Thrift on lichen-covered rock. Small plant of thrift, Armeria maritima, growing on rocks covered in the black lichen, Verrucaria maura, above the high tide mark of a shore. Thrift is a pink- flowered low growing downy perennial found along cliffs and shore lines. Verrucaria is an abundant lichen forming a thin black crust. It may be mistaken for tar stains when covering large areas of rock. Lichens are a composite organism made up of a fungus and an alga living together. The fungus provides a structure that prevents the alga from drying out. The photosynthetic alga provides food for the fungus in the form of carbohydrate. Photographed in North West Scotland.