EFFECT OF CARBON DIOXIDE 3OI 



normal geotropic movement which carried the tip of the specimen 

 below the horizontal position. Carbonic acid gas thus caused 

 an apparent reversal of normal geotropic response. Fresh 

 air was then introduced in the chamber, with the result that 

 the normal geotropic up-movement was renewed after an 

 interval of 5 minutes. 



In another experiment a stream of carbonic acid gas 

 was maintained throughout, the experiment lasting for more 



Fig. 178. Effect of CO2 on geotropic response of Tropaeolum. 



Enhancement of geotropic response followed by persistent 

 reversal under continued action of the gas. 



than an hour. It gave the same sequence of effects as 

 before — namely, enhancement at the first stage, a temporary 

 arrest at the second, and an actual reversal at the third 

 stage (fig. 178). The tip of the specimen under the con- 

 tinued action of gas persisted in its reversed position. 



The results described above relate to the action of 

 carbonic acid gas on the geotropic response of Tropaeohim. 

 In order to demonstrate the universality of the phenomenon, 

 further investigations were undertaken with a large number 

 of growing and of pulvinated organs. 



