20 THE POPULAR SCIENCE MONTHLY 



straight line and the positively heliotropic animal will move in a 

 straight line to the source of light. (It was assumed that in these 

 experiments the animal is under the influence of only one source of 

 light and positively heliotropic.) 



In a series of experiments I have shown that the heliotropic reac- 

 tions of animals are identical with the heliotropic reactions of plants. 

 It was known that sessile heliotropic plants bend their stems to the 

 source of light until the axis of symmetry of their tip is in the direction 

 of the rays of light. I found the same phenomenon in sessile animals, 

 e. g., certain hydroids and worms. Motile plant organs, e. g., the 

 swarm spores of plants, move to the source of light (or if they are nega- 

 tively 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 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 with the Bunsen-Roscoe law. As long as the inten- 

 sity of light or the mass of photochemical substances at the surface of 

 the animal is small, according to the law of Bunsen, it must take a 

 comparatively long time until the animal is automatically oriented by 

 the light, since according to this law the photochemical effect is equal 

 to the product of the intensity of the light into the duration of illu- 

 mination. If, however, the intensity of the light is strong or the active 

 mass of the photochemical substance great, it will require only a very 

 shoi't time until the difference in the mass of photochemical reaction 

 products on both sides of the animal reaches the value which is neces- 

 sary for the automatic turning to (or from) the light. The behavior 

 of the animals agrees with this assumption. If the light is sufficiently 

 strong the animals go in an almost straight line to the source of light; 

 if the intensity of light (or the mass of photosensitive substances on 

 the surface of the animal) is small the animals go in irregular lines, 

 but at last they also land at the source of light, since the directing 



