1. Fruit, even when immature, covered with hooked spines; stipules deeply toothed or lacerate.

  2. Stem glabrous or nearly so; fruit glabrous between the spines.

M. polymorpha

  2. Stem densely pubescent;  fruit ± pubescent between the spines and weakly glandular on the surface.

M. minima

1. Fruit without spines; stipules at most shallowly toothed.

  3. Corolla 1.8-2.2 (-2.5) mm long, yellow; plant low and ± prostrate or slightly ascending; fruit ± reniform, 1-seeded, nearly black when mature.

M. lupulina

  3. Corolla (7-) 8-10 (-11) mm long, blue-purple or (less often) yellow, cream, or variegated (white in albinos); plant erect; fruit curved to spirally coiled, several-seeded, green when mature.

M. sativa