Volvox in South Africa. 541 



of exact observations in the field are obvious, and so far the Flats 

 Volvox, unlike the Kietfontein one, has proved peculiarly intolerant 

 of culture conditions. 



Incidentally, observations of Volvox in Africa suggest an explana- 

 tion for the non-observance of Juveniles in V. globator -it seems that 

 these are formed in large numbers at one time, only when rain has 

 fallen and refilled the pool after a dry period, and that when oospores 

 germinate without drying, germination is much more sporadic. Con- 

 sequently the number of Juveniles present at any one time is compara- 

 tively small, and their presence proportionately difficult to detect. 



Observations on external conditions, in particular temperature 

 and hydrogen-ion concentration of the water, were carried on side by 

 side with those on Volvox. Here again more extensive observations 

 are necessary before it is possible to arrive at definite conclusions. 

 At present, however, the following facts seem to be fairly well 

 established : 



1. Effect of Temperature. Other conditions being favourable, 

 comparatively high temperatures are conducive to increased rapidity 

 and extent of development. The most luxuriant growths of Volvox 

 on the Cape Flats have always coincided with warm, bright weather. 

 In May 1931 a spell of unusually hot weather at the beginning of 

 winter, immediately following good rains which had caused the for- 

 mation of small pools, resulted in a very rich crop of Volvox. Again, 

 in November of the same year, one of the richest growths of Volvox 

 as yet observed on the Flats occurred in Belvedere Eoad Vlei III, 

 when the temperature of the water at midday was over 90 F. on 

 several successive days. In 1932 there were no very warm spells 

 during the winter ; in the vleis under observation Volvox was never 

 very abundant till well on in the season when, with warmer weather, 

 there came a succession of very rich crops in several pools e.g. De 

 Klip, Belvedere Eoad Vlei III and parts of Vlei II. 



2. Hydrogen-ion Concentration. At the Cape the water of those 

 vleis and ditches where Volvox occurs is always either approximately 

 neutral or slightly on the acid side, the pH ranging from 6-5 to 7-3, 

 most often from 6-7 to 7-0 ; on the other hand, the pools producing 

 Hydrodictyon are almost always on the alkaline side, pH 7-6 to 9-0, 

 and very rarely are the two algae found in the same vlei. Occa- 

 sionally H. africanum and V. capensis occur together, e.g. in the 

 Wood pool next to Dabchick Vlei, but so far the writer has never 

 found Volvox associated with the second form of Hydrodictyon 

 (H. indicum ?) which occurs on the Flats. On one occasion, in a 



