Ceratophyllum
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 Ceratophyllum submersum
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C. demersum |
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. The plant stems can reach over a meter in length in the aquarium. They do not tolerate drought. At intervals along nodes of the stem they produce rings of bright green leaves, which are narrow and often branched. The forked leaves feel brittle and stiff to the touch. 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 don't attract attention, with the male and female flowers on the same plant. Whilst growing best under good light it does not seem to cut out much light to plants below it, these seem to grow as normal. It will survive quite low light levels but growth is very slow or non existent. It is slower growing at lower temperatures and will form thicker leaves which give it the appearance of a different species. Either can be used as it will adapt to the temperature provided. 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. It grows continually in the tropical aquarium and if conditions are to its likening can form dense forests into which fry may escape, penetration being slow or denied to adult fish. Whilst light clumps will provide a refuge for harassed adults.
Fairly tough but long stems will snap if roughly handled. Propagation is by division and is easy as even the smallest piece of stem will eventually grow into a new plant. It excretes substances toxic to algae (allelopathic behaviour) and at good growth conditions it efficiently inhibits most algae growth.
Because of their appearance and their high oxygen production, they are often used in freshwater aquaria. Hornwort plants float in great numbers just under the surface. They offer excellent protection to fish-spawn, but also to snails, infected with bilharzia.
Hornwort, also known as coontail, is a very easy to grow plant that is tolerant of most water conditions and temperatures. It makes an excellent, tough to kill beginner's plant.
[edit] Relationships and classification
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.
The division of the genus into species is not completely settled. There are two main species:
- Ceratophyllum demersum - Common Hornwort or Rigid Hornwort
- Ceratophyllum submersum - Tropical Hornwort
More than 30 other species have been described, but many of them are probably mere variants of these.
