346 RESEARCHES ON IRRITABILITY OF PLANTS 



As in the response of animal muscle, so also in Mimosa 

 there is a range between minimal and maximal stimulations 

 where increasing amplitude of response occurs under 

 increasing intensity of stimulus. The range within which 

 the increasing effect is observed to take place is relatively 

 extended in the case of plants in a somewhat sub-tonic 

 condition. The range of variation is, however, restricted 

 in specimens which are highly excitable (p. 6i). 



Work Performed by the Plant 



During the responsive movement the pulvinus of 

 Mimosa can do the work of lifting a weight. The effect of 

 load on the response of Mimosa is similar to that on the 

 contractile response of muscle. In both, under increasing 

 load, the height of response undergoes a progressive 

 diminution with shortening of the period of recovery. 

 Within limits, the amount of work performed by the muscle 

 increases with the load. The same is true of the work 

 performed by the pulvinus of Mimosa. Thus it is found 

 that, while under a load of loo mg. the work performed was 

 1340 mm. mg., it became enhanced to 8666 mm. mg. under 

 a load of 2000 mg. 



The rate of work was found to be 7480 mm. mg. per 

 second (p. 57). 



Various Types of Response 



The response of the pulvinus of Mimosa exhibits charac- 

 teristics which are similar to those of the response of muscle. 

 Under normal conditions, and with sufficient intervening 

 periods of rest, the successive responses are found to be 

 of uniform height. Under conditions of incomplete re- 

 covery, however, the responses exhibit signs of fatigue. 

 The excitability of the plant in a sub-tonic condition is 

 enhanced by the action of stimulus itself. Under such 

 conditions the response exhibits a staircase increase. 



