Commentary Purg XXII 130-135

Many early commentators believed that this tree is upside down, with its roots in the air and its tip in or on the ground.  It seems better to understand that its 'branches' bend downward (rather than reaching upward, as do those of earthly trees) and are longer the higher they are found on the trunk, so as to prevent anyone from climbing.  However that matter may be resolved (and the text would seem to support this second view, as Scartazzini [DDP Scartazzini.Purg.XXII.133-134] argues), it seems clear that this tree is portrayed as being a shoot from the Tree of Life (Genesis 2:9).  While a good deal of debate surrounds this point, strong arguments for this identification are found in Scartazzini (DDP Scartazzini.Purg.XXII.131).  And see C.Purg.XXIV.115-117.

      For the applicability here (even if the tree is compared to a fir, an abete) of the medieval conception of the palm tree as delineated by Gregory the Great (Moralia II.437-438), see Hill (Hill.1967.1) and a more extensive treatment of the same hypothesis by Pertile (Pert.1998.2), pp. 166-78.