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Common orange lichen. Coloured scanning electron micrograph (SEM) of a section through the fruiting body (apothecium) of the common orange lichen (Xanthoria parietina). Lichens are organisms formed by the symbiotic association of a fungus (tube-like structures) and a green alga (green spheres) or cyanobacterium. They are able to produce nutrients through photosynthesis by the algal cells, and also absorb minerals from rain water or acidic digestion of the substrate. Fungal cells from the hymenium (orange) of the apothecium reproduce sexually to form ascospores. Xanthoria species are classified as foliose lichen due to their flat, leaf-like appearance. Magnification: x24 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)