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Genus Triadenum Family Hypericaceae

Description: Perennial herbs, rhizomatous, glabrous, with glandular canals or dots in various parts.

Leaves opposite, sessile, subsessile or petiolate; lamina ± broad, entire, venation pinnate, glands punctiform (on lamina and intramarginal).

Inflorescences terminal, sometimes also axillary, cymose, 2–15-flowered, or solitary flower. Flowers bisexual, tubular or campanulate at first, expanding to stellate for part of day; sepals 5, persistent; petals 5, partly imbricate or contorted, pink (often fleshy pink) or white, sometimes green-tinged, deciduous after anthesis; stamens 9, in 3 fascicles (bundles) each with 3 stamens, [or interpreted as 5 fascicles united to form apparently 3 (i.e., 2 ± 2 ± 1) with compound fascicles antisepalous]; filaments of each fascicle united for 1/5–2/3 of their length (1/5–2/3 connate); staminode fascicles (sterile stamen fascicles or fasciclodes) 3, yellow to orange, alternating with (inserted between) stamen fascicles; ovary 3-loculed; styles 3.

Fruit a septicidal 3-valved capsule, with longitudinal glandular vittae; seeds small, narrowly cylindric, carinate.


Flower
Photo J. R. Hosking

Distribution and occurrence: World: c. 6 species, mainly E Asia and eastern North America. Australia: 1 species naturalised rarely in N.S.W.

Triadenum is sometimes treated as a synonym of Hypericum. N.K.B. Robson, Studies in the genus Hypericum L. (Hypericaceae) 9. Addenda, corrigenda, keys, lists and general discussion, Phytotaxa 72: 1–111 (2012), provides reasons for Triadenum being generically distinct while molecular studies have found it is part of Hypericum, e.g. R. Ruhfel et al., Phylogeny of the clusioid clade (Malpighiales): Evidence from the plastid and mitochondrial genomes, Amer. J. Bot. 98(2): 306–325 (2011) and N.M. Nürk et al., Molecular phylogenetics and morphological evolution of St. John's wort (Hypericum; Hypericaceae), Mol. Phylogenet. Evol. 66(1): 1–16 (2013).

Text by P.G. Kodela, April 2017, based on Flora of North America Vol. 6 and Flora of China Vol. 13 (www.eFloras.org).
Taxon concept: Australian Plant Census (accessed April 2017)

One species in NSW: Triadenum japonicum

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