Page:Darwin - On the movements and habits of climbing plants.djvu/29

 the petioles of the lateral and terminal leaflets are sensitive. A loop of thread, weighing one-eighth of a grain, acted on the main petiole; but it took between two and three days to produce any effect. The leaves have the remarkable habit and power of spontaneously revolving, generally in vertical ellipses, in the same manner, but in a less degree, as will be described under C. microphylla.

Clematis calycina.—The young shoots are thin and flexible; one revolved, describing a broad oval, in 5 h. 30 m., and another in 6 h. 12 m.: they followed the course of the sun; but in all the species of the genus the course followed, if observed long enough, would no doubt be found to differ. This is a rather better twiner than the two last species: the stem, when a thin upright stick free from twigs was placed near, sometimes made two spiral turns round it; then, being arrested by the clasping of the petioles, it would run up for a space straight and then generally reversed its course and took one or two spiral turns in an opposite direction. This reversal of the spire occurred in all the foregoing species. The leaves are so small compared with those of most of the other species that the petioles at first seem ill-fitted for clasping. Nevertheless the main service of the revolving movement is to bring them into contact with surrounding objects, which are slowly but securely seized. The young petioles, which alone are sensitive, have their ends bowed a little downwards, so as to be in a slight degree hooked; ultimately the whole leaf becomes flat. I gently rubbed with a thin twig the lower surfaces of two young petioles; and in 2 h. 30 m. they were slightly curved downwards; in 5 h., after being rubbed, the end of one was bent completely back parallel to the basal portion; and in 4 h. subsequently it became nearly straight again. To show how sensitive the young petioles are, I may mention that I put, in order to mark them, short streaks of water-colour on their under sides; an infinitely thin crust was thus formed, but it sufficed in 24 h. to cause both to bend downwards. Whilst the plant is young, each leaf consists of three divided leaflets, which have barely distinct petioles, and these are not then sensitive; but when the plant is well grown, the two lateral and terminal leaflets have long petioles, and these now become sensitive and are capable of clasping in any direction any object.

When the petiole has clasped a twig, it undergoes some remarkable changes, which occur with the several other species, but in a less strongly marked manner, and will be here described once for