32 
Journal of Agricultural Research 
Vol. XXVI, No. I 
results indicate that in the case of corn, sorghum, watermelon, pumpkin, 
and soybean the heat absorbed by the leaf from the sun is quickly 
utilized in transpiration and rapidly disseminated into the surrounding 
air, so that the temperature of the leaves is always approximately that 
of the air. In the case of the leaves of cowpea and alfalfa the rate of 
transpiration is evidently rapid enough to reduce the temperature of 
the leaf slightly below that of the air. 
The results obtained in these experiments are in considerable contrast 
to those previously reported by several investigators. Askenasy (i) by 
placing a thermometer in close contact with the surface of the fleshy 
leaves of three species of Sempervivum found that they attained a 
temperature in the sunlight of i8° to 25 0 C. above that of the surrounding 
air. Under the same conditions, however, he found that the leaves of 
Aubrietia deltoidea and Gentiana cruciata showed a temperature of only 
4 0 to 7 0 C. above that of the air temperature. He attributed the high 
temperature of the fleshy leaves of Sempervivum to the fact that they 
carried on little transpiration and that the heat they absorb is not 
readily dissipated by air currents or radiation. These observations on 
the temperature of the leaves of Sempervivum were later verified by 
Ursprung (75). Blackman and Matthaei (2) by means of a thermo¬ 
electrical method found that the internal temperature of detached 
leaves of cherry laurel in direct sunshine was from 4 0 to 13 0 C. above 
that of the air temperature, the difference depending upon the position 
of the leaf relative to the rays of the sun and the time of day that the 
observations were made. Brown and Escombe (5, p. 83-83) indirectly 
calculated the temperature of the leaves of several plants from certain 
known energy relations between the leaf and the air. In the case of the 
sunflower the leaves showed a temperature of 17.3 0 C. when the air 
temperature was 16.9° C., and when the air temperature was 27.2 °C. 
the calculated temperature of the leaf was 25.4 0 C. Smith (14), using 
the modified thermoelectrical method of Matthaei (8) upon insolated 
leaves in the Tropics, found that the internal temperature of leaves 
of various types was 15° C. above that of the surrounding air, when the 
temperature of the latter was 25 0 to 28° C. Seeley (jo) obtained the 
temperature of the leaves of the garden strawberry by folding the leaf 
around the bulb of a thermometer. The temperature of the leaves 
obtained in this manner on clear days averaged 15 0 F. higher than the 
temperature of the air in a weather instrument shelter near by. 
In determining the temperature of a leaf, the atmospheric conditions 
surrounding it must always be taken into consideration. A leaf in 
direct sunshine freely exposed to a breeze always has a lower tempera¬ 
ture than one under like conditions of sunlight but in such a position 
as to be protected from air currents. It seems probable that the rela¬ 
tively high temperature of the leaves above that of the air surrounding 
them, as reported by Blackman and Matthaei and by Smith, was due to 
the fact that the leaves were boxed in by the apparatus used, so that the 
absorbed heat could not readily be disseminated. Under such condi¬ 
tions one would expect a much higher temperature for the leaves than 
if they were exposed to the freely circulating air. Smith ( 14) finds from 
his observations that breezes reduce the temperature of the leaves in 
sunlight by amounts varying from 2 0 to io° C., and that a thin leaf is 
much more noticeably affected than a thick one. 
When the atmospheric conditions during the day are comparatively 
mild and the rate of evaporation low, the leaves of most of the plants 
