Page:Insectivorous Plants, Darwin, 1899.djvu/49

 or of a human hair, $$\frac{8}{1000}$$ of an inch in length and weighing only $$\frac{1}{75740}$$ 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 tentacle 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 aggregation of the protoplasm. But the case is much more remarkable 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 exerted by the particle of hair, weighing only $$\frac{1}{78740}$$ 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, $$\frac{1}{50}$$ of an inch in length, and therefore much larger than those used in the above experiments, was not perceived when placed on my tongue ; and it is extremely doubtful whether any nerve in the human body, even if in an inflamed condition, would be in any way affected 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 by Repeated Touches.

We have already seen that, if the central glands are excited by being gently brushed, they transmit a motor impulse