Scientific name: Exacum zygomorpha English name: Vietnamese name: Other name:
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 x 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 x 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.