Page:Popular Science Monthly Volume 3.djvu/87

 leguminous plants at another; so that, in the course of thirty years, the author whose observations we are citing was witness of five or six such alternations.

It follows from all this that a plant, as was pointed out by the late Dean Herbert, does not necessarily grow in the situation best adapted for it, but where it can best hold its own against its hostile neighbors, and best sustain itself against unfavorable conditions generally.

The sources of success in the contest are manifold; they vary more or less in each individual case. Probably they are never exactly the same; nevertheless, there are certain circumstances which must always be operative in conducing to the victory. A few illustrations must suffice. It is easy to understand why first-comers, duly installed, should have an advantage over later visitants; why the more prolific should outnumber the less fertile; and how it is that a perennial plant has a better chance on any given spot, cœteris paribus, than an annual, whose progeny would find the ground occupied, and their chances of survival materially interfered with by their longer-lived neighbors.

Again, there is no difficulty in understanding why such plants as quitch (Triticum repens) or bearbine (Convolvulus sepium) hold their own so tenaciously, and so much to the prejudice of their neighbors. The long, creeping, underground stems, rooting, or capable of rooting, at every joint, give them an immense advantage over plants not so favorably organized. The ends of the shoots of the convolvulus, moreover, dilate into tubers, which are thrust into the ground, to form in the succeeding spring fresh centres of vegetation. A great rooting-power is obviously of great benefit; not less so is an extensive leaf-surface. It is not only that the copious feeding-roots absorb the available nourishment from the soil, not only that the wide leaf-surface avails itself of every ray of sunlight, every whiff of air that plays over it, and thus serves to build up the tissues of the plant to which the root or leaf respectively belongs, but they practically oust other plants less favorably circumstanced than themselves. The roots occupy the soil, and rob the weaker plants of their share of its resources. The tree with dense foliage shuts off from its lowlier neighbor much of the light and air necessary for its existence; and hence, in a measure, the absence of vegetation in pine-forests or under the shadow of dense woods. Some plants there are specially organized to resist and overcome these hostile conditions. Among them are the climbers, the twining plants, and those with tendrils of one sort or another. The bramble or wild-rose, with its slender, arching, hook-beset branches; the wild-hop, with its coils of cord-like sprays; the clematis, clinging on firmly by means of its leaf-stalks to any thing it can lay hold of; the ivy, grappling with the trunk of a tree—all these are, in some sense, weakly plants; they would be overweighted in the struggle with their stronger neighbors, if it were not for the special adaptation of