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Lichen. Coloured scanning electron micrograph (SEM) of a section through Caloplaca saxicola lichen. Lichens are organisms formed by the symbiotic association of a fungus (tube-like structures in image) and a green alga (spherical structures in image) 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 tendency to grow tight against the substrate. Lichen reproduce both sexually and asexually. Magnification: x650 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)