These ferns are characterized by root steles having 3–5 protoxylem poles and antheridia with 6–12 narrow, twisted or curved cells in their walls. Otherwise, their habitus is highly diverse, including plants with the typical fern fronds, others whose leaves resemble those of palm trees, and yet others again which have undivided leaves. They are tropical ferns, most diverse in Asia and the Pacific region.
Classification
In the molecular phylogenetic classification of Smith et al. in 2006, the Gleicheniales were placed in classPolypodiopsida. Three families, Dipteridaceae, Gleicheniaceae, and Matoniaceae were recognized. The linear sequence of Christenhusz et al., intended for compatibility with the classification of Chase and Reveal which placed all land plants in Equisetopsida, reclassifiedSmith's Polypodiopsida as subclass Polypodiidae and placed the Gleicheniales there. The circumscription of the order and its families was not changed, and that circumscription and placement in Polypodiidae has subsequently been followed in the classifications of Christenhusz and Chase and PPG I. The form taxonMicrophyllopteris is used for some Mesozoic Gleicheniales that cannot be reliably assigned to the present-day orders. The TriassicAntarctipteris and Gleichenipteris are sometimes ascribed to the Gleicheniaceae, but are probably better considered Gleicheniales incertae sedis. In historical treatments, the order has sometimes been treated as a subclass Gleicheniatae of the Pteridopsida, with the taxa treated as families here upranked to orders, so that a distinct subclass can be established for the leptosporangiate ferns. In other treatments, they were expanded to include the filmy ferns, as well as the similar-looking genusHymenophyllopsis. The resultant group was treated as class Gleichenopsida alongside the Pteridopsida, which would then be limited to the leptosporangiate ferns. However, this class is not monophyletic but rather a basalgrade, retaining ancient traits among the living ferns. Irrespective of their modern taxonomic treatment, the Gleicheniales were formerly included in the order Polypodiales. But the ferns in the loose sense are much too diverse a group to be shoehorned into one taxon at such a low rank.
