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Hammered shield lichen. Coloured scanning electron micrograph (SEM) of a section through a hammered shield lichen (Parmelia sulcata). Lichens are organisms formed by the symbiotic association of a fungus (thin tubular structures) and an alga (green spheres) or cyanobacterium. The algal cells use photosynthesis to produce nutrients which are absorbed by fungal hyphae that surround and penetrate the algae. Fungal hyphae also absorb minerals from rainwater or acidic digestion of the substrate. Parmelia lichen are classified as foliose due to their flat appearance and leaf-like structures. Magnification: x480 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)