Elms are deciduous and semi-deciduous trees making up the genus Ulmus, family Ulmaceae, found throughout the Northern Hemisphere from Siberia to Indonesia, Mexico to Japan. They have alternate, simple, single- or doubly-serrate leaves, usually asymmetric at the base and acuminate at the apex. Elms are hermaphroditic, having perfect flowers, and which, being wind-pollinated, are without petals. The fruit is a round samara. Elm wood was valued for its interlocking grain, and consequent resistance to splitting, with significant uses in wheels, chair seats and coffins. The wood is also resistant to decay when permanently wet, and hollowed trunks were widely used as water pipes during the medieval period in Europe.