290 LIGHT AND THE BEHAVIOR OF ORGANISMS 



they were negative in light intensities in which this organ- 

 ism had formerly always been found to be strongly posi- 

 tive. I then tested the colonies for the optimum and was 

 greatly surprised to find that they were negative to all 

 light intensities above 0.57 ca. m. In light from 0.57 to 

 0.29 ca. m., the lowest intensity to which they were ex- 

 posed, their reactions were indefinite. There was no indi- 

 cation of any positive reaction whatever. 



At different times a number of colonies were taken from 

 a given jar and half of them put into each of two similar 

 vessels containing equal amounts of water. One of the 

 vessels was then exposed to direct sunlight and the other 

 covered so as to exclude all light. After having been in 

 this condition a short time the reactions of the colonies in 

 the two vessels were compared by exposing both to the 

 same light intensity. In such cases it was always found 

 that the specimens which had been in direct sunlight were 

 negative to light of lower intensity than those which had 

 been in darkness. These results indicate that the colo- 

 nies had not become acclimated to the high light intensity. 

 But they do become acclimated under certain conditions, 

 judging from the observations of Oltmanns, who says 

 (1892, p. 190), that he covered two lots of Vol vox with 

 the same kind of prisms, July 31, in the evening. One of 

 these lots with its prism was kept in darkness until 9 A.M., 

 August I, the other was exposed to light. During the 

 following three days it was found that those which were 

 in darkness until 9 a.m. collected in regions of lower light 

 intensity than the others. Strasburger found the same to 

 be true with reference to the reactions of swarm spores. 

 It seems strange that the effect upon the optimum in 

 colonies exposed for so short a time could, as Oltmanns 

 states, be still observed after three days. 



There are some indications that when Volvox is negative 

 to light of low intensity it becomes positive when exposed 

 to a much higher intensity. This is shown by the follow- 

 ing observations : 



