The Initiation and Determination of Leaves.

Leaves are initiated sequentially in precisely ordered patterns throughout the vegetative phase of shoot development by the apical meristem, which maintains itself in the process as an organized unit of cells whose fates within the shoot are not yet determined. Leaves originate relatively uniformly as sim? ple peg-like outgrowths, and subsequently exhibit divergent patterns of determinate growth. The leaf has been character? ized as a developmental "ground state," an identity that is modified through the action of homeotic genes to generate floral organs (Coen and Meyerowitz, 1991). Yet leaves themselves are diverse, not only among different species of plants, but also within an individual. Plants frequently exhibit some degree of heterophylly, producing different leaves on the same shoot system. Alternative leaf forms on a single plant are of? ten sufficiently different with respect to morphology and anatomy that they could be said to have different "identities" in the same sense that leaf and petal are different identities. Furthermore, leaves are subdivided into regions of differing identities, such as the petiole and blade of a typical dicot leaf or the sheath and blade regions of a grass leaf. Thus, leaf de? termination, the process by which the characteristics of the leaf are fixed, is a complex problem in its own right. How is a leaf initiated, and how are its characteristics determined? Here, we explore these questions by reviewing selected aspects of early leaf development in angiosperms, discussing where possible the underlying mechanisms of control such as the roles played by genes and hormones. Excellent reviews with similar themes have been written previously (for example, Halperin, 1978; Lyndon, 1983).