Lichens

Composite organisms consisting of a fungus and a photosynthetic partner, either a green alga or a cyanobacterium, growing together in a symbiotic relationship. Lichens are named based on the fungal component. They are greenish gray, yellow, brown, or blackish in color, growing in leaflike, crustlike, or branching forms on exposed surfaces such as rocks, tree trunks, and walls. which plays the primary role in determining the lichen's form.