Spirodela Schleid.

Scientific Description

Family Lemnaceae.

Habit and leaf form. Much reduced, floating aquatic herbs. Plants of very peculiar form; thalloid (thallus symmetric or asymmetric, broadly ovate to elliptic, oblong-elliptic to obovate or slightly reniform, flat to biconvex, green or greenish, rather compressed; with 3–16 veins; with brown pigment cells and stellate crystals; often adhering to neighbouring plants; budding pouches 2, lateral). Leaves absent. Plants with roots (roots (1-)2–18 per thallus). Annual. Hydrophytic; free floating. Stem anatomy. Secondary thickening absent.

Reproductive type, pollination. Fertile flowers functionally male and functionally female. Unisexual flowers present. Plants monoecious. The unisexual flowers not conspicuously in separate aggregates (inflorescence solitary, of 1 male and 2 female flowers, borne in one of the budding pouches).

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’ (in groups of 2—3). Inflorescences spatheate (spathe reduced, membranous). Flowers minute. Perianth absent. Fertile stamens present, or absent (in female flowers). Androecium 1. Androecium exclusively of fertile stamens. Stamens 1. Anthers dehiscing transversely; bilocular; tetrasporangiate. Fertile gynoecium present, or absent (in male flowers). Gynoecium ostensibly 1 carpelled; ostensibly monomerous; ostensibly of one carpel; superior. Carpel 1–7 ovuled. Placentation basal. Ovules ascending; orthotropous, or anatropous, or hemianatropous.

Fruit and seed features. Fruit non-fleshy. The fruiting carpel indehiscent; a utricle, laterally winged towards the top. Fruit 1–5 seeded. Seeds endospermic, or non-endospermic; with starch. Embryo weakly differentiated to well differentiated. Cotyledons 1. Embryo straight. Testa longitudinally ribbed. Seedling. Mesocotyl absent. Seedling collar not conspicuous. Coleoptile absent. Seedling cataphylls absent. Primary root ephemeral.

Physiology, biochemistry. Photosynthetic pathway: C₃.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, Queensland, New South Wales, Victoria, Australian Capital Territory, and Tasmania. Northern Botanical Province and South-West Botanical Province.

Additional characters Thallus with a ventral scale (covering or fusing with the basal sheaths of the roots).