EFFECT OF CARBON DIOXIDE 



299 



vapour. I next describe the effect of the mild narcotic 

 carbonic acid gas on geotropic response. 



h 



Fig. 175. Effect of ether vapour on geotropism. n, the normal 

 reaction in air ; e, the reaction in an atmosphere of ether 

 vapour. 



Effect of Carbonic Acid Gas on Geotropic 

 Response of Growing Organs 



It has been explained that the intensity of geotropic 

 response is to a certain extent modified by the season. 

 I describe effects of application of CO2 to winter- and to 

 spring-specimens. 



Experiment 163. Effect of CO 2 on winter- specimen of 

 Tropaeolum. — The record given (fig. 176) shows the sluggish- 

 ness of response in the cold season. The latent period was 

 found to be 40 minutes (first part of the record not shown 

 in the figure). A geotropic curvature was then initiated 

 at a slow rate, as seen in the shghtlyinchned curve of ascent. 

 Carbonic acid gas was next passed into the plant-chamber ; 

 this caused a great enhancement of the geotropic reaction 

 in the course of about 3 minutes. The induced en- 

 hancement is clearly seen in the very erect curve, and in 

 the separation of the successive dots ; the acceleration 

 persisted for 20 minutes, after which the movement became 



