Hydrocharitaceae (incl. Najadaceae)
In accordance with recent molecular phylogenetic studies Najadaceae is here included in Hydrocharitaceae (APG III 2009, Chen & al. 2012).
1 Leaves orbicular (almost as wide as long), petiolate. Plants floating === 2
Leaves (much) longer than wide, sessile. Plants mostly submerged === 3
2 Stolon buds with 1 root. Leaves not spongy. Male and female plants present. Styles 6, 2-fid less than ½ length (native) === Hydrocharis
Stolon buds with 10 or more roots. Lower leaf surface very spongy (filled with aerenchymous tissue). Only female plants present (but rarely flowering). Styles 3-9, 2-fid nearly to base === Limnobium
3 Leaves all in basal rosette, (much) longer than 5 cm === 4
Leaves cauline, variably borne along stems, usually opposite or in whorls, (much) shorter than 5 cm === 5
4 Leaves sharply dentate along margin, very rigid. Flowers showy, 25-50 mm across (native) === Stratiotes
Leaves minutely denticulate near apex, flaccid. Flowers inconspicuous, at most 5 mm across === Vallisneria
5 Leaves strongly dentate, more or less widened at base and sheathing stem. Perianth absent. Stamens 1 (native) === Najas
Leaves subentire to very minutely serrate, at most slightly clasping stem. Perianth present. Stamens 3 or 9 (but only female plants encountered in Belgium in Elodea and Lagarosiphon) === 6
6 Lower leaves spirally arranged (not opposite nor whorled), always strongly recurved. Flowers inconspicuous (probably never produced in Belgium) === Lagarosiphon
All leaves opposite or in whorls of 3-6, strongly recurved or not. Flowers small to conspicuous, regularly produced === 7
7 Upper leaves in whorls of (3-)4-6(-8), up to 40 mm long and 5 mm wide, acute. Flowers conspicuous, up to 20 mm across === Egeria
Upper leaves in whorls of 3(-4), usually narrower and/or shorter. Flowers inconspicuous, ca. 5 mm across === Elodea
A non-native species from an additional genus, Hydrilla verticillata (L. f.) Royle, might have been overlooked. It much resembles Elodea nuttallii and Egeria densa but has leaves with minute prickles along the abaxial surface of the midvein and basal leafscales are fringed with orange-brown hairs. Hydrilla verticillata is often cultivated as an aquarium plant (“star-vine”, “oxygen plant”) and might escape. It is native in parts of Asia, Africa and Australia, as well as in the British Isles but has been reported as a xenophyte from several other European countries (see Wolff 1980).
APG III (2009) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants. APG III. Bot. J. Linn. Soc. 161(2): 105-121.
Chen L.-Y., Chen J.-M., Gituru R.W. & Wang Q.-F. (2012) Generic phylogeny, historical biogeography and character evolution of the cosmopolitan aquatic plant family Hydrocharitaceae. BMC Evolutionary Biology 12: 30 [available online at: http://www.biomedcentral.com/content/pdf/1471-2148-12-30.pdf]
Denys L., Packet J. & Van Landuyt W. (2004) Neofyten in het Vlaamse water: signalement van vaste waarden en rijzende sterren. Natuur.focus 3(4): 120-128.
Haynes R.R. (2000) Hydrocharitaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 22. Oxford University Press, New York-Oxford: 26-38.
Rich T.C.G. & Jermy A.C. (1998) Elodea / Lagarosiphon / Egeria densa / Hydrilla. In: Rich T.C.G. & Jermy A.C. (eds.), Plant crib: 317-318. BSBI, London.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Wolff P. (1980) Die Hydrilleae (Hydrocharitaceae) in Europa. Gött. Flor. Rundbr. 14: 33-52.