254 BRITISH PLANTS 
live completely submerged, on account of the diffi- 
culty of obtaining sufficient air. The plant can only 
grow in quantity when the rainfall is heavy and the 
atmosphere very humid. Sphagnum-bogs develop com- 
monly on wet, sloping rocks in mountainous regions. 
Here plenty of water is available at all times, but 
the slope of the rock allows the surplus water to drain 
away. 
When the plant is growing luxuriantly, it forms a deep, 
loose covering in which other plants find a difficulty in 
taking root, and, further, only those plants which can 
keep pace with the upward growth of the moss can exist. 
All other plants will be smothered. The most frequent 
plants are those with long straggling stems, rooted in 
the firmer peat below—e.g., Vaccinium Oxycoccus, Rubus 
Chamemorus, Empetrum nigrum, and monocotyledons 
with long narrow leaves, which can grow up between the 
moss—e.g., Narthecium, Eriophorum, sedges, and rushes. 
Erica Tetralix occurs occasionally ; also Drosera rotundi- 
folia, Pinguicula, and small creeping plants—e.g., Ana- 
gallis tenella, Campanula hederacea, and Selaginella 
selaginoides. 
6. Eriophorum-Moor. 
On the flat summits of moorland (1,250 to 2,000 feet), 
especially in the Pennines, vast areas are covered with 
an association of cotton-grass, locally known as moss- 
moors. The peat is always thick, sometimes reaching a 
depth of 30 feet, and saturated with the sourest water. 
The development of this association over large areas is 
dependent on very heavy and continuous rainfall—at 
least 40 inches per annum, since the only water which 
reaches it comes from above. Atsimilar altitudes where the 
rainfall is less a heather-moor develops. The Eriophorum- 
moor is also found at lower levels where water is abundant 
—e.g., the levels of Somerset. A very constant feature 
of these moss-moors is their shape. They are higher 
in the centre than at their edges, so that the general 
appearance is that of an inverted saucer. Bare peat 
is very common where water has denuded the sur- 
face. 
Eriophorum vaginatum is usually dominant, sometimes 
