Volvox in South Africa. 537 



observations of the response of Volvox to light, carried out in the 

 field or with large tub cultures imitating as far as possible natural 

 conditions, might prove very instructive in this connection. Con- 

 siderable difficulty, of course, attends such work in the field, and so 

 far experiments on phototropism in Volvox have been mainly con- 

 fined to the laboratory. The most detailed are those carried out by 

 Oltmanns (1917, p. 280), who attributes the sinking during the 

 quiescent period to geotaxis. This certainly seems sufficient explana- 

 tion for this part of the process ; even in preserved material, if the 

 tube containing it is shaken up and then left to settle, the colonies 

 can be seen dropping slowly, the axis often slightly oblique, but the 

 posterior pole, which with its closely packed somatic and numerous 

 reproductive cells is undoubtedly the densest part of the coenobium, 

 always on the under side. The specific gravity of the living colony 

 is evidently a little higher than that of the surrounding water. 



The movement was most beautifully seen on a large scale one 

 morning of February 1931. On that date Mr. Steer's fishpond* was 

 particularly rich in Volvox the strain inhabiting it was almost at 

 its optimum development the clear water in the centre of the pool 

 being green with Volvox. To the north of the pond is a fernery 

 protected by a roof of wooden slats an inch or so apart. The morning 

 sun shining on this roof threw its shadow on the water, making 

 alternate bars of light and shade. In each bar of light was a column 

 of moving green spheroids swimming upwards and moving about 

 near the surface, jostling one another as they moved. The light 

 reflected from the countless moving bodies gave the bars of light a 

 scintillating appearance, like motes in a ray of sunshine, most striking 

 even at a distance, while the bars of shadow, on closer examination, 

 were found to be practically free of Volvox. Among the mass of 

 upward-moving bodies others could be seen slowly sinking without 

 other movement. 



In the field it is seldom possible to observe the movement so well. 

 To begin with, at the Cape at any rate, there is seldom such a quantity 

 of Volvox in so small a space, and, further, conditions are much less 

 favourable for observation. Still, when Volvox has been particularly 

 abundant on the Flats, it has sometimes been possible, by wading 

 out into the middle of the pond and watching carefully, to see similar 



* A cement pool about 6 by 8 feet in area, 4 to 6 inches deep at one end, 2 to 3 

 feet at the other, with various water plants, Arrowhead, Cyperus, Limnanthemum, 

 etc., growing round the edge, the centre fairly clear, inhabited by tame frogs, 

 small fish from the Flats, and so on. 



VOL. XVI, PART 3. 35 



