Any of several fast-growing North American trees of the genus Populus. Members of the willow family, cottonwoods have heart-shaped, toothed leaves and cottony seeds. The dangling leaves clatter in the wind. The eastern cottonwood (P. deltoides) has thick glossy leaves. P. canadensis is a hybrid of P. deltoides and the Eurasian black poplar (P. nigra). The Alamo, or Fremont, cottonwood (P. fremontii), is the tallest of the group. See alsopoplar.