“This force is in the buds or, to speak more correctly, in the leaves. Each leaf is the seat of an active evaporation whose object is to rid the plant of the great quantity of water required for dissolving in the soil and then conveying to the leaves the nutritive substances present in the soil. This evaporation leaves a void in the cells that have given up the evaporated water. But this void is immediately filled from the neighboring cells, which give up their contents and receive in turn the contents of the next lower layers. From cell to cell, from fiber to fiber, from duct to duct, a similar transfer takes place at points farther and farther away from the evaporating surface, until the tip-ends of the rootlets are reached, where a continuous absorption makes good the loss of moisture by evaporation. The process reminds one somewhat of the working of our pumps, in which the piston leaves behind it a void that is immediately filled by the water in the pipe, which in its turn gets water from the bottom of the well. This liquid which ascends in every plant, absorbed by the spongioles of the rootlets and put in motion by the evaporation from the leaves, is called ascending sap, or crude sap. The sap is called ascending because it passes from below upward, from the roots to the branches; and it is called crude because it has not yet undergone the preparation that will turn it into the nutritive liquid of the plant. Thus we have learned our first lesson, namely: ascending sap is carried especially to those parts of the plant where buds are numerous, where leaves abound; it seeks by preference the ends of the branches, where evaporation is most active. “We know that the surface wood is the newest; it is formed of cells, fibers, and ducts whose cavities are free and whose walls are permeable. The interior wood is older; its cells, fibers, and ducts are encrusted, stopped up, decrepit, out of use. The liquid accordingly makes its way where circulation is possible, and ceases to flow where the passage is obstructed. That is to say, the ascent of the sap takes place through the sap-wood and chiefly through the outermost layers, or those of most recent formation. Repeated experiment leaves no doubt on this point. When a tree is cut down at the time of the sap’s greatest activity, we find the sap-wood moist and the older wood perfectly dry. Finally, in herbaceous plants the sap ascends through the whole body of the stem. Suspended during the winter on account of the absence of foliage, this ascent of the sap becomes remarkably brisk at the awakening of vegetation. Then it is that fruit-trees shed tears, so to speak, where the pruning-hook has left its mark; or, in other words, the ascending sap oozes from the openings of the severed ducts. These tears are especially noticeable in the grape-vine, where it has recently been trimmed.