Page:Darwin - On the movements and habits of climbing plants.djvu/27

 and Mohl class these plants with those which bear tendrils; but as a leaf is generally a defined object, the present classification has, at least, some plain advantages. There are other advantages, as leaf-climbers are intermediate in many respects between twiners and certain tendril-bearing plants. I have observed eight species of Clematis and seven of Tropæolum in order to discover what amount of difference there may be within the same genus; and the differences, as we shall see, are considerable.

.—C. glandulosa—The thin upper internodes revolve, moving against the course of the sun, precisely like those of a true twiner, at an average rate, judging from three revolutions, of 3 h. 48 m. The leading shoot immediately twined round a stick placed near it; but, after making an open spire of only one turn and a half, it ascended for a short space straight, and then reversed its spire and wound two turns in an opposite course. This was rendered possible by the straight piece between the opposed spires having become rigid. The simple, broad, ovate leaves of this tropical species, so unlike those of most of the other species of the genus, with their short thick petioles, seem but ill-fitted for any movement. Whilst twining up a vertical stick, no use is made of them. Nevertheless, if the footstalk of a young leaf be rubbed with a thin twig a few times on any side, it will in the course of a few hours bend to that side; afterwards it becomes straight again. The under side seemed to be the most sensitive; but the sensitiveness or irritability is but slight compared to that which we shall meet with in some of the following species; for a loop of string, weighing 1.64 grain, hanging for some days on a young footstalk, produced a scarcely perceptible effect. A sketch is here given of two young leaves which had naturally caught two twigs on each side of the stem. A forked twig placed so as to lightly press on the under side of a young footstalk caused it, in 12 h., to bend greatly, and ultimately to bent greatly, and ultimately to such an extent that the leaf passed to the opposite side of the stem; the forked stick having been removed, the leaf slowly recovered its proper position.