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 officinarum), 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 aérial 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 Alliwm ursinum, Geraniwm 
Robertianum, Mercurialis perennis, Scrophularia 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 débris 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 
Brachypodium gracile Fraxinus excelsior 
