Scanning electron microscope studies of cycad tracheids

SEM studies of pit membranes of cycad tracheids revealed porosities in some, but not most of the tracheid pits examined. Porosities, where present, are mostly varied in size and density. Porosities do not extend through pit membranes and appear limited to the first-formed portion of the primary wall. Because of inconsistency in porosity presence within a secondary xylem sample, as well as between stems and roots of a species, and among species, interpretation is difficult. Earlier reports of perforations or perforation plates in cycads are not confirmed, and may be the result of oxidative effects of maceration fluids. Forms of helical sculpture on the inner surfaces of tracheids in Encephalartos and Zamia represent a phenomenon newly reported for cycads. Helical sculpturing on secondary xylem elements of other vascular plants is usually interpreted as a xeromorphic feature, and this interpretation may be valid for cycads as well.