Mustard, in botany: A genus of plants belonging to the class of Tetradynamia, and to the order of Silicofera; and in the natural system ranged under the 39th order, Silicofera. The calyx consists of four expanding strap-shaped deciduous leaves; the unguis or base of the petals are straight; two glandules between the shorter stamens and pistil, also between the longer and the calyx. There are 17 species; the arvensis, orientalis, brasiliaca, alba, nigra, pyrenaica, pubescens, chinensis, juncea, eriocoides, allionii, hispanica, milfolia, incana, leavigata, cornua, and japonica. Three of these are natives of Britain; the alba, nigra, and arvensis.

1. The alba, or white mustard, which is generally cultivated as a salad herb for winter and spring use. This rises with a branched hairy stalk two feet high; the leaves are deeply jagged on their edges and rough. The flowers are disposed in loose spikes at the end of the branches, standing upon horizontal footstalks; they have four yellow petals in form of a cross, which are succeeded by hairy pods, that end with long, compressed, oblique beaks; the pods generally contain four white seeds.

2. The nigra, or common mustard, which is frequently found growing naturally in many parts of Britain, but is also cultivated in fields for the seed, of which the sauce called mustard is made. This rises with a branching stalk four or five feet high; the lower leaves are large, rough, and very like those of turnip; the upper leaves are smaller and less jagged. The flowers are small, yellow, and grow in spiked clusters at the end of the branches; they have four petals placed in form of a cross, and are succeeded by smooth four-cornered pods.

3. The arvensis, grows naturally on arable land in many parts of Britain. The seed of this is commonly sold under the title of Durham mustard-seed. Of this there are two varieties, if not distinct species; the one with cut, the other with entire leaves. The stalks rise two feet high; the leaves are rough; in the one they are jagged like turnip-leaves; in the other they are long and entire. The flowers are yellow; the pods are turgid, angular, and have long beaks.

Mustard, by its acrimony and pungency, stimulates the solids, and attenuates viscid juices; and hence it is deservedly recommended for exciting appetite, afflicting digestion, promoting the fluid secretions, and for the other purposes of the acid plants called antiscorbutic. It imparts its taste and smell in perfection to aqueous liquors, and by distillation with water yields an essential oil of great acrimony. To rectified spirit its seeds give out very little either of their smell or taste. Subjected to the press, they yield a considerable quantity of mild insipid oil, which is as free from acrimony as that. of almonds. They are applied as an external stimulant to benumbed or paralytic limbs; to parts affected with fixed rheumatic pains; and to the soles of the feet, in the low stage of acute diseases, for raising the pulse: in this intention, a mixture of equal parts of the powdered seeds and crumb of bread, with the addition sometimes of a little bruised garlic, are made into a cataplasm with a sufficient quantity of vinegar.