EUPHYCOPHYTA 



43 



groups, and usually lack pyrenoids. The species are not readily 

 determined taxonomically but as many are kept in pure culture 

 they are often given strain numbers. It seems that cells grown in 

 the dark differ from those grown in the Hght. Quiescent dark cells 

 are small and have a photosynthetic quotient (O2/CO2) of i. They 

 give rise to dividing dark cells which in turn give rise to 'Ught' cells 

 in daylight. The light cells are larger, poorer in chloroplast material 

 and less active photosynthetically (photosynthetic quotient = 3). 

 During the course of development the amoimt of deoxyribonucleic 

 acid increases. 'Light' cells give rise to a new generation of 'dark' 

 cells in the dark. Reproduction is by means of division into 2, 4, 8 

 on 6 autospores. Several species form a symbiotic association with 

 lower animals when they are known as Zoochlorella (cf. p. 427). 



Hydrodictyaceae : P^ J/a5/rwm {pedi^ foot; astrum, star). Fig. 19 



This alga occurs as a coenobium and it represents a more ad- 

 vanced stage of the simple condition seen in the fresh-water algae 



Fig. 19 Pediastrum. A, P. horyanum ( x 333). B, P. simplex yar. 

 duodeniarum ( x 333). Cj P- boryanum var. granulatum showing 

 liberation of zoospores. D, P. duplex with hypnospores. E, P. 

 horyanum gennination of tetrahedron. F, P. horyanum var. 

 granulatum, formation of new plate. (A, B, after Smith; C-F, after 



Fritsch.) 



