HELIOTROPISM or ANIMALS 20 



<>f l*(trllie*in chrysorrhcea is deli-nnined l>// tin- 

 direction of I lit- ra/fs of lit/Id, and not by differences in the 

 intensity of the light in different parts of space. Positive! \ 

 heliotropio animals are compelled to turn their oral pole 

 toward the source of light and to move in the direction of 

 the rays toward this source. 



-. The dependence of orientation <>n tin- refrangibility 

 of tlic rat/s. I shall now show that it is the more rcfrain/ilile 

 rni/* of flic rixilde *i>ech'nm wliicli ore chiefly concerned in 

 hrint/iin/ a/tout tJie orientation of tlie caterpillars of Por- 

 tliexiti chrysorrhcea. 



Experiment 1. If we place the test-tube on a table and 

 cover it with a box of dark-blue glass, the animals behave as 

 if the vessel were uncovered. Without exception, they move 

 in a strait / lit line to the window side of the vessel and remain 

 there. If instead of blue glass we use red, which to our 

 evt-s seerns much brighter than blue glass, no change occurs 

 in the orientation of the animals at first; after a long time, 

 however, the animals collect under the red glass on the win- 

 dow side of the vessel. In direct sunlight, however, orienta- 

 tion takes place more quickly. Exactly the same phenomena 

 are observed if an auimoniacal solution of copper is sub- 

 stituted for the blue glass, or a solution of potassium 

 bichromate for the ruby glass. This is also true in the 

 following experiments, where I may not always call special 

 attention to it. This experiment shows (1) that tJie more 

 refrain/ihle ratjx IK ire the saute effect as tin'.red i-ai/s, and 

 ('-) find ike less refrangible ra//x Itrhnj al>onf movement* in 

 tin- wane irai/ as the more rcfrain/ih/e ones, onfij their e( r eet 

 is less intense. The experiment also proves that it is wrong 

 to say, as do the anthropomorphists, that the animals "aiv 

 fond of" blue and "hate" red; for, were this true, the 

 animals should have been forced to move to the room side 

 of the test-tube when under the red glass, yet they moved 



