This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", 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..
Aquatic perennial herbs, with slender rootstocks. Leaves floating, petioled, ovate or orbicular, deeply cordate, entire or repand, or the primary ones different; flowers yellow or white, polygamous, umbellate at the summit of filiform stems at the bases of the petioles, or axillary, often accompanied by a cluster of thick elongated root-like tubers. Calyx 5-parted. Corolla nearly rotate, deeply 5-cleft, the lobes induplicate-valvate in the bud. sometimes fimbriate on the margins, glandular at the base. Stamens 5, inserted on the base of the corolla; filaments short; anthers sagittate, versatile. Ovary 1-celled; style short or none; stigma 2-lamellate. Capsule ovoid or oblong, indehisccnt or irregularly bursting. Seeds numerous or few, smooth or rough. [Greek, like Nymphaea.]
About 20 species, widely distributed in temperate and tropical regions. The following are the only ones known to occur in North America. Type species: Nymphoides ftava Hill.
Flowers white, accompanied by tufts of root-like tubers; native species. Floating leaves 1'-2' long; flowers 3"-6" broad; seeds smooth.
1 .N. lacunosum.
Floating leaves 2 -6' long; flowers 6"-10" broad; seeds rough.
2. N. aquaticum.
Flowers not accompanied by tufts of tubers; corolla bright yellow, 1 broad or more; introduced species.
3. N. nymphaeoides.
Rootstock buried in the mud, the roots long and fibrous. Primary leaves membranous, submerged, short-petioled; stems (stolons) filiform, greatly elongated, sometimes 10° long, bearing a short-petioled floating leaf, an umbel of flowers and a cluster of narrow tubers at its summit, or some of the floating leaves on petioles as long as the stems; floating leaves ovate-orbicular, purplish beneath, 1'-2' long, the basal sinus narrow or broad; pedicels slender; tubers linear-conic, 1/2'-1' long; flowers nearly white, 3"-6" broad; style none; capsule ovoid, covered by the connivent calyx-segments; seeds numerous, smooth.
In ponds. Nova Scotia to Florida, west to Ontario, Minnesota and Louisiana. July-Aug.
126. 1803. Limnanthemum trachyspermum A. Gray, Man. Ed.
Acad. Sci. 9: 12. 1889. Nymphoides aquaticum Kuntze, Rev. Gen. Pl.
Similar to the preceding species but stouter and larger. Floating leaves cordate-orbicular, thick, entire or repand, 2'-6' long, spongy, and with the petioles and stolons densely covered with minute pits; primary leaves spatulate; pedicels slender, 1-3' long; tubers linear-oblong, thicker; corolla white, 6"-10" broad; style none; seeds rough; capsule longer than the calyx.
In ponds, southern New Jersey and Delaware to Florida and Texas. May-Aug.
Menyanthes nymphaeoides L. Sp. Pl. 145. 1753.
Limnanthemum nymphaeoides Hoffm. & Link, Fl. Port. 1: 344. 1809.
Stems long, stout, creeping or buried in the mud, ascending to the surface of the water, branched, the upper nodes bearing a petioled leaf and a cluster of flowers, or the upper internodes very short and their leaves apparently opposite. Petioles stout, mostly longer than the ovate-orbicular firm blades, which are 2'-4' broad; pedicels stout, becoming 3'-6' long, not accompanied by tufts of tubers; flowers bright yellow, 1 in diameter, or more; corolla segments short-fringed; seeds with fimbriate margins.
Naturalized in ponds, District of Columbia. Native of Europe and Asia. May-July.