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.