What other parallels are there in the book of Mattie's unofficial asylum? What theme(s) is this motif of refuge used to express and develop?

The actual growth of the wisteria vine will be explained in the final chapter of the novel, and only then will it achieve its full symbolic significance. Yet even in this undeveloped metaphorical incarnation it proves a canny parallel to Turtle: just as the wisteria flourishes from its barren origin, Turtle now flourishes despite her harsh past. The later visit to Dr. Pelinowsky confirms this metaphor: like the wisteria vine, Turtle lay dormant during her period of deprivation, but this condition is now reversible.