Sap
The fluid that exists in living plants, for the most part inclosed in the cells of which they are built up. It varies much in composition in different parts of the same plant; and the nature of these variations must be explained. A plant growing in the soil pushes out roots and rootlets, and through them sucks in, from the damp earth, a large amount of water. This passes into the hairs of the root by gradual absorption, carrying with it some mineral compounds, such as common salt, phosphates, &c., in solution. These mineral substances are present in only very small proportions in the solutions; but, in course of time, a considerable quantity thus enters the plant, if the substances occur in the soil in forms suitable for absorption by plants. The water, with its dissolved mineral substances passes from the root-hairs into the root, and thence into the stem, remaining little changed in composition, and bearing the name of Crude Sap. This crude Sap flows upwards in the younger (i.e., outer) layers of wood of the stem, and in some plants it is possible to tap the stream, and to obtain nearly pure, and quite drinkable, water, when a hole is bored deep enough to reach the young layers of wood. The crude Sap is conveyed to the green parts of the plant; and there, especially in the leaves, it undergoes great changes. A quantity of the water is given off from the leaves by evaporation or transpiration, rendering the solutions denser in the leaves than in the stems. But the great change brought about in the stems. But the great change brought about in the Sap in the green parts of plants consists in the formation in these parts of various substances, of which the most easily detected is starch. This is present in solid grains in the cells containing the green substance or chlorophyll, when the green parts have been exposed for an hour to the action of daylight or to strong artificial light. Several substances, that in composition resemble starch to some extent, are formed in the cells containing chlorophyll, and some of them remain in solution in the cell Sap, while others assume a solid form. Other substances, resembling protoplasm in general composition, are also formed in the green parts of plants, and probably also in other parts; and these, too, are often dissolved in the Sap. Owing to the loss of water by evaporation, and to the addition of these new organic products, the Sap becomes "elaborated," being heavier and thicker than the crude Sap. It passes from the parts in which it was elaborated to all parts where nourishment is required, whether to supply the material consumed during growth, or to allow of the formation of new structures at the growing points of stems, and in the growth of leaves, flowers, and fruits. In many perennial plants, a large proportion of the new products is also transferred from the leaves to the stems, or to underground store-houses, e.g., tubers of Potato, roots of Parsnip and Turnip, bulb-scales of Lilies, &c. There are thus descending currents of elaborated Sap, in Dicotyledons, usually outside of the ascending currents of crude Sap. The elaborated Sap descends by two chief routes: the first, down which pass dissolved starch and compounds of a similar kind, is believed to be through the cellular tissues of the bark, and, to some extent, the medullary rays and the pith; the second route, down which the protoplasmic substance passes, is believed to be the soft-bast, or innermost layer of the bark. Down this the latter stream flows, especially by means of the "sievetubes"--slender tubes of long cells, end to end, that are separated, only partially, by sieve-like cross walls, the openings in which allow the protoplasm to pass from cell to cell. A practical result of the situation of the descending current is that if a ring of bark is removed, or a tight ring of any material (e.g., wire) is bound round a branch of any Dicotyledon of ordinary structure (e.g., an Apple-tree), the elaborated Sap is prevented from passing downwards; a thickened border is produced above the obstruction, and the crop of fruit on the branch is increased, since it gets all the food produced in the leaves of the branch, while duly supplied with crude Sap through the wood below the ring, which has been left uninjured. This process of ringing does not succeed in a Monocotyledon, or in a Dicotyledon with fibro-vascular bundles scattered among cellular pith, and in which the downward flow of Sap consequently remains unchecked, such as the species of Pepper (Piper). The causes of the upward flow of crude Sap are chiefly two, viz.: (1) in early spring, when growth begins, the absorption of fluid by root-hairs, probably due to chemical changes in the cell contents; and (2) afterwards, the free evaporation from the leaves, drawing fresh Sap upwards to supply what has been lost. Other forces also assist in a lesser degree; but to these reference need not now be made.