Pronounced li-ken, lichens aren't exactly plants and they're not exactly fungi. Lichens are actually a complex symbiotic partnership of several different organisms. A symbiotic partnership is one in which two, or more, different organisms interact in a mutually beneficial way.
Lichen, therefore, is a symbiosis between fungi and algae, or fungi and cyanobacteria, but most of the time between all three.
The fungus is the dominant partner in the relationship and is ultimately responsible for creating the structure of the lichen and determining what it looks like.
So why do these organisms need to live in partnership?
Fungi are heterotrophic which means they don't contain chlorophyll and can't photosynthesize or feed themselves in any way. Algae and cyanobacteria, conversely, can both photosynthesize - using energy from the sun to manufacture glucose (this makes them autotrophic) - but are unable to survive on their own outside of water. The perfect pair!
Fungi provide the stable condition on which algae and cyanobacteria can grow on land (a home), and the algae/cyanobacteria, in turn, provide simple sugars (food) to the fungus. Win, win.
So, why isn't lichen a plant?
A fungus is not a plant because its cell walls aren't comprised of cellulose and it doesn't contain chlorophyll wi