Page:Darwin - On the movements and habits of climbing plants.djvu/73

 leaflets (a, b, c, d) are highly sensitive, for a loop of thread weighing only the one-sixteenth of a grain caused them, in under 4 h., to become greatly curved: when the loop was removed, the petioles straightened themselves in about the same time. The petiole (e) was rather less sensitive; and in another specimen, in which the corresponding petiole bore rather larger leaflets, a loop of thread weighing one-eighth of a grain did not cause curvature until 18 h. had elapsed. Loops of thread weighing one-fourth of a grain, left suspended on all the lower petioles (f to l) during several days, produced no effect. Yet the three petioles f, g, and h are not quite insensible, for when left in contact with a stick for a day or two they slowly curled round it. So that the sensibility of the petiole gradually diminishes from the tendril-like extremities to the base. The internodes are not at all sensitive, which makes Mohl's statement that they are sometimes converted into tendrils the more surprising, not to say improbable.

The whole leaf, whilst young and sensitive, stands almost vertically upwards, as we have seen is the case with many tendrils. It is in continual movement, and one that I observed swept large, though irregular, ellipses, sometimes narrow, sometimes broad, with their longer axes directed to different points of the compass, at an average rate of about 2 h. for each revolution. The young internodes also, which bear the revolving leaves, likewise revolve irregularly in ellipses and spires; so that by these combined movements a considerable space is swept for a support. If the terminal and attenuated portion of the petiole fails in seizing any object, it ultimately bends downwards and inwards, and then soon loses all its irritability and power of movement. This bending down is of a very different nature from that which occurs with the extremities of the young leaves in many species of Clematis; for these, when thus bent or hooked, first acquire their full degree of sensitiveness.

Dicentra thalictrifolia.—In this allied plant the metamorphosis of the terminal leaflets has been complete, and they are converted into perfect tendrils. Whilst the plant was young, the tendrils appeared like modified branches, so that a distinguished botanist thought this was their nature; but in a full-grown plant, there can be no doubt, as I am assured by Dr. Hooker, that the tendrils are modified leaves. The tendrils, when of full size, are above 5 inches in length; they bifurcate twice, thrice, or even four times; their extremities are hooked, but blunt. All the branches of the tendrils are sensitive on all sides, but the basal