THE REGIONS OF GREATEST COLD 1097 
to allow of plant life. On high mountains, on the 
other hand, there is the same succession of day and 
night which prevails on the plains below, the height of 
the sun above the horizon being a question of latitude. 
On mountain-ranges situated within the Temperate 
Zone, such as the European Alps, and much more on 
those nearer the Equator, the day temperature in 
summer is very high wherever the sun strikes, and 
while plants may have to withstand at night a tem- 
perature comparable to that borne by the Arctic flora, 
they must endure by day the most intense insolation. 
Neither in the Arctic nor on the high hills does 
plant life cease merely on account of low temperature. 
Species belonging to many families venture even be- 
yond the limit of perpetual snow. The coldest known 
area on the earth’s surface lies in Siberia, actually 
within the limits of forest growth, and trees and herbs 
of many species survive winter temperatures which may 
fall below —60° C. (76 degrees of frost Fahrenheit). 
They freeze into solid lumps of ice without injury, and 
indeed the thawing process in spring is more dangerous 
to them than their congealment in autumn. Many of 
the high alpine plants are frozen solid every night 
only to be roasted alive by day; it seems amazing that 
any living organisms can endure under such circum- 
stances. Yet it is not only species confined to areas 
where such extremes exist, and specially adapted 
thereto, which can resist them successfully. In Central 
Europe the Common Chickweed and Common Daisy 
are often frozen solid, so that leaves and stems snap 
between the fingers like sealing-wax, yet with a rise of . 
temperature they continue growth quite unperturbed, 
just as they do in areas where frost is unknown. The 
