Hop (Hamulus lupulus), the only representative of a genus belonging to the Cannabinaceee, the same group as the hemp, is a herbaceous perennial, producing annually several twining shoots, often 15 or 20 feet long. It twines contrarily to the hands of a clock, has a very rough surface, and bears opposite, stalked, 3- to 5-lobed, coarsely serrate leaves, resembling those of the vine, but with a rough surface. The male (" seeders") and female flowers are on distinct plants, but the latter only are cultivated. The female inflorescence is a catkin or strobilus, with large bracts, each having in its axil two flowers each with a bracteole. The ovary and base of the bracts are covered with a yellow powder consisting of minute glands containing wax and the bitter acid principle lupulin (C32HM07), to which hops owe their value. In brewing, hops clarify the beer, give it an agreeable bitter flavour, and render it capable of being kept by checking the acetous fermentation. Hops were first systematically grown for market in England in the 16th century. They are cultivated chiefly in Kent, Sussex, and Herefordshire.