Common name. Pondweed Family.
Habit and leaf form. Aquatic herbs. Perennial (mainly), or annual (rarely); plants with neither basal nor terminal concentrations of leaves. Stem growth conspicuously sympodial, or not conspicuously sympodial. Hydrophytic; rooted. Leaves submerged, or submerged and floating. Heterophyllous (with clearly distinct submerged and floating leaves), or not heterophyllous. Leaves small, or medium-sized; alternate, or opposite, or whorled; when alternate, distichous; sometimes 3 per whorl; petiolate to sessile; sheathing. Leaf sheaths usually tubular; with free margins. Leaves simple. Leaf blades entire; linear, or oblong, or ovate; one-veined (Ruppia), or palmately veined to parallel-veined; cross-venulate, or without cross-venules. Leaves with stipules (commonly, the stipular appendages adhering to the sheath or not), or without stipules. Axillary scales present. Leaf blade margins entire. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Anemophilous (Potamogeton), or pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in racemes and in spikes. The terminal inflorescence unit racemose. Inflorescences pedunculate; axillary; mostly spikes, subumbellate racemes in Ruppia. Flowers ebracteate; small; regular; (2–)4 merous; cyclic; tricyclic. Perigone tube absent. Hypogynous disk absent. Perianth of ‘tepals’, or absent (in Ruppia, and always if the ‘perianth’ members are interpreted as staminal appendages — which seems not unreasonable); 4 (if the staminal appendages are interpreted as perianth). Androecium 4, or 2 (Ruppiaceae). Androecial members adnate (to the claws of the ‘perianth’ members, should these be so interpreted); all equal; free of one another; 2 -whorled (fairly clearly, in Potamogeton), or 1 -whorled. Androecium exclusively of fertile stamens. Stamens 4, or 2 (Ruppiaceae); oppositiperianthial (opposite the perianth, when this is present and regarded as such); filantherous and with sessile anthers, or with sessile anthers (depending on interpretation). Anthers dehiscing via longitudinal slits; extrorse; appendaged (if the ‘tepals’ are interpreted as outgrowths from the connective), or unappendaged. Gynoecium (3–)4(–8) carpelled. Carpels usually isomerous with the perianth (or with the stamens!). Gynoecium apocarpous; eu-apocarpous; superior. Carpel non-stylate, or stylate; apically stigmatic; 1 ovuled. Placentation marginal. Stigmas dry type; non-papillate; Group II type. Ovules pendulous; non-arillate; orthotropous to campylotropous.
Fruit and seed features. Fruit fleshy, or non-fleshy; an aggregate. The fruiting carpel indehiscent; drupaceous, or an achene, or baccate (Groenlandia). Dispersal usually by floating of the head of fruits. Fruit 1 seeded. Seeds non-endospermic; with starch. Cotyledons 1. Embryo achlorophyllous (two species of Potamogeton); slightly curved. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. First leaf dorsiventral. Primary root ephemeral.
Geography, cytology, number of species. World distribution: cosmopolitan. X = 8–10 (Ruppia), or 13–15. 100 species.