Page:Darwin - On the movements and habits of climbing plants.djvu/92

 the sun, in 3 h. 5 m., 2 h. 40 m., and 2 h. 50 m.; perhaps it might have travelled more quickly when nearly full-grown. The plant was placed in front of a window, and I ascertained that, as with twining stems so with these tendrils, the light accelerated the movement in one direction and retarded it in the other, the semicircle towards the light being performed in one instance in 15 m., and in a second instance in 20 m. less time than that required by the semicircle towards the dark end of the room. Considering the extreme tenuity of these tendrils, the action of the light on them is remarkable. The tendrils are long, and, as just stated, very thin, with the tip slightly curved or hooked. The concave side is extremely sensitive to a touch—even a single touch causing it to curl inwards; it subsequently straightens itself, and is again ready to act. A loop of soft thread weighing th of a grain caused the extreme tip to bend; at another time I tried to hang the same little loop on an inclined tendril, but three times it slid off; yet this extraordinarily slight degree of friction sufficed to make the tip curl. The tendril, though so sensitive, does not move very quickly after a touch, no conspicuous change being observable until 5 or 10 m. had elapsed. The convex side of the tip is not sensitive to a touch or to a suspended loop of thread. In one instance I observed a tendril revolving with the convex side of the tip forwards, and on coming into contact with a stick it merely scraped up and past the obstacle and was not able to clasp it; whereas tendrils revolving with the concave side of their tips forward promptly seize any object in their path.

Passiflora quadrangularis.—This is a very distinct species. The tendrils are thick, long, and stiff; they are sensitive to a touch only towards the extremity and on the concave surface. When a stick was so placed that the middle of the tendril came into contact with it, no curvature ensued. In the hothouse a tendril made two revolutions each in 2 h. 22 m.; in my cooler study one was completed in 3 h., and a second in 4 h. The internodes do not revolve; nor do those of the hybrid P. floribunda.

Tacsonia manicata.—Here again the internodes do not revolve. The tendrils are moderately thin and long; one made a narrow ellipse in 5 h. 20 m., and the next day a broad ellipse in 5 h. 7 m. The extremity being lightly rubbed on the concave surface, became just perceptibly curved in 7 m., clearly curved in 10 m., and hooked in 20 m.

We have seen that the tendrils in the last three families, namely