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. i8, 

 36). The gradients in size in different forms depend 



Fig, 20. — Axial gradient in cell size in alga Cladostephiis (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 



