68 THE SCOTTISH BOTANICAL REVIEW 



6. In dry, shallow soils, on limestone, chalk, or dolerite, 

 humus fails to accumulate, and this possibly leads in certain 

 cases to a scarcity of nitrogen. 



7. The relations of lime to humus must vary greatly from 

 point to point, according to the physiography in a limestone 

 country, since areas subject to drought alternate with areas 

 where lime and humus occur together in a deep, moist, or 

 marshy soil, where the degree of fertility would depend on 

 warmth and aeration, or stagnancy. In alpine regions in 

 particular these relations must be very complex, and plants 

 which appear to be calcicole may have markedly diverse 

 reasons for their choice of habitat. Some may require easily 

 available, inorganic nitrogen-compounds but be capable of 

 living in a soil subject to physical drought, a combination 

 limiting their distribution to dry, neutral, or faintly alkaline 

 soils where their dwarf habit would not influence them 

 adversely through competition. Others, having similar 

 requirements as to mineral food, but incapable of withstand- 

 ing drought, would need to compete with rank-growing 

 vegetation, or occupy humus-filled crevices in rocks with a 

 calcareous drainage ; while others again, with a small 

 demand for nitrogen but incapable of withstanding soil- 

 acidity, or of competition with a heath flora on an acid soil, 

 would be restricted to the driest limestone-habitats where 

 plant decay is accompanied by loss of nitrogen. 



Part IV. The Stable Tvpes of Vegetation 

 IN Britain. 



There are four chief stable types of vegetation in this 

 country : Moorland, Woodland, Heathland, and Grassland. 



Moorland covers extensive areas in the north and west of 

 the British Isles. It is there often the dominant type of 

 vegetation to sea-level, but further south and east only 

 covers wide stretches on mountain plateaux, or on low-lying 

 peat flats, where it has succeeded fenland through changes 

 leading to leaching of the surface or stagnation. The 

 formation of moorland peat depends chiefly on perennial 

 great atmospheric humidity, and rather low temperatures. 

 In the north-west, where these conditions have been pre- 



