A new species similar to M. alba and M. keithii, but distinguished by its solitary flowers that are not borne in an inflorescence, its single carpel per flower and single sessile monocarp per fruit.

Thailand: In Para Rubber plantation (Hevea brasiliensis (Kunth.) Mull. Arg.), Khlong Sok Village, Phanom District, Surat Thani Province, 15 December 2015, P. Chalermglin 581215 (holotype: BKF [dry and spirit collection]; isotypes: BK, QBG, PSU, K, SING [dry]).

Treelets or small trees, to ca. 5 m, main trunk slender. Young branches glabrous to sparsely pubescent, with short, appressed golden-brown hairs. Leaf laminas 9–13 cm long, 3.5–5 cm wide, length:width ratio 2.5–3.3, ovate to elliptic, papyraceous, glabrous and ± matt adaxially, glabrous abaxially; base obtuse to slightly cuneate; apex acute to slightly acuminate; primary vein glabrous ad- and abaxially; secondary veins 6–8 pairs per leaf, ± inconspicuous, glabrous ad- and abaxially; domatia absent; petioles 3.5–5 mm long, 1.1–1.4 mm in diameter, glabrous. Inflorescence reduced to a single flower. Flower pedicel ca. 4.5 mm long, ca. 1.5 mm in diameter, pubescent. Sepals ca. 2.5 mm long, ca. 2 mm wide, broadly ovate, pubescent abaxially. Outer petals 11.5–12.5 mm long, 7–8 mm wide, cream-coloured (turning yellowish in late-stage flowers), ovate, apex acute, margin not undulating with age, pubescent abaxially, sparsely pubescent adaxially. Inner petals ca. 9.5 mm long, ca. 3 mm wide apically, ca. 2 mm wide at claw, pale pink with whitish margins and near point of apical connivance (turning yellowish in late-stage flowers), clawed-triangular, hairs non-glandular. Stamens 0.9–1 mm long, 0.7–0.8 mm wide. Carpels solitary per flower, ca. 1.4 mm long; ovary ca. 0.9 mm long, ca. 0.5 mm wide, densely pubescent; stigma ca. 0.5 mm long, ca. 0.9 mm wide; ovules ca. 9 per carpel, in two columns. Monocarp solitary per fruit, ca. 52 mm long, ca. 32 mm in diameter, ellipsoid, smooth, without longitudinal ridge; stipe absent. Fruit pedicel ca. 6 mm long, ca. 4 mm in diameter. Seeds ca. 9 per monocarp, size unknown (fruiting specimen not preserved).

Figure 1. 

Mitrephora monocarpa sp. nov. (P. Chalermglin 581215). A Habit B leaves (left: adaxial; right: abaxial) C flower bud D young flower, shortly after separation of outer petals. Scale bar: 2 cm. Photos: P. Chalermglin.

Flowering was observed in June, July and December. As with all hermaphroditic-flowered Annonaceae species (Pang and Saunders 2014), M. monocarpa is protogynous. The flowers undergo visible change associated with the switch between the pistillate and staminate function: the outer petals are held at right-angles to the floral axis in pistillate-phase flowers (Fig. 2A, B), but become reflexed during the staminate phase (Fig. 2C–E). The stamens partially abscise as the thecae dehisce and ultimately remain suspended from the floral torus by their tracheary elements (inset in Fig. 2D; cf. Endress 1985). As the flower ages, the petals of both whorls begin to turn yellow and wilt (Fig. 2F) before abscising. Fruiting was observed in December.

Figure 2. 

Mitrephora monocarpa sp. nov. (P. Chalermglin 581215). A, B Pistillate-phase flowers C–E staminate-phase flowers (insert in D shows abscised stamens suspended by tracheary elements in the xylem) F late-stage flower, with petals turning yellow. Photos: P. Chalermglin.

Mitrephora monocarpa is only known from the type collection cited above, from a tropical rain forest over limestone, ca. 250 m elevation.

The specific epithet reflects the fact that the flower has only a single carpel and, hence, the fruit consists of a solitary monocarp.

Local name: Phrom Phanom.

The phylogenetic affinities of M. monocarpa are currently unknown, although morphologically it resembles two species, M. alba and M. keithii, with which it is broadly sympatric in Peninsular Thailand: all three species have a sparsely hairy indument on the twigs and leaves and comparatively small flowers. Mitrephora monocarpa is clearly distinguished from these species, however, as the flowers have only one carpel: M. alba, in contrast, has ca. 16 carpels per flower and M. keithii has 12–14 (Weerasooriya and Saunders 2010). Carpel number is variable in other species in the genus, although the smallest number previously recorded was four (in the Indochinese species M. calcarea Diels ex Weeras. & R.M.K.Saunders, the Bornean species M. kostermansii Weeras. & R.M.K.Saunders, the Philippine species M. lanotan (Blanco) Merr. and the Sumatran species M. rufescens Ridl.; Weerasooriya and Saunders 2010).

The fruits of M. monocarpa are easily distinguished from those of M. alba and M. keithii as they consist of only one monocarp (Fig. 3A, B); this monocarp is furthermore sessile, whereas those in M. alba and M. keithii are stipitate, with stipes that are 6–15 mm and 3–3.5 mm long, respectively.

Figure 3. 

Mitrephora monocarpa sp. nov. (P. Chalermglin 581215). A Immature fruit, composed of a single monocarp B partially mature fruit, with the exocarp turning yellow C dissected fruit, showing multiple seeds. Scale bar: 1 cm. Photos: P. Chalermglin.

Mitrephora monocarpa also resembles the Bornean species M. uniflora Weeras. & R.M.K.Saunders in possessing solitary flowers (Weerasooriya and Saunders 2010), presumably as a result of the evolutionary reduction of the thyrsoid inflorescence, evident in other Mitrephora species.