Leaves are the lungs of plants. The food taken in by the roots has to pass through the stem to the leaves to be acted upon by the air, before it becomes sap and is fit to be used for the growth of the plant. No portion of a plant is more varied in parts, forms, surface, and duration than the leaf.

No one can become familiar with leaves, and appreciate their beauty and variety, who does not study them upon the plants themselves. This chapter therefore will be devoted mainly to the words needed for leaf description, together with their application.

The Leaf.—In the axil of the whole leaf the bud forms for the growth of a new branch. So by noting the position of the buds, all the parts included in a single leaf can be determined. As a general thing the leaf has but one blade, as in the Chestnut, Apple, Elm, etc.; yet the Horse-chestnut has 7 blades, the Common Locust often has 21, and a single leaf of the Honey-locust occasionally has as many as 300. Figs. 17-58 ([Chapter VII.]) are all illustrations of single leaves, except [Fig. 43], where there are two leaves on a twig. A number of them show the bud by which the fact is determined (Figs. [25], [26, 31, 33, 34], [36, 40], etc.); others show branches which grew from the axillary buds, many of them fruiting branches (Figs. [37, 42, 43], [50], and [54]), one ([Fig. 51]) a thorny branch.

The cone-bearing plants (Figs. [59]-67) have only simple leaves. Each piece, no matter how small and scale-like, may have a branch growing from its axil, and so may form a whole leaf. A study of these figures, together with the observation of trees, will soon teach the student what constitutes a leaf.

Fig. 5.

Arrangement.—There are several different ways in which leaves are arranged on trees; the most common plan is the [alternate]; in this only one leaf occurs at a joint or node on the stem. The next in frequency is the [opposite], where two leaves opposite each other are found at the node. A very rare arrangement among trees, though common in other plants, is the [whorled], where more than two leaves, regularly arranged around the stem, are found at the node. When a number of leaves are bundled together,—a plan not rare among evergreens,—they are said to be fasciculated or in [fascicles]. The term scattered is used where alternate leaves are crowded on the stem. This plan is also common among evergreens.

Fig. 6.

Caution.—In some plants the leaves on the side shoots or spurs of a twig are so close together, the internodes being so short, that at first sight they seem opposite. In such cases, the leaf-scars of the preceding years, or the arrangement of the branches, is a better test of the true arrangement of the leaves. The twig of Birch shown in [Fig. 5] has alternate leaves.