Lichens growing on the bark of a rowan tree (Sorbus aucuparia) near the coast. Three types of lichen are seen here. The white lichen is Parmelia laevigata, the green lichen is Lobaria virens and the grey/brown lichen is Pannaria rubiginosa. These last two are good indicators of air quality, as they are intolerant even of small amounts of the pollutant sulphur dioxide. A lichen is a symbiotic organism comprising an alga and a fungus. The alga produces carbohydrates for the fungus by photosynthesis, and the fungus protects the alga from harsh conditions. Photographed near the Sound of Mull, Scotland.