| Ceratophyllum | |
|---|---|
| Ceratophyllum submersum | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| Order: | Ceratophyllales Link |
| Family: | Ceratophyllaceae Gray |
| Genus: | Ceratophyllum L. |
| Species | |
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See text |
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Ceratophyllum is a cosmopolitan genus of flowering plants, commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions. They are usually called hornworts, although this name is also used for unrelated plants of the division Anthocerotophyta.
Ceratophyllum grows completely submerged, usually, though not always, floating on the surface, and does not tolerate drought. The plant stems can reach 1–3 m in length. At intervals along nodes of the stem they produce rings of bright green leaves, which are narrow and often much-branched. The forked leaves are brittle and stiff to the touch in some species, softer in others. The plants have no roots at all, but sometimes they develop modified leaves with a rootlike appearance, which anchor the plant to the bottom. The flowers are small and inconspicuous, with the male and female flowers on the same plant. In ponds it forms thick buds (turions) in the autumn that sink to the bottom which give the impression that it has been killed by the frost but come spring these will grow back into the long stems slowly filling up the pond.[1][2][3][4]
Hornwort plants float in great numbers just under the surface. They offer excellent protection to fish-spawn, but also to snails, infected with bilharzia. Because of their appearance and their high oxygen production, they are often used in freshwater aquaria.
Ceratophyllum is considered unique enough to warrant its own family, Ceratophyllaceae, and its precise relationship to other angiosperms remains unclear. It was considered a relative of Nymphaeaceae and included in Nymphaeales in the Cronquist system but recent research has shown that it is not closely related to Nymphaeaceae or any other extant plant family. Some early molecular phylogenies suggested it was the sister group to all other angiosperms, but more recent ones have suggested that it is the sister group to either the monocots or the eudicots. The APG II system places the family in its own order, the Ceratophyllales.[5]
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| The phylogeny of the flowering plants, as of APG III (2009). |
The division of the genus into species is not completely settled. More than 30 species have been described, but many are probably just variants of these more widely accepted species:[1][2][6][7][8]
Of these, Ceratophyllum demersum is widespread, with a global distribution; the others all have more restricted ranges.