The Movements and Habits of Climbing PlantsDarwin, Charles
Science
The Movements and Habits of Climbing Plants
Darwin, Charles
Climbing plants; Geotropism; Plants; Plants -- Irritability and movements
_Bignonia unguis_.—The young shoots revolve, but less regularly and less
quickly than those of the last species. The stem twines imperfectly
round a vertical stick, sometimes reversing its direction, in the same
manner as described in so many leaf-climbers; and this plant though
possessing tendrils, climbs to a certain extent like a leaf-climber.
Each leaf consists of a petiole bearing a pair of leaflets, and
terminates in a tendril, which is formed by the modification of three
leaflets, and closely resembles that above figured (fig. 5). But it is a
little larger, and in a young plant was about half an inch in length. It
is curiously like the leg and foot of a small bird, with the hind toe cut
off. The straight leg or tarsus is longer than the three toes, which are
of equal length, and diverging, lie in the same plane. The toes
terminate in sharp, hard claws, much curved downwards, like those on a
bird’s foot. The petiole of the leaf is sensitive to contact; even a
small loop of thread suspended for two days caused it to bend upwards;
but the sub-petioles of the two lateral leaflets are not sensitive. The
whole tendril, namely, the tarsus and the three toes, are likewise
sensitive to contact, especially on their under surfaces. When a shoot
grows in the midst of thin branches, the tendrils are soon brought by the
revolving movement of the internodes into contact with them; and then one
toe of the tendril or more, commonly all three, bend, and after several
hours seize fast hold of the twigs, like a bird when perched. If the
tarsus of the tendril comes into contact with a twig, it goes on slowly
bending, until the whole foot is carried quite round, and the toes pass
on each side of the tarsus and seize it. In like manner, if the petiole
comes into contact with a twig, it bends round, carrying the tendril,
which then seizes its own petiole or that of the opposite leaf. The
petioles move spontaneously, and thus, when a shoot attempts to twine
round an upright stick, those on both sides after a time come into
contact with it, and are excited to bend. Ultimately the two petioles
clasp the stick in opposite directions, and the foot-like tendrils,
seizing on each other or on their own petioles, fasten the stem to the
support with surprising security. The tendrils are thus brought into
action, if the stem twines round a thin vertical stick; and in this
respect the present species differs from the last. Both species use
their tendrils in the same manner when passing through a thicket. This
plant is one of the most efficient climbers which I have observed; and it
probably could ascend a polished stem incessantly tossed by heavy storms.
To show how important vigorous health is for the action of all the parts,
I may mention that when I first examined a plant which was growing
moderately well, though not vigorously, I concluded that the tendrils
acted only like the hooks on a bramble, and that it was the most feeble
Public-domain text, read in full here on John Shaqi.
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