Members of the Caryophyllaceae and Hypericaceae, with similar foliage, are often mistaken for Gentianaceae, but Gentians can be readily distinguished by the sympetalous corolla, solitary style (sometimes cleft apically) or none, parietal placentation (it is free-central in Caryophyllaceae, axile in Hypericaceae), and 2 carpels. Many of our species of Gentianaceae have the flowers blue or bluish, a color absent in our Caryophyllaceae and Hypericaceae.
1. Leaves all reduced to tiny narrow scales, less than 3 (–5) mm long; stigmas decurrent on obscure style to top of the ovary.
1. Leaves all or mostly well developed; stigma distinct, not decurrent.
2. Cauline leaves in whorls of 4; corolla lobes with a conspicuous circular fringed nectar gland just below the middle; plant normally ca. 1 m or more tall.
2. Cauline leaves opposite; corolla with nectaries none or basal and without fringe; plant less than 1 m tall.
3. Corolla bright pink (rarely albino); style slender, ca. one fourth as long as the ovary or longer; stigmas linear or ovate.
4. Lobes of corolla 2–5.5 mm long, much shorter than the tube, pink (or whitish) at base; stigmas ± ovate, less than 1 mm long, less than half as long as the style; calyx lobes less than 1 mm broad.
4. Lobes of corolla 9–17 mm long, much longer than the tube, with yellow spot at base; stigmas linear (nearly or quite equaling the undivided portion of the style); calyx lobes mostly 1–2.3 mm broad.
3. Corolla blue, purplish, greenish, or white; style none or stubby, less than a fifth as long as ovary, with a large fan-like stigma, in Gentianopsis.
5. Petals 4, each with a spur ca. 2.5–4 (–5) mm long projecting at the base, greenish, often tinged with purplish, entire (not fringed).
5. Petals 5 and lacking a spur or 4 and fringed, blue, purplish, or white.
6. Pedicels mostly all distinctly longer than the flowers; flowers 4-merous, the corolla lobes fringed, spreading in sunshine; seeds covered with conspicuous papillae.
6. Pedicels none or much shorter than the flowers; flowers 5-merous, the corolla lobes entire, erect; seeds smooth.
7. Flowers 2.5–4.3 cm long, sessile or subsessile in mostly involucrate clusters; sinuses of calyx with a membranous connective between the lobes; corolla with distinct plaits [folds] between the lobes; seeds flattened and winged.
7. Flowers (1–) 1.3–2.3 cm long, on short but distinct pedicels; sinuses of calyx without membranous connective between the lobes; corolla lacking plaits between the lobes; seeds round.