There are over 275,000 different kinds of plants and most of them can be distinguished from each other by their leaves alone. Leaves originate as primordial in the buds regardless of their ultimate size and shape. When all is said and done, leaves usually consist of a stalk, the petiole, and a flattened blade, the lamina, which has a network of veins also known as the vascular bundles. Some leaves have a pair of appendages called stipules at the base of their petiole. In some cases, there is no petiole or stalk, and these leaves are called sessile. Deciduous trees generally lose their leaves once a year, after the growing season. Evergreens, or conifers, usually are only functional for two to seven years.
The overall arrangement of leaves with respect to the plant stem is called phyllotaxy. Leaves may be arranged in an alternate, opposite pattern if they are attached at the same node, or a whorled pattern if three or more are attached at a node. The leaf itself may be a simple leaf, which has an undivided blade; or a compound leaf, in which the blade is divided into leaflets in various ways. A pinnately compound leaf has leaflets in pairs along a central stalk—called the rachis.
A palmately compound leaf has all its leaflets attached at the same point at the end of the petiole. The leaflets of a pinnately compound leaf are sometimes subdivided into even smaller leaflets which makes a bipinnately compound leaf. The venation, or arrangement of vascular bundles, in a leaf blade or a leaflet may be either pinnate or palmate. A pinnately veined leaf has a main vein called the midrib with secondary veins branching out from it. However, in a palmately veined leaf, several veins branch out from the base of the blade—rather than from a central midrib. Monocot plants generally have leaves with parallel venation as compared with dicots, which have branching and diverging veins. The Ginkgo tree is special in that it has no midrib or other large veins. The veins fork evenly and progressively from the base of the blade out to the opposite margin of the leaf. This arrangement is called dichotomous (branching) venation.
