250 BEITISH PLANTS 



2. On moist soils : 



(d) Heather-Moor, peat deep. 



3. On wet soils : 



(e) Molinia cseruIea-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 CaUuna-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 Aira 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 



