Root vegetables are underground plant parts eaten by humans as food. Although botany distinguishes true roots (such as taproots and tuberous roots) from non-roots (such as bulbs, corms, rhizomes, and tubers, although some contain both hypocotyl and taproot tissue), the term "root vegetable" is applied to all these types in agricultural and culinary usage. Root vegetables are generally storage organs, enlarged to store energy in the form of carbohydrates. They differ in the concentration and the balance among starches, sugars, and other types of carbohydrate. Of particular economic importance are those with a high carbohydrate concentration in the form of starch; starchy root vegetables are important staple foods, particularly in tropical regions, overshadowing cereals throughout much of Central Africa, West Africa and Oceania, where they are used directly or mashed to make foods such as fufu or poi. Many root vegetables keep well in root cellars, lasting several months. This is one way of storing food for use long after harvest, which is especially important in nontropical latitudes, where winter is traditionally a time of little to no harvesting. There are also season extension methods that can extend the harvest throughout the winter, mostly through the use of polytunnels.