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Cecile Lumer, Susan E. Yost, The Reproductive Biology of Vincetoxicum nigrum (L.) Moench (Asclepiadaceae), a Mediterranean Weed in New York State, Bulletin of the Torrey Botanical Club, Vol. 122, No. 1 (Jan. - Mar., 1995), pp. 15-23
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Lumer et al. (1995) studied this question.
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