Tree lichen (Pseudevernia furfuracaea), coloured scanning electron micrograph (SEM). Lichens are organisms formed by the symbiotic association of a fungus and an alga or cyanobacterium. The algal cells use photosynthesis to produce nutrients which are absorbed by fungal hyphae that surround and penetrate the algae. Fungal hyphae also absorb minerals from rainwater or acidic digestion of the substrate. 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: x20 when printed 10 centimetres wide.