146 Herbert Eugene Walter 



With reference to an organism, directive light is resolvable 

 into unequalized intensity and non-directive light into equalized 

 intensity. 



Asymmetrical intensity in directive light is largely due to the 

 partial shadowing of that part of the body farthest away from the 

 source of the light. Slight changes in the position or direction 

 of the light may cause considerable changes in the symmetry and 

 the degree of the shadow effects and consequently in the relative 

 intensity of the light on different regions of the body of an organ- 

 ism. 



To different degrees of equalized or symmetrical intensity pla- 

 narians show considerable response, but the correlation between 

 their behavior and the degree of intensity is not so close as it is in 

 the case of asymmetrical intensity. 



Increase in intensity of non-directive light, continued exposure 

 to non-directive light of constant intensity, and change from dark- 

 ness to non-directive light, all tend to bring about apparent photo- 

 taxis similar to that occurring in directive light. 



Responses are more frequent on the part of planarians in inten- 

 sities approaching the optimum than in higher intensities, where 

 there is a tendency to inhibition. 



Relative differences in responses to various intensities are due 

 to specific differences between planarians. 



The physiological state of an organism together with the influ- 

 ence of known stimuli other than light are constant sources of 

 error in estimating reactions to light. These factors taken to- 

 gether play a more important part in planarian behavior than 

 light stimulus. 



Finally, the action of light upon planarians is a function of its 

 intensity, which, under certain conditions, is emphasized by the 

 direction of the light. 



2 TRIAL AND ERROR OR TROPISM ? 



It is apparent from the preceding section that light may have 

 two effects upon organisms. Of these, one is a kinetic effect, 

 arising from the intensity of the stimulus and resulting in a gen- 



