Panaxsiamensis J. Wen, a new species of the ginseng genus ( Panax , Araliaceae) from northern Thailand.

We herein describe a new species, Panaxsiamensis J. Wen, from the tropical monsoon forests in northern Thailand. Panaxsiamensis is characterized by a combination of characters including horizontally elongated rhizomes with thick internodes, 3-5 whorled leaves each with 7-9 sessile and lanceolate leaflets, lanceolate bracteoles not persisting at the fruiting stage, 2-locular ovaries, and red fruits with a black top. The new species is most closely related to Panaxzingiberensis C.Y. Wu & Feng from southeastern Yunnan province of China, sharing the character of sessile leaflets, but differing in that P.siamensis has well developed, elongated rhizomes (vs. compact, ginger-like rhizomes and rootstock in P.zingiberensis ), and 7-9 leaflets (vs. (3-) 5-7 leaflets in P.zingiberensis ). We also compare Panaxsiamensis to other related Asian Panax species, including P.assamicus Banerjee, P.bipinnatifidus Seem., P.pseudoginseng Wallich, and P.vietnamensis Ha & Grushv. The new taxon is preliminarily assessed as Vulnerable (VU D2), according to the IUCN Red List criteria. A taxonomic key is provided to facilitate the identification of P.siamensis and its close allies.