Systematic Characters of Devonian Ferns

Several groups of fern-like plants occur in the Middle and Upper Devonian and arc probably evolved from the Trimerophytina of Banks.The branching systems of these plants arc predominantly three-dimensional and are deceptively similar.All the plants are characterized by mesarch development of their primary xylem, but certain histological details permit their separation into at least three major groups â€" Progymnospermopsida, Cladoxylopsida, and Coenopteridopsida.The first group is somewhat better known anatomically than the others and is the least fern-like, probably evolving towards the gymnosperms.The second class is anatomically distinct from most other plant groups.The Devonian plants placed in Coenopteridopsida, with the exception of Bhacophyton, are mostly problematic.Utilizing data from these plants, several criteria are suggested as having value in the determination of the relationships of Devonian fern-like plants.In order of increasing value they are: cortical and epidermal histology, form of the primary xylem, presence (or absence) of secondary wood, and the structure of the protoxylem strands.The application of these criteria suggests that some, but not all, Devonian plants placed in Cladoxylopsida and Coenopteridopsida find their closest relationship with Carboniferous coenopterid ferns.No Devonian plant seems to be more closely related to modern ferns.