A distinct type of organism in which the thallus is composed of both fungal and algal cells in symbiotic association. The fungal partner (the mycobiont) is usually an ascomycete and is dominant to the alga (the phycobiont), which is a green or blue-green alga. Occasionally the fungus is a basidiomycete or a deuteromycete (imperfect fungus). The fungus forms the main part of the thallus, which is usually a stratified structure consisting of an upper and lower cortex of compact fungal tissue with a medulla in between of loosely woven hyphae. The algal cells are in a layer between the medulla and upper cortex and are closely surrounded by hyphae. This stratified type of structure is termed heteromerous. In some lichens, e.g. gelatinous lichens, the thallus consists of loosely woven hyphae and algal cells scattered in a jelly-like matrix. This unstratified structure is termed homoiomerous. The fungus obtains carbohydrates from the alga, while the alga receives water and nutrients from the fungus. The alga is also protected from desiccation by the surrounding fungal body. Lichen fungi are not found in the free-living state and attempts to culture them independently have not been successful (except in a few cases where the mycobiont is a basidiomycete). Algae very similar to those in lichens are found growing separately though it is uncertain if these are identical to the lichen algae.
Three main types of lichen are recognized by their growth habit. Crustose lichens grow closely attached to the substrate and usually lack distinct lobes. Foliose lichens are generally attached loosely to the substrate by rhizinae and the thallus has lobed leaflike extensions. Fruticose lichens are either erect and bushy or hanging and tassel-like, and are only attached at one point. Some lichens are intermediate in form. For example, Cladonia species initially form a basal crust from which arise erect branching structures (podetia). Reproduction may be by dispersal of fragments of the thallus containing both fungal and algal cells, by special vegetative reproductive bodies (see soredium
), or by fungal spores. The nature of the asci and ascospores produced by the fungal component are important in identification.
Lichens are extremely tolerant of almost total desiccation and can thus colonize exposed bare areas where other plants are unable to survive. They are however very slow growing, an average lichen possibly only extending by 1 mm a year. Some are believed to live for up to 4000 years. They have few commercial uses though some yield dyes, while others contain antibiotics, e.g. usnic acid from Usnea. Some of the larger species of arctic zones, e.g. reindeer moss (Cladonia rangiferina), are important as food for deer. The distribution of certain lichen species is used as an indicator of atmospheric pollution (see indicator species
). See also Lichenes