Hexatheca

Full name and orig. publication: Hexatheca C.B.Clarke, in A. & C.DC, Monogr. phan. 5/1: 193 (July 1883), non Sond. ex F.Muell. (1874) (Zannichelliaceae).

Etymology: From the Greek έξι, hexi = six, and ανδρος, andros = male, pertaining to the stamens. As the anthers of the two lower stamens are bithecous while those of the upper pair are monothecous, the androecium as a whole has six thecae.

Synonyms: -

Infrafamilial position: Didymocarpoideae-Trichosporeae-Didymocarpinae (Weber et al. 2013).

Description: Terrestrial shrubs or lignescent herbs. Stem erect, woody, usually unbranched. Leaves in a dense or spaced tuft at stem apex, opposite or in whorls of three, radially spreading, petiole long, short or ill-defined, lamina lanceolate to broad elliptical-acuminate. Cymes shortly or long pedunculate, with several flowers, bracteoles narrow-oblong to linear. Sepals free nearly to base, linear. Corolla white, with yellow mouth. Corolla infundibuliform, tube short, limb bilabiate, upper lip shorter, lobes rounded. Stamens 4, didynamous; filaments inserted at corolla base; anthers of the two lower stamens bithecous, those of the upper pair monothecous. Nectary absent. Ovary conical, pilose-villose; style short; stigma subbilobed. Fruit cylindrical, subcarnose, breaking irregularly into pieces.

Chromosome number: 2n = 34.

Species number: 4 (state of 2022).

Species names (given only in case of few species): (1) Hexatheca dolichopoda B.L.Burtt, (2) H. fulva C.B.Clarke, (3) H. johannis-winkleri Kraenzl., (4) H. longipedunculata S.Julia & Kiew.

Type species: Hexatheca fulva C.B.Clarke.

Distribution: Borneo (W Kalimantan, Sarawak to Sabah).

Ecology: Growing on sandstone or limestone rocks, usually with the stem held ± horizontally and with a fan of close-set leaves at the top; in erect-growing plants, however, the leaves are spaced.

Notes:
(1) The generic name refers to the curious organisation of the androecium: the anterior stamens are bithecous, the posterior ones are monothecous; thus the androecium as a whole has six thecae.
(2) Burtt (1989) treated the three species then known and referred H. minor Kraenzl. to Staurogyne (Acanthaceae). A fourth species was recently described in Julia et al. (2022).
(3) Based on the subcarnose-indehiscent or irregularly breaking fruits the genus was considered to be related to Cyrtandra by Clarke (1883) and Burtt (1963), but has to be referred to the Didymocarpoid Gesneriaceae (Wiehler & Burtt 1995) and Didymocarpoideae-Trichosporeae-Didymocarpinae, repectively (Weber et al. 2013).

Selected references: Burtt, Notes Roy. Bot. Gard. Edinburgh 46: 53-57 (1989), notes, new sp.; Julia et al., Garden's Bull. Singapore 74: 91-100 (2022), new sp.

Tax. details, cytology, and bibliography: Gesneriaceae Resource Centre (GRC). Royal Botanic Garden Edinburgh.

Page content and update responsibility/contact: Anton Weber.
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