1. Stipules present, conspicuous (though small), scarious, sometimes fused and forming a membranous collar between the leaf bases.
2. Petals absent; fruit 1-seeded, indehiscent; styles 2-forked or none and stigmas 2; leaves elliptic to narrowly obovate.
3. Plant a prostrate, matted weed, with flowers crowded in axillary clusters; leaves not red-dotted; stipules ciliate; sepals ca. 0.5–0.6 mm long.
3. Plant ± erect, with flowers on very slender terminal branches; leaves densely red-dotted, at least beneath; stipules not ciliate; sepals ca. 0.7–1 mm long.
2. Petals present (but shorter than the sepals); fruit a several-seeded capsule; styles and stigmas 3 or 5; leaves linear-filiform.
4. Styles and valves of capsule normally 5; petals white; leaves in 2 principal clusters at each node, appearing densely whorled.
4. Styles and valves of capsule normally 3; petals usually pink; leaves more clearly opposite (with partly connate stipules), but usually with axillary clusters.
1. Stipules absent (though leaf bases may meet around the stem).
5. Sepals (ca. 2–5 mm long) connate nearly or fully half their length to form a hard closely perigynous cup around the ovary and membranous fruit; fruit 1-seeded, indehiscent.
5. Sepals connate much more than half their length, or separate nearly or quite to their base, or over 2.5 cm long; fruit a few to many-seeded capsule.
6. Calyx of sepals separate nearly or quite to their base.
7. Petals clearly notched at the apex or deeply cleft (the corolla then appearing 10-parted), or rarely absent.
8. Styles 3–4 (very rarely to 6); capsules ± ovoid, 6 (–8)-cleft to the base.
8. Styles 5; capsules 10-cleft or 10-toothed, ovoid to cylindric.
9. Leaves various, but less than 1 cm broad; capsule cylindric.
9. Leaves ovate, the principal ones at least (1.5–) 2 cm broad; capsule ovoid.
7. Petals entire or toothed, but neither cleft nor merely notched at the tip.
10. Inflorescence umbellate; styles 3; capsules 6-cleft; plant withering in May or soon afterwards.
10. Inflorescence cymose, not umbellate, or flowers solitary; styles and capsules various; plant persisting beyond May.
11. Styles (as well as sepals and petals) (4–) 5, the capsules splitting by an equal number of teeth.
11. Styles 3 (fewer than sepals and petals), the capsules splitting by 3 or 6 teeth.
12. Leaves linear-subulate, the principal cauline ones subtending dense axillary clusters; plant entirely glabrous; capsule dehiscing into 3 valves.
12. Leaves ovate to elliptic or lanceolate, mostly without axillary tufts; plants puberulent at least on the stem; capsule with the 3 valves again split, resulting in a total of 6 teeth.
13. Ripe seeds minutely and regularly roughened (tuberculate) and unappendaged; leaves ovate-elliptic, acute to acuminate, but not over 7 (–9) mm long; petals shorter than sepals; annuals.
13. Ripe seeds smooth and shiny, with a pale appendage at the point of attachment; leaves lanceolate to broadly or narrowly elliptic, mostly over 10 mm long; petals exceeding sepals; perennials.
6. Calyx of sepals fused at least a fourth of their length.
14. Sepals fused ca. one-fourth to half their length (hence with prominent free tips), ± densely pilose, (1.6–) 3.5–5.5 (–7) cm long.
14. Sepals fused half their length or more, glabrous or pubescent, less than 3 cm long.
15. Calyx immediately subtended by closely appressed bracts (no naked pedicel evident between bracts and calyx).
16. Sepals each with (3–) 5–7 usually strong ribs, connate without membranous margins.
16. Sepals each with 1 (–3) main rib(s), connate by membranous ribless margins.
15. Calyx not subtended by bracts, or with at least a short pedicel visible above spreading bracts.
17. Styles 3–5 (or more), or flowers entirely staminate; calyx 10–30-nerved (or nerves obscure).
18. Calyx tubular or broadened below the middle; stems lacking glutinous zones on upper internodes, except in S. antirrhina with petals inconspicuous or lacking.
18. Calyx ± clavate, expanded above the middle; stems with glutinous zones on upper internodes and petals pink and showy (fresh flowers ca. 1 cm or more across).
17. Styles 2, the capsule opening by 4 teeth; flowers bisexual; calyx 5-nerved or very obscurely many-nerved.
19. Calyx less than 5 mm long.
19. Calyx at least 7 mm long.
20. Inflorescence crowded, the pedicels less than 5 mm long; calyx wingless.
20. Inflorescence open, the pedicels over 1 cm long; calyx with 5 green wings.