Sinapis L.
Sinapis is a genus of five species, native to Europe and the Mediterranean area. One species, Sinapis arvensis L., is a common native (perhaps rather archaeophytic) weed (Lambinon & Verloove 2012).
Molecular data shed new light on the genus. According to some authors (Al-Shehbaz & al. 2006) it is best considered part of a broadly circumscribed genus Brassica. Arias & Pires (2012), in turn, proposed redefining Sinapis as to include members of Brassica, including B. nigra. Pending further studies Sinapis is here treated in a conservative way.
1 Beak of fruit much flattened, 10-30 mm long, at least as long as the body of the siliqua. Siliqua usually with 4-8 seeds. Leaves deeply pinnately lobed === Sinapis alba
Beak of fruit conical or cylindrical, not distinctly flattened, 7-16 mm long, shorter than the body of the siliqua. Siliqua usually with 8-13 seeds. Leaves toothed or shallowly lobed (native) === S. arvensis
Literature:
Al-Shehbaz I. A., Beilstein M.A. & Kellogg E.A. (2006) Systematics and phylogeny of the Brassicaceae (Cruciferae): An overview. Pl. Syst. Evol. 259: 89-120. [available online at: http://www.telomereevolution.org/uploads/8/9/7/6/8976955/fulltext.pdf]
Arias T. & Pires J.C. (2012) A fully resolved chloroplast phylogeny of the brassica crops and wild relatives (Brassicaceae: Brassiceae): Novel clades and potential taxonomic implications. Taxon 61(5): 980-988. [available online at: https://www.researchgate.net/publication/259812488_A_fully_resolved_chlo...
Baillargeon G. (1986) Eine taxonomische Revision der Gattung Sinapis (Cruciferae: Brassiceae). Diss. Freie Universität Berlin.
Chater A.O. (1993) Sinapis. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 1 (2nd ed.). Cambridge University Press, Cambridge: 409-410.
Lambinon J. & Verloove F. (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Schulz O.E. (1919) Sinapis. In: Engler A. (ed.), Das Pflanzenreich 70(IV.105). Engelmann, Leipzig: 117-136. [available online at: https://www.biodiversitylibrary.org/item/56527#page/127/mode/1up]