190 The Phenomena of Morphogenesis 



(p. 131), for example, the last division of many (sometimes of all) of 

 the surface cells results in a small daughter cell at the apical end and a 

 larger one at the basal end. This initial difference is intensified during 

 the later development of these cells, for the smaller cell (a trichoblast) 

 sends out from its surface an elongate sac which becomes a root hair. 

 Such a structure is lacking in the larger cell (Cormack, 1949). The 

 beginning of this difference may be seen even before the last division, 

 for the cytoplasm at the apical end of the mother cell becomes much 

 more dense than that at the basal end. Differentiation between the two 

 daughter cells is thus related to the strongly polar character of the mother 



Fig. 8-3. Section through developing liverwort sporangium showing differentiation of 

 alternating spores and elater cells. ( From Goebel. ) 



cell. In Phalaris the epidermal cells contain a natural red pigment which 

 is deeper in color in the prospective root-hair cells, and they can be 

 distinguished early for this reason (Bloch, 1943b). In many plants the 

 surface cells of the root are all potentially alike, and the differentiation 

 of some cells into root hairs and others into hairless cells is not de- 

 termined at a differential cell division but by environmental factors. The 

 difference between these two types of root-hair determination may be 

 related to anatomical characters (Cormack, 1947). A close relation exists 

 between the distribution of cellulose-forming enzymes and the location 

 of the root hair on a surface cell ( Boysen-Jensen, 1950 ) . 



There are many somewhat similar cases of cellular differentiation. In 



