Memecylon potamicum R.D. Stone, 2020

Stone, Robert Douglas, 2020, New species of Memecylon (Melastomataceae) from Madagascar: treasures of the TEF Herbarium, Candollea 75 (2), pp. 219-239 : 232

Memecylon potamicum R.D. Stone , sp. nov. ( Fig. 7 ).

Holotypus: MADAGASCAR. Reg. Anosy [Prov. Toliara]: lieu de récolte Antamenaka, abord de la RN XI [parcel 1 of Andohahela National Park], bord de la rivière d’Emanara, 15.II.1965, fl., Service Forestier 25544 ( TEF!) .

Affinis Memecylon louveliano H. Perrier sed ab eo internodiis brevibus plerumque 0.8– 1.5 cm (non plusminusve 2.5 cm) longis, lamina foliari anguste elliptica 4.3–5.4 × 1.1–1.7 cm (non 5.5–7.5 × 2.5–3.5 cm) in sicco supra laete viridi subtus pallidiore (non obscure viridi vel nigricante) atque floribus minoribus hypantho-calyce 1–1.5 × 2 mm (non c. 2.5 × 2.5 mm) petalis c. 2 × 1.5 mm (non c. 2.8 × 2 mm) differt.

Evergreen tree; young branchlets subquadrangular, bark grayish white, with age becoming terete and longitudinally fissured; nodes thickened; internodes mostly 0.8–1.5 cm long (rarely to 2.5 cm). Leaves subcoriaceous, brittle when dry (fibrous sclereids very few or very short?), bright green on the adaxial surface, paler abaxially; petioles c. 3 mm long; blades narrowly elliptic, 4.3 –5.4 × 1.1– 1.7 cm, base cuneate, apex obtusely acuminate, acumen 6(–8) mm long; midnerve finely impressed adaxially, somewhat prominent abaxially; transverse veins ± visible, subprominent on both surfaces (especially beneath), oriented at an oblique angle relative to the midnerve, joined with the faint lateral nerves 1 mm from the margin. Cymes 0.5 cm long, glomerulate at the nodes below the leaves, grouping up to 6(–10) flowers per node; peduncle absent or to 1.5 mm long; axes contracted, marked by several pairs of squamiform, imbricate, ± persistent bracts. Flowers on slender pedicels 2–4 mm long; hypantho-calyx obconic to cupulopatellate, 1– 1.5 × 2 mm, margin shallowly sinuate-dentate; corolla before anthesis broadly conical to rounded-apiculate, c. 1.5 mm high; petals ± triangular, c. 2 × 1.5 mm, base auriculate above the short claw, apex acuminate-acute, midnerve visible as a dark line on the exterior (abaxial) side; stamens dolabriform, anthers 1.5 mm long, thecae fronto-ventral; connective dorsally incurved by the median oil-gland, posterior extremity acute, filaments 2.5–3 mm long; epigynous chamber shallow but with 8 membranous interstaminal partitions with additional lines beneath the scars of the petals and filaments; style 4.5 mm long. Fruit unknown.

Etymology. – The epithet potamicum is an adjective derived from Greek word potamos meaning “river” (in reference to the habitat of this plant).

Distribution and ecology. – The type and only known collection of M. potamicum is from southeastern Madagascar, at the extreme southern end of the central plateau ( Fig. 2 ). This locality is in the vicinity (or on the outskirts) of what is now parcel 1 of the Andohahela National Park. The habitat was evidently in riverine or gallery forest.

Conservation status. – Memecylon potamicum is known from a single location with an AOO of 4 km ². The species has not been seen for more than 50 years, but is presumed extant because the collecting locality was evidently near or adjacent to a large protected area (parcel 1 of Andohahela National Park). Lacking current data on population status and threats, M. potamicum is provisionally assessed as “Vulnerable” [VU D2] in accordance with IUCN Red List Categories and Criteria ( IUCN, 2012).

Notes. – Memecylon potamicum might be confused with M. louvelianum (a widespread and somewhat variable species of which there are several collections from the same vicinity, i.e., in or near the Andohahela National Park). The two species are evidently closely related, and one could consider the possibility that the material described here as M. potamicum is nothing more than a (rheophytic?) form of M. louvelianum with relatively small, narrow, obtusely acuminate leaves. However, M. potamicum is further distinguished by several other characters including its shorter internodes, leaves bright green on the adaxial surface in dried material (vs. dark green to blackish in M. louvelianum ), and smaller flowers.

Figures

Fig. 7. – Memecylon potamicum R.D. Stone. A. Flowering branches; B. Leaf; C. Floral bud; D. Open flower; E. Petal; F. Stamen.

Fig. 2. – Geographic distribution of Memecylon complanatum R.D. Stone (closed circle), M. convergens R.D. Stone (closed diamonds), M. fofifa R.D. Stone (open circles), M. longipetiolatum R.D. Stone (open square), M. paraxenum R.D. Stone (closed star), M. potamicum R.D. Stone (closed triangle), M. pseudogaleatum R.D. Stone (closed cross), M. subchartaceum R.D. Stone (open stars), and M. unguiculare R.D. Stone (closed square), mapped on the bioclimatic zones of Madagascar (after CORNET, 1974; see SCHATZ, 2000).