SEM study of xylem macerations of Ceratopteris thalictroides (L.) Brongn. (
Parkeriaceae, or Pteridaceae, tribe Ceratopterideae) has co-occurring vesse
l elements and tracheids in rhizormes and probably also in roots, a conditi
on newly reported for ferns. Vessel elements with helical thickening in the
root have perforation plates that consist of elliptical holes in primary w
alls, with no thinning or discontinuation of the gyres of secondary wall ba
nds; groups of perforations alternate with smaller zones of intact primary
wall. Perforations of this type are not visible with a light microscope, an
d thus the perforation plates differ from those previously figured for vasc
ular 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 r
elation to the perforation plates. End wall facets of rhizome vessels that
are perforation plates, have the same morphology as lateral walls; some per
foration plates also occur on lateral walls. Weblike pit membranes that con
tain pores of various sizes are present on some perforation plates and in t
he transitions between perforation plates and intact lateral wall pitting.
The presence of clearly differentiated vessels in Ceratopteris shows that v
essels are present even in a submersed aquatic fern. The xylem of Ceratopte
ris opens new questions about co-occurrence of vessel elements and tracheid
s in ferns, relative morphology, distribution, and abundance of the two cel
l types, and functional and evolutionary significance of vessel element pre
sence in a fern that has maximal moisture availability. (C)2000 Elsevier Sc
ience B.V All rights reserved.