TENDRIL (Fr. tear/rifle, from tendrr, tender, from Lat. teller, delicate: connected with tenuis, Gk. Tam* tanaos„ Skt. tuna, OH(1. flunni. Ger. (Mtn, AS. Pynne. Eng. thin), A slender, usually eylindrical organ of higher plants used for climbing. As to their origin, tendrils may occupy the position of leaflets, or of leaves, normally subtended by branches. or of branches usually subtended by leaves. The last two sorts, however, are not always easily distinguished. Some tendrils are coiled when young, unrolling as they mature: others are merely bent, or straight, or variously folded. At maturity they arc usually slightly booked near the apex.
Structurally, tendrils contain vascular bundles, surrounded by a cortex of thin walled and some what elongated parenchyma cells, covered by a delicate epidermis. (See HISTOLOGY.) Physio logically, they are sensitive to friction, often remarkably. Sensitiveness is ordinarily greatest on the concave side of the curved tip, though some tendrils are more sensitive on the flanks and others equally sensitive on all sides. Ten drils of Cyc'anthem under favorable conditions will respond to a stimulus in less than ten sec onds. those of Passitlora in about 30 seconds, while those of some other plants may only re spond after 30 to 90 minutes. The response con sists in a curvature of the tip. This is first. due to the shortening of cells on the concave side and the lengthening of those on the convex side. This
temporary curvature is maintained by the ac celerated growth of the convex side, so that the part of the tip beyond the object with which it has come into contact is coiled around the sup port, provided this be of sufficiently small diam eter. if large and rough the tip may be ap pressed to the surface, growing closely into the irregularities, and so adhere more or less firmly in that way. The petioles of some leaves, e.g. Clematis, show a similar but less acute sensi tiveness to friction, making one turn about a support. Uncaria has short conical hooks which are stimulated by contact and friction to grow in such a way that they enlarge greatly and cur•e about a support. After a tendril has laid hold of an appropriate support the portion be tween the support and the axis of the plant is brought under strain, to which it responds by unequal growth on one side. By this means it is thrown into a spiral with one or more points of reversal (a mechanical necessity when a once straight cylindrical body elongates on one side and so coils up). The coils develop mechanical tissues, and the most perfect, tendril thus becomes a strong, uniformly coiled, elastic spiral spring which supports the plant. See LIANAS.