Psilotopsida is a class of fern-like plants. As circumscribed by Smith et al. (2006) it contains two families, Psilotaceae and Ophioglossaceae, placed in orders Psilotales and Ophioglossales, respectively. The affinities of these two groups have long been unclear and a close relationship between them has only recently been confirmed through molecular systematic studies. Psilotopsida is the sister-group to all other ferns (including Marattiaceae and Equisetaceae). Scientific classification or biological classification is a method by which biologists group and categorize species of organisms. ... Divisions Green algae Chlorophyta Charophyta Land plants (embryophytes) Non-vascular plants (bryophytes) Marchantiophyta—liverworts Anthocerotophyta—hornworts Bryophyta—mosses Vascular plants (tracheophytes) †Rhyniophyta—rhyniophytes †Zosterophyllophyta—zosterophylls Lycopodiophyta—clubmosses †Trimerophytophyta—trimerophytes Pteridophyta—ferns and horsetails Seed plants (spermatophytes) †Pteridospermatophyta—seed ferns Pinophyta—conifers Cycadophyta—cycads Ginkgophyta—ginkgo Gnetophyta—gnetae Magnoliophyta—flowering plants... Classes Marattiopsida Osmundopsida Gleicheniopsida Pteridopsida A fern, or pteridophyte, is any one of a group of some twenty thousand species of plants classified in the Division Pteridophyta, formerly known as Filicophyta. ... Families Psilotaceae Tmesipteridaceae Zosterophyllopsida† Psilotales (the whisk ferns) is an order (i. ... Families and Genera Family Ophioglossaceae Ophioglossum Cheiroglossa Family Botrychiaceae Botrychium Botrypus Sceptridium Family Helminthostachiaceae Helminthostachys zeylanica The Ophioglossophyta (lit. ... Classes Psilotopsida Equisetopsida Marattiopsida Pteridopsida (Polypodiopsida) this dnt make sense A fern is any one of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. ... Species Psilotum nudum (L.) Beauvois Psilotum complanatum Sw. ... Genera Botrychium Cheiroglossa Helminthostachys Mankyua Ophioglossum Ophioglossaceae , the Adders tongue family, is a family of ferns, currently thought to be most closely related to Psilotaceae, the two together comprising the class Psilotopsida as the sibling group to the rest of the ferns. ... Families Psilotaceae Tmesipteridaceae Zosterophyllopsida† Psilotales (the whisk ferns) is an order (i. ... Families and Genera Family Ophioglossaceae Ophioglossum Cheiroglossa Family Botrychiaceae Botrychium Botrypus Sceptridium Family Helminthostachiaceae Helminthostachys zeylanica The Ophioglossophyta (lit. ... Orders Marattiales Class Marattiopsida is a group of ferns containing a single order, Marattiales, and family, Marattiaceae. ... Species Subgenus Equisetum Equisetum arvense - Field or Common Horsetail Equisetum bogotense - Andean Horsetail Equisetum diffusum - Himalayan Horsetail Equisetum fluviatile - Water Horsetail Equisetum palustre - Marsh Horsetail Equisetum pratense - Shade Horsetail Equisetum sylvaticum - Wood Horsetail Equisetum telmateia - Great Horsetail Subgenus Hippochaete Equisetum giganteum - Giant Horsetail Equisetum myriochaetum - Mexican Giant Horsetail Equisetum hyemale...

References

• Pryer, Kathleen M., Eric Schuettpelz, Paul G. Wolf, Harald Schneider, Alan R. Smith and Raymond Cranfill. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91:1582-1598 (online abstract here).
• Smith, A. R., K. M. Pryer, E. Schuettpelz, P. Korall, H. Schneider & P. G. Wolf. 2006. A classification for extant ferns. Taxon 55(3):705–731.