Common name. Eel-grass Family.
Habit and leaf form. Aquatic herbs. Perennial; plants with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves; rhizomatous (monopodial). Hydrophytic; marine; rooted. Leaves submerged; alternate; distichous; sessile; sheathing. Leaf sheaths with free margins, or with joined margins. Leaves edgewise to the stem, or with ‘normal’ orientation, or edgewise to the stem and with ‘normal’ orientation (???); simple. Leaf blades entire; flat; linear (ribbonlike); parallel-veined; without cross-venules. Leaves ligulate. Axillary scales present. Leaf blade margins entire. Leaves with a persistent basal meristem, and basipetal development. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male and functionally female, or functionally male, or functionally female. Unisexual flowers present. Plants monoecious, or dioecious (Phyllospadix). Pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in spikes. The terminal inflorescence unit racemose. Inflorescences axillary (enfolded in the subtending sheath, exserted only when fruiting); flattened spikes or spadices, the flowers on one side — males and females alternating in two ranks in the monoecious species. Flowers ebracteate; ebracteolate; small. Perigone tube absent. Perianth vestigial (if the marginal outgrowths from the axis — retincacula, which sometimes enclose the stamens — are interpreted as perianth), or absent. Fertile stamens present, or absent (female flowers). Androecium 1 (though interpretable as two fused members). Androecium exclusively of fertile stamens. Stamens 1; with sessile anthers. Anthers dehiscing via longitudinal slits; unilocular; tetrasporangiate (representing fusion of two bisporangiate members?). Pollen shed as single grains (becoming filamentous and up to 2 mm long when released). Pollen grains lacking exine, and dispersed in the sea as long filaments. Fertile gynoecium present, or absent (male flowers). Gynoecium ostensibly 1 carpelled (but subject to fanciful alternative interpretations); (?pseudo) monomerous; ostensibly of one carpel; superior. Carpel apically stigmatic; 1 ovuled. Placentation apical. Ovules sessile; pendulous; non-arillate; orthotropous.
Fruit and seed features. Fruit non-fleshy. The fruiting carpel indehiscent; an achene. Fruit 1 seeded. Seeds non-endospermic. Cotyledons 1. Embryo curved. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar conspicuous. Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Geography, cytology, number of species. World distribution: widespread coastal, but mainly extra-tropical, and lacking from West Africa, India, Malaysia and South America. X = 6, 10. 18 species.