42 PLANT-ANIMALS [OH. 



light. This mode of response we speak of as a 

 positively (+) phototropic response. 



C. paradoxa, which lives in a shadier situation, 

 responds to the light of the laboratory by an opposite 

 movement a negatively ( ) phototropic response 

 (Fig. 9, a). 



Fig. 9. Phototropism of C. paradoxa : the influence of light-intensity 

 on phototropic response, a. Mode of response when the light- 

 intensity is high. b. Mode of response when the light-intensity 

 is low. The glass troughs containing the animals are represented 

 (in plan) by oblongs. The troughs standing on a black ground 

 are represented by the shaded, those on a white ground by the 

 clear oblongs. The animals are indicated by dots, and the arrows 

 show the direction of the light. 



It is easy to prove, however, that neither the 

 positive phototropism of C. roscoifensis, nor the oppo- 

 site mode of reaction of C. paradoxa, is, in reality, an 

 inevitable reaction. Expose C. roscoffensis suddenly 

 to a bright light and it recoils (see Fig. 12) as we 

 ourselves in similar circumstances may recoil. Place 

 it in a dim light and it exhibits no phototropistic re- 



