At the end of ADA, Part I, chapter 38 Van offers his translation of a quatrain"

Lights in the rooms were going out.

Breathed fragrantly the rozy.

We sat together in the shade

Of a wide-branched beryozy

Ada remarks that "birch" (beryorzy) is what leaves the translator in "the lurch" -- referring to Van's inability to find a good English translation for beryozy (birches) that rhymes with "roses." Something is funny since 'the" would have worked as well as "a." (Rozy / beryozy is a hack rhyme in Russian.)

What puzzles me is the indefinite article "a" in the last line. Beryozy is plural, not singlular.

Any ideas what's going on here? A lapse in proof reading?

Also, does any one know whether Konstantin Romanov wrote the quatrain in question?