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 non- 

 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 sessilijiora) 

 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 (o), rare (r), very rare (vr), 

 local (1), occasional to abundant (o 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. 



