第29章 TENDRIL-BEARERS(6)

One large circle was swept at this latter unusually quick rate; but generally the circles or ellipses were small, and sometimes the course pursued was quite irregular.An internode, after making several revolutions, sometimes stood still for 12 hrs.or 18 hrs., and then recommenced revolving.Such strongly marked interruptions in the movements of the internodes I have observed in hardly any other plant.

The leaves bear four leaflets, themselves subdivided, and terminate in much-branched tendrils.The main petiole of the leaf, whilst young, moves spontaneously, and follows nearly the same irregular course and at about the same rate as the internodes.The movement to and from the stem is the most conspicuous, and I have seen the chord of a curved petiole which formed an angle of 59 degrees with the stem, in an hour afterwards making an angle of 106 degrees.The two opposite petioles do not move together, and one is sometimes so much raised as to stand close to the stem, whilst the other is not far from horizontal.The basal part of the petiole moves less than the distal part.The tendrils, besides being carried by the moving petioles and internodes, themselves move spontaneously; and the opposite tendrils occasionally move in opposite directions.By these combined movements of the young internodes, petioles, and tendrils, a considerable space is swept in search of a support.

In young plants the tendrils are about three inches in length: they bear two lateral and two terminal branches; and each branch bifurcates twice, with the tips terminating in blunt double hooks, having both points directed to the same side.All the branches are sensitive on all sides; and after being lightly rubbed, or after coming into contact with a stick, bend in about 10 m.One which had become curved in 10 m.after a light rub, continued bending for between 3 hrs.and 4 hrs., and became straight again in 8 hrs.or 9hrs.Tendrils, which have caught nothing, ultimately contract into an irregular spire, as they likewise do, only much more quickly, after clasping a support.In both cases the main petiole bearing the leaflets, which is at first straight and inclined a little upwards, moves downwards, with the middle part bent abruptly into a right angle; but this is seen in E.miniatus more plainly than in E.

scaber.The tendrils in this genus act in some respects like those of Bignonia capreolata; but the whole does not move from the light, nor do the hooked tips become enlarged into cellular discs.After the tendrils have come into contact with a moderately thick cylindrical stick or with rugged bark, the several branches may be seen slowly to lift themselves up, change their positions, and again come into contact with the supporting surface.The object of these movements is to bring the double-hooks at the extremities of the branches, which naturally face in all directions, into contact with the wood.I have watched a tendril, half of which had bent itself at right angles round the sharp corner of a square post, neatly bring every single hook into contact with both rectangular surfaces.The appearance suggested the belief, that though the whole tendril is not sensitive to light, yet that the tips are so, and that they turn and twist themselves towards any dark surface.Ultimately the branches arrange themselves very neatly to all the irregularities of the most rugged bark, so that they resemble in their irregular course a river with its branches, as engraved on a map.But when a tendril has wound round a rather thick stick, the subsequent spiral contraction generally draws it away and spoils the neat arrangement.So it is, but not in quite so marked a manner, when a tendril has spread itself over a large, nearly flat surface of rugged bark.We may therefore conclude that these tendrils are not perfectly adapted to seize moderately thick sticks or rugged bark.If a thin stick or twig is placed near a tendril, the terminal branches wind quite round it, and then seize their own lower branches or the main stem.The stick is thus firmly, but not neatly, grasped.What the tendrils are really adapted for, appears to be such objects as the thin culms of certain grasses, or the long flexible bristles of a brush, or thin rigid leaves such as those of the Asparagus, all of which they seize in an admirable manner.This is due to the extremities of the branches close to the little hooks being extremely sensitive to a touch from the thinnest object, which they consequently curl round and clasp.

When a small brush, for instance, was placed near a tendril, the tips of each sub-branch seized one, two, or three of the bristles; and then the spiral contraction of the several branches brought all these little parcels close together, so that thirty or forty bristles were drawn into a single bundle, which afforded an excellent support.

POLEMONIACEAE.--Cobaea scandens.--This is an excellently constructed climber.The tendrils on a fine plant were eleven inches long, with the petiole bearing two pairs of leaflets, only two and a half inches in length.They revolve more rapidly and vigorously than those of any other tendril-bearer observed by me, with the exception of one kind of Passiflora.Three large, nearly circular sweeps, directed against the sun were completed, each in 1 hr.15 m.; and two other circles in 1 hr.20 m.and 1 hr.23 m.Sometimes a tendril travels in a much inclined position, and sometimes nearly upright.The lower part moves but little and the petiole not at all; nor do the internodes revolve; so that here we have the tendril alone moving.

On the other hand, with most of the species of Bignonia and the Eccremocarpus, the internodes, tendrils, and petioles all revolved.