CHAP. XX ANIMALS AND PLANTS IN SOIL 79 



it may be mentioned that mole-heaps and ant-hills very often support 

 a vegetation somewhat different from that on the surrounding soil.^ 



Marine soil. A role obviously of less general significance, though 

 similar to that played by earthworms in relation to plant-life on terrestrial 

 soil, is played by species of Arenicola in relation to Zostera-vegetation 

 on the marine soil of European coasts.^ 



Saprophytic plants in the soil. 



A more important part is played by saprophytic plants in the soil, 

 especially by fungi and bacteria, than by animals. 



Fungi in soil. In all kinds of soil with abundant humus, fungal 

 mycelia live ; forest-soil in northern Europe in autumn, by its wealth 

 of Basidiomycetes, reveals the extent to which it is permeated by fungal 

 hyphae. But even when few or no fungi exhibit themselves above 

 ground, microscopical examination will certainly demonstrate their 

 presence in all humus-laden soil, even in acid heath-peat ; hyphae of 

 Cladosporium humifaciens occur in this, while the ^oots of Calluna and 

 other denizens have mycorrhizae, just like the majority of forest-trees 

 and some perennial herbs living on humus.^ Saccharomycetes hibernate 

 in soil.^ 



Bacteria in soil. These are of still greater importance than fungi. 

 They occur in all soil and in all water, in terrestrial soil, in the various 

 types of mud, in sahne and fresh water. In the uppermost layers of soil, 

 especially near human dweUings, they occur in millions upon millions ; 

 in soil occupied by vegetation their number increases with the depth 

 as far down as about half or three-quarters of a metre ; it then rapidly 

 decreases until, at a depth of live to six metres, there are as a rule no 

 bacteria : the soil has filtered them out of the percolating water. The 

 investigations of Adametz, according to Sacchse,^ gave the following 

 results : 



Number of Bacteria in one Gramme of Soil according to Adametz. 



Nature 0/ sou '^'P^ ^'X "/J."''""' ^'"""^^ l "<'"'-'- 



Sandy soil At the surface 380,000 



20-25 460.000 



Clay soil the surface 500,000 



20-25 464,000 



Other investigators have found in one gramme of soil up to a million 

 bacteria. The number of course depends upon various conditions. 



The number of species concerned is probably exceedingly great, 

 and we know that some of them play a prominent part in the biology of 

 soil. Some are aerobic, others anaerobic. There are present not only 

 ordinary putrefactive bacteria, many of which are of the highest signifi- 

 cance in regard to the composition of the air in soil, but also pathogenic 

 species, for example the tetanus germ (Bacillus tetani), as well as others, 

 including nitrifying and denitrifying bacteria, which cause the formation 

 of important chemical compounds in soil. Schlosing and Miintz were 



' Buchenau, 1876 ; Warming, 1804, 1906 ; P. E. Miillcr. iS()4 ; also sec p. 66. 

 ' Rosenvinge, 1889-90, see Warming, 1906; concerning Corophium sec Warming, 

 1906. ' See Chapter XXV. 



* C. E. Hansen, 1881. ' Sacchsc. 1S88. 



