|Pteridophyta, the Ferns|
Fern, or pteridophyte, is any one of a group of some twenty thousand species of plants classified in the Division Pteridophyta, formerly known by some as Filicophyta. A fern is defined as a vascular plant that lacks seeds, and that reproduces by shedding spores to initiate an alternation of generations. New fronds arise by circinate vernation (leaf formation by unrolling).
Fern life cycle
The life cycle of a typical fern consists of two distinct stages or generation phases (see alternation of generations), proceeding as follows:
- A sporophyte phase that produces spores by meiosis
- A spore grows by cell division into a haploid prothallus (a gametophyte phase)
- Prothallus produces gametes
- Male gamete fertilizes a female gamete
- The fertilized gamete (zygote) grows by cell division into a diploid sporophyte (the "fern")
A sporophytic fern consists of:
- Rhizome: Creeping stem, sometimes underground, absorbs nutrients, anchors plant
- Frond ("fern leaf"): green, photosynthesizes
- Spores develop on surface (usually underside)
- Petiole: stem-like part of leaf
- When young, it is curled into a fiddlehead
A gametophytic fern contains:
Evolution and Classification
Ferns first appear in the fossil record in the early Carboniferous epoch. By the Triassic the first evidence of ferns related to several modern families appears. The great fern radiation occurred in the late Cretaceous, where many modern families of ferns first appear.
Ferns have traditionally been grouped in the class Filices, but some modern classifications assign them their own division in the plant kingdom, which may be known as Pteridophyta.
Two related groups of plants, commonly known as ferns, are more distantly related to the main group of ferns: the whisk ferns (Psilophyta) and the adders-tongues, moonworts and grape-ferns (Ophioglossophyta). The Ophioglossophytes were formerly considered "true" ferns, and were grouped in the family Ophioglossaceae, but were subsequently found to be more distantly related. Some classification systems include the Psilopytes and Ophioglossophytes in division Pteridophyta, while others assign them separate divisions. Modern phylogeny indicates that the Ophioglossophytes, Psilopytes, and true ferns together constitute a monophyletic group, descended from a common ancestor.
The "true" ferns may be subdivided into four main groups, or classes (or orders if the ferns are considered as a class):
The last group includes most plants familiarly known as ferns. The Marattiopsida are a primitive group of tropical ferns with a large, fleshy rhizome, and are now thought to be a sister taxon to the main group of ferns, the leptosporangiate ferns, which include the other three groups listed above. Modern research indicates that the Osmundopsida diverged first from the common ancestor of the leptosporangiate ferns, followed by the Gleichenopsida.
A more complete classification scheme follows:
- Division: Pteridophyta
- Class: Marattiopsida
- Class: Osmundopsida
- Order: Osmundales (the flowering ferns)
- Class: Gleicheniopsida
- Class: Pteridopsida
- Subclass: Schizaeatae
- [heterosporous ferns]
- Subclass: Cyatheatae
- Subclass: Pteriditae
- Subclass: Polypoditae
- Order: Aspleniales (the spleenworts)
- Order: Athyriales (including the lady ferns, ostrich fern, maiden ferns, etc.)
- Order: Dryopteridales (the wood ferns and sword ferns)
- Order: Davalliales (including the rabbits-foot ferns and Boston ferns)
- Order: Polypodiales (including the rock-cap ferns or Polypodies)
Ferns are not as economically important as, say, cereal grains, with one possible exception. Ferns of the genus Azolla, which are very small, floating plants which do not look like ferns, and are called mosquito fern, are used as a biological fertilizer in the rice paddies of southeast Asia.
Other ferns with economic significance include:
- Dryopteris filix-mas -- male fern, used as a vermifuge
- Rumohra adiantoides -- floral fern, extensively used in the florist trade
- Osmunda regalis and Osmunda cinnamomea -- royal fern and cinnamon fern, respectively, the root fiber being used horticulturally; the fiddleheads of O. cinnamomea are also used as a cooked green
- Matteuccia struthiopteris -- ostrich fern, the fiddleheads used as a cooked green in North America
- Pteridium aquilinum -- bracken, the fiddleheads used as a cooked green in Japan
- Diplazium esculentum -- vegetable fern, a source of food for some native societies
- Tree ferns, used as building material in some tropical locales
In addition, a great many ferns are grown horticulturally. The leatherleaf fern or fancy fern, Rumohra adiantoides, is an especial favorite among florists who use its cut fronds in flower arrangements.
Several non-fern plants are called "ferns" and are sometimes popularly believed to be ferns in error. These include:
- "Asparagus fern" - This may apply to one of several species of the monocot genus Asparagus, which are flowering plants. A better name would be "fern asparagus".
- "Sweetfern" - This is a shrub of the genus Comptonia.
- "Air fern" - This is an unrelated aquatic plant that is harvested, dried, and dyed green, then sold as "living on air".
In addition, the book Where the Red Fern Grows has elicited many questions about the mythical "red fern" named in the book. There is no such known plant, although there has been speculation that the Oblique grape-fern, Sceptridium dissectum, could be referred to here, because it is known to appear on disturbed sites and its fronds may redden over the winter.
- Tree of Life Web Project: Filicopsida
- A classification of the ferns and their allies
- A fern book bibliography