The families of flowering plants.
Habit and leaf form. Aquatic herbs (with creeping rhizomes and leafy branches). Perennial; with neither basal nor terminal aggregations of leaves. Stem growth conspicuously sympodial, or not conspicuously sympodial. Hydrophytic (non-halophytic, by contrast with Ruppia); rooted. Leaves submerged, or submerged and floating. Heterophyllous (with clearly distinct submerged and floating leaves), or not heterophyllous. Leaves small, or medium-sized; alternate (mostly, usually), or opposite, or whorled (all opposite or in whorls of three in Groenlandia); when alternate, distichous; sometimes 3 per whorl; ‘herbaceous’, or membranous; pseudo- petiolate, or subsessile, or sessile; sheathing. Leaf sheaths usually tubular; with free margins. Leaves simple. Lamina entire; linear, or oblong, or ovate; one-veined, or palmately veined to parallel-veined; cross-venulate, or without cross-venules. Leaves stipulate (or ostensibly so, the sheaths of at least some leaves being either free and stipule-like, or fused to the leaf-base for most of their length), or exstipulate. Axillary scales (paired “intravaginal squamulae”) present. Lamina margins entire.
General anatomy. Accumulated starch exclusively ‘pteridophyte type’.
Leaf anatomy. Stomata absent.
The mesophyll without calcium oxalate crystals. Minor leaf veins without phloem transfer cells. Vessels absent.
Stem anatomy. Secondary thickening absent. Xylem without vessels. Sieve-tube plastids P-type; type II.
Root anatomy. Root xylem with vessels; vessel end-walls scalariform.
Reproductive type, pollination. Plants hermaphrodite. Pollination anemophilous, or ornithophilous.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in racemes. The ultimate inflorescence unit racemose. Inflorescences scapiflorous (usually emergent); pedunculate, axillary; mostly spikes; spatheate. Flowers ebracteate; small; regular; 4 merous; cyclic; tricyclic. Perigone tube absent. Hypogynous disk absent.
Perianthof ‘tepals’ (as usually interpreted), or absent (if the ‘perianth’ members are interpreted as staminal appendages — which seems not unreasonable); 4 (if the staminal appendages are interpreted as perianth); free (the members rounded, shortly clawed, valvate); 1 whorled.
Androecium4. Androecial members adnate (to the claws of the ‘perianth’ members, with these so interpreted); all equal; free of one another; 2 whorled (fairly clearly so), or 1 whorled. Androecium exclusively of fertile stamens. Stamens 4; oppositiperianthial (opposite the perianth, when this is 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. Endothecium developing fibrous thickenings. The endothecial thickenings girdling. Anther epidermis persistent. Microsporogenesis successive. The initial microspore tetrads isobilateral. Pollen polysiphonous. Pollen grains nonaperturate; 3-celled.
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 to basal (basal-ventral). Stigmas dry type; non-papillate; Group II type. Ovules pendulous; non-arillate; orthotropous; bitegmic; crassinucellate. Outer integument not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids pear-shaped. Hypostase present, or absent. Endosperm formation helobial. Embryogeny caryophyllad.
Fruit fleshy, or non-fleshy; an aggregate. The fruiting carpel indehiscent; nucular, or drupaceous, or an achene, or baccate (Groenlandia). Dispersal usually by floating of the head of fruits. Fruit 1 seeded. Seeds non-endospermic. Seeds with starch. Cotyledons 1. Embryo achlorophyllous (two species of Potamogeton); slightly curved. Testa without phytomelan; membranous.
Seedling.Germination phanerocotylar. Hypocotyl internode present (usually long). 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.
Physiology, biochemistry. Not cyanogenic. Alkaloids absent (3 species). Proanthocyanidins absent. Flavonols absent. Ellagic acid absent. Saponins/sapogenins absent. Anatomy non-C4 type (usually), or C4 type (or approaching it, in Potamogeton praelongus).
Geography, cytology. Frigid zone to tropical. Cosmopolitan. X = 13–15.
Taxonomy.Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Alismatales.
Species about 100. Genera 2; Groenlandia, Potamogeton.