Volvox in South Africa. 531 



dry up completely for several months of the year, during which they 

 become grass-covered flats. Even in the case of the Linyanti back- 

 waters, it is possible that the water dries up more or less completely, 

 since this river, like all the large tributaries of the Zambesi, is subject 

 to very great fluctuations in water-level throughout the year. 



Thus the life-cycle is dependent very largely on seasonal rainfall ; 

 moreover, as the vleis dry up and become shallower, Volvox is sub- 

 jected to very much higher temperatures than are the European 

 forms. At the other extreme, however, it is seldom subjected to 

 very low temperatures, rarely if ever having to cope with even a 

 few degrees of frost. The vleis in which Volvox is found may -dry up 

 very gradually, in which case it often happens that the life-cycle 

 has been completed, sometimes more than once, and only the resting 

 form remains, the ripened oospores being hidden in the soil at the 

 bottom of the vlei ; but very often the water dries up with astounding 

 ' rapidity, and Volvox may be cut off at the height of its development. 

 This occurs not only in regions of summer rainfall such as Kimberley, 

 where the great heat causes the pools to dry very soon after formation, 

 but also on the Cape Flats, where the beginning of summer is usually 

 marked by the coming of strong south-east winds, which lick up 

 the water from the remaining pools almost more rapidly. Instances 

 of this sudden drying of the vleis will be described in more detail 

 later. 



VOLVOX EOUSSELETII AND VOLVOX CAPENSIS I. THE ADULT FORM 



Distinguishing Features. 



Before describing the observations on the life-history and habits 

 of the two species of Volvox concerned, it will be as well to summarise 

 the outstanding characteristics which distinguish them from one 

 another. 



A. Volvox Rousseletii West (1910, p. 101, and 1918, p. 1). 



The colonies are globose when young, with slightly elongated 

 polar axis, usually becoming markedly egg-shaped when adult, the 

 sexual colonies more so than the asexual (Plate XVIII, B, C). 



The colonies are of three main types : (1) purely asexual, pro- 

 ducing from 1 to 16 daughter colonies, most often 6 to 8 arranged 

 alternately in two planes (Plate XX, A) : (2) male colonies, pro- 

 ducing from about one to several hundred sperm globoids ; and (3) 

 female colonies, producing from 60 to 300 or 400 oospores, average 



