ABSORBING AND EMITTING SURFACE. 169 
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 communication 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 ther- 
moscopic 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 greater in the former than in the latter 
case.* 
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 gener- 
ally smooth and even glazed on one surface ; they are usually 
covered on one or both sides with spicula, and they very com- 
monly 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 practicable 
to distinguish how far a reduction of temperature produced by 
vegetation is due to radiation, and how far to exhalation of the 
gaseous and watery fluids of the plant; for both processes 
usually go on together. But the frigorific effect of leafy strue- 
ture 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 consistence, when the 
temperature of the air a few feet above has not been brought 
down to the dew-point, still less to 32°, the degree of cold re- 
quired to congeal dew to frost. 
* See, on this particular point, and on the general influence of the forest on 
temperature, HUMBOLDT, Ansichten der Natur, i., 158. 
+ The leaves and twigs of plants may be reduced by radiation to a tempera- 
ture lower than that of the ambient atmosphere, and even be frozen when the 
air in contact with them is above 32°. Their temperature may be communicated 
