Lichen and Moss. Lichen and moss growing on beech tree bark. The tree's bark (brown, at centre) has spots of white crustose lichen. Light green fruticose lichen and a dark green moss can be seen around the exposed bark. Lichens grow in three different shapes. Crustose (flat crusts), foliose (flat and branching) and fruticose (branched and erect). Lichens are an example of symbiosis. Fungal and algal cells cohabit for mutual benefit and make up the body of the plant. Lichens only grow in areas of clean air and can therefore be used as indicators for atmospheric pollution. Photographed in Northumberland, England.