Homollea Arènes

Block, Petra De, 2018, Revision of the Madagascan endemic Homollea (Rubiaceae - Pavetteae), with description of two new species, European Journal of Taxonomy 423, pp. 1-24 : 4-5

Homollea Arènes

Notulae Systematicae 16: 15, f. 3 ( Arènes 1960).

Type species

Homollea longiflora Arènes.

Description

Shrubs or small trees; vegetative and reproductive parts glabrous or pubescent (externally); young shoots quadrangular with concave sides or flattened and bisulcate (when dry). Leaves decussate, shortly petiolate; petioles canaliculate above; blades coriaceous or subcoriaceous; margin (somewhat) revolute when dry; domatia present or absent; venation brochidodromous; midrib somewhat impressed above (at the leaf base), raised below; secondary veins somewhat raised above, raised below; higher order veins somewhat raised above; stipules interpetiolar, sheathing, sheats keeled, basally fused into a cone (1–5 mm long) with upper free parts (1–5 mm long) triangular with acute tips or ovate with acuminate tips, adaxial surface glabrous but with a continuous row of large (1–1.5 mm long) colleters, sometimes interspaced with a few hairs, at the base. Inflorescences pseudo-axillary, pedunculate, cymose, compact, consisting of 1–12 flowers; peduncle, axes, pedicels and bracts green; peduncle 0.5–13.5 cm long, quadrangular with concave sides or flattened and bisulcate (when dry); first order bracts with stipular parts consisting of small sheaths and foliar parts consisting of modified leaves; higher order bracts absent or strongly reduced; bracteoles absent. Flowers sessile or shortly pedicellate, 5-merous, hermaphrodite; calyx well developed, green; calyx tube 1–2.5 × 1.5–2.5 mm; calyx lobes much longer than tube, linear to narrowly spathuliform; corolla narrowly tubular (1–1.5 mm wide) but widening in the upper 5 mm below the throat, white; corolla lobes contorted to the left in bud and spreading at anthesis; stamens sessile, inserted in the corolla tube somewhat below the level of the throat, included for 1–4 mm in the corolla tube at anthesis; anthers linear, basimedifixed, sagittate, with sterile apical appendix to ca 0.5 mm long; disc annular, fleshy, glabrous; ovary cupular, 1–2 × 1.5–2 mm, bilocular; placentation axile; placentas peltate, attached at or somewhat below the middle of the septum, with 2–7 ovules arising from the upper margin of the placenta; style and stigma white; style slender, glabrous; stigma partly exserted at anthesis, slender; stigmatic lobes fused over their entire length, papillae situated along the lines of fusion of the stigmatic lobes; secondary pollen presentation present. Fruits drupaceous, spherical or subspherical, usually somewhat wider than high (not including calyx), not exceeding 1 cm in diameter, glossy and often strongly wrinkled in dried state, with persistent calyx, colour unknown (indicated as green on collector labels, only once suggested to be yellow or orange at maturity); exocarp glabrous or pubescent; mesocarp with well-developed vascular bundles; endocarp forming 2 hemispherical or hemi-ovate pyrenes which remain joined adaxially, either thin and crustaceous and without opening mechanism, or stony and breaking up along 4 preformed germination slits, rarely with the formation of separate stony dispersal units each including a single seed; seeds 1–3 per pyrene, (1–)2–6 per fruit, laterally compressed, reddish brown, hilar cavity shallow, elongated to linear; endosperm entire; embryo with foliaceous cotyledons; radicle inferior. Seed-coat thickened along the outer tangential wall and the upper parts of the radial walls; thickenings continuous but for a narrow ring-like intrusion from the cell lumen, visible in longitudinal section as two narrow canals running down from the outer edges of the cells; exotesta cells somewhat elongated around the hilum, but thickenings not taking part in the elongation; cell lumina filled with tannins ( De Block 1997; see also Bridson & Robbrecht 1985: f. 8, depicted for Paracephaelis trichantha (Baker) De Block , as Tarenna trichantha (Baker) Bremek. )). Pollen grains 3-zonocolporate; exine microreticulate to perforate, microgemmae present.

Distribution

Homollea is a genus of five species endemic to northern (Diana and Sava Regions, Antsiranana Province) and western Madagascar (Betsiboka, Boeny, Bongolava and Melaky Regions, Mahajanga Province). The species occur in lowland dry semideciduous or deciduous forest, on sandy soil or on limestone.

Identification key to the species of Homollea

1. Plants glabrous (young shoots, stipules and leaves, inflorescence, ovary, calyx, corolla and fruit); calyx tube glabrous inside ................................................................................................................ 2

– Plants moderately to densely pubescent (young shoots, stipules and leaves, inflorescence, ovary, calyx, corolla and fruit); calyx tube pubescent inside ...................................................................... 4

2. Bark of older shoots fawn or pale beige ............................................................... H. leandrii Arènes

– Bark of older shoots reddish brown .................................................................................................. 3

3. Leaves narrowly elliptic or narrowly ovate, (5.5–)7–12 × (1.3 –)1.7– 3(–4) cm; domatia often absent; inflorescence 3–12-flowered; blades of first order bracts linear, narrowly elliptic, narrowly ovate or, more rarely, ovate, with attenuate bases and petioles 3–6 mm long; pedicels 2–8 mm long; calyx lobes not ciliate at the base outside; secondary veins 7–12 on each side of midrib .. H. longiflora Arènes

– Leaves broadly obovate or broadly elliptic, rarely obovate, elliptic or ovate, 4–7 × 2.5–4 cm; ciliate pit domatia present; inflorescences 1–3-flowered; blades of first order bracts broadly ovate, with cordate or truncate bases and petioles 0–1 mm long; pedicels to 2 mm long; calyx lobes sparsely ciliate at the base outside (trichomes usually absent in fruiting stage); secondary veins 4–6(–7) on each side of midrib ................................................................................ H. furtiva De Block sp. nov.

4. Bark of older shoots fawn, greyish or dull brown, never reddish brown; young shoots, leaves and inflorescence parts pubescent with trichomes up to 0.5 mm long; leaves narrowly elliptic, narrowly obovate, linear or, more rarely, elliptic or obovate; first order bracts with shortly petiolate, linear, elliptic or ovate blades with cuneate to attenuate base; calyx lobes 5–8(–10) mm; corolla tube 15 –20 mm long; corolla lobes 8–11 mm long; 2–3 ovules per locule ................ H. perrieri Arènes

– Bark of older shoots reddish brown; young shoots, leaves and inflorescence parts pubescent with trichomes (0.3–) 0.5–1.5 mm long; leaves elliptic or, more rarely, ovate or obovate; first order bracts with sessile or subsessile, broadly ovate or ovate blades with cordate or truncate base; calyx lobes (9 –12–) 17–25 mm long; corolla tube 20–27 mm long; corolla lobes 13–18 mm long; 4–6 ovules per locule ................................................................................. H. septentrionalis De Block sp. nov.