Ferns

Lastly, the evolutionary status of two additional genera of modern plants must be considered: Psilotum and Tmesipteris. These free-sporing tropical and subtropical plants have very simple morphologies. In particular, species in the genus Psilotum superficially resemble plants of the Rhyniopsida in that their sporophytes consist of three-dimensional branched stems, with tiny scale-like appendages believed to be leaf derivatives. Moreover, like the Rhyniopsida, Psilotum does not have true roots. Thus, some botanists have suggested that Psilotum is a direct descendant of the Rhyniopsida. Others reject this hypothesis and point to the lack of a fossil record connecting these two groups. They suggest that Psilotum and Tmesipteris may have evolved by evolutionary simplification of an ancestor of the modern fern genus, Stromatopteris. Clearly, further research is needed to resolve the relationships of these fascinating, fern-like plants.