164 'Journal of Comparative Neurology and Psychology. 



variation seems to have been due largely to preceding exposure to 

 light. The threshold is higher in colonies previously exposed to 

 strong light than in those exposed to weak light. 



16. OPTIMUM. 



The optimum light intensity for practically all Volvox colonies 

 is somewhat lower than that of direct sunlight, 5000 ± candle 

 meters, but sometimes it is very much lower; it varies greatly. 

 This variation is clearly shown in the following observation. 



After a few very cloudy days the sun came out at 11 a. m., July 

 24, 1904, and the sky became exceptionally clear and remained 

 so the remainder of the day. At 2 p. m. Volvox colonies were 

 found in abundance freely exposed to the sunlight. Some of the 

 colonies were collected and taken to the laboratory where it was 

 accidentally discovered that they were negative in light intensities 

 in which this organism had formerly always been found to be 

 strongly positive. 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 candle meters. In light from 0.57 to 0.29 

 candle meters, the lowest intensity to which they were exposed, 

 their reactions were indefinite. There was no indication 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 exposure to light of high intensity 

 causes a lowering of the optimum. Oltmanns did not find this 

 to be true. He states ('92, p. 190) that he covered two lots of 

 Volvox with the same kinds 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 the darkness 

 until 9 a. m. collected in regions of lower light intensity than the 

 others. Strasburger found the same to be true with reference 



