174 VEGETATION OF THE PEAK DISTRICT [cH. 
(91 cm.) to about fifteen feet (457 cm.): commonly it is about 
ten feet (30°5 cm.) deep; whilst locally in hollows these depths 
may be exceeded. The lower layers are almost constantly wet, 
and hence act as an impervious substratum to the upper layers 
which, however, are sometimes very dry in summer owing to 
evaporation. 
(4) Altitude. The heather moor rarely exceeds 1500 feet 
(457 m.) in altitude: the cotton-grass moors ascend to 2000 feet 
(610 m.). Between 1500 feet and 1750 feet (533 m.), heather 
and cotton-grass frequently share dominance. 
(5) Exposure. The exposed ridges and peaks, from 
1500 feet upwards, are characterized by an association of 
bilberry (Vaccinium Myrtillus), whilst on the highest plateaux, 
retrogressive moors occur. 
In general, it may be said that heather moors are found in 
the drier, more sandy, shallower, and less elevated regions, that 
cotton-grass moors dominate the wetter, purer, and deeper peat 
at higher elevations, that bilberry moors occur on the highest 
and most exposed ridges, and that the natural drainage result- 
ing from the denudation of the peat of the cotton-grass moors 
on the highest plateaux and watersheds produces the associa- 
tions characterized by Vaccinium Myrtillus, Empetrum nigrum, 
and Rubus Chamaemorus. 
(6) Structure. The moorland plants possess certain mor- 
phological or structural peculiarities which enable them to thrive 
in their respective surroundings. 
The vegetative organs of Hriophorum vaginatum and E. an- 
gustifolium and many of their associates are well provided with 
aération canals which enable the underground parts to respire 
although they are embedded in peat which is, during the 
greater part of most years, supersaturated with water. Such 
plants are neither complete xerophytes nor complete hydro- 
phytes, but possess both xerophilous and hydrophilous characters. 
This peculiarity of moorland plants has been pointed out by 
Warming (1896: 177). They are frequently termed “bog 
xerophytes” or “swamp xerophytes” (see also Yapp, 1909: 
275—6). 
The root-systems of Calluna vulgaris, Vaccum Myrtillus, 
and Hmpetrum nigrum are superficial; and these plants have 
no aération canals. These facts seem to be obviously related 
