VIl] MOORLAND ASSOCIATIONS 173 



Other mosses 



Tetraphis pellucida Leptodontium flexifolium 



Dicranella crispa Splachnum sphaericum (on dung) 



D. cerviculata Aulacomnium palustre 



D. heteromalla Webera nutans 



Campylopus flexuosus Bryum pallens 



G. pyriformis Mnium subglobosum 



C. fragilis Hypnum fluitans 

 Dicranum Bonjeani H. exannulatum 



D. scoparium H. revolvens 

 Leucobryum glaucum H. falcatum 

 Rhacomitrium spp. (rare) H. stramineum 



FACTORS RELATED TO THE DISTRIBUTION OF THE 

 MOORLAND ASSOCIATIONS 



Factors which appear to be of importance in determining 

 the various plant associations of the moors are (1) the relative 

 amounts of sand and humus in the soil, (2) the amount of 

 water in the soil, (3) the depth of the peat, (4) the altitude 

 above sea-level, (5) the exposure to winds, (6) the anatomical 

 structure of the moorland plants, (7) a change in the nature 

 of the habitat, and perhaps (8) rainfall. 



(1) Sand and humus. The peat of the heather moors 

 yields a much greater quantity of silica (Si0 2 ) than the peat of 

 the cotton-grass moors. 



(2) Water. The peat of the heather moors possesses a 

 much lower physical water-content than the peat of the cotton- 

 grass moors ; and this fact illustrates the general rule that the 

 more humus a soil contains the more water it also contains. 



It would appear that the insoluble particles of silica are of 

 some importance in controlling the water-content, and thus of 

 importance in distinguishing the various associations. Corre- 

 lated with the water-content is the aeration of the peat ; and 

 the peat of the heather moor is much better aerated than that 

 of the cotton-grass moor. 



(3) Depth. The peat of the heather moor is shallower 

 than the peat of the cotton-grass moor. That of the heather 

 moor varies from a few inches to about four or five feet (122 or 

 152 cm.), and is commonly about a foot (30'5 cm.) in depth. 

 That of the cotton-grass moor varies from about three feet 



