NOTES FROM OAK AND BEECH FORESTS IN DENMARK. 245 



air and water ; it reacts neutral or faintly acid or faintly alkaline. 

 The vegetation of higher plants on this soil varies according to 

 the warmth and light of the place, the moisture of the soil, and 

 its content of available mineral matter." This soil, when not too 

 deeply shaded, carries a varied vegetation like that already given 

 for a good beech and oak forest. A full list of species will be 

 found in Warming's (Ecology of Plants (p. 332). 



" Mor " is formed when the organic refuse accumulates as a 

 coating above the mineral soil, the animal life being absent or 

 for various reasons unable to carry on the mixing of soil and 

 humus with sufficient energy. Fungi play a prominent part in 

 the conversion of the organic refuse on the surface of the soil 

 into humus, while bacteria play a minor part. Decomposition 

 proceeds slowly, extending often over many decades, and during 

 this process large quantities of humic acids (colloidal, non- 

 absorptive, saturated humus bodies) are developed. On being 

 carried down into the soil by rain water, these " acids " withdraw 

 from the upper soil-layers a large quantity of soluble mineral 

 matter (salts of iron and lime, alkaline bases, etc.), leaving the 

 upper soil-layers leached and poor, but depositing the absorbed 

 mineral matter at some distance below the surface, often in the 

 form of hard-pan. The uppermost humus layer or "mor" 

 varies, sometimes loose, sometimes felted together, but the 

 leached layer of soil under the " mor" is always close and dense. 

 The hard-pan layer also varies from a loose earthy texture to a 

 hard stony crust impenetrable by roots of plants; in this latter 

 condition it consists of grains of soil fused together by colloidal, 

 humic compounds. A "mor" soil is always strongly acid. 

 The soil-bacteria are greatly reduced in number, and have little 

 power to decompose cellulose and to break up the peptones, 

 and do not lead to development of nitric acid. In older 

 deposits of "mor," the nitrogen is so locked up that the plants 

 are starved as regards nitrogen compounds. Almost every 

 species growing on "mor" has mycorhiza (fungus-roots), in 

 which the lower organism is the main agent in acquiring 

 nitrogenous compounds for the roots of the higher plant. The 

 part played by these mycorhiza in supplying the mountain 

 pine {Fifiiis moniana) with atmospheric nitrogen is one of 

 P. E. Miiller's important contributions to the problem of 

 afforestation of sandy heaths. Other workers have demon- 

 strated the utility of mycorhiza in heather and other Ericaceae, 



