CHAPTER XXVI 
GRASSLAND ASSOCIATIONS 
1. Natural Pasture. 
On the more gentle slopes of hills and mountains the 
drainage is good, and the soil consequently dry. Hf an 
abundance of plant-food is available in the soil, grasses 
become established, and produce areas of natural pasture. 
Grassland of this kind is found at all altitudes from sea- 
level almost to the summits of the highest mountains, 
more especially where the rocks are igneous or calcareous. 
Sandstone hills rarely support a natural pasture, for 
plant-food is scarce, and the slopes become covered with 
grass-heath or calluna-heath (pp. 250, 252). 
The vegetation has to rely for its water-supply almost 
entirely on atmospheric precipitations—rain or dew. 
Periods of drought are therefore frequent, whilst strong 
drying winds prevail for a large part of the year. These 
factors tend to favour the development of a xerophytic 
type of vegetation, which becomes more pronounced at 
high altitudes. The natural pasture forms the great 
sheep-runs of all hilly districts, and the continual grazing 
has a certain influence on the vegetation, many of 
the taller plants being cut down to a few inches, and 
prevented from producing flowers and seed. 
The turf is usually dense,and compact, formed of the 
fibrous shallow roots of the grasses, which monopolize 
the water and food in the upper layers of the soil. he 
only herbs which can obtain a footing are perennial 
plants, whose long roots can explore the soil beneath 
the turf, or hemi-parasites (p. 126), which become fixed 
to the roots of the grasses and absorb much of their food 
from them—e.g., Huphrasia officinalis (eyebright) and 
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