Trithuria is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of minute aquatic herbs in the family Hydatellaceae

Hydatellaceae

Hydatellaceae are small, aquatic flowering plants. The family includes the genus Trithuria, which has been recently re-defined to include the genus Hydatella. The family consists of about a dozen species. These tiny , relatively simple, aquatic plants occur in Australasia and India. The simple...

. Recently the genus Hydatella was subsumed within Trithuria.

These diminutive, moss-like, aquatic plants are the closest living relatives of water-lilies and their allies. They reside near the point at which water-lilies (order Nymphaeales

Nymphaeales

Nymphaeales is an order of plants, which consists of water lilies and other aquatic plants.This order is considered to be a basal, or early diverging, group of angiosperms...

) diverged from other flowering plants. Together, the Nymphaeales+Hydatellaceae are sister to all other extant flowering plants except Amborella trichopoda.

Morphology places these plants near the base of the angiosperm clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

. This position is supported by ten synapomorphies

Synapomorphy

In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

:

• lack of a vascular cambium,
• lack of pericyclic sclerenchyma,
• anomocytic stomata,
• truncate anther connective,
• boat-shaped pollen,
• inner integument with two cell layers,
• palisade exotesta,
• seed operculum formed by cell enlargement in the inner integument,
• perisperm and
• hypogeal germination.