CONTROLS OF CELLULAR DIFFERENTIATION

Tissue patterning is an orderly, controlled expression of various potentials for cellular growth and differentiation. It is often assumed, or at least implied, that the capacity of plant cells to differentiate is unlimited and that each event is specified by the precise conditions the cells are in at some critical, early period of their development. This dogma is certainly not borne out by the discussions above, and an alternative set of generalizations is required. Thus, before the patterning processes that were arrived at in the previous chapters are summarized and discussed it is necessary to consider what developmental alternatives are available to plant cells and what determines their expression.

Totipotentiatility and limited developmental alternatives

Organized development, like other major functions of a plant, is limited to special tissues. The cells of these developing tissues are of distinct, differentiated types: they have unique characteristics and undergo only limited developmental changes, even in experimental conditions (Chapter 10). Furthermore, it is not just development but also its various component processes that are performed by cells of different specialization. Organogenesis occurs only in promeristematic, or embryonic, regions – in small cells with a dense cytoplasm, no obvious polarity, and slow growth. Most increase in volume or growth occurs in primary meristems where development is rapid, polarized and generally of limited duration. Cambia, whose cells differ from those of promeristems, are responsible for increase in diameter.