QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 275 



analysis is a chemical phenomenon and would follow the laws 

 governing chemical reactions. The failure to respond directly 

 is probably due, therefore, either to the presence of receptors of 

 such a nature that changes of axial position do not affect the 

 relative amount of stimulation received on opposite sides of the 

 animal (fig. 21), or to the indefinite transmission of the stimulus 

 from the end organs to the locomotor apparatus. 



There are then two factors which must be present to produce 

 phototaxis, one of these is resident in the stimulating agent, 

 the other is resident in the organism. A directive stimulant 

 may be ineffectual in producing orientation because of the ab- 

 sence in the reacting organism of a mechanism capable of direc- 

 tive response; similarly an animal with a perfectly developed 

 directive mechanism may fail to orient because of the absence of 

 directive distribution of the stimulating agent. The matter 

 may be summarized thus: 



Non-directive stimulant + indefinite response = photokinesis 



Non-directive stimulant + definite response 



proportional to the intensity of the stimulant = photokinesis 



Directive stimulant 4- indefinite response = photokinesis 



Directive stimulant + definite response pro- 

 portional to stimulus = phototaxis 

 The idea of a constantly acting stimulus which produces a 

 reaction proportional to its intensity was one of the fundamental 

 conceptions on which Loeb based his tropism theory. The fact 

 that the theory as he advanced it postulated bilaterality in the 

 responding organism has led to its abandonment by many authors 

 who have studied the asymmetrically sensitive Protozoa. There 

 is no doubt that in its original form the tropism theory does not 

 apply to the orientation of asymmetrical organisms. The pro- 

 portionality of the reaction to the intensity of the stimulant, 

 within the range of noiTnal physiological response, is however in 

 my opinion the essential basis of any reaction involving definite 

 orientation. Its method of expression varies in accordance with 

 the structural peculiarities of the reacting animal, but the under- 

 lying phenomena are nevertheless in all cases essentially the same. 

 If we include under phototaxis any reaction which involves a 



