The wheat plant is an annual, probably derived from a perennial; the ancestry of and precise distinctions between species are no longer always clear. For its early growth wheat thrives best in cool weather. Among the more ancient, and now less frequently cultivated, species are einkorn ( T. monococcum ), emmer ( T. dicoccum ), and spelt ( T. spelta ). Modern wheat varieties are usually classified as winter wheats (fall-planted and unusually winter hardy for grain crops) and spring wheats. Approximately three fourths of the wheat grown in the United States is winter wheat.
Flour from hard wheats (varieties evolved for the most part from T. aestivum ) contains a high percentage of gluten and is used to make bread and fine cakes. The hardest-kerneled wheat is durum ( T. durum ); its flour is primarily used in the manufacture of macaroni, spaghetti, and other pasta products. White- and soft-wheat varieties are paler and have starchy kernels; their flour is preferred for piecrust, biscuits, and breakfast foods. Wheat is used in the manufacture of whiskey and beer, and the grain, the bran (the residue from milling), and the vegetative plant parts make valuable livestock feed. Before the introduction of corn into Europe, wheat was the principal source of starch for sizing paper and cloth.