1g0 SOME BRITISH PLANT GROUPS 
to the mountains, each of them having a more or less 
definite limit below which (also above which, though 
our mountains are not high enough to render this 
point well marked) it is not found. 
Among the plant formations and associations of the 
lower grounds which we considered in Chapter II. it 
was noted that the controlling factors were mainly 
connected with the nature of the soil and the amount 
of the water-supply. Here on the mountains another 
factor, the climatic, comes in emphatically, and takes 
charge. The temperature of the atmosphere falls one 
degree centigrade for about every 200 feet of eleva- 
tion, so that a sharp frost on the lowlands may easily 
mean zero Fahrenheit on a 4,000-foot hill. The 
rarefaction of the atmosphere, too, tends to produce 
a much greater range of temperature, both diurnal 
and seasonal. Again, the velocity of the wind is much 
higher on the summits than on the plains, where 
friction is greatly increased by trees and other 
obstacles. These high winds have a very great 
cooling effect, as we may notice on our own bodies 
even in summer. In fact, as regards climatic change, 
an ascent of a thousand feet is comparable to a 
journey of several hundred miles northward. Anyone 
who has, on a winter tramp, been caught in a snow- 
storm on a 3,000-foot hill is forcibly reminded of what 
he has read of winter conditions in the Arctic regions. 
In ascending Ben Nevis we travel, in a sense, to the 
Arctic Circle. But the analogy is false, for conditions, 
especially in summer, are very different in the two 
places. The plants of our mountains have all the 
advantages of the high summer elevation of the sun, 
very different from the weak, sloping sunlight of the 
