The fact of a single touch or even of two or three touches not causing
inflection must be of some service to the plant; as during stormy
weather, the glands cannot fail to be occasionally touched by the tall
blades of grass, or by other plants growing near; and it would be a
great evil if the tentacles were thus brought into action, for the act
of re-expansion takes a considerable time, and until the tentacles are
re-expanded they cannot catch prey. On the other hand, extreme
sensitiveness to slight pressure is of the highest service to the
plant; for, as we have seen, if the delicate feet of a minute
struggling insect press ever so lightly on the surfaces of two or three
glands, the tentacles bearing these glands soon curl inwards and carry
the insect with them to the centre, causing, after a time, all the
circumferential tentacles to embrace it. Nevertheless, the movements of
the plant are not perfectly adapted to its requirements; for if a bit
of dry moss, peat, or other rubbish, is blown on to the disc, as often
happens, the tentacles clasp it in a useless manner. They soon,
however, discover their mistake and release such innutritious objects.
It is also a remarkable fact, that drops of water falling from a
height, whether under the form of natural or artificial rain, do not
cause the tentacles to move; yet the drops must strike the glands with
considerable force, more especially after the secretion has been all
washed away by heavy rain; and this often occurs, [page 36] though the
secretion is so viscid that it can be removed with difficulty merely by
waving the leaves in water. If the falling drops of water are small,
they adhere to the secretion, the weight of which must be increased in
a much greater degree, as before remarked, than by the addition of
minute particles of solid matter; yet the drops never cause the
tentacles to become inflected. It would obviously have been a great
evil to the plant (as in the case of occasional touches) if the
tentacles were excited to bend by every shower of rain; but this evil
has been avoided by the glands either having become through habit
insensible to the blows and prolonged pressure of drops of water, or to
their having been originally rendered sensitive solely to the contact
of solid bodies. We shall hereafter see that the filaments on the
leaves of Dionaea are likewise insensible to the impact of fluids,
though exquisitely sensitive to momentary touches from any solid body.
When the pedicel of a tentacle is cut off by a sharp pair of scissors
32