220 LICHENES. 



Geographical Relations. Abounding within the tro- 

 pics, and decreasing both in size and number towards the 

 poles. 



Properties. Their most general character is astrin- 

 gency. 



Officinal Plant. 

 Aspidium filix-mas. 



Amphigamiu. 

 60. LICHENES. 



Aerial perennial plants varying exceedingly in their 

 form, appearance, and texture, always constituting a 

 thallus, crust, or frond, (universal receptacle, Ach.) 

 which frequently spreads horizontally upon soil, rocks, 

 stones, the bark of trees and dead wood, and is pulveru- 

 lent, membraceous, coriaceous, gelatinous, filamentous, 

 and variously lobed and divided ; sometimes it is erect, 

 fructiculose and much branched, at other times pen- 

 dent. Variously coloured, rarely green, often the sub- 

 stance is simply composed of cellules, at other times the 

 cellules are mixed with fibres. Imperfect roots are 

 sometimes formed, but more for the purpose of fixing 

 the plant to its place of growth, than of deriving nutri- 

 ment, which appears to be afforded solely by the air. 

 Fructification is of two kinds, a powdery substance form- 

 ing indeterminate masses or collected into more or less 

 evident receptacles ; and what is considered a higher 

 state of fructification, apothecia or partial receptacles, 

 which have received different names, according to their 

 forms scutellce, peltcs, lirell<e, etc., etc. ; these for the 

 most part are sessile, perennial, and contain a somewhat 



