58 VEGETATION OF THE PEAK DISTRICT [CH. 
Influence of Shade on the Ground Vegetation 
There are practically no places within the oak woods where 
the shade is too great for the growth of a carpet of vegetation ; 
but where the beech (* Fagus sylvatica) has been introduced 
and planted in mass or even only in patches, such places occur. 
Where the shade is most dense under the beeches, practically 
no plants are to be found except a few mosses, and in autumn 
some of the larger fungi. Where the shade is less dense, a few 
grasses, such as Holcus mollis and Deschampsia flexuosa, may be 
scattered about; and mixed with them are a few shade-loving 
species, such as dog’s mercury (Mercurialis perennis) and wood 
sanicle (Sanicula europaea). 
The sycamore (* Acer Pseudoplatanus) also casts much shade ; 
but, as this is usually introduced singly among the indigenous 
oaks, the tree does not usually bring about a great change in 
the ground vegetation. 
The pine (* Pinus sylvestris) is frequently planted in mass ; 
and, when planted closely, much shade is thrown, and the 
original ground species tend to disappear. 
Of indigenous trees, the wych elm (U. glabra = U. montana) 
casts the greatest shade; but on the siliceous soils this species 
does not usually occur as a social tree except where planted. 
ALDER-WILLOW THICKETS 
In some of the valleys, there is, at the present time, no 
extensive tract of woodland at all. This is the case, for 
example, in the Edale Valley and around Chapel-en-le-Frith. 
There can be little doubt that, in all such cases, the primaeval 
woodland has been destroyed and the land put under cultivation. 
Trees, however, often line the banks of the streams which flow 
through such localities, and form narrow fringing thickets which 
may be a mile or so in length. The most abundant trees are 
the alder (Alnus glutinosa), willows (e.g. Salta fragilis and 
S. cinerea), and the ash (Fraaxinus eacelsior). These alder-willow 
thickets are to be regarded as the persisting and lingering 
remains of the alder-willow association which doubtless fringed 
practically all the streams when the latter flowed through the 
