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.

=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.

=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.

=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_.

(_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.

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.

=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.