Page:Darwin - On the movements and habits of climbing plants.djvu/38

 observations) is in this and the two last species, namely, 1 h. 47 m., 1 h. 46 m, and 1 h. 48 m. The present species cannot spirally twine, which seems mainly due to the rigidity of its stem. In a very young plant, which did not revolve, the petioles were not sensitive. In older plants the petioles of quite young leaves, and of leaves as much as an inch and a quarter in diameter, are sensitive. A moderate rub caused one to curve in 10 m., but others in 20 m.; the petioles became straight again in from 5 h. 45 m. to 8 h. Petioles which have naturally come into contact with a stick, sometimes take two turns round it. When clasped round a support, they become rigid and hard. The petioles are less sensitive to a weight than in the previous species; for loops of string weighing .82 of a grain did not cause any curvature, whilst a loop of double this weight (1.64 gr.) did act.

Tropæolum elegans.—I did not make many observations on this species. The short and stiff internodes revolve irregularly, and describe extremely small oval figures; one was completed in 3 h. A young petiole, when rubbed, became slightly curved in 17 m.; then much more so; and was nearly straight again in 8 h.

Tropæolum tuberosum.—The internodes on a plant nine inches high did not move at all; but on an older plant they moved irregularly, and made very small imperfect ovals. These movements could be detected only by being traced on a bell-glass placed over the plant. Sometimes the shoots stood still for hours; during some days they moved only in one direction in a crooked line; on other days they made small irregular spires or circles, one being completed in about 4 h. The movement of the apex of the shoot, from extreme point to point of the oval, was only about one inch or one and a half; yet this slight movement brought the petioles into contact with closely surrounding twigs, which were then clasped. With the lessened power of spontaneously revolving compared with the previous species, the sensitiveness of the petioles is likewise diminished. These, when rubbed a few times, did not become curved until half an hour had elapsed; the curvature increased during the next two hours, and then very slowly decreased; so that the petioles sometimes required 24 h. to become straight again. The petioles of very young leaves can act perfectly; one with the lamina only .15 of an inch in diameter, that is, about a twentieth of the full size, firmly clasped a thin twig: but leaves grown to one quarter of their full size can likewise act.

Tropæolum minus (?).—The internodes of a variety named