The Physiology of Flowering Plants - KHAM PHA MOI

Chapter 9Vegetative development9.1 IntroductionAs described in Chapter 8, plant growth results from a combinationof cell division, elongation and differentiation, initiated from groupsof cells known as meristems. During embryogenesis two meristemsare formed – the shoot apical meristem which gives rise to the shootsystem, and the root apical meristem which forms the root system.As these primary meristems develop, they give rise to more apicalmeristems which will form side branches or lateral roots, and lateralmeristems which result in an increase in girth (Fig. 9.1).Together, environmental and internal signals control the rateof growth, the activation of new meristems, and the differentiationof cells and tissues, producing the plant body within the frameworkof the basic ‘body plan’ of the plant. Meristem activity must be underprecise control to generate the specific structures of the plant, but atthe same time must be flexible enough to respond to environmentalsignals. Some of the controlling signals, and the genetic systems onwhich they act, are discussed below, with reference to the formationof the vegetative organs of the plant. The formation of reproductivestructures is covered in Chapter 11.9.2 The structure and activity of the shoot apicalmeristem9.2.1 Organ initiationDuring the vegetative phase of growth, the shoot apical meristem(SAM) produces stem, leaves and axillary buds in units known asphytomers. Each phytomer consists of a leaf (or leaves) joined to astem node, the associated axillary bud (or buds) in the leaf axil(s), andthe length of internode down to the next node (Fig. 9.1). The timeinterval between successive leaf initiations is termed the plastochron,of very variable length according to species, plant age and environmentalconditions, but taking at least a few days. The organ primordia