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J. F. Smith, J. C. Wolfram, K. D. Brown, C. L. Carroll, D. S. Denton, Tribal Relationships in the Gesneriaceae: Evidence from DNA Sequences of the Chloroplast Gene ndhF, Annals of the Missouri Botanical Garden, Vol. 84, No. 1 (1997), pp. 50-66
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Smith et al. (1997) studied this question.
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