Any of several species of plant grown for its acrid seeds and leaves, which are called MUSTARD GREENS. The MUSTARD plant belongs to the same family as broccoli, Brussels sprouts, collards, kale and kohlrabi. There are two major types of mustard seed-white (or yellow) and brown (or Asian). A third species, the black mustard seed, has been replaced for most purposes by the brown species because the latter can be grown and harvested more economically. Most mustard seed is ground into powder to make prepared mustard.