"Kuro-guwai" (Eleocharis kuroguwai) belongs to Cyperaceae (the Sedge family). It is a perennial herb that is distributed from the Kanto region of Honshu westward to Shikoku, Kyushu in Japan, and the Korean Peninsula. It grows in ponds and swamps and is 40 to 80 cm tall. The stem is hollow and round, with an internal wall filled with spongy material. The leaves are usually only in leaf sheaths and are attached to the base of the stem, and from July to October, small cylindrical spikes are produced at the top of the stem. The Japanese name "Kuro-guwai" comes from the tuber attached to the tip of the underground stem, which is blackish brown in color, similar to "kuwai" of Alismataceae (the Water-plantain family).