Fig. 46.—Climbing stem Fig. 47.—Climbing stem
of Honeysuckle. (× ¼.) of Convolvulus. (× ¼.)
The stem of the ivy climbs by means of little roots, which it puts out on the side furthest from the light ([Fig. 45]). These give out a sticky liquid which, on drying, cements them to the wall or tree.
Twining stems are much in advance of these. There seems to be something approaching intelligence in the manner in which the young stem of a hop, honeysuckle, or convolvulus, which at first grows straight up, begins to wander round and round, tracing a spiral path in the air until it touches a support. Then, however, as if the plant could feel, the movement below the point of contact stops; but the upper part of the stem still revolves and therefore twines round the support. The stems of the honeysuckle ([Fig. 46]) and the hop turn in the same direction as the hands of a clock. This is called the “clockwise” direction. On the other hand the convolvulus ([Fig. 47]) and most other twining stems are “counter-clockwise” climbers. The stem of the bittersweet revolves indifferently in either direction.
Sensitive clasping organs in their simplest form are seen in the twining leaf stalk of the clematis and tropœolum; the stem itself revolves as if to give its leaf stalks every opportunity of finding suitable supports. The leaf stalks seize these and twine round them.
Most wonderful of all climbing organs are the tendrils. They are well seen in [Fig. 48]. A part of the plant—sometimes a leaflet, as in the pea ([Fig. 28]); sometimes a branch, as in the passion flower; or a flower stalk, as in the vine—becomes modified into a thread, slight but strong. When the end of the thread touches and then twines round a support, the whole tendril forms itself into a spiral which, like a wire spring, draws the plant up to the support, and can yet lengthen and yield to the wind when necessary. In the middle of the tendril the direction of the spiral is reversed, so that the tendril can be straightened without being twisted.
The tendrils of the Virginian creeper do not twine, but on meeting a wall they form round red suckers at the end, and attach the plant ([Fig. 49]).
Fig. 48.—How a tendril grasps a support. Fig. 49.—Virginian Creeper.
The spiral is reversed at x. R, R, stem tendrils. (× ½.)