Integrative plant anatomy

Abstract

This chapter describes the origin and structure of the primary plant body. The postembryonic primary plant body arises from small, organized cell populations termed “meristems” that are located at the tips of the stems and the roots. Meristems are regions of active cell division, enlargement, and differentiation. The vegetative shoot apical meristem of angiosperms has a stratified organization, with two or more outer tunica layers in which cell divisions are primarily all anticlinal or oriented at right angles to the surface. Below the tunica layers is a region of corpus cells in which divisions occur in all planes. Active shoot apical meristems exhibit cytohistological zonation, in which the cells in the central zone are relatively large and divide infrequently as compared to the surrounding peripheral zone cells that divide more rapidly. A rib meristem underlies the central zone. Leaves are determinate organs initiated as groups of cells on the flanks of the shoot apex in precise and stable patterns. Leaf organogenesis commences with coordinated changes in the rate and pattern of cell division and expansion in different regions of the developing primordium.