vit] 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 
C. 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 (SiO,) 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 aération of the peat; and 
the peat of the heather moor is much better aérated 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 
