Page:Darwin - On the movements and habits of climbing plants.djvu/54

 account for the circumstance. Nevertheless the plant readily ascends a thin upright stick by its two opposite tendrils, both seizing the stick some way above, and afterwards spirally contracting. If the tendrils seize nothing, they do not contract spirally. Bignonia venusta ascended a vertical stick by spirally twining and by seizing it alternately with its two tendrils like a sailor pulling himself up a rope hand over hand; our present species pulls itself straight up, like a sailor seizing with both hands together the rope above his head.

The tendrils are almost identical in structure with those of the last species. They continue growing for some time, even after clasping an object, and when fully grown, though borne by a young plant, were 9 inches in length. The three divergent toes are shorter relatively to the tarsus than in the former species; they are blunt at their tips and but slightly hooked; they are not quite equal in length, one being rather longer than the others. The outer surfaces of the three toes are highly sensitive; for when lightly rubbed with a twig, they became perceptibly curved in 4 m. and greatly curved in 7 m.; in 7 h. they became straight again and ready to react. The tarsus, for a space of one inch close to the toes, is sensitive, but in a rather less degree than the toes; for after a slight rubbing this part required about twice as long a time to bend. Even the middle part of the tarsus, if acted on soon after the tendril has arrived at maturity, is sensitive to prolonged contact. After the tendrils have grown old, the sensitiveness is confined to the toes, when they will only curl very slowly round a stick. The maturity of the tendril is shown by the divergence of the three toes, at which period their outer surfaces first become irritable. The irritability of the tendril has little power of spreading from one part to another: thus, when a stick was caught by the part immediately beneath the three toes, these often remained sticking out, and never clasped the stick.

The tendrils revolve spontaneously. The movement begins before the tendril is converted into a grapnel by the divergence of the toes, and before any part has become sensitive; so that the revolving movement is at this early period quite useless. The movement is at this time slow, two ellipses being completed conjointly in 24 h. 18 m. When the tendril was mature, an ellipse was performed in 6 h.; so that even at this period the movement is much slower than that of the internodes. Large ellipses were swept, both in vertical and horizontal planes, by the tendrils.