Landoltia is a monospecific genus, probably native to SE-Asia and Australia but nowadays distributed throughout warm-temperate regions with mild winters and the subtropics (for instance in South America and South Africa). It is closely related with Spirodela and only recently segregated from it (Les & Crawford 1999). Its generic status, based on molecular phylogenetic research, was recently disputed by Ward & Hall (2010). Landoltia is morphologically very similar to Spirodela (several roots per frond) but phylogenetically closer to Lemna.
Literature:
Les D.H. & Crawford D.J. (1999) Landoltia (Lemnaceae), a new genus of duckweeds. Novon 9: 530-533.
Van Valkenburg J. & Pot R. (2008) Landoltia punctata (G. Mey.) D.H. Les & D.J. Crawford (Smal kroos), nieuw voor Nederland. Gorteria 33: 41-49.
Ward D.B. & Hall D.W. (2010) Keys to the flora of Florida – 25, Lemnaceae. Phytologia 92(2): 241-248.
Wiersema J.H. (2015) Application of the name Lemna punctata G. Mey., the type of Landoltia Les & D. J. Crawford. Plant Biol. (Stuttg.) 17, suppl 1: 5-9.