Lichens were once classified as single organisms—until the advent of microscopy, when the association of fungi with algae or cyanobacteria became evident.Although lichens had been assumed to consist of a single fungus species (usually an ascomycete) and a single photosynthetic partner, research suggests that many macrolichens also feature specific basidiomycete yeasts in the cortex of the organism.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work! Lichens are found worldwide and occur in a variety of environmental conditions.A diverse group of organisms, they can colonize a wide range of surfaces and are frequently found on tree bark, exposed rock, and as a part of biological soil crust.Lichens are long-lived and grow relatively slowly, and there is still some question as to how they propagate.Most botanists agree that the most common means of reproduction is vegetative; that is, portions of an existing lichen break off and fall away to begin new growth nearby.
The fungal filaments make up about 80% of the lichen body.
There is still some discussion about how to classify lichens, though many taxonomists rely on genetic analyses in addition to traditional morphological data.
Foliose lichens are large and leafy, reaching diameters of several feet in some species, and are usually attached to the substrate by their large platelike thalli at the centre.
Most lichens are very sensitive to air pollution, and like canaries in coal mines, may serve as indicators of air quality.
Lichen is composed of two or more dissimilar organisms that form a mutually beneficial (symbiotic) relationship to produce a new vegetative body that is called a thallus.