“We have seen that this gas, the plant’s chief source of nourishment, is composed of carbon combined with the breathable part of the air, or oxygen. Under the action of the sun’s light the leaves decompose this gas, liberating the oxygen in a condition henceforth fit for the respiration of animals and for combustion, while the carbon remains in the plant, mixes with the substances brought by the ascending sap, and with them becomes the nourishing liquid, the descending or elaborated sap, from which all future parts of the plant are to be formed. This liquid cannot be called wood, bark, leaf, flower, or fruit; it is not at all like any of these, and yet it is essentially a little of them all. An animal’s blood is neither flesh, bone, nor fleece; but bone, flesh, and fleece are of its substance. Likewise the elaborated sap is a liquid designed for the sustenance of all parts of the plant; it contains matter for fruit and wood, leaves and flowers, bark and buds. It is the plant’s blood; everything in the plant gets from it its nourishment, its wherewithal to develop. What a wonderful, what an incomprehensible process its production appears to us! In the crowded ranks of the leaf-cells, where one would suppose everything to be at rest, what activity, what transformations beyond the reach of human science! Liquids swell the cells, ooze from one to another, transpire, infiltrate, circulate, exchange their dissolved substances; vapors are exhaled, gases come, others go; the sun’s light separates what was united, unites what was separated, and the raw materials of the ascending sap combine henceforth with the materials of life. “The elaborated sap descends from the leaves to the twigs, from the twigs to the branches, from the branches to the stalk or trunk, and from the latter to the root, distributing itself here and there on its way. It circulates between the wood and the bark. It is this sap that, in the spring, when it is in great abundance, forms between the wood and the bark a thin layer of slightly viscous moisture and makes the bark easy to peel from its branch. Which of you in the month of May has not taken advantage of this peculiarity to peel off all in one piece a tube of bark from a very smooth twig of willow or lilac in order to make a whistle, trumpet, or other noisy plaything, the delight of boys of your age? “Nothing is easier than to prove the passage of sap from above downward. If you remove from a tree-trunk an annular band of bark, the nourishing liquid oozes and accumulates at the upper edge of the wound, but nothing of the sort takes place at the lower edge. Arrested thus by a break in its path, the sap accumulates above the uncovered ring and causes there an abundant growth of wood and bark, which piles up in the form of a thick circular swelling, while below the ring the trunk preserves its former size.