In "Ada, or Ardor" this apparently meaningless reference carries us from the inicial discoveries, to the last lines about the "flattened couple" (from the attic onto Atticus paper)  The links are myriads of interconnections...Would they make explicit the first reference to the gingko biloba in Pale Fire? 

If there is a connection with Goethe's poem (indicated by A.Sklyarenko), then we'll have to cope with the pair of lovers who, in the end, were turned into one, ie, "Vaniada." and the various ink-blots and pressed "maidenhair" trees or the "capillus-veneris".

 

1. "The express does not stop at Torfyanka, does it, Trofim?’
‘I’ll take you five versts across the bog,’ said Trofim, ‘the nearest is Volosyanka.’
His vulgar Russian word for Maidenhair; a whistle stop; train probably crowded.
Maidenhair. Idiot! Percy boy might have been buried by now! Maidenhair. Thus named because of the huge spreading Chinese tree at the end of the platform. Once, vaguely, confused with the Venus’-hair fern. She walked to the end of the platform in Tolstoy’s novel. First exponent of the inner monologue, later exploited by the French and the Irish. N’est vert, n’est vert, n’est vert. L’arbre aux quarante écus d’or, at least in the fall. Never, never shall I hear again her ‘botanical’ voice fall at biloba, ‘sorry, my Latin is showing.’ Ginkgo, gingko, ink, inkog. Known also as Salisbury’s adiantofolia, Ada’s infolio, poor Salisburia: sunk; poor Stream of Consciousness, marée noire by now. Who wants Ardis Hall!"

2. "On the way there he acquired his second walking stick: the Ardis Hall silver-knobbed one he had left behind in the Maidenhair station café. This was a rude, stout article with a convenient grip ..." 

3. Letter from Percy de Prey to Van Veen: "In a couple of days I must leave for a spell of military service abroad. If you desire to see me before I go I shall be glad to entertain you...at dawn tomorrow where the Maidenhair road crosses Tourbière Lane. If not..."

 

4. Ada's infolio:

Ancolie Bleue des Alpes, Ex en Valais, i.IX.69. From Englishman in hotel. ‘Alpine Columbine, color of your eyes.’

Epervière auricule. 25.X.69, Ex, ex Dr Lapiner’s walled alpine garden.

Golden [ginkgo] leaf: fallen out of a book’ The Truth about Terra’ which Aqua gave me before going back to her Home. 14.XII.69.

.......

[blue-ink blot shaped accidentally like a flower, or improved felt-pen deletion] (Compliquaria compliquata var. aquamarina. Ex, 15.I.70.

5. This tiny one...by the Poet Laureate Robert Brown...is called "Peter and Margaret."... I shall give you this valuable book for keeps.’ ...Lucette...scanning the fascinating flyleaf, his name on it, his bold flourish, and his own wonderful drawings in ink — a black aster (evolved from a blot), a doric column (disguising a more ribald design)*, a delicate leafless tree (as seen from a classroom window), and several profiles of boys (Cheshcat, Zogdog, Fancytart, and Ada-like Van himself).

 

6. ...Violet’s supreme achievement...produced on special Atticus* paper in a special cursive type... with the master copy bound in purple calf for Van’s ninety-seventh birthday, had been immediately blotted out by a regular inferno of alterations...One can even surmise that if our time-racked, flat-lying couple ever intended to die they would die, as it were, into the finished book, into Eden or Hades, into the prose of the book or the poetry of its blurb./... ‘Quel livre, mon Dieu, mon Dieu,’ Dr [Professor. Ed.] Lagosse exclaimed, weighing the master copy which the flat pale parents of the future Babes, in the brown-leaf Woods, a little book in the Ardis Hall nursery, could no longer prop up in the mysterious first picture: two people in one bed.

 

*: doric: Greek Attica as in Keats' "Attic shape". Related to the infolio the two young Veens found in the attic.  The manuscript in "Atticus paper"

.................................................................................................................

 

From the google I retrieved two Brian Boyd entries on the theme::
Nabokov's Pale fire: the magic of artistic discovery
Brian Boyd - 2001 - Literary Criticism - Boyd, Nabokov's Ada, 69. 12. “Supporting consonant”: a repeated consonant preceding ... jacaranda, maidenhair, ginkgo, and five generic “trees”) ends with ...
books.google.com.br/books?isbn=0691089574...

Brian Boyd - Ada, the Bog and the Garden: or, Straw, Fluff, and ...
de B Boyd - 2004 - Artigos relacionados Maidenhair. Thus named because of the huge spreading Chinese tree at the end of the ..... "Moving Van: The Neverland Veens of Nabokov's Ada," Contemporary ...muse.jhu.edu/demo/nabokov_studies/.../8.1boyd.html