Halophila Thouars

Scientific Description

Family Hydrocharitaceae.

Habit and leaf form. Aquatic herbs. Annual, or perennial; rhizomatous (rhizome creeping; nodes with 1-several simple roots, a leafy shoot and 2 scales, 1 scale enveloping the rhizome and 1 scale associated with the erect shoot; stems often scarcely developed, with 1–20 or more pairs of leaves). Hydrophytic; marine (or estuarine); rooted. Leaves submerged. Not heterophyllous. Leaves alternate (or paired), or whorled (then pseudo-verticillate); when alternate distichous; ‘herbaceous’; petiolate and sessile; sheathing to non-sheathing; simple; epulvinate. Leaf blades entire; linear to ovate, or oblong to elliptic; pinnately veined; cross-venulate, or without cross-venules. Leaves without stipules. Axillary scales present. Leaf blade margins shortly serrate to dentate, or entire. Leaves with a persistent basal meristem, and basipetal development. Leaf anatomy. Hairs absent, or present (rarely with scattered hairs). Stem anatomy. Secondary thickening absent.

Reproductive type, pollination. Fertile flowers functionally male, or functionally female, or functionally male and functionally female. Unisexual flowers present. Plants monoecious, or dioecious. Female flowers solitary (usually). Male flowers solitary (usually, breaking off after anthers have opened to float on the water surface). Floral nectaries absent. Pollinated by water.

Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’ (rarely, then 1 male and 1 female flower together); pedicellate (male flowers), or subsessile to sessile (female flowers); bracteate (the sessile ‘spathe’ comprised of 2 free, keeled, imbricate, membranous bracts); small; regular; 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present (in female flowers, cylindric), or absent (in male flowers). Perianth of ‘tepals’, or vestigial; 3; 1 -whorled. Fertile stamens present, or absent (female flowers). Androecium 3. Androecial members free of the perianth; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 3; isomerous with the perianth; with sessile anthers. Anthers dehiscing via longitudinal slits; extrorse; bilocular, or four locular; bisporangiate, or tetrasporangiate. Pollen shed in aggregates (as threadlike chains). Fertile gynoecium present, or absent (male flowers). Gynoecium 3–6 carpelled. The pistil 1 celled. Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary unilocular; 1 locular. Styles 3–6; free, or partially joined; simple; apical. Stigmas dry type; non-papillate; Group II type. Placentation laminar-dispersed. Ovules in the single cavity 12–100 (i.e. ‘many’); pendulous to ascending; non-arillate; orthotropous, or hemianatropous to anatropous.

Fruit and seed features. Fruit fleshy; indehiscent (often opening by decay); capsular-indehiscent (ovoid to globular, thin-walled). Capsules splitting irregularly (underwater). Dispersal by water. Fruit few- to numerous-seeded. Seeds endospermic, or non-endospermic; with starch. Cotyledons 1. Embryo straight. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; dorsiventrally flattened. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, Northern Territory, Queensland, New South Wales, Victoria, and Tasmania. Northern Botanical Province, Eremaean Botanical Province, and South-West Botanical Province.

Additional characters Fruit rostrate. Perianth of male flowers of ‘tepals’; 3. Perianth of female flowers vestigial (represented by a ring of minute tepals); 3.