Myriophyllum laxum

Herbs monoecious, aquatic, sometimes forming dense stands. Stems sometimes branched, to 1 m. Turions absent. Leaves usually alternate and/or in whorls of 3(or 4), sometimes opposite or irregular, heter­omorphic; petiole 0–3 mm; submersed leaves pectinate, elliptic to obovate in outline, 9–27(–31) × 6–18(–22) mm, segments (6–)8–12(–16), linear-filiform, longest segment (4–)8–17(–21) mm, surfaces with numerous black, ascidiate trichomes; emersed leaves pectinate to pinnatifid proximally, elliptic to obovate, spatulate, or oblanceolate, (0.6–)0.7–2.3(–2.7) × (0.1–)0.2–0.7(–1) mm, with black, ascidiate trichomes scattered on sur­faces and in axils. Inflorescences to 27 cm; flowers proximally pistillate, medially bisexual, distally staminate; bracteoles cream, lanceolate to triangular, 0.3–0.8 × 0.1–0.2(–0.3) mm, margins irregularly lobed, apex often narrowly apiculate. Staminate flowers: sepals cream, lanceolate to narrowly triangular, 0.1–0.3 ×0.1–0.2 mm; petals persistent, cream, suffused with ± red at tips, obovate to oblanceolate, 1.5–1.9 ×0.6–1.1 mm; stamens 4, filaments to 1.3 mm, anthers 1.2–1.5(–1.7) × 0.2–0.4(–0.6) mm. Pistillate flowers: sepals cream, lanceolate to ovate, 0.1–0.3 × 0.1–0.2 mm; petals caducous, cream, elliptic to obovate, 0.4–0.5 × 0.1–0.3 mm; pistils (0.5–)0.7–1.1(–1.3) mm, stigmas red to ± purple, to 0.5 mm. Fruits narrowly globose, 4-lobed. Mericarps brown to purple, cylindric to narrowly ovoid, 1–1.4 × (0.4–)0.6–0.8(–1.1) mm, transversely elliptic, abaxial surface rounded to shallowly 2-angled, densely tuberculate proximal to midpoint, tubercles crowded, relatively large, rarely with 2 shallow, partial longi­tudinal wings, tubercles often obscuring wings, ribs absent.

Phenology: Flowering and fruiting Jun–Oct.
Habitat: Oligotrophic waters, lakes, ponds, streams.
Elevation: 0–150 m.

Distribution

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Ala., Fla., Ga., La., Miss., N.C., S.C.

Discussion

Myriophyllum laxum is a coastal plain species that has a very restricted range in the southeastern United States. It is most similar vegetatively to M. humile, with which it shares a delicate habit. The two have historically been reported to overlap in range in Virginia; no specimens of the former have been seen from that state. In M. laxum the submersed leaves are usually whorled but often irregular and sometimes alternate; in M. humile the leaves are usually opposite or alternate and almost never whorled. The proximal half of mericarps of M. laxum is densely covered with large, mounded tubercles; mericarps of M. humile tend to be uniformly, sparsely to densely covered with smaller tubercles and have distinct tuberculate ridges typically running the entire length of the mericarp. The floral bracts are also very different in the two species and the flowers are much smaller in M. laxum.

Myriophyllum laxum has sometimes been confused with M. heterophyllum; however, they can be dis­tinguished by the number of submersed leaf segments with M. laxum typically having 8–12 versus 12–20 for M. heterophyllum. They can also be easily distin­guished by differences in floral and fruit characters. Myriophyllum laxum also interbreeds with M. heterophyllum (M. L. Moody and D. H. Les 2010; R. A. Thum et al. 2011), producing a hybrid that is often fertile, having floral and fruit characters very similar to those observed in M. laxum, but having highly variable leaf morphology typical of both parental species.

Selected References

None.

Lower Taxa

None.