Insectivorous Plants | Page 7

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
blade remain inflected over an object placed on the disc, depends on
various circumstances; namely on the vigour and age of the leaf, and,
according to Dr. Nitschke, on the temperature, for during cold weather
when the leaves are inactive, they re-expand at an earlier period than
when the weather is warm. But the nature of the object is by far the
most important circumstance; I have repeatedly found that the tentacles
remain clasped for a much longer average time over objects which
yield soluble nitrogenous matter than over those, whether organic or
inorganic, which yield no such matter. After a period varying from one
to seven days, the tentacles and blade re-expand, and are then ready to
act again. I have seen the same leaf inflected three successive times
over insects placed on the disc; and it would probably have acted a
greater number of times.
The secretion from the glands is extremely viscid, so that it can be
drawn out into long threads. It appears colourless, but stains little balls
of paper pale pink. An object of any kind placed on a gland always
causes it, as I believe, to secrete more freely; but the mere presence of
the object renders this difficult to ascertain. In some cases, however,
the effect was strongly marked, as when particles of sugar were added;
but the result in this case is probably due merely to exosmose. Particles
of carbonate and phosphate of ammonia and of some other salts, for
instance sulphate of zinc, likewise increase the secretion. Immersion in
a solution of one part of chloride of gold, or of some other salts, to 437
of water, excites the glands to largely increased secretion; on the other
hand, tartrate of antimony produces no such effect. Immersion in many
acids (of the strength of one part to 437 of water) likewise causes a
wonderful amount of [page 14] secretion, so that when the leaves are
lifted out, long ropes of extremely viscid fluid hang from them. Some
acids, on the other hand, do not act in this manner. Increased secretion
is not necessarily dependent on the inflection of the tentacle, for
particles of sugar and of sulphate of zinc cause no movement.
It is a much more remarkable fact that when an object, such as a bit of
meat or an insect, is placed on the disc of a leaf, as soon as the
surrounding tentacles become considerably inflected, their glands pour

forth an increased amount of secretion. I ascertained this by selecting
leaves with equal-sized drops on the two sides, and by placing bits of
meat on one side of the disc; and as soon as the tentacles on this side
became much inflected, but before the glands touched the meat, the
drops of secretion became larger. This was repeatedly observed, but a
record was kept of only thirteen cases, in nine of which increased
secretion was plainly observed; the four failures being due either to the
leaves being rather