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 surfaces 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.
Anyone who has rubbed precipitated chalk between his fingers will have
perceived 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 induced 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,
1/50 of an inch in length and weighing 1/8197 of a grain, or of a human
hair, 8/1000 of an inch in length and weighing only 1/78740 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 [page 33] 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 180o. 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 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 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
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