Wolffia Horkel & Schleid.
Wolffia is a genus with 11 species worldwide. It comprises the smallest flowering plants on earth. A single species is native in Belgium, W. arrhiza (L.) Horkel ex Wimm. (Lambinon & Verloove 2012). This is fairly rare, decreasing and considered at risk in some European countries (Schmitz & al. 2014). All species of Wolffia are much alike and in recent years it became clear that several exotic members of the genus have naturalized in Europe, initially without being noticed: W. australiana (Benth.) Hartog & Plas, W. columbiana Karsten and W. globosa (Roxb.) Hartog & Plas (Kirjakov & Velichkova 2013, Achterkamp & Soes 2014, Schmitz & al. 2014, Frank & al. 2020). Random sampling of Wolffia in Germany, the Netherlands and Poland demonstrated that W. columbiana now is much more frequent than native W. arrhiza (Schmitz & al. 2016). The presence in Europe of further non-native species is expected.
All these exotic species of Wolffia are probably introduced as contaminants via the horticultural trade (aquatic ornamentals).
Useful additional information on the genus with keys and many illustrations is available here: http://waynesword.palomar.edu/1wokey.htm.
1. Upper surface transparent green, more or less rounded, floating with the central portion of dorsal surface above water, the rest of plant body submersed. Stomates 1-10 (-30) === Wolffia columbiana
1. Upper surface dark green, distinctly flattened, floating with most of dorsal surface above water, only a small part of plant body submersed. Stomates 15-100 (native) === W. arrhiza
Literature
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Armstrong W.P. (1985) A status report on the genus Wolffia in California. Fremontia 13(1): 11-14.
Armstrong W.P. & Thorne R.F. (1984) The genus Wolffia (Lemnaceae) in California. Madrono 31(3): 171-179. [available online at: http://www.biodiversitylibrary.org/page/47946052#page/595/mode/1up]
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Frank D., Appenroth K.-J., Bog M. & Schmitz U. (2020) Man sieht nur was man kennt – Drei Zwergwasserlinsen-Arten der Gattung Wolffia Schleid. in Sachsen-Anhalt nachgewiesen. Mitt. florist. Kart. Sachsen-Anhalt (Halle) 25: 3–17. [available online at: http://botanischer-verein-sachsen-anhalt.de/wp-content/uploads/2021/01/F...
Kirjakov I. & Velichkova K. (2013) Wolffia globosa (Roxburgh) Hartog et Plas (Lemnaceae): a New Species in Bulgarian Flora. Journal of Biological & Scientific Opinion 1(4): 356-357. [available online at: https://www.researchgate.net/publication/269847473_WOLFFIA_GLOBOSA_ROXBU...
Lambinon J. & Verloove F. (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
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Landolt E. (1994) Taxonomy and Ecology of the Section Wolffia of the Genus Wolffia (Lemnaceae). Berichte des Geobotanischen Institutes der Eidg. Techn. Hochschule, Stiftung Rübel 60: 137-151. [available online at: http://www.e-periodica.ch/cntmng?pid=bgi-002:1994:60::181]
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Mazzeo N., Rodríguez R.R. & Rondanelli M. (1993) El género Wolffia Horkel ex Schleid. (Lemnaceae) en la flora acuática de Chile. Gayana, Bot. 50(2): 67-68. [available online at: https://archive.org/stream/gayana502univ#page/n3/mode/2up]
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Schmitz U., Köhler S. & Hussner A. (2014) First records of American Wolffia columbiana in Europe – Clandestine replacement of native Wolffia arrhiza? BioInvasions Records 3: 213-216. [available online at: http://www.reabic.net/journals/bir/2014/4/BIR_2014_Schmitz_Hussner.pdf]
Schmitz U., Köhler S. & Nesemann H. (2016) Neue Nachweise der Kolumbianischen Zwergwasserlinse Wolffia columbiana in Europa – Bei wie vielen vermeintlichen Vorkommen von Wolffia arrhiza handelt es sich in Wirklichkeit um den Neophyten? Veröff. Bochumer Bot. Ver. 8(1): 1-10. [available online at: https://www.botanik-bochum.de/publ/OVBBV8_1_Schmitz_Koehler_Nesemann_Wol...
Ward D.B. & Hall D.W. (2010) Keys to the flora of Florida – 25, Lemnaceae. Phytologia 92(2): 241-248. [available online at: http://www.biodiversitylibrary.org/page/47218126#page/251/mode/1up]