CHLOROCOCCALES 



39 



multinucleate, and it is in this older condition that the protoplast 

 divides and gives rise to numerous biflagellate zoospores which are 

 Hberated all together in a vesicle, usually in the early hours of the 

 morning. After a short motile phase the flagellae are withdrav^n 

 and a new vegetative phase commences. 



Isogamy and anisogamy are known, but there is no recorded 

 example of even primitive oogamy comparable to that found in 

 Chlamydomonas. Under certain conditions aplanospores are formed : 

 when this happens the parent gelatinizes and a "palmella" stage 

 results, the cells of which subsequently give rise to two to four 

 biflagellate gametes. It seems clear that the suppression of motility 

 has occurred several times in the Chlorococcaceae, a feature which 

 supports the idea of their polyphyletic origin. The aplanospore stage 

 also suggests how the genus Chlorella may have arisen. Under 

 normal conditions Chlorococciim reproduces by means of motile 

 zoospores, but when subjected to drought these bodies are non- 

 motile. In nutrient culture solutions of low concentration repro- 

 duction takes place by zoospores, whilst in highly concentrated 

 solutions the zoospores are replaced by aplanospores, so that it can 

 be concluded that the environment may affect the reproductive 

 mechanism to a considerable extent. C. humicolum is a very common 

 soil form (cf. p. 299). 



Chlorellaceae : Chlorella {chlor, green; ella, diminutive of 

 affection). Fig. 29. 

 The globular cells are non-motile, solitary or aggregated into 

 groups, and usually lack pyrenoids. They reproduce by division 



F E D 



A B 



Fig. 29. Chlorella vulgaris. A, single cell. B, division into four. C, final stage 

 of division into four daughter cells. D, first stage of division into eight. E, F, 

 second and third stages of division into eight daughter cells. (After Grintzesco.) 



