A fungus was a type of life-form which bore some similarities to plants.
Professor Clifford Jones studied fungi, specifically their overlooked nutritional value. He discovered that his edible fungi was poisonous to the giant maggots. (TV: The Green Death) Later, he and his wife Jo Grant went on an expedition to the Amazon in search of a particularly rare fungus. (TV: Planet of the Spiders)
Fungi could also be used as weapons. The Great Intelligence's robot Yetis were armed with guns which fired a deadly, cobweb-like fungus. (TV: The Web of Fear) The Ice Warriors used the fungal Martian Seed Pods in their attempt to conquer Earth. (TV: The Seeds of Death) The Tenth Doctor used a metal-eating fungus to destroy Baltazar's ship. (TV: The Infinite Quest)