The Present Flora of Britain. 13 



Climatic conditions cause two very distinct floras to be 

 represented in Britain. The lowland flora is in the main 

 the temperate flora of the neighbouring lowlands of 

 Belgium and France. The upland flora, on the other hand, 

 consists of numerous more or less isolated outliers of the 

 flora which overspreads the lowlands of the Arctic Regions 

 and occupies the mountains of Scandinavia. This latter 

 assemblage is found at higher and higher elevations as it is 

 traced southward, and is confined to hills sufficiently high 

 to have an average temperature approaching that met with 

 at the sea-level within the Arctic Circle. As the fall of 

 temperature is about i Q Fahr. for every 300 feet of elevation, 

 a sub-arctic climate is found over a considerable area in 

 Scotland, and on a certain number of isolated hills in 

 England, Wales, and Ireland. The seeds of the British 

 Alpine plants are invariably small and usually very minute, 

 a peculiarity that will be again alluded to. 



Local conditions govern the distribution of large groups 

 of species. First, there are the sea-coast plants, which are 

 all confined to a narrow belt near the sea. This flora is 

 very uniform throughout Britain, though some of the 

 species are found only on the south coast and a few only 

 on the east. 



The seeds of maritime plants are of various descriptions, 

 and often of large size. Many of them are scattered far 

 and wide by the sea, though the plants only establish 

 themselves where a suitable habitat occurs. Thus the 

 sea-coast flora includes a good many plants like the sea- 

 kale {Crambe maritima) , which tend to appear sporadically 

 wherever the habitat is suitable and to disappear again after 

 a few years — as though dispersal were easy, and the range 

 of the species was limited by climatic rather than by other 

 considerations. Many of the sand-dune or shingle-beach 



