We all know what lichens are. They're the standard example of a mutualistic association that we were all presented with in high school, an association of fungus and unicellular alga allowing both to survive long-term in situations that would normally be fatal for them both. More than 15,000 species of lichen have been described—or, rather, species of lichenised fungi, as names applied to lichens technically apply to the fungal member of the association (only a relatively small number of algae form lichen associations). Though these species can all be attributed to the Ascomycetes among the main fungal subdivisions*, they do not form a single clade within the asomycetes. Instead, it appears that the lichen lifestyle has been gained and/or lost on numerous occasions.
*Lichen-like associations are sometimes formed by other fungi such as Basidiomycetes but they lack the integrity of the ascomycetous examples. Lab workers have even been able to induce lichen-like associations between unicellular algae and colonial or hyphal bacteria such as myxobacteria and streptomycetes (Ahmadjian 1965).
Parmelia is a genus of foliose lichens which is found worldwide but has its highest diversity in Asia (Molina et al. 2004). Well over 1000 species have been assigned to the genus over the years but many (though not all) recent authors have tended towards a much more restricted circumscription of about forty species. True Parmelia, in this sense, is distinguished from other genera in the lichen family Parmeliaceae by its linear pseudocyphellae (pore-like structures in the upper-surface of the lichen's cortex) and its particularly small spores and conidia (conidia are reproductive structures like spores but produced asexually rather than sexually) (Elix 1993). ITS rDNA phylogeny is mostly consistent with many of the proposed segregate genera, including the restricted Parmelia, though it provides little information on their higher relationships (Crespo & Cubero 1998).
Parmelia achieves its highest diversity in temperate or boreal regions. The type species, P. saxatilis, is one of the world's most widespread lichen species, found in both the Arctic and the Antarctic, as well as cooler localities in between (Molina et al. 2004). Lichens can reproduce in one of two ways: small pieces of the thallus containing both algal cells and fungal hyphae may break off to grow directly into a new thallus elsewhere, or the lichen can release spores and/or conidia in the manner of other fungi. A germinating lichen spore will grow extremely slowly: even in laboratory cultures on agar, some lichen fungi will only reach a diameter of 1 mm within the course of a year when grown without algal symbionts(Ahmadjian 1965). Formation of the lichen association is dependent on the fungus randomly coming into contact with an alga, and growing lichen fungi will form exploratory hyphae around anything (even grains of sand) that they touch that might turn out to be an alga (Ahmadjian 1960). The low variety of algal species occuring in lichens appears to be dependent not on any direct attraction of the alga for the fungus, but on the alga's ability to resist digestion by the fungus' hyphae. Lichens are famed for their slow growth even after an association is established, and may increase in diameter by only a millimetre a year*, but the limiting factor is probably not so much their inherent growth abilities as that their favoured environments such as exposed on rocks may only allow growth for a minute part of the year.
*If you're thinking that that doesn't sound any greater than the rate for symbiont-less fungi that I cited above, remember that the latter rate applies to growth in the laboratory under theoretically optimal conditions; growth in the natural environment would be much, much slower.
Ahmadjian, V. 1960. The lichen association. Bryologist 63 (4): 250-254.
Ahmadjian, V. 1965. Lichens. Annual Review of Microbiology 19: 1-20.
Crespo, A., & O. F. Cubero. 1998. A molecular approach to the circumscription and evaluation of some genera segregated from Parmelia s. lat. Lichenologist 30 (4-5): 369-380.
Elix, J. A. 1993. Progress in the generic delimitation of Parmelia sensu lato Lichens (Ascomycotina: Parmeliaceae) and a synoptic key to the Parmeliaceae. Bryologist 96 (3): 359-383.
Molina, M. del C., A. Crespo, O. Blanco, H. T. Lumbsch & D. L. Hawksworth. 2004. Phylogenetic relationships and species concepts in Parmelia s. str. (Parmeliaceae) inferred from nuclear ITS rDNA and β-tubulin sequences. Lichenologist 36 (1): 37-54.