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Family Hydrocharitaceae

Synonyms: Halophilaceae APNI*
Najadaceae APNI*

Description: Freshwater or marine, annual or perennial, dioecious or monoecious aquatics with submerged, floating, or emergent leaves.

Leaves all basal or cauline, alternate, opposite, or whorled.

Inflorescences 1–many-flowered, submerged, floating, or emergent, spatheate, axillary. Flowers 2- or 3-merous, actinomorphic, bisexual or unisexual and if unisexual then the males usually in many-flowered inflorescences and the female flowers solitary. Perianth segments and stamens usually borne at the summit of a long hypanthium above the inferior ovary, the length of the hypanthium is related to the water depth. Perianth segments in 1 or 2 whorls, if 2 whorls then the inner usually petaloid, each whorl of 2 or 3 segments. Stamens 1–many, staminodes sometimes present. Ovary inferior; carpels fused, 2–15.

Fruit dry or fleshy, indehiscent or irregularly dehiscent; seeds numerous, variously ornamented.

Distribution and occurrence: World: 16 genera, c. 140 species, cosmopolitan. Australia: 13 genera, c. 30 species, all States.

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

Text by S. W. L. Jacobs (1993); edited KL Wilson (June 2010)
Taxon concept:

Taxa not yet included in identification key
Limnobium,    Najas

 Key to the genera 
1Leaves floating or emergent2
Leaves all submerged3
2Leaves floating; stolons absent; chasmogamous flowers more than 4 cm diam.; inner perianth segments white, maroon at the baseOttelia
Leaves emergent and/or floating; stolons present; flowers less than 4 cm diam.; inner perianth segments white
                       Back to 1
3Leaves ovate; plants marine or estuarineHalophila
Leaves linear; plants of freshwater
                       Back to 1
4Leaves all basalVallisneria
Leaves cauline
                       Back to 3
5Leaves alternate or scattered, laminas often recurved; all perianth segments about the same sizeLagarosiphon
Leaves whorled, laminas rarely recurved; inner perianth segments larger or about the same size as the outer segments
                       Back to 4
6Inner perianth segments much larger than outer segments: leaves mostly in whorls of 4 or 5Egeria
All perianth segments about the same size; leaves in whorls of 3–8
                       Back to 5
7Stamens 9; leaves usually in whorls of 3Elodea
Stamens 3; leaves usually in whorls of more than 3
                       Back to 6

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