THE WONDERS OF VEGETATION. 21 



feet, its summit corresponds only to the lower part 

 of the Alpine region, in the Alps and Pyrenees. 

 At this height trees have entirely disappeared ; but 

 a number of small plants unfold their petals on the 

 surface of the stones or rocks. These are the or- 

 ange-flowered poppy, the violet of Mount Cenis, the 

 blue-flowering astragal, and at the very summit the 

 rneadow-grass of the Alps, Gerard Euphorbia, and the 

 common nettle, which appears wherever man builds 

 a house. 



" On the northern slope we find the saxifrage, 

 which inhabits the Alpine summits amid perpetual 

 snows, and covers the icy shores of Spitzbergen." 



Thus, whether we travel from the hot countries of 

 the equator to the icy regions of the pole, or rise 

 from temperate plains to snowy mountain summits, 

 we find that the heat-giving power of the sun alone 

 governs the distribution of plants. Each plant seeks 

 its own degree of heat. The dwarf birch resists a cold 

 of 40 below zero, the orchids are frozen at 50 

 above it. On the other hand, each species requires a 

 certain amount of heat for germination ; and after 

 that, additional heat is necessary to enable it to flow- 

 er and ripen its seed. Our precious cereal, wheat, 

 refuses to ripen its golden ears, rich with bread that 

 makes the strength of man, unless it has 3600 of heat 

 in all, for the time from its first springing up till har- 

 vest ; and the darkling cluster of grapes cannot glad- 

 den the heart of man with its red wine, unless it have 

 5400. This is why each plant shows a decided pref- 

 erence for a particular locality and a particular temper- 



