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 venusta_.—The tendrils differ considerably from those of the
previous species. The lower part, or tarsus, is four times as long as
the three toes; these are of equal length and diverge equally, but do not
lie in the same plane; their tips are bluntly hooked, and the whole
tendril makes an excellent grapnel. The tarsus is sensitive on all
sides; but the three toes are sensitive only on their outer surfaces.
The sensitiveness is not much developed; for a slight rubbing with a twig
did not cause the tarsus or the toes to become curved until an hour had
elapsed, and then only in a slight degree. Subsequently they
straightened themselves. Both the tarsus and toes can seize well hold of
sticks. If the stem is secured, the tendrils are seen spontaneously to
sweep large ellipses; the two opposite tendrils moving independently of
one another. I have no doubt, from the analogy of the two following
allied species, that the petioles also move spontaneously; but they are
not irritable like those of _B. unguis_ and _B. Tweedyana_. The young
internodes sweep large circles, one being completed in 2 hrs. 15 m., and
a second in 2 hrs. 55 m. By these combined movements of the internodes,
petioles, and grapnel-like tendrils, the latter are soon brought into
contact with surrounding objects. When a shoot stands near an upright
stick, it twines regularly and spirally round it. As it ascends, it
seizes the stick with one of its tendrils, and, if the stick be thin, the
right—and left-hand tendrils are alternately used. This alternation
follows from the stem necessarily taking one twist round its own axis for
each completed circle.
The tendrils contract spirally a short time after catching any object;
those which catch nothing merely bend slowly downwards. But the whole
subject of the spiral contraction of tendrils will be discussed after all
the tendril-bearing species have been described.
_Bignonia littoralis_.—The young internodes revolve in large ellipses.
An internode bearing immature tendrils made two revolutions, each in 3
hrs. 50 m.; but when grown older with the tendrils mature, it made two
ellipses, each at the rate of 2 hrs. 44 m. This species, unlike the
preceding, is incapable of twining round a stick: this does not appear to
be due to any want of flexibility in the internodes or to the action of
the tendrils, and certainly not to any want of the revolving power; nor
can I account for the fact. Nevertheless the plant readily ascends a
thin upright stick by seizing a point above with its two opposite
tendrils, which then contract spirally. If the tendrils seize nothing,
they do not become spiral.
The species last described, ascended a vertical stick by twining spirally
and by seizing it alternately with its opposite tendrils, like a sailor
pulling himself up a rope, hand over hand; the present species pulls
itself up, like a sailor seizing with both hands together a rope above
his head.
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