IMPORTANCE FOR WOOD-PRODUCTION. 587 



litter, but also exerts itself wherever the roots of the trees pene- 

 trate. The presence of dead roots in the soil is not therefore a 

 matter of indifference and it may be doubted whether the 

 extraction of stumps is not prejudicial to the fertility of the 

 soil. 



Humus yields not only assimilable mineral nutriment, but 

 also the carbon-dioxide necessary for extracting nutriment frorn 

 the sub-soil ; it is therefore indispensable for the nourishment 

 of plants both in poor and rich soils. It is also obvious that 

 a scarcity of humus must be prejudicial to the production of 

 wood, owing to the enormous requirements of carbon-dioxide by 

 plants and the large quantity of this gas passed into the air 

 by decomposing organic matter. 



2. Mode of Decoiiq)ositio)i of Foi-est Litter. 



In describing the beneficial effects of forest litter and humus 

 the manner in which their decomposition is effected has been 

 pre-supposed, but now requires some explanation. It is well- 

 known that the decomposition of all organic matter can only be 

 effected by the agency of bacteria (fission-fungi). The necessary 

 conditions for the life of these organisms are the action of air, 

 and of a certain amount of moisture and heat, [whilst light is 

 prejudicial to them. — Tr.]. Hence it follows — as those factors 

 are not always equally effective, sometimes one and sometimes 

 another of them preponderating and some vegetable sub- 

 stances decomposing more readily than others — that the rate of 

 decomposition varies in different cases and the products of 

 decomposition themselves are variable. 



The comparative rate of the decomposition of forest litter and 

 humus is chiefly iufiueneed by the kind of soil- covering, soil, 

 locality, climate, nature of standing crop, &c. 



(a) Kind of Soil-Covering. — Soft and only slightly lignified 

 parts of plants decompose most rapidly. Thus, of broad-leaved 

 trees, the dead leaves of the hornbeam, ash and lime decompose 

 more rapidly than those of oak, beech and birch. Of conifers, 

 larch needles are soonest decomposed, then Scotch pine needles, 

 then those of silver-fir, and most slowly, those of spruce. It is 

 generally true, that dry leaves of broad-leaved trees decompose 



