At the whole plant level, growth of the plant body proceeds by the linear extension of stems and branches, the production of leaf or floral organs, and the elongation and branching of roots, mediated by apical, axillary, and lateral meristems. Secondary growth, or radial thickening mediated by cambial cells, contributes to body size increase in many plants, but will not be considered further in this chapter. Primary stem or leaf and root organ growth, here defined simply as an increase in volume, proceeds in two stages, which I will call phase I, during which cells multiply in cycles of growth and division; and phase II, during which cells cease dividing but expand until differentiation is completed. High rates of proliferation are observed in meristems, in young leaf and floral primordia, but not in stem cells and the stem cell niche (Fig. 1).
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