Insectivorous Plants | Page 8

torpid, or to the bits of meat being too small to
cause much inflection. We must therefore conclude that the central
glands, when strongly excited, transmit some influence to the glands of
the circumferential tentacles, causing them to secrete more copiously.
It is a still more important fact (as we shall see more fully when we
treat of the digestive power of the secretion) that when the tentacles
become inflected, owing to the central glands having been stimulated
mechanically, or by contact with animal matter, the secretion not only
increases in quantity, but changes its nature and becomes acid; and this
occurs before the glands have touched the object on the centre of the
leaf. This acid is of a different nature from that contained in the tissue
of the leaves. As long as the [page 15] tentacles remain closely
inflected, the glands continue to secrete, and the secretion is acid; so
that, if neutralised by carbonate of soda, it again becomes acid after a
few hours. I have observed the same leaf with the tentacles closely
inflected over rather indigestible substances, such as chemically
prepared casein, pouring forth acid secretion for eight successive days,
and over bits of bone for ten successive days.
The secretion seems to possess, like the gastric juice of the higher
animals, some antiseptic power. During very warm weather I placed
close together two equal-sized bits of raw meat, one on a leaf of the
Drosera, and the other surrounded by wet moss. They were thus left for
48 hrs., and then examined. The bit on the moss swarmed with
infusoria, and was so much decayed that the transverse striae on the
muscular fibres could no longer be clearly distinguished; whilst the bit
on the leaf, which was bathed by the secretion, was free from infusoria,
and its striae were perfectly distinct in the central and undissolved
portion. In like manner small cubes of albumen and cheese placed on

wet moss became threaded with filaments of mould, and had their
surfaces slightly discoloured and disintegrated; whilst those on the
leaves of Drosera remained clean, the albumen being changed into
transparent fluid.
As soon as tentacles, which have remained closely inflected during
several days over an object, begin to re-expand, their glands secrete less
freely, or cease to secrete, and are left dry. In this state they are covered
with a film of whitish, semi-fibrous matter, which was held in solution
by the secretion. The drying of the glands during the act of
re-expansion is of some little service to the plant; for I have often
observed that objects adhering to the leaves [page 16] could then be
blown away by a breath of air; the leaves being thus left unencumbered
and free for future action. Nevertheless, it often happens that all the
glands do not become completely dry; and in this case delicate objects,
such as fragile insects, are sometimes torn by the re-expansion of the
tentacles into fragments, which remain scattered all over the leaf. After
the re-expansion is complete, the glands quickly begin to re-secrete,
and as soon as full-sized drops are formed, the tentacles are ready to
clasp a new object.
When an insect alights on the central disc, it is instantly entangled by
the viscid secretion, and the surrounding tentacles after a time begin to
bend, and ultimately clasp it on all sides. Insects are generally killed,
according to Dr. Nitschke, in about a quarter of an hour, owing to their
tracheae being closed by the secretion. If an insect adheres to only a
few of the glands of the exterior tentacles, these soon become inflected
and carry their prey to the tentacles next succeeding them inwards;
these then bend inwards, and so onwards; until the insect is ultimately
carried by a curious sort of rolling movement to the centre of the leaf.
Then, after an interval, the tentacles on all sides become inflected and
bathe their prey with their secretion, in the same manner as if the insect
had first alighted on the central disc. It is surprising how minute an
insect suffices to cause this action: for instance, I have seen one of the
smallest species of gnats (Culex), which had just settled with its
excessively delicate feet on the glands of the outermost tentacles, and
these were already beginning to curve inwards, though not a single

gland had as yet touched the body of the insect. Had I not interfered,
this minute gnat would [page 17] assuredly have been carried to the
centre of the leaf and been securely clasped on all sides. We shall
hereafter see what excessively small doses of certain organic fluids and
saline solutions cause strongly marked inflection.
Whether insects alight on the leaves by mere chance, as a resting place,
or are attracted by the