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Rosette lichen. Coloured scanning electron micrograph (SEM) of a section through a rosette lichen (Physcia adscendens). Lichens are organisms formed by the symbiotic association of a fungus (blue-grey) and an alga (green spheres) or cyanobacterium. The algal cells use photosynthesis to produce nutrients which are absorbed by a mesh of fungal hyphae that surround and penetrate the algae. Fungal hyphae also absorb minerals from rainwater or acidic digestion of the substrate. Physcia lichen are classified as foliose due to their flat appearance and lobed branches. Magnification: x520 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)