A regular duty of the foresters in India is to cut the stems of climbing plants. These twining, trailing, rope-like creepers are, in fact, natural ropes, and are used as such in India, Burma, and other places. Sometimes they form natural bridges of living plants extending across a stream. The great suspension bridges in the valleys of the Himalayas are sometimes made without a single nail or plank. They are just three ropes (one for the feet and two to hold on by) made of jungle creepers. Crossing one of these swinging, swaying creeper-bridges is not an easy matter for those whose heads are unaccustomed to depths of hundreds of feet below them, especially if combined with a motion of the creeper-bridge sufficient in itself to produce violent seasickness. Yet the natives run across them with loads on their heads!
But it is not necessary to go to the tropics to find interesting and ingenious climbing plants.
There is a very common little British plant, Stellaria holostea (the Star of Bethlehem, Great Starwort, or Stitchwort), which is common in shady places, light woods, and by hedges. In the spring it grows very quickly, and the pairs of leaves are shut together over the growing point, so that the end of the stem is narrow and can insert itself between the leaves and twigs of the neighbouring plants. As soon as such a growing end gets out of the foliage into the light, each pair of leaves opens out and curves backwards, making a pair of broad, curved hooks excellently suited to hang the stem on to the leaves or twigs. Then another period of growth follows, and again a new pair of hook-like leaves opens out. The stem may be five or six feet long.
In a rather rare Speedwell (Veronica scutellata) a very similar method is used, but the leaves have special little backward-pointing teeth on their edges which assist in the attachment process.
But these leaves are not to be compared as regards perfection of mechanism with the tendrils by means of which plants climb. These tendrils are thin, flexible, twining threads, which may be formed by the modification of whole leaves, in other cases of leaflets, or sometimes of branches. Sweet Peas, Vetches, Passion-flowers, Vines, and many other plants possess them.
They are like twining plants in the way in which they revolve or twine so as to wrap themselves round anything which they touch. They move much faster than twining plants. A Cobæa tendril only takes twenty-five minutes to make a complete turn, Passion-flowers take from half to three-quarters of an hour, and the Vine tendril takes a little over an hour to make one complete turn.
But in one way they differ altogether, for they are sensitive to contact. If tickled, they contract and embrace closely the object which is touching them. They show a most extraordinary sensibility and sensitiveness.
As a matter of fact, these tendrils have a finer sense of touch and a much more delicate feeling of weight than any human being. They detect the weight of twenty-seven inches of a spider's thread.
It is, however, best to explain what happens. A half-grown curved tendril of the Passion-flower is perhaps the most interesting to experiment with, but any sort of tendril does quite well. If one very gently rubs the inner or concave side of its little hook, then in a very few minutes, or even seconds, the tendril distinctly curves. If this has happened naturally, as when for instance it has been rubbing upon a pea-stick, this curve makes it curl round the stick, and the more it touches the more it curls, until the whole tendril is wrapped round the support.
It is, of course, quite impossible to explain it all exactly: the sensitive part on the inside of the curve differs from the outside or convex part of the tendril; the former has a layer of elongated, thin-walled cells, full of the living matter, protoplasm, which are absent on the outer side. Immediately the tendril touches the stick, the outer convex surface begins to grow rapidly. It grows from forty to 200 times as fast as the inner side which touches the stick! Very soon after it has clasped the stick the tendril becomes woody and forms a strong, woody, spiral coil.