Wasabi is a perennial, root-like rhizome that is cylindrical in shape. A brownish-green skin covers its pale green flesh. The plant grows to about 18 in (46 cm) in height and produces leaves on long stems from the crown of the plant. As the plant ages, the leaves fall off and a rhizome, or creeping underground stem, is formed, from which new buds arise as modified stems. The modified stem is the part of the plant that is used. The highest quality wasabi, whose translated name is mountain hollyhock (also known as sawa wasabi ), thrives on cool water. It grows along the edges of cold mountain streams. When cultivated, rather than wild-crafted (harvested randomly from its natural growing places), it is grown on tree-shaded, terraced gravel beds covered by a thin layer of cool running mountain water or on artificially shaded gravel ridges formed in larger river beds. A lower quality wasabi (oka wasabi ) is grown in fields. There are two varieties of wasabi, Daruma —considered to have a more attractive appearance—and Mazuma —considered to have more heat. Wasabi is described as being "hot and fiery without burning," which changes to a sweetness that lingers in the mouth.