IMPORTANCE FOR WOOD-PRODUCTIOX. 581 



requisite amount of moisture. The action of hamus and litter 

 is in this respect threefold, viz. : The mechanical impediment it 

 aftbrds on slopes to rapid drainage of surface-water from 

 atmospheric precipitations, and the time thus allowed for the 

 water to sink into the soil-covering and the soil ; the sponge- 

 like action possessed by dead leaves and moss of absorbing and 

 retaining water, and the consequent reduced evaporation of 

 water from the soil. 



Without a sufficient supply of water, other productive agencies 

 of the soil are unavailing; it may therefore be affirmed that the 

 most important influence of the soil-covering on woody growth 

 is the supply of water it affords. Only about three-quarters of 

 the rain falling on the leaf-canopy of a forest actually reaches 

 the ground, much of it being broken into spray by the 

 branches and foliage and re-absorbed by the air. So much 

 the more important is it, that forest soil, as compared with 

 cleared land, should have the means of utilizing all rain-water 

 it secures. 



Most of the rain-water falling on a mountain-side, where 

 the soil has been deprived of litter, moss and humus and has 

 consequently become hard and compact, runs down to the 

 valleys, only a small part of it soaking into the soil. If, how- 

 ever, the spongy soil-covering has been preserved, much of the 

 rain penetrates it and is retained, gradually cbaining down into 

 the ground. This mechanical action of the soil- covering is 

 therefore highly important in mountain-forests. 



The water which the soil thus secures is also most thoroughly 

 retained by the absorptive action of the soil-covering; dry 

 coniferous needles can absorb 4 to 5 times their weight in 

 water, dry beech leaves 7 times, and mosses 6 to 10 times this 

 amount, without allowing it to dribble away. This absorptive 

 power of the soil-covering is further increased by that of humus 

 for water-vapour, which, becoming condensed in the cool soil, 

 increases the supply of moisture. Once the soil-covering is 

 thoroughly saturated with water from atmospheric precipita- 

 tions, it passes on the superfluous water to the subjacent soil 

 in the numerous interstices of which it is distributed, and thus 

 reaches the roots of the trees. 



When, however, a mossy soil-covering becomes very thick, 



