Any of about 15,000 species of thallophytic plants that consist of a symbiotic association of algae (usually green) and fungi (mostly Ascomycetes and Basidiomycetes). Lichens were once classified as single organisms until the advent of microscopy, when the association of algae and fungi became evident. There is still some discussion about how to classify lichens. It is not certain when fungi and algae came together to form lichens for the first time, but it was certainly after the mature development of the separate components. The basis of their relationship is the mutual benefit that they provide each other. Algae form simple carbohydrates that, when excreted, are absorbed by fungi cells and transformed into a different carbohydrate. Algae also produce vitamins that the fungi need. Fungi contribute to the symbiosis by absorbing water vapour from the air and by providing much-needed shade for the light-sensitive algae beneath.
Source: Encyclopaedia Britannica 2002