THE ELECTRO-MOTIVE RESPONSE OF PLANTS 



Stimulation, again, may be effected by the prick of a needle 

 or pin in the neighbourhood of A. Response to this also 

 occurs by the normal galvanometric negativity. Successive 

 pricks may thus give rise to successive responses. 



Or the specimen may be subjected to torsional vibration. 

 It is here held in the middle by a clamp, and stimulus of 

 torsional vibration is applied , , 



at one end. The stimulation 

 of A makes that end gal- 

 vanometrically negative, the 

 direction of the current 

 outside the circuit being 

 towards, and in the tissue 

 away from, A. Vibration of 

 B induces responsive nega- 

 tivity of B (fig. 19), the 



Current of response being (a) T he plant is clamped at c, between 



now reversed. In the cases A and B. 



(b} Responses obtained by alternately 



Current of response when 

 A is stimulated-* 



Citrrent of response -when 

 B is stimulated^* 



FIG. 19. The Torsional Vibrator 



stimulating the two ends. Stimula- 

 tion of A produces upward response ; 

 of B gives downward response. 



just described, it will be 

 noticed that stimulus is ap- 

 plied directly. This method 

 is, therefore, specially applicable when we wish to study the 

 excitability of such tissues as are not good conductors of 

 excitation, the method of transmitted stimulation being here, 

 therefore, inapplicable. 



In order to observe the effect of chemical stimulation, the 

 given agent sulphuric or hydrochloric acid is applied at 

 x at a short distance from the proximal contact. The trans- 

 mitted excitation is now again demonstrated by the induced 

 galvanometric negativity of that contact. It will thus be seen 

 that, whatever be the effective form of stimulus employed, 

 it gives rise to a definite and invariable electrical response 

 whose sign is always one of galvanometric negativity. 



It was shown, then, in the course of this chapter that 

 the excitatory change in ' sensitive ' plants is characterised 

 by contraction, negative turgidity variation, mechanical 

 depression of the leaf, and by the electrical" response of 



