HELIOTROPISM 



127 



move toward the window, gathering at the window end c of the 

 tube, although by so doing they go from the sunshine into the 

 shade. This shows that the effect of light consists in turning the head 

 of the animal, and subsequently its whole body, toward the source of 

 light, so that the symmetrical points of the photosensitive surface of the 

 body in this case the eyes - - are struck by the rays at the same 

 angle. The animals will remain at the window side of the tube at 

 c (Fig. 25). The experiment disproves the anthropomorphic idea that 

 the animals go to the brightest spot in space. 



It can also easily be shown that in these animals, just as in plants, 

 the more refrangible blue rays are more effective than the red rays, and 

 that the latter act like weak light. Let us suppose that a test-tube 

 containing the animals be placed on a table near the window 

 (ww, Fig. 26), through which diffused light D enters; and 

 that one half of the test-tube, namely, that near the 

 window, be covered with blue glass ab. At the beginning 

 of an experiment the animals are gathered at the 

 room end of the test-tube. They behave as if the 

 test-tube were entirely uncovered, and move toward 

 the window side of the test-tube, where they 

 remain. The same experiment may be repeated, 

 only with the difference, that the window 

 side ab of the test-tube is covered with 

 red instead of blue glass. The animals 

 now creep in the direction 

 of the window to that -^5^- 

 point in front of a where : ~ c ~^ 

 the light, filtered by the red 

 glass, begins to strike them. 

 Here they gather, migrat- 

 ing constantly in a narrow circle at the limit between red and diffused 

 light. The explanation of the latter experiment is as follows : As long 

 as the animals are at the room end of the test-tube, they are struck 

 simultaneously by the diffused daylight D which falls through the win- 

 dow, and by the weak light R which is reflected from the walls of the 

 room. Under these circumstances the animals are forced to turn their 

 heads toward the stronger source of light, namely, the window, 

 and consequently move toward it. - As soon as they reach the point 

 where the light from the window has to pass through the red glass, 

 before striking them, the light reflected from the walls of the room, 

 which contains the effective blue rays, is heliotropically more effective 

 than the light from the window, which has lost most of its heliotropi- 



FlG 



