Leaf morphogenesis in flowering plants.

Leaves, or organs derived from leaves, are the most morphologically diverse structures in a plant (Gifford and Foster, 1989). They occur as prominent photosynthetic structures; as inconspicuous fragments of tissue in tubers, rhizomes, and the stems of many cacti; as nonphotosynthetic storage organs in bulbs; and, in carnivorous plants, as complex structures specialized for catching and digesting small animais. Given this natural diversity, it is not surprising that there is some disagreement about what actually constitutes a leaf (Sattler and Rutishauser, 1992; Rutishauser, 1994). Most leaves have three more or less distinct parts: a leaf base, which may ensheathe the stem; a basal stalk, known as the petiole or rachis; and a dista1 portion, known as the blade or lamina, which is usually green and flat (Figure 1). Two major types of leaves, simple and compound, are distinguished by the shape of the blade. In a simple leaf, the blade is a unitary structure that may have a relatively smooth outline, or it may be moderately or very highly indented. In a compound leaf, the rachis produces a series of leaflets known as pinnae. Some leaves do not have all of these parts, and the form that each part takes in different species or within a single plant is highly variable. Severa1 other features are considered characteristic of leaves. Most leaves have dorsoventral asymmetry. That is, the side of the leaf toward the stem (the dorsal or adaxial surface) and the side oriented awayfrom the stem (the ventral or abaxial surface) are morphologically and anatomically different. Second, leaves are determinate structures, meaning that they do not grow indefinitely. Finally, leaves can usually be distinguished from other leaflike structures by the presence of one or more axillary buds at the junction between the base of the leaf and the stem. It is helpful to divide leaf development into several stages based on the time at which various features of the leaf become determined (Sylvester et al., 1996). During the first stage, the leaf primordium is initiated and acquires its identity as a leaf. During the second stage, the major parts of the leaf become determined and acquire their basic shape, and during the final phase, the histogenesis of the leaf is com-