276 LIGHT AND THE BEHAVIOR OF ORGANISMS 



below, near the slide, owing no doubt to the fact that the 

 source of Hght was somewhat above the level of the stage. 

 About half of the Euglenae were now evidently negative and 

 the rest positive, but half an hour later nearly all were nega- 

 tive, although there had been no change in light intensity 

 or temperature. When the temperature was reduced they 

 became still more strongly negative. After keeping the 

 temperature between 5° and 8° for three minutes, it was 

 rapidly raised to 22°; the Euglenae were now very strongly 

 positive. They fairly streamed toward the source of light. 

 The temperature was now again reduced and held at 5° for 

 a few minutes, during which the organisms were negative. 

 It was then slowly raised, and many became positive at 

 12°, after which it was once more reduced and held at 5° 

 for five minutes, during which the organisms were negative, 

 and then again slowly increased. Many of the Euglenae 

 now became positive at 8°. 



We have thus seen the same individuals within the 

 course of a few minutes in constant light intensity reverse 

 in the sense of reaction several times. We have seen them 

 come to rest as the temperature decreased and become active 

 again as it decreased still farther. W^e have seen them 

 change from a condition in which they were negative at 

 22° and positive at higher temperature to one in which 

 they were positive at 8° and negative at lower tempera- 

 ture. These observations were repeated many times under 

 different conditions with the same general results. Similar 

 changes in reactions to light were also repeatedly produced 

 by changes in temperature in different species of Chlamy- 

 domonas, Trachelomonas, Chlorogonium and Volvox. 



These results show: (i) That a decrease in heat energy 

 tends to cause a change in the sense of reaction to light 

 from positive to negative and an increase tends to cause a 

 change from negative to positive. A decrease in heat 

 energy, therefore, produces the same changes in the re- 

 actions to light as an increase in light energy. (2) That 

 the reactions to light of a given intensity depend not only 



