MECHANICAL RESPONSE TO STIMULUS 19 



leaflets, after each downward response, would sometimes 

 remain persistently closed, for an indefinite period, thus pre- 

 venting the continuation of the experiment. In cases where 

 the leaflets are completely closed, one naturally regards the 

 position as one of fatigue, or complete insensitiveness, 

 because no further mechanical response is then obtainable. 

 This depressed position, however, may not be indicative of 

 total want of sensibility, for the apparent absence of response 

 may really be due to the fact that further closure of the 

 leaflets is a mechanical impossibility. We may consider an 

 analogous instance in the case of animal tissues, muscle 

 floating in mercury for example. The tissue remains per- 

 sistently contracted after a single stimulus, and further 

 response is impossible. But if, again, the muscle be stimu- 

 lated while under tension, it responds to each stimulation, 

 the process of recovery being aided by the external tension. 



Practical importance of counterpoise. — Acting on 

 this idea, it appeared to me that if we applied an external 

 tension, the restoration of the leaflet to the natural outspread 

 position might be helped, and the difficulty solved of ob- 

 taining the uniform repetition of effects of successive stimuli 

 at brief and regular intervals of time. I therefore placed 

 a small sliding counterpoise on that arm of the lever which 

 was not attached to the leaflet. This was found to fulfil its 

 purpose. For in observing the effects of successive stimuli 

 on different leaflets, I found that while neighbouring leaflets, 

 not under tension, closed up after a few stimulations, and 

 gave no further response, the leaflet which was attached to 

 the lever, and which was under some slight tension, recovered 

 its normal outspread position in the course of three to five 

 minutes, and continued to respond in a normal manner to a 

 long series of successive stimuli. 



We shall now proceed to observe the actual responses 

 obtained. The object here is not to investigate the peculiar 

 or specific reaction of any one sensitive plant in particular, 

 but the effects found universally among motile plant-organs. 

 The occurrence of such effects in plants exhibiting all 



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