CHAPTER XVI 



ON EXCITATORY POLAR EFFECTS OF CURRENTS 



Hydro-mechanical theory of excitation in plants — Theory of protoplasmic change 

 — Crucial tests applied by means of polar excitation — Mono-polar and Bi-polar 

 methods of excitation — Advantages of study of polar excitation in plant-tissues 

 as compared with animal — Effects of feeble E. M.F. — Effect of moderately high 

 E.M.F. — Experiments with highly excitable tissues. 



Having observed, by means of mechanical responses, the 

 various excitatory effects which are caused in plants by 

 stimulation, and the influence of different agencies in modify- 

 ing these excitatory effects, it is now desirable to make an 

 inquiry into the manner in which excitation takes place, 

 and into the method by which it is transmitted to a distance. 

 There has been a great deal of uncertainty regarding this 

 subject, and the prevailing view is that which holds the trans- 

 mission of excitation to be due to the propagation of hydro- 

 static disturbance. 



Mechanical theory. — According to this theory, it is 

 supposed that stimulus causes a mechanical disturbance, 

 bringing about an alteration of the hydrostatic equilibrium. 

 The propagation of excitation in plants is thus regarded as 

 nothing more than the transmission of this hydro-mechanical 

 disturbance. 



We know, however, that the transmission of hydrostatic 

 disturbance takes place with relatively great rapidity, while 

 these excitatory effects in the case of plants travel sometimes 

 as slowly as I mm. or less per second. I have shown, more- 

 over, that its responses, both mechanical and electrical, 

 are profoundly modified by the physiological condition of the 

 plant. There is, for example, an optimum temperature at 

 which response is at a maximum, any change, whether above 



