lichen
/ˈlaɪkən/
noun
- A simple, slow-growing plant that looks like a crusty or leafy patch and grows on rocks, trees, or walls. It is made up of a fungus and an alga living together.
- Hikers often see bright orange lichen growing on mountain rocks.
- Lichen can survive in extreme environments like deserts and the Arctic.
- The old stone wall was covered in gray and green lichen.