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The families of flowering plants

L. Watson and M.J. Dallwitz

Potamogetonaceae Dum.

Excluding Ruppiaceae.

Habit and leaf form. Aquatic herbs (with creeping rhizomes and leafy branches). Perennial; without conspicuous 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 in the form of paired “intravaginal squamulae”, present. Lamina margins entire.

Leaf anatomy. Stomata absent. The mesophyll without crystals. Foliar vessels absent. Minor leaf veins without phloem transfer cells.

Axial (stem, wood) anatomy. Secondary thickening absent. The axial xylem without vessels.

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 units racemose. Inflorescences scapiflorous (usually emergent); pedunculate, axillary; mostly spikes; spatheate. Flowers ebracteate; small; regular; 4 merous; cyclic; tricyclic. Perigone tube absent. Hypogynous disk absent.

Perianth of ‘tepals’ (as usually interpreted), or absent (if the ‘perianth’ members are interpreted as staminal appendages — which seems not unreasonable); if the staminal appendages are interpreted as perianth, 4; free (the members rounded, shortly clawed, valvate); 1 whorled.

Androecium 4. 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, with this 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, phytochemistry. Anatomy non-C4 type (usually), or C4 type (or approaching it, in Potamogeton praelongus). Accumulated starch exclusively ‘pteridophyte type’. Not cyanogenic. Alkaloids absent (3 species). Saponins/sapogenins absent. Proanthocyanidins absent. Flavonols absent. Ellagic acid absent. Sieve-tube plastids P-type; type II.

Geography, cytology. Frigid zone to tropical. Cosmopolitan. X = 13–15.

Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG III core angiosperms; Superorder Lilianae; non-commelinid Monocot. APG IV Order Alismatales.

Species about 100. Genera 2; Groenlandia, Potamogeton.

Illustrations. • Le Maout and Decaisne: Potamogeton crispus and P. perfoliatus, with Ruppia. • Potamogeton javanicus: Thonner. • Potamogeton alpinus: as P. rufescens, Eng. Bot. 1402 (1869). • Potamogeton coloratus: as P. plantagineus, Eng. Bot. 1401 (1869). • Potamogeton filiformis: Eng. Bot. 1424 (1869). • Potamogeton natans: Eng. Bot. 1399 (1869). • Potamogeton x nitens: as P. nitens, Eng. Bot. 1407 (1869). • Potamogeton pectinatus: as P. flabellatus, Eng. Bot. 1421 (1869). • Potamogeton pectinatus (B. Ent.).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 15th April 2018.’.