Some 20,000 kinds of this unique association grow on earth, living in almost every environment, from within Antarctic rocks to the surface of desert soils. Lichens can withstand years of desiccation by simply becoming dormant. They revive again with miniscule amounts of water - in the desert, humid night air suffices. Some arctic lichens are estimated to be over 4,000 years old, among the world's oldest plants. What looked to early botanists like a single plant is actually a highly-organized association of hundreds to millions of microscopic algal cells woven into a fabric composed of fungal filaments, or hyphae.
Lichens come in three different forms.
Crustose lichens often form colorful patches on rocks, adhering so tightly to their growing substrate that they cannot be separated. The bodies of fruticose lichens, such as beard lichens hanging in clumps from tree branches, are formed of many slender stalks. Foliose lichens, common on the surface of soils in forests and on tree trunks, get their name from the upcurled edges of their thalli, which resemble drying leaves. New lichens grow from soredia, specialized pieces of the parent thallus consisting of a few algal cells enveloped by fungal hyphae. Soredia are designed to break away easily with the action of wind or rain.