150 Electromotive Properties of the Leaf of Dioncea. [Dec. 15, 



is therefore termed the " f undamental experiment." It consists in 

 measuring the successive differences of potential (cross differences) 

 which present themselves between two opposite points on the upper 

 and on the under surface of one lobe of the leaf, during periods which 

 precede, include, and follow the moment at which the opposite lobe 

 is mechanically or electrically excited. In this experiment it is found 

 that, provided that the conditions are favourable to the vigour of the 

 leaf, the variations of the cross difference (called the excitatory 

 variation) occur in the following order : — 



Before excitation (particularly Upper surface negative to 

 if the leaf has been previously under, 

 excited). 



At the moment of excitation. Sudden negativity of under 



surface, attaining its maximum 

 in about half a second, when the 

 difference amounts to not less 

 than y 1 -^ Daniell. 



After excitation. Rapidly increasing negativity 



of the upper surface, beginning 

 about 1*5" and culminating about 

 3" after excitation, and slowly 

 subsiding. 



This subsidence is not complete, for, as has been said, the lasting 

 difference between the two surfaces is augmented — the upper surface 

 becoming more negative after each excitation ("after-effect "). 



When by a similar method two points are taken for comparison on 

 opposite lobes, the phenomena are more complicated, but admit of 

 being explained as resulting from the more simple case above stated, 

 in which only a few strata of cells are interposed between the leading 

 off electrodes. . 



In Part V the relation of the leaf to different modes of excitation 

 is investigated. As regards electrical excitation the results are as 

 follows : — If a voltaic current is led across one lobe by non-polarisa- 

 ble electrodes applied to opposite surfaces (the other lobe being led off 

 as in the fundamental experiment) a response (excitatory variation) 

 occurs at the moment that the current is closed, provided that the 

 strength of the current is adequate, and not much more than adequate. 

 No response occurs at breaking the current. When a current of more 

 than adequate strength is used, and its direction is downwards, the 

 response at closing is followed by several others. This effect does not 

 happen when the current is directed upwards. To evoke a response 

 a current must be much stronger if directed upwards than if directed 

 downwards through the same electrodes. Weak currents cease to act 

 when their duration is reduced to t ^q" ; for stronger ones the limit is 



