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Usnea florida. The fruticose lichen Usnea florida. A lichen is a symbiotic organism comprising a fungus and an alga. The alga produces nutrients for the fungus by photosynthesis. Usnea florida is found on twigs and branches in the canopy of broad-leaved trees, especially oak. The distinctive disc-like structures are fruiting bodies. This lichen is a good indicator of air quality, as it is intolerant of high sulphur dioxide concentrations. It is now rare in the UK due to increasing levels of atmospheric pollution. Photographed in North Wales, UK. (KEYSTONE/SCIENCE PHOTO LIBRARY/CORDELIA MOLLOY/SCIENCE PHOTO LIBRARY)