40 



THE ALGAE 



Under dry conditions aplanospores are formed: when this oc- 

 curs the parent cell gelatinizes and a *palmella' stage results, the 

 cells of which subsequently give rise to 2 or 4 biflagellate gametes. 

 In culture solutions of low nutrient concentration reproduction 

 takes place by means of zoospores, whilst in highly concentrated 

 solutions they are replaced by aplanospores. The environment can 

 therefore affect the type of reproductive body. The production of 

 aplanospores suggests how the genus Chlorella may have arisen. 



Fig. 15 Chlorococcum humicolum. A-C, vegetative cells. 

 D-Fj zoospore formation ( x 800). (After Smith.) 



whilst the multinucleate state indicates a transition to a coenocytic 

 phase that is further developed in other genera {Protosiphon, 

 Hydrodictyon), 



Asexual reproduction is by zoospores or aplanospores and sexual 

 reproduction by biflagellate iso- or anisogametes. There is no evid- 

 ence of even a primitive oogamy such as is found in Chlamydo- 

 monas. 



The allied genus Trebouxia (Cystococcus) forms the algal com- 

 ponent in many hchens (see p. 427). The cells differ from those of 

 Chlorococcum in lacking an irregularly thickened wall and the 

 plastid is axile and not parietal. 



Characiaceae : Characium (a slip or cutting). Fig. 16 



Each plant is a soHtary imicell with a single parietal plastid, and 

 motile only in the reproductive phase. It may be supposed that it 

 has been derived from an ancestral motile organism that ceased to 



