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Mihai Costea, Darleen A. DeMason, Stem Morphology and Anatomy in Amaranthus L. (Amaranthaceae), Taxonomic Significance, The Journal of the Torrey Botanical Society, Vol. 128, No. 3 (Jul. - Sep., 2001), pp. 254-281
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Costea et al. (2001) studied this question.
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