Page:Darwin - On the movements and habits of climbing plants.djvu/101

 several plants a single touch, so slight as only just to move the highly flexible tendril, is enough to induce curvature. Passiflora gracilis has the most sensitive tendrils which I have seen: a hit of platina wire th of a grain in weight, gently placed on the concave point, caused two tendrils to become hooked, as did (and this perhaps is a better proof of sensitiveness) a loop of soft, thin cotton thread weighing nd of a grain, or about two milligrammes. With the tendrils of several other plants, loops weighing th of a grain sufficed. The point of the tendril of the Passiflora gracilis distinctly began to move in 25 seconds after a touch. Asa Gray saw movement in the tendrils of the Cucurbitaceous genus, Sicyos, in 30 seconds. The tendrils of some other plants, when lightly rubbed, move in a few minutes; in the Dicentra in half-an-hour; in the Smilax in an hour and a quarter or a half; and in the Ampelopsis still more slowly. The curling movement consequent on a single touch continues to increase for a considerable time, then ceases; after a few hours the tendril uncurls itself, and is again ready for action. When very light weights were suspended on tendrils of several plants and caused them to curve, these seemed to become accustomed to so slight a stimulus, and straightened themselves, as if the loops had been removed. It makes no difference, as far as I have seen, what sort of object a tendril touches, with the remarkable exception of drops of water in the case of the extremely sensitive tendrils of Passiflora gracilis and of the Echinocystis; hence we are led to infer that they have become habituated to showers of rain. As I made no observations with this view on other tendrils, I cannot say whether there are more cases of this adaptation. Moreover adjoining tendrils rarely catch each other, as we have seen with the Echinocystis and Passiflora, though I have seen this occur with the Bryony.

Tendrils of which the extremities are slightly curved or bowed are sensitive only on the concave surface; other tendrils, such as those of the Cobæa (though furnished with minute horny hooks) and those of Cissus discolor, are sensitive on all sides. Hence the tendril of this latter plant, when stimulated by a touch of equal force on opposite sides, does not bend. In the tendril of the Mutisia the inferior and lateral surfaces are sensitive, but not the upper surface. With branched tendrils, the several branches all act alike; but in the Hanburya the lateral spur-like branch does not acquire (for a reason which has been explained) its sensitiveness nearly so soon as the main branch. The lower or basal part of many tendrils is either not at all sensitive or sensitive only