Nelumbo is a genus of aquatic plants with large, showy flowers. Members are commonly called lotus, though the name is also applied to various other plants and plant groups, including the unrelated genus Lotus. Members outwardly resemble those in the family Nymphaeaceae ("water lilies"), but Nelumbo is actually very distant to that family. There are only two known living species of lotus; Nelumbo nucifera is native to East Asia, South Asia, Southeast Asia and probably Australia, and is better-known. It is commonly cultivated; it is eaten and used in traditional Chinese medicine. The other lotus is Nelumbo lutea, which is native to North America and the Caribbean. Horticultural hybrids have been produced between these two allopatric species. Description Ultrahydrophobicity The leaves of Nelumbo are highly water-repellent (i.e. they exhibit ultrahydrophobicity) and have given the name to what is called the lotus effect. Ultrahydrophobicity involves two criteria: a very high water contact angle between the droplet of water and the leaf surface, and a very low roll-off angle. This means that the water must contact the leaf surface at exactly one, minuscule point, and any manipulation of the leaf by changing its angle will result in the water droplet rolling off of the leaf. Ultrahydrophobicity is conferred by the usually dense layer of papillae on the surface of the Nelumbo leaves, and the small, robust, waxy tubules that protrude off each papilla. This helps reduce the area of contact between the water droplet and the leaf. Ultrahydrophobicity is said to confer a very important evolutionary advantage. As an aquatic plant with leaves that rest on the water's surface, the genus Nelumbo is characterized by its concentration of stomata on the upper epidermis of its leaves, unlike most other plants which concentrate their stomata on the lower epidermis, underneath the leaf. The collection of water on the upper epidermis, whether that be by rain, mist, or the nearby disturbance of water, is very detrimental to the leaf's ability to perform gas exchange through its stomata. Thus, Nelumbo's ultrahydrophobicity allows the water droplets to accumulate together very quickly, and then roll off of the leaf very easily at the slightest disturbance of the leaf, a process which allows its stomata to function normally without restriction due to blockage by water droplets. Thermoregulation A unique property of the genus Nelumbo is that it can generate heat, which it does by using the alternative oxidase