Page:The American Cyclopædia (1879) Volume IV.djvu/750

734 plants which may be called bog plants, and which belong to a peculiar group of vegetables composed essentially of woody tissue and living either in water or above water, according to local circumstances. The species of plants forming peat in our time do not thrive out of the bogs, neither do land plants invade the bogs and contribute by their remains to the composition of peat. The bog plants demand first, for their establishment and their growth, a shallow basin of water with an invariable level. A basin of this kind is generally prepared in advance by the deposition of a clay bottom, produced from the decomposition of water plants or plants living entirely under water, whose tissue from this cause is not woody or fibrous; for the woody tissue of the plants is derived from the atmosphere, chiefly by the respiration of the leaves, and constitutes a part of the compound generally proportionate to the degree of humidity and the amount of carbonic acid of the atmosphere wherein they live. The conferves and the charas which fix lime or silex in their tissue and feed fresh-water mollusks, sometimes in immense number, are especially the plants which by decomposition form a kind of clay. In some circumstances this vegetation has, by its remains and those of the shells, established a thickness of muddy clay of 2 to 6 in. in a year. When this bed of clay has rendered the basin water-tight, it becomes a prepared ground for the growth of other plants, which, rooting in the soft bottom, ascend upon long stems or long stalks to the surface, where they expand their leaves and open their flowers. These are at the same time aerial and water plants; their tissue is woody like that of some species of mosses, which appear at the same time, and floating at the surface absorb carbonic acid and water by their innumerable small leaves, and thus have in their compounds as large a proportion of woody matter as the hardest wood. Every year the remains of this vegetation are pressed down to the bottom, and successively heaped till they reach the surface of the water. Of course this first deposit becomes in time solid enough to receive other kinds of plants which root upon its surface: species of mosses, sedges, and trailing bushes; then larger shrubs, then trees, which, with the smaller species that continue to grow under them, increase each year by their debris the amount of material which, constantly heaped, constitutes what is improperly called the growth of the peat. This is the simplest and most ordinary proceeding in the formation of peat. The process of transformation of vegetable matter into peat is due to the presence of water, as is apparent from a consideration of the different modes of decomposition of vegetable matter as explained by chemistry. When wood is immersed in water and thus guarded against the action of the atmosphere or of the carbonic acid which causes its decay, it is preserved sound for a very long time. Roman constructions, even foundations of the lacustrine buildings of wood, have been dug out still solid and unimpaired by decay from lakes and swamps of Europe; but the wood has become entirely black. When the materials which enter into the composition of peat are growing in a deep basin of water, and their debris are heaped under water, the preservation of the matter against rapid decomposition is then the direct result of immersion. The growth of the peat in cases of this kind may be stopped at the water level, where by a more complete decomposition of the plants a coat of humus is formed, which being invaded by land plants is transformed into a prairie or a forest.—But more generally peat grows above the water level, and then the production of the matter and its protection against the influence of the atmosphere are essentially due to the agency of a peculiar kind of moss, the sphagnum. This moss, which in deep water vegetates in loose extensive mattings, extending over the surface like a vegetable carpet, becomes out of water transformed into compact tufts, and its long slender stems, then growing closely pressed against one another, are knit together. The sphagnum is in that state a veritable sponge, endowed with an extraordinary power of absorption. Not only does it draw the water from below by its long capillary stems, which, growing without interruption, are imbedded very deeply in the matter of the bog, but it especially imbibes it from atmospheric humidity by its innumerable small leaves, and thus is constantly saturated with water. Its growth is rapid; where more space is afforded, it extends its plants all around, covering the whole surface of the bogs and every kind of woody debris spread upon it. Its tufts go up the roots of the trees, and surround the standing trunks one to two feet high; they pass over the prostrate trees and their branches, and bury them under a thick carpet which preserves them against atmospheric influence. In foggy countries, as in Ireland and Germany, the sphagnum ascends steep slopes and builds its peat deposits from the plains to the tops of high mountains. This moss is indeed by itself a remarkable phenomenon in the economy of nature; being a kind of balancing power, absorbing the useless surplus of water from the ground, from the swamps, and from the atmosphere especially, using it in part for its growth, carefully husbanding it for the preservation and transformation of all the woody tissue, its own included, into peat, and in dry seasons distributing the remainder to feed the numerous springs which have their source in the bogs. Thus, the peat bogs, like the glaciers of some countries, feed mighty rivers. In the Mississippi, for example, the blackness of the water, which is preserved as far down as St. Louis, proves its origin. It is nothing but bog water; hence its extreme salubrity and its remarkable incorruptibility.—Along the low shores of some lakes and of the sea, the bog vegetation sometimes begins from