METHOD OF REVERSAL 



349 



2. The excitatory reaction of galvanometric negativity 



disappears on the return of the specimen to the 

 vertical position ; and, finally, 



3. The errors caused by the inaccurate reading of the 



angular scale and the physiological asymmetry 

 of the organ may be eliminated by the Method of 

 Reversal. When the plant is inclined to the right 

 through + 90°, the current of response flows in 

 one direction ; when it is inclined to the left 

 through — 90°, the direction of the responsive 

 current is reversed ; the experimental error of a 

 single determination is eliminated by taking the 

 mean of the two galvanometric deflections. 



(^ 



y 



Fig. 205. Diagrammatic representation of inducing the geo- 

 tropic reaciion of the shoot by the Method of Reversal. 

 Rotation through + 90° (right) makes a negative, while 

 rotation through — 90° (left) renders a positive. 



The details of the procedure will be understood from 

 the diagram given in fig. 205. The specimen is at first 

 vertical, with the two symmetrical contacts on its sides 

 A and B, the electrodes being connected in the usual manner 

 with the terminals T T' of the indicatinggalvanometer ; after 

 rotation through + 90° the upper side A becomes excited 

 and galvanometrically negative (right-hand figure). The 

 specimen is next rotated to — 90° ; A now becomes the 

 under, and B the upper and excited, side (left-hand figure). 

 The electromotive response is now reversed, B being 

 galvanometrically negative. The induced electromotive 



