DRAINAGE BY ROOTS OF TREES. 9205 
mechanical action of the roots as conductors of the superfluous 
humidity of the superficial earth to lower strata. The roots of 
trees often penetrate through subsoil almost impervious to 
water, and in such cases the moisture, which would otherwise 
remain above the subsoil and convert the surface-earth into a 
bog, follows the roots downwards and escapes into more porous 
strata or is received by subterranean canals or reservoirs.” 
When the forest is felled, the roots perish and decay, the orifices 
opened by them are soon obstructed, and the water, after having 
saturated the vegetable earth, stagnates on the surface and 
transforms it into ponds and morasses. Thus in La Brenne, a 
tract of 200,000 acres resting on an impermeable subsoil of 
argillaceous earth, which ten centuries ago was covered with 
forests interspersed with fertile and salubrious meadows and 
pastures, has been converted, by the destruction of the woods, 
into a vast expanse of pestilential pools and marshes. In 
Sologne the same cause has withdrawn from cultivation and 
human inhabitation not less than 1,100,000 acres of ground 
once well wooded, well drained, and productive. 
Tt is an important observation that the desiccating action of 
trees, by way of drainage or external conduction by the roots, 
is greater in the artificial than in the natural wood, and hence 
that the surface of the ground in the former is not characterized 
by that approach to a state of saturation which it so generally 
manifests in the latter. In the spontaneous wood, the leaves, 
fruits, bark, branches, and dead trunks, by their decayed 
material and by the conversion of rock into loose earth through 
the solvent power of the gases they develop in decomposition, 
cover the ground with an easily penetrable stratum of mixed 
vegetable aud mineral matter extremely favorable to the 
* “The roots of vegetables,” says d’Héricourt, ‘‘perform the office of 
draining in a manner analogous to that artificially practised in parts of Hol- 
land and the British islands. This method consists in driving deeply down 
into the soil several hundred stakes to the acre; the water filters down along 
the stakes, and in some cases as favorable results have been obtained by this 
means as by horizontal drains.” —Annales Forestiéres, 1857, p. 312. 
