The families of flowering plants.
Habit and leaf form. Non-rhizomatous, slender, aquatic herbs. Annual, or perennial (rarely); with neither basal nor terminal aggregations of leaves. Stem growth conspicuously sympodial, or not conspicuously sympodial. Halophytic and hydrophytic; marine, or non-marine (mostly in brackish water and salt marshes); rooted (rooting at the nodes). Leaves submerged, or submerged and floating (?). Not heterophyllous. Leaves small, or medium-sized; alternate, or opposite, or whorled (usually alternate except when subtending an inflorescence); when alternate, distichous; sessile; sheathing. Leaf sheaths with free margins. Leaves simple. Lamina entire; setaceous, or linear; one-veined; without cross-venules. Leaves exstipulate (?). Axillary scales (paired “intravaginal squamulae”) present. Lamina margins entire.
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 without vessels.
Reproductive type, pollination. Plants hermaphrodite. Pollination anemophilous, or by water.
Inflorescence, floral, fruit and seed morphology. Flowers solitary (rarely), or aggregated in ‘inflorescences’; usually in racemes, or in spikes, or in umbels. The ultimate inflorescence unit racemose. Inflorescences scapiflorous (at length long pedunculate, emergent or not?); terminal; short spikes or subumbelliform racemes, sometimes one- or few-flowered. Flowers ebracteate; small; regular. Hypogynous disk absent.
Perianthabsent (unless small staminal appendages are regarded as perianth segments).
Androecium 2. Androecial members all equal; free of one another; 1 whorled. Androecium exclusively of fertile stamens. Stamens 2; with sessile anthers. Anthers dehiscing via longitudinal slits; extrorse; ambiguously appendaged, or unappendaged (depending on whether their small appendages are interpreted as tepals: cf. Potamogeton, in which the corresponding structures are larger and more convincingly tepal-like). The initial microspore tetrads isobilateral. Pollen polysiphonous. Pollen grains nonaperturate; 3-celled.
Gynoecium (2–)4(–16) carpelled; apocarpous; eu-apocarpous; superior. Carpel non-stylate; apically stigmatic (the stigma peltate or umbonate); 1 ovuled. Placentation apical. Ovules pendulous; non-arillate; campylotropous; bitegmic; crassinucellate. Outer integument 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. Endosperm formation helobial. Embryogeny caryophyllad.
Fruit fleshy to non-fleshy (?); an aggregate. The fruiting carpel indehiscent; usually on a long, spirally twisted peduncle, drupaceous (with each drupelet becoming very long-stalked). Dispersal via the floating of the head of fruits?. Fruit 1 seeded. Seeds non-endospermic. Seeds with starch. Cotyledons 1. Embryo slightly curved, or straight. Testa without phytomelan; membranous.
Seedling.Germination phanerocotylar. Coleoptile absent. Primary root ephemeral.
Physiology, biochemistry. Not cyanogenic. Anatomy non-C4 type.
Geography, cytology. Temperate and sub-tropical. Widespread outside of frigid zones and the tropics. X = 8–10.
Taxonomy.Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Alismatales.
Species about 10. Genera 1; Ruppia.