Tendril climbers.—A slender coiling part that serves to hold a climbing plant to a support is known as a tendril. The free end swings or curves until it strikes some object, when it attaches itself and then coils and draws the plant close to the support. The spring of the coil also allows the plant to move in the wind, thereby enabling the plant to maintain its hold. Slowly pull a well-matured tendril from its support, and note how strongly it holds on. Watch the tendrils in a wind-storm. Usually the tendril attaches to the support by coiling about it, but the Virginia creeper and the Boston ivy (Fig. [170]) attach to walls by means of disks on the ends of the tendrils.
Fig. 170.—Tendril of Boston Ivy.
Since both ends of the tendril are fixed, when it finds a support, the coiling would tend to twist it in two. It will be found, however, that the tendril coils in different directions in different parts of its length. In Fig. [169], showing an old and stretched-out tendril, the change of direction in the coil occurred at a. In long tendrils of cucumbers and melons there may be several changes of direction.
Tendrils may represent either branches or leaves. In the Virginia creeper and the grape they are branches; they stand opposite the leaves in the position of fruit clusters, and sometimes one branch of a fruit cluster is a tendril. These tendrils are therefore homologous with fruit clusters, and fruit clusters are branches.
Fig. 171.—Leaves of Pea,—very large stipules, opposite leaflets, and leaflets represented by tendrils.
In some plants tendrils are leaflets (Chap. XI). Examples are the sweet pea and the common garden pea. In Fig. [171], observe the leaf with its two great stipules, petiole, six normal leaflets, and two or three pairs of leaflet tendrils and a terminal leaflet tendril. The cobaea, a common garden climber, has a similar arrangement. In some cases tendrils are stipules, as probably in the green briers (smilax).
The petiole or midrib may act as a tendril, as in various kinds of clematis. In Fig. [172], the common wild clematis or “old man vine,” this mode is seen.
Twiners.—The entire plant or shoot may wind about a support. Such a plant is a twiner. Examples are bean, hop, morning-glory, moon-flower, false bittersweet or waxwork (Celastrus), some honeysuckles, wistaria, Dutchman’s pipe, dodder. The free tip of the twining branch sweeps about in curves, much as the tendril does, until it finds support or becomes old and rigid.