Ferns are non-flowering plants that need moist habitats, but very few kinds actually have adapted to live in the water - with the exception of a few species in the genus Azolla. These aquatic ferns have slender, branched stems with bilobed leaves that resemble the leaves of cedars. As with other ferns, reproduction occurs through the production of spores, but the plant may spread very rapidly because the stems also break easily and each fragment becomes a new plant. These plants can double their weight in just a few days. A population may proliferate into a mat of floating plants, so sometimes it is called “duckweed fern.”
Leaves of the plant may become very reddish if growing in bright sunlight or in the presence of an excess of nutrients (possibly from pollution). This is due to the production of chemicals called anthocyanins. Ponds, muddy banks, and standing water in streams may be carpeted with a red (or green) layer of these ferns that can hide deeper water beneath.
A specific cyanobacterium, Anabaena azollae, occurs in cavities inside of the leaves and has a mutualistic relationship with the Azolla. Almost all samples of Azolla examined under a microscope will reveal this filamentous cyanobacterium (formerly called blue-green algae) within the fern. The photo below shows a torn section of Azolla with the filaments exposed.
This photo shows greater detail of the Anabaena. The Anabaena accomplishes nitrogen fixation (the conversion of nitrogen from the air into ammonia, which can be used by living organisms). In effect, the Anabaena serves as a producer of fertilizer within the Azolla plant!
The photo is of Anabaena separated from the Azolla. The fertilizing relationship was understood long ago in agricultural history, although the actual mechanisms have been studied only recently. In southeast Asia, Azolla was grown in rice paddies as a means of fertilizing the crops. Sometimes called "green manure," it has been estimated that the use of Azolla more than doubles rice production. Further, there is no need for crop rotation or the addition of other fertilizers to the soil, otherwise done at considerable cost. The plant also has been used as a feed for stock and as a control for mosquitoes because it closes the surface of the water and does not allow the larvae to breathe (leading to another common name--mosquito fern). On the other hand, the mat decreases production within the aquatic system and lowers oxygen levels in the water. This can have negative effects on other aquatic life, so Azolla may be considered a water weed.