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Constance I. Millar, Steven H. Strauss, M. Thompson Conkle, Robert D. Westfall, Allozyme Differentiation and Biosystematics of the Californian Closed-Cone Pines (Pinus subsect. Oocarpae), Systematic Botany, Vol. 13, No. 3 (Jul. - Sep., 1988), pp. 351-370
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Millar et al. (1988) studied this question.
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