TRANSCRIBER’S NOTE
- Some minor changes to the text are noted at the end of the book.
- Italic text is denoted by _underscores_.
- Bold text is denoted by =equal signs=.
- Small-cap text has been changed to ALL-CAP text.
- Footnote anchors are denoted by [number], and the footnotes have
been placed at the end of the book.
A PRACTICAL COURSE IN
BOTANY
WITH ESPECIAL REFERENCE TO ITS BEARINGS ON
AGRICULTURE, ECONOMICS, AND SANITATION
BY
E. F. ANDREWS
AUTHOR OF “BOTANY ALL THE YEAR ROUND”
WITH EDITORIAL REVISION BY
FRANCIS E. LLOYD
MACDONALD PROFESSOR OF BOTANY, McGILL UNIVERSITY,
FORMERLY OF ALABAMA POLYTECHNIC INSTITUTE
[Illustration]
NEW YORK ·:· CINCINNATI ·:· CHICAGO
AMERICAN BOOK COMPANY
COPYRIGHT, 1911, BY
E. F. ANDREWS.
ENTERED AT STATIONERS’ HALL, LONDON.
ANDREW’S PR. BOTANY.
W. P. 7
PREFACE
In preparing the present volume, the aim of the writer has been to
meet all the college entrance requirements and at the same time
to bring the study of botany into closer touch with the practical
business of life by stressing its relations with agriculture,
economics, and, in certain of its aspects, with sanitation. While
technical language has been avoided so far as the requirements of
scientific accuracy will permit, the student is not encouraged
to shirk the use of necessary botanical terms, out of a mere
superstitious fear of words because they happen to be a little new or
unfamiliar. Such a practice not only leads to careless and inaccurate
modes of expression, but tends to foster a slovenly habit of mind,
and in the long run causes the waste of more time and labor in the
search after roundabout, and often misleading, substitutes, than it
would require to master the proper use of a few new words and phrases.
In the choice of materials for experiment and illustration, the
endeavor has been to call for such only as are familiar and easily
obtained. The specimens for flower dissection have been selected
mainly from common cultivated kinds, because their wide distribution
makes them easy to obtain everywhere, while in cities and large
towns they are practically the only specimens available. Another
important consideration has been the desire to spare our native
wild flowers, or at least not to hasten the extinction with which
they are threatened by the ravages of Sunday excursionists and
summer tourists, to whose unthinking, but none the less destructive,
incursions, the automobile has laid open the most secret haunts
of nature. The influence of the public school teacher, and more
especially the teacher of botany, is the most potent factor from
which we can hope for aid in putting a stop to the relentless
persecution that has practically exterminated many of our choicest
wild plants and is fast reducing the civilized world to a depressing
monotony of weediness and artificiality. Except for purely systematic
and anatomical work, flowers can be studied to better purpose in
their living, active state than as dead subjects for dissection;
and the best way to show our interest in them, or to get the most
rational enjoyment out of them, is not, as a general thing, to
cut their heads off and throw them away to wither and die by the
roadside. The teacher, by instilling into the minds of the rising
generation a reverence for plant life, may do a great deal to aid
in the conservation of one of our chief national assets for the
gratification of the higher esthetic instincts. The fruits and
flowers of cultivation do not stand in the same need of protection,
since they are produced solely with a view to the use and pleasure of
man, and their propagation is provided for to meet all his demands.
To avoid too frequent interruptions of the subject matter, the
experiments are grouped together at the beginning or end of the
sections to which they belong, according as they are intended to
explain what is coming, or to illustrate what has gone before. A few
exceptions are made in cases where the experiment is such an integral
part of the subject that it would be meaningless if separated from
the context. Under no circumstances should those capable of being
performed in the schoolroom be omitted, as much of the information
which the book is intended to give is conveyed by their means. For
this reason, and also because the aim of the book is to present the
science from a practical rather than from an academic point of view,
the experiments outlined are for the most part of a simple, practical
nature, such as can be performed by the pupils themselves with a
moderate expenditure of ingenuity and money. The experience of the
writer has been that for the average boy or girl who wishes to get
a good general knowledge of the subject, but does not propose to
become a specialist in botany, the best results are often obtained
by the use of the simplest and most familiar appliances, as in
this way attention is not distracted from the experiment itself to
the unfamiliar apparatus for making it. In saying this, it is not
meant to underrate the value of a complete laboratory equipment,
but merely to emphasize the fact that the lack of it, while a
disadvantage, need not be an insuperable bar to the successful
teaching of botany. It is, of course, taken for granted that in
schools provided with a suitable laboratory outfit, teachers will be
prepared to supplement or to replace the exercises here outlined with
such others as in their judgment the subject may demand. There are as
many ideals in teaching as there are teachers, and the most that a
textbook can do is to present a working model which every teacher is
free to modify in accordance with his or her own method.
The writer takes pleasure in acknowledging here the many obligations
due to Professor Francis E. Lloyd, of the Botanical Department of the
Alabama Polytechnic Institute, at Auburn, Ala., for his valuable aid
in the revision of the manuscript, for the highly interesting series
of illustrations relating to phototropic movements, and for advice
and information on points demanding expert knowledge which have
contributed very materially to whatever merit this volume may possess.
Other members of the Auburn faculty to whom the author feels
especially indebted are Mr. C. S. Ridgeway, assistant in the
Botanical Department, Professor J. E. Duggar, of the Agricultural
Department, and Dr. B. B. Ross and Professor C. W. Williamson of the
Department of Chemistry. Acknowledgments are due also to Professor
George Wood of the Boys’ High School, Brooklyn, for suggestions
which have been of great assistance in the preparation of this work;
to Professor W. R. Dodson, of the University of Louisiana, for
illustrative material furnished, and to Professor William Trelease
for the loan of original material used in reproducing the beautiful
cuts from the Reports of the Missouri Botanical Garden, credit for
which is given in the proper place.
For original photographs and drawings by the author, and familiar
selections from well-known works, which can be generally recognized,
it has not been thought necessary to give special credit.
E. F. ANDREWS.
AUBURN, ALABAMA.
FULL-PAGE ILLUSTRATIONS
PLATE PAGE
1. A GROVE OF LIVE OAKS NEAR SAVANNAH, GEORGIA _Frontispiece_
2. CARRYING WATER OVER THE MISSISSIPPI LEVEE BY SIPHON TO
IRRIGATE RICE FIELDS 8
3. AËRIAL ROOTS OF A MEXICAN STRANGLING FIG 73
4. A FOREST OF BAMBOO 99
5. A GROUP OF CONIFERS 108
6. A WHITE OAK, SHOWING THE GREAT SPREAD OF BRANCHES 117
7. A TIMBER TREE SPOILED BY STANDING TOO MUCH ALONE 125
8. AN AMERICAN ELM, ILLUSTRATING DELIQUESCENT GROWTH 130
9. VEGETATION OF A MOIST, SHADY RAVINE 151
10. A MOSAIC OF MOONSEED LEAVES 179
11. HYBRID BETWEEN A RED AND A WHITE CARNATION 227
12. GOOSEBERRIES, SHOWING IMPROVEMENT BY SELECTION 251
13. THE EFFECTS OF IRRIGATION 272
14. A XEROPHYTE FORMATION OF YUCCAS AND SWITCH PLANTS 282
15. A GIANT TULIP TREE OF THE SOUTH ATLANTIC FOREST REGION 293
CONTENTS
CHAPTER I. THE SEED PAGE
I. THE STORAGE OF FOOD IN SEEDS 1
II. SOME PHYSIOLOGICAL PROPERTIES OF SEEDS 10
III. TYPES OF SEEDS 12
IV. SEED DISPERSAL 21
FIELD WORK 28
CHAPTER II. GERMINATION AND GROWTH
I. PROCESSES ACCOMPANYING GERMINATION 29
II. CONDITIONS OF GERMINATION 33
III. DEVELOPMENT OF THE SEEDLING 40
IV. GROWTH 47
FIELD WORK 52
CHAPTER III. THE ROOT
I. OSMOSIS AND THE ACTION OF THE CELL 53
II. MINERAL NUTRIMENTS ABSORBED BY PLANTS 58
III. STRUCTURE OF THE ROOT 61
IV. THE WORK OF ROOTS 65
V. DIFFERENT FORMS OF ROOTS 72
FIELD WORK 80
CHAPTER IV. THE STEM
I. FORMS AND GROWTH OF STEMS 81
II. MODIFICATIONS OF THE STEM 88
III. STEM STRUCTURE
A. MONOCOTYLS 96
B. HERBACEOUS DICOTYLS 102
C. WOODY STEMMED DICOTYLS 107
IV. THE WORK OF STEMS 112
V. WOOD STRUCTURE IN ITS RELATION TO INDUSTRIAL USES 118
VI. FORESTRY 124
FIELD WORK 128
CHAPTER V. BUDS AND BRANCHES
I. MODES OF BRANCHING 131
II. BUDS 138
III. THE BRANCHING OF FLOWER STEMS 141
FIELD WORK 145
CHAPTER VI. THE LEAF
I. THE TYPICAL LEAF AND ITS PARTS 147
II. THE VEINING AND LOBING OF LEAVES 154
III. TRANSPIRATION 160
IV. ANATOMY OF THE LEAF 164
V. FOOD MAKING 168
VI. THE LEAF AN ORGAN OF RESPIRATION 174
VII. THE ADJUSTMENT OF LEAVES TO EXTERNAL RELATIONS 177
VIII. MODIFIED LEAVES 189
FIELD WORK 194
CHAPTER VII. THE FLOWER
I. DISSECTION OF TYPES WITH SUPERIOR OVARY 196
II. DISSECTION OF TYPES WITH INFERIOR OVARY 204
III. STUDY OF A COMPOSITE FLOWER 210
IV. SPECIALIZED FLOWERS 214
V. FUNCTION AND WORK OF THE FLOWER 219
VI. HYBRIDIZATION 223
VII. PLANT BREEDING 230
VIII. ECOLOGY OF THE FLOWER
A. THE PREVENTION OF SELF-POLLINATION 235
B. WIND POLLINATION 239
C. INSECT POLLINATION 241
D. PROTECTIVE ADAPTATION 245
FIELD WORK 249
CHAPTER VIII. FRUITS
I. HORTICULTURAL AND BOTANICAL FRUITS 250
II. FLESHY FRUITS 255
III. DRY FRUITS 260
IV. ACCESSORY, AGGREGATE, AND MULTIPLE FRUITS 265
FIELD WORK 269
CHAPTER IX. THE RESPONSE OF THE PLANT TO
ITS SURROUNDINGS
I. ECOLOGICAL FACTORS 271
II. PLANT ASSOCIATIONS 277
III. ZONES OF VEGETATION 288
FIELD WORK 294
CHAPTER X. CRYPTOGAMS
I. THEIR PLACE IN NATURE 296
II. ALGÆ 299
III. FUNGI 303
A. BACTERIA 306
B. YEASTS 314
C. RUSTS 317
D. MUSHROOMS 323
IV. LICHENS 329
V. LIVERWORTS 334
VI. MOSSES 341
VII. FERN PLANTS 344
VIII. THE RELATION BETWEEN CRYPTOGAMS AND SEED PLANTS 354
IX. THE COURSE OF PLANT EVOLUTION 359
FIELD WORK 362
APPENDIX
1. SYSTEMATIC BOTANY 364
2. WEIGHTS, MEASURES, AND TEMPERATURES 367
[Illustration: PLATE 1.—Live oaks covered with Spanish moss
(_Tillandsia_).]
CHAPTER I. THE SEED
I. THE STORAGE OF FOOD IN SEEDS
MATERIAL.—In addition to the four food tests described in Exps.
1-6, there should be provided some raw starch, a solution of grape
sugar, the white of a hard-boiled egg, and any fatty substance,
such as lard or oil. For Exps. 8 and 9, a little diastase solution
will be necessary. “Taka” diastase, made from rice acted upon by a
fungus, can be obtained for a trifle at almost any drug store.
LIVING MATERIAL.—Grains of corn and wheat, and seeds of some kind
of bean, the larger the better. The “horse bean” (_Vicia faba_),
if it can be obtained, makes an excellent object for study, as the
cells are so large that they can be seen with the naked eye. For
showing the presence of proteins (aleurone grains) and oily matter,
use thin cross sections through the kernel of a castor bean or a
Brazil nut. Specimens for the study of the individual cell will be
found in the hairs growing on squash seedlings, in the epidermis of
one of the inner coats of an onion, in the roots of oat or radish
seedlings, or in the section of a young corn root.
A compound microscope will be required for this study.
=1. The economic importance of seeds.=—As a source of food to both
man and the lower animals, the importance of seeds can hardly be
overrated. All the flour, meal, rice, hominy, and other breadstuffs
sold in the market come from them, to say nothing of the fleece from
the cotton seed that clothes the greater part of the world, besides
furnishing a substitute for lard and an important food for cattle.
The oils and fats stored in nuts are also to be taken into account,
the peanut alone yielding the greater part of the so-called olive
oil of commerce. Since the value of our farm crops depends largely
upon the kind and quantity of these substances furnished by them, it
is worth our while, as a matter of economic as well as scientific
interest, to learn something about the nature of the different foods
contained in plants.
[Illustration: FIGS. 1-3.—The world’s three most important food
grains (magnified): 1, section of a rice grain; _a_, cuticle; _b_,
aleurone, or protein layer; _c_, starch cells; _d_, germ; 2, section
of a wheat grain; _k_, germ; _s_, starch; _a_, gluten; _t_, _t_,
_t_, layers of the seed coat; 3, section of a grain of corn; _c_,
husk; _e_, aleurone layer containing proteins; _eg_, yellowish,
horny endosperm, containing proteins and starch; _ew_, lighter
starchy endosperm: the darker part below is rich in oil and proteins,
and contains the _embryo_, consisting of the absorbing organ, or
_cotyledon_, _sc_; the rudimentary bud, _s_; and the root, _w_. (1,
from Circular 77, La. Exp. Station; 2, from Francé; 3, from Sachs.)]
=2. Why food is stored in seeds.=—The one purpose for which plants
produce their seed is to give rise to a new generation and so carry
on the life of the species. The seed is the nursery, so to speak, in
which the germ destined to produce a new plant is sheltered until
it is ready to begin an independent existence. But the young plant,
like the young animal, is incapable of providing for itself at first,
and would die unless it received nourishment from the mother plant
until it has formed roots and leaves so that it can manufacture food
for itself. Plants in general require very much the same food that
animals do, and they have the power, which animals have not, of
manufacturing it out of the crude materials contained in the soil
water and in the air. Such of these foods as are not needed for
immediate consumption, they store up to serve as a provision for the
young shoot when the seed begins to germinate.
[Illustration: FIGS. 4-7.—Sections of corn grains showing different
qualities of food contents: 4, 5, small germ and large proportion
of horny part, showing high protein; 6, 7, large germ and smaller
proportion of horny part, showing high oil content.]
=3. Food substances contained in seeds.=—There are four principal
classes of food stored in seeds: _sugars_, _starches_, _oils_, and
_proteins_. The first are held in solution and can be detected, if
in sufficient quantity, by the taste. The most important varieties
of this group are cane and grape sugar, the latter occurring most
abundantly in fruits, the former in roots and stems. Oil usually
occurs in the form of globules. It is very abundant in some seeds,
_e.g._ flax, castor bean, and Brazil nut. In the corn grain it is
found in the part constituting the germ, or embryo (Figs. 6, 7).
Starches and proteins occur in the form of small granules, which have
specific shapes in different plants (Figs. 8, 9). Those containing
proteins are called _aleurone_ grains, and are, as a rule, smaller
than the starch grains with which they are intermixed in the bean
and some other seeds. In wheat, corn, rice, and most grains they
form a layer just inside the husk, as shown in Fig. 10. This is the
reason why polished rice and finely bolted flour are less nutritious
than the darker kinds, from which this valuable food substance has
not been removed. The two most familiar kinds of proteins are the
_albumins_, of which the white of an egg is a well-known example, and
the _glutins_, which give to the dough of wheat flour and oatmeal
their peculiar gummy or “glutinous” structure.
[Illustration: FIGS. 8-9.—Different forms of starch grains: 8, rice;
9, wheat.]
=4. Organic foods.=—These four substances, starch, sugar, fats, and
proteins, with some others of less frequent occurrence, are called
_organic foods_, because they are produced, in a state of nature,
only through the action of organized living bodies, or, more strictly
speaking, of living vegetable bodies.
[Illustration: FIG. 10.—Transverse section near the outside of a
wheat grain: _e_, the husk; _a_, cells containing protein granules;
_s_, starch cells (_after_ Tschirch).]
=5. Our dependence upon plants.=—While the animal organism can digest
and assimilate these substances after they have been formed by
plants, it has no power to manufacture them for itself, and, so far
as we know at present, is wholly dependent upon the vegetable world
for these necessaries of life. In one sense the whole animal kingdom
may be said to be parasitic on plants. The wolf that eats a lamb is
getting his food indirectly from the grains and grasses consumed by
its victim, and the lion that devours the wolf that ate the lamb is
only one step further removed from a vegetable diet.
=6. The vegetable cell.=—If you will break open a well-soaked horse
bean and examine the contents with a lens, you will see that they
are composed of small oval or roundish granules packed together
like stones in a piece of masonry. These little bodies, called
_cells_, are the ultimate units out of which all animal and vegetable
structures are built up, as a wall is built of bricks and stones.
They differ very much from bricks and stones, however, in that they
are, or have been, living structures with their periods of growth,
activity, decline, and death, just like other living matter, as
will be seen by and by, when we come to look more particularly into
their life history. They consist usually of an inclosing membrane
which contains a living substance called _protoplasm_. This is the
essential part of the cell, and, so far as we know at present, the
physical basis of all life. Cells are commonly more or less rounded
in shape, though they take different forms according to the purpose
they serve. Sometimes, as in the fibers of cotton and the down of
young leaves, they are long and hairlike; when closely packed, they
often become angular by pressure, like those shown in Figs. 10, 11.
The cells composing the thick body of the bean are for the most
part starch and other substances stored up for food, which render
observation difficult. It will, therefore, be better to choose for a
study of the individual cell some kind that will show the essential
parts more distinctly.
[Illustration: FIG. 11—Typical cells: _n_, nucleus; _p_, protoplasm;
_w_, cell wall; _s_, sap.]
=7. Microscopic examination of a cell.=—Place under a high power
of the microscope a portion of fresh skin from one of the inside
scales of an onion, or a piece of the root tip of a very young corn
or oat seedling, and fix your attention on one of the individual
cells. Notice (1) the cell wall or inclosing membrane, _w_ (Fig. 11);
(2) the protoplasm, _p_, which may be recognized by its granular
appearance; (3) the _nucleus_, _n_; and (4) the cell sap, _s_. In
very young cells the protoplasm will be seen to fill most of the
interior; but in mature ones, like the large one on the right of the
figure, it forms a thin lining around the wall, with the nucleus
on one side, while the cell sap, composed of various substances in
solution, occupies the central portion. Though there is generally
an inclosing wall, this is not essential, its office being to give
strength and mechanical support by holding the contents together, as
an India-rubber bag holds water. It is the turgidity of the cell,
when distended with liquid, that gives firmness to herbaceous plants
and the tender parts of woody ones. This may be illustrated by
observing the difference between a rubber bag when quite full and
when only half full of water, or a football when partially and when
fully inflated. In its simplest form, however, the cell is a mere
particle of protoplasm, which has one part, constituting the nucleus,
a little more dense in appearance than the rest, but this kind is not
common in vegetable structures.
=8. How food substances get into the cells.=—As there are no openings
in the cell walls, the only way substances can get into a cell or
out of it is by soaking through the inclosing membrane, as will be
explained in a later chapter. Since starch, oil, and proteins, the
most important foods stored in seeds, are none of them soluble in the
cell sap, it is clear that they could not have got into the cells in
their present state, but must have undergone some change by which
they were rendered capable of passing through the cell wall.
=9. Digestion.=—The process by which this change is brought about
is known as _digestion_, from its similarity to the same function
in animals. Not only are foods, in the state in which we find them
stored in the seed, incapable of passing through the cell wall,
but the protoplasm, the living part of the cell, has no power to
assimilate and to utilize these substances as food until they have
been reduced to a soluble form in which they can be diffused freely
from cell to cell through any part of the plant. By _diffusion_ is
meant the gradual spread of soluble substances through the containing
medium, as when a lump of sugar or salt, dropped into a glass of
water, dissolves and slowly diffuses through the contents, imparting
a sweet or salty taste to the whole.
[Illustration: FIG. 12.—Starch grains of wheat in different stages of
disintegration under the action of a ferment (diastase), accompanying
germination: _a_, slightly corroded; _b_, _c_, and _d_, more advanced
stages of decomposition.]
During the process of digestion the different kinds of food are
acted upon and made soluble by certain chemical ferments, which are
secreted in plants for the purpose. The digestion of starch, the most
abundant of plant foods, is effected by diastase, a common ferment
obtained from germinating grains of barley, wheat, corn, rice, etc.
By the presence of diastase starch is converted into grape sugar,
a substance which is readily soluble in water, and which can be
diffused easily through the tissues of the plant to any part where it
is needed. In this way food travels from the leaf, where it is made,
to the seed, where the sugar is generally reconverted into starch and
stored up for future use, though sometimes, as in the sugar corn and
sugar pea, it remains in part unchanged. The kernels of this kind of
corn can be distinguished readily from those of the ordinary starch
corn, after maturity, by their wrinkled appearance, owing to their
greater loss of water in drying.
=10. Food tests.=—In order to tell whether any of the food substances
named occur in the seeds that we are going to examine, it will be
necessary to understand a few simple tests by which their presence
may be recognized. The chemicals required can be ordered ready for
use from a druggist or may be prepared in the laboratory as needed,
according to the directions given. Write in your notebook a brief
account of each experiment made, with the conclusions drawn from it.
EXPERIMENT 1. TO DETECT THE PRESENCE OF FATS.—Rub a small lump of
butter or a drop of oil on a piece of thin white paper. What is the
effect?
EXPERIMENT 2. ANOTHER TEST FOR FATS.—Place some macerated alcanna
root in a vessel with alcohol enough to cover it, and leave for
an hour. Add an equal bulk of water and filter. The solution will
stain fats, oils, and resins deep red.
[Illustration: PLATE 2.—Carrying water over the Mississippi levee
by siphon to irrigate rice fields. (_From_ Circular of La. Exp.
Station.)]
EXPERIMENT 3. TO SHOW THE PRESENCE OF STARCH.—Put a drop of iodine
solution on some starch. What change of color takes place? To
make iodine solution, add to one part of iodine crystals 4 parts
potassium iodide and 95 parts water. It should be kept in the
dark, as light decomposes it. Iodine colors starch blue, protein
substances light brown. In testing for starch, the solution should
be diluted till it is of a pale color, otherwise the stain will be
so deep as to appear black.
EXPERIMENT 4. A TEST FOR PROTEINS.—Place a small quantity of the
white of an egg, diluted with water, in a clean glass and add a few
drops of nitric acid; or drop some of the acid on the white of a
hard-boiled egg. What is the effect?
Nitric acid turns proteins yellow; if the color is indistinct, add
a drop of ammonia, when an orange color will ensue.
EXPERIMENT 5. ANOTHER TEST FOR PROTEINS.—Place on the substance to
be examined a drop of a saturated solution of cane sugar and water;
add a drop of pure sulphuric acid; if proteins are present, they
will be colored red. See also Exp. 3.
EXPERIMENT 6. A TEST FOR GRAPE SUGAR.—Heat a teaspoonful of
Fehling’s Solution to the boiling point in a test tube (a common
glass vial can be used by heating gradually in water) and pour in a
few drops of grape sugar solution. Heat again and observe the color
of the precipitate that forms.
Fehling’s Solution may be obtained of the druggist, or, if
preferred, it may be prepared in the laboratory as follows: (_a_)
Dissolve 173 grams of crystallized Rochelle salts and 125 grams
of caustic potash in 500 cc. of water; (_b_) dissolve 34.64 grams
crystallized copper sulphate in 500 cc. of water, and mix equal
parts as needed. (For English equivalents, see Appendix, Weights
and Measures.) The two mixtures must be kept separate till wanted
for use, or prepared fresh as needed.
Grape Sugar causes Fehling’s Solution to form a red precipitate.
EXPERIMENT 7. TO SHOW THE DIFFERENCE BETWEEN SUGAR AND STARCH IN
REGARD TO SOLUBILITY.—Mix some sugar with water and notice how
readily it dissolves. Try the same experiment with starch and
observe its different behavior.
EXPERIMENT 8. TO SHOW HOW STARCH IS DISINTEGRATED IN THE ACT OF
DIGESTION.—Place a few grains of starch on a slide, add a drop or
two of diastase solution, and observe under the microscope; the
starch granules will be seen to disintegrate and melt away. Even
with a hand lens it can be seen, from the greater clearness of the
liquid in comparison with a mixture of untreated starch and water,
that the grains have been dissolved.
EXPERIMENT 9. TO SHOW THAT DIASTASE CONVERTS STARCH INTO
SUGAR.—Make a paste of boiled starch so thin that it looks like
water. Pour a small quantity of it into each of two tubes, adding a
little diastase to one and leaving the other untreated. Keep in a
warm place for twenty-four hours, then test both tubes for starch,
as directed in Exp. 3, and note the result. If the diastase has not
acted, add a little more and watch.
Practical Questions
1. Name all the food and other economic products you can think of
that are derived from the seed of maize; from wheat; from flaxseed;
from cotton.
2. Mention some seeds from which medicines are procured.
3. Name all the seeds you can think of from which oil is obtained;
starch; some that are rich in proteins. (Exps. 1-5.)
4. Describe some of the ways in which these products are frequently
adulterated.
5. If you were raising corn to sell to a starch factory, what part
of the seed would you seek to develop? If to feed stock, what part?
Why, in each case? (3; Figs. 4-7.)
6. What grain feeds more human beings than does any other?
7. Name all the seeds you can think of that contain sugar in
sufficient quantity to be detected without chemical tests; that is,
by tasting alone.
8. Is “coal oil” a mineral or an organic substance? Explain, by
giving an account of its origin.
9. What is gluten? (3.) Name some grains that are especially rich
in it.
10. Which of our three chief food grains is a water plant? (See
Plate 2.) Which grows farthest south? Which farthest north? Which
one is of American origin?
II. SOME PHYSIOLOGICAL PROPERTIES OF SEEDS
MATERIAL.—Seeds of squash, pumpkin, or other melon; castor bean;
any kind of common kidney bean; grains of Indian corn.
APPLIANCES.—In the absence of gas, an alcohol or kerosene lamp may
be used for heating. A double boiler can easily be made by using
two tin vessels of different sizes. Partly fill the larger one with
water, set in it the smaller one with the substance to be heated,
and place over a burner. A pair of scales, a strong six-ounce
bottle, wire-netting, cord, and wax or paraffin should be provided.
EXPERIMENT 10. DO SEEDS IN THEIR ORDINARY QUIESCENT STATE CONTAIN
ANY WATER?—Place a number of beans, or grains of corn or wheat in
a glass bottle, making a small perforation in the cork to allow
the air to escape, and heat gently. Does any moisture form on the
glass?
A better test is to weigh two or three ounces of seeds, and heat
them in a double boiler or in oil to prevent scorching. Weigh at
intervals. If there is any loss of weight, to what is it due?
EXPERIMENT 11. DO SEEDS ABSORB WATER?—Soak a number of beans or
grains of corn in water for 12 to 24 hours and compare with dry
ones. What difference do you notice? To what cause is it due?
EXPERIMENT 12. HOW DID WATER GET INTO THE SOAKED SEEDS?—Dry gently
with a soft cloth some of the seeds used in the last experiment and
press them lightly to see if water comes out, and where. Place a
number of dry seeds of different kinds—squash, bean, castor bean,
quince, etc.—in warm water and notice whether any bubbles of air
form on them and at what point. Examine with a lens and see if this
point differs in any way from the rest of the seed cover. Does it
correspond with the point from which water exuded in the soaked
seeds? Could hard seeds like the squash, castor bean, buckeye, and
Brazil nut get water readily without an opening somewhere in the
coat?
EXPERIMENT 13. TO FIND OUT WHETHER WATER IS ABSORBED THROUGH THE
SEED COATS.—Place in moist sand or sawdust two rows of beans as
nearly as possible of the same size and weight, with the eye
pressed down to the substratum in one row and turned up in the
other, so that no moisture can enter through it. In the same way
arrange two rows of castor beans with the little end down in one
row and uppermost in the other. In the last set carefully break
away the spongy mass near the tip, without injuring the parts about
it. Watch and see in which rows water is absorbed most readily.
What change takes place in the spongy masses at the tips of those
castor beans on which they were left?
[Illustration: FIG. 13.—Effect of the expansion of seeds due to
absorption of water.]
EXPERIMENT 14. IS THE RATE OF GERMINATION AFFECTED BY THE PRESENCE
OR ABSENCE OF OPENINGS?—Seal up with wax or paraffin all the
openings of a number of air-dry peas or beans, and leave an equal
number of the same size and weight untreated. Be careful that the
sealing is absolutely water-tight, since otherwise the experiment
will be worthless. Plant both sets and keep under like conditions
of soil, temperature, and moisture. Do you see any difference in
the rate of germination of the two sets?
EXPERIMENT 15. DO SEEDS EXERT FORCE IN ABSORBING WATER?—Fill a
common six-ounce bottle as full as it will hold with dry peas,
beans, or grains of corn; then pour in water till the bottle is
full. Tie a piece of wire-netting or stout sackcloth over the top
to keep the seeds from being forced out. Bind both the neck and the
body of the bottle tightly with strong cords encircling it in both
a horizontal and vertical direction, and place under water in a
moderately warm temperature. Watch for results.
EXPERIMENT 16. IS THE FORCE EXERTED IN THE LAST EXPERIMENT A
MERELY MECHANICAL ONE, LIKE THE BURSTING OF A WATER PIPE, OR IS IT
PHYSIOLOGICAL AND THUS DEPENDENT ON THE FACT THAT THE SEEDS ARE
ALIVE?—To answer this question try Exp. 15 with seeds that have
been killed by heat or by soaking in formalin.
Practical Questions
1. Will a pound of pop corn weigh as much after being popped as
before? (Exp. 10.)
2. What causes the difference, if there is any? (Exp. 10.)
3. Does the tuft of downy hairs at the tip of wheat and oat grains
influence their water supply? The spongy covering of black walnuts
and almonds? The pithy inside layers of pecans and English walnuts?
(Exps. 12, 13.)
4. Why will seeds, as a general thing, germinate more readily after
being soaked? (Exps. 11, 14, 16.)
III. TYPES OF SEEDS
MATERIAL.—Dry and soaked grains of corn, wheat, or oats; bean,
squash, castor bean, and pine seed, or any equivalent specimens
showing the differences as to number of cotyledons and the
presence or absence of endosperm. Each student should be provided
with several specimens, both soaked and dry, of the kind under
consideration. Corn, beans, and wheat need to be soaked from 12
to 24 hours; squash and pumpkin from 2 to 5 days, and very hard
seeds, like the castor bean and morning-glory, from 5 to 10. If
such seeds are _clipped_, before soaking, that is, if a small piece
of the coat is chipped away from the end opposite the scar, or eye,
they will soften more quickly. Keep them in a warm place with an
even temperature till just before they begin to sprout, when the
contents become softened. Very brittle cotyledons may be softened
quickly by boiling for a few minutes.
No appliances are needed beyond the pupil’s individual outfit and
some of the food tests given in Section I of this chapter.
=11. Dissection of a grain of corn.=—Examine a dry grain of corn
on both faces. What differences do you notice? Sketch the grooved
side, labeling the hard, yellowish outer portion, _endosperm_, the
depression near the center, _embryo_, or _germ_.
Next take a grain that has been soaked for twenty-four hours. What
changes do you see? How do you account for the swelling of the
embryo? Remove the skin and observe its texture. Make an enlarged
sketch of a grain on the grooved side with the coat removed, labeling
the flat oval body embedded in the endosperm, _cotyledon_; the upper
end of the little budlike body embedded in the cotyledon, _plumule_,
the lower part, _hypocotyl_—words meaning, respectively, “seed leaf,”
“little bud,” and “the part under the cotyledon.” As this part has
not yet differentiated into root and stem, we cannot call it by
either of these names. The cotyledon, hypocotyl, and plumule together
compose the embryo. Pick out the embryo and sketch as it appears
under the lens. Crush it on a piece of white paper; what does it
contain?
[Illustration: FIGS. 14-16.—Dissection of a grain of corn: 14, soaked
grain, seen flatwise, cut away a little and slightly enlarged, so as
to show the embryo lying in the endosperm; 15, in profile section,
dividing the grain through the embryo and cotyledon; 16, the embryo
taken out whole. The thick mass is the cotyledon; the narrow body
projecting upwards, the plumule; the short projection at the base,
the hypocotyl (_after_ GRAY).]
Make a vertical section of another soaked grain at right angles to
its broader face, and sketch, labeling the parts as they appear in
profile. Make a cross section through the middle of another grain and
sketch, labeling the parts as before. What proportion of the grain is
endosperm and what embryo? Put a drop of iodine and of nitric acid
separately on pieces of the endosperm, and note the effects. Test the
seed coats and the cotyledon to see if they contain any starch.
Notice that the corn grain has but one cotyledon, hence such seeds
are said to be _monocotyledonous_, or one-cotyledoned. The grains are
not typical seeds, but are selected for examination because they are
large and easy to handle, can be obtained everywhere, and germinate
readily.
=12. Dissection of a bean.=—Sketch a dry bean as it lies in the pod,
showing its point of attachment and any markings that may appear on
its surface. Then take it from the pod and examine the narrow edge
by which it was attached. Notice the rather large scar (commonly
called the eye of the bean) where it broke away from the point of
attachment. This is the _hilum_. Near the hilum, look for a minute
round pore like a pinhole. This is called the _micropyle_, from a
Greek word meaning “a little gate,” because it is the entrance to
the interior of the seed coat. There was no micropyle observed in
the corn grain, because it is not a true seed but a fruit inclosing
a single seed. The inclosing membrane is the fruit skin, which has
become incorporated with the seed coat and taken its place as a
protective covering. Compare a soaked bean with a dry one; what
difference do you perceive? How do you account for the change in size
and hardness? Find the hilum and the micropyle in the soaked bean.
Lay it on one side and sketch, with the micropyle on top; then turn
toward you the narrow edge that was attached to the pod and sketch,
labeling all the parts. Make a section through the long diameter at
right angles to the flat sides, press it slightly open, and sketch
it. Notice the line or slit that seems to cut the section in half
longitudinally, and the small round object between the halves at one
end; can you tell what it is?
[Illustration: FIGS. 17, 18.—A kidney bean: 17, side view; 18, front
view, showing _h_, hilum, _m_, micropyle.]
[Illustration: FIG. 19.—Cotyledon of a bean, showing plumule.]
Slip off the coat from a whole bean and notice its texture. Hold it
up to the light and see if it shows any signs of veining. See whether
the scar at the hilum extends through the kernel, or marks only the
seed coat. Lay open the two flat bodies into which the kernel divides
when stripped of its coats, keeping them side by side, with the part
above the micropyle toward the top. Sketch their inner face and
label them _cotyledons_. Be careful not to break or displace the
tiny bud packed away between the cotyledons, just above the hilum.
Label the round portion of this bud, _hypocotyl_, and the upper, more
expanded part, _plumule_. Which way does the base of the hypocotyl
point; toward the micropyle, or away from it? Pick out this budlike
body entire and sketch as it appears under the lens. Open the plumule
with a pin and examine it with a lens; of what does it appear to
consist? Do you find any endosperm around the cotyledons, as in the
corn and oats? Break one of the soaked cotyledons, apply the proper
tests (Exps. 2, 3, 5), and report what substances it contains. Where
is the nourishment for the young plant stored? What part of the bean
gives it its value as food?
Notice that in the bean the embryo consists of three parts, the
hypocotyl, plumule, and the two cotyledons, which completely fill
the seed coats, leaving no place for endosperm. Seeds like the bean,
squash, and castor bean, which have two cotyledons, are said to be
_dicotyledonous_.
=13. The castor bean.=—Lay a castor bean on a sheet of paper before
you with its flat side down; what does it look like? The resemblance
may be increased by soaking the seed a few minutes, in order to swell
the two little protuberances at the small end. Can you think of any
benefit a plant might derive from this curious resemblance of its
seed to an insect?
Sketch the seed as it lies before you, labeling the protuberance
at the apex, _caruncle_. The caruncle is an appendage of the
seed-covering developed by various plants; its use is not always
clear. What appears to be its object in the castor bean? Refer to
Exp. 13 and see if there is any other purpose it might serve.
Turn the seed over and sketch the other side. Notice the colored
line or stripe that runs from the large end to the caruncle. This
is the _rhaphe_, and shows the position that would be occupied by
the seed stalk if it were present. Its starting point near the large
end, which is marked in fresh seeds by a slight roughness, is the
_chalaza_, or organic base of the seed, where the parts all come
together like the parts of a flower at their insertion on the stem.
Where was it situated in the common bean? How does this differ from
its position in the castor bean? Where the rhaphe ends, just at the
beak of the caruncle, you will find the hilum. The micropyle is
covered by the caruncle, which is an outgrowth around it.
[Illustration: FIGS. 20-22.—Castor bean (slightly magnified);
20, back view; 21, front view; _ch_, chalaza; _r_, rhaphe; _ca_,
caruncle; 22, vertical section; _en_, endosperm; _cc_, cotyledons;
_hy_, hypocotyl; _hi_, hilum; _m_, micropyle.]
Now cut a vertical section through a seed that has been soaked
for several days, at right angles to the broad sides, and sketch
it. Label the white, pasty mass within the seed coats, endosperm.
Can you make out what the narrow white line running through the
center of the endosperm, dividing it into two halves, represents?
Make a similar sketch of a cross section. Notice the same white
line running horizontally across the endosperm, dividing it into
two equal parts. To find out what these lines are, take another
seed (always use soaked seeds for dissection) and remove the coats
without injuring the kernel. Split the kernel carefully round the
edges, remove half the endosperm, and sketch the other half with the
delicate embryo lying on its inner face. You will have no difficulty
now in recognizing the lines in your drawings as sections of the
thin cotyledons. Where is the hypocotyl, and which way does its base
point? Remove the embryo from the endosperm, separate the cotyledons
with a pin, hold them up to the light, and observe their beautiful
texture. Sketch them under the lens, showing the delicate venation.
Is there any plumule?
Test the endosperm with a little iodine. Does it give a blue or a
brown reaction? Crush another bit of it on a piece of white paper and
see if it leaves a grease spot. What does this show that it contains?
Test the embryo in the same way, and see whether it contains any oil.
NOTE.—It should be borne in mind that the castor bean bears no
relation whatever to the true beans. It belongs to the spurge
family, which is botanically very remote from that of the peas and
beans.
[Illustration: FIGS. 23-25.—Seed of a squash; 23, seed from the
outside; 24, vertical section perpendicular to the broad side;
25, section parallel to the broad side, showing inner side of a
cotyledon; _a_, seed coat; _c_, cotyledons; _h_, hypocotyl; _p_,
plumule.]
=14. Study of a squash or gourd seed.=—How does the coat of a squash
seed differ from that of the bean? At the small end, look for two
dots, or pinholes, close together. Refer to your drawing of the bean
and see if you can make out, with the help of a lens, what they are.
The bean is a curved seed, which is bent so as to bring the hilum
close to the micropyle on one side. But by far the greater number
of seeds are _inverted_, or turned over on their stalks, as you
sometimes see huckleberry blossoms and bell flowers on their stems,
so that when the stalk breaks away from its attachment, the scar and
the micropyle come close together at one end, as in the squash seed.
[Illustration: FIG. 26.—Diagram of an inverted or anatropous seed,
showing the parts in section: _a_, outer coat; _b_, inner coat;
_c_, kernel; _d_, rhaphe; _ch_, chalaza; _h_, hilum; _m_, micropyle
(_After_ GRAY).]
Make a drawing of the outside of a seed, labeling all the parts you
have observed; then gently remove the hard coat, or _testa_, as it
is called. The thin, greenish covering that lines it on the inside
is the endosperm. How does it compare in quantity with that in the
corn and castor bean? How do the cotyledons compare in thickness
with those of the bean? Carefully separate them and draw, labeling
the parts as you make them out. The tiny pointed object between the
cotyledons at their point of union is the plumule; is it as well
developed as in the bean? Can you see any reason why seeds like the
pea and bean, which have cotyledons too thick and clumsy to do well
the work of true leaves, should have a well-developed plumule, while
those with thin cotyledons, like the squash and pumpkin, do not,
as a general thing, form a large plumule in the embryo? The little
projection in which the cotyledons end is the hypocotyl; which way
does it point? Where did you find the micropyle to be? Test the
cotyledons and some of the endosperm for food substances; what do you
find in them?
=15. Study of a pine seed.=—Remove one of the scales from a pine cone
and sketch the seed as it lies in place on the cone scale. Notice
its point of attachment to the scale, and look near this point for a
small opening, which you can easily recognize as the micropyle. The
seed with its wing looks very much like a fruit of the maple, but
differs from it in being a naked seed borne on the inner side of a
cone scale, without a pod or husk or outer covering of any kind, such
as beans and nuts and grains are provided with. Plants like the pine,
which bear their seed in this way, are called _Gymnosperms_, a word
that means “naked seeds,” in contradistinction to the _Angiosperms_,
which bear their seeds in pods or other closed envelopes.
[Illustration: FIGS. 27, 28.—Pitch pine seeds: 27, scale, or open
carpel, with one seed in place; 28, winged seed, removed. (_After_
GRAY.)]
[Illustration: FIG. 29.—Section of pine seed, showing the
polycotyledonous embryo (GRAY).]
Remove the coat from a seed that has been soaked for twenty-four
hours, and examine it with a lens. Does it consist of one or more
layers? Is there any difference in color between the inner and outer
layers? Look at the base of the hypocotyl for some loose, cobwebby
appendages. These are the remains of other embryos with certain
appendages belonging to them that were formed in the endosperm, but
failed to develop. Did you find remains of this kind in any of the
other seeds examined? Pick out the embryo from the endosperm and
test both for food substances. Which of these do you find? Which
are absent? How does the embryo differ from those already examined?
How many cotyledons are there? Make an enlarged sketch of a seed in
longitudinal section, labeling correctly all the parts observed.
=16. Comparison as to food value of seeds.=—Make in your notebook a
tabular statement after the model here given, of the food contents
found in the different seeds you have examined. Indicate the relative
quantity of each by writing under it, in the appropriate column, the
words, “much,” “little,” or “none,” as the case may be.
MODEL FOR RECORD OF SEEDS EXAMINED
+================+===========================================+
| | FOODS TESTED |
| SEEDS EXAMINED +----------+----------+----------+----------+
| | Starch | Sugar | Oil | Proteins |
+----------------+----------+----------+----------+----------+
| Corn | | | | |
+----------------+----------+----------+----------+----------+
| Wheat | | | | |
+----------------+----------+----------+----------+----------+
| Bean | | | | |
+----------------+----------+----------+----------+----------+
| Squash | | | | |
+----------------+----------+----------+----------+----------+
| Castor bean | | | | |
+----------------+----------+----------+----------+----------+
| Pine | | | | |
+----------------+----------+----------+----------+----------+
By far the greater number of seeds contain endosperm; that is, they
consist of an embryo with more or less nourishing matter stored
about it. Even in seeds which appear to have none, the endosperm is
present at some period during development, but is absorbed by the
cotyledons before germination.
=17. Manner of storing nourishment.=—In the various seeds examined,
we have seen that the nourishment for the young plant is either
stored in the embryo itself, as in the cotyledons of the bean, acorn,
squash, etc., or packed about them in the form of endosperm, as in
the corn, wheat, and castor bean.
=18. The number of cotyledons.=—Seeds are also classed according
to the number of their cotyledons, as having one, two, or many
cotyledons. The first two kinds make up the great class of
Angiosperms, which includes all the true flowering plants and forms
the most important part of the vegetation of the globe. The last
is characteristic of the great natural division of Gymnosperms, or
naked-seeded plants, of which we have had an example in the pine.
They are the most primitive type of living seed-bearing plants.
Though they are not so abundant now as in past ages, numbering only
about four hundred known species, they present many diversities of
form, which seem to ally them on the one hand with the lower, or
spore-bearing plants (ferns, mosses, etc.), and on the other hand
with the Angiosperms.
Practical Questions
1. Make a list of all the seeds you can find that have very thick
cotyledons, and underline those that are used as food by man or
beast.
2. Make a similar list of all the kinds with thin cotyledons and
more or less endosperm, that are used for food or other purposes.
3. Do you find a greater number of foodstuffs among the one kind
than the other?
4. How do the two kinds compare, as a general thing, in size and
weight?
5. From what part of the castor bean do we get oil? of the peanut?
of cotton seed? (Exps. 1-6.)
6. Is there any valid objection to the wholesomeness of peanut oil,
and of cottonseed lard as compared with hog’s lard? (1, 3.)
7. What is bran? Does it contain any nourishment? (11, 12; Exps.
1-6.)
8. What gives to Indian corn its value as food? to oats? wheat?
rice? (3; Exps. 1-6.)
9. Which of these grains has the larger proportion of endosperm to
embryo? (Figs. 1-3.)
10. Which contains the larger amount of starch in proportion to its
bulk, rice or Indian corn?
11. If you wished to produce a variety of corn rich in oil, you
would select seed for planting with what part well developed? (3;
Figs. 4-7.)
IV. SEED DISPERSAL
MATERIAL.—Fruits and seeds of any kind that show adaptations for
dispersal. Some common examples are: (1) Wind: ash, elm, maple,
ailanthus, milkweed, clematis, sycamore, linden, dandelion,
thistle, hawkweed. (2) Water: pecan, filbert, cranberry, lotus,
hickory nut, coconut—obtain one with the husk on, if possible. (3)
Animal agency (involuntary): cocklebur, tickseed, beggar-ticks,
burdock; (voluntary) almost all kinds of edible fruits, especially
the bright-colored ones—wild plums, cherries, haws, dogwood,
persimmons, etc. (4) Explosive and self-planting: witch-hazel,
wood sorrel, violet, crane’s-bill, wild vetch, peanut, medick,
stork’s-bill (Erodium).
EXPERIMENT 17. TO SHOW HOW SEEDS ARE DISPERSED BY WIND.—Take a
number of winged and plumed fruits and seeds, such as those of the
maple, ash, ailanthus, dandelion, clematis, milkweed, and trumpet
creeper; stand on a chair or table in a place where there is a
draft of air and let them all go. Which travel the farther, the
winged or the plumed kinds? Which sort is better fitted to aërial
transportation?
EXPERIMENT 18. DISPERSAL BY WATER.—Place in a bucket of water a
hazelnut, an acorn, an orange, a cranberry, a pecan, a hickory nut,
a fresh apple, and a coconut with the husk on. Which are the best
floaters? Cut open or break open the good swimmers, compare with
the non-floaters, and see to what peculiarity of structure their
floating qualities are due. In what situations do the cranberry and
the coconut grow? Can you see any advantage to a plant so situated
in producing fruits that float easily?
EXPERIMENT 19. DISPERSAL BY EXPLOSIVE CAPSULES.—Moisten slightly
some mature but unopened capsules of witch hazel, wood sorrel,
rabbit pea, or violet, and leave in a warm, dry place for fifteen
to forty-five minutes. What happens when the pods begin to dry?
Measure the distance to which the different kinds of seeds have
been ejected. Which were thrown farthest? What was the object of
the movement? What caused the explosion?
EXPERIMENT 20. THE USE OF ADHESIVE FRUITS.—Scatter broadcast
a handful of hooked or prickly seeds or fruits—cocklebur,
tickseed, beggar-ticks, bur grass, etc. Are they suited for wind
transportation? Drop one of them on your sleeve, or on the coat
of a fellow student; will it stay there? What would be the effect
if it became attached to the fur of a roaming animal? Is this a
successful mode of dissemination?
[Illustration: FIGS. 30-32.—30, A pod of wild vetch, with mature
valves twisting spirally to discharge the seed; 31, pod of
crane’s-bill discharging its seed; 32, capsules of witch-hazel
exploding.]
[Illustration: FIGS. 33-36.—Fruits adapted to wind dispersal: 33,
winged pod of pennycress; 34, spikelet of broom sedge; 35, akene of
Canada thistle; 36, head of rolling spinifex grass.]
=19. Agencies of dispersal.=—The means at nature’s disposal for this
purpose, as shown by the experiments just made, are four; namely,
wind, water, the explosion of capsules due to the withdrawal of
water, and the agency of animals, including man. The first three
are purely mechanical. The last, animal agency, is either voluntary
or involuntary, according as it is conscious and intentional, or
accidental merely. Man, of course, is the only consciously voluntary
agent. Of the four agencies named, animals and wind are the most
effective, and the greater number of adaptations observed will be
found to have reference to these.
[Illustration: FIG. 37.—Good quality of clover seed.]
[Illustration: FIG. 38.—Inferior quality of clover seed mixed with
“screenings.”]
[Illustration: FIG. 39.—Dodder on red clover, showing how the seeds
get mixed.]
=20. Involuntary dispersal.=—The lower animals may be voluntary
agents in a way, though not designedly so, as when a squirrel buries
nuts for his own use and then forgets the location of his hoard and
leaves them to germinate; or when a jaybird flies off with a pecan
in his bill, intending to crack and eat it, but accidentally lets
it fall where it will sprout and take root. Both man and the lower
animals are not only involuntary, but often unwilling agents of
dispersal. Some of the most troublesome weeds of civilization have
been unwittingly distributed by man as he journeyed from place to
place, carrying, along with the seed for planting his crops, the
various weed seeds, or “screenings,” as these mixtures are called
by dealers, with which they have been adulterated either through
carelessness and ignorance, or from unavoidable causes. The neglected
animals, also, that are allowed by short-sighted farmers to wander
about with their hair full of cockleburs and other adhesive weed
pests, are no doubt very unwilling carriers of those disagreeable
burdens.
=21. Tempting the appetite.=—This is the most important adaptation
to dispersal by animals. Have you ever asked yourself how it could
profit a plant to tempt birds and beasts to devour its fruit, as so
many of the bright berries we find in the autumn woods seem to do? To
answer this question, examine the edible fruits of your neighborhood
and you will find that almost without exception the seeds are hard
and bony, and either too small to be destroyed by chewing, and
thus capable of passing uninjured through the digestive system of
an animal; or, if too large to be swallowed whole, compelling the
animal, by their hardness or disagreeable flavor, to reject them.
In cases where the seeds themselves are edible and attractive, the
fruits are usually armed during the growing season with protective
coverings, like the bur of the chestnut and the astringent hulls
of the hickory nut and walnut. The acidity or other disagreeable
qualities of most unripe fruits serves a similar purpose, while their
green color, by making them inconspicuous among the foliage leaves,
tends still further to insure them against molestation.
[Illustration: FIGS. 40-42.—Adhesive fruits: 40, fruit of
hound’s-tongue; 41, akene of bur marigold; 42, fruit of bur grass
(cenchrus).]
=22. Voluntary agency.=—The cultivated fruits and grains owe their
distribution and survival almost entirely to the voluntary agency
of man. Dispersal by this means, whether intentional or accidental,
is purely artificial, and except in the case of a few annuals like
horseweed, bitterweed, ragweed, goosefoot, and other field pests that
have adjusted their season of growth and flowering to the conditions
of cultivation, is not correlated with any special modification of
the plants for self-propagation. On the contrary, many of the most
widely distributed weeds of cultivation, such as the oxeye daisy, the
rib grass, mayweed and bitterweed, possess very imperfect natural
means of dispersal, and are largely dependent for their propagation
on the involuntary agency of man.
=23. Use of the fruit in dispersal.=—It will be seen from the
foregoing observations that the fruit plays a very important part in
the work of dispersal, most of the adaptations for this purpose being
connected with it. In cases where a number of seeds are contained in
a large pod that could not conveniently be blown about by the breeze,
adaptations for wind dispersal are attached to the individual seeds,
as in the willow, milkweed, trumpet creeper, and paulonia; but as a
general thing, adaptations of the seed are for protection, the work
of dispersal being provided for by the fruit. In the case of the
large class of plants known as “tumbleweeds,” the whole plant body is
fitted to assist in the work of transportation. Such plants generally
grow in light soils and either have very light root systems, or are
easily broken from their anchorage and left to drift about on the
ground. The spreading, bushy tops become very light after fruiting,
so as to be easily blown about by the wind, dropping their seeds
as they go, until they finally get stranded in ditches and fence
corners, where they often accumulate in great numbers during the
autumn and winter.
[Illustration: FIG. 43.—A fruiting plant of winged pigweed
(_Cycloloma_), showing the bunchy top and weak anchorage of a typical
tumbleweed.]
[Illustration: FIG. 44.—Panicle of “old witch grass,” a common
tumbleweed.]
=24. The advantages of dispersal.=—Seed cannot germinate unless
they are placed in a suitable location as to soil, moisture, and
temperature. In order to increase the chances of securing these
conditions, it is clearly to the advantage of a species that its
seeds should be dispersed as widely as possible, both that the
seedlings may have plenty of room, and that they may not have to draw
their nourishment from soil already exhausted by their parents. The
farmer recognizes this principle in the rotation of crops, because
he knows that successive growths of the same plant will soon exhaust
the soil of the substances required for its nutrition, while they may
leave it richer in nourishment for a different crop.
[Illustration: FIG. 45.—Self-planting pod of peanut.]
=25. Self-planting seeds.=—Dispersal is not the only problem the
seed has to meet. The majority of seeds cannot germinate well on
top of the ground, and must depend on various agencies for getting
under the soil. Some of them do this for themselves. The seeds of the
stork’s-bill, popularly known as “filarees,” have a sharp-pointed
base and an auger-shaped appendage at the apex, ending in a
projecting arm (the “clock” of the filaree) by which it is blown
about by the wind with a whirling motion till it strikes a soft
spot, when it begins at once to bore its way into the ground. The
common peanut is another example. The blossoms are borne under the
leaves, near the base of the stem, and as soon as the seeds begin to
form, the flower stalks lengthen several inches, carrying the young
pods down to the ground, where they bore into the soil and ripen
their seeds.
Practical Questions
1. Name the ten most troublesome weeds of your neighborhood.
2. What natural means of dispersal have they?
3. Which of them owe their propagation to man?
4. Are there any tumbleweeds in your neighborhood?
5. Would you expect to find such weeds in a hilly or a well-wooded
region? (19, 23; Exp. 17.)
6. What situations are best fitted for their propagation? (19, 23;
Exp. 17.)
7. Make a list of all the fruits and seeds you can think of that
are adapted to dispersal by wind; by water; by animals.
8. By what means of dissemination, or protection, or both, is each
of the following distinguished: the squash; apple; fig; pecan;
poppy; bean; beggar-tick; linden; grape; rice; pepper; olive;
cranberry; jimson weed; thistle; corn; wheat; oats?
9. What is the agent of dispersion, or what the danger to be
provided against, in each case?
10. Could our cultivated fruits and grains survive in their present
state without the agency of man? (22.)
11. Name all the plants you can think of that bear winged seeds and
fruits; are they, as a general thing, tall trees and shrubs, or low
herbs?
12. Name all you can think of that bear adhesive seeds and fruits;
are they tall trees or low herbs?
13. Give a reason for the difference. (Exps. 17, 20.)
14. Why is the dandelion one of the most widely distributed weeds
in the world? (19; Exp. 17.)
15. Is the wool that covers cotton seed for dispersal or protection?
16. What advantage to the Indian shot (canna) is the excessive
hardness of its seeds? (21.)
17. What is the use to the species, of the bitter taste of lemon
and orange seed? (21.)
18. Why are the seeds of dates and persimmons and haws so hard?
(21.)
19. Do you find any edible seeds without protection? If so, account
for the want of it. (21, 22.)
20. Name some of the agencies that may assist in covering seeds
with earth.
21. Do you know of any seeds that bury themselves?
22. The seeds of weeds and other refuse found mixed with grain
sold on the market are known, commercially, as “screenings.” Wheat
brought to mills in Detroit showed screenings that contained, among
other things, seeds of black bindweed, green foxtail grass, yellow
foxtail, chess, oats, ragweed, wild mustard, corn cockle, and
pigweed. Can you mention some of the ways in which these foreign
substances may have gotten into the crop and suggest means for
keeping them out?
Field Work
The subjects treated in the foregoing chapter are, in general,
better suited to laboratory than to field work. There are some
details, however, which can be observed to advantage out of doors.
Many of the seeds found in your walks will show peculiarities of
shape and external markings and color that will invite observation.
Examine also the contents of different kinds you may meet with, as
to the presence or absence of endosperm and the arrangement and
development of the embryo. Note: (1) whether, as a general thing,
there is any difference in size and weight and amount of nourishing
matter in the two kinds; (2) the greater variety in the shape and
arrangement of the cotyledons in the albuminous kind, and in the
arrangement of the embryo; (3) the differences in the development
of the plumule in the two kinds,—and give a reason for the facts
observed.
Among the different seeds you may find, look for adaptations for
dispersal, and decide to what particular method each is suited.
Study the agencies by which various kinds may get covered with
soil. If the common stork’s-bill (_Erodium cicutarium_) grows in
your neighborhood, its seeds will well repay a little study, and if
there is a field of peanuts within reach, do not fail to pay it a
visit.