MOULY, A. (2024). Psydrax merrillii (Rubiaceae), a new combination for a Polynesian tree species. Candollea 79: 295–298. In English, English abstract.
Canthium merrillii (Setch.) Christoph. is a tree restricted to the Polynesian archipelagos of Samoa, Tonga, and Wallis and Futuna. The paleotropical genus Psydrax Gaertn., which belongs to the tribe Vanguerieae of the family Rubiaceae, comprises unarmed shrubs bearing stamens with anthers reflexed on the filaments, whereas the species of the genus Canthium Lam. are characterized by being spinescent shrubs. Canthium merrillii (Setch.) Christoph. is therefore transferred to Psydrax based on the absence of spines. A description and an illustration of the species are provided.
Submitted on June 16, 2024 – Accepted on October 21, 2024 – First published online on November 20, 2024
Introduction
Canthium merrillii (Setch.) Christoph. (Rubiaceae, Vanguerieae), originally described as Plectronia merrillii Setch. (Setchell, 1924), is a tree up to 15 m high restricted to the Polynesian archipelagos of Samoa, Tonga, and Wallis and Futuna, where it thrives in dry littoral forests to cloud forests from see level to 700 m of elevation.
Canthium Lam. is a genus characterized by its spinescent habit (Lantz & Bremer, 2004), mainly occurring in continental and insular tropical Asia and East Tropical Africa. In contrast, the species belonging to Psydrax Gaertn. are unarmed and have stamens with anthers reflexed on the filaments. Canthium merrillii is a completely unarmed tree displaying stamens as mentioned for Psydrax. Moreover, the species shows further typical characters of the Pacific Psydrax: corymbiform inflorescence, style ended in a stylar head complex, and fruits with verrucose pyrenes compressed at the insertion of the hilum (Meyer, 2017).
Whistler (2004) already made the combination in the genus Psydrax, cited the basionym and its author but without reference to its place of valid publication, which renders it not validly published under ICN Art. 41.5 (Turland et al., 2018). The valid combination is made below by giving a full and direct reference to the place of valid publication of the basionym.
Materials and methods
This contribution is mainly based on the revision of digital images of specimens kept at UC, especially those belonging to Setchell's herbarium. Collections from NOU and P were also studied, as well as digital images of specimens from AK, BISH, K, and US.
New combination
Psydrax merrillii (Setch.) Whistler ex Mouly, comb. nov. (Fig. 1).
≡ Plectronia merrillii Setch. in Pap. Dept. Marine Biol. Carnegie Inst. Wash. 20: 49. 1924. ≡ Canthium merrillii (Setch.) Christoph. in Bull. Bernice P. Bishop Mus. 128: 201. 1935. Holotypus: American Samoa. Tutuila Island: Tutuila, fl., III.1922, Setchell [leg. Babcock & Sütupe] 423 (UC [UC220154] image!; iso-: UC [UC220155, UC469852] image!).
- Psydrax merrillii (Setch.) Whistler, Rainforest Trees Samoa: 143. 2004 [comb. inval.].
Tree, up to 15 m high, branches cylindrical to laterally compressed, sometimes subquadrangular, unarmed, glabrous; bark brown-reddish then grey. Stipules 4.5–5.5 × 3.5–4 mm, triangular, connate at base, flat or slightly to deeply awned, persistent on young shoots. Leaves opposite, slightly coriaceous; lamina 8–16(–20) × 5–11(–12) cm, elliptic-ovate, base rounded to cuneate, apex obtuse, minutely mucronate; secondary nerves 5–7, slightly prominent underneath, tertiary nerves visible; domatia common, pocket-like, at the axil of secondary or tertiary nerves; petiole 0.9–1.3 cm long. Inflorescences 3–4 × 3–4 cm, axillary, paired at nodes, corymbiform, 15–30 flowers, axes thin and straight, topped by short navicular bracts, solitary or in pairs. Flowers hermaphroditic, fragrant; pedicel 2–8 mm long; hypanthium c. 2 mm in diam., turbinate, glabrous; calyx 5-lobed, glabrous outside, tube 0.4–0.8 mm long, lobes 0.5 mm long, triangular with obtuse apex; corolla cylindrical to campanulate, 5-lobed, pure white, turning yellow as it ages, tube 4–4.5 × 1.8–2.5 mm, mouth with a ring of moniliform trichomes, lobes 3.5–4 mm long, triangularlanceolate, reflexed, thick, margin slightly involute dorsally, papillose internally; stamens inserted at the corolla-lobes sinuses, exserted, filament 1.5–2 mm long, anthers 2.5–3 mm long, oblong, attached medio-basally and dorsally, reflexed; style elongated, 8–9 mm long, stigmatic head 1.5–2 mm long, cylindrical-ovoid, papillose; ovary bilocular, a single ovule per locule. Fruits 8–11 mm in diam., 3.5–4 mm thick, discoid, dark green to black when mature and fleshy in vivo, verrucose in sicco; pyrenes 2, c. 8–9.5 mm long, subcylindrical, thick, verrucose, crescent moon shaped. Seeds c. 7 mm long.
Distribution and ecology. – The plant is restricted to Upolu and Savai'i in Samoa, Tutuila in American Samoa, Tafahi in Tonga, and Futuna in Wallis and Futuna. It thrives from the dry littoral forests near the coast to the cloud forests at elevations of 5 to 700 m.
Additional specimens examined. – American Samoa. Tutuila Island: Pago Pago, 426 m, 18.III.1930, Diefenderfer 13 (BISH); s.l., VI–VII.1920, Setchell [leg. Sütupe] 518 (UC [UC215601]); West of Aoloaufou, 300 m, 1.X.1976, Whistler 3672 (US). Samoa. Savai'i Island: Gataivai (inland 1 mile), 50 m, 20.VIII.1968, Bristol 2269 (BISH); Taga, 5 m, 6.X.1931, Christophersen 2825 (BISH, P, US), 2835 (BISH); Salailua-Lataitai, 10 m, 19.X.1931, Christophersen 2860 (BISH); ibid., 17.IX.1931, Christophersen & Hume 2619 (BISH, P, US); Asau, just east of the Potlatch Lumber Mill, 20 m, 11.III.1974, Whistler 1787 (BISH). Upolu Island: Ridge above Malololelei, 700 m, 14.VIII.1929, Christophersen 242 (BISH); Vaea, 350 m, 28.VIII.1929, Christophersen 459 (BISH, P, US); near Afulilo waterfall in the Le Mafa area, 350 m, 11.II.1974, Whitsler 1571 (US); Ridge South of Mount Fao, 400 m, 2.I.1976, Whistler 3241 (US). Tonga. Tafahi Island: s.l., 530–540 m, 25.VII.1978, Buelow 1176 (AK); summit of the island, 19.XI.1987, Whistler 6255 (US); between Vai Kona and Vai Langita, 26.XI.1987, Whistler 6366 (US). Wallis and Futuna. Futuna Island: Nuku, Singave, 35 m, 1.VII.1974, Kirch s.n. (BISH); Nuku, Singave, along ridge leading to Osiiti, 30–40 m, 19.VII.1974, Kirch 160 (BISH); Mont Puke, 14.XI.2008, Munzinger et al. 5302 (K, NOU, P).
Acknowledgements
The author thanks Ana Penny at the University and Jepson Herbaria, University of California, Berkeley for providing digitized specimens.
Published by the Conservatoire et Jardin botaniques de Genève Open access article under Creative Commons Attribution Licence (CC BY 4.0)
