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Lichen. Coloured scanning electron micrograph (SEM) of Caloplaca saxicola lichen growing on a stone surface. 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. Caloplaca lichen are classified as crustose due to their crust-like appearance and preference to grow tight against the substrate. Lichen reproduce both sexually and asexually. Magnification: x1760 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)