PlantaeDB Logo
Login Sign-up

Genus Azolla in Family Salviniaceae

In botanical taxonomy, a genus (plural genera) is a rank used to group closely related species within a family. In the hierarchy, genus sits below family and above species.

Genera are defined by shared morphological, anatomical, and genetic characteristics (for example, features of flowers, fruits, seeds, or leaves) that indicate a close evolutionary relationship among the species they contain.

Each genus can include one or more species. Examples include Rosa (roses) and Solanum (nightshades, including tomato and eggplant).

Do you wish to read more about plant taxonomy? Click here!

Genus Description

Suggest a correction!

Azolla (Lam.) belongs to the Salviniaceae and includes about seven small aquatic ferns that float on still to slow-moving freshwater, forming mats with reduced, imbricate leaves and minute roots. Its species occur across the New World from the United States to Argentina, in Asia from Japan and Korea to tropical and subtropical Asia, and in Australia and New Zealand (PPGP I, 2016; WFO, 2024; POWO, 2024). Lectotypification of the generic name falls on Azolla filiculoides (Christenhusz et al., 2011).

The genus is distinguished by free-floating plants whose leaves are bilobed: a dorsal, chlorophyllous floating lobe and a ventral, thin, sometimes anthocyanous submerged lobe that encloses a specialized cavity that houses the cyanobacterial symbiont Nostoc azollae. Roots are few and fin. Azolla is heterosporous, bearing separate microsporangia and megasporangia in sori surrounded by indurated sporocarp walls; each megasporangium produces a single megaspore equipped with float chambers that enable buoyancy and dispersal (Christenhusz & Chase, 2014).

Diversity and range center in the Neotropics and East Asia; several species are regional endemics. A. filiculoides ranges widely in temperate to warm-temperate areas and A. pinnata occurs from East and Southeast Asia to Australasia. Species typically inhabit ponds, ditches, rice paddies, and slow streams at low to moderate elevations, occasionally forming dense surface mats. Morphological variation between and within species—especially in leaf lobe shape, indumentum, sporocarp arrangement, and megaspore ornamentation—has driven ongoing taxonomic refinement (Reeves & Liew, 2012; PPGP I, 2016).

Pollination and dispersal mechanisms are those of many aquatic ferns: microspores and megaspores are water-dispersed, with buoyancy facilitated by float structures. Azolla’s biology is strongly shaped by its symbiosis: Nostoc azollae fixes nitrogen in leaf cavities, supporting rapid growth on nutrient-poor waters and conferring a key ecological role in flooded agroecosystems (PPGP I, 2016).

Recent treatments reflect a broadly accepted species set, though rank and synonymy remain debated. A. microphylla has often been included in A. pinnata or treated as varietal, and New World taxa previously recognized (e.g., A. caroliniana) are frequently synonymized under A. filiculoides or A. microphylla depending on author; alternative concepts are discussed by Reeves & Liew (2012) and PPGP I (2016). Molecular phylogenies support Azolla’s monophyly but show limited resolution among major lineages, leaving reticulate histories plausible (Baker et al., 2015).

Human relevance is notable in rice cultivation, where selected strains are used as biofertilizers and green manure; some species are cultivated in ponds and water gardens, and several have become naturalized beyond native ranges (WFO, 2024). Conservation concerns arise where invasive mats affect water quality or biodiversity; data are uneven, with some regional taxa lacking current IUCN assessments. Continued integrative research to resolve species boundaries and distribution histories, combined with coordinated management of high-risk taxa, will improve both conservation and sustainable use of this unique fern lineage.

Pick a Species to see its components: