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 verna, and Sazifraga tridac- 
tylites, and of such dwarf perennials as Sedwm 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, 1t 18 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 
Phegopteris Robertiana 
Phyllitis Scolopendrium 
(=S. vulgare) 
Asplenium Trichomones 
A. Adiantum nigrum 
A. Ruta-muraria 
Cystopteris fragilis 
Helleborus viridis 
“H. foetidus” 
Cardamine impatiens 
Draba muralis 
Sedum Telephium 
Saxifraga hypnoides 
8. Tridactylites 
Poterium Sanguisorba 
Geranium lucidum 
G. sanguineum 
Hypericum montanum 
H. hirsutum 
Helianthemum Chamaecistus 
Viola hirta 
VY. sylvestris 
Pimpinella major 
Polemonium coeruleum 
“ Lithospermum officinale” 
Satureia Acinos 
S. vulgaris 
(=Calamintha Clinopodium) 
Origanum vulgare 
