Page:Darwin - On the movements and habits of climbing plants.djvu/51

 degree of spontaneous movement; for in one case they certainly described minute, irregular, vertical ellipses. The tendrils apparently curve themselves spontaneously to the same side with the petioles; but the movement was so slight that it may be passed over. From various causes, it was difficult to observe the movements of the petioles and tendrils in this and the two following species. The tendrils are so closely similar in all respects to those of the following species, that one description will suffice.

Bignonia unguis.—The young shoots revolve, but less regularly and less quickly than those of the last species. The stem twined imperfectly round a vertical stick, sometimes reversing its direction, exactly in the same manner as has been described in so many leaf-climbers; and this plant is in itself a leaf-climber, though possessing tendrils. Each leaf consists of a petiole bearing a pair of leaflets, and terminating in a tendril, which is exactly like that above figured, but a little larger. The whole tendril in a young plant was only about half an inch in length, and is very unlike most tendrils in shape. It curiously resembles the leg and foot of a small bird with the hind toe cut off. The straight leg or tarsus is longer than the three toes, which latter are of equal length, and, diverging, lie in the same plane; the toes terminate in sharp and hard claws, much curved downwards, exactly like the claws on a bird's foot. The whole tendril apparently represents three leaflets. The main petiole (but not the two sub-petioles of the lateral leaflets) is sensitive to contact with any object: even a small loop of thread after two days caused one to bend upwards. The whole tendrils, namely the tarsus and three toes, especially their under surfaces, are likewise sensitive to contact. Hence, when a shoot grows through branched twigs, its revolving movement soon brings the tendril into contact with some twig, and then all three toes bend (or sometimes one alone), and, after several hours, seize fast hold of the twig, exactly like a bird when perched. The tarsus, also, when it comes into contact with a twig, slowly bends, until the foot is carried quite round, and the toes pass on each side of the tarsus, or seize hold of it. If the main petiole bearing the leaflets comes into contact with a twig, it likewise bends round, until the tendril touches its own petiole or that of the opposite leaf, which is then seized. The petioles, and probably even the tendrils in a slight degree, move spontaneously; hence when a shoot attempted to twine round an upright stick, both petioles after a time came into contact with it, and the contact