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diff --git a/old/64601-0.txt b/old/64601-0.txt deleted file mode 100644 index 6cc820e..0000000 --- a/old/64601-0.txt +++ /dev/null @@ -1,18716 +0,0 @@ -The Project Gutenberg eBook of Elementary Botany, by George Francis -Atkinson - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: Elementary Botany - -Author: George Francis Atkinson - -Release Date: February 20, 2021 [eBook #64601] - -Language: English - -Character set encoding: UTF-8 - -Produced by: Sonya Schermann and the Online Distributed Proofreading Team - at https://www.pgdp.net (This file was produced from images - generously made available by The Internet Archive) - -*** START OF THE PROJECT GUTENBERG EBOOK ELEMENTARY BOTANY *** - - - - -Transcriber’s Notes: - - Underscores “_” before and after a word or phrase indicate _italics_ - in the original text. - Equal signs “=” before and after a word or phrase indicate =bold= - in the original text. - Small capitals have been converted to SOLID capitals. - Illustrations have been moved so they do not break up paragraphs. - Footnotes have been moved to the end of the chapter in which they - occur. - Typographical errors have been silently corrected. - - - - -[Illustration: CYCAS REVOLUTA (see page 311). (_Frontispiece._)] - - - - - ELEMENTARY BOTANY - - BY - GEORGE FRANCIS ATKINSON, PH.B. - _Professor of Botany in Cornell University_ - - _THIRD EDITION, REVISED_ - - [Illustration] - - NEW YORK - HENRY HOLT AND COMPANY - 1905 - - Copyright, 1898, 1905 - BY - HENRY HOLT AND COMPANY - - ROBERT DRUMMOND, PRINTER, NEW YORK - - - - -PREFACE. - - -The present book is the result of a revision and elaboration of the -author’s “Elementary Botany,” New York, 1898. The general plan of the -parts on physiology and general morphology remains unchanged. A number -of the chapters in the physiological part are practically untouched, -while others are thoroughly revised and considerable new matter is -added, especially on the subjects of nutrition and digestion. The -principal chapters on general morphology are unchanged or only slightly -modified, the greatest change being in a revision of the subject of -the morphology of fertilization in the gymnosperms and angiosperms in -order to bring this subject abreast of the discoveries of the past few -years. One of the greatest modifications has been in the addition of -chapters on the classification of the algæ and fungi with studies of -additional examples for the benefit of those schools where the time -allowed for the first year’s course makes desirable the examination of -a broader range of representative plants. The classification is also -carried out with greater definiteness, so that the regular sequence -of classes, orders, and families is given at the close of each of the -subkingdoms. Thus all the classes, all the orders (except a few in the -algæ), and many of the families, are given for the algæ, fungi, mosses, -liverworts, pteridophytes, gymnosperms, and angiosperms. - -But by far the greatest improvement has been in the complete -reorganization, rewriting, and elaboration of the part dealing with -ecology, which has been made possible by studies of the past few years, -so that the subject can be presented in a more logical and coherent -form. As a result the subject-matter of the book falls naturally into -three parts, which may be passed in review as follows: - -Part I. _Physiology._ This deals with the life processes of plants, -as absorption, transpiration, conduction, photosynthesis, nutrition, -assimilation, digestion, respiration, growth, and irritability. Since -protoplasm is fundamental to all the life work of the plant, this -subject is dealt with first, and the student is led through the study -of, and experimentation with, the simpler as well as some of the higher -plants, to a general understanding of protoplasm and the special way in -which it enables the plant to carry on its work and to adjust itself to -the conditions of its existence. This study also serves the purpose of -familiarizing the pupil with some of the lower and unfamiliar plants. - -Some teachers will prefer to begin the study with general morphology -and classification, thus studying first the representatives of the -great groups of plants, and others will prefer to dwell first on the -ecological aspects of vegetation. This can be done in the use of this -book by beginning with Part II or with Part III. - -But the author believes that morphology can best be comprehended after -a general study of life processes and functions of the different parts -of plants, including in this study some of the lower forms of plant -life where some of these processes can more readily be observed. The -pupil is then prepared for a more intelligent consideration of general -and comparative morphology and relationships. Even more important is -a first study of physiology before taking up the subject of ecology. -The great value to be derived from a study of plants in their relation -to environment lies in the ability to interpret the different states, -conditions, behavior, and associations of the plant, and for this -physiology is indispensable. It is true that a considerable measure -of success can be obtained by a good teacher in beginning with either -subject, but the writer believes that measure of success would be -greater if the subjects were taken up in the order presented here. - -Part II. _Morphology and life history of representative plants._ This -includes a rather careful study of representative examples among the -algæ, fungi, liverworts, mosses, ferns and their allies, gymnosperms -and angiosperms, with especial emphasis on the form of plant parts, -and a comparison of them in the different groups, with a comparative -study of development, reproduction, and fertilization, rounding out -the work with a study of life histories and noting progression and -retrogression of certain organs and phases in proceeding from the -lower to the higher plants. Thus, in the algæ a first critical study -is made of four examples which illustrate in a marked way progressive -stages of the plant body, sexual organs, and reproduction. Additional -examples are then studied for the purpose of acquiring a knowledge of -variations from these types and to give a broader basis for the brief -consideration of general relationships and classification. - -A similar plan is followed in the other great groups. The processes of -fertilization and reproduction can be most easily observed in the lower -plants like the algæ and fungi, and this is an additional argument in -favor of giving emphasis to these forms of plant life as well as the -advantage of proceeding logically from simpler to more complex forms. -Having also learned some of these plants in our study of physiology, -we are following another recognized rule of pedagogy, i.e., proceeding -from known objects to unknown structures and processes. Through -the study of the organs of reproduction of the lower plants and by -general comparative morphology we have come to an understanding of the -morphology of the parts of the flower, and of the true sexual organs -of the seed plants, and no student can hope to properly interpret the -significance of the flower, or the sexual organs of the seed plants who -neglects a careful study of the general morphology of the lower plants. - -Part III. _Plant members in relation to environment._ This part deals -with the organization of the plant body as a whole in its relation to -environment, the organization of plant tissues with a discussion of -the principal tissues and a descriptive synopsis of the same. This is -followed by a complete study from a biological standpoint of the -different members of the plant, their special function and their -special relations to environment. The stem, root, leaf, flower, etc., -are carefully examined and their ecological relations pointed out. This -together with the study of physiology and representatives in the groups -of plants forms a thorough basis for pure plant ecology, or the special -study of vegetation in its relation to environment. - -There is a study of the factors of environment or ecological factors, -which in general are grouped under the physical, climatic, and biotic -factors. This is followed by an analysis of vegetation forms and -structures, plant formations and societies. Then in order are treated -briefly forest societies, prairie societies, desert societies, arctic -and alpine societies, aquatic societies, and the special societies of -sandy, rocky, and marshy places. - -_Acknowledgments._ The author wishes to express his gratefulness to -all those who have given aid in the preparation of this work, or of -the earlier editions of Elementary Botany; to his associates, Dr. -E. J. Durand, Dr. K. M. Wiegand, and Professor W. W. Rowlee, of the -botanical department, and to Professor B. M. Duggar of the University -of Missouri, Professor J. C. Arthur of Purdue University, and Professor -W. F. Ganong of Smith College, for reading one or more portions of the -text; as well as to all those who have contributed illustrations. - -_Illustrations._ The large majority of the illustrations are new -(or are the same as those used in earlier editions of the author’s -Elementary Botany) and were made with special reference to the method -of treatment followed in the text. Many of the photographs were made -by the author. Others were contributed by Professor Rowlee of Cornell -University; Mr. John Gifford of New Jersey; Professor B. M. Duggar, -University of Missouri; Professor C. E. Bessey, University of Nebraska; -Dr. M. B. Howe, New York Botanical Garden; Mr. Gifford Pinchot, Chief -of the Bureau of Forestry; Mr. B. T. Galloway, Chief of the Bureau of -Plant Industry; Professor Tuomey of Yale University; and Mr. E. H. -Harriman, who through Dr. C. H. Merriam of the National Museum allowed -the use of several of his copyrighted photographs from Alaska. To those -who have contributed drawings the author is indebted as follows: to -Professor Margaret C. Ferguson, Wellesley College; Professor Bertha -Stoneman of Huguenot College, South Africa; Mr. H. Hasselbring of -Chicago; Dr. K. Miyake, formerly of Cornell University and now of -Doshisha College, Japan; and Professors Ikeno and Hirase of the Tokio -Imperial University. The author is also indebted to Ginn & Co., Boston, -for the privilege to use from his “First Studies of Plant Life” the -following figures: 28, 29, 46, 48, 49, 56, 62, 66, 67, 87, 102, 103, -422-426, 429, 430, 438-440, 443, 444, 448, 449, 452, 472-475. A few -others are acknowledged in the text. - -CORNELL UNIVERSITY, April, 1905. - - - - -TABLE OF CONTENTS. - - - PART I. PHYSIOLOGY. - - CHAPTER I. PAGE - PROTOPLASM. 1 - - CHAPTER II. - ABSORPTION, DIFFUSION, OSMOSE. 13 - - CHAPTER III. - HOW PLANTS OBTAIN WATER. 22 - - CHAPTER IV. - TRANSPIRATION, OR THE LOSS OF WATER BY PLANTS. 35 - - CHAPTER V. - PATH OF MOVEMENT OF WATER IN PLANTS. 48 - - CHAPTER VI. - MECHANICAL USES OF WATER. 56 - - CHAPTER VII. - STARCH AND SUGAR FORMATION. 60 - 1. The Gases Concerned. 60 - 2. Where Starch is Formed. 64 - 3. Chlorophyll and the Formation of Starch. 67 - - CHAPTER VIII. - STARCH AND SUGAR CONCLUDED; ANALYSIS OF PLANT SUBSTANCE. 73 - 1. Translocation of Starch. 73 - 2. Sugar, and Digestion of Starch. 75 - 3. Rough Analysis of Plant Substance. 79 - - CHAPTER IX. - HOW PLANTS OBTAIN THEIR FOOD, I. 81 - 1. Sources of Plant Food. 81 - 2. Parasites and Saprophytes. 83 - 3. How Fungi Obtain their Food. 86 - 4. Mycorhiza. 91 - 5. Nitrogen gatherers. 92 - 6. Lichens. 93 - - CHAPTER X. - HOW PLANTS OBTAIN THEIR FOOD, II. 97 - Seedlings, 97 - Digestion, 107 - Assimilation 109 - - CHAPTER XI. - RESPIRATION. 110 - - CHAPTER XII. - GROWTH. 118 - - CHAPTER XIII. - IRRITABILITY. 125 - - - PART II. MORPHOLOGY AND LIFE HISTORY OF REPRESENTATIVE PLANTS. - - CHAPTER XIV. - SPIROGYRA. 136 - - CHAPTER XV. - VAUCHERIA. 142 - - CHAPTER XVI. - ŒDOGONIUM. 147 - - CHAPTER XVII. - COLEOCHÆTE. 153 - - CHAPTER XVIII. - CLASSIFICATION AND ADDITIONAL STUDIES OF THE ALGÆ. 158 - - CHAPTER XIX. - FUNGI: MUCOR AND SAPROLEGNIA. 177 - - CHAPTER XX. - FUNGI CONTINUED (“Rusts” Uredineæ). 187 - - CHAPTER XXI. - THE HIGHER FUNGI. 195 - - CHAPTER XXII. - CLASSIFICATION OF THE FUNGI. 213 - - CHAPTER XXIII. - LIVERWORTS (Hepaticæ). 222 - Riccia, 222 - Marchantia. 226 - - CHAPTER XXIV. - LIVERWORTS CONTINUED. 231 - Sporogonium of Marchantia. 231 - Leafy-stemmed Liverworts. 236 - The Horned Liverworts. 240 - Classification of the Liverworts. 242 - - CHAPTER XXV. - MOSSES (Musci). 243 - Classification of Mosses. 248 - - CHAPTER XXVI. - FERNS. 251 - - CHAPTER XXVII. - FERNS CONTINUED. 262 - Gametophyte of Ferns. 262 - Sporophyte. 268 - - CHAPTER XXVIII. - DIMORPHISM OF FERNS. 273 - - CHAPTER XXIX. - HORSETAILS. 280 - - CHAPTER XXX. - CLUB MOSSES. 284 - - CHAPTER XXXI. - QUILLWORTS (Isoetes). 289 - - CHAPTER XXXII. - COMPARISON OF FERNS AND THEIR RELATIVES. 292 - Classification of the Pteridophytes. 295 - - CHAPTER XXXIII. - GYMNOSPERMS. 297 - - CHAPTER XXXIV. - FURTHER STUDIES ON GYMNOSPERMS. 311 - - CHAPTER XXXV. - MORPHOLOGY OF THE ANGIOSPERMS: TRILLIUM; DENTARIA. 318 - - CHAPTER XXXVI. - GAMETOPHYTE AND SPOROPHYTE OF ANGIOSPERMS. 325 - - CHAPTER XXXVII. - MORPHOLOGY OF THE NUCLEUS AND SIGNIFICANCE OF - GAMETOPHYTE AND SPOROPHYTE. 340 - - - PART III. PLANT MEMBERS IN RELATION TO ENVIRONMENT. - - CHAPTER XXXVIII. - THE ORGANIZATION OF THE PLANT. 349 - I. Organization of Plant Members. 349 - II. Organization of Plant Tissues. 356 - - CHAPTER XXXIX. - THE DIFFERENT TYPES OF STEMS. 365 - I. Erect Stems. 365 - II. Creeping, Climbing, and Floating Stems. 369 - III. Specialized Shoots and Shoots for Storage - of Food. 372 - IV. Annual Growth and Winter Protection of - Shoots and Buds. 374 - - CHAPTER XL. - FOLIAGE LEAVES. 383 - I. General Form and Arrangement of Leaves. 383 - II. Protective Modifications of Leaves. 392 - III. Protective Positions. 395 - IV. Relation of Leaves to Light. 397 - V. Leaf Patterns. 404 - - CHAPTER XLI. - THE ROOT. 410 - I. Function of Roots. 410 - II. Kinds of Roots. 415 - - CHAPTER XLII. - THE FLORAL SHOOT. 419 - I. The Parts of the Flower. 419 - II. Kinds of Flowers. 421 - III. Arrangement of Flowers, or Mode of - Inflorescence. 426 - - CHAPTER XLIII. - POLLINATION. 433 - - CHAPTER XLIV. - THE FRUIT. 450 - I. Parts of the Fruit. 450 - II. Indehiscent Fruits. 451 - III. Dehiscent Fruits. 452 - IV. Fleshy and Juicy Fruits. 454 - V. Reinforced, or Accessory, Fruits. 455 - VI. Fruits of Gymnosperms. 456 - VII. “Fruit” of Ferns, Mosses, etc. 457 - - CHAPTER XLV. - SEED DISPERSAL. 458 - - CHAPTER XLVI. - VEGETATION IN RELATION TO ENVIRONMENT. 464 - - CHAPTER XLVII. - CLASSIFICATION OF ANGIOSPERMS. 487 - - INDEX. 503 - - - - -PART I. - -PHYSIOLOGY. - - - - -CHAPTER I. - -PROTOPLASM.[1] - - -=1.= In the study of plant life and growth, it will be found -convenient first to inquire into the nature of the substance which we -call the living material of plants. For plant growth, as well as some -of the other processes of plant life, are at bottom dependent on this -living matter. This living matter is called in general _protoplasm_. - -=2.= In most cases protoplasm cannot be seen without the help of -a microscope, and it will be necessary for us here to employ one if we -wish to see protoplasm, and to satisfy ourselves by examination that -the substance we are dealing with _is_ protoplasm. - -=3.= We shall find it convenient first to examine protoplasm -in some of the simpler plants; plants which from their minute size -and simple structure are so transparent that when examined with the -microscope the interior can be seen. - -For our first study let us take a plant known as _spirogyra_, though -there are a number of others which would serve the purpose quite as -well, and may quite as easily be obtained for study. - - -Protoplasm in spirogyra. - -=4. The plant spirogyra.=—This plant is found in the water of -pools, ditches, ponds, or in streams of slow-running water. It is green -in color, and occurs in loose mats, usually floating near the surface. -The name “pond-scum” is sometimes given to this plant, along with -others which are more or less closely related. It is an _alga_, and -belongs to a group of plants known as _algæ_. If we lift a portion of -it from the water, we see that the mat is made up of a great tangle of -green silky threads. Each one of these threads is a plant, so that the -number contained in one of these floating mats is very great. - -Let us place a bit of this thread tangle on a glass slip, and examine -with the microscope and we will see certain things about the plant -which are peculiar to it, and which enable us to distinguish it from -other minute green water plants. We shall also wish to learn what these -peculiar parts of the plant are, in order to demonstrate the protoplasm -in the plant.[2] - -=5. Chlorophyll bands in spirogyra.=—We first observe the -presence of bands; green in color, the edges of which are usually -very irregularly notched. These bands course along in a spiral manner -near the surface of the thread. There may be one or several of these -spirals, according to the species which we happen to select for -study. This green coloring matter of the band is _chlorophyll_, and -this substance, which also occurs in the higher green plants, will -be considered in a later chapter. At quite regular intervals in the -chlorophyll band are small starch grains, grouped in a rounded mass -enclosing a minute body, the _pyrenoid_, which is peculiar to many algæ. - -[Illustration: Fig. 1. - -Thread of spirogyra, showing long cells, chlorophyll band, nucleus, -strands of protoplasm, and the granular wall layer of protoplasm.] - -=6. The spirogyra thread consists of cylindrical cells end to -end.=—Another thing which attracts our attention, as we examine a -thread of spirogyra under the microscope, is that the thread is made up -of cylindrical segments or compartments placed end to end. We can see a -distinct separating line between the ends. Each one of these segments -or compartments of the thread is a _cell_, and the boundary wall is in -the form of a cylinder with closed ends. - -=7. Protoplasm.=—Having distinguished these parts of the plant -we can look for the protoplasm. It occurs within the cells. It is -colorless (i.e., hyaline) and consequently requires close observation. -Near the center of the cell can be seen a rather dense granular body -of an elliptical or irregular form, with its long diameter transverse -to the axis of the cell in some species; or triangular, or quadrate in -others. This is the _nucleus_. Around the nucleus is a granular layer -from which delicate threads of a shiny granular substance radiate in a -starlike manner, and terminate in the chlorophyll band at one of the -pyrenoids. A granular layer of the same substance lines the inside of -the cell wall, and can be seen through the microscope if it is properly -focussed. This granular substance in the cell is _protoplasm_. - -=8. Cell-sap in spirogyra.=—The greater part of the interior -space of the cell, that between the radiating strands of protoplasm, is -occupied by a watery fluid, the “cell-sap.” - -=9. Reaction of protoplasm to certain reagents.=—We can employ -certain tests to demonstrate that this granular substance which we have -seen is protoplasm, for it has been found, by repeated experiments with -a great many kinds of plants, that protoplasm gives a definite reaction -in response to treatment with certain substances called reagents. Let -us mount a few threads of the spirogyra in a drop of a solution of -iodine, and observe the results with the aid of the microscope. The -iodine gives a yellowish-brown color to the protoplasm, and it can be -more distinctly seen. The nucleus is also much more prominent since it -colors deeply, and we can perceive within the nucleus one small rounded -body, sometimes more, the _nucleolus_. The iodine here kills and stains -the protoplasm. The protoplasm, however, in a living condition will -resist for a time some other reagents, as we shall see if we attempt -to stain it with a one per cent aqueous solution of a dye known as -_eosin_. Let us mount a few living threads in such a solution of eosin, -and after a time wash off the stain. The protoplasm remains uncolored. -Now let us place these threads for a short time, two or three minutes, -in strong alcohol, which kills the protoplasm. Then mount them in -the eosin solution. The protoplasm now takes the eosin stain. After -the protoplasm has been killed we note that the nucleus is no longer -elliptical or angular in outline, but is rounded. The strands of -protoplasm are no longer in tension as they were when alive. - -[Illustration: Fig. 2. Cell of spirogyra before treatment with iodine.] - -[Illustration: Fig. 3. Cell of spirogyra after treatment with alcohol -and iodine.] - -=10.= Let us now take some fresh living threads and mount them in -water. Place a small drop of dilute glycerine on the slip at one side -of the cover glass, and with a bit of filter paper at the other side -draw out the water. The glycerine will flow under the cover glass and -come in contact with the spirogyra threads. Glycerine absorbs water -promptly. Being in contact with the threads it draws water out of the -cell cavity, thus causing the layer of protoplasm which lines the -inside of the cell wall to collapse, and separate from the wall, -drawing the chlorophyll band inward toward the center also. The wall -layer of protoplasm can now be more distinctly seen and its granular -character observed. - -We have thus employed three tests to demonstrate that this substance -with which we are dealing shows the reactions which we know by -experience to be given by protoplasm. We therefore conclude that this -colorless and partly granular, slimy substance in the spirogyra cell -is protoplasm, and that when we have performed these experiments, and -noted carefully the results, we have _seen_ protoplasm. - -[Illustration: Fig. 4. Cell of spirogyra before treatment with -glycerine.] - -[Illustration: Fig. 5. Cells of spirogyra after treatment with -glycerine.] - - =11. Earlier use of the term protoplasm.=—Early - students of the living matter in the cell considered it - to be alike in substance, but differing in density; so - the term protoplasm was applied to all of this living - matter. The nucleus was looked upon as simply a denser - portion of the protoplasm, and the nucleolus as a still - denser portion. Now it is believed that the nucleus - is a distinct substance, and a permanent organ of the - cell. The remaining portion of the protoplasm is now - usually spoken of as the _cytoplasm_. - - In spirogyra then the cytoplasm in each cell consists - of a layer which lines the inside of the cell wall, - a nuclear layer, which surrounds the nucleus, and - radiating strands which connect the nucleus and wall - layers, thus suspending the nucleus near the center of - the cell. But it seems best in this elementary study to - use the term protoplasm in its general sense. - - -Protoplasm in mucor. - -=12.= Let us now examine in a similar way another of the simple -plants with the special object in view of demonstrating the protoplasm. -For this purpose we may take one of the plants belonging to the group -of _fungi_. These plants possess no chlorophyll. One of several species -of _mucor_, a common mould, is readily obtainable, and very suitable -for this study.[3] - -=13. Mycelium of mucor.=—A few days after sowing in some -gelatinous culture medium we find slender, hyaline threads, which are -very much branched, and, radiating from a central point, form circular -colonies, if the plant has not been too thickly sown, as shown in -fig. 6. These threads of the fungus form the _mycelium_. From these -characters of the plant, which we can readily see without the aid of a -microscope, we note how different it is from spirogyra. - -To examine for protoplasm let us lift carefully a thin block of -gelatine containing the mucor threads, and mount it in water on a -glass slip. Under the microscope we see only a small portion of the -branched threads. In addition to the absence of chlorophyll, which we -have already noted, we see that the mycelium is not divided at short -intervals into cells, but appears like a delicate tube with branches, -which become successively smaller toward the ends. - -=14. Appearance of the protoplasm.=—Within the tube-like thread -now note the protoplasm. It has the same general appearance as that -which we noted in spirogyra. It is slimy, or semi-fluid, partly -hyaline, and partly granular, the granules consisting of minute -particles (the _microsomes_). While in mucor the protoplasm has the -same general appearance as in spirogyra, its arrangement is very -different. In the first place it is plainly continuous throughout the -tube. We do not see the prominent radiations of strands around a large -nucleus, but still the protoplasm does not fill the interior of the -threads. Here and there are rounded clear spaces termed _vacuoles_, -which are filled with the watery fluid, cell-sap. The nuclei in mucor -are very minute, and cannot be seen except after careful treatment with -special reagents. - -[Illustration: Fig. 6. Colonies of mucor.] - -=15. Movement of the protoplasm in mucor.=—While examining the -protoplasm in mucor we are likely to note streaming movements. Often a -current is seen flowing slowly down one side of the thread, and another -flowing back on the other side, or it may all stream along in the same -direction. - -=16. Test for protoplasm.=—Now let us treat the threads with -a solution of iodine. The yellowish-brown color appears which is -characteristic of protoplasm when subject to this reagent. If we -attempt to stain the living protoplasm with a one per cent aqueous -solution of eosin it resists it for a time, but if we first kill the -protoplasm with strong alcohol, it reacts quickly to the application of -the eosin. If we treat the living threads with glycerine the protoplasm -is contracted away from the wall, as we found to be the case with -spirogyra. While the color, form and structure of the plant mucor is -different from spirogyra, and the arrangement of the protoplasm within -the plant is also quite different, the reactions when treated by -certain reagents are the same. We are justified then in concluding that -the two plants possess in common a substance which we call protoplasm. - -[Illustration: Fig. 7. Thread of mucor, showing protoplasm and -vacuoles.] - - -Protoplasm in nitella. - -=17.= One of the most interesting plants for the study of one -remarkable peculiarity of protoplasm is _Nitella_. This plant belongs -to a small group known as stoneworts. They possess chlorophyll, and, -while they are still quite simple as compared with the higher plants, -they are much higher in the scale than spirogyra or mucor. - -=18. Form of nitella.=—A common species of nitella is _Nitella -flexilis_. It grows in quiet pools of water. The plant consists of a -main axis, in the form of a cylinder. At quite regular intervals are -whorls of several smaller thread-like outgrowths, which, because of -their position, are termed “leaves,” though they are not true leaves. -These are branched in a characteristic fashion at the tip. The main -axis also branches, these branches arising in the axil of a whorl, -usually singly. The portions of the axis where the whorls arise are the -_nodes_. Each node is made up of a number of small cells definitely -arranged. The portion of the axis between two adjacent whorls is an -internode. These internodes are peculiar. They consist of but a single -“cell,” and are cylindrical, with closed ends. They are sometimes 5-10 -cm. long. - -[Illustration: Fig. 8. Portion of plant nitella.] - -=19. Internode of nitella.=—For the study of an internode of -nitella, a small one, near the end, or the ends of one of the “leaves” -is best suited, since it is more transparent. A small portion of the -plant should be placed on the glass slip in water with the cover glass -over a tuft of the branches near the growing end. Examined with the -microscope the green chlorophyll bodies, which form oval or oblong -discs, are seen to be very numerous. They lie quite closely side by -side and form in perfect rows along the inner surface of the wall. -One peculiar feature of the arrangement of the chlorophyll bodies is -that there are two lines, extending from one end of the internode to -the other on opposite sides, where the chlorophyll bodies are wanting. -These are known as neutral lines. They run parallel with the axis of -the internode, or in a more or less spiral manner as shown in fig. 9. - -=20. Cyclosis in nitella.=—The chlorophyll bodies are stationary -on the inner surface of the wall, but if the microscope be properly -focussed just beneath this layer we notice a rotary motion of particles -in the protoplasm. There are small granules and quite large masses of -granular matter which glide slowly along in one direction on a given -side of the neutral line. If now we examine the protoplasm on the other -side of the neutral line, we see that the movement is in the opposite -direction. If we examine this movement at the end of an internode -the particles are seen to glide around the end from one side of the -neutral line to the other. So that when conditions are favorable, such -as temperature, healthy state of the plant, etc., this gliding of the -particles or apparent streaming of the protoplasm down one side of -the “cell,” and back upon the other, continues in an uninterrupted -rotation, or _cyclosis_. There are many nuclei in an internode of -nitella, and they move also. - -=21. Test for protoplasm.=—If we treat the plant with a solution -of iodine we get the same reaction as in the case of spirogyra and -mucor. The protoplasm becomes yellowish-brown. - -[Illustration: Fig. 9. Cyclosis in nitella.] - -=22. Protoplasm in one of the higher plants.=—We now wish to -examine, and test for, protoplasm in one of the higher plants. Young -or growing parts of any one of various plants—the petioles of young -leaves, or young stems of growing plants—are suitable for study. -Tissue from the pith of corn (Zea mays) in young shoots just back of -the growing point or quite near the joints of older but growing corn -stalks furnishes excellent material. - -If we should place part of the stem of this plant under the microscope -we should find it too opaque for observation of the interior of the -cells. This is one striking difference which we note as we pass from -the low and simple plants to the higher and more complex ones; not -only in general is there an increase of size, but also in general an -increase in thickness of the parts. The cells, instead of lying end to -end or side by side, are massed together so that the parts are quite -opaque. In order to study the interior of the plant we have selected -it must be cut into such thin layers that the light will pass readily -through them. - -For this purpose we section the tissue selected by making with a razor, -or other very sharp knife, very thin slices of it. These are mounted in -water in the usual way for microscopic study. In this section we notice -that the cells are polygonal in form. This is brought about by mutual -pressure of all the cells. The granular protoplasm is seen to form a -layer just inside the wall, which is connected with the nuclear layer -by radiating strands of the same substance. The nucleus does not always -lie at the middle of the cell, but often is near one side. If we now -apply an alcohol solution of iodine the characteristic yellowish-brown -color appears. So we conclude here also that this substance is -identical with the living matter in the other very different plants -which we have studied. - -=23. Movement of protoplasm in the higher plants.=—Certain -parts of the higher plants are suitable objects for the study of the -so-called streaming movement of protoplasm, especially the delicate -hairs, or thread-like outgrowths, such as the silk of corn, or the -delicate staminal hairs of some plants, like those of the common -spiderwort, tradescantia, or of the tradescantias grown for ornament in -greenhouses and plant conservatories. - -Sometimes even in the living cells of the corn plant which we have just -studied, slow streaming or gliding movements of the granules are seen -along the strands of protoplasm where they radiate from the nucleus. -See note at close of this chapter. - -=24. Movement of protoplasm in cells of the staminal hair of -“spiderwort.”=—A cell of one of these hairs from a stamen of a -tradescantia grown in glass houses is shown in fig. 10. The nucleus is -quite prominent, and its location in the cell varies considerably in -different cells and at different times. There is a layer of protoplasm -all around the nucleus, and from this the strands of protoplasm extend -outward to the wall layer. The large spaces between the strands are, as -we have found in other cases, filled with the cell-sap. - -[Illustration: Fig. 10. Cell from stamen hair of tradescantia showing -movement of the protoplasm.] - - An entire stamen, or a portion of the stamen, having - several hairs attached, should be carefully mounted - in water. Care should be taken that the room be not - cold, and if the weather is cold the water in which - the preparation is mounted should be warm. With these - precautions there should be little difficulty in - observing the streaming movement. - -The movement is detected by observing the gliding of the granules. -These move down one of the strands from the nucleus along the wall -layer, and in towards the nucleus in another strand. After a little the -direction of the movement in any one portion may be reversed. - -=25. Cold retards the movement.=—While the protoplasm is moving, -if we rest the glass slip on a block of ice, the movement will become -slower, or will cease altogether. Then if we warm the slip gently, the -movement becomes normal again. We may now apply here the usual tests -for protoplasm. The result is the same as in the former cases. - -=26. Protoplasm occurs in the living parts of all plants.=—In -these plants representing such widely different groups, we find a -substance which is essentially alike in all. Though its arrangement -in the cell or plant body may differ in the different plants or in -different parts of the same plant, its general appearance is the same. -Though in the different plants it presents, while alive, varying -phenomena, as regards mobility, yet when killed and subjected to -well known reagents the reaction is in general identical. Knowing -by the experience of various investigators that protoplasm exhibits -these reactions under given conditions, we have demonstrated to -our satisfaction that we have seen protoplasm in the simple alga, -spirogyra, in the common mould, mucor, in the more complex stonewort, -nitella, and in the cells of tissues of the highest plants. - -=27.= By this simple process of induction of these facts -concerning this substance in these different plants, we have learned an -important method in science study. Though these facts and deductions -are well known, the repetition of the methods by which they are -obtained on the part of each student helps to form habits of scientific -carefulness and patience, and trains the mind to logical processes in -the search for knowledge. - -=28.= While we have by no means exhausted the study of protoplasm, -we can, from this study, draw certain conclusions as to its occurrence -and appearance in plants. Protoplasm is found in the living and growing -parts of all plants. It is a semi-fluid, or slimy, granular, substance; -in some plants, or parts of plants, the protoplasm exhibits a streaming -or gliding movement of the granules. It is irritable. In the living -condition it resists more or less for some time the absorption of -certain coloring substances. The water may be withdrawn by glycerine. -The protoplasm may be killed by alcohol. When treated with iodine it -becomes a yellowish-brown color. - - _Note._ In some plants, like elodea for example, it - has been found that the streaming of the protoplasm is - often induced by some injury or stimulus, while in the - normal condition the protoplasm does not move. - -FOOTNOTES: - -[1] For apparatus, reagents, collection and preservation of material, -etc., see Appendix. - -[2] If spirogyra is forming fruit some of the threads will be lying -parallel in pairs, and connected with short tubes. In some of the cells -there will be found rounded or oval bodies known as _zygospores_. These -may be seen in fig. 86, and will be described in another part of the -book. - -[3] The most suitable preparations of mucor for study are made by -growing the plant in a nutrient substance which largely consists of -gelatine, or, better, agar-agar, a gelatinous preparation of certain -seaweeds. This, after the plant is sown in it, should be poured into -sterilised shallow glass plates, called Petrie dishes. - - - - -CHAPTER II. - -ABSORPTION, DIFFUSION, OSMOSE. - - -=29.= We may next endeavor to learn how plants absorb water or -nutrient substances in solution. There are several very instructive -experiments, which can be easily performed, and here again some of the -lower plants will be found useful. - -=30. Osmose in spirogyra.=—Let us mount a few threads of this -plant in water for microscopic examination, and then draw under the -cover glass a five per cent solution of ordinary table salt (NaCl) -with the aid of filter paper. We shall soon see that the result is -similar to that which was obtained when glycerine was used to extract -the water from the cell-sap, and to contract the protoplasmic membrane -from the cell wall. But the process goes on evenly and the plant is not -injured. The protoplasmic layer contracts slowly from the cell wall, -and the movement of the membrane can be watched by looking through the -microscope. The membrane contracts in such a way that all the contents -of the cell are finally collected into a rounded or oval mass which -occupies the center of the cell. - -If we now add fresh water and draw off the salt solution, we can see -the protoplasmic membrane expand again, or move out in all directions, -and occupy its former position against the inner surface of the cell -wall. This would indicate that there is some pressure from within while -this process of absorption is going on, which causes the membrane to -move out against the cell wall. - -The salt solution draws water from the cell-sap. There is thus a -tendency to form a vacuum in the cell, and the pressure on the outside -of the protoplasmic membrane causes it to move toward the center of the -cell. When the salt solution is removed and the thread of spirogyra is -again bathed with water, the movement of the water is _inward_ in the -cell. This would suggest that there is some substance dissolved in the -cell-sap which does not readily filter out through the membrane, but -draws on the water outside. It is this which produces the pressure from -within and crowds the membrane out against the cell wall again. - -[Illustration: Fig. 11. Spirogyra before placing in salt solution.] - -[Illustration: Fig. 12. Spirogyra in 5% salt solution.] - -[Illustration: Fig. 13. Spirogyra from salt solution into water.] - -=31. Turgescence.=—Were it not for the resistance which the cell -wall offers to the pressure from within, the delicate protoplasmic -membrane would stretch to such an extent that it would be ruptured, and -the protoplasm therefore would be killed. If we examine the cells at -the ends of the threads of spirogyra we shall see in most cases that -the cell wall at the free end is arched outward. This is brought about -by the pressure from within upon the protoplasmic membrane which itself -presses against the cell wall, and causes it to arch outward. This is -beautifully shown in the case of threads which are recently broken. -The cell wall is therefore _elastic_; it yields to a certain extent to -the pressure from within, but a point is soon reached beyond which it -will not stretch, and an equilibrium then exists between the pressure -from within on the protoplasmic membrane, and the pressure from without -by the elastic cell wall. This state of equilibrium in a cell is -_turgescence_, or such a cell is said to be _turgescent_, or _turgid_. - -[Illustration: Fig. 14. Before treatment with salt solution.] - -[Illustration: Fig. 15. After treatment with salt solution.] - -[Illustration: Fig. 16. From salt solution placed in water. - -Figs. 14-16.—Osmosis in threads of mucor.] - -=32. Experiment with beet in salt and sugar solutions.=—We may now -test the effect of a five per cent salt solution on a portion of the -tissues of a beet or carrot. Let us cut several slices of equal size -and about 5mm in thickness. Immerse a few slices in water, a few in -a five per cent salt solution and a few in a strong sugar solution. -It should be first noted that all the slices are quite rigid when an -attempt is made to bend them between the fingers. In the course of one -or two hours or less, if we examine the slices we shall find that those -in water remain, as at first, quite rigid, while those in the salt and -sugar solutions are more or less flaccid or limp, and readily bend -by pressure between the fingers, the specimens in the salt solution, -perhaps, being more flaccid than those in the sugar solution. The salt -solution, we judge after our experiment with spirogyra, withdraws some -of the water from the cell-sap, the cells thus losing their turgidity -and the tissues becoming limp or flaccid from the loss of water. - -[Illustration: Fig. 17. Before treatment with salt solution.] - -[Illustration: Fig. 18. After treatment with salt solution.] - -[Illustration: Fig. 19. From salt solution into water again. - -Figs. 17-19.—Osmosis in cells of Indian corn.] - - -[Illustration: Fig. 20. Rigid condition of fresh beet section.] - -[Illustration: Fig. 21. Limp condition after lying in salt solution.] - -[Illustration: Fig. 22. Rigid again after lying again in water. - -Figs. 20-22.—Turgor and osmosis in slices of beet.] - -=33.= Let us now remove some of the slices of the beet from the -sugar and salt solutions, wash them with water and then immerse them in -fresh water. In the course of thirty minutes to one hour, if we examine -them again, we find that they have regained, partly or completely, -their rigidity. Here again we infer from the former experiment with -spirogyra that the substances in the cell-sap now draw water inward; -that is, the diffusion current is inward through the cell walls and the -protoplasmic membrane, and the tissue becomes turgid again. - -[Illustration: Fig. 23. Before treatment with salt solution.] - -[Illustration: Fig. 24. After treatment with salt solution.] - -[Illustration: Fig. 25. Later stage of the same. - -Figs. 23-25.—Cells from beet treated with salt solution to show -osmosis and movement of the protoplasmic membrane.] - -=34. Osmose in the cells of the beet.=—We should now make a -section of the fresh tissue of a red colored beet for examination with -the microscope, and treat this section with the salt solution. Here we -can see that the effect of the salt solution is to draw water out of -the cell, so that the protoplasmic membrane can be seen to move inward -from the cell wall just as was observed in the case of spirogyra.[4] -Now treating the section with water and removing the salt solution, the -diffusion current is in the opposite direction, that is inward through -the protoplasmic membrane, so that the latter is pressed outward until -it comes in contact with the cell wall again, which by its elasticity -soon resists the pressure and the cells again become turgid. - -=35. The coloring matter in the cell-sap does not readily escape -from the living protoplasm of the beet.=—The red coloring matter, -as seen in the section under the microscope, does not escape from the -cell-sap through the protoplasmic membrane. When the slices are placed -in water, the water is not colored thereby. The same is true when the -slices are placed in the salt or sugar solutions. Although water is -withdrawn from the cell-sap, this coloring substance does not escape, -or if it does it escapes slowly and after a considerable time. - -=36. The coloring matter escapes from dead protoplasm.=—If, -however, we heat the water containing a slice of beet up to a point -which is sufficient to kill the protoplasm, the red coloring matter -in the cell-sap filters out through the protoplasmic membrane and -colors the water. If we heat a preparation made for study under the -microscope up to the thermal death point we can see here that the red -coloring matter escapes through the membrane into the water outside. -This teaches that certain substances cannot readily filter through -the living membrane of protoplasm, but that they can filter through -when the protoplasm is dead. A very important condition, then, for -the successful operation of some of the physical processes connected -with absorption in plants is that the protoplasm should be in a living -condition. - -=37. Osmose experiments with leaves.=—We may next take the leaves -of certain plants like the geranium, coleus or other plant, and place -them in shallow vessels containing water, salt, and sugar solutions -respectively. The leaves should be immersed, but the petioles should -project out of the water or solutions. Seedlings of corn or beans, -especially the latter, may also be placed in these solutions, so that -the leafy ends are immersed. After one or two hours an examination -shows that the specimens in the water are still turgid. But if we lift -a leaf or a bean plant from the salt or sugar solution, we find that -it is flaccid and limp. The blade, or lamina, of the leaf droops as if -wilted, though it is still wet. The bean seedling also is flaccid, the -succulent stem bending nearly double as the lower part of the stem is -held upright. This loss of turgidity is brought about by the loss of -water from the tissues, and judging from the experiments on spirogyra -and the beet, we conclude that the loss of turgidity is caused by the -withdrawal of some of the water from the cell-sap by the strong salt -solution. - -=38.= Now if we wash carefully these leaves and seedlings, which -have been in the salt and sugar solutions, with water, and then immerse -them in fresh water for a few hours, they will regain their turgidity. -Here again we are led to infer that the diffusion current is now inward -through the protoplasmic membranes of all the living cells of the leaf, -and that the resulting turgidity of the individual cells causes the -turgidity of the leaf or stem. - -[Illustration: Fig. 26. Seedling of radish, showing root hairs.] - -=39. Absorption by root hairs.=—If we examine seedlings, which -have been grown in a germinator or in the folds of paper or cloths so -that the roots will be free from particles of soil, we see near the -growing point of the roots that the surface is covered with numerous -slender, delicate, thread-like bodies, the root hairs. Let us place a -portion of a small root containing some of these root hairs in water on -a glass slip, and prepare it for examination with the microscope. We -see that each thread, or root hair, is a continuous tube, or in other -words it is a single cell which has become very much elongated. The -protoplasmic membrane lines the wall, and strands of protoplasm extend -across at irregular intervals, the interspaces being occupied by the -cell-sap. - -[Illustration: Fig. 27. Root hair of corn before and after treatment -with 5% salt solution.] - -We should now draw under the cover glass some of the five per cent salt -solution. The protoplasmic membrane moves away from the cell wall at -certain points, showing that _plasmolysis_ is taking place, that is, -the diffusion current is outward so that the cell-sap loses some of its -water, and the pressure from the outside moves the membrane inward. -We should not allow the salt solution to work on the root hairs long. -It should be very soon removed by drawing in fresh water before the -protoplasmic membrane has been broken at intervals, as is apt to be the -case by the strong diffusion current and the consequent strong pressure -from without. The membrane of protoplasm now moves outward as the -diffusion current is inward, and soon regains its former position next -the inner side of the cell wall. The root hairs then, like other parts -of the plant which we have investigated, have the power of taking up -water under pressure. - -=40. Cell-sap a solution of certain substances.=—From these -experiments we are led to believe that certain substances reside in the -cell-sap of plants, which behave very much like the salt solution when -separated from water by the protoplasmic membrane. Let us attempt to -interpret these phenomena by recourse to diffusion experiments, where -an animal membrane separates two liquids of different concentration. - -=41. An artificial cell to illustrate turgor.=—Fill a small -wide-mouthed vial with a _very strong_ sugar solution. Over the mouth -tie firmly a piece of _bladder_ membrane. Be certain that as the -membrane is tied over the open end of the vial, the sugar solution -fills it in order to keep out air bubbles. Sink the vial in a vessel of -fresh water and leave it there for twenty-four hours. Remove the vial -and note that the membrane is arched outward. Thrust a sharp needle -through the membrane when it is arched outward, and quickly pull it -out. The liquid spurts out because of the inside pressure. - -[Illustration: FIG. 28. Puncturing a make-believe cell after -it has been lying in water.] - -[Illustration: FIG. 29. Same as Fig. 28 after needle is removed.] - -=42. Diffusion through an animal membrane.=—For this experiment -we may use a thistle tube, across the larger end of which should be -stretched and tied tightly a piece of a bladder membrane. A strong -sugar solution (three parts sugar to one part water) is now placed in -the tube so that the bulb is filled and the liquid extends part way in -the neck of the tube. This is immersed in water within a wide-mouth -bottle, the neck of the tube being supported in a perforated cork in -such a way that the sugar solution in the tube is on a level with the -water in the bottle or jar. In a short while the liquid begins to -rise in the thistle tube, in the course of several hours having risen -several centimeters. The diffusion current is thus stronger through the -membrane in the direction of the sugar solution, so that this gains -more water than it loses. - -We have here two liquids separated by an animal membrane, water on -the one hand which diffuses readily through the membrane, while on -the other is a solution of sugar which diffuses through the animal -membrane with difficulty. The water, therefore, not containing any -solvent, according to a general law which has been found to obtain in -such cases, diffuses more readily through the membrane into the sugar -solution, which thus increases in volume, and also becomes more dilute. -The bladder membrane is what is sometimes called a diffusion membrane, -since the diffusion currents travel through it. - -=43.= In this experiment then the bulk of the sugar solution is -increased, and the liquid rises in the tube by this pressure above -the level of the water in the jar outside of the thistle tube. The -diffusion of liquids through a membrane is _osmosis_. - -=44. Importance of these physical processes in plants.=—Now if -we recur to our experiment with spirogyra we find that exactly the -same processes take place. The protoplasmic membrane is the diffusion -membrane, through which the diffusion takes place. The salt solution -which is first used to bathe the threads of the plant is a stronger -solution than that of the cell-sap within the cell. Water therefore -is drawn out of the cell-sap, but the substances in solution in -the cell-sap do not readily move out. As the bulk of the cell-sap -diminishes the pressure from the outside pushes the protoplasmic -membrane away from the wall. Now when we remove the salt solution and -bathe the thread with water again, the cell-sap, being a solution of -certain substances, diffuses with more difficulty than the water, and -the diffusion current is inward, while the protoplasmic membrane moves -out against the cell wall, and turgidity again results. Also in the -experiments with salt and sugar solutions on the leaves of geranium, -on the leaves and stems of the seedlings, on the tissues and cells of -the beet and carrot, and on the root hairs of the seedlings, the same -processes take place. - -These experiments not only teach us that in the protoplasmic membrane, -the cell wall, and the cell-sap of plants do we have structures which -are capable of performing these physical processes, but they also show -that these processes are of the utmost importance to the plant; not -only in giving the plant the power to take up solutions of nutriment -from the soil, but they serve also other purposes, as we shall see -later. - -FOOTNOTE: - -[4] We should note that the coloring matter of the beet resides in -the cell-sap. It is in these colored cells that we can best see the -movement take place, since the red color serves to differentiate well -the moving mass from the cell wall. The protoplasmic membrane at -several points usually clings tenaciously so that at several places the -membrane is arched strongly away from the cell wall as shown in fig. -24. While water is removed from the cell-sap, we note that the coloring -matter does not escape through the protoplasmic membrane. - - - - -CHAPTER III. - -HOW PLANTS OBTAIN WATER. - - -In connection with the study of the means of absorption from the soil -or water by plants, it will be found convenient to observe carefully -the various forms of the plant. Without going into detail here, the -suggestion is made that simple thread forms like spirogyra, œdogonium, -and vaucheria; expanded masses of cells as are found in the thalloid -liverworts, the duckweed, etc., be compared with those liverworts, and -with the mosses, where leaf-like expansions of a central axis have been -differentiated. We should then note how this differentiation, from the -physiological standpoint, has been carried farther in the higher land -plants. - -=45. Absorption by Algæ and Fungi.=—In the simpler forms of -plant life, as in spirogyra and many of the algæ and fungi, the plant -body is not differentiated into parts.[5] In many other cases the only -differentiation is between the growing part and the fruiting part. In -the algæ and fungi there is no differentiation into stem and leaf, -though there is an approach to it in some of the higher forms. Where -this simple plant body is flattened, as in the sea-wrack, or ulva, it -is a _frond_. The Latin word for frond is _thallus_, and this name is -applied to the plant body of all the lower plants, the algæ and fungi. -The algæ and fungi together are sometimes called the _thallophytes_, -or _thallus plants_. The word thallus is also sometimes applied to the -flattened body of the liverworts. In the foliose liverworts and mosses -there is an axis with leaf-like expansions. These are believed by some -to represent true stems and leaves, by others to represent a flattened -thallus in which the margins are deeply and regularly divided, or in -which the expansion has only taken place at regular intervals. - -In nearly all of the algæ the plant body is submerged in water. In these -cases absorption takes place through all portions of the surface in -contact with the water, as in spirogyra, vaucheria, and all of the -larger seaweeds. Comparatively few of the algæ grow on the surfaces -of rocks or trees. In these examples it is likely that at times only -portions of the plant body serve in the process of absorption of water -from the substratum. A few of the algæ are parasitic, living in the -tissues of higher plants, where they are surrounded by the water or -liquids within the host. Absorption takes place in the same way in many -of the fungi. The aquatic fungi are immersed in water. In other forms, -like mucor, a portion of the mycelium is within the substratum, and -being bathed by the water or watery solutions absorbs the same, while -the fruiting portion and the aerial mycelium obtain their water and -food solutions from the mycelium in the substratum. In higher fungi, -like the mushrooms, the mycelium within the ground or decaying wood -absorbs the water necessary for the fruiting portion; while in the case -of the parasitic fungi the mycelium lies in the water or liquid within -the host. - -[Illustration: Fig. 30. Thallus of Riccia lutescens.] - -=46. Absorption by liverworts.=—In many of the plants termed -liverworts the vegetative part of the plant is a thin, flattened, more -or less elongated green body known as a thallus. - -_Riccia._—One of these, belonging to the genus riccia, is shown in -fig. 30. Its shape is somewhat like that of a minute ribbon which is -forked at intervals in a dichotomous manner, the characteristic kind of -branching found in these thalloid liverworts. This riccia (known as R. -lutescens) occurs on damp soil; long, slender, hair-like processes grow -out from the under surface of the thallus which resemble root hairs and -serve the same purpose in the processes of absorption. Another species -of riccia (R. crystallina) is shown in fig. 252. This plant is quite -circular in outline and occurs on muddy flats. Some species float on -the water. - -=47. Marchantia.=—One of the larger and coarser liverworts is figured -at 31. This is a very common liverwort, growing in very damp and muddy -places and also along the margins of streams, on the mud or upon -the surfaces of rocks which are bathed with the water. This is known -as _Marchantia polymorpha_. If we examine the under surface of the -marchantia we see numerous hair-like processes which attach the plant -to the soil. Under the microscope we see that some of these are similar -to the root hairs of the seedlings which we have been studying, and -they serve the purpose of absorption. Since, however, there are no -roots on the marchantia plant, these hair-like outgrowths are usually -termed here _rhizoids_. In marchantia they are of two kinds, one kind -the simple ones with smooth walls, and the other kind in which the -inner surfaces of the walls are roughened by processes which extend -inward in the form of irregular tooth-like points. Besides the hairs on -the under side of the thallus we note especially near the growing end -that there are two rows of leaf-like scales, those at the end of the -thallus curving up over the growing end, thus serving to protect the -delicate tissues at the growing point. - -[Illustration: Fig. 31. Marchantia plant with cupules and gemmæ; -rhizoids below.] - -[Illustration: Fig. 32. Portion of plant of Frullania, a foliose -liverwort.] - -[Illustration: Fig. 33. Portion of same more highly magnified, showing -overlapping leaves.] - -[Illustration: Fig. 34. Under side, showing forked under row of leaves -and lobes of lateral leaves.] - -=48. Frullania.=—In fig. 32 is shown another liverwort, which -differs greatly in form from the ones we have just been studying in -that there is a well-defined axis with lateral leaf-like outgrowths. -Such liverworts are called foliose liverworts. Besides these two quite -prominent rows of leaves there is a third row of poorly developed -leaves on the under surface. Also from the under surface of the axis we -see here and there slender outgrowths, the rhizoids, through which much -of the water is absorbed. - -[Illustration: Fig. 35. Foliose liverwort (bazzania) showing -dichotomous branching and overlapping leaves.] - -=49. Absorption by the mosses.=—Among the mosses, which are -usually common in moist and shaded situations, examples are abundant -which are suitable for the study of the organs of absorption. If we -take for example a plant of mnium (M. affine), which is illustrated in -fig. 36, we note that it consists of a slender axis with thin flat, -green, leaf-like expansions, Examining with the microscope the lower -end of the axis, which is attached to the substratum, there are seen -numerous brown-colored threads more or less branched. - -[Illustration: Fig. 36. Female plant (gametophyte) of a moss (mnium), -showing rhizoids below, and the tuft of leaves above, which protect the -archegonia.] - -=50. Absorption by the higher aquatic plants.=—Examples of -the water plants which are entirely submerged in water are the -water-crowfoots, some of the pondweeds, elodea or water-weeds, the -tape-grass, vallisneria, etc. In these plants all parts of the body -being submerged, they absorb water with which they are in contact. In -other aquatic plants, like the water-lilies, some of the pondweeds, the -duck-meats, etc., are only partially submerged in the water; the upper -surface of the leaf or of the leaf-like expansion being exposed to the -air, while the under surface lies in close contact with the water, and -the stems and the petioles of the leaves are also immersed in water. In -these plants absorption takes place through those parts in contact with -the water. - -=51. Absorption by the duck-meats.=—These plants are very curious -examples of the higher plants. - - _Lemna._—One of these is illustrated in fig. 37. - This is the common duckweed, _Lemna trisulca_. It - is very peculiar in form and in its mode of growth. - Each one of the lateral leaf-like expansions extends - outwards by the elongation of the basal part, which - becomes long and slender. Next, two new lateral - expansions are formed on these by prolification from - near the base, and thus the plant continues to extend. - The plant occurs in ponds and ditches and is sometimes - very common and abundant. It floats on the surface - of the water. While the flattened part of the plant - resembles a leaf, it is really the stem, no leaves - being present. This expanded green body is usually - termed a “frond.” A single rootlet grows out from the - under side and is destitute of root hairs. Absorption - of water therefore takes place through this rootlet and - through the under side of the “frond.” - -[Illustration: Fig. 37. Fronds of the duckweed (Lemna trisculca).] - -[Illustration: Fig. 38. Spirodela polyrhiza.] - -=52. Spirodela polyrhiza.=—This is a very curious plant, closely -related to the lemna and sometimes placed in the same genus. It occurs -in similar situations, and is very readily grown in aquaria. It reminds -one of a little insect as seen in fig. 38. There are several rootlets -on the under side of the frond. Absorption of water takes place here in -the same way as in lemna. - -=53. Absorption in wolffia.=—Perhaps the most curious of these -modified water plants is the little wolffia, which contains the -smallest specimens of the flowering plants. Two species of this genus -are shown in figs. 39-41. The plant body is reduced to nothing but a -rounded or oval green body, which represents the stem. No leaves or -roots are present. The plants multiply by “prolification,” the new -fronds growing out from a depression on the under side of one end. -Absorption takes place through the under surface. - -=54. Absorption by land plants.=—_Water cultures._—In connection -with our inquiry as to how land plants obtain their water, it will be -convenient to prepare some water cultures to illustrate this and which -can also be used later in our study of nutrition (Chapter IX). - -[Illustration: Fig. 39. Young frond of wolffia growing out of older -one.] - -[Illustration: Fig. 40. Young frond of wolffia separating from older -one.] - -[Illustration: Fig. 41. Another species of wolffia, the two fronds -still connected.] - -Chemical analysis shows that certain mineral substances are common -constituents of plants. By growing plants in different solutions of -these various substances it has been possible to determine what ones -are necessary constituents of plant food. While the proportion of the -mineral elements which enter into the composition of plant food may -vary considerably within certain limits, the concentration of the -solutions should not exceed certain limits. A very useful solution is -one recommended by Sachs, and is as follows: - -=55. Formula for water cultures=: - - Water 1000 cc. - Potassium nitrate 0.5 gr. - Sodium chloride 0.5 “ - Calcium sulphate 0.5 “ - Magnesium sulphate 0.5 “ - Calcium phosphate 0.5 “ - -The calcium phosphate is only partly soluble. The solution which is not -in use should be kept in a dark cool place to prevent the growth of -minute algæ. - -=56.= Several different plants are useful for experiments in water -cultures, as peas, corn, beans, buckwheat, etc. The seeds of these -plants may be germinated, after soaking them for several hours in warm -water, by placing them between the folds of wet paper on shallow trays, -or in the folds of wet cloth. The seeds should not be kept immersed in -water after they have imbibed enough to thoroughly soak and swell them. -At the same time that the seeds are placed in damp paper or cloth for -germination, one lot of the soaked seeds should be planted in good soil -and kept under the same temperature conditions, for control. When the -plants have germinated one series should be grown in distilled water, -which possesses no plant food; another in the nutrient solution, and -still another in the nutrient solution to which has been added a few -drops of a solution of iron chloride or ferrous sulphate. There would -then be four series of cultures which should be carried out with the -same kind of seed in each series so that the comparisons can be made on -the same species under the different conditions. The series should be -numbered and recorded as follows: - - No. 1, soil. - No. 2, distilled water. - No. 3, nutrient solution. - No. 4, nutrient solution with a few drops of iron solution added. - -[Illustration: Fig. 42. Culture cylinder to show position of -corn seedling (Hansen).] - -=57.= Small jars or wide-mouth bottles, or crockery jars, can be -used for the water cultures, and the cultures are set up as follows: A -cork which will just fit in the mouth of the bottle, or which can be -supported by pins, is perforated so that there is room to insert the -seedling, with the root projecting below into the liquid. The seed can -be fastened in position by inserting a pin through one side, if it is a -large one, or in the case of small seeds a cloth of a coarse mesh can -be tied over the mouth of the bottle instead of using the cork. After -properly setting up the experiments the cultures should be arranged -in a suitable place, and observed from time to time during several -weeks. In order to obtain more satisfactory results several duplicate -series should be set up to guard against the error which might arise -from variation in individual plants and from accident. Where there are -several students in a class, a single series set up by several will -act as checks upon one another. If glass jars are used for the liquid -cultures they should be wrapped with black paper or cloth to exclude -the light from the liquid, otherwise numerous minute algæ are apt to -grow and interfere with the experiment. Or the jars may be sunk in pots -of earth to serve the same purpose. If crockery jars are used they will -not need covering. - -=58.= For some time all the plants grow equally well, until the -nutriment stored in the seed is exhausted. The numbers 1, 3 and 4, in -soil and nutrient solutions, should outstrip number 2, the plants in -the distilled water. No. 4 in the nutrient solution with iron, having a -perfect food, compares favorably with the plants in the soil. - -=59. Plants take liquid food from the soil.=—From these experiments -then we judge that such plants take up the food they receive from the -soil in the form of a liquid, the elements being in solution in water. - -If we recur now to the experiments which were performed with the salt -solution in producing plasmolysis in the cells of spirogyra, in the -cells of the beet or corn, and in the root hairs of the corn and bean -seedlings, and the way in which these cells become turgid again when -the salt solution is removed and they are again bathed with water, we -shall have an explanation of the way in which plants take up nutrient -solutions of food material through their roots. - -[Illustration: Fig. 43. Section of corn root, showing rhizoids formed -from elongated epidermal cells.] - -=60. How food solutions are carried into the plant.=—We can see -how water and food solutions are carried into the plant, and we must -next turn our attention to the way in which these solutions are carried -farther into the plant. We should make a section across the root of a -seedling in the region of the root hairs and examine it with the aid -of a microscope. We here see that the root hairs are formed by the -elongation of certain of the surface cells of the root. These cells -elongate perpendicularly to the root, and become _3mm_ to _6mm_ long. -They are flexuous or irregular in outline and cylindrical, as shown in -fig. 43. The end of the hair next the root fits in between the adjacent -superficial cells of the root and joins closely to the next deeper -layer of cells. In studying the section of the young root we see that -the root is made up of cells which lie closely side by side, each with -its wall, its protoplasm and cell-sap, the protoplasmic membrane lying -on the inside of each cell wall. - -=61.= In the absorption of the watery solutions of plant food -by the root hairs, the cell-sap, being a more concentrated solution, -gains some of the former, since the liquid of less concentration flows -through the protoplasmic membrane into the more concentrated cell-sap, -increasing the bulk of the latter. This makes the root hairs turgid, -and at the same time dilutes the cell-sap so that the concentration is -not so great. The cells of the root lying inside and close to the base -of the root hairs have a cell-sap which is now more concentrated than -the diluted cell-sap of the hairs, and consequently gain some of the -food solutions from the latter, which tends to lessen the content of -the root hairs and also to increase the concentration of the cell-sap -of the same. This makes it possible for the root hairs to draw on -the soil for more of the food solutions, and thus, by a variation in -the concentration of the substances in solution in the cell-sap of -the different cells, the food solutions are carried along until they -reach the _vascular bundles_, through which the solutions are carried -to distant parts of the plant. Some believe that there is a rhythmic -action of the elastic cell walls in these cells between the root hairs -and the vascular bundles. This occurs in such a way that, after the -cell becomes turgid, it contracts, thus reducing the size of the cell -and forcing some of the food solutions into the adjacent cells, when -by absorption of more food solutions, or water, the cell increases in -turgidity again. This rhythmic action of the cells, if it does take -place, would act as a pump to force the solutions along, and would form -one of the causes of root pressure. - -=62. How the root hairs get the watery solutions from the -soil.=—If we examine the root hairs of a number of seedlings which -are growing in the soil under normal conditions, we shall see that a -large quantity of soil readily clings to the roots. We should note also -that unless the soil has been recently watered there is no free water -in it; the soil is only moist. We are curious to know how plants can -obtain water from soil which is not wet. If we attempt to wash off the -soil from the roots, being careful not to break away the root hairs, -we find, that small particles cling so tenaciously to the root hairs -that they are not removed. Placing a few such root hairs under the -microscope it appears as if here and there the root hairs were glued to -the minute soil particles. - -[Illustration: Fig. 44. Root hairs of corn seedling with soil particles -adhering closely.] - -=63.= If now we take some of the soil which is only moist, weigh -it, and then permit it to become quite dry on exposure to dry air, and -weigh again, we find that it loses weight in drying. Moisture has been -given off. This moisture, it has been found, forms an exceedingly thin -film on the surface of the minute soil particles. Where these soil -particles lie closely together, as they usually do when massed together -in the pot or elsewhere, this thin film of moisture is continuous from -the surface of one particle to that of another. Thus the soil particles -which are so closely attached to the root hairs connect the surface -of the root hairs with this film of moisture. As the cell-sap of the -root hairs draws on the moisture film with which they are in contact, -the tension of this film is sufficient to draw moisture from distant -particles. In this way the roots are supplied with water in soil which -is only moist. - -=64. Plants cannot remove all the moisture from the soil.=—If we -now take a potted plant, or a pot containing a number of seedlings, -place it in a moderately dry room, and do not add water to the soil we -find in a few days that the plant is wilting. The soil if examined will -appear quite dry to the sense of touch. Let us weigh some of this soil, -then dry it by artificial heat, and weigh again. It has lost in weight. -This has been brought about by driving off the moisture which still -remained in the soil after the plant began to wilt. This teaches that -while plants can obtain water from soil which is only moist or which is -even rather dry, they are not able to withdraw all the moisture from -the soil. - -[Illustration: Fig. 45. Experiment to show root pressure (Detmer).] - -=65. “Root pressure” or exudation pressure.=—It is a very common -thing to note, when certain shrubs or vines are pruned in the spring, -the exudation of a watery fluid from the cut surfaces. In the case of -the grape vine this has been known to continue for a number of days, -and in some cases the amount of liquid, called “sap,” which escapes is -considerable. In many cases it is directly traceable to the activity -of the roots, or root hairs, in the absorption of water from the soil. -For this reason the term _root pressure_ has been used to denote the -force exerted in supplying the water from the soil. But there are some -who object to the use of this term “root pressure.” The principal -objection is that the pressure which brings about the phenomenon -known as “bleeding” by plants is not present in the roots alone. This -pressure exists under certain conditions in all parts of the plant. The -term exudation pressure has been proposed in lieu of root pressure. It -should be remembered that the movement of water in the plant is started -by the pressure which exists in the root. If the term “root pressure” -is used, it should be borne clearly in mind that it does not express -the phenomenon exactly in all cases. - -=Root pressure may be measured.=—It is possible to measure -not only the amount of water which the roots will raise in a given -time, but also to measure the force exerted by the roots during root -pressure. It has been found that root pressure in the case of the -nettle is sufficient to hold a column of water about 4.5 meters (15 -ft.) high (Vines), while the root pressure of the vine (Hales, 1721) -will hold a column of water about 10 meters (36.5 ft.) high, and the -birch (Betula lutea) (Clark, 1873) has a root pressure sufficient to -hold a column of water about 25 meters (84.7 ft.) high. - -=66. Experiment to demonstrate root pressure.=—By a very simple -method this lifting of water by root pressure is shown. During the -summer season plants in the open may be used if it is preferred, but -plants grown in pots are also very serviceable, and one may use a -potted begonia or balsam, the latter being especially useful. The -plants are usually convenient to obtain from the greenhouses, to -illustrate this phenomenon. The stem is cut off rather close to the -soil and a long glass tube is attached to the cut end of the stem, -still connected with the roots, by the use of rubber tubing, as shown -in figure 45, and a very small quantity of water may be poured in to -moisten the cut end of the stem. In a few minutes the water begins to -rise in the glass tube. In some cases it rises quite rapidly, so that -the column of water can readily be seen to extend higher and higher up -in the tube when observed at quite short intervals. (To measure the -force of root pressure is rather difficult for elementary work. To -measure it see Ganong, Plant Physiology, pp. 67, 68, or some other book -for advanced work.) - -=67.= In either case where the experiment is continued for -several days it is noticed that the column of water or of mercury -rises and falls at different times during the same day, that is, the -column stands at varying heights; or in other words the root pressure -varies during the day. With some plants it has been found that the -pressure is greatest at certain times of the day, or at certain -seasons of the year. Such variation of root pressure exhibits what -is termed a periodicity, and in the case of some plants there is a -daily periodicity; while in others there is in addition an annual -periodicity. With the grape vine the root pressure is greatest in -the forenoon, and decreases from 12-6 P.M., while with the -sunflower it is greatest before 10 A.M., when it begins to -decrease. Temperature of the soil is one of the most important external -conditions affecting the activity of root pressure. - -FOOTNOTE: - -[5] See Chapter 38 for organization of members of the plant body. - - -CHAPTER IV. - -TRANSPIRATION, OR THE LOSS OF WATER BY PLANTS. - - -=68.= We should now inquire if all the water which is taken up in -excess of that which actually suffices for turgidity is used in the -elaboration of new materials of construction. We notice when a leaf -or shoot is cut away from a plant, unless it is kept in quite a moist -condition, or in a damp, cool place, that it becomes flaccid, and -droops. It wilts, as we say. The leaves and shoot lose their turgidity. -This fact suggests that there has been a loss of water from the shoot -or leaf. It can be readily seen that this loss is not in the form of -drops of water which issue from the cut end of the shoot or petiole. -What then becomes of the water in the cut leaf or shoot? - -=69. Loss of water from excised leaves.=—Let us take a handful of -fresh, green, rather succulent leaves, which are free from water on -the surface, and place them under a glass bell jar, which is tightly -closed below but which contains no water. Now place this in a brightly -lighted window, or in sunlight. In the course of fifteen to thirty -minutes we notice that a thin film of moisture is accumulating on the -inner surface of the glass jar. After an hour or more the moisture has -accumulated so that it appears in the form of small drops of condensed -water. We should set up at the same time a bell jar in exactly the same -way but which contains no leaves. In this jar there is no condensed -moisture on the inner surface. We thus are justified in concluding that -the moisture in the former jar comes from the leaves. Since there is no -visible water on the surfaces of the leaves, or at the cut ends, before -it may have condensed there, we infer that the water escapes from the -leaves in the form of _water vapor_, and that this water vapor, when -it comes in contact with the surface of the cold glass, condenses and -forms the moisture film, and later the drops of water. The leaves of -these cut shoots therefore lose water in the form of water vapor, and -thus a loss of turgidity results. - -[Illustration: Fig. 46. To show loss of water from leaves, the leaves -just covered.] - -[Illustration: Fig. 47. After a few hours drops of water have -accumulated on the inside of the jar covering the leaves.] - -=70. Loss of water from growing plants.=—Suppose we now take a -small and actively growing plant in a pot, and cover the pot and the -soil with a sheet of rubber cloth or flexible oilcloth which fits -tightly around the stem of the plant so that the moisture from the soil -or from the surface of the pot cannot escape. Then place a bell jar -over the plant, and set in a brightly lighted place, at a temperature -suitable for growth. In the course of a few minutes on a dry day a -moisture film forms on the inner surface of the glass, just as it did -in the case of the glass jar containing the cut shoots and leaves. -Later the moisture has condensed so that it is in the form of drops. If -we have the same leaf surface here as we had with the cut shoots, we -shall probably find that a larger amount of water accumulates on the -surface of the jar from the plant that is still attached to its roots. - -=71. Water escapes from the surfaces of living leaves in the form of -water vapor.=—This living plant then has lost water, which also -escapes in the form of water vapor. Since here there are no cut places -on the shoots or leaves, we infer that the loss of water vapor takes -place from the surfaces of the leaves and from the shoots. It is also -to be noted that, while this plant is losing water from the surfaces of -the leaves, it does not wilt or lose its turgidity. The roots by their -activity and pressure supply water to take the place of that which is -given off in the form of water vapor. This loss of water in the form of -water vapor by plants is _transpiration_. - -=72. A test for the escape of water vapor from plants.=—Make -a solution of cobalt chloride in water. Saturate several pieces of -filter paper with it. Allow them to dry. The water solution of cobalt -chloride is red. The paper is also red when it is moist, but when it -is thoroughly dry it is blue. It is very sensitive to moisture and the -moisture of the air is often sufficient to redden it. Before using dry -the paper in an oven or over a flame. - -=73.= Take two bell jars, as shown in fig. 49. Under one place a -potted plant, the pot and earth being covered by oiled paper. Or cover -the plant with a fruit jar. To a stake in the pot pin a piece of the -dried cobalt paper, and at the same time pin to a stake, in another -jar covering no plant, another piece of cobalt paper. They should both -be put under the jars at the same time. In a few moments the paper in -the jar with the plant will begin to redden. In a short while, ten or -fifteen minutes, probably, it will be entirely red, while the paper -under the other jar will remain blue, or be only slightly reddened. The -water vapor passing off from the living plant comes in contact with -the sensitive cobalt chloride in the paper and reddens it before there -is sufficient vapor present to condense as a film of moisture on the -surface of the jar. - -[Illustration: Fig. 48.] - -[Illustration: Fig. 49. - -Fig. 48.—Water vapor is given off by the leaves when attached to the -living plant. It condenses into drops of water on the cool surface of -the glass covering the plant. - -Fig. 49.—A good way to show that the water passes off from the leaves -in the form of water vapor.] - -=74. Experiment to compare loss of water in a dry and a humid -atmosphere.=—We should now compare the escape of water from the -leaves of a plant covered by a bell jar, as in the last experiment, -with that which takes place when the plant is exposed in a normal way -in the air of the room or in the open. To do this we should select -two plants of the same kind growing in pots, and of approximately the -same leaf surface. The potted plants are placed one each on the arms -of a scale. One of the plants is covered in this position with a bell -jar. With weights placed on the pan of the other arm the two sides are -balanced. In the course of an hour, if the air of the room is dry, -moisture has probably accumulated on the inner surface of the glass jar -which is used to cover one of the plants. This indicates that there has -here been a loss of water. But there is no escape of water vapor into -the surrounding air so that the weight on this arm is practically the -same as at the beginning of the experiment. We see, however, that the -other arm of the balance has risen. We infer that this is the result of -the loss of water vapor from the plant on that arm. Now let us remove -the bell jar from the other plant, and with a cloth wipe off all the -moisture from the inner surface, and replace the jar over the plant. We -note that the end of the scale which holds this plant is still lower -than the other end. - -=75. The loss of water is greater in a dry than in a humid -atmosphere.=—This teaches us that while water vapor escaped from -the plant under the bell jar, the air in this receiver soon became -saturated with the moisture, and thus the farther escape of moisture -from the leaves was checked. It also teaches us another very important -fact, viz., that plants lose water more rapidly through their leaves in -a dry air than in a humid or moist atmosphere. We can now understand -why it is that during the very hot and dry part of certain days plants -often wilt, while at nightfall, when the atmosphere is more humid, they -revive. They lose more water through their leaves during the dry part -of the day, other things being equal, than at other times. - -=76. How transpiration takes place.=—Since the water of transpiration -passes off in the form of water vapor we are led to inquire if this -process is simply _evaporation_ of water through the surface of the -leaves, or whether it is controlled to any appreciable extent by any -condition of the living plant. An experiment which is instructive in -this respect we shall find in a comparison between the transpiration of -water from the leaves of a cut shoot, allowed to lie unprotected in a -dry room, and a similar cut shoot the leaves of which have been killed. - -=77.= Almost any plant will answer for the experiment. For this -purpose I have used the following method. Small branches of the locust -(Robinia pseudacacia), of sweet clover (Melilotus alba), and of a -heliopsis were selected. One set of the shoots was immersed for a -moment in hot water near the boiling point to kill them. The other set -was immersed for the same length of time in cold water, so that the -surfaces of the leaves might be well wetted, and thus the two sets of -leaves at the beginning of the experiment would be similar, so far as -the amount of water on their surfaces is concerned. All the shoots were -then spread out on a table in a dry room, the leaves of the killed -shoots being separated where they are inclined to cling together. In -a short while all the water has evaporated from the surface of the -living leaves, while the leaves of the dead shoots are still wet on -the surface. In six hours the leaves of the dead shoots from which the -surface water had now evaporated were beginning to dry up, while the -leaves of the living plants were only becoming flaccid. In twenty-four -hours the leaves of the dead shoots were crisp and brittle, while those -of the living shoots were only wilted. In twenty-four hours more the -leaves of the sweet clover and of the heliopsis were still soft and -flexible, showing that they still contained more water than the killed -shoots which had been crisp for more than a day. - -=78.= It must be then that during what is termed transpiration -the living plant is capable of holding back the water to some extent, -which in a dead plant would escape more rapidly by evaporation. It is -also known that a body of water with a surface equal to that of a given -leaf surface of a plant loses more water by evaporation during the same -length of time than the plant loses by transpiration. - -=79. Structure of a leaf.=—We are now led to inquire why it is -that a living leaf loses water less rapidly than dead ones, and why -less water escapes from a given leaf surface than from an equal surface -of water. To understand this it will be necessary to examine the minute -structure of a leaf. For this purpose we may select the leaf of an ivy, -though many other leaves will answer equally well. From a portion of -the leaf we should make very thin cross-sections with a razor or other -sharp instrument. These sections should be perpendicular to the surface -of the leaf and should be then mounted in water for microscopic -examination.[6] - -=80. Epidermis of the leaf.=—In this section we see that the -green part of the leaf is bordered on what are its upper and lower -surfaces by a row of cells which possess no green color. The walls of -the cells of each row have nearly parallel sides, and the cross walls -are perpendicular. These cells form a single layer over both surfaces -of the leaf and are termed the _epidermis_. Their walls are quite stout -and the outer walls are _cuticularized_. - -[Illustration: Fig. 50. Section through ivy leaf showing communication -between stomate and the large intercellular spaces of the leaf, stoma -closed.] - -[Illustration: Fig. 51. Stoma open.] - -[Illustration: Fig. 52. Stoma closed. - -Figs. 51, 52.—Section through stomata of ivy leaf.] - -=81. Soft tissue of the leaf.=—The cells which contain the green -chlorophyll bodies are arranged in two different ways. Those on the -upper side of the leaf are usually long and prismatic in form and -lie closely parallel to each other. Because of this arrangement of -these cells they are termed the _palisade_ cells, and form what is -called the _palisade layer_. The other green cells, lying below, vary -greatly in size in different plants and to some extent also in the same -plant. Here we notice that they are elongated, or oval, or somewhat -irregular in form. The most striking peculiarity, however, in their -arrangement is that they are not usually packed closely together, but -each cell touches the other adjacent cells only at certain points. This -arrangement of these cells forms quite large spaces between them, the -intercellular spaces. If we should examine such a section of a leaf -before it is mounted in water we would see that the intercellular -spaces are not filled with water or cell-sap, but are filled with air -or some gas. Within the cells, on the other hand, we find the cell-sap -and the protoplasm. - -=82. Stomata.=—If we examine carefully the row of epidermal cells -on the under surface of the leaf, we find here and there a peculiar -arrangement of cells shown at figs. 51, 52. This opening through the -epidermal layer is a _stoma_. The cells which immediately surround the -openings are the _guard cells_. The form of the guard cells can be -better seen if we tear a leaf in such a way as to strip off a short -piece of the lower epidermis, and mount this in water. The guard cells -are nearly crescent-shaped, and the stoma is elliptical in outline. The -epidermal cells are very irregular in outline in this view. We should -also note that while the epidermal cells contain no chlorophyll, the -guard cells do. - -[Illustration: Fig. 53. Portion of epidermis of ivy, showing irregular -epidermal cells, stoma and guard cells.] - -=82=_a_. In the ivy leaf the guard cells are quite plain, but in -most plants the form as seen in cross-section is irregular in outline, -as shown in fig. 53_a_, which is from a section of a wintergreen leaf. -This leaf is interesting because it shows the characteristic structure -of leaves of many plants growing in soil where absorption of water by -the roots is difficult owing to the cold water, acids, or salts in the -water or soil, or in dry soil (see Chapters 47, 54, 55). The cuticle -over the upper epidermis is quite thick. This lessens the loss of water -by the leaf. The compact palisades of cells are in two to three cell -layers, also reducing the loss of water. - -=83. The living protoplasm retards the evaporation of water from the -leaf.=—If we now take into consideration a few facts which we have -learned in a previous chapter, with reference to the physical -properties of the living cell, we shall be able to give a partial -explanation of the comparative slowness with which the water escapes -from the leaves. The inner surfaces of the cell walls are lined with -the membrane of protoplasm, and within this is the cell-sap. These -cells have become turgid by the absorption of the water which has -passed up to them from the roots. While the protoplasmic membrane of -the cells does not readily permit the water to filter through, yet it -is saturated with water, and the elastic cell wall with which it is in -contact is also saturated. From the cell wall the water evaporates into -the intercellular spaces. But the water is given up slowly through the -protoplasmic membrane, so that the water vapor cannot be given off as -rapidly from the cell walls as it could if the protoplasm were dead. -The living protoplasmic membrane then which is only slowly permeable to -the water of the cell-sap is here a very important factor in checking -the too rapid loss of water from the leaves. - -[Illustration: Fig. 53_a_. - -Cross-section of leaf of wintergreen. _Cu._, cuticle; _Epid._, -epidermis; _v.d._, vascular duct; _Int. c. sp._, intercellular space; -_L. ep._, lower epidermis; _St._, stoma.] - -By an examination of our leaf section we see that the intercellular -spaces are all connected, and that the stomata, where they occur, open -also into intercellular spaces. There is here an opportunity for the -water vapor in the intercellular spaces to escape when the stomata are -open. - -=84. Action of the stomata.=—The guard cells serve an important -function in regulating transpiration. During normal transpiration the -guard cells are turgid and their peculiar form then causes them to arch -away from each other, allowing the escape of water vapor. When the air -becomes too dry transpiration is in excess of absorption by the roots. -The guard cells lose some of their water, and collapse so that their -inner faces meet in a straight line and close the stoma. Thus the rapid -transpiration is checked. Some evaporation of water vapor, however, -takes place through the epidermal cells, and if the air remains too -dry, the leaves eventually become flaccid and droop. During the day -the effect of sunlight is to increase certain sugars or salts in the -guard cells so that they readily become turgid and open the stomates, -but at night the cell-sap is less concentrated and the stomates are -usually closed. Light therefore favors transpiration, while in darkness -transpiration is checked. - -=85. Compare transpiration from the two surfaces of the leaf.=—This can -be done by using the cobalt chloride paper. This paper can be kept from -year to year and used repeatedly. It is thus a very simple matter to -make these experiments. Provide two pieces of glass (discarded glass -negatives, cleaned, are excellent), two pieces of cobalt chloride -paper, and some geranium leaves entirely free from surface water. Dry -the paper until it is blue. Place one piece of the paper on a glass -plate; place the geranium leaf with the under side on the paper. On the -upper side of the leaf now place the other cobalt paper, and next the -second piece of glass. On the pile place a light weight to keep the -parts well in contact. In fifteen or twenty minutes open and examine. -The paper next the under side of the geranium leaf is red where it lies -under the leaf. The paper on the upper side is only slightly reddened. -The greater loss of water, then, is through the under side of the -geranium leaf. This is true of a great many leaves, but it is not true -of all. - -=86. Negative pressure.=—This is not only indicated by the -drooping of the leaves, but may be determined in another way. If the -shoot of such a plant be cut underneath mercury, or underneath a strong -solution of eosin, it will be found that some of the mercury or eosin, -as the case may be, will be forcibly drawn up into the stem toward the -roots. This is seen on quickly splitting the cut end of the stem. When -plants in the open cannot be obtained in this condition, one may take -a plant like a balsam plant from the greenhouse, or some other potted -plant, knock it out of the pot, free the roots from the soil and allow -to partly wilt. The stem may then be held under the eosin solution and -cut. - -[Illustration: Fig. 54. Experiment to show lifting power of -transpiration.] - -[Illustration: Fig. 55. - -Estimation of the amount of transpiration. The tubes are filled with -water, and as the water transpires from the leaf surface its movement -in the tube from _a_ to _b_ can be measured. (After Mangin.)] - -=87. Lifting power of transpiration.=—Not only does transpiration -go on quite independently of root pressure, as we have discovered -from other experiments, but transpiration is capable of exerting a -lifting power on the water in the plant. This may be demonstrated in -the following way: Place the cut end of a leafy shoot in one end of a -U tube and fit it water-tight. Partly fill this arm of the U tube with -water, and add mercury to the other arm until it stands at a level in -the two arms as in fig. 54. In a short time we note that the mercury is -rising in the tube. - -=88. Root pressure may exceed transpiration.=—If we cover small -actively growing plants, such as the pea, corn, wheat, bean, etc., -with a bell jar, and place them in the sunlight where the temperature -is suitable for growth, in a few hours, if conditions are favorable, -we shall see that there are drops of water standing out on the margins -of the leaves. These drops of water have exuded through the ordinary -stomata, or in other cases what are called water stomata, through the -influence of root pressure. The plant being covered by the glass jar, -the air soon becomes saturated with moisture and transpiration is -checked. Root pressure still goes on, however, and the result is shown -in the exuding drops. Root pressure is here in excess of transpiration. -This phenomenon is often to be observed during the summer season in the -case of low-growing plants. During the bright warm day transpiration -equals, or may be in excess of, root pressure, and the leaves are -consequently flaccid. As nightfall comes on the air becomes more -moist, and the conditions of light are such also that transpiration -is lessened. Root pressure, however, is still active because the soil -is still warm. In these cases drops of water may be seen exuding from -the margins of the leaves due to the excess of root pressure over -transpiration. Were it not for this provision for the escape of the -excess of water raised by root pressure, serious injury by lesions, as -a result of the great pressure, might result. The plant is thus to some -extent a self-regulatory piece of apparatus so far as root pressure and -transpiration are concerned. - -=89. Injuries caused by excessive root pressure.=—Some varieties -of tomatoes when grown in poorly lighted and poorly ventilated -greenhouses suffer serious injury through lesions of the tissues. -This is brought about by the cells at certain parts becoming charged -so full with water through the activity of root pressure and lessened -transpiration, assisted also probably by an accumulation of certain -acids in the cell-sap which cannot be got rid of by transpiration. -Under these conditions some of the cells here swell out, forming -extensive cushions, and the cell walls become so weakened that they -burst. It is possible to imitate the excess of root pressure in the -case of some plants by connecting the stems with a system of water -pressure, when very quickly the drops of water will begin to exude from -the margins of the leaves. - -[Illustration: Fig. 56. - -The roots are lifting more water into the plant than can be given off -in the form of water vapor, so it is pressed out in drops. From “First -Studies Plant Life.”] - -=90.= It should be stated that in reality there is no difference -between transpiration and evaporation, if we bear in mind that -evaporation takes place more slowly from living plants than from dead -ones, or from an equal surface of water. - -=91.= The escape of water vapor is not the only function of the -stomata. The exchange of gases takes place through them as we shall -later see. A large number of experiments show that normally the stomata -are open when the leaves are turgid. But when plants lose excessive -quantities of water on dry and hot days, so that the leaves become -flaccid, the guard cells automatically close the stomata to check the -escape of water vapor. Some water escapes through the epidermis of many -plants, though the cuticularized membrane of the epidermis largely -prevents evaporation. In arid regions plants are usually provided -with an epidermis of several layers of cells to more securely prevent -evaporation there. In such cases the guard cells are often protected by -being sunk deeply in the epidermal layer. - -=92. Demonstration of stomates and intercellular air spaces.=—A -good demonstration of the presence of stomates in leaves, as well as -the presence and intercommunication of the intercellular spaces, can be -made by blowing into the cut end of the petiole of the leaf of a calla -lily, the lamina being immersed in water. The air is forced out -through the stomata and rises as bubbles to the surface of the water. -At the close of the experiment some of the air bubbles will still be -in contact with the leaf surface at the opening of the stomata. The -pressure of the water gradually forces this back into the leaf. Other -plants will answer for the experiment, but some are more suitable than -others. - -=92a. Number of stomata.=—The larger number of stomata are on the -under side of the leaf. (In leaves which float on the surface of the -water all of the stomata are on the upper side of the leaf, as in the -water-lily.) It has been estimated by investigation that in general -there are 40-300 stomata to the square millimeter of surface. In some -plants this number is exceeded, as in the olive, where there are 625. -In an entire leaf of Brassica rapa there are about 11,000,000 stomata, -and in an entire leaf of the sunflower there are about 13,000,000 -stomata. - -=92b. Amount of water transpired by plants.=—The amount of water -transpired by plants is very great. According to careful estimates a -sunflower 6 feet high transpires on the average about one quart per -day; an acre of cabbages 2,000,000 quarts in four months; an oak tree -with 700,000 leaves transpires about 180 gallons of water per day. -According to von Höhnel, a beech tree 110 years old transpired about -2250 gallons of water in one summer. A hectare of such trees (about 400 -on 2½ acres) would at the same rate transpire about 900,000 gallons, or -about 30,000 barrels in one summer. - -FOOTNOTE: - -[6] Demonstrations may be made with prepared sections of leaves, - - - - -CHAPTER V. - -PATH OF MOVEMENT OF WATER IN PLANTS. - - -=93.= In our study of root pressure and transpiration we have seen -that large quantities of water or solutions move upward through the -stems of plants. We are now led to inquire through what part of the -stems the liquid passes in this upward movement, or in other words, -what is the path of the “sap” as it rises in the stem. This we can -readily see by the following trial. - -=94. Place the cut ends of leafy shoots in a solution of some of -the red dyes.=—We may cut off leafy shoots of various plants and -insert the cut ends in a vessel of water to which have been added a few -crystals of the dye known as fuchsin to make a deep red color (other -red dyes may be used, but this one is especially good). If the study is -made during the summer, the “touch-me-not” (impatiens) will be found a -very useful plant, or the garden balsam, which may also be had in the -winter from conservatories. Almost any plant will do, however, but we -should also select one like the corn plant (zea mays) if in the summer, -or the petioles of a plant like caladium, which can be obtained from -the conservatory. If seedlings of the castor-oil bean are at hand we -may cut off some shoots which are 8-10 inches high, and place them in -the solution also. - -=95. These solutions color the tracts in the stem and leaves through -which they flow.=—After a few hours in the case of the impatiens, -or the more tender plants, we can see through the stem that certain -tracts are colored red by the solution, and after 12 to 24 hours there -may be seen a red coloration of the leaves of some of the plants -used. After the shoots have been standing in the solution for a few -hours, if we cut them at various places we will note that there are -several points in the section where the tissues are colored red. In -the impatiens perhaps from four to five, in the sunflower a larger -number. In these plants the colored areas on a cross-section of the -stem are situated in a concentric ring which separates more or less -completely an outer ring of the stem from the central portion. If we -now split portions of the stem lengthwise we see that these colored -areas continue throughout the length of the stem, in some cases even up -to the leaves and into them. - -[Illustration: Fig. 57. Broken corn stalk, showing fibrovascular -bundles.] - -=96.= If we cut across the stem of a corn plant which has been -in the solution, we see that instead of the colored areas being in -a concentric ring they are irregularly scattered, and on splitting -the stem we see here also that these colored areas extend for long -distances through the stem. If we take a corn stem which is mature, or -an old and dead one, cut around through the outer hard tissues, and -then break the stem at this point, from the softer tissue long strings -of tissue will pull out as shown in fig. 57. These strings of denser -tissue correspond to the areas which are colored by the dye. They are -in the form of minute bundles, and are called _vascular bundles_. - -=97.= We thus see that instead of the liquids passing through the -entire stem they are confined to definite courses. Now that we have -discovered the path of the upward movement of water in the stem, we are -curious to see what the structure of these definite portions of the -stem is. - -[Illustration: Fig. 58. - -Xylem portion of bundle. Cambium portion of bundle. Bast portion of -bundle. - -Section of vascular bundle of sunflower stem.] - -=98. Structure of the fibrovascular bundles.=—We should now make -quite thin cross-sections, either free hand and mount in water for -microscopic examination, or they may be made with a microtome and -mounted in Canada balsam, and in this condition will answer for future -study. To illustrate the structure of the bundle in one type we may -take the stem of the castor-oil bean. On examining these cross-sections -we see that there are groups of cells which are denser than the ground -tissue. These groups correspond to the colored areas in the former -experiments, and are the vascular bundles cut across. These groups are -somewhat oval in outline, with the pointed end directed toward the -center of the stem. If we look at the section as a whole we see that -there is a narrow continuous ring[7] of small cells situated at the -same distance from the center of the stem as the middle part of the -bundles, and that it divides the bundles into two groups of cells. - -=99. Woody portion of the bundle.=—In that portion of the bundle -on the inside of the ring, i.e., toward the “pith,” we note large, -circular, or angular cavities. The walls of these cells are quite thick -and woody. They are therefore called wood cells, and because they -are continuous with cells above and below them in the stem in such a -way that long tubes are formed, they are called woody vessels. Mixed -in with these are smaller cells, some of which also have thick walls -and are wood cells. Some of these cells may have thin walls. This -is the case with all when they are young, and they are then classed -with the fundamental tissue or soft tissue (parenchyma). This part of -the bundle, since it contains woody vessels and fibres, is the _wood -portion_ of the bundle, or technically the _xylem_. - -=100. Bast portion of the bundle.=—If our section is through a -part of the stem which is not too young, the tissues of the outer part -of the bundle will show either one or several groups of cells which -have white and shiny walls, that are thickened as much or more than -those of the wood vessels. These cells are _bast_ cells, and for this -reason this part of the bundle is the _bast_ portion, or the _phloem_. -Intermingled with these, cells may often be found which have thin -walls, unless the bundle is very old. Nearer the center of the bundle -and still within the bast portion are cells with thin walls, angular -and irregularly arranged. This is the softer portion of the bast, and -some of these cells are what are called _sieve_ tubes, which can be -better seen and studied in a longitudinal section of the stem. - -=101. Cambium region of the bundle.=—Extending across the center -of the bundle are several rows of small cells, the smallest of the -bundle, and we can see that they are more regularly arranged, usually -in quite regular rows, like bricks piled upon one another. These cells -have thinner walls than any others of the bundle, and they usually take -a deeper stain when treated with a solution of some of the dyes. This -is because they are younger, and are therefore richer in protoplasmic -contents. This zone of young cells across the bundle is the _cambium_. -Its cells grow and divide, and thus increase the size of the bundle. -By this increase in the number of the cells of the cambium layer, the -outermost cells on either side are continually passing over into the -phloem, on the one hand, and into the wood portion of the bundle, on -the other hand. - -=102. Longitudinal section of the bundle.=—If we make thin -longisections of the vascular bundle of the castor-oil seedling (or -other dicotyledon) so that we have thin ones running through a bundle -radially, as shown in fig. 59, we can see the structure of these parts -of the bundle in side view. We see here that the form of the cells is -very different from what is presented in a cross-section of the same. -The walls of the various ducts have peculiar markings on them. These -markings are caused by the walls being thicker in some places than in -others, and this thickening takes place so regularly in some instances -as to form regular spiral thickenings. Others have the thickenings in -the form of the rounds of a ladder, while still others have pitted -walls or the thickenings are in the form of rings. - -[Illustration: Fig. 59. - -Longitudinal section of vascular bundle of sunflower stem; spiral, -scalariform and pitted vessels at left; next are wood fibers with -oblique cross walls; in middle are cambium cells with straight cross -walls, next two sieve tubes, then phloem or bast cells.] - -=103. Vessels or ducts.=—One way in which the cells in side view -differ greatly from an end view, in a cross-section in the bundle, is -that they are much longer in the direction of the axis of the stem. The -cells have become elongated greatly. If we search for the place where -two of these large cells with spiral, or ladder-like, markings meet end -to end, we see that the wall which formerly separated the cells has -nearly or quite disappeared. In other words the two cells have now an -open communication at the ends. This is so for long distances in the -stem, so that long columns of these large cells form tubes or vessels -through which the water rises in the stems of plants. - -=104.= In the bast portion of the bundle we detect the cells of -the bast fibers by their thick walls. They are very much elongated and -the ends taper out to thin points so that they overlap. In this way -they serve to strengthen the stem. - -=105. Sieve tubes.=—Lying near the bast cells, usually toward -the cambium, are elongated cells standing end to end, with delicate -markings on their cross walls which appear like finely punctured plates -or sieves. The protoplasm in such cells is usually quite distinct, and -sometimes contracted away from the side walls, but attached to the -cross walls, and this aids in the detection of the sieve tubes (fig. -59.) The granular appearance which these plates present is caused by -minute perforations through the wall so that there is a communication -between the cells. The tubes thus formed are therefore called sieve -tubes and they extend for long distances through the tube so that there -is communication throughout the entire length of the stem. (The -function of the sieve tubes is supposed to be that for the downward -transportation of substances elaborated in the leaves.) - -=106.= If we section in like manner the stem of the sunflower we -shall see similar bundles, but the number is greater than eight. In -the garden balsam the number is from four to six in an ordinary stem -3-4_mm_ diameter. Here we can see quite well the origin of the vascular -bundle. Between the larger bundles we can see especially in free-hand -sections of stems through which a colored solution has been lifted by -transpiration, as in our former experiments, small groups of the minute -cells in the cambial ring which are colored. These groups of cells -which form strands running through the stem are _pro-cambium strands_. -The cells divide and increase just like the cambium cells, and the -older ones thrown off on either side change, those toward the center -of the stem to wood vessels and fibers, and those on the outer side to -bast cells and sieve tubes. - -=107. Fibrovascular bundles in the Indian corn.=—We should now -make a thin transection of a portion of the center of the stem of -Indian corn, in order to compare the structure of the bundle with that -of the plants which we have just examined. In fig. 60 is represented a -fibrovascular bundle of the stem of the Indian corn. The large cells -are those of the spiral and reticulated and annular vessels. This is -the woody portion of the bundle or xylem. Opposite this is the bast -portion or phloem, marked by the lighter colored tissue at _i_. The -larger of these cells are the sieve tubes, and intermingled with them -are smaller cells with thin walls. Surrounding the entire bundle are -small cells with thick walls. These are elongated and the tapering ends -overlap. They are thus slender and long and form fibers. In such a -bundle all of the cambium has passed over into permanent tissue and is -said to be closed. - -[Illustration: Fig. 60. - -Transection of fibrovascular bundle of Indian corn. _a_, toward -periphery of stem; _g_, large pitted vessels; _s_, spiral vessel; _r_, -annular vessel; _l_, air cavity formed by breaking apart of the cells; -_i_, soft bast, a form of sieve tissue; _p_, thin-walled parenchyma. -(Sachs.)] - -=108. Rise of water in the vessels.=—During the movement of the -water or nutrient solutions upward in the stem the vessels of the wood -portion of the bundle in certain plants are nearly or quite filled, -if root pressure is active and transpiration is not very rapid. If, -however, on dry days transpiration is in excess of root pressure, as -often happens, the vessels are not filled with the water, but are -partly filled with certain gases because the air or other gases in -the plant become rarefied as a result of the excessive loss of water. -There are then successive rows of air or gas bubbles in the vessels -separated by films of water which also line the walls of the vessels. -The condition of the vessel is much like that of a glass tube through -which one might pass the “froth” which is formed on the surface of -soapy water. This forms a chain of bubbles in the vessels. This chain -has been called Jamin’s chain because of the discoverer. - -=109.= Why water or food solutions can be raised by the plant -to the height attained by some trees has never been satisfactorily -explained. There are several theories propounded which cannot be -discussed here. It is probably a very complex process. Root pressure -and transpiration both play a part, or at least can be shown, as we -have seen, to be capable of lifting water to a considerable height. In -addition to this, the walls of the vessels absorb water by diffusion, -and in the other elements of the bundle capillarity comes also into -play, as well as osmosis. - -See Organization of Tissues, Chapter 38. - -=110. Flow of sap in the spring.=—The cause of the bleeding of -trees and the flow of sap in the spring is little understood. One of -the remarkable cases is the flow of sap in maple trees. It begins -in early spring and ceases as the buds are opening, and seems to be -initiated by alternation of high and low temperatures of day and night. -It has been found that the pressures inside of the tree at this time -are enormously increased during the day, when the temperature rises -after a cold night. This has led to the belief that the pressure is -caused by the expansion of the gases in the vascular ducts. The warming -up of the twigs and branches of the tree would take place rapidly -during the day, while the interior of the trunk would be only slightly -affected. The pressures then would cause the sap to flow downward -during the day, and at night the branches becoming cool, sap would flow -back again from the roots and trunk. - -Recent experiments by Jones _et al._ show that while some of the -pressure is due to the expansion of gas in the tree by the rise of -temperature, this cannot account for the enormous pressures which are -often present, for example, when after a rise in the temperature of 2° -C. there was an increase of 20 lbs. pressure. - -Then again, after the cessation of the flow in late spring there are -often as great differences between night and day temperatures. It -therefore seems reasonable to conclude that the expansion of gases by a -rise in temperature is not the direct cause. - -=Activities of the cells.=—It has been suggested by some that -the rise in temperature exercises an influence on the protoplasts, -or living cells, so that they are stimulated to a special activity -resulting in an exudation pressure from the individual cells, which is -known to take place. With the fall of temperature at night this -activity would cease and there might result a lessened pressure in -the cells. Since the specific activities of cells are known to vary -in different plants, and in the same plant at different seasons, some -support is gained for this theory, though it is generally believed that -the activities of the living cells in the stems are not necessary for -the upward flow of water. It must be admitted, however, that at present -we know very little about this interesting problem. - -FOOTNOTE: - -[7] This ring and the bundles separate the stem into two regions, -an outer one composed of large cells with thin walls, known as the -cortical cells, or collectively the _cortex_. The inner portion, -corresponding to what is called the pith, is made up of the same kind -of cells and is called the _medulla_, or _pith_. When the cells of -the cortex, as well as of the pith, remain thin walled the tissue is -called parenchyma. Parenchyma belongs to the group of tissues called -fundamental. - - - - -CHAPTER VI. - -MECHANICAL USES OF WATER. - - -=111. Turgidity of plant parts.=—As we have seen by the experiments on -the leaves, turgescence of the cells is one of the conditions which -enables the leaves to stand out from the stem, and the lamina of the -leaves to remain in an expanded position, so that they are better -exposed to the light, and to the currents of air. Were it not for this -turgidity the leaves would hang down close against the stem. - -[Illustration: Fig. 61. Restoration of turgidity (Sachs).] - -=112. Restoration of turgidity in shoots.=—If we cut off a living -stem of geranium, coleus, tomato, or “balsam,” and allow the leaves -to partly wilt so that the shoot loses its turgidity, it is possible -for this shoot to regain turgidity. The end may be freshly cut again, -placed in a vessel of water, covered with a bell jar and kept in a room -where the temperature is suitable for the growth of the plant. The -shoot will usually become turgid again from the water which is absorbed -through the cut end of the stem and is carried into the leaves where -the individual cells become turgid, and the leaves are again expanded. -Such shoots, and the excised leaves also, may often be made turgid -again by simply immersing them in water, as one of the experiments with -the salt solution would teach. - -=113.= Turgidity may be restored more certainly and quickly in a -partially wilted shoot in another way. The cut end of the shoot may be -inserted in a U tube as shown in fig. 61, the end of the tube around -the stem of the plant being made air-tight. The arm of the tube in -which the stem is inserted is filled with water and the water is -allowed to partly fill the other arm. Into this other arm is then -poured mercury. The greater weight of the mercury causes such pressure -upon the water that it is pushed into the stem, where it passes up -through the vessels in the stems and leaves, and is brought more -quickly and surely to the cells which contain the protoplasm and -cell-sap, so that turgidity is more quickly and certainly attained. - -=114. Tissue tensions.=—Besides the turgescence of the cells -of the leaves and shoots there are certain tissue tensions without -which certain tender and succulent shoots, etc., would be limp, and -would droop. There are a number of plants usually accessible, some at -one season and some at others, which may be used to illustrate tissue -tension. - -=115. Longitudinal tissue tension.=—For this in early summer one -may use the young and succulent shoots of the elder (sambucus); or the -petioles of rhubarb during the summer and early autumn; or the petioles -of richardia. Petioles of caladium are excellent for this purpose, and -these may be had at almost any season of the year from the greenhouses, -and are thus especially advantageous for work during late autumn or -winter. The tension is so strong that a portion of such a petiole -10-15_cm_ long is ample to demonstrate it. As we grasp the lower end of -the petiole of a caladium, or rhubarb leaf, we observe how rigid it is, -and how well it supports the heavy expanded lamina of the leaf. - -=116.= The ends of a portion of such a petiole or other object -which may be used are cut off squarely. With a knife a strip from -2-3_mm_ in thickness is removed from one side the full length of the -object. This strip we now find is shorter than the larger part from -which it was removed. The outer tissue then exerts a tension upon the -petiole which tends to shorten it. Let us remove another strip lying -next this one, and another, and so on until the outer tissues remain -only upon one side. The object will now bend toward that side. Now -remove this strip and compare the length of the strips removed with the -central portion. We find that they are much shorter now. In other words -there is also a tension in the tissue of the central portion of the -petiole, the direction of which is opposite to that of the superficial -tissue. The parts of the petiole now are not rigid, and they easily -bend. These two longitudinal tissue tensions acting in opposition to -each other therefore give rigidity to the succulent shoot. It is only -when the individual cells of such shoots or petioles are turgid that -these tissue tensions in succulent shoots manifest themselves or are -prominent. - -[Illustration: Fig. 62. Strip from dandelion stem made to imitate a -plant tendril.] - -=117.= To demonstrate the efficiency of this tension in giving -support, let us take a long petiole of caladium or of rhubarb. Hold it -by one end in a horizontal position. It is firm and rigid, and does not -droop, or but little. Remove all of the outer portion of the tissues, -as described above, leaving only the central portion. Now attempt to -hold it in a horizontal position by one end. It is flabby and droops -downward because the longitudinal tension is removed. - -=118. Longitudinal tension in dandelion stems.=—Take long -and fresh dandelion stems. Split them. Note that they coil. The -longitudinal tension is very great. Place some of these strips in fresh -water. They coil up into close curls because by the absorption of water -by the cells the turgescence of the individual cells is increased, and -this increases the tension in the stem. Now place them in salt water (a -5 per cent solution). Why do they uncoil? - -=119. To imitate the coiling of a tendril.=—Cut out a narrow -strip from a long dandelion stem. Fasten to a piece of soft wood, with -the ends close together, as shown in fig. 62. Now place it in fresh -water and watch it coil. Part of it coils one way and part another way, -just as a tendril does after the free end has caught hold of some place -for support. - -=120. Transverse tissue tension.=—To illustrate this one may take -a willow shoot 3-5_cm_ diameter and saw off sections about 2 cm long. -Cut through the bark on one side and peel it off in a single strip. Now -attempt to replace it. The bark will not quite cover the wood again, -since the ends will not meet. It must then have been held in transverse -tension by the woody part of the shoot. - - - - -CHAPTER VII. - -STARCH AND SUGAR FORMATION. - - -1. The Gases Concerned. - -=121. Gas given off by green plants in the sunlight.=—Let us take -some green alga, like spirogyra, which is in a fresh condition, and -place one lot in a beaker or tall glass vessel of water and set this in -the direct sunlight or in a well lighted place. At the same time cover -a similar vessel of spirogyra with black cloth so that it will be in -the dark, or at least in very weak light. - -[Illustration: Fig. 63. Oxygen gas given off by spirogyra.] - -[Illustration: Fig. 64. Bubbles of oxygen gas given off from elodea in -presence of sunlight. (Oels.)] - -=122.= In a short time we note that in the first vessel small -bubbles of gas are accumulating on the surface of the threads of the -spirogyra, and now and then some free themselves and rise to the -surface of the water. Where there is quite a tangle of the threads the -gas is apt to become caught and held back in larger bubbles, which on -agitation of the vessel are freed. - -If we now examine the second vessel we see that there are no bubbles, -or only a very few of them. We are led to believe then that sunlight -has had something to do with the setting free of this gas from the -plant. - -=123.= We may now take another alga-like vaucheria and perform the -experiment in the same way, or to save time the two may be set up at -once. In fact if we take any of the green algæ and treat them as -described above gas will be given off in a similar manner. - -=124.= We may now take one of the higher green plants, an aquatic -plant like elodea, callitriche, etc. Place the plant in the water with -the cut end of the stem uppermost, but still immersed, the plant being -weighted down by a glass rod or other suitable object. If we place the -vessel of water containing these leafy stems in the bright sunlight, -in a short time bubbles of gas will pass off quite rapidly from the -cut end of the stem. If in the same vessel we place another stem, from -which the leaves have been cut, the number of bubbles of gas given -off will be very few. This indicates that a large part of the gas is -furnished by the leaves. - -=125.= Another vessel fitted up in the same way should be placed -in the dark or shaded by covering with a box or black cloth. It will -be seen here, as in the case of spirogyra, that very few or no bubbles -of gas will be set free. Sunlight here also is necessary for the rapid -escape of the gas. - -=126.= We may easily compare the rapidity with which light of -varying intensity effects the setting free of this gas. After cutting -the end of the stem let us plunge the cut surface several times in -melted paraffine, or spread over the cut surface a coat of varnish. -Then prick with a needle a small hole through the paraffine or varnish. -Immerse the plant in water and place in sunlight as before. The gas now -comes from the puncture through the coating of the cut end, and the -number of bubbles given off during a given period can be ascertained by -counting. If we duplicate this experiment by placing one plant in weak -light or diffused sunlight, and another in the shade, we can easily -compare the rapidity of the escape of the gas under the different -conditions, which represent varying intensities of light. We see then -that not only is sunlight necessary for the setting free of this gas, -but that in diffused light or in the shade the activity of the plant in -this respect is less than in direct sunlight. - -=127. What this gas is.=—If we take quite a quantity of the -plants of elodea and place them under an inverted funnel which is -immersed in water, the gas will be given off in quite large quantities -and will rise into the narrow exit of the funnel. The funnel should be -one with a short tube, or the vessel one which is quite deep so that -a small test tube which is filled with water may in this condition be -inverted over the opening of the funnel tube. With this arrangement -of the experiment the gas will rise in the inverted test tube, slowly -displace a portion of the water, and become collected in a sufficient -quantity to afford us a test. When a considerable quantity has -accumulated in the test tube, we may close the end of the tube in -the water with the thumb, lift it from the water and invert. The gas -will rise against the thumb. A dry soft-pine splinter should be then -lighted, and after it has burned a short time, extinguish the flame by -blowing upon it, when the still burning end of the splinter should be -brought to the mouth of the tube as the thumb is quickly moved to one -side. The glowing of the splinter shows that the gas is _oxygen_. - -[Illustration: Fig. 65. Apparatus for collecting quantity of oxygen from -elodea. (Detmer.)] - -[Illustration: Fig. 66. Ready to see what the gas is.] - -=128.= It is better to allow the apparatus to stand several days -in the sunlight in order to catch a full tube of the gas. Or on a sunny -day carbon dioxide gas can be led into the water in the jar from a -generator, such an one as is used for the evolution of CO₂. The CO₂ -can be produced by the action of hydrochloric acid on bits of marble. -The CO₂ should not be run below the funnel. The test tube should be -fastened so that the light oxygen gas will not raise it off the funnel. -With the tube full of gas the test for oxygen can be made by lifting -the tube with one hand and quickly thrusting the glowing end of the -splinter in with the other hand. If properly handled, the splinter will -flame again. If it is necessary to keep the apparatus standing for more -than one day it is well to add fresh water in the place of most of the -water in the jar. Do not use leaves of land plants in this experiment, -since the bubbles which rise when these leaves are placed in water are -not evidence that this process is taking place. - -[Illustration: Fig. 67. The splinter lights again in the presence of -oxygen gas.] - - =129. Oxygen given off by green land plants - also.=—If we should extend our experiments to land - plants we should find that oxygen is given off by them - under these conditions of light. Land plants, however, - will not do this when they are immersed in water, but - it is necessary to set up rather complicated apparatus - and to make analyses of the gases at the beginning and - at the close of the experiments. This has been done, - however, in a sufficiently large number of cases so - that we know that all green plants in the sunlight, if - temperature and other conditions are favorable, give - off oxygen. - -=130. Absorption of carbon dioxide.=—We have next to inquire -where the oxygen comes from which is given off by green plants when -exposed to the sunlight, and also to learn something more of the -conditions necessary for the process. We know that water which has been -for some time exposed to the air and soil, and has been agitated, like -running water of streams, or the water of springs, has mixed with it a -considerable quantity of oxygen and carbon dioxide. - -If we boil spring water or hydrant water which comes from a stream -containing oxygen and carbon dioxide, for about 20 minutes, these -gases are driven off. We should set this aside where it will not be -agitated, until it has cooled sufficiently to receive plants without -injury. Let us now place some spirogyra or vaucheria, and elodea, or -other green water plant, in this boiled water and set the vessel in the -bright sunlight under the same conditions which were employed in the -experiments for the evolution of oxygen. No oxygen is given off. - -Can it be that this is because the oxygen was driven from the water in -boiling? We shall see. Let us take the vessel containing the water, -or some other boiled water, and agitate it so that the air will be -thoroughly mixed with it. In this way oxygen is again mixed with the -water. Now place the plant again in the water, set in the sunlight, and -in several minutes observe the result. No oxygen or but little is given -off. There must be then some other requisite for the evolution of the -oxygen. - -=132. The gases are interchanged in the plants.=—We will now -introduce carbon dioxide again in the water. This can be done by -leading CO₂ from a gas generator into the water. Broken bits of marble -are placed in the generator, acted upon by hydrochloric acid, and the -gas is led over by glass tubing. Now if we place the plant in the water -and set the vessel in the sunlight, in a few minutes the oxygen is -given off rapidly. - -=133. A chemical change of the gas takes place within the plant -cell.=—This leads us to believe then that CO₂ is in some way -necessary for the plant in this process. Since oxygen is given off -while carbon dioxide, a different gas, is necessary, it would seem that -a chemical change takes place in the gases within the plant. Since the -process takes place in such simple plants as spirogyra as well as in -the more bulky and higher plants, it appears that the changes go on -within the cell, in fact within the protoplasm. - -=134. Gases as well as water can diffuse through the protoplasmic -membrane.=—Carbon dioxide then is absorbed by the plant while -oxygen is given off. We see therefore that gases as well as water can -diffuse through the protoplasmic membrane of plants under certain -conditions. - - -2. Where Starch is Formed. - -We have found by these simple experiments that some chemical change -takes place within the protoplasm of the green cells of plants during -the absorption of carbon dioxide and the giving off of oxygen. We -should examine some of the green parts of those plants used in the -experiments, or if they are not at hand we should set up others in -order to make this examination. - -=135. Starch formed as a result of this process.=—We may take -spirogyra which has been standing in water in the bright sunlight for -several hours. A few of the threads should be placed in alcohol for a -short time to kill the protoplasm. From the alcohol we transfer the -threads to a solution of iodine in potassium iodide. We find that -at certain points in the chlorophyll band a bluish tinge, or color, -is imparted to the ring or sphere which surrounds the pyrenoid. In -our first study of the spirogyra cell we noted this sphere as being -composed of numerous small grains of starch which surround the pyrenoid. - -=136. Iodine used as a test for starch.=—This color reaction -which we have obtained in treating the threads with iodine is the -well-known reaction, or test, for starch. We have demonstrated then -that starch is present in spirogyra threads which have stood in the -sunlight with free access to carbon dioxide. - -If we examine in the same way some threads which have stood in the -dark for a few days we obtain no reaction for starch, or at best only -a slight reaction. This gives us some evidence that a chemical change -does take place during this process (absorption of CO₂ and giving off -of oxygen), and that starch is a product of that chemical change. - -=137. Schimper’s method of testing for the presence of starch.=—Another -convenient and quick method of testing for the presence of starch -is what is known as Schimper’s method. A strong solution of chloral -hydrate is made by taking 8 grams of chloral hydrate for every 5_cc_ -of water. To this solution is added a little of an alcoholic tincture -of iodine. The threads of spirogyra may be placed directly in this -solution, and in a few moments mounted in water on the glass slip and -examined with the microscope. The reaction is strong and easily seen. - -We should also examine the leaves of elodea, or one of the higher green -plants which has been for some time in the sunlight. We may use here -Schimper’s method by placing the leaves directly in the solution of -chloral hydrate and iodine. The leaves are made transparent by the -chloral hydrate so that the starch reaction from the iodine is easily -detected. - -The following is a convenient and safe method of extracting chlorophyll -from leaves. Fill a large pan, preferably a dishpan, half full of -hot water. This may be kept hot by a small flame. On the water float -an evaporating dish partly filled with alcohol. The leaves should be -first immersed in the hot water for several minutes, then placed in the -alcohol, which will quickly remove the chlorophyll. Now immerse the -leaves in the iodine solution. - -[Illustration: Fig. 68. Leaf of coleus showing green and white areas, -before treatment with iodine.] - -[Illustration: Fig. 69. Similar leaf treated with iodine, the starch -reaction only showing where the leaf was green.] - -=138. Green parts of plants form starch when exposed to light.=—Thus we -find that in the case of all the green plants we have examined, starch -is present in the green cells of those which have been standing for -some time in the sunlight where the process of the absorption of CO₂ -and the giving off of oxygen can go on, and that in the case of plants -grown in the dark, or in leaves of plants which have stood for some -time in the dark, starch is absent. We reason from this that starch is -the product of the chemical change which takes place in the green cells -under these conditions. The CO₂ which is absorbed by the plant mixes -with the water (H₂O) in the cell and immediately forms carbonic acid. -The chlorophyll in the leaf absorbs radiant energy from the sun which -splits up the carbonic acid, and its elements then are put together -into a more complex compound, starch. This process of putting together -the elements of an organic compound is a _synthesis_, or a _synthetic -assimilation_, since it is done by the living plant. It is therefore a -synthetic assimilation of carbon dioxide. Since the sunlight supplies -the energy it is also called _photosynthesis_, or _photosynthetic -assimilation_. We can also say carbon dioxide assimilation, or CO₂ -assimilation (see paragraph on assimilation at close of Chapter 10). - -=139. Starch is formed only in the green parts of variegated -leaves.=—If we test for starch in variegated leaves like the leaf -of a coleus plant, we shall have an interesting demonstration of the -fact that the green parts of plants only form starch. We may take -a leaf which is partly green and partly white, from a plant which -has been standing for some time in bright light. Fig. 68 is from a -photograph of such a leaf. We should first boil it in alcohol to remove -the green color. Now immerse it in the potassium iodide of iodine -solution for a short time. The parts which were formerly green are -now dark blue or nearly black, showing the presence of starch in -those portions of the leaf, while the white part of the leaf is still -uncolored. This is well shown in fig. 69, which is from a photograph of -another coleus leaf treated with the iodine solution. - - -3. Chlorophyll and the Formation of Starch. - -=140.= In our experiments thus far in treating of the absorption -of carbon dioxide and the evolution of oxygen, with the accompanying -formation of starch, we have used green plants. - -=141. Fungi cannot form starch.=—If we should extend our -experiments to the fungi, which lack the green color so characteristic -of the majority of plants, we should find that photosynthesis does not -take place even though the plants are exposed to direct sunlight. These -plants cannot then form starch, but obtain carbohydrates for food from -other sources. - -=142. Photosynthesis cannot take place in etiolated plants.=—Moreover -photosynthesis is usually confined to the green plants, and if by any -means one of the ordinary green plants loses its green color this -process cannot take place in that plant, even when brought into the -sunlight, until the green color has appeared under the influence of -light. - -This may be very easily demonstrated by growing seedlings of the -bean, squash, corn, pea, etc. (pine seedlings are green even when -grown in the dark), in a dark room, or in a dark receiver of some -kind which will shut out the rays of light. The room or receiver must -be quite dark. As the seedlings are “coming up,” and as long as they -remain in the dark chamber, they will present some other color than -green; usually they are somewhat yellowed. Such plants are said to be -_etiolated_. If they are brought into the sunlight now for a few hours -and then tested for the presence of starch the result will be negative. -But if the plant is left in the light, in a few days the leaves -begin to take on a green color, and then we find that carbon dioxide -assimilation begins. - -=143. Chlorophyll and chloroplasts.=—The green substance in -plants is then one of the important factors in this complicated -process of forming starch. This green substance is _chlorophyll_, -and it usually occurs in definite bodies, the chlorophyll bodies, or -_chloroplasts_. - - The material for new growth of plants grown in the - dark is derived from the seed. Plants grown in the dark - consist largely of water and protoplasm, the walls - being very thin. - -=144. Form of the chlorophyll bodies.=—Chlorophyll bodies vary in -form in some different plants, especially in some of the lower -plants. This we have already seen in the case of spirogyra, where the -chlorophyll body is in the form of a very irregular band, which courses -around the inner side of the cell wall in a spiral manner. In zygnema, -which is related to spirogyra, the chlorophyll bodies are star-shaped. -In the desmids the form varies greatly. In œdogonium, another of the -thread-like algæ, illustrated in fig. 144, the chlorophyll bodies -are more or less flattened oval disks. In vaucheria, too, a branched -thread-like alga shown in fig. 138, the chlorophyll bodies are oval in -outline. These two plants, œdogonium and vaucheria, should be examined -here if possible, in order to become familiar with their form, since -they will be studied later under morphology (see chapters on œdogonium -and vaucheria, for the occurrence and form of these plants). The form -of the chlorophyll body found in œdogonium and vaucheria is that which -is common to many of the green algæ, and also occurs in the mosses, -liverworts, ferns, and the higher plants. It is a more or less rounded, -oval, flattened body. - -[Illustration: Fig. 69_a_. - -Section of ivy leaf, palisade cells above, loose parenchyma, with large -intercellular spaces in center. Epidermal cells on either edge, with no -chlorophyll bodies.] - -=145. Chlorophyll is a pigment which resides in the chloroplast.=—That -the chlorophyll is a coloring substance which resides in the -chloroplastid, and does not form the body itself, can be demonstrated -by dissolving out the chlorophyll when the framework of the -chloroplastid is apparent. The green parts of plants which have been -placed for some time in alcohol lose their green color. The alcohol -at the same time becomes tinged with green. In sectioning such plant -tissue we find that the chlorophyll bodies, or chloroplastids as they -are more properly called, are still intact, though the green color is -absent. From this we know that chlorophyll is a substance distinct from -that of the chloroplastid. - -=146. Chlorophyll absorbs energy from sunlight for photosynthesis.=—It -has been found by analysis with the spectroscope that chlorophyll -absorbs certain of the rays of the sunlight. The energy which is thus -obtained from the sun, called _kinetic_ energy, acts on the molecules -of CH₂O₃, separating them into molecules of C, H, and O. (When the -CO₂ from the air enters the plant cell it immediately unites with -some of the water, forming carbonic acid = CH₂O₃.) After a series of -complicated chemical changes starch is formed by the union of carbon, -oxygen, and hydrogen. In this process of the reduction of the CH₂O₃ and -the formation of starch there is a surplus of oxygen, which accounts -for the giving off of oxygen during the process. - -=147. Rays of light concerned in photosynthesis.=—If a solution -of chlorophyll be made, and light be passed through it, and this -light be examined with the spectroscope, there appear what are called -absorption bands. These are dark bands which lie across certain -portions of the spectrum. These bands lie in the red, orange, yellow, -green, blue, and violet, but the bands are stronger in the red, which -shows that chlorophyll absorbs more of the red rays of light than of -the other rays. These are the rays of low refrangibility. The kinetic -energy derived by the absorption of these rays of light is transformed -into potential energy. That is, the molecule of CH₂O₃ is broken up, and -then by a different combination of certain elements starch is formed.[8] - -[8] In the formation of starch during photosynthesis the separated -molecules from the carbon dioxide and water unite in such a way that -carbon, hydrogen, and oxygen are united into a molecule of starch. This -result is usually represented by the following equation: CO₂ + H₂O = -CH₂O + O₂. Then by polymerization 6(CH₂O) = C₆H₁₂O₆ = grape sugar. -Then C₆H₁₂O₆-H₂O = C₆H₁₀O₅ = starch. It is believed, however, that the -process is much more complicated than this, that several different -compounds are formed before starch finally appears, and that the -formula for starch is much higher numerically than is represented by -C₆H₁₀O₅. - -=148. Starch grains formed in the chloroplasts.=—During -photosynthesis the starch formed is deposited generally in small grains -within the green chloroplast in the leaf. We can see this easily by -examining the leaves of some moss-like funaria which has been in the -light, or in the chloroplasts of the prothallia of ferns, etc. Starch -grains may also be formed in the chloroplasts from starch which was -formed in some other part of the plant, but which has passed in -solution. Thus the functions of the chloroplast are twofold, that of -photosynthesis and the formation of starch grains. - -=149.= In the translocation of starch when it becomes stored up in -various parts of the plant, it passes from the state of solution into -starch grains in connection with plastids similar to the chloroplasts, -but which are not green. The green ones are sometimes called -_chloroplasts_, while the colorless ones are termed _leucoplasts_, and -those possessing other colors, as red and yellow, in floral leaves, the -root of the carrot, etc., are called _chromoplasts_. - -=150. Photosynthesis in other than green plants.=—While -carbohydrates are usually only formed by green plants, there are some -exceptions. Apparent exceptions are found in the blue-green algæ, like -oscillatoria, nostoc, or in the brown and red sea weeds like fucus, -rhabdonia, etc. These plants, however, possess chlorophyll, but it is -disguised by another pigment or color. There are plants, however, which -do not have chlorophyll and yet form carbohydrates with evolution of -oxygen in the presence of light, as for example a purple bacterium, -in which the purple coloring substance absorbs light, though the rays -absorbed most energetically are not the red. - -[Illustration: Fig. 70. - -Cell exposed to weak diffused light showing chlorophyll bodies along -the horizontal walls.] - -[Illustration: Fig. 71. - -Same cell exposed to strong light, showing chlorophyll bodies have -moved to perpendicular walls. - -Figs. 70, 71.—Cell of prothallium of fern.] - -=151. Influence of light on the movement of chlorophyll -bodies.=—_In fern prothallia_.—If we place fern prothallia in weak -light for a few hours, and then examine them under the microscope, -we find that the most of the chlorophyll bodies in the cells are -arranged along the inner surface of the horizontal wall. If now the -same prothallia are placed in a brightly lighted place for a short -time most of the chlorophyll bodies move so that they are arranged -along the surfaces of the perpendicular walls, and instead of having -the flattened surfaces exposed to the light as in the former case, the -edges of the chlorophyll bodies are now turned toward the light. (See -figs. 70, 71.) The same phenomenon has been observed in many plants. -Light then has an influence on chlorophyll bodies, to some extent -determining their position. In weak light they are arranged so that the -flattened surfaces are exposed to the incidence of the rays of light, -so that the chlorophyll will absorb as great an amount as possible -of kinetic energy; but intense light is stronger than necessary, and -the chlorophyll bodies move so that their edges are exposed to the -incidence of the rays. This movement of the chlorophyll bodies is -different from that which takes place in some water plants like elodea. -The chlorophyll bodies in elodea are free in the protoplasm. The -protoplasm in the cells of elodea streams around the inside of the cell -wall much as it does in nitella and the chlorophyll bodies are carried -along in the currents, while in nitella they are stationary. - - - - -CHAPTER VIII. - -STARCH AND SUGAR CONCLUDED. ANALYSIS OF PLANT SUBSTANCE. - - -1. Translocation of Starch. - -=152. Translocation of starch.=—It has been found that leaves of -many plants grown in the sunlight contain starch when examined after -being in the sunlight for several hours. But when the plants are left -in the dark for a day or two the leaves contain no starch, or a much -smaller amount. This suggests that starch after it has been formed may -be transferred from the leaves, or from those areas of the leaves where -it has been formed. - -[Illustration: Fig. 72. - -Leaf of tropæolum with portion covered with corks to prevent the -formation of starch. (After Detmer.)] - -[Illustration: Fig. 73. - -Leaf of tropæolum treated with iodine after removal of cork, to show -that starch is removed from the leaf during the night.] - -To test this let us perform an experiment which is often made. We may -take a plant such as a garden tropæolum or a clover plant, or other -land plant in which it is easy to test for the presence of starch. Pin -a piece of circular cork, which is smaller than the area of the leaf, -on either side of the leaf, as in fig. 72, but allow free circulation -of air between the cork and the under side of the leaf. Place the plant -where it will be in the sunlight. On the afternoon of the following -day, if the sun has been shining, test the entire leaf for starch. The -part covered by the cork will not give the reaction for starch, as -shown by the absence of the bluish color, while the other parts of the -leaf will show it. The starch which was in that part of the leaf the -day before was dissolved and removed during the night, and then during -the following day, the parts being covered from the light, no starch -was formed in them. - -=153. Starch in other parts of plants than the leaves.=—We may -use the iodine test to search for starch in other parts of plants than -the leaves. If we cut a potato tuber, scrape some of the cut surface -into a pulp, and apply the iodine test, we obtain a beautiful and -distinct reaction showing the presence of starch. Now we have learned -that starch is only formed in the parts containing chlorophyll. We -have also learned that the starch which has been formed in the leaves -disappears from the leaf or is transferred from the leaf. We judge -therefore that the starch which we have found in the tuber of the -potato was formed first in the green leaves of the plant, as a result -of photosynthesis. From the leaves it is transferred in solution to -the underground stems, and stored in the tubers. The starch is stored -here by the plant to provide food for the growth of new plants from the -tubers, which are thus much more vigorous than the plants would be if -grown from the seed. - -=154. Form of starch grains.=—Where starch is stored as a reserve -material it occurs in grains which usually have certain characters -peculiar to the species of plant in which they are found. They vary -in size in many different plants, and to some extent in form also. -If we scrape some of the cut surface of the potato tuber into a pulp -and mount a small quantity in water, or make a thin section for -microscopic examination, we find large starch grains of a beautiful -structure. The grains are oval in form and more or less irregular in -outline. But the striking peculiarity is the presence of what seem to -be alternating dark and light lines in the starch grain. We note that -the lines form irregular rings, which are smaller and smaller until -we come to the small central spot termed the “hilum” of the starch -grain. It is supposed that these apparent lines in the starch grain are -caused by the starch substance being deposited in alternating dense -and dilute layers, the dilute layers containing more water than the -dense ones; others think that the successive layers from the hilum -outward are regularly of diminishing density, and that this gives the -appearance of alternating lines. The starch formed by plants is one of -the organic substances which are manufactured by plants, and it (or -glucose) is the basis for the formation of other organic substances in -the plant. Without such organic substances green plants cannot make any -appreciable increase of plant substance, though a considerable increase -in size of the plant may take place. - -NOTE.—The organic compounds resulting from photosynthesis, -since they are formed by the union of carbon, hydrogen, and oxygen in -such a way that the hydrogen and oxygen are usually present in the same -proportion as in water, are called _carbohydrates_. The most common -carbohydrates are sugars (cane sugar, C₁₂H₂₂O₁₁ for example, in beet -roots, sugar cane, sugar maple, etc.), starch, and cellulose. - -=155. Vaucheria.=—The result of carbon dioxide assimilation in -the threads of Vaucheria is not clearly understood. Starch is absent or -difficult to find in all except a few species, while oil globules are -present in most species. These oil globules are spherical, colorless, -globose and highly refringent. Often small ones are seen lying against -chlorophyll bodies. Oil is a _hydrocarbon_ (containing C, H, and O, but -the H and O are in different proportions from what they are in H₂O) -and until recently it was supposed that this oil in Vaucheria was the -direct result of photosynthesis. But the oil does not disappear when -the plant is kept for a long time in the dark, which seems to show -that it is not the direct product of carbon dioxide assimilation, and -indicates that it comes either from a temporary starch body or from -glucose. Schimper found glucose in several species of Vaucheria, and -Waltz says that some starch is present in Vaucheria sericea, while -in V. tuberosa starch is abundant and replaces the oil. To test for -oil bodies in Vaucheria treat the threads with weak osmic acid, or -allow them to stand for twenty-four hours in Fleming’s solution (which -contains osmic acid). Mount some threads and examine with microscope. -The oil globules are stained black. - - -2. Sugar, and Digestion of Starch.[9] - -=156.= It is probable that some form of sugar is always produced -as the result of photosynthesis. The sugar thus formed may be stored as -such or changed to starch. In general it may be said that sugar is most -common in the green parts of monocotyledonous plants, while starch is -most frequent in dicotyledons. Plant sugars are of three general kinds: -cane sugar abundant in the sugar cane, sugar beet, sugar maple, etc.; -glucose and fruit sugar, found in the fruits of a majority of plants, -and abundant in some, as in apples, pears, grapes, etc.; and maltose, a -variety produced in germinating seeds, as in malted barley. - -=157. Test for sugar.=—A very pretty experiment maybe made by taking -two test tubes, placing in one a solution of commercial grape sugar -(glucose), in the other one of granulated cane sugar, and adding to -each a few drops of Fehling’s solution.[10] After these tubes have -stood in a warm place for half an hour, it will be found that a bright -orange brown or cinnabar-colored precipitate of copper and cuprous -oxide has formed in the tube containing grape sugar, while the other -solution is unchanged. Grape sugar or glucose, therefore, reduces -Fehling’s solution, while cane sugar as such has no effect upon it. - -Cane sugar may be changed or converted to glucose by being boiled for a -short time with a dilute acid, or by adding Fehling’s solution to the -sugar solution and boiling. In the latter case the change is brought -about by the alkali and the precipitate of copper and cuprous oxide -forms. - -=158. Tests for sugar in plant tissue.=—(_a_) Scrape out a little -of the tissue from the inside of a ripe apple or pear, place it with a -little water in a test tube, and add a few drops of Fehling’s solution. -After standing half an hour the characteristic precipitate of copper -and cuprous oxide appears, showing that grape sugar is present in -quantity. - -Make thin sections of the apple and mount in a drop of Fehling’s -solution on a slide. After half an hour examine with the microscope. -The granules of cuprous oxide are present in the cells of the tissue in -great abundance. - -(_b_) Cut up several leaves of a young vigorous corn seedling, -cover with water in a test tube and boil for a minute. After the -decoction has cooled add the Fehling’s solution and allow to stand. -The precipitate will appear. For comparison take similar corn leaves, -remove the chlorophyll with alcohol and test with iodine. No starch -reaction appears. The carbohydrate in corn leaves is therefore glucose -and not starch. If now the corn seed be examined the cells will be -found to be full of starch grains which give the beautiful blue -reaction with iodine. This experiment shows that grape sugar is formed -in the leaves of the corn plant, but is changed to starch when stored -in the seed. - -(_c_) Take two leaves of bean seedling or coleus, test one for sugar -and the other for starch. Both are present. - -(_d_) Procure some maple sap in the spring, or in the winter months -make a decoction of the broken tips of young branches of the sugar -maple by boiling them in water in a test tube. To the sap or cool -decoction add Fehling’s solution. No precipitate appears after -standing. Now heat the same solution to the boiling point, and the -precipitate forms, showing the presence of cane sugar in the maple -sap which was converted to glucose and fruit sugar by boiling in the -presence of an alkali. - -(_e_) Scrape out some of the tissue from a sugar beet root, cover with -water in a test tube and add Fehling’s solution. No change takes place -after standing. Boil the same solution and the precipitate forms, -showing the presence of cane sugar, inverted to grape sugar and fruit -sugar by the hot alkali. - -=159. How starch is changed to sugar.=—We have seen that in -many plants the carbohydrate formed as the result of carbon dioxide -assimilation is stored as starch. This substance being insoluble -in water must be changed to sugar, which is soluble before it can -be used as food or transported to other parts of the plant. This -is accomplished through the action of certain enzymes, principally -diastase. This substance has the power of acting upon starch under -proper conditions of temperature and moisture, causing it to take up -the elements of water, and so to become sugar. - -This process takes place commonly in the leaves where starch is formed, -but especially in seeds, tubers (during the sprouting, etc.), and -other parts which the plant uses as storehouses for starch food. It is -probable that the same conditions of temperature and moisture which -favor germination or active growth are also favorable to the production -of diastase. - -=160. Experiments to show the action of diastase.=—(_a_) Place -a bit of starch half as large as a pea in a test tube, and cover with -a weak solution[11] (about ⅕ per cent) of commercial taka diastase. -After it has stood in a warm place for five or ten minutes test with -Fehling’s solution. The precipitate of cuprous oxide appears showing -that some of the starch has been changed to sugar. By using measured -quantities, and by testing with iodine at frequent intervals, it can -be determined just how long it takes a given quantity of diastase to -change a known quantity of starch. In this connection one should first -test a portion of the same starch with Fehling’s solution to show that -no sugar is present. - -(_b_) Repeat the above experiment using a little tissue from a potato, -and some from a corn seed. - -(_c_) Take 25 germinating barley seeds in which the radicle is just -appearing. Grind up thoroughly in a mortar with about three parts of -water. After this has stood for ten or fifteen minutes, filter. Fill a -test tube one-third full of water, add a piece of starch half the size -of a pea or less, and boil the mixture to make starch-paste. Add the -barley extract. Put in a warm place and test from time to time with -iodine. The first samples so treated will be blue, later ones violet, -brown, and finally colorless, showing that the starch has all -disappeared. This is due to the action of the diastase which was -present in the germinating seeds, and which was dissolved out and added -to the starch mixture. The office of this diastase is to change the -starch in the seeds to sugar. Germinating wheat is sweet, and it is a -matter of common observation that bread made from sprouted wheat is -sweet. - -(_d_) Put a little starch-paste in a test tube and cover it with saliva -from the mouth. After ten or fifteen minutes test with Fehling’s -solution. A strong reaction appears showing how quickly and effectively -saliva acts in converting starch to sugar. Successive tests with iodine -will show the gradual disappearance of the starch. - -=161. These experiments have shown us that diastase= from three -different sources can act upon starch converting it into sugar. The -active principle in the saliva is an _animal_ diastase (_ptyalin_), -which is necessary as one step in the digestion of starch food in -animals. The _taka_ diastase is derived from a fungus (Eurotium oryzæ) -which feeds on the starch in rice grains converting it into sugar -which the fungus absorbs for food. The _malt_ diastase and _leaf_ -diastase are formed by the seed plants. That in seeds converts the -starch to sugar which is absorbed by the embryo for food. That in the -leaf converts the starch into sugar so that it can be transported to -other parts of the plant to be used in building new tissue, or to be -stored again in the form of starch (example, the potato, in seeds, -etc.). The starch is formed in the leaf during the daylight. The light -renders the leaf diastase inactive. But at night the leaf diastase -becomes active and converts the starch made during the day. Starch is -not soluble in water, while the sugar is, and the sugar in solution is -thus easily transported throughout the plant. In those green plants -which do not form starch in their leaves (sugar beet, corn, and many -monocotyledons), grape sugar and fruit sugar are formed in the green -parts as the result of photosynthesis. In some, like the corn, the -grape sugar formed in the leaves is transported to other parts of the -plant, and some of it is stored up in the seed as starch. In others -like the sugar beet the glucose and fruit sugar formed in the leaves -flow to other parts of the plant, and much of it is stored up as cane -sugar in the beet root. The process of photosynthesis probably proceeds -in the same way in all cases up to the formation of the grape sugar and -fruit sugar in the leaves. In the beet, corn, etc., the process stops -here, while in the bean, clover, and most dicotyledons the process is -carried one step farther in the leaf and starch is formed. - - -3. Rough Analysis of Plant Substance. - -=162. Some simple experiments to indicate the nature of plant -substance.=—After these building-up processes of the plant, it is -instructive to perform some simple experiments which indicate roughly -the nature of the plant substance, and serve to show how it can be -separated into other substances, some of them being reduced to the -form in which they existed when the plant took them as food. For exact -experiments and results it would be necessary to make chemical analyses. - -=163. The water in the plant.=—Take fresh leaves or leafy shoots -or other fresh plant parts. Weigh. Permit them to remain in a dry room -until they are what we call “dry.” Now weigh. The plants have lost -weight, and from what we have learned in studies of transpiration this -loss in weight we know to result from the loss of water from the plant. - -=164. The dry plant material contains water.=—Take air-dry -leaves, shavings, or other dry parts of plants. Place them in a test -tube. With a holder rest the tube in a nearly horizontal position, -with the bottom of the tube in the flame of a Bunsen burner. Very -soon, before the plant parts begin to “burn,” note that moisture is -accumulating on the inner surface of the test tube. This is water -driven off which could not escape by drying in air, without the -addition of artificial heat, and is called “hygroscopic water.” - -=165. Water formed on burning the dry plant material.=—Light a -soft-pine or basswood splinter. Hold a thistle tube in one hand with the -bulb downward and above the flame of the splinter. Carbon will be -deposited over the inner surface of the bulb. After a time hold the -tube toward the window and look through it above the carbon. Drops of -water have accumulated on the inside of the tube. This water is formed -by the rearrangement of some of the hydrogen and oxygen, which is set -free by the burning of the plant material, where they were combined -with carbon, as in the cellulose, and with other elements. - -=166. Formation of charcoal by burning.=—Take dried leaves, and -shavings from some soft wood. Place in a porcelain crucible, and cover -about 3 cm. deep with dry fine earth. Place the crucible in the flame -of a Bunsen burner and let it remain for about fifteen minutes. Remove -and empty the contents. If the flame was hot the plant material will be -reduced to a good quality of charcoal. The charcoal consists largely of -carbon. - -=167. The ash of the plant.=—Place in the porcelain crucible -dried leaves and shavings as before. Do not cover with earth. Place the -crucible in the flame of the Bunsen burner, and for a moment place on -the porcelain cover; then remove the cover, and note the moisture on -the under surface from the escaping water. Permit the plant material to -burn; it may even flame for a time. In the course of fifteen minutes it -is reduced to a whitish powder, much smaller in bulk than the charcoal -in the former experiment. This is the ash of the plant. - -=168. What has become of the carbon?=—In this experiment the air -was not excluded from the plant material, so that oxygen combined with -carbon as the water was freed, and formed carbon dioxide, passing off -into the air in this form. This it will be remembered is the form in -which the plant took the carbon-food in through the leaves. Here the -carbon dioxide met the water coming from the soil, and the two united -to form, ultimately, starch, cellulose, and other compounds of carbon; -while with the addition of nitrogen, sulphur, etc., coming also from -the soil, still other plant substances were formed. - -=169.= The carbohydrates are classed among the non-nitrogenous -substances. Other non-nitrogenous plant substances are the organic -acids like oxalic acid (H₂C₂O₄), malic acid (H₂C₄H₄O₅), etc.; the fats -and fixed oils, which occur in the seeds and fruits of many plants. Of -the nitrogenous substances the proteids have a very complex chemical -formula and contain carbon, hydrogen, oxygen, nitrogen, sulphur, etc. -(example, _aleuron_, or proteid grains, found in seeds). The proteids -are the source of nitrogenous food for the seedling during germination. -Of the amides, _asparagin_ (C₄H₈N₂O₃) is an example of a nitrogenous -substance; and of the alkaloids, nicotin (C₁₀H₁₄N₂) from tobacco. - -All living plants contain a large per cent of water. According to -Vines “ripe seeds dried in the air contain 12 to 15 per cent of water, -herbaceous plants 60 to 80 per cent, and many water plants and fungi as -much as 95 per cent of their weight.” When heated to 100° C. the water -is driven off. The dry matter remaining is made up partly of organic -compounds, examples of which are given above, and inorganic compounds. -By burning this dry residue the organic substances are mostly changed -into volatile products, principally carbonic acid, water, and nitrogen. -The inorganic substances as a result of combustion remain as a white or -gray powder, the _ash_. - -The amount of the ash increases with the age of the plant, though the -percentage of ash may vary at different times in the different members -of the plant. The following table taken from Vines will give an idea of -the amount and composition of the ash in the dry solid of a few plants: - - CONTENT OF 1000 PARTS OF DRY SOLID MATTER. - - |Clover, in|Wheat,|Wheat,|Potato|Apples| Peas - | blossom |grain |straw |tubers| |(the seed) - ------------------+----------+------+------+------+------+---------- - Ash. | 68.3 | 19.7 | 53.7 | 37.7 | 14.4 | 27.3 - Potash. | 21.96 | 6.14 | 7.33 | 22.76| 5.14| 11.41 - Soda. | 1.39 | 0.44 | 0.74 | 0.99| 3.76| 0.26 - Lime. | 24.06 | 0.66 | 3.09 | 0.97| 0.59| 1.36 - Magnesium. | 7.44 | 2.36 | 1.33 | 1.77| 1.26| 2.17 - Ferric Oxide. | 0.72 | 0.26 | 0.33 | 0.45| 0.2 | 0.16 - Phosphoric Acid. | 6.74 | 9.26 | 2.58 | 6.53| 1.96| 9.95 - Sulphuric Acid. | 2.06 | 0.07 | 1.32 | 2.45| 0.88| 0.95 - Silica. | 1.62 | 0.42 |36.25 | 0.8 | 0.62| 0.24 - Chlorine. | 2.66 | 0.04 | 0.9 | 1.17| ....| 0.42 - ------------------+----------+------+------+------+------+---------- - -FOOTNOTES: - -[9] Paragraphs 156-160 were prepared by Dr. E. J. Durand. - -[10] Make up three stock solutions as follows: - (1) Copper sulphate 9 grams Water 250 cc. - (2) Caustic potash 30 grams Water 250 cc. - (3) Rochelle salts 49 grams Water 250 cc. - For Fehling’s solution take one volume of each of (1), (2), and (3), - and to the mixture add two volumes of water. - -[11] This solution of taka diastase should be made up cold. If it is - heated to 60° C. or over it is destroyed. - - - - -CHAPTER IX. - -HOW PLANTS OBTAIN THEIR FOOD. I. - - -1. Sources of Plant Food. - -=170. The necessary constituents of plant food.=—As indicated in -Chapter 3, investigation has taught us the principal constituents of -plant food. Some suggestion as to the food substances is derived by a -chemical analysis of various plants. In Chapter 8 it was noted that -there are two principal kinds of compounds in plant substances, the -organic compounds and the inorganic compounds or mineral substances. -The principal elements in the organic compounds are _hydrogen_, -_carbon_, _oxygen_ and _nitrogen_. The elements in the inorganic -compounds which have been found indispensable to plant growth are -_calcium_,[12] _potassium_, _magnesium_, _phosphorus_, _sulphur_ and -_iron_. (See paragraphs 54-58, and complete observations on water -cultures.) Other elements are found in the ash of plants; and while -they are not absolutely necessary for growth, some[13] of them are -beneficial in one way or another. - -=171.= The carbohydrates are derived, as we have learned, from -the CO₂ of the air, and water in the plant tissue drawn from the soil; -though in the case of aquatic plants entirely submerged, all the -constituents are absorbed from the surrounding water. - -=172. Food substances in the soil.=—Land plants derive their -mineral food from the soil, the soil received the mineral substances -from dissolving and disintegrating rocks. Nitrogenous food is chiefly -derived from the same source, but under a variety of conditions which -will be discussed in later paragraphs, but the nitrogen comes primarily -from the air. Some of the mineral substances, those which are soluble -as well as some of the nitrogenous substances, are found in solution in -the soil. These are absorbed by the plant, as needed, along with water, -through the root hairs. - -=173. Absorption of soluble substances.=—Since these substances -are dissolved in the water of the soil, it is not necessary for us -to dwell on the process of absorption. This in general is dwelt upon -in Chapter 3. It should be noted, however, that food substances in -solution, during absorption, diffuse through the protoplasmic membrane -independently of each other and also independently of the rate of -movement of the water from the soil into the root hairs and cells of -the root. - -When the cells have absorbed a certain amount of a given substance, -no more is absorbed until the concentration of the cell-sap in that -particular substance is reduced. This, however, does not interfere -with the absorption of water, or of other substances in solution by -the same cells. Plants have therefore a certain selective power in the -absorption of food substances. - -=174. Action of root hairs on insoluble substances. Acidity of root -hairs.=—If we take a seedling which has been grown in a germinator, -or in the folds of cloths or paper, so that the roots are free from the -soil, and touch the moist root hairs to blue litmus paper, the paper -becomes red in color where the root hairs have come in contact. This -is the reaction for the presence of an acid salt, and indicates that -the root hairs excrete certain acid substances. This acid property of -the root hairs serves a very important function in the preparation -of certain of the elements of plant food in the soil. Certain of the -chemical compounds of potash, phosphoric acid, etc., become deposited -on the soil particles, and are not soluble in water. The acid of the -root hairs dissolves some of these compounds where the particles of -soil are in close contact with them, and the solutions can then be -taken up by the roots. Carbonic acid and other acids are also formed in -the soil, and aid in bringing these substances into solution. - -=175.= This corrosive action of the roots can be shown by the -well-known experiment of growing a plant on a marble plate which is -covered by soil. In lieu of the marble plate, the peas may be planted -in clam or oyster shells, which are then buried in the soil of the -pot, so that the roots of the seedlings will come in contact with the -smooth surface of the shell. After a few weeks, if the soil be washed -from the marble where the roots have been in close contact, there will -be an outline of this part of the root system. Several different acid -substances are excreted from the roots of plants which have been found -to redden blue litmus paper by contact. Experiments by Czapek show, -however, that the carbonic acid excreted by the roots has the power of -directly bringing about these corrosion phenomena. The acid salts are -the substances which are most actively concerned in reddening the blue -litmus paper. They do not directly aid in the corrosion phenomena. In -the soil, however, where these compounds of potash, phosphoric acid, -etc., are which are not soluble in water, the acid salt (primary acid -potassium phosphate) which is most actively concerned in reddening -the blue litmus paper may act indirectly on these mineral substances, -making them available for plant food. This salt soon unites with -certain chlorides in the soil, making among other things small -quantities of hydrochloric acid. - -=176.= NOTE.—It is a general rule that plants cannot -take solid food into their bodies, but obtain all food in either a -liquid or gaseous state. The only exception to this is in the case of -the plasmodia of certain _Myxomycetes_ (Slime Moulds), and also perhaps -some of the Flagellates and other very low forms, which engulf solid -particles of food. It is uncertain, however, whether these organisms -belong to the plant or animal kingdom, and they probably occupy a more -or less intermediate position. - -=177. Action of nitrite and nitrate bacteria.=—Many of the -higher green plants prefer their nitrogenous food in the form of -nitrates. (Example, nitrate of soda, potassium nitrate, saltpetre.) -Nitrates are constantly being formed in soil by the action of certain -bacteria. The nitrite bacteria (Nitromonas) convert ammonia in the -soil to _nitrous acid_ (a _nitrite_), while at this point the nitrate -bacteria (Nitrobacter) convert the nitrites into nitrates. The fact -that this nitrification is going on constantly in soil is of the utmost -importance, for while commercial nitrates are often applied to the -soil, the nitrates are easily washed from the soil by heavy rains. -These nitrite and nitrate bacteria require oxygen for their activity, -and they are able to obtain their carbohydrates by decomposing organic -matter in the soil, or directly by assimilating the CO₂ in the soil, -deriving the energy for the assimilation of the carbon dioxide from -the chemical process of nitrification. This kind of carbon dioxide -assimilation is called _chemosynthetic_ assimilation. - - -2. Parasites and Saprophytes. - -=178. Parasites among the fungi.=—A parasite is an organism -which derives all or a part of its food directly from another living -organism (its host) and at the latter’s expense. The larger number -of plant parasites are found among the fungi (rusts, smuts, mildews, -etc.). (See Nutrition of the Fungi, paragraph 185.) Some of these are -not capable of development unless upon their host, and are called -_obligate_ parasites. Others can grow not only as parasites but at -other times can also grow on dead organic matter, and are called -_facultative_ parasites, i.e. they can choose either a parasitic life -or a saprophytic one. - -=179. Parasites among the seed plants.=—_Cuscuta._—There are, -however, parasites among the seed plants; for example, the dodder -(Cuscuta), parasitic on clover, and a great variety of other plants. -There is food enough in the seed for the young plant to take root and -develop a slender stem until it takes hold of its host. It then twines -around the stem of its host sending wedge-shaped haustoria into the -stem to obtain food. The part then in connection with the ground dies. - -The haustoria of the dodder form a complete junction with the vascular -bundles of its host so that through the vessels water and salts are -obtained, while through the junction of sieve tubes the elaborated -organic food is obtained. The union of the dodder with its host is like -that between a graft and the graft stock. The beech drops (Epiphegus) -is another example of a parasitic seed plant. It is parasitic on the -roots of the beech. - -[Illustration: Fig. 74. Dodder.] - -=180. The mistletoe= (Viscum album), which grows on the branches -of trees, sends its roots into the branches, and only the vessels -of the vascular system are fused according to some. If this is true -then it probably obtains only water and salts from its host. But the -mistletoe has green leaves and is thus able to assimilate carbon -dioxide and manufacture its own organic substances. It is claimed by -some, however, that the host derives some food from the parasite during -the winter when the host has shed its leaves, and if this is true it -would seem that organic food could also be derived during the summer -from the host by the mistletoe. - -=181. Saprophytes.=—A saprophyte is a plant which is enabled -to obtain its food, especially its organic food, directly from dead -animals or plants or from dead organic substances. Many fungi are -saprophytes, as the moulds, mushrooms, etc. (See Nutrition of the -Fungi.) - -=182. Humus saprophytes.=—The action of fungi as described in the -preceding chapter, as well as of certain bacteria, gradually converts -the dead plants or plant parts into the finely powdered brown substance -known as _humus_. In general the green plants cannot absorb organic -food from humus directly. But plants which are devoid of chlorophyll -can live saprophytically on this humus. They are known as _humus -saprophytes_. Many of the mushrooms and other fungi, as well as some -seed plants which lack chlorophyll or possess only a small quantity, -are able to absorb all their organic food from humus. It is uncertain -whether any seed plants can obtain all of their organic food directly -from humus, though it is believed that many can so obtain a portion -of it. But a number of seed plants, like the Indian-pipe (Monotropa) -and certain orchids, obtain organic food from humus. These plants lack -chlorophyll and cannot therefore manufacture their own carbohydrate -food. Not being parasitic on plants which can, as in the case of the -dodder and beech drops mentioned above, they undoubtedly derive their -organic food from the humus. But fungus mycelium growing in the humus -is attached to their roots, and in some orchids enters the roots and -forms a nutritive connection. The fungus mycelium can absorb organic -food from the humus and in some cases at least can transfer it over to -the roots of the higher plant (see Mycorhiza). - -=183. Autotrophic, heterotrophic, and mixotrophic plants.=—An -_autotrophic_ plant is one which is self-nourishing, i.e. it is -provided with an abundant chlorophyll apparatus for carbon dioxide -assimilation and with absorbing organs for obtaining water and salts. -Heterotrophic plants are not provided with a chlorophyll apparatus -sufficient to assimilate all the carbon dioxide necessary, so they -nourish themselves by other means. _Mixotrophic_ plants are those -which are intermediate between the other two, i.e. they have some -chlorophyll but not enough to provide all the organic food necessary, -so they obtain a portion of it by other means. Evidently there are all -gradations of mixotrophic plants between the two other kinds (example, -the mistletoe). - -=184. Symbiosis.=—Symbiosis means a living with or living -together, and is said of those organisms which live so closely in -connection with each other as to be influenced for better or worse, -especially from a nutrition standpoint. _Conjunctive_ symbiosis has -reference to those cases where there is a direct interchange of -food material between the two organisms (lichens, mycorhiza, etc.). -_Disjunctive_ symbiosis has reference to an inter-life relation without -any fixed union between them (example, the relations between flowers -and insects, ants and plants, and even in a broad sense the relation -between saprophytic plants in reducing organic matter to a condition -in which it may be used for food by the green plants, and these in -turn provide organic matter for the saprophytes to feed upon, etc.). -_Antagonistic_ symbiosis is shown in the relation of parasite to its -host, _reciprocal_ symbiosis, or _mutualistic_ symbiosis is shown in -those cases where both symbionts derive food as a result of the union -(lichens, mycorhiza, etc.). - - -3. How Fungi Obtain their Food. - -[Illustration: Fig. 75. Carnation rust on leaf and flower stem. From -photograph.] - -=185. Nutrition of moulds.=—In our study of mucor, as we have -seen, the growing or vegetative part of the plant, the mycelium, lies -within the substratum, which contains the food materials in solution, -and the slender threads are thus bathed on all sides by them. The -mycelium absorbs the watery solutions throughout the entire system of -ramifications. When the upright fruiting threads are developed they -derive the materials for their growth directly from the mycelium with -which they are in connection. The moulds which grow on decaying fruit -or on other organic matter derive their nutrient materials in the same -way. The portion of the mould which we usually see on the surface of -these substances is in general the fruiting part. The larger part of -the mycelium lies hidden within the substratum. - -=186. Nutrition of parasitic fungi.=—Certain of the fungi grow on -or within the higher plants and derive their food materials from them -and at their expense. Such a fungus is called a _parasite_, and there -are a large number of these plants which are known as _parasitic -fungi_. The plant at whose expense they grow is called the “_host_.” - -One of these parasitic fungi, which it is quite easy to obtain in -greenhouses or conservatories during the autumn and winter, is the -carnation rust (_Uromyces caryophyllinus_), since it breaks out in -rusty dark brown patches on the leaves and stems of the carnation (see -fig. 75). If we make thin cross-sections through one of these spots -on a leaf, and place them for a few minutes in a solution of chloral -hydrate, portions of the tissues of the leaf will be dissolved. After -a few minutes we wash the sections in water on a glass slip, and stain -them with a solution of eosin. If the sections were carefully made, and -thin, the threads of the mycelium will be seen coursing between the -cells of the leaf as slender threads. Here and there will be seen short -branches of these threads which penetrate the cell wall of the host and -project into the interior of the cell in the form of an irregular knob. -Such a branch is a _haustorium_. By means of this haustorium, which is -here only a short branch of the mycelium, nutritive substances are -taken by the fungus from the protoplasm or cell-sap of the carnation. -From here it passes to the threads of the mycelium. These in turn -supply food material for the development of the dark brown gonidia, -which we see form the dark-looking powder on the spots. Many other -fungi form haustoria, which take up nutrient matters in the way -described for the carnation rust. In the case of other parasitic fungi -the threads of the mycelium themselves penetrate the cells of the host, -while in still others the mycelium courses only between the cells of -the host (fungus of peach leaf curl for example) and derives food -materials from the protoplasm or cell-sap of the host by the process of -osmosis. - -[Illustration: Fig. 76. Several teleutospores, showing the variations -in form.] - -[Illustration: Fig. 77. Cells from the stem of a rusted carnation, -showing the intercellular mycelium and haustoria. Object magnified 30 -times more than the scale.] - -[Illustration: Fig. 78. Cell from carnation leaf, showing haustorium of -rust mycelium grasping the nucleus of the host. _h_, haustorium; _n_, -nucleus of host.] - -[Illustration: Fig. 79. Intercellular mycelium with haustoria entering -the cells. _A_, of Cystopus candidus (white rust); _B_, of Peronospora -calotheca. (De Bary.)] - -=187. Nutrition of the larger fungi.=—If we select some one of -the larger fungi, the majority of which belong to the mushroom family -and its relatives, which is growing on a decaying log or in the soil, -we shall see on tearing open the log, or on removing the bark or part -of the soil, as the case may be, that the stem of the plant, if it have -one, is connected with whitish strands. During the spring, summer, or -autumn months, examples of the mushrooms connected with these strands -may usually be found readily in the fields or woods, but during the -winter and colder parts of the year often they may be seen in forcing -houses, especially those cellars devoted to the propagation of the -mushroom of commerce. - -=188.= These strands are made up of numerous threads of the -mycelium which are closely twisted and interwoven into a cord or -strand, which is called a mycelium strand, or _rhizomorph_. These are -well shown in fig. 236, which is from a photograph of the mycelium -strands, or “spawn” as the grower of mushrooms calls it, of Agaricus -campestris. The little knobs or enlargements on the strands are the -young fruit bodies, or “buttons.” - -[Illustration: Fig. 80. Sterile mycelium on wood props in coal mine, 400 -feet below surface. - -(Photographed by the author.)] - -=189.= While these threads or strands of the mycelium in the -decaying wood or in the decaying organic matter of the soil are not -true roots, they function as roots, or root hairs, in the absorption -of food materials. In old cellars and on damp soil in moist places we -sometimes see fine examples of this vegetative part of the fungi, the -mycelium. But most magnificent examples are to be seen in abandoned -mines where timber has been taken down into the tunnels far below -the surface of the ground to support the rock roof above the mining -operations. I have visited some of the coal mines at Wilkesbarre, Pa., -and here on the wood props and doors, several hundred feet below the -surface, and in blackest darkness, in an atmosphere almost completely -saturated at all times, the mycelium of some of the wood destroying -fungi grows in a profusion and magnificence which is almost beyond -belief. Fig. 80 is from a flash-light photograph of a beautiful example -400 feet below the surface of the ground. This was growing over the -surface of a wood prop or post, and the picture is much reduced. On -the doors in the mine one can see the strands of the mycelium which -radiate in fan-like figures at certain places near the margin of -growth, and farther back the delicate tassels of mycelium which hang -down in fantastic figures, all in spotless white and rivalling the most -beautiful fabric in the exquisiteness of its construction. - -=190. How fungi derive carbohydrate food.=—The fungi being -devoid of chlorophyll cannot assimilate the CO₂ from the air. They -are therefore dependent on the green plants for their carbohydrate -food. Among the saprophytes, the leaf and wood destroying fungi -excrete certain substances (known as _enzymes_) which dissolve the -carbohydrates and certain other organic compounds in the woody or -leafy substratum in which they grow. They thus produce a sort of -extracellular digestion of carbohydrates, converting them into a -soluble form which can be absorbed by the mycelium. The parasitic -fungi also obtain their carbohydrates and other organic food from the -host. The mycelium of certain parasitic, and of wood destroying fungi, -excretes enzymes (_cytase_) which dissolve minute perforations in the -cell walls of the host and thus aid the hypha during its boring action -in penetrating cell walls. - -NOTE.—Certain wood destroying fungi growing in oaks absorb -tannin directly, i.e. in an unchanged form. One of the pine destroying -fungi (_Trametes pini_) absorbs the xylogen from the wood cells, -leaving the pure cellulose in which the xylogen was filtrated; while -_Polyporus mollis_ absorbs the cellulose, leaving behind only the wood -element. - - -4. Mycorhiza. - -=191.= While such plants as the Indian-pipe (Monotropa), some -of the orchids, etc., are _humus saprophytes_ and some of them are -possibly able to absorb organic food from the humus, many of them have -fungus mycelium in close connection with their roots, and these fungus -threads aid in the absorption of organic food. The roots of plants -which have fungus mycelium intimately associated in connection with -the process of nutrition, are termed _mycorhiza_. There is a mutual -interchange of food between the fungus and the host, a _reciprocal -symbiosis_. - -=192. Mycorhiza are of two kinds= as regards the relation of -the fungus to the root; _ectotrophic_ (or _epiphytic_), where the -mycelium is chiefly on the outside of the root, and _endotrophic_ (or -_endophytic_) where the mycelium is chiefly within the tissue of the -root. - -=193. Ectotrophic mycorhiza.=—Ectotrophic mycorhiza occur on the -roots of the oak, beech, hornbean, etc., in forests where there is a -great deal of humus from decaying leaves and other vegetation. The -young growing roots of these trees become closely covered with a thick -felt of the mycelium, so that no root hairs can develop. The terminal -roots also branch profusely and are considerably thickened. The fungus -serves here as the absorbent organ for the tree. It also acts on the -humus, converting some of it into available plant food and transferring -it over to the tree. - -=194. Endotrophic mycorhiza.=—These are found on many of the -humus saprophytes, which are devoid of chlorophyll, as well as on those -possessing little or even on some plants possessing an abundance, of -chlorophyll. Examples are found in many orchids (see the coral root -orchid, for example), some of the ferns (Botrychium), the pines, -leguminous plants, etc. In endotrophic mycorhiza the mycelium is more -abundant within the tissues of the root, though some of the threads -extend to the outside. In the case of the mycorhiza on the humus -saprophytes which have no chlorophyll, or but little, it is thought -by some that the fungus mycelium in the humus assists in converting -organic substances and carbohydrates into a form available for food -by the higher plant and then conducts it into the root, thus aiding -also in the process of absorption, since there are few or no root -hairs on the short and fleshy mycorhiza. The roots, however, of some -of these humus saprophytes have the power of absorbing a portion of -their organic compounds from the humus. It is thought by some, though -not definitely demonstrated, that in the case of the oaks, beeches, -hornbeans, and other chlorophyll-bearing symbionts, the fungus threads -do not absorb any carbohydrates for the higher symbiont, but that they -actually derive their carbohydrates from it.[14] But it is reasonably -certain that the fungus threads do assimilate from the humus certain -unoxidized, or feebly oxidized, nitrogenous substances (ammonia, for -example), and transfer them over to the host, for the higher plants -with difficulty absorb these substances, while they readily absorb -nitrates which are not abundant in humus. This is especially important -in the forest. It is likely therefore. - - -5. Nitrogen gatherers. - -[Illustration: Fig. 81. Root of the common vetch, showing root -tubercles.] - -=195. How clovers, peas, and other legumes gather nitrogen.=—It -has long been known that clover plants, peas, beans, and many other -leguminous plants are often able to thrive in soil where the cereals -do but poorly. Soil poor in nitrogenous plant food becomes richer in -this substance where clovers, peas, etc., are grown, and they are often -planted for the purpose of enriching the soil. Leguminous plants, -especially in poor soil, are almost certain to have enlargements, in -the form of nodules, or “root-tubercles.” A root of the common vetch -with some of these root-tubercles is shown in fig. 81. - -=196. A fungal or bacterial organism in these root-tubercles.=—If -we cut one of these root-tubercles open, and mount a small portion -of the interior in water for examination with the microscope, we -shall find small rod-shaped bodies, some of which resemble bacteria, -while others are more or less forked into forms like the letter Y, as -shown in fig. 82. These bodies are rich in nitrogenous substances, -or proteids. They are portions of a minute organism, of a fungus or -bacterial nature, which attacks the roots of leguminous plants and -causes these nodular outgrowths. The organism (Phytomyxa leguminosarum) -exists in the soil and is widely distributed where legumes grow. - -=197. How the organism gets into the roots of the legumes.=—This -minute organism in the soil makes its way through the wall of a root -hair near the end. It then grows down the interior of the root hair in -the form of a thread. When it reaches the cell walls it makes a minute -perforation, through which it grows to enter the adjacent cell, when it -enlarges again. In this way it passes from the root hair to the cells -of the root and down to near the center of the root. As soon as it -begins to enter the cells of the root it stimulates the cells of that -portion to greater activity. So the root here develops a large lateral -nodule, or “root-tubercle.” As this “root-tubercle” increases in size, -the fungus threads branch in all directions, entering many cells. The -threads are very irregular in form, and from certain enlargements it -appears that the rod-like bodies are formed, or the thread later breaks -into myriads of these small “bacteroids.” - -[Illustration: Fig. 82. Root-tubercle organism from vetch, old -condition.] - -[Illustration: Fig. 83. Root-tubercle organism from Medicago -denticulata.] - -=198. The root organism assimilates free nitrogen for its -host.=—This organism assimilates the free nitrogen from the air -in the soil, to make the proteid substance which is found stored in -the bacteroids in large quantities. Some of the bacteroids, rich in -proteids, are dissolved, and the proteid substance is made use of by -the clover or pea, as the case may be. This is why such plants can -thrive in soil with a poor nitrogen content. Later in the season some -of the root-tubercles die and decay. In this way some of the proteid -substance is set free in the soil. The soil thus becomes richer in -nitrogenous plant food. - -The forms of the bacteroids vary. In some of the clovers they are oval, -in vetch they are rod-like or forked, and other forms occur in some of -the other genera. - -=199.= NOTE.—So far as we know the legume tubercle -organism does not assimilate free nitrogen of the air unless it is -within the root of the legume. But there are microörganisms in the -soil which are capable of assimilating free nitrogen independently. -Example, a bacterium, _Clostridium pasteurianum_. Certain bacteria -and algæ live in _contact symbiosis_ in the soil, the bacteria fixing -free nitrogen, while in return for the combined nitrogen, the algæ -furnish the bacteria with carbohydrates. It seems that these bacteria -cannot fix the free nitrogen of the air unless they are supplied with -carbohydrates, and it is known that _Clostridium pasteurianum_ cannot -assimilate free nitrogen unless sugar is present. - - -6. Lichens. - -=200. Nutrition of lichens.=—Lichens are very curious plants -which grow on rocks, on the trunks and branches of trees, and on the -soil. They form leaf-like expansions more or less green in color, or -brownish, or gray, or they occur in the form of threads, or small -tree-like formations. Sometimes the plant fits so closely to the rock -on which it grows that it seems merely to paint the rock a slightly -different color, and in the case of many which occur on trees there -appears to be to the eye only a very slight discoloration of the bark -of the trunk, with here and there the darker colored points where fruit -bodies are formed. The most curious thing about them is, however, -that while they form plant bodies of various form, these bodies are -of a “dual nature” as regards the organisms composing them. The plant -bodies, in other words, are formed of two different organisms which, -woven together, exist apparently as one. A fungus on the one hand grows -around and encloses in the meshes of its mycelium the cells or threads -of an alga, as the case may be. - -[Illustration: Fig. 84. Frond of lichen (peltigera), showing rhizoids.] - -If we take one of the leaf-like forms known as peltigera, which grows -on damp soil or on the surfaces of badly decayed logs, we see that the -plant body is flattened, thin, crumpled, and irregularly lobed. The -color is dull greenish on the upper side, while the under side is white -or light gray, and mottled with brown, especially the older portions. -Here and there on the under surface are quite long slender blackish -strands. These are composed entirely of fungus threads and serve as -organs of attachment or holdfasts, and for the purpose of supplying -the plant body with mineral substances which are in solution in the -water of the soil. If we make a thin section of the leaf-like portion -of a lichen as shown in fig. 85, we shall see that it is composed of a -mesh of colorless threads which in certain definite portions contain -entangled green cells. The colorless threads are those of the fungus, -while the green cells are those of the alga. These green cells of the -alga perform the function of chlorophyll bodies for the dual organism, -while the threads of the fungus provide the mineral constituents of -plant food. The alga, while it is not killed in the embrace of the -fungus, does not reach the perfect state of development which it -attains when not in connection with the fungus. On the other hand the -fungus profits more than the alga by this association. It forms fruit -bodies, and perfects spores in the special fruit bodies, which are so -very distinct in the case of so many of the species of the lichens. -These plants have lived for so long a time in this close association -that the fungi are rarely found separate from the algæ in nature, but -in a number of cases they have been induced to grow in artificial -cultures separate from the alga. This fact, and also the fact that the -algæ are often found to occur separate from the fungus in nature, is -regarded by many as an indication that the plant body of the lichens is -composed of two distinct organisms, and that the fungus is parasitic on -the alga. - -[Illustration: Fig. 85. - -Lichen (peltigera), section of thallus; dark zone of rounded bodies -made up largely of the algal cells. Fungus cells above, and threads -beneath and among the algal cells.] - -=201.= Others regard the lichens as autonomous plants, that is, -the two organisms have by this long-continued community of existence -become unified into an individualized organism, which possesses a habit -and mode of life distinct from that of either of the organisms forming -the component parts. This community of existence between two different -organisms is called by some _mutualism_, or _symbiosis_. While the alga -enclosed within the meshes of the fungus is not so free to develop, -and probably does not attain the full development which it would alone -under favorable conditions, still it is very likely that it is often -preserved from destruction during very dry periods, within the tough -thallus, on the surface of bare rocks. - -[Illustration: Fig. 86. Section of fruit body or apothecium of lichen -(parmelia), showing asci and spores of the fungus.] - -FOOTNOTES: - -[12] Calcium is not essential for the growth of the fungi. - -[13] For example, silicon is used by some plants in strengthening -supporting tissues. Buckwheat thrives better when supplied with a -chloride. - -[14] Evidence points to the belief that certain cells of the host form -substances which attract, chemitropically, the fungus threads, and that -in these cells the fungus threads are more abundant than in others. -Furthermore in the vicinity of the nucleus of the host seems to be the -place where these activities are more marked. - - - - -CHAPTER X. - -HOW PLANTS OBTAIN THEIR FOOD, II. - - -Seedlings. - -=202.= It is evident from some of the studies which we have made -in connection with germination of seeds and nutrition of the plant -that there is a period in the life of the seed plants in which they -are able to grow if supplied with moisture, but may entirely lack any -supply of food substance from the outside, though we understand that -growth finally comes to a standstill unless they are supplied with food -from the outside. In connection with the study of the nutrition of the -plant, therefore, it will be well to study some of the representative -seeds and seedlings to learn more accurately the method of germination -and nutrition in seedlings during the germinating period. - -=203. To prepare seeds for germination.=—Soak a handful of -seeds (or more if the class is large) in water for 12 to 24 hours. -Take shallow crockery plates, or ordinary plates, or a germinator -with a fluted bottom. Place in the bottom some sheets of paper, and -if sphagnum moss is at hand scatter some over the paper. If the moss -is not at hand, throw the upper layer of paper into numerous folds. -Thoroughly wet the paper and moss, but do not have an excess of water. -Scatter the seeds among the moss or the folds of the paper. Cover -with some more wet paper and keep in a room where the temperature is -about 20°C. to 25°C. The germinator should be looked after to see that -the paper does not become dry. It may be necessary to cover it with -another vessel to prevent the too rapid evaporation of the water. The -germinator should be started about a week before the seedlings are -wanted for study. Some of the soaked seeds should be planted in soil in -pots and kept at the same temperature, for comparison with those grown -in the germinator. - -[Illustration: Fig. 87. Section of corn seed; at upper right of each is -the plantlet, next the cotyledon, at left the endosperm.] - -=204. Structure of the grain of corn.=—Take grains of corn that -have been soaked in water for 24 hours and note the form and difference -in the two sides (in all of these studies the form and structure of -the seed, as well as the stages in germination, should be illustrated -by the student). Make a longisection of a grain of corn through the -middle line, if necessary making several in order to obtain one which -shows the structures well near the smaller end of the grain. Note -the following structures: 1st, the hard outer “wall” (formed of the -consolidated wall of the ovary with the integuments of the ovules—see -Chapters 35 and 36); 2d, the greater mass of starch and other plant -food (the endosperm) in the centre; 3d, a somewhat crescent-shaped body -(the _scutellum_) lying next the endosperm and near the smaller end of -the grain; 4th, the remaining portion of the young embryo lying between -the scutellum and the seed coat in the depression. When good sections -are made one can make out the radicle at the smaller end of the seed, -and a few successive leaves (the plumule) which lie at the opposite -end of the embryo shown by sharply curved parallel lines. Observe the -attachment of the scutellum to the caulicle at the point of junction -of the plumule and the radicle. The scutellum is a part of the embryo -and represents a cotyledon. The endosperm is also called _albumen_, and -such a seed is _albuminous_. - -Dissect out an embryo from another seed, and compare with that seen in -the section. - -=205.= In the germination of the grain of corn the endosperm -supplies the food for the growth of the embryo until the roots are -well established in the soil and the leaves have become expanded and -green, in which stage the plant has become able to obtain its food from -the soil and air and live independently. The starch in the endosperm -cannot of course be used for food by the embryo in the form of starch. -It is first converted into a soluble form and then absorbed through -the surface of the scutellum or cotyledon and carried to all parts of -the embryo. An enzyme developed by the embryo acts upon the starch, -converting it into a form of sugar which is in solution and can thus -be absorbed. This enzyme is one of the so-called diastatic “ferments” -which are formed during the germination of all seeds which contain food -stored in the form of starch. In some seedlings, this diastase formed -is developed in much greater abundance than in others, for example, -in barley. Examine grains of corn still attached to seedlings several -weeks old and note that a large part of their content has been used up. -The action of diastase on starch is described in Chapter 8. - -=206. Structure of the pumpkin seed.=—The pumpkin seed has a -tough papery outer covering for the protection of the embryo plant -within. This covering is made up of the seed coats. When the seed is -opened by slitting off these coats there is seen within the “meat” -of the pumpkin seed. This is nothing more than the embryo plant. The -larger part of this embryo consists of two flattened bodies which -are more prominent than any other part of the plantlet at this time. -These two flattened bodies are the two first leaves, usually called -_cotyledons_. If we spread these cotyledons apart we see that they are -connected at one end. Lying between them at this point of attachment -is a small bud. This is the _plumule_. The plumule consists of the -very young leaves at the end of the stem which will grow as the seed -germinates. At the other end where the cotyledons are joined is a small -projection, the young root, often termed the _radicle_. - -=207. How the embryo gets out of a pumpkin seed.=—To see how the -embryo gets out of the pumpkin seed we should examine seeds germinated -in the folds of damp paper or on damp sphagnum, as well as some which -have been germinated in earth. Seeds should be selected which represent -several different stages of germination. - -[Illustration: Fig. 88. Germinating seed of pumpkin, showing how the -heel or “peg” catches on the seed coat to cast it off.] - -[Illustration: Fig. 89. Escape of the pumpkin seedling from the seed -coats.] - -=208. The peg helps to pull the seed coats apart.=—The root -pushes its way out from between the stout seed coats at the smaller -end, and then turns downward unless prevented from so doing by a hard -surface. After the root is 2-4_cm_ long, and the two halves of the seed -coats have begun to be pried apart, if we look in this rift at the -junction of the root and stem, we shall see that one end of the seed -coat is caught against a heel, or “peg,” which has grown out from the -stem for this purpose. Now if we examine one which is a little more -advanced, we shall see this heel more distinctly, and also that the -stem is arching out away from the seed coats. As the stem arches up -its back in this way it pries with the cotyledons against the upper -seed coat, but the lower seed coat is caught against this heel, and -the two are pulled gradually apart. In this way the embryo plant pulls -itself out from between the seed coats. In the case of seeds which are -planted deeply in the soil we do not see this contrivance unless we dig -down into the earth. The stem of the seedling arches through the soil, -pulling the cotyledons up at one end. Then it straightens up, the green -cotyledons part, and open out their inner faces to the sunlight, as -shown in fig. 90. If we dig into the soil we shall see that this same -heel is formed on the stem, and that the seed coats are cast off into -the soil. - -[Illustration: Fig. 90. Pumpkin seedling rising from the ground.] - -=209. Parts of the pumpkin seedling.=—During the germination of -the seed all parts of the embryo have enlarged. This increase in size -of a plant is one of the peculiarities of growth. The cotyledons have -elongated and expanded somewhat, though not to such a great extent -as the root and the stem. The cotyledons also have become green on -exposure to the light. Very soon after the main root has emerged from -the seed coats, other lateral roots begin to form, so that the root -soon becomes very much branched. The main root with its branches makes -up the root system of the seedling. Between the expanded cotyledons is -seen the plumule. This has enlarged somewhat, but not nearly so much as -the root, or the part of the stem which extends below the cotyledons. -This part of the stem, i.e., that part below the cotyledons and -extending to the beginning of the root, is called in all seedlings the -_hypocotyl_, which means “below the cotyledon.” - -=210. The common garden bean.=—The common garden bean, or the -lima bean, may be used for study. The garden bean is not so flattened -or broadened as the lima bean. It is rounded compressed, elongate -slightly curved, slightly concave on one side and convex on the other, -and the ends are rounded. At the middle of the concave side note the -distinct scar (the hilum) formed where the bean seed separates from -its attachment to the wall of the pod. Upon one side of this scar is a -slight prominence which is continued for a short distance toward the -end of the bean in the form of a slight ridge. This is the _raphe_, and -represents that part of the stalk of the ovule which is joined to the -side of the ovule when the latter is curved around against it (see -Chapter 36), and at the outer end of the raphe is the _chalaza_, -the point where the stalk is joined to the end of the ovule, best -understood in a straight ovule. Upon the opposite side of the scar -and close to it can be seen a minute depression, the _micropyle_. -Underneath the seed coat and lying between this point and the end of -the seed is the _embryo_, which gives greater prominence to the bean at -this point, but it is especially more prominent after the bean has been -soaked in water. Soak the beans in water and as they are swelling note -how the seed coats swell faster than the inner portion of the seed, -which causes them to wrinkle in a curious way, but finally the inner -portion swells and fills the seed coat out smooth again. Sketch a bean -showing all the external features both in side view and in front. Split -one lengthwise and sketch the half to which the embryo clings, noting -the young root, stem, and the small leaves which were lying between -the cotyledons. There is no endosperm here now, since it was all used -up in the growth of the embryo, and a large part of its substance was -stored up in the cotyledons. As the seed germinates the young plant -gets its first food from that stored in the cotyledons. The hypocotyl -elongates, becomes strongly arched, and at last straightens up, lifting -the cotyledons from the soil. As the cotyledons become exposed to the -light they assume a green color. Some of the stored food in them goes -to nourish the embryo during germination, and they therefore become -smaller, shrivel somewhat, and at last fall off. - -[Illustration: Fig. 91. Garden bean. - -_m_, micropyle; _h_, hilum or scar; _r_, raphe; _c_, point where -chalaza lies.] - -[Illustration: Fig. 92. Bean seed split open to show plantlet.] - -=211. The castor-oil bean.=—This is not a true bean, since it -belongs to a very different family of plants (Euphorbiaceæ). In the -germination of this seed a very interesting comparison can be made with -that of the garden bean. As the “bean” swells the very hard outer coat -generally breaks open at the free end and slips off at the stem end. -The next coat within, which is also hard and shining black, splits -open at the opposite end, that is at the stem end. It usually splits -open in the form of three ribs. Next within the inner coat is a very -thin, whitish film (the remains of the nucellus, and corresponding to -the perisperm) which shrivels up and loosens from the white mass, the -endosperm, within. In the castor-oil bean, then, the endosperm is not -all absorbed by the embryo during the formation of the seed. As the -plant becomes older we should note that the fleshy endosperm becomes -thinner and thinner, and at last there is nothing but a thin, whitish -film covering the green faces of the cotyledons. The endosperm has been -gradually absorbed by the germinating plant through its cotyledons and -used for food. - -[Illustration: Fig. 93. - -How the garden bean comes out of the ground. First the looped -hypocotyl, then the cotyledons pulled out, next casting off the seed -coat, last the plant erect, bearing thick cotyledons, the expanding -leaves, and the plumule between them.] - - -Arisæma triphyllum.[15] - -=212. Germination of seeds of jack-in-the-pulpit.=—The ovaries -of jack-in-the-pulpit form large, bright red berries with a soft pulp -enclosing one to several large seeds. The seeds are oval in form. Their -germination is interesting, and illustrates one type of germination of -seeds common among monocotyledonous plants. If the seeds are covered -with sand, and kept in a moist place, they will germinate readily. - -[Illustration: Fig. 94. Germination of castor-oil bean.] - -=213. How the embryo backs out of the seed.=—The embryo lies -within the mass of the endosperm; the root end, near the smaller end of -the seed. The club-shaped cotyledon lies near the middle of the seed, -surrounded firmly on all sides by the endosperm. The stalk, or petiole, -of the cotyledon, like the lower part of the petiole of the leaves, is -a hollow cylinder, and contains the younger leaves, and the growing end -of the stem or bud. When germination begins, the stalk, or petiole, of -the cotyledon elongates. This pushes the root end of the embryo out -at the small end of the seed. The free end of the embryo now enlarges -somewhat, as seen in the figures, and becomes the bulb, or corm, of the -young plant. At first no roots are visible, but in a short time one, -two, or more roots appear on the enlarged end. - -=214. Section of an embryo.=—If we make a longisection of the -embryo and seed at this time we can see how the club-shaped cotyledon -is closely surrounded by the endosperm. Through the cotyledon, then, -the nourishment from the endosperm is readily passed over to the -growing embryo. In the hollow part of the petiole near the bulb can be -seen the first leaf. - -[Illustration: Fig. 95. Seedlings of castor-oil bean casting the seed -coats, and showing papery remnant of the endosperm.] - -[Illustration: Fig. 96. Seedlings of jack-in-the-pulpit; embryo backing -out of the seed.] - -[Illustration: Fig. 97. - -Section of germinating embryos of jack-in-the-pulpit, showing young -leaves inside the petiole of the cotyledon. At the left cotyledon shown -surrounded by the endosperm in the seed; at right endosperm removed to -show the club-shaped cotyledon.] - -=215. How the first leaf appears.=—As the embryo backs out of -the seed, it turns downward into the soil, unless the seed is so lying -that it pushes straight downward. On the upper side of the arch thus -formed, in the petiole of the cotyledon, a slit appears, and through -this opening the first leaf arches its way out. The loop of the petiole -comes out first, and the leaf later, as shown in fig. 98. The petiole -now gradually straightens up, and as it elongates the leaf expands. - -[Illustration: Fig. 98. Seedlings of jack-in-the-pulpit, first leaf -arching out of the petiole of the cotyledon.] - -[Illustration: Fig. 99. Embryos of jack-in-the-pulpit still attached to -the endosperm in seed coats, and showing the simple first leaf.] - -[Illustration: Fig. 100. Seedling of jack-in-the-pulpit; section of the -endosperm and cotyledon.] - -=216. The first leaf of the jack-in-the-pulpit is a simple -one.=—The first leaf of the embryo jack-in-the-pulpit is very -different in form from the leaves which we are accustomed to see on -mature plants. If we did not know that it came from the seed of this -plant we would not recognize it. It is simple, that is it consists -of one lamina or blade, and not of three leaflets as in the compound -leaf of the mature plant. The simple leaf is ovate and with a broad -heart-shaped base. The jack-in-the-pulpit, then, as trillium, and some -other monocotyledonous plants which have compound leaves on the mature -plants, have simple leaves during embryonic development. The ancestral -monocotyledons are supposed to have had simple leaves. Thus there is in -the embryonic development of the jack-in-the-pulpit, and others with -compound leaves, a sort of recapitulation of the evolutionary history -of the leaf in these forms. - -=216=_a_. =Germination of the pea.=—Compare with the bean. -Note especially that the cotyledons are not lifted above the soil as in -the beans. Compare germination of acorns. - - -Digestion. - -=216=_b_. =To test for stored food substance in the seedlings -studied.=—The pumpkin, squash, and castor-oil bean are examples -of what are called oily seeds, since considerable oil is stored up -in the protoplasm in the cotyledons. To test for this, remove a -small portion of the substance from the cotyledon of the squash and -crush it on a glass slip in a drop or two of osmic acid.[16] Put on a -cover glass and examine with a microscope. The black amorphous matter -shows the presence of oil in the protoplasm. The small bodies which are -stained yellow are _aleurone_ grains, a form of protein or albuminous -substance. Both the oil and the protein substance are used by the -seedling during germination. The oil is converted into an available -food form by the action of an enzyme called _lipase_, which splits up -the fatty oil into glucose and other substances. Lipase has been found -in the endosperm of the castor-oil, cocoanut, and in the cotyledons -of the pumpkin, as well as in other seeds containing oil as a stored -product. The aleurone is made available by an enzyme of the nature of -trypsin. Test the endosperm of the castor-oil bean in the same way. -Make another test of both the squash and castor-oil seeds with iodine -to show that starch is not present. - -Test the cotyledon of the bean with iodine for the presence of -starch. If the endosperm of corn seed has not been tested do so now -with iodine. The endosperm consists largely of starch. The starch is -converted to glucose by a diastatic “ferment” formed by the seedling as -it germinates. Make a thin cross-section of a grain of wheat, including -the seed coat and a portion of the interior, treat with iodine and -mount for microscopic examination. Note the abundance of starch in the -internal portion of endosperm. Note a layer of cells on the outside of -the starch portions filled with small bodies which stain yellow. These -are aleurone grains. The cellulose in the cell walls of the endosperm -is dissolved by another enzyme called _cytase_, and some plants store -up cellulose for food. For example, in the endosperm of the _date_ the -cell walls are very much thickened and pitted. The cell walls consist -of reserve cellulose and the seedling makes use of it for food during -growth. - -=216=_c_. =Albuminous and exalbuminous seeds.=—In seeds -where the food is stored outside of the embryo they are called -_albuminous_; examples, corn, wheat and other cereals, Indian turnip, -etc. In those seeds where the food is stored up in the embryo they are -called _exalbuminous_; examples, bean, pea, pumpkin, squash, etc. - -=217. Digestion= has a well-defined meaning in animal physiology -and relates to the conversion of solid food, usually within the -stomach, into a soluble form by the action of certain gastric juices, -so that the liquid food may be absorbed into the circulatory system. -The term is not often applied in plant physiology, since the method -of obtaining food is in general fundamentally different in plants and -animals. It is usually applied to the process of the conversion of -starch into some form of sugar in solution, as glucose, etc. This we -have found takes place in the leaf, especially at night, through the -action of a diastatic ferment developed more abundantly in darkness. As -a result, the starch formed during the day in the leaves is digested -at night and converted into sugar, in which form it is transferred to -the growing parts to be employed in the making of new tissues, or it is -stored for future use; in other cases it unites with certain inorganic -substances, absorbed by the roots and raised to the leaf, to form -proteids and other organic substances. In tubers, seeds, parts of stems -or leaves where starch is stored, it must first be “digested” by the -action of some enzyme before it can be used as food by the sprouting -tubers or germinating seeds. - -For example, starch is converted to a glucose by the action of a -diastase. Cellulose is converted to a glucose by cytase. Albuminoids -are converted into available food by a tryptic ferment. Fatty oils are -converted into glucose and other products by lipase. - -Inulin, a carbohydrate closely related to starch, is stored up for -food in solution in many composite plants, as in the artichoke, the -root tuber of dahlia, etc. When used for food by the growing plant -it is converted into glucose by an enzyme, inulase. Make a section -of a portion of a dahlia tuber or artichoke and treat with alcohol. -The inulin is precipitated into sphæro crystals. (See also paragraphs -156-161 and 216_b_.) - -=218.= Then there are certain fungi which feed on starch or other -organic substances whether in the host or not, which excrete certain -enzymes to dissolve the starch, etc., to bring it into a soluble -form before they can absorb it as food. Such a process is a sort of -_extracellular digestion_, i.e., the organism excretes the enzyme and -digests the solid outside, since it cannot take the food within its -cells in the solid form. To a certain degree the higher plants perform -also extracellular digestion in the action of root hair excretion on -insoluble substances, and in the case of the humus saprophytes. But for -them soluble food is largely prepared by the action of acids, etc., in -the soil or water, or by the work of fungi and bacteria as described in -Chapter 9. - -=219. Assimilation.=—In plant physiology the term assimilation -has been chiefly used for the process of carbon dioxide assimilation -(= photosynthesis). Some objections have been raised against the use -of assimilation here as one of the life processes of the plant, since -its inception stages are due to the combined action of light, an -external factor, and chlorophyll in the plant along with the living -chloroplastid. So long, however, as it is not known that this process -can take place without the aid of the living plant, it does not seem -proper to deny that it is altogether not a process of assimilation. It -is not necessary to restrict the term assimilation to the formation -of new living matter in the plant cell; it can be applied also to -the synthetic processes in the formation of carbohydrates, proteids, -etc., and called synthetic assimilation. The sun supplies the energy, -which is absorbed by the chlorophyll, for splitting up the carbonic -acid, and the living chloroplast then assimilates by a synthetic -process the carbon, hydrogen, and oxygen. This process then can be -called _photosynthetic assimilation_. The nitrite and nitrate bacteria -derive energy in the process of nitrification, which enables them -to assimilate CO₂ from the air, and this is called _chemosynthetic -assimilation_. The inorganic material in the form of mineral salts, -nitrates, etc., absorbed by the root, and carried up to the leaves, -here meets with the carbohydrates manufactured in the leaf. Under -the influence of the protoplasm synthesis takes place, and proteids -and other organic compounds are built up by the union of the salts, -nitrates, etc., with the carbohydrates. This is also a process of -synthetic assimilation. These are afterward stored as food, or -assimilated by the protoplasm in the making of new living matter, or -perhaps without the first process of synthetic assimilation some of the -inorganic salts, nitrates, and carbohydrates meeting in the protoplasm -are assimilated into new living matter directly. - -FOOTNOTES: - -[15] In lieu of Arisæma make a practical study of the pea. See -paragraph 216_a_. - -[16] Dissolve a half gram of osmic acid in 50 _cc._ of water and keep -tightly corked when not using. - - - - -CHAPTER XI. - -RESPIRATION. - - -=220.= One of the life processes in plants which is extremely -interesting, and which is exactly the same as one of the life processes -of animals, is easily demonstrated in several ways. - -=221. Simple experiment to demonstrate the evolution of CO₂ during -germination.=—Where there are a number of students and a number of -large cylinders are not at hand, take bottles of a pint capacity and -place in the bottom some peas soaked for 12 to 24 hours. Cover with -a glass plate which has been smeared with vaseline to make a tight -joint with the mouth of the bottle. Set aside in a warm place for 24 -hours. Then slide the glass plate a little to one side and quickly -pour in a little baryta water so that it will run down on the inside -of the bottle. Cover the bottle again. Note the precipitate of barium -carbonate which demonstrates the presence of CO₂ in the bottle. Lower a -lighted taper. It is extinguished because of the great quantity of CO₂. -If flower buds are accessible, place a small handful in each of several -jars and treat the same as in the case of the peas. Young growing -mushrooms are excellent also for this experiment, and serve to show -that respiration takes place in the fungi. - -[Illustration: Fig. 101. Test for presence of carbon dioxide in vessel -with germinating peas. (Sachs.)] - -[Illustration: Fig. 102. Apparatus to show respiration of germinating -wheat.] - -=222.= If we now take some of the baryta water and blow our -“breath” upon it the same film will be formed. The carbon dioxide which -we exhale is absorbed by the baryta water, and forms barium carbonate, -just as in the case of the peas. In the case of animals the process by -which oxygen is taken into the body and carbon dioxide is given off is -_respiration_. The process in plants which we are now studying is the -same, and also is respiration. The oxygen in the vessel was partly used -up in the process, and carbon dioxide was given off. (It will be seen -that this process is exactly the opposite of that which takes place in -carbon dioxide assimilation.) - -=223. To show that oxygen from the air is used up while plants -respire.=—Soak some wheat for 24 hours in water. Remove it from -the water and place it in the folds of damp cloth or paper in a moist -vessel. Let it remain until it begins to germinate. Fill the bulb of -a thistle tube with the germinating wheat. By the aid of a stand and -clamp, support the tube upright, as shown in fig. 102. Let the small -end of the tube rest in a strong solution of caustic potash (one stick -caustic potash in two-thirds tumbler of water) to which red ink has -been added to give a deep red color. Place a small glass plate over the -rim of the bulb and seal it air-tight with an abundance of vaseline. -Two tubes can be set up in one vessel, or a second one can be set up in -strong baryta water colored in the same way. - -=224. The result.=—It will be seen that the solution of caustic -potash rises slowly in the tube; the baryta water will also, if that is -used. The solution is colored so that it can be plainly seen at some -distance from the table as it rises in the tube. In the experiment -from which the figure was made for the accompanying illustration, the -solution had risen in 6 hours to the height shown in fig. 102. In 24 -hours it had risen to the height shown in fig. 103. - -=225. Why the solution of caustic potash rises in the -tube.=—Since no air can get into the thistle tube from above or -below, it must be that some part of the air which is inside of the -tube is used up while the wheat is germinating. From our study of -germinating peas, we know that a suffocating gas, carbon dioxide, is -given off while respiration takes place. The caustic potash solution, -or the baryta water, whichever is used, absorbs the carbon dioxide. The -carbon dioxide is heavier than air, and so it settles down in the tube -where it can be absorbed. - -[Illustration: Fig. 103. Apparatus to show respiration of germinating -wheat.] - -[Illustration: Fig. 104. Pea seedlings; the one at the left had no -oxygen and little growth took place, the one at the right in oxygen and -growth was evident.] - -=226. Where does the carbon dioxide come from?=—We know it -comes from the growing seedlings. The symbol for carbon dioxide is -CO₂. The carbon comes from the plant, because there is not enough in -the air. Nitrogen could not join with the carbon to make CO₂. Some -oxygen from the air or from the protoplasm of the growing seedlings -(more probably the latter) joins with some of the carbon of the plant. -These break away from their association with the living substance and -unite, making CO₂. The oxygen absorbed by the plant from the air unites -with the living substance, or perhaps first with food substances, and -from these the plant is replenished with carbon and oxygen. After the -demonstration has been made, remove the glass plate which seals the -thistle tube above, and pour in a small quantity of baryta water. The -white precipitate formed affords another illustration that carbon -dioxide is released. - -[Illustration: Fig. 105. - -Experiment to show that growth takes place more rapidly in presence of -oxygen than in absence of oxygen. The two tubes in the vessel represent -the condition at the beginning of the experiment. At the close of the -experiment the roots in the tube at the left were longer than those -in the tube filled at the start with mercury. The tube outside of -the vessel represents the condition of things where the peas grew in -absence of oxygen; the carbon dioxide given off has displaced a portion -of the mercury. This also shows _anaerobic_ respiration.] - -=227. Respiration is necessary for growth.=—After performing -experiment in paragraph 221, if the vessel has not been open too -long so that oxygen has entered, we may use the vessel for another -experiment, or set up a new one to be used in the course of 12 to 24 -hours, after some oxygen has been consumed. Place some folded damp -filter paper on the germinating peas in the jar. Upon this place -one-half dozen peas which have just been germinated, and in which the -roots are about 20-25 _mm_ long. The vessel should be covered tightly -again and set aside in a warm room. A second jar with water in the -bottom instead of the germinating peas should be set up as a check. -Damp folded filter paper should be supported above the water, and on -this should be placed one-half dozen peas with roots of the same length -as those in the jar containing carbon dioxide. - -=228.= In 24 hours examine and note how much growth has taken -place. It will be seen that the roots have elongated but very little -or none in the first jar, while in the second one we see that the -roots have elongated considerably, if the experiment has been carried -on carefully. Therefore in an atmosphere devoid of oxygen very little -growth will take place, which shows that normal respiration with access -of oxygen (aerobic respiration) is necessary for growth. - -=229. Another way of performing the experiment.=—If we wish we -may use the following experiment instead of the simple one indicated -above. Soak a handful of peas in water for 12-24 hours, and germinate -so that twelve with the radicles 20-25 _mm_ long may be selected. Fill -a test tube with mercury and carefully invert it in a vessel of mercury -so that there will be no air in the upper end. Now nearly fill another -tube and invert in the same way. In the latter there will be some air. -Remove the outer coats from the peas so that no air will be introduced -in the tube filled with the mercury, and insert them one at a time -under the edge of the tube beneath the mercury, six in each tube, -having first measured the length of the radicles. Place in a warm -room. In 24 hours measure the roots. Those in the air will have grown -considerably, while those in the other tube will have grown but little -or none. - -=230. Anaerobic respiration.=—The last experiment is also an -excellent one to show _anaerobic_ respiration. In the tube filled -with mercury so that when inverted there will be no air, it will be -seen after 24 hours that a gas has accumulated in the tube which has -crowded out some of the mercury. With a wash bottle which has an exit -tube properly curved, some water may be introduced in the tube. Then -insert underneath a small stick of caustic potash. This will form a -solution of potash, and the gas will be partly or completely absorbed. -This shows that the gas was carbon dioxide. This evolution of carbon -dioxide by living plants when there is no access of oxygen is anaerobic -respiration (sometimes called intramolecular respiration). It occurs -markedly in oily seeds and especially in the yeast plant. - -[Illustration: Fig. 106. Test for liberation of carbon dioxide from -leafy plant during respiration. Baryta water in smaller vessel. -(Sachs.)] - -=231. Energy set free during respiration.=—From what we have -learned of the exchange of gases during respiration we infer that the -plant loses carbon during this process. If the process of respiration -is of any benefit to the plant, there must be some gain in some -direction to compensate the plant for the loss of carbon which takes -place. - -It can be shown by an experiment that during respiration there is a -slight elevation of the temperature in the plant tissues. The plant -then gains some heat during respiration. Energy is also manifested by -growth. - -=232. Respiration in a leafy plant.=—We may take a potted plant -which has a well-developed leaf surface and place it under a tightly -fitting bell jar. Under the bell jar there also should be placed a -small vessel containing baryta water. A similar apparatus should be set -up, but with no plant, to serve as a check. The experiment must be set -up in a room which is not frequented by persons, or the carbon dioxide -in the room from respiration will vitiate the experiment. The bell jar -containing the plant should be covered with a black cloth to prevent -carbon assimilation. In the course of 10 or 12 hours, if everything has -worked properly, the baryta water under the jar with the plant will -show the film of barium carbonate, while the other one will show none. -Respiration, therefore, takes place in a leafy plant as well as in -germinating seeds. - -=233. Respiration in fungi.=—If several large actively growing -mushrooms are accessible, place them in a tall glass jar as described -for determining respiration in germinating peas. In the course of 12 -hours test with the lighted taper and the baryta water. Respiration -takes place in fungi as well as in green plants. - -=234. Respiration in plants in general.=—Respiration is general -in all plants, though not universal. There are some exceptions in the -lower plants, notably in certain of the bacteria, which can only grow -and thrive in the absence of oxygen. - -[Illustration: Fig. 107. Fermentation tube with culture of yeast.] - -[Illustration: Fig. 108. Fermentation tube filled with CO₂ from action -of yeast in a sugar solution.] - -=235. Respiration a breaking-down process.=—We have seen that -in respiration the plant absorbs oxygen and gives off carbon dioxide. -We should endeavor to note some of the effects of respiration on the -plant. Let us take, say, two dozen dry peas, weigh them, soak for 12-24 -hours in water, and, in the folds of a cloth kept moist by covering -with wet paper or sphagnum, germinate them. When well germinated and -before the green color appears dry well in the sun, or with artificial -heat, being careful not to burn or scorch them. The aim should be to -get them about as dry as the seeds were before germination. Now weigh. -The germinated seeds weigh less than the dry peas. There has then been -a loss of plant substance during respiration. - -[Illustration: Fig. 108_a_. - -Yeast. Saccharomyces ceriviseæ. _a_, small colony; _b_, single cell -budding; _c_, single cell forming an ascus with four spores; _d_, -spores free from the ascus. (After Rees.)] - -=236. Fermentation of yeast.=—Take two fermentation tubes. Fill -the closed tubular parts of each with a weak solution of grape sugar, -or with potato decoction, leaving the open bulb nearly empty. Into the -liquid of one of the tubes place a piece of compressed yeast as large -as a pea. If the tubes are kept in a warm place for 24 hours bubbles -of gas may be noticed rising in the one in which the yeast was placed, -while in the second tube no such bubbles appear, especially if the -filled tubes are first sterilized. The tubes may be kept until the -first is entirely filled with the gas. Now dissolve in the liquid a -small piece of caustic potash. Soon the gas will begin to be absorbed, -and the liquid will rise until it again fills the tube. The gas was -carbon dioxide, which was chiefly produced during the anaerobic -respiration of the rapidly growing yeast cells. In bread making this -gas is produced in considerable quantities, and rising through the -dough fills it with numerous cavities containing gas, so that the bread -“rises.” When it is baked the heat causes the gas in the cavities to -expand greatly. This causes the bread to “rise” more, and baked in this -condition it is “light.” There are two special processes accompanying -the fermentation by yeast: 1st, the evolution of carbon dioxide as -shown above; and, 2d, the formation of alcohol. The best illustration -of this second process is the brewing of beer, where a form of the same -organism which is employed in “bread rising” is used to “brew beer.” - -=237. The yeast plant.=—Before the caustic potash is placed in -the tube some of the fermented liquid should be taken for study of -the yeast plant, unless separate cultures are made for this purpose. -Place a drop of the fermented liquid on a glass slip, place on this -a cover glass, and examine with the microscope. Note the minute oval -cells with granular protoplasm. These are the yeast plant. Note in -some a small “bud” at one side of the end. These buds increase in size -and separate from the parent plant. The yeast plant is one-celled, -and multiplies by “budding” or “sprouting.” It is a fungus, and some -species of yeast like the present one do not form any mycelium. Under -certain conditions, which are not very favorable for growth (example, -when the yeast is grown in a weak nutrient substance on a thin layer of -a plaster Paris slab), several spores are formed in many of the yeast -cells. After a period of rest these spores will sprout and produce the -yeast plant again. Because of this peculiar spore formation some place -the yeast among the sac fungi. (See classification of the fungi.) - -=238. Organized ferments and unorganized ferments.=—An organism -like the yeast plant which produces a fermentation of a liquid with -evolution of gas and alcohol is sometimes called a _ferment_, or -_ferment organism_, or an _organized_ ferment. On the other hand the -diastatic ferments or enzymes like diastase, taka diastase, animal -diastase (ptyalin in the saliva), cytase, etc., are _unorganized_ -ferments. In the case of these it is better to say _enzyme_ and leave -the word ferment for the ferment organisms. - -=239. Importance of green plants in maintaining purity of air.=—By -respiration, especially of animals, the air tends to become “foul” -by the increase of CO₂. Green plants, i.e., plants with chlorophyll, -purify the air during photosynthesis by absorbing CO₂ and giving off -oxygen. Animals absorb in respiration large quantities of oxygen and -exhale large quantities of CO₂. Plants absorb a comparatively small -amount of oxygen in respiration and give off a comparatively small -amount of CO₂. But they absorb during photosynthesis large quantities -of CO₂ and give off large quantities of oxygen. In this way a balance is -maintained between the two processes, so that the percentage of CO₂ in -the air remains approximately the same, viz., about four-tenths of one -per cent, while there are approximately 21 parts oxygen and 79 parts -nitrogen. - -=239a. Comparison of respiration and photosynthesis.= - - { Carbon dioxide is taken in by the plant and - { oxygen is liberated. - { Starch is formed as a result of the metabolism, - { or chemical change. - { The process takes place only in green plants, - Starch formation { and in the green parts of plants, that is, - or Photosynthesis. { in the presence of the chlorophyll. - { (Exception in purple bacterium.) - { The process only takes place under the - { influence of sunlight. - { It is a building-up process, because new plant - { substance is formed. - - { Oxygen is taken in by the plant and carbon - { dioxide is liberated. - { Carbon dioxide is formed as a result of the - { metabolism, or chemical change. - { The process takes place in all plants whether - Respiration. { they possess chlorophyll or not. - { (Exceptions in anaerobic bacteria). - { The process takes place in the dark as well as - { in the sunlight. - { It is a breaking-down process, because - { disintegration of plant substance occurs. - - - - -CHAPTER XII. - -GROWTH. - - -By growth is usually meant an increase in the bulk of the plant -accompanied generally by an increase in plant substance. Among the -lower plants growth is easily studied in some of the fungi. - -=240. Growth in mucor.=—Some of the gonidia (often called spores) -may be sown in nutrient gelatine or agar, or even in prune juice. If -the culture has been placed in a warm room, in the course of 24 hours, -or even less, the preparation will be ready for study. - -=241. Form of the gonidia.=—It will be instructive if we first -examine some of the gonidia which have not been sown in the culture -medium. We should note their rounded or globose form, as well as -their markings if they belong to one of the species with spiny walls. -Particularly should we note the size, and if possible measure them with -the micrometer, though this would not be absolutely necessary for a -comparison, if the comparison can be made immediately. Now examine some -of the gonidia which were sown in the nutrient medium. If they have not -already germinated we note at once that they are much larger than those -which have not been immersed in a moist medium. - -=242. The gonidia absorb water and increase in size before -germinating.=—From our study of the absorption of water or watery -solutions of nutriment by living cells, we can easily understand the -cause of this enlargement of the gonidium of the mucor when surrounded -by the moist nutrient medium. The cell-sap in the spore takes up more -water than it loses by diffusion, thus drawing water forcibly through -the protoplasmic membrane. Since it does not filter out readily, the -increase in quantity of the water in the cell produces a pressure from -within which stretches the membrane, and the elastic cell wall yields. -Thus the gonidium becomes larger. - -[Illustration: Fig. 109. Spores of mucor, and different stages of -germination.] - -=243. How the gonidia germinate.=—We should find at this time -many of the gonidia extended on one side into a tube-like process the -length of which varies according to time and temperature. The short -process thus begun continues to elongate. This elongation of the plant -is _growth_, or, more properly speaking, one of the phenomena of growth. - -=244. The germ tube branches and forms the mycelium.=—In the -course of a day or so branches from the tube will appear. This branched -form of the threads of the fungus is, as we remember, the mycelium. We -can still see the point where growth started from the gonidium. Perhaps -by this time several tubes have grown from a single one. The threads of -the mycelium near the gonidium, that is, the older portions of them, -have increased in diameter as they have elongated, though this increase -in diameter is by no means so great as the increase in length. After -increasing to a certain extent in diameter, growth in this direction -ceases, while apical growth is practically unlimited, being limited -only by the supply of nutriment. - -=245. Growth in length takes place only at the end of the thread.=—If -there were any branches on the mycelium when the culture was first -examined, we can now see that they remain practically the same distance -from the gonidium as when they were first formed. That is, the older -portions of the mycelium do not elongate. Growth in length of the -mycelium is confined to the ends of the threads. - -=246. Protoplasm increases by assimilation of nutrient -substances.=—As the plant increases in bulk we note that there -is an increase in the protoplasm, for the protoplasm is very easily -detected in these cultures of mucor. This increase in the quantity -of the protoplasm has come about by the assimilation of the nutrient -substance, which the plant has absorbed. The increase in the -protoplasm, or the formation of additional plant substance, is another -phenomenon of growth quite different from that of elongation, or -increase in bulk. - -=247. Growth of roots.=—For the study of the growth of roots we -may take any one of many different plants. The seedlings of such plants -as peas, beans, corn, squash, pumpkin, etc., serve excellently for this -purpose. - -=248. Roots of the pumpkin.=—The seeds, a handful or so, are -soaked in water for about 12 hours, and then placed between layers of -paper or between the folds of cloth, which must be kept quite moist but -not very wet, and should be kept in a warm place. A shallow crockery -plate, with the seeds lying on wet filter paper, and covered with -additional filter paper, or with a bell jar, answers the purpose well. - -The primary or first root (radicle) of the embryo pushes its way out -between the seed coats at the small end. When the seeds are well -germinated, select several which have the root 4-5 _cm_ long. With a -crow-quill pen we may now mark the terminal portion of the root off -into very short sections as in fig. 110. The first mark should be -not more than 1 _mm_ from the tip, and the others not more than 1mm -apart. Now place the seedlings down on damp filter paper, and cover -with a bell jar so that they will remain moist, and if the season is -cold place them in a warm room. At intervals of 8 or 10 hours, if -convenient, observe them and note the farther growth of the root. - -[Illustration: Fig. 110. Root of germinating pumpkin, showing region of -elongation just back of the tip.] - -=249. The region of elongation.=—While the root has elongated, -the region of elongation _is not at the tip of the root. It lies a -little distance back from the tip_, beginning at about 2mm from the tip -and extending over an area represented by from 4-5 of the millimeter -marks. The root shown in fig. 110 was marked at 10 A.M. on -July 5. At 6 P.M. of the same day, 8 hours later, growth had -taken place as shown in the middle figure. At 9 A.M. on the -following day, 15 hours later, the growth is represented in the lower -one. Similar experiments upon a number of seedlings give the same -result: the region of elongation in the growth of the root is situated -a little distance back from the tip. Farther back very little or no -elongation takes place, but growth in diameter continues for some time, -as we should discover if we examined the roots of growing pumpkins, or -other plants, at different periods. - -=250. Movement of region of greatest elongation.=—In the region -of elongation the areas marked off do not all elongate equally at the -same time. The middle spaces elongate most rapidly and the spaces -marked off by the 6, 7, and 8 _mm_ marks elongate slowly, those -farthest from the tip more slowly than the others, since elongation -has nearly ceased here. The spaces marked off between the 2-4 _mm_ -marks also elongate slowly, but soon begin to elongate more rapidly, -since that region is becoming the region of greatest elongation. Thus -the region of greatest elongation moves forward as the root grows, and -remains approximately at the same distance behind the tip. - -=251. Formative region.=—If we make a longitudinal section of the -tip of a growing root of the pumpkin or other seedling, and examine it -with the microscope, we see that there is a great difference in the -character of the cells of the tip and those in the region of elongation -of the root. First there is in the section a V-shaped cap of loose -cells which are constantly being sloughed off. Just back of this tip -the cells are quite regularly isodiametric, that is, of equal diameter -in all directions. They are also very rich in protoplasm, and have -thin walls. This is the region of the root where new cells are formed -by division. It is the _formative region_. The cells on the outside -of this area are the older, and pass over into the older parts of the -root and root cap. If we examine successively the cells back from this -_formative_ region we find that they become more and more elongated in -the direction of the axis of the root. The elongation of the cells in -this older portion of the root explains then why it is that this region -of the root elongates more rapidly than the tip. - -=252. Growth of the stem.=—We may use a bean seedling growing -in the soil. At the junction of the leaves with the stem there are -enlargements. These are the _nodes_, and the spaces on the stem between -successive nodes are the _internodes_. We should mark off several of -these internodes, especially the younger ones, into sections about 5 -_mm_ long. Now observe these at several times for two or three days, -or more. The region of elongation is greater than in the case of the -roots, and extends back farther from the end of the stem. In some young -garden bean plants the region of elongation extended over an area of 40 -_mm_ in one internode. See also Chapters 38, 39. - -=253. Force exerted by growth.=—One of the marvelous things -connected with the growth of plants is the force which is exerted by -various members of the plant under certain conditions. Observations on -seedlings as they are pushing their way through the soil to the air -often show us that considerable force is required to lift the hard soil -and turn it to one side. A very striking illustration may be had in the -case of mushrooms which sometimes make their way through the hard and -packed soil of walks or roads. That succulent and tender plants should -be capable of lifting such comparatively heavy weights seems incredible -until we have witnessed it. Very striking illustrations of the force -of roots are seen in the case of trees which grow in rocky situations, -where rocks of considerable weight are lifted, or small rifts in large -rocks are widened by the lateral pressure exerted by the growth of a -root, which entered when it was small and wedged its way in. - -=254. Zone of maximum growth.=—Great variation exists in the -rapidity of growth even when not influenced by outside conditions. In -our study of the elongation of the root we found that the cells just -back of the formative region elongated slowly at first. The rapidity of -the elongation of these cells increases until it reaches the maximum. -Then the rapidity of elongation lessens as the cells come to lie -farther from the tip. The period of maximum elongation here is the -_zone of maximum growth_ of these cells. - -[Illustration: Fig. 111. Lever auxanometer (Oels) for measuring -elongation of the stem during growth.] - -=255.= Just as the cells exhibit a zone of maximum growth, so -the members of the plant exhibit a similar zone of maximum growth. -In the case of leaves, when they are young the rapidity of growth -is comparatively slow, then it increases, and finally diminishes in -rapidity again. So it is with the stem. When the plant is young the -growth is not so rapid; as it approaches middle age the rapidity of -growth increases; then it declines in rapidity at the close of the -season. - -=256. Energy of growth.=—Closely related to the zone of maximum -growth is what is termed the energy of growth. This is manifested in -the comparative size of the members of a given plant. To take the -sunflower for example, the lower and first leaves are comparatively -small. As the plant grows larger the leaves are larger, and this -increase in size of the leaves increases up to a maximum period, when -the size decreases until we reach the small leaves at the top of the -stem. The zone of maximum growth of the leaves corresponds with the -maximum size of the leaves on the stem. The rapidity and energy of -growth of the stem is also correlated with that of the leaves, and the -zone of maximum growth is coincident with that of the leaves. It would -be instructive to note it in the case of other plants and also in the -case of fruits. - -=257. Nutation.=—During the growth of the stem all of the cells -of a given section of the stem do not elongate simultaneously. For -example the cells at a given moment on the south side are elongating -more rapidly than the cells on the other side. This will cause the -stem to bend slightly to the north. In a few moments later the cells -on the west side are elongating more rapidly, and the stem is turned -to the east; and so on, groups of cells in succession around the stem -elongate more rapidly than the others. This causes the stem to describe -a circle or ellipse about a central point. Since the region of greatest -elongation of the cells of the stem is gradually moving toward the apex -of the growing stem, this line of elongation of the cells which is -traveling around the stem does so in a spiral manner. In the same way, -while the end of the stem is moving upward by the elongation of the -cells, and at the same time is slowly moved around, the line which the -end of the stem describes must be a spiral one. This movement of the -stem, which is common to all stems, leaves, and roots, is _nutation_. - -=258.= The importance of nutation to twining stems in their search -for a place of support, as well as for the tendrils on leaves or stems, -will be seen. In the case of the root it is of the utmost importance, -as the root makes its way through the soil, since the particles of soil -are more easily thrust aside. The same is also true in the case of many -stems before they emerge from the soil. - - - - -CHAPTER XIII. - -IRRITABILITY. - - -=259.= We should now examine the movements of plant parts in -response to the influence of certain stimuli. By this time we have -probably observed that the direction which the root and stem take upon -germination of the seed is not due to the position in which the seed -happens to lie. Under normal conditions we have seen that the root -grows downward and the stem upward. - -=260. Influence of the earth on the direction of growth.=—When -the stem and root have been growing in these directions for a short -time let us place the seedling in a horizontal position, so that the -end of the root extends over an object of support in such a way that it -will be free to go in any direction. It should be pinned to a cork and -placed in a moist chamber. In the course of twelve to twenty-four hours -the root which was formerly horizontal has turned the tip downward -again. If we should mark off millimeter spaces beginning at the tip of -the root, we should find that the motor zone, or region of curvature, -lies in the same region as that of the elongation of the root. - -Knight found that the stimulus which influences the root to turn -downward is the force of gravity. The reaction of the root in response -to this stimulus is geotropism, a turning influenced by the earth. This -term is applied to the growth movements of plants influenced by the -earth with regard to direction. While the motor zone lies back of the -root-tip, the latter receives the stimulus and is the perceptive zone. -If the root-tip is cut off, the root is no longer geotropic, and will -not turn downward when placed in a horizontal position. Growth toward -the earth is _progeotropism_. The lateral growth of secondary roots is -_diageotropism_. - -[Illustration: Fig. 112. Germinating pea placed in a horizontal -position.] - -[Illustration: Fig. 113. In 24 hours gravity has caused the root to -turn downward. - -Figs. 112, 113.—Progeotropism of the pea root.] - -The stem, on the other hand, which was placed in a horizontal position -has become again erect. This turning of the stem in the upward -direction takes place in the dark as well as in the light, as we can -see if we start the experiment at nightfall, or place the plant in the -dark. This upward growth of the stem is also influenced by the earth, -and therefore is a case of geotropism. The special designation in the -case of upright stems is _negative geotropism_, or _apogeotropism_, or -the stems are said to be _apogeotropic_. If we place a rapidly growing -potted plant in a horizontal position by laying the pot on its side, -the ends of the shoots will soon turn upward again when placed in a -horizontal position. Young bean plants growing in a pot began within -two hours to turn the ends of the shoots upward. - -[Illustration: Fig. 114. Pumpkin seedling showing apogeotropism. -Seedling at the left placed horizontally, in 24 hours the stem has -become erect.] - -Horizontal leaves and shoots can be shown to be subject to the same -influence, and are therefore _diageotropic_. - -=261. Influence of light.=—Not only is light a very important -factor for plants during photosynthesis, it exerts great influence on -plant growth and movement. - -[Illustration: Fig. 115. Radish seedlings grown in the dark, long, -slender, not green.] - -[Illustration: Fig. 116. Radish seedlings grown in the light, shorter, -stouter, and green in color. Growth retarded by light.] - -=262. Growth in the absence of light.=—Plants grown in the dark -are subject to a number of changes. The stems are often longer, -more slender and weaker since they contain a larger amount of water -in proportion to building material which the plant obtains from -carbohydrates manufactured in the light. On many plants the leaves are -very small when grown in the dark. - -=263. Influence of light on direction of growth.=—While we are -growing seedlings, the pots or boxes of some of them should be placed -so that the plants will have a one-sided illumination. This can be -done by placing them near an open window, in a room with a one-sided -illumination, or they may be placed in a box closed on all sides but -one which is facing the window or light. In 12-24 hours, or even in a -much shorter time in some cases, the stems of the seedlings will be -directed toward the source of light. This influence exerted by the rays -of light is _heliotropism_, a turning influenced by the sun or sunlight. - -[Illustration: Fig. 117. Seedling of castor-oil bean, before and after -a one-sided illumination.] - -[Illustration: Fig. 118. Dark chamber with opening at one side to show -heliotropism. (After Schleichert.)] - -=264. Diaheliotropism.=—Horizontal leaves and shoots are -_diaheliotropic_ as well as _diageotropic_. The general direction which -leaves assume under this influence is that of placing them with the -upper surface perpendicular to the rays of light which fall upon them. -Leaves, then, exposed to the brightly lighted sky are, in general, -horizontal. This position is taken in direct response to the stimulus -of light. The leaves of plants with a one-sided illumination, as can be -seen by trial, are turned with their upper surfaces toward the source -of light, or perpendicular to the incidence of the light rays. In this -way light overcomes for the time being the direction which growth -gives to the leaves. The so-called “sleep” of plants is of course -not sleep, though the leaves “nod,” or hang downward, in many cases. -There are many plants in which we can note this drooping of the leaves -at nightfall, and in order to prove that it is not determined by the -time of day we can resort to a well-known experiment to induce this -condition during the day. The plant which has been used to illustrate -this is the sunflower. Some of these plants, which were grown in a box, -when they were about 35 _cm_ high were covered for nearly two days, -so that the light was excluded. At midday on the second day the box -was removed, and the leaves on the covered plants are well represented -by fig. 119, which was made from one of them. The leaves of the other -plants in the box which were not covered were horizontal, as shown by -fig. 120. Now on leaving these plants, which had exhibited induced -“sleep” movements, exposed to the light they gradually assumed the -horizontal position again. - -[Illustration: Fig. 119. Sunflower plant. Epinastic condition of leaves -induced during the day in darkness.] - -[Illustration: Fig. 120. Sunflower plant removed from darkness, leaves -extending under influence of light (diaheliotropism.)] - -=265. Epinasty and hyponasty.=—During the early stages of growth -of many leaves, as in the sunflower plant, the direction of growth is -different from what it is at a later period. The under surface of the -young leaves grows more rapidly in a longitudinal direction than the -upper side, so that the leaves are held upward close against the bud -at the end of the stem. This is termed _hyponasty_, or the leaves are -said to be _hyponastic_. Later the growth is more rapid on the upper -side and the leaves turn downward or away from the bud. This is termed -_epinasty_, or the leaves are said to be _epinastic_. This is shown by -the night position of the leaves, or in the induced “sleep” of the -sunflower plant in the experiment detailed above. The day position of -the leaves on the other hand, which is more or less horizontal, is -induced because of their irritability under the influence of light, the -inherent downward or epinastic growth is overcome for the time. Then -at nightfall or in darkness, the stimulus of light being removed, the -leaves assume the position induced by the direction of growth. - -[Illustration: Fig. 121. Squash seedling. Position of cotyledons in -light.] - -[Illustration: Fig. 122. Squash seedling. Position of cotyledons in the -dark.] - -=266.= In the case of the cotyledons of some plants it would seem -that the growth was hyponastic even after they have opened. The day -position of the cotyledons of the pumpkin is more or less horizontal, -as shown in fig. 121. At night, or if we darken the plant by covering -with a tight box, the leaves assume the position shown in fig. 122. - -While the horizontal position is the general one which is assumed -by plants under the influence of light, their position is dependent -to a certain extent on the intensity of the light as well as on the -incidence of the light rays. Some plants are so strongly heliotropic -that they change their positions all during the day. - -[Illustration: Fig. 123. Coiling tendril of bryony.] - -=267. Leaves with a fixed diurnal position.=—Leaves of some -plants when they are developed have a fixed diurnal position and are -not subject to variation. Such leaves tend to arrange themselves in a -vertical or paraheliotropic position, in which the surfaces are not -exposed to the incidence of light of the greatest intensity, but to the -incidence of the rays of diffused light. Interesting cases of the fixed -position of leaves are found in the so-called compass plants (like -Silphium laciniatum, Lactuca scariola, etc.). In these the horizontal -leaves arrange themselves with the surfaces vertical, and also pointing -north and south, so that the surfaces face east and west. - -=268. Importance of these movements.=—Not only are the leaves -placed in a position favorable for the absorption of the rays of light -which are concerned in making carbon available for food, but they -derive other forms of energy from the light, as heat, which is absorbed -during the day. Then with the nocturnal position, the leaves being -drooped down toward the stem, or with the margin toward the sky, or -with the cotyledons as in the pumpkin, castor-oil bean, etc., clasped -upward together, the loss of heat by radiation is less than it would be -if the upper surfaces of the leaves were exposed to the sky. - -=269. Influence of light on the structure of the leaf.=—In our -study of the structure of a leaf we found that in the ivy leaf the -palisade cells were on the upper surface. This is the case with a great -many leaves, and is the normal arrangement of “dorsiventral” leaves -which are diaheliotropic. Leaves which are paraheliotropic tend to have -palisade cells on both surfaces. The palisade layer of cells as we -have seen is made up of cells lying very close together, and they thus -prevent rapid evaporation. They also check to some extent the entrance -of the rays of light, at least more so than the loose spongy parenchyma -cells do. Leaves developed in the shade have looser palisade and -parenchyma cells. In the case of some plants, if we turn over a very -young leaf, so that the under side will be uppermost, this side will -develop the palisade layer. This shows that light has a great influence -on the structure of the leaf. - -=270. Movement influenced by contact.=—In the case of tendrils, -twining leaves, or stems, the irritability to contact is shown in a -movement of the tendril, etc., toward the object in touch. This causes -the tendril or stem to coil around the object for support. The stimulus -is also extended down the part of the tendril below the point of -contact (see fig. 123), and that part coils up like a wire coil spring, -thus drawing the leaf or branch from which the tendril grows closer to -the object of support. This coil between the object of support and the -plant is also very important in easing up the plant when subject to -violent gusts of wind which might tear the plant from its support were -it not for the yielding and springing motion of this coil. - -[Illustration: Fig. 124. Sensitive plant leaf in normal position.] - -[Illustration: Fig. 125. Pinnæ folding up after stimulus.] - -[Illustration: Fig. 126. Later all the pinnæ folded and leaf drooped.] - -=271. Sensitive plants.=—These plants are remarkable for the -rapid response to stimuli. Mimosa pudica is an excellent plant to study -for this purpose. - -=272. Movement in response to stimuli.=—If we pinch with the -forceps one of the terminal leaflets, or tap it with a pencil, the two -end leaflets fold above the “vein” of the pinna. This is immediately -followed by the movement of the next pair, and so on as shown in fig. -125, until all the leaflets on this pinna are closed, then the stimulus -travels down the other pinnæ in a similar manner, and soon the pinnæ -approximate each other and the leaf then drops downward as shown in -fig. 126. The normal position of the leaf is shown in fig. 124. If we -jar the plant by striking it or by jarring the pot in which it is grown -all the leaves quickly collapse into the position shown in fig. 126. -If we examine the leaf now we see minute cushions at the base of each -leaflet, at the junction of the pinnæ with the petiole, and a larger -one at the junction of the petiole with the stem. We shall also note -that the movement resides in these cushions. - -=273. Transmission of the stimulus.=—The transmission of the -stimulus in this mimosa from one part of the plant has been found to be -along the cells of the bast. - -=274. Cause of the movement.=—The movement is caused by a sudden -loss of turgidity on the part of the cells in one portion of the -pulvinus, as the cushion is called. In the case of the large pulvinus -at the base of the petiole this loss of turgidity is in the cells of -the lower surface. There is a sudden change in the condition of the -protoplasm of the cells here so that they lose a large part of their -water. This can be seen if with a sharp knife we cut off the petiole -just above the pulvinus before movement takes place. A drop of liquid -exudes from the cells of the lower side. - -=275. Paraheliotropism of the leaves of the sensitive plant.=—If -the mimosa plant is placed in very intense light the leaflets will -turn their edges toward the incidence of the rays of light. This is -also true of other plants in intense light, and is _paraheliotropism_. -Transpiration is thus lessened, and chlorophyll is protected from too -intense light. - -[Illustration: Fig. 126_a_. Leaf of Venus fly-trap (Dionæa muscipula), -showing winged petiole and toothed lobes.] - -[Illustration: Fig. 127. Leaf of Drosera rotundifolia, some of the -glandular hairs folding inward as a result of a stimulus.] - -We thus see that variations in the intensity of light have an important -influence in modifying movements. Variations in temperature also exert -a considerable influence, rapid elevation of temperature causing -certain flowers to open, and falling temperature causing them to close. - -=276. Sensitiveness of insectivorous plants.=—The Venus fly-trap -(Dionæa muscipula) and the sundew (drosera) are interesting examples of -sensitive plants, since the leaves close in response to the stimulus -from insects. - -=277. Hydrotropism.=—Roots are sensitive to moisture. They will -turn toward moisture. This is of the greatest importance for the -well-being of the plant, since the roots will seek those places in the -soil where suitable moisture is present. On the other hand, if the soil -is too wet there is a tendency for the roots to grow away from the -soil which is saturated with water. In such cases roots are often seen -growing upon the surface of the soil so that they may obtain oxygen, -which is important for the root in the processes of absorption and -growth. Plants then may be injured by an excess of water as well as by -a lack of water in the soil. - -=278. Temperature.=—In the experiments on germination thus -far made it has probably been noted that the temperature has much -to do with the length of time taken for seeds to germinate. It also -influences the rate of growth. The effect of different temperatures -on the germination of seed can be very well noted by attempting to -germinate some in rooms at various temperatures. It will be found, -other conditions being equal, that in a moderately warm room, or even -in one quite warm, 25-30 degrees centigrade, germination and growth -goes on more rapidly than in a cool room, and here more rapidly than in -one which is decidedly cold. In the case of most plants in temperate -climates, growth may go on at a temperature but little above freezing, -but few will thrive at this temperature. - -=279.= If we place dry peas or beans in a temperature of about -70° C. for 15 minutes they will not be killed, but if they have been -thoroughly soaked in water and then placed at this temperature they -will be killed, or even at a somewhat lower temperature. The same seeds -in the dry condition will withstand a temperature of 10° C. below, but -if they are first soaked in water this low temperature will kill them. - -=280.= In order to see the effect of freezing we may thoroughly -freeze a section of a beet root, and after thawing it out place it in -water. The water is colored by the cell-sap which escapes from the -cells, just as we have seen it does as a result of a high temperature, -while a section of an unfrozen beet placed in water will not color it -if it was previously washed. - -If the slice of the beet is placed at about -6° C. in a shallow glass -vessel, and covered, ice will be formed over the surface. If we examine -it with the microscope ice crystals will be seen formed on the outside, -and these will not be colored. The water for the formation of the -crystals came from the cell-sap, but the concentrated solutions in the -sap were not withdrawn by the freezing over the surface. - -=281.= If too much water is not withdrawn from the cells of many -plants in freezing, and they are thawed out slowly, the water which was -withdrawn from the cells will be absorbed again and the plant will not -be killed. But if the plant is thawed out quickly the water will not -be absorbed, but will remain on the surface and evaporate. Some will -also remain in the intercellular spaces, and the plant will die. Some -plants, however, no matter how slowly they are thawed out, are killed -after freezing, as the leaves of the pumpkin, dahlia, or the tubers of -the potato. - -=282.= It has been found that as a general rule when plants, or -plant parts, contain little moisture they will withstand quite high -degrees of temperature, as well as quite low degrees, but when the -parts are filled with sap or water they are much more easily killed. -For this reason dry seeds and the winter buds of trees, and other -plants, because they contain but little water, are better able to -resist the cold of winters. But when growth begins in the spring, and -the tissues of these same parts become turgid and filled with water, -they are quite easily killed by frosts. It should be borne in mind, -however, that there is great individual variation in plants in this -respect, some being more susceptible to cold than others. There is -also great variation in plants as to their resistance to the cold of -winters, and of arctic climates, the plants of the latter regions -being able to resist very low temperatures. We have examples also in -the arctic plants, and those which grow in arctic climates on high -mountains, of plants which are able to carry on all the life functions -at temperatures but little above freezing. - -For further discussion as to relation of plants to temperature, see -Chapters 46, 48, 49, and 53. - - - - -PART II. - -MORPHOLOGY AND LIFE HISTORY OF REPRESENTATIVE PLANTS. - - - - -CHAPTER XIV. - -SPIROGYRA. - - -=283.= In our study of protoplasm and some of the processes of -plant life we became acquainted with the general appearance of the -plant spirogyra. It is now a familiar object to us. And in taking up -the study of representative plants of the different groups, we shall -find that in knowing some of these lower plants the difficulties of -understanding methods of reproduction and relationship are not so great -as they would be if we were entirely ignorant of any members of the -lower groups. - -[Illustration: Fig. 128. Thread of spirogyra, showing long cells, -chlorophyll band, nucleus, strands of protoplasm, and the granular wall -layer of protoplasm.] - -=284. Form of spirogyra.=—We have found that the plant spirogyra -consists of simple threads, with cylindrical cells attached end to -end. We have also noted that each cell of the thread is exactly alike, -with the exception of certain “holdfasts” on some of the species. If -we should examine threads in different stages of growth we should find -that each cell is capable of growth and division, just as it is capable -of performing all the functions of nutrition and assimilation. The -cells of spirogyra then multiply by division. Not simply the cells at -the ends of the threads but any and all of the cells divide as they -grow, and in this way the threads increase in length. - -=285. Multiplication of the threads.=—In studying living material -of this plant we have probably noted that the threads often become -broken by two of the adjacent cells of a thread becoming separated. -This may be and is accomplished in many cases without any injury to the -cells. In this manner the threads or plants of spirogyra, if we choose -to call a thread a plant, multiply, or increase. In this breaking of a -thread the cell wall which separates any two cells splits. If we should -examine several species of spirogyra we would probably find threads -which present two types as regards the character of the walls at the -ends of the cells. In fig. 128 we see that the ends are plain, that is, -the cross walls are all straight. But in some other species the inner -wall of the cells presents a peculiar appearance. This inner wall at -the end of the cell is at first straight across. But it soon becomes -folded back into the interior of its cell, just as the end of an empty -glove finger may be pushed in. Then the infolded end is pushed partly -out again, so that a peculiar figure is the result. - -=286. How some of the threads break.=—In the separation of the -cells of a thread this peculiarity is often of advantage to the plant. -The cell-sap within the protoplasmic membrane absorbs water and the -pressure pushes on the ends of the infolded cell walls. The inner -wall being so much longer than the outer wall, a pull is exerted on -the latter at the junction of the cells. Being weaker at this point -the outer wall is ruptured. The turgidity of the two cells causes -these infolded inner walls to push out suddenly as the outer wall is -ruptured, and the thread is snapped apart as quickly as a pipe-stem may -be broken. - -[Illustration: Fig. 129. Zygospores of spirogyra.] - -=287. Conjugation of spirogyra.=—Under certain conditions, when -vegetative growth and multiplication cease, a process of reproduction -takes place which is of a kind termed sexual reproduction. If we select -mats of spirogyra which have lost their deep green color, we are likely -to find different stages of this sexual process, which in the case of -spirogyra and related plants is called _conjugation_. A few threads -of such a mat we should examine with the microscope. If the material -is in the right condition we see in certain of the cells an oval or -elliptical body. If we note carefully the cells in which these oval -bodies are situated, there will be seen a tube at one side which -connects with an empty cell of a thread which lies near as shown in -fig. 129. If we search through the material we may see other threads -connected in this ladder fashion, in which the contents of the cells -are in various stages of collapse from what we have seen in the growing -cell. In some the protoplasm and chlorophyll band have moved but little -from the wall; in others it forms a mass near the center of the cell, -and again in others we will see that the contents of the cell of one -of the threads has moved partly through the tube into the cell of the -thread with which it is connected. - -=289.= This suggests to us that the oval bodies found in the cells -of one thread of the ladder, while the cells of the other thread were -empty, are formed by the union of the contents of the two cells. In -fact that is what does take place. This kind of union of the contents -of two similar or nearly similar cells is _conjugation_. The oval -bodies which are the result of this conjugation are _zygotes_, or -_zygospores_. When we are examining living material of spirogyra in -this stage it is possible to watch this process of conjugation. Fig. -130 represents the different stages of conjugation of spirogyra. - -=290. How the threads conjugate, or join.=—The cells of two -threads lying parallel put out short processes. The tubes from two -opposite cells meet and join. The walls separating the contents of the -two tubes dissolve so that there is an open communication between the -two cells. The content of each one of these cells which take part in -the conjugation is a _gamete_. The one which passes through the tube to -the receiving cell is the _supplying gamete_, while that of the -receiving cell is the _receiving gamete_. - -[Illustration: Fig. 130. - - Conjugation in spirogyra; from left to right beginning in the upper -row is shown the gradual passage of the protoplasm from the supplying -gamete to the receiving gamete.] - -=291. How the protoplasm moves from one cell to another.=—Before -any movement of the protoplasm of the supplying cell takes place we can -see that there is great activity in its protoplasm. Rounded vacuoles -appear which increase in size, are filled with a watery fluid, and -swell up like a vesicle, and then suddenly contract and disappear. -As the vacuole disappears it causes a sudden movement or contraction -of the protoplasm around it to take its place. Simultaneously with -the disappearance of the vacuole the membrane of the protoplasm is -separated from a part of the wall. This is probably brought about by a -sudden loss of some of the water in the cell-sap. These activities go -on, and the protoplasmic membrane continues to slip away from the wall. -Every now and then there is a movement by which the protoplasm is moved -a short distance. It is moved toward the tube and finally a portion of -it with one end of the chlorophyll band begins to move into the tube. -About this time the vacuoles can be seen in an active condition in the -receptive cell. At short intervals movement continues until the content -of the supplying cell has passed over into that of the receptive cell. -The protoplasm of this one is now slipping away from the cell wall, -until finally the two masses round up into the one zygospore. - -=292. The zygospore.=—This zygospore now acquires a thick wall -which eventually becomes brown in color. The chlorophyll color fades -out, and a large part of the protoplasm passes into an oily substance -which makes it more resistant to conditions which would be fatal to the -vegetative threads. The zygospores are capable therefore of enduring -extremes of cold and dryness which would destroy the threads. They -pass through a “resting” period, in which the water in the pond may be -frozen, or dried, and with the oncoming of favorable conditions for -growth in the spring or in the autumn they germinate and produce the -green thread again. - -=293. Life cycle.=—The growth of the spirogyra thread, the -conjugation of the gametes and formation of the zygospore, and the -growth of the thread from the zygospore again, makes what is called a -complete _life cycle_. - -=294. Fertilization.=—While conjugation results in the fusion of -the two masses of protoplasm, fertilization is accomplished when the -nuclei of the two cells come together in the zygospore and fuse into a -single nucleus. The different stages in the fusion of the two nuclei of -a recently formed zygospore are shown in figure 131. - -In the conjugation of the two cells, the chlorophyll band of the -supplying cell is said to degenerate, so that in the new plant the -number of chlorophyll bands in a cell is not increased by the union of -the two cells. - -[Illustration: Fig. 131. Fertilization in spirogyra; shows different -stages of fusion of the two nuclei, with mature zygospore at right. -(After Overton.)] - -=295. Simplicity of the process.=—In spirogyra any cell of the -thread may form a gamete (excepting the holdfasts of some species). -Since all of the cells of a thread are practically alike, there is no -structural difference between a vegetative cell and a cell about to -conjugate. The difference is a physiological one. All the cells are -capable of conjugation if the physiological conditions are present. All -the cells therefore are potential gametes. (Strictly speaking the wall -of the cell is the _garnetangium_, while the content forms the gamete.) - -While there is sometimes a slight difference in size between the -conjugating cells, and the supplying cell may be the smaller, this is -not general. We say, therefore, that there is no differentiation among -the gametes, so that usually before the protoplasm begins to move one -cannot say which is to be the supplying and which the receiving gamete. - -=296. Position of the plant spirogyra.=—From our study then we -see that there is practically no differentiation among the vegetative -cells, except where holdfasts grow out from some of the cells -for support. They are all alike in form, in capacity for growth, -division, or multiplication of the threads. Each cell is practically -an independent plant. There is no differentiation between vegetative -cell and conjugating cell. All the cells are potential gametes. -Finally there is no structural differentiation between the gametes. -This indicates then a simple condition of things, a low grade of -organization. - -=297.= The alga spirogyra is one of the representatives of the -lower algæ belonging to the group called _Conjugatæ_. Zygnema with -star-shaped chloroplasts, mougeotia with straight or sometimes twisted -chlorophyll bands, belong to the same group. In the latter genus only -a portion of the protoplasm of each cell unites to form the zygospore, -which is located in the tube between the cells. - -[Illustration: Fig. 132. Closterium.] - -[Illustration: Fig. 133. Micrasterias.] - -[Illustration: Fig. 134. Xanthidium.] - -[Illustration: Fig. 135. Staurastrum.] - -[Illustration: Fig. 136. Euastrum.] - -[Illustration: Fig. 137. Cosmarium.] - -=298.= The desmids also belong to the same group. The desmids -usually live as separate cells. Many of them are beautiful in form. -They grow entangled among other algæ, or on the surface of aquatic -plants, or on wet soil. Several genera are illustrated in figures -132-137. - - - - -CHAPTER XV. - -VAUCHERIA. - - -=299.= The plant vaucheria we remember from our study in an -earlier chapter. It usually occurs in dense mats floating on the water -or lying on damp soil. The texture and feeling of these mats remind one -of “felt,” and the species are sometimes called the “green felts.” The -branched threads are continuous, that is there are no cross walls in -the vegetative threads. This plant multiplies itself in several ways -which would be too tedious to detail here. But when fresh bright green -mats can be obtained they should be placed in a large vessel of water -and set in a cool place. Only a small amount of the alga should be -placed in a vessel, since decay will set in more rapidly with a large -quantity. For several days one should look for small green bodies which -may be floating at the side of the vessel next the lighted window. - -[Illustration: Fig. 138. Portion of branched thread of vaucheria.] - -=300. Zoogonidia of vaucheria.=—If these minute floating green -bodies are found, a small drop of water containing them should be -mounted for examination. If they are rounded, with slender hair-like -appendages over the surface, which vibrate and cause motion, they very -likely are one of the kinds of reproductive bodies of vaucheria. The -hair-like appendages are _cilia_, and they occur in pairs, several of -them distributed over the surface. These rounded bodies are _gonidia_, -and because they are motile they are called _zoogonidia_. - -By examining some of the threads in the vessel where they occurred we -may have perhaps an opportunity to see how they are produced. Short -branches are formed on the threads, and the contents are separated from -those of the main thread by a septum. The protoplasm and other contents -of this branch separate from the wall, round up into a mass, and escape -through an opening which is formed in the end. Here they swim around in -the water for a time, then come to rest, and germinate by growing out -into a tube which forms another vaucheria plant. It will be observed -that this kind of reproduction is not the result of the union of two -different parts of the plant. It thus differs from that which is termed -sexual reproduction. A small part of the plant simply becomes separated -from it as a special body, and then grows into a new plant, a sort of -multiplication. This kind of reproduction has been termed _asexual -reproduction_. - -[Illustration: Fig. 139. Young antheridium and oogonium of Vaucheria -sessilis, before separation from contents of thread by a septum.] - -=301. Sexual reproduction in vaucheria.=—The organs which -are concerned in sexual reproduction in vaucheria are very readily -obtained for study if one collects the material at the right season. -They are found quite readily during the spring and autumn, and may be -preserved in formalin for study at any season, if the material cannot -be collected fresh at the time it is desired for study. Fine material -for study often occurs on the soil of pots in greenhouses during the -winter. While the zoogonidia are more apt to be found in material which -is quite green and freshly growing, the sexual organs are usually more -abundant when the threads appear somewhat yellowish, or yellow green. - -=302. Vaucheria sessilis; the sessile vaucheria.=—In this plant -the sexual organs are sessile, that is they are not borne on a stalk -as in some other species. The sexual organs usually occur several in a -group. Fig. 139 represents a portion of a fruiting plant. - -=303. Sexual organs of vaucheria. Antheridium.=—The antheridia -are short, slender, curved branches from a main thread. A septum is -formed which separates an end portion from the stalk. This end cell -is the _antheridium_. Frequently it is collapsed or empty as shown in -fig. 140. The protoplasm in the antheridium forms numerous small oval -bodies each with two slender lashes, the cilia. When these are formed -the antheridium opens at the end and they escape. It is after the -escape of these spermatozoids that the antheridium is collapsed. Each -spermatozoid is a male gamete. - -[Illustration: Fig. 140. Vaucheria sessilis, one antheridium between -two oogonia.] - -[Illustration: Fig. 141. Vaucheria sessilis; oogonium opening and -emitting a bit of protoplasm; spermatozoids; spermatozoids entering -oogonium. (After Pringsheim and Goebel.)] - -=304. Oogonium.=—The oogonia are short branches also, but -they become large and somewhat oval. The septum which separates the -protoplasm from that of the main thread is as we see near the junction -of the branch with the main thread. The oogonium, as shown in the -figure, is usually turned somewhat to one side. When mature the pointed -end opens and a bit of the protoplasm escapes. The remaining protoplasm -forms the large rounded egg-cell which fills the wall of the oogonium. -In some of the oogonia which we examine this egg is surrounded by a -thick brown wall, with starchy and oily contents. This is the -fertilized egg (sometimes called here the oospore). It is freed from -the oogonium by the disintegration of the latter, sinks into the mud, -and remains here until the following autumn or spring, when it grows -directly into a new plant. - -[Illustration: Fig. 142. Fertilization in vaucheria, _mn_, male -nucleus; _fn_, female nucleus. Male nucleus entering the egg and -approaching the female nucleus. (After Oltmans.)] - -=305. Fertilization.=—Fertilization is accomplished by the -spermatozoids swimming in at the open end of the oogonium, when one of -them makes its way down into the egg and fuses with the nucleus of the -egg. - -[Illustration: Fig. 143. Fertilization of vaucheria. _fn_, female -nucleus; _mn_, male nucleus. The different figures show various stages -in the fusion of the nuclei.] - -=306. The twin vaucheria (V. geminata).=—Another species of -vaucheria is the twin vaucheria. This is also a common one, and may be -used for study instead of the sessile vaucheria if the latter cannot -be obtained. The sexual organs are borne at the end of a club-shaped -branch. There are usually two oogonia, and one antheridium between them -which terminates the branch. In a closely related species, instead of -the two oogonia there is a whorl of them with the antheridium in the -center. - -=307. Vaucheria compared with spirogyra.=—In vaucheria we have a -plant which is very interesting to compare with spirogyra in several -respects. Growth takes place, not in all parts of the thread, but is -localized at the ends of the thread and its branches. This represents a -distinct advance on such a plant as spirogyra. Again, only specialized -parts of the plant in vaucheria form the sexual organs. These are -short branches. Farther there is a great difference in the size of the -two organs, and especially in the size of the gametes, the supplying -gametes (spermatozoids) being very minute, while the receptive gamete -is large and contains all the nutriment for the fertilized egg. In -spirogyra, on the other hand, there is usually no difference in size -of the gametes, as we have seen, and each contributes equally in the -matter of nutriment for the fertilized egg. Vaucheria, therefore, -represents a distinct advance, not only in the vegetative condition of -the plant, but in the specialization of the sexual organs. Vaucheria, -with other related algæ, belongs to a group known as the _Siphoneæ_, so -called because the plants are tube-like or _siphon_-like. - -[Illustration: Fig. 143_a_. Botrydium granulatum. _A_, the whole plant; -_B_, swarm spore; _C_, planogametes; _a_, a single gamete; _b_-_e_, two -gametes in process of fusion; _f_, zygote.] - -=308. Botrydium granulatum.=—An example of one of the simpler -members of the Siphoneæ is Botrydium granulatum. It is found sometimes -in abundance on wet ground which is colored green or red by its -presence, according to the stage of development. The plant body is long -pear-shaped, the smaller end attached to the ground by slender branched -rhizoids (Fig. 143). The protoplasm contains many nuclei and lines the -inside of the wall. When multiplication takes place large numbers of -small zoospores with one cilium each are formed in the protoplasm, and -escape at free end. Reproduction takes place by two-ciliated gametes, -which fuse in pairs to form zygospores. In dry seasons the protoplasm -in the pear-shaped plant passes down into the rhizoids and forms -small rounded _planospores_. All the stages of development are too -complicated to describe here. - - - - -CHAPTER XVI. - -ŒDOGONIUM. - - -=309.= Œdogonium is also an alga. The plant is sometimes -associated with spirogyra, and occurs in similar situations. Our -attention was called to it in the study of chlorophyll bodies. These we -recollect are, in this plant, small oval disks, and thus differ from -those in spirogyra. - -=310. Form of œdogonium.=—Like spirogyra, œdogonium forms simple -threads which are made up of cylindrical cells placed end to end. But -the plant is very different from any member of the group to which -spirogyra belongs. In the first place each cell is not the equivalent -of an individual plant as in spirogyra. Growth is localized or confined -to certain cells of the thread which divide at one end in such a way -as to leave a peculiar overlapping of the cell walls in the form of a -series of shallow caps or vessels (fig. 144), and this is one of the -characteristics of this genus. Other differences we find in the manner -of reproduction. - -=311. Fruiting stage of œdogonium.=—Material in the fruiting -stage is quite easily obtainable, and may be preserved for study in -formalin if there is any doubt about obtaining it at the time we need -it for study. This condition of the plant is easily detected because of -the swollen condition of some of the cells, or by the presence of brown -bodies with a thick wall in some of the cells. - -=312. Sexual organs of œdogonium. Oogonium and egg.=—The enlarged -cell is the oogonium, the wall of the cell being the wall of the -oogonium. (See fig. 145.) The protoplasm inside, before fertilization, -is the egg-cell. In those cases where the brown body with a thick -wall is present fertilization has taken place, and this body is the -_fertilized egg_, or _oospore_. It contains large quantities of an oily -substance, and, like the fertilized egg of spirogyra and vaucheria, is -able to withstand greater changes in temperature than the vegetative -stage, and can endure drying and freezing for some time without injury. - -[Illustration: Fig. 144. Portion of thread of œdogonium, showing -chlorophyll grains, and peculiar cap cell walls.] - -[Illustration: Fig. 145. Œdogonium undulatum, with oogonia and dwarf -males; the upper oogonium at the right has a mature oospore.] - -In the oogonium wall there can frequently be seen a rift near the -middle of one side, or near the upper end. This is the opening through -which the spermatozoid entered to fecundate the egg. - -=313. Dwarf male plants.=—In some species there will also be seen -peculiar club-shaped dwarf plants attached to the side of the oogonium, -or near it, and in many cases the end of this dwarf plant has an open -lid on the end. - -=314. Antheridium.=—The end cell of the dwarf male in such -species is the _antheridium_. In other species the spermatozoids are -developed in different cells (antheridia) of the same thread which -bears the oogonium, or on a different thread. - -[Illustration: Fig. 146. Zoogonidia of œdogonium escaping. At the right -one is germinating and forming the holdfasts, by means of which these -algæ attach themselves to objects for support. (After Pringsheim.)] - -=315. Zoospore stage of œdogonium.=—The egg after a period of -rest starts into active life again. In doing so it does not develop -the thread-like plant directly as in the case of vaucheria and -spirogyra. It first divides into four zoospores which are exactly like -the zoogonidia in form. (See fig. 152.) These germinate and develop -the thread form again. This is a quite remarkable peculiarity of -œdogonium when compared with either vaucheria or spirogyra. It is the -introduction of an intermediate stage between the fertilized egg and -that form of the plant which bears the sexual organs, and should be -kept well in mind. - -=316. Asexual reproduction.=—Material for the study of this stage -of œdogonium is not readily obtainable just when we wish it for study. -But fresh plants brought in and placed in a quantity of fresh water may -yield suitable material, and it should be examined at intervals for -several days. This kind of reproduction takes place by the formation -of _zoogonidia_. The entire contents of a cell round off into an oval -body, the wall of the cell breaks, and the zoogonidium escapes. It has -a clear space at the small end, and around this clear space is a row or -crown of cilia as shown in fig. 146. By the vibration of these cilia -the zoogonidium swims around for a time, then settles down on some -object of support, and several slender holdfasts grow out in the form -of short rhizoids which attach the young plant. - -[Illustration: Fig. 147. Portion of thread of œdogonium showing -antheridia.] - -[Illustration: Fig. 148. Portion of thread of œdogonium showing upper -half of egg open, and a spermatozoid ready to enter. (After Klebahn).] - -=317. Sexual reproduction. Antheridia.=—The antheridia are short -cells which are formed by one of the ordinary cells dividing into a -number of disk-shaped ones as shown in fig. 147. The protoplasm in each -antheridium forms two spermatozoids (sometimes only one) which are of -the same form as the zoogonidia but smaller, and yellowish instead of -green. In some species a motile body intermediate in size and color -between the spermatozoids and zoogonidia is first formed, which after -swimming around comes to rest on the oogonium, or near it, and develops -what is called a “dwarf male plant” from which the real spermatozoid is -produced. - -[Illustration: Fig. 149. Male nucleus just entering egg at left side.] - -[Illustration: Fig. 150. Male nucleus fusing with female nucleus.] - -[Illustration: Fig. 151. The two nuclei fused, and fertilization -complete. - -Figs. 149-151.—Fertilization in œdogonium. (After Klebahn).] - -=318. Oogonia.=—The oogonia are formed directly from one of the -vegetative cells. In most species this cell first enlarges in diameter, -so that it is easily detected. The protoplasm inside is the egg-cell. -The oogonium wall opens, a bit of the protoplasm is emitted, and the -spermatozoid then enters and fertilizes it (fig. 148). Now a hard brown -wall is formed around it, and, just as in spirogyra and vaucheria, it -passes through a resting period. At the time of germination it does -not produce the thread-like plant again directly, but first forms four -zoospores exactly like the zoogonidia (fig. 152). These zoospores then -germinate and form the plant. - -=319. Œdogonium compared with spirogyra.=—Now if we compare -œdogonium with spirogyra, as we did in the case of vaucheria, we find -here also that there is an advance upon the simple condition which -exists in spirogyra. Growth and division of the thread is limited to -certain portions. The sexual organs are differentiated. They usually -differ in form and size from the vegetative cells, though the oogonium -is simply a changed vegetative cell. The sexual organs are -differentiated among themselves, the antheridium is small, and the -oogonium large. The gametes are also differentiated in size, and the -male gamete is motile, and carries in its body the nucleus which fuses -with the nucleus of the egg-cell. - -[Illustration: Fig. 152. Fertilized egg of œdogonium after a period of -rest escaping from the wall of the oogonium, and dividing into the four -zoospores. (After Juranyi.)] - -But a more striking advance is the fact that the fertilized egg does -not produce the vegetative thread of œdogonium directly, but first -forms four zoospores, each of which is then capable of developing into -the thread. On the other hand we found that in spirogyra the zygospore -develops directly into the thread form of the plant. - -[Illustration: Fig. 153. Tuft of chætophora, natural size.] - -[Illustration: Fig. 154. Portion of chætophora showing branching.] - -=320. Position of œdogonium.=—Œdogonium is one of the true -thread-like algæ, green in color, and the threads are divided into -distinct cells. It, along with many relatives, was once placed -in the old genus conferva. These are all now placed in the group -_Confervoideæ_, that is, the _conferva-like algæ_. - -=321. Relatives of œdogonium.=—Many other genera are related -to œdogonium. Some consist of simple threads, and others of branched -threads. An example of the branched forms is found in chætophora, -represented in figures 153, 154. This plant grows in quiet pools or -in slow-running water. It is attached to sticks, rocks, or to larger -aquatic plants. Many threads spring from the same point of attachment -and radiate in all directions. This, together with the branching of the -threads, makes a small, compact, greenish, rounded mass, which is held -firmly together by a gelatinous substance. The masses in this species -are about the size of a small pea, or smaller. Growth takes place in -chætophora at the ends of the threads and branches. That is, growth is -apical. This, together with the branched threads and the tendency to -form cell masses, is a great advance of the vegetative condition of the -plant upon that which we find in the simple threads of œdogonium. - - - - -CHAPTER XVII. - -COLEOCHÆTE. - - -=322.= Among the green algæ coleochæte is one of the most -interesting. Several species are known in this country. One of these at -least should be examined if it is possible to obtain it. It occurs in -the water of fresh lakes and ponds, attached to aquatic plants. - -[Illustration: Fig. 155. Stem of aquatic plant showing coleochæte, -natural size.] - -[Illustration: Fig. 156. Thallus of Coleochæte scutata.] - -=323. The shield-shaped coleochæte.=—This plant (C. scutata) is -in the form of a flattened, circular, green plate, as shown in fig. -156. It is attached near the center on one side to rushes and other -plants, and has been found quite abundantly for several years in the -waters of Cayuga Lake at its southern extremity. As will be seen it -consists of a single layer of green cells which radiate from the center -in branched rows to the outside, the cells lying so close together as -to form a continuous plate. The plant started its growth from a single -cell at the central point, and grew at the margin in all directions. -Sometimes they are quite irregular in outline, when they lie quite -closely side by side and interfere with one another by pressure. If the -surface is examined carefully there will be found long hairs, the base -of which is enclosed in a narrow sheath. It is from this character that -the genus takes its name of coleochæte (sheathed hair). - -[Illustration: Fig. 157. Portion of thallus of Coleochæte scutata, -showing empty cells from which zoogonidia have escaped, one from each -cell; zoogonidia at the left. (After Pringsheim.)] - -[Illustration: Fig. 158. Portion of thallus of Coleochæte scutata, -showing four antheridia formed from one thallus cell; a single -spermatozoid at the right. (After Pringsheim.)] - -=324. Fruiting stage of coleochæte.=—It is possible at some -seasons of the year to find rounded masses of cells situated near the -margin of this green disk. These have developed from a fertilized egg -which remained attached to the plant, and probably by this time the -parent plant has lost its color. - -=325. Zoospore stage.=—This mass of tissue does not develop -directly into the circular green disk, but each of the cells forms a -zoospore. Here then, as in œdogonium, we have another stage of the -plant interpolated between the fertilized egg and that stage of the -plant which bears the gametes. But in coleochæte we have a distinct -advance in this stage upon what is present in œdogonium, for in -coleochæte the fertilized egg develops first into a several-celled mass -of tissue before the zoospores are formed, while in œdogonium only four -zoospores are formed directly from the egg. - -=326. Asexual reproduction.=—In asexual reproduction any of the -green cells on the plant may form zoogonida. The contents of a cell -round off and form a single zoogonidium which has two cilia at the -smaller end of the oval body, fig. 157. After swimming around for a -time they come to rest, germinate, and produce another plant. - -=327. Sexual reproduction.—Oogonium.=—The oogonium is formed by -the enlargement of a cell at the end of one of the threads, and then -the end of the cell elongates into a slender tube which opens at the -end to form a channel through which the spermatozoid may pass down to -the egg. The egg is formed of the contents of the cell (fig. 159). -Several oogonia are formed on one plant, and in such a plant as C. -scutata they are formed in a ring near the margin of the disk. - -[Illustration: Fig. 159. Coleochæte soluta; at left branch bearing -oogonium (_oog_); antheridia (_ant_); egg in oogonium and surrounded -by enveloping threads; at center three antheridia open, and one -spermatozoid; at right sporocarp, mature egg inside sporocarp wall.] - -[Illustration: Fig. 160. Two sporocarps still surrounded by thallus. -Thallus finally decays and sets sporocarp free.] - -[Illustration: Fig. 161. Sporocarp ruptured by growth of egg to form -cell mass. Cells of this sporophyte forming zoospores. - -Figs. 160, 161. C. scutata.] - -=328. Antheridia.=—In C. scutata certain of the cells of the -plant divide into four smaller cells, and each one of these becomes -an antheridium. In C. soluta the antheridia grow out from the end of -terminal cells in the form of short flasks, sometimes four in number or -less (fig. 159). A single spermatozoid is formed from the contents. It -is oval and possesses two long cilia. After swimming around it passes -down the tube of the oogonium and fertilizes the egg. - -=329. Sporocarp.=—After the egg is fertilized the cells of the -threads near the egg grow up around it and form a firm covering one -cell in thickness. This envelope becomes brown and hard, and serves -to protect the egg. This is the “fruit” of the coleochæte, and is -sometimes called a sporocarp (spore-fruit). The development of the cell -mass and the zoospores from the egg has been described above. - -Some of the species of coleochæte consist of branched threads, while -others form circular cushions several layers in thickness. These forms -together with the form of our plant C. scutata make an interesting -series of transitional forms from filamentous structures to an expanded -plant body formed of a mass of cells. - -=330. COMPARATIVE TABLE FOR SPIROGYRA, VAUCHERIA, ŒDOGONIUM, -COLEOCHÆTE.= - - -----------+-------------------------------------------------------- - | GAMETOPHYTE. (Bears the sexual organs and gonidia.) - -----------+-----------+---------+------------+--------------------- - |Vegetative | Growth. |Multipli- | Sexual Reproduction. - | Phase. | | cation.+--------------------- - | | | | Sexual Organs. - -----------+-----------+---------+------------+--------------------- - Spirogyra. |Simple |All |By breaking | Undifferentiated. - |threads of |cells |up of | Any cell of thread. - |cylindrical|divide |threads. | Conjugate by tube. - |cells. |and | | - | |grow. | | - -----------+-----------+---------+------------+--------------------- - |Branched |Limited |By | Differentiated. - |threads, |to ends |multiciliate|Antheridia|Oogonium, - Vaucheria. |continuous.|of |zoogonidia, |slender |large - | |threads |and other |cells on |on special - | |and |cells, from |special |rounded - | |branches.|terminal |branches. |cell, - | | |portions. | |branch, - | | | | opens - | | | | |and emits - | | | | |bit of - | | | | |protoplasm. - -----------+-----------+---------+------------+----------+---------- - |Simple |Limited |By oval | Differentiated. - |threads of |to |zoogonidia, |Antheridia|Oogonium, - Œdogonium. |cylindrical|certain |with crown |disk- |changed - |cells. |portions |of cilia. | shaped, |vegetative - | | of |Any cell |several |cell, opens - | |thread. |may form a |from one |and emits - | | |single |vegetative|bit of - | | |zoogonidium.|cell. |protoplasm. - | | | |Sometimes | - | | | |on dwarf | - | | | |males. | - -----------+-----------+---------+------------+----------+---------- - Coleochæte.|Branched |Terminal |By | Differentiated. - |threads, | or |zoogonidia |Antheridia|Oogonium, - |or compact |marginal.|with two |four or |enlarged - |circular | |cilia. |several |veg. cell, - |plates. | |Any cell |from |with long - | | |may form |single |tube through - | | |a single |veg. cell.|opening - | | |zoogonidium.| |of which - | | | | |spermatozoid - | | | | |enters. - | | | | |After - | | | | |fertilization - | | | | |wall of - | | | | |enveloping - | | | | |threads - | | | | |surrounds - | | | | |oogonium. - -----------+-----------+---------+------------+----------+---------- - - -----------+------------------------+-------------+----------- - | GAMETOPHYTE. | | - | (Bears the sexual | | - | organs and gonidia.) | SPOROPHYTE |How Veg. - -----------+------------------------+ Bears |Phase of - | Sexual Reproduction. | spores |Gametophyte - +------------------------+-------------+ is - | Gametes. | Fruit. |Developed. - -----------+------------------------+-------------+----------- - Spirogyra. | Undifferentiated. | Zygospore |Develops - | Entire contents of | Rests. |veg. phase - | conjugating cell. | |directly. - -----------+------------------------+-------------+----------- - | Differentiated. | Egg (or |Develops - |Small | Large | oospore). |veg. phase - Vaucheria. |two-ciliated | egg | Rests. |directly. - |spermatozoids.| cell. | | - -----------+--------------+---------+-------------+----------- - | Differentiated. | Egg (or |Divides - |Oval | Large | oospore). |into - Œdogonium. |spermatozoids | egg | Rests. |four cells; - |with | cell. | |each - |crown of | | |forms - |cilia. | | |zoospore - |Two from each | | |which - |antheridium. | | |develops - | | | |veg. phase - | | | |again. - -----------+--------------+---------+-------------+----------- - Coleochæte.| Differentiated. |Egg |Each forms - |Oval, | Large |(surrounded |a zoospore. - |biciliate | egg |by |Zoospore - |spermatozoid, | cell. |wall from |develops - |one from each | |gametophyte).|veg. phase - |antheridium. | |Rests. |again. - | | |Divides | - | | |and | - | | |grows to | - | | |form a mass | - | | |of cells. | - -----------+--------------+---------+-------------+----------- - - - - -CHAPTER XVIII. - -CLASSIFICATION AND ADDITIONAL STUDIES OF THE ALGÆ. - - -In order to show the general relationship of the algæ studied, the -principal classes are here enumerated as well as some of the families. -In some of the groups not represented by the examples studied above, a -few species are described which may serve as the basis of additional -studies if desired. The principal classes[17] of algæ are as follows: - - -Class Chlorophyceæ. - -=331.= These are the green algæ, so called because the chlorophyll -green is usually not masked by other pigments, though in some forms it -is. There are three subclasses. - -=332. Subclass PROTOCOCCOIDEÆ.=—In the Protococcoideæ are found -the simplest green plants. Many of them consist of single cells which -live an independent life. Others form “colonies,” loose aggregations -of individuals not yet having attained the permanency of even a simple -plant body, for the cells often separate readily and are able to form -new colonies. The colonies are often held together by a gelatinous -membrane, or matrix. Some are motile, while others are non-motile. A -few of the families are here enumerated. - -=333. Family Volvocaceæ.=—These are all motile, during the -vegetative stage. The individuals are single or form more or less -globose colonies. - -=334. The “red snow” plant (Sphærella nivalis).=—This is often -found in arctic and alpine regions forming a red covering over more or -less large areas of snow or ice. For this reason it is called the “red -snow plant.” - -=335. Sphærella lacustris=, a closely related species, is very -widely distributed in temperate regions along streams or on the borders -of lakes and ponds. Here in dry weather it is often found closely -adhering to the dry rock surface, and giving it a reddish color as -if the rock were painted. This is especially the case in the shallow -basins formed over the uneven surface of the rock near the water’s -edge. These places during heavy rains or in high water are provided -with sufficient water to fill the basins. During such times the red -snow plant grows and multiplies, loses its red color and becomes green, -and, being motile, is free swimming. It is a single-celled plant, -oval in form, surrounded by a gelatinous sheath and with two cilia or -flagella at the smaller end, by the vibration of which it moves (fig. -162). The single cell multiplies by dividing into two cells. When the -water dries out of the basin, the motile plant comes to rest, and many -of the cells assume the red color. To obtain the plant for study, -scrape some of the red covering from these rock basins and place it in -fresh spring water, and in a day or so the swarmers are likely to be -found. Under certain conditions small microzoids are formed. - -[Illustration: Fig. 162. - -Sphærella lacustris (Girod.) Wittrock. _A_, mature free swimming -individual with central red spot. _B_, division of mother individual to -form two. _C_, division of a red one to form four. _D_, division into -eight. _E_, a typical resting cell, red. _F_, same beginning to divide. -_G_, one of four daughter zoospores after swimming around for a time -losing its red color and becoming green. (After Hazen.)] - -=336. Chlamydomonas= is a very interesting genus of motile -one-celled green algæ, because the species are closely related to -the Flagellates among the lower animals. The plant is oval, with a -single chloroplast and surrounded by a gelatinous envelope through -which the two cilia or flagella extend. One-celled organisms of this -kind are sometimes called _monads_, i.e., a one-celled being. This -one has a gelatinous cloak and is, therefore, a _cloaked monad_ -(_Chlamydomonas_). The species often are found as a very thin green -film on fresh water. C. pulvisculus is shown in fig. 163. When it -multiplies the single cell divides into two, as shown in _B_. Sometimes -a non-motile palmella stage is formed, as shown in _C_ and _D_. -Reproduction takes place by gametes which are of unequal size, the -smaller one representing the sperm and the larger one the egg, as in -_E_ and _F_. These conjugate as in _G_ and _H_, the protoplasm of the -smaller one passing over into the larger one, and a zygospore is thus -formed. - -[Illustration: Fig. 163. - -_Chlamydomonas pulvisculus_ (Müll.) Ehrb. _A_, an old motile -individual; _n_, nucleus; _p_, pyrenoid; _s_, red eye spot; _v_, -contractile vacuole; _B_, motile individual has drawn in its cilia -and divided into two; _C_, mother plant has drawn in its cilia and -divided into four non-motile cells; _D_, pamella stage; _E_, female -gamete—egg; _F_, male gamete—sperm; _G_, early stage of conjugation; -_H_, zygospore with conjugating tube and empty male cell attached. -(After Wille.)] - -=337. Of those which form colonies=, Pandorina morum is widely -distributed and not rare. It consists of a sphere formed of sixteen -individuals enclosed in a thin gelatinous membrane. Each cell possesses -two cilia (or flagella), which extend from the broader end out through -the enveloping membrane. By the movement of these flagella the colony -goes rolling around in the water. When the plant multiplies each -individual cell divides into sixteen small cells, which then grow and -form new colonies. Reproduction takes place when the individual cells -of the young colonies separate, and usually a small individual unites -with a larger one and a zygospore is formed (see fig. 164). Eudorina -elegans is somewhat similar, but when the gametes are formed certain -mother cells divide into sixteen small motile males or sperms, and -certain other mother cells divide into sixteen large motile females or -eggs. These separate from the colonies, and the sperms pair with the -eggs and fuse to form zygospores. This plant as well as Chlamydomonas -pulvisculus foreshadows the early differentiation of sex in plants. - -[Illustration: Fig. 164. - -Pandorina morum (Müll.) Bory. I, motile colony; II, colony divided into -16 daughter colonies; III, sexual colony, gametes escaping; IV, V, -conjugating gametes; VI, VII, young and old zygospore; VIII, zygospore -forming a large swarm spore, which is free in IX; X, same large swarm -spore divided to form young colony. (After Pringsheim.)] - -[Illustration: Fig. 165. Pleurococcus (protococcus) vulgaris.] - -=338. Family Tetrasporaceæ.=—This family is well represented by -Tetraspora lubrica forming slimy green net-like sheets attached to -objects in slow-running water. It is really a single-celled plant. The -rounded cells divide by cross walls into four cells, and these again, -and so on, large numbers being held in loose sheets by the slime in -which they are imbedded. - -=339. Family Pleurococcaceæ.=—The members of this family are -all non-motile in the vegetative stage. They consist of single -individuals, or of colonies. Pleurococcus vulgaris (Protococcus -vulgaris) is a single-celled alga, usually obtained with little -difficulty. It is often found on the shaded, and cool, or moist side of -trees, rocks, walls, etc., in damp places. This plant is not motile. It -multiplies by fission (Fig. 165) into two, then four, etc. These cells -remain united for a time, then separate. Sometimes the cells are found -growing out into filaments, and it is thought by some that P. vulgaris -may be only a simple stage of a higher alga. Eremosphæra viridis is -another single-celled alga found in fresh water among filamentous -forms. The cells are large and globose. - -[Illustration: Fig. 166. - -Pediastrum boryanum. _A_, mature colony, most of the young colonies -have escaped from their mother cells; at _g_, a young colony is -escaping; _sp_, empty mother cells; _B_, young colony; _C_, same colony -with spores arranged in order. (After Braun.)] - -=340. Family Hydrodictyaceæ.=—These plants form colonies of -cells. Hydrodictyon reticulatum, the water net, is made up of large -numbers of cylindrical cells so joined at their ends as to form a large -open mesh or net. Pediastrum forms circular flat colonies, as shown in -fig. 166. Both of these plants are rather common in fresh-water pools, -the latter one intermingled with filamentous algæ, while the former -forms large sheets or nets. Multiplication in Hydrodictyon takes place -by the protoplasm in one of the cells dividing into thousands of minute -cells, which gradually arrange themselves in the form of a net, escape -together from the mother cell, and grow into a large net. In Pediastrum -multiplication takes place in a similar way, but the protoplasm in each -cell usually divides into sixteen small cells, and escaping together -from the mother cell arrange themselves and grow to full size (fig. -166). - -=341. The Conjugateæ= include several families of green algæ, -which probably should be included among the Chlorophyceæ. They have -probably had their origin from some of the more simple members of the -Protococcoideæ. They are represented by Spirogyra, Zygnema, and the -desmids, studied in Chapter 14. - -=342. Subclass CONFERVOIDEÆ.=—These are mostly filamentous algæ, -the filaments being composed of cells firmly united, and, with the -exception of the simplest forms, there is a definite growing point. A -few of the families are as follows: - -=343. Family Ulvaceæ.=—These contain the sea wracks, or sea -lettuce, like Ulva, forming expanded green, ribbon-like growths in the -sea. - -[Illustration: Fig. 167. - -Ulothrix zonata. _A_, base of thread. _B_, cells with zoospores, _C_, -one cell with zoospores escaping another cell with small biciliate -gametes escaping and some fusing to form zygospores, _E_, zoospores -germinating and forming threads: _F_, _G_, zygospore growing and -forming zoospores. (After Caldwell and Dodel-Port.)] - -=344. Family Ulotrichaceæ=, represented by Ulothrix zonata, not -uncommon in slow-running water or in ponds of fresh water attached -to rocks or wood. It consists of simple threads of short cells. -Multiplication takes place by zoospores. Reproduction takes place by -motile sexual cells (gametes) which fuse to form a zygospore (fig. 167). - -=345. Family Chætophoraceæ=, represented by Chætophora (in Chapter -15) and Drapernaudia in fresh water. - -=346. Family Œdogoniaceæ=, represented by Œdogonium (Chapter 16). - -=347. Family Coleochætaceæ=, represented by Coleochæte (Chapter -17). - -=348. Subclass SIPHONEÆ.=—There are several families. - -=349. Family Botrydiaceæ.=—This is represented by Botrydium -granulatum (Chapter 15, p. 146). - -=350. Family Vaucheriaceæ=, represented by Vaucheria (Chapter 15), -with quite a large number of species, is widely distributed. - - -Class Schizophyceæ (= Cyanophyceæ). - -[Illustration: Fig. 168. Glœocapsa.] - -=351. The Blue-Green Algæ=, or =Cyanophyceæ= form slimy -looking thin mats on damp wood or the ground, or floating mats or -scum on the water. The color is usually bluish green, but in some -species it is purple, red or brown. All have chlorophyll, but it is -not in distinct chloroplasts and is more or less completely guised -by the presence of other pigments. Two orders and eight families are -recognized. The following include some of our common forms: - -=352. ORDER COCCOGONALES (COCCOGONEÆ).=—Single-celled plants, -occurring singly or in colonies, in some forms forming short threads. -One of the two families is mentioned. - -=353. Family Chroococcaceæ.=—The plants multiply only through cell -division. Chroococcus, forms rounded, blue-green cells enclosed in a -thick gelatinous coat, in fresh water and in damp places; certain -species form “lichen-gonidia” in some genera of lichens. Glœocapsa is -similar to Chroococcus, but the colonies are surrounded by an additional -common gelatinous envelope (fig. 168); on damp rocks, etc. - -[Illustration: Fig. 169. - -_A_, Oscillatoria princeps, _a_, terminal cell; _b_, _c_, portions from -the middle of a filament. In _c_, a dead cell is shown between the -living cells; _B_, Oscillatoria froelichii, _b_, with granules along -the partition walls.] - -=354. ORDER HORMOGONALES (HORMOGONEÆ).=—Plants filamentous, -simple celled or with false or true branching, usually several celled -(Spirulina is single celled). Multiplication takes place through -_hormogones_, short sections of the threads becoming free; also through -resting cells. Two of the six families are mentioned. - -=355. Family Oscillatoriaceæ.=—This family is represented by -the genus Oscillatoria, and by several other genera common and widely -distributed. Oscillatoria contains many species. They are found on the -damp ground or wood, or floating in mats in the water. They often form -on the soil at the bottom of the pool, and as gas becomes entangled -in the mat of threads, it is lifted from the bottom and floated to -the surface of the water. The plant is thread-like, and divided up -into many short cells. The threads often show an oscillating movement, -whence the name _Oscillatoria_. - -=356. Family Nostocaceæ.=—This family is represented by Nostoc, -which forms rounded, slimy, blue-green masses on wet rocks. The -individual plants in the slimy ball resemble strings of beads, each -cell being rounded, and several of these arranged in chains as shown -in fig. 170. Here and there are often found larger cells (heterocysts) -in the chain. Nostoc punctiforme lives in the intercellular spaces -of the roots of cycads (often found in greenhouses), and in the -stems of Gunnera. N. sphæricum lives in the spaces between the cells -in many species of liverworts (in the genera Anthoceros, Blasia, -Pellia, Aneura, Riccia, etc.), and in the perforated cells of Sphagnum -acutifolium. Anabæna is another common and widely distributed genus. -The species occur in fresh or salt water, singly or in slimy masses. -Anabæna azollæ lives endophytically in the leaves of the water fern, -Azolla. - -[Illustration: Fig. 170. - -Nostoc linckii. _A_, filament with two heterocysts (_h_), and a large -number of spores (_sp_); _B_, isolated spore beginning to germinate; -_C_, young filament developed from spore. (After Bornet.)] - -[Illustration: Fig. 171. - -Bacteria. _A_, Bacillus subtilis. Spores in threads, unstained rods, -and stained rods showing cilia; _B_, Bacillus tetani, the tetanus -or lockjaw bacillus, found in garden soil and on old rusty nails. -Spores in club-shaped ends. _C_, Micrococcus; _D_, Sarcina; _E_, -Streptococcus; _F_, Spirillum. (After Migula.)] - - -Class Schizomycetes. - -=357. Bacteriales.=—The bacteria are sometimes classified -with the Cyanophyceæ, under the name Schizophyta, and represent the -subdivision Schizomycetes, or fission fungi, because many of them -multiply by a division of the cells just as the blue-green algæ do. -For example, Bacillus forms rods which increase in length and divide -into two rods, or it may grow into a long thread of many short rods. -Micrococcus consists of single rounded cells. Streptococcus forms -chains of rounded cells, Sarcina forms irregular cubes of rounded -cells, while others like Spirillum are spiral in form. Bacillus -subtilis may be obtained by making an infusion from hay and allowing -it to stand for several days. Bacillus tetani occurs in the soil, on -old rusty nails, etc. It is called the tetanus bacillus because it -causes a permanent spasm of certain muscles, as in “lockjaw.” This -bacillus grows and produces this result on the muscles when it occurs -in deep and closed wounds such as are caused by stepping on an old nail -or other object which pierces the flesh deeply. In such a deep wound -oxygen is deficient, and in this condition the bacillus is virulent. -Opening the wounds to admit oxygen and washing them out with a solution -of bichloride of mercury prevents the tetanus. Many bacteria are of -great importance in bringing about the decay of dead animal and plant -matter, returning it to a condition for plant food. (See also nitrate -and nitrite bacteria, Chapter IX.) While most bacteria are harmless -there are many which cause very serious diseases of man and animals, -as typhoid fever, diphtheria, tuberculosis, etc., while some others -produce disease in plants. Others aid in certain fermentations of -liquids and are employed for making certain kinds of wines or other -beverages. Some work in symbiosis with yeasts, as in the kephir yeast, -used in fermenting certain crude beverages by natives of some countries. - -=357=_a_. =Myxobacteriales (Myxobacteriaceæ -Thaxter[18]).=—These plants consist of colonies of bacteria-like -organisms, motile rods, which multiply by cross-division and secrete -a gelatinous substance or matrix which surrounds the colonies. They -form plasmodium-like masses which superficially resemble the slime -moulds. In the fruiting stage some species become elevated from the -substratum into cylindrical, clavate, or branched forms, which bear -cysts of various shapes containing the rods in a resting stage, or the -rods are converted into spore-like masses. Ex., Chondromyces crocatus -on decaying plant parts, Myxobacter aureus on wet wood and bark, -Myxococcus rubescens on dung, decaying lichens, paper, etc. - - -Class Flagellata. - -=358. The flagellates= are organisms of very low organization -resembling animals as much as they do plants. They are single celled -and possess two cilia or flagella, by the vibration of which they -move. Some are without a cell wall, while others have a well-defined -membrane, but it rarely consists of cellulose. Some have chromatophores -and are able to manufacture carbohydrates like ordinary green plants. -These are green in Euglena, and brown in Hydrurus. Some possess a -mouth-like opening and are able to ingest solid particles of food -(more like animals), while others have no such opening and absorb food -substances dissolved in water (more like plants). The Euglena viridis -is not uncommon in stagnant water, often forming a greenish film on the -water. - - -Class Peridineæ. - -=358=_a_. These are peculiar one-celled organisms provided with -two flagella and show some relationship to the Flagellates. They -usually are provided with a cellulose membrane, which in some forms -consists of curiously sculptured plates. In the higher forms this -cellulose membrane consists of two valves fitting together in such a -way as to resemble some of the diatoms. Like the Flagellates, some -have green chromatophores, which in some are obscured by a yellow or -brown pigment (resembling the diatoms), while still others have no -chlorophyll. The Peridineæ are abundant in the sea, while some are -found in fresh water. - - -Class Diatomaphyceæ (Bacillariales, Diatomaceæ). - -[Illustration: Fig. 171_a_. - -A group of Diatoms: _c_ and _d_, top and side views of the same form; -_e_, colony of stalked forms attached to an alga; _f_ and _g_, top and -side views of the form shown at _e_; _h_, a colony; _i_, a colony, the -top and side view shown at _k_ and _n_, forming auxospores. (After -Kerner.)] - -=358=_b_. =The diatoms= are minute and peculiar organisms -believed to be algæ. They live in fresh, brackish, and salt water. They -are often found covering the surface of rocks, sticks, or the soil -in thin sheets. They occur singly and free, or several individuals -may be joined into long threads, or other species may be attached to -objects by slender gelatinous stalks. Each protoplast is enclosed in a -silicified skeleton in the form of a box with two halves, often shaped -like an old-fashioned pill box, one half fitting over the other like -the lid of a box. It is evident that in this condition the plant cannot -increase much in size. - -They multiply by fission. This takes place longitudinally, i.e., in the -direction of the two halves or _valves_ of the box. Each new plant then -has a valve only on one side. A new valve is now formed over the naked -half, and fits inside the old valve. At each division the individuals -thus become smaller and smaller until they reach a certain point, when -the valves are cast off and the cell forms an _auxospore_, i.e., it -grows alone, or after conjugation with another, to the full size again, -and eventually provides itself with new valves. The valves are often -marked, with numerous and fine lines, often making beautiful figures, -and some are used for test objects for microscopes. - -The free forms are capable of movement. The movement takes place in the -longitudinal direction of the valves. They glide for some time in one -direction, and then stop and move back again. It is not a difficult -thing to mount them in fresh water and observe this movement. - -The diatoms have small chlorophyll plates, but the green color is -disguised by a brownish pigment called diatomin. The relationships of -the diatoms are uncertain, but some, because of the color, think they -are related to the Phæophyceæ. - - -Class Phæophyceæ. - -=359. The brown algæ. (Phæophyceæ).=—The members of this class -possess chlorophyll, but it is obscured by a brown pigment. The plants -are accessible at the seashore, and for inland laboratories may be -preserved in formalin (2½ per cent). (See also Chapter LVI.) - -[Illustration: Fig. 172. - -_A_, Ectocarpus siliculosus; _B_, branch with a young and a ripe -plurilocular sporangium; _E_, gametes fusing to form zygospore, (_B_, -after Thuret; _E_, after Berthold.)] - -=360. Ectocarpus.=—The genus Ectocarpus represents well some -of the simpler forms of the brown algæ (fig. 172). They are slender, -filamentous branched algæ growing in tufts, either epiphytic on other -marine algæ (often on Fucaceæ), or on stones. The slender threads are -only divided crosswise, and thus consist of long series of short cells. -The sporangia are usually plurilocular (sometimes unilocular), and -usually occur in the place of lateral branches. The zoospores escape -from the apex of the sporangium and are biciliate, and they fuse to -form zygospores. - -=361. Sphacelaria.=—The species of this genus represent an -advance in the development of the thallus. While they are filamentous -and branched, division takes place longitudinally as well as crosswise -(fig. 173). - -=362. Leathesia difformis= represents an interesting type because -the plant body is small, globose, later irregular and hollow, and -consists of short radiately arranged branches, the surface ones in the -form of short, crowded, but free, trichome-like green branches. This -trichothallic body recalls the similar form of Chætophora pisiformis -(Chapter 16) among the Chlorophyceæ. - -[Illustration: Fig. 173. Sphacelaria, portion of plant showing -longitudinal division of cells, and brood bud (plurilocular -sporangium).] - -[Illustration: Fig. 174. Laminaria digitata, forma cloustoni, North -Sea. (Reduced.)] - -=363. The Giant Kelps.=—Among the brown algæ are found the -largest specimens, some of the laminarias or giant kelps, rivaling in -size the largest land plants, and some of them have highly developed -tissues. _Postelsia palmæformis_ has a long, stout stem, from the -free end of which extend numerous large and long blades, while the -stem is attached to the rocks by numerous “root” like outgrowths, the -holdfasts. It occurs along the northern Pacific coast, and appears to -flourish where it receives the shock of the surf beating on the shore. -Several species of Laminaria occur on our north Atlantic coast. In L. -digitata, the stem expands at the end into a broad blade, which becomes -split into several smaller blades (fig. 174). _Macrocystis pyrifera_ -inhabits the ocean in the southern hemisphere, and sometimes is found -along the north American coast. It is said to reach a length of 200-300 -meters. - -=364. Fucus, or Rockweed.=—This plant is a more or less branched -and flattened thallus or “frond.” One of them, illustrated in fig. 119, -measures 15-30 _cm_ (6-12 inches) in length. It is attached to rocks -and stones which are more or less exposed at low tide. From the base -of the plant are developed several short and more or less branched -expansions called “holdfasts,” which, as their name implies, are organs -of attachment. Some species (F. vesiculosus) have vesicular swellings -in the thallus. - -The fruiting portions are somewhat thickened as shown in the figure. -Within these portions are numerous oval cavities opening by a circular -pore, which gives a punctate appearance to these fruiting cushions. -Tufts of hairs frequently project through them. - -[Illustration: Fig. 175. Portion of plant of Fucus showing conceptacles -in enlarged ends; and below the vesicles (Fucus vesiculosus).] - -[Illustration: Fig. 176. Section of conceptacle of Fucus, showing -oogonia, and tufts of antheridia.] - -=365. Structure of the conceptacles.=—On making sections of the -fruiting portions one finds the walls of the cavities covered with -outgrowths. Some of these are short branches which bear a large rounded -terminal sac, the oogonium, at maturity containing eight egg-cells. -More slender and much-branched threads bear narrowly oval antheridia. -In these are developed several two-ciliated spermatozoids. - -[Illustration: Fig. 177. Oogonium of Fucus with ripe eggs.] - -[Illustration: Fig. 178. Antheridia of Fucus, on branched threads.] - -=366. Fertilization.=—At maturity the spermatozoids and egg-cells -float outside of the oval cavities, where fertilization takes place. -The spermatozoid sinks into the protoplasm of the egg-cell, makes its -way to the nucleus of the egg, and fuses with it as shown in fig. 181. -The fertilized egg then grows into a new plant. Nearly all the brown -algæ are marine. - -[Illustration: Fig. 179. Antheridia of Fucus with escaping -spermatozoids.] - -[Illustration: Fig. 180. Eggs of Fucus surrounded by spermatozoids.] - -[Illustration: Fig. 181. - -Fertilization in Fucus; _fn_, female nucleus; _mn_, male nucleus; -_n_, nucleolus. In the left figure the male nucleus is shown moving -down through the cytoplasm of the egg; in the remaining figures the -cytoplasm of the egg is omitted. (After Strasburger.)] - -=367. The Gulf weed= (=Sargassum bacciferum=) in the warmer -Atlantic ocean unites in great masses which float on the water, whence -comes the name “Sargassum Sea.” The Sargassum grows on the coast where -it is attached to the rocks, but the beating of the waves breaks many -specimens loose and these float out into the more quiet waters, where -they continue to grow and multiply vegetatively. - -=368. Uses.=—Laminaria japonica and L. angustata are used as food -by the Chinese and Japanese. Some species of the Laminariaceæ are used -as food for cattle and are also used for fertilizers, while L. digitata -is sometimes employed in surgery. - -_Classification._—Kjellman divides the Phæophyceæ into two orders. - -=369. Order Phæosporales (Phæosporeæ)= including 18 families. -One of the most conspicuous families is the Laminariaceæ, including -among others the Giant Kelps mentioned above (Laminaria, Postelsia, -Macrocystis, etc.). - -=370. Order Cyclosporales (Cyclosporeæ).=—This includes one -family, the _Fucaceæ_ with Ectocarpus, Sphacelaria, Læathesia, Fucus, -Sargassum, etc. - - -Class Rhodophyceæ. - -=371. The red algæ (Rhodophyceæ).=—The larger number of the -so-called red algæ occur in salt water, though a few genera occur in -fresh water. The plants possess chlorophyll, but it is usually obscured -by a reddish or purple pigment. - -=372. Nemalion.=—This is one of the lower marine forms, though -its thallus is not one of the simplest in structure. The plant body -consists of a slender cylindrical branched shoot, sometimes very -profusely branched. The central strand is rather firm, while the cortex -is composed of rather loose filaments. - -[Illustration: Fig. 182. - -A red alga (Nemalion). _A_, sexual branches, showing antheridia (_a_); -carpogonium or procarp (_o_) with its trichogyne (_t_), to which are -attached two spermatia (_s_); _B_, beginning of a cystocarp (_o_), -the trichogyne (_t_) still showing; _C_, an almost mature cystocarp -(_o_), with the disorganizing trichogyne (_t_). (After Vines.)] - -=373. Batrachospermum.=—This genus occurs in fresh water, and the -species are found in slow-running water of shallow streams or ditches. -There is a central slender strand which is more or less branched, -and on these branches are whorls of densely crowded slender branches -occurring at regular intervals. The plants are usually very slippery. -Gonidia are formed on the ends of some of these branches in globose -sporangia, called monosporangia, since but a single spore or gonidium -is developed in each. Other branches often terminate in long slender -hyaline setæ. - -=374. Lemanea.=—This genus also occurs in fresh water. The -species develop only during the cold winter months in rapids of streams -or where the water from falls strikes the rocks and is thoroughly -aerated. They form tufts of greenish threads, cylindrical or whiplike, -which in the summer are usually much broken down. The threads are -hollow and have a firm cortex. These are the sexual shoots, and they -arise as branches from a sterile filamentous-branched, Chantransia-like -form. - -=375. Fertilization in the lower red algæ.=—The sexual organs in -the red algæ consist of antheridia and carpogonia. The antheridia are -usually borne in crowded clusters, or surfaces, and bear terminally -the small non-motile sperm cells. The carpogonium is a branch of one -or several cells, the terminal cell (procarp) extending into a long -slender process, the trichogyne. The sperm cell comes in contact with -the trichogyne, and in the case of Nemalion and some others the nucleus -has been found to pass down the inside and fuse with the nucleus of the -procarp. - -[Illustration: Fig. 183. - -_A_, part of a shoot showing whorls of branches with clusters of -carpospores. _B_, carpogonic branch or procarp. _c_, procarp cell; -_tr_, trichogyne. _C_, same with sperm (_sp_) uniting with trichogyne. -_D_, same with carpospores developing from procarp cell. _E_, male -branch with one-celled antheridia. _F_, same with some of antheridia -empty. (After Schmitz.)] - -From this point in the lower red algæ like Nemalion, Batrachospermum -and Lemanea the formation of the spores is very simple. The procarp -is stimulated to growth, and buds in different directions, producing -branched chains of spores (carpospores). The carpospores form a rather -compact cluster called the sporocarp, which means spore-fruit or -spore-fruit body. In Batrachospermum it is seen as a compact tuft in -the loose branching, in Nemalion it lies in the surface of the cortex, -while in Lemanea the sporocarps lie at different positions in the -hollow tube of the sexual shoot. - -[Illustration: Fig. 184. - -A red alga (Callithamnion), showing sporangium _A_, and the tetraspores -discharged _B_. (After Thuret.)] - -[Illustration: Fig. 185. Gracilaria, portion of frond, showing position -of cystocarps.] - -[Illustration: Fig. 186. Gracilaria, section of cystocarp showing -spores.] - -=376. Gonidia in the red algæ.=—The common type of gonidium -in the red algæ is found in the _tetraspores_. A single mother cell -divides into four cells arranged usually in the form of tetrads within -the _tetrasporangium_. In Callithamnion the tetrasporangium is exposed. -In Polysiphonia, Rhabdonia, Gracilaria, etc., it is imbedded in the -cortex. In Batrachospermum there are monosporangia, each monosporangium -containing a single gonidium, while in Lemanea, and according to some -also in Nemalion, gonidia are wanting. - -=377. Gracilaria.=—Gracilaria is one of the marine forms, and -one species is illustrated in fig. 185. It measures 15-20_cm_ or more -long, and is profusely branched in a palmate manner. The parts of the -thallus are more or less flattened. The fruit is a cystocarp, which -is characteristic of the Rhodophyceæ (Florideæ). In Gracilaria these -fruit bodies occur scattered over the thallus. They are somewhat -flask-shaped, are partly sunk in the thallus, and the conical end -projects strongly above the surface. The carpospores are grouped in -radiating threads within the oval cavity of the cystocarp. These -cystocarps are developed as a result of fertilization. Other plants -bear gonidia in groups of four, the so-called _tetraspores_. - -=378. Rhabdonia.=—This plant is about the same size as the -gracilaria, though it possesses more filiform branches. The cystocarps -form prominent elevations, while the carpospores lie in separated -groups around the periphery of a sterile tissue within the cavity. (See -figs. 187, 188.) Gonidia in the form of tetraspores are also developed -in Rhabdonia. - -[Illustration: Fig. 187. Rhabdonia, branched portion of frond showing -cystocarps.] - -[Illustration: Fig. 188. Section of cystocarp of rhabdonia, showing -spores.] - -=379. Fertilization of the higher red algæ.=—The process of -fertilization in most of the red algæ is very complicated, chiefly -because the fertilized egg-cell (procarp) does not develop the spores -directly, as in Nemalion, Lemanea, etc., but fuses directly, or by a -short cell or long filament with one or more auxiliary cells before -the sporocarp is finally formed. Examples are Rhabdonia, Polysiphonia, -Callithamnion, Dudresnaya, etc. (fig. 189). The auxiliary cell then -develops the sporocarp. See fig. 189 for conjugation of a filament from -the fertilized procarp with an auxiliary cell. - -[Illustration: Fig. 189. - -Dudresnaya purpurifera. _tr_, trichogyne, with spermatozoids attached; -_ct_, connecting-tube which grows out from below the base of the -trichogyne, and comes in contact with the fertile branches _f_, _f_; -_ct′_, young connecting-tube. (After Thuret and Bornet.)] - -=380. Uses of the red algæ.=—Many species produce a great amount -of gelatinous substance in their tissues, and several of these are used -for food, for the manufacture of gelatines and agar-agar. Some of these -are Gracilaria lichenoides and wrightii, the former species occurring -along the coast of India and China. The plant is easily converted into -gelatinous substance (agar-agar). Chondrus crispus, widely distributed -in the northern Atlantic is known as “Irish” moss and is used for food -and for certain medicinal purposes. Gigartina mamillosa in the Atlantic -and Arctic oceans is similarly employed. The following orders are -recognized in the red algæ: - -=381. Order Bangiales.=—Example, Bangia atropurpurea (= Conferva -atropurpurea) in springs and brooks in North America and Europe. -Porphyra contains a number of species forming broad, thin, leaf-like -purple sheets in the sea. - -=382. Order Nemalionales.=—Including Lemanea, Batrachospermum, -Nemalion, described above, and many others. - -=383. Order Gigartinales.=—In this order occurs the common -Iceland moss (Chondrus crispus) in the sea, and Rhabdonia and Gigartina -mentioned above. - -=384. Order Rhodomeniales.=—In this order occurs Gracilaria and -Polysiphonia mentioned above, also the beautiful marine forms like -Ceramium. - -=385. Order Cryptonemiales.=—Examples are Dudresnaya, Melobesia, -Corallina, etc., the last two genera include many species with a wide -distribution. - - -Class Charophyceæ, Order Charales. - -=386.= The Charales are by some thought to represent a distinct -class of algæ standing near the mosses, perhaps, because of the -biciliate character of the spermatozoids. There is one family, the -Characeæ. The plants occur in fresh and brackish water. Aside from the -peculiarity of the reproductive organs they are remarkable for the -large size of the cells of the internodes and of the “leaves,” and the -protoplasm exhibits to a remarkable degree the phenomenon of “cyclosis” -(see paragraphs 17-20). Three of the genera are found in North America -(Chara, Nitella (Fig. 8) and Tolypella). - -[Illustration: Fig. 172_a_. - -Reproductive organs of _Chara fragilis_. _A_, a central portion of a -leaf, _b_, with an antheridium, _a_, and a carpogonium, _s_, surrounded -by the spirally twisted enveloping cells; _c_, crown of five cells at -apex; β, sterile lateral leaflets; β′, large lateral leaflet near the -fruit; β″, bracteoles springing from the basal node of the reproductive -organs. _B_, a young antheridium, _a_, and a young carpogonium, _sk_; -_w_, nodal cell of leaf; _u_, intermediate cell between _w_ and the -basal-node cell of the antheridium; _l_, cavity of the internode of -the leaf; _br_, cortical cells of the leaf. _A_ × about 33; _B_ × 240. -(After Sachs.)] - -=386a.= The complicated structure of the sexual organs shows a -higher state of organization than any of the other living algæ known. -While the internodes in Nitella are composed of a single, stout cell, -some times a foot or more in length, the nodes in all are composed of -a group of smaller cells. From the lateral cells of this group lateral -axes (sometimes called leaves) arise in whorls. - -In Nitella the internodes are naked, but in most species of Chara -they are _corticated_, i.e., they are covered by a layer of numerous -elongated cells which grow downward from the nodes at the base of the -whorl of lateral shoots. - -=386b.= The sexual organs are situated at the nodes of the whorled -lateral shoots, and consist of antheridia and carpogonia. Most of the -plants are monœcious, and both antheridia and carpogonia are often -attached to the same node, the antheridium projecting downward while -the carpogonium is more or less ascending. The sexual organs are -visible to the unaided eye. The antheridium is a globose red body of -an exceedingly complicated structure. The sperms are borne in several -very long coiled slender threads which are divided transversely into -numerous cells. The carpogonium is oval or elliptical in outline, the -wall of which is composed of several closely coiled spiral threads -enclosing the large egg. - -FOOTNOTES: - -[17] In Engler & Prantl’s Pflanzenfamilien, Wille uses the term class -for these principal subdivisions of the algæ. Systematists are not yet -agreed upon a uniform use of the terms. - -[18] See Bot. Gaz., 17, 389, 1892. - - - - -CHAPTER XIX. - -FUNGI: MUCOR AND SAPROLEGNIA. - - -Mucor. - -=387.= In the chapter on growth, and in our study of protoplasm, -we have become familiar with the vegetative condition of mucor. We now -wish to learn how the plant multiplies and reproduces itself. For this -study we may take one of the mucors. Any one of several species will -answer. This plant may be grown by placing partially decayed fruits, -lemons, or oranges, from which the greater part of the juice has been -removed, in a moist chamber; or often it occurs on animal excrement -when placed under similar conditions. In growing the mucor in this way -we are likely to obtain Mucor mucedo, or another plant sometimes known -as Mucor stolonifer, or Rhizopus nigricans, which is illustrated in -fig. 191. This latter one is sometimes very injurious to stored fruits -or vegetables, especially sweet potatoes or rutabagas. Fig. 190 is from -a photograph of this fungus on a banana. - -=388. Asexual reproduction.=—On the decaying surface of the -vegetable matter where the mucor is growing there will be seen numerous -small rounded bodies borne on very slender stalks. These heads contain -the gonidia, and if we sow some of them in nutrient gelatine or agar -in a Petrie dish the material can be taken out very readily for -examination under the microscope. Or we may place glass slips close -to the growing fungus in the moist chamber, so that the fungus will -develop on them, though cultures in a nutrient medium are much better. -Or we may take the material directly from the substance on which it is -growing. After mounting a small quantity of the mycelium bearing these -heads, if we have been careful to take it where the heads appear -quite young, it may be possible to study the early stages of their -development. We shall probably note at once that the stalks or upright -threads which support the heads are stouter than the threads of the -mycelium. - -[Illustration: Fig. 190. Portion of banana with a mould (Rhizopus -nigricans) growing on one end.] - -These upright threads soon have formed near the end a cross wall which -separates the protoplasm in the end from the remainder. This end cell -now enlarges into a vesicle of considerable size, the head as it -appears, but to which is applied the name of _sporangium_ (sometimes -called gonidangium), because it encloses the _gonidia_. - -At the same time that this end cell is enlarging the cross wall is -arching up into the interior. This forms the _columella_. All the -protoplasm in the sporangium now divides into gonidia. These are -small-rounded or oval bodies. The wall of the sporangium becomes -dissolved, except a small collar around the stalk which remains -attached below the columella (fig. 192). By this means the gonidia are -freed. These gonidia germinate and produce the mycelium again. - -[Illustration: Fig. 191. Group of sporangia of a mucor (Rhizopus -nigricans) showing rhizoids and the stolon extending from an older -group.] - -=389. Sexual stage.=—This stage is not so frequently found, but -may sometimes be obtained by growing the fungus on bread. - -Conjugation takes place in this way. Two threads of the mycelium which -lie near each other put out each a short branch which is clavate in -form. The ends of these branches meet, and in each a septum is formed -which cuts off a portion of the protoplasm in the end from that of the -rest of the mycelium. The meeting walls of the branches now dissolve -and the protoplasm of each gamete fuses into one mass. A thick wall -is now formed around this mass, and the outer layer becomes rough and -brown. This is the _zygote_ or _zygospore_. The mycelium dies and it -becomes free often with the suspensors, as the stalks of these sexual -branches are called, still attached. This zygospore passes through -a period of rest, when with the entrance of favorable conditions of -growth it germinates, and usually produces directly a sporangium with -gonidia. This completes the normal life cycle of the plant. - -=390. Gemmæ.=—Gemmæ, as they are sometimes called, are often -formed on the mycelium. A short cell with a stout wall is formed on the -side of a thread of the mycelium. In other cases large portions of the -threads of the mycelium may separate into chains of cells. Both these -kinds of cells are capable of growing and forming the mycelium again. -They are sometimes called _chlamydospores_. - -[Illustration: Fig. 194. - -A mucor (Rhizopus nigricans); at left nearly mature sporangium with -columella showing within; in the middle is ruptured sporangium with -some of the gonidia clinging to the columella; at right two ruptured -sporangia with everted columella.] - -=390=_a_. The Mucorineæ according to their manner of zygospore -formation are of two kinds: 1st, the _homothallic_ (monœcious), in -which all of the colonies of thalli developed from different spores -are the same, and both gametes may be developed from the mycelium -from a single spore, as in Sporodinia grandis, a mould common on old -mushrooms; 2d, the _heterothallic_ (diœcious), in which certain plants -are of a male nature and small in comparison with those of perhaps a -female nature which are larger or more vigorous. When grown separately -each of these two kinds of thalli, or colonies of mycelium, produce -their own kind but only sporangia. If the two kinds are brought -together, however, branches from one conjugate with branches from -the other and zygospores are produced, as in Rhizopus nigricans, the -common bread or fruit mould. This is one reason why we rarely find this -fungus forming zygospores. (See Blakeslee, Sexual Reproduction in the -Mucorineæ, Proc. Am. Acad. Arts and Sci., =40=, 205-319, pl. 1-4, -1904.) - - -Water Moulds (Saprolegnia). - -=391.= The water moulds are very interesting plants to study -because they are so easy to obtain, and it is so easy to observe a type -of gonidium here to which we have referred in our studies of the algæ, -the motile gonidium, or zoogonidium. (See appendix for directions for -cultivating this mould.) - -=392. Appearance of the saprolegnia.=—In the course of a few days -we are quite certain to see in some of the cultures delicate whitish -threads, radiating outward from the body of the fly in the water. A few -threads should be examined from day to day to determine the stage of -the fungus. - -[Illustration: Fig. 195. Sporangia of saprolegnia, one showing the -escape of the zoogonidia.] - -=393. Sporangia of saprolegnia.=—The sporangia of saprolegnia -can be easily detected because they are much stouter than the ordinary -threads of the mycelium. Some of the threads should be mounted in fresh -water. Search for some of those which show that the protoplasm is -divided up into a great number of small areas, as shown in fig. 195. -With the low power we should watch some of the older appearing ones, -and if after a few minutes they do not open, other preparations should -be made. - -[Illustration: Fig. 196. Branch of saprolegnia showing oogonia with -oospores, eggs matured parthenogenetically.] - -[Illustration: Fig. 197. - -Downy mildew of grape (Plasmopora viticola), showing tuft of -gonidiophores bearing gonidia, also intercellular mycelium. (After -Millardet.)] - -[Illustration: Fig. 198. Phytophthora infestans showing peculiar -branches; gonidia below.] - -=394. Zoogonidia of saprolegnia.=—The sporangium opens at the end, and -the zoogonidia swirl out and swim around for a short time, when they -come to rest. With a good magnifying power the two cilia on the end may -be seen, or we may make them more distinct by treatment with Schultz’s -solution, drawing some under the cover glass. The zoogonidium is oval -and the cilia are at the pointed end. After they have been at rest for -some time they often slip out of the thin wall, and swim again, this -time with the two cilia on the side, and then the zoogonidium is this -time more or less bean-shaped or reniform. - -[Illustration: Fig. 199. - -Fertilization in saprolegnia, tube of antheridium carrying in the -nucleus of the sperm cell to the egg. In the right-hand figure a -smaller sperm nucleus is about to fuse with the nucleus of the egg. -(After Humphrey and Trow.)] - -[Illustration: Fig. 200. Branching hypha of Peronospora alsinearum.] - -[Illustration: Fig. 201. Branched hypha of downy mildew of grape -showing peculiar branching (Plasmopara viticola).] - -=395. Sexual reproduction of saprolegnia.=—When such cultures are -older we often see large rounded bodies either at the end of a thread, -or of a branch, which contain several smaller rounded bodies as shown -in fig. 196. These are the oogonia (unless the plant is attacked by a -parasite), and the round bodies inside are the egg-cells, if before -fertilization, or the eggs, if after this process has taken place. -Sometimes the slender antheridium can be seen coiled partly around the -oogonium, and one end entering to come in contact with the egg-cell. -But in some species the antheridium is not present, and that is the -case with the species figured at 196. In this case the eggs mature -without fertilization. This maturity of the egg without fertilization -is called _parthenogenesis_, which occurs in other plants also, but is -a rather rare phenomenon. - -[Illustration: Fig. 202. - -Gonidiophores and gonidia of potato blight (Phytophthora infestans). -_b_, an older stage showing how the branch enlarges where it grows -beyond the older gonidium. (After de Bary.)] - -[Illustration: Fig. 203. Gonidia of potato blight forming zoogonidia. -(After de Bary.)] - -=396.= In fig. 199 is shown the oogonium and an antheridium, -and the antheridium is carrying in the male nucleus to the egg-cell. -Spermatozoids are not developed here, but a nucleus in the antheridium -reaches the egg-cell. It sinks in the protoplasm of the egg, comes -in contact with the nucleus of the egg, and fuses with it. Thus -fertilization is accomplished. - - -Downy Mildews. - -=397.= The downy mildews make up a group of plants which are -closely related to the water moulds, but they are parasitic on land -plants, and some species produce very serious diseases. The mycelium -grows between the cells of the leaves, stems, etc., of their hosts, -and sends haustoria into the cells to take up nutriment. Gonidia are -formed on threads which grow through the stomates to the outside and -branch as shown in figs. 198-201. The gonidia are borne on the tips -of the branches. The kind of branching bears some relation to the -different genera. Fig. 200 is from Peronospora alsinearum on leaves of -cerastium; figs. 197 and 199 are Plasmopara viticola, the grape mildew, -while figs. 198 and 202 are from Phytophthora infestans which causes a -disease known as potato blight. The gonidia of peronospora germinate -by a germ tube, those of plasmopara first form zoogonidia, while in -phytophthora the gonidium may either germinate forming a thread, or -each gonidium may first form several zoogonidia, as shown in fig. 203. - -[Illustration: Fig. 204. - -Fertilization in Peronospora alsinearum; tube from antheridium carrying -in the sperm nucleus in figure at the left, female nucleus near; fusion -of the two nuclei shown in the two other figures. (After Berlese.)] - -[Illustration: Fig. 205. Ripe oospore of Peronospora alsinearum.] - -=398.= In sexual reproduction oogonia and antheridia are developed -on the mycelium within the tissues. Fig. 204 represents the antheridium -entering the oogonium, and the male nucleus fusing with the female -nucleus in fertilization. The sexual organs of Phytophthora infestans -are not sufficiently known. - -=399.= Mucor, saprolegnia, peronospora, and their relatives -have few or no septa in the mycelium. In this respect they resemble -certain of the algæ like vaucheria, but they lack chlorophyll. They are -sometimes called the alga-like fungi and belong to a large group called -_Phycomycetes_. - - - - -CHAPTER XX. - -FUNGI CONTINUED. - - -“Rusts” (Uredineæ). - -=400.= The fungi known as “rusts” are very important ones to -study, since all the species are parasitic, and many produce serious -injuries to crops. - -[Illustration: Fig. 206. Wheat leaf with red-rust, natural size.] - -[Illustration: Fig. 207. Portion of leaf enlarged to show sori.] - -[Illustration: Fig. 208. Natural size.] - -[Illustration: Fig. 209. Enlarged.] - -[Illustration: Fig. 210. Single sorus. - -Figs. 206, 207.—Puccinia graminis, red-rust stage (uredo stage). - -Figs. 208-210.—Black rust of wheat, showing sori of teleutospores.] - -=401. Wheat rust (Puccinia graminis).=—The wheat rust is one of -the best known of these fungi, since a great deal of study has been -given to it. One form of the plant occurs in long reddish-brown or -reddish pustules, and is known as the “red-rust” (figs. 206, 207). -Another form occurs in elongated black pustules, and this form is the -one known as the “black rust” (figs. 208-211). These two forms occur on -the stems, blades, etc., of the wheat, also on oats, rye, and some of -the grasses. - -[Illustration: Fig. 211. Head of wheat showing black rust spots on the -chaff and awns.] - -[Illustration: Fig. 212. Teleutospores of wheat rust, showing two cells -and the pedicel.] - -[Illustration: Fig. 213. Uredospores of wheat rust, one showing -remnants of the pedicel.] - -=402. Teleutospores of the black rust form.=—If we scrape off -some portion of one of the black pustules (sori), tease it out in water -on a slide, and examine with a microscope, we see numerous gonidia, -composed of two cells, and having thick, brownish walls as shown -in fig. 212. Usually there is a slender brownish stalk on one end. -These gonidia are called _teleutospores_. They are somewhat oblong or -elliptical, a little constricted where the septum separates the two -cells, and the end cell varies from ovate to rounded. The mycelium of -the fungus courses between the cells, just as is found in the case of -the carnation rust, which belongs to the same family (see Parag. 186). - -[Illustration: Fig. 214. Barberry leaf with two diseased spots, natural -size.] - -[Illustration: Fig. 215. Single spot showing cluster-cups enlarged.] - -[Illustration: Fig. 216. Two cluster-cups more enlarged, showing split -margin. - -Figs. 214-216.—Cluster-cup stage of wheat rust.] - -=403. Uredospores of the red-rust form.=—If we make a similar -preparation from the pustules of the red-rust form we see that instead -of two-celled gonidia they are one-celled. The walls are thinner -and not so dark in color, and they are covered with minute spines. -They have also short stalks, but these fall away very easily. These -one-celled gonidia of the red-rust form are called “uredospores.” The -uredospores and teleutospores are sometimes found in the same pustule. - -It was once supposed that these two kinds of gonidia belonged to -different plants, but now it is known that the one-celled form, the -uredospores, is a form developed earlier in the season than the -teleutospores. - -=404. Cluster-cup form on the barberry.=—On the barberry is found -still another form of the wheat rust, the “_cluster-cup_” stage. The -pustules on the under side of the barberry leaf are cup-shaped, the -cups being partly sunk in the tissue of the leaf, while the rim is more -or less curved backward against the leaf, and split at several places. -These cups occur in clusters on the affected spots of the barberry leaf -as shown in fig. 215. Within the cups numbers of one-celled gonidia -(orange in color, called æcidiospores) are borne in chains from short -branches of the mycelium, which fill the base of the cup. In fact the -wall of the cup (peridium) is formed of similar rows of cells, which, -instead of separating into gonidia, remain united to form a wall. These -cups are usually borne on the under side of the leaf. - -=405. Spermagonia.=—Upon the upper side of the leaves in the -same spot occur small, orange-colored pustules which are flask-shaped. -They bear inside, minute, rod-like bodies on the ends of slender -threads, which ooze out on the surface of the leaf. These flask-shaped -pustules are called _spermagonia_, and the minute bodies within them -_spermatia_, since they were once supposed to be the male element of -the fungus. Their function is not known. They appear in the spots at an -earlier time than the cluster-cups. - -[Illustration: Fig. 217. Section of an æcidium (cluster-cup) from -barberry leaf. (After Marshall-Ward.)] - -=406. How the cluster-cup stage was found to be a part of the wheat -rust.=—The cluster-cup stage of the wheat rust was once supposed -also to be a different plant, and the genus was called _æcidium_. -The occurrence of wheat rust in great abundance on the leeward side -of affected barberry bushes in England suggested to the farmers that -wheat rust was caused by barberry rust. It was later found that the -æcidiospores of the barberry, when sown on wheat, germinate and the -thread of mycelium enters the tissues of the wheat, forming mycelium -between the cells. This mycelium then bears the uredospores, and later -the teleutospores. - -=407. Uredospores can produce successive crops of -uredospores.=—The uredospores are carried by the wind to other -wheat or grass plants, germinate, form mycelium in the tissues, -and later the pustules with a second crop of uredospores. Several -successive crops of uredospores may be developed in one season, so -this is the form in which the fungus is greatly multiplied and widely -distributed. - -[Illustration: Fig. 218. - -Section through leaf of barberry at point affected with the cluster-cup -stage of the wheat rust; spermagonia above, æcidia below. (After -Marshall-Ward.)] - -[Illustration: Fig. 219. - -_A_, section through sorus of black rust of wheat, showing -teleutospores. _B_, mycelium bearing both teleutospores and -uredospores. (After de Bary.)] - -[Illustration: Fig. 220. Germinating uredospore of wheat rust. (After -Marshall-Ward.)] - -[Illustration: Fig. 221. Germ tube entering the leaf through a stoma.] - -=407a. Teleutospores the last stage of the fungus in the season.=—The -teleutospores are developed late in the season, or late in the -development of the host plant (in this case the wheat is the host). -They then rest during the winter. In the spring under favorable -conditions each cell of the teleutospore germinates, producing a -short mycelium called a _promycelium_, as shown in figs. 222, 223. -This promycelium is usually divided into four cells. From each cell a -short, pointed process is formed called a “_sterigma_.” Through this -the protoplasm moves and forms a small gonidium on the end, sometimes -called a _sporidium_. - -[Illustration: Fig. 222. Teleutospore germinating, forming promycelium.] - -[Illustration: Fig. 223. Promycelium of germinating teleutospore, -forming sporidia.] - -[Illustration: Fig. 224. Germinating sporidia entering leaf of barberry -by mycelium. - -Figs. 222-224.—Puccinia graminis (wheat rust). (After Marshall-Ward.)] - -=408. How the fungus gets from the wheat back to the barberry.=—If -these sporidia from the teleutospores are carried by the wind so that -they lodge on the leaves of the barberry, they germinate and produce -the cluster-cup again. The plant has thus a very complex life history. -Because of the presence of several different forms in the life cyle, it -is called a _polymorphic_ fungus. - -The presence of the barberry does not seem necessary in all cases for the -development of the fungus from one year to another. - -=409. Synopsis of life history of wheat rust.= - -_Cluster-cup stage on leaf of barberry._ - - Mycelium between cells of leaf in affected spots. - Spermagonia (sing. spermagonium), small flask-shaped - bodies sunk in upper side of leaf; contain “spermatia.” - Æcidia (sing. æcidium), cup-shaped bodies in under side - of leaf. - Wall or peridium, made up of outer layer of fungus - threads which are divided into short cells but - remain united. - At maturity bursts through epidermis of leaf; margin - of cup curves outward and downward toward surface - of leaf. - Central threads of the bundle are closely packed, but - free. Threads divide into short angular cells - which separate and become æcidiospores, with - orange-colored content. - Æcidiospores carried by the wind to wheat, oats, grasses, - etc. Here they germinate, mycelium enters at stomate, - and forms mycelium between cells of the host. - -_Uredo stage (red-rust) on wheat, oats, grasses, etc._ - - Mycelium between cells of host. - Bears uredospores (1-celled) in masses under epidermis, - which is later ruptured and uredospores set free. - Uredospores carried by wind to other individual hosts, - and new crops of uredospores formed. - -_Teleutospore stage (black rust), also on wheat, etc._ - - Mycelium between cells of host. - Bears teleutospores (2-celled) in masses (sori) under - epidermis, which is later ruptured. - Teleutospores rest during winter. In spring each cell - germinates and produces a promycelium, a short - thread, divided into four cells. - Promycelium bears four sterigmata and four gonidia - (or sporidia), which in favorable conditions pass - back to the barberry, germinate, the tube enters - between cells into the intercellular spaces of the - host to produce the cluster-cup again, and thus the - life cycle is completed. - -=410. Other examples of the rusts.=—Some of the rusts do great -injury to fruit trees and also to forest trees. The “cedar apples” are -abnormal growths on the leaves and twigs of the cedar stimulated by the -presence of the mycelium of a rust known as Gymnosporangium macropus. -The teleutospores are two-celled and are formed in the tissue of the -“cedar apple” or gall. The teleutosori are situated at quite regular -intervals over the surface of the gall at small circular depressions, -and can be easily seen in late autumn and during the winter. A quantity -of gelatine is developed along with the teleutospores. In early spring -with the warm spring rains the gelatinous substance accompanying the -teleutospores swells greatly, and causes the teleutospores to ooze -out in long, dull, orange-colored strings, which taper gradually to -a slender point and bristle all over the “cedar apple.” Here the -teleutospores germinate and produce the sporidia. The sporidia are -carried to apple trees where they infect leaves and even the fruit, -producing here the cluster-cups. There are no uredospores. - -G. globosum is another species forming cedar apples, but the gelatinous -strings of teleutospores are short and clavate, and the cluster-cups -are formed on hawthorns. G. nidusavis forms “witches brooms” or “birds -nests” in the branches of the cedar. The mycelium in the branches -stimulates them to profuse branching so that numerous small branches -are developed close together. The teleutosori form small pustules -scattered over the branches. G. clavipes affects the branches of cedar -only slightly deforming them or not at all, and the cluster-cups are -formed on fruits, twigs, and leaves of the hawthorns or quinces, the -cluster-cups being long, tubular, and orange in color. - - - - -CHAPTER XXI. - -THE HIGHER FUNGI. - - -=411. The series of the higher fungi.=—Of these there are two -large series. One of these is represented by the sac fungi, and the -other by the mushrooms, a good example of which is the common mushroom -(Agaricus campestris). - - -Sac Fungi (Ascomycetes). - -=412. The sac fungi= may be represented by the “powdery mildews”; -examples, uncinula, microsphæra, podosphæra, etc. Fig. 225 is from a -photograph of two willow leaves affected by one of these mildews. The -leaves are first partly covered with a whitish growth of mycelium, and -numerous chains of colorless gonidia are borne on short erect threads. -The masses of gonidia give the leaf a powdery appearance. The mycelium -lives on the outer surface of the leaf, but sends short haustoria into -the epidermal cells. - -=413. Fruit bodies of the willow mildew.=—On this same mycelium -there appear later numerous black specks scattered over the affected -places of the leaf. These are the fruit bodies (_perithecia_). If -we scrape some of these from the leaf, and mount them in water for -microscopic examination, we shall be able to see their structure. -Examining these first with a low power of the microscope, each one is -seen to be a rounded body, from which radiate numerous filaments, the -_appendages_. Each one of these appendages is coiled at the end into -the form of a little hook. Because of these hooked appendages this -genus is called _uncinula_. This rounded body is the _perithecium_. - -[Illustration: Fig. 225. Leaves of willow showing willow mildew. The -black dots are the fruit bodies (perithecia) seated on the white -mycelium.] - -=414. Asci and ascospores.=—While we are looking at a few of -these through the microscope with the low power, we should press on the -cover glass with a needle until we see a few of the perithecia rupture. -If this is done carefully we see several small ovate sacs issue, each -containing a number of spores, as shown in fig. 227. Such a sac is an -_ascus_, and the spores are _ascospores_. - -[Illustration: Fig. 226. Willow mildew; bit of mycelium with erect -conidiophores, bearing chain of gonidia; gonidium at left germinating.] - -[Illustration: Fig. 227. Fruit of willow mildew, showing hooked -appendages. Genus uncinula.] - -[Illustration: Fig. 228. Fruit body of another mildew with dichotomous -appendages. Genus microsphæra. - -Figs. 227-228.—Perithecia (perithecium) of two powdery mildews, -showing escape of asci containing the spores from the crushed fruit -bodies.] - -[Illustration: Fig. 229. Contact of antheridium and carpogonium -(carpogonium the larger cell); beginning of fertilization.] - -[Illustration: Fig. 230. Disappearance of contact walls of antheridium -and carpogonium, and fusion of the two nuclei.] - -[Illustration: Fig. 231. Fertilized egg surrounded by the enveloping -threads which grow up around it. - -Figs. 229-231.—Fertilization in sphærotheca; one of the powdery -mildews. (After Harper.)] - -=415. Number of spores in an ascus.=—The ascus is the most -important character showing the general relationship of the members of -the sac fungi. While many of the powdery mildews have a variable number -of spores in an ascus, a large majority of the ascomycetes have just 8 -spores in an ascus, while some have 4, others 16, and some an -indefinite number. The asci in a perithecium are more variable. In some -ascomycetes there is no perithecium. - -[Illustration: Fig. 231_a_. - -Edible Morel. Morchella esculenta. The asci, forming hymenium, cover -the pitted surface.] - -=416. The black fungi.=—These are very common on dead logs, -branches, leaves, etc., and may be collected in the woods at almost any -season. The perithecia are often numerous, scattered or densely crowded -as in Rosellinia. Sometimes they are united to form a crust which is -partly formed from sterile elements as in Hypoxylon, or they form black -clavate or branched bodies as in Xylaria. The black knot of the plum -and cherry is also an example. - -The lichens are mostly ascomycetes like the black fungi or cup fungi, -while a few are basidiomycetes. - -=417. The morels (Morchella).=—There are several species of -morels which are common in early spring on damp ground. Either one of -the species is suitable for use if it is desired to include this in -the study. Fig. 231a illustrates the Morchella esculenta. The stem is -cylindrical and stout. The fruiting portion forms the “head,” and it -is deeply pitted. The entire pitted surface is covered by the asci, -which are cylindrical and eight spored. A thin section may be made of -a portion for study, or a small piece may be crushed under the cover -glass. - -=418. The cup fungi.=—These fungi are common on damp ground or -on rotting logs in the summer. They may be preserved in 70 per cent -alcohol for study. Many of them are shaped like broad open cups or -saucers. The inner surface of the cup is the fruiting surface, and is -covered with the cylindrical asci, which stand side by side. A bit of -the cup may be sectioned or crushed under a cover glass for study. - - -Mushrooms (Basidiomycetes). - -=419. The large group of fungi= to which the mushroom belongs is -called the basidiomycetes because in all of them a structure resembling -a club, or basidium, is present, and bears a limited number of spores, -usually four, though in some genera the number is variable. Some place -the rusts (Uredineæ) in the same series (basidium series), because of -the short promycelium and four sporidia developed from each cell of the -teleutospore. - -=420. The gill-bearing fungi (Agaricaceæ).=—A good example for -this study is the common mushroom (Agaricus campestris). - -This occurs from July to November in lawns and grassy fields. The -plant is somewhat umbrella-shaped, as shown in fig. 232, and possesses -a cylindrical stem attached to the under side of the convex cap or -pileus. On the under side of the pileus are thin radiating plates, -shaped somewhat like a knife blade. These are the gills, or lamellæ, -and toward the stem they are rounded on the lower angle and are not -attached to the stem. The longer ones extend from near the stem to the -margin of the pileus, and the V-shaped spaces between them are occupied -by successively shorter ones. Around the stem a little below the gills -is a collar, termed the ring or annulus. - -[Illustration: Fig. 232. Agaricus campestris. View of under side -showing stem, annulus, gills, and margin of pileus.] - -[Illustration: Fig. 233. - -Agaricus campestris. Longitudinal section through stem and pileus. -_a_, pileus; _b_, portion of veil on margin of pileus; _c_, gill; _d_, -fragment of annulus; _e_, stipe.] - -[Illustration: Fig. 234. - -Portion of section of lamella of Agaricus campestris. _tr_, trama; -_sh_, subhymenium; _b_, basidium; _st_, sterigma (_pl._ sterigmata); -_g_, basidiospore.] - -[Illustration: Fig. 235. Portion of hymenium of Coprinus micaceus, -showing large cystidium in the hymenium.] - -=421. Fruiting surface of the mushroom.=—The surface of these -gills is the fruiting surface of the mushroom, and bears the gonidia -of the mushroom, which are dark purplish brown when mature, and thus -the gills when old are dark in color. If we make a thin section across -a few of the gills, we see that each side of the gill is covered with -closely crowded club-shaped bodies, each one of which is a _basidium_. -In fig. 234 a few of these are enlarged, so that the structure of -the gill can be seen. Each basidium of the common mushroom has two -spinous processes at the free end. Each one is a _sterig′ma_ (plural -_sterig′mata_), and bears a gonidium. In a majority of the members of -the mushroom family each basidium bears four spores. When mature these -spores easily fall away, and a mass of them gives a purplish-black -color to objects on which they fall, so that a print of the under -surface of the cap showing the arrangement of the gills can be obtained -by cutting off the stem, and placing the pileus on white paper for a -time. - -[Illustration: Fig. 236. Agaricus campestris. Soil washed from “spawn” -and “buttons,” showing the minute young “buttons” attached to the -strands of mycelium.] - -=422. How the mushroom is formed.=—The mycelium of the mushroom -lives in the ground, and grows here for several months or even years, -and at the proper seasons develops the mature mushroom plant. The -mycelium lives on decaying organic matter, and a large number of the -threads grow closely together forming strands, or cords, of mycelium, -which are quite prominent if they are uncovered by removing the soil, -as shown in fig. 236. - -[Illustration: Fig. 237. Agaricus campestris; sections of “buttons” of -different sizes, showing formation of gills and veil covering them.] - -=423.= From these strands the buttons arise by numerous threads -growing side by side in a vertical direction, each thread growing -independently at the end, but all lying very closely side by side. When -the buttons are quite small the gills begin to form on the under margin -of the knob. They are formed by certain of the threads growing downward -in radiating ridges, just as many of these ridges being started as -there are to be gills formed. At the same time, threads of the stem -grow upward to meet those at the margin of the button in such a manner -that they cover up the forming gills, and thus enclose the gills in a -minute cavity. Sections of buttons at different ages will show this, as -is seen in fig. 237. This curtain of mycelium which is thus stretched -across the gill cavity is the veil. As the cap expands more and more -this is stretched into a thin and delicate texture as shown in fig. -238. Finally, as shown in fig. 239, this veil is ruptured by the -expansion of the pileus, and it either clings to the stem as a collar, -or a portion of it remains clinging to the margin of the cap. When the -buttons are very young the gills are white, but they soon become pink -in color, and very soon after the veil breaks the spores mature, and -then the gills are dark brown. - -[Illustration: Fig. 238. Agaricus campestris; nearly mature plants, -showing veil still stretched across the gill cavity.] - -[Illustration: Fig. 239. Agaricus campestris; under view of two plants -just after rupture of veil, fragments of the latter clinging both to -margin of pileus and to stem.] - -[Illustration: Fig. 240. Agaricus campestris; plant in natural position -just after rupture of veil, showing tendency to double annulus on the -stem. Portions of the veil also dripping from margin of pileus.] - -[Illustration: Fig. 241. Agaricus campestris; spore print.] - -[Illustration: Fig. 242. “Fairy ring” formed by Agaricus arvensis -(photograph by B. M. Duggar). The mycelium spreads centrifugally each -year, consuming the available food, and thus the plants appear in a -ring.] - -[Illustration: Fig. 243. Amanita phalloides; white form, showing -pileus, stipe, annulus, and volva.] - -=424. Beware of the poisonous mushroom.=—The number of species -of mushrooms, or toadstools as they are often called, is very great. -Besides the common mushroom (Agaricus campestris) there are a large -number of other edible species. But one should be very familiar with -any species which is gathered for food, unless collected by one who -certainly knows what the plant is, since carelessness in this respect -sometimes results fatally from eating poisonous ones. - -[Illustration: Fig. 244. Amanita phalloides; plant turned to one side, -after having been placed in a horizontal position, by the directive -force of gravity.] - -=425.= A plant very similar in structure to the Agaricus -campestris is the Lepiota naucina, but the spores are white, and thus -the gills are white, except that in age they become a dirty pink. This -plant occurs in grassy fields and lawns often along with the common -mushroom. Great care should be exercised in collecting and noting the -characters of these plants, for a very deadly poisonous species, the -deadly amanita (Amanita phalloides) is perfectly white, has white -spores, a ring, and grows usually in wooded places, but also sometimes -occurs in the margins of lawns. In this plant the base of the stem is -seated in a cup-shaped structure, the _volva_, shown in fig. 243. One -should dig up the stem carefully so as not to tear off this volva if -it is present, for with the absence of this structure the plant might -easily be mistaken for the lepiota, and serious consequences would -result. - -[Illustration: Fig. 245. Edible Boletus. Boletus edulis. Fruiting -surface honey-combed on under side of cap.] - -=426. Tube-bearing fungi (Polyporaceæ).=—In the tube-bearing -fungi, the fruiting surface, instead of lying over the surface of -gills, lines the surface of tubes or pores on the under side of the -cap. The fruit-bearing portion therefore is “honey-combed.” The sulphur -polyporus (Polyporus sulphureus) illustrates one form. The tube-bearing -fungi are sometimes called “bracket” fungi, or “shelf” fungi, because -the pileus is attached to the tree or stump like a shelf or bracket. -One very common form in the woods is the plant so much sought by -“artists,” and often called Polyporus applanatus. It is hard and woody, -reddish brown, brown or grayish on the upper side, according to age, -and is marked by prominent and large concentric ridges. (This form is -probably P. leucophæus.) The under side is white and honey-combed by -numerous very minute pores. This plant is perennial, that is, it lives -from year to year. Each year a new layer is added to the under side, -and several new rings usually to the margin. If a plant two or three -years old is cut in two, there will be seen several distinct tube -layers or strata, each one representing a year’s growth. - -In some of these bracket fungi, each ring on the upper surface marks a -year’s growth as in the pine polyporus (P. pinicola). In the birch -polyporus (P. fomentarius) the tubes are quite large. It also occurs on -other trees. The beech polyporus (P. igniarius, also on other trees) -often becomes very old. I have seen one specimen over eighty years old. -Not all the tube-bearing fungi are bracket form. Some have a stem and -cap (see fig. 245). Some are spread on the surface of logs. - -[Illustration: Fig. 246. Coral fungus. Hydnum coralloides, spines -hanging down from branches.] - -=427. Hedgehog fungi (Hydnaceæ).=—These plants are bracket in -form or have a stem and cap, or are spread on the surface of wood; but -the finest specimens resemble coral masses of fungus tissue (example, -Hydnum, fig. 246). In most of them there are slender processes -resembling teeth, spines or awls, which depend from the under surface -(fig. 247). The fruiting surface covers these spines. - -=428. Coral fungi or fairy clubs (Clavariaceæ).=—These plants -stand upright from the wood, leaves, or soil, on which they grow -(example, Clavaria). The “coral” ones are branched, while the “fairy -clubs” are simple. The fruiting surface covers the entire exposed -surface of the plants (fig. 248). - -[Illustration: Fig. 247. Hydnum repandum, spines hanging down from -under side of cap.] - -[Illustration: Fig. 248. Clavaria botrytes.] - - - - -CHAPTER XXII. - -CLASSIFICATION OF THE FUNGI. - - -=429. Classification of the fungi.=—Those who believe that the fungi -represent a natural group of plants arrange them in three large series -related to each other somewhat as follows: - - The Gonidium Type or Series. The number of - gonidia in the sporangium is indefinite and - variable. It may be very large or very small, - or even only one in a sporangium. To this - series belong the lower fungi; examples: mucor, - saprolegnia, peronospora, etc. - - The Basidium Type or Series. The number of - gonidia on a basidium is limited and definite, - and the basidium is a characteristic structure; - examples: uredineæ (rusts), mushrooms, etc. - - The Ascus Type or Series. The number of spores - in an ascus is limited and definite, and the - ascus is a characteristic structure; examples: - leaf curl of peach (exoascus), powdery mildews, - black knot of plum, black rot of grapes, etc. - -=430.= Others believe that the fungi do not represent a natural -group, but that they have developed off from different groups of -the algæ by becoming parasitic. As parasites they no longer needed -chlorophyll, and consequently lost it. - -According to this view the lower fungi have developed off from the -lower algæ (saprolegnias, mucors, peronosporas, etc., being developed -off from siphonaceous algæ like vaucheria), and the higher fungi being -developed off from the higher algæ (the ascomycetes perhaps from the -Rhodophyceæ). - -=431. A very general outline of classification=,[19] according to -the former of these views, might be presented here to show the general -relationships of the fungi studied, with the addition of a few more -in orders not represented above. It should be borne in mind that the -author in presenting this view of classification does not necessarily -commit himself to it. It is based on that presented in Engler & -Prantl’s Pflanzenfamilien. There are three classes. - -[19] =Class Myxomycetes=, or =Mycetozoa=.—To this class belong the -“slime molds,” low organisms consisting of masses of naked protoplasm -which flows among decaying leaves and in decaying wood, coming to -the surface to fruit. The fruit in many cases resembles miniature -puff-balls, and these plants were formerly classed with the puff-balls. -The spores germinate by forming swarm spores which unite to form a -small plasmodium, which in turn grows to form a large plasmodium or -protoplasmic mass. It is doubtful if they are any more plant than -animal organisms. Examples: Trichia, Arcyria, Stemonitis, Physarum, -Ceratiomyxa, etc., on rotten wood; Plasmodiophora brassicæ is a -parasite causing club foot of cabbage, radishes, etc. It lives within -the roots, causing large knots and swellings on the same. - -[Illustration: Fig. 249. - -Chytrids. _A_, Harpochytrium hedenii, parasitic on spirogyra threads; -_a_, sickle-form plant; _b_, the sporangium part with escaping -zoospores; _c_, old plant proliferating by forming new sporangium -in the old empty one; _d_, zoospore; _e_, two young plants just -beginning to grow. _B_, Rhizophidium globosum parasitic on spirogyra. -Globose sporangium with delicate threads inside of the host, zoospores -escaping from one. _C_, Olpidium pendulum, parasitic in spirogyra cell. -Elliptical sporangium with slender exit tube through which zoospores -are escaping. _D_, Lagenidium rabenhorstii parasitic in spirogyra cell. -Two slender sporangia with exit tubes through which protoplasm escapes -forming a rounded mass at the end of tube, this protoplasm forming -biciliate zoospores.] - - -I. Class Phycomycetes (Alga-like Fungi). - - -1. SUBCLASS OOMYCETES. - -=432.= These are the egg-spore fungi. They include the water mold -(Saprolegnia), the downy mildew of the grape (Plasmopara), the potato -blight (Phytophthora), the white rust of cruciferous plants (Cystopus -= Albugo), the damping-off fungus (Pythium), and many parasites of -the algæ known as chytrids, as Olpidium, Rhizophidium, Lagenidium, -Chytridium, etc. - -The two following orders are sometimes placed in a separate subclass, -_Archimycetes_. - -[Illustration: Fig. 250. - -Monoblepharis insignis Thaxter. End of hypha bearing oogonium (_oog_) -and antheridium (_ant_). Sperms escaping from antheridium and creeping -up on the oogonium. (After Thaxter.)] - -=433. Order Chytridiales (Chytridineæ).=—These include the lowest -fungi. Many of them are parasitic on algæ and lack mycelium, the -swarm spore either with or without minute rhizoids, developing into -a globose sporangium (Rhizophidium, Chytridium, Olpidium, etc., fig. -249), or the swarm spore attached to the wall of the host develops into -a long sword-shaped body with a sterile base, which proliferates and -forms a new sporangium in the old one (Harpochytrium), or with slight -development of mycelium in aquatic plants (Cladochytrium). Some are -parasitic in leaves and stems of land plants. Synchytrium decipiens is -very common on the trailing legume, Amphicarpæa monoica. - -=434. Order Ancylistales (Ancylistineæ).=—The members of this -order have a slight development of mycelium and many are parasitic in -algæ (Lagenidium, fig. 249). - -=435. Order Saprolegniales (Saprolegniineæ).=—These include the -water molds (Saprolegnia). See Chapter XIX. - -=436. Order Monoblepharidales (Monoblepharidineæ).=—These are -peculiar water molds, related to the Saprolegniales, but motile sperm -cells are formed (Monoblepharis, etc., fig. 250). - -=437. Order Peronosporales (Peronosporineæ).=—These include the -downy mildews (Peronospora, Plasmopara, Phytopthora, etc.), and the -white rust of crucifers and other plants (Cystopus = Albugo), Chapter -XIX. - - -2. SUBCLASS ZYGOMYCETES. - -=438.= These are the conjugating fungi. - -=439. Order Mucorales (Mucorineæ).=—This includes the black mold -and its many relatives (Mucor, Rhizopus, etc.). Chapter XIX. - -=440. Order Entomophthorales (Entomophthorineæ).=—This order -includes the “fly fungus” (Empusa) and its many relatives parasitic on -insects. In the autumn and winter dead flies are often found stuck to -window-panes, with a white ring of the conidia around each fly. - - -II. Class Ascomycetes. (The ascus series.) - - -1. SUBCLASS HEMIASCOMYCETES. - -[Illustration: Fig. 251. - -Dipodascus albidus. _A_, thread with sexual organs, ascogonium and -antheridium; _B_, fertilized ascogonium developing ascus; _C_, ascus -with spores; _D_, conidia. (After Lagerheim.)] - -=441. Order Hemiascales (Hemiascineæ).=—Fungi with a well -developed, septate mycelium, but with a sporangium-like ascus, i.e., -a large and indefinite number of spores in the ascus. Examples: -Protomyces macrosporus in stems of Umbelliferæ, or P. polysporus in -Ambrosia trifida. These two are by some placed in the Ustilagineæ. -Dipodascus albidus grows in the exuding sap of Bromeliaceæ in Brazil -and the sap of the beech in Sweden. The ascus is developed as the -result of the fertilization of an ascogonium with an antheridium (see -fig. 251). - - -2. SUBCLASS PROTOASCOMYCETES. - -=442. The asci are well-defined= and usually with a limited and -definite number of spores (usually 8, sometimes 1, 2, 4, 16, or more). -Mycelium often well developed and septate. Asci scattered on the -mycelium, not associated in definite fields or groups. - -=443. Order Protoascales (Protoascineæ).=—The asci are separate -cells, or are scattered irregularly in loose wefts of mycelium. -No fruit body. (The yeast, Saccharomyces, see paragraph 237; and -certain mold-like fungi, some of which are parasitic on mushrooms, as -Endomyces, are examples.) - - -3. SUBCLASS EUASCOMYCETES. - -Asci associated in surfaces forming a hymenium, or in groups or -intermingled in the elements of a fruit body. Fruit body usually -present. - -The following four or five orders comprise the Discomycetes, according -to the usual classification. - -=444. Order Protodiscales (Protodiscineæ).=—The asci are exposed -and form large and indefinite groups, but there is no definite fruit -body. Examples: leaf curl of peach, plum pocket, etc. (Exoascus). - -=445. Order Helvellales (Helvellineæ).=—The asci form large -fields over the upper portion of the fruit body. This order includes -the morels (fig. 231_a_), helvellas, earth tongues (Geoglossum), etc. - -=446. Order Pezizales (Pezizineæ).=—The asci form a definite -field or fruiting surface surrounded on the sides and below by a wall -of fungus tissue, forming a fruit body in the shape of a cup. These are -known as the cup fungi (Peziza, Lachnea, etc.). - -=447. Order Phacidiales (Phacidiineæ).=—Fungi mostly saprophytic, -and fruit body similar to the cup fungi. Examples: Propolis in rotting -wood, Rhytisma forming black crusts on leaves (maple for example), -Urnula craterium, a large black beaker-shaped fungus on the ground. - -=448. Order Hysteriales (Hysteriineæ).=—Fungi with a more or less -elongated fruit body with an enclosing wall opening by a long slit. In -some forms the fruit body has the appearance of a two-lipped body; in -others it is shaped like a clam shell, the asci being inside. Example, -Hysterographium common on dry, dead, decorticated sticks. - -=449. Order Tuberales (Tuberineæ).=—The more or less rounded -fruit bodies are usually subterranean. The most important fungi in this -order are the truffles (Tuber). The mycelium of many species assists -in the formation of mycorhiza on the roots of oaks, etc., and several -species are partly cultivated, or protected, and collected for food. -This is especially the case with Tuber brumale and its forms; more than -a million francs worth of truffles are sold in France and Italy yearly. -Dogs and pigs are employed in the collection of truffles from the -ground. - -=450. Order Plectascales (Plectascineæ).=—The fruit body of these -plants is more or less globose, and contains the asci distributed -irregularly through the mycelium of the interior. Some are subterranean -(Elaphomyces), while others grow in decaying plants, or certain food -substances (Eurotium, Sterigmatocystis, Penicillium). Penicillium in -its conidial stage forms blue mold on fruit, bread, etc. - -The following four orders comprise the Pyrenomycetes, according to the -usual classification. - -=451. Order Perisporiales.=—The powdery mildews are good examples -of this order (Uncinula, Microsphæra, etc., Chapter XXI). - -=452. Order Hypocreales.=[20]—The fruit bodies are colorless, -or bright colored and entirely enclose the asci, sometimes opening -by an apical pore. Nectria cinnabarina has clusters of minute orange -oval fruit bodies, and is common on dead twigs. Cordyceps with a -number of species is parasitic on insects, and on certain subterranean -Ascomycetes, especially Elaphomyces (of the order _Plectascales_ = -_Plectascineæ_). - -=453. Order Dothidiales.=[21]—Fungi with black stroma formed of -mycelium in which are cavities containing the asci. The cavities are -usually shaped like a perithecium, but there is no wall distinct from -the tissue of the stroma (Dothidea, Phyllachora, on grasses). - -=454. Order Sphæriales.=[22]—These contain the so-called black -fungi, with separate or clustered, oval, fruit bodies, black in color. -The black wall encloses the asci, and usually opens by an apical pore. -Examples are found in the black knot of plum and cherry, black rot of -grapes, and in Rosellinia, Hypoxylon, Xylaria, etc., on dead wood. - -=455. Order Laboulbeniales (Laboulbineæ).=—These are peculiar -fungi attached to the legs and bodies of insects by a short stalk, and -provided with a sac-like fruit body which contains the asci. Example, -Laboulbenia. - - -III. Class Basidiomycetes. (The basidium series.) - - -1. SUBCLASS HEMIBASIDIOMYCETES. - -=456. Order Ustilaginales (Ustilagineæ).=—This order includes the -well-known smuts on corn, wheat, oats, etc. (Ustilago, Tilletia, etc.). - - -2. SUBCLASS ÆCIDIOMYCETES. - -=457. Order Uredinales=[23] (Uredineæ).—This order includes the -parasitic fungi known as rusts. Examples: wheat rust (Chapter XX), the -cedar apple, etc. - -The true Basidiomycetes include the following orders: - - -3. SUBCLASS PROTOBASIDIOMYCETES. - -=458. Order Auriculariales.=[24]—This order includes trembling -fungi in which the basidium is long and divided transversely into -usually four cells (example, Auricularia), and similar forms. Pilacre -petersii on dead wood represents an angiocarpous form. - -=459. Order Tremellales (Tremellineæ)=, trembling or gelatinous -fungi with the globose basidium divided longitudinally into four cells -(Tremella). - - -4. SUBCLASS EUBASIDIOMYCETES. - -=460. Order Dacryomycetales (Dacryomycetineæ).=—This order -includes certain fungi of a gelatinous or waxy consistency, usually -of bright colors. They resemble the Tremellales, but the basidia are -slender and fork into two long sterigmata. (Example, Dacryomyces.) -Gyrocephalus rufus is quite a large plant, 10-15 cm. high, growing on -the ground in woods. - -=461. Order Exobasidiales (Exobasidiineæ).=—The fungus causing -azalea apples is an example (Exobasidium). - -=462. Order Hymeniales (Hymenomycetineæ).=—In this order the -basidia are usually club-shaped and undivided, and bear usually four -spores on the end (sometimes two or six). There are several families. - -=463. Family Thelephoraceæ.=—The fruit bodies are more or less -membranous and spread over wood or the ground, or somewhat leaf-like, -growing on wood or the ground. The fruiting surface is nearly or quite -even, and occupies the under side of the leaf-like bodies (Stereum, -Thelephora) or the outside of the forms spread out on wood (Corticium, -Coniophora). - -=464. Family Clavariaceæ.=—This order includes the fairy clubs, -and some of the coral fungi. The larger number of species are in one -genus (Clavaria, fig. 248). - -=465. Family Hydnaceæ.=—The fungi of this order are known as -“hedgehog” fungi, because of the numerous awl-like teeth or spines over -which the fruiting surface is spread, as in Hydnum (figs. 246, 247). - -=466. Family Polyporaceæ.=—The tube-bearing fungi (Polyporus, -Boletus, etc., fig. 245). - -=467. Family Agaricaceæ.=—The gill-bearing fungi (Agaricus, -Amanita, etc., see Chapter XXI). - -The above five orders, according to the earlier classification (still -used at the present time by some), made up the order Hymenomycetes, -while the following five orders made up the Gasteromycetes. The -Hymenomycetes, according to this system, included those plants in which -the fruiting portion (hymenium) is either exposed from the first, or -if covered by a veil or volva (as in Agaricus, Amanita, etc.) this -ruptures and exposes the fruiting surface before, or at the time of, -the ripening of the spores, while the Gasteromycetes included those in -which the fruit body is closed until after the maturity of the spores. - -=468. Order Phallales (Phallineæ).=—The “stink-horn” fungi, or -“buzzard’s nose.” Usually foul-smelling fungi, the fruiting portion -borne aloft on a stout stalk, and dissolving (Dictyophora, Ithyphallus, -etc.). - -=469. Order Hymenogastrales (Hymenogastrineæ).=—The basidia form -a distinct hymenium which does not break down at maturity. Some of -the plants resemble Boletus or Agaricus in the way the fruit bodies -open (Secotium, etc.), while others open irregularly on the surface -(Rhizopogon) or like an earth star (Sclerogaster), or portions of the -surface become gelatinized (Phallogaster). The last-named one grows on -very rotten wood, while most of the others grow on the ground. - -=470. Order Lycoperdales (Lycoperdineæ).=—These include the -“puff-balls,” or “devil’s snuff-box” (Lycoperdon), and the earth stars -(Geaster). The basidia form a distinct hymenium, but at maturity the -entire inner portion of the plant (except certain peculiar threads, the -capillitium) disintegrates and with the spores forms a powdery mass. - -=471. Order Nidulariales (Nidulariineæ).=—These are known -as bird-nest fungi. The fruit body when mature is cup-shaped, -or goblet-shaped, and contains minute flattened circular bodies -(peridiola) containing the spores. The intermediate portions of the -fruit body disintegrate and set the peridiola free, which then lie in -the cup-shaped base like eggs in a nest. - -=472. Order Plectobasidiales (Plectobasidiineæ).=—The basidia -do not form a definite hymenium, but are interwoven with the threads -inside, or are collected into knot-like groups. (Examples: Calostoma, -Tulostoma, Astræus, Sphærobolus, etc.) - -=472a. Lichens.=—The plant body of the lichens (see paragraphs -200, 201) consists of two component parts, the one a fungus, the other -an alga. The fructification is that of the fungus. The fruit body -shows the lichens to be related some to the Ascomycetes, others to -the Hymenomycetes, and Gasteromycetes. They are usually classified as -a distinct class or order from the fungi, but a natural arrangement -would distribute them in several of the orders above. Their special -relationship with these orders has not been satisfactorily worked out. -For the present they are arranged as follows: - -=Ascolichenes.= - -_Pyrenocarpous lichens_ (those with a fruit body like the -Pyrenomycetes). - -_Gymnocarpous lichens_ (those with a fruit body like the Discomycetes). - -=Hymenolichenes= (those with a fruit body like the Hymenomycetes). - -=Gasterolichenes= (those with a fruit body like the -Gasteromycetes). - -From a vegetative standpoint there are two types according to the -distribution of the elements. - -1st. Where the fungal and algal elements are evenly distributed in the -plant body the lichen is said to be _homoiomerous_. There are two types -of these: - -_a. Filamentous lichens_, example, Ephebe pubescens. - -_b. Gelatinous lichens_, example, Collema (with the alga nostoc), -Physma (with the Chroococcaceæ). - -2d. Where the elements are stratified, as in Parmelia, etc., the lichen -is said to be _heteromerous_. In these there are three types: - -_a. Crustaceous lichens_, the plant body is in the form of a thin -incrustation on rocks, etc. - -_b. Foliaceous lichens_, the plant body is leaf-like and lobed and more -or less loosely attached by rhizoids: Parmelia, Peltigera, etc. - -_c. Fruticose lichens_, the plant body is filamentous or band-like and -branched, as in Usnea, Cladonia, etc. - -[Illustration: Fig. 251_a_. Rock lichen (Parmelia contigua).] - -FOOTNOTES: - -[20] As suborder in Engler and Prantl. - -[21] As suborder in Engler and Prantl. - -[22] As suborder in Engler and Prantl. - -[23] The Uredinales and Auriculariales in Engler and Prantl are placed -in order, Auriculariineæ. - -[24] The Uredinales and Auriculariales in Engler and Prantl are placed -in order, Auriculariineæ. - - - - -CHAPTER XXIII. - -LIVERWORTS (HEPATICÆ). - - -=473.= We come now to the study of representatives of another -group of plants, a few of which we examined in studying the organs of -assimilation and nutrition. I refer to what are called the liverworts. -Two of these liverworts belonging to the genus riccia are illustrated -in figs. 30, 252. - - -Riccia. - -=474. Form of the floating riccia (R. fluitans).=—The general -form of floating riccia is that of a narrow, irregular, flattened, -ribbon-like object, which forks repeatedly, in a dichotomous manner, -so that there are several lobes to a single plant. It receives its -name from the fact that at certain seasons of the year it may be found -floating on the water of pools or lakes. When the water lowers it comes -to rest on the damp soil, and rhizoids are developed from the under -side. Now the sexual organs, and later the fruit capsule, are developed. - -=475. Form of the circular riccia (R. crystallina).=—The circular -riccia is shown in fig. 252. The form of this one is quite different -from the floating one, but the manner of growth is much the same. -The branching is more compact and even, so that a circular plant is -the result. This riccia inhabits muddy banks, lying flat on the wet -surface, and deriving its soluble food by means of the little rootlets -(rhizoids) which grow out from the under surface. - -[Illustration: Fig. 252. Thallus of Riccia crystallina.] - -Here and there on the margin are narrow slits, which extend nearly to -the central point. They are not real slits, however, for they were -formed there as the plant grew. Each one of these V-shaped portions of -the thallus is a lobe, and they were formed in the young condition of -the plant by a branching in a forked manner. Since growth took place -in all directions radially the plant became circular in form. These -large lobes we can see are forked once or twice again, as shown by the -seeming shorter slits in the margin. - -=476. Sexual organs.=—In order to study the sexual organs we -must make thin sections through one of these lobes lengthwise and -perpendicular to the thallus surface. These sections are mounted for -examination with the microscope. - -=477. Archegonia.=—We are apt to find the organs in various -stages of development, but we will select one of the flask-shaped -structures shown in fig. 253 for study. This flask-shaped body we see -is entirely sunk in the tissue of the thallus. This structure is the -female organ, and is what we term in these plants the _archegonium_. It -is more complicated in structure than the oogonium. The lower portion -is enlarged and bellied out, and is the venter of the archegonium, -while the narrow portion is the neck. We here see it in section. The -wall is one cell layer in thickness. In the neck is a canal, and in -the base of the venter we see a large rounded cell with a distinct and -large nucleus. This cell is the _egg_ cell. - -=478. Antheridia.=—The antheridia are also borne in cavities sunk -in the tissue of the thallus. There is here no illustration of the -antheridium of this riccia, but fig. 259 represents an antheridium of -another liverwort, and there is not a great difference between the two -kinds. Each one of those little rectangular sperm mother cells in the -antheridium changes into a swiftly moving body like a little club with -two long lashes attached to the smaller end. By the violent lashing of -these organs the spermatozoid is moved through the water, or moisture -which is on the surface of the thallus. It moves through the canal of -the archegonium neck and into the egg, where it fuses with the nucleus -of the egg, and thus fertilization is effected. - -=479. Embryo.=—In the plants which we have selected thus far for -study, the egg, immediately after fecundation, we recollect, passed -into a resting state, and was enclosed by a thick protecting wall. -But in riccia, and in the other plants of the group which we are now -studying, this is not the case. The egg, on the other hand, after -acquiring a thin wall, swells up and fills the cavity of the venter. -Then it divides by a cross wall into two cells. These two grow, and -divide again, and so on until there is formed a quite large mass -of cells rounded in form and still contained in the venter of the -archegonium, which itself increases in size by the growth of the cells -of the wall. - -[Illustration: Fig. 253. - -Archegonium of riccia, showing neck, venter, and the egg; archegonium -is partly surrounded by the tissue of the thallus. (Riccia -crystallina.)] - -[Illustration: Fig. 254. - -Young embryo (sporogonium) of riccia, within the venter of the -archegonium; the latter has now two layers of cells. (Riccia -crystallina.)] - -=480. Sporogonium of riccia.=—The fruit of riccia, which is -developed from the fertilized egg in the archegonium, forms a rounded -capsule still enclosed in the venter of the archegonium, which grows -also to provide space for it. Therefore a section through the plant at -this time, as described for the study of the archegonium, should show -this capsule. The capsule then is a rounded mass of cells developed -from the egg. A single outer layer of cells forms the wall, and -therefore is sterile. All the inner cells, which are richer in -protoplasm, divide into four cells each. Each of these cells becomes -a spore with a thick wall, and is shaped like a triangular pyramid -whose sides are of the same extent as the base (tetrahedral). These -cells formed in fours are the _spores_. At this time the wall of the -spore-case dissolves, the spores separate from each other and fill the -now enlarged venter of the archegonium. When the thallus dies they are -liberated, or escape between the loosely arranged cells of the upper -surface. - -[Illustration: Fig. 255. Nearly mature sporogonium of Riccia -crystallina; mature spore at the right.] - -[Illustration: Fig. 256. - -Riccia glauca; archegonium containing nearly mature sporogonium. _sg_, -spore-producing cells surrounded by single layer of sterile cells, the -wall of the sporogonium.] - -=481. A new phase in plant life.=—Thus we have here in the -sporogonium of _riccia_ a very interesting phase of plant life, in -which the egg, after fertilization, instead of developing directly into -the same phase of the plant on which it was formed, grows into a quite -new phase, the sole function of which is the development of spores. -Since the form of the plant on which the sexual organs are developed -is called the _gametophyte_, this new phase in which the spores are -developed is termed the _sporophyte_. - -Now the spores, when they germinate, develop the _gametophyte_, or -thallus, again. So we have this very interesting condition of things, -the thallus (gametophyte) bears the sexual organs and the unfertilized -egg. The fertilized egg, starting as it does from a single-celled -stage, develops the sporogonium (sporophyte). Here the single-cell -stage is again reached in the spore, which now develops the thallus. - -=482. Riccia compared with coleochæte, œdogonium, etc.=—We have -said that in the sporogonium of riccia we have formed a new phase in -plant life. If we recur to our study of coleochæte we may see that -there is here possibly a state of things which presages, as we say, -this new phase which is so well formed in riccia. We recollect that -after the fertilized egg passed the period of rest it formed a small -rounded mass of cells, each of which now forms a zoospore. The zoospore -in turn develops the normal thallus (gametophyte) of the coleochæte -again. In coleochæte then we have two phases of the plant, each having -its origin in a one-celled stage. Then if we go back to œdogonium, -we remember that the fertilized egg, before it developed into the -œdogonium plant again (which is the gametophyte), at first divides into -_four_ cells which become zoospores. These then develop the œdogonium -plant. - - Note. Too much importance should not be attached to - this seeming homology of the sporophyte of œdogonium, - coleochæte, and riccia, for the nuclear phenomena - in the formation of the zoospores of œdogonium and - coleochæte are not known. They form, however, a very - suggestive series. - - -Marchantia. - -=483.= The marchantia (M. polymorpha) has been chosen for study -because it is such a common and easily obtained plant, and also for -the reason that with comparative ease all stages of development can -be obtained. It illustrates also very well certain features of the -structure of the liverworts. - -[Illustration: Fig. 257. Male plant of marchantia bearing -antheridiophores.] - -The plants are of two kinds, male and female. The two different organs, -then, are developed on different plants. In appearance, however, before -the beginning of the structures which bear the sexual organs they -are practically the same. The thallus is flattened like nearly all -of the thalloid forms, and branches in a forked manner. The color is -dark green, and through the middle line of the thallus the texture is -different from that of the margins, so that it possesses what we term a -midrib, as shown in figs. 257, 261. The growing point of the thallus is -situated in the little depression at the free end. If we examine the -upper surface with a hand lens we see diamond-shaped areas, and at the -center of each of these areas are the openings known as the stomates. - -[Illustration: Fig. 258. Section of antheridial receptacle from male -plant of Marchantia polymorpha, showing cavities where the antheridia -are borne.] - -=484. Antheridial plants.=—One of the male plants is figured at -257. It bears curious structures, each held aloft by a short stalk. -These are the antheridial receptacles (or male gametophores). Each -one is circular, thick, and shaped somewhat like a biconvex lens. The -upper surface is marked by radiating furrows, and the margin is -crenate. Then we note, on careful examination of the upper surface, -that there are numerous minute openings. If we make a thin section of -this structure perpendicular to its surface we shall be able to unravel -the mystery of its interior. Here we see, as shown in fig. 258, that -each one of these little openings on the surface is an entrance to quite -a large cavity. Within each cavity there is an oval or elliptical -body, supported from the base of the cavity on a short stalk. This is -an antheridium, and one of them is shown still more enlarged in fig. -259. This shows the structure of the antheridium, and that there are -within several angular areas, which are divided by numerous straight -cross-lines into countless tiny cuboidal cells, the _sperm mother -cells_. Each of these, as stated in the former chapter, changes into a -swiftly moving body resembling a serpent with two long lashes attached -to its tail. - -[Illustration: Fig. 259. Section of antheridium of marchantia, showing -the groups of sperm mother cells.] - -[Illustration: Fig. 260. Spermatozoids of marchantia, uncoiling and one -extended, showing the two cilia.] - -=485.= The way in which one of these sperm mother cells changes -into this spermatozoid is very curious. We first note that a coiled -spiral body is appearing within the thin wall of the cell, one end of -the coil larger than the other. The other end terminates in a slender -hair-like outgrowth with a delicate vesicle attached to its free end. -This vesicle becomes more and more extended until it finally breaks and -forms two long lashes which are clubbed at their free ends as shown in -fig. 260. - -[Illustration: Fig. 261. Marchantia polymorpha, female plants bearing -archegoniophores.] - -[Illustration: Fig. 262. Marchantia polymorpha, showing origin of -gametophore.] - -=486. Archegonial plants.=—In fig. 261 we see one of the female -plants of marchantia. Upon this there are also very curious structures, -which remind one of miniature umbrellas. The general plan of the -archegonial receptacle (or female gametophore), for this is what these -structures are, is similar to that of the antheridial receptacle, -but the rays are more pronounced, and the details of structure are -quite different, as we shall see. Underneath the arms there hang down -delicate fringed curtains. If we make sections of this in the same -direction as we did of the antheridial receptacle, we shall be able to -find what is secreted behind these curtains. Such a section is figured -at 266. Here we find the archegonia, but instead of being sunk in -cavities their bases are attached to the under surface, while the -delicate, pendulous fringes afford them protection from drying. An -archegonium we see is not essentially different in marchantia from -what it is in riccia, and it will be interesting to learn whether the -sporogonium is essentially different from what we find in riccia. - -=487. Homology of the gametophore of marchantia.=—To see the -relation of the gametophore to the thallus of marchantia take portions -of the thallus bearing the female receptacle. On the under side note -that the prominent midrib continues beyond the thin lateral expansions -and arches upward in the sinus or notch at the end, or at the side -where the branch of the thallus has continued to grow beyond. The stalk -of the gametophore is then a continuation of the midrib of the thallus. -On the apex of this are organized several radial growing points which -develop the digitate or ray-like receptacle. The gametophore is thus a -specialized branch of the thallus. When young, or in many cases when -nearly or quite mature, the gametophore, as one looks at the upper -surface of the thallus, appears to arise from the upper surface, as in -fig. 261. This is because the thin lateral expansions of the thallus -project forward and overlap in advance of the stalk. It is sometimes -necessary to tear these overlapping edges apart to see the real origin -of the gametophore. But in quite old plants these expanded portions are -farther apart and show clearly that the stalk arises from the midrib -below and arches upward in the sinus, as in fig. 262. - - - - -CHAPTER XXIV. - -LIVERWORTS CONTINUED. - - -[Illustration: Fig. 263. - -Archegonial receptacles of marchantia bearing ripe sporogonia. The -capsule of the sporogonium projects outside, while the stalk is -attached to the receptacle underneath the curtain. In the left figure -two of the capsules have burst and the elaters and spores are escaping.] - -=488. Sporogonium of marchantia.=—If we examine the plant shown -in fig. 181 we shall see oval bodies which stand out between the -rays of the female receptacle, supported on short stalks. These are -the sporogonia, or spore-cases. We judge at once that they are quite -different from those which we have studied in riccia, since those were -not stalked. We can see that some of the spore-cases have opened, the -wall splitting down from the apex in several lines. This is caused by -the drying of the wall. These tooth-like divisions of the wall now -curl backward, and we can see the yellowish mass of the spores in slow -motion, falling here and there. It appears also as if there were -twisting threads which aided the spores in becoming freed from the -capsule. - -[Illustration: Fig. 264. - -Section of archegonial receptacle of Marchantia polymorpha; ripe -sporogonia. One is open, scattering spores and elaters; two are still -enclosed in the wall of the archegonium. The junction of the stalk of -the sporogonium with the receptacle is the point of attachment of the -sporophyte of marchantia with the gametophyte.] - -[Illustration: Fig. 265. Elater and spore of marchantia. _sp_, spore; -_mc_, mother cell of spores, showing partly formed spores.] - -=489. Spores and elaters.=—If we take a bit of this mass of -spores and mount it in water for examination with the microscope, we -shall see that, besides the spores, there are very peculiar thread-like -bodies, the markings of which remind one of a twisted rope. These are -very long cells from the inner part of the spore-case, and their walls -are marked by spiral thickenings. This causes them in drying, and also -when they absorb moisture, to twist and curl in all sorts of ways. They -thus aid in pushing the spores out of the capsule as it is drying. - -=490. Sporophyte of marchantia compared with riccia.=—We must -recollect that the sporogonium in marchantia is larger than in riccia, -and that it is also not lying in the tissue of the thallus, but is only -attached to it at one side by a slender stalk. This shows us an -increase in the size and complex structure of this new phase of the -plant, the _sporophyte_. This is one of the very interesting things -which we have to note as we go on in the study of the higher plants. - -[Illustration: Fig. 266. - -Marchantia polymorpha, archegonium at the left with egg; archegonium at -the right with young sporogonium; _p_, curtain which hangs down around -the archegonia; _e_, egg; _v_, venter of archegonium; _n_, neck of -archegonium; _sp_, young sporogonium.] - -=491. Sporophyte dependent on the gametophyte for its -nutriment.=—We thus see that at no time during the development of -the sporogonium is it independent from the gametophyte. This new phase -of plants then, the sporophyte, has not yet become an independent -plant, but must rely on the earlier phase for sustenance. - -=492. Development of the sporogonium.=—It will be interesting -to note briefly how the development of the marchantia sporogonium -differs from that of riccia. The first division of the fertilized egg -is the same as in riccia, that is a wall which runs crosswise of the -axis of the archegonium divides it into two cells. In marchantia the -cell at the base develops the stalk, so that here there is a radical -difference. The outer cell forms the capsule. But here after the wall -is formed the inner tissue does not all go to make spores, as is the -case with riccia. But some of it forms the elaters. While in riccia -only the outside layer of cells of the sporogonium remained sterile, in -marchantia the basal half of the egg remains completely sterile and -develops the stalk, and in the outer half the part which is formed from -some of the inner tissue is also sterile. - -[Illustration: Fig. 267. - -Section of developing sporogonia of marchantia; _nt_, nutritive tissue -of gametophyte; _st_, sterile tissue of sporophyte; _sp_, fertile part -of sporophyte; _va_, enlarged venter of archegonium.] - -=493. Embryo.=—In the development of the embryo we can see all -the way through this division line between the basal half, which is -completely sterile, and the outer half, which is the fertile part. In -fig. 267 we see a young embryo, and it is nearly circular in section -although it is composed of numerous cells. The basal half is attached -to the base of the inner surface of the archegonium, and at this time -the archegonium still surrounds it. The archegonium continues to grow -then as the embryo grows, and we can see the remains of the shrivelled -neck. The portion of the embryo attached to the base of the archegonium -is the sterile part and is called the “foot,” and later develops the -stalk. The sporogonium during all the stages of its development derives -its nourishment from the gametophyte at this point of attachment at -the base of the archegonium. Soon, as shown in fig. 267 at the right, -the outer portion of the sporogonium begins to differentiate into -the cells which form the elaters and those which form spores. These -lie in radiating lines side by side, and form what is termed the -_archesporium_. Each fertile cell forms four spores just as in riccia. -They are thus called the mother cells of the spores, or spore mother -cells. - -=494. How marchantia multiplies.=—New plants of marchantia are -formed by the germination of the spores, and growth of the same to the -thallus. The plants may also be multiplied by parts of the old ones -breaking away by the action of strong currents of water, and when they -lodge in suitable places grow into well-formed plants. As the thallus -lives from year to year and continues to grow and branch the older -portions die off, and thus separate plants may be formed from a former -single one. - -[Illustration: Fig. 268. Marchantia plant with cupules and gemmæ; -rhizoids below.] - -=495. Buds, or gemmæ, of marchantia.=—But there is another -way in which marchantia multiplies itself. If we examine the upper -surface of such a plant as that shown in fig. 268, we shall see that -there are minute cup-shaped or saucer-shaped vessels, and within them -minute green bodies. If we examine a few of these minute bodies with -the microscope we see that they are flattened, biconvex, and at two -opposite points on the margin there is an indentation similar to that -which appears at the growing end of the old marchantia thallus. These -are the growing points of these little buds. When they free themselves -from the cups they come to lie on one side. It does not matter on what -side they lie, for whichever side it is, that will develop into the -lower side of the thallus, and forms rhizoids, while the upper surface -will develop the stomates. - - -Leafy-stemmed liverworts. - -=496.= We should now examine more carefully than we have done -formerly a few of the leafy-stemmed liverworts (called foliose -liverworts). - -[Illustration: Fig. 269. - -Section of thallus of marchantia. _A_, through the middle portion; _B_, -through the marginal portion; _p_, colorless layer; _chl_, chlorophyll -layer; _sp_, stomate; _h_, rhizoids; _b_, leaf-like outgrowths on -underside (Goebel).] - -=497. Frullania= (Fig. 32).—This plant grows on the bark of -logs, as well as on the bark of standing trees. It lives in quite dry -situations. If we examine the leaves we will see how it is able to do -this. We note that there are two rows of lateral leaves, which are very -close together, so close in fact that they overlap like the shingles -on a roof. Then, as the creeping stems lie very close to the bark of -the tree, these overlapping leaves, which also hug close to the stem -and bark, serve to retain moisture which trickles down the bark during -rains. If we examine these leaves from the under side as shown in fig. -34, we see that the lower or basal part of each one is produced into a -peculiar lobe which is more or less cup-shaped. This catches water and -holds it during dry weather, and it also holds moisture which the plant -absorbs during the night and in damp days. There is so much moisture in -these little pockets of the under side of the leaf that minute animals -have found them good places to live in, and one frequently discovers -them in this retreat. There is here also a third row of poorly -developed leaves on the under side of the stem. - -=498. Porella.=—Growing in similar situations is the plant known -as porella. Sometimes there are a few plants in a group, and at other -times large mats occur on the bark of a trunk. This plant, porella, -also has closely overlapping leaves in rows on opposite sides of the -stem, and the lower margin of each leaf is curved under somewhat as in -frullania, though the pocket is not so well formed. - -The larger plants are female, that is they bear archegonia, while -the male plants, those which bear antheridia, are smaller and the -antheridia are borne on small lateral branches. The antheridia -are borne in the axils of the leaves. Others of the leafy-stemmed -liverworts live in damp situations. Some of these, as Cephalozia, grow -on damp rotten logs. Cephalozia is much more delicate, and the leaves -are farther apart. It could not live in such dry situations where the -frullania is sometimes found. If possible the two plants should be -compared in order to see the adaptation in the structure and form to -their environment. - -[Illustration: Fig. 270. Thallus of a thalloid liverwort (blasia) -showing lobed margin of the frond, intermediate between thalloid and -foliose plant.] - -=499. Sporogonium of a foliose liverwort.=—The sporogonium of the -leafy-stemmed liverworts is well represented by that of several genera. -We may take for this study the one illustrated in fig. 274, but -another will serve the purpose just as well. We note here that it -consists of a rounded capsule borne aloft on a long stalk, the stalk -being much longer proportionately than in marchantia. At maturity the -capsule splits down into four quadrants, the wall forming four valves, -which spread apart from the unequal drying of the cells, so that the -spores are set free, as shown in fig. 276. Some of the cells inside -of the capsule develop elaters here also as well as spores. These are -illustrated in fig. 278. - -[Illustration: Fig. 271. Foliose liverwort, male plant showing -antheridia in axils of the leaves (a jungermannia).] - -[Illustration: Fig. 272. Antheridium of a foliose liverwort -(jungermannia).] - -[Illustration: Fig. 273. Foliose liverwort, female plant with rhizoids.] - -=500.= In this plant we see that the sporophyte remains attached to -the gametophyte, and thus is dependent on it for sustenance. This is -true of all the plants of this group. The sporophyte never becomes -capable of an independent existence, and yet we see that it is becoming -larger and more highly differentiated than in the simple riccia. - -[Illustration: Fig. 274. Fruiting plant of a foliose liverwort -(jungermannia). Leafy part is the gametophyte; stalk and capsule is the -sporophyte (sporogonium in the bryophytes). - -[Illustration: Fig. 275. Opening capsule showing escape of spores and -elaters. - -[Illustration: Fig. 276. Capsule parted down to the stalk. - -[Illustration: Fig. 277. Four spores from mother cell held in a group. - -[Illustration: Fig. 278. Elaters, at left showing the two spiral marks, -at right a branched elater. - -Figs. 275-278.—Sporogonium of liverwort (jungermannia) opening by -splitting into four parts, showing details of elaters and spores.] - - -The Horned Liverworts.[25] - -=501. The horned liverworts= take their name from the shape of -the sporogonium. This is long, slender, cylindrical, pointed, and very -slightly curved, suggesting the shape of a minute horn. Anthoceros is -one of the most common and widely distributed species. The plant grows -on damp soil or on mud. - - -Anthoceros. - -=502. The gametophyte.=—The gametophyte is thalloid. It is thin, -flattened, green, irregularly ribbon-shaped and branched. It lies on -the soil and is more or less crisped or wavy, or curled, the edges -nearly plane, or somewhat irregular, and with minute lobes, or notches, -especially near the growing end. The general form and branching can -be seen in fig. 279. Where the plants are much crowded the thallus is -more irregular, and often possesses numerous small lateral branches -in addition to the main lobes. Upon the under side are the slender -rhizoids, which attach to the soil. With a hand lens there can be seen -also upon the under side small dark, rounded and thickened spots, where -an alga (nostoc) is located. - - -Sexual Organs of Anthoceros. - -=502. The sexual organs of anthoceros= differ considerably from -those of the other liverworts studied. In the first place they are -immersed in the true tissue of the thallus, i.e., they do not project -above the surface. - -[Illustration: Fig. 279. - -Anthoceros gracilis. _A_, several gametophytes, on which sporangia -have developed; _B_, an enlarged sporogonium, showing its elongated -character and dehiscence by two valves, leaving exposed the slender -columella on the surface of which are the spores, _C_, _D_, _E_, _F_, -elaters of various forms, _G_, spores. (After Schiffner.)] - -=503. Antheridia.=—The antheridium arises from an internal cell -of the thallus, a cell just below the upper surface. This cell develops -usually a group of antheridia which lie in a cavity formed around this -cell as the thallus continues to grow. They are situated along the -middle line of the thallus, and can be seen by making a section in this -direction. The antheridia are oval or rounded, have a wall of one layer -of cells which contains the sperm cells, and each antheridium has a -slender stalk. The sperms are like those of the true liverworts. - -=504. Archegonia.=—The archegonia are also borne along the -middle line of the thallus. Each one arises at an early stage in the -development of the tissue of the thallus from a superficial cell, -but the archegonium does not project above the surface. The venter -therefore which contains the egg is deep down in the thallus, the wall -of the neck is formed from cells indistinguishable from the adjoining -cells of the thallus and opens at the surface. - - -Sporophyte of Anthoceros. - -=505. The Sporogonium.=—The sporogonium is developed from the -fertilized egg, fertilization resulting of course from the fusion of -one of the sperms with the nucleus of the egg. From the lower part -of the embryo certain cells elongate and push out like rhizoids into -the thallus (gametophyte), but never reach the outside so that the -sporogonium derives its nutriment from the gametophyte in a parasitic -manner like the true liverworts. It is surrounded at the base by a -sheath, an outgrowth of the gametophyte. - -=506. Growing point of the sporogonium.=—A remarkable thing -about the sporogonium of anthoceros, and its relatives, is that the -growing point instead of being situated at the free end is located near -the base, just above the nourishing foot. Thus the upper part of the -sporogonium is older. In the old sporogonia there may be ripe spores -near the free end, young ones near the middle, and undifferentiated -growing tissue near the base. A longitudinal section of a sporogonium -just as the spores are ripening will show this. - -=507. Structure of the sporogonium.=—A longitudinal section -of the sporogonium shows that the spore-bearing tissue occupies a -comparatively small portion of the sporogonium. In the section there -is a narrow layer (two cells thick) on either side and joined at the -top. In the entire sporogonium this fertile tissue is in the shape of -an inverted test tube situated inside of the sporogonium. The wall of -the sporogonium is about four cells thick. The sterile tissue inside -of the spore-bearing tube is the columella. The cells of the wall -contain chlorophyll, and there are true stomata with guard cells in the -epidermal layer. - -=508. Spores and elaters.=—In the spore-bearing tissue there -are two layers of cells (the archesporium). Each cell is a potential -mother cell. The cells, however, of alternate tiers do not form spores. -They elongate some what and are somewhat irregular and sometimes divide -or branch. They are supposed to represent rudimentary _elaters_. The -cells in the other tiers are actual mother cells, and each one forms -four spores. - -=509. The sporophyte of anthoceros= represents the highest type -found in the liverworts. The spongy green parenchyma forming the -wall, with the stomata in the epidermal layer, fits this tissue for -the process of photosynthesis, so that this part of the sporophyte -functions as the green leaf of the seed plants. It has been suggested -by some that if the rhizoids on the nourishing foot could only extend -outside and anchor in the soil, the sporophyte of anthoceros could live -an independent existence. But we see that it stops short of that. - - -Classification of the Liverworts. - - -CLASS HEPATICÆ. - -=510. Order Marchantiales.=[26]—There are two families represented -in the United States. - -Family Ricciaceæ, including Riccia and Ricciocarpus. - -Family Marchantiaceæ, including Marchantia, Fegatella (= Conocephalus), -Fimbriaria, Targionia, etc. - -=511. Order Jungermanniales.=[27]—There are two subdivisions -of this order. _The Anacrogynæ_ include chiefly thalloid forms with -continued apical growth, the archegonia back of the apical cell. -Examples: Blasia, Aneura, Pellia, etc. - -_The Acrogynæ_ include chiefly foliose forms, the archegonia arising -from the apical cell and in such cases interrupting apical growth. -Examples: Cephalozia, Frullania, Bazzania, Jungermannia, Ptilidium, -Porella, etc. - - -CLASS ANTHOCEROTES. - -=512. The Anthocerotes= have formerly been placed with the -Hepaticæ as an order. But because of their wide divergence from -the other liverworts in the development of the sexual organs, and -especially in the structure of the sporophyte, they are now by some -separated as a distinct class. There is one order. - -=Order Anthocerotales.=[28]—This includes one family -(Anthocerotaceæ) with Anthoceros and Notothylas in Europe and North -America, and Dendroceros in the tropics. The latter is epiphytic. - -FOOTNOTES: - -[25] May be used as an alternate study for marchantia. - -[26] As subclass in Engler and Prantl. - -[27] As subclass in Engler and Prantl. - -[28] As subclass in Engler and Prantl. - - - - -CHAPTER XXV. - -MOSSES (MUSCI). - - -=513.= We are now ready to take up the more careful study of the -moss plant. There are a great many kinds of mosses, and they differ -greatly from each other in the finer details of structure. Yet there -are certain general resemblances which make it convenient to take for -study almost any one of the common species in a neighborhood, which -forms abundant fruit. Some, however, are more suited to a first study -than others. (Polytrichum and funaria are good mosses to study.) - -=514. Mnium.=—We will select here the plant shown in fig. 280. -This is known as a mnium (M. affine), and one or another of the species -of mnium can be obtained without much difficulty. The mosses, as we -have already learned, possess an axis (stem) and leaf-like expansions, -so that they are leafy-stemmed plants also. Certain of the branches of -the mnium stand upright, or nearly so, and the leaves are all of the -same size at any given point on the stem, as seen in the figure. There -are three rows of these leaves, and this is true of most of the mosses. - -=515.= The mnium plants usually form quite extensive and pretty -mats of green in shady moist woods or ravines. Here and there among the -erect stems are prostrate ones, with two rows of prominent leaves so -arranged that it reminds one of some of the leafy-stemmed liverworts. -If we examine some of the leaves of the mnium we see that the greater -part of the leaf consists of a single layer of green cells, just as -is the case in the leafy-stemmed liverworts. But along the middle -line is a thicker layer, so that it forms a distinct midrib. This is -characteristic of the leaves of mosses, and is one way in which they -are separated from the leafy-stemmed liverworts, the latter never -having a midrib. - -[Illustration: Fig. 280. - -Portion of moss plant of Mnium affine, showing two sporogonia from one -branch. Capsule at left has just shed the cap or operculum; capsule at -right is shedding spores, and the teeth are bristling at the mouth. -Next to the right is a young capsule with calyptra still attached; next -are two spores enlarged.] - -=516. The fruiting moss plant.=—In fig. 280 is a moss plant “in -fruit,” as we say. Above the leafy stem a slender stalk bears the -capsule, and in this capsule are borne the spores. The capsule then -belongs to the _sporophyte phase_ of the moss plant, and we should -inquire whether the entire plant as we see it here is the sporophyte, -or whether part of it is gametophyte. If a part of it is gametophyte -and a part sporophyte, then where does the one end and the other begin? -If we strip off the leaves at the end of the leafy stem, and make a -longisection in the middle line, we should find that the stalk which -bears the capsule is simply stuck into the end of the leafy stem, and -is not organically connected with it. This is the dividing line, then, -between the gametophyte and the sporophyte. We shall find that here the -archegonium containing the egg is borne, which is a surer way of -determining the limits of the two phases of the plant. - -=517. The male and female moss plants.=—The two plants of mnium -shown in figs. 281, 282 are quite different, as one can easily see, -and yet they belong to the same species. One is a female plant, while -the other is a male plant. The sexual organs then in mnium, as in many -others of the mosses, are borne on separate plants. The archegonia are -borne at the end of the stem, and are protected by somewhat narrower -leaves which closely overlap and are wrapped together. They are similar -to the archegonia of the liverworts. - -[Illustration: Fig. 281. Female plant (gametophyte) of a moss (mnium), -showing rhizoids below, and the tuft of leaves above which protect the -archegonia.] - -[Illustration: Fig. 282. Male plant (gametophyte) of a moss (mnium) -showing rhizoids below and the antheridia at the center above -surrounded by the rosette of leaves.] - -The male plants of mnium are easily selected, since the leaves at the -end of the stem form a broad rosette with the antheridia, and some -sterile threads packed closely together in the center. The ends of the -mass of antheridia can be seen with the naked eye, as shown in fig. -282. When the antheridia are ripe, if we make a section through a -cluster, or if we merely tease out some from the end with a needle in -a drop of water on the slide, then prepare for examination with the -microscope, we can see the form of the antheridia. They are somewhat -clavate or elliptical in outline, as seen in fig. 284. Between them -there stand short threads composed of several cells containing -chlorophyll grains. These are sterile threads (paraphyses). - -=518. Sporogonium.=—In fig. 280 we see illustrated a sporogonium -of mnium, which is of course developed from the fertilized egg-cell of -the archegonium. There is a nearly cylindrical capsule, bent downward, -and supported on a long slender stalk. Upon the capsule is a peculiar -cap,[29] shaped like a ladle or spatula. This is the remnant of the -old archegonium, which, for a time surrounded and protected the young -embryo of the sporogonium, just as takes place in the liverworts. In -most of the mosses this old remnant of the archegonium is borne aloft -on the capsule as a cap, while in the liverworts it is thrown to one -side as the sporogonium elongates. - -[Illustration: Fig. 283. Section through end of stem of female plant of -mnium, showing archegonia at the center. One archegonium shows the egg. -On the sides are sections of the protecting leaves.] - -[Illustration: Fig. 284. Antheridium of mnium with jointed paraphysis -at the left; spermatozoids at the right.] - -=519. Structure of the moss capsule.=—At the free end on the moss -capsule as shown in the case of mnium in fig. 280, after the remnant -of the archegonium falls away, there is seen a conical lid which fits -closely over the end. When the capsule is ripe this lid easily falls -away, and can be brushed off so that it is necessary to handle the -plants with care if it is desired to preserve this for study. - -=520.= When the lid is brushed away as the capsule dries more we -see that the end of the capsule covered by the lid appears “frazzled.” -If we examine this end with the microscope we see that the tissue of -the capsule here is torn with great regularity, so that there are two -rows of narrow, sharp teeth which project outward in a ring around the -opening. If we blow our “breath” upon these teeth they will be seen to -move, and as the moisture disappears and reappears in the teeth, they -close and open the mouth of the capsule, so sensitive are they to the -changes in the humidity of the air. In this way all of the spores are -prevented to some extent from escaping from the capsule at one time. - -=521.= Note. If we make a section longitudinal of the capsule of -mnium, or some other moss, we find that the tissue which develops the -spores is much more restricted than in the capsule of the liverworts -which we have studied. The spore-bearing tissue is confined to a single -layer which extends around the capsule some distance from the outside -of the wall, so that a central cylinder is left of sterile tissue. This -is the columella, and is present in nearly all the mosses. Each of the -cells of the fertile layer divides into four spores. - -[Illustration: Fig. 285. Two different stages of young sporogonium of -a moss, still within the archegonium and wedging their way into the -tissue of the end of the stem. _h_, neck of archegonium; _f_, young -sporogonium. This shows well the connection of the sporophyte with the -gametophyte.] - -=522. Development of the sporogonium.=—The egg-cell after -fertilization divides by a wall crosswise to the axis of the -archegonium. Each of these cells continues to divide for a time, so -that a cylinder pointed at both ends is formed. The lower end of -this cylinder of tissue wedges its way down through the base of the -archegonium into the tissue of the end of the moss stem as shown in -fig. 285. This forms the foot through which the nutrient materials -are passed from the gametophyte to the sporogonium. The upper part -continues to grow, and finally the upper end differentiates into the -mature capsule. - -=523. Protonema of the moss.=—When the spores of a moss germinate -they form a thread-like body, with chlorophyll. This thread becomes -branched, and sometimes quite extended tangles of these threads are -formed. This is called the protonema, that is _first thread_. The older -threads become finally brown, while the later ones are green. From this -protonema at certain points buds appear which divide by close oblique -walls. From these buds the leafy stem of the moss plant grows. Threads -similar to these protonemal threads now grow out from the leafy stem, -to form the rhizoids. These supply the moss plant with nutriment, and -now the protonema usually dies, though in some few species it persists -for long periods. - - -Classification of the Mosses. - -CLASS MUSCINEÆ (MUSCI). - -=524. Order Sphagnales.=[30]—This order includes the peat mosses. There -is but one family (Sphagnaceæ) and but a single genus (Sphagnum). The -peat mosses are widely distributed over the globe, chiefly occurring -in moors, or “bogs,” usually low ground around the shores of lakes, -ponds, or along streams, but they often occur on wet dripping rocks in -cool shady places. Small ponds are sometimes filled in by their growth. -As the sphagnum growing in such an abundance of water only partially -decays, “ground” is built up rather rapidly, and the sphagnum remains -are known as “peat.” This “ground”-building peculiarity of sphagnum -sometimes enables the plant (often in conjunction with others) to fill -in ponds completely. (See Atoll Moor, Chapter LV.) - -The gametophyte of sphagnum, like that of all the mosses, is dimorphic, -but the first part (or protonema) which develops from the spores is -thalloid, and therefore more like the thallose liverworts. The leafy -axis (or gametophore) which develops from the thalloid form is very -characteristic (see Chapter LV). - -The archegonia are borne on the free end of the main axis, while the -antheridia are borne on short branches which are brightly colored, red, -yellow, etc. The sporophyte (sporogonium) is globose and possesses a -broad foot anchored in the end of a naked prolongation of the end of -the leafy gametophore. This naked prolongation of the gametophore looks -like the stalk of the sporogonium, but a study of its connection with -the sporogonium shows that it is part of the gametophyte, which is only -developed after the fertilization of the egg in the archegonium. In the -sporogonium there is a short columella, and the archesporium is in the -form of an inverted cup. - -=525. Order Andreæales.=[31]—This order includes the single genus -Andreæa. The plants are small but form extensive mats, growing on rocks -in arctic or alpine regions usually. They are sometimes found in great -abundance on bare, rather dry rocks on mountains. The protonema is -somewhat thalloid. The sporogonium opens by splitting longitudinally -into four valves. An elongated columella is present so that the -archesporium is shaped like an inverted test tube. - -=526. Order Archidiales.=[32]—This order contains the single genus -Archidium, and by some is placed as an aberrant genus in the Bryales. -There is no columella in the simple sporogonium. The archesporium -occupies all the internal part of the sporogonium, some cells being -fertile and others sterile. - -=527. Order Bryales.=[33]—These include the higher mosses, and a very -large number of genera and species. The protonema is filamentous and -branched except in a few forms where it is partly thalloid as in -Tetraphis (= Georgia). (Tetraphis pellucida is a common moss on very -rotten logs. The capsule has four prominent teeth.) In a few of the -lower genera (Phascum, Pleuridium, etc.) the capsule opens irregularly, -but in the larger number the capsule opens by a lid (operculum). A -cylindrical columella is present, and the archesporium is in the form -of a tube open at both ends. (Examples: Polytrichum, Bryum, Mnium, -Hypnum, etc.) - -=528.= TABLE SHOWING RELATION OF GAMETOPHYTE AND SPOROPHYTE IN THE -LIVERWORTS AND MOSSES. - - ---------------------------------------------------------------------- - GAMETOPHYTE. - (Prominent part of the plant. Leads an independent existence.) - -------------+--------------+---------------+------------------------- - | Vegetative | Vegetative | Sexual Organs. - | Stage. |Multiplication.| - -------------+--------------+---------------+------------------------- - Riccia. |Thallus |Sometimes by |Immersed by surrounding, - |flattened, |branching and |upward growth of thallus. - |ribbon-like, |dying away of +--------------+---------- - |forked, |older parts. |Antheridia, |Archegonia, - |or nearly | |with |with egg - |circular. | |spermatozoids.|in each. - -------------+--------------+---------------+------------------------- - Marchantia. |Thallus |By dying away | Borne on special - |flattened, |of older parts,| receptacles on - |ribbon-like, |and by gemmæ. | different plants. - |forked, | +--------------+---------- - |male and | |Antheridia, |Archegonia, - |female | |with |borne on - |plants bear | |spermatozoids |female - |gametophores. | |borne on |gametophore - | | |antheridio- | (or - | | | phores, |archegonio- - | | |or male | phore), - | | |gametophores. |each with - | | | |an egg. - -------------+--------------+---------------+------------------------- - Jungermannia |A plant with |By dying away | On different plants. - (or |apparent |of older parts.+--------------+---------- - Cephalozia,|leaves and | |Antheridia, |Archegonia, - Porella |stem; margins | |with |each with - etc.) |of thallus | |spermatozoids,|egg, on - |have become | |in axils of |female - |cut into | |leaves of |plant. - |lobes. Male | |male plant. | - |and female | | | - |plants. | | | - | | | | - | | | | - -------------+--------------+---------------+--------------+---------- - Mosses. |Plant with |By branching, | On different plants. - {Mnium, |apparent |by growth of +--------------+---------- - {Funaria, |leafy axis, |protonema from |Antheridia, |Archegonia, - {Polytrichum|3 rows of |axis, leaves, |with |each with - {etc. |leaves |or even |spermatozoids,|egg, on - |(similar to |sporogonium. |at end of |female - |jungermannia),|(In somegenera |stem of male |plant. - |borne on an |by gemmæ.) |plant. |(Calyptra - |earlier | | |found on - |protonemal | | |sporogonium - |stage Male | | |is remnant - |and female | | |of archeg- - |plants. | | | onium.) - | | | | - | | | | - -------------+--------------+---------------+--------------+---------- - - ----------------------------------------------------------+--------- - SPOROPHYTE. | - (Attached to gametophyte and dependent | - on it for nourishment.) | - ---------------+--------------+------------+--------------+Beginning - | Beginning | Sterile | Fertile | of - | of | Part. | Part. |Gameto- - | Sporophyte. | | | phyte. - ---------------+--------------+------------+--------------+ - Riccia. |Fertilized |Wall of |Central mass | - |egg. (Develops|sporogonium,|(archesporium)| - |sporogonium.) |of one-layer|develops .....| Spores. - | |cells. | | - ---------------+--------------+------------+--------------+--------- - Marchantia. |Fertilized |Sterile part|Central part | - |egg. (Develops|of stalked |of capsule | - |sporogonium.) |capsule is |(archesporium)| Spores. - | |stalk, wall |develops .....| - | |of capsule |and elaters. | - | |of several | | - | |layers, | | - | |elaters. | | - ---------------+--------------+------------+--------------+--------- - Jungermannia |Fertilized |Sterile part|Central part | - (or |egg. (Develops|of stalked |of capsule | - Cephalozia, |sporogonium.) |capsule is |(archesporium)| Spores. - Porella, | |stalk, wall |develops .....| - etc.) | |of capsule |and elaters. | - | |of several | | - | |layers, | | - | |elaters. | | - ----------------+--------------+------------+--------------+--------- - Mosses |Fertilized |Sterile part|Cylindrical | - { Mnium, |egg. (Develops|of stalked |layer of | - { Funaria, |sporogonium.) |capsule is |cells around | - { Polytrichum,| |stalk, wall |columella is | - { etc. | |of capsule |the | Spores. - | |of several |archesporium; | - | |layers, |it | - | |columella, |develops .....| - | |lid, teeth | | - | |etc., of | | - | |the highly | | - | |specialized | | - | |capsule. | | - ---------------+--------------+------------+--------------+--------- - -FOOTNOTES: - -[29] Called the calyptra. - -[30] As subclass in Engler and Prantl. - -[31] As subclass in Engler and Prantl. - -[32] As subclass in Engler and Prantl. - -[33] As subclass in Engler and Prantl. - - - - -CHAPTER XXVI. - -FERNS. - - -=529.= In taking up the study of the ferns we find plants which -are very beautiful objects of nature and thus have always attracted -the interest of those who love the beauties of nature. But they are -also very interesting to the student, because of certain remarkable -peculiarities of the structure of the fruit bodies, and especially -because of the intermediate position which they occupy within the -plant kingdom, representing in the two phases of their development the -primitive type of plant life on the one hand, and on the other the -modern type. We will begin our study of the ferns by taking that form -which is the more prominent, the fern plant itself. - -=530. The Christmas fern.=—One of the ferns which is very common -in the Northern States, and occurs in rocky banks and woods, is the -well-known Christmas fern (Aspidium acrostichoides) shown in fig. 286. -The leaves are the most prominent part of the plant, as is the case -with most if not all our native ferns. The stem is very short and -for the most part under the surface of the ground, while the leaves -arise very close together, and thus form a rosette as they rise and -gracefully bend outward. The leaf is elongate and reminds one somewhat -of a plume with the pinnæ extending in two rows on opposite sides of -the midrib. These pinnæ alternate with one another, and at the base of -each pinna is a little spur which projects upward from the upper edge. -Such a leaf is said to be pinnate. While all the leaves have the same -general outline, we notice that certain ones, especially those toward -the center of the rosette, are much narrower from the middle portion -toward the end. This is because of the shorter pinnæ here. - -[Illustration: Fig. 286. Christmas fern (Aspidium acrostichoides).] - -=531. Fruit “dots” (sorus, indusium).=—If we examine the under -side of such short pinnæ of the Christmas fern we see that there -are two rows of small circular dots, one row on either side of the -pinna. These are called the “fruit dots,” or sori (a single one is a -sorus). If we examine it with a low power of the microscope, or with -a pocket lens, we see that there is a circular disk which covers more -or less completely very minute objects, usually the ends of the latter -projecting just beyond the edge if they are mature. This circular disk -is what is called the _indusium_, and it is a special outgrowth of the -epidermis of the leaf here for the protection of the spore-cases. These -minute objects underneath are the fruit bodies, which in the case of -the ferns and their allies are called _sporangia_. This indusium in the -case of the Christmas fern, and also in some others, is attached to the -leaf by means of a short slender stalk which is fastened to the middle -of the under side of this shield, as seen in cross-section in fig. 292. - -=532. Sporangia.=—If we section through the leaf at one of the -fruit dots, or if we tease off some of the sporangia so that the stalks -are still attached, and examine them with the microscope, we can see -the form and structure of these peculiar bodies. Different views of a -sporangium are shown in fig. 293. The slender portion is the stalk, -and the larger part is the spore-case proper. We should examine the -structure of this spore-case quite carefully, since it will help us to -understand better than we otherwise could the remarkable operations -which it performs in scattering the spores. - -[Illustration: Fig. 287. Rhizome with bases of leaves, and roots of the -Christmas fern.] - -=533. Structure of a sporangium.=—If we examine one of the -sporangia in side view as shown in fig. 293, we note a prominent row -of cells which extend around the margin of the dorsal edge from near -the attachment of the stalk to the upper front angle. The cells are -prominent because of the thick inner walls, and the thick radial walls -which are perpendicular to the inner walls. The walls on the back of -this row and on its sides are very thin and membranous. We should make -this out carefully, for the structure of these cells is especially -adapted to a special function which they perform. This row of cells is -termed the _annulus_, which means a little ring. While this is not a -complete ring, in some other ferns the ring is nearly complete. - -[Illustration: Fig. 288. Rhizome of sensitive fern (Onoclea -sensibilis).] - -[Illustration: Fig. 289. Under side of pinna of Aspidium spinulosum -showing fruit dots (sori).] - -=534.= In the front of the sporangium is another peculiar group -of cells. Two of the longer ones resemble the lips of some creature, -and since the sporangium opens between them they are sometimes termed -the lip cells. These lip cells are connected with the upper end of the -annulus on one side and with the upper end of the stalk on the other -side by thin-walled cells, which may be termed connective cells, since -they hold each lip cell to its part of the opening sporangium. The -cells on the side of the sporangium are also thin-walled. If we now -examine a sporangium from the back, or dorsal edge as we say, it will -appear as in the left-hand figure. Here we can see how very prominent -the annulus is. It projects beyond the surface of the other cells of -the sporangium. The spores are contained inside this case. - -=535. Opening of the sporangium and dispersion of the spores.=—If -we take some fresh fruiting leaves of the Christmas fern, or of any one -of many of the species of the true ferns just at the ripening of the -spores, and place a portion of it on a piece of white paper in a dry -room, in a very short time we shall see that the paper is being dusted -with minute brown objects which fly out from the leaf. Now if we take -a portion of the same leaf and place it under the low power of the -microscope, so that the full rounded sporangia can be seen, in a short -time we note that the sporangium opens, the upper half curls backward -as shown in fig. 294, and soon it snaps quickly, to near its former -position, and the spores are at the same time thrown for a considerable -distance. This movement can sometimes be seen with the aid of a good -hand lens. - -[Illustration: Fig. 290. Four pinnæ of adiantum, showing recurved -margins which cover the sporangia.] - -[Illustration: Fig. 291. Section through sorus of Polypodium vulgare -showing different stages of sporangium, and one multicellular capitate -hair.] - -=536. How does this opening and snapping of the sporangium take -place?=—We are now more curious than ever to see just how this -opening and snapping of the sporangium takes place. We should now mount -some of the fresh sporangia in water and cover with a cover glass for -microscopic examination. A drop of glycerine should be placed at one -side of the cover glass on the slip so that the edge of the glycerine -will come in touch with the water. Now as one looks through the -microscope to watch the sporangia, the water should be drawn from under -the cover glass with the aid of some bibulous paper, like filter paper, -placed at the edge of the cover glass on the opposite side from the -glycerine. As the glycerine takes the place of the water around the -sporangia it draws the water out of the cells of the annulus, just as -it took the water out of the cells of the spirogyra as we learned some -time ago. As the water is drawn out of these cells there is produced -a pressure from without, the atmospheric pressure upon the glycerine. -This causes the walls of these cells of the annulus to bend inward, -because, as we have already learned, the glycerine does not pass -through the walls nearly so fast as the water comes out. - -[Illustration: Fig. 292. Section through sorus and shield-shaped -indusium of aspidium.] - -=537.= Now the structure of the cells of this annulus, as we have -seen, is such that the inner walls and the perpendicular walls are -stout, and consequently they do not bend or collapse when this pressure -is brought to bear on the outside of the cells. The thin membranous -walls on the back (dorsal walls) and on the sides of the annulus, -however, yield readily to the pressure and bend inward. This, as we -can readily see, pulls on the ends of each of the perpendicular walls -drawing them closer together. This shortens the outer surface of the -annulus and causes it to first assume a nearly straight position, then -curve backward until it quite or nearly becomes doubled on itself. The -sporangium opens between the lip cells on the front and the lateral -walls of the sporangium are torn directly across. The greater mass of -spores are thus held in the upper end of the open sporangium, and when -the annulus has nearly doubled on itself it suddenly snaps back again -in position. While treating with the glycerine we can see all this -movement take place. Each cell of the annulus acts independently, but -often they all act in concert. When they do not all act in concert, -some of them snap sooner than others, and this causes the annulus to -snap in segments. - -[Illustration: Fig. 293. Rear, side, and front views of fern -sporangium. _d_, _e_, annulus; _a_, lip cells.] - -[Illustration: Fig. 294. Dispersion of spores from sporangium of -Aspidium acrostichoides, showing different stages in the opening and -snapping of the annulus.] - -=538. The movements of the sporangium can take place in old and dried -material.=—If we have no fresh material to study the sporangium -with, we can use dried material, for the movements of the sporangia can -be well seen in dried material, provided it was collected at about the -time the sporangia are mature, that is at maturity, or soon afterward. -We take some of the dry sporangia (or we may wash the glycerine off -those which we have just studied) and mount them in water, and quickly -examine them with a microscope. We notice that in each cell of the -annulus there is a small sphere of some gas. The water which bathes the -walls of the annulus is absorbed by some substance inside these cells. -This we can see because of the fact that this sphere of gas becomes -smaller and smaller until it is only a mere dot, when it disappears in -a twinkling. The water has been taken in under such pressure that it -has absorbed all the gas, and the farther pressure in most cases closes -the partly opened sporangium more completely. - -=539.= Now we should add glycerine again and draw out the water, -watching the sporangia at the same time. We see that the sporangia -which have opened and snapped once will do it again. And so they may -be made to go through this operation several times in succession. We -should now note carefully the annulus, that is after the sporangia have -opened by the use of glycerine. So soon as they have snapped in the -glycerine we can see those minute spheres of gas again, and since there -was no air on the outside of the sporangia, but only glycerine, this -gas must, it is reasoned, have been given up by the water before it was -all drawn out of the cells. - -=540. The common polypody.=—We may now take up a few other ferns -for study. Another common fern is the polypody, one or more species of -which have a very wide distribution. The stem of this fern is also not -usually seen, but is covered with the leaves, except in the case of -those species which grow on the surface of rocks. The stem is slender -and prostrate, and is covered with numerous brown scales. The leaves -are pinnate in this fern also, but we find no difference between the -fertile and sterile leaves (except in some rare cases). The fruit dots -occupy much the same positions on the under side of the leaf that they -do in the Christmas fern, but we cannot find any indusium. In the place -of an indusium are club-shaped hairs as shown in fig. 291. The enlarged -ends of these clubs reaching beyond the sporangia give some protection -to them when they are young. - -=541. Other ferns.=—We might examine a series of ferns to see -how different they are in respect to the position which the fruit -dots occupy on the leaf. The common brake, which sometimes covers -extensive areas and becomes a troublesome weed, has a stout and smooth -underground stem (rhizome) which is often 12 to 20 _cm_ beneath the -surface of the soil. There is a long leaf stalk, which bears the -lamina, the latter being several times pinnate. The margins of the -fertile pinnæ are inrolled, and the sporangia are found protected -underneath in this long sorus along the margin of the pinna. The -beautiful maidenhair fern and its relatives have obovate pinnæ, and -the sori are situated in the same positions as in the brake. In other -ferns, as the walking fern, the sori are borne along by the side of the -veins of the leaf. - -=542. Opening of the leaves of ferns.=—The leaves of ferns -open in a peculiar manner. The tip of the leaf is the last portion -developed, and the growing leaf appears as if it was rolled up as in -fig. 287 of the Christmas fern. As the leaf elongates this portion -unrolls. - -=543. Longevity of ferns.=—Most ferns live from year to year, by -growth adding to the advance of the stem, while by decay of the older -parts the stem shortens up behind. The leaves are short-lived, usually -dying down each year, and a new set arising from the growing end of the -stem. Often one can see just back or below the new leaves the old dead -ones of the past season, and farther back the remains of the petioles -of still older leaves. - -[Illustration: Fig. 295. Cystopteris bulbifera, young plant growing -from bulb. At right is young bulb in axil of pinna of leaf.] - -=544. Budding of ferns.=—A few ferns produce what are called -bulbils or bulblets on the leaves. One of these, which is found -throughout the greater part of the eastern United States, is the -bladder fern (Cystopteris bulbifera), which grows in shady rocky -places. The long graceful delicate leaves form in the axils of the -pinnæ, especially near the end of the leaf, small oval bulbs as shown -in fig. 295. If we examine one of these bladder-like bulbs we see -that the bulk of it is made up of short thick fleshy leaves, smaller -ones appearing between the outer ones at the smaller end of the bulb. -This bulb contains a stem, young root, and several pairs of these -fleshy leaves. They easily fall to the ground or rocks, where, with -the abundant moisture usually present in localities where the fern is -found, the bulb grows until the roots attach the plant to the soil or -in the crevices of the rocks. A young plant growing from one of these -bulbils is shown in fig. 295. - -=545. Greenhouse ferns.=—Some of the ferns grown in -conservatories have similar bulblets. Fig. 296 represents one of these -which is found abundantly on the leaves of Asplenium bulbiferum. These -bulbils have leaves which are very similar to the ordinary leaf except -that they are smaller. The bulbs are also much more firmly attached to -the leaf, so that they do not readily fall away. - -=546.= Plant conservatories usually furnish a number of very -interesting ferns, and one should attempt to make the acquaintance of -some of them, for here one has an opportunity during the winter season -not only to observe these interesting plants, but also to obtain -material for study. In the tree ferns which often are seen growing in -such places we see examples of the massive trunks and leaves of some of -the tropical species. - -[Illustration: Fig. 296. Bulbil growing from leaf of asplenium (_A_, -bulbiferum).] - -=547. The fern plant is a sporophyte.=—We have now studied -the fern plant, as we call it, and we have found it to represent -the spore-bearing phase of the plant, that is the _sporophyte_ -(corresponding to the sporogonium of the liverworts and mosses). - -=548. Is there a gametophyte phase in ferns?=—But in the -sporophyte of the fern, which we should not forget is the fern plant, -we have a striking advance upon the sporophyte of the liverworts and -mosses. In the latter plants the sporophyte remained attached to the -gametophyte, and derived its nourishment from it. In the ferns, as we -see, the sporophyte has a root of its own, and is attached to the soil. -Through the aid of root hairs of its own it takes up mineral solutions. -It possesses also a true stem, and true leaves in which carbon -conversion takes place. It is able to live independently, then. Does -a gametophyte phase exist among the ferns? Or has it been lost? If it -does exist, what is it like, and where does it grow? From what we have -already learned we should expect to find the gametophyte begin with the -germination of the spores which are developed on the sporophyte, that -is on the fern plant itself. We should investigate this and see. - - - - -CHAPTER XXVII. - -FERNS CONTINUED. - - -Gametophyte of ferns. - -=549. Sexual stage of ferns.=—We now wish to see what the sexual -stage of the ferns is like. Judging from what we have found to take -place in the liverworts and mosses we should infer that the form of the -plant which bears the sexual organs is developed from the spores. This -is true, and if we should examine old decaying logs, or decaying wood -in damp places in the near vicinity of ferns, we should probably find -tiny, green, thin, heart-shaped growths, lying close to the substratum. -These are also found quite frequently on the soil of pots in plant -conservatories where ferns are grown. Gardeners also in conservatories -usually sow fern spores to raise new fern plants, and usually one can -find these heart-shaped growths on the surface of the soil where they -have sown the spores. We may call the gardener to our aid in finding -them in conservatories, or even in growing them for us if we cannot -find them outside. In some cases they may be grown in an ordinary room -by keeping the surfaces where they are growing moist, and the air also -moist, by placing a glass bell jar over them. - -[Illustration: Fig. 297. Prothallium of fern, under side, showing -rhizoids, antheridia scattered among and near them, and the archegonia -near the sinus.] - -=550.= In fig. 297 is shown one of these growths enlarged. Upon -the under side we see numerous thread-like outgrowths, the rhizoids, -which attach the plant to the substratum, and which act as organs for -the absorption of nourishment. The sexual organs are borne on the under -side also, and we will study them later. This heart-shaped, flattened, -thin, green plant is the _prothallium_ of ferns, and we should now give -it more careful study, beginning with the germination of the spores. - -[Illustration: Fig. 298. Spore of Pteris serrulata showing the -three-rayed elevation along the side of which the spore wall cracks -during germination.] - -[Illustration: Fig. 299. Spore of Aspidium acrostichoides with winged -exospore.] - -[Illustration: Fig. 300. Spore crushed to remove exospore and show -endospore.] - -=551. Spores.=—We can easily obtain material for the study of the -spores of ferns. The spores vary in shape to some extent. Many of them -are shaped like a three-sided pyramid. One of these is shown in fig. -298. The outer wall is roughened, and on one end are three elevated -ridges which radiate from a given point. A spore of the Christmas fern -is shown in fig. 299. The outer wall here is more or less winged. At -fig. 300 is a spore of the same species from which the outer wall has -been crushed, showing that there is an inner wall also. If possible we -should study the germination of the spores of some fern. - -=552. Germination of the spores.=—After the spores have been -sown for about one week to ten days we should mount a few in water for -examination with the microscope in order to study the early stages. If -germination has begun, we find that here and there are short slender -green threads, in many cases attached to brownish bits, the old walls -of the spores. Often one will sow the sporangia along with the spores, -and in such cases there may be found a number of spores still within -the old sporangium wall that are germinating, when they will appear as -in fig. 302. - -[Illustration: Fig. 301. Spores of asplenium; exospore removed from the -one at the right.] - -[Illustration: Fig. 302. Germinating spores of Pteris aquilina still in -the sporangium.] - -[Illustration: Fig. 303. Young prothallium of a fern (niphobolus).] - -=553. Protonema.=—These short green threads are called -_protonemal_ threads, or _protonema_, which means a _first thread_, and -it here signifies that this short thread only precedes a larger growth -of the same object. In figs. 302, 303 are shown several stages of -germination of different spores. Soon after the short germ tube emerges -from the crack in the spore wall, it divides by the formation of a -cross wall, and as it increases in length other cross walls are formed. -But very early in its growth we see that a slender outgrowth takes -place from the cell nearest the old spore wall. This slender thread is -colorless, and is not divided into cells. It is the first rhizoid, and -serves both as an organ of attachment for the thread, and for taking up -nutriment. - -=554. Prothallium.=—Very soon, if the sowing has not been so -crowded as to prevent the young plants from obtaining nutriment -sufficient, we will see that the end of this protonema is broadening, -as shown in fig. 303. This is done by the formation of the cell walls -in different directions. It now continues to grow in this way, the end -becoming broader and broader, and new rhizoids are formed from the -under surface of the cells. The growing point remains at the middle of -the advancing margin, and the cells which are cut off from either side, -as they become old, widen out. In this way the “wings,” or margins of -the little, green, flattened body, are in advance of the growing point, -and the object is more or less heart-shaped, as shown in fig. 297. Thus -we see how the prothallium of ferns is formed. - -=555. Sexual organs of ferns.=—If we take one of the prothallia -of ferns which have grown from the sowings of fern spores, or one of -those which may be often found growing on the soil of pots in -conservatories, mount it in water on a slip, with the under side -uppermost, we can then examine it for the sexual organs, for these are -borne in most cases on the under side. - -[Illustration: Fig. 304. Male prothallium of a fern (niphobolus), in -form of an alga or protonema. Spermatozoids escaping from antheridia.] - -[Illustration: Fig. 305. Male prothallium of fern (niphobolus), showing -opened and unopened antheridia; section of unopened antheridium; -spermatozoids escaping; spermatozoids which did not escape from the -antheridium.] - -=556. Antheridia.=—If we search among the rhizoids we see small -rounded elevations as shown in fig. 297 or 305 scattered over this -portion of the prothallium. These are the antheridia. If the prothallia -have not been watered for a day or so, we may have an opportunity of -seeing the spermatozoids coming out of the antheridium, for when the -prothallia are freshly placed in water the cells of the antheridium -absorb water. This presses on the contents of the antheridium -and bursts the cap cell if the antheridium is ripe, and all the -spermatozoids are shot out. We can see here that each one is shaped -like a screw, with the coils at first close. But as the spermatozoid -begins to move this coil opens somewhat and by the vibration of the -long cilia which are on the smaller end it whirls away. In such -preparations one may often see them spinning around for a long while, -and it is only when they gradually come to rest that one can make out -their form. - -[Illustration: Fig. 306. Section of antheridia showing sperm cells, and -spermatozoids in the one at the right.] - -[Illustration: Fig. 307. Different views of spermatozoids; in a quiet -condition; in motion (Adiantum concinnum).] - -[Illustration: Fig. 308. Archegonium of fern. Large cell in the center -is the egg, next is the ventral canal cell, and in the canal of the -neck are two nuclei of the canal cell.] - -[Illustration: Fig. 309. Mature and open archegonium of fern (Adiantum -cuneatum) with spermatozoids making their way down through the slime to -the egg.] - -[Illustration: Fig. 310. Fertilization in a fern (Marattia). _sp_, -spermatozoid fusing with the nucleus of the egg. (After Campbell.)] - -=557. Archegonia.=—If we now examine closely on the thicker part -of the under surface of the prothallium, just back of the “sinus,” we -may see longer stout projections from the surface of the prothallium. -These are shown in fig. 297. They are the archegonia. One of them in -longisection is shown in fig. 308. It is flask-shaped, and the broader -portion is sunk in the tissue of the prothallium. The egg is in the -larger part. The spermatozoids when they are swimming around over the -under surface of the prothallium come near the neck, and here they are -caught in the viscid substance which has oozed out of the canal of the -archegonium. From here they slowly swim down the canal, and finally one -sinks into the egg, fuses with the nucleus of the latter, and the egg -is then fertilized. It is now ready to grow and develop into the fern -plant. This brings us back to the sporophyte, which begins with the -fertilized egg. - - -Sporophyte. - -=558. Embryo.=—The egg first divides into two cells as shown in -fig. 228, then into four. Now from each one of these quadrants of the -embryo a definite part of the plant develops, from one the first leaf, -from one the stem, from one the root, and from the other the organ -which is called the foot, and which attaches the embryo to the -prothallium, and transports nourishment for the embryo until it can -become attached to the soil and lead an independent existence. During -this time the wall of the archegonium grows somewhat to accommodate the -increase in size of the embryo, as shown in figs. 312, 313. But soon -the wall of the archegonium is ruptured and the embryo emerges, the -root attaches itself to the soil, and soon the prothallium dies. - -[Illustration: Fig. 311. Two-celled embryo of Pteris serrulata. Remnant -of archegonium neck below.] - -The embryo is first on the under side of the prothallium, and the first -leaf and the stem curves upward between the lobes of the heart-shaped -body, and then grows upright as shown in fig. 314. Usually only one -embryo is formed on a single prothallium, but in one case I found a -prothallium with two well-formed embryos, which are figured in 315. - -=559. Comparison of ferns with liverworts and mosses.=—In the -ferns then we have reached a remarkable condition of things as compared -with that which we found in the mosses and liverworts. In the mosses -and liverworts the sexual phase of the plant (gametophyte) was the -prominent one, and consisted of either a thallus or a leafy axis, -but in either case it bore the sexual organs and led an independent -existence; that is it was capable of obtaining its nourishment from the -soil or water by means of organs of absorption belonging to itself, and -it also performed the office of photosynthesis. - -[Illustration: Fig. 312. Young embryo of fern (Adiantum concinnum) -in enlarged venter of the archegonium. _S_, stem; _L_, first leaf or -cotyledon; _R_, root; _F_, foot.] - -=560.= The spore-bearing phase (sporophyte) of the liverworts -and mosses, on the other hand, is quite small as compared with the -sexual stage, and it is completely dependent on the sexual stage for -its nourishment, remaining attached permanently throughout all its -development, by means of the organ called a foot, and it dies after the -spores are mature. - -=561.= Now in the ferns we see several striking differences. In -the first place, as we have already observed, the spore-bearing phase -(sporophyte) of the plant is the prominent one, and that which -characterizes the plant. It also leads an independent existence, and, -with the exception of a few cases, does not die after the development -of the spores, but lives from year to year and develops successive -crops of spores. There is a _distinct advance_ here in the _size_, -_complexity_, and _permanency_ of this phase of the plant. - -=562.= On the other hand the sexual phase of the ferns -(gametophyte), while it still is capable of leading an independent -existence, is short-lived (with very few exceptions). It is also much -smaller than most of the liverworts and mosses, especially as compared -with the size of the spore-bearing phase. The gametophyte phase or -stage of the plants, then, is decreasing in size and durance as the -sporophyte stage is increasing. We shall be interested to see if this -holds good of the fern allies, that is of the plants which belong to -the same group as the ferns. And as we come later to take up the study -of the higher plants we must bear in mind to carry on this comparison, -and see if this progression on the one hand of the sporophyte -continues, and if the retrogression of the gametophyte continues also. - -[Illustration: Fig. 313. Embryo of fern (Adiantum concinnum) still -surrounded by the archegonium, which has grown in size, forming the -“calyptra.” _L_, leaf; _S_, stem; _R_, root; _F_, foot.] - -[Illustration: Fig. 314. Young plant of Pteris serrulata still attached -to prothallium.] - -[Illustration: Fig. 315. Two embryos from one prothallium of Adiantum -cuneatum.] - - - - -CHAPTER XXVIII. - -DIMORPHISM OF FERNS. - - -=563.= In comparing the different members of the leaf series there -are often striking illustrations of the transition from one form to -another, as we have noted in the case of the trillium flower. This -occurs in many other flowers, and in some, as in the water-lily, these -transformations are always present, here showing a transition from the -petals to the stamens. In the bud-scales of many plants, as in the -butternut, walnut, currant, etc., there are striking gradations between -the form of the simple bud-scales and the form of the leaf. Some of the -most interesting of these transformations are found in the dimorphic -ferns. - -=564. Dimorphism in the leaves of ferns.=—In the common polypody -fern, the maidenhair, and in many other ferns, all the leaves are of -the same form. That is, there is no difference between the fertile leaf -and the sterile leaf. On the other hand, in the case of the Christmas -fern we have seen that the fertile leaves are slightly different from -the sterile leaves, the former having shorter pinnæ on the upper half -of the leaf. The fertile pinnæ are here the shorter ones, and perform -but little of the function of carbon conversion. This function is -chiefly performed by the sterile leaves and by the sterile portions of -the fertile leaves. This is a short step toward the division of labor -between the two kinds of leaves, one performing chiefly the labor of -carbon conversion, the other chiefly the labor of bearing the fruit. - -[Illustration: Fig. 316. Sensitive fern; normal condition of vegetative -leaves and sporophylls.] - -=565. The sensitive fern.=—This division of labor is carried to -an extreme extent in the case of some ferns. Some of our native ferns -are examples of this interesting relation between the leaves like -the common sensitive fern (Onoclea sensibilis) and the ostrich fern -(O. struthiopteris) and the cinnamon-fern (Osmunda cinnamomea). The -sensitive fern is here shown in fig. 316. The sterile leaves are large, -broadly expanded, and pinnate, the pinnæ being quite large. The fertile -leaves are shown also in the figure, and at first one would not take -them for leaves at all. But if we examine them carefully we see that -the general plan of the leaf is the same: the two rows of pinnæ which -are here much shorter than in the sterile leaf, and the pinnules, or -smaller divisions of the pinnæ, are inrolled into little spherical -masses which lie close on the side of the pinnæ. If we unroll one -of these pinnules we find that there are several fruit dots within -protected by this roll. In fact when the spores are mature these -pinnules open somewhat, so that the spores may be disseminated. - -[Illustration: Fig. 317. Sensitive fern; one fertile leaf nearly -changed to vegetative leaf.] - -There is very little green color in these fertile leaves, and what -green surface there is is very small compared with that of the broad -expanse of the sterile leaves. So here there is practically a complete -division of labor between these two kinds of leaves, the general plan -of which is the same, and we recognize each as being a leaf. - -[Illustration: Fig. 318. Sensitive fern, showing one vegetative leaf -and two sporophylls completely transformed.] - -=566. Transformation of the fertile leaves of onoclea to sterile -ones.=—It is not a very rare thing to find plants of the sensitive -fern which show intermediate conditions of the sterile and the -fertile leaf. A number of years ago it was thought by some that this -represented a different species, but now it is known that these -intermediate forms are partly transformed fertile leaves. It is a -very easy matter in the case of the sensitive fern to produce these -transformations by experiment. If one in the spring, when the sterile -leaves attain a height of 12 to 16 _cm_ (8-10 inches), cuts them away, -and again when they have a second time reached the same height, some -of the fruiting leaves which develop later will be transformed. A few -years ago I cut off the sterile leaves from quite a large patch of -the sensitive fern, once in May, and again in June. In July, when -the fertile leaves were appearing above the ground, many of them -were changed partly or completely into sterile leaves. In all some -thirty plants showed these transformations, so that every conceivable -gradation was obtained between the two kinds of leaves. - -[Illustration: Fig. 319. Normal and transformed sporophyll of sensitive -fern.] - -=567.= It is quite interesting to note the form of these changed -leaves carefully, to see how this change has affected the pinnæ and the -sporangia. We note that the tip of the leaf as well as the tips of all -the pinnæ are more expanded than the basal portions of the same. -This is due to the fact that the tip of the leaf develops later -than the basal portions. At the time the stimulus to the change in -the development of the fertile leaves reached them they were partly -formed, that is the basal parts of the fertile leaves were more or less -developed and fixed and could not change. Those portions of the leaf, -however, which were not yet completely formed, under this stimulus, or -through correlation of growth, are incited to vegetative growth, and -expand more or less completely into vegetative leaves. - -=568. The sporangia decrease as the fertile leaf expands.=—If -we now examine the sporangia on the successive pinnæ of a partly -transformed leaf we find that in case the lower pinnæ are not changed -at all, the sporangia are normal. But as we pass to the pinnæ which -show increasing changes, that is those which are more and more -expanded, we see that the number of sporangia decrease, and many of -them are sterile, that is they bear no spores. Farther up there are -only rudiments of sporangia, until on the more expanded pinnæ sporangia -are no longer formed, but one may still see traces of the indusium. -On some of the changed leaves the only evidences that the leaf began -once to form a fertile leaf are the traces of these indusia. In some of -these cases the transformed leaf was even larger than the sterile leaf. - -=569. The ostrich fern.=—Similar changes were also produced in -the case of the ostrich fern, and in fig. 319 is shown at the left a -normal fertile leaf, then one partly changed, and at the right one -completely transformed. - -=570. Dimorphism in tropical ferns.=—Very interesting forms -of dimorphism are seen in some of the tropical ferns. One of these -is often seen growing in plant conservatories, and is known as the -staghorn fern (Platycerium alcicorne). This in nature grows attached to -the trunks of quite large trees at considerable elevations on the tree, -sometimes surrounding the tree with a massive growth. One kind of leaf, -which may be either fertile or sterile, is narrow, and branched in a -peculiar manner, so that it resembles somewhat the branching of the -horn of a stag. Below these are other leaves which are different in -form and sterile. These leaves are broad and hug closely around the -roots and bases of the other leaves. Here they serve to catch and -retain moisture, and they also catch leaves and other vegetable matter -which falls from the trees. In this position the leaves decay and then -serve as food for the fern. - -[Illustration: Fig. 320. Ostrich fern, showing one normal sporophyll, -one partly transformed, and one completely transformed.] - - - - -CHAPTER XXIX. - -HORSETAILS. - - -=571.= Among the relatives of the ferns are the horsetails, so -called because of the supposed resemblance of the branched stems of -some of the species to a horse’s tail, as one might infer from the -plant shown in fig. 325. They do not bear the least resemblance to the -ferns which we have been studying. But then relationship in plants does -not depend on mere resemblance of outward form, or of the prominent -part of the plant. - -[Illustration: Fig. 321. Portion of fertile plant of Equisetum arvense -showing whorls of leaves and the fruiting spike.] - -=572. The field equisetum. Fertile shoots.=—Fig. 321 represents -the common horsetail (Equisetum arvense). It grows in moist sandy -or gravelly places, and the fruiting portion of the plant (for this -species is dimorphic), that is the portion which bears the spores, -appears above the ground early in the spring. It is one of the first -things to peep out of the recently frozen ground. This fertile shoot -of the plant does not form its growth this early in the spring. Its -development takes place under the ground in the autumn, so that with -the advent of spring it pushes up without delay. This shoot is from -10 to 20 _cm_. high, and at quite regular intervals there are slight -enlargements, the nodes of the stem. The cylindrical portions between -the nodes are the internodes. If we examine the region of the -internodes carefully we note that there are thin membranous scales, -more or less triangular in outline, and connected at their bases into a -ring around the stem. Curious as it may seem, these are the leaves of -the horsetail. The stem, if we examine it farther, will be seen to -possess numerous ridges which extend lengthwise and which alternate -with furrows. Farther, the ridges of one node alternate with those of -the internode both above and below. Likewise the leaves of one node -alternate with those of the nodes both above and below. - -[Illustration: Fig. 322. Peltate sporophyll of equisetum (side view) -showing sporangia on under side.] - -=573. Sporangia.=—The end of this fertile shoot we see possesses -a cylindrical to conic enlargement. This is the _fertile spike_, and we -note that its surface is marked off into regular areas if the spores -have not yet been disseminated. If we dissect off a few of these -portions of the fertile spike, and examine one of them with a low -magnifying power, it will appear like the fig. 322. We see here that -the angular area is a disk-shaped body, with a stalk attached to its -inner surface, and with several long sacs projecting from its inner -face parallel with the stalk and surrounding the same. These elongated -sacs are the _sporangia_, and the disk which bears them, together with -the stalk which attaches it to the stem axis, is the _sporophyll_, and -thus belongs to the leaf series. These sporophylls are borne in close -whorls on the axis. - -=574. Spores.=—When the spores are ripe the tissue of the -sporangium becomes dry, and it cracks open and the spores fall out. -If we look at fig. 323 we see that the spore is covered with a very -singular coil which lies close to the wall. When the spore dries this -uncoils and thus rolls the spore about. Merely breathing upon these -spores is sufficient to make them perform very curious evolutions by -the twisting of these four coils which are attached to one place of the -wall. They are formed by the splitting up of an outer wall of the spore. - -=575. Sterile shoot of the common horsetail.=—When the spores are -ripe they are soon scattered, and then the fertile shoot dies down. -Soon afterward, or even while some of the fertile shoots are still in -good condition, sterile shoots of the plant begin to appear above the -ground. One of these is shown in fig. 325. This has a much more slender -stem and is provided with numerous branches. If we examine the stem of -this shoot, and of the branches, we see that the same kind of leaves -are present and that the markings on the stem are similar. Since the -leaves of the horsetail are membranous and not green, the stem is green -in color, and this performs the function of photosynthesis. These green -shoots live for a great part of the season, building up material which -is carried down into the underground stems, where it goes to supply the -forming fertile shoots in the fall. On digging up some of these plants -we see that the underground stems are often of great extent, and that -both fertile and sterile shoots are attached to one and the same. - -[Illustration: Fig. 323. Spore of equisetum with elaters coiled up.] - -[Illustration: Fig. 324. Spore of equisetum with elaters uncoiled.] - -[Illustration: Fig. 325. Sterile plant of horsetail (Equisetum -arvensis).] - -=576. The scouring rush, or shave grass.=—Another common species -of horsetail in the Northern States grows on wet banks, or in sandy -soil which contains moisture along railroad embankments. It is the -scouring rush (E. hyemale), so called because it was once used for -polishing purposes. This plant like all the species of the horsetails -has underground stems. But unlike the common horsetail, there is but -one kind of aerial shoot, which is green in color and fertile. The -shoots range as high as one meter or more, and are quite stout. The new -shoots which come up for the year are unbranched, and bear the fertile -spike at the apex. When the spores are ripe the apex of the shoot dies, -and the next season small branches may form from a number of the nodes. - -=577. Gametophyte of equisetum.=—The spores of equisetum have -chlorophyll when they are mature, and they are capable of germinating -as soon as mature. The spores are all of the same kind as regards size, -just as we found in the case of the ferns. But they develop prothallia -of different sizes, according to the amount of nutriment which they -obtain. Those which obtain but little nutriment are smaller and develop -only antheridia, while those which obtain more nutriment become larger, -more or less branched, and develop archegonia. This character of an -independent prothallium (gametophyte) with the characteristic sexual -organs, and the also independent sporophyte, with spores, shows the -relationship of the horsetails with the ferns. We thus see that these -characters of the reproductive organs, and the phases and fruiting of -the plant, are more essential in determining relationships of plants -than the mere outward appearances. - - - - -CHAPTER XXX. - -CLUB MOSSES. - - -[Illustration: Fig. 326. Lycopodium clavatum, branch bearing two -fruiting spikes; at right sporophyll with open sporangium; single spore -near it.] - -=578.= What are called the “club mosses” make up another group -of interesting plants which rank as allies of the ferns. They are not -of course true mosses, but the general habit of some of the smaller -species, and especially the form and size of the leaves, suggest a -resemblance to the larger of the moss plants. - -=579. The clavate lycopodium.=—Here is one of the club mosses -(fig. 326) which has a wide distribution and which is well entitled to -hold the name of club because of the form of the upright club-shaped -branches. As will be seen from the illustration, it has a prostrate -stem. This stem runs for considerable distances on the surface of the -ground, often partly buried in the leaves, and sometimes even buried -beneath the soil. The leaves are quite small, are flattened-awl-shaped, -and stand thickly over the stem, arranged in a spiral manner, which -is the usual arrangement of the leaves of the club mosses. Here and -there are upright branches which are forked several times. The end of -one or more of these branches becomes produced into a slender upright -stem which is nearly leafless, the leaves being reduced to mere scales. -The end of this leafless branch then terminates in one or several -cylindrical heads which form the club. - -=580. Fruiting spike of Lycopodium clavatum.=—This club is the -fruiting spike or head (sometimes termed a _strobilus_). Here the -leaves are larger again and broader, but still not so large as the -leaves on the creeping shoots, and they are paler. If we bend down some -of the leaves, or tear off a few, we see that in the axil of the leaf, -where it joins the stem, there is a somewhat rounded, kidney-shaped -body. This is the spore-case or sporangium, as we can see by an -examination of its contents. There is but a single spore-case for each -of the fertile leaves (sporophyll). When it is mature, it opens by a -crosswise slit as seen in fig. 326. When we consider the number of -spore-cases in one of these club-shaped fruit bodies we see that the -number of spores developed in a large plant is immense. In mass the -spores make a very fine, soft powder, which is used for some kinds of -pyrotechnic material, and for various toilet purposes. - -[Illustration: Fig. 327. Lycopodium lucidulum, bulbils in axils of -leaves near the top, sporangia in axils of leaves below them. At right -is a bulbil enlarged.] - -=581. Lycopodium lucidulum.=—Another common species is figured -at 327. This is Lycopodium lucidulum. The habit of the plant is quite -different. It grows in damp ravines, woods, and moors. The older -parts of the stem are prostrate, while the branches are more or less -ascending. It branches in a forked manner. The leaves are larger than -in the former species, and they are all of the same size, there being -no appreciable difference between the sterile and fertile ones. The -characteristic club is not present here, but the spore-cases occupy -certain regions of the stem, as shown at 327. In a single season one -region of the stem may bear spore-cases, and then a sterile portion -of the same stem is developed, which later bears another series of -spore-cases higher up. - -=582. Bulbils on Lycopodium lucidulum.=—There is one curious way -in which this club moss multiplies. One may see frequently among the -upper leaves small wedge-shaped or heart-shaped green bodies but little -larger than the ordinary leaves. These are little buds which contain -rudimentary shoot and root and several thick green leaves. When they -fall to the ground they grow into new lycopodium plants, just as the -bulbils of cystopteris do which were described in the chapter on ferns. - -=583.= Note.—The prothallia of the species of lycopodium which -have been studied are singular objects. In L. cernuum a cylindrical -body sunk in the earth is formed, and from the upper surface there -are green lobes. In L. phlegmaria and some others slender branched, -colorless bodies are formed which according to Treub grow as a -saphrophyte in decayed bark of trees. Many of the prothallia examined -have a fungus growing in their tissue which is supposed to play some -part in the nutrition of the prothallium. - - -The little club mosses (selaginella). - -=584.= Closely related to the club mosses are the selaginellas. -These plants resemble closely the general habit of the club mosses, but -are generally smaller and the leaves more delicate. Some species are -grown in conservatories for ornament, the leaves of such usually having -a beautiful metallic lustre. The leaves of some are arranged as in -lycopodium, but many species have the leaves in four to six rows. Fig. -328 represents a part of a selaginella plant (S. apus). The fruiting -spike possesses similar leaves, but they are shorter, and their -arrangement gives to the spike a four-sided appearance. - -[Illustration: Fig. 328. Selaginella with three fruiting spikes. -(Selaginella apus.)] - -[Illustration: Fig. 329. Fruiting spike showing large and small -sporangia.] - -[Illustration: Fig. 330. Large sporangium.] - -[Illustration: Fig. 331. Small sporangium.] - -=585. Sporangia.=—On examining the fruiting spike, we find as -in lycopodium that there is but a single sporangium in the axil of a -fertile leaf. But we see that they are of two different kinds, small -ones in the axils of the upper leaves, and large ones in the axils of -a few of the lower leaves of the spike. The _microspores_ are borne -in the smaller spore-cases and the _macrospores_ in the larger ones. -Figures 329-331 give the details. There are many microspores in a -single small spore-case, but 3-4 macrospores in a large spore-case. - -=586. Male prothallia.=—The prothallia of selaginella are much -reduced structures. The microspores when mature are already divided -into two cells. When they grow into the mature prothallium a few more -cells are formed, and some of the inner ones form the spermatozoids, -as seen in fig. 332. Here we see that the antheridium itself is larger -than the prothallia. Only antheridia are developed on the prothallia -formed from the microspores, and for this reason the prothallia are -called _male prothallia_. In fact a male prothallium of selaginella is -nearly all antheridium, so reduced has the gametophyte become here. - -[Illustration: Fig. 332. Details of microspore and male prothallium -of selaginella; 1st, microspore; 2d, wall removed to show small -prothallial cell below; 3d, mature male prothallium still within the -wall; 4th, small cell below is the prothallial cell, the remainder is -antheridium with wall and four sperm cells within; 5th spermatozoid. -After Beliaieff and Pfeffer.] - -=587. Female prothallia.=—The female prothallia are developed -from the macrospores. The macrospores when mature have a rough, thick, -hard wall. The female prothallium begins to develop inside of the -macrospore before it leaves the sporangium. The protoplasm is richer -near the wall of the spore and at the upper end. Here the nucleus -divides a great many times, and finally cell walls are formed, so -that a tissue of considerable extent is formed inside the wall of the -spore, which is very different from what takes place in the ferns we -have studied. As the prothallium matures the spore is cracked at the -point where the three angles meet, as shown in fig. 334. The archegonia -are developed in this exposed surface, and several can be seen in the -illustration. - -[Illustration: Fig. 333. Section of mature macrospore of selaginella, -showing female prothallium and archegonia. After Pfeffer.] - -[Illustration: Fig. 334. Mature female prothallium of selaginella, -just bursting open the wall of macrospore, exposing archegonia. After -Pfeffer.] - -[Illustration: Fig. 335. Seedling of selaginella still attached to the -macrospore. After Campbell.] - -=588. Embryo.=—After fertilization the egg divides in such a way -that a long cell called a suspensor is cut off from the upper side, -which elongates and pushes the developing embryo down into the center -of the spore, or what is now the female prothallium. Here it derives -nourishment from the tissues of the prothallium, and eventually the -root and stem emerge, while a process called the “foot” is still -attached to the prothallium. When the root takes hold on the soil the -embryo becomes free. - - - - -CHAPTER XXXI. - -QUILLWORTS (ISOETES). - - -[Illustration: Fig. 336. Isoetes, mature plant, sporophyte stage.] - -=589.= The quillworts, as they are popularly called, are very -curious plants. They grow in wet marshy places. They receive their -name from the supposed resemblance of the leaf to a quill. Fig. 336 -represents one of these quillworts (Isoetes engelmannii). The leaves -are the prominent part of the plant, and they are about all that can -be seen except the roots, without removing the leaves. Each leaf, it -will be seen, is long and needle-like, except the basal part, which -is expanded, not very unlike, in outline, a scale of an onion. These -expanded basal portions of the leaves closely overlap each other, and -the very short stem is completely covered at all times. Fig. 338 is -from a longitudinal section of a quillwort. It shows the form of the -leaves from this view (side view), and also the general outline of the -short stem, which is triangular. The stem is therefore a very short -object. - -=590. Sporangia of isoetes.=—If we pull off some of the leaves of -the plant we see that they are somewhat spoon-shaped as in fig. 337. In -the inner surface of the expanded base we note a circular depression -which seems to be of a different texture from the other portions of the -leaf. This is a _sporangium_. Beside the spores on the inside of the -sporangium, there are strands of sterile tissue which extend across the -cavity. This is peculiar to isoetes of all the members of the class -of plants to which the ferns belong, but it will be remembered that -sterile strands of tissue are found in some of the liverworts in the -form of elaters. - -[Illustration: Fig. 337. Base of leaf of isoetes, showing sporangium -with macrospores. (Isoetes engelmannii.)] - -[Illustration: Fig. 338. Section of plant of Isoetes engelmannii, -showing cup-shaped stem, and longitudinal sections of the sporangia in -the thickened bases of the leaves.] - -=591.= The spores of isoetes are of two kinds, small ones -(microspores) and large ones (macrospores), so that in this respect -it agrees with selaginella, though it is so very different in other -respects. When one kind of spore is borne in a sporangium usually all -in that sporangium are of the same kind, so that certain sporangia -bear microspores, and others bear macrospores. But it is not uncommon -to find both kinds in the same sporangium. When a sporangium bears -only microspores the number is much greater than when one bears only -macrospores. - -=592.= If we examine some of the microspores of isoetes we see -that they are shaped like the quarters of an apple, that is they are of -the bilateral type as seen in some of the ferns (asplenium). - -=593. Male prothallia.=—In isoetes, as in selaginella, the -microspores develop only male prothallia, and these are very -rudimentary, one division of the spore having taken place before the -spore is mature, just as in selaginella. - -=594. Female prothallia.=—These are developed from the -macrospores. The latter are of the tetrahedral type. The development -of the female prothallium takes place in much the same way as in -selaginella, the entire prothallium being enclosed in the macrospore, -though the cell divisions take place after it has left the sporangium. -When the archegonia begin to develop the macrospore cracks at the three -angles and the surface bearing the archegonia projects slightly as in -selaginella. Absorbing organs in the form of rhizoids are very rarely -formed. - -=595. Embryo.=—The embryo lies well immersed in the tissue of the -prothallium, though there is no suspensor developed as in selaginella. - - - - -CHAPTER XXXII. - -COMPARISON OF FERNS AND THEIR RELATIVES. - - -=596. Comparison of selaginella and isoetes with the ferns.=—On -comparing selaginella and isoetes with the ferns, we see that the -sporophyte is, as in the ferns, the prominent part of the plant. It -possesses root, stem, and leaves. While these plants are not so large -in size as some of the ferns, still we see that there has been a great -advance in the sporophyte of selaginella and isoetes upon what exists -in the ferns. There is a division of labor between the sporophylls, -in which some of them bear microsporangia with microspores, and some -bear macrosporangia with only macrospores. In the ferns and horsetails -there is only one kind of sporophyll, sporangium, and spore in a -species. By this division of labor, or differentiation, between the -sporophylls, one kind of spore, the microspore, is compelled to form -a male prothallium, while the other kind of spore, the macrospore, is -compelled to form a female prothallium. This represents a progression -of the sporophyte of a very important nature. - -=597.= On comparing the gametophyte of selaginella and isoetes -with that of the ferns, we see that there has been a still farther -retrogression in size from that which we found in the independent and -large gametophyte of the liverworts and mosses. In the ferns, while it -is reduced, it still forms rhizoids, and leads an independent life, -absorbing its own nutrient materials, and assimilating carbon. In -selaginella and isoetes the gametophyte does not escape from the spore, -nor does it form absorbing organs, nor develop assimilative tissue. -The reduced prothallium develops at the expense of food stored by the -sporophyte while the spore is developing. Thus, while the gametophyte -is separate from the sporophyte in selaginella and isoetes, it is -really dependent on it for support or nourishment. - -=598.= The important general characters possessed by the ferns -and their so-called allies, as we have found, are as follows: The -spore-bearing part, which is the fern plant, leads an independent -existence from the prothallium, and forms root, stem, and leaves. The -spores are borne in sporangia on the leaves. The prothallium also leads -an independent existence, though in isoetes and selaginella it has -become almost entirely dependent on the sporophyte. The prothallium -bears also well-developed antheridia and archegonia. The root, stem, -and leaves of the sporophyte possess vascular tissue. All the ferns and -their allies agree in the possession of these characters. The mosses -and liverworts have well-developed antheridia and archegonia, and the -higher plants have vascular tissue. But no plant of either of these -groups possesses the combined characters which we find in the ferns and -their relatives. The latter are, therefore, the fern-like plants, or -_pteridophyta_. The living forms of the pteridophyta are classified as -follows into families or orders. (See page 295.) - -=599.= TABLE SHOWING RELATION OF GAMETOPHYTE AND SPOROPHYTE IN THE -PTERIDOPHYTES. - - -------------------------------------------------------------------- - GAMETOPHYTE. (Becoming smaller, mostly independent. - In selaginella and isoetes becoming dependent on the sporophyte.) - ---------------+-----------------------+---------------------------- - | Vegetative Part. | Sexual Organs. - ---------------+-----------------------+ - Ferns. |A green, thin, | Usually both kinds on - (Polypodiaceæ.)|expanded, | the same prothallium. - |heart-shaped growth, +---------------+------------ - |with rhizoids. |Antheridia with|Archegonia, - | |spermatozoids. |each with - | | |egg. - ----------------+-----------------------+---------------+------------ - Equisetum. |A green, thin, | Usually the two kinds - |expanded, | on different prothallia. - |lobed growth, +---------------+------------ - ||with rhizoids. |Antheridia, on |Archegonia - | |small male |on larger - | |prothallia, |female - | |with |prothallia, - | |spermatozoids. |each with - | | |an egg. - ---------------+-----------------------+---------------+------------ - Isoetes. |Colorless, rounded | - |mass of cells, | - |inside of spore wall, | - |usually no rhizoids, | On different prothallia. - |or but few. Two kinds. | - +-----------+-----------+---------------+------------ - |Small ones,|Large ones,|One |Few - |male. |female. |antheridium, |archegonia, - |Developed |Developed |much larger |in apex - |into small |from |than the |of oval, - |prothallial|nutriment |single |colorless, - |cell, and |stored in |prothallial |female - |antherid |macrospore |cell. |prothallium, - |cell while |from |Antheridium |each with - |still in |sporophyte.|with |egg. - |sporangium.| |spermatozoids. | - ---------------+-----------+-----------+---------------+------------ - Selaginella. |Colorless, rounded | - |mass of cells inside | - |of spore wall, no | - |rhizoids, or but few. | On different prothallia. - |Two kinds. | - +-----------+-----------+---------------+------------ - |Small ones,|Large ones,|One |Few - |male. |female. |antheridium, |archegonia, - |Developed |Developed |much larger |in apex - |into small |while |than the |of oval, - |prothallial|still in |single |colorless, - |cell, and |sporangium |prothallial |female - |antherid |and |cell. |prothallium, - |cell while |dependent |Antheridium |each with - |in |on |with |egg. - |sporangium.|sporophyte.|spermatozoids. | - ---------------+-----------+-----------+---------------+------------ - - ------------------------------------------------------------+------- - SPOROPHYTE. |Begin- - (Largest part of the plant. The fern plant. Independent | ning - of, and more hardy than, the gametophyte. | of - Usually perennial.) |Gameto- - | phyte. - ------------+-----------+-----------------+-----------------+------- - | Beginning | | | - | of |Vegetative Part. | Fruiting Part. | - |Sporophyte.| | | - ------------+-----------+-----------------+-----------------+------- - Ferns. |Fertilized |Root, stem, leaf.|Sporangia on | - (Polypod- |egg. | |leaf. All of one | - iaceæ.)|(Develops | |kind. Sporangium | - |into | | contains ....|Spores. - |fern | | | - |plant.) | | | - ------------+-----------+-----------------+-----------------+------- - Equisetum. |Fertilized |Root, stem, leaf.|Sporangia on | - |egg. | |sporophylls. All | - |(Develops | |of one kind. | - |into | |Sporangium | - |equisetum | | contains ....|Spores. - |plant.) | | | - ------------+-----------+-----------------+-----------------+------- - Isoetes. |Fertilized |Root, stem, leaf.|Sporangia of | - |egg. |Stem very short. |two kinds. Small | - |(Develops |Leaves bear |ones contain ....|Micro- - |into |sporangia in | |spores. - |isoetes |cavities at base;|Large ones | - |plant.) |outer leaves | contain ....|Macro- - | |usually bear | |spores. - | |macrosporangia, | | - | |inner ones | | - | |microsporangia. | | - ------------+-----------+-----------------+-----------------+------- - Selagniella.|Fertilized |Root, stem, leaf.|Sporangia of | - |egg. |Spore-bearing |two kinds. Small| - |(Develops |leaves grouped |ones contain ....|Micro- - |into |on the end of | |spores. - |selaginella|stem in a spike. |Large ones | - |plant.) |Lower ones bear | contain ....|Macro- - | |macrosporangia, | |spores. - | |upper ones bear | | - | |microsporangia. | | - ------------+-----------+-----------------+-----------------+------- - - -Classification of the Pteridophytes. - -Of the living pteridophytes four classes may be recognized. - - -CLASS FILICINEÆ.[34] - -This class includes the ferns. Four orders may be recognized. - -=600. Order Ophioglossales.= (One Family, Ophioglossaceæ).—This -order includes the grapeferns (Botrychium), so called because of the -large botryoid cluster of sporangia, resembling roughly a cluster -of grapes; and the adder-tongue (Ophioglossum), the sporangia being -embedded in a long tongue-like outgrowth from the green leaf. -Botrychium and Ophioglossum are widely distributed. The roots are -fleshy, nearly destitute of root hairs, and contain an endophytic -fungus, so that the roots are mycorhiza. The gametophyte is -subterranean, and devoid of chlorophyll. In Botrychium virginianum, -an endophytic fungus has been found in the prothallium. Another genus -(Helminthostachys) with one species is limited to the East Indies. - -=601. Order Marattiales= (One Family, Marattiaceæ).—These are -tropical ferns, with only four or five living genera (Marattia, Danæa, -etc.). They resemble the typical ferns, but the sporangia are usually -united, several forming a compound sporangium, or _synangium_. - -The Ophioglossales and Marattiales are known as eusporangiate ferns, -while the following order includes the leptosporangiate ferns. - -=602. Order Filicales.=—This order includes the typical ferns. -Eight families are recognized. - -_Family Osmundaceæ._—Three genera are known in this family. Osmunda -has a number of species, three of which are found in the Eastern United -States; the cinnamon-fern (O. cinnamomea), the royal fern (O. regalis), -and Clayton’s fern (O. claytoniana). No species of this family are -found on the Pacific coast. - -_Family Gleicheniaceæ._—These ferns are found chiefly in the tropics, -and in the mountain regions of the temperate zones of South America. -There are two genera, Gleichenia containing all but one of the known -species. - -_Family Matoniaceæ._—One genus, Matonia, in the Malayan region. - -_Family Schizæceæ._—These are chiefly tropical, but two species are -found in eastern North America, Schizæa pusilla and Lygodium palmatum, -the latter a climbing fern. - -_Family Hymenophyllaceæ._—These are known as the filmy ferns because -of their thin, delicate leaves. They grow only in damp or wet regions, -mostly in the tropics, but a few species occur in the southern United -States. - -_Family Cyatheaceæ._—These are known as the tree ferns, because of the -large size which many of them attain. They occur chiefly in tropical -mountainous regions, many of them palm-like and imposing because of the -large trunks and leaves. Dicksonia, Cyathea, Cibotium, Alsophila, are -some of the most conspicuous genera. - -_Family Parkeriaceæ._—There is a single species in this family -(Ceratopteris thalictroides), abundant in the tropics and extending -into Florida. It is aquatic. - -_Family Polypodiaceæ._—This family includes the larger number of -living ferns and many genera and species are found in North America. -Examples, Polypodium, Pteridium (= Pteris), Adiantum, etc. - -=603. Order Hydropterales (or Salviniales).=—The members of this -order are peculiar, aquatic ferns, some floating on the water (Azolla, -Salvinia), while others are anchored to the soil by roots (Marsilia, -Pilularia). They are known as water ferns. The sporangia are of two -kinds, one containing large spores (macrospores) and the other small -spores (microspores). They are therefore heterosporous ferns. - -_Family Salviniaceæ._—There are two genera, Salvinia and Azolla. - -_Family Marsiliaceæ._—Two genera, Marsilia and Pilularia. In this -family the sporangia are enclosed in a sporocarp, which forms a -pod-like structure. - - -CLASS EQUISETINEÆ.[35] - -=604. Order Equisetales.=—The single order contains a single -family, Equisetaceæ, among the living forms, and but a single genus, -Equisetum. There are about twenty-four species, with fourteen in the -United States (see Chapter XXIX). - - -CLASS LYCOPODIINEÆ.[36] - -=605. Order Lycopodiales.=—The first two families of this order -include the homosporous Lycopodiineæ, while the Selaginellaceæ are -heterosporous. - -_Family Lycopodiaceæ._—There are two genera. Lycopodium (club moss) -includes many species, most of them tropical, but a number in temperate -and subarctic regions. The gametophyte of many species is tuberous, -lacks chlorophyll, and in some there lives an endophytic fungus. -Phylloglossum with one species is found in Australia. - -_Family Psilotaceæ._—There are two genera. Psilotum chiefly in the -tropics has one species (P. triquetrum) in the region of Florida. - -_Family Selaginellaceæ._—These include the little club mosses, with -one genus, Selaginella (see Chapter XXX). - - -CLASS ISOETINEÆ. - -=606. Order Isoetales=, with one family Isoetaceæ and one genus -Isoetes (see Chapter XXXI). There are about fifty species, with about -sixteen in the United States. - -FOOTNOTES: - -[34] As class Filicales in Engler and Prantl. - -[35] As class Equisetales in Engler and Prantl. - -[36] As class Lycopodiales in Engler and Prantl. - - - - -CHAPTER XXXIII. - -GYMNOSPERMS. - - -The white pine. - -=607. General aspect of the white pine.=—The white pine (Pinus -strobus) is found in the Eastern United States. In favorable situations -in the forest it reaches a height of about 50 meters (about 160 feet), -and the trunk a diameter of over 1 meter. In well-formed trees the -trunk is straight and towering; the branches where the sunlight has -access and the trees are not crowded, or are young, reaching out in -graceful arms, form a pyramidal outline to the tree. In old and dense -forests the lower branches, because of lack of sunlight, have died -away, leaving tall, bare trunks for a considerable height. - -=608. The long shoots of the pine.=—The branches are of two -kinds. Those which we readily recognize are the long branches, so -called because the growth in length each year is considerable. The -terminal bud of the long branches, as well as of the main stem, -continues each year the growth of the main branch or shoot; while the -lateral long branches arise each year from buds which are crowded close -together around the base of the terminal bud. The lateral long branches -of each year thus appear to be in a whorl. The distance between each -false whorl of branches, then, represents one year’s growth in length -of the main stem or long branch. - -=609. The dwarf shoots of the pine.=—The dwarf branches are all -lateral on the long branches, or shoots. They are scattered over the -year’s growth, and each bears a cluster of five long, needle-shaped, -green leaves, which remain on the tree for several years. At the base -of the green leaves are a number of chaff-like scales, the previous bud -scales. While the dwarf branches thus bear green leaves, and scales, -the long branches bear only thin scale-like leaves which are not green. - -=610. Spore-bearing leaves of the pine.=—The two kinds of -spore-bearing leaves of the pine, and their close relatives, are so -different from anything which we have yet studied, and are so unlike -the green leaves of the pine, that we would scarcely recognize them as -belonging to this category. Indeed there is great uncertainty regarding -their origin. - -[Illustration: Fig. 339. Spray of white pine showing cluster of male -cones just before the scattering of the pollen.] - -=611. Male cones, or male flowers.=—The male cones are borne in -clusters as shown in fig. 339. Each compact, nearly cylindrical, or -conical mass is termed a cone, or flower, and each arises in place of a -long lateral branch. One of these cones is shown considerably enlarged -in fig. 340. The central axis of each cone is a lateral branch, and -belongs to the stem series. The stem axis of the cone can be seen -in fig. 341. It is completely covered by stout, thick, scale-like -outgrowths. These scales are obovate in outline, and at the inner -angle of the upper end there are several rough, short spines. They -are attached by their inner lower angle, which forms a short stalk -or petiole, and continues through the inner face of the scale as -a “midrib.” What corresponds to the lamina of the scale-like leaf -bulges out on each side below and makes the bulk of the scale. These -prominences on the under side are the sporangia (microsporangia). There -are thus two sporangia on a sporophyll (microsporophyll). When the -spores (microspores), which here are usually called pollen grains, are -mature, each sporangium, or anther locule, splits down the middle as -shown in fig. 342, and the spores are set free. - -[Illustration: Fig. 340. Staminate cone of white pine, with bud scales -removed on one side.] - -[Illustration: Fig. 341. Section of staminate cone, showing sporangia.] - -[Illustration: Fig. 342. Two sporophylls removed, showing opening of -sporangia.] - -[Illustration: Fig. 343. Pollen grain of white pine.] - -=612. Microspores of the pine, or pollen grains.=—A mature pollen -grain of the pine is shown in fig. 343. It is a queer-looking object, -possessing on two sides an air sac, formed by the upheaval of the outer -coat of the spore at these two points. When the pollen is mature, the -moisture dries out of the scale (or stamen, as it is often called here) -while it ripens. When a limb, bearing a cluster of male cones, is -jarred by the hand, or by currents of air, the split suddenly opens, -and a cloud of pollen bursts out from the numerous anther locules. The -pollen is thus borne on the wind and some of it falls on the female -flowers. - -[Illustration: Fig. 344. White pine, branch with cluster of mature -cones shedding the seed. A few young cones four months old are shown on -branch at the left. Drawn from photograph.] - -[Illustration: Fig. 345. Mature cone of white pine at time of -scattering of the seed, nearly natural size.] - -=613. Form of the mature female cone.=—A cluster of the -white pine cones is shown in fig. 344. These are mature, and the scales -have spread as they do when mature and becoming dry, in order that the -seeds may be set at liberty. The general outline of the cone is -lanceolate, or long oval, and somewhat curved. It measures about -10-15_cm_ long. If we remove one of the scales, just as they are -beginning to spread, or before the seeds have scattered, we shall find -the seeds attached to the upper surface at the lower end. There are -two seeds on each scale, one at each lower angle. They are ovate in -outline, and shaped somewhat like a biconvex lens. At this time the -seeds easily fall away, and may be freed by jarring the cone. As the -seed is detached from the scale a strip of tissue from the latter is -peeled off. This forms a “wing” for the seed. It is attached to one end -and is shaped something like a knife blade. On the back of the scale is -a small appendage known as the cover scale. - -[Illustration: Fig. 346. Sterile scale. Seeds undeveloped.] - -[Illustration: Fig. 347. Scale with well-developed seeds.] - -[Illustration: Fig. 348. Seeds have split off from scale.] - -[Illustration: Fig. 349. Back of scale with small cover scale.] - -[Illustration: Fig. 350. Winged seed free from scale. - -Figs. 346-350.—White pine showing details of mature scales and seed.] - -[Illustration: Fig. 351. Female cones of the pine at time of -pollination, about natural size.] - -=614. Formation of the female pine cone.=—The female flowers -begin their development rather late in the spring of the year. They -are formed from terminal buds of the higher branches of the tree. In -this way the cone may terminate the main shoot of a branch, or of the -lateral shoots in a whorl. After growth has proceeded for some time in -the spring, the terminal portion begins to assume the appearance of a -young female cone or flower. These young female cones, at about the -time that the pollen is escaping from the anthers, are long ovate, -measuring about 6-10 _mm_ long. They stand upright as shown in fig. 351. - -=615. Form of a “scale” of the female flower.=—If we remove one -of the scales from the cone at this stage we can better study it in -detail. It is flattened, and oval in outline, with a stout “rib,” if -it may be so called, running through the middle line and terminating -in a point. The scale is in two parts as shown in fig. 354, which is -a view of the under side. The small “outgrowth” which appears as an -appendage is the cover scale, for while it is smaller in the pine than -the other portion, in some of the relatives of the pine it is larger -than its mate, and being on the outside, covers it. (The inner scale is -sometimes called the ovuliferous scale, because it bears the ovules.) - -[Illustration: Fig. 352. Section of female cone of white pine, showing -young ovules (macrosporangia) at base of the ovuliferous scales.] - -[Illustration: Fig. 353. Scale of white pine with the two ovules at -base of ovuliferous scale.] - -[Illustration: Fig. 354. Scale of white pine seen from the outside, -showing the cover scale.] - -=616. Ovules, or macrosporangia, of the pine.=—At each of the -lower angles of the scale is a curious oval body with two curved, -forceps-like processes at the lower and smaller end. These are the -macrosporangia, or, as they are called in the higher plants, the -ovules. These ovules, as we see, are in the positions of the seeds on -the mature cones. In fact the wall of the ovule forms the outer coat of -the seed, as we will later see. - -[Illustration: Fig. 355. Branch of white pine showing young female -cones at time of pollination on the ends of the branches, and -one-year-old cones below, near the time of fertilization.] - -=617. Pollination.=—At the time when the pollen is mature the -female cones are still erect on the branches, and the scales, which -during the earlier stages of growth were closely pressed against one -another around the axis, are now spread apart. As the clouds of pollen -burst from the clusters of the male cones, some of it is wafted by -the wind to the female cones. It is here caught in the open scales, -and rolls down to their bases, where some of it falls between these -forceps-like processes at the lower end of the ovule. At this time the -ovule has exuded a drop of a sticky fluid in this depression between -the curved processes at its lower end. The pollen sticks to this, and -later, as this viscid substance dries up, it pulls the pollen close up -in the depression against the lower end of the ovule. This depression -is thus known as the _pollen chamber_. - -=618.= Now the open scales on the young female cone close up -again so tightly that water from rains is excluded. What is also very -curious, the cones, which up to this time have been standing erect, so -that the open scale could catch the pollen, now turn so that they hang -downward. This more certainly excludes the rains, since the overlapping -of the scales forms a shingled surface. Quantities of resin are also -formed in the scales, which exudes and makes the cone practically -impervious to water. - -=619.= The female cone now slowly grows during the summer and -autumn, increasing but little in size during this time. During the -winter it rests, that is, ceases to grow. With the coming of spring, -growth commences again and at an accelerated rate. The increase in size -is more rapid. The cone reaches maturity in September. We thus see that -nearly eighteen months elapse from the beginning of the female flower -to the maturity of the cone, and about fifteen months from the time -that pollination takes place. - -[Illustration: Fig. 356. - -Macrosporangium of pine (ovule). _int_, integument; _n_, nucellus; _m_, -macrospore; _pc_, pollen chamber; _pg_, pollen grain; _an_, axile row; -_spt_, spongy tissue. (After Ferguson.)] - -=620. Female prothallium of the pine.=—To study this we must -make careful longitudinal sections through the ovule (better made with -the aid of a microtome). Such a section is shown in fig. 358. The -outer layer of tissue, which at the upper end (point where the scale -is attached to the axis of the cone) stands free, is the ovular coat, -or _integument_. Within this integument, near the upper end, there is -a cone-shaped mass of tissue. This mass of tissue is the _nucellus_, -or the _macrosporangium_ proper. In the lower part of the nucellus in -fig. 356 can be seen a rounded mass of “spongy tissue” (_spt_), which -is a special nourishing tissue of the nucellus, or sporangium, around -the macrospore. Within this can be seen an axile row of three cells -(_an: m_). The lowest one, which is larger than the other two, is the -_macrospore_. Sometimes there are four of these cells in the axile row. -This axile row of three or four cells is formed by the two successive -divisions of a mother cell in the nucellus. So it would appear that -these three or four cells are all spores. - -Only one of them, however, the lower one, develops; the others are -disorganized and disappear. The nucleus of the macrospore now divides -several times to form several free nuclei in the now enlarging cavity, -much as the nucleus of the macrospore in Selaginella and Isoetes -divides within the spore. The development thus far takes place during -the first summer, and now with the approach of winter the very young -female prothallium goes into rest about the stage shown in fig. 358. -The conical portion of the nucellus which lies above is the nucellar -cap. - -[Illustration: Fig. 357. - -Pollen grains of pine. One of them germinating. _p_¹ and _p_², the two -disintegrated prothallial cells, = sterile part of male gametophyte; -_a.c._, central cell of antheridium; _v.n._, vegetative nucleus or tube -nucleus of the single-wall cell of antheridium; _s.g._, starch grains. -(After Ferguson.)] - -[Illustration: Fig. 358. - -Section of ovule of white pine. _int_, integument; _pc_, pollen -chamber; _pt_, pollen tube; _n_, nucleus; _m_, macrospore cavity.] - -=621. Male prothallia.=—By the time the pollen is mature the -male prothallium is already partly formed. In fig. 343 we can see two -well-formed cells. Two other cells are formed earlier, but they become -so flattened that it is difficult to make them out when the pollen -grain is mature. These are shown in fig. 357, _p_¹ and _p_², and they -are the only sterile cells of the male prothallium in the pines. The -large cell is the antheridium wall, its nucleus _v.n._ in fig. 357. The -smaller cell, _a.c._, is the central cell of the antheridium. During -the summer and autumn the male prothallium makes some farther growth, -but this is slow. The larger cell, called the vegetative cell or tube -cell, which is in reality the wall of the antheridium, elongates by -the formation of a tube, forming a sac, known as the pollen tube. It -is either simple or branched. It grows down into the tissue of the -nucellus, and at a stage represented in fig. 358, winter overtakes it -and it rests. At this time the central cell has divided into two cells, -and the vegetative nucleus is in the pollen tube. - -[Illustration: Fig. 359. - -Section of nucellus and endosperm of white pine. The inner layer -of cells of the integument shown just outside of nucellus; _arch_, -archegonium; _en_, egg nucleus. In the nucellar cap are shown three -pollen tubes. _vn_, vegetative nucleus or tube nucleus; _stc_, stalk -cell; _spn_, sperm nuclei, the larger one in advance is the one which -unites with the egg nucleus. The archegonia are in the endosperm or -female gametophyte. (After Ferguson.)] - -=622. The endosperm.=—In the following spring growth of all these -parts continues. The nuclei in the macrospore divide to form more, and -eventually cell walls are formed between them making a distinct tissue, -known as the _endosperm_. This endosperm continues to grow until a -large part of the nucellus is consumed for food. - -[Illustration: Fig. 360. - -Last division of the egg in the white pine cutting off the ventral -canal cell at the apex of the archegonium. _End_, endosperm; _Arch_, -archegonium.] - -=623. Female prothallium and archegonia.=—The endosperm is the -female prothallium. This is very evident from the fact that several -archegonia are developed in it usually on the side toward the pollen -chamber. The archegonia are sexual organs, and since the sexual organs -are developed on the gametophyte, therefore, the endosperm is the -female gametophyte, or prothallium. In fig. 359 are represented two -archegonia in the endosperm and the pollen tubes are growing down -through the nucellus. The archegonia are quite large, the wall is a -sheath or jacket of cells which encloses the very large egg which has a -large nucleus in the center. - -=624. Pollen tube and sperm cells.=—While the endosperm (female -prothallium) and archegonia are developing the pollen tube continues -its growth down through the nucellar cap, as shown in fig. 359. At -the same time the two cells which were formed in the pollen grain -(antheridium) from the central cell move down into the tube. One of -these is the “generative” cell, or “body” cell, and the other is called -the stalk cell, though it is more properly a sterile half of the -central cell. The nucleus of the generative cell, about the time the -archegonium is mature, divides to form two nuclei, which are the sperm -nuclei, and the one in advance is the larger, though it is much smaller -than the egg nucleus. - -=625. Fertilization.=—Very soon after the archegonia are mature -(early in June in the northern United States) the pollen tube grows -through into the archegonium and empties the two sperm nuclei, the -vegetative nucleus and the stalk cell, into the protoplasm of the large -egg. The larger of the two sperm nuclei at once comes in contact with -the very large egg nucleus and sinks down into a depression of the -same, as shown in fig. 361. These two nuclei, in the pines, do not fuse -into a resting nucleus, but at once organize the nuclear figure for the -first division of the embryo. Two nuclei are thus formed, and these -divide to form four nuclei which sink to the bottom of the archegonium -and there organize the embryo which pushes its way into the endosperm -from which it derives its food (fig. 362). - -[Illustration: Fig. 361. - -Archegonium of white pine at stage of fertilization, _en_, egg nucleus; -_spn_, sperm nucleus in conjugation with it; _nb_, nutritive bodies in -cytoplasm of large egg; _cpt_, cavity of pollen tube; _vn_, vegetative -nucleus or tube nucleus; _stc_, stalk cell; _spn_, second sperm -nucleus: _pr_, portion of prothallium or endosperm; _sg_, starch grains -in pollen tube. The sheath of jacket cells of the archegonium is not -shown. (After Ferguson.)] - -=626. Homology of the parts of the female cone.=—Opinions are -divided as to the homology of the parts of the female cone of the pine. -Some consider the entire cone to be homologous with a flower of the -angiosperms. The entire scale according to this view is a carpel, or -sporophyll, which is divided into the cover scale and the ovuliferous -scale. This division of the sporophyll is considered similar to that -which we have in isoetes, where the sporophyll has a ligule above the -sporangium, or as in ophioglossum, where the leaf is divided into a -fertile and a sterile portion. - -Others believe that the ovuliferous scale is composed of two leaves -situated laterally and consolidated representing a shoot in the axis of -the bract. There is some support for this in the fact that in certain -abnormal cones which show proliferation a short axis appears in the -axil of the bract and bears lateral leaves, and in some cases all -gradations are present between these lateral leaves on the axis and -their consolidation into an ovuliferous scale. In the normal condition -of the ovuliferous scale the axis has disappeared and the shoot is -represented only by the consolidated leaves, which would represent then -the macrosporophylls (or carpels) each bearing one macrosporangium -(ovule). - -[Illustration: Fig. 362. - -Pine seed, section of, _sc_, seed coat; _n_, remains of nucellus; -_end_, endosperm (= female gametophyte); _emb_, embryo = young -sporophyte. Seed coat and nucellus = remains of old sporophyte.] - -[Illustration: Fig. 363. Embryo of white pine removed from seed, showing -several cotyledons.] - -[Illustration: Fig. 364. Pine seedling just emerging from the ground.] - -One of the most interesting and plausible views is that of Celakovsky. -He believes that the axial shoot is reduced to two ovules, that the -ovules have two integuments, but the outer integument of each has -become proliferated into scales which are consolidated. In this -proliferation of the outer integument it is thrown off from the ovule -so that it only remains attached to one side and the larger part of the -ovule is thus left with only one integument. This view is supported -by the fact that in gingko, for example (another gymnosperm), the -outer integument (the “collar”) sometimes proliferates into a leaf. -Celakovsky’s view is, therefore, not very different from the second one -mentioned above. - -[Illustration: Fig. 365. White pine seedling casting seed coats.] - - - - -CHAPTER XXXIV. - -FURTHER STUDIES ON GYMNOSPERMS. - - -Cycas. - -[Illustration: Fig. 366. Macrosporophyll of Cycas revoluta.] - -=627.= In such gymnosperms as cycas, illustrated in the -frontispiece, there is a close resemblance to the members of the fern -group, especially the ferns themselves. This is at once suggested by -the form of the leaves. The stem is short and thick. The leaves have a -stout midrib and numerous narrow pinnæ. In the center of this rosette -of leaves are numerous smaller leaves, closely overlapping like bud -scales. If we remove one of these at the time the fruit is forming we -see that in general it conforms to the plan of the large leaves. There -are a midrib and a number of narrow pinnæ near the free end, the entire -leaf being covered with woolly hairs. But at the lower end, in place of -the pinnæ, we see oval bodies. These are the macrosporangia (ovules) -of cycas, and correspond to the macrosporangia of selaginella, and the -leaf is the macrosporophyll. - -=628. Female prothallium of cycas.=—In figs. 367, 368, are -shown mature ovules, or macrosporangia, of cycas. In 368, which is a -roentgen-ray photograph of 367, the oval prothallium can be seen. So in -cycas, as in selaginella, the female prothallium is developed entirely -inside of the macrosporangium, and derives the nutriment for its growth -from the cycas plant, which is the sporophyte. Archegonia are developed -in this internal mass of cells. This aids us in determining that it is -the prothallium. In cycas it is also called endosperm, just as in the -pines. - -[Illustration: Fig. 367. Macrosporangium of Cycas revoluta.] - -[Illustration: Fig. 368. Roentgen photograph of same, showing female -prothallium.] - -[Illustration: Fig. 369. A sporophyll (stamen) of cycas; sporangia in -groups on the under side. _b_, group of sporangia; _c_, open sporangia. -(From Warming.)] - -=629.= If we cut open one of the mature ovules, we can see the -endosperm (prothallium) as a whitish mass of tissue. Immediately -surrounding it at maturity is a thin, papery tissue, the remains of the -nucellus (macrosporangium), and outside of this are the coats of the -ovule, an outer fleshy one and an inner stony one. - -=630. Microspores, or pollen, of cycas.=—The cycas plant -illustrated in the frontispiece is a female plant. Male plants also -exist which have small leaves in the center that bear only -microsporangia. These leaves, while they resemble the ordinary leaves, -are smaller and correspond to the stamens. Upon the under side, as -shown in fig. 369, the microsporangia are borne in groups of three -or four, and these contain the microspores, or pollen grains. The -arrangement of these microsporangia on the under side of the cycas -leaves bears a strong resemblance to the arrangement of the sporangia -on the under side of the leaves of some ferns. - -=631. The gingko tree= is another very interesting plant belonging -to this same group. It is a relic of a genus which flourished in the -remote past, and it is interesting also because of the resemblance of -the leaves to some of the ferns like adiantum, which suggests that -this form of the leaf in gingko has been inherited from some fern-like -ancestor. - -[Illustration: Fig. 370. Zamia integrifolia, showing thick stem, -fern-like leaves, and cone of male flowers.] - -[Illustration: Fig. 371. Two spermatozoids in end of pollen tube of -cycas. (After drawing by Hirase and Ikeno.)] - -=632.= While the resemblance of the leaves of some of the -gymnosperms to those of the ferns suggests fern-like ancestors for the -members of this group, there is stronger evidence of such ancestry -in the fact that a prothallium can well be determined in the ovules. -The endosperm with its well-formed archegonia is to be considered a -prothallium. - -=633. Spermatozoids in some gymnosperms.=—But within the past two -years it has been discovered in gingko, cycas, and zamia, all belonging -to this group, that the sperm cells are well-formed spermatozoids. In -zamia each one is shaped somewhat like the half of a biconvex lens, and -around the convex surface are several coils of cilia. After the pollen -tube has grown down through the nucellus, and has reached a depression -at the end of the prothallium (endosperm) where the archegonia are -formed, the spermatozoids are set free from the pollen tube, swim -around in a liquid in this depression, and later fuse with the egg. In -gingko and cycas these spermatozoids were first discovered by Ikeno and -Hirase in Japan, and later in zamia by Webber in this country. In figs. -371-374 the details of the male prothallia and of fertilization are -shown. - -[Illustration: Fig. 372. Fertilization in cycas, small spermatozoid -fusing with the larger female nucleus of the egg. The egg protoplasm -fills the archegonium. (From drawings by Hirase and Ikeno.)] - -[Illustration: Fig. 373. Spermatozoid of gingko. Some abnormal forms -have a tail. (After Ikeno and Hirase.)] - -=634. The sporophyte in the gymnosperms.=—In the pollen grains -of the gymnosperms we easily recognize the characters belonging to the -spores in the ferns and their allies, as well as in the liverworts and -mosses. They belong to the same series of organs, are borne on the -same phase or generation of the plant, and are practically formed in -the same general way, the variations between the different groups not -being greater than those within a single group. These spores we have -recognized as being the product of the sporophyte. We are able then to -identify the sporophyte as that phase or generation of the plant formed -from the fertilized egg and bearing ultimately the spores. We see from -this that the sporophyte in the gymnosperms is the prominent part of -the plant, just as we found it to be in the ferns. The pine tree, then, -as well as the gingko, cycas, yew, hemlock-spruce, black spruce, the -giant redwood of California, etc., are sporophytes. - -While the sporangia (anther sacs) of the male flowers open and permit -the spores (pollen) to be scattered, the sporangia of the female -flowers of the gymnosperms rarely open. The macrospore is developed -within sporangium (nucellus) to form the female prothallium (endosperm). - -=635. The gametophyte has become dependent on the sporophyte.=—In -this respect the gymnosperms differ widely from the pteridophytes, -though we see suggestions of this condition of things in Isoetes and -Selaginella, where the female prothallium is developed within the -macrospore, and even in Selaginella begins, and nearly completes, its -development while still in the sporangium. - -In comparing the female prothallium of the gymnosperms with that -of the fern group we see a remarkable change has taken place. The -female prothallium of the gymnosperms is very much reduced in size. -Especially, it no longer leads an independent existence from the -sporophyte, as is the case with nearly all the fern group. It remains -enclosed within the macrosporangium (in cycas if not fertilized it -sometimes grows outside of the macrosporangium and becomes green), and -derives its nourishment through it from the sporophyte, to which the -latter remains organically connected. This condition of the female -prothallium of the gymnosperms necessitated a special adaptation of the -male prothallium in order that the sperm cells may reach and fertilize -the egg-cell. - -[Illustration: Fig. 374. - -Gingko biloba. _A_, mature pollen grain; _B_, germinating pollen grain, -the branched tube entering among the cells of the nucellus; _Ex_, -exine (outer wall of spore); _P_₁, prothallial cell; _P_₂, antheridial -cell (divides later to form stalk cell and generative cell); _P_₃, -vegetative cell; _Va_, vacuoles; _Nc_, nucellus. (After drawings by -Hirase and Ikeno.)] - -[Illustration: Fig. 375. - -Gingko biloba, diagrammatic representation of the relation of pollen -tube to the archegonium in the end of the nucellus. _pt_, pollen tube; -_o_, archegonium. (After drawing by Hirase and Ikeno.)] - -=636. Gymnosperms are naked seed plants.=—The pine, as we have -seen, has naked seeds. That is, the seeds are not enclosed within the -carpel, but are exposed on the outer surface. All the plants of the -great group to which the pine belongs have naked seeds. For this reason -the name “_gymnosperms_” has been given to this great group. - -[Illustration: Fig. 376. Spermatozoids of zamia in pollen tube; _pg_, -pollen grain; _a_, _a_, spermatozoids. (After Webber.)] - -[Illustration: Fig. 377. Spermatozoid of zamia showing spiral row of -cilia. (After Webber.)] - -=637. Classification of gymnosperms.=—The gingko tree has until -recently been placed with the pines, yew, etc., in the order _Pinales_, -but the discovery of the spermatozoids in the pollen tube suggests -that it is not closely allied with the Pinales, and that it represents -an order coordinate with them. Engler arranges the living gymnosperms -somewhat as follows: - - -Class Gymnospermæ. - - Order 1. Cycadales; family Cycadaceæ. Cycas, Zamia, etc. - Order 2. Gingkoales; family Gingkoaceæ. Gingko. - Order 3. Pinales (or Coniferæ); - family 1. Taxaceæ. - Taxus, the common yew in the eastern - United States, and Torreya, in the - western United States, are examples. - family 2. Pinaceæ. Sequoia (redwood of California), - firs, spruces, pines, cedars, cypress, etc. - Order 4. Gnetales. Welwitschia mirabilis, deserts of southwest - Africa; Ephedra, deserts of the Mediterranean - and of West Asia. Gnetum, climbers (Lianas), - from tropical Asia and America. - -=638.= TABLE SHOWING HOMOLOGIES OF SPOROPHYTE AND GAMETOPHYTE IN -THE PINE. - - TERMS CORRESPONDING TO THOSE USED IN PTERIDOPHYTES. COMMON TERMS. - - -------------+-------------------------------------------------- - | Sporophyte = Pine tree. - | Spore-bearing part = Male and female cones. - -------------+-------------------------------------------------- - Sporophyte | Microsporophyll = Stamen. - | Microsporangium = Pollen sac. - -------------+-------------------------------------------------- - | Microspore = Pollen grain. - | Mature microspore is = Mature pollen grain. - | rudimentary male - | prothallium with - | rudimentary - | antheridium - | Large cell (part of = Vegetative cell - | antheridium wall?) of pollen grain. - | Antheridium cell = Small cell of pollen - Male | grain. - gametophyte | Antheridium cell divides = Generative cell. - | to form stalk cell and - | central cell of - | antheridium - | (male sexual organ) - | Central cell of = Paternal cells, - | antheridium or generative cells. - | divides to form - | two sperm cells - -------------+-------------------------------------------------- - | Macrosporophyll = Ovuliferous scale (cover - | scale and carpellary - | outgrowth); or three - | carpels united into - Sporophyte | ovuliferous scale, - | the central one sterile - | (in axil of cover scale). - | Macrosporangium covered = Nucellus covered by - | by integument integument = ovule. - -------------+-------------------------------------------------- - | Macrospore (remains in = Large cell in center of - | sporangium) nucellus which develops - | embryo sac and endosperm - | (remains in nucellus). - Female | Female prothallium = Endosperm, in nucellus. - gametophyte | (in sporangium) - | Archegonia (female = Corpuscula, in endosperm. - | sexual organs) - | Egg = Maternal cell, or germ - | cell. - -------------+-------------------------------------------------- - | Egg (fertilized) = Germ cell. - | Young sporophyte = Pine embryo in nucellus - | and integument. - Young | Young sporophyte = Embryo | - sporophyte | In remains of = Endosperm | - | gametophyte = Nucellus | Seed. - | And sporangium = Integument | - | Surrounded by new | - | growth of old | - | sporophyte | - -------------+---------------------------------------+---------- - - - - -CHAPTER XXXV. - -MORPHOLOGY OF THE ANGIOSPERMS: TRILLIUM; DENTARIA. - - -Trillium. - -=639. General appearance.=—As one of the plants to illustrate -this group we may take the wake-robin, as it is sometimes called, -or trillium. There are several species of this genus in the United -States; the commonest one in the eastern part is the “white wake-robin” -(Trillium grandiflorum). This occurs in or near the woods. A picture of -the plant is shown in fig. 378. There is a thick, fleshy, underground -stem, or rhizome as it is usually called. This rhizome is perennial, -and is marked by ridges and scars. The roots are quite stout and -possess coarse wrinkles. From the growing end of the rhizome each year -the leafy, flowering stem arises. This is 20-30_cm_ (8-12 inches) in -height. Near the upper end is a whorl of three ovate leaves, and from -the center of this rosette rises the flower stalk, bearing the flower -at its summit. - -=640. Parts of the flower. Calyx.=—Now if we examine the flower -we see that there are several leaf-like structures. These are arranged -also in threes just as are the leaves. First there is a whorl of three, -pointed, lanceolate, green, leaf-like members, which make up the -_calyx_ in the higher plants, and the parts of the calyx are _sepals_, -that is, each leaf-like member is a _sepal_. But while the sepals are -part of the flower, so called, we easily recognize them as belonging to -the _leaf series_. - -=641. Corolla.=—Next above the calyx is a whorl of white or -pinkish members, in Trillium grandiflorum, which are also leaf-like in -form, and broader than the sepals, being usually somewhat broader at -the free end. These make up what is the _corolla_ in the higher plants, -and each member of the corolla is a _petal_. But while they are parts -of the flower, and are not green, their form and position would suggest -that they also belong to the leaf series. - -[Illustration: Fig. 378. Trillium grandiflorum.] - -=642. Andrœcium.=—Within and above the insertion of the corolla -is found another tier, or whorl, of members which do not at first -sight resemble leaves in form. They are known in the higher plants -as _stamens_. As seen in fig. 379 each stamen possesses a stalk (= -filament), and extending along on either side for the greater part of -the length are four ridges, two on each side. This part of the stamen -is the _anther_, and the ridges form the anther sacs, or lobes. Soon -after the flower is opened, these anther sacs open also by a split in -the wall along the edge of the ridge. At this time we see quantities of -yellowish powder or dust escaping from the ruptured anther locules. If -we place some of this under the microscope we see that it is made up of -minute bodies which resemble spores; they are rounded in form, and the -outer wall is spiny. They are in fact spores, the microspores of the -trillium, and here, as in the gymnosperms, are better known as _pollen_. - -[Illustration: Fig. 379. Sepal, petal, stamen, and pistil of Trillium -grandiflorum.] - -[Illustration: Fig. 380. - -Trillium grandiflorum, with the compound pistil expanded into three -leaf-like members. At the right these three are shown in detail.] - -=643. The stamen a sporophyll.=—Since these pollen grains are -the spores, we would infer, from what we have learned of the ferns -and gymnosperms, that this member of the flower which bears them is -a sporophyll; and this is the case. It is in fact what is called the -_microsporophyll_. Then we see also that the anther sacs, since they -enclose the spores, would be the sporangia (microsporangia). From this -it is now quite clear that the stamens belong also to the leaf series. -They are just six in number, twice the number found in a whorl of -leaves, or sepals, or corolla. It is believed, therefore, that there -are two whorls of stamens in the flower of trillium. - -=644. Gynœcium.=—Next above the stamens and at the center of the -flower is a stout, angular, ovate body which terminates in three long, -slender, curved points. This is the pistil, and at present the only -suggestion which it gives of belonging to the leaf series is the fact -that the end is divided into three parts, the number of parts in each -successive whorl of members of the flower. If we cut across the body of -this pistil and examine it with a low power we see that there are three -chambers or cavities, and at the junction of each the walls suggest to -us that this body may have been formed by the infolding of the margins -of three leaf-like members, the places of contact having then become -grown together. We see also that from the incurved margins of each -division of the pistil there stand out in the cavity oval bodies. These -are the _ovules_. Now the ovules we have learned from our study of the -gymnosperms are the _sporangia_ (here the macrosporangia). It is now -more evident that this curious body, the pistil, is made up of three -leaf-like members which have fused together, each member being the -equivalent of a sporophyll (here the macrosporophyll). This must be a -fascinating observation, that plants of such widely different groups -and of such different grades of complexity should have members formed -on the same plan and belonging to the same series of members, devoted -to similar functions, and yet carried out with such great modifications -that at first we do not see this common meeting ground which a -comparative study brings out so clearly. - -[Illustration: Fig. 381. Abnormal trillium. The nine parts of the -perianth are green, and the outer whorls of stamens are expanded into -petal-like members.] - -[Illustration: Fig. 382. Transformed stamen of trillium showing anther -locules on the margin.] - -=645. Transformations of the flower of trillium.=—If anything -more were needed to make it clear that the parts of the flower of -trillium belong to the leaf series we could obtain evidence from the -transformations which the flower of trillium sometimes presents. In -fig. 381 is a sketch of a flower of trillium, made from a photograph. -One set of the stamens has expanded into petal-like organs, with the -anther sacs on the margin. In fig. 380 is shown a plant of Trillium -grandiflorum in which the pistil has separated into three distinct and -expanded leaf-like structures, all green except portions of the margin. - - -Dentaria. - -=646. General appearance.=—For another study we may take a plant which -belongs to another division of the higher plants, the common “pepper -root,” or “toothwort” (Dentaria diphylla) as it is sometimes called. -This plant occurs in moist woods during the month of May, and is well -distributed in the northeastern United States. A plant is shown in fig. -383. It has a creeping underground rhizome, whitish in color, fleshy, -and with a few scales. Each spring the annual flower-bearing stem rises -from one of the buds of the rhizome, and after the ripening of the -seeds, dies down. - -The leaves are situated a little above the middle point of the stem. -They are opposite and the number is two, each one being divided into -three dentate lobes, making what is called a compound leaf. - -[Illustration: Fig. 383. Toothwort (Dentaria diphylla).] - -[Illustration: Fig. 384. Flower of the toothwort (Dentaria diphylla).] - -=647. Parts of the flower.=—The flowers are several, and they -are borne on quite long stalks (pedicels) scattered over the terminal -portion of the stem. We should now examine the parts of the flower -beginning with the calyx. This we can see, looking at the under side -of some of the flowers, possesses four scale-like sepals, which easily -fall away after the opening of the flower. They do not resemble leaves -so much as the sepals of trillium, but they belong to the leaf series, -and there are two pairs in the set of four. The corolla also possesses -four petals, which are more expanded than the sepals and are whitish in -color. The stamens are six in number, one pair lower than the others, -and also shorter. The filament is long in proportion to the anther, the -latter consisting of two lobes or sacs, instead of four as in trillium. -The pistil is composed of two carpels, or leaves fused together. So we -find in the case of the pepper root that the parts of the flower are -in twos, or multiples of two. Thus they agree in this respect with the -leaves; and while we do not see such a strong resemblance between the -parts of the flower here and the leaves, yet from the presence of the -pollen (microspores) in the anther sacs (microsporangia) and of ovules -(macrosporangia) on the margins of each half of the pistil, we are, -from our previous studies, able to recognize here that all the members -of the flower belong to the leaf series. - -=648.= In trillium and in the pepper root we have seen that the -parts of the flower in each apparent whorl are either of the same -number as the leaves in a whorl, or some multiple of that number. This -is true of a large number of other plants, but it is not true of all. A -glance at the spring-beauty (Claytonia virginiana), and at the anemone -(or Isopyrum biternatum, fig. 563) will serve to show that the number -of the different members of the flower may vary. The trillium and the -dentaria were selected as being good examples to study first, to make -it very clear that the members of the flower are fundamentally leaf -structures, or rather that they belong to the same series of members as -do the leaves of the plant. - -=649. Synopsis of members of the sporophyte in angiosperms.= - - Higher plant. - Sporophyte phase {Root. {Foliage leaves. - (or modern {Shoot. {Stem. {Perianth leaves. } - phase). { Leaf. {Spore-bearing leaves } - { with sporangia. } Flower. - {(Sporangia sometimes } - { on shoot.) } - - - - -CHAPTER XXXVI. - -GAMETOPHYTE AND SPOROPHYTE OF ANGIOSPERMS. - - -=650. Male prothallium of angiosperms.=—The first division which -takes place in the nucleus of the pollen grain occurs, in the case of -trillium and many others of the angiosperms, before the pollen grain -is mature. In the case of some specimens of T. grandiflorum in which -the pollen was formed during the month of October of the year before -flowering, the division of the nucleus into two nuclei took place soon -after the formation of the four cells from the mother cell. The nucleus -divided in the young pollen grain is shown in fig. 385. After this -takes place the wall of the pollen grain becomes stouter, and minute -spiny projections are formed. - -[Illustration: Fig. 385. Nearly mature pollen grain of trillium. The -smaller cell is the generative cell.] - -[Illustration: Fig. 386. Germinating spores (pollen grains) of -peltandra; generative nucleus in one undivided, in other divided to -form the two sperm nuclei; vegetative nucleus in each near the pollen -grain.] - -=651.= The larger cell is the vegetative cell of the prothallium, -while the smaller one, since it later forms the sperm cells, is -the generative cell. This generative cell then corresponds to the -central cell of the antheridium, and the vegetative cell perhaps -corresponds to a wall cell of the antheridium. If this is so, then the -male prothallium of angiosperms has become reduced to a very simple -antheridium. The farther growth takes place after fertilization. In -some plants the generative cell divides into the two sperm cells at -the maturity of the pollen grain. In other cases the generative cell -divides in the pollen tube after the germination of the pollen grain. -For study of the pollen tube the pollen may be germinated in a weak -solution of sugar, or on the cut surface of pear fruit, the latter -being kept in a moist chamber to prevent drying the surface. - -=652.= In the spring after flowering the pollen escapes from the -anther sacs, and as a result of pollination is brought to rest on -the stigma of the pistil. Here it germinates, as we say, that is, it -develops a long tube which makes its way down through the style, and in -through the micropyle to the embryo sac, where, in accordance with what -takes place in other plants examined, one of the sperm cells unites -with the egg, and fertilization of the egg is the result. - -[Illustration: Fig. 387. Section of pistil of trillium, showing -position of ovules (macrosporangia).] - -[Illustration: Fig. 388. Mandrake (Podophyllum peltatum).] - -=653. Macrospore and embryo sac.=—In trillium the three carpels -are united into one, and in dentaria the two carpels are also united -into one compound pistil. Simple pistils are found in many plants, for -example in the ranunculaceæ, the buttercups, columbine, etc. These -simple pistils bear a greater resemblance to a leaf, the margins of -which are folded around so that they meet and enclose the ovules or -sporangia. - -[Illustration: Fig. 389. Young ovule (macrosporangium) of podophyllum. -_n_, nucellus containing the one-celled stage of the macrospore; -_i.int_, inner integument; _o.int_, outer integument.] - -=654.= If we cut across the compound pistil of trillium we find -that the infoldings of the three pistils meet to form three partial -partitions which extend nearly to the center, dividing off three -spaces. In these spaces are the ovules which are attached to the -infolded margins. If we make cross-sections of a pistil of the -May-apple (podophyllum) and through the ovules when they are quite -young, we shall find that the ovule has a structure like that shown -in fig. 389. At _m_ is a cell much larger than the surrounding ones. -This is called the macrospore. The tissue surrounding it is called -here the nucellus, but because it contains the macrospore it must be -the macrosporangium. The two coats or integuments of the ovule are -yet short and have not grown out over the end of the nucellus. This -macrospore increases in size, forming first a cavity or sac in the -nucellus, the _embryo sac_. The nucleus divides several times until -eight are formed, four in the micropylar end of the embryo sac and -four in the opposite end. In some plants it has been found that one -nucleus from each group of four moves toward the middle of the embryo -sac. Here they fuse together to form one nucleus, the _endosperm -nucleus_ or _definitive nucleus_ shown in fig. 390. One of the nuclei -at the micropylar end is the egg, while the two smaller ones nearer -the end are the _synergids_. The egg-cell is all that remains of the -archegonium in this reduced prothallium. The three nuclei at the lower -end are the _antipodal_ cells. - -[Illustration: Fig. 390. Podophyllum peltatum, ovule containing mature -embryo sac; two synergids, and eggs at left, endosperm nucleus in -center, three antipodal cells at right.] - -[Illustration: Fig. 391. Macrospore (one-celled stage) of lilium.] - -=655. Embryo sac is the young female prothallium.=—In figs. -391-393 are shown the different stages in the development of the -embryo sac in lilium. The embryo sac at this stage is the young female -prothallium, and the egg is the only remnant of the female sexual -organ, the archegonium, in this reduced gametophyte. - -=656. Fertilization.=—When the pollen tube has reached the embryo -sac (paragraph 652) it opens and the two sperm cells are emptied near -the egg. The first sperm nucleus enters the protoplasm surrounding the -egg nucleus and uniting with the latter brings about fertilization. The -second sperm nucleus often unites with the endosperm nucleus (or with -one or both of the “polar nuclei”), bringing about what some call a -second fertilization. Where this takes place in addition to the union -of the first sperm nucleus with the egg nucleus it is called _double -fertilization_. The sperm nucleus is usually smaller than the egg -nucleus, but often grows to near or quite the size of the egg nucleus -before union. See figs. 394 and 395. - -[Illustration: Fig. 392. Two-and four-celled stage of embryo sac of -lilium. The middle one shows division of nuclei to form the four-celled -stage. (Easter lily.)] - -=657. Fertilization in plants is fundamentally the same as in -animals.=—In all the great groups of plants as represented by -spirogyra, œdogonium, vaucheria, peronospora, ferns, gymnosperms, and -in the angiosperms, fertilization, as we have seen, consists in the -fusion of a male nucleus with a female nucleus. Fertilization, then, in -plants is identical with that which takes place in animals. - -=658. Embryo.=—After fertilization the egg develops into a short -row of cells, the _suspensor_ of the embryo. At the free end the embryo -develops. In figs. 397 and 398 is a young embryo of trillium. - -=659. Endosperm, the mature female prothallium.=—During the -development of the embryo the endosperm nucleus divides into a great -many nuclei in a mass of protoplasm, and cell walls are formed -separating them into cells. This mass of cells is the _endosperm_, -and it surrounds the embryo. It is the _mature female prothallium_, -belated in its growth in the angiosperms, usually developing only when -fertilization takes place, and its use has been assured. - -[Illustration: Fig. 393. Mature embryo sac (young prothallium) of -lilium. _m_, micropylar end; _S_, synergids; _E_, egg; _Pn_, polar -nuclei; _Ant_, antipodals. (Easter lily.)] - -[Illustration: Fig. 394. Section through nucellus and upper part of -embryo sac of cotton at time of entrance of pollen tube. _E_, egg; _S_, -synergids; _P_, pollen tube with sperm cell in the end. (Duggar.)] - -=660. Seed.=—As the embryo is developing it derives its -nourishment from the endosperm (or in some cases perhaps from the -nucellus). At the same time the integuments increase in extent and -harden as the seed is formed. - -[Illustration: Fig. 395. Fertilization of cotton. _pt_, pollen tube; -_Sn_, synergids; _E_, egg, with male and female nucleus fusing. -(Duggar.)] - -[Illustration: Fig. 396. - -Diagrammatic section of ovary and ovule at time of fertilization in -angiosperm. _f_, funicle of ovule; _n_, nucellus; _m_, micropyle; _b_, -antipodal cells of embryo sac; _e_, endosperm nucleus; _k_, egg-cell -and synergids; _ai_, outer integument of ovule; _ii_, inner integument. -The track of the pollen tube is shown down through the style, walls of -the ovary to the micropylar end of the embryo sac.] - -=661. Perisperm.=—In most plants the nucellus is all consumed -in the development of the endosperm, so that only minute fragments -of disorganized cell walls remain next the inner integument. In some -plants, however, (the water-lily family, the pepper family, etc.,) -a portion of the nucellus remains intact in the mature seed. In such -seeds the remaining portion of the nucellus is the _perisperm_. - -=662. Presence or absence of endosperm in the seed.=—In many of -the angiosperms all of the endosperm is consumed by the embryo during -its growth in the formation of the seed. This is the case in the rose -family, crucifers, composites, willows, oaks, legumes, etc., as in the -acorn, the bean, pea and others. In some, as in the bean, a large part -of the nutrient substance passing from the endosperm into the embryo is -stored in the cotyledons for use during germination. In other plants -the endosperm is not all consumed by the time the seed is mature. -Examples of this kind are found in the buttercup family, the violet, -lily, palm, jack-in-the-pulpit, etc. Here the remaining endosperm in -the seed is used as food by the embryo during germination. - -[Illustration: Fig. 397. Section of one end of ovule of trillium, -showing young embryo in endosperm.] - -[Illustration: Fig. 398. Embryo enlarged.] - -[Illustration: Fig. 399. Seed of violet, external view, and section. -The section shows the embryo lying in the endosperm.] - -[Illustration: Fig. 400. Section of fruit of pepper (Piper nigrum), -showing small embryo lying in a small quantity of whitish endosperm -at one end, the perisperm occupying the larger part of the interior, -surrounded by pericarp.] - -=663. Outer parts of the seed.=—While the embryo is forming within the -ovule and the growth of the endosperm is taking place, where this is -formed, other correlated changes occur in the outer parts of the ovule, -and often in adjacent parts of the flower. These unite in making the -“seed,” or the “fruit.” Especially in connection with the formation of -the seed a new growth of the outer coat, or integument, of the ovule -occurs, forming the outer coat of the seed, known as the _testa_, while -the inner integument is absorbed. In some cases the inner integument -of the ovule also forms a new growth, making an inner coat of the seed -(rosaceæ). In still other cases neither of the integuments develops -into a testa, and the embryo sac lies in contact with the wall of -the ovary. Again an additional envelope grows up around the seed; an -example of this is found in the case of the red berries of the “yew” -(taxus), the red outer coat being an extra growth, called an _aril_. - -In the willow and the milkweed an aril is developed in the form of -a tuft of hairs. (In the willow it is an outgrowth of the funicle, -= stalk of the ovule, and is called a funicular aril; while in the -milkweed it is an outgrowth of the micropyle, = the open end of the -ovule, and is called a micropylar aril.) - -=664. Increase in size during seed formation.=—Accompanying this -extra growth of the different parts of the ovule in the formation of -the seed is an increase in the size, so that the seed is often much -greater in size than the ovule at the time of fertilization. At the -same time parts of the ovary, and in many plants, the adherent parts of -the floral envelopes, as in the apple; or of the receptacle, as in the -strawberry; or in the involucre, as in the acorn; are also stimulated -to additional growth, and assist in making the fruit. - -=665. Synopsis of the seed.= - - { {Aril, rarely present. - { { - { {Ovular coats (one or two usually - { { present), the _testa_. - { { - { {_Funicle_ (stalk of ovule), _raphe_ - { { (portion of funicle when bent on - { { to the side of ovule), - { {_micropyle_, _hilum_ (scar where - {_Ripened ovule._ { seed was attached to ovary). - { { - { {_Remnant of the nucellus_ (central - _The seed._ { { part of ovule); sometimes - { { nucellus remains as - { {_Perisperm_ in some albuminous - { seeds. - {_Endosperm_, present in albuminous seeds. - { - {_Embryo_ within surrounded by endosperm when this is - { present, or by the remnant of nucellus, and by the - { ovular coats which make the _testa_. In many seeds - { (example, bean) the endospermis transferred to the - { cotyledons which become fleshy(exalbuminous seeds). - -=666. Parts of the ovule.=—In fig. 401 are represented three -different kinds of ovules, which depend on the position of the ovule -with reference to its stalk. The funicle is the stalk of the ovule, -the hilum is the point of attachment of the ovule with the ovary, the -raphe is the part of the funicle in contact with the ovule in inverted -ovules, the chalaza is the portion of the ovule where the nucellus and -the integuments merge at the base of the ovule, and the micropyle is -the opening at the apex of the ovule where the coats do not meet. - -[Illustration: Fig. 401. - -_A_, represents a straight (orthotropous) ovule of polygonum; _B_, the -inverted (anatropous) ovule of the lily; and _C_, the right-angled -(campylotropous) ovule of the bean. _f_, funicle; _c_, chalaza; -_k_, nucellus; _ai_, outer integument; _ii_, inner integument; _m_, -micropyle; _em_, embryo sac.] - - -Comparison of Organ and Member. - -=667. The stamens and pistils are not the sexual organs.=—Before -the sexual organs and sexual processes in plants were properly -understood it was customary for botanists to speak of the stamens and -pistils of flowering plants as the sexual organs. Some of the early -botanists, a century ago, found that in many plants the seed would not -form unless first the pollen from the stamens came to be deposited on -the stigma of the pistil. A little further study showed that the pollen -germinated on the stigma and formed a tube which made its way down -through the pistil and into the ovule. - -This process, including the deposition of the pollen on the stigma, -was supposed to be fertilization, the stamen was looked on as the male -sexual organ, and the pistil as the female sexual organ. We have found -out, however, by further study, and especially by a comparison of the -flowering plants and the lower plants, that the stamens and pistils are -not the sexual organs of the flower. - -=668. The stamens and pistils are spore-bearing leaves.=—The -stamen is the spore-bearing leaf, and the pollen grains are not -unlike spores; in fact they are the small spores of the angiosperms. -The pistil is also a spore-bearing leaf, the ovule the sporangium, -which contains the large spore called an _embryo sac_. In the ferns -we know that the spore germinates and produces the green heart-shaped -prothallium. The prothallium bears the sexual organs. Now the fern leaf -bears the spores and the spore forms the prothallium. So it is in the -flowering plants. The stamen bears the small spores—pollen grains—and -the pollen grain forms the prothallium. The prothallium in turn forms -the sexual organs. The process is in general the same as it is in the -ferns, but with this special difference: the prothallium and the sexual -organ of the flowering plants are very much reduced. - -=669. Difference between organ and member.=—While it is not -strictly correct then to say that the stamen is a sexual organ, or male -organ, we might regard it as a _male member_ of the flower, and we -should distinguish between _organ_ and _member_. It is an _organ_ when -we consider _pollen production_, but it is not a sexual organ. When we -consider _fertilization_ it is _not a sexual organ, but a male member_ -of the flower which bears the small spore. - -The following table will serve to indicate these relations. - - Stamen = spore-bearing leaf = male member of flower. - Anther locule = sporangium. - Pollen grain = small spore = reduced male prothallium and - sexual organ. - -So the pistil is not a sexual organ, but might be regarded as the -female member of the flower. - - Pistil = spore-bearing leaf = female member of flower. - Ovule = sporangium. - Embryo sac = large spore = female prothallium containing the egg. - The egg = a reduced archegonium = the female sexual organ. - - -Progression and Retrogression in Sporophyte and Gametophyte. - -=670. Sporophyte is prominent and highly developed.=—In the -angiosperms then, as we have seen from the plants already studied, the -trillium, dentaria, etc., are sporophytes, that is they represent the -spore-bearing, or sporophytic, stage. Just as we found in the case of -the gymnosperms and ferns, this stage is the prominent one, and the -one by which we characterize and recognize the plant. We see also that -the plants of this group are still more highly specialized and complex -than the gymnosperms, just as they were more specialized and complex -than the members of the fern group. From the very simple condition -in which we possibly find the sporophyte in some of the algæ like -spirogyra, vaucheria, and coleochæte, there has been a gradual increase -in size, specialization of parts, and complexity of structure through -the bryophytes, pteridophytes, and gymnosperms, up to the highest types -of plant structure found in the angiosperms. Not only do we find that -these changes have taken place, but we see that, from a condition of -complete dependence of the spore-bearing stage on the sexual stage -(gametophyte), as we find it in the liverworts and mosses, it first -becomes free from the gametophyte in the members of the fern group, and -is here able to lead an independent existence. The sporophyte, then, -might be regarded as the modern phase of plant life, since it is that -which has become and remains the prominent one in later times. - -=671. The gametophyte once prominent has become degenerate.=—On -the other hand we can see that just as remarkable changes have come -upon the other phase of plant life, the sexual stage, or gametophyte. -There is reason to believe that the gametophyte was the stage of plant -life which in early times existed almost to the exclusion of the -sporophyte, since the characteristic thallus of the algæ is better -adapted to an aquatic life than is the spore-bearing state of plants. -At least, we now find in the plants of this group as well as in the -liverworts, that the gametophyte is the prominent stage. When we reach -the members of the fern group, and the sporophyte becomes independent, -we find that the gametophyte is decreasing in size, in the higher -members of the pteridophytes, the male prothallium consisting of only a -few cells, while the female prothallium completes its development still -within the spore wall. And in selaginella it is entirely dependent on -the sporophyte for nourishment. - -=672.= As we pass through the gymnosperms we find that the -condition of things which existed in the bryophytes has been reversed, -and the gametophyte is now entirely dependent on the sporophyte for -its nourishment, the female prothallium not even becoming free from -the sporangium, which remains attached to the sporophyte, while the -remnant of a male prothallium, during the stage of its growth, receives -nourishment from the tissues of the nucellus through which it bores its -way to the egg-cell. - -=673.= In the angiosperms this gradual degradation of the male -and female prothallia has reached a climax in a one-celled male -prothallium with two sperm cells, and in the embryo sac with no clearly -recognizable traces of an archegonium to identify it as a female -prothallium. The development of the endosperm subsequent, in most -cases, to fertilization, providing nourishment for the sporophytic -embryo at one stage or another, is believed to be the last remnant of -the female prothallium in plants. - -=674. The seed.=—The seed is the only important character -possessed by the higher plants (the gymnosperms and angiosperms) which -is not possessed by one or another of the lower great groups. With the -gradual evolution of the higher plants from the lower there has been -developed at certain periods organs or structural characters which -were not present in some of the lower groups. Thus the thallus is the -plant body of the algæ and fungi, so that these two groups of plants -are sometimes called _Thallophytes_. In the Bryophytes (liverworts and -mosses) the thallus is still present, but there is added the highly -organized archegonium in place of the simple female gamete or oogonium, -or carpogonium of the algæ and fungi, and the sporophyte has become -a distinct though still dependent structure. In the Pteridophytes -the thallus is still present as the prothallium, archegoina are also -present, and while both of these structures are retrograding the -sporophyte has become independent and has organized for the first time -a true vascular system for conduction of water and food. In the -gymnosperms and angiosperms the thallus is present in the endosperm; -distinct, though reduced, archegonia are present in most gymnosperms -and represented only by the egg in the angiosperms; the vascular system -is still more highly developed while the seed for the first time is -organized, and characterizes these plants so that they are called seed -plants, or _Spermatophytes_. - - -Variation, Hybridization, Mutation. - -=674a. Variation.=—It is a well-known fact that plants as well -as animals are subject to variation. Under certain conditions, some -of which are partly understood and others are unknown, the progeny of -plants differ in one or more characters from their parents. Some of -these variations are believed to be due to the influence of environment -(see Parts III and IV). Others are the result of the crossing of -individuals which show greater or lesser differences in one or more -characters, or the crossing of different species (_hybridization_). The -most profound variations are those which spring suddenly into existence -(_mutation_). - -=674b. Hybridization.=—Two different species are “crossed” where -the egg-cell of one species is fertilized by the sperm of another -species. The progeny resulting from such a cross is a _hybrid_. Hybrids -sometimes resemble one parent, sometimes another, sometimes both. Where -the parents differ only in respect to one character of an organ or -structure, there is a regular law in respect to the progeny if they are -self-fertilized. In the first generation all the individuals are alike -and resemble one of the parents, and the special differential character -of that parent is called the _dominant_ character. In the second -generation 75% possess the dominant character, while 25% resemble -the other original parent, and its differential character is called -_recessive_. These are _pure_ recessives, since successive generations, -if self-fertilized, are always recessive. Of the 75% which show the -dominant character in the second generation, one-third (or 25% of the -whole number) are pure dominants if self-fertilization is continued, -while 50% are really “cross breds” like the first generation, and -if self-fertilized split up again into approximately 25 dominants, -50 cross breds, and 25 recessives. This is what is called Mendel’s -law. Where the original parents differ in respect to more than one -character, the result is more complicated (see Mendel’s Principles of -Heredity; also de Vries, Das Spaltungsgesetz der Bastarde, Ber. d. -deutsch. bot. Gesell., 18, 83, 1900). - -=674c. Mutation.=—This term is applied to those variations which -appear so suddenly that some of the progeny of two like individuals -differ from all the others to a marked degree. Some of these mutations -are so different as to be regarded as new species. Some of the -primroses show mutations, and Œnothera gigas is a mutation from Œnothera -lamarkiana (see de Vries, Die Mutationstheorie, Leipzig). - -=675.= TABLE SHOWING HOMOLOGIES OF SPOROPHYTE AND GAMETOPHYTE IN -ANGIOSPERMS. - - TERMS CORRESPONDING TO THOSE USED IN PTERIDOPHYTES. COMMON TERMS. - - Sporophyte { = Higher plant. - Spore-bearing part { = Stamens and - { carpels. - -------------------------------------------------------------------- - { {Anther. - Sporophyte {Microsporophyll = Stamen {Filament. - { - {Microsporangium = Pollen sac, - { usually - { two or four. - -------------------------------------------------------------------- - {Microspore at maturity = Pollen grain. - { usually of 2 or 3 cells - { {young male prothallium} - - { - {1. Large cell (part of = Vegetative cell. - { antheridium wall?), with - { its nucleus surrounded - { by wall of spore - Male gametophyte {2. Small cell with nucleus, = Generative cell. - { no wall, floating in - { protoplasm of large cell - { is the central cell of - { antheridium - { (male sexual organ) - {Mature male prothallium = Pollen grain - { with tube. - {Antheridium cell divided, = Paternal cells, - { 2 sperm cells or generative - { cells. - -------------------------------------------------------------------- - { {Stigma. - {Macrosporophyll = {Carpel or {Style. - Sporophyte { {simple pistil {Ovary. - { - { - {Macrosporangium, covered = {Nucellus, covered - { by 1 or 2 coats { by 1 or 2 - { { coats = ovule. - -------------------------------------------------------------------- - {Macrospore, cell in end of = Uninuclear state - { macrosporangium, does of embryo sac. - { not become free, cavity - { enlarges - {Macrospore divides into - { 8 cells to form young - { female prothallium = Embryo sac. - { - Female gametophyte. {Remnant of archegonium, = Maternal cell, - { egg (female sexual organ) or germ cell. - {Growing part of prothallium = {Two polar nuclei - { { fused, making - { { endosperm - { { nucleus. - {Mature female prothallium = {Endosperm, - { { developed by - { { many divisions - { { of endosperm - { { nucleus. - -------------------------------------------------------------------- - Young sporophyte {After fecundation of egg, egg divides - surrounded by { to form embryo. Embryo in endosperm - parts of the { (sometimes latter nearly or quite - gametophyte and { absent) surrounded by coats = Seed. - new growth of old{ - sporophyte { - Young sporophyte surrounded by remnants of gametophyte and new - parts of old sporophyte (remains of endosperm and of nucellus, - and ovular coat) = the seed. - - - - -CHAPTER XXXVII. - -MORPHOLOGY OF THE NUCLEUS AND SIGNIFICANCE OF GAMETOPHYTE AND -SPOROPHYTE. - - -=676.= In the development of the spores of the liverworts, -mosses, ferns, and their allies, as well as in the development of the -microspores of the gymnosperms and angiosperms, we have observed that -four spores are formed from a single mother cell. These mother cells -are formed as a last division of the fertile tissue (archesporium) of -the sporangium. In ordinary cell division the nucleus always divides -prior to the division of the cell. In many cases it is directly -connected with the laying down of the dividing cell wall. - -[Illustration: Fig. 402. Forming spores in mother cells (Polypodium -vulgare).] - -[Illustration: Fig. 403. Spores just mature and wall of mother cell -broken (Asplenium bulbiferum).] - -=677. Direct division of the nucleus.=—The nucleus divides in two -different ways. On the one hand the process is very simple. The nucleus -simply fragments, or cuts itself in two. This is direct division. - -=678. Indirect division of the nucleus.=—On the other hand very -complicated phenomena precede and attend the division of the nucleus, -giving rise to a succession of nuclear figures presented by a definite -but variable series of evolutions on the part of the nuclear substance. -This is _indirect division_ of the nucleus, or _karyokinesis_. Indirect -division of the nucleus is the usual method, and it occurs in the -normal growth and division of the cell. The nuclear figures which are -formed in the division of the mother cell into the four spores are -somewhat different from those occurring in vegetative division, but -their study will serve to show the general character of the process. - -=679. Chromatin and linin of the nucleus.=—In figure 404 is -represented a pollen mother cell of the May-apple (podophyllum). The -nucleus is in the resting stage. There is a network consisting of very -delicate threads, the _linin_ network. Upon this network are numerous -small granules, and at the junction of the threads are distinct knots. -The nucleolus is quite large and prominent. The numerous small granules -upon the linin stain very deeply when treated with certain dyes used in -differentiating the nuclear structure. This deeply staining substance -is the _chromatin_ of the nucleus. - -[Illustration: Fig. 404. Pollen mother cell of podophyllum, resting -nucleus. Chromatin forming a network.] - -[Illustration: Fig. 405. Spirem stage of nucleus. _nu_, nuclear cavity; -_n_, nucleolus; _Sp_, spirem.] - -[Illustration: Fig. 406. Forming spindle, threads from protoplasm with -several poles, roping the chromosomes up to nuclear plate. - -(Figures 404-406 after Mottier.)] - -=680. The chromatin skein.=—One of the first nuclear figures in the -preparatory stages of division is the chromatin _skein_ or _spirem_. -The chromatin substance unites to form this. The spirem is in the form -of a narrow continuous ribbon, or band, woven into an irregular skein, -or gnarl, as shown in figure 405. This band splits longitudinally -into two narrow ones, and then each divides into a definite number -of segments, about eight in the case of podophyllum. Sometimes the -longitudinal splitting of the band appears to take place after the -separation into the chromatin segments. The segments remain in pairs -until they separate at the nuclear plate. - -[Illustration: Fig. 407. - -Karyokinesis in pollen mother cells of podophyllum. At the left the -spindle with the chromosomes separating at the nuclear plate; in the -middle figure the chromosomes have reached the poles of the spindle, -and at the right the chromosomes are forming the daughter nuclei. -(After Mottier.)] - -=681. Chromosomes, nuclear plate, and nuclear spindle.=—Each -one of these rod-like chromatin segments is a _chromosome_. -The pairs of chromosomes arrange themselves in a median plane -of the nucleus, radiating somewhat in a stellate fashion, forming -the _nuclear plate_, or _monaster_. At the same time threads of the -protoplasm (kinoplasm) become arranged in the form of a spindle, -the axis of which is perpendicular to the nuclear plate of chromosomes, -as shown in figure 407, at left. Each pair of chromosomes -now separate in the line of the division of the original spirem, -one chromosome of each pair going to one pole of the spindle, -while the other chromosome of each pair goes to the opposite pole. -The chromosomes here unite to form the daughter nuclei. Each of these -nuclei now divide as shown in figure 409 (whether the chromosomes in -this second division in the mother cell split longitudinally or divide -transversely has not been definitely settled), and four nuclei are -formed in the pollen mother cell. The protoplasm about each one of -these four nuclei now surrounds itself with a wall and the spores are -formed. - -[Illustration: Fig. 408. Different stages in the separation of divided -U-shaped chromosomes at the nuclear plate. (After Mottier.) In -podophyllum.] - -[Illustration: Fig. 409. Second division of nuclei in pollen mother -cell of podophyllum, chromosomes at poles.] - -[Illustration: Fig. 410. Chromosomes uniting at poles to form the -nuclei of the four spores. (After Mottier.)] - -=The number of chromosomes usually the same in a given species -throughout one phase of the plant.=—In those plants which have been -carefully studied, the number of chromosomes in the dividing nucleus -has been found to be fairly constant in a given species, through all -the divisions in that stage or phase of the plant, especially in the -embryonic, or young growing parts. For example, in the prothallium, or -gametophyte, of certain ferns, as osmunda, the number of chromosomes -in the dividing nucleus is always twelve. So in the development of the -pollen of lilium from the mother cells, and in the divisions of the -antherid cell to form the generative cells or sperm cells, there are -always twelve chromosomes so far as has been found. In the development -of the egg of lilium from the macrospore there are also twelve -chromosomes. - -=When fertilization takes place the number of chromosomes is doubled -in the embryo.=—In the spermatozoid of osmunda then, as well as in -the egg, since these are developed on the gametophyte, there are twelve -chromosomes each. The same is true in the sperm cell (generative cell) -of lilium, and also in the egg-cell. When these nuclei unite, as they -do in fertilization, the paternal nucleus with the maternal nucleus, -the number of chromosomes in the fertilized egg, if we take lilium as -an example, is twenty-four instead of twelve; the number is doubled. -The fertilized egg is the beginning of the sporophyte, as we have seen. -Curiously throughout all the divisions of the nucleus in the embryonic -tissues of the sporophyte, so far as has been determined, up to the -formation of the mother cells of the spores, the number of chromosomes -is usually the same. - -[Illustration: Fig. 411. Karyokinesis in sporophyte cells of -podophyllum (twice the number of chromosomes here that are found in the -dividing spore mother cells).] - -=682. Reduction of the number of chromosomes in the nucleus.=—If -there were no reduction in the number of chromosomes at any point in -the life cycle of plants, the number would thus become infinitely -large. A reduction, however, does take place. This usually occurs, -either in the mother cell of the spores or in the divisions of its -nucleus, at the time the spores are formed. In the mother cells a sort -of pseudo-reduction is effected by the chromatin band separating into -one half the usual number of nuclear segments. So that in lilium during -the first division of the nucleus of the mother cell the chromatin band -divides into twelve segments, instead of twenty-four as it has done -throughout the sporophyte stage. So in podophyllum during the first -division in the mother cell it separates into eight instead of into -sixteen. Whether a qualitative reduction by transverse division of the -spirem band, unaccompanied by a longitudinal splitting, takes place -during the first or second karyokinesis is still in doubt. Qualitative -reduction does take place in some plants according to Beliaieff and -others. Recently the author has found that it takes place in Trillium -grandiflorum during the second karyokinesis, and in Arisæma triphyllum -the chromosomes divide both transversely and longitudinally during -the first karyokinesis forming four chromosomes, and a qualitative -reduction takes place here. - -=683. Significance of karyokinesis and reduction.=—The precision -with which the chromatin substance of the nucleus is divided, when -in the spirem stage, and later the halves of the chromosomes are -distributed to the daughter nuclei, has led to the belief that this -substance bears the hereditary qualities of the organism, and that -these qualities are thus transmitted with certainty to the offspring. -In reduction not only is the original number of chromosomes restored, -it is believed by some that there is also a qualitative reduction of -the chromatin, i.e. that each of the four spores possesses different -qualitative elements of the chromatin as a result of the reducing -division of the nucleus during their formation. - -The increase in number of chromosomes in the nucleus occurs with the -beginning of the sporophyte, and the numerical reduction occurs at the -beginning of the gametophyte stage. The full import of karyokinesis and -reduction is perhaps not yet known, but there is little doubt that a -profound significance is to be attached to these interesting phenomena -in plant life. - -=684. The gametophyte may develop directly from the tissue of the -sporophyte.=—If portions of the sporophyte of certain of the -mosses, as sections of a growing seta, or of the growing capsule, be -placed on a moist substratum, under favorable conditions some of the -external cells will grow directly into protonemal threads. In some -of the ferns, as in the sensitive fern (onoclea), when the fertile -leaves are expanding into the sterile ones, protonemal outgrowths occur -among the aborted sporangia on the leaves of the sporophyte. Similar -rudimentary protonemal growths sometimes occur on the leaves of the -common brake (pteris) among the sporangia, and some of the rudimentary -sporangia become changed into the protonema. In some other ferns, as -in asplenium (A. filix-fœmina, var. clarissima), prothallia are borne -among the aborted sporangia, which bear antheridia and archegonia. In -these cases the gametophyte develops from the tissue of the sporophyte -without the intervention or necessity of the spores. This is _apospory_. - -[Illustration: Fig. 412. Apogamy in Pteris cretica.] - -=685. The sporophyte may develop directly from the tissue of the -gametophyte.=—In some of the ferns, Pteris cretica for example, -the embryo fern sporophyte arises directly from the tissue of the -prothallium, without the intervention of sexual organs, and in some -cases no sexual organs are developed on such prothallia. Sexual organs, -then, and the fusion of the spermatozoid and egg nucleus are not here -necessary for the development of the sporophyte. This is _apogamy_. -Apogamy occurs in some other species of ferns, and in other groups of -plants as well, though it is in general a rare occurrence except in -certain species, where it may be the general rule. - -=686. Types of nuclear division.=—The nuclear figures in the -vegetative cells are usually different from those in the spore -mother cells. In the spore mother cells there are two types of -nuclear division. (1) The first division in the mother cell is called -_heterotypic_. The early stages of this division usually extend over a -longer period than the second, and the figures are more complex. Before -the chromosomes arrive at the nuclear plate they are often in the form -of rings, or tetrads, or in the form of X, V, or Y, and the number is -usually one half the number in the preceding cells of the sporophyte. -(2) The _homotypic_ division immediately follows the heterotypic and -the figures are simpler, often the chromosomes being of a hook form, -or sometimes much stouter than in the heterotypic division. In the -vegetative cells (sometimes called somatic cells, or body cells in -contrast with reproductive cells) there is another type, called by some -the _vegetative type_. The chromosomes here are often in the form of -the letter U, and the figures are much simpler than in the heterotypic -division. In the somatic cells of the sporophyte, as stated above, -the number of chromosomes is double that found in the heterotypic -and homotypic divisions of the mother cells and in the somatic cells -of the gametophyte, Fig. 411 represents a late stage in the division -of somatic cells in the sporophyte of podophyllum. The root-tips of -various plants as the onion, lily, etc., are excellent places in which -to study nuclear division in the somatic cells of the sporophyte. - -=687. Comparison with animals.=—In animals there does not seem -to be anything which corresponds with the gametophyte of plants unless -the sperm cells and eggs themselves represent it. Heterotypic and -homotypic division with the accompanying reduction of the number of the -chromosomes takes place in animals usually in the mother cells of the -sperms and eggs. At the time of fertilization the number of chromosomes -is doubled, so that all the somatic cells (except in rare instances) -from the fertilized egg to the mother cells of sperms and eggs have the -doubled number of chromosomes. Reduction, therefore, takes place in -animals just prior to the formation of the gametes, while in plants it -takes place just prior to the formation of the gametophytes. - -=688. Perhaps there is not a fundamental difference between -gametophyte and sporophyte.=—This development of sporophyte, or -leafy-stemmed plant of the fern (parag. 685), from the tissue of the -gametophyte is taken by some to indicate that there is not such a great -difference between the gametophyte and sporophyte of plants as others -contend. In accordance with this view it has been suggested that the -leafy-stemmed moss plant, as well as the leafy stem of the liverworts, -is homologous with the sporophyte or leafy stem of the fern plant; that -it arises by budding from the protonema; and that the sexual organs are -borne then on the sporophyte. - - - - -PART III. - -PLANT MEMBERS IN RELATION TO ENVIRONMENT. - - - - -CHAPTER XXXVIII. - -THE ORGANIZATION OF THE PLANT. - - -I. Organization of Plant Members.[37] - -=689.= It is now generally conceded that the earliest plants to -appear in the world were very simple in form and structure. Perhaps the -earliest were mere bits of naked protoplasm, not essentially different -from early animal life. The simplest ones which are clearly recognized -as plants are found among the lower algæ and fungi. These are single -cells of very minute size, roundish, oval, or oblong, existing -during their growing period in water or in a very moist substratum -or atmosphere. Examples are found in the red snow plant (_Sphærella -nivalis_), the Pleurococcus, the bacteria; and among small colonies of -these simple organisms (Pandorina) or the thread-like forms (Spirogyra, -Œdogonium, etc.). It is evident that some of the life relations of such -very simple organisms are very easily obtained—that is, the adjustment -to environment is not difficult. All of the living substance is very -closely surrounded by food material in solution. These food solutions -are easily absorbed. Because of the minute size of the protoplasts and -of the plant body, they do not have to solve problems of transport of -food to distant parts of the body. When we pass to more bulky organisms -consisting of large numbers of protoplasts closely compacted together, -the problem of relation to environment and of food transport become -felt; the larger the organism usually the greater are these problems. -A point is soon reached at which there is a gain by a differentiation -in the work of different protoplasts, some for absorption, some for -conduction, some for the light relation, some for reproduction, and -so on. There is also a gain in splitting the form of the plant body -up into parts so that a larger surface is exposed to environment -with an economy in the amount of building material required. In this -differentiation of the plant body into parts, there are two general -problems to be solved, and the plant to be successful in its struggle -for existence must control its development in such a way as to preserve -the balance between them. (1) A ready display of a large surface to -environment for the purpose of acquiring food and the disposition of -waste. (2) The protection of the plant from injuries incident to an -austere environment. - -It is evident with the great variety of conditions met with in -different parts of the same locality or region, and in different parts -of the globe, that the plant has had very complex problems to meet and -in the solution of them it has developed into a great variety of forms. -It is also likely that different plants would in many cases meet these -difficulties in different ways, sometimes with equal success, at other -times with varied success. Just as different persons, given some one -piece of work to do, are likely to employ different methods and reach -results that are varied as to their value. While we cannot attribute -consciousness or choice to plants in the sense in which we understand -these qualities in higher animals, still there is something in their -“constitution” or “character” whereby they respond in a different -manner to the same influences of environment. This is, perhaps, -imperceptible to us in the different individuals of the same species, -but it is more marked in different species. Because of our ignorance of -this occult power in the plant, we often speak of it as an “inherent” -quality. - - Perhaps the most striking examples one might use to - illustrate the different line of organization among - plants in two regions where the environment is very - different are to be found in the adaptation of the - cactus or the yucca to desert regions, and the oak or - the cucurbits to the land conditions of our climate. - The cactus with stem and leaf function combined in a - massive trunk, or the yucca with bulky leaves expose - little surface in comparison to the mass of substance, - to the dry air. They have tissue for water storage and - through their thick epidermis dole it out slowly since - there is but little water to obtain from dry soil. - - The cucurbits and the oak in their foliage leaves - expose a very large surface in proportion to the mass - of their substance, to an atmosphere not so severely - dry as that of the desert, while the roots are able - to obtain an abundant supply of water from the moist - soil. The cactus and the yucca have differentiated - their parts in a very different way from the oak or the - cucurbits, in order to adapt themselves to the peculiar - conditions of the environment. - - When we say that certain plants have the power to - adapt themselves to certain conditions of environment, - we do not mean to say that if the cucurbits were - transferred to the desert they would take on the form - of the cactus or the yucca. They could do neither. - They would perish, since the change would be too great - for their organization. Nor do we mean, that, if the - cactus or yucca were transferred from the desert to our - climate, they would change into forms with thin foliage - leaves. They could not. The fact is that they are - enabled to live in our climate when we give them some - care, but they show no signs of assuming characters - like those of our vegetation. What we do mean is, that - where the change is not too great nor too sudden, some - of the plants become slightly modified. This would - indicate that the process of organization and change of - form is a very slow one, and is therefore a question of - time—ages it may be—in which change in environment - and adaptation in form and structure have gone on - slowly hand in hand. - -=690. Members of the plant body.=—The different parts into which -the plant body has become differentiated are from one point of view, -spoken of as members. It is evident that the simplest forms of life -spoken of above do not have members. It is only when differentiation -has reached the stage in which certain more or less prominent parts -perform certain functions for the plant that members are recognized. -In the algæ and fungi there is no differentiation into stem and leaf, -though there is an approach to it in some of the higher forms. Where -this simple plant body is flattened, as in the sea-wrack, or ulva, it -is a _frond_. The Latin word for frond is _thallus_, and this name is -applied to the plant body of all the lower plants, the algæ and fungi. -The algæ and fungi together are sometimes called _thallophytes_, or -_thallus plants_. The word thallus is also sometimes applied to the -flattened body of the liverworts. In the foliose liverworts and mosses -there is an axis with leaf-like expansions. These are believed by some -to represent true stems and leaves; by others to represent a flattened -thallus in which the margins are deeply and regularly divided, or in -which the expansion has only taken place at regular intervals. - -In the higher plants there is usually great differentiation of the -plant body, though in many forms, as in the duckweeds, it is in the -form of a frond. While there is a great variety in the form and -function of the members of the plant body, they are all reducible to a -few fundamental members. Some reduce these forms to three, the _root_, -_stem_, _leaf_; while others to two, the _root_, and _shoot_, which is -perhaps the best primary subdivision, and the shoot is then divided -into stem and leaf, the leaf being a lateral outgrowth of the stem, and -can be indicated by the following diagram: - - - {Stem. - {Shoot····{ - Plant body····{ {Leaf. - {Root. - - -KINDS OF SHOOTS. - -=691.= Since it is desirable to consider the shoot in its relation -to environment, for convenience in discussion we may group shoots into -four prominent kinds: (1) _Foliage shoots_; (2) _Shoots without foliage -leaves_; (3) _Floral shoots_; (4) _Winter conditions of shoots and -buds._ Topic (4) will be treated in Chapter XXXIX, section IV. - -[Illustration: Fig. 413. Lupinus perennis. Foliage shoot and floral -shoot.] - -=692. (1st) Foliage shoots.=—Foliage shoots are either aerial, -when their relation is to both light and air; or they are aquatic, -when their relation is to both light and water. They bear green -leaves, and whether in the air or water we see that light is one of -the necessary relations for all. Naturally there are several ways in -which a shoot may display its leaves to the light and air or water. -Because of the great variety of conditions on the face of the earth -and the multitudinous kinds of plants, there is the greatest diversity -presented in the method of meeting these conditions. There is to be -considered the problem of support to the shoot in the air, or in the -water. The methods for solving this problem are fundamentally different -in each case, because of the difference in the density of air and -water, the latter being able to buoy up the plant to a great degree, -particularly when the shoot is provided with air in its intercellular -spaces or air cavities. In the solution of the problem in the relation -of the shoot to aerial environment, stem and leaf have in most cases -coöperated;[38] but in view of the great variety of stems and their -modifications, as well as of leaves, it will be convenient to discuss -them in separate chapters. - -[Illustration: Fig. 413_a_. Burrowing type, the mandrake, a “rhizome.”] - -=693. (2d) Shoots without foliage leaves.=—These are subterranean -or aerial. Nearly all subterranean shoots have also aerial shoots, -the latter being for the display of foliage leaves (foliage-shoots), -and also for the display of flowers (flower-shoots). The subterranean -kinds bear scale leaves, i.e., the leaves not having a light relation -are reduced in size, being small, and they lack chlorophyll. Examples -are found in Solomon’s seal, mandrake (fig. 413_a_), etc. Here the -scale leaves are on the bud at the end of the underground stem from -which the foliage shoot arises. Aerial shoots which lack foliage -leaves are the dodder, Indian-pipe-plant, beech drops, etc. These -plants are saprophytes or parasites (see Chapter IX). Deriving their -carbohydrate food from other living plants, or from humus, they do not -need green leaves. The leaves have, therefore, probably been reduced -in size to mere scales, and accompanying this there has been a loss of -the chlorophyll. Other interesting examples of aerial shoots without -foliage leaves are the cacti where the stem has assumed the leaf -function and the leaves have become reduced to mere spines. The various -modifications which shoots have undergone accompanying a change in -their leaf relation will be discussed under stems in Chapter XXXIX. - -=694. (3d) Floral shoots.=—The floral shoot is the part of the -plant bearing the flower. As interpreted here it may consist of but a -single flower with its stalk, as in Trillium, mandrake, etc., or of -the clusters of flowers on special parts of the stem, termed flower -clusters, as the _catkin_, _raceme_, _spike_, _umbel_, _head_, etc. In -the floral shoot as thus interpreted there are several peculiarities to -observe which distinguish it from the foliage shoot and adapt it to its -life relations. - -The floral shoot in many respects is comparable to the foliage shoot, -as seen from the following peculiarities: - - (1) It usually possesses, beside the flowers, small - green leaves which are in fact foliage though they are - very much reduced in size, because the function of - the shoot as a foliage shoot is subordinated to the - function of the floral shoot. These small leaves on the - floral shoot are termed _bracts_. - - (2) It may be (_a_) unbranched, when it would consist - of a single flower, or (_b_) branched, when there would - be several to many flowers in the flower cluster. - - (3) The flower bud has the same origin on the shoot - as the leaf bud; it is either terminal or axillary, or - both. - - (4) The members of the flower belong to the leaf - series, i.e., they are leaves, but usually different - in color from foliage leaves, because of the different - life relation which they have to perform. Evidence - of this is seen in the transition of sepals, petals, - stamens, or pistils, to foliage leaves in many flowers, - as in the pond lily, the abnormal forms of trillium, - and many monstrosities in other flowers (see Chapter - XXXIV). - - (5) The position of the members of the flower on - its axis, though usually more crowded, in many cases - follows the same plan as the leaves on the stem. - -The various kinds of floral shoots or flower clusters will be discussed -in Chapter XLII, on the Floral Shoot. - - -II. Organization of Plant Tissues. - -=695.= A tissue is a group of cells of the same kind having a -similar position and function. In large and bulky plants different -kinds of tissue are necessary, not only because the work of the plant -can be more economically performed by a division of labor, but also -cells in the interior of the mass or at a distance from the source -of the food could not be supplied with food and air unless there -were specialized channels for conducting food and specialized tissue -for support of the large plant body. In these two ways most of the -higher plants differ from the simple ones. The tissues for conduction -are sometimes called collectively the _mestome_, while tissues for -mechanical support are called _stereome_. Division of labor has -gone further also so that there are special tissues for absorption, -assimilation, perception, reproduction, and the like. The tissues of -plants are usually grouped into three systems: (1) The Fundamental -System, (2) The Fibrovascular System, (3) The Epidermal System. Some of -the principal tissues are as follows: - - -1. THE FUNDAMENTAL SYSTEM. - -=696. Parenchyma.=—Tissue composed of thin-walled cells which -in the normal state are living. Parenchyma forms the loose and spongy -tissue in leaves, as well as the palisade tissue (see Chapter IV); the -soft tissue in the cortex of root and stem (Fig. 414); as well as that -of the pith, of the pith-rays or medullary rays of the stem; and is -mixed in with the other elements of the vascular bundle where it is -spoken of as wood parenchyma and bast parenchyma; and it also includes -the undifferentiated tissue (meristem) in the growing tips of roots and -shoots; also the “intrafascicular” cambium (i.e., between the bundles, -some also include the cambium within the bundle). - -=697. Collenchyma.=—This is a strengthening tissue often found -in the cortex of certain shoots. It also is composed of living cells. -The cells are thickened at the angles, as in the tomato and many other -herbs (fig. 414). - -=698. Sclerenchyma, or stone-tissue.=—This is also a -strengthening tissue and consists of cells which do not taper at the -ends and the walls are evenly thickened, sometimes so thick that the -inside (lumen) of the cell has nearly disappeared. Usually such cells -contain no living contents at maturity. Sclerenchyma is very common in -the hard parts of nuts, and underneath the epidermis of stems and -leaves of many plants, as in the underground stems of the bracken fern, -the leaves of pines (fig. 415), etc. - -[Illustration: Fig. 414. Transverse section of portion of tomato stem. -_ep_, epidermis; _ch_ chlorophyll-bearing cells; _co_, collenchyma; -_cp_, parenchyma.] - -[Illustration: Fig. 415. Margin of leaf of Pinus pinaster, transverse -section, _c_, cuticularized layer of outer wall of epidermis; -_i_, inner non-cuticularized layer; _c´_, thickened outer wall of -marginal cell; _g_, _i´_, hypoderma of elongated sclerenchyma; _p_, -chlorophyll-bearing parenchyma; _pr_, contracted protoplasmic contents. -×800. (After Sachs.)] - -[Illustration: Fig. 416. Section through a lenticel of Betula alba -showing stoma at top, phellogen below producing rows of flattened -cells, the cork. (After De Bary.)] - -=699. Cork.=—In many cases there is a development of “cork” -tissue underneath the epidermis. Cork tissue is developed by repeated -division of parenchyma cells in such a way that rows of parallel cells -are formed toward the outside. These are in distinct layers, soon lose -their protoplasm and die; there are no intercellular spaces and the -cells are usually of regular shape and fit close to each other. In -some plants the cell walls are thin (cork oak), while in others they -are thickened (beech). The tissue giving rise to cork is called “cork -cambium,” or phellogen, and may occur in other parts of the plant. For -example, where plants are wounded the living exposed parenchyma cells -often change to cork cambium and develop a protective layer of cork. -The walls of cork cells contain a substance termed _suberin_, which -renders them nearly waterproof. - -=700. Lenticels.=—These are developed quite abundantly underneath -stomates on the twigs of birch, cherry, beech, elder, etc. The -phellogen underneath the stoma develops a cushion of cork which presses -outward in the form of an elevation at the summit of which is the stoma -(fig. 416). The lenticels can easily be seen. - - -2. THE FIBROVASCULAR SYSTEM. - -=701. Fibrous tissue.=[39]—This consists of thick-walled cells, -usually without living contents which are elongated and taper at the -ends so that the cells, or fibers, overlap. It is common as one of the -elements of the vascular bundles, as wood fibers and bast fibers. - -=702. Vascular tissue, or tracheary tissue.=—This consists of the -vessels or ducts, and tracheides, which are so characteristic of the -vascular bundle (see Chapter V) and forms a conducting tissue for the -flow of water. The vascular tissue contains spiral, annular, pitted, -and scalariform vessels and tracheides according to the marking on the -walls (figs. 58, 59). These are all without protoplasmic contents when -mature. There are also thin-walled living cells intermingled called -wood parenchyma. In the conifers (pines, etc.) the tracheary tissue is -devoid of true vessels except a few spiral vessels in the young stage, -while it is characterized by tracheides with peculiar markings. These -marks on the tracheides are due to the “bordered” pits appearing as two -concentric rings one within the other. These can be easily seen in a -longitudinal section of wood of conifers. - -=703. Sieve tissue.=—This consists of elongated tubular cells -connected at the ends, the cross walls being perforated at the ends. -These are in the phloem part of the bundle, and serve to conduct -downwards the dissolved substances elaborated in the leaves. - -=704. Fascicular cambium.=—This is the living, cell-producing -tissue in the vascular bundle, which in the open bundle adds to the -phloem on one side and the xylem on the other. - - -3. THE EPIDERMAL SYSTEM. - -=705.= To the epidermal system belong the epidermis and the -various outgrowths of its cells in the form of hairs, or _trichomes_, -as well as the guard cells of the stomates, and probably some of the -reproductive organs. - -=706. The epidermis.=—The epidermis proper consists of a -single layer of external cells originating from the outer layer of -parenchyma cells at the growing apex of the stem or root. These cells -undergo various modifications of form. In many cases they lose their -protoplasmic contents. In many cases the outer wall becomes thickened, -especially in plants growing in dry situations or where they are -exposed to drying conditions. The epidermal cells generally become -considerably flattened, and are usually covered with a more or less -well developed waterproof cuticle, a continuous layer over the -epidermis. In many plants the cuticle is covered with a waxy exudation -in the form of a thin layer, or of rounded grains, or slender rods, -or grains and needles in several layers. These waxy coverings are -sometimes spoken of as “bloom” on leaves and fruit. - -=707. Trichomes.=—Trichome is a general term including various -hair-like outgrowths from the epidermis, as well as scales, prickles, -etc. These include root hairs, rhizoids, simple or branched hairs, -glandular hairs, glandular scales, etc. Glandular hairs are found on -many plants, as tomato, verbena, primula, etc.; glandular scales on -the hop; simple-celled hairs on the evening primrose, cabbage, etc.; -many-celled hairs on the primrose, pumpkin; branched hairs on the -shepherd’s-purse, mullein, etc., stellate hairs on some oak leaves. - -For stomates see Chapter IV. - - -4. ORIGIN OF THE TISSUES. - -=708. Meristem tissue.=—The various tissues consisting of cells -of dissimilar form are derived from young growing tissue known as -_meristem_. Meristem tissue consists of cells nearly alike in form, -with thin cell walls and rich in protoplasm. It is situated at the -growing regions of the plants. In the higher plants these regions in -general are three in number, the stem and root apex, and the cambium -cylinder beneath the cortex. Tissues produced from the stem and root -apex are called _primary_, those from the cambium _secondary_. In most -cases the main bulk of the plant is secondary tissue, while in the corn -plant it is all primary. - -[Illustration: Fig. 417. Section through growing point of stem, _d_, -dermatogen; _p_, plerome; periblem between. (After De Bary.)] - -=709. Origin of stem tissues.=—Just back of the apical meristem -in a longitudinal section of a growing point it can be seen that the -cells are undergoing a change in form, and here are organized three -formative regions. The outer layer of cells is called _dermatogen_ -(skin producer), because later it becomes the epidermis. The central -group of elongating cells is the _plerome_ (to fill). This later -develops the _central cylinder_, or _stele_, as it is called -(fig. 417). Surrounding the plerome and filling the space between it -and the dermatogen is the third formative tissue called the _periblem_, -which later forms the cortex (bark or rind), and consists of -parenchyma, collenchyma, sclerenchyma, or cork, etc., as the case may -be. It should be understood that all these different forms and kinds -of cells have been derived from meristem by gradual change. In the -mature stems, therefore, there are three distinct regions, the central -cylinder or stele, the cortex, and the epidermis. - -[Illustration: Fig. 418. Concentric bundle from stem of Polypodium -vulgare. Xylem in the center, surrounded by phloem, and this by the -endodermis. (From the author’s Biology of Ferns.)] - -=710. Central cylinder or stele.=—As the central cylinder is -organized from the plerome it becomes differentiated into the vascular -bundles, the pith, the pith-rays (medullary rays) which radiate from -the pith in the center between the bundles out to the cortex, and -the pericycle, a layer of cells lying between the central cylinder -and the cortex. The bundles then are farther organized into the -xylem and phloem portions with their different elements, and the -fascicular cambium (meristem) separating the xylem and phloem, as -described in Chapter V. Such a bundle, where the xylem and phloem -portions are separated by the cambium is called an open bundle (as -in fig. 58). Where the phloem and xylem lie side by side in the same -radius the bundle is a _collateral_ one. Dicotyledons and conifers are -characterized by open collateral bundles. This is why trees and many -other perennial plants continue to grow in diameter each year. -The cambium in the open bundle forms new tissue each spring and -summer, thus adding to the phloem on the outside and the xylem on -the inside. In the spring and early summer the large vessels in the -xylem predominate, while in late summer wood fibers and small vessels -predominate and this part of the wood is firmer. Since the vascular -bundles in the stem form a circle in the cylinder, this difference -in the size of the spring and late summer wood produces the “annual” -rings, so evident in the cross-section of a tree trunk. Branches -originate at the surface involving epidermis, cortex, and the bundles. - -In monocotyledonous plants (corn, palm, etc.) the bundles are not -regularly arranged to form a hollow cylinder, but are irregularly -situated through the stele. There is no meristem, or cambium, left -between the xylem and phloem portions of the bundle and the bundle is -thus _closed_ (as in fig. 60), since it all passes over into permanent -tissue. In most monocotyledons there is, therefore, practically no -annual increase in diameter of the stem. - -[Illustration: Fig. 419. Section of stem (rhizome) of Pteris aquilina. -_sc_, thick-walled sclerenchyma; _a_, thin-walled sclerenchyma; _par_, -parenchyma.] - -=711. Ferns.=—In the ferns and most of the Pteridophytes an -apical meristem tissue is wanting, its place being taken by a single -apical cell from the several sides of which cells are successively -cut off, though Isoetes and many species of Lycopodium have an apical -meristem group. In most of the Pteridophytes also the bundles are -_concentric_ instead of collateral. Fig. 418 represents one of the -bundles from the stem of the polypody fern. The xylem is in the center, -this surrounded by the phloem, the phloem by the phloem sheath, and -this in turn by the endodermis, giving a concentric arrangement of the -component tissues. A cross-section of the stem (fig. 419) shows two -large areas of sclerenchyma, which gives the chief mechanical support, -the bundles being comparatively weak. - -=712. Origin of root tissues.=—A similar apical meristem exists -in roots, but there is in addition a fourth region of formative -tissue in front of the meristem called _calyptrogen_ (fig. 420). This -gives rise to the “root cap” which serves to protect the meristem. -The vascular cylinder in roots is very different from that of the -stem. There is a solid central cylinder in which the groups of xylem -radiate from the center and groups of phloem alternate with them but -do not extend so near the center (fig. 421). As the root ages, changes -take place which obscure this arrangement more or less. Branches of -the roots arise from the central cylinder. In fern roots the apical -meristem is replaced by a single four-sided (tetrahedral) apical cell, -the root cap being cut off by successive divisions of the outer face, -while the primary root tissues are derived from the three lateral faces. - -[Illustration: Fig. 420. Median longitudinal section of the apex of a -root of the barley, Hordeum vulgare. _k_, calyptrogen; _d_, dermatogen; -_c_, its thickened wall; _pr_, periblem; _pl_, plerome; _en_, -endodermis; _i_, intercellular air-space in process of formation; _a_, -cell row destined to form a vessel; _r_, exfoliated cells of the root -cap. (After Strasburger.)] - -[Illustration: Fig. 421. Cross-section of fibrovascular bundle in -adventitious root of Ranunculus repens. _w_, pericycle; _g_, four -radial plates of xylem; alternating with them are groups of phloem. -This is a radial bundle. (After De Bary.)] - -=Function of the root cap.=—The root cap serves an important -function in protecting the delicate meristem or cambium at the tip of -the root. These cells are, of course, very thin-walled, and while there -is not so much danger that they would be injured from dryness, since -the soil is usually moist enough to prevent evaporation, they would be -liable to injury from friction with the rough particles of soil. No -similar cap is developed on the end of the stem, but the meristem here -is protected by the overlapping bud-scales. One of the most striking -illustrations of a root cap may be seen in the case of the Pandanus, or -screw-pine, often grown in conservatories (see fig. 447). On the prop -roots which have not yet reached the ground the root caps can readily -be seen, since they are so large that they fit over the end of the root -like a thimble on the finger. - -=713. Descriptive Classification of Tissues.= - - { Epidermis. - { - { { Simple hairs. - { { Many-celled hairs. - { { Branched hairs, often stellate. - Epidermal { Trichomes. { Clustered, tufted hairs. - System. ····{ { Glandular hairs. - { { Root hairs. - { { Prickles. - { - { Guard cells of stomates. - { Spiral vessels. - { Pitted vessels. - { Scalariform vessels. - { Xylem (wood). { Annular vessels. - { { Tracheides. - { { Wood fibers. - { { Wood parenchyma. - Fibrovascular { - System. ····{ Cambium (fascicular). - { - { { Sieve tubes. - { Phloem (bast). { Bast fibers. - { Companion cells. - { Bast parenchyma. - { Cork. - { Collenchyma. - { Cortex.····{ Parenchyma. - { { Fibers. - { { Milk tissue. - { - { Pith-ray.··{ Parenchyma. - { { Intrafascicular - { Stem and root. { cambium. - { { Pith.······{ Parenchyma. - { { { Sclerenchyma. - { { - Fundamental { { Bundle-sheath. - System. ····{ { - { { Endodermis. - { - { Leaves. { Palisade tissue. - { { Spongy parenchyma. - { - { Reproductive Organs (mainly fundamental). - -=714. Physiological Classification of Tissues.= - -_Formative Tissue._ - -Thin-walled cells composing the meristem, capable of division and from -which other tissues are formed. - -_Protective Tissue._ - -_Tegumentary System._—Epidermis, periderm, bark protecting the plant -from external contact. - -_Mechanical System._—Bast tissue, bast-like tissue, collenchyma, -sclerenchyma, afford protection against harmful bending, pulling, etc. - -_Nutritive Tissues._ - -_Absorptive System._—Root hairs and cells, rhizoids, aerial root -tissue, absorptive leaf glands, absorptive organs in seeds, haustoria -of parasites, etc. - -_Assimilatory System._—Assimilating cells in leaf and stem. - -_Conductive System._—Sieve tissue, tracheary tissue, milk tissue, -conducting parenchyma, etc. - -_Food-storing System._—Water reservoir, water tissue, slime tissue, -fleshy roots and stems, endosperm and cotyledons, etc. - -_Aerating System._—Air spaces and tubes, special air tissue, -air-seeking roots, stomates, lenticels, etc. - -_Secretory and Excretory System._—Water glands, digestive glands, -resin glands, nectaries, tannin, pitch and oil receptacles, etc. - -_Apparatus and Tissues for Special Duties._ - -Holdfasts. - -Tissues of movement, parachute hairs, floating tissue, hygroscopic -tissue, living tissue. - -For perceiving stimuli. - -For conducting stimuli, etc. - -FOOTNOTES: - -[37] =Suggestions to the teacher.=—In the study of the flowering plants -in the secondary school and in elementary courses three general topics -are suggested. 1st, the study of the form and members of the plant and -their arrangement, as in Chapters XXXVIII-XLV. 2d, the study of a few -plants representative of the more important families, in order that the -members of the plant, as studied under the first topic, may be seen in -correlation with the plant as a whole in a number of different types. -3d, the study of plants in their relation to environment, as in Chapter -XLVI. The first and second topics can be conducted consecutively in the -classroom and laboratory. The third topic can be studied at opportune -times during the progress of topics 1 and 2. For example, while -studying topic 1 excursions can be made to study winter conditions of -buds, shoots, etc., if in winter period, or the relations of leaves, -etc., to environment, if in the growing period. While studying topic 2 -excursions can be made to study flower relations, and also vegetation -relations to environment (see Chapters XLVI-LVII of the author’s -“College Text-book of Botany”). It is believed that a study of these -three general topics is of much more value to the beginning student -than the ordinary plant analysis and determination of species. - -[38] It is interesting to note that in some foliage shoots the stem is -entirely subterranean. See discussion of the bracken fern and sensitive -fern in Chapter XXXIX. - -[39] Some fibers occur also very frequently in the Fundamental System, -forming bundle-sheaths, or strands of mechanical tissue in the cortex. - - - - -CHAPTER XXXIX. - -THE DIFFERENT TYPES OF STEMS. WINTER SHOOTS AND BUDS. - - -I. Erect Stems. - -=715. Columnar type.=—The columnar type of stem may be simple or -branched. When branching occurs the branches are usually small and in -general subordinate to the main axis. The sunflower (Helianthus annuus) -is an example. The foliage part is mainly simple. The main axis remains -unbranched during the larger part of the growth-period. The principal -flowerhead terminates the stem. Short branches bearing small heads -then arise in the axils of a few of the upper leaves. In dry, poor -soil, or where other conditions are unfavorable, there may be only the -single terminal flowerhead, when the stem is unbranched. The mullein -is another columnar stem. The foliage part is rarely branched, though -branches sometimes occur where the main axis has become injured or -broken. The flower stem is terminal. The corn plant and the Easter lily -are good illustrations also of the columnar stem. - -Among trees the Lombardy poplar (Populus fastigiata) is an excellent -example of the columnar type. Though this is profusely branched, the -branches are quite slender and small in contrast with the main axis, -unless by some injury or other cause two large axes may be developed. -As the technical name indicates, the branching is fastigiate, i.e., the -branches are crowded close together and closely surround the central -axis. The royal palm and some of the tree ferns have columnar, simple -stems, but the large, wide-spreading leaves at the top of the stem give -the plant anything but a cylindrical habit. Some cedars and arbor-vitæ -are also columnar. - -The advantages of the columnar habit of stem are three: (1) That the -plant stands above other neighboring ones of equal foliage area and -thus is enabled to obtain a more favorable light relation; (2) where -large numbers of plants of the same species are growing close together, -they can maintain practically the same habit as where growing alone; -(3) the advantage gained by other types in their neighborhood in less -shading than if the type were spreading. The cylindrical type can, -therefore, grow between other types with less competition for existence. - -[Illustration: Fig. 422. Cylindrical stem of mullein.] - -[Illustration: Fig. 423. Conical type of larch.] - -=716. The cone type.=—This is well exampled in the larches, -spruces, the gingko tree, some of the pines, cedars, and other -gymnosperms. In the cone type, the main axis extends through the system -of branches like a tall shaft, i.e., the trunk is _excurrent_. The -lower branches are wide-spreading, and the branches become successively -shorter, usually uniformly, as one ascends the stem. The branching is -of two types: (1) the branches are in false whorls; (2) the branches are -distributed along the stem. To the first type belong the pines, Norway -spruce, Douglas spruce, etc. _The white pine_ is an exquisite example, -and in young and middle-aged trees shows the style of branching to -very good advantage. The branches are nearly horizontal, with a -slight sigmoid graceful curve, while towards the top the branches are -ascending. This direction of the branches is due to the light relation. -The few whorls at the top are ascending because of the strong light -from above. They soon become extended in a horizontal direction as the -main source of light is shifting to the side by the shading of the top. -The ascending direction first taken by the upper branches and their -subsequent turning downward, while the ends often still have a slight -ascending direction gives to the older branches their sigmoid curve. - -The young vernal shoots of the pines show some very interesting -growth movements. There are two growth periods: (1) the elongation of -the shoot, and (2) the elongation of the leaves. The elongation of the -shoot takes place first and is completed in about six weeks or two -months’ time. The direction of the shoot in the first period seems to -be entirely influenced by geotropism. It grows directly upward and -stands up as a very conspicuous object in strong contrast with the dark -green foliage of the more or less horizontal shoots. When the second -period of growth takes place, and the leaves elongate, the shoot bends -downward and outward in a lateral direction. - -The rate of growth of the pines can be very easily observed since each -whorl of branches (between the whorls of long shoots there are short -shoots bearing the needle leaves), whether on the main axis or on the -lateral branches, marks a year, the new branches arising each year -at the end of the shoot of the previous year. The rate of growth is -sometimes as high as twelve to twenty-four inches or more per year. - -The _spruces_ form a more perfect cone than the pines. The long -branches are mostly in whorls, but often there are intermediate ones, -though the rate of growth per year can usually be easily determined. In -the _hemlock-spruce_, the branching is distributed. The _larch_ has a -similar mode of branching, but it is deciduous, shedding its leaves in -the autumn, and it has a tall, conical form. - -It would seem that trees of the cone type possessed certain advantages -in some latitudes or elevations over other trees. (1) A conical tree, -like the spruces and larches and the pines, and hemlocks also, before -they get very old, meets with less injury during high winds than trees -of an oval or spreading type. The slender top of the tree where the -force of the wind is greatest presents a small area to the wind, while -the trunk and short slender branches yield without breaking. Perhaps -this is one reason why trees of this type exist in more northern -latitudes and at higher elevations in mountainous regions, and why -the spruce type reaches a higher latitude and altitude even than the -pines. (2) The form of the tree is such as to admit light to a large -foliage area, even where the trees are growing near each other. The -evergreen foliage, persistent for several years, on the wide-spreading -lower branches, probably affords some protection to the trees since -this cover would aid in maintaining a more equable temperature in the -forest cover than if the trees were bare during the winter. (3) There -is less danger of injury from the weight of snow since the greater -load of snow would lie on the lower branches. The form of the branches -also, especially in the spruces, permits them to bend downward without -injury, and if necessary unload the snow if the load becomes too heavy. - -=717. The oval type.=—This type is illustrated by the oak, -chestnut, apple, etc. The trees are usually deciduous, i.e., cast -their leaves with the approach of winter. The main axis is sometimes -maintained, but more often disappears (trunk is _deliquescent_), -because of the large branches which maintain an ascending direction, -and thus lessen the importance of the central axis which is so marked -in the cone type. Trees of this type, and in fact all deciduous trees, -exhibit their character or habit to better advantage during the winter -season when they are bare. Trees of this type are not so well adapted -to conditions in the higher altitudes and latitudes as the cone type, -for the reason given in the discussion of that type. The deciduous -habit of the oaks, etc., enables them to withstand heavy winds far -better than if they retained their foliage through the winter, even -were the foliage of the needle kind and adapted to endure cold. - -=718. The deliquescent type.=—The elm is a good illustration of -this type. The main axes and the branches fork by a false dichotomy, so -that a trunk is not developed except in the forest. The branches rise -at a narrow angle, and high above diverge in the form of an arch. The -chief foliage development is lofty and spreading. - -Trees possess several advantages over vegetation less lofty. They may -start their growth later, but in the end they outgrow the other kinds, -shade the ground and drive out the sun-loving kinds. - - -II. Creeping, Climbing, and Floating Stems. - -=719. Prostrate type.=—This type is illustrated by creeping or -procumbent stems, as the strawberry, certain roses, of which a good -type is one of the Japanese roses (Rosa wichuriana), which creeps very -close to the ground, some of the raspberries, the cucurbits like the -squash, pumpkin, melons, etc. These often cover extensive areas by -branching and reaching out radially on the ground or climbing over low -objects. The cucurbits should perhaps be classed with the climbers, -since they are capable of climbing where there are objects for support, -but they are prostrate when grown in the field or where there are no -objects high enough to climb upon. In the prostrate type, there is -economy in stem building. The plants depend on the ground for support, -and it is not necessary to build strong, woody trunks for the display -of the foliage which would be necessary in the case of an erect plant -with a foliage area as great as some of the prostrate stems. This -gain is offset, at least to a great extent, by the loss in ability to -display a great amount of foliage, which can be done only on the upper -side of the stem. - -[Illustration: Fig. 424. Prostrate type of the water fern (_marsilia_).] - -Other advantages gained by the prostrate stems are protection from -wind, from cold in the more rigorous climates, and some propagate -themselves by taking root here and there, as in certain roses, the -strawberry plant, etc. Some plants have erect stems, and then send -out runners below which take root and aid the plant in spreading and -multiplying its numbers. - -=720. The decumbent type.=—In this type the stem is first erect, -but later bends down in the form of an arch, and strikes root where the -tip touches the ground. Some of the raspberries and blackberries are of -this type. - -=721. The climbing type.=—The grapes, clematis, some roses, the -ivies, trumpet-creeper, the climbing bittersweet, etc., are climbing -stems. Like the prostrate type, the climbers economize in the material -for stem building. They climb over shrubs, up the trunks of trees and -often reach to a great height and acquire the power of displaying a -great amount of foliage by sending branches out on the limbs of the -trees, sometimes developing an amount of foliage sufficient to cover -and nearly smother the foliage of large trees; while the main stem of -the vine may be not over two inches in diameter and the trunk of the -supporting tree may be three feet in diameter. - -=722. Floating stems.=—These are necessarily found in aquatic -plants. The stems may be ascending or horizontal. The stems are -usually not very large, nor very strong, since the water bears them -up. The plants may grow in shallow water, or in water 10-12 feet or -more deep, but the leaves are usually formed at or near the surface of -the water in order to bring them near the light. Various species of -Potamogeton, Myriophyllum, and other plants common along the shores of -lakes, in ponds, sluggish streams, etc., are examples. Among the algæ -are examples like Chara, Nitella, etc., in fresh water; Sargassum, -Macrocystis, etc., in the ocean. In these plants, however, the plant -body is a thallus, which is divided into stem-like (_caulidium_) and -leaf-like (_phyllidium_) structures. - -=723. The burrowing type, or rhizomes.=—These are horizontal, -subterranean stems. The bracken fern, sensitive fern, the mandrake -(see fig. 413_a_), Solomon’s seal, Trillium, Dentaria, and the like, -are examples. The subterranean habit affords them protection from the -cold, the wind, and from injury by certain animals. Many of these stems -act as reservoirs for the storage of food material to be used in the -rapid growth of the short-lived aerial shoot. In the ferns mentioned, -the subterranean is the only shoot, and this bears scale leaves which -are devoid of chlorophyll, and foliage leaves which are larger, and the -only member of the plant body which is aerial. The foliage leaf has -assumed the function of the aerial shoot. The latter is not necessary -since flowers are not formed. The mandrake, Solomon’s seal, Trillium, -etc., have scale leaves on the fleshy underground stems, while foliage -leaves are formed on the aerial stems, the latter also bearing the -flowers. Some of the advantages of the rhizomes are protection from -injury, food storage for the rapid development of the aerial shoot, and -propagation. - -Many of the grasses have subterranean stems which ramify for great -distances and form a dense turf. For the display of foliage and for -flower and seed production, aerial shoots are developed from these -lateral upright branches. - - -III. Specialized Shoots and Shoots for Storage of Food.[40] - -=724. The bulb.=—The bulb is in the form of a bud, but the scale -leaves are large, thick, and fleshy, and contain stored in them food -products manufactured in the green aerial leaves and transported to -the underground bases of the leaves. Or when the bulb is aerial in its -formation, it is developed as a short branch of the aerial stem from -which the reserve food material is transported. Examples are found -in many lilies, as Easter lily, Chinese lilies, onion, tulip, etc. -The thick scale leaves are closely overlapped and surround the short -stem within (also called a _tunicated_ stem). In many lilies there -is a sufficient amount of food to supply the aerial stem for the -development of flower and seed. There are roots, however, from the bulb -and these acquire water for the aerial shoot, and when planted in soil -additional food is obtained by them. - -[Illustration: Fig. 425. Bulb of hyacinth.] - -[Illustration: Fig. 426. Corm of Jack-in-the-pulpit.] - -=725. Corm.=—A corm is a thick, short, fleshy, underground stem. -A good example is found in the jack-in-the-pulpit (Arisæma). - -=726. Tubers.=—These are thickened portions of the subterranean -stems. The most generally known example is the potato tuber (“Irish” -potato, not the sweet potato, which is a root). The “eyes” of the -potato are buds on the stem from which the aerial shoots arise when the -potato sprouts. The potato tuber is largely composed of starch which is -used for food by the young sprouts. - -=726=_a_. =Phylloclades.=—These are trees, shrubs, or -herbs in which the leaves are reduced to mere bracts and stems, -are not only green and function as leaves, but some or all of the -branches are flattened and resemble leaves in form as in Phyllanthus, -Ruscus, Semele, Asparagus, etc. The flowers are borne directly on -these flattened axes. The prickly-pear cactus (Opuntia) is also -a phylloclade. Examples of phylloclades are often to be found in -greenhouses. - -=727. Undifferentiated stems= are found in such plants as the -duckweed, or duckmeat (Lemna, Wolffia, etc. See Chapter III). - - -IV. Annual Growth and Winter Protection of Shoots and Buds.[41] - -=728. Winter conditions.=[42]—While herbs are subjected only to -the damp warm atmosphere of summer, woody plants are also exposed -during the cold dry winter, and must protect themselves against such -conditions. The air is dryer in winter than in summer; while at the -same time root absorption is much retarded by the cold soil. Then, too, -the osmotic activity of the dormant twig-cells being much reduced, the -water-raising forces are at a minimum. It is easy to see, therefore, -that a tree in winter is practically under desert conditions. Moreover, -it has been found by various investigators, contrary to the general -belief, that cold in freezing is only indirectly the cause of death. -The real cause is the abstraction of water from the cell by the ice -crystals forming in the intercellular spaces. Death ensues because the -water content is reduced below the danger-point for that particular -cell. It was formerly thought that on freezing, the cells in the tissue -were ruptured. This is not so. Ice almost never forms within the cell, -but in the spaces between. Freezing then is really a drying process, -and dryness, not cold, causes death in winter. To protect themselves -in winter, trees provide various waterproof coverings for the exposed -surfaces and reduce the activity of the protoplasm so that it will be -less easily harmed by the loss of water abstracted by the freezing -process. - -[Illustration: Fig. 427. Two-year-old twig of horse-chestnut, showing -buds and leaf-scars. (A twig with a terminal bud should have been -selected for this figure.)] - -=729. Protection of the twig.=—Woody plants protect the living -cells within the twigs by the production of a dull or rough corky bark, -or by a thick glossy epidermis over the entire surface. At intervals -occur small whitish specks called lenticels, which here perform nearly -the same function as do stomates in the leaf. - -=730. Bark of trunk.=—A similar service is performed by the -bark for the main trunk and branches of the tree. To admit of growth -in diameter the old bark is constantly being thrown off in strips, -flakes, etc., and replaced by a new but larger cylinder of young bark. -The external appearance thus produced enables experienced persons to -recognize many kinds of trees by the trunk alone. - -=731. Leaf-scars and bundle-scars.=—The presence of foliage -leaves during the winter would greatly increase the transpiring surface -without being of use to the plant; hence they are usually thrown off on -the approach of winter. The scars left by the fallen leaves are termed -leaf-scars. The small dots on the leaf-scars left by the vascular -bundles which extended through the petiole into the twig are termed -bundle-scars. Sometimes stipule-scars are left on each side of the -leaf-scar by the fallen stipules. - -=732. Nodes and internodes.=—The region upon a stem where a leaf -is borne is termed a node. The space between two nodes is an internode. - -[Illustration: Fig. 428.—Shoot of butternut showing leaf-scars, -axillary buds, and adventitious buds (buds coming from above the -axils).] - -[Illustration: Fig. 429.—Shoot and bud of white oak.] - -=733. Phyllotaxy.=—Investigation of a horse-chestnut or willow -twig will show that the leaf-scars occupy definite positions which -are constant for each plant but different for the two species. -The arrangement of the leaves on the stem in any plant is termed -phyllotaxy. In the horse-chestnut we find two scars placed at the same -node, but on opposite sides of the stem. Somewhat higher up we find two -more similarly placed, but in a position perpendicular to that of the -first pair. Such phyllotaxy is termed opposite. If in any plant several -leaves occur at a node, the phyllotaxy is whorled. If but one at each -node, as in the willow, the phyllotaxy is alternate. The opposite and -alternate types are very commonly met with. Closer observation will -show that in the willow, if a line be drawn connecting the successive -leaf-scars, it will pass spirally up the twig until at length a scar is -reached directly over the one taken as a starting-point. Such spiral -arrangement always accompanies alternate phyllotaxy. The section of the -spiral thus delineated is termed a cycle. We express the nature of the -cycle by the fractions ½, ⅓, ⅖, ⅜, ⁵/₁₃, etc., in which the numerator -denotes the number of turns around the stem in each cycle, and the -denominator the number of leaf-scars in the same distance. In a general -way we find in plants only such arrangements as are represented by -the fractions given above. These fractions show the curious condition -that the numerator and denominator of each is equal to the sum of -the numerator or denominator of the two preceding fractions. Much -speculation has been indulged in regarding the significance of these -definite laws of leaf arrangement. In part they may be due to the -desire that each leaf receive the maximum amount of light. Only certain -definite geometrical conditions will insure this. More likely it is due -to the economy of space allotted to the leaf-fundaments in the bud. -Here, again, geometrical laws govern this economy. The phyllotaxy is -nearly constant for a given species. - -=734. Buds.=—The growing point of the stem or branch together -with its leaf or flower fundaments and protective structures is termed -a bud. Winter buds on woody plants are terminal when inclosing the -growing point of the main axis of the twig; lateral when the growing -point is that of a branch of the main axis. Lateral buds are always -axillary, i.e., situated on the upper angle between a leaf and the main -axis. - -=735. Buds occupying special positions.=—Several species of trees -and shrubs produce more than one bud in each leaf-axil. The additional -ones are termed accessory or supernumerary buds. These may be lateral -to one another or they may be superposed as in the walnut or butternut. -In such cases some of the buds usually contain simply floral shoots and -are termed flower buds. In some species buds are frequently produced -on the side of the branches and trunk at some distance from the -leaf-axils, and entirely without regard for the latter; or more rarely -may occur upon the root. Such buds are termed adventitious, and are the -source of the feathery branchlets upon the trunks of the American elm. - -=736. Branching follows the phyllotaxy.=—Since the lateral or -branch-producing buds are always located in the axil of a leaf, the -branches necessarily follow the same arrangement upon the main axis -as do the leaves. Since, however, many of the axillary buds fail to -develop, this arrangement may be more or less obscured. - -[Illustration: Fig. 430. Bud of European elm in section, showing -overlapping of scales.] - -=737. Coverings of winter buds.=—These are of two sorts, hair -and cork, or scales. Buds protected simply by dense hair or sunk in -the cork of the twig are termed naked buds, and are comparatively -rare. Most species protect their buds by the addition of an imbricated -covering of closely appressed scales, the whole frequently being -rendered still more waterproof by the excretion of resin between the -scales or over the whole surface. The scales when studied carefully -are found to be much reduced leaves or parts of leaves. In some cases -they represent a modified whole leaf, when they are said to be laminar, -or a leaf-petiole, when they are petiolar, or stipular, when they are -much-specialized stipules of a leaf which itself is usually absent. The -latter type is much the less common. The form of the bud, the nature -and form of the scales, when combined with characters furnished by the -leaf- and bundle-scars, enable one to recognize and classify the winter -twigs of the various woody species. - -=738. Phyllotaxy of the bud-scales.=—Since the bud-scales are -leaves, they follow a definite phyllotaxy. This may or may not be -the same as that of the foliage leaves. Twigs with opposite leaves -have opposite bud-scales, or if with alternate leaves, then alternate -bud-scales, but the fractions vary. If the scales are stipular, then -there are of course two at each node. - -=739. Function of the bud coverings.=—It is popularly believed -that the scales and hairy coverings serve to keep the bud warm. -Research, however, shows this to be almost entirely erroneous, and that -the thin bud coverings are entirely inadequate to keep out the cold of -winter. They cannot keep the bud even a degree or two warmer than the -outside air, except when the changes are very rapid. Experiment also -shows that the modifying effect of the covering when the bud thaws -out is so slight as to be almost negligible. Indeed, a thermometer -bulb covered with scales taken from a horse-chestnut bud warmed up -more rapidly than a naked one when exposed to sunshine. The wool in -the horse-chestnut bud is not for the purpose of keeping it warm, but -to protect the young shoot from too great transpiration after the bud -opens the following spring. Research has also shown that such tempering -of the heat conditions is not especially beneficial to the plant, -as was once thought. Neither can we find the main function in the -prevention of water from entering the bud. This might be accomplished -in much simpler ways, even if we could demonstrate the desirability of -keeping the water out at all. - -The true functions of the bud-scales are two in number: Firstly, the -prevention of too great loss of water from the young and delicate -parts within; and secondly, the protection of these same parts from -mechanical injury. Without some such protection the delicate young -structures would be beaten off by the wind, or become the food for -hungry birds during the long winter months. - -[Illustration: Fig. 431. Opening buds of hickory.] - -=740. Opening of the buds.=—When the young shoot begins to -grow in the spring, the bud-scales are forced apart or open of their -own accord. During the young condition the shoot is very soft and -brittle, and also possesses a very thin, little cutinized epidermis. -In this condition it is especially liable to mechanical injury and to -injury from drying out. We find, therefore, a tendency for the inner -bud-scales to elongate during vernation, thus forming a tube around the -delicate tissue much like the opening out of a telescope. The young -leaves and internodes themselves are often provided with a woody or -hairy covering to retard transpiration. When the epidermis becomes more -efficient the hairy covering often falls away. - -In the case of naked buds protection is afforded in other ways: by -the protection of hairy covering, by physiological adaptation of the -tissue, or in many cases by the late appearance of the shoot in spring -after the very dry April and May winds have ceased. - -=741. Bud-scars, and how to tell the age of the plant.=—In -general the bud-scales when they fall away in the spring leave scars -termed scale-scars, and the whole aggregate of scale-scars makes up the -bud-scar. The position of the buds of previous winters is, therefore, -marked. It becomes an easy matter to determine the age of a branch, -since all that is necessary is to follow back from one bud-scar to -another, the portion of the stem between representing, except in rare -cases, one year’s growth. - -A woody plant grows in height only by the formation of new sections -of stem added to the apex or side of similar sections produced the -previous season, never, as is commonly supposed, by the further -elongation of the previous year’s growth. Hence a branch once formed -upon a tree is fixed as regards its distance from the ground. The -apparent rise of the branches away from the ground in forest trees is -an illusion caused by the dying away of the lower branches. - -=742. Definite and indefinite growth.=—With the opening of the -buds in spring, growth begins. In some cases, when all the members -for the season were formed, but still minute, within the bud, such -growth consists solely in the expansion of parts already formed; in -others only a few members are thus present to expand, while new ones -are produced by the growing point as the season progresses. In most -cases growth is completed by the middle of July, soon after which buds -are formed for next year’s growth. Such a method of growth is termed -definite. - -In a few woody plants, as, for example, sumach, locust, and raspberry, -growth continues until late in the autumn. In such cases the most -recently formed nodes and internodes are unable to become sufficiently -“hardened” before winter sets in, and are killed back more or less. -Next season’s shoot is a branch from some axillary bud. Such growth is -termed indefinite. - -[Illustration: Fig. 432. Three-year-old twig of the American ash, with -sections of each year’s growth showing annual rings.] - -=743. Structure of woody stems.=—If we make a cross-section of a -woody twig three general regions are presented to view. On the outside -is the rather soft, often greenish “bark,” so called, made up of -sieve tubes, ordinary parenchyma cells, and in many cases long fibrous -cells composing the “fibrous bark.” From a growing layer in this -region, termed the phellogen, the true corky bark of the older trunk is -formed. - -Next within the bark we find the so-called “woody” portion of the -twig. This is strong and resistant to both breaking and cutting. The -microscope shows it to be composed of the ordinary already known woody -elements,[43] wood fibers, for strengthening purposes, pitted and spiral -vessels as conducting tissue; and intermixed with these some living -parenchyma cells. A cross-section of the stem also shows narrow radial -lines through the wood. These are pith-rays, composed of vertical -plates of living parenchyma cells. These cells, unlike the others in -the wood, are elongated radially, not vertically. The height of the -pith-rays as well as their thickness varies with the species studied. -In the older trunk only the outer portion, a few inches in thickness, -remains light-colored and fresh, and is called sap-wood. The inner -wood is usually darker and harder, and is termed heart-wood. Living -parenchyma cells, in general, are present only in the sap-wood, and -in this almost solely the ascent of sap occurs. Dyestuffs and other -substances are frequently deposited in the walls of the heart-wood. - -The third region occupying the center of the twig is the pith. This is -composed ordinarily of angular, little elongated, parenchyma cells, -when mature mostly without cell-contents and filled with air. The pith -region in different trees is quite diversified. It may be hollow, -chambered, contain scattered thick-walled cells, have woody partitions, -or rarely be entirely thick-walled. - -The nature of the woody ring is rather perplexing at first; but -its origin is simple. We may conceive that it has developed from a -stem-type like the sunflower, in which the bundles, though separate, -are connected by a continuous cambium ring. In the woody twigs the -numerous bundles are closely packed together, and only separated by -the primary pith-rays extending from the pith to the cortex. Other -secondary pith-rays are produced within each bundle, but they usually -extend only part way from the cortex to the pith. The wood represents -the xylem of the bundle, and the sieve tubes of the bark, the phloem. - -=744. Growth in thickness.=—Although the year’s growth does not -increase in length after the first season has passed, it does increase -in diameter very much. From the size of an ordinary little twig it may -at length become a large tree trunk several feet in thickness. Only a -portion of the first year’s growth is produced by the growing point. -All the rest is a product of the cambium, a cylinder of wood being -added to the exterior of the old wood each season. The cambium, here, -as in the sunflower, lies between the phloem and the xylem, forming a -cylinder entirely around the stem. In spring, when active, it becomes -soft and delicate, thus enabling one to easily strip off the bark from -some trees, such as willow, etc., at that season. - -=745. Annual rings in woody stems.=—The wood produced by the -cambium each season is not homogeneous throughout, but is usually much -denser toward the outer part of the yearly cylinder, wood fibers here -predominating. In the inner portion vessels predominate, giving a much -more porous effect. The transition from one year’s growth to another is -very abrupt, giving rise to the appearance of rings in cross-section. -Since ordinarily in temperate climates but one cylinder of wood is -added each year, the number of rings will indicate the age of the trunk -or branch. This is not absolutely accurate, since in some trees under -certain conditions more than one ring may be produced in a summer. The -porous part of the ring is often termed “spring wood,” and the denser -portion “fall wood,” but since growth from the cambium ceases in most -trees by the middle of July, “summer wood” would be more appropriate -for the latter. It is mainly the alternation of the cylinders of the -spring and summer wood that gives the characteristic grain to lumber. -Pith-rays play an important part in wood graining only in a few woods, -as, for instance, in quartered oak. The reason for the production of -porous spring wood and dense summer wood is still one of the unsolved -problems of botany. - -FOOTNOTES: - -[40] Besides these specialized shoots for the storage of food, -food substances are stored in ordinary shoots. For example, in the -trunks of many trees starch is stored. With the approach of cold -weather the starch is converted into oil, in the spring it is converted -into starch again, and later as the buds begin to grow the starch is -converted into glucose to be used for food. In many other trees, on the -other hand, the starch changes to sugar on the approach of winter. - -[41] This topic was prepared by Dr. K. M. Wiegand. - -[42] See discussion of Tropophytes in Chapter XLVI. - -[43] Chapter V, and Organization of Tissues in Chapter XXXVIII. - - - - -CHAPTER XL. - -FOLIAGE LEAVES. - - -I. General Form and Arrangement of Leaves. - -=746. Influence of foliage leaves on the form of the stem.=—The -marked effect which foliage has upon the aspect of the plant or upon -the landscape is evident to all observers. Perhaps it is usual to -look upon the stem as having been developed for the display of the -foliage without taking into account the possibility that the foliage -may have a great influence upon the form or habit of the stem. It is -very evident, however, that the foliage exercises a great influence -on the form of the stem. For example, as trees increase in age and -size, the development of branches on the interior ceases and some of -those already formed die, since the dense foliage on the periphery -of the trees cuts off the necessary light stimulus. The tree, -therefore, possesses fewer branches and a more open interior. In the -forest also, the dense foliage above makes possible the shapely, -clean timber trunks. Note certain trees where by accident, or by -design, the terminal foliage-bearing branches have been removed that -foliage-bearing branches may arise in the interior of the tree system. - -Without foliage leaves the stems of green plants would develop a very -different habit from what they do. This development could take place in -three different directions under the influence of light: (1) The light -stimulus would induce profuse branching, so that there would be many -small branches. (2) The stem would develop fewer branches, but they -would be flattened. (3) Massive trunks with but few or no branches. In -fact, all these forms are found in certain green stems which do not -bear leaves. An example of the first is found in asparagus with its -numerous crowded slender branches. But such forms in our climate are -rare, since foliage leaves are more efficient. The second and third -forms are found among cacti, which usually grow in dry regions under -conditions which would be fatal to ordinary thin foliage leaves. - -=747. Relation of foliage leaves to the stem.=—In the study -of the position of the leaves on the stem we observe two important -modes of distribution: (1) the distribution along the individual -stem or branch which bears them, usually classed under the head of -_Phyllotaxy_; (2) the distribution of the leaves with reference to the -plant as a whole. - -=748. Phyllotaxy, or arrangement of leaves.=—In examining buds on -the winter shoots of woody plants, we cannot fail to be impressed with -some peculiarities in the arrangement of these members on the stem of -the plant. - -In the horse-chestnut, as we have already observed, the leaves are in -pairs, each one of the pair standing opposite its partner, while the -pair just below or above stand across the stem at right angles to the -position of the former pair. In other cases (the common bed-straw) the -leaves are in whorls, that is, several stand at the same level on the -axis, distributed around the stem. By far the larger number of plants -have their leaves arranged alternately. A simple example of alternate -leaves is presented by the elm, where the leaves stand successively on -alternate sides of the stem, so that the distance from one leaf to the -next, as one would measure around the stem, is exactly one half the -distance around the stem. This arrangement is one half, or the angle of -divergence of one leaf from the next is one half. In the case of the -sedges the angle of divergence is less, that is one-third. - -By far the larger number of those plants which have the alternate -arrangement have the leaves set at an angle of divergence represented -by the fraction two fifths. Other angles of divergence have been -discovered, and much stress has been laid on what is termed a law in -the growth of the stem with reference to the position which the leaves -occupy. Singularly by adding together the numerators and denominators -of the last two fractions gives the next higher angle of divergence. -Example: - - 1 + 2 3 2 + 3 5 - —- —- = —-; —- —- = —-; - 3 + 5 8 5 + 8 13 - -and so on. There are, however, numerous exceptions to this regular -arrangement, which have caused some to question the importance of any -theory like that of the “spiral theory” of growth propounded by Goethe -and others of his time. - -=749. Adaptation in leaf arrangement.=—As a result, however, of -one arrangement or another we see a beautiful adaptation of the plant -parts to environment, or the influence which environment, especially -light, has had on the arrangement of the leaves and branches of the -plant. Access to light and air are of the greatest importance to -green plants, and one cannot fail to be profoundly impressed with the -workings of the natural laws in obedience to which the great variety of -plants have worked out this adaptation in manifold ways. - -=750. Distribution of leaves with reference to the entire -plant.=—In this case, as in the former, we recognize that it is -primarily a light relation. As the plant becomes larger and more -branched the lower and inner leaves disappear. The trees and shrubs -have by far the larger number of leaves on the periphery of the branch -system. A comparison of different kinds of trees in this respect shows, -however, that there is great variation. Trees with dense foliage (elm, -Norway maple, etc.) present numerous leaves on the periphery which -admit but little light to the interior where leaves are very few or -wanting. The sugar maple and red maple do not cast such a dense shade -and there are more leaves in the interior. This is more marked in the -silver maple, and still more so in the locust (Gleditschia tricanthos). - -=751. Color of foliage leaves.=—The great majority of foliage -leaves are green in color. This we have learned (Chapter VII) is due -to the presence of a green pigment, chlorophyll, in the chloroplastids -thickly scattered in the cells of the leaf. We have also learned that -in the great majority of cases, the light stimulus is necessary for -the production of chlorophyll green. There are many foliage leaves -which possess other colors, as red (Rosa rubrifolia), purple (the -purple barberry, hazel, beech, birch, etc.), yellow (the golden oak, -elder, etc.); while many others have more or less deep tints of pink, -red, purple, yellow, when young. All of these leaves, however, possess -chlorophyll in addition to red, yellow, purple or other pigment. -These other pigments are sometimes developed in great quantity in the -cell-sap. They obscure the chlorophyll from view, but do not interfere -seriously with the action of light and the function of chlorophyll, and -perhaps in some cases serve as a screen to protect the protoplast. - -=752. Autumn colors.=—Foliage leaves of many trees display in -the autumn gorgeous colors. These colors are principally shades of -red or yellow, and sometimes purple. The autumn color is more marked -in some trees than in others. In the red maple, the red and scarlet -oak, the sourwood, etc., red predominates, though sometimes yellow -may be present with the red in a single leaf. Sugar maples, poplars, -hickories, etc., are principally yellow in autumn. The sweet gum has a -rich variety of color-red, purple, maroon, yellow; sometimes all these -colors are present on the same tree. - -The red and purple colors are found suffused in the cell-sap of -certain cells in the leaf much as we have found it in the cells of -the red beet. The yellow color is chiefly due to the disappearance -and degeneration of the chlorophyll while the leaf is in a moribund -state. A similar phenomenon is seen in the yellowing of crops when -the soil becomes too wet, or in the blanching of grass when covered -with a board, or of celery as the earth is ridged up over the leaves -in late summer and autumn. A number of different theories have been -advanced to explain autumn coloring, i.e., the appearance of the -red coloring matter. It has been attributed to the approach of cold -weather, and this has likely led to the erroneous belief on the part -of some that it is caused by frost. It very often precedes frost. Some -have attributed it to the action of the more oblique light rays during -autumn, and still others to the diminishing water-supply with the -approach of cool weather. The question is one which has not met as yet -with a satisfactory solution, and is certainly a very obscure one. It -is likely that the low temperature or the declining activities of the -leaf affect certain organic substances in the leaf and give rise to -the red color, and it is quite certain that in some years the display -is more brilliant than in others. The color is more striking in some -regions than in others and the different soil, as well as climate, has -been supposed to have some influence. The North American forests are -noted for the brilliant display of autumnal color. This is perhaps due -to some extent to the great variety or number of species which display -color. It would seem that there is some specific as well as individual -peculiarities in certain trees. Some individuals, for example, exhibit -brilliant colors every autumn, while others near of the same species -are more subdued. - -It has been shown by experiment that when sunlight passes through -red colors the temperature is slightly increased, and it has been -suggested that this may be of protection to the living substance which -has ceased working and is in danger of injury from cold. There does -not seem to be much ground for this suggestion, however. It certainly -could not protect the protoplasm of the leaf at night when the cold -is more intense, and during the day would only aggravate matters -by supplying an increased amount of heat, since extremes of heat -and cold in alternation are more harmful to plant life than uniform -cold. Especially would this be the case in alpine climates where the -alternation of heat and cold between day and night is extreme, and -brilliancy of the colors of alpine plants is well known. It seems -more reasonable to suppose that the red color acts as a screen, as -the chlorophyll is disappearing, to protect from the injurious action -of light, certain organic substances which are to be transferred back -from the leaf to the stem for winter storage. So in the case of many -stems in the spring or early summer when the young leaves often have -a reddish color, it is likely that it acts as a screen to protect the -living substance from the strong light at that season of the year until -the chlorophyll screen, which is weak in young leaves, becomes darker -in color and more effective, when the red color often disappears. - -=753. Function of foliage leaves.=—In general the function of -the foliage leaf as an organ of the plant is fivefold (see Chapters -IV, VII, VIII, XI), (1) that of carbon dioxide assimilation or -_photosynthesis_, (2) that of transpiration, (3) that of the synthesis -of other organic compounds, (4) that of respiration, and (5) that of -assimilation proper, or the making of new living substance. While none -of these functions are solely carried on in the leaf, it is the chief -seat of the first three of these processes, its form, position, and -structure being especially adapted to the purpose. Assimilation proper, -as well as respiration, probably take place equally in all growing or -active parts. - -=754. Parts of the leaf.=—All foliage leaves possess a _blade_ -or _lamina_, so called because of its _expanded_ and _thin_ character. -The blade is the essential part. Many leaves, however, are provided -with a stalk or _petiole_ by which the blade is held out at a greater -or lesser distance from the stem. Leaves with no petiole are _sessile_, -the blade is attached by one end directly on the stem. In some cases -the base of the blade is wrapped partly around the stem, or in others -it extends entirely around the stem and is _perfoliate_. Besides, many -leaves have short appendages, termed _stipules_, attached usually on -opposite sides of the petiole at its junction with the stem. In some -species of magnolia the stipules are so large that each one envelops -the entire portion of the bud which has not yet opened. Many leaves -possess outgrowths in the form of hairs, scales, etc. (See leaf -protection.) - -=755. Simple leaves.=—Simple leaves are those in which the -blade is plane along the edge, not divided. The edge may be entire or -indented (serrate) to a slight extent as in the elm. The form of the -simple leaf varies greatly but is usually constant for a given species, -or it may vary in shape in the same species on different parts of the -plant. Some of the terms applied to the outline of the leaf are ovate, -oval, elliptical, lanceolate, linear, needle-like, etc., but it is idle -for one to waste time on matters of minute detail in form until it -becomes necessary for those in the future who pursue taxonomic work. It -is evident that a simple leaf, except those of minute size, possesses -advantages over a divided leaf in the amount of surface it exposes to -the light. But in other respects it is at a disadvantage, especially -as it increases in size, since it casts a deeper shade and does not -admit of such a free circulation of air. It will be found, however, in -our study of the relation of leaves to light and air that the balance -between the leaf and its environment is obtained in the relation of the -leaves to each other. - -=756. Venation of leaves.=—A very prominent character of the -leaf is its “venation.” This is indicated by the presence of numerous -“veins,” indicated usually by narrow depressed lines on the upper -surface, and by more or less distinct elevated lines on the under -surface. There are two general types: (1) In the corn, Smilacina, -Solomon’s seal, etc., the veins extend lengthwise of the leaf and -are nearly parallel. Such leaves are said to be _parallel-veined_. -It is generally, though not always, a character of monocotyledenous -plants. (2) In the elm, rose, hawthorn, maple, oak, etc., the veins -are not all parallel. The larger ones either diverge from the base -of the blade (palmate leaf, maple), or the midvein extends through -the middle line of the leaf, while other prominent ones branch off -from this and extend, nearly parallel, toward the edge of the leaf -(pinnate venation). The smaller intermediate veins which are also -very distinct extend irregularly and branch and anastomose in such a -fashion as to give the figure of a net with very fine meshes. These -are _netted-veined_ leaves. These are characteristic of most of the -dicotyledenous plants. It is evident from what has been said of the -examples cited that there are two types of netted-veined leaves, the -_palmate_ and _pinnate_. - - NOTE. As we have already learned in Chapter - V the veins contain the vascular bundles of the - leaf. Through them the water and food solutions are - distributed to all parts of the leaf, and the return - current of food material elaborated in the leaf moves - back through the bast portion into the shoot. The veins - also possess a small amount of mechanical tissue. This - forms the framework of the leaf and aids in giving - rigidity to the leaf and in holding it in the expanded - position. The mechanical tissue in the framework - alone could not support the leaf. Turgescence of the - mesophyll is needed in addition. - -=757. Cut or lobed leaves.=—In many leaves, the indentations -on the margin are few and deep. Such leaves present several lobes -the proportionate size of which is dependent upon the depth of the -indentation or “incision.” Several of the maples, oaks, birches, the -poison ivy, thistles, the dandelion, etc., have lobed leaves. Where -the indentation reaches to or very near the midrib the leaf is said -to be cut. A study of various leaves will show all gradations from -simple leaves with plane edges to those which are cut or divided, as in -compound leaves, and the lobes are often variously indented. - -[Illustration: Fig. 433. Lobed leaves of oak forming a mosaic.] - -[Illustration: Fig. 434. Twice compound leaf. Leaflets arranged in one -plane, but open spaces permit free circulation of air through the large -leaf.] - -=758. Divided, or compound leaves.=—The rose, sumac, elder, -hickory, walnut, locust, pea, clover, American creeper, etc., are -examples of divided or compound leaves. The former are pinnately -compound, and the latter are palmately compound. The leaf of the -honey-locust is twice pinnately compound or bipinnate, and some are -three times pinnately compound.[44] It is evident that compound leaves -are only extreme forms of lobed or cut leaves and that the form of all -bears a definite relation to the primary venation. There has been a -reduction of mesophyll and of the area of smaller venation. - -_Unifoliate_ (for a single leaflet, as in orange and lemon where -the compound leaf is greatly reduced and consists of one pinna -attached to the petiole by a joint). _Bifoliate_ for one with two -leaflets; _trifoliate_ for one with three leaflets, as in the clover; -_plurifoliate_ for many leaflets. _Odd pinnate_ for a pinnate leaf with -one or more pairs of leaflets and one odd leaflet at the end. - -So leaves are _palmately bifoliate_, etc., _pinnately bifoliate_, etc. -_Decompound_ leaves are those where they are more than twice compound, -as _ternately decompound_ in the common meadow rue (Thalictrum). - -_Perfoliate_ leaves are seen in the bellwort (Uvularia), _connate -perfoliate_, as in some of the honeysuckles where the bases of opposite -leaves are joined together around the stem. _Equitant_ leaves are found -in the iris, where the leaves fit over one another at the base like a -saddle. - -=759.= These forms of leaves probably have some definite -significance. It is not quite clear why they should have developed as -they have; though it is possible to explain several important relations -of these forms to their environment. (1) The reduction of the surface -of the leaf, with the retention of the firmer portions, allows freer -movement of the air and affords the leaf greater protection from injury -during violent winds, just as the finely dissected leaves of some -water plants are less liable to injury from movement of the more dense -medium in which they live. It is possible that here we may have an -explanation of one of the factors involved in this reduction of leaf -surface. (2) In trees with compound leaves, like the hickory, walnut, -locust, ailanthus, etc., the midvein, and in the case of the Kentucky -coffee-tree (Gymnocladus) the primary lateral veins also, serve in -place of terminal branches of the stem. By the increase in the outline -of the leaf and the reduction of its surface between the larger veins, -the tree has attained the same leaf development that it would were the -larger veins replaced by stems bearing simple leaves. The tree as -it is, however, has the advantage of being able to cast off for the -winter period a layer of what otherwise would have been a portion of -the stem system, to retain which through the winter would use more -energy than with the present reduced stem system, and the stouter -stem is less liable to dry out. In the case of herbaceous plants, in -the case of plants like most of the ferns where the stem is on the -underground rootstock (Pteris), or a very short erect stem, as in the -Christmas fern, the leaf replaces the aerial stem, and the division (or -branching, as it is sometimes styled) of the leaf corresponds to the -branching of the stem. This is more marked in the gigantic exotics like -Cibotium regale, and in the tree ferns which have quite tall trunks, -the massive compound leaves replace branches. In the palms and cycads -are similar examples. Those who choose to observe can doubtless find -many examples close at hand. (3) While divided leaves have probably not -been evolved in response to the light relation, still their relation in -this respect is an important one, since if the leaf with its present -size were entire, it would cast too dense a shade on other leaves below. - -=760. General structure of the leaf.=—The general structure of -the leaf has been already studied (see Chapters IV, V, VII). It is -only necessary to recall the main points. The upper and lower surfaces -of the leaf are provided with a layer of cells usually devoid of -chlorophyll. The mesophyll of the leaf consists usually of a layer of -palisade cells beneath the epidermis, and the remainder consists of -loose parenchyma with large intercellular spaces. Through the mesophyll -course the “veins,” or fibrovascular strands, consisting of the xylem -and phloem portions and serving as conduits for water, salts, and -foodstuffs. In the epidermis are the stomata, each one protected by -the two guard cells. The guard cells as well as the mesophyll contain -chlorophyll. The stomata and the communicating intercellular spaces -furnish the avenues for the ingress and egress of gases, and for the -escape of water vapor. - -=761. Protection of leaves.=—There are many modifications of -the general plan of structure in different leaves, many of them being -adaptations for the protection of the leaf under adverse or trying -conditions. Many leaves are also capable of assuming certain positions -which afford them protection. The discussion of this subject may be -presented under two general heads: Protective modifications; protective -positions. - - -II. Protective Modification of Leaves. - -=762. General directions in which these modifications have taken -place.=—The usual type of foliage leaf selected is that of -deciduous trees or shrubs or of our common herbs. Such a leaf is -usually greatly expanded and thin in order to present as great a -surface as possible in comparison with its mass, since the kind of -work which the leaf has to do can be more effectually carried on when -it possesses this form. This form of leaf is best adapted for work in -regions where there is a medium amount of moisture such as exists in -the temperate zones. But since there are very great variations in the -climatic and soil conditions of these regions, and even greater changes -in desert and arctic regions, the type of leaf described is unsuited -for all. Its own life would be endangered, and it would also endanger -the life of the plant. Modifications have therefore taken place to -meet these conditions, or at least those plants whose leaves have -become modified in those directions which are suited to the surrounding -conditions have been able to persist. Excessive cold or heat, drought, -winds, intense light, rain, etc., are some of the conditions which -endanger leaves. The protective modifications of leaves may be grouped -under four general heads: (1) Structural adaptations; (2) Protective -covering; (3) Reduction of surface; (4) Elimination of the leaf through -the complete assumption of the leaf function by the stem. - -[Illustration: Fig. 435. Structure of leaf of Lactuca scariola. Upper -one grown in sunlight, palisade cells on both sides. Lower one grown in -shade, no palisade tissue.] - -=763.= (1) =Structural adaptations.=—The general structure -of the leaf presents certain features which are protective. The -palisade layer of cells found usually beneath the upper epidermis forms -a compact layer of long cells which not only acts as a light screen -cutting off a certain amount of the light, since too intense light -would be harmful; it also aids in lessening the loss of water from the -upper surface, where radiation is greater. The stomata are usually on -the under side of aerial leaves, and the mechanism which closes them -when the leaf is losing too much water is protective. As a protection -against intense light the number of palisade layers is sometimes -increased or the cells of this layer are narrow and long. This is often -beautifully shown when comparing leaves of the same plant grown in -strong light with those grown in the shade. The compass plant (Lactuca -scariola) affords an interesting example. The leaves grown in the light -are usually vertical, so that the light reaches both sides. Such leaves -often have all of the mesophyll organized into palisade cells (fig. -435), while leaves grown in the deep shade may have no palisade cells. - -=764.= (2) =Protective covering.=—_Epidermis and -cuticle._—The walls of the epidermal cells are much thickened in some -plants. Sometimes this thickening occurs in the outer wall, or both -walls may be thickened. Variation in this respect as well as the extent -of the thickening occur in different plants and are often correlated -with the extremes of conditions which they serve to meet. The cuticle, -a waxy exudation from the thick wall of the epidermis of many leaves, -also serves as a protection against too great loss of water, or against -the leaf becoming saturated with water during rains. The cabbage, -carnation, etc., have a well-developed cuticle. The effect of the -cuticle in shedding water can be nicely shown by spraying water on a -cabbage leaf or by immersing it in water. Sunken stomata also retard -the loss of water vapor. - -_Covers of hair or scales._—In many leaves certain of the cells of the -epidermis grow out into the form of hairs or scales of various forms, -and they serve a variety of purposes. When the hairs form a felt-like -covering as in the common mullein, some antennarias, etc., they lessen -the loss of water vapor because the air-currents close to the surface -of the leaf are retarded. Spines (see the thistles, etc.) also afford a -protection against certain animals. - -=765.= (3) =Reduction of surface.=—Reduction of leaf surface -is brought about in a variety of ways. There are two general modes: -(1st) Reduction of surface along with reduction of mass; (2d) Reduction -of surface inversely as the mass. Examples of the first mode are -seen in the dissected leaves of many aquatic plants. In this finely -dissected condition the mass of the leaf substance is much reduced as -well as the leaf surface, but the leaf is less liable to be injured -by movement of the water. In addition it has already been pointed out -that lobed and divided aerial leaves are much less liable to injury -from violent movements of the air, than if a leaf with the same general -outline were entire. The needle leaves of the conifers are also -examples, and they show as well structural provisions for protection -in the thick, hard cell-walls of the epidermis. To offset the reduced -surface there are numerous crowded leaves. Reduction of surface -inversely as the mass, i.e., the mass of the leaf may not be reduced -at all, or it may be more or less increased. In other words, there is -less leaf surface in proportion to the mass of leaf substance. It is -probable in many cases, example: the crowded, overlapping small scale -leaves of the juniper, arbor-vitæ, cypress, cassiope, pyxidanthera, -etc., that there has been a reduction in the size of the leaf, and at -the same time an increase in thickness. This with the crowding together -of the leaves and their thick cell-walls greatly lessens the radiation -of moisture and heat, thus protecting the leaves both in dry and cold -weather. The succulents, like “live-forever,” have a small amount of -surface in proportion to the mass of the leaf. In the yucca, though the -leaves are often large, they are very thick and expose a comparatively -small amount of surface to the dry air and intense sunlight of the -desert regions. The epidermal covering is also hard and thick. In -addition, such leaves, as well as those of many succulents, are so -thick they provide water storage sufficient for the plants, which -radiate so slowly from their surface. - -[Illustration: Fig. 436. A “Phylloclade,” leaves absent, stems -broadened to function as leaves, on the edges numerous flowers are -borne.] - -=766.= (4) =Elimination of the leaf.=—Perhaps the most -striking illustration of the reduction of leaf surface is in those -cases where the leaf is either completely eliminated as in certain -euphorbias, or in certain of the cacti where the leaves are thought -to be reduced to spines. Whether the cactus spine belongs to the -leaf series or not, the leaf as an organ for assimilation and -transpiration has been completely eliminated and the same is true in -the phylloclades. The leaf function has been assumed by the stem. The -stem in this case contains all the chlorophyll; is bulky, and provides -water storage. - - -III. Protective Positions. - -=767.= In many cases the leaves are arranged either in relation -to the stem, or to each other, or to the ground, in such a way as to -give protection from too great radiation of heat or moisture. In the -examples already cited the imbricated leaves of cassiope, pyxidanthera, -juniper, etc., come also under this head. In the junipers the leaves -spread out in the summer, while in the winter they are closely -overlapped. An interesting example of protective position is to be seen -in the case of the leaves of the white pine. During quite cold winter -weather the needles are appressed to the stem, and sometimes the trees -present a striking appearance in contrast with the spreading position -of the needles in summer. On windy days in winter, the needles turn -with the wind and become rigid in that position so that they remain in -a horizontal position for some time, often until the wind dies down, -or until milder weather. The following day, should there be a cold -strong wind from the opposite direction, the needles again assume a -leeward direction. In quiet weather appressed to the stem and in the -form of a brush there is less radiation of heat than if they diverged. -In strong winds by turning in the leeward direction the wind is not -driven between the needle bases and scales. Some plants, especially -many of those in arctic and alpine regions, have very short stems and -the leaves are developed near the ground, or the rock. Lying close on -the ground they do not feel the full force of the drying winds, there -is less radiation from them, and the radiation of heat from the ground -protects them. Many plants exhibit movement in response to certain -stimuli which place them in a position for protection. Some of these -examples have been discussed under the head of irritability (see -Chapter XIII). The night position of leaves and cotyledons presented -by many plants, but especially by many of the Leguminosæ, is brought -about by the removal of the light stimulus at evening. In many leaves, -when the light influence is removed, the influence of growth turns -the leaves downward, or the cotyledons of some plants upward. In this -vertical position of the leaf-blade there is less radiation of heat -during the cool night. The most striking cases of protection movements -are seen in the sensitive plant. As we have seen, the leaves of mimosa -close in a vertical position at midday if the light and heat are too -strong. Excessive transpiration is thus prevented. At night the vertical -position prevents excessive radiation of heat. The vertical or profile -position of the leaves of the compass plant already referred to not -only lessens transpiration, but the intense heat and light of the -midday sun is avoided. This profile position is characteristic of -certain plants in the dry regions of Australia, and the topmost leaves -of tropical forests. - - -IV. Relation of Leaves to Light. - -=768.= It is very obvious from our study of the function of the -foliage leaf that its most important relation to environment is that -which brings it in touch with light and air. It is necessary that light -penetrate the leaf tissue that the gases of the air and plant may -readily diffuse and that water vapor may pass out of the leaf. The thin -expanded leaf-blade is the most economical and efficient organ for leaf -work. We have seen that leaves respond to fight stimulus in such a way -as to bring their upper sides usually to face the source of fight, at -right angles to it or nearly so (_heliotropism_, see Chapter XIII). How -fully this is brought about depends on the kind of plant, as well as on -other elements of the environment, for as we have seen in our study of -leaf protection there is danger to some plants in any region, and to -other plants in certain regions that the intense light and heat may -harm the protoplast, or the chlorophyll, or both. - -[Illustration: Fig. 437. Mosaic form by trailing shoots of Panicum -variegatum, “ribbon-grass.”] - -The statement that leaves usually face the light at right angles is to -be taken as a generalized one. The source of the strongest illumination -varies on different days and again at different times of the day. -On cloudy days the zenith is the source of strongest illumination. -The horizontal position of a leaf, where there are no intercepting -lateral or superior objects would receive its strongest light rays -perpendicular to its surface. The fact is, however, that leaves on the -same stem, because of taller or shorter adjacent stems, are so situated -that the rays of greatest illuminating power are directed at some angle -between the zenith and horizon. Many leaves, then, which may have -their upper sides facing the general source of strongest illumination, -do not necessarily face the sun, and they are thus protected from -possible injury from intense light and heat because the direct rays of -sunlight are for the most part oblique. This does not apply, of course, -to those leaves which “follow the sun” during the day. Their specific -constitution is such that intense illumination is beneficial. - -The leaf is adjusted as well as may be in different species of varying -constitution, and under different conditions, to a certain balance in -its relation to the factors concerned. The problem then is to interpret -from this point of view the positions and grouping of leaves. Because -of the specific constitution of different plants, and because of a -great variety of conditions in the environment, we see that it is a -more or less complex question. - -[Illustration: Fig. 438. Sunflower with young head turned toward morning -sun.] - -=769. Day and night positions contrasted.=—In many plants the -day and night positions of the leaves are different. At night the -leaves assume a position more or less vertical, known as the _profile_ -position. This is generally regarded as a protective position, since -during the cool of the night the radiation of heat is less than if the -leaf were in a vertical position. In many of these plants, however, the -leaves in assuming the night position become closely appressed which -would also lessen the radiation. This peculiarity of leaves is largely -possessed by the members of the family Leguminoseæ (clovers, peas, -beans, etc.), and by the sensitive plants.[45] But it is also shared by -some other plants as well (oxalis, for example). The leaves of these -plants are usually provided with a mechanism which enables them to -execute these movements with ease. There is a cushion (_pulvinus_) of -tissue at the base of the petiole, and in the case of compound leaves, -at the base of the pinnæ and pinnules which undergoes changes in turgor -in its cells. The collapsing of the cells by loss of water into the -intercellular spaces causes the leaf to droop. When the cells regain -their turgor by the absorption of the water from the intercellular -spaces the leaf is raised to the horizontal, or day position. The light -stimulus induces turgor of the pulvinus, the disappearance of the -stimulus is accompanied by a loss of turgor. It is a remarkable fact -that in some sensitive plants, intense light stimuli are alarm signals -which result in the same movement as if the light stimulus were -entirely removed. As we know also contact or pressure stimulus, or -jarring produces the same result in “sensitive” plants like mimosa, -some species of rubus, etc. In many plants there is no well-developed -pulvinus, and yet the leaves show similar movements in assuming the -day and night positions. Examples are seen in the sunflower, and in -the cotyledons of many plants. A little observation will enable any -one interested to discover some of these plants.[46] In these cases the -night position is due to epinastic growth, and while this influence is -not removed during the day the light stimulus overcomes it and the leaf -is raised to the day position. - -[Illustration: Fig. 439. Same sunflower plant photographed just at -sundown.] - -=770. Leaves which rotate with the sun.=—During the growth period -the leaves of the sunflower as well as the growing end of the stem -respond readily to the direct sunlight. The response is so complete -that during sunny days the leaves toward the growing end of the stem -are drawn close together in the form of a rosette and the entire -rosette as well as the end of the stem are turned so that they face the -sun directly. In the morning under the stimulus of the rising sun the -rosette is formed and faces the east. All through the day, if the sun -continues to shine, the leaves follow it, and at sundown the rosette -faces squarely the western horizon. For a week or more the young -sunflower head will also face the sun directly and follow it all day as -surely as the rosette of leaves. At length, a little while before the -flowers in the head blossom, the head ceases to turn, but the rosette -of leaves and the stem also, to some extent, continue to turn with the -sun. When the leaves become mature they also cease to turn. This is -well shown in all three photographs (figs. 438-439). The lower leaves -on the stem being older have assumed the fixed horizontal position -usually characteristic of the plant with cylindrical habit. - -[Illustration: Fig. 440. Same plant a little older when the head does -not turn, but the stem and leaves do.] - -It is not true, as is commonly supposed, that the fully opened -sunflower head turns with the sun. But I have observed young heads -four or five inches in diameter rotate with the sun all day. This is -because the growing end of the stem as well as the young head responds -to the light stimulus. So there is some truth as well as a great deal -of fiction in the popular belief that the sunflower head follows the -sun. The young head will follow the sun all day even if all the leaves -are cut off, and the growing stem will also if all the leaves as well -as the flower head are cut away. Young seedlings will also turn even if -the cotyledons and plumule are cut off. - -This phenomenon of the rotation of leaves with the sun is much more -general than one would infer, as may be seen from a little careful -observation of rapidly growing plants on bright sunny days. In Alabama -I have observed beautiful rosettes of _Cassia marilandica_ rotate with -the sun all day. The peculiarity is very striking in the cotton plant, -especially when the rows extend north and south. In the forenoon or -afternoon it is most striking as the entire row shows the leaves tilted -up facing the sun. There are many of our weeds and common flowers of -field and garden which show this rotation of the leaves. Some of these -form rotating rosettes; while in others the leaves rotate independently -as in the sweet clover. - -=771. Fixed position of old leaves.=—In many of the cases cited -in the preceding paragraph, the rotation of the leaf only occurs -on sunny days. During cloudy days the leaves of the sunflower, for -example, are in a nearly horizontal position, or the lower ones may -be somewhat oblique, since the stronger illumination on such a plant -would be the oblique rays rather than the zenith rays. As the leaves -reach maturity also the epinastic growth is equalized by hyponastic -growth so that the growth movements bring the leaf to stand in a nearly -horizontal position, or that position in which it receives the best -illumination. In age, then, many leaves have a fixed position and this -corresponds with the position assumed on cloudy days. - -=772. Position on horizontal stems.=—On horizontal stems the -leaves have a horizontal position, and if such a stem is stood in an -erect position the appearance is very odd. If the leaf arises directly -from the horizontal stem, its petiole will be twisted part way around -in order to bring the face of the leaf uppermost. It is interesting -to observe the different relation of stem, petiole and blade and the -amount of twisting as the horizontal stem or vine trails over -irregularities in the surface, or climbs over and through other -vegetation. - -=773. Position of leaflets on divided leaves.=—An interesting -comparison can be made with entire, lobed, divided and dissected -leaves. The entire leaf usually lies in one plane, since usually the -problem of adjustment is the same for the entire surface. So the lobes -of a leaf usually lie all in the same plane as they would if the leaf -were entire. We find the same is true usually of the compound leaf. -It forms an incomplete mosaic. Some of the pieces having been removed -allow much of the light to pass through to leaves beneath. Leaves, -especially those of some size rarely lie in a flat plane. Some are more -or less depressed. Some curve downward. Compound leaves often curve -more or less and the leaflets often droop more or less in a graceful -fashion. It is interesting, however, that these far separated leaflets -all lie in the same general plane. This is because the area of the -leaf, if not too large, makes the problem of position with reference -to light much the same as if the leaf were entire. The leaflets or -divisions, though separated, are laminate, and they can work more -efficiently facing the light. But suppose we extend our observation to -the finely dissected capillary leaves of some of the parsley family -(Umbelliferæ), or to the upper leaves of the fennel-leaved thoroughwort -(Eupatorium fœniculaceum) among the aerial plants, and to Myriophyllum -among the aquatic plants. The divisions are thread-like or cylindrical. -One side of the leaflet is just as efficient when presented to the -light as another. As a result the leaflets are not arranged in the same -plane, but stand out in many directions. - -Occasionally one finds a divided or compound leaf in such a position -that one portion, because of being shaded above, receives the stronger -light stimulus from the side, while the other portion is lighted from -above. If this relation continues throughout the growth-period of the -leaf the leaflets of one portion may lie in a different plane from -those of the other portion. In such cases, some of the leaflets are -permanently twisted to bring them into their proper light relation. - - -V. Leaf Patterns. - -MOSAICS, OR CLOSE PATTERNS. - -[Illustration: Fig. 441. Fittonia showing leaves arranged to form -compact mosaic. The netted venation of the leaf is very distinctly -shown in this plant. (Photo by the Author.)] - -=774.= Where the leaves of a plant, or a portion of a plant, are -approximate and arranged in the form of a pattern, the leaves fitting -together to form a more or less even and continuous surface, such -patterns are sometimes termed “mosaics,” since the relation of leaves -to one another is roughly like the relation of the pieces of a mosaic. -A good illustration of a mosaic is presented by a greenhouse plant -Fittonia (fig. 441). The stems are prostrate and the erect branches -quite short, but it may have quite a wide system by the spreading of -the runners; the branches of such a length that the leaves borne near -the tips all fit together forming a broad surface of leaves so closely -fitted together often that the stems cannot be seen. The advantage of -a mosaic over a separate disposition of leaves at somewhat different -levels is that the leaves do not shade one another. Were all the light -rays coming down at right angles to the leaves, there would not be -any shading of the lower ones, but the oblique rays of light would be -cut off from many of the leaves. In the case of a mosaic all the rays -of light play upon all the leaves. Some of the mosaics which can be -observed are as follows: - -[Illustration: Fig. 442. Rosette pattern of leaves.] - -=775. Rosette pattern.=—The rosette pattern is presented by -many plants with “radial” leaves, or leaves which arise in a cluster -near the surface of the ground, and are thus more or less crowded in -their arrangement on the stem. The pretty gloxinia often presents fine -examples of a loose rosette. In the rosette pattern the petioles of -the lower leaves are longer than the upper ones, and the blade is thus -carried out beyond the inner leaves. The leaves being so crowded in -their attachment to the stem lie very nearly in the same plane. - -=776. Vines and climbers.=—Some of the most extensive mosaic -patterns are shown in creeping and climbing vines. A very common -example is that of the ivies trained on the walls of buildings, -covering in some instances many square yards of surface. Where the -vines trail over the ground or clamber over other vegetation, it is -interesting to observe the various patterns, and the distortion of -petioles brought about by turning of the leaves. Of examples found in -greenhouses, the Pellonia is excellent, and the trailing ribbon-grass -often forms loose mosaics. - -=777. Branch patterns.=—These patterns are very common. They -are often formed in the woods on the ends of branches by the leaves -adjusting themselves so as to largely avoid shading each other. Figure -443 illustrates one of them from a maple branch. It is interesting to -note the way in which the leaves fit themselves in the pattern, how in -some the petioles have elongated, while others have remained short. Of -course, it should be understood that the pattern is made during the -growth of the leaves. - -[Illustration: Fig. 443. Spray of leaves of striped maple, showing -different lengths of leafstalks.] - -[Illustration: Fig. 444. Cedar of Lebanon, strong light only from one -side of tree (Syria).] - -=778. The tree pattern.=—Mosaics are often formed by the exterior -foliage on a tree, though they are rarely so regular as some of those -mentioned above. Still it is common to see in some trees with drooping -limbs like the elm, beautiful and large mosaics. The weeping elm -sometimes forms a very close and quite even pattern over the entire -outer surface. In most trees the leaf arrangement is not such as to -form large patterns, but is more or less open. While the conifers do -not form mosaics there are many interesting examples of grouping of -foliage on branch systems into broadly expanded areas, as seen in the -branches of white pine trees, especially in the edge of a wood, or as -seen in the arbor-vitæ. - - -OTHER PATTERNS. - -=779. Imbricate pattern of short stems.=—This pattern is quite -common, and differs from the rosette in that the leaves are distributed -further apart on the stem so that the central ones are considerably -higher up than in the mosaic. The lower petioles are longer, as in -the rosette, so that the outer lower leaves extend further out. Some -begonias show fine imbricate patterns. - -[Illustration: Fig. 445. Imbricate pattern of leaves; Begonia.] - -=780. Spiral patterns.=—They are very common on stems of the -cylindrical type, which are unbranched, or but little branched. The -sunflower, mullein, chrysanthemum, as it is grown in greenhouses, the -Easter lily, etc., are examples. The spiral arrangement of the leaves -provides that each successive leaf on the stem, as one ascends the -stem, is a little to one side so that it does not cast shade on the leaf -just below. In some stems, according to the leaf arrangement (or -phyllotaxy), one would pass several times around in ascending the stem -before a leaf would be found directly above another, which would be -such a distance below that it would not be shaded to an appreciable -extent. Interesting observations can be made on different plants to -work out the relation of distance of leaves on the stem to length of -the upper and lower leaves; the number of vertical rows on the stem -compared to the width of the leaves; and the relation of these facts to -the problem of light supply. Related to the spiral pattern is that of -erect stems with opposite leaves. Here each pair is set at right angles -to the direction of the pair above or below. - -[Illustration: Fig. 446. Palm showing radiate arrangement of leaves and -the petiole of the leaf functions as stem in lifting leaf to the light.] - -=781. Radiate pattern.=—This pattern is present in many grasses -and related plants with narrow leaves and short stems. The leaves are -often very crowded at the base, but by radiating in all directions from -the horizontal to the vertical, abundant exposure to light is gained -with little shading. The dragon tree screw-pine, and plants grown in -greenhouses also illustrate this type. It is also shown in cycads, -palms, and many ferns, although these have divided leaves. - -[Illustration: Fig. 447. Screw-pine (Pandanus) showing prop roots and -radiate pattern of leaves.] - -=782. Compass plants.=—These plants with vertical leaf -arrangement, and exposure of both surfaces to the lateral rays of light -have been mentioned in other sections (Lactuca scariola). - -=783. Open patterns.=—Open patterns are presented by divided or -“branched” leaves. Where the leaves are very finely dissected, they may -be clustered in great profusion and yet admit sufficient light for some -depth below. Where the leaflets are broader, the leaves are likely to -be fewer in number and so arranged as to admit light to a great depth -so that successive leaves below on the same or adjacent stems may not -be too much shaded. On such plants, often the leaves lying next the -ground are entire or less divided. - -FOOTNOTES: - -[44] Some of the different terms used to express the kinds of compound -leaves are as follows: - -[45] The most remarkable case is that of the “telegraph” plant -(Desmodium gyrans). Aside from the day and night positions which the -leaves assume, there is a pair of small lateral leaflets to each leaf -which constantly execute a jerky motion, and swing around in a circle -like the second hand of a watch. - -[46] Seedlings are usually very sensitive to light and are good objects -to study. - - - - -CHAPTER XLI. - -THE ROOT. - - -I. Function of Roots. - -=784.= The most obvious function of the roots of ordinary -plants are two: 1st, To furnish anchorage and partial support, and -2d, absorption of liquid nutriment from the soil. The environmental -relation of such roots, then, in broad terms, is with the soil. It is -very clear that in some plants the root serves both functions, while in -other plants the root may fulfil only one of these requirements. - -The problems which the plant has to solve in working out these -relations are: - - (1) Permeation of the soil or substratum. - (2) Grappling the substratum. - (3) A congenial moisture or water relation. - (4) Distribution of roots for the purpose of reaching - food-laden soil. - (5) Exposure of surface for absorption. - (6) The renewal of the delicate structures for absorption. - (7) Aid in preparation of food from raw material. - (8) The maintenance of the required balance between the - environment as a whole and the increasing or changing - requirements of the plant. - -=785.= (1) =Permeation of the soil or substratum.=—The -fundamental divergence of character in the environmental relations of -root and stem are manifest as soon as they emerge from the germinating -seed. Under the influence of the same stimulus (_gravity_) the root -shows its geotropic character by growing downward, while the geotropic -character of the stem is shown in its upward growth. - -The medium which the root has to penetrate offers considerable -resistance, and the form of the root as well as its manner of growth is -adapted to overcome this difficulty. The slender, conical, penetrating -root-tip wedges its way between the minute particles of soil or into -the minute crevices of the rock, while the nutation of the root enables -it to search for the points of least resistance. The root-tips having -penetrated the soil, the older portions of the root continue this wedge -action by growth in diameter, though, of course, elongation of the old -parts of the root does not take place. It is the widening growth of the -tapering root that produces the wedge-like action. The crevices of the -rock are sometimes broadened, but the resistance here is so great, the -root is often greatly flattened out. - -=786.= (2) =Grappling the substratum.=—The mere penetration -of a single root into the soil gives it some hold on the soil and it -offers some resistance to a “pull” since it has wedged its way in -and the contact of soil particles offers resistance. The root hairs -formed on the first entering root growing laterally in great numbers -and applying themselves very closely to the soil particles, increase -greatly the hold of the plant on the soil, as one can readily see by -pulling up a young seedling. Lateral roots are soon formed, and as -these continue to extend and ramify in all directions, the hold is -increased until in the case of some of the larger plants the resistance -their hold would offer would equal many tons. Even in some of the -smaller shrubs and herbs the resistance is considerable, as one can -easily test by pulling with the hand. To obtain some idea of the amount -of resistance the roots of these smaller plants offer, they can be -tested by pulling with the ordinary spring scales. - -=787.= (3) =A congenial moisture, or water relation.=—In -general, the roots seek those portions of the soil provided with a -modicum of moisture. Usually a suitable moisture condition is present -in those portions of the soil containing the plant food. But if -portions of the soil are too dry and very nearby other portions -containing moisture, the roots grow mainly into the moist substratum -(_hydrotropism_). If the soil is too wet, the roots grow away from it -to soil with less water, or in some cases will grow to and upon the -surface of the soil. - -The roots need _aeration_, and where the supply of water is too great, -the air is shut out, and we know that corn, wheat, and many other -plants become “sickly” in low and undrained soil in wet seasons. This -can only be said in the case of our ordinary dry land plants, i.e., -those that occupy an intermediate position between _water-loving_ -plants and _dry-conditioned_ plants. This phase of the subject must be -reserved for special treatment. (See Chapter XLVI.) - -=788.= (4) =Distribution of roots for the purpose of reaching -food-laden soil.=—This is one of the essential relations of the -root in the case of the land plant, and probably accounts for the very -extensive ramification of the roots. To some extent it also explains -the different root systems in some plants. The pines, spruces, etc., -usually grow in regions where the soil is very shallow. The root -system does not extend deeply into the soil. It spreads laterally and -extends widely through the shallow surface soil and presents a very -different aspect from the stem system in the air. The root system of -the broad-leaved trees usually extends more deeply into the soil, while -of course, extending laterally to great distances. The hickory, walnut, -etc., especially have strong tap-roots which extend deeply into the -soil, and the root system of such a tree is more comparable in aspect, -if it were entirely uncovered, to the stem system in the air. The -tap-root is more pronounced in some trees than in others. It may be -that in the hickory and walnut the deep tap-root is important in -supplying the tree with water in dry seasons, especially when growing -on dry, gravelly soil which does not retain moisture on the surface -nor hold it within two or three feet of the surface. Experiment has -demonstrated, by pot culture of plants, that where soil rich in plant -food lies adjacent to poor soil, no matter in what part of the pot the -rich soil is, the greatest growth and branching of roots is in the rich -soil. - -=789.= (5) =Exposure of root surface for absorption.=—The -principal part of root absorption takes place in the young root and -the root hairs growing near the root-tip. The root-tips and root hairs -in their relation to the root systems on which they are borne are -not to be compared morphologically with the leaves and stem system. -But the root-tips and hairs are absorbing organs of the roots while -the main root system supports them, brings them into relation with -the soil and moisture, and conducts food and other substances to and -from them. One of the important relations of the leaf is that of -light, and since the source of light is restricted, i.e., it is not -equally strong from all sides, an expanded and thin leaf-blade is more -effective than an equal expenditure of plant material in the form of -thread-like outgrowths. It is different, however, with the plant food -dissolved in the soil water. It is equally accessible on all sides. A -greater surface for absorption is exposed with the same expenditure of -material by multiplication of the organs and a reduction in their size. -Numerous delicate root hairs present a greater absorbing surface than -if the same amount of material were massed into leaf-like expansions. -There is another important advantage also. Its slender roots and -thread-like root hairs allow greater freedom of circulation of water, -food solutions, and air than if the absorbing organs of the roots were -broadly expanded. - -=790.= (6) =The renewal of the delicate structures for -absorption.=—The delicate root hairs are easily injured. The thin -cell-walls through which food solutions flow become more or less choked -by the gradual deposit of substances in solution in the water, and -continued growth of the root in diameter forms a firmer epidermis and -cortex through which the solutions taken up by the root hairs would -pass with difficulty. For this reason new root hairs are constantly -being formed on the growing root-tip throughout the growing season, -and in the case of perennial plants, through each season of their -growth. - -=791.= (7) =Aid in preparation of food from raw -materials.=—For most plants the food obtained from the soil is -already in solution in the soil water. But there are certain substances -(examples, some of the chemical compounds of potash, phosphoric acid, -etc.) which are insoluble in water. Certain acids excreted by the -roots aid in making these substances soluble (see Chapter III). In -a number of plants the roots have become associated with fungus or -bacterial organisms which assist in the manufacture of nitrogenous food -substances, or even in the absorption of ordinary food solution from -the soil, or in making use of the decaying humus of the forest (see -Chapter IX). - -=792.= (8) =The maintenance of the required balance between -the environment and the increasing or changing requirements of the -plant.=—In this matter the entire plant participates. Mention is -made here only of the general relation which the root sustains to its -own environment and the increased burden placed upon it by the shoot. -The increase in the root system keeps pace with the increasing size of -the stem system. The roots become stronger, their ramifications wider, -and the number of absorbing rootlets more numerous. The observation -is sometimes offered that the correlation between the root system of -a plant, and the form of the stem system and position of the leaves, -is of such a nature that plants with a tap-root system have their -leaves so arranged as to shed the water to the center of the system, -while plants with a fibrous-root system have their leaves so arranged -as to shed the water outward. In support of this attention is called -to the radiate type of the leaf system of the dandelion, beet, etc. -In the second place the imbricate type as manifested in broad-leaved -trees, and in the overlapping branch systems of many pines, etc. One -should note, however, that in the former class the leaves are often -arranged to shed as much water outward as inward. As to the latter -class, there is need of experiment to determine whether these empirical -observations are correct, for the following reasons: 1st, Root and leaf -distribution are governed by other and more important laws, the root -being influenced by the location of food in the soil which usually -forms a very thin stratum while the shoot and leaf is mainly influenced -by light, and root distribution is much wider in a lateral direction -than that of the branches. 2d, In light rains the leaf surface holds -back practically all the rain which is then evaporated into the air -and lost to the root systems. 3d, In heavy and long-continued rains -the water breaks through the leaf system to such an extent that roots -under the tree would be as well supplied as those outside, and the -ground outside being saturated anyway, the roots do not need the -small additional water which may have been shed outward. 4th, It is -the habit of plants where left undisturbed (except in rare cases), to -grow in more or less dense formations or societies. Here there is no -opportunity for any appreciable centrifugal distribution of rainfall -and yet the root distribution is practically the same, except that the -root systems of adjacent plants are interlaced. - - -II. Kinds of Roots. - -=793. The root system.=—From the foregoing, it will be understood -that the roots of a plant taken together form the _root system_ of that -plant. In soil-roots in general we usually recognize two kinds of root -systems. - -=794. The fibrous-root system.=—Roots which are composed of -numerous slender branching roots resembling “fibers,” are termed -_fibrous_, or the plant is said to have a _fibrous-root system_. The -bean, corn, most grasses, and many other plants have fibrous-root -systems. - -=795. The tap-root system.=—Plants with a recognizable central -shaft-like root, more or less thickened and considerably stouter -than the lateral roots, are said to have _tap_ roots, or they have a -_tap-root system_. The dandelion, beet, carrot (see crown tuber) are -examples. The hickory, walnut, and some other trees have very prominent -tap-roots when young. The tap-root is maintained in old age, but the -lateral roots often become finally as large as the tap-root. Besides -tap-roots and fibrous-roots, which include the larger number, several -other kinds of roots are to be enumerated. - -=796. Aerial roots.=—Aerial roots are most abundantly developed -in certain tropical plants, especially in the orchids and aroids. Many -examples of these plants are grown in conservatories. The amount of -moisture is so great in these tropical regions that the roots are -abundantly supplied without the soil relation. Certain of the roots -hang free in the air and are provided with a special sheath of spongy -tissue called the _velamen_, through which moisture is absorbed from -the air. Other roots attach themselves to the trunk or branches of -the tree on which the orchid is growing, and furnish the support to -the _epiphyte_, as such plants are often called. Among the tangle -of these clinging roots falling leaves are caught. Here they decay -and nourishing roots grow from the clinging roots into this mass -of decaying leaves and supply some of the plant food. Aerial roots -sometimes possess chlorophyll. - -There are a number of plants, however, in temperate regions which -have aerial roots. These are chiefly used to give the stem support as -it climbs on trees or on walls. They are sometimes called clinging -roots. A common example is the climbing poison-ivy (Rhus radicans), the -trumpet creeper, etc. Such aerial roots are called _adventitious_ roots. - -=797. Bracing roots, or prop roots.=—These are developed in a -great variety of plants and serve to brace or prop the plant where -the fibrous-root system is insufficient to support the heavy shoot -system, or the shoot system branches so widely props are needed to hold -up the branches. In the common Indian corn several whorls of bracing -roots arise from the nodes near the ground and extend outward and -downward to the ground, though the upper whorls do not always succeed -in reaching the ground. The screw-pine so common in greenhouses affords -an excellent example of prop roots. The roots are quite large, and long -before the root reaches the soil the large root cap is evident. The -banyan tree of India is a classic example of prop roots for supporting -the wide-reaching branches. The mangrove in our own subtropical forests -of Florida is a nearer example. - -[Illustration: Fig. 448. Bracing roots of Indian corn.] - -[Illustration: Fig. 449. Buttresses of silk-cotton tree, Nassau.] - -=798. Buttresses= are formed at the junction of the root and -trunk, and therefore are part root and part stem. Splendid examples -of buttresses are formed on the silk-cotton tree. They are sometimes -formed on the elm and other trees in low swampy ground. - -=799. Fleshy roots, or root tubers.=—These are enlargements of -the root in the form of tubers, as in the sweet potato, the dahlia, -etc. They are storage reservoirs for food. Portions of the roots become -thick and fleshy and contain large quantities of sugar, as in the sweet -potato, or of _inulin_ (a carbohydrate) in the root tubers of the -dahlia and other composites. - -=800. Water-roots and roots of water plants.=—These are roots -which are developed in the water, or in the soil. Water-roots are -sometimes formed on land plants where the root comes in contact with a -body of water, or a stream. Water-roots usually possess no root hairs, -or but a few, as can be seen by comparing water-roots with soil-roots, -or by comparing roots of plants grown in water cultures. The greater -body of water in contact with the root and the more delicate epidermis -of the root render less necessary the root hairs. The duck-meats -(Lemna) are good examples of plants having only water-roots. Other -aquatic plants like the potamogetons, etc., have true roots which grow -into the soil and serve to anchor the plant, but they are not developed -as special organs of absorption, since the stem and leaves largely -perform this function. - -=801. Holdfasts.=—These are organs for anchorage which are not -true roots. These are especially well developed in some of the algæ -(Fucus, Laminaria, etc.). They are usually called _holdfasts_. The -holdfasts of the larger algæ are mainly for anchoring the plant. They -do not function as absorbing organs, and the structure is different -from that of true roots. - -=802. Haustoria or suckers= is a name applied to another kind of -holdfast employed by parasitic plants. In the dodder the haustorium -penetrates the tissue of the _host_ (the plant on which the parasite -grows), and besides furnishing a means of attachment, it serves as an -absorbing organ by means of which the parasite absorbs food from its -host. The parasitic fungi like the powdery mildews which grow on the -surface of their hosts have simple haustoria which serve both as organs -of attachment and absorption, while in the rusts which grow in the -interior of their hosts the haustoria are merely absorbing organs. - -=803. Rootlets, or rhizoids.=—Many of the algæ, liverworts and -mosses have slender, hair-like organs of attachment and absorption. -These plants do not have true roots. Because of the slender form and -small size of these organs, they are called _rhizoids_, or _rootlets_. -In form many of them resemble the root hairs of higher plants. - - - - -CHAPTER XLII. - -THE FLORAL SHOOT. - - -I. The Parts of the Flower. - -The portion of the stem on which the flowers are borne is the _flower_ -shoot or axis, or taken together with the flowers, it is known as the -_Flower Cluster_. - -=804. The flower.=—The flower is best understood by an -examination, first of one of the types known as a “complete” flower, -as in the buttercup, the spring-beauty, the blood-root, the apple, the -rose, etc. - -There are two sets of organs or members in the complete flower—(1) the -floral envelope; (2) the essential or necessary members or organs. - -The floral envelope when complete consists of—1st, an outer envelope, -the _calyx_, made up of several leaf-like structures (_sepals_), very -often possessing chlorophyll, which envelop all the other parts of the -flower when in bud; 2d, an inner envelope, the _corolla_, also made up -of several leaf-like parts (_petals_), usually bright colored and larger -than the sepals. The outer and inner floral envelopes are usually in -whorls (though in close spirals in many of the buttercup family, etc.), -and for reasons discussed elsewhere (Chapter XXXIV) represent leaves. -The essential or necessary members of the flower are also usually in -whorls and likewise represent leaves, but only in rare cases is there -any suggestion, either in their form or color, of a leaf relationship. -These members are in two sets: (1) The outer, or _andrœcium_, -consisting of a few or many parts (_stamens_); (2) the inner set, the -_gynœcium_, consisting of a few or many parts (_carpels_). - -=805. Purpose of the flower.=—While the ultimate purpose of -all plants is the production of seed or its equivalent through -which the plant gains distribution and perpetuation, the flower is -the specialized part of the seed plant which utilizes the food and -energies contributed by other members of the plant organization for the -production of seed. In addition to this there are definite functions -performed by the members of the flower, which come under the general -head of plant work, or flower work. - -=806. The calyx, or the sepals.=—These are chiefly protective, -affording protection to the young stamens and carpels in the flower -bud. Where the corolla is absent, sepals are usually present and then -assume the function of the petals. In a few instances the calyx may -possibly ultimately join in the formation of the fruit (examples: the -butternut, walnut, hickory). - -=807. The corolla, or petals.=—The petals are partly protective -in the bud, but their chief function where well developed seems to be -that of attracting insects, which through their visits to the flower -aid in “_pollination_,” especially “_cross pollination_.” - -=808. The stamens.=—The stamens (= microsporophylls) are -flower organs for the production of _pollen_, or _pollen-spores_ (= -microspores). The _stalk_ (not always present) is the _filament_, the -_anther_ is borne on the filament when the latter is present. The -anther consists of the _anther sacs_ or _pollen sacs_ (microsporangium) -containing the pollen-spores, and the _connective_, the sterile tissue -lying between and supporting the anther sac. The stamens are usually -separate, but sometimes they are united by their filaments, or by their -anthers. When the pollen is ripe they open by slits or pores and the -pollen is scattered; or in rarer cases the pollen mass (_pollinium_) is -removed through the agency of insects (see Insect pollination, Chap. -XLIII). - -=809. The pistil.=—The pistil consists of the “_ovary_,” the -_style_ (not always present), and the _stigma_. These are well shown in -a _simple pistil_, common examples of which are found in the buttercup, -marsh marigold, the pea, bean, etc. The simple pistil is equivalent to -a _carpel_ (= macrosporophyll), while the _compound pistil_ consists of -two or several carpels joined, as in the toothwort, trillium, lily, -etc. The _ovary_ is the enlarged part which below is attached to -the receptacle of the flower, and contains within the _ovules_. The -_style_, when present, is a slender elongation of the upper end of -the ovary. The _stigma_ is supported on the end of the style when the -latter is present. It is often on a capitate enlargement of the style -or extends down one side, or when the style is absent it is usually -seated directly on the upper end of the ovary. The stigmatic surface is -glutinous or “sticky,” and serves to hold the pollen-spores when they -come in contact with it. - -The _ovules_ are within the ovary and are arranged in different ways -in different plants. The pollen grain (or better pollen-spore = -microspore), after it has been transferred to the stigma, “germinates,” -and the pollen tube grows down through the tissue of the stigma and -style, or courses down the stylar canal until it reaches the ovule. -Here it usually enters the ovule (macrosporangium) at the _micropyle_ -(in some of the ament-bearing plants it enters at the _chalaza_), and -the sperm cells are emptied into the embryo sac in the interior of the -ovule. - -=810. Fertilization.=—One of the sperms unites with the egg -in the embryo sac. This is _fertilization_, and from the fertilized -egg the young embryo is formed still within the ovule. _Double -fertilization_,—the other sperm cell sometimes unites with one or both -of the “polar” nuclei which have united to form the “definitive” or -“endosperm” nucleus. As a result of fertilization, the embryo plant is -formed within the ovule, the coats of which enlarge by growth forming -the seed coats, and altogether forming the seed. (See Chapters XXXIV, -XXXV, XXXVI.) - - -II. Kinds of Flowers. - -=811. Absence of certain flower parts.=—The _complete_ flower -contains all the four series of parts. When any one of the series of -parts is lacking, the flower is said to be _incomplete_. Where only one -series of the floral envelopes is present the flowers are said to be -_apetalous_ (the petals are absent), examples: elm, buckwheat, etc. -Flowers which lack both floral envelopes are _naked_. When pistils are -absent but stamens are present the flowers are _staminate_, whether -floral envelopes are present or not; and so when stamens are absent and -pistils present the flower is _pistillate_. If both stamens and pistils -are absent the flower is said to be _sterile_ or _neutral_ (snowball, -marginal or showy flowers in hydrangea). Flowers with both stamens and -pistils, whether or not they have floral envelopes, are _perfect_ (or -hermaphrodite), so if only one of these sets of _essential organs_ -of the flower is present the flower is _imperfect_, or _diclinous_. -Sometimes the imperfect, or diclinous, flowers are on the same plant, -and the plant is said to be _monœcious_ (of one household). When -staminate flowers are on certain individual plants, and the pistillate -flowers of the same species are on other individuals, the plant is -_diœcious_ (or of two households). When some of the flowers of a plant -are diclinous and others are perfect, they are said to be _polygamous_. - -Many of these variations relating to the presence or absence of flower -parts in one way or another contribute to the well-being of the plant. -Some indicate a division of labor; thus in the neutral flowers of -certain species of hydrangea or viburnum, the showy petals serve to -attract insects which aid in the pollination of the fertile flowers. It -must not be understood, however, that all variations in plants which -results in new or different forms of flowers is for the good of the -species. For example, under cultivation the flowers of viburnum and -hydrangea sometimes are all neutral and showy. While such variations -sometimes contribute to the happiness of man, the plant has lost the -power of developing seed. In diclinous flowers cross pollination is -necessitated. - -=812. Form of the flower.=—The flower as a whole has _form_. -This is so characteristic that in general all flowers of the different -individuals of a species are of the same shape, though they may vary -in size. In general, flowers of closely related plants of different -species are of the same type as to form, so that often in the shape of -the flower alone we can see the relationship of kind, though the form -of the flower is not the most important nor always the sure index of -kinship. Since many flowers resemble certain familiar objects, names -are often used which relate to these objects. - -Flowers are said to be _regular_, or _irregular_. In a regular flower -all of the parts of a set or series are of the same shape and size, -while in irregular flowers the parts are of a different shape or size -in some of the sets. The flowers of the pea family (_Papilionaceæ_), -of the mint family (_Labiatæ_), of the morning glory, larkspur, -monkshood, etc., are irregular (fig. 450). The corolla usually gives -the characteristic form to the flower, and the name is usually applied -to the form of the corolla. - -[Illustration: Fig. 450. Several forms of flowers. Regular flowers. -_wh_, wheel-shaped corolla; _sa_, salver-shaped; _tub_, tubular-shaped. -Irregular flowers. _pa_, butterfly or papilionaceous; _per_, personate -or masked flower; _lab_, gaping or ringent corolla. The two latter are -called bilabiate flowers.] - -Some of the different forms are wheel-shaped or _rotate_ corolla when -the petals spread out at once like the spokes of a wheel, as in the -potato, tomato, or bittersweet; _salver-shaped_ when the petals spread -out at right angles from the end of a corolla tube, as in the phlox; -_bell-shaped_, or _campanulate_, as in the harebell or campanula; -_funnel-shaped_, as in the morning glory; _tubular_, when the ends of -the petals spread but little or none from the end of the corolla tube, -as in the turnip flower or in the disk florets of the composites. The -_butterfly_, or _papilionaceous_ corolla is peculiar as in the pea -or bean. The upper petal is the “banner,” the two lateral ones the -“wings,” and the two lower the “keel.” - -The _labiate_ corolla is characteristic of the mint family where the -gamosepalous corolla is unequally divided, so that the two upper lobes -are sharply separated from the three lower forming two “lips.” The -labiate corolla of the toad-flax, or snapdragon is _personate_, or -_masked_, because the lower lip arches upward like a palate and closes -the entrance to the corolla tube; that of the dead nettle (_Lamium_) is -_ringent_ or _gaping_, because the lips are spread wide apart. In some -plants the labiate corolla is not very marked and differs but slightly -from a regular form. - -The _ligulate_ or _strap-shaped_ corolla is characteristic of the -flowers of the dandelion or chicory, or of the ray flowers of other -composites (fig. 451). The lower part of the gamosepalous corolla is -tubular, and the upper part is strap-shaped, as if that part of the -tube were split on one side and spread out flat. - -These forms of the flower should be studied in appropriate examples. - -=813. Union of flower parts.=—In the buttercup flower all the -parts of each series are separate from one another and from other -series of parts. Each one is attached to the _receptacle_ of the -flower, which is a very much shortened portion of the flower axis. -The calyx being composed of separate and distinct parts is said to be -_polysepalous_, and the corolla is likewise _polypetalous_. The stamens -are _distinct_, and the pistils are _simple_. In many flowers, however, -there is a greater or lesser _union_ of parts. - -=814. Union of parts of the same series or cycle.=—The parts -_coalesce_, either slightly or to a great extent. Usually they are not -so completely coalesced but what the number of parts of the series can -be determined. Where the sepals are united the calyx is _gamosepalous_, -when the petals are united the corolla is _gamopetalous_. - -Union of the sepals or of the corolla is quite common, but union -of the stamens is rare except in a few families where it is quite -characteristic. When the stamens are united by their anthers, they -are _syngenœsious_. This is the case in most flowers of the composite -family. When all the stamens are united into one group by their -filaments, they are _monadelphous_ (one brotherhood), as in -hollyhock, hibiscus, cotton, marsh-mallow, etc. When they are united -by their filaments in two groups, they are _diadelphous_ (two -brotherhoods), as in the pea and most members of the pea family. In -most species of St. John’s wort (Hypericum), the stamens are united in -threes (_triadelphous_). - -=815. The carpels are often united.=—The pistil is then said to -be _compound_. Where the pistils are consolidated, usually the adjacent -walls coalesce and thus separate the cavity of each ovary. Each cavity -in the compound pistil is a _locule_. In some cases the adjacent walls -disappear so that there is one common cavity for the compound pistil -(examples: purslane, chickweeds, pinks, etc.). In a few cases there is a -false partition (example, in the toothwort and other crucifers). The -compound pistil is very often lobed slightly, so that the different -pistils can be discerned. More often the styles or stigmas are -distinct, and thus indicate the number of pistils united. - -=816. Union of the parts of different series.=—While in the -buttercup and many other flowers, all the different parts are inserted -on the torus or receptacle, in other flowers one series of parts may -be joined to another. This is _adnation_ of parts, or the two or more -series are _adnate_. In the morning glory the stamens are inserted -on the inner face of the corolla tube; the same is true in the mint -family, and there are many other examples. The insertion of parts, -whether free or adnate, is usually spoken of in reference to their -relation to the pistil. Thus, in the buttercup the floral envelopes and -stamens are all free and _hypogynous_, they are _below_ the pistil. -The pistil in this case is _superior_. In the cherry, pear, etc., the -petals and stamens are borne on the edge of the more or less elevated -tube of the calyx, and are said to be _perigynous_, i.e., around the -pistil. In the cranberry, huckleberry, etc., the calyx is for the -most part united with the wall of the ovary with the short calyx -limbs projecting from the upper surface. The petals and stamens are -inserted on the edge of the calyx above the ovary; they are, therefore, -_epigynous_, and the ovary being under the calyx, as it were, is -_inferior_. - - -III. Arrangement of Flowers, or Mode of Inflorescence. - -=817. Flowers are solitary or clustered.=—_Solitary flowers_ -are more simple in their arrangement, i.e., it is easier for us to -determine and name their relation to each other and to other parts of -the plant. They are either _axillary_, i.e., on short lateral shoots -in the axils of ordinary foliage leaves, or they are _terminal_, i.e., -they are borne on the end of the main axis of an ordinary foliage -shoot. In either case they are so far separated, and the foliage -leaves are so prominent, they do not form recognizable groups or -clusters. The manner of arrangement of flowers on the shoot is called -_inflorescence_, while the group of flowers so arranged is the _flower -cluster_. - -Two different modes of inflorescence are usually recognized in -the arrangement of flowers on the stem. (1) The _corymbose_, or -_indeterminate inflorescence_ (also indefinite inflorescence), in -which the flowers arise from axillary buds, and the terminal bud may -continue to grow. (2) The _cymose_ or _determinate inflorescence_ (also -_definite inflorescence_) in which the flowers arise from terminal -buds. This arrests the growth of the shoot in length. - -There are several advantages to the plant in the different modes of -inflorescence, chief among which is the massing of the flowers, thus -increasing the chances for effective pollination. - - -A. FLOWER CLUSTERS WITH INDETERMINATE INFLORESCENCE. - -=818. The simplest mode of indeterminate inflorescence= is where -the flowers arise in the axils of normal foliage leaves, while the -terminal bud, as in the florist’s smilax, the bellwort, moneywort, -apricot, etc., continues to grow. The flowers are _solitary_ and -_axillary_. In other cases which are far more numerous, the flowers are -associated into more or less definite clusters in which are a number -of recognizable types. The word type used in this sense, it should be -understood, does not refer to an original structure which is the -source of others. It merely refers to a mode of inflorescence which we -attempt to recognize, and about which we group those forms which have -a resemblance to one another. There are many forms of flower clusters -which do not conform to any one of our recognized types, and are very -puzzling. The evolution of the flower clusters has been _natural_, and -we cannot make them all conform to an _artificial_ classification. -These _types_ are named merely as a matter of convenience in the -expression of our ideas. The types usually recognized are as follows: - -=819. The raceme.=—The flower-shoot is more or less elongated, -and the leaves are reduced to a minute size termed _bracts_, while the -flowers on lateral axes are solitary in the axils of the bracts. The -reduction in the size of the leaves and the somewhat limited growth -of the shoot in length, makes the flowers more prominent, and brings -them into closer relation than if they were formed in the axils of -the leaves on the ordinary foliage shoot. The choke cherry, currant, -pokeweed, sourwood, etc., are examples of a raceme (fig. 569). In most -plants with the raceme type, while the inflorescence is indeterminate, -and the uppermost flowers (those toward the end of the main shoot) -are younger, still the period of flowering is somewhat restricted -and the raceme stops growing. In a few plants, however, as in the -common “shepherd’s-purse,” the raceme continues to grow throughout the -summer, so that the lower flowers may have ripened their seed while -the terminal portion of the raceme is still growing and producing -new flowers. Compound racemes are formed when by branching of the -flower-shoot there are several racemes in a cluster, as in the false -Solomon’s seal (Smilacina racemosa). - -=820. The panicle.=—The panicle is developed from the raceme type -by the branching of the lateral flower-axes forming a loose open flower -cluster, as in the _oat_. - -=821. The thyrsus= is a compact panicle of pyramidal form, as in -the lilac, horse-chestnut, etc. - -=822. The corymb.=—The corymb shows likewise an easy transition -from the raceme type, by the shortening of the main axis of -inflorescence, and the lengthening of the lower, lateral flower -peduncles so that the flower cluster is more or less flattened on -top. This represents the _simple corymb_. A _compound corymb_ is one -in which some of the flower peduncles branch again forming secondary -corymbs, as in the mountain-ash. It is like a panicle with the lower -flower stalks elongated. - -=823. The umbel.=—The umbel is developed from the raceme, or -corymb. The main flower-shoot remains very short or undeveloped with -several flowers on long peduncles arising close together around this -shortened axis, in the form of a whorl or cluster. Examples are found -in the milkweed, water pennywort (Hydrocotyle), the oxheart cherry, -etc. A _compound_ umbel is one in which the peduncles are branched, -forming secondary umbels, as in the caraway, parsnip, carrot, etc. - -=824. The spike.=—In the spike the main axis is long, and the -solitary flowers in the axils of the bracts are usually sessile, and -often very much crowded. The plaintain, mullein (fig. 422), etc., -are examples. The spike is a raceme, only the flowers are sessile -and crowded. In the grasses the flower cluster is branched, and the -branchlets bearing a few flowers are spikelets. - -=825. The head.=—When the flower axis is very much shortened -and the flowers crowded and sessile or nearly so, forming a globose -or compressed cluster, it is a _head_ or _capitulum_. The transition -is from a spike by the shortening of the main axis, as in the clover, -button bush (_Cephalanthus_), etc., or in the shortening of the -peduncles in an umbel, as in the daisy, dandelion, and other composite -flowers. In these the head is surrounded by an involucre, which in -the young head often envelopes the mass of flowers, thus affording -them protection. In some other composites (Lactuca, for example) the -involucre affords protection for a longer period, even while the seeds -are ripening. - -=826. The spadix.=—When the main axis of the flower cluster is -fleshy, the spike or head forms a _spadix_, as in the Indian turnip, -the skunk-cabbage, the calla, etc. The spadix is usually more or less -enclosed in a _spathe_, a somewhat strap-shaped leaf. - -=827. The catkin.=—A spike which is usually caducous, i.e., falls -away after the maturity of the flower or fruit, is called a catkin, -or an _ament_. The flower clusters of the alder, willow, (fig. 555), -poplar, and the staminate flower clusters of the oak, hickory, -hazel, birch, etc., are _aments_. So characteristic is this mode -of inflorescence that the plants are called _amentiferous_, or -_amentaceous_. - -[Illustration: Fig. 451. Head of sunflower showing centripetal -inflorescence of tubular flowers. (Photo by the Author.)] - -=828. Anthesis of flowers with indeterminate inflorescence.=—In -the anthesis of the raceme as well as in other corymbose forms the -lower (or outer) flowers being older, open first. The opening of the -flowers then takes place from below, upward; or from the outside, -inward toward the center of inflorescence. The _anthesis_, i.e., the -opening of the flowers of corymbose forms is said to be _centripetal_, -i.e., it progresses from outside, inward. The anthesis of the fuller’s -teazel is peculiar, since it shows both types. There are several -distinct advantages to the plant where anthesis extends over a period -of time, as it favors cross pollination, favors the formation of seed -in case conditions should be unfavorable at one period of anthesis, -distributes the drain on the plant for food, etc. - -[Illustration: Fig. 452. Heads of fuller’s teazel in different stages -of flowering.] - - -B. FLOWER CLUSTERS WITH DETERMINATE INFLORESCENCE. - -=829. The simplest mode of determinate inflorescence= is a plant -with a solitary terminal flower, as in the hepatica, the tulip, -etc. The leaves in these two plants are clustered in the form of a -rosette, and the aerial shoot is naked and bears the single flower at -its summit. Such a flower-shoot is a _scape_. As in the case of the -indeterminate inflorescence, so here the larger number of flower-shoots -are more complex and specialized, resulting in the evolution of flower -clusters or masses. Accompanying the association of flowers into -clusters there has been a reduction in leaf surface on the flower-shoot -so that the flowers predominate in mass and are more conspicuous. Among -the recognized modes of determinate inflorescence, the following are -the chief ones: - -=830. The cyme.=—In the cyme the terminal flower on the main axis -opens first and the remaining flowers are borne on lateral shoots, -which arise from the axils of leaves or bracts, below. These lateral -shoots usually branch and elongate so that the terminal flowers on all -the branches reach nearly the same height as the terminal flower on the -main shoot, forming a somewhat flattened or convex top of the flower -cluster. This is illustrated in the basswood flower. The anthesis of -the cyme is _centrifugal_, i.e., from the inside outward to the margin. -But it is often more or less mixed, since the lateral shoots if they -bear more than one flower are diminutive cymes and the terminal flower -opens before the lateral ones. Where the flower cluster is quite large -and the branching quite extensive, _compound cymes_ are formed, as in -the dogwood, hydrangea, etc. - -[Illustration: Fig. 453. Diagrams of cymose inflorescence. _A_, -dichasium; _B_, scorpioid cyme; _C_, helicoid cyme. (After -Strasburger.)] - -=831. The helicoid cyme.=—Where successive lateral branching -takes place, and always continues on the same side a curved flower -cluster is formed, as in the forget-me-not and most members of the -borage family. This is known as a _helicoid cyme_ (fig. 453, _C_). Each -new branch becomes in turn the “false” axis bearing a new branch on the -same side. - -=832. The scorpioid cyme.=—_A scorpioid cyme_ (fig. 453, _B_) is -formed where each new branch arises on alternate sides of the “false” -axis. - -=833. The forking cyme= is where each “false” axis produces two -branches opposite, so that it represents a false dichotomy (example, -the flower cluster of chickweed). - -=834.= Some of these flower clusters are peculiar and it is -difficult to see how the helicoid, or scorpioid, cymes are of any -advantage to the plant over a true cyme. The inflorescence of the -plant being determinate, if the flowering is to be extended over a -considerable period a peculiar form would necessarily result. In the -_helicoid cyme_ continued branching takes place on one side, and -the result in the forget-me-not is a continued inflorescence in its -effect like that of a continued raceme (compare shepherd’s-purse). -But we should not expect that all of the complex and specialized -structures from simple and generalized ones are beneficial to the -plant. In many plants we recognize evolution in the direction of -advantageous structures. But since the plant cannot consciously -evolve these structures, we must also recognize that there may be -phases of retrogression in which the structures evolved are not so -beneficial to the plant as the more simple and generalized ones of its -ancestors. Variation and change do not result in advancing the plant -or plant structures merely along the lines which will be beneficial. -The tendency is in all directions. The result in general may be -diagramed by a tree with divergent and wide-reaching branches. Some die -out; others remain subordinate or dormant; while still others droop -downward, showing a retrogression. But in this backward evolution -they do not return to the condition of their ancestors, nor is the -same course retraced. A new downward course is followed just as the -downward-growing branch follows a course of its own, and does not -return in the trunk. - - - - -CHAPTER XLIII. - -POLLINATION. - - -Origin of heterospory, and the necessity for pollination. - -=835. Both kinds of sexual organs on the same prothallium.=—In -the ferns, as we have seen, the sexual organs are borne on the -prothallium, a small, leaf-like, heart-shaped body growing in moist -situations. In a great many cases both kinds of sexual organs are -borne on the same prothallium. While it is perhaps not uncommon, in -some species, that the egg-cell in an archegonium may be fertilized -by a spermatozoid from an antheridium on the same prothallium, it -happens many times that it is fertilized by a spermatozoid from another -prothallium. This may be accomplished in several ways. In the first -place antheridia are usually found much earlier on the prothallium than -are the archegonia. When these antheridia are ripe, the spermatozoids -escape before the archegonia on the same prothallium are mature. - -=836. Cross fertilization in monœcious prothallia.=—By swimming -about in the water or drops of moisture which are at times present in -these moist situations, these spermatozoids may reach and fertilize -an egg which is ripe in an archegonium borne on another and older -prothallium. In this way what is termed cross fertilization is brought -about nearly as effectually as if the prothallia were diœcious, i.e. if -the antheridia and archegonia were all borne on separate prothallia. - -=837. Tendency toward diœcious prothallia.=—In other cases -some fern prothallia bear chiefly archegonia, while others bear only -antheridia. In these cases cross fertilization is enforced because -of this separation of the sexual organs on different prothallia. -These different prothallia, the male and female, are largely due to a -difference in food supply, as has been clearly proven by experiment. - -=838. The two kinds of sexual organs on different prothallia.=—In -the horsetails (equisetum) the separation of the sexual organs on -different prothallia has become quite constant. Although all the spores -are alike, so far as we can determine, some produce small male plants -exclusively, while others produce large female plants, though in some -cases the latter bear also antheridia. It has been found that when the -spores are given but little nutriment they form male prothallia, and -the spores supplied with abundant nutriment form female prothallia. - -=839. Permanent separation of sexes by different amounts of nutriment -supplied the spores.=—This separation of the sexual organs of -different prothallia, which in most of the ferns, and in equisetum, is -dependent on the chance supply of nutriment to the germinating spores, -is made certain when we come to such plants as isoetes and selaginella. -Here certain of the spores receive more nutriment while they are -forming than others. In the large sporangia (macrosporangia) only a few -of the cells of the spore-producing tissue form spores, the remaining -cells being dissolved to nourish the growing macrospores, which are few -in number. In the small sporangia (microsporangia) all the cells of the -spore-producing tissue form spores. Consequently each one has a less -amount of nutriment, and it is very much smaller, a microspore. The -sexual nature of the prothallium in selaginella and isoetes, then, is -predetermined in the spores while they are forming on the sporophyte. -The microspores are to produce male prothallia, while the macrospores -are to produce female prothallia. - -=840. Heterospory.=—This production of two kinds of spores -by isoetes, selaginella, and some of the other fern plants is -_heterospory_, or such plants are said to be _heterosporous_. -Heterospory, then, so far as we know from living forms, has originated -in the fern group. In all the higher plants, in the gymnosperms and -angiosperms, it has been perpetuated, the microspores being represented -by the pollen, while the macrospores are represented by the embryo sac; -the male organ of the gymnosperms and angiosperms being the antherid -cell in the pollen or pollen tube, or in some cases perhaps the pollen -grain itself, and the female organ in the angiosperms perhaps reduced -to the egg-cell of the embryo sac. - -=841. In the pteridophytes water serves as the medium for conveying -the sperm cell to the female organ.=—In the ferns and their allies, -as well as in the liverworts and mosses, surface water is a necessary -medium through which the generative or sperm cell of the male organ, -the spermatozoid, may reach the germ cell of the female organ. The -sperm cell is here motile. This is true in a large number of cases -in the algæ, which are mostly aquatic plants, while in other cases -currents of water float the sperm cell to the female organ. - -=842. In the higher plants a modification of the prothallium is -necessary.=—As we pass to the gymnosperms and angiosperms, -however, where the primitive phase (the gametophyte) of the plants -has become dependent solely on the modern phase (the sporophyte) of -the plant, surface water no longer serves as the medium through which -a motile sperm cell reaches the egg-cell to fertilize it. The female -prothallium, or macrospore, is, in nearly all cases, permanently -enclosed within the sporangium, so that if there were motile sperm -cells on the outside of the ovary, they could never reach the egg to -fertilize it. - -=843.= But a modification of the microspore, the pollen tube, -enables the sperm cell to reach the egg-cell. The tube grows through -the nucellus, or first through the tissues of the ovary, deriving -nutriment therefrom. - -=844.= But here an important consideration should not escape us. -The pollen grains (microspores) must in nearly all cases first reach -the pistil, in order that in the growth of this tube a channel may be -formed through which the generative cell can make its way to the egg -cell. The pollen passes from the anther locule, then, to the stigma of -the ovary. This process is termed _pollination_. - - -Pollination. - -=845. Self pollination, or close pollination.=—Perhaps very few -of the admirers of the pretty blue violet have ever noticed that there -are other flowers than those which appeal to us through the beautiful -colors of the petals. How many have observed that the brightly colored -flowers of the blue violet rarely “set fruit”? Underneath the soil -or débris at the foot of the plant are smaller flowers on shorter, -curved stalks, which do not open. When the anthers dehisce, they are -lying close upon the stigma of the ovary, and the pollen is deposited -directly upon the stigma of the same flower. This method of pollination -is _self pollination_, or _close pollination_. These small, closed -flowers of the violet have been termed “_cleistogamous_,” because they -are pollinated while the flower is closed, and fertilization takes -place as a result. - -But self pollination takes place in the case of some open flowers. -In some cases it takes place by chance, and in other cases by such -movements of the stamens, or of the flower at the time of the -dehiscence of the pollen, that it is quite certainly deposited upon the -stigma of the same flower. - -=846. Wind pollination.=—The pine is an example of -wind-pollinated flowers. Since the pollen floats in the air or -is carried by the “wind,” such flowers are _anemophilous_. Other -anemophilous flowers are found in other conifers, in grasses, sedges, -many of the ament-bearing trees, and other dicotyledons. Such plants -produce an abundance of pollen and always in the form of “dust,” so -that the particles readily separate and are borne on the wind. - -=847. Pollination by insects.=—A large number of the plants which -we have noted as being anemophilous are monœcious or diœcious, i.e. the -stamens and pistils are borne in separate flowers. The two kinds of -flowers thus formed, the male and the female, are borne either on the -same individual (monœcious) or on different individuals (diœcious). In -such cases cross pollination, i.e. the pollination of the pistil of -one flower by pollen from another, is sure to take place, if it is -pollinated at all. Even in monœcious plants cross pollination often -takes place between flowers of different individuals, so that more -widely different stocks are united in the fertilized egg, and the -strain is kept more vigorous than if very close or identical strains -were united. - -[Illustration: Fig. 454. Viola cucullata; blue flowers above, -cleistogamous flowers smaller and curved below. Section of pistil at -right.] - -=848.= But there are many flowers in which both stamens and -pistils are present, and yet in which cross pollination is accomplished -through the agency of insects. - -=859. Pollination of the bluet.=—In the pretty bluet the stamens -and styles of the flowers are of different length as shown in figures -455, 456. The stamens of the long-styled flower are at about the same -level as the stigma of the short-styled flower, while the stamens of -the latter are on about the same level as the stigma of the former. -What does this interesting relation of the stamens and pistils in the -two different flowers mean? As the butterfly thrusts its “tongue” down -into the tube of the long-styled flower for the nectar, some of the -pollen will be rubbed off and adhere to it. When now the butterfly -visits a short-styled flower this pollen will be in the right position -to be rubbed off onto the stigma of the short style. The positions -of the long stamens and long style are such that a similar cross -pollination will be effected. - -[Illustration: Fig. 455. Dichogamous flower of the bluet (Houstonia -cœrulea), the long-styled form.] - -[Illustration: Fig. 456. Dichogamous flower of bluet (Houstonia -cœrulea), the short-styled form.] - -=850. Pollination of the primrose.=—In the primroses, of which -we have examples growing in conservatories, that blossom during the -winter, we have almost identical examples of the beautiful adaptations -for cross pollination by insects found in the bluet. The general shape -of the corolla is the same, but the parts of the flower are in -fives, instead of in fours as in the bluet. While the pollen of the -short-styled primulas sometimes must fall on the stigma of the same -flower, Darwin has found that such pollen is not so potent on the -stigma of its own flower as on that of another, an additional provision -which tends to necessitate cross pollination. - -[Illustration: Fig. 457. Dichogamous flowers of primula.] - -In the case of some varieties of pear trees, as the Bartlett, it -has been found that the flowers remain largely sterile not only to -their own pollen, or pollen of the flowers on the same tree, but to -all flowers of that variety. However, they become fertile if cross -pollinated from a different variety of pear. - -=851. Pollination of the skunk’s cabbage.=—In many other flowers -cross pollination is brought about through the agency of insects, -where there is a difference in time of the maturing of the stamens and -pistils of the same flower. The skunk’s cabbage (Spathyema fœtida), -though repulsive on account of its fetid odor, is nevertheless a very -interesting plant to study for several reasons. Early in the spring, -before the leaves appear, and in many cases as soon as the frost is out -of the hard ground, the hooked beak of the large fleshy spathe of this -plant pushes its way through the soil. - -If we cut away one side of the spathe as shown in fig. 459 we shall -have the flowering spadix brought closely to view. In this spadix the -pistil of each crowded flower has pushed its style through between the -plates of armor formed by the converging ends of the sepals, and stands -out alone with the brush-like stigma ready for pollination, while the -stamens of all the flowers of this spadix are yet hidden beneath. The -insects which pass from the spadix of one plant to another will, in -crawling over the projecting stigmas, rub off some of the pollen which -has been caught while visiting a plant where the stamens are scattering -their pollen. In this way cross pollination is brought about. Such -flowers, in which the stigma is prepared for pollination before the -anthers of the same flower are ripe, are _proterogynous_. - -[Illustration: Fig. 458. Skunk’s cabbage.] - -[Illustration: Fig. 459. Proterogyny in skunk’s cabbage. (Photograph by -the author.)] - -[Illustration: Fig. 460. Skunk’s cabbage; upper flowers proterandrous, -lower ones proterogynous.] - - -=852.= Now if we observe the spadix of another plant we may see a -condition of things similar to that shown in fig. 460. In the flowers -in the upper part of the spadix here the anthers are wedging their way -through between the armor-like plates formed by the sepals, while the -styles of the same flowers are still beneath, and the stigmas are not -ready for pollination. Such flowers are _proterandrous_, that is, the -anthers are ripe before the stigmas of the same flowers are ready for -pollination. In this spadix the upper flowers are proterandrous, while -the lower ones are proterogynous, so that it might happen here that -the lower flowers would be pollinated by the pollen falling on them -from the stamens of the upper flowers. This would be cross pollination -so far as the flowers are concerned, but not so far as the plants -are concerned. In some individuals, however, we find all the flowers -proterandrous. - -=853. Spiders have discovered this curious relation of the flowers -and insects.=—On several different occasions, while studying -the adaptations of the flowers of the skunk’s cabbage for cross -pollination, I was interested to find that the spiders long ago had -discovered something of the kind, for they spread their nets here to -catch the unwary but useful insects. I have not seen the net spread -over the opening in the spathe, but it is spread over the spadix -within, reaching from tip to tip of either the stigmas, or stamens, or -both. Behind the spadix crouches the spider-trapper. The insect crawls -over the edge of the spadix, and plunges unsuspectingly into the dimly -lighted chamber below, where it becomes entangled in the meshes of the -net. - -Flowers in which the ripening of the anthers and maturing of the -stigmas occur at different times are also said to be _dichogamous_. - -=854. Pollination of jack-in-the-pulpit.=—The jack-in-the-pulpit -(Arisæma triphyllum) has made greater advance in the art of enforcing -cross pollination. The larger number of plants here are, as we have -found, diœcious, the staminate flowers being on the spadix of one -plant, while the pistillate flowers are on the spadix of another. In a -few plants, however, we find both female and male flowers on the same -spadix. - -=855.= The pretty bell-flower (Campanula rotundifolia) is -dichogamous and proterandrous (fig. 462). Many of the composites are -also dichogamous. - -=856. Pollination of orchids.=—But some of the most marvellous -adaptations for cross pollination by insects are found in the orchids, -or members of the orchis family. The larger number of the members of -this family grow in the tropics. Many of these in the forests are -supported in lofty trees where they are brought near the sunlight, -and such are called “epiphytes.” A number of species of orchids are -distributed in temperate regions. - -[Illustration: Fig. 461. A group of jacks.] - -=857. Cypripedium, or lady-slipper.=—One species of the -lady-slipper is shown in fig. 468. The labellum in this genus is shaped -like a shoe, as one can see by the section of the flower in fig. 468. -The stigma is situated at _st_, while the anther is situated at _a_, -upon the style. The insect enters about the middle of the boat-shaped -labellum. In going out it passes up and out at the end near the flower -stalk. In doing this it passes the stigma first and the anther last, -rubbing against both. The pollen caught on the head of the insect, will -not touch the stigma of the same flower, but will be in position to -come in contact with the stigma of the next flower visited. - -[Illustration: Fig. 462. Proterandry in the bell-flower (campanula). -Left figure shows the syngenœcious stamens surrounding the immature -style and stigma. Middle figure shows the immature stigma being pushed -through the tube and brushing out the pollen; while in the right-hand -figure, after the pollen has disappeared, the lobes of the stigma open -out to receive pollen from another flower.] - -=858. Epipactis.=—In epipactis, shown in fig. 469, the action is -similar to that of the blue iris. - -[Illustration: Fig. 463. Kalmia latifolia, showing position of anthers -before insect visits, and at the right the scattering of the pollen -when disturbed by insects. Middle figure section of flower.] - -=849.= In some of the tropical orchids the pollinia are set free -when the insect touches a certain part of the flower, and are thrown -in such a way that the disk of the pollinium strikes the insect’s head -and stands upright. By the time the insect reaches another flower the -pollinium has bent downward sufficiently to strike against the stigma -when the insect alights on the labellum. In the mountains of North -Carolina I have seen a beautiful little orchid, in which, if one -touches a certain part of the flower with a lead-pencil or other -suitable object, the pollinium is set free suddenly, turns a complete -somersault in the air, and lands with the disk sticking to the pencil. -Many of the orchids grown in conservatories can be used to demonstrate -some of these peculiar mechanisms. - -[Illustration: Fig. 464. Spray of leaves and flowers of cytisus.] - -[Illustration: Fig. 465. Flower of cytisus grown in conservatory. Same -flower scattering pollen.] - -=860. Pollination of the canna.=—In the study of some of the -marvellous adaptations of flowers for cross pollination one is led to -inquire if, after all, plants are not intelligent beings, instead of -mere automatons which respond to various sorts of stimuli. No plant -has puzzled me so much in this respect as the canna, and any one will -be well repaid for a study of recently opened flowers, even though it -may be necessary to rise early in the morning to unravel the mystery, -before bees or the wind have irritated the labellum. The canna flower -is a bewildering maze of petals and petal-like members. The calyx -is green, adherent to the ovary, and the limb divides into three, -lanceolate lobes. The petals are obovate and spreading, while the -stamens have all changed to petal-like members, called _staminodia_. -Only one still shows its stamen origin, since the anther is seen at one -side, while the filament is expanded laterally and upwards to form the -_staminodium_. - -[Illustration: Fig. 466. Spartium, showing the dusting of the pollen -through the opening keels on the under side of an insect. (From Kerner -and Oliver.)] - -=861.= The ovary has three locules, and the three styles are -usually united into a long, thin, strap-shaped style, as seen in the -figure, though in some cases three, nearly distinct, filamentous styles -are present. The end of this strap-shaped style has a peculiar curve on -one side, the outline being sometimes like a long narrow letter S. It -is on the end of this style, and along the crest of this curve, that -the stigmatic surface lies, so that the pollen must be deposited on the -stigmatic end or margin in order that fertilization may take place. - -[Illustration: Fig. 467. Cypripedium.] - -[Illustration: Fig. 468. Section of flower of cypripedium. _st_, -stigma; _a_, at the left stamen. The insect enters the labellum at -the center, passes under and against the stigma, and out through the -opening _b_, where it rubs against the pollen. In passing through -another flower this pollen is rubbed off on the stigma.] - -=862.= If we open carefully canna flower buds which are nearly -ready to open naturally, by unwrapping the folded petals and -staminodia, we shall see the anther-bearing staminodium is so wrapped -around the flattened style that the anther lies closely pressed against -the face of the style, near the margin _opposite that on which the -stigma lies_. - -[Illustration: Fig. 469. Epipactis with portion of perianth removed -to show details. _l_, labellum; _st_, stigma; _r_, rostellum; _p_, -pollinium. When the insect approaches the flower its head strikes the -disk of the pollinium and pulls the pollinium out. At this time the -pollinium stands up out of the way of the stigma. By the time the -insect moves to another flower the pollinia have moved downward so that -they are in position to strike the stigma and leave the pollen. At the -right is the head of a bee, with two pollinia (_a_) attached.] - -[Illustration: Fig. 470. Canna flowers with the perianth removed to -show the depositing of the pollen on the style by the stamen.] - -=863.= The walls of the anther locules which lie against the style -become changed to a sticky substance for their entire length, so that -they cling firmly to the surface of the style and also to the mass of -pollen within the locules. The result is that when the flower opens, -and this staminodium unwraps itself from the embrace of the style, -the mass of pollen is left there deposited, while the empty anther is -turned around to one side. - -=668.= Why does the flower deposit its own pollen on the style? -Some have regarded this as the act of pollination, and have concluded, -therefore, that cannas are necessarily self pollinated, and that cross -pollination does not take place. But why is there such evident care to -deposit the pollen on the side of the style away from the stigmatic -margin? If we visit the cannas some morning, when a number of the -flowers have just opened, and the bumblebees are humming around seeking -for nectar, we may be able to unlock the secret. - -=864.= We see that in a recently opened canna flower, the petal -which directly faces the style in front stands upward quite close to -it, so that the flower now is somewhat funnel-shaped. This front petal -is the _labellum_, and is the landing place for the bumblebee as he -alights on the flower. Here he comes humming along and alights on the -labellum with his head so close to the style that it touches it. But -just the instant that the bee attempts to crowd down in the flower the -labellum suddenly bends downward, as shown in fig. 468. In so doing the -head of the bumblebee scrapes against the pollen, bearing some of it -off. Now while the bee is sipping the nectar it is too far below the -stigma to deposit any pollen on the latter. When the bumblebee flies to -another newly opened flower, as it alights, some of the pollen of the -former flower is brushed on the stigma. - -=865.= One can easily demonstrate the sensitiveness of the -labellum of recently opened canna flowers, if the labellum has not -already moved down in response to some stimulus. Take a lead-pencil, or -a knife blade, or even the finger, and touch the upper surface of the -labellum by thrusting it between the latter and the style. The labellum -curves quickly downward. - -=866.= Sometimes the bumblebees, after sipping the nectar, will -crawl up over the style in a blundering manner. In this way the flower -may be pollinated with its own pollen, which is equivalent to self -pollination. Undoubtedly self pollination does take place often in -flowers which are adapted, to a greater or less degree, for cross -pollination by insects. - -[Illustration: Fig. 471. Pollination of the canna flower by bumblebee.] - -[Illustration: Canna flower. Pollen on style, stamen at left.] - - - - -CHAPTER XLIV. - -THE FRUIT. - - -I. Parts of the Fruit. - -=867. After the flower comes the fruit.=—With the perfection of -the fruit the seed is usually formed. This is the end towards which the -energies of the plant have been directed. While the seed consists only -of the ripened ovule and the contained embryo, the fruit consists of -the ripened ovary in addition, and in many cases with other accessory -parts, as calyx, receptacle, etc., combined with it. The wall of the -ripened ovary is called a _pericarp_, and the walls of the ovary form -the walls of the fruit. - -=868. Pericarp, endocarp, exocarp, etc.=—This is the part of the -fruit which envelops the seed and may consist of the carpels alone, or -of the carpels and the adherent part of the receptacle, or calyx. In -many fruits the pericarp shows a differentiation into layers, or zones -of tissue, as in the cherry, peach, plum, etc. The outer, which is -here soft and fleshy, is _exocarp_, while the inner, which is hard, is -the _endocarp_. An intermediate layer is sometimes recognized and is -called _mesocarp_. In such cases the skin of the fruit is recognized as -the _epicarp_. Epicarp and mesocarp are more often taken together as -exocarp. - -In general fruits are _dry_ or _fleshy_. Dry fruits may be grouped -under two heads. Those which open at maturity and scatter the seed are -_dehiscent_. Those which do not open are _indehiscent_. - - -II. Indehiscent Fruits. - -=869. The akene.=—The thin dry wall of the ovary encloses the -single seed. It usually does not open and free the seed within. Such a -fruit is an _akene_. An _akene_ is a dry, _indehiscent_ fruit. All of -the crowded but separate pistils in the buttercup flower when ripe make -a head of akenes, which form the fruit of the buttercup. Other examples -of akenes are found in other members of the buttercup family, also in -the composites, etc. The sunflower seed is a good example of an akene. - -[Illustration: Fig. 472. Seed, or akene, of buttercup.] - -[Illustration: Fig. 473. Fruit of red oak. An acorn.] - -=870. The samara.=—The winged fruits of the maple, elm, etc., are -indehiscent fruits. They are sometimes called key fruits. - -=871. The caryopsis= is a dry fruit in which the seed is -consolidated with the wall of the ovary, as in the wheat, corn, and -other grasses. - -=872. The schizocarp= is a dry fruit consisting of several locules -(from a _syncarpous gynœcium_). At maturity the carpels separate from -each other, but do not themselves dehisce and free the seed, as in the -carrot family, mallow family. - -=873. The acorn.=—The acorn fruit consists of the acorn and -the “cup” at the base in which the acorn sits. The cup is a curious -structure, and is supposed to be composed of an involucre of numerous -small leaves at the base of the pistillate flower, which become -consolidated into a hard cup-shaped body. When the acorn is ripe it -easily separates from the cup, but the hard pericarp forming the -“shell” of the acorn remains closed. Frost may cause it to crack, but -very often the pericarp is split open at the smaller end by wedge-like -pressure exerted by the emerging radicle during germination. - -[Illustration: Fig. 474. Germinating acorn of white oak.] - -=874. The hazelnut, chestnut, and beechnut.=—In these fruits a -crown of leaves (involucre) at the base of the flower grows around -the nut and completely envelops it, forming the husk or burr. When -the fruit is ripe the nut is easily shelled out from the husk. In the -beechnut and chestnut the burr dehisces as it dries and allows the nut -to drop out. But the fruit is not dehiscent, since the pericarp is -still intact and encloses the seed. - -=875. The hickory-nut, walnut, and butternut.=—In these fruits the -“shuck” of the hickory-nut and the “hull” of the walnut and butternut -are different from the involucre of the acorn or hazelnut, etc. In the -hickory-nut the “shuck” probably consists partly of calyx and partly of -involucral bracts consolidated, probably the calyx part predominating. -This part of the fruit splits open as it dries and frees the “nut,” the -pericarp being very hard and indehiscent. In the walnut and butternut -the “hull” is probably of like origin as the “shuck” of the hickory -nut, but it does not split open as it ripens. It remains fleshy. The -walnut and butternut are often called _drupes_ or _stone-fruits_, but -the fleshy part of the fruit is not of the same origin as the fleshy -part of the true drupes, like the cherry, peach, plum, etc. - - -III. Dehiscent Fruits. - -=876. Of the dehiscent fruits= several prominent types are -recognized, and in general they are sometimes called _pods_. There is a -single carpel (simple pistil), and the pericarp is dry (gynœcium -_apocarpous_); or where there are several carpels united the pistil is -compound (gynœcium _syncarpous_). - -[Illustration: Fig. 475. Diagrams illustrating three types (in -cross-section) of the dehiscence of dry fruits. _Loc_, loculicidal; -_Sep_, Septicidal, Septifragal.] - -[Illustration: Fig. 476. Fruit of sweet pea; a pod.] - -=877. The capsule.=—When the capsule is _syncarpous_ it may -dehisce in three different ways: 1st. When the carpels split along -the line of their union with each other longitudinally (_septicidal -dehiscence_), as in the azalea or rhododendron. 2d. When the carpels -_split down the middle line_ (_loculicidal dehiscence_), as in the -fruit of the iris, lily, etc. 3d. When the carpels open by pores -(_poricidal dehiscence_), as in the poppy. Some syncarpous capsules -have but one locule, the partitions between the different locules when -young having disappeared. The “bouncing-bet” is an example, and the -seeds are attached to a central column in four rows corresponding to -the four locules present in the young stage. - -=878. A follicle= is a capsule with a single carpel which splits -open along the ventral or upper suture, as in the larkspur, peony. - -=879. The legume, or true pod=, is a capsule with a single carpel -which splits along both sutures, as the pea, bean, etc. As the pod -ripens and dries, a strong twisting tension is often produced, which -splits the pod suddenly, scattering the seeds. - -=880. The silique.=—In the toothwort, shepherd’s-purse, and -nearly all of the plants in the mustard family the fruit consists of -two united carpels, which separate at maturity, leaving the partition -wall persistent. Such a fruit is a _silique_; when short it is a -_silicle_, or _pouch_. - -=881. A pyxidium, or pyxis=, is a capsule which opens with a lid, -as in the plantain. - - -IV. Fleshy and Juicy Fruits. - -=882. The drupe, or stone-fruit.=—In the plum, cherry, peach, -apricot, etc., the outer portion (exocarp) of the pericarp (ovary) -becomes fleshy, while the inner portion (endocarp) becomes hard and -stony, and encloses the seed, or “pit.” Such a fruit is known as a -drupe, or as a stone-fruit. In the almond the fleshy part of the fruit -is removed. - -[Illustration: Fig. 477. Drupe, or stone-fruit, of plum.] - -=883. The raspberry and blackberry.=—While these fruits are -known popularly as “berries,” they are not berries in the technical -sense. Each ovary, or pericarp, in the flower forms a single small -fruit, the outer portion being fleshy and the inner stony, just as -in the cherry or plum. It is a _drupelet_ (little drupe). All of the -drupelets together make the “berry,” and as they ripen the separate -drupelets cohere more or less. It is a collection, or aggregation, of -fruits, and consequently they are sometimes called _collective fruits_, -or _aggregate fruits_. In the raspberry the fruit separates from the -receptacle, leaving the latter on the stem, while the drupelets of -the blackberry and dewberry adhere to the receptacle and the latter -separates from the stem. - -=884. The berry.=—In the true berry both exocarp (including -mesocarp) and endocarp are fleshy or juicy. Good examples are found -in cranberries, huckleberries, gooseberries, currants, snowberries, -tomatoes, etc. The calyx and wall of the pistil are adnate, and in -fruit become fleshy so that the seeds are imbedded in the pulpy juice. -The seeds themselves are more or less stony. In the case of berries, -as well as in strawberries, raspberries, and blackberries, the fruits -are eagerly sought by birds and other animals for food. The seeds being -hard are not digested, but are passed with the other animal excrement -and thus gain dispersal. - - -V. Reinforced, or Accessory, Fruits. - -When the torus (receptacle) is grown to the pericarp in fruit, the -fruit is said to be _reinforced_. The torus may enclose the pericarps, -or the latter may be seated upon the torus. - -[Illustration: Fig. 478. Fruit of raspberry.] - -=885. In the strawberry= the receptacle of the flower becomes -larger and fleshy, while the “seeds,” which are akenes, are sunk in the -surface and are hard and stony. The strawberry thus differs from the -raspberry and blackberry, but like them it is not a true berry. - -=886. The apple, pear, quince, etc.=—In the flower the calyx, -corolla, and stamens are perigynous, i.e., they are seated on the -margin of the receptacle, or torus, which is elevated around the -pistils. In fruit the receptacle becomes consolidated with the wall -of the ovary (with the pericarp). The torus thus _reinforces_ the -pericarp. The torus and outer portion of the pericarp become fleshy, -while the inner portion of the pericarp becomes papery and forms the -“core.” The calyx persists on the free end of the fruit. Such a fruit -is called a _pome_. The receptacle, or torus, of the rose-flower, -closely related to the apple, is instructive when used in comparison. -The rose-fruit is called a “hip.” - -=887. The pepo.=—The fruit of the squash, pumpkin, cucumber, -etc., is called a _pepo_. The outer part of the fruit is the receptacle -(or torus), which is consolidated with the outer part of the -three-loculed ovary. The calyx, which, with the corolla and stamens, -was epigynous, falls off from the young fruit. - - -VI. Fruits of Gymnosperms. - -The fruits of the gymnosperms differ from nearly all of the angiosperms -in that the seed formed from the ripened ovule is naked from the first, -i.e., the ovary, or carpel, does not enclose the seed. - -=888. The cone-fruit= is the most prominent fruit of the -gymnosperms, as can be seen in the cones of various species of pine, -spruce, balsam, etc. - -=889. Fleshy fruits of the gymnosperms.=—Some of the fleshy -fruits resemble the stone-fruits and berries of the angiosperms. The -_cedar_ “_berries_,” for example, are fleshy and contain several seeds. -But the fleshy part of the fruit is formed, not from pericarp, since -there is no pericarp, but from the outer portion of the ovules, while -the inner walls of the ovules form the hard stone surrounding the -endosperm and embryo. An examination of the pistillate flower of the -cedar (juniper) shows usually three flask-shaped ovules on the end -of a fertile shoot subtended by as many bracts (carpels?). The young -ovules are free, but as they grow they coalesce, and the outer walls -become fleshy, forming a berry-like fruit with a three-rayed crevice -at the apex marking the number of ovules. The red fleshy fruit of the -yew (taxus) resembles a drupe which is open at the apex. The stony -seed is formed from the single ovule on the fertile shoot, while the -red cup-shaped fleshy part is formed from the outer integument of the -ovule. The so-called “aril” of the young ovule is a rudimentary outer -integument. - -The fruit of the maidenhair tree (ginkgo) is about the size of a plum -and resembles very closely a stone-fruit. But it is merely a ripened -ovule, the outer layer becoming fleshy while the inner layer becomes -stony and forms the pit which encloses the embryo and endosperm. -The so-called “aril,” or “collar,” at the base of the fruit is the -rudimentary carpel, which sometimes is more or less completely expanded -into a true leaf. The fruit of cycas is similar to that of ginkgo, but -there is no collar at the base. In zamia the fruit is more like a cone, -the seeds being formed, however, on the under sides of the scales. - - -VII. The “Fruit” of Ferns, Mosses, etc. - -=890. The term “fruit”= is often applied in a general or popular -sense to the groups of spore-producing bodies of ferns (_fruit dots_, -or _sori_), the spore-capsules of mosses and liverworts, and also to -the fruit-bodies, or spore-bearing parts, of the fungi and algæ. - - - - -CHAPTER XLV. - -SEED DISPERSAL. - - -=891. Means for dissemination of seeds.=—During late summer -or autumn a walk in the woods or afield often convinces us of the -perfection and variety of means with which plants are provided for the -dissemination of their seeds, especially when we discover that several -hundred seeds or fruits of different plants are stealing a ride at -our expense and annoyance. The hooks and barbs on various seed-pods -catch into the hairs of passing animals and the seeds may thus be -transported considerable distances. Among the plants familiar to us, -which have such contrivances for unlawfully gaining transportation, -are the beggar-ticks or stick tights, or sometimes called bur-marigold -(bidens), the tick-treefoil (desmodium), or cockle-bur (xanthium), and -burdock (arctium). - -[Illustration: Fig. 479. Bur of bidens or bur-marigold, showing barbed -seeds.] - -[Illustration: Fig. 480. Seed pod of tick-treefoil (desmodium); at the -right some of the hooks greatly magnified.] - -=892.= Other plants like some of the sedges, etc., living on the -margins of streams and of lakes, have seeds which are provided with -floats. The wind or the flowing of the water transports them often to -distant points. - -=893.= Many plants possess attractive devices, and offer a -substantial reward, as a price for the distribution of their seeds. -Fruits and berries are devoured by birds and other animals; the seeds -within, often passing unharmed, may be carried long distances. Starchy -and albuminous seeds and grains are also devoured, and while many -such seeds are destroyed, others are not injured, and finally are -lodged in suitable places for growth, often remote from the original -locality. Thus animals willingly or unwillingly become agents in the -dissemination of plants over the earth. Man in the development of -commerce is often responsible for the wide distribution of harmful as -well as beneficial species. - -[Illustration: Fig. 481. Seeds of geum showing the hooklets where the -end of the style is kneed.] - -=894.= Other plants are more independent, and mechanisms are -employed for violently ejecting seeds from the pod or fruit. The -unequal tension of the pods of the common vetch (Vicia sativa) when -drying causes the valves to contract unequally, and on a dry summer day -the valves twist and pull in opposite directions until they suddenly -snap apart, and the seeds are thrown forcibly for some distance. In the -impatiens, or touch-me-not as it is better known, when the pods are -ripe, often the least touch, or a pinch, or jar, sets the five valves -free, they coil up suddenly, and the small seeds are thrown for several -yards in all directions. During autumn, on dry days, the pods of the -witch hazel contract unequally, and the valves are suddenly spread -apart, and the seeds are hurled away. - -Other plants have seeds provided with tufts of pappus, or hair-like -masses, or wing-like outgrowths which serve to buoy them up as they are -whirled along, often miles away. In late spring or early summer the -pods of the willow burst open, exposing the seeds, each with a tuft of -white hairs making a mass of soft down. As the delicate hairs dry, they -straighten out in a loose spreading tuft, which frees the individual -seeds from the compact mass. Here they are caught by currents of air -and float off singly or in small clouds. - -[Illustration: Fig. 482. Touch-me-not (Impatiens fulva); side and front -view of flower below; above unopened pod, and opening to scatter the -seed.] - -=895. The prickly lettuce.=—In late summer or early autumn the -seeds of the prickly lettuce (Lactuca scariola) are caught up from the -roadsides by the winds, and carried to fields where they are unbidden -as well as unwelcome guests. This plant is shown in fig. 483. - -=896. The wild lettuce.=—A related species, the wild lettuce -(Lactuca canadensis) occurs on roadsides and in the borders of fields, -and is about one meter in height. The heads of small yellow or purple -flowers are arranged in a loose or branching panicle. The flowers are -rather inconspicuous, the rays projecting but little above the apex of -the enveloping involucral bracts, which closely press together, forming -a flowerhead more or less flask-shaped. - -[Illustration: Fig. 483. Lactuca scariola.] - -At the time of flowering the involucral bracts spread somewhat at the -apex, and the tips of the flowers are a little more prominent. As the -flowers then wither, the bracts press closely together again and the -head is closed. As the seeds ripen the bracts die, and in drying bend -outward and downward, around the flower stem below, or they fall away. -The seeds are thus exposed. The dark brown achenes stand over the -surface of the receptacle, each one tipped with the long slender -beak of the ovary. The “pappus,” which is so abundant in many of the -plants belonging to the composite family, forms here a pencil-like -tuft at the tip of this long beak. As the involucral bracts dry and -curve downward, the pappus also dries, and in doing so bends downward -and stands outward, bristling like the spokes of a small wheel. It is -an interesting coincidence that this takes place simultaneously with -the pappus of all the seeds of a head, so that the ends of the pappus -bristles of adjoining seeds meet, forming a many-sided dome of a -delicate and beautiful texture. This causes the beaks of the achenes to -be crowded apart, and with the leverage thus brought to bear upon the -achenes they are pried off the receptacle. They are thus in a position -to be wafted away by the gentlest zephyr, and they go sailing away on -the wind like a miniature parachute. As they come slowly to the ground -the seed is thus carefully lowered first, so that it touches the ground -in a position for the end which contains the root of the embryo to come -in contact with the soil. - -=897. The milkweed, or silkweed.=—The common milkweed, or -silkweed (Asclepias cornuti), so abundant in rich grounds, is -attractive not only because of the peculiar pendent flower clusters, -but also for the beautiful floats with which it sends its seeds -skyward, during a puff of wind, to finally lodge on the earth. - -[Illustration: Fig. 484. Milkweed (Asclepias cornuti); dissemination of -seed.] - -=898.= The large boat-shaped, tapering pods, in late autumn, are -packed with oval, flattened, brownish seeds, which overlap each other -in rows like shingles on a roof. These make a pretty picture as the pod -in drying splits along the suture on the convex side, and exposes them -to view. The silky tufts of numerous long, delicate white hairs on the -inner end of each seed, in drying, bristle out, and thus lift the seeds -out of their enclosure, where they are caught by the breeze and borne -away often to a great distance, where they will germinate if conditions -become favorable, and take their places as contestants in the battle -for existence. - -=899. The virgin’s bower.=—The virgin’s bower (Clematis -virginiana), too, clambering over fence and shrub, makes a show of -having transformed its exquisite white flower clusters into -grayish-white tufts, which scatter in the autumn gusts into hundreds -of arrow-headed, spiral plumes. The achenes have plumose styles, and -the spiral form of the plume gives a curious twist to the falling seed -(fig. 485). - -[Illustration: Fig. 485. Seed distribution of virgin’s bower -(clematis).] - - - - -CHAPTER XLVI. - -VEGETATION IN RELATION TO ENVIRONMENT.[47] - - -I. Factors Influencing Vegetation Types. - -=900.= All plants are subject to the influence of environment -from the time the seed begins to germinate until the seed is formed -again, or until the plant ceases to live. A suitable amount of warmth -and moisture is necessary that the seed may germinate. Moisture may -be present, but if it is too cold, germination will not take place. -So in all the processes of life there are several conditions of the -environment, or the “outside” of plants, which must be favorable for -successful growth and reproduction. Not only is this true, but the -surroundings of plants to a large extent determine the kind of plants -which can grow in particular localities. It is also evident that the -reaction of environment on plants has in a large measure caused them -to take on certain forms and structures which fit them better to exist -under local conditions. In other cases where plants have varied by -mutation (p. 338) some of the new forms may be more suited to the -conditions of environment than others and they are more apt to survive. -These conditions of environment acting on the plant are _factors_ which -have an important determining influence on the existence, habitat, -habit, and form of the plant. These factors are sometimes spoken of -as _ecological factors_, and the study of plants in this relation is -sometimes spoken of as ecology,[48] which means a study of plants in -their home or a study of the household relations of plants. These -factors are of three sorts: 1st, physical factors; 2d, climatic -factors; 3d, biotic factors. - -=901. Physical factors.=—Some of these factors are water, light, -heat, wind, chemical or physical condition of the soil, etc. _Water_ -is a very important factor for all plants. Even those growing on land -contain a large percentage of water, which we have seen is rapidly lost -by transpiration, and unless water is available for root absorption -the plant soon suffers, and aquatic plants are injured very quickly by -drying when taken from the water. Excess of soil water is injurious to -some plants. _Light_ is important in photosynthesis, in determining -direction of growth as well as in determining the formation of suitable -leaves in most plants, and has an influence in the structure of the -leaf according as the light may be strong, weak, etc. _Heat_ has great -influence on plant growth and on the distribution of plants. The -growth period for most vegetation begins at 6° C. (= 43° F.), or in -the tropics at 10°-12° C., but a much higher temperature is usually -necessary for reproduction. Some arctic algæ, however, fruit at 1.8° C. -The upper limit favorable for plants in general is 45°-50° C., while -the optimum temperature is below this. Very high temperatures are -injurious, and fatal to most plants, but some algæ grow in hot springs -where the temperature reaches 80°-90° C. Some desert plants are able to -endure a temperature of 70° C., while some flowering plants of other -regions are killed at 45° C. Some plants are specifically susceptible -to cold, but most plants which are injured by freezing suffer because -the freezing is a drying process of the protoplasm (see p. 374). _Wind_ -may serve useful purposes in pollination and in aeration, but severe -winds injure plants by causing too rapid transpiration, by felling -trees, by breaking plant parts, by deforming trees and shrubs, and by -mechanical injuries from “sand-blast.” _Ground covers_ protect plants -in several ways. Snow during the winter checks radiation of heat from -the ground so that it does not freeze to so great a depth, and this is -very important for many trees and shrubs. It also prevents alternate -freezing and thawing of the ground, which “heaves” some plants from -the soil. Leaves and other plant remains mulch the soil and check -evaporation of water. The influence of the _chemical condition_ of the -soil is very marked in alkaline areas where the concentration of salt -in the soil permits a very limited range of species. So the physical -and mechanical conditions of the soil influence plants because the -moisture content of the ground is so closely dependent on its physical -condition. Rocky and gravelly soil, other things being equal, is dry. -Clay is more retentive of moisture than sand, and moisture also varies -according to the per cent of humus mixed with it, the humus increasing -the percentage of moisture retained. - -=902. Climatic factors.=—These factors are operative over very -wide areas. There are two climatic factors: rainfall or atmospheric -moisture, and temperature. A very low annual rainfall in warm or -tropical countries causes a desert; an abundance of rain permits the -growth of forests; extreme cold prevents the growth of forests and -gives us the low vegetation of arctic and alpine regions. - -=903. Biotic factors.=—These are animals which act favorably -in pollination, seed distribution, or unfavorably in destroying or -injuring plants, and man himself is one of the great agencies in -checking the growth of some plants while favoring the growth of others. -Plants also react on themselves in a multitude of ways for good or -evil. Some are parasites on others; some in symbiosis (see p. 85) aid -in providing food; shade plants are protected by those which overtop -them; mushrooms and other fungi disintegrate dead plants to make humus -and finally plant food; certain bacteria by nitrification prepare -nitrates for the higher plants (see p. 83). - - -II. Vegetation Types and Structures. - -=904. Responsive type of vegetation.=—In studying vegetation in -relation to environment we are more concerned with the form of the -plants which fits them to exist under the local conditions than we are -with the classification of plants according to natural relationships. -Plants may have the same vegetation type, grow side by side, and still -belong to very different floristic types. For example, the cactus, -yucca, three-leaved sumac, the sage-brush, etc., have all the same -general vegetation type and thrive in desert regions. The red oaks, the -elms, many goldenrods, trillium, etc., have the same general vegetation -type, but represent very different floristic types. The latter plants -grow in regions with abundant rainfall throughout the year, where -the growing season is not very short and temperature conditions are -moderate. Some goldenrods grow in very sandy soil which dries out -quickly. These have fleshy or succulent leaves for storing water, -and while they are of the same floristic type as goldenrods growing -in other places, the vegetation type is very different. The types of -vegetation which fit plants for growing in special regions or under -special conditions, they have taken on in response to the influence -of the conditions of their environment. While we find all gradations -between the different types of vegetation, looking at the vegetation -in a broad way, several types are recognized which were proposed by -Warming as follows: - -=905. Mesophytes.=—These are represented by land plants under -temperate or moderate climatic and soil conditions. The normal -land vegetation of our temperate region is composed of mesophytes, -that is, the plants have mesophytic structures during the growing -season. The deciduous forests or thickets of trees and shrubs with -their undergrowth, the meadows, pastures, prairies, weeds, etc., are -examples. In those portions of the tropics where rainfall is great the -vegetation is mesophytic the year around. - -=906. Xerophytes.=—These are plants which are provided with -structures which enable them to live under severe conditions of -dryness, where the air and soil are very dry, as in deserts or -semideserts, or where the soil is very dry or not retentive of -moisture, as in very sandy soil which is above ground water, or in -rocky areas. Since the plants cannot obtain much water from the soil -they must be provided with structures which will enable them to -retain the small amount they can absorb from the soil and give it off -slowly. Otherwise they would dry out by evaporation and die. Some -of the structures which enable xerophytic plants to withstand the -conditions of dry climate and soil are lessened leaf surface, increase -in thickness of leaf, increase in thickness of cuticle, deeply sunken -stomates, compact growth, also succulent leaves and stems, and in some -cases loss of the leaf. Evergreens of the north temperate and the -arctic regions are xerophytes. - -=907. Hydrophytes.=—These are plants which grow in fresh water -or in very damp situations. The leaves of aerial hydrophytes are very -thin, have a thin cuticle, and lose water easily, so that if the air -becomes quite dry they are in danger of drying up even though the roots -may be supplied with an abundance of water. The aquatic plants which -are entirely submerged have often thin leaves, or very finely divided -or slender leaves, since these are less liable to be torn by currents -of water. The stems are slender and especially lack strengthening -tissue, since the water buoys them up. Removed from the water they -droop of their own weight, and soon dry up. The stems and leaves have -large intercellular spaces filled with air which aids in aeration and -in the diffusion of gases. Some use the term _hygrophytes_. - -=908. Halophytes.=—These are salt-loving plants. They grow in -salt water, or in salt marshes where the water is brackish, or in -soil which contains a high per cent of certain salts, for example the -alkaline soils of the West, especially in the so-called “Bad Lands” -of Dakota and Nebraska, and in alkaline soils of the Southwest and -California. These plants are able to withstand a stronger concentration -of salts in the water than other plants. They are also found in soil -about salt springs. - -=909. Tropophytes.=[49]—Tropophytes are plants which can live as -mesophytes during the growing season, and then turn to a xerophytic -habit in the resting season. Deciduous trees and shrubs, and perennial -herbs of our temperate regions, are in this sense tropophytes, while -many are at the same time mesophytes if they exist in the portions of -the temperate region where rainfall is abundant. In the spring and -summer they have broad and comparatively thin leaves, transpiration -goes on rapidly, but there is an abundance of moisture in the soil, -so that root absorption quickly replaces the loss and the plant does -not suffer. In the autumn the trees shed their leaves, and in this -condition with the bare twigs they are able to stand the drying effect -of the cold and winds of the winter because transpiration is now at a -minimum, while root absorption is also at a minimum because of the cold -condition of the soil. Perennial herbs like trillium, dentaria, the -goldenrods, etc., turn to xerophytic habit by the death of their aerial -shoots, while the thick underground shoot which is also protected by -its subterranean habit carries the plant through the winter. - -=910.= While these different vegetation types are generally -dominant in certain climatic regions or under certain soil conditions, -they are not the exclusive vegetation types of the regions. For -example, in desert or semidesert regions the dominant vegetation -type is made up of xerophytes. But there is a mesophytic flora even -in deserts, which appears during the rainy season where temperature -conditions are favorable for growth. This is sometimes spoken of as the -rainy-season flora. The plants are annuals and by formation of seed can -tide over the dry season. So in the region where mesophytes grow there -are xerophytes, examples being the evergreens like the pines, spruces, -rhododendrons; or succulent plants like the stonecrop, the purslane, -etc. Then among hydrophytes the semiaquatics are really xerophytes. The -roots are in water, and absorption is slow because there are no root -hairs, or but few, and the aerial parts of the plant are xerophytic. - - -III. Plant Formations. - -=911.= The term plant formation is applied to associations of -plants of the same kind, though there is a great difference in the use -of the word by different writers which leads to some confusion.[50] It -is sometimes applied to an association of individuals of a species, or -of several species occupying a rather definite area of ground where the -soil conditions are not greatly different (individual formation); by -others it is applied to the plants of a definite physiographic area, as -a swamp, moor, strand, or beach, bank, rock hill, clay hill, ravine, -bluff, etc. (principal formation); and in a broad sense it is applied -to the plants of climatic regions, of those in bodies of water, etc. -(general formations). Space here is too limited to discuss all these -kinds of formations, but the nature of the general formations will be -pointed out. The general formations may be grouped into four divisions: - - 1st. Climatic formations. - 2d. Edaphic formations. - 3d. Aquatic formations. - 4th. Culture formations. - -=912. Climatic formations.=—Climatic influences extend over -wide regions, so that climate controls the general type of vegetation -of a region. In the sense of control there are two climatic factors, -temperature and moisture, especially soil moisture. Temperature exerts -a controlling influence over the vegetation type only where the total -heat during the period of growth and reproduction is very low. This -occurs in polar lands and at high elevations where the climate is -alpine. In the temperate and tropical regions of the globe moisture, -not heat, controls the general vegetation type. These vegetation types -in general are coincident with rainfall distribution, and Schimper -recognizes here three types, which with the arctic-alpine type would -make four climatic formations as follows: - -1st. _The woodland formation._—This formation is characterized by -trees and shrubs, and it is what is called a _close_ formation. By this -it is meant that so far as the climate is concerned the conditions are -favorable for the development of trees and shrubs in such abundance -that they become the dominant vegetation type of the region and grow -close together. Other plants, as herbs, grasses, etc., occur, but -they grow as subordinate elements of the general vegetation type, and -as undergrowth. The land portion of the globe, therefore, outside of -arctic and alpine regions, where the annual precipitation is 40 to 60 -or more inches, is the area for woodland formation. In some places, -the eastern part of England, for example, the annual precipitation is -25 to 30 inches, but the cool temperature permits a forest growth. It -is true there are places where forests do not grow,—where man cuts -them down, for example. But if cultivated lands in this region were -allowed to go to waste, they would in time grow up to forest again. -So there are swamps where the soil is too wet for trees, or sandy or -rocky areas where there is not a sufficient amount of soil or water to -support forest trees. But here it is the soil conditions, not climatic -conditions, which prevent the development of the forest. But we know -that swamps are being filled in and the ground gradually becoming -higher and drier, and that soil is slowly accumulating in rocky areas, -so that in time if left to natural forces these places would become -forested. So this area of heavy annual rainfall is a _potential_ forest -area. These areas are determined by warm currents of moisture-laden -air from the ocean moving over cooler land areas where the moisture -is precipitated. In general these areas are along the coasts of great -continents and on mountains. Therefore the interior of a continent is -apt to be dry because most of the moisture has been precipitated before -it reaches the interior. Deserts or steppes are therefore usually near -the interior of continents. Some exceptions to this general rule are -found: central South America, which is a region of exceptional rainfall -because the moisture-laden winds here come from the warmest part of the -ocean; the desert region west of the Andes mountains, where the winds -are not favorable; southern California, where the winds come chiefly -from a cooler portion of the Pacific ocean and move over an area of -high temperature, etc. - -[Illustration: Fig. 486. Typical prairie scene, a few miles west of -Lincoln, Nebraska. (Bot. Dept., Univ. Nebraska.)] - -2d. _Grassland formation._—Grasses form the dominant vegetation -type where the annual rainfall is approximately 15 to 25 inches. In -true grasslands the formation is a close one since there is still a -sufficient amount of moisture to provide for all the plants which can -stand on the ground. Yet there is not enough moisture to permit the -growth of forest as the dominant type without aid and protection by -man. The so-called prairie regions are examples. Trees and shrubs do -occur, but they cannot compete successfully with the grasses because -the climatic conditions are favorable for the latter and unfavorable -for the former. On the border line between forest and prairie the line -of division is not a clear-cut one because conditions grade from one to -the other. The two formations are somewhat mixed, like the outposts of -contending armies, arms of the forest or prairie extending out here and -there. In the United States the prairies extend from Illinois to about -the 100th meridian, and beyond this to the foothills of the Rockies and -southwest to the Sonora Nevada desert the region is drier, the rainfall -varying from 10 to 20 inches. This is the area of the Great Plains, -and while grasses of the bunch type are dominant, they make a more or -less open formation because the moisture is not sufficient to supply -all the plants which could be crowded on the ground, each individual -tuft needing an area of ground surrounding it on which it can draw -for moisture. Such a formation is an open one, and in this respect is -similar to desert formations. - -[Illustration: Fig. 487. Winter range in northwestern Nevada, showing -open formation; white sage (Eurotia lanata) in foreground, salt-bush -(Atriplex confertifolia) and bud-sage (Artemisia spinescens) at base -of hill, red sage (Kochia americana) on the higher slope. (After -Griffiths, Bull. 38, Bureau Plant Ind., U. S. Dept. Agr.)] - -3d. _Desert formations._—These occur where the annual rainfall is -still lower, 10 to 4 inches or even less, 2 to 3 inches, while in -one place in Chili it is as low as ½ inch. In the great Sahara desert -it is about 8 inches, while in the Sonora Nevada desert in the -southwestern United States it is 4 to 8 inches. Here the formation is -an open one. In the forest and prairie formations the plants compete -with each other for occupancy of the ground, since climatic conditions -are favorable, so that the struggle against climate is not severe. -But in the desert plants do not compete with each other; since the -climate is so austere, the struggle is against the climate. Hence -plants stand at some distance from each other because the roots need -the moisture from the ground for some distance around them. There is -not enough moisture for all the plants that begin, and those which get -the start take the moisture away from the intervening ones, which then -die. Since the struggle is against the adverse conditions of climate -and not a competition between plants to occupy the ground, no one -floristic type dominates as in the case of the grasses and forests of -the grassland and woodland formations, but grassland and woodland types -grow together. So we find grasses, trees, and shrubs growing without -competition in the desert. The dominant vegetation type is xerophytic. - -[Illustration: Fig. 488. Northern limit of tree growth, Alaska. -(Copyright, 1899, by E. H. Harriman.)] - -4th. _Arctic-alpine formation._ This formation extends from the limit -of tree growth to the region of perpetual ice and snow. The forest -here comes in competition with climate, with the severe cold of the -long winter night, so that tree growth is limited, and on the border -line with the woodland formation the trees are stunted, bent to one -side by the heavy snows, or the tops are killed by the cold wind. The -arctic zone of plant growth is sometimes spoken of as the “cold waste,” -since conditions here are somewhat similar to those in the desert, the -extreme cold exercising a drying effect on vegetation, and the -vegetation type then is largely xerophytic. - -=913. Edaphic[51] formations.=—Edaphic formations may occur -in any of the climatic-formation areas. They are controlled by the -condition of soil or ground. The condition of the soil is unfavorable -for the growth of the general vegetation type of that region, or is -more favorable for another vegetation type, so that soil conditions -overcome the climatic conditions. These areas include swamps, moors, -the strand or beach, rocky areas, etc., as well as oases in the desert, -warm oases in the arctic zone, river bottoms in the prairie and plains -region, alkaline areas, etc. The edaphic formations may be close or -open according to the nature of the soil. The edaphic formations then -are infiltrated in the climatic formations, the different vegetation -types fitting together like pieces of mosaic, which can be seen in some -places from a mountain top, or if one could take a bird’s-eye view of -the landscape or from a balloon. - -=914. Aquatic formations.=—These are made up of water plants and -are of two general kinds: fresh-water plant formations in ponds, lakes, -streams; and salt-water plant formations in the ocean and inland salt -seas. - -=915. Culture formations.=—Culture formations are largely -controlled by man, who destroys the climatic or edaphic formation and -by cultivation protects cultivated types, or by allowing land to go to -“waste” permits the growth of weeds, though weeds are often abundant -in the culture areas. In general the culture formations may be grouped -into two subdivisions: 1st, the vegetation of cultivated places; and -2d, the vegetation of waste places, as abandoned fields, roadsides, etc. - - -IV. Plant Societies. - -=916. Plant societies= are somewhat definite associations of the -vegetation of an area marked by physiographic conditions. A single -plant society is nearly if not altogether identical with a “_principal -formation_,” but is a more popular expression, and besides includes all -the plants growing on the area, while in the use of the term “principal -formation” we have reference mainly to the dominant plants and the most -conspicuous subordinate species. - -=917. Complex character of plant societies.=—In their broadest -analysis all plant societies are complex. Every plant society has one -or several dominant species, the individuals of which, because of -their number and size, give it its peculiar character. The society may -be so nearly pure that it appears to consist of the individuals of a -single species. But even in those cases there are small and conspicuous -plants of other species which occupy spaces between the dominant ones. -Usually there are several or more kinds in the same society. The larger -individuals come into competition for first place in regard to ground -and light, the smaller ones come into competition for the intervening -spaces for shade, and so on down in the scale of size and shade -tolerance. Then climbing plants (lianas) and epiphytes (lichens, algæ, -mosses, ferns, tree orchids, etc.) gain access to light and support by -growing on other larger and stouter members of the society. - -Parasites (dodder, mistletoes, rusts, smuts, mildews, bacteria, etc.) -are present, either actually or potentially, in all societies, and in -their methods of obtaining food sap the life and health of their hosts. -Then come the scavenger members, whose work it is to clean house, as it -were, the great army of saprophytic fungi (molds, mushrooms, etc.), and -bacteria ready to lay hold on dead and dying leaves, branches, trunks, -roots, etc., disintegrate them, and reduce them to humus, where other -fungi change them into a form in which the larger members of the plant -society can utilize them as plant food and thus continue the cycle of -matter through life, death, decay, and into life again. Mycorhiza (see -Chapter IX) or other forms of mutualistic symbiosis occur which make -atmospheric nitrogen available for food, or shorten the path from humus -to available food, or the humus plants feed on the humus directly. -Nor should we leave out of account the myriads of nitrate and nitrite -bacteria (see Chapter IX) which make certain substances in the soil -available to the higher members of the society. Most plant societies -are also benefited or profoundly influenced in other ways by animals, -as the flower-visiting insects, birds which feed on injurious insects, -the worms which mellow up the soil and cover dead organic matter so -that it may more thoroughly decay. In short, every plant society is -a great cosmos like the universe itself of which it is a part, where -multitudinous forms, processes, influences, evolutions, degenerations, -and regenerations are at work. - -=918. Forest Societies.=[52]—Each different climatic belt or -region has its characteristic forest. For example, the forests of -the Hudsonian zone in North America are different from those of -the Canadian zone, and these in turn different from those in the -transition zone (mainly in northern United States). The forests of -the Rocky mountains and of the Pacific coast differ from those of the -Alleghanian, Carolinian (mainly middle United States) or Austroriparian -(southern United States) areas. Finally, tropical forests are -strikingly different from those of other regions. Similar variations -occur in the forests of other regions of the globe. The character of -these forests depends largely on climatic factors. The character of -the forest varies, however, even in the same climatic area, dependent -on soil conditions, or success in seeding and ground-gaining of the -different species in competition, etc. - -=919. General structure of the forest.=—Structurally the forest -possesses three subdivisions: the floor, the canopy, and the interior. -The floor is the surface soil, which holds the rootage of the trees, -with its covering of leaf-mold and carpet of leaves, mosses, or other -low, more or less compact vegetation. The canopy is formed by the -spreading foliage of the tree crowns, which, in a forest of an even -and regular stand, meet and form a continuous mass of foliage through -which some light filters down into the interior. Where the stand is -irregular, i.e., the trees of different heights, the canopy is said to -be “compound” or “storied.” Where it is uneven, there are open places -in the canopy which admit more light, in which case the undergrowth -may be different. The interior of the forest lies between the canopy -and the floor. It provides for aeration of the floor and interior -occupants, and also room for the boles or tree trunks (called by -foresters the wood mass of the forest) which support the canopy and -provide the channels for communication and food exchange between the -floor and canopy. The canopy manufactures the carbohydrate food and -assimilates the mineral and proteid substances absorbed by the roots in -the soil; and also gets rid of the surplus water needed for conveying -food materials from the floor to the place where they are elaborated. -It is the seat where energy is created for work, and also the place for -seed production. - -[Illustration: Fig. 489. Mature forest of redwood (Sequoia -sempervirens). (Bureau of Forestry, U. S. Dept. Agr., Bull. 38.)] - -=920. Longevity of the forest.=—The forest is capable of -self-perpetuation, and, except in case of unusual disaster or the -action of man, it should live indefinitely. As the old trees die they -are gradually replaced by younger ones. So while trees may come and -trees may go, the forest goes on forever. - -=921. Autumn colors.=—One of the striking effects produced by -the deciduous forests is that of the autumn coloring of the leaves. -It is more pronounced in the forests of the United States than in -corresponding life zones in the eastern hemisphere because of the -greater number of species. With the disintegration of the chlorophyll -bodies, other colors, which in some cases were masked by the green, -appear. In other cases decomposition products result in the formation -of other colors, as red, scarlet, yellow, brown, purple, maroon, etc., -in different species. These coloring substances to some extent are -believed to protect the nitrogenous substances in the leaf from injury. -The colors absorb the sun’s rays, which otherwise might destroy these -nitrogenous substances before they have passed back through the petiole -of the leaf into the stem, where they may be stored for food. The -gorgeous display of color, then, which the leaves of many trees and -shrubs put on is one of the many useful adaptations of the plants. - -=922. Importance of the forest in the disposal of rainfall.=—The -importance of the forest in disposing of the rainfall is very great. -The great accumulation of humus on the forest floor holds back the -water both by absorption and by checking its flow, so that it does not -immediately flow quickly off the slopes into the drainage system of the -valley. It percolates into the soil. Much of it is held in the humus -and soil. What is not retained thus filters slowly through the soil -and is doled out more gradually into the valley streams and mountain -tributaries, so that the flood period is extended, and its injury -lessened or entirely prevented, because the body of water moving at any -one time is not dangerously high. The winter snow is shaded and in the -spring melts slowly, and the spring freshets are thus lessened. The -action of the leaves and humus in retarding the flow of the water -prevents the washing away of the soil; the roots of trees bind the soil -also and assist in holding it. - -=923. Absence of forest encourages serious floods.=—The great -floods of the Mississippi and its tributaries are due to the rapidity -with which heavy rainfall flows from the rolling prairies of the west, -and from the deforested areas west of the Alleghany system. The serious -floods in recent years in some of the South Atlantic States are in -part due to the increasing area of deforestation in the Blue Ridge and -southern Alleghany system. - -=924. The prairie and plains societies.=—These are to be found -in the grassland formation. In the prairies “meadows” are formed in -the lower ground near river courses where there is greater moisture -in soil. The grasses here are principally “sod-formers” which have -creeping underground stems which mat together, forming a dense sod. On -the higher and drier ground the “bunch” grasses, like buffalo-grass, -beard-grass, or broom-sedge, etc., are dominant, and in the drier -regions as one approaches desert conditions the vegetation gradually -takes on more the character of the desert, so that in the plains -sage-brush, the prickly-pear cactus, etc., occur. Besides the dominant -vegetation of the society there are subordinate species, and the -societies are especially marked by a spring and autumn flora of -conspicuous flowering plants which are mixed with the grasses. - -=925. Desert societies.=—These are composed of plants which -possess a form or structure which enables them to exist in a very -dry climate where the air is very dry and the soil contains but -little moisture. The true desert plants are perennial. The growth and -flowering period occurs during the rainy season, or those portions -of the rainy season when the temperature is favorable, and they rest -during the very dry season and cold. Characteristic desert plants are -the cacti with thick succulent green stems or massive trunks, the -leaves being absent or reduced to mere spines which no longer function -in photosynthesis; yuccas with thick, narrow and long leaves with a -firm and thick cuticle; small shrubs or herbs with compact rounded -habit and small thick gray leaves. All of these structures conserve -moisture. The mesquite tree is one of the common trees in portions -of the Sonora Nevada desert. Besides the true desert plants, desert -societies have a rainy-season flora consisting of annuals, which can -germinate, vegetate, flower, and seed during the period of rain and -before the ground moisture has largely disappeared, and these pass the -resting period in seed. - -[Illustration: Fig. 490. Desert vegetation, Arizona, showing large -succulent trunks of cactus with shrubs and stunted trees. Open -formation. (Photograph by Tuomey.)] - -[Illustration: Fig. 491. Polar tundra with scattered flowers, Alaska. -(Copyright by E. H. Harriman.)] - -[Illustration: Fig. 492. Perennial rosette plant from alpine flora of -the Andes, showing short stem, rosette of leaves, and large flower. -(After Schimper.)] - -=926. Arctic-alpine societies.=—The most striking of the arctic -plant societies are the “polar tundra,” extensive mats of vegetation -largely made up of mosses, lichens, etc., only partially decayed -because of the great cold of the subsoil, and perhaps also because -of humus acid in the partially decayed vegetation. These tundras -are brightened by numerous flowering plants which are characterized -by short stems, a rosette of leaves near the ground, and by large -bright-colored flowers. Heaths, saxifrages, and dwarf willow abound. -Alpine plant societies are similar to the arctic, although some of the -conditions are more severe than in the arctic region. This is -principally due to the fact that during the summer while the plants are -growing they are subject to a high temperature during the day and a -very low temperature at night, whereas during the summer in arctic -regions while the plants are growing there is continuous warmth for -growth and continuous light for photosynthesis. Five types of alpine -plants are recognized by some. 1st. _Elfin tree._ This type has short, -gnarled, often horizontal stems, as seen in pines, birches, and other -trees growing in alpine heights. 2d. _The alpine shrubs._ In the -highest alpine belts they are dwarfed and creeping, richly branched and -spreading close to the ground, while at lower belts they are more like -lowland shrubs. 3d. _The cushion type._ The branching is very profuse -and the branches are short and touch each other on all sides, forming -compact masses (examples saxifrages, androsace, mosses, etc.). 4th. -_Rosette plants._ These are perennial, short stems and very strong -roots, and play an important part in the alpine meadows. 5th. _Alpine -grasses._ These usually have much shorter leaves than grasses of the -lowlands and consequently form a low sward. - -=927. Edaphic plant societies.=—These are equivalent to edaphic -plant formations, and the vegetation is of course controlled by the -peculiar conditions of the soil. There are a number of different -kinds of edaphic plant societies determined by the character of the -physiographic areas. 1st. _Sphagnum moors._ These are formed in shallow -basins originally with more or less water. The growth of the sphagnum -moss along with other vegetation and its partial decay in the water -builds up ground rapidly so that in course of time the pond may be -completely filled in. This filling in proceeds from the shore toward -the center, and in the early stages of course there would be a pond -in the center. The partial decay of vegetation creates an excess of -humus acid which retards absorption by the roots. The conditions are -such, then, as require aerial structures for retarding the loss of -water, and plants growing in such moors are usually xerophytes. Some of -the plants are identical with those growing in the arctic tundra. 2d. -_Sand_[53] _strand of beach._ The quantity of sand with very little or -no admixture of humus or plant food makes it difficult for plants to -obtain a sufficient amount of water even where rainfall is abundant. -The same may be said of the sand dunes farther back from the shore. The -plants of these areas are then usually xerophytes. Some of the plants -accustomed to growing in such localities are American sea-rocket, -seaside spurge, bugseed, sea-blite, sea-purslane, the sandcherry, dwarf -willow, marram-grass, certain species of beard-grass, etc. 3d. _Rocky -shores or areas._ Here lichens and mosses first grow, later to be -followed by herbs, grasses, shrubs, and trees, as decayed plant remains -accumulate in the rock crevices. 4th. _Shores of ponds, or swamp -moors._ Here the vegetation often takes on a zonal arrangement if the -ground gradually slopes to the shore and out into the pond. In Fig. 493 -is shown zonal distribution of plants. The different kinds of plants -are drawn into these zones by the varying amount of ground water in -the soil, or the varying depth of the water on the margin of the pond -as one proceeds from the land towards the deeper water. On the border -lines or tension lines between the different zones the plants are -struggling to occupy here ground which is suitable for each adjacent -individual formation. Other edaphic societies are those of marl ponds, -alkaline areas, oases in deserts, warm oases in arctic lands, the -forested areas along river bottoms in prairie or plains regions, etc. - -[Illustration: Fig. 493. Macrophytes in the upper zone of the photic -region. Ascophyllum and Fucus at low tide, Hunter’s Island, New York -City. (Photograph by M. A. Howe.)] - -[Illustration: Fig. 494. - -Zonal distribution of plants, South Shore, Cayuga Lake.] - -=928. Aquatic plant societies.=—In general we might distinguish -three kinds, 1st. _Fresh-water plant societies_, with floating algæ -like spirogyra, œdogonium, etc., the floating duck-meats, riccias; the -plants of the lily type with roots and stems attached to the bottom -and leaves floating on the surface, like the water-lily and certain -pondweeds, and finally the completely submerged ones like certain -pondweeds, the bassweed (Chara), etc. 2d. _Marine plant societies_, -which are made up mostly of the red and brown algæ or “seaweeds,” -though some green algæ and flowering plants also occur. 3d. _The salt -marshes_ where the water is brackish and there is usually a luxuriant -growth of marsh-grasses. - -FOOTNOTES: - -[47] For a fuller discussion of this subject by the author see Chapters -XLVI-LVII of his “College Text-book of Botany” (Henry Holt & Co.). - -[48] =οῖκος= = house, and =λόγος= = discourse. - -[49] Term used by Schimper. - -[50] See the author’s “College Text-book of Botany.” Chapter XLIX. - -[51] =ἔδαφος= = ground. - -[52] For a full discussion of forest societies see Chapter L in the -author’s “College Text-book of Botany.” - -[53] See Chapter LIV of the author’s “College Text-book of Botany.” - - - - -CHAPTER XLVII. - -CLASSIFICATION OF THE ANGIOSPERMS. - - -Relation of Species, Genus, Family, Order, etc. - -=929. Species.=—It is not necessary for one to be a botanist in -order to recognize, during a stroll in the woods where the trillium -is flowering, that there are many individual plants very like each -other. They may vary in size, and the parts may differ a little in -form. When the flowers first open they are usually white, and in age -they generally become pinkish. In some individuals they are pinkish -when they first open. Even with these variations, which are trifling -in comparison with the points of close agreement, we recognize the -individuals to be of the _same kind_, just as we recognize the corn -plants, grown from the seed of an ear of corn, as of the same kind. -Individuals of the same kind, in this sense, form a _species_. The -white wake-robin, then, is a species. - -But there are other trilliums which differ greatly from this one. The -purple trillium (T. erectum) shown in fig. 495 is very different from -it. So are a number of others. But the purple trillium is a species. It -is made up of individuals variable, yet very like one another, more so -than any one of them is like the white wake-robin. - -=930. Genus.=—Yet if we study all parts of the plant, the -perennial rootstock, the annual shoot, and the parts of the flower, we -find a great resemblance. In this respect we find that there are -several species which possess the same general characters. In other -words, there is a relationship between these different species, a -relationship which includes more than the individuals of one kind. It -includes several kinds. Obviously, then, this is a relationship with -broader limits, and of a higher grade, than that of the individuals -of a species. The grade next higher than species we call _genus_. -Trillium, then, is a genus. Briefly the characters of the genus -trillium are as follows: - -[Illustration: Fig. 495. Trillium erectum (purple form), two plants -from one rootstock.] - -=931. Genus trillium.=—Perianth of six parts: sepals 3, -herbaceous, persistent; petals colored. Stamens 6 (in two whorls), -anthers opening inward. Ovary 3-loculed, 3-6-angled; stigmas 3, -slender, spreading. Herbs with a stout perennial rootstock, with -fleshy, scale-like leaves, from which the low annual shoot arises, -bearing a terminal flower and 3 large netted-veined leaves in a whorl. - - _Note._—In speaking of the genus the present usage - is to say trillium, but two words are usually employed - in speaking of the species, as Trillium grandiflorum, - T. erectum, etc. - -=932. Genus erythronium.=—The yellow adder-tongue, or dogtooth -violet (Erythronium americanum), shown in fig. 496, is quite different -from any species of trillium. It differs more from any of the species -of trillium than they do from each other. The perianth is of six parts, -light yellow, often spotted near the base. Stamens are 6. The ovary is -obovate, tapering at the base, 3-valved, seeds rather numerous, and the -style is elongated. The flower stem, or scape, arises from a scaly bulb -deep in the soil, and is sheathed by two elliptical-lanceolate, mottled -leaves. The smaller plants have no flower and but one leaf, while the -bulb is nearer the surface. Each year new bulbs are formed at the end -of runners from a parent bulb. These runners penetrate each year deeper -into the soil. The deeper bulbs bear the flower stems. - -=933. Genus lilium.=—While the lily differs from either the -trillium or erythronium, yet we recognize a relationship when we -compare the perianth of six colored parts, the 6 stamens, and the -3-sided and long 3-loculed ovary. - -[Illustration: Fig. 496. Adder-tongue (erythronium). At left below -pistil, and three stamens opposite three parts of the perianth. Bulb at -the right.] - -=934. Family Liliaceæ.=—The relationship between genera, as -between trillium, erythronium, and lilium, brings us to a still higher -order of relationship, where the limits are broader than in the genus. -Genera which are thus related make up the _family_. In the case of -these genera the family has been named after the lily, and is the lily -family, or _Liliaceæ_. - -=935. Order, class, group.=—In like manner the lily family, the -iris family, the amaryllis family, and others which show characters of -close relationship are united into an _order_ which has broader limits -than the family. This order is the lily order, or order _Liliales_. The -various orders unite to make up the _class_, and the classes unite to -form a _group_. - -=936. Variations in usage of the terms class, order, etc.=—Thus, -according to the system of classification adopted by some, the -angiosperms form a _group_. The group angiosperms is then divided into -two _classes_, the _monocotyledones_ and _dicotyledones_. (It should -be remembered that all systematists do not agree in assigning the -same grade and limits to the classes, subclasses, etc. For example, -some treat of the angiosperms as a class, and the monocotyledons -and dicotyledons as subclasses; while others would divide the -monocotyledons and dicotyledons into classes, instead of treating each -one as a class or as a subclass. Systematists differ also in usage as -to the termination of the ordinal name; for example, some use the word -_Liliales_ for _Liliifloræ_, in writing of the order.) - -[Illustration: Fig. 497. - -_A._ Cross-section of the stem of an oak tree thirty-seven years old, -showing the annual rings. _rm_, the medullary rays; _m_, the pith -(medulla). _B._ Cross-section of the stem of a palm tree, showing the -scattered bundles.] - -=937. Monocotyledones.=—In the monocotyledons there is a single -cotyledon on the embryo; the leaves are parallel-veined; the parts -of the flower are usually in threes; endosperm is usually present in -the seed; the vascular bundles are usually closed, and are scattered -irregularly through the stem as shown by a cross-section of the stem -of a palm (fig. 497), or by the arrangement of the bundles in the corn -stem (fig. 57). Thus a single character is not sufficient to show -relationship in the class (nor is it in orders, nor in many of the -lower grades), but one must use the sum of several important characters. - -=938. Dicotyledones.=—In the dicotyledons there are two -cotyledons on the embryo; the venation of the leaves is reticulate; -the endosperm is usually absent in the seed; the parts of the flower -are frequently in fives; the vascular bundles of the stem are -generally open and arranged in rings around the stem, as shown in the -cross-section of the oak (fig. 497). There are exceptions to all the -above characters, and the sum of the characters must be considered, -just as in the case of the monocotyledons. - -=939. Taxonomy.=—This grouping of plants into species, genera, -families, etc., according to characters and relationships is -_classification_, or _taxonomy_. - -To take Trillium grandiflorum for example, its position in the system, -if all the principal subdivisions should be included in the outline, -would be indicated as follows: - - Group, Angiosperms. - Class, Monocotyledones. - Order, Liliales. - Family, Liliaceæ. - Genus, Trillium. - Species, grandiflorum. - -In the same way the position of the toothwort would be indicated as -follows: - - Group, Angiosperms. - Class, Dicotyledones. - Order, Papaverales. - Family, Cruciferæ. - Genus, Dentaria. - Species, diphylla. - -But in giving the technical name of the plant only two of these names -are used, the genus and species, so that for the toothwort we say -_Dentaria diphylla_, and for the white wake-robin we say _Trillium -grandiflorum_. - -=940. Kingdom and Subkingdom.=—Organic beings form altogether two -kingdoms, the Animal Kingdom and the Plant Kingdom. The Plant Kingdom -is then divided into a number of subkingdoms as follows: 1st, -Subkingdom Thallophyta, the thallus plants, including the Algæ and -Fungi; 2d, Subkingdom Bryophyta, the moss-like plants, including the -Liverworts and Mosses; 3d, Subkingdom Pteridophyta, the fern-like -plants, including Ferns, Lycopods, Equisetum, Isoetes, etc.; 4th, -Subkingdom Spermatophyta, the seed plants, including Gymnosperms and -Angiosperms. Subkingdoms are divided into groups of lower order down to -the classes. So there are subclasses, subfamilies or tribes, subgenera, -and even subspecies. But taking the principal taxonomic divisions from -the greater to the lesser rank, the order would be as follows: - - Plant Kingdom. - Subkingdom, Spermatophyta. - Group (not used in a definite sense). - Class, Gymnospermæ. - Order, Pinales. - Family, Pinaceæ. - Genus, Pinus. - Species, strobus, or, in full, - Pinus strobus, the white pine. - - -Group Angiospermæ. - - -I. CLASS MONOCOTYLEDONES. - -=941. Order Pandanales.=—Aquatic or marsh plants. The cattail -flags (Typha) and the bur-reeds (Sparganium), each representing a -family. The name of the order is taken from the tropical genus Pandanus -(the screw-pine often grown in greenhouses). - -=942. Order Naiadales.=—Aquatic or marsh herbs. Three families -are mentioned here. - -The pondweed family (Naiadaceæ), named after one genus, Naias. The -largest genus is Potamogeton, the species of which are known as -pondweeds. Ruppia occidentalis occurs in saline ponds in Nebraska, and -R. maritima along the seacoast and in saline districts in the interior. - -The water-plantain family (Alismaceæ) includes the water-plantain -(Alisma) and the arrow-leaves (Sagittaria). - -The tape-grass family (Vallisneriaceæ) includes the tape-grass, or -eel-grass (the curious Vallisneria spiralis). - -=943. Order Graminales.=—Two families. - -The grass family (Gramineæ), the grasses and grains. - -The sedge family (Cyperaceæ), the sedges. - -=944. Order Palmales=, with one family, Palmaceæ, includes the -palms, abundant in the tropics and extending into Florida. Cultivated -in greenhouses. - -=945. Order Arales.= - -The arum family (Araceæ). Flowers in a fleshy spadix. Examples: Indian -turnip (Arisæma), sweet-flag (Acorus), skunk-cabbage (Spathyema). - -The duckweed family (Lemnaceæ). (Examples: Lemna, Spirodela, Wolffia. -See paragraphs 51-53.) - -=946. Order Xyridales=, from the genus Xyris, the yellow-eyed -grass family (Xyridaceæ). Species mostly tropical, but a few in North -America. Other examples are the pipewort family (Eriocaulaceæ, example, -Eriocaulon septangulare), the pineapple family (Bromeliaceæ, example, -the pineapple cultivated in Florida); the Florida moss or hanging moss -(Tillandsia usneoides); the spiderwort family (Commelinaceæ), including -the spiderwort (Tradescantia, several species in North America); the -pickerel-weed family (Pontederiaceæ), including the genus Pontederia in -borders of ponds and streams. - -=947. Order Liliales.=—Some of the families are as follows: - -The rush family (Juncaceæ, example, Juncus), with many species, plants -of usually swamp habit. - -The lily family (Liliaceæ, examples: Lilium, Allium = Onion, -Erythronium, Yucca). - -The iris family (Iridaceæ, examples: Iris, the blue-flag, fleur-de-lis, -etc.). - -The lily-of-the-valley family (Convallariaceæ, examples: -lily-of-the-valley, Trillium, etc.) - -The amaryllis family (Amaryllidaceæ, examples: Narcissus, the daffodil; -Cooperia, in southwestern United States). - -=948. Order Scitaminales.=—This order includes the large showy -cultivated Canna of the canna family. - -=949. Order Orchidales.= Example, the orchid family (Orchidaceæ) -with Cypripedium, Orchis, etc. - - -II. CLASS DICOTYLEDONES. - -SERIES 1. CHORIPETALÆ. Petals wanting (Apetalæ, or -Archichlamydæ of some authors), or present and distinct from one -another (Polypetalæ, or Metachlamydæ). - -=950. Order Casuarinales=, confined to tropical seacoasts -(example, Casuarina). - -=951. Order Piperales= includes the lizard’s-tail family -(Saururaceæ), Saururus cernuus, lizard’s-tail, in the eastern United -States. - -=952. Order Salicales.=—Shrubs or trees, flowers in aments. -Includes the willows and poplars (Salix and Populus of the willow -family, Salicaceæ). - -=953. Order Myricales.=—Shrubs or small trees. Includes the -sweet-gale (Myrica gale) in wet places in northern United States and -British North America, Myrica cerifera forming thickets on sand dunes -along the Atlantic coast, and the sweet-fern (Comptonia peregrina = C. -asplenifolia) in the eastern United States in dry soil of hillsides. - -=954. Order Leitneriales.=—Shrubs or trees. Includes the -cork-wood, Leitneria floridana (Leitneriaceæ). - -=955. Order Juglandales.=—Trees, staminate flowers in aments. The -walnut family (Juglandaceæ, examples: walnut, butternut, etc. Juglans; -hickory, Hicoria = Carya). - -=956. Order Fagales.=—Trees and shrubs. Flowers in aments, or the -pistillate ones with an involucre which forms a cup in fruit, as in the -acorn of the oak. - -The birch family (Betulaceæ, examples: Betula, birch; Corylus, -hazelnut; Alnus, alder, etc.). - -The beech family (Fagaceæ = Cupuliferæ, examples: Fagus, beech; -Castanea, chestnut; Quercus, oak). - -=957. Order Urticales.=—Trees, shrubs, or herbs. Examples: the -elm family (Ulmaceæ), the mulberry family (Moraceæ), and the nettle -family (Urticaceæ). - -=958. Order Santalales=, herbs or shrubs, mostly parasitic. - -The mistletoe family (Loranthaceæ), with the American mistletoe -(Phoradendron flavescens), parasitic on deciduous trees in the South -Atlantic, Central, and Gulf States (N. J. to Ind. Ter.). - -The sandalwood family (Santalaceæ, example, the bastard toad-flax, -Comandra umbellata), widely distributed in North America. - -=959. Order Aristolochiales.=—Herbs or vines with heart-shaped or -kidney-shaped leaves. The birthwort family (Aristolochiaceæ, example, -Aristolochia serpentaria, the Virginia snake-root, eastern United -States; wild ginger, or heart-leaf, Asarum canadense, eastern North -America.) - -=960. Order Polygonales.=—Examples: the buckwheat family -(Polygonaceæ), including buckwheat (Fagopyrum), and numerous species -of Polygonum, known as smartweed, water-pepper, tear-thumb, bindweed, -knotweed, prince’s-feather, etc. - -=961. Order Chenopodiales.=—Herbs. There are several families; -one of the largest is the goosefoot family (Chenopodiaceæ). The genus -Chenopodium includes many species, known as goosefoot, lamb’s-quarters, -etc. Here belong also the Russian thistle (Salsola tragus) and the -saltwort (S. kali). The former is sometimes a troublesome weed in the -central and western United States, naturalized from Europe. The latter -occurs along the Atlantic coast on seabeaches. Atriplex occurs in salty -or alkaline soil, also the glasswort (Salicornia herbacea), the bugseed -(Corispermum). The pokeweed family (Phytolaccaceæ), the Amaranth family -(Amaranthaceæ), the purslane family (Portulacaceæ, including the -purslane or “pursley,” Portulaca oleracea, and the spring-beauty, -Claytonia virginica), and the pink family (Caryophyllaceæ), belong here. - -=962. Order Ranales.=—Herbs, shrubs, or trees. Examples are: - -The water-lily family (Nymphæaceæ), with the yellow water-lily (Nymphæa -advena = Nuphar advena) and the white water-lily (Castalia odorata = -Nymphæa odorata). - -The magnolia family (Magnoliaceæ), including the magnolias -(Magnolia) and the tulip-tree (Liriodendron). The crowfoot family -(Ranunculaceæ), with the buttercups, hepatica, clematis, etc. - -=963. Order Papaverales.=—Mostly herbs. Examples are: - -The poppy family (Papaveraceæ), including the opium or garden poppy -(Papaver somniferum), the blood-root (Sanguinaria canadensis), the -Dutchman’s-breeches (Bicuculla cucullaria = Dicentra cucullaria), -squirrel’s-corn (Bicuculla canadensis = D. canadensis). - -The mustard family (Cruciferæ), including the toothwort (Dentaria), -shepherd’s-purse (Bursa bursa-pastoris = Capsella bursa-pastoris), the -cabbage, turnip, etc. - -=964. Order Sarraceniales.=—Insectivorous plants. - -The pitcher-plant family (Sarraceniaceæ). Examples: Sarracenia -purpurea, the pitcher-plant, in peat-bogs, northern and eastern North -America. - -The sundew family (Droseraceæ). Examples: Drosera rotundifolia, and -other sundews. - -=965. Order Rosales.=—Herbs, shrubs or trees. Seventeen families -are given in the eastern United States. Examples: - -The riverweed family (Podostemaceæ), containing the riverweed -(Podostemon). - -The saxifrage family (Saxifragaceæ), containing a number of species. -Example, Saxifraga virginiensis. - -The gooseberry family (Grossulariaceæ), including the wild and the -cultivated gooseberry. - -The witch-hazel family (Hamamelidaceæ), including the witch-hazel -(Hamamelis), in eastern North America, and the sweet gum (Liquidambar -styraciflua). - -The plane-tree family (Platanaceæ), with the plane-tree, or buttonwood -(Platanus occidentalis), eastern North America. (Other species occur in -western United States.) - -The rose family (Rosaceæ), including roses, spiræas, raspberries, -strawberries, the shrubby cinquefoil (Dasiphora fruticosa), etc. - -The apple family (Pomaceæ), including the apple, mountain-ash, pear, -June-berry (or shadbush, also service-berry), the hawthorns (Cratægus). - -The plum family (Drupaceæ), including the cherries, plums, peaches, etc. - -The pea family (Papilionaceæ), including the pea, bean, clover, vetch, -lupine, etc., a very large family. - -=966. Order Geraniales.=—Herbs, shrubs, or trees. Nine families -in the eastern United States. Examples: - -The geranium family (Geraniaceæ), with the cranesbill (Geranium -maculatum) and others. - -The wood-sorrel family (Oxalidaceæ), with the wood-sorrel (Oxalis -acetosella) and others. - -The flax family (Linaceæ). Example, flax (Linum vulgaris). - -The spurge family (Euphorbiaceæ). Plants with a milky juice, and -curious, degenerate flowers. Examples: the castor-oil plant (Ricinus), -the spurges (many species of Euphorbia). - -=967. Order Sapindales.=—Mostly trees or shrubs. Twelve families -in the eastern United States. Example: - -The sumac family (Anacardiaceæ), containing the sumacs in the genus -Rhus. Examples: the poison-ivy (R. radicans), a climbing vine, in -thickets and along fences, in eastern United States. Sometimes -trained over porches. The poison-oak (R. toxicodendron), a low shrub. -Poison-sumac or poison-alder (R. vernix = R. venenata), sometimes -called “thunderwood,” or dogwood, is a large shrub or small tree, very -poisonous. The smoke-tree (Cotinus cotinoides) belongs to the same -family, and is often planted as an ornamental tree. The maple family -(Aceraceæ), including the maples (Acer). - -The buckeye family (Hippocastanaceæ), including the horse-chestnut -(Æsculus hippocastanum), much planted as a shade tree along streets. -Also there are several species of buckeye in the same genus. - -The jewelweed family (Balsaminaceæ), including the touch-me-not -(Impatiens biflora and aurea) in moist places. The garden balsam (Imp. -balsamea) also belongs here. - -=968. Order Rhamnales.=—Shrubs, vines, or small trees. There are -two families, the buckthorn (Rhamnaceæ), the grape family (Vitaceæ), -including the grapes (Vitis), the American ivy (Parthenocissus -quinquefolia = Ampelopsis quinquefolia), in woods and thickets, eastern -North America, and much planted as a trailer over porches. The Japanese -ivy (P. tricuspidata = A. veitchii) used as a trailer on the sides of -buildings belongs here. - -=969. Order Malvales.=—Herbs, shrubs, or trees. - -The linden family (Tiliaceæ). Example, the basswood or American linden -(Tilia americana.) - -The mallow family (Malvaceæ), including the hollyhock, the mallows, -rose of Sharon (Hibiscus), etc. - -=970. Order Parietales=, with seven families in the eastern United -States. The St. John’s wort (Hypericum) and the violets each represent -a family. The violets (Violaceæ) are well-known flowers. - -=971. Order Opuntiales.=—These include the cacti (Cactaceæ), -chiefly growing in the dry or desert regions of America. - -=972. Order Thymeleales=, with two families and few species. - -=973. Order Myrtales.=—Land, marsh, or aquatic plants. The most -conspicuous are in the evening primrose family (Onagraceæ), including -the fireweeds, or willow herbs (Epilobium), and the evening primrose -(Onagra biennis = Œnothera biennis). - -=974. Order Umbellales.=—Herbs, shrubs, or trees, flowers in -umbels. - -The ginseng family (Araliaceæ). This includes the spikenards and -sarsaparillas in the genus Aralia, and the ginseng (or “sang”), Panax -quinquefolium. - -The carrot family (Umbelliferæ). This family includes the wild carrot -(Daucus carota), the poison-hemlock (Cicuta), the cultivated carrot and -parsnip, and a large number of other genera and species. - -The dogwood family (Cornaceæ). The flowering dogwood (Cornus florida), -abundant in eastern North America, is an example. - -SERIES 2. GAMOPETALÆ (= Sympetalæ or Metachlamydæ). Petals -partly or wholly united, rarely separate or wanting. - -=975. Order Ericales.=—There are six families in eastern United -States. Examples: - -The wintergreen family (Pyrolaceæ), including the shin-leaf (Pyrola -elliptica). - -The Indian-pipe family (Monotropaceæ), with the Indian-pipe (Monotropa -uniflora) and other humus saprophytes. (See paragraphs 182-191.) - -The heath family (Ericaceæ). Examples: Labrador tea (Ledum), in bogs -and swamps in northern North America. The azaleas, with several -species widely distributed, are beautiful flowering shrubs, and many -varieties are cultivated. The rhododendrons are larger with larger -flower clusters, also beautiful flowering shrubs. R. maximum in the -Alleghany Mountains and vicinity, from Nova Scotia to Ohio and Georgia. -R. catawbiense, usually at somewhat higher elevations, Virginia to -Georgia. The mountain laurel (Kalmia latifolia) and other species rival -the rhododendrons and azaleas in beauty. The trailing arbutus (Epigæa -repens) in sandy or rocky woods is a well-known small trailing shrub -in eastern North America. The sourwood (Oxydendrum arboreum) is a tree -with white racemes of flowers in August, and scarlet leaves in autumn. -The spring or creeping wintergreen (Gaultheria procumbens) is a small -shrub with aromatic leaves, and bright red spicy berries. - -The huckleberry family (Vaccinaceæ) includes the huckleberries -(example, Gaylussacia resinosa, the black or high-bush huckleberry, -eastern United States), the mountain cranberry (Vitis-Idæa vitisidæa -= Vaccinium vitisidæa) in the northern hemisphere; the bilberries and -blueberries (of genus Vaccinium); the cranberries (examples: the large -American cranberry, Oxycoccus macrocarpus and the European cranberry, -Oxycoccus oxycoccus, in cold bogs of northern North America, the latter -also in Europe and Asia). - -=976. Order Primulales.=—Two families here. The primrose family -(Primulaceæ) contains the loosestrifes (Steironema), star-flower -(Trientalis), etc. - -=977. Order Ebenales.=—Of the four families, the ebony family -(Ebenaceæ) contains the well-known persimmon (Diospyros virginiana) and -the storax family (Styracaceæ) with the silverbell, or snowdrop tree -(Mohrodendron carolinum). - -=978. Order Gentianales.=—Herbs, shrubs, vines, or trees. Six -families in the United States. - -The olive family (Oleaceæ) includes the common lilac (Syringa), the ash -trees (Fraxinus), the privet (Ligustrum). - -The gentian family (Gentianaceæ) among other genera includes the -gentians (Gentiana). - -The milkweed family (Asclepiadaceæ) contains plants mostly with a milky -juice. Asclepias with many species is one of the most prominent genera. - -=979. Order Polemoniales.=—Mostly herbs, rarely shrubs and trees. -Fifteen families in the eastern United States. - -The morning glory family (Convolvulaceæ) includes the bindweeds -(Convolvulus), the morning glory (Ipomæa), etc. - -The dodder family (Cuscutaceæ) includes the dodders, or “love-vines.” -There are nearly thirty species in the United States. The stems are -slender and twine around other plants upon which they are parasitic -(see paragraph 179). - -The phlox family (Polemoniaceæ). The most prominent genus is Phlox. -Over forty species occur in North America. - -The borage family (Boraginaceæ) includes the heliotrope (Heliotropium), -the hound’s-tongue (Cynoglossum), the forget-me-not (Myosotis), and -others. - -The vervain family (verbenaceæ) contains the verbenas. - -The mint family (Labiatæ) contains the mints (Mentha), skull-cap -(Scutellaria), dead-nettles (Lamium). - -The potato family (Solanaceæ) includes the ground-cherry (Physalis), -the nightshades (Solanum), the tomato (Lycopersicon), tobacco -(Nicotiana). - -The figwort family (Scrophulariaceæ) includes the common mullein -(Verbascum), the monkey-flower (Mimulus), the toad-flax (Linaria), -turtle’s-head (Chelone), and many other genera and species. - -The bladderwort family (Lentibulariaceæ) includes the curious bog or -aquatic plants with finely dissected leaves, and with bladders in which -insects are caught (Utricularia). - -The trumpet-creeper family (Bignoniaceæ) includes the trumpet-creeper -(Bignonia), the catalpa tree, and others. - -=980. Order Plantaginales= with one family (Plantaginaceæ) -includes the plantains (Plantago). - -=981. Order Rubiales= with three families is represented by the -madder family (Rubiaceæ) with the bluets (Houstonia), the button-bush -(Cephalanthus), the partridge-berry (Mitchella), the bedstraws -(Galium), etc. - -The honeysuckle family (Caprifoliaceæ) with the elder (Sambucus), the -arrowwoods and cranberry trees (Viburnum), the honeysuckles (Lonicera), -etc. - -=982. Order Valerianales= with two families includes the teasel -family (Dipsacaceæ). Example, Fuller’s teasel (Dipsacus). - -=983. Order Campanulales= with five families, the corolla usually -gamopetalous. - -The gourd family (Cucurbitaceæ) includes the pumpkin, squash, melon, -and a few feral species. Example, the star-cucumber (Sicyos angulatus), -in moist places in eastern and middle United States. - -The bell-flower family (Campanulaceæ) includes the hare-bells or -bell-flowers (Campanula), the lobelias (example, Lobelia cardinalis, -the cardinal-flower), etc. - -The chicory family (Cichoriaceæ) includes the chicory or succory -(Cichorium intybus, known also as blue-sailors), the oyster-plant or -salsify (Tragopogon porrifolius), the dandelion (Taraxacum taraxacum = -T. densleonis), the lettuce (Lactuca), the hawkweed (Hieraceum), and -others. - -The ragweed family (Ambrosiaceæ) includes the ragweeds (Ambrosia), the -cockle-bur (Xanthium), and others. - -The thistle family (Compositæ) includes the thistle (Carduus), asters -(Aster), goldenrods (Solidago), sunflowers (Helianthus), eupatoriums or -joepye-weeds, thoroughworts (Eupatorium), cone-flowers or black-eyed -Susans (Rudbeckia), tickseed (Coreopsis), bur-marigold or beggar-ticks -or devil’s-bootjack (Bidens), chrysanthemums, etc. - - - - -INDEX. - - - Absorption, 13, 22-28 - Aceraceæ, 497 - Acorn, 451 - Acorus, 493 - Æcidiomycetes, 218 - Æcidiospore, 189 - Æsculus hippocastanum, 498 - Agaricaceæ, 199, 219 - Agaricus arvensis, 206 - Agaricus campestris, 200-207 - Akene, 451 - Albumen, 98 - Albuminous, 98, 108 - Alder, 495 - Algæ, 136-176 - Algæ, absorption by, 22 - Alismaceæ, 493 - Alpine formation, 474 - Alpine plant societies, 483 - Amanita phalloides, 207, 208 - Amaranth, 495 - Amaryllidaceæ, 494 - Aments, 429 - American mistletoe, 495 - Ampelopsis, 498 - Ancylistales, 215 - Andreales, 249 - Andrœcium, 319, 419 - Anemophilous, 435 - Angiosperms, morphology of, 318-348; - classification, 487 - Antheridiophore, 227 - Antheridium, 144, 149, 155, 176, 223, 228, - 240, 245, 246, 266, 287, 433 - Anthesis, 429 - Anthoceros, 240, 241 - Anthocerotales, 242 - Anthocerotes, 242 - Apogamy, 346 - Apogeotropic (ap″o-ge″o-trop´ic), 126 - Apogeotropism (ap″o-ge-ot′ro-pism), 126 - Apple, 456, 497 - Apple family, 497 - Aquatic formations, 475 - Aquatic plant societies, 486 - Araceæ, 493 - Archegonia (ar-che-go′ni-a), 223, 229, 233, 241, 244-246, - 267, 288, 291, 307, 308 - Archegoniophore, 229 - Archegonium, 433 - Archesporium (ar″che-spo´ri-um), 235 - Archidiales, 249 - Arctic formation, 481 - Aril, 457 - Arisæma, 493 - Arisæma triphyllum, 442, 443 - Aristolochiales, 495 - Arrow leaf, 492 - Arum family, 493 - Asclepias, 500 - Asclepias cornuti, 462 - Ascomycetes (as-co-my-ce′tes), 195-198, 216-218 - Ascus, 190, 213 - Ash of plants, 79, 80 - Ash tree, 500 - Aspidium acrostichoides, 253, 257 - Assimilation, 67, 109 - Aster, 502 - Atriplex, 495 - Auriculariales, 218 - Autotrophic plants, 85 - Azalea, 499 - Azolla, 296 - - Bacteria, 164, 165 - Bacteria, nitrite and nitrate, 83 - Bacteriales, 164, 165 - Bacteroid, 93 - Bangiales, 175 - Basidiomycetes (ba-sid″i-o-my-ce′tes), 199-208, 218 - Basidium, 201, 213 - Bast, 50-52 - Batrachospermum, 171-173, 175 - Bazzania, 25 - Beard-grasses, 480 - Bedstraws, 501 - Beechnut, 452 - Beet, osmose in, 15, 16, 17, 18 - Begonia, 407 - Bellflower, 501 - Berry, 454, 455, 456 - Betulaceæ, 495 - Bicuculla, 496 - Bidens, 458 - Bignonia, 501 - Bilberries, 500 - Biotic factors, 466 - Birch, 495 - Bird’s-nest fungi, 220 - Blackberry, 454 - Black fungi, 198 - Bladderwort, 501 - Blasia, 164, 236 - Bloodroot, 496 - Bluets, 436, 437, 501 - Boletus, 209 - Boletus edulis, 209 - Boraginaceæ, 500 - Botrychium, 295 - Botrydiaceæ, 162 - Botrydium granulatum, 146, 162 - Broom-sedge, 480 - Brown algæ, 167-170 - Bryales, 349 - Buckeye family, 498 - Buckthorn, 498 - Buckwheat, 495 - Buds, winter condition of, 374-377 - Buffalo-grass, 480 - Bug seed, 495 - Bulb, 372 - Bunch-grasses, 480 - Butternut, 452, 494 - Buttonbush, 501 - Buttonwood, 497 - - Cacti, 395, 498 - Callithamnion, 173 - Calyptrogen, 361 - Cambium, 50, 52, 358, 363 - Campanula rotundifolia, 442, 444, 510 - Campanulales, 501 - Canna, 445-449, 494 - Capsella bursa-pastoris, 496 - Capsule, 453 - Carbohydrate, 71, 75, 80, 90 - Carbon dioxide, 62-67, 110-113 - Cardinal-flower, 501 - Carpogonium, 172, 176 - Carrot family, 499 - Caryophyllaceæ, 496 - Caryopsis, 451 - Cassia marilandica, 402 - Cassiope, 395 - Castalia odorata, 496 - Castor-oil plant, 497 - Catalpa, 501 - Catkin, 428 - Cattail-flag, 492 - Caulidium, 371 - Cedar apples, 194 - Cell, 3; - artificial 20 - Cell-sap, 3, 40 - Ceratopteris thalictroides, 296 - Chætophora, 151, 162 - Chætophoraceæ, 162 - Chara, 176 - Charales, 176 - Chemical condition of soil, 466 - Chemosynthetic assimilation, 109 - Chenopodiales, 495 - Chenopods, 495 - Chestnut, 452, 494 - Chicory family, 502 - Chlamydomonas, 159, 160 - Chlamydospores, 180 - Chloral hydrate, 65, 87 - Chlorophyceæ, 158 - Chlorophyll, 2, 67, 72 - Chloroplast, 68, 69, 71 - Christmas fern, 251-253 - Chromoplast, 71 - Chromosomes, 342-345 - Chroococcaceæe, 163 - Chrysanthemum, 502 - Chytridiales, 215 - Cichoriaceæ, 502 - Cichorium intybus, 502 - Clavaria botrytes, 212 - Clavariaceæ, 210, 219 - Claytonia virginica, 496 - Cleistogamous, 435 - Clematis virginiana, 462, 463, 496 - Climatic factors, 466 - Climatic formations, 470 - Clostridium pasteurianum, 93 - Clover, 497 - Club mosses, 284, 289 - Coccogonales, 163 - Cocklebur, 502 - Cold wastes, 474 - Coleochætaceæ, 162 - Coleochæte, 153-156, 226 - Collenchyma, 356, 363 - Comandra, 495 - Compass plants, 409 - Compositæ, 502 - Comptonia asplenifolia, 494 - Cone-fruit, 456 - Confervoideæ, 162 - Coniferæ, 316 - Conjugation, 137, 141, 160, 162, 179 - Convallariaceæ, 494 - Cooperia, 494 - Cordyceps, 218 - Coreopsis, 502 - Cork, 357, 363 - Corm, 373 - Cortex, 50 - Corymb, 427 - Cotyledon, 99-101 - Cranberry, 500 - Cratægus, 497 - Crowfoot family, 496 - Cruciferæ, 496 - Cryptonemiales, 175 - Cucurbitaceæ, 501 - Culture formations, 470, 475 - Cultures, water, 28, 29 - Cup fungi, 199 - Cupuliferæ, 495 - Cuscuta, 83, 500 - Cushion type of vegetation, 483 - Cuticle, 43 - Cyanophyceæ, 163 - Cyatheaceæ, 295 - Cycadales, 316 - Cycas, 311, 312, 457 - Cyclosis, 9, 10 - Cyclosporales, 171 - Cyme, 430, 432 - Cyperaceæ, 493 - Cypripedium, 443, 447, 494 - Cystocarp, 174 - Cystopteris bulbifera, 260 - Cystopus, 215 - Cytase, 92, 108 - Cytisus, 445 - Cytoplasm (cy′to-plasm), 5 - - Dacryomycetales, 219 - Dahlia, 108 - Dandelion, 502 - Dasiphora fruticosa, 497 - Daucus carota, 499 - Dehiscence, 453 - Dentaria, 322-324 - Dentaria diphylla, 496 - Dermatogen, 359 - Desert formation, 473 - Desert societies, 480 - Desmodium, 458 - Desmodium gyrans, 399 - Diadelphous (di″a-del′phous), 425 - Diageotropism (di″a-ge-ot′ro-pism), 126 - Diaheliotropic (di″a-he″li-o-trop′ic), 127 - Diaheliotropism (di″a-he″li-ot′ro-pism), 127 - Diastase, 77, 78, 108, 116 - Diatoms, 166 - Dichogamous (di-chog′a-mous), 437, 442 - Dicentra, 496 - Dicotyledons, 494 - Dictyophora, 219 - Diffusion, 13-20 - Digestion, 107, 108, 109 - Dimorphism of ferns, 273-280 - Diœcious, 435 - Dionæa muscipula, 133 - Dipodascus, 216 - Dipsacus, 501 - Discomycetes, 217 - Dodder, 83, 84, 500 - Dogwood, 499 - Dothidiales, 218 - Downy mildews, 185 - Drosera rotundifolia, 133, 496 - Drupaceæ, 497 - Drupe, 454 - Duckweeds, 26, 28 - Dudresnaya, 175 - Dunes, 484 - - Ebenales, 500 - Ecological factors, 464 - Ecology (sometimes written œcology), 464 - Ectocarpus, 167 - Edaphic formations, 475 - Elaphomyces, 217, 218 - Elder, 501 - Elm family, 495 - Elodea, 61-63 - Embryo of ferns, 269-272 - Embryo sac, 326-328 - Empusa, 215 - Endocarp, 450 - Endomyces, 216 - Endosperm, 103, 105, 107, 306, 309; - nucleus, 327, 329-334 - Entomophthorales, 215 - Enzyme, 92, 98, 116, 117 - Epidermal system, 358 - Epidermis, 358, 359, 363 - Epigæa repens, 499 - Epigynous, 425 - Epilobium, 498 - Epinastic (ep-i-nas′tic), 129 - Epinasty (ep′i-nas-ty), 129 - Epipactis, 444, 447 - Epiphegus, 84 - Epiphytes, 416 - Equisetales, 296 - Equisetineæ, 296 - Equisetum, 280-283 - Ericaceæ, 499 - Ericales, 499 - Erythronium, 493 - Etiolated plants (e′ti-o-la″ted), 68 - Euascomycetes, 217 - Eubasidiomycetes, 219 - Eupatorium, 403, 502 - Euphorbiaceæ, 497 - Eurotium oryzæ, 78 - Evening primrose family, 498 - Exalbuminous, 108 - Exoascus, 217 - Exobasidiales, 219 - Exocarp, 450 - - Fagales, 494 - Fehling’s solution, 75, 76 - Ferment, 98, 108, 116 - Ferns, 251-279, 292, 457; - classification of, 295 - Fertilization, 307, 308, 328, 329, 140, 145, - 169, 172, 174, 197, 421 - Fibrovascular bundles, 49-54 - Figwort family, 501 - Filicales, 295 - Filicineæ, 295 - Fittonia, 404 - Flagellates, 83, 165 - Flax, 497 - Flower cluster, 419 - Flower, form of, 422; - parts of, 419; - union of parts, 424 - Flowers, arrangements of, 426; - kinds of, 421 - Follicle, 453 - Forest, formations 471; - societies, 477 - Forests, relation to rainfall, 479 - Fresh-water societies, 486 - Frond, 352 - Fruit, 450-457; - parts of, 450 - Frullania, 25, 236 - Fucus, 168-170 - Fungi, absorption by, 22; - classification of, 213-222; - nutrition of, 86-90; - respiration in, 115 - - Gametangium (gam″et-an′gi-um), 140 - Gamete (gam′ete), 138, 139 - Gametophore (gam′et-o-phore), 230, 248 - Gametophyte (gam′et-o-phyte), 225, 226, 244, 245, 250, 262, 270, - 283, 292, 294, 305, 314, 317, - 336-339, 340-348, 434 - Gamopetalous (gam″o-pet′a-lous), 424 - Gamosepalous (gam-o-sep′a-lous), 424 - Gas in plants, 60-64 - Gasteromycetes, 219 - Gemmæ, 179, 235 - General formations, 470 - Gentian, 500 - Geotropism (ge-ot′ro-pism), 125-127, 410 - Geraniaceæ, 497 - Geraniales, 497 - Geranium family, 497 - Germ, 459 - Gigartinales, 175 - Gingko, 313-315, 457 - Gingkoales, 316 - Ginseng, 499 - Glasswort, 495 - Gleicheniaceæ, 295 - Glucose, 108. See sugar. - Gnetales, 316 - Gonidia, 118, 143, 172, 174, 178-184 - Gonidiangium (go″nid-an′gi-um), 178 - Gonidium, 213 - Gooseberry, 496 - Goosefoot family, 495 - Gracilaria, 173, 174, 175 - Graminales, 492 - Gramineæ, 492 - Grape, 498 - Grass family, 492 - Grassland formation, 471 - Green algæ, 158 - Growth, 118-124, 380 - Gulfweed, 170 - Gymnosperms, 311, 456 - Gymnosporangium, 194 - Gynœcium, 320, 419, 451, 452 - Gyrocephalus, 219 - - Halophytes, 468 - Harpochytrium, 214, 215 - Haustorium, 87, 88 - Hawkweed, 502 - Hawthorn, 497 - Hazelnut, 452, 495 - Head, 428 - Heart-leaf, 495 - Heath family, 499 - Heliotrope, 500 - Heliotropism (he-li-ot′ro-pism), 127-131, 133, 397 - Helvellales, 217 - Hemiascomycetes, 216 - Hemibasidiomycetes, 218 - Hepaticæ, 242 - Heterospory (het″er-os′po-ry), 434 - Heterothallic, 180 - Heterotrophic plants, 85 - Hickory, 494 - Hickory-nut, 452 - Hilum, 101, 102 - Hippocastanaceæ, 498 - Holdfasts, 418 - Hollyhock, 498 - Homothallic, 180 - Honeysuckle, 501 - Hormogonales, 163 - Horse-chestnut, 498 - Horsetails, 280-283 - Houstonia cœrulea, 437 - Huckleberry, 499 - Humus saprophytes, 85, 91 - Hybridization, 338 - Hydnaceæ, 210, 219 - Hydnum coralloides, 210 - Hydnum repandum, 211 - Hydrocarbon, 75 - Hydrodictyaceæ, 161 - Hydrophytes, 468 - Hydropterales, 295 - Hydrotropism (hy-drot′ro′pism), 133, 134, 412 - Hygrophytes, 468 - Hymeniales, 219 - Hymenogastrales, 219 - Hymenomycetes, 219 - Hymenomycetineæ, 219 - Hymenophyllaceæ, 295 - Hypericum, 498 - Hypocotyl (hy′po-co″tyl), 101 - Hypocreales, 217 - Hypogenous, 425 - Hyponastic (hy-po-nas′tic), 129 - Hyponasty (hy′po-nas-ty), 129 - Hysteriales, 217 - - Impatiens, 498 - Impatiens fulva, 460 - Indian-pipe, 499 - Indian turnip, 493 - Indusium, 252 - Inflorescence, 426 - Insectivorous plants, 133, 496 - Integument, 304 - Intramolecular respiration, 113, 114 - Inulase, 108 - Inulin, 108, 417 - Iodine, 65 - Ipomœa, 500 - Iridaceæ, 493 - Iris, 493 - Irritability, 125-135 - Isoetales, 296 - Isoetes, 289-291, 292 - Isoetineæ, 296 - Ivy, 498 - - Jack-in-the-pulpit, 373 - Jewelweed, 498 - Juglandales, 494 - June-berry, 497 - Jungermanniales, 242 - - Kalmia latifolia, 444 - Karyokinesis, 341-344 - Kelps, 168 - Kingdom, 492 - - Labiatæ, 423, 501 - Laboulbeniales, 218 - Labrador tea, 499 - Lactuca canadensis, 460 - Lactuca scariola, 409, 460, 461 - Lagenidium, 214, 215 - Laminaria, 168, 169 - Lamium, 424, 501 - Larch, 367 - Laurel, 499 - Leaf patterns, 404 - Leathesia difformis, 168 - Leaves, form and arrangement, 383-391; - function of, 387; - protective modifications of, 392; - protective positions, 395; - reduction of surface, 394; - relation to light, 397; - structure of, 40-43, 131, 391, 393 - Legumes, 92, 93, 453 - Leguminosæ (= Papilionaceæ), 396, 399 - Leitneria floridana, 494 - Leitneriales, 494 - Lemanea, 171, 173, 175, 492 - Lemna, 418 - Lemna trisulca, 26, 27 - Lenticel, 357, 358 - Lepiota naucina, 208 - Lettuce, 502 - Leucoplast, 71 - Lichens, 86, 93-95, 220, 221 - Light, 465 - Liliaceæ, 490, 493 - Liliales, 490, 493 - Lilium, 489-493 - Linaria vulgaris, 501 - Linden, 498 - Linum vulgaris, 497 - Lipase, 108 - Liquidambar, 496 - Liriodendron, 496 - Live-forever, 394 - Liverworts, 222-239; - absorption by, 23-25; - classification of, 242 - Lobelia, 501 - Lupinus perennis, 353 - Lycoperdales, 220 - Lycopodiaceæ, 296 - Lycopodiales, 296 - Lycopodiineæ, 296 - Lycopodium, 284-286 - - Macrosporangium, 94, 302, 304, 311, 312, 321 - Macrospore, 287, 290, 326-328, 434 - Magnolia, 496 - Mallow family, 498 - Malvales, 498 - Maple family, 497 - Marchantia, 24, 226-236 - Marchantiales, 242 - Marine plant societies, 486 - Marratiales, 295 - Marsilia, 370 - Marsiliaceæ, 296 - Matoniaceæ, 295 - Medicago denticulata, 92 - Medulla, 50 - Members of the flower, 335 - Members of the plant, 349-353 - Meristem, 359 - Mesocarp, 450 - Mesophytes, 467 - Microsporangia, 294, 299 - Microspore, 287, 290, 299, 312, 435 - Microsporophylls, 299, 320, 420 - Milkweed family, 500 - Mimosa, 132, 396 - Mimulus, 501 - Mint family, 501 - Mistletoe, 84, 495 - Mitchella, 501 - Mixotrophic plants, 85 - Mnium, 243-246 - Molds, nutrition of, 86-90 - Molds, water, 181 - Monadelphous, 424 - Monoblepharidales, 215 - Monoblepharis, 215 - Monocotyledons, 490, 492 - Monœcious, 435 - Monotropa uniflora, 499 - Morchella, 198, 199 - Morel, 198, 199 - Morning-glories, 500 - Mosaics, 405 - Mosses, 243-248, 457; - absorption by, 25; - classification of, 248 - Mucor, 6, 7, 15, 118, 119, 177-180, 215 - Mucorales, 215 - Mulberry, 495 - Mullein, 366, 394, 501 - Mushrooms, 199-208 - Mustard family, 496 - Mutation, 338 - Mutualism, 95 - Mycelium, 6, 86-90 - Mycetozoa, 213, 214 - Mycorhiza, 86, 91, 92, 217 - Myosotis, 500 - Myrica cerifera, 494 - Myrica gale, 494 - Myricales, 494 - Myriophyllum, 403 - Myrtales, 498 - Myxobacteriales, 165 - Myxomycetes, 83, 213, 214 - - Naiadaceæ, 492 - Naiadales, 492 - Naias, 492 - Nemalion, 171, 172, 175 - Nemalionales, 175 - Nettle, 495 - Nicotiana, 501 - Nidulariales, 220 - Nitella, 8, 9, 176 - Nitrobacter, 83 - Nitrogen, 92, 93 - Nitromonas, 83 - Nostocaceæ, 164 - Nucellus, 304 - Nucleus, 3, 4; - morphology of, 340-345 - Nuphar advena, 496 - Nutation, 123, 124 - Nymphæa odorata, 496 - - Oak, 495 - Oak family, 495 - Œdogoniaceæ, 162 - Œdogonium, 147-151, 350 - Œnothera biennis, 498 - Œnothera gigas, 338 - Œnothera lamarkiana, 338 - Olpidium, 214, 215 - Onagar biennis, 498 - Onagraceæ, 498 - Onoclea sensibilis, 254, 273-278 - Oogonium, 144, 150, 155 - Oomycetes, 214, 215 - Ophioglossales, 295 - Ophioglossum, 295 - Opuntiales, 498 - Orchidaceæ, 494 - Orchidales, 494 - Orchids, 442 - Oscillatoriaceæ, 163 - Osmosis, 13-20 - Osmundaceæ, 295 - Ostrich fern, 279 - Ovule, 302, 321, 334, 421 - Oxalis, 497 - Oxycoccus, 500 - Oxydendrum arboreum, 501 - Oxygen, 63, 110-113 - - Palisade cells, 41, 43 - Palmaceæ, 493 - Palmales, 493 - Palms, 408 - Pandanales, 492 - Pandanus, 492 - Pandorina, 160, 350 - Panicle, 427 - Papaverales, 496 - Papilionaceæ, 423, 497 - Parasites, 83, 84, 86 - Parasitic fungi, nutrition of, 86-90 - Parenchyma, 50, 356, 363 - Parietales, 498 - Parkeriaceæ, 296 - Parmelia, 96 - Parthenogenesis, 184 - Partridge-berry, 501 - Pea, 497 - Pea family, 497 - Pear, 456 - Pediastrum, 161 - Pellia, 164 - Pellonia, 405 - Peltigera, 94, 95 - Pepo, 456 - Pericycle, 360 - Peridineæ, 166 - Perigynous, 425 - Perisperm, 331, 332 - Perisporiales, 217 - Peronospora, 183, 215 - Peronosporales, 215 - Persimmon, 500 - Pezizales, 217 - Phacidiales, 217 - Phæophyceæ, 167 - Phæosporales, 171 - Phallales, 219 - Phloem, 50-52, 360, 361, 363 - Phlox family, 500 - Phoradendron flavescens, 495 - Photosynthesis, 67, 68, 70, 117 - Phycomycetes (Phy″co-my-ce′tes), 214, 215 - Phyllidium, 371 - Phylloclades, 373, 395 - Phyllotaxy, 375, 384 - Physical condition of soil, 465 - Physical factors, 465 - Phytolaccaceæ, 495 - Phytomyxa leguminosarum, 92 - Phytophthora, 182, 184, 215 - Pickerel-weed, 493 - Pilularia, 296 - Pinales, 216 - Pine, white, 297-310 - Piperales, 494 - Pitcher-plant, 496 - Pith, 50 - Plant food, sources of, 81 - Plant formations, 496 - Plant substance, analysis of, 79, 80 - Plantaginales, 501 - Plantago, 501 - Plasmolysis (plas-mol′y-sis), 19 - Plasmopara, 183, 215 - Plectascales, 217 - Plectobasidiales, 220 - Pleurococcaceæ, 161 - Pleurococcus, 161 - Plum family, 497 - Plumule, 99 - Podostemon, 496 - Poison-hemlock, 499 - Poison-ivy, 497 - Poison-oak, 497 - Poisonous mushrooms, 207, 208 - Poison-sumac, 497 - Pokeweed, 495 - Polemoneales, 500 - Pollen grain, 299, 305 - Pollination, 303, 304, 420, 430, 433-449 - Pollinium, 420 - Polygonales, 495 - Polygonum, 495 - Polypodiaceæ, 296 - Polyporaceæ, 209, 219 - Polyporus, 209, 210 - Polyporus mollis, 92 - Polyporus sulphureus, 209 - Pomaceæ, 497 - Pondweeds, 492 - Poppy, 496 - Porella, 237 - Portulaca, 495 - Potamogeton, 492 - Potato, 501 - Powdery mildews, 195-198, 217 - Primrose, 498, 500 - Primula, 438 - Primulales, 500 - Procarp, 172, 174, 175 - Progeotropism (pro″ge-ot′ro-pism), 126 - Promycelium (pro″my-ce′li-um), 192 - Proterandrous, 441, 442 - Proterandry, 444 - Proterogenous, 441, 442 - Proterogeny, 440 - Prothallium, 265, 287, 288, 291, 292, 304, 305, - 311, 325, 328, 335, 433, 434 - Protoascales, 216 - Protoascomycetes, 216 - Protobasidiomycetes, 218 - Protococcoideæ, 158, 162 - Protodiscales, 217 - Protomyces, 216 - Protonema (pro″to-ne′ma), 248, 264 - Protoplasm, 1-12, 42-43, 342; - movement of, 7-11 - Psilotaceæ, 296 - Pteridophytes, 295, 434 - Pteris cretica, 346 - Puccinia, 187 - Puff-balls, 220 - Pumpkin, 501 - Purslane, 495 - Pyrenoid, 2, 3 - Pyrenomycetes, 217 - Pyrola, 499 - Pyxidium, 453 - - Quercus, 495 - Quillworts, 289-291 - Quince, 456 - - Raceme, 427 - Radicle, 99 - Ragweed, 502 - Rainy-season flora, 481 - Ranales, 496 - Ranunculaceæ, 496 - Raspberry, 454, 455 - Red algæ, 171, 174; - uses of, 175 - Reproduction, 137, 143, 149, 154, 155, 179, 185, 186 - Respiration, 110-116, 117 - Rhamnales, 498 - Rhizoids, 24-26 - Rhizome, 354 - Rhizomorph (rhi′zo-morph), 89 - Rhizophidium, 214, 215 - Rhizopus, 177-180, 215 - Rhododendron, 499 - Rhodomeniales, 175 - Rhodophyceæ, 171 - Rhus radicans, 416, 497 - Riccia, 23, 164, 222-226 - Ricinus, 497 - Riverweed, 496 - Root, function of, 410-418 - Root hairs, absorption by, 19, 30, 32 - Root hairs, action on soil, 82 - Root pressure, 33, 34, 45 - Root, structure of, 30, 361, 362 - Root-tubercles, 92 - Roots, kinds of, 415 - Rosaceæ, 497 - Rosales, 496 - Rose family, 497 - Rosette, 405 - Rosette plants, 483 - Rubiales, 501 - Rudbeckia, 502 - Rusts, 187-194 - - Salicaceæ, 494 - Salix, 494 - Salsify, 502 - Salviniaceæ, 296 - Samara, 451 - Sandalwood, 495 - Sanguinaria, 496 - Santalales, 495 - Sap, rise of, 53, 54 - Sapindales, 497 - Saprolegnia, 181-184 - Saprolegniales, 215 - Saprophytes, 83-85 - Sargassum, 170 - Sarraceniales, 496 - Sarsaparilla, 499 - Saxifrage, 496 - Schizæaceæ, 295 - Schizocarp, 451 - Schizomycetes, 164 - Schizophyceæ, 163 - Sclerenchyma, 356-357, 361, 363 - Scouring rush, 282 - Screw-pine, 409, 492 - Scrophulariaceæ, 501 - Sedge family, 492 - Seed, dispersal of, 458-463 - Seed plants, 338 - Seed, structure of, 98, 102 - Seedlings, 97-107 - Seeds, 330-334 - Selaginella, 286-288, 292 - Selaginellaceæ, 296 - Sensitive fern, 273 - Sensitive plants, 132, 396, 399 - Sexual organs, 144, 147 - Shadbush, 497 - Shepherd’s-purse, 496 - Shoot, floral, 419, 432 - Shoots, 353-355; - types of, 361-373; - winter condition of, 374-377 - Sieve tissue, 358, 363 - Sieve tubes, 52, 53 - Silique, 453 - Silk-cotton tree, 417 - Silver bell, 500 - Siphoneæ, 146, 162 - Skunk’s cabbage, 439-442 - Slime molds, 83 - Smoke-tree, 497 - Societies, 475 - Solanum, 501 - Solidago, 502 - Sourwood, 499 - Spadix, 428 - Spartium, 446 - Spathyema fœtida, 438, 493 - Spermagonia, 190 - Spermatophytes, 338 - Sphacelaria, 168 - Sphærella lacustris, 158, 159 - Sphærella nivalis, 158, 350 - Sphæriales, 218 - Sphagnales, 248 - Sphagnum, 164 - Spiderwort, 11, 493 - Spike, 428 - Spirodela polyrhiza, 27 - Spirogyra, 1-5, 13, 14, 60, 72, 136-140, 350 - Sporangia, 178-182 - Sporangium, 253-258, 281, 290 - Spores, 225, 256-258, 263, 264, 281 - Sporocarp, 173 - Sporogonium (spo″ro-go′ni-um), 224, 231, 233, 234, 237, 238, - 239, 241, 246, 247, 248 - Sporophyll, 274, 281, 292 - Sporophyte (spo′ro-phyte), 225, 226, 232, 234, 237-239, 241, 242, - 250, 261, 268, 270, 283, 292, 294, 314, - 315, 317, 336-339, 340-348 434 - Spurge family, 497 - Squash, 501 - Staminodium, 446 - Starch, formation of, 68, 70-74; - changed to sugar, 77, 78; - translocation of, 73; - digestion of, 75 - Stems, types of, 365-373 - Stems, woody, structure of, 381-382 - Stoma (pl. stomata) (sto′ma-ta), 42-44, 46 - Strawberry, 455, 497 - Sugar, test for, 75, 76 - Sumac, 497 - Sundew, 133, 496 - Sunflower, 399-401, 502 - Sweet gum, 496 - Symbiosis, 85, 86, 92-95 - Synergids (syn´er-gids), 327, 330 - Syngenœsious, 424 - Synthetic assimilation, 67 - - Tape-grass, 493 - Taraxacum densleonis, 502 - Teasel, 501 - Telegraph-plant, 399 - Teleutospore, 188 - Temperature, 134, 135, 465 - Tetrasporaceæ, 161 - Tetraspores, 173, 174 - Thallophytes, 352 - Thallus, 352 - Thelephoraceæ, 219 - Thistle family, 502 - Thunderwood, 497 - Thyrsus, 427 - Tilia, 498 - Tillandsia, 493 - Tissue, tensions of, 57-59 - Tissues, classification of, 363, 364; - kinds of, 356-359; - organization of, 356-362 - Toad-flax, 501 - Tomato, 501 - Tradescantia, 493 - Tragopogon, 502 - Trailing arbutus, 499 - Trametes pini, 90 - Transpiration, 35-46 - Tremellales, 218, 219 - Triadelphous, 425 - Trillium, 318-322, 494 - Trumpet-creeper, 501 - Tuberales, 217 - Tubers, 373 - Tundra, 481 - Turgescence, 14, 15 - Turgor, 20; - restoration of, 56, 57 - Typha, 493 - - Ulmaceæ, 495 - Ulmus americana, 495 - Ulothrix, 162 - Ulotrichaceæ, 162 - Ulvaceæ, 162 - Umbel, 428 - Umbellales, 498 - Uredinales, 218 - Uredineæ, 187-194, 218 - Uredospore, 189 - Uromyces caryophyllinus, 87 - Urticales, 495 - Ustilaginales, 218 - Ustilagineæ, 218 - Utricularia, 501 - - Vaccinium, 499 - Vacuoles, 7, 8 - Valerianales, 501 - Vallisneria spiralis, 493 - Variation, 338 - Vascular tissue, 358, 363 - Vaucheria, 142-146 - Vaucheriaceæ, 162 - Vegetation types, 464 - Venus fly-trap, 133 - Verbascum, 501 - Verbena, 501 - Vessels, 52, 53 - Vetch, 92, 497 - Viburnum, 501 - Vicia sativa, 459 - Viola cucullata, 436 - Violaceæ, 498 - Virgin’s bower, 462, 463 - Viscum album, 84 - Vitaceæ, 498 - Volvocaceæ, 158 - - Walnut, 452, 494 - Water, 465; - flow of, in plants, 53, 54 - Water-lilies, 496 - Water-plantain, 493 - White pine, 396 - Wild carrot, 499 - Willow family, 494 - Wind, 471 - Wintergreen, 499; - leaf of, 43 - Witch-hazel, 496 - Wolffia, 28 - Woodland formation, 470 - - Xerophytes, 467 - Xylem, 50, 52, 360, 361, 363 - Xylogen, 92 - Xyridales, 493 - - Yeast, 216; - fermentation of, 115, 116 - Yucca, 480, 493 - - Zamia, 313, 316, 457 - Zoogonidia, 143, 149, 178-184 - Zoospore, 149, 154 - Zygomycetes, 215 - Zygospore, 2, 138-140, 157, 160, 179, 180 - Zygote (zy′gote), 138, 179 - - - - -TWO NOTABLE NATURE BOOKS. - - - FERNS - - A Manual for the Northeastern States. 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Institute of Technology. - _Advanced Course._ 537 pp. 8vo. 2.00 - The same. _Briefer Course._ 415 pp. 12mo. 1.20 - The same. _Elementary Course._ 423 pp. 12mo. 1.00 - - =Finance.= By Prof. HENRY CARTER ADAMS, - University of Michigan. - _Advanced Course._ 573 pp. 8vo. 3.50 - - HENRY HOLT & CO. - =29 West 23d Street, New York= - =378 Wabash Avenue, Chicago= - VIII, ’02 - - - - - CHEMISTRY - - Cairns’s Quantitative Chemical Analysis - Revised and enlarged by Dr. E. WALLER. - 417 pp. 8vo. $2.00, _net_. - - Cohen’s Physical Chemistry for Biologists - Translated by Dr. MARTIN FISCHER, - Chicago University. 343 pp. 12mo, $1.75, _net_. - - Congdon’s Qualitative Analysis - By Prof. ERNEST A. CONGDON, - Drexel Institute. 64 pp. _Interleaved._ - 8vo. 60c., _net_. - - Nicholson and Avery’s Exercises in Chemistry - With Outlines for the Study of Chemistry. - To accompany any elementary text. - By Prof. H. H. 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It has passed through - several editions in England, and has been translated - into German (being the elementary text-book in the - University of Leipsic), French, and Italian. - - =Remsen and Randall’s Experiments= - (_for the “Introduction”_). 50c., _net_. - =Elements of Chemistry= (_Elementary_). - X + 272 pp. 12mo. 80c., _net_. - =Laboratory Manual= - (_for the “Elements”_). 40c., _net_. - - Torrey’s Elementary Chemistry - By JOSEPH TORREY, Jr., - Harvard. 437 pp. 12mo. $1.25, _net_. - - White’s Qualitative Analysis - By Prof. JOHN WHITE, - Univ. of Nebraska. 96 pp. 8vo. 80c., _net_. - - Woodhull and Van Arsdale’s Chemical Experiments - By Prof. JOHN F. WOODHULL - and M. B. VAN ARSDALE, - Teachers’ College, New York City. - 136 pp. 12mo. 60c., _net_. - Extremely simple experiments in - the chemistry of daily life. - - HENRY HOLT & CO. - =29 West 23d Street, New York= - =378 Wabash Avenue, Chicago= - VIII, ’05 - - - - - CHAMBERLIN & SALISBURY’S - - GEOLOGY - - By THOMAS C. CHAMBERLIN and ROLLIN D. SALISBURY, - Professors in the University of Chicago. - (_American Science Series._) 2 vols. 8vo. - _Vol. I. Geological Processes and their - Results._ XIX + 654 pp. $4.00. - _Vol. II. Earth History._ [_In preparation._] - - This is a notable scientific work by two of the - highest authorities on the subject in the United - States, and yet written in a style so simple that - it can be clearly understood by the intelligent - reader who has had little previous training in - the subject. - - =Chas. D. Walcott=, _Director of U. S. - Geological Survey_:—I am impressed with the - admirable plan of the work and with the thorough - manner in which geological principles and - processes and their results have been presented. - The text is written in an entertaining style - and is supplemented by admirable illustrations, - so that the student cannot fail to obtain a - clear idea of the nature and work of geological - agencies, of the present status of the science, - and of the spirit which actuates the working - geologist. - - =Henry S. Williams=, _Professor in Yale - University_.—I believe it is the best treatise - on this part of the subject which we have seen in - America. - - =R. S. Woodward=, _Professor in Columbia - University_:—It is admirable for its science, - admirable for its literary perfection, and - admirable for its unequalled illustrations. - - =T. C. Hopkins=, _Professor in Syracuse - University_:—It gives us the most advanced - thought on all the great questions of dynamical - and structural geology to be found in geological - literature. - - =H. Foster Bain=, _U. S. Geological - Survey_:—The book is pre-eminently a teaching - book and I have no doubt that it will at once - become the standard American text-book on geology. - - =William N. Rice=, _Professor in Wesleyan - University_:—The book is full of new ideas. It - is one of the indispensable books for the library - of every working geologist and every one who - wishes to be an up-to-date teacher of geology. - - =T. A. Jaggar, Jr.=, _Assistant Professor - in Harvard University_:—The book appears to - be an excellent statement of modern American - geology, with abundant new illustrative material, - based upon the most recent work of government and - other surveys. It is especially satisfactory to - have in hand a geological volume which does not - attempt to cover the whole field. Modern geology - is much too large a subject to be condensed into - a single volume. - - HENRY HOLT & CO. - =29 West 23d Street, New York= - =378 Wabash Avenue, Chicago= - VIII, ’05 - - - - -THE METRIC SYSTEM. - - -[Illustration: 10-centimeter rule. The upper edge is in millimeters, -the lower in centimeters and half centimeters.] - - UNITS. THE MOST COMMONLY USED DIVISIONS AND MULTIPLES. - - { _Centimeter_ (cm), 1/100 meter; _Millimeter_ (mm), - { 1/1000 meter; _Micron_ (μ), 1/1000 millimeter. -THE METER, for { The micron is the unit in micrometry. - LENGTH { _Kilometer_, 1000 meters; used in measuring roads and - { other long distances. - - { _Milligram_ (mg), 1/1000 gram. -THE GRAM, for { _Kilogram_, 1000 grams, used for ordinary masses, like - WEIGHT { groceries, etc. - -THE LITER, for { _Cubic Centimeter_ (cc), 1/1000 liter. This is more - CAPACITY { common than the correct form, Milliliter. - -_Divisions_ of the _units_ are indicated by Latin prefixes: _deci_, -1/10; _centi_, 1/100; _milli_, 1/1000. - -_Multiples_ are designated by Greek prefixes: _deka_, 10 times; -_hecto_, 100 times; _kilo_, 1000 times; _myria_, 10,000 times. - - -TABLE OF METRIC AND ENGLISH MEASURES. - -METER = 100 centimeters, 1000 millimeters, 1,000,000 microns, -39.3704 inches. - -Millimeter (mm) = 1000 microns, 1/10 millimeter, 1/1000 meter, 1/25 -inch, approximately. - -MICRON (μ) (unit of measure in micrometry) = 1/1000 mm, -1/1000000 meter (0.000039 inch), 1/25000 inch, approximately. - -Inch (in.) = 25.399772 mm (25.4 mm, approx.). - -LITER = 1000 milliliters or 1000 cubic centimeters, 1 quart -(approx.). - -Cubic centimeter (cc or cctm) = 1/1000 liter. - -Fluid ounce (8 fluidrachms) = 29.578 cc (30 cc, approx.). - -GRAM = 15.432 grains. - -Kilogram (kilo) = 2.204 avoirdupois pounds (2⅕ pounds, approx.). - - Ounce Avoirdupois (437½ grains) = 28.349 grams } (30 grams, - Ounce Troy or Apothecaries’ (480 grains) = 31.103 grams } approx.). - - -TEMPERATURE. - - To change Centigrade to Fahrenheit: (C. × ⁹/₅) + 32 = F. - For example, to find the equivalent of 10° Centigrade, - C. = 10°, (10° × ⁹/₅) + 32 = 50° F. - - To change Fahrenheit to Centigrade: (F. - 32°) × ⁵/₉ = C. - For example, to reduce 50° Fahrenheit to Centigrade, - F. = 50°, and (50° - 32°) × ⁵/₉ = 10° C.; - or - 40° Fahrenheit to Centigrade, - F. = - 40°, (- 40° - 32°) = - 72°, - whence - 72° × ⁵/₉; = - 40° C. - - —_From “The Microscope” (by S. 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