vi ELECTROMOTIVE ACTION IN VEGETABLE CELLS 25 



without involving the surrounding tissue. In this respect again 

 we are reminded of the polar after-currents in muscle. 



It thus becomes intelligible that, according to the situation of 

 the leading-off points on the opposite surfaces of a partially 

 " modified " leaf, the excitatory variations discharged by a trans- 

 mitted excitation may be directly opposite in character, the 

 diphasic variation in the modified tract presenting different signs 

 from that in the tract that is unmodified (normal). 



Munk, as we have seen, assumed the precise seat of the 

 excitation on the leaf of Dioncea to be without significance to 

 the character of the electrical variation discharged, and concluded 

 that the propagation of the changes which underlie the excitatory 

 effect (movement) must be so rapid that they begin, as it were, 

 simultaneously at all points. Burden -Sanderson's investiga- 

 tions show that this theory (which is a priori improbable 

 if the. excitatory movements of plants correspond with the 

 excitation of protoplasmic parts) is as a fact inadequate. It is 

 evident that if the view advanced by 

 Munk were correct, there should be no 

 galvanic effect of excitation with a sym- 

 metrical lead-off from upper or under 

 surface of both lobes, even when one 

 side only was stimulated (Fig. 147). A 

 variation under these conditions is only 

 to be expected when the activity of the 



two lobes differs either in degree, or in the moment of its com- 

 mencement ; much as an electrical P.D. can only occur between 

 two points of a muscle when the physiological state of the two 

 points is different, or when the same state is developed in them 

 at different times. 



And we find experimentally that this method of leading off is 

 invariably followed by galvanic effects of excitation, as appears at 

 once from the curves, Fig. 148, a, b. 



Fig. 147 represents a leaf led off symmetrically from the 

 under surface, and excited with opening shocks. The galvano- 

 meter is replaced by a capillary electrometer, the effects being 

 photographically recorded. The directly excited lobe is invariably 

 at first negative, and subsequently positive to the other, giving 

 rise to a diphasic variation similar in character to that which 

 results on leading off two points from the normally isoelectric 



