Polypodiales

The order Polypodiales encompasses the major lineages of polypod ferns, which comprise more than 80% of today's fern species. They are found in many parts of the world including tropical, semitropical and temperate areas. Polypodiales are unique in bearing sporangia with a vertical annulus interrupted by the stalk and stomium. These sporangial characters were used by Johann Jakob Bernhardi to define a group of ferns he called the "Cathetogyratae"; the Pteridophyte Phylogeny Group has suggested reviving this name as the informal term cathetogyrates, to replace the ambiguously circumscribed term "polypods" when referring to the Polypodiales. The sporangia are born on stalks 1–3 cells thick and are often long-stalked. (In contrast, the Hymenophyllales have a stalk composed of four rows of cells.) The sporangia do not reach maturity simultaneously. Many groups in the order lack indusia, but when present, they are attached either along the edge of the indusium or in its center. Both Polypodiales and Cyatheales differ from other ferns in having a photoreceptor called a neochrome, which allows them to perform photosynthesis better in low-light conditions, such as in the shadows on the forest floor. The common ancestor of the two groups appears to have derived the neochrome via horizontal gene transfer from a hornwort. Their gametophytes are green, usually heart-shaped, and grow at the surface (rather than underground, as in Ophioglossales). The order Polypodiales was first described by Link in 1833. The circumscription of the order has changed over time as ferns have been classified in many different ways (see the review by Christenhusz and Chase, 2014). Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era. They referred to the ferns (now including horsetails) as monilophytes, dividing them into four groups, with the vast majority of species being placed in a taxon they called "Polypodiopsida". The four-fold grouping has persisted through subsequent systems, despite changes in nomenclature.