| Noun | 1. | Psilopsida - whisk ferns; comprising the family Psilotaceae or Psilotatae: vascular plants with no roots, partial if any leaf differentiation, and rudimentary spore sacs class - (biology) a taxonomic group containing one or more orders division Pteridophyta, Pteridophyta - containing all the vascular plants that do not bear seeds: ferns, horsetails, club mosses, and whisk ferns; in some classifications considered a subdivision of Tracheophyta order Psilotales, Psilotales - lower vascular plants having dichotomously branched sporophyte divided into aerial shoot and rhizome and lacking true roots |