Lichens are a symbiotic association
   of photosynthetic microorganisms held in a mass of fungal hyphae.
<P>
   The   photosynthetic   partners   are   typically   green   algae   or
   cyanobacteria.
<P>
   The  fungal  component  is  most  often  an  ascomycete,  but  several
   basidiomycete lichens are known.