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 cottonwoods and <WSD>willows</WSD>. 
Meaning of the word marked with <WSD> tags:  Any of various deciduous trees or shrubs in the genus "Salix", in the willow family Salicaceae, found primarily on moist soils in cooler zones in the northern hemisphere.