(botany) An order of ferns in the subclass Ophioglossidae.

An order of the class Polypodiopsida known as the adder's-tongue ferns. It is a small group with only 3 genera and about 80 species. Two genera, Ophioglossum and Botrychium, are widely distributed in tropical and temperate regions and have about the same number of species; the third genus, Helminthostachys, is represented by a single species confined to southeastern Asia and Polynesia. These are considered the most primitive of the present-day ferns. No fossils have been reported for this group.

The chromosome number is high in the species of Ophioglossum. Ophioglossum petiolatum, a tropical species, has a chromosome count of over 1000, the largest number observed in a naturally occurring species of vascular plants. This group is distinguished from other ferns by the arrangement of the sporogenous tissue in the characteristic fertile spike of the sporophyte. The leaves are erect or merely bent over in bud and not circinate. The gametophyte is a small, nongreen, fleshy, subterranean saprophyte, associated with an endophytic fungus. The group appears to be an evolutionary dead end. See also Fungi; Leaf; Polypodiales; Polypodiopsida; Pteropsida.