Animal Instincts and Tropisms 261 



organisms like the swarmspores of algae move to or 

 from the source of light as do free-swimming animals. 



3. The writer suggested in 1897* that the light acts 

 chemically in the heliotropic reactions and in 1912 that 

 the heliotropic reactions probably follow the law of 

 Bunsen and Roscoe, 2 and it was possible to confirm 

 this idea by direct experiments. 3 This law states 

 that the photochemical effect of light equals i t where i 

 is the intensity of the light and t the duration of illumi- 

 nation. The experiments were carried out on young 

 regenerating polyps of Eudendrium by measuring the 

 time required to cause fifty per cent, of the polyps 

 to bend to the source of light. The intensity of light 

 was varied by altering the distance of the source of 

 light from the polyps. Table VI gives the result. 



TABLE VI 



Distance between Polyps 

 and Source of Light 



Time Required to Cause Fifty Per Cent, of the 

 Polyps to Bend towards the Source of Light 



OBSERVED 



CALCULATED FROM 

 BUNSEN- ROSCOE LAW 



Metres 



0.25 

 0.50 

 i.oo 

 1.50 



Minutes 



10 



between 35 and 40 



150 



between 360 and 420 



Minutes 



40 

 1 60 



360 



1 Loeb, J., Arch. f. d. ges. PhysioL, 1897, Ixvi., 439. 



1 Loeb, J., The Mechanistic Conception of Life, Chicago, 1912, p. 27. 



3 Loeb, J., and Ewald, W. F., Zentralbl.f. PhysioL, 1914, xxvii., 1165. 



