PELAGIC PLANT LIFE 



319 



their cells diminishing by being divided and increasing again 

 owing to the formation of auxospores (see Fig. 220), it is 



Fig. 220.— Colonies of Thalassiothrix nitzschiowes (^f^). 

 (/, With long cells shortly after auxospore formation ; b, with shorter and thicker cells. 



difficult to show in the case of many species to what extent 

 variations are due to adaptation and regulation of their floating 

 power, though in the case of some chain-forming species it is 



Fig. 221. — Parts of two chains of Chmtoceras decipiens (^f^). 

 a. From the Atlantic off the coast of Spain, April 1910 ; b, from Christiania-Fjord, March 191 1. 



evident enough. Chcetoceras decipiejis, one of the commonest 

 species in the northern Atlantic, consists of straight chains 

 of flattened, almost rectangular cells, every one of which is 



