Rigid Hornwort
The genus Ceratophyllum comprises four species (and several subspecies) of aquatic herbs widely distributed in freshwater habitats across the globe. Hornworts lack roots (although plants may be anchored by leafless branches) and float beneath the surface of still or slow-moving water bodies. In autumn the plants sink to protect themselves from low temperatures.
Individual plants, which can be up to three metres long, tend to branch extensively. One of their most distinctive features is the forked leaves, which give the plants their scientific and common names. These leaves, which are arranged in whorls around the stems, do not have cuticles or stomata.
Hornworts are monoecious, bearing inconspicuous male and female flowers in alternating axils. The stamens, which are not clearly differentiated into filament and anther, break off male flowers and float to the surface, showering the pollen (which has the same density as water) on to the female flowers. The fruits are a spiny achene with a persistent style, and are dispersed by both water currents and birds. Hornworts reproduce readily from pieces of stem.
Ceratophyllum demersum can form very substantial floating masses that can block waterways, outcompete other aquatic plants and cause problems for hydroelectric schemes. Positive ecological benefits can arise as the dense growth provides a sheltered habitat for young fish, insect larvae and other invertebrates. However, the habitats created by hornworts can be a concern in areas where mosquito larvae are carriers for malaria and snails for bilharzia. Chinese grass carp are used to control Ceratophyllum.
The popularity of Ceratophyllum as an aquarium plant has contributed to its global spread, and potential as an invasive species. Ceratophyllum demersum is one of the few living plants to have been taken into space; in January 1998 it was included in a community of aquatic organisms flown aboard the Space Shuttle Endeavour.
Ceratophyllum can be used to monitor heavy metal pollution as it accumulates traces of lead and cadmium. As a result, it has been used as a means of phytoremediation. Due to its free-floating habit it requires high levels of nutrients in the water, removing large quantities of nitrogen from its immediate environment. It has also been shown to have an allelopathic effect on phytoplankton and cyanobacteria.
Ceratophyllum is the only genus in the order Ceratophyllales. Once it was considered a specialized member of the Nymphaeales, subsequent molecular analyses show that it is the sister group to the eudicots.
Further reading
Judd WS 2016. Plant systematics: a phylogenetic approach. Sinauer.
Mabberley DJ 2008. Mabberley's Plant-Book. Cambridge University Press.
James Penny