Division: Pteridophyta (Filicophyta)
Class: Pteridopsida (Filicopsida)
The order Marsileales is most definitively delineated by heterospory, which is exclusive among true ferns to the Marsileales. This order also includes most of the truly aquatic ferns, although not all, Ceratopteris of the Pteridales being the most notable exception.
Although the three families of the Marsileales differ widely in morphology, recent studies have indicated that they should be considered a monophyletic grouping. Other proposals suggest a relationship between the Marsileales and the Schizeales, or between the Marsileales and the Hymenophyllales. However, the evidence is not really strong, athough there is some genetic evidence indicating a possible though ancient alliance with the Schizeales.
The probable derivation of the order would have begun with plants with chromosome counts based on n=11. First to diverge was probably Azolla, which retained the n=11 and further resulted in a group with n=12. The main stem then probably reduced to n=10. The families Salviniaceae and Marsileaceae then next diverged, with the count further reducing to n=9 in Salvinia and doubling to n=20 in the Marsileaceae. The Marsilaceae then experienced changes in some species to n=22 and n=16.
Families of Marsileales:
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This page was last revised on 11-10-1997.