DEAD PRODUCTS OF TREES. 171 
whereas, in the former, they often interpose a complete canopy 
between the ground and the sky, and materially interfere with 
both processes. 
Dead Products of Trees. 
Besides this various action of standing trees, considered as in- 
organic matter, the forest exercises, by the annual moulting of 
its foliage, still another influence on the temperature of the 
earth, and, consequently, of the atmosphere which rests upon it. 
If we examine the constitution of the superficial soil in a primi- 
tive or an old and undisturbed artificially planted wood, we 
find, first, a deposit of undecayed leaves, twigs, and seeds, lying 
in loose layers on the surface ; then, more compact beds of the 
same materials in incipient, and,as we descend, more and more 
advanced, stages of decomposition ; then, a mass of black mould, 
in which traces of organic structure are hardly discoverable 
except by microscopic examination ; then, a stratum of mmeral 
soil, more or less mixed with vegetable matter carried down 
into it by water, or resulting from the decay of roots; and, 
finally, the inorganic earth or rock itself. Without this deposit 
of the dead products of trees, this latter would be the superfi- 
cial stratum, and as its powers of absorption, radiation, and con- 
duction of heat would differ essentiaily from those of the layers 
with which it has been covered by the droppings of the forest, 
it would act upon the temperature of the atmosphere, and be 
acted on by it, in a very different way from the leaves and 
mould which rest upon it. Dead leaves, still entire, or partially 
decayed, are very indifferent conductors of heat, and, therefore, 
though they diminish the warming infiuence of the summer 
sun on the soil below them, they, on the other hand, prevent the 
escape of heat from that soil in winter, and, consequently, in 
cold climates, even when the ground is not covered by a pro- 
tecting mantle of snow, the earth does not freeze to as great a 
depth in the wood as in the open field. 
