This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Annual or perennial, mainly tufted herbs, with sessile leaves, and terminal cymose or capitate, rarely axillary and solitary, white flowers. Sepals 5. Petals 5, entire or scarcely emarginate, rarely none. Stamens 10. Styles generally 3 (rarely 2-5). Ovary 1-celled, many-ovuled. Capsule globose or oblong, dehiscent at the apex by as many valves or teeth as there are styles, or twice as many. Seeds reniform-globose or compressed. [Latin, sand, in allusion to the habitat of many species.]
About 150 species, of wide geographic distribution. Type species: Arenaria serpyllifolia L. Valves of the capsule 2-cleft or 2-toothed, sometimes appearing as if double the number of the styles.
Leaves ovate or oblong.
Sepals acuminate; annual herbs of waste places.
Capsule flask-shaped, firm.
Capsule nearly cylindric, papery.
Sepals obtuse or scarcely acute; perennial; arctic.
Leaves subulate or setaceous.
Cymes very dense; stems 1' - 4' tall.
Cymes loose; stems 4'-15' tall.
Valves of the capsule entire (Genus Alsine Wahl., Alsinopsis Small).
Leaves rigid, subulate or setaceous.
Arctic or alpine, densely tufted, 1'-3' high.
Flowers s"-8" broad.
Flowers 2"-4" broad.
Neither arctic nor alpine, tufted but diffuse, 4"-16" high.
Leaves densely imbricated; pine barren species.
Leaves fascicled in the axils.
Petals longer and capsule shorter than the sepals.
Petals shorter and capsule longer than the sepals.
Leaves soft, herbaceous; narrowly linear or filiform.
Sepals prominently ribbed; southern.
Sepals not ribbed; alpine and northern.