Reading the whole chapter you'll see that the tree here implied is the tree of knowing Good and Evil. Ἐν μέσῳ could mean "under it", or you can read the sentence in connection with 3.3 (τοῦ ξύλου, ὅ ἐστιν ἐν μέσῳ τοῦ παραδείσου), i.e. the sentence may mean not "in the midst of the trees of the garden", as the English translator has it, but "under the tree which is in the midst of the garden". In any case, I think that Brenton's translation misses the point here. Grammatically it is probable, since a noun in singular sometimes can refer to many beings of the same kind (e.g. we can say to paidion meaning all children). Yet here the whole chapter is about the particular tree, and is simultaneously ironical, (since the firstborn tried to hide from God in the very place of their sin) and prophetic, showing the need of exiting a paradise that had become, instead of the original habitation of man, the place of the origin of sin.