Androzani tree

Androzani trees were one of the universe's most efficient fuel sources, as well as an intelligent species.

Biology
Androzani trees normally looked like, but could shape themselves into new forms. A group of trees formed themselves into a tower, complete with doors, windows and thrones. The trees could also create features like Christmas ornaments; the Wooden King was grown inside one.

Androzani trees could separate their "lifeforce", turning into balls of light. Though this lifeforce could hover in the air, it couldn't travel through space without a suitable host. Men were "weak", while women were "strong", but children were inadequate either way. This was based on the logic of alien races residing in a "mothership", a maternal term. (TV: The Doctor, the Widow and the Wardrobe)

History
The forest on a planet knew that, in 5345, the harvest rangers from Androzani Major would melt down the forest for fuel. They also knew that the Eleventh Doctor would come to the planet, so they planned to escape using him. They formed themselves into a tower with a globe to travel off planet. A crown was made, which acted as an interface to allow the lifeforce to enter a person. The Wooden Queen was also in the tower to help the process.

In 5345, Cyril Arwell came to the planet via a portal from 1941 created by the Doctor. The Wooden King was created to lure him to the tower, but once he got there he was found to too weak to transport the forest. The Doctor and Lily, who had followed him, were also found to be too weak. Their mother Madge was suitable and the forest entered her just before they were harvested by acid rain. Using her own mind, she was able to pull the globe through the Time Vortex and landed in 1941 England, while the forest dispersed into the Vortex. (TV: The Doctor, the Widow and the Wardrobe)