The Continuous Measurement of Photosynthesis, Respiration, and Transpiration of Alfalfa and Wheat Growing under Field Conditions.

The apparatus and experimental technique which are described in this paper have been developed to determine continuously, automatically, and throughout the life of a plant, if desired, the carbon dioxide exchange and the transpiration in a chamber in which plants are growing under field conditions. This discussion give.s a general resume of our methods, including, first, a description of the equipment; second, illustrations of its application to alfalfa and wheat; third, a statistical analysis of the data, showing the magnitude of the errors and variability involved; and fourth, some interesting observations on the relation of light intensity and temperature to the rate of photosynthesis, respiration, and transpiration, which these methods have made clear. The lack of an adequate method for the rapid and continuous determination of atmospheric carbon dioxide has been responsible for the somewhat limited employment of the carbon dioxide exchange in the measurement of photosynthesis of plants and respiration of both plants and animals. The best methods available-including the gasometric procedure of BENEDICT (2) ; the volumetric method of JOHNSON and WALKER (5) ; the electrometric method of SPOEHR and MCGEE (8); and the gravimetric methods-have left much to be desired either of convenience or accuracy for this purpose. Since the atmosphere contains only 310 parts per million by volume of carbon dioxide, the gasometric and gravimetric methods have been handicapped by insufficient material, and the volumetric and electrometric methods have needed more efficient absorbers to permit the use of larger air samples. The latter need recently has been ifiled by the absorbers of MARTIN and GREEV1 (6), HEINICKE and HOFFMAN (4), and THOMAS (9). A unique method of determining carbon dioxide exchange has recently been devised by McALISTER (7). After this paper was submitted for publication, a paper by WAUGH (10) appeared, describing a hand-operated conductivity appa-