GEOGRAPHICAL DISTRIBUTION OP PLANTS. 



551 



Thus the group of ferns, mosses, lichens, and fungi in the equatorial zone constitutes 

 of the vegetation on the plains, and |th of that on the mountains, while in the tem 

 perate zone it forms one half of the whole number of plants, and in the frigid zone almost 

 the entire vegetation belongs to this family. The group of grasses constitutes, in the 

 equatorial zone, ^th of the whole number of plants that exist in it, in the temperate zone 

 -^th, and in the frigid zone -j^th, the maximum ratio being attained in the latter. The 

 social plants, or those which live together, covering large tracts of country, like the 

 common heath which is spread over the sand-hills of Jutland, Holstein, Hanover, West 

 phalia, and Holland, are comparatively rare within the tropics, and are only found on 

 the sea coasts and upon elevated plains. 



In the cold and inhospitable climate of high northern latitudes, where the ground is 

 frozen hard during nine months in the year, and covered with snow several feet deep, the 

 vegetable tribes are few in number, stunted in their appearance, and of analogous species 

 in the polar regions of Europe, Asia, and America, the continents there being nearly 

 united. Mosses and lichens the cryptogamia of Linnaeus, and acotyledones of Jussieu 

 form one of the chief botanical features of the arctic zone, and extend in small and 

 isolated tufts as far as travellers have been able to penetrate to the north. One species 

 occurs in great abundance on the southern confines of this zone the reindeer moss 



