The turnip (Brassica rapa var. rapa) is a root vegetable commonly grown in temperate climates worldwide for its white, bulbous taproot. Small, tender, varieties are grown for human consumption, while larger varieties are grown as feed for livestock. Turnip leaves are sometimes eaten as "turnip greens". Turnips are high in Dietary Fibre, Vitamin C, Vitamin B6, Folate, Calcium, Potassium and Copper. The turnip is a vital source of vitamins and this is why it is valued. The turnip was a well-established crop in Hellenistic and Roman times, which leads to the assumption that it was brought into cultivation earlier. Pliny the Elder writes that he considered the turnip one of the most important vegetables of his day, rating it "directly after cereals or at all events after the bean, since its utility surpasses that of any other plant." Pliny praises it as a source of fodder for farm animals, and because "prevents the effects of famine" for humans (N.H. 18.34).