A. B. Plowman — Electromotive Force in Plants. 95 



The first systematic study of the electrical properties of 

 plants was made by Prof. J. Burdon Sanderson, in a series of 

 experiments on the electromotive properties of the leaf of 

 Dionsea.* His work was characterized by more than usual 

 care, and while some of his conclusions do not seern to be fully 

 justified by the results, this is in many respects the most satis- 

 factory piece of work in biological physics up to the close of 

 the last century. In order to avoid as far as possible any 

 mechanical injury to the plant, Sanderson used contacts of a 

 paste made of pure clay worked up with a dilute solution of 

 common salt. This mixture was contained in glass tubes 

 leading off to a contact with a saturated solution of zinc sul- 

 phate, which in turn was in contact with rods of pure zinc, 

 thus forming so-called "non-polarizable electrodes." With 

 such an arrangement there must be a certain amount of diffu- 

 sion between the dilute NaCI solution in the clay and the, 

 saturated ZnS0 4 solution, and in all probability there is more 

 or less diffusion at the leaf-contact itself. So that while this 

 form of contact is probably the best yet devised, it certainly 

 leaves very much to be desired in the way of freedom from 

 possible inherent electromotive force. Where the differences 

 of potential to be measured are of the order of 1X10" 4 volt, 

 or even much less, it is highly essential that all electromotive 

 force be eliminated from the contact terminals. Sanderson's 

 use of terms in describing electrical conditions is. often some- 

 what vague and confusing. Thus he speaks of relative " posi- 

 tivity" and "negativity" of parts, but fails to state definitely 

 in what sense these terms are used. For example, when he 

 says that "the upper surface is positive to the lower, but that 

 under certain conditions this may change to relative negativ- 

 ity," we have no means of knowing whether he is speaking 

 of the internal circuit or the external circuit. Just as in a 

 Daniell cell, the copper element is positive with reference to the 

 external circuit, and the zinc is positive with reference to the 

 internal circuit, so here such a statement as the one quoted 

 above does not give any information regarding the direction of 

 the current. 



The more important conclusions arrived at by Sanderson 

 were, in effect, as follows : 



I. In a healthy leaf of Dionsea, in the unexcited state, there 

 is a difference of electrical potential between the upper and 

 under parts of the lobe ; the midrib and the lobes ; and also 

 between different parts of the midrib. 



II. When the leaf is excited this difference of potential 

 between the two surfaces undergoes a sudden reversal of sign, 

 sometimes preceded by a momentary increase of difference in 



* Proc. Royal Soc, London, 1881 and 1882. Nature, x, 1874. 



