32 DKOSEKA EOTUNDIFOLIA. Chap. H. 



into contact witli the glands, and caused this rapid 

 movement. Accordingly I added to some distilled 

 ^vater a pinch of a quite innocent substance, namely, 

 precipitated carbonate of lime, which consists of an 

 impalpable powder ; I shook the mixture, and thus got 

 a fluid like thin milk. Two leaves were immersed in 

 it, and in 6 m. almost every tentacle was much 

 inflected. I placed one of these leaves under the 

 microscope, and saw innumerable atoms of lime ad- 

 hering to the external surface of the secretion. Some, 

 however, had penetrated it, and were lying on the sur- 

 faces of the glands ; and no doubt it was these particles 

 which caused the tentacles to bend. When a leaf is im- 

 mersed in water, the secretion instantly swells much ; 

 and I presume that it is ruptured here and there, so 

 that little eddies of water rush in. If so, we can under- 

 stand how the atoms of chalk, which rested on the 

 surfaces of the glands, had j)enetrated the secretion. 

 Anyone who has rubbed precipitated chalk between 

 his fingers will have perceived how excessively fine 

 tlic powder is. No doubt there must be a limit, beyond 

 wliich a particle would be too small to act on a gland ; 

 ]»ut what this limit is, I know not. I have often seen 

 iilnx'S and dust, which had fallen from the air, on the 

 ghmds of plants kept in my room, and these never 

 induced any movement ; but then such particles lay 

 on the surface of the secretion and never reached the 

 ghind itself. 



Finally, it is an extraordinary fact that a little 

 bit of soft thread, -^V of an inch in length and weigh- 

 ing -^\j-^ of a grain, or of a human hair, ■y-J'-j^-^ of an 

 inch in length and weighing only --^i— ^ of a grain 

 (•000822 milligramme), or particles of precipitated 

 chalk, after resting for a short time on a gland, 

 should induce some change in its cells, exciting them 



