Absorption of Heat by Leaves. 345 



during the night, were it not for the fact that the blanket of 

 dew which covers them cuts down this radiation to two thirds 

 of its former value. 



In the experiments upon the absorption of heat by the leaves, 

 considerable individual difference was found in different speci- 

 mens of the same species of leaf. In stating the results there- 

 fore I shall give the range of variation, where necessary. The 

 following table gives the percentages of heat absorbed and 

 transmitted by single leaves of the species named. 



Name of Leaf. Transmits Absorbs 



Ash 19*7 percent 80-3 per cent 



Elm. 18-23 77-82 



Maple 16-20 80-84 



Wild Cherry.. '.. 15-18 82-85 



Horse Chestnut.. 19 81 



Lilac 14-18 82-86 



Mullein (V. thapsus) 17 83 



Burdock (A. lappa) 14 86 



Chicory (Chicoiium intybus) 17 83 



Rose leaves 28-31 69-72 



Petals of red rose (cultivated) 33 67 



" " White rose... 27 73 



" " Yellow rose 24 76 



" " Oenothera speciosa - 28 73 



" " Tradescantia Vir- 



ginica 31 69 



The above table would seem to indicate that if dark heat be 

 allowed to pass through a leaf, rather more than 80 per cent is 

 absorbed by the leaf and somewhat less than 20 per cent trans- 

 mitted. The heat passes through exactly as readily when it 

 enters by the lower surface, as when it enters the upper. 



The absorption of heat by leaves is highly selective. Thus 

 a single elm leaf in the path of the dark radiant heat transmits 

 20 per cent of the heat which entered it. If now the heat 

 which has passed through the first leaf be allowed to fall upon 

 a second, it will be found that only 78 per cent of it will be 

 transmitted. A third leaf will transmit over 83 per cent of 

 the heat which passed through the second, and the effect of a 

 fourth leaf is hardly noticeable. This may in some measure 

 account for the somewhat remarkable fact that there is no very 

 great difference between the absorbing abilities of such leaves 

 as mullein and wild cherry, although the former are thick 

 tough leaves and the latter very thin. By dissolving the chlo- 

 rophyl out of leaves and again testing them, as has been ex- 

 plained under methods of experimenting, it was found that 

 this substance absorbs but little of the dark heat. Thus, wild 

 cherry leaves transmitted 9* per cent and chicory (C. intybus.) 



