130 DYNAMICS OF LIVING MATTER 



cells, which are provided with cilia, and move out from the algae. 

 Strassburger showed that these swarm spores may be heliotropic and 

 move in the direction of the rays of light to or away from the source of 

 light. The blue rays are more effective in this case than the red rays. 



3. THE CONTROL OF THE PRECISION AND SENSE OF HELIOTROPIC 



REACTIONS IN ANIMALS 



When a large number of animals are tested for heliotropic reactions, 

 we find that there are two problems to be solved : the first, to account 

 for the variations in the degree of heliotropic sensitiveness ; the second, 

 to account for the variation in the sense of heliotropism. As far as the 

 first factor is concerned, we find animals that are not heliotropic at all, 

 animals that show a slight degree of heliotropism, and others that are 

 so pronounced in their heliotropism, that their motions, and indirectly 

 their whole existence, is only a function of light. As far as the sense 

 of heliotropism is concerned, we find positively and negatively helio- 

 tropic animals. What causes these differences? We started with 

 the assumption that the heliotropic reactions are caused by a chemical 

 effect of light; in all such reactions time plays a role. We assume, 

 furthermore, that if light strikes the two sides of a symmetrical organ- 

 ism with unequal intensity, the velocity or the character of the chemical 

 reactions in the photosensitive elements of both sides of the body is 

 different; that in consequence of this difference the muscles, or con- 

 tractile elements, on one side of the organism are in a higher state of 

 tension than their antagonists. The consequence is a curvature or a 

 bending of the head. With this assumption it becomes at once obvious 

 what is responsible for the variation in the intensity of heliotropism. 

 Let us consider for the time being only positively heliotropic animals. 

 Some of these, e.g. winged plant lice or the caterpillars of Porthesia 

 (immediately after leaving the nest), may be called most intensely posi- 

 tively heliotropic, inasmuch as they move toward the source of light 

 in as straight a line as their structural imperfections permit. If they 

 deviate from the direction of the rays of light for only a short time, 

 possibly less than a second, the difference in the tension of the muscles 

 on both sides of the body becomes so great that it suffices to turn their 

 heads automatically again toward the source of light. If one side of 

 the body alone be exposed to the light for only a fraction of a second, 

 the light causes such a difference in the chemical substances formed 

 on both sides of the body that the threshold for the difference in the 

 tone of the muscles is exceeded, and the bending of the head must occur. 

 If, however, the light does not increase the reaction velocity as much 



