166 ABSOEBING AND EMITTING SURFACE. 



can be placed "upon a flat piece of ground, without itself exposing 

 a greater sui'face than it covers. This applies, of course, to forest 

 trees and their leaves, and indeed to all vegetables, as well as to 

 other prominent bodies. If we suppose forty trees to be planted 

 on an acre, one being situated in the centre of every square of 

 two rods the side, and to grow until their branches and leaves 

 everywhere meet, it is evident that, when in full fohage, the 

 trunks, branches and leaves would present an amount of ther- 

 moscopic surface much greater than that of an acre of bare 

 earth ; and besides this, the fallen leaves lying scattered on the 

 ground, would somewhat augment the sum total.* On the other 

 hand, the growing leaves of trees generally form a succession of 

 stages, or, loosely speaking, layers, corresponding to the annual 

 growth of the branches, and more or less overlying each other. 

 This disposition of the fohage interferes with that free commu- 

 nication between sun and sky above and leaf-surface below, on 

 which the amount of radiation and absorption of heat depends. 

 From all these considerations, it appears that though the efiective 

 thermoscopic surface of a forest in full leaf does not exceed that 

 of bare ground in the same proportion as does its measured super- 

 ficies, yet the actual quantity of area capable of receiving and 

 emitting heat must be gi-eater in the former than in the latter 

 case.f 



It must further be remembered that the form and texture of a 

 given surface are important elements in determining its thermo- 

 scopic character. Leaves are porous, and admit air and light 

 more or less freely into their substance; they are generally 

 smooth and even glazed on one surface ; they are usually cov- 

 ered on one or both sides with spiculse, and they very commonly 

 present one or more acuminated points in their outline — all cir- 

 cumstances which tend to augment their power of emitting heat 

 by reflection or radiation. Direct experiment on growing trees 



* " The "Washington elm at Cambridge — a tree of no extraordinary size — 

 was some years ago estimated to produce a crop of seven millions of leaves, 

 exposing a surface of two hundred thousand square feet, or about five acres of 

 foliage." — Gray, First Lessons in Botany and Vegetable Physiology. 



f See, on this particular point, and on the general influence of the forest on 

 temperature, Humboldt, Ansichten der Natur, i., 158. 



