Growth and Differentiation of the Shoot

Abstract

The aerial portion of the seedling considered in the previous chapter is generally known as the shoot system. The main axis of the shoot is the stem, which provides support for leaves and bear flowers and fruits at a later stage in its ontogeny. The stem is terminated by a self-perpetuating population of small, isodiametric, rapidly dividing cells known as the shoot apical meristem. It is this meristem that gives rise to the leaf primordia and produces the tissues that contribute to the increase in length of the stem. Additional shoot apical meristems are formed in the axils of leaves or adventitiously on cultured explants. The shoot apical meristem is often protected by leaf primordia, and in this configuration, it is known as the bud. One of the outstanding characteristics of the shoot apical meristem is that as a center of postembryonic development, it functions as an organizer that determines the fate of its own derivatives and in the process continues to produce leaves or floral meristems repeatedly throughout the life of the plant. In other words, the shoot apical meristem is capable of indeterminate growth. Because of the close relationship between the shoot apical meristem and its immediate derivatives such as the leaf, internode, node, and axillary bud and the reiterative manner in which they are cut off during the vegetative life of the plant, the activity of this meristem has been likened to a modular construction. Thus, what comes out of the meristem is a module, and a modular unit comprising the leaf, internode, node, and axillary bud is termed a phytomer or metamer. In a general sense, the shoot apical meristem can be considered analogous to the stem cell population that programs the continued growth of certain structures such as the hair or nails in animals, although the latter are not akin to modules.