A lichen consists of 2 or more partners that live together symbiotically, with both of them benefiting from the alliance. One partner is a fungus. The other is either an alga – 90% of known lichen – commonly Trebouxia, then Trentepohlia or a cyanobacterium – about 8%, commonly Nostoc.

The alga or cyanobacterium is able to use sunlight to produce essential nutrients by photosynthesis that feed both partners. The fungus creates a body, called a thallus, in which they both live.

The fungus also produces chemical compounds that may act as sunscreen to protect the photosynthetic partner.

There are more than 1,700 species of lichen in the British Isles. Approximately 30,000 species of lichen have been described and identified worldwide. The algal partners in lichens can be found living on their own in nature, as free-living species in their own right.

The fungal partners in British lichens are recognizable Ascomycetes or Basidiomycetes. However, they have come to need the right kind of algal partner in order to survive. Unlike other fungi or indeed their algal partner, they cannot survive on their own. Lichens have a variety of different growth forms. The simplest lichens are crusts of loosely mixed fungal hyphae and algae. Others are more complex, with leafy or shrubby forms like miniature trees, also having specialised structures to attach them to a surface.