78 THE SCOTTISH BOTANICAL REVIEW 



Another type of grassland found in this country, the 

 "alpine pasture " of Robert Smith (34), may closely approach 

 the stable types, and perhaps in some cases might be 

 classed along with them. In its usual development, how- 

 ever, it shows clearly its origin in periodical flushing of the 

 surface of the hill-slopes with water, and there is indeed 

 every stage between alpine grassland and the "moorland 

 flushes" described in the "Vegetation of Caithness." The 

 centre of distribution of the grassland of these moorland 

 flushes lies in the migratory grassland of the higher sandy 

 alluvia of the moorland streams. The typical alpine grass- 

 land is best developed on the slopes which rise above the 

 moorland, and consequently uninfluenced b}- acid drainage 

 from above. Its chief development apparently occurs on the 

 slopes receiving the most heat and the least atmospheric 

 precipitation, i.e. the slopes where the upper limits of the 

 moorland follow the lowest contours. In its typical develop- 

 ment Nardiis stricta is scarce or absent and the prominent 

 grasses are Festuca vivipara, Anthoxanthuin odoratuin, and 

 alpine forms of Aim caspitosa with Alchemilla alpina and 

 other plants. As noted b}- W. G. Smith, it always shows the 

 greatest development where the slopes are formed of the 

 finest rain-wash derived from mica-schists or other rocks 

 which suffer deep and complete mechanical division through 

 weathering, and especially where these rocks contain more than 

 the usual small amount of soluble alkaline bases (3). Where 

 sparsely developed, the grassland is restricted to the slopes 

 immediately beneath the crags from which it receives the 

 surface run-off of water and rain-wash, and wide stretches of 

 alpine grassland are always surmounted by extensive areas of 

 crag or rock-debris. Many of the smaller grassland stretches 

 migrate rapidly with the changing spread of surface waters, 

 and all owe their persistence to the delay in leaching by 

 atmospheric precipitation, induced by flushing of the surface 

 and the finely divided nature of the soil. 



The following scheme of classification shows some of the 



possible relations of the stable types of vegetation in Britain : — 



I. Arctic-alpine Region — the sanctuary of plant-formations 



from a former more widely extended arctic-alpine and tundra 



province. The province is ver}- limited at present in this 



