A lichen was a type of vegetative life.
When James T. Kirk returned to Organia (which was at the time confined by a thought-shield) in 2269, it resembled a rocky wasteland, "like that of a planet which had lost its last beetle and shred of lichen a million years before." (TOS novel: Spock Must Die!)
A species of lichen native to Vulcan was a powerful hallucinogen if ingested or smoked. In 2247, Spock and David Rabin rescued hostages held by Sered by throwing these lichens into a fire in their hideout. (TOS novel: Vulcan's Forge)