GKOTROPIC TORSION 



rm 



Kffect of coal gun on the haUntw : Kriteriuifnt 197. — 

 The method of balance describetl above opens out new- 

 possibilities in regard t(t investigations on the relative modi- 

 fications of geotropic and phototropic excitabilities by a 

 given external change. Traces of coal gas are known to 

 enhance the phototropic excitability of an organ while 

 continued absence of oxygen is found to depress it. The 

 experiment I am going to describe shows: (1) the enhance- 

 ment of phototropic excitability on the introduction of coal 

 gas, and (2) the depressing effect of excess of coal gas and of 

 the absence of oxygen. After obtaining the normal curve 

 of geotropic torsion, light was applied below^ at - L, and 

 exact balance was obtained in the course of two minutes as 

 seen in the top of the curve becoming horizontal. Coal 



gas was now introduced 

 in the plant-chamber at 

 C. This induced an en- 

 hancement of phototropic 

 effect with resulting 

 over-balance seen in the 

 reversal of torsion. This 

 enhancement persisted for 

 more than three minutes. 

 By this time the plant- 

 chamber was completely 

 filled with coal gas, and 

 the resulting depression of 

 -L. Application of coal gas at c, at first phototropic action is Seen 



caused enhancement of phototropic action . ,, i „ 



,,,,,,. m tht: second upset of 



with resulting reversal. Prolonged applica- '^ 



tion induced depression of phototropic re- the balance, this time in 



action, geotropic action thus becoming favour of geotropic tor- 



predominant. ^.^^ ^^..^^ ^^^^ j^ ^^^^^^ 



seem that the cells which respond to light are situated 

 nearer the surface of the organ than those which react to 

 geotropic stimulus. Hence an agent which acts on the organ 



Fiii. 183.— Eflfec: of coal gas on photo_ 

 geotropic balance. Geotropic torsion, G, is 

 exactly balanced by opposing action of light 



