542 Annals of the South African Museum.. 



ditch at the side of the road near the farm Vaderlandsche Rietvlei, 

 a very rich growth of the latter Hydrodictyon was found, the pH 

 of the water being 8-3. A mile or so along the road Volvox was 

 abundant in the ditch, while Hydrodictyon was not present, the pH 

 here being 7-3. 



Even more striking is the case of De Klip, a granite outcrop in the 

 Flats. The granite forms a ring of fair-sized bosses round which 

 is a chain of small pools, while two larger pools lie between the 

 granite bosses. The former, in which the pH ranged from 6-7 to 

 7-3, yielded fine crops of Volvox and no Hydrodictyon, while the 

 position was reversed in the inner pools where the pH was 7-6 to 

 8-5 ; here only a few yards separated the pools. 



The one exception to this rule as to the acidity of the water 

 inhabited by Volvox is that already mentioned as occurring in the 

 ditch near Dabchick Vlei (p. 533), which was accompanied by patho- 

 logical symptoms in the Volvox. On other occasions the pH of the 

 water from this ditch was that normally found in Volvox pools, and 

 the colonies were quite healthy. 



It was thought that changes in the hydrogen-ion concentration 

 might possibly have some connection with the periodic disappear- 

 ances of Volvox, but so far the observations made do not support 

 this theory. The fluctuations in the pH of any one pool throughout 

 the season were comparatively small ; for example, in Belvedere Road 

 Vlei III A, from May to November the pH ranged from 6-4 to 7-1, 

 the neutral point being approached five or six times during the 

 season, always after heavy rain. In the main pool the range was 

 similar, although even when the two pools were continuous the pH 

 values of the two were seldom identical. In Vlei II the pH ranged 

 from 6-5 to 7-2, in Vlei I (Ackerman's) 6-4 to 7-4, while the remaining 

 Volvox vleis all gave similar values. 



So far, then, observations of external conditions offer no full 

 explanation of the termination of a growth phase in Volvox. Occa- 

 sionally it is possible to point to unduly high temperature as a possible 

 cause, but the probability seems to be that there is an inherent 

 time-limit to the duration of the alga, at any rate in South African 

 waters under natural conditions. Probably the duration of any 

 one phase is modified by external factors, but these are only secondary 

 in their effect. Given favourable conditions, the Volvox oospores 

 germinate, a brief asexual phase is followed by a sexual one, and 

 this in turn by a second asexual phase which may continue for a 

 shorter or longer time, but ultimately ends, and the alga disappears 



