STABLE AND MIGRATORY PLANT FORMATIONS 143 



debris. Grassland is, however, frequently built up on moor- 

 land shingle stretches by the dumping of sods carried down 

 stream. Such sods usually consist of the resistant upper 

 layers of peat, matted with roots, but the original peat flora 

 always dies off when subjected to this treatment, and gives 

 place to Carex Goodenovii, Nardiis stricta, or other species. 

 The sods gradually accumulate until a somewhat uneven 

 surface becomes covered with grasses, sedges, and mosses, 

 and eventually, when deserted by ordinary floods, forms a 

 fine pasture in which Anthoxanthtiin, Festiica, and Agrostis 

 are prominent species. 



From gravels and shingles to the wide stretches of coarse 

 rock debris of the alpine screes and mountain plateaux might 

 appear but a short step, so far as the nature of the surface as 

 a habitat for plants is concerned. The material which forms 

 this debris has not, however, been through the mill of picking 

 for wear and tear that has eliminated all softer material in 

 the former case. The material is angular, formed by frost, 

 and subject to further splitting and reduction by the same 

 agent. The surfaces, though rougher than the pebbles, are 

 less permanent as a basis for lichens, and are further subject 

 to frequent abrasion from slipping or sand blast. Where 

 long undisturbed, on the flatter summits, lichen growth is, 

 however, rapid on account of the brief spells of drought, 

 and may extend to the foliaceous and fruticose types of 

 development. 



The barrenness of screes and mountain top debris, like 

 that of shingles, is, no doubt, partly due to superficial 

 drought preventing the growth of shallow-rooted perennials 

 and especially their seedlings. It is noteworthy that the 

 plants of shingles and porous rock debris are often such as 

 have fruits liable to lodge near the surface owing to their 

 size or adhesiveness preventing them from being carried deep 

 into the debris. Small seeds would have little chance of 

 survival on germinating. Other plants of these places have 

 highly developed underground rhizomes or roots, with 

 great powers of searching for water and for replacing 

 subaerial parts which undergo destruction. The barrenness 

 of screes is further due as much to instability inducing land- 

 slip as to the porous nature of the surface, while that of the 



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