Differentiation 191 



Ricinus the secretory cells are formed, in the young meristem or in later 

 development, by differential cell division (Bloch, 1948), though conform- 

 ing to no sharp pattern. Once formed, these cells continue to divide, pro- 

 ducing rows of similar cells in a cell lineage. This may even persist in 

 tissue culture. 



The differentiation between chlorophyll-bearing and colorless cells in 

 the leaf of Sphagnum results from a differential division, preceded by a 

 polar movement of the cytoplasm (Zepf, 1952). The origin of elaters in 



Fig. 8-4. Trochodendron. Section of leaf with a large branching sclereid. (From 

 Foster. ) 



the capsule of liverworts is similar, a cell of the archesporial tissue divid- 

 ing into a spore mother cell and an elater cell ( Fig. 8-3 ) . 



Certain trichosclereids develop in much the same manner. These cells, 

 which in Monstera become very long and thick-walled, are set apart at 

 the last division of certain cells of the meristematic cortex. The smaller 

 daughter cell (this time at the basal end), possessing a relatively larger 

 nucleus, develops into the sclereid and the other into a typical parenchyma 

 cell. Although the sclereid begins in this case as the smaller daughter cell, 

 it soon sends out one or more processes which grow longitudinally be- 



