It is not so obvious that the leaves spread out their broad surfaces into the air for the same purpose precisely as that for which the roots diffuse their fibres through the soil. The only difference is, that while the roots suck in chiefly _liquid_, the leaves inhale almost solely _gaseous_ food. _In the sunshine, the leaves are continually absorbing carbonic acid from the air and giving off oxygen gas._ That is to say, they are continually appropriating carbon from the air.[4] _When night comes, this process ceases, and they begin to absorb oxygen and to give off carbonic acid._ But this latter process does not go on so rapidly as the former, so that, on the whole, plants when growing gain a large portion of carbon from the air. The actual quantity, however, varies with the season, with the climate, and with the kind of tree. The proportion of the whole carbon contained by a plant, which has been derived from the air, is greatly modified also by the quality of the soil in which it grows, and by the comparative abundance of liquid food which happens to be within reach of its roots. It has been ascertained, however, that in our climate, on an average, not less than from one-third to three-fourths of the entire quantity of carbon contained in the crops we reap from land of average fertility, is really obtained from the air. [4] Since carbonic acid, as shewn in the previous chapter, consists only of carbon and oxygen, they retain the carbon and reject the oxygen. We see then why, in arctic climates, where the sun once risen never sets again during the entire summer, vegetation should almost rush up from the frozen soil—the green leaf is ever gaining from the air and never losing, ever taking in and never giving off carbonic acid, since no darkness ever interrupts or suspends its labours. How beautiful, too, does the contrivance of the expanded leaf appear! The air contains only one gallon of carbonic acid in 2500, and this proportion has been adjusted to the health and comfort of animals to whom this gas is hurtful. But to catch this minute quantity, the tree hangs out thousands of square feet of leaf in perpetual motion, through an ever-moving air; and thus, by the conjoined labours of millions of pores, the substance of whole forests of solid wood is slowly extracted from the fleeting winds. The green stem of the young shoot, and the green stalks of the grasses, also absorb carbonic acid as the green of the leaf does, and thus a larger supply is afforded when the growth is most rapid, or when the short life of the annual plant demands much nourishment within a limited time. SECTION II.—OF THE GROWTH AND SUBSTANCE OF PLANTS. In this way the perfect plant derives its food from the soil and from the air; but perfect plants arise from seeds; and the study of the entire life—the career, so to speak—of a plant, presents many interesting and instructive subjects of consideration.