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Monoecious, very rarely dioecious, small to minute aquatic annuals, floating at the surface of the water, or floating just below the surface whereby only a very small part of the frond is exposed to the air, or completely submerged and then rising to the surface in the flowering period. In taxa with only 1 budding pouch (; subfam. Wolffioideae) the inflorescence is borne in a median or lateral dorsal flowering cavity (an exception is extra-Mal. Wolffiopsis which has 2 dorsal flowering cavities), without a spathe and consisting of 1 female and 1 male flower. In taxa with 2 budding pouches (, ) the inflorescence is surrounded by a spathe and consists of 1 female and 2 male flowers (female flower rarely absent). Ovules orthotropous, amphitropous or anatropous. Fruit symmetric () or asymmetric, 1-4-seeded, globose or laterally compressed, winged or without wings. Seeds smooth or ribbed (), with little or no endosperm;


America: present Asia-Tropical: Hawaiian Is: present Mahe Atoll: present Maldives, Indian Ocean: present New Caledonia: present Pacific:, Fiji (Fijipresent); Hawaiipresent; Samoa (Samoapresent); Tonga (Tongapresent) Pacific Islands: present Polynesia: present South Africa: present Tropical Africa: present Tropics of Africa: present all over the world: present oceanic islands: subtropical and tropical parts of America: present
There are 6 genera with c. 30 spp. all over the world, obviously introduced in oceanic islands (see under dispersal). The genera Spirodela, Lemna, and Wolffia are widely distributed in the temperate and tropical zones; the other genera have a more restricted range. Wolffiella occurs in the subtropical and tropical parts of America and in South Africa, Pseudowolffia is restricted to tropical Africa, and Wolffiopsis has been found in the tropics of Africa and America. See DEN HARTOG & VAN DER PLAS ().
In Malesia the first three mentioned genera occur with together 6 spp. As Lemnaceous plants are distinctly under-collected, their precise distribution is very incompletely known, and consequently range extensions may be expected. For this reason 3 spp. are added here which have not yet been found in Malesia, but which may be found in future.
In oceanic islands Lemnaceae are very scarce and possibly all introduced according to GUPPY. From the Pacific islands the following are known to me from collections: Hawaiian Is. Spirodela polyrhiza and Lemna perpusilla (HELLER 1895, LE ROY TOPPING and DEGENER 1927). L.perpusilla is, besides in Hawaii, also collected in Polynesia already during the Wilkes’ Exploring Exp. from Fiji (non vidi), and further known from New Caledonia (last century), Samoa (1893) and Tonga (1926). Spirodela punctata is also known from Fiji and New Caledonia. This is also recently collected on Mahe Atoll (Maldives, Indian Ocean).


Dispersal of the whole plant as well as the seeds over short distances may be effectuated by birds; Wolffia brasiliensis was found by WEDDELL on the feathers of a shot bird (). Over very short distances transport by amphibians is possible (). Water currents transport the plants and seeds as well. Transport over long distances is restricted by the fact that desiccation of the plants is a distinctly limiting factor. Seeds sink after coming free. Several islands or groups of islands have populations which are characterized by a particular pattern of pigmentation (Spirodela punctata in Fiji), a large number of roots (S. punctata of Java), or a particular shape (Lemna perpusilla of New Guinea). This 'raciation' implies that there is no regular exchange between the various populations, and that the island populations have obviously largely developed from isolated clones. This, and the fact of an endemic species in India (Wolffia microscopica), pleads against longdistance dispersal. In the oceanic islands of the Pacific (New Caledonia and Fiji) and the Indian Ocean (Maldives) Spirodela punctata occurs, but this species is notorious for being dispersed by man, e.g. in the rice fields of northern Italy and is said by DAUBS to be introduced in America. GUPPY concluded () to its introduction into the Pacific islands and excluded dispersal of Lemnaceae over wide seas.


Two subfamilies can be recognized within the Lemnaceae, the Lemnoideae and the Wolffi-oideae. The Lemnoideae are characterized by the presence of roots, 2 budding pouches, an inflorescence with 2 male flowers and a membranous spathe. The Wolffioideae have only 1 budding pouch and an inflorescence with 1 male flower; roots and spathe are absent. See for our new system of Lemnaceae C. DEN HARTOG & F. VAN DER PLAS ().


Because of their high rate of vegetative reproduction and the ease with which they can be cultured, Lemnaceae are often used in plant physiological experiments ().


Oxalate of lime (raphides; clustered crystals) is present in many, but by no means in all, members of the family. So-called myriophyllin cells (see sub Haloragaceae) occur in species of Spirodela, Wolffia and Wolffiella. The flavonoid constituents were investigated thoroughly. Four main types of flavonoid compounds occur in the family and each species and genus is said to be characterized by a distinct pattern of flavonoids. According to J. W. MCCLURE & R. E. AISTON () and B. L. TURNER () Spirodela shows the most complex pattern of flavonoid compounds; anthocyanins, flavonols, flavones and C-glycoflavones are present. In Lemna the property to produce flavonols is lost. The only flavonoid compounds observed in Wolffiella are flavonols. Three species of Wolffia were found to contain only flavonols and two species to produce only flavones and glycoflavones. Two lines of reductive biochemical evolution, both starting with Spirodela are evident: Spirodela → Lemna → Wolffia p.p. and Spirodela → Wolffiella → Wolffia p.p. The American authors assume that the evolution of taxa most probably followed the same pathway and that Wolffia, as defined by morphology, represents a biphyletic group. Regrettably, members of the genera Pseudowolffia and Wolffiopsis were not included in these studies. The pectic substances of cell walls of Lemnaceae contain galacturonic acid, apiose and xylose (). Such apiose-containing pectines occur in all members of the family investigated hitherto. This striking chemical feature, however, seems to be linked with ecology rather than with systematics ().
Most authors assume intimate relationships between Araceae and Lemnaceae. The chemical characters so far known from both families () are in perfect agreement with such a hypothesis. The fact, however, that chemical information about these families is still scanty implies that such a statement indicates lack of negative evidence, rather, than strong positive evidence. — R. HEGNAUER.