Chap. II.] THE EFFECTS OP REPEATED TOUCHES. 29 



or of a human hair, tAt of an inch in length and weighing 

 only rrfiff of a grain (.000822 milligram), or particles of 

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

 should induce some change in its cells, exciting them to 

 transmit a motor impulse throughout the whole length of the 

 pedicel, consisting of about twenty cells, to near its base, 

 causing this part to bend, and the tentficle to sweep through 

 an angle of above 180. That the contents of the cells of the 

 glands, and afterwards those of the pedicels, are affected in a 

 plainly visible manner by the pressure of minute particles, we 

 shall have abundant evidence when we treat of the aggre- 

 gation of the protoplasm. But the case is much more re- 

 markable than as yet stated ; for the particles are supported 

 by the viscid and dense secretion; nevertheless, even smaller 

 ones than those of which the measurements have been given, 

 when brought by an insensibly slow movement, through the 

 means above specified, into contact with the surface of a 

 gland, act on it, and the tentacle bends. The pressure ex- 

 erted by the particle of hair, weighing only Tshns of a grain 

 and supported by a dense fluid, must have been inconceivably 

 slight. We may conjecture that it could hardly have equalled 

 the millionth of a grain ; and we shall hereafter see that far 

 less than the millionth of a grain of phosphate of ammonia in 

 solution, when absorbed by a gland, acts on it and induces 

 movement. A bit of hair, A of an inch in length, and there- 

 fore much larger than those used in the above experiments, 

 was not perceived when placed on my tongue ; and it is -ex- 

 tremely doubtful whether any nerve in the human body, 

 even if in an inflamed condition, would be in any way af- 

 fected by such a particle supported in a dense fluid, and 

 slowly brought into contact with the nerve. Yet the cells of 

 the glands of Drosera are thus excited to transmit a motor 

 impulse to a distant point, inducing movement. It appears 

 to me that hardly any more remarkable fact than this has 

 been observed in the vegetable kingdom. 



The Inflection of Exterior Tentacles, when their Glands 

 are excited hy Repeated Touches. 



We have already seen that, if the central glands are ex- 

 cited by being gently brushed, they transmit a motor impulse 



