Circumscription of the genus Medicago (Leguminosae) by seed characters.

Delimitation of Medicago from its close relatives in tribe Trifolieae subtribe Trigonellinae remains controversial. Scanning electron microscope studies of the seed surface of Medicago, Trigonella, and Melilotus showed consistent differences between Medicago and the other two genera. Smooth and rough seeds occur in all three genera, the rough seeds proving taxonomically useful. The roughness of Medicago seeds is due to rugose surface bumps (usually wrinkled, always with rounded sides), except in the monotypic section Orbiculares. The roughness of seeds of Trigonella, Melilotus, and Medicago section Orbiculares is due to tuberculate surface bumps. The structure of the Malpighian layer shows completely parallel taxonomic differences. The Malpighian cells are traversed by the light line, and the length of the portion of the cell above the light line (a) and that below the light line (b) distinguishes smooth, rugose, and tuberculate seeds. In smooth seeds (of Medicago, Melilotus, and Trigonella), a and b are ...