DEAINAGE BY ROOTS OF TREES. 201 



by evaporation before tlie water can be carried ofE by infiltration, 

 while the soil of a neighboring grove will remain half saturated 

 for weeks together. Sandy soils frequently rest on a tenacious 

 subsoil, at a moderate depth, as is usually seen in the pine plains 

 of the United States, where pools of rain-water collect in shght 

 depressions on the surface of earth the upper stratum of which is 

 as porous as a sponge. In the open grounds such pools are very 

 soon dried up by the sun and wind ; in the woods they remain 

 unevaporated long enough for the water to diffuse itself laterally 

 until it finds, in the subsoil, crevices through which it may escape, 

 or slopes which it may follow to their outcrop or descend along 

 them to lower strata. 



Drainage iy Hoots of Trees. 



Becquerel notices a special function of the forest to which I 

 have already alluded, but to which sufficient importance has not, 

 until very recently, been generally ascribed. I refer to the me- 

 chanical action of the roots as conductors of the superfluous hu- 

 midity 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 salu- 

 brious meadows and pastures, has been converted, by the destruc- 

 tion of the woods, into a vast expanse of pestilential pools and 

 marshes. In Solome the same cause has withdrawn from culti- 



* "The roots of vegetables," says d'Hericourt, "perform the office of drain- 

 ing in a manner analogous to that artificially practised in parts of Holland 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 hori- 

 zontal drains." — Annates Forestiires, 1857, p. 313. 



