78 VEGETATION OF THE PEAK DISTRICT [CH. 
this district, the physical properties of the soil and accompanying 
conditions play a more important part in determining the 
character of the plant associations and the distribution of 
species than the chemical composition of the soil.” It has, 
however, to be remembered that the district investigated by 
Woodhead is quite destitute of calcareous soils. My own 
observations lead me to believe that in western Europe the 
presence or comparative absence of calcareous compounds in 
the soil is, directly or indirectly, a factor of prime importance 
in the distribution both of plant communities and of species, 
that within the limits of any particular district possessing only 
calcareous soils the water-content of the soil is a factor of great 
importance, and that ‘within the limits of any particular nen- 
calcareous area two soil factors have to be considered, namely, 
the amount of water and the amount of acidic humus. This 
view is essentially in harmony with that taken up by Graebner 
(1895, 1901, 1909, 1910), and by Nilsson (1902). Graebner 
has maintained that natural divisions of vegetation will only be 
reached by basing the classification on the richness or poverty 
of the soil, whilst the water-content of the soil furnishes a 
useful factor for the subdivision of the vegetation-divisions 
thus obtained. It is probable, however, in districts such as the 
higher Alps and in tropical countries, that other master-factors 
come into play having a more important effect on vegetation . 
than the presence or absence of lime in the soil; and probably 
the same occurs in some aquatic habitats, such as rapidly 
flowing streams. 
The following is a list of the constituent plants of the ash 
(Fraxinus excelsior) woods and of the oak (Quercus sessiliflora) 
woods of the Peak District. The frequency of each species is 
indicated by letters in the two columns :—dominant (d), sub- 
dominant (s), abundant (a), occasional (0), rare (r), very rare (vr), 
local (1), occasional to abundant (0 to a), etc. Herbaceous 
species which are confined or almost confined to the more 
shady parts of the woods are printed in thick type, and those 
preferring marshy places are printed in italics. Species which 
are not indigenous but which have been planted either in the 
woods or on the sites of former woods are preceded by an 
asterisk. 
