54 LIGHT AND THE BEHAVIOR OF ORGANISMS 



sity on the surface as De Candolle held, but to the direction 

 in which the rays pass through the tissue. HeHotropism, 

 to some of those who agreed with Sachs, meant orientation 

 due to direction of rays through the tissue, to others merely 

 orientation due to ray direction in general. 



(2) Darwin in 1880 said orientation in plants is due to 

 modification of circumnutation. It is regulated by differ- 

 ence of intensity on opposite surfaces, probably changes of 

 intensity, and he used the term heliotropism to indicate 

 this. 



(3) In 1888 Loeb maintained that orientation in animals 

 is controlled by the direction in which the rays pass through 

 the tissue, that is, in the same way in which Sachs had said 

 it was controlled in plants. In 1889 he still held that light 

 reactions in plants and animals are governed by the same 

 lawa. But now he says symmetrically located points on 

 the photosensitive surface must be struck by light at the 

 same angle. " Light automatically puts the plant or the 

 animal into such a position that the axis of symmetry of 

 the body, or organ, falls into the direction of the rays of 

 light." Heliotropism is however used not only to express 

 this explanation of orientation, which differs materially 

 from that of Sachs, but also to indicate movement toward 

 or from the source of light. In his later work, he abandons 

 the idea of the importance of the angle between the sen- 

 sitive surface and the light rays and substitutes the idea 

 that it is relative intensity on opposite sides which governs 

 orientation. Thus heliotropism received a new significa- 

 tion. His most recent views are expressed in the following 

 quotations (1906, pp. 135, 138): " Heliotropism covers only 

 those cases where the turning to light is compulsory and 

 irresistible, and is brought about automatically or mechani- 

 cally by the light itself. ... If the current curves of 

 radiating energy, e.g., light rays, strike an animal on one 

 side only, or on one side more strongly than on the sym- 

 metrical side, the velocity or the kind of chemical reactions 

 in the symmetrical photosensitive points of both sides of 



