BUNSEN-ROSCOE LAW — ANIMAL HELIOTROPISM 189 



The previous treatment of the colonies was similar to that 

 used in the experiments of Loeb and Ewald. After cutting ofT 

 the existing polyps the stems were allowed to lie for about twenty- 

 eight hours exposed to diffused light. They were then placed 

 in the dark for from eighteen to twenty hours, by which time the 

 hydranths had usually regenerated. The exposure was then made 

 as soon as possible. 



In all 3873 hydranths were used or an average of 82 to a dish. 

 Of these 1671 were available for the determinations, i.e., they 

 bent to the light as a result of a short or prolonged exposure. 

 The remaining 2202 hydranths were not available for the deter- 

 minations, being either refractory or being originally placed 

 parallel to the direction of the rays of light, facing either to the 

 back or front. 



At the beginning of an experiment the hydranths bent towards 

 the light were counted, as well as the total number present in 

 the dish. After the exposure the dishes were allowed to remain 

 in the dark for from two to three hours, during which time the 

 heliotropic bending occurred.^ They were then replaced in light 

 in the same relative position as during the exposure and the 

 polyps turned or bent towards the light were counted. The 

 difference between this number and the number originally turned 

 to the front gave the number of hydranths caused to bend by the 

 exposure. The dish was then allowed to remain in the light 

 for from two to three hours longer and the polyps turned to the 

 light at the end of this period were once more counted. This 

 value less the number turned to the front before the original 

 exposure was taken as 100 per cent in computing the percentage 

 which had been caused to bend by the initial exposure. 

 . The following example will indicate the method of calculation 

 and also prove the fact that the number of polys which bend to 

 the light increases with the duration of exposure. 



'' The very fact that the bending occurred in the dark and not while the 

 organisms were exposed to the light should in itself suffice to prove the untena- 

 bility of the anthropomorphic explanations of heliotropic reactions by "trial 

 and error" or by hypothetical sensations of brightness. 



