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 we re examined with scanning electron microscopy (SEM). All species have trans verse to somewhat oblique simple perforation plates on end walls of vessels ; presence of tracheids could not be determined with SEM. Lateral walls pos sess scalariform pitting except for S, bigelovii, in which pitting is oppos ite. Pit membranes are generally intact, but pits on some areas of lateral walls contain remnants of pit membranes; these remnants chiefly take the fo rm of strands or threads that traverse the pit apertures. Areas of pits wit h such porose pit membranes may be regarded as incipient perforation prates , like lateral perforation plates or incipient perforation plates we have r eported in ferns. These lateral perforation plates, as well as the simple e nd wall perforation plates, may aid in rapid uptake of water during short p eriods when water is available; studies on water uptake patterns of homophy llous Selaginella species are needed to examine this possibility.