Enhalus Rich.

Scientific Description

Family Hydrocharitaceae.

Habit and leaf form. Aquatic herbs. Perennial; plants with neither basal nor terminal concentrations of leaves; rhizomatous (rhizome creeping, with short internodes, covered by fibrous remains of old leaves). Hydrophytic; marine; rooted. Leaves submerged. Not heterophyllous. Leaves alternate (tufted, usually 2–6 per shoot); distichous; ‘herbaceous’; sessile; sheathing. Leaf sheaths with free margins. Leaves simple; epulvinate. Leaf blades entire; linear (strap-like); many-veined; parallel-veined; cross-venulate, or without cross-venules. Leaves without stipules. Axillary scales present (coarse, simple roots arising from axillary buds on undersurface and vegetative branches or flowering stems arising from axillary buds on upper surface). Leaves with a persistent basal meristem, and basipetal development. Stem anatomy. Secondary thickening absent.

Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Unisexual flowers present. Plants dioecious. Female flowers solitary (reaching water surface by elongation of the peduncle but submerged at high tide; peduncle becoming coiled and contracted after anthesis, straightening in fruit). Male flowers aggregated in ‘inflorescences’ (breaking off just before anthesis, the open flowers floating on the surface). Floral nectaries absent. Pollinated by water (on water surface).

Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; in cymes. The terminal inflorescence unit (when flowers clustered) cymose. Inflorescences scapiflorous, or not scapiflorous; axillary; long-pedunculate; spatheate (in male inflorescence spathe bracts 2, shortly connate at the base, somewhat imbricate above). Flowers bracteate (forming the ‘spathe’ of solitary female flower, bracts 2, almost free bracts, persistent); small; regular; 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present, or absent. Perianth with distinct calyx and corolla; 6; 2 -whorled; isomerous; different in the two whorls; white. Calyx 3; 1 -whorled; polysepalous; regular. Corolla 3; 1 -whorled; polypetalous; white. Petals clawed, or sessile. 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; filantherous to with sessile anthers (subsessile). Anthers dehiscing via short slits; latrorse; bilocular; bisporangiate, or tetrasporangiate. Pollen very large. Fertile gynoecium present, or absent (male flowers). Gynoecium 6 carpelled. The pistil 1 celled (with 6 placentae). Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary unilocular; 1 locular (but with deeply intruding partial partitions). Styles 6; free, or partially joined; simple (almost to base); apical. Stigmas 12; 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; dehiscent; a capsule (ovoid, acuminate). Capsules splitting irregularly (underwater). Dispersal by water. Fruit 8–14 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, Northern Territory, and Queensland. Northern Botanical Province.

Additional characters Perianth of male flowers with distinct calyx and corolla; 6. Perianth of female flowers with distinct calyx and corolla; 6.