52 PRACTICAL FORESTRY 
decomposing, furnishes materials upon which plants 
feed. A virgin forest-floor is full of nourishing plant- 
food. Humus forms a mulch or blanket over the sur- 
face which holds the soil and prevents leaching and 
washing. 
The nature of humus varies in different parts of 
the world. Its formation is dependent upon warmth 
and moisture, and is aided by various fungi which live 
in it, and by animals, such as earthworms, which 
ventilate and work it. In northern regions vegetable 
matter accumulates in the form of peat, and fails to 
decompose as rapidly as it forms because of the cold 
and absence of soil-fauna. In temperate regions it 
disintegrates much more rapidly, and in tropical dis- 
tricts, in the presence of sufficient moisture, the pro- 
cess of humification is very rapid. In the forests of 
the northeastern United States the litter on the sur- 
face is called “ duff.” 
While the accumulation of such material protects 
the surface of the soil and holds moisture, trees will 
grow with more rapidity in regions such as exist in 
the tropics, where the humus disintegrates quickly, 
and constantly supplies the root system of the forest 
with a great abundance of the proper kind of food 
material. In dry and windy countries, and in woods 
where there is insufficient shade, the formation of 
humus is hindered. The litter is dried by the sun and 
