Source : Webster's Revised Unabridged Dictionary (1913)

Lichen \Li"chen\ (l[imac]"k[e^]n; 277), n. [L., fr. Gr.
   leichh`n.]
   1. (Bot.) One of a class of cellular, flowerless plants,
      (technically called {Lichenes}), having no distinction of
      leaf and stem, usually of scaly, expanded, frond-like
      forms, but sometimes erect or pendulous and variously
      branched. They derive their nourishment from the air, and
      generate by means of spores. The species are very widely
      distributed, and form irregular spots or patches, usually
      of a greenish or yellowish color, upon rocks, trees, and
      various bodies, to which they adhere with great tenacity.
      They are often improperly called {rock moss} or {tree
      moss}.

   Note: A favorite modern theory of lichens (called after its
         inventor the Schwendener hypothesis), is that they are
         not autonomous plants, but that they consist of
         ascigerous fungi, parasitic on alg[ae]. Each lichen is
         composed of white filaments and green, or greenish,
         rounded cells, and it is argued that the two are of
         different nature, the one living at the expense of the
         other. See {Hyph[ae]}, and {Gonidia}.

   2. (Med.) A name given to several varieties of skin disease,
      esp. to one characterized by the eruption of small,
      conical or flat, reddish pimples, which, if unchecked,
      tend to spread and produce great and even fatal
      exhaustion.

Source : WordNet®

Lichenes
     n : comprising the lichens which grow symbiotically with algae;
         sometimes treated as an independent group more or less
         coordinate with Algae and Fungi [syn: {division Lichenes}]