Page:Popular Science Monthly Volume 25.djvu/666

650 probability is, that the plant or animal will continue to develop in that direction till it diverges widely from the original form. The struggle for existence will cause all the imperfect forms to be killed off, and only those will survive which are best suited to the altered conditions of life. Once let an organism begin to vary in any one direction, and there is no telling where or when it will stop. This much is certain, that it never ceases until the best results possible have been attained.

The chief characteristic, then, of the convolvulus family is the climbing habit. The origin of this habit is found in the fact that sunlight and air are two things needful for a plant's proper growth and development. In situations where these two things are found in limited quantities, plants with climbing habits and animals with arboreal instincts will abound. In Brazil, for instance, where immense tracts are covered with a dense forest-growth, it is noticed that all forms of animal life have become adapted to residence in trees. Many of them live there entirely. Monkeys seldom leave the tree-tops. Lizards and snakes and insects are there, and even man himself is often found living among the branches. So, too, plants form immensely long stems, reaching in many cases to the tops of trees a hundred feet high. The extraordinary development of climbing powers has been gradually acquired in the course of ages. In times and places where vegetation was not dense, and where the struggle for light was not great, plants of erect habit succeeded well. Then it was a conflict to see which could grow tallest. But when a weak plant found that, by taking hold of its tall and erect neighbor and by clinging to it, it could reach the sunlight much easier and by an expenditure of much less material than by growing erect itself, it was a great step on the road. This habit, being transmitted from one generation to the next, kept on improving. Less and less rigid, more and more flexuous stems ensued, and the delicate climbing vines of modern times are the results of this necessity of reaching sunlight with as little waste of material as possible.

There are many methods adopted by plants to climb. While some of them reach upward by means of tendrils developed at the ends of stems or leaves, others twist their petioles round the support, and still others twine their stems round other stems that may come in their way. This last is the method adopted by those of the Convolvulaceæ which climb at all. For even in this family there are some species which are erect in growth. The Calystegia spithamœa is one of them. Others do not grow up into the air, but trail along the ground or over low plants, and thus secure their due share of sunlight. Others, again, climb freely, and this is the case with the dodder.

The climbing bitter-sweet is said to sometimes strangle the trees upon which it grows. The constriction caused by its growing stem is so great as to cut off the supply of sap from the roots, and cause the