262 Animal Instincts and Tropisms 



We must therefore conclude that the heliotropic 

 curvature of the polyps is determined by a photochemi- 

 cal action of the light. The light brings about or 

 accelerates a chemical reaction which follows the Bunsen- 

 Roscoe law. As soon as the product of this reaction 

 on one side of the polyp exceeds that on the other by 

 a certain quantity, the bending occurs. When the 

 product it is the same for symmetrical spots of the 

 organism no bending can result. This is what our 

 theory suggested. 



It is very difficult to prove directly the applicability 

 of the Bunsen-Roscoe law for free-moving animals, 

 but it can be shown that intermittent light is as effect- 

 ive as constant light of the same intensity, provided 

 that the total duration of the illumination by the 

 intermittent light is equal to that of the constant light, 

 and the duration of the intermission is sufficiently small 

 (Talbot's law). Talbot's law is in reality only a modi- 

 fication of the Bunsen-Roscoe law. Ewald has proved 

 in a very elegant way the applicability of Talbot's 

 law to the orientation of the eyestalk of Daphnia.* 

 This makes it probable that the law of Bunsen-Roscoe 

 underlies generally the heliotropic reaction of animals. 



It is of importance for the theory of the identity 

 of the heliotropism of animals and plants that in the 

 latter organisms the law of Bunsen and Roscoe is also 

 applicable. This had been shown previously by 



1 Ewald, W. F., Science, 1913, xxxviii., 236. 



