The oxygen in the air is necessary to the respiration of animals, and to the support of combustion (burning of bodies). The nitrogen serves principally to dilute the strength, so to speak, of the pure oxygen, in which gas, if unmixed, animals would live and combustibles burn with too great rapidity. The small quantity of carbonic acid affords an important part of their food to plants, and the watery vapour in the air aids in keeping the surfaces of animals and plants in a moist and pliant state; while, in due season, it descends also in refreshing showers, or studs the evening leaf with sparkling dew. There is a beautiful adjustment in the constitution of the atmosphere to the nature and necessities of living beings. The energy of the pure oxygen is tempered, yet not too much weakened, by the admixture of nitrogen. The carbonic acid, which alone is noxious to life, is mixed in so minute a proportion as to be harmless to animals, while it is still beneficial to plants; and when the air is overloaded with watery vapour, it is provided that it shall descend in rain. These rains at the same time serve another purpose. From the surface of the earth there are continually ascending vapours and exhalations of a more or less noxious kind; these the rains wash out from the air, and bring back to the soil, at once purifying the atmosphere through which they descend, and refreshing and fertilizing the land on which they fall. CHAPTER III. Structure of plants—Mode in which their nourishment is obtained—Growth and substance of plants— Production of their substance from the food they imbibe—Mutual transformations of starch, sugar, and woody fibre. From the compound substances, described in the preceding chapter, plants derive the greater portion of the carbon, hydrogen, oxygen, and nitrogen, of which their organic part consists. The living plant possesses the power of absorbing these compound bodies, of _decomposing_ them in the interior of its several vessels, and of _recompounding_ their elements in a different way, so as to produce new substances,—the ordinary products of vegetable life. Let us briefly consider the general structure of plants, and their mode of growth. SECTION I.—OF THE STRUCTURE OF PLANTS, AND THE MODE IN WHICH THEIR NOURISHMENT IS OBTAINED. A perfect plant consists of three several parts,—a root which throws out arms and fibres in every direction into the soil,—a trunk which branches into the air on every side,—and leaves which, from the ends of the branches and twigs, spread out a more or less extended surface into the surrounding air. Each of these parts has a peculiar structure and a special function assigned to it.