Lichens
Author: Ray Bowers

Description:
Lichens are two organisms that grow together as a symbiotic relationship . This relationship is a very efficient parasitic-mutual relationship. An algae produces food through photosynthesis and some types of algae can also use nitrogen in the air to produce nitrogen compounds. This production of nitrogen compounds is known as nitrogen fixation . The algae reproduces fast enough so that more cells are produced than are digested by the fungus. The fungus provides a surface for the algae to grow on, as well as minerals that the fungus dissolves from the surface on which it grows or substrate.

The body of the lichen is called the thallus. The thallus may have a variety of colors due to the algae that usually grows as a layer just below the surface and the fungus itself. The thallus may appear as a crust that is closely attached to a substrate, a leaf-like or blade-like structure that is loosely attached to the substrate, or as a branched structure that is attached to the substrate by a stalk or freely growing over the substrate. The lower surface of the thallus has hair-like threads, called rhizines, which attach the lichen to the substrate. Small cups or disk sometimes appear on the surface of the thallus that are called apothecia. The apothecia may be a different color than the thallus. The size can vary from less than 1mm to 400mm in diameter.

Geographic range:
Lichens can be found on all continents, and they are found throughout New Mexico.

Habitat:
Lichens can be found in all terrestrial habitats from the poles to the equatorial jungles. They can be found growing on rocks on the tops of mountains or on the sides of trees at sea level. Lichens are one of the first organisms to appear on newly formed volcanic rock. Lichens can be found on a variety of substrates from bare soil, rock or old tombstones to trees or old wooden buildings. They can be found in very dry or very humid environments.

Reproduction and Development:
The fungus component of a lichen can reproduce sexually by means of spores released from the apothecia. The most common type of reproduction is asexual. This may be in the form of soredia which are powdery grains made of a few algae cells surrounded by fungus threads; or by fragmentation, where a piece of lichen breaks off and start growing in a different location. The alga component is not able to live on its own in the environment.

Conservation:
Some lichens are very sensitive to air pollution, and are being used to determine air quality. Habitat loss due to urbanization has caused some lichens to disappear from some areas. Foot or vehicular traffic can damage soil lichens.

Other info:
Some lichens have been used as dyes. The Klamath tribe of California used lichens to color porcupine quills. Some lichen pigments will change color in the presence of acids or bases, and have been used in test paper to identify the presence of acids or bases. Some lichens produce chemicals, such as usnic acid, that are antibiotic. The Native Americans have used this quality and preparations with these chemicals are still sold in Europe . In Japan the lichen known as rock tripe is considered a delicacy.

Lichens play an important role in the arctic as forage for herbivores; one type of lichen is called reindeer moss. Lichens are often the first organism to colonize a new surface such as bare rock. Their presence may produce enough soil so that other plants can make a foot-hold. Lichens grow very slowly, and can be used to date past geological events.

Taxonomy:
This taxonomy is not based on how the lichens are related, but on growth forms:

Crustose Lichens have the thallus growing closely attached to the substrate.
Foliose Lichens have the thallus as a leaf-like or blade-like in structure with branching lobes that is loosely attached to the substrate.
Fruticose Lichens have a thallus that is shrubby with branches that are round or flat in cross section. They be attached at the base of a stalk or free growing.

References:
Corbridge, James N. and William A. Weber. 1998. Rocky Mountain Lichen Primer. Niwot , Colorado: University Press of Colorado.

Fink, Bruce. 1935. The Lichen Flora of the United States . Ann Arbor : The University of Michigan Press.

Hale, Mason E. and Mariette Cole. 1988. Lichens of California . Berkeley : University of California Press.

Shuttleworth, Floyd S. and Herbert S. Zim. 1987. Mushrooms and Other Non-Flowering Plants. New York : Golden Press.

Stern, Kinsley R. 1997. Introductory Plant Biology. Dubuque , Iowa : Wm. C. Brown Publishing.