STABLE AND MIGRATORY PLANT FORMATIONS 63 



where the soil consists largely of limestone fragments. Plants 

 with these requirements should therefore be more abundant 

 in limestone districts than elsewhere. Damp crevices in 

 limestone, rain-wash, screes, and the neutral humus of well- 

 lighted woods on limestone slopes, and deep, sandy alluvia 

 flanking calcareous waters are the most likely habitats to 

 meet these conditions. 



Shallow-rooted plant associations, indicating the presence 

 of lime in the surface waters, are frequently met with in 

 calcareous "flushes" where, owing to the slope of the 

 ground, waters derived from calcareous springs, or from the 

 surface run-off from calcareous rocks, periodically flush areas 

 which otherwise would be sterile, through atmospheric 

 leaching or an acid condition of the soil (26). The soil of 

 these calcareous flushes must often be well aerated, owing to 

 the periodic nature of the flow of waters, and their high 

 oxygen content. Many of them are only flushed during 

 wet weather, and the soil at other times being well aerated 

 and warm (dry, calcareous flushes), may thus favour the 

 conditions for the production of nitrates. 



Where deep residual soils overlie limestone, or where deep 

 soils, overlying other rocks, contain abundant lime in solu- 

 tion brought from higher levels, the vegetation may well be 

 luxuriant, as noted by Hilgard, since the lime promotes 

 rapid humification and nitrification in the soil. This luxuri- 

 ance of the vegetation may be lasting in character, provided 

 the climate ensures a continuous supply of humus on the one 

 hand, and alkaline ground- waters on the other. The super- 

 ficial layers of the soil in such cases may contain but little 

 lime, and the shallow-rooted ground associations would then 

 be no index to its presence. 



5. Plants of a distinctly xerophytic habit, and often dwarf in 

 stature, including a number of phanerogams and mosses that 

 are chiefly confined to dry limestone habitats or chalk, where 

 the soil is excessively calcareous, dry, and poor in humus. 



On exposed limestone surfaces subject to drought, or on 

 chalk, humus fails to accumulate under our existing climate 

 unless a leached soil is formed (3). Such limestone surfaces 

 contrast with the leached surfaces of other well-drained or 

 porous soils where acid humus accumulates. The physical 



