World Tree

World Trees were a type of interstellar tree famous for their long lifespans, huge size, and sentience.

Biology
The distribution of World Trees was managed by the Grand Arboretum, who looked after their seeds so they could not fall into the wrong hands. Upon being planted, a World Tree would become conscious and start growing at a rapid pace, remembering and absorbing the knowledge of all those it came across. As they could live for tens of thousands of years, they became cultural touchstones and living records for species across the universe.

Upon first sprouting, a World Tree looked like a small twisted red bush, but would grow to a five-foot-tall red tree within a matter of hours. In Earth terms, it would become the biggest tree on the planet by the end of its first day, taller than the tallest skyscraper within a week, and large enough to tear the planet in two by the size of its roots within a month. As a result, they could only grow on specially selected host worlds. (AUDIO: The World Tree)

History
One day, the Grand Arboretum lost one of its World Tree seeds, at which point it floated aimlessly through space for over a thousand years. Instead of burning up on impact with Earth, however, it landed safely in Nora Wicker's garden on 19 August 1987, where it was discovered by the Eleventh Doctor. Realising it would destroy Earth if left uncontrolled, he saw no option but it keep it held in a Time Bubble until the Grand Arboretum found a suitable host planet, but accidentally captured Nora's cottage in the time loop as well.

Despite the Doctor offering Nora the chance the escape the bubble after 16,921 loops, she refused and promised to look after the World Tree so it would not feel alone and instead told it stories about her late husband Ernie. The Doctor promised to pick her up and return her to her own time when the tree was eventually ready to collect. (AUDIO: The World Tree)