REGULATION OF REACTIONS 289 



environmental conditions. They tend to become attuned, 

 as Strasburger puts it, to the Hght intensity of their en- 

 vironment; they become accUmated. The sensitiveness and 

 the optimum, as well as the reactions in general, at any 

 given time, depend upon the preceding exposure of the or- 

 ganism. This is well illustrated by the behavior of Volvox 

 as observed by the writer. 



On July 30, 1904, at 5 p.m., it was found that Volvox, 

 which had been collected at 6 a.m. and kept in the dark all 

 day, responded definitely to light of 0.16 ca. m. intensity, 

 and quite definitely to light of 0.14 ca. m. This is the 

 lowest intensity to which any response was obtained at 

 any time. Specimens collected shortly after 12 M., July 14 

 and 15 respectively, and tested as soon as brought into 

 the laboratory, responded to light of 0.50 to 0.83 ca. m. 

 The sky was clear on both of these days, but the organ- 

 isms were found among the water plants in more or less 

 shaded places. 



It was found at different times that after being exposed 

 to direct sunlight a few moments the colonies did not 

 respond even to an intensity as high as 500 ca. m. We 

 have thus observed the threshold to vary from 0.14 to 

 500 ca. m., and this variation seems to have been due 

 largely to preceding exposure to light. The threshold is 

 higher in colonies pre\dously exposed to strong light than 

 in those exposed to weak light. 



The optimum light intensity for practically all Volvox 

 colonies is somewhat lower than that of direct sunlight, 

 5000 ± ca. m., but sometimes it is very much lower; it 

 varies greatly. This variation is clearly shown in the fol- 

 lowing observation : 



After a few very cloudy days the sun came out at il 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 



