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.