INTERNAL TEMPERATURE OF LEAVES. 243 



Thus the leaf, which in direct sunhght attained a temperature 

 of 42 '3°, was immediately brought down to 27*9° by being 

 shaded, and this is a sample of what normally occurred. It 

 is noteworthy, however, that very rarely did the leaf tempera- 

 ture sink in the shade to that of the surrounding air. Readings, 

 such as those given in Table XIV., where the leaf in the shade 

 has a temperature of 1-1° C. above the temperature of the 

 surrounding air (10.48 a.m.), are typical. The usual shade 

 reading was between 1° and 2° C. above the surrounding air. 

 On certain occasions it rose higher than this, the highest 

 reading recorded in the shade being 4*1° C. above that of the 

 surrounding air. This occurred on January 27, 1906, when 

 the shade temperature of the air was 28*2 C, almost the 

 highest temperature recorded during the whole of the observa- 

 tions. Thus, this high temperature in the shade was on a 

 very cloudless morning when the reflected radiation was very 

 bright. On one occasion only, October 31, 1906, did the 

 temperature in the shade sink below that of the surrounding 

 air. This was the case of the young flaccid red leaf, with 

 little chlorophyll, of Amherstia nobilis, a special case which 

 will be dealt with later. 



It seems then in general that, in direct sunlight, green 

 leaves of various thicknesses and textures attain a temperature 

 of from 11°-15° C. above that of the surrounding air, while in 

 the shade they are usually l°-2° C, and sometimes as much as 

 4° C. higher than the air. The heating up is no doubt due to 

 the absorption of radiation by the colouring matter of the leaf. 

 Part of this is used up to provide energy for transpiration and 

 assimilation, but in direct sunlight much more is absorbed 

 than can be used, and the excess raises the temperature of the 

 leaf. Further, in the open air on bright sunny days there is, 

 as a rule, sufficient reflected radiation (direct sunlight being 

 shut off) to raise the temperature of the leaf l°-2° C, and 

 sometimes more above the surrounding air. Only on dull or 

 wet days does the leaf temperature remain the same as that 

 of the surrounding air. 



Effect of the Thickness of a Leaf. 

 It seems somewhat surprising that the thickness and texture 

 of the leaf should have so httle influence on this result. There 

 7(1)09 ^ (32) 



