Family: Water Fern [Salviniaceae]


Azolla

The Azollaceae Family has now been subsumed into this brand-new family, the Salviniaceae Family.

The Water Ferns (Azollas) number about half-a-dozen in the World but only one appears in the British Isles, Azolla Feliculoides, and that is an escape from garden ponds (usually a throw-away). Many Azollas are semi-tropical. Water Fern (Azolla Feliculoides) appears similar to Duckweed in that it floats and carpets still freshwater to such an extent that oxygen has no chance to diffuse into the water from the air, and as a result kills fish. It is a controlled weed that is gaining increased prominence in the British Isles from garden pond escapees. It may often grow with Duckweeds of various kinds and or Wollfia.

It grows in spring with remarkable speed, doubling in mass in just a few days. This remarkable ability is bestowed upon it by it being in a mutually beneficial symbiotic relationship with a cyanobacterium (or blue-algae) called Anabaena Azollae which fixes nitrogen for it enabling the phenomenal growth rate. The cyanobacteria are in bright green filamentous strands.

Each individual plant of Azolla Feliculoides consists of a short branched stem with minute angular overlapping leaves which are at first bright green. The plants have pendulous roots that dangle in the water whilst the plant floats on water. The plants clump together to form a continuous carpet on the water resembling floating moss. The angular leaves are bright green at first but exposed to strong sunlight or heat-stress develop a pinkish, reddish, orangish or purplish colouration at the edges due to deoxyanthocyanins.

Symbiotic relationships between cyanobacteria (algae) and fungi are well known, the result being called a lichen, but this is the only symbiotic relationship between an aquatic fern and algae known.

Azollas are able to fix nitrogen, unlike a great many other plants (except the Pea Family which can also fix nitrogen). It needs phosphorus fertiliser in order to grow, which is usually supplied in ample needs by run-off from farm-land. It dies back in low temperatures in Winter, but may survive by means of submerged buds. The roots dangle free into the depths of the shallow waters.

Dredged or removed mechanically from shallow waters it can be used as a source of compost rich in available nitrogen and phosphorus.



[AZOLLA] Water Fern

Water Fern (Azola filiculoides). Photo: © RWD

Family: Water Fern [Salviniaceae]

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