258 BRITISH PLANTS 



Thesium humifusum (bastard toadflax). On very poor 

 soils, or exceptionally dry ones, the turf is less dense, and 

 a few annuals find room to grow — e.g., Linum catharticum 

 (purging flax) and Arenaria serpyllifoUa (thyme-leaved 

 sandwort) on the dry chalk-downs. Most of the peren- 

 nial herbs are rosette-plants — e.g., Hieracium Pilosella 

 and Rumex Acetosella — 'Or have prostrate stems — e.g., 

 Galium saxatile and wild thyme. Not only are these 

 plants protected from drying winds by placing their 

 leaves close to the ground, but the growth of other 

 plants in their immediate . neighbourhood is to a large 

 extent prevented. Mole-heaps, earth thrown out from 

 rabbit-burrows, and other areas of disturbed ground, pos- 

 sess quite a different flora from the surrounding pasture, 

 weeds of cultivation, both annual and perennial, being 

 common. 



The constitution of the flora of a natural pasture varies 

 with the chemical nature of the soil and the altitude. 

 On the very rich soils formed by the disintegration of 

 igneous and metamorphic rocks the dominant grasses 

 are much the same at all levels, but in the alpine regions 

 the commonest plants associated with them are alpines 

 not found in the lower regions. 



Lowland and sub-alpine pastures on rich soil occur up 

 to an altitude of about 2,000 feet, but their vertical 

 range depends on the amount of water present in the 

 soil ; on very dry soils they do not extend so high. The 

 dominant grasses are : Anthoxanthum odoratum (sweet 

 vernal-grass), Agrostis vulgaris (fine bent-grass), Festtica 

 ovina (sheep's-fescue), and Aira flexuosa (waved hair- 

 grass). The latter two grasses are well adapted for dry 

 conditions, their leaves being rolled inwards, so that the 

 lower surface, bearing stomata, are protected from the 

 wind (p. 43). Nardus stricta, found occasionally here, 

 but more commonly at higher levels, has similar rolled 

 leaves. Other grasses frequently found are : Poa pra- 

 tensis (smooth meadow-grass) and Triodia decwmhens 

 (heath-grass). Of rosette-plants, the commonest are : 

 Hieracium Pilosella (mouse-ear hawkweed), Achillea 

 Millefolium (yarrow, milfoil), Rumex Acetosella (sheep's- 

 Borrel), Carlina vulgaris (carline - thistle), and Luzula 

 campestris (field wood-rush). In many plants the leaves 

 close to the ground are large, those on the erect stems 



