GEOGRAPHY OF PLANTS. 



271 



which, however, the mountains only, the southern shores, and 

 the alluvial land, are distinguished, and the heights above 

 the sea are given. Wahlenberg's Map of Lapland, in his 

 Flora La/pponica^ is much more correct. Maclure has given a 

 Map of the constitution of the Mountains in the Free States 

 of North America, (Geographische Ephemeriden). I know 

 not that any person has given a more pleasant and instructive 

 account of the soils and mountains of his country, in relation 

 to their Floras, than the excellent Villar, respecting the Alps 

 which divide Italy from Switzerland, in the Preface to the 

 Histoire des Plantes de Dcmphine. 



398. 



This brings us to a very important circumstance, which 

 must be taken into account in every examination of the 

 causes of the growth of plants, namely, the height of their 

 station above the level of the sea. As, upon the whole, the 

 temperature on the highest mountain tops seems to be the 

 same with the temperature at the polar circle, it is commonly 

 believed, that under the snow-line^ and near to it, the same 

 vegetation is found as in the pol^r regions. The limit of 

 perpetual snow, under the Equator, is at the height of 15,000 

 feet ; in the 35th degree of Ni Lat. it is at 10,800 ; in the 45th 

 degree, at 8400 ; in the 50th degree, at 6000 ; in the 60th de- 

 gree, at 3000 ; in the 70th degree, at from 1200 to 2000 feet 

 above the level of the sea ; and, at the 75th degree of N. Lat,, 

 the snow- line lies almost upon the ground In general, it 

 has been ascertained by observations, that the same vegeta- 

 tion is produced at the same distance from the snow-line. It 

 must be considered, however, that towards the pole, the sum- 

 mer is shorter, but hotter, than under the snow-line upon the 

 tropical mountains, where winter and summer cause no 

 change of temperature. On this account, a better vegetation 

 must be produced in the polar regions during summer, espe- 

 cially as the plants are there exposed to the uninterrupted 

 light of the sun : while, on the other hand, from the uniforni 

 temperature on the highest tropical mountains, throughout 

 the year, a very different Flora is there produced from that 



