SEM studies of vessels in ferns 14. Ceratopteris

SEM study of xylem macerations of Ceratopteris thalictroides (L.) Brongn. (Parkeriaceae, or Pteridaceae, tribe Ceratopterideae) has co-occurring vessel elements and tracheids in rhizormes and probably also in roots, a condition newly reported for ferns. Vessel elements with helical thickening in the root have perforation plates that consist of elliptical holes in primary walls, with no thinning or discontinuation of the gyres of secondary wall bands; groups of perforations alternate with smaller zones of intact primary wall. Perforations of this type are not visible with a light microscope, and thus the perforation plates differ from those previously figured for vascular plants [Bailey, I.W., 1944. Am. J. Pot. 31, 421-428] in which helical elements have perforation plates in which the bands show alterations with relation to the perforation plates. End wall facets of rhizome vessels that are perforation plates, have the same morphology as lateral walls; some perforation plates also occur on lateral walls. Weblike pit membranes that contain pores of various sizes are present on some perforation plates and in the transitions between perforation plates and intact lateral wall pitting. The presence of clearly differentiated vessels in Ceratopteris shows that vessels are present even in a submersed aquatic fern. The xylem of Ceratopteris opens new questions about co-occurrence of vessel elements and tracheids in ferns, relative morphology, distribution, and abundance of the two cell types, and functional and evolutionary significance of vessel element presence in a fern that has maximal moisture availability. (C)2000 Elsevier Science B.V All rights reserved.