As we get up to the level of perpetual snow on the higher mountains, or go northward within the Arctic Circle, the conditions under which plant life exists become very severe. It has been pointed out that in spite of a superficial similarity, wide disparity exists between the sets of conditions prevailing in the two kinds of habitat just mentioned. In the Arctic the winter is continuously dark and the summer continuously light; and in summer the sun is never far above the horizon, so that the temperature remains low, though it rises amply far enough above freezing-point 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 temperature 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 beyond 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 circumstances. 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 main difficulty induced by cold would appear to be the withdrawal of available water; if that goes on for too long, life ceases. Of course the suspension of activities which accompanies freezing cannot continue indefinitely, and in the cold regions of the Earth plants are found only where for a sufficient portion of the year the maximum temperature rises above freezing-point enough to allow of ordinary vital functions being resumed. A curious point in this power of resistance in plants to extremes of temperature is that they display no obvious protective adaptations. “Our present powers of investigation,” Schimper concludes,[12] “do not enable us to