14 COMPARATIVE ELECTRO-PHYSIOLOGY 



Munk attempted to explain the complicated electrical 

 effects which he observed in Dioncea by assuming the existence 

 of two different kinds of electro-motive elements, affected in 

 opposite ways, the maximum changes being initiated in one 

 set earlier than in the other. In this way, he thought, it 

 might be possible to explain the occurrence of positive and 

 negative variations, holding that the upper parenchymatous 

 layer of the leaf, and the upper midrib, went through the 

 negative, and the under layer and the under midrib through 

 the positive change, 



Burdon Sanderson, in his ' fundamental experiment ' on the 

 lamina of Dioncm, had his led-off circuit connected with the 

 upper and lower surfaces of one lobe, stimulus being applied at 

 the other. In the experiments described in the ' Phil. Trans.' 

 of 1882, he found that the upper or inner surface of the leaf 

 become positive on excitation. This he regarded as the 

 true excitatory change. The upper contact now, however, 

 after a certain interval became negative, a change which 

 Burdon Sanderson designated as the after-effect. This after- 

 effect he ascribed to the electrical variations caused by that 

 movement of water which had been observed by Kunkel. 

 But with regard to the preceding positive variation he says : 



' The excitatory disturbance which immediately follows 



(excitation is an explosive molecular change, which by 

 Tthe mode~oF7ts origin/ the suddenness of its incidence, 

 I and the rapidity of its propagation is distinguished from 

 every other phenomena except the one with which I 

 have identified it, namely, the corresponding process in 

 the excitable tissues of animals. . . . The direction of 

 the excitatory effect in the fundamental experiment is 

 such as to indicate that in excitation excited cells 

 become positive to unexcited, whereas in animal tissues 

 excited parts always become negative to unexcited.' l 



In a subsequent series of experiments, however, given in 

 1 Phil. Trans.' for 1888 Burdon Sanderson finds the reaction 



1 rhil. l^rans. vol. clxxiii. 



