HELIOTROPISM 131 



or if the mass of photosensitive substances is less, it will require a longer 

 orie-sided exposure of the animal before the threshold for creating 

 a difference in the tension of the muscles on both sides of the body is 

 reached. Such animals will also move toward the source of light, 

 but they do not move so directly in the direction of the rays of light as 

 the strongly hehotropic forms, but much more irregularly. Finally, if the 

 light does not accelerate the reaction velocity in the animal at all, or 

 if the proper photosensitive substances are not present, or the proper 

 products are not formed in the photochemical reaction, the animal 

 will not appear in the slightest degree heliotropic. 



The correctness of this view can, I believe, be demonstrated, by 

 exposing animals which in strong light are intensely positively helio- 

 tropic to weak light. If in the strong light they move in as straight 

 a line toward the source of light as the imperfections of their locomo- 

 tions permit, a low intensity of Hght can be found where they still go 

 toward the source of light, but where their progressive motion follows 

 the direction of the rays of light much less accurately. In the weaker 

 light the acceleration of the photochemical reactions is less than in 

 strong light, hence the time during which an animal can deviate from 

 the direction of the rays, exposing only one side of its body to the source 

 of light, becomes longer. The same result can be obtained by putting 

 these animals behind a red screen. This explains, also, the influence 

 of temperature upon the precision with which the hehotropic animals 

 follow the direction of the rays of light. Within certain limits the 

 precision with which such animals move in the direction of the rays 

 increases with the temperature. 



If it be true that the immediate effect of the light in causing the 

 heliotropic reactions is of a chemical nature, we should expect that it 

 must be possible by the use of chemicals to control the precision and 

 sense of the hehotropic reactions. I have recently found facts * which 

 prove the correctness of this supposition. It may be of importance 

 that these chemicals are such as may be formed by the organism itself. 

 The experiments were made on fresh-water Crustaceans, Gammarus 

 pulex, Daphnia, and Cyclops. If Gammarus are left to themselves, 

 they may be found in any part of the aquarium chnging to sohd bodies ; 

 but if they are disturbed by transferring them from one vessel to another, 

 or by merely stirring the water in the vessel in which they are, they 

 become, transitorily at least, negatively hehotropic. It is possible, 

 however, to make them at once intensely positively heliotropic, by adding 

 certain chemicals to the water, e.g. esters. If the negatively hehotropic 

 Gammarus are in a glass jar containing 50 c.c. of tap water, they become 



* Loeb, University of California Publications, Physiology, Vol. 2, p. I, 1904. 



