§?f ht S" 



Fig. 23 Ulothrix zonata. A, B, rhizoid formation. C, liberation of 

 swarmers into vesicle ( x 375). D, germination of aplanospores in 

 the cell ( X 250). E, liberation of gametes ( x 375). F, escape of 

 zoospores ( x 375). G, akinetes of U. idiospora. H, palmelloid 

 condition. I, schema to illustrate the different types of filaments and 

 swarmers. J, K, aplanospores ( x 400). L, zoospore formation 

 ( x 400). M, banded chloroplasts in a portion of the vegetative 

 filament ( x 400). (A, B, I, after Gross; C-F, J-M, after West; G, 



H, after Fritsch.) 



rhizoids may grow out from the cells or else the filaments become 

 branched. This suggests one way at least in which the branched 

 habit may have evolved from the simple filament, in this case prob- 

 ably representing an attempt to increase the absorbing surface in 

 order to counteract the deficiency of salts. The cells of the different 



50 



