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(Polypodiales) The largest order of the Filicinae containing about 9000 species. Its members differ from those of the Marattiales and Ophioglossales in the development of the sporangium, which originates from one initial cell, while in the other orders it develops from a group of initials. Although secondary vascular tissue is not clearly developed in any of the Filicinae some members of the Filicales, the tree ferns, attain heights of 10 m. In these species bands of schlerenchyma strengthen the stem. The lamina of the fern frond usually shows differentiation into palisade and spongy mesophyll but in the filmy ferns the leaf is only one cell thick. The larger homosporous families of the Filicales include (according to one of many classifications): the Osmundaceae (royals ferns) with some 20 species in 3 genera; the Gleicheniaceae (staghorn ferns) with some 150 species in 5 genera; the Polypodiaceae with about 1050 species in some 55 genera; the Schizaeaceae, including the climbing ferns, with about 160 species in 6 genera; the Adiantaceae, including the maidenhair ferns, with about 850 species in some 20 genera; the Cyatheaceae, including the brackens, filmy ferns, and tree ferns, with about 1500 species; and the Aspleniaceae, including the spleen-worts and lady ferns, with about 1900 species in some 75 genera. The heterosporous families all contain aquatic ferns and are often classified into separate orders, the Marsileales, comprising the Marsileaceae (about 70 species in 3 genera), and the Salviniales, comprising the Salviniaceae (10 species in the genus Salvinia) and the Azollaceae (5 species in the genus Azolla).
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