Lichen represent a fascinating combination of organisms working together to form some familiar and unfamiliar growths on a variety of substrates. This 6-page fact sheet written by James Stevenson, Lara B. Milligan, and Jennifer L. Gillett-Kaufman and published by the UF/IFAS Entomology and Nematology Department defines and explains these organisms.
https://edis.ifas.ufl.edu/in1296