Palantir of Fortune "Minor Works" edition

[Guinevere]

It's the description of Niggle's picture:

It had begun with a leaf caught in the wind, and it became a tree; and the tree grew, sending out innumerable branches, and thrusting out the most fantastic roots.

("innumerable" and "fantastic" were the clues )
I remember this because it reminds me of Tolkien's description in the foreword of how the LotR developped .