Individuals with white flowers may be expected in all the species in which the corolla is ordinarily blue or purplish. Interspecific hybrids occur rarely.

1. Plaits [folds] between the corolla lobes distinctly longer than the lobes, toothed or erose; calyx lobes minutely ciliate.

G. andrewsii

1. Plaits distinctly shorter than the corolla lobes, entire or toothed; calyx lobes smooth, papillose, or ciliate.

2. Stem puberulent, especially on the angles; calyx lobes linear, broader at the very base; corolla open at anthesis.

G. puberulenta

2. Stem glabrous; calyx lobes broadest above the base (or linear in G. linearis and G. rubricaulis); corolla nearly closed at anthesis.

3. Calyx lobes distinctly keeled basally on the underside and in one species also minutely ciliate; apex of plaits between the corolla lobes ± strongly erose or at least bifid; plants only in the southern half of the Lower Peninsula.

4. Leaf blades (at least the upper) (1.7-) 2.7-5 cm broad; calyx lobes and leaves not ciliate; flowers white, marked with greenish.

G. alba

4. Leaf blades less than 2 cm broad; calyx lobes (and at least the involucral leaves) densely ciliate; flowers mostly blue.

G. saponaria

3. Calyx lobes without a keel or cilia (at most minutely papillose or scabrous); apex of plaits between the corolla lobes broadly triangular, terminating in a single off-center tooth (sometimes with a much smaller subsidiary tooth on each side); plants only in the Upper Peninsula and north half of the Lower Peninsula (though farther south in Ontario).

5. Leaves deep green, linear or nearly so, not over 1 cm broad; calyx lobes green throughout, ± exposed by the spreading, narrow involucral leaves.

G. linearis

5. Leaves ± pale green, lanceolate to ovate, at least the larger ones over 1 cm broad; calyx lobes green at most toward apex, usually well concealed by the broad, enveloping involucral leaves.

G. rubricaulis