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 fleshy herbs. Stems prostrate, creeping, usually little branched; leaves opposite, sessile, numerous. Flowers axillary, solitary, perfect, yellow, on slender bracted peduncles. Calyx narrowly obconic, its 4 segments narrow. Petals conspicuous, surpassing the calyx-segments. Stamens 4; filaments elongated, very slender; anthers ovoid. Ovary 4-celled; united styles filiform, elongated; stigma 4-lobed; ovules numerous. Capsules club-shaped, curved at the base at a right angle to the peduncle, about as long as the persistent calyx, crowned by a 4-lobed stylopodium. [Derivation as in Ludwigia.]
Two species of the southeastern United States, the first typical.
Peduncles much longer than the leaves; calyx-lobes linear-lanceolate.
Peduncles shorter than the leaves; calyx-lobes lanceolate.
Ludivigia arcuata Walt. Fl. Car. 89. 1788.
Creeping or floating, rooting at the nodes, glabrous or somewhat appressed-pubescent, little branched, 3'-12' long. Leaves opposite, oblanceolate, sessile, leathery, smooth, obtusish or acute at the apex, narrowed at the base, 6"-12" long; flowers 6"-10" broad on filiform, 2-bracted peduncles much longer than the leaves; calyx-lobes linear-lanceolate, acuminate, shorter than the obovate petals; filaments and filiform style about 2" long; capsule club-shaped, somewhat curved, glabrous, 4"-5" long, about equalling the calyx-lobes.
Similar to the preceding species, creeping, glabrous. Leaves oblong-oblanceolate, acutish at the apex, narrowed to the sessile or nearly sessile base; flowers about 1/2' broad, on slender peduncles shorter than the leaves; calyx-lobes lanceolate to ovate-lanceolate; filaments about 1" long; filaments and stout style about 1" long; capsule a little longer than the calyx-lobes.