Ghalk Grassland 175 



varies from place to place, partly owing to minor varia- 

 tions of the depth and nature of the soil, partly to the 

 incidence of exposure and of grazing. 



On the North Downs of Surrey and Kent the grassland 

 is more frequently mown, and in place of the sheep 

 pasture dominated by Festuca ovina, a mixed grass as- 

 sociation is present in which Bromus erectus, Trisetum 

 Jlavescens or Brachypodium gracile (sylvaticum) are 

 frequently dominant. 



The soil of the chalk grassland is typically very 

 shallow, often not exceeding an inch (2'5 cm.) 

 in depth. If the whole depth of soil, down 

 to the weathering rock, be taken together, estimations 

 show a very high percentage of calcium carbonate; but 

 the upper layers which accompany the greater part of the 

 intricate network of roots and rhizomes of which the 

 turf is composed, are tjrpically extremely poor in lime. 

 Corresponding with this very different nature of the 

 surface layer from that of the underlying soil, two groups 

 of plants, whose root systems are hardly competitive, may 

 be distinguished^. 



The lower, calcareous, soil is tenanted only by roots of 

 plants such as the rock-rose (Helianthemum Chamipcistus) , 

 the salad-burnet {Poterium Sanguisorha), the squinancy- 

 wort (Asperula cynancMca), by the tubers of orchids, and 

 generally by the underground systems of most of the 

 plants which may be called calcicole. The upper non- 

 calcareous layer, on the other hand, which is richer in 

 humus but more exposed to drought, is occupied by the 

 underground parts of shallow-rooted plants such as 

 Festuca ovina, which occur in all dry grassland. 



In the non-calcareous upper layer actual heath plants 



such as Calluna vulgaris and Potentilla 



heatii " erecta sometimes occur. The mixture of 



these with deeper rooting calcicoles gives 



' This is an example of a type of association which has been dis- 

 tinguished by Woodhead as complementary (Woodhead, 1905, p. 345). 



