Plants annual, winter annual, or perennial. Taproots usually slender, perennial taxa often rhizomatous, rooting at nodes. Stems prostrate to ascending or erect, simple or branched, terete or 4-angled. Leaves sometimes connate basally into sheath, often sessile; blade 1-veined, linear or lanceolate to ovate or deltate, succulent (S. crassifolia [gemmae], S. fontinalis, S. humifusa, and S. irrigua) or not, apex acute or obtuse. Inflorescences terminal, open cymes, rarely axillary (S. alsine, S. americana) or umbellate (S. umbellata), or terminal or axillary solitary flowers; bracts paired (1 in S. dicranoides), foliaceous, scarious and reduced, or absent. Pedicels erect, sometimes reflexed in fruit, glabrous or pubescent, not glandular. Flowers usually bisexual (S. dicranoides unisexual); perianth and androecium hypogynous or weakly perigynous; hypanthium cup- or disc-shaped; sepals (4-)5, distinct, green, occasionally purple tinged (S. irrigua) or red proximally (S. pallida), lanceolate to ovate-triangular, 2-12 mm, herbaceous (rarely coriaceous), margins often white, scarious, apex acute, acuminate, or obtuse, not hooded; petals (1-)5 or absent, white (sometimes translucent in S. borealis), not clawed, blade apex 2-fid usually for 3- 5 its length (S. holostea occasionally laciniate); nectaries at base of filaments opposite sepals usually present, disc sometimes prominent; stamens (1-)5 or 10 or absent, arising from nectariferous disc (prominent in S. dicranoides and S. irrigua) at ovary base; filaments distinct; staminodes absent; styles [2-]3(-5), capitate to clavate, 0.2-7 mm, glabrous proximally; stigmas [2-]3(-5), terminal or subterminal, papillate (30×). Capsules globose to conic, opening by 3 or 6, occasionally 4, 8, or 10 ascending to recurved valves; carpophore present or absent. Seeds (1-)3-20+, yellow-brown to dark brown, globose to ellipsoid, laterally compressed, rarely shiny, papillate or rugose, rarely smooth, marginal wing absent, appendage absent. x = 10, 11, 12, 13, 15.
Fls solitary in the forks of the stem, or in terminal cymes; sep 5; pet 5, bifid, often deeply so, or lacking; stamens mostly 10, sometimes fewer; styles mostly 3(4) but 5 in S. aquatica; ovules numerous; capsule ordinarily dehiscent by twice as many valves as there are styles, but in S. aquatica with 5 apically notched or shortly bifid valves; low annual or perennial herbs with mostly rather small, opposite, exstipulate lvs; נmost commonly=13. (Alsine, Myosoton) 100+, widespread, esp. N. Temp.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.