Healing leaves

In Eden, the tree of life confers immortality (Gen 2:9; 3:22-24). The New Jerusalem picks up on the tree of life motif, but there it has a different purpose. In the New Jerusalem the leaves have medicinal properties (Rev 22:2). And that goes back to Ezekiel's new Edenic vision (Ezk 47:12).

The reason for the difference is presumably twofold:

i) In Revelation, eternal life is conferred by the atonement of Christ. 

ii) Eden is an unfallen world, so the tree's function is to extend nature. By contrast, the New Jerusalem is a world fallen and redeemed, so the tree's function is restorative. Not to prolong the status quo permanently, but to reverse it and cure it.