QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 251 



2. Theories of orientation 



a. Relation of phototaxis and photokinesis. Among organisms 

 that are visibly affected by light, there seem to be two distinct 

 types of response. In some forms light appears to act as a stim- 

 uhis to activity without any directive effect on position or loco- 

 motion. In other forms, light produces a definite orientation 

 usually accompanied, in motile organisms, by locomotion toward 

 or away from the source of light. For the first type of reaction, 

 I have used the term 'photokinesis' (Engelmann '83); for the 

 second, or directive response, 'phototaxis' (Davenport '97). 



Holt and Lee ('01) maintained that the motor reactions of 

 organisms to stimulation by light were not of two kinds, namelj^ 

 reactions to the intensity of light and reactions to the directions 

 of its rays, but that hght acted as a stimulus through its inten- 

 sity alone, the direction of its rays serving merely to modify the 

 intensity. With this conclusion I am in entire agreement. 

 The terms 'phototaxis' and 'photokinesis,' as used in the fol- 

 lowing discussion, are applied not to reactions to ray direction 

 and intensity respectively but to different types of reactions to 

 the intensity of light. 



It appears from the work of Bancroft ('13) on Euglena, that 

 light may operate as a stimulus either through its constant 

 intensity, or through changes of intensity. In Euglena at least 

 the mechanisms responding to constant intensity and to changes 

 of intensity are distinct and separately modifiable. It should 

 be clearly borne in mind throughout the following discussion 

 that the term 'photokinesis' is not used synonomously with 

 'Untershiedsempfindlichkeit' but to include any activity induced 

 by light, ivhich does not result in orientation, whether the activity 

 be produced by constant intensity or by changes of intensity. 



Probably the fundamental response to light is of the kinetic 

 sort, that is, activity induced by light without the operation from 

 within the organism of factors which screen or intensify its direct 

 action, or of factors which indirectly distribute its effects. In 

 such cases movement persists until fatigue ensues, or if there 

 is a region of the environment where the stimulating agent is 



