302 CHAP. XXVII. ANAESTHETICS AND GEOTROPISM 



Experiment 165. Response of the peduncle of Tuberose. — 

 This specimen gave a normal geotropic response, though it 

 was relatively sluggish. The latent period was 45 minutes. 

 The effect of CO^ was a preliminary enhancement, followed 

 by arrest and reversal of normal geotropic response. On 

 the substitution of fresh air in the plant-chamber, the normal 

 up-movement was once more restored. Continued action of 

 carbon dioxide is thus seen to produce in the Tuberose 

 a reversal of geotropic action similar to that observed in 

 Tropaeolum.^ 



In regard to the reversal of geotropic movement under 

 carbon dioxide, it is to be borne in mind that geotropic 

 response is ultimately due to induced variation in the rate 

 of growth. It has been shown that all narcotics, including 

 carbonic acid gas, ether, and chloroform, induce variation in 

 the rate of growth, an acceleration at the first stage, an 

 arrest at the second, and a reversed contractile response at 

 the third stage. Corresponding to these are the accelera- 

 tion, arrest, and reversal of geotropic movement. The effect 

 of CO2 is therefore by no means unique, but the common 

 reaction under all narcotic agents. 



Effect of Carbonic Acid Gas on Geotropic Response 



OF PULVINATED OrGAN 



The geotropic excitabihty of the upper half of the 

 pulvinus of Mimosa, as previously explained, is very much 

 less than that of the lower half. It thus happens that the 

 leaf of Mimosa is in a state of equilibrium in a horizontal, 

 the so-called dia-geotropic, position. But if the plant be 

 inverted so that the relatively more excitable lower half 

 is above, the geotropic excitation and the resulting curvature 

 are greatly enhanced, the leaf becoming continuously 

 erected in this inverted position. The experiment may be 



1 J. Lynn observed {New Phytologist, Aug. 19, 1921) a reversal of 

 geotropic curvature in the hypocotyl of Helianihus annuus under the action 

 of carbonic acid gas. He suggests in explanation a far-fetched theory of 

 hydrion concentration, for which no experimental verification has been 

 adduced. 



