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The watery contents of a plant; an indefinite and undescriptive term little used by botanists.CH

Sap, a term applied to the juices of living plants.CH

Sap is composed of water containing mineral salts absorbed from the soil, and organic substances chiefly constructed within living cells. The water taken from the soil by the roots or other absorbing organs may contain potassium, sodium, magnesium, calcium, iron, and nitrates, phosphates, sulfates, and chlorides. The different processes and the different reactions that take place in separate tracts of tissue are responsible for the fact that the sap is not alike in composition throughout the body of the plant.CH

The mineral elements and their salts may be found in nearly all saps. The limits of this note do not permit the enumeration of the large number of organic substances which may be found in the sap of various species. The more important of such compounds may be grouped under the acids, sugars, or carbohydrates and proteins. Many plants are of economic importance because of the materials dissolved in the sap. The sap of the sugar maple, for example, contains over 3 1/2 per cent of sugar, while the sugar-beet and sugar-cane have a sap in which the proportion is very much higher.CH

The popular expression of "ascent of sap" refers to the fact that water entering the living cells of the roots is forced into the woody tissues or non-living elements through which it passes upward to the leaves at a rate which may vary from a few inches to over a yard an hour. (See Transpiration.) The forces operative and the mechanism of the flow are not perfectly understood. Among other facts of interest it may be mentioned that the sap-current may pass through dead sections of stem, although it is equally certain that the activities of the living cells furnish at least a part of the motive power.CH

The flow of sap from the sugar maple and other trees in the early spring, before the soil has thawed and while it is yet too cold for the living matter of the plant to show any great activity, is not due to the bleeding pressure, but to the expansion of the gases and liquids in the trunk and branches of the tree due to the direct warming action of the sun's rays. During the daytime the bubbles of air in the wood-cells become heated and expand, driving the sap from the wood-cells into the auger-hole which has been bored into the tree. At night the trunk of the tree cools slowly and the flow ceases, to be begun again next day.CH

The exudation pressure by which water or sap is forced from the living cells is exhibited in the bleeding which ensues when stems and branches are cut away. The pressure which produces bleeding is often called root-pressure; although it is exerted by any part of the plant. Bleeding is exhibited by a large number of trees at the beginning of the growing season, and is also especially noticeable in the vine, dahlia, castor-oil plant, calla, nicotiana, and corn.CH

The amount of bleeding exhibited by any plant may be found if the stem is cut and bent over in such manner that the end is thrust into a tumbler or small vessel, which will serve to collect the escaping sap.CH

Interesting records of measurement of the amount of bleeding are available. A specimen of Betula papyracea gave off over sixty-three pounds of water in twenty-four hours; an Agave americana yielded twelve and one-half pounds in twenty-four hours. The pulque of Mexico is the preparation of sap which collects in the center of the mature rosette of agave when a cavity is cut into it.CH

The range of concentration of sap as denoted by its osmotic properties varies widely. Cacti and other succulents have a sap which would set up a pressure of only three to twelve atmospheres. Spinose desert shrubs may have a sap which would set up a pressure of over a hundred atmospheres. The concentration is greatest in mature leaves, and in the lilac this may be from twelve to twenty-five atmospheres, while the roots of the same plant may not show more than four or six atmospheres.CH

This article contains a definition from the Glossary of Gardening Terms.
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