Page:Darwin - On the movements and habits of climbing plants.djvu/104

 when the stem spirally ascends a thin upright stick, but they can seize any twig or branch lying beneath them; but when the stem spirally ascends a somewhat thicker stick, a slight degree of sensitiveness in the petioles is brought into play, and they wind their tendrils round the stick. In B. unguis and B. Tweedyana the sensitiveness, as well as the power of movement, in the petioles is greatly augmented; and the tendrils and petioles are thus inextricably wound together round thin upright sticks; but the stem, in consequence, does not twine so well: B. Tweedyana, in addition, emits aërial roots which adhere to the stick. In B. venusta the tendrils have lost the bird's-foot structure, and are converted into long three-pronged grapnels; these exhibit a conspicuous power of spontaneous movement; the petioles, however, have lost their sensitiveness. The stem can spirally twine round an upright stick, and is aided in its ascent by the tendrils alternately seizing the stick some way above and then spirally contracting. In this and all the following species the tendrils spirally contract after seizing any object. In B. littoralis and B. Chamberlaynii the tendrils, which have the same structure as in B. venusta, and the non-sensitive petioles and the internodes all spontaneously revolve. The stem, however, cannot spirally twine, but ascends an upright stick by both tendrils, seizing it above. In B. littoralis the tips of the tendrils become developed into adhesive disks. In B. speciosa and B. picta we have similar powers of movement, but the plant cannot spirally twine round a stick; it can, however, ascend by clasping it with one or both of its unbranched tendrils, on their own level; and these exhibit the strange, apparently useless, habit of continually inserting their pointed ends into minute crevices and holes. In B. capreolata the stem twines in an imperfect manner; the much-branched tendrils revolve in a capricious manner, and they have the power of bending in a conspicuous manner from the light to the dark; their hooked extremities, even whilst immature, crawl into any crevice, or, when mature, seize any thin projecting point; in both cases they develope adhesive disks, which have the power of enveloping by growth the finest fibres.

In the allied Eccremocarpus the internodes, petioles, and tendrils all spontaneously revolve together; its much-branched tendrils resemble those of Bignonia capreolata, but they do not turn from the light; and their bluntly hooked extremities, which arrange themselves so neatly to any surface, do not form adhesive disks; they act best when each seizes a few thin stems, like the culms of a grass, which they afterwards draw together by their spiral