62 THE SCOTTISH BOTANICAL REVIEW 



chiefly obtain their nitrogen from inorganic ammonium 

 salts. 



The importance of lime, in determining fen vegetation, is 

 possibly due to its powers of retarding the transformation of 

 inorganic ammonium and potassium salts into less assimil- 

 able combinations with humus derivatives, such as occur 

 where lime is wanting and acid humus accumulates. Where 

 the latter begins, inorganic compounds of nitrogen are largely 

 superseded by combinations of nitrogen and humus, and 

 moorland plants take possession. This may ensue from 

 changes in the physiography cutting off the supply of 

 calcareous waters and inducing stagnancy, or from a lowering 

 of the water-level followed by leaching of the surface layers 

 of the humus (3) (26) (13). Deeper rooted fenland species 

 may then continue to live for a time side by side with 

 shallower rooted phanerogams, mosses, and lichens from the 

 moorland. 



The scarcity of inorganic nitrogen compounds in acid bog- 

 waters suggests that the numerous species of algae, and some 

 mosses, e.g. the Sphagnacese, may be capable of using some of 

 the more oxidised, soluble compounds of humus with 

 ammonia, such as ammonium crenate, for the purposes of 

 obtaining nitrogen, while the densely tufted species of mosses, 

 with a highly developed tomentum of rhizoids, may be 

 partially saprophytic for similar purposes. If these mosses 

 had far to search for their inorganic food, one would not 

 expect the closely interwoven and limited extension of the 

 rhizoids that characterises the species that live in humus. 

 The tomentum of PolytricJmvi strictum has so close a re- 

 semblance to a mycorhiza that it may well be believed to 

 function in a similar manner. Micorhizae have been detected 

 in several species of hepatics, and are probably also present 

 in mosses (35), 



4. Mesophytes that flourish in moist, well-aerated soils 

 adapted to the formation of nitrates. In this climate porous 

 soils need to be sufficiently deep, and well provided with 

 humus, to retain moisture and supply nitrogen compounds, 

 and with sufficient carbonate of lime to prevent acidity. 



Habitats having these soil-characters will only occur where 

 there is an abundant supply of lime in the soil-waters, or 



