Lemnaceae

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. Fronds either solitary or connected in small groups by short to very short hyaline or rarely elongate green stipes (fig. 1 Aa, 1 Ba), symmetric or asymmetric, with reniform, round, linear-lanceolate or angular dorsal outline, green, with or without red or brown pigment cells, sometimes with both types of pigment; base symmetric or asymmetric, obtuse, emarginate or narrowing into the stipe; apex symmetric or asymmetric, round, obtuse or acute; margin entire or slightly dentate; dorsal side flat to slightly convex, smooth or with one or more small papillae; ventral side flat to strongly inflated; somewhere in the median provided with a ‘node’ (fig. 1 Ac). There the roots, nerves, new fronds, and flowers emerge. Nerves 0-1-∞, running towards the apex. New fronds attached to the node of the mother frond by means of a ‘stipe’ which is sometimes hardly visible and is connate with their ventral side. Daughter frond sometimes (in Spirodela) provided with 2 basally connate, roundish scales inserted at the base of the stipe (fig. 1 Ad, 2 e), unequal, one connate with the ventral side as far as its node. Roots several, one or none, unbranched, growing downward from the node; in root-producing species the root(s) closely enveloped by a sheath, which during growth is circumscissile-dehiscing, leaving a basal sheath (in some species soon hardly visible) and a ‘calyptra’ on top. Budding pouches 2 (fig. 1 Ab) or 1 (fig. 1 Cb); if there is one budding pouch this is basal, median, dorso-ventrally flattened or funnel-shaped and it produces only new fronds; when there are 2 budding pouches these are lateral, one on either side of the axis, dorso-ventrally flattened and produce new fronds, one pouch may give rise to an inflorescence. In taxa with only 1 budding pouch (fig. 1 Cj; 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 (fig. 3 k, 6 e-f) the inflorescence is surrounded by a spathe and consists of 1 female and 2 male flowers (female flower rarely absent). Perianth none. — Male flower consisting of 1 stamen, anther uni- (fig. 8 d) or bilocular (fig. 2 f), apically or transversely dehiscent; filament short or long and slender; pollen grains 17-21 μØ, spinose. — Female flower consisting of 1 globular ovary with a short persistent style (fig. 3 k, 6 f), and containing 1-4 ovules. Ovules orthotropous, amphitropous or anatropous. Fruit symmetric (fig. 5 d-e) or asymmetric, 1-4-seeded, globose or laterally compressed, winged or without wings. Seeds smooth or ribbed (fig. 4 l), with little or no endosperm; operculum and chalaza prominent. Distribution. 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 (Blumea 18, 1970, 355-368).