168 DEAD PEODUCTS OF TREES. 



when tlie eartli is colder than the air, and transmit it in the con-, 

 trary dii'ection when the temperature of the earth is higher than 

 that of the atmosphere. Of course, then, as conductors, they 

 tend to equalize the temperature of the earth and the air. 



In countries where the questions I am consideruig have the 

 greatest practical importance, a very large proportion, if not a 

 majority, of the trees are of deciduous foliage, and their radiating 

 as well as their shading surface is very much greater in summer 

 than in winter. In the latter season, they little obstruct the re- 

 ception of heat by the ground or the radiation from it ; whereas, 

 in the former, they often interpose a complete canopy between 

 the ground and the sky, and materially interfere with both pro- 

 cesses. 



Dead Products of Trees. 



Besides this various action of standing trees considered as inor- 

 ganic matter, the forest exercises, by the annual moulting of its 

 foKage, 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 primitive or 

 an old and undisturbed artificiaUy 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 mineral 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 superficial stratum, and as its powers of ab- 

 sorption, radiation and conduction of heat would differ essentially 

 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 en- 

 tire, or partially decayed, are very indifferent conductors of heat, 

 and, therefore, though they diminish the warming influence of 



