30 ELECTRO-PHYSIOLOGY CHAP. 



does not give the smallest support to such a purely gratuitous 

 hypothesis, and that it fails, moreover, to account for the differences 

 of potential which Burdon- Sanderson has shown to exist regularly 

 between upper and under surface, we are thrown back on the 

 scarcely less arbitrary assumption of a constant (chemical) 

 difference, and resulting electrical P.D., between the upper and 

 lower halves of each parenchyma cell on the upper surface 

 of the leaf, comparable in some measure with the difference in 

 potential between free end and base of the mucous cells. But 

 it is obvious that neither structure nor arrangement of the single 

 cells agrees with even this presumption. Burdon - Sanderson's 

 view is undoubtedly the more probable, that the surface of a 

 single ell is individually, and under all conditions, isoelectric. 

 It is hardly necessary to add that no current can result from 

 the mere contiguity of two cell-bodies surrounded with cellulose 

 walls, and thus completely separated, when the one is altered 

 equally in all its parts with respect to the other ; any more than 

 a muscle-current appears when a fibre that is equally active at 

 all points is brought into contact with another fibre in the 

 resting state. But whenever the bodies of adjacent cells are 

 directly united by processes, and so form a physiological whole 

 (i.e. continuity of substance), any chemical differences arising 

 within the plasma of the cell-aggregate must produce current that 

 can be led off externally. 



Many investigations in recent years concur to show that the 

 bodies of vegetable cells are frequently, and perhaps always, con- 

 nected directly through their cellulose sheaths by means of fine 

 processes, as also occurs in many animal tissues. If this may be 

 assumed of the cells in the Dioncecu leaf (it has been directly 

 proved for the cells of the excitable cushion of Mimosa by 

 Gardiner and Haberlandt), and if the same continuity exists 

 between the excitable plasma of the upper and the non-excitable 

 plasma of the lower parenchyma cells, then all the electrical 

 manifestations so far described may be referred to differences of 

 potential between upper and lower cells, which are not merely 

 contiguous, but are in direct protoplasmic connection, and happen 

 to be in unlike and varying physiological states. 



From this point of view it would not be without interest to 

 test the electromotive reactions of vegetable organs in other cases 

 where chemical differences between different layers of cells might 



