74 VEGETATION OF THE PEAK DISTRICT [CH. 



4. Rocky knolls. The very driest parts of the ash woods 

 occur on the rocky knolls. Here the soil is extremely shallow; 

 and in places the bare rock protrudes. Trees and shrubs are 

 absent; and the absence of shade allows of the growth of 

 saxicolous lichens and bryophytes, of such ephemeral species as 

 Arenaria serpyllifolia, Erophila vema, and Saxifraga tridac- 

 tylites, and of such dwarf perennials as Sedum acre and Thymus 

 Serpyllum. Such a community does not, except in a topo- 

 graphical sense, belong to a woodland association at all, and 

 is to be regarded as an outlier of another association. 



Limestone screes and cliffs also occur in the midst of the 

 ash woods. These, if damp, become in time clothed with the 

 vegetation of the ash woods ; and, by comparing several such 

 localities, it is possible to gain some idea of a progressive 

 succession from bare screes and cliffs to a closed ash association. 

 Such a succession supplies the reason, a historical one, why 

 such plants as the mossy saxifrage (Saxifraga hypnoides) and 

 the limestone polypody (Phegopteris Robertiana) are sometimes 

 found on old screes in the midst of existing ash woods. 



Comparison of the Woodland Plants 

 OF THE Southern Pennines 



The numerous trees and shrubs which occur in the ash 

 woods and which are absent from the oak and birch woods have 

 already been mentioned (see page 71). To the species of this 

 class there mentioned, the following herbaceous plants may be 

 added : — 



Polypodium vulgare Poterium Sanguisorba 



Phegopteris Robertiana Geranium lucidum 



Phyllitis Scolopendrium G. sanguineum 



( = S. vulgare) Hypericum montanum 



Asplenium Trichomones H. hirsutum 



A. Adiantum nigrum Helianthemum Chamaecistus 



A. Ruta-muraria Viola hirta 



Cystopteris fragilis V. sylvestris 



Helleborus viridis Pimpinella major 



" H. foetidus " Polemonium coeruleum 



Cardamine impatiens " Lithospermum officinale" 



Draba muralis Satureia Acinos 



Sedum Telephium S. vulgaris 



Saxifraga hypnoides ( = Calamintha Clinopodium) 



S. TridactyUtes Origanum vulgare 



