Vestal, Paul A. A. G. Bush Science Center; Department of Biology, Rollins College, Winter Park, Florida.
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 (see illustration), 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 plants are homosporous and eusporangiate; that is, spore sacs develop from groups of epidermal cells. Each sporangium produces a large number of spores, as many as 15,000 in some species. 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 (illus. a) of the sporophyte (spore-producing generation). The leaves are erect or merely bent over in bud and not circinate. The gametophyte (gamete-producing generation) 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
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