370 S. O. MAST 



cation whatever of orientation. But after approximately one 

 minute it could be seen clearly that they were swimming slowly 

 from the light. Gradually they became more and more active, 

 and as they became more active they became more strongly 

 negative, until at 4.50 p.m. they were swimming rapidly and 

 fairly directly from the source of light. x\.t 4.55 p.m., they were 

 moved 4 m. from the window and placed in diffuse light. Here 

 they promptly became strongly positive. They were then re- 

 turned to direct sunlight near the window, when they again 

 promptly became strongly negative. These changes in illumi- 

 nation were repeated several times and the same results were 

 obtained each time. 



It is thus evident that strong illumination tends to make these 

 organisms negative and that weak- illumination tends to make 

 them positive. Under certain conditions, however, just the 

 opposite holds. In previous work this was observed only once 

 (Mast, '07, p. 165). In the observations now under considera- 

 tion it was, however, repeatedly observed both in Volvox and 

 in Pandorina, and the conditions under which it occurs have 

 been discovered. This is demonstrated by the results obtained 

 in the observations described below: 



On July 13, Volvox colonies were collected at 7.30 a.m., in a 

 pool fully exposed to direct sunlight. They were taken to the 

 laboratory, and put into two finger-bowls. One bowl was put 

 into total darkness, the other was exposed in strong diffuse day- 

 light. Thus one group of colonies remained light-adapted, 

 while the other group became dark-adapted. At 3 p.m., light- 

 adapted colonies and dark-adapted colonies were put, respectively, 

 into each of six rectangular observation aquaria. These were 

 then exposed in the dark-room in light from the 250-watt lamp, 

 one aquarium containing dark- and one containing light-adapted 

 colonies side by side in each of the following illuminations, 62.5 

 m.c, 250 m.c, and 4,000 m.c. (fig. 1). The dark-adapted colonies 

 were strongly positive in all the illuminations and the light- 

 adapted were negative in all, but they were clearly more strongly 

 negative in the lowest intensity than they were in the highest. 

 In 4,000 m.c. they were only slightly negative; in 250 m.c. 



