20 Macfarlane. — Contributions to the History of 



round each margin about one-sixteenth of an inch from its 

 edge. By repeated mechanical irritation of the hairs after 

 the leaf had closed, the same reflexing movement of the 

 margins has occurred. Instructive results were got from a 

 lot of plants that the writer expressed from Wilmington. 

 Most of them during transit suffered severe concussions 

 though the leaf substance was not ruptured. They were at 

 once planted out, but seven-eighths of the leaves had shut 

 tightly, and their margins were reflexed, though nothing Was 

 enclosed. A very copious viscid acid secretion, moreover, 

 had been poured out. Other plants, whose roots had been 

 washed clean of earth, had been placed loosely in a large 

 preserve bottle, and at least half of the leaves were in the 

 same condition as the last, due also to knocking against each 

 other in transit. The above, coupled with similar behavior 

 under chemical and electrical stimulus, points to the belief 

 that complete tightening of the leaf is of a tetanic nature, 

 and can only be accomplished by repeated stimulus, either of 

 a mechanical, chemical, electrical or other kind. 



Leaves that were isolated from plants in an atmosphere at 

 76 F.were found to retain their normal irritability from twenty 

 to thirty minutes. Thereafter they became flaccid, and about 

 one hour after removal did not respond to stimulus. Whole 

 plants, from whose roots the soil had been washed away, 

 showed irritability in their leaves for one and a half to two 

 hours, according to the size, vigor, and root development of 

 each. . Entire plants, that had been washed clean of soil and 

 kept in a moist, tightly- sealed glass vessel, were highly irrit- 

 able on removal after six clays. 



Leaf Closure. 

 (b) By Heat Stimuli. 



My experiments on the effect of dry heat have not been so 

 extensive as to permit expression of a definite opinion, but 

 the action of water, at different temperatures, has been tried 

 on twenty-seven leaves. 



Darwin says : " A leaf was cut off and suddenly plunged 

 perpendicularly into boiling water. I expected that the lobes 



