The Works of Charles Darwin: Vol 18: The Movements and Habits of Climbing Plants by Paul H Barrett

Author:Paul H Barrett [Barrett, Paul H]
Language: eng
Format: epub
Tags: Literary Collections, American, General
ISBN: 9780814717950
Google: VOUTCgAAQBAJ
Publisher: NYU Press
Published: 1987-11-15T16:16:06+00:00

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. The long, straight, tapering main stem of the tendril of the Cobaea bears alternate branches; and each branch is several times divided, with the finer branches as thin as very thin bristles and extremely flexible, so that they are blown about by a breath of air; yet they are strong and highly elastic. The extremity of each branch is a little flattened, and terminates in a minute double (though sometimes single) hook, formed of a hard, translucent, woody substance, and as sharp as the finest needle. On a tendril which was eleven inches long I counted ninety-four of these beautifully constructed little hooks. They readily catch soft wood, or gloves, or the skin of the naked hand. With the exception of these hardened hooks, and of the basal part of the central stem, every part of every branchlet is highly sensitive on all sides to a slight touch, and bends in a few minutes towards the touched side. By lightly rubbing several subbranches on opposite sides, the whole tendril rapidly assumed an extraordinarily crooked shape. These movements from contact do not interfere with the ordinary revolving movement. The branches, after becoming greatly curved from being touched, straighten themselves at a quicker rate than in almost any other tendril seen by me, namely in between half an hour and an hour. After the tendril has caught any object, spiral contraction likewise / begins after an unusually short interval of time, namely, in about twelve hours.

Before the tendril is mature, the terminal branchlets cohere, and the hooks are curled closely inwards. At this period no part is sensitive to a touch; but as soon as the branches diverge and the hooks stand out, full sensitiveness is acquired. It is a singular circumstance that immature tendrils revolve at their full velocity before they become sensitive, but in a useless manner, as in this state they can catch nothing. This want of perfect coadaptation, though only for a short time, between the structure and the functions of a climbing plant is a rare event. A tendril, as soon as it is ready to act, stands, together with the supporting petiole, vertically upwards.

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