Myriophyllum ussuriense
Myriophyllum laxum
Russian water-milfoil, terrestrial water milfoil, Ussurian milfoil, Ussurian water-milfoil
loose watermilfoil, Piedmont water-milfoil
often branched, to 0.6 m. Turions present, ± brown, narrowly cylindrical, with gradual transition from foliage leaves to highly reduced turion leaves, (4–)7–12(–20)× 0.5–2(–3) mm, apex rounded to truncate;
leaves often pectinate proximally and entire to 3-fid distally, strongly appressed to axis, lanceolate to narrowly elliptic or ovate in outline, (1.5–)2–4(–6.5) × (0.2–)0.3–2(–2.5) mm;
segments 0–6(–10), longest segment 0.5–2 mm, basal segment less than or equal to 1/2 central axisof leaf, apex ± acute or rounded, brown, long-necked, ascidiate trichomes in axils present.
sometimes branched, to 1 m. Turions absent.
opposite or in whorls of 3(or 4), heteromorphic;
petiole 0–9 mm; submersed leaves usually pectinate, sometimes 2- or 3-lobed, ovate to widely ovate or trullate in outline, (1.3–)5–22(–26) × (0.3–)3–28(–35) mm, segments(0–)4–12(–14), distinctly alternate, lobed to linear-filiform, longest segment (0.5–)2–20(–25) mm; emersed leaves usually linear, spatulate, or 2- or 3-lobed, sometimes pectinate proximally, (1.7–)2.5–9(–10.5) × 0.3–3.5(–5) mm, segments (0–)2–8(–12), lobed to linear-filiform.
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.
to 12 cm;
flowers usually unisexual, rarely bisexual;
bracteoles cream to stramineous, lanceolate, elliptic, ovate, or obovate, (0.2–)0.3–0.7(–0.9) × (0.1–)0.2–0.4(–0.5) mm, margins entire, irregular, dentate, glandular, or lobed.
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.
sepals cream, elliptic to lanceolate, 0.5–0.7 × 0.2–0.5 mm;
petals persistent, cream, sometimes apically suffused with purple, widely oblanceolate, 1.2–2.5 × 0.7–1.2 mm;
stamens 8, filaments to 1.4 mm, anthers 0.9–1.8 × 0.2–0.4 mm.
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.
sepals and petals rudimentary or absent;
pistils to 0.7 mm, stigmas white, to 0.3 mm.
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.
subglobose, 4-lobed.
narrowly globose, 4-lobed.
brown, obovate, 0.8 × 0.6 mm, abaxial surface rounded, minutely tuberculate, wings and ribs absent.
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.
= [14] 21.
Myriophyllum ussuriense
Myriophyllum laxum
Plants of Myriophyllum ussuriense typically grow in a semi-terrestrial habit in shallow water or on saturated sediments to a height of 20 cm. Shoots often have swollen stem bases that taper dramatically towards the apex. In some populations, extensive production of erect shoots from rhizomes produce dense stands. The floral bracts are distinctive, being opposite or alternate and elongate with usually 2–8 relatively short segments. Dimorphism in size between staminate and pistillate flowers of M. ussuriense is distinctive. Although most populations appear to be unisexual with staminate plants predominating and pistillate plants rare (O. Ceska et al. 1986), the latter are extremely small with a vestigial perianth and are easily overlooked, indicating that monoecy may be more common than thought in this species. S. Ueno and Y. Kadono (2001) reported that seven of 80 populations of M. ussuriense in Japan had some monoecious plants. No fruit was found despite an extensive examination of available material.
Submersed plants have pectinate leaves that are extremely delicate with usually fewer than 12 straight segments. A useful characteristic of some leaves is that the central axis terminates in a right-angled bifurcation.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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