This is our only genus of Boraginaceae with small blue flowers and smooth shiny nutlets (although the latter are ± hidden in the calyx). While the genus is a familiar and easily recognized one, the species are often difficult to distinguish. In cases of doubt, as always, try both choices. Corolla size can be difficult to measure on dried specimens, and the last few flowers produced at the end of the bloom season will be smaller than normal. There are pubescence differences that are difficult to describe but help one with experience or adequate material to compare for texture, angle, and other aspects of the hairs.

All of our species except two are Old World introductions, the showier ones as escapes from cultivation and the tiny-flowered ones as weeds.

1. Hairs on calyx all straight at the tip, ± appressed.

2. Style distinctly shorter than nutlets and calyx; calyx lobes (at least at anthesis) about as long as the tube or slightly longer; corolla limb less than 5 mm broad.

M. laxa

2. Style exceeding the nutlets and usually the calyx; calyx lobes shorter than tube; corolla limb 5-9.5 (-10) mm broad.

M. scorpioides

1. Hairs at least toward base of calyx hooked at the tip, ± spreading.

3. Pedicels (at anthesis, or certainly as fruit matures) at least as long as the calyx; corolla ca. 2.5-9 mm wide; calyx lobes ± spreading around the fruit; plant usually biennial, when well developed with several stems shortly decumbent at their bases.

4. Limb of corolla ± cupped, ca. 2.5-4 mm broad.

M. arvensis

4. Limb of corolla flat, 5-10 mm broad.

M. sylvatica

3. Pedicels shorter than calyx (even in fruit); corolla ca. 1-2 mm wide; calyx lobes erect (± parallel beyond the fruit); plant annual, with ± straight slender stems.

5. Calyx appearing 2-lipped at least when mature, the lobes unequal (2 lower lobes longer than 3 upper); corolla white; pedicels with mostly slightly spreading (not closely appressed) ± curved hairs lacking hooked tips; nutlets 1.1-1.2 mm broad.

M. verna

5. Calyx lobes equal or subequal; corolla blue or yellowish turning blue; pedicels with some hooked hairs or with all hairs closely appressed and straight; nutlets ca. 0.7-0.8 mm broad.

6. Stems bearing flowers only on upper part, above any leaves or bracts; pedicels with only very closely appressed straight hairs; stems with only straight-tipped hairs; corolla yellow when young, turning blue.

M. discolor

6. Stems bearing flowers nearly to the base, the lowest ones subtended by bracts (or leaves); pedicels and stems mostly with some spreading hook-tipped hairs; corolla blue.

M. stricta