Volvox in South Africa. 563 



If, as has been done in the case of both F. Rousseletii from Riet- 

 fontein and of F. capensis from the Cape Flats, a single Volvox strain 

 be kept constantly under observation throughout a development 

 period, the number is found to vary greatly, according partly to the 

 age of the strain, partly to external conditions. The following 

 observations illustrate this point: 



1. V. Rousseletii Mr. Steer's original culture from Eietfontein 



soil. 



22nd February 1930 (a fortnight after the culture was started). 

 Volvox abundant, entirely asexual. Daughter colonies most often 

 8, but often more, up to 13 (Plate XVIII, A). Size of mature 

 individual very large. Largest seen 2058 x 2037 p,, with 9 daughters ; 

 many over 1500 /JL. 



Sth March. Many young sexual colonies present, males specially 

 abundant. Daughter colonies 4 to 8. 



23rd March. Mainly sexual. Asexual with 2 to 6 daughters 

 (Plate XVIII, C). 



28th March. Sexual colonies very few. Asexual with 1 to 3, 

 rarely 4, daughters. Size much smaller (Plate XVIII, D). 



The colonies had become much smaller in size, with far fewer 

 daughters. It continued thus reproducing vegetatively for some 

 time, then entirely disappeared, only to reappear in the same culture 

 some weeks later. 



A similar sequence of events was observed later in the case of an 

 offshoot of this culture which had established itself in Mr. Steer's 

 fishpond, though here the number of daughter colonies per coenobium 

 never rose so high. Towards the end of the period of activity the 

 average number of daughter colonies dropped to 3-4. 



2. F. capensis, on the Cape Flats. 



V. capensis appears to be particularly sensitive to temperature 

 changes. On the Flats it appears in the winter months, since then 

 only do pools form in this area. Early in the season the maximum 

 number of daughter colonies is 8, often fewer, but if there is a spell 

 of warm sunny days the number rises considerably. In 1931 some 

 vleis persisted unusually late in the season, right into November, 

 in which month there was a spell of very hot weather. During this 

 the activity increased enormously, as many as 20 daughters having 

 been counted in one colony, while 13, 16, 17, and 19 daughters were 

 common. The temperature of the water at noon was frequently 

 over 90 F. just below the surface, some 6 cooler at a depth of 8 or 9 



