Description: Small aquatic ferns, free-floating; rhizome short-creeping, usually bearing simple or branched roots, sometimes hairs.
Fronds in 2 alternate rows, green or reddish, 2-lobed, the upper lobe thick and photosynthetic with a central cavity containing a colony of the nitrogen-fixing blue-green alga Anabaena azollae, the lower lobe thin and submerged, and, when fertile, bearing 2–4 sporocarps; vernation circinate.
Sporocarps of 2 sizes, the smaller with a single megaspore, the larger with numerous microspores. Plants rarely fertile, usually multiplying by fragmentation.
Distribution and occurrence: World: 6 species, widespread in tropical & temperate regions. Australia: 2 species, all States except W.A.
Text by Peter G. Wilson
Taxon concept:
| | Key to the species | |
| 1 | Roots in fully grown plants branched with small, fine side branches | Azolla pinnata |
| Roots in fully grown plants simple with no hair-like branches | Azolla filiculoides |
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