Sow-thistle, in Botany, a genus of plants belonging to the class of syngenesia, and to the order of polygamia aquilis. There are 13 species; four of these are natives of Britain.—1. Palustris, marsh sow-thistle. The stem is erect, from six to ten feet high, branched and hairy towards the top: the leaves are firm, broad, half pinnated, serrated, and sharp-pointed; the lower ones sagittate at the base: the flowers are of a deep yellow, large, and dispersed on the tops of the branches: the calyx is rough. It is frequent in marshes, and flowers in July or August.—2. Arvensis, corn sow-thistle. The leaves are alternate, runcinate, and heart-shaped at the base; the root creeps under ground; the stem is three or four feet high, and branched at the top. It grows in corn-fields, and flowers in August.—3. Oleracea, common sow-thistle. The stalk is succulent, pistular, and a cubit high or more; the leaves are broad, embracing the stem, generally deeply sinuated, smooth, or prickly at the edges; the flowers are of a pale yellow, numerous, in a kind of umbel, and terminal; the calyx is smooth. It is frequent in waste places and cultivated grounds.—4. Alpinae, blue-flowered sow-thistle. The stem is erect, purplish, branched, or simple, from three to six feet high: the leaves are large, smooth, and sinuated; the extreme segment large and triangular: the flowers are blue, and grow on hairy viscid pedicels, in long spikes: the calyx is brown. This species is found in Northumberland.