Have you ever walked through the woods and seen a crusty-looking shelf growing out of a tree? That is actually a lichen, a complex organism comprised of a fungus and its partner algae, the two parts interweaving to form one.
The word lichen appeared around 1600, and was drawn from the Greek leikhēn, meaning "what eats around itself." This is a good meaning for what the word represents, as lichen is comprised of fungus cells that literally surround the algae cells to form a mutualistic bond. Because of the crusty look of a lichen, the name has also been given to a number of skin diseases that present the same patchy look as the fungus-algae.