152 COMPARATIVE ELECTRO-PHYSIOLOGY 



even after it has been set free. Recovery is thus, in such 

 a case, indefinitely delayed. In other words, a molecularly 

 over-strained substance exhibits a persistent after-effect. 



Turning next to plant response, we find a similar per- 

 sistence of the after-effect to occur in consequence of over- 

 stimulation. And first we shall take the simplest case 

 that in which the tissue is directly stimulated. Here the 

 specimen was petiole of cauliflower, and increasing stimuli 



' 5 7h 10 izi 



FIG. 106. Photographic Record exhibiting Persistent Galvanometric 

 Negativity in Plant Tissue after Strong Stimulation 



Stimuli applied at intervals of three minutes. Vertical line = 'I volt. 



were applied, at intervals of three minutes, by means of a 

 gradually increasing angle of torsional vibration. It will be 

 noticed that whereas the electrical recovery from moderate 

 stimulation as seen in the first of the series is complete, 

 it becomes, with increasing stimulus, more and more in- 

 complete (fig. 1 06). In other words, the tissue, after strong 

 stimulation, is seen to exhibit an after-effect of residual 

 galvanometric negativity, which is really due to incomplete 

 molecular recovery, in consequence of over-strain. 



In the cases just given, stimulation was applied directly. 



