GEOGRAPHICAL DISTRIBUTION. 



101 



projecting through the snow, without any limitation as to 

 height, and ascending far above the snow-line ; they also 

 occur at above 16,000 feet on Chimborazo. Among the spe- 

 cies found at the greatest elevations hitherto reached by man, 

 none is more common than the familiar Lecidea geograpliica ; 

 it occurs far above the line of perpetual snow on the Alps, 

 and is the last type of vegetation met with on the Andes 

 and Himalayas, and on the deserts of Nova Zembla. 



The chief habitats of Lichens are the bark of trees, rocks 

 or stones, and the soil ; but they likewise grow frequently on 

 the decayed stumps or rotten hollows of old trees, on wooden 

 palings, shrubs, mosses and Jungermannise, perennial or 

 evergreen leaves, the mortar of walls, and sometimes on the 

 thallus of other species. Of corticolous species, some are 

 characteristic of the rugged bark of old trees, as various 

 Parmelias, Ramalinas, or Stictas ; others prefer the smooth 

 bark of young trees, as many of the Graphidece, which are 

 also frequently to be found on the smoothest stems of 

 shrubs. Some Lichens are to be found chiefly on trees, 

 such as the ash or oak, which grow on the sides of public 

 highways, as Parmelia parietina, P. stellaris, P. pulveru* 

 lenta, and P. olivacea ; others occur only or chiefly in our 

 forests, as Usnea barbata, Sticta pulmonaria, and various 



