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Lichen. Coloured scanning electron micrograph (SEM) of the spores (green) of Cladonia sp. lichen colonising a twig. The ball-shaped spores are produced asexually and known as soredia. 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. They can reproduce both sexually, as fungal only spores, or asexually, as balls of fungal and algal soredia. Cladonia are fruticose lichen, which are lichen with free standing erect structures. Magnification: x10 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)