The most common fungi in lichens are usually species of Ascomycetes, or a few Basidiomycetes. The usual algal partners are either species of green algae Chlorophyta or blue-green bacteria of the family Cyanophyceae. In general, the fungal partner cannot live without its phycobiont, but the algae is often capable of living freely in moist soil or water. The largest lichens can form a thallus up to 3 ft (1 m) long, although most lichens are smaller than a few inches or centimeters in length. Lichens can be very colorful, ranging from bright reds and oranges, to yellows and greens, and white, gray, and black hues.
Most lichens grow very slowly. Lichens in which the phycobiont is a blue-green bacterium have the ability to fix nitrogen gas into ammonia. Some lichens can commonly reach ages of many centuries, especially
species living in highly stressful environments, such as alpine or arctic tundra.
Lichens can grow on diverse types of substrates. Some species grow directly on rocks, some on bare soil, and others on the bark of tree trunks and branches. Lichens often grow under exposed conditions that are frequently subjected to periods of drought, and sometimes to extremes of hot and cold. Lichen species vary greatly in their tolerance of severe environmental conditions. Lichens generally respond to environmental extremes by becoming dormant, and then quickly becoming metabolically active again when they experience more benign conditions.
Lichens are customarily divided into three growth forms, although this taxonomy is one of convenience, and is not ultimately founded on systematic relationships. Crustose lichens form a thallus that is closely appressed to the surface upon which they are growing. Foliose lichens are only joined to their substrate by a portion of their thallus, and they are somewhat leaf-like in appearance. Fruticose lichens rise above their substrate, and are much branched and bushy in appearance.
Most lichens regenerate asexually as lichen symbioses, and not by separate reproduction of their mycobiont and phycobiont. Reproduction is most commonly accomplished by small, specialized fragments of thallus known as soredia, consisting of fungal tissue enclosing a small number of algal cells. The soredia generally originate within the parent thallus, then grow out through the surface of the thallus, and detach as small bits of tissue that are dispersed by the wind or rain. If the dispersing soredium is fortunate enough to lodge in a favorable microenvironment, it develops into a new thallus, genetically identical to the parent.