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Last updated on April 18, 2014 at 1:21 EDT
Lichen-covered boulders in the Superstition Mountains close
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Lichen-covered boulders in the Superstition Mountains close to the Phoenix metropolitan area.

May 26, 2010
Lichen-covered boulders in the Superstition Mountains close to the Phoenix metropolitan area.

Lichens are actually made up of two plants: an algae and a fungus, living in a symbiotic relationship. In desert environments, lichen will dry out completely and remain dormant until rain or dewfall provides enough moisture to make them active again. This ability allows lichens to survive some of the harshest environments on the planet. Lichens are very sensitive to air pollution. Scientists rely on them as a bioindicator species, like a natural early warning system. [Image 4 of 5 related images. See Image 5.]

More about this Image: Plant biologist Thomas Nash has spent a career studying lichens, the crusty mats of blue and green and orange often seen covering desert rocks. The Lichen Herbarium at Arizona State University is home to 90,000 specimens collected over 30 years by Nash and his colleagues. Supported by the National Science Foundation, Nash and 70 scientists from 16 countries have spent more than 14 years documenting all the lichen species found in the Sonoran Desert region. More than 40 percent of the lichens known to North America live in the Sonoran Desert and surrounding regions. They recently completed volume I of the first comprehensive account of all lichen species known from the Southwest, including large parts of northwestern Mexico.