Themes of Plant Biology
Seedless Vascular Plants
Two living genera, Psilotum and Tmesipteris, with only two species of the former and less than 30 of the latter, constitute the entire phylum. Both genera are weeds in the tropics and subtropics. Tmesipteris is confined to the islands of the South Pacific, including Australia and New Zealand whereas Psilotum is more widespread, reaching parts of the southern United States.
The sporophyte of Psilotum looks like a survivor from the Devonian age; it has no leaves nor roots, a protoxylem, a dichotomously branching green stem with small scales that bears bright yellow synangia (formed from three fused sporangia) on short lateral branches. It is homosporous and the spores develop into bisexual gametophytes that resemble pieces of sporophyte rhizome.
While many of the morphological and anatomical features fit, data from current RNA sequencing and other chemical analyses cause many botanists to reject the long held belief that Psilotum is a living ancestor of the seed plants. Instead, Psilotum appears to many botanists to be a descendent of the ferns by loss and simplification of structures. There are no known fossils of the Psilotophyta. So much for appearances.
