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‘Relative Transpiration’ as a Measure of the Intrinsic Transpiring Power of the Plant

‘Relative Transpiration’ as a Measure of the Intrinsic Transpiring Power of the Plant HE term ' relative transpiration ' was first employed by Livingston in 1906 (8) as the expression for the intrinsic transpiring power of a plant. It was defined as the amount of water lost per hour from one square cm. of plant surface, divided by the amount lost per hour from one square cm. of a water surface under similar atmospheric conditions, and this number, of course, represents the area of water surface which would evaporate water at the same rate as unit area of the plant. Since 1906 the methods of measurement of evaporation and the units employed have been modified, but the conception of' relative remains unaltered and has been used by various investigators, e. g. Shreve (12) and Livingston and Hawkins (10). The process of transpiration is essentially, of course, the evaporation of water from a wet surface, and consequently changes in atmospheric conditions, such as temperature, humidity and air movement, which cause an increase or decrease in evaporation, also produce similar effects upon transpiration. Thus among the factors influencing transpiration an important part is played by atmospheric conditions, and therefore most measurements of transpiration are markedly affected by these external conditions. In an experimental http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Annals of Botany Oxford University Press

‘Relative Transpiration’ as a Measure of the Intrinsic Transpiring Power of the Plant

Abstract

HE term ' relative transpiration ' was first employed by Livingston in 1906 (8) as the expression for the intrinsic transpiring power of a plant. It was defined as the amount of water lost per hour from one square cm. of plant surface, divided by the amount lost per hour from one square cm. of a water surface under similar atmospheric conditions, and this number, of course, represents the area of water surface which would evaporate water at the same rate as unit area of the plant. Since 1906 the methods of measurement of evaporation and the units employed have been modified, but the conception of' relative remains unaltered and has been used by various investigators, e. g. Shreve (12) and Livingston and Hawkins (10). The process of transpiration is essentially, of course, the evaporation of water from a wet surface, and consequently changes in atmospheric conditions, such as temperature, humidity and air movement, which cause an increase or decrease in evaporation, also produce similar effects upon transpiration. Thus among the factors influencing transpiration an important part is played by atmospheric conditions, and therefore most measurements of transpiration are markedly affected by these external conditions. In an experimental
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