Description: Perennial, aquatic herbs. rhizomes thick, woody, densely covered with fibres formed from the bases of the vascular bundles of otherwise decayed leaves, bearing simple roots which frequently terminate in storage tubers. Stems simple, axillary within the leaf tufts.
Leaves radical, arising in tufts from the rhizome, more or less linear, flattened distally, thickened and spongy towards the base, emergent portions erect to floating.
Inflorescence a terminal raceme, few to many flowers, usualy dense to spike-like. Pedicels shorter than the flowers, persistent, spreading to upcurved in fruit. Flowers small, bisexual; perianth segments 6, in 2 similar alternate rows; stamens 6; ovules one per carpel.
Fruiting carpels follicle-like, indehiscent, 1-seeded; fruits falling at maturity. Seed narrow-elliptic, basal and erect.
Distribution and occurrence: World: 8 species; Australia 8 species (native) all States.
Text by Louisa Murray Feb 2017
Taxon concept: S. von Mering & J.W. Kadereit in O. Seberg et al. (eds), Diversity, Phylogeny, and Evolution in the Monocotyledons: 73 (2010); following Australian Plant Census (CHAH 2014) Aston, Helen I. (1995) A revision of the tuberous-rooted species of Triglochin L. (Juncaginaceae) in Australia. Muelleria 8(3):331-364.
| ||Key to the species|| |
|1||Tubers small, mostly 4.5–13 mm long, often nearly globular, clustered closely beneath the rhizome; fruits 7–9.6 mm long, very broadly obovoid with base contracted and stalk-like; mature carpels usually 6, ventrally attached; dorsal surface of mature capel more or less flat, shallowly indented, or shallowly rounded, never keeled and ridged||Cycnogeton microtuberosum|
|Tubers mostly larger, ellipsoid to narrowly so or long cylindrical, distanced from rhizome; fruits not as above; mature carpels keeled or ridged (except in Cycnogeton dubium)||2|
|2||Mature carpels free||Cycnogeton dubium|
|Mature carpels ventrally attached along at least one-fifth of their length (includes beak)|
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|3||Leaves linear, usually submerged and isolateral, thin textured; leaf sheaths narrow, mostly tightly inrolled with the width of each usually less than one quarter of the leaf width; plant of clear flowing streams||Cycnogeton rheophilum|
|Leaves linear-tapered, emergent, dorsiventral, spongy in texture; leaf sheaths broader, gradually incurved, the width of each usually one third or more of the leaf width, both sheaths overlapping|
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|4||Fruits small, usually 3–5 mm long and globular in outline or to 8.5 mm long and more ellipsoid, c. 230–1000 per infrutescence, tightly touching, 14–27 on each centimetre of rachis length; mature carpels each with a prominent narrowly obtuse dorsal keel and 2 prominent lateral shoulder keels||Cycnogeton multifructum|
|Fruits not as above, c. 1–320 per infrutescence, mostly loosely or not touching, 2.6–11 on each centimetre of rachis length; mature carpels each with variably prominent to near absent dorsal and lateral keels|
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