76 THE SCOTTISH BOTAMCAL REVIEW 



Apart from grass-moor, or grass-heath, the various acid 

 types of grassland can hardly be considered to have more 

 than a temporar}- significance, except where their habitat is 

 governed by migratory factors of surface change. Nardus 

 stricta is normal to the flood-rims of streams and acid 

 "flushes" in the moorland, but often rapidly invades an}- 

 northern or high-lying area, where peat is undergoing denuda- 

 tion or has but lately been removed. The distribution of 

 Nardus grassland in this countr)' appears to be mainl}- 

 governed by the extent of acid drainage from moorland peat 

 and the hard, leached ground from which peat has been 

 denuded. Peat subject to snow lie is also usually covered 

 by Nardus. Nardus has no value as pasture, and the 

 trampling and manuring of cattle or rabbits tends rapidly to 

 its disappearance. Aira flexuosa^ on the other hand, gener- 

 ally avoids flushed ground and afi'ects soils which have been 

 long subjected to atmospheric leaching. Its normal habitat 

 is grass-heath and the acid-enduring woodlands of the moor- 

 land region. It rarely forms true grasslands, either by 

 itself or in conjunction with other species, except where the 

 migrator}' factors have lately been in operation.' Thus 

 while Nardus affects frequently flushed moorland alluvia, 

 Aira is abundant on the older, deserted leached terraces. 



The so-called neutral and alkaline grasslands (3) (33) of 

 Festuca, Agrostis, etc., usually form pasturelands. Their per- 

 manence depends either on constant grazing, treading, and 

 manuring b}' cattle, which is favourable to their establish- 

 ment, or on physiographic or migratory factors setting limits 

 to the occupation of the ground b}- woodland, heath, or 

 moorland. Where the}^ form pastureland, one or another 

 type ma}- be artificially established on almost an}- class of 

 soil, after a short tillage ; but where they occur naturally, 

 physiographic and edaphic factors are always prominent. 



The climate of this country favours pastureland in places 

 where the growth of woodland is retarded by the shallowness 

 of the soil-cap, wind exposure, periodic flushing or flooding, 

 or by the situation forming a favourite resort of animals (3) ; 

 but for the persistence of pasture on the same spot, it is 



' Dr. W. G. Smith points out that Ai7-a flexuosa is often a tussock -plant on 

 new soils exposed to leaching, e.g. waste heaps. 



