The early stages of differentiation, considered in Chapter 13, are characterized by the blocking out of regions within which cells are relatively homogeneous and behave similarly. In contrast, the later stages of differentiation show highly localized specializations, often with adjacent cells differing markedly in the developmental changes they undergo. Therefore, in studying these stages of differentiation it is essential to pay close attention to the events taking place in individual cells, and this will be the first task. When this has been done, it will be necessary to return to the higher levels of organization to consider the interrelations that keep the differentiating cells as part of an organized system.

CELLULAR CHANGES DURING DIFFERENTIATION

In contrast to the body of higher animals with its large number of differentiated cell types, relatively few cell types, possibly no more than twelve, are differentiated in the body of the vascular plant. However, to describe the cellular events related to differentiation of even this small number of cell types would fill many pages. Rather than do this, attention will be concentrated on the differentiation of the tracheary elements of the xylem, the tracheids and vessel members, and the sieve elements and companion cells of the phloem. These differentiate in the primary body from procambium and in the secondary body from cambium, and because the sequence of differentiation is generally similar in both cases, it usually will not be necessary to make distinctions between them in the following description.