Trees of the Northern United States. A. C. Apgar
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.
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.
Forms.—Leaves can readily be divided into the three following groups with regard to their general outline:
1. Broadest at the middle. Orbicular, about as broad as long and rounded. Oval, about twice as long as wide, and regularly curved. Elliptical, more than twice as long as wide, and evenly curved. Oblong, two or three times as long as wide, with the sides parallel. Linear, elongated oblong, more than three times as long as wide. Acerose, needle-shaped, like the leaf of the Pine-tree.
2. Broadest near the base. Deltoid, broad and triangular. Ovate, evenly curved, with a broad, rounded base. Heart-shaped or cordate, similar to ovate, but with a notch at the base. Lanceolate, shaped like the head of a lance. Awl-shaped, shaped like the shoemaker's curved awl. Scale-shaped, short, rounded, and appressed to the stem. The Arbor-vitæ has both awl-shaped and scale-shaped leaves.
3. Broadest near the apex. Obovate, same as ovate, but with the stem at the narrow end. Obcordate, a reversed heart-shape. Oblanceolate, a reversed lanceolate. Wedge-shaped or cuneate, having a somewhat square end and straight sides like a wedge.
These words are often united to form compound ones when the form of the leaf is somewhat intermediate. The term which most nearly suits the general form is placed at the end; thus lance-ovate indicates a leaf between lanceolate and ovate, but nearer ovate than lanceolate; while ovate-lanceolate indicates one nearer lanceolate.
Bases.—Oftentimes leaves are of some general form, but have a peculiar base, one that would not be expected from the statement of shape. An ovate leaf which should have a rounded base might have a tapering one; it would then be described as ovate with a tapering base. A lanceolate leaf should naturally have a tapering base, but might have an abrupt one. Many leaves, no matter what their general form may be, have more or less notched bases; such bases are called cordate, deeply or slightly, as the case may be; and if the lobes at base are elongated, auriculate. If the basal lobes project outward, the term halberd-shaped is used. Any form of leaf may have a base more or less oblique.
Points.—The points as well as the bases of leaves are often peculiar, and need to be described by appropriate terms. Truncate indicates an end that is square; retuse, one with a slight notch; emarginate, one with a decided notch; obcordate, with a still deeper notch; obtuse, angular but abrupt; acute, somewhat sharpened; acuminate, decidedly sharp-pointed; bristle-pointed and awned, with a bristle-like tip; spiny-pointed, with the point sharp and stiff (Holly); mucronate, with a short, abrupt point.
Margins.—Entire, edge without notches; repand, slightly wavy; sinuate, decidedly wavy; dentate, with tooth-like notches; serrate, with notches like those of a saw; crenate, with the teeth rounded; twice serrate, when there are coarse serrations finely serrated, as on most Birch leaves; serrulate, with minute serrations; crenulate, with minute crenations. Leaves can be twice crenate or sinuate-crenate. Revolute indicates that the edges are rolled over.
When a leaf has a few great teeth, the projecting parts are called lobes, and the general form of the leaf is what it would be with the notches filled in. In the description of such leaves, certain terms are needed in describing the plan of the notches, and their depth and form.
Leaves with palmate veining are palmately lobed or notched; those with pinnate veining are pinnately lobed or notched. While the term lobe is applied to all great teeth of a leaf, whether rounded or pointed, long or short, still there are four terms sometimes used having special signification with reference to the depth of the notches. Lobed indicates that the notches extend about one fourth the distance to the base or midrib; cleft, that they extend one half the way; parted, about three fourths of the way; and divided, that the notches are nearly deep enough to make a compound leaf of separate leaflets.
So leaves may be palmately lobed, cleft, parted or divided, and pinnately lobed, cleft, parted or divided. The term pinnatifid is often applied to pinnately cleft leaves. The terms entire, serrate, crenate, acute-pointed, etc., are applied to the lobes as well as to the general margins of leaves.
Surface.—The following terms are needed in describing the surface of leaves and fruit.
Glabrous, smooth; glaucous, covered with a whitish bloom which can be rubbed off (Plum); rugous, wrinkled; canescent, so covered with minute hairs as to appear silvery; pubescent, covered with fine, soft, plainly seen hairs; tomentose, densely covered with matted hairs; hairy, having longer hairs; scabrous, covered with stiff, scratching points; spiny, having stiff, sharp spines; glandular-hairy, having the hairs ending in glands (usually needing a magnifying glass to be seen).
Texture.—Succulent, fleshy; scarious, dry and chaffy; punctate, having translucent glands, so that the leaf appears,