"Dwarfing is a horticultural process, requiring not only skill and knowledge of plants, but great patience.  The tree selected is either a specimen dwarfed by nature or a tiny seedling.  By training, tying, pruning and grafting, the specimens are shaped and kept in miniature form.  Fine examples of the art, which the Japanese call "bon-sai," are not only curiosities.  They are replicas of ancient trees on a tiny scale.  Their wind-blown tops, projecting roots, and graceful spreading branches exactly reproduce the effect of forest giants.  Maples, bamboos, cherries, pines, hollies, oaks, azaleas, junipers, and other evergreens are employed.  They remain in the open air, most of the year.  In everything but size and hardiness, these tiny trees resemble a forest of century old specimens developed by nature, instead of by art." 1

1     "Dwarfing Horticultural Process," The Oak Creek Times and The Yampa Leader (Oak Creek, Routt County, Colorado), January 5, 1939 pg. 3.  No other citation is given for this info.