18 ELECTRO-PHYSIOLOGY CHAP. 



hairs are touched with a fine brush, or if two platinum electrodes 

 are plunged downwards into the leaf, and lead in the current from 

 an induction coil. The phenomena vary according as the leaf 

 is stimulated at different parts of its upper surface. Excitation 

 of the centre appears to be the most effective, being followed by 

 a negative variation after an interval of ^ ^ sec. 



Munk, on leading off from the two ends of the mid-rib on 

 the lower surface of the leaf, without, or even with, compensation 

 of the current flowing from base to apex, invariably observed a 

 diphasic variation, i.e. a positive preceded by an initial negative 

 variation, and this is equally the case when all visible excitatory 

 movement of the leaf is wanting. Sometimes Munk even saw a 

 positive effect before the initial negative phase, resulting in a 

 complicated triphasic variation ; this only appeared when the 

 excited leaf exhibited an actual movement. When there was 

 at first no difference of potential between the two leading -off 

 points, the diphasic variation still appeared with excitation, the 

 mirror moving rapidly in the direction of an ascending current, 

 and then giving a much weaker deflection in the opposite direc- 

 tion. On leading off from two points of the under surface of 

 the leaf, taken on the same " transverse line," median to the 

 " principal longitudinal line," the excitation either produced a 

 pure positive variation, or at most gave a trace of initial nega- 

 tivity. All these electrical processes fall mainly within the 

 mechanical, and easily detected, latent period, i.e. the interval 

 between the moment of excitation and beginning of the final 

 movement of the leaf. From the standpoint of Munk's theoretical 

 construction of the leaf of Dioncea out of electromotive elements 

 (" peripolar " cells), there are three alternative explanatkms of the 

 two successive and opposite phases of which each variation 

 consists ; and these are so closely interwoven with the facts 

 actually observed by Munk, that, as Burdon- Sanderson has 

 pointed out, it is exceedingly difficult to separate observation 

 from theory, 



(1) In the diphasic variation, as in the diphasic action current 

 of nerve and muscle, the electromotive elements at the two 

 contacts may not be simultaneously affected by the excitatory 

 change (negative variation of E.M.F.) ; (2) the elements may 

 all be affected simultaneously, and in the same direction 

 which would be opposite in the two phases ; or (3), as Munk 



