Toward the end of The Magician's Nephew, Aslan told Digory and Polly to bury all the magic rings, and they buried it in a circle around the place where Digory planted the core of the apple he brought from Narnia to cure his Mom.
Quote from the last chapter of the book:
"I've got them all," said Polly. "Look, it's all right, I'm wearing gloves. Let's bury them."
"Yes, let's. I've marked the place where I buried the core of the Apple yesterday."
Then Polly came over the wall and they went together to the place. But, as it turned out, Digory need not have marked the place. Something was already coming up. It was not growing so that you could see it grow as the new trees had done in Narnia; but it was already well above ground. They got a trowel and buried all the magic rings in a circle round it.
My question: While the last chapter explained how the Wardrobe came from the tree, did C.S. Lewis explain, either in the Narnia books, his letters, interviews, or his other writings, what became of the buried rings?