Exacum zygomorpha Aver. & K.S.Nguyen, 2014

Averyanov, Leonid V., Nguyen, Khang Sinh & Nguyen, Hiep Tien, 2014, Checklist of mycoheterotrophic species of the genus Exacum (Gentianaceae) and new species, E. zigomorpha, from northern Vietnam, Phytotaxa 183 (2), pp. 108-113 : 110-111

5. Exacum zygomorpha Aver. & K.S.Nguyen , sp. nov. Figs 1–3 .

Small achlorophyllous herb with erect, slender, branched stem; leaves, opposite, scale–like; flowers hardly opening, white, asymmetric; calyx decussate, bisymmetric, 1–1.2 cm across; corolla of 4 petals, 1–1.2 cm long, zygomorphic; petals narrowly oblong, 2.5–3 mm wide, joined at the base; androecium slightly zygomorphic, one-side turned; anthers yellow, 4 × 1 mm long; ovary of 2 carpels, 1-locular, almost globular 1.5 mm in diam.; style thread-like, 8–8.5 mm long; stigma entire.

Type: — VIETNAM. Nghe An Prov., Ky Son Distr., Nam Can Municipality, Tham Hin village , eastern slopes of Phu Xai Lai Leng Mountain . Primary humid broad-leaved forest on steep mountain slopes composed of sandstone and gray shale at elev. 2000–2300 m around point 19°12’13.57”N 104°11’35.66”E. October 24, 2013, L.Averyanov, N. T.Hiep, N. S.Khang, L. M.Tuan, N. A.Trang & L. H.Dan, CPC 6183 (holotype LE!, isotype Herbarium of the Center for Plant Conservation, Hanoi).

Small achlorophyllous pure white mycoheterotrophic herb 5–8 cm tall. Roots numerous, clustering on short underground plagiotropic rhizome, fleshy, finely longitudinally wrinkled, separated by narrowing restrictions into fusiform portions 0.5–1 cm long, 1–2 mm in diam. Stem erect, slender, glabrous, branched like a simple cyme into 3 floriferous branches at the middle or in its lower portion, terete to indistinctly quadrangular, 1–2 mm in diam., indistinctly ridged, slightly swollen below nodes. Leaves numerous, opposite, scale–like, triangular, obtuse to acute, (1–)1.5–3 × 1–1.5 mm, internodes (3–)5–15(–20) mm long. Flowers terminal, solitary, 4-merous, asymmetric (zygomorphic–like), pure white, anthers dull light yellow. Calyx campanulate, slightly bisymmetric (having biaxial symmetry), 1–1.2 cm across, consist of 4 paired opposite, decussate, fleshy, spreading scales with distinct midvein, fused at the base; two outer scales narrowly triangular, acute, 5–6 × 2.5–3 mm; inner scales smaller and broader, shortly attenuate, with membranous, finely irregularly incised margin. Corolla of 4 petals, 10–12 mm long, zygomorphic; petals erect, connivent and obliquely one side turned, narrowly oblong, entire, 2.5–3 mm wide with 3 nerves, blunt or round at apex, joined on 2 mm from the base, forming globular tube 2 mm in diam., embracing ovary.Androecium slightly zygomorphic, stamens inserted at throat of corolla tube below sinus between petals, 8 mm long, shorter than petals (on 1–2 mm), all one-side turned; filaments subterete, 3.5–4 mm long; anthers 4 mm long, slightly curved, introrse, dorsifixed, sagittate, 1 mm wide at the base, narrowing to the apex, incompletely 2-locular, dehiscing by apical pores or abortive, indehiscent. Ovary of 2 carpels, 1-locular, broadly ovoid to globular 1.5 mm in diam., attenuate at the apex into distinctly bent, terete, thread-like style 8–8.5 mm long; stigma terminal, small, entire, truncate, discoid. Placentation free central with massive 4-lobed column bearing many ovules, divided to the apex into 4 short narrow sterile stalks. Fruits not seen.

Etymology: —Species name refers to distinct zygomorph–like asymmetry of its flowers.

Distribution: — Vietnam (Nghe An). Only one locality in Phu Xai Lai Leng Mountain on the border of the provinces Xiangkhouang ( Laos) and Nghe An ( Vietnam).

Ecology: —Primary humid shady broad-leaved and mixed forests (with Cunninghamia konishii ) on soils rich in humus derived from sandstone and shaly rocks at elev. 2000–2300 m, usually along ridge edges. It flowers from October–November. Very rare.

Notes: —New species has obviously near-affinities to Exacum paucisquama but it differs in its branching stem, bisymmetric (having biaxial symmetry) decussate calyx, flowers not opening widely, pure white, distinctly asymmetric flowers and a 1-celled ovary. Connivent curved petals, hanging over anthers, hardly opening, an unattractive corolla and abortive anthers lacking pollen observed in the new species as well as the development of the ovary and numerous ovules suggests the seed formation may be by apomixis without pollination. The new taxon exhibits a pronounced trend in asymmetric floral formation well-expressed in other Exacum species. Asymmetry of the flower in the new species is a result of secund bending of petals and stamens (of basically an actinomorphic corolla and androecium) on one side along the bisectrix of the perpendicular axes of the bisymmetric calyx (fig. 1I, J, 2G). The one–celled ovary of E. zygomorpha is unique in the genus. The ovary has free central placentation with 4-lobed, massive, column-like placenta divided to the apex into 4 short narrow sterile stalks (fig. 2A–F).

A blue-flowered specimen from Vietnam collected within the Hoang Lien Son Range near Sapa town in Lao Cai province (Poilane 12648) are certainly very similar to E. paucisquama while distinctly differing in having a branching stem, as well as apiculate petals and therefore probably represent a separate undescribed species. This taxon needs additional collections and observations from Hoang Lien Son mountain area to better understand this unusual plant .

type — CPC 6183 by L.Averyanov and T.Maisak.

Figures

FIGURE 1. A, B: Flowering plants. C: Opened flower, half side view. D: Flower with artificially recurved and flattened petals and stamens. E: Flowers, side views. F: Outer sepal. G: Inner sepal. H: Flower, side view with sagittal section of the calyx. I: Flowers, side view with sagittal section of the calyx and two petals removed from place of their junction into tube. J: Flower, side view, sagittal section. K: Removed and flattened petals, view from outside. L: Removed and flattened petals, view from inside. M: Flattened sepal, adaxial surface. N: Stamens, abaxial, adaxial and side views. O: Ovary with removed corolla and sagittal section of the calyx. All drawn from the

FIGURE 2. A–F: Ovary sections: A—transversal section of ovary along line 5 (designated on fig C); B—transversal section of ovary along line 5 (designated on fig. C) with ovules removed; C—longitudinal tangential ovary section along line 1 (designated on fig. B); D—longitudinal tangential ovary section along line 2 (designated on fig. B); E—sagittal radial ovary section along line 3 (designated on fig. B); F—sagittal radial ovary section along line 4 (designated on fig. B). G: Floral diagram; x and y—symmetry axes of bisymmetric (biaxial) decussate calyx; z—approximate symmetry axis and bending direction of zygomorphic corolla, stamens and style. All drawn from the type—CPC 6183 by L.Averyanov and T.Maisak.

FIGURE 3. A: Flowering plants in native habitat. B: Flower bud at the beginning of anthesis. C–E: open flowers, views from different sides. All photos from the type—CPC 6183 by Khang Sinh Nguyen, correction and design by L.Averyanov.