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.
Typographical errors have been silently corrected.
AN INTRODUCTION TO NATURE STUDY.
[Illustration]
MACMILLAN AND CO., LIMITED
LONDON · BOMBAY · CALCUTTA
MELBOURNE
THE MACMILLAN COMPANY
NEW YORK · BOSTON · CHICAGO
ATLANTA · SAN FRANCISCO
THE MACMILLAN CO. OF CANADA, LTD.
TORONTO
AN INTRODUCTION
TO
NATURE-STUDY
BY
ERNEST STENHOUSE, B.SC. (LOND.)
ASSOCIATE OF THE ROYAL COLLEGE OF SCIENCE, LONDON; JOINT-AUTHOR
WITH A. T. SIMMONS, B.SC. OF “SCIENCE OF COMMON LIFE”
MACMILLAN AND CO., LIMITED
ST. MARTIN’S STREET, LONDON
1910
First Edition 1903.
Reprinted 1904 (twice), 1905, 1906, (with additions) 1908, 1910.
GLASGOW: PRINTED AT THE UNIVERSITY PRESS
BY ROBERT MACLEHOSE AND CO. LTD.
PREFACE.
One of the most encouraging of recent educational movements is the
increasing importance attached, both in this country and abroad,
to what is called Nature-Study. It is evident that the instruction
contemplated differs as widely, on the one hand, from the traditional
object-lessons on polar bears and ironclads, as it differs from formal
Biology on the other. This difference is abundantly shown, not only
by the circulars and syllabuses issued by our own Board of Education,
but by the publications of the leading educational authorities of
Europe and America. The aim of Nature-Study, as thus laid down, is not
primarily the acquisition of the facts of natural history: it is rather
a training in methods of open-eyed, close, and accurate observation,
especially of familiar animals and plants, which shall teach the
student to _see_ what he looks at, and to _think about_ what he sees.
It is in a spirit of entire agreement with these views that this book
has been written. No previous knowledge of Biology on the part of the
reader is assumed, and technical terms have as far as possible been
dispensed with. In drawing up the course, I have had in mind throughout
the attitude of an intelligent youth of sixteen, and the work will be
found to be well within the powers of such a student. Teachers will,
however, find no difficulty in adapting the exercises to the needs of
younger pupils.
Care has been taken to select, as types for study, animals and plants
which are at the same time representative and easily obtainable,[1] and
I have been further guided in the selection by the Board of Education
Syllabuses of the King’s Scholarship Examination and Section I. of
the Elementary Stage of General Biology, the subjects of which are
included in the volume. The book has, however, a considerably wider
scope than is indicated in these syllabuses, and will therefore, I
hope, be found useful not only in schools and training-colleges, and
to examination candidates, but also to members of field clubs and to
students of natural history generally. It has been necessary to arrange
the chapters with some attempt at logical sequence, but it is not
supposed that this order will be adhered to in practice; by the aid of
the monthly nature-calendar, together with numerous cross-references,
it will be found easy to take up the work at any point.
The chapters are divided into sections, each of which consists of
two parts: First, precise instructions for practical observations
and experiments, designed to exercise the reasoning faculties of the
students; and, second, a descriptive portion, in which the meaning and
relation of the results obtained are discussed. At the end of each
chapter is a number of additional exercises, either original or taken
from past examination papers. Of the latter class, questions to which
dates are affixed have been set by the Board of Education, while those
marked “N.F.U.” are selected from National Froebel Union tests. In
many cases, the exercises provide subjects for further observation and
experiment, as well as for written description.
Much trouble has been taken in the selection of the illustrations,
many of which have been expressly drawn or photographed for this book.
Through the kindness of the publishers I have been able to include
illustrations from Strasburger’s _Text-Book of Botany_, Parker and
Haswell’s _Text-Book of Zoology_, _The Cambridge Natural History_, and
other books; and Mr. Ernest Evans has courteously consented to the use
of a number of figures from his _Botany for Beginners_. The following
illustrations have been prepared from photographs supplied by Mr. J.
C. Shenstone, F.L.S., Vice-President of the Essex Field Club: Figs.
27, 57, 65, 67 to 71, 74, 75, 80, 81, 83, 84, 85, 87, 89, 92, 94, 95,
102 to 110, 120, 136, 145, 149, 152 and 153; while Figs. 180, 196,
200, 201, 203, 205, and 211 are reproduced, by permission, from Pike’s
_Woodland, Field, and Shore_ (Religious Tract Society).
Finally, I must acknowledge gratefully the continuous help which,
at every stage in the preparation of the book, I have received from
Professor R. A. Gregory and Mr. A. T. Simmons, B.Sc.—help as valuable
as it was generous.
The issue of a new edition has provided the opportunity of adding a
section on School Journeys, originally contributed by me as an article
to _The School World_, and reprinted here by kind permission of the
Editors of that journal. For the illustrative sketch-map (Fig. 237),
and for Figs. 19, 21 and 138, I am indebted to my friend Mr. T. D.
Tuton Hall.
E. STENHOUSE.
CONTENTS.
_PART I. PLANT LIFE._
CHAPTER PAGE
I. SEEDS AND THEIR EARLY STAGES OF GROWTH, 1
II. HOW A GREEN PLANT FEEDS, 26
III. THE FORMS AND DUTIES OF LEAVES, 37
IV. BUDS. THE HISTORY OF A TWIG, 55
V. HOW STEMS DO THEIR WORK, 67
VI. SOME COMMON FLOWERS, 88
VII. GRASSES, 125
VIII. COMMON FOREST TREES, 140
IX. FRUITS: HOW SEEDS ARE SCATTERED, 165
X. FERNS AND HORSETAILS, 183
XI. MOSSES, MUSHROOMS, AND MOULDS, 199
_PART II. ANIMAL LIFE._
XII. THE RABBIT: A TYPICAL MAMMAL, 211
XIII. HOW A RABBIT LIVES, 222
XIV. SOME OTHER MAMMALS, 246
XV. THE PIGEON: A TYPICAL BIRD, 265
XVI. THE DEVELOPMENT AND EDUCATION OF THE CHICK, 282
XVII. SOME FAMILIAR BRITISH BIRDS, 301
XVIII. FROGS AND TADPOLES, 332
XIX. THE HABITS AND LIFE-HISTORIES OF COMMON INSECTS, 349
XX. SOME CRUSTACEANS, MOLLUSCS, AND WORMS, 372
XXI. FIELD-WORK. THE SCHOOL JOURNEY, 388
MONTHLY NATURE CALENDAR, 400
_PART I._ PLANT LIFE
CHAPTER I. SEEDS AND THEIR EARLY STAGES OF GROWTH.
1. THE STRUCTURE OF SEEDS WITH TWO COTYLEDONS.
1. =Preparation of the seeds.=—Obtain several seeds of the broad bean,
pea, mustard, yellow lupine, vegetable marrow, and sycamore; soak
them in cold or slightly warm water until they are soft enough to be
cut through easily with a sharp knife. The time necessary will vary
with different seeds according to the size of the seeds, and with the
temperature of the water. The beans should be left in the water for a
few days. When the seeds are soft enough, examine one or two of each,
and in the meantime put about six of each (except the mustard) in damp
sawdust in a warm place. Put the mustard seeds on damp flannel in a
saucer.
2. =The outside of a broad bean.=—Notice the flattened oval shape, with
an indentation at one place (Fig. 1). What is the colour of the skin
(_seed-coat_) of the bean seed? Is all the skin of this colour? A black
_scar_ extends along the edge from the indentation for about ¾ in. What
is this scar? If beans in the pod can be obtained, see that the scar
is the place of attachment of the seed stalk. Make drawings to scale,
showing side and edge-views of the seed. Wipe the bean dry and then
squeeze it gently. Notice that a drop of water comes out at a point at
one end of the stalk scar. There is evidently a little hole here. This
little hole is called the _micropyle_. Mark its position by a dot on
the drawing.
3. =The inside of a broad bean.=—With a sharp knife cut the seed-coat
open, beginning at the side of the seed furthest from the micropyle,
and carefully remove the seed-coat. Notice that near the micropyle
the seed-coat forms a funnel-shaped depression, and that the point of
the funnel is at the micropyle. Does anything fit into the funnel?
A little cone may be seen to fill the funnel; this conical body is
called the _radicle_. Make a drawing of the seed after the removal of
the seed-coat. Look at the edge opposite the radicle and notice that a
crack divides the body of the seed into halves. Put the point of your
knife blade into the crack, and gently force the halves apart. They
come apart without tearing, showing that they are naturally separate,
although they fit so closely together.
These two swollen bodies are called the _cotyledons_. Separate them
and see, at the point where they join the radicle, a little curved
rod, evidently a continuation of the radicle, lying between them. This
rod is the _plumule_. Take off one cotyledon, and make a drawing of
the inner face of the other cotyledon, with the adhering plumule and
radicle (Fig. 2).
4. =Starch present in the cotyledons of the bean.=—Scrape the
inner surface of a cotyledon and then pour on it a drop of iodine
solution.[2] Is there any change? Pour also a drop of iodine solution
on a piece of laundry-starch. Is a similar blue colour formed? What
substance is probably present in the cotyledons of the bean?
5. =The pea.=—Examine a pea in a similar manner. Make drawings showing
the _stalk-scar_, the _micropyle_, and the _plumule_ and _radicle_ with
their manner of connection with the _cotyledons_. Does the end of the
radicle point towards the micropyle? How many cotyledons has this seed?
What shape and colour are they? Do they contain starch?
6. =The seed of the yellow lupine.=—Compare this with the bean and the
pea, and find out how many cotyledons it has, and whether they contain
starch. Can you find the plumule? It is very small, but occupies a
position similar to that of the plumule of the bean. Does the end of
the radicle point to the micropyle?
7. =The vegetable marrow seed.=—Notice the peculiar shape (somewhat
like a pocket-flask) of the seed, and the thickened margin which runs
round it. Carefully cut the seed-coat away so as not to injure the part
inside. How many cotyledons are present? What is their colour? Do they
contain starch? Can you see the plumule and radicle clearly? If not, do
not decide that they are absent, but leave the question to be settled
later, when you watch a vegetable marrow seed “come up.”
8. =The mustard seed.=—Notice how much smaller this seed is than the
others. With a balance, find how many mustard seeds are equal in weight
to one bean seed. Observe the stickiness of the seed-coat of the soaked
seed, and then remove it carefully with needles, exposing two thin
plates, each one folded on itself, and one tucked inside the other,
like two sheets of note-paper. These are the cotyledons; it seems that
the smallness of the seed may be mainly due to the small size of the
cotyledons. What is their colour? Remember these characters and try,
when you watch the young plants come up later, to find an explanation
of them.
9. =The sycamore fruit.=—The seed of the sycamore is enclosed in a case
which has a wing attached to it. The wing, the case, and the enclosed
seed together constitute the _fruit_ of the sycamore. The fruits occur
in pairs (Fig. 137). Notice that a cord runs out to each fruit from the
stalk on which the pair of fruits is borne. Make a drawing of a pair of
fruits, then separate the fruits.
10. =The sycamore seed.=—Cut open a fruit. Can you see anything between
the seed and the fruit-case? Would the hairy covering of the seed tend
to keep it warm during the winter? Why? Why do you prefer to wear
flannel in winter and linen in summer? Flannel is more fluffy than
linen.
Remove the seed-coat carefully. Running down one side you will see
a little curved rod. This is the radicle. Gently raise it with the
point of your knife. Notice that the rest of the seed seems to consist
of a green part, which is curled up. Uncoil the curls carefully. You
find that they are two green leaves, fixed at the top of the radicle.
These are the cotyledons. In the seed each cotyledon is first folded
in two across the middle and then coiled up. Make a sketch showing the
coils (Fig. 4). Can you see the plumule? It is just at the top of the
radicle, where the cotyledons are fixed on.
=Plants are living things.=—One of our foremost naturalists[3] tells
us that when he goes out into the woods, or into one of those fairy
forests which we call fields, he finds himself welcomed by a glad
company of friends, everyone with something interesting to tell. Such a
feeling would be quite impossible to one who did not vividly recognise
the fact that plants are alive; for it is precisely this recognition
or its absence which makes the observation of the forms and habits of
plants fascinating or the reverse. Let the Nature-Student, then, at
the outset of his work, keep the idea of =life= inseparably bound up
with his every thought about plants. It may at first require a little
effort, but before long it will enable him to understand how the
friendship of the more silent half of animate nature may form one of
the great pleasures of life.
=The study of seeds.=—The manifestation of life is so striking, and the
changes in form and size take place so rapidly, in the germination of
seeds, that the study of plants cannot better be commenced than with
this stage of their growth. The method has also the logical virtue of
beginning at the beginning, or nearly so.
These early changes can be well observed by taking various common
seeds, soaking them in water until they are soft, and then allowing
them to germinate in damp sawdust, taking a few out at intervals
and noting their progress. The growth of the seeds takes place more
rapidly if they are kept in a warm room, but in any case some days will
probably elapse before much change is noticeable in them.
During the interval of waiting, some of the seeds themselves should
be carefully examined, and =drawings= of all the parts should be
made. The drawing ought on no account to be omitted. It compels the
student’s attention to details which would otherwise pass unnoticed;
and a careful sketch is a much better record of an observation than
any amount of description alone could be. The drawing need not be
elaborate; an outline pencil-sketch to scale will usually be sufficient.
[Illustration: FIG. 1.—A Broad-Bean seed. _A_, side view; _B_, edge
view: _st. sc._, stalk-scar; _m_, micropyle. (× ⅔.)]
=The seed of the broad bean.=—The seed of the broad bean (Fig. 1) is
large, having a diameter of perhaps an inch and a half, and a thickness
of half an inch. In shape it is oval, but at one region the edge is
indented, and a black scar (_st. sc._) runs from the indentation along
the edge for a distance of about three-quarters of an inch. This scar
is the place of attachment of the stalk which formerly carried the
seed in the bean-fruit (pod). It may be called the =stalk-scar=. If
a soaked bean is wiped dry and then gently squeezed, a small drop of
water escapes from the end of the stalk-scar nearest the indentation.
The hole out of which the water comes is very small and difficult to
see, but its position is thus made clear. This hole (_m_) is called the
=micropyle=,—a word meaning the “little gate.”
The bean seed is covered by a tough brown skin, the =seed-coat= (Fig.
2, _s.c._), a funnel-shaped depression in which leads to the micropyle
(_m_). The depression is occupied by a part of the seed which is shaped
like a conical peg and called the =radicle= (_R_); the point of the
radicle is directed toward the micropyle. The great body of the seed
is composed of two fleshy, cream-coloured lobes, easily wedged apart
by inserting a knife-blade between them; these fleshy lobes are the
=cotyledons= (_Cot._). Between them, and continuous with the radicle,
is a small yellow body, the =plumule= (_pl._). The relations of the
radicle, plumule and cotyledons are best seen by removing one cotyledon
(Fig. 2).
[Illustration: FIG. 2.—Broad-Bean seed, seen from the inside, after the
removal of half the seed-coat and one cotyledon. _Cot._, the inner face
of remaining cotyledon; _C′_, area of attachment of other cotyledon;
_m_, micropyle; _pl_, plumule; _R_, radicle; _S.c._, seed-coat; _st.
sc._, stalk-scar. (× 1.)]
A scraped cotyledon at once turns blue when a drop of dilute iodine
solution is poured on it, thus showing the presence of =starch=. We
shall see in Chapter II. what use the growing seedling makes of the
starchy food which is stored in its cotyledons.
=The seed of the pea.=—Except in size and shape the seed of the pea
is very similar to the bean seed. Its form is spherical, and the
scar left by the stalk which formerly attached it to the wall of the
pea-pod (Fig. 3) is plainly to be seen. Pointing towards the micropyle
is the peg-like radicle; the plumule lies between the hemispherical
cotyledons. As before, the cotyledons can be proved to contain starch,
by the blue colour which is formed when a drop of iodine solution is
poured on the scraped surface.
[Illustration: FIG. 3.—Pods and Seeds of Pea. (× ½.)]
=The seed of the yellow lupine.=—The seed of the yellow lupine is about
as large as a pea, but it is slightly flattened in shape. The seed-coat
is prettily mottled; when it is removed, the greater part of the seed
is found to consist of two cotyledons. They are somewhat swollen, but
the stored food is not starch. The plumule and radicle occupy positions
similar to those of the bean and pea.
=The vegetable marrow seed.=—This seed has a rather curious shape, and
somewhat resembles a pocket-flask. It is flattened, and the border
of the seed-coat is thickened and of silky appearance, the rest of
the “skin” having some resemblance to kid. The two cotyledons, which
compose the greater part of the seed, are white and only slightly
fleshy. The plumule and radicle are at the pointed end of the seed, and
are difficult to see.
=The mustard seed.=—In comparing the mustard seed with those already
described, one is struck with the great difference in size. An average
broad-bean seed weighs about 600 times as much as the mustard seed.
While the two fleshy cotyledons make up the bulk of the seed of the
bean, pea, lupine and vegetable marrow, the cotyledons of the mustard
seed are thin and leaf-like. They are folded on themselves, one inside
the other (as at _g_, Fig. 61), and enclose the radicle. The characters
of the cotyledons account very largely for the small size of the
mustard seed. It will be seen, when the growth of the young plants is
watched, that the difference is associated with the special duties
which the cotyledons perform in the various cases.
[Illustration: FIG. 4.—Sycamore Fruit, cut through in the plane of the
wing. _s.c._, seed-coat (indicated by a thick, broken line); _f.w._,
fruit-wall; _h_, layer of fine hairs; _R_, radicle; _pl._, plumule;
_cot. 1_, _cot. 2_, cotyledons (diagrammatically shaded). (× 2.)]
=The sycamore seed.=—What is generally called the seed of the sycamore
is really a =fruit=. The fruits are in pairs (Figs. 33 and 137), and
each half consists of a flat wing and a rounded case in which the
seed itself is enclosed. The round seed-cases of the two fruits are
connected together. When they come apart, a scar marks the place where
they were formerly in contact, and a little cord runs out to each fruit
from the stalk on which the pair of fruits is borne.
Between the sycamore seed and the wall of its case is a layer of fine
hair (_h_, Fig. 4), which forms a warm nest for the seed in winter. The
seed is surrounded by a thin, brown seed-coat, and consists mainly of
two cotyledons, but these are very different from any yet described.
Each is a green leaf, measuring, when unfolded, about an inch in
length. It is first folded across the middle of its length, and then
rolled up into a close coil with its fellow. The coils are very plainly
to be seen when the seed coat is removed, or when the whole seed is
cut through, by a sharp knife, in the plane of the wing. Running down
one side of the seed is a green rod, the radicle (_R_, Fig. 4). The
two cotyledons (_cot. 1_ and _cot. 2_) spring from its upper end, and
between them is the tiny plumule (_pl._)
The sycamore seed bears more resemblance to the mustard seed than to
the others, but it is on a much larger scale. In each of these two
seeds the cotyledons are plainly leaves, while in the others their
nature is disguised by the great accumulation of stored food in them.
2. THE EARLY STAGES OF GROWTH OF SEEDS WITH TWO COTYLEDONS.
If the seeds which were sown in damp sawdust and on flannel are kept
warm they will soon be ready for study. You should remember that at
present your object is not so much to rear the plants as to find out
_how_ they grow. As soon, therefore, as any sign of growth is to be
seen when you take a seed out, you should begin to examine them at
regular intervals, taking one or two out every day and leaving the rest
to continue their development. Keep the sawdust damp, but not wet.
1. =The pea and bean.=—(_a_) _General development._—Very soon the
seed-coat splits at the micropyle-end of the stalk-scar, and the end
of the radicle protrudes. Does the radicle grow upwards or downwards?
Observe that even if the seed was so planted that the micropyle was at
the top, the radicle turns over and grows straight down. Turn over a
seedling and see if you can persuade the radicle to grow upwards. Open
a seed when the radicle is about an inch long, and see what the plumule
is doing. It is still enclosed in the seed-coat, and lies between the
cotyledons, but is larger than at first. As the growth proceeds the
cotyledons begin to separate near the top of the radicle, and you can
get a glimpse of the plumule.
(_b_) _The growth of the radicle._—Day by day the radicle becomes
longer. Is it _all_ growing longer, or does the increase in length take
place more at one part than another? To answer this question, take
five or six inches of cotton thread and moisten the middle part with
Indian ink. Lay the seed on a flat ruler, so that the radicle lies
over an inch divided into—say—tenths. Hold the thread tight, and press
the inked part gently on the radicle, making about five marks at equal
intervals from the point upwards. The ink will dry almost immediately.
Then carefully replant the seed, taking care not to injure the radicle.
After a few days take it out again, lay it once more on the ruler, and
measure the distance between the marks.
_The radicle is evidently the young root._
(_c_) _The root-cap._—Hold up the radicle to the light, and examine its
tip with a lens. Try to see that the tip is covered by a little cap,
somewhat like a very small thimble. This is called the _root-cap_.
(_d_) _The root-hairs._—Hold a seed, with the radicle about an inch
long, against a dark surface. Is the surface of the radicle smooth, or
can you see any fluffiness on it? Is all the radicle fluffy, or only a
part? Which part? As you examine older and older seedlings notice how
much of the radicle is fluffy, and where the fluffy part is. The fluffy
appearance is caused by fine, closely-set hairs, called _root-hairs_.
(_e_) _The plumule._—How soon after planting does the plumule become
free? Does it grow upwards or downwards? _The plumule is evidently the
young stem._
As soon as the young stem is old enough mark it with Indian ink as
you marked the young root, and replant it to find if there is any
difference in the rates of growth of its different parts.
(_f_) _The fate of the cotyledons._—From time to time examine the
cotyledons and notice that as the seedling grows larger they become
more and more shrunken. Something is evidently being taken from them,
perhaps to feed the young plant. We shall inquire into this by further
experiment (Chapter II.).
Do the cotyledons remain in their original position, or are they
carried upwards with the growing stem or downwards with the growing
root?
2. =The yellow lupine.=—In the same way observe how this seed grows. Do
the cotyledons shift their position or change in colour? Do they become
leaf-like? How do they differ from later-formed leaves? What becomes of
them at last? What becomes of the seed-coat?
3. =The mustard seed.=—Notice that, soon after the radicle has come out
of the seed-coat, a sort of hump forms at its upper end, and at length
the cotyledons are pulled out of the seed-coat and turn up towards the
light. What is their colour? Observe that the two cotyledons are soon
raised on the end of a little stalk. Like the cotyledons of the yellow
lupine they are plainly leaves. Notice their shape. Are they of equal
size? Why not? When they are about three inches above the seed-coat
gently separate them and notice the little bud between them. Draw the
seedling. How large can you get a mustard seedling to grow on damp
flannel? Plant a few mustard seeds on earth, and notice the difference
between the shape of the cotyledons or seed-leaves and that of the
leaves which appear later. What becomes of the cotyledons?
4. =The vegetable marrow seed.=—Make similar observations upon the
vegetable marrow seeds, noticing particularly whether the cotyledons
remain in their original positions and shrink up as the plant increases
in size, or whether they are pulled out of the seed-coat by the
elongating stem, and become green and leafy. How does the plant hold
down its seed-coat whilst it pulls out its cotyledons?
5. =The sycamore seed.=—From what you have seen of the cotyledons of
the sycamore seed, will you expect them to behave like those of the
mustard seed, or like those of the pea and bean? Even in the seed they
are green, and plainly leaves. How do they escape from the seed-coat?
What is their shape? Do they come out before or after the radicle? Do
they get any larger as the stem grows? How large can you get a sycamore
seedling to grow in damp sawdust? As large as a seedling of pea or
bean? Plant some sycamore seeds on earth and compare the shape of the
cotyledons with that of the next-formed leaves. How soon do the “true”
leaves appear after the cotyledons have escaped from the seed? Do
any “true” leaves grow on the plants in sawdust? What becomes of the
cotyledons at last?
=The embryo.=—The plumule, radicle, and cotyledons, which have now been
seen in the seed, form the =embryo= of the plant. The adult plant will
be wholly formed by the growth and development of these parts, and we
must now follow carefully the changes which take place when the seed
germinates, and try to find out what becomes of each part. It is better
to put the seeds at first in damp sawdust rather than in earth, as the
young roots can then be more readily cleaned and observed. With small
seeds the early stages of growth are better seen if damp flannel is
used.
=Germination.=—Under the influence of moisture and warmth the embryo
in the seed begins to swell and unfold its parts. The radicle makes
its appearance first (Fig. 5), breaking through the seed-coat at the
micropyle; it is the young root. The radicle always grows downwards,
that is, toward the centre of the earth. If the seed lies in such a
position that the micropyle is directed upwards, the point of the
radicle turns over and grows downwards as soon as it escapes from the
seed-coat. As the young root becomes longer and thicker (Fig. 6) the
seed-coat opens more and more, showing the cotyledons beneath, and
these, too, are gradually forced apart.
[Illustration: FIG. 5.—An early stage in the germination of a
Broad-Bean seed. _R_, radicle; _s.c._, seed-coat. (× ⅔.)
FIG. 6.—A slightly later stage in the germination of a Broad-Bean seed.
_cot._, cotyledon; _pl_, plumule; _R_, radicle; _s.c._, seed-coat. (×
⅔.)]
[Illustration: FIG. 7.—Mustard Seedling, showing root-hairs and
cotyledons. (× ½.)]
=The cotyledons.=—During the germination of various seeds, a very
marked difference in the behaviour of the cotyledons is to be seen.
In the case of the =broad bean and pea= the cotyledons remain in
their original positions, partially enclosed by the split seed-coat.
Presently a hump (Figs. 6 and 11) forms at the upper end of the
radicle, as if the plant were making an effort to pull its plumule out
of the seed. It soon succeeds (Fig. 12), and the plumule turns up to
the light. It is the young stem. At its end is a little bud, formed by
a number of small, overlapping, green leaves which surround the growing
point. Henceforth the stem grows upwards, that is in a direction
precisely opposite to that of the root’s growth. Both stem and root are
attached to the cotyledons, which gradually shrivel up as the stem and
root become larger and larger.
When, however, the seed of the =mustard=, or =sycamore=, germinates the
cotyledons behave very differently (Figs. 7 and 8). Soon after the
root has become well established the cotyledons come quite out of the
seed-coat and unfold themselves. Instead of remaining on or under the
surface of the ground they are carried upwards at the end of a stalk
toward the light, and for some time the little plant appears to consist
of root, stalk, and cotyledons only. If, however, the cotyledons are
gently pressed apart, a tiny bud is seen between them. This evidently
corresponds to the bud at the end of the stem of the bean or pea.
In the case of the =lupine= (Fig. 9) or =vegetable marrow= (Fig. 10)
the cotyledons appear to combine these two conditions. They are swollen
and contain stored food; yet they come out of the seed-coat early,
become green, and open out to the light. They are evidently leaves,
though their shape differs from that of the later leaves.
[Illustration: FIG. 8.—Three stages in the growth of a Sycamore
Seedling. _cot._, cotyledons; _fol._, first pair of foliage leaves.
(Slightly reduced.)]
The germinating vegetable-marrow seed possesses a curious contrivance
for pulling its cotyledons out of the seed-coat. This is a peg (_p_,
Fig. 10) which develops at the top of the radicle, and holds down the
lower half of the seed-coat whilst the other half is forced upwards to
allow the cotyledons to be withdrawn.
[Illustration: FIG. 9.—Three stages in the growth of the Yellow
Lupine. On the right the cotyledons are still enclosed in the mottled
seed-coat. In the middle plant the cotyledons are spreading out; the
first foliage leaves have not yet unfolded. On the left, the first two
foliage leaves are unfolding, and the cotyledons have spread out flat.
(Slightly reduced.)]
After a little thought a possible explanation of these differences
in the cotyledons suggests itself. It may be that, in the case of
the mustard and sycamore, leaves are required as early as possible,
while the bean and pea have no immediate need for leaves because their
cotyledons contain so much stored food. The cotyledons of these plants
shrivel up as the seedling grows, and this seems to indicate that
during its early stages the plant lives upon this food. In Chapter II.
we shall make experiments to see if this explanation is the true one.
If so, the lupine and vegetable-marrow seeds evidently rely partly upon
their stored food and partly upon setting the cotyledons to work as
leaves, whilst the plant is still very young.
[Illustration: FIG. 10.—Germinating Vegetable Marrow seed. _p_, the peg
by which the seed-coat (_s.c._) is held down to allow the cotyledons
(_cot._) to be withdrawn. (× 1.) (After Bailey.)]
[Illustration: FIG. 11.—A germinating Pea; _cot_, cotyledon; _pl_,
plumule; _R_, radicle; _r.h._, root-hairs; _S.c._, seed-coat. The
radicle has been marked with Indian ink at intervals of ¹/₁₀”.]
=The true leaves.=—The cotyledons are really =makeshift leaves=, which
are already formed in the seeds. Even when they expand and become green
they do not live long, but as soon as the next few leaves are well
established, shrivel up and wither. The true or “foliage” leaves first
make their appearance as a bud which surrounds the growing point of the
stem. As this part of the stem increases in length, the foliage leaves
become separated from each other and spread out to the light and air.
=The lengthening of the stem and root.=—Unless an experiment to test
the truth of the matter is really made, it might be supposed that the
different parts of the stem and root of the seedling grow in length
at the same rate. This can be tested by marking the stem and root
with lines of Indian ink at equal distances. In one experiment with
a pea seedling five lines were marked upon the young root at regular
intervals of one-tenth of an inch, beginning at the tip (Fig. 11).
The seedling was carefully replanted and examined again a few days
later. Between the tip and the first mark there was then (Fig. 12) a
distance of seven-tenths of an inch; that is, this part had grown to
seven times its former length. The second interval was four times as
long as before, the third was one and a half times as long, while
the fourth and fifth intervals had not increased in length at all.
Such experiments prove that the root grows in length either at or
just behind the tip. When a young stem is treated in the same way the
lengthening is found to take place more evenly.
[Illustration: FIG. 12.—The Pea seedling of Fig. 11, a few days later.
_cot_, cotyledons; _pl_, plumule; _R_, radicle; _S.c._, seed-coat. (×
1.)]
[Illustration: FIG. 13.—The tip of a root, showing the root-cap.
(Magnified.)]
=Rootlets.=—After a time the radicle begins to put out branches called
=rootlets=. These come off the main root in rows. In some cases
rootlets make their appearance whilst the radicle is still very short,
as in the vegetable marrow of Fig. 10, but in others the radicle may be
a few inches long before it produces rootlets.
=The root cap.=—The tip of the root, and of each of its branches, is
covered by a little cap, shaped somewhat like a thimble (Fig. 13).
This protects the tender growing point from the friction of particles
of soil, and is continually renewed by growth from within as its outer
layers are worn away.
=Root hairs.=—When a young root is held against a dark background it
appears fluffy. This appearance is caused by a large number of very
fine hairs upon its surface. The hairs are not found all over the
root and its branches, but only for a short distance a little way
behind the tips (Figs. 7 and 11). These root hairs are of very great
importance to the plant, as will be seen in Chapter II.
3. THE STRUCTURE OF GRAINS OF MAIZE AND WHEAT.
1. =Preparation of the seeds.=—Soak grains of maize (Indian corn) and
wheat in water until they are soft. The grains of maize will need
soaking for several days. Plant about a dozen of each in damp sawdust,
and in the meantime examine others.
2. =The maize grain.=—A grain of maize is really a fruit, as a pea-pod
is. A pea-pod contains several seeds; a maize fruit contains only one
seed, which fills it. Notice the shape of the grain—flattened, rounded
along one edge, and bluntly pointed at the opposite edge (Fig. 14).
Notice a whitish patch on one of the flattened sides; a ridge (_E_)
down the middle of this marks the position of the _embryo_. Cut through
the grain lengthwise, so as to divide the _embryo_ into two equal
parts, and examine the cut surface (Fig. 15). Identify:
(_a_) The _embryo_, lying somewhat obliquely, and to one side. The
radicle (_rad._) is directed towards the pointed end of the grain, and
the plumule (_pl._) towards the rounded end.
(_b_) The _endosperm_ (_end_): a mass of material outside the embryo,
and forming at least half of the grain.
(_c_) The _scutellum_ (_scm_): a plate lying between the endosperm and
the embryo.
(_d_) The _coats_ of the seed and fruit, surrounding the whole.
Draw. Add a drop of iodine solution to the cut surface of the
half-grain. The endosperm turns blue. What does this indicate?
3. =The wheat grain.=—A grain of wheat is also a one-seeded fruit.
Notice the groove along one side, and—on the opposite side near one
end—the white patch marking the position of the embryo. At the other
end is a tuft of very fine hairs. Cut the grain lengthwise, so as to
divide the white patch into two equal parts, and make out the embryo,
endosperm, and scutellum (Fig. 16). Draw. Test the endosperm with
iodine solution. Does it contain starch?
=Grains of maize and wheat.=—A grain of maize or wheat is really a
one-seeded fruit. In other words, the grain consists not only of the
seed with its seed-coat, but also of the seed-case. In this respect it
resembles the fruit of the sycamore (p. 8). When, however, a grain of
maize or wheat is carefully examined, it is found to differ greatly
from all the seeds hitherto mentioned. A =maize grain= is somewhat
flattened, and rather pointed along one edge (Fig. 14). On one flat
side, near the pointed end, may be seen a whitish patch, and, along the
middle line of this, a ridge which marks the position of the _embryo_.
[Illustration: FIG. 14.—Maize grain, showing the position of the embryo
(_E_). (× 4.)]
[Illustration: FIG. 15.—A longitudinal section of a Maize grain,
through the middle line of the embryo. _end_, endosperm; _pl_, plumule;
_rad_, radicle; _rt_, origin of a root; _scm_, scutellum. (× 4.)]
A =wheat grain= has a different shape. It is oval, with a deep groove
running down one side. One end is clothed with a tuft of very fine
hairs; near the other end, on the side opposite to the groove, is a
white patch beneath which is the embryo.
These differences in shape are of small importance, for the two grains
are really of very similar structure, as may be seen when they are cut
through lengthwise with a sharp knife, in a direction which divides
the embryo along its middle line. It is then plain that each grain
consists of two principal parts: the embryo and the endosperm (Figs.
15 and 16). The =embryo= lies to one side and in the lower half of the
seed. At its upper end the young stem and at its lower end the young
root—each still enclosed in protecting sheaths—are easily seen. The
greater part of the seed is quite outside the embryo; it is a mass of
food called the =endosperm=, which has been stored up for the use of
the young plant during its earliest stages of germination. This food
mass at once turns blue when a drop of iodine solution is placed upon
it, showing that it contains a considerable amount of starch. It is the
endosperm which is made into flour when corn is ground.
Lying between the embryo and endosperm is a flat plate called the
=scutellum=. Covering embryo, scutellum, and endosperm is the seed-coat
proper, and outside that come the various layers of the fruit case.
[Illustration: FIG. 16.—Diagram of a longitudinal section through the
middle line of a wheat grain.]
In the seeds previously examined, the embryo—consisting of plumule,
radicle, and cotyledons—was seen to fill the seed-coat completely. In
some cases the cotyledons were found to be more or less swollen with
stored food-material. In the maize and wheat, however, the embryo forms
only a comparatively small proportion of the seed, the bulk of which
consists of stored food called endosperm. This is a difference of some
importance. Still more important, however, is the fact that the two
cotyledons, which were so conspicuous a feature of the other seeds,
cannot apparently be seen at all in these seeds. Are there then no
cotyledons in the seeds of the maize and wheat? If there are, how and
when do they appear, and what is their number?
4. THE EARLY STAGES OF GROWTH OF MAIZE AND WHEAT.
1. =The roots.=—Watch for the appearance of the roots. Is there, as in
the seedlings previously studied, one principal root, or are there soon
several, all apparently of equal or nearly equal importance? Do the
roots grow straight down as before, or do they spread horizontally?
2. =The stem and the cotyledon.=—Notice that a rod, somewhat thicker
than a root, grows out near the origin of the roots, and curves upwards
towards the light. When this is about an inch long on a maize seedling,
slit it open carefully, and observe that it consists of a pale outer
sheath and a green core. The sheath is the single _cotyledon_; the
green core is the young _stem_ enclosed in a young _foliage leaf_. Cut
open the grain and notice how the endosperm has shrivelled. As the
seedlings become larger watch the young stem growing out at the end of
its sheathing cotyledon. What is the length of the cotyledon when the
stem first appears (_a_) in the maize, (_b_) in the wheat?
3. =The foliage leaves.=—As soon as a foliage leaf unfolds make a
drawing of its shape. Contrast it with the young foliage leaves of
the other seedlings. Hold up the leaves to the light and compare the
arrangement of their veins.
=How maize and wheat seeds grow.=—When the maize and wheat fruits
have been kept in damp sawdust for a few days, the seeds—one in each
fruit—begin to germinate. As a rule the wheat plants have grown to a
height of some inches before the roots and stems of the maize plant
have emerged from the seeds.
=The roots.=—As might be expected, the first signs of life make their
appearance at the white scar which indicates the position of the embryo
or young plant. Instead of one main root growing out, several little
roots make their appearance almost at the same time. They do not grow
as directly downwards as the radicle of a pea or bean (Fig. 6), but
tend to spread in a horizontal direction (Fig. 17). It is clear that in
this way the roots are more independent of each other than if they grew
directly downward side by side.
=The cotyledon and the stem.=—Growing out from the seed close to the
roots is another rod (Fig. 17, _C_), rather thicker than the roots,
which at once curves upwards to the light. It is pale green in colour.
This is the =cotyledon= or first leaf. When it is carefully slit open,
it is found to be a hollow sheath, enclosing a bright green core. In
the seedlings which are left undisturbed, the core at last breaks
through the tip of the cotyledon. It consists of the young =stem= and
its surrounding foliage leaves. As the growth of the plant continues,
these sheathing leaves unfold themselves into the long narrow blades
characteristic of grass leaves (Fig. 98). The bottom of each leaf is
tubular and forms a sheath round the stem.
[Illustration: FIG. 17.—Young wheat seedling. _C_, cotyledon; _r_, _r_,
_r_, _r_, roots. (× 3.)]
=The endosperm.=—The endosperm, which at first made up more than
half the seed, gradually shrivels up as the little plant continues
its growth. The food material which it contains is absorbed by
the scutellum and is passed on to afford the plant the necessary
nourishment for those early stages when it is too young to feed itself.
By the time the first few foliage leaves are well developed, all that
remains of the grain is an empty husk.
=Comparisons and contrasts.=—The examination of these seeds and
seedlings will enable the student to see that differences, which at
the first glance appear great, are often of only minor importance;
while apparently small variations may prove, on closer inspection,
to be caused by deeply-seated differences of structure and habits of
life. He should always set himself the questions, “In what ways do such
and such objects resemble each other; and in what ways do they differ
from each other? Which of the differences and resemblances are of most
importance?” He should also notice that a mere difference of size is
often of very small consequence.
Above all, the student should get into the habit of asking the
=reasons= for the differences and resemblances which he notices in his
nature-study. To learn what these reasons are he must observe closely,
think carefully, and then make experiments to test the accuracy of his
conclusions. “Be sure you are right; then look again”[4] should be his
motto.
It is at once plain that the seedlings fall into two classes, according
to the number of cotyledons or seed-leaves which they possess. The
wheat and maize have only one such seed-leaf, while the mustard,
bean, pea, sycamore, and vegetable marrow have two each. We shall
see later on that one-seed-leaved plants differ from those with two
seed-leaves not only in the number of their cotyledons, but also in the
characters of their leaves and flowers and in their method of growth.
These differences are so constant and so important that botanists have
agreed to call all plants of the first class (such as maize and wheat)
=Monocotyledons=, and plants of the second class =Dicotyledons=.
One of these differences is that the main roots of dicotyledons
are formed directly by the growth of their radicles; while in
monocotyledons there is, after a short time, no such main root to be
found, but several roots of almost equal size spring from the base of
the stem and spread outwards in all directions (Fig. 17).
Both maize and wheat seeds contain—_outside_ the embryo—a large store
of food called endosperm (Figs. 15 and 16), which is not seen in any
of the dicotyledonous seeds described in this chapter. This is not a
very important difference, for, if we examined a very large number
of dicotyledon seeds, we should find that most of them possessed
endosperm. On the other hand, many monocotyledonous seeds are destitute
of endosperm. Only after observing a very large number of facts is it
safe to make general statements.
Confining our attention to dicotyledons, we are impressed by the great
variation in size of the cotyledons. Those of the bean and pea are
swollen with food material and form a large proportion of the bulk of
the seed. As a consequence, the seedling has enough food to enable it
to grow into quite a sturdy little plant before it needs any foliage
leaves. The cotyledons of the mustard and sycamore, however, are thin,
and they unfold almost immediately into green leaves, and set to work
to help to maintain the plant until the first foliage leaves can be
formed. The cotyledons of the lupine (Fig. 9) and vegetable marrow
(Fig. 10) serve a double purpose. They not only contain a store of food
ready to hand, but they also set to work early to make new food, until
the new leaves are sufficiently advanced to take up their duties. It
should be remembered that cotyledons are makeshift leaves.
EXERCISES ON CHAPTER I.
1. Make a collection of the seeds of various trees; try to
find, in each seed, the cotyledons, radicle, and plumule.
Which of the seeds contain stored starch?
2. Soak pine and larch seeds in water for several days
and then sow them, with a covering of half an inch of soil.
Make notes of the number, shape, size, and behaviour of the
cotyledons. How large are the seedlings at the end of the
first season?
3. Make similar observations on the growth of sycamore,
ash, and beech. Cover the seeds with an inch of soil.
4. Plant seeds of oak and chestnut two inches deep, and
make drawings and notes of the stages of growth.
5. Investigate the structure and method of germination of
a barley seed, and find out whether barley is a dicotyledon
or a monocotyledon.
6. Make experiments to discover the effects, upon the
germination of various seeds, of differences of temperature,
moisture, and light, and write full accounts of the results
obtained.
7. Draw from memory a young seedling of maize, and notice
its chief peculiarities. (1898)
8. Draw the seedling of the sycamore in two or more
stages, and add short notes. (1898)
9. Draw the root of any seedling that you have studied,
giving its name. Mark the exact position of the root-hairs.
(1898)
10. Open the nut provided. Draw what is to be found in
it in one or two positions. Name the parts and give short
explanations. (1901)
11. Explain, with drawings, how certain seedlings withdraw
their seed-leaves from the seed-coat. (1901)
12. Describe and explain as far as you can the principal
changes to be observed during the germination of a bean or
pea. (1901)
13. Describe the germination of a bean, and compare it
with that of a grain of wheat. (1898)
14. Describe the structure of a grain of wheat, and
contrast it with that of an acorn. (1896)
15. Plant seeds in wet, sticky soil (so that the air
cannot easily get to them), and compare their growth with
that of similar seeds in a light, open soil.
16. Two acorns are allowed to germinate, one in the neck
of a bottle full of water, and the other in an ordinary
flower pot. What differences will be noted in the two plants
as they grow? (Certificate, 1904)
FOOTNOTES:
[1] If any difficulty is found in procuring plant-specimens, they
may be obtained by post from the British Botanical Association,
Ltd., Holgate, York. The names of dealers prepared to supply various
animal-specimens are mentioned in the text.
[2] Made by dissolving one or two crystals of potassium iodide in about
half a pint of water, and then adding iodine in small quantities until
the solution is the colour of sherry.
[3] Lord Avebury, _The Pleasures of Life_ (Macmillan).
[4] Prof. Comstock.