Page:Darwin - On the movements and habits of climbing plants.djvu/89

 in 3 h. 12 m.; in a second plant the same course was followed, and the two were completed in 3 h. 41 m.; in a third plant the internodes followed the sun, and made two circles in 3 h. 47 m. The average rate of these six revolutions was 1 h. 40 m. The stem shows no tendency to twine spirally round a support; but the allied tendril-hearing genus Paullinia is said (Mohl, S. 4) to be a twiner. By the revolving movement, the flower-peduncles, which stand up above the end of the shoot, are carried round and round; but when the internodes were securely tied, the long and thin peduncles themselves were seen to be in continued and sometimes rapid movement from side to side. They swept a wide space, but only occasionally moved in a moderately regular elliptical course. By these combined movements one of the two short hooked tendrils, sooner or later, catches hold of some twig or branch, and then it curls round and securely grasps it. These tendrils are, however, but slightly sensitive; for by rubbing their under surfaces only a slight movement was slowly produced. I hooked a tendril on to a twig; and in 1 h. 45 m. it had curved considerably inwards; in 2 h. 30 m. it formed a ring; and in from 5 to 6 hours from being first hooked, it closely grasped the stick. A second tendril acted at nearly the same rate; but I observed one that took 24 hours before it curled twice round a thin twig. Tendrils which have caught nothing spontaneously curl, after the interval of several days, closely up into a helix. Those which have curled round some object soon become a little thicker and tougher. The long and thin main peduncle, though spontaneously moving, is not sensitive and never clasps a support. It never contracts spirally. Such contraction would apparently have been of service to the plant in climbing; nevertheless it climbs pretty well without this aid. The seed-capsules, though light, are of enormous size (hence its English name of Balloon-vine), and as two or three are carried on the same peduncle, the tendrils arising close to them may possibly be of service in preventing these balloons from being dashed to pieces by the wind. In the hothouse they served simply for climbing.

The position of the tendrils alone suffices to show their homological nature; but in two instances one of the tendrils produced at its tip a flower; this, however, did not prevent the tendril acting properly and curling round a twig. In a third case the two lateral branches which ought to have existed as tendrils, both produced flowers like the central branch, and had quite lost their tendril-structure.