20 ELECTRO-PHYSIOLOGY CHAP. 



surfaces of each lobe of the leaf, external and internal (or upper 

 and lower), give a different electrical reaction, so that on leading 

 off from opposite points of the upper and lower surface, there is 

 found to be current, either so that the latter is positive to the 

 former (which Burdon-Sanderson at first held to be normal) or 

 vice versa. The degree of positivity, and corresponding magnitude 

 of P.D. and of the leaf-current, in the first case, depend, as soon 

 appears, essentially upon the physiological state of the leaf, and 

 above all upon the previous excitation. 



If, after compensating the current of rest, mechanical or 

 other stimuli are sent in moderately rapid succession into a leaf 

 of which the under surface is already positive, there is without 



FIG. 142. 



exception a marked rise of positivity in the excited surface. It 

 is only by degrees, during the subsequent period of rest, that the 

 current diminishes, until with the same lead-off there is complete 

 isoelectricity, and finally, as has been said, a current in the 

 opposite direction, corresponding with negativity of the under 

 (external), and positivity of the upper (internal), surface of the 

 leaf a state which, according to Burdon-Sanderson's later observa- 

 tions, must be regarded as normal in the leaf which has remained 

 a long time unexcited. In this instance the leaf-current must, in 

 regard to the internal surface, be regarded as outgoing. 



Excitation in such a case is naturally .followed by the 

 opposite alterations, as in an initially ingoing leaf-current : the 

 positive upper surface becomes suddenly negative to the lower 

 surface, so that the leaf-current is once more ingoing. It should 

 also be remarked that the lower surface of the leaf is the less 

 positive and more negative, in proportion with the time that has 

 elapsed since the last excitation. The current of rest, on the 

 contrary, is at all times dependent on the previous excitation of 



