Tree

A tree was a woody form of plant that grew on several planets. Generally forests were composed (at least partly) of trees. The forests of Eden were dense even in the small section captured by Tryst's Continuous Event Transmuter. (DW: Nightmare of Eden)

Intelligent or sentient trees
Whilst on Earth in the 1820s the Rani mined a section of forest with mines that turned humans into trees. Following their transformation the former-humans retained some level of consciousness. (DW: The Mark of the Rani)

By 5,000,000,000 many trees that had originated on Earth had achieved sentience and humanoid form. (DW: The End of the World)