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.