Verrucaria spp. pictured above is a lichen that forms a thin, black, tar-like layer on rock surfaces. Verrucaria sometimes grows inside rock crevices.

The diagram above offers an inside look at the structure of lichen. The upper cortex is the strong outer covering provided by the fungus. The algal layer lies below the upper cortex. The medulla stores the nutrients produced by the algae. Click the diagram for a larger view.