4 STUDIES IN GENERAL PHYSIOLOGY 



with those which have been shown to be of paramount 

 influence in plants. 



Aside from the problem of proving by suitable experi- 

 ments the stated propositions, it is also necessary for us to 

 show what role the orientation toward the light plays in the 

 economy of life of an animal. I shall therefore first describe 

 the experimental proofs of the identity of animal heliot- 

 ropism with plant heliotropism, and then show by individual 

 examples what role heliotropism plays in the economy of life 

 of animals. To discuss the latter point it will be necessary 

 also to describe briefly the other forms of irritability pos- 

 sessed by an animal. 



In a short article which appeared in January, 1888, I 

 described the principal laws upon which depends the orien- 

 tation of animals to light, and the identity of these laws with 

 those governing plant heliotropism. 1 



II. THE ESSENTIAL PHENOMENA AND LAWS OF HELIOTROPISM 



IN PLANTS 



Assuming that the reader is acquainted with the orienta- 

 tion of plants toward a source of light, it will suffice at this 

 place to call attention briefly to the essential facts which bear 

 upon our subject. In so doing I shall follow the presenta- 

 tion given by J. von Sachs in his lectures on plant physi- 

 ology. 2 



Straight stems or roots of growing plants bend when light 

 falls on them on one side only, or with greater intensity on 

 one side than on the other, until their tips lie in the direc- 

 tion of the rays of light. Those organs which turn toward 

 the source of light are called positively heliotropic; those 

 which turn from the light, negatively heliotropic. 



i " Die Orientierung der Thiere gegen das Licht (thierischer Heliotropismus)," 

 Sitzungsberichte der Wiirzburger physikalisch-medicinischen Gesellschaft, January, 



1888. 



2 Vorlesungen iiber Pflanzen-Physiologie, 2d ed. (Leipzig, 1887). 



