FIG. 6. (Drosera rotundifolia.) Diagram showing one of the exterior
tentacles closely inflected; the two adjoining ones in their ordinary
position.)
tentacles is in any way excited; for the surrounding ones remain
unaffected. In the accompanying outline (fig. 6) we see one tentacle,
on which a particle of meat had been placed, thus bent towards the
centre of the leaf, with two others retaining their original position.
A gland may be excited by being simply touched three or four times, or
by prolonged contact with organic or inorganic objects, and various
fluids. I have distinctly seen, through a lens, a tentacle beginning to
bend in ten seconds, after an object had been [page 12] placed on its
gland; and I have often seen strongly pronounced inflection in under
one minute. It is surprising how minute a particle of any substance,
such as a bit of thread or hair or splinter of glass, if placed in
actual contact with the surface of a gland, suffices to cause the
tentacle to bend. If the object, which has been carried by this
movement to the centre, be not very small, or if it contains soluble
nitrogenous matter, it acts on the central glands; and these transmit a
motor impulse to the exterior tentacles, causing them to bend inwards.
Not only the tentacles, but the blade of the leaf often, but by no
means always, becomes much incurved, when any strongly exciting
substance or fluid is placed on the disc. Drops of milk and of a
solution of nitrate of ammonia or soda are particularly apt to produce
this effect. The blade is thus converted into a little cup. The manner
in which it bends varies greatly. Sometimes the apex alone, sometimes
one side, and sometimes both sides, become incurved. For instance, I
placed bits of hard-boiled egg on three leaves; one had the apex bent
towards the base; the second had both distal margins much incurved, so
that it became almost triangular in outline, and this perhaps is the
commonest case; whilst the third blade was not at all affected, though
the tentacles were as closely inflected as in the two previous cases.
The whole blade also generally rises or bends upwards, and thus forms a
smaller angle with the footstalk than it did before. This appears at
first sight a distinct kind of movement, but it results from the
incurvation of that part of the margin which is attached to the
footstalk, causing the blade, as a whole, to curve or move upwards.
The length of time during which the tentacles as [page 13] well as the
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