The gametophyte-sporophyte junction: unequivocal hints for two evolutionary lines of archegoniate land plants

New results have added to our knowledge about the gametophyte-sporophyte junction in bryophytes, hornworts and pteridophytes and its significance for archegoniate land plant evolution. Bryophyta (liverworts and mosses) show a uniform structure the sporophyte foot is separated from the surrounding gametophyte tissue by a placental space. At maturity the space is in most cases filled with residues of collapsed cells of gametophyte origin. The taxa with a placental space represent the bryophyte lineage of land plant evolution, indicating a common ancestor of the bryophyte groups Hepaticophytina and Bryophytina. To emphasize the separate position of Fossombronia within liverworts, the new class Fossombroniopsida is proposed. In Anthocerotophyta and Psilotopsida sporophyte haustorial cells intermingle resp. interdigitate with gametophyte transfer cells, and electron-dense fibrillar material fills the interface. In Lycopodiopsida, Equisetopsida and Pteridopsida, a direct contact exists between gametophyte and sporophyte placental layers. In some cases, sporophyte placental cells tend to interdigitate between gametophyte placental cells. No residues of collapsed gametophyte cells are found in the interface between the two generations.