Melanoxerus Kainul. & B. Bremer

Kainulainen, Kent, 2021, A taxonomic revision of Melanoxerus (Rubiaceae), with descriptions of three new species of trees from Madagascar, Candollea 76 (1), pp. 105-116 : 106-107

Melanoxerus Kainul. & B. Bremer in Taxon 63: 828. 2014.

Typus: Melanoxerus suavissimus ( Homolle ex Cavaco) Kainul. & B. Bremer ( Gardenia suavissima Homolle ex Cavaco.  Euclinia suavissima ( Homolle ex Cavaco) J.- F. Leroy).

Deciduous or semi-deciduous shrubs or trees up to 26 m tall. Sympodial plagiotropic branching, usually with much reduced distal shoot internodes, terminally clustered leaves, and scarious stipules covering the terminal buds. Leaves opposite, petiolate, glossy, papyraceous to subcoriaceous, cordiform to obovate, drying dark brown to black. Stipules triangular, caducous. Flowers bisexual. Inflorescences terminal, with few or solitary flowers. Calyces shortly tubular; lobes narrowly triangular, caducous. Corollas funnelform or campanulate to broadly urceolate; lobes contorted in bud, overlapping to the left, spreading horizontally at anthesis. Styles club-shaped, ribbed, with secondary pollen presentation in the distal end. Pollen dispersed in tetrads. Fruits fleshy-indehiscent, globose to ovoid, with a thick mesocarp and an exocarp covered with conspicuous lenticels. Seeds flattened, irregularly angled, horizontally inserted and immersed in soft pulp.

Distribution, habitat and ecology. – Melanoxerus is endemic to Madagascar and distributed across most of the southern and western regions (i.e. Anosy, Androy, Atsimo-Andrefana, Menabe, Melaky, Boeny, and Sofia Regions), as well as in northernmost Madagascar in the DIANA and SAVA Regions. Melanoxerus occurs in vegetation types ranging from subarid thickets to subhumid semi-deciduous forests, from sea level to about 800 m in elevation, and on various substrates from coastal sands to laterite, including skeletal soils in karstic limestone formations (tsingy). It is absent from the Central Highlands and the humid east coast. Although there are exceptions, in general the species distributions reflect the ecoregions of Madagascar (cf. OLSON et al., 2001), with M. antsirananensis being found in the dry deciduous forests of the north; M. atropurpureus in the inland dry deciduous forests of the west; M. maritimus in dry deciduous forest on coastal sands; and M. suavissimus in the dry spiny thicket and succulent woodlands of the southwest ( Fig. 1 ).

Little is known about the ecology of this genus, and no information on pollination or seed dispersal has been recorded for the specimens that I have examined. The flowers are large and ± fleshy, with funnelform to broadly urceolate corollas that are externally white (sometimes tinged purple or pale green) and internally purple or white with green, red, or purple markings. Pollen is usually deposited on the upper part of the style (secondary pollen presentation), which is club-shaped with longitudinal ridges. The flowers are strongly fragrant with a sweet smell like jasmine.

The fruits are large, indehiscent “berries” with a fleshy pulp and are most likely an adaptation to endozoochory. The seeds are presumably dispersed by lemurs, but it is possible that seed dispersal, at least in part, has relied on the recently extinct Malagasy megafauna, as has been suggested for Adansonia L. ( BAUM, 1995) – a genus which also has large indehiscent fruits, and a distribution in Madagascar that is similar to that of Melanoxerus . The leaves of Melanoxerus appear to be attractive to herbivores because in the specimens seen in this study the leaves are usually damaged. Although not documented, insects are probably responsible for part of the damages, although slugs (pers. obs.) and lemurs of the genus Propithecus (information on the label of Meyers & Boltz 110) have been observed feeding on leaves of M. antsirananensis .

Key to the species of Melanoxerus

1. Leaves broadly lanceolate to obovate; flowers solitary or not; corolla tube campanulate to broadly urceolate; corolla throat glabrous; mature fruits with lenticels 1–2.5 mm in diam. ............................................................................ 2

1a. Leaves cordiform or ovate to lanceolate; flowers solitary; corolla tube funnelform; corolla throat pubescent; mature fruits with lenticels 0.1–1 mm in diam. ....................... 3

2. Leaves obovate, 7–18 × 4.5–8.5 cm; corolla throat white with red, purple or violet (and sometimes also green) spots ............................................ 1. M. antsirananensis

2a. Leaves broadly lanceolate, 10–30 × 6.5–14.5 cm; corolla throat uniformly purple to violet ... 2. M. atropurpureus

3. Leaves broadly ovate to cordiform; leaf length/width ratio 4:3 to 1:1; corolla lobes rounded to indented ............................................................. 3. M. maritimus

3a. Leaves ovate to lanceolate; leaf length/width ratio 3:1 to 3:2; corolla lobes acute ...................... 4. M. suavissimus

Figures

Fig.1.– Distribution of Melanoxerus antsirananensis Kainul. (yellow circles), M. atropurpureus Kainul. (pink circles), M. maritimus Kainul. (triangles), and M. suavissimus (Homolle ex Cavaco) Kainul. & B. Bremer (stars) in Madagascar.[mapped on the bioclimatic zones of Madagascar (after CORNET, 1974; see SCHATZ, 2000)]