Description from Flora of China
Herbs annual, biennial, or perennial, rarely subshrubs or shrubs, often glaucous. Trichomes absent or simple. Stems erect or ascending, simple or branched, leafy or rarely leafless. Basal leaves petiolate, rosulate or not, simple, entire, dentate, lyrate-pinnatifid, or pinnatisect. Cauline leaves petiolate or sessile, base cuneate, attenuate, auriculate, sagittate, or amplexicaul, margin entire, dentate, or lobed. Racemes ebracteate, elongated in fruit. Fruiting pedicels ascending, divaricate, or reflexed. Sepals ovate or oblong, erect, ascending, or rarely spreading, base of lateral pair saccate or not. Petals yellow, rarely white or pink; blade obovate, spatulate, or rarely oblanceolate, apex obtuse or emarginate; claw distinct, subequaling or longer than sepals. Stamens 6, tetradynamous; anthers ovate or oblong, obtuse at apex. Nectar glands 4, median and lateral, rarely 2 and lateral. Ovules 4-50 per ovary. Fruit dehiscent siliques, linear or rarely oblong, terete, 4-angled, or latiseptate, sessile or shortly stipitate, segmented; valvular segment dehiscent, 4-46-seeded, longer than terminal segment, smooth or torulose, valves with a prominent midvein and obscure lateral veins; terminal segment seedless or 1(-3)-seeded; replum rounded; septum complete, translucent or opaque, veinless or with a distinct midvein; style obsolete or distinct; stigma capitate, entire or 2-lobed. Seeds uniseriate or rarely biseriate, wingless, globose or rarely oblong, plump or rarely slightly flattened; seed coat reticulate, mucilaginous or not when wetted; cotyledons conduplicate.
Brassica includes the most important vegetables and oilseed plants of the Brassicaceae, and China is the center where human selection has created numerous cultivars, more so than elsewhere in the world. Most of these were described by Liberty H. Bailey as species based primarily on minor differences in leaf morphology. In fact, Bailey (Gent. Herb. 1: 53-108. 1922; 2: 211-267. 1930; 4: 319-330. 1940) recognized 25 crop species of Brassica (including two presently assigned to Sinapis and ten as "new"), of which 23 species names clearly belong to only six species (nos. 1-5 of the present account, plus B. carinata A. Braun). Critical study of all of Bailey's types by one of the present authors (Al-Shehbaz), along with comprehensive cytological, crossing, and molecular studies conducted by numerous researchers over the past several decades, reveal that all of Bailey's "species and infraspecific taxa" clearly belong to four species: B. juncea (2n = 36), B. napus (2n = 38), B. oleracea (2n = 18), and B. rapa (2n = 20).
Cultivated forms (or taxa) with the same chromosome number are indistinguishable in fruit, seed, and flower characters, and they interbreed freely and produce fully fertile offspring. Furthermore, such forms often lose their identity outside of cultivation and become basically indistinguishable from the weedy forms of the species to which they belong. Because the Chinese Brassica are maintained only in cultivation as distinct crops and have well-established Chinese names, they have been recognized in most of Chinese floras as distinct species. However, they are best treated as varieties, just as the numerous and morphologically far more diversified forms of B. oleracea are recognized worldwide (see below). As many as 18 species of Brassica have been recognized in China, but the easternmost native range of the genus hardly reaches C Asia. On the basis of the enormous array of cultivated infraspecific taxa of B. juncea and B. rapa in China, it is evident that these two species have been domesticated there for thousands of years.
About 40 species: primarily in the Mediterranean region, especially SW Europe and NW Africa; six species in China.