2 ALPINE PLANTS. 
explanation. In the first place, good drainage 
enables the rain or other surface-water to 
descend more freely and escape more rapidly 
from the soil. 
There is no doubt that the admission of 
frequently-renewed supplies of air into the 
soil is favourable to its fertility. When rain 
falls upon the surface it makes its way into 
the fissures, expelling the air which previously 
filled them; when the rain ceases the water 
runs off by means of the drainage, and as it 
leaves the pores of the soil empty above it, 
the air follows and fills with a renewed supply 
the numerous cavities from which the descent 
of the water had driven it. Where the land 
remains full of water no such renewal of air 
can take place. 
A word as to the temperature of the rain 
as it falls through the atmosphere. 
If this be higher than that of the surface 
soil, the latter is heated by it; and if the 
rain be copious and sink easily into the sub- 
soil, it will carry this warmth with it 
to the drainage. Thus it is that the drainage 
is most valuable, and the under-soil in well- 
drained places is warmer, because the evapor- 
