Commentary Purg XXXII 51

The griffin now binds the temo (shaft), by means of which he had been drawing the chariot, to the tree.  Most discussants currently believe that this instrument represents Christ's cross.  The first commentators argued that, since Adam's sin had been disobedience, this scene showed the obedience to which Christ enjoined his Church, a reasonable enough understanding.  While embracing it, Benvenuto (DDP Benvenuto.Purg.XXXII.43-51) reports that some others believe that this ligature is symbolic of the Incarnation, while still others believe it is related to the cross.  This last interpretation eventually became dominant, and remains so today.

      It was only with Scartazzini (DDP Scartazzini.Purg.XXXII.51), however, that the most likely source for this tercet, the fourteenth chapter of the apocryphal Gospel of Nicodemus, which Scartazzini claims is also the source of the lengthy paraphrase found in the gloss to the passage by Francesco da Buti (DDP Buti.Purg.XXXII.43-51), came to light.  Any number of medieval legends were developed from this text (see Longfellow [DDP Longfellow.Purg.XXXII.51]).  In it, Adam's non-canonical son Seth visits the gates of the garden of Eden, now a wasteland, in search of some oil to ease Adam's aching head.  Seth is not allowed to enter by the guardian angel, but does see a very tall tree, denuded of its leaves.  The angel gives him a branch from the tree (it is the Tree of the Knowledge of Good and Evil) which he brings back to Adam, who has died before Seth returns to Limbo with the branch.  Planted, it soon supplied the wood that would serve for the crucifixion of Jesus.

      For a brief discussion of the 'Legend of the Wood of the Cross' as being applicable to Dante's phrasing here see Moore (Moor.1903.1), pp. 219-20.  For these legends see Mussafia (Muss.1869.1).  It is remarkable that the Gospel of Nicodemus has not made its way into other commentaries to this tercet, since it has been a staple of commentators since Torraca (DDP Torraca.Inf.IV.54) as a probable source for Dante's sense of the harrowing of hell, witnessed by Virgil, as it is described in [Inf IV 52-63].