Simple and compound leaves: reduction or multiplication?

Flowering plants can have either simple or compound leaves. Compound leaves have an interesting evolutionary history, and arose independently in the ferns, cycads and flowering plants. The compound structure may be a consequence of simple leaves that are subdivided into separate leaflets; inherently different from simple leaves, and intermediate between shoots and leaves; or the leaf ground state, with suppression and reduction leading to simple leaves. The knox genes have been extensively studied to determine their role in leaf initiation and development. Preliminary evidence indicates that the simple and compound leaf types, though superficially similar at the time of initiation, have differences that arise in the preprimordium stage in the shoot apical meristem. Compound leaves in peas and tomato probably have independent origins from an ancestrally simple state in the basal angiosperms. Genetic and molecular analyses of leaf morphogenesis in diverse organisms will help determine whether independent origins of compound leaves gave rise to similar structures.