50 RESEARCHES ON IRRITABILITY OF PLANTS 



we calibrate other positions of the scale in terms of this 

 unit. In this manner the scale is marked so as to indicate 

 intensities of "i, -5, 1, 2, 3, 4, 5, and so on. The calibration 

 is carried out by means of a ballistic galvanometer. In 

 subsequent chapters we shall employ these practical units, 

 which will thus have a definite significance. 



Having shared the prevailing belief that the sensitive- 

 ness of the plant was very feeble compared with that of 

 the animal, I was considerably surprised to find that the 

 intensity of induction-shock which is barely sufficient to 

 induce sensation in man is quite enough to cause excitatory 

 fall in a Mimosa of moderate sensitiveness. Indeed, I found 

 that in the case of a highly excitable specimen an intensity 

 only one-tenth of this was sufficient to excite it. In other 

 words, under this particular test Mimosa may prove ten 

 times as sensitive as a human subject ! Later on I shall 

 give details of measurements which will show that, as far 

 as electric mode of stimulation is concerned, the plant is 

 in no way inferior to the animal in sensitiveness. 



Summary 



The extent of responsive fall in Mimosa increases with 

 increasing intensity of stimulus. The rate of movement 

 is also greater under stronger stimulus. 



The rate of responsive movement becomes slower under 

 fatigue. In a given case the normal maximum rate of 

 movement of 50 mm. per second was reduced to 8 mm. 

 under fatigue. 



Temperature enhances the rate of movement. A rate 

 of 10 mm. per second at a temperature of 22 C. was found 

 enhanced to 105 mm. per second when the temperature was 

 raised to 28 C. 



In a typical case of Mimosa, in summer, the latent 

 period was found to be one-tenth of a second. The maxi- 

 mum contraction was attained in 3 seconds and the recovery 

 completed in 15 minutes. The rate of recovery was relatively 



