4 THE LICHENS OF PERTHSHIRE 
area still remains.. Drainage, however, is steadily reducing its 
size, and very little is left entirely unaltered in character. 
CLIMATE. 
; d 
by reduction of radiation, causes mildness, and similarly a dull 
. 
sky in summer, by cutting off the sun’s rays, produces coolness. 
ds pr on from wind, a large portion of Perth- 
shire is particularly favoured, the high mountains in the west 
produce an elective screen from the strong winds and gales 
course, very exposed and wind-swept, and are subjected a 
bleak and arctic climate Several summits e within the 
super-arctic zone n. During the winter the snow is 
of Watso 
often almost completely blown off into the hollows and corries, 
leaving the exposed crags and highest rocks little or no protection 
from the rigorous conditions which prevail. It is here that some 
lichens seem to be most at home, especially members of the genus 
Gyrophora. 
Turning to the consideration of rainfall we find, as would be 
expected, an extraordinary difference between the east and the 
west. € latter region being very mountainous, and situated 
on the great watershed of Scotland, it receives a full share of the 
are blowing in from the Atlantic. Further to the east these - 
winds are descending currents, and having now been deprived of 
