Page:Darwin - On the movements and habits of climbing plants.djvu/57

 fissure. I fully expected, from the analogy of B. capreolata and B. littoralis, that the tip would have developed itself into an adhesive disk; but I could never detect even a trace of this process. Improbable as the view may be, I am led to suspect that this habit in the tendril of inserting its tip into dark holes and crevices has been inherited by the plant after having lost the power of forming adhesive disks.

Bignonia picta.—This species closely resembles the last in the structure and movements of its tendrils. I casually examined a fine growing plant of the allied B. Lindleyi, and this apparently behaves in all respects in the same manner.

Bignonia capreolata.—We now come to a species having tendrils of a different type: but first for the internodes. A young shoot made three large revolutions, following the sun, at an average rate of 2 h. 23 m. The stem is thin and flexible, and I have seen one make four regular spiral turns round a thin upright stick, ascending, of course, from right to left, and therefore in a reversed direction compared with the first-described species; but afterwards, from the interference of the tendrils, it ascended either straight up the stick or in an irregular spire. These tendrils are highly remarkable. In a young plant they were about 2½ inches in length, and much branched, the five chief branches apparently representing two pairs of leaflets and a terminal one; each branch is bifid or more commonly trifid toward its extremity, with all the points blunt but distinctly hooked. A tendril when lightly rubbed bends to that side, and subsequently becomes straight again; but a loop of thread weighing ¼th of a grain produced no effect. The terminal branches of a tendril twice became in 10 m. slightly curved when touching a stick; and in 30 m. the tips curled quite round the stick: the basal part is less sensitive. The tendrils revolve in an apparently capricious manner, sometimes not at all, or very slightly, but at other times they describe large regular ellipses. I could detect no spontaneous movement in the petioles.

At the same time that the tendrils are revolving more or less regularly, another remarkable movement first begins; the tendrils slowly begin to bend from the light towards the darkest side of the house. I repeatedly changed the position of my plants, and the successively formed tendrils always ended by pointing, some little time after the revolving movement had quite ceased, to the darkest side. But when I placed a thick post near a tendril, and between it and the light, the tendril pointed in that direction. In two instances a pair of leaves stood so that one tendril was directed