The continued growth of the plant body depends upon the production of new cells by mitotic activity in its meristematic regions. One might predict that this would result in a homogeneous cell population, because mitosis ordinarily leads to the formation of identical sister cells. It is obvious, however, that the plant body does not consist of such a uniform assemblage of cells. Rather, it is composed of diverse specialized cells arranged in patterns having functional significance. If this were not the case, the plant could function, at best, in only a very restricted manner. The phenomenon of differentiation, as this production of diverse cell types in definite patterns is called, has been alluded to in earlier chapters because it is almost impossible to consider growth apart from it. Now it is necessary to turn attention specifically to the phenomenon itself, one of the major topics of interest in modern developmental biology.

GENETIC CORRELATES OF DIFFERENTIATION

The diversity of differentiated cell types might suggest that genetic changes must be involved in differentiation. The preponderance of evidence, however, indicates that cellular diversity within the organism is accomplished in a framework of genetic homogeneity. The most striking evidence in support of this principle is to be found in the well-known regeneration phenomena characteristic of plants, which will be discussed fully in Chapter 17. Roots, shoots, and in many cases whole plants are often regenerated from fully differentiated cells either as a normal process or as a result of wounding or some other stimulus.