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Common orange lichen (Xanthoria parietina), coloured scanning electron micrograph (SEM). The circular dish- like structures are fruiting bodies known as apothecia. Lichens are organisms formed by the symbiotic association of a fungus and a green alga or cyanobacterium. They are able to produce nutrients through photosynthesis by the algal cells, and also absorb minerals from rainwater or acidic digestion of the substrate. Xanthoria lichen are classified as foliose due to their branches and flattened leaf-like structures. This lichen is found on trees, rocks and walls. It is normally orange-yellow in colour due to the presence of the pigment parietin. Magnification: x21 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)