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 serpyllifolia (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), Festuca 
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 decumbens 
(heath-grass). Of rosette-plants, the commonest are: 
Hieracium Pilosella (mouse-ear hawkweed), Achillea 
Millefolium (yarrow, milfoil), Rumeaw Acetosella (sheep’s- 
sorrel), 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 
