Page:Darwin - On the movements and habits of climbing plants.djvu/62

 movements is that the double hooks at the extremities of the branches, which naturally face in all directions, may be brought into contact with the wood. I have watched a tendril, which had bent itself at right angles abruptly round the sharp corner of a post, neatly bring every single hook into contact with both 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 opaque surface. Ultimately the branches arrange and fit 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 thus arranged itself round a rather thick smooth stick, the subsequent spiral contraction generally spoils the neat arrangement, and draws the tendril from its support. So it is, but not in quite so marked a manner, when a tendril has spread itself over the rugged bark of a thick trunk; for in this case the spiral contraction of the opposite branches sometimes draws the opposed hooks firmly to their supports. Hence we may conclude that these tendrils are not perfectly adapted to seize smooth moderately thick sticks or rugged bark. When a thin stick or twig is placed near a tendril, its terminal branches wind quite round it and seize their own lower branches or main stem; and the stick is thus firmly, but not neatly, grasped. The extremities of the branches, close to the little double hooks, have a strong tendency to curl inwards, and are excited to this movement by contact with the thinnest objects. This accounts for the tendrils apparently preferring such objects as excessively thin culms of a grass, or the long flexible bristles of a brush, or the thin rigid leaves of an Asparagus, all which objects they seized in an admirable manner; for the tips of each sub-branch seized one, two, or three of the bristles, for instance, 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, and afforded an excellent support.

.—Cobæa scandens.—This is an admirably constructed climber. The terminal portion of the petiole, which forms the tendril, was in one very fine specimen eleven inches in length, with the basal part bearing two pairs of leaflets, only two and a half inches in length. The tendril of the Cobæa revolves more rapidly and vigorously than in any other plant observed by me, with the exception of one Passiflora. It made three fine large, nearly