Modern botanicals also list Artemisia cina or Levant wormwood as the definitive source; this is also backed up by statements in formularies published in the heyday of santonin use such as King's American Dispensatory.
1. de Kraker, J.W.; Franssen, M.C.; Dalm M.C.; de Groot, A.; Bouwmeester, H.J.; “Biosynthesis of germacrene A carboxylic acid in chicory roots. Demonstration of a cytochrome P450 (+)-germacrene a hydroxylase and NADP+-dependent sesquiterpenoid dehydrogenase(s) involved in sesquiterpene lactone biosynthesis”. (2001). Plant Physiol. 125(4): 1930-40.
Barton, D.H.R.; Moss, G.P.; Whittle, J.A.; “Investigations on the Biosynthesis of Steroids and Terpenoids. Part I. A Preliminary Study of the Biosynthesis of Santonin”. (1968). J. Chem. Soc. 338(8): 1813-18.