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Nymphaeaceae is a family of flowering plants, commonly called water lilies. They live as rhizomatous aquatic herbs in temperate and tropical climates around the world. The family contains five genera with about 70 known species. Water lilies are rooted in soil in bodies of water, with leaves and flowers floating on or emergent from the surface. The leaves are round, with a radial notch in Nymphaea and Nuphar, but fully circular in Victoria and Euryale.
Water lilies are a well studied clade of plants because their large flowers with multiple unspecialized parts were initially considered to represent the floral pattern of the earliest flowering plants, and later genetic studies confirmed their evolutionary position as basal angiosperms. Analyses of floral morphology and molecular characteristics and comparisons with a sister taxon, the family Cabombaceae, indicate, however, that the flowers of extant water lilies with the most floral parts are more derived than the genera with fewer floral parts. Genera with more floral parts, Nuphar, Nymphaea, Victoria, have a beetle pollination syndrome, while genera with fewer parts are pollinated by flies or bees, or are self- or wind-pollinated. Thus, the large number of relatively unspecialized floral organs in the Nymphaeaceae is not an ancestral condition for the clade.
Horticulturally, water lilies have been hybridized for temperate gardens since the 19th century, and the hybrids are divided into three groups: hardy, night-blooming tropical, and day-blooming tropical water lilies. Hardy water lilies are hybrids of Nymphaea species from the subgenus Castalia; night-blooming tropical water lilies are developed from the subgenus Lotos; and the day-blooming tropical plants arise from hybridization of plants of the subgenus Brachyceras.
Water lilies do not have surface leaves in the winter time, and therefore the gases in the rhizome lacunae access equilibrium with the gases of the sediment water. The leftover of internal pressure is embodied by the constant streams of bubbles that outbreak when rising leaves are ruptured in the spring. Methane is still penetrating the roots and rhizome in the summer, moving fast up the petioles, and goes out over the emergent leaves into the atmosphere.
