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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 Byseus antiquitatis. Stones which are constantly shaded and moist are covered with another byssus, of a beautiful deep green; it is the Byssus velutina, L.

The lichens, which ordinarily occur upon walls and rocks, are the Lichen caleareus, 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

bles suitable to the temperature of the localities, provided they be frequently watered by rain.

The circumstances which subject sand to the power of vegetation do not everywhere exist; there are even vast countries where the earth appears condemned to present to its inhabitants. nothing but a dry and burnt surface. Such are those immense plains of Africa, those dreadful deserts, the countries of silence and of death, which man traverses only with fear, but which Nature may yet, by means of certain local circumstances, bring to a state of life, as she has done in many other places. The most efficacious, in fact the only means of doing this, is the presence of water. We already know, that several great rivers carry their waters through them, such as the Nile in Egypt, and the Niger in a part of the Sahara. The springs which feed them, enlarged by the rains, occasion, every year, considerable overflowings. These superabundant waters deposit, upon the lands which have been inundated by them, a mud which, by being mingled with sand, acquires a great degree of fertility; in other places they form seas, lakes, and pools, which carry the principles of life into those countries of death.

A new order of plants meets us upon the edges, and at the surface of these lakes. We can easily imagine, that those which have established vegetation upon the sandy or stony soils could not here fulfil the same object, and we shall see this all-powerful Nature overcoming with time, the obstacles which oppose themselves to its operation. When the waters have covered a piece of ground, plants almost immediately begin to appear; they are more or less abundant, according to circumstances If these waters are running like those of rivers, or agitated like those of great lakes, vegetation only exists upon their edges; but if they be tranquil, stagnant, and of little depth, plants grow in them more numerously, and with more rapidity; they at first cover the surface of the waters, and occupy, from the simplicity of their organisation, the same order as those which grow upon rocks; they are merely very delicate, interwoven filaments, without roots, and without apparent fructification. They precede the growth of more perfect vegetables, and prepare the soil which is to receive them,-an operation which we may equally observe without leaving our houses. If we examine neglect

ed or abandoned basins of water, we find them covered with a greenish scum, which, for a long time, was considered as consisting of impurities thrown out to the surface of the water, but which, if observed with more attention, we shall easily be convinced, belongs to the vegetable kingdom. The substances of which this scum consists are designated by the names of conferva and byssi. Duckweeds (lemnæ) and callitriches accompany or succeed them. These plants, which are destitute of roots, form, by their interlacement, a sort of floating sward, the remains of which are precipitated to the bottom of the water, and constitute the soil destined to receive plants of a superior rank. After this potamogetons, charæ, and myriophylla line the interior of basins and lakes, extend themselves into meadows constantly covered with water, and reserved for the nourishment of a great number of aquatic animals.

In proportion as the bottom is raised, more vigorous species appear above the water, and develope those beautiful corollas, the brilliancy of which vies with that of the flowers of our gardens. The liquid plain is converted into a parterre embellished with tufts of floating ranunculi, naiads, hydrocharides, vallisneriæ, surmounted by the ample calices of silver, gold or azure of the nelumbos, and nuphars, with broad and varnished leaves, while the sagittariæ, flowering junci, menyantheses, huttoniæ, &c. form upon their edges an elegant and varied border, to which are joined beautiful veronica, cnanthæ, phyllandræ, surmounted by salicariæ, bidentes, cupatoriæ, &c.

Thus the waters, as well as the bare and stony part of the globe, are peopled with vegetables, which convert into marshes those liquid plains upon which have formerly floated the barks of fishermen. These waters gain in surface what they lose in depth, and carry fertility to all the surrounding grounds. In proportion as they are lowered, we see beginning to grow those species which in some measure hold an intermediate place between aquatic and terrestrial plants, such as large gramineæ, reeds, poas, carices, scirpi, rushes, typhæ, &c., but no plant contributes more to the conversion of these marshes into pasture grounds, than the prevalence of certain species of mosses, especially sphagna, which rise in yearly layers above one another, and daily increase in thickness as well as in extent. If these

waters, absorbed by the power of vegetation, are not fed by springs in proportion to their loss, this marshy soil will by degrees be dried up, and will be covered in time with fertile mea

dows and trees of all sorts, and will then be fit for cultivation.

What I have here said with respect to the gradual progress of vegetation is in no degree conjectural: we find its proof at almost every step, as well in the bosom of the earth as at its surface, especially in soil which has not been overturned by recent revolutions. In how many places do we not meet, beneath the bed of vegetable or argillaceous earth, ancient peat-bogs extended over strata of sand or heaps of rolled stones; an evident proof that this soil has formerly been traversed by the waters of rivers, or occupied by those of lakes. The vast marshes of the Somme furnish us with one example among a thousand. The soil is often covered, as M. Girard has observed, with a layer of earth adapted for vegetation, about two feet in its greatest thickness; the height of the bed of peat on which it rests is from six to ten feet thick between Amiens and Pecquigny; it increases to thirty feet opposite the villages of L'Etoile and Long, beyond which it gradually diminishes. The low part of the city of Amiens, according to the observations of M. Sellier, is built upon a bed of peat, which is sometimes more than twelve feet thick; it rests upon a bed of marl, which is itself supported by a bed of sand and pebbles, mixed with marine shells. This vast formation has therefore been long occupied by great lakes, as is proved by the discovery which has been made of several boats and Roman arms preserved in the peat at different depths.

We are not permitted, to follow the establishment of vegetation in the depths of the ocean; but if marine plants, like land or fresh-water ones, required to be implanted in an earthy or muddy soil, we should scarcely conceive how they could resist the destructive action of those roaring waves which incessantly overturn and drive before them every obstacle that comes in their way, sweeps the bottom of the seas, and heaps upon the shores the debris of rocks. To struggle with impediments so powerful, marine plants would require a peculiar mode of existence; nature has therefore awarded them a more solid base than that of a mobile sand, continually tossed about by the impetuous movements of the waters; it has fixed their abode

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