Key points are not available for this paper at this time.
Bruce G. Baldwin, Michael J. Sanderson, J. Mark Porter, Martin F. Wojciechowski, Christopher S. Campbell, Michael J. Donoghue, The its Region of Nuclear Ribosomal DNA: A Valuable Source of Evidence on Angiosperm Phylogeny, Annals of the Missouri Botanical Garden, Vol. 82, No. 2 (1995), pp. 247-277
Baldwin et al. (Sun,) studied this question.