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Lichens are unique organisms on Earth because they result from a partnership between mushrooms and algae or cyanobacteria. Mushrooms provide a home for algae and protect it from ultra-violet rays, and the algae nourish the mushrooms using photosynthesis. Lichens are often classified based on their morphology. Lichens have no roots, and get their nutrients directly from the air. They are therefore extremely sensitive to atmospheric pollution, and their diversity diminishes the closer you get to large urban centres.