The Mass Factor in the Energy Relations of Leaves

In a recent paper the writer (4) emphasized the variability of the reflection factor in the energy relations of leaves, and pointed out the fact that reflection is frequently large, and not to be neglected in determining the absorption coefficient as was done by Brown and Escombe (1, pp. 75 and 91) in their fundamental studies of the energetics of the leaf. There is evident need of a reinvestigation of the problems of energy exchange between the leaf and its environment, and the internal transformations of energy in the processes of transpiration and photosynthesis, with due regard to the variability of the reflection and absorption of energy in all regions of the spectrum, from the far infra-red to the limits of the solar ultra-violet. Attention is called to an error in the previous paper (4, p. 604) which states that the Brown and Escombe studies were made only on Catalpa Mgnonioides. While some of their important measurements were made on the hypostomatous leaves of this species, in several sections of their work numerous species were used for studies of photosynthesis, thermal emissivity, and respiration. The equation used by Brown and Escombe to calculate the heating effects of the absorbed radiation, on the supposition that there were no dissipation of energy by internal work, is Ba/ms = t in degrees C. rise per minute, in which B is the total incident radiation per square cm. per minute, a the coefficient of absorption of energy, m the mass of the square centimeter of leaf lamina, and s the specific heat of the leaf substance. The values substituted for these symbols were as follows : B 0.8 calorie, a = 0.78, m = 0.020 gm., and s = 0.879. The values used for m and s were obtained from measurements made on the leaves of Helianthus annuus. The value