I came upon the following information in Maurice Hewlett's essay "The Crystal Vase," which first appeared in the December, 1919, issue of The London Mercury. The essay begins as a discussion of the merits of various writers of published letters, journals, and diaries. However, it soon turns into an appreciation of the journals of Dorothy Wordsworth -- William's sister and Coleridge's friend. Hewlett notes: "We may see deeply into ourselves, but she sees deeply into a deeper self than most of us can discern."
Coleridge was with them most days, or they with him [in the late winter of 1798]. Here is a curious point to note. Dorothy records:
"March 7th. -- William and I drank tea at Coleridge's. . . . Observed nothing particularly interesting. . . . One only leaf upon the top of a tree -- the sole remaining leaf -- danced round and round like a rag blown by the wind."
And Coleridge has in Christabel:
The one red leaf, the last of its clan,
That dances as often as dance it can,
Hanging so light, and hanging so high,
On the topmost twig that looks up at the sky.
Maurice Hewlett, A Green Shade: A Country Commentary (1920).
I have since discovered that her brother and Coleridge relied upon Dorothy Wordsworth's observations of nature on more than one occasion, putting them to use in their poetry. But it is best to go to the source and read her journals, which are indeed wonderful.
"Hampstead Heath, Looking Towards Harrow at Sunset" (1823)