Midribs of cycad pinnae

Comparisons of midribs of three cycad genera,Chigua, Cycas, andStangeria, the only three genera characterized by midribs in the pinnae, show that those ofChigua andStangeria are very similar to each other and quite unlike those ofCycas. Midribs ofCycas include a single, median vein, and the pinnae of these species lack lateral veins. Pinnae midribs ofChigua andStangeria include several (2–5 and 2–8, respectively) longitudinal parallel veins, and dichotomizing lateral veins arising from the midrib. Pinnae of other cycads, includingZamia, with whichChigua 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 midrib condition inChigua andStangeria 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 mesarch protoxylem poles. It would appear that the midrib has been derived independently at least three times within the cycads, once in each major group.