w- a. ^ b cells 

 f I to KtlJ" bKimoTyied -- ■ 



5 days 



2c/ays 



1 2 



Length of exposure to drought 



Sdays 



Fig. 205 Effect of exposure to drought on A, Hormidium ; B, 'Hor- 

 midiunC stage of Prasiola', C, Zygogonium ericetorum. (After Fritsch 



and Haines.) 



in the permeability of the cell wall, whilst the access of 

 moisture normally brings about changes in the reverse direc- 

 tion. The majority of cells which do plasmolyse lack the 

 characteristic granules, mainly of fat, that are to be found in 

 most terrestrial algae. 

 (5) Those cells which survive after drought do not contain any 

 vacuoles and possess instead a rigid, highly viscous proto- 

 plast which is in a gel condition. This is the normal state of 

 the vegetative cells of Pleurococcus and the cells in the ^Hor- 

 midium' stage of Prasiola. 



380 



