Volvox in South Africa. 543 



completely, until germination of oospores again initiates a period 

 of activity. If a rise in temperature coincides with a period of 

 renewed activity, then the greater warmth is beneficial, speeding up 

 the life-processes and resulting in a specially rich growth both as 

 regards numbers and size of individual colonies. 



Mainx (1929, p. 205), working with cultures of F. aureus in earth- 

 decoction, found that slight changes in temperature or alkalinity 

 of the culture medium might hasten the onset of the sexual phase, 

 but that in general low temperatures delayed it. The observations 

 made on the Cape Flats suggest that the same would apply to F. 

 capensis. 



Very often, of course, the time-limit is determined by the drying 

 up of the vlei. For instance, in 1931 a vlei near Ottery Road, Wyn- 

 berg, was under observation. Volvox was in good condition, the 

 vlei containing plenty of water, when a south-east wind sprang up 

 and continued for some days, sweeping across the flats and licking 

 up the water in all the unprotected vleis. The vlei was visited on 

 Monday, 2nd November ; the south-easter was blowing lightly, the 

 vlei, though low, was continuous. Two days later (4th November) 

 at 6 p.m. it was again visited ; the south-easter, though still not very 

 strong, had been incessant, and the vlei was reduced to a few isolated 

 pools, very rich in Volvox in beautiful condition (sexual and asexual), 

 and swarming with tadpoles, mayfly and dragon-fly larvae, and minute 

 fish, probably the young of the Cape minnow (Galaxias punctifer). 

 By 11 a.m. next day even these pools had disappeared, and save for 

 two or three patches of churned-up mud and dying tadpoles no 

 free water remained. Yet the previous day Volvox was at the height 

 of its development. 



However, before attempting a complete solution of the problem 

 of the behaviour of Volvox, detailed observations over long periods 

 are necessary, not only observations of Volvox itself, but also of 

 the organisms associated with it, the changing composition of the 

 surrounding medium, its hydrogen-ion concentration, temperature, 

 and weather conditions in general, in relation to its occurrence in 

 Nature, supplemented by carefully controlled culture experiments. 



Constitution and Size of the Coenobium. 



As in Volvox globator, V. Rousseletii and V. capensis are built up 

 of a very large number of cells arranged in a single layer round the 

 periphery of a hollow, prolate spheroid. 



