Myriophyllum is a nearly cosmopolitan genus of ca. 45 species. It is by far best represented in Australia with 36 species (of which 31 are endemic; see Mabberley 2008). Three species are native in Belgium: Myriophyllum alternifolium DC., M. spicatum L. and M. verticillatum L. (Lambinon & al. 2004) but only the second is more or less widespread. Wann (1997) cite two species that are sometimes cultivated for ornament in Europe (or as aquarium plants): Myriophyllum aquaticum and native M. spicatum. Both readily become invasive outside their natural distribution range. Jäger & al. (2008) provide a more detailed overview.
Myriophyllum species are often very reluctant to flower and reproduction often is solely clonal. Alas, inflorescences are usually required for an accurate identification (see also Stace 2010). Several attempts have been made to identify non-flowering plants of Myriophyllum (see Wimmer 1997, De Beer & De Vlaeminck 2008). However, this is impossible in the absence of a microscope. In the key beneath only flowering plants can be reliably identified. In addition, under each entry supplementary features for the potential distinction of vegetative plants are provided.
1. Uppermost floral bracts deeply serrate to pinnatisect === 2
1. Uppermost floral bracts entire or shallowly serrate === 3
2. Emergent leaves glaucous, with numerous sessile, translucent, globular glands. Plant dioecious (only female plants observed so far in Belgium) === 1. Myriophyllum aquaticum
2. Emergent leaves green, with scattered sessile glands. Plant monoecious (upper flowers male, lower florets female) (native) === M. verticillatum
3. Stamens 4. Uppermost floral bracts much exceeding flowers === 2. M. heterophyllum
3. Stamens 8. Uppermost floral bracts shorter than flowers (native) === M. alternifolium and M. spicatum
Literature:
Aiken S.G. (1981) A conspectus of Myriophyllum (Haloragaceae) in North America. Brittonia 33: 57-89.
De Beer D. & De Vlaeminck R. (2008) Myriophyllum heterophyllum,
een nieuwe invasieve waterplant. Dumortiera 94: 8-13.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Moody M.L. & Les D.H. (2010) Systematics of the aquatic angiosperm genus Myriophyllum (Haloragaceae). Syst. Bot. 35(1): 121-139.
Orchard A.E. (1981) A revision of South American Myriophyllum. Brunonia 4: 27-65.
Orchard A.E. (1985) Myriophyllum (Haloragaceae) in Australasia. II. The Australian species. Brunonia 8: 173-291.
Schindler A.K. (1905) Haloragaceae. In: Engler A. (ed.), Das Pflanzenreich 23(V.255). Engelmann, Leipzig: 1-133.
Sell P. & Murrell G. (2009) Flora of Great Britain and Ireland. Vol. 3 Mimosaceae – Lentibulariaceae. Cambridge University Press, Cambridge: XXVIII + 595 p.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.
Wann J.D. (1997) Myriophyllum. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 358.
Wimmer W. (1997) Myriophyllum heterophyllum Michaux in Niedersachsen und Bremen sowie seine Bestimmung im vegetativen Zustand. Flor. Rundbr. 31: 23-31.