XVI International Botanical Congess

Phylogenetic studies based on molecular methods and on comparative morphology indicate that vascular plants are monophyletic. The group shares a common origin with bryophytes, and although the exact nature of this relationship remains unclear, hornworts and mosses are most probably the closest relatives. Within vascular plants there is a basic dichotomy between lycophytes (lycopsids) and euphyllophytes (ferns, horsetails, seed plants). This relationship has strong support from comparative morphology, the fossil record, and many, but not all, molecular studies. Whereas relationships among lycophytes are comparatively well resolved, there is still much conflict among basal elements of the euphyllophyte clade. Filicales, seed plants, and horsetails are clearly monophyletic, but their relationships with each other and with various extinct groups are poorly resolved. Phylogenetic difficulties are exacerbated by the rapid diversification of euphyllophytes during the Late Devonian.