20 



PLANT RESPONSE 



first, the normal contractile effect, and secondly, the relaxation 

 due to fatigue, in a manner exactly similar to that which 

 characterises the fatigue-reversals of Mimosa and of skeletal 

 muscles. In the present case, the india-rubber attained its 

 maximum contraction in the course of two minutes, after 

 which there was a continuous relaxation. 



In this response of india-rubber, and in its fatigue-reversal, 

 we have an analogy with the response of living animal tissues, 



such as muscle, so close as to 

 compel us to the conclusion 

 that both alike are phenomena 

 of molecular response. A 

 mere contraction of the india- 

 rubber might have been sup- 

 posed to be due to the specific 

 action of heat on that sub- 

 stance. And had this been 

 all, successive thermal shocks 

 would have had the effect of 

 continuously increasing the 

 contraction, till a limit was 

 reached. But if, on the other 

 hand, molecular excitability 

 be a factor in the process 

 of response, we might then 

 expect, under certain condi- 

 tions of fatigue, a loss of 

 molecular excitability. That 

 this is actually the case is shown by the fatigue-reversal, 

 attended with relaxation, which is seen in the figure. 



A muscle, again, in this condition of fatigue-relaxation, 

 after a period of rest — during which the molecules have 

 time for recovery from their state of strain — becomes 

 once more excitable without any visible change. We found 

 a like phenomenon occurring in the case of fatigued and 

 relaxed Mimosa, under continuous stimulation. In that case 

 it was, as we saw, a period of rest of about eight or ten 



Fig. 62. Automatic Record of Fatigue 

 in the Contractile Response of India- 

 rubber under Rapidly Succeeding 

 Thermal Shocks 



The first effect of stimulus is contrac- 

 tion, but this passes into relaxation 

 under continued stimulation. The 

 time-marks represent intervals of 

 half a minute. 



