Robert B. Withrow 391 



cent, an increase in cloudy weather during the winter accounts for 

 much of the decrease in available radiation. However, data compiled 

 by Kimball {2j) have shown that the maximum intensity on a clear 

 day in December is within 10 per cent of the highest intensities attained 

 in June. This generalization applies to data for clear cloudless days 

 from Alaska to Puerto Rico. Thus, greenhouse crops in the winter in 

 the midwestern and northeastern portions of the United States are sub- 

 jected to short days and relatively low daylight intensities, both of which 

 greatly limit the nutrient requirements of the plants. 



THERMAL EFFECTS 



The absorption of sunlight by leaves and other portions of the shoot 

 and by the soil surface frequently results in shoot and root tempera- 

 tures significantly higher than the ambient air temperature. This rise 

 in temperature due to the absorption of radiant energy may become 

 sufficiently high to significantly accelerate thermal reactions and has 

 been shown to alter markedly the transpiration rate; it undoubtedly 

 affects enzymatic reactions and causes changes in form and rate of 

 growth. When means of heat dissipation are restricted, lethal tempera- 

 tures may be reached. 



Horizontal leaf surfaces, at midday during clear weather in the tem- 

 perate zone, are exposed to sunlight intensities ranging from 1.2 to 1.5 

 gram calories per square centimeter, minute. And of this total incident 

 energy, 20 to 30 per cent is reflected by thin leaves, a much smaller pro- 

 portion is transmitted, and the remainder is absorbed. Most of the ab- 

 sorbed energy is degraded to heat and manifests itself as a temperature 

 rise of the exposed portions of the shoot, while only a small portion, 

 less than 5 per cent, is used in photosynthesis. 



Curtis (9) has found that the temperature of citrus leaves in intense 

 sunlight may be 10 to 15 degrees above the air temperature and that all 

 leaves exposed to intense sunlight had a higher temperature than that 

 of the surrounding air, regardless of wind velocity or relative humidity, 

 although high wind velocities could reduce the temperature by a matter 

 of 10 degrees. Miller and Saunders (^5) have reported a diurnal vari- 

 ation in leaf temperature and transpiration rate of corn, both of which 

 attain maxima coinciding with the maximum of sunlight intensity. 



