Petrocodon

Full name and orig. publication: Petrocodon Hance, J. Bot. 21: 167 (June 1883).

Etymology: From the Greek πετρος, petros = rock, stone, and κωδων, kōdon = bell, "the bell of the rocks".

Synonyms: Calcareoboea C.Y.Wu ex H.W.Li (1982), Didymocarpus Wall. p.p. (excl. type), Dolicholoma D.Fang & W.T.Wang (1983), Lagarosolen W.T.Wang (1984), Paralagarosolen Y.G.Wei (2004), Tengia Hance (1883).

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

Description: Perennial rosette herbs with stout rootstock. Leaves petiolate, lamina oblong to lanceolate, or elliptical to suborbicular, margin entire or dentate, indumentum variable. Pair-flowered cymes long-pedunculate-scapose, with one, several or many flowers in subumbellate arrangement; bracteoles small, linear to elliptical, or relatively large, leaf-like and forming a kind of involucre (the former genus Calcareoboea). Sepals almost free to base, linear to lanceolate. Corolla actinomorphic (the former Tengia), or zygomorphic, funnel-shaped or urceolate-campanulate (Petrocodon in its orig. sense, the former Tengia), either with 5 nearly equal lobes, lobes triangular, acute, more rarely rounded, limb actinomorphic (the former Tengia) or bilabiate, upper lip 2-lobed, lower lip 3-lobed (most species), or (the former Calcareoboea) upper lip 4-sect and lower lip entire; colour white, pink, lilac, blue or (Calcareoboea) bright red. Stamens 5 (the former Tengia), 4 (see Notes) or 2; filaments inserted near the base, in the middle or in the upper part of the corolla tube; anthers coherent; staminodes 0, 1 or 3. Nectary annular, cupular or cyathiform. Ovary slender ovoid, elliptical or cylindrical; stigma inconspicuous, capitate or disciform, rarely 2-lobed. Capsule slender cylindrical, 2-valved, loculicidally dehiscent, or 4-valved (the former Dolicholoma, Paralagarosolen and Tengia). Seeds (where known) tubercled or verrucose.

Chromosome number: 2n = 20 (P. hancei, Cao et al. 2003).

Species number: Around 45, new species are continuously described.

Type species: Petrocodon dealbatus Hance.

Species names (only given in case of few species).

Distribution: NE Thailand (P. bonii), Laos (same sp.), S and SE China (Guangdong, Guangxi, Guizhou, Hunan, Hubei, Yunnan), N Vietnam.

Ecology: Usually growing on rocks, in rock crevices and similar habitats in shaded, moist places.

Notes:
(1) Petrococodon, described in 1883, remained for more than 120 years as a monotypic genus (see, e.g., Wang W.T. et al. 1990, 1998, Weber 2004). A second species (P. ferrugineus Y.G.Wei) was described in 2007. Based on the molecular-phylogenetic studies of Wang Y.Z. et al. (2010, 2011) and Weber et al. (2011), Petrocodon was considerably enlarged by the inclusion of five genera (Calcareoboea, Dolicholoma, Lagarosolen, Paralagarosolen, Tengia) and some odd species of Didymocarpus. In the treatment of Weber et al. (2011) c. 20 species were referred to the genus. Since then, about 25 new species have been added.
(2) By the new definition, Petrocodon became a very variable genus, which is difficult to circumscribe. All species are rosette plants, with scapose, 1- to many-flowered cymes, calyx with lobes (almost) free to base, corolla with 5-sect limb (rarely lobes rounded) and usually undivided stigma.
(3) The great diversity in symmetry, shape, size and colouration of the corolla obviously reflects adaptations to different pollinators. These are apparently various groups of insects, but with the inclusion of Calcareoboea also birds seem to be involved in pollination. With respect to pollination syndromes (different forms of melittophily, psycho- and/or sphingophily, ornithophily), Petrocodon represents one of the most varied genera of Old World Gesneriaceae.
(4) The former genus Tengia, with actinomorphic flowers and 5 fertile stamens, was formerly considered as a morphologically primitive and ancient genus (Wang W.T. et al. 1990, 1992, 1998). Burtt (1970) was the first to relate Tengia to the genus Petrocodon and, based on additional examples, to consider the actinomorphic flower symmetry as a secondary condition in Gesneriaceae. The molecular studies of Wang Y.Z. et al. (2010, 2011) and Weber (2011) are in full agreement with Burtt's opinion.
(5) Until recently, Petrocodon was only known to include – with the exception of P. scopulorum, formerly in Tengia; see Note (4) – only diandrous species. However, starting with the description of P. hunanensis (Xu et al. 2015), several species have come to knowledge in which 4 fertile stamens are present. The androecium thus is no longer a key character defining the genus.

Selected references: Burtt, Notes Roy. Bot. Gard. Edinburgh 30: 1-10 (1970), notes on actinomorphic fls.; Cao, L.M. et al., Guihaia 23(4): 331-333 (2003), chromos., Wang W.T. et al., Fl. Reipubl. Pop. Sin, vol. 69 (1990); Wang W.T. et al., Edinburgh J. Bot.49: 5-74 (1992), keys, notes on actinomorph. fls.; Wang W.T. et al. in Wu & Raven (eds.), Fl. China 18: 348 (1998); Wang Y.Z. et al., Taxon 49(4): 1044–1052 (2010), mol. syst., actinomorph. fls.; Wang Y.Z. et al., J. Syst. Evol. 49: 50–64 (2011), mol. syst.; Weber, in Kubitzki & Kadereit, Fam. Gen. Vasc. Pl., vol 7 (2004); Weber et al., Phytotaxa 23: 49–67 (2011), molec. syst., redef. of Petrocodon; Yu et al., Phytotaxa 195: 65–72 (2015), descr. of first tetrandrous species.

Illustrations: The type species (P. dealbatus) is depicted under Petrocodon - trad. sense as well as under M in the below plate. In this plate also the former generic placement of the depicted species is indicated. More illustrations can be found under the synonyms given above in blue.

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

Page content and update responsibility/contact: Anton Weber.
Last update: