Ludwigia peploides (Kunth) P. H. Raven subsp. montevidensis (Spreng.) P. H. Raven, water primrose, yellow water weed. Aquatic perennial herb, fibrous–rooted, aquatic phase branches floating on water to creeping and ascending on shoreline, < 40 cm tall; shoots to 300 cm long, with cauline leaves, sparsely pubescent to glabrate, ± glutinous on young growth; adventitious roots nodal.
Stems ± cylindric, < 4 mm diameter, glossy pale yellowish green, internodes to 20 mm long; the submerged stems light green tinged pink or rose, with large air cavities (aerenchyma) in cortex.
Leaves helically alternate, simple, petiolate, with stipules; stipules 2, attached to base of petiole on raised base, broadly ovate, < 1 mm long, truncate at base, fleshy below midpoint and secreting liquid on new growth, green and orangish below and reddish above midpoint; petiole hemicylindric, to 22 mm long and indistinct from leaf base, lower margins sometimes colorless and minutely serrate; blade oblanceolate to elliptic or spatulate, < 15—70 × < 5—25 mm, long–tapered at base, entire or sparsely short–ciliate on margins, acute to obtuse at tip having a small glandular point on the lower surface, conspicuously pinnately veined with principal veins slightly raised on both surfaces, surfaces glossy and glabrous or with scattered short hairs on the lower surface aging glabrescent.
Inflorescence flowers solitary and axillary (leafy raceme), essentially glabrous to sparsely pilose; pedicel erect, to 35 × 1 mm and spreading in fruit, with 2 subopposite bracteoles at base of ovary, bracteoles ovate to inverted heart–shaped, 1.5—1.7 × 1.3—1.4 mm, wet (secreting), aging purplish red, sparsely pilose, persistent in fruit.
Flower bisexual, radial, 15—23 mm across; hypanthium absent, nectary at the base of each stamen, arc–shaped mound bracketing filament on inside and adjacent to other filaments, densely short–villous; sepals 5, spreading, acute–triangular, ca. 5 × 2—2.3 mm, green with reddish margins, with 3 parallel veins, glabrous or sparsely pilose on lower surface; petals 5, obovate to fan–shaped, 11—13 mm long, bright yellow, truncate to shallowly notched at top, glabrous, pinnately veined with paired lateral veins; stamens 10 in 2 sets, unequal within a flower, sometimes some anthers not fertile; filaments 3.3—4.2 mm long, yellow; anthers exserted, dorsifixed, dithecal, (0.8—)1.6—1.8 mm long, anthers of outer whorl < inner whorl, light yellow, longitudinally dehiscent; pollen light yellow, permanently united in tetrads, sticky and held in a mass by minute threads (viscin threads); pistil 1; ovary inferior, straight, weakly 5–angled, ca. 7 × 1.5 mm, green, subglabrous with scattered short papillae, 5–chambered, each chamber packed with a single stack of many ovules; style much thicker than filaments, 6 mm long and extended above anthers, light yellow, base of style 0.8 mm diameter increasing to funnel–like top 2 mm diameter; stigma low 5–lobed with rim around.
Fruit capsule, ± indehiscent, many–seeded, ± cylindric, 29—34 × 3.5—4 mm, straight to somewhat curved, top truncate with 5 U–shaped to crescent–shaped hairy ridges (= persistent nectary), hard with seeds embedded in fruit wall, glabrescent; sepals persistent, ascending to spreading, ovate, ca. 8 × 3 mm, 3–veined; style abscised.
Seed rhomboid–polyhedron, 1.5—1.9 × ca. 1.5 mm, tan, with mostly flat rectangular faces but roundish ends where seeds touched.
A. C. Gibson