Lichens are not plants, Lichens are a complex life form that is a symbiotic partnership of two separate organisms, a fungus and an alga. The dominant partner is the fungus, which gives the lichen the majority of its characteristics. The alga can be either a green alga or a blue-green alga. Lichens are a food source for herbivores like reindeer, provide nesting material for birds.
Source: FOREST SERVICE
U.S. Department of Agriculture
