VN writes in LRL (p.283):
"Note two typical Chekhov moves: first, Sorin sings a few bars of a Schubert song, then checks himself and tells with a laugh the nasty thing somebody once said about his singing voice; second, then when Nina and Treplev are left alone they kiss and immediately after she asks, "What's that tree there?" The answer, an elm. "Why is the tree so dark?" she goes on. These trifles disclose better than anything invented before Chekhov: the wistful helplessness of human beings — the old man who made a mess of his life, the delicate girl who will never be happy."

In ADA (part One, ch.14) we read: "But she shook her head, she shook her bent head, while still twisting and twining her daisies.
‘Well,’ he said, getting up, ‘I must be going. Good-bye, everybody. Good-bye, Ada. I guess it’s your father under that oak, isn’t it?’
‘No, it’s an elm,’ said Ada.
Van looked across the lawn and said as if musing — perhaps with just a faint touch of boyish show-off:
‘I’d like to see that Two-Lice sheet too when Uncle is through with it..."

And here enters ADA On Line (and I only just now realized that the big chain around an oak may hide a deliberate wordplay "chain/chêne," not a coincidence, thus enchaining Pushkin, Chateaubriand, Chekhov, Shakespeare: wrong trees and wrong lineages):*

92.31-33: “I guess it’s your father under that oak, isn’t it? “No, it’s an elm”: “isn’t it?” corrected from 1969, “isn’t?”

Ada’s evasion of Greg’s helpless advances echoes Act I of Chekhov’s The Seagull (Chayka, 1896), where Treplyov manages to kiss Nina, only for her to change the subject immediately: “What kind of tree is that?” “It’s an elm.” (“Eto kakoe derevo?” “Vyaz.”) Greg is not the only one to mistake the tree: earlier, Ada, in a much more interested mood, notes for Van: “We can squirm from here into the front hall by a secret passage, but I think we are supposed to go and look at the grand chêne which is really an elm” (53-54).

Nabokov, who had once lived with his family at 6 Elm Park Gardens in London, installs Sebastian Knight at the London address of 36 Oak Park Gardens and plays with the common confusion (“a couple of elms, not oaks, in spite of the street-name’s promise,” RLSK 37). He would later recall: “Among fifty college students whom I once happened to ask (in planned illustration of the incredible ignorance concerning natural objects that characterizes young Americans of today) the name of a tree, an American elm, that they could see through the classroom window, none was able to identify it: some hesitantly suggested it might be an oak, others were silent” (EO 3.9).

[ ]The “oak”-“elm” confusion, so soon after Ada’s mistranslation of Shakespeare into French, recalls her mistranslation of Marvell into French, where “the palm, the oak or bays” of Marvell’s “The Garden” become lost in Ada’s version (65.06-13). MOTIF: Ada’s taxonomy; under tree; wrong tree.

And I cannot wait to read all the notes on the item wrong tree: 50.06; 53.34-54.01; 92.31-33; 522.15-16;

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* 398.28-29: “and the big chain around the trunk of the rare oak, Quercus ruslan Chat.”