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 

 Chamcemorus, 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. At similar 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 



