A deciduous tree of the genus Populus (family Salicaceae) with wind-pollinated flowers borne in catkins and in some species leaf stalks (petioles) that are flattened, causing the leaves to twist in the wind, making it seem that the tree is trembling. There are 25–35 species native to the northern hemisphere, many grown for timber or ornament. Populus tremula is aspen, P. tremuloides quaking (trembling) aspen, P. alba white poplar, and P. deltoides eastern cottonwood.