LEAF TEMPERATURES AND THE COOLING OF LEAVES BY RADIATION

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Abstract
Measurements of leaf temps., by means of thermocouples threaded into the leaves, demonstrate, both in laboratory and in the field, that leaves may be quickly cooled to 2-3[degree]C below air temp. by radiation in the infra-red to cold objects at varying distances or to a clear sky. This loss of heat by radiation is independent of the temp of the atmosphere in the immediate neighborhood of the leaf, as the N and O of the air are almost completely transparent to these radiations and there is not enough water vapor or CO2 present close to the leaves to have a great effect on this radiation. Changes in leaf temps. as great as 10[degree]C in 1 min. may be brought about by natural or artificial changes in light intensity or in rates of air movement. Data reported by other investigators have led to the conclusion that leaves in direct sunlight may be cooled by transpiration to temps. below that of the air, but such a conclusion is considered questionable because it is found that the methods which had been used to measure air temp. are likely to give readings that are too high by 2[degree]-12[degree]C.