poles connected by a sheet of protoxylem tracheids mixed, apparently, with parenchyma. Upon the separation of a trace, the outer pole is reconstituted within a longitudinal distance of 2 or 3 mm, and the protoxylem poles again begin their radial separation which will culminate in the divergence of another leaf trace at a higher level. References External links Introduction to the Progymnosperms Davis, Paul and Kenrick, Paul; Fossil Plants. Smithsonian Books (in association with the Natural History Museum of London), Washington, D.C. (2004). Late Devonian plants Late Devonian first appearances Mississippian extinctions Prehistoric plants