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Elso S. Barghoorn, Jr., The Ontogenetic Development and Phylogenetic Specialization of Rays in the Xylem of Dicotyledons-III. The Elimination of Rays, Bulletin of the Torrey Botanical Club, Vol. 68, No. 5 (May, 1941), pp. 317-325
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Elso S. Barghoorn (1941) studied this question.
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