44S LIFE MOVEMENTS IN PLANTS 



the neutral point, N, and A. This natural current may be 

 allowed to remain, the action current due to geotropism 

 being superposed ott, it; or the natural current may be 

 neutralised by means of a potentiometer and the reflected 

 spot of light brought to zero of the scale. 



hiduced electric variation on upper side of the organ : 

 Experiment 170. — While the sepal is held vertical, the 

 stalk is displaced through +90=' so that the point A is 

 above. Geotropic stimulation is at once followed by a 

 responsive current which flows through the galvanometer 

 from N to A, the upper side of the organ thus exhibiting 

 excitatory reaction of galvanometric negativity (Right-hand 

 figure of 166). When the stalk is brought back to vertical 

 position geotropic stimulation disappears, and with it the 

 responsive current. 



Electric response of the lower side: Experiment 171. — The 

 stal'k is now displaced through - 90^ ; the point A, which 

 under rotation through +90** pointed upwards, is now 

 made to point downwards. The direction of the current 

 of response is now found to have undergone a reversal ; 

 it now flows from A on the lower side to the neutral 

 point N ; thus under geotropic action the lower side of 

 the organ exhibits galvanometric positivity indicative of 

 increase of turgor and expansion (Left-hand figure 166).* 



Having thus found that the upper side of the organ 

 under geotropic stimulus becomes galvanometrically nega- 

 tive, and the lower side, galvanometrically positive, we 

 make electric connections with two diametrically opposite 

 points of the shoot A and B, and subject the organ 

 to alternate rotation through + 90° and - 90°. The electro- 

 motive changes induced at the two sides now became 

 algebraically summated. I employ two methods for geotropic 



* For detailed account cf . Chapter XLIII. 



