A REEXAMINATION OF THE APPLICABILITY OF THE 



BUNSEN-ROSCOE LAW TO THE PHENOMENA 



OF ANIMAL HELIOTROPISM 



JACQUES LOEB AND HARDOLPH WASTENEYS 



Rockefi'llc^- Iiislitule for Medical Research, New York 



It has been shown by a number of botanists that the heho- 

 tropic reactions of plants obey the Bunsen-Roscoe law whereby 

 the heliotropic effect is determined by the product of the inten- 

 sity into the duration of illumination. The reactions of free 

 swimming animals to light are generally too quick to permit an 

 examination of the validity of this law, but Ewald^ has shown 

 that if the efficiency of intermittent and constant light is com- 

 pared in such forms it is found that both have equal efficiency 

 when the product of duration into intensity of illumination is 

 equal in both cases (Talbot's law.) This proof is in reality also 

 a proof of the fact that the heliotropic reaction is determined by 

 the product of intensity into duration of illumination. It should 

 also be mentioned that the striking results of Bradley Patten^ 

 on the direction of movements of the negatively heliotropic 

 larvae of the blowfly under the influence of two lights of unequal 

 intensity strongly suggest the validity of some such law as that 

 of Bunsen and Roscoe for heliotropic reactions. 



It is, how^ever, desirable to have a direct proof for the appli- 

 cability of this law to heliotropic reactions of animals. For such 

 a proof we are compelled to turn to sessile animals. Loeb and 

 Ewald^ have made some preliminary experiments on the hydroid 

 Eudendrium which is positively heliotropic and their obeerva- 

 tions agreed with the Bunsen and Roscoe law. The number of 



- Ewald, \V. F., Science, 1913, 38, 236. 



2 Patten, B., Am. Jour. Physiol., 191.5, 88, 313. 



3 Loeb, J., and Ewald, W. F., Zentralbl. f. Physiol., 1914, 27, 1165. 



187 



