100 
where rain does not fall for years. The monsoons on the 
southern coasts of China, at Canton and Macao, produce so 
low a temperature that the leaves of plants are withered. 
3. The east coasts of continents and isolated masses of 
land in the Northern Hemisphere are colder than the west 
coasts of the same latitudes. 
Hence it becomes necessary, neglecting latitude, to con- 
nect places having the same mean annual temperature, by 
what are called isothermal lines; which lines, although, in 
the Tropics and near the Equator, they run parallel with the 
parallels of latitude, yet, in the Temperate Zone, run very 
irregularly ; and, as a rule, in the interior of continents fall 
south of the latitude, and on the coasts rise north of it ; 
while on the west coast they range to the south, and on the 
east coast rise to the north. The isothermal lines, therefore, 
indicate, with tolerable accuracy, the general vegetation of 
particular countries, but are of little use when we apply them 
to the growth of any particular plant, as of wheat ; for the 
isothermal heat includes the heat of the whole year. But 
some plants are nearly at rest during winter, and the sur- 
rounding temperature has little influence on them ; so that 
the mean temperature of different seasons, and of single 
months, is chiefly to be observed. For when plants unfold 
their leaves in spring, when they blossom in summer, and 
form their fruit in the autumn, every thing depends on their 
receiving, during these important periods, that degree of 
temperature which is appointed them by nature. 
Thus barley, oats, and spelt are cultivated in the north of 
Europe at very high latitudes; at 69 deg. and even at 70 deg. 
north latitude, as at Lyngen, Alten, and on the frontiers 
of Norway, Sweden, and Russia ; while on the plateau of 
Southern Peru, on the shores of the great lake of Titicaca, 
where is perpetual spring, and which is clothed with rich and 
beautiful vegetation, at a height of 12,000 feet, only oats 
