The lichen, although considered a single organism, is a collective construct of both a fungi and a photosynthetic organism(often green algae) which, able to survive in some of the most extreme environments on earth, exist in an inextricably linked symbiotic partnership.Generally oriented in a way such that the fungus, at times even penetrating the cell walls of their photosynthetic complement, envelops the phototroph, who provides food for the protection providing fungi.Their are lichens where the cyanobacterium functions as the photobiont,or when cyanobacterium and green algae collaborate to form a tripart lichen. Lichenism has proved to be an extremely useful adaptation for fungi, being adopted by approximately 20 percent of the species.Lichens often reproduce asexually via the dispersion of diaspores containing fungal and algal cells or simple fragmentation. The fungus, although intimately entwined with its symbiotically linked photobiont, can still reproduce sexually via meiosis and, after fertilization, the propagation of spores who are independent fungi until such time as they find a photosynthetic partner. A lichen can be seen growing over the bark of the above tree.


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