HELIO TROP1SM 1 2 5 



then move only in this direction. If the positively heliotropic polyp 

 of Eudendrium could be transformed into a free-swimming animal, it 

 would be compelled to swim automatically toward the source of light. 



It had been known since man began to use artificial light that cer- 

 tain animals, especially insects, show a tendency to fly or creep to the 

 flame. The explanation generally given of this phenomenon was an- 

 thropomorphic; it was assumed that the animals fly into the flame 

 because they are fond of light, or that they are driven by curiosity, 

 or that they are afraid of the dark. It seemed to me that we had no 

 right to see in this tendency of animals to fly into flame the expres- 

 sion of an emotion, but that this might be a purely mechanical or com- 

 pulsory effect of the light, identical with the heliotropic curvature 

 observed in plants. I believed that the essential effect of the light 

 upon these animals might consist in a compulsory automatic turning 

 of the head toward the source of light, corresponding to the turning 

 of the head, or the tip, of a plant stem toward the light ; and that the 

 process of moving toward the source of light was only a secondary 

 phenomenon. It seemed to me also that if the stem of the plant could 

 suddenly acquire the power of locomotion, it would act exactly like the 

 animals which fly into the flame.* 



I have since been able to prove directly that this deduction is cor- 

 rect. Eudendrium furnishes us the opportunity of observing the same 

 organism in rapid succession as a free-moving and as a sessile organ- 

 ism. In an early stage of development the larvae of Eudendrium are 

 ciliated pelagic organisms which swim actively. When these larvae 

 are in an aquarium which receives its light from one side only, they swim 

 at once toward that side and remain there as long as the direction of 

 the rays of light remains unchanged. If the aquarium is turned, they 

 also turn at once, and swim toward the lighted side of the aquarium. 

 This condition does not last long, for the larva soon attaches itself, 

 or rather adheres, to a solid body, and immediately afterward a polyp 

 grows out from the end opposite that which is attached to the solid 

 body. As soon as the polyp grows out, it undergoes a positively helio- 

 tropic curvature, as described above, provided that the light continues 

 to fail into the aquarium from one side only. It is thus possible to see 

 the same individual behave in twenty-four hours, first, like an insect 

 that is attracted by the light, and then like a heliotropic plant. I men- 

 tioned before that the heliotropic curvature of the stem of Eudendrium 

 occurs much more rapidly behind a blue than behind a red screen, if 



* The first paper on the identification of the flying of animals into the light with the 

 heliotropic curvatures <>f plants appeared in January, 1888. Sitzungsberichte der Wiirz- 

 burger med. physik. Gesellsch., 1888. The same number contained also a preliminary notice 

 (m the identity of geotropism in animals and plants. 



