Stems 10-100 cm, minutely glandular-pilose distally. Leaf blades 2-3.5(-8.5) × 1-2(-4.4) cm. Pedicels 1-2(-3) cm, minutely glandular-pilose. Flowers: sepals 4-6 mm, to 9 mm in fruit; petals 4-7 mm, mostly exceeding sepals. Capsules 5-10 mm, usually slightly exceeding calyx. 2n = 20(-) (Asia), 28 (Europe, Asia), 29 (Europe). Flowering spring-fall. Stream banks, low woods, marshes, meadows, occasionally cultivated areas; 100-700 m; introduced; B.C., Ont., Que.; Conn., Del., Ill., Ind., Iowa, Kans., Ky., Md., Mass., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., Tenn., Vt., Va., Wash., W.Va., Wis.; Europe; temperate Asia. Reports of Myosoton aquaticum from Louisiana appear to be based on misidentified specimens of Stellaria cuspidata Willdenow ex Schlechtendal subsp. prostrata (Baldwin) J. K. Morton.
Although occurring over a wide area, Myosoton aquaticum is often noted as rare or occasional in particular states or provinces. Very few collections of this species from the flora area were made prior to 1900; two of the first gatherings were from port areas (Baltimore, Maryland, and as a ballast plant in Philadelphia, Pennsylvania, in 1877). Its presence outside the Japanese Pavillion at the Philadelphia Centennial Grounds in 1878 (Scribner 50 and 51, MO) suggests an escape from an intentional introduction.
Perennial herb 10 cm - 1 m tall Stem: prostrate with upright or ascending tips, loosely branched, glandular-hairy above, rooting at the lower nodes. Leaves: opposite, mostly stalkless (the lower short-stalked), 2 - 8 cm long, 1 - 4 cm wide, egg-shaped to lance- egg-shaped with a pointed tip, one-veined. Inflorescence: a terminal, leafy, open cluster (cyme) of a few flowers subtended by paired bracts. Flowers: white, with five deeply notched petals seeming as ten. Stalk 1 - 3 cm long, minutely glandular-hairy. Stamens ten. Styles five, alternate with the sepals. Sepals: five, distinct, green, 4 - 6 mm long (to 9 mm in fruit), egg-shaped with a pointed tip, minutely glandular-hairy. Petals: five, white, 4 - 7 mm long, deeply notched (giving the appearance of ten petals), longer than the sepals. Fruit: a dehiscent capsule (opening by five valves), 0.5 - 1 cm long, egg-shaped, on a reflexed stalk. Seeds numerous, blackish brown, under 1 mm long, kidney-shaped, laterally compressed.
Similar species: The closely related genus Stellaria differs from Myosoton by typically having three, not five, styles.
Flowering: late May to late October
Habitat and ecology: Introduced from Europe. Occasionally found in pastures as a weed, but more often found along streams in moist soil.
Occurence in the Chicago region: non-native
Etymology: Aquaticum means "living in or near water."