Luke 13:19

Our Lord was this mustard-seed, when he was buried in the earth; and He became a tree, when he ascended into heaven; but a tree that overshadowed the whole creation, in the branches of which the birds of heaven rested; that is, the powers of heaven, and all such as by good works have raised themselves from the earth. The apostles are the branches, to repose in whose bosoms we take our flight, borne on the wings of Christian virtue. Let us sow this seed (Christ) in the garden of our hearts, that the grace of good works may flourish, and you may send forth the various perfumes of every virtue. (St. Ambrose)