The Azaleas of North America, by Alfred Rehder (forming the second portion of A Monograph of Azalea, Rhododendron Subgenus Anthodendron, by Ernest Henry Wilson and Alfred Rehder; Publ. Arnold Arboretum No. 9, 1921), is the last and really only thorough revision of the group. But the nomenclatural rules followed by Rehder differed somewhat from those now in force, so that two widespread species cannot retain the names under which they appear in Rehder’s account.