W. 1981. Observations on ptyxis, phenology, and trichomes in the Cycadales and thensystematic implications. Amer. J. Bot. 68: 1104–1114.———. 1981. Observations on ptyxis, phenology, and trichomes in the Cycadales and their systematics implications. American Journal of Botany 68: 1104–...