HELIOTROPISM OF ANIMALS 35 



mals, when a few have collected in a spot the others on 

 arriving hold fast to the sides of those already there. An 

 animal at rest acts upon a creeping one as a convex edge. 

 On the other hand, I have never observed that the animals 

 within the cubical box collect on concave edges. From this 

 it follows that the friction of gliding over the convex corners 

 is the source of the stimulation which compels the animal to 

 come to rest there ; in moving over the concave corners this 

 friction, of course, does not take place. 



These three forms of irritability control mainly the daily 

 life of the animals. We find them in great numbers in 

 fruit trees and bushes, where they pass the winter in their 

 nests; as soon as the warm weather comes, they leave their 

 nests. Positive heliotropism and negative geotropism com- 

 pel them to creep upward to the tips of branches, and contact- 

 irritability holds them fast on the small buds. We can 

 easily show that neither smell nor a special mystical 

 "instinct" leads the animals to the buds, as we are able to 

 compel them by the aid of light to starve in close proximity 

 to food. The animals move to the window side or to the 

 top of a test-tube in which they are kept. If then a branch 

 covered with buds is pushed into the test-tube on the room 

 side, the animals nevertheless remain where light and gravi- 

 tation have compelled them to go and are holding them. If, 

 however, they once are on the buds, the latter act as a 

 stimulus which may be even stronger than the light. It is 

 in such a case impossible to draw the animals away from the 

 food by means of light. 



All these forms of irritability can best be demonstrated 

 on animals which have just left the nest in which they have 

 spent the winter, and which have not yet eaten anything. 

 As soon as they have eaten and are about to moult, their 

 irritability decreases, and at the time of moulting it is almost 

 impossible to show any effect of light or gravity upon them. 



