sensitive filaments were now quite insensible. After 24 hrs. this leaf
recovered its sensibility, but was still rather torpid. A leaf exposed
in a large bottle for only 3 m. to ten drops was rendered insensible.
After 52 m. it recovered its sensibility, and when one of the filaments
was touched, the lobes closed. It began [page 305] to reopen after 20
hrs. Lastly another leaf was exposed for 4 m. to only four drops of the
ether; it was rendered insensible, and did not close when its filaments
were repeatedly touched, but closed when the end of the open leaf was
cut off. This shows either that the internal parts had not been
rendered insensible, or that an incision is a more powerful stimulus
than repeated touches on the filaments. Whether the larger doses of
chloroform and ether, which caused the leaves to close slowly, acted on
the sensitive filaments or on the leaf itself, I do not know.
Cyanide of potassium, when left in a bottle, generates prussic or
hydrocyanic acid. A leaf was exposed for 1 hr. 35 m. to the vapour thus
formed; and the glands became within this time so colourless and
shrunken as to be scarcely visible, and I at first thought that they
had all dropped off. The leaf was not rendered insensible; for as soon
as one of the filaments was touched it closed. It had, however,
suffered, for it did not reopen until nearly two days had passed, and
was not even then in the least sensitive. After an additional day it
recovered its powers, and closed on being touched and subsequently
reopened. Another leaf behaved in nearly the same manner after a
shorter exposure to this vapour.]
On the Manner in which Insects are caught. We will now consider the
action of the leaves when insects happen to touch one of the sensitive
filaments. This often occurred in my greenhouse, but I do not know
whether insects are attracted in any special way by the leaves. They
are caught in large numbers by the plant in its native country. As soon
as a filament is touched, both lobes close with astonishing quickness;
and as they stand at less than a right angle to each other, they have a
good chance of catching any intruder. The angle between the blade and
footstalk does not change when the lobes close. The chief seat of
movement is near the midrib, but is not confined to this part; for, as
the lobes come together, each curves inwards across its whole breadth;
the marginal spikes however, not becoming curved. This move- [page 306]
ment of the whole lobe was well seen in a leaf to which a large fly had
been given, and from which a large portion had been cut off the end of
268