Fungus

Fungus was a type of life-form which bore some similarities to plants.

Professor Clifford Jones studied fungi, specifically their overlooked nutritional value. He later discovered that his edible fungi was poisonous to the giant maggots. (DW: The Green Death) Later, he and his wife Jo Grant went on an expedition to the Amazon in search of a particularly rare fungus. (DW: 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. (DW: The Web of Fear) The Ice Warriors used the fungal Martian Seed Pods in their attempt to conquer Earth. (DW: The Seeds of Death) The Tenth Doctor used a metal-eating fungus to destroy Baltazar's ship. (DW: The Infinite Quest)