30 PLANT RESPONSE . 



such cases exist. What these conditions are will be detailed 

 in the next chapter, where it will also be shown that excita- 

 tion of an organ may take place, even where there is little 

 mechanical indication of the fact, owing to antagonistic and 

 balanced contractions. 



Electrical response. — It is my intention, in the course of 

 the present work, to offer a complete demonstration of all the 

 phenomena of excitation in plants, by means of mechanical 

 response alone. But the conclusions to which we shall be led 

 by the study of this response will receive irrefragable support, 

 if they can also be established independently by some mode 

 of investigation altogether different. Such a mode of in- 

 quiry, namely the electrical, and the conclusions to which it 

 leads, will be fully described in the companion volume to this 

 work, on the Electro-Physiology of Plants. Meanwhile it is 

 convenient in this place to enter upon a short elucidation of 

 the principle of that method, in order that we may be able, 

 while considering the results of mechanical response, to make 

 casual references to confirmatory results of independent ob- 

 servations obtained by means of the electrical method. 



It has been said that under the action of stimulus excited 

 cells undergo contraction, and that owing to the consequent 

 expulsion of water, the turgidity of the tissue is diminished. 

 Thus one expression of the molecular change induced by 

 stimulus is a negative variation of turgidity. But this 

 molecular change may also be detected by means of other 

 concomitant physical changes. For instance, the electrical 

 level or potential of a given point may, owing to the ex- 

 citatory molecular change, undergo variation, relatively to 

 another point which is unexcited. A hydraulic model will 

 serve to make this point clear (fig. 22). 



Let us imagine a flexible pipe of india-rubber, with bent 

 ends of glass-tube, filled with water, and held in the middle 

 by a clamp c. It is also supported in the stable horizontal 

 position by spiral springs. If a single blow, say upwards, 

 be now given to the end A, the level of the pipe at that end 

 will be raised, and there will be a resultant flow of water from 



