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.