Iiifiuence of Climate on Cultivation. 
495 
ficient to raise as full crops as the land will grow, a comparatively 
moderate space is allotted to each plant. On the other hand, in 
the north and west, where the soil and climate are so favourable 
as to enable the plants to work up a large amount of nitrogenous 
manures and to produce crops of 40 tons to the acre, almost 
as large a space is assigned to the plants of the Swede as to 
mangold-wurzel in the south. 
The Swede turnip seems to possess greater powers of resist- 
ing the droughts of summer than the other varieties. As sug- 
gested in the case of mangold-wurzel, its more glossy and waxy 
leaves may prevent or moderate evaporation to some extent. The 
little repute in which the common yellows are held in the south is 
perhaps a good deal owing to their perspiring more moisture 
through their leaves. The purple-top yellow seems to exceed 
all the others in this respect, if we are to ascribe its extreme 
susceptibility to drought and heat to this cause. On the other 
hand, in moist seasons it yields an abundant produce and bears 
copious manurings of nitrogenous substances. The white globe 
variety of turnips seems to resist drought more effectually than 
any of the common yellow varieties. But Swedes thrive well 
in a moist atmosphere, in which the quantity of water trans- 
pired through the leaves must be small in comparison to that 
which takes place where the climate is dry. 
Potato. — This plant illustrates in a very striking manner the 
influence of climate and cultivation on its produce. Brought 
from the cooler climates in the elevated regions of the Andes, it 
has succeeded admirably in temperate latitudes. Its leaves, 
however, seem as easily injured by frosts as when it was first 
introduced into Europe — a fact which betrays its tropical origin. 
In hot countries, the vegetative powers of the plant seem 
chiefly directed towards the formation of stems, leaves, and 
seeds. In higher latitudes, on the contrary, a larger portion of 
its juices are directed to the formation of starchy roots. Even in 
the northern states of America the plant has a great tendency to 
throw out a profusion of stems, at the expense of a diminished 
crop of tubers, in which the proportion of starch is somewhat 
scanty. In the British Islands the plant seems to reach its acme 
in yielding food for man. Here, and probably more in the 
north than the south, the climate seems most happily constituted 
for its growth. 
The moist and warm parts of the island have suffered most 
from the mysterious disease which has preyed upon the crop more 
or less since 1845. Heat and moisture, though not the cause of the 
disease, evidently lend increased virulence to its attacks. The 
quality and quantity of the produce in the British Islands is, 
however, more regulated by the character and nature of the soil 
