17. Now that the leaves of the tree we speak of are not valueless but are a source of health to the nations is testified by St. John in the Apocalypse, where he says: And He shewed me a river of water of life, bright as crystal, proceeding out of the throne of God and of the Lamb; in the midst of the street of it and on either side of the river the tree of life, bearing twelve manner of fruits, yielding its fruit every month: and the leaves of the tree are for the healing the nations Apoc. xxii. 1..