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Raven, Curtis, and Evert. Biology of Plants, third edition. New York: Worth Publishing, 1981.

Stern and Kingsley. Introduction to Plant Biology, fourth edition. Dubuque, Iowa: William C. Brown Publishers, 1988.

Miller, C. N. Jr. "The origin of modern conifer families." Origin and Evolution of Gymnosperms. Ed. C. B. Beck. New York: Columbia University Press, 1988.

Campbell, Neil A. Biology, fourth edition. Menlo Park, California: Bejnamin/Cummings Publishing Company, Inc., 1996.

Gymnosperm database homepage: http://home.earthlink.net/~earlecj/