3l8 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT 



Amalgamation of two zinc rods with mercury reduces the 

 electric difference, but cannot altogether ehminate it. 



I have been able to wipe out the difference of potential 

 between two pieces of the same metal, say, of platinum, 

 making a voltaic couple by immersing them in dilute com- 

 mon salt solution. The circuit is kept complete for 24 hours, 

 and the potential of the two electrodes by this process is 

 nearly equahsed. Perfect equahty is secured by repeated 

 warming and cooHng of the solution and by sending through 

 the circuit an alternating current which is gradually reduced 

 to zero. I have by this means been able to obtain two 

 electrodes which are iso-electric. The specially prepared 

 electrodes (made of gold or platinum wire) are put in con- 

 nection with the plant through kaolin paste moistened with 

 normal sahne solution. Care should be taken to put an 

 opaque cover over the plant-holder, so as to guard against 

 any possible photo-electric action ; moistened blotting- 

 paper maintains the closed chamber (not shown in fig. 190) 

 in a uniform humid condition. 



Three Different Methods of Observation 



For serving definite experimental purposes I have 

 employed three different methods : (i) The Method of 

 Contact with Sensitive and Indifferent Points ; (2) the 

 Method of Axial Rotation ; and (3) that of Vertical 

 Rotation. 



Method of contact with sensitive and indifferent points. — 

 This is the most perfect method, for in it the indifferent 

 distant point is unaffected by geotropic action. An example 

 will make the matter clear. In the Water-Lily (Nymphaea) 

 it is the peduncle which is sensitive to geotropic stimulus. 

 One electric contact is made at an indifferent point on 

 a sepal, which is always kept vertical ; the other is made 

 at a point A on one side of the peduncle (fig. 193). 



Experiment 175. Induced electric variation on upper 

 side of the organ. — The sepal being held vertical, the peduncle 



