10. I have just said that the ribs--and might have said, farther, the stalk that sustains them--are knit out of the _tissue_ of the leaf. But what is the leaf tissue itself knit out of? One would think that was nearly the first thing to be discovered, or at least to be thought of, concerning plants,--namely, how and of what they are made. We say they 'grow.' But you know that they can't grow out of nothing;--this solid wood and rich tracery must be made out of some previously existing substance. What is the substance?--and how is it woven into leaves.--twisted into wood? 11. Consider how fast this is done, in spring. You walk in February over a slippery field, where, through hoar-frost and mud, you perhaps hardly see the small green blades of trampled turf. In twelve weeks you wade through the same field up to your knees in fresh grass; and in a week or two more, you mow two or three solid haystacks off it. In winter you walk by your currant-bush, or your vine. They are shrivelled sticks--like bits of black tea in the canister. You pass again in May, and {46} the currant-bush looks like a young sycamore tree; and the vine is a bower: and meanwhile the forests, all over this side of the round world, have grown their foot or two in height, with new leaves--so much deeper, so much denser than they were. Where has it all come from? Cut off the fresh shoots from a single branch of any tree in May. Weigh them; and then consider that so much weight has been added to every such living branch, everywhere, this side the equator, within the last two months. What is all that made of? 12. Well, this much the botanists really know, and tell us,--It is made chiefly of the breath of animals: that is to say, of the substance which, during the past year, animals have breathed into the air; and which, if they went on breathing, and their breath were not made into trees, would poison them, or rather suffocate them, as people are suffocated in uncleansed pits, and dogs in the Grotta del Cane. So that you may look upon the grass and forests of the earth as a kind of green hoar-frost, frozen upon it from our breath, as, on the window-panes, the white arborescence of ice. 13. But how is it made into wood? The substances that have been breathed into the air are charcoal, with oxygen and hydrogen,--or, more plainly, charcoal and water. Some necessary earths,--in smaller quantity, but absolutely essential,--the trees get from the ground; but, I believe all the charcoal they want, and most of the water, from the air. Now the question is, where and how do they take it in, and digest it into wood? {47}