During the winter nearly all leaves are folded in various ways in a bud for protection from the elements. Nature shows herself in some of her wisest moods in the selection of methods to accomplish this. In some buds, notably those of the horse-chestnut, the bud is coated with a sticky substance to protect the tender young leaves inside. In others there is a hard outer coat, as in the hickory, impregnable to the most driving sleet, others again have the leaf rolled so tightly and pointed so sharply at the end, as in the beech, that water cannot cling to the bud nor soak in, until the warmth of spring gives the signal for the annual miracle of the bursting out of foliage. Leaf buds are sometimes hard to find on certain plants, as they are formed at the base of a leafstalk and covered by it during the growing season. It is only as the leaf falls in the autumn that the hollow base of its stalk is seen to have hidden during the summer the young bud for the following season. The plane tree or sycamore is a good example of a plant where no leaf buds can be found until the falling of the leaves in autumn.
The forms of leaves are infinite in their variety, and the reasons for some of their peculiarities in this respect are not yet understood. The average netveined leaf is obviously composed of a blade ([Figure 25]), and at the base a stalk known as a petiole. Sometimes at the base of the petiole—which is lacking in many leaves—there are two tiny leaflike appendages, called stipules, which are of no apparent use to the plant, and, as if in recognition of this fact, they often fall off long before autumn. In some plants, however, stipules are permanent, while in certain others they are never found, as, for instance, in the horse-chestnut tree.
Fig. 25. Simple leaf with blade, leafstalk (petiole), and two stipules at the base. Margins of the leafblade serrate or saw-toothed. Fig. 26. Leaf with a sagittate base, or shaped like an arrowhead, the lobes pointing downward, and with entire margins. Fig. 27. Retuse or emarginate tip, somewhat indented. Fig. 28. With the base auriculate or with rounded basal lobes. Fig. 29. Hastate, like an arrowhead but the lobes pointing outward. Fig. 30. With cuneate base (wedge-shaped). Fig. 31. Cuspidate tip with a usually hard and stiff point. Fig. 32. Perfoliate, the leaf bases joined and the stem passing through them. Fig. 33. Truncate, the top flattened. Fig. 34. Pinnately lobed, with deep indentations cut toward the midrib. Fig. 35. Palmately lobed, out toward the top of the leafstalk.
The outline of leaves is as varied as nature itself. Some of the common kinds are shown in drawings ([Figures 13-24]), which tell more of the story than pages of description could do. Their margins, too, their tips, their bases ([Figures 25-35]), all parts of them, in fact, are so variable and yet in each kind of plant so uniform, that in the description of the plants of any region the botanist has used these characteristics of leaves as one method of identifying the particular plant in hand.