Page:Darwin - On the movements and habits of climbing plants.djvu/91

 millegrammes) placed most gently on the tip, thrice plainly caused it to curve; as twice did a bent bit of thin platina wire weighing th of a grain; but this latter weight, when left suspended, did not suffice to cause permanent curvature. These trials were made under a bell-glass, so that the loops of thread and wire were not agitated by the wind. The movement after a touch is very rapid: I took hold of the lower part of several tendrils and then touched with a thin twig their concave tips, and watched them carefully through a lens; the tips plainly began to bend in the following times—31, 25, 32, 31, 28, 39, 31, and 30 seconds; so that the movement was generally perceptible in half a minute after the touch, but once plainly in 25 seconds. One of the tendrils which thus became bent in 31 seconds had been touched two hours previously and had coiled into a helix; thus in this interval it had straightened itself and had perfectly recovered its sensibility.

I repeated the experiment made on the Echinocystis, and placed several plants of this Passiflora so close together that the tendrils were repeatedly dragged over each other; but no curvature ensued. I likewise repeatedly flirted small drops of water from a brush on many tendrils, and syringed others so violently that the whole tendril was dashed about, but they never became curved. The impact from the drops of water on my hand was felt far more plainly than that from the loops of thread (weighing nd of a grain) when allowed to fall on it; and these loops, which caused the tendrils to become curved, had been placed most gently on them. Hence it is clear, either that the tendrils are habituated to the touch of other tendrils and to that of drops of rain, or that they are sensitive only to prolonged though excessively slight pressure. To show the difference in the kind of sensitiveness in different plants and likewise to show the force of the syringe used, I may add that the lightest jet from it instantly caused the leaves of a Mimosa to close; whereas the loop of thread weighing nd of a grain, when rolled into a ball and gently placed on the glands at the bases of the leaflets of the Mimosa, caused no action. Had I space, I could advance much more striking cases in plants both belonging to the same family, of one being excessively sensitive to the lightest pressure if prolonged, but not to a brief impact; and of another plant equally sensitive to impact, but not to slight though prolonged pressure.

Passiflora punctata.—The internodes do not move; but the tendrils regularly revolve. One that was about half-grown and very sensitive made three revolutions, opposed to the course of