The name of this family of plants has in modern times evolved into brassicaceae, which comes from the Latin word brassica, referring to cabbages and related vegetables. The alternate, older name, though, is cruciferae, which means “cross-bearing.” This describes the four petals of mustard flowers, which resemble a cross.
We Are Family: Trying to classify life, here within the vast kingdom of plants, into distinct categories has been going on for centuries. Constant adjustment of one plant or another moving between categories has been going on since the beginning of taxonomy, and indeed right to this day because there are simply no hard rules for what belongs in a plant family. Often rankings are made because certain plants can hybridize with others or because have similar physical systems (they reproduce the same way for example), but again there are no overarching rules. Often the names of families come from a prominent genus in the family, but here we instead have a struggle for a name for a family that contains some quite different plants. “Cruciferae,” as we now know, refers to the mustard plant. “Brassicaceae,” though, refers to cabbages and the many vegetables that sprang from it, and brassicaceae seems to have won the day for now. Whatever you call it, the family has 338 genera and some 3,700 species within it, and provides some of the staples of human consumption.
The family includes agricultural crops, among which many are prominent vegetables such as: cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, savoy, kohlrabi, and gai lan, turnip, napa cabbage, bomdong, bok choy and rapini, rocket salad/arugula, garden cress, watercress and radish. A few members of the family are used for spices such as horseradish, brassica, wasabi and white, Indian and black mustard (Sinapis alba, B. juncea and B. nigra respectively). The family may also provide the largest volume of vegetable oils of any species.
Plants in the mustard family are usually herbs and can be annuals, biennials, or perennials. The leaves are normally simple and alternately arranged, and many are peppery. The flowers are normally cruciform (e.g., in the form of a cross) with four petals and four sepals; the plants are commonly referred to as “crucifers” or “cruciferous” for that reason. The flowers are usually white, yellow, or lavender. The seeds are produced in dry podlike fruits, often with a partition between the halves; long thin fruits are known as siliques, and short rounded fruits are known as silicles. The tiny seeds of the mustard plant, of course, when compared to the size they grow has been well noted in history. (See here.)