Tar lichen (Verrucaria sp.), coloured scanning electron micrograph (SEM). The image shows a section through the lichen and the substrate rock. The circular structure (centre) is a fruiting body known as a perithecium. Lichens are organisms formed by the symbiotic association of a fungus and a green alga 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. Verrrucaria lichen are commonly found on seashore rocks and have a black, oily appearance. Magnification: x52 when printed 10 centimetres wide.