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Lichen. Coloured scanning electron micrograph (SEM) of a section through lichen (Xanthoria flammea). Lichens are organisms formed by the symbiotic association of a fungus and a green alga or cyanobacterium. The fungus grows a network of tube-like hyphae that extract nutrients from photosynthesising algae (green spheres). The hyphae can also absorb minerals from rainwater or acidic digestion of the substrate. Xanthoria lichen are classified as foliose due to their branched appearance and flattened leaf-like structures. Magnification: x800 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)