538 Annals of the South African Museum. 



behaviour in the case of V. capensis in both the Ottery and Wetton 

 Road vleis. 



Henneguy (1876, p. 288) states that in F. aureus the mature 

 female colonies exhibit negative phototropism, actually swimming 

 away from the light. This has not been observed in the case of the 

 African species ; indeed, colonies red with ripening oospores have 

 been watched joining in the mass movement, but as they grow older 

 their activity decreases and they tend to collect at the bottom of the 

 water. But when watching a dish of Volvox brought into the light 

 it is obvious that the horizontal movement is not invariably directly 

 towards the light. Some colonies at first swim in other directions, 

 occasionally even directly away from the light ; eventually, however, 

 all become similarly orientated, and move towards the illuminant. 



Sequence of Asexual and Sexual Phases. 



In both F. Rousseletii and F. capensis the appearance of the various 

 types of colonies, provided external conditions are favourable, 

 follows the same sequence : 



1. Asexual Phase. Soon after rain has fallen and pools have 

 formed if the weather is warm, within four or five days "Juv- 

 eniles," resulting from the germination of oospores, appear (Plates 

 XLVII and XL VIII). They are quickly replaced by normal asexual 

 colonies, which by the third or fourth generation have the full com- 

 plement of cells and sometimes a large number of daughter colonies 

 (cf. Plate XVIII, A). This asexual phase may be prolonged, all the 

 daughters produced being themselves asexual, or it may be short 

 and soon succeeded by the sexual phase, the duration apparently 

 depending to a great extent on the temperature. 



2. Sexual Phase. The sexual phase begins with the appearance 

 among the asexual colonies of occasional sexual individuals; the 

 proportion of sexual to asexual colonies rapidly increases, until the 

 former may actually outnumber the asexual colonies, which are, 

 however, always present (Plate XVIII, B). In F. capensis the 

 number of sexual colonies is relatively smaller than in F. Rousseletii 

 (Plate XII, D). In the latter, at the beginning of the sexual phase, 

 male colonies are often present in large numbers, sometimes out- 

 numbering the female colonies. Usually, however, the latter pre- 

 dominate, while, as the phase advances, the number of male colonies 

 falls rapidly (Plate XVIII, B). It is usually at this stage that mixed 

 colonies occasionally appear ; in F. Rousseletii these contain asexual 



