Systematic implications of the Psilotaceae

Abstract

To assess the taxonomic relationships between the Psilotaceae and the Filicales ( = Polypodiales), certain factors including character divergence, gaps in the record, categorical consistency, conservatism of characters, and use of factual vs. hypothetical data have been given consideration. Although it has been proposed that the aerial shoots of Psilotaceae are homologous to fronds, and the appendages to pinnae, and that the Psilotaceae should be classified as Filicales,Psilotum andTmesipteris show a number of profound differences from Filicales. These involve the structure of the aerial organs, vascular patterns, fertile appendages, and sporangia. In view of the taxonomic criteria used, and the nature of the character differences, it is concluded that the Psilotaceae do not warrant inclusion in Filicales. InsteadPsilotum andTmesipteris constitute a separate line of evolution, and should be treated as a class, the Psilopsida, of Tracheophyta, equivalent to Equisetopsida, Lycopodiopsida, and Polypodiopsida.