808. Transpiration relates to that important office performed by the leaves and other green organs, whereby pure water is separated from the crude sap and given off into the air. It takes place chiefly through the stomata, and is greatest by day and in a warm, dry atmosphere.
809. Upon the activity of transpiration depends also the amount of absorption. It not only makes room for the fluids from below to enter, but by disturbing their equilibrium, it creates an upward tendency, as the flame of a lamp draws the fluid up the wick. All the mineral and organic constituents of the sap are of course left in the plant.
810. The quantity of pure water transpired by plants is immense. A forest makes a damp atmosphere for miles around. Dr. Hales, in a series of instructive experiments in transpiration, ascertained that a sunflower three and a half feet high, with a surface of 5, 616 square inches, transpired from 20 to 30 oz. in twelve hours; a cabbage, 15 to 25 oz. in the same time - equal to the transpiration of a dozen laboring men. We may easily
811. Experiment with a single leaf recently plucked, say of Podophyllum. Insert its petiole in a narrow-mouthed goblet of water, and around it fill the mouth with dry cotton to restrain evaporation. Over the whole place a bell-glass and expose to the sunshine. The vapor transpired will condense on the bell-glass, equaling (save the solid matters) the loss in the goblet.