The stem of any of our common trees consists of three parts,—the pith in the centre, the wood surrounding the pith, and the bark which covers the whole. The pith consists of bundles of minute hollow tubes, laid horizontally one over the other; the wood and inner bark, of long tubes bound together in a vertical position, so as to be capable of carrying liquids up and down between the roots and the leaves. When a piece of wood is sawn across, the ends of these tubes may be distinctly seen. The branch is only a prolongation of the stem, and has a similar structure.
The root, immediately on leaving the trunk or stem, has also a similar structure; but as the root tapers away, the pith gradually disappears, the bark also thins out, the wood softens, till the white tendrils, of which its extremities are composed, consist only of a colourless spongy mass, full of pores, but in which no distinction of parts can be perceived. In this spongy mass the vessels or tubes which descend through the stem and root lose themselves, and by them these spongy extremities are connected with the leaves.
The leaf is an expansion of the twig. The fibres which are seen to branch out from the base over the inner surface of the leaf are prolongations of the vessels of the wood. The green exterior portion of the leaf is, in like manner, a continuation of the bark in a very thin and porous form. The green of the leaf, though full of pores, especially on the under part, yet also consists of, or contains, a collection of tubes or vessels, which stretch along its surface, and communicate with those of the bark.
Most of these vessels in the living plant are full of sap, and this sap is in almost continual motion. In spring and autumn the motion is more rapid, and in winter it is sometimes scarcely perceptible; yet the sap is supposed to be rarely quite stationary in every part of the tree.
From the spongy part of the root the sap ascends through the vessels of the wood, till it is diffused over the inner surface of the leaf by the fibres which the wood contains. Hence, by the vessels in the green of the leaf, it is returned to the bark, and through the vessels of the inner bark it descends to the root.
Every one understands why the roots send out fibres in every direction through the soil,—it is in search of water and of liquid food, which the spongy fibres suck in and send forward with the sap to the upper parts of the tree. It is to aid these roots in procuring food that, in the art of culture, such substances are mixed with the soil where these roots are, as are supposed to be necessary, or at least favourable, to the growth of the plant.
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.
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.