Leaves

The basic angiosperm leaf is composed of a leaf base, two stipules, a petiole, and a blade (lamina). The leaf base is the slightly expanded area where the leaf attaches to the stem. The paired stipules, when present, are located on each side of the leaf base and may resemble scales, spines, glands, or leaflike structures. The petiole is a stalk that connects the blade with the leaf base. The blade is the major photosynthetic surface of the plant and appears green and flattened in a plane perpendicular to the stem.

When only a single blade is inserted directly on the petiole, the leaf is called simple. Simple leaves may be variously lobed along their margins. The margins of simple leaves may be entire and smooth or they may be lobed in various ways. The coarse teeth of dentate margins project at right angles, while those of serrate margins point toward the leaf apex. Crenulate margins have rounded teeth or scalloped margins. Leaf margins of simple leaves may be lobed in one of two patterns, pinnate or palmate. In pinnately lobed margins the leaf blade (lamina) is indented equally deep along each side of the midrib (as in the white oak, Quercus alba; Fagaceae), and in palmately lobed margins the lamina is indented along several major veins (as in the red maple, Acer rubrum; Sapindaceae). A great variety of base and apex shapes also are found.

Many leaves contain only some of these leaf parts; for example, many leaves lack a petiole and so are attached directly to the stem (sessile), and others lack stipules (exstipulate). In compound leaves, a blade has two or more subunits called leaflets: in palmately compound leaves, the leaflets radiate from a single point at the distal end of the petiole; in pinnately compound leaves, a row of leaflets forms on either side of an extension of the petiole called the rachis. Some pinnately compound leaves branch again, developing a second set of pinnately compound leaflets (bipinnately compound). The many degrees of compoundness in highly elaborated leaves, such as bipinnately or tripinnately compound, cause these leaves to often appear to be shoot systems. It is always possible to distinguish them, however, because axillary buds are found in the angle between the stem and the petiole (axil) of pinnately or palmately compound leaves but not in the axils of leaflets.

The three patterns of leaf arrangement on stems in angiosperms are alternate, opposite (paired), and whorled. In alternate-leaved plants, the leaves are single at each node and borne along the stem alternately in an ascending spiral. In opposite-leaved plants, the leaves are paired at a node and borne opposite to each other. A plant has whorled leaves when there are three or more equally spaced leaves at a node.