Family Podostemaceae

Podostemaceae Rich. ex C.A. Agardh

Including Marathrinae (Marathraceae) Dum., Tristichaceae Willis, Philocrenaceae Bong.

Habit and leaf form. Vegetatively diverse and peculiarly modified emergent aquatic herbs; laticiferous (sometimes with laticifers or latex cells), or non-laticiferous and without coloured juice; resinous (sometimes with resin cells), or not resinous. Plants of very peculiar vegetative form; more or less thalloid (lichenoid, seaweed-like (e.g., ‘fucoid’), ‘bryophytic’ or ‘filmy-fern’-like, sometimes more or less resolvable into modified stems and leaves). Leaves well developed (but then minute, on secondary shoots, without axillary buds), or much reduced, or absent. Plants rootless (often with root-like but chlorophyllus plagiotropic basal branches, anchored by specialized basal branches or ‘haptera’ or attached to the substrate by numerous hairs); autotrophic. Annual (often), or perennial. Hydrophytic (flowering and fruiting aerially at times of low water); non-marine; rooted (growing on rocks in fast-flowing rivers or cataracts). Leaves when present, minute; alternate; imbricate, or not imbricate; sessile; non-sheathing; simple, or compound; sometimes much dissected.

General anatomy. Plants with laticifers (cells or coenocytes), or without laticifers. Plants with silica bodies (these commonly so abundant in the outer tissues as to maintain the form of desiccated thalli), or without silica bodies (?).

Stem anatomy. Cork cambium absent. Nodes unilacunar. Primary vascular tissue greatly reduced, the xylem often lacking or represented by a few tracheids with annular or spiral thickenings. Secondary thickening absent. Xylem when present, with tracheids; without vessels.

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Pollination anemophilous, or entomophilous (or cleistogamous).

Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; when aggregated, in cymes (these often spiciform). The ultimate inflorescence unit when flowers aggregated, cymose. Inflorescences spatheate (in Podostemoideae, the spathes enclosing up to twenty flowers). Flowers (bi-) bracteolate (these subtending or enclosing the flowers, and in Podostemoideae modified to form the spathe); small; regular to very irregular (dorsiventrally flattened to varying degrees); cyclic; tricyclic to polycyclic.

Perianth sepaline, or petaline, or of ‘tepals’, or vestigial, or absent (when present, usually regarded as apetalous); when present, 1, or 2–3(–5), or 5–50 (rarely to ‘many’); free, or joined (especially in Tristichoideae); 1 whorled; sepaloid, or petaloid.

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). Endothecium developing fibrous thickenings. Microsporogenesis successive, or simultaneous. The initial microspore tetrads tetrahedral, or isobilateral, or decussate. Tapetum glandular. Pollen shed in aggregates, or shed as single grains; when aggregated, in diads. Pollen grains aperturate (usually), or nonaperturate (occasionally); when aperturate, 3 aperturate, or 4–9 aperturate (rarely); colpate, or colporate, or foraminate (rarely); 2-celled (in 5 genera).

Gynoecium (1–)2(–3) carpelled. The pistil (1–)2(–3) celled. Gynoecium syncarpous; synovarious to synstylovarious; superior. Ovary (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; bitegmic; tenuinucellate. Outer integument contributing to the micropyle. Embryo-sac development reduced Allium-type (with variations on this). Embryogeny solanad.

Fruit non-fleshy; dehiscent; a capsule. Capsules septicidal. Seeds non-endospermic (there being no double fertilization); very small (often with a mucilaginous testa). Cotyledons 2. Embryo straight.

Seedling. Germination phanerocotylar. Primary root ephemeral.

Physiology, biochemistry. Aluminium accumulation not found.

Geography, cytology. Sub-tropical to tropical. Pantropical and warm North America. X = 10.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Podostemiflorae; Podostemales. Cronquist’s Subclass Rosidae; Podostemales. APG 3 core angiosperms; core eudicot; Superorder Rosanae; fabid; Order Malpighiales.

Species 130. Genera 48; Angolaea, Apinagia, Butumia, Castelnavia, Ceratolacis, Cladopus, Crenias, Crenias, Dalzellia, Devillea, Dicraeanthus, Diplobryum, Djinga, Endocaulos, Farmeria, Hydrobryopsis, Hydrobryum, Indotristicha, Jenmaniella, Lawia, Ledermanniella, Leiothylax, Letestuella, Lonchostephus, Lophogyne, Macrarenia, Macropodiella, Malaccotristicha, Marathrum, Monostylis, Mourera, Oserya, Paleodicraeia, Podostemum, Polypleurella, Polypleurum, Rhyncholacis, Saxicolella, Sphaerothylax, Stonesia, Thelethylax, Torrenticola, Tristicha, Tulasneantha, Weddellina, Willisia, Winklerella, Zehnderia, Zeylandium.

Illustrations.
• Technical details: Tristicha (Thonner).