Page:Darwin - On the movements and habits of climbing plants.djvu/84

 by the next genus, Cissus. This genus, as we shall immediately see, produces well-developed tendrils and ordinary bunches of flowers; but there is no gradation between the two states. If the genus Vitis were unknown, the boldest believer in the modification of species would never, I suppose, have surmised that the same individual plant, at the same period of growth, would have yielded every possible gradation between ordinary flower-stalks for the support of the flowers and fruit, and tendrils used exclusively for climbing. But the vine clearly gives us this case; and it seems to me as striking and curious an instance of transition as can well be conceived.

Cissus discolor.—The young shoots show no more movement than can be accounted for by daily variations in the action of the light. The tendrils, however, revolve with much regularity, following the sun, and, in the plants observed by me, swept circles of about 5 inches in diameter. Five circles were completed in the following times:—4 h. 45 m., 4 h. 50 m., 4 h. 45 m., 4 h. 30 m., and 5 h. The same tendril continues revolving during three or four days. The tendrils are from 3½ to 5 inches in length; they are formed of a long foot-stalk, bearing two short branches, which in old plants again bifurcate. The two branches are not of quite equal length; and, as with the vine, the longer one has a scale at its base. The tendril stands vertically upwards; the extremity of the shoot is bent abruptly downwards; and this position is probably of service in keeping it out of the way of the revolving tendril.

The two branches whilst young are highly sensitive; for I found a touch with a pencil so gentle as only just to move the tendril which was borne at the end of a long flexible shoot, sufficed to cause it to become perceptibly curved in four or five minutes; the tendril became straight again in rather above one hour. A loop of soft thread weighing one-seventh of a grain was thrice tried, and caused the tendrils to become curved in 30 or 40 m.: half this weight produced no effect. The long foot-stalk is much less sensitive, for slight rubbing produced no effect; but prolonged contact with a stick caused it to bend. The two terminal branches are sensitive on all sides; if a number of tendrils be just touched on different sides, two branches of the one on their inner sides, two on their outer sides, or both branches on the same side, in about a quarter of an hour they present a curiously different appearance. If a branch be touched at the same time with equal force on opposite sides, both sides are equally stimulated and there is no movement. At the beginning of my work, and before