Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Footnotes have been moved to the end of the chapter in which they
occur.
Typographical errors have been silently corrected.
[Illustration: CYCAS REVOLUTA (see page 311). (_Frontispiece._)]
ELEMENTARY BOTANY
BY
GEORGE FRANCIS ATKINSON, PH.B.
_Professor of Botany in Cornell University_
_THIRD EDITION, REVISED_
[Illustration]
NEW YORK
HENRY HOLT AND COMPANY
1905
Copyright, 1898, 1905
BY
HENRY HOLT AND COMPANY
ROBERT DRUMMOND, PRINTER, NEW YORK
PREFACE.
The present book is the result of a revision and elaboration of the
author’s “Elementary Botany,” New York, 1898. The general plan of the
parts on physiology and general morphology remains unchanged. A number
of the chapters in the physiological part are practically untouched,
while others are thoroughly revised and considerable new matter is
added, especially on the subjects of nutrition and digestion. The
principal chapters on general morphology are unchanged or only slightly
modified, the greatest change being in a revision of the subject of
the morphology of fertilization in the gymnosperms and angiosperms in
order to bring this subject abreast of the discoveries of the past few
years. One of the greatest modifications has been in the addition of
chapters on the classification of the algæ and fungi with studies of
additional examples for the benefit of those schools where the time
allowed for the first year’s course makes desirable the examination of
a broader range of representative plants. The classification is also
carried out with greater definiteness, so that the regular sequence
of classes, orders, and families is given at the close of each of the
subkingdoms. Thus all the classes, all the orders (except a few in the
algæ), and many of the families, are given for the algæ, fungi, mosses,
liverworts, pteridophytes, gymnosperms, and angiosperms.
But by far the greatest improvement has been in the complete
reorganization, rewriting, and elaboration of the part dealing with
ecology, which has been made possible by studies of the past few years,
so that the subject can be presented in a more logical and coherent
form. As a result the subject-matter of the book falls naturally into
three parts, which may be passed in review as follows:
Part I. _Physiology._ This deals with the life processes of plants,
as absorption, transpiration, conduction, photosynthesis, nutrition,
assimilation, digestion, respiration, growth, and irritability. Since
protoplasm is fundamental to all the life work of the plant, this
subject is dealt with first, and the student is led through the study
of, and experimentation with, the simpler as well as some of the higher
plants, to a general understanding of protoplasm and the special way in
which it enables the plant to carry on its work and to adjust itself to
the conditions of its existence. This study also serves the purpose of
familiarizing the pupil with some of the lower and unfamiliar plants.
Some teachers will prefer to begin the study with general morphology
and classification, thus studying first the representatives of the
great groups of plants, and others will prefer to dwell first on the
ecological aspects of vegetation. This can be done in the use of this
book by beginning with Part II or with Part III.
But the author believes that morphology can best be comprehended after
a general study of life processes and functions of the different parts
of plants, including in this study some of the lower forms of plant
life where some of these processes can more readily be observed. The
pupil is then prepared for a more intelligent consideration of general
and comparative morphology and relationships. Even more important is
a first study of physiology before taking up the subject of ecology.
The great value to be derived from a study of plants in their relation
to environment lies in the ability to interpret the different states,
conditions, behavior, and associations of the plant, and for this
physiology is indispensable. It is true that a considerable measure
of success can be obtained by a good teacher in beginning with either
subject, but the writer believes that measure of success would be
greater if the subjects were taken up in the order presented here.
Part II. _Morphology and life history of representative plants._ This
includes a rather careful study of representative examples among the
algæ, fungi, liverworts, mosses, ferns and their allies, gymnosperms
and angiosperms, with especial emphasis on the form of plant parts,
and a comparison of them in the different groups, with a comparative
study of development, reproduction, and fertilization, rounding out
the work with a study of life histories and noting progression and
retrogression of certain organs and phases in proceeding from the
lower to the higher plants. Thus, in the algæ a first critical study
is made of four examples which illustrate in a marked way progressive
stages of the plant body, sexual organs, and reproduction. Additional
examples are then studied for the purpose of acquiring a knowledge of
variations from these types and to give a broader basis for the brief
consideration of general relationships and classification.
A similar plan is followed in the other great groups. The processes of
fertilization and reproduction can be most easily observed in the lower
plants like the algæ and fungi, and this is an additional argument in
favor of giving emphasis to these forms of plant life as well as the
advantage of proceeding logically from simpler to more complex forms.
Having also learned some of these plants in our study of physiology,
we are following another recognized rule of pedagogy, i.e., proceeding
from known objects to unknown structures and processes. Through
the study of the organs of reproduction of the lower plants and by
general comparative morphology we have come to an understanding of the
morphology of the parts of the flower, and of the true sexual organs
of the seed plants, and no student can hope to properly interpret the
significance of the flower, or the sexual organs of the seed plants who
neglects a careful study of the general morphology of the lower plants.
Part III. _Plant members in relation to environment._ This part deals
with the organization of the plant body as a whole in its relation to
environment, the organization of plant tissues with a discussion of
the principal tissues and a descriptive synopsis of the same. This is
followed by a complete study from a biological standpoint of the
different members of the plant, their special function and their
special relations to environment. The stem, root, leaf, flower, etc.,
are carefully examined and their ecological relations pointed out. This
together with the study of physiology and representatives in the groups
of plants forms a thorough basis for pure plant ecology, or the special
study of vegetation in its relation to environment.
There is a study of the factors of environment or ecological factors,
which in general are grouped under the physical, climatic, and biotic
factors. This is followed by an analysis of vegetation forms and
structures, plant formations and societies. Then in order are treated
briefly forest societies, prairie societies, desert societies, arctic
and alpine societies, aquatic societies, and the special societies of
sandy, rocky, and marshy places.
_Acknowledgments._ The author wishes to express his gratefulness to
all those who have given aid in the preparation of this work, or of
the earlier editions of Elementary Botany; to his associates, Dr.
E. J. Durand, Dr. K. M. Wiegand, and Professor W. W. Rowlee, of the
botanical department, and to Professor B. M. Duggar of the University
of Missouri, Professor J. C. Arthur of Purdue University, and Professor
W. F. Ganong of Smith College, for reading one or more portions of the
text; as well as to all those who have contributed illustrations.
_Illustrations._ The large majority of the illustrations are new
(or are the same as those used in earlier editions of the author’s
Elementary Botany) and were made with special reference to the method
of treatment followed in the text. Many of the photographs were made
by the author. Others were contributed by Professor Rowlee of Cornell
University; Mr. John Gifford of New Jersey; Professor B. M. Duggar,
University of Missouri; Professor C. E. Bessey, University of Nebraska;
Dr. M. B. Howe, New York Botanical Garden; Mr. Gifford Pinchot, Chief
of the Bureau of Forestry; Mr. B. T. Galloway, Chief of the Bureau of
Plant Industry; Professor Tuomey of Yale University; and Mr. E. H.
Harriman, who through Dr. C. H. Merriam of the National Museum allowed
the use of several of his copyrighted photographs from Alaska. To those
who have contributed drawings the author is indebted as follows: to
Professor Margaret C. Ferguson, Wellesley College; Professor Bertha
Stoneman of Huguenot College, South Africa; Mr. H. Hasselbring of
Chicago; Dr. K. Miyake, formerly of Cornell University and now of
Doshisha College, Japan; and Professors Ikeno and Hirase of the Tokio
Imperial University. The author is also indebted to Ginn & Co., Boston,
for the privilege to use from his “First Studies of Plant Life” the
following figures: 28, 29, 46, 48, 49, 56, 62, 66, 67, 87, 102, 103,
422-426, 429, 430, 438-440, 443, 444, 448, 449, 452, 472-475. A few
others are acknowledged in the text.
CORNELL UNIVERSITY, April, 1905.
TABLE OF CONTENTS.
PART I. PHYSIOLOGY.
CHAPTER I. PAGE
PROTOPLASM. 1
CHAPTER II.
ABSORPTION, DIFFUSION, OSMOSE. 13
CHAPTER III.
HOW PLANTS OBTAIN WATER. 22
CHAPTER IV.
TRANSPIRATION, OR THE LOSS OF WATER BY PLANTS. 35
CHAPTER V.
PATH OF MOVEMENT OF WATER IN PLANTS. 48
CHAPTER VI.
MECHANICAL USES OF WATER. 56
CHAPTER VII.
STARCH AND SUGAR FORMATION. 60
1. The Gases Concerned. 60
2. Where Starch is Formed. 64
3. Chlorophyll and the Formation of Starch. 67
CHAPTER VIII.
STARCH AND SUGAR CONCLUDED; ANALYSIS OF PLANT SUBSTANCE. 73
1. Translocation of Starch. 73
2. Sugar, and Digestion of Starch. 75
3. Rough Analysis of Plant Substance. 79
CHAPTER IX.
HOW PLANTS OBTAIN THEIR FOOD, I. 81
1. Sources of Plant Food. 81
2. Parasites and Saprophytes. 83
3. How Fungi Obtain their Food. 86
4. Mycorhiza. 91
5. Nitrogen gatherers. 92
6. Lichens. 93
CHAPTER X.
HOW PLANTS OBTAIN THEIR FOOD, II. 97
Seedlings, 97
Digestion, 107
Assimilation 109
CHAPTER XI.
RESPIRATION. 110
CHAPTER XII.
GROWTH. 118
CHAPTER XIII.
IRRITABILITY. 125
PART II. MORPHOLOGY AND LIFE HISTORY OF REPRESENTATIVE PLANTS.
CHAPTER XIV.
SPIROGYRA. 136
CHAPTER XV.
VAUCHERIA. 142
CHAPTER XVI.
ŒDOGONIUM. 147
CHAPTER XVII.
COLEOCHÆTE. 153
CHAPTER XVIII.
CLASSIFICATION AND ADDITIONAL STUDIES OF THE ALGÆ. 158
CHAPTER XIX.
FUNGI: MUCOR AND SAPROLEGNIA. 177
CHAPTER XX.
FUNGI CONTINUED (“Rusts” Uredineæ). 187
CHAPTER XXI.
THE HIGHER FUNGI. 195
CHAPTER XXII.
CLASSIFICATION OF THE FUNGI. 213
CHAPTER XXIII.
LIVERWORTS (Hepaticæ). 222
Riccia, 222
Marchantia. 226
CHAPTER XXIV.
LIVERWORTS CONTINUED. 231
Sporogonium of Marchantia. 231
Leafy-stemmed Liverworts. 236
The Horned Liverworts. 240
Classification of the Liverworts. 242
CHAPTER XXV.
MOSSES (Musci). 243
Classification of Mosses. 248
CHAPTER XXVI.
FERNS. 251
CHAPTER XXVII.
FERNS CONTINUED. 262
Gametophyte of Ferns. 262
Sporophyte. 268
CHAPTER XXVIII.
DIMORPHISM OF FERNS. 273
CHAPTER XXIX.
HORSETAILS. 280
CHAPTER XXX.
CLUB MOSSES. 284
CHAPTER XXXI.
QUILLWORTS (Isoetes). 289
CHAPTER XXXII.
COMPARISON OF FERNS AND THEIR RELATIVES. 292
Classification of the Pteridophytes. 295
CHAPTER XXXIII.
GYMNOSPERMS. 297
CHAPTER XXXIV.
FURTHER STUDIES ON GYMNOSPERMS. 311
CHAPTER XXXV.
MORPHOLOGY OF THE ANGIOSPERMS: TRILLIUM; DENTARIA. 318
CHAPTER XXXVI.
GAMETOPHYTE AND SPOROPHYTE OF ANGIOSPERMS. 325
CHAPTER XXXVII.
MORPHOLOGY OF THE NUCLEUS AND SIGNIFICANCE OF
GAMETOPHYTE AND SPOROPHYTE. 340
PART III. PLANT MEMBERS IN RELATION TO ENVIRONMENT.
CHAPTER XXXVIII.
THE ORGANIZATION OF THE PLANT. 349
I. Organization of Plant Members. 349
II. Organization of Plant Tissues. 356
CHAPTER XXXIX.
THE DIFFERENT TYPES OF STEMS. 365
I. Erect Stems. 365
II. Creeping, Climbing, and Floating Stems. 369
III. Specialized Shoots and Shoots for Storage
of Food. 372
IV. Annual Growth and Winter Protection of
Shoots and Buds. 374
CHAPTER XL.
FOLIAGE LEAVES. 383
I. General Form and Arrangement of Leaves. 383
II. Protective Modifications of Leaves. 392
III. Protective Positions. 395
IV. Relation of Leaves to Light. 397
V. Leaf Patterns. 404
CHAPTER XLI.
THE ROOT. 410
I. Function of Roots. 410
II. Kinds of Roots. 415
CHAPTER XLII.
THE FLORAL SHOOT. 419
I. The Parts of the Flower. 419
II. Kinds of Flowers. 421
III. Arrangement of Flowers, or Mode of
Inflorescence. 426
CHAPTER XLIII.
POLLINATION. 433
CHAPTER XLIV.
THE FRUIT. 450
I. Parts of the Fruit. 450
II. Indehiscent Fruits. 451
III. Dehiscent Fruits. 452
IV. Fleshy and Juicy Fruits. 454
V. Reinforced, or Accessory, Fruits. 455
VI. Fruits of Gymnosperms. 456
VII. “Fruit” of Ferns, Mosses, etc. 457
CHAPTER XLV.
SEED DISPERSAL. 458
CHAPTER XLVI.
VEGETATION IN RELATION TO ENVIRONMENT. 464
CHAPTER XLVII.
CLASSIFICATION OF ANGIOSPERMS. 487
INDEX. 503
PART I.
PHYSIOLOGY.
CHAPTER I.
PROTOPLASM.[1]
=1.= In the study of plant life and growth, it will be found
convenient first to inquire into the nature of the substance which we
call the living material of plants. For plant growth, as well as some
of the other processes of plant life, are at bottom dependent on this
living matter. This living matter is called in general _protoplasm_.
=2.= In most cases protoplasm cannot be seen without the help of
a microscope, and it will be necessary for us here to employ one if we
wish to see protoplasm, and to satisfy ourselves by examination that
the substance we are dealing with _is_ protoplasm.
=3.= We shall find it convenient first to examine protoplasm
in some of the simpler plants; plants which from their minute size
and simple structure are so transparent that when examined with the
microscope the interior can be seen.
For our first study let us take a plant known as _spirogyra_, though
there are a number of others which would serve the purpose quite as
well, and may quite as easily be obtained for study.
Protoplasm in spirogyra.
=4. The plant spirogyra.=—This plant is found in the water of
pools, ditches, ponds, or in streams of slow-running water. It is green
in color, and occurs in loose mats, usually floating near the surface.
The name “pond-scum” is sometimes given to this plant, along with
others which are more or less closely related. It is an _alga_, and
belongs to a group of plants known as _algæ_. If we lift a portion of
it from the water, we see that the mat is made up of a great tangle of
green silky threads. Each one of these threads is a plant, so that the
number contained in one of these floating mats is very great.
Let us place a bit of this thread tangle on a glass slip, and examine
with the microscope and we will see certain things about the plant
which are peculiar to it, and which enable us to distinguish it from
other minute green water plants. We shall also wish to learn what these
peculiar parts of the plant are, in order to demonstrate the protoplasm
in the plant.[2]
=5. Chlorophyll bands in spirogyra.=—We first observe the
presence of bands; green in color, the edges of which are usually
very irregularly notched. These bands course along in a spiral manner
near the surface of the thread. There may be one or several of these
spirals, according to the species which we happen to select for
study. This green coloring matter of the band is _chlorophyll_, and
this substance, which also occurs in the higher green plants, will
be considered in a later chapter. At quite regular intervals in the
chlorophyll band are small starch grains, grouped in a rounded mass
enclosing a minute body, the _pyrenoid_, which is peculiar to many algæ.
[Illustration: Fig. 1.
Thread of spirogyra, showing long cells, chlorophyll band, nucleus,
strands of protoplasm, and the granular wall layer of protoplasm.]
=6. The spirogyra thread consists of cylindrical cells end to
end.=—Another thing which attracts our attention, as we examine a
thread of spirogyra under the microscope, is that the thread is made up
of cylindrical segments or compartments placed end to end. We can see a
distinct separating line between the ends. Each one of these segments
or compartments of the thread is a _cell_, and the boundary wall is in
the form of a cylinder with closed ends.
=7. Protoplasm.=—Having distinguished these parts of the plant
we can look for the protoplasm. It occurs within the cells. It is
colorless (i.e., hyaline) and consequently requires close observation.
Near the center of the cell can be seen a rather dense granular body
of an elliptical or irregular form, with its long diameter transverse
to the axis of the cell in some species; or triangular, or quadrate in
others. This is the _nucleus_. Around the nucleus is a granular layer
from which delicate threads of a shiny granular substance radiate in a
starlike manner, and terminate in the chlorophyll band at one of the
pyrenoids. A granular layer of the same substance lines the inside of
the cell wall, and can be seen through the microscope if it is properly
focussed. This granular substance in the cell is _protoplasm_.
=8. Cell-sap in spirogyra.=—The greater part of the interior
space of the cell, that between the radiating strands of protoplasm, is
occupied by a watery fluid, the “cell-sap.”
=9. Reaction of protoplasm to certain reagents.=—We can employ
certain tests to demonstrate that this granular substance which we have
seen is protoplasm, for it has been found, by repeated experiments with
a great many kinds of plants, that protoplasm gives a definite reaction
in response to treatment with certain substances called reagents. Let
us mount a few threads of the spirogyra in a drop of a solution of
iodine, and observe the results with the aid of the microscope. The
iodine gives a yellowish-brown color to the protoplasm, and it can be
more distinctly seen. The nucleus is also much more prominent since it
colors deeply, and we can perceive within the nucleus one small rounded
body, sometimes more, the _nucleolus_. The iodine here kills and stains
the protoplasm. The protoplasm, however, in a living condition will
resist for a time some other reagents, as we shall see if we attempt
to stain it with a one per cent aqueous solution of a dye known as
_eosin_. Let us mount a few living threads in such a solution of eosin,
and after a time wash off the stain. The protoplasm remains uncolored.
Now let us place these threads for a short time, two or three minutes,
in strong alcohol, which kills the protoplasm. Then mount them in
the eosin solution. The protoplasm now takes the eosin stain. After
the protoplasm has been killed we note that the nucleus is no longer
elliptical or angular in outline, but is rounded. The strands of
protoplasm are no longer in tension as they were when alive.
[Illustration: Fig. 2. Cell of spirogyra before treatment with iodine.]
[Illustration: Fig. 3. Cell of spirogyra after treatment with alcohol
and iodine.]
=10.= Let us now take some fresh living threads and mount them in
water. Place a small drop of dilute glycerine on the slip at one side
of the cover glass, and with a bit of filter paper at the other side
draw out the water. The glycerine will flow under the cover glass and
come in contact with the spirogyra threads. Glycerine absorbs water
promptly. Being in contact with the threads it draws water out of the
cell cavity, thus causing the layer of protoplasm which lines the
inside of the cell wall to collapse, and separate from the wall,
drawing the chlorophyll band inward toward the center also. The wall
layer of protoplasm can now be more distinctly seen and its granular
character observed.
We have thus employed three tests to demonstrate that this substance
with which we are dealing shows the reactions which we know by
experience to be given by protoplasm. We therefore conclude that this
colorless and partly granular, slimy substance in the spirogyra cell
is protoplasm, and that when we have performed these experiments, and
noted carefully the results, we have _seen_ protoplasm.
[Illustration: Fig. 4. Cell of spirogyra before treatment with
glycerine.]
[Illustration: Fig. 5. Cells of spirogyra after treatment with
glycerine.]
=11. Earlier use of the term protoplasm.=—Early
students of the living matter in the cell considered it
to be alike in substance, but differing in density; so
the term protoplasm was applied to all of this living
matter. The nucleus was looked upon as simply a denser
portion of the protoplasm, and the nucleolus as a still
denser portion. Now it is believed that the nucleus
is a distinct substance, and a permanent organ of the
cell. The remaining portion of the protoplasm is now
usually spoken of as the _cytoplasm_.
In spirogyra then the cytoplasm in each cell consists
of a layer which lines the inside of the cell wall,
a nuclear layer, which surrounds the nucleus, and
radiating strands which connect the nucleus and wall
layers, thus suspending the nucleus near the center of
the cell. But it seems best in this elementary study to
use the term protoplasm in its general sense.
Protoplasm in mucor.
=12.= Let us now examine in a similar way another of the simple
plants with the special object in view of demonstrating the protoplasm.
For this purpose we may take one of the plants belonging to the group
of _fungi_. These plants possess no chlorophyll. One of several species
of _mucor_, a common mould, is readily obtainable, and very suitable
for this study.[3]
=13. Mycelium of mucor.=—A few days after sowing in some
gelatinous culture medium we find slender, hyaline threads, which are
very much branched, and, radiating from a central point, form circular
colonies, if the plant has not been too thickly sown, as shown in
fig. 6. These threads of the fungus form the _mycelium_. From these
characters of the plant, which we can readily see without the aid of a
microscope, we note how different it is from spirogyra.
To examine for protoplasm let us lift carefully a thin block of
gelatine containing the mucor threads, and mount it in water on a
glass slip. Under the microscope we see only a small portion of the
branched threads. In addition to the absence of chlorophyll, which we
have already noted, we see that the mycelium is not divided at short
intervals into cells, but appears like a delicate tube with branches,
which become successively smaller toward the ends.
=14. Appearance of the protoplasm.=—Within the tube-like thread
now note the protoplasm. It has the same general appearance as that
which we noted in spirogyra. It is slimy, or semi-fluid, partly
hyaline, and partly granular, the granules consisting of minute
particles (the _microsomes_). While in mucor the protoplasm has the
same general appearance as in spirogyra, its arrangement is very
different. In the first place it is plainly continuous throughout the
tube. We do not see the prominent radiations of strands around a large
nucleus, but still the protoplasm does not fill the interior of the
threads. Here and there are rounded clear spaces termed _vacuoles_,
which are filled with the watery fluid, cell-sap. The nuclei in mucor
are very minute, and cannot be seen except after careful treatment with
special reagents.
[Illustration: Fig. 6. Colonies of mucor.]
=15. Movement of the protoplasm in mucor.=—While examining the
protoplasm in mucor we are likely to note streaming movements. Often a
current is seen flowing slowly down one side of the thread, and another
flowing back on the other side, or it may all stream along in the same
direction.
=16. Test for protoplasm.=—Now let us treat the threads with
a solution of iodine. The yellowish-brown color appears which is
characteristic of protoplasm when subject to this reagent. If we
attempt to stain the living protoplasm with a one per cent aqueous
solution of eosin it resists it for a time, but if we first kill the
protoplasm with strong alcohol, it reacts quickly to the application of
the eosin. If we treat the living threads with glycerine the protoplasm
is contracted away from the wall, as we found to be the case with
spirogyra. While the color, form and structure of the plant mucor is
different from spirogyra, and the arrangement of the protoplasm within
the plant is also quite different, the reactions when treated by
certain reagents are the same. We are justified then in concluding that
the two plants possess in common a substance which we call protoplasm.
[Illustration: Fig. 7. Thread of mucor, showing protoplasm and
vacuoles.]
Protoplasm in nitella.
=17.= One of the most interesting plants for the study of one
remarkable peculiarity of protoplasm is _Nitella_. This plant belongs
to a small group known as stoneworts. They possess chlorophyll, and,
while they are still quite simple as compared with the higher plants,
they are much higher in the scale than spirogyra or mucor.
=18. Form of nitella.=—A common species of nitella is _Nitella
flexilis_. It grows in quiet pools of water. The plant consists of a
main axis, in the form of a cylinder. At quite regular intervals are
whorls of several smaller thread-like outgrowths, which, because of
their position, are termed “leaves,” though they are not true leaves.
These are branched in a characteristic fashion at the tip. The main
axis also branches, these branches arising in the axil of a whorl,
usually singly. The portions of the axis where the whorls arise are the
_nodes_. Each node is made up of a number of small cells definitely
arranged. The portion of the axis between two adjacent whorls is an
internode. These internodes are peculiar. They consist of but a single
“cell,” and are cylindrical, with closed ends. They are sometimes 5-10
cm. long.
[Illustration: Fig. 8. Portion of plant nitella.]
=19. Internode of nitella.=—For the study of an internode of
nitella, a small one, near the end, or the ends of one of the “leaves”
is best suited, since it is more transparent. A small portion of the
plant should be placed on the glass slip in water with the cover glass
over a tuft of the branches near the growing end. Examined with the
microscope the green chlorophyll bodies, which form oval or oblong
discs, are seen to be very numerous. They lie quite closely side by
side and form in perfect rows along the inner surface of the wall.
One peculiar feature of the arrangement of the chlorophyll bodies is
that there are two lines, extending from one end of the internode to
the other on opposite sides, where the chlorophyll bodies are wanting.
These are known as neutral lines. They run parallel with the axis of
the internode, or in a more or less spiral manner as shown in fig. 9.
=20. Cyclosis in nitella.=—The chlorophyll bodies are stationary
on the inner surface of the wall, but if the microscope be properly
focussed just beneath this layer we notice a rotary motion of particles
in the protoplasm. There are small granules and quite large masses of
granular matter which glide slowly along in one direction on a given
side of the neutral line. If now we examine the protoplasm on the other
side of the neutral line, we see that the movement is in the opposite
direction. If we examine this movement at the end of an internode
the particles are seen to glide around the end from one side of the
neutral line to the other. So that when conditions are favorable, such
as temperature, healthy state of the plant, etc., this gliding of the
particles or apparent streaming of the protoplasm down one side of
the “cell,” and back upon the other, continues in an uninterrupted
rotation, or _cyclosis_. There are many nuclei in an internode of
nitella, and they move also.
=21. Test for protoplasm.=—If we treat the plant with a solution
of iodine we get the same reaction as in the case of spirogyra and
mucor. The protoplasm becomes yellowish-brown.
[Illustration: Fig. 9. Cyclosis in nitella.]
=22. Protoplasm in one of the higher plants.=—We now wish to
examine, and test for, protoplasm in one of the higher plants. Young
or growing parts of any one of various plants—the petioles of young
leaves, or young stems of growing plants—are suitable for study.
Tissue from the pith of corn (Zea mays) in young shoots just back of
the growing point or quite near the joints of older but growing corn
stalks furnishes excellent material.
If we should place part of the stem of this plant under the microscope
we should find it too opaque for observation of the interior of the
cells. This is one striking difference which we note as we pass from
the low and simple plants to the higher and more complex ones; not
only in general is there an increase of size, but also in general an
increase in thickness of the parts. The cells, instead of lying end to
end or side by side, are massed together so that the parts are quite
opaque. In order to study the interior of the plant we have selected
it must be cut into such thin layers that the light will pass readily
through them.
For this purpose we section the tissue selected by making with a razor,
or other very sharp knife, very thin slices of it. These are mounted in
water in the usual way for microscopic study. In this section we notice
that the cells are polygonal in form. This is brought about by mutual
pressure of all the cells. The granular protoplasm is seen to form a
layer just inside the wall, which is connected with the nuclear layer
by radiating strands of the same substance. The nucleus does not always
lie at the middle of the cell, but often is near one side. If we now
apply an alcohol solution of iodine the characteristic yellowish-brown
color appears. So we conclude here also that this substance is
identical with the living matter in the other very different plants
which we have studied.
=23. Movement of protoplasm in the higher plants.=—Certain
parts of the higher plants are suitable objects for the study of the
so-called streaming movement of protoplasm, especially the delicate
hairs, or thread-like outgrowths, such as the silk of corn, or the
delicate staminal hairs of some plants, like those of the common
spiderwort, tradescantia, or of the tradescantias grown for ornament in
greenhouses and plant conservatories.
Sometimes even in the living cells of the corn plant which we have just
studied, slow streaming or gliding movements of the granules are seen
along the strands of protoplasm where they radiate from the nucleus.
See note at close of this chapter.
=24. Movement of protoplasm in cells of the staminal hair of
“spiderwort.”=—A cell of one of these hairs from a stamen of a
tradescantia grown in glass houses is shown in fig. 10. The nucleus is
quite prominent, and its location in the cell varies considerably in
different cells and at different times. There is a layer of protoplasm
all around the nucleus, and from this the strands of protoplasm extend
outward to the wall layer. The large spaces between the strands are, as
we have found in other cases, filled with the cell-sap.
[Illustration: Fig. 10. Cell from stamen hair of tradescantia showing
movement of the protoplasm.]
An entire stamen, or a portion of the stamen, having
several hairs attached, should be carefully mounted
in water. Care should be taken that the room be not
cold, and if the weather is cold the water in which
the preparation is mounted should be warm. With these
precautions there should be little difficulty in
observing the streaming movement.
The movement is detected by observing the gliding of the granules.
These move down one of the strands from the nucleus along the wall
layer, and in towards the nucleus in another strand. After a little the
direction of the movement in any one portion may be reversed.
=25. Cold retards the movement.=—While the protoplasm is moving,
if we rest the glass slip on a block of ice, the movement will become
slower, or will cease altogether. Then if we warm the slip gently, the
movement becomes normal again. We may now apply here the usual tests
for protoplasm. The result is the same as in the former cases.
=26. Protoplasm occurs in the living parts of all plants.=—In
these plants representing such widely different groups, we find a
substance which is essentially alike in all. Though its arrangement
in the cell or plant body may differ in the different plants or in
different parts of the same plant, its general appearance is the same.
Though in the different plants it presents, while alive, varying
phenomena, as regards mobility, yet when killed and subjected to
well known reagents the reaction is in general identical. Knowing
by the experience of various investigators that protoplasm exhibits
these reactions under given conditions, we have demonstrated to
our satisfaction that we have seen protoplasm in the simple alga,
spirogyra, in the common mould, mucor, in the more complex stonewort,
nitella, and in the cells of tissues of the highest plants.
=27.= By this simple process of induction of these facts
concerning this substance in these different plants, we have learned an
important method in science study. Though these facts and deductions
are well known, the repetition of the methods by which they are
obtained on the part of each student helps to form habits of scientific
carefulness and patience, and trains the mind to logical processes in
the search for knowledge.
=28.= While we have by no means exhausted the study of protoplasm,
we can, from this study, draw certain conclusions as to its occurrence
and appearance in plants. Protoplasm is found in the living and growing
parts of all plants. It is a semi-fluid, or slimy, granular, substance;
in some plants, or parts of plants, the protoplasm exhibits a streaming
or gliding movement of the granules. It is irritable. In the living
condition it resists more or less for some time the absorption of
certain coloring substances. The water may be withdrawn by glycerine.
The protoplasm may be killed by alcohol. When treated with iodine it
becomes a yellowish-brown color.
_Note._ In some plants, like elodea for example, it
has been found that the streaming of the protoplasm is
often induced by some injury or stimulus, while in the
normal condition the protoplasm does not move.
FOOTNOTES:
[1] For apparatus, reagents, collection and preservation of material,
etc., see Appendix.
[2] If spirogyra is forming fruit some of the threads will be lying
parallel in pairs, and connected with short tubes. In some of the cells
there will be found rounded or oval bodies known as _zygospores_. These
may be seen in fig. 86, and will be described in another part of the
book.
[3] The most suitable preparations of mucor for study are made by
growing the plant in a nutrient substance which largely consists of
gelatine, or, better, agar-agar, a gelatinous preparation of certain
seaweeds. This, after the plant is sown in it, should be poured into
sterilised shallow glass plates, called Petrie dishes.
CHAPTER II.
ABSORPTION, DIFFUSION, OSMOSE.
=29.= We may next endeavor to learn how plants absorb water or
nutrient substances in solution. There are several very instructive
experiments, which can be easily performed, and here again some of the
lower plants will be found useful.
=30. Osmose in spirogyra.=—Let us mount a few threads of this
plant in water for microscopic examination, and then draw under the
cover glass a five per cent solution of ordinary table salt (NaCl)
with the aid of filter paper. We shall soon see that the result is
similar to that which was obtained when glycerine was used to extract
the water from the cell-sap, and to contract the protoplasmic membrane
from the cell wall. But the process goes on evenly and the plant is not
injured. The protoplasmic layer contracts slowly from the cell wall,
and the movement of the membrane can be watched by looking through the
microscope. The membrane contracts in such a way that all the contents
of the cell are finally collected into a rounded or oval mass which
occupies the center of the cell.
If we now add fresh water and draw off the salt solution, we can see
the protoplasmic membrane expand again, or move out in all directions,
and occupy its former position against the inner surface of the cell
wall. This would indicate that there is some pressure from within while
this process of absorption is going on, which causes the membrane to
move out against the cell wall.
The salt solution draws water from the cell-sap. There is thus a
tendency to form a vacuum in the cell, and the pressure on the outside
of the protoplasmic membrane causes it to move toward the center of the
cell. When the salt solution is removed and the thread of spirogyra is
again bathed with water, the movement of the water is _inward_ in the
cell. This would suggest that there is some substance dissolved in the
cell-sap which does not readily filter out through the membrane, but
draws on the water outside. It is this which produces the pressure from
within and crowds the membrane out against the cell wall again.
[Illustration: Fig. 11. Spirogyra before placing in salt solution.]
[Illustration: Fig. 12. Spirogyra in 5% salt solution.]
[Illustration: Fig. 13. Spirogyra from salt solution into water.]
=31. Turgescence.=—Were it not for the resistance which the cell
wall offers to the pressure from within, the delicate protoplasmic
membrane would stretch to such an extent that it would be ruptured, and
the protoplasm therefore would be killed. If we examine the cells at
the ends of the threads of spirogyra we shall see in most cases that
the cell wall at the free end is arched outward. This is brought about
by the pressure from within upon the protoplasmic membrane which itself
presses against the cell wall, and causes it to arch outward. This is
beautifully shown in the case of threads which are recently broken.
The cell wall is therefore _elastic_; it yields to a certain extent to
the pressure from within, but a point is soon reached beyond which it
will not stretch, and an equilibrium then exists between the pressure
from within on the protoplasmic membrane, and the pressure from without
by the elastic cell wall. This state of equilibrium in a cell is
_turgescence_, or such a cell is said to be _turgescent_, or _turgid_.
[Illustration: Fig. 14. Before treatment with salt solution.]
[Illustration: Fig. 15. After treatment with salt solution.]
[Illustration: Fig. 16. From salt solution placed in water.
Figs. 14-16.—Osmosis in threads of mucor.]
=32. Experiment with beet in salt and sugar solutions.=—We may now
test the effect of a five per cent salt solution on a portion of the
tissues of a beet or carrot. Let us cut several slices of equal size
and about 5mm in thickness. Immerse a few slices in water, a few in
a five per cent salt solution and a few in a strong sugar solution.
It should be first noted that all the slices are quite rigid when an
attempt is made to bend them between the fingers. In the course of one
or two hours or less, if we examine the slices we shall find that those
in water remain, as at first, quite rigid, while those in the salt and
sugar solutions are more or less flaccid or limp, and readily bend
by pressure between the fingers, the specimens in the salt solution,
perhaps, being more flaccid than those in the sugar solution. The salt
solution, we judge after our experiment with spirogyra, withdraws some
of the water from the cell-sap, the cells thus losing their turgidity
and the tissues becoming limp or flaccid from the loss of water.
[Illustration: Fig. 17. Before treatment with salt solution.]
[Illustration: Fig. 18. After treatment with salt solution.]
[Illustration: Fig. 19. From salt solution into water again.
Figs. 17-19.—Osmosis in cells of Indian corn.]
[Illustration: Fig. 20. Rigid condition of fresh beet section.]
[Illustration: Fig. 21. Limp condition after lying in salt solution.]
[Illustration: Fig. 22. Rigid again after lying again in water.
Figs. 20-22.—Turgor and osmosis in slices of beet.]
=33.= Let us now remove some of the slices of the beet from the
sugar and salt solutions, wash them with water and then immerse them in
fresh water. In the course of thirty minutes to one hour, if we examine
them again, we find that they have regained, partly or completely,
their rigidity. Here again we infer from the former experiment with
spirogyra that the substances in the cell-sap now draw water inward;
that is, the diffusion current is inward through the cell walls and the
protoplasmic membrane, and the tissue becomes turgid again.
[Illustration: Fig. 23. Before treatment with salt solution.]
[Illustration: Fig. 24. After treatment with salt solution.]
[Illustration: Fig. 25. Later stage of the same.
Figs. 23-25.—Cells from beet treated with salt solution to show
osmosis and movement of the protoplasmic membrane.]
=34. Osmose in the cells of the beet.=—We should now make a
section of the fresh tissue of a red colored beet for examination with
the microscope, and treat this section with the salt solution. Here we
can see that the effect of the salt solution is to draw water out of
the cell, so that the protoplasmic membrane can be seen to move inward
from the cell wall just as was observed in the case of spirogyra.[4]
Now treating the section with water and removing the salt solution, the
diffusion current is in the opposite direction, that is inward through
the protoplasmic membrane, so that the latter is pressed outward until
it comes in contact with the cell wall again, which by its elasticity
soon resists the pressure and the cells again become turgid.
=35. The coloring matter in the cell-sap does not readily escape
from the living protoplasm of the beet.=—The red coloring matter,
as seen in the section under the microscope, does not escape from the
cell-sap through the protoplasmic membrane. When the slices are placed
in water, the water is not colored thereby. The same is true when the
slices are placed in the salt or sugar solutions. Although water is
withdrawn from the cell-sap, this coloring substance does not escape,
or if it does it escapes slowly and after a considerable time.
=36. The coloring matter escapes from dead protoplasm.=—If,
however, we heat the water containing a slice of beet up to a point
which is sufficient to kill the protoplasm, the red coloring matter
in the cell-sap filters out through the protoplasmic membrane and
colors the water. If we heat a preparation made for study under the
microscope up to the thermal death point we can see here that the red
coloring matter escapes through the membrane into the water outside.
This teaches that certain substances cannot readily filter through
the living membrane of protoplasm, but that they can filter through
when the protoplasm is dead. A very important condition, then, for
the successful operation of some of the physical processes connected
with absorption in plants is that the protoplasm should be in a living
condition.
=37. Osmose experiments with leaves.=—We may next take the leaves
of certain plants like the geranium, coleus or other plant, and place
them in shallow vessels containing water, salt, and sugar solutions
respectively. The leaves should be immersed, but the petioles should
project out of the water or solutions. Seedlings of corn or beans,
especially the latter, may also be placed in these solutions, so that
the leafy ends are immersed. After one or two hours an examination
shows that the specimens in the water are still turgid. But if we lift
a leaf or a bean plant from the salt or sugar solution, we find that
it is flaccid and limp. The blade, or lamina, of the leaf droops as if
wilted, though it is still wet. The bean seedling also is flaccid, the
succulent stem bending nearly double as the lower part of the stem is
held upright. This loss of turgidity is brought about by the loss of
water from the tissues, and judging from the experiments on spirogyra
and the beet, we conclude that the loss of turgidity is caused by the
withdrawal of some of the water from the cell-sap by the strong salt
solution.
=38.= Now if we wash carefully these leaves and seedlings, which
have been in the salt and sugar solutions, with water, and then immerse
them in fresh water for a few hours, they will regain their turgidity.
Here again we are led to infer that the diffusion current is now inward
through the protoplasmic membranes of all the living cells of the leaf,
and that the resulting turgidity of the individual cells causes the
turgidity of the leaf or stem.
[Illustration: Fig. 26. Seedling of radish, showing root hairs.]
=39. Absorption by root hairs.=—If we examine seedlings, which
have been grown in a germinator or in the folds of paper or cloths so
that the roots will be free from particles of soil, we see near the
growing point of the roots that the surface is covered with numerous
slender, delicate, thread-like bodies, the root hairs. Let us place a
portion of a small root containing some of these root hairs in water on
a glass slip, and prepare it for examination with the microscope. We
see that each thread, or root hair, is a continuous tube, or in other
words it is a single cell which has become very much elongated. The
protoplasmic membrane lines the wall, and strands of protoplasm extend
across at irregular intervals, the interspaces being occupied by the
cell-sap.
[Illustration: Fig. 27. Root hair of corn before and after treatment
with 5% salt solution.]
We should now draw under the cover glass some of the five per cent salt
solution. The protoplasmic membrane moves away from the cell wall at
certain points, showing that _plasmolysis_ is taking place, that is,
the diffusion current is outward so that the cell-sap loses some of its
water, and the pressure from the outside moves the membrane inward.
We should not allow the salt solution to work on the root hairs long.
It should be very soon removed by drawing in fresh water before the
protoplasmic membrane has been broken at intervals, as is apt to be the
case by the strong diffusion current and the consequent strong pressure
from without. The membrane of protoplasm now moves outward as the
diffusion current is inward, and soon regains its former position next
the inner side of the cell wall. The root hairs then, like other parts
of the plant which we have investigated, have the power of taking up
water under pressure.
=40. Cell-sap a solution of certain substances.=—From these
experiments we are led to believe that certain substances reside in the
cell-sap of plants, which behave very much like the salt solution when
separated from water by the protoplasmic membrane. Let us attempt to
interpret these phenomena by recourse to diffusion experiments, where
an animal membrane separates two liquids of different concentration.
=41. An artificial cell to illustrate turgor.=—Fill a small
wide-mouthed vial with a _very strong_ sugar solution. Over the mouth
tie firmly a piece of _bladder_ membrane. Be certain that as the
membrane is tied over the open end of the vial, the sugar solution
fills it in order to keep out air bubbles. Sink the vial in a vessel of
fresh water and leave it there for twenty-four hours. Remove the vial
and note that the membrane is arched outward. Thrust a sharp needle
through the membrane when it is arched outward, and quickly pull it
out. The liquid spurts out because of the inside pressure.
[Illustration: FIG. 28. Puncturing a make-believe cell after
it has been lying in water.]
[Illustration: FIG. 29. Same as Fig. 28 after needle is removed.]
=42. Diffusion through an animal membrane.=—For this experiment
we may use a thistle tube, across the larger end of which should be
stretched and tied tightly a piece of a bladder membrane. A strong
sugar solution (three parts sugar to one part water) is now placed in
the tube so that the bulb is filled and the liquid extends part way in
the neck of the tube. This is immersed in water within a wide-mouth
bottle, the neck of the tube being supported in a perforated cork in
such a way that the sugar solution in the tube is on a level with the
water in the bottle or jar. In a short while the liquid begins to
rise in the thistle tube, in the course of several hours having risen
several centimeters. The diffusion current is thus stronger through the
membrane in the direction of the sugar solution, so that this gains
more water than it loses.
We have here two liquids separated by an animal membrane, water on
the one hand which diffuses readily through the membrane, while on
the other is a solution of sugar which diffuses through the animal
membrane with difficulty. The water, therefore, not containing any
solvent, according to a general law which has been found to obtain in
such cases, diffuses more readily through the membrane into the sugar
solution, which thus increases in volume, and also becomes more dilute.
The bladder membrane is what is sometimes called a diffusion membrane,
since the diffusion currents travel through it.
=43.= In this experiment then the bulk of the sugar solution is
increased, and the liquid rises in the tube by this pressure above
the level of the water in the jar outside of the thistle tube. The
diffusion of liquids through a membrane is _osmosis_.
=44. Importance of these physical processes in plants.=—Now if
we recur to our experiment with spirogyra we find that exactly the
same processes take place. The protoplasmic membrane is the diffusion
membrane, through which the diffusion takes place. The salt solution
which is first used to bathe the threads of the plant is a stronger
solution than that of the cell-sap within the cell. Water therefore
is drawn out of the cell-sap, but the substances in solution in
the cell-sap do not readily move out. As the bulk of the cell-sap
diminishes the pressure from the outside pushes the protoplasmic
membrane away from the wall. Now when we remove the salt solution and
bathe the thread with water again, the cell-sap, being a solution of
certain substances, diffuses with more difficulty than the water, and
the diffusion current is inward, while the protoplasmic membrane moves
out against the cell wall, and turgidity again results. Also in the
experiments with salt and sugar solutions on the leaves of geranium,
on the leaves and stems of the seedlings, on the tissues and cells of
the beet and carrot, and on the root hairs of the seedlings, the same
processes take place.
These experiments not only teach us that in the protoplasmic membrane,
the cell wall, and the cell-sap of plants do we have structures which
are capable of performing these physical processes, but they also show
that these processes are of the utmost importance to the plant; not
only in giving the plant the power to take up solutions of nutriment
from the soil, but they serve also other purposes, as we shall see
later.
FOOTNOTE:
[4] We should note that the coloring matter of the beet resides in
the cell-sap. It is in these colored cells that we can best see the
movement take place, since the red color serves to differentiate well
the moving mass from the cell wall. The protoplasmic membrane at
several points usually clings tenaciously so that at several places the
membrane is arched strongly away from the cell wall as shown in fig.
24. While water is removed from the cell-sap, we note that the coloring
matter does not escape through the protoplasmic membrane.
CHAPTER III.
HOW PLANTS OBTAIN WATER.
In connection with the study of the means of absorption from the soil
or water by plants, it will be found convenient to observe carefully
the various forms of the plant. Without going into detail here, the
suggestion is made that simple thread forms like spirogyra, œdogonium,
and vaucheria; expanded masses of cells as are found in the thalloid
liverworts, the duckweed, etc., be compared with those liverworts, and
with the mosses, where leaf-like expansions of a central axis have been
differentiated. We should then note how this differentiation, from the
physiological standpoint, has been carried farther in the higher land
plants.
=45. Absorption by Algæ and Fungi.=—In the simpler forms of
plant life, as in spirogyra and many of the algæ and fungi, the plant
body is not differentiated into parts.[5] In many other cases the only
differentiation is between the growing part and the fruiting part. In
the algæ and fungi there is no differentiation into stem and leaf,
though there is an approach to it in some of the higher forms. Where
this simple plant body is flattened, as in the sea-wrack, or ulva, it
is a _frond_. The Latin word for frond is _thallus_, and this name is
applied to the plant body of all the lower plants, the algæ and fungi.
The algæ and fungi together are sometimes called the _thallophytes_,
or _thallus plants_. The word thallus is also sometimes applied to the
flattened body of the liverworts. In the foliose liverworts and mosses
there is an axis with leaf-like expansions. These are believed by some
to represent true stems and leaves, by others to represent a flattened
thallus in which the margins are deeply and regularly divided, or in
which the expansion has only taken place at regular intervals.
In nearly all of the algæ the plant body is submerged in water. In these
cases absorption takes place through all portions of the surface in
contact with the water, as in spirogyra, vaucheria, and all of the
larger seaweeds. Comparatively few of the algæ grow on the surfaces
of rocks or trees. In these examples it is likely that at times only
portions of the plant body serve in the process of absorption of water
from the substratum. A few of the algæ are parasitic, living in the
tissues of higher plants, where they are surrounded by the water or
liquids within the host. Absorption takes place in the same way in many
of the fungi. The aquatic fungi are immersed in water. In other forms,
like mucor, a portion of the mycelium is within the substratum, and
being bathed by the water or watery solutions absorbs the same, while
the fruiting portion and the aerial mycelium obtain their water and
food solutions from the mycelium in the substratum. In higher fungi,
like the mushrooms, the mycelium within the ground or decaying wood
absorbs the water necessary for the fruiting portion; while in the case
of the parasitic fungi the mycelium lies in the water or liquid within
the host.
[Illustration: Fig. 30. Thallus of Riccia lutescens.]
=46. Absorption by liverworts.=—In many of the plants termed
liverworts the vegetative part of the plant is a thin, flattened, more
or less elongated green body known as a thallus.
_Riccia._—One of these, belonging to the genus riccia, is shown in
fig. 30. Its shape is somewhat like that of a minute ribbon which is
forked at intervals in a dichotomous manner, the characteristic kind of
branching found in these thalloid liverworts. This riccia (known as R.
lutescens) occurs on damp soil; long, slender, hair-like processes grow
out from the under surface of the thallus which resemble root hairs and
serve the same purpose in the processes of absorption. Another species
of riccia (R. crystallina) is shown in fig. 252. This plant is quite
circular in outline and occurs on muddy flats. Some species float on
the water.
=47. Marchantia.=—One of the larger and coarser liverworts is figured
at 31. This is a very common liverwort, growing in very damp and muddy
places and also along the margins of streams, on the mud or upon
the surfaces of rocks which are bathed with the water. This is known
as _Marchantia polymorpha_. If we examine the under surface of the
marchantia we see numerous hair-like processes which attach the plant
to the soil. Under the microscope we see that some of these are similar
to the root hairs of the seedlings which we have been studying, and
they serve the purpose of absorption. Since, however, there are no
roots on the marchantia plant, these hair-like outgrowths are usually
termed here _rhizoids_. In marchantia they are of two kinds, one kind
the simple ones with smooth walls, and the other kind in which the
inner surfaces of the walls are roughened by processes which extend
inward in the form of irregular tooth-like points. Besides the hairs on
the under side of the thallus we note especially near the growing end
that there are two rows of leaf-like scales, those at the end of the
thallus curving up over the growing end, thus serving to protect the
delicate tissues at the growing point.
[Illustration: Fig. 31. Marchantia plant with cupules and gemmæ;
rhizoids below.]
[Illustration: Fig. 32. Portion of plant of Frullania, a foliose
liverwort.]
[Illustration: Fig. 33. Portion of same more highly magnified, showing
overlapping leaves.]
[Illustration: Fig. 34. Under side, showing forked under row of leaves
and lobes of lateral leaves.]
=48. Frullania.=—In fig. 32 is shown another liverwort, which
differs greatly in form from the ones we have just been studying in
that there is a well-defined axis with lateral leaf-like outgrowths.
Such liverworts are called foliose liverworts. Besides these two quite
prominent rows of leaves there is a third row of poorly developed
leaves on the under surface. Also from the under surface of the axis we
see here and there slender outgrowths, the rhizoids, through which much
of the water is absorbed.
[Illustration: Fig. 35. Foliose liverwort (bazzania) showing
dichotomous branching and overlapping leaves.]
=49. Absorption by the mosses.=—Among the mosses, which are
usually common in moist and shaded situations, examples are abundant
which are suitable for the study of the organs of absorption. If we
take for example a plant of mnium (M. affine), which is illustrated in
fig. 36, we note that it consists of a slender axis with thin flat,
green, leaf-like expansions, Examining with the microscope the lower
end of the axis, which is attached to the substratum, there are seen
numerous brown-colored threads more or less branched.
[Illustration: Fig. 36. Female plant (gametophyte) of a moss (mnium),
showing rhizoids below, and the tuft of leaves above, which protect the
archegonia.]
=50. Absorption by the higher aquatic plants.=—Examples of
the water plants which are entirely submerged in water are the
water-crowfoots, some of the pondweeds, elodea or water-weeds, the
tape-grass, vallisneria, etc. In these plants all parts of the body
being submerged, they absorb water with which they are in contact. In
other aquatic plants, like the water-lilies, some of the pondweeds, the
duck-meats, etc., are only partially submerged in the water; the upper
surface of the leaf or of the leaf-like expansion being exposed to the
air, while the under surface lies in close contact with the water, and
the stems and the petioles of the leaves are also immersed in water. In
these plants absorption takes place through those parts in contact with
the water.
=51. Absorption by the duck-meats.=—These plants are very curious
examples of the higher plants.
_Lemna._—One of these is illustrated in fig. 37.
This is the common duckweed, _Lemna trisulca_. It
is very peculiar in form and in its mode of growth.
Each one of the lateral leaf-like expansions extends
outwards by the elongation of the basal part, which
becomes long and slender. Next, two new lateral
expansions are formed on these by prolification from
near the base, and thus the plant continues to extend.
The plant occurs in ponds and ditches and is sometimes
very common and abundant. It floats on the surface
of the water. While the flattened part of the plant
resembles a leaf, it is really the stem, no leaves
being present. This expanded green body is usually
termed a “frond.” A single rootlet grows out from the
under side and is destitute of root hairs. Absorption
of water therefore takes place through this rootlet and
through the under side of the “frond.”
[Illustration: Fig. 37. Fronds of the duckweed (Lemna trisculca).]
[Illustration: Fig. 38. Spirodela polyrhiza.]
=52. Spirodela polyrhiza.=—This is a very curious plant, closely
related to the lemna and sometimes placed in the same genus. It occurs
in similar situations, and is very readily grown in aquaria. It reminds
one of a little insect as seen in fig. 38. There are several rootlets
on the under side of the frond. Absorption of water takes place here in
the same way as in lemna.
=53. Absorption in wolffia.=—Perhaps the most curious of these
modified water plants is the little wolffia, which contains the
smallest specimens of the flowering plants. Two species of this genus
are shown in figs. 39-41. The plant body is reduced to nothing but a
rounded or oval green body, which represents the stem. No leaves or
roots are present. The plants multiply by “prolification,” the new
fronds growing out from a depression on the under side of one end.
Absorption takes place through the under surface.
=54. Absorption by land plants.=—_Water cultures._—In connection
with our inquiry as to how land plants obtain their water, it will be
convenient to prepare some water cultures to illustrate this and which
can also be used later in our study of nutrition (Chapter IX).
[Illustration: Fig. 39. Young frond of wolffia growing out of older
one.]
[Illustration: Fig. 40. Young frond of wolffia separating from older
one.]
[Illustration: Fig. 41. Another species of wolffia, the two fronds
still connected.]
Chemical analysis shows that certain mineral substances are common
constituents of plants. By growing plants in different solutions of
these various substances it has been possible to determine what ones
are necessary constituents of plant food. While the proportion of the
mineral elements which enter into the composition of plant food may
vary considerably within certain limits, the concentration of the
solutions should not exceed certain limits. A very useful solution is
one recommended by Sachs, and is as follows:
=55. Formula for water cultures=:
Water 1000 cc.
Potassium nitrate 0.5 gr.
Sodium chloride 0.5 “
Calcium sulphate 0.5 “
Magnesium sulphate 0.5 “
Calcium phosphate 0.5 “
The calcium phosphate is only partly soluble. The solution which is not
in use should be kept in a dark cool place to prevent the growth of
minute algæ.
=56.= Several different plants are useful for experiments in water
cultures, as peas, corn, beans, buckwheat, etc. The seeds of these
plants may be germinated, after soaking them for several hours in warm
water, by placing them between the folds of wet paper on shallow trays,
or in the folds of wet cloth. The seeds should not be kept immersed in
water after they have imbibed enough to thoroughly soak and swell them.
At the same time that the seeds are placed in damp paper or cloth for
germination, one lot of the soaked seeds should be planted in good soil
and kept under the same temperature conditions, for control. When the
plants have germinated one series should be grown in distilled water,
which possesses no plant food; another in the nutrient solution, and
still another in the nutrient solution to which has been added a few
drops of a solution of iron chloride or ferrous sulphate. There would
then be four series of cultures which should be carried out with the
same kind of seed in each series so that the comparisons can be made on
the same species under the different conditions. The series should be
numbered and recorded as follows:
No. 1, soil.
No. 2, distilled water.
No. 3, nutrient solution.
No. 4, nutrient solution with a few drops of iron solution added.
[Illustration: Fig. 42. Culture cylinder to show position of
corn seedling (Hansen).]
=57.= Small jars or wide-mouth bottles, or crockery jars, can be
used for the water cultures, and the cultures are set up as follows: A
cork which will just fit in the mouth of the bottle, or which can be
supported by pins, is perforated so that there is room to insert the
seedling, with the root projecting below into the liquid. The seed can
be fastened in position by inserting a pin through one side, if it is a
large one, or in the case of small seeds a cloth of a coarse mesh can
be tied over the mouth of the bottle instead of using the cork. After
properly setting up the experiments the cultures should be arranged
in a suitable place, and observed from time to time during several
weeks. In order to obtain more satisfactory results several duplicate
series should be set up to guard against the error which might arise
from variation in individual plants and from accident. Where there are
several students in a class, a single series set up by several will
act as checks upon one another. If glass jars are used for the liquid
cultures they should be wrapped with black paper or cloth to exclude
the light from the liquid, otherwise numerous minute algæ are apt to
grow and interfere with the experiment. Or the jars may be sunk in pots
of earth to serve the same purpose. If crockery jars are used they will
not need covering.
=58.= For some time all the plants grow equally well, until the
nutriment stored in the seed is exhausted. The numbers 1, 3 and 4, in
soil and nutrient solutions, should outstrip number 2, the plants in
the distilled water. No. 4 in the nutrient solution with iron, having a
perfect food, compares favorably with the plants in the soil.
=59. Plants take liquid food from the soil.=—From these experiments
then we judge that such plants take up the food they receive from the
soil in the form of a liquid, the elements being in solution in water.
If we recur now to the experiments which were performed with the salt
solution in producing plasmolysis in the cells of spirogyra, in the
cells of the beet or corn, and in the root hairs of the corn and bean
seedlings, and the way in which these cells become turgid again when
the salt solution is removed and they are again bathed with water, we
shall have an explanation of the way in which plants take up nutrient
solutions of food material through their roots.
[Illustration: Fig. 43. Section of corn root, showing rhizoids formed
from elongated epidermal cells.]
=60. How food solutions are carried into the plant.=—We can see
how water and food solutions are carried into the plant, and we must
next turn our attention to the way in which these solutions are carried
farther into the plant. We should make a section across the root of a
seedling in the region of the root hairs and examine it with the aid
of a microscope. We here see that the root hairs are formed by the
elongation of certain of the surface cells of the root. These cells
elongate perpendicularly to the root, and become _3mm_ to _6mm_ long.
They are flexuous or irregular in outline and cylindrical, as shown in
fig. 43. The end of the hair next the root fits in between the adjacent
superficial cells of the root and joins closely to the next deeper
layer of cells. In studying the section of the young root we see that
the root is made up of cells which lie closely side by side, each with
its wall, its protoplasm and cell-sap, the protoplasmic membrane lying
on the inside of each cell wall.
=61.= In the absorption of the watery solutions of plant food
by the root hairs, the cell-sap, being a more concentrated solution,
gains some of the former, since the liquid of less concentration flows
through the protoplasmic membrane into the more concentrated cell-sap,
increasing the bulk of the latter. This makes the root hairs turgid,
and at the same time dilutes the cell-sap so that the concentration is
not so great. The cells of the root lying inside and close to the base
of the root hairs have a cell-sap which is now more concentrated than
the diluted cell-sap of the hairs, and consequently gain some of the
food solutions from the latter, which tends to lessen the content of
the root hairs and also to increase the concentration of the cell-sap
of the same. This makes it possible for the root hairs to draw on
the soil for more of the food solutions, and thus, by a variation in
the concentration of the substances in solution in the cell-sap of
the different cells, the food solutions are carried along until they
reach the _vascular bundles_, through which the solutions are carried
to distant parts of the plant. Some believe that there is a rhythmic
action of the elastic cell walls in these cells between the root hairs
and the vascular bundles. This occurs in such a way that, after the
cell becomes turgid, it contracts, thus reducing the size of the cell
and forcing some of the food solutions into the adjacent cells, when
by absorption of more food solutions, or water, the cell increases in
turgidity again. This rhythmic action of the cells, if it does take
place, would act as a pump to force the solutions along, and would form
one of the causes of root pressure.
=62. How the root hairs get the watery solutions from the
soil.=—If we examine the root hairs of a number of seedlings which
are growing in the soil under normal conditions, we shall see that a
large quantity of soil readily clings to the roots. We should note also
that unless the soil has been recently watered there is no free water
in it; the soil is only moist. We are curious to know how plants can
obtain water from soil which is not wet. If we attempt to wash off the
soil from the roots, being careful not to break away the root hairs,
we find, that small particles cling so tenaciously to the root hairs
that they are not removed. Placing a few such root hairs under the
microscope it appears as if here and there the root hairs were glued to
the minute soil particles.
[Illustration: Fig. 44. Root hairs of corn seedling with soil particles
adhering closely.]
=63.= If now we take some of the soil which is only moist, weigh
it, and then permit it to become quite dry on exposure to dry air, and
weigh again, we find that it loses weight in drying. Moisture has been
given off. This moisture, it has been found, forms an exceedingly thin
film on the surface of the minute soil particles. Where these soil
particles lie closely together, as they usually do when massed together
in the pot or elsewhere, this thin film of moisture is continuous from
the surface of one particle to that of another. Thus the soil particles
which are so closely attached to the root hairs connect the surface
of the root hairs with this film of moisture. As the cell-sap of the
root hairs draws on the moisture film with which they are in contact,
the tension of this film is sufficient to draw moisture from distant
particles. In this way the roots are supplied with water in soil which
is only moist.
=64. Plants cannot remove all the moisture from the soil.=—If we
now take a potted plant, or a pot containing a number of seedlings,
place it in a moderately dry room, and do not add water to the soil we
find in a few days that the plant is wilting. The soil if examined will
appear quite dry to the sense of touch. Let us weigh some of this soil,
then dry it by artificial heat, and weigh again. It has lost in weight.
This has been brought about by driving off the moisture which still
remained in the soil after the plant began to wilt. This teaches that
while plants can obtain water from soil which is only moist or which is
even rather dry, they are not able to withdraw all the moisture from
the soil.
[Illustration: Fig. 45. Experiment to show root pressure (Detmer).]
=65. “Root pressure” or exudation pressure.=—It is a very common
thing to note, when certain shrubs or vines are pruned in the spring,
the exudation of a watery fluid from the cut surfaces. In the case of
the grape vine this has been known to continue for a number of days,
and in some cases the amount of liquid, called “sap,” which escapes is
considerable. In many cases it is directly traceable to the activity
of the roots, or root hairs, in the absorption of water from the soil.
For this reason the term _root pressure_ has been used to denote the
force exerted in supplying the water from the soil. But there are some
who object to the use of this term “root pressure.” The principal
objection is that the pressure which brings about the phenomenon
known as “bleeding” by plants is not present in the roots alone. This
pressure exists under certain conditions in all parts of the plant. The
term exudation pressure has been proposed in lieu of root pressure. It
should be remembered that the movement of water in the plant is started
by the pressure which exists in the root. If the term “root pressure”
is used, it should be borne clearly in mind that it does not express
the phenomenon exactly in all cases.
=Root pressure may be measured.=—It is possible to measure
not only the amount of water which the roots will raise in a given
time, but also to measure the force exerted by the roots during root
pressure. It has been found that root pressure in the case of the
nettle is sufficient to hold a column of water about 4.5 meters (15
ft.) high (Vines), while the root pressure of the vine (Hales, 1721)
will hold a column of water about 10 meters (36.5 ft.) high, and the
birch (Betula lutea) (Clark, 1873) has a root pressure sufficient to
hold a column of water about 25 meters (84.7 ft.) high.
=66. Experiment to demonstrate root pressure.=—By a very simple
method this lifting of water by root pressure is shown. During the
summer season plants in the open may be used if it is preferred, but
plants grown in pots are also very serviceable, and one may use a
potted begonia or balsam, the latter being especially useful. The
plants are usually convenient to obtain from the greenhouses, to
illustrate this phenomenon. The stem is cut off rather close to the
soil and a long glass tube is attached to the cut end of the stem,
still connected with the roots, by the use of rubber tubing, as shown
in figure 45, and a very small quantity of water may be poured in to
moisten the cut end of the stem. In a few minutes the water begins to
rise in the glass tube. In some cases it rises quite rapidly, so that
the column of water can readily be seen to extend higher and higher up
in the tube when observed at quite short intervals. (To measure the
force of root pressure is rather difficult for elementary work. To
measure it see Ganong, Plant Physiology, pp. 67, 68, or some other book
for advanced work.)
=67.= In either case where the experiment is continued for
several days it is noticed that the column of water or of mercury
rises and falls at different times during the same day, that is, the
column stands at varying heights; or in other words the root pressure
varies during the day. With some plants it has been found that the
pressure is greatest at certain times of the day, or at certain
seasons of the year. Such variation of root pressure exhibits what
is termed a periodicity, and in the case of some plants there is a
daily periodicity; while in others there is in addition an annual
periodicity. With the grape vine the root pressure is greatest in
the forenoon, and decreases from 12-6 P.M., while with the
sunflower it is greatest before 10 A.M., when it begins to
decrease. Temperature of the soil is one of the most important external
conditions affecting the activity of root pressure.
FOOTNOTE:
[5] See Chapter 38 for organization of members of the plant body.
CHAPTER IV.
TRANSPIRATION, OR THE LOSS OF WATER BY PLANTS.
=68.= We should now inquire if all the water which is taken up in
excess of that which actually suffices for turgidity is used in the
elaboration of new materials of construction. We notice when a leaf
or shoot is cut away from a plant, unless it is kept in quite a moist
condition, or in a damp, cool place, that it becomes flaccid, and
droops. It wilts, as we say. The leaves and shoot lose their turgidity.
This fact suggests that there has been a loss of water from the shoot
or leaf. It can be readily seen that this loss is not in the form of
drops of water which issue from the cut end of the shoot or petiole.
What then becomes of the water in the cut leaf or shoot?
=69. Loss of water from excised leaves.=—Let us take a handful of
fresh, green, rather succulent leaves, which are free from water on
the surface, and place them under a glass bell jar, which is tightly
closed below but which contains no water. Now place this in a brightly
lighted window, or in sunlight. In the course of fifteen to thirty
minutes we notice that a thin film of moisture is accumulating on the
inner surface of the glass jar. After an hour or more the moisture has
accumulated so that it appears in the form of small drops of condensed
water. We should set up at the same time a bell jar in exactly the same
way but which contains no leaves. In this jar there is no condensed
moisture on the inner surface. We thus are justified in concluding that
the moisture in the former jar comes from the leaves. Since there is no
visible water on the surfaces of the leaves, or at the cut ends, before
it may have condensed there, we infer that the water escapes from the
leaves in the form of _water vapor_, and that this water vapor, when
it comes in contact with the surface of the cold glass, condenses and
forms the moisture film, and later the drops of water. The leaves of
these cut shoots therefore lose water in the form of water vapor, and
thus a loss of turgidity results.
[Illustration: Fig. 46. To show loss of water from leaves, the leaves
just covered.]
[Illustration: Fig. 47. After a few hours drops of water have
accumulated on the inside of the jar covering the leaves.]
=70. Loss of water from growing plants.=—Suppose we now take a
small and actively growing plant in a pot, and cover the pot and the
soil with a sheet of rubber cloth or flexible oilcloth which fits
tightly around the stem of the plant so that the moisture from the soil
or from the surface of the pot cannot escape. Then place a bell jar
over the plant, and set in a brightly lighted place, at a temperature
suitable for growth. In the course of a few minutes on a dry day a
moisture film forms on the inner surface of the glass, just as it did
in the case of the glass jar containing the cut shoots and leaves.
Later the moisture has condensed so that it is in the form of drops. If
we have the same leaf surface here as we had with the cut shoots, we
shall probably find that a larger amount of water accumulates on the
surface of the jar from the plant that is still attached to its roots.
=71. Water escapes from the surfaces of living leaves in the form of
water vapor.=—This living plant then has lost water, which also
escapes in the form of water vapor. Since here there are no cut places
on the shoots or leaves, we infer that the loss of water vapor takes
place from the surfaces of the leaves and from the shoots. It is also
to be noted that, while this plant is losing water from the surfaces of
the leaves, it does not wilt or lose its turgidity. The roots by their
activity and pressure supply water to take the place of that which is
given off in the form of water vapor. This loss of water in the form of
water vapor by plants is _transpiration_.
=72. A test for the escape of water vapor from plants.=—Make
a solution of cobalt chloride in water. Saturate several pieces of
filter paper with it. Allow them to dry. The water solution of cobalt
chloride is red. The paper is also red when it is moist, but when it
is thoroughly dry it is blue. It is very sensitive to moisture and the
moisture of the air is often sufficient to redden it. Before using dry
the paper in an oven or over a flame.
=73.= Take two bell jars, as shown in fig. 49. Under one place a
potted plant, the pot and earth being covered by oiled paper. Or cover
the plant with a fruit jar. To a stake in the pot pin a piece of the
dried cobalt paper, and at the same time pin to a stake, in another
jar covering no plant, another piece of cobalt paper. They should both
be put under the jars at the same time. In a few moments the paper in
the jar with the plant will begin to redden. In a short while, ten or
fifteen minutes, probably, it will be entirely red, while the paper
under the other jar will remain blue, or be only slightly reddened. The
water vapor passing off from the living plant comes in contact with
the sensitive cobalt chloride in the paper and reddens it before there
is sufficient vapor present to condense as a film of moisture on the
surface of the jar.
[Illustration: Fig. 48.]
[Illustration: Fig. 49.
Fig. 48.—Water vapor is given off by the leaves when attached to the
living plant. It condenses into drops of water on the cool surface of
the glass covering the plant.
Fig. 49.—A good way to show that the water passes off from the leaves
in the form of water vapor.]
=74. Experiment to compare loss of water in a dry and a humid
atmosphere.=—We should now compare the escape of water from the
leaves of a plant covered by a bell jar, as in the last experiment,
with that which takes place when the plant is exposed in a normal way
in the air of the room or in the open. To do this we should select
two plants of the same kind growing in pots, and of approximately the
same leaf surface. The potted plants are placed one each on the arms
of a scale. One of the plants is covered in this position with a bell
jar. With weights placed on the pan of the other arm the two sides are
balanced. In the course of an hour, if the air of the room is dry,
moisture has probably accumulated on the inner surface of the glass jar
which is used to cover one of the plants. This indicates that there has
here been a loss of water. But there is no escape of water vapor into
the surrounding air so that the weight on this arm is practically the
same as at the beginning of the experiment. We see, however, that the
other arm of the balance has risen. We infer that this is the result of
the loss of water vapor from the plant on that arm. Now let us remove
the bell jar from the other plant, and with a cloth wipe off all the
moisture from the inner surface, and replace the jar over the plant. We
note that the end of the scale which holds this plant is still lower
than the other end.
=75. The loss of water is greater in a dry than in a humid
atmosphere.=—This teaches us that while water vapor escaped from
the plant under the bell jar, the air in this receiver soon became
saturated with the moisture, and thus the farther escape of moisture
from the leaves was checked. It also teaches us another very important
fact, viz., that plants lose water more rapidly through their leaves in
a dry air than in a humid or moist atmosphere. We can now understand
why it is that during the very hot and dry part of certain days plants
often wilt, while at nightfall, when the atmosphere is more humid, they
revive. They lose more water through their leaves during the dry part
of the day, other things being equal, than at other times.
=76. How transpiration takes place.=—Since the water of transpiration
passes off in the form of water vapor we are led to inquire if this
process is simply _evaporation_ of water through the surface of the
leaves, or whether it is controlled to any appreciable extent by any
condition of the living plant. An experiment which is instructive in
this respect we shall find in a comparison between the transpiration of
water from the leaves of a cut shoot, allowed to lie unprotected in a
dry room, and a similar cut shoot the leaves of which have been killed.
=77.= Almost any plant will answer for the experiment. For this
purpose I have used the following method. Small branches of the locust
(Robinia pseudacacia), of sweet clover (Melilotus alba), and of a
heliopsis were selected. One set of the shoots was immersed for a
moment in hot water near the boiling point to kill them. The other set
was immersed for the same length of time in cold water, so that the
surfaces of the leaves might be well wetted, and thus the two sets of
leaves at the beginning of the experiment would be similar, so far as
the amount of water on their surfaces is concerned. All the shoots were
then spread out on a table in a dry room, the leaves of the killed
shoots being separated where they are inclined to cling together. In
a short while all the water has evaporated from the surface of the
living leaves, while the leaves of the dead shoots are still wet on
the surface. In six hours the leaves of the dead shoots from which the
surface water had now evaporated were beginning to dry up, while the
leaves of the living plants were only becoming flaccid. In twenty-four
hours the leaves of the dead shoots were crisp and brittle, while those
of the living shoots were only wilted. In twenty-four hours more the
leaves of the sweet clover and of the heliopsis were still soft and
flexible, showing that they still contained more water than the killed
shoots which had been crisp for more than a day.
=78.= It must be then that during what is termed transpiration
the living plant is capable of holding back the water to some extent,
which in a dead plant would escape more rapidly by evaporation. It is
also known that a body of water with a surface equal to that of a given
leaf surface of a plant loses more water by evaporation during the same
length of time than the plant loses by transpiration.
=79. Structure of a leaf.=—We are now led to inquire why it is
that a living leaf loses water less rapidly than dead ones, and why
less water escapes from a given leaf surface than from an equal surface
of water. To understand this it will be necessary to examine the minute
structure of a leaf. For this purpose we may select the leaf of an ivy,
though many other leaves will answer equally well. From a portion of
the leaf we should make very thin cross-sections with a razor or other
sharp instrument. These sections should be perpendicular to the surface
of the leaf and should be then mounted in water for microscopic
examination.[6]
=80. Epidermis of the leaf.=—In this section we see that the
green part of the leaf is bordered on what are its upper and lower
surfaces by a row of cells which possess no green color. The walls of
the cells of each row have nearly parallel sides, and the cross walls
are perpendicular. These cells form a single layer over both surfaces
of the leaf and are termed the _epidermis_. Their walls are quite stout
and the outer walls are _cuticularized_.
[Illustration: Fig. 50. Section through ivy leaf showing communication
between stomate and the large intercellular spaces of the leaf, stoma
closed.]
[Illustration: Fig. 51. Stoma open.]
[Illustration: Fig. 52. Stoma closed.
Figs. 51, 52.—Section through stomata of ivy leaf.]
=81. Soft tissue of the leaf.=—The cells which contain the green
chlorophyll bodies are arranged in two different ways. Those on the
upper side of the leaf are usually long and prismatic in form and
lie closely parallel to each other. Because of this arrangement of
these cells they are termed the _palisade_ cells, and form what is
called the _palisade layer_. The other green cells, lying below, vary
greatly in size in different plants and to some extent also in the same
plant. Here we notice that they are elongated, or oval, or somewhat
irregular in form. The most striking peculiarity, however, in their
arrangement is that they are not usually packed closely together, but
each cell touches the other adjacent cells only at certain points. This
arrangement of these cells forms quite large spaces between them, the
intercellular spaces. If we should examine such a section of a leaf
before it is mounted in water we would see that the intercellular
spaces are not filled with water or cell-sap, but are filled with air
or some gas. Within the cells, on the other hand, we find the cell-sap
and the protoplasm.
=82. Stomata.=—If we examine carefully the row of epidermal cells
on the under surface of the leaf, we find here and there a peculiar
arrangement of cells shown at figs. 51, 52. This opening through the
epidermal layer is a _stoma_. The cells which immediately surround the
openings are the _guard cells_. The form of the guard cells can be
better seen if we tear a leaf in such a way as to strip off a short
piece of the lower epidermis, and mount this in water. The guard cells
are nearly crescent-shaped, and the stoma is elliptical in outline. The
epidermal cells are very irregular in outline in this view. We should
also note that while the epidermal cells contain no chlorophyll, the
guard cells do.
[Illustration: Fig. 53. Portion of epidermis of ivy, showing irregular
epidermal cells, stoma and guard cells.]
=82=_a_. In the ivy leaf the guard cells are quite plain, but in
most plants the form as seen in cross-section is irregular in outline,
as shown in fig. 53_a_, which is from a section of a wintergreen leaf.
This leaf is interesting because it shows the characteristic structure
of leaves of many plants growing in soil where absorption of water by
the roots is difficult owing to the cold water, acids, or salts in the
water or soil, or in dry soil (see Chapters 47, 54, 55). The cuticle
over the upper epidermis is quite thick. This lessens the loss of water
by the leaf. The compact palisades of cells are in two to three cell
layers, also reducing the loss of water.
=83. The living protoplasm retards the evaporation of water from the
leaf.=—If we now take into consideration a few facts which we have
learned in a previous chapter, with reference to the physical
properties of the living cell, we shall be able to give a partial
explanation of the comparative slowness with which the water escapes
from the leaves. The inner surfaces of the cell walls are lined with
the membrane of protoplasm, and within this is the cell-sap. These
cells have become turgid by the absorption of the water which has
passed up to them from the roots. While the protoplasmic membrane of
the cells does not readily permit the water to filter through, yet it
is saturated with water, and the elastic cell wall with which it is in
contact is also saturated. From the cell wall the water evaporates into
the intercellular spaces. But the water is given up slowly through the
protoplasmic membrane, so that the water vapor cannot be given off as
rapidly from the cell walls as it could if the protoplasm were dead.
The living protoplasmic membrane then which is only slowly permeable to
the water of the cell-sap is here a very important factor in checking
the too rapid loss of water from the leaves.
[Illustration: Fig. 53_a_.
Cross-section of leaf of wintergreen. _Cu._, cuticle; _Epid._,
epidermis; _v.d._, vascular duct; _Int. c. sp._, intercellular space;
_L. ep._, lower epidermis; _St._, stoma.]
By an examination of our leaf section we see that the intercellular
spaces are all connected, and that the stomata, where they occur, open
also into intercellular spaces. There is here an opportunity for the
water vapor in the intercellular spaces to escape when the stomata are
open.
=84. Action of the stomata.=—The guard cells serve an important
function in regulating transpiration. During normal transpiration the
guard cells are turgid and their peculiar form then causes them to arch
away from each other, allowing the escape of water vapor. When the air
becomes too dry transpiration is in excess of absorption by the roots.
The guard cells lose some of their water, and collapse so that their
inner faces meet in a straight line and close the stoma. Thus the rapid
transpiration is checked. Some evaporation of water vapor, however,
takes place through the epidermal cells, and if the air remains too
dry, the leaves eventually become flaccid and droop. During the day
the effect of sunlight is to increase certain sugars or salts in the
guard cells so that they readily become turgid and open the stomates,
but at night the cell-sap is less concentrated and the stomates are
usually closed. Light therefore favors transpiration, while in darkness
transpiration is checked.
=85. Compare transpiration from the two surfaces of the leaf.=—This can
be done by using the cobalt chloride paper. This paper can be kept from
year to year and used repeatedly. It is thus a very simple matter to
make these experiments. Provide two pieces of glass (discarded glass
negatives, cleaned, are excellent), two pieces of cobalt chloride
paper, and some geranium leaves entirely free from surface water. Dry
the paper until it is blue. Place one piece of the paper on a glass
plate; place the geranium leaf with the under side on the paper. On the
upper side of the leaf now place the other cobalt paper, and next the
second piece of glass. On the pile place a light weight to keep the
parts well in contact. In fifteen or twenty minutes open and examine.
The paper next the under side of the geranium leaf is red where it lies
under the leaf. The paper on the upper side is only slightly reddened.
The greater loss of water, then, is through the under side of the
geranium leaf. This is true of a great many leaves, but it is not true
of all.
=86. Negative pressure.=—This is not only indicated by the
drooping of the leaves, but may be determined in another way. If the
shoot of such a plant be cut underneath mercury, or underneath a strong
solution of eosin, it will be found that some of the mercury or eosin,
as the case may be, will be forcibly drawn up into the stem toward the
roots. This is seen on quickly splitting the cut end of the stem. When
plants in the open cannot be obtained in this condition, one may take
a plant like a balsam plant from the greenhouse, or some other potted
plant, knock it out of the pot, free the roots from the soil and allow
to partly wilt. The stem may then be held under the eosin solution and
cut.
[Illustration: Fig. 54. Experiment to show lifting power of
transpiration.]
[Illustration: Fig. 55.
Estimation of the amount of transpiration. The tubes are filled with
water, and as the water transpires from the leaf surface its movement
in the tube from _a_ to _b_ can be measured. (After Mangin.)]
=87. Lifting power of transpiration.=—Not only does transpiration
go on quite independently of root pressure, as we have discovered
from other experiments, but transpiration is capable of exerting a
lifting power on the water in the plant. This may be demonstrated in
the following way: Place the cut end of a leafy shoot in one end of a
U tube and fit it water-tight. Partly fill this arm of the U tube with
water, and add mercury to the other arm until it stands at a level in
the two arms as in fig. 54. In a short time we note that the mercury is
rising in the tube.
=88. Root pressure may exceed transpiration.=—If we cover small
actively growing plants, such as the pea, corn, wheat, bean, etc.,
with a bell jar, and place them in the sunlight where the temperature
is suitable for growth, in a few hours, if conditions are favorable,
we shall see that there are drops of water standing out on the margins
of the leaves. These drops of water have exuded through the ordinary
stomata, or in other cases what are called water stomata, through the
influence of root pressure. The plant being covered by the glass jar,
the air soon becomes saturated with moisture and transpiration is
checked. Root pressure still goes on, however, and the result is shown
in the exuding drops. Root pressure is here in excess of transpiration.
This phenomenon is often to be observed during the summer season in the
case of low-growing plants. During the bright warm day transpiration
equals, or may be in excess of, root pressure, and the leaves are
consequently flaccid. As nightfall comes on the air becomes more
moist, and the conditions of light are such also that transpiration
is lessened. Root pressure, however, is still active because the soil
is still warm. In these cases drops of water may be seen exuding from
the margins of the leaves due to the excess of root pressure over
transpiration. Were it not for this provision for the escape of the
excess of water raised by root pressure, serious injury by lesions, as
a result of the great pressure, might result. The plant is thus to some
extent a self-regulatory piece of apparatus so far as root pressure and
transpiration are concerned.
=89. Injuries caused by excessive root pressure.=—Some varieties
of tomatoes when grown in poorly lighted and poorly ventilated
greenhouses suffer serious injury through lesions of the tissues.
This is brought about by the cells at certain parts becoming charged
so full with water through the activity of root pressure and lessened
transpiration, assisted also probably by an accumulation of certain
acids in the cell-sap which cannot be got rid of by transpiration.
Under these conditions some of the cells here swell out, forming
extensive cushions, and the cell walls become so weakened that they
burst. It is possible to imitate the excess of root pressure in the
case of some plants by connecting the stems with a system of water
pressure, when very quickly the drops of water will begin to exude from
the margins of the leaves.
[Illustration: Fig. 56.
The roots are lifting more water into the plant than can be given off
in the form of water vapor, so it is pressed out in drops. From “First
Studies Plant Life.”]
=90.= It should be stated that in reality there is no difference
between transpiration and evaporation, if we bear in mind that
evaporation takes place more slowly from living plants than from dead
ones, or from an equal surface of water.
=91.= The escape of water vapor is not the only function of the
stomata. The exchange of gases takes place through them as we shall
later see. A large number of experiments show that normally the stomata
are open when the leaves are turgid. But when plants lose excessive
quantities of water on dry and hot days, so that the leaves become
flaccid, the guard cells automatically close the stomata to check the
escape of water vapor. Some water escapes through the epidermis of many
plants, though the cuticularized membrane of the epidermis largely
prevents evaporation. In arid regions plants are usually provided
with an epidermis of several layers of cells to more securely prevent
evaporation there. In such cases the guard cells are often protected by
being sunk deeply in the epidermal layer.
=92. Demonstration of stomates and intercellular air spaces.=—A
good demonstration of the presence of stomates in leaves, as well as
the presence and intercommunication of the intercellular spaces, can be
made by blowing into the cut end of the petiole of the leaf of a calla
lily, the lamina being immersed in water. The air is forced out
through the stomata and rises as bubbles to the surface of the water.
At the close of the experiment some of the air bubbles will still be
in contact with the leaf surface at the opening of the stomata. The
pressure of the water gradually forces this back into the leaf. Other
plants will answer for the experiment, but some are more suitable than
others.
=92a. Number of stomata.=—The larger number of stomata are on the
under side of the leaf. (In leaves which float on the surface of the
water all of the stomata are on the upper side of the leaf, as in the
water-lily.) It has been estimated by investigation that in general
there are 40-300 stomata to the square millimeter of surface. In some
plants this number is exceeded, as in the olive, where there are 625.
In an entire leaf of Brassica rapa there are about 11,000,000 stomata,
and in an entire leaf of the sunflower there are about 13,000,000
stomata.
=92b. Amount of water transpired by plants.=—The amount of water
transpired by plants is very great. According to careful estimates a
sunflower 6 feet high transpires on the average about one quart per
day; an acre of cabbages 2,000,000 quarts in four months; an oak tree
with 700,000 leaves transpires about 180 gallons of water per day.
According to von Höhnel, a beech tree 110 years old transpired about
2250 gallons of water in one summer. A hectare of such trees (about 400
on 2½ acres) would at the same rate transpire about 900,000 gallons, or
about 30,000 barrels in one summer.
FOOTNOTE:
[6] Demonstrations may be made with prepared sections of leaves,
CHAPTER V.
PATH OF MOVEMENT OF WATER IN PLANTS.
=93.= In our study of root pressure and transpiration we have seen
that large quantities of water or solutions move upward through the
stems of plants. We are now led to inquire through what part of the
stems the liquid passes in this upward movement, or in other words,
what is the path of the “sap” as it rises in the stem. This we can
readily see by the following trial.
=94. Place the cut ends of leafy shoots in a solution of some of
the red dyes.=—We may cut off leafy shoots of various plants and
insert the cut ends in a vessel of water to which have been added a few
crystals of the dye known as fuchsin to make a deep red color (other
red dyes may be used, but this one is especially good). If the study is
made during the summer, the “touch-me-not” (impatiens) will be found a
very useful plant, or the garden balsam, which may also be had in the
winter from conservatories. Almost any plant will do, however, but we
should also select one like the corn plant (zea mays) if in the summer,
or the petioles of a plant like caladium, which can be obtained from
the conservatory. If seedlings of the castor-oil bean are at hand we
may cut off some shoots which are 8-10 inches high, and place them in
the solution also.
=95. These solutions color the tracts in the stem and leaves through
which they flow.=—After a few hours in the case of the impatiens,
or the more tender plants, we can see through the stem that certain
tracts are colored red by the solution, and after 12 to 24 hours there
may be seen a red coloration of the leaves of some of the plants
used. After the shoots have been standing in the solution for a few
hours, if we cut them at various places we will note that there are
several points in the section where the tissues are colored red. In
the impatiens perhaps from four to five, in the sunflower a larger
number. In these plants the colored areas on a cross-section of the
stem are situated in a concentric ring which separates more or less
completely an outer ring of the stem from the central portion. If we
now split portions of the stem lengthwise we see that these colored
areas continue throughout the length of the stem, in some cases even up
to the leaves and into them.
[Illustration: Fig. 57. Broken corn stalk, showing fibrovascular
bundles.]
=96.= If we cut across the stem of a corn plant which has been
in the solution, we see that instead of the colored areas being in
a concentric ring they are irregularly scattered, and on splitting
the stem we see here also that these colored areas extend for long
distances through the stem. If we take a corn stem which is mature, or
an old and dead one, cut around through the outer hard tissues, and
then break the stem at this point, from the softer tissue long strings
of tissue will pull out as shown in fig. 57. These strings of denser
tissue correspond to the areas which are colored by the dye. They are
in the form of minute bundles, and are called _vascular bundles_.
=97.= We thus see that instead of the liquids passing through the
entire stem they are confined to definite courses. Now that we have
discovered the path of the upward movement of water in the stem, we are
curious to see what the structure of these definite portions of the
stem is.
[Illustration: Fig. 58.
Xylem portion of bundle. Cambium portion of bundle. Bast portion of
bundle.
Section of vascular bundle of sunflower stem.]
=98. Structure of the fibrovascular bundles.=—We should now make
quite thin cross-sections, either free hand and mount in water for
microscopic examination, or they may be made with a microtome and
mounted in Canada balsam, and in this condition will answer for future
study. To illustrate the structure of the bundle in one type we may
take the stem of the castor-oil bean. On examining these cross-sections
we see that there are groups of cells which are denser than the ground
tissue. These groups correspond to the colored areas in the former
experiments, and are the vascular bundles cut across. These groups are
somewhat oval in outline, with the pointed end directed toward the
center of the stem. If we look at the section as a whole we see that
there is a narrow continuous ring[7] of small cells situated at the
same distance from the center of the stem as the middle part of the
bundles, and that it divides the bundles into two groups of cells.
=99. Woody portion of the bundle.=—In that portion of the bundle
on the inside of the ring, i.e., toward the “pith,” we note large,
circular, or angular cavities. The walls of these cells are quite thick
and woody. They are therefore called wood cells, and because they
are continuous with cells above and below them in the stem in such a
way that long tubes are formed, they are called woody vessels. Mixed
in with these are smaller cells, some of which also have thick walls
and are wood cells. Some of these cells may have thin walls. This
is the case with all when they are young, and they are then classed
with the fundamental tissue or soft tissue (parenchyma). This part of
the bundle, since it contains woody vessels and fibres, is the _wood
portion_ of the bundle, or technically the _xylem_.
=100. Bast portion of the bundle.=—If our section is through a
part of the stem which is not too young, the tissues of the outer part
of the bundle will show either one or several groups of cells which
have white and shiny walls, that are thickened as much or more than
those of the wood vessels. These cells are _bast_ cells, and for this
reason this part of the bundle is the _bast_ portion, or the _phloem_.
Intermingled with these, cells may often be found which have thin
walls, unless the bundle is very old. Nearer the center of the bundle
and still within the bast portion are cells with thin walls, angular
and irregularly arranged. This is the softer portion of the bast, and
some of these cells are what are called _sieve_ tubes, which can be
better seen and studied in a longitudinal section of the stem.
=101. Cambium region of the bundle.=—Extending across the center
of the bundle are several rows of small cells, the smallest of the
bundle, and we can see that they are more regularly arranged, usually
in quite regular rows, like bricks piled upon one another. These cells
have thinner walls than any others of the bundle, and they usually take
a deeper stain when treated with a solution of some of the dyes. This
is because they are younger, and are therefore richer in protoplasmic
contents. This zone of young cells across the bundle is the _cambium_.
Its cells grow and divide, and thus increase the size of the bundle.
By this increase in the number of the cells of the cambium layer, the
outermost cells on either side are continually passing over into the
phloem, on the one hand, and into the wood portion of the bundle, on
the other hand.
=102. Longitudinal section of the bundle.=—If we make thin
longisections of the vascular bundle of the castor-oil seedling (or
other dicotyledon) so that we have thin ones running through a bundle
radially, as shown in fig. 59, we can see the structure of these parts
of the bundle in side view. We see here that the form of the cells is
very different from what is presented in a cross-section of the same.
The walls of the various ducts have peculiar markings on them. These
markings are caused by the walls being thicker in some places than in
others, and this thickening takes place so regularly in some instances
as to form regular spiral thickenings. Others have the thickenings in
the form of the rounds of a ladder, while still others have pitted
walls or the thickenings are in the form of rings.
[Illustration: Fig. 59.
Longitudinal section of vascular bundle of sunflower stem; spiral,
scalariform and pitted vessels at left; next are wood fibers with
oblique cross walls; in middle are cambium cells with straight cross
walls, next two sieve tubes, then phloem or bast cells.]
=103. Vessels or ducts.=—One way in which the cells in side view
differ greatly from an end view, in a cross-section in the bundle, is
that they are much longer in the direction of the axis of the stem. The
cells have become elongated greatly. If we search for the place where
two of these large cells with spiral, or ladder-like, markings meet end
to end, we see that the wall which formerly separated the cells has
nearly or quite disappeared. In other words the two cells have now an
open communication at the ends. This is so for long distances in the
stem, so that long columns of these large cells form tubes or vessels
through which the water rises in the stems of plants.
=104.= In the bast portion of the bundle we detect the cells of
the bast fibers by their thick walls. They are very much elongated and
the ends taper out to thin points so that they overlap. In this way
they serve to strengthen the stem.
=105. Sieve tubes.=—Lying near the bast cells, usually toward
the cambium, are elongated cells standing end to end, with delicate
markings on their cross walls which appear like finely punctured plates
or sieves. The protoplasm in such cells is usually quite distinct, and
sometimes contracted away from the side walls, but attached to the
cross walls, and this aids in the detection of the sieve tubes (fig.
59.) The granular appearance which these plates present is caused by
minute perforations through the wall so that there is a communication
between the cells. The tubes thus formed are therefore called sieve
tubes and they extend for long distances through the tube so that there
is communication throughout the entire length of the stem. (The
function of the sieve tubes is supposed to be that for the downward
transportation of substances elaborated in the leaves.)
=106.= If we section in like manner the stem of the sunflower we
shall see similar bundles, but the number is greater than eight. In
the garden balsam the number is from four to six in an ordinary stem
3-4_mm_ diameter. Here we can see quite well the origin of the vascular
bundle. Between the larger bundles we can see especially in free-hand
sections of stems through which a colored solution has been lifted by
transpiration, as in our former experiments, small groups of the minute
cells in the cambial ring which are colored. These groups of cells
which form strands running through the stem are _pro-cambium strands_.
The cells divide and increase just like the cambium cells, and the
older ones thrown off on either side change, those toward the center
of the stem to wood vessels and fibers, and those on the outer side to
bast cells and sieve tubes.
=107. Fibrovascular bundles in the Indian corn.=—We should now
make a thin transection of a portion of the center of the stem of
Indian corn, in order to compare the structure of the bundle with that
of the plants which we have just examined. In fig. 60 is represented a
fibrovascular bundle of the stem of the Indian corn. The large cells
are those of the spiral and reticulated and annular vessels. This is
the woody portion of the bundle or xylem. Opposite this is the bast
portion or phloem, marked by the lighter colored tissue at _i_. The
larger of these cells are the sieve tubes, and intermingled with them
are smaller cells with thin walls. Surrounding the entire bundle are
small cells with thick walls. These are elongated and the tapering ends
overlap. They are thus slender and long and form fibers. In such a
bundle all of the cambium has passed over into permanent tissue and is
said to be closed.
[Illustration: Fig. 60.
Transection of fibrovascular bundle of Indian corn. _a_, toward
periphery of stem; _g_, large pitted vessels; _s_, spiral vessel; _r_,
annular vessel; _l_, air cavity formed by breaking apart of the cells;
_i_, soft bast, a form of sieve tissue; _p_, thin-walled parenchyma.
(Sachs.)]
=108. Rise of water in the vessels.=—During the movement of the
water or nutrient solutions upward in the stem the vessels of the wood
portion of the bundle in certain plants are nearly or quite filled,
if root pressure is active and transpiration is not very rapid. If,
however, on dry days transpiration is in excess of root pressure, as
often happens, the vessels are not filled with the water, but are
partly filled with certain gases because the air or other gases in
the plant become rarefied as a result of the excessive loss of water.
There are then successive rows of air or gas bubbles in the vessels
separated by films of water which also line the walls of the vessels.
The condition of the vessel is much like that of a glass tube through
which one might pass the “froth” which is formed on the surface of
soapy water. This forms a chain of bubbles in the vessels. This chain
has been called Jamin’s chain because of the discoverer.
=109.= Why water or food solutions can be raised by the plant
to the height attained by some trees has never been satisfactorily
explained. There are several theories propounded which cannot be
discussed here. It is probably a very complex process. Root pressure
and transpiration both play a part, or at least can be shown, as we
have seen, to be capable of lifting water to a considerable height. In
addition to this, the walls of the vessels absorb water by diffusion,
and in the other elements of the bundle capillarity comes also into
play, as well as osmosis.
See Organization of Tissues, Chapter 38.
=110. Flow of sap in the spring.=—The cause of the bleeding of
trees and the flow of sap in the spring is little understood. One of
the remarkable cases is the flow of sap in maple trees. It begins
in early spring and ceases as the buds are opening, and seems to be
initiated by alternation of high and low temperatures of day and night.
It has been found that the pressures inside of the tree at this time
are enormously increased during the day, when the temperature rises
after a cold night. This has led to the belief that the pressure is
caused by the expansion of the gases in the vascular ducts. The warming
up of the twigs and branches of the tree would take place rapidly
during the day, while the interior of the trunk would be only slightly
affected. The pressures then would cause the sap to flow downward
during the day, and at night the branches becoming cool, sap would flow
back again from the roots and trunk.
Recent experiments by Jones _et al._ show that while some of the
pressure is due to the expansion of gas in the tree by the rise of
temperature, this cannot account for the enormous pressures which are
often present, for example, when after a rise in the temperature of 2°
C. there was an increase of 20 lbs. pressure.
Then again, after the cessation of the flow in late spring there are
often as great differences between night and day temperatures. It
therefore seems reasonable to conclude that the expansion of gases by a
rise in temperature is not the direct cause.
=Activities of the cells.=—It has been suggested by some that
the rise in temperature exercises an influence on the protoplasts,
or living cells, so that they are stimulated to a special activity
resulting in an exudation pressure from the individual cells, which is
known to take place. With the fall of temperature at night this
activity would cease and there might result a lessened pressure in
the cells. Since the specific activities of cells are known to vary
in different plants, and in the same plant at different seasons, some
support is gained for this theory, though it is generally believed that
the activities of the living cells in the stems are not necessary for
the upward flow of water. It must be admitted, however, that at present
we know very little about this interesting problem.
FOOTNOTE:
[7] This ring and the bundles separate the stem into two regions,
an outer one composed of large cells with thin walls, known as the
cortical cells, or collectively the _cortex_. The inner portion,
corresponding to what is called the pith, is made up of the same kind
of cells and is called the _medulla_, or _pith_. When the cells of
the cortex, as well as of the pith, remain thin walled the tissue is
called parenchyma. Parenchyma belongs to the group of tissues called
fundamental.
CHAPTER VI.
MECHANICAL USES OF WATER.
=111. Turgidity of plant parts.=—As we have seen by the experiments on
the leaves, turgescence of the cells is one of the conditions which
enables the leaves to stand out from the stem, and the lamina of the
leaves to remain in an expanded position, so that they are better
exposed to the light, and to the currents of air. Were it not for this
turgidity the leaves would hang down close against the stem.
[Illustration: Fig. 61. Restoration of turgidity (Sachs).]
=112. Restoration of turgidity in shoots.=—If we cut off a living
stem of geranium, coleus, tomato, or “balsam,” and allow the leaves
to partly wilt so that the shoot loses its turgidity, it is possible
for this shoot to regain turgidity. The end may be freshly cut again,
placed in a vessel of water, covered with a bell jar and kept in a room
where the temperature is suitable for the growth of the plant. The
shoot will usually become turgid again from the water which is absorbed
through the cut end of the stem and is carried into the leaves where
the individual cells become turgid, and the leaves are again expanded.
Such shoots, and the excised leaves also, may often be made turgid
again by simply immersing them in water, as one of the experiments with
the salt solution would teach.
=113.= Turgidity may be restored more certainly and quickly in a
partially wilted shoot in another way. The cut end of the shoot may be
inserted in a U tube as shown in fig. 61, the end of the tube around
the stem of the plant being made air-tight. The arm of the tube in
which the stem is inserted is filled with water and the water is
allowed to partly fill the other arm. Into this other arm is then
poured mercury. The greater weight of the mercury causes such pressure
upon the water that it is pushed into the stem, where it passes up
through the vessels in the stems and leaves, and is brought more
quickly and surely to the cells which contain the protoplasm and
cell-sap, so that turgidity is more quickly and certainly attained.
=114. Tissue tensions.=—Besides the turgescence of the cells
of the leaves and shoots there are certain tissue tensions without
which certain tender and succulent shoots, etc., would be limp, and
would droop. There are a number of plants usually accessible, some at
one season and some at others, which may be used to illustrate tissue
tension.
=115. Longitudinal tissue tension.=—For this in early summer one
may use the young and succulent shoots of the elder (sambucus); or the
petioles of rhubarb during the summer and early autumn; or the petioles
of richardia. Petioles of caladium are excellent for this purpose, and
these may be had at almost any season of the year from the greenhouses,
and are thus especially advantageous for work during late autumn or
winter. The tension is so strong that a portion of such a petiole
10-15_cm_ long is ample to demonstrate it. As we grasp the lower end of
the petiole of a caladium, or rhubarb leaf, we observe how rigid it is,
and how well it supports the heavy expanded lamina of the leaf.
=116.= The ends of a portion of such a petiole or other object
which may be used are cut off squarely. With a knife a strip from
2-3_mm_ in thickness is removed from one side the full length of the
object. This strip we now find is shorter than the larger part from
which it was removed. The outer tissue then exerts a tension upon the
petiole which tends to shorten it. Let us remove another strip lying
next this one, and another, and so on until the outer tissues remain
only upon one side. The object will now bend toward that side. Now
remove this strip and compare the length of the strips removed with the
central portion. We find that they are much shorter now. In other words
there is also a tension in the tissue of the central portion of the
petiole, the direction of which is opposite to that of the superficial
tissue. The parts of the petiole now are not rigid, and they easily
bend. These two longitudinal tissue tensions acting in opposition to
each other therefore give rigidity to the succulent shoot. It is only
when the individual cells of such shoots or petioles are turgid that
these tissue tensions in succulent shoots manifest themselves or are
prominent.
[Illustration: Fig. 62. Strip from dandelion stem made to imitate a
plant tendril.]
=117.= To demonstrate the efficiency of this tension in giving
support, let us take a long petiole of caladium or of rhubarb. Hold it
by one end in a horizontal position. It is firm and rigid, and does not
droop, or but little. Remove all of the outer portion of the tissues,
as described above, leaving only the central portion. Now attempt to
hold it in a horizontal position by one end. It is flabby and droops
downward because the longitudinal tension is removed.
=118. Longitudinal tension in dandelion stems.=—Take long
and fresh dandelion stems. Split them. Note that they coil. The
longitudinal tension is very great. Place some of these strips in fresh
water. They coil up into close curls because by the absorption of water
by the cells the turgescence of the individual cells is increased, and
this increases the tension in the stem. Now place them in salt water (a
5 per cent solution). Why do they uncoil?
=119. To imitate the coiling of a tendril.=—Cut out a narrow
strip from a long dandelion stem. Fasten to a piece of soft wood, with
the ends close together, as shown in fig. 62. Now place it in fresh
water and watch it coil. Part of it coils one way and part another way,
just as a tendril does after the free end has caught hold of some place
for support.
=120. Transverse tissue tension.=—To illustrate this one may take
a willow shoot 3-5_cm_ diameter and saw off sections about 2 cm long.
Cut through the bark on one side and peel it off in a single strip. Now
attempt to replace it. The bark will not quite cover the wood again,
since the ends will not meet. It must then have been held in transverse
tension by the woody part of the shoot.
CHAPTER VII.
STARCH AND SUGAR FORMATION.
1. The Gases Concerned.
=121. Gas given off by green plants in the sunlight.=—Let us take
some green alga, like spirogyra, which is in a fresh condition, and
place one lot in a beaker or tall glass vessel of water and set this in
the direct sunlight or in a well lighted place. At the same time cover
a similar vessel of spirogyra with black cloth so that it will be in
the dark, or at least in very weak light.
[Illustration: Fig. 63. Oxygen gas given off by spirogyra.]
[Illustration: Fig. 64. Bubbles of oxygen gas given off from elodea in
presence of sunlight. (Oels.)]
=122.= In a short time we note that in the first vessel small
bubbles of gas are accumulating on the surface of the threads of the
spirogyra, and now and then some free themselves and rise to the
surface of the water. Where there is quite a tangle of the threads the
gas is apt to become caught and held back in larger bubbles, which on
agitation of the vessel are freed.
If we now examine the second vessel we see that there are no bubbles,
or only a very few of them. We are led to believe then that sunlight
has had something to do with the setting free of this gas from the
plant.
=123.= We may now take another alga-like vaucheria and perform the
experiment in the same way, or to save time the two may be set up at
once. In fact if we take any of the green algæ and treat them as
described above gas will be given off in a similar manner.
=124.= We may now take one of the higher green plants, an aquatic
plant like elodea, callitriche, etc. Place the plant in the water with
the cut end of the stem uppermost, but still immersed, the plant being
weighted down by a glass rod or other suitable object. If we place the
vessel of water containing these leafy stems in the bright sunlight,
in a short time bubbles of gas will pass off quite rapidly from the
cut end of the stem. If in the same vessel we place another stem, from
which the leaves have been cut, the number of bubbles of gas given
off will be very few. This indicates that a large part of the gas is
furnished by the leaves.
=125.= Another vessel fitted up in the same way should be placed
in the dark or shaded by covering with a box or black cloth. It will
be seen here, as in the case of spirogyra, that very few or no bubbles
of gas will be set free. Sunlight here also is necessary for the rapid
escape of the gas.
=126.= We may easily compare the rapidity with which light of
varying intensity effects the setting free of this gas. After cutting
the end of the stem let us plunge the cut surface several times in
melted paraffine, or spread over the cut surface a coat of varnish.
Then prick with a needle a small hole through the paraffine or varnish.
Immerse the plant in water and place in sunlight as before. The gas now
comes from the puncture through the coating of the cut end, and the
number of bubbles given off during a given period can be ascertained by
counting. If we duplicate this experiment by placing one plant in weak
light or diffused sunlight, and another in the shade, we can easily
compare the rapidity of the escape of the gas under the different
conditions, which represent varying intensities of light. We see then
that not only is sunlight necessary for the setting free of this gas,
but that in diffused light or in the shade the activity of the plant in
this respect is less than in direct sunlight.
=127. What this gas is.=—If we take quite a quantity of the
plants of elodea and place them under an inverted funnel which is
immersed in water, the gas will be given off in quite large quantities
and will rise into the narrow exit of the funnel. The funnel should be
one with a short tube, or the vessel one which is quite deep so that
a small test tube which is filled with water may in this condition be
inverted over the opening of the funnel tube. With this arrangement
of the experiment the gas will rise in the inverted test tube, slowly
displace a portion of the water, and become collected in a sufficient
quantity to afford us a test. When a considerable quantity has
accumulated in the test tube, we may close the end of the tube in
the water with the thumb, lift it from the water and invert. The gas
will rise against the thumb. A dry soft-pine splinter should be then
lighted, and after it has burned a short time, extinguish the flame by
blowing upon it, when the still burning end of the splinter should be
brought to the mouth of the tube as the thumb is quickly moved to one
side. The glowing of the splinter shows that the gas is _oxygen_.
[Illustration: Fig. 65. Apparatus for collecting quantity of oxygen from
elodea. (Detmer.)]
[Illustration: Fig. 66. Ready to see what the gas is.]
=128.= It is better to allow the apparatus to stand several days
in the sunlight in order to catch a full tube of the gas. Or on a sunny
day carbon dioxide gas can be led into the water in the jar from a
generator, such an one as is used for the evolution of CO₂. The CO₂
can be produced by the action of hydrochloric acid on bits of marble.
The CO₂ should not be run below the funnel. The test tube should be
fastened so that the light oxygen gas will not raise it off the funnel.
With the tube full of gas the test for oxygen can be made by lifting
the tube with one hand and quickly thrusting the glowing end of the
splinter in with the other hand. If properly handled, the splinter will
flame again. If it is necessary to keep the apparatus standing for more
than one day it is well to add fresh water in the place of most of the
water in the jar. Do not use leaves of land plants in this experiment,
since the bubbles which rise when these leaves are placed in water are
not evidence that this process is taking place.
[Illustration: Fig. 67. The splinter lights again in the presence of
oxygen gas.]
=129. Oxygen given off by green land plants
also.=—If we should extend our experiments to land
plants we should find that oxygen is given off by them
under these conditions of light. Land plants, however,
will not do this when they are immersed in water, but
it is necessary to set up rather complicated apparatus
and to make analyses of the gases at the beginning and
at the close of the experiments. This has been done,
however, in a sufficiently large number of cases so
that we know that all green plants in the sunlight, if
temperature and other conditions are favorable, give
off oxygen.
=130. Absorption of carbon dioxide.=—We have next to inquire
where the oxygen comes from which is given off by green plants when
exposed to the sunlight, and also to learn something more of the
conditions necessary for the process. We know that water which has been
for some time exposed to the air and soil, and has been agitated, like
running water of streams, or the water of springs, has mixed with it a
considerable quantity of oxygen and carbon dioxide.
If we boil spring water or hydrant water which comes from a stream
containing oxygen and carbon dioxide, for about 20 minutes, these
gases are driven off. We should set this aside where it will not be
agitated, until it has cooled sufficiently to receive plants without
injury. Let us now place some spirogyra or vaucheria, and elodea, or
other green water plant, in this boiled water and set the vessel in the
bright sunlight under the same conditions which were employed in the
experiments for the evolution of oxygen. No oxygen is given off.
Can it be that this is because the oxygen was driven from the water in
boiling? We shall see. Let us take the vessel containing the water,
or some other boiled water, and agitate it so that the air will be
thoroughly mixed with it. In this way oxygen is again mixed with the
water. Now place the plant again in the water, set in the sunlight, and
in several minutes observe the result. No oxygen or but little is given
off. There must be then some other requisite for the evolution of the
oxygen.
=132. The gases are interchanged in the plants.=—We will now
introduce carbon dioxide again in the water. This can be done by
leading CO₂ from a gas generator into the water. Broken bits of marble
are placed in the generator, acted upon by hydrochloric acid, and the
gas is led over by glass tubing. Now if we place the plant in the water
and set the vessel in the sunlight, in a few minutes the oxygen is
given off rapidly.
=133. A chemical change of the gas takes place within the plant
cell.=—This leads us to believe then that CO₂ is in some way
necessary for the plant in this process. Since oxygen is given off
while carbon dioxide, a different gas, is necessary, it would seem that
a chemical change takes place in the gases within the plant. Since the
process takes place in such simple plants as spirogyra as well as in
the more bulky and higher plants, it appears that the changes go on
within the cell, in fact within the protoplasm.
=134. Gases as well as water can diffuse through the protoplasmic
membrane.=—Carbon dioxide then is absorbed by the plant while
oxygen is given off. We see therefore that gases as well as water can
diffuse through the protoplasmic membrane of plants under certain
conditions.
2. Where Starch is Formed.
We have found by these simple experiments that some chemical change
takes place within the protoplasm of the green cells of plants during
the absorption of carbon dioxide and the giving off of oxygen. We
should examine some of the green parts of those plants used in the
experiments, or if they are not at hand we should set up others in
order to make this examination.
=135. Starch formed as a result of this process.=—We may take
spirogyra which has been standing in water in the bright sunlight for
several hours. A few of the threads should be placed in alcohol for a
short time to kill the protoplasm. From the alcohol we transfer the
threads to a solution of iodine in potassium iodide. We find that
at certain points in the chlorophyll band a bluish tinge, or color,
is imparted to the ring or sphere which surrounds the pyrenoid. In
our first study of the spirogyra cell we noted this sphere as being
composed of numerous small grains of starch which surround the pyrenoid.
=136. Iodine used as a test for starch.=—This color reaction
which we have obtained in treating the threads with iodine is the
well-known reaction, or test, for starch. We have demonstrated then
that starch is present in spirogyra threads which have stood in the
sunlight with free access to carbon dioxide.
If we examine in the same way some threads which have stood in the
dark for a few days we obtain no reaction for starch, or at best only
a slight reaction. This gives us some evidence that a chemical change
does take place during this process (absorption of CO₂ and giving off
of oxygen), and that starch is a product of that chemical change.
=137. Schimper’s method of testing for the presence of starch.=—Another
convenient and quick method of testing for the presence of starch
is what is known as Schimper’s method. A strong solution of chloral
hydrate is made by taking 8 grams of chloral hydrate for every 5_cc_
of water. To this solution is added a little of an alcoholic tincture
of iodine. The threads of spirogyra may be placed directly in this
solution, and in a few moments mounted in water on the glass slip and
examined with the microscope. The reaction is strong and easily seen.
We should also examine the leaves of elodea, or one of the higher green
plants which has been for some time in the sunlight. We may use here
Schimper’s method by placing the leaves directly in the solution of
chloral hydrate and iodine. The leaves are made transparent by the
chloral hydrate so that the starch reaction from the iodine is easily
detected.
The following is a convenient and safe method of extracting chlorophyll
from leaves. Fill a large pan, preferably a dishpan, half full of
hot water. This may be kept hot by a small flame. On the water float
an evaporating dish partly filled with alcohol. The leaves should be
first immersed in the hot water for several minutes, then placed in the
alcohol, which will quickly remove the chlorophyll. Now immerse the
leaves in the iodine solution.
[Illustration: Fig. 68. Leaf of coleus showing green and white areas,
before treatment with iodine.]
[Illustration: Fig. 69. Similar leaf treated with iodine, the starch
reaction only showing where the leaf was green.]
=138. Green parts of plants form starch when exposed to light.=—Thus we
find that in the case of all the green plants we have examined, starch
is present in the green cells of those which have been standing for
some time in the sunlight where the process of the absorption of CO₂
and the giving off of oxygen can go on, and that in the case of plants
grown in the dark, or in leaves of plants which have stood for some
time in the dark, starch is absent. We reason from this that starch is
the product of the chemical change which takes place in the green cells
under these conditions. The CO₂ which is absorbed by the plant mixes
with the water (H₂O) in the cell and immediately forms carbonic acid.
The chlorophyll in the leaf absorbs radiant energy from the sun which
splits up the carbonic acid, and its elements then are put together
into a more complex compound, starch. This process of putting together
the elements of an organic compound is a _synthesis_, or a _synthetic
assimilation_, since it is done by the living plant. It is therefore a
synthetic assimilation of carbon dioxide. Since the sunlight supplies
the energy it is also called _photosynthesis_, or _photosynthetic
assimilation_. We can also say carbon dioxide assimilation, or CO₂
assimilation (see paragraph on assimilation at close of Chapter 10).
=139. Starch is formed only in the green parts of variegated
leaves.=—If we test for starch in variegated leaves like the leaf
of a coleus plant, we shall have an interesting demonstration of the
fact that the green parts of plants only form starch. We may take
a leaf which is partly green and partly white, from a plant which
has been standing for some time in bright light. Fig. 68 is from a
photograph of such a leaf. We should first boil it in alcohol to remove
the green color. Now immerse it in the potassium iodide of iodine
solution for a short time. The parts which were formerly green are
now dark blue or nearly black, showing the presence of starch in
those portions of the leaf, while the white part of the leaf is still
uncolored. This is well shown in fig. 69, which is from a photograph of
another coleus leaf treated with the iodine solution.
3. Chlorophyll and the Formation of Starch.
=140.= In our experiments thus far in treating of the absorption
of carbon dioxide and the evolution of oxygen, with the accompanying
formation of starch, we have used green plants.
=141. Fungi cannot form starch.=—If we should extend our
experiments to the fungi, which lack the green color so characteristic
of the majority of plants, we should find that photosynthesis does not
take place even though the plants are exposed to direct sunlight. These
plants cannot then form starch, but obtain carbohydrates for food from
other sources.
=142. Photosynthesis cannot take place in etiolated plants.=—Moreover
photosynthesis is usually confined to the green plants, and if by any
means one of the ordinary green plants loses its green color this
process cannot take place in that plant, even when brought into the
sunlight, until the green color has appeared under the influence of
light.
This may be very easily demonstrated by growing seedlings of the
bean, squash, corn, pea, etc. (pine seedlings are green even when
grown in the dark), in a dark room, or in a dark receiver of some
kind which will shut out the rays of light. The room or receiver must
be quite dark. As the seedlings are “coming up,” and as long as they
remain in the dark chamber, they will present some other color than
green; usually they are somewhat yellowed. Such plants are said to be
_etiolated_. If they are brought into the sunlight now for a few hours
and then tested for the presence of starch the result will be negative.
But if the plant is left in the light, in a few days the leaves
begin to take on a green color, and then we find that carbon dioxide
assimilation begins.
=143. Chlorophyll and chloroplasts.=—The green substance in
plants is then one of the important factors in this complicated
process of forming starch. This green substance is _chlorophyll_,
and it usually occurs in definite bodies, the chlorophyll bodies, or
_chloroplasts_.
The material for new growth of plants grown in the
dark is derived from the seed. Plants grown in the dark
consist largely of water and protoplasm, the walls
being very thin.
=144. Form of the chlorophyll bodies.=—Chlorophyll bodies vary in
form in some different plants, especially in some of the lower
plants. This we have already seen in the case of spirogyra, where the
chlorophyll body is in the form of a very irregular band, which courses
around the inner side of the cell wall in a spiral manner. In zygnema,
which is related to spirogyra, the chlorophyll bodies are star-shaped.
In the desmids the form varies greatly. In œdogonium, another of the
thread-like algæ, illustrated in fig. 144, the chlorophyll bodies
are more or less flattened oval disks. In vaucheria, too, a branched
thread-like alga shown in fig. 138, the chlorophyll bodies are oval in
outline. These two plants, œdogonium and vaucheria, should be examined
here if possible, in order to become familiar with their form, since
they will be studied later under morphology (see chapters on œdogonium
and vaucheria, for the occurrence and form of these plants). The form
of the chlorophyll body found in œdogonium and vaucheria is that which
is common to many of the green algæ, and also occurs in the mosses,
liverworts, ferns, and the higher plants. It is a more or less rounded,
oval, flattened body.
[Illustration: Fig. 69_a_.
Section of ivy leaf, palisade cells above, loose parenchyma, with large
intercellular spaces in center. Epidermal cells on either edge, with no
chlorophyll bodies.]
=145. Chlorophyll is a pigment which resides in the chloroplast.=—That
the chlorophyll is a coloring substance which resides in the
chloroplastid, and does not form the body itself, can be demonstrated
by dissolving out the chlorophyll when the framework of the
chloroplastid is apparent. The green parts of plants which have been
placed for some time in alcohol lose their green color. The alcohol
at the same time becomes tinged with green. In sectioning such plant
tissue we find that the chlorophyll bodies, or chloroplastids as they
are more properly called, are still intact, though the green color is
absent. From this we know that chlorophyll is a substance distinct from
that of the chloroplastid.
=146. Chlorophyll absorbs energy from sunlight for photosynthesis.=—It
has been found by analysis with the spectroscope that chlorophyll
absorbs certain of the rays of the sunlight. The energy which is thus
obtained from the sun, called _kinetic_ energy, acts on the molecules
of CH₂O₃, separating them into molecules of C, H, and O. (When the
CO₂ from the air enters the plant cell it immediately unites with
some of the water, forming carbonic acid = CH₂O₃.) After a series of
complicated chemical changes starch is formed by the union of carbon,
oxygen, and hydrogen. In this process of the reduction of the CH₂O₃ and
the formation of starch there is a surplus of oxygen, which accounts
for the giving off of oxygen during the process.
=147. Rays of light concerned in photosynthesis.=—If a solution
of chlorophyll be made, and light be passed through it, and this
light be examined with the spectroscope, there appear what are called
absorption bands. These are dark bands which lie across certain
portions of the spectrum. These bands lie in the red, orange, yellow,
green, blue, and violet, but the bands are stronger in the red, which
shows that chlorophyll absorbs more of the red rays of light than of
the other rays. These are the rays of low refrangibility. The kinetic
energy derived by the absorption of these rays of light is transformed
into potential energy. That is, the molecule of CH₂O₃ is broken up, and
then by a different combination of certain elements starch is formed.[8]
[8] In the formation of starch during photosynthesis the separated
molecules from the carbon dioxide and water unite in such a way that
carbon, hydrogen, and oxygen are united into a molecule of starch. This
result is usually represented by the following equation: CO₂ + H₂O =
CH₂O + O₂. Then by polymerization 6(CH₂O) = C₆H₁₂O₆ = grape sugar.
Then C₆H₁₂O₆-H₂O = C₆H₁₀O₅ = starch. It is believed, however, that the
process is much more complicated than this, that several different
compounds are formed before starch finally appears, and that the
formula for starch is much higher numerically than is represented by
C₆H₁₀O₅.
=148. Starch grains formed in the chloroplasts.=—During
photosynthesis the starch formed is deposited generally in small grains
within the green chloroplast in the leaf. We can see this easily by
examining the leaves of some moss-like funaria which has been in the
light, or in the chloroplasts of the prothallia of ferns, etc. Starch
grains may also be formed in the chloroplasts from starch which was
formed in some other part of the plant, but which has passed in
solution. Thus the functions of the chloroplast are twofold, that of
photosynthesis and the formation of starch grains.
=149.= In the translocation of starch when it becomes stored up in
various parts of the plant, it passes from the state of solution into
starch grains in connection with plastids similar to the chloroplasts,
but which are not green. The green ones are sometimes called
_chloroplasts_, while the colorless ones are termed _leucoplasts_, and
those possessing other colors, as red and yellow, in floral leaves, the
root of the carrot, etc., are called _chromoplasts_.
=150. Photosynthesis in other than green plants.=—While
carbohydrates are usually only formed by green plants, there are some
exceptions. Apparent exceptions are found in the blue-green algæ, like
oscillatoria, nostoc, or in the brown and red sea weeds like fucus,
rhabdonia, etc. These plants, however, possess chlorophyll, but it is
disguised by another pigment or color. There are plants, however, which
do not have chlorophyll and yet form carbohydrates with evolution of
oxygen in the presence of light, as for example a purple bacterium,
in which the purple coloring substance absorbs light, though the rays
absorbed most energetically are not the red.
[Illustration: Fig. 70.
Cell exposed to weak diffused light showing chlorophyll bodies along
the horizontal walls.]
[Illustration: Fig. 71.
Same cell exposed to strong light, showing chlorophyll bodies have
moved to perpendicular walls.
Figs. 70, 71.—Cell of prothallium of fern.]
=151. Influence of light on the movement of chlorophyll
bodies.=—_In fern prothallia_.—If we place fern prothallia in weak
light for a few hours, and then examine them under the microscope,
we find that the most of the chlorophyll bodies in the cells are
arranged along the inner surface of the horizontal wall. If now the
same prothallia are placed in a brightly lighted place for a short
time most of the chlorophyll bodies move so that they are arranged
along the surfaces of the perpendicular walls, and instead of having
the flattened surfaces exposed to the light as in the former case, the
edges of the chlorophyll bodies are now turned toward the light. (See
figs. 70, 71.) The same phenomenon has been observed in many plants.
Light then has an influence on chlorophyll bodies, to some extent
determining their position. In weak light they are arranged so that the
flattened surfaces are exposed to the incidence of the rays of light,
so that the chlorophyll will absorb as great an amount as possible
of kinetic energy; but intense light is stronger than necessary, and
the chlorophyll bodies move so that their edges are exposed to the
incidence of the rays. This movement of the chlorophyll bodies is
different from that which takes place in some water plants like elodea.
The chlorophyll bodies in elodea are free in the protoplasm. The
protoplasm in the cells of elodea streams around the inside of the cell
wall much as it does in nitella and the chlorophyll bodies are carried
along in the currents, while in nitella they are stationary.
CHAPTER VIII.
STARCH AND SUGAR CONCLUDED. ANALYSIS OF PLANT SUBSTANCE.
1. Translocation of Starch.
=152. Translocation of starch.=—It has been found that leaves of
many plants grown in the sunlight contain starch when examined after
being in the sunlight for several hours. But when the plants are left
in the dark for a day or two the leaves contain no starch, or a much
smaller amount. This suggests that starch after it has been formed may
be transferred from the leaves, or from those areas of the leaves where
it has been formed.
[Illustration: Fig. 72.
Leaf of tropæolum with portion covered with corks to prevent the
formation of starch. (After Detmer.)]
[Illustration: Fig. 73.
Leaf of tropæolum treated with iodine after removal of cork, to show
that starch is removed from the leaf during the night.]
To test this let us perform an experiment which is often made. We may
take a plant such as a garden tropæolum or a clover plant, or other
land plant in which it is easy to test for the presence of starch. Pin
a piece of circular cork, which is smaller than the area of the leaf,
on either side of the leaf, as in fig. 72, but allow free circulation
of air between the cork and the under side of the leaf. Place the plant
where it will be in the sunlight. On the afternoon of the following
day, if the sun has been shining, test the entire leaf for starch. The
part covered by the cork will not give the reaction for starch, as
shown by the absence of the bluish color, while the other parts of the
leaf will show it. The starch which was in that part of the leaf the
day before was dissolved and removed during the night, and then during
the following day, the parts being covered from the light, no starch
was formed in them.
=153. Starch in other parts of plants than the leaves.=—We may
use the iodine test to search for starch in other parts of plants than
the leaves. If we cut a potato tuber, scrape some of the cut surface
into a pulp, and apply the iodine test, we obtain a beautiful and
distinct reaction showing the presence of starch. Now we have learned
that starch is only formed in the parts containing chlorophyll. We
have also learned that the starch which has been formed in the leaves
disappears from the leaf or is transferred from the leaf. We judge
therefore that the starch which we have found in the tuber of the
potato was formed first in the green leaves of the plant, as a result
of photosynthesis. From the leaves it is transferred in solution to
the underground stems, and stored in the tubers. The starch is stored
here by the plant to provide food for the growth of new plants from the
tubers, which are thus much more vigorous than the plants would be if
grown from the seed.
=154. Form of starch grains.=—Where starch is stored as a reserve
material it occurs in grains which usually have certain characters
peculiar to the species of plant in which they are found. They vary
in size in many different plants, and to some extent in form also.
If we scrape some of the cut surface of the potato tuber into a pulp
and mount a small quantity in water, or make a thin section for
microscopic examination, we find large starch grains of a beautiful
structure. The grains are oval in form and more or less irregular in
outline. But the striking peculiarity is the presence of what seem to
be alternating dark and light lines in the starch grain. We note that
the lines form irregular rings, which are smaller and smaller until
we come to the small central spot termed the “hilum” of the starch
grain. It is supposed that these apparent lines in the starch grain are
caused by the starch substance being deposited in alternating dense
and dilute layers, the dilute layers containing more water than the
dense ones; others think that the successive layers from the hilum
outward are regularly of diminishing density, and that this gives the
appearance of alternating lines. The starch formed by plants is one of
the organic substances which are manufactured by plants, and it (or
glucose) is the basis for the formation of other organic substances in
the plant. Without such organic substances green plants cannot make any
appreciable increase of plant substance, though a considerable increase
in size of the plant may take place.
NOTE.—The organic compounds resulting from photosynthesis,
since they are formed by the union of carbon, hydrogen, and oxygen in
such a way that the hydrogen and oxygen are usually present in the same
proportion as in water, are called _carbohydrates_. The most common
carbohydrates are sugars (cane sugar, C₁₂H₂₂O₁₁ for example, in beet
roots, sugar cane, sugar maple, etc.), starch, and cellulose.
=155. Vaucheria.=—The result of carbon dioxide assimilation in
the threads of Vaucheria is not clearly understood. Starch is absent or
difficult to find in all except a few species, while oil globules are
present in most species. These oil globules are spherical, colorless,
globose and highly refringent. Often small ones are seen lying against
chlorophyll bodies. Oil is a _hydrocarbon_ (containing C, H, and O, but
the H and O are in different proportions from what they are in H₂O)
and until recently it was supposed that this oil in Vaucheria was the
direct result of photosynthesis. But the oil does not disappear when
the plant is kept for a long time in the dark, which seems to show
that it is not the direct product of carbon dioxide assimilation, and
indicates that it comes either from a temporary starch body or from
glucose. Schimper found glucose in several species of Vaucheria, and
Waltz says that some starch is present in Vaucheria sericea, while
in V. tuberosa starch is abundant and replaces the oil. To test for
oil bodies in Vaucheria treat the threads with weak osmic acid, or
allow them to stand for twenty-four hours in Fleming’s solution (which
contains osmic acid). Mount some threads and examine with microscope.
The oil globules are stained black.
2. Sugar, and Digestion of Starch.[9]
=156.= It is probable that some form of sugar is always produced
as the result of photosynthesis. The sugar thus formed may be stored as
such or changed to starch. In general it may be said that sugar is most
common in the green parts of monocotyledonous plants, while starch is
most frequent in dicotyledons. Plant sugars are of three general kinds:
cane sugar abundant in the sugar cane, sugar beet, sugar maple, etc.;
glucose and fruit sugar, found in the fruits of a majority of plants,
and abundant in some, as in apples, pears, grapes, etc.; and maltose, a
variety produced in germinating seeds, as in malted barley.
=157. Test for sugar.=—A very pretty experiment maybe made by taking
two test tubes, placing in one a solution of commercial grape sugar
(glucose), in the other one of granulated cane sugar, and adding to
each a few drops of Fehling’s solution.[10] After these tubes have
stood in a warm place for half an hour, it will be found that a bright
orange brown or cinnabar-colored precipitate of copper and cuprous
oxide has formed in the tube containing grape sugar, while the other
solution is unchanged. Grape sugar or glucose, therefore, reduces
Fehling’s solution, while cane sugar as such has no effect upon it.
Cane sugar may be changed or converted to glucose by being boiled for a
short time with a dilute acid, or by adding Fehling’s solution to the
sugar solution and boiling. In the latter case the change is brought
about by the alkali and the precipitate of copper and cuprous oxide
forms.
=158. Tests for sugar in plant tissue.=—(_a_) Scrape out a little
of the tissue from the inside of a ripe apple or pear, place it with a
little water in a test tube, and add a few drops of Fehling’s solution.
After standing half an hour the characteristic precipitate of copper
and cuprous oxide appears, showing that grape sugar is present in
quantity.
Make thin sections of the apple and mount in a drop of Fehling’s
solution on a slide. After half an hour examine with the microscope.
The granules of cuprous oxide are present in the cells of the tissue in
great abundance.
(_b_) Cut up several leaves of a young vigorous corn seedling,
cover with water in a test tube and boil for a minute. After the
decoction has cooled add the Fehling’s solution and allow to stand.
The precipitate will appear. For comparison take similar corn leaves,
remove the chlorophyll with alcohol and test with iodine. No starch
reaction appears. The carbohydrate in corn leaves is therefore glucose
and not starch. If now the corn seed be examined the cells will be
found to be full of starch grains which give the beautiful blue
reaction with iodine. This experiment shows that grape sugar is formed
in the leaves of the corn plant, but is changed to starch when stored
in the seed.
(_c_) Take two leaves of bean seedling or coleus, test one for sugar
and the other for starch. Both are present.
(_d_) Procure some maple sap in the spring, or in the winter months
make a decoction of the broken tips of young branches of the sugar
maple by boiling them in water in a test tube. To the sap or cool
decoction add Fehling’s solution. No precipitate appears after
standing. Now heat the same solution to the boiling point, and the
precipitate forms, showing the presence of cane sugar in the maple
sap which was converted to glucose and fruit sugar by boiling in the
presence of an alkali.
(_e_) Scrape out some of the tissue from a sugar beet root, cover with
water in a test tube and add Fehling’s solution. No change takes place
after standing. Boil the same solution and the precipitate forms,
showing the presence of cane sugar, inverted to grape sugar and fruit
sugar by the hot alkali.
=159. How starch is changed to sugar.=—We have seen that in
many plants the carbohydrate formed as the result of carbon dioxide
assimilation is stored as starch. This substance being insoluble
in water must be changed to sugar, which is soluble before it can
be used as food or transported to other parts of the plant. This
is accomplished through the action of certain enzymes, principally
diastase. This substance has the power of acting upon starch under
proper conditions of temperature and moisture, causing it to take up
the elements of water, and so to become sugar.
This process takes place commonly in the leaves where starch is formed,
but especially in seeds, tubers (during the sprouting, etc.), and
other parts which the plant uses as storehouses for starch food. It is
probable that the same conditions of temperature and moisture which
favor germination or active growth are also favorable to the production
of diastase.
=160. Experiments to show the action of diastase.=—(_a_) Place
a bit of starch half as large as a pea in a test tube, and cover with
a weak solution[11] (about ⅕ per cent) of commercial taka diastase.
After it has stood in a warm place for five or ten minutes test with
Fehling’s solution. The precipitate of cuprous oxide appears showing
that some of the starch has been changed to sugar. By using measured
quantities, and by testing with iodine at frequent intervals, it can
be determined just how long it takes a given quantity of diastase to
change a known quantity of starch. In this connection one should first
test a portion of the same starch with Fehling’s solution to show that
no sugar is present.
(_b_) Repeat the above experiment using a little tissue from a potato,
and some from a corn seed.
(_c_) Take 25 germinating barley seeds in which the radicle is just
appearing. Grind up thoroughly in a mortar with about three parts of
water. After this has stood for ten or fifteen minutes, filter. Fill a
test tube one-third full of water, add a piece of starch half the size
of a pea or less, and boil the mixture to make starch-paste. Add the
barley extract. Put in a warm place and test from time to time with
iodine. The first samples so treated will be blue, later ones violet,
brown, and finally colorless, showing that the starch has all
disappeared. This is due to the action of the diastase which was
present in the germinating seeds, and which was dissolved out and added
to the starch mixture. The office of this diastase is to change the
starch in the seeds to sugar. Germinating wheat is sweet, and it is a
matter of common observation that bread made from sprouted wheat is
sweet.
(_d_) Put a little starch-paste in a test tube and cover it with saliva
from the mouth. After ten or fifteen minutes test with Fehling’s
solution. A strong reaction appears showing how quickly and effectively
saliva acts in converting starch to sugar. Successive tests with iodine
will show the gradual disappearance of the starch.
=161. These experiments have shown us that diastase= from three
different sources can act upon starch converting it into sugar. The
active principle in the saliva is an _animal_ diastase (_ptyalin_),
which is necessary as one step in the digestion of starch food in
animals. The _taka_ diastase is derived from a fungus (Eurotium oryzæ)
which feeds on the starch in rice grains converting it into sugar
which the fungus absorbs for food. The _malt_ diastase and _leaf_
diastase are formed by the seed plants. That in seeds converts the
starch to sugar which is absorbed by the embryo for food. That in the
leaf converts the starch into sugar so that it can be transported to
other parts of the plant to be used in building new tissue, or to be
stored again in the form of starch (example, the potato, in seeds,
etc.). The starch is formed in the leaf during the daylight. The light
renders the leaf diastase inactive. But at night the leaf diastase
becomes active and converts the starch made during the day. Starch is
not soluble in water, while the sugar is, and the sugar in solution is
thus easily transported throughout the plant. In those green plants
which do not form starch in their leaves (sugar beet, corn, and many
monocotyledons), grape sugar and fruit sugar are formed in the green
parts as the result of photosynthesis. In some, like the corn, the
grape sugar formed in the leaves is transported to other parts of the
plant, and some of it is stored up in the seed as starch. In others
like the sugar beet the glucose and fruit sugar formed in the leaves
flow to other parts of the plant, and much of it is stored up as cane
sugar in the beet root. The process of photosynthesis probably proceeds
in the same way in all cases up to the formation of the grape sugar and
fruit sugar in the leaves. In the beet, corn, etc., the process stops
here, while in the bean, clover, and most dicotyledons the process is
carried one step farther in the leaf and starch is formed.
3. Rough Analysis of Plant Substance.
=162. Some simple experiments to indicate the nature of plant
substance.=—After these building-up processes of the plant, it is
instructive to perform some simple experiments which indicate roughly
the nature of the plant substance, and serve to show how it can be
separated into other substances, some of them being reduced to the
form in which they existed when the plant took them as food. For exact
experiments and results it would be necessary to make chemical analyses.
=163. The water in the plant.=—Take fresh leaves or leafy shoots
or other fresh plant parts. Weigh. Permit them to remain in a dry room
until they are what we call “dry.” Now weigh. The plants have lost
weight, and from what we have learned in studies of transpiration this
loss in weight we know to result from the loss of water from the plant.
=164. The dry plant material contains water.=—Take air-dry
leaves, shavings, or other dry parts of plants. Place them in a test
tube. With a holder rest the tube in a nearly horizontal position,
with the bottom of the tube in the flame of a Bunsen burner. Very
soon, before the plant parts begin to “burn,” note that moisture is
accumulating on the inner surface of the test tube. This is water
driven off which could not escape by drying in air, without the
addition of artificial heat, and is called “hygroscopic water.”
=165. Water formed on burning the dry plant material.=—Light a
soft-pine or basswood splinter. Hold a thistle tube in one hand with the
bulb downward and above the flame of the splinter. Carbon will be
deposited over the inner surface of the bulb. After a time hold the
tube toward the window and look through it above the carbon. Drops of
water have accumulated on the inside of the tube. This water is formed
by the rearrangement of some of the hydrogen and oxygen, which is set
free by the burning of the plant material, where they were combined
with carbon, as in the cellulose, and with other elements.
=166. Formation of charcoal by burning.=—Take dried leaves, and
shavings from some soft wood. Place in a porcelain crucible, and cover
about 3 cm. deep with dry fine earth. Place the crucible in the flame
of a Bunsen burner and let it remain for about fifteen minutes. Remove
and empty the contents. If the flame was hot the plant material will be
reduced to a good quality of charcoal. The charcoal consists largely of
carbon.
=167. The ash of the plant.=—Place in the porcelain crucible
dried leaves and shavings as before. Do not cover with earth. Place the
crucible in the flame of the Bunsen burner, and for a moment place on
the porcelain cover; then remove the cover, and note the moisture on
the under surface from the escaping water. Permit the plant material to
burn; it may even flame for a time. In the course of fifteen minutes it
is reduced to a whitish powder, much smaller in bulk than the charcoal
in the former experiment. This is the ash of the plant.
=168. What has become of the carbon?=—In this experiment the air
was not excluded from the plant material, so that oxygen combined with
carbon as the water was freed, and formed carbon dioxide, passing off
into the air in this form. This it will be remembered is the form in
which the plant took the carbon-food in through the leaves. Here the
carbon dioxide met the water coming from the soil, and the two united
to form, ultimately, starch, cellulose, and other compounds of carbon;
while with the addition of nitrogen, sulphur, etc., coming also from
the soil, still other plant substances were formed.
=169.= The carbohydrates are classed among the non-nitrogenous
substances. Other non-nitrogenous plant substances are the organic
acids like oxalic acid (H₂C₂O₄), malic acid (H₂C₄H₄O₅), etc.; the fats
and fixed oils, which occur in the seeds and fruits of many plants. Of
the nitrogenous substances the proteids have a very complex chemical
formula and contain carbon, hydrogen, oxygen, nitrogen, sulphur, etc.
(example, _aleuron_, or proteid grains, found in seeds). The proteids
are the source of nitrogenous food for the seedling during germination.
Of the amides, _asparagin_ (C₄H₈N₂O₃) is an example of a nitrogenous
substance; and of the alkaloids, nicotin (C₁₀H₁₄N₂) from tobacco.
All living plants contain a large per cent of water. According to
Vines “ripe seeds dried in the air contain 12 to 15 per cent of water,
herbaceous plants 60 to 80 per cent, and many water plants and fungi as
much as 95 per cent of their weight.” When heated to 100° C. the water
is driven off. The dry matter remaining is made up partly of organic
compounds, examples of which are given above, and inorganic compounds.
By burning this dry residue the organic substances are mostly changed
into volatile products, principally carbonic acid, water, and nitrogen.
The inorganic substances as a result of combustion remain as a white or
gray powder, the _ash_.
The amount of the ash increases with the age of the plant, though the
percentage of ash may vary at different times in the different members
of the plant. The following table taken from Vines will give an idea of
the amount and composition of the ash in the dry solid of a few plants:
CONTENT OF 1000 PARTS OF DRY SOLID MATTER.
|Clover, in|Wheat,|Wheat,|Potato|Apples| Peas
| blossom |grain |straw |tubers| |(the seed)
------------------+----------+------+------+------+------+----------
Ash. | 68.3 | 19.7 | 53.7 | 37.7 | 14.4 | 27.3
Potash. | 21.96 | 6.14 | 7.33 | 22.76| 5.14| 11.41
Soda. | 1.39 | 0.44 | 0.74 | 0.99| 3.76| 0.26
Lime. | 24.06 | 0.66 | 3.09 | 0.97| 0.59| 1.36
Magnesium. | 7.44 | 2.36 | 1.33 | 1.77| 1.26| 2.17
Ferric Oxide. | 0.72 | 0.26 | 0.33 | 0.45| 0.2 | 0.16
Phosphoric Acid. | 6.74 | 9.26 | 2.58 | 6.53| 1.96| 9.95
Sulphuric Acid. | 2.06 | 0.07 | 1.32 | 2.45| 0.88| 0.95
Silica. | 1.62 | 0.42 |36.25 | 0.8 | 0.62| 0.24
Chlorine. | 2.66 | 0.04 | 0.9 | 1.17| ....| 0.42
------------------+----------+------+------+------+------+----------
FOOTNOTES:
[9] Paragraphs 156-160 were prepared by Dr. E. J. Durand.
[10] Make up three stock solutions as follows:
(1) Copper sulphate 9 grams Water 250 cc.
(2) Caustic potash 30 grams Water 250 cc.
(3) Rochelle salts 49 grams Water 250 cc.
For Fehling’s solution take one volume of each of (1), (2), and (3),
and to the mixture add two volumes of water.
[11] This solution of taka diastase should be made up cold. If it is
heated to 60° C. or over it is destroyed.
CHAPTER IX.
HOW PLANTS OBTAIN THEIR FOOD. I.
1. Sources of Plant Food.
=170. The necessary constituents of plant food.=—As indicated in
Chapter 3, investigation has taught us the principal constituents of
plant food. Some suggestion as to the food substances is derived by a
chemical analysis of various plants. In Chapter 8 it was noted that
there are two principal kinds of compounds in plant substances, the
organic compounds and the inorganic compounds or mineral substances.
The principal elements in the organic compounds are _hydrogen_,
_carbon_, _oxygen_ and _nitrogen_. The elements in the inorganic
compounds which have been found indispensable to plant growth are
_calcium_,[12] _potassium_, _magnesium_, _phosphorus_, _sulphur_ and
_iron_. (See paragraphs 54-58, and complete observations on water
cultures.) Other elements are found in the ash of plants; and while
they are not absolutely necessary for growth, some[13] of them are
beneficial in one way or another.
=171.= The carbohydrates are derived, as we have learned, from
the CO₂ of the air, and water in the plant tissue drawn from the soil;
though in the case of aquatic plants entirely submerged, all the
constituents are absorbed from the surrounding water.
=172. Food substances in the soil.=—Land plants derive their
mineral food from the soil, the soil received the mineral substances
from dissolving and disintegrating rocks. Nitrogenous food is chiefly
derived from the same source, but under a variety of conditions which
will be discussed in later paragraphs, but the nitrogen comes primarily
from the air. Some of the mineral substances, those which are soluble
as well as some of the nitrogenous substances, are found in solution in
the soil. These are absorbed by the plant, as needed, along with water,
through the root hairs.
=173. Absorption of soluble substances.=—Since these substances
are dissolved in the water of the soil, it is not necessary for us
to dwell on the process of absorption. This in general is dwelt upon
in Chapter 3. It should be noted, however, that food substances in
solution, during absorption, diffuse through the protoplasmic membrane
independently of each other and also independently of the rate of
movement of the water from the soil into the root hairs and cells of
the root.
When the cells have absorbed a certain amount of a given substance,
no more is absorbed until the concentration of the cell-sap in that
particular substance is reduced. This, however, does not interfere
with the absorption of water, or of other substances in solution by
the same cells. Plants have therefore a certain selective power in the
absorption of food substances.
=174. Action of root hairs on insoluble substances. Acidity of root
hairs.=—If we take a seedling which has been grown in a germinator,
or in the folds of cloths or paper, so that the roots are free from the
soil, and touch the moist root hairs to blue litmus paper, the paper
becomes red in color where the root hairs have come in contact. This
is the reaction for the presence of an acid salt, and indicates that
the root hairs excrete certain acid substances. This acid property of
the root hairs serves a very important function in the preparation
of certain of the elements of plant food in the soil. Certain of the
chemical compounds of potash, phosphoric acid, etc., become deposited
on the soil particles, and are not soluble in water. The acid of the
root hairs dissolves some of these compounds where the particles of
soil are in close contact with them, and the solutions can then be
taken up by the roots. Carbonic acid and other acids are also formed in
the soil, and aid in bringing these substances into solution.
=175.= This corrosive action of the roots can be shown by the
well-known experiment of growing a plant on a marble plate which is
covered by soil. In lieu of the marble plate, the peas may be planted
in clam or oyster shells, which are then buried in the soil of the
pot, so that the roots of the seedlings will come in contact with the
smooth surface of the shell. After a few weeks, if the soil be washed
from the marble where the roots have been in close contact, there will
be an outline of this part of the root system. Several different acid
substances are excreted from the roots of plants which have been found
to redden blue litmus paper by contact. Experiments by Czapek show,
however, that the carbonic acid excreted by the roots has the power of
directly bringing about these corrosion phenomena. The acid salts are
the substances which are most actively concerned in reddening the blue
litmus paper. They do not directly aid in the corrosion phenomena. In
the soil, however, where these compounds of potash, phosphoric acid,
etc., are which are not soluble in water, the acid salt (primary acid
potassium phosphate) which is most actively concerned in reddening
the blue litmus paper may act indirectly on these mineral substances,
making them available for plant food. This salt soon unites with
certain chlorides in the soil, making among other things small
quantities of hydrochloric acid.
=176.= NOTE.—It is a general rule that plants cannot
take solid food into their bodies, but obtain all food in either a
liquid or gaseous state. The only exception to this is in the case of
the plasmodia of certain _Myxomycetes_ (Slime Moulds), and also perhaps
some of the Flagellates and other very low forms, which engulf solid
particles of food. It is uncertain, however, whether these organisms
belong to the plant or animal kingdom, and they probably occupy a more
or less intermediate position.
=177. Action of nitrite and nitrate bacteria.=—Many of the
higher green plants prefer their nitrogenous food in the form of
nitrates. (Example, nitrate of soda, potassium nitrate, saltpetre.)
Nitrates are constantly being formed in soil by the action of certain
bacteria. The nitrite bacteria (Nitromonas) convert ammonia in the
soil to _nitrous acid_ (a _nitrite_), while at this point the nitrate
bacteria (Nitrobacter) convert the nitrites into nitrates. The fact
that this nitrification is going on constantly in soil is of the utmost
importance, for while commercial nitrates are often applied to the
soil, the nitrates are easily washed from the soil by heavy rains.
These nitrite and nitrate bacteria require oxygen for their activity,
and they are able to obtain their carbohydrates by decomposing organic
matter in the soil, or directly by assimilating the CO₂ in the soil,
deriving the energy for the assimilation of the carbon dioxide from
the chemical process of nitrification. This kind of carbon dioxide
assimilation is called _chemosynthetic_ assimilation.
2. Parasites and Saprophytes.
=178. Parasites among the fungi.=—A parasite is an organism
which derives all or a part of its food directly from another living
organism (its host) and at the latter’s expense. The larger number
of plant parasites are found among the fungi (rusts, smuts, mildews,
etc.). (See Nutrition of the Fungi, paragraph 185.) Some of these are
not capable of development unless upon their host, and are called
_obligate_ parasites. Others can grow not only as parasites but at
other times can also grow on dead organic matter, and are called
_facultative_ parasites, i.e. they can choose either a parasitic life
or a saprophytic one.
=179. Parasites among the seed plants.=—_Cuscuta._—There are,
however, parasites among the seed plants; for example, the dodder
(Cuscuta), parasitic on clover, and a great variety of other plants.
There is food enough in the seed for the young plant to take root and
develop a slender stem until it takes hold of its host. It then twines
around the stem of its host sending wedge-shaped haustoria into the
stem to obtain food. The part then in connection with the ground dies.
The haustoria of the dodder form a complete junction with the vascular
bundles of its host so that through the vessels water and salts are
obtained, while through the junction of sieve tubes the elaborated
organic food is obtained. The union of the dodder with its host is like
that between a graft and the graft stock. The beech drops (Epiphegus)
is another example of a parasitic seed plant. It is parasitic on the
roots of the beech.
[Illustration: Fig. 74. Dodder.]
=180. The mistletoe= (Viscum album), which grows on the branches
of trees, sends its roots into the branches, and only the vessels
of the vascular system are fused according to some. If this is true
then it probably obtains only water and salts from its host. But the
mistletoe has green leaves and is thus able to assimilate carbon
dioxide and manufacture its own organic substances. It is claimed by
some, however, that the host derives some food from the parasite during
the winter when the host has shed its leaves, and if this is true it
would seem that organic food could also be derived during the summer
from the host by the mistletoe.
=181. Saprophytes.=—A saprophyte is a plant which is enabled
to obtain its food, especially its organic food, directly from dead
animals or plants or from dead organic substances. Many fungi are
saprophytes, as the moulds, mushrooms, etc. (See Nutrition of the
Fungi.)
=182. Humus saprophytes.=—The action of fungi as described in the
preceding chapter, as well as of certain bacteria, gradually converts
the dead plants or plant parts into the finely powdered brown substance
known as _humus_. In general the green plants cannot absorb organic
food from humus directly. But plants which are devoid of chlorophyll
can live saprophytically on this humus. They are known as _humus
saprophytes_. Many of the mushrooms and other fungi, as well as some
seed plants which lack chlorophyll or possess only a small quantity,
are able to absorb all their organic food from humus. It is uncertain
whether any seed plants can obtain all of their organic food directly
from humus, though it is believed that many can so obtain a portion
of it. But a number of seed plants, like the Indian-pipe (Monotropa)
and certain orchids, obtain organic food from humus. These plants lack
chlorophyll and cannot therefore manufacture their own carbohydrate
food. Not being parasitic on plants which can, as in the case of the
dodder and beech drops mentioned above, they undoubtedly derive their
organic food from the humus. But fungus mycelium growing in the humus
is attached to their roots, and in some orchids enters the roots and
forms a nutritive connection. The fungus mycelium can absorb organic
food from the humus and in some cases at least can transfer it over to
the roots of the higher plant (see Mycorhiza).
=183. Autotrophic, heterotrophic, and mixotrophic plants.=—An
_autotrophic_ plant is one which is self-nourishing, i.e. it is
provided with an abundant chlorophyll apparatus for carbon dioxide
assimilation and with absorbing organs for obtaining water and salts.
Heterotrophic plants are not provided with a chlorophyll apparatus
sufficient to assimilate all the carbon dioxide necessary, so they
nourish themselves by other means. _Mixotrophic_ plants are those
which are intermediate between the other two, i.e. they have some
chlorophyll but not enough to provide all the organic food necessary,
so they obtain a portion of it by other means. Evidently there are all
gradations of mixotrophic plants between the two other kinds (example,
the mistletoe).
=184. Symbiosis.=—Symbiosis means a living with or living
together, and is said of those organisms which live so closely in
connection with each other as to be influenced for better or worse,
especially from a nutrition standpoint. _Conjunctive_ symbiosis has
reference to those cases where there is a direct interchange of
food material between the two organisms (lichens, mycorhiza, etc.).
_Disjunctive_ symbiosis has reference to an inter-life relation without
any fixed union between them (example, the relations between flowers
and insects, ants and plants, and even in a broad sense the relation
between saprophytic plants in reducing organic matter to a condition
in which it may be used for food by the green plants, and these in
turn provide organic matter for the saprophytes to feed upon, etc.).
_Antagonistic_ symbiosis is shown in the relation of parasite to its
host, _reciprocal_ symbiosis, or _mutualistic_ symbiosis is shown in
those cases where both symbionts derive food as a result of the union
(lichens, mycorhiza, etc.).
3. How Fungi Obtain their Food.
[Illustration: Fig. 75. Carnation rust on leaf and flower stem. From
photograph.]
=185. Nutrition of moulds.=—In our study of mucor, as we have
seen, the growing or vegetative part of the plant, the mycelium, lies
within the substratum, which contains the food materials in solution,
and the slender threads are thus bathed on all sides by them. The
mycelium absorbs the watery solutions throughout the entire system of
ramifications. When the upright fruiting threads are developed they
derive the materials for their growth directly from the mycelium with
which they are in connection. The moulds which grow on decaying fruit
or on other organic matter derive their nutrient materials in the same
way. The portion of the mould which we usually see on the surface of
these substances is in general the fruiting part. The larger part of
the mycelium lies hidden within the substratum.
=186. Nutrition of parasitic fungi.=—Certain of the fungi grow on
or within the higher plants and derive their food materials from them
and at their expense. Such a fungus is called a _parasite_, and there
are a large number of these plants which are known as _parasitic
fungi_. The plant at whose expense they grow is called the “_host_.”
One of these parasitic fungi, which it is quite easy to obtain in
greenhouses or conservatories during the autumn and winter, is the
carnation rust (_Uromyces caryophyllinus_), since it breaks out in
rusty dark brown patches on the leaves and stems of the carnation (see
fig. 75). If we make thin cross-sections through one of these spots
on a leaf, and place them for a few minutes in a solution of chloral
hydrate, portions of the tissues of the leaf will be dissolved. After
a few minutes we wash the sections in water on a glass slip, and stain
them with a solution of eosin. If the sections were carefully made, and
thin, the threads of the mycelium will be seen coursing between the
cells of the leaf as slender threads. Here and there will be seen short
branches of these threads which penetrate the cell wall of the host and
project into the interior of the cell in the form of an irregular knob.
Such a branch is a _haustorium_. By means of this haustorium, which is
here only a short branch of the mycelium, nutritive substances are
taken by the fungus from the protoplasm or cell-sap of the carnation.
From here it passes to the threads of the mycelium. These in turn
supply food material for the development of the dark brown gonidia,
which we see form the dark-looking powder on the spots. Many other
fungi form haustoria, which take up nutrient matters in the way
described for the carnation rust. In the case of other parasitic fungi
the threads of the mycelium themselves penetrate the cells of the host,
while in still others the mycelium courses only between the cells of
the host (fungus of peach leaf curl for example) and derives food
materials from the protoplasm or cell-sap of the host by the process of
osmosis.
[Illustration: Fig. 76. Several teleutospores, showing the variations
in form.]
[Illustration: Fig. 77. Cells from the stem of a rusted carnation,
showing the intercellular mycelium and haustoria. Object magnified 30
times more than the scale.]
[Illustration: Fig. 78. Cell from carnation leaf, showing haustorium of
rust mycelium grasping the nucleus of the host. _h_, haustorium; _n_,
nucleus of host.]
[Illustration: Fig. 79. Intercellular mycelium with haustoria entering
the cells. _A_, of Cystopus candidus (white rust); _B_, of Peronospora
calotheca. (De Bary.)]
=187. Nutrition of the larger fungi.=—If we select some one of
the larger fungi, the majority of which belong to the mushroom family
and its relatives, which is growing on a decaying log or in the soil,
we shall see on tearing open the log, or on removing the bark or part
of the soil, as the case may be, that the stem of the plant, if it have
one, is connected with whitish strands. During the spring, summer, or
autumn months, examples of the mushrooms connected with these strands
may usually be found readily in the fields or woods, but during the
winter and colder parts of the year often they may be seen in forcing
houses, especially those cellars devoted to the propagation of the
mushroom of commerce.
=188.= These strands are made up of numerous threads of the
mycelium which are closely twisted and interwoven into a cord or
strand, which is called a mycelium strand, or _rhizomorph_. These are
well shown in fig. 236, which is from a photograph of the mycelium
strands, or “spawn” as the grower of mushrooms calls it, of Agaricus
campestris. The little knobs or enlargements on the strands are the
young fruit bodies, or “buttons.”
[Illustration: Fig. 80. Sterile mycelium on wood props in coal mine, 400
feet below surface.
(Photographed by the author.)]
=189.= While these threads or strands of the mycelium in the
decaying wood or in the decaying organic matter of the soil are not
true roots, they function as roots, or root hairs, in the absorption
of food materials. In old cellars and on damp soil in moist places we
sometimes see fine examples of this vegetative part of the fungi, the
mycelium. But most magnificent examples are to be seen in abandoned
mines where timber has been taken down into the tunnels far below
the surface of the ground to support the rock roof above the mining
operations. I have visited some of the coal mines at Wilkesbarre, Pa.,
and here on the wood props and doors, several hundred feet below the
surface, and in blackest darkness, in an atmosphere almost completely
saturated at all times, the mycelium of some of the wood destroying
fungi grows in a profusion and magnificence which is almost beyond
belief. Fig. 80 is from a flash-light photograph of a beautiful example
400 feet below the surface of the ground. This was growing over the
surface of a wood prop or post, and the picture is much reduced. On
the doors in the mine one can see the strands of the mycelium which
radiate in fan-like figures at certain places near the margin of
growth, and farther back the delicate tassels of mycelium which hang
down in fantastic figures, all in spotless white and rivalling the most
beautiful fabric in the exquisiteness of its construction.
=190. How fungi derive carbohydrate food.=—The fungi being
devoid of chlorophyll cannot assimilate the CO₂ from the air. They
are therefore dependent on the green plants for their carbohydrate
food. Among the saprophytes, the leaf and wood destroying fungi
excrete certain substances (known as _enzymes_) which dissolve the
carbohydrates and certain other organic compounds in the woody or
leafy substratum in which they grow. They thus produce a sort of
extracellular digestion of carbohydrates, converting them into a
soluble form which can be absorbed by the mycelium. The parasitic
fungi also obtain their carbohydrates and other organic food from the
host. The mycelium of certain parasitic, and of wood destroying fungi,
excretes enzymes (_cytase_) which dissolve minute perforations in the
cell walls of the host and thus aid the hypha during its boring action
in penetrating cell walls.
NOTE.—Certain wood destroying fungi growing in oaks absorb
tannin directly, i.e. in an unchanged form. One of the pine destroying
fungi (_Trametes pini_) absorbs the xylogen from the wood cells,
leaving the pure cellulose in which the xylogen was filtrated; while
_Polyporus mollis_ absorbs the cellulose, leaving behind only the wood
element.
4. Mycorhiza.
=191.= While such plants as the Indian-pipe (Monotropa), some
of the orchids, etc., are _humus saprophytes_ and some of them are
possibly able to absorb organic food from the humus, many of them have
fungus mycelium in close connection with their roots, and these fungus
threads aid in the absorption of organic food. The roots of plants
which have fungus mycelium intimately associated in connection with
the process of nutrition, are termed _mycorhiza_. There is a mutual
interchange of food between the fungus and the host, a _reciprocal
symbiosis_.
=192. Mycorhiza are of two kinds= as regards the relation of
the fungus to the root; _ectotrophic_ (or _epiphytic_), where the
mycelium is chiefly on the outside of the root, and _endotrophic_ (or
_endophytic_) where the mycelium is chiefly within the tissue of the
root.
=193. Ectotrophic mycorhiza.=—Ectotrophic mycorhiza occur on the
roots of the oak, beech, hornbean, etc., in forests where there is a
great deal of humus from decaying leaves and other vegetation. The
young growing roots of these trees become closely covered with a thick
felt of the mycelium, so that no root hairs can develop. The terminal
roots also branch profusely and are considerably thickened. The fungus
serves here as the absorbent organ for the tree. It also acts on the
humus, converting some of it into available plant food and transferring
it over to the tree.
=194. Endotrophic mycorhiza.=—These are found on many of the
humus saprophytes, which are devoid of chlorophyll, as well as on those
possessing little or even on some plants possessing an abundance, of
chlorophyll. Examples are found in many orchids (see the coral root
orchid, for example), some of the ferns (Botrychium), the pines,
leguminous plants, etc. In endotrophic mycorhiza the mycelium is more
abundant within the tissues of the root, though some of the threads
extend to the outside. In the case of the mycorhiza on the humus
saprophytes which have no chlorophyll, or but little, it is thought
by some that the fungus mycelium in the humus assists in converting
organic substances and carbohydrates into a form available for food
by the higher plant and then conducts it into the root, thus aiding
also in the process of absorption, since there are few or no root
hairs on the short and fleshy mycorhiza. The roots, however, of some
of these humus saprophytes have the power of absorbing a portion of
their organic compounds from the humus. It is thought by some, though
not definitely demonstrated, that in the case of the oaks, beeches,
hornbeans, and other chlorophyll-bearing symbionts, the fungus threads
do not absorb any carbohydrates for the higher symbiont, but that they
actually derive their carbohydrates from it.[14] But it is reasonably
certain that the fungus threads do assimilate from the humus certain
unoxidized, or feebly oxidized, nitrogenous substances (ammonia, for
example), and transfer them over to the host, for the higher plants
with difficulty absorb these substances, while they readily absorb
nitrates which are not abundant in humus. This is especially important
in the forest. It is likely therefore.
5. Nitrogen gatherers.
[Illustration: Fig. 81. Root of the common vetch, showing root
tubercles.]
=195. How clovers, peas, and other legumes gather nitrogen.=—It
has long been known that clover plants, peas, beans, and many other
leguminous plants are often able to thrive in soil where the cereals
do but poorly. Soil poor in nitrogenous plant food becomes richer in
this substance where clovers, peas, etc., are grown, and they are often
planted for the purpose of enriching the soil. Leguminous plants,
especially in poor soil, are almost certain to have enlargements, in
the form of nodules, or “root-tubercles.” A root of the common vetch
with some of these root-tubercles is shown in fig. 81.
=196. A fungal or bacterial organism in these root-tubercles.=—If
we cut one of these root-tubercles open, and mount a small portion
of the interior in water for examination with the microscope, we
shall find small rod-shaped bodies, some of which resemble bacteria,
while others are more or less forked into forms like the letter Y, as
shown in fig. 82. These bodies are rich in nitrogenous substances,
or proteids. They are portions of a minute organism, of a fungus or
bacterial nature, which attacks the roots of leguminous plants and
causes these nodular outgrowths. The organism (Phytomyxa leguminosarum)
exists in the soil and is widely distributed where legumes grow.
=197. How the organism gets into the roots of the legumes.=—This
minute organism in the soil makes its way through the wall of a root
hair near the end. It then grows down the interior of the root hair in
the form of a thread. When it reaches the cell walls it makes a minute
perforation, through which it grows to enter the adjacent cell, when it
enlarges again. In this way it passes from the root hair to the cells
of the root and down to near the center of the root. As soon as it
begins to enter the cells of the root it stimulates the cells of that
portion to greater activity. So the root here develops a large lateral
nodule, or “root-tubercle.” As this “root-tubercle” increases in size,
the fungus threads branch in all directions, entering many cells. The
threads are very irregular in form, and from certain enlargements it
appears that the rod-like bodies are formed, or the thread later breaks
into myriads of these small “bacteroids.”
[Illustration: Fig. 82. Root-tubercle organism from vetch, old
condition.]
[Illustration: Fig. 83. Root-tubercle organism from Medicago
denticulata.]
=198. The root organism assimilates free nitrogen for its
host.=—This organism assimilates the free nitrogen from the air
in the soil, to make the proteid substance which is found stored in
the bacteroids in large quantities. Some of the bacteroids, rich in
proteids, are dissolved, and the proteid substance is made use of by
the clover or pea, as the case may be. This is why such plants can
thrive in soil with a poor nitrogen content. Later in the season some
of the root-tubercles die and decay. In this way some of the proteid
substance is set free in the soil. The soil thus becomes richer in
nitrogenous plant food.
The forms of the bacteroids vary. In some of the clovers they are oval,
in vetch they are rod-like or forked, and other forms occur in some of
the other genera.
=199.= NOTE.—So far as we know the legume tubercle
organism does not assimilate free nitrogen of the air unless it is
within the root of the legume. But there are microörganisms in the
soil which are capable of assimilating free nitrogen independently.
Example, a bacterium, _Clostridium pasteurianum_. Certain bacteria
and algæ live in _contact symbiosis_ in the soil, the bacteria fixing
free nitrogen, while in return for the combined nitrogen, the algæ
furnish the bacteria with carbohydrates. It seems that these bacteria
cannot fix the free nitrogen of the air unless they are supplied with
carbohydrates, and it is known that _Clostridium pasteurianum_ cannot
assimilate free nitrogen unless sugar is present.
6. Lichens.
=200. Nutrition of lichens.=—Lichens are very curious plants
which grow on rocks, on the trunks and branches of trees, and on the
soil. They form leaf-like expansions more or less green in color, or
brownish, or gray, or they occur in the form of threads, or small
tree-like formations. Sometimes the plant fits so closely to the rock
on which it grows that it seems merely to paint the rock a slightly
different color, and in the case of many which occur on trees there
appears to be to the eye only a very slight discoloration of the bark
of the trunk, with here and there the darker colored points where fruit
bodies are formed. The most curious thing about them is, however,
that while they form plant bodies of various form, these bodies are
of a “dual nature” as regards the organisms composing them. The plant
bodies, in other words, are formed of two different organisms which,
woven together, exist apparently as one. A fungus on the one hand grows
around and encloses in the meshes of its mycelium the cells or threads
of an alga, as the case may be.
[Illustration: Fig. 84. Frond of lichen (peltigera), showing rhizoids.]
If we take one of the leaf-like forms known as peltigera, which grows
on damp soil or on the surfaces of badly decayed logs, we see that the
plant body is flattened, thin, crumpled, and irregularly lobed. The
color is dull greenish on the upper side, while the under side is white
or light gray, and mottled with brown, especially the older portions.
Here and there on the under surface are quite long slender blackish
strands. These are composed entirely of fungus threads and serve as
organs of attachment or holdfasts, and for the purpose of supplying
the plant body with mineral substances which are in solution in the
water of the soil. If we make a thin section of the leaf-like portion
of a lichen as shown in fig. 85, we shall see that it is composed of a
mesh of colorless threads which in certain definite portions contain
entangled green cells. The colorless threads are those of the fungus,
while the green cells are those of the alga. These green cells of the
alga perform the function of chlorophyll bodies for the dual organism,
while the threads of the fungus provide the mineral constituents of
plant food. The alga, while it is not killed in the embrace of the
fungus, does not reach the perfect state of development which it
attains when not in connection with the fungus. On the other hand the
fungus profits more than the alga by this association. It forms fruit
bodies, and perfects spores in the special fruit bodies, which are so
very distinct in the case of so many of the species of the lichens.
These plants have lived for so long a time in this close association
that the fungi are rarely found separate from the algæ in nature, but
in a number of cases they have been induced to grow in artificial
cultures separate from the alga. This fact, and also the fact that the
algæ are often found to occur separate from the fungus in nature, is
regarded by many as an indication that the plant body of the lichens is
composed of two distinct organisms, and that the fungus is parasitic on
the alga.
[Illustration: Fig. 85.
Lichen (peltigera), section of thallus; dark zone of rounded bodies
made up largely of the algal cells. Fungus cells above, and threads
beneath and among the algal cells.]
=201.= Others regard the lichens as autonomous plants, that is,
the two organisms have by this long-continued community of existence
become unified into an individualized organism, which possesses a habit
and mode of life distinct from that of either of the organisms forming
the component parts. This community of existence between two different
organisms is called by some _mutualism_, or _symbiosis_. While the alga
enclosed within the meshes of the fungus is not so free to develop,
and probably does not attain the full development which it would alone
under favorable conditions, still it is very likely that it is often
preserved from destruction during very dry periods, within the tough
thallus, on the surface of bare rocks.
[Illustration: Fig. 86. Section of fruit body or apothecium of lichen
(parmelia), showing asci and spores of the fungus.]
FOOTNOTES:
[12] Calcium is not essential for the growth of the fungi.
[13] For example, silicon is used by some plants in strengthening
supporting tissues. Buckwheat thrives better when supplied with a
chloride.
[14] Evidence points to the belief that certain cells of the host form
substances which attract, chemitropically, the fungus threads, and that
in these cells the fungus threads are more abundant than in others.
Furthermore in the vicinity of the nucleus of the host seems to be the
place where these activities are more marked.
CHAPTER X.
HOW PLANTS OBTAIN THEIR FOOD, II.
Seedlings.
=202.= It is evident from some of the studies which we have made
in connection with germination of seeds and nutrition of the plant
that there is a period in the life of the seed plants in which they
are able to grow if supplied with moisture, but may entirely lack any
supply of food substance from the outside, though we understand that
growth finally comes to a standstill unless they are supplied with food
from the outside. In connection with the study of the nutrition of the
plant, therefore, it will be well to study some of the representative
seeds and seedlings to learn more accurately the method of germination
and nutrition in seedlings during the germinating period.
=203. To prepare seeds for germination.=—Soak a handful of
seeds (or more if the class is large) in water for 12 to 24 hours.
Take shallow crockery plates, or ordinary plates, or a germinator
with a fluted bottom. Place in the bottom some sheets of paper, and
if sphagnum moss is at hand scatter some over the paper. If the moss
is not at hand, throw the upper layer of paper into numerous folds.
Thoroughly wet the paper and moss, but do not have an excess of water.
Scatter the seeds among the moss or the folds of the paper. Cover
with some more wet paper and keep in a room where the temperature is
about 20°C. to 25°C. The germinator should be looked after to see that
the paper does not become dry. It may be necessary to cover it with
another vessel to prevent the too rapid evaporation of the water. The
germinator should be started about a week before the seedlings are
wanted for study. Some of the soaked seeds should be planted in soil in
pots and kept at the same temperature, for comparison with those grown
in the germinator.
[Illustration: Fig. 87. Section of corn seed; at upper right of each is
the plantlet, next the cotyledon, at left the endosperm.]
=204. Structure of the grain of corn.=—Take grains of corn that
have been soaked in water for 24 hours and note the form and difference
in the two sides (in all of these studies the form and structure of
the seed, as well as the stages in germination, should be illustrated
by the student). Make a longisection of a grain of corn through the
middle line, if necessary making several in order to obtain one which
shows the structures well near the smaller end of the grain. Note
the following structures: 1st, the hard outer “wall” (formed of the
consolidated wall of the ovary with the integuments of the ovules—see
Chapters 35 and 36); 2d, the greater mass of starch and other plant
food (the endosperm) in the centre; 3d, a somewhat crescent-shaped body
(the _scutellum_) lying next the endosperm and near the smaller end of
the grain; 4th, the remaining portion of the young embryo lying between
the scutellum and the seed coat in the depression. When good sections
are made one can make out the radicle at the smaller end of the seed,
and a few successive leaves (the plumule) which lie at the opposite
end of the embryo shown by sharply curved parallel lines. Observe the
attachment of the scutellum to the caulicle at the point of junction
of the plumule and the radicle. The scutellum is a part of the embryo
and represents a cotyledon. The endosperm is also called _albumen_, and
such a seed is _albuminous_.
Dissect out an embryo from another seed, and compare with that seen in
the section.
=205.= In the germination of the grain of corn the endosperm
supplies the food for the growth of the embryo until the roots are
well established in the soil and the leaves have become expanded and
green, in which stage the plant has become able to obtain its food from
the soil and air and live independently. The starch in the endosperm
cannot of course be used for food by the embryo in the form of starch.
It is first converted into a soluble form and then absorbed through
the surface of the scutellum or cotyledon and carried to all parts of
the embryo. An enzyme developed by the embryo acts upon the starch,
converting it into a form of sugar which is in solution and can thus
be absorbed. This enzyme is one of the so-called diastatic “ferments”
which are formed during the germination of all seeds which contain food
stored in the form of starch. In some seedlings, this diastase formed
is developed in much greater abundance than in others, for example,
in barley. Examine grains of corn still attached to seedlings several
weeks old and note that a large part of their content has been used up.
The action of diastase on starch is described in Chapter 8.
=206. Structure of the pumpkin seed.=—The pumpkin seed has a
tough papery outer covering for the protection of the embryo plant
within. This covering is made up of the seed coats. When the seed is
opened by slitting off these coats there is seen within the “meat”
of the pumpkin seed. This is nothing more than the embryo plant. The
larger part of this embryo consists of two flattened bodies which
are more prominent than any other part of the plantlet at this time.
These two flattened bodies are the two first leaves, usually called
_cotyledons_. If we spread these cotyledons apart we see that they are
connected at one end. Lying between them at this point of attachment
is a small bud. This is the _plumule_. The plumule consists of the
very young leaves at the end of the stem which will grow as the seed
germinates. At the other end where the cotyledons are joined is a small
projection, the young root, often termed the _radicle_.
=207. How the embryo gets out of a pumpkin seed.=—To see how the
embryo gets out of the pumpkin seed we should examine seeds germinated
in the folds of damp paper or on damp sphagnum, as well as some which
have been germinated in earth. Seeds should be selected which represent
several different stages of germination.
[Illustration: Fig. 88. Germinating seed of pumpkin, showing how the
heel or “peg” catches on the seed coat to cast it off.]
[Illustration: Fig. 89. Escape of the pumpkin seedling from the seed
coats.]
=208. The peg helps to pull the seed coats apart.=—The root
pushes its way out from between the stout seed coats at the smaller
end, and then turns downward unless prevented from so doing by a hard
surface. After the root is 2-4_cm_ long, and the two halves of the seed
coats have begun to be pried apart, if we look in this rift at the
junction of the root and stem, we shall see that one end of the seed
coat is caught against a heel, or “peg,” which has grown out from the
stem for this purpose. Now if we examine one which is a little more
advanced, we shall see this heel more distinctly, and also that the
stem is arching out away from the seed coats. As the stem arches up
its back in this way it pries with the cotyledons against the upper
seed coat, but the lower seed coat is caught against this heel, and
the two are pulled gradually apart. In this way the embryo plant pulls
itself out from between the seed coats. In the case of seeds which are
planted deeply in the soil we do not see this contrivance unless we dig
down into the earth. The stem of the seedling arches through the soil,
pulling the cotyledons up at one end. Then it straightens up, the green
cotyledons part, and open out their inner faces to the sunlight, as
shown in fig. 90. If we dig into the soil we shall see that this same
heel is formed on the stem, and that the seed coats are cast off into
the soil.
[Illustration: Fig. 90. Pumpkin seedling rising from the ground.]
=209. Parts of the pumpkin seedling.=—During the germination of
the seed all parts of the embryo have enlarged. This increase in size
of a plant is one of the peculiarities of growth. The cotyledons have
elongated and expanded somewhat, though not to such a great extent
as the root and the stem. The cotyledons also have become green on
exposure to the light. Very soon after the main root has emerged from
the seed coats, other lateral roots begin to form, so that the root
soon becomes very much branched. The main root with its branches makes
up the root system of the seedling. Between the expanded cotyledons is
seen the plumule. This has enlarged somewhat, but not nearly so much as
the root, or the part of the stem which extends below the cotyledons.
This part of the stem, i.e., that part below the cotyledons and
extending to the beginning of the root, is called in all seedlings the
_hypocotyl_, which means “below the cotyledon.”
=210. The common garden bean.=—The common garden bean, or the
lima bean, may be used for study. The garden bean is not so flattened
or broadened as the lima bean. It is rounded compressed, elongate
slightly curved, slightly concave on one side and convex on the other,
and the ends are rounded. At the middle of the concave side note the
distinct scar (the hilum) formed where the bean seed separates from
its attachment to the wall of the pod. Upon one side of this scar is a
slight prominence which is continued for a short distance toward the
end of the bean in the form of a slight ridge. This is the _raphe_, and
represents that part of the stalk of the ovule which is joined to the
side of the ovule when the latter is curved around against it (see
Chapter 36), and at the outer end of the raphe is the _chalaza_,
the point where the stalk is joined to the end of the ovule, best
understood in a straight ovule. Upon the opposite side of the scar
and close to it can be seen a minute depression, the _micropyle_.
Underneath the seed coat and lying between this point and the end of
the seed is the _embryo_, which gives greater prominence to the bean at
this point, but it is especially more prominent after the bean has been
soaked in water. Soak the beans in water and as they are swelling note
how the seed coats swell faster than the inner portion of the seed,
which causes them to wrinkle in a curious way, but finally the inner
portion swells and fills the seed coat out smooth again. Sketch a bean
showing all the external features both in side view and in front. Split
one lengthwise and sketch the half to which the embryo clings, noting
the young root, stem, and the small leaves which were lying between
the cotyledons. There is no endosperm here now, since it was all used
up in the growth of the embryo, and a large part of its substance was
stored up in the cotyledons. As the seed germinates the young plant
gets its first food from that stored in the cotyledons. The hypocotyl
elongates, becomes strongly arched, and at last straightens up, lifting
the cotyledons from the soil. As the cotyledons become exposed to the
light they assume a green color. Some of the stored food in them goes
to nourish the embryo during germination, and they therefore become
smaller, shrivel somewhat, and at last fall off.
[Illustration: Fig. 91. Garden bean.
_m_, micropyle; _h_, hilum or scar; _r_, raphe; _c_, point where
chalaza lies.]
[Illustration: Fig. 92. Bean seed split open to show plantlet.]
=211. The castor-oil bean.=—This is not a true bean, since it
belongs to a very different family of plants (Euphorbiaceæ). In the
germination of this seed a very interesting comparison can be made with
that of the garden bean. As the “bean” swells the very hard outer coat
generally breaks open at the free end and slips off at the stem end.
The next coat within, which is also hard and shining black, splits
open at the opposite end, that is at the stem end. It usually splits
open in the form of three ribs. Next within the inner coat is a very
thin, whitish film (the remains of the nucellus, and corresponding to
the perisperm) which shrivels up and loosens from the white mass, the
endosperm, within. In the castor-oil bean, then, the endosperm is not
all absorbed by the embryo during the formation of the seed. As the
plant becomes older we should note that the fleshy endosperm becomes
thinner and thinner, and at last there is nothing but a thin, whitish
film covering the green faces of the cotyledons. The endosperm has been
gradually absorbed by the germinating plant through its cotyledons and
used for food.
[Illustration: Fig. 93.
How the garden bean comes out of the ground. First the looped
hypocotyl, then the cotyledons pulled out, next casting off the seed
coat, last the plant erect, bearing thick cotyledons, the expanding
leaves, and the plumule between them.]
Arisæma triphyllum.[15]
=212. Germination of seeds of jack-in-the-pulpit.=—The ovaries
of jack-in-the-pulpit form large, bright red berries with a soft pulp
enclosing one to several large seeds. The seeds are oval in form. Their
germination is interesting, and illustrates one type of germination of
seeds common among monocotyledonous plants. If the seeds are covered
with sand, and kept in a moist place, they will germinate readily.
[Illustration: Fig. 94. Germination of castor-oil bean.]
=213. How the embryo backs out of the seed.=—The embryo lies
within the mass of the endosperm; the root end, near the smaller end of
the seed. The club-shaped cotyledon lies near the middle of the seed,
surrounded firmly on all sides by the endosperm. The stalk, or petiole,
of the cotyledon, like the lower part of the petiole of the leaves, is
a hollow cylinder, and contains the younger leaves, and the growing end
of the stem or bud. When germination begins, the stalk, or petiole, of
the cotyledon elongates. This pushes the root end of the embryo out
at the small end of the seed. The free end of the embryo now enlarges
somewhat, as seen in the figures, and becomes the bulb, or corm, of the
young plant. At first no roots are visible, but in a short time one,
two, or more roots appear on the enlarged end.
=214. Section of an embryo.=—If we make a longisection of the
embryo and seed at this time we can see how the club-shaped cotyledon
is closely surrounded by the endosperm. Through the cotyledon, then,
the nourishment from the endosperm is readily passed over to the
growing embryo. In the hollow part of the petiole near the bulb can be
seen the first leaf.
[Illustration: Fig. 95. Seedlings of castor-oil bean casting the seed
coats, and showing papery remnant of the endosperm.]
[Illustration: Fig. 96. Seedlings of jack-in-the-pulpit; embryo backing
out of the seed.]
[Illustration: Fig. 97.
Section of germinating embryos of jack-in-the-pulpit, showing young
leaves inside the petiole of the cotyledon. At the left cotyledon shown
surrounded by the endosperm in the seed; at right endosperm removed to
show the club-shaped cotyledon.]
=215. How the first leaf appears.=—As the embryo backs out of
the seed, it turns downward into the soil, unless the seed is so lying
that it pushes straight downward. On the upper side of the arch thus
formed, in the petiole of the cotyledon, a slit appears, and through
this opening the first leaf arches its way out. The loop of the petiole
comes out first, and the leaf later, as shown in fig. 98. The petiole
now gradually straightens up, and as it elongates the leaf expands.
[Illustration: Fig. 98. Seedlings of jack-in-the-pulpit, first leaf
arching out of the petiole of the cotyledon.]
[Illustration: Fig. 99. Embryos of jack-in-the-pulpit still attached to
the endosperm in seed coats, and showing the simple first leaf.]
[Illustration: Fig. 100. Seedling of jack-in-the-pulpit; section of the
endosperm and cotyledon.]
=216. The first leaf of the jack-in-the-pulpit is a simple
one.=—The first leaf of the embryo jack-in-the-pulpit is very
different in form from the leaves which we are accustomed to see on
mature plants. If we did not know that it came from the seed of this
plant we would not recognize it. It is simple, that is it consists
of one lamina or blade, and not of three leaflets as in the compound
leaf of the mature plant. The simple leaf is ovate and with a broad
heart-shaped base. The jack-in-the-pulpit, then, as trillium, and some
other monocotyledonous plants which have compound leaves on the mature
plants, have simple leaves during embryonic development. The ancestral
monocotyledons are supposed to have had simple leaves. Thus there is in
the embryonic development of the jack-in-the-pulpit, and others with
compound leaves, a sort of recapitulation of the evolutionary history
of the leaf in these forms.
=216=_a_. =Germination of the pea.=—Compare with the bean.
Note especially that the cotyledons are not lifted above the soil as in
the beans. Compare germination of acorns.
Digestion.
=216=_b_. =To test for stored food substance in the seedlings
studied.=—The pumpkin, squash, and castor-oil bean are examples
of what are called oily seeds, since considerable oil is stored up
in the protoplasm in the cotyledons. To test for this, remove a
small portion of the substance from the cotyledon of the squash and
crush it on a glass slip in a drop or two of osmic acid.[16] Put on a
cover glass and examine with a microscope. The black amorphous matter
shows the presence of oil in the protoplasm. The small bodies which are
stained yellow are _aleurone_ grains, a form of protein or albuminous
substance. Both the oil and the protein substance are used by the
seedling during germination. The oil is converted into an available
food form by the action of an enzyme called _lipase_, which splits up
the fatty oil into glucose and other substances. Lipase has been found
in the endosperm of the castor-oil, cocoanut, and in the cotyledons
of the pumpkin, as well as in other seeds containing oil as a stored
product. The aleurone is made available by an enzyme of the nature of
trypsin. Test the endosperm of the castor-oil bean in the same way.
Make another test of both the squash and castor-oil seeds with iodine
to show that starch is not present.
Test the cotyledon of the bean with iodine for the presence of
starch. If the endosperm of corn seed has not been tested do so now
with iodine. The endosperm consists largely of starch. The starch is
converted to glucose by a diastatic “ferment” formed by the seedling as
it germinates. Make a thin cross-section of a grain of wheat, including
the seed coat and a portion of the interior, treat with iodine and
mount for microscopic examination. Note the abundance of starch in the
internal portion of endosperm. Note a layer of cells on the outside of
the starch portions filled with small bodies which stain yellow. These
are aleurone grains. The cellulose in the cell walls of the endosperm
is dissolved by another enzyme called _cytase_, and some plants store
up cellulose for food. For example, in the endosperm of the _date_ the
cell walls are very much thickened and pitted. The cell walls consist
of reserve cellulose and the seedling makes use of it for food during
growth.
=216=_c_. =Albuminous and exalbuminous seeds.=—In seeds
where the food is stored outside of the embryo they are called
_albuminous_; examples, corn, wheat and other cereals, Indian turnip,
etc. In those seeds where the food is stored up in the embryo they are
called _exalbuminous_; examples, bean, pea, pumpkin, squash, etc.
=217. Digestion= has a well-defined meaning in animal physiology
and relates to the conversion of solid food, usually within the
stomach, into a soluble form by the action of certain gastric juices,
so that the liquid food may be absorbed into the circulatory system.
The term is not often applied in plant physiology, since the method
of obtaining food is in general fundamentally different in plants and
animals. It is usually applied to the process of the conversion of
starch into some form of sugar in solution, as glucose, etc. This we
have found takes place in the leaf, especially at night, through the
action of a diastatic ferment developed more abundantly in darkness. As
a result, the starch formed during the day in the leaves is digested
at night and converted into sugar, in which form it is transferred to
the growing parts to be employed in the making of new tissues, or it is
stored for future use; in other cases it unites with certain inorganic
substances, absorbed by the roots and raised to the leaf, to form
proteids and other organic substances. In tubers, seeds, parts of stems
or leaves where starch is stored, it must first be “digested” by the
action of some enzyme before it can be used as food by the sprouting
tubers or germinating seeds.
For example, starch is converted to a glucose by the action of a
diastase. Cellulose is converted to a glucose by cytase. Albuminoids
are converted into available food by a tryptic ferment. Fatty oils are
converted into glucose and other products by lipase.
Inulin, a carbohydrate closely related to starch, is stored up for
food in solution in many composite plants, as in the artichoke, the
root tuber of dahlia, etc. When used for food by the growing plant
it is converted into glucose by an enzyme, inulase. Make a section
of a portion of a dahlia tuber or artichoke and treat with alcohol.
The inulin is precipitated into sphæro crystals. (See also paragraphs
156-161 and 216_b_.)
=218.= Then there are certain fungi which feed on starch or other
organic substances whether in the host or not, which excrete certain
enzymes to dissolve the starch, etc., to bring it into a soluble
form before they can absorb it as food. Such a process is a sort of
_extracellular digestion_, i.e., the organism excretes the enzyme and
digests the solid outside, since it cannot take the food within its
cells in the solid form. To a certain degree the higher plants perform
also extracellular digestion in the action of root hair excretion on
insoluble substances, and in the case of the humus saprophytes. But for
them soluble food is largely prepared by the action of acids, etc., in
the soil or water, or by the work of fungi and bacteria as described in
Chapter 9.
=219. Assimilation.=—In plant physiology the term assimilation
has been chiefly used for the process of carbon dioxide assimilation
(= photosynthesis). Some objections have been raised against the use
of assimilation here as one of the life processes of the plant, since
its inception stages are due to the combined action of light, an
external factor, and chlorophyll in the plant along with the living
chloroplastid. So long, however, as it is not known that this process
can take place without the aid of the living plant, it does not seem
proper to deny that it is altogether not a process of assimilation. It
is not necessary to restrict the term assimilation to the formation
of new living matter in the plant cell; it can be applied also to
the synthetic processes in the formation of carbohydrates, proteids,
etc., and called synthetic assimilation. The sun supplies the energy,
which is absorbed by the chlorophyll, for splitting up the carbonic
acid, and the living chloroplast then assimilates by a synthetic
process the carbon, hydrogen, and oxygen. This process then can be
called _photosynthetic assimilation_. The nitrite and nitrate bacteria
derive energy in the process of nitrification, which enables them
to assimilate CO₂ from the air, and this is called _chemosynthetic
assimilation_. The inorganic material in the form of mineral salts,
nitrates, etc., absorbed by the root, and carried up to the leaves,
here meets with the carbohydrates manufactured in the leaf. Under
the influence of the protoplasm synthesis takes place, and proteids
and other organic compounds are built up by the union of the salts,
nitrates, etc., with the carbohydrates. This is also a process of
synthetic assimilation. These are afterward stored as food, or
assimilated by the protoplasm in the making of new living matter, or
perhaps without the first process of synthetic assimilation some of the
inorganic salts, nitrates, and carbohydrates meeting in the protoplasm
are assimilated into new living matter directly.
FOOTNOTES:
[15] In lieu of Arisæma make a practical study of the pea. See
paragraph 216_a_.
[16] Dissolve a half gram of osmic acid in 50 _cc._ of water and keep
tightly corked when not using.
CHAPTER XI.
RESPIRATION.
=220.= One of the life processes in plants which is extremely
interesting, and which is exactly the same as one of the life processes
of animals, is easily demonstrated in several ways.
=221. Simple experiment to demonstrate the evolution of CO₂ during
germination.=—Where there are a number of students and a number of
large cylinders are not at hand, take bottles of a pint capacity and
place in the bottom some peas soaked for 12 to 24 hours. Cover with
a glass plate which has been smeared with vaseline to make a tight
joint with the mouth of the bottle. Set aside in a warm place for 24
hours. Then slide the glass plate a little to one side and quickly
pour in a little baryta water so that it will run down on the inside
of the bottle. Cover the bottle again. Note the precipitate of barium
carbonate which demonstrates the presence of CO₂ in the bottle. Lower a
lighted taper. It is extinguished because of the great quantity of CO₂.
If flower buds are accessible, place a small handful in each of several
jars and treat the same as in the case of the peas. Young growing
mushrooms are excellent also for this experiment, and serve to show
that respiration takes place in the fungi.
[Illustration: Fig. 101. Test for presence of carbon dioxide in vessel
with germinating peas. (Sachs.)]
[Illustration: Fig. 102. Apparatus to show respiration of germinating
wheat.]
=222.= If we now take some of the baryta water and blow our
“breath” upon it the same film will be formed. The carbon dioxide which
we exhale is absorbed by the baryta water, and forms barium carbonate,
just as in the case of the peas. In the case of animals the process by
which oxygen is taken into the body and carbon dioxide is given off is
_respiration_. The process in plants which we are now studying is the
same, and also is respiration. The oxygen in the vessel was partly used
up in the process, and carbon dioxide was given off. (It will be seen
that this process is exactly the opposite of that which takes place in
carbon dioxide assimilation.)
=223. To show that oxygen from the air is used up while plants
respire.=—Soak some wheat for 24 hours in water. Remove it from
the water and place it in the folds of damp cloth or paper in a moist
vessel. Let it remain until it begins to germinate. Fill the bulb of
a thistle tube with the germinating wheat. By the aid of a stand and
clamp, support the tube upright, as shown in fig. 102. Let the small
end of the tube rest in a strong solution of caustic potash (one stick
caustic potash in two-thirds tumbler of water) to which red ink has
been added to give a deep red color. Place a small glass plate over the
rim of the bulb and seal it air-tight with an abundance of vaseline.
Two tubes can be set up in one vessel, or a second one can be set up in
strong baryta water colored in the same way.
=224. The result.=—It will be seen that the solution of caustic
potash rises slowly in the tube; the baryta water will also, if that is
used. The solution is colored so that it can be plainly seen at some
distance from the table as it rises in the tube. In the experiment
from which the figure was made for the accompanying illustration, the
solution had risen in 6 hours to the height shown in fig. 102. In 24
hours it had risen to the height shown in fig. 103.
=225. Why the solution of caustic potash rises in the
tube.=—Since no air can get into the thistle tube from above or
below, it must be that some part of the air which is inside of the
tube is used up while the wheat is germinating. From our study of
germinating peas, we know that a suffocating gas, carbon dioxide, is
given off while respiration takes place. The caustic potash solution,
or the baryta water, whichever is used, absorbs the carbon dioxide. The
carbon dioxide is heavier than air, and so it settles down in the tube
where it can be absorbed.
[Illustration: Fig. 103. Apparatus to show respiration of germinating
wheat.]
[Illustration: Fig. 104. Pea seedlings; the one at the left had no
oxygen and little growth took place, the one at the right in oxygen and
growth was evident.]
=226. Where does the carbon dioxide come from?=—We know it
comes from the growing seedlings. The symbol for carbon dioxide is
CO₂. The carbon comes from the plant, because there is not enough in
the air. Nitrogen could not join with the carbon to make CO₂. Some
oxygen from the air or from the protoplasm of the growing seedlings
(more probably the latter) joins with some of the carbon of the plant.
These break away from their association with the living substance and
unite, making CO₂. The oxygen absorbed by the plant from the air unites
with the living substance, or perhaps first with food substances, and
from these the plant is replenished with carbon and oxygen. After the
demonstration has been made, remove the glass plate which seals the
thistle tube above, and pour in a small quantity of baryta water. The
white precipitate formed affords another illustration that carbon
dioxide is released.
[Illustration: Fig. 105.
Experiment to show that growth takes place more rapidly in presence of
oxygen than in absence of oxygen. The two tubes in the vessel represent
the condition at the beginning of the experiment. At the close of the
experiment the roots in the tube at the left were longer than those
in the tube filled at the start with mercury. The tube outside of
the vessel represents the condition of things where the peas grew in
absence of oxygen; the carbon dioxide given off has displaced a portion
of the mercury. This also shows _anaerobic_ respiration.]
=227. Respiration is necessary for growth.=—After performing
experiment in paragraph 221, if the vessel has not been open too
long so that oxygen has entered, we may use the vessel for another
experiment, or set up a new one to be used in the course of 12 to 24
hours, after some oxygen has been consumed. Place some folded damp
filter paper on the germinating peas in the jar. Upon this place
one-half dozen peas which have just been germinated, and in which the
roots are about 20-25 _mm_ long. The vessel should be covered tightly
again and set aside in a warm room. A second jar with water in the
bottom instead of the germinating peas should be set up as a check.
Damp folded filter paper should be supported above the water, and on
this should be placed one-half dozen peas with roots of the same length
as those in the jar containing carbon dioxide.
=228.= In 24 hours examine and note how much growth has taken
place. It will be seen that the roots have elongated but very little
or none in the first jar, while in the second one we see that the
roots have elongated considerably, if the experiment has been carried
on carefully. Therefore in an atmosphere devoid of oxygen very little
growth will take place, which shows that normal respiration with access
of oxygen (aerobic respiration) is necessary for growth.
=229. Another way of performing the experiment.=—If we wish we
may use the following experiment instead of the simple one indicated
above. Soak a handful of peas in water for 12-24 hours, and germinate
so that twelve with the radicles 20-25 _mm_ long may be selected. Fill
a test tube with mercury and carefully invert it in a vessel of mercury
so that there will be no air in the upper end. Now nearly fill another
tube and invert in the same way. In the latter there will be some air.
Remove the outer coats from the peas so that no air will be introduced
in the tube filled with the mercury, and insert them one at a time
under the edge of the tube beneath the mercury, six in each tube,
having first measured the length of the radicles. Place in a warm
room. In 24 hours measure the roots. Those in the air will have grown
considerably, while those in the other tube will have grown but little
or none.
=230. Anaerobic respiration.=—The last experiment is also an
excellent one to show _anaerobic_ respiration. In the tube filled
with mercury so that when inverted there will be no air, it will be
seen after 24 hours that a gas has accumulated in the tube which has
crowded out some of the mercury. With a wash bottle which has an exit
tube properly curved, some water may be introduced in the tube. Then
insert underneath a small stick of caustic potash. This will form a
solution of potash, and the gas will be partly or completely absorbed.
This shows that the gas was carbon dioxide. This evolution of carbon
dioxide by living plants when there is no access of oxygen is anaerobic
respiration (sometimes called intramolecular respiration). It occurs
markedly in oily seeds and especially in the yeast plant.
[Illustration: Fig. 106. Test for liberation of carbon dioxide from
leafy plant during respiration. Baryta water in smaller vessel.
(Sachs.)]
=231. Energy set free during respiration.=—From what we have
learned of the exchange of gases during respiration we infer that the
plant loses carbon during this process. If the process of respiration
is of any benefit to the plant, there must be some gain in some
direction to compensate the plant for the loss of carbon which takes
place.
It can be shown by an experiment that during respiration there is a
slight elevation of the temperature in the plant tissues. The plant
then gains some heat during respiration. Energy is also manifested by
growth.
=232. Respiration in a leafy plant.=—We may take a potted plant
which has a well-developed leaf surface and place it under a tightly
fitting bell jar. Under the bell jar there also should be placed a
small vessel containing baryta water. A similar apparatus should be set
up, but with no plant, to serve as a check. The experiment must be set
up in a room which is not frequented by persons, or the carbon dioxide
in the room from respiration will vitiate the experiment. The bell jar
containing the plant should be covered with a black cloth to prevent
carbon assimilation. In the course of 10 or 12 hours, if everything has
worked properly, the baryta water under the jar with the plant will
show the film of barium carbonate, while the other one will show none.
Respiration, therefore, takes place in a leafy plant as well as in
germinating seeds.
=233. Respiration in fungi.=—If several large actively growing
mushrooms are accessible, place them in a tall glass jar as described
for determining respiration in germinating peas. In the course of 12
hours test with the lighted taper and the baryta water. Respiration
takes place in fungi as well as in green plants.
=234. Respiration in plants in general.=—Respiration is general
in all plants, though not universal. There are some exceptions in the
lower plants, notably in certain of the bacteria, which can only grow
and thrive in the absence of oxygen.
[Illustration: Fig. 107. Fermentation tube with culture of yeast.]
[Illustration: Fig. 108. Fermentation tube filled with CO₂ from action
of yeast in a sugar solution.]
=235. Respiration a breaking-down process.=—We have seen that
in respiration the plant absorbs oxygen and gives off carbon dioxide.
We should endeavor to note some of the effects of respiration on the
plant. Let us take, say, two dozen dry peas, weigh them, soak for 12-24
hours in water, and, in the folds of a cloth kept moist by covering
with wet paper or sphagnum, germinate them. When well germinated and
before the green color appears dry well in the sun, or with artificial
heat, being careful not to burn or scorch them. The aim should be to
get them about as dry as the seeds were before germination. Now weigh.
The germinated seeds weigh less than the dry peas. There has then been
a loss of plant substance during respiration.
[Illustration: Fig. 108_a_.
Yeast. Saccharomyces ceriviseæ. _a_, small colony; _b_, single cell
budding; _c_, single cell forming an ascus with four spores; _d_,
spores free from the ascus. (After Rees.)]
=236. Fermentation of yeast.=—Take two fermentation tubes. Fill
the closed tubular parts of each with a weak solution of grape sugar,
or with potato decoction, leaving the open bulb nearly empty. Into the
liquid of one of the tubes place a piece of compressed yeast as large
as a pea. If the tubes are kept in a warm place for 24 hours bubbles
of gas may be noticed rising in the one in which the yeast was placed,
while in the second tube no such bubbles appear, especially if the
filled tubes are first sterilized. The tubes may be kept until the
first is entirely filled with the gas. Now dissolve in the liquid a
small piece of caustic potash. Soon the gas will begin to be absorbed,
and the liquid will rise until it again fills the tube. The gas was
carbon dioxide, which was chiefly produced during the anaerobic
respiration of the rapidly growing yeast cells. In bread making this
gas is produced in considerable quantities, and rising through the
dough fills it with numerous cavities containing gas, so that the bread
“rises.” When it is baked the heat causes the gas in the cavities to
expand greatly. This causes the bread to “rise” more, and baked in this
condition it is “light.” There are two special processes accompanying
the fermentation by yeast: 1st, the evolution of carbon dioxide as
shown above; and, 2d, the formation of alcohol. The best illustration
of this second process is the brewing of beer, where a form of the same
organism which is employed in “bread rising” is used to “brew beer.”
=237. The yeast plant.=—Before the caustic potash is placed in
the tube some of the fermented liquid should be taken for study of
the yeast plant, unless separate cultures are made for this purpose.
Place a drop of the fermented liquid on a glass slip, place on this
a cover glass, and examine with the microscope. Note the minute oval
cells with granular protoplasm. These are the yeast plant. Note in
some a small “bud” at one side of the end. These buds increase in size
and separate from the parent plant. The yeast plant is one-celled,
and multiplies by “budding” or “sprouting.” It is a fungus, and some
species of yeast like the present one do not form any mycelium. Under
certain conditions, which are not very favorable for growth (example,
when the yeast is grown in a weak nutrient substance on a thin layer of
a plaster Paris slab), several spores are formed in many of the yeast
cells. After a period of rest these spores will sprout and produce the
yeast plant again. Because of this peculiar spore formation some place
the yeast among the sac fungi. (See classification of the fungi.)
=238. Organized ferments and unorganized ferments.=—An organism
like the yeast plant which produces a fermentation of a liquid with
evolution of gas and alcohol is sometimes called a _ferment_, or
_ferment organism_, or an _organized_ ferment. On the other hand the
diastatic ferments or enzymes like diastase, taka diastase, animal
diastase (ptyalin in the saliva), cytase, etc., are _unorganized_
ferments. In the case of these it is better to say _enzyme_ and leave
the word ferment for the ferment organisms.
=239. Importance of green plants in maintaining purity of air.=—By
respiration, especially of animals, the air tends to become “foul”
by the increase of CO₂. Green plants, i.e., plants with chlorophyll,
purify the air during photosynthesis by absorbing CO₂ and giving off
oxygen. Animals absorb in respiration large quantities of oxygen and
exhale large quantities of CO₂. Plants absorb a comparatively small
amount of oxygen in respiration and give off a comparatively small
amount of CO₂. But they absorb during photosynthesis large quantities
of CO₂ and give off large quantities of oxygen. In this way a balance is
maintained between the two processes, so that the percentage of CO₂ in
the air remains approximately the same, viz., about four-tenths of one
per cent, while there are approximately 21 parts oxygen and 79 parts
nitrogen.
=239a. Comparison of respiration and photosynthesis.=
{ Carbon dioxide is taken in by the plant and
{ oxygen is liberated.
{ Starch is formed as a result of the metabolism,
{ or chemical change.
{ The process takes place only in green plants,
Starch formation { and in the green parts of plants, that is,
or Photosynthesis. { in the presence of the chlorophyll.
{ (Exception in purple bacterium.)
{ The process only takes place under the
{ influence of sunlight.
{ It is a building-up process, because new plant
{ substance is formed.
{ Oxygen is taken in by the plant and carbon
{ dioxide is liberated.
{ Carbon dioxide is formed as a result of the
{ metabolism, or chemical change.
{ The process takes place in all plants whether
Respiration. { they possess chlorophyll or not.
{ (Exceptions in anaerobic bacteria).
{ The process takes place in the dark as well as
{ in the sunlight.
{ It is a breaking-down process, because
{ disintegration of plant substance occurs.
CHAPTER XII.
GROWTH.
By growth is usually meant an increase in the bulk of the plant
accompanied generally by an increase in plant substance. Among the
lower plants growth is easily studied in some of the fungi.
=240. Growth in mucor.=—Some of the gonidia (often called spores)
may be sown in nutrient gelatine or agar, or even in prune juice. If
the culture has been placed in a warm room, in the course of 24 hours,
or even less, the preparation will be ready for study.
=241. Form of the gonidia.=—It will be instructive if we first
examine some of the gonidia which have not been sown in the culture
medium. We should note their rounded or globose form, as well as
their markings if they belong to one of the species with spiny walls.
Particularly should we note the size, and if possible measure them with
the micrometer, though this would not be absolutely necessary for a
comparison, if the comparison can be made immediately. Now examine some
of the gonidia which were sown in the nutrient medium. If they have not
already germinated we note at once that they are much larger than those
which have not been immersed in a moist medium.
=242. The gonidia absorb water and increase in size before
germinating.=—From our study of the absorption of water or watery
solutions of nutriment by living cells, we can easily understand the
cause of this enlargement of the gonidium of the mucor when surrounded
by the moist nutrient medium. The cell-sap in the spore takes up more
water than it loses by diffusion, thus drawing water forcibly through
the protoplasmic membrane. Since it does not filter out readily, the
increase in quantity of the water in the cell produces a pressure from
within which stretches the membrane, and the elastic cell wall yields.
Thus the gonidium becomes larger.
[Illustration: Fig. 109. Spores of mucor, and different stages of
germination.]
=243. How the gonidia germinate.=—We should find at this time
many of the gonidia extended on one side into a tube-like process the
length of which varies according to time and temperature. The short
process thus begun continues to elongate. This elongation of the plant
is _growth_, or, more properly speaking, one of the phenomena of growth.
=244. The germ tube branches and forms the mycelium.=—In the
course of a day or so branches from the tube will appear. This branched
form of the threads of the fungus is, as we remember, the mycelium. We
can still see the point where growth started from the gonidium. Perhaps
by this time several tubes have grown from a single one. The threads of
the mycelium near the gonidium, that is, the older portions of them,
have increased in diameter as they have elongated, though this increase
in diameter is by no means so great as the increase in length. After
increasing to a certain extent in diameter, growth in this direction
ceases, while apical growth is practically unlimited, being limited
only by the supply of nutriment.
=245. Growth in length takes place only at the end of the thread.=—If
there were any branches on the mycelium when the culture was first
examined, we can now see that they remain practically the same distance
from the gonidium as when they were first formed. That is, the older
portions of the mycelium do not elongate. Growth in length of the
mycelium is confined to the ends of the threads.
=246. Protoplasm increases by assimilation of nutrient
substances.=—As the plant increases in bulk we note that there
is an increase in the protoplasm, for the protoplasm is very easily
detected in these cultures of mucor. This increase in the quantity
of the protoplasm has come about by the assimilation of the nutrient
substance, which the plant has absorbed. The increase in the
protoplasm, or the formation of additional plant substance, is another
phenomenon of growth quite different from that of elongation, or
increase in bulk.
=247. Growth of roots.=—For the study of the growth of roots we
may take any one of many different plants. The seedlings of such plants
as peas, beans, corn, squash, pumpkin, etc., serve excellently for this
purpose.
=248. Roots of the pumpkin.=—The seeds, a handful or so, are
soaked in water for about 12 hours, and then placed between layers of
paper or between the folds of cloth, which must be kept quite moist but
not very wet, and should be kept in a warm place. A shallow crockery
plate, with the seeds lying on wet filter paper, and covered with
additional filter paper, or with a bell jar, answers the purpose well.
The primary or first root (radicle) of the embryo pushes its way out
between the seed coats at the small end. When the seeds are well
germinated, select several which have the root 4-5 _cm_ long. With a
crow-quill pen we may now mark the terminal portion of the root off
into very short sections as in fig. 110. The first mark should be
not more than 1 _mm_ from the tip, and the others not more than 1mm
apart. Now place the seedlings down on damp filter paper, and cover
with a bell jar so that they will remain moist, and if the season is
cold place them in a warm room. At intervals of 8 or 10 hours, if
convenient, observe them and note the farther growth of the root.
[Illustration: Fig. 110. Root of germinating pumpkin, showing region of
elongation just back of the tip.]
=249. The region of elongation.=—While the root has elongated,
the region of elongation _is not at the tip of the root. It lies a
little distance back from the tip_, beginning at about 2mm from the tip
and extending over an area represented by from 4-5 of the millimeter
marks. The root shown in fig. 110 was marked at 10 A.M. on
July 5. At 6 P.M. of the same day, 8 hours later, growth had
taken place as shown in the middle figure. At 9 A.M. on the
following day, 15 hours later, the growth is represented in the lower
one. Similar experiments upon a number of seedlings give the same
result: the region of elongation in the growth of the root is situated
a little distance back from the tip. Farther back very little or no
elongation takes place, but growth in diameter continues for some time,
as we should discover if we examined the roots of growing pumpkins, or
other plants, at different periods.
=250. Movement of region of greatest elongation.=—In the region
of elongation the areas marked off do not all elongate equally at the
same time. The middle spaces elongate most rapidly and the spaces
marked off by the 6, 7, and 8 _mm_ marks elongate slowly, those
farthest from the tip more slowly than the others, since elongation
has nearly ceased here. The spaces marked off between the 2-4 _mm_
marks also elongate slowly, but soon begin to elongate more rapidly,
since that region is becoming the region of greatest elongation. Thus
the region of greatest elongation moves forward as the root grows, and
remains approximately at the same distance behind the tip.
=251. Formative region.=—If we make a longitudinal section of the
tip of a growing root of the pumpkin or other seedling, and examine it
with the microscope, we see that there is a great difference in the
character of the cells of the tip and those in the region of elongation
of the root. First there is in the section a V-shaped cap of loose
cells which are constantly being sloughed off. Just back of this tip
the cells are quite regularly isodiametric, that is, of equal diameter
in all directions. They are also very rich in protoplasm, and have
thin walls. This is the region of the root where new cells are formed
by division. It is the _formative region_. The cells on the outside
of this area are the older, and pass over into the older parts of the
root and root cap. If we examine successively the cells back from this
_formative_ region we find that they become more and more elongated in
the direction of the axis of the root. The elongation of the cells in
this older portion of the root explains then why it is that this region
of the root elongates more rapidly than the tip.
=252. Growth of the stem.=—We may use a bean seedling growing
in the soil. At the junction of the leaves with the stem there are
enlargements. These are the _nodes_, and the spaces on the stem between
successive nodes are the _internodes_. We should mark off several of
these internodes, especially the younger ones, into sections about 5
_mm_ long. Now observe these at several times for two or three days,
or more. The region of elongation is greater than in the case of the
roots, and extends back farther from the end of the stem. In some young
garden bean plants the region of elongation extended over an area of 40
_mm_ in one internode. See also Chapters 38, 39.
=253. Force exerted by growth.=—One of the marvelous things
connected with the growth of plants is the force which is exerted by
various members of the plant under certain conditions. Observations on
seedlings as they are pushing their way through the soil to the air
often show us that considerable force is required to lift the hard soil
and turn it to one side. A very striking illustration may be had in the
case of mushrooms which sometimes make their way through the hard and
packed soil of walks or roads. That succulent and tender plants should
be capable of lifting such comparatively heavy weights seems incredible
until we have witnessed it. Very striking illustrations of the force
of roots are seen in the case of trees which grow in rocky situations,
where rocks of considerable weight are lifted, or small rifts in large
rocks are widened by the lateral pressure exerted by the growth of a
root, which entered when it was small and wedged its way in.
=254. Zone of maximum growth.=—Great variation exists in the
rapidity of growth even when not influenced by outside conditions. In
our study of the elongation of the root we found that the cells just
back of the formative region elongated slowly at first. The rapidity of
the elongation of these cells increases until it reaches the maximum.
Then the rapidity of elongation lessens as the cells come to lie
farther from the tip. The period of maximum elongation here is the
_zone of maximum growth_ of these cells.
[Illustration: Fig. 111. Lever auxanometer (Oels) for measuring
elongation of the stem during growth.]
=255.= Just as the cells exhibit a zone of maximum growth, so
the members of the plant exhibit a similar zone of maximum growth.
In the case of leaves, when they are young the rapidity of growth
is comparatively slow, then it increases, and finally diminishes in
rapidity again. So it is with the stem. When the plant is young the
growth is not so rapid; as it approaches middle age the rapidity of
growth increases; then it declines in rapidity at the close of the
season.
=256. Energy of growth.=—Closely related to the zone of maximum
growth is what is termed the energy of growth. This is manifested in
the comparative size of the members of a given plant. To take the
sunflower for example, the lower and first leaves are comparatively
small. As the plant grows larger the leaves are larger, and this
increase in size of the leaves increases up to a maximum period, when
the size decreases until we reach the small leaves at the top of the
stem. The zone of maximum growth of the leaves corresponds with the
maximum size of the leaves on the stem. The rapidity and energy of
growth of the stem is also correlated with that of the leaves, and the
zone of maximum growth is coincident with that of the leaves. It would
be instructive to note it in the case of other plants and also in the
case of fruits.
=257. Nutation.=—During the growth of the stem all of the cells
of a given section of the stem do not elongate simultaneously. For
example the cells at a given moment on the south side are elongating
more rapidly than the cells on the other side. This will cause the
stem to bend slightly to the north. In a few moments later the cells
on the west side are elongating more rapidly, and the stem is turned
to the east; and so on, groups of cells in succession around the stem
elongate more rapidly than the others. This causes the stem to describe
a circle or ellipse about a central point. Since the region of greatest
elongation of the cells of the stem is gradually moving toward the apex
of the growing stem, this line of elongation of the cells which is
traveling around the stem does so in a spiral manner. In the same way,
while the end of the stem is moving upward by the elongation of the
cells, and at the same time is slowly moved around, the line which the
end of the stem describes must be a spiral one. This movement of the
stem, which is common to all stems, leaves, and roots, is _nutation_.
=258.= The importance of nutation to twining stems in their search
for a place of support, as well as for the tendrils on leaves or stems,
will be seen. In the case of the root it is of the utmost importance,
as the root makes its way through the soil, since the particles of soil
are more easily thrust aside. The same is also true in the case of many
stems before they emerge from the soil.
CHAPTER XIII.
IRRITABILITY.
=259.= We should now examine the movements of plant parts in
response to the influence of certain stimuli. By this time we have
probably observed that the direction which the root and stem take upon
germination of the seed is not due to the position in which the seed
happens to lie. Under normal conditions we have seen that the root
grows downward and the stem upward.
=260. Influence of the earth on the direction of growth.=—When
the stem and root have been growing in these directions for a short
time let us place the seedling in a horizontal position, so that the
end of the root extends over an object of support in such a way that it
will be free to go in any direction. It should be pinned to a cork and
placed in a moist chamber. In the course of twelve to twenty-four hours
the root which was formerly horizontal has turned the tip downward
again. If we should mark off millimeter spaces beginning at the tip of
the root, we should find that the motor zone, or region of curvature,
lies in the same region as that of the elongation of the root.
Knight found that the stimulus which influences the root to turn
downward is the force of gravity. The reaction of the root in response
to this stimulus is geotropism, a turning influenced by the earth. This
term is applied to the growth movements of plants influenced by the
earth with regard to direction. While the motor zone lies back of the
root-tip, the latter receives the stimulus and is the perceptive zone.
If the root-tip is cut off, the root is no longer geotropic, and will
not turn downward when placed in a horizontal position. Growth toward
the earth is _progeotropism_. The lateral growth of secondary roots is
_diageotropism_.
[Illustration: Fig. 112. Germinating pea placed in a horizontal
position.]
[Illustration: Fig. 113. In 24 hours gravity has caused the root to
turn downward.
Figs. 112, 113.—Progeotropism of the pea root.]
The stem, on the other hand, which was placed in a horizontal position
has become again erect. This turning of the stem in the upward
direction takes place in the dark as well as in the light, as we can
see if we start the experiment at nightfall, or place the plant in the
dark. This upward growth of the stem is also influenced by the earth,
and therefore is a case of geotropism. The special designation in the
case of upright stems is _negative geotropism_, or _apogeotropism_, or
the stems are said to be _apogeotropic_. If we place a rapidly growing
potted plant in a horizontal position by laying the pot on its side,
the ends of the shoots will soon turn upward again when placed in a
horizontal position. Young bean plants growing in a pot began within
two hours to turn the ends of the shoots upward.
[Illustration: Fig. 114. Pumpkin seedling showing apogeotropism.
Seedling at the left placed horizontally, in 24 hours the stem has
become erect.]
Horizontal leaves and shoots can be shown to be subject to the same
influence, and are therefore _diageotropic_.
=261. Influence of light.=—Not only is light a very important
factor for plants during photosynthesis, it exerts great influence on
plant growth and movement.
[Illustration: Fig. 115. Radish seedlings grown in the dark, long,
slender, not green.]
[Illustration: Fig. 116. Radish seedlings grown in the light, shorter,
stouter, and green in color. Growth retarded by light.]
=262. Growth in the absence of light.=—Plants grown in the dark
are subject to a number of changes. The stems are often longer,
more slender and weaker since they contain a larger amount of water
in proportion to building material which the plant obtains from
carbohydrates manufactured in the light. On many plants the leaves are
very small when grown in the dark.
=263. Influence of light on direction of growth.=—While we are
growing seedlings, the pots or boxes of some of them should be placed
so that the plants will have a one-sided illumination. This can be
done by placing them near an open window, in a room with a one-sided
illumination, or they may be placed in a box closed on all sides but
one which is facing the window or light. In 12-24 hours, or even in a
much shorter time in some cases, the stems of the seedlings will be
directed toward the source of light. This influence exerted by the rays
of light is _heliotropism_, a turning influenced by the sun or sunlight.
[Illustration: Fig. 117. Seedling of castor-oil bean, before and after
a one-sided illumination.]
[Illustration: Fig. 118. Dark chamber with opening at one side to show
heliotropism. (After Schleichert.)]
=264. Diaheliotropism.=—Horizontal leaves and shoots are
_diaheliotropic_ as well as _diageotropic_. The general direction which
leaves assume under this influence is that of placing them with the
upper surface perpendicular to the rays of light which fall upon them.
Leaves, then, exposed to the brightly lighted sky are, in general,
horizontal. This position is taken in direct response to the stimulus
of light. The leaves of plants with a one-sided illumination, as can be
seen by trial, are turned with their upper surfaces toward the source
of light, or perpendicular to the incidence of the light rays. In this
way light overcomes for the time being the direction which growth
gives to the leaves. The so-called “sleep” of plants is of course
not sleep, though the leaves “nod,” or hang downward, in many cases.
There are many plants in which we can note this drooping of the leaves
at nightfall, and in order to prove that it is not determined by the
time of day we can resort to a well-known experiment to induce this
condition during the day. The plant which has been used to illustrate
this is the sunflower. Some of these plants, which were grown in a box,
when they were about 35 _cm_ high were covered for nearly two days,
so that the light was excluded. At midday on the second day the box
was removed, and the leaves on the covered plants are well represented
by fig. 119, which was made from one of them. The leaves of the other
plants in the box which were not covered were horizontal, as shown by
fig. 120. Now on leaving these plants, which had exhibited induced
“sleep” movements, exposed to the light they gradually assumed the
horizontal position again.
[Illustration: Fig. 119. Sunflower plant. Epinastic condition of leaves
induced during the day in darkness.]
[Illustration: Fig. 120. Sunflower plant removed from darkness, leaves
extending under influence of light (diaheliotropism.)]
=265. Epinasty and hyponasty.=—During the early stages of growth
of many leaves, as in the sunflower plant, the direction of growth is
different from what it is at a later period. The under surface of the
young leaves grows more rapidly in a longitudinal direction than the
upper side, so that the leaves are held upward close against the bud
at the end of the stem. This is termed _hyponasty_, or the leaves are
said to be _hyponastic_. Later the growth is more rapid on the upper
side and the leaves turn downward or away from the bud. This is termed
_epinasty_, or the leaves are said to be _epinastic_. This is shown by
the night position of the leaves, or in the induced “sleep” of the
sunflower plant in the experiment detailed above. The day position of
the leaves on the other hand, which is more or less horizontal, is
induced because of their irritability under the influence of light, the
inherent downward or epinastic growth is overcome for the time. Then
at nightfall or in darkness, the stimulus of light being removed, the
leaves assume the position induced by the direction of growth.
[Illustration: Fig. 121. Squash seedling. Position of cotyledons in
light.]
[Illustration: Fig. 122. Squash seedling. Position of cotyledons in the
dark.]
=266.= In the case of the cotyledons of some plants it would seem
that the growth was hyponastic even after they have opened. The day
position of the cotyledons of the pumpkin is more or less horizontal,
as shown in fig. 121. At night, or if we darken the plant by covering
with a tight box, the leaves assume the position shown in fig. 122.
While the horizontal position is the general one which is assumed
by plants under the influence of light, their position is dependent
to a certain extent on the intensity of the light as well as on the
incidence of the light rays. Some plants are so strongly heliotropic
that they change their positions all during the day.
[Illustration: Fig. 123. Coiling tendril of bryony.]
=267. Leaves with a fixed diurnal position.=—Leaves of some
plants when they are developed have a fixed diurnal position and are
not subject to variation. Such leaves tend to arrange themselves in a
vertical or paraheliotropic position, in which the surfaces are not
exposed to the incidence of light of the greatest intensity, but to the
incidence of the rays of diffused light. Interesting cases of the fixed
position of leaves are found in the so-called compass plants (like
Silphium laciniatum, Lactuca scariola, etc.). In these the horizontal
leaves arrange themselves with the surfaces vertical, and also pointing
north and south, so that the surfaces face east and west.
=268. Importance of these movements.=—Not only are the leaves
placed in a position favorable for the absorption of the rays of light
which are concerned in making carbon available for food, but they
derive other forms of energy from the light, as heat, which is absorbed
during the day. Then with the nocturnal position, the leaves being
drooped down toward the stem, or with the margin toward the sky, or
with the cotyledons as in the pumpkin, castor-oil bean, etc., clasped
upward together, the loss of heat by radiation is less than it would be
if the upper surfaces of the leaves were exposed to the sky.
=269. Influence of light on the structure of the leaf.=—In our
study of the structure of a leaf we found that in the ivy leaf the
palisade cells were on the upper surface. This is the case with a great
many leaves, and is the normal arrangement of “dorsiventral” leaves
which are diaheliotropic. Leaves which are paraheliotropic tend to have
palisade cells on both surfaces. The palisade layer of cells as we
have seen is made up of cells lying very close together, and they thus
prevent rapid evaporation. They also check to some extent the entrance
of the rays of light, at least more so than the loose spongy parenchyma
cells do. Leaves developed in the shade have looser palisade and
parenchyma cells. In the case of some plants, if we turn over a very
young leaf, so that the under side will be uppermost, this side will
develop the palisade layer. This shows that light has a great influence
on the structure of the leaf.
=270. Movement influenced by contact.=—In the case of tendrils,
twining leaves, or stems, the irritability to contact is shown in a
movement of the tendril, etc., toward the object in touch. This causes
the tendril or stem to coil around the object for support. The stimulus
is also extended down the part of the tendril below the point of
contact (see fig. 123), and that part coils up like a wire coil spring,
thus drawing the leaf or branch from which the tendril grows closer to
the object of support. This coil between the object of support and the
plant is also very important in easing up the plant when subject to
violent gusts of wind which might tear the plant from its support were
it not for the yielding and springing motion of this coil.
[Illustration: Fig. 124. Sensitive plant leaf in normal position.]
[Illustration: Fig. 125. Pinnæ folding up after stimulus.]
[Illustration: Fig. 126. Later all the pinnæ folded and leaf drooped.]
=271. Sensitive plants.=—These plants are remarkable for the
rapid response to stimuli. Mimosa pudica is an excellent plant to study
for this purpose.
=272. Movement in response to stimuli.=—If we pinch with the
forceps one of the terminal leaflets, or tap it with a pencil, the two
end leaflets fold above the “vein” of the pinna. This is immediately
followed by the movement of the next pair, and so on as shown in fig.
125, until all the leaflets on this pinna are closed, then the stimulus
travels down the other pinnæ in a similar manner, and soon the pinnæ
approximate each other and the leaf then drops downward as shown in
fig. 126. The normal position of the leaf is shown in fig. 124. If we
jar the plant by striking it or by jarring the pot in which it is grown
all the leaves quickly collapse into the position shown in fig. 126.
If we examine the leaf now we see minute cushions at the base of each
leaflet, at the junction of the pinnæ with the petiole, and a larger
one at the junction of the petiole with the stem. We shall also note
that the movement resides in these cushions.
=273. Transmission of the stimulus.=—The transmission of the
stimulus in this mimosa from one part of the plant has been found to be
along the cells of the bast.
=274. Cause of the movement.=—The movement is caused by a sudden
loss of turgidity on the part of the cells in one portion of the
pulvinus, as the cushion is called. In the case of the large pulvinus
at the base of the petiole this loss of turgidity is in the cells of
the lower surface. There is a sudden change in the condition of the
protoplasm of the cells here so that they lose a large part of their
water. This can be seen if with a sharp knife we cut off the petiole
just above the pulvinus before movement takes place. A drop of liquid
exudes from the cells of the lower side.
=275. Paraheliotropism of the leaves of the sensitive plant.=—If
the mimosa plant is placed in very intense light the leaflets will
turn their edges toward the incidence of the rays of light. This is
also true of other plants in intense light, and is _paraheliotropism_.
Transpiration is thus lessened, and chlorophyll is protected from too
intense light.
[Illustration: Fig. 126_a_. Leaf of Venus fly-trap (Dionæa muscipula),
showing winged petiole and toothed lobes.]
[Illustration: Fig. 127. Leaf of Drosera rotundifolia, some of the
glandular hairs folding inward as a result of a stimulus.]
We thus see that variations in the intensity of light have an important
influence in modifying movements. Variations in temperature also exert
a considerable influence, rapid elevation of temperature causing
certain flowers to open, and falling temperature causing them to close.
=276. Sensitiveness of insectivorous plants.=—The Venus fly-trap
(Dionæa muscipula) and the sundew (drosera) are interesting examples of
sensitive plants, since the leaves close in response to the stimulus
from insects.
=277. Hydrotropism.=—Roots are sensitive to moisture. They will
turn toward moisture. This is of the greatest importance for the
well-being of the plant, since the roots will seek those places in the
soil where suitable moisture is present. On the other hand, if the soil
is too wet there is a tendency for the roots to grow away from the
soil which is saturated with water. In such cases roots are often seen
growing upon the surface of the soil so that they may obtain oxygen,
which is important for the root in the processes of absorption and
growth. Plants then may be injured by an excess of water as well as by
a lack of water in the soil.
=278. Temperature.=—In the experiments on germination thus
far made it has probably been noted that the temperature has much
to do with the length of time taken for seeds to germinate. It also
influences the rate of growth. The effect of different temperatures
on the germination of seed can be very well noted by attempting to
germinate some in rooms at various temperatures. It will be found,
other conditions being equal, that in a moderately warm room, or even
in one quite warm, 25-30 degrees centigrade, germination and growth
goes on more rapidly than in a cool room, and here more rapidly than in
one which is decidedly cold. In the case of most plants in temperate
climates, growth may go on at a temperature but little above freezing,
but few will thrive at this temperature.
=279.= If we place dry peas or beans in a temperature of about
70° C. for 15 minutes they will not be killed, but if they have been
thoroughly soaked in water and then placed at this temperature they
will be killed, or even at a somewhat lower temperature. The same seeds
in the dry condition will withstand a temperature of 10° C. below, but
if they are first soaked in water this low temperature will kill them.
=280.= In order to see the effect of freezing we may thoroughly
freeze a section of a beet root, and after thawing it out place it in
water. The water is colored by the cell-sap which escapes from the
cells, just as we have seen it does as a result of a high temperature,
while a section of an unfrozen beet placed in water will not color it
if it was previously washed.
If the slice of the beet is placed at about -6° C. in a shallow glass
vessel, and covered, ice will be formed over the surface. If we examine
it with the microscope ice crystals will be seen formed on the outside,
and these will not be colored. The water for the formation of the
crystals came from the cell-sap, but the concentrated solutions in the
sap were not withdrawn by the freezing over the surface.
=281.= If too much water is not withdrawn from the cells of many
plants in freezing, and they are thawed out slowly, the water which was
withdrawn from the cells will be absorbed again and the plant will not
be killed. But if the plant is thawed out quickly the water will not
be absorbed, but will remain on the surface and evaporate. Some will
also remain in the intercellular spaces, and the plant will die. Some
plants, however, no matter how slowly they are thawed out, are killed
after freezing, as the leaves of the pumpkin, dahlia, or the tubers of
the potato.
=282.= It has been found that as a general rule when plants, or
plant parts, contain little moisture they will withstand quite high
degrees of temperature, as well as quite low degrees, but when the
parts are filled with sap or water they are much more easily killed.
For this reason dry seeds and the winter buds of trees, and other
plants, because they contain but little water, are better able to
resist the cold of winters. But when growth begins in the spring, and
the tissues of these same parts become turgid and filled with water,
they are quite easily killed by frosts. It should be borne in mind,
however, that there is great individual variation in plants in this
respect, some being more susceptible to cold than others. There is
also great variation in plants as to their resistance to the cold of
winters, and of arctic climates, the plants of the latter regions
being able to resist very low temperatures. We have examples also in
the arctic plants, and those which grow in arctic climates on high
mountains, of plants which are able to carry on all the life functions
at temperatures but little above freezing.
For further discussion as to relation of plants to temperature, see
Chapters 46, 48, 49, and 53.
PART II.
MORPHOLOGY AND LIFE HISTORY OF REPRESENTATIVE PLANTS.