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Janet C. Barber, Javier Francisco Ortega, Arnoldo Santos-Guerra, Aguedo Marrero, Robert K. Jansen, Evolution of Endemic Sideritis (Lamiaceae) in Macaronesia: Insights from a Chloroplast DNA Restriction Site Analysis, Systematic Botany, Vol. 25, No. 4 (Oct. - Dec., 2000), pp. 633-647
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Barber et al. (2000) studied this question.
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