The root and tuber crops are all perennials or biennials because of the storage root. The perennials contain large amounts of starch, which is their chief contribution to the diet, while the biennials contain very little. The perennials generally contribute some protein to the diet as well as starch, but this varies among species. Roots and tubers generally contain vitamin C, but only carrots and sweet potatoes contain useful amounts of vitamin A. Many roots and tubers contain toxic substances or antinutritional factors. As a general rule, it is best to cook them before eating. Roots and tubers are extremely important crops on a worldwide basis and should be included on every farm.