ELECTRO-PHYSIOLOGY 



urges that it succeeds even when applied to a leaf of which the 

 tissues are fully saturated by long immersion in water, " so that 

 there can be no further absorption of water from the electrodes." 

 The chief point insisted on by Kunkel is the fact that 

 electromotive action appears only with rapid, and not with slow, 

 flexion of a green stalk. But without denying that electrical 

 effects might be caused in dead or living parts of plants by rapid 

 and adequate movements of water (due to purely mechanical 

 causes), it is equally certain that they are not under all condi- 

 tions solely due to the migration of water. This is sufficiently 

 plain from the experiments on excitable leaves, to which we shall 

 return later. Above all, an explanation is needed of the differ- 

 ences in potential (often permanent and very considerable) which 

 frequently make their appearance in certain vegetable organs, and 

 which, from the point of view we have been discussing, would 

 present insuperable difficulties to Kunkel; for it is hardly a 

 satisfactory or probable explanation that derives the strong 

 " current of rest," amounting to O'l Dan. (which Kunkel found 

 in the leaf of Mimosa, on leading off from the upper border 

 of the excitable pulvinus at the base of the common [primary] 

 leaf-stalk, and from one of the two strong thorns near the inser- 

 tion of the leaf), from " the diffusion currents obtained, even in 

 the resting state, on moistening certain portions of tissues that 

 are peculiarly adapted to rapid alteration in their charge of 

 water, taking it up and giving it out quickly in large quantity." 

 Kunkel, however, concerned himself little with the electromotive 

 reactions of excitable parts of plants (which are theoretically of 

 fundamental importance), and his researches in this direction are 

 superseded by the later investigations of Burdon- Sanderson. 



The intrinsically smaller differences of potential which Kunkel 

 observed in different species of green leaves must equally, ac- 

 cording to Haake, be referred to vital physiological processes. In 

 the first place, there is an obvious relation between the electro- 

 motive manifestations and respiration. When suitable leaves or 

 stalks were enclosed in a glass tube, with the electrodes inserted 

 into one end, while gases were led through the other (Fig. 137), there 

 was invariably a rapid diminution of the original P.D. between 

 mid-rib (close to its entrance into the stalk) and mesophyll (near 

 the centre of the leaf), if the oxygen was completely driven off 

 by moist hydrogen. With the readmission of air the current 



