Hydrilla Rich.

Scientific Description

Family Hydrocharitaceae.

Habit and leaf form. Aquatic herbs. Annual, or perennial; plants with neither basal nor terminal concentrations of leaves; stoloniferous, regenerating vegetatively by bulbil-like buds. Hydrophytic; non-marine; rooted. Leaves submerged. Heterophyllous, or not heterophyllous. Leaves minute to medium-sized; whorled (mostly); 3–8 per whorl (or rarely more); ‘herbaceous’; sessile; simple; epulvinate. Leaf blades entire; oblong, or elliptic to linear; one-veined; cross-venulate, or without cross-venules. Leaves without stipules. Axillary scales present (usually 2 per leaf, translucent, irregularly fringed). Leaf blade margins finely serrate to dentate (i.e. serrulate to denticulate). Leaves with a persistent basal meristem, and basipetal development. Leaf anatomy. Hairs absent. 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 (reaching water surface by elongation of perigone tube); with staminodes (3), or without staminodes. Male flowers solitary (shed in bud, free-floating on water surface at anthesis). Floral nectaries absent. Pollinated by water.

Inflorescence and flower features. Flowers solitary; shortly pedicellate (male flowers), or sessile (female flowers); bracteate (forming the axillary ‘spathe’ of 2 connate bracts, splitting or rupturing at apex as flower emerges); small; regular; 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present (female flowers), or absent (male flowers). Perianth with distinct calyx and corolla; 6; 2 -whorled; isomerous; different in the two whorls. Calyx 3; 1 -whorled; polysepalous; regular. Corolla 3; 1 -whorled; polypetalous; white, or red (in male flowers, transparent in female flowers). 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; shortly filantherous. Anthers dehiscing via short slits (explosive); latrorse; bisporangiate, or tetrasporangiate. Pollen shed as single grains (spherical). Fertile gynoecium present, or absent (male flowers). Gynoecium 3 carpelled. The pistil 1 celled (with 3 placentae). Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary unilocular; 1 locular (but with deeply intruding partial partitions). Styles 3; free, or partially joined; simple; apical. Stigmas dry type; adaxially papillate; Group II type. Placentation laminar-dispersed, or parietal. Ovules in the single cavity 12–100 (i.e. ‘many’); pendulous to ascending; non-arillate; orthotropous (rarely), or hemianatropous to anatropous.

Fruit and seed features. Fruit fleshy, or non-fleshy; spinose, or not spinose; indehiscent; capsular-indehiscent. Dispersal by water. Fruit 2–6 seeded. Seeds fusiform; endospermic, or non-endospermic; with starch. Cotyledons 1. Embryo straight. Testa smooth. 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 Australian Capital Territory. Northern Botanical Province and South-West Botanical Province.