2 54 LIGHT AND THE BEHAVIOR OF ORGANISMS 



move toward or from a source of light are usually termed 

 phototactic, those which orient but do not move as photo- 

 tropic, and those which do not orient but still react have 

 been termed photopathic. The adjectives positive and 

 negative are ordinarily used in connection with these terms 

 to signify whether the organisms go or bend toward the 

 source of light or in the opposite direction ; or whether they 

 collect in regions of higher intensity or in those of lower 

 intensity. The terms mentioned above have however been 

 used not only to signify direction of movement, but also 

 to designate the nature of the stimulation and the response 

 as set forth in Part I, under definitions of tropisms. 



Loeb says (1906, p. 135): '' Heliotropism covers only 

 those cases where the turning to the light is compulsory 

 and irresistible, and is brought about automatically or 

 mechanically by the light itself. On the other hand, there 

 are compulsory and mechanical reactions to light which 

 are not cases of heliotropism; namely, the reaction to sud- 

 den changes in the intensity of light. ... In the former 

 case the results are a function of the constant intensity, 

 in the latter a function of the quotient of the change of 

 intensity over time." All cases of orientation are con- 

 sidered by him to be due to heliotropism, i.e., to the effect 

 of light by virtue of its " constant intensity." All other 

 reactions to light are due to changes of intensity. In this 

 class Loeb puts the contraction of the tubicolous annelids 

 Serpula and Spirographis and the collection of Planaria in 

 regions of low light intensity. These reactions, he says, 

 are due to Unterschiedsempfindlichkeit — sensibility to dif- 

 ference of Intensity; those resulting In orientation are not. 



Davenport (1897, pp. 210, 211) maintains that "Two 

 kinds of effects are produced by light: one by the direction 

 of its ray — phototactic; the other by the difference In illu- 

 mination of parts of the organism — photopathic. . . . 

 Light acts directly either through difference In Intensity on 

 the two sides of the organism, or by the course the rays 

 take through the organism." Here again we have two 



