54 THE CLIMATAL DISTRIBUTION OF BRITISH PLANTS. 
it; in Wales, however, it is reached by all the higher moun- 
tains. In the Pennines, the Lake District, and the South of 
Scotland, all the high mountains reach the Arctic region, 
while in Mid-Scotland it descends as low as 1,000 feet, and 
in the extreme north to within two or three hundred feet 
of the sea level. 
In comparing the flora of the Arctic with that of the Agrarian 
region, by far the most striking fact is the great reduction 
in the number of species. In the three Agrarian zones the 
total number cannot be less than 1,300, while in all the three 
zones of the Arctic region the flora scarcely numbers 350 
species. 
Watson considers that the upward limit of the Common 
Bracken Fern (Pteris aguilina) marks the division between 
the Agrarian and the Arctic regions. 
A great number of common trees and shrubs do not 
extend higher than the Agrarian region, among these are 
the Oak, Ash, Elm, Beech, Holly, Yew, Hawthorn, Gorse, 
and Ivy. Indeed, almost the only trees and shrubs the 
Arctic region can boast are the Scotch Pine, the Mountain 
Ash, the Sweet Gale, the Birch, and the many species of 
Willow. 
Another class of plants which distinguish by their absence 
the Arctic region are the weeds of cultivated ground. 
For marking out the limits of the three zones into which 
he divided the Arctic region, Watson adopted the upward 
limits of three common moorland plants, namely, the Heather 
(Calluna vulgaris), and two common Heaths (Erica tetralix 
and E. cinerea). Above the limit of Bracken we are in zone 
4, the Infer-arctic, this extends as far as the Ericas grow; 
when they disappear we are in zone 5, the Mid-arctic; and 
when, about 1,000 feet higher up, the Heather disappears, 
we are in the highest zone of all, the Super-arctic. 
Zone 5, the Mid-arctic, occurs on Snowdon and the 
higher mountains in its neighbourhood, also on the higher 
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