I. PHOTOTAXIS AND PHOTOPATHY. 



The motor reactions of organisms to light, so far as known at present, are of 

 two kinds: phototactic and photopathic. In both intensity of the light, not the 

 direction of the rays, is the determining factor. All those reactions in which the 

 direction of movement is determined by an orientation of the organism which is 

 brought about by the light are phototactic;* and all those reactions in which the 

 movement, although due to the stimulation of light, is not definitely directed through 

 the orientation of the organism are photopathic. f 



Photopathic reactions are supposedly due to the fact that certain animals are 

 forced by light so to orient themselves that symmetrical points on the surface of the 

 body are equally stimulated. This, the so-called "orientation theory" of Loeb ('93, 

 p. 86) and of Verworn ('99, p. 499 et seq.), accounts for those cases in which animals 

 seek or avoid light. Evidently a bilaterally symmetrical organism receives equal 

 stimulation on symmetrical points when the long axis of its body is parallel with 

 the rays of light, the head being directed either toward or away from the source of 

 light. When, as a result of orientation with the head toward the light, an animal 

 moves toward the source of light the reaction is said to be positively phototactic; 

 when the movement is away from the light it is negatively phototactic. These 

 reactions may be either primary or secondary. They are primary when the 

 animal moves in the axis of the rays, and secondary when, prevented from so doing, 

 it moves at right angles to the axis, but at the same time into regions of increasing 

 (positive secondary phototaxis) or diminishing (negative secondary phototaxis) inten- 

 sity. 



An organism which selects a particular intensity of light and confines its move- 

 ments to the region illuminated with that intensity is photopathic. t The so-called 

 " optimal intensity " is usually found by means of a phototactic reaction. Photopathic 



*See Holt and Lee (:01) for the theory of phototactic reaction. 



t It is to be noted that the terms phototaxis and photopathy are here given new meanings. Previously photo- 

 taxis has been applied to those reactions which were supposed to be determined by the direction of the rays of light, 

 and photopathy to those due to differences in the intensity of the light. 



t Strassburger ('78, p. 572) has described this kind of reaction for the swarm-spores of Ulothrix and Hsema- 

 tococcus. 



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