28 DROSERA ROTUNDIFOLIA. [Chap. IL 



the glands, or in some cases the tentacles may not have been 

 in an active condition. In order to excite movement, it is 

 indispensable that the particles should actually rest on the 

 glands; for a touch once, twice, or even thrice repeated by 

 any hard body, is not suflScient to excite movement. 



Another experiment, showing that extremely minute par- 

 ticles act on the glands when immersed in water, may here 

 be given. A grain of sulphate of quinine was added to an 

 ounce of water, which was not afterwards filtered; and, on 

 placing three leaves in ninety minims of this fluid, I was 

 much surprised to find that all three leaves were greatly in- 

 flected in 15 m.: for I knew from previous trials that the 

 solution does not act so quickly as this. It immediately oc- 

 curred to me that the particles of the undissolved salt, which 

 were so light as to float about, might have come into contact 

 with the glands, and caused this rapid movement. Accord- 

 ingly I added to some distilled water a pinch of a quite inno- 

 cent 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 adhering to the external surface 

 of the secretion. Some, however, had penetrated it, and 

 were lying on the surface of the glands; and no doubt it was 

 these particles which caused the tentacles to bend. When a 

 leaf is immersed 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 understand 

 how the atoms of chalk, which rested on the surfaces of the 

 glands, had penetrated the secretion. Any one who has 

 rubbed precipitated chalk between his fingers will have per- 

 ceived how excessively fine the powder is. No doubt there 

 must be a limit, beyond which a particle would be too small 

 to act on a gland ; but what this limit is I know not. I have 

 often seen fibres and dust, which had fallen from the air, on 

 the glands of plants kept in my room, and these never in- 

 duced any movement; but then such particles lay on the 

 surface of the secretion and never reached the gland itself. 



Finally, it is an extraordinary fact that a little bit of soft 

 thread, iV of an inch in length and weighing nVr of a grain. 



