Most species in the salicaceae (willow) family are large trees or shrubs, including cottonwoods and aspens, but some, often those in high elevation regions, are small, just a few inches tall. Leaves are usually alternate and often arranged spirally. Flowers are produced in narrow or branched clusters, and they usually contain only pistils or only stamens, on different plants. Almost all US salicaceae species are in just two genera, populus (cottonwood, or poplar) and salix (willow).