第46章 TENDRIL-BEARERS--(continued)(12)

Tendrils revolve by the curvature of their whole length, excepting the sensitive extremity and the base, which parts do not move, or move but little.The movement is of the same nature as that of the revolving internodes, and, from the observations of Sachs and H.de Vries, no doubt is due to the same cause, namely, the rapid growth of a longitudinal band, which travels round the tendril and successively bows each part to the opposite side.Hence, if a line be painted along that surface which happens at the time to be convex, the line becomes first lateral, then concave, then lateral, and ultimately again convex.This experiment can be tried only on the thicker tendrils, which are not affected by a thin crust of dried paint.The extremities are often slightly curved or hooked, and the curvature of this part is never reversed; in this respect they differ from the extremities of twining shoots, which not only reverse their curvature, or at least become periodically straight, but curve themselves in a greater degree than the lower part.In most other respects a tendril acts as if it were one of several revolving internodes, which all move together by successively bending to each point of the compass.There is, however, in many cases this unimportant difference, that the curving tendril is separated from the curving internode by a rigid petiole.With most tendril-bearers the summit of the stem or shoot projects above the point from which the tendril arises; and it is generally bent to one side, so as to be out of the way of the revolutions swept by the tendril.In those plants in which the terminal shoot is not sufficiently out of the way, as we have seen with the Echinocystis, as soon as the tendril comes in its revolving course to this point, it stiffens and straightens itself, and thus rising vertically up passes over the obstacle in an admirable manner.

All tendrils are sensitive, but in various degrees, to contact with an object, and curve towards the touched side.With several plants a single touch, so slight as only just to move the highly flexible tendril, is enough to induce curvature.Passiflora gracilis possesses the most sensitive tendrils which I have observed: a bit of platina wire 0.02 of a grain (1.23 mg.) in weight, gently placed on the concave point, caused a tendril to become hooked, as did a loop of soft, thin cotton thread weighing one thirty-second of a grain (2.02 mg.) With the tendrils of several other plants, loops weighing one sixteenth of a grain (4.05 mg.) sufficed.The point of a tendril of Passiflora gracilis began to move distinctly in 25seconds after a touch, and in many cases after 30 seconds.Asa Gray also saw movement in the tendrils of the Cucurbitaceous genus, Sicyos, in 30 seconds.The tendrils of some other plants, when lightly rubbed, moved in a few minutes; with Dicentra in half-an-hour; with Smilax in an hour and a quarter or half; and with 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 to act.When the tendrils of several kinds of plants were caused to bend by extremely light weights suspended on them, they seemed to grow accustomed to so slight a stimulus, and straightened themselves, as if the loops had been removed.It makes no difference what sort of object a tendril touches, with the remarkable exception of other tendrils and drops of water, as was observed with the extremely sensitive-tendrils of Passiflora gracilis and of the Echinocystis.I have, however, seen tendrils of the Bryony which had temporarily caught other tendrils, and often in the case of the vine.

Tendrils of which the extremities are permanently and slightly curved, are sensitive only on the concave surface; other tendrils, such as those of the Cobaea (though furnished with horny hooks directed to one side) and those of Cissus discolor, are sensitive on all sides.Hence the tendrils of this latter plant, when stimulated by a touch of equal force on opposite sides, did not bend.The inferior and lateral surfaces of the tendrils of Mutisia are sensitive, but not the upper surface.With branched tendrils, the several branches act alike; but in the Hanburya the lateral spur-like branch does not acquire (for excellent reasons which have been explained) its sensitiveness nearly so soon as the main branch.With most tendrils the lower or basal part is either not at all sensitive, or sensitive only to prolonged contact.We thus see that the sensitiveness of tendrils is a special and localized capacity.It is quite independent of the power of spontaneously revolving; for the curling of the terminal portion from touch does not in the least interrupt the former movement.In Bignonia unguis and its close allies, the petioles of the leaves, as well as the tendrils, are sensitive to a touch.

Twining plants when they come into contact with a stick, curl round it invariably in the direction of their revolving movement; but tendrils curl indifferently to either side, in accordance with the position of the stick and the side which is first touched.The clasping movement of the extremity is apparently not steady, but undulatory or vermicular in its nature, as may be inferred from the curious manner in which the tendrils of the Echinocystis slowly crawled round a smooth stick.