animals, as distinguished from inorganic things, or minerals; and hence it is called organic structure. The cell is "the simplest form of organic life," whether vegetable or animal. Cells are the units of which the most complicated organic tissues are made up. They are essentially different, then, from the particles into which an inorganic or mineral substance may be divided. Each of these particles has all the properties of the mass from which it was separated; and if we divide it into yet smaller particles, the same will be true of those; and so on indefinitely. But if we cut a cell in two, the halves do not have the properties of the whole; they are not smaller units, but the fragments of a unit.

148. The Food of Plants. Plants get their food from the earth and the air, but much the greater part from the air. We know that there are plants which flourish in the most barren soil, or even upon the naked rock, and that some live and grow suspended in the air, and having no contact with the earth. Even those which demand a rich soil are indirectly indebted to the air for what they draw from the earth. The soil owes its fertility to the decomposition of organic matter; and this organic matter was originally produced by plants which had no rich soil to draw from, but were dependent mainly upon the air.

The atmosphere, then, is the storehouse from which plants directly or indirectly obtain nearly all their food. The portion which is purely of earthy origin is always insignificant, and often it is nothing at all. In fact, plants give to the earth far more than they get from it. This is illustrated by the accumulation of vegetable organic matter in the soil wherever vegetation is undisturbed from year to year. In uncultivated fields and in primeval forests we often find a great depth of rich mould. The more rank and luxuriant the vegetation, the more

rapidly this deposit increases, showing that the plants not only restore to the soil all that they have drawn from it, but are continually transferring fresh matter from the aerial storehouses to the earth.

But while the soil is enriched by undisturbed vegetation, it is impoverished by agriculture. The farmer carries away the crop from the field, with all that it has

taken from both the earth and the air. The land cannot yield in this way year after year, unless he follows the example of nature, and restores to the soil an equivalent for what he removes. This he can do by the use of

manure.

149. The Earthy Portion of the Plant.-If we burn wood or any other vegetable substance, almost all of it is dissipated into air. But a little ashes will remain; and these represent the earthy or inorganic portion of the plant. They consist mainly of alkaline chlorides, potash, soda, silica, metallic phosphates, calcic and magnesic carbonates, and ferric and manganic oxides. These are dissolved in the water which soaks through the soil and which is taken up by the roots of the plant. Much of the water is evaporated through the leaves, but the substances which it held in solution remain behind, and thus gradually accumulate in the tissues of the plant.

Since the plant must obtain from the soil the inorganic materials it needs, it is evident that it will flourish only in a soil containing those materials. This explains why certain plants thrive only in certain situations. A locality may be fertile for some species of vegetation and barren for others. The pines, which need little alkaline matter, will flourish in a sandy soil containing little alkali; but the maples and elms, which require a good deal of potash, cannot live in such a soil.

We have said that the farmer, who carries away the produce of the field with all that it has drawn from the

soil, must restore in the form of manure an equivalent for what he removes, or the field will soon become impoverished.

"A medium crop of wheat takes from one acre of ground about 12 pounds; a crop of beans, about 20 pounds; and a crop of beets, about 11 pounds of phosphoric acid, besides a very large quantity of potash and soda. It is obvious that such a process tends continually to exhaust arable land of the mineral substances useful to vegetation which it contains, and that a time must come, when, without supplies of such mineral matters, the land would become unproductive from their abstraction. . . . In the neighborhood of large and populous towns, for instance, where the interest of the farmer and marketgardener is to send the largest possible quantity of produce to market, consuming the least possible quantity on the spot, the want of saline principles in the soil would very soon be felt, were it not that for every wagon-load of greens and carrots, fruit and potatoes, corn and straw, that finds its way into the city, a wagon-load of dung, containing each and every one of these principles locked up in the several crops, is returned to the land, and proves enough, and often more than enough, to replace all that has been carried away from it." BOUSSIN

GAULT.

This renewal of fertility is sometimes attained by letting the field lie fallow, or uncultivated, for one or more years. The inorganic materials of the soil are mainly furnished by the gradual disintegration of the rocks; and while the field lies fallow this process is going on, under the influence of the oxygen and carbonic acid of the air, aided by the rains and changes of temperature. In this way, fresh portions of the rocks or of their ruins are rendered soluble and thus fitted for the nourishment of plants.

An alternation of crops may answer the same purpose as letting the field lie fallow. For instance, wheat and potatoes may be raised on the ground in alternate years. The wheat requires a large amount of silica and alkaline matter, while the potatoes take up no silica. The renewal of the soluble silica in the soil therefore goes on, while the potatoes are growing, as it would if the field were fallow.

Some soils abound in silicates so readily decomposed, that in every one or two years a sufficient supply for a crop of wheat becomes soluble. In Hungary there are large districts where wheat and tobacco have been raised alternately upon the same soil for centuries, the land never receiving back any of the mineral matter which is carried away with the crops. On the other hand, there are fields in which the amount of soluble silica required for a single crop of wheat is not separated from the insoluble masses in the soil in less than three or four years.

150. The Organic Portion of the Plant. When we burnt the vegetable matter, much the greater part of it passed off into the air. This was the organic portion of the plant. It was taken from the air, and the burning has given it back in the very form in which the plant found it there.

We have learned that all the vegetable tissues, however varied in their texture and consistency, are almost entirely made up of the same kind of cells. The substance of which these cells are made is called cellulose, or woody fibre; and it is a compound of carbon, hydrogen, and oxygen, C12H2O10. We have also learned that the plant contains small quantities of albuminous compounds (139), which, in addition to the elements just named, contain nitrogen. These four elements, then, are essential to the growth of the plant, and must be contained in its food.

151. The Plant gets Hydrogen and Oxygen from Water. The hydrogen and the oxygen of the cellulose are obtained from the water which the plant takes in, not only through its roots, but through its leaves. It will be noticed that water contains hydrogen and oxygen in the same proportions as cellulose does.

152. The Plant gets Carbon from Carbonic Acid.We have learned that carbon is a solid, and insoluble in water. Even if it were reduced to the finest powder and mixed with the water, it could not be taken up by the plant, since nothing but liquids and gases can pass through the walls of the cells. It must, then, be furnished to the plant in some liquid or gaseous compound, like the carbonic acid, which, as we have seen, is one of the gases mixed in the atmosphere.

Now if a leafy plant be placed under a glass vessel and set in the sunshine, and a stream of carbonic acid be made to pass slowly over it, it is found that a part of the carbonic acid is removed and replaced by oxygen. The plant absorbs the carbonic acid, decomposes it, retains the carbon, and exhales the oxygen.

We conclude, then, that it is from the carbonic acid in the atmosphere that plants get their carbon. In the leaves, under the influence of sunlight, the carbonic acid is decomposed, the carbon stored away in the plant, and the oxygen given back to the air.

Since carbonic acid is soluble in water, it is probable that plants also obtain a part of their carbon through the roots. Plants which live under water must get all their carbon from the gas dissolved in the water.

153. The Plant gets Nitrogen from Ammonia.We have learned (123) that there is ammonia in the atmosphere, and (37) that this gas is very soluble in water. Hence it is washed out of the air by the rain, and, thus dissolved in water, is taken up by the roots of plants.