Page:Darwin - On the movements and habits of climbing plants.djvu/63

 circular sweeps, against the sun, each in 1 h. 15 m., and two others in 1 h. 20 m. and 1 h. 23 m. Sometimes it travels in a much inclined position, and sometimes nearly upright. The lower part moves but little, and the basal portion or petiole, which bears the leaflets, not at all; nor do the internodes revolve; so that here we have the tendril alone moving. With most of the species of Bignonia and with Eccremocarpus, the internodes, tendrils, and petioles all revolve. The long, straight, tapering main stem of the tendril of the Cobæa bears alternate branches; and each branch is several times divided, with the finer branches as thin as very thin bristles, extremely flexible, so that they are blown about by a breath of air, yet strong and highly elastic. The extremity of each branch is a little flattened, and terminates in a minute double (but sometimes single) hook, formed of hard, transparent, woody substance, and as sharp as the finest needle. On the eleven-inch tendril I counted ninety-four of these beautifully constructed little hooks. They readily catch soft wood, or gloves, or the skin of the hands. Excepting these hardened hooks, and excepting the basal part of the central stem of the tendril, every part of every branch is highly sensitive on all sides to a slight touch, and bends in a few minutes towards the touched side. By lightly rubbing several branches on different and opposite sides, the whole tendril rapidly assumes 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, the spiral contraction also begins after an unusually short interval of time, namely, in about twelve hours.

Before the tendril is mature, the terminal branches cohere and the hooks are curled closely inwards: at this period no part is sensitive to a touch; but as soon as all the branches have diverged and the hooks stand out, full sensitiveness is acquired. It is a singular circumstance that the immature tendril, before becoming sensitive, begins to revolve at its full velocity: this movement must be useless as the tendril in this state can catch nothing: it is a rare instance of a want, though only for a short time, of perfect coadaptation in the structure and functions of a climbing-plant. The petiole with the tendril perfectly matured, but with the leaflets still quite small, stands at this period vertically upwards, the young growing shoot or axis being thrown to one side. The