Sentence:  Instead there were the white of aspens, streaks of branch and slender trunk glistening from the green of leaves, and the darker green of oaks, and through the middle of this forest, from wall to wall, ran a winding line of brilliant green which marked the course of <WSD>cottonwoods</WSD> and willows. 
Meaning of the word marked with <WSD> tags:  A tree from one of number of species of tree in the genus "Populus" (poplars), typically growing along watercourses, with fluffy catkins.