Significance of Tropisms for Psychology 37 



it is only with this sort of reactions that we have to deal in the 

 psychology of the will. 



II 



Some experiments on winged plant lice may serve as an 

 introduction to the methods of prescribing to animals the 

 direction of their progressive movements. 



In order to obtain the material, potted rose bushes or Cine- 

 rarias infected with plant lice are brought into a room and 

 placed in front of a closed window. If the plants are allowed 

 to dry out, the aphids (plant lice), previously wingless, change 

 into winged insects. After this metamorphosis the animals 

 leave the plants, fly to the window, and there creep upward 

 on the glass. They can then easily be collected by holding a 

 test-tube underneath and touching one animal at a time from 

 above with a pen or scalpel, which causes the animals to drop 

 into the test-tube. In this manner a sufficiently large number, 

 perhaps twenty-five or fifty suitable subjects for the experi- 

 ment, may be obtained. With these animals it may be demon- 

 strated that the direction of their movement toward the light 

 is definitely determined — provided that the animals are healthy 

 and that the light is not too weak. The experiment is so arranged 

 that only a single source of light, e.g., artificial light, is used. 



The animals place themselves with their heads toward the 

 source of light and move toward it in as straight a line as the 

 imperfection of their locomotor apparatus allows, approaching as 

 near to the source of light as their prison (the test-tube) permits. 

 When they reach that end of the test-tube which is directed 

 toward the source of light, they remain there, stationary, in a 

 closely crowded mass. If the test-tube is turned 180"^ the 

 animals again go straight toward the source of light until the 

 interference of the glass stops their further progressive move- 

 ments.^ It can be demonstrated in these animals that the 



1 Loeb, Der Heliotropismus der Tiere und seine Uebereinstimmung mit dem 

 Heliotropismus der Pfiamen, Wurzburg, 1889. Translated in Studies in General 

 Physiology, 1906. 



