THE FOREST AND SPRINGS 371 



winter. The fact may be accepted as almost certain in the case of Scotch pine and larch, 

 and it is probable even of the spruce, the tree of our coimtry whose foliage is the thickest. 

 Influence of Forests on the Infiltration of Water. — In the preceding paragraph 

 the influence of conditions on the quantity of water which reaches the soil has been 

 shown. It remains for us now to examine how forests modify the conditions of the 

 feeding of springs, in working on the deep infiltration of the water as far as the sub- 

 terranean sheet, of which springs constitute the overflow. 



Of the water reaching the ground, one part runs along the surface and arrives directly 

 at the water courses in the form of streams. Those waters which run along the sur- 

 face without penetrating into the soil are called "wild waters" — and "coeflScient in 

 surface flow," is the term (le nombre) which expresses their relative importance. 



A second part returns directly to the atmosphere, in a gaseous condition, as a result 

 of the phenomenon of physical evaporation. 



A third part, after having penetrated the superficial strata of the soil, is extracted 

 from it by suction of roots which carry it into the body of the plants. This water is 

 partiaUy utilized in forming vegetable tissues, but the greater part returns to the atmos- 

 phere in gaseous form by the stomata of leaves, after having brought into the latter 

 the mineral elements necessary to the growth of the plant. This important phenomenon 

 is called physiological evaporation; it carries away from the soil considerable quantities 

 of water which have already penetrated to greater or lesser depths according to the 

 dimensions of the vegetation. One understands in fact that the zone thus drained 

 is quite near to the surface in the case of grass or cereals with superficial roots, while 

 it can be fairly deep in the case of forests whose roots penetrate very far down in 

 permeable soils. 



Lastly, a fourth part having penetrated, thanks to the permeabihty of the soil, 

 manages to pass through the entire depth of the region where roots can inhale, or from 

 which, by means of capillary action, it can raise itself into the region where the roots 

 are active. It penetrates deeper and deeper into the soU until, encountering the obstacle 

 of an impermeable stratum, it accumulates and founds a subterranean sheet of free 

 water. It is this sheet which dispenses itself outwards when the conditions stated 

 in Paragraph I of this chapter have been fulfUled. If the lie of the ground is such 

 that the water cannot reach the open air, the subterranean sheet is then exploited 

 by means of wells, and it has been proposed ^^ to call the sheet nearest to the surface a 

 "phreatic sheet," because it is this one that feeds the wells, their depth being naturally 

 limited to the level of the upper part of the highest subterranean sheet, and not gener- 

 ally getting beyond it even when this level sinks. 



Before proceeding further, it is necessary to establish an essential distinction between 

 springs in mountainous regions and those in a country of plains. 



In the mountains the surface flow plays such an important part in the question we 

 are about to consider that we need only concern ourselves with this phenomenon, after 

 setting aside those of evaporation and permeabihty. The influence of the loss due 

 to superficial running waters ("wild waters") surpasses all others. This special and 

 most simple case is the only one upon wliich one can formulate absolutely certain con- 

 clusions. It is, moreover, much the most important; springs are infinitely more numer- 

 ous, abundant, and useful in the mountains than in the plains. Springs in the plains 

 are either fed by waters which have filtered down from the mountains or else they have 

 virtually no influence on the regular course of waters on account of their feeble outflow. 

 The rainfall in low regions is, in fact, too feeble, after the levy made on it by agricultural 

 vegetation generally speaking, to allow remaining a sufficient proportion to feed the 

 springs. Often indeed the soil of the plains bears crops, ^ which, in order to develop, 



" Daubree "Les Eaux Souterraines," Vol. I, p. 19 (Paris, Dunod, pub., 1887). 



