of secretion. They are extremely sensitive to various stimulants,
namely repeated touches, the pressure of minute particles, the
absorption of animal matter and of various fluids, heat, and galvanic
action. A tentacle with a bit of raw meat on the gland has been seen to
begin bending in 10 s., to be strongly incurved in 5 m., and to reach
the centre of the leaf in half an hour. The blade of the leaf often
becomes so much inflected that it forms a cup, enclosing any object
placed on it.
A gland, when excited, not only sends some influence down its own
tentacle, causing it to bend, but likewise to the surrounding
tentacles, which become incurved; so that the bending place can be
acted on by an impulse received from opposite directions, [page 263]
namely from the gland on the summit of the same tentacle, and from one
or more glands of the neighbouring tentacles. Tentacles, when
inflected, re-expand after a time, and during this process the glands
secrete less copiously, or become dry. As soon as they begin to secrete
again, the tentacles are ready to re-act; and this may be repeated at
least three, probably many more times.
It was shown in the second chapter that animal substances placed on the
discs cause much more prompt and energetic inflection than do inorganic
bodies of the same size, or mere mechanical irritation; but there is a
still more marked difference in the greater length of time during which
the tentacles remain inflected over bodies yielding soluble and
nutritious matter, than over those which do not yield such matter.
Extremely minute particles of glass, cinders, hair, thread,
precipitated chalk, &c., when placed on the glands of the outer
tentacles, cause them to bend. A particle, unless it sinks through the
secretion and actually touches the surface of the gland with some one
point, does not produce any effect. A little bit of thin human hair
8/1000 of an inch (.203 mm.) in length, and weighing only 1/78740 of a
grain (.000822 mg.), though largely supported by the dense secretion,
suffices to induce movement. It is not probable that the pressure in
this case could have amounted to that from the millionth of a grain.
Even smaller particles cause a slight movement, as could be seen
through a lens. Larger particles than those of which the measurements
have been given cause no sensation when placed on the tongue, one of
the most sensitive parts of the human body.
235