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T. PLANT'S AN ID Tlt EYES.
The quantity of plants, shrubs, bushes, and timber that grows on most lands, and especially in tropical forests, in the course of one century, must be enormous. , Were these masses of vegetation deposited in a sea, they would pile up into a hill of considerable size; but though timber grows and decays for thousands of years, yet no such wood mountains are found to pile on the sites of forests, even within the tropics. . It might have been expected that the masses of solid matter which are every day derived from the decay of terrestrial plants and animals would contribute to augment the amount of soil on the earth's surface. It must, therefore, awaken your surprise, when you learn that the vegetable mould which clothes the globe does not grow in thickness.
This statement ought to awaken not only your surprise, but
your attention. Ask yourself, what comes, then, of all the vegetable masses, and of all the animal matter, that rot in forests and woodlands, every year, over the extent of the globe: The answer of science is, that a portion of this vegetable and animal mass is volatilized into the air, and that the rest is carried away by running water, in which it either sinks into the earth, or flows towards the sea. By this process, the same ingredier:ts enter again and again into the composition of a variety and a succession of organic beings in vegetable and animal life. It is well known that thousands of carcasses of terrestrial animals, and immense forests of drift timber, are every century floated into the sea, where both are imbedded in subaqueous deposits. Nevertheless, the vegetable mould on the earth’s surface is kept in equilibrium. The principal elements tha chemists have found in plants are the three gases, hydrogen, carbon, and oxygen. Vegetables and animals derive them from water and from the atmosphere. But whence do water and the atmosphere derive them, in order to supply plants with them They derive them from the putrefaction of vegetable substances and animal matter, from the decay of rocks as the result of weathering and abrasion, and also from the agency of mineral
* which bring constant supplies from the interior of the earth.
In our chapter on Aqueous Agency, we considered the tendency of running water to scoop out gullies in the soil, and to carry the detritus towards the sea. This operation of streams and rivulets is counteracted by the power of vegetation. Vegetation counteracts the operations of running water in two ways. 1. It is in some degree antagonistic to the transporting power of rivers, and may be considered as reconstructive. The agency of vegetable life, by absorbing various gases from the atmosphere, causes a large mass of solid matter to accumulate on the surface of the land. Such a mass must, alone, constitute a great counterpoise to all the earthy detritus transported by the aqueous agents of decay. 2. The influence of vegetation is conservative, and tending to retard the waste of land. You constantly witness in a field, where a rivulet flows from a well, that the green swari through which the water runs prevents the soil fiom being
carried away. Upon sloping ground, also, it is seen that a cover-
II. PEAT AND MOSS,
Peat has not only a conservative influence to protect the underlying soil, but it has a reconstructive power, by which it augments the mass of vegetation on the earth. Peat is not the product of aly one species of plant, butt consists of any herb or moss that is capable of growing in moist and cold situations, and that has also the property of throwing up new shoots at its upper part, while its lower portions are decaying. Peat, as the remains of dead plants, is a product intermediate loetween vegetable matter and ligmite. It is first a plant, and it is turned into a lignite by a gradual and prolonged action of the water that covers it. Beds of peat are seldom, perhaps never, found within the tropics. Even in Spain and in the south of France they very rarely occur. In proportion as you advance from the tropics towards the north, beds of peat become more frequent, and its substance becomes more inflammable. This is also true in latitudes between the tropics and the south pole. The depth of peat soil and the number of peat bogs depend on the physical condition of the district. On the declivities of mountains, the depth of a peat bed rarely exceeds four feet. But in low and hollow grounds, into which peaty matter is constantly carried down by running water, the depth of a peat bog becomes forty feet and more. The extent of some of these bogs in the north of Europe is enormous. On the Shannon in Ireland there is a bog, or moss, that is fifty miles long and two or three-miles wide. France, between the city of Nantes and the sea, there is, about the mouth of the Loire, a bog that is one hundred and fifty miles in circumference. Many of the peat mosses are known to occupy the sites of ancient forests of pines and oaks, some of which have disappeared even Y, thin periods of history. Such bogs are formed by the fall of trees, and by the stagnation of water. Had these trees fallen in warm climates, the woody matter would be removed either by insects or by putrefaction; but, in Europe, having fallen in low, or moist situations, they are preserved by water. The agency of peat mosses is not always conservative of the surface Boil. In some districts where the climate is cold and moist, b :gs occasionally grow to be agents of destruction. A peat bog acts like a sponge. It absorbs water in large quantities, and swells to the height of many yards above the surrounding soil. The capillary action of the turf fibres enables it to retain the fluid for some length of time, but not for ever. It frequently bursts, and then a violent inundation follows, and the muddy torrent, as instanced more than once in Ireland, scoops out ravines in the slopes of hills, bears away blocks and timber, and scatters them over the plains, or deposits them in the nearest lake or sea. If such a moss bursts near an arm of the sea, that part of the sea becomes the receptacle of drift peat. On many coasts in the Baltic Sea and the German Ocean, we constantly meet with sections in which alternations of clay and sand with differ...,v1ayers of peat are of frequent occurrence. At the bottom of many of the mosses in Holland, the remains of ships, of oars, &c., have been found; and in the valley of the Somme in France, there was found, in the lowest tier of the bog, a boat loaded with bricks. These two facts prove that, at one period and that very late, these mosses were navigable lakes or arms of the Sea.
...thing, to stones, to wood, or to other sea-weeds.
III. SEA-WEEDS AND FUUOIDS.
The “unfathomed caves of ocean” bear not only “full many a gem,” but also full many a plant. Beneath the surface of the mighty sea, a magnificent vegetable world extends through all latitudes and longitudes, wherever a ray of light can penetrate, This world of vegetation is more wide-spread than the verdasi covering of the dry land. . Marine plants are exceedingly numerous, but they are divided into two great classes, or groups. First, the jointed kind, which embrace the species called CoNFERVA, very small plants which consist of simple tubes—some of them even microscopic. Secondly, the disjointed group, consisting of such sea-weeds as dulse, laver, Wrack, and all the gigantic species which either luxuriate in submarine forests, or float like green meadows or boundless prairies, in the ocean.
The sea-weeds that grow in comparatively small depths near the shore, are both the most limited in number, and the least extensive in distribution. The fuci that grow at the greatest depth of the ocean are both the greater in number, and the wider in extent of range. This circumstance suggests a correspondence of latitude, in the amount of vegetation, between height on land and depth in the ocean. On land, the lower the height or the latitudeis, the more luxuriant is the growth of trees. And, in the ocean, the go the depth, the more abundant is the growth of marine plants. .
The seeds of marine plants are produced in their native elemeat; they can therefore remain immersed in it for an indefinite period without any injury to their power of fructification. This fact enables us to account easily for their wide-spread diffusionin every zone where uncongenial climates or contrary currents donotinterfere with their distribution. Sea-weeds have well-contrived facilities for sowing themselves over the body of the ocean. First, they generally have hollow pod-like receptacles in which their seeds are lodged. Thése pods give them buoyancy for floating. Other seed vessels have this power of floating in consequence of filaments which are attached to them. Secondly, a very large number of species are enveloped with mucous or adhesive matter, like that which surrounds the eggs of fish. This mucus not only prevents them from injury, but serves to attach them to rocks or to floating bodies. These Bea-weeds fix their roots to anything and everyThis circumstance shows that they must derive all their nutriment from the water of the sea, and from the air contained in the water. Thirdly, naturalists have shown that these thalassophytes, or seaplants, are what is technically called proliferous ; that is, they are
In so full of power to propagate their kind, that the smallest frag
ment of a branch of them can develapitsélf into a perfect plant.
Some of the gigantic sea-weeds grow up to the surface of the ocean, and appear Hike green meadows of immeasurable extent. These oecur on each side of the equator in the Atlantic, Pacific, and Indian Oceans. The most extensive bank in the Atlantic is a little to the west of the meridian of Fayal, one of the Azores, between 35° and 36° of north latitude. When CoIUMBUs camé to it, he comparedit to a vast inundated field of grass, and he states that the weeds were so thick as to retard the progress of
|the yessel. This part of the Atlantic is called Mar de Sargasso,
or the Gulph-weed sea. It includes two banks of Fucus connected
. These insmense fields of marine plants must, like land vegetation, suffer decay-and-their decayed remains, as they subside and sink to the sea bottom, must, in the course of centuries, produce considerable beds, of vegetable matter. In Holland, a submarine peat was dug up that was formed by the decay of sea-weeds. You have now been introduced to the agency which vegetation ‘exerts in the formation and in the conservation of the soil that forms the green surface of the earth. The principles and the facts which have been thus briefly intimated, you must now apply not only to the superficial covering of the globe, but also to the structure of the crust of the earth, as formed at different geological periods. As you walk or ride over plains or mountains, an entire vegetable world may be lying in ruins under your feet. Geology has demonstrated that, at different periods or epochsin our world's history, vegetation has played a distinguished part, both in rank, luxuriance, and in extensive distribution. Different geologists have their respective systems for dividing the epochs of ancient vegetation. M. ADoLPHE BRONGNIART divides them into four. The first begins with the earliest traces of vegetable life, and terminates with the coal formation. The second concludes with the -triassic. The third comprises the oolite and chalk. The fourth ends with the tertiary period. Count STERN
tion of the coal period.
present themselves as arborescent plants with branched trunks, sixty or seventy feet high. The third kind of vegetation is that of horse-tails, or the Equisetaceae, distinguished in the engraving by having jointed and furrowed trunks and branches. With us, the largest plants of this kind attain but a very few feet in height, but in the coal measures they are found with arbqrescent or tree-like trunks, ten feet high and five or six inches in diameter. These three families of plants form about three-fourths of the vegetaThe remainder consists of cone-bearing trees, and of vast quantities of a kind, apparently, like the cactus. Of the entire number of species discovered in the carboniferous rocks, two-thirds belong to a vegetation like the Fern. In these coal forests, there were no plants bearing flowers, no
trees bearing fleshy, juicy fruits, no kind of grass, and no birds.
It used to be thought that it was a forest without a single living thing to move in it; but lately the skeleton of a reptile has been discovered in rocks much older than the coal series. One remarkable characteristic of the vegetation of the coal period is the uniformity or monotony of its plants. In our age we find that different countries, in different climates, produce different plants; but, in the carboniferous era, the same plants grew in Germany, Belgium, France, England, North America, and
*ERG, by uniting the second and third of these epochs, reduces the periods of ancient vegetation to three. His divisions are— j, the vegetation of islands; 2. that of sea coasts; and 3. that of continents. e e The earliest vegetation of the globe, and that which terminated in the carboniferous period, was simple but very magnificent. An ideal landscape of the earth during the carboniferous age is represented in fig. 97. Look at the forest represented in this engraving. The plants and trees are different from , all vegetable products that you now see. There is nothing like it in the temperate zones, nor within the tropics. The vegetation consists of ferns — but ferns not herbaceous as in our cold climate, but ferns which grow in the form of trees of considerable height, with palm-like, unbranched trunks. The next kind of vegetation is that of club mosses, or the Lycopodiaceae. With us, these club mosses are dwarf plants,
Australia. This fact proves a remarkable uniformity of climat at that period. When North America was discovered, there were found in it only two wild plants that agreed with the vegetation of Europe. But of 53 kinds of plants found in the North American coal beds, 35 are common in the European coalfields. However luxuriant this vegetation of the carboniferous era was: all the species of its plants, and almost all their genera, passed away before the second period of vegetation set in. A few ferns entered into the second era, but all the palms and calamites disappeared. The first flora, therefore, which was universally diffused over all the dry land of the angient globe, was especially distinguished from the second, which is regarded as the flora of the triassic, the oolite, and the wealden, group of plants. This second family passed imperceptibly into the third, which comprises 'the plants of the tertiary formations. In the trias the characters of vegetation are altered by the complete disappearance of the
small in size and few in number, but in the coal formations they
cactus tribe, by a diminution of the proportion of ferns, and by &
she appearance of a few new species. Of this triassic vegetation very
is of immense interest to the geologist, as it often encloses speci-
wood, will prepare you for understanding the fucoid fossils which
are found in ancient rocks, and for the vegetable remains found in the coal series. You have only to imagine layers of peat and deposits of drift wood to become bituminised, and the different seams of sand and mud between them to become consolidated by pressure from above and heat from below, to be able to account for their carbonisation, and for the structure of a genuine coal, formation.