1. Nelumbo Adans. (lotus)

Plants perennial, herbaceous aquatics, with extensive, branching, spongy rhizomes, rooting at the nodes and bearing large, banana-shaped tubers (produced in autumn), producing milky latex. Leaves spirally alternate, mostly long-petiolate, the petiole to 3 m or more, relatively stiff and stout, reaching the water surface or emergent to about 80 cm, sometimes prickly. Stipules apparently absent, but perhaps represented by small, sheathing scales at the petiole bases. Leaf blades 20–60(–80) cm in diameter, centrally peltate, more or less circular, the margin entire but slightly undulate, usually with a shallow sinus containing a small, broadly triangular, blunt to sharp point, usually also with a narrow, pale to yellowish or brownish differentiated band, the upper surface flat (in floating blades) to moderately concave (in emergent blades) about the central attachment point (visible as a pale, scarlike mark), glabrous, often somewhat glaucous, the undersurface pale but with small brown spots, the main veins numerous, palmate from the petiolar attachment, mostly branched dichotomously 2 or more times, raised on the blade undersurface, connected by a network of finer crossveins. Inflorescences of solitary flowers, these long-stalked directly from the nodes of the rhizome, emergent (usually slightly overtopping the emergent leaves), hypogynous, more or less perfect, actinomorphic. Perianth showy, of numerous, spirally arranged, overlapping tepals; the outermost 3–5 tepals 0.8–1.4 cm long, more or less ovate, somewhat concave, often green or greenish-tinged; grading into the median tepals, these 6.0–8.5 cm long, obovate; grading into the innermost ones, these somewhat shorter, lanceolate, grading into the stamens. Stamens numerous, arranged in a dense, overlapping spiral, the filament 6–12 mm long, slender, the anther 9–18 mm long, attached at the base, orangish yellow, with a terminal appendage, this 3–7 mm long, club-shaped, to narrowly club-shaped, usually somewhat arched or curved, pale yellow. Pistils several to numerous, simple, free from one another, embedded in deep pits along the upper surface of a large, flat-topped, conic, spongy, expanded receptacle, this continuing to expand as the fruits mature, becoming nodding, dark brown, and somewhat leathery to hardened at fruiting. Style 1 per ovary, very short, the stigma protruding through an apical pore in the receptacle pit, more or less disc-shaped, slightly concave and with a central pore. Ovule 1 per carpel, the placentation apical. Aggregate fruits consisting of nuts embedded in the long-persistent, greatly expanded receptacle, these 9–15 mm long, ovoid to nearly spherical, sometimes somewhat flattened, indehiscent, hard-walled, brown to nearly black. Two species, Northern Hemisphere, but in the Old World also south to Australia.

The flowers of Nelumbo are unusual in both their morphology and function. The conic, flat-topped, enlarged receptacle is thermogenic, that is, it has been shown to generate heat and to moderate the flower temperature to about 31 degrees Celsius independent of the ambient temperature. This apparently is related to two main functions, increased release of volatile, odoriferous compounds produced by the flower tissues and the breakdown of starch in the receptacle through an alternate respiratory pathway (see Vogel and Hadacek [2004] and Watling et al. [2006] for reviews). The stamens produce unusual, club-shaped appendages that also contain starch. Although it might be suspected that these appendages would function as a lure for pollinators, foraging of beetles or other insects upon these structures has not been observed in nature. Thus, these structures may have lost their former function in attracting insects. C. R. Robertson (1889a) recorded a series of more than 30 insect visitors to flowers of N. lutea in Illinois, mainly bees and flies, but later observers (Sohmer and Sefton, 1978; Schneider and Buchanan, 1980) also noted visitation by various beetles. Unlike in the true water lilies, in which pollination usually involves entrapment of beetles in flowers that close overnight, no such specialized pollination mechanism has been observed to function in Nelumbo.

Today, the principal use of Nelumbo in the United States is as an ornamental aquatic in ponds and lakes. Cultivation of lotus for various uses goes back to prehistoric times. However, the plants spread very aggressively, forming a dense, thick layer of intertwined rhizomes and tubers, and can become a nuisance, both because of excessive shading of the water and interference in canoe and other small boat traffic. Hall and Penfound (1944) estimated that a small patch in Tennessee enlarged radially by more than 45 feet during a single growing season. Steyermark (1963) noted that the rhizomes and tubers are eaten by wildlife, especially beavers, and that some waterfowl eat the fruits. He further noted that the plants form excellent cover and habitat for fish, waterfowl, and other animals. Native Americans made extensive use of N. lutea for food, boiling or baking the starchy tubers and cooking the young foliage. The immature fruits were eaten raw prior to hardening of the fruit wall; mature fruits were parched, dehulled, and the seeds cooked or ground into a flour (Steyermark, 1963; Moerman, 1998). Native foods enthusiasts continue to use the foliage, tubers, and young fruits in similar fashion (J. Phillips, 1998). Tubers of N. nucifera are cooked and sliced as a starchy vegetable in Asian cuisines. They also are dried and powdered for use in teas and herbal tonics.

The seeds of Nelumbo in their hard-walled fruits are extremely long-lived. Steyermark (1963) noted that in Missouri seeds of N. lutea had germinated in the bottoms of St. Louis County when areas that had been farmed for more than 40 years were allowed to go fallow. He also stated that other seeds that had been dormant for as long as 200 years retained the ability to germinate. However, since that time, other studies involving N. nucifera have greatly extended knowledge of seed longevity in the group. Shen-Miller et al. (2002) excavated seeds from deposits in a dry lakebed that were carbon-dated to be up to 466 years old and were able to successfully germinate them and to grow plants from them (although plants exhibited reduced vigor). Even more amazingly, Shen-Miller and her associates had earlier managed to germinate lotus seeds up to 1,300 years old. The seeds also are unusual in two other features: the embryo is green within the seed prior to germination, and the radicle portion of the embryo aborts prior to maturity, so the seedling never develops an initial taproot.

Although several additional taxa have been described, the prevailing view has been that Nelumbo contains only two morphologically similar species native to the New World and the Old World respectively. Borsch and Barthlott (1994) analyzed morphological variation in the genus and concluded that these two taxa should be considered subspecies of a single species. However, their careful analysis did disclose a pattern of subtle differences between the two taxa for a number of features, mostly overlapping quantitative differences in characters such as anther appendage length and perianth size. Additionally, the two taxa have been resolved in molecular studies involving both restriction fragment analysis of the mitochondrial genome (Kanazawa et al. (1998) and rbcL sequence data (C. L. Anderson et al., 2005). The situation requires more intensive study, but for now it seems prudent to continue to recognize two species (Wiersema, 1997).