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Evidence of polar auxin flow in 375 million-year-old fossil wood¹

²Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701 USA; ³Department of Biology, Faculty of Science and Science Education, University of Haifa–Oranim, Tivon 36006, Israel

In living woody seed plants (conifers and dicotyledons), when various obstacles such as buds and branches disrupt the axial polar auxin flow, auxin whirlpools are formed that induce the differentiation of circular tracheary elements in the secondary xylem. Identical circular patterns also occur at the same positions in the wood of the 375 million-year-old Upper Devonian fossil progymnosperm Archaeopteris. We propose that this is the earliest clear fossil evidence of polar auxin flow. Such spiral patterns do not occur in the primary xylem of the ca. 390–385 million-year-old Lower Devonian fossil land plants, fossil progymnosperms, Psilotum nudum, living ferns, and current seed plants that we examined. This discovery reveals an exciting potential for plant fossils to provide structural evidence of evolutionarily diagnostic physiological and developmental mechanisms and for the use of a combination of fossil evidence and developmental biology to characterize evolutionary patterns in terms of genetic changes in growth regulation.

Key Words: Archaeopteris • fossil wood • polar auxin flow • spiral xylem • vascular differentiation

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