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Establishment of Vegetation at the Surface of the Globe * WE have seen vegetation covering, with verdure and flowers, all parts
of our globe; we have seen it extending itself from the bottom of the valleys to the most elevated places, resisting, in the plains, the burning rays of the sun, struggling upon the mountains with the frosts, bursting forth every summer from beneath the snows, and only stopping short at the zone of perpetual ice. But how does this vegetation come to cover the nakedness of rocks, to fix the mobility of sands, to implant itself in the strong gravel, to convert immense lakes into marshes, and these again into forests and fields ? for such was, and such still is, the surface of the globe, in all places destitute of vegetation, whether in islands which have newly sprung from the bosom of the waters, or in tracts where the soil has been overturned by particular accidents, or deprived, by other circumstances, of their ancient verdure; such, also, do we find it, if we remove the layer, more or less thick, of earth which clothes it. This earth is, therefore, of new formation, as well as the vegetation which it supports ; it has not been formed simultaneously with the rock on which it rests, or with the bed of sand which it covers.
This important observation is commonly overlooked. Accus tomed to see the same flowers re-appear at each return of spring, the same meadows clothed again in fresh verdure, we scarcely
* Of all the branches of Natural History, undoubtedly Botany is that which has hitherto (mineralogy and geology now dividing with it the public attention), in Britain, been the most generally cultivated : hence every where we find splendid gardens and conservatories ; and numerous works on botany are daily issuing from the press. Distinguished botanists have not been wanting in England: and Scotland, although behind in this science, has given to England several young, intelligent, and active botanists,—to Europe its greatest botanist, our illustrious countryman Brown : but, strange to say, the only naturalists who have actively embarked in the botany of Scotland, have been principally Englishmen or foreigners. Scotland offers still a fine and unexplored field to the’philosophic botanist,-in the investigation of the physical and geographical distribution of its land and aquatic plants. He who shall undertake this highly interesting investigation, must be intimately acquainted with the facts and reasonings of meteorology and hydrography, with the details and views of geology; and the ardent inquirer into the geography of plants cannot expect to illustrate it, without also knowing the na. tural history of animals; and, finally, he must be familiar with the use of the barometer, and other instruments.-Edit.
reflect upon the origin of this beautiful and abundant vegetation, or rather referring it to the period of the general creation of beings, it seems to us to lose itself in the mysterious obscurity of the formation of the universe ; and we thus find ourselves discharged, as it were, from the task of inquiring, by what means nature has everywhere diffused this precious mould, the source of riches and of life, and which yet is but the residue of generations heaped upon generations. Here an objection presents itself, which appears, in part, to destroy what I have advanced. If vegetable earth, it may be said, is necessary to the existence of plants, it must have been created previously to their existence, and can only receive what it had itself furnished them.
Such has been the error, which, during a long series of ages, has prevented our understanding one of the greatest operations of nature, and which, although continually before our eyes, has only escaped our observation from the little attention which we have bestowed upon an order of plants considered as little worthy of regard from their homeliness of aspect, their diminutive size, and the simplicity of their composition; but when the piercing eye of genius determined their relations in the natural order of things, when it recognised the functions which they had to fulfil, and the rank which they occupy in the general system of vegetation, they assumed a character of grandeur, which directed the attention toward their existence. It has been discovered, that, so far from requiring vegetable earth for their subsistence, they have furnished it by their decomposition, in small quantity it is true, but yet sufficient for the reception of plants of an order somewhat higher, and to which, in proportion as the vegetable earth increases, succeed vegetables much more vigorous.
To explain what we have to say upon this subject, we must fix our notice, for a moment, upon those plants which I have said to be the basis of vegetation. Although very common throughout nature, they have scarcely been remarked. They everywhere invest walls, rocks, humid places, and the trunks of trees; they attach themselves to all substances, however little they may be favoured by circumstances. The rays of the sun, and dry and cold winds, are as much inimical to them, as shade and humidity are favourable. These plants bear the names of conferva, byssi, and lichens. To them succeed mosses, hepaticæ, lycopo OCTOBER-DECEMBER 1826.
diaceæ, fungi, &c. They constitute a great and important family in the natural order of vegetation. Linnæus has named them cryptogamous plants, from the circumstance that the mode of fecundation, by which they are reproduced, is very little · known.
The byssi are plants which present themselves only under the form of a powdery tissue, or of a filamentous down, variously coloured ; they attach themselves chiefly to moist substances, dry up in the rays of an ardent sun, and leave behind them only formless and blackish spots. The conferva belong to stagnant waters and inundated lands; they are composed of capillary elongated filaments, simple or articulated. The lichens are sometimes nothing else than prominent blackish points, scattered upon a greenish or greyish ground ; at other times they are simple or branched lines, which have the semblance either of alphabetical characters, or of a sort of geographical chart, marked upon a very thin smooth membrane, applied to the bark of trees. Other species attach themselves to rocks, forming plats of various colours, leprous, granular, or powdery crusts; or assuming a greater degree of development, spread out into rosaceous expansions of a foliaceous aspect, with laciniated or lobated margins. Some of them rise from a scaly crust, in the form of simple stems, or ramify into small elegant shrubs, dilated at their summits into little cups, which are either simple or proliferous, and which are furnished upon their edges with fungous tubercles, of a brown or blackish colour, or of a beautiful scarlet red. Others present themselves under a very different form, falling from the trees in long intermingled filaments, like horse's hair or tufted locks; some of a greyish green, others of a beautiful gold yellow, orange or lemon. I shall not extend my remarks upon this class of plants, with which we shall have to form an acquaintance in another place, when we come to treat of the natural families. Here we shall speak of them only with relation to the great functions which nature has confided to them for the establishment of vegetation.
When we remark the hardness, the dryness, and the bareness of rocks, we should scarcely imagine that their summit might one day be crowned with forests ; and yet this great work is carried on every day under our eyes, and even in the midst of our habitations. We observe the walls covered with greenish spots, which grow from humidity, and which the light and heat reduce to black and tenacious spots; these are so many byssi which have essayed to establish vegetation there, as well as upon the most polished statues and marbles ; it is they which impress the seal of age upon our old castles and gothic edifices. Elsewhere, particularly upon rough stones, we see spreading out into broad plats those lichens of various colours, like the ulcerous crusts which corrode the skin of animals; they scoop out and corrode the surface of rocks, and deposit in the vacuities which they have formed, the portion of earth produced by their destruction. Although in very small quantity, this earth suffices to administer to the development of lichens of a higher order. Their debris, added to those of the former, furnish a small layer of earth sufficient for the existence of mosses of an inferior order, to which, in like manner, succeed more vigorous species *.
Already a turfy layer invests the tops of walls and the surface of rocks; it increases from year to year by the remains of the vegetables which it nourishes; its pulverulent particles are retained by the dense and tufted roots, and stems of mosses ; the moisture is long preserved in it; the layer of earth grows thicker; gramineæ, and other herbaceous plants, with low stems, begin to establish themselves, such as semperviva, drabæ, saxifrages, dandelions, some gerania, &c. The soil increases in proportion as the generations succeed each other ; it is converted, through time, into a meadow, visited by a great number of animals. Plants, with ligneous stems, announce that this newly formed soil will quickly receive larger vegetables, the multipli
• Those who have not directed their attention to the study of nature, will, perhaps, be very much astonished to be told, that all those black or greenish spots which invest the surface of statues and walls exposed to humidity, are true plants. These plats are formed by a byssus, to which Linnæus has given the name of Byssus antiquitatis. Stones which are constantly shaded and moist are covered with another byssus, of a beautiful deep green ; it is the Byrsus velutina, L.
The lichens, which ordinarily occur upon walls and rocks, are the Lichen calca. reus, pertusus, tartareus, candelarius, parellus, saxatilis, centrifugus, crispus, omphalodes, parietinus, pustulatus, &c.
The mosses which occur upon old walls are the Mnium setaceum, capillare, &c.; Bryum apocarpum, striatum, rurale, truncatulum, murale, cæspititium ; Hypnum sericeum, serpens, myosuroides, &c.
cation of which must ultimately establish immense forests in a soil which might be thought to have been condemned to perpetual sterility.
Such, upon these arid rocks, is the development of vegetation, begun by simple byssi, and some lichens, propagated by tufts of mosses, augmented by herbaceous plants. Their accumulated remains have formed this vegetable mould, now sufficiently thick that the most vigorous trees may drive their roots into it. Following in this manner the progress of vegetation, we have convinced ourselves, that vegetable earth is nothing else than the result of the annual decomposition of vegetables, and that without them it could not have existed ; that nature alone, and not human industry, could have deposited it upon the rock, or the old wall where we have observed it, and where its formation is in a manner executed under our eyes.
We shall not yet leave those forests, whose establishment we have followed, from the humble grass or the creeping moss, to the production of the largest vegetables. What an abundance of earth is furnished every year, by the fall of their leaves, and the other remains of vegetation! It is from this vast magazine, incessantly renewed, that nature derives the substances necessary for fertilizing the plains and valleys. To transport these materials, she makes use of the vehicle of water, of those tempestuous rains which precipitate themselves in torrents, or descend in sheets from the summits of the mountains into the deepest valleys. These waters carry with them the spoils of vegetation, and cover with them the plains which are frequently sterile, cretaceous, sandy, or stony; their fertilization, without this means, might have cost Nature ages of labour.
But the plants which lay the foundations of vegetation upon the rocks, being destitute of roots, could not exist upon arid and mobile sand, to fix the mobility of which, another order of vegetables is required"; this also has been produced. In place of byssi and lichens, which require a fixed and solid base, we find, as the first plants, several species of gramineæ and cyperaceæ, whose filiform and cespitose roots are interlaced with one another, bury themselves in the sand, bind it together, mingle their remains with it, and render it adapted for the reception of vegeta