In nature, lichen are a composite organism consisting of fungus and photosynthetic bacteria or algae (a photobiont) living in a symbiotic relationship. Lichens are named based on the type of fungal component they contain.
There over 20,000 known species and over 3,000 are known in Australia.
Lichen take on biochemistry and forms quite differently to their individual fungal or bacterial components.
There are different types of lichen:
These stand erect or pendulous and are three-dimensional.
These are essentially two dimensional and firmly attached to the surface they live on. It's basically impossible to view the underside of these lichen.
These are halfway between crustose and fruticose. But essentially flat though they do have some obvious three dimensional aspects to them.