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May 26, 2009
News Article: Richmond Times-Dispatch

Published: May 26, 2009

If you come across a rock in its natural setting, you will likely find it covered by lichens, proof that nature is alive.

What are lichens? Lichen is not a single organism, it is made up of two parts: a fungus (mushroom is a familiar example) and either green algae or blue-green algae.

These two parts have a mutually beneficial (symbiotic) relationship. The fungus provides the housing. The algae provide the nutrients through photosynthesis. The threadlike chains of cells that form the body of the lichen can grow into rock crevices, leak organic acid that dissolves minerals, and provide additional nutrition.

DNA studies have shown that fungus is so different from plants (trees, grass, flowers, and such) that biologists have put it in a separate kingdom. Plant and fungus kingdoms are in the domain of Eucarya (the animal kingdom belongs here also). Algae belong to the domain of Bacteria. Lichen, therefore, consists of members of two different domains, and neither is a plant.

Crustose lichens grow on rocks, making them appear polished.

We can't see their undersides because crustose lichens are so tightly bound to the rock. Lichens are nature's pioneers in breaking down rocks. Crustose lichens, because of their efficient pairing with the rock, are super-pioneers.

Crustose lichens often provide spectacular colorful displays like modern abstract art.

Much of the coloring of lichens result from their biochemistry, but humidity in the area can make a difference. Dark crustose lichen growing on an exposed rock surface might turn bright yellow in crevices where higher humidity persists even under a hot sun.

Foliose lichens have undersides that are more easily detached from the rock. Common, green-gray foliose lichen found growing on rocks in the mid-Atlantic region is Flavoparmelia baltimorensis. A closely related species, F. caperata, prefers tree trunks. They are both flat lying, and the individual body is a few centimeters across.

In the Potomac River gorge near Washington, lichen biologists Mason Hale and his coworker James Lawrey measured the rate of growth of different lichens One crustose lichen, Lecanora, grows less than 1 millimeter per year. F. baltimorensis grows faster at a few millimeters a year while F. caperata grows even faster.

The use of the size of lichen patches to estimate age of exposure of a rock is called lichenometry.

On your walk, look for another foliose lichen, Umbilicaria, on rocks. It is dark gray to brown, a few centimeters across, and is fittingly described as "over-fried potato chips." Human footfalls crush the holdfast that ties the lichen to the rock. Over time, this lichen found along the Potomac River, has gone from common to rare; now found only where people do not frequent.

Virginia science Standards of Learning: K.9; 1.8; 2.5; 4.5; LS.5; LS.7; LS.9; BIO.5.

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