Phylogenetic analyses of land plants up through the mid 1990's generally favored a basal position for the liverworts among the various extant land plant groups. Several lines of evidence lent support from the fossil record to this contention, including, 1) the production of permanent tetrads in several modern liverworts, 2) mesofossil remains (Lower Devonian) that are reminiscent of liverworts and 3) the ultrastructural similarities between Ordovician and Silurian cryptospores and the spores of certain members of the liverwort order Sphaerocarpales. The ultrastructural similarities are, however, found in dyads, for which no modern counterparts are known among the non vascular plants. Comparisons with coeval fossil tetrads revealed few similarities with the Sphaerocarpales. More recent analyses have begun to consider the hornworts as at least an equally probable sister taxon to all other land plants. There is a conspicuous dearth of information on spore wall ultrastructure in modern hornworts. In this poster, we present information on the spore wall of representatives of four distinct genera - Notothylas, Phaeoceros, Anthoceros and Dendroceros. The specimens examined have diverse ultrastructure. Notothylas and Phaeoceros have walls composed of at least three separate layers - the inner of which is fibrillar - as well as some kind of structural modification at the equator (cingulum) that forms a zone of weakness. These similarities support preliminary molecular phylogenies that unite these genera as sister taxa. The structural details, however, differ significantly. The species of Anthoceros examined has fewer layers, no apparent equatorial zone of weakness and a fibrillar/tubular wall nearly throughout. Dendroceros has a single layered vermiculate wall around its endosporic gametophyte that is quite distinct from that of the other three genera.

Key words: Anthocerophyta, hornworts, spores, ultrastructure