Conductivity under Geotropic Stimulation. 351 



Experimental Proofs. 

 Method. 



The hypothesis was tested by some preliminary experiments with the 

 Kohlrausch bridge, and it was found that the turning of the root horizontal 

 always caused an immediate decrement of the resistance, which was inter- 

 preted, as in McClendon's experiments (5), as increase in permeability to ions. 

 This decrement of resistance was distinctly less in the upper side of the root 

 than in the lower. A more accurate apparatus, which will now be described, 

 was then set up, and the results are given below. 



The Kohlrausch modification of the Wheatstone bridge consists in (1) the 

 use of an alternating current which prevents local electrolysis in the tissue ; 

 (2) the use of a telephone by which the position of the sliding contact is 

 regulated so that the least amount of sound is heard in the telephone. This 

 " silence " point or position of minimum sound is the point at which the 

 resistances have the well known relationship to one another. Fig. 3 shows 



H E 



Fig. 3. 



the arrangement of the apparatus diagrammatical ly, AB being the metre 

 bridge, C the sliding contact, D the telephone, E the induction coil, F a 

 single accumulator, G- a plug key for putting the accumulator into or out of 

 action, H a tapping key, K a resistance box, and P the plant or unknown 

 resistance. By means of the tapping key the injurious effects of the passage 

 of the current through the tissue are minimised. When the silence point is 

 obtained the position of C is read off and the resistance calculated from the 

 ratio AG : CB : : P : P. 



The resistance to be determined was that of the tissue at one side of the 

 root-tip of Vicia Faba from 1 mm. to 2 mm. from the apex. The ends of two 

 fine platinum wires were bent at right angles, the bent ends being 0'75 mm. 

 to 1-5 mm. in length. About 1-5 cm. of the root was covered with paraffin 

 wax melted at a temperature of 45° C. As the root was dipped into the wax 



