lichen
n. C / Un. a small plant that grows on rocks, trees, or walls. It is actually a mix of two different living things working together.
n. a composite organism that arises from the symbiotic relationship between a fungus and a photosynthetic partner, such as an alga or cyanobacterium.
The old stone wall was covered in green lichen.
Because lichen is very sensitive to air quality, scientists often use it to measure pollution levels in a forest.
The lichen's ability to survive in extreme environments, such as the polar regions or the surface of a rock, makes it a subject of interest for astrobiologists studying potential life on other planets.
Borrowed from Latin līchēn, from Ancient Greek λειχήν (leikhḗn), from λείχω (leíkhō, “to lick”). Originally used of liverwort; the modern sense first recorded 1715.