There are about 3,600 described species of lichens in North America.
Lichens are symbiotic partnerships between multiple organisms (a fungus and a green alga or cyanobacteria (blue-green alga)) that share one body. Chlorophyll in the alga and or cyanobacteria give the lichens their green color and the ability to use sunlight to make their food through photosynthesis.
Lichens absorb toxins and pollutants such as heavy metals, carbon, sulfur, or other pollutants, making them important bioindicators of air quality.
For more information, see HGIC 2354, Spanish Moss, Lichens, and Slime Molds and Lichens.