:When summing up recent events:
The tree does seem to be suspiciously friendly and sane for an ancient being or ancient artefact, let alone both in one. Even the only other vaguely sane ancient being, the balloon-jellyfish godling, wasn't this friendly until she was punched in the face a few times and given a pretty good Shut Up, Hannibal speech by a sapient brain tumor, and she doesn't seem to have been nearly as old.
...that was a weird sentence. -- RecklessPrudence
It wasn't every day that a companion went and rescued herself. And it was even less often that they turned up with allies. The Doctor looked over the motley crew. There was a brain in a tank/mecha body that seemed to be completely taken over by a tumour, a gigantic tree with a face, and a floating thing that looked like the unlikely progeny of a jellyfish and a dirigible.
"Well done, Holly," said the Doctor. "I know I warned you about wandering off, so... how...?" he circled a finger at the general assembly. "This station is millions of years old. None of them should be alive."
The tree creaked something that could have been words, but it seemed wont to hold Holly like some kind of treasured doll.