"Always, always," it sighed, "faithful beyond any man's deserving. I will keep the color of your eyes when no other in the world remembers your name. There is no immortality but a tree's love.
The Last Unicorn by Peter S. Beagle

A wizard tries to free himself with magic after being tied to a tree. Unfortunately he is not very good at spells, so instead of loosing the ropes, the tree comes alive and falls madly in love with the wizard.