Potamogetonaceae Dum.
Including Ruppiaceae Horan., Ruppiaceae (Kunth) Hutch.
Habit and leaf form. Aquatic herbs. Perennial (mainly), or annual (rarely); with neither basal nor terminal aggregations 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. Lamina entire; linear, or oblong, or ovate; one-veined (Ruppia), or palmately veined to parallel-veined; cross-venulate, or without cross-venules. Leaves stipulate (commonly, the stipular appendages adhering to the sheath or not), or exstipulate. Axillary scales 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 (Potamogeton, Ruppia). Vessels absent.
Stem anatomy. Secondary thickening absent. Xylem without vessels. Sieve-tube plastids P-type; type II.
Root anatomy. Root xylem with vessels, or without vessels (Ruppia). Vessel end-walls scalariform.
Reproductive type, pollination. Fertile flowers hermaphrodite. Plants hermaphrodite. Anemophilous (Potamogeton), or pollinated by water.
Inflorescence, floral, fruit and seed morphology. 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. Endothecium developing fibrous thickenings. The endothecial thickenings girdling. Anther epidermis persistent. Microsporogenesis successive. The initial microspore tetrads isobilateral. 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. Stigmas dry type; non-papillate; Group II type. Ovules pendulous; non-arillate; orthotropous to campylotropous; bitegmic; crassinucellate. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids pear-shaped. Hypostase present, or absent.
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. Seeds with starch. Cotyledons 1. Embryo achlorophyllous (two species of Potamogeton); slightly curved. Testa without phytomelan; membranous.
Seedling. Germination phanerocotylar. Hypocotyl internode present (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 (Potamogeton (many species), Ruppia), or C4 type (or approaching it, in Potamogeton praelongus).
Geography, cytology. Frigid zone to tropical. Cosmopolitan. X = 8–10 (Ruppia), or 13–15.
Taxonomy. Subclass Monocotyledonae. Superorder Alismatiflorae; Zosterales. APG (1998) Monocot; non-commelinoid; Alismatales. Species 100. Genera 3; Groenlandia, Potamogeton, Ruppia.
Illustrations. • Ruppia maritima. • Potamogeton pectinatus. • Technical details (Potamogeton, Ruppia). • Technical details (Potamogeton).
Cite this publication as: ‘L. Watson and M. J. Dallwitz (1992 onwards). The Families of Flowering Plants: Descriptions, Illustrations, Identification, and Information Retrieval. Version: 14th December 2000. http://biodiversity.uno.edu/delta/’. Dallwitz (1980), Dallwitz, Paine and Zurcher (1993, 1995, 2000), and Watson and Dallwitz (1991) should also be cited (see References).
