Lichen, an organism made up of an alga and a fungus. The alga, in some species, is a green alga; in other species, a blue-green alga. The fungus usually belongs to the same class of living things as morels and truffles. There are about 25,000 species of lichens. Biologists name lichens according to the species of fungus involved. Lichens are found in all parts of the world, even in polar regions and at high altitudes, growing on rocks, wood, or firm soil. Lichens grow very slowly. Most lichens are gray-green, but they may also be white, yellow, yellow-green, red, orange, brown, or black.

The partnership between algae and fungi in forming lichen is an example of symbiosis. Traditionally, biologists have considered the symbiosis to be mutualism (a mutually beneficial association), with the algae making food for the lichen through photosynthesis and the fungi providing protection for the algae and obtaining water and minerals for the lichen. Some biologists believe that in many environments the algae could survive without the fungi, and that the fungi are parasites on the algae. Whatever the nature of the association, lichens can survive in some environmental conditions in which neither the algae nor the fungi could live alone.

When algal cells and fungal cells inter-mesh they produce a structure called a thallus, which forms the visible body of the lichen. In many lichens, algal and fungal cells in the thallus make chemical compounds that cannot be made by either algae or fungi independently. The shape of the thallus is used to classify lichens into three main types:

Crustose Lichens

such as brown crust lichen, are crustlike and can attach to rocks.

Foliose Lichens

such as dog-tooth lichen, have flat, leaflike structures.

Fruticose Lichens

such as old man's beard and reindeer moss, have erect or hanging, branchlike structures.

Certain lichens reproduce asexually from broken-off pieces of thallus or from soredia (structures consisting of a few algal cells surrounded by fungal cells). In other lichens, fungal cells reproduce sexually by forming spores, which develop into fungi. When wind or rain transports suitable algae to these fungi, a lichen develops.

Lichens aid in the breakdown of rock and wood to form soil. Certain lichens are able to remove nitrogen from the air and convert it into a form that plants can use as a nutrient. Lichens are very sensitive to atmospheric pollution and are used to monitor environmental damage. Reindeer and caribou depend on lichens for food. Humans have used lichens as food in emergencies.

Lichens have some commerical value as a source of litmus (a dye used as a chemical indicator), of perfume fixatives, and of usnic acid (an antibiotic).

Lichens make up the division Mycophycophyta. The algae are of the phylum Chlorophyta or Cyanophyta. Most of the fungi are of the division Ascomycota; some are of the division Basidiomycota or Deuteromycota.