MOORLAND ASSOCIATIONS 249 



3. At any altitude where lime is quite absent from the 

 soil, as on dry, sandy, or pebbly heaths, where any lime 

 at one time present has been washed into the subsoil. 



Peat seldom accumulates directly on limestone or 

 chalk, for here the conditions are all in favour of bacterial 

 action. Plenty of oxygen and lime is present, and the 

 soil is warm, except at very high altitudes. When peat 

 does form in a limestone district, it can usually be traced 

 to the presence of a capping of some other material on 

 the limestone. The great bogs of Ireland, for example, 

 lie chiefly on limestone covered with boulder-clay. 



The plant-remains in moorland-peat, in contrast to 

 those in marshy peat, are well preserved. A section 

 through a thick deposit will often show a well-marked 

 stratification of the material from which the peat is 

 formed. At one level Sphagnum may predominate, at 

 another cotton-grass, whilst remains of birch and pine 

 are frequent. From these different layers we can gain, 

 not only an idea of the former vegetation, but also of the 

 changes in climate which the district has undergone. 



Humous acids are formed in all peaty soils, and, if 

 present in large quantities, the absorption of water is 

 rendered difficult, and the vegetation is of a pronounced 

 xerophytic type, as on damp heather-moors and cotton- 

 grass bogs. Here the dominant plants are either heath- 

 like plants with small evergreen rolled leaves (Figs. 9, 12), 

 or grass-like plants with erect cylindrical leaves. Mineral 

 food is scarce, especially the nitrates and phosphates of lime, 

 potash, and magnesia, and plants which can obtain food 

 from other sources than the mineral substances in the soil 

 have a big advantage over their neighbours. Carniv- 

 orous plants, for example, which obtain a large pro- 

 portion of their food from the bodies of insects, are 

 common in the wetter parts ; whilst others e.g., all the 

 heaths, cranberries, and some grasses obtain organic 

 food through the agency of fungi (mycorhiza) which 

 become attached to their roots. 



The various associations of moorland-plants may be 

 summarized as follows : 



1. On dry, poor soils, drainage good : 



(a) Grass-Heath, peat thin. 



(b) Calluna-Heath, peat thicker. 



(c) Vaccinium-Moor, alpine. 



