REVERSION IN ORIENTATION TO LIGHT 371 



they were clearly more strongly negative. One of the aquaria 

 was then moved nearer to the lamp into an illumination of 1(3,000 

 m.c. The colonies immediately became positive, although not 

 strongly. The dark-adapted colonies were not tested in this 

 intensity, but judging from results obtained in numerous other 

 experiments, one of which is described in detail in the preceding 

 pages, they probably would have been negative. 



These results seem to indicate that long exposure in strong 

 light produces changes in Volvox .which make it negative in 

 weak and positive in strong light. The following results ob- 

 tained in observations on Pandorina support this conclusion. 



August 4 was a bright clear day. At 11.30 a.m., Pandorina 

 was taken in abundance from a small puddle at the edge of the 

 collecting pond. This puddle had been fully exposed to direct 

 sunlight all forenoon. The specimens collected were taken to the 

 laboratory and some of them were immediately exposed to direct 

 sunlight in an observation aquarium, surrounded by water which 

 was changed from time to time so as to prevent excessive rise 

 in temperature. They were strongly positive and remained so. 

 At 3 P.M. the aquarium was placed on the stage of a microscope 

 in direct sunhght, and a beam of direct sunhght, concentrated 

 by means of the concave mirror, was thrown up through the 

 aquarium. The colonies promptly aggregated in this extremely 

 intense illumination forming a dense mass. At 5 p.m. many colo- 

 nies were lying on the bottom inactive, but all the rest were 

 still fairly strongly positive. These results indicate clearly that 

 under certain conditions Pandorina cannot be made negative 

 by exposure to intense light. The specimens used in this experi- 

 ment were unfortunately not tested in weaker light, but judging 

 from the results of other observations they probably would have 

 been negative. For example, at 3 p.m., August 11, colonies 

 adapted to strong diffuse light were exposed in an illumination 

 of 16,000 m.c. They were inactive for a few moments and 

 then became definitely positive. The illumination was then 

 considerably reduced. The colonies continued to proceed toward 

 the light, but only for a few moments, wheh practicall}^ all of 

 them turned through 180 degrees and proceeded from the light. 



