In the Green Park, London, a blue police box hovered just above the ground.  In its state of semi-materialization nobody below could see or even feel it.  But the box’s two occupants could see outside.  They watched as a tree grew rapidly before their eyes, sprouting from a sapling into a giant, black poplar.

“See how tenaciously it lives?”  Said the Fourth Doctor.  “How nothing—not the coming of war or famine—can divert it from its purpose?”

“My people revered such trees as great warriors,” said Leela, “for on many battlefields, they were the last to remain standing.”

The Doctor laughed.