Ceratophyllum
Template:Short description Template:Automatic taxobox
Ceratophyllum is a cosmopolitan genus of flowering plants commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions. It is the only extant genus in the family Ceratophyllaceae,<ref name=WCSP_36893>Kew World Checklist of Selected Plant FamiliesTemplate:Dead link</ref> itself the only extant family in the order Ceratophyllales.<ref name=APG4>Template:Cite journal</ref> They are usually called coontails or hornworts, although hornwort is also used for unrelated plants of the division Anthocerotophyta.
Description
Ceratophyllum grows completely submerged, usually, though not always, floating on the surface of the water. 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. Roots are completely absent and are missing even in the embryonic stage,<ref>Competition Science Vision nov 2003</ref> but sometimes they develop modified leaves with a rootlike appearance, which anchor the plant to the bottom. Stomata are not present on any species.<ref>Frontiers | Refined Interpretation of the Pistillate Flower in Ceratophyllum Sheds Fresh Light on Gynoecium Evolution in Angiosperms</ref> 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, which sink to the bottom and give the impression that the plant has been killed by the frost; but come spring, these will grow back into the long stems, slowly filling up the pond.<ref name=foc>Flora of China: Ceratophyllum</ref><ref name=fna>Flora of North America: Ceratophyllum</ref><ref name=blamey>Blamey, M. & Grey-Wilson, C. (1989). Flora of Britain and Northern Europe. Template:ISBN</ref><ref name=rhs>Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan Template:ISBN.</ref>
Taxonomy
Ceratophyllum was first described by Carl Linnaeus in 1753 with Ceratophyllum demersum as the type species.<ref name="Tropicos" /> Ceratophyllum is considered distinctive enough to warrant its own family, Ceratophyllaceae, but was considered a relative of Nymphaeaceae and included in Nymphaeales in the Cronquist system. 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 research suggests that it is the sister group to the eudicots. This notion was supported upon sequencing of the C. demersum genome in 2020.<ref>Template:Cite journal</ref> The APG IV system placed the family in its own order, the Ceratophyllales, and gives the following cladogram.<ref name=WCSP_36893/><ref name=APGIII2009>Template:Cite journal</ref><ref name=mobot>Angiosperm Phylogeny Web: Ceratophyllales</ref><ref name=APG4/>
Species
The subgeneric division of the genus Ceratophyllum into its appropriately recognized species, subspecies, and varieties is not settled. More than 30 species have been described and published. A narrow interpretation of this work rejects over 23 of these taxa as variants, accepting only 7 species. This narrow interpretation lumps to the point of failing to give these potential species the taxonomic importance of even being named on a subspecific or varietal level. The genus as narrowly defined in this manner contains the following seven species:<ref name=WCSP_36893/><ref name=foc/><ref name=fna/><ref name=grin>Germplasm Resources Information Network: Ceratophyllum</ref><ref name=fe>Flora Europaea: Ceratophyllum</ref><ref name = "POWO, 2023" />
- Ceratophyllum australe Griseb.
- Ceratophyllum demersum L. (rigid/common hornwort) – cosmopolitan
- Ceratophyllum echinatum A.Gray (spiny hornwort) – North America
- Ceratophyllum muricatum Cham. (prickly hornwort) – Near-cosmopolitan
- Ceratophyllum platyacanthum Cham. – Europe and Asia
- Ceratophyllum submersum L. (soft/tropical hornwort) – Europe, Middle-East, Central Asia, northern and central Africa, Florida, and Dominican Republic
- Ceratophyllum tanaiticum Sapjegin
Phylogeny
A 2018 phylogenetic analysis of the above species gives the below cladogram. It finds support for 4 species being monophyletic, while 3 others are paraphyletic. It found C. australe to be a divergent lineage of C. tanaiticum, rendering C. tanaiticum paraphyletic. Nonetheless, it still regards C. australe as a separate species due to significant morphological differences and its geographic isolation from the rest of C. tanaiticum. C. demersum and C. platyacanthum were recovered as paraphyletic with respect to each other, and as such they could possibly be considered one species, although their morphologies and ecologies are distinct.<ref>Template:Cite journal</ref>