The Mechanistic Conception of Life 



29 



heliotropic away from it), and the same is observed in motile 

 animals. In plants only the more refrangible rays from green to 

 blue have these heliotropic effects, while the red and yellow 

 rays are little or less effective; and the same is true for the 

 heliotropic reactions of animals. 



It has been shown by Blaauw for the heliotropic curvatures 

 of plants that the product of the intensity of a source of light 

 into the time required to induce a heliotropic curvature is a 



Fig. 20. — Positive heliotropism of a marine worm (Spirographis). (From 

 nature.) The light fell into the aquarium from one side only and the worms all 

 bent their heads toward the source of light, as the stems of positively heliotropic 

 plants would do under the same conditions. 



constant; and the same result was obtained simultaneously 

 by another botanist, Froschl. It is thus proved that the Bunsen- 

 Roscoe law controls the heliotropic reactions of plants. The 

 same fact had already been proved for the action of light on our 

 retina. 



The direct measurements in regard to the applicability of 

 Bunsen's law to the phenomena of animal heliotropism have not 

 yet been made. But a number of data point to the probability 

 that the law holds good here also. The first of these facts is 

 the identity of the light reactions of plants and animals. The 

 second is at least a rough observation which harmonizes mth 

 the Bunsen-Roscoe law. As long as the intensity of light or 

 the mass of photochemical substances at the surfaces of the 



