Transpiration

Transpiration, the loss of water in the form of vapour from the general surface of a plant, especially through the stomata. Stephen Hales determined that a sunflower with 5,616 square inches of leaf-surface loses 30 fluid ounces in 12 hours' daylight. Transpiration is most active in dry, warm air, and in the light, as then the actively-assimilating guard-cells of the stomata become most turgid and curved. Transpiration sets up a current from the roots, Which has been measured, by adding a solution of lithium-citrate, and then using the spectroscope, as 118 centimetres an hour in the tobacco-plant. This current travels by way of the xylem of the alburnum, the tracheal system of which contains broken columns ("chapelets de Jamin") of air and water. The air is rarefied by the removal of its oxygen, becoming, in the tops of trees, almost pure nitrogen; and the active protoplasmic cells of the medullary rays, in their rapidly-alternating katabolic and anabolic processes, liberating and absorbing water, act on the tracheal elements alternately as force-pumps and suction-pumps. As these tracheal elements have their transverse septa at one level in each radial series but not at the same level in the contiguous series: the water travels upwards round the stem, step by step, the pits in the radial walls of the vessels being its chief channel. This complex process has been successfully investigated by Sachs, Elfving, Robert Hartig, and, above all Emil Godlewski.