Glossa Ordinaria

In which rod, no doubt the blessed Virgin Mary is predicted, who sprung from the stock of Jesse and David and fecundated by the Holy Spirit, brought forth a new flower of human flesh, becoming a virgin-mother. (Pope St. Leo the Great Sermon 4) Or it could be that a flower refers to Christ; and it is probably implied indirectly by "rod" that he has royal dignity as well, a rod being a symbol of kingship. (St. Cyril of Alexandria) Or it could be that Jesse is the root, David the tree that through its branch, that is, Mary, has produced fruit, that is Christ.