process were to go on unchecked, every hour, in an unbroken +geometrical progression, the progeny of a single bacterium would, +in 24 hours, number nearly 17 million; in 25 hours, 34 million; in +26 hours, 68 million, and in five days they would cover the entire +surface of the globe, land and sea, to a depth of 3 feet! In ordinary +standard milk sold by dairymen, and containing, when examined, less +than 10,000 microbes to the cubic centimeter,—about 20 drops,—the +number was found to have increased after 24 hours to 600 million. +It is comforting to know, however, that the majority of these are +of the harmless kinds which are the active agents in the making of +buttermilk and cheese. + +The effects of their rapid multiplication will be better appreciated +when we consider that bacteria are the smallest of known living +creatures. If 1000 of the influenza bacilli were spread out in +a single layer with their sides touching, but not overlapping, +they would not take up more room than one of the periods used in +punctuating this book; and a coccus concerned in a tubercular disease +prevalent among cattle in South America has recently been discovered, +of which double that number could be accommodated in the same space. + +[Illustration: FIGS. 445, 446.—Milk (highly magnified): 445, pure, +fresh milk, showing fat globules; 446, milk that has stood for hours +in a warm room in a dirty dish, showing fat globules and many forms +of bacteria.] + +=352. Distribution of bacteria.=—Ordinary air, when free from +dust, contains, on the average, not more than five germs to the +liter—equal to about 1 for every 12 cubic inches. Pathogenic, or +disease-producing, germs seldom occur in ordinary fresh air, and +even when present are, under ordinary circumstances, harmful only to +people whose bodies, by reason of physical weakness or unhygienic +habits, offer a congenial soil for their multiplication. Numerous +instances are known in which perfectly healthy persons have carried +about infectious disease germs in their bodies and even transmitted +them to others without experiencing any inconvenience, or even being +aware of their presence. Among others, the germs of pneumonia, +diphtheria, and tuberculosis are often found in the mouth, nose, +and sputum of perfectly healthy persons. There are also a number +of bacteria that are regular inhabitants of the mouth, some of +which are the cause of decayed teeth and foul breath. One form of +bacterium, concerned in the production of inflammation and abscesses +(_Staphylococcus_) is so constantly present on the human epidermis +that one authority has declared it impossible to sterilize the +skin so thoroughly as to free it entirely of this microbe. It is +ordinarily not harmful unless it comes in contact with open wounds +and abrasions. + +=353. The economic importance of bacteria.=—It is hard to say +whether these organisms concern us most on account of the damages +attributable to them on the one hand, or the benefits we owe them +on the other. If they were all as harmful as the pathogenic kinds, +life would hardly be possible on the globe, while without their +presence life as we know it would have ceased to be possible long +ago. They are nature’s great army of scavengers, the sole agents of +decomposition, without which dead organic matter would be subject +only to the slow changes by which the rocks and mineral matter of the +earth’s crust are disintegrated, and the undecomposed bodies of the +vast procession of plants and animals that have existed since life +first began on our globe would long ago have cumbered its surface to +such an extent as to render impossible the continued development of +life such as we know. + +=354. Sterilization= is the process of ridding a substance of living +microörganisms. To do this effectively, the process must be repeated +several times at intervals, so as to give any spores that may have +survived previous applications time to pass into the vegetative +state, when their power of resistance is diminished and they are more +easily destroyed. The incubation period, as the time required for +the germination of the spores is called, is different for different +kinds of bacteria; hence the importance, from a sanitary point of +view, of a thorough knowledge of their life history. + +=355. Disinfection= is sterilization on a large scale, and the same +principles apply to both. Heat is the safest disinfectant for objects +that will bear it, if continued long enough and repeated often enough +at a sufficiently high temperature. Freezing will destroy some kinds +of germs and check or retard the development of nearly all, but +is not to be relied on as a permanent germicide, since even among +flowering plants there are many kinds, not only of seeds, but of +perennial vegetative forms that are capable of enduring an arctic +temperature of many degrees below freezing for long continued periods. + +Chemical disinfectants act usually as microbe poisons, and are +unsuitable as sterilizers for food, though valuable in the +purification of houses, clothing, and utensils—especially the +instruments employed in surgical operations. + +The prevention of the growth of bacteria, especially in wounds +and surgical incisions, whether by means of chemical or physical +agencies, is known as _antisepsis_. + + + Practical Questions + + 1. Why should a person recovering from an ague continue for some + time after to take quinine every third or every seventh day? (350, + 354.) + + 2. Name some of the principal diseases produced by bacteria. + + 3. What is the principle to be acted on in the avoidance of such + diseases? (Exps. 94, 95.) + + 4. Are the same means equally effective for prevention and for + cure? (354, 355; Exps. 93-95.) + + 5. Why is “fresh air” beneficial in a sick room? (352; Exp. 94.) + + 6. Does it act as a disinfectant, or as a mere diluent of the + infected air of the room? (352.) + + 7. Why ought preserved fruits and vegetables to be scalding hot + when put into the can? (355.) + + 8. Why is it necessary to exclude the air from them? (Exps. 93, 94.) + + 9. Reconcile question 8 with question 5. + + 10. Why does the use, for drinking purposes, of water that has been + boiled render a person less liable to infectious diseases? (355.) + + 11. Was the old-fashioned practice of handing the baby round to be + promiscuously kissed by friends and neighbors a good one for the + baby? (352.) + + 12. Why is the spitting habit to be condemned? The use of common + drinking cups in schoolrooms and other public places? (352.) + + 13. Is it proper from a sanitary point of view that roommates at + a boarding school, or even members of the same family, should use + soap, towels, and other articles of the toilet in common? (352.) + + + B. YEASTS + + MATERIAL.—A piece of fresh baker’s yeast, some warm water, and a + little honey or sugar solution; a pipette, or a medicine dropper; + three or four clean pint bottles or preserve jars. + + To raise a crop of yeast fungi for observation, rub one fourth of a + fresh yeast cake in water enough to make a paste; add one pint of + water, with a tablespoonful of honey or sugar, and stir well. + + EXPERIMENT 96. WHAT CONDITIONS FAVOR THE GROWTH OF YEAST?—Pour + equal parts of the liquid made as directed (see Material) into each + of three pint bottles, stopper lightly, and label. Put (1) in a + warm, dark place; (2) in a cool, dark place; and (3) in a bright + light in a warm place. Observe at intervals of a few hours the + changes that occur in each. Notice the bubbles that rise from the + liquid. In which bottle do they form most rapidly? Lower a lighted + match into it, or transfer some of the gas with a pipette into a + vessel containing limewater, and tell what it is. Taste some of the + fermenting liquid. Is it sweet? What has become of the sugar that + was put into it? + +=356. Yeasts and ferments.=—Yeasts belong to a very different order +of fungi from the bacteria, but on account of their simplicity of +structure and the similarity of their action to that of some of the +latter, it is usual to consider them together. They are the active +agents of fermentation, and include a large number of species. The +kind used for household purposes is the same as that employed in +making beer. Of this species there are many varieties, each one of +which gives a characteristic taste to the beer made from it; and +brewers, by paying attention to the cultivation of yeasts, give their +product the special flavors peculiar to the different brands. This +kind of yeast is not known to exist except in a state of cultivation, +and probably owes its survival and present condition of development +to a symbiosis with man, on account of its usefulness in bread +making, and still more, perhaps, to its part in the gratification of +his bibulous propensities, for among savage tribes the manufacture of +fermented liquors is practiced long before the wholesome art of bread +making. + +[Illustration: FIGS. 447-449.—Forms of common yeast (_Saccharomyces +cerevisiæ_): 447, brewers’ yeast; 448, household yeast (the large +grains are starch); 449, yeast from beer sediment, showing budding. +(Figs. 447, 448 × 250; Fig. 449 × 1270.)] + +There are other yeasts existing in a state of nature, such as those +on the surface of fruits, which cause the latter, under certain +circumstances, to ferment and decay. For this reason artificial +ferments are not needed in making wine and other alcoholic liquors +from fruits. Fermentation is also caused by certain forms of +bacteria, as in the formation of vinegar and the souring of milk. +Such bacteria often contaminate the yeast ferments. + +=357. Microscopic examination.=—Place a drop of the cultural +liquid on a slide and examine under the highest power of the +microscope. What do you see? These egg-shaped bodies are yeast +plants, unicellular organisms like the pleurococcus. Do you see any +chlorophyll? Are the yeasts parasitic? How do you know? What do they +live on? (Suggestion: What food substance that has disappeared was +put into the culture liquid?) In getting their nourishment from the +sugar, these fungi disintegrate it into alcohol and carbon dioxide, +which is a process of fermentation. It is the bubbles of gas that +were seen rising in the liquid which cause beer to effervesce and +bread to rise. They permeate the dough and by their expansion produce +the sponginess peculiar to leavened bread. Look for a cell with a +bud forming on it; from what part does it appear to grow? Where a +number of buds remain for some time attached to the mother cell (Fig. +449), they form a _colony_. Make a sketch of a single cell and of a +colony of two or more adherent ones, labeling all the parts. If the +cell wall cannot be made out clearly, run a little glycerine, or salt +water, under the cover glass with a medicine dropper. What causes the +contents of the cell to contract and leave the wall? (56, 59.) + +=358. Reproduction.=—From time to time buds break away from the +mother cell and form new individuals or colonies of their own. This +process is called multiplication by budding, and is only another form +of cell division. + +Whenever reproduction takes place by other means than seeds or +spores, it is said to be _vegetative_. This sort of reproduction +is not confined to unicellular plants, but exists also among the +phanerogams, the propagation of species by means of buds, tubers, +rootstocks, runners, grafting, and the like being variations of +the same process. On the other hand, yeasts and bacteria and the +unicellular algæ have the power, under extreme conditions, to form +resting spores, which sometimes lie dormant for years and resume +their activity when favorable conditions return. + + + Practical Questions + + 1. When is fermentation useful to man? + + 2. What is the effect on canned fruits and vegetables if yeast + cells get into them? + + 3. Why does milk turn sour in warm weather? (350, 351; Exp. 96.) + + 4. Why do buttermilk and clabber spoil if left standing too long? + (345, 356.) + + 5. What causes bread to be “heavy”? (356, 357.) + + 6. Why will dough not rise unless kept in a warm place? (Exp. 96.) + + 7. Why is beer kept cold during fermentation? (350, 356.) + + + C. RUSTS + + MATERIAL.—A leaf of wheat affected with red rust; a leaf or a stalk + with black rust. Some barberry leaves with yellowish pustules on + the under side, which under the lens look like clusters of minute + white corollas. These are popularly known as “cluster cups.” As the + spots on barberry occur in spring, the red rust in summer, and the + black rust in autumn, gather the specimens as they can be found, + and preserve for use. + +[Illustration: FIGS. 450, 451.—Leaf of wheat affected with orange +leaf-rust (_Puccinia rubigo-vera_), uredo stage: 450, upper side of +leaf; 451, under side.] + + The orange leaf, or brown, rust (_Puccinia rubigo-vera_) is more + common in some parts of the country than the ordinary wheat rust + (_Puccinia graminis_), but the two are so much alike that the + directions given will do for either. If the orange leaf-rust (so + named from its color, and not from any connection with orange + leaves, the logical relation of the words being orange leaf-rust, + and not orange-leaf rust) is used, the cups and pustules should be + looked for on plants of the borage family—comfrey, hound’s-tongue, + etc. The orange leaf-rust of apple is caused by a fungus which will + serve to illustrate the same class of parasites. The “teleuto” + stage of this will be found on cedar trees, in the excrescences + commonly known as “cedar apples”; the “cluster cups” on the leaves + of apple and haw trees affected with the disease. + +=359. Red rust.=—Uredo stage. Examine a leaf of “red rusted” wheat +under the lens, and notice the little oblong brown dots that cover +it. These are clusters of spore cases, and are the only part that +appears above the surface. Viewed under the microscope, the red rust +will be seen to consist of a mycelium (see Fig. 452), which ramifies +through the tissues of the leaf and bears clusters of single-celled +reddish spores that break through the epidermis and form the reddish +brown spots and streaks from which the disease takes its name. These +spores, falling upon other leaves, germinate in a few hours and form +new mycelia, from which, in six to ten days, fresh spores arise. +Formerly this was thought to complete the life history of the fungus, +to which the name of _Uredo_ was given. It is now known, however, +that the red rust is merely a stage in the life cycle of the plant, +and to this stage the old name uredo is applied, the spores being +called _uredospores_. + +=360. Black rust.=—Teleuto stage. Next examine with a lens a part +of the plant attacked by black rust. Do you observe any difference +except in the color? Do the two kinds of rust attack all parts +of the plant equally? If not, what part does each seem to affect +more particularly? At what season does the black rust appear most +abundantly? Place a section of the diseased part under the microscope +and notice that the difference in color is due to a preponderance +of long, two-celled bodies with very thick, black walls (Fig. 453). +These are called _teleutospores_, a word meaning “final spores,” +because they are formed only toward the end of the season. They are +developed from the same mycelium with the uredospores, and are not +a product of the latter, but collateral with them and belong to the +same stage in the life history of the fungus. After they appear, +the uredospores cease to be developed at all, and only the dark +teleutospores are produced. These remain on the culms in the stubble +fields over winter, ready to begin the work of reproduction in +spring. For this reason the teleutos are popularly known as “winter +spores” in contradistinction to the uredos, or “summer spores,” whose +activity is confined to the warm months. + +[Illustration: FIG. 452.—Uredo spores of wheat rust (_Puccinia +graminis_), magnified. (_From_ COULTER’S “Plant Structures.”)] + +[Illustration: FIG. 453.—Teleutospores of wheat rust, magnified. +(_From_ COULTER’S “Plant Structures.”)] + +[Illustration: FIG. 454.—Teleutospore germinating and forming +sporidia, _s_, _s_. (_From_ COULTER’S “Plant Structures.”)] + +It was formerly supposed that black rust was caused by a different +fungus from that producing red rust, and to it the name _Puccinia_ +was given. This has been retained as a general designation for all +fungi undergoing these two phases, and the particular form of fungus +that we are now considering is known in all its stages as _Puccinia +graminis_. + +=361. The nonparasitic stage.=—The formation of teleutospores +completes that portion of the life history of the fungus during which +it is parasitic on wheat and grasses of different kinds. In spring +they begin to germinate on the ground, each cell producing a small +filament, from which arise in turn several small branches. Upon the +tip of each of these branches is developed a tiny sporelike body +called a _sporidium_ (Fig. 454), which continues the generation of +the rust fungus through the next stage of its existence. The filament +which bears these sporidia is not parasitic, but when the sporidia +ripen and the spores contained in them are scattered by the wind, +there begins a second parasitic phase, which forms the most curious +part of this strange life history. + +=362. The æcidium.=—Examine next the under side of some barberry +leaves (or comfrey, etc., if orange leaf-rust is used) for clusters +of small whitish bodies that appear under the lens like little white +corollas with yellow anthers in the center. Examine a section of one +of these under the microscope and notice that the yellow substance +is composed of regular layers of colored spores. The corolla-like +receptacles containing them, popularly known as “cluster cups,” +are borne on a mycelium produced from the spores described in the +last paragraph. This mycelium is parasitic on barberry or other +leaves, according to the kind of fungus, and was long believed to be +a distinct plant, to which the name _Æcidium_ (pl., _Æcidia_) was +given. This term is now applied to the cluster cups, and those fungi +which at any period of their life history produce them are called +æcidium fungi. + +[Illustration: FIG. 455.—Section through a barberry leaf, showing on +the upper side two spermogonia, _s_, _s_; and on the under side, an +æcidium, _æ_.] + +=363. Spermogonia.=—On the upper surface of the leaves that bear +the æcidia, notice some small black dots hardly larger than pin +points, but which, when sufficiently magnified, appear as little +flask-shaped bodies (Fig. 455) under the epidermis. These are known +as _spermogonia_, or _pycnidia_. When mature, they break through +the epidermis so that the necks protrude, and discharge a quantity +of minute cells or spores, very like some that, later on, we shall +find playing an important part in the reproductive processes of +certain other fungi, and of mosses and liverworts. In the rust fungi, +however, their function is not understood. They may possibly be +survivals of organs which were once active in the life processes of +the plant, but have become useless under changed conditions. Do such +organs throw any light on the evolutionary history of plants? + +[Illustration: FIG. 456.—A species of “cedar apple” +(_Gymnosporangum_), showing the uredo-teleuto stage of the apple rust +fungus. (_From_ a photograph by Prof. F. E. Lloyd.)] + +=364. Connection between barberry and wheat rust.=—With the discharge +of the æcidium spores, the part of the life cycle of the fungus +spent on the barberry comes to an end, and it is ready to begin the +uredo-teleuto stage over again as soon as it finds a suitable host. +Where there are no barberries, it is capable of propagating without +them, either by adapting itself to some other host plant, or by +omitting the æcidium stage altogether. The parasitic habit being an +acquired one, the fungus, like some animal organisms that we know of, +can often be “educated” by force of circumstances into tolerating, +and even thriving upon, foods which under other circumstances it +would reject. The wheat rust is known to be capable of propagating +year after year in the uredo stage, the spores surviving through +the winter on volunteer grains and grasses; and in no other country +in the world does rust do greater damage to the wheat crop than in +Australia, where the barberry is practically unknown. This power of +accommodation possessed by many parasites is one of the difficulties +the agriculturist has to contend with in the development of rustproof +varieties. + +=365. Polymorphism.=—Plants that pass through different stages in +their life history are said to be _polymorphic_, that is, of many +forms. The habit is very common among the lower forms of vegetation. +The fact that one or more of the phases are sometimes omitted, as +the æcidium phase of wheat rust in warm climates, suggests the idea +that it may be of use in helping the plant to tide over difficult +conditions. Besides giving better chances of obtaining nourishment, +it probably has the same effect as cross fertilization among +flowering plants, in imparting increased strength and vitality to the +succeeding generation. Wheat rust produced from barberry æcidia is +said to be much more vigorous—and consequently more destructive—than +when derived from a uredo that has reproduced itself for several +generations. + +=366. The damage done by rust= to the host is through the destruction +of its tissues by the mycelium. The chlorophyll is destroyed so that +the plant can no longer manufacture food, and is too starved to +produce good grain. There are many varieties of wheat rust, which +have been found on twenty-seven different kinds of grain. Most +of them are specialized to a particular host plant and will not, +ordinarily (364), infest any other. Has this fact any bearing upon +the production of rustproof varieties? + + + Practical Questions + + 1. Is a farmer wise to leave scabby and mildewed weeds and bushes + in the neighborhood of his grain fields? (364, 365.) + + 2. Are there any objections to the presence of volunteer grain + stalks along roadsides and in fence corners during winter? (364.) + + 3. Should cedar trees be allowed to grow near an apple orchard? + Give a reason for your answer. (p. 317, Material.) + + 4. Should diseased plants be plowed under? (361.) + + 5. What disposition should be made of them? + + 6. Ought diseased fruits to be left hanging on the tree? + + 7. Why is it necessary to pick over and discard from a crate or bin + all decaying fruits and vegetables? + + 8. Does a rotation of crops tend to prevent the spread of disease + in plants? Give reasons for your answer. + + 9. Are rustproof varieties to be relied on indefinitely? (364.) + + + D. MUSHROOMS + + MATERIAL.—Any kind of gilled mushroom in different stages of + development, with a portion of the substratum on which it grows, + containing some of the so-called spawn. The common mushroom sold in + the markets (_Agaricus campestris_) can usually be obtained without + difficulty. Full directions for cultivating this fungus are given + in Bulletin 53 of the U. S. Department of Agriculture. From 6 to 12 + hours before the lesson is to begin, cut the stem from the cap of a + mature specimen, close up to the gills, lay it, gills downward, on + a piece of clean paper, cover with a bowl or pan to keep the spores + from being blown about by the wind, and leave until a print (Fig. + 466) has been formed. + +=367. Mushrooms and toadstools.=—The popular distinction which limits +the term “mushroom” to a single species, the _Agaricus campestris_, +and classes all others as toadstools, has no sanction in botany. All +mushrooms are toadstools and all toadstools are mushrooms, whether +poisonous or edible. The real distinction is between mushrooms and +puffballs, the former term being more properly applied to fungi which +have the spore-bearing surface exposed. + +=368. Examination of a typical specimen.=—The most highly specialized +of the fungi, and the easiest to observe on account of their size and +abundance, are the mushrooms that are such familiar objects after +every summer shower. The _gilled_ kind—those with the rayed laminæ +under the cap—are usually the most easily obtained. Specimens should +be examined as soon after gathering as possible, since they decay +very quickly. + +=369. The mycelium.=—Examine some of the white fibrous substance +usually called spawn through a lens. Notice that it is made up of +fine white threads interlacing with each other, and often forming +webby mats that ramify to a considerable distance through the +substratum of rotten wood or other material upon which the fungus +grows. This webby structure, often mistaken for root fibers, is +the thallus or true vegetative body of the plant, the part rising +above ground, and usually regarded as the mushroom, being only the +fruit, or reproductive organ. Place some of the mycelium under the +microscope and notice that it is composed of delicate filaments made +up of single cells placed end to end, as in Spirogyra (341). These +filaments are called _hyphæ_. + +=370. The button.=—Look on the mycelium for one of the small round +bodies called buttons (Fig. 457). These are the beginning of the +fruiting body popularly known as the mushroom, and are of various +sizes, some of the youngest being barely visible to the naked eye. +After a time they begin to elongate and make their way out of the +substratum. + +[Illustration: FIG. 457.—Mycelium of a mushroom (_Agaricus +campestris_), with young buttons (fruiting organs) in different +stages: 1, 2, 3, 4, 5, sections of fructification at successive +periods of development; _m_, mycelium; _st_, stipe; _p_, pileus; _l_, +gill, or lamina; _v_, veil.] + +[Illustration: FIG. 458.—Diagram of unexpanded _Amanita_, showing +parts: _a_, volva; _b_, pileus; _c_, gills; _d_, veil; _e_, stipe; +_m_, mycelium.] + +=371. The veil and the volva.=—Make a vertical section through the +center of one of the larger buttons after it is well above ground, +and sketch. Notice whether it is entirely enveloped from root to cap +in a covering membrane—the _volva_ (Fig. 458, _a_)—or whether the +enveloping membrane extends only from the upper part of the stem to +the margin of the cap—the _veil_ (Fig. 458, _d_); whether it has both +veil and volva, or finally, whether it is naked, that is, devoid of +both. + +=372. The stipe, or stalk.=—Notice this as to length, thickness, +color, and position; that is, whether it is inserted in the center +of the cap or to one side (excentric), or on one edge (lateral). +Observe the base, whether bulbous, tapering, or straight, and whether +surrounded by a cup, or merely by concentric rings or ragged bits +of membrane (the remains of the volva). Look for the _annulus_ or +ring (remains of the veil) near the insertion of the stipe into the +cap, and if there is one, notice whether it adheres to the stipe, or +moves freely up and down (Fig. 459, _a_); whether it is thick and +firm, or broad and membranous so that it hangs like a sort of curtain +round the upper part of the stipe (Fig. 467, _a_). Break the stem +and notice whether it is hollow or solid; observe also the texture, +whether brittle, cartilaginous, fibrous, or fleshy. + +[Illustration: FIG. 459.—Parasol mushroom (_Lepiota procera_), +showing movable annulus: _st_, stipe; _a_, annulus, or ring; _u_, +umbo; _p_, _p_, floccose patches left by volva.] + +[Illustration: FIG. 460.—Chanterelle (_Cantharellus cibarius_), with +infundibuliform pileus and decurrent gills.] + +=373. The pileus, or cap.=—Observe this as to color and surface, +whether dry, or moist and sticky; smooth, or covered with scurf or +scales left by the remains of the volva, as it was stretched and +broken up by the expanding cap (Fig. 459, _p_, _p_). Note also the +size and shape, whether conical, expanded, funnel-shaped (Fig. 460), +or _umbonate_—having a protuberance at the apex (Fig. 459)—or whether +the margin is turned up at the edge (revolute, Fig. 467), or under +(involute, Fig. 459). + +=374. The gills, or laminæ.=—Look at the under surface and notice +whether the gills are broad or narrow, whether they extend straight +from stem to margin, or are rounded at the ends, or curved, toothed, +or lobed in any way. Notice their attachment to the stipe, whether +_free_, not touching it at all; _adnate_, attached squarely to the +stem at their anterior ends; or _decurrent_, running down on the stem +for a greater or less distance (Fig. 460). + +[Illustration: FIGS. 461-463.—Section of a gilled mushroom: 461, +through one side, showing sections of the pendent gills, _g_, _g_ +(slightly magnified); 462, one of the gills more enlarged, showing +the central tissue of the trama, _tr_, and the broad border formed by +the hymenium, _h_; 463, a small section of one side of a gill very +much enlarged, showing the club-shaped basidia, _b_, _b_, standing +at right angles to the surface, bearing each two small branches with +a spore, _s_, _s_, at the end. The sterile paraphyses, _p_, are seen +mixed with the basidia.] + +[Illustration: FIGS. 464, 465.—A tube fungus (_Boletus edulis_): 464, +entire; 465, section, showing position of the tubes.] + +=375. The hymenium.=—Cut a tangential section through one side of +the pileus and sketch the section of the gills as they appear under +a lens, or a low power of the microscope. Notice that the blade +consists of a central portion called the _trama_ (_tr_, Fig. 462) +and a somewhat thickened portion, _h_, constituting the _hymenium_, +or spore-bearing surface. Now examine, under a high power, a small +section from the edge of a gill, including a bit of the trama. +Notice that this last consists of a tissue of mycelial cells (Fig. +463) covered by the hymenium, or spore-bearing membrane, which is +thickly clothed with a layer of elongated, club-shaped cells (_b_, +_b_ and _p_, _p_, Fig. 463) set upon it at right angles to the +surface. Some of these put out from two to four, or in some species +as many as eight, little prongs, each bearing a spore (_s_, _s_, +Fig. 463), while others remain sterile. The spore-bearing cells +are called _basidia_; the sterile ones, _paraphyses_; and the +whole spore-bearing surface together, the _hymenium_, from a Greek +word meaning a membrane. It is from the presence of this expanded +fruiting membrane that the class of mushrooms we are considering gets +its botanical name, _Hymenomycetes_, membrane fungi. The hymenium +is not always borne on gills, but is arranged in various ways which +serve as a convenient basis for distinguishing the different orders. +In the tube fungi, to which the edible boletus belongs (Figs. 464, +465), the basidia are placed along the inside of little tubes that +line the under side of the pileus, giving it the appearance of a +honeycomb. In another order, the porcupine fungi, they are arranged +on the outside of projecting spines or teeth, while in the morelles +they are held in little cups or basins. + +[Illustration: FIG. 466.—Spore print of a gilled mushroom.] + +[Illustration: FIG. 467.—Deadly agaric (_Amanita phalloides_), +showing the broad pendent annulus, _a_, formed by the ruptured veil; +the cup at the base, _c_, and floccose patches on the pileus, left by +the breaking up of the volva.] + +=376. Spore prints.=—When the gills are ripe, they shed their spores +in great abundance. Take up the pileus that was laid on paper, +as directed under Material, on page 323, and examine the print +made by the discharged spores; it will be found to give an exact +representation of the under side of the pileus. + +=377. The spores.=—Notice the color of the spores as shown in the +print. This is a matter of importance in distinguishing gill-bearing +fungi, which are divided into five sections according to the color of +the spores. One source of danger, at least, to mushroom eaters would +be avoided if this difference was always attended to, for the deadly +amanita (_Amanita phalloides_) and the almost equally dangerous fly +mushroom (_A. muscaria_) both have white spores, while the favorite +edible kind (_Agaricus campestris_), though white-gilled when young, +produces dark, purple-brown spores that cannot fail to distinguish it +clearly for any one who will take the trouble to make a print. + +=378. Economic properties.=—Most of the wood-destroying fungi belong +to this and allied orders. They are among the worst enemies the +forester has to deal with (140), and millions of feet of lumber are +destroyed every year by them. + +[Illustration: FIG. 468.—Portion of the root of a maple affected with +rot caused by the mycelium of a fungus that has penetrated to its +interior.] + +Over seven hundred kinds of fungi growing in the United States have +been described as edible, but the evil repute into which the whole +class has been brought by the poisonous qualities of a few species, +and the difficulty, to any but an expert, of distinguishing between +these and the harmless kinds, has caused them to be generally +neglected as articles of diet. While they are pleasant relishes and +furnish an agreeable variety to our daily fare, their food value has +been greatly exaggerated. They contain a large proportion of water, +often over 90 per cent, and the most valued of them, the _Agaricus +campestris_, is about equivalent to cabbage in nutrient properties. + + + Practical Questions + + 1. Why are mushrooms generally grown in cellars? (186, 343.) + + 2. Name any fungi you know of that are good for food or medicine or + any other purpose. + + 3. Name the most dangerous ones you know of. + + 4. Do you find fungi most abundant on young and healthy trees, or + on old, decrepit ones? Account for the difference. (141, 343, 378.) + + 5. Do you ever find them growing on perfectly sound wood anywhere? + + 6. Are they ever beneficial to a tree? (86.) + + 7. Is it wise to leave old, unhealthy trees and decaying trunks in + a timber lot? + + + IV. LICHENS + + MATERIAL.—Specimens can be found almost everywhere, growing on + rocks, walls, logs, stumps, and trees. Some of the more common + kind are: _Parmelia_, recognizable by the shallow spore cups borne + on the upper surface of the thallus; _Cladonia_, by the little + stalked receptacles, like goblets, in which its spores are held; + _Physcia_, by its bright orange color. Where practicable, it is + well to have several different kinds for comparison. Iceland moss + (_Cetraria islandica_) can generally be obtained from the grocers, + and is a good example of an intermediate form between foliaceous + and fruticose lichens. + + If the specimens are very dry, they will be too brittle to handle + conveniently, and should be moistened by soaking a short time in + water. This will render them quite flexible and also bring out the + green color more clearly. + +[Illustration: FIG. 469.—Foliaceous lichens: _A_, _Xanthoria +(Physcia) parietina_; _B_, _Parmelia conspersa_; _a_, spore cups.] + +=379. Examination of a typical specimen.=—The commonest kind of +lichens, and generally the most easily obtained, are those that +grow on rocks and tree trunks in flat, spreading patches. Their +margins are much dented and curled, giving them a somewhat leaflike +appearance, whence they are called “foliaceous” lichens. This broad, +expanded body is the thallus, or vegetative part, as distinguished +from its reproductive part. Examine carefully the thallus of your +specimen. Note the size and shape of the indentations. Is there any +order or regularity about them, such as was observed in the lobing of +leaves? Is there any difference in color between the upper and under +sides? What other differences do you notice? Do you see anything +like hairs, or rootlets, on the under side? Mount one of them in +water and place under the microscope. What does it look like? Compare +with one of the hairs from a leaf of mullein, gromwell, blueweed, or +other hairy plant, with the hypha of a fungus mycelium, and with your +study of the root hair in 67 (_a_). Is it a hair or a root? These +rootlike hairs are called _rhizoids_, and serve to anchor the lichen +to its substratum. Look on the upper side for little cup-shaped or +saucer-shaped receptacles. On what part of the thallus are they +situated? Examine with a lens and see if you can make out what they +contain. These cups are the spore cases. The lichen fungus belongs to +the division of sac fungi, which produce their spores in closed sacs, +or cups. + +[Illustration: FIG. 470.—Portion of the thallus of a lichen, +magnified, showing imprisoned algæ.] + +=380. Structure of the thallus.=—Make a thin section through a +thallus and place under the microscope. Notice the small green bodies +enveloped in the hyphæ of the fungus. Are they most abundant near the +upper or the lower epidermis? Has their green color anything to do +with this, and with the difference in color between the two surfaces +of the thallus? (184.) Do they look like chlorophyll granules? Can +you tell what they are? Compare with your study of the unicellular +algæ (337) and with Fig. 429. Does this throw any light on their real +nature? + +[Illustration: FIG. 471.—Artificial lichen mycelium, _m_, made by +sowing spores of a fungus, _sp_, among alga cells, _a_.] + +=381. The lichen thallus a composite body.=—You will probably have no +difficulty in making out that these small round bodies are green algæ +of some kind, but of what species will depend upon the kind of lichen +with which it is associated. In Cladonia and the bearded lichen +(Fig. 473), it is a protococcus; in other forms, a pleurococcus or a +nostoc—and so on, each species of lichen fungus being specialized to +a certain form of alga. The great botanist, De Bary, showed that it +is even possible to produce a lichen thallus artificially by sowing +the spores of a fungus among the cells of the particular alga with +which it is able to unite. The spores will germinate without the +alga, but soon perish unless they come in contact with the right +one. It is thus made clear that the lichen plant as a whole is a +combination of elements belonging to two distinct orders, the algæ +and fungi, but so closely associated as to constitute practically a +single individual. + +=382. Slavery, or partnership?=—Now, what can be the object of this +peculiar association? Is it a symbiosis, or a case of enslavement? +The fungi, as we know, are all parasites, unable to manufacture +their own food or to exist at all except at the expense of other +organisms, living or dead. But the lichens have refined upon the +gross rapacity of their order, and instead of indiscriminately +destroying the hosts that furnish their nourishment, have used their +victims to better purpose by converting them into contented, well-fed +slaves! The imprisoned algæ perform for them the same service that +the chlorophyll bodies do for the higher plants, and so the lichen +fungi have the advantage of other parasites in getting their food +manufactured at home, so to speak. And while the algæ have to do +double work in order to feed both themselves and their masters, +the fungus, in return, shelters them against cold and drought, and +prolongs their growing period by giving them a more continuous supply +of moisture and food materials, which it draws from the substratum by +means of its rhizoids. In this way both plants are enabled to live in +situations that neither could occupy without the other. + +[Illustration: FIG. 472.—A crustaceous lichen (_Graphis elegans_) +growing on holly: _A_, natural size; _B_, slightly magnified.] + +=383. Reproduction.=—The multiplication of the lichen algæ is +exclusively vegetative. The fungus, on the other hand, reproduces +normally by spores, and the fruiting bodies found on the thallus +originate from the fungus mycelium. + +=384. Classification.=—To be strictly accurate, the two kinds of +vegetable bodies that make up the lichen thallus would probably have +to be classified separately, as algæ or fungi, respectively, but as +fructification is the generally accepted basis of classification, +and the plant body is too intimately permeated with both kinds of +tissue to be divided, each lichen body as a whole is classed with +its particular kind of fungus. The entire group, on account of the +distinctive characters that mark it, is placed in a separate order +of its own. This includes three principal divisions, distributed +according to the shape of the thallus, and its habit of growth: +(1) _Crustaceous_, those that adhere closely to the substratum, as +if glued or inscribed on it; (2) _Foliaceous_, with a broad, more +or less lobed and leaflike thallus that adheres loosely to the +substratum by means of rhizoids springing from its under surface; (3) +_Fruticose_, with branching, stemlike thallus attached at the base +like a regularly rooting plant (Figs. 473, 474). Among these are the +Iceland moss, used as an article of food by man, and the reindeer +moss (_Cladonia rangiferina_), which is the chief sustenance of the +reindeer. + +[Illustration: FIGS. 473, 474.—Fruticose lichens: 473, _Usnea +barbata_, bearded lichen; 474, _Cladonia rangiferina_, reindeer moss: +_A_, sterile; _B_, fruiting portion.] + + + Practical Questions + + 1. Have lichens any economic value? (384.) + + 2. In what way are they most useful? (320.) + + 3. Do you find them, as a general thing, on healthy young trees and + boughs, or on old ones, and those showing signs of decay? + + 4. Do you ever find them growing on trees or other objects in + densely inhabited areas,—cities, large towns, and manufacturing + centers? + + 5. Do they grow more thickly on the shady (northern) side of + rocks, walls, and trees growing in the open, than on the sunny and + (presumably) warmer sides? + + 6. Mention some ways in which a growth of lichens might be + beneficial to a tree. + + 7. In what ways could it be harmful? + + + V. LIVERWORTS + + MATERIAL.—Liverworts can generally be found growing with mosses + in damp, shady places, and are easily recognized by their flat, + spreading habit, which gives them the appearance of green lichens. + _Marchantia polymorpha_ (Fig. 475), one of the largest and best + specimens for study, is common in shady, damp ground throughout + the states. It is diœcious, and specimens bearing both male and + female organs should be provided. _Lunularia_, a smaller species + that can be recognized by the little crescent-shaped receptacles on + some of the divisions of the thallus, is abundant in greenhouses on + the floor, or on the sides of pots and boxes kept in damp places; + but the spore-bearing receptacles are seldom or never present, the + species being an introduced one and possibly rendered sterile by + changed conditions. _Anthoceros_ (Fig. 426) and leafy liverworts, + such as that shown in Fig. 484, also make good examples for study. + + EXPERIMENT 97. WHY ARE THE UPPER AND UNDER SIDES OF A LIVERWORT + DIFFERENT?—Plant a growing branch of marchantia, or of any flat, + spreading liverwort, in moist earth so that the upper side will lie + next the soil, and watch for a week or two, noting the changes that + take place. What would you infer from these as to the cause of any + differences that may have been observed between the two surfaces? + +=385. Examination of a typical liverwort=—The thallus.—The broad, +flat, branching organ that forms the body of the plant is the +thallus. Examine the end of each branch; what do you find there? Are +the two forks into which the apex of the branches divides equal or +unequal? Compare the growing end with the distal one; does it proceed +from a true root? Notice that as the lower end dies, the growing +branches go on increasing and reproducing the thallus. + +[Illustration: FIGS. 475, 476.—Umbrella liverwort (_Marchantia +polymorpha_): 475, portion of a female thallus about natural size, +showing dichotomous branching; _f_, _f_, archegonial or female +receptacles; _r_, rhizoids; 476, portion of a male thallus bearing an +antheridial disk or receptacle, _d_, and gemmæ, _g_, _g_.] + +[Illustration: FIG. 477.—A portion of the upper epidermis of +marchantia, magnified, showing rhomboidal plates with a stoma in +each.] + +Do you find anything like a midrib? If so, trace it through the +branches and body of the thallus; where does it end? Does it seem +to be formed like the midrib of a leaf? Hold a piece of the thallus +up to the light and see if you can detect any veins. Is it of the +same color in all parts, and if there is a difference, can you give +a reason for it? Examine the upper surface with a lens. Peel off a +piece of the epidermis, place it under a low power of the microscope, +or between two moistened bits of glass, and hold up to the light, +keeping the upper surface toward you; what is its appearance? +Observe a tiny dot near the center of the rhomboidal areas into which +the epidermis is divided and compare it with your drawings of stomata +(181, 183). What would you judge that these dots are for? While +differing in structure from the stomata of leaves, they serve the +same purposes and may be regarded a more rudimentary form of the same +organ. + +=386. Rhizoids.=—Wash the dirt from the under side of a thallus and +examine with a lens; how does it differ from the upper surface? Do +you see anything like roots? Place one in a drop of water under the +microscope. Compare with similar organs found on the lichen (379). +What are they? Would rhizoids be of any use on the upper side? +stomata on the under side? + +=387. Gemmæ.=—Look along the upper surface for little saucer-shaped +(in lunularia, crescent-shaped) cupules (_g_, _g_, Fig. 476). Notice +their shape and position, whether on a midrib or near the margin. +Examine the contents with a lens and see if you can tell what they +are. These little bodies, called _gemmæ_, are of the nature of +buds, by which the plant propagates itself vegetatively somewhat +as the onion and the tiger lily do by means of bulblets. Sow some +of the gemmæ on moist sand, cover them with a tumbler to prevent +evaporation, and watch them develop the thalloid structure. + +=388. The fruiting receptacles.=—Procure, if possible, thalli with +upright pedicels bearing flattened enlargements at the top (Figs. +475, 476). These are thallus branches modified into receptacles +containing the reproductive organs, which, in marchantia, are +diœcious, the two kinds growing on separate thalli. Notice their +difference in shape, one kind being slightly lobed or scalloped, the +other rayed like the spokes of a wheel. The first kind are known as +_antheridial_, or male, receptacles; the second as _archegonial_, or +female. + +=389. The antheridia.=—Examine one of the male receptacles on +both surfaces and in vertical section. Notice the tiny egg-shaped +bodies sunk in little cavities between the lobes just under the +upper epidermis (Fig. 478). These are antheridia. When mature, they +rupture at the apex, and multitudes of extremely small bodies, called +_antherozoids_, or _spermatozoids_, are discharged from them. + +[Illustration: FIG. 478.—Longitudinal section of a male receptacle of +marchantia polymorpha, magnified: _a_, antheridia; _t_, thallus; _s_, +ventral scales; _r_, rhizoids.] + +[Illustration: FIG. 479.—Under side of an archegonial receptacle +enlarged. The archegonia are borne among the hairs on the under +surface, which is presented to view in the figure; _f_, a spore case.] + +=390. Archegonia.=—Next examine one of the female receptacles. Look +on the under surface, between the narrow divisions of the receptacle, +for radiating rows of flask-shaped bodies with their necks turned +downward, and all surrounded by a toothed sheath or involucre +(Fig. 479). These bodies are the archegonia, or female organs, and +correspond, loosely speaking, to the ovaries of flowering plants. If +the receptacle is a mature one, the archegonia will be replaced by +the ripe spore cases (_sporangia_), as at _f_, Fig. 479. + +Make enlarged drawings of the upper surface of a male and a female +receptacle, and of a vertical section of each, passing through +an antheridium in the male, and an archegonial row in the female +receptacle. Label the parts observed in each. + +=391. Minute study of an archegonium.=—Place under the microscope +a very thin, longitudinal section through a ray of a receptacle +containing a young archegonium, and observe that the latter consists +of a lower portion, the _venter_, _v_, Fig. 480, and an upper part, +the neck, which is perforated by the _neck canal_, _ca_. The venter +contains the _egg cell_, _o_, and the ventral canal cell, _vc_. The +neck canal is filled with small cells which, at maturity, dissolve +into a mucilaginous substance that swells on being wet and discharges +itself through the top of the neck, leaving an open passage to the +venter, where the egg cell is ready to be fertilized. + +[Illustration: FIGS. 480, 481.—480, young archegonium of M. +polymorpha; _v_, ventral portion; _o_, egg cell; _vc_, ventral canal +and cells; _ca_, neck canal with cells; 481, the same ready for +fertilization after discharge of the mucilaginous fluid.] + +Make a drawing of the section as seen under the microscope, labeling +all the parts. + +=392. Fertilization.=—In the liverworts, and in cryptogams generally, +this process has to take place under water, as the antherozoids are +motile only in a liquid, but the amount required is so small that a +few drops of rain or dew will enable them to make their journey to +the archegonium. The mucilaginous substances discharged from the neck +canal attract them to the mouth of the opening, one or more of them +penetrates to the egg cell, and fertilization is accomplished. Do you +see any analogies between this and the same function among flowering +plants? (250, 251.) + +=393. The spore case.=—After fertilization the egg becomes an +_oöspore_, capable of producing a new plant. Instead, however, of +separating from the mother plant and giving rise to an independent +growth, it germinates within the archegonium and produces there a +small, stalked body, called a _sporogonium_, or _sporophyte_, which +at length ripens into a spore case, as shown at _f_, Fig. 479. At +maturity this capsule-like sporophyte ruptures at the apex, and +discharges a mass of spores, mingled with elongated filaments +called _elators_, which, by their elastic movements, assist in +disseminating the spores. These latter, on germinating, produce, +not a simple sporophyte like that which bore them, but the thallus +of the liverwort with all its complicated arrangement of antheridia +and archegonia and vegetative organs that seem to foreshadow, by the +analogies they suggest, the coming of the higher plants. + +=394. Sexual and asexual reproduction.=—We find here a very marked +change from the simple reproductive processes observed in the algæ +and fungi. In the forms thus far considered, this function was +carried on mainly by simple vegetative fission or budding, with a +more or less irregular intervention of resting spores. If only one +kind of spore is concerned, reproduction is said to be _asexual_. +When two different kinds of cells, the egg and sperm cell, unite to +form an oöspore, as in the liverworts, reproduction is said to be +_sexual_. While sexual reproduction takes place to some extent among +both algæ and fungi, the prevailing method among thallophytes is +asexual, and may be carried on in three different ways: by fission +(and budding), by resting spores, and by conjugation. + +Representing the plant body by _A_ and the resting spores by _a_, +the primitive asexual processes may be expressed to the eye by the +accompanying formulas:— + + (1) Fission and budding: _A_ → _A_ → _A_ → _A_ → + (2) Resting spores: _A a_ → _A a_ → _A a_ → + (3) Conjugation: _A_ + _A_ → _a_ → _A_ + _A_ → _a_ → + +In (3), as was seen in the conjugating cells of the spirogyra (342), +the method is a little more complicated, showing an approach toward +the sexual process. In each of these cases, however, there is only +one kind of cell concerned, while in the liverworts there are not +only different kinds, technically known as _gametes_, but specialized +organs, archegonia and antheridia, for producing them. The thallus +body bearing these organs is termed the _gametophyte_, because it +bears the gametes, or sexual organs,—the suffix _phyte_ meaning a +plant; for example, _epiphyte_, on or upon plants; _spermophyte_, +or _spermatophyte_, seed plant; _sporophyte_, spore plant. The +_sporophyte_, produced within the archegonium, bears simple nonsexual +spores that are capable of germinating independently. Structurally it +is a separate, individual organism, though it does not appear as such +in this class, but lives inclosed in the archegonium, as a parasite +on the mother plant. + +=395. Alternation of generations.=—If we represent the sporophyte +by _S_, the thallus, or gametophyte, by _G_, the female gamete, or +egg cell, by _fg_, the antherozoids (male gametes) by _mg_, the +fertilized egg cell, or oöspore, resulting from their union by _oös_, +and the asexual spores discharged from the sporophyte by _o_, this +complicated mode of reproduction may be expressed diagrammatically as +follows:— + + _fg_ _fg_ + ╱ ╲ ╱ ╲ + _G_ _oös_→_s_→_o_→_G_ _oös_→_s_→_o_→_G_→etc. + ╲ ╱ ╲ ╱ + _mg_ _mg_ + +A glance at the diagram will show a continual interchange of the +sexual and asexual modes of reproduction, in which each generation +gives rise to its _opposite_, the asexual sporophyte producing +the sexual gametophyte, and this in turn, through its gametes, +giving rise to the asexual sporophyte. This regular recurrence +in genealogical succession of two differing forms is what is +meant by the expression “alternation of generations.” Analogous +processes occur also among some of the thallophytes, but as there +is no well-defined differentiation of sporophyte and gametophyte, +alternation proper may be regarded as beginning with the bryophytes. +The subject is a complicated one and somewhat difficult to grasp, +but it is important to form a correct idea of it and to fix clearly +in mind the different modes of reproduction as we proceed from the +lower to the higher forms of vegetation, since in this way alone can +their biological relationships and their order of succession in the +evolutionary scale be made intelligible. + + + VI. MOSSES + + MATERIAL.—One of the most widely distributed of mosses is the + Sphagnum, or peat moss, so generally used by florists in packing + plants for shipment, and it can be obtained from them at almost all + times. It is rather difficult, however, to find specimens with the + fruiting organs, since they are rarely to be met with except in + late autumn or early spring. Other common forms are _Polytrichum_, + _Funaria_, and _Mnium_, any of which will meet all essential + conditions of the study outlined in the text. + +[Illustration: FIGS. 482, 483.—Protonema of a moss: 482, germinating +spore; 483, protonema; _kn_, buds; _r_, rhizoids; _s_, spore.] + +=396. The protonema or thallus stage.=—In mosses the sexual, or +gametophyte generation differs from that of liverworts in undergoing +two phases. The germinating cells of the sporophyte do not develop +immediately into the leafy stem, which is the typical gametophyte +of true mosses, but produce first a filamentous, creeping structure +called the _protonema_ (Fig. 483), that spreads over the ground +and forms the tangled green felt usually observed where mosses are +growing. Place a few of these filaments on a slide in water, and +examine under the microscope. Do they remind you of any of the +forms of algæ? Look near the base of the branches for knots or +enlargements, like those seen at _kn_, Fig. 483. These are buds +from which the leafy moss stems will develop. Do they correspond +to anything observed among the thallophytes? Notice the rootlike +filaments that extend under ground; how do they differ from the ones +above ground? Why are they colorless? How do you know that they are +not true roots? [67 (_a_), 379.] Sketch one of each kind of filament +sufficiently enlarged to show the cells composing it. + +A protonema that arises directly from the spore is said to be +_primary_, while those which sometimes spring from rhizoids above +ground, or from stems or leaves, are _secondary_. The fact that a +protonema can bud from parts of the fruiting stems shows that the two +do not belong to different generations, but are merely successive +stages of a single generation, and both together compose the +gametophyte. + +=397. The leafy stage.=—In their fully developed state the true +mosses show a marked advance in organization over the liverworts. +There is a distinct differentiation of the growing axis into stem +and leaves, though no true roots are formed. The leaves are arranged +spirally, on upright stems, while in the liverworts the vegetative +body is either a flat, spreading thallus, or the leaves are arranged +horizontally on opposite sides of a prostrate, or more or less +inclined, axis. Sometimes a second set occurs, on the upper side of +the axis, but in this case the leaves are usually much smaller and +inclined to the horizontal arrangement, as shown in Fig. 484. + +[Illustration: FIG. 484.—Scapania, a liverwort with leafy thallus, +approaching the form of mosses and lycopodiums. (_From_ COULTER’S +“Plant Structures.”)] + +[Illustration: FIG. 485.—Fruiting receptacle of a moss (_Phascum +cuspidatum_), bearing both antheridia, _an_, and archegonia, _ar_, at +the bifurcated apex; _b_, leaves; _p_, paraphyses.] + +[Illustration: FIG. 486.—Fruiting stem of a moss (_Polytrichum +commune_) with ripe capsules: _s_, seta, or footstalk; _c_, capsule +with calyptra; _f_, capsule after the calyptra has fallen away; _d_, +operculum, or lid.] + +=398. The reproductive organs.=—The antheridia and archegonia are +borne in groups at the end either of the main axes, or of lateral +branches (Figs. 485, 486), but as a rule only one archegonium is +fertilized, so the mature sporogonia are solitary. The plants may be +either diœcious or monœcious, as in Fig. 485; and in the latter case, +the reproductive organs may be borne on the same, or on different, +receptacles. The antheridia and the archegonia are both mixed with +club-shaped hairs called paraphyses (Fig. 485). + +=399. The sporophyte.=—An examination of the fruiting capsule of any +of the true mosses will show that it consists of a long footstalk, +the _seta_, _s_, Fig. 486, bearing a capsule, or ripened sporogonium, +_f_, which is at first surmounted by a cap or hood, known as the +_calyptra_, _c_. The hood represents the excessively developed and +often highly specialized wall of the archegonium. It falls away at +maturity, and the spores are discharged through an opening made by +the removal of the _operculum_, or lid, _d_. The spores and the +capsule are both developed from the fertilized egg (oöspore), within +the archegonium, in much the same manner as in the liverworts, and +together constitute the sporophyte, or asexual generation. It never +leads a completely independent existence, but remains a partial +parasite on the mother plant, though the lower part of the young +sporogonium is usually provided with stomata and chlorophyll so +that it is capable of manufacturing food. In this respect it shows +a distinct advance on the corresponding phase of the liverworts—if +we except the single genus _Anthoceros_, which alone among the +liverworts has the cells of the sporogonium provided with chlorophyll. + +=400. Alternation of generations.=—The process of reproduction +in mosses is so closely similar to that of liverworts that it is +unnecessary to repeat the details. There are some minor variations, +but in all essentials the processes are the same and may be +represented to the eye by the same formula. + +=401. Relative position of mosses and liverworts in the line of +evolution.=—Though mosses, as a rule, show a higher degree of +organization than liverworts, in both generations, their development +has been _away_ from the general course of evolution followed by +the higher plants. This, as will be seen later, tends towards a +decreasing complexity of the gametophyte with increasing complexity +of the sporophyte, while the mosses show increasing complexity of +_both_. Like the order of birds in the animal kingdom, they form a +highly specialized and somewhat isolated group. While they may be +regarded as descendants from a common ancestral stock with the ferns +and club mosses, they have been switched off, so to speak, on a +side track of the great evolutionary trunk line, and their advance +on this side track has carried them to a point more remote from the +course along which the higher forms of plant life have traveled than +the distant junction at which they branched off from their less +progressive kindred, the humble liverworts. + + + VII. FERN PLANTS + + MATERIAL.—Any kind of fern in the fruiting stage. Several different + varieties should be cultivated in the schoolroom for observation. + While gathering specimens, look along the ground under the fronds, + or in greenhouses where ferns are cultivated, among the pots and + on the floor, for a small, heart-shaped body like that represented + in Figs. 501, 502, called a _prothallium_. It is found only in + moist and shady places, and care should be taken in collecting + specimens, as in their early stages the prothallia bear a strong + resemblance to certain liverworts found in the same situations. The + best way is for each class to raise its own specimens by scattering + the spores of a fern in a glass jar, on the bottom of which is a + bed of moist sand or blotting paper. Cover the jar loosely with a + sheet of glass and keep it moist and warm, and not in too bright a + light. Spores of the sensitive ferns (_Onoclea_) will germinate in + from two to ten days, according to the temperature. Those of the + royal fern (_Osmunda_) germinate promptly if sown as soon as ripe, + but if kept even for a few weeks are apt to lose their vitality. + The spores of sensitive fern can be kept for six months or longer, + while those of the bracken (_Pteris_) and various other species + require a rest before germinating, so that in these cases it is + better to use spores of the previous season. + +[Illustration: FIGS. 487-491.—A fern plant: 487, fronds and +rootstock; 488, fertile pinna: _s_, _s_, sori; 489, cross section of +a stipe, showing ends of the fibrovascular bundles; 490, a cluster of +sporangia, magnified; 491, a single sporangium still more magnified, +shedding its spores.] + +=402. Study of a typical fern.=—Observe the size and general outline +of the fronds, and note whether those of the same plant are all +alike, or if they differ in any way, and how. Observe the shape +and texture of the divisions or pinnæ composing the frond, their +mode of attachment to the rachis, and whether they are simple, or +notched, or branched in any way. Hold a pinna up to the light and +notice the veining. Is it like any of the kinds described in 171, +172? In what respect is it different? This forked venation is a very +general characteristic of ferns. When the forks do not reticulate +or intercross, the veins are said to be free; are they free in your +specimen, or reticulated? Make a sketch, labeling the primary +branches of the frond, _pinnæ_ (sing., _pinna_), the secondary ones, +if any, _pinnules_, and the common stalk that supports them, _stipe_. +Note the color, texture, and surface of the stipe. If any appendages +are present, such as hairs, chaff, or scales (in Pteris, nectar +glands), notice whether they are equally distributed. If not, where +are they most abundant? + +Examine the mode of attachment of the stipes to their underground +axis. Break one away and examine the scar. Compare with your drawings +of leaf scars and with Fig. 105. Do the stipes grow from a root or a +rhizome? How do you know? Do you find any remains of leafstalks of +previous years? How does the rootstock increase in length? Measure +some of the internodes; how much did it increase each year? Cut a +cross section and look for the ends of the fibrovascular bundles. +Trace their course through several internodes. Do they run straight, +or do they turn or bend in any way at the nodes? If so, where do they +go? Do you see anything like roots? Where do they originate? Put one +of them under the microscope and find out whether they are roots or +hairs. + +True roots are first developed in the pteridophytes. Since those of +the fern spring from an underground stem, to what class of roots do +they belong? (83.) + +=403. Minute study of a fern stem.=—Place a very thin section of +a fern rhizoma, or of the stipe of a frond, under the microscope. +Except in very young stems the vascular bundles are arranged in a +ring, or sometimes in two or more rings (Fig. 492), with plates of +strengthening tissue, _l_, _l_, between the inner and outer rings. +Notice the inner epidermal layer of hard brown tissue, and within +that, the soft parenchyma, which fills the rest of the interior. Test +it with iodine and observe how rich in starch it is. If the section +of a petiole is under observation, the details will be somewhat +different; would you expect to find as much starch in the stipe as in +the rootstock? Why, or why not? + +Make a longitudinal section of a rhizome through the point where +a leafstalk is attached and trace the course of the bundles. This +will be facilitated if the specimen has stood in eosin solution a +few hours. Make enlarged drawings of both sections, labeling all the +parts. + +[Illustration: FIG. 492.—Diagram of a cross section through the +stem of a fern (_Pteris_): _s_, _s_, _s_, rings of fibrovascular +bundles; _l_, _l_, plates of strengthening tissue, with a ring of +fibrovascular bundles between them; _lp_, zone of strengthening +fibers; _r_, cortex; _e_, epidermis.] + +[Illustration: FIGS. 493-494.—Parts of fertile pinnæ: 493, of +_polypodium_, enlarged, showing the sori without indusium; 494, of +_pellea_, showing indusium formed by the revolute margin.] + +Clearly differentiated conducting bundles occur in the mosses, +but they are of much simpler structure than in the pteridophytes, +consisting usually of a single central strand, and are found +more frequently in the leaves than in the stems. A true vascular +structure appears first in the pteridophytes, whence these plants are +distinguished as _vascular cryptogams_. + +=404. Fructification.=—Examine the back of a fruiting frond; what +do you find there? These dots are the _sori_ (sing., _sorus_), or +spore clusters, and the fronds or pinnæ bearing them are said to be +_fertile_. Are there any differences of size, shape, etc., between +the fertile and the sterile fronds of your specimen? between the +fertile and the sterile pinnæ? On what part of the frond are the +fertile pinnæ borne? Notice the shape and position of the sori, +and their relation to the veins, whether borne at the tips, in the +forks, on the upper side (toward the margin), or the lower (toward +the midrib). Look for a delicate membrane (_indusium_) covering the +sori, and observe its shape and mode of attachment. If the specimen +under examination is a polypodium, there will be no indusium; if a +maidenhair, or a bracken, it will be formed of the revolute margin +of the pinna. In lady fern and Christmas fern (_Aspidium_), the sori +frequently become confluent, that is, so close together as to appear +like a solid mass. Sketch a fertile pinna as it appears under the +lens, bringing out all the points noted. + +[Illustration: FIGS. 495-496.—Christmas fern (_Aspidium_): 495, part +of a fertile frond, natural size; 496, a pinna enlarged, showing the +sori confluent under the peltate indusia.] + +[Illustration: FIGS. 497-500.—Spores of pteridophytes, magnified: +497, a fern spore; 498, 499, two views of a spore of a club moss; +500, spore of a common horsetail (_Equisetum arveuse_).] + +=405. The spore cases.=—Look under the indusium at the cluster +of little stalked circular appendages (Fig. 490). These are the +_sporangia_, or spore cases, in which the reproductive bodies are +borne. Place one of them under the microscope, and it will be found +to consist of a little stalked circular body like a tennis racket +(Fig. 491), surrounded by a jointed ring called the _annulus_. Watch +a few moments and see if you can find out the use of the annulus. If +not, warm the slide and you will probably see the ring straighten +itself with a sudden jerk, rupturing the wall of the sporangium and +discharging the spores with considerable force. If this does not +happen, add a drop of strong glycerine to a specimen mounted in +water; the rupture will be apt to follow quickly. What causes it, in +either case? [56, (1); Exp. 19.] + +=406. The sporophyte.=—The spores found in such abundance on the +fertile pinnæ; are all alike, and each one is capable of germinating +and continuing the work of reproduction as effectually as the sexual +spores of the bryophytes. The fertile frond, or part of a frond, on +which they are borne is called a _sporophyll_ (spore-bearing leaf), +and the entire plant is the _sporophyte_, which, with its crop of +spores, makes up one generation. + +It is important to observe that in the ferns and in all pteridophytes +the sporophyte is the conspicuous and highly organized body that +is commonly recognized as the normal growing plant; while with the +bryophytes just the reverse holds true,—the sexual generation, +or gametophyte, represents the normal plant structure, while the +sporophyte is an insignificant appendage which never attains an +independent existence. Broadly speaking, in bryophytes, it is a spore +fruit; in the pteridophytes and spermatophytes a highly developed +plant. + +[Illustration: FIGS. 501, 502.—Prothallium of a common fern +(_Aspidium_): 501, under surface, showing rhizoids, _rh_, antheridia, +_an_, and archegonia, _ar_; 502, under surface of an older +gametophyte, showing rhizoids, _rh_, young sporophyte, with root, +_w_, and leaf, _b_.] + +=407. The gametophyte.=—When one of these asexual spores germinates, +it produces, not a fern plant like the one that bore it, but a small, +heart-shaped body like that shown in Fig. 501. Examine one of these +bodies carefully with a lens. Observe that there are no veins nor +fibrovascular bundles, and the whole body of the plant seems to +consist of one uniform tissue. Compare it with the forked apex of +a branching thallus of a liverwort. Do you perceive any points of +similarity? The two are, in fact, morphologically the same. This +heart-shaped body is called a _prothallium_, and is the gametophyte +of the fern. It may be of different shapes, and in some species is +branching and filamentous, like the protonema of a moss. Generally, +however, it is flat and more or less two-lobed, as shown in Fig. +501. It is small and inconspicuous and very short-lived, being of +importance only in connection with the work of reproduction. + +Look with your lens for a cluster of small, bottle-shaped bodies +just below the deep cleft in the heart. If you cannot make out what +they are, put a thin section through a part of the prothallium +containing one under the microscope, and you will see that they are +the archegonia. Lower down among the rhizoids, near the pointed base, +will be found the antheridia. In some species the prothalli are +diœcious, one kind bearing antheridia, the other archegonia, but this +is rare among the true ferns. + +[Illustration: FIG. 503.—Young archegonium of a fern, magnified: _K_, +neck canal cell; _K′_, ventral canal cell: _O_, egg cell.] + +=408. Fertilization.=—This process is the same in all essentials as +in the bryophytes. As in other cryptogams, it can take place only +under water,—a circumstance which points to an aquatic origin for +this sub-kingdom, and through them to the entire flora of the globe. +The archegonia differ somewhat in shape from those of the liverworts +and mosses, but a section under the microscope will show that they +consist of essentially the same parts. On account of the similarity +of these organs, the pteridophytes and bryophytes are often classed +together as _Archegoniates_. + +=409. Alternation of generations.=—Among the section of ferns that +we have been considering, the order of alternation corresponds in +all essentials to that prevailing among the bryophytes, and may +be represented by the same formula. The chief difference is in the +relatively much greater importance of the sporophyte, which may be +expressed by putting it first:— + + _fg_ _fg_ + ╱ ╲ ╱ ╲ + _S_→_o_→_G_ _oös_→_S_→_o_→_G_ _oös_→_S_→_o_→_G_ etc. + ╲ ╱ ╲ ╱ + _mg_ _mg_ + +But some of the pteridophytes—of which the Selaginella offers a +conspicuous example—have differentiated their asexual spores (_o_ +of the formula) into two kinds, large and small, known respectively +as _megaspores_ and _microspores_. The prothallia developed by the +former bear archegonia containing female gametes only; those by the +latter, antheridia containing male gametes—while in the diœcious +bryophytes, the archegonial and antheridial thalli are produced by +spores of the same kind. + +[Illustration: FIGS. 504.-508.—A kind of pteridophyte (_Selaginella +martensii_) with its organs of fructification: 504, a fruiting +branch; 505, a microsporophyll with a microsporangium, showing +microspores through a rupture in the wall; 506, a megasporophyll +with a megasporangium; 507, megaspores; 508, microspores. (_From_ +COULTER’S “Plant Structures.”)] + +The differentiation of the asexual spores in the higher pteridophytes +gives rise to corresponding changes in the sporangia that bear +them, and even in the sporophylls themselves, one kind bearing +microsporangia only, the other megasporangia. In this way the +differentiation of sex is pushed back, step by step, until it +virtually begins with the sporophyte, or asexual generation. + +Using the same terms as before, and representing the microspores +by the abbreviation _mo_, the megaspores by _Mo_, the archegonial +gametophyte by _arG_, the antheridial by _anG_, the formula may be +modified to express this more complicated process of alternation, as +follows:— + + _Mo_→_arG_→_fg_ _Mo_→_arG_→_fg_ + ╱ ╲ ╱ ╲ + _s_ _oös_→_S_ _oös_→_S_ etc. + ╲ ╱ ╲ ╱ + _mo_→_anG_→_mg_ _mo_→_anG_→_mg_ + + +Comparing this formula with the preceding, it will be seen that the +increased complexity affects the sporophyte at the expense of the +gametophyte, which has now become a mere dependent on the former. + +=410. Advantages of alternation.=—This roundabout mode of +reproduction would hardly have been developed unless it had been of +some benefit to the plants in which it occurs. The chief advantage +seems to be in more rapid multiplication and consequently better +chance to propagate the species, as compared with the slow process +of sexual reproduction were the plant confined to that method alone. +Only one plant is produced by each oöspore, and if this were a +gametophyte with its limited number of archegonia, multiplication +would be slow; but the sporophyte with its millions of spores, +each capable of producing a new individual, enables the species +to multiply indefinitely. At the same time the interposition of a +gametophyte, or sexual generation, secures the introduction of a new +strain with effects analogous to those of cross fertilization. + +[Illustration: FIG. 509.—Part of the fruiting stem of a scouring +rush, _Equisetum limosum_, showing the cone-like spore cluster. +(_After_ GRAY.)] + +=411. Classification of pteridophytes.=—In our study of this group, +the ferns have been taken as the type because they are the most +familiar and most widely distributed of all the vascular cryptogams. +But while they exceed in numbers, both of individuals and species, +all the other orders combined, they form only one division of three +great groups that make up the class Pteridophyta. These groups are: +(1) ferns, under which are included, besides the true ferns, two +widely differing orders, with the grape ferns and adder’s-tongue in +one, and the water ferns in the other; (2) the club mosses, embracing +the two subdivisions of _Lycopodium_ and _Selaginella_; (3) the +horsetail family, including horsetails and scouring rushes. Orders +(2) and (3) are grouped together as cone-bearing (strobilaceous) +pteridophytes, because their sporangia are clustered in oblong heads, +or _strobiles_ (Fig. 509), somewhat like the cones of the pine. The +orders of pteridophytes differ greatly among themselves, but agree +in possessing certain characteristics that point to their derivation +from a common ancestry. + +=412. Distinction between pteridophytes and bryophytes.=—In passing +from the Thallophytes and Bryophytes to the vascular cryptogams, we +cross the widest chasm in the vegetable kingdom—a gap relatively as +great as that between vertebrates and invertebrates among animals. +The most important modifications that discriminate the two groups +are: (1) the presence in Pteridophytes of a highly organized vascular +system accompanied by a well-marked differentiation of the plant body +into root and stem; (2) increased importance and complexity of the +sporophyte with proportionate diminution of the gametophyte. + +While vessels for conducting water occur in some of the bryophytes +(403), a well-defined vascular system and true roots are met with +first in the Pteridophytes. The change in the relative importance +of sporophyte and gametophyte is so marked that in Selaginella, the +genus which approaches nearest in structure to the seed-bearing +plants, the suppression of the gametophyte has proceeded so far that +it never leads an independent existence at all and is difficult even +to recognize as a distinct individual. + + + Practical Questions + + 1. Have ferns any economic use—that is, are they good for food, + medicines, etc.? + + 2. What is their chief value? + + 3. Under what ecological conditions do they grow? + + 4. Are they often attacked by insects, or by blights and disease of + any kind? + + 5. Of what advantage is it to ferns to have their stems + underground, in the form of rootstocks? (321.) + + 6. What causes the young frond of ferns to unroll? (54, 98.) + + 7. Name the ferns indigenous to your neighborhood. + + 8. Which of these are most ornamental, and to what peculiarities of + structure do they owe that quality? + + 9. Are cultivated ferns usually raised from the spores or in some + other way? Why? + + 10. After the great eruption of Krakatoa in 1883, by which the + vegetation of the island was completely destroyed, ferns were the + first plants to reappear. Explain why. (19; Exp. 17.) + + + VIII. THE RELATION BETWEEN CRYPTOGAMS AND SEED PLANTS + +=413. No break in the chain of life.=—The great gap that was once +supposed to exist between the cryptogams and phanerogams has been +bridged over by the discovery of analogies in the reproductive +processes of the two groups that connect them together as successive +links in one continuous chain of vegetable life. It is therefore very +important to have a clear understanding of the nature and meaning of +these processes, for the chief turning points in the life history of +the different groups of plants are connected with them, their natural +relationships to each other, and their distribution according to +their respective places in the evolutionary scale, being determined +largely by a comparison of their modes of continuing the life of the +group. + +=414. Alternation of generations in seed plants.=—This process, so +conspicuous among Bryophytes and Pteridophytes, and not unknown +among Thallophytes, is universal among seed plants (Spermatophytes) +also, though in so masked a form that it is not easy to recognize +without a more detailed study than would be practicable within the +limits of a book like this. Briefly, we may say that the stamens of +spermatophytes, and the pistils, or rather the carpels, which we +have seen to be transformed leaves (298), represent the sporophylls +(406) of the higher pteridophytes. The pollen sacs and ovules are +sporangia, bearing microspores and megaspores (409), represented +respectively by the pollen grains in the anther and the embryo sac +in the ovule. These go through a series of microscopic changes in +the body of the ovule analogous to the production of the oöspore +in the archegonia of ferns and liverworts, but the process is so +obscure that to an ordinary observer the pollen grains and the ovule +appear to be the real gametes, and were long supposed to be such. +The fertilized germ cell in the embryo sac (251) corresponds to +an oöspore; the embryo sac with the endosperm found in all seeds +(previous to its absorption by the cotyledons) is a rudimentary +gametophyte; and the embryo in the matured seed is the undeveloped +sporophyte, destined, after germination and further growth, to +produce a new generation with its recurrent cycle of alternating +phases. + +[Illustration: FIG. 510.—Diagrammatic section through the ovule of a +gymnosperm belonging to the spruce family: _i_, integument covering +the ovule; _e_, endosperm (corresponding to female gametophyte), +which fills the embryo sac, containing two archegonia, _a_; _o_, egg +cell; _p_, pollen grains; _t_, pollen tubes entering the neck, _c_, +of the archegonia.] + +In the gymnosperms,—pines, yews, cycads, etc.,—which represent the +most ancient and primitive type of existing seed-bearing plants, +the similarity of these processes to those of certain of the +pteridophytes is very striking, and it was through the study of +these that the sequences of the process were traced in the much +more obscure form in which they occur among the angiosperms. From +the endosperm in the seeds of gymnosperms archegonia were found to +be developed (Fig. 510) in much the same way as in Selaginella, +from the prothallium, thus showing the endosperm to be a modified +and greatly reduced gametophyte. In some cases, it has even been +found to protrude a little way out of the embryo sac and to take +on a slightly greenish tinge—another reminiscence of its origin. +Fertilization, too, takes place in precisely the same manner as in +the pteridophytes, except that in all but the ginkgo and the cycads, +the fertilizing cells in the pollen grains are non-motile, and find +their way to the ovule by growing down into the embryo sac with the +pollen tube, instead of swimming to it—an adaptation probably brought +about in response to changed condition during the course of evolution +from aquatic to terrestrial life. + +The analogies between the sequence of alternations in the two classes +will be made clearer by a comparison of the accompanying diagrams. +The corresponding terms applied to the various organs stand in +the same vertical row. Diagram (1) shows the process as it takes +place in the more highly developed Pteridophytes; diagram (2) the +corresponding phases in angiosperms. + + + PTERIDOPHYTES + + _mospl_→_mic_→_mo_→_anG_→_ant_→_mg_→ + ╱ ╲ + (1) _S_ _öos_→_S_ + ╲ ╱ + _Mospl_→_Mgc_→_Mo_→_arG_→_arc_→_fg_→ + +_mospl_, microsporophyll; _mic_, microsporangium; _mo_, microspores; +_anG_, male gametophyte; _ant_, antheridia; _mg_, antherozoids. The +letters in the lower line stand for the corresponding female organs. + + + SPERMATOPHYTES + + _st_→_an_→_pol_→_fc_→ _not_ →_gc_→ + ╱ _developed_ ╲ + (2) _S_ _öos_→_S_ + ╲ _developed_ ╱ + _p_ →_ov_→_em_→_end_→ _only in_ →_ec_→ + _gymnosperms_ + +_st_, stamen; _an_, anther; _pol_, pollen; _fc_, food cells in pollen +grain; _gc_, generative cell; _p_, pistil; _ov_, ovules; _em_, embryo +sac; _end_, endosperm; _ec_, egg cell. + + +=415. Disappearance of the gametophyte.=—The seed is a comparatively +recent development in plant evolution. It has no counterpart anywhere +among the cryptogams, but is strictly characteristic of the three +great orders of Spermophytes: Monocotyl, Dicotyl, and Gymnosperms, +which compose the greater part of the vegetation of the globe. +Structurally, it is a matured sporangium containing a rudimentary +sporophyte (the embryo), and a reduced gametophyte (the embryo sac), +which, under the form of endosperm, has dwindled to an insignificance +that makes it difficult to recognize it as a phase in an alternation +of generations. + +=416. Significance of the sporophyte.=—The gametophyte is obviously a +more ancient and primitive structure than the sporophyte, which first +becomes prominent in the ferns and their allies. The sudden and +violent break in the succession of vegetable life that accompanies +the appearance of the pteridophytes (412) is probably to be explained +by the development of a land flora and the necessity of adaptation to +life in a new medium. The fact that no living cell, whether vegetable +or animal, can absorb nourishment except in a liquid form, seems to +point to an aquatic origin more or less remote for all life. This +inference is further strengthened, in the case of plants, by the +fact that even in so highly organized a group as the pteridophytes, +fertilization cannot take place except in water. Such a requirement +would manifestly be a great disadvantage to land plants, and one of +the first steps in response to the demands of a new habitat would be +to get rid, as far as possible, of the primitive gametophyte with +its outgrown adaptations to a liquid medium, and to transfer the +greater part of the work of reproduction to the asexual generation, +in which the problem of fertilization did not have to be directly +met, the asexual spores germinating without it. The greater the +number of these produced, the better the chance that at least some of +the gametes developed from them would meet the difficult conditions +of fertilization, and the survival of the species be assured. At the +same time, in order to meet the requirements of terrestrial life +successfully, and to provide for continuing the sexual generation, +correlative changes would have to take place in the gametophyte by +which the increasing uncertainty of fertilization due to structural +changes in the sporophyte, and the absence of a liquid medium for +the conveyance of free swimming antherozoids would be avoided. This +necessity has been met by the development of the pollen tube, which +bores its way to the egg cell, carrying with it the generative cells, +which in seed plants have taken the place of the more primitive +antherozoids. With the concomitant reduction of the gametophyte and +development of the seed habit, the adaptation to land conditions has +been made complete. + +Roughly speaking, it may be said: (1) that Thallophytes are +predominantly aquatic; (2) Archegoniates (Bryophytes and +Pteridophytes), amphibious; (3) Spermophytes, terrestrial; (4) that +the seed habit is a response to terrestrial conditions; and (5) +that the increased development of the sporophyte was a necessary +adaptation to meet those conditions. + + + IX. THE COURSE OF PLANT EVOLUTION + +=417. Plant genealogy.=—It has been shown by a study of existing +forms of plant life that there is no hard and fast line of division +anywhere between the different groups, but that they are all +connected by ties of kinship more or less defined, according to their +distance from a common ancestral stock. The geological record points +to the same conclusion, and our classification of them into families, +orders, and species is merely a very imperfect genealogical table +of their supposed pedigrees. This does not mean, however, that we +can assert positively that such and such a species is derived from +such or such another, but that both are descended from some common +intermediate form more or less remote. While we have reason to +believe that the flowering plants are derived through pteridophyte +and bryophyte types from some of the green algæ, no direct connection +has ever been traced between any particular kind of flowering +plant and any particular kind of alga,—or between a liverwort and +an alga, for that matter,—and probably never will be, because the +intermediate forms die out, or pass on by variation into other lines +of development. But while this is true, all the evidence we possess +does go to show that, since the beginning of life on the globe, there +has been a general progressive evolution from lower and simpler to +higher and more complex forms. + +=418. Retrogressive evolution.=—While the general course of evolution +has been upward and onward, the movement has not always followed +a straight line, but, like a mountain road, shows many windings +and deviations from the direct route. The monocotyls furnish a +conspicuous example of this departure from the general law of +progression. It was formerly supposed, on account of their greater +simplicity of structure, that they were a more ancient type than +dicotyls, but recent investigations point to the conclusion that +they are a later offshoot, derived from some primitive form of +aquatic dicotyl, and represent, not an ancient and primitive stock, +but a case of retrogressive evolution from a higher type. Strong +presumptions in favor of this view are: (1) that various species of +dicotyls show an unequal development of the seed leaves, amounting, +in the bryony, to complete abortion of one of them, while some +monocotyl seeds show morphological characters that can best be +explained as survivals, or inheritances, from a dicotyl ancestor; +(2) the structural resemblances between gymnosperms and dicotyls are +closer than between gymnosperms and monocotyls, which could hardly +be the case if the latter were the more ancient; (3) the geological +record does not show them to have appeared before dicotyls; (4) the +number of cotyledons furnishes no criterion as to the relative age +of any plant group, since all three types are represented among the +pteridophytes, where plants are found bearing one, two, or more +cotyledons. + +The theory of their comparatively recent origin from an aquatic +ancestor is further borne out by the many points of similarity +between their internal structure and that of hydrophytes (318), and +also by the great proportion of aquatic plants among them, amounting +to thirty-three per cent, while in dicotyls the proportion is only +four per cent. Can you give any reasons, from your examination of +their internal structure (113, 114), for believing that the line of +development which they have followed is a less effective one for +meeting conditions now existing on the globe than that attained by +dicotyls? + +We should remember, too, that while progressive evolution implies +successful adjustment to surroundings, it is possible to conceive of +a state, as our planet approaches the period of cosmic debility and +decay, when the conditions of existence may become progressively more +and more unfavorable. In this case the course of evolution would be +reversed, the higher types gradually dying out as the struggle for +life became more severe, and the tendency would be constantly toward +lower and simpler forms, until finally all life would become extinct +on our planet. We have no right, however, to assume that during such +a course of retrogressive evolution the same forms would be repeated +in reverse order as have already appeared, because there is no reason +to believe that the conditions brought about by planetary decline and +“old age” would be the same as those attending planetary birth and +adolescence. + +[Illustration: FIG. 511.—Diagram showing the supposed course of plant +evolution.] + +=419. Explanation of the diagram.=—An attempt to show the general +course of plant evolution up to the present time is made in the +accompanying diagram. The four great divisions, Thallophytes, +Bryophytes, Pteridophytes, and Spermatophytes, are represented by +spaces between four horizontal lines arranged one above the other in +the order of their succession in time and complexity of organization. +It should be borne in mind that these dividing lines are not sharply +defined in nature, but overlap or indent the territory between them +with varying degrees of irregularity, like the coast line on a +map. The relative positions of the different orders we have been +considering are represented by upright and diagonal lines, the +general course of which, as indicated by the arrows, is intended to +give an idea of the trend of evolutionary progress in the particular +group represented by each line. No one of these lines is made to +originate directly in any other, because, with the possible exception +of the monocotyls, we have no authority for asserting that any such +direct connection exists between plants as we know them, but only +that certain types give evidence of descent from a common ancestry. +This lack of certainty is expressed by placing the point of origin +for any given line in more or less close proximity to the one which +is supposed to be the nearest living representative of the common +ancestor. The line of ferns, for instance, is depicted as originating +in the region of the bryophytes, somewhere in the neighborhood of +the liverworts, but the two lines nowhere come in contact, because +there is no evidence that any fern, living or fossil, is directly +descended from any particular kind of liverwort known to us. With +these explanations, the diagram shows, in a rough way, the generally +accepted view of plant relationships as based on the evidence at +present before us. But in questions of this sort it is wise to keep +in mind the blunt remark of a famous old American statesman, that +“only fools and dead people never change their opinions.” + + + Field Work + + 1. If you live in the country, study the appearance of plants + affected with blights, smuts, rusts, and mildews, and learn to + recognize the different kinds of disease by their signs. Notice + which kinds are most prevalent in your neighborhood, and what + plants are most affected by them. + + 2. Notice the different kinds of mushrooms you find growing wild. + Observe the difference between those that grow on the ground and + those that grow on logs, stumps, and trees; between those found + in the woods and those in open ground. Find out how those on the + ground get their nourishment. Uncover the mycelium, and notice + the extent of its surface. Examine the soil and find out if it + contains anything upon which they could feed. Note the prevalence + of shelf fungi on trees. Examine the condition of the wood where + they grow, and decide in what ways they injure their hosts. Notice + whether they abound most on healthy or on decaying trunks and + boughs, and decide whether this is because the fungus prefers that + kind of host, or whether the injury it does causes the decay, or + whether both causes operate together. Notice what fungi grow on + different trees, and study their preferences in this respect. + + 3. Observe the different kinds of lichens found in your walks + and try to distinguish the three classes. Which kind are most + abundant in your neighborhood? Which least so? Note the situations + in which you find each kind growing, whether on stumps, trees, + rocks, or the ground. Consider how the algæ and fungi aid each + other in the different positions; could either, for instance, exist + independently on bald rocks? Notice on what kind of trees the + different lichens seem to thrive best and on which poorly or not at + all, and whether the character of the bark—rough, smooth, scaly—has + anything to do with their choice of a habitat. + + + + +APPENDIX + + + + + SYSTEMATIC BOTANY + + +=Taxonomy, or systematic botany=, deals with the family relationships +of plants in the order of their nearness or remoteness with regard +to a common line of descent. Its chief value is the insight it +gives into the course of plant evolution and into the nature of +the modifications that differentiate each group from the ancestral +type. While it is not advisable to spend too much time in the mere +identification of species, a sufficient number should be examined +and described to familiarize the student with the distinctive +characteristics of the principal botanical orders. + +=Principles of classification.=—All the known plants in the world, +numbering not less than one hundred and twenty thousand species +of the seed-bearing kind alone, are ranged according to certain +resemblances of structure, into a number of great groups known as +families or orders. The names of these families are distinguished by +the ending _aceæ_; the rose family, for instance, are the _Rosaceæ_; +the pink family, _Caryophyllaceæ_; the walnut family, _Juglandaceæ_, +etc. Each of these families is divided into lesser groups called +_genera_ (singular, _genus_), characterized by similarities showing +a still greater degree of affinity than that which marks the larger +groups or orders; and finally, when the differences between the +individual plants of a kind are so small as to be disregarded, they +are considered to form one species; all the common morning-glories, +for instance, of whatever shade or color, belong to the species +_Ipomea purpurea_. The small differences that arise within a species +as to the color and size of flowers, and other minor points, +constitute mere varieties, and have no special names applied to them. +The line between varieties and species is not clearly defined, and +in the nature of things can never be, since progressive development, +through unceasing change, is the law of all life. + +In botanical descriptions, the name both of the species and the genus +is given, just as in designating a person, like Mary Jones or John +Robinson, we give both the surname and the Christian name. The genus, +or generic name, answers to the surname, and that of the species to +the Christian name—except that in botanical nomenclature the order +is reversed, the generic, or surname, coming first, and the specific +or individual name last; for example, _Ipomea_ is the generic, or +surname, of the morning-glories, and _purpurea_ the specific one. + +=How to use the key.=—Any good manual will answer the purpose. Gray’s +“School and Field Book” is, perhaps, the best available at present +for the states east of the Mississippi. Reference to the floral +analyses in sections =I-IV= of Chapter VII will make its use clear. +Suppose, for instance, we want to find out to what botanical species +the morning-glory or the sweet potato belongs. Turning to the key, +we find the sub-kingdom of Phænerogams—flowering or seed-bearing +plants—divided into two great classes, Angiosperms and Gymnosperms, +as explained in 18. A glance will show that our specimen belongs +to the former class. Angiosperms, again, are divided into the two +subclasses of Dicotyledons and Monocotyledons (18, 171). We at +once recognize our plant, by its net-veined leaves and pentamerous +flowers, as a dicotyledon (171, 229), and turning again to the key, +we find this subclass divided into three great groups: Sympetalous +(211), called also Monopetalous and Gamopetalous; Apopetalous, or +Polypetalous (211), and Apetalous—having no petals or corolla. A +glance will refer our blossom to the sympetalous or monopetalous +group, which we find divided into two sections, characterized by +the superior or inferior ovary (218, 225). Further examination will +show that the morning-glory belongs to the former class, which is +in turn divided into two sections, according as the corolla is +_regular_, or _more or less irregular_. We see at once that we must +look for our specimen in the group having regular corollas. This we +find again subdivided into four sections, according to the number and +position of the stamens, and we find that the morning-glory falls +under the last of these,—“Stamens as many as the lobes or parts of +the corolla and alternate with them.” A very little further search +brings us to the family _Convolvulaceæ_, and turning to that title +in the descriptive analysis, we find under the genus, _Ipomea_, a +full description of the common morning-glory, in the species _Ipomea +purpurea_, and of the sweet potato in the species _Ipomea batatas_. + +=Making collections.=—Mere labeled aggregations of species are not +recommended, but the collection of examples illustrating special +points in morphology and plant variation may be made with profit; +such, for instance, as the adaptations observed in tendrils and +stipular appendages, the various modifications of leaves and stems +to serve other than their normal purposes, or the different forms +of leaves and flowers on the same stem, or on different plants of +the same species. A collection made with some specific object in +view would also be instructive, and might prove of great value; for +instance, to get together examples of all the troublesome weeds of +a locality for the purpose of studying their habits and devising +means for their eradication; or of all the native useful plants, with +detailed analyses of their economic properties, and observations on +their habits and the practicability of further developing them. In +short, wherever collecting is carried on, it should be done with some +object other than the mere identification of species, which often +results in greater detriment to the wild plants of a neighborhood +than profit to the collector. + + + + + WEIGHTS, MEASURES, AND TEMPERATURES + + +As the metric system of weights and measures and the Centigrade +appraisement of temperatures are universally employed in scientific +works, the following tables showing the equivalents in our common +English standards of those in most frequent use, are given for the +convenience of students who have not already familiarized themselves +with the subject. The values given are approximate only, but will +answer for all practical purposes, except in cases where very +great exactitude is required. The micron, or micrometer, is used +principally by scientific investigators for measuring extremely +minute objects seen under the microscope. + + + MEASURES OF LENGTH + + ================================+===================================== + METRIC | ENGLISH EQUIVALENTS + -----------------+--------------+------------------------------------- + Kilometer | km. | ⅔ of a mile. + -----------------+--------------+------------------------------------- + Meter | m. | 39 inches. + -----------------+--------------+------------------------------------- + Decimeter | dm. | 4 inches. + -----------------+--------------+------------------------------------- + Centimeter | cm. | ⅖ of an inch. + -----------------+--------------+------------------------------------- + Millimeter | mm. | ¹⁄₂₅ of an inch. + -----------------+--------------+------------------------------------- + Micron | µ | ¹⁄₂₅₀₀₀ of an inch. + -----------------+--------------+------------------------------------- + + + CAPACITY + + -----------------+---------+------------------------------------------ + Liter | l. | 61 cubic inches, or 1 quart, U.S. measure + -----------------+---------+------------------------------------------ + Cubic centimeter | cc. | ¹⁄₁₆ of a cubic inch. + -----------------+---------+------------------------------------------ + + + WEIGHT + + -----------------+---------------+------------------------------------ + Kilogram | kg., or kilo. | 2⅕ pounds. + -----------------+---------------+------------------------------------ + Gram | gm. | 15½ grains avoirdupois. + | | ¹⁄₂₈ of an ounce avoirdupois. + =================+===============+==================================== + + + METRIC AND ENGLISH SCALES + +[Illustration: 10 CENTIMETERS = 1 DECIMETER + +100 MILLIMETERS + +4 INCHES] + + + TEMPERATURE EQUIVALENTS + +The next table gives the Fahrenheit equivalent, in round numbers, for +every tenth degree Centigrade from absolute zero to the boiling point +of water. To find the corresponding F. for any degree C., multiply +the given C. temperature by nine, divide by five, and add thirty-two. +Conversely, to change F. to C. equivalent, subtract thirty-two, +multiply by five, and divide by nine. + + Cent. Fahr. + ---------------- + 100 212 + 90 194 + 80 176 + 70 158 + 60 140 + 50 122 + 40 104 + 30 86 + 20 68 + 10 50 + 0 32 + −10 14 + −20 −4 + −30 −22 + −40 −40 + −50 −58 + −100 −148 + ———————————————— + Absolute zero. + -273 -459 + + + + +FOOTNOTES: + +[1] Vines, “Lectures on the Physiology of Plants,” p. 282. See also +Sachs, “Physiology of Plants.” + +[2] Marshall Ward, “The Oak.” + + + + +INDEX + +(The numbers, unless otherwise designated, refer to paragraphs.) + + + Aborted, 220, 291. + + Absorption, 58, 71, 72; Exp. 39. + selective, 60. + + Accessory buds, 158. + + Accessory fruits, 302. + + Adaptation, 206, 237. + + Adhesive fruits, 20; Exp. 20. + + Adjustment of leaves, 196-202. + + Adnate, 374. + + Adventitious buds, 65, 158. + + Adventitious roots, 37, 83. + + Æcidium, 362. + + Aëration, 319. + + Aërial roots, 88. + + Aggregate fruits, 301, 303. + + Air space, 114, 116, 184. + + Akene, 234, 296, 302, 305. + + Albumin, 3. + + Albuminous, 56. + + Albuminous seed, _i.e._, containing endosperm; + Field work, p. 28. + + Aleurone, 3. + + Algæ, 333, 336-342. + + Alternate leaves, 168. + + Alternation of generations, 395, 400, 409, 414. + + Analogous, 108. + + Anatropous, Fig. 26. + + Angiosperms, 15, 18; Fig. 511. + + Annuals, 91. + + Annulus, 372, 405. + + Anther, 213, 235; Figs. 270-274. + + Antheridia, 389, 394, 398, 407. + + Antheridial, 388. + + Antherozoids, 389, 392, 395, 416. + + Antisepsis, 355. + + Arch of the hypocotyl, 42, 44. + + Archegonia, 390, 394, 407, 408. + + Archegonial, 388. + + Archegoniates, 408, 416. + + Archegonium, 391, 394, 398. + + Asexual generation, 395, 399, 409, 416. + + Asexual reproduction, 394, 395. + + Asexual spore, 395, 407, 409, 410, 416. + + Assurgent, 95. + + Axial placenta, 216, 300. + + Axil, 100, 166. + + Axillary buds, 145. + + Axis, 64, 65, 79, 152, 156, 159, 161. + + + Bacillus, 348, 349. + + Bacteria, 333, 345, 347-353. + + Bark, 118, 119, 122, p. 128, (3). + + Basidia, 375. + + Bast, 116, 119, 122. + + Berry, 291. + + Biennial, 92. + + Bilabiate, 237, 243. + + Bilateral regularity, 219. + + Bilateral zonation, 326. + + Black rust, 360. + + Blade of leaf, 165. + + Biogenetic law, 253. + + Biological factors, 309. + + Bordered pits, 114, 117; Fig. 123. + + Boreal, 329. + + Bract, 161. + + Bryophytes, 334, 385-401. + + Bud scales, 147-149. + + Buds, 145, 155-158. + + Bulb, 107. + + Button (of mushroom), 370. + + + Calyptra, 399. + + Calyx, 211. + + Cambium, 115, 116, 120, 123. + + Cap, 372, 373. + + Capillarity, 136; Exp. 53. + + Capitate, 220. + + Caprification, 279, 305. + + Caprifig, 279. + + Capsule, 298. + + Carbon, 27, 28, 62. + + Carbon dioxide, 29, 63, 185, 186, 187, 189; Exps. 23, 25. + + Carpels, 216, 288. + + Caruncle, 13. + + Catkin, 161. + + Caulicle, 46. + + Cedar apples, Fig. 456. + + Cell, 6, 7. + collecting, 184. + companion, 114. + + Cell sap, 7, 110. + + Cell wall, 7, 183. + + Central cylinder, 67. + + Central placenta, 216, 300. + + Chalaza, 13. + + Chlorophyll, 186, 341, 366. + + Chlorophyll bodies, 184, 186, 382. + + Cion, 65. + + Classification, 90, 252, 283, 343, 384, 411, 417. + + Cleistogamic flowers, 272. + + Climatic zones, 329. + + Climbing stems, 96-98. + + Clipped seed, p. 12 (material). + + Closed bundle, 114. + + Close-fertilized, 272. + + Cluster cups, 362. + + Coccus (pl. cocci), 339, 348. + + Coiled inflorescence, 162. + + Collective fruits, 304. + + Colony, 316, 337, 357. + + Color of flowers, 276. + + Compass plants, 199. + + Complete flower, 219. + + Composite, 235, 381. + + Composite flower, 236. + + Compound leaf, 178. + + Conduplicate, Figs. 159, 160. + + Confluent, 404. + + Conifers, 117, 327. + + Conjugation, 342, 394. + + Corolla, 211. + + Cortex, 64, 115, 122. + + Corymb, 161. + + Cotyledon, 11, 12, 18. + + Cross cut, 133. + + Cross fertilization, 255. + + Cross pollination, 255. + + Crustaceous lichen, 384. + + Cryptogam, 332. + + Crystalloids, 60. + + Culture medium, 347; p. 306 (material). + + Cycle, 217, 219, 229. + + Cycle of growth, 50. + + Cyme, 162. + + Cymose inflorescence, 162. + + Cypress knees, 319. + + + Deciduous, 203. + + Declined, 95. + + Decurrent, 374. + + Definite annual growth, 153. + + Definite inflorescence, 160, 162. + + Dehiscent fruits, 283, 298. + + Deliquescent, 144. + + Determinate growth, 153. + + Determinate inflorescence, 160, 162. + + Diadelphous, 239. + + Diastase, 9. + + Dichogamy, 269. + + Dichotomous, 152; Fig. 155. + + Dicotyl, 42, 115, 116, 171, 220. + + Dicotyledonous, 12. + + Differentiate, 245, 345, 409. + + Diffusion, 9, 57. + + Digestion, 9. + + Dimorphic, 270. + + Dimorphism, 270. + + Dimorphous, 270. + + Diœcious, 268. + + Disinfection, 355. + + Disk flower, 233. + + Dispersal of seed, 19-25. + + Dominant, 257, 258. + + Dormant buds, 157. + + Dorsal; Figs. 390, 391. + + Drupe, 292. + + Dry fruits, 283, 293-300. + + Duct, 67, 111, 114. + + + Ecological factors, 310. + + Ecology, 266, 308, 310. + + Edgings, 134. + + Egg cell, 251, 391. + + Elators, 393. + + Embryo, 11. + + Embryology, 253. + + Embryo sac, 251. + + Endodermis, 67 (b). + + Endosperm, 11, 13, 14, 16, 17, 414. + + Epicotyl, 45, 46, 47. + + Epidermis, 64, 115, 122, 183. + + Epigynous, 225, 230. + + Epiphyte, 87, 394. + + Essential constituents, 62. + + Essential organs, 212. + + Evolution, 242, 245, 265, 334, 335, 401, 414, 415, 417, 418, 419. + + Evolutionary, 253, 413. + + Excentric attachment, 372. + + Excurrent, 144, 154. + + + Factors, 54, 265, 310. + + Fall of the leaf, 203. + + Fascicled roots, 80, 81. + + Fats, 1, 3, 4. + + Feather-veined, 172. + + Ferments, 9, 356. + + Fertile, 404. + + Fertile flower, 267. + + Fertilization, 247, 251, 252, 392, 408, 416. + + Fibrous roots, 37, 78, 80, 81. + + Fibrovascular bundle, 67, 114, 116, 176, 288. + + Fig wasp, 279. + + Filament of the stamen, 213; + a hairlike appendage, 341, 361, 369, 393, 396. + + Filamentous algæ, 340, 341. + + Fission, 338, 394. + + Fleshy fruits, 283, 288-292. + + Floral envelopes, 211. + + Foliaceous lichen, 379, 384. + + Follicle, 298. + + Forestry, 139-142. + + Forked stems, 152. + + Formation, 316. + + Free, 218, 374. + + Free central placenta, 216. + + Free gills, 374. + + Free ovary, 218. + + Free veining, 402. + + Freezing, 33. + + Frog’s spit, 340. + + Frond, 402. + + Fruit, 282. + + Fruticose lichen, 384. + + Function, 41. + + Fungi, 333, 343, 344, 345, 346, 378. + + Fungus, 86, 364. + + + Gametes, 394. + + Gametophyte, 394, 395, 396, 406, 407, 410, 412, 414, 415, 416. + + Gemmæ, 387. + + Generative cell, 249, 416. + + Geophilous, 321. + + Geotropism, 51, 52, 53. + + Germ, 2, 11. + + Germ cell, 251, 414. + + Germination, 32, 35; Exps. 25, 26-29. + + Germs, 352, 355. + + Gills (of mushroom), 374. + + Girdling, 131. + + Glutin, 3. + + Gourd, 14, 290. + + Grain, 11, 297. + + Grain of timber, 133, 134, 135. + + Gravity, 52. + + Growth, 48-52, 179. + + Guard cell, 183. + + Gymnosperms, 15, 18, 117, 414. + + Gymnosporangium, Fig. 456. + + + Halophyte, 317, 323. + + Haustoria, 85. + + Hay bacillus, 348, 349. + + Head, 161. + + Heartwood, 131. + + Heliotropic, 200. + + Heliotropism, 198. + + Herbaceous, 90, 94, 115, 116. + + Heredity, 264, 265. + + Hilum, 12, 13, 14. + + Homologous, 108. + + Host plant, 85. + + Humus, 75, 86. + + Hybrid, 256. + + Hybridization, 256, 257, 263. + + Hydrophytes, 317, 318, 319. + + Hymenium, 375. + + Hymenomycetes, 375. + + Hyphæ (sing. hypha), 369, 380. + + Hypocotyl, 11, 12, 14, 46. + arched, 42, 44. + straight, 44. + + Hypogynous, 218, 225. + + + Imbibition, 136. + + Imperfect flower, 219, 231, 267. + + Impure hybrid, 258, 259. + + In-breeding, 254. + + Incomplete flower, 219. + + Incubation, 354. + + Indefinite annual growth, 153. + + Indefinite inflorescence, 160, 161. + + Indefinite number of parts, 229. + + Indehiscent fruit, 283, 294. + + Indeterminate growth, 153. + + Indeterminate inflorescence, 160, 161. + + Indusium, 404. + + Inferior ovary, 221, 225. + + Inflorescence, 159. + + Insectivorous plants, 208-210. + + Internode, 46, 110; Exp. 35. + + Invasion, 328. + + Inverted seed, 14. + + Involucre, 161, 232. + + Involute, 373; Fig. 251. + + Iodine solution, Exp. 3. + + Irregular flower, 219, 237. + + Irritability, 201. + + + Joint, 110, 113. + + + Keel, 238. + + Knots, 137. + + + Lamina, 209. + + Laminæ, 368, 374. + + Lateral, 372, 398. + + Lateral buds, 145. + + Leaf attachment, 167. + + Leaf cups, 202. + + Leaf scars, 146. + + Leaf traces, 146. + + Legume, 299. + + Lenticels, 106, 118, 288. + + Lichen, 379. + + Life cycle, 359, 364. + + Loam, 75. + + Lobing, 177; Figs. 210-212. + + Locule, 216. + + Loment, Fig. 394. + + Lyrate, Fig. 197. + + + Medulla, 119, 122. + + Medullary rays, 64, 116, 121, 122, 134, 135. + + Megasporangia, 409. + + Megaspore, 409, 414. + + Mendel’s law, 258. + + Mesophyte, 317, 324. + + Metabolism, 193. + + Microbe, 351, 355. + + Micrococcus, 339. + + Micropyle, 12, 13, 14, 15, 45. + + Microsporangia, 409. + + Microspore, 409, 414. + + Midrib, 172. + + Mixed forest, 139, 324. + + Modification, 100-108, 206, 207, 289. + + Molecule, 136. + + Monadelphous, 239. + + Monocotyl, 110, 112, 171, 217, 221, 418. + + Monocotyledonous, 11. + + Monœcious, 268. + + Monopetalous, 211. + + Monosepalous, 211. + + Morphology, 108. + of the flower, 244. + + Mosaic (leaf), 197. + + Mosses, 334, 396-401. + + Muck, 75. + + Multiple fruit, 304, 305. + + Mushroom, 333, 367. + + Mutation, 264. + + Mycelium, 343, 359, 369. + + Mychorrhiza, 86. + + + Neck canal, 391. + + Net-veined, 171. + + Neuter, 267. + + Neutral flower, 231, 267. + + Nitrogen, 62, 63, 188. + + Nitrogenous food, 188. + + Node, 46, 65, 110, 113. + + Nucleus, 7, 341. + + Numerical plan, 217, 229. + + Nut, 295. + + Nutriment, 3, 186. + + Nutrition, 50, 54, 179, 193. + + Nyctitropic, 200. + + + Obsolete, 220. + + Oil, 1, 3, 8. + + Oöspore, 393, 394, 395. + + Open bundle, 116. + + Operculum, 399. + + Opposite leaves, 168. + + Organ, 41. + + Organic foods, 4. + + Organs of reproduction, 40. + of vegetation, 40. + + Osmosis, 56, 57. + + Ovary, 214, 216, 223. + + Ovule, 216. + + Oxidation, 27; Exps. 21, 22. + + Oxygen, 62, 63, 186, 187; Exps. 22, 66. + + + Palisade cells, 184. + + Palmate veining, 172. + + Panicle, Fig. 171. + + Papilionaceous, 237, 238. + + Pappus, 234. + + Parallel veining, 171. + + Paraphyses, 375, 398. + + Parasitic, 5, 345, 364. + + Parasitic plants, 85, 343, 382. + + Parenchyma, 110, 114, 115. + + Parietal, 216. + + Pathogenic, 352, 353. + + Pedicel, 159. + + Peduncle, 159, 288. + + Pentamerous, 229. + + Pepo, 290. + + Perennial, 93. + + Perfect flower, 219. + + Perianth, 211. + + Pericarp, 288. + + Perigynous, Figs. 301, 302. + + Persistent, 166. + + Petals, 211. + + Petiole, 165. + + Phanerogams, 331, 332. + + Phloem, 114, 116. + + Photosynthesis, 186, 192, 193. + + Phototropism, 195. + + Phyllotaxy, 168, 169. + + Pileus, 373. + + Pinna, 402. + + Pinnate veining, 172. + + Pinnule, 402. + + Pioneer plant, 316, 319, 320. + + Pistil, 212, 214, 223, 228, 240. + + Pistillate, 267. + + Pitcher plant, 209. + + Pith, 110, 115, 116, 119, 121, 122. + + Pitted ducts, 114. + + Placenta, 216, 288, 298, 300. + + Plant society, 316. + + Plasmolysis, 59. + + Pleurococcus, 337. + + Plicate, 155. + + Plumule, 11, 12, 14, 45, 46. + + Pod, 298. + + Pollen, 213. + + Pollen grains, 213. + + Pollen sac, 213. + + Pollen tubes, 249, 250. + + Pollination, 215, 247. + + Polycotyledons, 15, 45. + + Polymorphic, 365. + + Polymorphism, 365. + + Polypetalous, 211. + + Polysepalous, 211. + + Pome, 288. + + Prefoliation, 155. + + Primary, 396. + + Primary root, 42, 79. + + Pronuba, 278. + + Prostate, 95. + + Protection, 199, 204, 207, 280, 287. + + Proteins, 3, 8, 33, 188, 204. + + Prothallium, 407. + + Protonema, 396. + + Protoplasm, 6, 7, 57, 58, 67, 110, 116. + + Pteridophytes, 335, 411, 412. + + Puccinia, 360. + + Pure dominant, 258, 259. + + Pure forest, 139, 324. + + Pure recessive, 258, 259. + + Pycnidia, 363. + + + Quartered cut, 135. + + + Raceme, 161. + + Rhachis, 178. + + Radial section, 132, 135. + + Radicle, 46. + + Rhaphe, 13. + + Ray, 161, 391. + + Ray flowers, 231. + + Receptacle, 211, 288, 289, 388, 390, 398. + + Recessive, 257, 258. + + Red rust, 359. + + Regular flower, 219. + + Reproduction, 338, 351, 358, 383. + + Respiration, 30, 31, 191, 192. + + Resting spore, 338, 342, 358, 394. + + Reticulation, 172, 402. + + Retrogressive evolution, 418. + + Revolute, 373, 404. + + Rhizoids, 379, 386. + + Rhizome, 105. + + Ringing, 127. + + Rings of growth, 122, 123, 134, 135. + + Rogue, 260. + + Root cap, 39. + + Root hairs, 38, 67. + + Root pressure, Exp. 49. + + Root pull, 69. + + Rootstock, 105. + + Root system, 89. + + Root tubercles, 63, 300. + + Rosette, 197. + + Rotation of crops, 24, 327. + + Runner, 95. + + + Samara, 296. + + Sap movement, 125, 126, 128, 129. + + Saprophyte, 86. + + Sapwood, 131. + + Scale leaves, 101, 106, 107, 147-149, 207. + + Scape, 107, 159. + + Scorpioid inflorescence, 162; Figs. 173-176. + + Screenings, 20; p. 28, Qn. 22. + + Secondary roots, 37, 42, 79. + + Seed, 11-18, 332, 415. + + Seed coat, 12, 14, 15, 43. + + Seedless fruits, 285, 286. + + Seedlings, 36, 42, 43, 45. + + Seed plants, 331, 414. + + Seed vessel, 282. + + Selection, 260, 265, 286. + artificial, 262. + natural, 261. + + Self-fertilization, 254, 271. + + Sepals, 211. + + Sessile, 167, 214. + + Seta, 399. + + Sexual generation, 395, 396, 406, 410, 416. + + Sexual reproduction, 394, 395, 410. + + Sheath, 67, 116. + + Shrinking of timber, 136. + + Sieve tube, 114. + + Slabs, 134. + + Sleep movements, 200. + + Soils, 75, 77. + + Sori, 404. + + Spathe, 221. + + Specialization, 237. + + Spermatophytes, 331, 335, 394, 414. + + Spermatozoid, 389. + + Spermogonia, 363. + + Spike, 161. + + Spirillum, 348. + + Spirogyra, 341. + + Sporangia, 390, 405. + + Spore, 332, 349, 350, 377, 406, 410. + + Spore case, 390, 393, 405. + + Spore print, 376. + + Sporidium, 361. + + Sporogonium, 393, 399. + + Sporophyll, 406, 414. + + Sporophyte, 393-395, 399, 406, 410, 412, 414, 416. + + Sport, 264. + + Stamen, 212, 213. + + Staminate, 267, 268. + + Staminodia, 244. + + Standard, 238. + + Starch, 3, 4, 187, 204, 288; Exps. 69, 70. + + Stems, 90-99. + + Sterile flower, 267. + + Sterilization, 354. + + Stigma, 214. + + Stigmatic surface, 223. + + Stimulus, 98, 186, 201. + + Stipe, 240, 372, 402. + + Stipule, 149, 165, 166. + + Stolon, 95. + + Stoma, 181, 182, 183. + + Stomata, 181, 182. + + Stone fruit, 292. + + Storage of food, 2, 3, 4, 17, 70, 103, 104-107, 287. + + Strangling fig, 88. + + Strobile, 411. + + Strobiliaceous, 411. + + Style, 214. + + Succession, 327. + + Sugars, 3, 4, 204, 288. + + Summer spores, 360. + + Sundew, 210. + + Superior ovary, 218, 221, 225. + + Supernumerary buds, 158. + + Suppressed, 220. + + Survival of the fittest, 261. + + Suture, 216, 298, 299. + + Swarm spore, 349. + + Swelling of timber, 136. + + Symbiosis, 309, 382. + + Symmetrical flower, 219. + + Sympetalous, 211. + + Syncarpous, 300. + + Synsepalous, 211. + + Systematic botany, _see_ Appendix. + + + Tangential cut, 132, 134. + + Tap root, 79. + + Teleutospore, 360. + + Tendril, 96, 97. + + Terminal bud, 145, 154. + + Testa, 14. + + Thallophytes, 333. + + Thallus, 333, 341, 343, 379, 380, 381, 385. + + Tillage, 76. + + Tissue, 60, 61. + + Toadstools, 367. + + Toxins, 345. + + Tracheids, 114, 117. + + Trailing, 95. + + Trama, 375. + + Transpiration, 179, 180. + + Trifoliolate, Figs. 215, 216. + + Trimerous, 217. + + Trimorphic, 270. + + Tuber, 106. + + Tumbleweeds, 23. + + Turgidity, 7. + + Turgor, 179. + + Twining, cause of, 98; Exp. 55. + + Twining stems, 96; Exp. 54. + + Type, 18, 260, 263, 265, 336, 411. + + + Umbel, 161. + + Umbonate, 373. + + Underground stems, 104-107. + + Unicellular, 337. + + Unisexual, 267. + + Uredo, 359. + + Uredospore, 359, 360. + + + Variation, 263, 264, 265. + + Vascular bundles, 111. + + Vascular cryptogams, 403, 411, 412. + + Vascular cylinder, 64. + + Vascular system, 111, 113, 335. + + Vegetative reproduction, 358. + + Veil, 371. + + Veins, 173-176. + + Venter, 391. + + Ventral, Figs. 390, 391. + + Vernation, 155. + + Vessels, 111. + + Vexillum, 238, 239. + + Vibrio, 348. + + Vitality of seeds, 34; Exp. 30. + + Volva, 371. + + + Water roots, 39, 84. + + Whorled leaves, 168. + + Wind pollination, 274, 275. + + Wings, 238. + + Winter spores, 360. + + + Xerophyte, 317. + + Xerophyte societies, 317, 320-322. + + Xylem, 114, 116. + + + Yeast, 356. + + Yeast colony, 357. + + Yellow trumpets, 209. + + Yucca, 278. + + Yucca moth, 278. + + + Zonation, 325, 327. + bilateral, 326. + concentric, 326. + horizontal, 326. + vertical, 326. + + Zones of vegetation, 325. + + + + +TRANSCRIBER’S NOTE + + + Obvious typographical errors and punctuation errors have been + corrected after careful comparison with other occurrences within + the text and consultation of external sources. + + Some hyphens in words have been silently removed, some added, + when a predominant preference was found in the original book. + + Except for those changes noted below, all misspellings in the text, + and inconsistent or archaic usage, have been retained. + + p. 45: ‘many of them has’ amended to ‘many of them have’ + p. 281: ‘are adpated’ amended to ‘are adapted’ + p. 291: ‘and as it can, moveover’ amended to ‘and as it can, moreover’ + p. 354: ‘eruption of Krakatao’ amended to ‘eruption of Krakatoa’ + + + + +*** END OF THE PROJECT GUTENBERG EBOOK 78430 *** |