"Yama-ai" (Mercurialis leiocarpa) belongs to the Euphorbiaceae (the Spurge family). It is a perennial herb that is distributed to Honshu, Shikoku, Kyushu and the Ryukyu Islands of Japan, also Taiwan, the Korean Peninsula and China. This herb grows in mountain forest edges or along streams, and can reach 30-40 cm in height. "Yama-ai" spreads by rhizomes and forms clumps. The leaves are oblong-lanceolate, serrated and arranged oppositely. It is dioecious. The peduncles are borne at the terminal axils and bloom small flowers from March to April. In Japan, the leaves and rhizomes was used for dyeing in ancient days.