38 



POPULAR HISTORY Oh' LICHENS. 



chiefly confined to, and form a characteristic coating of, 

 the older trees of our forests, which thus become "mossed 

 with age." In appearance they most closely approximate 

 the characters of higher plants. Some species appear pos- 

 sessed of a distinct stem or axis with numerous divergent 

 branches : but these have none of the structural characters 

 of the stems or branches of the Phanerogamia, or Flower- 

 ing Plants ; they are wholly cellular. Filamentous species 

 are comparatively limited in geographical range, and are 

 chiefly found at low altitudes ; they are susceptible of few 

 economical applications. As the crustaceous and foliace- 

 ous, so the fruticulose and filamentous thallus are closely 

 allied. All these forms pass into each other by insensible 

 gradations, giving rise to great irregularities in the charac- 

 ter of the thallus of Lichens. Filamentous species are 

 usually adherent to bases of support by means of a central 

 point. 



If we tear or cut across any common foliaceous or fru- 

 ticulose species, we shall find it, by the naked eye, to con- 

 sist of three distinct tissues, viz. superiorly or externally of 

 a tough, membranaceous or leathery, impervious, dense cor- 

 tical layer ; immediately subjacent or internal to this, of a 

 thin, bright-green, gonidic layer; and inferiorly or inter- 



