populus monilifera

from The Collaborative International Dictionary of English v.0.48

Cottonwood \Cot"ton*wood`\ (-w[oo^]d`), n. (Bot.)
   An American tree of the genus {Populus} or poplar, having the
   seeds covered with abundant cottonlike hairs; esp., the
   {Populus monilifera} and {Populus angustifolia} of the
   Western United States.
   [1913 Webster]