CHAPTER XXIX 
VEGETATION OF ROCKS AND WALLS 
I. Alpine Rocks. 
Tue alpine zone in this country extends from the limit of 
tree-growth (usually about 2,000 feet) to the summits of 
the highest mountains. It is on the sheltered rock-ledges 
in this region that the typical alpine plant, whose char- 
acters have been described in Chapter IV., reaches its 
greatest development. Some are restricted to this zone, 
and all have their headquarters here. In the west of 
Ireland, however, their distribution is different : some— 
e.g., the mountain-avens (Dryas octopetala), rose-root 
(Sedum Rhodiola), and yellow mountain-saxifrage (Sazi- 
fraga aizoides)—grow on rocks at the sea-level, whilst 
others never reach the 2,000-feet line. It is very difficult 
to understand why this should be so, for the climate is 
milder and more humid than any other part of the British 
Isles. The strong winds and gales which prevail for the 
greater part of the year, however, prevent the growth of 
many lowland forms, and alpines, naturally adapted by 
their tufted or rosette habit for an environment similar 
to this, might live there free from competition. Although 
this may account for their presence at sea-level, it sheds 
no light on their absence from the higher zones, for compe- 
tition does not drive the alpine up a mountain-side ; it 
merely restricts its distribution to the higher levels. 
Alpine plants occur in all situations—in exposed, wind- 
swept places, on the dry upper slopes (pp. 255, 259), and on 
wet peaty soil (p. 253)—but all the plants of these habitats, 
except those of the wettest bogs, grow also on rocky 
ledges and clefts. Here the surface-soil is very thin, but 
the cracks and crevices are filled with soil, and into this 
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