A lichen is a partnership of an alga and a fungus. These form fixed combinations that are described as lichen species. These have fixed shapes and lifecycles. Makes you wonder, doesn' t it?
Not as unusual as you may think either, as it is thought, on the ground of some pretty good arguments, that the cells of any plant and animal started as partnerships of different, bacteria like life forms.
In lichens the partnership is looser. The alga and the fungus can be separated and cultured apart. They don't like that, they do a lot better together. Lichens can grow in very difficult places. The individual partners would die there!