ORIGIN AND DETECTION OF ELECTRO-MOTIVE CHANGES 393 



currents upon vital activity afford no safe guide as to the action of the 

 weak internal currents 1 . These may, however, exert a distinct stimulating 

 action, and if prolonged may represent a considerable total expenditure of 

 energy. 



Variations of the electrical current in an external circuit indicate auto- 

 genie or aitiogenic changes within the plant, but unfortunately the origin of 

 the change of potential or of the altered resistance producing the modified 

 current is usually unknown and is in all cases difficult to determine. Never- 

 theless the ease and exactness with which the external currents can be 

 measured render them of great value as indicators of internal changes. 

 Even in animal physiology, however, where much work has been done in 

 this direction, but little is known as to the function of the electrical currents 

 observed 2 . Hence we need only discuss the electrical currents and changes 

 of potential so far as is necessary to show their general character and their 

 relationship with other vital processes. The action of stimuli upon the 



FIG. 68. Testing-apparatus for electrical currents. The glass tube (a) can be separated into two halves at (c). 

 Gases can be led into and out of the two halves by the tube at (/>). The electrodes (e} are attached by air-tight 

 india-rubber caps (k) to the side tubes (6, 6). 



production of electricity agrees as regards the influence of summation, 

 intensity, and conjoint action with that of stimuli in general. Variations 

 of the strength of the current due to polarization effects or to changes 

 of resistance are naturally of less importance than those produced by an 

 increased difference of potential due to greater electrical activity. 



Methods. These have already been developed very fully in animal physiology 3 . 

 The brushes of the non-polarizable electrodes (Fig. 68) are moistened with a dilute 

 solution (0-05 per cent.) of sodium chloride, or with spring-water, care being taken to 

 wash away any traces of zinc sulphate that may diffuse through from the tube con- 

 taining the carbon electrode. Haake 4 used a tube (Fig. 68) which could be separated 



1 Cf. Enler, Meddelanden fran Stockholms Hb'gskolas Botaniska Institut, 1899, Bd. II ; 

 Lemstrdm, Electricity in Agriculture, 1904; Ewart, 1. c., pp. 88-93. 



2 Biedermann, I.e., p. 273. 



3 See Hermann, Physiolog. Practicum, 1898, p. 75; Biedermann, Elektrophysiologie, 1895; 

 Burdon-Sanderson, Kunkel, and Haake, 1. c. The addition of a drop of water, or the mere appli- 

 cation of the electrode may produce a transitory current. Hence careful control is required. 



* Haake, Flora, 1892, p. 461. 



