Heterosporous ferns are a monophyletic lineage comprising five extant genera that fall into two clades: Marsileaceae, including Marsilea, Pilularia and Regnellidium; and Salviniaceae, including Azolla and Salvinia. The Marsileaceae have an amphibious habit, whereas the Salviniaceae are exclusively floating aquatics. Observations from light and scanning electron microscopy indicate that the megaspores of each of the heterosporous fern genera have a uniquely modified outer spore wall (perine) above the aperture, which is referred to as an acrolamella, and they also differ substantially in the details of their perine ultrastructure. The megaspores of Marsileaceae are dispersed freely into water and possess an ephemeral gelatinous outer perine that acts as a flexible floating apparatus. In Salviniaceae, the megaspores are retained in the sporangium wall and have either a completely (Salvinia) or partly (Azolla) alveolate outer perine, which also functions in providing buoyancy. In Azolla, parts of the indusium and the sporangium wall form, together with the megaspore body, an extraordinary megaspore apparatus, making these spores the most complex among living land plants. As with the megaspores, the microspores of Marsileaceae are freely released in water and possess a gelatinous perine layer, whereas, the microspores of Salviniaceae are retained in massulae (Azolla), or within the microsporangium (Salvinia). The evolution of mega- and microspore characters is inferred using a phylogeny obtained from an independent analysis of nucleotide sequence data. Because extant genera in both clades each exhibit multiple unique perine structures, it is not possible to definitively reconstruct spore wall ultrastructure characters in the common ancestor of heterosporous ferns. To help resolve this dilemma, approaches using comparative developmental data and the integration of fossil heterosporous ferns will be outlined.

Key words: character evolution, heterospory, Marsileaceae, Salviniaceae, sexual reproduction, spore morphology