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Title: Elementary Botany
Author: Atkinson, George Francis
Language: English
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[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
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FERNS
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and the story of each individual is interesting
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CHAMPLIN’S YOUNG FOLKS’
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Earlier Volumes of Champlin’s Young Folks’ Cyclopædia.
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AMERICAN SCIENCE SERIES
_All prices are_ NET _unless marked_
RETAIL. _Details of the books will be found in
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=Physics.= By Prof. GEORGE F. BARKER,
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_Advanced Course._ 902 pp. 8vo. $3.50
=Chemistry.= By Prest. IRA REMSEN,
Johns Hopkins University.
Chemistry. _Advanced Course._ 850 pp. 8vo. 2.80
College Chemistry, xx + 669 pp. 8vo. 2.00
Chemistry. _Briefer Course._
(_New Edition_, 1901.) 435 pp. 12mo. 1.10
Chemistry. _Elementary Course._ 272 pp. 12mo. 80c.
Laboratory Manual (_to Elementary Course_).
196 pp. 12mo. 40c.
Chemical Experiments. By Prof. REMSEN
and Dr. W. W. RANDALL. (_For Briefer
Course._) _No_ blank pages for notes.
158 pp. 12mo. 50c.
=Astronomy.= By Prof. SIMON NEWCOMB
of Johns Hopkins and EDWARD S. HOLDEN,
late Director of the Lick Observatory, California.
_Advanced Course._ 512 pp. 8vo. 2.00
The same. _Briefer Course._ 352 pp. 12mo. 1.12
The same. _Elementary Course._
By E. S. HOLDEN, 446 pp. 12mo. 1.20
=Geology.= By Profs. THOMAS C. CHAMBERLIN
and ROLLIN D. SALISBURY,
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=General Biology.= By Prof. W. T. SEDGWICK,
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_Revised and Enlarged._ 231pp. 8vo. 1.75
=Botany.= By Prof. C. E. BESSEY,
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The Human Body and the Effects of Narcotics.
261 pp. 12mo. 1.20
=Psychology.= By Prof. WILLIAM JAMES
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CHEMISTRY
Cairns’s Quantitative Chemical Analysis
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Cohen’s Physical Chemistry for Biologists
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8vo. 60c., _net_.
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Noyes’s (W. A.) Organic Chemistry
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This book is used in hundreds of schools and
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=Elements of Chemistry= (_Elementary_).
X + 272 pp. 12mo. 80c., _net_.
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Woodhull and Van Arsdale’s Chemical Experiments
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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.
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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. H. Gage) by permission._
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