Rellimia is one of the best known of the progymnosperms, although the anatomy of its wood and tree structure has yet to be fully described. The occurrence of Rellimia in Devonian strata makes it one of the earliest plants known to have developed wood. This study examines pyrite permineralizations from the Catskill region of New York and elucidates anatomical details of the secondary xylem using light and scanning electron microscopy. The wood is pycnoxylic with tall, narrow rays that are typically uniseriate, although biseriate and multiseriate rays can be present. Ray cells are slightly rectangular, and the rays are homocellular. All cells appear to have thin walls and appear to be ray parenchyma. Tracheids have crowded, circular-bordered pits on all walls, including the long, tapering end walls. Bordered pits are in alternate spirals across the face walls and are often hexagonal in outline. Fractures through the bordered pit pair reveal details of the wall and pyrite casts that fill the pit canal and pit chamber during fossilization. Bordered pit pairs have crossed pit apertures, with the pit canal being longer than the shallow pit chamber. Preserved pit membranes and secondary walls show microfibrillar arrangement. Anatomical characteristics of the known wood of progymnosperm taxa are compared and discussed in light of the group's position in lignophyte evolution.
- Secondary xylem