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Lichen. Coloured scanning electron micrograph (SEM) of a section through lichen (Caloplaca saxicola). This image shows the reproductive structure, known as the apothecium, which forms a concave disc. Within the disc are raised hair- like cells (orange) that form sexually produced fungal spores. The main body of the lichen (thallus) is formed by the symbiotic association of a fungus and a green alga or cyanobacterium. Lichens are able to produce nutrients through photosynthesis by the algal cells, and can also absorb minerals from rainwater or acidic digestion of the substrate. Magnification: x240 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)