Measurement of Transpiration

Because transpiration itself is hard to measure directly, plant physiologists rely on indirect methods that track a proxy quantity instead. Ganong's potometer measures the rate of water absorbed by a cut twig and treats that absorbed volume as effectively equal to the volume transpired. The apparatus consists of a horizontal graduated tube bent at both ends - one bent end wide, the other narrow - with a water reservoir (fitted with a stopcock) attached near the wide end; a leafy twig is fixed into the wide end through a split cork, the narrow end is dipped into a beaker of coloured water, and an air bubble is deliberately introduced into the graduated section near the narrow end. Once the apparatus is set in bright sunlight, ongoing transpiration by the twig draws water up through the tube, and the introduced air bubble is seen to move steadily toward the twig; the bubble's rate of travel along the graduated scale gives a direct, readable measure of the rate of water absorption, which is taken to equal the rate of transpiration. A second, simpler method uses cobalt chloride (CoCl2) paper: dry strips of the blue, moisture-sensitive salt are placed on the upper and lower surfaces of a healthy leaf and immediately sealed under glass slides; the strip on the lower surface turns pink first, as CoCl2 becomes hydrate …