Canti VII-XIII

We see asquint, like those whose twisted sight
can make out only the far-off" he said
"for the King of All still grants us that much light."

These are the words of Farinata Degli Uberti, an enemy of Dante's who explains to him why it is the damned can foresee the future but don't have any knowledge of what is happening in the present in the living world. This is further punishment for them because it means that when Judgement Day arrives, and there is no more "future" their knowledge will be useless.
Farinata is one of the Heretics who denied God's existence and did not believe in immortality. the Heretics are similar to the Pagans in Limbo because they both are blind to God's light. It is ironic therefore that they have this ability to see the future. The reason that the Pagans are in Limbo and the Heretics are in "lower Hell" is that the Pagans were never given a chance to believe because they came before Christ. The Heretics and Farinata however lived when Christianity had already spread over much of the world and chose to not believe in God even though they were taught and warned against the dangers of doing so. As Dante sympathized with the souls of the great poets in Limbo, so he sympathizes with Farinata who, though he was an enemy of Florence redeemed himself in Dante's eyes by defending Florence against being destroyed at the Diet of Empoli.
"Briefly is the answer given: when
Out of the flesh from which it tore itself,
The violent spirit comes to punishment,
Minos assigns it to the seventh shelf.
It falls into the wood, and landing there,
Wherever fortune flings it, it strikes root,
And there it sprouts lusty as any tare,
Shoots up a sapling, and becomes a tree.
The Harpies, feeding on its leaves then, give it
Pain and pain's outlet simultaneously.
Canto XIII (100)
This is the punishment for those souls which committed violence against themselves in the form of suicide and self destruction. I found the end of this quote to be most interesting because they are punished by being encased in trees, and can only express themselves when they bleed as they Harpies wound them. As all the other souls are tormented in a way that is similar to their sins, the souls violent against themselves are punished by being repeatedly wounded. Because in life they felt their only means of expression was to destroy themselves, this belief is made a reality in Hell.