38 PRACTICAL FORESTRY 
mulch, which protects the mineral soil from the de- 
structive action of sun and wind. This humus must 
be of the proper nature, and must disintegrate in pro- 
portion to the amount of leaf and litter fall. It must 
not accumulate in immense masses, such as the peat 
or duff beds of the North, but must decompose into 
materials of such a nature that the roots may be con- 
stantly and regularly fed during the growing season. 
This decomposition depends upon warmth and mois- 
ture, but also, however, upon the bacteria and fungi, 
as well as animals, which live in the soil. <A peat-bed 
in which these are not present is sour and unpro- 
ductive. 
The root-fibrils absorb the moisture of the soil in 
large quantities. This moisture passes through the 
tree and evaporates from the leaves. Water is the 
vehicle which carries the mineral materials which the 
tree requires in the process of growth. These min- 
eral matters are often in very dilute solution, so that 
the amount of water thus used is immense. In the 
process of food absorption by the root-fibrils the 
tree is often, if not in the majority of cases, aided by 
the fungi in the humus, which attach themselves to 
the roots. This is a sort of mutualism called sym- 
biosis. In the case of the alders and leguminous 
trees, nodules of bacteroids are much in evidence, 
while with the beech, spruce, and other trees the 
