THE PROBLEM OF PATTERN SPECIFICATION

The form of living organisms is one of their outstanding characteristics: though organisms are complex, their various parts bear predictable, repeated relations to one another. It is this regularity, or the deviation from a random distribution of the various parts, that will be referred to as a patterned structure (Child, 1941; Spemann, 1938; Sinnott, 1963; Wolpert, 1971). The regularity of form is characteristic not only of external shape but also of the microscopic arrangements of the various types of cells and tissues (Fig. 1.1). Furthermore, patterns are apparent in the temporal as well as the spatial relations. The following chapters are concerned with the processes that generate cellular patterns in plant tissues, with the basis of organized, in contrast to tumorous, structure.

Mature biological form is necessarily the result of the non-random distribution of the developmental processes. Form can be changed and even disrupted without preventing the continuation of growth and differentiation: examples are the regenerative events following wounding, the development of callus and tumor tissues and the effects of various mutations. It follows that it is possible to separate the ‘building blocks’ of development, the processes of growth and differentiation, from the regulation of their spatial and temporal occurrence (Wolpert, 1971). Thus discussions of patterning can often accept as given the molecular and cellular processes of growth and differentiation, fascinating though they are, and deal primarily with the determination of their relative location, in both time and space.