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Common orange lichen (Xanthoria parietina) coloured scanning electron micrograph (SEM). The image shows a section through a fruiting body, known as the apothecium. Lichens are organisms formed by the symbiotic association of a fungus (tube-like structures) and a green alga 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 within the apothecium, known as the hymenium (orange), reproduce sexually to form ascospores. Magnification: x24 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)