Thalassia K.D.Koenig

Scientific Description

Family Hydrocharitaceae.

Habit and leaf form. Aquatic herbs. Perennial; plants with neither basal nor terminal concentrations of leaves; rhizomatous (rhizome monopodial, with 1 short simple root and 1 membranous scale at each node). Hydrophytic; marine. Leaves submerged. Not heterophyllous. Leaves alternate (tufted, 2–6 on short lateral shoots); distichous; sessile; sheathing (sheath pale-coloured). Leaf sheaths with free margins. Leaves simple; epulvinate. Leaf blades entire; linear (ribbon-like); 9–17 -nerved (with many short brown markings (tannin cells) parallel with the veins); parallel-veined; cross-venulate. Leaves without stipules. Axillary scales present. Leaf blade margins entire (although apex very finely serrulate). 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. Floral nectaries absent. Pollinated by water.

Inflorescence and flower features. Flowers solitary; pedicellate to subsessile; bracteate (‘spathe’ submerged, 1 or 2 per shoot in male plants, solitary in female plants; spathe bracts 2, connate on 1 side in male plants, connate on both sides in female plants, persistent); small; regular; 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present (in female flowers, elongate, partially persistent). Perianth of ‘tepals’; 3; 1 -whorled; uncoloured or light-brown, with numerous coloured tannin cells. Fertile stamens present, or absent (female flowers). Androecium 3–12. Androecial members free of the perianth; free of one another; 1–4 -whorled (the whorls trimerous). Androecium exclusively of fertile stamens. Stamens 3–12; isomerous with the perianth to polystemonous; alterniperianthial, or oppositiperianthial. Anthers dehiscing via longitudinal slits; latrorse; bilocular, or four locular (or trilocular); bisporangiate, or tetrasporangiate. Pollen shed in aggregates (as threadlike chains, forming pollen tubes before reaching the stigmas). Fertile gynoecium present, or absent (male flowers). Gynoecium 6–8 carpelled. The pistil 1–3 celled (unilocular, imperfectly bi- or trilocular). Gynoecium syncarpous; synovarious; inferior. Ovary unilocular; 1 locular (but with deeply intruding partial partitions). Styles 6–8; forked; apical. Stigmas 12–16 (stigmata 2–6 times as long as the style, each with 2 longitudinal grooves on the underside); dry type; non-papillate; Group II type. Placentation laminar-dispersed, or basal. 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 (globose, echinate). Capsules splitting irregularly (underwater, the pericarp bursting into a number of irregular, stellately spreading valves). Dispersal by water. Fruit few-seeded. Seeds conical with a thickened basal portion; scantily 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 and Queensland. Northern Botanical Province and Eremaean Botanical Province.

Additional comments. Ecologically very important for its great production of biomass.

Additional characters Fruit shortly rostrate. Stigmas the stigmatic area linear. Perianth of male flowers of ‘tepals’; 3. Perianth of female flowers of ‘tepals’; 3.