Family Hydrocharitaceae
Synonyms: Halophilaceae APNI*
Najadaceae APNI*
Description: Freshwater or marine, annual or perennial aquatics with submerged, floating, or emergent leaves, dioecious, monoecious, or with bisexual flowers, usually attached to the substrate, sometimes free, often stoloniferous or rhizomatous. Stems often branched.
Leaves all basal or cauline, alternate, opposite, or whorled, sessile or petiolate, sometimes with a sheathing base (Najas); veins usually ± parallel, connected by numerous cross-veins; stipules membranous or absent.
Inflorescences 1–many-flowered (usually as one or more monochasia if flowers not solitary), submerged, floating, or emergent, axillary, generally spatheate. Flowers 2- or 3-merous, usually ± actinomorphic, sometimes very small and inconspicuous (Najas), spatheate (female flowers sometimes without a spathe in Najas), bisexual or unisexual and if unisexual then the males usually in many-flowered inflorescences and the female flowers solitary, bisexual flowers often cleistogamous. Perianth segments and stamens usually borne at the summit of a long hypanthium (not in Najas) above the inferior ovary. The length of the hypanthium is related to the water depth. Perianth segments in 1 or 2 whorls (perianth absent or falling early in Najas), if 2 whorls then the inner usually petaloid, often showy, each whorl of (1) 2 or 3 segments. Stamens 1–many (and then often in 6 whorls of 3), staminodes sometimes present. Ovary inferior; carpels (1–)3–15, unilocular; styles 1–(3–15), often once divided, rarely twice divided, sometimes entire.
Fruit dry or fleshy, berry-like or capsular, indehiscent or irregularly dehiscent; seeds numerous, variously ornamented, without endosperm.
Distribution and occurrence: World: 16 genera, c. 130 species, cosmopolitan. Australia: 13 genera, 36 species, all States.
External links:
Angiosperm Phylogeny Website (Family: Hydrocharitaceae, Order: Alismatales)
Wikipedia Najas was formerly in a monotypic family, Najadaceae.
Text by S. W. L. Jacobs (1993); edited KL Wilson (June 2010). Updated K.L. Gibbons, 2 Jun. 2023.
Taxon concept: APG IV, Australlian Plant Census [accessed 1 June 2023]. Description adapted from Fl. NSW 4 (1993); VicFlora, Flora of Australia Online [both accessed 29 May- 2 Jun. 2023;, Christenhusz, Fay & Chase (2017) Plants of the World. Kew Publishing: Surrey.
| | Key to the genera | |
| 1 | Male flowers with 3–9 stamens and perianth segments in two whorls of 3; leaves with entire margins, or if finely toothed then in whorls | 2 |
| Male flowers with a single stamen and perianth segment; leaves with finely toothed margins, in subopposite pairs | Najas |
| 2 | Leaves floating or emergent | 3 |
| Leaves all submerged
Back to 1 | 4 |
| 3 | 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, greenish-white or yellowish
Back to 2 | 9 |
| 4 | Leaves ovate; plants marine or estuarine | Halophila |
| Leaves linear; plants of freshwater
Back to 2 | 5 |
| 5 | Leaves all basal | Vallisneria |
| Leaves cauline
Back to 4 | 6 |
| 6 | 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 5 | 7 |
| 7 | 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 6 | 8 |
| 8 | Stamens 9; leaves usually in whorls of 3 | Elodea |
| Stamens 3; leaves usually in whorls of more than 3
Back to 7 | Hydrilla |
| 9 | Undersurface of lamina with an orbicular or ovate pad of honeycomb-like aerenchymatous tissue smaller than and clearly delimited from the rest of the lamina; male flowers with staminal filaments free; female flowers with petals well-developed | Hydrocharis |
| Undersurface of lamina mostly covered with well-developed large-cell aerenchyma; male flowers with staminal filaments connate at least 1/2 their length; female flowers with petals usually undeveloped
Back to 3 | Limnobium |
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