O come, Thou Root of Jesse's tree,
An ensign of Thy people be;
Before Thee rulers silent fall;
All peoples on Thy mercy call.
The antiphon today is "O Radix Jesse" (O Root of Jesse), where we pray - with more than a little impatience - for our ruler and guide to come to us.
O Radix Jesse, qui stas in signum populorum, super quem continebunt reges os suum, quem Gentes deprecabuntur; veni ad liberandum nos, jam noli tardare.
O Root of Jesse, which standest for an ensign of the people,
at Whom the kings shall shut their mouths, Whom the Gentiles shall seek,
come to deliver us, do not tarry!
I cannot find a source for this medieval Tree of Jesse. I chose it over the usual "blossom from the branch from the root" depictions, because it looks like an ensign - the army's standard, the rallying point of the troops, to be defended at all costs - which is carried before the soldiers and leads them into battle.