The Reversal of Tropisms loi 



negative upon diminution of the light. Ordinarily 

 when these forms are placed in a dish and exposed 

 to a window they keep running and hopping in the 

 direction of the light. If now the dish is carried 

 back into a darker part of the room the whole troop 

 of Orchestias will turn about and flock to the nega- 

 tive end of the dish. After being kept for a half 

 hour in the darker part of the room the amphipods 

 again become positive. If they are now carried to 

 a part of the room in which the light is still less 

 intense the negative response appears once more. If 

 the Orchestias are brought back into more intense 

 light they almost immediately become positive again ; 

 and the positive reaction appears the more quickly 

 the stronger the light. Usually the transition from 

 weak to strong light causes positive forms to be- 

 come negative. In the Orchestias we find just the 

 reverse. The transition from positive to negative 

 or the reverse occurs so quickly that it is scarcely 

 possible that change in light attunement plays a part 

 in the transformation. 



One might expect, a priori, to find tropisms occa- 

 sionally reversed by temperature, so profoundly does 

 this factor influence all forms of behavior, especially 

 in the lower organisms. Strasburger found that the 

 swarm spores of many algae are positive at higher 

 temperatures and negative at lower ones. Massart 

 finds that the flagellate Chromulina is positive at 20° 

 C, but negative at 5° C. According to Loeb the 

 larvse of Polygordius which were negative at i$° 



