I 88 BIOLOGICAL LECTURES. 



the leaf be not further irritated by some instrument, or by a 

 caught animal, it slowly relaxes after 12-15 hours; but if 

 additional stimuli are given, the leaf gradually and firmly 

 tightens up till its margins become recurved. Prolonged 

 stimuli, either of a mechanical, chemical, or electrical nature, 

 start eventually (after 8-10 hours usually) the flow of an acid 

 secretion from glands that cover both halves of the leaf. It is 

 clear, therefore, that complete contraction of Dionaa leaf can 

 only be effected by a summation of stimuli ; and such stimuli 

 may either be partially or entirely mechanical, chemical, or 

 electrical. I have further shown, contrary to prevailing 

 ophiions, that not merely the three hairs of each leaf-half, but 

 the entire surface is irrito-contractile. Thus, a minute bit of 

 ice or a drop of hot water placed on any part of the leaf away 

 from the hairs, also a forceps' pinch, a mechanical shock, and 

 many chemical agents, excite to closure. 



We have already said that the time-interval between two 

 shocks may be too short to start contraction, but it is equally 

 true that too great a time interval is attended with no visible 

 change ; in other words, the piling on of the second stimulus 

 to the first, if too long delaved, is insufficient to effect closure. 

 With an interval between of 50-60 seconds, at a temperature 

 of i4"-2i° C, contraction takes place after a second excitation; 

 and I stated that if the interval were increased to 90-120 

 seconds, the strength of the two was almost entirely cancelled. 

 Through tlic kindness, however, of my friend Mr. Aldrich 

 Pennock, I have studied ]ilants in his hot-houses at high tem- 

 peratures (35--40°C.), and find that the effects of previous 

 stimuli can be retained for 4 minutes at least. l^ut a 

 second stimulus applied 4 minutes after the first causes no 

 visible movement of closure ; a third, 4 minutes after the 

 second is similar; and not till the sixth stimulus does an 

 almost inappreciable contraction of the halves take place. 

 Summation of successive stimuli from the eighth to the 

 twelfth gives added force — sufficient to bring the halves 

 together. Here, then, is an irritable tissue that steadily 

 contracts, or prepares for contraction, through a period of 

 from 30-45 minutes. 



