Take an ordinary soda-water syphon, with or without a wine-glassful of brandy, and empty it till only a few drops remain in the bottom. Then the bottle is full of gas; and that gas, which will rush out with a spurt when you press the knob, is the stuff that plants eat--the raw material of life, both animal and vegetable. The tree grows and lives by taking in the carbonic acid from the air, and solidifying its carbon; the animal grows and lives by taking the solidified carbon from the plant, and converting it once more into carbonic acid. That, in its ideally simple form, is the Iliad in a nutshell, the core and kernel of biology. The whole cycle of life is one eternal see-saw. First the plant collects its carbon compounds from the air in the oxidized state; it deoxidizes and rebuilds them: and then the animal proceeds to burn them up by slow combustion within his own body, and to turn them loose upon the air, once more oxidized. After which the plant starts again on the same round as before, and the animal also recommences _da capo_. And so on _ad infinitum_. But the point which I want particularly to emphasize here is just this: that trees and plants don't grow out of the ground at all, as most people do vainly talk, but directly out of the air; and that when they die or get consumed, they return once more to the atmosphere from which they were taken. Trees undeniably eat carbon. Of course, therefore, all the ordinary unscientific conceptions of how plants feed are absolutely erroneous. Vegetable physiology, indeed, got beyond these conceptions a good hundred years ago. But it usually takes a hundred years for the world at large to make up its leeway. Trees don't suck up their nutriment by the roots, they don't derive their food from the soil, they don't need to be fed, like babies through a tube, with terrestrial solids. The solitary instance of an orchid hung up by a string in a conservatory on a piece of bark, ought to be sufficient at once to dispel for ever this strange illusion--if people ever thought; but of course they don't think--I mean Other People. The true mouths and stomachs of plants are not to be found in the roots, but in the green leaves; their true food is not sucked up from the soil, but is inhaled through tiny channels from the air; the mass of their material is carbon, as we can all see visibly to the naked eye when a log of wood is reduced to charcoal: and that carbon the leaves themselves drink in, by a thousand small green mouths, from the atmosphere around them.