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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.

But how about the juice, the sap, the qualities of the soil, the manure required? is the incredulous cry of Other People. What is the use of the roots, and especially of the rootlets, if they are not the mouths and supply-tubes of the plants? Well, I plainly perceive I can get ‘no forrarder,’ like the farmer with his claret, till I’ve answered that question, provisionally at least; so I will say here at once, without further ado–the plant requires drink as well as food, and the roots are the mouths that supply it with water. They also suck up a few other things as well, which are necessary indeed, but far from forming the bulk of the nutriment. Many plants, however, don’t need any roots at all, while none can get on without leaves as mouths and stomachs. That is to say, no true plantlike plants, for some parasitic plants are practically, to all intents and purposes, animals. To put it briefly, every plant has one set of aerial mouths to suck in carbon, and many plants have another set of subterranean mouths as well, to suck up water and mineral constituents.

Have you ever grown mustard and cress in the window on a piece of flannel? If so, that’s a capital practical example of the comparative unimportance of soil, except as a means of supplying moisture. You put your flannel in a soup-plate by the dining-room window; you keep it well wet, and you lay the seeds of the cress on top of it. The young plants, being supplied with water by their roots, and with carbon by the air around, have all the little they need below, and grow and thrive in these conditions wonderfully. But if you were to cover them up with an air-tight glass case, so as to exclude fresh air, they’d shrivel up at once for want of carbon, which is their solid food, as water is their liquid.

The way the plant really eats is little known to gardeners, but very interesting. All over the lower surface of the green leaf lie scattered dozens of tiny mouths or apertures, each of them guarded by two small pursed-up lips which have a ridiculously human appearance when seen through a simple microscope. When the conditions of air and moisture are favourable, these lips open visible to admit gases; and then the tiny mouths suck in carbonic acid in abundance from the air around then. A series of pipes conveys the gaseous food thus supplied to the upper surface of the leaf, where the sunlight falls full upon it. Now, the cells of the leaf contain a peculiar green digestive material, which I regret to say has no simpler or more cheerful name than chlorophyll; and where the sunlight plays upon this mysterious chlorophyll, it severs the oxygen from the carbon in the carbonic acid, turns the free gas loose upon the atmosphere once more through the tiny mouths, and retains the severed carbon intact in its own tissues. That is the whole process of feeding in plants: they eat carbonic acid, digest it in their leaves, get rid of the oxygen with which it was formerly combined, and keep the carbon stored up for their own purposes.