[The Squatter of Amon Rûdh]

As luck would have it I've found a very telling comment about Leaf by Niggle from the man himself:

Leaf by Niggle arose suddenly and almost complete. It was written down almost at a sitting, and very nearly in the form in which it appears. Looking at it myself now from a distance I should say that, in addition to my tree-love (it was originally called The Tree), it arose from my own pre-occupation with The Lord of the Rings, the knowledge that it would be finished in great detail or not at all, and the fear (near certainty) that it would be 'not at all'. The war had arisen to darken all horizons. But no such analyses are a complete explanation of a short story.

(from Letter #199 to Caroline Everett, 24 June 1957)

Of course the big eye-opener is the association of Niggle's picture with The Lord of the Rings rather than the Silmarillion, although it's equally applicable to the older work. It's easy to forget how long it took Tolkien to write his novel, and equally easy to imagine him despairing of ever finishing it.