Virgil and Dante come upon a dark forest filled with old, gnarled trees and devoid of any greenery. Here are the harpies, horrible part-woman part-bird monsters of Greek mythology. The harpies roost in the trees and release terrible shrieks. Virgil tells Dante that they are now in the second ring of the seventh circle of hell. Dante hears screaming all around him but does not see anyone. He looks around, confused.
Dante takes the harpies from Greek mythology, where they are also fearsome monsters. All of the screaming that Dante hears is evidence of the great suffering of souls around him that cannot speak to identify themselves.
Virgil tells Dante to pluck a small branch from a tree. When Dante does this, the tree cries out in pain, asking him "Why dost thou rend my bones?" (13.35) and bleeds. The tree explains that all the trees in the forest were once humans. Virgil apologizes to the tree but says that only by plucking a branch could Dante believe that the trees used to be people. Virgil asks the tree to tell Dante who it used to be.
In being transformed into a tree, this soul has lost its identity to some degree. Speaking to Dante, though, might offer it an opportunity to attain some kind of individual fame in Dante's narrative.
The tree says that it will speak because of Virgil's kind words and answers that it was the man who held the keys to Frederick II's heart and advised the emperor. Envy drove people against him, though, who convinced Frederick himself to distrust him. He then killed himself. Though he does not identify himself by name, he is Pier delle Vigne. Pier says that he was never unfaithful to Frederick and asks for Dante to heal his reputation on earth. Virgil encourages Dante to ask Pier more questions.
Virgil's powerful words persuade the otherwise reticent tree to speak. Pier begs Dante to ameliorate his bad reputation back on earth, because his reputation is the only part of him that survives outside of hell.
Dante says that he cannot think of anything more to ask Pier, because he is so stirred by pity. So, Virgil asks Pier to tell Dante how he became a tree, and whether any of the trees in this forest will ever be released from this form. According to Pier, when someone commits suicide, his or her soul is sent by Minos to the seventh circle of hell. There, it falls somewhere in the forest, sprouts "like a corn of wheat," (13.99) and grows into a tree. The harpies then feed on the trees' leaves, which causes the trees great pain.
Dante's excessive pity incapacitates and almost paralyzes him (just as it earlier made him faint around Francesca). The suicides' punishment is fitting in that, having disdained their bodies by enacting their own death, they are now separated from their bodies and transformed into something else (trees).
Pier continues to explain that on Judgment Day, the souls will be reunited with their former bodies, but the bodies will hang upon the trees, rather than being truly reintegrated with their souls. Suddenly, Dante hears a loud noise and turns to see two naked men sprinting through the forest, chased by hounds. One cries out for death to come, and the other teases the first (whom he calls Lano) before hiding in a bush. The hounds find the man in the bush and tear him limb from limb.
Judgment Day, the culmination of God's divine justice, perfects hell's punishments by reuniting bodies and souls. However, the suicides are fittingly doomed to be always separated from the bodies they so willingly threw away in killing themselves.
Virgil guides Dante to the bush, which is itself trying to speak. It cries out in pain (its leaves and branches have been torn in the commotion), asking Jacomo (the man who hid in the bush) why he tried to hide there. Virgil asks the bush who it once was. The bush asks Virgil and Dante to gather its scattered leaves and says that it was a Florentine who hanged himself.
Even in the bush's transformed and injured state, it still has a voice and is able to speak. However, it does not attain fame because it does not tell Dante its name. It is possible that the soul avoids naming itself because of shame at having killed itself.