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 quite absent, owing to the 
absence of sufficient 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.g., Huphrasia officinalis 
and Poa annua, at 3,980 feet in Perthshire. Many alpines 
are found in the neighbourhood of alpine streams and 
rills. 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: Arabis alpina (Skye), Draba 
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 campestris (Perth), Saxifraga 
cernua (Ben Lawers, 3,800), S. rivularis (Ben Nevis, Ben 
Lawers, Cairn Gorm, etc., 3,500 to 3,900), Hrigeron 
alpinum (Perth, 2,500 to 3,500), Gnaphalium norvegicum 
(Perth, Forfar, and Aberdeen), Lactuca alpina (Aberdeen), 
Menzesia cerulea (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, 4 to 2 feet high—Saliz Arbuscula and 
S. lanata (Perth); Luzula arcuata (summits of several 
mountains—e.g., 4,290, Ben Macdhui, Banffshire), Juncus 
biglumis, Carex alpina, C. rupestris, C. atrofusca (Perth, 
2,600), Alopecurus alpinus, Phleum alpinum, Poa laxa. 
Other alpines confined to Scotland, but more widely 
spread than the foregoing, are: Cherleria sedoides, Sagina 
