50 



TROPISMS 



experiment the positively heliotropic animals are in the 

 direct sunlight at a, they promptly move toward the win- 

 dow, gathering at the window end c of the tube, although 

 by so doing they go from the sunshine into the shade. 

 This experiment shows also that it is not the intensity 



Fia. 16. Showing that positively heliotropic animals will move from sunlight into shade 

 if in so doing the illumination of the two eyes remains the same. 



gradient of light in the dish which makes positively helio- 

 tropic animals move to the light, but that difference in 

 intensity on both sides of the animal which is caused by 

 the screening effect of the animal 's own body. The same 

 holds true for chemotropism. 



Thus far we have discussed positively heliotropic 



