Stephen R. Downie, Esmeralda Llanas, Deborah S. Katz-Downie, Multiple Independent Losses of the rpoC1 Intron in Angiosperm Chloroplast DNA's, Systematic Botany, Vol. 21, No. 2 (Apr. - Jun., 1996), pp. 135-151
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Downie et al. (1996) studied this question.