Azolla nilotica

Azolla nilotica is a medium-sized floating fern from the family Salviniaceae, found naturally in the Nile River and across eastern and central Africa. It grows up to 32 cm long, with a horizontal, branched, hairy rhizome about 2 mm thick. Roots spring in bundles from the nodes, unlike other Azolla species, which have single roots. Each leaf has a green upper lobe up to 2 mm long, with a rounded or broadly pointed tip, a knobby middle, and a broad translucent edge. The lower lobe is colorless, smaller, thinner, and contains a cavity housing a symbiotic cyanobacterium (variously called *Anabaena azollae*, *Nostoc azollae*, or *Trichormus azollae*) that fixes nitrogen into nitrate. The plant is heterosporous, producing two types of spores: larger macrospores and minute microspores. Sporocarps grow in groups of four in the axils of older leaves, and can be any mix of macrosporocarps and clusters of microsporocarps (massulae). Unlike other *Azolla* species, *A. nilotica* never turns red, even late in the growing season.

The species name *nilotica* refers to its collection from the Nile. German botanist Georg Heinrich Mettenius first published the name in 1865, crediting French botanist Joseph Decaisne. Its distribution spans eastern Africa from southern Sudan and South Sudan through the Democratic Republic of Congo, Gabon, Rwanda, Burundi, Uganda, Kenya, Tanzania, Malawi, Mozambique, Zambia, and Zimbabwe. Late Holocene fossil spores have been found in the Nile Delta among *Cyperus papyrus*, and it may have gone extinct there when large papyrus swamps were drained for agriculture.

The fern grows in stagnant or slow-moving water—temporary pools, waterholes, and lake edges—and can survive on drying mud. It occurs from sea level up to 1650 m in altitude, but cannot tolerate temperatures below 10 °C, so it is unlikely to naturalize outside the tropics. Because of its high nitrate content, it is used as a green fertilizer, like other *Azolla* species, and is increasingly accepted for feeding livestock, especially in Nigeria and other West African countries.