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
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 10,000 highly qualified scientists and 46 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information