other substance containing soluble nitrogenous matter, the lobes,
instead of remaining concave, thus including a concavity, slowly press
closely together throughout their whole breadth. As this takes place,
the margins gradually become a little everted, so that the spikes,
which at first intercrossed, at last project in two parallel rows. The
lobes press against each other with such force that I have seen a cube
of albumen much flattened, with distinct impressions of the little
prominent glands; but this latter circumstance may have been partly
caused by the corroding action of the secretion. So firmly do they
become pressed together that, if any large insect or other object has
been caught, a corresponding projection on the outside of the leaf is
distinctly visible. When the two lobes are thus completely shut, they
* According to Dr. Curtis, in Boston Journal of Nat. Hist, vol. i
1837, p. 123. [page 308]
resist being opened, as by a thin wedge driven between them, with
astonishing force, and are generally ruptured rather than yield. If not
ruptured, they close again, as Dr. Canby informs me in a letter, with
quite a loud flap. But if the end of a leaf is held firmly between the
thumb and finger, or by a clip, so that the lobes cannot begin to
close, they exert, whilst in this position, very little force.
I thought at first that the gradual pressing together of the lobes was
caused exclusively by captured insects crawling over and repeatedly
irritating the sensitive filaments; and this view seemed the more
probable when I learnt from Dr. Burdon Sanderson that whenever the
filaments of a closed leaf are irritated, the normal electric current
is disturbed. Nevertheless, such irritation is by no means necessary,
for a dead insect, or a bit of meat, or of albumen, all act equally
well; proving that in these cases it is the absorption of animal matter
which excites the lobes slowly to press close together. We have seen
that the absorption of an extremely small quantity of such matter also
causes a fully expanded leaf to close slowly; and this movement is
clearly analogous to the slow pressing together of the concave lobes.
This latter action is of high functional importance to the plant, for
the glands on both sides are thus brought into contact with a captured
insect, and consequently secrete. The secretion with animal matter in
solution is then drawn by capillary attraction over the whole surface
270