Fern

Ferns (Pteridophyta)
A tree fern unrolling a new frond
Scientific classification
Kingdom: Plantae Division: Pteridophyta
Classes
Marattiopsida Osmundopsida Gleicheniopsida Pteridopsida

A fern, or pteridophyte, is any one of a group of some twenty thousand species of plants classified in the Division Pteridophyta, formerly known as Filicophyta. A fern is defined as a vascular plant that does not produce seeds, but reproduces by spores to initiate an alternation of generations. New sporophyte fronds typically arise by circinate vernation (that is, "leaf" formation by unrolling).

Contents

1 Fern life cycle 2 Fern structure 3 Evolution and Classification 4 Economic uses 5 Misunderstood names

Fern life cycle

The life cycle of a typical fern consists of two distinct stages or phases (see alternation of generations for an explanation of the terminology), proceeding as follows:

1. A sporophyte (diploid) phase produces spores by meiosis
2. A spore grows by cell division into a haploid prothallus (a gametophyte phase)
3. Prothallus produces gametes
4. Male gamete fertilizes a female gamete in the prothallus
5. The fertilized gamete (diploid zygote) grows by cell division into a sporophyte (the "fern" plant)

Fern structure

A sporophytic fern consists of:

• Rhizome: Creeping stem, sometimes underground, absorbs nutrients, anchors plant
• Frond ("fern leaf"): green, photosynthesises
  • Spores develop on surface (usually underside)
• Petiole: stem-like part of leaf
  • When young, it is curled into a fiddlehead
• Fibrous root system

A gametophytic fern contains:

• Thallus: green, photosynthesises
  • contains gametes
• Rhizoids: root-like stem

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 appeared. The "great fern radiation" occurred in the late Cretaceous, when many modern families of ferns first appeared.

Ferns have traditionally been grouped in the Class Filices, but modern classifications assign them their own division in the plant kingdom, called Pteridophyta.

Two related groups of plants, commonly known as ferns, are actually more distantly related to the main group of "true" ferns. These are the whisk ferns (Psilophyta) and the adders-tongues, moonworts, and grape-ferns (Ophioglossophyta). The Ophioglossophytes were formerly considered true ferns and 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 to 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 true ferns are considered as a class):

• Marattiopsida
• Osmundopsida
• Gleicheniopsida
• Pteridopsida

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
    • Order: Marattiales
    • Order: Christenseniales
  • Class: Osmundopsida
    • Order: Osmundales (the flowering ferns)
  • Class: Gleicheniopsida
    • Subclass: Gleicheniatae
      • Order: Gleicheniales (the forked ferns)
      • Order: Dipteridales
      • Order: Matoniales
    • Subclass: Hymenophyllatae
      • Order: Hymenophyllales (the filmy ferns)
    • Subclass: Hymenophyllopsitae
      • Order: Hymenophyllopsidales
  • Class: Pteridopsida
    • Subclass: Schizaeatae
      • Order: Schizeales (including the climbing ferns)
    • [heterosporous ferns]
      • Order: Marsileales (Hydropteridales) (the water-clovers, mosquito fern, water-spangle)
    • Subclass: Cyatheatae
      • Order: Cyatheales (the tree ferns)
      • Order: Plagiogyriales
      • Order: Loxomales
    • Subclass: Pteriditae
      • Order: Lindseales
      • Order: Pteridales (including the brakes and maidenhair ferns)
      • Order: Dennstaedtiales (the cup ferns)
      • Order: Hypolepidales (including bracken)
    • 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)

Economic uses

Ferns are not of major economic importance, 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, taking advantage of their ability to fix nitrogen from the air into compounds that can then be used by other plants.

Other ferns with some economic significance include:

• Dryopteris filix-mas (male fern), used as a vermifuge
• Rumohra adiantoides (floral fern), extensively used in the florist trade
• Osmunda regalis (royal fern) and Osmunda cinnamomea (cinnamon fern), the root fiber being used horticulturally; the fiddleheads of O. cinnamomea are also used as a cooked vegetable
• Matteuccia struthiopteris (ostrich fern), the fiddleheads used as a cooked vegetable in North America
• Pteridium aquilinum (bracken), the fiddleheads used as a cooked vegetable 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 in horticulture.

Misunderstood names

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.