Differentiation 195 



The proportions of these intracellular structures often change during 

 differentiation. Thus in moss protonemata the nucleus increases in size 

 from the tip of the caulonema backward but the nucleolus decreases for 

 the first few cells. The nucleus also gradually changes from a spherical to 

 a spindle shape, and other changes are evident (Bopp, 1955). Intra- 

 cellular diversity is particularly conspicuous in the differentiation of large 

 coenocvtic bodies, as in certain algae. 



Differentiation of Histological Patterns. Differentiated cells rarely occur 

 separately but are grouped into tissues. Endodermis, vascular tissues, 

 and many others are familiar examples. These tissue patterns begin 

 to appear in the embryo (Miller and Wetmore, 1945; Fig. 8-6; Spurr, 

 1949; Esau, 1954), grow more diverse in the seedling, and reach their 

 maximum differentiation in the mature plant. Nowhere else are the com- 

 plexities of differentiation so evident as in the development of these 



O.OMM 



Fig. 8-6. Beginnings of differentiation in early embryo of Phlox. Successive stages, 

 showing origin of central procambial core. (From Miller and Wetmore.) 



histological patterns. Specialized as these may be, each constitutes an 

 element in an integrated whole. 



Wall Relationships. One of the basic elements in histological pattern 

 is the relationship of cells to each other. This is determined primarily by 

 the position of new walls in dividing cells relative to walls in adjacent 

 cells. In most tissues a new wall is so placed that it does not come op- 

 posite a neighboring cell partition, and the cells are thus "staggered" 

 in position, like bricks in a building (p. 47). In a few cases, however, 

 walls in adjacent cells are exactly opposite, so that they extend in con- 

 tinuous lines across the tissue (Fig. 8-7). This is particularly evident in 

 cork and in regenerating tissue at wounds. This arrangement lends itself 

 well to the development of aerenchyma, since when such cells pull 

 apart at their corners a larger volume of intercellular space results than if 

 the walls were staggered. The two types of pattern may well be seen 



