mustard, common name for the Cruciferae, a large family chiefly of herbs of north temperate regions. The easily distinguished flowers of the Cruciferae have four petals arranged diagonally ("cruciform") and alternating with the four sepals. Most of the nearly 50 genera indigenous to the United States are found in the West. The family includes numerous weeds and wildflowers, e.g., peppergrass, toothwort, and shepherd's-purse. The Cruciferae, often rich in sulfur compounds and in vitamin C, include important food and condiment plants, many cultivated from ancient times. Especially important are the herbs of the genus Brassica, e.g., rape, rutabaga, turnip, mustard, and numerous varieties of the cabbage species. Cress, watercress, horse-radish, and radish are also of this family. A few species are cultivated as ornamentals, e.g., candytuft, rose of Jericho, wallflower, and types of stock, rocket, and alyssum. Woad was formerly an important dye source. The herbs of the family that are called mustard are species of Brassica native to Europe and W Asia. Most important commercially are the black mustard ( B. nigra ) and white mustard ( B. alba ). These are yellow-flowered annuals naturalized in the United States; the black mustard is often a weed infesting grainfields, as is also the charlock, or wild mustard ( B. arvensis ). The black and the white mustard resemble each other and are used more or less similarly. They are cultivated for the seeds, which are ground and used as a condiment, usually mixed to a paste with vinegar or oil, sometimes with spices or with an admixture of starch to reduce the pungency. (The pungency of mustard does not develop until it is moistened.) Mustards are also grown as salad plants and for greens, as are the Indian, or leaf, mustard ( B. juncea ) and the Chinese mustard, or bok-choi ( B. chinensis ). The white mustard is used in some places as forage for sheep and as green manure. Black mustard seeds are more pungent than the white and yield a yellowish, biting oil (mustard oil) that has also been useful in medicine. Mustard is classified in the divison Magnoliophyta, class Magnoliopsida, order Capparales, family Cruciferae.
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