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Comparison of midribs of three cycad genera, Chigua, Cycas, and Stangeria, the only three genera characterized by midribs in the pinnae, show that those of Chigua and Stangeria are very similar to each other and quite unlike those of Cycas. Midribs of Cycas include a single, median vein, and the pinnae of these species lack lateral veins. Pinnae midribs of Chigua and Stangeria include several (2-5 and 2-8, respectively) longitudinal parallel veins, and dichotomizing lateral veins arising from the midrib. Pinnae of other cycads including Zamia, with which Chigua appears to be most closely allied, exhibit evenly spaced, longitudinally parallel, dichotomizing veins, a character considered to be primitive. All veins in cycad leaves have a single mesarch protoxylem pole. The midrid condition in Chigua and Stangeria represents an advanced state in comparison to that in the leaf of the Marattiales, for example, where there is a single veined midrib but with numerous mesearch protoxylem poles. It would appear that the midrid has been derived independently at least three times within the cycads, once in each major group.
Brittonia © 1996 New York Botanical Garden Press