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 réle the orientation toward the light plays in the 
economy of life of an animal. Ishall therefore first describe 
the experimental proofs of the identity of animal heliot- 
ropism with plant heliotropism, and then snow 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.* 
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.’ 
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. 
1“ Die Orientierung der Thiere gegen das Licht (thicrischer Heliotropismus),” 
Sitzungsberichte der Wirzburger physikalisch-medicinischen Gesellschaft, January, 
Loss. 
2 Vorlesungen tiber Pfhlanzen-Physiologie, 2d ed. (Leipzig, 1887). 
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