Influence of Climate on Cultivation. 159 
an expensive crop to raise, and, when done so to such an extent 
as a fourth of the arable land, it demands a correspondingly larger 
breadth of the higher priced grains. This is a resource which 
neither the Scotch nor the West of England farmer can fall back 
upon. Literally speaking, they cannot afford to grow so large a 
breadth of turnips as the Norfolk farmer, in consequence of not 
being able to grow so large a breadth of barley and wheat, which 
are the most valuable grain crops. Other compensations, how- 
ever, intervene, and enable the Scotch farmer to pay as high a 
rent for land of equal fertility as the Norfolk farmer can do. 
Chemistry teaches us the changes which matter undergoes in 
passing from the inorganic to the organic kingdom ; the laws of 
physiology, the functions of the various organs, as well as the 
nature and uses of the products that are formed ; the laws of 
climate, the conditions under which the different kinds of plants 
flourish or produce their various products in greatest abundance 
and perfection. ^ 
Other things being equal, the force and rapidity of vegetable 
growth is in proportion to the temperature of the atmosphere and 
soil. It is well known that plants grow with greater vigour in 
summer than in spring — in tropical countries than in temperate. 
The higher temperature enables plants to digest or assimilate a 
larger quantity' of food in a given time. Plants are therefore far 
more grateful for a supply of manure when applied for summer 
than for spring growth. The vegetables raised by the market- 
gardener are said to be " forced by manure," because the common 
kinds are grown during the colder })eriod of the year. 
But not only have plants the power of digesting a larger quan- 
tity of manure during the warmer season, but they seem also to 
have much greater powers of absorbing food from the atmosphere. 
This may arise from two causes. First, it is highly probable 
that ammonia, a most active agent in producing vegetable growth, 
exists in greater abundance during the warmer season — its quantity 
in the atmosphere being to some extent regulated by that of its 
water or vapour, which is always greatest in sum.mer. The 
second cause of the greater powers possessed by plants of absorbing 
ammonia during the warmer season is perhaps the increased 
vigour of the plants. 
When manure is abundant, the agriculturist endeavours to 
select those plants which are capable of expanding in size in 
proportion to the liberality of the treatment. This can only be 
fully accomplished when their period of growth extends over the 
warmer season. These conditions are fulfilled in the meadow 
under irrigation, or in a field of cabbages ; both grow so long as 
the temperature is sufficient for vegetable growth, and both 
possess great capacities of growth under liberal treatment. 
