ECOLOGY OF WOODLAND PLANTS. 843 
is Deschampsia flexuosa, the dense, dry, wiry tussocks of which 
form an unfavourable medium for the germination of the seeds 
of Scilla; and although the two species are frequently found 
together, Scilla very rarely forms those unbroken sheets so 
characteristic of the moist areas and when associated with Holcus 
mollis. The thinning-out of this species in woods with a very 
shallow sandy soil, as in many of the woods in the gritstone 
area, is very striking. 
A. comparison of the soil-map with the bracken-map shows 
that a change of soils and accompanying conditions do not offer 
here a barrier to this species. 
Fig. 5 shows the distribution of common grasses. The grass 
vegetation is composed mainly of two species. In the moister 
parts, where the soils consist of fairly thick humus over loam, is 
Holcus mollis. In the drier parts to the north, especially where 
the ground is in rounded mounds with shallow, well-drained, 
sandy soil mixed with humus, Deschampsia flexuosa dominates. 
Along with this are scattered such xerophytes as Galium 
saxatile, Linn., Vaccinium Myrtillus, Linn., Calluna Erica, DC., 
and Teucrium Scorodonia, Linn. Here and there Holcus makes 
deep inroads into this area, occupying chiefly the moister humus- 
covered hollows. 
Tt will be seen, on comparing maps 2, 3, and 5, that Holcus 
mollis, Bracken (P. aquilina), and Bluebell (S. festalis) often 
occupy the same area and appear to be in competition with each 
other, but closer examination shows this is not the case. We 
have here a well-marked society or sub-association, the species 
of which are admirably adapted to each other's requirements. 
Holcus is a surface-plant (fig. 6, p. 344), its long rhizomes 
running in the loose leaf-mould, as may be determined by the ease 
with which it is uprooted. Beneath this in the deeper humus 
are the rhizomes of the Bracken, often running along the upper 
surface of or just within the loam, forming a distinct Bracken 
layer, the decay of its fronds contributing an annual quota for 
its higher associates ; while in the firm loam below are the bulbs 
of Scilla, though often we find in the Holcus and Bracken layers 
young bulbs on their way downwards. Their soil requirements, 
their modes of life, their periods of active vegetative growth, their 
times of flowering and fruiting, are for the most part different. 
The unbroken sheets of blue when Scilla is in flower in early 
spring, followed in the summer by equally continuous sheets of 
Pteris, form the most striking features in the woodland vegetation 
