“Now what would you expect to find in this liquid if you collected some of it as it trickles in the form of tears either from the vine or from a fruit-tree? Many things, doubtless, you will say, since this precious liquid is the prime source of all that the plant contains in itself. If such is your thought, undeceive yourselves: ascending sap is little more than clear water, and often it is very difficult for science to prove beyond a doubt the presence in it of various substances in solution, so minute a fraction of the whole do they compose. Among these substances the most frequent are compounds of potash, of lime, of carbonic acid gas, traces of phosphates, and compounds of nitrogen or ammonia. In short, the liquid from which the plant is to derive its nourishment is the weakest sort of broth, composed of an enormous quantity of water and a very small proportion of dissolved substances. These inconsiderable substances are the only or almost the only things utilized by the plant; and the water that has collected them in filtering through the soil, and has then carried them from the roots to the leaves through the sap-wood, the water that forms almost the whole of the ascending sap, is destined, as soon as the journey is accomplished, to leave the plant and return as vapor to the atmosphere whence it descended in the form of rain.” CHAPTER XXIII 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.