Page:Darwin - On the movements and habits of climbing plants.djvu/37

 clasp either their own stem or the supporting stick. The petioles and filaments often become, after a time, in some degree spirally contracted. In these spontaneous movements, and in the abortion of their laminæ, the sensitive filaments present a much nearer approach to the condition of tendrils than do the petioles of any other leaf-climber observed by me.

Tropæolum azureum.—An upper internode made four revolutions, following the sun, at an average rate of 1 h. 47 m. The stem twined spirally in the same irregular manner as in the last species; it produced no filaments or rudimentary leaves. The petioles of the young leaves are very sensitive: a single very light rub with a twig caused one to move perceptibly in 5 m., and another in 6 m.; the former petiole became bent at right angles in 15 m., and became straight again in between 5 h. and 6 h. A loop of thread weighing ⅛th of a grain caused a petiole to curve.

Tropæolum pentaphyllum.—The plant observed by me had not the power of spirally twining, which seemed due, not to the want of flexibility in the stem, but rather to continual interference from the clasping petioles. An upper internode made three revolutions, following the sun, at an average rate of 1 h. 46 m. The main purpose of the revolving movement in all the species is manifestly to bring the petioles into contact with some supporting object. The petiole of a young leaf, after a slight rub, became curved in 6 m.; another, on a cold day, in 20 m.; but others generally in from 8 m. to 10 m.: the curvature usually increased greatly in from 15 m. to 20 m. The petioles became straight again in between 5 h. and 6 h., and on one occasion in 3 h. When a petiole had fairly clasped a stick, it could not on the removal of the stick recover itself; but the free upper part of a petiole, which had already clasped a stick by its basal part, still had the power of movement. A loop of thread weighing ⅛th of a grain certainly caused a petiole to curve; but the stimulus was not sufficient, the loop remaining suspended, to cause a permanent flexure. If a much heavier loop be placed in the angle between the petiole and the stem, it produces no effect; whereas we have seen that the angle between the stem and petiole of Clematis montana is sensitive.

Tropæolum peregrinum.—In a very young plant the internodes did not revolve, resembling in this respect a young twining plant. The four upper internodes in an older plant made three irregular revolutions, in a course opposed to the sun, at an average rate of 1 h. 48 m. It is remarkable how nearly the same the average rate of revolution (taken, however, but from few