第28章 TENDRIL-BEARERS(5)

I have stated that after a tendril has come into contact with a stick, it bends round it in about half an hour; but I repeatedly observed, as in the case of B.speciosa and its allies, that it often again loosed the stick; sometimes seizing and loosing the same stick three or four times.Knowing that the tendrils avoided the light, Igave them a glass tube blackened within, and a well-blackened zinc plate: the branches curled round the tube and abruptly bent themselves round the edges of the zinc plate; but they soon recoiled from these objects with what I can only call disgust, and straightened themselves.I then placed a post with extremely rugged bark close to a pair of tendrils; twice they touched it for an hour or two, and twice they withdrew; at last one of the hooked extremities curled round and firmly seized an excessively minute projecting point of bark, and then the other branches spread themselves out, following with accuracy every inequality of the surface.I afterwards placed near the plant a post without bark but much fissured, and the points of the tendrils crawled into all the crevices in a beautiful manner.To my surprise, I observed that the tips of the immature tendrils, with the branches not yet fully separated, likewise crawled just like roots into the minutest crevices.In two or three days after the tips had thus crawled into the crevices, or after their hooked ends had seized minute points, the final process, now to be described, commenced.

This process I discovered by having accidentally left a piece of wool near a tendril; and this led me to bind a quantity of flax, moss, and wool loosely round sticks, and to place them near tendrils.The wool must not be dyed, for these tendrils are excessively sensitive to some poisons.The hooked points soon caught hold of the fibres, even loosely floating fibres, and now there was no recoiling; on the contrary, the excitement caused the hooks to penetrate the fibrous mass and to curl inwards, so that each hook caught firmly one or two fibres, or a small bundle of them.The tips and the inner surfaces of the hooks now began to swell, and in two or three days were visibly enlarged.After a few more days the hooks were converted into whitish, irregular balls, rather above the 0.05th of an inch (1.27 mm.) in diameter, formed of coarse cellular tissue, which sometimes wholly enveloped and concealed the hooks themselves.The surfaces of these balls secrete some viscid resinous matter, to which the fibres of the flax, &c., adhere.When a fibre has become fastened to the surface, the cellular tissue does not grow directly beneath it, but continues to grow closely on each side; so that when several adjoining fibres, though excessively thin, were caught, so many crests of cellular matter, each not as thick as a human hair, grew up between them, and these, arching over on both sides, adhered firmly together.As the whole surface of the ball continues to grow, fresh fibres adhere and are afterwards enveloped; so that I have seen a little ball with between fifty and sixty fibres of flax crossing it at various angles and all embedded more or less deeply.Every gradation in the process could be followed--some fibres merely sticking to the surface, others lying in more or less deep furrows, or deeply embedded, or passing through the very centre of the cellular ball.The embedded fibres are so closely clasped that they cannot be withdrawn.The outgrowing tissue has so strong a tendency to unite, that two balls produced by distinct tendrils sometimes unite and grow into a single one.

On one occasion, when a tendril had curled round a stick, half an inch in diameter, an adhesive disc was formed; but this does not generally occur in the case of smooth sticks or posts.If, however, the tip catches a minute projecting point, the other branches form discs, especially if they find crevices to crawl into.The tendrils failed to attach themselves to a brick wall.

I infer from the adherence of the fibres to the discs or balls, that these secrete some resinous adhesive matter; and more especially from such fibres becoming loose if immersed in sulphuric ether.This fluid likewise removes small, brown, glistening points which can generally be seen on the surfaces of the older discs.If the hooked extremities of the tendrils do not touch anything, discs, as far as Ihave seen, are never formed; but temporary contact during a moderate time suffices to cause their development.I have seen eight discs formed on the same tendril.After their development the tendrils contract spirally, and become woody and very strong.Atendril in this state supported nearly seven ounces, and would apparently have supported a considerably greater weight, had not the fibres of flax to which the discs were attached yielded.

From the facts now given, we may infer that though the tendrils of this Bignonia can occasionally adhere to smooth cylindrical sticks and often to rugged bark, yet that they are specially adapted to climb trees clothed with lichens, mosses, or other such productions;and I hear from Professor Asa Gray that the Polypodium incanum abounds on the forest-trees in the districts of North America where this species of Bignonia grows.Finally, I may remark how singular a fact it is that a leaf should be metamorphosed into a branched organ which turns from the light, and which can by its extremities either crawl like roots into crevices, or seize hold of minute projecting points, these extremities afterwards forming cellular outgrowths which secrete an adhesive cement, and then envelop by their continued growth the finest fibres.

Eccremocarpus scaber (Bignoniaceae).--Plants, though growing pretty well in my green-house, showed no spontaneous movements in their shoots or tendrils; but when removed to the hot-house, the young internodes revolved at rates varying from 3 hrs.15 m.to 1 hr.13 m.