158 SOILS. 



ranean or " root " mycelia of the larger fungi, toadstools, 

 mushrooms, which are commonly found about dead stumps 

 and other deposits of decaying vegetable and animal offai. 

 All these being dependent upon the presence of air for their 

 life functions, remain within such distance from the surface as 

 will afford adequate aeration ; the depth reached depending 

 upon the perviousness^of the soil and subsoil. In the humid 

 region this will usually be within a foot of the surface, but in 

 the arid may reach to several feet. Ultimately these organ- 

 isms contribute their substance to the store of humus in the 

 land. 



On the surface of moist soils we frequently find a copious growth of 

 green fibrils, which may be either those of algae, such as Oscillaria, or 

 the early stages (prothallia) of moss vegetation. This vegetation has 

 been credited with absorption of nitrogen from the air, thus enriching 

 the soil; but later researches have shown this effect to be due to 

 symbiotic bacteria (see above p. 156). 



Animal Agencies. Darwin first suggested that wherever the 

 common earthworm (Liunbricus) finds the conditions of ex- 

 istence, it exerts a most important influence in the formation 

 of the humous surface-soil layer; and the limitation imposed 

 upon these conditions by the subsoil has doubtless a great deal 

 to do with the sharp demarcation we often find between it and 

 the surface soil. Briefly stated, the earthworm nourishes itself 

 by swallowing, successively, portions of the surrounding earth, 

 digesting a part of its organic matter and then ejecting the un- 

 digested earth in the form of " casts/' such as may be seen by 

 thousands on the surface of the ground during or after a rain. 

 Darwin (The Formation of Vegetable Mold, 1881), has cal- 

 culated from actual observation that in humid climates and in 

 a ground fairly stocked with these worms, the soil thus brought 

 up may amount to from one-tenth to two-tenths of an inch 

 annually over the entire surface; so that in half a century the 

 entire surface foot might have been thus worked over. Aside 

 from the mechanical effect thus achieved in loosening the soil, 

 and the access of air and water permitted by their burrows, the 

 chemical effects resulting from their digestive process, and the 

 final return of their own substance to the soil mass; also their 



