SOIL AND SUBSOIL. 157 



DISTRIBUTION OF THE HUMUS WITHIN THE SURFACE SOIL. 



The uniform distribution of the humus-contents of the sur- 

 face soil, as shown in sections of the same, is by no means easil\ 

 accounted for. The roots from which its substance is so largely 

 derived are not so universally distributed as to account for 

 it ; but least of all can the rapid disappearance of the leaf-fall 

 and other vegetable offal from the surface be accounted for 

 without some outside agencies. Of these, the action of fun- 

 gous vegetation, and of insects and earthworms, are doubtless 

 the chief ones. 



Fungi. \Yhen we examine a decaying root, we find radiat- 

 ing from it a zone of deeper tint, as though from a colored 

 solution penetrating outward. Rut since under normal con- 

 ditions humus is insoluble, this explanation cannot stand. 

 Microscopic examination, however, reveals that the outside 

 limit of this zone is also the limit to which the fungous fibrils 

 concerned in the process extend ; and as these fibrils are much 

 more finely distributed and much more numerous than the 

 roots of any plant, it is natural that the humus resulting from 

 their decomposition should be more evenly distributed than 

 the roots themselves. 1 



Such fungous growth is not, however, confined to dead and 

 decaying roots only. A large number of trees and shrubs, 

 among them pines and firs, beeches, aspen and many others, 

 also the heaths, and woody plants associated with them, appear 

 to depend largely tor their healthy development, notably in 

 northern latitudes, upon the co-operation ("symbiosis") of 

 fungous fibrils that " infest " their roots, enabling them to 

 assimilate, indirectly, the decaying 1 organic (and inorganic) 

 matter which would otherwise be unavailable, and at the same 

 time converting that matter into their own substance. Fun- 

 gous growths thus mediate both the decomposition and 

 rehabilitation of the vegetable debris. 



The vegetative fibrils (mycelia) of several kinds of molds 

 are constantly present in the soil, and while consuming the 

 dead tissue of the higher plants, spread their own substance 

 throughout the soil mass. The same is true of the subter- 



1 Kosticheff, Formation ami Properties of Humus; in abstract T<>ur. ('hem. Soc. 

 .1891, p. 61 1. 



