"The first purpose of the spontaneous revolving movement," says Darwin, "is to enable the shoot to find its support. This is admirably effected by the revolutions carried on night and day, a wider and wider circle being swept as the shoot increases in length. This movement likewise explains how it is that plants twine." The hop and the honeysuckle always move in the same direction as the hands of a watch (Fig. 66). They follow the sun. The bean, jasmine, wistaria, and convolvulus turn always in the contrary direction to the hands of a watch, or against the sun. A few vines - notably the bittersweet - seem indifferent which way they twine, and one species studied by Darwin, the Scypanthus elegans, can revolve first one way and then the other; can, in fact, "reverse" like an expert waltzer.
Bind-weed (Convolvulus). Twining "against the sun".
Hop-vine (Humulus Lupulus). Twining "with the sun".
Fig. 66. - Bind-weed and hop-vine. (From the Vegetable World).
But the great majority of those which have been studied twine always the same way, and as a rule plants near of kin wind about their supports in the same direction. The speed of the revolving movement varys greatly. The convolvulus and the bean sweep completely around the circle in less than two hours. On the other hand, some plants take twenty-four hours for a single revolution, and one sluggard was found which seemed unable to get around in less than forty-eight hours. The rate of speed seems to have little to do with the thickness of the vine, for the woody shoots of the wistaria are found to traverse the circle faster than do the slender herbaceous tips of the morning-glories.
In all the leaf-climbers and tendril-bearers whose habits have been investigated, the young internodes revolve, but there movements are less regular than those of the twiners.
The tender shoots of that familiar leaf-climber, the clematis, while growing vigorously in spring, make small oval revolutions, moving always in the same direction as the hands of a watch. Later in the season the vine-tips travel more fitfully and slowly through a very small circle, and by midsummer their movements have almost ceased.
But the leaf-stalks have acquired a high degree of sensitiveness, as if to make up for the failing powers of the shoots.
While the leaf is yet so young that its blade - or flat, green surface - has attained but one-sixth of its full size, its stalk is so well developed that the whole affair has somewhat the disproportioned and lanky appearance of a few-days'-old colt. At this stage of growth the sensitiveness of the leafstalk is at its highest, and the tender blade is bent downward, so that the whole leaf has a hook-like form (Fig. 67).
When the growth of the plant or an impulse from the wind brings the hook into such a position that it catches on a twig the sensitive stalk feels the pressure and begins to curve. Darwin experimented upon one species of clematis with a stick placed so as to press lightly against one of its young leaf-stalks. He found that the leaf-stalk curled completely around the stick in the course of twelve hours, and though, after twenty-four hours, the stick was removed, the young stalk never subsequently straightened itself.
Fig. 67. - Scaling-hooks of the Virginia creeper (Ampelopsis quin-quefolia) and of the wild clematis (Clematis Virginiana).
After the clasping leaf-stalk has made sure of its hold, it is subjected to some remarkable alterations. It literally "undergoes a change of heart," so that, though the stalk in its days of youth and freedom was flexible, and could be snapped easily, the clasping coil is wonderfully tough.
The purpose of this change evidently is to fit the leaf-stem to give the branch firm and durable support.
The ways of the tendril-bearing vines may be readily studied by observation of two among them which are familiar to all mankind - the grapevine and its graceful cousin, the Virginia creeper. Nature ages ago set an example of thrift akin to that which beats its swords into ploughshares when cruel war is done. She is wont to adopt the same organ in various ways, so that it can fulfil various tasks in various conditions. Thus vine tendrils are leaf-stalks, or flower-stalks, as the case may be, altered over into fitness for their new work of clasping and clinging. Those of the great majority of vines are transformed leaf-stalks, and now and then betray their true nature by bearing at their extremities partly-grown or imperfectly formed leaves. Those of the grape and the Virginia creeper are altered flower-stalks, and occasionally reveal their origin by developing into what are known as "flower-tendrils." These, like Bottom the weaver, undertake all roles, bearing a bunch of flowers midway, and having coiling, sensitive tips. And among those borne by the grape the vine-dresser finds every gradation, from the tendril with a solitary blossom half-way along its length to the bunch of flowers or grapes ending in a tendril coil. But whether they are leaf-stems or flower-stems by nature the conduct of all tendrils is much the same. "Both kinds spontaneously revolve," says Darwin, "and at about the same rate. Both, when touched, bend quickly toward the touched side. And both kinds soon after grasping a support contract spirally, and then increase greatly in thickness and strength." A vigorous grape-tendril is often several inches in length, and forks once or twice. Its branches move independently of one another, and in bright July days they traverse their circle in from two to three hours. After a tendril has revolved for a time it bends toward the dark, so that if a grapevine be planted against a wall the tendrils reach toward it, and in a vineyard they generally point toward the north. "The tendrils of the Virginia creeper exhibit," says Darwin, "no marked or regular revolving movement, though they show a decided tendency to turn from the light toward the dark." But the vital force which they save by thus living in comparative quiet and ease seems expended in movements, few and slight, yet strangely like those of a reasoning creature. These tendrils have generally several branches, each instinct with vitality. When they meet with a flat surface they all turn toward it, spread themselves as far apart as possible, and bring their hooked tips into close contact with it. "In effecting this," says Darwin, "the several branches after touching the surface often rise up, place themselves in a new position, and again come down into contact with it. In the course of about two days after a tendril has arranged its branches so as to press upon any surface, its curved tips swell, become bright-red, and form on their under sides little disks, or cushions, with which they adhere firmly." As these disks soon fasten themselves to smooth surfaces, naturalists believe that the vine secretes a little resinous vegetable cement, by means of which the tendril tip sticks fast to the spot which it has selected. After it has made sure of its hold, the tendril contracts spirally, and by so doing it draws up the branch upon which it grows. Coiled thus, it is as elastic as a steel spring, and when the main stalk of the tendril is pulled the strain is distrib-uted equally among all the attached disks. Withered branches of the Virginia creeper continue to cling to their supporting wall years after their death, despite the tugging winds of many winters and the softening influences of the rains of many springs. Darwin experimented upon a single lateral branchlet of a tendril supposed to be at least ten years old, and found that it supported a weight of two pounds. "The whole tendril had five disk-bearing branches, of equal thickness, and apparently of equal strength, so that, after having been exposed during ten years to the weather, it would probably have resisted a strain of ten pounds".
But a tendril which has found no support undergoes no further development. It was "adapted" to catch and cling, and as it has failed to fulfil its office there is no further use for it in the vine's economy. For, whatever society may do, Nature tolerates no shirks. The useless tendril, in the course of a week or two, shrivels into a slender thread, drops off, and drifts away like summer leaves in October.