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)
Wikipedia
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 | |
| 1 | Leaves floating or emergent | 2 |
| Leaves all submerged | 3 |
| 2 | Leaves floating; stolons absent; chasmogamous flowers more than 4 cm diam.; inner perianth segments white, maroon at the base | Ottelia |
| Leaves emergent and/or floating; stolons present; flowers less than 4 cm diam.; inner perianth segments white
Back to 1 | Hydrocharis |
| 3 | Leaves ovate; plants marine or estuarine | Halophila |
| Leaves linear; plants of freshwater
Back to 1 | 4 |
| 4 | Leaves all basal | Vallisneria |
| Leaves cauline
Back to 3 | 5 |
| 5 | Leaves alternate or scattered, laminas often recurved; all perianth segments about the same size | Lagarosiphon |
| Leaves whorled, laminas rarely recurved; inner perianth segments larger or about the same size as the outer segments
Back to 4 | 6 |
| 6 | Inner perianth segments much larger than outer segments: leaves mostly in whorls of 4 or 5 | Egeria |
| All perianth segments about the same size; leaves in whorls of 3–8
Back to 5 | 7 |
| 7 | Stamens 9; leaves usually in whorls of 3 | Elodea |
| Stamens 3; leaves usually in whorls of more than 3
Back to 6 | Hydrilla |
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