IRRITO-COXTRACTILITY IN PLANTS. 191 



additional contraction. I may best illustrate by a concrete 

 example selected at random from many others. A leaflet 

 stimulated fell through 42^ in 45 seconds, a second stimulus 

 increased the angle to 69° within 38 seconds after rest, a third 

 stimulus increased it to 81° after 33 seconds, and a fourth to 

 84° after 27 seconds. 



Thus, by four successive stimuli applied during a period of 

 143 seconds, the leaf described an angular movement of 84°. 

 But, as I have already stated, the period of ma.ximal movement 

 after first excitation occurs between the seventh and twentieth 

 seconds. Taking advantage of this fact, by shortening the time 

 interval between the stimuli, the same amount of contraction 

 can be got in a much shorter time. A leaf was irritated and 

 allowed to contract for 22 seconds, when it had fallen through 

 38° ; it was again irritated, and allowed to contract for 20 

 seconds when it had fallen through 61°. Again irritated it 

 fell through an additional 15°, and after the fourth stimulus 

 had fallen through 81°. 



The above time-intervals remain wonderfully constant in all 

 active leaves when the environmental conditions remain 

 constant, but a distinct shortening of the latent period by | 

 to \ second was noted on a moist, close and warm morning in 

 early July after a thunder-storm of the previous evening. It 

 should also be stated that when the plants are continuously 

 exposed to such high shade temperatures as 35°- 40° C, they 

 become smaller in size and irregular in action. 



We now turn to thermal stimuli, and I may at once state 

 that ice particles produce a very marked effect on this and all 

 other sensitive plants. When a piece weighing \-\ grain is 

 placed at the junction of three leaflets a longer latent period 

 than for mechanical stimuli ensues, but when once started 

 contraction steadily proceeds till the ice has melted and the 

 resulting water has attained a temperature that fails to excite 

 the protoplasm. If by aid of a pipette or blotting paper the 

 water is sipped off and a fresh bit of ice is placed, contraction 

 will go on till the nyctitropic position has been reached. But 

 as in the case of mechanical stimuli more localized action can 

 be started, for if ice be placed not on the pulvini but near 



