The third of the ‘O’ Antiphons is based upon Isaiah 11:1 and Isaiah 11:10. Jesse was King David’s father the prophet Micah had prophesied that the Messiah would be of the house of David and would be born in David’s city, Bethlehem (Micah 5:1) . Christ is the "shoot of Jesse" by his descent from Jesse’s son, King David. For the early and medieval Church Christ’s heredity was all-important. The Tree of Jesse was a means of illustrating the actual royal descent of Jesus, especially for royalty and the nobility, including those who had joined the clergy:
1 Et egredietur virga de radice Jesse, et flos de radice ejus ascendet
1 And there shall come forth a rod out of the root of Jesse, and a flower shall rise up out of his root. (Isaiah 11:1)
It’s sung below by Peter Morton (tenor), and St John’s College Choir Cambridge conducted by David Hill. Enjoy :-).