Tree lichen. Coloured scanning electron micrograph (SEM) of Pseudevernia furfuracaea lichen. Lichens are organisms formed by the symbiotic association of a fungus and an alga or cyanobacterium. The algal cells use photosynthesis to produce sugars which are absorbed by fungal hyphae that surround and penetrate the algae. Fungal hyphae also absorb minerals from rainwater or acidic digestion of the surface. Pseudevernia lichen are classified as fruticose due to their erect, shrub-like appearance. They are commonly found on tree surfaces, only appearing in regions with little air pollution. Magnification: x64 when printed 10 centimetres wide.