540 Annals of the South African Museum. 



length of the periods during which Volvox was present in any one vlei 

 or part of a vlei varied from two to six weeks, most often lasting 

 for from three to five weeks. The time elapsing between disappear- 

 ance and reappearance at the same spot likewise varied ; usually 

 periods of from one to two weeks during which no Volvox could be 

 found separated each two successive phases, but in one vlei on one 

 occasion Volvox disappeared for as long as five weeks. In the latter 

 case the prolonged disappearance was both preceded and followed 

 by comparatively lengthy (four and and six weeks respectively) 

 periods of great activity, during which large numbers of oospores 

 were produced. 



(3) " Juvenile " colonies, resulting from the germination of oospores, 

 were collected at the beginning of the season only. Judging from 

 observations made elsewhere (notably near Kimberley), the obvious 

 explanation of the reappearance of Volvox after each total disappear- 

 ance is, that some of the oospores formed during the previous phase 

 ripen quickly and germinate at once without undergoing a period of 

 drying, and it was hoped that this would be demonstrable by the 

 presence of Juveniles at the beginning of the successive phases, but 

 none were found. 



Nevertheless, the failure to detect Juveniles cannot be regarded 

 as proof of their absence. So far, whenever Juveniles have been 

 collected, it has been soon after the pool has formed (3 to 5 days) 

 and usually while it is still small. Hence, firstly, the many oospores 

 dormant in the soil have all, after a period of dryness, been suddenly 

 re-immersed in water under the same conditions, and it is only to be 

 expected that large numbers would in such circumstances germinate 

 simultaneously, whereas when oospores germinate without previous 

 drying it is probable that germination is sporadic, spread over a 

 longer time, and depending in part on the age of the various 

 oospores. Secondly, the very small size of the Juveniles (about that 

 of large Eudorina colonies) must be taken into consideration in 

 larger expanses of water the difficulty of collection and detection is 

 enormously increased unless they are present in very great numbers. 



The present field observations then can only be taken as showing 

 that several successive growths of Volvox capensis may occur in a 

 given piece of water during the season. The probability is that 

 this is due to the germination of oospores formed during the preceding 

 phase, of which some at least may undoubtedly germinate without- 

 previous drying, but more exact investigation is necessary to prove 

 it, since so far field observations give negative results. The difficulties 



