DESCENDING SAP

“Ascending sap, a liquid composed of a large quantity of water and a very small proportion of dissolved nutritive substances, is absorbed in the ground by the roots and carried to the leaves through the sap-wood. It is not yet a nutritive fluid for the plant; it becomes so in the foliage by a double process. First, on being distributed to the leaves, which furnish a vast surface for evaporation, it exhales its superabundant water in the form of vapor and thus concentrates its usable ingredients. Then, under the influence of the sun’s rays and through the medium of the green matter contained in the leaves, it undergoes modifications that work a fundamental change in its character. Among the processes here taking place, one of the best known is the decomposition of the carbonic acid gas taken from the air by the leaves and from the soil by the roots.

“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 [113]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. [114]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.

“A tight ligature, by compressing and obstructing the passages through which the nutritive fluid has to pass, causes the formation of a similar swelling above the line of stoppage. You may have seen a sapling, bound too tightly to the stake intended for its support, strangled by its own growth if the gardener has forgotten to loose the band in time. Little by little the trunk swells above this band, which is finally overgrown by the bark and even hidden within its substance. Indeed, it is not rare to find a tree with its trunk caught fast in a narrow [115]passage, as for example in the crevice of a rock, and swollen above the obstacle into an unsightly excrescence. The stoppage of the sap in its downward course explains this phenomenon.

“If the tree-trunk is not completely encircled by the stricture, if somewhere there is a strip of bark left free to serve as a passage, the nourishing juice takes this way to get around the obstacle, and so pursues its course to the roots. Then the tree continues to live. But if the barrier is absolutely insuperable, as in the case of an unyielding ligature or when the tree has been girdled, the sap cannot descend to the roots to nourish them; and with the death of these the end of the tree is not far distant.

“An important lesson remains to be drawn from these details concerning the circulation of this nutritive liquid in plants. Henceforth, when we fasten a plant to its prop or supporting stake, we shall be careful not to tie the string too tight or else to loosen it at the proper time, since otherwise we should run the risk of strangling the plant and so causing its death.”