Helictosperma, Block & Rakotonasolo & Ntore & Sylvain G. Razafimandimbison & Janssens, 2018

Block, Petra De, Rakotonasolo, Franck, Ntore, Salvator, Sylvain G. Razafimandimbison, & Janssens, Steven, 2018, Four new endemic genera of Rubiaceae (Pavetteae) from Madagascar represent multiple radiations into drylands, PhytoKeys 99, pp. 1-66 : 1

Helictosperma gen. nov.

Diagnosis.

Differing from Exallosperma by the shorter calyx lobes (3-9 mm vs. 12-16 mm long), the shorter corolla tubes (0.7-1.4 cm vs. 2.7-3.6 cm long), the completely exserted anthers at anthesis (vs. included in the corolla tube except for the tips), the pollen with microreticulate to perforate tectum (vs. psilate tectum), the fruits containing a single stony pyrene that opens into four valves, and the single seed that is rolled-in on itself like a giant pill-millipede (vs. fruits containing 2 hemi-ovoid pyrenes not opening into 4 valves, each with 1 laterally flattened, bean-shaped seed).

Type species.

Helictosperma malacophylla (Drake) De Block

Shrubs or small trees, with Terminalia -branching pattern, branching modules consisting of a long-shoot, horizontal in orientation, never bearing inflorescences and relatively smooth, and an inflorescence-bearing short-shoot with short internodes, erect in orientation, densely beset with corky stipular remnants and alternating vegetative and reproductive nodes; vegetative parts glabrous or pubescent. Leaves grouped terminally on short-shoots, deciduous, petiolate with petioles long, slender and canaliculate above; blades papyraceous; domatia present; margins not revolute; bases rounded, subcordate, cordate or unequal, more rarely truncate or obtuse. Stipules keeled, with a dense row of large colleters interspaced with hairs at the base but otherwise glabrous on the inner surface, dimorphic: in vegetative nodes consisting of truncate or triangular sheaths forming a cone and topped by needle-like awns, in inflorescence-bearing nodes consisting of ovate sheaths with acute or shortly acuminate tips. Inflorescences seemingly terminal but actually pseudo-axillary on erect short-shoots, pedunculate, pauci- or multiflorous, cymose with trichotomous branching; all parts (axes, bracts, bracteoles, pedicels) glabrous or pubescent; bracts and bracteoles well-developed, linear. Flowers hermaphroditic, pentamerous, pedicellate; all parts (ovary, calyx, corolla) glabrous or pubescent outside; secondary pollen presentation present. Calyx well-developed; tube short; lobes much longer than tube. Corolla white, turning yellowish with age; tube narrowly cylindrical; lobes contorted to the left in bud and spreading at anthesis, oblong, with blunt and emarginate tips. Stamens inserted in the sinuses of the corolla lobes at the level of the throat; filaments short; anthers completely exserted from the corolla tube at anthesis, basifixed, with sagittate base and short sterile apical appendix. Disc annular, fleshy, glabrous. Ovary cup-shaped, bilocular; placentation axile, with 3 ovules arising on top of a small placenta attached to the lower half of the septum. Style and stigma exserted from the corolla tube at anthesis; stigmatic lobes fused over their entire length except for the very tips, receptive zone on the adaxial surfaces of the free tips and along the lines of fusion of the lobes. Fruits drupaceous, spherical, pubescent or glabrous, crowned by the persistent calyx, containing 1 pyrene; pyrene crustaceous, spherical, formed by the outer convex parts of the two locules (the septum remaining membraneous and pushed to the side by the developing seed), opening along 4 preformed longitudinal germination slits of which 2 run down the margins of the locules and 2 are perpendicular to those, containing 1 seed; seed spherical, rolled-in on itself in the shape of a giant pill-millipede; hilum ovate, profound, moderate annulus around hilum present; exotesta cells with continuous plate-like thickenings along the outer tangential and upper parts of the radial walls, annulus formed by strongly elongated exotesta cells; endotesta consisting of crushed cell layers with many crystals; endosperm entire. Pollen grains 3-zonocolporate, exine microreticulate to perforate, supratectal elements absent.

A genus with 2 species, endemic to western and northern Madagascar, occurring on calcareous soil.

Etymology.

The genus is named for the shape of the seeds, which are rolled-in on themselves in the shape of giant pill-millipedes.

Key to the species of Helictosperma

1	Vegetative and reproductive parts densely covered with erect or spreading hairs; inflorescences consisting of 25-90 flowers, 2.5-8 × 2-7 cm; calyx lobes 3-5 × 1-1.5 mm; corolla tube pubescent outside; corolla lobes with ciliate margins	H. malacophylla
-	Vegetative and reproductive parts usually glabrous but, if pubescent, then hairs appressed; inflorescences consisting of (1-)5-15(-20) flowers, up to 3 × 2 cm; calyx lobes 7-9 × 1.5-2.5 mm; corolla tube glabrous outside; corolla lobes without ciliate margins	H. poissoniana