Commentary Par XXVII 115-120

The Crystalline Sphere rules the temporal relationships among the parts of the rest of the universe.  Dante employs the word testo ('flowerpot'; in modern Italian 'baking dish'), a hapax when having this sense (but see [Inf XV 89] and [Purg VI 29] for its use with the meaning 'text') to portray the ninth (and invisible) sphere as the container of all time, with its invisible roots here, displaying its leaves, pushed downward, in the visible portions of the rest of the spheres (the stars and planets).  (The Crystalline Sphere's 'likeness' to a flowerpot would seem to be based on the fact that we cannot see the 'roots of time,' just as we cannot see the root system of a plant when it is in a pot.)

The author of the Codice Cassinese (DDP Cassinese.Par.XXVII.115) was perhaps the first to point to Dante's source here, Aristotle’s Physics (IV.x-xiii); Francesco Torraca (DDP Torraca.Par.XXVII.118-120) appears to have been the first commentator to cite Dante's citation of that passage in Convivio (Conv.IV.ii.6).