In a five-kingdom classification system, lichens are put somewhat awkwardly into the kingdom fungi. Although the fungal components of lichens are in fact ascomcetes and basid-iomycetes, the role of these fungi in the lichen collective has physiologically changed them to the point where they deserve their own taxon. Together lichen partners produce acids and pigments that neither can produce on its own; together they make the "tissuelike" substance that turns rock to soil and expands the domain of the living.
In contrast to the diverse fungal members of lichens (estimated at 25,000), the photo-synthetic partners are relatively few. Most common are Trebouxia and Pseudotrebouxia:
walled, immobile cells that can no longer survive without their fungal brethren. Made of members of two separate kingdoms, one or two (depending on if the photosynthetic component is algal) also have a symbiotic history from bacteria coming together; the existence of fungi as a phylum, while logical, shows that branches on the tree of life merge as well as split.
—Lynn Margulis and Dorion Sagan
See also: Classification, Biological; Coevolution; Conservation Biology; Five Kingdoms of Nature; Pollution; Soil; Topsoil Formation; Topsoil, Loss of
Brodo, Irwin M., Sylvia Duran Sharnoff, and Stephen Sharnoff, 2001. Lichens of North America. New Haven: Yale University Press; Kendrick, Bryce. 1992. The Fifth Kingdom. Sidney, British Columbia: Mycologue Publications; Margulis, Lynn, and Karlene Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3d ed. New York: W. H. Freeman.
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