138 VEGETATION OF THE PEAK DISTRICT [CH. 



of Somerset has been previously described (Moss, 1907 a : 49) ; 

 and, of the species of plants there mentioned, all except the 

 Welsh poppy (Meconopsis cambrica) occur in Derbyshire, though 

 in this locality the scaly fern (Ceterach oficinarum), the harts- 

 tongue {Scolopendrium vulgare), the yew {Taxus baccata), and 

 the whitebeam {Pyrus Aria) are much rarer than in Somerset. 



Here, the screes are never of great depth ; and very often 

 plants, whose aerial parts appear above the loose talus, are 

 rooted in the soil below. Such soil does not differ materially 

 from the rest of the soil of the limestone slopes, but, being 

 covered by stones, evaporation is less intense. Thus, a few 

 moisture-loving species, such as Allium ursinum, Geranium 

 Robertianum, Mercurialis perennis, Scrophidaria nodosa, and 

 Valeriana sambucifolia, apparently occur on the older screes. 

 Closer examination, however, proves that all these plants are 

 really rooted in the soil below the screes. There are, in fact, 

 no true " lithophytes " on the screes of Derbyshire or Somerset, 

 except perhaps the lichens and some of the mosses that grow 

 on the bare rocks and stones themselves. 



The screes, however, are interesting as they furnish examples 

 of open associations. Doubtless, in most cases, woodland or 

 scrub or grassland characterized the hill-slopes which are now 

 covered by the screes before the latter fell away from the rocky 

 escarpment above. The debris would destroy the original plant 

 associations ; and the new surface would thus afford a suitable 

 habitat for the invasion of plants from the neighbouring 

 associations. Newly formed screes, since they have very little 

 vegetation, may be regarded as edaphic deserts. In fact, 

 probably all open plant associations, in all non-arctic or non- 

 alpine districts, which have a mean annual rainfall of fifteen 

 inches (28 cm.) or more, may be so regarded. Only those plants 

 which have long subaerial organs are able to colonize the newer 

 screes. Where the screes are continually, though perhaps 

 slowly, accumulating, the plant associations remain in an open 

 condition. On such new screes, the following plants have been 

 observed thinly scattered about : — 



Phegopteris Robertiana Corylus Avellana (dwarfed) 



( = Polypodium calcareum) Geranium Robertianum 



Arrhenatherum avenaceum Teucrium Scorodonia 



Braxjhypodium gracile Fraxinus excelsior 



