Mechanisms of Plant Reproduction: Questions and Approaches.

It is a daunting but exciting task to attempt both an orientation to plant development and an overview of the reviews in this special issue on plant reproduction. Our purpose in this arti- cle isto provide a guide to the key points of plant development and reproduction, pointing out not only the general themes that are common among angiosperms but also some of the exceptional species and events that illustrate the diversity of types adapted to specific selective forces. We view the eluci- dation of developmental mechanisms in plant reproduction as both a goal in itself and a prelude to a more comprehensive appreciation of the evolutionary steps that have led to the diver- sity of existing angiosperm species. Severa1 previous reviews (Walbot, 1985; Goldberg, 1988) discuss more fully the earlier state of the art of many aspects of plant development and reproduction. Evolution has hit upon at least two distinctly different strate- gies for being multicellular: the animal strategy, in which the organism-and, within limits, its cells-can move, and the plant strategy, in which both the organism and its cells are largely fixed in place. These two strategies underlie everything the organism does, from the way it responds to predators and obtains nutrition to the way it reproduces. For example, in animals, the entire body plan and the organ systems are gener- ally established during embryo development. As organs grow, they receive new cells from stem cell populations whose derivatives have a limited range of possible pathways of dif- ferentiation. By contrast, all postembryonic plant structures emerge from apical meristems that arise early in embryonic development. These meristems serve as reservoirs of cells throughout plant development, replenishing themselves while giving rise to lateral meristems, which in turn develop into or- gans such as leaves (see Steeves and Sussex, 1989). Although there is continuous organogenesis from the shoot apical meristem, this cell population matures over time. The meristem produces juvenile Ieaves initially, followed by adult leaves; in maize, and possibly other plants as well, the same apical meristem earlier gave rise to embryonic leaves (cotyledons). Reproductive development begins with the tran- sition of the apical meristem from producing vegetative