18 THE WONDERS OF VEGETATION. 



still find the mountain ash, the pine-tree and all the 

 conifers. 



At last even these plants cease to grow the oak, 

 the hazel and the poplar at 60 North latitude ; the 

 birch and lime at 63 ; and the conifers are not found 

 higher than 67. Beyond 70 only a few stunted wil- 

 lows grow here and there close to the ground. Fur- 

 ther north, beyond 75, not a tree is to be found, 

 shrubs and plants even have disappeared and cereals 

 can no longer exist, for even barley and oats are not 

 found beyond 70. 



The local physiognomy of plants depends thus 

 upon the normal temperature of each climate. The 

 same principle is applied to the elevation at which 

 plants grow, and by combining the two, we are enabled 

 to understand in its entirety the distribution of plants 

 over our globe. 



Instead of travelling from the equator towards 

 the poles, let us simply ascend a high mountain and 

 we will find that the classes of plants appear in the 

 same order, following the thermometric ladder of al- 

 titudes. We know that the higher we rise in the at- 

 mosphere, the lower we find the temperature ; and this 

 fall is so rapid that the ascent of a few minutes in a 

 balloon, or of a few hours upon a mountain side, takes 

 us through all the different degrees of temperature 

 from 70 or 80 of heat on the plains to the freezing- 

 point, and below in the higher regions of the atmos- 

 phere. In consequence of this law all the mountains 

 of the globe have a lower temperature at their sum- 

 mit than at their base ; and we find among their vege- 



