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L. E.  Urbatsch, Baldwin, B. G., and Donoghue, M. J., ?Phylogeny of the Coneflowers and Relatives (Heliantheae: Asteraceae) Based on Nuclear rDNA Internal Transcribed Spacer (ITS) Sequences and Chlorplast DNA Restriction Site Data?, Systematic Botany, vol. 25, pp. 539-565, 2000.