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 1° 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 
