1(50 Influence of Climate on Cultivation. 
The amount of food which plants can derive from the atmo- 
sphere appears to be regulated bj principles which are not very 
dissimilar from those which liold with respect to the capacities of 
plants for manure. The capabilities of plants for absorbing food 
from the atmosphere depend upon the extent of surface of 
healthy succulent leaf which they can maintain during the season 
of growth. Under favourable circumstances, grass and clover 
send forth a perpetual succession of leaves, which renders them 
much less dependent on a supply of food to their roots than the 
wheat plant. Or in the case of the natural vegetation of forests, 
which yearly produces a great amount of organic matter, through 
the large surface of leaf exposed during the warm season. 
The suitability of climate for forage crops consists in its 
capability of maintaining them in health throughout the grow- 
ing season. Within certain limits, the strength of vegetation 
depends upon temperature, if a due supply of moisture exists. 
When moisture is deficient, assimilation is retarded or prevented, 
and the most abundant supplies of food are of little avail in pro- 
moting growth. Much of the skill of the agriculturist consists in 
selecting those plants which are best adapted to the climate, as 
well as in adopting those particular acts of cultivation that serve 
to compensate for deficiencies of climate. 
One of the principal properties that render soils fertile is their 
power of absorbing and retaining moisture, and thus furnishing 
a steady supply to the plants that grow upon it. One, also, of 
the chief objects of cultivation is to increase this power, and by 
this means increase their productiveness. As we shall have occa- 
sion to point out, climate greatly influences the maxims that 
guide agriculturists in this matter. 
The physical or mechanical properties of soils are chiefly 
concerned in the distribution of the natural vegetation, such as 
trees and grass. Blowing sands on the sea-coast are tenanted 
by those species of grasses that can resist the extreme aridity of 
the medium in which their roots are fixed. It is only on deep loams 
that the finer and more luxuriant grasses flourish in dry climates. 
These can sustain growth during the heats and droughts of 
summer : except by the operations of drainage and marling, art 
can do little to increase the absorbent powers of a field under 
grass. Deep-rooting plants, such as clover, sanfoin, and 
lucerne, are less under the influence of drought, and are con- 
sequently greater objects of culture in dry dimates. On the 
other hand, the humid climate of the West of England renders it 
particularly genial to the growth of the shallow-rooted grasses, 
which become an economical means of restoring fertility to land 
exhausted by cropping. 
The influence of climate on the growth of the common perennial 
