UNIVERSALITY OF SENSITIVENESS IN PLANTS 39 



found fully described elsewhere, but the results may be 

 summarised as follows : On simultaneous excitation of two 

 points, the current of response flows in the tissue from the 

 more excitable point a to the less excitable point B ; con- 

 versely, if the direction of the responsive current is from A to 

 B, the point A may be taken as the more excitable. By 

 means of the unfailing discrimination of the differences of 

 excitability in a tissue which this method renders possible, it 

 will be shown in the course of the present work that many ot 

 the anomalies of growth-curvature receive a most complete 

 and satisfactory explanation. 



Fig. 28. Response of Selenium to the Stimulus of Light 

 (Conductivity variation method.) 



The electrical response given by plant-tissues in general, 

 as described in this chapter, is obtained by means of the 

 difference of electrical potential or electromotive variation, 

 induced as between the excited and unexcited portions of the 

 tissue. 



There is another method of detection by means of 

 those changes of electrical conductivity which are concomi- 

 tant to excitation in the substance under experiment. 1 It 

 should be borne in mind that the various responses, obtained 

 by the mechanical, by the electromotive, or by the conduc- 

 tivity-variation method, are merely different expressions of 



1 Bose, Molecular Changes produced in Matter by Electric Waves, Roy. 

 Soc. Proc. 1 00. 



