'For thus Isaiah announced, in his eleventh chapter: “There shall come forth a rod out of the stem of Jesse, and a flower shall rise up out of his root, and the Holy Spirit shall rest upon him.” The stem and root is the generation of Jesse or David, in particular the Virgin Mary; the rod and flower is Christ. Now, just as unlikely, nay incredible, a thing it is that a fair branch and flower should spring from a dry and withered stem and root, just so unlikely was it that Mary the Virgin should become the mother of such a child. For, I take it, she is called a stem and root, not only because she became a mother in a supernatural manner and without violation of her virginity, even as it is above nature to make a branch grow out of a dead tree-stump, — but also for the following reason: Of yore, in the days of David and Solomon, the royal stem and line of David had been green and flourishing, fortunate in its great glory, might and riches, and famous in the eyes of the world. But in the latter days, when Christ was to come, the priests had usurped this honor and were the sole rulers, while the royal line of David had become so impoverished and despised it was like unto a dead stem, so that there was no hope nor likelihood that a king descended therefrom would ever attain to any great glory. But when all seemed most unlikely-comes Christ, and is born of the despised stem, of the poor and lowly maiden! The rod and flower springs from her whom Sir Annas’ or Caiaphas’ daughter would not have deigned to have for her humblest lady’s maid. Thus God’s work and His eyes are in the depths, but man’s only in the height.
Luther's Works, American Edition, vol. 21, 'The Magnificat' (Concordia, St Louis, 1956), pp299-300.