VEGETATION OF ROCKS AND WALLS 287 



the plants send their exceptionally long roots in search 

 of food and water. The rainfall is heavy on the mountain- 

 top, and out of reach of the wind the plants suffer little 

 from lack of water. But they are subjected to intense 

 cold during the night and brilliant illumination in the day- 

 time, whilst the air is rarefied. The first factor hinders 

 absorption, and the others favour transpiration, so the 

 plants are typical xerophytes, with rosettes of leaves or 

 cushion-habit. Geophytes are very rare, owing to the 

 absence of sufiicient soil in which the plants could hiber- 

 nate, whilst the short vegetative period prevents the 

 establishment of most annuals. Indeed, there is no true 

 alpine annual in this country, although lowland forms 

 may exist at the highest levels-^e.gr., Euphrasia officinalis 

 and Poa annua, at 3,980 feet in Perthshire. Many alpines 

 are found in the neighbourhood of alpine streams and 

 rUls. Here plenty of water is available at all times, but 

 the climatic factors mentioned above are as evident as 

 on the rock-ledges, and the habit of the plants is the 

 same. 



Some of the rarest British plants, last survivors of an 

 arctic climate (see p. 210), are found at high altitudes. 

 The Highlands of Scotland are specially noteworthy in 

 this respect. The following plants are found in a few 

 localities only in Scotland : Arabia alpina (Skye), Draha 

 rupestris (Ben Lawers, Perthshire, 3,000 to 3,980 feet), 

 Arenaria rubella (Perth, 2,700 to 3,800), Sagina nivalis 

 (Perth, 3,100 to 3,900), 8. Boydii (Braemar), Astra- 

 galus alpinus, Oxytropis campesfris (Perth), Saxifraga 

 cernua (Ben Lawers, 3,800), 8. rivularis (Ben Nevis, Ben 

 Lawers, Cairn Gorm, etc., 3,500 to 3,900), Erigeron 

 alpinum (Perth, 2,500 to 3,500), Gnaphalium norvegicum 

 (Perth, Forfar, and Aberdeen), Lactuca alpina (Aberdeen), 

 Menziesia coeridea (Perth, 2,350 to 2,460), Gentiana nivalis 

 (Perth, 2,400 to 3,450), Myosotis pyrenaica (Perth, 2,400 to 

 3,450), Veronica fruticans (Perth, 1,200 to 3,600); dwarf 

 species of willow, |- to 2 feet high — 8alix Arbuscula and 

 8. lanata (Perth) ; Luzula arcuata (summits of several 

 mountains — e.g., 4,290, Ben Macdhui, Aberdeen), Juncv^ 

 higlumis, Garex alpina, G. rupestris, G. atrofusca (Perth, 

 2,600), Alopecurus alpinus, Phleum alpinum, Poa laxa. 

 Other alpines confined to Scotland, but more widely 

 spread than the foregoing, are : Gherleria sedoides, 8agina 



