The Birch family as now understood, comprises six genera and about seventy-five species, mostly natives of the northern hemisphere. Some authors include this family with the Oak or Beech family under the name of Cupuliferae. The chief distinction is the arrangement of the pistillate flowers. The Birch family has the pistillate flowers in aments while the Oak family has the pistillate flowers subtended by an involucre which becomes a bur or cup in fruit.
Proceedings of the Iowa Academy of Sciences
©1900 Iowa Academy of Science, Inc.
Fitzpatrick, T. J. and Fitzpatrick, M. F. L.
"Betulaceae of Iowa,"
Proceedings of the Iowa Academy of Science, 8(1), 169-177.
Available at: https://scholarworks.uni.edu/pias/vol8/iss1/26