(Polypodiopsida) A class of the *Pteropsida
containing about 10 000 species of chiefly tropical herbaceous plants, the ferns, most of which may be distinguished by their characteristic *fronds
. All ferns, except the heterosporous water ferns, are homosporous and normally bear the spores on the abaxial surface of the frond in distinct groups of sporangia called sori (see sorus
). The free-living gametophyte requires humid conditions to survive and is a vulnerable part of the life cycle. If often resembles a heart-shaped liverwort, but filamentous and subterranean gametophytes also occur. It is thought the Filicinae evolved from plants similar to Psilophyton (see Psilopsida
). The Filicinae contains two subclasses, the Leptosporangiatae, in which the sporangium develops from a single cell, and the Eusporangiatae, where the sporangium develops from a number of cells. The *Filicales
belong to the Leptosporangiatae and the *Ophioglossales
to the Eusporangiatae.
The Filicinae have a rich fossil record going back to the Devonian and made up a large part of the Carboniferous flora. Many forms became extinct during the Palaeozoic and two extinct orders are recognized, the Cladoxylales and the Coenopteridales. Both of these appear to have been predominantly homosporous.