DESCENDING SAP

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“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.