COOLING EFFECT OF TRANSPIRATION 211 



and mesophytes. Askenasy (1875) found that Sempervivum 

 alpinum attained a leaf temperature of 49*3° C and 

 Sempervivum arenarium of 48' 7° C. in the sun, with air 

 temperature 31° C. Under the same conditions the leaf 

 temperature of the non-succulent Aubretia deltoides was 

 35° C. Stahl (1909) also registers high values for the 

 temperature of succulents, e.g. for Opuntia monacantha 

 51*7° C. when exposed normal to the sun's rays, and 42' 7° C. 

 even in profile position. 



How much of these very high temperatures is to be 

 referred to reduced transpiration we cannot say ; a part 

 at least of the excess must follow the more extensive 

 absorption of heat rays by such thick leaves. 



We can get at the effectiveness of the cooling effect of 

 transpiration in another way. In an experiment of Brown 

 and Escombe (1905) a sunflower leaf transpired at the rate 

 of 3'96 grm. per 100 sq. cm. per hour in the sun. Taking 

 the latent heat of steam as 590 calories (at 18° C), this means 

 that the energy used up in transpiration is 0*389 calorie per 

 square centimetre per minute. Now. Brown and Wilson 

 (1905) have determined the thermal emissivity — that is, the 

 loss of heat by radiation, conduction, and convection — of 

 the green leaf. For Helianthus the value, in a gentle breeze 

 of 5 miles an hour, is 0*038 calorie per square centimetre 

 per minute per 1° C. excess of leaf temperature above that 

 of the surrounding air, or from the two surfaces of the leaf 

 0*076. In a leaf 10° C. in excess of the air temperature, the 

 thermal emissivity would be 0*76 calorie. Transpiration 

 at the rate taken would therefore account for 30 per cent, 

 of the total loss of heat from the leaf. In perfectly still 

 air the value for the thermal emissivity from both sides of 

 the leaf is 0*3 cal. If we suppose the transpiration to 

 be reduced to one-half of the previous value it will now 

 account for 40 per cent, of the total loss of heat. These 

 figures illustrate the very important effect which tran- 

 spiration must have in cooling the leaf. 



When we try to apply these results to xerophytic plants 

 we find difficulties due to insufficient data. We may, 



