1158. The food of plants consists of carbon, oxygen, hydrogen, nitrogen, and various earthy and saline substances; and these they obtain from earth, air, and water. The atmospheric air consists of oxygen, nitrogen, and a small proportion of carbonic acid gas. It also contains hydrogen in the form of aqueous vapour and ammonia. The soil contains all the various kinds of earthy and mineral matters, and a proportion of organic matters; and water not only supplies plants with hydrogen and oxygen, when it is decomposed, but it serves as a medium for dissolving various salts and other substances, and enabling the spongioles of the roots to absorb them.