Page:Darwin - On the movements and habits of climbing plants.djvu/67

 tendril itself bends and revolves in harmony with the internode, a considerably wider space than that here specified (and represented one-half reduced) is swept. Dutrochet observed an ellipse completed in 1 h. 20 m.; I saw one completed in 1 h. 30 m. The direction followed is variable, either with or against the sun.

Dutrochet asserts that the petiole of the leaf spontaneously moves, as well as the young internodes and tendrils; but he does not say that he secured the internodes; when this was done, I never detected any movement in the petiole, except to and from the light.

The tendrils, on the other hand, when the internodes and petioles were secured, described irregular spires or regular ellipses, exactly like those made by the internodes. A young tendril, only 1⅛ inch in length, revolved. Dutrochet has shown that when a plant is placed in a room, so that the light enters laterally, the internodes travel much quicker to the light than from it: on the other hand, he asserts that the tendril itself moves from the light towards the dark side of the room. With due deference to this great observer, I think he was mistaken, owing to his not having secured the internodes. I took a young plant with highly sensitive tendrils, and tied the petiole so that the tendril alone could move; it completed a perfect ellipse in 1 h. 30 m.; and I then turned the plant half round, so that the opposite side faced the light, but this made no change in the direction of the succeeding ellipse. The next day I watched a plant similarly secured until the tendril (which was highly sensitive) made an ellipse in a line exactly to and from the light; the movement was so great that the tendril bent itself down at the two ends of its elliptical course into a line a little beneath the horizon, thus travelling more than 180 degrees; but the curvature was fully as great towards the light as towards the dark side of the room. I believe Dutrochet was misled by not having secured the internodes, and by having observed a plant of which the internodes and tendrils, from inequality of age, no longer curved or moved in harmony together.

Dutrochet made no observations on the sensitiveness of the tendrils; these, whilst young and about an inch in length, with the leaflets on the petiole only partially expanded, are highly sensitive; a single light touch with a twig on the inferior or concave surface near the tip caused them quickly to bend, as did occasionally a loop of thread weighing one-seventh of a grain. The upper or convex surface is barely or not at all sensitive. After bending from a touch the tendril straightened itself in