For Cycas
Johnson & Wilson. 1990. In Kramer, K.U., and P.S. Green. 1990. Pteridophytes and Gymnosperms. In K. Kubitzki, ed., The families and genera of vascular plants. Springer-Verlag, Berlin. Google Scholar
Jones, D.L. 1993. Cycads of the world. Smithsonian Institution Press, Washington, DC. 312 pp. Google Scholar