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Ceratophyllum
'''''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 leaf|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 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.[Flora of China: ''Ceratophyllum''][Flora of North America: ''Ceratophyllum''][Blamey, M. & Grey-Wilson, C. (1989). ''Flora of Britain and Northern Europe''. ISBN 0-340-40170-2][Huxley, A., ed. (1992). ''New RHS Dictionary of Gardening''. Macmillan ISBN 0-333-47494-5.]
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 aquarium|aquaria.
Relationships and classificationCeratophyllum 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.[Angiosperm Phylogeny Web: Ceratophyllales]
The current phylogeny is:
{{clade| style=font-size:75%;line-height:75%
|label1=Angiospermae
|1={{clade
|1=''Amborella''
|2=Nymphaeaceae
|3=Austrobaileyales
|4=
}}
}}
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:[Germplasm Resources Information Network: ''Ceratophyllum''][Australian Plant Name Index: ''Ceratophyllum''][Flora Europaea: ''Ceratophyllum''][
]
- ''Ceratophyllum demersum'' L. (Rigid Hornwort or Common Hornwort)
- ''Ceratophyllum echinatum'' A.Gray (Spineless Hornwort)
- ''Ceratophyllum muricatum'' Cham. (Prickly Hornwort)
- ''Ceratophyllum platyacanthum'' Cham.
- ''Ceratophyllum submersum'' L. (Soft Hornwort or Tropical Hornwort)
Of these, ''Ceratophyllum demersum'' is widespread, with a global distribution; the others all have more restricted ranges.
References
Image:Ceratophyllum demersum2.gif|''Ceratophyllum demersum''
Image:Ceratophyllum demersum3.jpg|''Ceratophyllum demersum''
Category:Angiosperms
Category:Aquatic plants
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