lichen

noun

• 1a simple slow-growing plant that typically forms a low crustlike, leaflike, or branching growth on rocks, walls, and trees.

  Lichens are composite plants consisting of a fungus that contains photosynthetic algal cells. Their classification is based upon that of the fungal partner, which in most cases belongs to the subdivision Ascomycotina, and the algal partners are either green algae or cyanobacteria. Lichens obtain their water and nutrients from the atmosphere and can be sensitive indicators of atmospheric pollution

• 2 [usually with modifier] a skin disease in which small pimples or bumps occur close together.