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Synonyms: Zannichelliaceae APNI*

Description: Aquatic annuals or perennials, bisexual, dioecious or monoecious, often rhizomatous, sometimes salt-tolerant. Stems erect, floating or creeping, or plant submerged with slender capillary stems; stems often producing buds specialized for vegetative reproduction (turions).

Leaves alternate (often sub-opposite subtending inflorescences), opposite or (sometimes in Zannichellia) pseudowhorled, simple, submerged and/or floating (if both present then leaves usually dimorphic); base sometimes sheathing; margins entire or curled; ligule present or absent (ligule sometimes longer than sheath).

Inflorescence a (usually emergent) pedunculate spike with 2–many flowers, the peduncle sometimes becoming spiral or bent in fruit; or flowers solitary or clustered, axillary. Flowers bisexual or unisexual, often very small. Perianth with 4 segments (sometimes interpreted as a connective outgrowth), cup-like, reduced to a few small scales or absent. Stamens 1–4, often 2 or 4 and epipetalous, sometimes fused into a single anther mass or stamens 1 (Althenia, Zannichellia); anthers sessile or on filaments, generally dehiscing by longitudinal slits. Ovary superior, carpels 1–9, free.

Fruit usually aggregate, a drupelet (or drupe-like), a nutlet or achene, or (rarely) a berry.

Distribution and occurrence: World: 3 genera, 100 species, cosmopolitan. Australia: 2 genera, c. 12 species, all States.

External links:
Angiosperm Phylogeny Website (Family: Potamogetonaceae, Order: Alismatales)
Wikipedia

Ruppia is often placed in a monotypic family, Ruppiaceae. It is here not included in the Potamogetonaceae family description, pending curatorial review by herbarium staff. K.L. Gibbons, 6 Jun. 2023.