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W. M. M. Eddie, T. Shulkina, J. Gaskin, R. C. Haberle, R. K. Jansen, Phylogeny of Campanulaceae S. Str. Inferred from Its Sequences of Nuclear Ribosomal DNA, Annals of the Missouri Botanical Garden, Vol. 90, No. 4 (Autumn, 2003), pp. 554-575
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Eddie et al. (2003) studied this question.
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