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Title: An Introduction to Nature-study
Author: Stenhouse, Ernest
Language: English
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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.
CHAPTER II. HOW A GREEN PLANT FEEDS.
5. THE FOOD WHICH A GREEN PLANT OBTAINS FROM THE SOIL.
1. =A plant cannot grow permanently in damp sawdust or clean
sand.=—Notice that the seedlings which were grown in damp sawdust
presently wither and die, while those which were grown in soil
flourish, and, with proper care, come to maturity. Obtain some clean
sand, and, to be sure that there is nothing in it which water can
dissolve, wash the sand in several changes of clean water. Germinate
some seeds in the sand, keeping it damp. The resulting plants in this
case also wither and die. Evidently soil contains some plant-food which
the plant cannot obtain from sawdust or clean sand. What is this food?
[Illustration: FIG. 18.—Porcelain crucible heated by Bunsen burner.]
2. =The amount of water and mineral matter in plants.=—Take a healthy
plant, say a bean plant, and weigh it. Then dry it thoroughly in the
oven and weigh it again. It will be found very much lighter; the
difference in weight represents the water which has been driven off.
Burn the dried plant. When the flame goes out notice the black charcoal
which is obtained. Continue the heating and observe that at last
nothing is left but a little grey ash. This experiment can be performed
over an ordinary fire by using an old shovel or a tile, but if you can
use a porcelain crucible (without lid) and a Bunsen burner (Fig. 18)
you will get better results. If a chemical balance is available, weigh
the ash and compare it with the weight of ash obtained from an ordinary
bean seed, such as that which gave rise to the plant you have used.
The ash from the plant is much greater than that got from the seed.
This extra ash must have been taken from the soil during the plant’s
growth.
3. =A nutritive solution.=—Make, or ask a dispensing chemist to make,
the following solution:
Potassium nitrate (consists of _potassium_, _nitrogen_,
and _oxygen_), 1 gram.
Sodium chloride (consists of _sodium_ and _chlorine_), ½ ”
Calcium sulphate (consists of _calcium_, _sulphur_,
and _oxygen_), ½ ”
Magnesium sulphate (consists of _magnesium_,
_sulphur_, and _oxygen_), ½ ”
Calcium phosphate (consists of _calcium_, _phosphorus_,
and _oxygen_), ½ ”
Water, 1 litre.
(A few drops of a dilute solution of sulphate, or chloride, of _iron_
should be added.)
Water, with this solution, a plant growing in wet sand, and when it is
well grown, dry and burn it. As much ash is obtained as from a plant of
the same size grown in soil. Notice the difference between such a plant
and one which has had water only supplied to it.
4. =Water culture.=—Fix two similar young plants in corks as shown
in Fig. 19, and put the corks into two bottles, the first of which
contains pure water and the second the nutritive solution, and let the
roots of the seedlings dip into the liquids. Cover the outsides of
the bottles with rolls of paper to keep out the light. Notice that
the plant living in the nutritive solution thrives, while the other
presently withers. Dry and burn the former, and observe that it yields
more ash than does a seed such as that from which it sprang.
5. =Plants obtain their mineral food from the soil by their roots.=—As
the roots are the only parts of the plant which are in contact with the
nutritive solution, or which (under ordinary conditions) are in the
soil, the mineral matter must be taken in by the roots.
6. =The root-hairs.=—Take up a seedling which has been growing in
damp sand, and observe the small particles of sand adhering to the
root-hairs (p. 17). The hairs of a plant’s root and rootlets apply
themselves very closely to particles of soil (Fig. 20), and the mineral
food (dissolved in water) passes into the hairs and so gets into the
root and thence to the other parts of the plant.
7. =Roots as storehouses of food.=—Examine, before the plants flower,
the roots of a turnip, a carrot, and a radish, and notice how greatly
they are swollen. You know that these roots are valued as foods; of
what use do you think the stored food is _to the plants themselves_?
=The food of a young seedling.=—When such a seed as that of a bean is
germinated in damp sawdust or wet clean sand, and kept in a warm and
light place, it puts out a radicle, which grows downwards and becomes
the main root, and a stem which grows upwards and bears green leaves.
After a time the main root branches, giving off side roots, which
spread in all directions through the sawdust or sand. The main root and
the rootlets bear very fine fluffy hairs for a short length, which is
situated just behind their points (Fig. 11), and these root hairs come
into very close contact with particles of damp sawdust or sand (Fig.
20), the moisture passing into them and thus reaching the main root,
from which it is distributed to the various parts of the plant. The
stem likewise flourishes, growing in length and thickness, and putting
out new leaves.
All this time the young bean plant is living on the food material
stored up in its cotyledons (p. 6); and if the sand or sawdust is
kept moist, with even pure water, this seed food is at first quite
sufficient. When at last the seed food is all used up, however, and all
that remains of the cotyledons is a shrivelled skin, the plant begins
to droop and wither from lack of food.
=Plants obtain food from the soil.=—Contrast this with the condition of
a seedling which has been grown in soil. It still flourishes, even when
the seed food is used up, for it is drawing up food from the soil—food
which could not be obtained from the damp sawdust or clean sand.
That the plant really has taken up some solid matter from the soil can
be proved by a few simple experiments. A plant which has been growing
in soil for some time after its seed food is used up is dried and
burnt, and the ashes are weighed. The weight of ash or mineral matter
thus obtained is found to be considerably greater than that of the ash
obtained from an ungerminated seed, or from a seedling grown in damp
sawdust or sand which has only been supplied with pure water.
=The mineral food of plants.=—The composition of the ash obtained
from various plants has been carefully determined by chemists, and in
this manner they have been able to find out what substances must be
present in soil in order that the plant may obtain all the mineral
food it requires. A mixture of potassium nitrate (nitre), sodium
chloride (common salt), calcium sulphate (plaster of Paris), magnesium
sulphate (Epsom salts), calcium phosphate, and chloride (or sulphate)
of iron—dissolved in water in the proportions specified on p. 27—has
been found to supply the necessary elements of the mineral food in a
form which the plant can readily use. That such a mixture is capable
of supporting the plant, while water alone is incapable of doing so,
may be seen by growing a plant—in the manner shown in Fig. 19—in this
solution. If, in addition, the plant is supplied with light and fresh
air, it will grow in a perfectly healthy and normal manner. If any of
the constituents (except the common salt) are omitted, the plant will
suffer. On the other hand, a plant which is growing in pure water will
presently die, from the lack of the necessary mineral food.
[Illustration: FIG. 19.—Plant growing in a nutritive solution of salts.
The bottle should be covered with a roll of paper to keep out the
light.]
=The work of the roots.=—These experiments show that water—of which
a large proportion of a plant consists—and the mineral constituents
of its food (dissolved in the soil-water) are taken up from the soil
by the roots. In ordinary soil the rootlets spread out on all sides,
dividing and subdividing, seeking for this very weak solution of
mineral salts. Even when soil appears practically dry, a very thin film
of moisture covers each little particle of earth, and the root hairs
become closely applied to these little particles (Fig. 20), so that the
water passes through their walls and gradually makes its way to the
main root, the stem, and the leaves.
[Illustration: FIG. 20.—Tip of a root hair with adhering particles of
soil. (× 240.)]
Roots sometimes perform other duties in addition to those of fixing
the plant in the soil and providing it with water and mineral food.
It is usual, for example, for =biennial plants=—which produce flowers
and seeds in their second year, and then die—to take in much more food
during their first season than they require at the time, and to store
up the surplus in readiness for the great effort of the second year.
These reserve materials are often stored in the roots, which then
become swollen and fleshy, like those of the turnip, radish, and carrot.
6. THE FOOD WHICH A GREEN PLANT OBTAINS FROM THE AIR.
1. =Plants contain much carbon.=—Char a stick, and notice the black
charcoal which is formed. Charcoal is an impure form of carbon.
2. =Carbon dioxide gas is formed when wood burns.=—Fasten a shaving
or a splinter of wood on a piece of wire, light it, and lower it into
a clean glass jar. When the wood has burned for a few seconds take it
out, and pour a little clear lime-water into the jar. The lime-water
turns milky. Similarly, pour a little lime-water into a jar in which
nothing has been burning, and notice that it remains clear. There is
evidently a difference in the nature of the air of the two jars. The
difference is caused by the burning of the wood, during which some of
the carbon unites with the oxygen of the air in the jar, forming an
invisible gas, called _carbon dioxide_. Carbon dioxide can always be
detected by the milkiness it causes in clear lime-water.
3. =Carbon dioxide present in ordinary air.=—Pour some clear lime-water
into a blue saucer and let it stand exposed to the air for half an
hour, then examine it. A white scum has formed on the surface of the
lime-water. Stir with a glass rod; the solution becomes milky. The
scum and the milkiness are produced by the union of the lime with
carbon dioxide from the air. Carbon (in the form of carbon dioxide) is
therefore present in the air.
4. =No carbon in the food solution.=—Examine again the list of
elements (p. 27), which compose the mineral salts which have been found
to replace satisfactorily the food which a plant obtains from the soil.
_There is no carbon in it._ A plant evidently does not depend on the
soil for the carbonaceous part of its food. From what other source can
a plant obtain its carbon? Carbon has just been proved to be present in
the air. Does the plant obtain its carbon from the air?
5. =Green leaves contain starch after exposure to sunlight.=—Take a
green leaf from a plant which has been exposed to the sunlight, boil
the leaf in water for a minute or two to kill it. Then put it in
methylated spirit until the leaf-green is dissolved out. When the leaf
is bleached rinse it in water, then put it into a dilute solution of
iodine (p. 2) and notice that it becomes blue or purplish brown. The
formation of this colour proves the presence of _starch_ in the leaf.
6. =Starch contains carbon.=—Char a piece of laundry starch and observe
the charcoal formed.
7. =Green leaves do not contain starch after being left in the dark for
24 hours.=—Keep a leafy plant in the dark for 24 hours and then test
a leaf as in the previous experiment. No starch can be detected. Put
the plant in the sunlight for an hour or two and test another leaf. It
contains starch. Plainly, starch is only formed in leaves if they are
exposed to light, and any starch previously present disappears when the
plant is kept in the dark.
8. =A green plant kept in air from which the carbon dioxide has been
removed will not form starch in its leaves.=—Obtain a large glass
bottle, such as those used by confectioners, and fit it with a cork
or india-rubber stopper through which passes a glass tube bent[5] as
in Fig. 21. Care should be taken to make all the joints tight, and it
may be necessary to soak the cork in melted paraffin to ensure this.
Pack the bend of the tube loosely with pieces of soda lime, and in
the bottle place a small jar containing lumps or a strong solution of
caustic soda. When the apparatus is ready, place in the bottle a small
plant or a leafy twig in water (fuchsia answers very well), which has
been kept in the dark for 24 hours. The caustic soda in the jar very
soon absorbs all the carbon dioxide which is present, and the soda lime
in the bend of the tube prevents any carbon dioxide from getting into
the bottle from the outside air. Place the jar in bright sunlight for
a few hours and then test a leaf for starch. None can be detected. It
is plain that one of the carbonaceous food stuffs—starch—is not formed
in the leaves of plants unless the plant is grown (in the light) in air
containing carbon dioxide.
9. =Seedlings are at first independent of light.=—Germinate pea or bean
seeds in wet sand or sawdust in the dark. Notice that for some time
the seedlings grow almost as well as when in the light. Soon, however,
the stem becomes long, weak and straggling, and the leaves are pale in
colour, even if the plant is supplied with mineral food.
10. =The formation of sugar in a germinating pea-seed.=—Take up a
pea-seedling when the stem is one or two inches long, and chew the
partly shrunken cotyledons. Notice the slightly sweet taste. Contrast
this with the taste of an ungerminated seed.
=A plant contains much carbon.=—When a piece of wood or other part
of a plant is strongly heated it first blackens or chars, showing
the presence of a large proportion of charcoal or impure carbon. On
continued heating, this carbon “burns away.” In the process of burning
it unites with some of the oxygen of the air, and forms a colourless,
invisible gas known as =carbon dioxide=. Though this substance cannot
be seen, its presence can be easily detected by means of clear
lime-water, which, when exposed to the gas, absorbs it and becomes
milky owing to the formation of a white precipitate of chalk. If, for
example, a splinter of wood is burnt in a glass jar, and a little clear
lime-water is immediately afterwards poured into the jar and shaken up,
a milkiness at once proclaims the presence of carbon dioxide.
=The air contains carbon.=—If lime-water is poured into a blue saucer
and left exposed to the air for half an hour, a white scum of chalk
is seen to have formed on its surface, showing that carbon dioxide is
present in the atmosphere. It is important that the student should
realise this presence of carbon—as invisible carbon dioxide gas—in
the air. Although the proportion is very small, amounting to only 3
parts of carbon dioxide in 10,000 parts of fresh country air, it is of
incalculable importance to plants, and indirectly to ourselves and all
other animals.
=A green plant obtains its carbon from the air.=—Since the parts of
a plant contain much carbon, and the food which a plant obtains from
the soil need not contain any carbon; while the air, on the other
hand, does contain carbon, it seems likely that a plant obtains
its carbonaceous food from the air. This surmise is confirmed by
experiments. One of the most easily recognisable of plant products
containing carbon is =starch=, for it yields a very characteristic
blue, or purplish-brown, colour when treated with iodine solution. By
means of this test starch can easily be proved to be present in the
green leaves of a plant which has been exposed to the air and sunlight.
The leaf is first killed by being boiled in water for a minute or two,
and then its green colouring matter is dissolved out by immersion in
alcohol (methylated spirit). The bleached leaf is rinsed in water and
then put in iodine solution, and the blue or purplish-brown colour
which is formed shows the presence of starch. There is a marked
difference when a leaf, which has been kept in the dark for twenty-four
hours, is similarly tested. In this case no starch can be detected.
One compound of carbon—_i.e._ starch—may thus be recognised easily; and
if we found that a leaf made no starch when supplied only with air from
which the carbon dioxide had been removed, this fact would be strong
evidence in favour of the conclusion that a green plant obtains its
carbon from the carbon dioxide of the air. To test this, a large bottle
is fitted, by means of a tightly fitting cork or stopper, with a tube
containing lumps of soda lime, a substance which eagerly absorbs carbon
dioxide from air. A small jar of caustic soda is placed inside the
bottle (Fig. 21). A green plant or a leafy twig, which has been kept
for twenty-four hours in the dark to free it from starch, is then put
in the bottle, and the whole exposed to sunlight for a few hours. At
the end of this time it is found, on testing the leaves, that no starch
has been formed. By this, and other experiments, botanists have proved
that _green plants obtain all their carbon from the carbon dioxide of
the air, and that sunlight is indispensable for the process_. We shall
examine this question more fully when we study leaves (Chapter III.).
[Illustration: FIG. 21.—Experiment to prove that leaves do not make
starch unless the air with which they are supplied contains carbon
dioxide. _S_, tube containing lumps of soda lime; _S′_, jar containing
a solution of caustic soda.]
=The carbonaceous food of a young seedling.=—Just as a bean or pea
seedling is for a time independent of an outside supply of mineral
food—its roots needing only to be supplied with water—so there is
enough carbonaceous food also stored up in the seed to satisfy for some
time the needs of the growing plant—the stored starch being gradually
changed into sugar and absorbed. For this reason a young seedling will
live healthily in the dark. When, however, the seed food is exhausted,
or nearly so, the plant draws upon the store of carbon, which is
present as carbon dioxide in the air, for a renewal of the starch and
allied substances which are necessary to it. As it cannot make use of
this atmospheric carbon in the dark, it must henceforth be supplied
with sunlight or it will not thrive. Plants kept in the dark after
their seed-food is exhausted are pale in colour and unhealthy. Their
stems grow long and straggling (Fig. 22), but are usually too weak to
stand upright.
EXERCISES ON CHAPTER II.
1. Make experiments to discover the effects, upon
seedlings growing in a nutritive solution (p. 27), of each
of the following modifications in the composition of the
solution: (_a_) omit the sodium chloride; (_b_) omit the
potassium nitrate; (_c_) omit the compound of iron; (_d_)
omit the magnesium sulphate; (_e_) substitute sodium nitrate
for potassium nitrate.
2. Explain how it is that a green plant cannot carry on
its nutrition in darkness. (1892)
3. What part of its food does a green plant obtain from
the air? In what form, and under what conditions, is it
taken in? (1889)
4. Describe the method of water cultures, and give the
general results of a set of experiments. (1898)
5. Give experimental proof that green plants require to be
fed with combined nitrogen. (1897)
6. What are the necessary conditions for the formation of
starch in a plant? Mention experiments which support your
statements. (1896)
7. Explain the influence of light on a growing plant.
Illustrate your answer by reference to the changes in a
ripening and germinating bean.
(King’s Schol. 1903)
8. How can it be proved experimentally that a green plant
draws some, but not the whole, of its nourishment from the
air? (1904)
[Illustration: FIG. 22.—Two mustard seedlings of equal age. _E_, grown
in the dark; _N_, grown in ordinary daylight.]
FOOTNOTE:
[5] Cheap glass tubing can easily be made soft enough to bend by
heating it in an ordinary batswing gas burner.
CHAPTER III. THE FORMS AND DUTIES OF LEAVES.
7. THE FORMS OF LEAVES.
1. =The shapes.=—Make a collection of the leaves of a large number
of different plants, for example, elm, beech, lime, oak, birch, ash,
blackberry, pine, yew, horse chestnut, rose, holly, woodsorrel, grass.
Lay each in turn flat in your notebook and trace the shape of the leaf
blade by passing the point of your pencil round the edge. Measure the
length and greatest width; write down these dimensions. Is the greatest
width at, above, or below, the middle of the leaf blade?
Most of the leaves are flattened green plates. Those of the pine and
yew are long and needle-shaped. Do you know any other leaves like these?
2. =The veins.=—What enables the leaf to keep stretched out? Turn it
over and notice the “veins” on the lower side. Do they act like the
ribs of an umbrella? Fill in the positions of the main veins in your
drawings. Are the veins parallel to each other in any of your leaves?
Write a list of as many leaves as you can find which have parallel
veins.
3. =Skeleton leaves.=—Put some leaves in a saucer with a little _soft_
water, and allow them to rot. Clean away the soft stuff from time to
time by _gently_ brushing the leaves with an old tooth-brush. Notice
the “skeleton” which remains. Skeleton leaves may be made much more
quickly by soaking the leaves for some time in a _weak_ solution of
bleaching powder. Wash them well before drying.
4. =The colour.=—Is the green deeper on the upper or the lower surface
of a leaf? Which surface usually receives more light?
5. =The apex.=—In how many of your leaves is the apex (_a_) pointed,
(_b_) blunt, (_c_) rounded?
6. =The margin.=—Examine the edge or margin of each leaf. In how many
is it (_a_) quite plain, (_b_) hairy, (_c_) wavy, (_d_) saw-edged,
(_e_) doubly saw-edged, (_f_) spiny? Do you find spines on holly leaves
which are so high on the tree as to be out of the reach of cattle? What
is the use of the spines?
7. =Blackberry leaves.=—Gather several leaves from a blackberry bush.
Notice that in addition to the “saw cuts,” the margins of some are cut
into slightly, while others are divided quite to the midrib, the leaf
being thus cut into two or more _leaflets_. Select specimens which form
a gradual series between the “simple” leaf and the “compound” leaf
(consisting of three or five leaflets) and draw them.
8. =The horse chestnut leaf.=—Draw the compound leaf of the horse
chestnut, and draw an even curved line joining the points of the
leaflets. You can imagine that the compound leaf may have been formed
by a leaf of this shape being cut into until it was divided into seven
complete leaflets.
9. =The rose leaf.=—Draw an imaginary simple leaf such as may have been
the original form from which the compound leaf of the rose was derived.
Notice the difference in the arrangement of the main veins of the
leaves of the horse chestnut and the rose. Does this account for the
leaflets coming off at the sides of the midrib in the rose leaf, and
springing from one point, like fingers from the palm of the hand, in
the case of the horse chestnut?
10. =Sycamore and ivy leaves.=—If the large indentations in these
leaves were continued to the midrib, would the compound leaves thus
formed be of the type of the rose leaf or of that of the horse chestnut
leaf?
11. =The leaf-stalk.=—What attaches the leaf-blade to the stem or
branch of the plant? Can you see signs of the main veins joining the
top of the leaf-stalk? Do you know any plant with the blades of the
leaves fixed _directly_ on the stem, _i.e._ without any leaf-stalks?
12. =Stipules.=—Examine the rose leaf again and notice the two
leaf-like outgrowths at the bottom of the stalk. These are called
_stipules_. Make a list of as many leaves as you can find which have
stipules. How many leaves can you find with a _sheath_ at the bottom of
the stalk?
13. =The leaf of the sweet pea.=—Notice the large stipules of this
compound leaf (Fig. 28). What are the tendrils? Do you think they may
be mainly the larger veins of the upper leaflets? Is the leaf of the
type of the rose or of the horse chestnut leaf?
14. =Other compound leaves.=—Compare and contrast ash, lupine,
woodsorrel, strawberry, and other compound leaves with those of the
rose and horse chestnut.
=A leaf.=—The leaves of different plants vary much in size and shape,
but in general a leaf is a thin, broad, and more or less oval =blade=
of green colour, attached by a leaf =stalk= to the stem or branch. In
some cases, however, the leaf stalk is absent and the blade is attached
directly to the stem or branch.
=The veins.=—The leaf is kept taut by a number of branching ribs,
somewhat as the silk of an opened umbrella is stretched tightly by the
ribs. The ribs or “veins” of the leaf run beneath the skin, but are
generally nearer the lower surface than the upper, and are easily seen
when the leaf is turned over. If a leaf is allowed to rot in a little
soft water, the skin and the soft green stuff of the interior decay and
leave these veins as a white “skeleton” (Fig. 23). The process may be
assisted by _gently_ brushing the leaf from time to time. A skeleton
leaf may be obtained still more quickly by putting the leaf in a _weak_
solution of bleaching powder until the skin and interior are soft
enough to be brushed away. Care should be taken to use a weak solution,
or the veins also will be rotted. The skeleton should be well washed in
water before drying.
The arrangement of the veins in a leaf varies widely, but it falls
broadly into two classes, according as the main veins run parallel
or nearly parallel to each other (Fig. 24), or form a less regular
network (Fig. 23). The =venation= of a leaf is curiously associated
with the number of cotyledons possessed by the seedling; for nearly
all dicotyledons (p. 23) have net-veined leaves, while the leaves of
monocotyledons are almost invariably parallel-veined. Careful drawings
of several typical leaves should be made, and the principal veins
indicated on them.
[Illustration: FIG. 23.—Net-veined leaf of a dicotyledon (White Thorn).
(× ½)]
[Illustration: FIG. 24.—Parallel venation of the leaf of a monocotyledon
(Solomon’s Seal). (× 1)]
=The shapes of leaves.=—Although the blade of a leaf is most commonly
flattened, and roughly oval in outline, there are several exceptions.
The leaves of pine, spruce, larch, and yew are needle-shaped; those
of grasses (Figs. 102-110) are very long in proportion to their
width; while the leaves of many moorland plants are rolled up into
hollow cylinders. There is some reason—could we find it—for every
such variation, and the significance of some of these shapes will
be referred to later (p. 47). When the _dimensions_ of leaves are
carefully measured, the proportion of the length to the width will
be found to vary much in the leaves of different plants, but will be
found to be pretty constant for the same sort of plant. This holds
good, too, for the position of the greatest width (_e.g._ at, above,
or below, the middle of the blade), the form of the _apex_ of the
blade (blunt, pointed, spiny, or rounded), the nature of the _margin_
(smooth and “entire,” hairy, saw-edged, doubly saw-edged, lobed,
etc.), and the extent and positions of the larger _indentations_.
Thus, while any particular elm leaf (Fig. 124) is probably slightly
different from every other elm leaf in the world, it resembles every
other elm leaf more than it does any leaf from any other plant than an
elm. No leaf of this shape ever grows on an oak tree or a sycamore.
Thus, in spite of minor variations there is a wonderful =conformity
to type=, and the student will find that by carefully examining the
shape, venation, margin, apex, etc., of all his leaves, and above all
by _drawing_ them, he will soon be able to recognise them at sight. It
is by doing this and noticing in each case the methods of folding and
arrangement of young leaves in the bud that it may be possible in the
future to explain some variations which are at present not understood.
It has already been seen that the peculiar forms of the leaves known
as _cotyledons_ are associated to some extent with the shape and size
of the seeds containing them, and with the amount, if any, of the food
stored in them.
=Simple and compound leaves.=—Blackberry leaves (Fig. 76) well repay
close examination. Some of the leaves on the bush will be found to
be simple—having one blade only on the leaf stalk. Here and there,
however, a leaf may be discovered which is so deeply cut into along one
side, that it is almost completely divided into two leaflets; and other
leaves will easily be found which consist of three or five leaflets,
much resembling the leaves of the rose (Fig. 25), a near relative of
the blackberry. Here, then, we have a plant which produces simple or
compound leaves according to its needs. It seems as if the blackberry
were still trying, as an experiment, a device which the rose tree has
found so advantageous as to have adopted for good. Some other plants,
the ash, for example, have compound leaves broadly similar to the rose
leaf—the leaflets springing in pairs from the sides of the midrib.
The compound leaf of the horse chestnut (Fig. 26) is of a different
type, for the seven leaflets all arise from one point; and the leaves
of the lupine (Fig. 9) are arranged on the same plan. When the venation
is examined, the reason for this becomes plain. In these cases the
main veins all diverge from the top of the leaf stalk; whereas in the
rose and ash the midrib gives rise to side ribs in pairs. The leaflets
are naturally arranged so that one of the larger veins shall support
each. The next question arising is, “What causes the differences in
the methods of branching of the midrib?” At present this is a mystery.
Compound leaves consisting of _three_ leaflets are found in woodsorrel,
strawberry (Fig. 50), clover, etc.
[Illustration: FIG. 25.—Compound leaf of the Rose. _L_, leaflets; _P_,
leaf-stalk; _st_, stipules. (× ½)]
[Illustration: FIG. 26.—Compound leaf of the Horse Chestnut. (× ⅙)]
=Intermediate leaves.=—The ivy (Fig. 27) and sycamore (Fig. 33) have
leaves which seem intermediate between truly simple and truly compound
leaves. From the arrangement of the veins it is seen that they approach
the horse chestnut type more than that of the rose. On the other hand,
if the deep indentations of the oak leaf (Fig. 113) were carried to
the midrib, the simple leaf would be divided into leaflets arranged,
somewhat like those of the rose or ash leaf, along the sides of the
midrib.
[Illustration: FIG. 27.—Ivy. (× ¹/₁₀)]
[Illustration: FIG. 28.—Compound leaf of Pea. _Fl_, flower-stalk; _Sp_,
stipules; _T_, tendrils. (× ½)]
=Stipules.=—At the bases of many leaf stalks, close to the stem, are
leaf-like outgrowths called =stipules=. They are well seen in the rose
(Fig. 25) and pea (Fig. 28). Some leaves have a =sheath= at the bottom
of the stalk, partially enclosing the stem.
=Tendrils.=—The pea also affords an interesting case of leaflets
being modified to do special work. Here the upper leaflets seem to
have remained undeveloped except for their main veins, and these have
acquired a remarkable power of twining round suitable objects and so
supporting the stem. Many other plants have tendrils, but these are not
always modified leaflets.
8. HOW LEAVES ARE ARRANGED ON THE STEM.
1. =Opposite leaves.=—Examine a deadnettle plant (Fig. 92). Do the
leaves come off the stem haphazard? How many come off at each level?
Are both leaves on the same side of the stem, or opposite each other?
Are the two leaves at the next level above just over these, or do the
directions cross? Do the leaves get as much light or more than they
would if each pair were just over the pair next below? How many other
plants do you know which have leaves arranged in this manner? Examine
leafy twigs of sycamore, horse chestnut (Fig. 40), and ash. Are the
stalks of the lower leaves of these twigs of the same length as those
of the upper ones? Is it any advantage for the lower leaves to have
longer leaf stalks?
2. =Box leaves.=—What is the arrangement in the box? Examine
particularly the young leaves near the end of the twig. Are the lower
ones twisted? Can you suggest a reason for this twisting? Can you find
any twigs in which no twisting has taken place? Are these untwisted
twigs so placed that they are equally exposed to the light on all sides?
3. =Alternate leaves.=—Examine the arrangement of the leaves on a
wallflower stem. They come off _alternately_, each springing from a
rib on the stem. How many ribs are there? Look at the bottom of the
stem, where the leaves have fallen off, and notice that each has left
a scar. Mark one of the scars with your pencil and then count how many
scars you pass before coming to another on the same rib. How many
times do you wind round the stem in doing this? You pass five leaves
and wind spirally round the stem twice. This is always the case in the
wallflower.
Examine leafy twigs of oak and pear trees. Here, too, the leaves are
_alternate_, and every sixth leaf is above the first, and a line
joining all the leaf-bases or scars between the first and sixth leaves
would wind spirally round the stem twice.
What is the arrangement in the elm and lime?
4. =Leaves which form a rosette.=—Examine plants of primrose (Fig. 81)
and daisy. The leaves in these cases spring from close to the ground
and form a rosette. Notice that the bottom of the leaf blade is much
narrower than the upper part. Is any saving of material obtained by
this arrangement?
5. =The position of branches and buds.=—Look in the upper angle between
a leaf and the stem in all your specimens. This angle is called the
_axil_ of the leaf. Can you see a _bud_ in the axil of the leaf? Can
you find that a bud or a side branch ever arises in any other position?
(The former positions of fallen leaves are marked by scars.)
To the beginner in nature-study leaves seem in the majority of cases to
be arranged on the stem of a plant in a haphazard and confusing manner,
and it is only after very careful observation that a definite order and
regularity is seen to be always maintained.
=Nodes and internodes.=—The level at which a leaf springs from the stem
is called a =node= (Lat. _nodus_, a knot), and the length between two
consecutive nodes is called an =internode= (Lat. _inter_, between).
=Opposite leaves.=—It is best to begin the study of leaf-arrangement
by examining some such plant as the =deadnettle= (Fig. 92). The leaves
come off in pairs: two at the same level, set opposite to each other.
The next pair above or below springs from the stem in a direction at
_right angles_ to the first—a device which allows the leaves to get a
more equal share of light than if each pair were placed directly over
the next below.
A similar arrangement is adopted by various other plants, including
the =horse chestnut= (Fig. 40), =sycamore= (Fig. 34), =box=, =privet=,
etc., but in some instances it is disguised. =Box= twigs afford an
interesting example of this. Those twigs which are equally, or almost
equally, illuminated on all sides, have their leaves arranged in pairs
at right angles to each other like those of the deadnettle. Some twigs,
however, receive the light from one direction only, and in these
cases the leaves turn themselves until they face the light; so that
at a casual glance the pairs of leaves seem to lie all in the same
plane. One only needs to examine the end of the twig, where the leaves
are just unfolding, to see that the arrangement is really in pairs
alternately at right angles. In the case of the _privet_ the efforts of
the leaves to face the light often cause the stem itself to be twisted
between the leaf-levels.
=The alternate, or spiral, arrangement.=—Perhaps the commonest
leaf-arrangement is one in which only one leaf is given off at any
particular node, the next leaf being a little further round the stem,
and so on. As a result, an ink line or a piece of thread joining the
leaf-bases would wind spirally round the stem. In the case of the
=wallflower=, =oak=, =pear=, and many others, such a line would wind
spirally _twice_ round the stem before coming to a leaf vertically
above the first, and in so doing it would pass _five_ leaves. This may
be shortly described as the ²/₅ arrangement. A less common one is ⅜,
where in winding spirally round the stem 3 times, 8 leaves would be
passed.
=Efforts of leaves to obtain light.=—It would be difficult to imagine
any order of insertion which would secure a more _equal_ distribution
of light to each leaf than the spiral arrangement; but here also cases
of leaves twisting in order to face the light are not uncommon. =Lime=
leaves very often turn for this reason, so as to lie in almost the same
plane—adopting a device similar to that of the box and privet described
above. Further, the lime leaves arrange themselves at such angles that
there is very little overlapping. Elm twigs also often exhibit similar
instances of a mutual accommodation of leaves to each other’s light
supply.
The lower leaves on a =horse chestnut= twig have longer stalks than
those nearer the end (Fig. 40). This enables the leaves to stand well
out to the light and escape the overshadowing of those above.
=The positions of branches.=—A branch of the stem of a flowering plant
always arises as a bud in the upper angle between a leaf and the stem.
This position is called by botanists the =axil of the leaf=, from the
Latin word _axilla_, the arm-pit. Clearly, then, the arrangement of the
branches is primarily dependent upon that of the leaves, and we shall,
for example, never find “opposite” branches on a tree which bears its
leaves on the “alternate” system. It is easy to notice, however, that
not all the buds develop into branches. In other words there are many
buds which remain dormant, and the final arrangement of the branches is
often somewhat irregular on this account. But wherever an ordinary bud
or a branch occurs, we may be perfectly sure that there was once a leaf
immediately below, even if the leaf-scar can no longer be seen.
=Economy of leaf surface.=—All these things seem to indicate that a
good supply of light is of the greatest importance to leaves, and this
conclusion is supported by the fact that leaves are usually either
narrow or actually cut away in places where the light cannot reach
them. The leaves of the daisy and of the primrose (Fig. 81), for
example, all spring from nearly the same point, and form a rosette.
Evidently there would be a certain amount of overlapping at the
leaf-bases, unless the blades there were very narrow, as they are.
Again, the greatly-indented leaves of the ivy are often arranged
so that a point of one leaf fits over an indentation of another—a
beautiful example of plant economy.
9. THE WORK OF LEAVES.
1. =In sunlight leaves make starch.=—Expts. 5, 7, and 8 (Sec. 6) have
already proved (_a_) that leaves of a plant growing in ordinary air and
exposed to the sunlight make starch; (_b_) that in the dark this starch
somehow disappears; (_c_) that in air destitute of carbon dioxide
leaves are unable to make starch even in sunlight.
2. =The parts of a leaf which are not exposed to light do not make
starch.=—Keep a plant, say of tropæolum—or, if not convenient, a single
leaf (Fig. 31)—in the dark for 24 hours to free the leaves from starch.
Split a small cork and pin the halves on opposite sides of a leaf,
and then expose the plant to bright sunlight for an hour or two. (If
a single leaf is used let the end of the stalk dip into water.) Take
off the cork, kill the leaf with boiling water, dissolve out the green
colouring matter with methylated spirit, rinse, and test with iodine
solution. The part from which the light was excluded remains bleached,
and therefore contains no starch; while the rest of the leaf becomes
blue or purplish brown owing to the presence of starch.
3. =Parts of a leaf which are not green do not form starch in
sunlight.=—Take a variegated leaf from a plant (_e.g._ the variegated
geranium or maple) which has been in bright sunlight for some hours.
Apply the usual test for starch. The parts which were originally green
contain starch; the originally white parts remain bleached.
4. =Leaves supplied with carbon dioxide, and exposed to sunlight, give
off oxygen gas.=—(_a_) Take a bunch of fresh watercress or any green
water-weed and put it in a beaker or glass jar. Cover the plant with an
inverted funnel which is shorter than the beaker. Now fill the beaker
with ordinary tap water or river water (_not_ distilled water), so that
the end of the neck of the funnel is covered. Completely fill a narrow
test tube with water, close it with the thumb, and invert it over the
neck of the funnel. If this has been done carefully the test tube will
still be full of water. Expose the arrangement (Fig. 29) to bright
sunlight, and notice the bubbles of gas which are given off from the
plant and collect at the top of the tube. When a few inches of gas
have collected, raise the test tube, close it with the thumb whilst
still under water, and hold it mouth upwards. In the meantime, light a
splinter of wood with the other hand. When it is well burning, blow out
the light, remove the thumb from the test tube, and plunge the glowing
splinter into the gas. It bursts into flame again, showing that the gas
is _oxygen_.
[Illustration: FIG. 29.—Experiment to prove that green leaves supplied
with carbon dioxide, and exposed to sunlight, give off oxygen gas.]
[Illustration: FIG. 30.—Experiment to prove that tap-water, or
river-water, contains dissolved carbon dioxide.]
(_b_) Repeat the experiment, (i) placing the apparatus in the dark,
(ii) without using any plant. No gas collects in the test tube.
(_c_) To show that the water used contains carbon dioxide in solution,
completely fill a gallon can or a large flask with similar water, and
attach a cork and a delivery tube which has also been filled with
water—dipping the end of the tube into a little clear lime-water (Fig.
30). Put the same quantity of lime-water into another vessel for
comparison, and then heat the can. Gas is given off, and as it bubbles
through the lime-water the liquid is gradually turned milky.
5. =Leaves wither in sunlight unless supplied with water.=—(_a_) Cut
off a leafy twig and leave it exposed to sunlight for an hour or two;
notice the change in the appearance of the leaves.
(_b_) Put a similar twig in the dark for the same length of time; again
notice the leaves. Is the difference due to a difference in light or
to one of heat? (_c_) To test this, keep, if possible, a similar twig
in the dark in a warm place. Do the leaves wither as much as in (_a_)?
(_d_) Smear with vaseline the _under_ surface of some of the leaves of
such a twig and again expose to sunlight. Do the smeared leaves remain
fresh longer than the others?
(_e_) Cut off the end of a twig with a sharp knife whilst it is under
water, and leave it exposed to sunlight, dipping in water. The leaves
remain fresh. How do you explain these differences?
6. =In sunlight, leaves give off water.=—Take a piece of cardboard
about 4 in. square and make a small hole in the middle. Pass the end
of a leafy twig through the hole and make up with wax any chinks
between the twig and the card. Put the card on a tumbler containing
water, so that the end of the twig dips under water; and invert on the
card—covering the leafy end of the twig—a second tumbler which is clean
and dry. Put the apparatus in the sunshine and notice the mistiness
(or even visible drops of water) forming on the inside of the upper
tumbler. Where does this moisture come from?
7. =The skin of a leaf is perforated by little pores.=—Dip a fresh
laurel leaf into boiling water in a beaker or tumbler. Can you see
bubbles of air escaping from the leaf? Are they to be seen on both
surfaces of the leaf, or only on one? Which?
Examine both surfaces of box leaves with a strong lens, and try to see
the little dots (pores) on the lower surface.
The student who has performed the experiments described in this
section, and who has thought about the results obtained, cannot but
have gained some insight into the main duties of leaves. The meaning of
these results must now be discussed.
=The formation of starch in leaves.=—When the green leaves of a plant
are exposed to sunlight in ordinary air—that is in air containing a
certain proportion of carbon dioxide—the leaf forms starch in its
interior, and the starch can be detected by applying the iodine test
(p. 34). When part of a leaf is protected from the light, as by
pinning the halves of a split cork on opposite sides of it (Fig. 31),
no starch is formed in the shaded parts, but only in the regions which
are exposed to the light. Further, if a variegated leaf is treated in
the same way, starch can be detected only in those parts of the leaf
which were originally green; the parts which were white are free from
starch. It is plain that it is the green colouring matter which puts
the energy of the sunlight at the disposal of the leaf and enables it
to manufacture starch.
[Illustration: A B
FIG. 31.—_A_, Tropæolum leaf, on which have been pinned the halves of a
split cork (_C_). (× ½.)
_B_, the same leaf tested for starch with iodine solution, after
exposure to sunlight for an hour. The part shielded from the light
remains bleached; the rest of the leaf has turned blue.]
At least three conditions are therefore necessary for the formation of
starch in leaves: (1) the green colouring matter; (2) sunlight; (3)
carbon dioxide.
=Oxygen is liberated when leaves form starch.=—Carbon dioxide gas,
which has been seen to be indispensable for the manufacture of starch
in leaves, consists of carbon, or charcoal, chemically united with
the gas oxygen. The green-stuff of the interior of the leaf makes the
starch by causing this carbon to combine with water which has come
up from the roots, but it returns to the air the unnecessary oxygen.
Water plants, the leaves of which are not directly exposed to the air,
use carbon dioxide which the water has dissolved from the air. They
also give off the surplus oxygen, after fixing the carbon. This is the
explanation of the bubbles of gas which, in sunlight, are often seen
rising from the plants in an aquarium. By such an arrangement as is
described in Expt. 9, 4, this evolved gas can be collected, and proved
to be oxygen.
[Illustration: FIG. 32.—Experiment to prove that green leaves, exposed
to sunlight, give off water.]
=The use of water to the leaves.=—It is common knowledge that if a twig
is allowed to become dry its leaves hang limply and wither; but if the
twig is allowed to dip into water the leaves will keep fresh and crisp
for a considerable time. This necessity for supplying the twig with
water seems to indicate that leaves give off water, and that this is so
may be proved by a few simple experiments. Two tumblers may be arranged
as in Fig. 32: separated by a card through which passes the end of a
leafy twig. The end of the twig dips into water in the lower tumbler.
In order to prevent water vapour from passing from the lower tumbler
to the upper, the chinks between the twig and the card are sealed with
paraffin wax.
When this arrangement is placed in the sunlight, a dew soon collects
on the inside of the inverted upper tumbler. This water must have been
given off in the form of vapour from the leaves. That the loss of water
from leaves is due rather to the light than to the heat of the sunshine
may be shown by keeping leafy twigs in the dark. The leaves keep fresh
much longer than when placed in the light, even if they are kept in as
warm a place.
=The pores of the leaf-surface.=—An ordinary leaf remains fresh much
longer if its lower surface is smeared with vaseline. The explanation
of this lies in the fact that the waterproof skin of a leaf is
perforated by a multitude of little pores, especially on the lower
surface. In most leaves, indeed, the pores are confined to the lower
surface. Smearing the surface with vaseline blocks up these pores and
thus prevents the escape of water vapour from the interior of the leaf.
These little mouths (known as _stomata_)[6] open in the light and close
in the dark. During the daytime, therefore, the air (containing its
small proportion of carbon dioxide) has free access to the interior
of the leaf through the stomata, and, on the other hand, any water
which the leaf does not require can escape in the form of vapour. A
leaf requires water not only because all its mineral food (p. 29) is
brought to it dissolved in water, but also because water as well as
carbon dioxide is required for the manufacture of the starch and other
plant-foods.
=How plant-food is distributed.=—The water which comes up from the
roots is distributed to the various parts of the leaf through the
_veins_. These are therefore not only supports, which stretch out
the soft leaf-stuff to the light and air, but they also form a very
complete network of _irrigating channels_ or water pipes. Further, the
starch and other foods which a leaf makes are drained off into the
stem through other minute channels which are bound up with the water
pipes. The starch, for example, is changed into a kind of sugar which
dissolves in water and drains away. From the stem, the food solutions
are distributed to all the parts where growth is taking place.
EXERCISES ON CHAPTER III.
1. Make drawings of as many cases as possible of economy
of leaf surface.
2. Grow various plants, _e.g._ Tropæolum, Geranium,
Fuchsia, Mustard, etc., in the window, and notice the effect
which the direction of the light has upon the positions of
the leaves.
3. Smear with vaseline the lower surfaces of various
growing leaves, and on the following day test the leaves for
starch, comparing each with an unsmeared leaf from the same
plant.
4. How is the transpiration of water from a green leaf
effected and controlled? Discuss the uses of transpiration.
(1897)
5. Put the same quantity of water into each of two similar
test-tubes, and let the end of a leafy twig dip into one.
Weigh the tubes, place them together in the sun for an hour,
weigh again, and estimate roughly the weight of water lost
by one square inch of leaf surface per hour. Compare various
plants in this respect. Repeat the experiments, (_a_) in a
moderate light, (_b_) in the dark.
6. Make a list of plants in which the leaves are so
arranged as (_a_) to conduct rain-water towards the base
of the main stem, (_b_) to cause rain-water to fall to the
ground from the outside of the foliage. Try to discover
whether the difference has any relation to the arrangement
of the roots.
7. Under what conditions can plants use carbon dioxide as
a source of food? Mention experimental and other proofs of
the principal statements made. (1905)
8. What part of its food does a green plant obtain from
the air? In what form and under what conditions is it taken
in? (King’s Scholarship, 1905)
FOOTNOTE:
[6] Greek, _stoma_, a mouth.
CHAPTER IV. BUDS. THE HISTORY OF A TWIG.
10. THE STRUCTURE AND USES OF BUDS.
1. =A typical bud.=—Split a cabbage, or a lettuce “heart,” down the
middle, and observe how the leaves are arranged round the conical end
of the stalk. The leaves which are fixed lowest on the stalk are the
largest, and they cover the outside of the bud. The leaves are smaller
and smaller as they are fixed nearer and nearer the end of the stalk,
until round the tip they are almost too small to be recognised as
leaves. A _bud_ is the tip or “growing point” of a stem or branch,
together with the young leaves which crowd round it. Draw the section.
2. =A sycamore twig.=—Look in the axils (p. 47) of the leaves of a
sycamore twig in summer or autumn, and notice the buds. Can you see any
buds on the lower parts of the twig—which bore leaves twelve months or
longer ago? Did these also arise in the axils of leaves? How can you be
sure they did? Tie a piece of string, or tape, round this twig and look
at it often, making a note of every change which you see in it.
3. =The fall of the leaf.=—On what date did you first notice that the
leaves of the sycamore were falling? Did the first leaves fall from the
twigs at, or near, the ends of the branches, or lower down near the
trunk? Examine the scar left by the first leaf which falls from your
twig. What is its shape? What is the meaning of the dark dots on the
scar? Gently take off the leaf nearest to the scar. Does it come off
easily? Are there any dark dots on this scar? Can you see anything on
the end of the leaf stalk corresponding to the dots? Do you think the
dots are the ends of the food pipes referred to on p. 53?
Make a drawing of a fallen leaf and write down on the drawing at the
proper place, the colour of that part of the leaf, or better still,
colour the drawing with water-colour paints. Collect good specimens of
the autumn leaves of as many other trees as possible, and make coloured
drawings of them.
On what date did you first notice that all the leaves were shed? Make
similar notes for other trees.
4. =The structure of a sycamore bud.=—In winter take off a sycamore
twig which has a big bud at the end, and examine it. The bud is clothed
with overlapping scales of a pale green colour. Make a drawing of the
bud, double the size, and then take off the scales one by one and count
them. Look with a lens at the upper end of one of the largest scales.
Can you see any trace of a rudimentary leaf blade? When the last scale
is removed there remains on the end of the twig a tiny tuft of delicate
green leaves surrounded by a little down. Count them, and notice how
each is folded up fanwise. In some buds you may find a bunch of green
flowers in the middle. These buds are larger than those containing
leaves only.
Cut a bud across with a sharp knife, and examine the cut surface with
a lens to see more clearly how the scales and young foliage leaves are
arranged. Draw what you see.
5. =How the bud bursts.=—In spring watch your twig very carefully to
be sure you do not miss any stage of the opening of the buds. Make
drawings every day or two of the end bud, when it has begun to unfold.
Notice how the scales fall off—those on the outside first—and how the
foliage leaves push open the upper scales and come out. Watch the way
in which each leaf opens its folds and spreads itself out flat.
The buds may be made to open earlier by _cutting_ off a twig about two
or three feet long and keeping it in water in a warm room.
6. =Starch is stored up in the twigs.=—Cut off a twig and pour a drop
of iodine solution on the cut end. What does the blue colour indicate?
7. =The growth of the bud.=—Watch your bud growing, and notice that the
tip of the twig—which was surrounded by the young leaves—elongates so
that each pair of leaves is soon separated from the next pair. Notice
the _rings_ of _scars_ which the scales left when they fell off.
8. =A year’s growth.=—Look down the twig until you find the rings or
scars which were left _last spring_, when the scales fell off the
expanding terminal bud. A year ago the end of the twig was at this
point, so that the length between one such set of rings and the next
marks a year’s growth.
9. =Side branches.=—The buds on the length formed last summer but one
may have grown out into side twigs. Try to find the leaf scars below
each of these side twigs.
10. =A horse chestnut twig and its buds.=—Examine in the same way a
horse chestnut twig and trace its history. The buds are larger than
those of the sycamore, and each is covered with a shining, sticky
layer of resin. What do you think is the use of the resin? Put a bud
in water, and when you take it out notice how the water runs off and
leaves the bud dry.
Pull one of the terminal buds to pieces. The scales will come apart
more easily if the bud is soaked for some time in methylated spirit,
to dissolve out the resin. Notice the thick layer of down inside the
scales. What do you think is its use? Scrape the down gently, and
carefully clean it away from the little foliage leaves in the middle.
See how the leaflets of each leaf are folded. In some terminal buds you
may also find a little pink flower-spray.
Tie a piece of string round a growing twig, so that you can recognise
it, and watch all the stages of the expansion of the terminal bud, the
unfolding of the leaves, and the elongation of the tip (Figs. 37, 39
and 40). Cut off other twigs two or three feet long, in February or
early March, and keep them in water in a warm room.
11. =Other buds.=—Examine the buds of the beech, lilac, violet, dock,
fern, etc., and make drawings showing (1) how the leaves are arranged
with regard to each other, and (2) how each leaf is folded or rolled.
As a rule these points can be easily made out by examining with a lens
the cut surface of a bud which has been cut across with a sharp knife;
but the buds should also be examined at frequent intervals when they
are unfolding.
=A typical bud.=—An excellent idea of the structure of a typical bud
can be obtained by splitting down the middle an ordinary cabbage,
or a lettuce “heart,” and examining the manner in which the leaves
crowd round and cover the conical end of the stalk. Around the tip or
“growing point” the leaves are extremely small and tender. They are
overlapped by slightly larger leaves, which spring from the stalk a
little lower down. These in their turn are covered by still larger
leaves, inserted at a yet lower level, and so on—the largest and oldest
leaves folding over the smaller and more recently formed. The growing
point of a stem, or of a branch of a stem, surrounded by a cluster of
immature leaves, is called a =bud=.
=The history of a sycamore twig.=—The student who would know the
meaning of the various marks and scars on the surface of a twig, should
select one and follow carefully for a year the fate of the buds which
it bears. It is convenient to begin by studying a twig on a sycamore
tree. It may be marked for ease of recognition by tying a piece of tape
on it. If several students are working, each should write his name or
number on a luggage-label and fix this to his twig.
[Illustration: FIG. 33.—Sycamore leaves and fruits. (From a photograph
by Mr. A. Flatters.) (× ¹/₁₂.)]
=The positions of the buds.=—In summer the younger twigs of the
sycamore tree are covered with large five-pointed leaves (Fig. 33).
The leaves come off in pairs, each pair being at right angles to the
pair next above or below. Every leaf is engaged throughout the day in
building up—by means of the green stuff in its interior—starch and
other foods (p. 50), and in giving off excess water, in the form of
invisible vapour, through its stomata (p. 53). In the axil (p. 47) of
each leaf is a little bud, called from its position an =axillary= bud
(Fig. 41, _B_), and at the very tip of the twig is a larger =terminal=
bud.
=Autumn colours and the fall of the leaf.=—As the summer wanes, the
soil becomes colder, and the chilled roots lose much of their power of
absorbing moisture. It is plain that if the leaves continued giving off
water when no fresh supplies were forthcoming the tree would suffer.
How is the danger to be met? Starch and other foods have already been
stored up in quantity sufficient to supply the needs of the winter and
the early spring. The leaves have finished their work, and one by one
they fall off. But this does not take place until careful preparation
has been made. Their green colouring matter breaks up; the part which
may still be useful to the plant drains into the stem, leaving little
heaps of yellow grains in the leaves. Or a special colouring matter
may be formed, which, united in various ways with the materials of the
dying leaf, gives the warm shades of red, orange, and purple which make
the woods so beautiful in autumn. When all is ready, a layer of cork
(Fig. 41, _C_) forms at the junction of the leaf stalk and the twig so
that no raw wound may be left; the leaf-base splits across, just above
the cork layer, and the leaf flutters to the ground, there to rot and
make rich leaf mould.
=The leaf scars.=—The former position of each leaf is now marked by a
curved scar (_l.s._ Fig. 34), and a row of brown dots (_v.b._) in the
scar still shows where the food-pipes bent outwards from the twig into
the leaf. A line (_a_) stretches across and joins the two scars of each
pair.
=The buds.=—Just above every scar is the bud which arose in the axil of
the fallen leaf. It is covered with overlapping scales of light green
colour. At the end of the twig is a single terminal bud, similar in
appearance to the axillary buds, but of larger size. It is instructive
to take the terminal bud of a twig to pieces. The outer scales are
tough and green, while the inner ones are thinner and have a beautiful
silvery appearance. Usually there are fourteen scales. Each is long and
narrow and bears at its upper end the rudiment of a leaf blade, which
cannot usually be seen well without a lens. The scale is really a leaf
which has been arrested in its development.
[Illustration: FIG. 34.—Sycamore twig in winter. _l.s._, leaf-scars;
_v.b._, ends of food-pipes; _R_, rings of scars left by scales of last
winter’s terminal bud. (Slightly reduced.)]
[Illustration: FIG. 35.—Cross section of a Sycamore bud. (× 7.)]
When all the scales have been removed, there remains a tiny tuft of
delicate green foliage leaves surrounded by a little down. Each leaf
is folded fanwise, both for convenience of packing, and to protect its
tender tissues from the cold and the damp when the bud is expanding.
The complicated folding of these leaves is well shown in Fig. 35,
which is a magnified sketch of a cross section through a sycamore
bud. The large sheaths surrounding the young leaves are four of the
overlapping scales. In a large terminal bud a bunch of green flowers
may often be found.
=The bursting of the buds.=—About the middle of April the tree wakens
to new life. The stored food makes its way to the terminal buds;
invigorated by the rich sugary sap the young leaves swell and push
forward, burst apart the scales, and open out their folds to the
light and air, as if eager to get to work at the earliest possible
moment. The scales fall off to the ground, leaving close-set rings
of scars; the growing point elongates, and new leaves—which were
indistinguishable in the bud—grow out and expand. During the summer,
food is plentiful, and a little bud appears in the axil of every leaf.
Only with the autumn does the activity of the tree slow down. Except
for some two pairs at the tip, the newest leaves now remain stunted.
They form scales and close round the tender growing point protectingly,
in readiness for the winter.
[Illustration: FIG. 36.—Sycamore twig in winter. (× ⅛.)]
If necessary, the axillary buds could have behaved as the terminal
bud did, in which case they would have grown out into side twigs.
They usually remain small, however, until next year (Fig. 36), for
the leaves are so busy making food, and the terminal bud is so busy
growing in length, that no energy can be spared for their further
development. If any accident had befallen the terminal growing point,
one or more of the axillary buds would have grown out into side twigs.
Gardeners take advantage of such reserve buds when they clip off the
ends of twigs to make a plant grow “bushy.”
[Illustration: FIG. 37.—Horse Chestnut twig in winter. (× ¹/₁₁.)]
[Illustration: FIG. 38.—The terminal part of a Horse Chestnut twig in
winter. _l.s._, leaf-scars; _v.b._, ends of food-pipes. (× ⅔.)]
=A year’s growth.=—When the bud scales drop off they leave, as we have
seen, a series of closely-set rings of scars. The distance between one
set of rings and the next (as at R., Fig. 34), therefore represents a
year’s growth. The student should get a twig two or three feet long and
find out for himself, by examining the marks on it, what the twig did
last year, two years ago, and three years ago. With care he will be
able to say definitely in which year any fairly recent side-twig began
to grow out from its bud.
=A horse chestnut twig.=—A winter twig of horse chestnut (Fig. 37)
is very similar in its general features to what we have seen in the
sycamore. The buds are in pairs at right angles to each other, and
below each bud is a large corky leaf scar (_l.s._ Fig. 38), with the
positions of the former food-pipes marked by black dots (_v.b._). These
buds, however, are larger than those of the sycamore, and each is
covered with a shining layer of resin, to keep out insects and the rain.
We may slip the end of a penknife under the bud scales and remove them
one by one. The first few are thin and papery, and soaked in resin.
Those next inside are woody and much thicker. Next comes a layer of
papery scales, inside that a coat of cottony down, then another soft
papery layer, and lastly a thick pad of down. When we carefully scrape
away this down, we find—warm and cosy in its midst—a tuft of little
objects with a most quaint resemblance to hands clad in woollen gloves.
We remove one of these, and on scraping it gently with a knife we see
that the “hand” has seven fingers; and it presently becomes clear that
each finger is a tiny green leaflet folded on itself, and that the hand
is a young leaf. If we take off these gloved leaves in turn one by one,
we find as we proceed that they become smaller, until they are almost
too small to be distinguished in their fluffy nest. And when all the
down and the baby leaves are scraped away, the tender growing point
of the twig is left alone at the summit of a little cone, with steps
showing where the leaves were.
Had the twig been left undisturbed on the tree, the bud would have
awakened in spring and begun to grow (Fig. 39).[7] The scales and
the down would have been shed, leaving only the rings of scars as a
memento of the winter sleep; the growing point would have pushed on
and on, lengthening perhaps a foot or more in three weeks; the leaves
would have opened their bright green fingers to the spring air, and
begun their work (Fig. 40), only to cease when in the autumn they too
dropped off, and the new buds tucked themselves up in their beds to go
to sleep. The leaves of the horse chestnut fall off, as do those of the
sycamore, owing to a “separation-layer” arising at the base of the leaf
stalk (Fig. 41), and in this case each leaflet also becomes separately
detached in the same way.
[Illustration: FIG. 39.—Later stage of the Horse Chestnut twig of Fig.
37. (× ¹/₉.)]
[Illustration: FIG. 40.—The later development of the terminal bud of
the twig of Figs. 37 and 39. (× ⅕.)]
=Other forms of buds.=—Surrounding the young silky-fringed leaves of
the =beech= bud are several crimson membranous scales which are really
the stipules (p. 43) of undeveloped leaves. The thin, soft parts of the
leaf blade are sharply pleated (Fig. 42) between the side veins which
spring from the midrib.
The way in which the branching of a tree depends on the buds is well
shown in a lilac. The terminal bud does not usually develop, so that
each of the two lateral buds just below grows out into a branch,
producing the characteristic “forking.” In the lilac bud every
gradation between scales and ordinary foliage leaves may be seen.
[Illustration: FIG. 41.—Longitudinal section of twig of Sycamore.
_A_, base of leaf; _B_, axillary bud; _C_, cork layer. (× 2.) (From a
photograph by Mr. A. Flatters.)]
[Illustration: FIG. 42.—Cross section through leaf-bud of Beech. (× 6.)]
In the =violet= bud the two margins of the leaf are rolled inwards
towards the midrib; while in the =dock= they are rolled backwards.
Young =fern= leaves (Fig. 146) are not folded from side to side like
the examples referred to above, but are rolled into a tight coil from
apex to base. It is the upper surface of the leaf (frond) which is to
the inside. As the leaf grows, the coil straightens out.
EXERCISES ON CHAPTER IV.
1. Where and when are the buds of common English trees
formed? (1901)
2. Show, by describing and drawing one example, that the
branch of a tree may preserve a record of past seasons in
the bark. (1901)
3. Draw an unopened bud of sycamore. Of what parts is it
composed, and how are the parts arranged? (1901)
4. What can be seen inside a large bud of horse chestnut?
On what part of the branch are the largest buds to be found?
(1898)
5. Upon what does the method of branching of a tree
depend? Give examples.
6. Examine pollarded trees; what is the cause of their
peculiarities?
7. Examine buds of various trees in spring, and try
to discover in which cases the bud scales are (_a_)
modifications of entire leaves, (_b_) modified leaf stalks,
and (_c_) modified stipules.
8. Examine and describe the method of protection, during
the summer, of the axillary buds of the plane.
9. Take a poplar shoot during late summer, and examine
the corky separation-layer forming at the base of each
leaf. Write an account of its use, and make drawings of its
appearance.
10. The “heart” of a cabbage or lettuce is of lighter
colour, sweeter taste, and more tender texture than the
external leaves. How do you explain these differences?
(King’s Schol., 1902)
11. Describe and explain the effect of clipping a privet
hedge.
12. Give an account of the changes in appearance in any
common leaf during the whole period of its growth. Explain
briefly what part the leaf plays in the life of the plant.
(Certificate, 1904)
13. Explain precisely how you would decide whether a given
specimen consisted of (_a_) one compound leaf, or (_b_) a
twig bearing several simple leaves.
FOOTNOTE:
[7] In a large terminal bud the cone often ends in a tiny pink spray,
which gives rise to a branch of beautiful flowers when the bud unfolds.
In this case no further growth of the main axis of the twig takes place.
CHAPTER V. HOW STEMS DO THEIR WORK
11. THE DUTIES OF STEMS.
1. =The shapes of stems.=—Cut across a deadnettle stem and a wallflower
stem and examine the shape of the sections. The former is square, the
latter is five-ribbed. Is there any relation between the form of the
stem and the arrangement of the leaves?
2. =The “bleeding” of stems.=—Cut through the lower part of a
scarlet-runner plant in spring. Can you see any water escaping from
that part of the stem still in the ground? Similarly, cut back a
sunflower stem when it is from ½ to 1 inch thick. Does the water
exude from all parts of the cut surface equally, or does it come from
definite channels? To see this better, dry the cut end with blotting
paper and examine the surface with a lens. “Bleeding” is best seen in
vine stems; if a young vine is available, cut it back in spring and
observe the large escape of water.
3. =The water-current travels along definite channels.=—Colour some
water with red ink and put in it the stalks of cut white flowers such
as snowdrops or narcissi. The stalks and flowers soon become veined
with red. Cut the stalks across and see that the strands in the
interior are also coloured. The coloured water has evidently travelled
along definite channels.
4. =The food-channels in a soft stem.=—(_a_) Take a stout piece of the
stem of a deadnettle, including three nodes (p. 45), and slit it down
in a line between the end pairs of leaves. Then boil the stem in water
until the internal tissue is soft. Take the stem out and carefully
scrape away all the soft material until the woody strands can be well
seen. These are the _food-channels_. Notice their arrangement in the
stem, and the manner in which they run out to the leaves.
(_b_) Similarly, examine the strands in a piece of sunflower stem, and
in an old cabbage stalk.
5. =The path of the water-current in a woody stem.=—Take a leafy twig
of elder or laurel and, from the part of the shoot below the leaves,
remove a ring of about an inch of the bark and the soft tissues which
lie beneath it, so as to expose the wood. Put the end of the twig
(below the ring) in water. The leaves remain fresh and crisp, showing
that the water travels either along the wood or the pith, or along both.
6. =The water travels along the wood.=—Put a similar leafy twig of
elder dipping in water which has been coloured by red ink. Expose
to sunlight, and after an hour or two cut open the twig and observe
which parts are coloured. The bark and the soft tissues between it and
the wood are not coloured. The wood is stained red. The pith is not
coloured.
7. =The path of the leaf-made food.=—(_a_) Take a leafy branch of a
tree—_e.g._ willow—and near the bottom remove a ring of the bark and
the soft tissues lying between bark and wood. Put the twig in water, so
that the ringed part and a few inches above shall be immersed. After a
time roots are produced _above_ the cut. If any arise from the stripped
part they are few in number and much shorter than those above.
(_b_) Place a similar but uninjured twig in water, and notice that
the new roots are produced _at the end_. What is the reason for the
difference?
=The duties of stems.=—No one can study leaves without being impressed
by the great importance to the plant of the work which they do. Even
the casual observation that year after year thousands of fresh leaves
make their appearance would indicate this. And when it is learned that
throughout the day every leaf is busily engaged in decomposing carbon
dioxide, and joining up the carbon with the water and mineral compounds
which have come up from the root—thus forming sugars, starch and a
host of other valuable substances—some general idea is obtained that
the trunk and spreading boughs of a tree may, after all, be of minor
importance, and may exist mainly to help the leaves to perform their
duties as perfectly as possible.
This is the true view to take of the stem and its branches; their
duties are (1) to bear the leaves and spread them out, so that these
will receive as much sunshine and fresh air as possible; (2) to supply
the leaves with the water and mineral substances which they require for
their work; and (3) to receive from the leaves and distribute to the
rest of the plant the food materials which the leaves have prepared.
A stem, together with the leaves and branches which it bears, is called
a =shoot=.
It will be found that almost all the variations in the structure and
habits of stems are connected with the arrangements of the leaves. For
example, the weight of the mass of leaves borne by a forest tree is
very great, and they offer a great resistance to the wind; the trunk
and its branches, therefore, are correspondingly strong and stout.
Again, when a stem is ribbed or ridged in any particular manner it will
generally be found that the ridges have a definite relation to the
leaves and to their points of insertion on the stem. This has been seen
(p. 44) to be the case in the stem of the wallflower.
The tip of a stem or branch is occupied by a =bud=. As the tip
elongates (Fig. 40), the outer leaves of the bud become separated by
internodes, and new leaves continually arise around the growing point.
=The path of the water in the stem.=—It may easily be shown that
water travels up the stem to the leaves. It is common knowledge that
slightly withered leaves or flowers become fresh and crisp again if
the twig or stalk is put in water. If the stalks of white flowers—say
snowdrops or narcissi—are put into water coloured with red ink, the
flowers become veined with red, and, if the stalks are cut into, red
strands may be seen. This experiment not only shows that water has
travelled up the stalk, but it also shows that the water has passed
along =definite channels=. When a vine stem is cut back in the spring,
the water wells out rapidly from the end of that part of the stalk
which is still in the ground. This =bleeding=, as it is called, may
also be seen, though to a smaller extent, in stems of sunflower,
scarlet-runner, and other plants. By drying the cut surface with
blotting paper, and then examining it with a lens, it may be seen
that the water escapes from the ends of certain little tubes; and it
is possible in a boiled or rotting stem (Expt. 11. 4) to follow the
distribution of the strands containing these tubes. The strands run
along the length of the stem, =outside the pith=. Cross strands connect
the main ones, chiefly at the leaf-levels (nodes); and other strands
run out into the leaves to form the veins.
The tubes which convey the water current are in that part of the strand
which is nearest the pith, and they become woody. Thus, if the current
year’s growth of, say, a horse chestnut or an elder twig is cut across,
a thin ring of =wood= is seen surrounding the soft pith. Outside the
wood are the softer tissues, surrounded by the bark. A ring of the bark
and soft tissues beneath it may be entirely removed from a growing
twig, leaving the wood exposed, but the leaves above remain crisp and
fresh, showing that this treatment has not interfered with their water
supply. The water, therefore, travels along either the wood or the
pith. That the wood and not the pith conducts the water may be shown by
putting a leafy twig in water coloured with red ink. Only the wood is
stained.
=The path of the food made by the leaves.=—The plant food which the
leaves make is drained off into the stem, and distributed to the
parts where growth is taking place. This food travels in the soft
tissue—called the =bast=—which lies below the bark. This fact can be
shown indirectly by removing a ring of bast from the lower part of
a branch, say of willow, and putting the branch into water. When at
length the cutting puts out roots, these spring from the top of the
ring. If any spring from the stripped part they are markedly smaller,
and fewer in number. In an uninjured cutting, which is of course
supplied with prepared food along all its length by the bast vessels,
such roots spring from the cut end. The supply of leaf-made food can
also be cut off by ligaturing a twig below the leaves, as by twisting
a wire or cord tightly round it. In such a case growth usually ceases
in the part of the twig below the strangled part, while the upper part
of the twig, to which the leaf-made food is now restricted, grows much
more luxuriantly than before. Gardeners often produce unusually fine
fruits by ligaturing the lower parts of the twigs on which the fruits
are ripening.
12. HOW STEMS ARE STRENGTHENED.
1. =The formation of wood.=—(i.) In summer take a horse chestnut twig
of three or four years’ growth. Cut through it with a sharp knife at
the following places, and trim the cut ends flat:
(_a_) Near the apex;
(_b_) at the middle of the current year’s growth;
(_c_) near the bottom of the current year’s growth;
(_d_) about the middle of last year’s growth;
(_e_) ” ” the previous year’s growth.
Make a drawing of what you see in each case:—In (_a_) the twig is
covered on the outside by a green _skin_. In the middle is the soft
_pith_. Between the two is a ring of separate _strands_. In (_b_) and
(_c_) the strands have joined up, and a distinct, though thin, layer of
_wood_ surrounds the pith. _Bast_ and other soft tissues lie between
the wood and the bark. (_d_) has two layers of wood. (_e_) has three
layers of wood.
(ii.) Split each length longitudinally. Why is it easier to do this
than to cut the twig across? In which direction does the grain of
the wood run? Make out in each piece the pith, strands, or layers
of wood, bast, etc., and skin or bark. You can tear off the bast in
ribbon-like shreds. See how the strands run out into the young leaves.
Cut lengthwise through the junction between the main twig and any side
twigs, and notice that corresponding parts are continuous.
2. =The strength of a grass stem.=—Notice the relatively enormous
strength of a straw and other grass stems.
Burn a straw and observe the tube of mineral matter which is left
behind. Examine a piece of bamboo; is it hollow or solid?
=Woody stems.=—To enable them to bear the weight of the leaves and
branches, and to withstand the force of the wind, the stems of plants
are strengthened in various ways. Most commonly this is effected by the
formation of =wood= in the walls of the water-vessels.
Even in succulent stems, such as that of the sunflower, the strands of
vessels are stiffened by the long and narrow wood pipes which run along
them; and when the strands join up to form a complete cylinder a very
strong column is the result. Engineers make use of the same device,
knowing that the same amount of material will bear a far greater stress
when made into a hollow cylinder than it will in any other form.
=The thickening of woody stems.=—In dicotyledons (p. 23) and
gymnosperms (p. 163) the cylinder, which is formed by the joining-up
of the conducting strands, consists of three layers. The innermost of
these is the =wood=, and the outermost is =bast=. Between them is a
very delicate layer called =cambium=, which is continually dividing
and forming more wood on its inner side and more bast on its outer
side. The wood is hard and resists pressure, while the bast is soft and
is squeezed against the inside of the bark by the expanding wood (Fig.
43).
The formation of new wood and bast takes place vigorously during the
summer, at the expense of the food which is manufactured by the leaves
and travels along the bast to the active cambium. As autumn comes,
the activity of the tree slows down, and the new wood is formed of
closer texture. In winter the process stops altogether; but with the
warmth and the plentiful food-supply of spring the formation of new,
open-textured wood is resumed.
[Illustration: FIG. 43.—Portion of a four-year-old stem of the Pine,
cut in winter. 1, 2, 3, 4, the four successive annual rings of the
wood; _b_, bast; _br_, bark; _c_, cambium; _f_, spring wood; _i_,
junction of wood of successive years; _m_, pith; _ms_, medullary rays;
_s_, autumn wood. (× 4.)]
This difference in texture between the autumn wood and the later spring
wood is quite visible to the naked eye (Fig. 44), and gives rise to a
series of =annual rings=, each of which represents a year’s growth.
Thus, a cross cut through the four year old part of a branch (Fig. 43)
shows four layers of wood, one of which was formed each year. The length
formed last year has two layers of wood (if we look in summer), and the
current year’s growth has one layer, all of which has been formed since
spring.
=The advantages of secondary thickening.=—The formation of secondary
wood and bast is very important. The new wood is required (1) to
provide additional water-vessels to supply the demands of an increasing
number of leaves, and (2) to give the necessary increase of mechanical
strength to the growing tree. Again, the increase in the quantity
of food manufactured by the leaves makes a larger quantity of bast
necessary for its distribution. Last of all, new tissues are required
to take up the duties of those which are old and worn-out.
[Illustration: FIG. 44.—Cross section of Larch stem, showing annual
rings.]
In forest trees, the central pith becomes almost obliterated and the
old stem practically consists of wood, bast, and bark. The various
annual rings, and the bast, of a woody stem are joined together by a
number of radiating horizontal spokes called =medullary rays= (Fig.
43, _ms_, _ms′_, _ms″_, _ms‴_). These conduct water and food materials
across the stem from layer to layer. The beautiful lines and patches
called “silver grain,” which may be seen in oak furniture, consist of
the medullary rays exposed in radial-longitudinal section.
=Bark.=—The very young stem is surrounded by a thin waterproof skin,
perforated by stomata (p. 53) as the skin of a leaf is; and the part
immediately below the skin possesses leaf-green, and can therefore
decompose carbon dioxide as a leaf does (p. 51). An old stem, on the
other hand, is covered by a layer of tough bark (_br_, Fig. 43), which
splits from time to time, owing to the stretching which is caused
by the increasing thickness of the wood. The bark begins as a layer
of =cork=, which forms on the outer side of the bast. The cork cuts
off the food supply from all the external tissues, which die. Bark,
therefore, consists of the cork and the dead layers outside it.
=Grass stems.=—The great strength of grass stems—so apparent when we
try to bend a straw—is largely due to _silica_, the substance of which
rock-crystal and ordinary sand are composed. When a straw is burnt,
this remains as a hollow cylinder of mineral matter. The great strength
of the cylindrical form has already been referred to. It is very well
seen also in a bamboo stem, and anyone who has blunted the edge of his
knife on a piece of bamboo will appreciate the additional hardness
which is given by the presence of mineral matter.
13. CLIMBING STEMS.
1. =Hooking stems.=—Examine a bramble or a wild rose plant in a hedge.
Why does it need to climb? How does it climb? Pull a branch and notice
by what means it resists the pull. What is the shape of the _prickles_?
Do they point upwards or downwards? On what parts of the plant are
the prickles found? Notice how easily a prickle may be pushed off,
sideways. Is it as easy to tear it off lengthways? Is the prickle a
little branch, or merely an extension of the rind?
Contrast a prickle with the _thorn_ of the hawthorn. The thorn does not
come off easily, and it contains a woody core which is continuous with
the wood of the branch. Cut lengthwise through the thorn and the branch
which bears it to see this. The thorn is a short pointed branch; it
arises in the axil of a leaf, and sometimes bears leaves itself.
2. =The ivy.=—Observe how the climbing stem of the ivy is attached to a
wall or tree. It puts out a line of _roots_ on its shaded side. These
roots give out a sticky fluid, which, on hardening, fixes them to the
wall.
3. =Twining stems.=—Watch the growth of a convolvulus seedling. At
first the young stem grows straight up, but soon the tip begins to
move round and round. Try to find out how long it takes to describe
one revolution. Put a long stick in the ground near the plant and
notice how, when the revolving stem touches the stick, the spiral is
henceforth described round the support and the stem consequently clings
to it. Lay your watch face-upward, and notice whether the stem moves in
the same direction as the hands (the “clockwise” direction), or in the
opposite (“counter-clockwise”) direction.
Make similar observations on the hop, honeysuckle, and scarlet runner,
and note the results.
4. =Leaf climbers.=—Examine a climbing tropœolum (often, though
wrongly, called “nasturtium”). Which parts of the plant clasp the
support? Watch a young plant coming up. Does the stem revolve before
the leaf stalks come in contact with the support? Compare the
_clematis_.
5. =Tendril climbers.=—Examine plants of sweet pea, bryony, vine,
passion flower, and cucumber. Try to find out in each case which part
has been modified to form the tendrils. Watch a plant day by day until
a free tendril grasps a support, and notice how it becomes spirally
coiled. Can you straighten the tendril by pulling, without leaving any
kinks? Why? Is the spiral of the tendril continuous, or does it change
its direction in the middle? Make a _continuous_ spiral with wire, and
notice the kinks formed when the wire is straightened by pulling the
ends. Which is better for the plant in a gale of wind, a continuous or
a reversed spiral? Why?
Notice the sucker-like tendrils of the Virginian creeper, and the way
in which they fix the plant to the wall.
Plants whose stems are not strong enough to stand erect without support
must adopt some special means of spreading out their leaves to the
light and air. One of the commonest devices of such plants is that of
climbing up other and stronger plants, walls, trellis-work, etc.
=Scramblers and climbers.=—In the simplest cases the plant simply
scrambles over other plants. Many brambles and roses are merely
scramblers, but more often they are true climbers, weaving themselves
among their neighbours by the help of hooked =prickles= (Fig. 76).
The prickles point backwards, and therefore anchor the twigs firmly,
as is very evident on trying to pull a branch out of the hedge. A
prickle may easily be broken off by a side push, for it is merely an
outgrowth of the rind, and does not contain any woody core. It is so
attached, however, that it resists much greater force in the lengthwise
direction—a manifest advantage to the plant.
[Illustration: FIG. 45.—Ivy climbing up a wall. _R_, aërial roots. (×
⅓.)]
The differences between such a prickle and a =thorn= like that of the
hawthorn should be carefully noticed. A thorn is really a little twig
which has remained short and become pointed at the end. It has a core
of wood which is continuous with the wood of the branch bearing it.
That a thorn is really a little branch is shown by its origin in the
axil of a leaf, and by its often giving rise to leaves and buds.
[Illustration: FIG. 46.—Climbing stem of Honeysuckle. (× ¼.)]
[Illustration: FIG. 47.—Climbing stem of Convolvulus. (× ¼.)]
The stem of the ivy climbs by means of little =roots=, which it puts
out on the side furthest from the light (Fig. 45). These give out a
sticky liquid which, on drying, cements them to the wall or tree.
=Twining= stems are much in advance of these. There seems to be
something approaching intelligence in the manner in which the young
stem of a hop, honeysuckle, or convolvulus, which at first grows
straight up, begins to wander round and round, tracing a spiral path
in the air until it touches a support. Then, however, as if the plant
could feel, the movement below the point of contact stops; but the
upper part of the stem still revolves and therefore twines round the
support. The stems of the honeysuckle (Fig. 46) and the hop turn in the
same direction as the hands of a clock. This is called the “clockwise”
direction. On the other hand the convolvulus (Fig. 47) and most other
twining stems are “counter-clockwise” climbers. The stem of the
bittersweet revolves indifferently in either direction.
=Sensitive clasping organs= in their simplest form are seen in the
_twining leaf stalk_ of the clematis and tropœolum; the stem itself
revolves as if to give its leaf stalks every opportunity of finding
suitable supports. The leaf stalks seize these and twine round them.
Most wonderful of all climbing organs are the =tendrils=. They are
well seen in Fig. 48. A part of the plant—sometimes a leaflet, as in
the pea (Fig. 28); sometimes a branch, as in the passion flower; or a
flower stalk, as in the vine—becomes modified into a thread, slight
but strong. When the end of the thread touches and then twines round
a support, the whole tendril forms itself into a spiral which, like a
wire spring, draws the plant up to the support, and can yet lengthen
and yield to the wind when necessary. In the middle of the tendril
the direction of the spiral is reversed, so that the tendril can be
straightened without being twisted.
The tendrils of the Virginian creeper do not twine, but on meeting a
wall they form round red _suckers_ at the end, and attach the plant
(Fig. 49).
[Illustration: FIG. 48.—How a tendril grasps a support. The spiral is
reversed at _x_.]
[Illustration: FIG. 49.—Virginian Creeper. _R_, _R_, stem tendrils. (×
½.)]
14. CREEPING AND UNDERGROUND STEMS.
1. =A creeping stem.=—Examine a plant of the ground ivy. Is the stem
strong enough to stand upright? How does it spread out its leaves to
the light and air? The stem grows along the ground, and at intervals
it gives off a pair of leaves which grow upwards, and a tuft of roots
which grow down to the ground.
2. =A runner.=—Is the “runner” of the strawberry of the same nature,
_i.e._ is it a continuous stem like that of the ground ivy? When
the plant is carefully examined, the creeping “stem” is seen to be
a _branch_ arising in the axil of a leaf. The branch runs along the
ground for a little distance, and then roots itself and gives off a
number of leaves. In the axil of one of these another branch arises and
runs on in the same direction. The same branch does _not_ run on and on.
3. =A stolon.=—Follow carefully the underground part of the couch grass
and make out its connection with the main shoot. It is a branch like
the runner of the strawberry, which arises in the axil of a leaf, and
extends only to the next shoot.
Compare the stolons of the cinquefoil.
4. =A potato tuber.=—Examine a potato tuber (the part which is eaten).
Notice the “eyes.” These are buds, with scale leaves. _Leaves never
occur on roots_, so that the potato must be an underground stem. Put a
pin into every eye, and wind a thread round the tuber along the bases
of the pins. It forms a spiral. Cut the potato into halves, and pour a
drop of iodine solution on the cut surface. What is the meaning of the
blue dots which at once make their appearance? Plant a potato in warm,
moist earth, and when it has sprouted notice that each bud (eye) has
given rise to a branch.
5. =Bulbs.=—Cut an onion or snowdrop bulb down the middle, and draw
what you see, marking on your drawing the outer scale leaves, the
swollen bases of last year’s leaves, the young leaves in the middle,
the short, thickened stem, and the roots. Also cut other bulbs across
and again draw. What is the similarity and what is the difference
between these bulbs and such a bud as a cabbage?
Also examine hyacinth, tulip and daffodil bulbs. Put them in glasses
with water touching their bases, and watch them grow. What do they live
upon?
6. =A crocus corm.=—Obtain a few crocus “bulbs” in the early winter.
Observe the tough outer tunic springing round the edge of a circular
scar on the base. If there are any roots they come off from the scar.
Take off the tunics from one “bulb” and observe the bud or buds at
the top of the white mass inside. Other tunics cover the buds. Cut
lengthwise through the mass of the “bulb” so as to bisect the largest
bud. Separate the parts of the bud with a needle and notice (_a_)
the thin outer leaves, (_b_) the young foliage leaves, (_c_) the
flower-sheath and flower. Pour a drop of iodine solution on the cut
white mass (the stem) below the bud. It turns blue. Why?
What is the principal difference between a crocus “bulb” and the bulb
of an onion, hyacinth, or tulip? A true bulb is mainly composed of
swollen leaves or leaf bases; in the crocus the thick, rounded stem
makes up most of the bulk. It is better, therefore, to speak of a
crocus _corm_, to indicate the difference.
Plant the remaining corms and examine them at intervals for a year.
Notice the formation of the roots, the lengthening of the buds, the
formation of the flowers, _the activity of the foliage leaves after
flowering_ (why?), the withering of the roots and leaves in summer, and
the growth of the enlarged base of the branch into next year’s corm.
=Creeping stems.=—Instead of climbing, many stems find that the best
method of spreading out their leaves is to creep along or under the
ground, and give off leaves and roots at intervals. Not only does this
device prevent the leaves of one node from interfering with the light
and air supply of those of the next, but the plant is continually
coming in contact with a fresh lot of soil. The ground ivy is an
instructive example of this method of growth. The stem creeps along
the ground, and at every node it gives off a pair of leaves which grow
upwards, and a tuft of roots which grow down into the ground.
[Illustration: FIG. 50.—Runner of Strawberry. (× ⅓.)]
The =runner= of the strawberry (Fig. 50) appears at the first glance to
grow in a similar manner. As a matter of fact, however, the apparent
stem is a _branch_ arising in the axil of one of the leaves of the last
node. The branch runs along the ground and gives rise to a new shoot,
and from this another branch, springing from the axil of a leaf, forms
another runner. The same branch does not run on from shoot to shoot.
The =stolon= of the couch grass (Fig. 51) is somewhat similar. The
erect stem of the plant is divided, as usual, into nodes or knots (from
which the narrow, sheathing leaves arise) and internodes. Branches
(stolons) spring in the axils of the lower leaves, turn downwards, and
run on underneath the soil, taking root again at some distance from the
parent plant.
[Illustration: FIG. 51.—Stolon of Couch Grass. (× ⅙.)]
[Illustration: FIG. 52.—Creeping underground stem of Solomon’s Seal.
_a_, bud of next year’s aërial growth; _b_, scar of this year’s growth;
_c_, _d_, _e_, scars of aërial growth of previous years; _w_, roots. (×
¾.)]
=Underground stems.=—Although the stem is usually that part of the
axis of a plant which is above ground, there are many exceptions. The
bracken fern, daisy, coltsfoot, Solomon’s seal (Fig. 52), and many
other plants have stems which are ordinarily buried in the ground,
giving off leaves above and roots below. In some cases such underground
stems become much swollen with stored food-material—manufactured by
the leaves in excess of immediate requirements. In the potato, for
example, certain underground branches of the stem store up starch to
such an extent that their ends become fleshy, ovoid masses some inches
in thickness (Fig. 53). The true nature of these =tubers= is revealed
by the buds or “eyes” which spring upon them. The buds are arranged
spirally—as may be seen by sticking a pin into each and joining up the
pins with thread—and when the tubers are kept in a warm, moist place,
each bud grows out into a new leafy shoot.
[Illustration: FIG. 53.—Part of a Potato plant, showing the old tuber
(dark) and several new ones. (× ⅙.)]
The structure of a =bulb= is easily made out in the onion, tulip (Fig.
54), hyacinth, or daffodil. When such a bulb is cut down the middle,
it is seen to be mainly composed of leaves or leaf-bases, swollen with
stored food. Inside these are the young leaves and the flower bud,
which would have expanded next season; and on the outside are a few
thin scale leaves. All these leaves spring from a fleshy button at the
base, which gives off roots below. The button is the flattened _stem_.
When a plant produces a bulb it will generally be found that it flowers
either very early or very late in the season; that is, at a period
which would not be very favourable for the work of the leaves. The
flower (Fig. 55) is produced at the expense of the stored food in the
bulb—made in excess of the requirements of the previous season. After
the plant has flowered, the new leaves work until they have made enough
food for next season’s flower, and then they also die.
[Illustration: FIG. 54.—Longitudinal section of Tulip bulb. _zk_,
modified stem; zs, scale leaves; _v_, terminal bud; _k_, young bud;
_w_, roots. (× 1.)]
[Illustration: FIG. 55.—Daffodil. (× ¹/₉.)]
The so-called bulb of the crocus is technically known as a =corm= (Fig.
56). It differs from a true bulb in consisting mainly of a fleshy,
rounded stem in which the surplus food made by last year’s leaves is
stored up. The plant is thus able to flower early, without waiting
for the new leaves to supply food. The swollen stem bears one or more
buds, and the whole is surrounded by tough tunics of scales. When the
corm begins to grow, roots are put out from the base, and the flowers
and—later—the leaves of the buds expand. The foliage leaves continue
their work after flowering, and the food which they make accumulates in
the base of the former bud, which becomes swollen to form the new corm
for next year’s flower. The leaves then die down, their bases becoming
the tunics of the new corm.
[Illustration: FIG. 56.—Crocus corm, seen from the side, from below,
and in longitudinal section. _c′_, base of bud, which will grow into
next year’s corm; _fd.ch._, food channel; _flr_, flower bud; _l_, young
leaves; _rts_, roots; _t₁_, _t₂_, _t₃_, _t₄_, tunics. (× ½.)]
EXERCISES ON CHAPTER V.
1. Mention experiments, which prove that organic
substances are formed in the leaves, and distributed to
other parts of a green flowering plant. By what channels are
they distributed? (1895)
2. Draw a cross section through the stem of a flowering
plant selected by yourself. Explain the uses of the chief
things seen in the section. (1897)
3. Mention experiments or observations which show by what
tissues water ascends to the leaves, and nutritive substance
descends from the leaves. (1897)
4. By what tissues does water pass along the stem of a
tree to the leaves? Give proofs of your statements. (1898)
5. Describe the effect of a tight ligature upon a growing
hazel stem. (1898)
6. What proofs can be given that the stem of a tree draws
nourishment from the leaves? (1898)
7. Show, by describing and drawing one example, that the
branch of a tree may preserve a record of past seasons in
its wood. (1901)
8. Mention an experiment which shows that organic
substance formed in the leaves travels down the stem outside
the cambium. (1901)
9. Obtain thin sections (_e.g._ plane-shavings) of as
many different kinds of wood as possible, and gum them into
a book, writing under each section the name of the wood
and the direction (transverse, radial-longitudinal, or
tangential-longitudinal) of the section.
10. Make yourself familiar with the appearance and
characters of the different kinds of timber used in
carpentry and joinery.
11. Notice the light-brown spots on the bark of twigs of
apple and horse chestnut. These are called _lenticels_; they
are breathing-pores. In how many other trees can you find
lenticels?
12. Observe whether an ivy stem puts out roots only where
they can become fixed to a support, or indiscriminately.
13. Make a list of plants which you have observed to climb
by twining stems, and note whether they are clockwise or
counter-clockwise climbers.
14. Make careful drawings of all the tendrils you can
find, and try to discover which part of the plant has been
modified to form the tendril.
15. Gently stroke a tendril of the passion flower and
write an account of any resulting movement.
16. Mention any three climbing plants which grow wild in
this country, and explain in each case how the plant climbs.
(1895)
17. What is the difference between (_a_) a thorn, (_b_) a
leaf-spine, and (_c_) a prickle? Give examples. (1896)
18. Enumerate and briefly describe the principal varieties
of tendrils, and explain how they act. (1897)
19. If a wire is fastened tightly about a growing
branch of a common tree, and left for two or three years,
what effect will be produced, and how can the effect be
explained? (1904)
20. What processes of vegetable growth are accompanied by
the presence of sugar? Give examples from plants within your
own experience. (King’s Scholarship, 1904)
21. What are the chief uses of the vessels of a herbaceous
stem? Mention observations and experiments in support of
your statements. (1905)
22. How can you demonstrate experimentally that food
substances, formed in the leaves of a tree, descend to the
branches below? (1905)
CHAPTER VI. SOME COMMON FLOWERS.
15. A TYPICAL FLOWER.
=I. The wallflower.=—After noticing the general habit of growth of a
wallflower plant (Fig. 57), and especially the shape and venation of
the leaves, make out the following parts in one of its flowers. On the
top of the flower-stalk (called the _receptacle_) are:
[Illustration: FIG. 57.—Wallflower. (× ⅙.)]
(_a_) Four small, narrow, purplish leaves, called _sepals_. The four
sepals together constitute the _calyx_. Take off the sepals one by one.
Notice that two opposite sepals are bulged out at their bases, forming
pouches containing nectar. Try to get out a small drop of nectar on the
point of a pencil and taste it.
(_b_) Four showy leaves arranged in the form of a Maltese cross,
called _petals_. They are yellow, or red, or purplish in colour, are
delicately scented, and have beautiful velvety surfaces. The four
petals together constitute the _corolla_. Take off the petals one by
one.
(_c_) Six _stamens_, each consisting of a greenish stalk or _filament_
surmounted by a yellow, boat-shaped body, called the _anther_. The
anther is a four-chambered box containing an enormous number of tiny
yellow grains called _pollen grains_. Two of the stamens are shorter,
and are fixed at a lower level on the receptacle than the remaining
four. Take off the stamens one by one.
(_d_) A central _pistil_, shaped somewhat like a slender bottle. At
the top, where the cork would come in a real bottle, is the notched
_stigma_, slightly sticky. The neck is called the _style_, and the part
corresponding to the body of the bottle is the _ovary_. Tear open the
ovary with a needle to see the _ovules_, which in an undisturbed flower
would have become seeds.
Watch bees visiting flowers. Does each bee confine itself to one kind
(species) of flower at each journey, or does it visit several kinds
indiscriminately? Try to discover what the bees are doing. Avoid
alarming them.
=The work of flowers.=—The roots, stem, and leaves of a plant do a
great deal of work, but, as it is performed for the benefit of the
plant itself, it is all, in a sense, selfish work. Plants, however,
like animals, grow old in time, and at last die. If they are not to
become extinct it is evident that they must devote part of their
energies to producing new individuals, and to sending these forth into
the world as well equipped as possible for the battle of life. This
unselfish and self-sacrificing part of a plant’s life-work is called
=reproduction=; in the higher plants it is carried out by flowers.
=The structure of a wallflower blossom.=—The flowers of different
groups of plants vary greatly in structure, but a good general idea of
the arrangement of the parts of a flower can be obtained by examining
the blossoms of a wallflower plant (Fig. 58). Other flowers may
afterwards be compared and contrasted.
[Illustration: FIG. 58.—Wallflower. _A_, branch, bearing leaves and
flowers; _B_, flower; _C_, longitudinal section of flower; _D_, stamens
and pistil; _E_, fruit; _S_, transverse section of stem, (× ½.)]
There are evidently at least eight leaves in the flower, but, unlike
the green foliage leaves, these are not arranged spirally, but stand
at nearly the same level on the end—called the =receptacle=—of the
flower-stalk. The most external leaves are four in number, small,
narrow, and purplish in colour. Each of these leaves is called a
=sepal=, and the four sepals together constitute the =calyx= of the
flower. Two opposite sepals are pouched at the base, forming pockets,
in which a sugary fluid, called nectar, collects. Before the bud
opens, the calyx is the only part of the flower which is visible. It
is probably developed in the wallflower solely for the protection of
the more delicate structures within. Next, inside the sepals, placed
alternately with them, and standing a little higher on the receptacle,
are four showy leaves arranged in the form of a Maltese cross. These
leaves are called =petals=, and the four petals together form the
=corolla=. The petals are delicately scented, and their surfaces have a
beautiful velvety sheen.
[Illustration: FIG. 59.—Cross section through a Wallflower bud. _Sep_,
sepals; _Pet_, petals; _lg.st._, anther of a long stamen; _sh. st._,
anther of a short stamen; _pol.sac._, pollen sacs; _Ov_, wall of ovary;
_Ovl_, ovule. (× 8.)]
When the sepals and petals are removed, there remain standing on the
receptacle six =stamens= surrounding a centrally-placed =pistil=
(Fig. 58, D). The stamens are the male part, and the pistil is the
female part, of the flower. Each stamen consists of a greenish stalk
or =filament=, surmounted by a yellow boat-shaped body called the
=anther=. The anther is a box with four compartments (Fig. 59). When it
is ripe, each compartment contains an enormous number of tiny, yellow
grains called =pollen grains=; and when the anther bursts (as it does
as soon as the flower opens) its inner face is covered by the yellow
dust of the pollen.
The =pistil= bears a rough resemblance to a slender bottle, and
consists of three distinct parts. The neck of the “bottle,” called
the =style=, is short in the wallflower, and differs from an ordinary
bottle-neck in being solid instead of tubular. At the top of the neck,
where the cork would come in a real bottle, is a body called the
=stigma=. The stigma of the wallflower pistil is hairy, notched, and
slightly sticky, from the presence of a sugary solution which forms
upon it. The part of the pistil, corresponding to the body of the
bottle, is the =ovary=. It contains four rows of little white =ovules=,
which are destined to become seeds capable of growing up and forming
new wallflower plants.
The relations of the parts of the flower are well seen in Fig. 59.
=Fertilisation.=—In order that an ovule may become a seed, its contents
must mix with the contents of a pollen grain. The fusion of the two
constitutes =fertilisation=. For fertilisation to take place, the
pollen grain must first of all gain access to the stigma of the pistil.
If this be prevented the flowers will wither without forming ripe
seeds. (This may be proved easily by Expts. =18=, 3 and =21=, 2.) The
sugary solution at the top of the stigma stimulates the pollen grains
to growth, and each puts out a long tube which grows down the style.
The living matter of the grains keeps near the tips of the tubes as
these continue their journey down the style. At length the tubes enter
the ovary and find the ovules. Each ovule has at one end a minute pore
(the micropyle—p. 6), and a pollen tube finds this and enters it. The
living matter of the pollen tube fuses with that of the ovule in the
neighbourhood of the pore, and fertilisation is effected. It is now
easy to understand that the comparatively insignificant stamens and
pistil are the all-important parts of a flower.
=How the wallflower advertises.=—Botanists have proved that a flower
produces more, and also better, seeds when it is fertilised by pollen
from another flower of the same species. This is called =cross
fertilisation=. The wallflower relies upon bees for the transference of
the pollen from one flower to another; and it is solely to attract them
that the petals are so delicately scented and brilliantly coloured, and
that sweet nectar collects in the sepal-pouches. The gaily coloured
petals are therefore advertisement placards which are hung out to
attract the attention of bees. A bee comes to a wallflower for the
sake of both nectar and pollen—the “bee-bread.” As the bee thrusts its
proboscis down between stamens and pistil in search of the sweet liquid
in the pouches, its head is pretty certain to come in contact with,
and to brush off, some of the pollen dust hanging loose on the inner
faces of the anthers. When the bee flies off to another wallflower and
continues its search for nectar, it almost invariably leaves some of
the pollen, from the first flower, on the hairy and sticky stigma of
the second.
In almost all cases when a flower is brightly coloured it depends upon
the help of insects for cross fertilisation.
16. THE WALLFLOWER FAMILY.
[Illustration: FIG. 60.—Shepherd’s Purse. (× ½.)]
1. =Shepherd’s purse.=—Compare the shepherd’s purse (Fig. 60) with the
wallflower. The flower is very much smaller, and white, but the parts
have the same arrangement as in the wallflower, viz., four sepals,
four petals arranged in the form of a cross, six stamens (two short
and four long), and a central pistil, all arranged separately on the
receptacle. Look down the plant, and notice that in the oldest (lowest)
flowers, everything but the pistil has dropped off, and that this
has become greatly enlarged to form a _fruit_. Cut some fruits open,
both lengthwise and crosswise, and observe that each consists of two
pocket-like chambers, separated by a thin partition on which the seeds
are borne. Notice the manner in which the oldest fruits have opened
naturally.
2. =Other relatives of the wallflower.=—Compare also the flowers and
fruits of the stock and candytuft, and also cress, mustard, radish
(Fig. 61), cabbage, and turnip, which have been allowed to “run to
seed.” Examine the roots of the turnip and radish.
Make a note of the earliest dates on which you see the above plants in
flower.
=The wallflower family.=—The plants of the family to which the
wallflower belongs are of very great importance to mankind; for
while not one of them is poisonous, many are extremely valuable as
food-crops. They are all _dicotyledons_; that is, their seeds contain
two cotyledons or “makeshift leaves,” as has already (Chapter I.)
been seen in the case of the mustard. The net-like venation of the
leaves of the full-grown plant also indicates this. There is a great
family likeness between the flowers of this group, and they are easily
recognised (Fig. 61 _a_, _c_) by the cross-shaped corolla and the six
stamens (two short and four long). All the parts of the flower—sepals,
petals, stamens, and pistil—are fixed separately on the top of the
flower stalk or receptacle. The cross-arrangement of the petals has
led to these plants being called =Crucifers= (cross-bearers). The
=shepherd’s purse= (Fig. 60)—so-called from the shape of its fruit—is
a common weed with small, white flowers. All stages of the flower may
generally be found on the same plant. While at the top the buds may
still be unopened, the flowers below have been fertilised (in this case
generally self-fertilised); the sepals, petals, and stamens—having
fulfilled their duties—have fallen off; the ovules have become seeds,
and the ovule-box or ovary has become a seed-box or fruit, consisting
of two bags separated by a partition which bears two rows of seeds on
each side (Fig. 62).
[Illustration: FIG. 61.—Wild Radish. _a_, flower (nat. size); _b_,
petal; _c_, stamens and pistil (× 2); _d_, pistil (× 2); _e_, fruit (×
1); _f_, cross section of fruit; _g_ and _h_, embryo (mag.)]
[Illustration: FIG. 62.—_A_, Fruits of Shepherd’s Purse (× ½); _B_, a
single open fruit (mag.); _C_, cross section of _B_.]
[Illustration: FIG. 63.—Root of Turnip. (× ⅓.)]
[Illustration: FIG. 64.—Root of Radish. (× ½.)]
=Useful crucifers.=—The turnip and the radish are largely cultivated
for their roots (Figs. 63 and 64), and are then taken out of the
ground at the end of their first season. As these plants naturally
flower in their second year of growth and then die, they are called
=biennials=. The production of flowers and fruit is a great strain on a
plant, and it is to prepare for the effort that the turnip and radish
store so much food in their roots during the first year as to give them
a globular and spindle shape respectively. A carrot is not a crucifer,
but it also adopts this device.
The cabbage is grown for its leaves. Varieties of the cabbage are
Brussels sprouts, broccoli, and cauliflower; it is the very small
flower-buds of the last two which are eaten. Cress and white mustard
are eaten in the seedling stage. The seeds of the black mustard are
ground and eaten as a condiment.
17. THE BUTTERCUP FAMILY.
1. =The buttercup.=—Notice the habit of growth, characters of the
leaves, etc. Is the buttercup a dicotyledon? Make this observation with
all flowering plants. (See, however, Chap. VIII., p. 163.)
In the flowers of a buttercup (Fig. 65) make out:
(_a_) The _calyx_ of five green, separate sepals; they are
the only parts to be seen in young, unopened buds. Take off
the sepals of a fully-opened flower one by one.
(_b_) The _corolla_ of five, golden-yellow, separate
petals, alternate with the sepals. Notice the nectary—a
little pocket—near the base of the upper surface of each
petal. Take off the petals one by one and observe that they
are fixed on the receptacle, a little higher than the sepals.
(_c_) The large number of separate _stamens_, inserted
still higher on the receptacle.
(_d_) On the top of the receptacle the large number of
separate, flask-shaped bodies, which together make up the
_pistil_. Each of these is called a _carpel_.
Watch bees and other insects visiting buttercups and notice how they
stand on the flower to obtain the nectar from the nectaries. Does a bee
on leaving go to another buttercup, or does it change to another kind
of flower?
In a flower from which the sepals, petals, and stamens have fallen,
notice the compound _fruit_ (Fig. 66), consisting of ripened carpels.
Open a carpel with a needle and pick out the single seed.
2. =Other plants of the buttercup family.=—Notice that in the anemone
(Fig. 67) and marsh marigold (Fig. 68), the sepals appear to be absent
(the three leaves under the anemone flower are not parts of the flower;
they are called _bracts_). The apparent petals are really the sepals;
it is the corolla which is absent. Observe the large number of stamens,
and notice that the pistil consists of several separate carpels.
In what kind of ground have you seen these plants growing wild?
[Illustration: FIG. 65.—Buttercup. (× ¹/₁₀.)]
=The buttercup family.=—The buttercup (Fig. 65) and its relatives
resemble the crucifers (1) in being dicotyledons (as is indicated (p.
40) by the venation of the leaves), and (2) in the fact that, of the
parts which compose the flower, each group is arranged separately on
the receptacle. For example, the stamens are not connected with either
the calyx, the corolla or the pistil. Having noted these points of
resemblance, however, we are met by some important differences. In the
buttercup there are usually five sepals and five petals; there may
be twenty or more stamens; and the pistil is not a single structure,
but consists of a number of separate parts, each of which is called
a =carpel=, and contains a single ovule. The pistil of a crucifer
consists of only two carpels; these are welded together into a single
structure, and until the fruit ripens a slight notch in the stigma is
the only external indication that the pistil is in two parts.
[Illustration: FIG. 66.—_a_, Compound fruit of Buttercup (× 2½); _b_, a
carpel (× 4); _c_, carpel in longitudinal section (× 4).]
[Illustration: FIG. 67.—Anemone. (× ⅓.)]
Insects visit buttercups for the sake of nectar and pollen, and whilst
creeping about the flower transfer pollen to the stigmas of the
carpels. The insects may have brought some of this pollen from other
flowers, and then cross-fertilisation is caused. If the pollen is
derived from the same flower self-fertilisation is the result.
After fertilisation the ovules become seeds; the sepals, petals, and
stamens drop off; and the carpels swell up, forming a dry compound
fruit (Fig. 66), consisting of several nutlets.
Two other common plants of this family are the =anemone= (Fig. 67) and
=marsh marigold= (Fig. 68). In neither of these cases has the flower
any petals, but the calyx has taken on the appearance of a corolla. In
the marsh marigold it is large and yellow; in the anemone it is white
or purple. The three green leaves immediately beneath the flower of the
anemone are called _bracts_. They should not be mistaken for sepals.
[Illustration: FIG. 68.—Marsh Marigold. (× ⅙.)]
The plants of the buttercup family are as generally poisonous as the
crucifers are wholesome. Monkshood is especially poisonous, and its
root has been mistaken, with fatal results, for that of horse-radish.
It is often noticed that grazing cattle avoid the buttercups in a
field. The bitter and disagreeable taste of the leaves is of course a
valuable protection to the plant.
18. THE PEA FAMILY.
1. =The garden pea.=—Notice again the habit of the plant: its compound,
net-veined leaves with large stipules, and its method of climbing by
tendrils which are modified leaflets. Examine the flower and make out
(_a_) the _calyx_ of five united sepals; (_b_) the curiously shaped
_corolla_. The large upper petal is called the _standard_, the two at
the side are the _wings_, and the lowest (really two locked together)
is the _keel_; (_c_) the ten _stamens_. One (opposite the standard) is
separate; the remaining nine have the lower parts of their filaments
united to form a tube. Slit open the filament-tube and remove the
stamens, noticing how they are attached to the other parts of the
flower; (_d_) the _pistil_. Slit open the ovary and see the ovules in
it. Watch the various stages of the formation of the fruit (pod).
[Illustration: FIG. 69.—Vetch. (× ⅓.)]
Watch bees visiting the flowers. The insect alights on the wings, and
its weight pulls them down and lowers the keel, bringing the stamens
against the bee’s body.
Cut a complete flower down the middle with a sharp knife, and notice
that calyx, corolla, and stamens seem not to be inserted separately on
the receptacle, but to spring from a common base.
2. =Other plants of the pea family.=—Examine also bean, vetch (Fig.
69), meadow vetchling (Fig. 70), clover (Fig. 71), laburnum (Fig. 73),
and broom. Compare the habits of growth of the plants, and notice that
they all have the same peculiar shape of flower. Dissect a flower of
each. Notice that in laburnum and broom the ten stamens are all united.
In clover the flowers are in _heads_. Notice how the leaflets of the
clover plant close at sunset.
3. =Fertilisation.=—Dig up several red clover plants in early summer
and pot them. Cover about half the plants with gauze, so fixed on wire
frames that insects cannot get inside, and then put all the plants
together where they will get plenty of sun. Water them regularly, and
notice which plants ripen seed. How do you account for the differences?
[Illustration: FIG. 70.—Meadow Vetchling. (× ¼.)]
[Illustration: FIG. 71.—Red Clover. (× ⅙.)]
[Illustration: FIG. 72.—Bird’s-foot Trefoil. 1, flowering branch (× ⅔);
2, flower; 3, pistil and stamens; 4, pistil (× 1¹/₉); 5, fruit (× ⅔); 6,
corolla; _a_, standard; _b_, wings; _c_, keel; 7, diagram of flower.]
[Illustration: FIG. 73.—Flowering branch of Laburnum; _st_, standard;
_w_, wings; _k_, keel; 1, 2, 3, the flower from different points of
view. (× ½.)]
=The pea family.=—Plants of the pea family are found in all quarters
of the earth. They are of very diverse size and habit of growth; the
laburnum, for example, is a tree; the gorse is a bush; the broad
bean has a strong, erect, herbaceous stem; the pea is a weak-stemmed
climbing plant; the clovers are small herbs with flowers forming
“heads.” Most of the members of the family agree in having a
“butterfly-shaped” corolla (Figs. 72 and 73), which consists of three
well-marked parts, viz., a large =standard=, a pair of =wings=, and two
closely-connected petals which form a boat-shaped =keel=. There are ten
stamens, and the filaments of nine of these usually cohere to form a
tube surrounding the ovary. In the laburnum, gorse, and a few others,
all the ten stamens are united. When a bee visits the flower, in search
of nectar, it alights on the “wings” of the flower, and its weight
depresses these and pulls down the keel. The anthers of the stamens
are so placed with respect to the keel that this results in a mass of
pollen being scraped off the anthers and forced out at the beak of the
keel, or in the stamens being suddenly liberated and scattering pollen
on the bee. The pollen sticks to the bee’s body, and some of it is
almost certainly transferred to the stigma of the next flower visited.
The quaint shape of the corolla is thus definitely adapted to the
visits of insects; for the nectar is so placed that to obtain it the
insect must carry off some of the pollen.
The calyx, corolla, and stamens are not obviously—as in the wallflower
and buttercup—inserted separately on the receptacle, but seem to spring
from a common base.
The fruit is a =pod= (Fig. 3), which opens when ripe along both margins
and liberates the seeds. Laburnum seeds are poisonous, but the seeds
of many other plants of the family (peas, beans, lentils, etc.) are
valuable foods.
19. THE ROSE FAMILY.
1. =The wild rose= (Fig. 75).—With a sharp knife cut vertically through
the middle of a wild rose. Notice that the receptacle forms a deep
cup, and that the carpels of the pistil are enclosed in the cup. From
the edge of the cup spring the five sepals, five petals, and numerous
stamens. What is the great difference between a rose and a buttercup?
Trace the formation of the succulent fruit or hip from the receptacle
cup of the flower. Cut through a rose hip, and observe the ripened
carpels in the interior.
2. =The blackberry.=—Similarly examine a blackberry flower (Fig. 76).
This is still more like a buttercup, but—as in the rose and the pea—the
calyx, corolla, and stamens seem to spring from a common base, and not
to be inserted separately on the receptacle.
Trace the formation of the compound fruit, and notice that each part is
like a little plum or cherry.
3. =The cherry.=—Similarly examine cherry blossom (Fig. 77). The
pistil consists of one carpel, and is fixed at the bottom of the
receptacle-cup, while the calyx, corolla, and stamens are fixed on the
margin of the cup. Trace the origin of each part of the fruit.
Compare the plum and apricot.
[Illustration: FIG. 74.—Apple Blossom. (× ⅓.)]
[Illustration: FIG. 75.—Wild Rose. (× ⅓.)]
4. =The apple and pear.=—Cut vertically through an apple blossom (Fig.
74) or pear blossom (Fig. 78), and notice that the ovary is embedded in
the receptacle, and that calyx, corolla, and stamens are fixed on the
top of this. Cut across the ovary and see the five divisions (carpels).
The eatable part of the fruit is the swollen receptacle. Compare the
hawthorn.
=The wild rose.=—The plants of the rose family are, most commonly,
woody trees or shrubs. The leaves are provided with stipules in nearly
all cases. The =wild rose= (Fig. 75) may be taken as a type of the
group. It bears a superficial resemblance to a buttercup, but on
dissection considerable difference in the arrangement of the parts is
seen. In the rose, the receptacle is urn-shaped; from the margin of the
urn spring the five sepals, five petals, and numerous stamens; while
inside the urn the separate carpels of the pistil are inserted. In the
=blackberry= (Fig. 76), =raspberry=, and =strawberry= the receptacle is
knob-shaped, and the carpels are arranged on the outside of the knob,
somewhat as in the buttercup. Here again, however, the calyx, corolla,
and stamens differ from those of the buttercup in seeming to arise from
a common base.
[Illustration: FIG. 76.—Blackberry. 1, flowering branch (× ⅓); 2,
longitudinal section of flower (× 1); 3, fruit (× ⅓); 4, diagram of
flower.]
[Illustration: FIG. 77.—Cherry. 1, flowering branch (× ⅔); 2,
longitudinal section of flower; 3, longitudinal section of fruit.]
The differences between the rose, blackberry, raspberry, and strawberry
are more marked when the pistil has become a fruit. The fleshy part of
the rose hip is the urn-shaped receptacle which encloses the ripened
carpels. In the case of the blackberry and raspberry the receptacle
is dry, and is surrounded by the compound fruit (Fig. 76, 3) of the
several bodies like little plums or cherries. The eatable part of the
strawberry fruit (Fig. 144) is the swollen receptacle, on the outside
of which are the little yellow nutlets derived from the carpels of the
flower.
In the =cherry= (Fig. 77), =plum=, and =apricot= the pistil consists
of only one carpel, which is enclosed in the urn-like receptacle.
After fertilisation, the greater part of the wall of the ovary becomes
fleshy, and one of the two ovules contained in it becomes a seed. The
“stone” is formed from the innermost part of the ovary wall.
The =apple= (Fig. 74) and =pear= (Fig. 78) have their five carpels
embedded in the receptacle, and the rest of the flower stands on this
part. The eatable portion of the fruit is the swollen receptacle. The
=hawthorn= has usually only two carpels, and in the fruit the part
derived from the receptacle becomes hard and horny. In other respects
it is very similar to the apple.
[Illustration: FIG. 78.—Pear. 1, flowering branch (× ⅔); 2,
longitudinal section of flower; 3, longitudinal section of fruit; 4,
diagram of flower.]
The rose family is widely distributed, especially in temperate regions.
20. THE PARSLEY FAMILY.
1. =The poison hemlock.=—Examine this plant (Fig. 79) very carefully,
remembering that it is poisonous. Notice the general habit of growth;
the characters of the sheathing, compound leaves; the hollow ribbed
stem; and also the arrangement of the flowers, which is characteristic
of the family. From the top of a main flower-stalk several smaller
stalks come off together, like the ribs of an umbrella. From the top of
each of these spring the stalklets which bear the small white flowers.
Notice the _bracts_ at the points of origin of the stalks. Examine
the flowers, and watch insects visiting them. Which insects are most
commonly found on the flowers?
Compare the cow parsnip, the water hemlock, carrot, parsley, parsnip,
and celery, carefully noting the points of resemblance and difference.
[Illustration: FIG. 79.—Poison Hemlock. (× ¼.)]
[Illustration: FIG. 80.—Hedge Parsley. (× ⅙.)]
=The parsley family.=—The plants of this family may be recognised
easily by the arrangement of the flowers. Several stalks spring
together from the top of the main flower stalk and each of these again
gives rise at its tip to a number of smaller stalks, at the ends of
which the small flowers are borne (Figs. 79 and 80). The flowers are
fertilised by the aid of insects, and as the nectar is on the surface
it is accessible to small insects such as flies, beetles, etc. The
flowers are rendered more conspicuous by being placed close together.
The stems are usually hollow, and the leaves are alternate, and
generally compound, with sheathing bases. Many of the plants of this
family are very poisonous, and such should be carefully distinguished
and whenever possible exterminated.
The =poison hemlock= (Fig. 79) varies in height from two to seven
feet. It has a hollow stem which is spotted with purple in the lower
part, and when bruised the leaves give off a smell like that of mice.
Cattle are often poisoned by eating the plant in hay, and children
have been poisoned even by blowing whistles made from the stem. The
=water-hemlock= is extremely poisonous. It grows along the sides of
pools. The stem is hollow, and the leaflets of the compound leaves
are finely toothed. The root is a cluster of fleshy swellings, and
has unfortunately a rather pleasant taste. Other poisonous plants of
the family are the =water dropwort= and the =fool’s-parsley=. Among
the harmless and useful members of the group are =celery= (when
cultivated), =carrot=, =parsnip=, and =parsley=.
21. THE PRIMROSE AND COWSLIP.
1. =The primrose.=—Examine the habit (Fig. 81) of the plant, its
underground stem, its spoon-shaped leaves—arranged in a rosette—and
the manner in which the flowers spring from the stem. In the flower
make out (_a_) the _calyx_, 5-pointed and with united sepals; (_b_)
the _corolla_, consisting of 5 petals united below into a tube. Tear
down the corolla-tube to see (_c_) the 5 _stamens_ inserted on the
corolla-tube. In some (“thrum-eyed”) flowers the anthers are at the top
of the tube; in others (“pin-eyed”), they are halfway down; (_d_) the
_pistil_, consisting of stigma, style, and ovary. In thrum-eyed flowers
the style is short and the stigma is halfway down the corolla-tube;
while in pin-eyed flowers the style is long and the stigma is at the
top of the tube. Do you find both pin-eyed and thrum-eyed flowers on
the same plant, or does one plant bear only one kind?
2. =Fertilisation.=—Cover up a plant of each kind with gauze, to keep
insects from the flowers, and notice whether the covered flowers ripen
seeds like the others.
3. =The cowslip.=—Compare the cowslip (Fig. 83), and notice that the
main stalk gives off from the same point several smaller stalks,
each of which bears a flower. Observe that the cowslip also has both
pin-eyed and thrum-eyed forms of flowers.
[Illustration: FIG. 81.—Primrose. (× ¼.)]
=The primrose and cowslip.=—In the primrose we have flowers of a
type differing from all those previously considered in this chapter.
Not only are the sepals joined together to form a five-toothed
=calyx-tube=, but the five petals are also joined together to form a
=corolla-tube=, and the stamens are fixed on the corolla-tube. There
are two kinds of primroses, known to country children as pin-eyed
and thrum-eyed flowers respectively (Fig. 82). The two forms grow
on separate plants. In a pin-eyed primrose the style is long, and
the stigma—looking somewhat like the head of a pin—is at the top of
the corolla-tube; while the stamens are halfway down. The thrum-eyed
primroses have their stamens at the top, while the stigma of the pistil
is halfway down the tube, exactly opposite the place where, in the
pin-eyed form, the stamens are inserted. This curious state of things
was a great puzzle to botanists until Darwin cleared up the mystery. A
bee, thrusting its proboscis down a pin-eyed primrose in search of the
nectar at the bottom, dusts it with pollen about halfway down—just in
the place which will come in contact with the stigma when the animal
visits a thrum-eyed flower. And the pollen from the thrum-eyed form
adheres to the part of the bee which will presently touch the stigma of
a long-styled flower. This beautiful and simple arrangement makes it
practically certain that each primrose shall be fertilised by pollen
from the other form. In the thrum-eyed form, however, it is possible
for pollen to fall upon the stigma and produce self-fertilisation.
[Illustration: FIG. 82.—Chinese Primrose. _L_, long-styled (“pin-eyed”)
flower; _K_, short-styled (“thrum-eyed”) flower; _G_, stigma; _S_,
anthers.]
[Illustration: FIG. 83.—Cowslip. (× ⅙.)]
It is obvious that the cowslip (Fig. 83) is closely related to
the primrose. The difference lies chiefly in the character of the
flower-stalk. In the cowslip this is long, and it bears at its top
several stalklets, each of which ends in a flower. As in the primrose,
cross-fertilisation is secured by some flowers being pin-eyed (long
styled) and others thrum-eyed (short styled).
22. THE DAISY AND ITS RELATIVES.
1. =The daisy.=—Take up several daisy plants (Fig. 84) entire, and
wash away the soil from the roots. Notice how the stems—some of which
are underground—are connected together. Draw a leaf. What advantage is
it to the plant to have leaves of the shape noticed? Cut vertically
through the “head,” and notice that what is usually called the “flower”
really consists of a large number of small flowers.
[Illustration: FIG. 84.—Daisy. (× ¼.)]
The central or _disc flowers_ are tubular. Which disc flowers
open first, those near the middle or those nearer the edge of the
disc? Pick off a flower and notice the 5-toothed corolla. Tear the
corolla down with a needle, and observe the tiny stamens (5) fixed
on the corolla-tube. The anthers are joined together. Notice the
divided stigma of the pistil. The white and pink _ray flowers_ have
strap-shaped corollas. They have no stamens, but each has a pistil
like that of a disc flower. What do you think is the object of the ray
flowers being so conspicuous? Why do they close over the disc at night?
Notice the large number of green _bracts_ below the disc.
2. =The dandelion.=—Compare the dandelion (Fig. 85). Notice that all
the flowers are strap-shaped, like the ray flowers of the daisy. Pull
one out, and make out the strap-like corolla, the five stamens with
joined anthers, and the double stigma (Fig. 86, 2). Notice the tuft of
fine hairs below the corolla and above the knob-like ovary. The tuft of
hairs is the top of the calyx-tube.
When the flowers have been fertilised, the yellow corollas wither, and
each calyx-tube elongates until it is about an inch long, the tuft
of fine hairs being still at the top (4). Blow a “clock,” and notice
how easily the fruits are detached from the disc and how slowly they
settle. What advantage is this to the plant?
[Illustration: FIG. 85.—Dandelion. (× ¼.)]
3. =The thistle.=—Compare the thistle (Fig. 87). The bracts are very
prickly. Is this an advantage? Are the flowers tubular or strap-shaped?
Examine the fruits (“thistle down”) and compare with those of dandelion.
=The daisy.=—What is generally called the “flower” of the daisy (Fig.
84) really consists of a very large number of small separate flowers
set close together on a flattened disc or receptacle. The group of
flowers is called a =head=. On the lower surface of the head are
several green leaves, or bracts, which protect the bud before it opens.
The flowers of a daisy-head are of two kinds. The white or pink straps
set round the edge of the head are the corollas of the ray flowers.
They have no stamens, but a pistil with a divided stigma is present in
each. The disc flowers are yellow and tubular. The corolla consists of
five united petals. On it are fixed five stamens, the anthers of which
are joined together. The pistil is like that of a ray flower. No calyx
is present in either the ray or disc flowers of the daisy.
The daisy is fertilised by the aid of insects which are attracted by
the strap-like corollas of the ray flowers.
[Illustration: FIG. 86.—Dandelion. 1, two heads and a leaf (× ⅔); 2, a
single flower (× 2); 3, fruit (× ⁸/₃); 4, receptacle, with one fruit.]
The =dandelion= “flower” (Fig. 85) is also really a head consisting of
a great many separate flowers. There are often between two and three
hundred of these little flowers present in one head. Below the head
are several green bracts, and these protect the flowers both in the
bud and at night (Fig. 86). The flowers of the dandelion are all of
one type. The corolla is strap-shaped (Fig. 86, 2), and of a beautiful
yellow colour to attract insects. On the end of the strap may be seen
five small teeth which indicate that it really consists of five united
petals. At the base of the corolla is a tuft of fine hairs, which is
the top of the calyx-tube, and at the bottom of the flower is a little
white knob—the ovary. The five stamens are fixed on the inside of the
corolla tube. Their anthers are united to form a tube through which the
upper part of the style, and the forked stigma protrude.
[Illustration: FIG. 87.—Thistle. (× ¼.)]
[Illustration: FIG. 88.—Tubular flower of Thistle (magnified).]
It will be noticed that a dandelion flower is practically like a ray
flower of a daisy, with the addition of calyx and stamens.
The =thistle= (Fig. 87) is another common member of the family. Its
bracts are prickly, and are a protection from the attacks of animals.
The flowers (Fig. 88) are all tubular. The common thistle distributes
its fruit by a plume of radiating fine hairs—the calyx. The fruit is
commonly known as “thistle down.”
The =Compositae=, as plants of this family are called, are found in all
parts of the world. The family is the largest in the vegetable kingdom,
and many of the plants included in it are of considerable importance.
23. THE FOXGLOVE FAMILY.
1. =The foxglove.=—Examine a flowering plant of foxglove (Fig. 89).
Notice the general habit of growth. In the flower make out the
five-lobed calyx, the irregular corolla with five petals joined to form
a tube, the four stamens (two long and two short) fixed on the corolla
tube (Fig. 90), and the form and attachment of the pistil. Watch bees
visiting the flower. 2. =The speedwell.=—Compare the speedwell (Fig.
91), and notice that the corolla is more nearly regular than is the
case with the foxglove, that it consists of _four_ combined petals, and
that _two_ stamens are fixed upon it.
3. =The musk.=—Compare the musk. Dissect a flower and notice the forms
and positions of the parts. Especially examine the pistil with its
two-lobed stigma. With a hair, carefully touch one of the lobes of the
stigma of a growing flower and watch how the lobes close. Do the lobes
open again? Put a little pollen on, and watch to see if this time the
lobes open again after closing.
Watch insects visiting the flowers and try to make out how the
pollination of the stigmas is brought about.
=The foxglove family.=—In the primrose and the disc-flowers of the
daisy are seen examples of regular, dicotyledonous flowers with five
petals fused to form a corolla-tube, and with five stamens inserted on
the corolla. In the strap-shaped flowers of the dandelion the corolla
is irregular, but still consists of five fused petals and bears the
five stamens.
[Illustration: FIG. 89.—Foxglove, (× ⅙.)]
The =foxglove= (Fig. 89) and its relatives have also irregular corollas
of joined petals on which the stamens are fixed; but the stamens
are usually only four in number, two being long and two short as
in Fig. 90, _b_. In the foxglove the stamens ripen and shed their
pollen before the pistil of the same flower is mature. This prevents
self-fertilisation, but bumble bees in passing from one flower to
another convey the ripe pollen of the younger flowers to the stigmas of
flowers which are ready for fertilisation.
[Illustration: FIG. 90.—Foxglove. _a_, flower; _b_, corolla cut open
and spread out; _c_, calyx and pistil; _d_, fruit; _e_, section of
fruit. (× ⅔.)]
The pretty blue =speedwell= (Fig. 91) is closely related to the
foxglove, but its corolla has only four lobes instead of five, and some
times these seem of almost equal size at the first glance. Generally,
however, the corolla is very plainly irregular. The speedwell has only
two stamens, while =mullein= has five.
[Illustration: FIG. 91.—Speedwell. (× ½.)]
=Calceolaria=, =musk=, =gloxinia=, and =snapdragon=—other members of
the family—are often cultivated in gardens.
The flowers of the musk are especially interesting, because they
show so well what is called =irritability=,—the power which all
living things possess of acting in a definite manner in response to a
definite irritation or stimulus. We have already seen good examples
of plant-irritability in the way a climbing stem winds itself round
a support. The stigma of the musk flower has two flaps. If these
are touched with a hair or bristle they quickly close together, but
presently open again as if they had found out that they had been
tricked. When, however, a little pollen is put on the flaps they close
finally, for their whole object is accomplished.
Some of the plants in this family are poisonous, the foxglove being
especially so in all its parts.
24. THE LABIATES.
1. =The deadnettle.=—Examine a deadnettle plant (Fig. 92). Notice
the habit of growth, and write down a description of the shape and
appearance of the stem and leaves. What is the shape of the flower? How
many sepals, petals, and stamens has it? Do the stamens ripen first, or
does the pistil? What insects do you find visiting the flower? Try to
find out how they pollinate the stigma.
[Illustration: FIG. 92.—White Deadnettle. (× ⅙.)]
2. =Other labiates.=—Compare the sage (especially in respect of its
relation to bees), rosemary, thyme, marjoram, and mint, and distinguish
between their various flowers, leaves, and scents.
=The labiates.=—The deadnettle is a type of an easily recognisable
family of plants. The stem is square in section, and the leaves are
arranged upon it in opposite pairs at right angles to each other. The
plants are hairy and have distinctive odours. The aroma of =thyme=,
=mint=, =marjoram=, =sage=, etc., has led to the plants being used for
flavouring food. None of the labiates is poisonous.
The shape of the flower is very characteristic, and is specially
adapted to the visits of bees. The flowers are so modified that the
lowest part of the corolla forms a platform on which the bee may
conveniently alight, while the upper petals unite into an arched roof
which protects the pistil and stamens.
[Illustration: FIG. 93.—Pollination of the Sage by a bumble bee. For
explanation see text. (× 1.)]
The mechanism of cross-pollination is particularly well shown in the
case of the =sage= (Fig. 93). The flower contains four stamens, but two
of these have lost their use, and the others are modified in a strange
manner. The whole stamen has somewhat the shape of a capital T, and at
each end of the cross-piece is a pollen box. Usually the cross-piece
(_c_, Fig. 93, 3) is not at right angles to the filament, but is swung
up—the junction acts as a hinge—until it is nearly vertical (Fig. 93,
4). The pollen box _s_, which is at the lower end of the cross-piece
_c_ when this is vertical, contains hardly any pollen. The entrance to
the honey tube is thus guarded by two pillars, the filaments (_f_) of
the stamens; and the lower pollen box (_s_) of the cross-piece of each
stamen is directly in front of the bee’s head as it stands on the lower
lip of the flower. When it pushes forward its head to reach the nectar
it comes in contact with the lower pollen boxes, and the cross pieces
swing round on their hinges, bringing the upper pollen boxes down with
a smack on the bee’s back (Fig. 93, 1), and sprinkling it liberally
with pollen dust. Having shed their pollen the stamens shrivel up, and
the pistil comes to maturity. As the pistil ripens, the stigma arches
over (Fig. 93, 2) so as to scrape along the back of any bee visiting
the flower for the nectar, and thus to wipe off the pollen which has
been brought from a younger flower.
25. THE LILY AND SNOWDROP FAMILIES.
1. =The hyacinth.=—Take up a plant (Fig. 94) entire and notice the
underground bulb with roots springing from its lower surface, and the
long narrow leaves. Is the venation of the leaves parallel or net-like?
Is the hyacinth a monocotyledon or a dicotyledon? See the bract at the
base of each flower-stalklet.
Examine the flower. Its leaves cannot be distinguished into calyx
and corolla, but are quite similar to each other in size, shape, and
colour. They are therefore called the _perianth_. The perianth leaves
are united to form a tube. Tear down the tube to see the six stamens
fixed on it. Are they all on the same level? Examine the pistil and cut
the ovary across to see the ovules in the three joined carpels. Which
is fixed at the higher level, the perianth or the base of the pistil?
2. =Other plants of the lily family.=—Examine also the white lily,
tulip, star of Bethlehem, and lily of the valley, and notice that in
spite of small differences they are all monocotyledons (how do you know
this?) and all have the _perianth fixed below the ovary_.
3. =The snowdrop.=—Compare and contrast the snowdrop (Fig. 95). Make
out that it is a monocotyledon, but that its _perianth is inserted
above the ovary_. This is the great point of difference from the lily
family. Notice also how the stamens are fixed.
[Illustration: FIG. 94.—Wild Hyacinth. (× ¹/₁₀.)]
[Illustration: FIG. 95.—Snowdrops. (× ¼.)]
4. =Other plants of the snowdrop family.=—Examine the daffodil (Figs.
55 and 96) and narcissus. Arising from the short perianth tube of the
daffodil is a longer one which is often mistaken for a corolla; it is
called the _corona_. None of the flowers hitherto described contains
anything corresponding to a corona. The corona in the narcissus is
short. As in the snowdrop, the perianth is fixed _above_ the ovary.
Observe how the stamens are fixed, and notice the dry leaf beneath the
flower.
=The lily family.=—Either the =wild hyacinth= (Fig. 94) or the
cultivated single hyacinth may be taken as a good representative of
this family. The first point which strikes the student on examining the
general “habit” of the plant is the character of the long sheathing
leaves. Their veins do not form an obvious network, such as is seen in
the leaves of dicotyledons, but run lengthwise and roughly parallel
to each other in the manner characteristic of grasses and other
monocotyledons (p. 40). The leaves are narrow, and are not divided into
blade and stalk; they and the flower stalk spring from an underground
bulb (p. 84) which consists chiefly of the swollen leaf-bases of
a previous season. A separate calyx and corolla are not to be
distinguished in the flower; the six leaves being all alike in size,
shape, and colour. These six leaves hence receive a special name, and
are called the =perianth=. The perianth leaves are united into a tube,
on the inside of which the six stamens are arranged in two series of
three each. In the middle of the flower, and fixed _above the insertion
of the perianth_, is the pistil, which consists of three united carpels.
[Illustration: FIG. 96.—Daffodil flower, cut down the middle. (× ⅔.)]
It will be noticed that the parts of the flower are in threes. There
are six united perianth leaves (three inner and three outer), six
stamens (also in two series), and three united carpels. This is very
common—though by no means universal—in monocotyledons.
All the plants of the lily family—including the tulips, the true
lilies, lily of the valley, asparagus, onion, etc.—agree in being
monocotyledons, and in their flowers having a conspicuous perianth (for
attracting insects) and six stamens, and in the ovary being above the
insertion of the perianth.
=The snowdrop family.=—Plants of this family are very similar to those
of the lily family; in fact in only one respect can any sharp line of
demarcation be drawn between the two groups; in the snowdrop and its
relatives the other parts of the flower stand _upon the ovary_ (Fig.
96). The flowers of some plants of the family, _e.g._ the daffodil,
possess a tubular outgrowth of the perianth, which is called a
=corona=. It is often mistaken for a corolla.
26. THE ARUM LILY.
1. =The cuckoo-pint.=—Examine the habit of the plant, and then cut open
the “flower.” You will probably find a number of small flies inside.
Examine the central rod and make out the stamens and pistils on it.
Which ripen first?
=The arum= “=lily=,” with its humble relative the cuckoo-pint (Fig.
97), merits special mention; for, in the first place, it is not a lily
at all, and secondly, it furnishes an extremely interesting example of
pistils coming to maturity before anthers, which is rather rare.
What is generally called the “flower” of the cuckoo-pint, or “lords and
ladies,” consists of a big curled leaf with a purple rod sticking up in
the middle. Near the bottom of the rod, but hidden from sight by the
lower part of the leaf, the true flowers arise. The chamber containing
them is shut in by a series of stiffish hairs which point downwards.
Below the hairs the rod supports a series of anthers and, near the
bottom of the chamber, several pistils. On cutting open the chamber
one nearly always finds a number of small flies, covered with pollen,
which they have brought from another arum. The flies get in easily
enough, but once in they are prisoners, for the down-pointing hairs
prevent them from getting out again. The pistils near the bottom of the
rod ripen, and are fertilised by the pollen the flies have brought.
After a time the anthers above ripen and shed their welcome pollen
on the hungry captives. Soon after this the hairs at the top of the
chamber shrivel up, and the flies, once more covered with pollen, are
at liberty to return to the outer world and, untaught by experience, to
repeat the experiment on another “flower.”
[Illustration: FIG. 97.—_A_, Cuckoo-pint. (× ¼.) In _B_, the front of
the leaf is cut away to show the rod _sp_ (_C_). _F_, female flowers;
_M_, male flowers; _st_, undeveloped male flowers.]
EXERCISES ON CHAPTER VI.
1. Describe the arrangement of the stamens in the
wallflower, the sage, and the primrose.
2. Describe the pistil of the deadnettle, the primrose, and
the shepherd’s purse. (1895)
3. In what respects does the flower of the buttercup differ
from that of the wild rose?
4. Explain the arrangement and form of the stamens in some
flower selected by yourself. Describe the structure and
contents of the anther. (1897)
5. Where is the pollen of a flower formed? What is its use? (1898)
6. Name one or two plants which do not ripen seed if insects
are excluded, and show why they do not. (1898)
7. Name two plants which would be in flower in each of the
months from March to August inclusive, and state in what
localities they would be found? (N.F.U.)
8. In what ways are insects attracted to visit flowers? Give
examples, with an explanation and drawing in each case, of
any special structure which may be a means of attraction. (N.F.U.)
9. Name ten flowering plants which may be found, (_a_) in a
shady wood, (_b_) in a meadow, in late spring.
10. Name any fresh specimens of flowers which usually could
be obtained growing wild in March, June, and October
respectively. (N.F.U.)
11. What wild flowers would you expect to find in early
April in your part of the country? In what kinds of places
would you look for them? (N.F.U.)
12. Name the parts of any flower you have examined which
are concerned with the production of seed.
(King’s Scholarship, 1904)
CHAPTER VII. GRASSES.
27. THE EVERYDAY LIFE OF A GRASS.
1. =General features.=—Pull up a sod of couch grass or of Yorkshire
fog and clear away the earth as well as possible. Notice the fibrous,
creeping branches or _stolons_, and the bunches of fine roots. In
summer the plant sends up also erect branches called _haulms_, which
bear leaves and flowers. Examine the shape of the leaves and notice
their parallel venation, which indicates (p. 40) that the plant is a
monocotyledon.
Observe how the leaves are borne on the haulms. Each leaf arises at a
knot (node) which is slightly swollen. The first part of the leaf is
a _sheath_ which encloses the haulm up to perhaps the next knot. Then
the leaf-blade stands out from the haulm. Turn back the blade slightly
and notice at this point a little strap, the _ligule_ (Fig. 98), on the
upper surface of the leaf-blade. Notice the variation in the size and
shape of the ligules of different kinds of grass.
Pull the haulm until it gives way, and draw the broken part out of the
leaf-sheaths. Chew the end, and notice the taste of sugar. Cut across
haulms of various ages. The young haulms are solid; the old ones are
hollow except at the nodes, where there is a horizontal shelf. Notice
this also in a bamboo cane, which is the haulm of a large tropical
grass. In spring, mark a young haulm, so that you can recognise it, and
measure it day by day. Note the measurements made on various days, and
also the date of opening of the flowers.
2. =Different grasses.=—Learn to recognise different grasses, not only
by the way in which the clusters of flowers are borne on the haulms
(Figs. 102 to 110) and by their colours, but also by the general
appearance of the plant when it is not flowering; the size and shape of
the leaves, the character of the ligules, the manner of rooting, etc.
Again germinate grains of wheat, and especially notice the single
cotyledon, the leaves, and roots (p. 21).
=Importance of grasses.=—It is impossible to over-estimate the value
to man of the various grasses. From the earliest times he has obtained
his staple food from the grains of such cereals as wheat, barley,
oats, rice, and maize; and his flocks and herds have grazed upon the
nutritious herbage of the plains. The numberless applications of such
large tropical grasses as the bamboo and the sugar-cane are also
well known. Sugar occurs very generally in grasses. It can easily be
recognised even in ordinary meadow-grass by pulling out the upright
“stem” from the sheathing leaves, and chewing the tender end.
[Illustration: FIG. 98.—Part of a Grass Stem and Leaf. _h_, haulm;
_s_, part of leaf-blade; _l_, ligule; _v_, leaf-sheath; _k_, node-like
swelling at the base of the leaf-sheath. (× ½.).]
=The general characters of grasses.=—In this country, grasses are
usually herbs not more than three or four feet high. They have abundant
leaves, which are long, narrow, and pointed at the end; they have
parallel veins like the leaves of most other monocotyledons. The roots,
too, are of the usual monocotyledonous type—springing in bunches from
the base of the stem, not branching from a main tap-root. Most grasses
spread by prostrate creeping branches or =stolons= (p. 83), which
spring from the axils of leaves and then run along beneath the soil.
At some little distance from the parent shoot, the stolon forms roots
below and a new shoot above. Other stolons arise in the axils of the
leaves of the new shoots, and so the process is repeated. In this way
such grasses become thoroughly established in the soil, and—if they
are of the undesirable species which are classed as “weeds”—cause much
trouble. This habit of growth is, however, useful in binding together
the sand and earth of embankments, etc., into a compact mass. The new
shoots of stolon-bearing grasses are of two kinds. Throughout the
greater part of the year they consist of tufts of leaves; but, in the
spring and summer, there are also produced erect branches—commonly
known as stems, but better called =haulms=—which bear leaves in the
“alternate” manner. The point at which a leaf springs from the haulm is
called a knot or node; it is usually swollen. The distance between two
knots is, of course, an internode. Extra roots are often given off from
the lower nodes of the haulm.
The lowest part of the =leaf= is a cylindrical, and generally
split, sheath (Fig. 98), which closely embraces the haulm for some
distance—often for the length of an internode. The sheath protects and
supports the soft, growing internode, inside it. At the top of the
sheath is the blade of the leaf, which stands out from the haulm. At
the junction of the blade and sheath, on the upper surface of the leaf,
is a little membranous outgrowth called a =ligule= (Fig. 98, _l_). It
often varies greatly in size and shape in different species of grasses,
and is a valuable means of distinguishing between them. Its use is not
certainly known. The student should examine the ligule of every grass
he studies.
The haulm, or flowering branch, is for some time short and solid,
and its nodes are so close together that the sheath of a lower leaf
may overlap several upper leaves. The haulm is thus protected from
the rough weather of the early spring. In the meantime, the flowers
have been developing at its upper end. When they are almost ready to
open, the haulm begins to grow very rapidly; its internodes elongate
so quickly that the internal tissues are torn and the haulm becomes a
hollow straw, except at the nodes, where there are horizontal shelves.
The haulm hardens and stiffens as it grows, so that, when the spikelets
of flowers at its upper end open, they are carried high above the
leaves on a slender but very strong rod, which bends and dances in the
wind, and allows the pollen to be detached and blown to the stigmas of
other flowers.
28. THE REPRODUCTION OF A GRASS.
1. =The arrangement of the spikelets.=—Gather ears (flowering haulms)
of several different kinds of grasses and notice the arrangement of the
flowers. In the oat (Fig. 99), the meadow grasses (Fig. 103), fescues
(Fig. 102), and others, the nodding oval bodies, called _spikelets_,
are borne on delicate stalks which spread outwards from the haulm. In
the foxtails (Figs. 104 and 105), timothy (Fig. 106), and sweet vernal
grass (Fig. 110) the spikelets are on short stalks, which can, however,
be seen on bending the ear sharply on itself. In wheat, barley, couch
grass, rye, and rye grasses (Fig. 109) the spikelets are devoid of
stalks and are set close along the haulm. _Be quite sure you understand
what is meant by a spikelet._ 39 spikelets are shown in Fig. 99.
[Illustration: FIG. 99.—Panicle of Oats. (× ¼.)]
2. =The arrangement and structure of the flowers.=—Take an open
spikelet from the ear of a grass (_e.g._ oat) which has large flowers,
and examine it. At the bottom are two boat-shaped leaves—really
bracts—called _glumes_. Remove them carefully. Above them will be
found two or more flowers, each flower enclosed in two other leaves.
The outer of these two leaves is called the _outer pale_; the inner
is the _inner pale_. From the middle of the back of the outer pale of
the oat springs a bristle called an _awn_. Remove the outer pale and
make out the three _anthers_ on long filaments, and the ovary with two
branching feathery _stigmas_ and short styles. Fig. 100 is a diagram of
a spikelet, which makes clear the relation of the glumes, pales, and
flowers. Fig. 101, _A_, is a spikelet of meadow fescue. Its outer pales
differ from those of the oat in not bearing awns. Otherwise it is very
similar. Fig. 101, _B_, shows the appearance (magnified) of a flower
of meadow fescue from which the outer pale has been removed. The two
little scales seen in front of the ovary possibly represent a perianth.
Be careful not to confuse the terms ear, spikelet, and flower. The ear
consists of a number of spikelets, and each spikelet consists of glumes
and one or more flowers.
Similarly dissect spikelets and flowers of other grasses, making notes
of the lengths of the stalks of the spikelets, and the presence (and
lengths) or absence of awns.
3. =The use of the awns.=—Try to find out the use of the awns. Are they
generally rough or smooth? Have you ever seen grass “seeds” sticking in
the wool of sheep? What kept them attached?
4. =Grain.=—Examine several kinds of grass “seeds” and make out that
they not only consist of the entire ripened ovary and are therefore
_fruits_, but that usually the pales also remain attached to them.
5. =The embryo and endosperm.=—Again cut through soaked grains of maize
and wheat, and make out the embryo and endosperm (p. 19).
=Grass flowers.=—Grasses are true flowering plants; but because they
depend on the wind for the transference of pollen to the stigmas,
they do not pander to the taste of bees and butterflies by secreting
nectar, and hence have no need to display those advertisement placards
which we call petals. For this reason their flowers are not generally
recognised as such. It requires a little care to make out the parts
of the flower and to understand the manner in which the flowers are
arranged among themselves.
[Illustration: FIG. 100—Diagram of a Grass Spikelet. _g_, the glumes;
_p₁_ and _p₂_, the outer and inner pales; _B_, flower.]
The whole group of flowers borne by any one haulm is generally called
the =ear= or =panicle=. The ear in its turn consists of several bodies
called spikelets. The appearance of the ear varies greatly in different
grasses (Figs. 102 to 110) according as the stalks of the spikelets are
long and spread outwards from the haulm, as in the meadow-grasses (Fig.
103), oats (Figs. 99 and 108), fescues (Fig. 102), and others; short,
as in the foxtails (Figs. 104 and 105), timothy (Fig. 106), and sweet
vernal grass (Fig. 110); or absent altogether, as in the wheat, barley,
rye, rye grasses (Fig. 109), and couch grass.
A single =spikelet= of a grass is shown diagrammatically in Fig. 100.
At the bottom are two boat-shaped bracts called =glumes= (_g_, _g_),
which almost or entirely cover the spikelet before it opens. When the
glumes have been removed a few flowers remain. Each one is protected by
two leaves called =pales=: outer (_p₁_) and inner (_p₂_) respectively.
The flower (_B_), as a rule, has _three stamens_, the anthers being
borne on long filaments and dangling out to the wind (Fig. 101), and a
pistil with two branching feathery _stigmas_. In many grasses the outer
pale bears a bristle called an =awn=, well seen in “bearded” wheat,
barley, and upright brome grass.
Fig. 101, _A_, represents a single spikelet of meadow fescue, with two
open flowers. The outer pales of this grass are not awned. Fig. 101,
_B_, shows a single flower of the same grass from which the outer pale
has been removed. The two little scales seen in front of the ovary (and
at _e_ in Fig. 100) possibly represent all that is left of a perianth
(p. 121).
=Fertilisation.=—By the time the flowers have opened, the haulm has
usually grown so tall that they are lifted well above the leaves. The
stamens hang their anthers loosely out to the wind, and, as the slender
haulm sways in the breeze, the pollen is readily detached and carried
to other flowers, perhaps miles away.
It is in order to catch the wind-borne pollen-grains that the stigmas
of grasses are branched and feathery. A great waste of pollen is
obviously entailed by this method. But on the other hand it must
be remembered that insect-pollinated flowers have to pay for their
privileges by storing nectar in cunningly hidden pockets, and by
advertisement-expenses, all of which the grasses avoid.
[Illustration: FIG. 101.—Meadow Fescue. _A_, spikelet with two open
flowers (× 1½); _B_, a flower from which the outer pale has been
removed (× 6).]
The ovule is fertilised in the usual way, by a pollen tube growing down
the style from a grain on the stigma. As a result of fertilisation the
ovary becomes a fruit containing one seed which fills it. In many cases
(_e.g._ barley and oats) the pales still remain in position and adhere
to the fruit. When the awn is present it may play an important part in
distributing the “seeds” by clinging to the hides of animals, etc.
Sometimes the awn is of use in fixing the grain in position on the
ground until the seed has germinated.
The structure of a grass seed has already (p. 19) been studied in the
wheat and maize. The developing embryo (p. 21) lives upon a store of
food called the endosperm (Fig. 16) until its roots and leaves are
sufficiently advanced to make food for themselves.
29. SOME COMMON GRASSES.
1. =The fescues.=—Gather plants of _meadow fescue_, and examine the
habit of growth (in tufts), the broad leaves, and the nodding panicles
and the flowers. Also examine the “seeds.”
Compare _sheep’s fescue_ (Fig. 102), and notice the very fine leaves.
The outer pales are awned.
2. =The meadow grasses.=—Examine the various species of meadow grass
(Fig. 103). They can be distinguished from each other by the characters
of the ligules. Notice the tree-like habit of the panicles and compare
them with the fescues. The meadow grasses never bear awns. Notice the
woolly “webs” at the bases of the “seeds.”
3. =The foxtails.=—In _meadow foxtail_ (Fig. 104) notice the prostrate
stolons and large succulent leaves. Does the grass grow in tufts? Why
not? What is the earliest date on which you have seen it in flower?
Double the ear on itself to see the short stalks of the spikelets. How
many flowers are there in each spikelet? Notice the silky awns of the
flowers.
Compare the _slender foxtail_ (Fig. 105), which is a troublesome weed.
4. =Timothy.=—Compare the leaves and flowers of timothy or “meadow
catstail” (Fig. 106) with the meadow foxtail. The ear is green and
rough, and the flowers are awnless. What is the date of flowering?
5. =Yorkshire fog.=—Notice the woolly covering of this weed (Fig. 107).
This covering and its bitter flavour make it distasteful to cattle.
Observe the “kneed” awn of the flower. Dig up and shake off the earth
from a sod to see the stolons.
6. =Wild oat= (Fig. 108). Examine the large spikelets and make out the
long twisted awn of the flower. Compare this weed with the _cultivated
oat_ (Fig. 99) and with the _yellow oat grass_.
7. =The perennial rye grass.=—Notice that the ear is flattened, and
that the spikelets are without stalks and have only one glume. The leaf
is glossy and has a prominent midrib and a flattened sheath.
[Illustration: FIG. 102.—Sheep’s Fescue. (× ⅙.)]
[Illustration: FIG. 103.—Smooth-stalked Meadow Grass. (× ⅙.)]
8. =Sweet-scented vernal grass.=—Notice the tufted habit of growth
and the characters of the leaves. Chew the stalk and notice the sweet
odour of new-mown hay. What is the date of flowering? Make out that the
flower has two stamens only.
9. =Rushes and sedges.=—Rushes and sedges are sometimes mistaken for
grasses. Examine the stems, the flowers, and the leaves (look for
ligules), and tabulate as many differences from the true grasses as
possible.
The student should learn to recognise the common grasses and to
distinguish the useful species from the weeds. When the grasses are in
flower there is not much difficulty in doing this, but the habit of
growth—the characters of the leaves and roots, and of the stolons of
the perennial species—should also be noticed carefully, as during the
greater part of the year these alone can be depended upon.
[Illustration: FIG. 104.—Meadow Foxtail. (× ⅙.)]
[Illustration: FIG. 105.—Slender Foxtail. (× ⅙.)]
=The fescues= fall into two groups according as their leaves are broad
or narrow. The =meadow fescue= is a good example of the former group.
It is found in meadows and pastures and has long broad leaves. =Sheep’s
fescue= (Fig. 102) is a good example of the second group of fescues.
It has very fine—almost bristle-shaped—leaves. Like the meadow fescue
it grows in tufts; it inhabits high lands and downs, especially in
limestone districts. The nodding spikelets are borne on fairly long
stalks, and the panicles are somewhat like those of the meadow grasses
(Fig. 103). The flowers of sheep’s fescue, however, bear short awns.
=The meadow grasses= much resemble the fescues in general appearance,
but the panicles (Fig. 103) are rather more tree-like—the stalks of the
spikelets spreading more horizontally—and the flowers are never awned.
The various species can be distinguished by their ligules; for example,
the ligule of the =smooth-stalked meadow grass= (Fig. 103) is blunt,
while that of the =rough-stalked= species is long and pointed. The
=annual meadow grass= is a weed to be found almost everywhere.
[Illustration: FIG. 106.—Timothy Grass. (× ⅙.)]
=The foxtails= are easily recognised by the tail-like appearance of the
ears (Figs. 104 and 105). The =meadow foxtail= (Fig. 104) is a valuable
grass with broad, long, and succulent leaves. It spreads by means of
prostrate stolons. The spikelets have short stalks, as can be seen on
bending the ear on itself, and each spikelet contains only one flower.
The silky awns give the ear a silvery grey colour. The grass flowers in
early spring. The =slender foxtail= (Fig. 105) is a most injurious weed
of cornfields. It can be distinguished from the meadow foxtail by its
less vigorous appearance, by the thinner and more pointed ear, and by
the black patches on the ear. It is often called “black bent.”
=Timothy grass= or “meadow catstail” (Fig. 106) has a general
resemblance to meadow foxtail, but its ears are rough to the touch and
green in colour. It also flowers much later in the year (July) and its
pales are awnless; the two grasses are therefore easily distinguished.
Timothy grows abundantly in clay soils, forming fairly close tufts.
[Illustration: FIG. 107.—Yorkshire Fog. (× ⅙.)]
[Illustration: FIG. 108.—Wild Oat. (× ⅙.)]
=Yorkshire fog= (Fig. 107) is a rank weed which is distasteful to
cattle, partly because it is covered with hairs and is difficult to
wet, and partly because it has a bitter flavour. It is pale in colour
and soft. It spreads rapidly by means of creeping stolons.
=Oat= grasses are of several species; one is cultivated as a cereal,
others are valued as forage for cattle, while others still are
troublesome weeds. The =wild oat= (Fig. 108) is a common weed in
cornfields. It is an annual, from two or three feet high, with large
spikelets forming a loose panicle, the flowers having awns twice as
long as the spikelets. The cultivated oat is supposed to be a variety
of the wild oat. The panicle of the =yellow oat grass= is oblong, and
has erect spikelets.
[Illustration: FIG. 109.—Perennial Rye Grass. (× ⅙.)]
[Illustration: FIG. 110.—Sweet-scented Vernal Grass. (× ⅙.)]
The =perennial rye grass= (Fig. 109) is very easily recognised by
its flattened ears of alternate, stalkless spikelets. Each spikelet
has only one glume. The leaves are dark-green and glossy, and the
leaf-sheaths are flattened instead of being round, as is the case with
most grasses. This is one of the most valuable of forage grasses.
=Sweet-scented vernal grass= (Fig. 110) does not occur very abundantly
in meadows and pastures, but the fragrance of new-mown hay is almost
wholly attributed to it. The odour can be distinctly perceived when a
stalk of the grass is chewed. The leaves are somewhat hairy and are
broad and flat. The spikelets are borne on short stalks. The flowers
open early and are remarkable in possessing only two stamens instead of
three.
=Rushes= and =sedges= are monocotyledonous plants which are often
erroneously called grasses. The stems of =rushes= are vivid green,
round, and pointed, and contain a distinct pith. The flowers (Fig.
111) are never in spikelets; they contain six stamens, and the pistil
has three long stigmas. The =sedges=, like grasses, have narrow,
pointed leaves, but the ligules are either very small or absent, and
the leaf-sheaths are not split. The flowers are often in spikelets
with glumes, as are grass flowers. The stems of sedges are solid and
triangular.
[Illustration: FIG. 111.—A Rush. _a_, flowering part of stem; single
flower (_b_) and pistil (_c_) more highly magnified.]
The flowers of rushes in many respects resemble those of the lily
family. Indeed it is supposed that the ancestors, not only of the lily
family, but also (along a different line) of the grasses and sedges,
were primitive and now extinct rushes. The lilies have developed the
rush perianth more and more as they have increasingly depended on
insects for pollination; while in the grasses and sedges the perianth
has gradually dwindled because these plants found that wind-pollination
was sufficient for their needs.
EXERCISES ON CHAPTER VII.
1. What grasses are grown as corn in your part of the
country? How can you recognise the grasses (_a_) before
flowering, (_b_) in the ear, (_c_) by the straw?
2. What forage grasses are most cultivated in your part of
the country? Make a list of the earliest dates on which
you have seen each in flower.
3. Examine grains of wheat, barley, oats, and rye, as
harvested. In which of these are the pales present? What
is the “beard” of barley?
4. Make a collection of “seeds” of various grasses,
and write down the characters by which they may be
distinguished from each other.
5. What are the commonest weeds you have seen in corn
fields? Do the corn and the weeds flower at the same time
or not? Is the time of flowering an advantage (_a_) to the
weeds, (_b_) to the farmer?
6. In what important respects do wind-fertilized flowers
differ from insect-fertilized flowers? Give examples of
each. (1898)
CHAPTER VIII. COMMON FOREST TREES.
30. THE OAK FAMILY.
1. =The oak.=—(_a_) _Habits of growth._—Examine an oak tree growing
in an exposed situation. What is its approximate height? Estimate the
diameter of the trunk at (_a_) the ground level, (_b_) at heights of 1,
2, 3, 4, etc., feet. At what height do the principal boughs come off?
About what angle do the boughs make with the trunk? Are they straight,
evenly curved, or zig-zag? Contrast in these respects an oak growing in
a plantation. Try to account for the differences observed.
(_b_) _The bark._—Examine the barks in oaks of various ages. Do the
ridges and furrows caused by the splitting of the bark form any
definite pattern, or are they arranged anyhow? Find a piece which shows
the pattern well, and make a drawing of it.
(_c_) _Method of branching._—Carefully observe the tree from a
distance, and notice how the great boughs spring from the trunk, and
in their turn give rise to smaller and smaller branches. Notice the
shape of a tuft of the smallest twigs against the sky; then close your
eyes and try to recall the picture. A still better way is to make a
careful drawing of the tree, not attempting to put in details, but
paying special attention to the trunk and main branches, and to the
general massing of the twigs. What is the general “expression” of the
tree? Would you call it, _e.g._, graceful, formal, sturdy, delicate,
stiff, or sombre? The method of branching is best seen in the winter or
spring, when the tree is leafless.
Examine a twig in winter or spring, and notice the position of the
buds. Round the tip there may be three or more crowded buds. The one
at the tip generally dies, and those just behind grow out into lateral
branches. Can you see any trace of this having been the case with the
big boughs? On a twig make out—from the marks on the outside—the growth
of last year, two, three, and four years ago. Cut these lengths across
with a sharp knife, and count the rings of wood.
(_d_) _The leaves._—Watch a marked twig from spring to summer, and
notice how short are the new shoots which come from the buds, and
consequently how close together the new leaves are. Does this account
for the crown of foliage on the tree being so dense? Make a drawing of
an oak leaf. At what time of the year do (i) the leaf-buds open, (ii)
the leaves fall from the tree? In winter, try to find an oak which has
not shed its leaves; is it a young tree or an old one?
(_e_) _The flowers._—In May notice (i) the hanging catkins, each of
which is a bunch of _male flowers_ containing stamens, but no pistil;
(ii) the groups of small _female flowers_ which arise in the axils of
two or three of the upper foliage leaves; each contains a pistil, but
no stamens. Notice the cup of scaly bracts, which surrounds the lower
part of a female flower. Does the tree flower before or after the leaf
buds have expanded?
(_f_) _The fruit._—Trace the development of the acorn from the female
flower, and the change of the covering of scales into the woody cup. If
possible, compare for some years the yield of acorns by a selected old
oak. Germinate an acorn.
(_g_) _Associated animals._—Make as full a list as possible of the
animals which you have seen obtaining food or shelter from the oak.
What was each doing when you saw it? Look for _galls_—the so-called
“oak apples”—and cut them open to see the insect inside. Also examine
the other kinds of galls found on the leaves and catkins.
[Make similar observations, sketches, and notes of other forest-trees,
in addition to the special observations mentioned below, and learn to
recognise their “expressions,” methods of branching, bark, leaves,
flowers, and fruit.]
2. =The beech.=—Notice the smooth, olive-grey bark, and the “fluted
column” appearance of the base of the trunk of many beeches; the long,
wavy boughs; the brown, sharp leaf-buds; the smooth, silky-fringed
leaves; the scarcity, or absence, of vegetation beneath the tree; the
hanging, globular catkins of male flowers; the pairs of female flowers,
surrounded by prickly scales. Make notes of the dates of flowering and
opening of the leaf-buds. Trees may be considered in flower when the
stamens can be seen. Trace the development of the fruit or “mast.” Each
pair of flowers gives rise to two three-sided nuts, enclosed in a woody
cup or husk. It is covered with hard bristles, and splits into four
parts when ripe.
Compare the _sweet chestnut_.
3. =The birch.=—Observe the graceful appearance of the tree; the
slender trunk, with its bark streaked with brown, yellow, and silver;
the purple-brown colour and wiriness of the young twigs; the shape
and size of the dark green leaves, and the cylindrical, many-flowered
catkins. The male catkins appear in the autumn, the smaller female
catkins in the following spring. Examine the two kinds of flowers. Find
an old birch-cone and break it open to see the winged fruits. What is
the use of the wings?
4. =The hazel and alder.=—In what situations do these trees grow?
Compare them with the birch. Notice that in both hazel and alder the
male flowers are in long, dangling catkins. The groups of female
flowers of the hazel look almost like leaf-buds, but can be recognised
by the spreading red stigmas. The female flowers of the alder form
distinct cones. Is the flowering part of the twig of this year’s or
last year’s growth? Look in the autumn for the cones and catkins which
will expand next spring. Trace the development of the fruit.
=The oak.=—The student cannot better commence the study of forest
trees than by selecting one as a type, and making himself thoroughly
familiar with its life-history, and with its appearance at all times
of the year. Other trees should then be compared and contrasted point
by point, is and with each other. The oak answers admirably as such a
central type. It is perhaps the best-known of all British forest trees,
not only from its wide distribution, but also from its historical and
legendary associations.
[Illustration: FIG. 112.—The Oak.]
The oak (Fig. 112) flourishes in exposed and sunny situations,
especially where the soil is well drained. It can be recognised by
the zig-zag and wide-spreading boughs, which often spring from the
trunk almost horizontally. A very strong form of trunk is plainly
necessary to support such branches, and in an old solitary oak it may
often be seen that the bole is very thick at the ground line, and then
rapidly narrows, until at a height about equal to the base-diameter it
may be only half or one-third the thickness. Above this point it is
practically cylindrical up to the origin of the boughs, where it is
swollen. An oak growing in a wood, or plantation, is of very different
form. Its trunk is tall and straight, and the larger boughs are not
given off until near the top. To secure the necessary light, the tree
used its energy in growing in length, and in keeping pace with its
neighbours rather than in spreading laterally. In such a crowded oak
the lateral buds generally remain undeveloped, while the end of the
shoot pushes onwards and upwards to the light. The opposite is the case
when there is plenty of room and light on all sides. Then it is usually
the terminal bud which dies, while the lateral buds grow out into
branches, forming the “knee-joints” which were once so greatly valued
for shipbuilding.
The bark of the oak is very rugged, with ridges and furrows running
almost vertically.
[Illustration: FIG. 113.—The Oak; leaves, flowers and fruit. _A_,
flowering branch (× ¼); _B_, a male flower (magnified); _C_, stamens
(magnified); _D_, a female flower (magnified); _E_, acorns; _F_, cup of
acorn; _G_, _H_, seed.]
The young shoots which are formed when the buds expand in spring are
short, and the leaves are set closely together. As a result, the oak’s
foliage is characteristically dense and thick. The leaves are very
distinctive; their general shape is oval, and the margin is deeply and
irregularly lobed (Fig. 113).
If in early spring we go out in the woods and fix on an old oak tree
(the oak hardly ever flowers before it is 50 years old), we shall
probably see the =flowers= on some of the young twigs. The _female
flowers_—one to five on each flower-stalk—are near the end of the
twig, while the male flowers arise lower down. The female flowers are
destitute of stamens, each consisting practically of a single pistil,
partially enclosed in two envelopes, the lower of which ultimately
becomes the familiar “cup” of the acorn. The stigma has three
spreading lobes for receiving pollen. The _male stalks_ or catkins,
hang down from the lower part of the twig, and every stalk bears about
a dozen flowers. The male flowers have each from 5 to 12 stamens, but
they have no pistil. The stamens produce pollen in the usual way, and
when they burst, the wind blows the loose pollen from the stamens and
scatters it in the air. Some of the pollen dust is almost certain to be
wafted to the stigmas of the female flowers, and the pollen grains put
out tubes, and in due course fertilise the ovules.
[Illustration: FIG. 114.—The Beech.]
Most of our forest trees resemble the oak in being pollinated by the
aid of the wind. It is evident that for the process to be successful
the trees must flower early in the spring, before the foliage has
become so thick as to be in the way of the pollen and prevent it from
reaching the stigmas of the female flowers. It is also important that
the pollen may be easily detached, and it is for this purpose that the
male flowers of the oak and similar trees hang down in the familiar
=catkin= fashion.
[Illustration: FIG. 115.—The Beech. 1, flowering branch (× ⅔); 2, a
male flower (× 4); 3, a female flower cut through longitudinally (× 2);
4, cross section of ovary; 5, cup and fruits (× ⅔); 6, fruit.]
After fertilisation the female flower changes into an =acorn= (Fig.
113, _E_), a nut enclosed in a cup. The nut contains a single large
seed, which on germination grows up into a new oak.
=Galls= of various kinds are to be found on most oaks. These are
excrescences caused by certain insects having laid their eggs in the
soft tissues beneath the surface. More than fifty species of insects
obtain their food from the oak. Some of these will be referred to in a
later chapter.
[Illustration: FIG. 116.—The Birch.]
=The beech.=—The beech (Fig. 114) is easily recognised by the
olive-grey, smooth bark, and by the shape of the base of the trunk,
which usually has the appearance of being formed by the union of
several separate columns. When well grown the tree is lofty, and bears
a wide-spreading crown of branches which, when clothed with leaves in
summer, casts a dense shade. The ground beneath the tree is generally
destitute of other vegetation. The winter-buds are long and pointed;
they expand in May, the new shoots at first drooping, but straightening
out in a fortnight or so. The leaves (Fig. 115) are broad, thin, and
glossy, and are fringed with fine, silky hairs. Young beeches, like
young oaks, often retain their leaves through the winter.
The tree is in flower by the time the foliage has fully developed. The
male flowers are borne in small, rounded catkins (_a_, Fig. 115). The
female flowers (3, Fig. 115) are in pairs. Each pair is surrounded by
prickly scales, and gives rise after fertilisation to two three-sided
and pointed nuts enclosed in a woody cup or husk. The husk is covered
with hard, blunt prickles, and when ripe splits into four parts.
Beeches do not harbour many insects, but squirrels frequent them for
the sake of the nuts. The prickly husks protect the fruit from being
eaten before it is ripe.
[Illustration: FIG. 117.—The Birch; leaves, flowers and fruit. 1,
branch with male (_b_) and female (_a_) catkins (× ⅔); 2, bract with
three male flowers (× 2); 3, bract with three female flowers (× 4); 4,
ripe cone (× ⅔); 5, fruit (× ⁴/₃).]
The =Spanish=, or =sweet chestnut= is allied to the oak and beech. It
has long, narrow leaves; it flowers in July. Its fruit is enclosed in a
prickly husk, which splits into four when the nuts are ripe in October.
=The birch.=—The birch (Fig. 116) is a slender, graceful tree, with
bark streaked with brown, yellow, and silvery patches. Its leaves (Fig.
117) are rather small, and their glossy, dark-green colour contrasts
pleasantly with the brownish-purple of the young twigs. Both male and
female flowers are arranged in catkins (Fig. 117). The male flowers
appear in autumn, but do not open until spring, when the newly-formed
female flowers are ready. The female catkins (_a_, Fig. 117) are much
smaller than the male (_b_).
[Illustration: FIG. 118.—The Hazel; leaf, flowers and fruit. 1, a
flowering branch (× ⅔); 2, a male flower (× 2); 3, a stamen (× 4); 4, a
female flower cut through longitudinally; 5, fruit with cup (× ⅔); 6,
fruit without cup; a foliage leaf.]
[Illustration: FIG. 119.—The Alder; leaf, flowers and fruit. 1, Branch
with male catkins (_a_) and female cones (_b_) (× ⅔); 2, male flowers
(× 2⅔); 3, female cone; 4, two female flowers; 5, ripe cone (× ⅔); 6, a
fruit (× 1).]
The =hazel= and =alder= develop their flowers in the year preceding
their opening. The female flowers of the hazel (Fig. 118) look somewhat
like buds, but may be distinguished by the red stigmas. The fruit is
a nut, enclosed by a sheath of soft bracts. The seeds are largely
dispersed by squirrels. The alder grows on the banks of streams; its
female flowers (Fig 119, _b_) are arranged in short catkins, which may
be called cones. The ripe cone (5, Fig. 119) contains two nuts at the
base of each scale. The fruits fall into the stream, and float away,
perhaps to germinate at a considerable distance.
31. THE WILLOW AND THE POPLAR.
1. =The willow.=—About the third week in March, examine willow trees,
and notice the soft, round, silky bodies which spring alternately
on the young twigs. On some trees these are broad and yellow; they
are _male catkins_ (Fig. 120). Pick off a flower and see the stamens
(generally two stamens to each flower) inserted on a small silky bract
(Fig. 121, _B_). With a lens look for the honey cup at the bottom of
the bract.
On other trees notice the long, narrow, silvery _female catkins_. Pick
off a flower to see the single pistil with the forked style, also on
a small, honeyed bract. Have you ever seen bees visiting the flowers?
Are the catkins as conspicuous when the trees are in leaf? Is it an
advantage to the trees to flower before the leaves come out? In June,
examine the ripened female catkins. Pull out a tuft of the hairy seeds
and dry it in the sun, noticing how they form a fluffy mass. Blow the
mass of seeds. How do you think the seeds are dispersed?
2. =The poplar.=—Find male and female poplars and examine their
flowers. Are the female flowers pollinated by insects or by the wind?
Is self-fertilisation ever possible with willows and poplars? Why not?
Which appear first, the flowers, or the leaves of poplars? Why?
Examine the leaves of the English poplar. Why do they turn over so
easily, even with a very slight breeze? Are both sides of the leaf of
the same colour?
Compare the _Lombardy poplar_ with the English poplar.
=The willow.=—Many species of willow are known, but the sallow
willow—called the =saugh tree= in Scotland—is common in coppices and
hedges. It has purplish brown branches, and large, broad, downy leaves.
In the willow the male and female flowers are produced on different
trees, so that self-fertilisation is obviously impossible. The flowers
of the male willow form broad yellow catkins which cling closely to
the twig (Fig. 121). They are often called “golden palms.” Each flower
consists of two stamens, borne on a silky scale which has a tiny honey
cup at the base. The female flowers form long, narrow, and silvery
catkins; each flower is merely a single pistil with a forked stigma,
and, like a male flower, is supported on a small honeyed bract. The
flowers appear in March, before the leaves, and the catkins are very
conspicuous. They are visited by bees for the sake of the honey and
pollen, pollination being thus effected. In June, the female catkins
are ripe. Each ovary has now become a fruit, which opens and liberates
the silky seeds. When the seeds dry, their fine hairs cling together
so that a light fluffy mass is formed, which can be blown to great
distances by the wind.
[Illustration: FIG. 120.—Twig of male Willow, with catkins. (× ⅙)]
[Illustration: FIG. 121.—The Willow. _A_, flowering male-shoot (× ⅔);
_B_, male flower with bract (magnified); _C_, female cone; _D_, _E_,
female flowers (magnified); _F_, fruit (× ⅔); _G_, the same magnified;
_H_, seed (magnified).]
=The poplar.=—The various poplars are also _completely unisexual_, that
is, any one tree bears either male or female flowers, but not both. In
this case also the flowers appear before the leaves; but the flowers
are pollinated by the aid of the wind, not by insects, and nectaries
are therefore not formed. The catkins are not very much like those of
the willows in appearance, but the fruit and seeds are very similar.
Both willows and poplars are fond of the banks of streams. The English
poplars are graceful trees which can be recognised at a distance by the
manner in which the broad leaves turn on their long stalks, exposing
the lighter-coloured under-surface at the slightest breath of wind.
This is especially noticeable with the =aspen= or trembling poplar
(Fig. 122). The =Lombardy poplar= is a tall, stiff tree.
[Illustration: FIG. 122.—The Poplar. 1, male catkin (× ⅔); 2, female
catkin (× ⅔); 3, male flower (× 2); 4, female flower (× 4); 5, the same
in longitudinal section; 6, fruit; 7, the same after opening; 8, seeds
(× 3); 9, diagram of male flower.]
32. THE ELM, LIME, AND ASH.
1. =The elm.=—Notice the straight trunk, rough bark, and slender
branches of the common elm. Examine the flowers in early spring and
notice that each contains both stamens and pistil. When do the flowers
open? When do the leaves expand? Draw a leaf and a fruit. What is the
use of the broad plate of the fruit?
2. =The lime.=—Observe the straight trunk, smooth bark, and general
_pyramid_ shape of the tree. Draw a leaf, and notice that it is pointed
and is larger on one side of the midrib than on the other. Notice that
the flower stalks spring from leaves (bracts) differing in shape (Fig.
126) and colour from ordinary leaves. Draw one of these leaves with
its attached flower stalk. When do the flowers open? Try to discover
whether bees haunt the flowers. Find out what is the use to the fruit
of the long leafy bract.
3. =The ash.=—Look in winter for the black buds and the flattened
tips of the twigs. When do the flowers open? Notice their rich purple
colour. Examine a flower and make out the two stamens and the pistil.
Trace the formation of the fruit, which is winged and hangs in bunches
called _keys_. What is the use of the wing? When do the leaves expand?
Are they simple or compound? Draw one. What is its colour? When do the
leaves fall?
[Illustration: FIG. 123.—The Elm.]
=The elm.=—The elm (Fig. 123) is usually a lofty tree, easily
recognised at a distance by its straight trunk, slender branches,
and rounded masses of foliage. The bark is rough and very corky. The
tree flowers early in the spring before the leaves are expanded. The
flowers are purple and contain both stamens and pistil. Each fruit
(Fig. 124) is a flat plate with a rounded seed box in the middle; it is
distributed by the wind. The seeds of the common elm do not often ripen
in this country. The leaves (Fig. 124) are rough to the touch, and have
very prominent veins. They do not fall until late autumn.
[Illustration: FIG. 124.—The Elm; leaves, flowers and fruit. 1,
flowering branch (× ⅔); 2, branch with leaves; 3, a flower (× 2); 4,
the same, cut through longitudinally; 5, a fruit (× ⅔).]
=The lime.=—The lime tree (Fig. 125) has a straight smooth trunk.
The tree generally spreads at the base, and tapers to a blunt apex.
The leaves are bright green, heart-shaped, and pointed, and plainly
larger on one side of the midrib than on the other. The yellowish-green
flowers are in bunches, carried on a stalk which springs from
the middle of a long, narrow bract (Fig. 126). The flowers are
complete—calyx, corolla, stamens, and pistil being all present—and are
pollinated by bees, which visit them for the sake of nectar, being
attracted by the sweet scent.
[Illustration: FIG. 125.—Lime Trees.]
[Illustration: FIG. 126.—The Lime. _A_, group of flowers on stalk
(_a_), springing from bract (_b_) (× ⅔); _B_, longitudinal section of
fruit (magnified).]
[Illustration: FIG. 127.—The Ash.]
=The ash= (Fig. 127) is a very graceful tree, and its compound leaves,
with leaflets springing from the sides of the midrib, give the foliage
a characteristically feathery appearance. The bark is ashen-grey in
colour. The tips of the twigs are curiously flattened, and the winter
buds are jet black. The flowers are small, with two purple-black
stamens and a pistil. They open in April before the leaves appear,
forming close clusters. The fruits are long and flat, and hang together
in bunches which are popularly called =keys=. They are only detached
by high winds, and are then blown to considerable distances. The leaves
appear rather late—about the end of May—and are shed early. They
are compound, each consisting of seven or more leaflets arranged in
opposite pairs along the midrib, with a single leaflet at the end.
The =mountain ash= or Rowan tree has leaves somewhat like those of the
ash proper, but in other respects it is quite different, as it belongs
to the rose family (p. 103).
33. THE SYCAMORE AND HORSE CHESTNUT.
1. =The sycamore.=—Observe the size and general shape of the tree.
Examine the twigs in winter and watch the buds open in spring. Draw a
leaf. Why are the leaves usually so sticky in warm weather? Notice the
hanging sprays of green flowers. Are both stamens and pistil present
in the same flower? Are the flowers visited by bees? Why? Follow the
development of the fruit. Draw a pair of fruits. What is the use of the
wing on a fruit? Do the fruits fall off themselves or are they torn
off by gales? Look for seedling sycamores in the woods; also germinate
seeds in garden soil and again observe the various stages.
Compare the _plane_, and notice the pyramidal shape of the tree. The
leaves are very similar in outline to those of the sycamore, but the
flowers are clustered into balls which dangle on long stalks. In
summer, pull off a leaf and observe how the axillary bud is covered by
the cup-shaped base of the leaf-stalk.
2. =The horse chestnut.=—Notice the general shape and method of
branching. These characters, the large buds in winter, and the shape
and size of the leaves in spring and summer, render the tree easily
recognisable at all seasons. Trace the connection between the method
of branching and arrangement of the leaves and buds on the twigs. Note
the date of flowering, and of the unfolding and shedding of the leaves.
Examine the flowers; are they pollinated by insects or by the wind?
Trace the development of the fruit.
[Illustration: FIG. 128.—The Sycamore.]
=The sycamore= (Fig. 128) is one of the best-known trees in this
country, and often grows to a great height. Its leaves (Fig. 33) are
large and five-pointed, the main veins spreading from the top of the
leaf stalk. When young they are of a beautiful red colour. In hot
weather the leaves become very sticky with a sugary syrup called
=honey-dew=. The sycamore is indeed a species of maple, and is closely
related to the sugar maple of North America. In May, the flowers hang
from the twigs in drooping green clusters. They contain both stamens
and pistils, and are visited by bees for the sake of the honey in which
they abound. The fruits or =keys= (Figs. 137 and 4) occur in pairs, or
sometimes three together. Each has a flat, membranous wing, by means of
which it is easily transported by the wind.
In Scotland the sycamore is often called the =plane=. The leaves of the
plane and sycamore are somewhat similar in shape, but the trees belong
to different families. The plane can be identified by its globular
heads of flowers and subsequent balls of seeds, which hang on long
stalks from the twigs.
[Illustration: FIG. 129.—The Horse Chestnut.]
=The horse chestnut.=—The horse chestnut differs from all our other
forest trees of equal size in bearing brilliantly coloured and
conspicuous flowers. The flowers are complete, containing not only
stamens and pistil, but also a calyx and a beautiful pink or white
corolla. The tree has a striking appearance at all seasons of the year.
In winter, the thick twigs, the large terminal buds, and the opposite
lateral buds already described on pp. 62-64 are very conspicuous,
and give an instructive clue to the method of branching. The buds
open early and the new shoots rapidly lengthen, so that the tree is
a mass of foliage whilst most neighbouring trees are still bare. The
leaves (Fig. 26) are compound, consisting of seven large, spoon-shaped
leaflets which spring from a common point at the end of the leaf-stalk.
The flowers open in May. The fruit is ripe in October and then falls to
the ground, its prickly husk splitting into three parts to liberate the
rounded seeds. The leaves fall early, leaving large scars (Fig. 38),
which have some resemblance to the hoof-marks of a horse.
34. CONE-BEARING TREES.
1. =The Scotch pine.=—Notice the shape of the tree: the tall straight
stem and rugged bark, and the dark tufts of foliage. Does the tree bear
leaves all the year round? Does it ever shed its leaves? Are they shed
at any special time of the year? Examine a leafy twig; observe that
the leaves are needle-shaped and come off in pairs. At the apex is a
terminal bud which will continue the length of the twig next season;
the lateral buds below will grow out into twigs at the same time. Try
to make out which parts of the twig grew during last year, and which
during the two previous years.
Notice the cones. The young female cones (_b_, Fig. 130, 1) are erect,
and their scales separate slightly in spring to allow the pollen to
enter. Afterwards they hang down (_c_) whilst the seeds are ripening.
Three years after pollination the scales come apart again to let the
seeds fall out. Examine the seeds from a ripe cone and notice the
attached wings (Fig. 130, 4). Examine cones of various ages. In spring
look for the pointed cones of male flowers (Fig. 130, 1, _a_) which
produce the abundant pollen.
2. =The spruce fir.=—Compare the spruce fir. Notice the conical
_Christmas tree_ shape and the large spreading branches near the
ground. Compare the leaves and cones with those of the pine.
3. =The larch.=—Compare the larch with the pine and spruce. Notice the
drooping boughs, the alternate tufts of leaves, and the small cones
arranged in a row along the twig. Is the larch an evergreen?
=Cone-bearing trees.=—The cone-bearing trees, such as the pines and
firs, are true flowering plants, but of a type which is very different
from any hitherto described. The flowers are peculiar, and form cones,
the male flowers producing pollen and the female flowers ovules. The
ovules, however, are not enclosed in ovaries, but are naked, so that
the pollen gains access to the ovule directly, and is not received on a
stigma. The female cone consists very largely of smooth scales, a pair
of ovules being borne by each scale near its base. The pollen grains
of these trees are rendered particularly buoyant by being blown out at
the sides into little air-filled bladders, and are thus easily carried
by the wind. When the pollen falls on the female cone, the grains slide
down the smooth scales and very likely come in contact with the ovules
at the bottom. Each ovule has a sticky drop of gum at its end, and the
pollen is caught in the gum. Such pollen grains as roll off the upper
scales are almost certain to fall on a lower one and reach the ovules.
=The Scotch pine.=—This is a large tree, with a dome-shaped crown of
foliage. The bark is rough and scaly. Its foliage leaves (Fig. 130) are
long and needle-shaped, and occur in pairs, each pair being carried
by a very short branch. The leaves do not fall off each winter, as do
those of most forest trees, but remain on the branches for three years
or more, so that the younger twigs are clothed with foliage at all
seasons. The cones of male flowers (Fig. 130, 1, _a_) are found at
the base of some of the shoots of the current year. The female cones
(Fig. 130, 1, _b_) are formed round the _ends_ of the young twigs.
They consist mainly of overlapping woody scales, each being knobbed
on its exposed surface and bearing a couple of ovules near the base,
where it springs from the axis of the cone. The young cones are erect,
and in spring (when clouds of pollen are blowing about) their scales
separate slightly to admit the pollen in the chinks between them. After
pollination the scales close again and the cone hangs down (Fig. 130,
1, _c_). The ovules receive the pollen in May, but the actual union, or
fertilisation, does not take place until June of the following year.
The seeds become mature two years after fertilisation. When they are
ripe each bears a thin wing which has split off from the upper surface
of the scale. The scales now separate, and the winged seeds fall out,
to be distributed by the wind. When all the seeds have fallen, the
empty cones drop off the tree.
[Illustration: FIG. 130.—The Scotch Pine. 1, branch with male (_a_) and
female (_b_, _c_) cones; _c_, cone (× ⅔): 2, two views of a stamen (×
2): 3, scales bearing two ovules (× 2): 4, scale with two seeds (_a_),
wing (_b_), seed (_c_): 5, seed in longitudinal section (× 1½).]
Owing to the length of time necessary for the ripening of the seeds,
cones of various ages may be found on the tree all the year round.
=The spruce fir.=—The spruce has a characteristic conical shape, which
is familiar in Christmas trees. The branches are long, and spread
horizontally. The foliage leaves are needle-shaped and four-sided in
section, but are shorter than those of the pine, and are borne singly.
The tree is “evergreen” in the sense that the pine is so, that is, the
leaves are not shed all together, but gradually.
The method of pollination, fertilisation, and distribution of the
winged seeds is similar to that of the pine. In the spruce, however,
the seeds are ripe in October of the year in which the cones are
pollinated.
=The larch.=—The larch is a cone-bearer, and a near relative of the
pines and firs, but it differs from them in shedding its leaves
annually. The leaves are (as is usual in cone-bearing trees)
needle-shaped, but they are very thin and they grow in tufts on short
alternate spurs. The cones are small and are arranged in a row on the
twig. Their scales do not fit so closely together as those of the pine
and fir cones. The larch tree has a conical form, but can readily be
distinguished from the spruce by its _drooping_ boughs, and absence of
leaves in winter.
=Gymnosperms.=—The pines, firs, and larches are neither dicotyledons
nor monocotyledons, but belong to a class of plants which botanists
call =gymnosperms=, in allusion to the fact that their ovules are not
enclosed in ovaries, like those of other flowering plants, but are
_naked_. This is the most ancient and primitive group of flowering
plants known. In fact they form a connecting link between higher
flowering plants and the group to which the ferns and horsetails—which
are still more primitive—belong.
EXERCISES ON CHAPTER VIII.
1. What trees are the first to put on leaves in England?
Which flower before leafing? Which trees are the latest
to come into leaf? When do evergreens, for the most part,
change their leaves? (N.F.U.)
2. Describe the fruit and seed of four of our commonest
trees, explaining in each case how the seeds are
distributed. (N.F.U.)
3. Make observations to find which of the following trees
endure shade most readily—oak, beech, elm, sycamore,
chestnut.
4. What do you suppose are the advantages and disadvantages
of shading and crowding trees, as regards the quality of
the resulting timber?
5. Study young pines and firs, and find out during which
period of life they grow most rapidly.
6. Which trees have you found growing (_a_) in heavy clay
soils, (_b_) in sandy soils?
7. Describe any observations upon six common British trees
which could be made during a walk in early spring. (N.F.U.)
CHAPTER IX. FRUITS: HOW SEEDS ARE SCATTERED.
35. PLANTS WHICH SOW THEIR OWN SEEDS.
1. =The fruit of the wallflower.=—Examine wallflower fruits and make
out that each consists of the ripened pistil. Does the fruit open of
itself? How many chambers does it consist of? Where are the seeds
attached? Are they blown off at last by the wind? Draw the fruit. It is
called a _siliqua_.
2. Compare with this the fruit of _shepherd’s purse_ (Fig. 62), and
_penny cress_ (Fig. 132), and notice that they are of the type of the
wallflower fruit, but are much broader in proportion to the length.
Such a fruit is called a _silicula_. Draw.
3. =The fruit of the pea.=—Examine a ripe pea-pod and compare it with
(_a_) the pistil of an unfertilised flower, (_b_) a half-ripe pod. How
many carpels have taken part in forming the pod? How many seeds (peas)
does the pod contain? Leave a pod on the plant until the shell becomes
dry, to find out how the fruit opens. Does it open along one edge only,
or along both? How are the seeds attached? Such a pod is called a
_legume_. Draw it.
Compare and draw the legumes of the _broad bean_, _French bean_,
_scarlet runner_, _laburnum_ (remember its seeds are poisonous), and
_bird’s foot trefoil_. The legume of the bird’s foot trefoil bursts
open suddenly and scatters the seeds in the air. Is the scattering of
the seeds any advantage? Why?
4. =The fruit of the field geranium.=—Make out that five carpels are
grouped around a central rod. Examine fruits which have opened. About
noon on a bright, sunny day gently touch a ripe fruit with a small
brush, and watch the carpels spring back from the rod and jerk the
seeds into the air. Compare and contrast this fruit with a siliqua,
silicula, and legume respectively.
5. =The fruit of the poppy.=—Examine a poppy head. The top of the
fruit is the stigma. Observe below this a line of small holes running
round the fruit. Draw. Shake the fruit, and notice that seeds fall out
through the holes. Cut the fruit across to see the large number of
small seeds inside. How does the fruit hang on the growing plant? Does
the wind shake it and liberate the seeds?
6. =The fruit of the pansy and violet.=—Watch the ripening of the
fruits on the plants. Observe that the ovary swells up into an
egg-shaped body which afterwards splits into three boat-shaped valves
containing seeds. Try to make out why the seeds are one by one shot out
as the sides of the valves dry. Put a ripe fruit before the fire and
watch the process. Imitate it by placing a pea between two flat rulers
and pressing the rulers together.
=The origin of a fruit.=—When the ovules of a flower have been
fertilised (p. 92) by the pollen tubes they change into seeds
which have the remarkable power of growing up—in favourable
circumstances—into plants resembling that which produced the seeds.
This is not, however, the only result of fertilisation. Whilst the
ovules are changing into ripe seeds, those parts of the flower—the
stamens, corolla, and calyx—which have finished their work wither and
fall off, though the calyx sometimes remains. Other parts—the pistil
and sometimes the receptacle (p. 90)—take on new duties, and become
gradually modified in order to protect or scatter the seeds.
Thus, the tender wall of the pistil often becomes a woody, leathery,
or juicy seed-case; while the receptacle, or top of the flower stalk,
may become fleshy and swollen with sugary pulp, as a bait for birds
and other animals. In any case we give the name of fruit to all such
altered and persistent parts together with the seeds which accompany
them. A pea pod, for example, is as truly a fruit as a plum, and a
poppy-head as a strawberry.
What a fruit is like depends to a great extent upon the characters of
the pistil which gave rise to it. If, for example, the pistil consists
of several separate carpels, the ripened carpels or fruits will also
be separate. When, on the other hand, the pistil is composed of united
carpels, these will remain united, at least until the fruit is ripe.
Then in some cases they come apart.
[Illustration: FIG. 131.—_A_, fruits of wallflower (× ⅕); _B_, siliqua
(× 1); _C_, siliqua open.]
[Illustration: FIG. 132.—Fruits (siliculae) of Penny Cress. (Nat.
size.)]
The part of a fruit which is derived from the walls of the pistil is
called the =pericarp=.
=The fruit of the wallflower.=—After fertilisation the stamens, petals,
and sepals of the wallflower drop off, leaving the pistil alone on
the top of the flower stalk. The pistil increases greatly in size
(Fig. 131, _B_) during the ripening of the seeds. At last its wall
(the pericarp) splits into two flaps. These become free at the bottom,
exposing a central plate which bears the rows of seeds. When the seeds
are quite ripe, a slight breeze is sufficient to shake them off,
and they fall to the ground to take their chance of finding a place
favourable for germination.
A fruit with a dry pericarp, which opens of itself when the seeds are
ripe, is called a =capsule=. This particular kind of capsule—consisting
of two carpels which come apart at maturity, leaving a central
partition bearing seeds—is known as a =siliqua=. When it is short in
proportion to its length, as in the shepherd’s purse (Fig. 62) and
penny cress (Fig. 132), it is distinguished as a =silicula=.
=The fruits of the pea tribe.=—The pod of the pea (Fig. 3) and its
relatives is a capsule of another kind. It consists of one carpel only,
and opens, when ripe, along both back and front margins to liberate
the seeds. Such a fruit is called a =legume=. In the young fruit the
pericarp is somewhat fleshy and succulent, but it becomes dry during
ripening. The legume of the bird’s foot trefoil bursts open suddenly,
and throws the seeds to a considerable distance.
=The fruit of the field geranium= (Fig. 133) is a long capsule composed
of five carpels arranged round a central column (_A_). When the seeds
are ripe, the carpels suddenly spring from the rod, remaining attached
only at their upper ends (_B_), and fling the seeds into the air. The
method may be watched by stroking the fruits very gently with a small
brush, when they open in the manner described. The experiment is most
likely to succeed in dry, sunny weather, about the middle of the day.
=The fruit of the poppy.=—The pistil of the poppy swells during
ripening into a large, globular capsule known as the poppy head. The
top of the fruit (Fig. 134) is the persistent stigma. Just below this,
a line of small holes like windows runs round the head. As the fruit
hangs inverted on the top of the flower stalk it is shaken about by the
wind, and the tiny seeds fall out through the windows.
[Illustration: FIG. 133.—Field Geranium (× ¼).]
[Illustration: FIG. 134.—_A_, capsule of poppy; _B_, cross section of
capsule (× ¼); _C_, seeds.]
=The fruit of the violet or pansy.=—The arrangement by which the
violet (Fig. 134) or pansy sows its seeds is most interesting. After
fertilisation the ovary swells into a great egg-shaped capsule, in the
inside of which the seeds are arranged in three rows. When the seeds
have become ripe and hard, the capsule splits down the side along three
lines, and is thus divided into three parts. These open outwards, and
bend back as shown in Fig. 135. Each is a boat-shaped valve. The seeds
inside are thus exposed to the air, and they soon dry and become ready
for scattering.
[Illustration: FIG. 135.—Violet. _F_, explosive fruit; _S_, seeds being
shot out of fruit.]
Then the curved sides of each valve begin to straighten and come
together, and naturally allow less and less room for the hard, smooth
seeds inside. The pressure of the sides of each valve on the seeds
inside it becomes greater and greater, until one by one they are shot
out to a considerable distance. If a ripe violet ovary be warmed before
the fire the whole operation may easily be watched. The process may
be imitated by putting a pea between two flat rulers and pressing the
rulers together, when the pea may be shot to a distance of several
yards.
=Why fruits are scattered.=—A flowering plant is practically confined
for life to the place where it first sprang up, so that it is unable
to go about and select favourable situations for its offspring. On the
other hand, if the seeds simply fell to the ground beneath the parent
plant, the seedlings would generally be so crowded together that they
would interfere very much with each other’s growth. In addition, they
would often be under a great disadvantage, because the parent plant
would keep so much light from them. Hence, very many plants have some
special arrangement for scattering their seeds, so that some at least
of the seeds will have a chance of falling in a place where they will
obtain plenty of light, air, and good soil.
In this section we have studied examples of devices by which plants sow
their own seeds. We shall see next that other plants call in the help
of the wind and of animals.
36. SEEDS SOWN BY THE WIND.
1. =The fruit of the dandelion.=—Examine the manner in which the
tiny flowers of the dandelion are grouped together to form the head
(p. 113); and make out the various parts of the flower, especially
the _ovary_—a little white knob at the bottom of the flower, and the
_calyx-tube_—forming a tuft of fine hairs above the ovary. Trace the
development of the fruits: the withering of the corollas and stamens,
the elongation of the calyx-tube, and the expansion of the tuft of
hairs to form a parachute. Blow a dandelion “clock” (the head of
fruits), and notice how the parachutes float the fruits in the air.
Examine “thistle down” and contrast it with the dandelion fruit.
2. =The fruit of the willow.=—Examine the ripened catkins of a female
willow in June, and notice how each fruit has split into halves, which
come apart and expose the silky seeds inside. Pull a tuft of seeds out
and dry them in the sun. Notice that they wriggle and writhe about and
gradually become entangled together into a woolly mass, which is easily
blown away.
3. =The elm fruit.=—About the end of April look for elm fruits. Notice
the flat green plate (wing) with the rounded swelling near the middle
(Fig. 124, 5). Cut open the fruit and see that the swelling is caused
by a single seed. How does the flat wing aid in the distribution of the
seed?
Compare the winged fruits of the _ash_ and _sycamore_ (Fig. 137).
Do these winged fruits drop from the trees easily, or are they torn off
by gales?
Take a pair of sycamore fruits. Cut off the wing from one and let it
and an uninjured fruit fall at the same instant from a height. Which
reaches the ground first? Why?
4. =Pine and fir cones.=—Examine and compare pine and fir cones of
various ages. Break open a ripe cone and see the scales with the pairs
of naked seeds, each seed bearing a thin, papery wing which has split
off from the upper surface of the scale.
[Illustration: FIG. 136.—“Clock” of Dandelion Fruits. (× ¼.)]
=Wind-sown seeds.=—Many plants depend on the wind for the dispersal of
their seeds, and consequently the seeds are provided with outgrowths of
various kinds, which increase the surface greatly without adding much
to the weight, and, acting like parachutes, offer increased resistance
to the air and thus prevent the seeds from falling quickly to the
ground. In some cases the outgrowth is part of the pericarp, in others
it is an appendix carried by the seed itself, while in the lime it is
the bract upon which the flowers were formerly borne.
=The dandelion fruit.=—The fruit of the dandelion (Fig. 136) affords
one of the best possible examples of wind-dispersal. It will be
remembered (p. 113) that what is commonly called the flower of the
dandelion is really a head of perhaps 300 complete flowers: each with
a hairy calyx-tube, a yellow, strap-shaped corolla, five stamens, and
a pistil. When the flowers have been fertilised, the yellow corollas
and the stamens wither, the ovary increases in size with the ripening
of the single nutlet in its interior, and each calyx-tube elongates
until it is about an inch in length, the tuft of fine hairs being still
at its upper end. The attachment of the fruit (Fig. 86, 4) to the disc
(receptacle) is so slight when the seed is ripe that a very gentle puff
of air is sufficient to overcome it. The tuft of hairs at the upper
end has by this time expanded until it acts like a parachute, which
supports the tiny fruit for a long time in the air.
The common =thistle=—a relative of the dandelion—also distributes its
fruit by means of a tuft of fine hairs derived from the calyx. In this
case the hairs radiate from the seed. Such “thistle down” is commonly
found floating through the air in summer.
[Illustration: FIG. 137.—A pair of Sycamore Fruits. (× ⅔.)]
=The willow fruit.=—The catkins of the female willow are ripe in June.
Each catkin consists of a large number of tiny pods derived from the
ovaries of the flowers (p. 151). Each fruit splits into halves, which
bend back from each other (Fig. 121, _F_), exposing the silky seeds
within to the warmth of the sun. The seeds (_H_) turn and twist about
as they dry, and gradually entangle themselves together into a light,
woolly mass, which is easily blown to great distances by the wind. The
willow and the dandelion, therefore, use very similar devices to ensure
the dispersal of their seeds, although these plants are not at all
nearly related.
=The fruits of the elm, sycamore, and ash.=—These common forest trees
bear fruits with the seed attached to a flat plate, which is an
outgrowth of the pericarp (p. 167). In the elm fruit (Fig. 124, 5) the
plate is green and oval, and the seed forms a rounded swelling at,
or near, its middle. The fruits of the =sycamore= generally grow in
pairs (Figs. 33 and 137). Each half consists of a single seed with an
attached membranous plate, and the two seed-boxes of each pair are in
contact. The fruits of the =ash= hang from the twigs in bunches called
“keys,” each fruit on a separate little stalk. The plates which bear
the seed are long, narrow, and oval in shape.
It is plain that such plates, or wings, expose a relatively large
surface to the air and prevent the fruit from falling to the ground as
quickly as it otherwise would. The fruit is thus often blown to a great
distance before it finally settles and the seed germinates. The action
of the wing may be well shown by cutting off the plate from a sycamore
fruit and letting it and one with an attached wing fall at the same
instant from a height. The seed without a wing comes straight down as a
pea would; but the winged seed spins in the air and settles more slowly.
In the case of the round downy fruits of the =lime=, a similar service
is performed by the bract (Fig. 126), upon which the flowers were
carried.
=Pine and fir cones.=—If a ripe pine, or fir cone is broken open, it
will be seen that each seed is attached to a thin, papery wing (Fig.
130, 4), which has split off the upper surface of the scale bearing the
seeds. The winged seeds are shaken out of the cone by the wind, and
blown away.
=Trees alone bear winged seeds.=—Winged seeds would be useless to any
but fairly high trees, because if they were formed on the low plant
they would fall to the ground long before the wind could catch them
properly. It is also interesting to find that such seeds are generally
attached so firmly that they are only broken off by gales strong enough
to carry them a considerable distance.
On the other hand, the tiny plumed fruit of the dandelion or thistle is
so very light in comparison with the surface exposed to the air that it
takes quite a long time to fall even a few inches.
37. SEEDS WHICH ARE SCATTERED BY ANIMALS.
1. =Hooked fruits.=—Examine plants of _herb bennet_ (wood avens) in
summer and autumn, and find the fruits. Brush your sleeve against the
fruits, and notice how they cling to the cloth. Examine them with a
lens and observe the hooks at the ends of the styles.
Compare _goosegrass_ (cleavers) and find the hooks on the fruits.
2. =Nuts.=—Examine a _hazel_ nut. Notice the sheathing bracts at the
base of the fruit. Crack the nut and examine the broken edge of the
shell (pericarp) with a lens. Make out the three layers which compose
it. How many seeds are present? Cut the seed (kernel) across and see
that the bulk of it consists of two cotyledons (Chap. I.). Does the
fruit open of itself, if undisturbed?
Compare the acorn of the _oak_. Trace the development of the acorn from
the female flower, noticing that the cup is developed from a wrinkled
disc surrounding the lower part of the flower. Cut across the ovary
in June and notice that there are six ovules in it. In the ripe fruit
observe that the cup separates easily from the nut. Remove the shell
(pericarp) of the nut. How many seeds does it contain? What do you
think has become of the other five ovules? Cut through the seed and
observe the two cotyledons.
Compare the fruits of the _beech_. Notice that they are three-sided
nuts (each being a seed enclosed in a woody pericarp); and that two
nuts occur together, surrounded by a bristly woody cup which splits,
when the nuts are ripe, into four valves.
3. =Stone fruits.=—Examine a ripe _plum_. Cut it open and crack the
stone to see the single seed. Notice that the pericarp consists of
three layers as in the hazel nut, but that here the middle layer is
soft and fleshy, and the outer layer is the “skin.” The stone is the
inner layer of the pericarp. Is there any special means of liberating
the seed? Compare the _cherry_, and examine the seed and the three
layers of the pericarp.
Examine the fruits of the _blackberry_ and _raspberry_, and observe
that each consists of several small stone-fruits arranged on the
receptacle.
4. =Berries.=—_The gooseberry._—Notice the stalk at the bottom of the
fruit, and, at the top, the withered remains of the calyx. Cut across a
half-ripe gooseberry and observe the thick, fleshy pericarp enclosing
the seeds. Treat a ripe gooseberry in the same way, and observe that
the pericarp has now for the most part become a soft pulp, in which the
seeds are embedded. The rest of the pericarp is a membranous skin. Is
there any special means of liberating the seeds?
Compare _grapes_, _currants_, _oranges_, and _vegetable marrow_ fruits,
and notice that the structure of these resembles that of the gooseberry.
5. =The apple.=—Cut across the receptacle of the flower (apple blossom)
and notice how the five carpels are buried in it (p. 106). Trace the
formation of the fruit, and see how the _receptacle_ becomes larger
and larger during ripening. At the top of the ripe fruit observe the
withered remains of the calyx. Cut the apple through across the middle
to see the _core_. This consists of the horny walls of the five carpels
and the contained seeds (pips). From what part of the flower is the
fleshy, eatable part of the apple derived? Compare the _pear_.
6. =The rose hip.=—Examine fruits of the wild rose. Notice that they
are urn-shaped. On the flat rim of the urn observe the five scars
left by the sepals (or, in some cases, the sepals themselves). In the
opening of the urn see a tuft of greyish hairs. Cut the fruit down from
top to bottom through the middle to see the thick, fleshy wall of the
urn and the contained _nutlets_. From what part of the flower is the
fleshy wall of the hip derived? Examine a nutlet. What does the tuft
of hairs seen in the mouth of the urn consist of? Open a nutlet with a
needle and pick out the seed.
7. =The strawberry.=—Cut a strawberry down the middle and notice at the
base the persistent calyx, in the inside the fleshy receptacle, and on
the outside the yellowish nutlets. Open a nutlet with a needle and pick
out the seed.
=The help of animals.=—It has been seen in Chapter VI. that insects
play a very important part in the fertilisation of many flowers. Very
many plants also call in the aid of animals at a later stage for the
dispersal of the seeds, and the devices by which this aid is obtained
are often very ingenious.
=Hooked fruits.=—Sometimes after a country walk the reader has probably
found small fruits and seeds sticking to his clothes. They have become
attached to the cloth by means of small hooks which they carry. The
fruit of the =herb bennet= (wood avens) is a good example of this
device. When the stigma of the fruit breaks away, a little hook is
left at the top of the style (Fig. 138). Goosegrass or cleavers—a
common hedgerow plant—and many others have also hooked fruits. Sheep or
cattle, grazing near such plants are very likely to brush against them
and carry off the fruits and seeds in their hair. They may not again
reach the ground until they have been carried far from the place where
they grew.
=The position of hooked fruits.=—It is evident that these little hooks
would be quite useless if the fruits grew out of the reach of animals.
Such hooked fruits are never found, for instance, on high trees.
[Illustration: FIG. 138.—_A_, Aggregate Fruit of Wood Avens (nat.
size); most of the stigmas have broken off. _a_, _a_, _a_, fruits with
stigmas still attached; _c_, calyx; _B_, single fruit with stigma still
attached (magnified); _C_, single hooked fruit after loss of stigma
(magnified).]
=Nuts.=—A fruit which has a dry, woody pericarp and does not open of
itself is called a =nut=. The fruits of the hazel, oak, and beech are
good examples. The shell (pericarp) of the =hazel= nut (Fig. 139)
is composed of three layers, and encloses a single seed, or kernel.
The nut of the =oak= (Fig. 113) is called an acorn. Its lower part
lies in a cup developed from a wrinkled disc by which the lower part
of the female flower was surrounded. When the fruit is ripe the nut
easily separates from the cup. The acorn contains one seed only, which
consists largely of two swollen cotyledons. If the ovary of the
female flower of the oak is cut across in June six ovules are found
in it. As in the case of the hazel only one ovule is allowed to reach
maturity; the rest are sacrificed in order that the remaining one may
be more perfectly developed. In the =beech=, two three-sided nuts occur
together, surrounded by a bristly, woody cup, which splits into four
valves when the nuts are ripe.
[Illustration: FIG. 139.—_A_, group of Hazel Nuts (× ½); _B_,
longitudinal section of fruit.]
=The dispersal of nuts.=—=Squirrels= and other nut-eating animals are
instrumental in the dispersal of the seeds in a somewhat indirect
manner. They have a habit of storing up nuts and seeds in holes; but in
their active life these animals often forget where their larder is. The
seeds, thus left to themselves, sprout and grow into trees. The unripe
nuts of the beech are protected from the attacks of squirrels by the
hard bristles on the outer husk.
[Illustration: FIG. 140.—_A_, Plum; _B_, longitudinal section; _C_,
cross section; _E_, _M_, _En_, outer, middle, and inner layers of
pericarp; _S_, seed. (× ½.)]
=Stone fruits.=—A ripe =plum= (Fig. 140) or =cherry= (Fig. 77), like
a nut, consists of a seed and a pericarp; but here the pericarp is
specially modified to tempt animals and at the same time to protect the
seed from them. The inner pericarp-layer is a hard, woody shell called
the stone; the middle part swells up to form a juicy, sweet mass;
while the outer layer constitutes the “skin,” and is often beautifully
coloured.
[Illustration: FIG. 141.—Blackberry. (× 1.)]
=Blackberries= (Fig. 141) and =raspberries= consist of several small
fruits of the plum type, which are arranged round an axis derived from
the receptacle of the flower.
=Berries.=—The =gooseberry= (Fig. 142) is a type of this class of
fruit. It contains several seeds. The pericarp is thick and fleshy
before the fruit is ripe, but during ripening the greater part of it
becomes a soft, sweet pulp in which the seeds are embedded. The rest of
the pericarp is a membranous skin. =Grapes=, =currants=, =oranges=, and
=vegetable marrow= fruits are also berries. The vegetable marrow has
obviously a general resemblance to a half-ripe gooseberry but is on a
much larger scale.
[Illustration: FIG. 142.—_A_, Gooseberry; _B_, longitudinal section;
_C_, cross section. (× ⅔.)]
=The apple.=—In the apple (Fig. 143) we have an example of a fruit
in the formation of which the receptacle has taken a large share.
Even in the flower, the five carpels of the pistil are buried in the
receptacle, and as the seeds (pips) ripen, the =receptacle= swells
until it composes the greater part of the fruit and at last becomes
sweet and fleshy. The carpels with the contained seeds constitute the
=core=. The withered sepals are still to be seen at the top of the
fruit. The structure of the =pear= is quite similar to that of the
apple.
[Illustration: FIG. 143.—_A_, Apple; _B_, longitudinal section; _C_,
cross section. (× ⅓.)]
[Illustration: FIG. 144.—_A′_, Strawberry; _B′_, longitudinal section;
_Ac_, carpel, _R_, swollen receptacle. (× ⅔.)]
=The fruit of the wild rose.=—The fruits of the wild rose are called
=hips=. They are urn-shaped, and on the flat rim of the urn five scars
show the former position of the sepals.
In some cases the sepals themselves have remained. The narrower mouth
of the urn is filled by a tuft of greyish hairs which, when the fruit
is cut open, are seen to be the styles of the carpels. The carpels are
hairy, and stand on the bottom and sides of the urn-cavity. Each carpel
contains a seed. Comparison with the flower shows that the red, fleshy
urn is the developed =receptacle=.
=The strawberry.=—In the strawberry (Fig. 144) the eatable part of the
fruit is again the swollen and juicy receptacle. In this case it has
grown up on the _inside_ of the carpels—the little, yellow nutlets
(carpels) lying on the surface. Each carpel contains a seed.
=Why some fruits are sweet.=—The delicious flavours of sweet fruits
have been developed as baits for the allurement of such animals as are
likely to scatter the seeds to the best advantage. Consider such fruits
as cherries, currants, and rose-hips. Birds—such as thrushes—find
them very nice to eat, and are ready to carry them away to consume at
their leisure. The birds eat the sweet, fleshy part, but in the case
of a stone-fruit they drop the stone and leave the seed to germinate.
A seed which is in danger of being swallowed is either hairy like the
rose-carpels (and therefore rejected because it irritates the mouth
unpleasantly), or it is enclosed in a hard or horny case, upon which
the animal’s digestive juices are unable to act. When the seed is
dropped it is no worse for its experience, and with good luck grows up
into a plant.
[Illustration: FIG. 145.—Holly. The red colour of the fruits contrasts
strongly with the dark-green of the leaves. (× ⅙.)]
=Eatable fruits generally conspicuous.=—In order that birds and other
animals may easily find the luscious fruits which they may eat as
a reward for scattering the seeds, such fruits are almost always
displayed very conspicuously, and are brilliantly coloured. Oranges,
plums, red currants, apples, etc., illustrate this fact well. =Red= is
perhaps the commonest colour of eatable fruits, because it contrasts so
strongly with the green colour of the foliage (Fig. 145).
EXERCISES UPON CHAPTER IX.
1. Name and define the different kinds of self-opening
fruits. From what common plants could you collect examples
of these during an Autumn walk? (N.F.U.)
2. Describe any rough, prickly, or hairy seeds or fruits,
explaining the form and nature of the outgrowths and their
use. (N.F.U.)
3. What wild fruits and wild flowers would you expect to
find in September in your part of the country? In what
kinds of places would you look for them? (N.F.U.)
4. Describe and draw the fruit of a field geranium, and
point out the uses of some of its peculiarities. (1898)
5. Describe the seed vessels of a pansy. Draw one entire,
and also burst open. How does it scatter the seeds? (1898)
6. Shortly describe the fruit of an apple or pear, and of a
cherry or plum. Point out the chief differences between
them. (1901)
7. How are the seeds of cherry, field geranium, and pine or
birch dispersed? (1901)
8. Give examples of seeds which are dispersed by the aid of
birds or other animals, explaining in each case how the
dispersal is effected. (1893)
9. Why is it an advantage to some fruits to be (_a_)
brightly coloured, (_b_) sweet? Give examples.
10. A school museum contains, among other things, some
dandelion fluff, a dish of marrowfat peas, a few nodules
of garlic, and some hawthorn berries. How could you
employ these to illustrate a lesson on plant germination?
(Certificate 1903)
CHAPTER X. FERNS AND HORSETAILS.
38. FERNS.
1. =The male-fern.=—(_a_) _Habit of growth._—In summer dig up a plant
of the common male-fern (Fig. 146) and wash the soil from the roots.
Make out the short, stumpy, creeping _stem_, covered with the hairy
bases of old leaves; the slender matted _roots_ springing from the
leaf-bases; the large, compound _leaves_ or _fronds_ of the current
year, and the coiled young leaves which have not yet expanded.
(_b_) _The stem._—Remove the large leaves, leaf-bases, and roots from
the stem, and examine it. Notice how the youngest leaves are grouped
round the apex of the stem. Cut across the other end of the stem, and
notice the cut ends of the _conducting strands_ embedded in a softer
_ground-tissue_. Cut the stem in halves lengthwise and carefully scrape
away the ground-tissue of one half to see how the harder conducting
strands are connected together. If you spoil this, try again on the
other half, after boiling it until it is softened.
(_c_) _The leaves._—Make a drawing of one of the expanded leaves.
Notice that the leaf consists of a number of leaflets, and that these
are again cut up into segments, or are at least deeply lobed. Notice
the brown hairs clothing the leaf-stalk and the midrib.
Notice the crozier-like coiling of the young fronds, and make out that
when they uncoil the inside of the coil becomes the upper surface of
the leaf.
(_d_) _The reproductive organs._—Examine the lower surface of
full-grown fronds, and notice the brown rounded patches. Dry some
leaves bearing these patches and shake them over a sheet of paper.
Collect the brown dust which falls from the patches; it consists of
minute grains called _spores_.
(_e_) _The prothallus._—Sow some spores on damp soil sheltered from
the direct rays of the sun. Keep the soil moist, and notice that in a
few weeks the spores have grown into small, flat, green, heart-shaped
plants. Each of these is called a _prothallus_. Pick off prothalli of
different ages with a needle, float them in water, and compare the
various stages of growth. In old prothalli notice a young fern plant
springing up from the lower surface. Observe that as these become
larger the prothalli which bear them shrivel up and die.
2. =The bracken, fern.=—Dig up a bracken fern, wash the earth from the
roots, and compare it, point for point, with the male-fern. Notice:
(_a_) The long cylindrical, branching, underground _stem_. Does it
lie deeper in the ground than the stem of the male-fern? Cut the
stem across and compare the section with Fig. 148, noting (i) the
two concentric rings of separate _vascular strands_ (_s_). (ii) The
_strengthening material_, arranged as a brown zone (_lp_) just below
the skin, and as an incomplete ring (_ll_) between the two rings of
vascular strands. (iii) The softer _ground-tissue_ (_R_), in which the
vascular and strengthening strands are embedded. Draw the external
appearance of the stem (natural size), and also the cross section (4
times natural size).
(_b_) _The roots._—Describe the roots. Do they appear to arise from any
particular region of the stem?
(_c_) _The leaves._—From what part of the stem do the leaves arise? How
many come up each year? What is the appearance of a young leaf before
it expands? How does it differ from that of the male-fern? Draw a young
leaf, and also an expanded one. Is the leaf simple or compound? Does it
branch? Does the leaf of the male-fern branch?
(_d_) _The spores._—Examine the under side of the leaf, and notice the
absence of the brown patches seen in the male-fern. Notice that the
edge of the bracken frond is folded over like a hem. Dry a frond, and
then run the point of your pencil under the fold and observe the brown
dust (spores) which is removed.
(_e_) _The prothalli._—Sow some bracken spores on damp earth and keep
in a shaded place. Notice that the prothalli produced are very similar
to those of the male-fern. Watch their development and the growth, on
old prothalli, of a new generation of ferns.
3. =The hart’s tongue fern.=—Examine the hart’s tongue fern, and notice
how it differs from the two previous types. Are the leaves simple or
compound? Observe the brown trenches on the lower surface of the leaf.
Try to get out some spores by running the point of a pencil along the
trenches of a dried leaf. Sow the spores and try to raise prothalli.
=The daily life of a fern.=—The everyday life of a fern is very similar
to that of a flowering plant; for the organs by which it obtains food
are—broadly speaking—of the same type. Ferns are lovers of damp and
shady situations. The plant obtains its mineral food from the soil by
means of its roots; and its spreading green leaves, or fronds, enable
it, with the help of the sunlight, to decompose the carbon dioxide
of the air and to build up starch, sugars, and other carbonaceous
compounds.
=The male-fern.=—The common male-fern (which, by the way, has no sex
whatever) may be taken as a type of the group. It occurs abundantly in
woods and hedgerows. The =stem=, or =rhizome=, is short and stumpy;
it grows obliquely upwards, and does not branch. It is covered with
old leaf-bases, which are clothed with brown, scaly hairs. The stem
consists of a rather soft ground-substance, in which is embedded a
hollow cylindrical network of conducting strands. In a cross section
these appear as a somewhat irregular ring of dots (Fig. 146, 2, _a_).
When the soft ground-tissue is carefully scraped away, the network of
strands is left as a skeleton, with large diamond-shaped meshes.
The thin wiry =roots= spring from the bases of the leaf-stalks, just
where these come off the stem.
[Illustration: FIG. 146.—The Male-Fern. 1, Illustration showing
general habit (× ¹/₁₀); _a_, young leaves: 2, cross section of stem
showing conducting bundles _a_: 3, portion of leaf; _b_, spore-boxes;
_a_, covering scale: 4, longitudinal, and 5, cross section of group
of spore-boxes; _a_, leaf; _b_, scale; _c_, spore-boxes: 6, a single
spore-box; _a_, stalk; _c_, spring; _d_, spores.]
The =leaves= or fronds of the male-fern arise near the growing-point
(the upper end) of the stem. Leaves of almost all ages are present.
Those which surround the growing-point are mere rudiments; while
slightly older ones (Fig. 146, 1, _a_) are tightly rolled up into a
coil. During their growth the coils unfold, the inside of the coil
becoming the upper surface of the leaf. A fully expanded leaf is
roughly triangular in shape. The leaf-stalk and the midrib are covered
with brown, scaly hairs; and the blade of the leaf is divided into
several distinct leaflets, which are arranged on the midrib in two
rows. The leaflets are also in many cases sub-divided into separate
segments.
=The spores.=—On the lower surface of many of the segments of a
male-fern frond may be seen a number of small, brown, kidney-shaped
scales (Fig. 146, 3, _a_). Each scale is attached to the leaf by a
short stalk, and the structure thus bears a rough resemblance to a
little umbrella. Attached to the bottom of the short “handle” of the
“umbrella” are several tiny boxes, somewhat like pill-boxes. These are
shown at _b_ (Fig. 146, 3), and highly magnified at 4 and 5. Fig. 146,
6, represents a single box, more highly magnified. When ripe, each box
contains about fifty minute grains, which may thus be likened to the
pills in the pill-boxes. These “pills” are called =spores=. Summing up
thus far, we may say that the spores are formed in spore-boxes, which
are attached by stalks to the lower surface of the frond, each group of
spore-boxes being covered by a protective scale.
=The scattering of the spores.=—When the spores are ripe, each box
(Fig. 146, 6) becomes dry, and is ultimately burst by the sudden
straightening of a spring (_c_) which is coiled round its edge. The
force of the uncoiling of the spring is sufficient to jerk the spores
(_d_) out of the box into the air, and they may be carried for some
distance by the wind before they at length reach the ground. Once
there, however, each spore, under favourable conditions, begins to
grow, and gives rise to a plant which, curiously enough, is not in the
least like the parent fern plant which produced the spore.
=The difference between a spore and a seed.=—It is important to notice
that the spore is produced by a purely non-sexual process. In this
respect it differs widely from the seed of a flowering plant, which,
it will be remembered (p. 92), results from the union of the living
matter of a pollen grain with that of an ovule.
[Illustration: FIG. 147.—The Male-Fern. _A_, Prothallus seen from below;
_an_, male organs; _ar_, female organs; _rh_, hairs. _B_, Prothallus
with young fern attached to it; _b_, the first leaf; _w_, the primary
root (about 5 times nat. size).]
=The prothallus.=—The new plant, which is produced when a fern spore
germinates, is called a prothallus (Fig. 147, _A_). It is a flat, filmy
little plant, of the form which is generally called heart-shaped.
It has neither stem nor roots, but, as it contains green colouring
matter like that of leaves, and as it puts out on its lower surface
little hairs (_rh_) which take up watery solutions from the soil, it
is in no danger of starvation, and is quite capable of taking care of
itself and leading an independent existence. This tiny plant (a large
fern prothallus is perhaps half the size of a 3d. piece), in contrast
with its parent, produces sexual organs. Some of these organs (_an_)
give rise to male cells and others (_ar_) to female cells. The male
cells are excessively small, and can only be seen by high powers of
the microscope. When they are ripe they swim about in a drop of rain
or dew, as if they were little animals, and find their way to the
female cells, which they fertilise. The =embryo= which results from the
union grows up (Fig. 147, _B_) into an ordinary fern plant, one being
borne by each prothallus. In its young stages it is _parasitic_ on the
prothallus, _i.e._ it depends entirely upon the prothallus for its
nutrition; but it soon develops a little leaf (_b_) and a root (_w_),
and henceforth feeds itself. When the young fern is well established
in independent life, the prothallus shrivels up and dies. The
subsequently-formed roots of the fern are not branches of the primary
root, but spring from the bases of the leaves.
=Alternation of generations.=—In the life-history of the fern there are
thus two very different generations. The first generation—the ordinary
fern, which is non-sexual—produces, by means of spores, the sexual
generation, called the prothallus. The prothallus gives rise, in its
turn, by a sexual process, to the obvious fern plant. Each generation
therefore resembles, not its parent, but its grandparent.
[Illustration: FIG. 148.—Cross section of underground stem (rhizome) of
Bracken Fern. _s_, Conducting strands; _l_, _lp_, strengthening tissue;
_R_, ground-tissue; _e_, skin. (× 7.)]
[Illustration: FIG. 149.—Leaf of the Bracken Fern. (× ¹/₁₂.)]
The =bracken= or =brake fern= differs in several respects from the male
fern. Its cylindrical stem (rhizome) creeps along horizontally beneath
the ground, branching at intervals, by a division of its growing point
into two. Conducting strands run along the stem and into the leaves
and may be seen in cross section (Fig. 148, _s_) to form two somewhat
irregular, concentric rings. Strands and plates of strengthening tissue
(_l_) accompany them, and a cylindrical zone (_lp_) of similar material
also occurs just beneath the outside skin (_e_) of the stem. The great
advantage of having supporting structures arranged as hollow cylinders
has already (p. 72) been referred to. The rest of the stem consists of
softer packing or ground-tissue, in which starch is often stored.
[Illustration: FIG 150.—Hart’s Tongue Fern. (× ⅙.)]
[Illustration: FIG. 151.—Hart’s Tongue Fern. Part of a section through
the fertile portion of a leaf; _sg_, spore-boxes; _i_, _i_, protecting
flaps. (× 25.)]
The sappy stalk of a young bracken leaf is very sturdy; when it is
only six inches high the stalk may be already half an inch across at
the bottom, and half that thickness where it curls over at the top
to form a crook. The end of the leaf-stalk divides into three, each
bearing a frond which, at this stage, is coiled up tightly, looking
somewhat like a green caterpillar. At a later stage, the middle one of
the three branches commonly divides again into three, and the branching
may continue until the fully expanded leaf (Fig. 149) is very complex.
The spore-boxes and contained =spores= of the bracken are very similar
to those of the male fern, but they are not collected in patches, like
those shown in Fig. 146, 3. Instead, they are arranged in a row along
the margin of the lower surface of the frond-segment; and the margin is
turned over—like a hem—so as to cover them in.
The spores are liberated in the usual way (p. 187) when ripe; and each
germinates, under favourable conditions, to form a sexual =prothallus=.
Each prothallus gives rise to an =embryo=, which is at first parasitic
upon it, but presently grows up into an ordinary bracken.
=The hart’s tongue fern= (Fig. 150) has simple and undivided leaves,
which, as usual, bear spore-boxes upon the lower surface. In this case
the spore-boxes are produced in trenches, which appear to the naked eye
as oblique brown lines. Each trench is covered in by a pair of thin
flaps (_i_, Fig. 151). The life-history closely resembles that already
described for the male-fern and bracken.
39. THE COMMON HORSETAIL.
1. =Habit of growth.=—Carefully dig up a plant of the common horsetail.
Notice (_a_) the deep, creeping, underground _stems_; (_b_) the thin
wiry _roots_ springing from the stem; (_c_) the two kinds of upright
shoots or _haulms_: one kind (Fig. 152)—to be found in summer—being
thin, and giving rise to tiers of green branches, which come off like
the ribs of an umbrella; the other kind (Fig. 153), coming up in March,
being paler in colour, without branches, and bearing little _cones_ at
the apex.
2. =The underground stem.=—Make out that the stem is distinctly divided
into nodes—marked by toothed leaf-sheaths—and internodes. Does it
branch? Notice that some of the branches are swollen, forming _tubers_.
3. =The roots.=—These are thin and wiry, and come off from the stem. Do
they come off at the nodes or at the internodes?
4. =The branched haulms.=—Observe that each is divided into
nodes—marked by toothed leaf-sheaths—and internodes. Are the internodes
smooth or ridged? From what parts of the haulms do the _lateral
branches_ arise? Carefully tear down a leaf-sheath to see its relation
to the branches. Notice that the branches themselves branch repeatedly.
What is the colour of the branches?
5. =The cone-bearing haulms.=—Do these haulms appear earlier or later
than the others? What is their colour? Does a cone-bearing haulm give
rise to branches at the nodes? Examine the cone with a lens and see
that it is covered with hexagonal scales. Does the hexagonal shape
allow the scales to be more closely packed? Take off a scale with a
needle and notice, with the help of a lens, the _spore-boxes_ attached
to its inner surface.
[Illustration: FIG. 152.—Sterile Haulms of Horsetail.]
[Illustration: FIG. 153.—Fertile Haulms of Horsetail.]
6. =The prothallus.=—Dry a ripe cone and shake it over paper to collect
the spores. Sow these on moist soil, and try to raise prothalli.
=The general appearance of the horsetail.=—The common horsetail (Fig.
154), which may be found in hedgerows and cornfields, is not in any
respect showy. It rarely exceeds a few feet in height, and does not
attract attention by any display of bright colours. The plant has
stiff, jointed stems or =haulms=, standing gracefully erect and bearing
rings of small united leaves at intervals. The branches arise just
above the leaves, and alternate with them: several coming off at each
level, somewhat like the ribs of an umbrella. The whole plant has thus
a rather stiff and formal aspect.
[Illustration: FIG. 154.—Common Horsetail. (× ⅙.) 1, Fertile haulms,
terminating in cones (_a_): 2, a sterile haulm; _a_, tubers: 3, scale
of cone with spore-boxes (mag.): 4, scale with ruptured spore-boxes
(mag.): 5, 6, 7, spores (mag.).]
The haulms are very plainly of two kinds; for some, which are of a pale
colour, do not branch, and are developed merely to bear the pretty
little =cones= (Fig. 154, 1, _a_) which appear at their upper ends. As
the purpose of the cones is to give rise to the next generation, the
haulms which bear them may be called the fertile haulms. They will be
described presently, when the reproduction of the plant is considered.
=The sterile haulms.=—The erect branching shoots (Fig. 154, 2), bear
no organs of reproduction, and are hence referred to as the sterile
haulms. Their work is to provide the whole plant with carbonaceous
food, and it is on this account that their branches are of the
characteristic green tint. In most green plants the manufacture of
carbonaceous food (Chapter III.) takes place mainly in the leaves.
In the horsetail, however, the leaves are small and of very little
importance in this respect, and the work is carried on by the sterile
haulms and their branches.
=The underground stem and roots.=—A large part of the horsetail plant
is hidden beneath the surface of the ground, and often penetrates
to a great depth. In ordinary language these underground parts are
called roots. They are, however, subterranean =stems=, as is shown
quite plainly by the fact that they bear small leaf-sheaths like those
on the stems above ground. In the common horsetail the underground
stems often become swollen in parts by the formation of _tubers_
(Fig. 154), as is the case with the potato. The true =roots= are very
slender and thread-like, and spring from the nodes (p. 45) of the
underground stems. They penetrate the soil in all directions, seeking
for water, which they take up by the fine hairs which clothe them, like
velvet-pile, a little behind their points. The stem and its haulms,
like those of grasses, are stiffened by _silica_, which is deposited
in the outer layers. This is of course obtained from the soil. It
is not absolutely necessary to the life of the plant, but it is
nevertheless very useful, as it enables the haulms to stand upright and
spread out their branches to the light and air.
The sterile branches are thus concerned with the horsetail’s daily
life. The roots provide it with water and mineral food, and the green
branches supply the necessary carbonaceous matter. Stem, branches, and
roots are permeated with a complete system of canals, through which the
food substances find their way to the various centres of activity.
=The reproduction of the horsetail.=—It has been seen that some of the
haulms do not bear branches, but are set aside for the production of
the cones. These =fertile haulms= come up in March, before the green
sterile haulms appear. When the =cones= are carefully examined they
are seen to be covered by a number of shield-shaped scales. To the
inner side of each scale (Fig. 154, 3, 4) are attached from five to ten
boxes, each containing a large number of little grains or =spores=. The
boxes burst open when they are ripe, about the end of March, liberating
the spores; and the fertile haulms, having performed the one duty for
which they were developed, at once die down.
If the spores fall in a favourable situation they germinate, and each
gives rise to a new plant. The new plant is, however, not a horsetail,
but a small filmy =prothallus=, somewhat like the prothallus of a fern.
Some of the prothalli produce male cells, while others give rise to
female cells. When the minute male cells are ripe, they are set free,
and are able, by means of fine, lashing threads, to swim towards the
female cells of a neighbouring prothallus, through a drop of dew or
rain. The two cells fuse together and give rise to a little =embryo=,
which in due course grows up into a new plant—an ordinary horsetail
with stems and branches and roots like its grandparent.
=The advantage of an alternation of generations.=—The life-history
of the horsetail is evidently very similar to that of a fern, each
exhibiting a well-marked alternation of generations. There is reason
for believing that the prothallus-generation is the original form, and
that the ordinary fern and horsetail were developed simply to scatter
spores at intervals, and so to give new plants the advantages of fresh
soil.
[Illustration: FIG. 155.—Calamite: an extinct plant allied to the
Horsetails (greatly reduced).]
=Living and extinct horsetails.=—None of the British horsetails is of
great height, although one species may attain to six feet. Some of the
tropical members of the family, however, are very much larger than
this, reaching even forty feet. In spite of the last-named fact it is
quite plain that the horsetails have had their day, and are dying out.
If we wish to form an idea of what they were at the height of their
prosperity, we must carry our minds back to the long distant age when
our coal was being formed; when so much of this country as then existed
was low-lying swamp, covered with exuberant vegetation. Then the
horsetails and their relatives were stately forest trees, and at the
head of the vegetable kingdom. Some of them towered to a height of over
ninety feet. Nor did mere height constitute the only difference between
them and their degenerate descendants. Many of these old-world giants
had already found out the device, since invented afresh by more modern
plants, of thickening their stems and roots with secondary wood, and of
giving rise to bark like that of our present-day forest trees. The wood
of these =Calamites= (Fig. 155), as they are called by geologists, had
already reached much the same stage of development as is found to-day
in such a tree as the yew. The cones often show great variation from
the comparatively simple form found in the modern horsetail, but they
are essentially of the same type.
=Flowering and flowerless plants.=—Botanists divide plants into two
great groups—=flowering= plants, which reproduce themselves by means of
pollen grains and ovules; and =flowerless= plants, which still retain
the primitive marriage customs of their ancestors. The flower is a
comparatively recent invention in the history of plant life, and its
success is shown by the dominant position in the plant world which the
flowering plants now occupy.
Many of the flowerless plants—like the horsetails—have fallen behind
their competitors. The ferns, in spite of their conservatism, still
hold their own, and seem in no danger of extinction. Many other
flowerless plants maintain their position by sheer force of numbers.
Yet others have become completely extinct, and can now be known only by
their fossil remains. But these latter are sometimes so distinct that
stems, roots, and spore-boxes can be seen with all their sharpness of
outline unimpaired; the delicate tracery of frond and leaf is visible,
as clear and fresh as if made yesterday. And such rocky herbaria tell
us in unmistakable terms that our forest trees and other flowering
plants are after all mere parvenus and upstarts.
EXERCISES ON CHAPTER X.
1. Explain the formation of the green scales which are
frequently seen on the surface soil of a fernery. Whence
do they arise? What happens to them if they are allowed to
grow? (King’s Scholarship, 1902)
2. Point out the differences between the fronds of a fern
and the leaves of most flowering plants. (1901)
3. Show how the spore-producing plant of a fern is attached
to the prothallus, and trace the early development of the
former.
4. Make a list of the ferns which you have seen growing
wild, and state exactly in what kinds of places they were
growing.
5. Make experiments to prove that the prothallus of a fern
is capable of manufacturing starch.
6. Make drawings of the spore-bearing leaves of all the
ferns you can find, marking in each case the position of
the spore-groups.
7. In what situations have you found horsetails growing?
What other plants were growing near?
8. What is the difference between a seed and a spore?
9. Describe the situation in which the main stem of the
bracken is found, its mode of growth, and the simpler
facts of its structure. (1904)
10. In what respects do you consider the rhizome of a fern
(_a_) similar; (_b_) dissimilar to the root of
a tree?
(Certificate, 1905)
11. Where are the spores of a fern formed, and how are they
dispersed? What do they produce on germination? (1905)
CHAPTER XI. MOSSES, MUSHROOMS, AND MOULDS.
40. LIVERWORTS AND TRUE MOSSES.
1. =A common liverwort.=—Look along the sides of a brook or a well,
and try to find a flat green plant with numerous lobed and overlapping
branches. Each branch is perhaps half an inch across. This is one
of the commonest liverworts (_Pellia_). Notice the prominent midrib
running along each branch. In spring, observe the “frilled” appearance
of the end, caused by the small new branches.
Pull the plant up, and notice that it is attached to the soil by a
large number of fine _hairs_ which spring from the lower surface of the
midrib.
In February, or March, examine the upper surface of the growing plant
with a lens, and notice the small, dark-green balls, mounted on short,
thick stalks. Examine these at intervals until May, and notice that the
stalks then grow rapidly until they are two or three inches long. Each
is white, and still bears the black ball (the _capsule_) on its summit.
When the stalk is full-grown, the capsule opens—its wall splitting into
four parts—to liberate the _spores_. How soon, after the liberation of
the spores, do the capsule and its stalk die down?
2. =A common moss.=—Separate a single plant from a tuft of the common
moss (_Funaria_) which grows, almost everywhere, on the ground and on
walls. Notice that the plant consists of a _stem_ perhaps half an inch
high, thickly covered with small simple green _leaves_. The moss is
fixed in the soil by a tuft of fine _hairs_.
On some of the plants notice a thin stalk (about half an inch long)
springing from the top of the stem; and on the end of the stalk an
ovoid _capsule_ or spore-box. In cases where the stalk is not yet
full-grown, notice that the capsule is covered by a conical _hood_,
somewhat like a candle-extinguisher.
Select a plant bearing a ripe capsule, and warm it gently before the
fire—holding it over a sheet of paper—to dry it. Examine the mouth of
the capsule with a lens, and try to see the teeth which surround the
opening. Then shake the capsule over moist soil in a small flower-pot
to scatter the _spores_; cover with a sheet of glass and keep in a warm
room. In a few days notice that the soil is covered with fine green
threads. Ultimately new moss plants will grow from these.
=The life-history of a liverwort.=—One of the commonest and simplest
of this class of plants is known to botanists as _Pellia_. It may
generally be found growing by the sides of streams or old wells. It
has neither stem, leaves, nor root, but consists of flat, green,
overlapping lobes which fork at their ends. It branches very freely,
and in spring the new branches give the ends of the lobes a frilled
appearance. A rather prominent midrib runs along each branch. The plant
is attached to the soil by a large number of fine hairs which spring
from the lower surface of the midrib.
The whole appearance of this liverwort is very suggestive of the
prothallus (Fig. 147) of a fern; and, indeed, it corresponds to a
prothallus, not only in its general structure and mode of life, but
also in bearing sexual organs of a very similar type. It was seen in
Chapter X. that a fern prothallus at length produces an embryo, which
grows up and forms the spore-bearing generation which is what people
usually understand by a “fern”.
The liverwort also gives rise to a spore-bearing generation, but in
this case it consists merely of a small, round, spore-box, dark-green
in colour, which is carried on the summit of a white stalk—the whole
looking somewhat like a stout pin. In February, or early March, the
spore-boxes may be seen upon the upper surface of the plant as small
balls, perhaps one-sixteenth of an inch in diameter, protruding from
the mouth of a little pocket in which their early stages are passed.
About May, their stalks lengthen so rapidly that in a few days the
spore-boxes are lifted to a height of two or three inches. Then each
box opens by its wall splitting into four, and the spores are liberated
to germinate and form new liverworts. Having thus shed the spores, the
box and stalk die down.
The complete life-cycle of a liverwort thus includes two unlike
generations, as that of a fern does; but it is a different generation
which attains the greater development in the two cases. The ordinary
fern is the spore-bearing generation; the “ordinary” liverwort is the
sexual generation, and its sporing offspring is not a separate plant at
all, but a mere stalked box, almost entirely dependent upon its parent.
[Illustration: FIG. 156.—A Moss. _A_, a plant with spore-case still
covered by hood (_c_) (× 1). _B_, a plant with ripe spore-case (_k_);
_s_, stalk; _d_, lid; _rh_, hairs (× 1). _C_, mature spore-case with
lid (_d_) removed; _p_, fringe of teeth (× 3).]
=The mosses.=—A moss-plant appears at first sight to be very similar
in its general features to one of the higher plants; for it has a
little stem, bearing flattened green leaves which build up carbonaceous
food in the usual way. It has no true roots, but fine hairs penetrate
the soil and do the work of roots by taking up solutions of mineral
food. It is all the more remarkable, therefore, to find from its
method of reproduction that the moss-plant belongs to the generation
which corresponds to the prothallus stage of a fern’s life-history,
and not to the leafy, sporing generation which it somewhat resembles
superficially.
The sexual organs of a moss are essentially of the same type as those
which a prothallus bears, and here also the tiny male cells gain access
to the female cells by swimming through a drop of rain or dew. The
result of fertilisation is an embryo which, however, grows up to form,
not a plant with stem and leaves, but a stalked spore-case only.
In a tuft of the moss _Funaria_, which is so common in all country
places, several stages of development of the spore-cases may usually
be seen. Fig. 156 represents a moss which is very similar to _Funaria_
but larger. In _A_, the spore-case is still covered by a conical
hood (_c_). In _B_, the stalk (_s_) has grown much longer, and the
hood has dropped off. The spore-case or capsule (_k_) is now seen to
be a pear-shaped body, closed by the lid (_d_). When the spores are
ripe, the lid becomes detached. The mouth of the nodding capsule is
still blocked, however, by a number of teeth (Fig. 156, _C_, _p_)
which remain close together in damp weather. In dry weather the teeth
separate, and allow the spores to fall out. When a moss spore falls in
a favourable situation it germinates to form a fine, branching network
of green threads, from which new moss-plants arise as buds.
The obvious plant of a moss—like that of a liverwort—therefore
corresponds to the prothallus generation of a fern or horsetail.
Its sporing generation, corresponding to the obvious fern-plant or
horsetail, is a stalked spore-case, which is always more or less
dependent on its parent and dies down as soon as its work of scattering
the spores is accomplished.
41. THE COMMON MUSHROOM.
1. =Habit of growth.=—In what situations do you find mushrooms growing?
In what kinds of weather and at what time of the year are they most
abundant? Take up a mushroom with a trowel, and carefully wash the
earth from the lower part to see the tangle of white threads (called
the _mycelium_), which is the underground part of the plant. See that
several of these run into the bottom of the stalk. On the underground
mycelium look for young mushrooms in the “button” stage. Look around,
and try to get a series of mushrooms showing all stages from the
smallest buttons to fully-opened specimens.
2. =Structure.=—Draw a side view (natural size) of a full-grown
mushroom, showing the _stalk_ and _cap_. Running round the stalk notice
a ragged flap, called the _collar_. What is the height of the collar
from the base of the stalk? Examine younger mushrooms to find what
the collar really is. Notice that in young specimens a membrane or
_veil_ stretches from the edge of the cap to the stalk, and that as
the mushroom grows larger this veil is torn away from the cap (Fig.
157), and remains as a ragged flap (the collar) on the stalk. In young
specimens, therefore, the lower side of the cap is completely shut in
by the veil, while in fully-grown ones it is exposed.
3. =The gills and spores.=—Cut the stalk across at the top, and make a
drawing of the lower surface of the cap. Notice the radiating vertical
plates (the _gills_), and their flesh-coloured or dark-brown tint. Be
careful to show the exact arrangement of the gills in, say, a quarter
of the drawing. Cut off the cap of a mushroom which has brown gills
and lay it, gills down, on a sheet of paper; cover it with a tumbler
to shield it from draughts, and leave it for a day. Then take off the
cap (being careful not to smear it along the paper), and observe the
radiating brown lines. On touching them, it will be seen that the
lines consist of fine brown dust. The particles of dust are _spores_.
They have evidently fallen from the gills.
4. =The source of the mushroom’s food.=—What is the colour of the
mushroom? Cut through the stalk and cap in various directions and
notice the pure white “flesh” of the interior. Do you think it likely
that the plant can obtain carbonaceous food from the air? Why not?
Dry a mushroom and burn it carefully. Does it contain _carbon_? Where
must the carbon have come from, if not from the air. Can you find any
decaying vegetable or animal matter in the soil in which mushrooms grow?
5. =Toadstools.=—Carefully compare the common mushroom with other
gilled fungi which look somewhat like it. Notice particularly the
following characters: the colour, shape, and texture of the surface of
the cap; the colour and arrangement of the gills; the proportions of
the length and thickness of the stalk to the diameter of the cap; the
presence or absence of a collar on the stalk; the presence or absence
of a cup, or scaly swelling, at the base of the stalk.
The following =precautions= are necessary in selecting mushrooms for
food: Never eat a “button” mushroom; in this stage wholesome and
poisonous mushrooms cannot be properly distinguished from each other.
Reject all mushrooms which show signs of a _cup_ or a scaly swelling at
the base of the stalk—especially if they have also _white spores_ and a
collar.
=The common meadow mushroom.=—The common mushroom (Fig. 157) may easily
be found in meadows, especially in autumn and after damp weather
in summer. The part which rises above the surface of the ground
consists of a stout =stalk=, about three inches long and perhaps
three-quarters of an inch thick. On the upper end of the stalk is a
circular horizontal =cap=, convex and smooth above and concave below;
with a diameter about equal to the length of the stalk. The interior
of the stalk and cap is composed of a firm white fleshy substance.
The underground part of the plant consists of a tangle of fine white
threads, most of which are woven together into strands; and it is easy
to see that several of these strands run into the base of the stalk.
Each of the fine threads is called a =hypha=, and the network which
they together compose is known as the =mycelium=. It has indeed been
found by microscopic examination that the whole plant is built up of
such hyphae—closely packed together in the stem and cap, more loosely
aggregated in the underground strands. The stalk of the full-grown
mushroom bears, a little above its middle, a ragged ring of tissue—the
=collar=. This ring is the remains of a membrane or =veil= which in the
younger stages reached to the edge of the cap, completely enclosing its
lower surface; but which was torn off the cap (Fig. 157) as the stalk
of the “button” mushroom elongated.
[Illustration: FIG. 157.—Common Meadow Mushroom. To the right, mushroom
in “button” stage (× ½).]
=The reproduction of the mushroom.=—The lower side of the cap of a
mature mushroom bears a very large number of radiating plates called
the =gills=. These are at first pink, but the colour afterwards changes
to a dark brown. Each gill produces an immense number of extremely fine
=spores=, too small to be seen individually by the naked eye. When,
however, the cap of a mushroom is laid, gills down, on a sheet of paper
and protected from draughts, a “spore print” is obtained; the dust of
the fallen spores marks the positions of the gills in brown radial
lines. Quite recently it has been found possible to raise new mushroom
plants by germinating the spores, and there can be no doubt that this
is the natural method of reproduction.
=The method of life of the mushroom.=—The mushroom, like all the
plants we have studied, thus consists of parts which perform one or
other of two duties; that of feeding the plant, and that of continuing
the race. In this case the food is supplied _entirely_ by the
underground mycelium; while the aërial part, consisting of stalk and
cap, is concerned only with the work of scattering the spores. There is
nothing corresponding to the leaf-green of the higher plants, and the
mushroom is therefore unable to make use of the carbon dioxide of the
air (p. 34) as the source of the carbonaceous food which is necessary
for its life. Carbonaceous and mineral food alike are obtained from the
soil by the underground mycelium. The plant is, in short, dependent
upon carbonaceous food which has been previously built up by some
other plant or animal, and can therefore grow only in soil containing
decaying animal or vegetable matter. It would be quite unable to
subsist upon the nutritive solution of salts which was seen (p. 29) to
suffice (when supplemented by fresh air) for green plants.
=Fungi.=—The mushroom belongs to a class of plants called the Fungi,
all of which obtain their carbonaceous food ready-made from some
other plant or animal, living or dead. For this reason many fungi are
parasitic upon other living plants. Most of the diseases of crops and
of forest trees are due to fungi. The moulds which destroy food are
fungi, and the dry rot which ruins timber is caused by organisms of the
same class.
=Toadstools.=—The common meadow mushroom is a justly-esteemed article
of food, but some other gilled fungi, which are intensely =poisonous=,
bear a sufficiently close resemblance to mushrooms to render them
dangerous. The student should therefore take every opportunity of
examining these, and of comparing them with the edible mushroom in the
manner described on p. 204.
42. MOULDS.
1. =The growth of moulds.=—Damp three small slices of bread, and cover
them with tumblers to prevent them from drying. Expose one slice to
ordinary daylight; keep another in a dark place at about the same
temperature; and submit the third to the direct rays of bright sunlight
as much as possible.
2. =White mould= (_Mucor_).—In one or two days observe that the bread
is covered with white fleecy threads. Some of these may grow to a
height of an inch or more, and end in small black knobs which can be
seen with the naked eye. The knobs contain _spores_. Examine the mould
with the help of a lens. On which of the three pieces of bread is there
most mould, and on which the least?
Mix freshly-boiled and strained juice of stewed fruit (preferably
colourless or nearly so) with an equal quantity of water. Half fill a
small glass with the mixture, and with the point of a needle add a few
spores from the mould on the bread. Cover the glass, avoid shaking it,
and examine day by day. Notice the delicate threads (_hyphae_) of the
_mycelium_ which spreads over the liquid and sends down branches below
the surface. Observe also the hyphae which grow up into the air and
bear spore-knobs at the end. Similarly, sow some of the spores on a
little of the nutritive solution of salts (p. 27) contained in a clean
glass. Do these spores also develop into moulds? Why not?
Scratch through the skin of a ripe fruit (_e.g._ plum, apple, grape,
etc.) with a needle, and lay the fruit aside with the scratch upward.
Scratch a similar fruit in the same way, and rub into the scratch a few
mould-spores before laying it aside. With these put a third fruit which
has a perfectly whole skin. Compare the fruits after a few days.
3. =Blue mould= (_Penicillium_).—After some days the white mould on the
bread will probably be crowded out by a blue or greenish velvety mould
called _Penicillium_. Examine it with a lens. As with _Mucor_, make
experiments as to the action of light upon this mould; the germination
of its spores (which form a greenish powder on the ends of the short
aërial hyphae) in fruit-juice; and the action of the spores upon ripe
fruits.
=Common moulds.=—In the dust floating about in the air the spores of
certain moulds are almost always present. When these spores fall upon
materials which—like bread, fruit, old leather, etc.—are capable of
affording them suitable food-substances, they germinate and form the
woolly growths which are familiar to everyone.
=The white mould= (_Mucor_).—What is known familiarly as white mould,
and botanically as =Mucor=, is very convenient for study on account of
its abundance and large size. If a piece of bread is kept in a damp
atmosphere it generally becomes covered, after a day or two, with a
fleecy growth of the white threads of this mould. These may attain
a height of an inch or more, and many of them bear at their ends a
small black knob, in which the spores are formed. When mature, the
knobs burst open, and the fine spores are scattered in the air. The
various parts of the mould are best seen in position by scattering
some of the spores, or a little dust from a shelf, upon the surface
of clear, colourless fruit-juice in a glass vessel, and following the
stages of growth with a lens. It then becomes clear that the mould—like
the mushroom—consists of (_a_) a buried tangle or mycelium of hyphal
threads which take in the plant’s food, and of (_b_) an aërial part
which scatters the spores. The mould also resembles the mushroom in not
containing the green colouring matter possessed by the higher plants,
and therefore in being dependent upon ready-made organic food. Provided
with this, it can grow freely even in the dark.
[Illustration: FIG. 158.—_Penicillium._ An aërial hypha with its
terminal spore-brushes. (× 360).]
=Blue mould= (_Penicillium_).—It is generally found that the fleecy
hyphae of _Mucor_, which first cover the damp bread, are crowded out
in a short time by the growth of another mould, which is known to
botanists as =Penicillium=. This also consists of a mycelium, which
penetrates the nutritive substance, and of aërial hyphae which produce
spores. Here, however, the spores are not borne in cases, but break off
from the ends of brush-like branches of the aërial hyphae (Fig. 158).
The spores of _Penicillium_ are greenish-blue in colour, and it is from
this circumstance that the mould receives its common name. The colour
is, however, quite different from the leaf-green which gives the higher
plants the power, in sunlight, of decomposing the carbon dioxide of the
air and building up their own carbonaceous food—a power which neither
_Penicillium_ nor any other fungus possesses.
=The smallest plants.=—The description of microscopic organisms is
beyond the scope of this book, but the student ought to realise that
by far the greater number of plants are quite invisible to the naked
eye. Some of these—to be found in every pond and ditch—are green, and,
though they are of very simple structure, they obtain their food in
a manner substantially resembling that adopted by a flowering-plant
or a fern. Others, like the yeast plant, are fungi, and are subject
to the limitations in food-supply which are characteristic of that
class. The smallest of all plants are the =bacteria=; they are almost
inconceivably minute, yet they possess an influence upon the health and
wellbeing of mankind which it is impossible to over-estimate.
EXERCISES ON CHAPTER XI.
1. Explain in what respects a moss plant resembles and
differs from the prothallus of a fern.
2. What structure in a moss corresponds, as regards
reproduction, to an ordinary fern plant?
3. What do you mean by the term “alternation of
generations”? Explain your answer by references to ferns
and mosses.
4. How does a mushroom resemble and differ from a green
plant in its method of obtaining food?
5. Why does a piece of damp bread become mouldy when it is
exposed to the air?
6. Describe simple experiments which prove that the dust of
ordinary air contains living particles.
7. Mention common plants which are wafted about by currents
of air in a dwelling-house, and point out changes which
such plants may set up in articles of human food. (1905)
_PART II._ ANIMAL LIFE
CHAPTER XII. THE RABBIT: A TYPICAL MAMMAL.
43. THE RABBIT: HABITS AND APPEARANCE.
1. =The habits of the wild rabbit.=—In what places have you known
wild rabbits to have a _warren_? In what kind of ground is a rabbit
warren generally found? How can you recognise it? Are all the holes
of the warren of similar size, or can you distinguish between main
entrances and “bolt-holes”? Look for smooth paths, perhaps nine inches
wide, which lead to the main entrances and intersect each other, so
as to form “runs.” Watch the animals feeding and playing; to do this
successfully it will be necessary to keep very still and silent; avoid
walking on the runs. What do the rabbits eat? Do they walk, or hop?
How do they run? Notice how conspicuous is the white tail of a running
rabbit. In June look for a nest of young rabbits. The position of such
a nest may often be recognised, when the doe is away from home, by a
smooth patch of earth with which she has covered up the hole. Dig up
this very carefully and notice how the nest is lined. Examine the young
ones without hurting them, and then cover them up again.
What is the _colour_ of a wild rabbit? Does the colour render the
animal less conspicuous? Are tame rabbits so often of this colour? Why
not? Watch a tame rabbit, noticing especially the movements of its
nostrils, whiskers, and ears, and its method of feeding. Try to see how
it gnaws the bars of its hutch.
2. =Fur.=—Examine a dead rabbit. With what is the skin covered? What
are the differences between the fur of a rabbit, the hair of a dog,
and the wool of a sheep? Is fur curly? Does it lie flat on the skin?
Does it consist of different sizes of hair—short, fine hairs and long,
thicker ones? What other animals do you know which have fur?
3. =The head.=—(_a_) _The whiskers._—On what parts of the head are
the longest hairs found? How do these differ, apart from length, from
ordinary hairs? What other common animals have whiskers? Do such
animals often make their way through narrow passages? What do you
suppose is the use of the whiskers?
(_b_) _The skull._—Feel the bones of the head through the skin, and
make out the rounded brain-case, the ridge above and the arch below
each eye, and the positions of the jaws.
(_c_) _The external ears._—Examine the large ear-flaps. Notice that the
upper parts are thin and almost transparent, and that the lower parts
are gristly and lead into the interior of the head.
(_d_) _The eyes._—Notice that the eyes are at the _sides_ of the head.
Is this position an advantage to the rabbit? Examine, in each eye,
the upper and lower lids; and also, in the angle of the eye next the
nose, the _third eyelid_—a fold of white skin. Take hold of this fold
with the forceps and see that it is easily pulled over the eyeball.
Look in a mirror and see the little fleshy body which occupies a
similar position in your own eye; this corresponds to the rabbit’s
third eyelid. In the visible part of the rabbit’s eyeball notice the
round dark _pupil_ in the middle (contrast the pupil of a cat’s eye);
the coloured ring (the _iris_) surrounding the pupil; and the “white”
(called the _sclerotic_) surrounding the iris.
(_e_) _The lips and nostrils._—Notice how the upper lip is split in
the middle line, so as to show the front teeth. What is the use of the
split? Notice the grooves passing from the upper lip to the nostrils.
(_f_) _The inside of the mouth._—Open the rabbit’s mouth and notice:
(i) The two gnawing or _incisor teeth_ in the lower jaw, and the pair
of strongly-grooved incisors which are so conspicuous in the upper jaw.
Just behind the large, upper incisors two smaller incisors may be felt
with the finger. Separated by a wide space from the incisors are the
_grinding teeth_—six on each side in the upper jaw, and five on each
side in the lower jaw.
(ii) The _tongue_, lying between the halves of the lower jaw.
(iii) The hard _ridges_ running across the fore half of the roof of the
mouth.
(iv) The _hairiness_ of the inside of each cheek, between the incisor
and grinding teeth.
4. =The neck.=—Notice how the neck enables the head to be turned freely
in various directions without the body being moved. Feel through the
skin and identify:
(_a_) In front of the neck, the _trachea_ or windpipe, with the
_larynx_ or “voice box” at its upper end between the halves of the
lower jaw.
(_b_) The bones of the neck-part of the _spine_.
5. =The trunk.=—Feel through the skin and make out the _spine_, the
_breast-bone_, and the curved _ribs_ which connect these. The spine,
breast-bone, and ribs together form a bony cage which encloses the
fore-part (the _thorax_) of the body. The hinder and larger part of the
body (the _abdomen_) is not protected by ribs, but the bones of the
spine are largest and stoutest in this part of the body.
6. =The limbs.=—How many limbs has the rabbit? Which pair is the
longer? Is the difference in length an assistance in leaping? Make out
the main divisions of each limb, and, by feeling through the skin, the
manner of attachment of its bones to the bones of the body (Fig. 161).
7. =The tail.=—Notice the length, shape, position, and colour of the
tail. The chain of bones inside it is a prolongation of the spine.
=Methods of studying animals.=—In studying animals, methods similar
in principle to those described in previous chapters for plants should
be employed. A real knowledge of the =habits= of animals can only be
obtained by watching them as closely as possible in their natural
surroundings. It is often very difficult to get sufficiently close
to shy animals to see them distinctly without causing alarm, and a
good field-glass is a valuable help in such cases. A great deal of
first-hand knowledge of wild life can, however, be gained without such
aids if the student will learn to move quietly and silently, and to
remain motionless as soon as he is in a good position for observation.
The books of such masters of woodcraft as Richard Jefferies, William
J. Long, Ernest Thompson Seton, and W. H. Hudson give charming
descriptions of the methods of tracking and studying wild animals, and
should be carefully read by every field-naturalist. Observations made
in the field should be at once recorded, with the date, in a note book.
Many animals can be kept for some time, without cruelty, in
confinement; and a more intimate knowledge of certain of their habits
can be thus obtained. The practice of keeping =pets= is, however, to be
encouraged only when every possible care is taken to secure the comfort
of the captives.
To understand the internal structure of an animal, =dissection= is
necessary. This consists in exposing and separating the internal organs
of the dead animal from each other, in order to notice their mutual
relation. A careful outline drawing of every dissection should be
made and preserved. The student should make it a rule never to kill
_any_ animal unless for some useful purpose, and then to do so in the
quickest and most painless manner.
=The habits of the rabbit.=—Wild rabbits live in =burrows= or
underground passages which they excavate, by means of their strong
feet, in the soil of sandbanks, fields, woods, etc. The animals
are sociable, and the burrows belonging to any one community are
collectively known as a =warren=. The passages of the warren
communicate with the outside world by means of openings, some of which
are in common use, while others seem to be used mainly as “bolt-holes”
in cases of sudden alarm. A rabbit which is bolting to its burrow
exposes the white underside of its tail, and thus acts as a danger
signal and guide to its fellows. When a rabbit is startled, or puzzled
by seeing some unusual object, it generally thumps the ground smartly
with its long hind-foot; other rabbits in the neighbourhood are thereby
warned.
[Illustration: FIG. 159.—Rabbits.]
It is common to find, outside a rabbit warren, a number of intersecting
paths, perhaps nine inches wide, and worn smooth by the patter of
little feet. These are the highways, or “runs,” which lead from the
holes of the warren to the various feeding-grounds.
Rabbits breed very rapidly; it has been estimated that in five years a
single pair might have about a million descendants, were it not for the
countless mishaps to which rabbits are exposed. The young are born and
suckled in a special shallow burrow, which the doe excavates and lines
with dry leaves, fur, etc. When she leaves the nest for any purpose she
covers up the entrance with soil.
The proportions of the parts of the body of a very young rabbit are
markedly different from those of the adult. The head is relatively
larger, the tail longer, and the ears shorter; while the hind limbs
and fore limbs are of almost equal length. During the first six months
of its life the animal gradually takes on the proportions of the
adult—with small head, long ears, large hind legs with long feet, and
small, upwardly-turned tail.
A rabbit is able to stand upright on its hind legs, and to maintain
itself in this position for a considerable time. It thus obtains a
wider view and a greater choice of food.
The great difference in the length of the fore and hind legs gives
the animal a characteristic gait. “In a freely moving rabbit,” says
Jefferies, “both fore-feet stop when the hinder come up—one hinder
foot slightly behind the other, and rather wide apart.” Rabbits are
exclusively vegetarian feeders, living on green herbs and on the tender
shoots and bark of shrubs and young trees.
=The external characters of the rabbit.=—The outside of the rabbit’s
body is almost entirely covered with =fur=. This consists of two
kinds of hair—coarse and fine. The coarser hairs are fewer in number
and longer than the fine ones, which they protect from wet. The fine
hairs are extremely closely-set, and stand straight out from the skin.
In a seal-skin coat only the short hairs of the fur are to be found;
the longer hairs have been removed by the dressing process. The great
warmth of furs is due to the air which is entangled between the fine,
close hairs. It is generally the case that fur-bearing animals are
exposed to either mud or wet by their method of life; in spite of this
fact, while the hair of a dog soon becomes wetted by rain, and the
wool of a sheep retains a great deal of dirt, furred animals are noted
for keeping their coats clean and dry.
The longest hairs of the rabbit are the stiff =whiskers= which stand
out from the upper lip, the cheeks, and above the eyes. They are
extremely sensitive to touch, and are of great assistance to the animal
in finding its way through the dark burrows.
The =colour= of the wild rabbit is greyish brown, except on the belly
and under the tail, where it is white. This colour harmonises well
with the surroundings, and renders the animal much less noticeable.
Wild rabbits are exposed to so many enemies, that individuals which
happen to be born with conspicuously coloured fur have generally
but a poor chance of surviving and leaving offspring to inherit
their disadvantages. There is thus in each generation a =natural
selection= of the animals which are best protected, by their colour,
from observation. Among tame rabbits, on the other hand, =protective
colouration= is of very little importance, and one variation of colour
is as likely as another to be transmitted by =heredity= to the next
generation.
=The regions of the body.=—For convenience and precision in describing
animals it is customary to use the words =anterior= and =posterior=
to indicate the fore, or head, end, and the hind, or tail, end
respectively. The belly-surface is said to be =ventral=, and the back
=dorsal=.
The body of a rabbit obviously consists of head, trunk, a short tail,
and four limbs. The general arrangement of the bony framework, or
=skeleton=, (Fig. 161) which supports the softer parts may be felt
through the skin. The skeleton consists of (1) the _skull_; (2) the
spine or _vertebral column_ (generally spoken of as the backbone),
placed dorsally, and reaching from the anterior end of the neck into
the tail; (3) the ventral _sternum_ or breast-bone, which is connected
with the vertebral column by means of curved _ribs_; (4) the bones of
the two pairs of limbs, with the shoulder-and hip-bones to which they
are attached. The bones will be studied in more detail in the next
chapter.
The trunk is divided into two regions—an anterior =thorax=, or chest,
(enclosed in the bony cage formed by the ribs, sternum, and the
adjoining part of the vertebral column), and a posterior =abdomen=. The
two cavities are divided from each other by a fleshy partition called
the _diaphragm_.
=Organs of special sense.=—In addition to the sense of touch which is
possessed by the whole surface of the body, the rabbit has organs which
enable it to distinguish objects by sight, sounds, scents, and taste.
Its sense of =smell= is so keen that, to be successful in snaring
rabbits, “in walking to the spot selected for the snare it is best to
avoid even stepping on the run, and while setting it up to stand back
as far as convenient and lean forward. The grass that grows near must
not be touched by the hand, which seems to impart a very strong scent.
The stick that has been carried in the hand must not be allowed to fall
across the run; and be careful that your handkerchief does not drop
out of your pocket on or near it. If a bunch of grass grows very tall
and requires parting, part it with the end (not the handle) of your
stick.”[8]
The shape of the =ear-flaps=, which can be turned in different
directions (Fig. 159), enables the rabbit to catch very slight sounds.
The =eyes= are placed on the _sides_ of the head, so that the animal
has a wide field of view, and an enemy approaching from behind is
not likely to be unnoticed. The eye of the rabbit is very similar in
structure to our own, but it possesses one useful adjunct which ours
has lost, in the shape of a _third eyelid_—an opaque flap of skin which
lies in the inner angle, and can be drawn over the eyeball at will.
The little fleshy nodule in the corresponding position of the human eye
is a rudiment of a similar structure.
=The rabbit a gnawing animal.=—Rabbits are much addicted to gnawing
young trees for the sake of the bark and the softer juicy tissues
between bark and wood. Even tame rabbits exhibit the same instinct by
gnawing their wooden hutches, although these are of no use as food.
The gnawing is done by the sharp teeth, called =incisors=, which are
conspicuous in the middle of both upper and lower jaw. In the upper jaw
two incisors (which are so deeply grooved that they look like four)
are visible even when the mouth is closed, owing to the split in the
middle of the upper lip. The hardest part of the tooth (a substance
called _enamel_) is at the front. Behind this the tooth is composed of
a softer, bony material called _dentine_; and the back of the tooth
consists of still softer dentine. The result of these differences in
the composition of the various parts of the tooth is that the gnawing
of hard substances wears away the back of the tooth most, the middle
part next, and the front least of all; and thus a sharp chisel-edge
is always maintained. Moreover, _the teeth of the rabbit never stop
growing_, so that they never become appreciably shorter through use.
Immediately behind the two visible incisors of the upper jaw, another
and smaller pair can be felt by the finger; the enamel-faces of these
are directed backwards towards the cavity of the mouth. The softer and
more easily worn faces of the two pairs of upper incisors are thus in
contact, and are continually worn down to form a groove. Into this
groove the two incisors of the lower jaw bite. The incisor teeth stand
well out from the jaws, and the split upper lip can be drawn back, so
that the lips are not injured by gnawing. In the hinder part of the
mouth, where the chewing, or =mastication=, of the food takes place,
are six flatter but cross-ridged =grinding teeth= on each side of
the upper jaw, and five on each side of the lower jaw. The insides of
the cheeks are protected from sharp splinters of wood by a patch of
=hair= on each side, which extends from the region of the incisors to
the grinding teeth behind. The roof of the mouth is protected by hard
cross-ridges, and the tongue by tough skin.
=Rodents.=—Rabbits and hares, rats and mice, and squirrels are said to
be rodents (Lat. _rodo_, I gnaw), as they not only agree in the gnawing
habit, but also in other very important respects.
=Mammals.=—Rodents, and all other animals which suckle their young,
are included by naturalists in the class Mammalia. These animals agree
further in breathing air, in having warm blood, and in being more or
less completely covered with hair.
=Vertebrates.=—Mammals, birds, reptiles, amphibians, and fishes are
grouped together to form the sub-kingdom Vertebrata. They are given
this name because they all possess a spinal, or vertebral, column
(which usually consists largely of a chain of bones), running below the
dorsal surface (p. 217) of the body, from neck to tail.
=The position of the rabbit in the animal kingdom.=—It is clear from
the above that the rabbit is, in the first place, a =vertebrate=
animal; it belongs, secondly, to the _mammalian_ class of vertebrates,
and thirdly, to the _rodent_-order of mammals.
EXERCISES ON CHAPTER XII.
1. In what respects does the hind-foot of a rabbit differ
from the fore-foot? What is the use of the difference?
2. Describe the way in which a rabbit runs. What precautions
does it take when feeding in an open place? (1901)
3. Make observations of the habits and external characters
of hares, and compare them, point for point, with those of
rabbits.
4. Describe the inside of a rabbit’s mouth, and explain the
advantages of any peculiar features to be seen in it.
5. Mention the two chief constituents of a rabbit’s tooth.
Describe by what means the edges of a rabbit’s incisor
teeth are kept sharp.
6. Make a list of all the vertebrate animals with which you
are acquainted. Why are they called vertebrates?
7. By what external characters would you recognise that an
animal was a mammal?
FOOTNOTE:
[8] Jefferies, _The Amateur Poacher_ (Smith, Elder & Co.).
CHAPTER XIII. HOW A RABBIT LIVES.
44. THE SKELETON AND MUSCLES
1. =The examination of the bones.=—In a boiled rabbit clear away the
flesh from the bones. Before separating a bone, notice carefully how
it is attached to neighbouring bones. Notice also, especially in the
limbs, the attachment of the bundles of flesh (_muscles_) to the bones,
the ends of each muscle being fixed to separate bones. Notice that the
places of attachment of the largest muscles are marked by ridges or
roughnesses on the bones.
2. =The skull.=—Observe the two rounded knobs at the back of the skull,
which fit into hollows on the first bone (vertebra) of the vertebral
column. In the skull notice the saw-like boundaries of the various
bones, and make out: the great, rounded _brain-case_; the external
openings of the _ears_; the _eye-sockets_; the snout, with the two
_nasal chambers_; and the upper and lower _jaws_. Before separating the
lower jaw observe carefully how it is hinged on the arches which run
below the eyes, and notice the great muscle on each side which moves
it. Examine the _teeth_ in detail, and remove them one by one from
their sockets. Notice the bony shelf or palate separating the nasal
chambers from the cavity of the mouth. Draw a side view of the skull.
Shave off the top of the brain-case with a sharp knife, and examine the
_brain_.
3. =The vertebral column.=—Make out that each of the bones (vertebrae)
which compose the vertebral column is really a ring, and that the whole
column is therefore a bony tube. In this tube is enclosed the spinal
cord, a backward continuation of the brain. Examine and draw vertebrae
from the various parts of the column, and notice how they vary in shape
and size. Compare vertebrae from (_a_) the neck, (_b_) the region of
the chest (notice the attachment of the ribs), (_c_) the abdominal
region, (_d_) the region of the hips (four united vertebrae; leave
these for the present in position between the hip-bones), (_e_) the
tail.
4. =The ribs and sternum.=—Examine the manner of attachment of the
anterior (p. 217) pairs of ribs to the sternum or breast-bone. Some of
the more posterior ribs are free at their ventral ends.
5. =The bones of the fore-limb.=—Examine the bones in order and make
out: (_a_) the triangular shoulder-blade, (_b_) the single long bone
of the upper arm (its upper, rounded end fitting into a socket in the
shoulder-blade; its lower end forming, with the bones of the fore-arm,
the elbow joint), (_c_) the two bones of the fore-arm (these lie side
by side; notice the peg which makes it impossible for the hinged
elbow-joint to be bent back beyond a straight line), (_d_) the small
bones of the wrist, (_e_) the bones of the hand.
6. =The bones of the hind-limb.=—Notice that the limb as a whole is
divided into parts—upper leg, leg, ankle and foot—which correspond to
the divisions of the fore-limb. Make out: (_a_) that as the bone of
the upper arm fits into a socket in the shoulder-blade, so the bone
of the upper leg has a ball-shaped end which works in a socket of the
hip-bone. Notice that the two hip-bones are joined together ventrally,
and that they are fixed to the welded vertebrae of the hip-region. Does
this give increased strength to the hind-limb? Identify: (_b_) the two
bones of the leg (between knee and ankle) and notice that they differ
from the corresponding bones of the arm in being fused together in the
lower part; (_c_) the bones of the ankle, and (_d_) the bones of the
foot.
7. =The structure of a long bone.=—Take the long bones of the upper
arms and upper legs, and examine them further. Break one across the
middle. Is it solid or hollow? What is the advantage of its being
hollow? Is the tube empty or does it contain marrow? Are the ends of
the bone hollow or solid? Place one of the bones on a bright hot fire.
Is there any sign of burning? Does all the bone burn away? Carefully
remove what is left, and compare the brittleness of the “burnt” bone
with that of an unburnt bone. Place another bone for several days in
a cupful of water to which has been added about two teaspoonfuls of
strong hydrochloric acid. Then take it out, wash it; and try to bend
it. Continue the treatment until the bone is soft enough to tie in a
knot. Can you tie an ordinary bone in a knot? Why not? Put a “burnt”
bone in similar dilute acid. Does it dissolve? What do you think gives
a bone its hardness?
8. =Bones moved by muscle.=—Stretch out one of your arms and grasp
the middle of the upper arm firmly with the other hand. Now bend the
elbow, and notice that the great muscle (the _biceps_) of the front of
the upper arm thickens and becomes shorter. Straighten the arm again,
and notice that the muscle becomes thinner and longer again. Examine
Fig. 160, which shows how the upper end of the biceps is attached at
_a_ to the shoulder-blade and its lower end to one of the bones of the
fore-arm at _P_. If the biceps muscle shortens, the fore-arm must be
pulled up, because the shoulder remains stationary.
Carefully notice the various movements of which your arm is
capable—_e.g._ extension of the arm; bending (flexure) on the
elbow-joint; rotation of the fore-arm, so that either the palm or the
back of the hand can be turned upwards; and grasping. Observe your
power of touching the tip of the little finger with that of the thumb.
If a human skeleton is accessible watch how one (which?) of the two
long bones of the fore-arm rotates when the hand is turned over. How
many of these movements can the rabbit make?
=The uses of the skeleton.=—The bodies of vertebrate animals (p. 220)
are supported, and their more delicate parts protected from injury,
by an internal framework called the skeleton. In some of the simpler
fishes this is composed of gristle or “cartilage”; but in more highly
developed animals it consists almost entirely, in the adult state,
of bone. Further, almost all the bones are connected with strands or
bundles of red flesh called =muscle=, which the animal is able to
shorten or “contract” at will. When a muscle shortens, the bones to
which its ends are fixed are of necessity pulled nearer to each other.
If the bone at one end of the muscle remains stationary, that at the
other end is moved into a new position. This is, generally speaking,
the manner in which the numberless movements of the limbs, head, etc.,
are made.
[Illustration: FIG. 160.—The action of the Biceps muscle. _a_, the
attachment of the muscle to the shoulder-blade; _P_, attachment of the
muscle to the fore-arm; _F_, elbow; _W_, hand.]
=The movements of the limbs.=—An excellent illustration of this
relation between the bones and muscles is seen in the bending of the
arm at the elbow. When the arm is bent, a great mass of flesh, the
biceps muscle, in front of the upper arm may be observed to become much
thicker. The muscle (Fig. 160) thickens because it becomes shorter. Its
upper end is attached to the corner of the shoulder-blade (_a_), which
remains stationary; while its lower end is connected at _P_ with one
of the bones of the fore-arm. The shortening of the muscle therefore
draws up the fore-arm. The elbow-joint, about which the motion takes
place, is a very perfect hinge. Several other forms of joint are also
found in various parts of the body. It is seen, when the skeleton is
examined, that in every case the characters of the joints and the
attachment of the muscles are most admirably adapted to the movements
with which they are associated.
=The study of the skeleton.=—The study of the rabbit’s skeleton is not
only highly interesting in itself, but necessary for the intelligent
appreciation of the animal’s life. And when it is compared with the
skeletons of other familiar animals, a common plan of structure is
found which illustrates in the most convincing manner the kinship which
often exists between very dissimilar creatures. Such a comparison
shows that, almost bone for bone, the skeleton of a rabbit corresponds
with that of a man or a horse, and even with that of a bird or a frog.
Mounted skeletons are shown in most natural-history museums, and the
student should, whenever possible, examine and compare them.[9] He can
himself, however, easily separate the bones from a boiled rabbit, and
make out their main features and relationships.
=The rabbit’s skull and backbone=—The bones of the head are
collectively known as the =skull=. This consists of (1) a large brain
case; (2) the cavities for the organs of special sense, viz., (_a_)
a pair of nasal chambers (in which the organ of smell is located) in
front: their hinder ends open into the top of the throat; (_b_) the
eye-sockets at the sides, and (_c_) the flask-shaped chambers for the
internal ears at the sides of the hinder end of the brain-case; (3)
the jaws: the upper jaw is rigidly attached to the brain-case, but the
lower jaw is hinged at each side on the hinder end of the bony arch
which runs below the eye. Both upper and lower jaws bear teeth, which
are fixed in sockets. On each side the upper jaw contains two incisor
teeth (p. 219) and—much further back—six grinding teeth. In the lower
jaw are one incisor and five grinding teeth on each side.
[Illustration: FIG. 161.—The Rabbit. The skeleton in position, seen
from the left side.]
The =vertebral column= or backbone is a chain of bony rings or
_vertebrae_ which runs dorsally (p. 217) from the hinder end of the
skull to the tail, and forms a long tube containing the spinal cord—a
backward continuation of the brain. On the anterior face of the first
vertebra are two hollows into which a pair of knobs on the skull fit.
Although, with the exception of the first and second, all the vertebrae
are formed on essentially similar lines, there is considerable
variation in shape and size in the different regions of the spine, as
may be seen from Fig. 161. There are seven neck vertebrae, and this
number is remarkably constant in mammals (p. 220). Following these
are the chest vertebrae, which bear pairs of =ribs=. The majority of
the ribs curve round and join on to a ventral bar of bone called the
=sternum= or breast bone; so that the cavity of the chest, containing
those very important organs the heart and lungs, is enclosed in a
protective bony cage. The vertebrae of the abdominal region of the body
are very large and stout. Between them and the tail-bones are four
fused vertebrae, forming a mass which on each side gives attachment to
the large hip-bone.
=The bones of the rabbit’s limbs.=—The fore and hind-limbs of the
rabbit are obviously comparable: the upper arm corresponding to the
upper leg, the elbow to the knee, the fore-arm to the “leg,” the wrist
to the ankle, and the fore-foot to the hind-foot. This similarity
becomes even more apparent when the limb-skeletons are examined.
Commencing in each limb at the end nearest the body we find a single
long bone. In the fore-limb the rounded, upper end of this bone
works in a socket at the anterior angle of the =shoulder-blade=, a
triangular plate on each side which overlies the chest dorsally; in
the hind-limb the rounded end of the corresponding bone works in a
cup in the =hip-bone=. Again, between the elbow and the wrist are two
bones lying side by side; and between the knee and the ankle are two
corresponding bones, although here they are only separate in their
upper parts. Similarly, the wrist bones correspond to the ankle bones,
and the bones of what may be called the fingers to those of the toes.
Certain of the ankle bones are, however, much elongated: obviously a
great advantage to an animal which progresses by a series of hops—owing
to the increased leverage which is thereby given to the hind-foot. The
rabbit’s fore-foot bears five toes, the hind-foot four.
=The structure of a long bone.=—The long bones of the limbs are hollow
except at the ends. Strength and lightness are thus secured by a
device (the hollow cylinder), which has already (p. 72) been seen to
be adopted by the supporting structures of plants, and is also copied
by human engineers. The cavity of the tube is filled by marrow, which
supplies the bone with food. The hard, bony tube itself is partly
composed of mineral matter and partly of animal matter. The mineral
matter is left as a white, brittle framework when the animal matter
is burnt away; while on the other hand the mineral part—which gives
the bone its rigidity—may be dissolved out by immersing the bone in
dilute hydrochloric acid, leaving the organic tissue as a soft flexible
substance having the shape of the original bone.
45. HOW THE RABBIT DIGESTS ITS FOOD.
[Illustration: FIG. 162.—Experiment to show that starch solution will
not pass through a thin membrane. _b_, beaker; _w_, water; _p.p._,
parchment paper; _t.t._, stem of thistle funnel; _s_, starch solution
in head of thistle funnel.]
1. =A solution of starch will not pass through a thin membrane.=—Rub up
with water as much starch as will lie on a shilling, so as to form a
thin “cream,” and then pour on it about a cupful of boiling water. The
starch swells up and largely dissolves in the water. Add a few drops of
the starch solution to about half a pint of water, stir, and test it
by adding a little iodine solution (p. 2, footnote). A beautiful blue
colour is obtained, showing that the test is a very delicate one. Now
take a thistle funnel and with a file cut through the stem about six
inches below the head. Wet a piece of parchment paper or thin bladder
(having previously held it up to the light to be sure there are no
holes in it), and tie it tightly across the mouth of the funnel. Fill
the head and about an inch of the stem with the starch solution. This
can easily be done by means of a “canula,” such as is used for filling
fountain pens. Now put the funnel into a beaker of water, in the manner
shown in Fig. 162, and put the arrangement aside for a few hours.
After that time add iodine solution to the water in the beaker. No
blue colour is formed, showing that no starch has passed through the
membrane.
2. =A solution of sugar will pass through a thin membrane.=—A delicate
test for certain varieties of sugar (not, however, table sugar) is a
liquid known as Fehling’s solution.[10] Place a particle of honey in a
test tube with a teaspoonful of Fehling’s solution, and put the test
tube into a vessel of boiling water. Notice that in a short time the
blue colour of the solution disappears and the liquid becomes red and
turbid.
Now repeat Experiment =45, 1=, but instead of starch solution use
honey dissolved in water. To show that some of the honey-sugar has
passed through the membrane, take about a teaspoonful of the water in
the beaker, and put it in a test tube with twice as much Fehling’s
solution. Heat as before, and notice the red turbidity. If table sugar
is used it may be recognised by the sweet taste of the water after the
experiment.
3. =The action of saliva on starch.=—(_a_) Chew a piece of india-rubber
to induce a free flow of saliva, and collect the liquid. In one test
tube put half a teaspoonful of starch solution; in a second tube put a
mixture of equal quantities of starch solution and saliva; in a third
put saliva alone. Keep the tubes at blood heat for twenty minutes.
Then add a little water to each tube and divide its contents into two
parts. Test one part of each for starch with iodine solution, and the
other part for sugar with Fehling’s solution. The first tube contains
only starch. The second now contains no starch, but shows the presence
of sugar. The third contains neither starch nor sugar. _Evidently the
saliva has changed the starch of the second tube into sugar._
(_b_) Repeat the experiment, but keep the mixture of starch and saliva
in a _cold_ place. No sugar is produced.
(_c_) Repeat the experiment as in (_a_), but use saliva which has been
heated to boiling in a test tube. No sugar is formed.
=The necessity for food.=—It is common knowledge that a rabbit, like
every other animal, must have a regular supply of food if it is to
continue healthy, and that it would soon die outright if food were
withheld. The reason for this is that the living substance of the
animal’s body is incessantly wasting away. The rabbit cannot move a
muscle except at the expense of the living substance of the muscle,
and the more active is its life, the more rapidly does its body waste.
It is to counteract this continual waste by continual formation of new
living substance that food is taken.
But the succulent plants which the rabbit eats are not suddenly
transformed into animal muscle and bone, and so forth, when they are
swallowed. They have first of all to undergo a process which is called
=digestion=. This takes place in a tube—the digestive canal—which runs
from end to end of the body. The digestive canal of the rabbit is
coiled in a somewhat intricate manner. That of the frog is, however,
much simpler and more typical of vertebrates (p. 220) generally, and
will serve equally well in so elementary a consideration of digestion
as the present.[11]
=The frog’s digestive canal.=—The hinder end of the frog’s mouth opens
(Fig. 163) into the =gullet= (_gul._), a short wide tube which leads to
a capacious bag called the =stomach= (_st._). The termination of this
is continuous with a coiled narrow tube called the =small intestine=
(_s. in._), which passes suddenly into a much wider =large intestine=
or rectum. The rectum opens to the exterior, at the hinder end of the
body, by an aperture (_an._) known as the anus. In addition to the
digestive tube proper, two large digestive glands, the liver and the
pancreas, should be carefully noticed. The =liver= (_lr._) is a large,
dark red organ, consisting of two lobes which lie at the sides of the
stomach. It makes a digestive fluid called _bile_. The =pancreas=
(_pn._) is an elongated body which lies in the loop between the stomach
and the first portion—called the _duodenum_ (_du._)—of the small
intestine. It makes a digestive fluid called the _pancreatic juice_.
In the frog both the bile and the pancreatic juice are discharged into
the duodenum by one tube or duct (_b.d._). Small digestive glands also
occur in the inner wall of the stomach; these discharge a fluid called
_gastric juice_ into the cavity of the stomach.
[Illustration: FIG. 163.—The Frog, dissected from the left side. _gul_,
gullet; _st_, stomach; _du_, duodenum; _s. int_, small intestine;
_an._ anus; _lr_, liver; _pn_, pancreas; _b.d_, bile duct; _bl_,
urinary bladder; _c.art_, _l.au_, _s.v_, _v_, parts of the heart; _cn.
3_, “body” of 3rd vertebra; _eus t._ Eustachian tube; _gl_, glottis,
leading to _l.lng_, left lung, and _r.lng_, right lung; _n.a. 1_, arch
of 1st vertebra; _p.na_, internal opening of nostril; _sp.cd_, spinal
cord; _spl._ spleen; _tng_, tongue; _kd_, kidney; _ur_, ureter; _vo.t_,
vomerine teeth.]
=The rabbit’s digestive canal=—In its main features the digestive canal
of the rabbit resembles that of the frog; here also the mouth opens
into a long tube consisting of gullet, a bag-like stomach, a small
intestine, and a large intestine. There are also a liver and a pancreas
which discharge their fluids—by separate ducts, however,—into the first
part of the small intestine; and the stomach is supplied with gastric
juice by small glands in its inner wall. There are, however, certain
differences besides those of size in the digestive tubes of the two
animals. In the first place, a fluid called _saliva_ is poured into the
rabbit’s mouth by the ducts of salivary glands which occur near the
mouth. Secondly, the coils of the small intestine are very much more
complicated than in the frog. And lastly, at the junction of small and
large intestines there is given off, in the rabbit and in herbivorous
mammals generally (when these have simple stomachs,—p. 261), a great,
spirally-constricted tube which ends blindly in a finger-like process.
=How the rabbit digests its food.=—A rabbit needs food to repair the
constant waste of substance which the activities of its life entail;
and the same is true of every other living thing, be it plant or
animal. Now, every part of a rabbit’s body is irrigated and drained by
the finest branches of a system of pipes through which blood is always
flowing; and it is in this blood-stream that the food is conveyed to
the muscles and other organs which are to be repaired. The food finds
its way into the blood when that fluid is flowing through the small
vessels which lie in the thickness of the wall of the digestive tube.
Before food can gain access to the blood it must be in a condition
in which it is capable of diffusing through the thin membrane which
separates them. =Digestion is the process which renders food soluble
and diffusible=, and hence capable of passing into the blood. The
food of animals is of several different kinds. A few of these are
soluble and diffusible at the time they are eaten, but most of them are
neither, and therefore require treating according to their nature. This
is why so many different fluids are poured into the rabbit’s digestive
canal. Saliva, gastric juice, bile, and pancreatic juice are each
capable of acting upon certain constituents of the food and rendering
them soluble and diffusible.
It would be beyond the scope of this book to consider the work of
these fluids in detail, but the =action of saliva= is not only fairly
typical, but it can easily be imitated outside the body. =Starch= is a
very common constituent of vegetable foods, and its presence or absence
is readily determined by the blue colour which it gives with a solution
of iodine. Starch is quite insoluble in cold water, but when treated
with hot water it swells up and, to a great extent, dissolves. But a
mere solution of starch cannot get into the blood, for it is incapable
(Expt. =45, 1=) of passing through a thin membrane. On the other
hand, if starch is mixed with saliva, and the mixture is kept at the
temperature of the body, it is found in a short time that the starch
has been changed into sugar, which is not only soluble, but readily
diffuses through a thin membrane. In other words, starch _as such_
is useless to the rabbit as food; only after it has been digested by
conversion into sugar can it be used by the body.
Something very similar often takes place in plants. It was seen (p. 33)
that the cotyledons of a pea become sweet during germination. Starch
is a convenient form of food for storing in the cotyledons of a pea,
the endosperm of the maize seed, the short stem of the crocus-corm,
and so on; but before the plant can use it as food the starch must be
made soluble and diffusible by being changed into sugar. In plants the
change is brought about, not by saliva, but by a substance known as
_diastase_.
The pancreatic juice continues the change of starch into sugar which
is commenced by the saliva, but it also digests other food-stuffs as
well. Similarly, gastric juice and bile are each concerned with the
digestion of certain foods. The result of the action—separate and
combined—of the digestive fluids is that, when a rabbit eats no more
food than it requires, all the useful parts of the food are absorbed
into the blood, and then distributed to the tissues by the blood as it
flows through them.
46. THE CIRCULATION OF THE BLOOD.
1. =Evidence of the circulation.=—(_a_) _The arterial pulse._—Feel your
pulse at the wrist and count the number of beats per minute.
(_b_) _The beats of the heart._—Similarly count how many times your
heart beats per minute. Put your ear over the heart of a friend, and
listen to the sounds of the heart. How many different sounds can you
distinguish?
(_c_) _The valves of the veins._—Press your thumb on your arm inside
the elbow, and then move it down the arm towards the wrist. Notice the
small knots which rise under the skin between the point of pressure and
the wrist.
2. =The structure of a sheep’s heart.=[12]—Procure from the butcher
a sheep’s heart with, if possible, the lungs still attached. If the
thin transparent bag which naturally surrounds the heart has been
left on, carefully cut it away. Make out the main external features
of the heart before cutting into it. The shape is conical, the apex
of the cone being posterior, the base (where the blood-vessels are
attached) anterior. The ventral face is rounded; the dorsal face is
flatter. Compare Fig. 164. The line of fat (_3_) marks the line of
division between two chambers, the right (_R.V._) and left (_L.V._)
_ventricles_. Feel that the right ventricle yields to pressure
more readily than the left. Two more chambers, the right and left
_auricles_, are situated at the basal (thick) end. _R.A._ and _L.A._
are flaps of the right and left auricles respectively. Identify the
great vessels _SVC_ and _IVC_ which discharge into the right auricle.
Cut them open to see the entrance. Then lay open the right auricle.
Pass your finger down and notice that the right auricle communicates
freely with the right ventricle. Observe that you can from the right
ventricle pass your finger into the tube _P.A._; make out that this
goes to the lungs. Lay open the left auricle, and see that it receives
vessels _from_ the lungs. Pass your finger from the left auricle into
the left ventricle, and notice that this latter chamber leads into the
_Aorta_, _Ao._ (_A´o´._ a branch of _Ao._). Notice that the walls of
the blood-vessels in connection with the two auricles are collapsed,
while those (_P.A._ and _Ao._) leading from the two ventricles are more
elastic and remain open. Which of the two auricles has the thicker
wall? Which of the two ventricles has the thicker wall? Can you pass
your finger from one auricle to the other? From one ventricle to the
other? Cut away the auricles and pour water into the ventricles. Then
squeeze the heart gently, and notice the flaps which rise to close the
openings into the ventricles. Could blood pass from the ventricles to
the auricles? Why not? Lay open the bases of the vessels _P.A._ and
_Ao._ leading from the right and left ventricles respectively, and
notice the pockets of thin membrane which are attached there. Open them
out gently with the point of a pencil. Put the heart under the tap and
let water trickle down the vessels towards the ventricles; notice how
the pockets stand out as they fill with water. What would be the effect
of blood trying to pass from _P.A._ or _Ao._ back into their respective
ventricles?
=The circulation of the blood.=—The blood of such an animal as the
rabbit is contained in a system of closed tubes called blood-vessels,
through which the fluid is continually flowing. The regular flow of
the blood in one direction is maintained by the action of the =heart=,
a four-chambered organ which is situated in the chest, between the
lungs. The heart is muscular, and, like other muscles (p. 225), has
the power of “contracting” in definite directions. The contraction of
the walls of the chambers of the heart lessens their capacity, and
therefore drives the blood out of them, the direction of flow being
determined by valves. The vessels which carry blood _to_ the heart are
called =veins=; those which transmit the blood which is pumped _out
of_ the heart are called =arteries=. The arteries branch into smaller
and smaller tubes which supply the various organs of the body, and the
small arterial branches divide again and again in the organs until
their finest ramifications form a close-meshed network of vessels with
excessively thin walls, through which diffusion can readily take place
between the tissues and the blood. These finest blood-vessels are
called =capillaries=. They can be studied in the transparent web of the
frog’s foot with the aid of a low power of the microscope. The blood
can then be seen flowing, at a speed which varies with the size of the
vessel, its course being rendered obvious by the tiny oval particles
(red corpuscles) which are suspended in it. The smallest capillaries
are so thin-walled that they appear to be merely channels in the
substance of the web, and the corpuscles creep along in single file.
But these channels unite to form larger vessels with obvious walls, and
these unite again and again until a main vein is formed in which the
blood, with its suspended corpuscles, rushes along in a swift torrent
towards the heart.
=The heart.=—The beginner will find the sheep’s heart more convenient
for examination than the rabbit’s, on account of its larger size;
but apart from some difference in the arrangement of the great
blood-vessels opening into them, the two hearts are broadly similar.
The heart (Fig. 164) consists of four chambers. Two of these, the
=auricles=, are receiving-chambers, and are placed at the thick,
anterior end of the heart. Into the right auricle open the great veins
(_SVC_, _IVC_,) which bring blood from all parts of the body except
the lungs; the left auricle receives only blood from the lungs. Each
auricle opens into a more posterior chamber called a =ventricle=, the
right auricle opening into the right ventricle and the left auricle
into the left ventricle. The ventricles pump blood into the arteries.
The blood from the right ventricle is sent into the artery (_P.A._)
which supplies the capillaries of the lungs; while the blood of the
left ventricle is forced into the aorta (_Ao._), a great artery which
branches and supplies with blood all parts of the body except the
lungs. The two auricles contract simultaneously, forcing their contents
into the flaccid ventricles. Then both the filled ventricles contract
at once, and pump blood into the great arteries, flaps of membrane
between the auricles and ventricles preventing a backward flow into the
auricles. Similarly the bases of the great arteries are provided with
membranous pockets which readily admit the blood from the ventricles
when these contract, but entirely prevent a return of blood to the
ventricles. The appearance of these four sets of valves, as seen
from above, is shown in Fig. 165. After the contraction of the two
ventricles there is a short rest, then the auricles contract again and
the whole process is repeated.
[Illustration: FIG. 164.—Heart of Sheep, in position between the lungs.
_R.A._, appendage of right auricle; _L.A._, appendage of left auricle;
_R.V._, right ventricle; _L.V._, left ventricle; _S.V.C._, _I.V.C._,
great veins from the system generally; _P.A._, artery to lungs; _Ao._,
aorta; _A´o´._, branch of aorta; _L._, lung; _Tr._, windpipe, leading
to lungs; 2, 3, 4, fat.]
[Illustration: FIG. 165.—The orifices of the heart seen from above,
the auricles and great vessels being cut away. _PA_, valves at base
of artery to lungs; _Ao_, valves at base of aorta; _R.A.V._, orifice
between right auricle and ventricle, with its valves _l.v. 1_, _l.v.
2_, and _l.v. 3_; _L.A.V._, orifice between left auricle and ventricle,
with its valves _m.v. 1_ and _m.v. 2_.]
[Illustration: FIG. 166.—Diagrammatic sections of veins with valves. In
the upper figure the blood is supposed to be flowing in the direction
of the arrow, towards the heart; in the lower, back towards the
capillaries.]
[Illustration: FIG. 167.—A vein laid open to show a pair of
pouch-shaped valves.]
The =sounds= which are heard when the ear is placed over another
person’s heart are often compared to the syllables _lub-dup_. The “lub”
is partly caused by the contraction of the ventricles; the “dup,” which
immediately follows, is caused by the sudden closure of the valves at
the bases of the great arteries. The throb of the heart, which can be
felt from the outside, is really the thrust of the apex of the heart
against the chest-wall at each beat. The sudden forcing of blood into
the already-full but elastic arteries causes a wave to travel along
these vessels, which can readily be felt, or even seen, at places where
a fairly large artery lies just beneath the skin. This arterial wave is
called the =pulse=. Many of the veins are provided with pouch-shaped
valves which permit the blood to flow freely towards the heart, but
which bar the passage of blood in the opposite direction. Their action
will readily be understood from Figs. 166 and 167.
=The importance of the capillaries.=—The capillaries are the least
conspicuous part of the circulatory system, but they are by far the
most important. The heart, arteries, and veins exist merely to renew
constantly the blood which flows through these minute channels. The
excessive thinness of the walls of the capillaries makes it possible
for a ready exchange to take place between the living tissue and the
blood, and the vessels themselves form a network of such extremely
close texture that it is practically impossible to prick any living
part of the body with a fine needle without puncturing some of them and
“drawing blood.” The work of the blood in supplying the various organs
of the body with food has already been referred to. We have next to see
how this all-important fluid is of service in supplying the organs with
oxygen.
47. RESPIRATION.
1. =Carbon dioxide is formed when flesh burns.=—Dry a piece of meat and
attach it to the end of a wire. Then light it, and when it is burning
vigorously lower it into a clean glass jar. When the flame goes out
remove the charred flesh, and at once pour a little clear lime-water
into the jar and shake up. The lime-water turns milky, showing the
presence of carbon dioxide gas in the jar. Examine what is left of the
meat. It is charred, showing that meat contains carbon. How was the
carbon dioxide formed during the burning?
2. =Carbon dioxide is formed by the living body.=—Breathe through a
glass tube into clear lime-water, so that the air you expel from your
lungs bubbles through the liquid. Does the lime-water remain clear,
or turn milky? Does the air you breathe out contain a considerable
quantity of carbon dioxide gas?
=Burning and life.=—When a piece of the flesh of any animal has been
dried it may easily be set on fire. The burning is caused by the union
of the constituents of the flesh with some of the oxygen of the air
to form various gases. One of these gases is carbon dioxide, formed
by the combination of the carbon of the flesh with oxygen. Carbon is
present in all the parts of animals and plants, as is evident from
the separation of charcoal (an impure form of carbon) in the first
stages of burning; the carbon dioxide gas which is formed may easily be
recognised by the milkiness which it produces in clear lime-water. The
liberation of heat, and the formation of carbon dioxide, which always
accompany the burning of animal and plant tissues, are worthy of very
careful attention.
[Illustration: FIG. 168.—Experiment to prove that expired air contains
carbon dioxide.]
It is well known that the body of a living animal such as a rabbit
or a man is always _warm_; and the experiment (Fig. 168) of passing
through clear lime-water the air breathed out from the lungs shows, by
the milkiness produced, that the animal is also constantly producing
carbon dioxide during its life. Is life, then, always accompanied
by a peculiar form of burning, in which the living substance of the
body is the combustible material? It seems so, and the experiments of
physiologists tend to confirm this view.
=The necessity for breathing.=—The energy which enables a muscle to
contract is derived from the oxidation—the slow burning—of part of its
substance, just as the energy which enables a steam-engine to move
is derived from the burning of fuel in the boiler fires. The fires
soon go out, and the engine stops, unless fresh fuel is added from
time to time and a plentiful supply of air is available. Similarly,
a muscle loses its power of contracting, a gland that of secreting,
the brain that of thinking, unless the waste matters resulting from
previous activities are cleared away and replaced by fresh food and
fresh oxygen. Wherever vital action is taking place, whether in a
contracting muscle, a secreting gland, or a thinking brain, there is
continual consumption of oxygen and continual production of waste
material, chiefly carbon dioxide. In the higher animals the renewal of
oxygen and the removal of waste material are performed by the blood.
Blood-vessels are to the body what rivers and canals are to a country:
they act as highways for the transport of material. We may perhaps
carry the analogy a little farther and find in the red corpuscles of
the blood the rough equivalents of boats or canal-barges, for they
carry with them tiny loads of oxygen. As the blood creeps along the
narrow channels in an active tissue the red corpuscles relinquish their
oxygen, and the fluid portion of the blood takes up carbon dioxide.
The blood continues its course, and sooner or later arrives at a place
where it can obtain a fresh supply of oxygen and get rid of its surplus
carbon dioxide. In the rabbit this exchange takes place as the blood
is passing through the capillaries of the =lungs=. There the blood
is separated from the air by a membrane so delicate that gases can
readily pass through it; and, hence, on leaving the lungs the blood has
got rid of the waste carbon dioxide, and its red corpuscles are laden
with fresh oxygen. _This exchange of useless carbon dioxide for oxygen
constitutes respiration or breathing._
=The mechanism of respiration.=—In active animals the air inside the
lungs soon becomes vitiated, unless there is some means of changing
it. Under ordinary conditions a man changes the air in his lungs from
thirteen to fifteen times a minute. He does this quite automatically,
and without thinking about it. Every four seconds or so a set of
muscles contracts and pulls his ribs upwards and outwards; another
muscular contraction pulls down the floor of his chest at the same
time. As a consequence the cavity is much enlarged. The lungs follow
the movements of the walls of the chest, and some thirty cubic inches
of air are sucked in. Immediately the ribs fall back to their former
position, the chest-floor rises, and air is driven out. Then after a
short pause the process is repeated. It should be noticed that only
about thirty cubic inches of air are changed at each respiration,
although the capacity of the human lungs averages about 230 cubic
inches. All mammals breathe in much the same way.
=Plants and animals.=—There are considerably more points of similarity
than of difference between plants and animals. In every case the vital
activities are accompanied by an oxidation of living substance, and
from this fact arises the necessity for food and oxygen. The breathing
of plants is essentially like that of animals, and consists in taking
in oxygen and giving out carbon dioxide; though the _mechanism_ of
respiration is—except in the lowest plants and animals—entirely
different in the two cases. It is in the sources from which they obtain
their =food= that plants and animals are most unlike. An animal must be
supplied with food which has already been prepared by some other living
thing; and it is obvious that the food even of carnivorous animals can
ultimately be traced back to plants, for the flesh-eater preys upon
the vegetarian. Animals are therefore entirely dependent upon plants
for food. In this sense the saying “all flesh is grass” is full of
significance.
Green plants (Chapter II.) are quite independent of all other forms
of life, and can build up their substance from water, mineral matter,
and the carbon dioxide of the air. The taking in of carbon dioxide
and giving out of oxygen by green plants has nothing whatever to do
with respiration; it is part of their process of feeding. Green
plants breathe in the usual manner—by taking in oxygen and giving
out carbon dioxide. It should, however, be noticed that the peculiar
method by which a green plant obtains its carbonaceous food is of the
highest importance to animal life; for by this process the amount of
injurious carbon dioxide in the air is considerably lessened, while the
proportion of life-supporting oxygen in it is greatly increased.
Fungi (Chapter XI.) seem to be intermediate, as regards their method
of feeding, between green plants and animals. They require their
carbonaceous food in an organic form, that is, already prepared by
other living things; but they can obtain the other elements of their
food from mineral salts and water.
The thoughtful student will be increasingly impressed by the extent to
which the plant and animal kingdoms are dependent upon each other, and
by the manner in which each utilises the waste products of the other
for carrying on its own life processes.
EXERCISES ON CHAPTER XIII.
1. Of what parts does the skull of a rabbit consist? What is
the use of each part?
2. Describe one of the long bones of a rabbit’s leg. To what
features does it owe its strength?
3. The bones of the skeleton are useful (1) as affording
points of attachment for the muscles; (2) as affording
protection for delicate tissues and organs. Give examples
of each of these uses. Do not give the technical names for
the various muscles. (King’s Schol., 1902)
4. Draw and describe one of the middle joints of the
backbone of a quadruped, and explain the uses of the
various parts. (1898)
5. What is meant by “digestion”? Why must food be digested?
Where does digestion take place?
6. Give full practical instructions for demonstrating the
chief properties of saliva, and its action upon various
kinds of food. (1897)
7. Prove that the action of human saliva upon starch is not
due to living particles contained in it. (1898)
8. What are the chief uses of the blood? Why is it necessary
that it should be kept in motion? (1901)
9. Where would you look for the Aorta in a sheep’s heart?
What valves are found in it? How does it differ in
appearance and feel from a large vein? (1901)
10. What kinds of valves are found in a sheep’s heart, and
where are they placed? Describe a valve of each kind. (1898)
11. How does air breathed out from the lungs differ from
common air? How can the differences be demonstrated? (1898)
12. Describe the process by which a mammal renews the air in
its lungs.
13. What is meant by “respiration”? Why is respiration
necessary?
14. Point out the most remarkable differences between the
nutrition of a green plant and that of an animal. (1897)
15. Name organisms which can derive nourishment from carbon
dioxide, from sugar, and from the dead bodies of animals.
(1898)
16. What are the simplest functions which distinguish living
animal matter from inanimate matter? (King’s Schol. 1903)
17. Explain as fully as you can how food taken into the
stomach acts upon organs, such as the brain, which are not
closely connected with the stomach. (1904)
18. The uses of bone are, generally speaking, to protect
delicate structures, to support weight and to gain
leverage. Illustrate this statement by a simple
description of one example of each type. (Certificate, 1904)
19. The flow of liquids through the body is regulated in
certain localities by valves. Explain the action of a
valve, and indicate where they are to be found in the
body. (Certificate, 1904)
20. In which kind of blood-vessel can the pulse be felt? In
which kinds can it not be felt? Explain the reason of the
difference. (Certificate, 1905)
21. How is oxygen conveyed from the lungs to the various
parts of the body? Describe what could be observed if a
drop of blood were spread out on a piece of glass and
examined under a microscope. (King’s Schol. 1905)
FOOTNOTES:
[9] See footnote, p. 231.
[10] Prepared by adding to a solution of copper sulphate first tartaric
acid, and then caustic soda until the blue mixture is clear. It may be
obtained from a chemist if the materials are not available.
[11] NOTE TO TEACHERS.—A general dissection of a frog should be made
and exhibited to the class. Detailed instructions for such a dissection
will be found in Marshall’s _The Frog_ (Smith, Elder & Co.) or in
Huxley and Martin’s _Elementary Practical Biology_ (Macmillan). The
frog’s heart continues to beat for some time after the death of the
animal, and may be shown as an illustration of the next section of this
chapter. Teachers who are unskilled in dissection may obtain prepared
dissections, mounted skeletons, etc., from Newmann & Co., 84 Newman
Street, London, W.
[12] More detailed instructions will be found in Foster and Shore’s
_Physiology for Beginners_ (Macmillan).
CHAPTER XIV. SOME OTHER MAMMALS.
48. THE CAT AND DOG.
1. =The external characters of the cat.=—Carefully and gently examine a
cat, and make notes of the following characters:
(_a_) _Hair._—What is the covering of the body like? Is the hair
like that of a rabbit, _i.e._ fur (p. 216)? Are the whiskers very
noticeable? On what parts of the head do they grow? Are the fur and
large whiskers in any way connected with the animal’s habits?
(_b_) _Eyes._—Look at a cat’s eyes in a bright light. Is the pupil (p.
212) round or slit-like? Keep the cat in the dark for a few minutes and
then look again at the pupil of the eye; has it changed in form? Is the
change of any advantage to the cat?
(_c_) _Teeth._—Gently open the cat’s mouth and examine the teeth.
Notice the sharp, pointed teeth behind the incisors (p. 219); they are
called the _canine_ teeth. Has a rabbit any canine teeth? Why does a
cat, and not a rabbit, need such teeth? Feel the remaining teeth with
your finger; are they flat like those of a rabbit, or sharp-edged? How
are the characters of the teeth associated with the kind of food? Watch
a cat eating; does it chew its food or swallow it “in lumps”?
(_d_) _Tongue._—Pass your finger-end over the cat’s tongue; is it rough
or smooth? In which direction of motion of the finger does the tongue
feel roughest? Would the roughness be of any help to the animal in
licking meat from bones?
(_e_) _The limbs._—Measure the legs. Are the fore and hind limbs of
equal length, slightly unequal, or very unequal? How do they compare in
this respect with the limbs of the rabbit? Make out the main bones by
feeling through the skin, and especially notice where the ankle-joint
is. Examine the toes and notice how, when you gently squeeze them just
behind the ends, the sharp _claws_ protrude, and go back into a kind of
sheath when the pressure is removed. Can the animal put out its claws
and draw them back at will?
2. =The habits of the cat.=—(_a_) _Food._—What kind of food does the
cat prefer, animal or vegetable? Does it bolt its food greedily, or
does it eat deliberately and daintily? Have you ever known cats to hunt
other animals? Do they hunt singly, or do several cats join together
to hunt? How do they approach the prey; do they try to run it down by
speed, or do they creep up slyly and then spring? What is the use of
the claws? How does a cat drink?
(_b_) _Locomotion._—Watch a cat moving slowly; does it walk or hop? Try
to find out the order in which it puts its feet down. How does it run?
Is it nimble or clumsy? Does the cat walk with the whole sole of its
foot on the ground, or does it walk “on its toes”?
(_c_) _Likes and dislikes._—Do you consider a cat sociable, _i.e._
fond, in general, of the society of other cats? Do cats, as a rule,
show much appreciation of the difference between right and wrong? Are
they as affectionate as dogs? Have you ever heard of any cat trying to
remain in a house after the family had removed to another house? Are
cats fonder of warm or of cool places in a house? Do they like getting
wet? Do they pay much attention to personal cleanliness? How do they
wash themselves? Do you think the tongue is used as a comb? How? Can
you know whether a cat is pleased or angry (i) by the appearance and
movements of the tail, (ii) by the sounds which it makes?
(_d_) _Intelligence._—Write accounts of cases of great intelligence
which you have known cats to show.
(_e_) _Voice._—How do you describe a cat’s voice? How does the voice
vary according to the animal’s mood?
(_f_) _Play._—How does a cat play?
3. =Kittens.=—(_a_) _Appearance._—Are kittens helpless or active
when they are born? Is there any very marked difference between the
proportions of the body and limbs and those of a full-grown cat? At
what age is a cat full-grown?
(_b_) _Play._—How does a kitten play? How does it pretend to “stalk” a
small object, such as a ball of wool? In the same way that a full-grown
cat stalks a mouse? Why does the kitten adopt this method before it has
any experience of hunting?
(_c_) _Education._—Watch a cat with its kittens, and describe any
actions which seem like education. Have you ever seen a cat teach its
kittens to fight? Have you ever seen it punish a kitten for disobeying
a call?
4. =The external characters of the dog.=—Examine a dog in the same way,
and compare it point by point with the cat as regards the following
characters:
(_a_) _Hair._—Notice that the dog is covered, not with fur, but with
rough hair, and that the whiskers are not very large. How are these
differences associated with differences in habit?
(_b_) _Eyes._—Notice that the pupil is always round, although it is
smaller in a strong light than in a weak one.
(_c_) _Teeth._—Compare the teeth with those of the cat, and notice
that they are of similar form—with strong interlocking canines and a
sharp-edged tooth on each side of the upper jaw which clips against
a similar tooth in the lower jaw—but that the cat’s teeth are more
pointed.
(_d_) _Tongue._—Notice that the dog’s tongue is much smoother than the
cat’s.
(_e_) _Limbs._—Examine and measure the limbs, comparing them with those
of the cat, rabbit, and man. Notice that the claws are blunter than the
cat’s, and that they cannot be withdrawn into sheaths. How many toes
are there on the feet?
5. =The habits of the dog.=—(_a_) _Food._—Does the dog prefer animal
or vegetable food? Watch a dog gnawing a bone, and observe the use of
the clipping teeth. Does a dog eat greedily or deliberately? Does it
chew its food or swallow it in lumps? Do dogs hunt singly or in packs?
Do they stalk the prey stealthily, as cats do, or do they try to run
it down by speed? Whenever possible watch a pack of hounds; upon what
sense—hearing, sight, or smell—do the hounds rely most? How does a dog
drink?
(_b_) _Locomotion._—Does a slowly-moving dog hop or walk? Try to find
out the order in which it puts its feet down. How does it run? Does a
dog walk flat-footed or on its toes?
(_c_) _Likes and dislikes._—Do you consider dogs sociable or otherwise?
Do they seem to know the difference between right and wrong? Are dogs
affectionate? Are they more attached to the people or to the houses to
which they are accustomed? What can you learn of a dog’s feelings, by
the movements of its tail? Write accounts of instances, which you know
to be true, illustrating its likes and dislikes. What expression of the
human face seems to you most like the snarl of an angry dog? What are
the resemblances?
(_d_) _Intelligence._—Write accounts of evidence of intelligence, or
reasoning power, which you have observed in dogs.
(_e_) _Voice._—What is the ordinary voice of the dog? What other sounds
do dogs make, and what do they mean?
(_f_) _Play._—How do dogs play? Do you think dogs have any sense of
humour, or are able to appreciate a joke for the joke’s sake? Have you
ever seen any expression resembling a smile on a dog’s face?
6. =Puppies.=—(_a_) _Appearance._—Are puppies blind and helpless when
they are born, or are they active? How soon can they see? Are the
proportions of the body and limbs markedly different from those of the
full-grown dog? At what age is a dog full-grown?
(_b_) _Play._—Does a puppy play in the same manner as a kitten? What
differences have you noticed? Have these differences any connection
with the methods of catching the prey of the adult animals?
(_c_) _Education._—Have you ever seen a puppy being taught to do
anything by its mother? Write full accounts of such cases.
7. =Different breeds of dogs.=—Make notes of your observations of as
many different breeds of dogs as possible, _e.g._ collies, terriers,
retriever, fox-hound, pointer, spaniel, etc., and describe the
resemblances and differences in size, form, habits, intelligence, etc.
[Illustration: FIG. 169.—The Cat.]
=The external characters of the cat and dog.=—The cat and the dog are
so commonly kept as pets that they are perhaps more easily examined
than any other animals. In addition, they are so closely related and
yet exhibit so many differences that they afford a valuable exercise in
the methods of =comparison and contrast= which are at the foundation
of all successful work in Nature-Study. Several differences are at
once apparent on even a casual inspection. The body of the =cat= (Fig.
169) is covered with soft, smooth _fur_, and its head is provided
with long, sensitive whiskers. In both of these respects it resembles
the rabbit and other mammals which are in the habit of creeping along
narrow and dark passages. It is commonly said that cats can see in the
dark. Although this is not altogether true—for no animal can see in
total darkness—the cat’s eyes have the power of adapting themselves
remarkably to the intensity of the light. In a dim light, the curtain,
or iris, which surrounds the pupil (the dark, central window through
which light enters the eye) is drawn back so as to admit as much light
as possible; whereas in a very bright light the curtain is so nearly
closed that the pupil is merely a narrow, vertical slit. On the other
hand, the =dog= (Fig. 170), which is not fond of dark passages, has
its body clothed with _rough hair_, instead of fur; and its whiskers
are not nearly so long as those of the cat. Again, although the iris
of a dog’s eye alters in size to regulate the amount of light entering
the eye, the pupil is always round, and the change of size is much
less marked than in the cat. Another very noticeable difference is in
the =claws= at the ends of the toes, corresponding to the nails at the
ends of our own fingers and toes. In the cat these are sickle-shaped
and extremely sharp, and are kept drawn completely back into sheaths
when they are not required. The dog’s claws are blunt, and cannot be
retracted. Both the dog and the cat rest the weight of the body upon
the toes—not upon the sole of the foot—when walking or running.
[Illustration: FIG. 170.—Foxhound.]
There is also a very marked difference in the =tongues= of the two
animals. The tongue of the cat is rough, with small points directed
backwards. These points are of great help in licking the flesh from
bones; and they also serve as a comb when the cat—which is fastidious
about the cleanliness of its fur—“washes” itself. The tongue of the dog
is smooth and moist.
=The inherited habits of the cat and dog.=—Very obvious differences
are also to be seen in the habits of the two animals, and these would
be somewhat difficult to explain if we confined our attention to the
domesticated animals only, which live under artificial conditions.
When, however, we consider the wild relatives of the cat and dog, many
of the differences become full of significance. The wild animals of
the cat family, almost without exception, are either solitary or live
in pairs; whereas the wild dogs (wolves, jackals, etc.) live in packs.
The habits which these respective methods of life entailed have become
so firmly implanted in the nature of the race that even now, after
thousands of generations of domestication, they may be traced. Such
inherited habits, which are not dependent upon, or may be at variance
with, present conditions of life, are called =instincts=.
We will first see how the ancestral custom of living in organised packs
has left its impress upon the instincts of the domesticated dog. The
first essential to the success of any community of animals—whether
these are bees or rooks, wolves or men—is that all the creatures
composing it shall conform to certain rules, which have for their
object the good of the community and not merely that of the individual.
Acts which promote the wellbeing of the society as a whole are good,
and are directly or indirectly rewarded. Acts which tend to injure the
society as such are bad, and inevitably bring punishment either to
the individual offender or, what is worse, to his pack. A distinction
between right and wrong is thus established which would be impossible
to any animal living so solitary a life that its acts affected only
itself. In this manner were aroused the social instinct, the love
of praise and the dread of shame, the lifelong attachment to early
friends, and almost all the other qualities which have so endeared the
dog to mankind; because these qualities resulted naturally from the
ancestral pack-life. Left to itself, the dog loses its nerve, for it is
by nature unfitted for a solitary life. What more woe-begone animal is
ever seen than a lost dog?
Contrast the cat in these respects. It is at heart an outlaw, like its
wild ancestors: recognising, in general, no motives but those of its
own ease and gratification. Its social instinct is almost absent; and
though it sometimes displays affection to people who pet it, the cat
is, as a rule, more attached to places than to persons. It retains,
too, the independence and versatility which are developed by a solitary
life. A lost cat can usually take care of itself and find sufficient
food; and cases are not uncommon of cats leaving comfortable homes and
choosing to live wild lives in the woods.
Ancestral methods of =hunting= also account largely for certain
differences in the domestic cat and dog. The solitary ancestral cats,
like the lions and tigers of to-day, sprang suddenly upon the prey at
the end of silent, stealthy stalking. How firmly this method has become
fixed in the character of the race may be seen still in the manner in
which a cat stalks a mouse or a small bird, or even, in play, a dead
leaf. At the final spring the sharp, sickle-shaped claws are used to
hold down the victim. The solitary hunter is able to devour its capture
at leisure, and the domestic cat is still distinguished by its dainty
and deliberate manner of feeding.
The wild dogs hunt in a quite different manner. The whole pack joins
in the chase, the trail being followed by the sense of smell. There is
no attempt at concealment, no stealthy stalking; but an open reliance
upon speed, endurance, and numbers, rather than upon cunning. If one
dog loses the scent another picks it up and gives the signal. Mutual
help is thus the secret of success in hunting. But this is at an end
when the prey is killed. The victim is torn to pieces and devoured
greedily, each animal eating as rapidly as possible, for in most cases
there is not enough to satisfy them all. And the well-fed domestic dog
still betrays the ancestral necessity for hurried eating in the manner
of bolting his food.
=Methods of expressing feeling.=—Animals express their feelings in
various ways—by the voice, by the face, by the tail, and by the general
attitude of the body. Perhaps no animal’s feelings are more readily
recognised by man than the dog’s. “It is a remarkable fact,” says
Darwin,[13] “that the dog, since being domesticated, has learnt to bark
in at least four or five distinct tones. Although barking is a new art,
no doubt the wild parent-species of the dog expressed their feelings
by cries of various kinds. With the domesticated dog we have the bark
of eagerness, as in the chase; that of anger, as well as growling; the
yelp or howl of despair, as when shut up; the baying at night; the
bark of joy, as when starting on a walk with his master; and the very
distinct one of demand or supplication, as when wishing for a door
or window to be opened.” The movements of the tail are also full of
meaning, and capable of expressing several different moods. It seems
likely that in a dog the movements of the tail were originally of use
chiefly to signal to the rest of the pack. The use of the movements of
a cat’s tail is not very clear, although these also differ according to
the animal’s feelings.
=Carnivores.=—Dogs and cats, with several other mammals, which are
mostly flesh-eaters, are called =Carnivores=. They have never fewer
than four distinct toes on each foot, and the claws (nails) are often
capable of being withdrawn into sheaths. The teeth (Fig. 171) are
characterised by the large, interlocking canines, which are conical,
curved, and pointed; and by one of the cheek teeth on each side having
a sharp, cutting edge which bites against the similar tooth of the
other jaw, almost in the manner of the blades of scissors. The seals
and walruses, however, which are carnivores adapted to living in water,
have no such clipping teeth.
[Illustration: FIG. 171.—Skull of Wolf, seen from left side.]
49. THE BAT.
1. =Habits.=—At what time of the year have you seen bats flying about?
Do they fly in broad daylight, or only in the evening? How can you
distinguish a bat’s flight from that of a bird? Have you ever heard a
bat squeak? Upon what does it feed? Are flying insects plentiful in
winter? Have you ever seen a bat drink? How does it drink? What does
the bat do in winter? Try to find a sleeping bat in winter in a barn, a
hollow tree, or a belfry. What is its position?
2. =Appearance.=—Examine a sleeping bat or a stuffed specimen. What
is its body covered with, hair or feathers? Is it a bird or a mammal?
Is the hair soft and furry? How large are the ears? Are the whiskers
large or small? What are the wings like? Can you see any fingers?
How many fingers are there? Do any of the fingers bear claws? Do the
fingers support the wings? Which are longer, the fore limbs or the hind
limbs? Do the toes of the hind limbs bear claws? What is their use? If
possible, put a live bat on the ground; does it walk easily? Apart from
the wings, what animal does the bat seem most to resemble? Do you know
of any other flying mammal?
=The Bat= (Figs. 172 and 184_a_).—On summer evenings in most country
districts of Britain bats may be seen flitting about catching insects.
The flight is peculiar, and somewhat suggestive of that of a butterfly,
so that even in the dusk the animal may be distinguished easily from
a small bird. “Bats drink on the wing like swallows, by sipping the
surface as they play over pools and streams. They love to frequent
waters, not only for the sake of drinking, but on account of insects,
which are found over them in the greatest plenty.”[14]
[Illustration: FIG. 172.—The Bat. (× ⅓.)]
The voice of the bat is a shrill squeak, of so high a pitch that many
people cannot hear it at all. The animal is probably quite blind, but
it has keen powers of scent and hearing, and in avoiding obstacles
seems to be greatly aided by patches of specially sensitive skin on the
face, and by delicate whiskers. During the day it lurks in dark corners
of barns, church-towers, hollow trees, etc., hanging head downward by
the hooked claws of its hind feet.
On close examination the body of the bat is seen to be covered with
soft =fur=, a character which at once proclaims the animal to be not a
bird—as it has been incorrectly considered—but a =mammal=. The fur is
often of a bright chestnut colour. The ears are large and practically
devoid of hair, and are so thin as to be almost transparent. The wings
are thin folds of skin, which are attached in front to the long arms,
are supported by the greatly elongated fingers, and reach to the hind
limbs. Another membrane passes between the hind limbs, and in common
species is also supported by the tail. The thumb is free, and bears a
claw which is of some assistance in climbing. The hind limbs are small.
The structure of the bat is obviously but little adapted to walking,
and the animal moves about very awkwardly when on the ground, although
it can rise on the wing again without much difficulty.
The bat is rarely to be seen abroad after the middle of November, for
as the cold weather approaches and insects become scarce, it suspends
itself by its hind claws in some dark and sheltered corner, and goes to
sleep for the winter, reappearing in early spring.
50. HOOFED MAMMALS.
1. =The external characters of the sheep.=—(_a_) _Wool._—How does the
covering of a sheep’s body differ from the fur of a cat or rabbit,
and from the hair of the dog? Examine a lock of “raw” wool; is it at
all greasy? Dip it in water; is it much wetted? Are the fibres easily
entangled together? Why is woollen clothing so warm?
(_b_) _Teeth._—Obtain a sheep’s head from the butcher, and examine the
teeth. Notice the absence of incisors and canines in the upper jaw, and
of canines in the lower jaw. Observe the thickened pad on the surface
of the upper jaw, against which the lower incisors bite. Are the cheek
teeth flat, or sharp-edged like the cheek teeth of carnivores? Watch a
sheep feeding, and notice how it bites the grass. How is the lower jaw
moved during the chewing of the cud?
(_c_) _Horns._—Which sheep have the largest horns, the males (rams) or
the females (ewes)? If you can find a cast horn notice whether it is
hollow or solid. What is the use of horns?
(_d_) _Limbs._—Is there any marked difference in the lengths of fore
and hind limbs? Notice the _hoofs_ which cover those parts of the feet
touching the ground. How many hoofs are there on each foot?
2. =The habits of the sheep.=—Are sheep solitary or do they live in
flocks? What kind of ground do they seem to prefer, flat or hilly? Are
sheep on a hillside easily seen from a distance? Why not? Are sheep
nimble or clumsy? Can they run very fast? Would a flock of sheep be
safer from, say, wolves on a rocky hillside or on an open plain? In a
grazing flock of sheep notice whether most of the animals have their
heads turned in the same direction. Has the direction any relation to
the direction of the wind? Can you explain this?
In a running flock of sheep does any one animal act as leader? Is the
leader a ram or a ewe? Is it a lamb or an old animal? Do the rest of
the flock imitate the actions of the leader, and, for example, leap
over a wall at the same place in single file? Have you ever noticed
that if one animal jumps at a certain place, all the following sheep
jump at the same place? Have you ever seen sheep fighting? Were the
combatants rams or ewes? How did they fight?
What is the voice of a sheep like?
3. =Lambs.=—(_a_) _Appearance._—At what time of the year are lambs
born? Are they helpless or active? Have they long legs? What advantage
to the lamb is length of limb?
(_b_) _Play._—Watch lambs playing. Do they show a preference for any
eminence, _e.g._ a rock, in the neighbourhood? What is the meaning of
this preference? How do lambs fight? Have you ever seen a ewe stamp
with her fore-feet when anyone approached her lamb? At what age is a
sheep full grown?
=The sheep.=—The sheep (Fig. 173) differs in several important respects
from any of the animals previously mentioned. It is of course a mammal,
for it suckles its young, and its body is covered by hair; but the hair
is of that warm fleecy kind which is called =wool=. The fibres of wool
are seen under the microscope to be rough and scaly. For this reason
they can be spun into loosely-textured threads which entangle a great
deal of air; woollen garments are thus very bad conductors of heat. The
wool upon the sheep’s body is slightly greasy, from a substance which
is given off by the skin and protects the animal from rain. The toes
of the sheep are not armed with ordinary nails or claws, but claws are
represented by horny masses called =hoofs=, which encase the ends of
the toes, and upon which the whole weight of the body is thrown. The
sheep, especially the male (ram), is often provided with weapons in the
form of hollow =horns=, which grow upon its forehead.
[Illustration: FIG. 173.—Sheep.]
[Illustration: FIG. 174.—Section of Skull of Sheep. (After Flower.)]
[Illustration: FIG. 175.—Stomach of a Ruminant opened to show the
internal structure. For description, see text.]
=Method of feeding.=—The sheep is a strict vegetarian, living largely
upon grass. It grips the grass between its lower incisor teeth and a
hard pad on the upper jaw; there are no incisor teeth in the upper
jaw. Neither upper nor lower jaw bears canine teeth; and the cheek
teeth, which are used for chewing the food, have their crowns ridged
lengthwise. The arrangement of the teeth of the sheep is shown in Fig.
174. During grazing, however, the food is not at once chewed, but
is simply mixed with a large quantity of saliva and swallowed, the
chewing-process being performed at a later period. It is obviously a
great advantage to an animal, which in the wild state is liable at any
moment to be attacked by enemies, to be able to stow away its food
quickly, and afterwards masticate it at leisure. This is rendered
possible, in oxen, sheep, goats, deer, and the few other animals which
chew the cud, by the peculiar form of stomach shown in Fig. 175. The
hastily swallowed food is passed into the large paunch _b_ and into the
compartment _c_. When the animal finds an opportunity of “ruminating”
or chewing the cud, the food is returned to the mouth in small
quantities at a time, and is there finely divided by the cheek teeth.
In this condition it is again swallowed, and makes its way at once into
the compartment _d_, where it is strained between leaf-like folds and
then passed into the last chamber _e_, and thence to the intestines, to
undergo the final processes of digestion.
=Ruminants.=—Animals which, like the sheep, oxen, goats, deer, etc.,
ruminate or chew the cud in this manner, are called =ruminants=. In
all ruminants the weight of the body is supported by the tips of the
third and fourth toes of the feet, the remaining toes either having
completely disappeared or remaining very small (Fig. 176). The tips of
the toes are encased in horny hoofs, which represent greatly enlarged
claws or nails. The ruminants therefore belong to what may be called
the =even-toed hoofed mammals=. Pigs are also even-toed hoofed mammals,
but they are not ruminants, because, having simple stomachs, they do
not chew the cud.
[Illustration: FIG. 176.—Bones of fore-foot of Red Deer. (After
Flower.)]
[Illustration: FIG. 177.—Bones of fore-foot of Horse. (After Flower.)]
=Other hoofed mammals.=—In the horse and donkey the reduction of the
number of toes has gone still further; for these animals have now only
the third or middle toe of each foot left (Fig. 177); and because it
has to carry the whole weight of the body, it has become very large and
stout; its tip is encased in a hoof.
The =hoofed mammals= are therefore divided into two groups:
(1) the _odd-toed_, including the horse and ass (they have simple
stomachs, and therefore do not chew the cud); and
(2) the _even-toed_, including (_a_) ruminants like the sheep, etc.,
and (_b_) such non-ruminants as the pigs and their relatives.
=The inherited habits of the sheep.=—As in the cases of the dog and
cat, so in the sheep, the true explanation of several curious habits is
to be found in the manner of life of the wild ancestors; for it must
be remembered that domestication, however kindly an animal may take
to it, is an artificial condition of life. Wild sheep live in flocks,
as a rule in cold and mountainous districts; and some organisation is
necessary if they are not to be at the mercy of savage carnivores. An
old and experienced ram is generally in charge of the flock, and in
case of alarm he leads the way to a more inaccessible position. The
rest of the flock follow in single file, closely imitating his every
movement, leaping without hesitation—and therefore saving valuable
time—wherever he has leapt. The survival of this instinct in domestic
sheep may be observed whenever a flock is travelling along the road.
Even young lambs still display a decided preference for rocks,
hillocks, and other elevated positions. Wild sheep, being so liable
to sudden interruption when grazing, are enabled by their compound
stomachs to swallow food quickly and postpone the chewing process to a
more favourable opportunity. Sentinels usually keep watch, and warn the
flock of approaching enemies by stamping their hoofs on the ground, an
action which may still be seen whenever a ewe fears danger to her lambs.
=The play and education of the young.=—The young of mammals are usually
under the care of the mother for education and protection until they
are nearly adult; and it is generally found that the longest infancy
(in proportion to the natural life of the animal) occurs in the most
intelligent races. The importance of the play and education period
is very great, for it not only gives the young animal an opportunity
of training its natural faculties in comparative safety, by exercise
of various kinds and by games with companions of its own age and
strength; but it allows the mother to impart, by direct instruction,
some of the experience which she has personally gained during her
life. The extent of this maternal education is greater than has been
generally supposed; and every opportunity should be taken of observing
and recording cases of it.[15]
The play of young animals also in many cases gives important clues to
lost ancestral habits, which are not now to be seen in adult life. The
inherited tendency of animals to repeat, during their own development,
the history of their race is very great; some striking cases of this
will be considered in later chapters.
EXERCISES ON CHAPTER XIV.
1. Make a list of cases which you have observed of
protective colouration in mammals, specifying (_a_) the
colour of the animal, (_b_) the colour of its surroundings.
2. Describe any cases which you have observed of mammals
having differently-coloured coats in summer and winter. Of
what use is the change in colour?
3. Make a list of the mammals you know from observation to
walk (_a_) flat-footed, (_b_) on their toes, (_c_) on the
tips of their toes.
4. Make a list of the mammals which habitually hop, walk,
fly, and swim respectively; and find out, by observation
if possible, how the structure is adapted to the method of
life.
5. Study the habits of the mole, and try to discover by what
modifications it is enabled to burrow so rapidly.
6. What is the difference in the ways in which cows and
horses get up and lie down?
7. For what purposes do the following mammals use their
tails—cows, squirrels, rabbits? What mammals do you know
which are without tails?
8. Observe and describe the differences—apart from
speed—between walking, trotting, and galloping, in the
case of the horse.
9. Describe how you have tamed any wild animal. Why is it
easier to tame a young animal than an old one?
10. Describe cases of mammals showing antipathy to certain
colours. How do you explain the dislike?
11. Which domestic mammals are (_a_) most shy, (_b_) most
inquisitive, (_c_) most gentle, (_d_) most suspicious,
(_e_) most intelligent?
12. Of what shape is the pupil of the eye in a dog, a cat,
and a sheep? Does the shape of the pupil change from time
to time in any of these animals? (1898)
13. Describe the fore-foot of a cat, a dog, and a sheep. The
bones of the foot need not be described in detail. Draw a
footprint of each animal (fore-foot), and show how each
part of the print is produced. (1898)
14. Give a short account of the life of a bat. When and
where does it seek its food? How does it pass the winter?
(1898)
15. Give illustrations (from your own experience if
possible) of the curiosity of dogs and cats. Show that
their curiosity is necessary to their welfare. (1901)
16. Mention animals which are nocturnal (only coming forth
at night), animals which burrow in the ground, animals
which are solitary, and animals which are social. (1901)
17. How are young cats treated by their mother when they are
helpless, when they first run about freely, and when they
are able to get their own food? (1903)
18. Mention some of the peculiarities which serve in most
cases to distinguish the hind limb of a quadruped from the
fore limb. (1905)
19. The habits of an animal can be inferred from its teeth.
To what extent is this statement true of (_a_) the cat,
(_b_) the rabbit? (King’s Scholarship, 1905)
FOOTNOTES:
[13] _The Descent of Man._ Cheap edition, 1s. (Murray).
[14] Gilbert White, _The Natural History of Selborne_.
[15] See Lloyd Morgan’s _Habit and Instinct_ (Arnold), Thompson-Seton’s
_Wild Animals I have known_ (Nutt), and Long’s _School of the Woods_
(Ginn).
CHAPTER XV. THE PIGEON: A TYPICAL BIRD.
51. HABITS AND STRUCTURE OF THE PIGEON.
1. =General observations upon the dovecote pigeon.=—Watch a group of
pigeons. What is the shape of the body? With what is the body covered?
What is the colour of the feathers? Does the bird walk or hop? How many
walking limbs has it? Has it any other means of moving from place to
place, in addition to walking? How many wings has it? Are the wings
anterior or posterior (p. 217) to the legs? Watch a pigeon preening its
feathers; why does it apply its bill so frequently to its tail? Can the
bird bend its neck easily in all directions? On what do pigeons feed?
How do they pick up their food? Do they chew the food? What is the
voice of the pigeon like? At what time of the year do pigeons moult?
How many eggs does the hen-pigeon lay? What is the colour of the eggs?
What are newly-hatched pigeons like? How do the parent birds feed them?
Try to find a wood-pigeon’s nest. What is its shape? What is it made
of? Is the top open or closed-in?
2. =The external characters.=—Closely examine a dead pigeon in respect
of the following features:
(_a_) _Feathers._—In what direction do the free ends of the feathers
point? Where are the longest feathers?
(_b_) _The head._—What is the general shape of the head? Notice the
horny _bill_; is it blunt, pointed, or hooked? Does it bear teeth?
Observe the _cere_, a whitish patch of swollen skin at each side of
the base of the upper beak, and surrounding the _nostril_. Examine
the large _eyes_, and notice the upper and lower eyelids and the
transparent third eyelid. Find the _ear-opening_, a little below and
behind the level of the eye, and hidden by the small feathers of the
neck. Open the mouth and see the pointed _tongue_.
(_c_) _The neck._—In the feathered bird this appears short; it will be
better seen after the removal of the feathers (contour feathers) which
cover it.
(_d_) _The trunk._—What is the apparent shape of the trunk? Where is
its heaviest part?
(_e_) _The wings._—Open out the wings fully and measure the distance
from tip to tip; compare it with the length of the trunk. Which
surface, upper or lower, of the expanded wing is rounded (convex)
and which is hollowed (concave)? Put up an umbrella and, holding it
by the handle, move it quickly (i) away from you, (ii) towards you.
In which direction of motion does the air offer more resistance? Is
it an advantage for the expanded wing to be convex above and concave
below? Why? Feel the wing-bones through the skin, and notice that the
wing-skeleton consists of three parts corresponding to the bones of
your upper-arm, fore-arm, and hand (with the wrist) respectively; and
that they are bent on each other in the form of a Z when the wings
are folded. The large _quill-feathers_ attached to the segment which
corresponds to the hand and wrist are called _primaries_; count them.
Those attached to the fore-arm are called _secondaries_; count them.
Find the tuft of smaller feathers which spring, at the front edge
of the wing, from the thumb; this tuft is called the _thumb-wing_.
The primaries and secondaries collectively may be called the rowing
feathers. Notice the smaller feathers which, on both the upper and
lower surface of the wing, cover the quills of the primaries and
secondaries; these smaller feathers are called the _wing-coverts_.
(_f_) _The legs._—Feel the leg-bones through the skin, and make out
the various segments. Is the whole of the hind-limb feathered? Which
part is devoid of feathers? With what is this part covered? How many
toes are there? Do any point backwards? Stretch out the legs and notice
that the toes open. Bend the ankle-joint and notice that the toes close
automatically. Of what use is this in perching?
(_g_) _The tail._—Examine the tail and notice the large quill-feathers
attached to it; count them. As they are largely used for steering
during flight they are sometimes called the steering feathers.
3. =The feathers.=—Examine the arrangement of the feathers. The short
feathers which clothe the body generally are called _contour feathers_
because they determine the contour of the unplucked bird; remove one
or two and place them aside. The feathers which cover the bases of
the large quills are called _coverts_—wing-coverts or tail-coverts
according to their position. Remove one or two for future examination.
Examine the arrangement of the _quill-feathers_ of the wings, and
notice how the vane or web of one partially overlaps the next. The
vane is supported by a shaft. Are the two sides of the vane of equal
width? Is the narrower side directed forwards or backwards? Pull out a
quill-feather from each wing and compare them. How could you recognise,
if you did not know, whether any particular feather had come from the
right or from the left wing? Pull out a quill-feather from the tail;
are the sides of the vane of equal width?
4. =Examination of various feathers.=—Examine in detail a
_quill-feather_ from the wing. Make out:
(_a_) _The quill._—Is it hollow or solid? Try to bend it and notice its
great strength. Observe the small hole at its base.
(_b_) _The shaft._—This is the prolongation of the quill, and carries
the web or vane. Is the shaft hollow or solid? Notice the small tuft of
down on the inner face of the feather, at the junction of the quill and
shaft.
(_c_) _The vane._—Hold up the feather to the light and examine the
vane with a lens. Notice that the vane consists of a number of laths
which spring from the shaft; these are called _barbs_. Try to separate
the barbs, and observe that they offer considerable resistance to the
pull, as if they were somehow fastened together. Examine a barb with a
strong lens and observe the finer branches, called _barbules_, which
it bears. The barbules on one side of the barb carry little hooks,
while those of the other side bear flanges, on which fit the hooks of
barbules carried by the next barb below. The hooks and flanges cannot
be seen without a microscope.
(_d_) _Other feathers._—Compare the _covert_ and _contour feathers_
with the quill feathers. Observe, in the contour feathers, the less
perfect interlocking of the barbs of those parts which are covered by
other feathers.
Pluck the pigeon, and notice the hair-like structures left in the skin.
They are called _filoplumes_. Pull one out and examine it with a lens;
it bears a few barbs at its upper end, but these do not interlock.
Examine a nestling-pigeon, and notice the _down-feathers_ which clothe
it temporarily. Each is at first covered at the base with a horny
sheath. The barbs do not interlock. The down-feathers are pushed off by
the growth of the permanent feathers.
5. =The plucked pigeon.=—Observe that feathers do not grow on all parts
of the body, but are confined to definite _feather-tracts_, which can
be recognised by the scars left by the quills. Notice especially the
sockets of the large wing-and tail-quills. Make out the _oil-gland_,
a small knob just above the tip of the true tail. It furnishes a
lubricating fluid used in preening the feathers. Examine more closely
the different regions of the body—the neck, the joints of the limbs,
etc., which were disguised by the feathers. The filoplumes have already
been seen. Feel the great _muscles of the breast_, and the edge of the
_sternum_ or breast-bone in the middle line. Just in front of this feel
the soft crop, with the grain which is probably present in it.
Before boiling the bird in order to separate and examine the bones,
open the _crop_ and inspect its contents. Cut away the enormous muscles
of the breast (why are they so large?) and open the body-cavity behind
and remove the internal organs, being careful not to break any bones.
6. =The skeleton.=—Boil the bird until the flesh can be easily removed.
A small nail-brush will be found useful in cleaning the bones. Keep
as many of these in contact as possible, and make out and examine the
following parts:
(_a_) _The skull_, with the rounded brain-case, horny beak, and large
eye-sockets. Notice, at the back of the skull, the _single_ knob, which
fits into a hollow on the first vertebra.
(_b_) _The backbone._—Notice the long neck, the fusion of many of
the vertebrae in the trunk-region, and, at the end of the tail, the
“ploughshare bone.” This last supports the tail-quills.
(_c_) _The breast-bone_, or sternum, produced ventrally into a thin
plate called the _keel_ (to which were attached the great muscles of
the breast), and connected by ribs to the backbone.
(_d_) The position of the _shoulder joint_, and the socket for the bone
of the upper-arm. Notice the =V=-shaped “merrythought.”
(_e_) _The bones of the fore-limb_ (wing).—Make out the parts belonging
to the upper-arm, fore-arm, and wrist and hand, and notice that the
bones of the wrist and hand are firmly fused together to give increased
strength.
(f) _The large hip-bones._—Notice their forward extension and fusion
with the joined vertebrae of this region, and the resulting increased
firmness of this part.
(_g_) The bones of the _hind-limbs_.
7. =Pneumatic bones.=—Examine the bone of the upper-arm of the pigeon
and notice, just below the head of the bone, a small hole which leads
into the interior. Break the bone across; is it hollow or solid? Does
the inside contain marrow? How does it differ from the corresponding
bone of a rabbit?
8. =Different breeds.=—Examine various breeds of domestic pigeons, and
compare them with the common variety, as regards colour, shape, method
of flight, extent of feathering of the hind-limbs, etc.
=Birds.=—It is probable that no class of animals has been more studied
than that of the birds; and the reasons for this are not difficult to
find. The wonderful power of flight—rivalled only by the insects—and
the perfect adaptation of structure to this power; the habits,
always interesting and in many cases showing a curious parallelism
to human institutions; the beautiful voices of song birds; the grace
of movement; the mysteries of migration; and, it may be added, the
remarkable story of the origin of birds which has been revealed by
modern zoology,—form a combination of characters, at once familiar and
elusive, which has greatly stimulated human sympathy and imagination.
It is well to begin the study of birds by first taking one familiar
species and examining it somewhat closely, and afterwards comparing
and contrasting other members of the class. Such a convenient type for
study is found in the dovecote pigeon.
[Illustration: FIG. 178.—Pigeons.]
=The pigeon.=—The body of the common dovecote pigeon (Fig. 178) is
ovoid in shape, tapering gently at the neck and appearing, in the
living animal, to pass insensibly into the head. The fore-limbs are
modified to form a pair of =wings=, which are set on a little above and
in front of the centre of gravity of the body. Except during flight the
weight of the body is entirely supported by the hind-limbs or =legs=,
which are fixed almost exactly below the centre of gravity of the body,
_i.e._ in the best imaginable position. Each foot bears four toes, the
first of which is directed backwards. Each toe is armed with a claw.
The body generally is clothed with =feathers=, remarkable structures
which are as characteristic of birds as hairs are (p. 220) of mammals.
The great majority of the feathers are small, overlapping and
plate-like, with their free ends pointing backwards; and they form
a light, warm, and smooth covering which is admirably adapted to the
animal’s needs. Certain large feathers, carried by the wings and tail,
are used in flight. The legs are feathered to the ankle-joint, but the
feet are covered only with scales. The general colour of the dovecote
pigeon is a slaty blue.
The head is rounded, and terminates in front in a horny =bill=, which
does not bear teeth. At the base of the upper beak there is, on
each side, a whitish patch of swollen skin called the =cere=, which
surrounds the opening of the =nostril=. The =eyes= are large and
round; in addition to upper and lower eyelids, each is provided with
a transparent =third eyelid=, which can be flicked rapidly across the
eyeball. Birds generally have very powerful sight and depend more upon
this sense than upon any other. There are no external ears, but when
the feathers are separated, a little below and behind the eyes, a pair
of apertures leading to the internal =ears= may be seen.
=Habits.=—The pigeon lives upon grain, which it picks up by means of
its horny bill. The length and flexibility of the neck are a great
help in feeding. The food is swallowed immediately and passed into
a large bag called the =crop=, which is really a dilatation of the
lower part of the gullet. Here it is macerated for some time before it
passes onward to the true stomach. The hinder part of the stomach is
called the =gizzard=; it has very thick walls and a hard horny lining.
In the gizzard the food is ground up by the aid of small stones which
the pigeon swallows for the purpose. The stomach is followed by a
much-coiled intestine, in which the process of digestion is completed.
The pigeon differs from many birds in walking instead of hopping.
When perching, it bends its legs at the ankles, an action which
automatically closes the toes in such a manner that the perch is
grasped behind by the first toe and in front by the second, third, and
fourth. The weight of the body is sufficient to maintain a tight clasp
upon the perch, so that the bird is able to sleep comfortably in this
position.
[Illustration: FIG. 179.—A Falcon. The regions of a bird’s body.]
The hen-pigeon lays two white =eggs=, which are sat upon by the
parents for fourteen days and thereby kept at a temperature of about 40
degrees Centigrade. The young birds then break through the shell and
are hatched. As the chicks are at first quite helpless and unable to
feed themselves, the parents supply them with a milky fluid secreted by
their crops. This is sometimes called “pigeon’s milk.” Newly hatched
pigeons are covered with fine feathers called =down=, which are pushed
off by the development of the permanent feathers beneath them. The
nests of wood-pigeons (Fig. 180) are usually built in trees; they are
somewhat rough structures, composed of twigs, and open at the top.
[Illustration: FIG. 180.—Wood-pigeon’s nest.]
=The wings.=—A bird’s wings are full of interest, from whatever point
of view they are considered. Even now, the exact manner in which they
and the tail are used to bring about the many different movements of
flight is not fully understood; but a general idea may be obtained by
an examination of the bones and feathers and the careful observation of
flying birds.
[Illustration: FIG. 181.—The skeleton of the limbs and tail of a
flying-bird. _OA_, bone of upper arm; _OS_, bone of upper leg; _Rd_,
_Ul_, bones of fore-arm; _T_, _Fi_, bones of leg; _HW_, _MH_, bones of
wrist; _F_, _F_, _F_, bones of fingers; _Z_, _Z_, _Z´_, _MP_, bones
of foot; _Scp_, shoulder blade; _Cr_, keel on sternum (_St_); _Pg_,
ploughshare bone.]
The =bones= of the fore-limb and the neighbouring regions show a
remarkable modification of the primitive plan. The bones of the upper
arm _OA_, (Fig. 181) and fore-arm (_Rd._, _Ul._,) are arranged much
like the corresponding bones of the rabbit; but those of the wrist and
hand are unlike anything we have yet examined. The fourth and fifth
fingers have entirely disappeared; and the first (thumb), second, and
third, though they can be still recognised (Figs. 181, _F_, _F_, _F_
and 184_a_), have become consolidated with what remains of the wrist
(_HW_, _MH_) to form a firm mass which is in no danger of “giving”
during the powerful down-stroke of the wing. The head of the bone of
the upper arm fits into a socket (_gl.cv._, Fig. 182) at the junction
of the shoulder-blade (_scp._) and another and stouter bone (_cor._)
which runs back and is attached to the breast-bone or sternum (_st._).
The sternum in its turn is connected by means of ribs with the spinal
column, which is strengthened here by the fusion of some of its bones.
The ventral part of the sternum is produced into a large plate called
the _keel_ (_car._, Fig. 182), which gives attachment to the great
muscles of the breast, used in the movements of the wings. So great is
the development of these muscles in flying birds that in the pigeon
they have a weight equal to one-fifth the total weight of the body. The
three segments of the wing, corresponding to the upper-arm, fore-arm,
and wrist and hand respectively are, in the position of rest, bent
upon each other like the letter =Z=. The large quill =feathers= of the
wing, fixed along the hinder border of the limb, are in two series
(Fig. 179). Those attached to the fore-arm are called _secondaries_;
while those attached to the wrist and hand are known as _primaries_. A
smaller tuft of feathers borne by the thumb is called the bastard-wing
or _thumb-wing_. The smaller feathers covering the bases of the
wing-quills are called _wing-coverts_.
[Illustration: FIG. 182.—Pigeon. The bones of the trunk. _actb_, socket
for bone of upper leg; _car_, keel of sternum; _cd.v_, vertebrae of
tail; _cor_, coracoid; _fur_, merrythought; _gl.cv_, socket for bone of
upper arm; _pyg.st_, ploughshare bone; _scp_, shoulder blade; _s.scr_,
fused vertebrae of hip region; _st_, sternum; _st.r_, _vr.r_, rib;
_th.v.1_, first, and _th.v.5_, last thoracic vertebra. (× ⅓.)]
=The hind-limbs and tail.=—The hind-limbs, or legs, of birds are the
sole support of the body except during flight, and a relatively great
weight is thus thrown upon them. To provide for this, the skeleton of
the hip region is exceptionally strong. Not only are the bones of the
spine here welded together into one solid mass, but the hip-bones
extend forward much further than is the case in quadrupeds, and are
firmly attached to the fused vertebrae. Each hip-bone possesses a
socket (_actb_., Fig. 182) into which fits the head of the bone of the
upper leg. Another fusion of bones is to be noticed at the end of the
tail, resulting in the _ploughshare-bone_ (Fig. 182, _pyg. st._) which
supports the large _steering feathers_ of the tail. The quills of these
feathers are covered at their bases by the _tail-coverts_.
Just above the end of the tail of a plucked pigeon is to be seen a
small conical body, the _oil-gland_, from which the bird obtains a
fluid which it applies to the feathers during the preening process.
=The feathers.=—Feathers do not grow upon all parts of a bird’s body,
but are restricted to certain definite areas or “_feather tracts_”
which, however, differ in arrangement in different species.
[Illustration: FIG. 183.—Pigeon. _A_, portion of a wing feather; _cal_,
quill; _rch_, shaft; _B_, filoplume; _C_, nestling-down.]
One of the large feathers of the wing of the pigeon is illustrated
in Fig. 183, _A_. It consists of a hollow, horny =quill= (_cal._),
prolonged into a solid =shaft= (_rch._) which supports the web or
=vane=. The outer face of the feather is slightly convex and smooth;
while the inner surface is somewhat concave and rough, and carries a
little tuft of down at the junction of quill and shaft. The growing
feather is nourished by a conical projection of the deeper part of the
skin, which fits into a small hole (_inf. umb._) at the base of the
quill. It is easily shown that the vane of the feather is composed,
on each side, of a large number of parallel laths which spring from
the shaft. These laths are called =barbs=. Considerable resistance is
felt when an attempt is made to pull the barbs apart, but the manner
of their connection cannot be clearly seen without the aid of the
microscope. A fairly low magnifying power, however, shows (Fig. 184)
that each barb bears extremely delicate threads—=barbules=—on each
side, arranged on the barb much as the barbs are arranged on the shaft.
The barbules on one side of each barb (the side furthest from the
quill) are seen to carry tiny _hooks_; while the barbules of the other
side of the barb are furnished with _flanges_. The hooked barbules of
one barb cross the flanged barbules of the next and interlock with
them; so that relatively great force is required to pull the barbs
apart and destroy the continuity of the web. The quill-feathers of the
tail, and the coverts, have webs of similar structure.
[Illustration: FIG. 184.—Structure of a Feather. _A_, small portion
of a feather with pieces of two barbs, each having to the left three
hooked barbules, and to the right a number of flanged barbules; _B_,
hooklet of one barbule interlocking with flange of another barbule;
_C_, two adjacent flanged barbules; _D_, a hooked barbule. (After
Pycraft.)]
The =contour-feathers= are used for protection and warmth rather than
for flight, and hence have less-perfectly interlocking barbs. The
hair-like structures seen on the skin of the plucked bird are extremely
simple feathers called =filoplumes= (Fig. 183, _B_). Each consists
of a long, slender axis, bearing at the end a few barbs which do not
interlock.
The =down-feathers= (Fig. 183, _C_), which form the temporary covering
of nestling pigeons, also bear barbs which do not interlock.
At regular intervals, either once or twice a year, birds =moult=, that
is, shed most or all of their feathers and grow new ones. The moulting
usually takes place gradually and symmetrically: a flight feather from
each wing, for example, being dropped at the same time.
=Flight.=—The outstretched wing of a pigeon has a relatively great
area; and is markedly convex on the upper surface and concave on
the lower, resembling in this respect an open umbrella. The great
difference in the resistance which the inside and outside of an open
umbrella respectively oppose to the wind is familiar to everyone, and
illustrates the advantage of the dome-shape of the outstretched wing.
The force which the great breast-muscles put into the down-stroke of
the wing is enormous in comparison with the weight of the body, and
is sufficient to push the bird upwards and forwards in the air. The
gently tapering shape and smooth surface of the feathered bird diminish
the resistance of the air. When the wing is raised again for the next
stroke, the quill-feathers are turned a little edgewise, so that the
air slips between them; just as an oarsman “feathers” his oar to lessen
the resistance to the blade in the return-stroke. The direction of the
wing-stroke can be altered in accordance with the direction of air
currents, and the fan of tail-feathers is capable of being opened or
closed, raised or lowered, and turned at various angles to act as a
rudder.
But the method of flight which perhaps most of all excites the
observer’s admiration is =soaring=, in which the bird seems to remain
almost passive, with outstretched wings and spread tail, and mounts
automatically in a spiral course. Exactly how the soaring is performed,
only the birds themselves know; but it must be similar in essence to
the arrangement of the sails of a yacht so as to select that component
of the force of the breeze which will drive the boat in the required
direction, even though this be almost “in the teeth of the wind.” Such
spiral soaring may be seen to perfection, among common British birds,
in the skylark (p. 316). A breeze is necessary for soaring.
In =hovering=, the motion of the wings is extremely rapid, and the bird
remains poised in one place. The kestrel (p. 330) derives its common
name of “windhover” from its habit of using this method of flight when
looking for food.
=The pigeon’s air supply.=—We know that a man breathes more quickly
when he is taking active exercise than at ordinary times; and we might,
from this and similar observations, expect to find the most perfect
breathing organs in animals which lead the most active lives. Of all
vertebrate animals, birds probably perform the most work in proportion
to their size; and it is not surprising, therefore, to learn that they
have special facilities for quickly replacing fouled air by fresh.
The =lungs= are comparatively small, but they are not the only organs
of respiration (p. 242); for the windpipe opens also into several
=air-sacs=, which supply nearly all parts of the body with air, and
even communicate with the interior of certain bones. The bone of the
pigeon’s upper-arm, for example, is hollow and contains air. By means
of the system of air-sacs, the air in the lungs is _completely_ renewed
at each respiratory act, and thus the waste carbon-dioxide in the blood
can be exchanged for fresh oxygen much more completely than it is in
mammals. One result of this is that the blood of birds is much warmer
than that of mammals. The air-sacs are also an assistance to flight by
rendering the body more buoyant.
=Different breeds of pigeons.=—The various breeds of domestic pigeons
furnish one of the best examples of the changes in structure which
breeders can bring about, after several generations, by careful
crossing. In spite of the great differences between, say, the
=tumbler=, with its habit of turning head-over-heels in the air;
the =pouter=, with its exaggerated crop; the =Jacobin=, with its
neck-feathers reversed; the =fantail=, with its large number of erect
tail-feathers; and others,—there can be no doubt that all these
varieties have been artificially produced from the =rock-pigeon=; and
it is curious to observe occasional reversions to the characters of
the ancestral stock. “The rock-pigeon is of a slaty blue, with white
loins.... The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black bars.
Some semi-domestic breeds, and some truly wild breeds, have, besides
the two black bars, the wings chequered with black. These several
marks do not occur together in any other species of the whole family.
Now, in every one of the domestic breeds, taking thoroughly well-bred
birds, all the above marks, even to the white edging of the outer
tail-feathers, sometimes concur perfectly developed. Moreover, when two
birds belonging to two or more distinct breeds are crossed, none of
which are blue or have any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters.”[16] This
is a striking instance of the tendency exhibited by living things to
revert to ancestral characters, even though these latter may have lain
dormant for hundreds of generations.
EXERCISES ON CHAPTER XV.
1. Why does a bird require a long neck, and a keel upon the
breast bone? (1898)
2. Draw the bones of a bird’s wing, and mark the places of
insertion of the largest quills. (1898)
3. What bones carry the primary and secondary quills of a
bird’s wing, and the quills of the tail? Illustrate your
answer by a drawing. (1901)
4. Point out the chief differences between the skulls of a
quadruped and a bird. (1901)
5. Describe one of the large feathers of a bird’s wing. (1898)
6. Draw general plans of the fore-limbs of a mammal and a
bird. Letter the principal bones, so as to show how they
correspond in the two animals. (1901)
7. If the skull of a bird were placed before you, what
features would enable you to recognise it with certainty? (1901)
8. How does the fact that a bird stands on two legs affect
the skeleton of the trunk? (1897)
[Illustration: FIG. 184_a_.—Comparison of Bat and Bird.]
9. Point out the chief differences between the wing of a
bird and that of a bat.
10. Explain how the barbs of a quill feather are attached
to one another. Describe a feather at the time when it is
forcing its way through the skin. (1905)
11. What peculiarities distinguish feathers which aid in
flight from feathers which merely prevent loss of heat? (1906)
12. Describe the wing of a bird, both as to skeleton and as
to external appearance. Compare the skeletal parts with
those of the corresponding structures in a Mammal. (1908)
FOOTNOTE:
[16] Darwin, _The Origin of Species_ (Murray). Cheap edition, 1s.
CHAPTER XVI. THE DEVELOPMENT AND EDUCATION OF THE CHICK.
52. A HEN’S EGG.
Obtain three or four hen’s-eggs, and make the following observations
upon them:
1. =External appearance.=—What is the _colour_ of the egg? What is
its _shape_? How does the shape differ from that of a cricket ball or
other sphere? The shape of an egg is said to be ovoid. [What is the
difference between an ovoid and an oval?] Measure, with a tape, the
length and breadth of the egg, and also its distance round, (i) in the
direction of its greatest length, (ii) in the direction of its greatest
width. What advantages has this shape? Could a hen sit so comfortably
upon her eggs if they had sharp corners? Put an egg upon the table
and roll it gently. Does it roll in a straight line? Does it roll far
before coming to rest? Compare the rolling of a cricket ball or other
sphere. Is it an advantage that an egg is not likely to roll far, or in
a straight line? Why?
2. =The strength of the shell.=—Hold an egg with your thumb at one end
and your first or second finger at the other, and press exactly in the
line of the length of the egg. You cannot break the shell.
3. =Structure.=—(_a_) _The shell._—Tap the egg gently at the middle
of its broad end until the shell cracks. Then carefully remove small
pieces of the shell and notice the _shell-membrane_, a tough skin
which is closely applied to the inside of the shell. Snip through the
membrane in the middle of the broad end; notice the _air-chamber_
which lies beneath it. Observe the inner membrane which separates the
air-chamber from the inside of the egg. Hold a piece of shell up to
the light, and notice the small, almost transparent dots. The shell is
perforated by very small pores, through which the air can pass.
(_b_) _The white of the egg._—Tap the shell so as to crack it all round
at its widest part; raise bits of shell carefully and see the membrane
here. Tear through the membrane and notice that in this region there
is no air-space, but the white lies just beneath the shell-membrane.
Separate the halves of the shell, notice the position and shape of the
yolk, and then let the contents of the egg fall gently into a basin.
Observe the appearance, colour, and transparency of the white, and try
to distinguish two tangled cords of firmer white—the _balancers_ (Fig.
185)—arising close to the yellow yolk.
(_c_) _The yolk._—What is the shape of the yolk as it lies in the
basin? How does it differ from the shape of a yolk suspended naturally
in the white? What is the cause of the change of shape? Notice
carefully a small paler patch in the middle of the upper surface. This
is the lightest part of the yolk, so that the yolk always settles with
this part uppermost after any turning of the egg, and therefore the
pale patch is always more directly exposed to the heat of the hen’s
body (during incubation) than is any other part of the yolk. Prick the
yolk and notice that the yellow, fluid contents flow out. You have
evidently pierced the thin bag which formerly preserved the shape.
(_d_) _A hard-boiled egg._—Boil an egg in water for five minutes,
and then chip round the shell in the direction of the length and,
with a sharp knife, cut the whole egg into halves along this plane.
Make a drawing of the section, indicating the shell, shell-membrane,
air-chamber, white, and yolk in position. Observe that the white is no
longer transparent and fluid, but an opaque, white, and elastic solid.
Try to peel off the white in layers. They will probably break off
short, but you may be able to see that the white is deposited in spiral
sheets around the yolk.
=The hen’s egg.=—A hen’s egg bears a somewhat similar relation to the
adult bird that the maize or bean seed (Chapter I.) bears to the adult
plant; for the egg contains
(1) a speck of living matter which little by little grows and becomes
marked off, by orderly arrangement, to form the various regions and
organs of the adult animal; and
(2) a store of food, which enables the young chick to develop in
security, without being hampered, whilst still weak and helpless, by
the necessity of earning its own living. The changes by which the speck
of living matter becomes the perfect chick will be considered in the
next section. We must now examine the structure of the egg itself, and
see what provision it contains for the nourishment and safety of the
developing bird.
The egg is ovoid in =shape=, one end being distinctly broader than the
other. This shape has the advantage of preventing the egg from rolling
very far when placed upon a slightly inclined surface, and it is worthy
of notice that the eggs of birds which lay on cliffs and other exposed
situations are usually more elongated and pointed than others, so
that when stirred they do not roll away, but simply describe a small
circle and come to rest again. In the case of eggs which are laid
in cup-shaped nests (Fig. 197) the pear-shape lends itself to close
packing, and thus allows the eggs to be more easily kept warm by the
parent bird.
The =shell= (_sh._, Fig. 185) is composed of a chalky material, and is
perforated by small pores, through which the developing chick obtains
fresh oxygen from the air, and gives off its surplus carbon dioxide
(p. 242). A small piece of shell easily snaps, but the shape of the
complete shell so distributes an outside pressure, especially one in
the direction of the long axis, that relatively great force is required
to break it. The shell is lined by a thin, parchment-like =membrane=
(_sh. m._). At the broad end of the egg this membrane is double, and
the two layers enclose an =air-chamber= (_a_).
Inside the shell-membrane are the white or albumin, and the yolk.
The =white= (_alb´._) is a viscous, transparent fluid. Its innermost
part (_alb._), which immediately surrounds the yolk, is of thicker
consistency than the rest, and is prolonged into two twisted cords
called the =balancers= (_ch._), which suspend the yolk in position.
[Illustration: FIG. 185.—Semi-diagrammatic view of a fowl’s egg at the
time of laying. _a_, air-space; _alb_, dense layer of albumin; _alb´_,
more fluid albumin; _bl_, germinal disc; _ch_, balancers; _sh_, shell;
_sh.m_, shell-membrane; _sh.m 1_, _sh.m 2_, its two layers separated to
enclose air-space; _yk_, yolk. (After Marshall.) (× 1.)]
The =yolk= is a golden-yellow fluid enclosed in a thin, elastic
membrane and hence preserving a spherical shape. On its upper surface
(but under the membrane) is a small circular patch (_bl._) of paler
colour. This patch, called the =germinal disc=, is about ⅛” in
diameter; it contains the living matter from which the chick will be
formed; the rest of the yolk, and the white, are simply a store of
inert food which is used up by the growing chick.
=The work of the balancers.=—In order that the small living patch of
pale yolk, the germinal disc, may grow and develop into the chick it
must be kept warm. When eggs are hatched in the natural manner, the
heat is supplied by the body of the sitting hen, and the upper part of
the egg is consequently kept warmer than the rest. It is important that
the living part of the egg shall always lie nearest the hen’s body and
thus be kept warm, and this is secured by an arrangement as effective
as it is simple. The yolk is lightest in the neighbourhood of the
germinal disc, and therefore always lies with this part uppermost. If
the egg is slowly turned over, the yolk remains “right side up.” If it
is turned over quickly, the yolk soon swings round into its original
position. The twisted cords of white, the balancers, not only sling
up the yolk and guard it from being thrown to one side by a sudden
movement, but they also prevent it from rotating too quickly—and so
possibly injuring the delicate body of the young chick—when it rights
itself after the egg has been turned over.
The balancers are rendered necessary by the hen’s habit of turning her
eggs two or three times a day. It is often supposed that the eggs are
turned in order to keep them equally warm on all sides, but this is
unnecessary. Most probably the egg is turned in order to alter slightly
the young chick’s position from time to time, and allow its parts to
grow naturally, unimpeded by the other contents of the egg.
53. THE DEVELOPMENT OF THE CHICK.
1. =A simple incubator.=—Eggs are best incubated in the natural
manner, that is, by the warmth of the hen’s body; but if a sitting
hen cannot be obtained, an ordinary water-oven, such as is used in
chemical laboratories, may be made to answer. It should be heated
by a self-regulating burner, and kept at a temperature of about 40°
C. The eggs should be turned two or three times a day, and the air
of the oven should be kept moist by sprinkling water upon pieces of
cloth, blotting paper, or hay, kept with the eggs. Spring is the most
favourable time of the year for making the observations, as eggs laid
at other seasons are not always in a condition to produce chicks.
2. =How to mark the eggs.=—The most instructive changes take place
during the first five days of incubation. If all the stages of the
first five days are to form the subject of one lesson, an egg should
be marked “5” with pencil, and then put into the incubator or under
the hen five days before the lesson; a day later, an egg numbered
“4” should be put in, and so on. The numbers will then indicate the
length of incubation at the time of the lesson, and the eggs should be
examined in order, from 1 to 5. If one egg is to be examined each day,
five should be put in the incubator at the same time; no numbering will
then be required.
3. =How to examine the eggs.=—Have ready a basin of water, heated
slightly above the temperature of the hand (_i.e._ to about 40° C.),
and dissolve table-salt in it in the proportion of a level teaspoonful
of salt to a pint of water. The young chicks will keep alive longer in
this solution than in ordinary water. Tap the shell in the middle of
its broad end, and open the air-chamber (_a_, Fig. 185) completely.
Then crack the shell in the middle of the length and, keeping the
length of the egg horizontal, cut transversely round the middle of
the shell with scissors in a vertical plane, until the halves are on
the point of coming apart. Then lower the egg into the warm saline
solution, pull the halves of the shell apart, and float out the
contents. Examine the embryo carefully, making out as much as possible,
and then snip round it with a pair of fine scissors to remove it from
the yolk; float it into a watch-glass and cover it with weak alcohol
(equal parts of water and spirits of wine). Examine it with a lens.
After it has remained for a day in weak alcohol, put the embryo into
strong alcohol in a small bottle (writing the age on a label), and
preserve it.
Notice the gradual absorption of the white of the egg as development
proceeds.
4. =Chick after one day’s incubation.=—Notice that the embryo is now to
be distinguished as a streak crossing the germinal disc in a direction
at right angles to the long axis of the egg. Notice a rounded swelling
at one end of the embryo; this is the _head_. Place the egg before
you with the broad end to your left, and observe that the head of the
embryo points away from you.
5. =Chick after two days’ incubation.=—Observe the increase in size of
the embryo; make a note of its length. The head and neck of the embryo
are now almost covered by a very thin transparent bag which has grown
over it from the sides. This bag is called the _amnion_; it is filled
with fluid, and protects the embryo from jars. Remove the amnion and
notice the large _head_; it is now twisted so that its left side lies
against the yolk, while the rest of the embryo still lies “face-down.”
Observe the large _eye_ on the right side of the head; the left eye
cannot be seen without turning the head over. Notice the _heart_, a
small red dot which by help of a lens can be seen to beat rapidly.
Surrounding the embryo is a circular network of _blood-vessels_ which
bring food from the yolk to the heart, to be distributed to the various
parts of the body. How large is the circular area of blood-vessels?
6. =Chick after three days’ incubation.=—The white of the egg is
distinctly shrunken, and the network of blood-vessels is much larger
than before. Remove the amnion and notice the marked increase in size
of the embryo, especially of the _head_. The right side of the head and
neck are still turned towards the shell. They are now quite free from
the yolk, but the body of the embryo communicates with the yolk by a
short, wide tube, the _yolk-stalk_. Try to see a small pit, a little
above and behind the large _eye_. This is the beginning of the right
_ear_. Measure the embryo and the width of the surrounding network of
blood-vessels. Watch, through a lens, the beating of the _heart_.
7. =Chick after four days’ incubation.=—Carefully cut open the amnion
to see the embryo better. Observe that the young chick is still more
completely folded off from the yolk, and that the _yolk-stalk_ is
consequently narrower than before. The head is so strongly bent upon
itself that the snout almost touches the tail. The body also has now
turned over so as to lie with its left side on the yolk. Observe the
two pairs of small buds which are the _rudiments of the limbs_.
8. =Chick after five days’ incubation.=—Cut open the _amnion_, and
notice the great increase in size of the embryo, and especially the
enormous development of the _head_. The _limbs_ now show signs of
division into segments. Observe, under the hinder end of the body, a
small, bladder-like outgrowth, the _allantois_, which in the later
stages grows rapidly and spreads all round the inside of the shell. It
is the breathing organ of the chick.
9. =Effect of varnishing an egg.=—Varnish an egg, and leave it under
the hen with unvarnished eggs for the whole period of incubation (21
days). The varnished egg does not develop, because the varnish closes
the pores of the shell and prevents the embryo from breathing.
[Illustration: FIG. 186.—Hen’s Egg after 1 day’s incubation. _a_,
air-chamber; _hd_, head of embryo; _v.a._, area in which yolk
blood-vessels will appear later; _yk_, yolk. (× 1.)]
=The early development of the chick.=—When the hen’s egg is exposed to
a temperature of about 40°C., the =germinal disc=—the circular patch of
living matter which lies, somewhat like a small inverted watch-glass,
upon the upper surface of the yolk—grows larger, and its parts are
gradually modified to form the various regions of the chick’s body. The
white and yolk of the egg are used up during the process, and by the
time they are absorbed, the young bird is in a condition to break the
shell and take up the activities of an independent life. To watch the
orderly appearance and gradual development of the different systems of
organs is most fascinating, and fortunately the observation of the main
features presents no great difficulty if the foregoing instructions are
followed.
The first signs of the chick are to be seen (Fig. 186), towards the
end of the =first day= of incubation, in a streak which crosses the
germinal disc in a direction at right angles to the long axis of the
egg. One end of the streak is distinctly rounded, and this is the
=head= end (_hd._). In almost all cases, if the egg be placed so that
its broad end is to the observer’s left, the head of the embryo will be
directed away from him.
=A day later=, the embryo is markedly larger, and is partly covered
by a double membrane called the =amnion=.[17] Surrounding the embryo
is now (Fig. 188) a network of fine blood-vessels which ramify over
the yolk and carry small particles of yolk to feed the growing organs.
At the end of the second day the network could be about covered by a
sixpenny piece. On each side, its veins join to form a single small
tube running to the =heart= (_ht._), a tiny red dot which is situated
behind the head and can be seen, by the help of a lens, to be beating
rapidly. The head has grown more than the rest of the embryo, owing to
the rapid development of the primitive brain, and is bent forwards.
The head has turned over a little, so that its right side is directed
towards the egg-shell; the rest of the embryo still lies “face-down.”
The right =eye= (_e._) can easily be seen as a dark spot on the side of
the head.
[Illustration: FIG. 188.—Hen’s Egg after 2 days’ incubation. The amnion
has been removed. _a_, air-chamber; _au_, beginning of right ear; _e_,
right eye; _ht_, heart; _v.a._, network of yolk blood-vessels; _yk_,
yolk. (× 1.)]
By the end of the =third day= (Fig. 189) the head and neck are raised
distinctly above the yolk, and the tail is also more plainly marked
off, so that the body is connected with the yolk only by a short wide
tube, the =yolk-stalk= (_yk.st._, Fig. 187, _B_). The neck as well
as the head has now turned so as to lie with the right side directed
towards the shell. The commencement of the =ear= is to be seen on each
side as a small pit (_au._) a little above and behind the large eye.
[Illustration: FIG. 189.—Hen’s Egg after 3 days’ incubation. The amnion
has been removed. References as in Fig. 188. (× 1.)]
[Illustration: FIG. 190.—Chick Embryo after 4 days’ incubation. The
amnion has been removed. _all_, allantois; _am_, cut edge of amnion;
_f.l._, fore-limb; _h.l._, hind-limb; _ht_, heart; _t_, tail. (After
Duval.) (× 2.)]
On the =fourth day= the embryo turns to lie entirely on its left side.
It becomes more completely folded off from the yolk, and the connecting
yolk-stalk is a narrower tube than before. The previously formed organs
increase in size and complexity. The disproportionate size of the head,
owing to the great development of the brain, is still more marked than
before, and the head is so strongly bent upon itself that the snout
almost touches the tail. One of the most noteworthy features of the
fourth day is the appearance of the =limbs=. As yet these are merely
a pair of small buds (_fl._, _hl._, Fig. 190), and no joints can be
detected in them.
[Illustration: FIG. 191.—Hen’s Egg after 5 days’ incubation. _a_,
air-chamber; _all_, allantois; _am_, amnion; _ar. vasc._, area of yolk
blood-vessels; _emb_, embryo; _yk_, yolk. (After Duval.) (× 1.)]
[Illustration: FIG. 192.—Diagrams illustrating the method of
development of the allantois. _al_, allantois; _am_, amnion fold;
_am.c._, amniotic cavity; _em_, embryo; _yk_, yolk. (After Foster and
Balfour.)]
By the end of the =fifth day= (Fig. 191) the head is enormous, and the
limbs now show signs of being divided into definite segments. A thin
bladder-like structure—the =allantois= (_all._), which appeared on the
fourth day—has grown out from the lower part of the body, behind the
yolk-stalk. Its mode of origin is well shown in Fig. 192. It rapidly
increases in size, and soon extends over the embryo (Figs. 192 _B_,
and 191) and becomes closely applied to the shell-membrane. Air passes
through the pores of the shell, and its oxygen is taken up by the blood
which circulates in the vessels of the allantois. At the same time,
waste carbon dioxide is able to escape from the blood to the outer
air. The allantois is therefore the =breathing organ= of the developing
chick. If the egg is varnished and the shell thus rendered air-tight,
the embryo dies of suffocation at an early stage.
[Illustration: FIG. 193.—Chick after 9 days’ incubation. (After Duval.)
(× 1.)]
The chief organs of the bird are now established, and the later
development may be sketched more briefly. By the end of the =ninth day=
(Fig. 193) the white of the egg is almost used up; the yolk, however,
is still large, and is connected with the chick’s body by the narrow
yolk-stalk. It thus appears that the white is not directly absorbed
by the chick, but is first taken up by the yolk and afterwards passed
on by the yolk blood-vessels which run to the heart. By this time,
too, the allantois has spread at least halfway round the inside of the
shell, that a supply of oxygen adequate to the increased needs of the
animal may be obtained from the air. The chick has now a characteristic
bird-like appearance; the beak has appeared; feathers have begun to
sprout; the neck is long and slender; and the segments of the limbs,
including the fingers and toes, are well defined.
About the =fourteenth day= the chick turns so as to lie lengthwise in
the shell, with its head near the broad end. The yolk-sac dwindles
in size, and at last, about the twentieth day, it is drawn into the
interior of the body. Now the chick becomes restless, and—usually
on the =twenty-first day=—thrusts its beak through the inner
shell-membrane into the air-chamber at the broad end of the egg.
For the first time it draws refreshing air into its lungs, and is
stimulated to break the shell by a knob on its beak, and to creep out
into the world.
54. THE EDUCATION AND LATER GROWTH OF THE CHICK.[18]
1. =Hatching.=—On the 20th or 21st day of incubation remove three or
four eggs from the clutch under the hen, and keep them in a warm cosy
place so that you may watch the process of hatching. Can you hear the
chick tapping the inside of the shell? Could it have been _taught_ to
tap? Which part of the shell cracks first? When the shell has cracked,
can you hear the chick chirping? Could it have been taught to chirp?
Imitate the chirp and listen for a response. Keep the newly-hatched
chicks in a warm, soft place. How soon do they recover from the
exhaustion of hatching? Do the chicks show a liking for warm corners?
Are they afraid of being touched gently?
2. =Locomotion.=—How soon do the chicks begin to run about? Do they
stumble upon obstacles, or do they avoid or leap over them? Do they use
their wings in running, or in jumping down a small step? Put a chick
into a basket, and lower the basket quickly through the air, being
careful not to let the chick fall out. Does it move its wings? How?
3. =Feeding and drinking.=—Lay a few grains of soaked wheat upon the
ground before a chick which has not yet been fed. Does it peck at
them instinctively? Tap a grain with the point of a pencil; does the
chick now peck at it? Does it hit the grain at the first try? Does it
ever strike at a grain which is out of reach? Watch a hen with her
chicks; does she teach them to peck at food? How? Do the chicks know
instinctively the difference between food and grains of sand, or have
they to learn? Try if a chick can distinguish between a small worm and
a bit of red worsted. Are young chicks afraid of large insects?
Take a chick, which has not yet drunk, to a small pool (a few drops) of
water. Does it seem to know the use of the water? Induce it to peck at
the water; does it now drink? How?
4. =The crouching-instinct.=—Clap your hands suddenly and loudly near
young chicks. How do the chicks behave? Does this behaviour render them
less conspicuous?
5. =Non-recognition of hen.=—Having made the above observations on
chicks hatched apart from the hen, put the chicks among their brothers
and sisters with the parent hen. Do the new-comers respond, as readily
as the others, to the clucking, etc., of the hen?
6. =Maternal training and protection.=—How many different meanings can
you recognise in the different sounds of the hen’s voice? How does she
call and protect the chicks when any danger threatens them?
7. =The voice of the chick.=—Notice the different sounds by which a
chick expresses pleasure, alarm, distress, etc.
8. =Preening and scratching.=—At what age do chickens begin to preen
their feathers, and to scratch the ground? Try to discover if these are
instinctive activities, or whether they have to be learnt from the hen.
9. =Change in plumage.=—At what age are the down feathers replaced by
the true plumage? At what age is it possible to distinguish the young
cocks from the young hens?
10. =External characters, etc., of adults.=—Notice the flowing
tail-feathers, hackles (the long feathers on the neck and loins), comb,
and spurs of the cock-birds, and their absence or small size in the
hens. What differences are there as regards voice? How does the hen
announce that she has laid an egg? Which sex is the more pugnacious?
=The recently hatched chick.=—The newly-hatched chick is clothed with
fine down-feathers (pp. 273 and 277). It is generally exhausted by
its struggles to escape from the shell, but it soon recovers, and is
able to run about freely on the second day after hatching. Young birds
which, like the chick, are active immediately after hatching, are said
to be =precocious=.
=Instinct and education.=—From the first, the chick performs certain
movements which are obviously =instinctive=, that is, which have not
been acquired by any process of imitation or instruction. Even in
the egg, it may be heard chirping soon after it has taken its first
breath of air; and the complex activities of walking, running, jumping
over obstacles, and, later, preening the feathers and scratching
the ground—each of which involves the nicest adjustment of several
muscles—are also instinctive. Certain other powers have to be learnt;
the hen, for example, lifts and drops before the chick a particle of
food which she wishes it to seize, and it soon learns to peck. At
first its pecks generally fall a little short of the objects, but
presently it becomes very adroit at catching food. “A chick a day or
so old will catch a running fly at from the seventh to the twelfth
shot.”[19] There does not appear to be much instinctive recognition of
the difference between objects which are valuable as food and those
which are the reverse. Such discrimination comes only by experience,
and to a very young chick a piece of red worsted and a small worm are
equally attractive before being seized. In the same way, the nature of
water has to be learnt. Chicks peck at drops of water, but do not seem
to know the use of water until their beaks are wetted, when they drink
instinctively in the usual manner.
The use of the wings is also a matter of instinct, but it is acquired
somewhat late, and is assisted by parental encouragement and example
even in birds which are noted for their powers of flight. Fowls do
not fly much, but chicks may be observed to use their wings as an aid
in running or jumping. Prof. Lloyd Morgan[20] quotes an interesting
experiment which shows well how deeply the reliance on flight is
stamped in bird-nature. “If a chick a day or two old be placed in a
basket, held firmly in the hand, and then lowered rapidly through
the air, the fledgling will stretch out his little immature wings in
such an attitude as would make them break the fall were they fully
developed; or will, if he be a little older, flap them with flight-like
action, in either case showing an instinctive response.”
=Hens and chickens.=—Chickens which have been hatched in the natural
manner are, from the first, under the control of the hen (Fig. 194),
and her anxious care for their welfare has always been considered one
of the most beautiful examples of the maternal instinct. In their
defence her courage is unbounded; should she discover a succulent
worm or other dainty morsel, her ordinary complacent cluck changes
to a note of invitation, to which the brood at once responds; and at
her danger-signal the chickens run to take refuge under her wings, or
crouch motionless against the ground. The chirp of a chick also is
capable of expressing several states of feeling, such as contentment,
pleasure, alarm, and distress. These different notes are not only
perfectly intelligible to the hen, but are familiar to anyone who has
had experience of poultry.
[Illustration: FIG. 194.—Hen and Chickens.]
In the face of these well-known instances of the perfect understanding
between the hen and her chickens, it is somewhat surprising to find
that chickens, hatched apart from the hen and then placed with the
rest of the brood after a few days, at first pay very little attention
to the hen, except for their instinctive tendency to nestle into warm
places, and do not seem to understand her clucks.
=Adult fowls.=—The down-feathers are soon shed and replaced by the
true plumage, and the chicken gradually takes on the appearance of
the adult. A marked difference is generally to be seen between the
cocks and hens (Fig. 195) of any breed of poultry. The cock is usually
provided with flowing tail-feathers, hackles (elongated feathers on
the neck and loins), a prominent comb, and “spurs”—features which are
either absent or much less perfectly developed in the hens. As great
differences exist between the voices of the two sexes, the crow by
which the pugnacious male challenges his rivals, and the cackle of the
hen when she has laid an egg, being familiar examples.
[Illustration: FIG. 195.—Cock and Hens.]
EXERCISES ON CHAPTER XVI.
1. Point out the differences in appearance and position
between the white and yolk of a fowl’s egg. What becomes
of each during the development of the egg?
2. When does the heart of a chick begin to beat? Describe
its appearance at this time, and explain what must be done
to expose it to view.
3. Explain exactly where the chick is to be found in a
fertile egg which has been incubated for three days. How
big is a chick of that age? Of what colour is it? Does it
give any signs of life? (1898)
4. What is the amnion? Where, in a developing chick, is it
to be found, what is its appearance, when and how is it
formed, and what is its use?
5. Explain the nature and use of the “balancers” of an egg.
6. Explain how a developing chick breathes.
7. What new organs are first seen distinctly on the fourth
and fifth days respectively of incubation of a hen’s egg?
Explain exactly how you would expose them to view, and
describe their appearance.
8. What is the period of incubation of a duck’s egg? Observe
and describe exactly how the hatching duckling breaks open
the shell and escapes from it.
9. Observe and describe a duckling’s first attempt to swim.
Is the action instinctive or not?
10. Make other observations upon ducklings, comparing them
in as many respects as possible with chicks, and write
careful accounts of the results you obtain.
11. How does a fowl’s egg which has been hatched for four
days differ from a fresh-laid egg? Describe the new
structures which have formed in it, so far as they can be
made out by the unaided eye. (1904)
12. Where does the chick begin to form in the egg? Explain
the arrangement which brings it as near as possible to the
body of the sitting hen. (1905)
13. What is the use of the cloudy masses attached to
opposite sides of the yolk of a fowl’s egg? (1906)
FOOTNOTES:
[17] The amnion originates, early in the second day, as a double fold
of the yolk-surface in front of the embryo. Similar folds arise round
the sides and tail, forming a low wall (Fig. 187, _A_); the folds
gradually grow over the embryo (Fig. 187, _B_) until, during the
fourth day, they meet (Fig. 187, _C_) and enclose it in a protective
transparent bag, containing a watery fluid.
[Illustration: FIG. 187.—Diagrams to illustrate the method of
development of the amnion. The embryo and the rest of the yolk are
supposed to be seen in median longitudinal section; the head end is to
the right. For convenience, the yolk-membrane (_yk.m._) is represented
at some distance from its contents. _am_, folds of amnion; _am.c._,
amniotic cavity; _em_, embryo; _yk_, yolk; _yk.st._, yolk-stalk. (After
Foster and Balfour.)]
[18] I desire to acknowledge my indebtedness, in drawing up this
section, to Prof. Lloyd Morgan’s _Habit and Instinct_ (Arnold).
[19] Lloyd Morgan’s _Habit and Instinct_ (Edward Arnold).
[20] Lloyd Morgan’s _Habit and Instinct_ (Edward Arnold).
CHAPTER XVII. SOME FAMILIAR BRITISH BIRDS.
55. THE THRUSH FAMILY.
1. =The song-thrush, mavis, or throstle.=—Throstles are to be seen
throughout the year. Take every opportunity of observing their habits,
and make careful notes of these at the time. Especially attend to the
following characters:
(_a_) _General appearance._—What is the average size of the bird? What
is the length of the tail? At what angle is the tail held? Does the
bird move it in any special manner? What is the prevailing colour of
the body? Notice the light-coloured and spotted breast. Compare the
male and female as regards size, colouration, etc.
(_b_) _Habits._—What situations do throstles chiefly inhabit? Do they
keep near the edge of the wood, or have you often seen them inside
thick woods? Does the bird hop, or walk, when on the ground? Upon what
does it feed? Have you ever seen it cracking the shell of a snail?
If you find a heap of snail-shells near a stone, watch, at a little
distance, for the throstle coming to crack snail-shells on the stone.
Does it also eat worms, or fruit? Describe a throstle’s beak. How would
you describe the flight—as high or low, swift or slow, straight, in
wide curves, or undulating? Does the bird perch on trees? How does it
use its toes in perching?
(_c_) _Song._—In what months does the throstle sing? Does it sing late
at night and early in the morning? How does it compare with other
birds in this respect? In what positions have you seen the bird
singing—when flying, perching, or on the ground? Try to write down the
syllables which seem to you most like the throstle’s song, and notice
how the phrases are repeated. Notice that throstles sing less during
August than in autumn. Listen, in September or October, for young
thrushes learning to sing, and contrast their efforts with the song of
the adult birds.
(_d_) _The nest._—At what time of the year do throstles pair and build
their nests? In what situations have you found the nests? Write a
description of the shape, size, and materials of the first throstle’s
nest you find. Visit it frequently, try not to disturb the birds, and
note the dates of laying of the eggs. How many eggs are laid? What
are their size and colour? How are they arranged in the nest? Do the
broad or the narrow ends point inwards? What are the advantages of this
arrangement (i) for purposes of packing, (ii) when the eggs hatch? Do
both birds sit on the eggs? If only one, which? What is the time of
incubation?
(_e_) _The young._—Are the young birds naked and helpless, or
feathered, at hatching? How do the parent-birds feed them? What is the
appearance of the fledglings? In what respect do they (i) resemble,
(ii) differ from, the parents? How long do the birds remain in the nest
after hatching? Write an account of any education you have observed
them to receive from the parents.
{Make similar observations and notes on other birds, in addition to
the special observations mentioned below, and learn to recognise their
appearance, flight, song, nest, and eggs. A field-glass will be found
of great assistance.}
2. =The missel-thrush, or storm-cock.=—Compare the missel-thrush with
the song-thrush. Notice its larger size, the brighter colour of its
spotted breast, and its somewhat less musical song. Listen for its song
during, or immediately after, a storm, even in winter. In the nesting
season observe the marked courage and pugnacity of the bird.
3. =The blackbird.=—Compare the blackbird with the thrushes in
respect of appearance, food, habits, and song. Notice the difference
in the colouration of the male and female blackbird. Observe that
the young bird has a spotted breast like that of a thrush, but that
old blackbirds are not spotted. Have you noticed a blackbird show a
preference for singing in any particular place? What observations
would lead you to suppose that the blackbird is a near relative of the
thrushes?
4. =Fieldfares.=—Distinguish these birds from thrushes. Notice that
they arrive about October, and frequent fields in large parties.
=The thrush family.=—For many reasons the thrush family forms a
convenient starting point in the study of the habits of common British
birds. It includes some of our very finest songsters and therefore most
popular birds; many of its members are abundant in most parts of the
country, so that nearly every student has the opportunity of observing
them at first hand; the birds are not specially shy; and, lastly, the
family affords excellent material for the method of comparison and
contrast, which, be it repeated, is the essence of all sound work in
nature-study.
The =song-thrush=, which is also known as the =throstle= and =mavis=,
is a shapely bird, easily recognisable by its grey, spotted breast
(Fig. 196), which may be seen all the year round in wooded parts of
the country. The male and female are very similar in appearance, and
of almost equal size, measuring 8 or 9 inches from head to tail. The
thrush generally feeds on the ground, hopping along on a pair of sturdy
legs and feet, seizing and pulling out worms which incautiously show
themselves at the mouths of their burrows, and catching insects and
snails. A thrush usually has a favourite stone on which it cracks the
shells of snails, and it is not uncommon to find, near the stone, a
little heap of broken snail-shells—the remains of many a feast. In
winter, when the summer diet can no longer be obtained, the thrush
subsists largely upon the fruit of hawthorn, mistletoe, etc. Its beak,
as is usual with birds, is distinctly adapted to the capture of its
food, being, in this case, narrow, round, and fairly long. The thrush
is a strong and rather rapid flier.
[Illustration: FIG. 196.—Song-Thrush. (× ⅙).]
Thrushes begin to sing at the first signs of spring, pairing and
commencing the work of nest-building in February or early March.
The throstle’s song is surpassed by very few birds; and many people
consider it equal, if not superior, to the nightingale’s. The song
begins before sunrise, and may be heard when almost all other birds
have retired to rest. The thrush has a curious habit of repeating
each strain two or three times, “lest,” as Browning says, “you should
think he never could recapture the first fine careless rapture.”
Many attempts have been made to imitate the song by words; Mr. R.
Kearton[21] quotes with approval the following rendering by the famous
Scottish naturalist Macgillivray:
“Qui qui qui kweeu quip,
Tiurru tiurru chipiwi,
Too-tee, too-tee, chiu choo,
Chirri chirri chooee
Quiu qui qui.”
But, as Mr. Kearton remarks, “no human words can ever represent,
especially in cold type, the passionate vehemence, the sprightliness,
or the tender pleading of a thrush’s song.”
[Illustration: FIG. 197.—Nest of Song-Thrush. (× ⅓).]
The nest of the thrush (Fig. 197) is a massive structure, deep and
cup-shaped, and open at the top. It is built of small twigs and grass,
and is plastered with mud or cow-dung until the inside is smooth and
hard. It is usually placed in an evergreen bush or tree, but may
also be found in cavities in tree-trunks or in holes in walls, or
even in sheltered places on the ground. The eggs number from 4 to 6;
they are about an inch long, are sky-blue in colour, and marked with
black spots, which are most numerous near the thick end. The eggs are
arranged with their pointed ends inwards; this method not only allows
them to be packed more closely together and therefore to be more
easily covered by the sitting bird, but is also of convenience at the
time of hatching, as the young birds usually emerge (p. 294) at the
thick end. The eggs hatch after a fortnight’s incubation. The young
thrushes are at first blind, almost naked, and are quite helpless. They
make rapid progress, however, and are sufficiently fledged to leave the
nest in about a fortnight after hatching. The fledgling throstle (Fig.
198) is lighter in colour than the adult, and much more distinctly
spotted. In October the young birds may be heard learning to sing;
their first efforts are hesitating and uncertain, but by tireless
practice and careful imitation of the old birds they presently come
into full possession of their delightful powers.
[Illustration: FIG. 198.—Young Song-Thrushes in Nest.]
The =missel=, or =mistletoe=, =thrush= (Fig. 199) much resembles the
song-thrush in appearance and manner of life, but is distinctly larger,
measuring about 10½ inches from head to tail. It has a very pleasing
song, which may be heard, even in the depth of winter, when all other
song-birds are mute. The missel thrush’s habit of singing vigorously
during storms has led to its alternative name of “=storm-cock=.” The
bird is noted for its pugnacity in the breeding season, and for the
courage with which it defends its nest and young; any other birds
approaching the nest are promptly attacked and driven to a distance.
The nest is generally built in a tree, where a branch springs from the
trunk. It is made of dried grass, moss, and wool, and the hen lays from
4 to 6 pale-green eggs, which are speckled with brown.
[Illustration: FIG. 199.—Mistletoe-Thrush. (× ⅙.)]
[Illustration: FIG. 200.—Blackbird (Hen). (× ⅙.)]
The =blackbird= (Fig. 200) may easily be distinguished from the
thrush by its colour—the male being black, with a yellow beak, and
the female blackish brown, slightly mottled below. The blackbird is
slightly larger than the thrush, but in shape and habits the two
birds are very similar; they haunt the same woods, live upon similar
food (_i.e._ worms, insects, snails and, in winter, fruit) and
build their nests in similar situations. The blackbird’s song is,
however, quite distinctive, and consists of mellow, flute like notes,
without the repetition which is so characteristic of the throstle and
missel-thrush. The blackbird is fond of perching, when singing, upon a
bare branch which commands a good view of the surroundings. The nest
(Fig. 201) is much like that of the song thrush, but the inside has a
soft lining of fine grass instead of being hardened with mud. The 4, 5,
or occasionally 6 eggs are bluish-green in colour, and are marked with
_blurred_ brown spots.
[Illustration: FIG. 201.—Blackbird’s Nest.]
Curiously enough, young blackbirds, like young thrushes, are very
distinctly spotted. There can be no doubt that blackbirds and thrushes
are descended from the same stock, and that their ancestors had spotted
breasts. For some reason, the blackbird has lost its family colours,
although, as is so often the case among animals, it is still compelled
to bear the marks of its ancestry during its infancy. Young blackbirds
also receive a musical education from their parents, and may often be
heard practising their song in September or October.
The =fieldfare=, another near relative of the thrush, arrives in
this country about the beginning of October, and stays with us for
the winter, returning in spring to its nesting places among the
pines and firs of Norway. The general colour of the bird is grey,
with reddish-brown on the wings, and its breast is speckled in the
thrush-manner. Fieldfares frequent fields in large parties, especially
in the evenings, but are readily alarmed.
=Other birds of the thrush family.=—The redwing, wheat-ear, whin-chat,
stone-chat, redstart, robin, nightingale, and hedge-sparrow are other
members of the thrush family which can here be only referred to. They
have many points of resemblance, one of the most interesting of which
is that the young birds are invariably spotted. The bill is usually
rather long, stout, and straight, the staple food being insects,
worms, etc., though fruit is also eaten, especially in winter. The
nests are typically cup-shaped, and the eggs greenish or blue, with
or without spots. The young birds are quite helpless and almost naked
when hatched, and can only open their mouths to be fed by the parents.
Even after they have left the nest they need careful and continuous
teaching by the parents before they can be made to understand that
in future they will have to obtain their own food. Birds of this
family, on account of the variable character of their food, are
not so markedly migratory as many others, although they generally
move further southward in winter, as the supply of insects becomes
scarce—their place being taken by birds which have spent the summer
in more northerly regions. This habit is very obvious in the case of
the fieldfare and redwing, which visit us for the winter and return to
Norway and Sweden for the summer. The nightingale, on the other hand,
arrives here from the south in April to build its nest and rear its
young.
56. THE SWALLOW FAMILY, THE SKYLARK, AND CROWS.
1. =The swallow.=—(_a_) _General appearance._—Notice, first of all, the
bird’s colouration, as this readily distinguishes the swallow from the
martins. The whole of the upper surface is steel-blue; the only white
is on the ventral surface. A black band stretches across the breast;
the throat and forehead are pink. Observe the long, deeply forked tail,
and the broad, short bill.
(_b_) _Habits._—What are the earliest and latest dates on which you
have seen swallows? Do the birds seem to prefer the neighbourhood of
water? Have you ever seen them (i) perching on trees, (ii) on the
ground? Why do swallows occasionally alight on the ground? Do they seem
comfortable on the ground? Observe the small and weak feet; are these
adapted to a terrestrial life? Do swallows feed on the ground? What do
they eat? How are the insects caught? What is the advantage of the very
wide gape? How would you describe the flight of the swallow?
(_c_) _Voice._—Does the swallow sing? Describe its voice. Have you
heard it when the bird was on the wing, or only when it was at rest?
What kind of sound is uttered by the swallow when it is alarmed or
angry?
(_d_) _Nest._—In what situations have you found swallows’ nests? (Do
not confuse the nest with that of the house-martin, which lays white,
unspotted eggs.) How soon after their arrival do the birds begin to
build? What is the shape of the nest? Of what materials is it composed?
How many eggs does it contain? What is their colour?
(_e_) _Young._—Look for the newly-hatched young about the last week in
June, and describe their appearance. How soon after hatching do they
leave the nest? Do they at once feed themselves, or are they fed by the
parents? Keep the nest under observation after the young birds have
left it, and notice whether the old birds rear another brood the same
summer.
In September look for the annual congregation of swallows which
precedes their departure for Africa.
2. =The house-martin.=—Distinguish the house-martin from the swallow
by the patch of white on the upper tail-coverts (Fig. 179), and by the
feathered toes. Make notes of the dates of arrival and departure of the
birds. Do house-martins occupy old nests or build fresh ones? What are
the position, material, shape, and size of the nest? How does it differ
from the swallow’s nest? Is the martin of sociable or solitary habits?
Watch the birds building new nests or repairing old ones, and describe
the process. Examine and count the pure-white eggs. What are the
newly-hatched young like? How many broods have you known one pair of
martins to rear in one season? Have you ever known martins to be turned
out of their nests by sparrows? Watch for the autumn congregation.
3. =The sand-martin.=—Notice the small size of the bird, the
mouse-colour of the upper parts, and the black feathers of the wings
and tail. Note the dates of arrival and departure. Is the bird often
seen near houses? Look for the nesting-holes in cliffs, banks, etc.
Examine the holes, and notice that they lead into tunnels which slope
slightly upwards. What is the use of the slope? If possible examine the
nest at the end of the tunnel, and compare it and the eggs with those
of the swallow and house-martin. Observe the peculiar jerky flight.
Does the bird sing? Describe its voice.
4. =The skylark.=—Does the skylark frequent woods or open ground?
Describe the appearance, size, and colouration of the bird, and try to
see the long toes. Especially notice the long claw of the hind-toe.
Have you ever seen the bird perching on trees or bushes? Are the feet
well adapted for perching? Does the skylark run or hop? In what month
does it begin to sing? In March look for larks’ nests in hollows and
ruts of fields. What are the materials of the nest? Describe the number
and appearance of the eggs. Listen to the song, and try to say in
what respects it differs from that of other birds. Does the bird sing
only when flying, or also when on the ground? Describe the skylark’s
flight, and study its method of soaring (p. 278). Observe that the
birds collect into flocks in autumn. Do they leave the country for
the winter? How do they spend the winter? Do they sing in winter? If
possible examine the beak; for what kind of food do you think it best
adapted?
5. =The rook.=—Notice the size, shape, and colouration of the bird. Do
rooks live merely in pairs or in large communities? Do they build on
the ground or in trees? Are the trees high or low? What is the largest
number of rooks’ nests you have seen in one tree? Watch the birds
repairing and building nests in February and March. Of what are the
nests composed? Have you ever seen rooks stealing sticks from other
rooks’ nests? Are the birds quarrelsome? Where do they feed? Do they
hop or walk? Do they keep together when feeding? Do all the birds of
one rookery feed at the same time, or do some remain in the trees? Why?
What is the shape of the beak? How is this associated with the food of
the bird? Do the farmers in your neighbourhood consider rooks useful
or the reverse? Why? What is the voice of the rook like? How does it
vary to express warning, anger, etc. Describe any observations which
lead you to suppose that a colony of rooks has a code of laws. Describe
the flight of the rook; what other birds fly in a somewhat similar
manner? In spring, look beneath the trees for the broken shells of eggs
which may have fallen from the nests. What is their colour? Describe
the appearance and habits of the young birds. Where do rooks spend the
winter? Have you ever seen them visiting and inspecting their nests in
winter?
6. =Other birds of the crow family.=—With the rook compare and contrast
the crow, raven, jackdaw, magpie, and jay.
=The swallow family.=—Swallows and martins are popular with all lovers
of birds, for many reasons. Their arrival is welcomed as visible
evidence of the approach of summer; their graceful and rapid flight
delights the eye; their obvious liking for the neighbourhood of human
dwellings wins our sympathy; and, lastly, they claim our gratitude by
their incalculable services in keeping down insect pests.
The =swallow= (Fig. 202) may readily be distinguished from the martins
by the absence of white from its upper parts, which are glossy and
of a steel-blue colour. The forehead, chin, and throat are of a pink
or chestnut colour; the ventral surface is white, with a black band
crossing the neck. The tail is long, and very deeply forked. The bill
is short and broad, and the gape stretches nearly to the eyes—a great
advantage to a bird living on insects which are caught flying. The feet
are very small and weak, but as the swallow spends most of its time in
the air, and rarely alights on the ground except to collect materials
for its nest, it does not need very sturdy hind-limbs, and the feet are
chiefly used for perching.
[Illustration: FIG. 202.—The Swallow. (× ¼.)]
Swallows arrive in this country about the middle of April, and about a
month later proceed to build their nests in the chimneys of houses and
on the rafters of barns and outhouses. The nest is basin-shaped and
open at the top; it is chiefly composed of mud, which the birds collect
on the ground, place in position, and allow to dry. Short straws are
used to bind the mud together, and the nest is lined with soft grass
and feathers. The hen lays from 4 to 7 eggs, which are a translucent
white, with reddish-brown or grey markings. The first brood of young is
usually hatched about the end of June, but a second or even a third
brood may be raised before the parents depart for the south. Gilbert
White[22] gives the following charming account of the training of young
swallows: “For a day or two they are fed on the chimney-top, and then
are conducted to the dead, leafless bough of some tree, where, sitting
in a row, they are attended with great assiduity, and may then be
called _perchers_. In a day or two more they become _flyers_, but are
still unable to take their own food; therefore they play about near
the place where the dams are hawking for flies; and, when a mouthful
is collected, at a certain signal given, the dam and the nestling
advance, rising towards each other, and meeting at an angle; the young
one all the while uttering such a little quick note of gratitude and
complacency, that a person must have paid very little regard to the
wonders of Nature that has not often remarked this feat.” When the
young bird has learnt to feed itself “it at once associates with the
first brood of house-martins; and with them congregates, clustering on
sunny roofs, towers, and trees.... All the summer long is the swallow a
most instructive pattern of unwearied industry and affection; for from
morning to night, while there is a family to be supported, she spends
the whole day in skimming close to the ground, and executing the most
sudden turns and quick evolutions.... When a fly is taken a smart snap
from her bill is heard, resembling the noise at the shutting of a watch
case; but the motion of the mandibles is too quick for the eye.”
The swallow drinks whilst flying, sipping the water from the surface
of pools. Its song is a delicate and pleasing warble, which is uttered
both at rest and during flight. The voice becomes a squeak when the
bird is alarmed or angry. Just before the autumn migration, swallows
perch in crowds on roofs, hedges, and the branches of trees. They leave
for Africa about the beginning of October.
The =house-martin= may be distinguished from the swallow by the patch
of white upon the upper tail-coverts, and by its feathered toes. Like
the swallow, it arrives in this country about the middle of April. The
birds soon begin to repair their old nests and to construct new ones
under the eaves of houses. The nest is largely composed of clay or
mud, which the birds collect from damp places in the road, etc., place
in position against the wall, and allowed to harden. The completed
nest is somewhat bag-shaped, having only a small hole at the top; it
is lined with soft grass. The eggs are white, without spots. It is
worthy of notice when a wild bird lays white eggs, they are hidden
from sight by the shape or position of the nest, as in the case of
the martins, or the nest is built in an inaccessible place. Exposed
eggs are usually made more or less inconspicuous by being coloured
or spotted. House-sparrows (Fig. 208) have often been known to expel
the defenceless martins from their nests, and to lay their own eggs
therein. The eggs of the sparrow may be recognised by their grey
colour, and by the brown blotches with which they are marked.
The structure of the martin, like that of the swallow, is admirably
adapted to a life in the air, and to a diet of flying insects—the bird
having powerful wings, small and weak feet, and a soft and short but
widely-opening beak. House-martins leave us about the beginning of
October.
The =sand-martin= is distinctly smaller than either the swallow or
house-martin, and it may be further distinguished from them by the
“mouse-colour” of its upper parts. It usually arrives in this country
about a fortnight in advance of its two relatives, and also departs
before them, leaving in August or September. These birds live in
colonies in tunnels which they excavate in banks or friable cliffs
(Fig. 203)—generally in the near vicinity of water. The excavation may
be three feet in length; it slopes slightly upwards to the nest (which
is placed at the end of the tunnel), in order that rain water may not
collect in it. The eggs are pure white, and five or six in number. The
birds are exclusively insectivorous; they do not sing, but utter a
little twitter. The flight is very characteristic, consisting of “odd
jerks and vacillations, not unlike the motions of a butterfly.”
[Illustration: FIG. 203.—Sand Martins’ Nesting Holes.]
[Illustration: FIG. 204.—The Skylark. (× ⅓.)]
=The skylark.=—At the first glance, it is obvious that the skylark
(Fig. 204) is adapted for a life spent largely upon the ground. The
large toes lie flat on the earth, and the long claw of the hind toe
is plainly not very suitable for grasping the twigs of trees in the
act of perching. In fact, the skylark only occasionally perches,
on low bushes. Walking or running on the ground, it lives upon
grubs, caterpillars, flies, worms, etc., and also upon seeds, which
its strong beak enables it to extract from the husks. The body is
brown, the wings being streaked with black, and the dull-white throat
and breast being marked with brown spots—a colouration which is a
considerable protection. The nest is built in March, in a hollow or
rut on the ground; it is lined with dry grass. The four or five eggs
are greenish-grey, and spotted with brown. Whilst the hen sits upon
her eggs (Fig. 205) the cock-bird either hunts for food or sings his
delightful song as he soars in the air. The wonderful powers of soaring
of this bird have already (p. 278) been referred to. When the lark
drops to the ground he alights at a little distance from the nest,
and then runs up to it through the grass. By this device he avoids
revealing the position of his mate and young.
[Illustration: FIG. 205.—Skylark at Home.]
Skylarks rarely sing in the depth of winter; in autumn they form
large flocks which patrol the fields until spring in search for
food—rendering incalculable services to mankind by the destruction of
insect-larvae. During the coldest weather of winter the birds crouch
under banks and hedges.
=The rook.=—The rook (Fig. 206)—often called the crow—is a rather
large bird, measuring about 17 inches from head to tail. Its plumage
is black, but as it becomes adult the feathers covering the face and
nostrils are shed, leaving the skin of these parts bare. The bird
has long and pointed wings, and is a strong flier. The bill is stout
and almost straight. Rooks are no songsters, their harsh “caw” being
destitute of musical qualities. When heard at a distance, however, the
cry “becomes a confused noise or chiding; or rather a pleasing murmur,
very engaging to the imagination, and not unlike the cry of a pack of
hounds in hollow, echoing woods, or the rushing of the wind in tall
trees, or the tumbling of the tide upon a pebbly shore.”[23]
[Illustration: FIG. 206.—The Rook. (× ¹/₁₀.)]
One of the most interesting features of rook-ways is the habit of
living in organised communities.[24] It is true that a primitive form
of social life is found among swallows, martins, and (in winter) larks,
reminding us of the gregarious habits of the rabbit (Chapter XII.)
among mammals; but a rook-society is rather to be compared with the
pack-life of the wild dogs (p. 252), which depends for its integrity
upon the observance of certain rules of conduct by its members.
Naturalists have long known that such a code of laws is in force in
every rookery, but what these laws are we do not fully understand. Here
is a profitable and interesting field of observation for the student.
It has sometimes been observed that certain birds are prevented by the
rest from building until the nests already commenced are finished, and
cases have been known of birds being expelled from the rookery, or even
put to death, after a consultation or “trial,” presumably for some
breach of law. On the other hand, it must be admitted that rooks are
generally arrant thieves, not scrupling to rob their neighbours’ nests
of building-materials to save themselves trouble. They are also very
quarrelsome.
[Illustration: FIG. 207.—Rooks’ Nests in Trees.]
The nests are built on high trees (Fig. 207); they are composed of
sticks and turf, and are lined with moss and soft grass. As each of
several adjacent trees often contains a large number of nests, a sort
of rook-town called a “rookery” is formed. This is added to from year
to year, as the number of the birds increases. During the summer the
rookery forms the headquarters of the community, but about August or
September, before the leaves are shed, the birds leave their nests, to
spend the winter in thicker or evergreen woods, often at some distance.
At intervals during the winter they revisit their spring nests. They
come back to the rookery in February or March, and at once set about
repairing old nests or building new ones. The male bird begins to feed
the female even before the bluish-green eggs are laid, and is a pattern
of domestic virtue throughout the period of incubation. For some time
after hatching, the young birds are fed by the parents. Rooks feed on
the ground in flocks, a few birds remaining behind in the trees as
sentinels to give the alarm when danger threatens. The food consists
very largely of the larvae of injurious insects, especially the grubs
of the cockchafer beetle, and although the birds undoubtedly eat corn
and other young crops, they do far more good than harm.
=Other birds of the crow family.=—The crow, raven, jackdaw, magpie, and
jay are so similar in many respects to the rook and to each other that
naturalists include them all in one group, which they call the crow
family.
=The passerine order.=—The _families_ of the birds hitherto considered
in this chapter, with the flycatchers, wrens, chats, tits, finches,
linnets, starlings, etc., are further grouped together to form what is
called an _order_. The order which consists of these families derives
its name =passerine= from the sparrow[25] (Fig. 208), perhaps the
commonest of the birds included in it. It is probably in this order
that bird structure attains its most perfect development. Again, the
birds of the crow family are generally considered to stand at the
head of the order; if this view is correct, rooks must be regarded as
occupying, among birds, a position which roughly corresponds to man’s
place in the class of mammals.
Passerine birds are nearly all singers, and their toes are generally
adapted for perching. They feed upon insects, seeds (especially the
seeds of grasses), and soft fruit. Their beaks vary in form according
to the nature of the food upon which they most depend, as is well shown
by the soft, widely opening bills of the swallows, the strong but
slender beaks of the thrushes, larks, and crows, and the short, hard
beaks of such seed-eaters as the canary, sparrow, and other finches.
Birds which are exclusively, or almost exclusively, insectivorous are
compelled to migrate to warmer countries to obtain food during the
winter. The eggs of passerine birds are usually coloured or spotted,
and the young birds are hatched in a helpless and almost naked
condition, being carefully tended and fed by both parents until they
are able to fend for themselves. A few familiar birds of other orders
will now be briefly considered.
[Illustration: FIG. 208.—House Sparrows. (× ⅕.)]
[Illustration: FIG. 209.—The Swift. (× ⅙.)]
57. THE SWIFT AND THE CUCKOO.
1. =The swift.=—Distinguish the swift from the swallow by its larger
size, its sooty black colour (except for the dull white of the throat),
and its long, bowed wings. In what respect does the swift resemble the
swallow? Watch for the appearance of swifts about the end of April. Do
the birds arrive singly, in pairs, or in flocks? Notice the rapid turns
and twists of the flying bird. Does it as a rule fly near the ground,
or at somewhat great heights? Have you ever noticed the bird perching,
or settling on the ground? Have you seen it clinging to walls with its
feet? Were these the walls of houses or barns, or of high towers? What
actions would lead you to suppose that the nests are built in high
towers? Are more swifts to be seen in the evening, or during the day?
Can you explain this? Watch during the first half of August, and make a
note of the day on which swifts were last seen.
2. =The cuckoo.=—What is the earliest date on which you have heard the
cuckoo? Imitate the cry by putting your hollowed hands together and
blowing between the thumbs. With practice you will be able to deceive
the bird into thinking that another cuckoo is present, and it will
approach near enough to be seen distinctly. Estimate the size of the
bird, and write down a careful description of its form, colouration,
and method of flight. Try to see that two of its toes point forwards
and two backwards. Have you ever seen cuckoos mobbed by small birds?
Carefully examine as many nests of small birds as possible, and try to
find one containing an egg slightly larger than the rest and perhaps
differently coloured. This will probably be a cuckoo’s egg. Visit the
nest at frequent intervals, both before and after the hatching of the
eggs, and write descriptions of the eggs, the appearance and size of
the young birds, what becomes of each, and the behaviour of the old
birds. What is the latest date on which you have heard the cuckoo?
=The swift.=—Though the swift bears a strong general resemblance to
the swallow, the two birds are not at all closely related, but belong
to different orders. The swallow, like all other passerine birds (p.
320), has 12 tail-feathers, and its first toe is separately movable
and directed backwards. The swift, on the other hand, has only 10
tail-quills, and all its toes point forwards. There are, however, more
obvious differences, which readily enable one bird to be distinguished
from the other, even at a distance. The swift is, with the exception
of a dull-white patch on the throat, of a sooty black hue over all the
body, and in flight its long and powerful wings take the form of a bent
bow (Fig. 209). The swallow flies in bold, sweeping curves, while the
more rapid swift turns sharply in the air, in pursuit of the flying
insects upon which it feeds, in a manner suggestive of the flight of a
bat. While, again, the swallow flies much near the ground, the swift
is usually seen at greater heights. The harsh scream of the swift is,
moreover, in marked contrast with the swallow’s pleasing song.
Swifts do not willingly alight on the ground, even to collect materials
for their nests, nor do they perch on trees or roofs, the arrangement
of their toes being quite unsuitable for either action, and the length
of the wings being such that, even with toes of the normal type,
walking would be almost impossible. The nest, a somewhat rude structure
of grass and feathers, is generally built in high towers or other tall
buildings. The eggs are two in number; they are more conical in shape
than those of the swallow, and, as the position of the nest renders
them independent of protective colouration, are white. The hen-bird
sits on them during the day, but generally leaves the nest and flies
abroad in the evening to hunt for insects. Only one brood is reared,
and the birds take their departure early in August, to spend the winter
in South Africa or Madagascar.
=The cuckoo.=—The cuckoo is placed by naturalists, not among the
passerine birds, but in the order to which the swift belongs. The
reasons for this cannot be fully considered here, but it may be
mentioned that the cuckoo has only 10 tail-feathers, and that it
differs from all passerine birds in having its fourth toe, as well as
the first, directed backwards, the second and third pointing forwards
(Fig. 210).
The cuckoo has a total length of about 14 inches. The upper surface
of the body, and the throat, are grey; the rest of the lower surface
is white, crossed with black bars. The wings are large and powerful,
and the thighs are covered with long feathers. In general appearance,
as well as in manner of flight, the cuckoo bears a great resemblance
to a hawk, which may perhaps be the reason why small birds so often
join forces and attack it. A keen-eyed observer will, however, at once
distinguish the birds by the head and the bill, both of which are
markedly longer in the cuckoo than in the hawk. Cuckoos arrive here
in April, the males usually appearing first. The well-known call, to
which the bird owes its name, is uttered by the male only; the voice of
the female is quite different, and is often compared to the sound of
bubbling water. The cuckoo feeds entirely upon insects; it is believed
to be the only bird which eats the hairy caterpillar of the tiger-moth
(p. 369).
[Illustration: FIG. 210.—The Cuckoo, (× ¹/₇.)]
Of all bird-habits, probably none has excited more interest than the
manner of life of the young cuckoo. Its parents build no nest, but
depend entirely upon other birds for rearing their offspring. The
female cuckoo lays her egg upon the ground and then carries it in her
bill to a nest which contains similarly coloured eggs, if such a nest
is to be found. If not, another convenient nest is selected. It is said
that the male sometimes renders assistance by distracting the attention
of the rightful owners in the meantime. In most cases the owners of
the nest, apparently unaware of the trick, sit upon the strange egg
with the rest, and in due course the young birds are hatched. The young
cuckoo, whilst still blind and naked, wriggles itself under its foster
brothers and hoists them over the edge of the nest. Far from resenting
the crime, the duped parents now devote their energies to feeding the
murderer, and continue their attentions until after it is fully fledged
(about a fortnight after hatching) and perhaps several times their
combined size.
[Illustration: FIG. 211.—Cuckoo’s Egg in Greenfinch’s Nest (Cuckoo’s
Egg on the right).]
The small size of the cuckoo’s egg—about one-fourth of what might be
expected from so large a bird—and its general similarity in colour to
their own eggs, both aid in deceiving the victimised birds in the first
instance, and the disproportionate size of their supposed offspring
shortly after hatching apparently does not arouse their suspicions. It
is believed that any particular female cuckoo lays only eggs of one
type and deposits them, as far as possible, in nests of the mimicked
species—the species, probably, by which she herself was reared.
During the summer, therefore, she visits one such nest after another,
until she has disposed of all her eggs. Further, it is supposed that
her offspring inherit her tendency to lay eggs of the particular
colour, and therefore to prey in their turn upon the species of their
foster-parents. In this country, the meadow pipit, pied wagtail,
robin, and reed-warbler are perhaps the most usual victims of the
cuckoo’s parasitism, but the nests of other species (Fig. 211) are not
uncommonly selected.
The old cuckoos depart about the end of July, apparently leaving the
young ones to find their way south alone.
58. THE DUCK.
1. =Tame ducks.=—Closely observe ducks, both on land and when they are
swimming. Where is the heaviest part of the body? Are the legs attached
in front of, directly below, or behind the centre of gravity? Is the
position of the legs an advantage or a disadvantage (i) in walking,
(ii) in swimming? How does the foot differ from the feet of the birds
previously mentioned? Between which toes is the web stretched? How
are the toes held when the bird lifts its foot in walking? Does it
walk gracefully? How does it swim? Are the legs moved together or
alternately in swimming? Notice how the web folds up when the foot is
moved forwards, and spreads out for the back-stroke. What does the duck
eat? How is the food obtained? Why are a duck’s feathers so little
wetted by the water? Watch, and describe exactly, how the bird preens
its feathers.
How can you distinguish the male (drake) from the female duck, (i) by
appearance, (ii) by the voice? Do the colours of the drake’s plumage
differ at different periods of the year? What relation have the changes
of plumage to the moulting-season? What differences can you observe in
the methods of moulting of ducks and fowls? Is there any time of the
year when (i) ducks, (ii) fowls are unable to fly? Why? When and where
do ducks lay their eggs? Describe the appearance of the eggs. Are young
ducks helpless or precocious (p. 296), naked or clothed, at hatching?
Can they feed themselves? Can they swim?
Examine a dead duck. Notice the thick covering of down-feathers next
the skin. What is its use? Examine and draw a foot, and see how the web
folds between the toes. Observe the soft, sensitive skin on the outside
of the beak; what do you suppose is its use? Notice the shape of the
beak. Open it to see the horny plates fringing its inner edge, and
notice how these, with other plates on the thick, fleshy tongue, form a
strainer.
=The duck.=—The birds of the duck order—which includes also the geese
and swans—differ in many important respects from all our previous
types, and it is interesting to observe how perfectly these differences
are adapted to the manner of life. On land, the ungainly waddle, which
is entailed by the insertion of the hind limbs so far back on the
body (Fig. 212), shows that the duck is not in an entirely congenial
element; in the water the bird is a model of graceful movement,
perfectly balanced, progressing smoothly by alternate strokes of its
webbed feet, and altering its course to any desired direction with the
utmost ease. The duck feeds largely upon the small animals which abound
in the water of ponds, and in the mud of the sides, and the beak is
beautifully fitted for the duties it has to perform. It is covered
on the outside by a soft and highly sensitive skin, which enables the
bird to detect with certainty the presence of its prey in the mud;
the inner edge of the beak is provided with horny plates which, with
similar plates situated on the edge of the thick, fleshy tongue, form a
very efficient strainer, by means of which the useless water and thin
mud can be forced out at the sides of the mouth and separated from the
worms, etc., which were taken into the mouth at the same time. A thick
coat of down-feathers, which lies next the skin and contains a great
deal of entangled air, forms a non-conducting layer which prevents the
undue escape of the heat of the body, and saves the bird from becoming
chilled when in the water. Ducks are, moreover, careful to keep their
feathers well oiled, and may often be seen preening themselves—applying
the bill alternately to the oil-gland on the tail and to the feathers.
The completeness with which water flows off a duck’s back is proverbial.
[Illustration: FIG. 212.—Ducks.]
During the greater part of the year the brilliant plumage of the drake
forms a striking contrast to the sober brown and grey feathers of the
female. Another point of difference is that in the male the four middle
tail-feathers are curled upwards. From July to October, however, before
the moult takes place, the two sexes are very similar in appearance.
Birds of the duck order moult in a somewhat different manner from most
other birds, in that all the quill-feathers are shed at once, instead
of in pairs. Until the new feathers develop, flight is of course out
of the question, and the birds remain as secluded as possible in the
meantime.
The eggs are white and greasy-looking; they are laid in a rough,
open-topped nest, lined with down, which is placed on the ground.
The wild duck covers up the eggs when she leaves the nest. The young
are active immediately after hatching. It has been noticed that the
eggs of precocious birds are generally larger, in proportion to the
size of the parent, than those of birds which are naked and helpless
at hatching—the larger store of egg-food allowing a more complete
development of the young bird in the shell. Further, the nests of
precocious birds are, as a rule, less elaborately constructed.
59. TWO COMMON HAWKS.
1. =The sparrow-hawk.=—Watch for sparrow-hawks near farms. Notice the
general resemblance of the bird to the cuckoo, but distinguish them
by the short head and beak of the hawk. Observe the _bluish-grey_
colouration of the upper parts. Describe the flight of the hawk and, if
possible, its method of catching its prey.
2. =The kestrel or windhover.=—Distinguish the kestrel from the
sparrow-hawk (i) by the _reddish_ colour of its upper parts, and (ii)
by its habit of hovering in mid-air. Have you ever known the kestrel to
prey upon small birds? Upon what does it feed?
=The sparrow hawk and kestrel.=—These two birds are the only British
hawks which the average nature-student is likely to see during
a country walk; and as one of them is given to preying on young
game-birds, chickens, etc., while the other as generally confines
itself to animals which are universally regarded as vermin, it is
important to be able to distinguish them at sight.
[Illustration: FIG. 213.—The Nest of the Sparrow Hawk.]
The =sparrow-hawk= (Fig. 213) attains a length of 13 inches; the female
is slightly larger than the male. The upper parts of the body are
_bluish-grey_ in colour; the lower parts are buffish white, and crossed
with brown bars. The head is short and round; the bill is hooked and
sharp, as in birds of prey generally, and the toes are armed with sharp
claws. The bird is often to be seen near farms, lurking behind hedges
and waiting for an opportunity of dashing upon chickens or other small
birds and carrying them off.
The =kestrel=, which in size and shape much resembles the sparrow-hawk,
is really a species of falcon (Fig. 179). It is unrivalled among common
British birds in its power of remaining poised in one position in
mid-air by that rapid motion of the wings which is called hovering, a
power which has earned for it the name of “windhover.” The kestrel,
though in reality one of the farmer’s best friends, from its wholesale
destruction of field-mice, voles, and many injurious insects, is often
ignorantly confused with the sparrow-hawk. The _reddish_ hue of the
plumage of the kestrel’s upper parts, together with the hovering habit,
ought to render such a mistake impossible to observant eyes, and to
secure the bird from a persecution which may reasonably be directed
against the sparrow-hawk.
EXERCISES ON CHAPTER XVII.
1. Extend the foregoing methods of study to the following
passerine birds:—Starling, finches, wagtails, pipits,
nuthatch, tits, warblers, wrens, and flycatchers, and make
notes of the observations.
2. Try to discover reasons for grouping (_a_) woodpeckers,
nightjars, and kingfishers with swifts and cuckoos; (_b_)
pheasants and grouse with fowls.
3. Compare geese and swans with ducks, and make notes of as
many points of resemblance and difference as possible.
4. Compare and contrast owls with hawks.
5. Arrange the above birds in lists according to (_a_)
food, and the characters of the beak; (_b_) characters of
feet and arrangement of toes; (_c_) nests (open-topped,
covered, built in holes or tunnels); (_d_) colour and
number of eggs; (_e_) condition of young at time of
hatching.
6. How many birds do you know which (_a_) spend only the
summer, (_b_) spend only the winter, (_c_) stay all the
year, in this country? State, in each instance, upon what
food the bird most depends.
FOOTNOTES:
[21] _Our Bird Friends_ (Cassell).
[22] _The Natural History of Selborne._
[23] _The Natural History of Selborne._
[24] A delightful account is given in the story of “Silverspot”
(Thompson-Seton’s _Wild Animals I Have Known_).
[25] Latin, _passer_, a sparrow.
CHAPTER XVIII. FROGS AND TADPOLES.
60. THE LIFE OF THE FROG.
1. =Manner of life.=—Where have you found frogs? Are they commonest in
dry or in damp situations? At what time of the year have you seen them
actually in water? How do frogs move about? Do they walk or hop? Chase
a frog, and notice that its hops become shorter as it is pursued. Upon
what does the frog feed? Have you ever seen frogs abroad in the depth
of winter? Are insects common in winter? How do you suppose frogs spend
the winter? What are the principal enemies of frogs?
2. =External characters.=—Catch a frog; is its body dry or moist? Does
it feel cold or warm? How does it behave when caught? Does it soon
become tame if well treated? Put a frog under an inverted pickle-bottle
and observe it carefully. What is its _size_? What was the size of the
smallest frog you have seen? At what time of the year can the smallest
frogs be seen? What is the _colour_ of the frog? Put one frog with
dark-coloured leaves and soil, and another with light-coloured leaves,
and try to find out if the skin of the first becomes lighter and that
of the second darker in colour. Is it an advantage to the frog to be
able to change slightly in colour? Why? What is the position of the
frog when at rest? Notice the shortness of the body, the hump on the
back, the relative lengths of the limbs, the 4 unwebbed fingers, the 5
webbed toes, and the absence of a tail. What is the advantage of the
long hind-limbs? Mention other leaping animals whose hind-limbs are
longer than the fore-limbs. Is a short-bodied animal (_i.e._ one with
its four limbs pretty close together) less likely to be injured by
the fall, after a leap, than a long-bodied animal? Why? Put the frog
into water, and watch it swim. What is the use of the webs between the
toes? Examine the _head_. Notice the very wide stretch of the mouth,
the small openings of the nostrils, the large prominent eyes, and the
ear-drum—a dark-coloured disc on each side, a little below and behind
the eye.
3. =Breathing.=—Watch the up-and-down movements of the floor of the
mouth, by which the animal pumps air, through its nostrils, in and out
of its mouth-cavity.
4. =Method of feeding.=—Put live insects, small worms, etc., under the
upturned bottle enclosing the frog, and watch the animal’s method of
feeding. Try to see how it uses its tongue. What is the advantage of
the wide gape of the mouth? Can you get the frog to accept dead insects?
5. =The inside of the mouth.=—To kill a frog painlessly, soak about
a teaspoonful of chloroform on cotton wool, and put it with the frog
under a bottle or tumbler. After 15 or 20 minutes the frog will be
quite dead.
(_a_).—Open the jaws widely, and examine the inside of the mouth.
Notice how the rounded _eyeballs_ project into the mouth-cavity when
they are pressed from the outside. Pass your finger-end round the
margins of the jaws, and feel the row of fine _teeth_ borne by the
upper jaw; the lower jaw is destitute of teeth. Feel also, in the roof
of the mouth, two small patches of teeth (_Vo._, Fig. 216); these are
carried by small bones called the vomers, and are therefore called the
_vomerine teeth_. Just to the outside of the vomerine teeth and in
front of the eyeball, notice on each side the internal opening of the
_nostril_ (_Ch._, Fig. 216); pass into one nostril a stiff bristle,
and observe that it emerges at the external nostril. Pull forward the
_tongue_ (_T_, Fig. 216) and observe that it is attached at its front
end; feel how sticky the tongue is. Behind the eyes, and at the angles
of the jaws, notice the openings of the Eustachian tubes (_E_, Fig.
216); push a stiff bristle into one and observe, from the outside, that
the end of the bristle presses the inner surface of the ear-drum.
(_b_).—If the roof of the mouth has dried, moisten it with water, and
place a very small cork-shaving or a tiny snip of paper on it, far back
between the eyeballs. Notice that the shaving travels slowly down the
throat. This experiment ought to be made soon after the frog has been
killed.
6. =The bones.=—Feel, through the body-wall, the various parts of the
skeleton, making out the skull, backbone, limb-bones and their manner
of attachment, and breast-bone (protecting the heart). Observe the
_absence of ribs_.
7. =The skin.=—Examine the skin of the dead frog, and notice that it is
_damp_ or even somewhat slimy, and that it differs from the skin of a
mammal or bird in being _naked_, bearing neither hairs, feathers, nor
scales. Pinch up the skin, and notice how very loosely it is attached
to the underlying body-wall. Snip through the thin skin, turn it back
from the body-wall, and see the network of _blood-vessels_ upon its
inner surface.
=The habits of the frog.=—The common grass frog (Fig. 214) is to be
found in abundance, from early spring to October, in ditches, marshy
land, and other damp places. As winter approaches, frogs generally
bury themselves in the mud at the bottom of ponds, etc., and remain
there in a state of torpor until the spring, when they emerge and the
females lay their eggs. Frogs are said to be “cold-blooded” because
their temperature never varies much from that of their surroundings.
Birds and mammals, on the other hand, maintain an almost constant
temperature,—a healthy man’s blood, for example, being just as warm
in the depth of winter as it is on a hot summer day. So long as they
are not actually frozen hard, therefore, frogs can endure the winter
cold without much inconvenience. Although the frog is essentially a
land animal, it is quite at home in the water, and swims gracefully
and easily by the help of the webs which connect the long toes of its
hind feet. On land, it progresses by long leaps, its limbs and body
being well adapted to this habit. As is usual in leaping animals, the
hind legs and feet are markedly longer than the fore limbs; and the
shortness of the body enables the two pairs of limbs to be brought
together to break the shock of the fall.
[Illustration: FIG. 214.—The Frog. (× 1.)]
=Food.=—The frog’s method of catching its prey is very interesting,
and is graphically described by Dr. Hans Gadow[26] as follows:—“The
food, which consists chiefly of insects, snails, and worms, must be
moving to excite interest; then the frog, whose favourite position is
half squatting, half supported by the arms, erects itself and, facing
the insect, turns round upon its haunches, adjusts its position anew
by a shifting of the legs, and betrays its mental agitation by a few
rapid movements of the throat. All this time the prey is watched
intently until it moves; then there follows a jump, a flap of the
tongue, and the insect is seen no more.” This flap of the tongue is
well illustrated in Fig. 215. The frog’s tongue is free behind, but
is attached, by its anterior end, close to the middle of the lower
jaw,—an arrangement which enables it to be flicked out to its full
length. Further, it is covered by a glutinous secretion, which sticks
tenaciously to the prey.
[Illustration: FIG. 215.—Three Stages of the Movement of the Tongue of
a Frog. (× 1.)]
The frog is fortunate also in the extremely wide gape of its mouth,
which stretches, almost literally, from ear to ear. Once in the mouth,
the captive is prevented from escaping by the teeth. Of these there
are two sets; a row of fine teeth is present along the greater part of
the margin of the upper jaw, and, in addition, two small patches of
teeth—the vomerine teeth (_Vo._, Fig. 216)—occur on the roof of the
mouth. Fairly large insects are promptly gulped down into the stomach;
those which, owing to their minute size, escape being swallowed in
the ordinary manner, are slowly but surely forced down the throat by
the incessant lashing of thousands of tiny threads—too small to be
seen except by high powers of the microscope—which are carried by the
skin of the roof of the mouth. The action of these invisible threads
may easily be seen if a frog’s mouth be opened widely and a small
cork-shaving be placed near the top of the throat (Expt. =60=, 5, _b_).
The shaving steadily travels backwards and is soon lost to sight.
It should be clearly understood that frogs and toads are of
incalculable value in keeping down insect pests, and deserve systematic
protection for this if for no other reason.
=How a frog breathes.=—When a resting frog is watched, the floor of
the mouth is seen to be raised and lowered alternately. It is a common
belief among children that these movements are a sign that the animal
is “getting ready to spit.” Frogs do not spit, however, and the action
is simply a part of the breathing process, which is performed in the
following manner. The mouth being closed, the nostrils are opened,
and, by alternate up-and-down movements of the floor of the mouth,
the air present in the mouth-cavity is completely replaced by fresh
air. The nostrils are then closed, and the slit-like glottis (_gl._,
Fig. 163) which leads to the lungs (_l. lng._, _r. lng._) is opened.
The foul air is forced out of the lungs and mixed with the pure air
in the mouth-cavity. Then, immediately, the floor of the mouth is
raised—pumping the mixed air into the lungs—and the glottis is closed.
In the =lungs= an exchange takes place between the oxygen of the
refreshed air, and the surplus carbon dioxide in the blood which is
circulating in the capillaries (p. 242) of the walls of the lungs. In
the meantime the nostrils have again been opened, and the first stages
of the process are already being repeated.
[Illustration: FIG. 216.—Inside of mouth of Edible Frog. _Ch_, internal
opening of nostril; _E_, opening of Eustachian tube; _S_, opening
leading into vocal sac; _T_, tongue; _Vo_, vomerine teeth. (× 1.)]
A considerable part of a frog’s breathing is carried on, in these
first stages, through the thin membrane forming the =roof of the
mouth-cavity=. This is richly supplied with blood capillaries, and is
therefore admirably adapted for the exchange of gases which constitutes
respiration. Moreover, the =skin= covering the general surface of the
body has also a very abundant blood-supply, and forms yet a third
respiratory organ; so that it is practically impossible to drown a frog
in ordinary water—in water, that is, which contains dissolved air. This
power of breathing through the skin is of great importance to a frog
during the winter sleep at the bottom of a pond.
=The frog’s skin.=—The frog’s skin is kept moist by a slimy fluid which
is continually being discharged from small glands in its substance.
The moisture not only facilitates skin-breathing as described above,
but its evaporation keeps the body cool even in hot weather—a matter
of vital importance to an animal to which a temperature of 40° C. or
so is fatal. A third most interesting property of the frog’s skin is
its power of changing somewhat in colour to match the colour of its
surroundings. The change depends upon an alteration in form and size
of certain small brown specks imbedded in the thickness of the skin.
When the animal finds itself in dark-coloured surroundings these specks
enlarge, and the skin as a whole takes on a darker hue. On a light
background the reverse change takes place.
=A case of evolution.=—The advantage which accrues to a frog from being
thus rendered less conspicuous to enemies and prey alike is obvious,
and there is no difficulty in picturing to ourselves the probable
manner in which the advantage was developed. Widely different races
of animals have colour-specks in the skin, and we may assume that
frogs and their ancestors have possessed such spots for countless
generations. Now suppose that, ages ago, a frog happened to be born
with the power of altering very slightly the size of his colour-specks.
If this power rendered him less conspicuous, in however slight a
degree, than his neighbour frogs he would, other things being equal,
be more likely than they to escape from enemies, grow to maturity, and
in due course have sons and daughters. All his offspring, and they
might number thousands, would tend to inherit, to varying extents, the
power of changing colour. In some—perhaps in most—the power would be
practically absent; in others it might be equal to that of the parent;
while in a few instances it would probably be greater. In any case,
those frogs of the second generation which had the greatest power of
changing their colour would be the most likely to survive the keen
struggle for existence and therefore to leave offspring. The survival
of the “fittest” frogs of each generation, and the transmission to
the next generation, in ever-increasing intensity, of their favouring
accomplishment, would naturally result at last in the production of
a race of animals in which, as in the frogs of to-day, the power of
changing colour is universal.
[Illustration: FIG. 217.—Head of Male Edible Frog, seen from the left
side, showing inflated vocal sacs. (× 1.)]
=The laying of the eggs.=—About the end of March the frogs resort in
great numbers to shallow ponds and ditches, pair with much croaking,
and the females lay their eggs or “spawn.” Both sexes croak, and the
male of the edible frog, though not of the common grass frog, is able
to make more noise in virtue of a pair of vocal sacs (Fig. 217), which
he can inflate with air from the mouth (Fig. 216), and which act as
resonators. After spawning, the frogs leave the water, abandoning the
eggs to their fate, and resume their ordinary terrestrial life, until
approaching winter prompts them to hide in the mud and go to sleep.
61. THE DEVELOPMENT OF THE FROG.
1. =A simple aquarium.=—A simple aquarium, in which the development of
the frog from the egg may be watched, is easily made. Obtain a fairly
large basin, and cover the bottom with sand, mud, and stones from a
pond. Arrange these so that the bottom shall shelve from the surface of
the water at one side to a depth of 3” or 4” at the other. Put in some
stones covered with green slime, which will almost certainly be found
in the pond, plant a few water-weeds, and allow the water to clear.
2. =Frog spawn.=—Having prepared the aquarium, obtain, towards the end
of March (or earlier in a mild spring) a handful of frog spawn from a
pond or ditch. It forms a mass of jelly in which the true eggs—small
balls about ¹/₁₀” in diameter—are imbedded. Put this in the aquarium
and examine it carefully every day, making the observations described
below. If possible, obtain a pair of spawning frogs, and place them in
a bucket with a little water, so that the earliest stages also may be
studied.
3. =The eggs before hatching.=—Observe the globe of jelly which
surrounds each egg. Try to pick it up between your finger and thumb. Do
you think it is of any use in protecting the eggs from being eaten by
fishes, birds, etc.? Remove the jelly as completely as possible from
one egg, put the egg in a watch-glass with a little water, and examine
it carefully with a strong lens. To which of the stages shown in Fig.
218 does it correspond? If possible, treat a newly-laid egg in the same
manner and examine it every hour or so through the day. Make notes of
the time elapsing between the various stages shown in Fig. 218.
4. =Hatching.=—Notice that the developing eggs change from the
spherical to the ovoid form. What is the shape when the embryo begins
to move? Notice the appearance of a neck and a tail. At this stage the
embryo makes its way out of the jelly, or hatches. The jelly may now
be thrown away, as it is of no further use. How does the embryo behave
immediately after hatching? Put one in a watch-glass and with a lens
try to see the _sucker_ by which it attaches itself to water-weeds,
etc.
5. =Tadpolehood.=—Examine the tadpoles at frequent intervals by the
help of a lens, and write careful accounts of the changes which take
place in them. Notice particularly the fine threads—the external
_gills_—which grow out from the sides of the neck. How many are there?
After a time they shrivel up and are replaced by other gills which
cannot be seen. Write a description of a tadpole at the time when it
begins to feed. How large is it?
Count the tadpoles in the aquarium at intervals of a few days,
carefully removing all dead ones. Supply fresh green slime from the
pond from time to time. Have you any reason to suppose that tadpoles
are cannibals?
How soon do the tadpoles come to the surface to breathe air? Describe
the appearance of one when it begins to do this. Has it any legs? How
many can you see? In what order do the legs grow out?
Notice now the dwindling of the tail. (It does not drop off, as is so
often stated.) The tadpole has become a frog. What percentage of the
original eggs have developed into frogs?
[Illustration: FIG. 218.—The early development of the Frog. _mi_,
small black cells; _mg_, large white yolk-cells; _ect_, black cells
overspreading yolk-cells; _yk.pl_, yolk-plug; _md.gr_, groove of
commencing nervous system; _md.f_, right margin of groove; _br.cl_,
depressions marking position of future gill-slits; _stdm_, pit which
will become mouth; _t_, tail; _br.1_, _br.2_, external gills; _e_, eye;
_sk_, sucker. (× 5.)]
=The early development of the frog’s egg.=—A frog’s egg is a little
spherical mass about one-tenth of an inch in diameter. When laid, it
is covered by a thin gelatinous layer which soon swells up in the
water to form a transparent globe of about half-an-inch diameter, in
the centre of which the true egg can be seen. This jelly is extremely
slippery and difficult to grasp, and is consequently an efficient
protection against the attacks of birds, fishes, insect-larvae, etc.,
and even of parasitic fungi and other small organisms. The jelly also
acts as a float. At the time of laying, each egg consists of a black
and a white portion. In the lower, or white, part there is a store of
food, on which the little embryo subsists until it acquires a mouth
and begins to fend for itself. The development begins in the upper, or
black, part of the egg, and may easily be watched with a lens. And to
be appreciated properly, the changes should be watched, and not merely
read about. The student should get, if possible, some freshly-laid
frog’s eggs, and remove the gelatinous investment from one or two. It
will require care to do this without injuring the eggs. They should now
be put, with a little fresh water, into a watch-glass, and carefully
examined at intervals of half an hour or so. Soon a little pit makes
its appearance in the middle of the black half, and gradually extends
until it becomes a groove, which little by little reaches quite round
the egg (Fig. 218, _A_). In the meantime another groove begins to form
at right angles to the first, and, in its turn, grows down round the
egg (_B_). If we compare the whole egg to the earth, and the middle
of the black half to the North Pole, these two grooves may be said to
be along meridians at right angles to each other. The third groove
(_C_) may be considered as along a parallel of latitude, but it is
somewhat to the north of the Equator. These first three grooves deepen
until the whole egg is cut up into eight separate pieces. The sight of
the apparently lifeless speck dividing itself up in this regular and
orderly manner, “while you wait,” is an intellectual treat which should
not be missed. The cleavage of the egg goes on rapidly, but in a less
regular manner now, until the whole is cut up into a hollow sphere of
segments (_F_), black and small (_mi_) in the northern hemisphere,
whitish and larger (_mg_) in the south.
It is worth while to pause here to consider how these early changes are
assisted by the peculiar condition of the egg at the time of laying.
The southern hemisphere of the egg is laden with a store of food. The
food is dead, and acts as a mechanical hindrance to the activity of
the living matter. In the northern half of the egg but little food
is present to impede its activity, and it is plainly important that
this half shall receive as much warmth as possible from the uncertain
sunshine of early spring. Two circumstances ensure this. In the first
place, the food-laden region is the heaviest part of the egg, so
that the latter—buoyed up as it is by the jelly—tends to float with
its most “alive” part upwards. Secondly, this upper part is coloured
black,—a great advantage, since black objects absorb heat readily.
Both these peculiarities therefore favour the more rapid development
of the “northern” hemisphere. Hence the third cleft is to the north
of the egg’s equator, instead of being halfway between the upper and
lower poles, and hence, too, at the close of segmentation the northern
segments are smaller and more numerous than the southern. In the case
of the hen’s egg (Chapter XVI.), the amount of stored food contained
in the egg (the yolk) is so enormous that segmentation is confined
entirely to a small patch on the upper surface. Another result of the
relatively small amount of stored food in the frog’s egg is that the
tadpole is compelled to turn out and earn its own living at a stage
when the chick’s inherited fortune is still considerable.
=The tadpole.=—The spherical mass soon becomes ovoid, and is divided
into head and trunk by a neck-constriction (_J_). An occasional wriggle
shows that the creature is alive. Shortly afterwards a tail grows out
from the hinder end of the trunk, giving the animal something of the
appearance of a fish (_L_). In this stage the tadpole makes its way out
of the jelly, and thus hatches, about a fortnight after the laying of
the eggs. The little creature is quite helpless; it has no mouth (the
egg food is not yet exhausted, however), and its attempts at swimming
are still feeble and uncertain. In this defenceless condition (Fig.
219, 1) it will be seen to attach itself to the water-weed of the
aquarium by means of a sucker (Fig. 218, _L_, _sk_) on the underside
of its head. It is in a very favourable position for examination, and
by help of a lens, two—soon there are three—pairs of fine, thread-like
outgrowths can be distinguished on the sides of the neck (Fig. 219,
2 and 2_a_). These are the =external gills=. In a few days the mouth
breaks through, and the animal begins to nibble at the vegetation with
little horny jaws, and soon swims about the aquarium with confidence.
[Illustration: FIG. 219.—The Frog. Stages in the life-history, from
the newly-hatched Tadpoles (1) to the young Frog (8). (× 1.) 2_a_ is a
magnified view of 2.]
The gills are the organs by which the young tadpole breathes, and
each little gill-thread is seen, when viewed with a low power of the
microscope, to contain tiny loops of blood-vessels. Every time the
tadpole uses his jaws, wags his tail, or, in short, does anything at
all, he uses up some oxygen and produces some carbon dioxide. The water
of the aquarium contains dissolved oxygen—green water-weeds are kept
in aquaria for the express purpose of liberating oxygen (p. 51)—and
this oxygen makes its way through the excessively thin membrane which
divides the blood of the gills from the water, to be swept away by the
current of the blood to the various parts of the body. The carbon
dioxide produced is brought to the gills by the blood stream, and
passes through the membrane into the water, where it is utilised by
the water-weed as food (p. 51). Animal and plant are thus mutually
serviceable.
A series of slits soon opens through the sides of the neck, and along
their margins are formed folds which are usually called the =internal
gills=. The external gills dwindle and shrivel up as the internal gills
are being formed, and at the same time a flap of skin grows backwards
from each side of the head and covers over the slits so that they
cannot be seen. Presently the two flaps fuse at their edges—except at
one point on the left side, where a spout is left—and so enclose a
chamber. The water which enters the tadpole’s mouth pours through the
gill-slits, into the chamber, and out through the spout. As it swills
over the folds of the internal gills there is an exchange of carbon
dioxide for oxygen in the manner just described.
The tadpole in the meantime is growing strong and active, and the tail
has grown out to form a powerful organ, the sinuous motion of which
propels the animal with relatively great speed through the water.
From the point of view of the biologist, perhaps the most interesting
feature of this stage of tadpolehood is the almost entire correspondence
of the structure with that of a fish, although the adult frog is not
in any sense a fish. This curious state of things is explained by
supposing that frogs have descended from fish-like ancestors, and that
every frog, in the course of its development, is under the necessity of
repeating, in a more or less modified manner, the chief stages of its
ancestral history. As Marshall happily expressed it,[27] a frog during
its development climbs up its own genealogical tree.
=The metamorphosis.=—Just as the external gills are replaced by
internal gills, so these, in their turn, are replaced by =lungs=,
and advanced tadpoles frequently come to the surface of the water to
breathe air. Limbs have now grown out from the sides of the body, and
the webbed hind feet considerably assist the tail in swimming. Changes
take place in nearly all the internal organs, fitting the animal for
its life on land; and these changes are so extensive that there is
necessarily a short period when the creature is neither tadpole nor
frog, and is incapable of feeding. The tail, however, which would be
useless to the terrestrial, leaping frog, is gradually =absorbed=, and
forms a store of nutriment during the transformation.
The gills shrivel up, and the slits close; the outer layer of the skin
(including the horny jaws) is thrown off, the hind limbs lengthen, and
the animal leaves the water—a frog.
EXERCISES ON CHAPTER XVIII.
1. Make observations upon toads. In what respects do they
differ in appearance from frogs? How do they move about?
2. Are the skins of toads dry or moist? In what situations
have you found toads? How do they protect themselves from
the heat of the summer sun?
3. At what time of the year, and in what places, do toads
lay their eggs? Compare the voice with that of a frog. Do
toads inflate any part of the body when they sing? Compare
the vocal sacs with those of frogs.
4. Look for toad-spawn in the spring. It forms long,
gelatinous ropes in which the eggs are embedded. How large
are the eggs? In what respects do they differ from frogs’
eggs?
5. Keep toads’ eggs in an aquarium, and carefully compare
all the stages of development with those of the frog.
6. Count how many caterpillars you can persuade a toad to
eat “at one sitting.” What would be one result of the
extermination of toads and frogs?
7. Describe carefully what happens when a frog leaps. Point
out the special arrangements which enable a frog to leap
safely. (1897)
8. Describe the ordinary process of feeding of a frog, and
show how it is assisted by the peculiar structure of the
frog’s mouth and tongue. (1895)
9. Explain the special use of the wide gape of the frog.
Where are the teeth of the frog situated? (1898)
10. Mention some remarkable features of the mouth of a frog,
and try to show that they are adapted to meet special
needs. (1901)
11. Describe the process of filling the lungs with air, as
observed in the frog and the rabbit. (1896)
12. What changes take place in a tadpole during the first
week after hatching? Illustrate your answer by drawings. (1898)
13. Describe the appearance of one of the gills of a very
young tadpole as seen by a low magnifying glass. (1901)
14. Describe the appearance, size, and mode of life of a
tadpole about a fortnight after hatching. On what does it
feed? Describe its mouth carefully. (1897)
15. Compare a fresh-hatched tadpole with one in which the
hind limbs have recently appeared. (1898)
16. Trace the history of a tadpole from hatching to the time
when the tail begins to grow less. What does it feed upon
during this time? (1898)
17. Relate the life-history of the frog, from the time of
hatching to the end of the first summer. (1897)
18. Give some account of the habits of a young tadpole a few
days after hatching, especially with respect to locomotion
and feeding. Make a drawing of such a tadpole in side
view, three times the natural length. (1904)
19. Describe the hind leg of a frog, and explain the ways in
which it is used. (1905)
20. Show how a full-grown frog is enabled to live either in
air or in water. (1906)
FOOTNOTES:
[26] _The Cambridge Natural History_, Vol. VIII. (Macmillan.)
[27] Marshall’s _The Frog_ (Smith, Elder & Co.).
CHAPTER XIX. THE HABITS AND LIFE-HISTORIES OF COMMON INSECTS.
62. THE COCKROACH: A TYPICAL INSECT.
1. =Habits.=—In what places have you seen cockroaches? Are they often
to be seen during the day, or do they, in general, come forth only at
night? What is the colour of the body? Put a live cockroach under a
tumbler, and watch its movements. In what position is the head held?
Notice the long _feelers_; how are they used? Look at the lower part
of the head, and try to see the _palps_, which resemble small feelers.
How many legs has the cockroach? Watch the rhythmical movement of the
hinder part of the body. Soak a small piece of bread in milk or water,
and put it under the tumbler; watch the cockroach feed. How does it use
the palps? Notice that the jaws move from side to side. How does the
insect clean itself?
2. =External characters.=—Put a cockroach into a test-tube, and dip
the tube into boiling water; this kills the insect instantly. Notice
the thin “shell” which covers the outside of the animal. Make out that
the body consists of (_a_) _head_, carrying the eyes, feelers, and
jaws; (_b_) _thorax_, carrying the wings and legs; and (_c_) _abdomen_.
Examine them in turn.
(_a_) _The head._—Notice the vertical position, the black,
kidney-shaped _eyes_, the _feelers_, the “upper” lip (_labrum_), and
the small, blackish _mandibles_ at the sides of the labrum. Pass your
knife-point close behind the labrum and into the mouth, and see the
“lower” lip (_labium_), which lies behind the knife, _i.e._ behind the
mouth. Cut off the head and carefully strip off the labium. Put it on a
sheet of paper and examine it with a lens, comparing it with Fig. 222
(_Mx._ 2). Examine the same head from behind with a lens and see the
pair of _first maxillae_. Notice how they are attached to the head, and
then remove one and compare it with Fig. 222 (_Mx._ 1). Work one of
the mandibles backwards and forwards with the point of a pin, and then
remove it and examine it with a lens to see the toothed inner margin.
(_b_) _The thorax._—Notice the overlapping fore-wings (_wing-covers_).
Pull them aside with forceps; notice that they are narrow and rather
stiff, make out their points of attachment at the corners of the second
segment of the thorax, and then cut them off with scissors. Pull out
the delicate _hind-wings_ in the same way, carefully noticing their
fanlike method of folding, and their points of attachment to the
corners of the third segment of the thorax. Stretch one out, cover it
with a piece of glass to flatten it, and then draw it, marking the
lines of folding. (Notice that some cockroaches of the common species
found in kitchens are destitute of wings, and have only very small
wing-covers. These are females.) After removal of the wing-covers and
wings, the three segments of the thorax are very distinct. Notice that
each segment bears a pair of _legs_. Take off one of the legs and draw
it. How many joints has it? What is the use of its bristles?
(_c_) _The abdomen._—Observe the line which runs along the dorsal
middle line of the abdomen; this marks the position of the _heart_.
Examine the method of telescoping of the segments of the abdomen, and
the soft membrane which connects the dorsal and ventral plates of each
segment. Notice that the abdomen bears neither wings nor legs. Observe
the pair of short palp-like bodies (_cer._, Fig. 223) at the end of the
abdomen, and between them, in the male, a pair of more slender styles.
In the female observe the boat-shaped “_brood-chamber_” (Fig. 223,
_st._ 7), on the ventral surface.
(_d_) _The spiracles._—In the thin membrane between the dorsal and
ventral plates, at the junction of two abdominal segments, look for a
small hole (spiracle) leading into the interior of the body. How many
abdominal spiracles can you find on each side? Observe the two larger
spiracles on each side of the thorax, between the first and second, and
the second and third legs.
=What is an insect.=—The word insect is so commonly applied to animals
having no claim whatever to the title, that it is advisable to point
out at once some of the features which distinguish insects from
other animals with which they are often confused. Insects, spiders,
crustaceans, centipedes, and their near relatives, all have jointed
bodies and legs, which are covered by a continuous suit of armour
of a substance called =chitin=. In some cases, as in lobsters and
crabs, this is for the most part hardened by mineral matter to form a
stout shell, being soft and flexible only at the joints where ease of
movement is required. In other cases the layer of chitin may remain
thin and delicate, and all gradations between the two extremes may
be found. Though soft where movement of one part on another takes
place, the chitin is always firm enough, elsewhere, not only to form
a protection for internal organs, but also to afford attachment
to the muscles which move the body and limbs. It is therefore a
=skeleton=, but as it is on the outside of the body it is called an
_exoskeleton_, to distinguish it from the internal skeleton (p. 224)
of the vertebrata. Animals such as insects, spiders, centipedes, and
crustaceans, which have jointed bodies and legs, and are covered by
a chitinous exoskeleton, are called =arthropods=. Insects may be at
once distinguished from all other arthropods by the _single_ pair
of feelers and the _six_ legs. Many of them, but not all, possess
wings—structures which are found in no other arthropods. Insects are
always air-breathers when adult.
=The cockroach.=—The cockroach is not a general favourite, but it
displays so well the essential features of insect structure that it
affords an excellent introduction to the study of the more popular
members of the class, which in many respects are highly specialised. It
is also easily obtained, and of fairly large size.
The body of the cockroach (Fig. 220) is very distinctly divided into
three regions: (1) the head (Fig. 221) which carries the feelers, the
eyes (_ey._) and the jaws (_man._, _max.¹_, and _max.²_); (2) the
thorax, separated from the head by the slender neck, and bearing the
legs and the wings; and (3) the abdomen, which bears neither jaws nor
limbs.
[Illustration: FIG. 220.—Cockroach, male, seen from above. (× 1.)]
The =head= (Fig. 221) hangs nearly vertically from the neck. The large,
compound =eyes= (_ey._) are somewhat kidney-shaped; the long, jointed
=feelers= are set in sockets just below the eyes. The front of the head
is smooth and rounded; hinged on its lower edge is a flap which hangs
down in front of the mouth, and is called the _labrum_ or upper lip.
Just behind the labrum, and attached to the side and front plates of
the head, is the first pair of =jaws=, the _mandibles_ (_man._, Fig.
221; _Mn._, Fig. 222); they work from side to side, and their inner
edges, which bite against each other, are strongly toothed. The second
pair of jaws, called the _first pair of maxillae_ (_max.¹_, Fig. 221;
_Mx._ 1, Fig. 222) is situated behind the mandibles. Each first maxilla
consists of a two-jointed base and two branches; the inner branch bites
against the inner branch of the first maxilla of the other side, while
the outer and longer branch forms the _maxillary palp_, which acts as
a small feeler. The two _second maxillae_ (the third pair of jaws) are
broadly like the first maxillae, but are partially fused to form the
_labium_ or lower lip (Fig. 222, _Mx._ 2), which hangs down behind
the mouth. The outer branches of the second maxillae are called the
_labial palps_ (_Lab. Pa._). It is well to acquire clear notions of the
arrangement of the jaws in the cockroach, in order to understand the
great modification which the mouth-parts of many other insects have
undergone. It is important to notice that the jaws of all arthropods
work from side to side, not vertically as do the jaws of vertebrates.
[Illustration: FIG. 221.—Cockroach. The head and its appendages seen
from the left side. _cerv._, one of the neck-plates; _ey._, eye;
_gen._, side plate of head; _man._, mandible; _max.¹_, first pair of
maxillae; _max.²_, second pair of maxillae (labium). (× 5.)]
The =thorax= consists of three segments, each of which bears a pair
of =legs=, upon which the weight of the resting or walking insect is
supported. It has been found, by instantaneous photography, that in
a walking insect the weight is carried at any instant by the first
and third legs of one side and the second leg of the other side. At
the next step the body is carried by the remaining three legs. In the
American (Fig. 220) and German cockroaches both sexes possess two
pairs of =wings=, which are fixed at the front angles of the second and
third segments of the thorax. The hind-wings, which alone are used in
flight, are folded up fanwise when not in use, and are covered by the
smaller fore-wings, which are generally called the =wing-covers=. In
the common cockroach of this country, only the male has well developed
wing-covers and wings. In the female the wing-covers remain small,
while the wings themselves have disappeared.
[Illustration: FIG. 222.—Jaws of the Cockroach. _Mn._, mandibles; _Mx._
1, first pair of maxillae; _Mx. Pa._, maxillary palp (outer branch);
_La._, _Ga._, inner branch; _Mx._ 2, second pair of maxillae (labium);
_Lab. Pa._, labial palp (outer branch); _La._, _Ga._, inner branch. (×
8.)]
The =abdomen= consists of ten segments, although only eight can be
clearly seen without dissection. The “telescopic” arrangement of the
segments is well shown in Fig. 223. Where one segment joins the next,
the chitin remains thin and flexible. In each segment the chitin forms
a dorsal (p. 217) and a ventral plate, which are joined together at
the sides by a flexible membrane. The chitinous covering of the upper
surface of the abdomen is so transparent that the =heart=, a median
tube, may be seen through it.
=How an insect breathes.=—If the sides of the abdominal segments be
carefully examined, a small aperture will be seen perforating the
thin layer of chitin between the dorsal and ventral plates, at the
point where each segment joins the next; and, in the thorax, larger
but similar openings will be found on each side between the first
and second and the second and third legs. These holes are called
=spiracles= (Fig. 223, _spir._); they lead into a complex system of
air-tubes which ramify throughout the whole system and supply the
organs with oxygen. The tubes are prevented from collapsing by a spiral
lining thread of chitin. This peculiar method of respiration, which
is characteristic of insects, should be carefully contrasted with the
manner of breathing in a rabbit or a man (p. 242) and in a tadpole
(p. 344). It ought to be borne in mind, however, that the essence
of respiration is the same in all living things, and consists in a
replacement of excess carbon dioxide by fresh oxygen (p. 242); the
difference lies merely in the manner of effecting the exchange. In the
rabbit, the blood, carrying the excess of carbon dioxide, is brought to
the air (in the lungs); in the tadpole it is exposed, in the gills, to
the dissolved air of the surrounding water; while in the cockroach the
air is carried directly to the tissues needing fresh oxygen. The air
of the tubes is renewed by a rhythmic action of the abdomen, which can
readily be observed in the living insect.
=The internal organs.=—Figs. 163 and 223, which represent a general
dissection of a frog and of a cockroach respectively, exhibit in the
most striking manner the essential differences in the structure of
vertebrates and arthropods. The fact that the skeleton of the frog is
wholly internal and that of the cockroach wholly external, has already
been mentioned. It will be seen, further, that while in the frog the
central nervous system (the brain and spinal cord) lies entirely dorsal
to the digestive canal, in the cockroach the great nerve chain (Fig.
223, _n_, _n_,) is mainly ventral—the only dorsal part of the central
nervous system being the so called brain (_brn._), which is connected
with the ventral chain by a ring surrounding the gullet (_gul._).
The heart of the frog is ventral to the digestive canal; that of the
cockroach is the most dorsally placed organ of the body.
[Illustration: FIG. 223.—Cockroach; general dissection of female from
the left side. _abd._ 1, first, and _abd._ 5, fifth abdominal segments;
_brn._, “brain”; _cer._, cercus; _c.gl._, glands which form the
egg-case; _cr._, crop; _f._, feeler; _giz._, gizzard; _gul._, gullet;
_int._, intestine; _lab. pa._, labial palp; _mx. pa._, maxillary palp;
_n_, _n_, nerve chain; _r. ov._, right, and _l. ov._, left ovary; _sal.
du._, salivary duct; _sal. gl._, salivary gland; _spir._, spiracles;
_st._ 7, brood-chamber; _th._ 1, first, _th._ 2, second, and _th._ 3,
third thoracic segments. (× 2.)]
=Habits and life-history.=—Cockroaches infest kitchens and pantries;
they are of social habits, and hide together in crevices during the
day, but come forth at night to feed. They are not at all fastidious
as to diet, but are especially fond of starchy foods, which they are
able to digest (p. 234) by means of the fluid formed in their large
salivary glands (Fig. 223, _sal. gl._). The eggs are laid sixteen at
a time in a little oblong case, which the female carries about in a
boat-shaped receptacle (_st._ 7, Fig. 223) at the end of her abdomen,
until she finds a suitable place in which to deposit it. When the
little cockroaches hatch, they are quite white, but except for the
absence of wings they closely resemble the parents in form, and run
about and feed freely from the first. The chitinous exoskeleton is
shed from time to time as the animal increases in size, a new coat, at
first soft and wrinkled, but rapidly stretching and hardening, having
previously formed beneath the old one. Shortly before the last moult,
the wing-covers and wings begin to grow out from the angles of the
second and third thoracic segments. In this manner the young animal
gradually takes on the form and dimensions of the adult.
=The position of the cockroach in the insect class.=—The cockroach
is familiarly known as the “black-beetle,” but the name is a very
misleading one, because the true beetles differ from cockroaches
not only in structure, but also in life history. Cockroaches are
grouped with earwigs, grasshoppers, crickets, and locusts in the
order =Orthoptera=,[28] a term alluding to the fanlike folding of the
hind-wings.
63. A WATER BEETLE.[29]
1. =The habits of Dytiscus.=—Search a pond for water beetles. Put them
in a wide-mouthed bottle with some of the weeds to be found in the
pond, and take them home and observe their habits. Among the larger
beetles—especially from ponds with a clear surface (not covered with
duckweed, etc.)—will probably be seen some with a yellow band round
the edge of the upper surface. These are _Dytisci_. Notice the ovoid,
smooth body, the breadth of the hind legs, and—in the male—the cup on
each fore leg. Try to see how the legs of each pair are used. On what
does the animal feed? Take a small piece of meat in a pair of forceps
and hold it near the beetle’s head. If the animal does not notice it,
stroke one of the feelers with the meat; how does the beetle behave?
What is the use of the feelers? Notice how the beetle rises to the
surface immediately it stops paddling. Which end of the body sticks out
of the water? Why does the beetle need to come to the surface? In April
look for larvae (Fig. 224, _A_), and describe their appearance and
habits.
2. =External characters.=—Kill a Dytiscus by dropping it for a moment
into boiling water. Examine it first from the dorsal surface, noting
the almost unbroken oval outline, and the firmness of the armour.
Notice that the inner edges of the two wing-covers fit closely together
in the middle line. How does the beetle compare in this respect with
the cockroach? Observe the small triangular plate between the anterior
ends of the wing-covers; open out the covers and wings to find of which
segment of the thorax it is a part. Examine the wing-covers and wings
closely; notice that the wings are folded transversely as well as
longitudinally. Spread and pin out one wing-cover and wing of one side,
and draw a dorsal view of the animal. Dissect out the jaws and compare
them with the jaws of the cockroach (Fig. 222). Examine and draw one
leg of each pair.
=The bordered little diver.=—In nearly every English pond there at
times occurs a beetle—an inch or more in length—which is known to
naturalists as _Dytiscus marginalis_. The name is somewhat cumbrous,
but it would be difficult to find a more appropriate one; for “bordered
little diver”—the plain English of the scientific term—indicates
at once the peculiarities which soonest strike the observer: the
creature’s skill in diving, and the yellow band which runs round the
edge of the olive-green body. The outline of the beetle is an almost
unbroken oval, and it is worth noticing that either such a boat-shape
(Fig. 224, _B_), or the cigar-shape of the torpedo is adopted by almost
all actively swimming aquatic animals. The resistance of the water is
further lessened by the smoothness of the beetle’s armour, which forms
a hard shell enclosing the body. As in all insects, the body is divided
into head, thorax, and abdomen. The eyes are large, and are so arranged
that the animal can at the same time see objects both above and below—a
great advantage to a creature living so active a life. The feelers are
very sensitive organs of touch, and possibly of smell also. The jaws
consist of one pair of mandibles and two pairs of maxillae, and are
shaped much like those of the cockroach; they are powerful, and render
Dytiscus a very formidable enemy to the more peaceable inhabitants of
the pond.
[Illustration: FIG. 224.—A beetle of the Dytiscus family. _A_, larva;
_B_, adult insect (male).]
The thorax consists of the usual three segments, but only the first
and a small triangular area of the second are to be seen until the
wing-covers are pulled aside. These last differ from the wing-covers
of the cockroach not only in being much harder and stronger, but also
in their inner edges meeting accurately along the middle line of the
body. The delicate, filmy wings, which alone are of use in flight,
are folded transversely as well as longitudinally. On the lower side
of the thorax are three pairs of legs; they are very interesting and
are worth examining in some detail. In the male, the first leg on each
side is furnished with little circular areas which were at one time
believed to be suckers. It is now known, however, that they give off
a sticky substance which adheres firmly to any object clasped by the
legs. The middle pair of legs seems to be used chiefly for steering. In
both sexes the third or last pair of legs is modified to form a pair
of sculls. Ordinary land-beetles can move their legs in a vertical
direction, as well as in a horizontal one; but the hind legs of
water-beetles are jointed to the thorax in such a manner that they can
only move backwards and forwards, not up and down. The resemblance of
the hind leg to an oar does not stop here, however. On one side there
is a fringe of stiff hairs, forming the blade of the oar. The joint
carrying the hairs is so arranged that the beetle can “feather” its
oar—by turning the edge of the blade to the water—at each stroke.
Occasionally, usually after sunset, the beetle quits his watery
home, and “wheels his droning flight” in search of pastures new. His
flights are, however, only temporary and merely from one pond to
another. Although Dytiscus thus normally lives in water, very cursory
observation only is needed to see that he cannot exist without a
regular supply of fresh air. He no sooner stops paddling than his
body rises naturally to the surface, and as the tail is lighter than
the head, it rises out of the water. The wing-covers are now raised
a little, so that the space between them and the wings is put into
communication with the outside air. The impure gas contained in this
space is soon replaced by a bubble of fresh pure air, the wing-cases
are lowered, and the “little diver” plunges once more into the depths.
The water is prevented by hairs from getting into the air-space below
the wing-cases, and the true wings are thus kept always dry. The
spiracles (p. 355) are in communication with the air-space, so that the
animal is enabled to remain below the surface for a relatively long
time.
=The life-history of Dytiscus.=—In March or April the female Dytiscus
lays her eggs in slits which she cuts in the submerged stems of
pond-weeds, and the eggs hatch in about three weeks. The creature which
emerges from the egg is of active habits, but is not at all like the
parent in appearance. A young animal which leads an independent and
self-supporting life, and differs markedly in structure from the adult,
is called a =larva=. Thus a tadpole is a larval frog, and a caterpillar
is the larva of a butterfly or moth. The larva of Dytiscus when of full
size is about 2 inches in length. Like other larvae of its family (Fig.
224, _A_), it has six slender legs, which serve both for swimming and
for crawling over the bottom of the pond, and its head is provided with
a pair of sickle-shaped mandibles, with which it seizes its prey. Each
mandible is grooved on its inner side, the groove being converted into
a tube by a membrane which covers it in. The savage larva sucks the
blood of its victim until literally nothing is left but the shrivelled
husk. At the end of the Dytiscus larva’s tail are two appendages which
are fringed with hair. When the creature wishes to breathe it comes
to the surface, and the tip of its tail protrudes out of the water.
As each of the appendages just mentioned is pierced with a hole which
leads into one of the two main air-tubes of the body, an interchange of
vitiated for pure air readily takes place.
When the larva is about six weeks old, it leaves the pond and buries
itself in the soil on the banks. Its exoskeleton is shed, and a thin,
transparent layer of chitin—the “pupa-skin”—takes its place. In this
condition the animal sinks into a state of torpor, and apparently
becomes as motionless as a mummy. In this resting stage it is called a
=pupa=. The pupal stage is necessary for the completion of the great
changes—commenced some time previously—which must take place before
the larva can acquire the structure of the adult. The pupal stage lasts
two or three weeks, and when at last the creature emerges from its cell
it is a beetle like its parents.
In consisting of three well-marked stages, the life-history of a beetle
thus differs essentially from that of a cockroach. All beetles agree
with Dytiscus in this respect, though in manner of life almost every
conceivable variation is found. The beetle-order of insects receives
its scientific name—=Coleoptera=[30]—from the sheathing character of
the strong and closely-fitting wing-covers. The wings themselves are
large, and folded in a somewhat complex manner. The mouth parts greatly
resemble those of the cockroach.
64. BUTTERFLIES AND MOTHS.[31]
1. =Cabbage-white butterflies.=—(_a_) _The eggs._—In May or September,
search the leaves of cabbages, turnips, and other crucifers (p. 95) for
the tiny eggs of cabbage-white butterflies. Do the eggs occur singly
or in clusters? Are they found on the upper or the lower surface of
the leaf? Cut off a piece of leaf which carries eggs, put it under
a tumbler, and examine it every day until the larvae (caterpillars)
emerge from the eggs.
(_b_) _The larva._—Have ready a “breeding-cage,” _i.e._ a box
measuring, say, 18” × 8” × 6”, one large face of which is of perforated
zinc or fine wire gauze, and the other of glass; the box should be
without bottom, so that it can be placed over a food-plant. Put the
larvae, with leaves of the plant on which they were found, under the
box, and observe them carefully. Replace all rotted or soiled leaves by
fresh ones. It is best to keep the food-plant in a bottle of wet sand
inside the case; the leaves then remain fresh for several days.
Describe the appearance of the caterpillar; notice its general
worm-like form and absence of wings. In the _head_, observe the small
feelers, and watch the action of the mandibles in feeding. Behind the
head come the three segments of the _thorax_; notice that each bears
a pair of short, jointed legs. How many segments can you see in the
_abdomen_? Which of the abdominal segments bear legs or feet? How do
these differ from the thoracic legs? Look for the spiracles (p. 355) at
the sides of the body. Which segments have spiracles? Kill a full-fed
caterpillar by immersing it in methylated spirit, or by putting it into
a small box with a few drops of ether (_a most inflammable liquid_) or
chloroform. When it is dead, examine it more closely. Try to make out
the mouth-parts clearly; they are best seen from the back of the head.
The labium (p. 353) is here represented by a conical body called the
_spinneret_, out of which come the silken threads used to protect the
pupa.
(_c_) _The pupa._—When the larva is full-fed, it seeks out a sheltered
place, and fixes itself in position by means of silk threads which
issue from the spinneret; the larval skin is shed and replaced by a
pupal skin, and the animal remains quiescent until the change from
larva to butterfly is complete. Examine the manner of attachment of the
pupa to its support. Kill a pupa by immersion in methylated spirit,
or by dipping it for a moment in boiling water; carefully strip off
the skin, and make a drawing of the animal. Especially notice the
arrangement of the wings and legs.
(_d_) _The imago or winged insect._—When the internal organs of the
butterfly are completed, the pupal skin splits, and the perfect insect
comes out. Look for pupae between September and March, or in July,
and try to see the transformation. Notice the method of flight of
the butterfly, and the position in which the wings are held when it
settles. At what time of the day, and in what kind of weather, have
you seen cabbage-whites flying? Kill a butterfly by putting it into a
bottle containing crushed laurel leaves, or a few lumps of potassium
cyanide (_a deadly poison_) wrapped in blotting paper, and examine it
closely.
(i) _The head._—Notice the large, compound eyes, the
knobbed feelers, the long and coiled _proboscis_, and the
labial palps (which look like tusks).
(ii) The thorax.—The true form of the thorax and abdomen
is concealed by the hairs which clothe the body. Wet the
body with methylated spirit to make the hairs lie down.
Notice that the first and third segments of the thorax are
very small, while the middle segment, which carries the
fore-wings, is large. Observe that the fore-wings are not
modified into wing-covers, but are, generally speaking,
much like the hind-wings. Determine the sex of the specimen
by means of Fig. 225. Notice that, when you touch a wing,
a little white dust comes off on your finger; the dust
consists of very small scales. Do both surfaces of the
wings bear scales? With a rather stiff camel-hair brush,
brush the scales from the wings of one side. What markings
have been removed, and what new markings have been made
clear, by the removal of the scales? How many legs has the
butterfly?
(iii) _The abdomen._—Count the segments.
2. =The tiger moth.=—In early summer examine lettuce, strawberry, and
nettle leaves for “woolly bears”—the hairy caterpillars of the tiger
moth. Keep these in the breeding-cage with the food-plants, and observe
and describe their appearance and habits. How do they behave when
alarmed? Watch them spin their cocoons (pupa cases) about the end of
June, and describe the process. Examine the pupa, and state in what
respects it differs from the caterpillar. About the end of July watch
for the emergence of the perfect moth.
Notice the position of the wings of a resting moth. Are they held like
those of a butterfly? Kill and examine the moth, and compare it further
with a butterfly. Especially notice
(_a_) That the feelers of the tiger moth are not knobbed at the
ends, but are either thread-like (in the female) or comb-like (in
the male); and
(_b_) That a bristle at the base of the hind-wing hooks on a
catch in the fore-wing of the same side.
3. =The vapourer moth.=—About the end of June examine rose trees, fruit
trees, willows, oaks, etc., for caterpillars of the vapourer moth.
They may be recognised (Fig. 228) by the reddish warts and the tufts
of hair which stand out from various parts of the body. Notice that
the caterpillars fall into two groups according to size, being, when
full-grown, about 2 in. and 1¼ in. long respectively. Keep the larvae
in a breeding-cage until they pupate and change into moths; notice
that the moths (the females) which come from the large caterpillars
have no wings; while the male moths, which are derived from the small
caterpillars, have well-developed wings.
=A typical butterfly.=—The great beauty of butterflies and moths, and
the ease with which the stages of their wonderful life-history can be
followed, have made these insects favourite objects of study among
naturalists of all ages.
Among the commonest of butterflies are the well-known =cabbage-whites=,
the caterpillars of which work so much havoc upon crops of cruciferous
plants (p. 95). The eggs are laid in May and September upon the leaves,
and soon hatch out into small larvae called caterpillars, which feed
voraciously and grow rapidly. The larval skin is shed from time to time
as it becomes too small. The =caterpillar= (Fig. 225, _A_) is somewhat
worm-like in appearance, but insect characters may easily be recognised
in it. The _head_ is small and shiny. It carries six eye-spots, but
the large compound eyes of the adult are not yet visible. A pair of
short feelers is present; and the mouth-parts are obviously comparable
with those of the cockroach, although the labium (p. 353) has become
converted into a _spinneret_, which gives out the silk threads used for
the protection of the pupa. The mandibles, which are used in gnawing
leaves, are stout and toothed; the first maxillae are rather small.
Behind the head come the three segments of the _thorax_, each of which
bears a pair of short jointed legs; wings have not, as yet, developed.
The _abdomen_ really consists of ten segments, although only nine
can be seen without dissection. Segments 3 to 6 of the abdomen bear
short, unjointed legs called _pro-legs_ or _cushion feet_, and the last
segment bears a pair of appendages called the _anal feet_. Spiracles
are to be seen on the first eight abdominal segments. In the larva of
the green-veined cabbage-white, each spiracle is reddish and surrounded
by a yellow border.
[Illustration: FIG. 225.—Cabbage White Butterfly. _A_, larva; _B_,
pupa; _C_, perfect insect. (All × ⅔.) (_B_ and _C_ from photographs by
Mr. A. Flatters)]
When the caterpillar has attained its full size, it stops feeding and
seeks out a sheltered place—often a chink in a wall. Silken threads are
given off by the spinnerets until a little heap of silk is formed into
which the hooked end of the abdomen is fixed. Then a girdle of silk is
made, passed round the thicker fore-part of the body, and so attached
to the wall that the animal is supported in an upright position. The
larval skin now splits and is peeled off, and the =pupa=, or chrysalis,
stage (Fig. 225, _B_) is entered upon. All the external parts of the
butterfly are complete at the time of pupation, but profound changes
are still necessary in the internal organs; it is to allow these
changes to take place in tranquillity that the resting, or pupal, stage
is interposed between larval and adult life.
At last the new organs are ready for their work, the pupal skin cracks,
and the =perfect insect= (Fig. 225, _C_) emerges. The head now
carries a pair of large compound eyes, and two slender feelers with
knobbed ends. The jaws also have been remodelled in accordance with
the completely different manner of life upon which the insect is now
entering. The mandibles are now only doubtfully recognisable; the first
pair of maxillae are elongated and grooved, and are closely applied
to each other to form a long tube called the _proboscis_ (Fig. 226,
_Mx._ 1), which, when not in use for sucking up the sweet juices of
flowers, is kept coiled up beneath the head; the palps of the labium
(_Lab. Pa._) project like tusks on the sides of the head. The thorax
is provided with two pairs of broad wings, which are covered with
minute overlapping scales—forming a delicate “bloom” which is readily
detached by rough handling. In many butterflies and moths the scales
are gorgeously coloured and arranged in symmetrical patterns. The name
=Lepidoptera=,[32] which is applied to this order of insects, was
suggested by the scaly covering of the wings. When the butterfly is
at rest the wings are either fully expanded horizontally, or are held
vertically over the back, the upper surfaces of the fore wings being in
contact. The first segment of the thorax, which bears the first of the
three pairs of legs, is greatly reduced; the third segment also is but
small.
[Illustration: FIG. 226.—_A_, Head of a Lepidopterous insect; _B_, the
labium; _Ant._, feeler; _E_, eye; _Lab. Pa._, labial palp; _Mx._ 1,
proboscis; _Mx. Pa._, maxillary palp (magnified).]
In short, the whole organisation of a butterfly is definitely adapted
to the special duties which belong to this period of its life. The
growing stage is over; the sole object of life is now to seek a
mate and—in the case of the female—to lay the eggs in a place where
the future larvae may find plenty of food. Ease of locomotion and
conspicuousness are secured by the broad and brilliantly-coloured
wings; the peculiar manner of flight is considerable protection against
the attacks of birds; large eyes aid in the recognition of the mate;
and a concentrated and easily-digested food is supplied by the nectar
of flowers, and made accessible by the long, sucking proboscis—the
service of cross-pollination (p. 92) often being unconsciously rendered
in return for the sweet draught.
The common species of cabbage-white butterflies spend the winter as
pupae; the perfect insects emerge in April, lay their eggs, and then
die. The caterpillars pupate and a second generation of butterflies
appears, their offspring reaching the pupa stage about the end of
autumn.
=Moths.=—Moths pass through a life-history which is identical, in its
broad features, with that of butterflies. The larva which hatches from
the egg is a caterpillar, whose life is spent in feeding and growing.
At the same time the external features of the adult are gradually
taking form under the skin. When at last the full larval size is
attained, and a resting stage is necessary for the perfection of the
internal organs, the caterpillar’s skin splits, and is shed, and the
animal becomes a pupa or chrysalis. A moth-pupa is generally somewhat
egg-shaped (Fig. 227), whereas the pupa of a butterfly is usually
conical, though there are many exceptions.
The winged moth, which at length emerges from the pupal skin, differs
from a butterfly in certain obvious respects. Its body is usually broad
and thick; its feelers are either comb-like or thread like, not knobbed
at the ends; the two wings of one side are in most cases secured
together at the base by one or more bristles on the hind-wing hooking
over a catch on the fore-wing; in rest, the wings usually slope and
are not fully extended. Whereas butterflies usually fly only in the
sunshine, moths often fly by night, and the flowers which night-flying
moths frequent for nectar are as a rule white and strongly-scented,
and close during the day. In finding their mates, moths seem to depend
largely upon the sense of smell, which is probably lodged in the
feelers.
[Illustration: FIG. 227.—Stages of Tiger Moth. _A_, Caterpillar, from
left side; _B_, pupa (removed from cocoon), ventral view; _C_, perfect
insect (female). (From a photograph by Mr. A. Flatters.) (× ⁵/₇.)]
The life-history of a typical moth is well exemplified by the =Tiger
Moth= (Fig. 227), which is easily reared in captivity. The larva—often
called the “woolly bear,” from its thick covering of hair—may be found
in early summer on the leaves of lettuce, strawberry, nettle, and other
plants. It pupates about the end of June, working its hair, together
with silk spun by the spinneret, into a _cocoon_, in which the resting
stage is passed. About a month later the perfect moth emerges; its
fore-wings are beautifully mottled with cream colour and chocolate
brown; the hind-wings are red, with metallic violet spots. The feelers
of the male are comb-like, and are probably very sensitive organs of
smell, by means of which he seeks out his mate. The female’s feelers
are thread-like.
In the =Vapourer Moth= (Fig. 228), whose “looping” flight may often be
observed even in the streets of towns during the day, the two sexes
are remarkably different from each other. The male (_C_) alone can fly;
the female (_D_) is wingless, and is confined for the whole of her
short adult life to the place where she emerged from the cocoon. Here
she lays her eggs and then dies. Neither she nor her mate is capable of
feeding.
[Illustration: FIG. 228.—Stages of Vapourer Moth. _A_, Larva (male);
_B_, male pupa; _C_, male moth; _D_, female moth. (From a photograph by
Mr. A. Flatters.) (× ⅚.)]
It would be difficult to find a more striking example of the fact that
the one duty of the adult moth is reproduction. The female vapourer
is even debarred from the privilege of choosing favourable places for
her eggs; but a compensation for this disadvantage lies in the agility
of the larvae (_A_), which are able to migrate without difficulty to
another plant whenever food becomes scarce. As Prof. Miall remarks:[33]
“Whatever the larva can do for itself, the female can safely leave
undone; but what the larva cannot do, by reason of sluggishness or
restricted diet, the parent must provide for. Hence activity and
intelligence in the one lead to degeneration in the other.... Wings are
to insects what spores are to ferns, plumed seeds to dandelions, and
hooked seeds to burrs—a ready means of dispersal.”
=Other insects.=—Space does not permit of more than a reference to
other insects, and the work of this chapter is to be regarded as the
merest introduction to the study of this fascinating class of animals.
The chief remaining orders are the =Neuroptera=, including May flies,
dragon flies, and caddis flies; the =Hemiptera=, among which are
included the various “bugs,” water boatmen, plant lice, etc.; the
=Diptera= (two-winged flies), such as the house-fly, gnat, harlequin
fly, daddy-long-legs, etc.; and the =Hymenoptera=, including bees,
wasps, ants, gall flies (p. 146), and ichneumons. Many of these
insects have aquatic larvae which may be found in ponds; and their
life-histories should be studied in aquaria and careful notes made of
the transformations.[34]
EXERCISES ON CHAPTER XIX.
1. Examine the following animals, and find out (_a_) which
are arthropods, (_b_) which are insects: tortoise, spider,
grasshopper, lobster, earwig, centipede. Give reasons for
your conclusions.
2. Keep a grasshopper under a tumbler with a small sod of
grass. Observe its habits, and find out how it “chirps.”
Compare its structure with that of the cockroach.
3. Compare a cockchafer with a water-beetle. In what order
of insects would you place the cockchafer, and why?
4. Compare other water-beetles with Dytiscus, and try to
trace their life-history.
5. Observe the habits and examine the structure of the
water-boatman. What reasons can you find for excluding it
from the beetle-order?
6. Examine a daddy-long-legs, and try to find the two
stumps which are all that remain of the hind-wings.
7. Look for “blood worms” (larvae of the harlequin fly) in
horse-troughs and sluggish streams in summer; keep them in a
saucer of water with a few dead leaves. Observe their habits
and describe the appearance of the pupa. What kind of insect
emerges from the pupa?
8. Keep caddis-worms in an aquarium and describe their habits.
9. Examine the leaves of stinging nettles for caterpillars
in June, and try to rear butterflies from them. Carefully
notice from which kind of caterpillar each butterfly is
derived.
10. Look for caterpillars of the Privet Hawk Moth on
privets and lilacs on August and September evenings. Keep
some, with earth and twigs of the food plant, in a covered
flower-pot, and observe their method of pupation.
11. Compare the colouration of the wings of butterflies
and moths with that of the plants they most frequent, and
describe any cases of protective colouration which you find.
FOOTNOTES:
[28] Greek: _orthos_, straight; _pteron_, a wing.
[29] See Footnote, p. 372.
[30] Greek: _koleos_, a sheath; _pteron_, a wing.
[31] Living eggs, larvae, and pupae of Lepidoptera may be obtained from
Mr. H. W. Head, Burniston, near Scarborough, and other dealers.
[32] Greek: _lepis_, a scale; _pteron_, a wing.
[33] _Injurious and Useful Insects_ (Bell).
[34] Miall’s _Injurious and Useful Insects_ (Bell), and _Natural
History of Aquatic Insects_ (Macmillan), are strongly recommended as
guides to further work.
CHAPTER XX.
SOME CRUSTACEANS, MOLLUSCS, AND WORMS.
65. SOME COMMON CRUSTACEANS.
1. =The crayfish and lobster.=—(_a_) _Habits._—Readers in limestone
districts will probably be able to find crayfishes in the streams,
and the habits of the animals in their natural surroundings should
be observed and described. Other readers will be able to obtain live
specimens from dealers.[35]
Place the animal on the table or desk. Notice that the body consists of
an anterior, unsegmented portion, the _cephalothorax_ (corresponding
to the head _plus_ the thorax of an insect), and a posterior, jointed
_abdomen_. Watch the movements of the stalked eyes, the feelers and
legs; allow the claws of the largest pair of legs (the “pincers”) to
grasp a pencil. Put the crayfish in a white dish, with an inch or two
of water, and by means of a pipette discharge a few drops of water
containing some suspended colouring matter, such as carmine or indigo,
near the point of attachment of the last walking leg. Describe the
movements of the coloured water. When the animal is startled, notice
the sudden vertical flexure of the abdomen, by means of which the body
is pulled backward in the water. Feed the crayfish with meat or worms,
and try to see the action of the jaws.
(_b_) _External characters._—Kill the crayfish instantaneously by
dropping it into boiling water. Compare the animal with a cockroach (p.
349). Examine more closely the stalked eyes, the two pairs of feelers,
the _four_ pairs of walking legs and the single pair of large pincers,
and the fusion of the head and thorax to form the cephalothorax. Count
the segments of the abdomen.
(_c_) _The appendages._—Examine the ventral surface of the abdomen,
and notice that each segment, except the last, bears a pair of small
jointed organs; these are called _swimmerets_. Remove one of the pair
carried by the last segment but two, examine and draw it; make out that
it consists of a stalk and two branches. Notice that the branches of
the swimmerets of the last segment but one are expanded, and form, with
the last segment, the _tail fin_.
Study the appendages of the cephalothorax from behind forwards. They
consist of (i) four pairs of legs used for walking, and a larger pair
of pincers; (ii) three pairs of foot-jaws or _maxillipedes_; (iii) two
pairs of _maxillae_; (iv) one pair of _mandibles_; (v) two pairs of
_feelers_.
(_d_) _The gills._—With strong scissors cut off the side part of the
exoskeleton of the cephalothorax, and notice the plume-like _gills_ in
the _gill-chamber_ thus laid open. Move the adjoining legs, and see
that some of the gills also move.
2. =The crab.=—Obtain a crab and compare it, point by point, with
the crayfish, or lobster. Notice the great width of the shell of the
cephalothorax; much of this width is due to the gill-covers, which
stand out from the sides of the true body. Look for the opening of the
gill-chamber at the base of the pincers. Make out the stalked eyes (in
sockets), the two pairs of feelers, and the five pairs of legs. Notice
how the animal runs; in what respects does the method differ from the
gait of the crayfish? Examine the ventral surface of a dead specimen,
and notice that the abdomen is tucked under the cephalothorax. Stretch
out the abdomen; compare it with that of the crayfish, and count the
segments. Notice the absence of the tail fin; it is not required,
as the crab does not swim. Extend the abdomen, and make drawings of
the animal (i) from above, and (ii) from below, and label the parts.
Carefully remove the foot-jaws and the true jaws, and compare them
with the corresponding appendages of the crayfish. With strong
scissors cut off one of the gill-covers, and examine the gills. Observe
that in the crab the gills are not in any case attached to appendages;
they all spring from the sides of the body-wall.
Tabulate the respects in which you have found the crayfish and crab (i)
to agree with, (ii) to differ from, insects.
[Illustration: FIG. 229.—Crayfish, seen from the right side. The
appendages are numbered in Arabic and the abdominal segments in Roman
numerals. _ant._ 1, first, and _ant._ 2, second, feelers of the right
side; _c.th._, cephalothorax; _E_, eye; _g.c._, gill-cover. (× ⅔.)]
The student who has worked through Chapter XIX. will at once recognise
in a crayfish, or a lobster, an animal possessing many features in
common with the insects, and will find it interesting to try to
discover for himself why it is not called an insect, but is placed by
naturalists in a different class.
=The crayfish.=—The crayfish (Fig. 229), or the lobster (which agrees
closely in structure with the crayfish), is obviously an =arthropod=
(p. 351), for it is covered by an exoskeleton of chitin, and has hollow
jointed limbs and a segmented body. In these respects it agrees with
the insects. On the other hand, it plainly possesses at least five
pairs of legs, and has two pairs of feelers (_ant._ 1 and _ant._ 2,
Fig. 229), whereas no insect has more than three pairs of legs (when
adult), or more than one pair of feelers.
[Illustration: FIG. 230.—Crayfish; the right gill-chamber and gills as
seen after removal of the right gill-cover. _a₁_, _a₂_, feelers; 6-13,
sixth to thirteenth appendages; xiv, xv, first and second abdominal
segments; _E_, eye; _ep._ 5, scoop on second maxilla; _g.ch._,
gill-chamber; _pdb._ 7, gill attached to seventh appendage; _pdb._ 12,
gill attached to twelfth appendage; _plb._ 13, gill on body-wall above
thirteenth appendage. (Slightly reduced.)]
These differences are apparent at the first glance, and closer
examination reveals even greater contrasts. The three primary divisions
of the body into head, thorax, and abdomen, which are so characteristic
of insects, are not obvious in the crayfish; for the head and thorax
are here fused into one mass, the =cephalothorax=[36] (Fig. 229,
_c.th._), which is covered by a shield called the =carapace=. Moreover,
in the crayfish _every_ segment except the last bears a pair of
=appendages=, which vary in form in different regions of the body
according to their duties, but which can all be shown, by careful
comparison, to be modifications of one primitive form, which is
=Y=-shaped and consists of a basal stalk and two branches. This form
of appendage is well seen in the _swimmerets_ (17, Fig. 229) of the
abdominal region; further forwards the appendages become _walking legs_
(9-13, Fig. 229); next come three pairs which combine the characters
of legs and jaws, and are called _maxillipedes_ (8, Fig. 229); then
are the true jaws—two pairs of _maxillae_, and one pair of biting and
crushing _mandibles_; and lastly, in front of the jaws, two pairs of
_feelers_ (_ant._ 2 and _ant._ 1). It is believed that the first pair
of feelers corresponds to the single pair of feelers of the cockroach;
that the jaws correspond to the jaws; that the maxillipedes of the
crayfish are the equivalents of the walking legs of the cockroach;
while the remaining appendages of the crayfish have no representatives
in the insect.
Lastly, the crayfish differs essentially from the insect in its
method of =respiration= (p. 355), for it is an aquatic animal and
breathes _dissolved_ oxygen. It therefore possesses neither lungs nor
spiracles but =gills= (Fig. 230). These are situated at the sides of
the true body-wall, in gill-chambers formed by the downgrowth of the
sides of the carapace. The gills are delicate plumes, containing fine
blood-vessels, so that an exchange of gases readily takes place between
the blood and the surrounding water. On each side, a scoop (Fig. 230,
_ep._ 5) on the second maxilla is continually baling water out of the
front of the gill-chamber, fresh water flowing in from behind to take
its place. Certain of the gills are attached to the legs, so that the
motion of the legs also is some assistance to respiration.
[Illustration: FIG. 231.—Crab; _A_, from above, _B_, from below, _ant._
1, first, and _ant._ 2, second feelers; _abd._ 3, third, and _abd._ 7,
seventh abdominal segments; _E_, eye; _l._ 1, pincers; _l._ 5, last
walking leg; _mxp_, third maxillipede. (× ⅙.)]
=The crab= (Fig. 231) is markedly different in shape from the crayfish
or lobster, but is nevertheless easily seen to be built on essentially
the same lines of structure. It also consists of twenty segments,
of which the first thirteen are fused to form a cephalothorax; and
the appendages of this region are quite comparable with those of
the crayfish. The great width of the shell is largely due to the
gill-covers, which stand out from the sides of the body much further
than do those of the crayfish. As a result the crab finds it easy to
run side-first. The crab is essentially a walking, not a swimming,
animal, and the abdomen—upon which the crayfish and lobster so much
depend in swimming—is in the crab reduced in size and kept tucked under
the cephalothorax.
[Illustration: FIG. 232.—The Wood-louse. (× 1.)]
=Crustaceans.=—These facts are sufficient to show that the title
“insect” cannot with any propriety be given to either the crayfish
or the crab, unless, indeed, the term is to be applied to all
arthropods indiscriminately. The crayfish, lobsters, shrimps, prawns,
crabs, barnacles, and many less-familiar animals, are placed in the
=crustacean= class of arthropods. Crustaceans have usually a distinct
head, thorax, and abdomen; but some of the thoracic segments may be
fused with the head to form a cephalothorax (p. 375). Like other
arthropods, the animals are covered with an armour of chitin, and in
many cases this is so hardened, except at the joints, by mineral matter
that it becomes a rigid shell or crust. The head bears two pairs of
feelers in addition to the jaws; and the segments of the thorax and
abdomen are provided with appendages which are variously modified as
jaw-feet, legs, swimmerets, etc. Typically, the animals breathe by
gills and are aquatic, but forms are known which are able to live
comfortably on land if the gills are kept moist. One of the most
interesting examples of this is found in the common =wood-louse= (Fig.
232), which lurks under stones and logs in damp and dark situations,
and breathes by plate-like gills on the abdominal segments.
=Other arthropods.=—Other familiar arthropods, which are neither
insects nor crustaceans, are spiders and centipedes. The spiders belong
to the =arachnid class=, and the centipedes to the =myriapod class= of
arthropods.
66. SOME COMMON MOLLUSCS.[37]
1. =The fresh-water mussel.=—(_a_) _Habits._—Study the habits of the
living animal (Fig. 233) in the aquarium. Notice the muscular _foot_
which is protruded between the halves (_valves_) of the shell, and by
means of which the mussel slowly makes its way along the sandy bottom.
With a pipette, discharge some water, coloured with carmine or indigo,
close to the more pointed end of the shell. Where is the coloured water
taken in, and where is it expelled? Notice that the shell closes when
the animal is handled.
(_b_) _The regions of the shell._—The rounded end is anterior, the more
pointed end posterior, the straight hinge-line dorsal, and the gape
ventral. Notice the knob-like _umbo_ on each side near the anterior
end of the hinge; it is the oldest part of the valve. Make out the
concentric lines of growth, showing the successive positions of the
margin. Draw the shell from the right side and from above.
(_c_) _General structure._—Kill the mussel by putting it for a few
minutes into hot, but not boiling, water. Notice that the valves of
the shell now gape apart somewhat. Hold up the animal and see, lining
the shell, a thin soft membrane, the _mantle_. Carefully separate
the mantle from one valve and notice, near each end, a thick white
pillar which passes from one valve to the other. These pillars are the
_closing muscles_. Pass the blade of a knife between the mantle and
valve, and cut through each closing muscle quite near to the valve.
Turn the valve back and remove it from the other valve by cutting
through the elastic _ligament_ (at the hinge) with strong scissors.
Clean the inside of the separated valve and examine it; notice (i) the
line of attachment of the mantle; (ii) the impressions of the closing
muscles; (iii) the lines of shifting of the closing muscles—triangular
depressions stretching from the umbo to the muscles.
Examine the animal as it lies in the other valve. Make out: (i) The
right and left lobes of the _mantle_ which line the valves and (in the
natural position of the mussel) hang down from the sides of the body;
(ii) the two plate-like _gills_ on each side, lying between the mantle
and the foot; (iii) the median _foot_; (iv) the reddish, triangular
_palps_ surrounding the _mouth_—an aperture at the anterior end of the
foot. Push a stout pin into the mouth and upwards into the gullet. Make
a drawing showing the relative positions of the parts.
2. =The garden snail.=—(_a_) _Habits._—Where have you seen snails? At
what time of the year are they most active? Upon what do they feed?
Place a snail upon a lettuce leaf and carefully watch its method of
eating. How does it move about? Put a live snail upon a sheet of glass,
and look through the glass to see the wave-like action of the flat
sole—the _foot_—by means of which it moves.
Have you ever found snails in winter? How much of the animal was
visible? How is the mouth of the shell closed in winter?
(_b_) _General appearance._—Make a drawing of the animal from the
left side and another from the right side, and notice the differences
between the two. Is the shell placed over the middle of the animal, or
does it lie to one side? How many turns has the spiral of the shell? In
a fully expanded snail observe the fleshy “collar” round the margin of
the shell; it is the edge of the _mantle_. Notice the rounded _head_,
the two pairs of _tentacles_, the _eye-spot_ at the tip of each of the
larger and upper tentacles, the _mouth_, and, near the base of the
shell on the right side, the _respiratory pore_, which opens into the
_lung_. Touch various parts of the body in turn to see if they are
irritable; how are the tentacles retracted? Can the animal close and
open its respiratory pore at will?
3. =An air-breathing pond-snail.=—Obtain several fresh-water snails and
study their habits in glass aquaria. The commonest fresh-water snails
are species of _Limnaea_; identify some of these by comparison with
Fig. 234, _A_, and try to make out the parts already seen in the garden
snail. Watch the action of the mouth as the animal feeds on the green
scum which often collects on the sides of aquaria. Notice how it creeps
over the glass or along the surface-film of the water. When the snail
comes to the surface it replaces the air of its lung by fresh air,
and a bubble may often be seen escaping from the respiratory opening
under the lip of the shell. Look for the spawn (_egg-masses_) of this
snail on leaves or on the sides of the aquarium, and examine the eggs
frequently with a hand lens.
[Illustration: FIG. 233.—Fresh-water Mussel. _A_, from left side; _B_,
from behind. _ft_, foot; _in. sph._, inhalant siphon; _ex. sph._,
exhalant siphon; _lg_, ligament; _m_, mantle; _um_, umbo. (× ⅔.) (After
Howes.)]
=Molluscs.=—Molluscs are soft-bodied animals, in most cases protected
by a hard, external shell, but they differ essentially from crustaceans
and all other arthropods in not being segmented, and in not possessing
jointed limbs. Familiar and instructive examples of two classes of the
group are found in the fresh-water mussel and the garden snail.
The =fresh-water mussel= (Fig. 233) is to be found in streams, along
the bed of which it ploughs its way by means of a muscular =foot=
(_ft._). The body is enclosed in a brown shell, which consists of
halves called =valves=, hinged together along the straight, dorsal
edge by an elastic =ligament= (_lg._). The action of the ligament is
to separate the valves slightly unless they are forcibly held together
by the contraction of closing muscles which run from one valve to
the other. Hence the shell of a dead mussel always gapes open. The
rounded end of the shell is anterior, the more pointed end posterior.
The oldest part of each valve is the =umbo= (_um._, Fig. 233), a
knob just in front of the ligament; and concentric lines surrounding
the umbo mark successive positions of the margin of the valve as the
animal increased in size. The valves are formed by the activity of the
=mantle lobes= (_m._)—a pair of delicate membranes which hang down from
the sides of the body. The foot is a median prolongation of the body
itself, and on each side a pair of plate-like =gills= lies between the
foot and the mantle lobe of that side. “Thus the whole animal has been
compared to a book, the back being represented by the hinge, the covers
by the valves, the fly-leaves by the mantle lobes, the first two and
the last two pages by the gills, and the remainder of the leaves by the
foot.”[38]
When the living mussel is undisturbed, the mantle folds project
slightly at the hinder end of the shell, their edges being so placed
in contact that they form two short tubes. A current of water flows in
at the lower of these (_in. sph._, Fig. 233), carrying to the mouth
a supply of food-particles, and to the gills and mantle a store of
dissolved oxygen; while an outgoing current leaves by the upper tube
(_ex. sph._).
The =garden snail= (Fig. 234, _B_) seems at first sight to have but
little resemblance to a mussel; but it also is a mollusc—consisting of
a soft, unsegmented body, which is produced ventrally into a =foot=,
and is protected by a =shell= formed by the activity of a =mantle fold=
of the body. In this case, however, the shell is one piece, and is
spirally coiled. The snail has also a distinct =head=, which bears
two pairs of tentacles; at the tip of each of the longer and upper
tentacles (_t_) is an eye (_e_). The animal crawls about by wave-like
contractions of the muscular foot; it feeds upon vegetation, which
it rasps into small particles by means of a toothed tongue, and then
swallows. The snail is entirely adapted to a terrestrial life, and
breathes air—the mantle fold under the shell enclosing a =lung chamber=
with blood-vessels in its walls, which opens to the exterior by a
respiratory pore (_p.o._, Fig. 234) on the right side. The snail spends
the winter, in a state of torpor, under logs or stones, the body being
entirely retracted into the shell, the mouth of which is closed by a
plate of hardened slime.
[Illustration: FIG. 234.—_A_, A Fresh-water Snail (_Limnaea_); _B_,
Garden Snail. _e_, _e_, eyes; _p.o._, respiratory pore; _t_, _t_,
tentacles, (× 1.)]
[Illustration: FIG. 235.—Slug, _p.a._, respiratory pore. (× 1.)]
=Slugs= (Fig. 235) are of very similar structure, but in them the shell
has almost disappeared, even the trace which remains being concealed by
the mantle fold.
Among the commonest of fresh-water snails are various species of
=Limnaea= (Fig. 234, _A_). They may be found abundantly in ponds,
and are often kept in aquaria, where they perform a useful service
by devouring the minute plants which are apt to accumulate to an
undesirable extent and form green scum on the sides. These snails,
like the garden snail, breathe air, and often come to the surface to
take a fresh supply into their lung chambers. Some other water snails,
however, breathe _dissolved_ oxygen by means of gills beneath the shell.
67. EARTHWORMS.
1. =External characters.=—Dig up several earthworms from the soil and
examine them. What is the _length_ of the largest and of the smallest
specimens? What is the _thickness_ of the body? Watch the worms
crawling about, and describe the method of locomotion. Has the animal
any legs? Do the length and thickness of any one worm vary during its
movements? Is the variation connected with locomotion? How? Can you
distinguish between the fore (anterior) and hind (posterior) ends, and
between the upper (dorsal) and lower (ventral) surfaces? How do they
differ?
Kill a large worm by immersion in methylated spirit and examine it more
closely. Notice the segmented character of the body, and estimate the
number of segments. Which part of the body has the largest segments?
Observe the swollen appearance of segments 32 to 37; this region is
called the _clitellum_. Notice the _mouth_ (overhung by a short lobe)
in the first segment, and the _vent_ in the last segment. On the
ventral surface of segment 14 try to see a pair of small pores; these
pores are the openings through which the _eggs_ are discharged from the
body. Pull the worm gently between your fingers, and notice the bristly
feel; in which direction of motion is this most apparent? Examine the
ventral surface with a strong lens in a good light, and try to see four
double rows of very small _bristles_.
2. =Habits.=—Examine the surface of the ground of a garden or lawn at
night by help of a lantern or lamp, being careful to tread lightly,
and observe the actions of any worms you see. Are more worms to be seen
at night than during the day? Do the worms seem disturbed by the light
of the lantern? Try to grasp one; is it easily caught? Why not? Where
does the worm retreat? If you can grasp a worm before it has time to
withdraw completely into its burrow, observe the difficulty of drawing
it out without tearing it. Try if the worms are disturbed by a loud
shout (be careful not to blow upon them when shouting), or by a heavy
stamp of your foot.
Carefully lay open several _burrows_ and notice:
(_a_) Whether the mouths of the burrows are plugged in
any way, and, if leaves are used for this purpose, whether
the leaves have been dragged into the hole (i) by the broad
ends, (ii) by the narrow ends, or (iii) by the sides; do you
find any signs of intelligence in the method adopted?
(_b_) The length and width of the burrow and the character
of its lining.
(_c_) The end of the burrow; is it enlarged?
Examine _worm castings_. Mark out a square yard of surface and collect,
dry, and weigh all the castings found on this area in a certain time,
say a month. Such observations should be made at different periods
of the year, and over different kinds of soil, and comparisons made.
Estimate the weight brought up per acre by worms during a year. Are the
particles composing the castings very fine, fine, or coarse?
Place worms in glass-covered flower-pots with earth of different
degrees of firmness, and observe the methods of burrowing in loose and
in firmly compacted earth respectively. Place pieces of leaf of carrot,
onion, and cabbage on the surface, and at night observe how the worms
grasp the pieces and drag them into the burrows.
=Earthworms.=—Few people except naturalists have any idea of the vast
number of earthworms living in the surface soil of this and most other
countries, or of the importance of the work which they do.
The common earthworm (Fig. 236) is much simpler in structure than any
of the animals previously considered in this book. It is roughly
cylindrical in shape, though somewhat flattened on the ventral surface.
It is divided into about 150 segments, which are marked on the exterior
by grooves running round the body. The mouth is an opening in the first
segment (Fig. 236, 1) and is overhung by a short fleshy lobe. The worm
crawls along by alternate elongation and shortening of its body, being
aided by short bristles which are directed backwards and act as pivots;
limbs are entirely absent. The animal is very sensitive to touch,
even to the vibrations of the ground; but it is stone-deaf, only just
capable of distinguishing between light and darkness, and has very
little sense of smell.
[Illustration: FIG. 236.—Earthworm, seen from right side. 1, 15, 33,
first, fifteenth, and thirty-third segments. (× ½.)]
During the day, the earthworm generally remains in its burrow in the
soil, with its head just inside the entrance. Its method of forming the
burrow depends upon the texture of the ground. In loose soil the earth
is simply pushed aside, but where the material is too compact for this,
the animal actually eats its way through. The burrow is lined with
soft earth or little stones, and is plugged at the mouth with leaves
or other convenient objects. The animal was found by Darwin to display
distinct intelligence in its manner of drawing leaves into the mouth of
its burrow, seizing them in most cases by their narrow ends, so that
they could be pulled in with as little difficulty as possible. Objects
are generally grasped between the lobe, which overhangs the mouth,
and the lower part of the first segment, the hold being maintained by
a sucking action. The inner end of the burrow is enlarged to allow
the worm room to turn round. At night, the fore part of the body is
protruded in search of food, the tail being generally retained in the
burrow, ready for instantaneous retreat in case of alarm.
The earthworm feeds upon leaves—which are first softened by a fluid
discharged over them, and then sucked into pieces small enough to
be swallowed, for the animal has no jaws—and upon the half-decayed
organic matter which is always present in ordinary soil. The soil
itself is swallowed in large quantities; the nutritious portion is
extracted, and the undigested matter deposited upon the surface of
the ground, near the mouth of the burrow, in the form of =castings=.
As a result of numerous experiments, Darwin estimated the weight of
castings thus thrown up by earthworms on an acre of land as 15 tons
annually. The following passage is worthy of very careful attention:
“When we behold a wide, turf-covered expanse, we should remember that
its smoothness, on which so much of its beauty depends, is mainly due
to all the inequalities having been slowly levelled by worms. It is a
marvellous reflection that the whole of the superficial mould over any
such expanse has passed, and will again pass, every few years through
the bodies of worms. The plough is one of the most ancient and most
valuable of man’s inventions; but long before he existed the land was
in fact regularly ploughed, and still continues to be thus ploughed
by earthworms. It may be doubted whether there are many other animals
which have played so important a part in the history of the world as
have these lowly organised creatures.”[39]
The earthworm lays its eggs in a small cocoon formed by the hardening
of a viscid material which is discharged by a swollen part of the body
called the clitellum, extending from the 32nd to the 37th segments.
After the formation of the cocoon the worm moves backwards, and the
eggs leave the body by small pores on the ventral surface of segment
14, as this region passes the cocoon. A small amount of food-material
is also enclosed in the cocoon, and forms a store of nutriment for the
young worms during their early development.
EXERCISES ON CHAPTER XX.
1. Compare the legs of a cockroach with those of a
crayfish and a vertebrate. (1900)
2. Describe the respiratory organs of the crayfish. How
are they continually supplied with fresh water? (1898)
3. Examine a spider. How many legs has it? Of what
divisions does its body consist? Why do you consider that a
spider is not an insect?
4. Make observations, and write descriptions, of the
habits of spiders, paying special attention to the methods
of construction of the webs, the manner of catching prey in
different cases, and the care of the young by the parents.
5. Compare a centipede with an insect, pointing out the
features of resemblance and difference. (1897)
6. How does a pond-mussel open and close its shell? (1900)
7. Compare an oyster with a fresh-water mussel, and try to
find points of resemblance and difference, making careful
notes and sketches of these. How many closing muscles has
the oyster? Are its gills plate-like? Why do you consider
the oyster a mollusc?
8. Where is the lung of a snail situated? How do we know
that it is really a lung? (1897)
9. How does an earthworm resemble and differ from a
caterpillar? (1900)
10. Explain the action of an earthworm in the formation of
vegetable mould. (1897)
11. Classify the animal described below:
A land animal with a long narrow soft body and no legs;
two pairs of tentacles on the head; a breathing hole on one
side? (1904)
12. Give the characters by which insects can be
distinguished from crustaceans. (1905)
FOOTNOTES:
[35] Living specimens may be obtained from Mr. T. Bolton, 25 Balsall
Heath Road, Birmingham.
[36] Greek, _kephalon_, a head.
[37] See footnote, p. 372.
[38] Parker and Haswell’s _Text-book of Zoology_ (Macmillan).
[39] Darwin’s _Vegetable Mould and Earthworms_ (Murray).
CHAPTER XXI. FIELD-WORK.
An animal or a plant must be studied from several points of view before
its manner of life can be understood in any real sense. It must, for
example, be regarded, first, as a complicated piece of machinery, every
part of which is beautifully fitted for the performance of a special
duty; it must also be considered as an _individual_, having likes or
dislikes—or at least tendencies—which are to some extent peculiar to
itself; finally, it must be considered in its relation to other animals
and plants, and to its surroundings. Field-work is especially concerned
with the last of these methods of study—the observation of living
things under natural conditions,—and this ought constantly to be borne
in mind. To make nature-study a pretext for uprooting locally-rare
plants and robbing birds’ nests is indefensible. The commonest plants
are usually the most instructive, and afford ample material for the
beginner’s experimental work; while the pleasure of finding and
describing (perhaps photographing) a bird’s nest, and of keeping the
eggs and young under observation, is something unknown to the mere
collector of eggs. The life of both plant and animal is sacred in the
eyes of every nature-student worthy of the name. At the same time,
no sentimentality ought to prevent the destruction of undoubtedly
noxious insects and weeds. In general, however, specimens should be
killed only for the purpose of a leisurely examination of structure,
which would otherwise be impossible, or to make needed additions to the
teaching-collection of a school museum.
Generally speaking, collecting is of very doubtful value, except to
experts. Insects and common plants may, however, be collected without
scruple by the beginner, though it is worth remembering that the
most perfect specimens of butterflies and moths are those reared in
captivity from the eggs, larvae, or pupae.
The student is strongly recommended to make a sketch map of some
small area to which he has easy access, and to record upon this the
positions of features of special interest. Such a map may, in the
first instance, be copied on an enlarged scale from an ordnance map of
the neighbourhood, which may be obtained at the local free library.
It will be advisable to duplicate the drawing by means of one of the
many appliances for such work, and to keep one copy each for trees,
flowering plants, birds’ nests, etc.—the position of each object, or of
a well-defined group, being carefully marked by a small number (Fig.
237), and the reference, with the date, being filled in on the margin.
The varying character of the ground—sandy, marshy, clayey, etc.—should
be indicated by diagrammatic shading or colouring. By following this
method the student will more clearly realise that different plants are
dependent upon different conditions of soil, drainage, etc.; and that,
_e.g._, plants at home on a bleak moor, in a hedge, in marshy land or
in water respectively, are characteristically modified so that they can
make the best of their special conditions of life. In this manner he
will, almost unconsciously, gain wider views of the relationships which
exist between the facts learned from his more detailed observations.
It would be a distinct gain to biological science if field clubs also
would adopt some such plan, each member undertaking to fill in upon his
map information of the animal and plant life of the area allotted to
him. The co-operation of various clubs, and the systematic arrangement
of information thus obtained, would result in a store of knowledge of
the highest value.
Many animals are so shy that they can only be approached with
difficulty. In such cases it is especially necessary for the observer
to move quietly and silently, and, when a favourable position is
reached, to remain as motionless as possible, preferably with his
back to the sun. If a field-glass can be obtained it will be of great
assistance, but some practice will probably be required before it can
be used easily.
It is well to start each ramble with some definite object of study in
view—either trees, grasses, flowers, fruits, birds, caterpillars, or
pond life, for example, and to be provided with pill-boxes, bottles,
etc., according to circumstances. A notebook, pencil, pocket knife, and
hand lens should, however, _always_ be carried, and observations should
be recorded on the spot.
=Pond life.=—Practically the only method of gaining a real knowledge
of aquatic organisms is by the help of the aquarium. Specimens may be
obtained by means of a net, or of a small wide-mouthed bottle tied to
a stick. A pickle bottle is convenient for carrying home the material
collected. The conditions of a natural pond should be imitated as
closely as possible in the aquarium. At first, some little difficulty
will probably be found in obtaining the necessary balance between the
animals and plants of the aquarium. When this has been reached, it will
only rarely be necessary to change the water, provided that dead or
sickly specimens are promptly removed.
THE SCHOOL JOURNEY.
In the organisation of school journeys so much depends upon local
conditions of various kinds, so much must of necessity be left to the
initiative of the teacher, that it is manifestly impossible here to do
more than enunciate certain general principles. In the first place,
it should be borne in mind constantly that the primary object of the
school journey is the cultivation of habits of thoughtful observation;
and that the chief danger to be guarded against is that out-of-focus
condition into which the mind, like the eye, inevitably falls when
it is concerned with too many things at once. To obviate this danger
the teacher should go over the route in advance, noting carefully the
features, physical and otherwise, which afford material for observation
and investigation by the class. The order in which these features
may be best studied should be decided upon, and a scheme of several
visits, each to be concerned with one special subject of study, can
then be drawn up. Such a preliminary survey should suggest a plan by
which every member of the class may be allotted a definite task—to find
something or do something, or to solve some problem on the spot.
These principles may be best illustrated by a special example, but it
will be obvious that the same ideas, with modifications in detail,
may be applied in any district. The sketch-map (Fig. 237) illustrates
a walk through Healey Dell, near Rochdale, Lancashire. The rocks
which are exposed at various places along the route belong to the
Carboniferous formation, and are composed of shale, coal, or millstone
grit.
The object of the =first journey= will be in most cases to familiarise
the class with the “lie of the land” and the most obvious features of
the scenery. As a preparation, lessons should be given on the points
of the compass and the various methods of finding the direction of
the north. The simplest of these is by the use of the compass: it
being remembered that the needle points about 16° to the west of true
north. A second method depends on the fact that at noon the sun is in
the south and that therefore (because the hour hand of a watch makes
two revolutions in the twenty-four hours) the north and south line
approximately bisects the angle between twelve and the hour hand,
if the latter is pointed to the sun when the watch is horizontal.
Incidentally, the method of finding the pole-star might be also
explained to the class. Further, each pupil should be encouraged to
find out how many steps he takes, on the average, in pacing a given
measured distance. If the general direction of the walk is north and
south, as in the example, it will be found best to begin the first
journey at the south end (in this case from Shawclough Station), since
to most children it is easier to conceive of a journey northward than
in any other specified direction. Throughout the ramble constant
reference should be made to the direction of the route and the relative
positions of well-marked features of the landscape. The distances
between certain points should be estimated, and, whenever possible,
measured (by pacing), and notes made by the class. For example, from
Shawclough to Ending the distances and directions are roughly: ¼-mile
W.N.W., ⅓-mile N.N.W., ¼-mile E.N.E. In the first journey also the
class should be made to notice where the ground slopes most and where
least (the direction and angle of slope should be estimated in a few
cases), and the names of neighbouring woods, farms, etc., should be
learnt. The direction of flow of the river and the various bends in
its course should also be noted, and reference made to the route by
which, after joining those of other rivers, its waters ultimately reach
the sea. Afterwards, the pupils should write an account of the journey
and, in the higher classes, should be encouraged to draw a sketch-map,
however crude, from memory.
[Illustration: FIG. 237.—Sketch-map of Healey Dell, Rochdale.
1, birches; 2, willows; 3, beech; 4, potholes; 5, two waterfalls;
6, shallows, and vertical concave bank; 7, “Fairies’ Chapel”; 8,
stratification of rocks; 9, mud deposits; 10, waterfall; 11, docks; 12,
sloping trees; 17, weir; 14, railway viaduct; 15, horse chestnut; 16,
aqueduct; 17, sycamore; 18, elm; 19, elm; 20, beech; 21, pond life;
22, well; 23, shale; 24, sycamore; 25, oak, bearing leaves in winter;
26, flagstones; 27, beeches, with wood-pigeons’ and magpies’ nests;
28, hawthorn hedge; 29, solitary oak; 30, gutter, with _Pellia_; 31,
blackberry bush; 32, stunted oak.]
[Illustration: G. A. Close, photographer, Rochdale.
FIG. 238.—Waterfall, Healey Dell, Rochdale (marked “10” in Fig. 237).]
Before the =second journey= each pupil should be provided with a blank
sketch-map of the route. This may, in the first instance, be copied or
traced from the six-inch Ordnance Map, and then duplicated in large
numbers. Only the route and river, and a few of the more conspicuous
landmarks, should be indicated on the maps as given to the class:
details should be filled in, on the spot, by the pupils. The object of
the second journey may conveniently be the study of the river and its
work, and for this purpose it will be advisable to follow the stream
in the direction of its flow. Variations in the speed of the current,
and in the width of the stream and the hardness of the rocks or banks
between which the water flows, should be noted, and the relations
between cause and effect elicited by questioning. The hardness of the
rocks at 4 has prevented the channel from being widened to a greater
extent by the water, and accounts for the rapidity of the flow. A glass
of water collected here is found to contain much suspended gravel.
The considerable loss of weight of bodies in water is noteworthy, as
explaining the great size of the stones which may be transported
by rivers. The scouring action of such stones is shown in the fine
“potholes” at 4 and below the waterfall (Fig. 238) at 10, and has
resulted also in the quaint stone portico of the “Fairies’ Chapel”
under the right (west) bank of the river at 7. Again, the difference
in the rates of erosion of hard and soft rocks has had much to do with
forming the waterfalls at 5 and 10. Where the stream is wider and the
flow slower (as at 6 and 9, and below 10), may be noticed sand and
mud deposits; and where the stream makes a bend it is found that the
slowest flow and the maximum deposit are on the convex bank; while the
concave bank is worn almost vertical (as at 6 and other places) by the
swifter rush of the water, and may be undercut to such an extent as
to cause the bank to give way. In this manner a river is constantly
changing its course. The weir at 13, and the old water-wheel still to
be seen in the ruined mill below, suggest remarks on the motive power
of water, and on the circumstances which may cause the old industries
of a district to be superseded by new ones. Along the rest of the
route the bed of the river is less steep and its banks exhibit less
variation, but they still afford plenty of material for study. The
railway viaduct at 14 and an aqueduct at 16 suggest at least a casual
reference to the derivation of the terms. Before the pupils are asked
to write a “composition” on the ramble, a revision lesson on the
features noticed should be given, and the accuracy of the entries on
the sketch-maps checked by comparison with an enlarged map drawn by the
teacher, or with a large “parish plan” of the Ordnance Survey, on the
scale of 25.3 inches to the mile.
It will be well to devote two or more journeys to the study of the
=trees= along the route. One journey should be taken in the spring,
before the leaves are out, and another in the summer, when the foliage
is well developed. It is far better to study three kinds of trees in
some detail than to risk confusion at the beginning by attending to a
dozen. In Healey Dell the commonest trees are beech, oak, and sycamore,
and these serve admirably as an introduction to tree lore. If the first
tree-journey be taken in the summer, the leaves of some three abundant
species should be compared and contrasted, and each pupil should secure
good specimens, to be drawn and preserved afterwards. The presence of
a little bud in the “axil” (the upper angle between leaf and twig) of
most of the leaves should be pointed out by the teacher; and since the
arrangement of the buds (and therefore of the subsequent branches of
the twig) thus depends on the positions of the leaves, this last point
is of considerable interest. In the sycamore, the leaves are in pairs
at right angles to each other; in the beech and oak they are single
and alternate, but much more crowded together in the oak than in the
beech. The bark of the three trees is equally distinctive, and with
the method of branching (obscured when the foliage is thick) serves to
identify the trees from a distance in the winter. In winter and spring
the interest of a tree is centred in its buds, and there are few things
which more richly repay study. In spring, attention should also be
given to the flowers—generally arranged in catkins—of common trees.
Separate sketch-maps should be used as records of the positions of the
more notable trees or plantations along the route. Any tendency to
vandalism on the part of the pupils by tearing off branches should, of
course, be sternly repressed; especially interesting twigs should, on
occasion, be _cut_ off by the teacher only, for later study.
There is much diversity of opinion as to the way in which =flowers=
may be best studied in a limited number of school journeys. In most
cases it will perhaps be impracticable to attempt more than teaching
the names and calling attention to the habitat of the commonest. This,
though a necessary introduction to the subject, tends to degenerate
into a mere exercise of the memory, and in itself possesses little
educational value. It should be supplemented by a detailed examination
of a typical flower—say a buttercup—and by the explanation of the work
of each part. Once the pupil has understood that the single duty of a
flower is the production of healthy seeds, and has been led to notice
how, by the aid of ingenious devices, the up-to-date plants have learnt
to call in the aid of insects, while the more conservative families
still rely on the aid of the wind, he will be eager to discover for
himself “how the thing works.” With young children it is folly to
attempt any but the very broadest principles of classification of
flowers; but quite young children can appreciate the advance from
flowers without petals, through flowers with separate petals, to those
with petals united to form a tube (thus restricting the nectar more
and more to “useful” insects); and so understand the advantage which a
primrose has over a buttercup, and a buttercup over an oak flower.
At least one journey should be given, in the autumn, to the study
of the dispersion of =fruits and seeds=. The pupils should provide
themselves with empty match-boxes or chip pill-boxes. In this ramble
the class may with advantage be divided into four groups. Group A will
collect examples of fruits and seeds which are dispersed by the wind;
Group B, of fruits which by means of hooks or otherwise become attached
to the hides of grazing animals, and are carried far from the place
where they grow; Group C will collect fruits which tempt animals to eat
them for the sake of sweet pulp (in these cases the pupils should find
out _a_ how the fruit is made conspicuous, _b_ how the seeds themselves
are protected from being injured by the animals); while Group D will
search for specimens of plants which sow their own seeds.
There still remains abundant material for study in this walk, and
mention only can be made of the sticklebacks, frog-spawn, snails,
caddis-worms, dragon fly larvae, blood-worms—of the “things creeping
innumerable, both small and great beasts”—which have been found in
the river, ponds, and wells along our route, and have been used to
stock the aquarium; of the rabbits and birds; of the nests of ants
and wild bees and wasps; of a certain blackberry bush (31) rich in
interesting leaves; and of a thousand and one other things which, under
the guidance of a judicious leader, may well be the means of teaching
children to see what they look at and to think about what they see. For
this is the first and last object of the school journey.
MONTHLY NATURE CALENDAR.[40]
JANUARY.
PLANT LIFE.
=General.=—As in December (p. 422).
=Plants usually in flower.=—Shepherd’s purse, daisy,
snowdrop, and a few others.
ANIMAL LIFE.
=General.=—As in December (p. 422).
=Mammals.=—Bats (p. 255) reappear at the end of the month.
=Birds.=—Missel thrush (p. 306) sings.
=Insects.=—Winter pupae of butterflies and moths
(Chap. XIX.) may be found.
FEBRUARY.
PLANT LIFE.
=General.=—As in December.
=Plants usually in flower.=—Shepherd’s purse, daisy,
snowdrop, hazel, and a few others. Hedges are now clipped;
study the effect which this treatment has upon subsequent
growth.
=Corn.=— Barley and oats are sown.
=Liverworts.=—Spore-cases of _Pellia_ (p. 200) become
visible.
ANIMAL LIFE.
=General.=—As in December.
=Mammals.=—Bats reappear.
=Birds.=—Thrushes and blackbirds pair and begin to build;
study differences in song (pp. 304 and 308). Rooks repair
and build nests (p. 318).
=Frogs=. (p. 332) reappear.
=Insects.=—Pupae of cabbage-white butterflies and other
Lepidoptera may be found.
MARCH.
PLANT LIFE.
=General Work for Spring Months.=—Study the
germination of seeds and the early stages of growth of
the new plants (Chap. I.); the structure and methods of
unfolding of buds (Chaps. IV. and VIII.); the movements
of young twining stems; “bleeding” of stems, and paths of
food-currents (Chap. V.); and examine and collect spring
flowers (Chaps. VI., VII., and VIII.).
=Plants usually in flower=—Shepherd’s purse, marsh marigold,
wild plum, daisy, dandelion, daffodil, snowdrop, hazel,
alder, willow, poplar, elm, and others.
=Horsetail.=—Fertile haulms (p. 195) appear.
=Corn.=—Barley and oat sowing continued.
ANIMAL LIFE.
=General Work for Spring Months.=—Study the
development of frogs and toads (Chap. XVIII.) and insects
(Chap. XIX.); the development and education of the chick
(Chap. XVI.); the nesting-habits of birds (Chap. XVII.); the
education and play of lambs and other young mammals (Chap.
XIV.); the habits of molluscs (Chap. XX.); and the work of
earthworms (Chap. XX.). Stock aquaria.
=Mammals.=—Lambs are born.
=Birds.=—Thrushes, blackbirds, skylarks, rooks, and other
birds lay eggs. Fieldfares and redwings begin to depart.
=Frogs and Toads= lay their eggs.
=Insects.=—_Dytiscus_ (p. 361) lays eggs. Pupae of
cabbage-white butterflies and other Lepidoptera may be
found.
APRIL.
PLANT LIFE.
=Plants usually in flower.=—Wallflower,
shepherd’s purse, buttercup, anemone, marsh marigold,
common vetch, plum, pear, strawberry, primrose, cowslip,
daisy, dandelion, speedwell, red deadnettle, daffodil, wild
hyacinth, wild tulip, rushes, arum, annual meadow grass,
oak, birch, alder, willow, poplar, elm, ash, and others.
=Trees which unfold their leaf-buds.=—Larch, horse chestnut,
beech, elm, sycamore.
Elm =fruits= (p. 154) are abundant; study the method of
dispersal (p. 173).
=Corn.=—Barley and oat sowing continued.
ANIMAL LIFE.
=Birds.=—Sand-martins, cuckoos, swallows,
house-martins, nightingales, swifts, and other birds arrive.
Kestrels and sparrow-hawks nest. Fieldfares and redwings
depart.
=Frogs and Toads.=—Tadpoles hatch.
=Insects.=—Cabbage-white butterflies (p. 362) and other
Lepidoptera emerge from pupae. Caterpillars are plentiful.
Moths may be taken on willow flowers at beginning of month.
Larvae of _Dytiscus_ (p. 361) and other aquatic insects may
be found in ponds.
=Molluscs.=—Garden snails (p. 379) reappear.
MAY.
PLANT LIFE.
=Plants usually in flower.=—Wallflower,
shepherd’s purse, buttercup, anemone, marsh marigold,
laburnum, common vetch, red clover, white clover, broom,
cherry, apple, pear, strawberry, hawthorn, primrose,
cowslip, daisy, dandelion, speedwell, red deadnettle, white
deadnettle, lily of the valley, wild hyacinth, star of
Bethlehem, wild tulip, rushes, sedges, arum, sweet-scented
vernal grass, slender foxtail, meadow foxtail, annual meadow
grass, perennial rye grass, oak, beech, birch, willow, elm,
horse-chestnut, ash, sycamore, and others.
Most =forest trees= are now in leaf.
=Liverworts.=—Spore-cases of _Pellia_ (p. 200) open.
ANIMAL LIFE.
=Birds.=—Swallows build nests. Most birds are
incubating eggs (Chap. XVI.).
=Frogs and Toads.=—Tadpoles may be found in ponds.
=Insects.=—Moths and butterflies are common. Cabbage-whites
lay eggs. Caterpillars of tiger-moth (p. 364) and other
Lepidoptera may be found. Aquatic insect-larvae in ponds.
JUNE.
PLANT LIFE.
=General Work for Summer Months.=—Study the
forms and duties of leaves (Chap. III.); and the thickening
of stems (Chap. V.). Examine, identify, and collect grasses
(Chap. VII.) and summer flowers (Chap. VI.), and make
observations upon cross-pollination of flowers by insects
(Chap. VI.). Study the development and structure of fruits
(Chap. IX.); and the life-history of ferns (Chap. X.).
Compare and contrast mushrooms and toadstools (Chap. XI.).
=Plants usually in flower.=—Wallflower, shepherd’s purse,
buttercup, anemone, meadow vetchling, common vetch, red
clover, white clover, broom, wild rose, hawthorn, poison
hemlock, cow parsnip, carrot, daisy, dandelion, speedwell,
mullein, snapdragon, thyme, sage, red deadnettle, white
deadnettle, lily of the valley, wild hyacinth, star of
Bethlehem, rushes, sedges, sweet-scented vernal grass,
slender foxtail, meadow foxtail, Timothy grass, Yorkshire
fog, wild oat, annual meadow grass, smooth-stalked meadow
grass, rough-stalked meadow grass, meadow fescue, sheep’s
fescue, perennial rye grass, couch grass, lime, sycamore,
and others.
Lime =leaves= unfold.
=Fruits= of strawberry, willow, and other plants are ripe.
Haymaking begins.
ANIMAL LIFE.
=Mammals.=—Examine young wild rabbits (Chap. XII.).
=Birds.=—Most birds are hatching eggs. First brood of
swallows hatched at end of month. Drake’s plumage becomes
similar to duck’s (p. 329).
=Frogs and Toads.=—Tadpoles reach full size.
=Insects.=—All stages of Lepidoptera very abundant.
Caterpillars of tiger-moth (p. 364) pupate at end of month.
Vapourer caterpillars (p. 364) may be found at end of month.
Larvae of aquatic insects in ponds; pupae of _Dytiscus_ (p.
361) in soil on banks.
JULY.
PLANT LIFE.
=Plants in flower.=—Shepherd’s purse, candytuft,
buttercup, meadow vetchling, red clover, white clover,
wild rose, blackberry, poison hemlock, water hemlock,
cow parsnip, carrot, hedge parsley, fool’s parsley,
water dropwort, daisy, dandelion, thistle, foxglove,
speedwell, snapdragon, mullein, musk, mint, thyme, sage,
red deadnettle, white deadnettle, rushes, slender foxtail,
Timothy grass, Yorkshire fog, wild oat, annual meadow
grass, smooth-stalked meadow grass, rough-stalked meadow
grass, meadow fescue, sheep’s fescue, couch grass, Spanish
chestnut, lime, and others.
=Fruits= of gooseberry, cherry, raspberry, strawberry, and
other plants are ripe.
=Ferns.=—Spores of male-fern, bracken, and hart’s tongue are
ripe (Chap. X.).
=Fungi.=—Mushrooms (p. 203) and toadstools may be found.
ANIMAL LIFE.
=Mammals.=—Lambs are weaned.
=Birds.=—Many birds are hatching second broods. Old cuckoos
begin to depart at end of month. Poultry moult. Young wild
ducks may be seen in water meadows.
=Frogs and Toads.=—Young animals leave ponds.
=Insects.=—Cabbage-white caterpillars pupate. Tiger moths
(p. 369) emerge from pupae at end of month. Caterpillars of
vapourer moth (p. 364) may be found.
AUGUST.
PLANT LIFE.
=Plants in flower.=—Shepherd’s purse, candytuft,
buttercup, meadow vetchling, red clover, white clover, wild
rose, blackberry, water hemlock, cow parsnip, carrot, hedge
parsley, fool’s parsley, water dropwort, daisy, dandelion,
thistle, foxglove, speedwell, snapdragon, mullein, musk,
mint, thyme, sage, red deadnettle, white deadnettle, rushes,
slender foxtail, Timothy grass, Yorkshire fog, wild oat,
annual meadow grass, couch grass, and others.
=Leaves= of horse chestnut begin to fall.
=Fruits= of cherry, plum, raspberry, and other plants are ripe.
=Corn.=—Wheat, oats, and barley harvests begin.
=Ferns.=—Spores of male-fern, bracken, and hart’s tongue are ripe.
=Fungi.=—Mushrooms and toadstools may be found.
ANIMAL LIFE.
=Birds.=—Swifts and old cuckoos depart. Many
rooks go into winter quarters. Nightingales begin to depart.
Sand-martins congregate. Song-birds are comparatively silent.
=Insects.=—All stages of vapourer moth may be found.
SEPTEMBER.
PLANT LIFE.
=General Work for Autumn Months.=—Study the
storage of food in twigs, underground stems, bulbs, etc.
(Chaps. IV. and V.); collect good specimens of leaves
showing autumn colours, observe the phenomena of leaf-fall
and the formation of vegetable mould, and notice the order
in which forest trees become leafless (Chaps. IV. and
VIII.). Study the development and structure of fruits,
and the methods of dispersal of seeds (Chap. IX.). Make
collections of dry fruits (Chap. IX.), and of the “seed” of
useful and injurious grasses (Chap. VII.).
=Plants usually in flower.=—Shepherd’s purse, buttercup,
meadow vetchling, red clover, white clover, blackberry,
hedge parsley, water dropwort, daisy, dandelion, thistle,
foxglove, speedwell, snapdragon, musk, mint, red deadnettle,
white deadnettle, slender foxtail, annual meadow grass, and
others.
Ash and horse chestnut =leaves= fall.
=Fruits= of apple, pear, plum, blackberry, and other plants
are ripe.
=Corn.=—Wheat-sowing begins.
=Fungi.=— Mushrooms and toadstools may be found.
ANIMAL LIFE.
=General Work for Autumn Months.=—Study the
various methods by which animals prepare for the winter:
_e.g._ migration of birds, hibernation of bats, frogs,
insects, etc.; change of colour or thickness of coat,
storage of food, etc.
=Birds.=—Swallows and house-martins congregate. Sand-martins
and nightingales depart. Rooks go into winter quarters.
Young song-birds may be heard learning to sing.
=Insects.=—Moths may be taken on ivy blossoms, etc. Eggs of
cabbage-white butterflies may be found. Caterpillars are
mostly full-fed and ready to pupate. Pupae of vapourer moth
may be found.
OCTOBER.
PLANT LIFE.
=Plants usually in flower.=—Shepherd’s purse,
white clover, daisy, dandelion, snapdragon, red deadnettle,
white deadnettle, slender foxtail, and others.
Most =forest-trees= shed their leaves.
=Fruits= of apple, pear, plum, and other plants are ripe.
=Corn.=—Wheat-sowing continued.
ANIMAL LIFE.
=Birds.=—Young song-birds may be heard learning
to sing. Fieldfares and redwings arrive. Swallows and
house-martins depart. Drakes reassume their distinctive
plumage.
=Frogs= hibernate (p. 334).
=Insects.=—October is the best month for pupa hunting.
NOVEMBER.
PLANT LIFE.
=Plants usually in flower.=—Shepherd’s purse,
daisy, white deadnettle, and others.
Most =forest-trees= are now leafless.
=Fruits= of hawthorn, rose, holly, mistletoe, etc., are ripe.
=Corn.=—Wheat-sowing continued.
ANIMAL LIFE.
=Mammals.=—Bats hibernate (p. 257).
=Birds.=—Larks patrol fields in flocks (p. 318).
=Insects.=—Pupae may be found.
DECEMBER.
PLANT LIFE.
=General Work for Winter Months.=—Arrange
collections of flowers, grasses, leaves, etc. Study the
methods of branching, and the bark, of trees, and make
drawings of typical examples (Chap. VIII.). Examine bulbs
and corms, (Chap. V.) and grow them in water-glasses.
Trace the water-conducting strands in the flower-stalks of
snowdrop, narcissus, etc. (Chap. V.).
=Flowers= of daisy, white deadnettle, and a few others may
be found.
=Fruits= of mistletoe, holly, etc., are ripe.
ANIMAL LIFE.
=General Work for Winter Months.=—Prepare
skeletons, etc., and study the structure and manner of life
of rabbits, poultry, and pigeons (Chaps. XII., XIII., and
XV.). In snowy weather, examine and draw the footprints
of domestic and other animals (Chap. XIV.). Place grain,
crumbs, suet, and other food for birds, and identify those
which come to feed (Chap. XVII.).
=Birds.=—Missel-thrush sings. Sparrow-hawks may be seen near
farms.
=Insects.=—Pupae may be found.
FOOTNOTE:
[40] No attempt has been made to compile a complete calendar, the
events noticed being those only which are considered in this book.
TYPICAL EXAMINATION PAPERS.
The National Froebel Union.
ELEMENTARY CERTIFICATE. NATURE KNOWLEDGE.—
Paper I.
(_Questions 1 and 2 and_ ANY TWO _others to be answered._)
1. Describe the flower provided, and draw it in longitudinal
section. Explain how cross-pollination is ensured in this
flower; give drawings to illustrate.
2. Give and plan out the subject-matter for one or more
Lessons, to be given to children of seven or eight years of
age, on the Dispersal of Fruits and Seeds by Animals.
3. Describe the structure of an Acorn and of a Wheat grain,
and contrast these two seeds. Give enlarged drawings of each.
4. What is the work of the _root_ of a plant, and how is this
work carried out? What different forms of _roots_ are to be
found? Give examples and make a rough sketch of each.
5. Describe the _bud_ in winter, and its method of unfolding
in spring, of _any two_ of the following trees:—Oak, Beech,
Sycamore. Make sketches to illustrate.
6. Make as complete a list as you can of Flowering Plants
which grow in ponds; state how these plants have adapted
themselves to their habitat, and say at what time of the year
each of them flowers. What plants would you expect to find
growing round the margin of the pond?
Paper II.
(_Questions 1 and 2_ MUST _be answered, and_ ANY THREE _others._)
1. Draw up Notes of Lessons on _one_ of the following
subjects:—“A Mole,” “Domestic Fowls,” “A Cat.” State the age
of the children, and the method you would pursue.
2. What is a _ruminant_? Give as many groups of ruminating
animals as you can and the habits of _one_.
3. Give the life-history of _one_ of the following, with
illustrations:—a Bee, a Caddis-fly, a Spider, a Butterfly.
4. Give the life and habits of the Squirrel.
5. How does a Starling differ from other Birds?
6. Draw a common Snail and a Slug. Give a short account of
their life-histories.
7. Give instances of _protective colouring_ amongst (1)
Insects, (2) Birds, (3) Mammals, in this country.
Board of Education.
TEACHERS’ CERTIFICATE EXAMINATION.
GENERAL ELEMENTARY SCIENCE.
PART II.
11. Give an example in each case of a plant with—(_a_) Plumed
fruits or seeds. (_b_) Winged fruits or seeds. (_c_) Climbing
stem covered with hooks. (_d_) Flowers which come out before
the leaves. (_e_) Flowers in which the stamens are united to
form a tube.
12. Show in the case of any two British wild plants the
special means they possess for survival in the struggle for
existence.
13. Name five of the earliest flowering wild plants in your
neighbourhood, in the order in which they flower, and mention
the chief characteristics of the flower in each case.
14. Describe the life-history of a fern so far as it can be
observed by the naked eye and with the aid of a pocket lens.
15. Describe, with the help of drawings, the work of a bee
in its mode both of collecting pollen and honey and of
fertilising flowers.
16. Give a short account of the structure of a bird’s wing.
How are the wings made use of during flight?
17. Give an account of some simple experiments you would
employ to demonstrate the phenomena of respiration in animals
and plants.
Board of Education.
PRELIMINARY EXAMINATION FOR THE CERTIFICATE.
PART II.
ELEMENTARY SCIENCE.—SYLLABUS NO. II.
(_You should answer_ SIX _questions._)
1. Describe with the aid of a drawing the various structures
seen by means of a pocket lens in a section across the middle
region of a grain of wheat.
2. How would you measure the rate of transpiration of water
from a small plant or a leafy stem?
3. How can it be shown that the root responds to external
influences of moisture, light and gravity?
4. Give a brief account of the function of the green leaf in
the nutrition of plants.
5. Compare by drawings the leaves of broad bean and garden
pea. Then discuss the means by which the two plants obtain
mechanical support.
6. When the bulb of a thermometer is placed in a jar of soaked
and germinating seeds, what temperature change is observed?
Explain the cause of this.
7. Describe experiments which show in what respects the air is
affected in composition by passing through the lungs.
8. Describe with the help of drawings the structure of the
flowers of the hazel or willow and show how they are adapted
for cross-pollination.
9. What is meant by root pressure, and how would you
demonstrate it? Illustrate your answer by drawings.
10. What are the conditions of the soil which make it a
suitable medium for healthy root-action and vigorous plant
growth? Conversely under what conditions of soil would the
plant fail to thrive or die?
Board of Education (South Kensington).
GENERAL BIOLOGY.
SECTION I. OF STAGE I.
(_You are permitted to answer only_ EIGHT _questions._)
1. Write what you can of the habits of the common House
Fly and of the common Clothes Moth; draw figures of their
appearance at different stages of the life-history.
2. Where and when do you find Frog’s eggs? Of what use is the
jelly with which they are surrounded?
3. How does the Tadpole swim, and how does the Frog swim? How
does the Frog jump, and how does it catch a fly?
4. Contrast the characters of the mouth (including teeth if
present) in the Frog, Bird, Cat, Rabbit and Sheep.
5. Describe the characteristic modes of locomotion in the
Bird, Dog, Rabbit and Bat, and point out any peculiarities of
the skeleton which are related to these habits.
6. Describe the heart of the Sheep, and account so far as you
can for any differences you can point out between the various
chambers.
7. In what way are bees useful to flowers? Explain in any one
example you choose what happens when a bee visits the flower.
8. Describe the roots of a pea or bean. What importance do you
attach to the different parts you mention?
9. What is starch? How would you show whether or not it was
present in a leaf? What conditions are necessary in order that
the leaf may produce it?
10. Describe the fruit of _either_ the Sycamore _or_ the
Poppy, and explain the uses of the different parts in
dispersing the seed.
11. Describe how you would proceed in arranging an experiment
to enable you to study the germination of a seed. Give a brief
account of the process of germination of any seed you may
select.
12. Describe and sketch the specimen provided, and explain, as
far as you can, the use of the different parts.
INDEX.
Acorn, 141, 144, 146, 175, 177
Air, carbon dioxide in, 31, 33
Air-chamber of hen’s egg, 283, 285
Air sacs of birds, 279
Albumin, 285
Alder, 142, 149
Allantois, 289, 293
Alternation of generations, 189, 196
Amnion, 288, 290
Anemone, 97, 98
Animals and plants, resemblances and differences, 243
Annual, meadow grass, 135; rings, 73
Anterior, 217
Anther of stamen, 91
Aorta, 236, 238
Appendages of crayfish and lobster, 373, 375
Apple, 104, 106; fruit of, 176, 179
Apricot, 106
Aquarium, a simple, 340
Arachnids, 378
Arteries, 237, 238
Arteries, pulse in, 235, 239
Arthropods, 351
Arum, 122
Ash, 153, 156; fruit of, 171, 173; mountain, 157
Aspen, 152
Auricles of heart, 235, 237
Awn, 129, 130, 131
Axil of leaf, 45, 47
Backbone, of pigeon, 269;
of rabbit, 217, 222, 227
Bacteria, 209
Balancers of hen’s egg, 283, 285
Bark, 74
Bast, 71, 72
Bat, 255, 256
Bean seed, 1, 2, 5;
germination of, 9, 13;
starch in, 2
Beech, 142, 145, 147;
fruit of, 175, 178
Beetle, a water, 357
Beetles, 362
Berries, 175, 179
Biennials, 31, 96
Bile, 232, 234
Birch, 142, 147, 148
Birds, 269
Bird’s foot trefoil, 102
Black bent, 135
Blackberry, 103, 105;
fruit of, 175, 179;
leaves, 38, 41, 105
Blackbird, 302, 307
Bleeding of stems, 67, 70
Blood, 233, 235, 242;
circulation of, 235;
red corpuscles of, 237, 242
Blue mould, 207, 208
Bone, structure of a long, 223, 228
Bones, of birds, 269;
of pigeon, 274;
of rabbit, 222
Box leaves, 44, 45
Bracken fern, 184, 189
Bracts of anemone, 97, 99;
of daisy, 111;
of dandelion, 113;
of lime, 152, 154, 155, 174
Branches, position of, 45, 47
Breast bone, of pigeon, 269;
of rabbit, 217, 223, 227
Breathing, definition of, 242;
necessity for, 241;
of cockroach, 355;
of crustaceans, 376, 377;
of fresh-water mussel, 381;
of frog, 333, 337;
of mammals, 242, 243;
of snails, 382, 383;
of tadpole, 344
Breathing organ of chick-embryo, 289, 294
Broad bean seed, 1, 2, 5
Broccoli, 96
Broom, 101
Brussels sprouts, 96
Bud, a typical, 55, 58
Buds, 55, 69;
bursting of, 56, 61, 64;
of ash, 153, 156;
of horse chestnut, 57, 63;
of sycamore, 56, 60;
position of, 45, 50
Bulb, 81, 84
Burrows of earthworms, 384, 385
Buttercup, 96, 97;
family, 96, 97
Butterflies, cabbage-white, 362, 365
Calamites, 196
Calceolaria, 116
Calyx, 90
Cambium, 72
Candytuft, 94
Canine teeth, 246, 248, 255
Capillaries, 237, 240
Capsule, 168
Carbon, contained in plants, 31, 33;
contained in starch, 32
Carbon dioxide, formed by living body, 240, 241;
formed when flesh burns, 240, 241;
formed when wood burns, 31;
in air, 31, 33
Carnivores, 254
Carpel, 96, 98
Carrot, 96, 108
Castings of earthworms, 384, 386
Cat, 246, 250, 252, 254
Catkin, 146
Catkins, of alder, 142, 149;
of beech, 142, 148;
of birch, 142, 148;
of hazel, 142, 149;
of oak, 141, 144, 145;
of poplar, 151;
of willow, 150, 151
Caterpillar, of cabbage-white butterfly, 362, 365;
of tiger-moth, 364, 369;
of vapourer-moth, 364, 370
Cauliflower, 96
Celery, 108
Centipedes, 378
Cephalothorax of crab, 373, 376;
of crayfish and lobster, 373, 375
Cherry, 103, 105, 106;
fruit of, 175, 179
Chestnut, horse, 38, 42, 47, 57, 63, 65, 157, 159;
Spanish or sweet, 148
Chick, development of, 286, 289;
education of, 295, 296;
embryo of, 287, 289;
hatching of, 294, 295
Chitin, 251
Chrysalis, 366
Claws of cat, 247, 251
Cleavers, 175, 177
Climbing stems, 75
Clitellum of earthworm, 383, 386
“Clock” of dandelion, 112, 171, 172
Clover, 101, 102
Cockroach, 349, 351;
habits of, 356;
life-history of, 356;
position of in insect class, 357;
structure of, 355
Cocoon of earthworm, 386;
of tiger-moth, 364, 369
Coleoptera, 362
Collar of mushroom, 203, 205;
of garden snail, 379
Collecting, 389, 390
Colour, of flowers, 93;
of fruits, 182
Compositae, 114
Cone-bearing trees, 160, 161
Cones of alder, 149;
of horsetail, 191, 192, 194, 195;
of larch, 161, 163;
of Scotch pine, 160, 162, 171, 174;
of spruce fir, 163, 171, 174
Convolvulus, 76, 78, 79
Cork, 75
Corm, 81, 85
Corolla, 91
Corona, 120, 122
Cotyledons, 13, 16, 22, 23;
of bean, 2, 6
Cowslip, 109, 110
Crayfish, 372, 374
Creeping stems, 80, 82
Cress, 94, 96
Crocus corm, 81, 85
Crop of pigeon, 271
Cross fertilisation, 92, 102, 110, 118, 122, 151
Crow, 312, 318, 320;
family, 312, 320
Crucifers, 95
Crustaceans, 372, 377
Cuckoo, 322, 324;
cuckoo-pint, 122
Currants, 176, 179
Cushion-feet of caterpillar, 365
Daffodil, 84, 85, 120, 122
Daisy, 111, 112
Dandelion, 112, 113;
fruit of, 171, 172
Deadnettle, 117
Deer, 261
Dentine, 219
Development of chick, 286, 289;
of frog, 340
Diaphragm, 218
Diastase, 234
Dicotyledons, 23, 40, 72
Digestion, 231, 233;
definition of, 233
Digestive canal of frog, 231;
of rabbit, 232
Diptera, 370
Dispersal of insects, 370;
of seeds, 165
Dissection, 214
Dog, 248, 251, 252, 254
Donkey, 261
Dorsal, 217
Down feathers, 273, 277
Dropwort, water, 108
Duck, 326, 327
Dytiscus, habits and structure of, 357, 358;
life-history of, 361
Ear, of grass, 130
Ears, of rabbit, 218
Earthworms, 383, 384
Education of young animals, 248, 249, 262
Eggs, of cabbage-white butterfly, 362;
of cockroach, 357;
of Dytiscus, 361;
of earthworm, 386;
of frog, 339, 340, 341;
of hen, 282, 284;
of Limnaea, 380
Elm, 152, 153;
fruit of, 171, 173
Embryo, 12;
of chick, 287, 289;
of fern, 188, 191;
of horsetail, 195
Enamel, 219
Endosperm, 22
Evolution, a case of, 338
Extinct horsetails, 196
Eyes, of butterfly, 363, 367, 368;
of cat, 246, 250;
of cockroach, 348, 352;
of crab, 373;
of crayfish and lobster, 372;
of dog, 248, 251;
of pigeon, 271;
of rabbit, 218;
of snail, 379, 382
Falcon, 272, 330
Fantail pigeon, 280
Feathers of pigeon, 265, 266, 267, 270, 275, 276
Feelers, of butterfly, 363, 367;
of cockroach, 349, 352;
of crayfish and lobster, 372, 373, 374, 375;
of Dytiscus, 358, 359;
of moth, 364, 368, 369
Fehling’s solution, 230
Ferns, 183;
bracken, 184, 189;
hart’s tongue, 185, 190, 191;
male fern, 183, 185
Fertilisation, 92;
of ferns, 188;
of forest trees, 146;
of foxglove, 116;
of grasses, 131;
of horsetail, 195;
of moss, 202;
of oak, 145;
of primrose, 109;
of red clover, 101;
of Scotch pine, 162
Fescues, 132, 134
Fieldfare, 303, 309
Field work, 388
Filament of stamen, 91
Fir, spruce, 161, 163
Flight of bird, 278
Flower, of a grass, 128, 129;
of wallflower, 88, 89
Flowerless plants, 197
Flowers, work of, 89
Food, carbonaceous, of plants, 34, 35;
mineral, of plants, 29;
necessity of, 230;
obtained from air, 31, 34;
obtained from soil, 26, 29;
of mushroom, 204, 206;
of young seedling, 28
Fool’s parsley, 108
Fowls, 296, 299
Foxglove, 114, 115;
family, 114, 115
Foxtails, 132, 134, 135
Frog, development of, 340;
digestive canal of, 231;
life of, 332, 334
Fronds of ferns, 65, 183, 184, 186, 189
Fruit, 166
of apple, 176, 179;
of ash, 171, 173;
of beech, 175, 178;
of blackberry, 175, 179;
of buttercup, 98;
of cherry, 175, 179;
of dandelion, 171, 172;
of elm, 171, 173;
of field geranium, 165, 168;
of gooseberry, 175, 179;
of hazel, 175, 177;
of herb bennet, 174, 177;
of lime, 174;
of oak, 175, 177;
of pansy, 166, 169;
of pea, 7, 165, 168;
of pear, 176, 180;
of penny-cress, 165, 168;
of plum, 175, 179;
of poppy 166, 168;
of raspberry, 175, 179;
of rose, 176, 180;
of shepherd’s purse, 165;
of strawberry, 176, 180, 181;
of sycamore, 3, 171, 173;
of vegetable marrow, 176, 179;
of violet, 166, 169;
of wallflower, 165, 167;
of willow, 171, 173;
of wood avens, 174, 177
Fruits, colour of, 182; hooked, 174, 177
Funaria, 199, 202
Fungi, 206, 244
Fur,
of bat, 256;
of cat, 246, 250;
of rabbit, 212, 216
Galls, 141, 146
Gastric juice, 232, 233
Geranium, fruit of, 165, 168
Germinal disc, 285, 287, 289
Germination, 12, 21
Gills,
of crab, 374;
of crayfish and lobster, 373, 376;
of fresh-water mussel, 379, 381;
of mushroom, 203, 205;
of tadpole, 341, 344, 346
Gizzard of pigeon, 271
Gloxinia, 116
Glumes, 129, 130
Goats, 261
Gooseberry, 175, 179
Goose grass, 175, 177
Gorse, 101
Grapes, 176, 179
Grasses, 125
Grass,
flowers of, 128, 129;
reproduction of, 128
Grass stems, 72, 75
Grinding teeth of rabbit, 219
Gullet, 231, 232
Gymnosperms, 72, 163
Hair,
of cat, 246, 250;
of dog, 248, 251;
of sheep, 258
Hart’s tongue fern, 185, 190, 191
Hatching,
of chick, 294, 295;
of tadpole, 340, 344
Haulms,
of grasses, 125, 127;
of horsetail, 191, 192, 193, 194
Hawks, 329
Hawthorn, 106
Hazel, 142, 149; fruit of, 175, 177
Head,
of clover, 101, 102;
of daisy, 113;
of dandelion, 113
Heart,
beats of, 235, 239;
of chick-embryo, 288, 291;
of cockroach, 350, 355, 356;
sheep’s, structure of, 235, 237;
valves of, 236, 238, 239
Hemiptera, 370
Hemlock, poison, 106, 107; water, 108
Hen, eggs of, 282, 284
Herb-bennet, fruit of, 174, 177
Heredity, 217, 339
Hip bones of rabbit, 223, 228
Hip of rose, 176, 180
Honey-dew, 158
Honeysuckle, 78
Hoofed mammals, 257
Hoofs, 258, 259, 261
Hooked fruits, 174, 177
Hooking stems, 75
Horns, 257, 259
Horse, limbs of, 261
Horse chestnut, 157, 159;
buds of, 57, 63;
leaves of, 38, 42, 47, 64
Horsetail, 191
House-martin, 311, 315
House-sparrows, 315, 320
Hovering, 279, 331
Hyacinth, 84, 119, 120
Hymenoptera, 370
Hypha of mushroom, 205
Hyphae of moulds, 208, 209
Imago, 363
Incisor teeth of rabbit, 213, 219
Incubator, a simple, 286
Insect, definition of, 351
Insects, 349
Instinct, 252, 296
Internodes, 45
Intestines, 231, 232
Iodine solution, 2
Irritability, 116
Ivy, leaves, 38, 43; stem, 75, 77, 78
Jacobin pigeon, 280
Jaws,
of cockroach, 349, 352, 354;
of crayfish, 375;
of rabbit, 222, 226;
of sheep, 257, 259
Keel,
of bird’s breast-bone, 274;
of pea flower, 100, 102
Kestrel, 329, 330
Keys,
of ash, 153, 156;
of sycamore, 159
Kittens, 248
Labial palps,
of cockroach, 353;
of Lepidoptera, 367
Labiates, 117
Labium,
of caterpillar, 363, 365;
of cockroach, 349, 353
Labrum of cockroach, 349, 352
Laburnum, 101, 102
Lambs, 258, 262
Lark, 311, 316
Larva,
of cabbage-white butterfly, 362, 365;
of Dytiscus, 361;
of tiger-moth, 364, 369;
of vapourer-moth, 364, 370
Leaf fall, 55, 59
Leaf of a grass, 127
Leaves, arrangement of, 44;
of blackberry, 38, 41, 105;
of box, 44, 45;
of ferns, 65, 183, 184, 185, 186, 189, 191;
of horse chestnut, 38, 42, 47, 64;
of ivy, 38, 43;
of rose, 38, 42;
of sweet pea, 39, 43;
of sycamore, 38, 42, 58, 157, 158;
shapes of, 37, 40;
simple and compound, 41;
skeleton, 37, 39;
veins of, 37, 39;
work of, 48
Legume, 165, 168
Ligule, 125, 127
Lily family, 119, 120
Limbs, movement of, 225;
of cat, 246;
of chick-embryo, 288, 292;
of dog, 248;
of pigeon, 266, 270;
of rabbit, 216, 223, 228;
of sheep, 259, 261
Lime, 152, 154, 155; fruit of, 174
Limnaea, 379, 382
Liver, 232, 233
Liverwort, 199, 200
Lobster, 372, 374
Locomotion,
of cat, 247, 251;
of crab, 377;
of dog, 249, 251;
of earthworm, 383, 385;
of fresh-water mussel, 380;
of garden snail, 379, 382;
of insects, 353, 360, 368;
of rabbit, 216
Lombardy poplar, 150, 152
“Lords and Ladies,” 122
Lupine seed, 3, 7, 11, 14
Maize seed, 18, 19; germination of, 21
Male fern, 183, 185
Mammals, 220
Mandibles,
of caterpillar, 365;
of cockroach, 349, 352;
of crayfish and lobster, 373, 375
Mantle of fresh-water mussel, 378, 381;
of garden snail, 379, 381
Maps, use of, in Nature-study, 389
Marjoram, 118
Marsh marigold, 97, 98
Martin,
house, 311, 315;
sand, 311, 315
Mast of beech, 142
Mastication, 219
Mavis, 301, 303
Maxillae,
of cockroach, 350, 353;
of crayfish and lobster, 373, 375;
of Lepidoptera, 367
Maxillary palps of cockroach, 353
Maxillipedes of crayfish and lobster, 373, 375
Meadow,
catstail, 132, 136;
fescue, 132, 134;
flower of, 131;
foxtail, 132, 134, 135;
grasses, 132, 135;
vetchling, 101
Medullary rays, 74
Metamorphosis of frog, 346
Mineral matter in plants, 26, 28, 29
Mint, 118
Missel-thrush, 302, 306
Molluscs, 378
Monkshood, 99
Monocotyledons, 23, 40
Mosses, 199, 201
Moth, 368;
tiger-moth, 364, 369;
vapourer-moth, 364, 369
Moulds, 207, 208
Moulting of birds, 277, 327, 329
Mountain ash, 157
Mucor, 207, 208
Mullein, 116
Muscles, action of, 224, 225
Mushroom, 203
Musk, 115, 116
Mussel, fresh-water, 378, 380
Mustard, 94, 96; seed, 3, 7, 11
Mycelium,
of moulds, 208, 209;
of mushroom, 203, 205
Myriapods, 378
Narcissus, 120
Natural selection, 217, 338
Nectary, 90
Neuroptera, 370
Nodes, 45
Nuts, 175, 177
Oak, 140, 142;
family, 140;
fruit of, 175, 177
Oat, 133, 136
Oranges, 176, 179
Orthoptera, 357
Ovary, 92
Ovule, 92
Oxen, 261
Oxygen, liberated by leaves, 48, 51
Pales, 129, 130
Pancreas, 232, 233
Pancreatic juice, 232, 234
Panicle, 130
Pansy, fruit of, 166, 169
Parsley, 108;
family, 106, 107;
fool’s, 108;
hedge, 107
Parsnip, 108
Passerine order of birds, 320
Pea, 99;
family, 99, 101;
fruit of, 7, 165, 168;
leaves of, 39, 43;
seed, 2, 7;
seed, germination of, 9, 13
Pear, 104, 106; fruit of, 176, 180
Pellia, 199, 200
Penicillium, 207, 208
Perching of pigeon, 271
Perennial rye grass, 133, 137
Perianth, 119, 121
Pericarp, 167
Petal, 91
Pets, 214
Pigeon, 265, 270
Pigeons, different breeds of, 279
Pine cones, 160, 162, 171, 174
Pine, Scotch, 160, 161
Pistil, 91
Plane, 157, 159
Plants and animals, resemblances and differences, 243
Plants, respiration of, 243
Play of young animals, 248, 249, 258, 262
Plough-share bone of pigeon, 269
Plum, 106; fruit of, 175, 179
Plumule of bean, 2, 6
Pod, 103
Poison hemlock, 106, 107, 108
Pollen, 91
Pond-life, 340, 357, 378, 379, 390
Pond-snails, 379, 382
Pouter pigeon, 280
Poplar, 150, 151
Poppy, fruit of, 166, 168
Posterior, 217
Potato, 81, 84
Poultry, 296, 299
Precocious birds, 296, 329
Prickles, 75, 77
Primrose, 108, 109; fertilisation of, 109
Proboscis of butterfly, 367
Pro-legs of caterpillar, 365
Protective colouration,
of frog, 332;
of rabbit, 217
Prothallus,
of ferns, 184, 185, 188, 191;
of horsetail, 192, 195
Pupa,
of cabbage-white butterfly, 363, 366;
of Dytiscus, 361;
of tiger-moth, 364, 369
Rabbit, 211; digestion in, 233
Radicle,
of bean, 2, 6;
of lupine, 7;
of pea, 2, 7;
of sycamore, 4
Radish, 94, 95
Raspberry, 105, 175, 179
Receptacle, 90
Red, clover, 101;
fertilisation of, 101;
corpuscles of blood, 237, 242
Reproduction,
of a grass, 128;
of a moss, 201;
of ferns, 187, 188;
of flowering plants, 89;
of horsetail, 195;
of liverwort, 201;
of moulds, 208, 209;
of mushroom, 205
Respiration, 240;
definition of, 242;
of chick-embryo, 294;
of cockroach, 355;
of crustaceans, 376, 377;
of fresh-water mussel, 381;
of frog, 337;
of mammals, 242, 243;
of pigeon, 279;
of snail, 379, 382, 383;
of tadpole, 344
Resting stage,
of butterfly, 363, 366;
of Dytiscus, 361
Ribs of rabbit, 217, 223, 227
Rock pigeon, 280
Rodents, 220
Rook, 312, 318
Root cap, 17
Root, hairs, 17, 28, 30; lengthening of, 16
Rootlets, 17
Roots, 17, 21, 28;
as storehouses of food, 28, 30;
climbing, 75, 78;
work of, 28, 30
Rose, 103, 104;
family, 103;
fruit of, 176, 180;
leaves of, 38, 42
Rowan tree, 157
Ruminants, 261
Runner, 80, 82
Rushes, 133, 138
Rye grass, 133, 137
Saliva, 233, 234
Sallow willow, 150
Sand-martin, 311, 315
Saugh tree, 150
Scales of Lepidoptera, 364, 367
Scotch pine, 160, 161
Secondary thickening, 73
Sedges, 133, 138
Seeds,
germination of, 9, 12;
of broad bean, 1, 5, 9, 13;
of maize, 18, 19;
of mustard, 3, 7, 11;
of pea, 2, 7;
of sycamore, 3, 8, 11, 13;
of vegetable marrow, 3, 7, 11;
of wheat, 18, 19;
of yellow lupine, 3, 7, 11
Sepal, 90
Sheep, 257, 258
Sheep’s,
fescue, 132, 133, 134;
heart, structure of, 235, 237
Shell of hen’s egg, 282, 284
Shell,
of fresh-water mussel, 378, 381;
of garden snail, 379, 382;
of slug, 382
Shepherd’s purse, 93, 95; fruit of, 165
Shoot, 69
Shoulder blade of rabbit, 223, 228
Silicula, 165, 168
Siliqua, 165, 168
Skeleton,
leaves, 37, 39;
of arthropods, 351;
of pigeon, 268;
of rabbit, 217, 222, 226;
uses of, 224
Skin of frog, 334, 338
Skull,
of pigeon, 268;
of rabbit, 217, 222, 226
Skylark, 311, 316
Slender foxtail, 134, 135
Smell-sense of moths, 369; of rabbit, 218
Snapdragon, 116
Snowdrop, 120, 122
Soaring, 278
Song-thrush, 301, 303
Sounds of heart, 238
Spanish chestnut, 148
Sparrow, 315, 320
Sparrow hawk, 329, 330
Spawn,
of frog, 339, 340, 341;
of Limnaea, 380
Speedwell, 115, 116
Spiders, 378
Spikelet, 128, 130
Spinneret of caterpillar, 363, 365
Spiracles,
of cabbage-white caterpillar, 363, 365;
of cockroach, 350, 355
Spores,
of ferns, 184, 185, 187, 190, 191;
of horsetail, 195;
of liverwort, 199, 201;
of moss, 200, 202;
of Mucor, 207, 208;
of mushroom, 203, 205;
of Penicillium, 207, 209
Spruce fir, 161, 163; cones, 171, 174
Squirrels, 178, 220
Stamen, 91
Standard, 100, 102
Starch, 229;
action of saliva on, 230, 234;
formation of, in leaves, 32, 34;
in cotyledons, 2, 6;
in leaves, 48, 50;
in twigs, 56;
test for, 2, 32
Stem, lengthening of, 16
Stems,
bleeding of, 67, 70;
climbing, 75;
creeping, 80, 82;
duties of, 68;
food channels in, 67, 68, 69, 71;
hooking, 75;
of grasses, 72, 75;
of ivy, 75, 77, 78;
strengthening of, 71, 72, 75;
thickening of, 72;
twining, 76, 79;
underground, 80, 83
Sternum,
of pigeon, 269;
of rabbit, 217, 223, 227
Stigma, 91
Stipules, 39, 43
Stock, 94
Stolon, 81, 83, 125, 126
Stomach, 231, 232; of ruminant, 260
Stomata, 53
Stone fruits, 175, 179
Storm-cock, 302, 307
Strawberry, 105; fruit of, 176, 180, 181
Style of flower, 91
Sugar, 230
Sugar, formation of,
in germinating pea, 33;
in grasses, 126
Swallow, 310, 313; family, 310, 312
Sweet chestnut, 148
Sweet-scented vernal grass, 133, 137
Swift, 321, 322
Swimmerets of crayfish and lobster, 373, 375
Sycamore, 157, 158;
buds of, 56, 60;
fruit of, 3, 8, 171, 173;
leaves of, 38, 42, 58, 157, 158;
seeds of, 3, 8, 11, 13
Tadpoles, 341, 344
Tail,
of rabbit, 215;
of tadpole, 346, 347
Teeth, of cat, 246, 255;
of dog, 248, 255;
of frog, 333, 336;
of rabbit, 219;
of sheep, 257, 259
Tendrils, 43, 76, 79
Tentacles of snail, 379, 382
Thistle, 112, 114
Thorax,
of cockroach, 350, 353;
of rabbit, 218
Thorns, 75, 78
Throstle, 301, 303
Thrush, 301, 303;
family, 301, 303;
missel, 302, 306
Thyme, 118
Tiger-moth, 364, 369
Timothy grass, 132, 135, 136
Toadstools, 204, 206
Tongue,
of cat, 246, 252;
of frog, 333, 336
Tuber, 81, 84, 191, 194
Tumbler pigeon, 279
Turnip, 94, 95
Twining stems, 76, 79, 80
Valves,
of heart, 236, 238, 239;
of veins, 235, 239
Vapourer-moth, 364, 369
Vegetable marrow,
seed of, 3, 7, 11, 14;
fruit of, 176, 179
Vegetable mould, formation of, by earthworms, 384, 386
Veins,
of leaves, 37, 39;
valves of, 235, 239
Ventral, 217
Ventricles of heart, 235, 237
Vertebrae,
of pigeon, 269;
of rabbit, 223, 227
Vertebral column, 217, 222, 227
Vertebrates, 220, 355
Vetch, 100
Violet, fruit of, 166, 169
Wallflower, 88, 89;
family, 93, 94;
fruit of, 165, 167
Warren of rabbit, 211, 214
Water,
amount of, in plants, 26;
culture, 27;
dropwort, 108;
hemlock, 108
Wheat seed, 18, 19; germination of, 21
Whiskers,
of bat, 256;
of cat, 246, 250;
of rabbit, 217
White,
mould, 207, 208;
of egg, 283, 285
Wild oat, 133, 136
Willow, 150; fruit of, 171, 173
Windhover, 329, 331
Wind-sown seed, 172
Wing-covers,
of cockroach, 350, 354;
of beetles, 358, 359, 362
Winged seeds, 174
Wings,
of bat, 257;
of cockroach, 350, 353;
of pea flower, 100, 102;
of pigeon, 266, 270, 273
Wood 70, 71, 72;
avens, fruit of, 174, 177;
louse, 377
Wool, 257, 258
“Woolly bear,” 364, 369
Worm castings, 384, 386
Worms, 383
Yellowoat Grass, 137
Yolk of hen’s egg, 283, 285
Yorkshire fog, 125, 132, 136
GLASGOW: PRINTED AT THE UNIVERSITY PRESS
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A HEALTH READER
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