第27章 TENDRIL-BEARERS(4)

The tendrils, by their own movement and by that of the internodes, slowly travelled over the surface of the wood, and when the apex came to a hole or fissure it inserted itself; in order to effect this the extremity for a length of half or quarter of an inch, would often bend itself at right angles to the basal part.I have watched this process between twenty and thirty times.The same tendril would frequently withdraw from one hole and insert its point into a second hole.I have also seen a tendril keep its point, in one case for 20hrs.and in another for 36 hrs., in a minute hole, and then withdraw it.Whilst the point is thus temporarily inserted, the opposite tendril goes on revolving.

The whole length of a tendril often fits itself closely to any surface of wood with which it has come into contact; and I have observed one bent at right angles, from having entered a wide and deep fissure, with its apex abruptly re-bent and inserted into a minute lateral hole.After a tendril has clasped a stick, it contracts spirally; if it remains unattached it hangs straight downwards.If it has merely adapted itself to the inequalities of a thick post, though it has clasped nothing, or if it has inserted its apex into some little fissure, this stimulus suffices to induce spiral contraction; but the contraction always draws the tendril away from the post.So that in every case these movements, which seem so nicely adapted for some purpose, were useless.On one occasion, however, the tip became permanently jammed into a narrow fissure.Ifully expected, from the analogy of B.capreolata and B.littoralis, that the tips would have been developed into adhesive discs; but Icould never detect even a trace of this process.There is therefore at present something unintelligible about the habits of this plant.

Bignonia picta.--This species closely resembles the last in the structure and movements of its tendrils.I also casually examined a fine growing plant of the allied B.Lindleyi, and this apparently behaved in all respects in the same manner.

Bignonia capreolata.--We now come to a species having tendrils of a different type; but first for the internodes.A young shoot made three large revolutions, following the sun, at an average rate of 2hrs.23 m.The stem is thin and flexible, and I have seen one make four regular spiral turns round a thin upright stick, ascending of course from right to left, and therefore in a reversed direction compared with the before described species.Afterwards, from the interference of the tendrils, it ascended either straight up the stick or in an irregular spire.The tendrils are in some respects highly remarkable.In a young plant they were about 2.5 inches in length and much branched, the five chief branches apparently representing two pairs of leaflets and a terminal one.Each branch is, however, bifid or more commonly trifid towards the extremity, with the points blunt yet distinctly hooked.A tendril bends to any side which is lightly rubbed, and subsequently becomes straight again; but a loop of thread weighing 0.25th of a grain produced no effect.On two occasions the terminal branches became slightly curved in 10 m.after they had touched a stick; and in 30 m.the tips were curled quite round it.The basal part is less sensitive.The tendrils revolved in an apparently capricious manner, sometimes very slightly or not at all; at other times they described large regular ellipses.I could detect no spontaneous movement in the petioles of the leaves.

Whilst the tendrils are revolving more or less regularly, another remarkable movement takes place, namely, a slow inclination from the light towards the darkest side of the house.I repeatedly changed the position of my plants, and some little time after the revolving movement had ceased, the successively formed tendrils always ended by pointing to the darkest side.When I placed a thick post near a tendril, between it and the light, the tendril pointed in that direction.In two instances a pair of leaves stood so that one of the two tendrils was directed towards the light and the other to the darkest side of the house; the latter did not move, but the opposite one bent itself first upwards and then right over its fellow, so that the two became parallel, one above the other, both pointing to the dark: I then turned the plant half round; and the tendril which had turned over recovered its original position, and the opposite one which had not before moved, now turned over to the dark side.

Lastly, on another plant, three pairs of tendrils were produced at the same time by three shoots, and all happened to be differently directed: I placed the pot in a box open only on one side, and obliquely facing the light; in two days all six tendrils pointed with unerring truth to the darkest corner of the box, though to do this each had to bend in a different manner.Six wind-vanes could not have more truly shown the direction of the wind, than did these branched tendrils the course of the stream of light which entered the box.I left these tendrils undisturbed for above 24 hrs., and then turned the pot half round; but they had now lost their power of movement, and could not any longer avoid the light.

When a tendril has not succeeded in clasping a support, either through its own revolving movement or that of the shoot, or by turning towards any object which intercepts the light, it bends vertically downwards and then towards its own stem, which it seizes together with the supporting stick, if there be one.A little aid is thus given in keeping the stem secure.If the tendril seizes nothing, it does not contract spirally, but soon withers away and drops off.If it seizes an object, all the branches contract spirally.