146 
FORM OF LEAVES IMPORTANT. 
what augment the sum total.* On the other hand, the grow¬ 
ing 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 foliage interferes with that free communica¬ 
tion 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 
effective thermoscopic surface of a forest in full leaf does not 
exceed that of bare ground in the same proportion as does its 
measured superficies, yet the actual quantity of area capable 
of receiving and emitting heat must be greater 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 
thermoscopic 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 covered on one or both sides with spiculse, and they 
very commonly present one or more acuminated points in their 
outline—all circumstances which tend to augment their power 
of emitting heat by reflection or radiation. Direct experiment 
on growing trees is very difficult, nor is it in any case prac¬ 
ticable to distinguish how far a reduction of temperature pro¬ 
duced by vegetation is due to radiation, and how far to exha¬ 
lation of the fluids of the plant in a gaseous form; for both 
processes usually go on together. But the frigorific effect of 
leafy structure is well observed in the deposit of dew and the 
occurrence of hoarfrost on the foliage of grasses, and other 
small vegetables, and on other objects of similar form and con- 
* “ 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.”— Gkay, First Lessons in Botany and Vegetable Physi¬ 
ology, as quoted by Coultas, What may be learned from a Tree , p. 34. 
t See, on this particular point, and on the general influence of the 
forest on temperature, Humboldt, Ansichten der Natur , i, 158. 
