20 "For the Church is one, although she embraces greater and greater multitudes in the course of her prolific growth. So the sun has many rays, but one light; a tree has many branches, but one trunk rooted firmly in the ground; and when many streams issue from a single source, though their number seems to come directly from the abundance of flowing water, still there is only one source. Shut out a ray of the sun: the unity of its light has not been sundered; tear a branch from a tree: that branch no longer puts forth shoots; block a stream from its source: that stream dries up.
Source: Meminisse Iuvat (Vatican.va)