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James W. Hardin, George Doerksen, Dale Herndon, Melinda Hobson, Freeman Thomas, Pollination Ecology and Floral Biology of Four Weedy Genera in Southern Oklahoma, The Southwestern Naturalist, Vol. 16, No. 3/4 (Feb. 18, 1972), pp. 403-412
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Hardin et al. (1972) studied this question.
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