Hydatellaceae

Hydatellaceae is the botanical name for a family of flowering plants containing the genus Trithuria, which has been recently re-classified to include the genus Hydatella^[1]. The family consists of about a dozen species. These tiny (few cm tall), relatively simple, aquatic plants are found only in Australasia and India. The simple leaves are concentrated around a short stem basally. The plants are submerged and emergent aquatic annuals, rooted in the substrate below the water.

The members of this plant family are monoecious or dioecious and are likely wind-pollinated (anemophilous), water-pollinated (hydrophilous) or self-pollinating (autogamous). Flower-like reproductive units (which may be pseudanthia) are composed of small collections of minute stamen- and/or pistil-like structures that may each represent very reduced individual flowers. The non-fleshy fruits are follicles or achenes.^[2]

Hydatellaceae was for many years assumed to be close relatives of the grasses and sedges and were sometimes included in the family Centrolepidaceae. Even as recently as 2003, the APG II system assigned Hydatellaceae to the grass order Poales in the commelinid monocots. However, research based on DNA and morphology by Saarela et al. indicates that Hydatellaceae is the living sister group of the water lilies (Nymphaeaceae and Cabombaceae) and thus represents one of the most ancient lineages of flowering plants.^[3]

This realignment indicates that earlier classifications were misleading, a consequence of the apparently reduced vegetative and reproductive morphology of these plants. It also represents the first time in recent history that a plant family has been ejected from the monocots.^[4] The APG II system has not yet been formally updated to incorporate the recent findings on Hydatellaceae. However, the Angiosperm Phylogeny Website has now expanded the order Nymphaeales to include Hydatellaceae ^[5] a classification that is consistent with the findings of Saarela et al.^[3]

References

^ Sokoloff, Dmitry D., Margarita V. Remizowa, Terry D. Macfarlane, Paula J. Rudall. 2008. Classification of the early-divergent angiosperm family Hydatellaceae: one genus instead of two, four new species and sexual dimorphism in dioecious taxa. Taxon 57: 179-200.
^ T.D. Macfarlane, L. Watson and N.G. Marchant (Editors) (2000 onwards). Western Australian Genera and Families of Flowering Plants. Western Australian Herbarium. Version: August 2002. FloraBase: Hydatellaceae. Accessed 20 March 2007.
^ ^a ^b Saarela, Jeffery M., Hardeep S. Rai, James A. Doyle, Peter K. Endress, Sarah Mathews, Adam D. Marchant, Barbara G. Briggs & Sean W. Graham. 2007. Hydatellaceae identified as a new branch near the base of the angiosperm phylogenetic tree. Nature 446:312-315.
^ Graham, S. A New Understanding of the Early Evolution of Flowering Plants. University of British Columbia Botanical Garden press release, 14 March 2007.
^ Stevens, P. F. (2001 onwards). Nymphaeales. Angiosperm Phylogeny Website. Version 7, May 2006. Accessed 21 March 2007.

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