The order Salviniales (formerly known as the Hydropteridales and including the former Marsileales) is an order of

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...

s in the class

Polypodiopsida A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except th ...

Description

Salviniales are all aquatic and differ from all other ferns in being

heterosporous Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isos ...

, meaning that they produce two different types of

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

s (megaspores and microspores) that develop into two different types of

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

s (female and male gametophytes, respectively), and in that their gametophytes are endosporic, meaning that they never grow outside the spore wall and cannot become larger than the spores that produced them. The megasporangia each produce a single megaspore. In being heterosporus with endosporic gametophytes they are more similar to

seed plant A spermatophyte (; ), also known as phanerogam (taxon Phanerogamae) or phaenogam (taxon Phaenogamae), is any plant that produces seeds, hence the alternative name seed plant. Spermatophytes are a subset of the embryophytes or land plants. They inc ...

s than to other ferns. The fertile and sterile leaves are dimorphic, taking on a different shape, and leaves bear

anastomosing An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be normal ...

veins.

Aerenchyma Aerenchyma or aeriferous parenchyma or lacunae, is a modification of the parenchyma to form a spongy tissue that creates spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and t ...

is frequently present in roots, shoots, and petioles (leaf stalks). The ferns of this order vary radically in form and do not look particularly fern-like. Species of the family

Salviniaceae Salviniaceae (), is a family of heterosporous ferns in the order Salviniales. The Salviniaceae contain the two genera ''Azolla'' and ''Salvinia'', with about 20 known species in total. The oldest records of the family date to the Late Cretaceous ...

are natant (floating), while those of the family

Marsileaceae Marsileaceae () is a small family of heterosporous aquatic and semi-aquatic ferns, though at first sight they do not physically resemble other ferns. The group is commonly known as the "pepperwort family" or as the "water-clover family" because ...

are rooted. However, the natant species may temporarily grow on wet mud during times of low water, and the Marsileaceae may grow as emergent species, depending on species and location. The group has also the smallest known genomes of all ferns. One genus, ''

Azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like other typical ferns but more rese ...

'', is amongst the fastest growing plants on earth and caused a cooling of the climate in the

Azolla event The Azolla event is a scenario hypothesized to have occurred in the middle Eocene epoch, around , when blooms of the freshwater fern ''Azolla'' are thought to have happened in the Arctic Ocean. As they sank to the stagnant sea floor, over a per ...

about 50 million years ago. There is a well-known

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

member of the Marsileales, '' Hydropteris'' (''incertae sedis'').

Classification

In the molecular phylogenetic classification of Smith et al. (2006), the Salviniales were placed in the

leptosporangiate ferns The Polypodiidae, commonly called leptosporangiate ferns, formerly Leptosporangiatae, are one of four subclasses of ferns, and the largest of these, being the largest group of living ferns, including some 11,000 species worldwide. The group has ...

, class

. Two families,

and

, were recognized. The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009) which placed all land plants in Equisetopsida, reclassified Smith's Polypodiopsida as subclass

Polypodiidae Polypodiidae may refer to: * Polypodiidae (cnidarians), a zoological family that only includes ''Polypodium hydriforme'', a cnidarian parasite of fish eggs *Polypodiidae (plant), a botanical subclass of Equisetopsida sensu lato that includes the le ...

and placed the Salviniales there. The circumscription of the order and its families was not changed, and that circumscription and placement in Polypodiidae has subsequently been followed in the classifications of Christenhusz and Chase (2014) and PPG I (2016). The likely

phylogenic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relationships between the two families and five genera of the Salviniales are shown in the following diagram.

References

* C. Michael Hogan. 2010
''Fern''. Encyclopedia of Earth. National council for Science and the Environment
Washington, DC {{Taxonbar, from=Q7175204 Fern orders