Trithuria

Lore & Background

Trithuria was described by Joseph Dalton Hooker in 1858 with the type species Trithuria submersa. Until DNA sequence data and a reinterpretation of morphology proved otherwise, these plants were believed to be monocots related to the grasses (Poaceae). The genus was initially placed in Centrolepidaceae (now synonymous with Restionaceae) but was separated as its own family Hydatellaceae by Ulrich Hamann in 1976, though its affinity remained obscure until 2007 when it was identified as a basal angiosperm lineage. The family is now placed in the order Nymphaeales, though an alternative order Hydatellales exists but is not widely accepted.

Trithuria exhibits a remarkable similarity to Centrolepis, and species of both genera were mistaken for members of the other. The plants are submerged and emergent aquatics, rooted in the substrate, just a few cm tall. Most species are ephemeral aquatics that flower in vernal pools when water draws down, but several are submerged perennials in shallow lakes. Individual species are cosexual or dioecious, and are either wind-pollinated or self-pollinating; two predominantly apomictic species are also known. Hydatellaceae are unusual among angiosperms in allocating nutrients to the embryo before fertilization, a trait similar to gymnosperms.

Reader's Guide

Trithuria holds significant evolutionary importance as the closest living relative of the water-lily families Nymphaeaceae and Cabombaceae, together forming the order Nymphaeales, which is the sister group of all flowering plants except Amborellales. Its divergence from the rest of Nymphaeales occurred soon after that order split from its sister taxon, though the crown clade evolved relatively recently in the early Miocene (~19 Ma). The genus is also notable for its unique reproductive morphology: stamens are centered and surrounded by pistils, a condition otherwise known only in two members of Triuridaceae. The 'flowers' may instead represent condensed inflorescences or non-flowers. Additionally, Hydatellaceae are unusual among angiosperms in provisioning nutrients to the embryo before fertilization, a trait otherwise seen in gymnosperms. The family was long misclassified as monocots related to grasses until molecular and morphological evidence in 2007 revealed its true basal angiosperm placement. With 13 described species (likely an underestimate), Trithuria is distributed across India, Australia, and New Zealand, with most species in Australia. Conservation status varies: Trithuria lanterna is Least Concern, while Trithuria inconspicua is critically endangered.

Did You Know?

• Trithuria is one of only two plant groups (besides two species of Triuridaceae) in which the stamens are centered and surrounded by the pistils.
• Hydatellaceae allocate nutrients to the embryo before fertilization, a trait otherwise found only in gymnosperms among seed plants.
• Until 2007, Trithuria was thought to be a monocot related to grasses; DNA data revealed it is a basal angiosperm lineage.
• The generic name Trithuria comes from Greek words meaning 'three' and 'window', referencing the dehiscence of the capsule fruit.