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.