52 GUIDE TO THE STUDY^ OF LICHENS. 



SECTION V. 



THE OCCUERENCE AND DISTRIBUTION OF 

 LICHENS. 



The actual number of species of lichens occurring 

 on the entire globe is not known. According to Ny- 

 lander, about 1,500 species were recognized in 1858 ; 

 between 4,000 and 5,000 species and varieties were 

 recognized in 1892, according to Hue. The United 

 States contains perhaps between 800 and 1,000 species. 

 It must, however, be remembered, that all estimates 

 are more or less unreliable. Many years of careful 

 study are required in order to get at reliable data re- 

 garding the actual number of lichen-species. 



I. THE LATITUDINAL AND ALTITCDINAL DISTRIBU- 

 TION OF LICHENS. 



Lichens extend to the far north, where, with a few 

 mosses, they form the most advanced outposts of vege- 

 table life ; they extend within a short distance of the 

 northern snow limit. In the high arctic regions they 

 constitute the characteristic flora of the country. Here 

 they occur mostly as earth-lichens and rock-lichens, dif- 

 ferent species growing in close association, forming what 

 are technically known as " lichen-tundra." The great 

 majority of species belong to the genera Cetraria, 

 Gladonia, and Evernia, The cause for their occur- 



