Page images

most potent of manures, the most concentrated vegetable food, — which the rain immediately washes down to the soil.

It appears, then, that all the essential materials for vegetation are furnished by the atmosphere, and are brought to the plant in the most available form ; that plants actually derive their nourishment from this source ; and it might be shown, that all the constituents of vegetables (excepting the small earthy portion, that many species can do without) have, at some period, belonged to the atmosphere. Hence, vegetables may not improperly be considered as formed out of air, or as so much solidified atmosphere! They represent an amount of matter which the force of organization has withdrawn from the air, and confined for a time to the surface.

If this be true, as it certainly is, we might hastily infer, that the soil merely serves as a foothold to plants; and that any given vegetable does in fact obtain all its nourishment directly from the atmosphere. The too unguarded statement of so distinguished a chemist as Professor Liebig seems to tend to this conclusion, and has been thus understood ; partly, as we think, from some misapprehension of his real meaning. Indeed, this view would appear to be fully borne out by such facts and considerations as the following.

Vegetation must have had a beginning. Science and sacred history equally point to a time, when no vegetable matter existed in the soil which overspread the earth's surface. The first plants must have grown without the aid of either vegetable or animal matter; they must have derived their whole organic food directly from the air. Nor are we left to speculations which go back to the origin of vegetation. This process is often repeated on a small scale, even in our own times ; as when small volcanic islands are raised from the sea, and the bare inorganic soil is gradually clothed, first, perhaps, with humble lichens, then with mosses, serns, and rushes, and at last with common herbage, shrubs, and trees, - forming, in time, a vast accumulation of vegetable matter, where none at first existed. So, likewise, after an eruption of molten lava or volcanic ashes from Etna, heated so as to destroy every vestige of organic matter that we can imagine to have existed therein, the soil thus produced is often covered with rich vegetation in less than a century.

or waves.

In all these cases, no vegetable matter has been conveyed to these perfectly sterile mineral soils, except the minute portion contained in the seeds, &c., wafted thither by winds

And yet a vast quantity has been produced, and is represented not only by the existing vegetation, but by the rich mould imparted to the soil by the decay of previous generations. What is the source of this vegetable matter? The necessary materials exist in the air. Plants possess the peculiar faculty of drawing them from the air. The air must have furnished the whole.

We may submit this view to the test of actual experiment, by causing any common seed of known weight – a garden bean, for example - to germinate, as it readily will, on pounded Aints, watered with rain-water alone. When the plant has attained the fullest development of which it is capable under such circumstances, it will be found to weigh many times as much as the seed from which it sprang. It may be supposed to derive a portion of its weight from the flinty soil. Let its ashes, therefore, be deducted, and its carbon alone taken into the account. This element, which may have increased perhaps fifty or a hundred fold, can have been derived only from the carbonic acid brought to the plant by the rain-water and the air.

Many striking confirmations of this fact fall under the notice of the agriculturist. He well knows, that, when an impoverished soil is laid down to grass, the longer it remains in undisturbed vegetation, the richer in vegetable mould does the soil become. In our forests, also, a vast amount of vegetable matter is accumulated in the trunks of the trees. Do they draw this from a stock in the soil ? How is it, then, that the vegetable mould in the soil increases with the age

of the forest ? As a whole, they must draw from the air not only the carbon which their trunks contain, but an additional quantity, which they impart to the soil, in the annual fall of leaves, &c.*

This point may be more strikingly presented by directing our attention to the case of a single tree. Let us take, for example, one of those gigantic Oregon Pines, which were noticed in a recent number of this Review, t - that tree in every

* These points are well stated by Professor Johnston, in his Agricultural Chemistry, Lect. IV., from which we have taken many of the foregoing il. lustrations.

North American Revier, Vol. LIX., p. 223.

whose prostrate trunk Douglas found to be two hundred and fifty feet in length, and tapering regularly from a base of fiftyeight feet in circumference. The solid contents of this trunk we may estimate, in round numbers, at 17,000 cubic feet. The weight of its wood, when seasoned, supposing it to have the specific gravity of our White Pine, — which it somewhat resembles, — would be 714,000 pounds. Of this, four pounds

thousand must be allowed for the ashes, the mineral constituents which have certainly been derived from the earth (2856 lbs.) ; leaving for its proper organic materials 711,144 pounds.

Taken as mere dry pine wood, half of this amount is carbon, namely, 355,572 pounds. But this tree abounds in turpentine and resin, — substances which contain a much greater proportion of carbon. We allow, to make round numbers, 428 pounds for the excess of carbon in this form. Disregarding the other elements, then, this tree has accumuluted 356,000 pounds, or 178 tons, of carbon or charcoal in its trunk. Whence has this large amount been derived ? From the ground ? A thousand years ago, when the seed germinated which has given rise to this giant tree, the soil of the limited space its roots occupy could not, certainly, have contained the millionth part of the carbon which is now stored up in its trunk. It may not have contained a particle; for the Pine will thrive upon a sandy, sterile soil ; and these very trees, says Douglas, were actually growing “ in nearly pure sand." In any given year, this tree must have returned to the soil, by its falling leaves and cones, quite as much as it took from it. Was it furnished with carbon from the falling leaves or decaying trunks of neighbouring trees and plants? A portion, certainly, may have been so supplied, in any given case ; but this only removes the question a step further back. Besides, this tree stood in a forest of similar Pines, – many of them equally, and some still more gigantic and venerable, — all of which have been accumulating carbon for hundreds of years at the same rate. There can be but one answer to this question. The immense stock of carbon, not only of this tree, but of the whole forest, has been drawn from the air.

And in a similar manner we might show, that the whole amount of all the organic elements of all plants was originally derived from the air. But does it therefore follow,

that each and every plant actually draws all its food directly from the air ? In certain cases, undoubtedly. The Epiphytes, or Air-plants, having no connection with the soil, live necessarily and directly on the atmosphere. So do those plants which first vegetate on volcanic earth, bare rocks, naked walls, or in pure sand. But we must particularly remark, that only a comparatively small number of plants will grow, or continue to live, under such circumstances; and, especially, none of the vegetables useful as food for man, or the higher animals, will thus thrive and come to maturity.

We have also seen, that the plants which do grow in this manner, as well as the undisturbed forest and grass land, deposit vegetable mould, and thus gradually make a soil rich in organized matter. Is this vegetable mould useless, and so left to escape, by slow decomposition, into the air from which it has been so laboriously withdrawn ? Certainly not. Nature does not thus waste her strength, or relinquish the advantages she has gained over inorganic nature. The races of plants that may thus grow at the entire expense of the air gradually form a soil of vegetable mould during their life, which they increase in their decay; and their successors live more healthsully upon the inheritance, being supported partly upon what they industriously take from the air, and partly upon the ancestral accumulation of vegetable mould. In this way, each successive generation may enrich the soil ; for, although drawing perhaps a large part of its nourishment from this store, yet it annually returns a portion by its dead leaves ; and when it dies, it bequeaths to the soil not only all that it took from it, but all that it drew from the air.

It is in this manner, especially, that the humble lichens, mosses, ferns, and other plants which short-sighted man terms useless, play an essential part in the economy of nature. They can live directly upon the air. Their minute seeds, quite invisible to the naked eye, and in number far surpassing man's power of computation, - light almost as the air itself, are widely scattered by the winds over mountain and plain, and lodged upon every naked rock, or stagnant pool, or tract of barren sand, where all they need is moisture to excite and maintain their growth. Some, like the lichens, require even little of this. They attach themselves to dry rocks, or plains of lava, which are washed only by

the occasional shower ; and here they make the earliest inroads upon barrenness. Not only do the accumulated remains of successive generations fill the crevices with fertile mould, and the water, which it holds like a sponge, by its freezing and thawing, aids in the disintegration of the rock ; but many of them create, from aërial elements, oxalic acid, a powerful solvent, which, as it is gradually set free, acts upon and excavates the stony surface to which the plant firmly adheres. Thus the dying lichen digs for itself in the solid rock a sepulchre, in which its dust inay rest. Well did Linnæus, in his lively fancy, term the lichens vernaculi, or bond-slaves, chained, as it were, to the rocks which they labor to cover with soil for the benefit of others, though they derive from it no nourishment for themselves.

Next, certain tribes of mosses take possession of this scanty soil, and more rapidly increase its amount ; while others, of aquatic habits, convert shallow pools into quaking bogs, and create in time vast accumulations of vegetable matter in the form of peat. These are truly servi, as Linnæus termed them ; servants to the nobler plants which soon succeed them. For then come the ferns, (the novaccolæ, or colonists,) after lichens have furnished them a foothold upon the rock, or mosses given them one upon the

spongy and unstable morass. And these are the

precursors of grasses, rushes, and other plants of more immediate importance to man and the higher animals ; but which never would come to perfection, if left, like their humble but indispensable predecessors, to draw an unaided subsistence directly from the inorganic world. It is thus that the bare earth is at length enabled to support the sturdy forest, or the rich field of waving grain.

While it is strictly true, therefore, that all the organic elements, or the food of plants, have been originally derived from the air, it is not true, that what is contained in any given plant, in any one crop we raise, is directly drawn from the atmosphere alone. "A plant grows partly at the expense of the vegetable matter in the soil accumulated by preceding vegetables, and partly at that of the air. When it reaches maturity, or when winter arrives, it dies; and of its decaying structure, a part is resolved into air, and returned to the atmosphere ; a part remains, and is incorporated with the soil. If that which remains be greater than VOL. LX.No. 126.


« PreviousContinue »