AbstractWithin the bryophytes scanning electron microscopy has revealed an abundance of surface features useful in elucidating affinities at generic and specific levels. Appraisal of the broader systematic significance of such data requires transmission electron microscope (TEM) studies of the underlying developmental processes. TEM has made an incisive contribution to the understanding of interrelationships between bryophytes, charalian green algae and tracheophytes. New ultrastructural information should be incorporated into modern schemes of bryophytic classification. The presence of proximal triplet extensions to the two flagellar bases in the spermatids of bryophytes is the only attribute at the ultrastructural level which draws mosses, hepatics and Anthocerotae together. Although cellular organization is basically similar to that in tracheophytes, an increasing list of cytological features and developmental processes peculiar to each of these groups is strongly indicative of wide phyletic differences and also rules direct lines of descent between charalian algae and vascular plants.