The Order Marsileales was formerly known as the Hydropteridales, but by modern rules of botanic nomenclature, the name has been changed. Formerly, the species in this order were all placed in the same family for fairly superficial reasons of heterosporosity and aquatic habitat, but modern analysis has borne out their relationship.
The ferns of this order vary radically in form from one another, and do not look particularly fern-like. All are heterosporous, with megaspores that may be regarded as analogous to female spores, and microspores that may be regarded as analogous to male spores, although this is not a technically correct definition.
Species of the families Azollaceae and Salviniaceae are natant (floating), while those of the family Marsileaceae are rooted. However, the natant species may temporarily grow on wet mud during times of low water, and the Marsileaceae may grow as emergent species, depending on species and location.
There is a well-known fossil member of the Marsileales, Hydropteris (incertae sedis).
Individual specimen entries are published in the sample database supplied with The Compleat Botanica for species or varieties of this supra-generic taxon.
For a description of the methodology followed in establishing this hierarchy see the note Nomenclature used in The Compleat Botanica.
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