MECHANICAL RESPONSE TO STIMULUS I I 



The shock of stimulus causes molecular derangement in 

 the tissue of the plant, and it is this fundamental mole- 

 cular change that finds expression in mechanical movement. 

 It finds independent expression also in electrical move- 

 ment. For the conspicuous display of mechanical response 

 certain peculiar structural arrangements are, as has been 

 said, advantageous ; but for the exhibition of electrical 

 response, the molecular change itself, which is concomitant 

 to excitation, is the only condition. This subject of the 

 electrical response of plants, however, I treat in detail else- 

 where.' For the present we are concerned only with the 

 question of mechanical response to stimulus. We have not 

 only to determine the existence of such response, but also to 

 ascertain under what conditions it occurs, and by what means 

 it is brought about. 



The whole sequence of molecular events initiated by 

 stimulus and expressed as mechanical response, may be very 

 simply illustrated by means of an india-rubber model. We 

 take a piece of stretched india-rubber, attached to a recording 

 lever. The rubber is enclosed in a tube in which there is also 

 enclosed a spiral of thin German-silver wire, by which the india- 

 rubber may be subjected to the momentary action of heat. 

 The quantity of heat generated is regulated by the strength 

 and duration of an electrical current flowing through the 

 heating wire. This application may be uniform for successive 

 experiments, or increased at will. 



Longitudinal response. — The thermal stimulus causes a 

 molecular rearrangement in the substance of the india-rubber, 

 in consequence of which the piece becomes shorter and 

 broader. This sudden longitudinal shortening is recorded by 

 the lever as the first half of the responsive movement. As 

 the substance gradually recovers from the effect of the 

 momentary stimulation, the molecules return to their normal 

 position, with a concomitant restoration of the india-rubber to 

 its original form. During this second half of the process, we 



1 Bose, Response in the Living and Non-Living (Messrs. Longmans, Green 

 & Co.). Bose, Electro-Physiology of Plants. 



