NYMPHAEALES

the water-lily order of flowering plants, belonging to the class called dicotyledon (characterized by two seed leaves). The order is composed of eight genera and 70 species in five families, all being herbaceous (nonwoody) perennials. The order's members are found in quiet freshwater habitats throughout most of the world. The aesthetic appeal of the water lilies, especially those of the genera Nymphaea and Victoria, has led to their worldwide cultivation. The Nymphaeaceae, or water-lily family, has about 50 species. The other families in the order are the related Barclayaceae; the Cabombaceae, or fanworts; the Ceratophyllaceae, or hornworts; and the Nelumbonaceae. All plants of the Nymphaeales order are aquatic. In the genus Ceratophyllum, the plants are either rooted or free-floating but submerged; in the other genera, they are attached to pond or stream bottoms, but their leaves and flowers may be either submerged, floating, or standing above the water. They range in size from the small, fragile fanwort (Cabomba), which has floating leaves less than 0.3 cm (1 inch) in diameter, to the hornwort (Ceratophyllum), with richly branched stems reaching 3.54.5 m (1215 feet) in length, to Victoria, which has floating circular, shieldlike leaves 2 m (6.5 feet) across and flowers 40 cm (about 15 inches) in diameter. Some water lilies (Nymphaea) produce leaves 50 cm (about 18 inches) wide that, in one summer, can cover an area 2.5 m (8 feet) in diameter. Many species of water lilies are suited for pools, aquatic gardens, and aquariums, where they are prized for their attractive foliage and showy flowers . The submerged leaves, the starchy, horizontal creeping rhizomes, and the protein-rich seeds of the larger species have been used as food by humans throughout history. The petioles (leaf stalks) serve as cover for fish and holdfasts for their spawn, and hoofed wildlife also browse on the plant parts. The emergent leaves provide cover for aquatic birds, and the seeds of many species are food for fish and fowl. Several members of Nymphaeales provide an essential link in plant successionfrom pond to swamp to wet prairie. Some species of water lilies may grow so rankly as to clog lakes and irrigation ditches. Some Nymphaeales genera have extensive, buried or surficial, creeping rhizomes (rootlike horizontal stem structures). Others have short, erect, buried stems, either with or without associated floating horizontal branches. The blades of variously shaped leaves that emerge from the submerged rhizomes may spread over the surface of the water or stand above the water by means of elongate petioles. Most of the 35 species and many hybrids of Nymphaea of the Nymphaeaceae are cultivated; their flowers are the most showy of the Nymphaeales. The fragrant N. odorata, native to the eastern United States, with 13-centimetre (5-inch) white flowers, and its cultivars (horticultural varieties) are widely grown in parks, gardens, and natural ponds in warm temperate regions. The yellow water lily (Nuphar), also of the Nymphaeaceae, is noted for conspicuous sepals and tall-stemmed flowers. The genus Barclaya, with four species, constitutes the Barclayaceae. It is distinguished from the Nymphaeaceae by an extended perianth tube (combined sepals and petals) arising from the top of the ovary and by stamens that are joined basally. Barclayaceae is native to tropical Asia and Indonesia. The water shield (Brasenia schreberi) and the seven species of Cabomba, the fanworts, constitute the Cabombaceae. Several species of Cabomba are distributed in eastern and southeastern North America and in much of northern South America. The family Nelumbonaceae consists of just two species, the sacred lotus and the American lotus, and is considered by some authorities to be a separate order called Nelumbonales. The Ceratophyllaceae, consisting of the genus Ceratophyllum, differs from other families in the Nymphaeales in that the plants' ferny leaves are borne at intervals on the submerged stem. Flowers of Ceratophyllum not only lack sepals and petals, but stamens (male) and the pistil (female) are borne in separate flowers and on separate plantsa feature that is unique in this order. Pollen, liberated by higher-placed male flowers, settles in water to reach a channel in the female flower, facilitating pollen germination. All the genera in the order are perenniali.e., they live through more than one growing season. Among members of Nymphaeales, forms of vegetative reproduction occur in Nymphaea mexicana, which spreads by runners, and N. micrantha of western Africa, which produces detachable plantlets at the junction of the petiole and blade. Sexual reproduction, however, is the dominant means of propagation. Flowers arise singly from submerged nodes. Except for Ceratophyllum (in which flowers are water-pollinated) and Euryale of the Nymphaeaceae (which is self-pollinated in closed flowers), flowers rise to the surface of the water or higher and are insect-pollinated. All flowers of this order are radially symmetrical. Three types of fruits are produced in this order. A spiny nutlet is formed in Ceratophyllum, while in the Cabombaceae the single carpel matures into a follicle (opening along one side). Leathery, several-chambered berries, which eventually shed their seeds, are produced by the Nymphaeaceae and the Barclayaceae; they mature under water. The seeds of all genera are distributed passively, with most being released to float in water until they become anchored in the soil substratum. See also fanwort; hornwort; water lily.