Thalassodendron Hartog

Scientific Description

Family Cymodoceaceae.

Habit and leaf form. Herbs. Perennial; plants with neither basal nor terminal concentrations of leaves; rhizomatous (rhizome sympodial, robust, somewhat woody, with 1 or 2 erect stems arising at every fourth internode and 1–5 branched roots arising at the internode preceeding the stem-bearing internodes; stems simple or with few branches). Hydrophytic; marine; rooted. Leaves submerged; alternate (in a terminal cluster); sessile; sheathing (sheath compressed, narrowed at the base, shed with the blade). Leaf sheaths with free margins. Leaves simple; epulvinate. Leaf blades entire; linear; 13–27 -nerved; parallel-veined. Leaves ligulate (at junction of sheath and blade). Axillary scales present. Leaf blade margins more or less spinulose, with a denticulate apex. 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. Plants often viviparous, or not viviparous. Pollinated by water.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’ (paired); terminating a short lateral shoot; bracteate (bracts 4–6, enclosing the inflorescence, the inner 2 bracts differing in shape between male and female plants, the innermost bract of female inflorescence succulent); small. Perianth absent. Fertile stamens present, or absent (female plants). Androecium 2. Androecial members coherent (the two dorsally united, the anthers paired on a common filament, attached at the same height). Androecium exclusively of fertile stamens. Stamens 2. Anthers unilocular, or bilocular; tetrasporangiate; appendaged. The anther appendages apical. Pollen grains lacking exine, and dispersed in the sea as long filaments. Fertile gynoecium present, or absent (male plants). Gynoecium 2 carpelled; apocarpous; eu-apocarpous; superior. Carpel stylate; apically stigmatic; 1 ovuled. Placentation apical. Styles each forked. Ovules pendulous; non-arillate; orthotropous.

Fruit and seed features. Fruit fleshy, or non-fleshy; an aggregate. The fruiting carpel indehiscent; nucular (the ‘false fruit’ composed of the 2 fertilized ovaries, one of which usually abortive, and the fleshy inner bract). Seeds non-endospermic. Cotyledons 1.

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