Movement ensues if a gland is momentarily touched three or four times;
but if touched only once or twice, [page 264] though with considerable
force and with a hard object, the tentacle does not bend. The plant is
thus saved from much useless movement, as during a high wind the glands
can hardly escape being occasionally brushed by the leaves of
surrounding plants. Though insensible to a single touch, they are
exquisitely sensitive, as just stated, to the slightest pressure if
prolonged for a few seconds; and this capacity is manifestly of service
to the plant in capturing small insects. Even gnats, if they rest on
the glands with their delicate feet, are quickly and securely embraced.
The glands are insensible to the weight and repeated blows of drops of
heavy rain, and the plants are thus likewise saved from much useless
movement.
The description of the movements of the tentacles was interrupted in
the third chapter for the sake of describing the process of
aggregation. This process always commences in the cells of the glands,
the contents of which first become cloudy; and this has been observed
within 10 s. after a gland has been excited. Granules just resolvable
under a very high power soon appear, sometimes within a minute, in the
cells beneath the glands; and these then aggregate into minute spheres.
The process afterwards travels down the tentacles, being arrested for a
short time at each transverse partition. The small spheres coalesce
into larger spheres, or into oval, club-headed, thread- or
necklace-like, or otherwise shaped masses of protoplasm, which,
suspended in almost colourless fluid, exhibit incessant spontaneous
changes of form. These frequently coalesce and again separate. If a
gland has been powerfully excited, all the cells down to the base of
the tentacle are affected. In cells, especially if filled with dark red
fluid, the first step in the [page 265] process often is the formation
of a dark red, bag-like mass of protoplasm, which afterwards divides
and undergoes the usual repeated changes of form. Before any
aggregation has been excited, a sheet of colourless protoplasm,
including granules (the primordial utricle of Mohl), flows round the
walls of the cells; and this becomes more distinct after the contents
have been partially aggregated into spheres or bag-like masses. But
after a time the granules are drawn towards the central masses and
unite with them; and then the circulating sheet can no longer be
distinguished, but there is still a current of transparent fluid within
the cells.
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