Its young flowering branches grow out towards the light away from the main stem, and the yellow withered stem in autumn rests upon the hedge just as a piece of string laid upon it might do.
The Bramble and Rose manage to get a support in very much the same way, but in Great Britain the Bramble generally grows in open ground and its branches take root.
The peculiar, curved-back prickles of the Bramble and its arching sideways growth would of course hang it on to any horizontal branches in the neighbourhood. Kerner measured the length of the stem of a Bramble which had interwoven itself into the boughs of a tree, and found that it was over twenty feet long, although it was only one-third of an inch thick. In Chile one often finds hedges of Brambles ten to fifteen feet in height, which have been formed by the aid of other plants, and also by the way in which the branches become entangled with one another.
Some Climbing Roses act in a very similar way, especially if grown on trellis, but the flower shoots always turn to the light like those of the Galium.
But it is the creepers and lianes of the tropical forests that are the most remarkable of all climbing plants. They twine round the stems and hang in great loops and grotesque folds from the branches. Sometimes in the dense shade it may be difficult to see the main stem, for it is quite thin, though as strong as a piece of steel wire. It often happens, when hurrying through a rather open part of the forest after game, that one's leg suddenly catches in a thin, spiny, wiry stem of Smilax or some such creeper. The first that one knows of the creeper is when a quarter of an inch of the spine is buried in one's flesh.
Away up amongst the branches and foliage far above one's head, leaves and flowers are developed on numerous branches which have vigorously pushed out as soon as they got near the sunlight, this tough, spiny, thread-like stem being their only connexion with the ground.
The development of these climbing plants is probably connected with the dense shade of forests. In such places a young stem growing up will become long and drawn out; its tip will droop over and hang downwards. But there is a curious peculiarity in the growth of all stems. The stem generally grows more rapidly at any one time on one side, say on the north, and therefore bends over to the opposite side. After a time it will be growing most rapidly on the eastern side and then its head points westwards, and so on. The result is that the tip of the stem swings in an irregular circle round the stem itself. Its head turns to every point of the compass in succession. Supposing a stone is tied to the end of a piece of string, and one swings the stone horizontally in a circle, then, if an upright stick is put in the ground and the string comes against it, the string will coil itself round the stick because the stone goes on swinging horizontally.
Our young climbing plant in the shade of the forest acts in exactly the same way. If there is any trunk of a suitable size, it will in the course of its revolving or sweeping round first touch and then coil itself round and round the trunk.[138] Of these twining stems, one of the most interesting and beautiful is the common Hop. The young shoots or suckers which come from the ground may be seen waving their stems helplessly round in the air. If they cannot find something to cling to, then they form weak limp curves, but if one such shoot touches a pole it very soon obtains a hold, wraps itself round the support, and easily climbs up to a height of many feet. But the Hop is worth examining closely. If one passes the fingers along the stem, it feels rough and prickly. With the aid of a hand-lens, a whole series of most exquisite little hooks will be discovered. They are like small pimples with two or three very fine and minute, sharp grappling-hooks on the top. These prevent the stem from slipping off. It is also helped in climbing by its leaves, which curve outwards, and are also provided with grappling prickles on the under side. At the top of the stem the young leaves are close together, and folded near the point, so as not to interfere with the tip finding its way in and out of a trellis-work or amongst branches.
These grappling-hooks on the Hop are as perfect in their way, though by no means so beautiful and elegant as those which are found in the climbing palm, Desmoncus, so well described by Kerner in his Natural History of Plants. It is one of the rotang palms which reach lengths of 600 feet, though their stem may be no more than 1-1/3 to 2 inches thick. The leaflets towards the end of the leaf are transformed into strong spiny barbs which are exquisitely adapted to hang on to other plants. In many places, thickets in which these rotang palms have developed are so matted and tangled together that it is quite impossible even to cut into them, and they are practically impenetrable.