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Lichen. Coloured scanning electron micrograph (SEM) of lichen (Cladonia sp.) growing on moss (green). The ball-shaped structures attached to the moss are asexually produced spores known as soredia which have begun to colonise the moss. 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. Magnification: x144 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)