SEM studies on vessels of the homophyllous species of Selaginella

Metaxylem vessel elements from macerated xylem of rhizomes and rhizophores of Selaginella arizonica, Selaginella bigelovii, and Selaginella oregana were examined with scanning electron microscopy (SEM). All species have transverse to somewhat oblique simple perforation plates on end walls of vessels; presence of tracheids could not be determined with SEM. Lateral walls possess scalariform pitting except for S. bigelovii, in which pitting is opposite. Pit membranes are generally intact, but pits on some areas of lateral walls contain remnants of pit membranes; these remnants chiefly take the form of strands or threads that traverse the pit apertures. Areas of pits with such porose pit membranes may be regarded as incipient perforation plates, like lateral perforation plates or incipient perforation plates we have reported in ferns. These lateral perforation plates, as well as the simple end wall perforation plates, may aid in rapid uptake of water during short periods when water is available; studies on water uptake patterns of homophyllous Selaginella species are needed to examine this possibility.