762 



FOREST-LITTER. 



covering of dead leaves, which reduces the temperature during 

 that season must be injurious, and there would be no growth 

 at all in the spruce, silver-fir and beech forests of the Bavarian 

 plateau. 



Mayr's observations prove, that during the months May- 

 August, the following temperatures prevail in July : 



Hence it follows : that the soil- covering of litter, by itself, 

 and when united to the cover of the trees, cools the soil 

 considerably; that quite low temperatures suffice for the 

 activity of the roots of trees, so that the cooling of the soil 

 during the season of growth is not prejudicial : the con- 

 sequent elevation of the temperature of the soil in winter, is 

 indifferent to the plants, but involves a continuous chemical 

 decomposition of the litter, whenever this temperature is 

 above freezing point. 



In soils containing humus, according to Frank, fungi which 

 form a mycorhiza on the roots of most forest trees, are always 

 present, they are absent in soil deprived of humus [or 

 artificially sterilised by burning. Tr.] and a long time passes 

 before mycorhizae are produced. Owing to this symbiosis of 

 plants and fungi, the former not only derive nutritive matter 

 through the humus, but are enabled to obtain nitro- 

 genous substance indirectly from the atmosphere. 



(d) Fertility of the Soil. 



Humus improves the productivity of soils, chiefly by its 

 physical power of absorption, and also by its own decompo- 

 sition and conversion into nutritive material. Water and 

 water-vapour are absorbed by humus, as well as the most 



