STRUCTURE OF LICHENS

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About 18,000 to 20,000 species of lichens have been identified. Scientists typically classify lichens based on the fungal partner, and the lichen name is the same as the scientific name of the fungal partner. By far the most common fungi found in lichens are sac fungi, or ascomycetes, which produce reproductive spores in special cells shaped like sacs. In about two dozen tropical lichens, the fungal component is a club fungus, or basidiomycete. The club fungi, which include common mushrooms and toadstools, produce their reproductive spores in special cells that are shaped like clubs. 

The algal partner of a lichen is usually composed of green algae in the form of single cells or chains of cells. Green algae contain chlorophyll, the primary light-absorbing pigment necessary for photosynthesis. This pigment is housed in saclike structures called chloroplasts, the sites of photosynthesis. In some dark-colored, gelatinous lichens, cyanobacteria (also known as blue-green algae) are the algal component. Usually a lichen contains a single species of algae, but in a few cases both a green alga and a cyanobacterium live in different parts of the same lichen.

A typical lichen has a three-layered structure. A middle layer containing algal cells entwined in fungal hyphae is sandwiched between two layers of fungal tissue. This three-layered structure is arranged into one of three basic growth forms. Crustose lichens grow as flattened crusts with the bottom layer cemented to the surface of a rock or tree bark. Foliose lichens have a leaflike appearance, with a distinct upper and lower surface exposed to the air. Foliose lichens often form large, flaky patches on tree trunks. Fruticose lichens grow in hairlike, shrubby strands on the ground or hanging from tree branches. Each strand is tubular, with the typical three-layered structure surrounding a hollow core. A common fruticose lichen is old man’s beard, which hangs in wispy clumps from tree limbs and resembles moss. 

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