52 PLANT GEOGRAPHY 



the dark powder found in a hollow willow. This is 

 neutral, or alkaline, containing ammonia, but little humic 

 acid. The acid humus of peat-bogs, on the other hand, 

 is the result of the action of anaerobic bacteria, those 

 living in water-logged soil and not requiring free oxygen. 

 Where, however, lime and other bases are abundant, a 

 "mild," i.e. alkaline, peat may form, as in the "black 

 soil " of our English fens. This alkalinity may, in fact, 

 be considered as characteristic of " fens/' as distinguished 

 from acid " moor " lands. 



Earthworms not only swallow and digest large 

 quantities of vegetable matter, as well as the mineral 

 constituents of soil, adding the resultant " casts " as a 

 finely divided humus to the soil, but by their burrowing 

 they admit air into the surface soil. This is important 

 in the life-history of many of the bacteria. These 

 micro-organisms hardly occur beyond a small depth 

 from the surface. Some of them, such as the widely 

 distributed Azotobacter chroococcum Beijk., oxidise 

 organic carbohydrate, and thus obtain energy which 

 they employ in " fixing," i.e. combining, free atmo- 

 spheric nitrogen. Others, the nitroso- and nitro- 

 bacteria, transform respectively ammonia-compounds in 

 the soil into nitrites; and nitrites into nitrates, which 

 can be utilised by the roots of the higher plants. Others 

 again, mostly anaerobic, are denitrifying, reducing, that 

 is, nitrates into nitrites, ammonia or gaseous nitrogen. 

 Yet another type, such as Pseudomonas radioicola 

 Moore, lives upon waste root-tissue in nodular swellings 

 on the roots of Leguminosce and some other plants, and 

 fixes atmospheric nitrogen as nitrate which is available 

 for the use of the higher plant. 



In the humus of dark woods or on sandy heaths, soils, 

 that is, deficient in nitrates, the roots of many of our 

 forest-trees, both broad-leaved, like the Beech, and 

 coniferous, those of the brown saprophytes growing 

 beneath their shade, such as Monotropa and the Bird's- 

 nest Orchis, and those of Heaths and other xerophytic 

 plants, are invested with a felt of fungal mycelial 

 threads. These mycorhizce, as they are termed, belong 

 to various groups of fungi: they frequently replace the 

 root-hairs of the plant; and they may, or may not, 

 penetrate the surface of the root which they invest. 

 Not only has their presence been proved to be beneficial 



