234 COMPARATIVE ELECTRO-PHYSIOLOGY 



first positive and then negative. These latter again, after 

 previous excitation, become so modified as to show only 

 positive changes. And lastly, using the experimental 

 arrangement of symmetrical contacts, a diphasic variation 

 is obtained positive followed by negative on the under- 

 surface, instead of the upper, of the lobe excited. No theory 

 is advanced, however, by which a comprehensive explanation 

 might be afforded of these apparently anomalous results. 



But from the generalisations which I have already esta- 

 blished, regarding the electrical signs of the hydro-positive 

 and true excitatory effects respectively, and from the results 

 of certain experiments on ordinary leaves which I shall 

 presently describe, it will be found easy to arrive at a true 

 explanation of the various observations related by Burdon 

 Sanderson, which would otherwise have appeared inexplic- 

 able. The fact that hydrostatic disturbance induces galvano- 

 metric positivity, and that true excitation induces negativity, 

 has already been clearly demonstrated under conditions from 

 which all possible sources of complication had been elimi- 

 nated (p. 61). The experimental arrangement adopted by 

 Burdon Sanderson, however, laboured under the double dis- 

 advantage, not only of a liability to confuse the hydro- 

 positive and true excitatory effects, but also of the com- 

 plexity arising from the differential excitability of the 

 responding organ. It is only indeed by the closest analysis 

 that it is possible to discriminate, in his results, between such 

 as are due to true excitation and those arising from the hydro- 

 positive effect. 



The various electrical phenomena which are possible in 

 an anisotropic organ in consequence of the hydro-positive 

 and excitatory effects respectively, may be clearly exhibited, 

 as I have already shown, by means of the mechanical 

 response of the leaf of Mimosa. With regard to this, we 

 have seen (pp. 59, 60) that direct stimulation of the pulvinus 

 induces a negative mechanical response, or fall of the leaf, 

 by the greater contraction of the more excitable lower half 

 of the organ. The corresponding electrical variation would 



