第25章 TENDRIL-BEARERS(2)

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, Iconcluded that the tendrils acted only like the hooks on a bramble, and that it was the most feeble and inefficient of all climbers!

Bignonia Tweedyana.--This species is closely allied to the last, and behaves in the same manner; but perhaps twines rather better round a vertical stick.On the same plant, one branch twined in one direction and another in an opposite direction.The internodes in one case made two circles, each in 2 hrs.33 m.I was enabled to observe the spontaneous movements of the petioles better in this than in the two preceding species: one petiole described three small vertical ellipses in the course of 11 hrs., whilst another moved in an irregular spire.Some little time after a stem has twined round an upright stick, and is securely fastened to it by the clasping petioles and tendrils, it emits aerial roots from the bases of its leaves; and these roots curve partly round and adhere to the stick.

This species of Bignonia, therefore, combines four different methods of climbing generally characteristic of distinct plants, namely, twining, leaf-climbing, tendril-climbing, and root-climbing.

In the three foregoing species, when the foot-like tendril has caught an object, it continues to grow and thicken, and ultimately becomes wonderfully strong, in the same manner as the petioles of leaf-climbers.If the tendril catches nothing, it first slowly bends downwards, and then its power of clasping is lost.Very soon afterwards it disarticulates itself from the petiole, and drops off like a leaf in autumn.I have seen this process of disarticulation in no other tendrils, for these, when they fail to catch an object, merely wither away.

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.

The tendrils are similar in structure to those of the last species.