4] PHOTOTAXIS AND PHOTOPATHY 209 



required, to travel 18 cm. in full light, 15% longer time than 

 the same individual required in light \ as strong. Since the 

 increased time was only 15% instead of 300%, as it should be 

 were rate proportional to intensity, it seems probable -a con- 

 clusion confirmed by the direct observation of the organisms in 

 the trough that the slower rate in the weaker light is due to 

 less precise orientation. How would the rate be influenced by 

 two lights of different intensities acting from opposite direc- 

 tions ? Upon this matter we have no experimental data. 



Light, then, serves to orient the organism ; but how ? This 

 again leads us to the general question of the cause of the tactic 

 response, a question which must be referred to a later chap- 

 ter. Certain special considerations may, however, be introduced 

 here. Let us first think of the way in which light acts on the 

 negatively phototactic (and photopathic ?) earthworm. Repre- 



LOW LIGHT ATTUNEMENT 



LOW LIGHT ATTUNEMENT 



FIG. 63. Diagram, representing sunlight (SS) falling upon an elongated, bilateral 

 organism (represented by the arrow) whose head is at A. (Original.) 



sent the worm by an arrow whose head indicates the head end 

 (Fig. 63, A). Let solar; rays SS fall upon it horizontally and 

 perpendicularly to its axis. Then the impinging ray strikes it 

 laterally, or, in other words, it is illuminated on one side and 

 not on the other. Since, now, the protoplasm of both sides is 

 attuned to an equal intensity of light, that which is the less 

 illuminated is nearer its optimum intensity. Its protoplasm is in 

 a phototonic condition. That which is strongly illuminated has 

 lost its phototonic condition. Only the darkened muscles, then, 

 are capable of normal contraction ; the brightly illuminated 

 ones are relaxed. Under these conditions the organism curves 

 towards the darker side ; and since its head region is the most 

 sensitive, response begins there. Owing to a continuance of 

 the causes, the organism will continue to turn from the light 

 until both sides are equally illuminated ; i.e. until it is in the 

 light ray. Subsequent locomotion will carry the organism in a 



