CHAPTER XXIX 



VEGETATION OF WOKS AND WALLS 



I. Alpine Rocks. 



The 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 {Saxi- 

 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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