968 MR. T. W. WOODHEAD ON THE 
We will now consider some of the modifications noted in the 
more important species characterizing the several associations of 
the Huddersfield distriet. 
(a) DOMINANT SPECIES. 
PTERIS AQUILINA, Linn. (Pteridium aquilinum Kuan). 
Bracken. 
It has been already shown ihat the distribution of Bracken is 
extensive in this district. It is the dominant plart of the 
undergrowth in the Coal-Measure Oak woods, where it forms 
large and characteristic masses, but when the dominant trees 
are Sycamore, Elm, and Beech, or a dense shrubby undergrowth 
of Hazel or Elder, their deep shade tends to restrict its distri- 
bution. Outside these woods, it is confined mainly to the 
hedgerows, perhaps a relie of a former more extended distribution. 
It ascends into the Gritstone area, where, in the dry shallow 
soils, it is limited by competition with rhizomatous plants such 
as Ling and Bilberry. In the open Bireh-Oak woods along 
the edges of the Gritstone Plateau many observations were 
made to determine the positions these rhizomes occupy with 
relation to each other. It was found that, in 95 per cent. of the 
plants examined, the rhizomes of Bracken were from four to six 
inches nearer the surface than was found to be the case ina 
corresponding number of observations in the Coal- Measure Oak 
woods. In areas where Bracken is associated with Ling and 
Bilberry, its rhizomes were found to be distinctly below those of 
the latter; while the rhizomes of Ling, though generally two to 
three inches above those of the Bracken, were not uncommonly 
found at the same level, and often they were distinctly interlaced. 
Here Bracken rarely forms continuous sheets, but is broken up 
into more or less isolated patches, as shown in the Heather area 
in the map of Armitage Bridge Woods (fig. 8, p. 349) ; and this is 
a characteristie feature in all dry Oak and Birch woods wherever 
these plants are associated; they form, as we have seen, a com- 
petitive association, and sometimes one, sometimes the other 
species dominates. In such situations, therefore, Bracken is 
much more exposed to adverse conditions, such as higher level, 
dry shallow soils, strong winds, much less protection by trees; 
and the rhizomes being more superficial, they are more likely 
to suffer injury through cold. As might naturally be expected, 
