Description from Flora of China
Antidesma acutisepalum Hayata; A. ambiguum Pax & K. Hoffmann; A. delicatulum Hutchinson; A. filipes Handel-Mazzetti; ?A. gracillimum Gage; A. hiiranense Hayata; A. japonicum var. acutisepalum (Hayata) Hurusawa; A. japonicum var. densiflorum Hurusawa; A. neriifolium Pax & K. Hoffmann; A. pentandrum Merrill var. hiiranense (Hayata) Hurusawa.
Shrubs or small trees 2-8 m tall; young twigs delicate, pubescent when young, glabrescent. Stipules linear, 2-5 × 0.7-1 mm, caducous; petiole 2-10 mm, pubescent to glabrous; leaf blade elliptic or oblong-elliptic to oblong-lanceolate, rarely obovate, 3.5-13 × (1-)1.5-4.5 cm, papery to subleathery, glabrous except for midvein sometimes pubescent, shiny and drying light olive to grayish green on both sides, base acute, obtuse, or rounded, apex acuminate or acute, sometimes caudate, mucronulate; midvein impressed or flat adaxially, lateral veins 5-12 pairs, tertiary veins weakly percurrent to reticulate. Inflorescences terminal or axillary, slender, lax-flowered, 2-6 cm, fruiting 4-10 cm, males unbranched to 6-branched, females unbranched or 2-branched at most, axes glabrous to puberulent. Male flowers: pedicels (0.5-)1-1.5 mm, fruiting (2-)3-6 mm, puberulent to glabrous; sepals 3-5, free or nearly so, 0.4-0.7 mm, ovate to triangular, glabrous; disk cushion-shaped, fully or partially enclosing bases of filaments and pistillode, glabrous; stamens 3-5, 1-2 mm; rudimentary ovary absent, flat, clavate, cylindric, or 3-fid. Female flowers: calyx as in male; disk glabrous; ovary glabrous; stigmas 2-5. Drupes ellipsoid, laterally compressed, 5-6(-8) × 4-6(-8) mm; style (sub)terminal. Fl. Apr-Aug, fr. Jun-Sep.
The seeds yield up to 48% oil.
Antidesma ambiguum is a new synonym here. In the key accompanying the protologue, the two species are solely distinguished by pubescent vs. glabrescent young twigs; both of these occur in A. japonicum.
The concept of Antidesma pentandrum (here treated as a synonym of A. montanum ) in some respects bridges the gap between A. japonicum and A. montanum. Further studies in the northern geographic range of the genus might show the need to subsume A. japonicum under A. montanum.
Sometimes 1 or 2 staminodes may be found in female flowers. This occurs in some other species of Antidesma but is very rare.
The Chinese material all belongs to Antidesma japonicum var. japonicum ; A. japonicum var. robustius Airy Shaw differs most obviously by the midrib which is distinctly raised adaxially; it is endemic to Thailand.
Open forests in humid valleys, rarely in scrub on limestone; 300-1700 m. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hubei, Hunan, Jiangsu, Jiangxi, Qinghai, Sichuan, Taiwan, Xizang, Yunnan, Zhejiang [Japan, Malaysia, Thailand, Vietnam].