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 diy 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 

 bilbeiTy {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-gi-ass moors 

 on the highest plateaux and watersheds produces the associa- 

 tions characterized by Vaccinium Myrtillus, Empetrum nigrum, 

 and Ruhus Chamaetnorus. 



(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 EriopJiorum vaginatum and E. an- 

 gustifolium and many of their associates are well provided with 

 aeration 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 vulgaHs, Vaccinium Myrtillus, 

 and Empetrum nigrum are superficial ; and these plants have 

 no aeration canals. These facts seem to be obviously related 



