THE ALGAE 



i6i 



siphon, which originate by the divisions of an apical cell (Fig. 151). The 

 pericentral cells are cut off successively from the central cells by a series of 



Cortical 

 siphons 



Fig. 149. — Polysiphonia nigrescens. Part of vegetative 

 thallus showing dichotomous growth. 



Central 

 siphon 



M 



ytgff^ 



A 



B 



Fig. 150. — Polysiphonia sp. Young branch of thallus. A, Surface view showing cortical 

 cells. B, Optical section showing central siphon and pericentral siphons. 



tangential longitudinal divisions which begin at one side and continue in 

 both directions round the cells (Fig. 152). 



