Androzani tree

Androzani trees were one of the universe's most efficient fuel sources. In 5345 the Eleventh Doctor, Madge Arwell and her children, Cyril and Lily, visited a forest of Androzani trees on an unnamed planet just before they were harvested by acid rain. Madge carried the trees' life force to safety through the Time Vortex. (DW: The Doctor, the Widow and the Wardrobe)