Description from Flora of China
Herbs, mostly perennial, minute, aquatic, floating or submersed, reduced to small green bodies called fronds corresponding partly to leaf and partly to stem. Roots absent or 1-21, filiform, arising from lower surface of frond; root hairs absent. Fronds 1 to many coherent together, orbicular or oblong to lanceolate and flat or convex (gibbous) on lower surface, or globose to ovoid, 0.4-15 mm, with or without veins; daughter fronds successively formed from 1 basal cavity or 2 basal pouches detaching or remaining attached for many weeks, bases of pouches sometimes surrounded by a small, membranous scale (prophyll) covering bases of roots. Node (point of origin of veins, roots, and daughter fronds) situated ca. 1/3 from base toward apex. Turions (in some species) formed under unfavorable conditions, frondlike but smaller, compact, sinking to bottom. Flowers (interpreted as inflorescences by some authors) rarely seen in most species, 1(or 2), emerging from one of pouches or from a cavity near median line on upper surface, sometimes surrounded by a small, utricular, membranous scale (corresponding to a spathe in the Araceae); perianth absent; stamens 1 or 2; pollen grains ulcerate, exine spinulose; ovary 1, bottle-shaped, 1-7-ovuled, tapering into short style; stigma funnel-shaped. Fruit indehiscent, opening by bursting.
In the opinion of the present authors, it is too early to decide if the Lemnaceae should be included within the Araceae. While this placement has been supported by some phylogenetic studies, only a few species of Lemnaceae have been investigated so far. Before the whole spectrum of variability of molecular characteristics of the Lemnaceae is known, it would be unwise to change the status. The Lemnaceae are a well-characterized entity of taxa, whereas the Araceae are a heterogeneous group. It is very difficult to find common characters in the Lemnaceae and Araceae. The flowering organ is not at all decisive. It is not even certain if the flowering organ of the Lemnaceae is a flower or a reduced inflorescence. It is also very difficult to find special common characters between Lemnaceae and the genera that are placed nearest to them according to phylogenetic studies.
Different species of Lemnaceae are often found sympatric at the same habitats; sometimes they are associated with various kinds of aquatic ferns and vascular plants. They usually grow gregariously and with rapid reproduction (up to doubling within 24 hours). The fronds of all species are commonly utilized as high-quality feed for domestic animals, fish, and fowl.
Li Hen. 1979. Lemnaceae. In: Wu Cheng yih & Li Hen, eds., Fl. Reipubl. Popularis Sin. 13(2): 206-211.
Five genera and 38 species: worldwide in aquatic ecosystems; four genera and eight (possibly nine) species in China.
(Authors: Li Heng (李恒 Li Hen); Elias Landolt)