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Sieve Cells in Phloem of a Middle Devonian Progymnosperm

Phloem tissue from a Middle Devonian member of the Aneurophytales (Progymnospermopsida) is described. This may be the oldest firm evidence of conducting elements of the phloem, extending our knowledge of this tissue back some 35 million years. The discovery indicates a close phylogenetic relation between progymnosperms and gymnosperms and provides a basis for investigating patterns of specialization in the phloem of these groups of plants.