26 LIGHT AND THE BEHAVIOR OF ORGANISMS 



that the law put forward by Sachs for the heliotropism 

 of plants, namely, that the direction of the rays of light 

 determines the orientation, holds good also for animals." 

 Elsewhere in the same paper he states this law explicitly 

 as follows (1905, p. 5): "Sachs came to the conclusion 

 that the direction in which the rays of light penetrate the 

 plant tissue determines the orientation of the plant toward 

 light." This statement of the law is correct, but it should 

 be emphasized that Sachs also said " that in heliotropic 

 curvatures the important point is not at all that the one 

 side of the part of the plant is illuminated more strongly 

 than the other." There is evidently much confusion here 

 in the application of Sachs' theory. 



Do Loeb's conclusions in this paper show " that the law 

 put forward by Sachs for heliotropism of plants . . . 

 holds good also for animals"? He writes (1905, p. 28): 

 " From what has been said, no one, I believe, will doubt 

 that the direction of the progressive movements of the 

 caterpillars of Porthesia chrysorrhoea is determined by the 

 direction of the rays of light, and not by differences in 

 the intensity of the light in different parts of space. Posi- 

 tively heliotropic 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." And (1905, p. 53), " The 

 direction of the rays, and not the distribution of the intensity 

 of the light, in the test-tube, therefore, determines the direction 

 of the progressive movements.''' From these quotations it is 

 evident that Loeb means ray direction in general in opposi- 

 tion to difference in intensity in the field. He proved that 

 under the conditions of his experiments the direction of 

 motion is not governed by the difference of intensity in 

 the field. But this has nothing to do with Sachs' theory, 

 for this theory does not consider the effect of ray direction 

 in the field or " distribution of the intensity in the test- 

 tube." Sachs, as stated above, says very definitely that 

 it is the direction in which the rays pass through the tissue 

 and not difference of light intensity on opposite sides of the 



