38 CHLOROPHYCEAE 



fusion taking place whilst still enclosed or else after they have 

 escaped. The resulting zygote is motile and quadriflagellate. 

 Swarmers are also known which do not fuse, and it has been 

 suggested that these develop from haploid races which have arisen 

 apogamously, but a simpler explanation would be to regard them 

 as zoospores. In any case the principal phase in the life cycle would 

 seem to be diploid. Resting cells are also known in which the walls 

 are thick and stratified. Species have been reported from mosses 

 and algae as well as angiosperms, and as many of them have a 

 decided pathogenic action they must at least be facultative parasites. 



*Chlorococcaceae : Chlorococcum {chloro, green; coccum^ berry) 

 (Cystococcus). Fig. 28. 



Much confusion has existed over this genus, as many of the 

 species formerly described are now known to be phases in the life 



Fig. 28. Chlorococcum humicolum. 

 (After Smith.) 



A-F, various stages in the hfe histor>^ ( x 800). 



cycles of species from other genera. Some of the species have been 

 segregated into the genus Trebouxia, the cells of which form the 

 algal component of several lichens (cf. p. 296). The plants are non- 

 motile spherical cells which vary much in size, occurring singly or 

 else forming a stratum on the soil. There is no eye-spot or con- 

 tractile vacuole ; the chloroplast is parietal, and there may be one or 

 more pyrenoids. The cell walls are two-layered with a thin inner 

 layer and an outer gelatinous one which is sometimes lamellose. 

 The young cells are uninucleate but the adult ones are commonly 



