CHAPTER X 



EFFECT OF MECHANICAL STIMULATION ON GROWTH 



Amongst the mechanical stimuh which induce excitatory 

 contraction in Mimosa may be mentioned the irritation 

 caused by rough contact, by a prick or by a wound. Friction 

 causes moderate stimulation, from which the excited 

 pulvinus recovers within a short time. But a prick or a 

 cut induces a far more intense and persistent excitation ; 

 the recovery becomes protracted, and the overstimulated 

 pulvinus remains contracted for a long period. 



I now describe the effect of mechanical irritation on 

 growing organs, which will be found to be essentially 

 similar to that on the pulvinus. For moderate stimulation 

 I employ rough contact or friction ; for more intense 

 stimulation, a prick or a cut. 



Effect of Mechanical Stimulation 



Experiment 50. — ^The experiment was carried out with 

 the peduncle of Zephyranthes, which had a normal rate of 

 growth of o • 18 [JL per second. It was subjected to mechani- 

 cal stimulation, the surface being rubbed with a piece of 

 cardboard. This caused a retardation of growth, the 

 depressed rate being o-ii (jl per second, or three-fifths the 

 normal rate. The normal rate of growth was restored 

 after this moderate stimulation within a comparatively 

 short period of rest. After 15 minutes the rate became 

 0-14 (JL per second, and complete recovery was attained 

 after an hour when the rate became o-i8 y. per second, as 



