Vessels have hitherto been reported only in a few ferns ((Pteridium, Marsilea() on the basis of light microscopy. These ferns occur in habitats markedly seasonal with respect to moisture availability. Light microscopy cannot adequately resolve presence of pit membranes in pits or perforation plates of vessels, but SEM reveals pit membranes in vessels prepared by means of maceration and sectioning. Care must be taken to identify any artifacts in membrane absence, tearing, and distortion. Nevertheless, we can report with certainty vessel elements in which end walls not only lack pit membranes but have perforations different in size and shape from the pits of lateral walls (Astrolepis, Dicranopteris, Microgramma, Polystichum, Vittaria, Woodsia(). All of these experience prolonged drought. In addition, other fern genera of dry habitats have vessels with perforation plates morphologically like lateral wall except that pit membrane remnants (often porose, weblike, or vestigial) are present in perforations whereas nonperforated pit membranes occur on lateral walls.

Key words: ferns, pit membrane, tracheids, vessels, xylem