Podostemaceae Kunth

Scientific Description

Common name. River-weed Family.

Habit and leaf form. Aquatic herbs. Plants of very peculiar form; more or less thalloid (lichenoid or seaweed-like, or ‘bryophytic’). Leaves well developed (but minute, on secondary shoots, without axillary buds), or much reduced, or absent. Plants with roots (creeping over the substrate and bearing ‘haptera’ or holdfasts that attach the plant), or rootless. Annual (often), or perennial. Hydrophytic; rooted (growing on rocks in rivers or cataracts). Leaves when present, minute; alternate; imbricate; sessile; non-sheathing; simple, or compound; sometimes much dissected. Stem anatomy. Nodes unilacunar. Secondary thickening absent (vascular tissue greatly reduced).

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite.

Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; in cymes. The terminal inflorescence unit when flowers aggregated, cymose. Inflorescences cymes, often spiciform; spatheate (in Podostemoidae, the spathes enclosing up to twenty flowers). Flowers bracteolate; small; regular to very irregular (dorsiventrally flattened to varying degrees); cyclic; tricyclic to polycyclic. Perianth sepaline, or petaline, or vestigial; when present, 1, or 2–3(–5), or 5–50 (to ‘many’); 1 -whorled; free, or joined. Androecium 1, or 2–100 (to ‘many’). Androecial members free of the perianth; coherent (usually with basally connate filaments), or free of one another; 1–5 -whorled (? — to ‘several’ whorled). Androecium exclusively of fertile stamens. Stamens 1–30. Anthers tetrasporangiate (usually with the microsporangia aligned in a row). Pollen shed in aggregates, or shed as single grains; when aggregated, in diads. Gynoecium (1–)2(–3) carpelled. The pistil (1–)3 celled. Gynoecium syncarpous; synovarious to synstylovarious; superior. Ovary unilocular, or plurilocular; (1–)2(–3) locular. Styles (1–)2(–3); partially joined; apical. Placentation when unilocular, free central; when 2(–3)-locular, axile. Ovules 2–50 per locule (to ‘many’); anatropous.

Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule. Seeds non-endospermic (there being no double fertilization); small. Cotyledons 2. Embryo straight. Seedling. Germination phanerocotylar.

Physiology, biochemistry. Aluminium accumulation not found.

Geography, cytology, number of species. World distribution: pantropical and warm North America. X = 10. 130 species.