Lemnaceae Martinov
Very small gregarious aquatic perennials, usually < 5 mm long, floating on, or just under, surface of still or slowly moving water, often together forming a complete cover; consisting of ± flattened leafless green platelets, with or without roots, each new platelet arising from a bud in a horizontal pouch on an older platelet. Inflorescence rarely seen, produced in a similar pouch, of 1-2- (3) male flowers maturing successively, and 1 female flower, the whole enclosed within or associated with a membranous spathe. Male flower a single stamen; filament projecting well beyond spathe; pollen sacs 1-2, rounded, splitting transversely; pollen globular. Female flower of rounded ovary and short erect style; carpel single, ovules 1-6, basal. Fruit flattened, sometimes ± winged. Seeds relatively large, smooth or ribbed. Genera 6, widespread in temperate and tropical countries.
The discovery of Wolffia over a century after Lemna and nearly twenty years after the adventive Spirodela suggests similar status, but we feel it is so minute that it might easily have been overlooked by earlier collectors. We treat it as indigenous, but with reservations.
