250 BRITISH PLANTS 



2. On moist soils : 



(d) Heather-Moor, peat deep. 



3. On wet soils : 



(e) Molinia caerulea-Bog, a modification of the 



grass-heath. 

 (/) Erica Tetralix - Moor, a modification of the 



heather-moor. 

 (g) Myrica Gale-Bog, intermediate between the 



marsh and the heather-moor. 

 (h) Eriophorum-Moor (cotton-grass moor), on flat 



summits where rainfall is high. 

 (i) Sphagnum-Bog, on constantly wet rocks, etc., 



where water is not very rich in humous acids. 



1. Grass-Heath. 



On well-drained soils possessing a thin covering of 

 peaty material a grass-community becomes established. 

 This grass-heath is similar in many respects to the natural 

 pasture, but is at once distinguished by the presence of 

 Calluna vulgaris (heather) and other heath-plants. It 

 occurs on the steeper slopes of mountains, and occa- 

 sionally on gravelly soil in the lowlands. Where the 

 slope of the ground is more gradual, peat accumulates 

 to a greater extent, and Calluna competes successfully 

 with the grasses, with the result that an association 

 intermediate between the grass-heath and the Calluna- 

 heath is formed. A similar intermediate association may 

 develop through the artificial drainage of a heather- 

 moor. The grass-heath in some cases arises as a sub- 

 stituted association when a Calluna-heath is used as 

 grazing-land. The heather is destroyed to a greater or 

 less extent, and grasses take its place ; but it soon reverts 

 to its original condition if cattle are kept off the land. 



Festuca-Agrostis-Anthoxanthum Association. On the 

 richer soils the dominant grasses are those of the natural 

 pasture : Festuca ovina, Agrostis vulgaris, Anthoxanthum 

 odoratum, and Air a flexuosa. Calluna is always very 

 abundant, but in exposed places is usually dwarf. Ulex 

 europceus (gorse) is also occasionally very abundant. Of 

 typical heath-plants associated with the foregoing, Vac- 

 cinium Myrtillus (bilberry, whortleberry), Erica cinerea 



