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The cottonwoods are three species of poplars in the section Aegiros of the genus Populus, native to North America, Europe and western Asia. The name is also commonly used for any species of poplar in the United States, including some poplars not in the section Aegiros, notably the Swamp cottonwood Populus heterophylla (in the section Leucoides) and the Black and Narrowleaf Cottonwoods P. trichocarpa and P. angustifolia (balsam poplars in the section Tacamahaca). Those in section Aegiros are large deciduous trees 20-45 m tall, distinguished by thick, deeply fissured bark, and triangular-based to diamond-shaped leaves, green on both sides (without the whitish wax on the undersides of balsam poplar leaves), and without any obvious balsam scent in spring. An important feature of the leaves is the petiole which is flattened sideways, so that the leaves have a particular type of movement in the wind. The aspens (Populus section Populus) share this characteristic, but not the balsam poplars.
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Nevada, USA
