ORIGIN AND DEVELOPMENT OF HEATHER MOORLAND. 119 



the soil become incorporated with it. The soil of a heavily- 

 manured garden or farm is rich in humus, which is rapidly dis- 

 integrated and decomposed, largely through the agency of bacteria. 

 In forests and under close masses of vegetation (e.g., heather) the 

 plant remains decompose slowly, and tend to accumulate as raw 

 humus. Under such conditions light and air are deficient, and 

 physical disintegration proceeds slowly, while bacterial action is 

 largely replaced by the slower action of true fungi. The leaf 

 mould of gardeners is raw humus collected in woods, while peat of 

 various kinds is the raw humus accumulated under heather, cotton- 

 grass, bog-moss, or other moorland plants. Raw humus may be 

 formed on soils naturally rich or poor, and in all cases it masks the 

 true action of the soil. The effects of raw humus on vegetation 

 have been described by Grebe. ^ A layer of moderate thickness 

 cuts off the underlying soil from oxidation and other atmospheric 

 action, so that roots of plants are imperfectly aerated. Under heath 

 vegetation the raw humus, combined with the roots of plants, 

 forms a felted mass which interferes with the circulation of air 

 and water in the soil ; it hinders evaporation of superfluous water 

 in spring, while in summer it forms a dry felt which prevents dew 

 and light rains from reaching the soil. The raw humus, in short, 

 renders the soil unfavourable to plants which require fresh soU, 

 active aeration, and sustained moisture. The formation of humus 

 is accompanied by the development of organic acids, which in the 

 case of raw humus are imperfectly oxidised, and their accumulation 

 gives rise to changes prejudicial to the growth of most plants. 

 The heath vegetation is adapted to growth in moor-humus by its 

 modified xerophytic character and by the shallow growth of under- 

 ground parts. The adaptations of heath-plants are dealt with in a 

 paper by L. C. Miall {Nature, Win., 1898, pp. 377 and 401), and 

 in this or the special works of Warming or Kerner the details will 

 be found. Two forms of heather vegetation are recorded on the 

 Scottish maps, of which the one with shallow peat is the drier; 

 the other is the bog or moss, with deep peat. 



The formation of moor-pan may precede or follow on the accumu- 

 lation of raw humus. Its effect is to assist in masking the true 

 fertility of the soil. Graebner's account of moor-pan is the best we 

 have seen, and throws considerable light on the formation of 

 heaths and peat. Moor-pan, as we translate the German word 



' Zeitschrift fur Forst- und Jagdwesen, xxviii. (1896), pp. 513-542. 



