Page:Darwin - On the movements and habits of climbing plants.djvu/33

 of C. calycina; but in older plants it has spread to the three sub-petioles. In C. viticella it has spread to the petioles of the seven leaflets, and to the subdivisions of the basi-lateral sub-petioles. In this latter species the sensitiveness has diminished in the basal part of the main petiole, in which alone it resided in C. montana, and has accumulated in the abruptly bent terminal portion.

Clematis flammula.—The shoots, which are rather thick, straight, and stiff, whilst growing vigorously in the spring, made small oval revolutions, following the sun in their course. Four were made at an average rate of 3 h. 45 m. The longer axis of the oval, described by the extreme tip, was directed at right angles to the line joining the opposite leaves; its length was in one case only 1, and in another case 1 inch; so that the young leaves are moved a very short distance. The shoots of the same plant observed in midsummer, when growing not so quickly, did not revolve at all. I cut down another plant in the early summer, so that by August 1st it had formed new and moderately vigorous shoots; these, when observed under a bell-glass, were on some days quite stationary, and on other clays moved to and fro only about the eighth of an inch. Consequently the revolving power is here much enfeebled, and under unfavourable circumstances is completely lost. This species must depend on the probable, though not ascertained, spontaneous movements of its leaves, on the rapid growth of its shoots, and on movements from the wind, for coming into contact with surrounding objects: hence, perhaps, it is that the petioles have acquired, as we shall see, in compensation a high degree of sensitiveness.

The petioles are bowed downwards, and have the same general hook-like form as in C. viticella. The medial petiole and lateral sub-petioles are sensitive, especially the much-bent terminal portion. As the sensitiveness is here greater than in any other species of the genus observed by me, and is in itself remarkable, I will give fuller details. The petioles, when so young that they have not separated from each other, are not sensitive; when the lamina of a leaflet has grown to quarter of an inch in length (that is, about one-sixth of its full size), the sensitiveness is highest; but at this period the petioles are much more fully developed proportionally than the laminæ of the leaves. Full-grown petioles are not in the least sensitive. A thin stick placed so as to press lightly against a petiole, bearing a leaflet a quarter of an inch in length, caused the petiole to bend in 3 h. 15 m.; in another case a petiole curled completely round a stick in 12 h. These petioles