Fig. 7.

There is one variety of alternation, called two-ranked, which is quite characteristic of certain trees; that is, the leaves are so flattened out as to be in one plane on the opposite sides of the twig ([Fig. 6]). The Elm-trees form good examples of two-ranked alternate leaves, while the Apple leaves are alternate without being two-ranked. Most leaves spread from the stem, but some are appressed, as in the Arbor-vitæ ([Fig. 7]). In this species the branches are two-ranked.

Parts of Leaves.—A [complete leaf] consists of three parts: the blade, the thin expanded portion; the petiole, the leafstalk; and the stipules, a pair of small blades at the base of the petiole. The petiole is often very short and sometimes wanting. The stipules are often absent, and, even when present, they frequently fall off as soon as the leaves expand; sometimes they are conspicuous. Most Willows show the stipules on the young luxuriant growths.

Fig. 8.

Veining.—The leaves of most trees have a distinct framework, the central line of which is called a midrib; sometimes the leaf has several other lines about as thick as the midrib, which are called ribs; the lines next in size, including all that are especially distinct, are called veins, the most minute ones being called veinlets ([Fig. 8]).

Kinds.—Leaves are [simple] when they have but one blade; compound when they have more than one. Compound leaves are [palmate] when all the blades come from one point, as in the Horse-chestnut; pinnate when they are arranged along the sides, as in the Hickory. Pinnate leaves are of two kinds: [odd-pinnate], when there is an odd leaflet at the end, as in the Ash, and [abruptly pinnate] when there is no end leaflet.

Many trees have the leaves twice pinnate; they are either [twice odd-pinnate] or [twice abruptly pinnate]. The separate blades of a compound leaf are called leaflets. Leaves or leaflets are sessile when they have no stems, and petiolate when they have stems.

When there are several ribs starting together from the base of a blade, it is said to be radiate-veined or [palmate-veined]. When the great veins all branch from the midrib, the leaf is [feather-veined or pinnate-veined]. If these veins are straight, distinct, and regularly placed, the leaf is said to be [straight-veined]. The Chestnut is a good example. Leaves having veinlets joining each other like a net are said to be netted-veined. All the trees with broad leaves in the northern United States, with one exception, have netted-veined foliage. A leaf having its veinlets parallel to one another is said to be [parallel-veined or -nerved]. The Ginkgo-tree, the Indian Corn, and the Calla Lily have parallel-veined leaves. The narrow leaves of the cone-bearing trees are also parallel-veined.