METABOLIC GRADIENTS 75 



gives place to an apical region, consisting of several or 

 many cells, and in the course of this change the apical 

 cell itself becomes relatively smaller, and a gradient of 

 increasing size extends from the apical region (Figs. 18, 

 36). The gradients in size in different forms depend 



20 



FIG. 20. Axial gradient in cell size in alga Cladostephus (from 

 Pringsheim) . 



upon the relation between frequency of division and 

 growth in size of the apical cell, and this relation shows a 

 characteristic range in each form. Even where the 

 whole plant body is a single multinucleate cell, the 

 apical regions of stem and branches are undoubtedly 

 physiologically growing tips. In the higher plants the 



