140 DYNAMICS OF LIVING MATTER 



antiopa). "Since the head is the portion stimulated by light, it is 

 natural to suspect that the eyes are the particular parts concerned. Loeb 

 has pointed out that the orientation of an organism in light is depend- 

 ent upon the equal stimulation of symmetrical points on its body. 

 Should the eyes be the parts stimulated, any interference with one of 

 these ought to result in a disturbance of the direction of the butterfly's 

 locomotion. Thus if the cornea of one eye were blackened, the insect 

 in locomotion, being positively phototropic, ought to move as though 

 that eye were in shade; namely, in a circle, with the unaffected eye 

 toward the center. Specimens prepared by blacking the cornea of one 

 eye showed the expected response. When the right eye was covered, 

 the insects crept or flew in a circle, with the left side invariably toward 

 the center; and the reverse took place when the other eye alone was 

 blackened. These circus movements agree with those observed by 

 Holmes in other positively phototropic Arthropods." These data ex- 

 plain why in a field of force which affects the chemical processes in an 

 animal neither too little nor too much, the animal is turned automati- 

 cally until symmetrical points of its surface are struck equally by- the 

 lines of force. As soon as this occurs the animals must keep this 

 orientation, and therefore have no further choice in the direction of 

 their motions. 



Whether the oral pole is turned toward the source of the lines of 

 force or away from it, depends upon whether the energy which streams 

 along the lines of force alters the chemical reactions in such a way 

 as to increase the tone of the muscles (or the contractile protoplasm) 

 connected with the stimulated elements, or to decrease it. 



The light rays are not the only lines of force which bring about an 

 automatic orientation of animals; the galvanic current curves act as 

 lines of force, and we speak in that case of galvanotropic orientation, 

 or galvanotropism. A number of plants and animals are oriented 

 automatically by the lines of gravitation emanating from the center 

 of the earth, and are compelled to put their axes or planes of symmetry 

 into a vertical direction (geotropism). While in these cases the current 

 curves are very marked, the same cannot be said in regard to the lines 

 of force in a field of diffusion. The lines of diffusion determined by the 

 particles emanating from a center of diffusion should be straight lines, 

 but in reality currents of air or liquids cause disturbances of these 

 ideal lines. It thus happens that in the case of chemotropism we can 

 at the best expect only an approximate orientation. 



There are some other tropismlike reactions of animals and plants 

 which we shall discuss here, although they do not strictly belong in this 

 chapter; namely, stereotropism and rheotropism. 



