250 BRITISH PLANTS 
2. On moist soils : 
(d) Heather-Moor, peat deep. 
3. On wet soils : 
(e) Molinia czerulea-Bog, a modification of the 
grass-heath. 
(f) Erica Tetralix- Moor, a modification of the 
heather-moor. 
(g) Myrica Gale-Bog, intermediate between the 
marsh and the heather-moor. 
(hk) Eriophorum-Moor (cotton-grass moor), on flat 
summits where rainfall is high. 
(‘) 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 Aira flexuosa. Calluna is always very 
abundant, but in exposed places is usually dwarf. Ulex 
europeus (gorse) is also occasionally very abundant. Of 
typical heath-plants associated with the foregoing, Vac- 
cinium Myrtillus (bilberry, whortleberry), Hrica cinerea 
