Differing from Helictosperma malacophylla by the pauciflorous inflorescences [(1–)5–15(–20) vs. 25–90 flowers], the larger calyx lobes (7–9 × 1.5–2.5 mm vs. 3–5 × 1–1.5 mm), the glabrous corolla tube, the corolla lobes without ciliate margins and the usually glabrous vegetative and reproductive parts, but, if pubescent, then hairs appressed (vs. erect or spreading in H. malacophylla).

MADAGASCAR. Antsiranana Province, Analamerana, along Ambatabe river, 41 m, 7 Jan 2002 (fl.), De Block, Rakotonasolo & Randriamboavonjy 1095 (holotype: BR!; isotypes: BR!, K!, MO!, P!, TAN!, UPS!).

Shrub 1.5–4 m tall, more rarely small tree to 4 m tall, dbh to 7 cm; young shoots somewhat quadrangular and bisulcate, dark brown, glabrous or sparsely to densely covered with appressed hairs; older branches brown, pale or greyish-brown. Leaves often immature at time of flowering, 3–10 × 2–6 cm, ovate, rarely elliptic or obovate; blades papyraceous, drying brown to dark brown, more rarely greenish, hardly discolorous, glabrous or with midrib and secondary nerves sparsely to densely covered with appressed hairs, more rarely also higher order nerves pubescent on the lower surface, glabrous or sparsely to moderately covered with appressed hairs on the upper surface; base rounded, subcordate, cordate or unequal, more rarely truncate or obtuse; apex acuminate, acumen 3–10(–15) mm long; hair tuft or ciliate pit domatia present, sometimes also in the axils of secondary nerves; midrib and secondary nerves raised on the lower leaf surface; midrib impressed in the basal half on the upper leaf surface; 5–8 secondary nerves on each side of the midrib. Petioles glabrous to densely covered with short appressed hairs, 5–35 mm long. Stipules caducous, glabrous or sparsely to densely covered with appressed hairs outside, but rapidly becoming corky and losing the pubescence; stipules of vegetative nodes with sheaths 1.5–2.5 mm long and awns 2–5 mm long, those of inflorescence-bearing nodes ovate with acute to shortly acuminate tips, 4–7 mm long. Inflorescences consisting of (1–)5–15(–20) flowers, up to 3 × 2 cm; peduncle, inflorescence axes and pedicels glabrous or moderately to densely covered with appressed hairs; peduncle 0.5–3.5 cm long; first order axes up to 1.2 cm long; first order bracts with stipular parts narrowly triangular and leaves long-petiolate and identical in shape and size to the vegetative leaves or somewhat smaller; second order bracts of the central axis often similar to the first order bracts but leaves considerably smaller and narrower with acute to attenuate base, more rarely identical in shape to vegetative leaves with cordate or rounded base, up to 3.5 × 2.5 cm; second order bracts of lateral axes, higher order bracts and bracteoles linear, glabrous, ciliate or sparsely to moderately covered with appressed or spreading hairs on both surfaces, no colleters present inside; bracts up to 2.2 cm long; bracteoles subopposite on the pedicel, 0.2–1.2 cm long; first order branching often shifted above the first order bracts (up to 1 cm higher); bracts often adnate to axis for up to 5 mm. Flowers pedicellate, pedicels 1–6 mm long. Calyx green; tube 0.75–1 mm long, glabrous or more rarely moderately to densely covered with appressed hairs outside, glabrous and without colleters inside; lobes erect, leaf-like, 7–9 × 1.5–2.5 mm, glabrous inside and outside but with margins ciliate or more rarely sparsely covered with appressed hairs outside (mostly in basal half or along veins), bases not overlapping but closely joining, tips acute to obtuse. Corolla tube 5–14 mm long, ca. 1 mm in diameter at the base, ca. 2 mm in diameter at the throat, glabrous outside, densely covered with erect hairs except at the base and at the throat inside; lobes 4–5 × 3–3.5 mm, glabrous on both surfaces, margins not ciliate. Anthers 3–4 mm long; filaments 1–1.5 mm long. Ovary 1–1.5 mm long, faintly ribbed longitudinally when dry, green, glabrous or more rarely moderately to densely covered with appressed hairs. Style and stigma white, exserted from the corolla tube for 4–7 mm at anthesis; style densely covered with spreading, upwardly directed hairs over the whole length except for a further 2–3 mm. Fruits 5–7 mm in diameter (persistent calyx not included), with faint longitudinal ribs, glabrous or more rarely moderately to densely covered with appressed hairs, drying blackish and glossy when ripe; seeds ca. 5 mm in diameter, dark brown.

Lowland dry deciduous and semi-deciduous forest on limestone; alt. 0–450 m.

Helictosperma poissoniana is known from the Diana Region (Antsiranana Province) and from the Boeny and Melaky Regions (Mahajanga Province). Fig. 14C.

Flowering: October–January, May; Fruiting: January–December.

Hazontaka (Rakotovao 4081-RN); maroampotatra (Rakotovao 3914-RN); pitsopitsoka (Randriamiera 6722-RN); refeko (Leandri 573); tsarepepana (dialect Antakarana; Humbert 19013); voanievitra (Rakotovao 6240-RN).

The three flowering specimens from the Tsingy de Bemaraha (Leandri 573 & 578; Jongkind 3415) have longer flowers (corolla tube 13–14 mm long) than all other specimens of this species (corolla tube 5–9 mm long). – Some specimens in the P herbarium were annotated as Tarenna poissoniana Homolle (e.g. Poisson 21). Capuron (1973) discussed this species in his unpublished treatment of the Madagascan Rubiaceae under the same name.

Near Threathened: NT. The extent of occurrence (EOO) of Helictosperma poissoniana is estimated to be 70,048 km², which exceeds the upper limits for any threat category but its area of occupancy (AOO) is 198 km², which falls within the limits for the Endangered category under the sub-criterion B2. The species occurs in seven locations and in three protected areas: Namoroka Strict Nature Reserve, Bemaraha National Park and Ankarana Special Reserve. Helictosperma poissoniana is widespread but threathened locally as a result of the reduction of its habitat through slash-and-burn agriculture, illegal logging and fires to improve grazing. Furthermore, artisanal sapphire mining in Ankarana Special Reserve is a serious problem. Based on the above observations, the species is assessed as Near Threathened.

MADAGASCAR. Antsiranana Province: Massif de l’Ankarana, 5 Nov 1990 (fl.), Bardot-Vaucoulon 238 (P); Massif de l’Ankarana, 17 Nov 1990 (fl., fr.), Bardot-Vaucoulon 303 (K, P); plateau de l’Ankarana, W de Mahamasina (Antanatsimanaja), 23 Apr 1963 (fr.), Capuron 22670-SF (BR, P, TEF); près de Marotaolana, Anivorano Nord, 4 Nov 1966 (fr.), Capuron 24543-SF (BR, P, TEF); district Ambilobe, village Ambilomagodro, km 114, montagne d’Ambohibe, grès de l’Isalo, 300 m, 8 Feb 1960 (fr.), Cours & Humbert 5705 (P); Ankarana, close to Apondrabe river, 82 m, 26 May 1999 (fr.), De Block, Rapanarivo & Randriamboavonjy 1042 (BR, G, K, MO, P, TAN, WAG); Ankarana, following the dry river Apondrabe, close to Mahamasina, 82 m, 27 May 1999 (fr.), De Block, Rapanarivo & Randriamboavonjy 1057 (BR, K, MO, P, TAN); Analamerana, along Ambatabe River, 41 m, 7 Jan 2002 (fl.), De Block, Rakotonasolo & Randriamboavonjy 1092 (BR, MO, TAN, UPS); Ankarana, near Mahamasina, perte d’eau, 82 m, 15 Jan 2002 (fr.), De Block, Rakotonasolo & Randriamboavonjy 1242 (BR, G, K, MO, TAN, WAG); Ankarana Special Reserve, c. 5 km NW of park village near Besaboba river, 90 m, 25 Apr 1993 (fr.), Harder, Merello, Razafimandimbison & Razafindrabaeza 1704 (MO, P, TAN); Diego-Suarez, Jan 1945 (fl.), Homolle 305 (P); Ambodimagodro, plateau de l’Ankarana, Dec 1938–Jan 1939, 250 m (fl.), Humbert 19013 (P); plateau de l’Analamera, 50–400 m, Jan 1938 (fl.), Humbert 19184 (P); collines et plateaux calcaires de l’Ankarana du Nord, 30–350 m, 24 Jan–29 Feb 1960 (fr.), Humbert 32468 (BR, P); collines et plateaux calcaires de l’Ankarana du Nord, colline S du jardin botanique 8, 30–350 m, 24 Jan–29 Feb 1960 (fr.), Humbert 32626 (BR, P); collines et plateaux calcaires de l’Ankarana du Nord, 30–350 m, 24 Jan–29 Feb 1960 (fr.), Humbert 32832 (P); Ankarana du Nord, Mar 1962 (fr.), Keraudren 1687 (P); Ankarana Réserve Spéciale, close to camp des Anglais, 180 m, 18 Feb 1994 (fr.), Lewis, McDonagh, Andrianarisata, Randriamabolona, Andiratsiferama & Bled 1125 (BR, K, MO, P, WAG); Réserve Spéciale d’Ankarana, Ambondromifehy, 11 Jan 2008 (fr.), Rakotonasolo 1164 (K); Mahajanga Province: Beanka, partie sud, Sarodrano, relevé linéaire B30, 429 m, 5 Mar 2012 (fr.), Bolliger, Hanitrarivo & Rakotozafy 278 (BR, G); forêt de Marohogo, près du village de Marohogo, 7 Apr 1965 (fr.), Capuron 24091-SF (BR, P, TEF); Soalala District, Réserve Naturelle Intégrale VIII, Tsingy de Namoroka, c. 40 km S of Soalala, 130 m, 3 Feb 2000 (fr.), Davis, Rakotonasolo & Wilkin 2533 (BR, K, TAN); district Antsalova, Tsingy de Bemaraha, Réserve Naturelle IX, near Ambodiria, 150 m, 17 Mar 2004 (fr.), Davis & Rakotonasolo 3122 (BR, K); forêt de Marohogo, 22 m, 13 Feb 1999 (fr.), De Block & Rakotonasolo 797 (BR, C, G, K, MO, P, TAN, TEF, WAG); forêt de Marohogo, 22 m, 13 Feb 1999 (fr.), De Block & Rakotonasolo 798 (BR, C, G, K, MO, P, TAN, WAG); environs de Majunga, 2–15 m, 28–30 Dec 1924 (fl.), Humbert 4046 (BR, P); Tsingy de Bemaraha, N of Manambolo river, 50 m, 28 Nov 1996 (fl.), Jongkind, Andriantiana & Razanatsoa 3258 (BR, K, WAG); Tsingy de Bemaraha, N of Manambolo river, 50 m, 6 Dec 1996 (fl.), Jongkind, Andriantiana & Razanatsoa 3415 (BR, K, WAG); Réserve Naturelle IX, Bemaraha, Antsingy Nord, 22 Nov 1932 (fl.), Leandri 573 (P); calcaires de l’Antsingy, vers Ambodiriana, E d’Antsalova, 100–150 m, 9 Feb 1960 (fr.), Leandri & Saboureau 3072 (BR, P); Antsingy d’Antsalova, Tsingy de Bemaraha, Réserve Naturelle IX, Jan 1975 (fr.), Morat 4837 (P, TAN); Beanka, partie nord, bord de la rivière Bokarano, 187 m, 18 Dec 2011 (fr.), Nusbaumer, Bolliger, Hanitrarivo & Rakotozafy 3202 (BR, G); environs de Majunga, May 1908 (fl.), Perrier de la Bâthie 3266 (P); Namoroka, Andranomavo, Ambongo, Oct 1905 (fl.), Perrier de la Bâthie 3634 (BR, P); environs de Majunga, May 1908 (fr.), Perrier de la Bâthie 3766 (P); Kamakama, sur le plateau de l’Ankarana, Oct 1901 (fl.), Perrier de la Bâthie 3777 (P); Majunga, 22 Dec 1904 (fr.), Poisson 21 (P); Antsalova, Réserve Naturelle Intégrale IX, Tsingy de Bemaraha, Ambodiriana, 14 Mar 2004 (fr.), Rakotonasolo, Davis & Maurin 767 (BR, K, TAN); Réserve Naturelle VIII, Tsingy de Namoroka, canton Andranomavo, district Soalala, 30 Apr 1952 (fr.), Rakotovao 3914-RN (P); Réserve Naturelle VIII, Tsingy de Namoroka, canton Andranomavo, district Soalala, 10 Jun 1952 (fr.), Rakotovao 4081-RN (P); Réserve Naturelle VIII, Tsingy de Namoroka, canton Andranomavo, district Soalala, 20 Nov 1953 (fr.), Rakotovao 5672-RN (BR, P); Ambatafolaka, Réserve Naturelle VIII, Tsingy de Namoroka, canton Andranomavo, district Soalala, 4 Feb 1954 (fr.), Rakotovao 6240-RN (BR, P, TEF); Beanka, partie centrale, Andoloposa, 358 m, 26 Mar 2012 (fr.), Rakotozafy, Bolliger & Hanitrarivo 97 (BR, G); Boeny, canton Andranomavo, district Soalala, 13 Oct 1954 (fl.), Randriamiera 6722-RN (P, TEF); Boeny, canton Andranomavo, district Soalala, 18 Jan 1955 (fr.), Randriamiera 7070-RN (BR, P, TEF); Boeny, canton Andranomavo, district Soalala, 25 Feb 1957 (fr.), Randriamiera 8771-RN (BR, P, TEF); Boeny, canton Andranomavo, district Soalala, 15 Apr 1957 (fr.), Randriamiera 8795-RN (BR, P, TEF).