THE HELIOTROPISM OF ANIMALS AND ITS IDENTITY 

 WITH THE HELIOTROPISM OF PLANTS 1 



I. INTRODUCTION 



I INTEND to show in the following pages that animal 

 movements depend upon light in the same way as the move- 

 ments of plants. 



It is a well-known fact that animals, when light falls on 

 them, move toward the source of light, like the moth, or 

 move away from it, like the earthworm. It is also well 

 known that certain plant organs have a tendency to turn 

 toward or from the source of light when illuminated from 

 one side only. While the conditions which govern the 

 behavior of plants toward light have been well analyzed, 

 especially by Sachs, little has been done to investigate the 

 conditions upon which depend the movements of animals 

 toward a source of light. It is the purpose of this paper 

 to fill this gap, and to enumerate the facts which show that 

 in reality the animal motions called forth by light depend 

 upon the same circumstances as the motions which light 

 produces in plants. 



The effects of light which we intend to study are purely 

 mechanical, inasmuch as they consist in changes in position, 

 as well as in the direction and the sense of the progressive 

 movements of living animals. Consequently we shall regard 

 as essential such circumstances as can help to explain the 

 mechanical effects of the light. These circumstances, as in 

 the case of all stimulations, are of a double origin: first, 

 those belonging to the stimulus in this case the light; and, 



i Pamphlet, Wurzburg, 1889. 



1 



