There are many species of the aspen, most of which attain considerable size. Their foliage and wood greatly resemble each other, and most of them are of very rapid growth, but are equalled if not excelled by far more valuable timber trees, and hence in this country are not valued so much for timber, because for the same labor we obtain a much better article. The seeds are covered with a cotton-like down, which becomes a great nuisance by being continually blown over everything, so that for shade, when planted near a house, the male tree should always be preferred.