ORIGIN AND DETECTION OF ELECTRO-MOTIVE CHANGES 389 



currents are of common occurrence, although to detect them delicate 

 measurements are usually required. These are usually made by laying 

 two non-polarizable electrodes on the regions to be examined, and placing 

 them in circuit with a galvanometer whose deflection indicates the passage 

 of a current from the region of higher potential to that of lower potential. 

 A positive result gives, however, no indication as to how the potential 

 differences are produced. 



The continuance of the current in the external circuit shown by the 

 permanent deflection of the galvanometer indicates that the difference is 

 continually maintained, and that a return current flows in the plant in such 

 a direction as to form a complete internal and external circuit. A break in 

 the external circuit must exert some influence upon the internal circuit, but 

 the internal currents are hardly likely to cease as when a battery circuit is 

 broken, for a difference of potential between two points must always produce 

 a current of electricity if the resistance of the intervening medium is not too 

 high. The direction and intensity of these currents will largely depend upon 

 the shape, arrangement, and conductivity of the intervening tissues, which 

 may be such as to permit of the continual circulation of electrical currents in 

 plants. The only evidence at our disposal is, however, derived from observa- 

 tions made upon the currents led off and measured in circuits external to 

 the plant. 



A variety of factors may induce variations of electrical potential in 

 plants, and if the sum of the processes producing a rise of potential is equal 

 to that of those tending to diminish the potential no external modification 

 will be made manifest. In general the visible differences of potential appear 

 to result directly or indirectly from metabolism, and to a very much less 

 degree from the imbibition of water and its passage through capillary tubes. 



All chemical changes in which ions take part involve also electrical 

 changes. In a galvanic cell, for instance, the electrical charges imparted 

 to the plates maintain the difference of potential, which induces flow in 

 the external circuit. When oxidation or reduction takes place in tissues 

 separated by an intervening conducting space, it is usually possible to lead 

 off an external current, and the same is even possible when two reacting 

 substances are brought into contact by diffusion l . In plants, therefore, we 

 have all the conditions for the production of electrical currents. 



It is possible that the protoplasmic membranes may allow some ions to 

 pass but not others, and in this way, or even by retarding the speed of 

 certain ions, a difference of potential may be produced capable of giving rise 



1 Haake (Flora, 1892, p. 465) observed a pronounced deflection of a galvanometer connected 

 with the ends of a strip of filter-paper at the moment when copper sulphate and ferrocyanide of 

 potassium met by diffusion and interacted. Cf. also Dubois, Centralbl. f. Physiol., 1901, Bd. xiv, 

 p. 32. 



