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CHAPTER Y.

THE FOOD OF PLANTS—ITS NATURE AND SOURCES.

287. Having shortly described the elements of vege-table matter, and enumerated the principal compoundsof those elements which are found in plants, we mayat once proceed to consider the sources of the food ofplants; that is to say, the means naturally providedto insure to them a due supply of the various substancesnecessary to their growth, namely, oxygen, hydrogen,carbon, nitrogen, and the various earthy and saline sub-stances which are always found in plants.

288. There are only two sources whence it is possiblefor plants to derive these matters, namely, the air andthe soil; let us inquire what substances they can obtainin this manner, and how they avail themselves of thefood thus offered to them.

289. It has been already stated that the air at alltimes contains a small quantity of carbonic acid gas(28); it likewise always contains a still more minuteproportion of ammonia, which is constantly beingformed by decay. Here, then, we see that the air con-tains the four elements of organic matter; and when, inaddition to these facts, we remember that the air isalways more or less damp, it is easy to understand that