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88 On Electricity in Plants^ afid on Vegeto-terrestrial Currents, 



sometimes 90 degrees^ and the needle may even whirl round. It 

 is hence evident that the bark forms a voltaic couple^ the outer 

 surface or parenchyma being the positive side and the inner cov^- 

 ered with the cambium the negative. The action soon ceases 

 when the bark is removed from the branch. This is shown by 

 taking a piece of the bark of a young branch of alder, removing 

 the epidermis and putting it between two plates of platinum and 

 the whole under a pressure of 2 kilograms between two pieces of 

 wood; a current is immediately produced; but on repeating the 

 trials at short intervals, being careful to depolarize the platinum 

 plates each time, the effects diminish rapidly, although the sur- 

 faces may be moistened with distilled water, — and sometimes they 

 are not sensible after half an hour. These electric properties 

 diminish much more rapidly than in the muscles of cold-blooded 

 animals in which they remain several hours after death. 



When the bark is not separated from the branch, these proper- 

 ties continue for several days, proving that their disappearance 

 should be attributed to chemical changes which take place in the 

 sap through contact with the air. 



The bark is in reality, therefore, a voltaic couple, which loses 

 rapidly its electric property when detached from the wood and 

 left exposed to the air. It follows, that in the reaction upon the 

 sap of the layer of air that adheres to the surface of the platinum, 

 there ought to be somewhat analogous effects, as regards intensity, 

 to those which take place by contact widi the atmosphere. The 

 layer of hygrometric moisture adhering to the surface of the 

 platinum has no influence, since the effects are the same if the 

 needles have been previously heated to redness. 



In recapitulation, — we see, that from the pith to the cambium, 

 the ligneous layers are less and less positive with reference to the 

 pith, while from the cambium to the epidermis, the cortical beds 

 and parenchyma are more and more positive, or at least comport 

 themselves thus. In the production of the derived currents, this 

 inversion in the electric effects corresponds with the position of 

 the cellular tissue in the bark and in the wood ; in the bark it is 

 external, in the wood internal, and in each case it is positive. 



Where then, do the electric properties of the bark reside? Is 

 it in the different layers which compose the cortical system, or 

 only in the epidermis and cambium? The following experi- 

 ments, if not completely settling this question^ tend to its elu- 

 cidation. 



If we take a branch of alder in full sap, and make a transverse 

 section, then insert one of the needles between the bark and 

 the wood and the other into the parenchyma after removing the 

 epidermis, the needle deviates a certain number of degrees, say 

 20^, If we now withdraw the second needle which was placed 

 in the periphery, remove the green layer with an ivory knife 



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