\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.