3. Bidens beckii Torr. ex Spreng. (water marigold)

Megalodonta beckii (Torr. ex Spreng.) Greene

Pl. 284 g, h; Map 1151

Plants perennial herbs, submerged aquatics (except for the stem tips and heads). Stems 50–200 cm or more long, glabrous. Leaves of two dissimilar kinds, both kinds sessile or nearly so, glabrous, the submerged leaves opposite or sometimes in whorls of 3, the blade 1.5–3.5 cm long, fan-shaped to nearly circular in outline, repeatedly dichotomously dissected into narrowly linear (threadlike) segments, grading abruptly into the emergent leaves, which are 0.5–4.0 cm long, lanceolate to narrowly oblong-elliptic, unlobed, tapered to a sharply pointed tip, the margins finely to coarsely and sharply toothed. Inflorescences of solitary terminal heads, occasionally with 1 or a few additional heads from the uppermost leaf axils, the heads radiate, not nodding at fruiting. Involucre with the outer series of 5 or 6 bracts 5–8 mm long, spreading to loosely ascending, oblong to obovate, glabrous; the inner series of 7–9 bracts 7–10 mm long, ovate, glabrous. Chaffy bracts narrowly lanceolate, usually purplish-tinged at the tip. Ray florets present, 6–8, the corolla showy, 10–15 mm long, yellow. Disc florets 10–40, the corollas 5–6 mm long, light yellow. Pappus of (2–)3–6 awns 13–30 mm long (elongating as the fruit matures), these with downward-pointed barbs toward the tip, more or less spreading at fruiting. Fruits 10–15 mm long, linear, more or less circular in cross-section, brownish yellow to yellowish brown, the surfaces smooth, glabrous. 2n=36. July–October.

Uncommon, known only from a single historical collection from the city of St. Louis (northern U.S. south to Oregon, Illinois, and Maryland; Canada). Lakes, submerged aquatics.

Steyermark (1963) discussed the ambiguity in whether George Engelmann’s 1846 St. Louis record was collected in Missouri or whether it actually originated from just across the Mississippi River in the American Bottoms area of St. Clair County, Illinois. In the absence of data to the contrary, the record is accepted here. Regardless of its historical status, the species appears to have become extirpated from the St. Louis metropolitan region. This unusual species is relatively uncommon throughout its range and apparently disappears from ponds and lakes quickly following hydrological disturbances. Roberts (1985) noted that the species produces specialized modified stem segments known as turions that facilitate vegetative dispersal of plants and also act as a mechanism for overwintering. He discussed that these are morphologically similar to turions produced in another genus of submerged aquatics, Myriophyllum (Haloragaceae). Vegetatively, however, B. beckii resembles more closely the genus Ceratophyllum (Ceratophyllaceae), which is common in still waters in Missouri. Members of Ceratophyllum tend to have the dichotomously parted leaves divided fewer times than in B. beckii, and the margins tend to have minute teeth. Ceratophyllum also produces minute axillary flowers that develop into individual achenes with a spiny tip and margins.

Roberts (1985) followed Sherff (1955) in segregating the species into its own genus, Megalodonta Greene, based on its morphological adaptations to an aquatic lifestyle, differences in phytochemistry, and an unusual chromosome number. Preliminary molecular phylogenetic studies (Kimball and Crawford, 2004) did not uphold this generic segregation, although they failed to resolve details of the relationships among B. beckii and its relatives in Bidens and Coreopsis.